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: 1

[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 (COO⁻_rM_r/z^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⁺(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 (COO⁻_rM_r/z^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⁺(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.