U.S. patent application number 13/884279 was filed with the patent office on 2013-10-24 for compositions.
This patent application is currently assigned to IMERYS MINERALS LIMITED. The applicant listed for this patent is John Claude Husband, David Robert Skuse, Per Svending. Invention is credited to John Claude Husband, David Robert Skuse, Per Svending.
Application Number | 20130280545 13/884279 |
Document ID | / |
Family ID | 43431457 |
Filed Date | 2013-10-24 |
United States Patent
Application |
20130280545 |
Kind Code |
A1 |
Husband; John Claude ; et
al. |
October 24, 2013 |
Compositions
Abstract
Compositions such as filled and coated papers may include
microfibrillated cellulose and inorganic particulate material.
Inventors: |
Husband; John Claude; (St.
Austell, GB) ; Svending; Per; (Kungalv, SE) ;
Skuse; David Robert; (Truro, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Husband; John Claude
Svending; Per
Skuse; David Robert |
St. Austell
Kungalv
Truro |
|
GB
SE
GB |
|
|
Assignee: |
IMERYS MINERALS LIMITED
Par, Cornwall
GB
|
Family ID: |
43431457 |
Appl. No.: |
13/884279 |
Filed: |
November 9, 2011 |
PCT Filed: |
November 9, 2011 |
PCT NO: |
PCT/GB11/52181 |
371 Date: |
July 10, 2013 |
Current U.S.
Class: |
428/464 ;
106/506; 162/181.1; 162/181.2; 162/181.3; 162/181.6; 162/181.8;
428/452; 428/535; 428/537.5 |
Current CPC
Class: |
D21H 17/25 20130101;
D21H 19/52 20130101; D21H 19/22 20130101; Y10T 428/31703 20150401;
D21H 11/04 20130101; D21H 23/48 20130101; D21H 19/38 20130101; Y10T
428/31982 20150401; Y10T 428/31993 20150401; D21H 17/63 20130101;
D21H 27/10 20130101; D21H 19/34 20130101 |
Class at
Publication: |
428/464 ;
162/181.1; 162/181.3; 162/181.2; 162/181.8; 162/181.6; 106/506;
428/537.5; 428/535; 428/452 |
International
Class: |
D21H 17/63 20060101
D21H017/63 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 15, 2010 |
GB |
1019288.8 |
Aug 5, 2011 |
GB |
1113559.7 |
Claims
1-26. (canceled)
27. A paper product comprising a co-processed microfibrillated
cellulose and inorganic particulate material composition, wherein
the paper product has: i) a first tensile strength greater than a
second tensile strength of the paper product devoid of the
co-processed microfibrillated cellulose and inorganic particulate
material composition; ii) a first tear strength greater than a
second tear strength of the paper product devoid of the
co-processed microfibrillated cellulose and inorganic particulate
material composition; and/or iii) a first burst strength greater
than a second burst strength of the paper product devoid of the
co-processed microfibrillated cellulose and inorganic particulate
material composition; and/or iv) first sheet light scattering
coefficient greater than a second sheet light scattering
coefficient of the paper product devoid of the co-processed
microfibrillated cellulose and inorganic particulate material
composition; and/or v) a first porosity less than a second porosity
of the paper product devoid of the co-processed microfibrillated
cellulose and inorganic particulate material composition; and/or
vi) a first z-direction (internal bond) strength greater than a
second z-direction (internal bond) strength of the paper product
devoid of the co-processed microfibrillated cellulose and inorganic
particulate material composition.
28. The paper product of claim 27, further comprising a paper
coating composition which comprises a functional coating for liquid
packaging, barrier coatings, or printed electronics
applications.
29. The paper product of claim 27, further comprising a second
coating comprising a polymer, a metal, an aqueous composition, or a
combination thereof.
30. The paper product of claim 27, further having a first moisture
vapour transmission rate (MVTR) lower than a second MVTR of the
paper product devoid of the co-processed microfibrillated cellulose
and inorganic particulate material composition.
31. The paper product of claim 27, wherein the paper comprises from
about 0.5 wt. % to about 50 wt. % of the co-processed
microfibrillated cellulose and inorganic particulate material
composition.
32. The paper product of claim 27, wherein the paper comprises from
about 25 wt. % to about 35 wt. % of the co-processed
microfibrillated cellulose and inorganic particulate material
composition.
33. The paper product of claim 27, which is coated with a paper
coating composition, where the coated paper product has a first
gloss greater than a second gloss of the coated paper product
devoid of the co-processed microfibrillated cellulose and inorganic
particulate material composition.
34. The paper product of claim 27, further comprising a coating
composition which comprises a co-processed microfibrillated
cellulose and inorganic particulate material composition.
35. The paper product of claim 34, wherein the inorganic
particulate material composition of the coating composition is
kaolin.
36. The paper product of claim 27, further comprising one or more
functional coatings on the paper product.
37. The paper product of claim 36, wherein the one or more
functional coatings is a polymer, a metal, an aqueous composition,
or a combination thereof.
38. The paper product of claim 36, wherein the one or more
functional coatings is a liquid barrier layer.
39. The paper product of claim 36, wherein the functional coating
is a printed electronics layer.
40. A papermaking composition comprising a co-processed
microfibrillated cellulose and inorganic particulate material
composition, wherein the papermaking composition has: (i) a first
cationic demand lower than a second cationic demand of the
papermaking composition devoid of the co-processed microfibrillated
cellulose and inorganic particulate material composition; and/or
(ii) a first, first-pass retention greater than a second,
first-pass retention of the papermaking composition devoid of the
co-processed microfibrillated cellulose and inorganic particulate
material composition; and/or (iii) a first ash retention greater
than a second ash retention of the papermaking composition devoid
of the co-processed microfibrillated cellulose and inorganic
particulate material composition.
41. A papermaking composition comprising a co-processed
microfibrillated cellulose and inorganic particulate material
composition, wherein the papermaking composition is substantially
devoid of retention aids.
42. A paper product comprising a co-processed microfibrillated
cellulose and inorganic particulate material composition, wherein
the paper product has a first formation index lower than a second
formation index of the paper product devoid of the co-processed
microfibrillated cellulose and inorganic particulate material
composition.
43. The paper product of claim 27, wherein the inorganic
particulate material comprises an alkaline earth metal carbonate or
sulphate, such as calcium carbonate, magnesium carbonate, dolomite,
gypsum, a hydrous kandite clay such as kaolin, halloysite or ball
clay, an anhydrous (calcined) kandite clay such as metakaolin or
fully calcined kaolin, talc, mica, huntite, hydromagnesite, ground
glass, perlite or diatomaceous earth, or combinations thereof.
44. The paper product of claim 27, wherein the microfibrillated
cellulose has a d.sub.50 ranging from about 25 .mu.m to about 250
.mu.m, more preferably from about 30 .mu.m to about 150 .mu.m, even
more preferably from about 50 .mu.m to about 140 .mu.m, still more
preferably from about 70 .mu.m to about 130 .mu.m, and most
preferably from about 50 .mu.m to about 120 .mu.m.
45. The paper product of claim 27, wherein the microfibrillated
cellulose has a monomodal particle size distribution.
46. The paper product of claim 27, wherein the microfibrillated
cellulose has a multimodal particle size distribution.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to compositions, such as
filled and coated papers, comprising microfibrillated cellulose and
inorganic particulate material.
BACKGROUND OF THE INVENTION
[0002] Inorganic particulate materials, for example an alkaline
earth metal carbonate (e.g. calcium carbonate) or kaolin, are used
widely in a number of applications. These include the production of
mineral containing compositions which may be used in paper
manufacture, paper coating, or polymer composite production. In
paper and polymer products such fillers are typically added to
replace a portion of other more expensive components of the paper
or polymer product. Fillers may also be added with an aim of
modifying the physical, mechanical, and/or optical requirements of
paper and polymer products. Clearly, the greater the amount of
filler that can be included, the greater potential for cost
savings. However, the amount of filler added and the associated
cost saving must be balanced against the physical, mechanical and
optical requirements of the final paper or polymer product. Thus,
there is a continuing need for the development of fillers for paper
or polymers which can be used at a high loading level without
adversely effecting the physical, mechanical and/or optical
requirements of paper products. There is also a need for the
development of methods for preparing such fillers economically.
[0003] The present invention seeks to provide alternative and/or
improved fillers for paper or polymer products which may be
incorporated in the paper or polymer product at relatively high
loading levels whilst maintaining or even improving the physical,
mechanical and/or optical properties of the paper or polymer
product. The present invention also seeks to provide an economical
method for preparing such fillers. As such, the present inventors
have surprisingly found that a filler comprising microfibrillated
cellulose and an inorganic particulate material can be prepared by
economical methods and can be loaded in paper or polymer products
at relatively high levels whilst maintaining or even improving the
physical, mechanical and/or optical properties of the final paper
or polymer product.
[0004] Further, the present invention seeks to address the problem
of preparing microfibrillated cellulose economically on an
industrial scale. Current methods of microfibrillating cellulosic
material require relatively high amounts of energy owing in part to
the relatively high viscosity of the starting material and the
microfibrillated product, and a commercially viable process for
preparing microfibrillated cellulose on an industrial scale has
hitherto before proved elusive.
SUMMARY OF THE INVENTION
[0005] According to a first aspect, the present invention is
directed to an article comprising a paper product comprising a
co-processed microfibrillated cellulose and inorganic particulate
material composition and one or more functional coatings on the
paper product.
[0006] According to a second aspect, the present invention is
direct to a paper product comprising a co-processed
microfibrillated cellulose and inorganic particulate material
composition, wherein the paper product has: (i) a first tensile
strength greater than a second tensile strength of the paper
product devoid of the co-processed microfibrillated cellulose and
inorganic particulate material composition; (ii) a first tear
strength greater than a second tear strength of the paper product
devoid of the co-processed microfibrillated cellulose and inorganic
particulate material composition; and/or iii) a first burst
strength greater than a second burst strength of the paper product
devoid of the co-processed microfibrillated cellulose and inorganic
particulate material composition; and/or iv) a first sheet light
scattering coefficient greater than a second sheet light scattering
coefficient of the paper product devoid of the co-processed
microfibrillated cellulose and inorganic particulate material
composition; and/or v) a first porosity less than a second porosity
of the paper product devoid of the co-processed microfibrillated
cellulose and inorganic particulate material composition; and/or
vi) a first z-direction (internal bond) strength greater than a
second z-direction (internal bond) strength of the paper product
devoid of the co-processed microfibrillated cellulose and inorganic
particulate material composition.
[0007] According to a third aspect, the present invention is
directed to a coated paper product, wherein the coating comprises a
co-processed microfibrillated cellulose and inorganic particulate
material composition, and wherein the coated paper product has: i.
a first gloss greater than a second gloss of the coated paper
product comprising a coating composition devoid of the co-processed
microfibrillated cellulose and inorganic particulate material
composition; and/or ii. a first stiffness greater than a second
stiffness of the coated paper product comprising a coating
composition devoid of the co-processed microfibrillated cellulose
and inorganic particulate material composition; and/or iii. a first
barrier property which is improved compared to a second barrier
property of the coated paper product comprising a coating
composition devoid of the co-processed microfibrillated cellulose
and inorganic particulate material composition.
[0008] According to a fourth aspect, the present invention is
directed to a polymer composition comprising a co-processed
microfibrillated cellulose and inorganic particulate material
composition.
[0009] According to a fifth aspect, the present invention is
directed to a papermaking composition comprising a co-processed
microfibrillated cellulose and inorganic particulate material
composition, wherein the papermaking composition has a first
cationic demand lower than a second cationic demand of the
papermaking composition devoid of the co-processed microfibrillated
cellulose and inorganic particulate material composition.
[0010] According to a sixth aspect, the present invention is
directed to a papermaking composition comprising a co-processed
microfibrillated cellulose and inorganic particulate material
composition, wherein the papermaking composition is substantially
devoid of retention aids.
[0011] According to a seventh aspect, the present invention is
directed to a paper product comprising a co-processed
microfibrillated cellulose and inorganic particulate material
composition, wherein the paper product has a first formation index
lower than a second formation index of the paper product devoid of
the co-processed microfibrillated cellulose and inorganic
particulate material composition.
DETAILED DESCRIPTION OF THE INVENTION
[0012] As used herein, "co-processed microfibrillated cellulose and
inorganic particulate material composition" refers to compositions
produced by the processes for microfibrillating fibrous substrates
comprising cellulose in the presence of an inorganic particulate
material as described herein.
[0013] Unless otherwise stated, "functional coating" refers to a
coating or coatings applied to the surface of a paper product to
modify, enhance, upgrade and/or optimize one or more non-graphical
properties of said paper product (i.e., properties primarily
unrelated to the graphical properties of the paper). In
embodiments, the functional coating is not one which comprises a
co-processed microfibrillated cellulose and inorganic particulate
material composition. For example, the functional coating may be a
polymer, a metal, an aqueous composition, a liquid barrier layer or
a printed electronics layer.
Paper Products
[0014] In certain embodiments, the paper products comprise a
co-processed microfibrillated cellulose and inorganic particulate
material composition incorporated into the paper pulp (e.g., in the
paper base as a filler composition). For example, the paper
products may comprise at least about 0.5 wt. %, at least about 5
wt. %, at least about 10 wt. %, at least about 15 wt. %, at least
about 20 wt. %, at least about 25 wt. %, at least about 30 wt. %,
or at least about 35 wt. % of a co-processed microfibrillated
cellulose and inorganic particulate material composition, based on
the total weight of the paper product. Generally, the paper
products will comprise no more than about 50 wt. %, for example, no
more than about 45 wt. %, or no more than about 40 wt. % of a
co-processed microfibrillated cellulose and inorganic particulate
material composition. In a particular embodiment, the paper product
comprises from about 25% to about 35% wt. % of a co-processed
microfibrillated cellulose and inorganic particulate material
composition. The fibre content of the co-processed microfibrillated
cellulose and inorganic particulate material composition may be at
least about 2 wt. %, at least about 3 wt. %, at least about 4 wt.
%, at least about 5 wt. %, at least about 6 wt. %, at least about 7
wt. %, at least about 8 wt. %, at least about 10 wt. %, at least
about 11 wt. %, at least about 12 wt. %, at least about 13 wt. %,
at least about 14 wt. % or at least about 15. wt. %. Generally, the
fibre content of the co-processed microfibrillated cellulose and
inorganic particulate material composition will be less than about
25 wt. %, for example, less than about 20 wt. %.
[0015] After co-processing to form the co-processed
microfibrillated cellulose and inorganic particulate material
composition, additional inorganic particulate may be added (e.g.,
by blending or mixing) to reduce the fibre content of the
co-processed microfibrillated cellulose and inorganic particulate
material composition.
[0016] In particular embodiments, the paper products comprising a
co-processed microfibrillated cellulose and inorganic particulate
material composition have a lower porosity as compared to the paper
products produced without (i.e., devoid of) the co-processed
microfibrillated cellulose and inorganic particulate material
composition. For instance, the porosity of the paper products
comprising a co-processed microfibrillated cellulose and inorganic
particulate material composition may have a porosity about 10% less
porous, about 20% less porous, about 30% less porous, about 40%
less porous, or about 50% less porous than a porosity of the paper
products devoid of the co-processed microfibrillated cellulose and
inorganic particulate material composition. Such a reduction in
porosity may provide improved coating hold-out for coated paper
products comprising a co-processed microfibrillated cellulose and
inorganic particulate material. Such a reduction in porosity may
enable a reduction in coat weight for coated paper products
comprising a co-processed microfibrillated cellulose and inorganic
particulate material without compromising the physical and/or
mechanical properties of the coated paper product.
[0017] In an embodiment, porosity is determined using a Bendtsen
Model 5 porosity tester in accordance with SCAN P21, SCAN P60, BS
4420 and Tappi UM 535.
[0018] In other embodiments, the paper products comprising a
co-processed microfibrillated cellulose and inorganic particulate
material composition have a tensile strength about 2% greater,
about 5% greater, about 10% greater, about 15% greater, about 20%
greater, or about 25% greater than a tensile strength of the paper
products devoid of a co-processed microfibrillated cellulose and
inorganic particulate material composition (e.g., the paper product
has the same filler loading).
[0019] In further embodiments, the paper products comprising a
co-processed microfibrillated cellulose and inorganic particulate
material composition have a tear strength about 2% greater, about
5% greater, about 10% greater, about 15% greater, about 20%
greater, or about 25% greater than a tear strength of the paper
products devoid of a co-processed microfibrillated cellulose and
inorganic particulate material composition (e.g., the paper product
has the same filler loading). Such low porosity, strong paper
products may comprise functional papers such as gaskets, grease
proof papers, linerboard for plasterboard, flame retardant papers,
wall papers, laminates, or other functional paper products.
[0020] In an embodiment, tensile strength is determined using a
Testometrics tensile tester according to SCAN P16.
[0021] In further embodiments, the paper products comprising a
co-processed microfibrillated cellulose and inorganic particulate
material composition have a z-direction (internal bond) strength
about 2% greater, about 5% greater, about 10% greater, about 15%
greater, about 20% greater, or about 25% greater than a z-direction
(internal bond) strength of the paper products devoid of a
co-processed microfibrillated cellulose and inorganic particulate
material composition (e.g., the paper product has the same filler
loading).
[0022] In an embodiment, z-direction (internal bond) strength is
determined using a Scott bond tester according to TAPPI T569.
[0023] In certain embodiments, the paper products comprising a
co-processed microfibrillated cellulose and inorganic particulate
material composition may be coated. Particular embodiments of the
coated paper products comprising a co-processed microfibrillated
cellulose and inorganic particulate material composition may have
an increased gloss as compared to the coated paper product devoid
of the co-processed microfibrillated cellulose and inorganic
particulate material composition. For example, the coated paper
products comprising a co-processed microfibrillated cellulose and
inorganic particulate material composition may have a gloss about
5% greater, about 10% greater, or about 20% greater than the coated
paper products devoid of the co-processed microfibrillated
cellulose and inorganic particulate material composition.
[0024] In an embodiment, gloss is determined in accordance with
TAPPI method T 480 om-05 (Specular gloss of paper and paperboard at
75 degrees).
[0025] In other embodiments, the coated paper products comprising a
co-processed microfibrillated cellulose and inorganic particulate
material composition may have improved print properties such as
print gloss, snap, print density, picking speed or percent missing
dots.
[0026] In other embodiments, the coated paper products comprising a
co-processed microfibrillated cellulose and inorganic particulate
material composition may have a lower moisture vapour transmission
rate (MVTR, tested in accordance with a modified version of TAPPI
T448 using silica gel as the desiccant and a relative humidity of
50%) as compared to the coated paper product devoid of the
co-processed microfibrillated cellulose and inorganic particulate
material composition. For example, the coated paper products
comprising a co-processed microfibrillated cellulose and inorganic
particulate material composition may have a MVTR about 2% less,
about 4% less, about 6% less, about 8% less, about 10% less, about
12% less, about 15% less, or about 20% less than the coated paper
products devoid of the co-processed microfibrillated cellulose and
inorganic particulate material composition (e.g., the coated paper
product has the same filler loading).
[0027] In certain embodiments, the paper products comprising a
co-processed microfibrillated cellulose and inorganic particulate
material composition may serve as a base for functional coatings
such as coatings for liquid packaging, barrier coatings, and
coatings for printed electronics. The paper products comprising a
co-processed microfibrillated cellulose and inorganic particulate
material composition provide a smooth surface for the functional
coatings to be applied on. For example, the paper products may
include a barrier coating comprising a polymer, a metal, an aqueous
composition (e.g., a water-based barrier layer), or a combination
thereof.
[0028] The aqueous composition may comprise one or more of the
inorganic particulate materials described herein. For example, the
aqueous composition may comprise kaolin, such as platy kaolin or
hyper-platy kaolin. By `platy` kaolin is meant kaolin a kaolin
product having a high shape factor. A platy kaolin has a shape
factor from about 20 to less than about 60. A hyper-platy kaolin
has a shape factor from about 60 to 100 or even greater than 100.
"Shape factor", as used herein, is a measure of the ratio of
particle diameter to particle thickness for a population of
particles of varying size and shape as measured using the
electrical conductivity methods, apparatuses, and equations
described in U.S. Pat. No. 5,576,617, which is incorporated herein
by reference. As the technique for determining shape factor is
further described in the '617 patent, the electrical conductivity
of a composition of an aqueous suspension of orientated particles
under test is measured as the composition flows through a vessel.
Measurements of the electrical conductivity are taken along one
direction of the vessel and along another direction of the vessel
transverse to the first direction. Using the difference between the
two conductivity measurements, the shape factor of the particulate
material under test is determined.
[0029] In some embodiments, the paper products comprising a
co-processed microfibrillated cellulose and inorganic particulate
material composition provide a low permeability surface for
application of the functional coatings such that there is little or
no penetration of the functional coating into the paper product.
Thus, thinner, fewer, and/or non-polymeric functional coatings
might be used to achieve a desired function (e.g., barrier
function). In certain embodiments, the coated papers products
comprising a co-processed microfibrillated cellulose and inorganic
particulate material composition may have improved oil resistance
(as measured using an oil based-solution of Sudan Red IV in dibutyl
phthalate using an IGT printing unit) as compared to the coated
paper product devoid of the co-processed microfibrillated cellulose
and inorganic particulate material composition. For example, the
coated paper products comprising a co-processed microfibrillated
cellulose and inorganic particulate material composition may have
an oil resistance which is about 2% greater, about 4% greater,
about 6% greater, about 8% greater, or about 10% greater than the
coated paper products devoid of the co-processed microfibrillated
cellulose and inorganic particulate material composition (e.g., the
coated paper product has the same filler loading).
Improved Paper Making and Sheet Properties
[0030] In some embodiments, the paper products comprising a
co-processed microfibrillated cellulose and inorganic particulate
material composition allow for improved processes for making such
paper products. For instance, by including a co-processed
microfibrillated cellulose and inorganic particulate material
composition in the paper furnish, the wet end processing of the
paper base may not require pre-treatment (e.g., addition of
cationic polymers). In addition, as compared to a paper furnish
including microfibrillated cellulose, a paper furnish including a
co-processed microfibrillated cellulose and inorganic particulate
material composition has lower or no change in cationic demand,
improved retention, and improved formation. In some embodiments in
which retention is improved by the co-processed microfibrillated
cellulose and inorganic particulate material composition used in
the paper product, use of retention aids may be reduced or
eliminated and damage to the paper products resulting from the
retention aids may be avoided.
[0031] Cationic demand of a sample of papermaking furnish is
indicated by the amount of highly charged cationic polymer required
to neutralize its surface. A streaming current test may be used to
determine cationic demand, based on the amount of cationic titrant
(e.g., poly-DADMAC) required to reach a zero signal. Another way to
determine the endpoint is by evaluating the zeta potential after
each incremental addition of titrant. Another strategy for
determining cationic demand is to mix the sample with a known
excess of cationic titrant, filter to remove the solids, and then
back-titrate to a color endpoint (colloidal titration). In
embodiments, the cationic demand of a papermaking furnish
comprising the co-processed microfibrillated cellulose and
inorganic particulate material composition is comparable to or less
than the cationic demand of a papermaking furnish devoid of the
co-processed microfibrillated cellulose and inorganic particulate
material composition (e.g., the paper furnish has the same filler
loading).
[0032] In an embodiment, cationic demand (also known as `anionic
charge`) is measured using a Mutek PCD 03 Titrator in accordance
with the method described below in the `Examples`.
[0033] Retention is a general term for the process of keeping fine
particles and fibre fines within the web of paper as it is being
formed. First-pass retention gives a practical indication of the
efficiency by which these fine materials are retained in the web of
paper as it is being formed. In certain embodiments, the first-pass
retention of a paper furnish comprising the co-processed
microfibrillated cellulose and inorganic particulate material
composition is greater, for example, at least about 2% greater,
about 5% greater, or about 10% greater than a paper furnish devoid
of the co-processed microfibrillated cellulose and inorganic
particulate material composition (e.g., the paper furnish has the
same filler loading). In an embodiment, first-pass retention is
determined on the basis of the solids measurement in the headbox
(HD) and in the white water (WW) tray and is calculated according
to the following formula:
Retention<[(HB.sub.solids-WW.sub.solids)/HB.sub.solids].times.100
[0034] Ash retention (as determined by incineration) during paper
formation may be improved in paper products formed from a paper
furnish comprising the co-processed microfibrillated cellulose and
inorganic particulate material composition compared to a paper
furnish devoid of the co-processed microfibrillated cellulose and
inorganic particulate material composition (e.g., the paper furnish
has the same filler loading). In embodiments, as retention during
paper formation formed from a paper furnish comprising the
co-processed microfibrillated cellulose and inorganic particulate
material composition is at least about 5%, at least about 10%, at
least about 15%, at least about 20%, or at least about 25% greater
than a paper furnish devoid of the co-processed microfibrillated
cellulose and inorganic particulate material composition (e.g., the
paper furnish has the same filler loading).
[0035] In an embodiment, ash retention is determined following the
same principles as first-pass retention, but based on the weight of
the ash component in the headbox (HB) and in the white water (WW)
tray, and is calculated according to the following formula:
Ash retention=[(HB.sub.ash-WW.sub.ash)/HB.sub.ash].times.100
[0036] Paper formation is the resulting non-uniform distribution of
fibers, fiber fragments, mineral fillers, and chemical additives on
the paper forming web. Formation may be characterized by the
small-scale basis weight variation in the plane of the paper sheet.
Another way of describing formation is the variability of the basis
weight of paper. The uneven structure of paper may be seen with the
naked eye at length scales ranging from fractions of a millimeter
to a few centimeters. In certain embodiments, the formation index
(PTS) of a paper furnish comprising the co-processed
microfibrillated cellulose and inorganic particulate material
composition is at least about 5% less, about 10% less, about 15%
less, about 20%, or about 25% less than a paper furnish devoid of
the co-processed microfibrillated cellulose and inorganic
particulate material composition (e.g., the paper furnish has the
same filler loading).
[0037] In an embodiment, formation index (PTS) is determined using
the DOMAS software developed by PTS in accordance with the
measurement method described in section 10-1 of their handbook,
DOMAS 2.4 User Guide'.
[0038] In other embodiments, a paper board product comprising a
co-processed microfibrillated cellulose and inorganic particulate
material composition may have improved foldability and/or crack
resistance.
[0039] Paper products comprising a co-processed microfibrillated
cellulose and inorganic particulate material composition also may
have a combination of improved sheet properties. For example, the
paper product sheets comprising a co-processed microfibrillated
cellulose and inorganic particulate material composition have
improved strength properties and improved formation. Without being
bound by a particular theory, such a combination is surprising
because it is believed that additional refining or fibrillation
undesirably damages paper formation due to reduced stability that
leads to a propensity to flocculate, but may increase paper sheet
strength.
[0040] In other embodiments, the paper product sheets comprising a
co-processed microfibrillated cellulose and inorganic particulate
material composition have improved tensile strength, tear strength
and z-direction strength (internal bond). This is surprising since
normally in pulp refining, as tensile strength increases, tear
strength and/or z-directional strength will decrease. For example,
paper product sheets comprising a co-processed microfibrillated
cellulose and inorganic particulate material composition may have a
tensile strength which is at least about 2% greater, at least about
3% greater, at least about 4% greater, at least about 5% greater,
at least about 6% greater, at least about 7% greater, at least
about 8% greater, at least about 9%, at least about 10% greater, at
least about 12% greater, at least about 15% greater, or at least
about 20% greater than paper product sheets devoid of the
co-processed microfibrillated cellulose and inorganic particulate
material composition (e.g., the paper product sheet has the same
filler loading). In other embodiments, paper product sheets
comprising a co-processed microfibrillated cellulose and inorganic
particulate material composition may have a tear strength which is
at least about 5% greater, at least about 10% greater, at least
about 15% greater, at least about 20% greater, or at least about
25% greater than paper product sheets devoid of the co-processed
microfibrillated cellulose and inorganic particulate material
composition (e.g., the paper product sheet has the same filler
loading). In other embodiments the paper product sheets comprising
a co-processed microfibrillated cellulose and inorganic particulate
material composition have a combination of improved tensile
strength and improved tear strength. For example, paper product
sheets comprising a co-processed microfibrillated cellulose and
inorganic particulate material composition may have a tensile
strength which is from about 2% to about 10% greater than paper
product sheets devoid of the co-processed microfibrillated
cellulose and inorganic particulate material composition, and a
tear strength from about 5% to about 25% greater than paper product
sheets devoid of the co-processed microfibrillated cellulose and
inorganic particulate material composition.
[0041] In an embodiment, tear strength is determined in accordance
with TAPPI method T 414 om-04 (Internal tearing resistance of paper
(Elmendorf-type method).
[0042] In other embodiments, the paper product sheets comprising a
co-processed microfibrillated cellulose and inorganic particulate
material composition have improved tensile strength and improved
scatter (i.e., optical) properties, e.g., sheet light scattering
and sheet light absorption. Again, this is surprising since
normally, as tensile strength increases, sheet light scatter
decreases. In certain embodiments the paper product sheets
comprising a co-processed microfibrillated cellulose and inorganic
particulate material composition may have a sheet light scattering
coefficient (in m.sup.2kg.sup.-1, measured using filters 8 and 10)
which is at least about 2% greater, at least about 3% greater, at
least about 4% greater, at least about 5% greater, at least about
6% greater, at least about 7% greater, at least about 8% greater,
at least about 9% greater, or at least about 10% greater than paper
product sheets devoid of the co-processed microfibrillated
cellulose and inorganic particulate material composition (e.g., the
paper product sheet has the same filler loading). In other
embodiments the paper product sheets comprising a co-processed
microfibrillated cellulose and inorganic particulate material
composition have a combination of improved tensile strength and/or
improved tear strength, and improved light scattering. For example,
paper product sheets comprising a co-processed microfibrillated
cellulose and inorganic particulate material composition may have a
tensile strength which is from about 2% to about 10% greater than
paper product sheets devoid of the co-processed microfibrillated
cellulose and inorganic particulate material composition, and/or a
tear strength from about 5% to about 25% greater than paper product
sheets devoid of the co-processed microfibrillated cellulose and
inorganic particulate material composition, and a sheet light
scattering coefficient (in m.sup.2kg.sup.-1, measured using filters
8 and 10) which is from about 2% to about 10% greater, for example,
from about 2% to about 5% greater than paper product sheets devoid
of the co-processed microfibrillated cellulose and inorganic
particulate material composition (e.g., the paper product sheet has
the same filler loading).
[0043] In an embodiment, sheet light scattering and absorption
coefficients are measured using reflectance data from an Elrepho
instrument: R inf=reflectance of stack of 10 sheets, Ro=reflectance
of 1 sheet over a black cup, and these values and the substance
(gm.sup.-2) of the sheet are inputted into the Kubelka-Munk
equations described in "Paper Optics" by Nils Pauler, (published by
Lorentzen and Wettre, ISBN 91-971-765-6-7), p. 29-36.
[0044] Bursting strength is widely used as a measure of resistance
to rupture in many kinds of paper. In certain embodiments, the
paper product sheets comprising a co-processed microfibrillated
cellulose and inorganic particulate material composition may have a
burst strength which is at least about 5% greater, at least about
10% greater, at least about 15% greater, at least about 20%
greater, or at least about 25% greater than paper product sheets
devoid of the co-processed microfibrillated cellulose and inorganic
particulate material composition (e.g., the paper product sheet has
the same filler loading).
[0045] In an embodiment, Burst Strength is determined using a
Messemer Buchnel burst tester according to SCAN P 24.
[0046] In certain embodiments, such improved paper product sheet
properties may be achieved in paper product sheets comprising a
co-processed microfibrillated cellulose and inorganic particulate
material composition including microfibrillated cellulose having a
d.sub.50 ranging from about 25 .mu.m to about 250 .mu.m, more
preferably from about 30 .mu.m to about 150 .mu.m, even more
preferably from about 50 .mu.m to about 140 .mu.m, still more
preferably from about 70 .mu.m to about 130 .mu.m, and most
preferably from about 50 .mu.m to about 120 .mu.m. In particular
embodiments, the microfibrillated cellulose of the co-processed
microfibrillated cellulose and inorganic particulate material
composition has a high steepness (as defined below) directed
towards a desired d.sub.50. In one embodiment, a steep particle
size distribution of the microfibrillated cellulose may be produced
by microfibrillation of the fibrous substrate comprising cellulose
in the presence of the inorganic particulate material in a batch
process in which the resulting co-processed microfibrillated
cellulose and inorganic particulate material composition having the
desired microfibrillated cellulose steepeness may be washed out of
the micrifibrillation apparatus with water or any other liquid.
[0047] In certain embodiments, the microfibrillated cellulose of
the co-processed microfibrillated cellulose and inorganic
particulate material composition has a monomodal particle size
distribution. In other embodiments, the microfibrillated cellulose
of the co-processed microfibrillated cellulose and inorganic
particulate material composition has a multimodal particle size
distribution produced by, for example, less or partial
microfibrillation of the fibrous substrate comprising cellulose in
the presence of the inorganic particulate material.
Coatings
[0048] In certain embodiments, the coatings may comprise a
co-processed microfibrillated cellulose and inorganic particulate
material composition. The coatings comprising a co-processed
microfibrillated cellulose and inorganic particulate material
composition may also be used as functional papers such as those
used for liquid packaging, barrier coatings, or printed electronics
applications. For example, the functional coating may be a barrier
layer, e.g., a liquid barrier layer, or the functional coating may
be a printed electronics layer.
[0049] The coating comprising a co-processed microfibrillated
cellulose and inorganic particulate material composition may be
applied to a paper product to produce a paper product or paper
coating having greater strength properties (e.g., tensile strength,
tear strength and stiffness), greater gloss, and/or improved print
properties (e.g., print gloss, snap, print density, or percent
missing dots). For example, the paper product coated with a coating
comprising a co-processed microfibrillated cellulose and inorganic
particulate material composition may have a tensile strength about
5% greater, about 10% greater, or about 20% greater than a tensile
strength of the paper product coated with a coating devoid of a
co-processed microfibrillated cellulose and inorganic particulate
material composition. In certain embodiments, the paper product
coated with a coating comprising a co-processed microfibrillated
cellulose and inorganic particulate material composition may have a
tear strength about 5% greater, about 10% greater, or about 20%
greater than a tear strength of the paper product coated with a
coating devoid of a co-processed microfibrillated cellulose and
inorganic particulate material composition. In certain embodiments,
the paper product coated with a coating comprising a co-processed
microfibrillated cellulose and inorganic particulate material
composition may have a stiffness about 5% greater, about 10%
greater, or about 20% greater than a stiffness of the paper product
coated with a coating devoid of a co-processed microfibrillated
cellulose and inorganic particulate material composition. In some
embodiments, the paper product coated with a coating comprising a
co-processed microfibrillated cellulose and inorganic particulate
material composition may have a gloss about 5% greater, about 10%
greater, or about 20% greater than a gloss of the paper product
coated with a coating devoid of a co-processed microfibrillated
cellulose and inorganic particulate material composition. In some
embodiments, the paper product coated with a coating comprising a
co-processed microfibrillated cellulose and inorganic particulate
material composition may have a barrier property which is improved
compared to barrier property of the paper product coated with a
coating devoid of a co-processed microfibrillated cellulose and
inorganic particulate material composition. The barrier property
may be selected from the rate at which one or more of oxygen,
moisture, grease and aromas pass (i.e., transmitted) pass through
the coated paper product. The coating comprising a co-processed
microfibrillated cellulose and inorganic particulate material
composition may therefore slow down or ameliorate (i.e., decrease)
the rate at which one or more of oxygen, moisture, grease and
aromas pass through the coated paper product.
[0050] In embodiments, tensile strength, tear strength and gloss
are determined in accordance with the methods described above.
[0051] In embodiments, stiffness (i.e., elastic modulus) is
determined in accordance with the stiffness measurement method
described in J. C. Husband, L. F. Gate, N. Norouzi, and D. Blair,
"The Influence of kaolin Shape Factor on the Stiffness of Coated
Papers", TAPPI Journal, June 2009, p. 12-17 (see in particular the
section entitled `Experimental Methods`); and J. C. Husband, J. S.
Preston, L. F. Gate, A. Storer, and P. Creaton, "The Influence of
Pigment Particle Shape on the In-Plane tensile Strength Properties
of Kaolin-based Coating Layers", TAPPI Journal, December 2006, p.
3-8 (see in particular the section entitled `Experimental
Methods`).
[0052] In an embodiment, the inorganic particulate material is
kaolin. Advantageously, the kaolin is a platy kaolin or a
hyper-play kaolin.
Dispersible Compositions
[0053] In certain embodiments, the co-processed microfibrillated
cellulose and inorganic particulate material composition may be in
the form of a dry or substantially dry, re-dispersable composition,
as produced by the processes described herein or by any other
drying process known in the art (e.g., freeze-drying). The dried
co-processed microfibrillated cellulose and inorganic particulate
material composition may be easily dispersed in aqueous or
non-aqueous medium (e.g., polymers).
[0054] Thus, in accordance with the third aspect of the present
invention, there is provided a polymer composition comprising the
co-processed microfibrillated cellulose and inorganic particulate
material composition described herein.
[0055] The polymer composition may comprise at least about 0.5 wt.
%, at least about 5 wt. %, at least about 10 wt. %, at least about
15 wt. %, at least about 20 wt. %, at least about 25 wt. %, at
least about 30 wt. %, or at least about 35 wt. % of a co-processed
microfibrillated cellulose and inorganic particulate material
composition, based on the total weight of the polymer composition.
Generally, the polymer will comprise no more than about 50 wt. %,
for example, no more than about 45 wt. %, or no more than about 40
wt. % of a co-processed microfibrillated cellulose and inorganic
particulate material composition. In a particular embodiment, the
polymer composition comprises from about 25% to about 35% wt. % of
a co-processed microfibrillated cellulose and inorganic particulate
material composition. The fibre content of the co-processed
microfibrillated cellulose and inorganic particulate material
composition may be at least about 2 wt. %, at least about 3 wt. %,
at least about 4 wt. %, at least about 5 wt. %, at least about 6
wt. %, at least about 7 wt. %, at least about 8 wt. %, at least
about 10 wt. %, at least about 11 wt. %, at least about 12 wt. %,
at least about 13 wt. %, at least about 14 wt. % or at least about
15. wt. %. Generally, the fibre content of the co-processed
microfibrillated cellulose and inorganic particulate material
composition will be less than about 25 wt. %, for example, less
than about 20 wt. %.
[0056] The polymer may comprise any natural or synthetic polymer or
mixture thereof. The polymer may, for example, be thermoplastic or
thermoset. The term "polymer" used herein includes homopolymers
and/or copolymers, as well as crosslinked and/or entangled
polymers.
[0057] Polymers, including homopolymers and/or copolymers,
comprised in the polymer composition of the present invention may
be prepared from one or more of the following monomers: acrylic
acid, methacrylic acid, methyl methacrylate, and alkyl acrylates
having 1-18 carbon atoms in the alkyl group, styrene, substituted
styrenes, divinyl benzene, diallyl phthalate, butadiene, vinyl
acetate, acrylonitrile, methacrylonitrile, maleic anhydride, esters
of maleic acid or fumaric acid, tetrahydrophthalic acid or
anhydride, itaconic acid or anhydride, and esters of itaconic acid,
with or without a cross-linking dimer, trimer, or tetramer,
crotonic acid, neopentyl glycol, propylene glycol, butanediols,
ethylene glycol, diethylene glycol, dipropylene glycol, glycerol,
cyclohexanedimethanol, 1,6 hexanediol, trimethyolpropane,
pentaerythritol, phthalic anhydride, isophthalic acid, terephthalic
acid, hexahydrophthalic anyhydride, adipic acid or succinic acids,
azelaic acid and dimer fatty acids, toluene diisocyanate and
diphenyl methane diisocyanate. Copolymers comprising methyl
methacrylate and styrene monomers are preferred.
[0058] The polymer may be selected from one or more of
polymethylmethacrylate (PMMA), polyacetal, polycarbonate,
polyacrylonitrile, polybutadiene, polystyrene, polyacrylate,
polypropylene, epoxy polymers, unsaturated polyesters,
polyurethanes, polycyclopentadienes and copolymers thereof.
Suitable polymers also include liquid rubbers, such as
silicones.
[0059] Preparation of the polymer compositions of the present
invention can be accomplished by any suitable mixing method known
in the art, as will be readily apparent to one of ordinary skill in
the art.
[0060] Such methods include blending of the individual components
or precursors thereof and subsequent processing in a conventional
manner. Certain of the ingredients can, if desired, be pre-mixed
before addition to the compounding mixture.
[0061] In the case of thermoplastic polymer compositions, such
processing may comprise melt mixing, either directly in an extruder
for making an article from the composition, or pre-mixing in a
separate mixing apparatus. Dry blends of the individual components
can alternatively be directly injection moulded without pre-melt
mixing.
[0062] The polymer composition can be prepared by mixing of the
components thereof intimately together. The said co-processed
microfibrillated cellulose and inorganic particulate material
composition may then be suitably blended with the polymer and any
desired additional components, before processing as described
above.
[0063] For the preparation of cross-linked or cured polymer
compositions, the blend of uncured components or their precursors,
and, if desired, the co-processed microfibrillated cellulose and
inorganic particulate material composition and any desired
non-perlite component(s), will be contacted under suitable
conditions of heat, pressure and/or light with an effective amount
of any suitable cross-linking agent or curing system, according to
the nature and amount of the polymer used, in order to cross-link
and/or cure the polymer.
[0064] For the preparation of polymer compositions where the
co-processed microfibrillated cellulose and inorganic particulate
material composition and any desired other component(s) are present
in situ at the time of polymerisation, the blend of monomer(s) and
any desired other polymer precursors, co-processed microfibrillated
cellulose and inorganic particulate material composition and any
other component(s) will be contacted under suitable conditions of
heat, pressure and/or light, according to the nature and amount of
the monomer(s) used, in order to polymerise the monomer(s) with the
perlite and any other component(s) in situ.
The Fibrous Substrate Comprising Cellulose
[0065] The fibrous substrate comprising cellulose may be derived
from any suitable source, such as wood, grasses (e.g., sugarcane,
bamboo) or rags (e.g., textile waste, cotton, hemp or flax). The
fibrous substrate comprising cellulose may be in the form of a pulp
(i.e., a suspension of cellulose fibres in water), which may be
prepared by any suitable chemical or mechanical treatment, or
combination thereof. For example, the pulp may be a chemical pulp,
or a chemithermomechanical pulp, or a mechanical pulp, or a
recycled pulp, or a papermill broke, or a papermill waste stream,
or waste from a papermill, or a combination thereof. The cellulose
pulp may be beaten (for example in a Valley beater) and/or
otherwise refined (for example, processing in a conical or plate
refiner) to any predetermined freeness, reported in the art as
Canadian standard freeness (CSF) in cm.sup.3. CSF means a value for
the freeness or drainage rate of pulp measured by the rate that a
suspension of pulp may be drained. For example, the cellulose pulp
may have a Canadian standard freeness of about 10 cm.sup.3 or
greater prior to being microfibrillated. The cellulose pulp may
have a CSF of about 700 cm.sup.3 or less, for example, equal to or
less than about 650 cm.sup.3, or equal to or less than about 600
cm.sup.3, or equal to or less than about 550 cm.sup.3, or equal to
or less than about 500 cm.sup.3, or equal to or less than about 450
cm.sup.3, or equal to or less than about 400 cm.sup.3, or equal to
or less than about 350 cm.sup.3, or equal to or less than about 300
cm.sup.3, or equal to or less than about 250 cm.sup.3, or equal to
or less than about 200 cm.sup.3, or equal to or less than about 150
cm.sup.3, or equal to or less than about 100 cm.sup.3, or equal to
or less than about 50 cm.sup.3. The cellulose pulp may then be
dewatered by methods well known in the art, for example, the pulp
may be filtered through a screen in order to obtain a wet sheet
comprising at least about 10% solids, for example at least about
15% solids, or at least about 20% solids, or at least about 30%
solids, or at least about 40% solids. The pulp may be utilised in
an unrefined state, that is to say without being beaten or
dewatered, or otherwise refined.
[0066] The fibrous substrate comprising cellulose may be added to a
grinding vessel or homogenizer in a dry state. For example, a dry
paper broke may be added directly to the grinder vessel. The
aqueous environment in the grinder vessel will then facilitate the
formation of a pulp.
The Inorganic Particulate Material
[0067] The inorganic particulate material may, for example, be an
alkaline earth metal carbonate or sulphate, such as calcium
carbonate, magnesium carbonate, dolomite, gypsum, a hydrous kandite
clay such as kaolin, halloysite or ball clay, an anhydrous
(calcined) kandite clay such as metakaolin or fully calcined
kaolin, talc, mica, huntite, hydromagnesite, ground glass, perlite
or diatomaceous earth, or magnesium hydroxide, or aluminium
trihydrate, or combinations thereof.
[0068] A preferred inorganic particulate material for use in the
method according to the first aspect of the present invention is
calcium carbonate. Hereafter, the invention may tend to be
discussed in terms of calcium carbonate, and in relation to aspects
where the calcium carbonate is processed and/or treated. The
invention should not be construed as being limited to such
embodiments.
[0069] The particulate calcium carbonate used in the present
invention may be obtained from a natural source by grinding. Ground
calcium carbonate (GCC) is typically obtained by crushing and then
grinding a mineral source such as chalk, marble or limestone, which
may be followed by a particle size classification step, in order to
obtain a product having the desired degree of fineness. Other
techniques such as bleaching, flotation and magnetic separation may
also be used to obtain a product having the desired degree of
fineness and/or colour. The particulate solid material may be
ground autogenously, i.e. by attrition between the particles of the
solid material themselves, or, alternatively, in the presence of a
particulate grinding medium comprising particles of a different
material from the calcium carbonate to be ground. These processes
may be carried out with or without the presence of a dispersant and
biocides, which may be added at any stage of the process.
[0070] Precipitated calcium carbonate (PCC) may be used as the
source of particulate calcium carbonate in the present invention,
and may be produced by any of the known methods available in the
art. TAPPI Monograph Series No 30, "Paper Coating Pigments", pages
34-35 describes the three main commercial processes for preparing
precipitated calcium carbonate which is suitable for use in
preparing products for use in the paper industry, but may also be
used in the practice of the present invention. In all three
processes, a calcium carbonate feed material, such as limestone, is
first calcined to produce quicklime, and the quicklime is then
slaked in water to yield calcium hydroxide or milk of lime. In the
first process, the milk of lime is directly carbonated with carbon
dioxide gas. This process has the advantage that no by-product is
formed, and it is relatively easy to control the properties and
purity of the calcium carbonate product. In the second process the
milk of lime is contacted with soda ash to produce, by double
decomposition, a precipitate of calcium carbonate and a solution of
sodium hydroxide. The sodium hydroxide may be substantially
completely separated from the calcium carbonate if this process is
used commercially. In the third main commercial process the milk of
lime is first contacted with ammonium chloride to give a calcium
chloride solution and ammonia gas. The calcium chloride solution is
then contacted with soda ash to produce by double decomposition
precipitated calcium carbonate and a solution of sodium chloride.
The crystals can be produced in a variety of different shapes and
sizes, depending on the specific reaction process that is used. The
three main forms of PCC crystals are aragonite, rhombohedral and
scalenohedral (e.g., calcite), all of which are suitable for use in
the present invention, including mixtures thereof.
[0071] Wet grinding of calcium carbonate involves the formation of
an aqueous suspension of the calcium carbonate which may then be
ground, optionally in the presence of a suitable dispersing agent.
Reference may be made to, for example, EP-A-614948 (the contents of
which are incorporated by reference in their entirety) for more
information regarding the wet grinding of calcium carbonate.
[0072] In some circumstances, minor additions of other minerals may
be included, for example, one or more of kaolin, calcined kaolin,
wollastonite, bauxite, talc or mica, could also be present.
[0073] When the inorganic particulate material of the present
invention is obtained from naturally occurring sources, it may be
that some mineral impurities will contaminate the ground material.
For example, naturally occurring calcium carbonate can be present
in association with other minerals. Thus, in some embodiments, the
inorganic particulate material includes an amount of impurities. In
general, however, the inorganic particulate material used in the
invention will contain less than about 5% by weight, preferably
less than about 1% by weight, of other mineral impurities.
[0074] The inorganic particulate material used during the
microfibrillating step of the method of the present invention will
preferably have a particle size distribution in which at least
about 10% by weight of the particles have an e.s.d of less than 2
.mu.m, for example, at least about 20% by weight, or at least about
30% by weight, or at least about 40% by weight, or at least about
50% by weight, or at least about 60% by weight, or at least about
70% by weight, or at least about 80% by weight, or at least about
90% by weight, or at least about 95% by weight, or about 100% of
the particles have an e.s.d of less than 2 .mu.m.
[0075] Unless otherwise stated, particle size properties referred
to herein for the inorganic particulate materials are as measured
in a well known manner by sedimentation of the particulate material
in a fully dispersed condition in an aqueous medium using a
Sedigraph 5100 machine as supplied by Micromeritics Instruments
Corporation, Norcross, Ga., USA (telephone: +1 770 662 3620;
web-site: www.micromeritics.com), referred to herein as a
"Micromeritics Sedigraph 5100 unit". Such a machine provides
measurements and a plot of the cumulative percentage by weight of
particles having a size, referred to in the art as the `equivalent
spherical diameter` (e.s.d), less than given e.s.d values. The mean
particle size d.sub.50 is the value determined in this way of the
particle e.s.d at which there are 50% by weight of the particles
which have an equivalent spherical diameter less than that d.sub.50
value.
[0076] Alternatively, where stated, the particle size properties
referred to herein for the inorganic particulate materials are as
measured by the well known conventional method employed in the art
of laser light scattering, using a Malvern Mastersizer S machine as
supplied by Malvern Instruments Ltd (or by other methods which give
essentially the same result). In the laser light scattering
technique, the size of particles in powders, suspensions and
emulsions may be measured using the diffraction of a laser beam,
based on an application of Mie theory. Such a machine provides
measurements and a plot of the cumulative percentage by volume of
particles having a size, referred to in the art as the `equivalent
spherical diameter` (e.s.d), less than given e.s.d values. The mean
particle size d.sub.50 is the value determined in this way of the
particle e.s.d at which there are 50% by volume of the particles
which have an equivalent spherical diameter less than that d.sub.50
value.
[0077] In another embodiment, the inorganic particulate material
used during the microfibrillating step of the method of the present
invention will preferably have a particle size distribution, as
measured using a Malvern Mastersizer S machine, in which at least
about 10% by volume of the particles have an e.s.d of less than 2
.mu.m, for example, at least about 20% by volume, or at least about
30% by volume, or at least about 40% by volume, or at least about
50% by volume, or at least about 60% by volume, or at least about
70% by volume, or at least about 80% by volume, or at least about
90% by volume, or at least about 95% by volume, or about 100% of
the particles by volume have an e.s.d of less than 2 .mu.m.
[0078] Unless otherwise stated, particle size properties of the
microfibrillated cellulose materials are as are as measured by the
well known conventional method employed in the art of laser light
scattering, using a Malvern Mastersizer S machine as supplied by
Malvern Instruments Ltd (or by other methods which give essentially
the same result).
[0079] Details of the procedure used to characterise the particle
size distributions of mixtures of inorganic particle material and
microfibrillated cellulose using a Malvern Mastersizer S machine
are provided below.
[0080] Another preferred inorganic particulate material for use in
the method according to the first aspect of the present invention
is kaolin clay. Hereafter, this section of the specification may
tend to be discussed in terms of kaolin, and in relation to aspects
where the kaolin is processed and/or treated. The invention should
not be construed as being limited to such embodiments. Thus, in
some embodiments, kaolin is used in an unprocessed form.
[0081] Kaolin clay used in this invention may be a processed
material derived from a natural source, namely raw natural kaolin
clay mineral. The processed kaolin clay may typically contain at
least about 50% by weight kaolinite. For example, most commercially
processed kaolin clays contain greater than about 75% by weight
kaolinite and may contain greater than about 90%, in some cases
greater than about 95% by weight of kaolinite.
[0082] Kaolin clay used in the present invention may be prepared
from the raw natural kaolin clay mineral by one or more other
processes which are well known to those skilled in the art, for
example by known refining or beneficiation steps.
[0083] For example, the clay mineral may be bleached with a
reductive bleaching agent, such as sodium hydrosulfite. If sodium
hydrosulfite is used, the bleached clay mineral may optionally be
dewatered, and optionally washed and again optionally dewatered,
after the sodium hydrosulfite bleaching step.
[0084] The clay mineral may be treated to remove impurities, e.g.
by flocculation, flotation, or magnetic separation techniques well
known in the art. Alternatively the clay mineral used in the first
aspect of the invention may be untreated in the form of a solid or
as an aqueous suspension.
[0085] The process for preparing the particulate kaolin clay used
in the present invention may also include one or more comminution
steps, e.g., grinding or milling. Light comminution of a coarse
kaolin is used to give suitable delamination thereof. The
comminution may be carried out by use of beads or granules of a
plastic (e.g. nylon), sand or ceramic grinding or milling aid. The
coarse kaolin may be refined to remove impurities and improve
physical properties using well known procedures. The kaolin clay
may be treated by a known particle size classification procedure,
e.g., screening and centrifuging (or both), to obtain particles
having a desired d.sub.50 value or particle size distribution.
The Microfibrillatinq Process
[0086] In accordance with the first aspect of the invention, there
is provided a method of preparing a composition for use as a filler
in paper or as a paper coating, comprising a step of
microfibrillating a fibrous substrate comprising cellulose in the
presence of an inorganic particulate material. According to
particular embodiments of the present methods, the
microfibrillating step is conducted in the presence of an inorganic
particulate material which acts as a microfibrillating agent.
[0087] By microfibrillating is meant a process in which
microfibrils of cellulose are liberated or partially liberated as
individual species or as smaller aggregates as compared to the
fibres of the pre-microfibrillated pulp. Typical cellulose fibres
(i.e., pre-microfibrillated pulp) suitable for use in papermaking
include larger aggregates of hundreds or thousands of individual
cellulose microfibrils. By microfibrillating the cellulose,
particular characteristics and properties, including but not
limited to the characteristic and properties described herein, are
imparted to the microfibrillated cellulose and the compositions
including the microfibrillated cellulose.
[0088] The step of microfibrillating may be carried out in any
suitable apparatus, including but not limited to a refiner. In one
embodiment, the microfibrillating step is conducted in a grinding
vessel under wet-grinding conditions. In another embodiment, the
microfibrillating step is carried out in a homogenizer. Each of
these embodiments is described in greater detail below.
[0089] Wet-Grinding
[0090] The grinding is suitably performed in a conventional manner.
The grinding may be an attrition grinding process in the presence
of a particulate grinding medium, or may be an autogenous grinding
process, i.e., one in the absence of a grinding medium. By grinding
medium is meant a medium other than the inorganic particulate
material which is co-ground with the fibrous substrate comprising
cellulose.
[0091] The particulate grinding medium, when present, may be of a
natural or a synthetic material. The grinding medium may, for
example, comprise balls, beads or pellets of any hard mineral,
ceramic or metallic material. Such materials may include, for
example, alumina, zirconia, zirconium silicate, aluminium silicate
or the mullite-rich material which is produced by calcining
kaolinitic clay at a temperature in the range of from about
1300.degree. C. to about 1800.degree. C. For example, in some
embodiments a Carbolite.RTM. grinding media is preferred.
Alternatively, particles of natural sand of a suitable particle
size may be used.
[0092] Generally, the type of and particle size of grinding medium
to be selected for use in the invention may be dependent on the
properties, such as, e.g., the particle size of, and the chemical
composition of, the feed suspension of material to be ground.
Preferably, the particulate grinding medium comprises particles
having an average diameter in the range of from about 0.1 mm to
about 6.0 mm and, more preferably, in the range of from about 0.2
mm to about 4.0 mm. The grinding medium (or media) may be present
in an amount up to about 70% by volume of the charge. The grinding
media may be present in amount of at least about 10% by volume of
the charge, for example, at least about 20% by volume of the
charge, or at least about 30% by volume of the charge, or at least
about 40% by volume of the charge, or at least about 50% by volume
of the charge, or at least about 60% by volume of the charge.
[0093] The grinding may be carried out in one or more stages. For
example, a coarse inorganic particulate material may be ground in
the grinder vessel to a predetermined particle size distribution,
after which the fibrous material comprising cellulose is added and
the grinding continued until the desired level of microfibrillation
has been obtained. The coarse inorganic particulate material used
in accordance with the first aspect of this invention initially may
have a particle size distribution in which less than about 20% by
weight of the particles have an e.s.d of less than 2 .mu.m, for
example, less than about 15% by weight, or less than about 10% by
weight of the particles have an e.s.d. of less than 2 .mu.m. In
another embodiment, the coarse inorganic particulate material used
in accordance with the first aspect of this invention initially may
have a particle size distribution, as measured using a Malvern
Mastersizer S machine, in which less than about 20% by volume of
the particles have an e.s.d of less than 2 .mu.m, for example, less
than about 15% by volume, or less than about 10% by volume of the
particles have an e.s.d. of less than 2 .mu.m
[0094] The coarse inorganic particulate material may be wet or dry
ground in the absence or presence of a grinding medium. In the case
of a wet grinding stage, the coarse inorganic particulate material
is preferably ground in an aqueous suspension in the presence of a
grinding medium. In such a suspension, the coarse inorganic
particulate material may preferably be present in an amount of from
about 5% to about 85% by weight of the suspension; more preferably
in an amount of from about 20% to about 80% by weight of the
suspension. Most preferably, the coarse inorganic particulate
material may be present in an amount of about 30% to about 75% by
weight of the suspension. As described above, the coarse inorganic
particulate material may be ground to a particle size distribution
such that at least about 10% by weight of the particles have an
e.s.d of less than 2 .mu.m, for example, at least about 20% by
weight, or at least about 30% by weight, or at least about 40% by
weight, or at least about 50% by weight, or at least about 60% by
weight, or at least about 70% by weight, or at least about 80% by
weight, or at least about 90% by weight, or at least about 95% by
weight, or about 100% by weight of the particles, have an e.s.d of
less than 2 .mu.m, after which the cellulose pulp is added and the
two components are co-ground to microfibrillate the fibres of the
cellulose pulp. In another embodiment, the coarse inorganic
particulate material is ground to a particle size distribution, as
measured using a Malvern Mastersizer S machine such that at least
about 10% by volume of the particles have an e.s.d of less than 2
.mu.m, for example, at least about 20% by volume, or at least about
30% by volume or at least about 40% by volume, or at least about
50% by volume, or at least about 60% by volume, or at least about
70% by volume, or at least about 80% by volume, or at least about
90% by volume, or at least about 95% by volume, or about 100% by
volume of the particles, have an e.s.d of less than 2 .mu.m, after
which the cellulose pulp is added and the two components are
co-ground to microfibrillate the fibres of the cellulose pulp
[0095] In one embodiment, the mean particle size (d.sub.50) of the
inorganic particulate material is reduced during the co-grinding
process. For example, the d.sub.50 of the inorganic particulate
material may be reduced by at least about 10% (as measured by a
Malvern Mastersizer S machine), for example, the d.sub.50 of the
inorganic particulate material may be reduced by at least about
20%, or reduced by at least about 30%, or reduced by at least about
50%, or reduced by at least about 50%, or reduced by at least about
60%, or reduced by at least about 70%, or reduced by at least about
80%, or reduced by at least about 90%. For example, an inorganic
particulate material having a d.sub.50 of 2.5 .mu.m prior to
co-grinding and a d.sub.50 of 1.5 .mu.m post co-grinding will have
been subject to a 40% reduction in particle size. In certain
embodiments, the mean particle size of the inorganic particulate
material is not significantly reduced during the co-grinding
process. By `not significantly reduced` is meant that the d.sub.50
of the inorganic particulate material is reduced by less than about
10%, for example, the d.sub.50 of the inorganic particulate
material is reduced by less than about 5%.
[0096] The fibrous substrate comprising cellulose may be
microfibrillated in the presence of an inorganic particulate
material to obtain microfibrillated cellulose having a d.sub.50
ranging from about 5 to .mu.m about 500 .mu.m, as measured by laser
light scattering. The fibrous substrate comprising cellulose may be
microfibrillated in the presence of an inorganic particulate
material to obtain microfibrillated cellulose having a d.sub.50 of
equal to or less than about 400 .mu.m, for example equal to or less
than about 300 .mu.m, or equal to or less than about 200 .mu.m, or
equal to or less than about 150 .mu.m, or equal to or less than
about 125 .mu.m, or equal to or less than about 100 .mu.m, or equal
to or less than about 90 .mu.m, or equal to or less than about 80
.mu.m, or equal to or less than about 70 .mu.m, or equal to or less
than about 60 .mu.m, or equal to or less than about 50 .mu.m, or
equal to or less than about 40 .mu.m, or equal to or less than
about 30 .mu.m, or equal to or less than about 20 .mu.m, or equal
to or less than about 10 .mu.m.
[0097] The fibrous substrate comprising cellulose may be
microfibrillated in the presence of an inorganic particulate
material to obtain microfibrillated cellulose having a modal fibre
particle size ranging from about 0.1-500 .mu.m and a modal
inorganic particulate material particle size ranging from 0.25-20
.mu.m. The fibrous substrate comprising cellulose may be
microfibrillated in the presence of an inorganic particulate
material to obtain microfibrillated cellulose having a modal fibre
particle size of at least about 0.5 .mu.m, for example at least
about 10 .mu.m, or at least about 50 .mu.m, or at least about 100
.mu.m, or at least about 150 .mu.m, or at least about 200 .mu.m, or
at least about 300 .mu.m, or at least about 400 .mu.m.
[0098] The fibrous substrate comprising cellulose may be
microfibrillated in the presence of an inorganic particulate
material to obtain microfibrillated cellulose having a fibre
steepness equal to or greater than about 10, as measured by
Malvern. Fibre steepness (i.e., the steepness of the particle size
distribution of the fibres) is determined by the following
formula:
Steepness=100.times.(d.sub.30/d.sub.70)
[0099] The microfibrillated cellulose may have a fibre steepness
equal to or less than about 100. The microfibrillated cellulose may
have a fibre steepness equal to or less than about 75, or equal to
or less than about 50, or equal to or less than about 40, or equal
to or less than about 30. The microfibrillated cellulose may have a
fibre steepness from about 20 to about 50, or from about 25 to
about 40, or from about 25 to about 35, or from about 30 to about
40.
[0100] The grinding is suitably performed in a grinding vessel,
such as a tumbling mill (e.g., rod, ball and autogenous), a stirred
mill (e.g., SAM or IsaMill), a tower mill, a stirred media detritor
(SMD), or a grinding vessel comprising rotating parallel grinding
plates between which the feed to be ground is fed.
[0101] In one embodiment, the grinding vessel is a tower mill. The
tower mill may comprise a quiescent zone above one or more grinding
zones. A quiescent zone is a region located towards the top of the
interior of tower mill in which minimal or no grinding takes place
and comprises microfibrillated cellulose and inorganic particulate
material. The quiescent zone is a region in which particles of the
grinding medium sediment down into the one or more grinding zones
of the tower mill.
[0102] The tower mill may comprise a classifier above one or more
grinding zones. In an embodiment, the classifier is top mounted and
located adjacent to a quiescent zone. The classifier may be a
hydrocyclone.
[0103] The tower mill may comprise a screen above one or more grind
zones. In an embodiment, a screen is located adjacent to a
quiescent zone and/or a classifier. The screen may be sized to
separate grinding media from the product aqueous suspension
comprising microfibrillated cellulose and inorganic particulate
material and to enhance grinding media sedimentation.
[0104] In an embodiment, the grinding is performed under plug flow
conditions. Under plug flow conditions the flow through the tower
is such that there is limited mixing of the grinding materials
through the tower. This means that at different points along the
length of the tower mill the viscosity of the aqueous environment
will vary as the fineness of the microfibrillated cellulose
increases. Thus, in effect, the grinding region in the tower mill
can be considered to comprise one or more grinding zones which have
a characteristic viscosity. A skilled person in the art will
understand that there is no sharp boundary between adjacent
grinding zones with respect to viscosity.
[0105] In an embodiment, water is added at the top of the mill
proximate to the quiescent zone or the classifier or the screen
above one or more grinding zones to reduce the viscosity of the
aqueous suspension comprising microfibrillated cellulose and
inorganic particulate material at those zones in the mill. By
diluting the product microfibrillated cellulose and inorganic
particulate material at this point in the mill it has been found
that the prevention of grinding media carry over to the quiescent
zone and/or the classifier and/or the screen is improved. Further,
the limited mixing through the tower allows for processing at
higher solids lower down the tower and dilute at the top with
limited backflow of the dilution water back down the tower into the
one or more grinding zones. Any suitable amount of water which is
effective to dilute the viscosity of the product aqueous suspension
comprising microfibrillated cellulose and inorganic particulate
material may be added. The water may be added continuously during
the grinding process, or at regular intervals, or at irregular
intervals.
[0106] In another embodiment, water may be added to one or more
grinding zones via one or more water injection points positioned
along the length of the tower mill, or each water injection point
being located at a position which corresponds to the one or more
grinding zones. Advantageously, the ability to add water at various
points along the tower allows for further adjustment of the
grinding conditions at any or all positions along the mill.
[0107] The tower mill may comprise a vertical impeller shaft
equipped with a series of impeller rotor disks throughout its
length. The action of the impeller rotor disks creates a series of
discrete grinding zones throughout the mill.
[0108] In another embodiment, the grinding is performed in a
screened grinder, preferably a stirred media detritor. The screened
grinder may comprise one or more screen(s) having a nominal
aperture size of at least about 250 .mu.m, for example, the one or
more screens may have a nominal aperture size of at least about 300
.mu.m, or at least about 350 .mu.m, or at least about 400 .mu.m, or
at least about 450 .mu.m, or at least about 500 .mu.m, or at least
about 550 .mu.m, or at least about 600 .mu.m, or at least about 650
.mu.m, or at least about 700 .mu.m, or at least about 750 .mu.m, or
at least about 800 .mu.m, or at least about 850 .mu.m, or at or
least about 900 .mu.m, or at least about 1000 .mu.m.
[0109] The screen sizes noted immediately above are applicable to
the tower mill embodiments described above.
[0110] As noted above, the grinding may be performed in the
presence of a grinding medium. In an embodiment, the grinding
medium is a coarse media comprising particles having an average
diameter in the range of from about 1 mm to about 6 mm, for example
about 2 mm, or about 3 mm, or about 4 mm, or about 5 mm.
[0111] In another embodiment, the grinding media has a specific
gravity of at least about 2.5, for example, at least about 3, or at
least about 3.5, or at least about 4.0, or at least about 4.5, or
least about 5.0, or at least about 5.5, or at least about 6.0.
[0112] In another embodiment, the grinding media comprises
particles having an average diameter in the range of from about 1
mm to about 6 mm and has a specific gravity of at least about
2.5.
[0113] In another embodiment, the grinding media comprises
particles having an average diameter of about 3 mm and specific
gravity of about 2.7.
[0114] As described above, the grinding medium (or media) may
present in an amount up to about 70% by volume of the charge. The
grinding media may be present in amount of at least about 10% by
volume of the charge, for example, at least about 20% by volume of
the charge, or at least about 30% by volume of the charge, or at
least about 40% by volume of the charge, or at least about 50% by
volume of the charge, or at least about 60% by volume of the
charge.
[0115] In one embodiment, the grinding medium is present in amount
of about 50% by volume of the charge.
[0116] By `charge` is meant the composition which is the feed fed
to the grinder vessel. The charge includes of water, grinding
media, fibrous substrate comprising cellulose and inorganic
particulate material, and any other optional additives as described
herein.
[0117] The use of a relatively coarse and/or dense media has the
advantage of improved (i.e., faster) sediment rates and reduced
media carry over through the quiescent zone and/or classifier
and/or screen(s).
[0118] A further advantage in using relatively coarse grinding
media is that the mean particle size (d.sub.50) of the inorganic
particulate material may not be significantly reduced during the
grinding process such that the energy imparted to the grinding
system is primarily expended in microfibrillating the fibrous
substrate comprising cellulose.
[0119] A further advantage in using relatively coarse screens is
that a relatively coarse or dense grinding media can be used in the
microfibrillating step. In addition, the use of relatively coarse
screens (i.e., having a nominal aperture of least about 250 um)
allows a relatively high solids product to be processed and removed
from the grinder, which allows a relatively high solids feed
(comprising fibrous substrate comprising cellulose and inorganic
particulate material) to be processed in an economically viable
process. As discussed below, it has been found that a feed having a
high initial solids content is desirable in terms of energy
sufficiency. Further, it has also been found that product produced
(at a given energy) at lower solids has a coarser particle size
distribution.
[0120] As discussed in the `Background` section above, the present
invention seeks to address the problem of preparing
microfibrillated cellulose economically on an industrial scale.
[0121] Thus, in accordance with one embodiment, the fibrous
substrate comprising cellulose and inorganic particulate material
are present in the aqueous environment at an initial solids content
of at least about 4 wt %, of which at least about 2% by weight is
fibrous substrate comprising cellulose. The initial solids content
may be at least about 10 wt %, or at least about 20 wt %, or at
least about 30 wt %, or at least about at least 40 wt %. At least
about 5% by weight of the initial solids content may be fibrous
substrate comprising cellulose, for example, at least about 10%, or
at least about 15%, or at least about 20% by weight of the initial
solids content may be fibrous substrate comprising cellulose.
[0122] In another embodiment, the grinding is performed in a
cascade of grinding vessels, one or more of which may comprise one
or more grinding zones. For example, the fibrous substrate
comprising cellulose and the inorganic particulate material may be
ground in a cascade of two or more grinding vessels, for example, a
cascade of three or more grinding vessels, or a cascade of four or
more grinding vessels, or a cascade of five or more grinding
vessels, or a cascade of six or more grinding vessels, or a cascade
of seven or more grinding vessels, or a cascade of eight or more
grinding vessels, or a cascade of nine or more grinding vessels in
series, or a cascade comprising up to ten grinding vessels. The
cascade of grinding vessels may be operatively linked in series or
parallel or a combination of series and parallel. The output from
and/or the input to one or more of the grinding vessels in the
cascade may be subjected to one or more screening steps and/or one
or more classification steps.
[0123] The total energy expended in a microfibrillation process may
be apportioned equally across each of the grinding vessels in the
cascade. Alternatively, the energy input may vary between some or
all of the grinding vessels in the cascade.
[0124] A person skilled in the art will understand that the energy
expended per vessel may vary between vessels in the cascade
depending on the amount of fibrous substrate being microfibrillated
in each vessel, and optionally the speed of grind in each vessel,
the duration of grind in each vessel, the type of grinding media in
each vessel and the type and amount of inorganic particulate
material. The grinding conditions may be varied in each vessel in
the cascade in order to control the particle size distribution of
both the microfibrillated cellulose and the inorganic particulate
material. For example, the grinding media size may be varied
between successive vessels in the cascade in order to reduce
grinding of the inorganic particulate material and to target
grinding of the fibrous substrate comprising cellulose.
[0125] In an embodiment the grinding is performed in a closed
circuit. In another embodiment, the grinding is performed in an
open circuit. The grinding may be performed in batch mode. The
grinding may be performed in a re-circulating batch mode.
[0126] As described above, the grinding circuit may include a
pre-grinding step in which coarse inorganic particulate ground in a
grinder vessel to a predetermined particle size distribution, after
which fibrous material comprising cellulose is combined with the
pre-ground inorganic particulate material and the grinding
continued in the same or different grinding vessel until the
desired level of microfibrillation has been obtained.
[0127] As the suspension of material to be ground may be of a
relatively high viscosity, a suitable dispersing agent may
preferably be added to the suspension prior to grinding. The
dispersing agent may be, for example, a water soluble condensed
phosphate, polysilicic acid or a salt thereof, or a
polyelectrolyte, for example a water soluble salt of a poly(acrylic
acid) or of a poly(methacrylic acid) having a number average
molecular weight not greater than 80,000. The amount of the
dispersing agent used would generally be in the range of from 0.1
to 2.0% by weight, based on the weight of the dry inorganic
particulate solid material. The suspension may suitably be ground
at a temperature in the range of from 4.degree. C. to 100.degree.
C.
[0128] Other additives which may be included during the
microfibrillation step include: carboxymethyl cellulose, amphoteric
carboxymethyl cellulose, oxidising agents,
2,2,6,6-Tetramethylpiperidine-1-oxyl (TEMPO), TEMPO derivatives,
and wood degrading enzymes.
[0129] The pH of the suspension of material to be ground may be
about 7 or greater than about 7 (i.e., basic), for example, the pH
of the suspension may be about 8, or about 9, or about 10, or about
11. The pH of the suspension of material to be ground may be less
than about 7 (i.e., acidic), for example, the pH of the suspension
may be about 6, or about 5, or about 4, or about 3. The pH of the
suspension of material to be ground may be adjusted by addition of
an appropriate amount of acid or base. Suitable bases included
alkali metal hydroxides, such as, for example NaOH. Other suitable
bases are sodium carbonate and ammonia. Suitable acids included
inorganic acids, such as hydrochloric and sulphuric acid, or
organic acids. An exemplary acid is orthophosphoric acid.
[0130] The amount of inorganic particulate material and cellulose
pulp in the mixture to be co-ground may vary in a ratio of from
about 99.5:0.5 to about 0.5:99.5, based on the dry weight of
inorganic particulate material and the amount of dry fibre in the
pulp, for example, a ratio of from about 99.5:0.5 to about 50:50
based on the dry weight of inorganic particulate material and the
amount of dry fibre in the pulp. For example, the ratio of the
amount of inorganic particulate material and dry fibre may be from
about 99.5:0.5 to about 70:30. In an embodiment, the ratio of
inorganic particulate material to dry fibre is about 80:20, or for
example, about 85:15, or about 90:10, or about 91:9, or about 92:8,
or about 93:7, or about 94:6, or about 95:5, or about 96:4, or
about 97:3, or about 98:2, or about 99:1. In a preferred
embodiment, the weight ratio of inorganic particulate material to
dry fibre is about 95:5. In another preferred embodiment, the
weight ratio of inorganic particulate material to dry fibre is
about 90:10. In another preferred embodiment, the weight ratio of
inorganic particulate material to dry fibre is about 85:15. In
another preferred embodiment, the weight ratio of inorganic
particulate material to dry fibre is about 80:20.
[0131] The total energy input in a typical grinding process to
obtain the desired aqueous suspension composition may typically be
between about 100 and 1500 kWht.sup.-1 based on the total dry
weight of the inorganic particulate filler. The total energy input
may be less than about 1000 kWht.sup.-1, for example, less than
about 800 kWht.sup.-1, less than about 600 kWht.sup.-1, less than
about 500 kWht.sup.-1, less than about 400 kWht.sup.-1, less than
about 300 kWht.sup.-1, or less than about 200 kWht.sup.-1. As such,
the present inventors have surprisingly found that a cellulose pulp
can be microfibrillated at relatively low energy input when it is
co-ground in the presence of an inorganic particulate material. As
will be apparent, the total energy input per tonne of dry fibre in
the fibrous substrate comprising cellulose will be less than about
10,000 kWht.sup.-1, for example, less than about 9000 kWht.sup.-1,
or less than about 8000 kWht.sup.-1, or less than about 7000
kWht.sup.-1, or less than about 6000 kWht.sup.-1, or less than
about 5000 kWht.sup.-1, for example less than about 4000 kWht-1,
less than about 3000 kWht.sup.-1, less than about 2000 kWht.sup.-1,
less than about 1500 kWht.sup.-1, less than about 1200 kWht.sup.-1,
less than about 1000 kWht.sup.-1, or less than about 800
kWht.sup.-1. The total energy input varies depending on the amount
of dry fibre in the fibrous substrate being microfibrillated, and
optionally the speed of grind and the duration of grind.
[0132] Homogenizing
[0133] Microfibrillation of the fibrous substrate comprising
cellulose may be effected under wet conditions in the presence of
the inorganic particulate material by a method in which the mixture
of cellulose pulp and inorganic particulate material is pressurized
(for example, to a pressure of about 500 bar) and then passed to a
zone of lower pressure. The rate at which the mixture is passed to
the low pressure zone is sufficiently high and the pressure of the
low pressure zone is sufficiently low as to cause microfibrillation
of the cellulose fibres. For example, the pressure drop may be
effected by forcing the mixture through an annular opening that has
a narrow entrance orifice with a much larger exit orifice. The
drastic decrease in pressure as the mixture accelerates into a
larger volume (i.e., a lower pressure zone) induces cavitation
which causes microfibrillation. In an embodiment, microfibrillation
of the fibrous substrate comprising cellulose may be effected in a
homogenizer under wet conditions in the presence of the inorganic
particulate material. In the homogenizer, the cellulose
pulp-inorganic particulate material mixture is pressurized (for
example, to a pressure of about 500 bar), and forced through a
small nozzle or orifice. The mixture may be pressurized to a
pressure of from about 100 to about 1000 bar, for example to a
pressure of equal to or greater than 300 bar, or equal to or
greater than about 500, or equal to or greater than about 200 bar,
or equal to or greater than about 700 bar. The homogenization
subjects the fibres to high shear forces such that as the
pressurized cellulose pulp exits the nozzle or orifice, cavitation
causes microfibrillation of the cellulose fibres in the pulp.
Additional water may be added to improve flowability of the
suspension through the homogenizer. The resulting aqueous
suspension comprising microfibrillated cellulose and inorganic
particulate material may be fed back into the inlet of the
homogenizer for multiple passes through the homogenizer. In a
preferred embodiment, the inorganic particulate material is a
naturally platy mineral, such as kaolin. As such, homogenization
not only facilitates microfibrillation of the cellulose pulp, but
also facilitates delamination of the platy particulate
material.
[0134] A platy particulate material, such as kaolin, is understood
to have a shape factor of at least about 10, for example, at least
about 15, or at least about 20, or at least about 30, or at least
about 40, or at least about 50, or at least about 60, or at least
about 70, or at least about 80, or at least about 90, or at least
about 100. Shape factor, as used herein, is a measure of the ratio
of particle diameter to particle thickness for a population of
particles of varying size and shape as measured using the
electrical conductivity methods, apparatuses, and equations
described in U.S. Pat. No. 5,576,617, which is incorporated herein
by reference.
[0135] A suspension of a platy inorganic particulate material, such
as kaolin, may be treated in the homogenizer to a predetermined
particle size distribution in the absence of the fibrous substrate
comprising cellulose, after which the fibrous material comprising
cellulose is added to the aqueous slurry of inorganic particulate
material and the combined suspension is processed in the
homogenizer as described above. The homogenization process is
continued, including one or more passes through the homogenizer,
until the desired level of microfibrillation has been obtained.
Similarly, the platy inorganic particulate material may be treated
in a grinder to a predetermined particle size distribution and then
combined with the fibrous material comprising cellulose followed by
processing in the homogenizer.
[0136] An exemplary homogenizer is a Manton Gaulin (APV)
homogenizer.
[0137] After the microfibrillation step has been carried out, the
aqueous suspension comprising microfibrillated cellulose and
inorganic particulate material may be screened to remove fibre
above a certain size and to remove any grinding medium. For
example, the suspension can be subjected to screening using a sieve
having a selected nominal aperture size in order to remove fibres
which do not pass through the sieve. Nominal aperture size means
the nominal central separation of opposite sides of a square
aperture or the nominal diameter of a round aperture. The sieve may
be a BSS sieve (in accordance with BS 1796) having a nominal
aperture size of 150 .mu.m, for example, a nominal aperture size
125 .mu.m, or 106 .mu.m, or 90 .mu.m, or 74 .mu.m, or 63 .mu.m, or
53 .mu.m, 45 .mu.m, or 38 .mu.m. In one embodiment, the aqueous
suspension is screened using a BSS sieve having a nominal aperture
of 125 .mu.m. The aqueous suspension may then be optionally
dewatered.
The Aqueous Suspension
[0138] The aqueous suspensions of this invention produced in
accordance with the methods described above are suitable for use in
a method of making paper or coating paper.
[0139] As such, the present invention is directed to an aqueous
suspension comprising, consisting of, or consisting essentially of
microfibrillated cellulose and an inorganic particulate material
and other optional additives. The aqueous suspension is suitable
for use in a method of making paper or coating paper. The other
optional additives include dispersant, biocide, suspending aids,
salt(s) and other additives, for example, starch or carboxy methyl
cellulose or polymers, which may facilitate the interaction of
mineral particles and fibres during or after grinding.
[0140] The inorganic particulate material may have a particle size
distribution such that at least about 10% by weight, for example at
least about 20% by weight, for example at least about 30% by
weight, for example at least about 40% by weight, for example at
least about 50% by weight, for example at least about 60% by
weight, for example at least about 70% by weight, for example at
least about 80% by weight, for example at least about 90% by
weight, for example at least about 95% by weight, or for example
about 100% of the particles have an e.s.d of less than 2 .mu.m.
[0141] In another embodiment, the inorganic particulate material
may have a particle size distribution, as measured by a Malvern
Mastersizer S machine, such that at least about 10% by volume, for
example at least about 20% by volume, for example at least about
30% by volume, for example at least about 40% by volume, for
example at least about 50% by volume, for example at least about
60% by volume, for example at least about 70% by volume, for
example at least about 80% by volume, for example at least about
90% by volume, for example at least about 95% by volume, or for
example about 100% by volume of the particles have an e.s.d of less
than 2 .mu.m.
[0142] The amount of inorganic particulate material and cellulose
pulp in the mixture to be co-ground may vary in a ratio of from
about 99.5:0.5 to about 0.5:99.5, based on the dry weight of
inorganic particulate material and the amount of dry fibre in the
pulp, for example, a ratio of from about 99.5:0.5 to about 50:50
based on the dry weight of inorganic particulate material and the
amount of dry fibre in the pulp. For example, the ratio of the
amount of inorganic particulate material and dry fibre may be from
about 99.5:0.5 to about 70:30. In an embodiment, the ratio of
inorganic particulate material to dry fibre is about 80:20, or for
example, about 85:15, or about 90:10, or about 91:9, or about 92:8,
or about 93:7, or about 94:6, or about 95:5, or about 96:4, or
about 97:3, or about 98:2, or about 99:1. In a preferred
embodiment, the weight ratio of inorganic particulate material to
dry fibre is about 95:5. In another preferred embodiment, the
weight ratio of inorganic particulate material to dry fibre is
about 90:10. In another preferred embodiment, the weight ratio of
inorganic particulate material to dry fibre is about 85:15. In
another preferred embodiment, the weight ratio of inorganic
particulate material to dry fibre is about 80:20.
[0143] In an embodiment, the composition does not include fibres
too large to pass through a BSS sieve (in accordance with BS 1796)
having a nominal aperture size of 150 .mu.m, for example, a nominal
aperture size of 125 .mu.m, 106 .mu.m, or 90 .mu.m, or 74 .mu.m, or
63 .mu.m, or 53 .mu.m, 45 .mu.m, or 38 .mu.m. In one embodiment,
the aqueous suspension is screened using a BSS sieve having a
nominal aperture of 125 .mu.m.
[0144] It will be understood therefore that amount (i.e., % by
weight) of microfibrillated cellulose in the aqueous suspension
after grinding or homogenizing may be less than the amount of dry
fibre in the pulp if the ground or homogenized suspension is
treated to remove fibres above a selected size. Thus, the relative
amounts of pulp and inorganic particulate material fed to the
grinder or homogenizer can be adjusted depending on the amount of
microfibrillated cellulose that is required in the aqueous
suspension after fibres above a selected size are removed.
[0145] In an embodiment, the inorganic particulate material is an
alkaline earth metal carbonate, for example, calcium carbonate. The
inorganic particulate material may be ground calcium carbonate
(GCC) or precipitated calcium carbonate (PCC), or a mixture of GCC
and PCC. In another embodiment, the inorganic particulate material
is a naturally platy mineral, for example, kaolin. The inorganic
particulate material may be a mixture of kaolin and calcium
carbonate, for example, a mixture of kaolin and GCC, or a mixture
of kaolin and PCC, or a mixture of kaolin, GCC and PCC.
[0146] In another embodiment, the aqueous suspension is treated to
remove at least a portion or substantially all of the water to form
a partially dried or essentially completely dried product. For
example, at least about 10% by volume of water in the aqueous
suspension may be removed from the aqueous suspension, for example,
at least about 20% by volume, or at least about 30% by volume, or
least about 40% by volume, or at least about 50% by volume, or at
least about 60% by volume, or at least about 70% by volume or at
least about 80% by volume or at least about 90% by volume, or at
least about 100% by volume of water in the aqueous suspension may
be removed. Any suitable technique can be used to remove water from
the aqueous suspension including, for example, by gravity or
vacuum-assisted drainage, with or without pressing, or by
evaporation, or by filtration, or by a combination of these
techniques. The partially dried or essentially completely dried
product will comprise microfibrillated cellulose and inorganic
particulate material and any other optional additives that may have
been added to the aqueous suspension prior to drying. The partially
dried or essentially completely dried product may be stored or
packaged for sale. The partially dried or essentially completely
dried product may be optionally re-hydrated and incorporated in
papermaking compositions and other paper products, as described
herein.
Paper Products and Processes for Preparing Same
[0147] The aqueous suspension comprising microfibrillated cellulose
and inorganic particulate material can be incorporated in
papermaking compositions, which in turn can be used to prepare
paper products. The term paper product, as used in connection with
the present invention, should be understood to mean all forms of
paper, including board such as, for example, white-lined board and
linerboard, cardboard, paperboard, coated board, and the like.
There are numerous types of paper, coated or uncoated, which may be
made according to the present invention, including paper suitable
for books, magazines, newspapers and the like, and office papers.
The paper may be calendered or super calendered as appropriate; for
example super calendered magazine paper for rotogravure and offset
printing may be made according to the present methods. Paper
suitable for light weight coating (LWC), medium weight coating
(MWC) or machine finished pigmentisation (MFP) may also be made
according to the present methods. Coated paper and board having
barrier properties suitable for food packaging and the like may
also be made according to the present methods.
[0148] In a typical papermaking process, a cellulose-containing
pulp is prepared by any suitable chemical or mechanical treatment,
or combination thereof, which are well known in the art. The pulp
may be derived from any suitable source such as wood, grasses
(e.g., sugarcane, bamboo) or rags (e.g., textile waste, cotton,
hemp or flax). The pulp may be bleached in accordance with
processes which are well known to those skilled in the art and
those processes suitable for use in the present invention will be
readily evident. The bleached cellulose pulp may be beaten,
refined, or both, to a predetermined freeness (reported in the art
as Canadian standard freeness (CSF) in cm.sup.3). A suitable paper
stock is then prepared from the bleached and beaten pulp.
[0149] The papermaking composition of the present invention
typically comprises, in addition to the aqueous suspension of
microfibrillated cellulose and inorganic particulate material,
paper stock and other conventional additives known in the art. The
papermaking composition of the present invention may comprise up to
about 50% by weight inorganic particulate material derived from the
aqueous suspension comprising microfibrillated cellulose and
inorganic particulate material based on the total dry contents of
the papermaking composition. For example, the papermaking
composition may comprise at least about 2% by weight, or at least
about 5% by weight, or at least about 10% by weight, or at least
about 15% by weight, or at least about 20% by weight, or at least
about 25% by weight, or at least about 30% by weight, or at least
about 35% by weight, or at least about 40% by weight, or at least
about 45% by weight, or at least about 50% by weight, or at least
about 60% by weight, or at least about 70% by weight, or at least
about 80% by weight of inorganic particulate material derived from
the aqueous suspension comprising microfibrillated cellulose and
inorganic particulate material based on the total dry contents of
the papermaking composition. The microfibrillated cellulose
material may have a fibre steepness of greater than about 10, for
examples, from about 20 to about 50, or from about 25 to about 40,
or from about 25 to 35, or from about 30 to about 40. The
papermaking composition may also contain a non-ionic, cationic or
an anionic retention aid or microparticle retention system in an
amount in the range from about 0.1 to 2% by weight, based on the
dry weight of the aqueous suspension comprising microfibrillated
cellulose and inorganic particulate material. It may also contain a
sizing agent which may be, for example, a long chain alkylketene
dimer, a wax emulsion or a succinic acid derivative. The
composition may also contain dye and/or an optical brightening
agent. The composition may also comprise dry and wet strength aids
such as, for example, starch or epichlorhydrin copolymers.
[0150] In accordance with the eighth aspect described above, the
present invention is directed to a process for making a paper
product comprising: (i) obtaining or preparing a fibrous substrate
comprising cellulose in the form of a pulp suitable for making a
paper product; (ii) preparing a papermaking composition from the
pulp in step (i), the aqueous suspension of this invention
comprising microfibrillated cellulose and inorganic particulate
material, and other optional additives (such as, for example, a
retention aid, and other additives such as those described above);
and (iii) forming a paper product from said papermaking
composition. As noted above, the step of forming a pulp may take
place in the grinder vessel or homogenizer by addition of the
fibrous substrate comprising cellulose in a dry state, for example,
in the form of a dry paper broke or waste, directly to the grinder
vessel. The aqueous environment in the grinder vessel or
homogenizer will then facilitate the formation of a pulp.
[0151] In one embodiment, an additional filler component (i.e., a
filler component other than the inorganic particulate material
which is co-ground with the fibrous substrate comprising cellulose)
can be added to the papermaking composition prepared in step (ii).
Exemplary filler components are PCC, GCC, kaolin, or mixtures
thereof. An exemplary PCC is scalenohedral PCC. In an embodiment,
the weight ratio of the inorganic particulate material to the
additional filler component in the papermaking composition is from
about 1:1 to about 1:30, for example, from about 1:1 to about 1:20,
for example, from about 1:1 to about 1:15, for example from about
1:1 to about 1:10, for example from about 1:1 to about 1:7, for
example, from about 1:3 to about 1:6, or about 1:1, or about 1:2,
or about 1:3, or about 1:4, or about 1:5. Paper products made from
such papermaking compositions may exhibit greater strength compared
to paper products comprising only inorganic particulate material,
such as for example PCC, as filler. Paper products made from such
papermaking compositions may exhibit greater strength compared to a
paper product in which inorganic particulate material and a fibrous
substrate comprising cellulose are prepared (e.g., ground)
separately and are admixed to form a paper making composition.
Equally, paper products prepared from a papermaking composition
according to the present invention may exhibit a strength which is
comparable to paper products comprising less inorganic particulate
material. In other words, paper products can be prepared from a
paper making composition according to the present at higher filler
loadings without loss of strength.
[0152] The steps in the formation of a final paper product from a
papermaking composition are conventional and well know in the art
and generally comprise the formation of paper sheets having a
targeted basis weight, depending on the type of paper being
made.
[0153] Additional economic benefits can be achieved through the
methods of the present invention in that the cellulose substrate
for making the aqueous suspension can be derived from the same
cellulose pulp formed for making the papermaking composition and
the final paper product. As such, and in accordance with the ninth
aspect described above, the present invention is directed to a an
integrated process for making a paper product comprising: (i)
obtaining or preparing a fibrous substrate comprising cellulose in
the form of a pulp suitable for making a paper product; (ii)
microfibrillating a portion of said fibrous substrate comprising
cellulose in accordance with the first aspect of the invention to
prepare an aqueous suspension comprising microfibrillated cellulose
and inorganic particulate material; (iii) preparing a papermaking
composition from the pulp in step (i), the aqueous suspension
prepared in step (ii), and other optional additives; and (iv)
forming a paper product from said papermaking composition.
[0154] Thus, since the cellulose substrate for preparing the
aqueous suspension has already been prepared for the purpose of
making the papermaking compositions, the step of forming the
aqueous suspension does not necessarily require a separate step of
preparing the fibrous substrate comprising cellulose.
[0155] Paper products prepared using the aqueous suspension of the
present invention have surprisingly been found to exhibit improved
physical and mechanical properties whilst at the same time enabling
the inorganic particulate material to be incorporated at relatively
high loading levels. Thus, improved papers can be prepared at
relatively less cost. For example, paper products prepared from
papermaking compositions comprising the aqueous suspension of the
present invention have been found to exhibit improved retention of
the inorganic particulate material filler compared to paper
products which do not contain any microfibrillated cellulose. Paper
products prepared from papermaking compositions comprising the
aqueous suspension of the present invention have also been found to
exhibit improved burst strength and tensile strength. Further, the
incorporation of the microfibrillated cellulose has been found to
reduce porosity compared to paper comprising the same amount of
filler but no microfibrillated cellulose. This is advantageous
since high filler loading levels are generally associated with
relatively high values of porosity and are detrimental to
printability.
Paper Coating Composition and Coating Process
[0156] The aqueous suspension of the present invention can be used
as a coating composition without the addition of further additives.
However, optionally, a small amount of thickener such as
carboxymethyl cellulose or alkali-swellable acrylic thickeners or
associated thickeners may be added.
[0157] The coating composition according to the present invention
may contain one or more optional additional components, if desired.
Such additional components, where present, are suitably selected
from known additives for paper coating compositions.
[0158] Some of these optional additives may provide more than one
function in the coating composition. Examples of known classes of
optional additives are as follows:
(a) one or more additional pigments: the compositions described
herein can be used as sole pigments in the paper coating
compositions, or may be used in conjunction with one another or
with other known pigments, such as, for example, calcium sulphate,
satin white, and so-called `plastic pigment`. When a mixture of
pigments is used, the total pigment solids content is preferably
present in the composition in an amount of at least about 75 wt %
of the total weight of the dry components of the coating
composition; (b) one or more binding or cobinding agents: for
example, latex, which may, optionally, be carboxylated, including:
a styrene-butadiene rubber latex; an acrylic polymer latex; a
polyvinyl acetate latex; or a styrene acrylic copolymer latex,
starch derivatives, sodium carboxymethyl cellulose, polyvinyl
alcohol, and proteins; (c) one or more cross linkers: for example,
in levels of up to about 5% by weight; e.g., glyoxals, melamine
formaldehyde resins, ammonium zirconium carbonates; one or more dry
or wet pick improvement additives: e.g., in levels up to about 2%
by weight, e.g., melamine resin, polyethylene emulsions, urea
formaldehyde, melamine formaldehyde, polyamide, calcium stearate,
styrene maleic anhydride and others; one or more dry or wet rub
improvement and abrasion resistance additives: e.g., in levels up
to about 2% by weight, e.g., glyoxal based resins, oxidised
polyethylenes, melamine resins, urea formaldehyde, melamine
formaldehyde, polyethylene wax, calcium stearate and others; one or
more water resistance additives: e.g., in levels up to about 2% by
weight, e.g., oxidised polyethylenes, ketone resin, anionic latex,
polyurethane, SMA, glyoxal, melamine resin, urea formaldehyde,
melamine formaldehyde, polyamide, glyoxals, stearates and other
materials commercially available for this function; (d) one or more
water retention aids: for example, in levels up to about 2% by
weight, e.g., sodium carboxymethyl cellulose, hydroxyethyl
cellulose, PVOH (polyvinyl alcohol), starches, proteins,
polyacrylates, gums, alginates, polyacrylamide bentonite and other
commercially available products sold for such applications; (e) one
or more viscosity modifiers and/or thickeners: for example, in
levels up to about 2% by weight; e.g., acrylic associative
thickeners, polyacrylates, emulsion copolymers, dicyanamide,
triols, polyoxyethylene ether, urea, sulphated castor oil,
polyvinyl pyrrolidone, CMC (carboxymethyl celluloses, for example
sodium carboxymethyl cellulose), sodium alginate, xanthan gum,
sodium silicate, acrylic acid copolymers, HMC (hydroxymethyl
celluloses), HEC (hydroxyethyl celluloses) and others; (f) one or
more lubricity/calendering aids: for example, in levels up to about
2% by weight, e.g., calcium stearate, ammonium stearate, zinc
stearate, wax emulsions, waxes, alkyl ketene dimer, glycols; one or
more gloss-ink hold-out additives: e.g., in levels up to about 2%
by weight, e.g., oxidised polyethylenes, polyethylene emulsions,
waxes, casein, guar gum, CMC, HMC, calcium stearate, ammonium
stearate, sodium alginate and others; (g) one or more dispersants:
the dispersant is a chemical additive capable, when present in a
sufficient amount, of acting on the particles of the particulate
inorganic material to prevent or effectively restrict flocculation
or agglomeration of the particles to a desired extent, according to
normal processing requirements. The dispersant may be present in
levels up to about 1% by weight, and includes, for example,
polyelectrolytes such as polyacrylates and copolymers containing
polyacrylate species, especially polyacrylate salts (e.g., sodium
and aluminium optionally with a group II condensed sodium
phosphate, non-ionic surfactants, alkanolamine and other reagents
commonly used for this function. The dispersant may, for example,
be selected from conventional dispersant materials commonly used in
the processing and grinding of inorganic particulate materials.
Such dispersants will be well recognised by those skilled in this
art. They are generally water-soluble salts capable of supplying
anionic species which in their effective amounts can adsorb on the
surface of the inorganic particles and thereby inhibit aggregation
of the particles. The unsolvated salts suitably include alkali
metal cations such as sodium. Solvation may in some cases be
assisted by making the aqueous suspension slightly alkaline.
Examples of suitable dispersants include: water soluble condensed
phosphates, e.g., polymetaphosphate salts [general form of the
sodium salts: (NaPO.sub.3).sub.x] such as tetrasodium metaphosphate
or so-called "sodium hexametaphosphate" (Graham's salt);
water-soluble salts of polysilicic acids; polyelectrolytes; salts
of homopolymers or copolymers of acrylic acid or methacrylic acid,
or salts of polymers of other derivatives of acrylic acid, suitably
having a weight average molecular mass of less than about 20,000.
Sodium hexametaphosphate and sodium polyacrylate, the latter
suitably having a weight average molecular mass in the range of
about 1,500 to about 10,000, are especially preferred; (h) one or
more antifoamers and defoamers: for example, in levels up to about
1% by weight, e.g., blends of surfactants, tributyl phosphate,
fatty polyoxyethylene esters plus fatty alcohols, fatty acid soaps,
silicone emulsions and other silicone containing compositions,
waxes and inorganic particulates in mineral oil, blends of
emulsified hydrocarbons and other compounds sold commercially to
carry out this function; (i) one or more optical brightening agents
(OBA) and fluorescent whitening agents (FWA): for example, in
levels up to about 1% by weight, e.g., stilbene derivatives; (j)
one or more dyes: for example, in levels up to about 0.5% by
weight; (.kappa.) one or more biocides/spoilage control agents: for
example, in levels up to about 1% by weight, e.g., oxidizing
biocides such as chlorine gas, chlorine dioxide gas, sodium
hypochlorite, sodium hypobromite, hydrogen, peroxide, peracetic
oxide, ammonium bromide/sodium hypochlorite, or non-oxidising
biocides such as GLUT (Glutaraldehyde, CAS No 90045-36-6), ISO
(CIT/MIT) (Isothiazolinone, CAS No 55956-84-9 & 96118-96-6),
ISO (BIT/MIT) (Isothiazolinone), ISO (BIT) (Isothiazolinone, CAS No
2634-33-5), DBNPA, BNPD (Bronopol), NaOPP, CARBAMATE, THIONE
(Dazomet), EDDM--dimethanol (O-formal), HT--Triazine (N-formal),
THPS--tetrakis (O-formal), TMAD--diurea (N-formal), metaborate,
sodium dodecylbenene sulphonate, thiocyanate, organosulphur, sodium
benzoate and other compounds sold commercially for this function,
e.g., the range of biocide polymers sold by Nalco; (l) one or more
levelling and evening aids: for example, in levels up to about 2%
by weight, e.g., non-ionic polyol, polyethylene emulsions, fatty
acid, esters and alcohol derivatives, alcohol/ethylene oxide,
calcium stearate and other compounds sold commercially for this
function; (m) one or more grease and oil resistance additives: for
example, in levels up to about 2% by weight, e.g., oxidised
polyethylenes, latex, SMA (styrene maleic anhydride), polyamide,
waxes, alginate, protein, CMC, and HMC.
[0159] Any of the above additives and additive types may be used
alone or in admixture with each other and with other additives, if
desired.
[0160] For all of the above additives, the percentages by weight
quoted are based on the dry weight of inorganic particulate
material (100%) present in the composition. Where the additive is
present in a minimum amount, the minimum amount may be about 0.01%
by weight based on the dry weight of pigment.
[0161] The coating process is carried out using standard techniques
which are well known to the skilled person. The coating process may
also involve calendaring or supercalendering the coated
product.
[0162] Methods of coating paper and other sheet materials, and
apparatus for performing the methods, are widely published and well
known. Such known methods and apparatus may conveniently be used
for preparing coated paper. For example, there is a review of such
methods published in Pulp and Paper International, May 1994, page
18 et seq. Sheets may be coated on the sheet forming machine, i.e.,
"on-machine," or "off-machine" on a coater or coating machine. Use
of high solids compositions is desirable in the coating method
because it leaves less water to evaporate subsequently. However, as
is well known in the art, the solids level should not be so high
that high viscosity and leveling problems are introduced. The
methods of coating may be performed using an apparatus comprising
(i) an application for applying the coating composition to the
material to be coated and (ii) a metering device for ensuring that
a correct level of coating composition is applied. When an excess
of coating composition is applied to the applicator, the metering
device is downstream of it. Alternatively, the correct amount of
coating composition may be applied to the applicator by the
metering device, e.g., as a film press. At the points of coating
application and metering, the paper web support ranges from a
backing roll, e.g., via one or two applicators, to nothing (i.e.,
just tension). The time the coating is in contact with the paper
before the excess is finally removed is the dwell time--and this
may be short, long or variable.
[0163] The coating is usually added by a coating head at a coating
station. According to the quality desired, paper grades are
uncoated, single-coated, double-coated and even triple-coated. When
providing more than one coat, the initial coat (precoat) may have a
cheaper formulation and optionally coarser pigment in the coating
composition. A coater that is applying coating on each side of the
paper will have two or four coating heads, depending on the number
of coating layers applied on each side. Most coating heads coat
only one side at a time, but some roll coaters (e.g., film presses,
gate rolls, and size presses) coat both sides in one pass.
[0164] Examples of known coaters which may be employed include,
without limitation, air knife coaters, blade coaters, rod coaters,
bar coaters, multi-head coaters, roll coaters, roll or blade
coaters, cast coaters, laboratory coaters, gravure coaters,
kisscoaters, liquid application systems, reverse roll coaters,
curtain coaters, spray coaters and extrusion coaters.
[0165] Water may be added to the solids comprising the coating
composition to give a concentration of solids which is preferably
such that, when the composition is coated onto a sheet to a desired
target coating weight, the composition has a rheology which is
suitable to enable the composition to be coated with a pressure
(i.e., a blade pressure) of between 1 and 1.5 bar.
[0166] Calendering is a well known process in which paper
smoothness and gloss is improved and bulk is reduced by passing a
coated paper sheet between calender nips or rollers one or more
times. Usually, elastomer-coated rolls are employed to give
pressing of high solids compositions. An elevated temperature may
be applied. One or more (e.g., up to about 12, or sometimes higher)
passes through the nips may be applied.
[0167] Coated paper products prepared in accordance with the
present invention and which contain optical brightening agent in
the coating may exhibit a brightness as measured according to ISO
Standard 11475 which is at least 2 units greater, for example at
least 3 units greater compared to a coated paper product which does
not comprise microfibrillated cellulose which has been prepared in
accordance with the present invention. Coated paper products
prepared in accordance with the present invention may exhibit a
Parker Print Surf smoothness measured according to ISO standard
8971-4 (1992) which is at least 0.5 .mu.m smoother, for example at
least about 0.6 .mu.m smoother, or at least about 0.7 .mu.m
smoother compared to a coated paper product which does not comprise
microfibrillated cellulose which has been prepared in accordance
with the present invention.
[0168] For the avoidance of doubt, the present application is
directed to the subject-matter described in the following numbered
paragraphs:
1. A paper product comprising a paper coating composition including
a co-processed microfibrillated cellulose and inorganic particulate
material composition, wherein the paper product has: i) a first
tensile strength greater than a second tensile strength of the
paper product comprising the paper coating composition devoid of
the co-processed microfibrillated cellulose and inorganic
particulate material composition; ii) a first tear strength greater
than a second tear strength of the paper product comprising the
paper coating composition devoid of the co-processed
microfibrillated cellulose and inorganic particulate material
composition; and/or iii) a first gloss greater than a second gloss
of the paper product comprising the paper coating composition
devoid of the co-processed microfibrillated cellulose and inorganic
particulate material composition and/or iv) a first burst strength
greater than a second burst strength of the paper product
comprising the paper coating composition devoid of the co-processed
microfibrillated cellulose and inorganic particulate material
composition; and/or v) first sheet light scattering coefficient
greater than a second sheet light scattering coefficient of the
paper product comprising the paper coating composition devoid of
the co-processed microfibrillated cellulose and inorganic
particulate material composition; and/or vi) a first porosity less
than a second porosity of the paper product comprising the paper
coating composition devoid of the co-processed microfibrillated
cellulose and inorganic particulate material composition. 2. The
paper product of paragraph 1, wherein the paper coating composition
comprises a functional coating for liquid packaging, barrier
coatings, or printed electronics applications. 3. The paper product
of paragraph 1 or 2, further comprising a second coating comprising
a polymer, a metal, an aqueous composition, or a combination
thereof. 4. The paper product of paragraphs 1, 2 or 3, further
having a first moisture vapour transmission rate (MVTR) greater
than a second MVTR of the paper product comprising the paper
coating composition devoid of the co-processed microfibrillated
cellulose and inorganic particulate material composition. 5. The
paper product of any of paragraphs 1-4, wherein the paper comprises
from about 25 wt. % to about 35 wt. % of the co-processed
microfibrillated cellulose and inorganic particulate material
composition.
Microfibrillation in the Absence of Grindable Inorganic Particulate
Material
[0169] In another aspect, the present invention is directed to a
method for preparing an aqueous suspension comprising
microfibrillated cellulose, the method comprising a step of
microfibrillating a fibrous substrate comprising cellulose in an
aqueous environment by grinding in the presence of a grinding
medium which is to be removed after the completion of grinding,
wherein the grinding is performed in a tower mill or a screened
grinder, and wherein the grinding is carried out in the absence of
grindable inorganic particulate material.
[0170] A grindable inorganic particulate material is a material
which would be ground in the presence of the grinding medium.
[0171] The particulate grinding medium may be of a natural or a
synthetic material. The grinding medium may, for example, comprise
balls, beads or pellets of any hard mineral, ceramic or metallic
material. Such materials may include, for example, alumina,
zirconia, zirconium silicate, aluminium silicate or the
mullite-rich material which is produced by calcining kaolinitic
clay at a temperature in the range of from about 1300.degree. C. to
about 1800.degree. C. For example, in some embodiments a
Carbolite.RTM. grinding media is preferred. Alternatively,
particles of natural sand of a suitable particle size may be
used.
[0172] Generally, the type of and particle size of grinding medium
to be selected for use in the invention may be dependent on the
properties, such as, e.g., the particle size of, and the chemical
composition of, the feed suspension of material to be ground.
Preferably, the particulate grinding medium comprises particles
having an average diameter in the range of from about 0.5 mm to
about 6 mm. In one embodiment, the particles have an average
diameter of at least about 3 mm.
[0173] The grinding medium may comprise particles having a specific
gravity of at least about 2.5. The grinding medium may comprise
particles have a specific gravity of at least about 3, or least
about 4, or least about 5, or at least about 6.
[0174] The grinding medium (or media) may be present in an amount
up to about 70% by volume of the charge. The grinding media may be
present in amount of at least about 10% by volume of the charge,
for example, at least about 20% by volume of the charge, or at
least about 30% by volume of the charge, or at least about 40% by
volume of the charge, or at least about 50% by volume of the
charge, or at least about 60% by volume of the charge.
[0175] The fibrous substrate comprising cellulose may be
microfibrillated to obtain microfibrillated cellulose having a
d.sub.50 ranging from about 5 to .mu.m about 500 .mu.m, as measured
by laser light scattering. The fibrous substrate comprising
cellulose may be microfibrillated to obtain microfibrillated
cellulose having a d.sub.50 of equal to or less than about 400
.mu.m, for example equal to or less than about 300 .mu.m, or equal
to or less than about 200 .mu.m, or equal to or less than about 150
.mu.m, or equal to or less than about 125 .mu.m, or equal to or
less than about 100 .mu.m, or equal to or less than about 90 .mu.m,
or equal to or less than about 80 .mu.m, or equal to or less than
about 70 .mu.m, or equal to or less than about 60 .mu.m, or equal
to or less than about 50 .mu.m, or equal to or less than about 40
.mu.m, or equal to or less than about 30 .mu.m, or equal to or less
than about 20 .mu.m, or equal to or less than about 10 .mu.m.
[0176] The fibrous substrate comprising cellulose may be
microfibrillated to obtain microfibrillated cellulose having a
modal fibre particle size ranging from about 0.1-500 .mu.m, as
measured by laser light scattering. The fibrous substrate
comprising cellulose may be microfibrillated in the presence to
obtain microfibrillated cellulose having a modal fibre particle
size of at least about 0.5 .mu.m, for example at least about 10
.mu.m, or at least about 50 .mu.m, or at least about 100 .mu.m, or
at least about 150 .mu.m, or at least about 200 .mu.m, or at least
about 300 .mu.m, or at least about 400 .mu.m.
[0177] The fibrous substrate comprising cellulose may be
microfibrillated to obtain microfibrillated cellulose having a
fibre steepness equal to or greater than about 10, as measured by
Malvern (laser light scattering). Fibre steepness (i.e., the
steepness of the particle size distribution of the fibres) is
determined by the following formula:
Steepness=100.times.(d.sub.30/d.sub.70)
[0178] The microfibrillated cellulose may have a fibre steepness
equal to or less than about 100. The microfibrillated cellulose may
have a fibre steepness equal to or less than about 75, or equal to
or less than about 50, or equal to or less than about 40, or equal
to or less than about 30. The microfibrillated cellulose may have a
fibre steepness from about 20 to about 50, or from about 25 to
about 40, or from about 25 to about 35, or from about 30 to about
40.
[0179] In one embodiment, the grinding vessel is a tower mill. The
tower mill may comprise a quiescent zone above one or more grinding
zones. A quiescent zone is a region located towards the top of the
interior of a tower mill in which minimal or no grinding takes
place and comprises microfibrillated cellulose and inorganic
particulate material. The quiescent zone is a region in which
particles of the grinding medium sediment down into the one or more
grinding zones of the tower mill.
[0180] The tower mill may comprise a classifier above one or more
grinding zones. In an embodiment, the classifier is top mounted and
located adjacent to a quiescent zone. The classifier may be a
hydrocyclone.
[0181] The tower mill may comprise a screen above one or more grind
zones. In an embodiment, a screen is located adjacent to a
quiescent zone and/or a classifier. The screen may be sized to
separate grinding media from the product aqueous suspension
comprising microfibrillated cellulose and to enhance grinding media
sedimentation.
[0182] In an embodiment, the grinding is performed under plug flow
conditions. Under plug flow conditions the flow through the tower
is such that there is limited mixing of the grinding materials
through the tower. This means that at different points along the
length of the tower mill the viscosity of the aqueous environment
will vary as the fineness of the microfibrillated cellulose
increases. Thus, in effect, the grinding region in the tower mill
can be considered to comprise one or more grinding zones which have
a characteristic viscosity. A skilled person in the art will
understand that there is no sharp boundary between adjacent
grinding zones with respect to viscosity.
[0183] In an embodiment, water is added at the top of the mill
proximate to the quiescent zone or the classifier or the screen
above one or more grinding zones to reduce the viscosity of the
aqueous suspension comprising microfibrillated cellulose at those
zones in the mill. By diluting the product microfibrillated
cellulose at this point in the mill it has been found that the
prevention of grinding media carry over to the quiescent zone
and/or the classifier and/or the screen is improved. Further, the
limited mixing through the tower allows for processing at higher
solids lower down the tower and dilute at the top with limited
backflow of the dilution water back down the tower into the one or
more grinding zones. Any suitable amount of water which is
effective to dilute the viscosity of the product aqueous suspension
comprising microfibrillated cellulose may be added. The water may
be added continuously during the grinding process, or at regular
intervals, or at irregular intervals.
[0184] In another embodiment, water may be added to one or more
grinding zones via one or more water injection points positioned
along the length of the tower mill, the or each water injection
point being located at a position which corresponds to the one or
more grinding zones. Advantageously, the ability to add water at
various points along the tower allows for further adjustment of the
grinding conditions at any or all positions along the mill.
[0185] The tower mill may comprise a vertical impeller shaft
equipped with a series of impeller rotor disks throughout its
length. The action of the impeller rotor disks creates a series of
discrete grinding zones throughout the mill.
[0186] In another embodiment, the grinding is performed in a
screened grinder, preferably a stirred media detritor. The screened
grinder may comprise one or more screen(s) having a nominal
aperture size of at least about 250 .mu.m, for example, the one or
more screens may have a nominal aperture size of at least about 300
.mu.m, or at least about 350 .mu.m, or at least about 400 .mu.m, or
at least about 450 .mu.m, or at least about 500 .mu.m, or at least
about 550 .mu.m, or at least about 600 .mu.m, or at least about 650
.mu.m, or at least about 700 .mu.m, or at least about 750 .mu.m, or
at least about 800 .mu.m, or at least about 850 .mu.m, or at or
least about 900 .mu.m, or at least about 1000 .mu.m.
[0187] The screen sizes noted immediately above are applicable to
the tower mill embodiments described above.
[0188] As noted above, the grinding is performed in the presence of
a grinding medium. In an embodiment, the grinding medium is a
coarse media comprising particles having an average diameter in the
range of from about 1 mm to about 6 mm, for example about 2 mm, or
about 3 mm, or about 4 mm, or about 5 mm.
[0189] In another embodiment, the grinding media has a specific
gravity of at least about 2.5, for example, at least about 3, or at
least about 3.5, or at least about 4.0, or at least about 4.5, or
least about 5.0, or at least about 5.5, or at least about 6.0.
[0190] As described above, the grinding medium (or media) may be in
an amount up to about 70% by volume of the charge. The grinding
media may be present in amount of at least about 10% by volume of
the charge, for example, at least about 20% by volume of the
charge, or at least about 30% by volume of the charge, or at least
about 40% by volume of the charge, or at least about 50% by volume
of the charge, or at least about 60% by volume of the charge.
[0191] In one embodiment, the grinding medium is present in amount
of about 50% by volume of the charge.
[0192] By `charge` is meant the composition which is the feed fed
to the grinder vessel. The charge includes water, grinding media,
the fibrous substrate comprising cellulose and any other optional
additives (other than as described herein).
[0193] The use of a relatively coarse and/or dense media has the
advantage of improved (i.e., faster) sediment rates and reduced
media carry over through the quiescent zone and/or classifier
and/or screen(s).
[0194] A further advantage in using relatively coarse screens is
that a relatively coarse or dense grinding media can be used in the
microfibrillating step. In addition, the use of relatively coarse
screens (i.e., having a nominal aperture of least about 250 um)
allows a relatively high solids product to be processed and removed
from the grinder, which allows a relatively high solids feed
(comprising fibrous substrate comprising cellulose and inorganic
particulate material) to be processed in an economically viable
process. As discussed below, it has been found that a feed having a
high initial solids content is desirable in terms of energy
sufficiency. Further, it has also been found that product produced
(at a given energy) at lower solids has a coarser particle size
distribution.
[0195] As discussed in the `Background` section above, the present
invention seeks to address the problem of preparing
microfibrillated cellulose economically on an industrial scale.
[0196] Thus, in accordance with one embodiment, the fibrous
substrate comprising cellulose is present in the aqueous
environment at an initial solids content of at least about 1 wt %.
The fibrous substrate comprising cellulose may be present in the
aqueous environment at an initial solids content of at least about
2 wt %, for example at least about 3 wt %, or at least about at
least 4 wt %. Typically the initial solids content will be no more
than about 10 wt %.
[0197] In another embodiment, the grinding is performed in a
cascade of grinding vessels, one or more of which may comprise one
or more grinding zones. For example, the fibrous substrate
comprising cellulose may be ground in a cascade of two or more
grinding vessels, for example, a cascade of three or more grinding
vessels, or a cascade of four or more grinding vessels, or a
cascade of five or more grinding vessels, or a cascade of six or
more grinding vessels, or a cascade of seven or more grinding
vessels, or a cascade of eight or more grinding vessels, or a
cascade of nine or more grinding vessels in series, or a cascade
comprising up to ten grinding vessels. The cascade of grinding
vessels may be operatively inked in series or parallel or a
combination of series and parallel. The output from and/or the
input to one or more of the grinding vessels in the cascade may be
subjected to one or more screening steps and/or one or more
classification steps.
[0198] The total energy expended in a microfibrillation process may
be apportioned equally across each of the grinding vessels in the
cascade. Alternatively, the energy input may vary between some or
all of the grinding vessels in the cascade.
[0199] A person skilled in the art will understand that the energy
expended per vessel may vary between vessels in the cascade
depending on the amount of fibrous substrate being microfibrillated
in each vessel, and optionally the speed of grind in each vessel,
the duration of grind in each vessel and the type of grinding media
in each vessel. The grinding conditions may be varied in each
vessel in the cascade in order to control the particle size
distribution of the microfibrillated cellulose.
[0200] In an embodiment the grinding is performed in a closed
circuit. In another embodiment, the grinding is performed in an
open circuit.
[0201] As the suspension of material to be ground may be of a
relatively high viscosity, a suitable dispersing agent may
preferably be added to the suspension prior to grinding. The
dispersing agent may be, for example, a water soluble condensed
phosphate, polysilicic acid or a salt thereof, or a
polyelectrolyte, for example a water soluble salt of a poly(acrylic
acid) or of a poly(methacrylic acid) having a number average
molecular weight not greater than 80,000. The amount of the
dispersing agent used would generally be in the range of from 0.1
to 2.0% by weight, based on the weight of the dry inorganic
particulate solid material. The suspension may suitably be ground
at a temperature in the range of from 4.degree. C. to 100.degree.
C.
[0202] Other additives which may be included during the
microfibrillation step include: carboxymethyl cellulose, amphoteric
carboxymethyl cellulose, oxidising agents,
2,2,6,6-Tetramethylpiperidine-1-oxyl (TEMPO), TEMPO derivatives,
and wood degrading enzymes.
[0203] The pH of the suspension of material to be ground may be
about 7 or greater than about 7 (i.e., basic), for example, the pH
of the suspension may be about 8, or about 9, or about 10, or about
11. The pH of the suspension of material to be ground may be less
than about 7 (i.e., acidic), for example, the pH of the suspension
may be about 6, or about 5, or about 4, or about 3. The pH of the
suspension of material to be ground may be adjusted by addition of
an appropriate amount of acid or base. Suitable bases included
alkali metal hydroxides, such as, for example NaOH. Other suitable
bases are sodium carbonate and ammonia. Suitable acids included
inorganic acids, such as hydrochloric and sulphuric acid, or
organic acids. An exemplary acid is orthophosphoric acid.
[0204] The total energy input in a typical grinding process to
obtain the desired aqueous suspension composition may typically be
between about 100 and 1500 kWht.sup.-1 based on the total dry
weight of the inorganic particulate filler. The total energy input
may be less than about 1000 kWht.sup.-1, for example, less than
about 800 kWht.sup.-1, less than about 600 kWht.sup.-1, less than
about 500 kWht.sup.-1, less than about 400 kWht.sup.-1, less than
about 300 kWht.sup.-1, or less than about 200 kWht.sup.-1. As such,
the present inventors have surprisingly found that a cellulose pulp
can be microfibrillated at relatively low energy input when it is
co-ground in the presence of an inorganic particulate material. As
will be apparent, the total energy input per tonne of dry fibre in
the fibrous substrate comprising cellulose will be less than about
10,000 kWht.sup.-1, for example, less than about 9000 kWht.sup.-1,
or less than about 8000 kWht.sup.-1, or less than about 7000
kWht.sup.-1, or less than about 6000 kWht.sup.-1, or less than
about 5000 kWht.sup.-1, for example less than about 4000 kWht-1,
less than about 3000 kWht.sup.-1, less than about 2000 kWht.sup.-1,
less than about 1500 kWht.sup.-1, less than about 1200 kWht.sup.-1,
less than about 1000 kWht.sup.-1, or less than about 800
kWht.sup.-1. The total energy input varies depending on the amount
of dry fibre in the fibrous substrate being microfibrillated, and
optionally the speed of grind and the duration of grind.
[0205] The following procedure may be used to characterise the
particle size distributions of mixtures of minerals (GCC or kaolin)
and microfibrillated cellulose pulp fibres.
[0206] Calcium Carbonate
[0207] A sample of co-ground slurry sufficient to give 3 g dry
material is weighed into a beaker, diluted to 60 g with deionised
water, and mixed with 5 cm.sup.3 of a solution of sodium
polyacrylate of 1.5 w/v % active. Further deionised water is added
with stirring to a final slurry weight of 80 g.
[0208] Kaolin
[0209] A sample of co-ground slurry sufficient to give 5 g dry
material is weighed into a beaker, diluted to 60 g with deionised
water, and mixed with 5 cm.sup.3 of a solution of 1.0 wt % sodium
carbonate and 0.5 wt % sodium hexametaphosphate. Further deionised
water is added with stirring to a final slurry weight of 80 g.
[0210] The slurry is then added in 1 cm.sup.3 aliquots to water in
the sample preparation unit attached to the Mastersizer S until the
optimum level of obscuration is displayed (normally 10-15%). The
light scattering analysis procedure is then carried out. The
instrument range selected was 300RF: 0.05-900, and the beam length
set to 2.4 mm.
[0211] For co-ground samples containing calcium carbonate and fibre
the refractive index for calcium carbonate (1.596) is used. For
co-ground samples of kaolin and fibre the R1 for kaolin (1.5295) is
used.
[0212] The particle size distribution is calculated from Mie theory
and gives the output as a differential volume based distribution.
The presence of two distinct peaks is interpreted as arising from
the mineral (finer peak) and fibre (coarser peak).
[0213] The finer mineral peak is fitted to the measured data points
and subtracted mathematically from the distribution to leave the
fibre peak, which is converted to a cumulative distribution.
Similarly, the fibre peak is subtracted mathematically from the
original distribution to leave the mineral peak, which is also
converted to a cumulative distribution. Both these cumulative
curves may then be used to calculate the mean particle size
(d.sub.50) and the steepness of the distribution
(d.sub.30/d.sub.70.times.100). The differential curve may be used
to find the modal particle size for both the mineral and fibre
fractions.
EXAMPLES
[0214] Unless otherwise specified, paper properties were measured
in accordance with the following methods: [0215] Burst strength:
Messemer Buchnel burst tester according to SCAN P 24. [0216]
Tensile strength: Testometrics tensile tester according to SCAN P
16. [0217] Bendtsen porosity: Measured using a Bendtsen Model 5
porosity tester in accordance with SCAN P21, SCAN P60, BS 4420 and
Tappi UM 535. [0218] Bulk: This is the reciprocal of the apparent
density as measured according to SCAN P7. [0219] ISO Brightness:
The ISO brightness of handsheets was measured by means of an
Elrepho Datacolour 3300 brightness meter fitted with a No. 8 filter
(457 nm wavelength), according to ISO 2470: 1999 E. [0220] Opacity:
The opacity of a sample of paper is measured by means of an Elrepho
Datacolor 3300 spectro-photometer using a wavelength appropriate to
opacity measurement. The standard test method is ISO 2471. First, a
measurement of the percentage of the incident light reflected is
made with a stack of at least ten sheets of paper over a black
cavity (Rinfinity). The stack of sheets is then replaced with a
single sheet of paper, and a second measurement of the percentage
reflectance of the single sheet on the black cover is made (R). The
percentage opacity is then calculated from the formula: Percentage
opacity=100.times.R/Rinfinity. [0221] Tear strength: TAPPI method T
414 om-04 (Internal tearing resistance of paper (Elmendorf-type
method)). [0222] Internal (z-direction) strength using a Scott bond
tester according to TAPPI T569. [0223] Gloss: TAPPI method T 480
om-05 (Specular gloss of paper and paperboard at 75 degrees) may be
used. [0224] Stiffness: The stiffness measurement method described
in J. C. Husband, L. F. Gate, N. Norouzi, and D. Blair, "The
Influence of kaolin Shape Factor on the Stiffness of Coated
Papers", TAPPI Journal, June 2009, p. 12-17 (see in particular the
section entitled `Experimental Methods`); and J. C. Husband, J. S.
Preston, L. F. Gate, A. Storer, and P. Creaton, "The Influence of
Pigment Particle Shape on the In-Plane tensile Strength Properties
of Kaolin-based Coating Layers", TAPPI Journal, December 2006, p.
3-8 (see in particular the section entitled `Experimental
Methods`). [0225] L&W Bending resistance (force required to
bend a sheet through a given angle in mN: measured according to
SCAN-P29:84. [0226] Cationic demand (or anionic charge): measured
in Mutek PCD 03; samples were titrated with Polydadmac (average
molecular weight of about 60000) with conc. 1 mEq/L (purchased from
PTE AB/Selcuk Dolen). The pulp mixture was filtered before the
determination but not the white water samples. Before sample
testing a calibration test is run to check the approximate
consumption of polyelectrolyte. In sample testing the
polyelectrolytes are dosed in batches (about 10 times) with 30 s
intervals. [0227] Sheet light scattering and absorption
coefficients are measured using reflectance data from the Elrepho
instrument: R inf=reflectance of stack of 10 sheets, Ro=reflectance
of 1 sheet over a black cup. These values and the substance
(gm.sup.-2) of the sheet are inputted into the Kubelka-Munk
equations described in "Paper Optics" by Nils Pauler, (published by
Lorentzen and Wettre, ISBN 91-971-765-6-7), p. 29-36. [0228]
First-pass retention is determined on the basis of the solids
measurement in the headbox (HD) and in the white water (WW) tray
and is calculated according to the following formula:
Retention=[(HBsolids-WWsolids)/HBsolids].times.100 [0229] Ash
retention is determined following the same principles as first-pass
retention, but based on the weight of the ash component in the
headbox (HB) and in the white water (WW) tray, and is calculated
according to the following formula: Ash
retention=[(HBash-WWash)/HBash].times.100 [0230] Formation index
(PTS) is determined using the DOMAS software developed by PTS in
accordance with the measurement method described in section 10-1 of
their handbook, DOMAS 2.4 User Guide'
Example 1
Preparation of Co-Processed Filler
[0231] Composition 1
[0232] The starting materials for the grinding work consisted of a
slurry of pulp (Northern bleached kraft pine) and a ground calcium
carbonate (GGC) filler, Intracarb 60.TM., comprising about 60% by
volume of particles less than 2 .mu.m. The pulp was blended in a
Cellier mixer with the GCC to give a nominal 6% addition of pulp by
weight. This suspension, which was at 26.5% solids, was then fed
into a 180 kW stirred media mill containing ceramic grinding media
(King's, 3 mm) at a medium volume concentration of 50%. The mixture
was ground until an energy input between 2000 and 3000 kWht.sup.-1
(expressed on pulp alone) had been expended and then the
pulp/mineral mixture was separated from the media using a 1 mm
screen. The product had a fibre content (by ashing) of 6.5 wt %,
and a mean fibre size (D50) of 129 .mu.m as measured using a
Malvern Mastersizer S.TM.. The fibre psd steepness
(D30/D70.times.100) was 31.7.
[0233] Composition 2
[0234] The preparation of this filler followed the procedure
outlined in composition 1. The pulp was blended in a Cellier mixer
with the Intracarb 60 to give a 20% addition of pulp. This
suspension, which was at 10-11% solids, was then fed into a 180 kW
stirred media mill containing ceramic grinding media (King's, 3 mm)
at a medium volume concentration of 50%. The mixture was ground
until an energy input between 2500 and 4000 kWht.sup.-1 (expressed
on pulp alone) had been expended and then the pulp/mineral mixture
was separated from the media using a 1 mm screen. The product had a
fibre content (by ashing) of 19.7 wt %, and a mean fibre size (D50)
of 79.7 .mu.m as measured using a Malvern Mastersizer S.TM.. The
fibre psd steepness (D30/D70.times.100) was 29.3. Before addition
to the paper machine the fibre content was reduced to 11.4 wt % by
blending in an approximately 50/50 ratio with GCC (Intracarb
60.TM.).
Example 2
Preparation of Basepaper
[0235] A blend of 80% by weight of eucalyptus pulp (Sodra Tofte)
refined to 27.degree. SR at 4.5% solids and 20% by weight of
softwood kraft (Sodra Monsteras) pulp refined to 26.degree. SR at
3.5% solids was prepared in pilot scale equipment. This pulp blend
was used to make a continuous reel of paper using a pilot scale
paper machine running at 800 m min.sup.-1. The stock was fed to the
twin wire roll former via a 13 mm slot from a UMV10 headbox. The
target grammage of the paper was 75 gm.sup.-2 and fillers and
loading levels are set out in Table 1.
TABLE-US-00001 TABLE 1 Uncoated basepaper properties before
calendering Filler IC60 control Comp. 1 Comp. 2 Loading, wt % 19.9
27.8 27.9 28.5 Grammage, 74.5 74.1 77.8 71.9 gm.sup.-2 Tensile
strength 34.0 26.5 26.9 29.4 Nm g.sup.-1 Bendtsen porosity, 735 749
367 296 cm.sup.3 min.sup.-1
[0236] A 2-component retention aid system was used consisting of a
cationic polyacrylamide, Percol 47NS.TM., (BASF) at a dose of
300-380 g t.sup.-1 and a microparticle bentonite, Hydrocol SH.TM.
at 2 kg t.sup.-1. The press section consists of one double felted
roll press running at a linear load of 10 kN m.sup.-1 followed by
two Metso SymBelt presses with the shoe length of 250 mm running at
600 and 800 kN m.sup.-1 respectively. The rolls in the two shoe
presses are inverted in relation to each other.
[0237] The paper was dried using heated cylinders.
Application of a Barrier Coating
[0238] A coating was applied to each of the basepapers. The
formulation consisted of 100 parts of a high shape factor kaolin
(Barrisurf HX.TM.) and 100 parts of a styrene-butadiene copolymer
latex (DL930.TM., Styron). The solids content was 50.1 wt % and the
Brookfield 100 rpm viscosity was 80 mPas. Coatings were applied by
hand using a suitable wirewound rod to give a coat weight of 13-14
gm.sup.-2. Drying was accomplished using a hot air dryer.
Example 3
[0239] The coated papers of Example 2 were then tested for moisture
vapour transmission rate (MVTR) over 2 days. The method was based
on TAPPI T448 but used silica gel as the dessicant and a relative
humidity of 50%. The amount of moisture transferred through the
paper was measured over the first and second days and then
averaged. Results are summarized in Table 2.
[0240] The papers were also tested for oil resistance using an
oil-based solution of Sudan Red IV in dibutyl phthalate using an
IGT printing unit. A controlled volume of the fluid (5.8 .mu.l) was
applied to the paper using a syringe and passed through the
printing nip at a pressure of 5 kgf and a speed of 0.5 m s.sup.-1.
The area covered by the fluid stain was measured using image
analysis and used as an indication of the ability of the coating to
resist penetration by oil-based fluids. Results are summarized in
Table 2.
TABLE-US-00002 TABLE 2 Coated basepaper properties Filler IC60
control Comp. 1 Comp. 2 Loading, wt % 19.9 27.8 27.9 28.5 MVTR 44.1
40.4 40.4 36.3 gm.sup.-2/day Stain area, pixels 62592 70855 73749
75672
[0241] These results show that the paper containing co-ground
filler at the highest fibre level (composition 2) has a lower
moisture vapour transmission rate than the control.
[0242] Coated papers on both compositions 1 and 2 have higher stain
areas indicating improved fluid resistance.
Example 4
Preparation of Co-Processed Filler
[0243] Composition 3
[0244] The starting materials for the grinding work consisted of a
slurry of pulp (Botnia pine) and a ground calcium carbonate filler,
Intracarb 60.TM.. The pulp was blended in a Cellier mixer with the
Intracarb to give a nominally 20 wt % addition of pulp. This
suspension, which was at 10-11% solids, was then fed into a 180 kW
stirred media mill containing ceramic grinding media (King's, 3 mm)
at a medium volume concentration of 50%. The mixture was ground
until an energy input between 2500 and 4000 kWht.sup.-1 had been
expended and then the pulp/mineral mixture was separated from the
media using a 1 mm screen. The product had a fibre content (by
ashing) of 19.7 wt %, and a mean fibre size (D50) of 79.7 .mu.m as
measured using a Malvern Mastersizer S.TM.. The fibre psd steepness
(D30/D70.times.100) was 29.3. Before addition to the paper machine
(see Example 5 below) the fibre content was reduced by blending 9
parts by weight of the composition containing 19.7 wt % fibre with
23 parts of fresh Intracarb 60 to give a fibre content, measured by
ash, of 5.8 wt %.
[0245] Composition 4
[0246] A second filler composition was prepared by blending 50
parts by weight of composition 3, containing 19.7 wt % fibre, with
50 parts of fresh Intracarb 60 to give a fibre content, measured by
ash, of 11.4 wt %.
Example 5
Preparation of Paper
[0247] A blend of 80% by weight of eucalyptus pulp (Sodra Tofte)
refined to 27.degree. SR at 4.5% solids and 20% by weight of
softwood kraft (Sodra Monsteras) pulp refined to 26.degree. SR at
3.5% solids was prepared in pilot scale equipment. This pulp blend
was used to make a continuous reel of paper using a pilot scale
paper machine running at 800 m min.sup.-1. The stock was fed to the
twin wire roll former via a 13 mm slot from a UMV10 headbox. The
target grammage of the paper was 75 gm.sup.-2 and fillers and
loading levels are set out in Table 1. A 2-component retention aid
system was used consisting of a cationic polyacrylamide, Percol
47NS.TM., (BASF) at a dose of 300-380 g t.sup.-1 and a
microparticle bentonite, Hydrocol SH.TM. at 2 kg t.sup.-1. The
press section consists of one double felted roll press running at a
linear load of 10 kN m.sup.-1 followed by two Metso SymBelt presses
with the shoe length of 250 mm running at 600 and 800 kN m.sup.-1
respectively. The rolls in the two shoe presses are inverted in
relation to each other.
[0248] The paper was dried using heated cylinders.
[0249] Table 3 below lists the wet end measurements made during the
papermaking stage. Paper properties are summarised in Table 4.
[0250] These data show that the co-ground fillers do not
significantly contribute to the anionic trash in the white water
recirculation, and do not have a detrimental effect on total
retention, whist improving the ash retention. Finally, the
formation of the paper is improved by the addition of co-ground
filler.
TABLE-US-00003 TABLE 3 Paper machine parameters IC60 Control Comp.
3 Comp. 4 Loading, wt % 19.9 27.8 27.4 28.5 Retention aid dose, g
t.sup.-1 300 380 380 380 Cationic demand of 0.0225 0.0195 0.0195
0.0210 white water, .mu.eq g.sup.-1 Total 1st pass retention, 72.4
73.9 74.1 70.8 wt % Ash retention, wt % 43.7 35.1 51.1 44.7
Formation index, PTS 842 800 636 668
TABLE-US-00004 TABLE 4 Paper properties IC60 control Comp. 3 Comp.
4 Loading, wt % 19.9 27.8 27.4 28.5 Grammage, 74.5 74.1 77.3 71.9
gm.sup.-2 Burst strength 19.3 15.5 18.1 19.8 index, Nm g.sup.-1
Tensile 34.0 26.5 27.4 29.4 strength index, Nm g.sup.-1 Tear
strength 4.12 3.41 3.83 4.12 index, Nm g.sup.-1 Scott bond 136.6
122.2 134.2 131.8 strength, Jm.sup.-2 Sheet light 61.5 (F8) 68.0
(F8) 69.9 (F8) 71.3 (F8) scattering 58.0 (F10) 63.8 (F10) 65.4
(F10) 66.2 (F10) coefficient, m.sup.2kg.sup.-1, filters 8 and 10
Sheet light 0.381 (F8) 0.385 (F8) 0.407 (F8) 0.419 (F8) absorption
0.136 (F10) 0.143 (F10) 0.160 (F10) 0.170 (F10) coefficient,
m.sup.2kg.sup.-1, filters 8 and 10
[0251] These results show that the papers containing co-ground
filler (compositions 3 and 4) have an unusual combination of
strength properties. Normally in pulp refining, if tensile strength
increases, tear decreases. In these examples, both tensile and tear
strength increase at the same time. Scott bond internal strength
also improves.
[0252] Normally, if tensile strength increases, sheet light scatter
decreases. In this instance, both increase.
Example 6
Preparation of Co-Ground Filler
[0253] The starting materials for the grinding work consisted of a
slurry of pulp (Botnia pine) and a ground calcium carbonate filler,
Intracarb 60.TM.. The pulp was blended in a Cellier mixer with the
GCC to give a 20% addition of pulp. This suspension, which was at
8.8% solids, was then fed into a 180 kW stirred media mill
containing a ceramic grinding media (King's, 3 mm) at a media
volume concentration of 50%. The mixture was ground until an energy
input between 2500 kWht.sup.-1 had been expended and then the
pulp/mineral mixture was separated from the media using a 1 mm
screen. The product had a fibre content (by ashing) of 19.0 wt %,
and a mean fibre size (d.sub.50) of 79 .mu.m as measured using a
Malvern Mastersizer S.TM.. The fibre psd steepness
(d.sub.30/d.sub.70.times.100) was 30.7.
Example 7
Preparation of Base Paper
[0254] A blend of 56% by weight of Fibria eucalyptus pulp refined
to 33 SR (100 kWh/t), 14% Botnia RMA 90 softwood kraft pulp beaten
to 31 SR, and 30% by weight of coated woodfree broke containing 50%
by weight of GCC (Royal Web Silk) was prepared at 3% solids in
water using a pilot scale hydrapulper.
[0255] This pulp blend was used to make a continuous reel of paper
using a pilot scale Fourdrinier machine running at 12 m min.sup.-1.
The target grammage of the paper was 73-82 gm.sup.-2 and fillers
and loading levels are set out in Table 1. A cationic polymeric
retention aid (Percol E622, BASF) was added at a dose of 200 g
t.sup.-1 (10% loading) or 300 g t.sup.-1 (15-20% loading). The
paper was dried using heated cylinders.
[0256] The basepaper was calendered for 1 nip on machine using a
steel roll calendar at 20 kN pressure. The properties of the papers
after calendering are summarised in Table 5.
[0257] These results show that the paper containing co-ground
filler has higher burst and tensile strength than the control. The
bending resistance is also increased. The porosity however, is much
reduced. The sheets containing the highest amount of coground
filler have improved surface smoothness to those containing the
control chalk.
TABLE-US-00005 TABLE 5 Uncoated woodfree basepaper properties after
calendering Control Base 2 Base 3 5% broke Base 1 5% broke 5% broke
filler 5% broke filler filler filler 10% IC60* 10% Ex 6 15% Ex. 6
20% Ex 6 Loading, wt % 15.1 15.8 19.7 23.4 Grammage, gm.sup.-2 72.8
74.4 77.6 82.2 Geometric mean 33.3 35.0 31.4 33.8 tensile strength
Nm g.sup.-1 Burst strength 19.9 22.2 21.2 21.4 Nm g.sup.-1
Geometric mean 3.22 3.41 4.15 4.2 bending force, L&W, mN
Bendtsen 1202 842 592 577 porosity, cm.sup.3 min.sup.-1 Bendtsen
350 340 342 286 smoothness cm.sup.3 min.sup.-1 Wireside ISO
Brightness 76.7 76.6 77.5 78.0 Opacity, % 80.6 80.6 84.4 85.9
*Intracarb 60 .TM.
Example 8
[0258] A coating mix was prepared according to the following
formulation: [0259] 85 parts ultrafine ground calcium carbonate
(Carbital 95.TM.) comprising about 95% by volume of particles less
than 2 .mu.m [0260] parts fine glossing kaolin (Hydragloss 90.TM.
KaMin) [0261] 11 pph styrene-butadiene-acrylonitrile latex
(DL920.TM., Styron) [0262] 0.3 pph CMC (Finnfix.TM., CP Kelco)
[0263] 1 pph calcium stearate (Nopcote C104).
[0264] The pH was adjusted to 8.0 with NaOH and the solids to 65.5
wt %. The viscosity, measured using a Brookfield viscometer at 100
rpm was 270 mPas. This was applied to samples of the basepapers in
Table 5 using a laboratory coater (Heli-Coater.TM.) at a speed of
600 m min.sup.-1. Coat weights of between 7.0 and 12.0 gm.sup.-2
was applied and adjusted by control of blade displacement.
[0265] After conditioning at 23.degree. C. and 50% RH, all the
coated paper samples produced were then supercalendered for 10 nips
using a Perkins laboratory calendar. The pressure was 50 bar at a
roll temperature of 65.degree. C. and a speed of 40 m
min.sup.-1.
[0266] The coated and calendered strips were then tested for
smoothness (Parker Print Surf, ISO 8971-4), 75.degree. TAPPI gloss
(T480), and coverage using a burn-out procedure followed by image
analysis of the grey level image. The procedure involves treating
the paper with an alcoholic solution of ammonium chloride, followed
by heating to 200.degree. C. for 10 minutes to char the basepaper
fibres. The grey level of the paper is a measure of the ability of
the coating layer to cover the blackened fibres. Values for grey
level close to 0 indicate poor coverage (black) whilst higher
values indicate higher whiteness and therefore better coverage.
[0267] Results for a coat weight of 12 gm.sup.-2 are summarised in
Table 6.
[0268] Samples of the coated paper were also tested for their
printing properties. Papers were printed using an IGT Printing Unit
at a speed of 0.5 m s.sup.-1 and a pressure of 500N. A magenta
sheetfed offset ink was used, applying a volume of 0.1 cm.sup.3.
The gloss of the printed ink layer was measured using a Hunterlab
75.degree. glossmeter according to the TAPPI T480 standard. The ink
density was measured using a Gretag Spectroeye.TM. densitometer.
The picking speed of the coating was measured with the IGT Printing
Unit in acceleration mode using a standard low viscosity oil. The
printing speed was accelerated from 0-6 m s.sup.-1 and the distance
on the coated strip when damage first occurred was measured and
quoted as a printing velocity. Higher values mean that the coating
is stronger.
TABLE-US-00006 TABLE 6 Coated paper properties PPS Burn- smooth-
out, Dry 75.degree. ness average Print Print pick Loading, TAPPI
.mu.m, grey gloss, den- velocity Base wt % gloss 1000 Pa level
75.degree. sity cm s.sup.-1 Control 15.1 64 1.29 111.6 70 1.50 183
Base 1 15.8 63 1.21 114.6 70 1.51 194 Base 2 19.7 71 1.17 140.9 77
1.53 191 Base 3 23.4 68 1.30 129.9 75 1.46 198
[0269] The results show that substituting a co-ground filler
containing microfibrillated cellulose for a standard GCC filler
gives improvements in coated sheet quality when the paper is
subsequently coated. The coated paper surface has higher gloss,
better smoothness and the coated layer has better coverage
according to the burnout test (higher grey level values). Printing
properties are also improved with the ink layer having a higher
gloss. It was also found that the dry pick strength increased when
filler containing microfibrillated cellulose was used in the
base.
Example 9
Preparation of Co-Ground Filler
[0270] The starting materials for the grinding work consisted of a
slurry of pulp (Botnia pine) and a ground calcium carbonate filler,
Polcarb 60.TM., comprising about 60% by volume of particles less
than 2 .mu.m. The pulp was blended in a Cellier mixer with the
Polcarb to give a 20% addition of pulp. This suspension, which was
at 8.7% solids, was then fed into a 180 kW stirred media mill
containing a ceramic grinding media (King's, 3 mm) at a media
volume concentration of 50%. The mixture was ground until an energy
input between 2500 kWht.sup.-1 had been expended and then the
pulp/mineral mixture was separated from the media using a 1 mm
screen. The product had a fibre content (by ashing) of 20.7 wt %,
and a mean fibre size (d.sub.50) of 79 .mu.m as measured using a
Malvern Mastersizer S.TM.. The fibre psd steepness
(d.sub.30/d.sub.70.times.100) was 29.5.
Example 10
Preparation of Basepaper
[0271] A blend of 40% by weight of Pressurised groundwood pulp, 40%
Botnia RMA 90 softwood kraft pulp beaten to 31 SR and 20% by weight
of coated LWC broke containing 50/50 GCC/kaolin was prepared at 3%
solids in water using a pilot scale hydrapulper.
[0272] This pulp blend was used to make a continuous reel of paper
using a pilot scale Fourdrinier machine running at 16 m min.sup.-1.
The target grammage of the paper was 38-43 gm.sup.-2 and fillers
and loading levels are set out in Table 7. A cationic polymeric
retention aid (Percol 230L, BASF) was added at a dose of 200 g
t.sup.-1 (10% loading) or 300 g t.sup.-1 (15-20% loading). The
paper was dried using heated cylinders.
[0273] The basepaper was calendered for 1 nip on machine using a
steel roll calendar at 20 kN pressure. The properties of the papers
after calendering are summarised in Table 7.
[0274] These results show that the paper containing co-ground
filler has higher burst and tensile strength than the control. The
bending resistance is also increased. The porosity however, is much
reduced. The sheets containing the highest amount of co-ground
filler have improved surface smoothness to those containing the
control chalk.
TABLE-US-00007 TABLE 7 Uncoated basepaper properties after
calendering Base 1 Base 2 Base 3 Control 5% 5% broke 5% broke 5%
broke filler broke filler filler filler 6% Polcarb 60 5% Ex 9 10%
Ex. 9 14% Ex 9 Loading, wt % 11.2 10.1 15.4 18.8 Grammage,
gm.sup.-2 38.2 38.2 42.0 43.0 Geometric 26.8 32.4 30.4 28.4 mean
tensile strength Nm g.sup.-1 Burst strength 14.8 17.4 16.0 15.4 Nm
g.sup.-1 Geo. mean 3.22 3.41 4.15 4.2 bending force, L&W, mN
Bendtsen 1202 842 592 577 porosity, cm.sup.3 min.sup.-1 Bendtsen
350 340 342 286 smoothness cm.sup.3 min.sup.-1 Wireside ISO
Brightness 76.7 76.6 77.5 78.0 Opacity, % 80.6 80.6 84.4 85.9
Example 11
[0275] A coating mix was prepared according to the following
formulation: [0276] 60 parts fine ground calcium carbonate
(Carbital 90.TM.) comprising about 90% by volume of particles less
than 2 .mu.m [0277] 40 parts fine Brazilian kaolin (Capim DG.TM.)
[0278] 8 pph styrene-butadiene-acrylonitrile latex (DL920.TM.,
Styron) [0279] 4 pph starch (Cargill C*film) [0280] 1 pph calcium
stearate (Nopcote C104).
[0281] The pH was adjusted to 8.0 with NaOH and the solids to 67.5
wt %. The viscosity, measured using a Brookfield viscometer at 100
rpm was 270 mPas. This was applied to samples of the basepapers in
Table 7 using a laboratory coater (Heli-Coater.TM.) at a speed of
600 m min.sup.-1. Coat weights of between 7.0 and 12.0 gm.sup.-2
was applied and adjusted by control of blade displacement.
[0282] After conditioning at 23.degree. C. and 50% RH, all the
coated paper samples produced in Examples 3 and 4 were then
supercalendered for 10 nips using a Perkins laboratory calendar.
The pressure was 50 bar at a roll temperature of 65.degree. C. and
a speed of 40 m min.sup.-1.
[0283] The coated and calendered strips were then tested for
smoothness (Parker Print Surf, ISO 8971-4), 75.degree. TAPPI gloss
(T480), and coverage in accordance with Example 8 above.
[0284] Samples of the coated paper were also tested for their
printing properties in accordance with Example 8 above.
[0285] Results interpolated to a coat weight of 10 gm.sup.-2 are
summarised in Table 8.
TABLE-US-00008 TABLE 8 Coated paper properties 75.degree. PPS
Burn-out, Print Loading, TAPPI smoothness average gloss, Base wt %
gloss .mu.m, 1000 Pa grey level 75.degree. Control 11.2 48 1.36
142.3 62 Base 1 10.1 50 1.35 135.9 62 Base 2 15.4 54 1.17 161.0 66
Base 3 18.8 52 1.20 148.5 65
[0286] The results show that substituting a co-ground filler
containing microfibrillated cellulose for a standard chalk filler
gives improvements in coated sheet quality when the paper is
subsequently coated. The coated paper surface has higher gloss,
better smoothness and the coated layer has better coverage
according to the burnout test (generally higher grey level values).
Printing properties are also improved with the ink layer having a
higher gloss.
Example 11
[0287] 400 g of unrefined bleached softwood kraft pulp (Botnia Pine
RM90) was soaked in 20 litres of water for 6 hours, then slushed in
a mechanical mixer. The stock so obtained was then poured into a
laboratory Valley beater and refined under load for 28 mins to
obtain a sample of refined pulp beaten to 525 cm.sup.3 Canadian
Standard Freeness (CSF).
[0288] The pulp was then dewatered using a consistency tester
(Testing Machines Inc.) to obtain a pad of wet pulp at between
23.0-24.0 wt % solids. This was then used in co-grinding
experiments as detailed below:
[0289] 143 g of a slurry of Carbital 60HS.TM. (solids 77.7 wt %;
about 60% by volume of particles less than 2 .mu.m) was weighed
into a grinding pot. 51.0 g of wet pulp was then added and mixed
with the carbonate. 1485 g of King's 3 mm grinding media was then
added followed by 423 g water to give a media volume concentration
of 50%. The mixture was ground together at 1000 rpm until an energy
input of 5,000-12,500 kWh/ton (expressed on fibre) had been
expended. The product was separated from the media using a 600
.mu.m BSS screen. The solids content of the resulting slurry was
between 22.0-25.0 wt % and a Brookfield viscosity (100 rpm) of
1400-2930 mPas. The fibre content of the product was analysed by
ashing at 450.degree. C. and the size of the mineral and pulp
fractions measured using a Malvern Mastersizer.
[0290] Further samples based on the same GCC and pulp were prepared
using similar conditions but at higher pulp addition levels. The
sample properties are listed in Table 9.
TABLE-US-00009 TABLE 9 Conditions and properties of co-ground MFC -
GCC slurries Brookfield MFC D50, viscosity, wt % MFC Energy .mu.m,
Solids 100 rpm, Sample on mineral kWh/t MFC (Malvern) wt % mPa s 1
11.1 7500 41.6 22.0 2930 2 10.9 10,000 16.5 23.9 1685 3 10.9 12,500
12.5 25.0 1405 4 17.2 5,000 43 14.9 1815 5 15.7 10,000 16.4 17.4
1030 6 15.3 12,500 12.3 18.4 960 7 24.1 12,500 11.7 13.5 1055
Example 12
[0291] 131 g of a slurry of Barrisurf HX.TM. (solids 53.0 wt %;
shape fator=100) was weighed into a grinding pot. 33.0 g of wet
pulp at 22.5 wt % solids was then added and mixed with the kaolin.
1485 g of King's 3 mm grinding media was then added followed by 429
g water to give a media volume concentration of 50%. The mixture
was ground together at 1000 rpm until an energy input of between
5000 and 12,500 kWh/ton (expressed on fibre) had been expended. The
products were separated from the media using a 600 .mu.m BSS
screen. The solids content of the resulting slurries was between
13.5-15.9 wt % and Brookfield viscosity (100 rpm) values between
1940 and 2600 mPas. The fibre content of the products was analysed
by ashing at 450.degree. C. and the size of the mineral and pulp
fractions measured using a Malvern Mastersizer.
[0292] Further samples based on the same kaolin and pulp were
prepared using similar conditions but at higher pulp addition
levels. The sample properties are listed in Table 10.
TABLE-US-00010 TABLE 10 Conditions and properties of co-ground MFC
- kaolin slurries Brookfield wt % viscosity, MFC Energy MFC D50,
.mu.m, Solids 100 rpm, Sample on mineral kWh/t MFC (Malvern) wt %
mPa s 8 12.6 5000 52.2 13.5 2632 9 13.0 7500 34.3 14.3 2184 10 12.5
10,000 23 14.6 1940 11 13.4 12,500 18.2 15.9 2280 12 18.6 5000 42.5
14.1 4190 13 16.6 7500 24.8 16.2 4190 14 15.9 10,000 17 16.0 3156
15 16.4 12,500 13.6 16.1 2332 16 22.5 5000 41.9 14.3 6020 17 21.2
7500 28.2 14.4 5220 18 21.4 10,000 16.5 14.8 3740 19 20.0 12,500
11.9 18.1 4550 20 27.7 7500 31.4 13.6 4750 21 28.4 10,000 21.4 15.6
5050 22 32.3 12,500 13.6 17.4 6490
Example 13
[0293] Portions of the above slurries were applied onto a
polyethylene terephthalate film (Terinex Ltd.) using a 150 .mu.m
film thickness wirewound rod (Sheen Instruments Ltd, Kingston, UK).
The coatings were dried by the application of a hot air gun. The
dried coatings were removed from the PET film and cut into barbell
shapes 4 mm wide using a cutter designed for rubber testing. The
tensile properties of the coatings were measured using a tensile
tester (Testometric 350., Rochdale, UK). The procedure is described
in the article by J. C. Husband, J. S. Preston, L. F. Gate, A.
Storer. and P. Creaton, "The Influence of Pigment Particle Shape on
the In-Plane tensile Strength Properties of Kaolin-based Coating
Layers", TAPPI Journal, December 2006, p. 3-8 (see in particular
the section entitled `Experimental Methods`). The tensile strength
of the coated films was calculated from the load at break and the
elastic modulus from the initial slope of the stress vs. strain
curve. The procedure is described in the article by J. C. Husband,
L. F. Gate, N. Norouzi, and D. Blair, "The Influence of kaolin
Shape Factor on the Stiffness of Coated Papers", TAPPI Journal,
June 2009, p. 12-17 (see in particular the section entitled
`Experimental Methods`).
[0294] The results for the mechanical properties are summarised in
Tables 11 and 12.
TABLE-US-00011 TABLE 11 mechanical properties of co-ground MFC -
GCC coatings wt % MFC on Energy Tensile strength, Elastic modulus,
Sample mineral kWh/t MFC MPa GPa 1 11.1 7500 0.78 0.44 2 10.9
10,000 0.90 0.68 3 10.9 12,500 0.74 0.65 4 17.2 5,000 0.68 0.35 5
15.7 10,000 1.33 0.75 6 15.3 12,500 1.36 0.83 7 24.1 12,500
[0295] These results show that a combination of MFC and high aspect
ratio kaolin can produce strength and elastic modulus values. The
elastic modulus would translate directly into improved coated paper
stiffness, for example.
TABLE-US-00012 TABLE 12 Conditions and properties of co-ground MFC
- Barrisurf HX coating wt % MFC Energy Tensile strength, Elastic
modulus, Sample on mineral kWh/t MFC MPa GPa 8 12.6 5000 1.93 1.29
9 13.0 7500 2.96 1.68 10 12.5 10,000 2.55 1.66 11 13.4 12,500 2.41
1.69 12 18.6 5000 2.25 1.45 13 16.6 7500 3.27 2.14 14 15.9 10,000
4.31 2.64 15 16.4 12,500 2.98 2.16 16 22.5 5000 2.91 2.11 17 21.2
7500 5.71 2.94 18 21.4 10,000 5.95 2.91 19 20.0 12,500 3.26 2.53 20
27.7 7500 6.62 2.86 21 28.4 10,000 5.53 2.54 22 32.3 12,500 5.33
2.67
* * * * *
References