SE525893C2 - Light-stable lignin-containing paper, method of making paper light-stable and the use of particulate titanium dioxide in combination with radical removal to inhibit yellowing in paper - Google Patents

Abstract

Lignin-containing papers are rendered light stable when they contain TiO2 in combination with an amine hindered radical scavenger.

Description

25 The traditional market for high-yield pulps is newsprint and advertising paper for short-term use. However, there is a strong interest in using bleached mechanical pulps in higher quality paper grades. Paper made from mechanical pulp has attractive optical properties and printing properties.

Advances in pulp and bleaching techniques have made it possible to produce high-yield pulps of suitable strength and brightness for use in high-quality printing and writing papers, which currently use almost exclusively bleached chemical pulp. However, the rapid light-induced yellowing of lignin in high yield pulps is a significant remaining obstacle to their wider use. A. Cockram, (CT MP in Fine Papers., 1989 International Mechanical Pulping Conference Proceedings, 1989, Helsinki, p. 20) has estimated that if the time it takes for light-induced yellowing in these papers could be increased by 3-6 months, the potential market for bleached TMP and CTMP can be expanded by 0.6-2.2 million tonnes per year.

Most of the discoloration is due to the lignin, which undergoes photochemical reactions that form colored groups when exposed to near ultraviolet (UV) light (wavelength 300-400 nm). Many methods for inhibiting yellowing in mechanical masses have been tried. These can generally be classified into two main groups: blocking the yellowing reaction by lignin modification and the addition of chemicals that either stop or redirect photochemistry to avoid colored products. This has been reviewed in detail by C. Heitner in "Photochemistry of Lignocellulosic Materials", C. Heitner, J .C. Scaiano, ed .; ACS Sym. Looks. 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, ed., Tappi Press, Atlanta, 1996, p. 183-211. WO 99/05108 discloses that yellowing inhibitors based on hindered amines, nitroxides, hydroxylamines and alkoxyamines can be used together with an organically based ultraviolet absorbent (UVA) to give lignin-containing paper light stability comparable to that of paper made from ligands. 10 15 20 25 30. . . . . '' O II ninfri kraftmassa. However, the cost of UVA is not insignificant and can be difficult to justify for printing and writing paper.

Pigmented coatings and fillers improve the gloss, color and printability of the paper. They can also improve the brightness stability, depending on the type and amount of pigment used. The pigment gives some light stability, either by scattering or absorbing incident UV light. Clay and calcium carbonate provide brightness stability mainly through the scattering mechanism, as they have high reflectance but low absorption of UV light.

Robert Johnson (CTMP in fi ne papers: on-machine surface treatments for improved brightness stability, Tappi Journal, May 1991, p. 209) has investigated the use of TiOz to improve the brightness stability of papers containing up to 50% CTMP. With 50% CTMP in the base sheet, a brightness stability equivalent to wood-free paper is achieved if the sheet is coated with 14 g / m 2 / side with a mixture where 6% of the pigment was rutile TiO 2. Thin coatings or "lick coatings" (5 g / mz / side), where 10% of the pigment is TiOZ and the rest is clay or CaCO ). The effect of TiO 2 on paper made from 100% mechanical pulp has not been investigated.

DESCRIPTION OF THE INVENTION The present invention provides a method that can be used to make light-stable lignin-containing papers using TiO 2 pigments and a radical scavenger. Light-stable lignin-containing paper or board can be produced with this method.

The invention also provides a light stable lignin-containing paper containing TiO 2 and a radical scavenger. In accordance with one aspect of the invention, there is provided a light stable lignin-containing paper having a particulate titanium dioxide content and a radical scavenger capable of inhibiting yellowing of the paper.

According to another aspect of the invention there is provided a method of making lignin-containing paper light stable, comprising: a) mixing particulate titanium dioxide and a radical scavenger in the paper, in an amount sufficient to inhibit yellowing of the paper, or b) coating the paper with a coating composition containing particulate titanium dioxide and a radical scavenger, in an amount sufficient to inhibit yellowing of the paper.

DETAILED DESCRIPTION OF THE INVENTION AND DESCRIPTION OF PREFERRED EMBODIMENTS Advances in printing and packaging technology place ever higher demands on paper quality, in particular optical properties and surface properties.

According to the present invention, it has been found that the combined use of titanium dioxide and a radical scavenger in lignin-containing sheets of paper or in a coating on a lignin-containing sheet of paper inhibits the yellowing of the paper sheet and gives an inhibitory effect greater than the sum of the individual effects of titanium dioxide and the radical scavenger. i) Paper: The lignin-containing papers according to the invention are papers with a high lignin content where yellowing takes place as a result of modifications of the lignin in the paper.

These papers are made from pulp having a predominant proportion of high yield pulp produced in mechanical pulp processes, in particular rock abrasive pulp, mechanical refiner pulp, thermomechanical pulp, chemithermomechanical pulp and chemimechanical pulp, which are produced in yields higher than 80% by weight. High yield indicates high retention of lignin and these pulps normally have a lignin content (Klason lignin) of 15 to 35% by weight, based on the weight of the pulp. The lignin content varies between hardwood and banana wood and between different masses. In general, the pulps produced from hardwood have a lignin content of 15 to 25% by weight of the pulp and pulps made from softwood have a lignin content of 20 to 35% by weight of the pulp.

The pulp is usually bleached with, for example, peroxide, and the bleaching does not remove the lignin.

Paper can be formed from such bleached high yield pulp as the sole pulp component or the bleached high yield pulp can be mixed with chemical pulp, eg kra fi pulp.

In general, the high yield pulp will comprise more than 10% by weight, preferably more than 50%, more preferably 80 to 100% by weight of the pulp component of the paper. ii) TiO2: TiO2 is the most effective light scattering pigment available commercially, due to its high refractive index. It is thus known that TiOZ can be used as a filler, usually together with other pigments such as clay or malt or precipitated calcium carbonate, to improve the optical properties such as brightness and opacity.

In addition to improving the optical properties, pigmented coatings improve surface properties such as gloss, smoothness, color, print detail and light density. TiO 2 can also be used in pigmented coatings along with clay, calcium carbonate and other pigments. Such coatings can be used on or in the machine together with various types of equipment including, but not limited to, blade, rod or knife knives or glue presses for glue coating. The surface coatings for the chemicals and pigments give close to 100% retention, which reduces deposits. in the wet area, wear on the paper machine's cloth and load on the drain.

In addition to its strong scattering properties, TiOz is a strong absorber of near UV light. TiO 2 can therefore stabilize the brightness of the paper both by scattering and absorbing UV light.

TiOz is found in rutile and anatase crystalline forms. Although both forms have high scattering ability and high absorption of near UV light, the rutile form is preferred according to the present invention. The rutile form is more stable with respect to degassing and has higher absorption of close UV light.

The way in which TiO 2 behaves is strongly influenced by the average size of the particles. The present invention utilizes both the UV absorbing and dispersing properties of TiO 2. Particle sizes from 0.01 to 1.5 μm are those normally used. Particle sizes in the range of 0.1 to 1.5 microns are preferred because of their better dispersion ability, and in this range, particle sizes from 0.16 to 0.28 microns are even more preferred.

TiO 2 is suitably used in the paper in an amount of 0.05 to 10% by weight, preferably 0.5 to 15% by weight and even more preferably 1 to 10% by weight, based on the kiln dry fiber weight. iii) Radical transducer: Radical transducer is used together with TiO 2 either in the base sheet or in the coating, to improve the brightness stability of the lignin-containing paper. Both of these agents show better brightness stability as the dosage increases up to a limiting value. In accordance with the invention, it has been found that the combination of these two agents provides better brightness stability than is possible if only one is used alone.

Thiols, ascorbates and hindered amine compounds are radical scavengers that have a reducing effect on light-induced yellowing. The hindered amine compounds as described by Seltzer et al., McGarry et al. and Yuan et al. are preferred due to their superior thermal stability and absence of unpleasant odor.

(Seltzer et al. (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).

In common papermaking practices, pigments are used which contain as much as 40% by weight of the total mass of the paper when used as a filler, and up to 50% by weight when used as a coating. It is technically possible to use TiO 2 as the only pigment. However, preferred methods use TiO 2 charges of 0.05 to 50% by weight, preferably 2 to 30% by weight of the total pigments. Suitable pigments for use with TiO 2 as filler or coating include, but are not limited to, clay and malt or precipitated calcium carbonate as filler.

It is known in the art that radical scavengers in the form of hindered amines improve the brightness stability of lignin-containing papers. These compounds are exemplified by the piperidine and pyrrolidine structures A to D: 3 R H 3 A CH 3 | CH3 Y 'F H1 B R} i | Ra [XPÜm / p CHQ I CHS Y m H1 CHQ I CHQ Y n, 'f “wfïïkä fra-f» en, | a, ° Y m where: R 1 is selected from hydrogen, hydroxyl, primary, secondary, tertiary or quaternary amino, alkyl, alkoxy, aminoalkylalkylene oxides of the general formula [-O- (CH 2) 4 and N is 1-20. The alkyl or alkyl moiety of R 1 has from 1 to 12 carbon atoms and may be straight chain or branched. The alkyl or alkyl moiety may be unsubstituted or substituted by hydroxyl (OH), carboxyl (COOH) or carboxylate groups (C00, MzÉ / z) where M is a metal ion from the first, second or third group of the periodic table, or Zn Cu, Ni or Co; or M is a group N 'L (R6) 4 where R0 is an alkyl of 1-8 carbon atoms or benzyl; and r and z are each integers of 1, 2 or 3.

R 2 and R 2; may be the same or different and are selected from hydrogen, hydroxyl (OH), carboxyl (COOH) or carboxylate groups (C00, Mz fl / z) where M is a metal ion from the first, second or third group of the Periodic Table, or Zn, Cu, Ni or Co; or M is a group N * (R 6) 4 where R 3 is an alkyl of 1-8 carbon atoms or benzyl, and r and z are both integers of 1, 2 or 3. Y is oxyl or hydroxyl, and X is an inorganic or organic anion, such as carbonate, borates, bicarbonate, chloride, bisulfate, sulfate, phthalate, acetate, citrate, oxalate, ascorbate, phosphonate, phosphate, nitrate, bromide, bisulfite fi t, sul fi t, benzoate, tartrate, acrylate, polyacrylate, fumarate, maleate, itaconate, glycolate, gluconate, malate, mandelate, tiglate, polymetakiylat, a carboxylate of nitrilotriacetic acid, hydroxy etyletylendiamintriättiksyra, ethylenediaminetetraacetic acid or dietylentriaminpentaät- acid, ettdietylentriaminpentametylenfosfonat, an alkylsulfonate or an aryl sulfonate 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, usually m and p are integers of 1, 2 el ler 3.

The radical amine radical scavenger is suitably used in an amount of 0.05 to 5% by weight, preferably 0.1 to 2% by weight and most preferably 0.1 to 1.0% by weight based on the oven weight. iv) Organic absorber for ultraviolet light: In preferred embodiments, an organic absorbent for ultraviolet light (UVA) is used, eg o-hydroxybenzophenone or o-hydroxyphenylbenzotriazole, in addition to titanium dioxide and radical scavengers and this has been found to further limit yellowing-CH.

The organic absorber for ultraviolet light (UVA) is suitably used in an amount of 0.1 to 0.8% by weight, preferably 0.1 to 0.4% by weight, based on the oven dry fi overweight.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagram showing the relationship between the PC number and the content of TiO2 at different coating weights, Fig. 2 is a diagram showing the relationship between the PC Fig. 3 is a diagram showing the relationship between the PC number and the content of radical transducers at different contents of TiO 2, Fig. 4 is a diagram showing the relationship between the PC number and the content of TiO 2 at a constant content of radical scavengers, Fig. 5 is a diagram showing the relationship between the PC number and accelerated exposure to light at different contents of TiO 2 and TiO 2 in combination with a radical scavenger in a coating, and Fig. 6 is a diagram similar to Fig. 5, but using TiO 2 as a filler instead of as a coating.

EXAMPLES The present invention will now be illustrated by the following examples. Machine-made papers containing mechanical pulp were coated with pigmented mixtures with or without TiO 2 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 TiO 2 slurry was used in the experiments. Two different coating mixtures were used. The effect of the radical scavenger (RS) on the yellowing inhibition was tested by incorporating the inhibitor as an additive into the mixture. The effect of TiO 2 was investigated by replacing some of the calcium carbonate in the coating mixture.

Accelerated photolysis was performed in an exposure chamber equipped with cold white fl uorescent lamps. The light intensity in the chamber was about 50 times greater than that of normal office light. ISO brightness was measured according to ISO standard 2470, using a Technidyne Micro TB-1C rectometer. The light stability of the paper is expressed in a change either in ISO brightness or in the PC number as an av mction of the accelerated irradiation time. Brightness data was converted to post-color number (PC-number) according to the following two equations: PC z (Wyeñer '(lf / brittle) X. K / s = (i-Roqz / znoo (2) Where k and s are the absorption and scatter coefficients of the paper, respectively, and Roo is the ISO brightness expressed as a fraction value.As a rough estimate, the PC number is directly proportional to the chromophores formed during the light-induced yellowing. is, the less the paper has turned yellow.% of TiOz, RS and UVA are given as% by weight, based on the oven weight fi overweight.

Example 1 A 49 g / m 2 base paper, with an initial brightness of 72%, was coated with various amounts of pigmented coating paint using a cylindrical laboratory blade coater (CLC). The base sheet contained 97% by weight of alkaline peroxide bleached mechanical pulp (APMP) and 3% by weight of kraft pulp. The pigmented coating color contained in% by weight, 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 coating weight and the amount of TiO 2 included in the coating.

Fig. 1 shows the PC number obtained after 12 days of accelerated exposure as a function of the TiO 2 content for coating weights of 4, 9 and 14 g / m 2 / side. These weights roughly correspond to light, medium and heavy coatings. The different values of the interceptors for each line show that clay / calcium carbonate coating as such, in the absence of TiO 2, gives some inhibition of the yellowing, and that this inhibition is improved with increasing coating weight. . Addition of TiO 2 to any coating weight further increases the yellowing inhibition. Inhibition is improved with increasing TiO 2 addition up to a maximum between 5-6% on fi children. Although yellowing slows, it still occurs at these maximum levels of TiOZ.

Example 2 A coated paper as described in Example 1 was prepared with 9 g / m 2 / side coating weight. Samples containing TiO 2 and the citric acid salt of 4-hydroxy-2,2,6,6-tetramethyl-N-hydroxypiperidine as radical scavengers were tested for light stability.

Fig. 2 shows the resulting diagram of the PC number after 12 days of accelerated exposure to the amount of TiO 2 added. As noted in Example 1, the yellowing inhibition is maximized when only TiO 2 alone is used at a TiO 2 content between 5 and 6%. Fig. 2 also shows that the addition of radical scavengers to a coating paint containing TiO 2 improves the yellowing inhibition. Fig. 2 also shows that the addition of a radical scavenger to the coating paint improves the yellowing inhibition under conditions where no further benefit can be obtained by increasing the amount of TiO 2.

Fig. 3 shows a diagram of the PC number after 12 days of exposure to the amount of radical calorific handles in the coating, at a coating weight of 9 g / m 2 and different contents of TiO 2. The yellowing inhibition is obtained by adding radical scavengers alone and reaches a maximum when applied at about 0.5% / side. This maximum level can be improved by including TiO 2 in the coating.

Otvcn c OQO 10 15 20 '= 25 f * e' \ '7 J “w * i 13 Example 3 Coated paper as described in Example 1 was prepared with a coating weight of 14 g / m 2 / side and containing 0.3% of the radical scavenger 4-hydroxy-2,2,6,6-tetramethyl-N-hydroxypiperidine citrate and various amounts of TiO 2.

The diagram in Fig. 4 shows that complete inhibition of yellowing after 12 days of accelerated exposure was possible using 0.31% of the radical scavenger with 3.5% TiO 2.

Example 4 A 57 g / m 2 machine-made paper made from 100% peroxide bleached thermomechanical pulp of softwood, with an initial ISO brightness of 78%, was treated with a pigmented formulation in a laboratory applicator. The coating weight was 4 g / m 2 / side. The pigmented mixture consisted of 80% malt calcium carbonate and 20% delaminated clay, starch, latex and some minor additives.As shown in Fig. 5, the light stability increased 12% when the calcium carbonate was replaced with TiO Doubling the amount of TiOz to 2.3% gave no further improvement in light stability.Addition of 1.0% of the radical scavenger together with 1.15% TiOz gave a light stability that was better than possible if used TiOZ only.

Example 5 A paper made from a 57 g / m 2 base sheet with a 4 g / m 2 coating, as described in Example 4, was prepared. In addition to a control sample, samples containing 0.6% TiO 2, 1% radical scavenger and both 0.6% TiO 2 and 1% radonon were irradiated with tin: - u o o dical scavengers for 12 days. The values of the PC number are shown in Table 1. The data show that the effect of adding TiO 2 and radical scavengers together at the different levels (smaller increase in the PC number) is greater than the sum of their individual effects.

Table 1 Change in PC number for BTMP paper with 4 g / mz / side coating Sample PC number Sum of individual effects Effect of addition when TiOZ and of TiOz are added and radical scavengers radical scavengers together (Pckonaoli - Pcriozl * (Pckonuoii - PCRs ) PCmmii - PCrioz + Rs Control 16.92 0.6% TiOz 14.80 7.69 9.58 1% RS 11.35 0.6% TiOz 7.34 + 1% RS Example 6 Coated paper with a 49 g / m 2 base sheet and 9 g / m 2 / side coating were prepared as described in Example 1. The samples were prepared, containing the following yellowing inhibitors in the coating color: control sample (no inhibitor), 1.5% TiO 2 plus 0.4% radical scavenger and 0, 3% organic UVA (a 60/40 mixture of the hydroxyphenylbenzotriazole structures E and F below, where n = 5-9), 1.5% TiO 2 plus 0.4% radical scavenger and 0.3% organic UVA.

Ivcøu OO 0000 00 oi I 0 f 'n ~ -. "' Fw (W 'v 15 §II = {' ë" I =. Få 'o; v. N ø n QI nn - nen ø oo QWz Oß / É : E .Q Na "HO ZZ o O_E \ CNQ F The values for the PC number after 12 days of accelerated exposure are shown in Table 2. The data show that the effect (small increase in PC number) of the addition of TiO2, radical transducer and organic UVA together at indicated levels is greater than the individual effects of radical scavengers + organic UVA and TiO 2.

Table 2 Change in PC number for mechanical paper with 9 g / mz / side coating Sample PC number The sum of individual effects Effect of addition when adding TiOz and of TiOz and radical scavenger radical scavenger together (Pckonuoil - PCrio2) + (Pckonuoli 'Pcxs fl iva ) pCkonuoii - PCr1o2 + Rs + UvA Control 6.06 pl, 5% TiO2 5.81 2.79 3.26 0.4% RS + 3.52 0.3% UVA 1.5% TiO2 + 2.8 _ 0.4% RS + 0.3% UVA oøuon 10 16 nu v 1 aquoovou UQ n oo Example 7 A standard 100 g / mz sheet made of 100% peroxide bleached thermomechanical pulp of softwood, initial brightness 73%, was prepared either with or without rutile TiO 2 as filler. The TiO 2 contained in the sheet was 13.7% based on oven dry fiber. Control sheets and filled sheets were also treated with 1.0% radical scavenger in a laboratory application. As shown in Fig. 6, TiO 2 in the paper improves the light stability even when it was included in the paper as a filler, just as in the surface treatment.

The radical scavenger improved the light stability of the paper, but a combination of TiO 2 and radical scavenger gave much better yellowing inhibition.

Claims (29)

5 10 15 20 'ÉÃ 2, 30 t; . none o 17 o; I I 0 o o ø ø Q Q o c a o o A c o u: Patent claim Light-stable lignin-containing paper containing particulate titanium dioxide and a radical scavenger that inhibits the yellowing of the paper. The paper of claim 1, wherein said radical scavenger is an obstructed arnin. Paper according to claim 1 or 2, wherein said titanium dioxide is rutile titanium dioxide having a particle size of 0.01 to 1.5 μm and is present in an amount of 0.05 to 10% by weight based on the oven weight of the oven. ' 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-sized paper sheets and said particle size is 0.1 to 1.5 μm. Paper according to claim 3, * wherein said titanium dioxide is in an amount of 1 to 5% by weight, based on the oven-dry erv overweight, and said particle size is 0.16 to 0.28 μm. Paper according to any one of claims 1-5, wherein said radical colony handle is present in an amount of 0.05 to 5% by weight, based on the oven weight fi overweight. Paper according to any one of claims 1-5, wherein said radical scavengers are present in an amount of 0.1 to 2% by weight, based on the oven weight. Paper according to any one of claims 1 to 5, wherein said radical scavenger is present in an amount of 0.1 to 1.0% by weight, based on the oven weight. Paper according to any one of claims 1-8, wherein said radical scavenger is selected from piperidines or pyrrolidines of formula A, B, C and D:. a c n n. p a n a n a o ø Q o nu H1 R H3 Å CH3 | CH3 Y + F31 B Ra | i | Ra lXPÜm / p CH3 | CHS Y m H1 CH 2,, if Y n, * Rziü fl a [XW-vv cH,, en, ° Y m where: R the general formula _ [- O- (CH2) ,,] N where n is 1 to 4 and N is 1 to 20 and the alkylene or alkyl moiety in R] has from 1 to 12 carbon atoms and may be a straight chain or a branched chain, the alkylene or alkyl moieties may be unsubstituted or substituted by hydroxyl (OH), carboxyl (COOH) or carboxylate groups (COO ', MzA / z) where M is a metal ion from the first, second or third group of the Periodic Table, or Zn, Cu , Ni or Co; or M is N + (R 6) 4 where R 1 is an alkyl of 1 to 8 carbon atoms or benzyl; and r and z are each integers of 1, 2 or 3; R; and R 3 may be the same or different and are selected from hydrogen, hydroxyl (OH), carboxyl (COOH) or carboxylate groups (C00, MZÉ / Z) where M is a metal ion from the first, second or third group of the Periodic Table, or Z , Cu, Ni or Co; and M is 10 15 20 25 r 'fw .r- ~ e' 19. ' a group N + (R6) 4 where R6 is an 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 between 1 and 20. Paper according to any one of claims 1-9, wherein said titanium dioxide and radical scavenger are present as filler in the paper. A paper according to any one of claims 1-9, wherein said titanium dioxide and said radical scavenger are present in a pigment coating on at least one side of the paper. A paper according to any one of claims 1 to 11, further including an organic absorbent for ultraviolet light together with said titanium dioxide and radical scavenger IC. Paper according to any one of claims 1 to 12, made from lignin-containing pulp having a lignin content of 15 to 35% by weight, based on the weight of the oven-dry fi berlignin in the lignin-containing pulp. Paper according to any one of claims 1 to 12, made from a pulp comprising a high yield pulp selected from mechanical pulp, rock abrasive pulp, thermomechanical pulp, chemithermomechanical pulp and chememechanical pulp in an amount of 80 to 100% by weight, based on the total weight of the pulp . A method of making lignin-containing paper light stable comprising; 0 0 0 Q l II OIIO II II 10 15 20 20. Incorporating into said paper an amount of particulate titanium dioxide and a radical scavenger, in an amount sufficient to inhibit the yellowing of paper, or b) coating the paper with a coating mixture having a content of particulate titanium dioxide and a radical scavenger, in an amount sufficient to inhibit the yellowing of the paper. The method of claim 15, wherein said radical scavenger is a hindered amine. A method according to claims 15 or 16, wherein said titanium dioxide is rutile titanium dioxide having a particle size of 0.01 to 1.5 μm and is present in an amount of 0.05-10% by weight, based on the weight of oven-sized pulp fibers in the paper. The method of claim 17, wherein said titanium dioxide is present in an amount of 0.5 to 6% by weight, based on the weight of kiln-sized pulp fibers. The method of claim 17, wherein said titanium dioxide is present in an amount of 1 to 5% by weight, based on the furnace fi overweight, and said particle size is 0.16 to 0.28 hr. A method according to any one of claims 15-19, wherein said radical scavenger is present in an amount of 0.05 to 5% by weight, based on the oven dry weight. A method according to any one of claims 15-19, wherein said radical scavenger is present in an amount of 0.1 to 1.0% by weight, based on the oven dry weight. 21. . . - ~ v u 2. °. - -. . . . . . .- A process according to any one of claims 15-21, wherein said radical scavenger is selected from piperidines and pyrrolidines of the formula A, B, C and D: H1 'Zvfbrä ^ CHS | CH3 v + H1 XP- B R iii H3 l lm / p CHQ 'CHS Y m H1 R na c CH,, cH, v H1 * H2 i' R ”lxriml, en,, eu, ° Y m where: R; has been selected from hydrogen, hydroxyl, primary, secondary, tertiary or quaternary amino, alkyl, alkoxy, aminoalkylalkylene oxide according to the general formula [-O- (CH 2) 2] N where n is 1 to 4 and N is 1 to 20 and alkylene or the alkyl moiety in R] has from 1 to 12 carbon atoms and may be straight chain or branched, the alkylene or alkyl moiety may be unsubstituted or substituted by hydroxyl (OH), carboxyl (COOH) or carboxylate groups (COO ', Mzl fl / z ), where M is a metal ion from the first, second or third group of the periodic table, or Zn, Cu, Ni or Co; or M is N + (R 6) 4 where R 6 is an alkyl of 1-8 carbon atoms or benzyl; and r and z are each integers of 1, 2 or 3; 00000 10 15 20: 25 9 I o w ø a o c o n p Q I Q o II R; and R 3 may be the same or different and is selected from hydrogen, hydroxyl (OH), carboxyl (COOH) or carboxylate groups (CO, MZÉ / Z), where M is a metal ion from the first, second or third group in the periodic table, or Zn, Cu, Ni or Co; or M is a group N + (R 6) 4 where R 0 is an 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. A method according to any one of claims 15-22, wherein said titanium dioxide and radical scavengers are incorporated as fillers in the paper in accordance with a). A method according to any one of claims 15-22, wherein said titanium dioxide and said radical scavenger are present in said coating formulation according to b). A method according to any one of claims 15-24, further including an organic absorbent for ultraviolet light together with said titanium dioxide and radical scavenger. A method according to any one of claims 15 to 25, wherein said paper is prepared from a lignin-containing pulp having a lignin content of 15 to 35% by weight, based on the weight of kiln-tonic lignin in the lignin-containing pulp. A method according to any one of claims 15 to 25, wherein said paper is prepared from a pulp comprising a high yield pulp selected from mechanical pulp, rock abrasive pulp, thermomechanical pulp, chemithermomechanical pulp and chemimechanical pulp in an amount of at least 10% by weight , based on the total weight of the mass. 23 o Q u a o u. Q n o o n o o O o o o a n o Q o o O O II 28. Use of particulate titanium dioxide together with a radical scavenger in lignin-containing paper to inhibit yellowing. Use according to claim 28, wherein said radical scavenger is a hindered amine.