Title:
Inhibition of yellowing in papers
Kind Code:
A1


Abstract:
Lignin-containing papers are rendered light stable when they contain TiO2 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)
Application Number:
10/240726
Publication Date:
07/03/2003
Filing Date:
10/08/2002
Assignee:
YUAN ZHIRUN
SCHMIDT JOHN A
MCGARRY PETER F
HEITNER CYRIL
Primary Class:
Other Classes:
8/495, 162/158, 162/162, 162/181.4
International Classes:
D21H21/14; D21H17/07; D21H17/67; (IPC1-7): D21H19/36; D21H19/46
View Patent Images:



Primary Examiner:
FORTUNA, JOSE A
Attorney, Agent or Firm:
NORTON ROSE FULBRIGHT CANADA LLP (MONTREAL, QC, CA)
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: 3embedded image in which: R1 is selected from hydrogen, hydroxyl, primary, secondary, tertiary or quaternary amino, alkyl, alkoxy, aminoalkyl alkylene oxide of the general formula [—O—(CH2)n]N where n is 1 to 4 and N is 1 to 20. The alkyl or alkyl moiety in R1 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 (COOrMr/zz+) 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+(R6)4 where R6 is alkyl of 1 to 8 carbon atoms or benzyl; and r and z are each integers of 1, 2 or 3; R2 and R3 can be the same or different and are selected from hydrogen, hydroxyl (OH), carboxyl (COOH) or carboxylate (COOrMr/zz+) 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+(R6)4 where R6 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: 4embedded image in which: R1 is selected from hydrogen, hydroxyl, primary, secondary, tertiary or quaternary amino, alkyl, alkoxy, aminoalkyl alkylene oxide of the general formula [—O—(CH2)n]N where n is 1 to 4 and N is 1 to 20. The alkyl or alkyl moiety in R1 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 (COOrMr/zz+) 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+(R6)4 where R6 is alkyl of 1 to 8 carbon atoms or benzyl; and r and z are each integers of 1, 2 or 3; R2 and R3 can be the same or different and are selected from hydrogen, hydroxyl (OH), carboxyl (COOH) or carboxylate (COOrMr/zz+) 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+(R6)4 where R6 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 TiO2 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/m2/side with a formulation where 6% of the pigment was rutile TiO2. Thin surface treatments or “lick coatings” (5 g/m2/side), where 10% of the pigment is TiO2 and the remainder is clay or CaCO3, also improved the brightness stability, but not as much as the fully coated sheet (14 g/m2/side). The effect of TiO2 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 TiO2 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 TiO2, 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) TiO2:

[0021] TiO2 is the most efficient light-scattering pigment commercially available, due to its high refractive index. Thus, it is known in the art that TiO2 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 TiO2 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, TiO2 is a strong absorber of near UV light. Thus, TiO2 can stabilize paper brightness by both scattering and absorbing UV light.

[0024] TiO2 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 TiO2 is strongly influenced by its average particle size. The current invention exploits both UV absorbing and scattering properties of TiO2. 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 TiO2 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 TiO2 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 TiO2 as the only pigment. However, preferred methods use TiO2 loadings of 0.05% to 50%, more preferably 2 to 30%, by weight of the total pigments. Suitable pigments for use with TiO2 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: 1embedded image

[0033] in which:

[0034] R1 is selected from hydrogen, hydroxyl, primary, secondary, tertiary or quaternary amino, alkyl, alkoxy, aminoalkyl alkylene oxide of the general formula [—O—(CH2)n]N where n is 1 to 4 and N is 1 to 20. The alkyl or alkyl moiety in R1 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 (COOrMr/zx+) 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+(R6)4 where R6 is alkyl of 1 to 8 carbon atoms or berizyl, and r and z are each integers of 1, 2 or 3;

[0035] R2 and R3 can be the same or different and are selected from hydrogen, hydroxyl (OH), carboxyl (COOH) or carboxylate (COOrMr/zz+) 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+(R6)4 where R6 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 diethylenetriaminepentamethylenephosphonate, 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 TiO2, at different coating weights;

[0043] FIG. 2 is a plot demonstrating the relationship between PC number and content of TiO2 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 TiO2;

[0045] FIG. 4 is a plot demonstrating the relationship between PC number and content of TiO2 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 TiO2 and TiO2 in combination with a radical scavenger in a coating; and

[0047] FIG. 6 is a plot similar to FIG. 5 but employing the TiO2 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 TiO2 or a radical scavenger (RS). The citric acid salt of 4-hydroxy-2,2,6,6-tetramethyl-N-hydroxypiperidine was used as the radical scavenger. Rutile TiO2 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 TiO2 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)after−(k/s)before)×100 (1)

k/s=(1−R)2/2R (2)

[0050] Where k and s are the absorption and scattering coefficients of the paper, respectively, and R 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 TiO2, RS and UVA is by weight based on the weight of oven dried fibres.

Example 1

[0052] A 49 g/m2 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 TiO2 incorporated in the coating.

[0053] FIG. 1 plots the PC number obtained after 12 days accelerated exposure as a function of TiO2 content for coat weights of 4, 9 and 14 g/m2/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 TiO2, affords some inhibition of yellowing, and that this inhibition improves with increasing coat weight. Adding TiO2 at any coat weight further improves the yellowing inhibition. Inhibition improves with increasing TiO2 addition up to a maximum between 5% to 6% on fiber. Although yellowing slows, it still occurs at these maximum levels of TiO2.

Example 2

[0054] A coated paper as described in Example 1 was prepared with a 9 g/m2/side coat weight. Samples containing TiO2 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 TiO2 added. As noted in Example 1, when TiO2 alone is used, yellowing inhibition is maximized at a TiO2 content between 5% and 6%. FIG. 2 also shows that adding radical scavenger to a coating color containing TiO2 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 TiO2.

[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/m2 and different contents of TiO2. 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 TiO2 in the coating.

Example 3

[0057] Coated paper as described in Example 1 was prepared with a 14 g/m2/side coat weight containing 0.3% of the radical scavenger 4-hydroxy-2,2,6,6-tetramethyl-N-hydroxypiperidine citrate and various amounts of TiO2.

[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% TiO2.

Example 4

[0059] A 57 g/m2 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/m2 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 CaCO3 with TiO2 which corresponds to 1.15% TiO2 charge on fiber, increased the brightness stability. Doubling the amount of TiO2 to 2.3% provided no further improvement in brightness stability. Adding 1.0% of the radical scavenger together with 1.15% TiO2 gave a brightness stability better than that possible using TiO2 alone.

Example 5

[0060] A paper made from a 57 g/m2 base sheet with a 4 g/m2 coating, as described in Example 4, was prepared. In addition to the control, samples containing 0.6% TiO2, 1% radical scavenger and both 0.6% TiO2 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 TiO2 and radical scavenger together at the indicated levels (smaller increase in PC number) is greater than the sum of their individual effects. 1

TABLE 1
Change in PC Number for BTMP paper with 4g/m2/side coating
Sum of individual effects of
adding TiO2and radical
scavengerEffect of adding TiO2 and radical
(PCcontrol-PCTiO2) + (PCcontrol-scavenger together
SamplePC numberPCRS)PCcontrol- PCTiO2+RS
Control16.92
0.6% TiO214.807.699.58
1% RS11.35
0.6% TiO2 +7.34
1% RS

Example 6

[0061] Coated paper using a 49 g/m2 base sheet and 9 g/m2/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% TiO2 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% TiO2 plus 0.4% radical scavenger and 0.3% organic UVA. 2embedded image

[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 TiO2, 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 TiO2. 2

TABLE 2
Change in PC Number for mechanical paper with 9 g/m2/side coating
Sum of individual effects ofEffect of adding TiO2
number adding TiO2 and radicaland radical scavenger
scavengertogether
PC(PCcontrol-PCTiO2) +PCcontrol -
SampleNumber(PCcontrol-PCRS+UVA)PCTiO2+RS+UVA
Control6.06
1.5% TiO25.812.793.26
0.4% RS +3.52
0.3% UVA
1.5% TiO2 +2.8
0.4% RS +
0.3% UVA

Example 7

[0063] A 100 g/m2 standard handsheet made from 100% peroxide-bleached softwood thermomechanical pulp, initial brightness 73%, was prepared either with or without rutile TiO2 as filler. The TiO2 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, TiO2 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 TiO2 and radical scavenger offered much better yellowing inhibition.