U.S. patent application number 10/240726 was filed with the patent office on 2003-07-03 for inhibition of yellowing in papers.
Invention is credited to Heitner, Cyril, McGarry, Peter F, Schmidt, John A, Yuan, Zhirun.
Application Number | 20030121630 10/240726 |
Document ID | / |
Family ID | 26893788 |
Filed Date | 2003-07-03 |
United States Patent
Application |
20030121630 |
Kind Code |
A1 |
Yuan, Zhirun ; et
al. |
July 3, 2003 |
Inhibition of yellowing in papers
Abstract
Lignin-containing papers are rendered light stable when they
contain TiO.sub.2 in combination with an amine hindered radical
scavenger.
Inventors: |
Yuan, Zhirun;
(Pointe-Claire, CA) ; Schmidt, John A; (Ile
Bizard, CA) ; McGarry, Peter F; (Ile Bizard, CA)
; Heitner, Cyril; (Pierrefonds, CA) |
Correspondence
Address: |
OGILVY RENAULT
1981 MCGILL COLLEGE AVENUE
SUITE 1600
MONTREAL
QC
H3A2Y3
CA
|
Family ID: |
26893788 |
Appl. No.: |
10/240726 |
Filed: |
October 8, 2002 |
PCT Filed: |
April 19, 2001 |
PCT NO: |
PCT/CA01/00543 |
Current U.S.
Class: |
162/135 ;
162/158; 162/162; 162/181.4; 8/495 |
Current CPC
Class: |
D21H 17/07 20130101;
D21H 17/675 20130101; D21H 21/143 20130101 |
Class at
Publication: |
162/135 ;
162/181.4; 162/158; 162/162; 8/495 |
International
Class: |
D21H 019/36; D21H
019/46 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 19, 2000 |
US |
60198447 |
Claims
1. A light stable lignin-containing paper having a content of
particulate titanium dioxide and a radical scavenger effective to
inhibit yellowing of the paper.
2. A paper according to claim 1, wherein said radical scavenger is
a hindered amine radical scavenger.
3. (Amended) A paper according to claim 1, wherein said titanium
dioxide is rutile titanium dioxide having a particle size of 0.01
to 1.5 microns and is present in an amount of 0.05 to 10%, by
weight, based on the weight of oven dry pulp fibre in the
paper.
4. A paper according to claim 3, wherein said titanium dioxide is
in an amount of 0.5 to 6%, by weight, based on the weight of oven
dry pulp fiber and said particle size is 0.1 to 1.5 microns.
5. A paper according to claim 3, wherein said titanium dioxide is
in an amount of 1 to 5%, by weight, based on the weight of oven dry
fibre and said particle size is 0.16 to 0.28 microns.
6. (Amended) A paper according to claim 1, wherein said radical
scavenger is present in an amount of 0.05 to 5%, by weight, based
on the weight of oven dry fibre.
7. (Amended) A paper according to claim 1, wherein said radical
scavenger is present in an amount of 0.1 to 2%, by weight, based on
the weight of oven dry fibre.
8. (Amended) A paper according to claim 1, wherein said radical
scavenger is present in an amount of 0.1 to 1.0%, by weight, based
on the weight of oven dry fibre.
9. (Amended) A paper according to claim 1, wherein said radical
scavenger is selected from piperidines and pyrolidines of formula
A, B, C and D: 3in which: R.sub.1 is selected from hydrogen,
hydroxyl, primary, secondary, tertiary or quaternary amino, alkyl,
alkoxy, aminoalkyl alkylene oxide of the general formula
[--O--(CH2).sub.n].sub.N where n is 1 to 4 and N is 1 to 20. The
alkyl or alkyl moiety in R.sub.1 has from 1 to 12 carbons, and may
be straight chain or branched. The alkyl and alkyl moieties may be
unsubstituted or substituted with hydroxyl (OH), carboxyl (COOH) or
carboxylate (COO.sup.-.sub.rM.sub.r/z.sup.z+) groups with M being a
metal ion from the 1st, 2nd or 3rd group of the periodic table, or
Zn, Cu, Ni or Co; or M is N.sup.+(R.sub.6).sub.4 where R.sub.6 is
alkyl of 1 to 8 carbon atoms or benzyl; and r and z are each
integers of 1, 2 or 3; R.sub.2 and R.sub.3 can be the same or
different and are selected from hydrogen, hydroxyl (OH), carboxyl
(COOH) or carboxylate (COO.sup.-.sub.rM.sub.r/z.sup.z+) groups with
M being a metal ion from the 1st, 2nd or 3rd group of the periodic
table, or Zn, Cu, Ni or Co; or M is a group N.sup.+(R.sub.6).sub.4
where R.sub.6 is alkyl of 1 to 8 carbon atoms or benzyl, and r and
z are each integers of 1, 2 or 3; Y is oxyl or hydroxyl; and X is
an inorganic or organic anion, p and m are integers of 1 to 20.
10. (Amended) A paper according to claim 1, wherein said titanium
dioxide and radical scavenger are present with filler in the
paper.
11. (Amended) A paper according to claim 1, wherein said titanium
dioxide and said radical scavenger are present in a pigment coating
on at least one side of the paper.
12. (Amended) A paper according to claim 1, further including an
organic ultra violet absorber with said titanium dioxide and
radical scavenger.
13. (Amended) A paper according to claim 1, derived from a
lignin-containing pulp having a lignin content of 15 to 35%, by
weight, based on the weight of oven dry fibre lignin in the
lignin-containing pulp.
14. (Amended) A paper according to claim 1, derived from a pulp
which comprises a high yield pulp selected from refiner mechanical
pulp, stone groundwood pulp, thermomechanical pulp,
chemithermo-mechanical pulp and chemimechanical pulp in an amount
of 80 to 100%, by weight, based on the total pulp weight.
15. A method of rendering a lignin-containing paper light stable
comprising: a) incorporating in said paper a content of particulate
titanium dioxide and a radical scavenger, in an amount effective to
inhibit yellowing of the paper, or b) coating the paper with a
coating formulation having a content of particulate titanium
dioxide and a radical scavenger, in an amount effective to inhibit
yellowing of the paper.
16. A method according to claim 15, wherein said radical scavenger
is a hindered amine.
17. (Amended) A method according to claim 15, wherein said titanium
dioxide is rutile titanium dioxide having a particle size of 0.01
to 1.5 microns and is present in an amount of 0.05 to 10%, by
weight, based on the weight of oven dry pulp fiber in the
paper.
18. A method according to claim 17, wherein said titanium dioxide
is in an amount of 0.5 to 6%, by weight, based on the weight of
oven dry pulp fiber.
19. A method according to claim 17, wherein said titanium dioxide
is in an amount of 1 to 5%, by weight, based on the weight of oven
dry fibre and said particle size is 0.16 to 0.28 microns.
20. (Amended) A method according to claim 15, wherein said radical
scavenger is present in an amount of 0.05 to 5%, by weight, based
on the weight of oven dry fibre.
21. (Amended) A method according to claim 15, wherein said radical
scavenger is present in an amount of 0.1 to 1.0%, by weight, based
on the weight of oven dry fibre.
22. (Amended) A method according to claim 15, wherein said radical
scavenger is selected from piperidines and pyrrolidines of formula
A, B, C and D: 4in which: R.sub.1 is selected from hydrogen,
hydroxyl, primary, secondary, tertiary or quaternary amino, alkyl,
alkoxy, aminoalkyl alkylene oxide of the general formula
[--O--(CH2).sub.n].sub.N where n is 1 to 4 and N is 1 to 20. The
alkyl or alkyl moiety in R.sub.1 has from 1 to 12 carbons, and may
be straight chain or branched. The alkyl and alkyl moieties may be
unsubstituted or substituted with hydroxyl (OH), carboxyl (COOH) or
carboxylate (COO.sup.-.sub.rM.sub.r/z.s- up.z+) groups with M being
a metal ion from the 1st, 2nd or 3rd group of the periodic table,
or Zn, Cu, Ni or Co; or M is N.sup.+(R.sub.6).sub.4 where R.sub.6
is alkyl of 1 to 8 carbon atoms or benzyl; and r and z are each
integers of 1, 2 or 3; R.sub.2 and R.sub.3 can be the same or
different and are selected from hydrogen, hydroxyl (OH), carboxyl
(COOH) or carboxylate (COO.sup.-.sub.rM.sub.r/z.sup.z+) groups with
M being a metal ion from the 1st, 2nd or 3rd group of the periodic
table, or Zn, Cu, Ni or Co; or M is a group N.sup.+(R.sub.6).sub.4
where R.sub.6 is alkyl of 1 to 8 carbon atoms or benzyl, and r and
z are each integers of 1, 2 or 3; Y is oxyl or hydroxyl; and X is
an inorganic or organic anion, p and m are integers of 1 to 20.
23. (Amended) A method according to claim 15, wherein said titanium
dioxide and scavenger are incorporated with filler in the paper in
accordance with a).
24. (Amended) A method according to claim 15, wherein said titanium
dioxide and said radical scavenger are present in said coating
formulation in accordance with b).
25. (Amended) A method according to claim 15, further including an
organic ultraviolet absorber with said titanium dioxide and radical
scavenger.
26. (Amended) A method according to claim 15, wherein said paper is
derived from a lignin-containing pulp having a lignin content of 15
to 35%, by weight, based on the weight of oven dry fibre lignin in
the lignin-containing pulp.
27. (Amended) A method according to claim 15, wherein said paper is
derived from a pulp which comprises a high yield pulp selected from
refiner mechanical pulp, stone groundwood pulp, thermomechanical
pulp, chemithermomechanical pulp and chemimechanical pulp in an
amount of at least 10%, by weight, based on the total pulp
weight.
28. (Cancel) Use of particulate titanium dioxide in conjunction
with a radical scavenger in a lignin-containing paper to inhibit
yellowing.
29. (Cancel) Use according to claim 28, wherein said radical
scavenger is a hindered amine radical scavenger.
Description
TECHNICAL FIELD
[0001] The present invention relates to light stable
lignin-containing papers and to methods of rendering
lignin-containing papers light stable; more especially the
invention provides a method that can be used in paper mills to
produce light stable lignin-containing papers by the application of
titanium dioxide and a radical scavenger, which can be added to the
base paper sheet or as a component of pigment coating formulations
employed to coat the paper.
BACKGROUND ART
[0002] Numerous processes are known to convert various types of
wood, recycled paper and other fibrous raw materials into pulp
suitable for making paper. In general, these processes can be
categorized as mechanical pulping, chemical pulping and
combinations thereof.
[0003] In mechanical pulping processes, such as the processes which
produce refiner mechanical pulp (RMP) and thermomechanical pulp
(TMP) processes, the fibres are separated by a combination of heat
and mechanical force. Stone groundwood pulp (GWD) is produced by
grinding logs between heavy stones. Such processes produce pulps at
a lower cost, at a yield of about 95%, by wt., based on starting
wood. Chemithermo-mechanical pulp (CTMP) and chemimechanical pulp
(CMP) processes apply some degree of chemical treatment to improve
the strength properties of the pulp. The yield is somewhat reduced
by the chemical treatment, but it is still substantially higher,
usually greater than 80%, by wt., than that from a purely chemical
pulping process. These high-yield mechanical pulps are bleached, if
desired, with chemicals that do not remove lignin, such as hydrogen
peroxide or sodium dithionite.
[0004] The traditional market of high-yield pulps is newsprint and
short-life advertising papers. However, there is intensive interest
in using bleached mechanical pulps in value-added paper grades.
Paper made from mechanical pulp has attractive optical and printing
properties. Advances in pulping and bleaching technology have made
it possible to produce high-yield pulps with suitable strength and
brightness for use in high-value printing and writing grades of
paper, which currently use bleached chemical pulp almost
exclusively. However, rapid light-induced yellowing of lignin in
high-yield pulps remains a significant impediment to their broader
use. A. Cockram, (CTMP in Fine Papers., 1989 International
Mechanical Pulping Conference Proceedings, 1989, Helsinki., p. 20),
estimated that, if the time taken for light-induced yellowing of
these papers could be increased by 3-36 months, the potential
market for bleached TMP and CTMP would be expanded by 0.6-2.2
million tons per year.
[0005] Most of the discoloration is due to lignin, which undergoes
photochemical reactions that form coloured groups when exposed to
near ultra-violet (UV) light (wavelength 300-400 nm). Many methods
to inhibit the yellowing of mechanical pulps have been attempted.
These can be generally classified into two main groups: blocking
the yellowing reactions through lignin modification; and adding
chemicals that either stop or redirect the photochemistry to avoid
colored products. This has been completely reviewed by C. Heitner
in "Photochemistry of Lignocellulosic Materials", C. Heitner, J. C.
Scaiano, eds,:ACS Sym. Ser. 531, 1-25 (1993), and "Chemistry of
Brightness Reversion and its Control, Chapter 5", Pulping
Bleaching-Principles and Practice, C. W. Dence, D. W. Reeve, eds.,
Tappi Press, Atlanta, 1996, pp 183-211. International publication
WO 99/05108 discloses that yellowing inhibitors based on hindered
amines, nitroxides, hydroxylamines and alkoxyamines can be used
together with an organic-based ultra violet absorber (UVA) to give
lignin-containing papers light stability comparable to that found
in papers made from lignin-free kraft pulps. However, the cost of
the UVA is not trivial and may be difficult to justify for printing
and writing papers.
[0006] Pigmented coatings and fillers improve paper gloss, color,
and printability. They can also improve brightness stability,
depending on type and amount of the pigments used. The pigments
provide some brightness stability, either by scattering or
absorbing incident UV light. Clay and calcium carbonate provide
brightness stability primarily by the scattering mechanism, since
they have a high reflectance but a low absorption of UV light.
[0007] Robert Johnson (CTMP in fine papers: on-machine surface
treatments for improved brightness stability, Tappi Journal, May
1991, p. 209) examined the use of TiO.sub.2 to improve the
brightness stability of papers containing up to 50% CTMP. With 50%
CTMP in the base sheet, brightness stability equivalent to
wood-free papers was achieved when the sheet was coated at 14
g/m.sup.2/side with a formulation where 6% of the pigment was
rutile TiO.sub.2. Thin surface treatments or "lick coatings" (5
g/m.sup.2/side), where 10% of the pigment is TiO.sub.2 and the
remainder is clay or CaCO.sub.3, also improved the brightness
stability, but not as much as the fully coated sheet (14
g/m.sup.2/side). The effect of TiO.sub.2 on papers made from 100%
mechanical pulps has not been examined.
DISCLOSURE OF THE INVENTION
[0008] The present invention provides a method that can be used to
produce light stable lignin-containing papers using TiO.sub.2
pigment and a radical scavenger. Light stable lignin-containing
papers or paperboards can be produced with this method.
[0009] The invention also provides a light stable lignin-containing
paper containing TiO.sub.2, and a radical scavenger.
[0010] In accordance with one aspect of the invention there is
provided a light stable lignin-containing paper having a content of
particulate titanium dioxide and a radical scavenger effective to
inhibit yellowing of the paper.
[0011] In accordance with another aspect of the invention there is
provided a method of rendering a lignin-containing paper light
stable comprising: a) incorporating in said paper a content of
particulate titanium dioxide and a radical scavenger, in an amount
effective to inhibit yellowing of the paper, or b) coating the
paper with a coating formulation having a content of particulate
titanium dioxide and a radical scavenger, in an amount effective to
inhibit yellowing of the paper.
DETAILED DESCRIPTION OF THE INVENTION AND DESCRIPTION OF PREFERRED
EMBODIMENTS
[0012] Advances in printing and packaging technology are placing
greater demands on paper quality, particularly optical and surface
properties.
[0013] In this invention it is found that the combined use of
titanium dioxide and a radical scavenger in a lignin-containing
paper sheet or in a coating on a lignin-containing paper sheet,
inhibits yellowing of the paper sheet, and more especially produces
an inhibiting effect greater than the summation of the individual
effects of the titanium dioxide and the radical scavenger.
[0014] i) Paper
[0015] The lignin-containing papers of the invention, are papers of
high lignin content in which yellowing occurs as a result of
modifications in the lignin in the paper.
[0016] More especially these are papers derived from pulp having as
a major component a high yield pulp produced in mechanical pulping
processes, more especially stone groundwood pulp, refiner
mechanical pulp, thermomechanical pulp, chemithermomechanical pulp
and chemimechanical pulp, which are produced in yields greater than
80%, by wt. High yield indicates high retention of lignin and these
pulps typically have a lignin content (Klason lignin) of 15 to 35%,
by weight, based on the weight of the pulp. Lignin content varies
as between hardwoods and softwoods and between species. In general
these pulps derived from hardwoods have a lignin content of 15-25%,
by weight, of the pulp and the pulps derived from softwoods have a
lignin content of 20 to 35%, by weight of the pulp.
[0017] Typically the pulp will be bleached, for example, with
peroxide, the bleaching does not remove the lignin.
[0018] The paper may be formed from such bleached high yield pulp
as the sole pulp component or the bleached high yield pulp may be
co-mixed with a chemical pulp, for example, kraft pulp.
[0019] In general, the high yield pulp will comprise more than 10%,
by weight, and preferably more than 50%, more preferably 80% to
100%, by weight, of the pulp component of the paper.
[0020] ii) TiO.sub.2:
[0021] TiO.sub.2 is the most efficient light-scattering pigment
commercially available, due to its high refractive index. Thus, it
is known in the art that TiO.sub.2 can be used as a filler, usually
together with other pigments such as clay or ground or precipitated
calcium carbonate, to improve optical properties such as brightness
and opacity.
[0022] Pigmented coatings, in addition to enhancing optical
properties, improve surface properties such as gloss, smoothness,
color, printing detail, and brilliance. The TiO.sub.2 may also be
employed in pigmented coatings together with clay, calcium
carbonate and other pigments. Such coatings can be applied on or
off machine with various kinds of equipment including, but not
restricted to, blade, rod or airknife coaters, or film coating size
presses. Surface application of chemicals and pigments provides
close to 100% retention, which reduces wet-end deposits, wear of
paper machine clothing and effluent loads.
[0023] In addition to its superlative scattering properties,
TiO.sub.2 is a strong absorber of near UV light. Thus, TiO.sub.2
can stabilize paper brightness by both scattering and absorbing UV
light.
[0024] TiO.sub.2 occurs in rutile and anatase crystal forms. While
both forms have high scattering power and high near UV absorption,
the Futile form is preferred in this invention. The rutile form is
more stable with regard to outgassing and has a higher near UV
absorption.
[0025] The behaviour of TiO.sub.2 is strongly influenced by its
average particle size. The current invention exploits both UV
absorbing and scattering properties of TiO.sub.2. Particle sizes
from 0.01 to 1.5 microns are typically employed. Particle sizes
ranging from 0.1 to 1.5 microns are preferred due to their better
scattering ability, and within this range, particle sizes from 0.16
to 0.28 microns are even more preferred.
[0026] The TiO.sub.2 is suitably employed in the paper in an amount
of 0.05 to 10%, preferably 0.5 to 15% and more preferably 1 to 10%,
by weight based on the oven dry weight of fibre.
[0027] iii) Radical Scavengers:
[0028] Radical scavengers are used in conjunction with the
TiO.sub.2 either in the base sheet or in a coating, to improve the
brightness stability of the lignin-containing papers. Both agents
show better brightness stability as dosages are increased up to a
limiting value. In accordance with the invention it has been found
that combining these two agents provides better brightness
stability than is possible using either agent alone.
[0029] Thiols, ascorbate and hindered amine compounds are radical
scavengers that have some efficacy at reducing light-induced
yellowing.
[0030] The hindered amine compounds such as described by Seltzer et
al., McGarry et al. and Yuan et al. are preferred because of their
superior thermal stability and lack of objectionable odor. (Seltzer
et al (International publication WO 99/05108) McGarry et al, (J.
Pulp Paper Sci., (2000), 26(2), 59-66) and Yuan et al (Proceedings:
PAPTAC 87th Annual Meeting, Montreal, 2001, (PAPTAC) C43-C52).
[0031] In typical papermaking practice, pigments comprise as much
as 40%, by weight, of total mass of the paper when used as a
filler, and up to 50%, by weight, when applied as a coating. It is
technically possible to use TiO.sub.2 as the only pigment. However,
preferred methods use TiO.sub.2 loadings of 0.05% to 50%, more
preferably 2 to 30%, by weight of the total pigments. Suitable
pigments for use with TiO.sub.2 in the filler or coating include
but are not restricted to clay and ground or precipitated calcium
carbonate, as extenders.
[0032] It is known in the art that hindered amine radical
scavengers improve the brightness stability of lignin-containing
papers. These compounds are exemplified by the piperidine and
pyrrolidine structures A through D: 1
[0033] in which:
[0034] R.sub.1 is selected from hydrogen, hydroxyl, primary,
secondary, tertiary or quaternary amino, alkyl, alkoxy, aminoalkyl
alkylene oxide of the general formula [--O--(CH2).sub.n].sub.N
where n is 1 to 4 and N is 1 to 20. The alkyl or alkyl moiety in
R.sub.1 has from 1 to 12 carbons, and may be straight chain or
branched. The alkyl and alkyl moieties may be unsubstituted or
substituted with hydroxyl (OH), carboxyl (COOH) or carboxylate
(COO.sup.-.sub.rM.sub.r/z.sup.x+) groups with M being a metal ion
from the 1st, 2nd or 3rd group of the periodic table, or Zn, Cu, Ni
or Co; or M is a group N.sup.+(R.sub.6).sub.4 where R.sub.6 is
alkyl of 1 to 8 carbon atoms or berizyl, and r and z are each
integers of 1, 2 or 3;
[0035] R.sub.2 and R.sub.3 can be the same or different and are
selected from hydrogen, hydroxyl (OH), carboxyl (COOH) or
carboxylate (COO.sup.-.sub.rM.sub.r/z.sup.z+) groups with M being a
metal ion from the 1st, 2nd or 3rd group of the periodic table, or
Zn, Cu, Ni or Co; or M is a group N.sup.+(R.sub.6).sub.4 where
R.sub.6 is alkyl of 1 to 8 carbon atoms or benzyl, and r and z are
each integers of 1, 2 or 3;
[0036] Y is oxyl or hydroxyl; and
[0037] X is an inorganic or organic anion, such as carbonate,
borates, bicarbonate, chloride, bisulfate, sulfate, formate,
acetate, citrate, oxalate, ascorbate, phosphonate, phosphate,
nitrate, bromide, bisulfite, sulfite, benzoate, tartrate, acrylate,
polyacrylate, fumarate, maleate, itaconate, glycolate, gluconate,
malate, mandelate, tiglate, polymethacrylate, a carboxylate of
nitrilotriacetic acid, hydroxyethylethylenediaminetriacetic acid,
ethylenediaminetetraacetic acid or of diethylenetriaminepentaacetic
acid, a diethylenetriaminepentam- ethylenephosphonate, an
alkylsulfonate or an arylsulfonate and p is an integer of 1 to 20,
preferably 1 to 10, more preferably 1 to 5 and most preferably 1, 2
or 3 and m is an integer of 1 to 20, preferably 1 to 10, more
preferably 1 to 5 and most preferably 1, 2 or 3, typically m and p
may be integers 1, 2 or 3.
[0038] The hindered amine radical scavenger is suitably employed in
an amount of 0.05 to 5%, preferably 0.1 to 2% and more preferably
0.1 to 1.0%, by weight, based on the weight of oven dried
fibres.
[0039] iv) Organic Ultra Violet Absorber:
[0040] In preferred embodiments, an organic ultra violet absorber
(UVA), for example, o-hydroxybenzophenone or
o-hydroxyphenylbenzotriazole is employed in addition to the
titanium dioxide and the radical scavenger and this is found to
further inhibit yellowing.
[0041] The UVA is suitably employed in an amount of 0.1 to 0.8%,
preferably 0.1 to 0.4%, by wt. based on the weight of oven dry
fibres.
BRIEF DESCRIPTION OF DRAWINGS
[0042] FIG. 1 is a plot demonstrating the relationship between the
PC number and the content of TiO.sub.2, at different coating
weights;
[0043] FIG. 2 is a plot demonstrating the relationship between PC
number and content of TiO.sub.2 at different contents of radical
scavenger;
[0044] FIG. 3 is a plot demonstrating the relationship between PC
number and content of radical scavenger at different contents of
TiO.sub.2;
[0045] FIG. 4 is a plot demonstrating the relationship between PC
number and content of TiO.sub.2 at a fixed content of radical
scavenger;
[0046] FIG. 5 is a plot demonstrating the relationship between PC
number and accelerated exposure to light for different contents of
TiO.sub.2 and TiO.sub.2 in combination with a radical scavenger in
a coating; and
[0047] FIG. 6 is a plot similar to FIG. 5 but employing the
TiO.sub.2 as filler rather than in a coating.
EXAMPLES
[0048] The present invention will be illustrated by the following
examples. Machine-made papers containing mechanical pulp were
coated with pigmented formulations with or without TiO.sub.2 or a
radical scavenger (RS). The citric acid salt of
4-hydroxy-2,2,6,6-tetramethyl-N-hydroxypipe- ridine was used as the
radical scavenger. Rutile TiO.sub.2 slurry was used in the
experiments. Two different coating formulations were used. The
effect of RS on the yellowing inhibition was tested by
incorporating the inhibitor as an additive in the formulation. The
effect of TiO.sub.2 was examined by substituting it for a portion
of the calcium carbonate in the coating formulations.
[0049] Accelerated photolysis was done in an exposure chamber
equipped with cool-white fluorescent lamps. The light intensity in
the chamber was approximately 50 times that of normal office light.
ISO brightness was determined according to ISO standard 2470, using
a Technidyne Micro TB-1C reflectometer. The light stability of the
paper is presented by the change in either ISO brightness or PC
number as a function of accelerated irradiation time. The
brightness data were converted to post-color number (PC number)
according to the following two equations:
PC=((k/s).sub.after-(k/s).sub.before).times.100 (1)
k/s=(1-R.sub..infin.).sup.2/2R.sub..infin. (2)
[0050] Where k and s are the absorption and scattering coefficients
of the paper, respectively, and R.sub..infin. is the ISO brightness
expressed as a fractional value. To a rough approximation, PC
number is directly proportional to the chromophores formed during
light-induced yellowing. Therefore the smaller the PC number, the
less the paper has yellowed.
[0051] The % of TiO.sub.2, RS and UVA is by weight based on the
weight of oven dried fibres.
Example 1
[0052] A 49 g/m.sup.2 base paper, with an initial brightness of
72%, was coated with various amounts of a pigmented coating color
using a blade cylindrical laboratory coater (CLC). The base sheet
contained in wt. %, 97% alkaline peroxide bleached mechanical pulp
(APMP) pulp and 3% kraft pulp. The pigmented coating color
contained, in wt. %, 55% clay, 45% ground calcium carbonate,
starch, latex, and coating additives. The initial ISO brightness
varied from 74.5% to 81.0% depending on the total coat weight and
amount of TiO.sub.2 incorporated in the coating.
[0053] FIG. 1 plots the PC number obtained after 12 days
accelerated exposure as a function of TiO.sub.2 content for coat
weights of 4, 9 and 14 g/m.sup.2/side. These weights correspond
approximately to light, medium and heavy coating. The different
values of the intercepts for each line show that the clay/calcium
carbonate coating itself, in the absence of TiO.sub.2, affords some
inhibition of yellowing, and that this inhibition improves with
increasing coat weight. Adding TiO.sub.2 at any coat weight further
improves the yellowing inhibition. Inhibition improves with
increasing TiO.sub.2 addition up to a maximum between 5% to 6% on
fiber. Although yellowing slows, it still occurs at these maximum
levels of TiO.sub.2.
Example 2
[0054] A coated paper as described in Example 1 was prepared with a
9 g/m.sup.2/side coat weight. Samples containing TiO.sub.2 and the
citric acid salt of
4-hydroxy-2,2,6,6-tetramethyl-N-hydroxypiperidine as radical
scavenger were tested for light stability.
[0055] FIG. 2 shows the resulting plot of PC number after 12 days
accelerated exposure versus the amount of TiO.sub.2 added. As noted
in Example 1, when TiO.sub.2 alone is used, yellowing inhibition is
maximized at a TiO.sub.2 content between 5% and 6%. FIG. 2 also
shows that adding radical scavenger to a coating color containing
TiO.sub.2 enhances the yellowing inhibition. FIG. 2 also shows that
adding radical scavenger to the coating color improves yellowing
inhibition under conditions where no additional benefit can be
obtained by increasing the amount of TiO.sub.2.
[0056] FIG. 3 shows plots of PC number after 12 days exposure
versus the amount of radical scavenger in the coating, at a coating
weight of 9 g/m.sup.2 and different contents of TiO.sub.2. The
yellowing inhibition achieved by adding radical scavenger alone
reaches a maximum at an application of about 0.5%/side. This
maximum level can be improved by including TiO.sub.2 in the
coating.
Example 3
[0057] Coated paper as described in Example 1 was prepared with a
14 g/m.sup.2/side coat weight containing 0.3% of the radical
scavenger 4-hydroxy-2,2,6,6-tetramethyl-N-hydroxypiperidine citrate
and various amounts of TiO.sub.2.
[0058] The plot in FIG. 4 shows that complete inhibition of
yellowing after 12 days accelerated exposure was possible using
0.3% of the radical scavenger with 3.5% TiO.sub.2.
Example 4
[0059] A 57 g/m.sup.2 machine-made paper made from 100%
peroxide-bleached softwood thermomechanical pulp, initial ISO
brightness 78%, was treated with a pigmented formulation by a
laboratory film applicator. The coat weight was 4 g/m.sup.2 per
side. The pigmented formulation consisted of 80% ground calcium
carbonate and 20% delaminated clay, starch, latex, and some minor
additives. As shown in FIG. 5, replacing 12% of the CaCO.sub.3 with
TiO.sub.2 which corresponds to 1.15% TiO.sub.2 charge on fiber,
increased the brightness stability. Doubling the amount of
TiO.sub.2 to 2.3% provided no further improvement in brightness
stability. Adding 1.0% of the radical scavenger together with 1.15%
TiO.sub.2 gave a brightness stability better than that possible
using TiO.sub.2 alone.
Example 5
[0060] A paper made from a 57 g/m.sup.2 base sheet with a 4
g/m.sup.2 coating, as described in Example 4, was prepared. In
addition to the control, samples containing 0.6% TiO.sub.2, 1%
radical scavenger and both 0.6% TiO.sub.2 and 1% radical scavenger
were irradiated for 12 days. The PC number values are listed in
Table 1. The data show that the effect of adding TiO.sub.2 and
radical scavenger together at the indicated levels (smaller
increase in PC number) is greater than the sum of their individual
effects.
1TABLE 1 Change in PC Number for BTMP paper with 4g/m.sup.2/side
coating Sum of individual effects of adding TiO.sub.2and radical
scavenger Effect of adding TiO.sub.2 and radical
(PC.sub.control-PC.sub.TiO2) + (PC.sub.control- scavenger together
Sample PC number PC.sub.RS) PC.sub.control- PC.sub.TiO2+RS Control
16.92 0.6% TiO.sub.2 14.80 7.69 9.58 1% RS 11.35 0.6% TiO.sub.2 +
7.34 1% RS
Example 6
[0061] Coated paper using a 49 g/m.sup.2 base sheet and 9
g/m.sup.2/side coating were prepared as described in Example 1.
Samples were prepared containing the following yellowing inhibitors
in the coating colour: control (no inhibitor), 1.5% TiO.sub.2 plus
0.4% radical scavenger and 0.3% organic UVA (a 60/40 mixture of
hydroxyphenylbenzotriazole structures E and F below, where n=5-9),
1.5% TiO.sub.2 plus 0.4% radical scavenger and 0.3% organic UVA.
2
[0062] The PC number values after 12 days of accelerated exposure
are tabulated in Table 2. The data show that the effect (smaller
increase in PC number) of adding TiO.sub.2, radical scavenger, and
organic UVA together at the indicated levels is greater than the
sum of the individual effects of radical scavenger +organic UVA and
TiO.sub.2.
2TABLE 2 Change in PC Number for mechanical paper with 9
g/m.sup.2/side coating Sum of individual effects of Effect of
adding TiO.sub.2 number adding TiO.sub.2 and radical and radical
scavenger scavenger together PC (PC.sub.control-PC.sub.TiO2) +
PC.sub.control - Sample Number (PC.sub.control-PC.sub.RS+UVA)
PC.sub.TiO2+RS+UVA Control 6.06 1.5% TiO.sub.2 5.81 2.79 3.26 0.4%
RS + 3.52 0.3% UVA 1.5% TiO.sub.2 + 2.8 0.4% RS + 0.3% UVA
Example 7
[0063] A 100 g/m.sup.2 standard handsheet made from 100%
peroxide-bleached softwood thermomechanical pulp, initial
brightness 73%, was prepared either with or without rutile
TiO.sub.2 as filler. The TiO.sub.2 retained in the sheets was 13.7%
based on o.d. fiber. The control and filled sheets were also
treated with 1.0% radical scavenger by a laboratory film
applicator. As shown in FIG. 6, like surface treatment, TiO.sub.2
also improved the paper brightness stability when it was retained
in the sheet as filler. The radical scavenger improved the paper
brightness stability, but a combination of TiO.sub.2 and radical
scavenger offered much better yellowing inhibition.
* * * * *