U.S. patent application number 15/310324 was filed with the patent office on 2017-05-25 for grinding method and grinding medium.
The applicant listed for this patent is FiberLean Technologies Limited. Invention is credited to Jean Andre Alary, Andreas Borger, Tafadzwa Motsi, Neil Rowson, David Robert Skuse, Thomas Richard Skuse.
Application Number | 20170145635 15/310324 |
Document ID | / |
Family ID | 50943248 |
Filed Date | 2017-05-25 |
United States Patent
Application |
20170145635 |
Kind Code |
A1 |
Motsi; Tafadzwa ; et
al. |
May 25, 2017 |
GRINDING METHOD AND GRINDING MEDIUM
Abstract
A method for manufacturing microfibrillated cellulose, a
particulate grinding medium suitable for use in said method, a
material which wears rough, and a method for making said
particulate grinding medium.
Inventors: |
Motsi; Tafadzwa; (Par
Cornwall, GB) ; Skuse; David Robert; (Truro Cornwall,
GB) ; Alary; Jean Andre; (L'Isle sur la Sorgue,
FR) ; Borger; Andreas; (Goritschach, AT) ;
Rowson; Neil; (Redditch Worcestershire, GB) ; Skuse;
Thomas Richard; (Birmingham West Midlands, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
FiberLean Technologies Limited |
Par Cornwall |
|
GB |
|
|
Family ID: |
50943248 |
Appl. No.: |
15/310324 |
Filed: |
May 14, 2015 |
PCT Filed: |
May 14, 2015 |
PCT NO: |
PCT/EP2015/060724 |
371 Date: |
November 10, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
D21D 1/00 20130101; D21H
11/18 20130101; C04B 35/10 20130101; C04B 35/48 20130101; C04B
2235/3229 20130101; C04B 2235/3217 20130101 |
International
Class: |
D21H 11/18 20060101
D21H011/18; C04B 35/10 20060101 C04B035/10; C04B 35/48 20060101
C04B035/48; D21D 1/00 20060101 D21D001/00 |
Foreign Application Data
Date |
Code |
Application Number |
May 15, 2014 |
EP |
14290145.3 |
Claims
1.-44. (canceled)
45. A method for manufacturing microfibrillated cellulose, the
method comprising a step of microfibrillating a fibrous substrate
comprising cellulose by grinding in the presence of a particulate
grinding medium which is to be removed after the completion of
grinding, wherein the particulate grinding medium has a specific
gravity of at least about 3.5, and wherein at the beginning of
grinding the particulate grinding medium has a surface roughness of
at least about 0.5 .mu.m.
46. A method for manufacturing microfibrillated cellulose, the
method comprising a step of microfibrillating a fibrous substrate
comprising cellulose by grinding in the presence of a particulate
grinding medium which is to be removed after the completion of
grinding, wherein the particulate grinding medium has a specific
gravity of at least about 3.5, and wherein at the beginning of
grinding the particulate grinding medium has a mean coefficient of
friction of at least about 0.10.
47. The method according to claim 45, wherein at the beginning of
grinding the particulate grinding medium also has a mean
coefficient of friction of at least about 0.10.
48. The method according to claim 47, wherein after the completion
of grinding, the surface roughness is at least about 90% of the
surface roughness at the beginning of grinding.
49. The method according to claim 47, wherein after the completion
of grinding the mean coefficient of friction is at least about 90%
of the mean coefficient of friction at the beginning of
grinding.
50. The method according to claim 47, wherein after the completion
of grinding, the surface roughness is at least the same as the
surface roughness at the beginning of grinding.
51. The method according to claim 47, wherein after the completion
of grinding the mean coefficient of friction is at least the same
as the mean coefficient of friction at the beginning of
grinding.
52. The method according to claim 47, wherein the particulate
grinding medium wears rough during grinding, further wherein after
the completion of grinding, the surface roughness is greater than
the surface roughness at the beginning of grinding.
53. The method according to 47, wherein the particulate grinding
medium wears rough during grinding, and further wherein, after the
completion of grinding, the mean coefficient of friction is greater
than the mean coefficient of friction at the beginning of
grinding.
54. The method according to claim 52, wherein the surface roughness
increases by at least 5% during grinding and/or the mean
coefficient of friction increases by at least 5% during
grinding.
55. The method according to claim 53, wherein the mean coefficient
of friction increases by at least 5% during grinding.
56. The method according to claim 47, wherein the particulate
grinding medium is a ceramic grinding medium.
57. The method according to claim 56, wherein the ceramic grinding
medium is formed of a material comprising alumina, zirconia,
zirconium silicate, yttria, ceria, or yttria and/or ceria
stabilized zirconia, and mixtures thereof.
58. The method according to 47, wherein at the beginning of
grinding the particulate grinding medium has a surface roughness of
from about 1.0 .mu.m to about 5.0 .mu.m.
59. The method according to claim 47, wherein at the beginning of
the grinding the particulate grinding medium has a mean coefficient
of friction of from about 0.15 to about 0.50.
60. The method according to claim 47, wherein the particulate
grinding medium comprises rod-shaped particles having an aspect
ratio of equal to or greater about than 1.5:1, for example, equal
to or greater than about 2:1.
61. The method according to claim 45, wherein the grinding is
performed in the presence of an inorganic particulate material
which is not to be removed after completion of grinding, producing
a microfibrillated cellulose comprising said inorganic particulate
material.
62. The method according to claim 46, wherein the grinding is
performed in the presence of an inorganic particulate material
which is not to be removed after completion of grinding, producing
a microfibrillated cellulose comprising said inorganic particulate
material.
63. The method according to claim 47, wherein the grinding is
performed in the presence of an inorganic particulate material
which is not to be removed after completion of grinding, producing
a microfibrillated cellulose comprising said inorganic particulate
material.
64. The method according to claim 63, wherein the microfibrillating
step is conducted in an aqueous environment.
65. The method according to claim 63, wherein the grinding is
performed in one or more grinding vessels.
66. The method according to claim 65, wherein the grinding is
performed in one or more grinding vessels, wherein the grinding
vessel is a stirred media detritor.
67. The method according to claim 66, wherein the total amount of
energy used in the method is less than that used in a comparable
method in which the particulate grinding medium has at the
beginning of grinding: (i) a surface roughness which is less rough;
or (ii) a lesser mean coefficient of friction; or both (i) and
(ii).
68. A particulate ceramic grinding medium having (i) a surface
roughness of at least about 0.5 .mu.m, or (ii) a mean coefficient
of friction of at least about 0.10, or both (i) and (ii), wherein
the grinding medium is formed by sintering a composition comprising
at least one of zirconia (ZrO.sub.2) and alumina
(Al.sub.2O.sub.3).
69. The particulate ceramic grinding medium according to claim 68,
wherein the composition further comprises ceria
(Ce.sub.2O.sub.3).
70. The particulate ceramic grinding medium according to claim 69,
wherein the composition comprises from about 5 to about 25 wt. %
ceria, based on the total weight of the composition.
71. The particulate ceramic grinding medium according to claim 70,
wherein the composition comprises from about 10 to about 20 wt.
%.
72. The particulate ceramic grinding medium according to claim 69,
wherein the composition comprises up to about 90 wt. % ceria
stabilized zirconia.
73. The particulate ceramic grinding medium according to claim 72,
further comprising at least about 10 wt. % alumina.
74. The particulate ceramic grinding medium according to claim 68,
wherein the composition comprises at least 90 wt. % alumina.
75. The particulate ceramic grinding medium according to claim 68,
wherein the composition comprises at least 95 wt. % alumina.
76. The particulate ceramic grinding medium according to claim 68,
wherein the composition comprises at least 99.5 wt. % alumina.
77. A particulate grinding medium, wherein the grinding medium
wears rough during grinding, in a method comprising a step of
microfibrillating a fibrous substrate comprising cellulose by
grinding in the presence of said particulate grinding medium which
is to be removed after the completion of grinding.
78. The particulate grinding medium according to claim 25, wherein
the particulate grinding medium has: (i) a surface roughness of at
least about 0.5 .mu.m; or (ii) a mean coefficient of friction of at
least about 0.1 O; or both (i) and (ii).
79. The particulate ceramic grinding medium according to claim 68,
wherein the grinding medium wears rough during grinding, in a
method comprising a step of microfibrillating a fibrous substrate
comprising cellulose by grinding in the presence of said
particulate grinding medium which is to be removed after the
completion of grinding.
80. The particulate ceramic grinding medium according to claim 68,
wherein the particulate ceramic grinding medium has a specific
gravity of at least 3.5 to about 6.5.
81. A particulate grinding medium having a (i) a surface roughness
of at least about 1.6 um, or (ii) a mean coefficient of friction of
at least about 0.25, or both (i) and (ii).
82. A method for making a particulate ceramic grinding medium
according to claim 68, the method comprising: a) obtaining,
providing or making a composition comprising raw materials suitable
for making the ceramic grinding medium; b) mixing the composition
comprising raw materials, forming a mixture; c) combining the
mixture with binder, forming a bound mixture; d) granulating the
bound mixture by mixing the bound mixture composition over a period
of time during which the mixing speed is reduced; and e) sintering
the granulated composition.
83. The method according to claim 82, further comprising the step
of drying the granulated composition.
84. The method according to claim 83, further comprising the step
of shaping the granulated composition.
85. The method according to claim 84, further comprising the step
of sizing the granulated composition.
86. The method according to claim 68, wherein steps b) to d) are
performed in a mixer equipped with an impeller, wherein the
impeller speed during step b) is greater than the impellor speed
during steps c) and d), and wherein the impeller speed during step
c) is equal to or greater than the impellor speed during step
d).
87. The method according to claim 82, wherein an initial mixing
speed in step b) is at least about 150% greater than a final mixing
speed in step d).
88. The method according to claim 87, wherein the initial mixing
speed is between about 2750 and 3250 rpm, and the final mixing
speed is between about 600 and 1200 rpm.
89. A method of manufacturing microfibrillated cellulose by
microfibrillating a fibrous substrate comprising cellulose by
grinding in the presence of a particulate grinding medium without
replenishing the method with fresh grinding media, wherein at the
beginning of the grinding the particulate grinding medium has: (i)
a surface roughness of at least about 0.5 um; or (ii) a mean
coefficient of friction of at least about 0.1 O; or both (i) and
(ii).
90. A method of simultaneously manufacturing (a) microfibrillated
cellulose and (b) a roughened particulate grinding medium,
comprising grinding a fibrous substrate comprising cellulose by
grinding in the presence of a particulate grinding medium which has
at the beginning of grinding: (i) a surface roughness of at least
about 0.5 um; or (ii) a mean coefficient of friction of at least
about 0.1; or both (i) and (ii).
91. The particulate ceramic grinding medium according to claim 68,
wherein the grinding medium is obtainable by a method comprising:
a) obtaining, providing or making a composition comprising raw
materials suitable for making the ceramic grinding medium; b)
mixing the composition comprising raw materials, forming a mixture;
c) combining the mixture with binder, forming a bound mixture; d)
granulating the bound mixture by mixing the bound mixture over a
period of time during which the mixing speed is reduced; and e)
sintering the granulated composition.
92. The particulate ceramic grinding medium according to claim 91,
further comprising the step of drying the granulated
composition.
93. The particulate ceramic grinding medium according to claim 92,
further comprising the step of shaping the granulated
composition.
94. The particulate ceramic grinding medium according to claim 93,
further comprising the step of sizing the granulated composition.
Description
TECHNICAL FIELD
[0001] The present invention is directed to method for
manufacturing microfibrillated cellulose, to a particulate grinding
medium suitable for use in said method, to materials which wear
rough, and to a method for making said particulate grinding
medium.
BACKGROUND OF THE INVENTION
[0002] Methods and compositions comprising microfibrillated
cellulose are described in WO-A-2010/131016. Paper products
comprising such microfibrillated cellulose have been shown to
exhibit excellent paper properties, such as paper strength. The
methods described in WO-A-2010/131016 also enable the production of
microfibrillated cellulose economically.
[0003] Despite the benefits seen in WO-A-2010/131016, there is
ongoing need to further improve the economics of producing
microfibrillated cellulose on an industrial scale, and to develop
new processes for producing microfibrillated cellulose. It would
also be desirable to be able to further develop or enhance one or
more properties of microfibrillated cellulose.
SUMMARY OF THE INVENTION
[0004] According to a first aspect, the present invention is
directed to a method for manufacturing microfibrillated cellulose,
said method comprising a step of microfibrillating a fibrous
substrate comprising cellulose by grinding in the presence of a
particulate grinding medium which is to be removed after the
completion of grinding, wherein the particulate grinding medium has
a specific gravity of at least about 3.5, and wherein at the
beginning of grinding the particulate grinding medium has: (i) a
surface roughness of at least about 0.5 .mu.m; or (ii) a mean
coefficient of friction of at least about 0.10; or both (i) and
(ii).
[0005] According to a second aspect, the present invention is
directed to the use of a particulate grinding medium having a
specific gravity of at least about 3.5 and i) a surface roughness
of at least 0.5 .mu.m, or (ii) a coefficient of friction of at
least about 0.10, or both (i) and (ii), in the manufacture of
microfibrillated cellulose.
[0006] According to a third aspect, the present invention is
directed to a particulate ceramic grinding medium having (i) a
surface roughness of at least about 0.5 .mu.m, or (ii) a mean
coefficient of friction of at least about 0.10, or both (i) and
(ii), wherein the grinding medium is formed by sintering a
composition comprising at least one of zirconia (ZrO.sub.2) and
alumina (Al.sub.2O.sub.3).
[0007] According to a fourth aspect, the present invention is
directed to a particulate grinding medium which wears rough during
grinding, for example, during grinding in a method comprising a
step of microfibrillating a fibrous substrate comprising cellulose
by grinding in the presence of said particulate grinding medium
which is to be removed after the completion of grinding. The
grinding medium may be a ceramic grinding medium.
[0008] According to a fifth aspect, the present invention is
directed to a particulate grinding medium having a (i) a surface
roughness of at least about 1.6 .mu.m, or (ii) a mean coefficient
of friction of at least about 0.25, or both (i) and (ii). The
grinding medium may be a ceramic grinding medium.
[0009] According to a sixth aspect, the present invention is
directed to a method for making a particulate ceramic grinding
medium according to the third, fourth and fifth aspects, or used in
the first and second aspects, said method comprising:
[0010] a. obtaining, providing or making a composition comprising
raw materials suitable for making the ceramic grinding medium;
[0011] b. mixing the composition comprising raw materials, forming
a mixture;
[0012] c. combining the mixture with binder, forming a bound
mixture;
[0013] d. granulating the bound mixture composition by mixing the
bound mixture over a period of time during which the mixing speed
is reduced;
[0014] e. optionally drying the granulated composition;
[0015] f. optionally shaping the granulated composition;
[0016] g. optionally sizing the granulated composition; and
[0017] h. sintering the granulated composition.
[0018] According to a seventh aspect, there is provided a material
which roughens or wears rough when agitated in the presence of a
fibrous substrate comprising cellulose.
[0019] According to an eighth aspect, there is provided an
unpolished particulate grinding media having a surface roughness
which increases by at least 1% when subject to abrasive
contact.
[0020] According to a ninth aspect, there is provided a polished
particulate grinding media having a surface roughness which
increases by at least 20% when subject to abrasive contact.
[0021] According to a tenth aspect, there is provided a method of
manufacturing microfibrillated cellulose by microfibrillating a
fibrous substrate comprising cellulose by grinding in the presence
of a particulate grinding medium without replenishing the method
with fresh grinding media, wherein at the beginning of the grinding
the particulate grinding medium has: (i) a surface roughness of at
least about 0.5 um; or (ii) a mean coefficient of friction of at
least about 0.10; or both (i) and (ii).
[0022] According to an eleventh aspect, there is provided a method
of simultaneously manufacturing (a) microfibrillated cellulose and
(b) a roughened particulate grinding medium, comprising grinding a
fibrous substrate comprising cellulose by grinding in the presence
of a particulate grinding medium which has at the beginning of
grinding: (i) a surface roughness of at least about 0.5 um; or (ii)
a mean coefficient of friction of at least about 0.10; or both (i)
and (ii).
[0023] According to a twelfth aspect, there is provided the use of
a particulate grinding medium having i) a surface roughness of at
least about 0.5 um, or (ii) a mean coefficient of friction of at
least about 0.10, or both (i) and (ii), in the manufacture of
microfibrillated cellulose to reduce the energy input per unit
amount of microfibrillated cellulose produced.
[0024] According to a thirteenth aspect, there is provided the use
of a particulate grinding medium having i) a surface roughness of
at least about 0.5 um, or (ii) a mean coefficient of friction of at
least about 0.10, or both (i) and (ii), in the manufacture of
microfibrillated cellulose to improve one or more properties of the
microfibrillated cellulose.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] FIG. 1 is a graph comparing burst strength of paper
comprising microfibrillated cellulose produced using grinding media
according to an embodiment of the present invention and a
comparative grinding media.
DETAILED DESCRIPTION OF THE INVENTION
[0026] Generally, the present invention is related to
modifications, for example, improvements, to the methods and
compositions described in WO-A-2010/131016, the entire contents of
which are hereby incorporated by reference.
The Microfibrillating Method
[0027] In accordance with the first aspect of the present
invention, the method comprises a step of microfibrillating a
fibrous substrate comprising cellulose by grinding in the presence
of a particulate grinding medium which is to be removed after the
completion of grinding. By "microfibrillating" is meant a process
in which microfibrils of cellulose are liberated or partially
liberated as individual species or as small aggregates as compared
to the fibres of the pre-microfibrillated pup. Typical cellulose
fibres (i.e., pre-microfibrillated pulp) suitable for use in
papermaking include larger aggregates of hundreds or thousands of
individual cellulose fibrils. By microfibrillating the cellulose,
particular characteristics and properties, including the
characteristics and properties described herein, are imparted to
the microfibrillated cellulose and the compositions comprising the
microfibrillated cellulose.
[0028] In certain embodiments, the particulate grinding medium has
a specific gravity of at least about 3.5. At the beginning of
grinding the grinding medium has: (i) a surface roughness of at
least about 0.5 .mu.m; or (ii) a mean coefficient of friction of at
least about 0.10; or both (i) and (ii). By "particulate grinding
medium" is meant a medium other than the inorganic particulate
material which, in certain embodiments, is co-ground with the
fibrous substrate comprising cellulose. Advantageously, it has been
found that a particulate grinding medium have a relatively rough
surface facilitates, e.g., enhances, the production of microfibrils
during the manufacture of microfibrillated cellulose. It is
believed that microfibrils are formed due to the intimate
interaction of the particulate grinding media surface which has a
relatively rough texture and cellulose fibres during the grinding
process. Without wishing to be bound by theory, it is thought that
the mechanism of microfibril production is due to the relatively
rough surface of the particulate grinding media `hooking` and
`tearing` and/or `delayering` cellulose during grinding. The
interaction between the particulate grinding media and cellulose
that results in microfibrillated cellulose may include
media-cellulose collisions, shear of cellulose between media
particulates or between media particulates and grinder wall.
[0029] As used herein, the term "at the beginning of grinding" is
referring to the condition of the grinding medium before it has
been used in a grinding process.
[0030] Surface roughness may be determined by optical
interferometry, i.e., the measurement of the surface topography of
a test surface of the particulate grinding medium relative to a
reference surface, as carried out by an optical interferometer. In
certain embodiments, surface roughness is determined in accordance
with the following method. A representative sample of the
particulate grinding medium is obtained and placed in an
interferometer coupled to an optical microscope. A suitable
interferometer is an Omniscan MicroXAM2. A suitable optical
microscope is a Keyence Optical Microscope. A representative sample
consists of 5 individual particles (e.g., beads) of the particulate
grinding medium to be analysed, selected at random from any given
batch of particulate grinding medium. A surface roughness for each
individual particle is determined at two different locations on the
surface, and the 10 results (i.e., two per particle) averaged. The
size of the surface area analysed at each location on each particle
is constant. A suitable interferometer operating procedures is
provided in Appendix 1. In certain embodiments, surface roughness
is determined in accordance with the interferometer operating
procedure provided in Appendix 1, or any other suitable procedure
which provides essentially the same result.
[0031] Mean coefficient of friction may be determined by
tribometry, i.e., the measurement of friction on a surface, as
carried out with a tribometer. A tribometer measures the magnitude
of friction and wear as surfaces are rubbed over each other. In
certain embodiments, mean coefficient of friction is determined in
accordance with the following method. Three individual specimens
(e.g., beads) of the particulate grinding medium to be analysed are
obtained, and each specimen subjected to three identical runs in a
tribometer. A friction coefficient is determined for each run,
giving nine friction coefficient measurements (i.e., three for each
specimen). A mean coefficient of friction is obtained by adding
together the nine friction coefficient measurements and dividing by
nine. A suitable tribometer operating procedure is provided in
Appendix 2. In certain embodiments, mean coefficient of friction is
determined in accordance with the tribometer operating procedure
provided in Appendix 2, or any other suitable procedure which
produces essentially the same result.
[0032] In certain embodiments, the particulate grinding medium has
a surface roughness of from about 0.5 .mu.m to about 5.0 .mu.m, for
example, from about 0.5 .mu.m to about 4.0 .mu.m, or from about 0.5
.mu.m to about 3.0 .mu.m, or from about 0.5 .mu.m to about 2.5
.mu.m, or from about 0.5 .mu.m to about 2.0 .mu.m, or from about
0.5 .mu.m to about 1.5 .mu.m, or from about 0.5 .mu.m to about 1.0
.mu.m, or from about 0.55 .mu.m to about 5.0 .mu.m, or from about
0.6.mu. to about 5.0 .mu.m, or from about 0.65 .mu.m to about 5.0
.mu.m, or from about 0.7 .mu.m to about 5.0 .mu.m, or from about
0.75 .mu.m to about 5.0 .mu.m, or from about 0.8 .mu.m to about 5.0
.mu.m, or from about 0.85 .mu.m to about 5.0 .mu.m, or from about
0.90 to about 5.0 .mu.m, or from about 0.95 .mu.m to about 0.5
.mu.m, or from about 1.0 .mu.m to about 5.0 .mu.m. In certain
embodiments, the surface roughness is equal to or less than about
5.0 .mu.m, for example, equal to or less than about 4.5 .mu.m, or
equal to or less than about 4.0 .mu.m, or equal to or less than
about 3.5 .mu.m, or equal to or less than about 3.0 .mu.m, or equal
to or less than about 2.8 .mu.m, or equal to or less than about 2.6
.mu.m, or equal to or less than about 2.4 .mu.m, or equal to or
less than about 2.2 .mu.m, or equal to or less than about 2.0
.mu.m, or equal to or less than about 1.8 .mu.m, or equal to or
less than about 1.6 .mu.m, or equal to or less than about 1.4
.mu.m, or equal to or less than about 1.2 .mu.m, or equal to or
less than about 1.0 .mu.m.
[0033] In certain embodiments, the particulate grinding medium has
a surface roughness of at least about 0.55 .mu.m, for example, at
least about 0.6 .mu.m, or at least about 0.65 .mu.m, or at least
about 0.7 .mu.m, or at least about 0.75 .mu.m, or at least about
0.8 .mu.m, or at least about 0.85 .mu.m, or at least about 0.9
.mu.m, or at least about 0.95 .mu.m, or at least about 1.0 .mu.m,
or at least about 1.05 .mu.m, or at least about 1.1 .mu.m, or at
least about 1.15 .mu.m, or at least about 1.2 .mu.m, or at least
about 1.25 .mu.m, or at least about 1.3 .mu.m, or at least about
1.35 .mu.m, or at least about 1.4 .mu.m, or at least about 1.45
.mu.m, or at least about 1.5 .mu.m, or at least about 1.55 .mu.m,
or at least about 1.6 .mu.m, or at least about 1.65 .mu.m, or at
least about 1.7 .mu.m, or at least about 1.75 .mu.m, or at least
about 1.8 .mu.m, or at least about 1.85 .mu.m, or at least about
1.9 .mu.m, or at least about 1.95 .mu.m, or at least about 2.0
.mu.m, or at least about 2.05 .mu.m, or at least about 2.1 .mu.m,
or at least about 2.15 .mu.m, or at least about 2.2 .mu.m, or at
least about 2.25 .mu.m, or at least about 2.3 .mu.m, or at least
about 2.35 .mu.m, or at least about 2.4 .mu.m, or at least about
2.45 .mu.m, or at least about 2.5 .mu.m, or at least about 2.55
.mu.m, or at least about 2.6 .mu.m, or at least about 2.65 .mu.m,
or at least about 2.7 .mu.m, or at least about 2.75 .mu.m, or at
least about 2.8 .mu.m, or at least about 2.85 .mu.m, or at least
about 2.9 .mu.m, or at least about 2.95 .mu.m, or at least about
3.0 .mu.m.
[0034] In certain embodiments, for example, certain embodiments of
the fifth aspect, the particulate grinding medium has a surface
roughness of at least about 1.6 .mu.m, for example, from about 1.6
.mu.m to about 5.0 .mu.m, or at least about 1.7 .mu.m, or at least
about 1.8 .mu.m, or at least about 1.9 .mu.m, or at least about 2.0
.mu.m, or at least about 2.1 .mu.m, or at least about 2.2 .mu.m, or
at least about 2.3 .mu.m, or at least about 2.4 .mu.m, or at least
about 2.5 .mu.m, or at least about 2.6 .mu.m, or at least about 2.7
.mu.m, or at least about 2.8 .mu.m, or at least about 2.9 .mu.m, or
at least about 3.0 .mu.m, or at least about 3.1 .mu.m, or at least
about 3.2 .mu.m, or at least about 3.3 .mu.m, or at least about 3.4
.mu.m, or at least about 3.5 .mu.m, or at least about 3.6 .mu.m, or
at least about 3.7 .mu.m, or at least about 3.8 .mu.m, or at least
about 3.9 .mu.m, or at least about 4.0 .mu.m. In certain
embodiments, the surface roughness is equal to or less than about
5.0 .mu.m, for example, equal to or less than about 4.5 .mu.m, or
equal to or less than about 4.0 .mu.m.
[0035] In certain embodiments, the particulate grinding medium has
a mean coefficient of friction of from about 0.10 to about 0.50,
for example, from about 0.15 to about 0.50, or from about 0.175 to
about 0.50, or from about 0.20 to about 0.50, or from about 0.225
to about 0.50, or from about 0.25 to about 0.50, or from about
0.275 to about 0.50, or from about 0.30 to about 0.50, or from
about 0.325 to about 0.50, or from about 0.35 to about 0.50, or
from about 0.375 to about 0.50, or from about 0.40 to about
0.50.
[0036] In certain embodiments, the mean coefficient of friction is
equal to or less than about 0.50, for example, equal to or less
than about 0.48, or equal to or less than about 0.46, or equal to
or less than about 0.44, or equal to or less than about 0.42, or
equal to or less than about 0.40, or equal to or less than about
0.39, or equal to or less than about 0.38, or equal to or less than
about 0.37, or equal to or less than about 0.36, or equal to or
less than about 0.35.
[0037] In certain embodiments, the mean coefficient of friction is
at least about 0.15, for example, at least about 0.175, or at least
about 0.20, or at least about 0.225, or at least about 0.25, or at
least about 0.275, or at least about 0.30.
[0038] In certain embodiments, grinding medium has a surface
roughness of from about 0.5 .mu.m to about 5.0 .mu.m and a mean
coefficient of friction of from about 0.10 to about 0.50, for
example, a surface roughness of from about 0.75 .mu.m to about 5.0
.mu.m and a mean coefficient of friction of from about 0.10 to
about 0.50, or a surface roughness of from about 1.0 .mu.m to about
5.0 .mu.m and a mean coefficient of friction of from about 0.10 to
about 0.50, or a surface roughness of from about 1.0 .mu.m to about
5.0 .mu.m and a mean coefficient of friction of from about 0.10 to
about 0.50, or a surface roughness of from about 0.5 .mu.m to about
5.0 .mu.m and a mean coefficient of friction of from about 0.20 to
about 0.50, or a surface roughness of from about 0.5 .mu.m to about
5.0 .mu.m and a mean coefficient of friction of from about 0.25 to
about 0.50, or a surface roughness of from about 0.5 .mu.m to about
5.0 .mu.m and a mean coefficient of friction of from about 0.30 to
about 0.50, or a surface roughness of from about 0.75 .mu.m to
about 4.0 .mu.m and a mean coefficient of friction of from about
0.20 to about 0.40, or a surface roughness of from about 0.75 .mu.m
to about 3.5 .mu.m and a mean coefficient of friction of from about
0.25 to about 0.40.
[0039] In certain embodiments, for example, certain embodiments of
the fifth aspect, the particulate grinding medium has a mean
coefficient of friction of at least about 0.26, for example, at
least about 0.28, or at least about 0.30, or at least about, or at
least about 0.32, or at least about 0.34, or at least about 0.36,
or at least about 0.38, or at least about 0.40, or at least about
0.42, or at least about 0.44, or at least about 0.46, or at least
about 0.48, or at least about 0.50. In certain embodiments, the
coefficient of friction is no greater than about 0.80, for example,
no greater than about 0.75, or no greater than about 0.70, or no
greater than about 0.65, or no greater than about 0.60, or no
greater than about 0.55.
[0040] In certain embodiments, after the completion of grinding,
the surface roughness of the particulate grinding medium is at
least about 90% of the surface roughness at the beginning of
grinding, for example, at least about 92% of the surface roughness,
or at least about 94% of the surface roughness, or at least about
96% of the surface roughness or at least about 98% of the surface
roughness, or at least about 99% of the surface roughness at the
beginning of grinding, as determined in accordance with the methods
described herein. As used herein, the term "after the completion of
grinding" is referring to the condition of the grinding medium
following use in a method according to the first aspect of the
invention, i.e., a method for manufacturing microfibrillated
cellulose comprising a step of microfibrillating a fibrous
substrate comprising cellulose by grinding in the presence of the
grinding medium, for example, a method for manufacturing
microfibrillated cellulose having a fibre steepness of from 20 to
50. Said method may be conducted in the presence or absence of
grindable inorganic particulate material.
[0041] In certain embodiments, after the completion of grinding,
the surface roughness is at least the same as the surface roughness
at the beginning of grinding, as determined in accordance with the
methods described herein. In certain embodiments, the particulate
grinding medium wears rough during grinding, such that, after
completion of grinding, the surface roughness is greater than the
surface roughness at the beginning of grinding, as determined in
accordance with the methods described herein. For example, in
certain embodiments, the surface roughness increases by at least
about 1% during grinding (i.e., the surface roughness at the end of
the grinding process is at least about 1% greater than the surface
roughness at the beginning of the grinding process), or increases
by at least about 2% during grinding, or increases by at least 3%
during grinding, or increases by at least 4% during grinding, or
increases by at least 5% during grinding, or increases by at least
6% during grinding, or increases by at least 7% during grinding, or
increases by at least 8% during grinding, or increases by at least
9% during grinding, or increases by at least 10% during grinding,
or increases by at least 11% during grinding, or increases by at
least 12% during grinding, or increases by at least 13% during
grinding, or increases by at least 14% during grinding, or
increases by at least 15% during grinding, or increases by at least
16% during grinding, or increases by at least 17% during grinding,
or increases by at least 18% during grinding, or increases by at
least 19% during grinding, or increases by up to about 20% during
grinding. The provision and use of a grinding medium which wears
rough (or at least retains at least 90% of its initial surface
roughness) during grinding is contrary to conventional grinding
media, which would normally smoothen during grinding. The provision
and use of a grinding medium which already possesses a surface
roughness greater than that of conventional grinding media and
which additionally wears rough during the grinding process may
provide additional benefits, such as, for example, continued
savings in total energy input during the grinding process, and/or
additional improvements in one or more properties, e.g., a strength
property of the microfibrillated cellulose and/or paper products
(e.g., burst strength) comprising the microfibrillated cellulose,
and/or less, or even no, need to replenish the grinding process
with fresh grinding media having the required surface roughness
and/or coefficient of friction.
[0042] Thus, in certain embodiments, there is provided a method of
manufacturing microfibrillated cellulose by microfibrillating a
fibrous substrate comprising cellulose by grinding in the presence
of a particulate grinding medium, as described herein, without
replenishing the method with fresh grinding media, wherein at the
beginning of the grinding the particulate grinding medium has: (i)
a surface roughness of at least about 0.5 um; or (ii) a mean
coefficient of friction of at least about 0.10; or both (i) and
(ii).
[0043] Further, according to certain embodiments, there is provided
a method of simultaneously manufacturing (a) microfibrillated
cellulose and (b) a roughened particulate grinding medium,
comprising grinding a fibrous substrate comprising cellulose by
grinding in the presence of a particulate grinding medium, as
described herein, which has at the beginning of grinding: (i) a
surface roughness of at least about 0.5 um; or (ii) a mean
coefficient of friction of at least about 0.10; or both (i) and
(ii).
[0044] Alternatively or additionally, advantageously additionally,
in certain embodiments, after the completion of grinding, the mean
coefficient of friction is at least about 90% of the mean
coefficient of friction at the beginning of grinding, for example,
at least about 92% of the mean coefficient of friction at the
beginning of grinding, or at least about 94% of the mean
coefficient of friction at the beginning of grinding, or at least
about 96% of the mean coefficient of friction at the beginning of
grinding, or at least about 98% of the mean coefficient of friction
at the beginning of grinding, or at least about 99% of the mean
coefficient of friction at the beginning of grinding, as determined
in accordance with the methods described herein. In certain
embodiments, after the completion of grinding, the mean coefficient
of friction is at least the same as the surface roughness at the
beginning of grinding, as determined in accordance with the methods
described herein. In certain embodiments, the particulate grinding
medium wears rough during grinding, such that, after completion of
grinding, the mean coefficient of friction is greater than the mean
coefficient of friction at the beginning of grinding, as determined
in accordance with the methods described herein. For example, in
certain embodiments, the mean coefficient of friction increases by
at least about 1% during grinding, or increases by at least about
2% during grinding, or increases by at least 3% during grinding, or
increases by at least 4% during grinding, or increases by at least
5% during grinding, or increases by at least 6% during grinding, or
increases by at least 7% during grinding, or increases by at least
8% during grinding, or increases by at least 9% during grinding, or
increases by at least 10% during grinding, or increases by at least
11% during grinding, or increases by at least 12% during grinding,
or increases by at least 13% during grinding, or increases by at
least 14% during grinding, or increases by at least 15% during
grinding, or increases by at least 16% during grinding, or
increases by at least 17% during grinding, or increases by at least
18% during grinding, or increases by at least 19% during grinding,
or increases by up to about 20% during grinding.
[0045] It will be further understood that in certain embodiments, a
relatively small number of particles (e.g., five or less particles
in a representative sample of 100 particles) having a surface
roughness less than 0.5 .mu.m and/or a mean coefficient of less
than 0.10 may be present as a by product of the process by which
the particles of the grinding medium are made or handled.
[0046] In certain embodiments, the particulate grinding medium has
a specific gravity of at from about 3.5 to about 8.0, for example,
from about 3.5 to about 7.0, or from about 3.5 to about 6.5, or a
specific gravity of at least about 3.6, or at least about 3.7, or
at least about 3.8, or at least about 3.9, or at least about 4.0,
or at least about 4.1, or at least about 4.2, or at least about
4.3, or at least about 4.4, or at least about 4.5, or at least
about 4.6, or at least about 4.7, or at least about 4.8, or at
least about 4.9, or at least about 5.0, or at least about 5.1, or
at least about 5.2, or at least about 5.3, or at least about 5.4,
or at least about 5.5, or at least about 5.6, or at least about
5.6, or at least about 5.7, or at least about 5.8, or least about
5.9, or at least about 6.0. Higher specific gravities are preferred
since such grinding media have a reduced, or even no, tendency to
elutriate from the grinding vessel, e.g., a tower mill, during
manufacture of the microfibrillated cellulose. In addition, higher
specific gravities allow for an increase in mill productivity and
utilization. This is because denser media result in higher motor
power draw (i.e., greater motor efficiency); there is more energy
transferred to the particles per unit time within the grinder
volume when using higher specific gravity media. As a result, the
time to reach a target energy or particle size is reduced.
[0047] In certain embodiments, the particulate grinding medium
comprises, consists essentially of, or consists of, particles
having a particle size in the range of from about 0.5 mm to about
15 mm, for example, from about 0.5 mm to about 12 mm, for example,
from about 1 mm to about 10 mm, or from about 1 mm to about 8 mm,
or from about 1 mm to about 6 mm, or from about 1 mm to about 5 mm,
or from about 1 mm to about 4 mm, or from about 1 mm to about 3 mm.
The term `particle size` used in this context is understood by
persons of skill in the art to mean that the particles pass through
a sieve having an aperture size corresponding to the size of the
particles. Thus, by way of example, passing particles through a
sieve having an 8 mm aperture size would produce grinding medium
particles with a particle size of no greater than 8 mm. Similarly,
a grinding medium having a particle size of from about 1 mm to
about 3 mm means that the grinding medium could be obtained using
screens with aperture sizes of about 1 mm (minimum) and about 3 mm
(maximum), respectively.
[0048] The particulate grinding medium may be formed of natural or
synthetic material, for example, are formed of a dense, hard
mineral, ceramic or metallic material suitable for use as a
grinding media. In certain embodiments, the particulate grinding
medium is a ceramic grinding medium. Such materials include
alumina, zirconia, zirconium silicate, yttria, ceria, or yttria
and/or ceria stabilized zirconia, and mixtures thereof. In certain
embodiments, the particulate ceramic grinding medium may have a
composite structure of more than one material, e.g., alumina and
zirconia, or alumina and zirconium silicate, or alumina and
mullite. In certain embodiments, the particulate grinding medium
does not consist exclusively of mullite. In certain embodiments,
the particulate grinding medium does not contain mullite.
[0049] The particulate grinding medium may be formulated to
restrict the SiO.sub.2 content to a specific low level, e.g., less
than about 4 weight %, and preferably not more than about 2 weight
%. The particulate grinding medium may contain no more than 10
weight percent iron oxide, for example, no more than 8 weight %
iron oxide, or no more than 6 weight % iron oxide, or no more than
4 weight % iron oxide, or no more than 2 weight % iron oxide, or no
more than 1 weight % iron oxide.
[0050] In certain embodiments of the first or second aspects, the
particulate grinding medium is a particulate grinding medium
according to the third aspect, as described in detail below.
[0051] The particulate grinding media may comprise particles of any
suitable shape, e.g., balls, beads, cylpebs, pellets, rods, discs,
cubes, toroids, cones, and the like.
[0052] In certain embodiments, the particulate grinding media
comprises substantially spherical particles, e.g., balls and/or
beads. For example, the grinding media may comprise at least 10% by
weight of substantially spherical particles, or may comprise at
least 20% by weight of substantially spherical particles, or may
comprise at least 30% by weight of substantially spherical
particles, or may comprise at least 40% by weight of substantially
spherical particles, or may comprise at least 50% by weight of
substantially spherical particles, or may comprise at least 60% by
weight of substantially spherical particles, or may comprise at
least 70% by weight substantially spherical particles, or may
comprise at least 80% by weight of substantially spherical
particles, or may comprise at least 90% by weight of substantially
spherical particles, or may comprise essentially only (e.g., 95% by
weight or more, or at least 99% by weight) substantially spherical
particles.
[0053] In certain embodiments, the grinding medium comprises
rod-shaped particles, for example, rod-shaped particles having an
aspect ratio of equal to or greater than about 2:1.
[0054] The rod-shaped particles are solid bodies which have an axis
running the length of the body about which an outer surface is
defined, and opposite end surfaces. The outer surface and the
opposite end surfaces together define the body. In certain
embodiments, the lengthwise axis is substantially rectilinear, by
which we mean that the line representing the shortest distance
between the two ends falls completely within the body. In other
embodiments, the rod-shaped particles may take an arcuate form in
which the axis is curvilinear and the line representing the
shortest distance does not fall completely within the body.
Mixtures of rod-shaped bodies having a rectilinear axis and bodies
having an arcuate form are contemplated, as are embodiments in
which substantially all (for example 90% by weight or 95% by weight
or 99% by weight) of the rod-shaped particles of aspect ratio of
2:1 or more either have the rectilinear form or have the arcuate
form.
[0055] In certain embodiments, the cross section of the rod-shaped
particles is substantially constant along the length of the
particle. By "substantially constant" is meant that the major
dimension of the cross-section does not vary by, for example, more
than 20% or by more than 10% or by more than 5%. In another
embodiment, the cross-section of the rod-shaped particles varies
along the length of the particle by, for example, by more than 20%.
For example, the body of the rod-shaped particle may take the form
of a barrel in which the cross-section at each of the ends of the
body of the particle is less than a cross-section measured between
the ends; or for example, the body of the rod-shaped particle may
take the form of an inverse barrel in which the cross-section at
each of the ends of the particle is greater than a cross-section
measured between the ends. The cross-sectional shape of the
rod-shaped particles may be symmetrical or asymmetrical. For
example, the cross-sectional shape may be circular or substantially
circular, or may be substantially ovoid. Other shapes include
angular shapes such as triangles, squares, rectangles, stars (five
and six-pointed), diamonds, etc. The boundary between the outer
lengthwise surface and the opposite end surfaces may be angular,
i.e. having a discrete sharp boundary, or non-angular, i.e. being
rounded or radiused. The end surfaces may be flat, convex or
concave.
[0056] As previously noted, the aspect ratio of the rod-shaped
particles is advantageously 2:1 or more than 2:1. The aspect ratio
is to be understood as the ratio of the longest dimension of the
particle to the shortest dimension. For present purposes, the
longest dimension is the axial length of the rod-shaped particles.
Where the particle has a constant cross-section along its length,
the shortest dimension for purposes of defining the aspect ratio is
the largest dimension of the cross-section which passes through the
geometric centre of the particle cross-section. Where the
cross-section varies along the length of the particle, the shortest
dimension for purposes of defining the aspect ratio is the largest
dimension at the point at which the cross-section is at a maximum.
Where the particle has an irregular shaped cross-section, the
shortest dimension for the purposes of defining the aspect ratio is
the maximum transverse dimension perpendicular to the axis of the
rod-shaped particle. An example of suitable rod-shaped particles
for use in certain embodiments of the invention are particles
having a substantially rectilinear axis and a substantially
circular cross section.
[0057] Another example of suitable rod-shaped particles for use in
certain embodiments of the invention are particles having a arcuate
form and a substantially circular cross-section. In both these
examples, the boundary between the outer lengthwise surface and the
opposite end surfaces is rounded and the ends are generally flat or
convex. In certain embodiments, the rod-shaped particles have an
aspect ratio of 2.5:1 or more than 2.5:1, or an aspect ratio of 3:1
or more than 3:1, or an aspect ratio of 4:1 or more than 4:1, or an
aspect ratio of 5:1 or more than 5:1, or an aspect ratio of 6:1 or
more than 6:1. The aspect ratio may be 10:1 or less than 10:1, or
may be 9:1 or less than 9:1 or may be 8:1 or less than 8:1 or may
be 7:1 or less than 7: or may be 6:1 or less than 6:1 or may be 5:1
or less than 5:1. The aspect ratio may be in the range of from 2:1
to 10:1 or may be in the range of from 2:1 to 5:1 or may be in the
range 3:1 to 8:1 or may be in the range of from 3:1 to 6:1.
[0058] In certain embodiments, the axial length of the rod-shaped
particles ranges from about 1 mm to about 5 mm, or from about 2 mm
to about 4 mm. In another embodiment, the rod length is less than
about 3 mm.
[0059] In certain embodiments, the grinding media may comprise
(i.e., in addition to the rod-shaped particles having an aspect
ratio of 2:1 or more) other particles selected from rod-shaped
particles having an aspect ratio less than 2:1 and particles having
other shapes such as spheres, cylpebs, cubes, discs, toroids,
cones, and the like. For example, the grinding media may comprise
at least 10% by weight of rod-shaped particles having an aspect
ratio of 2:1 or more, or may comprise at least 20% by weight of
rod-shaped particles having an aspect ratio of 2:1 or more, or may
comprise at least 30% by weight of rod-shaped particles having an
aspect ratio of 2:1 or more, or may comprise at least 40% by weight
of rod-shaped particles having an aspect ratio of 2:1 or more, or
may comprise at least 50% by weight of rod-shaped particles having
an aspect ratio of 2:1 or more, or may comprise at least 60% by
weight of rod-shaped particles having an aspect ratio of 2:1 or
more, or may comprise at least 70% by weight of rod-shaped
particles having an aspect ratio of 2:1 or more, or may comprise at
least 80% by weight of rod-shaped particles having an aspect ratio
of 2:1 or more, or may comprise at least 90% by weight of
rod-shaped particles having an aspect ratio of 2:1 or more, or may
comprise essentially only (e.g. 95% by weight or more) rod-shaped
particles having an aspect ratio of 2:1 or more. It will be further
understood that in certain embodiments of the invention, a
relatively small number of shaped particles having an aspect ratio
smaller than 2:1 may be present as a by-product of the process by
which the particles are made or handled. Similarly, rod-shaped
particles having a relatively high aspect ratio such as, for
example, greater than about 10:1, may be added to the grinding
process, in which case these rods may snap to their own preferred
length during the grinding process. It will also be understood that
as the grinding process progresses the shape of at least some of
the rod-shaped particles may evolve such that the ends round off,
and the aspect ratio lowers, and in some cases the virgin
rod-shaped particles may eventually become small spheres, so a
typical mature grinder may contain rods, worn rods and even
spheres. Thus, a "worked-in" sample of rod-shaped particles which
originally had an aspect ratio at least 2:1 or more may contain a
majority (if worked long enough) of particles somewhat different in
shape to the rod-shaped particles comprised in the virgin media.
The grinder may be topped up with fresh media comprising rod-shaped
particles having an aspect ratio of 2:1 or more.
[0060] 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.
[0061] 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. The microfibrillated step may be carried out in an aqueous
environment, i.e., under wet-grinding conditions. In another
embodiment, the microfibrillating step is carried out in a
homogenizer.
[0062] In certain embodiments, the microfibrillating process, e.g.,
grinding, is carried out in the presence of grindable inorganic
particulate material. In certain embodiments, the grinding is
carried out in the absence of grindable inorganic particulate
material.
[0063] The grinding medium 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. In certain embodiments, the
grinding medium is present in an amount from about 30 to about 70%
by volume of the charged, for example, from about 40 to about 60%
by volume of the charge, for example, from about 45 to about 55% by
volume of the charge.
[0064] By `charge` is meant the composition which is the feed fed
to the grinder vessel. The charge includes water (when present),
grinding media, fibrous substrate comprising cellulose and
inorganic particulate material (when present), and any other
optional additives (when present) as described herein.
[0065] The grinding may be performed in a vertical mill or a
horizontal mill.
[0066] In certain embodiments, the grinding is 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.
[0067] In one embodiment, the grinding vessel is a vertical mill,
for example, a stirred mill, or a stirred media detritor, or a
tower mill.
[0068] The vertical 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.
[0069] 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 (when present)
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.
[0070] 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.
[0071] 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.
[0072] 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 (when present) inorganic
particulate material and to enhance grinding media
sedimentation.
[0073] In another embodiment, the grinding is performed in a
screened grinder, for example, a stirred media detritor. The
screened grinder may comprise one or more screen(s) sized to
separate grinding media from the product aqueous suspension
comprising microfibrillated cellulose and inorganic particulate
material. 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, or at least about 1250 .mu.m, or at least about 1500
.mu.m. In certain embodiments, the screened grinder may comprise
one or more screen(s) having a nominal aperture size of up to about
4000 .mu.m, for example, up to about 3500 .mu.m, or up to about
3000 .mu.m, or up to about 2500 .mu.m, or up to about 2000
.mu.m.
[0074] In certain embodiments, 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.
[0075] As described herein, the total amount of energy used in the
method (i.e., total energy input) may be less than that used in a
comparable method in which the particulate grinding medium has at
the beginning of grinding (i) a surface roughness which is less
rough and/or (ii) a lesser mean coefficient of friction than that
required by the method of the first aspect of the present
invention. As such, the present inventors have surprisingly found
that a cellulose pulp can be microfibrillated at relatively lower
energy input when it is ground in the presence of particulate
grinding medium having i) a surface roughness of at least about 0.5
.mu.m, or (ii) a mean coefficient of friction of at least about
0.10, or both (i) and (ii). In other words, the particulate
grinding medium may be used in order to reducing the energy input
per unit amount of microfibrillated cellulose produced. Further, as
described above, in certain embodiments, the use of a particulate
grinding medium having i) a surface roughness of at least about 0.5
.mu.m, or (ii) a mean coefficient of friction of at least about
0.10, or both (i) and (ii) may improve one or more properties of
the microfibrillated cellulose, e.g., a strength property of the
microfibrillated cellulose and/or paper products (e.g., burst
strength) comprising the microfibrillated cellulose.
[0076] When present, 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, perlite or diatomaceous earth, or magnesium
hydroxide, or aluminium trihydrate, or combinations thereof.
[0077] In certain embodiments, the inorganic particulate material
comprises or is calcium carbonate. Hereafter, certain embodiments
of 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.
[0078] The particulate calcium carbonate used in certain
embodiments of 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 autogeneously, 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.
[0079] Precipitated calcium carbonate (PCC) may be used as the
source of particulate calcium carbonate in certain embodiments of
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 certain
embodiments 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, all of which are suitable for use in certain
embodiments of the present invention, including mixtures
thereof.
[0080] 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.
[0081] 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.
[0082] When the inorganic particulate material 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 certain
embodiments of the invention will contain less than about 5% by
weight, preferably less than about 1% by weight, of other mineral
impurities.
[0083] 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.
[0084] In certain embodiments, at least about 50% by weight of the
particles have an e.s.d of less than 2 .mu.m, for example, at least
about 55% by weight of the particles have an e.s.d of less than 2
.mu.m, or at least about 60% by weight of the particles have an
e.s.d of less than 2 .mu.m
[0085] 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 (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.
[0086] 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.
[0087] Thus, in another embodiment, the inorganic particulate
material may have a particle size distribution, as measured by the
well known conventional method employed in the art of laser light
scattering, 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.
[0088] In certain embodiments, at least about 50% by volume of the
particles have an e.s.d of less than 2 .mu.m, for example, at least
about 55% by volume of the particles have an e.s.d of less than 2
.mu.m, or at least about 60% by volume of the particles have an
e.s.d of less than 2 .mu.m
[0089] Details of the procedure that may be used to characterise
the particle size distributions of mixtures of inorganic particle
material and microfibrillated cellulose using the well known
conventional method employed in the art of laser light scattering
are provided in WO-A-2010/131016 at page 40, line 32 to page 41,
line 34, the entire contents of which are hereby incorporated by
reference.
[0090] 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.
[0091] Kaolin clay used in certain embodiments of 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.
[0092] 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.
[0093] 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.
[0094] 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.
[0095] The process for preparing the particulate kaolin clay used
in certain embodiments of 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.
[0096] The relative amounts of inorganic particulate material and
cellulosic material, including microfibrillated cellulose, 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 cellulosic
material, 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
cellulosic material. For example, the ratio of the amount of
inorganic particulate material and cellulosic material may be from
about 99.5:0.5 to about 70:30. In certain embodiments, the ratio of
inorganic particulate material to cellulosic material 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.
[0097] In certain embodiment, the microfibrillated cellulose
obtainable by the method of the first aspect comprises up to about
80% by weight water, for example, up to about 75% water, or up to
about 70%, or up to about 65% by weight water, or up to about 60%
by weight water, or up to about 55% by weight water, or up to about
50% by weight water, or up to about 45% by weight water, or up to
about 40% by weight water, or up to about 35% by weight water, or
up to about 30% by weight water, or up to about 25% by weight
water.
[0098] In certain embodiments, microfibrillated cellulose
obtainable by the method of the first aspect comprises from about
50 to about 70% by weight water, for example, from about 55 to
about 65% by weight water, or from about 60 to about 70% by weight
water, or from about 60 to about 65% by weight water, or from about
65 to about 70% by weight water.
[0099] The microfibrillated cellulose obtainable by the method of
the first aspect may comprise other optional additives including,
but not limited to, 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.
[0100] In certain embodiments in which a grindable inorganic
particulate is present, the fibrous substrate comprising cellulose
and inorganic particulate material are present in the aqueous
environment at an initial solids content of at least about 2 wt %,
of which at least about 2% by weight is fibrous substrate
comprising cellulose, for example, an initial solids content of
from about 2% by weight to about 20% by weight, or from about 4% by
weight to about 15% by weight, or from about 5% by weight to about
12% by weight, or from about 7% by weight to about 10% by weight.
In such embodiments, 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. In certain embodiments, no more
than about 40% by weight of the initial solids content is fibrous
substrate comprising cellulose, for example, no more than about 30%
by weight of the initial solids content is fibrous substrate
comprising cellulose, or no more than about 25% by weight of the
initial solids content is fibrous substrate comprising
cellulose
[0101] The grinding process may include a pre-grinding step in
which coarse inorganic particulate is 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.
[0102] As the suspension of material to be ground may be of a
relatively high viscosity, a suitable dispersing agent may be added
to the suspension prior to or during 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.
[0103] 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.
[0104] In certain embodiments, the product of the process 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, 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 product of the grinding
process may be removed. Any suitable technique can be used to
remove water from the product 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 optionally
inorganic particulate material and any other optional additives
that may have been added 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.
[0105] 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.
[0106] 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.
[0107] 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)
[0108] 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.
[0109] A suitable procedure for characterising the particles size
distribution of microfibrillated cellulose, and mixtures of
inorganic particulate material and microfibrillated cellulose, is
described in WO-A-2010/131016, at page 40, line 32 to page 41 line,
34.
Grinding Medium According to Third, Fourth and Fifth Aspects,
Optionally for Use in First and Second Aspects
[0110] The particulate ceramic grinding medium of the third aspect
has (i) a surface roughness of at least about 0.5 .mu.m, or (ii) a
mean coefficient of friction of at least about 0.10, or both (i)
and (ii). The grinding medium is formed by sintering a composition
comprising at least one of zirconia (ZrO.sub.2), e.g.,
ceria-stabilised zirconia, and alumina (Al.sub.2O.sub.3).
[0111] In certain embodiments, the composition comprises zirconia
(ZrO.sub.2), meaning that the particulate ceramic grinding medium
formed by sintering such a composition will contain a zirconia
phase.
[0112] In certain embodiments, the composition further comprises
from about 5 wt. % to about 25 wt. % ceria (Ce.sub.2O.sub.3), based
on the total weight of the composition, for example, from about 10
wt. % to about 20 wt. % ceria, or from about 12 wt. % to about 18
wt. % ceria, or from about 10 wt. % to about 15 wt. % ceria, or
from about 11 wt. % to about 14 wt. % ceria, or from about 11 wt. %
to about 13 wt. % ceria. Additionally, the composition may comprise
at least about 40 wt. % zirconia, for example, from about 40 wt. %
to about 90 wt. % zirconia, or from about 40 wt. % to about 80 wt.
% zirconia, or from about 50 wt. % to about 70 wt. % zirconia, or
from about 55 wt. % to about 70 wt. % zirconia, or from about 60
wt. % to about 75 wt. % zirconia, or from about 65 wt. % to about
75 wt. % zirconia, or from about 65 wt. % to about 70 wt. %
zirconia, based on the total weight of the composition.
Additionally, the composition may comprise up to about 40 wt. %
alumina, for example, up to about 30 wt. % alumina, or from about 1
wt. % to about 40 wt. % alumina, or from about 5 wt. % to about 30
wt. % alumina, or from about 10 wt. % to about 25 wt. % alumina, or
from about 10 wt. % to about 20 wt. % alumina, or from about 12 to
about 20 wt. % alumina, or from about 14 wt. % to about 20 wt. %
alumina, or from about 14 to about 18 wt. % alumina.
[0113] In embodiments in which the composition comprises ceria and
zirconia, or ceria, zirconia and alumina, the ceria and zirconia
may be in the form of a ceria-stabilised zirconia. In certain
embodiments, the ceria-stabilized zirconia comprises from about 10
wt. % to about 20 wt. % ceria, and up to about 90 wt. % zirconia,
based on the total weight of the ceria stabilized zirconia, for
example, from about 12 to about 18 wt. % ceria and up to about 88
wt. % zirconia, or from about 14 wt. % to about 16 wt. % and up to
about 86 wt. % zirconia, or up to about 85 wt. % zirconia, or up to
about 84 wt. % zirconia.
[0114] In certain embodiments, the ceria-stabilised zirconia
comprises no more than about 2 wt. % iron oxide, for example, no
more than about 1 wt. % iron oxide, or no more than about 0.75 wt.
% iron oxide, or no more than about 0.5 wt. % iron oxide, or from
about 0.1 wt. % to about 0.75 wt. % iron oxide, or from about 0.2
wt. % to about 0.6 wt. % iron oxide.
[0115] In certain embodiments, the composition comprises at least
about 10 wt. % alumina with the balance ceria-stabilised zirconia
(which may comprise a minor amount of iron oxide, as described
above) in which the ceria-stabilized zirconia contains relative
amounts of ceria and zirconia as described above. In certain
embodiments, the composition comprises from about 10 wt. % to about
30 wt. % alumina, with the balance ceria stabilized zirconia, for
example about 15 wt. % to about 25 wt. % alumina, with the balance
ceria-stabilized zirconia.
[0116] In certain embodiments, the composition comprises from about
15 wt. % to about 25 wt. % alumina, from about 10 wt. % to about 15
wt. % ceria, and from about 50 wt. % to about 75 wt. %
zirconia.
[0117] In certain embodiments, the particulate ceramic grinding
medium is formed by sintering a composition comprising at least
about 90 wt. % alumina, for example, at least about 95 wt. %
alumina, or at least about 99 wt. % alumina, or at least about 99.5
wt. % alumina, or at least about 99.9 wt. %, or substantially 100
wt. % alumina. For example, the particulate grinding medium may be
made by sintering an alumina-containing material, such as, for
example, technical grade alumina, bauxite or any other suitable
combination of oxides thereof.
[0118] In certain embodiments, the particulate ceramic grinding
medium according to the third and fourth aspects, is obtainable by
a method comprising:
[0119] a. obtaining, providing or making a composition comprising
raw materials suitable for making the ceramic grinding medium;
[0120] b. mixing the composition comprising raw materials, forming
a mixture;
[0121] c. combining the mixture with binder and/or solvent, forming
a bound mixture;
[0122] d. granulating the bound mixture by mixing the bound mixture
over a period of time during which the mixing speed is reduced;
[0123] e. optionally drying the granulated composition;
[0124] f. optionally shaping the granulated composition;
[0125] g. optionally sizing the granulated composition; and
[0126] h. sintering the granulated composition.
[0127] In certain embodiments, the raw materials in step b) of the
method are homogenized, e.g., by mixing, forming a homogenized
composition. By `homogenized` is meant that the mixture of raw
materials has a uniform composition throughout. In such
embodiments, the homogenized composition is combined with binder
and/or solvent in step c), forming a bound homogenized composition,
which is granulated in step d) by mixing the bound homogenized
composition over a period of time during which the mixing speed is
reduced.
[0128] The binding agent and/or solvent is one of those well known
in the industry. Possible binding agents include, for example,
methyl cellulose, polyvinyl butyrals, emulsified acrylates,
polyvinyl alcohols, polyvinyl pyrrolidones, polyacrylics, starch,
silicon binders, polyacrylates, silicates, polyethylene imine,
lignosulfonates, alginates, etc. In certain embodiments, a
polyvinyl alcohol binder is used.
[0129] Possible solvents may include, for example, water, alcohols,
ketones, aromatic compounds, hydrocarbons, etc.
[0130] Other additives well known in the industry may be added as
well. For example, lubricants may be added, such as ammonium
stearates, wax emulsions, oleic acid, Manhattan fish oil, stearic
acid, wax, palmitic acid, linoleic acid, myristic acid, and lauric
acid. Plasticizers may also be used, including polyethylene glycol,
octyl phthalates, and ethylene glycol.
[0131] In certain embodiments, homogenizing comprises mixing the
composition comprising raw materials for a suitable period of time
such that the mixture of raw materials has a uniform composition
throughout. In certain embodiments, step c) comprises mixing the
homogenized composition with the binder and/or solvent. In certain
embodiments, the mixing speed during step b) is greater than the
mixing step in step c), and an initial mixing speed in step d) is
no greater than a final mixing speed in step c).
[0132] In certain embodiments, mixing or homogenizing in step b)
comprises mixing the composition comprising raw materials for a
period of time from about 1 minute to about 60 minutes, for
example, from about 1 minute to about 30 minutes, or from about 1
minute to about 20 minutes, or from about 1 minute to about 10
minutes, or from about 2 minutes to about 10 minutes, or from about
2 minutes to about 8 minutes, or from about 2 minutes to about
minutes. Typically, the mixing speed is held constant during step
b).
[0133] In certain embodiments, combining, e.g. mixing, the mixture
or homogenized composition with the binder and/or solvent may be
carried over a period of time of from about 30 seconds to about 30
minutes, for example, from about 30 seconds to about 20 minutes, or
from about 30 seconds to about 10 minutes, or from about 1 minute
to about 8 minutes, or from about 1 minute to about 5 minutes, or
from about 2 minutes to about 5 minutes, or from about 2 minutes to
about 4 minutes. As described above, the mixing speed during step
c) is preferably less than the mixing speed in step b), and
optionally at least the same as or greater than the initial mixing
speed in step d). The binder and/or solvent may be added slowly
during this step, e.g., continuously, or intermittently, preferably
continuously. Alternatively, the all of the binder and/or solvent
may be added at the beginning of mixing.
[0134] In certain embodiments, granulating the homogenized, bound
composition, comprises mixing the composition over a period of time
during which the mixing speed is gradually or stepwise reduced. A
suitable period of time may be from about 1 minute to about 60
minutes, for example, from about 2 minutes to about 30 minutes, or
from about 3 minutes to about 20 minutes, or from about 4 minutes
to about 15 minutes, or from about 4 minutes to about 12 minutes,
or from about 4 minutes to about 10 minutes, or from about 4
minutes to about 8 minutes. During the suitable period of time, the
mixing speed may be reduced, e.g., stepwise, such that the final
mixing speed is at least about 25% less than the initial mixing
speed in step d), for example, at least about 30% less, or at least
about 35% less, or at least about 40% less, or at least about 45%
less than the initial mixing speed in step d).
[0135] In certain embodiments, an initial mixing speed in step b)
is at least about 150% greater than a final mixing speed in step
d), for example, at least about 175% greater, or at least about
190% greater, or at least about 200% greater, or at least about
210% greater.
[0136] The various mixing stages may be performed in any suitable
mixing apparatus, for example, a mixer equipped with an impeller.
An exemplary mixing apparatus is an Eirich mixer type RV02E
equipped with a pin type impellor.
[0137] In certain embodiments, the initial impeller speed in step
b) is between about 2750 and 3250 rpm, and the final impeller speed
in step d) is between about 600 and 1200 rpm. In certain
embodiments, the impeller speed in step b) is between about 2750
and 3250 rpm, and the impeller speed during step c) is between
about 2000 and 2500 rpm. In such embodiments, the initial impeller
speed in step d) is no greater than, preferably less than the
impeller speed during step c), for example, less than about 2000
rpm, or less than about 1900 rpm, or less than about 1800 rpm. In
such embodiments, the final impellor speed in step d) may be less
than about 1500 rpm, for example, less than about 1200 rpm, or less
than about 1000 rpm, or less than about 800 rpm. The final mixing
speed, e.g., final impeller speed, may be held constant for a
period of time ranging from about 1 minute to about 10 minutes, for
example, from about 1 minute to about 8 minutes.
[0138] Following granulation, the granulated composition may be
removed from the mixer and dried. For example, at a temperature of
up to about 120.degree. C. for a suitable period of time, e.g.,
from about 10 minutes to about 5 hours, or from about 30 minutes to
about 2 hours. Before or during drying the granulated composition
may be shaped, e.g., to form rod-shaped particles.
[0139] The optionally dried composition may then be subjected to a
sizing process, e.g., by sieving. An appropriately sized sieve may
be selected corresponding to the desired size of particulate
grinding medium.
[0140] The particulated composition is then sintered at a suitable
sintering temperature. Suitable sintering temperatures range from
about 1200.degree. C. to about 1700.degree. C. The well time during
sintering may range from about 1 hour to about 24 hours, for
example, from about 2 hours to about 12 hours, or from about 2
hours to about 8 hours, or from about 2 hours to about 6 hours, or
from about 3 hours to about 5 hours, or from about 3.5 hours to
about 4.5 hours.
[0141] For embodiments in which the particulate ceramic grinding
media is formed from a composition comprising at least ceria and
zirconia, the sintering temperature is advantageously from about
1400.degree. C. to about 1500.degree. C., for example, from about
1425.degree. C. to about 1475.degree. C., or from about
1440.degree. C. to about 1460.degree. C., and a dwell time of from
about 2 hours to about 6 hours, for example, from about 3 hours to
about 5 hours, or from about 3.5 hours to about 4.5 hours.
[0142] For embodiments in which the particulated composition is
formed from a composition comprising at least about 90 wt. %
alumina, the sintering temperature is advantageously from about
1500.degree. C. to about 1700.degree. C., for example, from about
1550.degree. C. to about 1650.degree. C., or from about
1575.degree. C. to about 1625.degree. C., and a dwell time of from
about 2 hours to about 6 hours, for example, from about 3 hours to
about 5 hours, or from about 3.5 hours to about 4.5 hours.
[0143] In certain embodiments, the particulate grinding medium of
the third aspect may have a surface roughness and/or mean
coefficient as described above in connection with the particular
grinding medium used in the method according to the first aspect of
the present invention. As such, in certain embodiments, the
grinding medium of the third aspect wears rough during the during
the grinding process in which it is to be used, for example, during
grinding in a method according to the first aspect of the present
invention described herein.
[0144] Also provided, in accordance with the fourth aspect, is a
particulate grinding medium which wears rough during the grinding
process in which it is to be used, for example, during grinding in
a method according to the first aspect of the present invention
described herein. In certain embodiments, the particulate grinding
medium of the fourth aspect may have a surface roughness and/or
mean coefficient as described above in connection with the
particular grinding medium used in the method according to the
first aspect of the present invention. In certain embodiments, the
particulate grinding medium which wears rough during grinding has,
at the beginning of grinding, (i) a surface roughness of at least
about 0.5 .mu.m, or (ii) a mean coefficient of friction of at least
about 0.10, or both (i) and (ii). Said particulate grinding medium
may be formed of natural or synthetic material, for example, formed
of a dense, hard mineral, ceramic or metallic material suitable for
use as a grinding media. In certain embodiments, the particulate
grinding medium is a ceramic grinding medium. Such materials
include alumina, zirconia, zirconium silicate, yttria, ceria, or
yttria and/or ceria stabilized zirconia, and mixtures thereof.
[0145] In certain embodiments, the particulate grinding medium
according to the third and fourth aspects has a specific gravity of
at least about 3.5, for example, a specific gravity of from about
3.5 to about 8.0, for example, from about 3.5 to about 7.0, or from
about 3.5 to about 6.5, or a specific gravity of at least about
3.6, or at least about 3.7, or at least about 3.8, or at least
about 3.9, or at least about 4.0, or at least about 4.1, or at
least about 4.2, or at least about 4.3, or at least about 4.4, or
at least about 4.5, or at least about 4.6, or at least about 4.7,
or at least about 4.8, or at least about 4.9, or at least about
5.0, or at least about 5.1, or at least about 5.2, or at least
about 5.3, or at least about 5.4, or at least about 5.5, or at
least about 5.6, or at least about 5.6, or at least about 5.7, or
at least about 5.8, or least about 5.9, or at least about 6.0.
[0146] In certain embodiments, the particulate grinding medium is
used in the manufacture of microfibrillated cellulose. In certain
embodiments, particulate grinding medium is used for improving one
or more properties of the microfibrillated cellulose and/or for
reducing the energy input per unit amount of microfibrillated
cellulose produced.
[0147] In certain embodiments, the particulate grinding medium
according to the fourth and fifth aspects are used in a method for
manufacturing microfibrillated cellulose, said method comprising a
step of microfibrillating a fibrous substrate comprising cellulose
by grinding in the presence of the particulate grinding medium
which is to be removed after the completion of grinding.
[0148] In certain embodiments, a material is provided which wears
rough or roughens when agitated in the presence of a fibrous
substrate comprising cellulose. In certain embodiments, the
material, in particulate form, wears rough or roughens when ground
in the presence of a fibrous substrate comprising cellulose to
produce microfibrillated cellulose, as described herein. For the
avoidance of doubt, the material which wears rough or roughens is
other than the inorganic particulate material described herein
which according to certain embodiments may be co-ground with the
fibrous substrate comprising cellulose. In certain embodiments, the
material is a grinding media, for example, a grinding media
according to certain embodiments described herein. By "wears rough"
or "roughens" is meant that surface of the material measurably
roughens following agitation. The increase in surface roughness may
be visually discernible or determined in accordance with the
methods described herein. In certain embodiments, the material has
a specific gravity of at least about 3.5.
[0149] According to certain embodiments, provided is an unpolished
particulate grinding media having a surface roughness which
increases by at least about 1% when subject to abrasive contact. By
"unpolished" is meant that the grinding media has not been
subjected to any polishing treatment (i.e., to smoothen its
surface) prior to its use as a grinding media. The increase in
surface roughness may be determined in accordance with the methods
described herein. In certain embodiments, the unpolished
particulate grinding media has a surface roughness of at least
about 0.5 .mu.m, and/or (ii) a mean coefficient of friction of at
least about 0.10 prior to abrasive contact. Abrasive contact may be
an autogenous process (e.g., agitation in a mill or other suitable
grinding apparatus) or may be conducted in the presence of another
material, for example, another grinding media which, following
abrasive contact, is separable from the unpolished particulate
grinding media or, for example, a fibrous substrate comprising
cellulose which, during abrasive contact, may be ground producing
microfibrillated cellulose (e.g., microfibrillated cellulose
according to embodiments described herein).
[0150] In certain embodiments, the surface roughness increases by
at least about 5%, or at least about 10%, or at least about 15%, or
at least about 20%, or at least about 25%, or at least about 30%,
or at least about 35%, or at least about 40%, or at least about
45%, or at least about 50%. In certain embodiments, the material
has a specific gravity of at least about 3.5. In certain
embodiments, the unpolished particulate grinding media, prior to
abrasive contact, has a surface roughness of at least about 2.0
.mu.m, and/or (ii) a mean coefficient of friction of at least about
0.20, for example, a surface roughness of at least about 2.2 .mu.m,
or a surface roughness of at least about 2.4 .mu.m, or a surface
roughness of at least about 2.6 .mu.m, or a surface roughness of at
least about 2.8 .mu.m, or a surface roughness of at least about 3.0
.mu.m.
[0151] In certain embodiments, provided is a polished particulate
grinding media having a surface roughness which increases by at
least about 20% when subject to abrasive contact. By "polished" is
meant that the grinding media has been subjected to a polishing
treatment (i.e., to smoothen its surface) prior to its use as a
grinding media. The increase in surface roughness may be determined
in accordance with the methods described herein. In certain
embodiments, the polished particulate grinding media has a surface
roughness of at least about 0.5 .mu.m, and/or (ii) a mean
coefficient of friction of at least about 0.10 prior to abrasive
contact. Abrasive contact may be an autogenous process (e.g.,
agitation in a mill or other suitable grinding apparatus) or may be
conducted in the presence of another material, for example, another
grinding media which, following abrasive contact, is separable from
the polished particulate grinding media or, for example, a fibrous
substrate comprising cellulose which, during abrasive contact, may
be ground producing microfibrillated cellulose (e.g.,
microfibrillated cellulose according to embodiments described
herein). In certain embodiments, the surface roughness increases by
at least about 25%, or at least about 30%, or at least about 35%,
or at least about 40%, or at least about 45%, or at least about
50%. In certain embodiments, the material has a specific gravity of
at least about 3.5. In certain embodiments, the polished
particulate grinding media, prior to abrasive contact, has a
surface roughness of at least about 1.4 .mu.m, and/or (ii) a mean
coefficient of friction of at least about 0.08, or at least about
0.10, for example, a surface roughness of at least about 1.6 .mu.m,
or a surface roughness of at least about 1.8 .mu.m, or a surface
roughness of at least about 1.9 .mu.m.
Method of Making Particulate Grinding Medium
[0152] In certain embodiments, the particulate grinding medium may
be made by any suitable method in which a particulate grinding
having (i) a surface roughness of at least about 0.5 .mu.m, or (ii)
a mean coefficient of friction of at least about 0.10, or both (i)
and (ii), is produced.
[0153] The method may comprise forming a particulate grinding
medium which has a surface roughness of less than 0.5 .mu.m and/or
a mean coefficient of friction less than 0.10, and subjecting the
particulate grinding medium to a surface roughening step such that
the surface roughness is at least about 0.5 .mu.m, and/or the mean
coefficient of friction is at least about 0.10, at the end of the
surface roughening step. For example, a particulate grinding medium
initially not meeting the surface roughness and/or mean coefficient
of friction requirements of the first aspect may be co-ground with
an abrasive material, such as a micro abrasive powder (e.g., a
fused alumina micro abrasive powder, in a grinding vessel, such as
a planetary mill.
[0154] Advantageously, the particulate grinding medium of the first
aspect (as well as the second, third, fourth and fifth aspects) may
be made by a process comprising:
[0155] a. obtaining, providing or making a composition comprising
raw materials suitable for making the ceramic grinding medium;
[0156] b. mixing the composition comprising raw materials, forming
a mixture;
[0157] c. combining the mixture with binder, forming a bound
mixture;
[0158] d. granulating the bound mixture by mixing the bound mixture
over a period of time during which the mixing speed is reduced;
[0159] e. optionally drying the granulated composition;
[0160] f. optionally shaping the granulated composition;
[0161] g. optionally sizing the granulated composition; and
[0162] h. sintering the granulated composition.
[0163] In certain embodiments, the raw materials in step b) of the
method are homogenized, e.g., by mixing, forming a homogenized
composition. In such embodiments, the homogenized composition is
combined with binder and/or solvent in step c), forming a bound
homogenized composition, which is granulated in step d) by mixing
the bound homogenized composition over a period of time during
which the mixing speed is reduced.
[0164] Further embodiments and details of such a process are
described above in connection with making a particulate grinding
medium according to the third and/or fourth aspects.
Paper Products and Processes for Preparing Same
[0165] The composition obtainable by the first aspect of the
present invention comprising microfibrillated cellulose and (when
present) 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
certain embodiments of 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 certain embodiments of 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.
[0166] 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 certain embodiments of 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.
[0167] The papermaking composition typically comprises, in addition
to the composition comprising microfibrillated cellulose and (when
present) inorganic particulate material, paper stock and other
conventional additives known in the art. For example, a papermaking
composition may comprise up to about 50% by weight inorganic
particulate material derived from the composition 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, or at least about 90% by
weight of inorganic particulate material derived from the
composition comprising microfibrillated cellulose and inorganic
particulate material, based on the total dry contents of the
papermaking composition. 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.
[0168] Paper products according to certain embodiments of the
present invention may be made by a process 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
composition of certain embodiments of this invention comprising
microfibrillated cellulose and (when present) 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 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 will then facilitate the
formation of a pulp.
[0169] An additional filler component (i.e., a filler component
other than the inorganic particulate material which may be
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. Paper products made from such papermaking compositions may
exhibit greater strength (e.g., improved burst strength) compared
to paper products comprising microfibrillated cellulose made by a
comparable process in which the particulate grinding medium used in
the process has at the beginning of grinding (i) a surface
roughness which is less rough and/or (ii) a lesser mean coefficient
of friction than that required by the method of the first aspect of
the present invention. Similarly, paper products prepared from a
papermaking composition according to certain embodiments of the
present invention comprising inorganic particulate 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 certain
embodiments of the present invention at higher filler loadings
without loss of strength.
[0170] 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.
EXAMPLES
Example 1
[0171] Raw material compositions as described in Table 1 were each
filled into an Eirich mixer type RV02E equipped with a pin type
impeller and de-agglomerated and homogenized for 4 minutes at an
impeller speed of 3000 rpm. In a second step, in each case, the
impeller speed was lowered to 2200 rpm and binder solution (a 0.5
wt. % PVA solution in water) was added over a period of 3 minutes.
In a mixing step (i.e., granulation), the impeller speed was
stepwise reduced to a final speed to form beads. Details of the
final step mixing for each composition are provided in Tables 2A
and 2B.
[0172] The beads were removed from the mixer and dried at
60.degree. C. for 1 hour. The dried material was sieved. In each
case, a size fraction was used for sintering. For the 100% alumina
beads (Sample 2A) the sintering temperature was 1600.degree. C. and
a dwell time of 4 hours. For the ceria/yttria/alumina beads
(Samples 2B and 2C) the sintering temperature was 1450.degree. C.
and a dwell time of 4 hours.
TABLE-US-00001 TABLE 1 Amount of Raw material material in Sample
composition mixer 2A 100 wt. % alumina 5.5 kg 2B 20 wt. %
alumina/80 wt. % 6.5 kg of 14.5 wt. % ceria stabilised zirconia* 2C
50 wt. % alumina/50 wt. % 6.5 hg of 14.5 wt. % ceria stabilised
zirconia* *15.5 wt. % Ce.sub.2O.sub.3, 84 wt. % ZrO.sub.2, 0.5 wt.
% Fe.sub.2O.sub.3
TABLE-US-00002 TABLE 2A Impeller speed, rpm-Sample 2A Time, min
1750 2 1500 2 1100 2
TABLE-US-00003 TABLE 2B Impeller speed, rpm-Samples 2B & 2C
Time, min 1750 2 1500 2 1000 5 750 5
[0173] The formulated beads of Sample 2B have a specific gravity of
5.57; formulated beads of Sample 2C have a specific gravity of
4.74.
Example 2
[0174] Beads samples 2B and 2C and a comparative zirconia media
were used to prepare microfibrillated cellulose.
Ingredients Used in the Production of Microfibrillated
Cellulose:
[0175] unrefined Botnia pulp [0176] ground calcium carbonate having
a particle size distribution such that about 60 wt. % of the
particles have an p.s.d. of less than 2 .mu.m [0177] grinding media
2B and 2C [0178] comparative zirconia grinding media having a
surface roughness of less than 0.5 .mu.m
Grinding Conditions:
[0178] [0179] Target total solids and POP (Percentage Of
Pulp--percentage of the filler dry weight that is pulp): 9% and 20%
POP respectively [0180] Target total solids and POP: 15% and 20%
POP respectively [0181] Target MVC (media volume concentration):
45% [0182] 1000 rpm [0183] Energy input--2000, 2500 and 3500
kWh/t
[0184] Each media type was split into 3 equal portions. Each
portion was then used for grinding at only one specific energy
level for 8 batches without mixing the different portions.
[0185] Microfibrillated cellulose samples produced were analysed as
follows: [0186] Particle size distribution was determined using
Malvern `S` instrument, in accordance with the method described
above, [0187] Total solids content and POP of samples were
measured
[0188] The products prepared according to the above procedures were
evaluated as fillers in handsheets. Generally, a batch of bleached
chemical pulp comprising 70 parts eucalyptus and 30 parts northern
bleached softwood pulp was beaten in a valley beater to give a CSF
of 520 cm.sup.3. After disintegration and dilution to 2% thick
stock, the fibre was diluted to 0.3 wt. % consistency for sheet
making.
[0189] Filler slurry (comprising the microfibrillated cellulose and
calcium carbonated particulate) was added together with retention
aid (Ciba, Percol 292, 0.02 wt. % on furnish). Handsheets were made
to a basis weight of 80 gm.sup.-2 using a British handsheet mold
according to standard methods (e.g. SCAN C 26:76 (M 5:76). Sheets
were prepared at approximately 15 and 25 parts inorganic
particulate loading and the burst strength value at 20% inorganic
particulate loading interpolated from these data. The burst at 20%
loaded was expressed as a percentage of the unfilled value, and
then the normalized for comparison.
[0190] Paper burst strength was determined using a Messemer Buchnel
burst tester according to SCAN P24.
[0191] Results are summarised in FIG. 1. It is seen that
microfibrillated cellulose produced using media samples 2B and 2C
gave better strength improvement when incorporated in paper
compared to microfibrillated cellulose produced using the zirconia
media. Moreover, microfibrillated cellulose produced using media
samples 2B and 2C at an energy input of 2000 kWh/t gave better
strength improvement than microfibrillated cellulose produced using
zirconia media at a higher energy input of 2500 kWh/t.
Example 3--Analysis of Media after Grinding
[0192] Beads were collected after every other grind and
analysed/characterised using an interferometer and tribometer, in
accordance with the methods described in Appendices 1 and 2 below.
The interferometer was used to characterize the media surface
roughness and the tribometer was used to determine the coefficient
of friction of the media when rubbed over a dry fibre pad (made
from softwood, Botnia pine).
[0193] The interferometer used was a phase shifting interferometer
which uses monochromatic light (Omniscan MicroXAM2) to measure the
media surface roughness and topography.
[0194] A Longshore Systems Engineering tribometer was used to
determine the coefficient of friction of the bead samples.
[0195] Results are summarised in Table 3.
TABLE-US-00004 TABLE 3 S.sub.a St. Mean Coef. of .mu.m Err Friction
(.mu.) Zirconia Used 0.36 0.03 0.164 Media 2B As Received 6.10 0.86
0.466 After 2 grinds 3.48 0.32 0.346 After 4 grinds 5.68 0.73 0.323
After 6 grinds 3.49 0.14 0.309 After 8 grinds 3.11 0.29 0.264 Media
2C As Received 4.57 0.75 0.321 After 2 grinds 2.95 0.23 0.370 After
4 grinds 2.91 0.30 0.290 After 6 grinds 2.15 0.14 0.283 After 8
grinds 2.93 0.27 0.229 S.sub.a = average surface roughness
(arithmetic mean)
APPENDIX 1
Interferometer Operation
(Omniscan MicroXAM2)
[0196] 1. Switch power on [0197] 2. Boot up PC [0198] 3. Stick down
5 specimen particles onto a glass slide (measure roughness at two
locations of each particle) [0199] 4. Locate each specimen particle
directly under the light beam, preferably focusing directly on top
of the particle. An image will appear on the screen/monitor, which
will not be clear (blurry) [0200] 5. Alter the light intensity so
that there is a red spot in the middle of the picture (the red spot
should not cover the full image on the screen) [0201] 6. Check if
the red spot becomes smaller on moving the lens down (anti
clockwise on dial) towards the particle. The image becoming more
out of focus. [0202] 7. Then bring the lens back up to the position
it was before, and then turn the light intensity down so that the
red dot is much smaller and less defined. [0203] 8. Then slowly
keep moving the lens up (clockwise on dial) until the image comes
into focus (the particle surface becomes more defined). Turn the
light intensity down if needed so the red light is more sparse and
less bold [0204] 9. When the image is in focus, tare the position
of the lens on the control box [0205] 10. Then run the sample
(after entering the correct file name), abort the process when it
is clear the picture has completely come out of focus [0206] 11.
With the image that is displayed you can crop out any anomalies by
using the crop button on the left, and right clicking to select
`make main image`. Then save. [0207] 12. Read off the required
value from the image [0208] 13. Repeat for second area of particle.
[0209] 14. Repeat for each particle. [0210] 15. Average the 10
readings obtained.
APPENDIX 2
Tribometer Operating Procedure
Sphere on Flat Friction Measurements
[0210] [0211] 1. Switch power on to PC, monitor and Tribometer.
[0212] 2. Boot up PC. [0213] 3. Press [Start] on the Tribometer
touch screen interface (TSI). [0214] 4. Press the Start button on
the rear of the Tribometer controller. [0215] 5. Press [Proceed] on
the TSI. [0216] 6. Press [Linear] on the TSI. [0217] 7. Open DSC
Toolkit software. [0218] 8. In the DSC Toolkit, select device which
is the tribometer (which is the Normal Load strain gauge). [0219]
9. Open DSC Toolkit software. [0220] 10. In the DSC Toolkit select
device which is the tribometer (which is the Lateral Load strain
gauge). [0221] 11. Adhere probe to screw with metric thread of M2,
M3, M4, M5 or M6, depending on the sphere diameter. [0222] 12.
Attach screw to Tribometer beam via the appropriate adaptor. [0223]
13. Add brass weights until the target Normal Load is achieved.
[0224] 14. Immobilise substrate (which is a dry fibre pad made from
softwood, Botnia pine) on the lower plate. [0225] The fibre pad is
trimmed to dimensions suitable for using the in-built system for
immobilising samples. [0226] 15. Using the TSI, input the Start
position and End position; these should be the same value,
corresponding to where you wish the friction measurement to begin
on your sample. [0227] 16. Using the TSI, input the desired sliding
velocity. [0228] 17. Using the TSI, set the number of cycles to 1.
[0229] 18. Press [Logging] on the DSC Toolkit software. [0230] 19.
Set the Log Interval to 10 ms. [0231] 20. Specify the filename and
directory. [0232] 21. Lower the crosshead until the sphere makes
compressive contact with the substrate, and the Normal Load reaches
zero. [0233] 22. Using the TSI, press [Begin Motion]. [0234] 23.
Press [Start] on the DSC Toolkit software. [0235] 24. Using the
TSI, press [Run]. [0236] 25. Once the cycle has completed, Press
[Stop] on the DSC Toolkit software. [0237] 26. Raise the crosshead
until the sphere is clear of the substrate.
* * * * *
References