WO2007009221A1

WO2007009221A1 - Enhanced brightness and brightness stability of lignocellulosic materials

Info

Publication number: WO2007009221A1
Application number: PCT/CA2006/001146
Authority: WO
Inventors: Zhirun Yuan; Thomas Q. Hu; John A. Schmidt
Original assignee: Pulp and Paper Research Institute of Canada; FPInnovations
Current assignee: FPInnovations
Priority date: 2005-07-15
Filing date: 2006-07-12
Publication date: 2007-01-25
Anticipated expiration: 2008-01-15

Abstract

The present invention provides a method for the production of lignocellulosic materials, in particular, lignin-containing or lignin-free papers or paperboards, with a higher and more stable brightness. The enhanced brightness and brightness stability is achieved using pigmented coatings that also contain a water-soluble phosphine or phosphonium compound possessing at least one phosphorus hydroxymethyl bond/linkage. The pigments can also be pretreated with the phosphine or phosphonium compounds before addition as a filler or preparation of the coating colour. One example of such a water-soluble phosphine is tris(hydroxymethyl)phosphine (THP), P(CH2OH)3. One example of such a phosphonium compound is tetrakis(hydroxymethyl)phosphonium sulfate (THPS), [P(CH2OH)4]2SO4. There is a synergy between THPS and the pigment on the brightness gain and on the brightness/light stabilization of the papers or paperboards coated or filled with the pigment.

Description

ENHANCED BRIGHTNESS AND BRIGHTNESS STABILITY OF LIGNOCELLULOSIC MATERIALS

TECHNICAL FIELD The present invention relates to a method that can be used in paper mills to enhance brightness and brightness stability of lignocellulosic materials which have a pigment coating or pigment filler, that contains, or in which the pigment has been treated with, certain water-soluble phosphines or phosphonium compounds.

BACKGROUND ART Lignocellulosic materials such as wood are the raw materials used for the production of pulps and papers, hi order to make papers, lignocellulosic materials such as wood are first reduced to pulps (discrete fibres) by a mechanical or chemical pulping process. In mechanical pulping, pulps are produced in a yield of 90-98% (with retention of lignin) mainly through the action of mechanical forces. In chemical pulping, pulps are produced through the action of the pulping chemicals such as sodium hydroxide and sodium sulphide at elevated temperatures. One example of mechanical pulps is the so- called thermomechanical pulp (TMP) produced from the thermomechanical pulping process. One example of chemical pulps is the so-called kraft pulp produced from the kraft pulping process. Chemical pulps are often bleached with oxidative bleaching chemicals to remove the residual lignin in the pulps and to provide the bleached pulps with high brightness and brightness stability. Mechanical pulps are bleached, if desired, with chemicals that do not remove lignin significantly or at all, such as hydrogen peroxide or sodium dithionite [Dence and Reeve, Pulp Bleaching - Principles and Practice, Tappi Press: Atlanta, p. 457-512, 1996]. Alkaline hydrogen peroxide, in the presence of peroxide stabilizers such as sodium silicate and magnesium sulfate, is capable of bleaching mechanical pulps such as spruce TMP from an initial brightness of 55-60% to 70-80% ISO. However, alkaline peroxide bleaching becomes much less efficient as the bleached brightness increases. A high chemical charge must be applied to obtain, for example, a brightness of > 75% ISO for spruce TMP. This not only increases cost, but also reduces the pulp yield and produces effluents with high dissolved organics and thus chemical oxygen demand (COD) [Soteland et al., 1988 International Pulp Bleaching Conference Proceedings, Tappi Press: Altanta, p. 231, 1988].

The traditional markets of high-yield mechanical pulps are newsprint and short- life advertising papers. However, there is a significant interest in using bleached mechanical pulps in high-brightness coated paper grades. Advances in pulping technology have made it possible to produce high-yield pulps with suitable strength and surface properties for use in these grades, which currently use bleached chemical pulp almost exclusively. However, both peroxide-bleached and dithionite-bleached pulps are highly unstable; they rapidly turn yellow (lose the brightness gained from bleaching) when exposed to light and/or heat or during storage [Leary, J. Pulp Paper Sci. 20(6): Jl 54-160, 1994]. Such a brightness reversion remains a significant impediment to the broader use of bleached mechanical pulps.

Besides improving gloss, color, and printability, pigment coating can increase the brightness and brightness stability of lignin-containing and lignin-free papers. The extent of the improvement depends on the coat weight, the pigments used, and the relative brightness of the coating color and the basesheet [Yuan et al., 2004 TAPPI Coating Conference Proceedings, Baltimore, MD, USA]. The pigments provide some brightness stability by either scattering or absorbing incident UV light. Common pigments used for the coating of papers include titanium dioxide (TiO₂), clay, precipitated calcium carbonate (PCC), and ground calcium carbonate (GCC). The above-mentioned pigments are also commonly used as fillers in various grades of papers to replace more expansive fibers or to obtain improved optical properties and printability. For example, pigment filler improves the paper brightness if the brightness of the filler itself is higher than that of fibers. The application of fillers is especially important when opacity is needed at a low basis weight.

PCT WO 2004/070110 Al, published August 19, 2004, discloses the bleaching and brightness stabilization of lignocellulosic materials with water-soluble phosphines or phosphonium compounds; however, no pigment coating was used in these applications. In addition, water-soluble phosphines with phosphorus hydroxyalkyl bond/linkages such as tris(hydroxypropyl)phosphine (THPP), P(CH₂CH₂CH₂OH)₃, were as effective as those with phosphorus hydroxymethyl bond/linkages, such as tris(hydroxymethyl)phosphine (THP), P(CH₂OH)₃, or as tetrakis(hydroxymethyl)phosphonium chloride (THPC), [P(CH₂OH)₄]Cl. About three points of ISO brightness increase was obtained when a bleached CTMP sheet was impregnated with 2-6.4% THPC [Davidson et al., J. Wood Chem. Tech. 11(4): 419-437 (1991)]. Again, no pigment coating was used in conjunction with THPC.

Aqueous solutions of tetrakis(hydroxymethyl)phosphonium sulphate (THPS), [P(CH₂OH)₄]₂SO₄, are acidic (pH -3.2) due to the small dissociation of THPS to THP, formaldehyde and sulfuric acid [Ellzey, Textile Chem. Color. 10(5): 12 (1978); Frank et al., Textile Res. J. 52(11): 678-693 (1982)]. Higher pH will promote the release of formaldehyde from THPS or THP. Formaldehyde, an irritant for the respiratory tract, can react with sodium bisulfite to form formaldehyde-bisulfite adduct, or it can be reduced with a reducing agent such as sodium hydrosulfite or sodium borohydride. Unexpectedly and surprisingly, it has been discovered that certain water-soluble phosphines or phosphonium salts, when combined with pigment coating or when used to treat pigment, increased the light scattering power of the pigment and consequently provide a synergistic increase in brightness to lignocellulosic materials such as papers coated or filled with the pigment. The present invention is based on such a surprising discovery.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a method of producing lignin-free or lignin-containing paper or paperboard of higher and more stable brightness.

It is a further object of this invention to provide lignin-free or lignin-containing paper or paperboard of higher and more stable brightness.

It is a still further object of this inventon to provide a method of improving the brightness of lignin-free or lignin-containing paper or paperboard which contains filler or has a pigment coating.

The present invention provides a method that can be used to produce lignin- containing or lignin-free papers or paperboards with a higher and more stable brightness through a pigment coating or addition of pigment filler, that contains, or in which the pigment has been treated with, certain water-soluble phosphines or phosphonium compounds. The phosphine or phosphonium compounds should possess at least one phosphorus hydroxymethyl bond/linkage. There is a synergy between the said phosphine or phosphonium salt and the pigment on the brightness gain and brightness/light stabilization of papers or paperboards coated or filled with the pigment or filler.

This invention also provides the lignin-free or lignin-containing paper or paperboard of higher and more stable brightness, which is produced by this method.

The advancing technologies of printing and packaging have placed greater demands on the surface properties of the paper sheet. To meet the more stringent requirements, many paper surfaces are now coated with suitable formulations to improve gloss, color, printing detail, and brightness. The coating can be applied either on- machine or off-machine. During the coating process, the web is given a generous application of coating, the excess of which is usually removed by either a blade or an air knife. Application of chemicals to the paper surfaces provides close to 100% retention, which is advantageous since it reduces the problems of wet-end deposits, increases the life of machine clothing, and reduces the chemicals in the white water to assist in environmental improvement. Alternatively, pigments are also used as fillers to replace more expansive fibers or to obtain improved optical properties and printability. The application of fillers is especially important when opacity is needed at a low basis weight.

The inorganic pigment is the most abundant component in a coating color and it is also the most important factor affecting the properties of the coating color. Two desirable benefits of a pigmented coating or filling are increased brightness and opacity, which result from the high light scattering power of the pigments. Light scattering coefficient is the most important property for pigments. Common inorganic pigments include titanium dioxide (TiO₂) clay, precipitated calcium carbonate (PCC), and ground calcium carbonate (GCC), silica and silicate, Talc, and alumina trihydrate.

The phosphines or phosphonium salts described in WO 2004/070110 Al or a pigmented coating can separately increase the brightness and brightness stability of lignin-containing papers. Surprisingly, certain phosphines or phosphonium salts described hereinafter, when combined with or used to pretreat pigments, increase synergistically the brightness and brightness stability of lignin-containing or lignin-free, pigment coated or filled papers. In particular, tetrakis(hydroxymethyl)phosphonium sulfate (THPS) and a pigmented coating can further improve brightness and brightness stability over the enhancements obtained by using THPS or a pigmented coating alone.

DISCLOSURE OF THE INVENTION

In accordance with one aspect of the invention, there is provided a method for the synergistic brightness enhancement and brightness stabilization of papers and paperboards comprising pigmented coatings that contain, or in which the pigment has been treated with a water-soluble phosphine or phosphonium compound of formula (A):

Z [X] im- (A)

wherein t is zero or 1 ; when t = 0, R₄R₅PY₂ is absent and R₃ is bonded to the P of the R₁R₂PY₁ group; R₅ is absent, an alkylene group (CH₂)_S (s = 1 to 12) interrupted by 0 to 6 oxygen (O) atoms or secondary amino (NR') groups, and/or substituted by a zero to 2s number of a hydroxyl, alkyl, aryl, thio, thioether, amino, ester, amide, carboxyl and/or carboxylate groups, or a phenylene group substituted by a zero to 4 number of a hydroxyl, alkyl, aryl, thio, thioether, amino, ester, amide, carboxyl, carboxylate, and/or sulfonate groups; or preferably R₅ is an alkylene group (CH₂)s (s = 1 to 4) where the carbon chain is optionally interrupted by one or two oxygen (O) atom(s); wherein when y = l, n = z ^:= m = 0, Yi, Y₂ and X are absent; R₁, R₂ and R₃, or R₁, R₂, R₃, R₄ and R₅ groups are collectively selected such that the molecule has an overall solubility of at least 0.001 g/L; Ri is a hydroxymethyl (HOCH₂) group, R₂ and R₃, or R₂,

R₃ and R₄ are independently selected from hydrogen, optionally substituted linear or branched alkyl groups, or optionally substituted aryl groups, the optional substitution referring to the presence of substituents selected from ether, amino, hydroxy, ester, thioether, amide, carbonyl, carboxyl, and carboxylate moieties; wherein when X is present, X is an inorganic or organic anion, and the value of m is < 5; the total charge of yn = zm; Y₁ is a hydroxymethyl group (CH₂OH) and t is zero or 1; Ri, R₂ and R₃, or R₁, R₂, R₃, R₄ and Y₂ are independently selected from hydrogen, a Lewis acid such as boron trifluoride (BF₃), optionally substituted linear or branched alkyl groups, or optionally substituted aryl groups, the optional substitution referring to the presence of substituents selected from ether, amino, hydroxy, ester, thioether, amide, carbonyl, carboxyl, and carboxylate moieties.

In formula (A), the integers y, m, n and z have, in particular, the following values: n is 0 to 2, y is 1 to 5, z is 0 to 2, and m is 0 to 5. In another aspect of the invention, there is provided a paper or paperboard coated with a pigment and a compound of formula (A) defined herein.

In another aspect of the invention, there is provided a paper or paperboard filled with a filler and a compound of formula (A) defined herein.

In another aspect of the invention, there is provided a paper or paperboard filled or coated with a filler or a pigment that has been treated with a compound of formula (A) defined herein.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to the present invention, synergistic brightness enhancement and brightness stabilization of lignin-containing or lignin-free papers or paperboards can be achieved through a pigment coating or addition of pigment filler, that contains, or in which the pigment has been treated with, certain water-soluble phosphines or phosphonium compounds. The phosphine or phosphonium compounds should possess at least one phosphorus hydroxymethyl bond/linkage, preferably with a water-soluble tertiary phosphine or a quaternary phosphonium compound possessing at least one phosphorus hydroxymethyl bond/linkage.

The preferred embodiments of the compounds of formula (A) have the following characteristics: a) Y₁ and Y₂ are both absent, Ri is a hydroxymethyl (HOCH₂) group, R₂ and R₃, or R₂, R₃ and R₄ are independently hydrogen, an alkyl group (R) or an ether group

(OR) with R being (CH₂)_qH (q = 1 to 12) interrupted by 0 to 6 oxygen (O) atoms or secondary amino (NR') groups, and/or substituted by a zero to (2q + 1) number of a hydroxyl, thio, thioether, amino, ester, amide, carboxyl and/or carboxylate groups. R' is either hydrogen or an optionally substituted linear or branched alkyl group or optionally substituted aryl group; wherein optional substitution refers to the presence of one or more substituents selected from ether, amino, hydroxy, ester, thioether, amide, carbonyl, carboxyl, and carboxylate moieties; b) Yi and Y₂ are both absent, Ri is a hydroxymethyl (HOCH₂) group, R₂ and R₃, or R₂, R₃ and R₄ are independently hydrogen, an alkyl group (R) or an ether group (OR) with R being CH₂(CH₂)_qH (q = 0 to 5) interrupted by 0 to 3 oxygen (O) atoms or secondary amino (NR') groups, and/or substituted by a zero to (2q + 1) number of a hydroxyl, thio, thioether, amino, ester, amide, carboxyl and/or carboxylate groups; c) Y] and Y₂ are both absent, Ri is a hydroxymethyl (HOCH₂) group, and R₂ is the same as R₃ in the molecule; d) Y₁ and Y₂ are both absent, R₁, R₂ and R_3> or R₁, R₂, R₃ and R₄ are all hydroxymethyl (CH₂OH) groups; e) Yi is a hydroxymethyl group (CH₂OH) and t is zero or 1, Ri, R₂ and R₃, or Ri, R₂, R₃, R₄ and Y₂ are independently hydrogen, a Lewis acid such as boron trifluoride (BF₃), an alkyl group (R) or an ether group (OR) with R being (CH₂)_qH (q = 1 to 12) interrupted by 0 to 6 oxygen (O) atoms or secondary amino (NR') groups, and/or substituted by a zero to (2q + 1) number of a hydroxyl, thio, thioether, amino, ester, amide, carboxyl and/or carboxylate groups. R' is either hydrogen or an optionally substituted linear or branched alkyl group or optionally substituted aryl group; wherein optional substitution refers to the presence of substituents selected from ether, amino, hydroxy, ester, thioether, amide, carbonyl, carboxyl, and carboxylate moieties; f) Y₁ is a hydroxymethyl group (CH₂OH) and t is zero or 1, R₁, R₂ and R₃, or R₁, R₂, R₃, R₄ and Y₂ are independently hydrogen, a Lewis acid such as boron trifluoride (BF₃), an alkyl group (R) or an ether group (OR) with R being CH₂(CH₂)_qH (q = 0 to 5) interrupted by 0 to 3 oxygen (O) atoms or secondary amino (NR') groups, and/or substituted by a zero to (2q + 1) number of a hydroxyl, thio, thioether, amino, ester, amide, carboxyl and/or carboxylate groups; g) Y₁ is a hydroxymethyl group (CH₂OH) and t is zero or 1, and at least one ofR₃, R₄ and Y₂ is a hydroxymethyl (CH₂OH) group.

In the phosphonium compounds of formula (A), X is suitably selected from chloride, sulfate, hydroxide, hydrosulfite, phosphate, carbonate, bicarbonate, bisulfate, alkoxide, formate, acetate, citrate, oxalate, ascorbate, ethylenediaminetetraacetate or diethylenetriaminepentaacetate. The compounds of formula (A) for use in the invention need to be water-soluble and the variables in formula (A) are selected so that the compounds (A) have an overall water solubility of at least 0.001 g/L.

Further examples of preferred phosphine and phosphonium compounds for use in the invention are indicated below: Phosphines:

The R₁, R₂ and R₃ groups being collectively selected such that the molecule has an overall solubility of at least 0.001 g/L. Where R₁ is a hydroxymethyl (HOCH₂) group, R₂ and R₃ are independently selected from hydrogen and optionally substituted linear or branch alkyl groups, or optionally substituted aryl groups. Where optional substitution can refer to the presence of substituents selected from ether, amino, hydroxy, ester, thioether, amide, carbonyl, carboxyl, and carboxylate moieties. In a more preferred embodiment, R₁ is a hydroxymethyl (HOCH₂) group, and R₂ and R₃ are independently an alkyl group (R) or an ether group (OR) with R being (CH₂)_qH (q = 1 to 12) interrupted by 0 to 6 oxygen (O) atoms or secondary amino (NR') groups, and/or substituted by a zero to (2q + 1) number of a hydroxyl, thio, thioether, amino, ester, amide, carboxyl and/or carboxylate groups.

In even more preferred embodiments, R₁ is a hydroxymethyl (HOCH₂) group, R₂ and R₃ are independently an alkyl group (R) or an ether group (OR) with R being CH₂(CH₂)_qH (q = 0 to 5) interrupted by 0 to 3 oxygen (O) atoms or secondary amino

(NR'), and/or substituted by a zero to (2q + 1) number of a hydroxyl, thio, thioether, amino, ester, amide, carboxyl and/or carboxylate groups. R' is either hydrogen or an optionally substituted linear or branched alkyl group or optionally substituted aryl group.

Where optional substitution can refer to the presence of substituents selected from ether, amino, hydroxy, ester, thioether, amide, carbonyl, carboxyl, and carboxylate moieties.

In yet even more preferred embodiments, R₁ is a hydroxymethyl (HOCH₂) group, and at least one of R₂ and R₃ is also a CH₂OH group. hi a most preferred embodiment, the water-soluble phosphine is the commercially available compound, tris(hydroxymethyl)phosphine (THP), P(CH₂OH)₃. Diphosphines and Bisphosphines:

The R₁, R₂, R₃, R₆ and R₇ groups being collectively selected such that the molecule has an overall solubility of at least 0.001 g/L.

Where R₁ is a hydroxymethyl (HOCH₂) group, R₂, R₃ and R₇ are independently selected from hydrogen, optionally substituted linear or branched alkyl groups, or optionally substituted aryl groups. Where optional substitution can refer to the presence of substituents selected from ether, amino, hydroxy, ester, thioether, amide, carbonyl, carboxyl, and carboxylate moieties. hi more preferred embodiments, the diphosphine compound is of C₂ or C_s symmetry.

In preferred embodiments, R₁ is a hydroxymethyl (HOCH₂) group, R₂, R₃ and R₇ are independently hydrogen, an alkyl group (R) or an ether group (OR) with R being (CH₂)_qH (q = 1 to 12) interrupted by 0 to 6 oxygen (O) atoms or secondary amino (NR') groups, and/or substituted by a zero to (2q + 1) number of a hydroxyl, thio, thioether, amino, ester, amide, carboxyl and/or carboxylate groups. R' is either hydrogen or an optionally substituted linear or branched alkyl group or optionally substituted aryl group. Where optional substitution can refer to the presence of substituents selected from ether, amino, hydroxy, ester, thioether, amide, carbonyl, carboxyl, and carboxylate moieties.

In more preferred embodiments, R₁ is a hydroxymethyl (HOCH₂) group, R₂, R₃ and R₇ are independently hydrogen, an alkyl group (R) or an ether group (OR) with R being CH₂(CH₂)_qH (q = 0 to 5) interrupted by 0 to 3 oxygen (O) atoms or secondary amino (NR') groups, and/or substituted by a zero to (2q + 1) number of a hydroxyl, thio, thioether, amino, ester, amide, carboxyl and/or carboxylate groups.

In even more preferred embodiments, R₁ is a hydroxymethyl (HOCH₂) group, and R₂ is the same as R₃. In most preferred embodiments, R₁, R₂, R₃ and R₇ are all hydroxymethyl (CH₂OH) groups.

R₆ is absent; an alkylene group (CH₂)_S (s = 1 to 12) interrupted by 0 to 6 oxygen (O) atoms or secondary amino (NR') groups, and/or substituted by a zero to 2s number of a hydroxyl, alkyl, aryl, thio, thioether, amino, ester, amide, carboxyl and/or carboxylate groups; or a phenylene group substituted by a zero to 4 number of a hydroxyl, alkyl, aryl, thio, thioether, amino, ester, amide, carboxyl, carboxylate, and/or sulfonate groups.

In preferred embodiments, R₆ is an alkylene group (CH₂)_S (s = 1 to 4), where the carbon chain is optionally interrupted by one or two oxygen (O) atom(s).

Phosphonium Compounds:

Wherein X is an inorganic or organic anion such as, but not limited to, chloride, sulfate, hydroxide, hydrosulfite, phosphate, carbonate, bicarbonate, bisulfate, alkoxide, formate, acetate, citrate, oxalate, ascorbate, ethyl enediaminetetraacetate or diethylene- triaminepentaacetate, and the value of m is < 5; the total charge of yn = zm.

Where Ri is a hydroxymethyl group (CH₂OH); and R₂, R₃ and R₇ are independently selected from hydrogen, a Lewis acid such as boron trifluoride (BF₃), optionally substituted linear or branched alkyl groups, or optionally substituted aryl groups. Where optional substitution can refer to the presence of substituents selected from ether, amino, hydroxy, ester, thioether, amide, carbonyl, carboxyl, and carboxylate moieties.

In preferred embodiments, Rj is a hydroxymethyl group (CH₂OH); and R₂, R₃ and R₇ are independently hydrogen, a Lewis acid such as boron trifluoride (BF₃), an alkyl group (R) or an ether group (OR) with R being (CH₂)_qH (q = 1 to 12) interrupted by 0 to 6 oxygen (O) atoms or secondary amino (NR') groups, and/or substituted by a zero to (2q + 1) number of a hydroxyl, thio, thioether, amino, ester, amide, carboxyl and/or carboxylate groups. R' is either hydrogen or an optionally substituted linear or branched alkyl group or optionally substituted aryl group. Where optional substitution can refer to the presence of substituents selected from ether, amino, hydroxy, ester, thioether, amide, carbonyl, carboxyl, and carboxylate moieties.

In more preferred embodiments, R₁ is a hydroxymethyl group (CH₂OH); and R₂, R₃ and R₇ are independently hydrogen, a Lewis acid such as boron trifluoride (BF₃), an alkyl group (R) or an ether group (OR) with R being CH₂(CH₂)_qH (q = 0 to 5) interrupted by 0 to 3 oxygen (O) atoms or secondary amino (NR') groups, and/or substituted by a zero to (2q + 1) number of a hydroxyl, thio, thioether, amino, ester, amide, carboxyl and/or carboxylate groups.

In a most preferred embodiment, the phosphonium compound is the commercially-available tetrakis(hydroxymethyl)phosphonium sulphate (THPS), [P(CH₂OH)₄J₂SO₄, or tetrakis(hydroxymethyl)phosphonium chloride (THPC), [P(CH₂OH)₄]Cl.

Diphosphonium and Bisphosphonium Compounds:

n+

R. R₄

R₁ — P- -R₆-P R₇ z [Xf-

R? R₈

Wherein X is an inorganic or organic anion such as, but not limited to, chloride, sulfate, hydroxide, hydrosulfite, phosphate, carbonate, bicarbonate, bisulfate, alkoxide, formate, acetate, citrate, oxalate, ascorbate, ethylenediaminetetraacetate or diethylenetriaminepentaacetate, and the value of m is < 5; the total charge of yn = zm. Where Rj is a hydroxymethyl group (CH₂OH); and R₂, R₃, R₄, R₇ and R₈ are independently selected from hydrogen, a Lewis acid such as boron trifluoride (BF₃), optionally substituted linear or branched alkyl groups, or optionally substituted aryl groups. Where optional substitution can refer to the presence of substituents selected from ether, amino, hydroxy, ester, thioether, amide, carbonyl, carboxyl, and carboxylate moieties.

In preferred embodiments, R₁ is a hydroxymethyl group (CH₂OH); and R₂, R₃, R₄,

R₇ and R₈ are independently hydrogen, a Lewis acid such as boron trifluoride (BF₃), an alkyl group (R) or an ether group (OR) with R being (CH₂)_qH (q = 1 to 12) interrupted by 0 to 6 oxygen (O) atoms or secondary amino (NR') groups, and/or substituted by a zero to (2q + 1) number of a hydroxyl, thio, thioether, amino, ester, amide, carboxyl and/or carboxylate groups. R' is either hydrogen or an optionally substituted linear or branched alkyl group or optionally substituted aryl group. Where optional substitution can refer to the presence of substituents selected from ether, amino, hydroxy, ester, thioether, amide, carbonyl, carboxyl, and carboxylate moieties.

In more preferred embodiments, R₁ is a hydroxymethyl group (CH₂OH); and R₂, R₃, R₄, R₇ and R₈ are independently hydrogen, a Lewis acid such as boron trifluoride (BF₃), an alkyl group (R) or an ether group (OR) with R being CH₂(CH₂)_qH (q = 0 to 5) interrupted by 0 to 3 oxygen (O) atoms or secondary amino (NR') groups, and/or substituted by a zero to (2q + 1) number of a hydroxyl, thio, thioether, amino, ester, amide, carboxyl and/or carboxylate groups.

In most preferred embodiments, Ri is a hydroxymethyl group (CH₂OH); and at least one OfR₄, R₇ and R₈ is also a hydroxymethyl (CH₂OH) group.

Unless indicated otherwise, terms indicated hereinafter have the following meanings in this specification: i) alkyl and alkyl moieties are straight chain or branched and have 1 to 12, preferably 1 to 6 and more preferably 1 to 4 carbon atoms; alkyl moieties contemplates the alkyl portions of thioether, amide, ether and ester substituents; ii) aryl and aryl moieties and arylene have 6 to 14 carbon atoms and are preferably phenyl or phenylene; aryl moieties contemplates the aryl portions of thioether, amide, ether and ester substituents; iii) water-soluble means, with reference to the compounds of formula (A) that the compounds have an overall water solubility of at least 0.001 g/L. iv) filler refers to mineral pigments used in papermaking or coating processes. THPS can also increase the light scattering powers of the inorganic pigments or fillers even before they are added into the coating or papermaking process.

Paper or paperboard "treated" with filler and a compound (A) as described herein contemplates the filler and the compound (A) being incorporated in the pulp from which the paper or paperboard is formed, or that the paper or paperboard is coated with a composition of the filler and a compound (A).

Similarly, indications of the filler being present in the paper or paperboard contemplates the filler being incorporated in the pulp from which the paper or paperboard is formed, or that the paper or paperboard is coated with a composition of the filler.

The present invention is illustrated but not limited by the following examples. Unless otherwise specified, machine-made papers containing mechanical pulp were coated with pigmented formulations with or without THPS or THPP. For comparison, the basesheet was also treated with water or THPS solution only. The effect of THPS or THPP on brightness and brightness stabilization was studied by incorporating the THPS or THPP as an additive in the coating formulation. The brightness of the coated sheets was then monitored for 20-30 days under two different conditions: storage in the dark at room temperature or exposure to normal office lighting with sunlight passing through a window. ISO brightness was determined according to ISO standard 2470, using a Technidyne Micro TB-IC refiectometer.

In some cases, a laboratory hot soft-nip calendar was used to calendar the coated sheets in order to see the effect of calendaring on brightness and light scattering coefficient of the coated paper. Example 1 - Application of THPS or THPP and/or pigment coating to a 100% BTMP paper

A 57 g/m² 100% bleached TMP (BTMP) basesheet, with an initial brightness of 79.8% ISO, was treated with a pigmented formulation using a laboratory coater at 10 5 m/min. The coat weight was 5 g/m² per side and the coated sheets were dried by a cylinder dryer at 105°C for 3 minutes. The pigmented formulation consisted of 80% ground calcium carbonate (GCC), 20% delaminated clay, starch, latex, and some minor additives. The pH of the coating color was controlled at 8.0. For comparison, the basesheet was also treated with THPS solution at the same pH. The THPS or THPP o charge was 1% w/w on o.d. fibers.

Table 1 shows the ISO brightness values of the various papers after the papers have been stored for 30 days, either in the dark (inside a drawer) at room temperature (-23⁰C) or exposed to normal office lighting 24 hours a day plus the sunlight passing through a window (light intensity = 40-70 foot candles). 5 Coating alone increased ISO brightness by 1.4 points compared to the untreated basesheet control, after 30 days of storage in the dark. Treatment with THPS alone increased ISO brightness by 1.5 points. The sum of the individual effects, a brightness gain of 2.9 points, was 35% less than the ISO brightness gain of 4.4 points obtained when THPS was included in the coating colour. Thus, adding THPS to the pigment coating o provided a synergistic brightness increase during dark storage.

Including THPS in the pigment coating also provided a synergistic effect in stabilizing the brightness to light exposure. The ISO brightness of the sample treated with coating alone was 2.8 points higher than that of the untreated control after 30 days of light exposure. The ISO brightness of the sample treated with THPS alone was 1.8 5 points higher than that of the control. The sum of these effects is 27% less than the 6.3 points difference in ISO brightness that occurred when THPS was included in the pigment coating.

In contrast to the results observed with THPS, combining THPP and the pigment coating provided a smaller brightness increase to the coated paper than using pigment o alone, and a brightness/light stabilizing effect similar to that using pigment alone. Table 1. ISO Brightness values (%) of the various papers and the difference in ISO brightness values over the control sheet (shown in bracket) after 30 days of ambient dark storage or light exposure.

Paper Ambient dark storage Ambient light exposure

_

Control 79.8

Pigment coating 81.2 (1 •4) 74.0 (2.8)

THPS 81.3 (1 .5) 73.0 (1.8)

Pigment coating + THPS 84.2 (4.4) 77.5 (6.3)

Pigment coating + THPP 80.9 (1 •1) 74.2 (3.0)

Example 2 - Application of THPS and/or pigment coating to a 100% BTMP paper

A 57 g/m² 100% BTMP basesheet, with an initial brightness of 79.0% ISO, was treated with a pigmented formulation using a laboratory CLC6000 coater at 800m/min. The CLC6000 coater is equipped with an infrared dryer. The coat weight was 8 g/m² per side. The pigmented formulation consisted of one pigment at a time, together with some latex and starch as binders. The pH of the coating color was controlled at 7.0. For comparison, the basesheet was also treated with THPS solution at the same pH. The THPS charge was 0.5% w/w on o.d. fibers. Table 2.1 shows the optical properties and the synergism between THPS and individual pigment. The brightness after coating did not change with time as in example 1 since the paper has been dried at high temperature with the infrared dryer at the CLC coater. THPS applied alone on the basesheet gave a negligible brightness increase at a charge of 0.5%. Coating with various pigments led to an ISO brightness gain ranging from 0.5 to 5.8 points, with the GCC giving the least and the TiO₂ the most. There is a synergism between THPS and the pigment, i.e., the sum of the individual brightness gain from THPS and a pigment alone was less than the ISO brightness gain obtained when THPS was used together with the pigment in the coating colour.

Table 2.1 also shows that the synergistic increase in brightness is due to the increase of light scattering coefficient of the pigment coating, especially for TiO₂ and clay, by THPS. The absorption coefficient of the coated papers did not change with the addition of THPS. There is a synergism between THPS and the pigments in increasing the light scattering ability of the pigments, i.e., the sum of the individual scattering increase from THPS and a pigment alone, was less than the increase in scattering obtained when THPS was used together with the pigment in the coating colour. Table 2.1 Sheet optical properties anc I the synergism between ' THPS and the pigments

Sample Bright. ΔB* Synergism Scatttering Δs** Synergism Absorption

% in Bright. Coeff. in S Coeff.

Basesheet with water 79.0 68.85 1.78

Basesheet+0.5%THPS 79.3 0.3 69.11 0.26 1.74

GCC 82.3 3.3 77.66 8.81 1.31

GCC+0.5%THPS 83.1 4.1 0.5 79.31 10.46 1. 39 1.29

Clay 79.6 0.6 75.20 6.35 1.49

Clay+0.5%THPS 80.9 1.9 1.0 80.21 11.36 4. 75 1.44

TiO₂ 84.8 5.8 106.44 37.59 1.18

TiO₂+0.5%THPS 86.5 7.5 1.4 113.89 45.04 7. 19 1.13

* ΔB = brightness of sheet with a pigment and/or THPS - brightness of base sheet witr. L water;

**Δs = scattering of sheet with ; i pigment and/or THPS - scattering of base sheet with water.

Table 2.2 shows the effect of calendaring with a hot soft-nip calendar to a PPS roughness about 1 μm on the optical properties of the papers coated with clay, clay and THPS, TiO₂, and TiO₂ and THPS, respectively. The difference in ISO brightness between the "clay + 0.5% THPS" coated paper and the clay coated paper after calendaring (2.4 ISO points) is larger than that before calendaring (1.3 ISO points). Similarly, the difference in ISO brightness between the "TiO₂ + 0.5% THPS" coated paper and the TiO₂ coated paper after calendaring (2.3 ISO points) is larger than that before calendaring (1.7 ISO points).

Table 2.2. Effect of calendaring on the optical properties of the coated papers.

Clay Clay + 0.5% TiO₂ TiO₂ + 0.5%

THPS THPS

Before calendaring:

ISO Brightness, % 79.6 80.9 84.8 86.5

Light scat. Coeff., m²/kg 75.20 80.21 106.44 113.89

Absorb. Coeff., m²/kg 1.49 1.44 1.18 1.13

Brightness gain due to THPS 1.3 1.7

Scattering gain due to THPS 5.01 7.45

After calendaring:

ISO Brightness, % 74.6 77.0 81.3 83.6

Light scat. Coeff., m²/kg 53.88 56.69 77.46 84.25

Absorb. Coeff., m²/kg 1.85 1.71 1.45 1.36

Brightness gain due to THPS 2.4 2.3

Scattering gain due to THPS 2.81 6.79 Table 2.3 shows the brightness values of various papers before and after 20 days of ambient light exposure. The paper coated with THPS and TiO₂ showed a higher synergistic brightness enhancement effect after the light exposure (11.4 — 0.6 - 8.7 = 2.1 ISO points) than that before the light exposure (7.5 - 0.3 - 5.8 = 1.4 ISO points). Thus, the combination of THPS and the pigment TiO₂ not only provides a synergistic brightness enhancement to the paper, but it also provides more significant, synergistic brightness/light stabilization to the paper.

Table 2.3. ISO Brightness values (%) of the various papers and the difference in

ISO brightness values over the control sheet (shown in bracket) before and after 20 days of ambient light exposure.

Paper Brightness before light Ambient light exposure exposure

Control 79.0 73.2

TiO₂ coating 84.8 (5.8) 81.9 (8.7)

THPS 79.3 (0.3) 73.8 (0.6)

TiO₂ + THPS coating 86.5 (7.5) 84.6 (11.4)

Claims

CLAIMS:

1. A method of enhancing brightness and brightness stabilization of paper or paperboard which comprises incorporating in the paper or paperboard, or coating the paper or paperboard with, a filler and a compound (A):

wherein t is zero or 1 ; when t = 0, R₄R₅PY₂ is absent and R₃ is bonded to the P of the RiR₂PYj group; R₅ is absent, an alkylene group (CH₂)_S (s = 1 to 12) interrupted by 0 to 6 oxygen (O) atoms or secondary amino (NR') groups, and/or substituted by a zero to 2s number of a hydroxyl, alkyl, aryl, thio, thioether, amino, ester, amide, carboxyl and/or carboxylate groups, or a phenylene group substituted by a zero to 4 number of a hydroxyl, alkyl, aryl, thio, thioether, amino, ester, amide, carboxyl, carboxylate, and/or sulfonate groups; or preferably R₅ is an alkylene group (CH₂)s (s = 1 to 4) where the carbon chain is optionally interrupted by one or two oxygen (O) atom(s); wherein when y = l, n = z = m = 0, Yi, Y₂ and X are absent; R₁, R₂ and R₃, or R₁, R₂, R₃, R₄ and R₅ groups are collectively selected such that the molecule has an overall solubility of at least 0.001 g/L; Ri is a hydroxymethyl (HOCH₂) group, R₂ and R₃, or R₂, R₃ and R₄ are independently selected from hydrogen, optionally substituted linear or branched alkyl groups, or optionally substituted aryl groups, the optional substitution referring to the presence of substituents selected from ether, amino, hydroxy, ester, thioether, amide, carbonyl, carboxyl, and carboxylate moieties; wherein when X is present, X is an inorganic or organic anion, and the value of m is < 5; the total charge of yn = zm; Yi is a hydroxymethyl group (CH₂OH) and t is zero or 1; Ri, R₂ and R₃, or R₁, R₂, R₃, R₄ and Y₂ are independently selected from hydrogen, a Lewis acid such as boron trifiuoride (BF₃), optionally substituted linear or branched alkyl groups, or optionally substituted aryl groups, the optional substitution referring to the presence of substituents selected from ether, amino, hydroxy, ester, thioether, amide, carbonyl, carboxyl, and carboxylate moieties.

2. A method according to claim 1, which comprises incorporating in the paper or 5 paperboard said filler and said compound (A).

3. A method according to claim 1, which comprises coating the paper or paperboard with said filler and said compound (A).

o 4. A method according to claim 1 , 2 or 3, wherein said paper or paperboard is lignin- free.

5. A method according to claim 1, 2 or 3, wherein said paper or paperboard is lignin- containing. 5

6. A method according to any one of claims 1 to 5, wherein Y₁ and Y₂ are both absent, R₁ is a hydroxymethyl (HOCH₂) group, R₂ and R₃, or R₂, R₃ and R₄ are independently hydrogen, an alkyl group (R) or an ether group (OR) with R being (CH₂)_qH (q = 1 to 12) interrupted by 0 to 6 oxygen (O) atoms or secondary amino (NR') 0 groups, and/or substituted by a zero to (2q + 1) number of a hydroxyl, thio, thioether, amino, ester, amide, carboxyl and/or carboxylate groups. R' is either hydrogen or an optionally substituted linear or branched alkyl group or optionally substituted aryl group; wherein optional substitution refers to the presence of one or more substituents selected from ether, amino, hydroxy, ester, thioether, amide, carbonyl, carboxyl, and carboxylate 5 moieties.

7. A method according to any one of claims 1 to 5, wherein Y₁ and Y₂ are both absent, R₁ is a hydroxymethyl (HOCH₂) group, R₂ and R₃, or R₂, R₃ and R₄ are independently hydrogen, an alkyl group (R) or an ether group (OR) with R being o CH₂(CH₂)_qH (q = 0 to 5) interrupted by 0 to 3 oxygen (O) atoms or secondary amino

(NR') groups, and/or substituted by a zero to (2q + 1) number of a hydroxyl, thio, thioether, amino, ester, amide, carboxyl and/or carboxylate groups.

8. A method according to any one of claims 1 to 5, wherein Y₁ and Y₂ are both absent, R₁ is a hydroxymethyl (HOCH₂) group, and R₂ is the same as R₃ in the molecule.

9. A method according to any one of claims 1 to 5, wherein Y₁ and Y₂ are both 5 absent, R₁, R₂ and R₃, or R₁, R₂, R₃ and R₄ are all hydroxymethyl (CH₂OH) groups.

10. A method according to any one of claims 1 to 5, wherein Y₁ is a hydroxymethyl group (CH₂OH) and t is zero or 1, R₁, R₂ and R₃, or Ri, R₂, R₃, R₄ and Y₂ are independently hydrogen, a Lewis acid such as boron trifluoride (BF₃), an alkyl group (R) o or an ether group (OR) with R being (CH₂)_qH (q = 1 to 12) interrupted by 0 to 6 oxygen (O) atoms or secondary amino (NR') groups, and/or substituted by a zero to (2q + 1) number of a hydroxyl, thio, thioether, amino, ester, amide, carboxyl and/or carboxylate groups, R' is either hydrogen or an optionally substituted linear or branched alkyl group or optionally substituted aryl group; wherein optional substitution refers to the presence 5 of substituents selected from ether, amino, hydroxy, ester, thioether, amide, carbonyl, carboxyl, and carboxylate moieties.

11. A method according to any one of claims 1 to 5, wherein Y₁ is a hydroxymethyl group (CH₂OH) and t is zero or 1, R₁, R₂ and R₃, or R₁, R₂, R₃, R₄ and Y₂ are o independently hydrogen, a Lewis acid such as boron trifluoride (BF₃), an alkyl group (R) or an ether group (OR) with R being CH₂(CH₂)_qH (q = 0 to 5) interrupted by 0 to 3 oxygen (O) atoms or secondary amino (NR') groups, and/or substituted by a zero to (2q + 1) number of a hydroxyl, thio, thioether, amino, ester, amide, carboxyl and/or carboxylate groups. 5

12. A method according to any one of claims 1 to 5, wherein Y₁ is a hydroxymethyl group (CH₂OH) and t is zero or 1, and at least one of R₃, R₄ and Y₂ is a hydroxymethyl (CH₂OH) group.

0 13. A method according to any one of claims 1 to 12, wherein a formaldehyde- reducing agent such as sodium hydrosulfite or sodium borohydride, or a formaldehyde- reacting agent such as sodium bisulfite is also incorporated into or coated onto the paper or paperboard.

14. A method according to claim 1, 2 or 3, wherein X is present and is selected from chloride, sulfate, hydroxide, hydrosulfϊte, phosphate, carbonate, bicarbonate, bisulfate, alkoxide, formate, acetate, citrate, oxalate, ascorbate, ethylenediaminetetraacetate and diethylenetriaminepentaacetate.

15. A method according to any one of claims 1 to 5, wherein said compound (A) is tris(hydroxymethyl)phosphine.

16. A method according to any one of claims 1 to 5, wherein said compound (A) is tetrakis(hydroxymethyl)phosphonium sulphate or chloride.

17. A paper or paperboard treated with a filler and a compound (A):

Z [X] m¹ - (A)

wherein t is zero or 1 ; when t = 0, R₄R₅PY₂ is absent and R₃ is bonded to the P of the R₁R₂PY₁ group; R₅ is absent, an alkylene group (CH₂)_S (s = 1 to 12) interrupted by 0 to 6 oxygen (O) atoms or secondary amino (NR') groups, and/or substituted by a zero to 2s number of a hydroxyl, alkyl, aryl, thio, thioether, amino, ester, amide, carboxyl and/or carboxylate groups, or a phenylene group substituted by a zero to 4 number of a hydroxyl, alkyl, aryl, thio, thioether, amino, ester, amide, carboxyl, carboxylate, and/or sulfonate groups; or preferably R₅ is an alkylene group (CH₂)s (s = 1 to 4) where the carbon chain is optionally interrupted by one or two oxygen (O) atom(s); wherein when y = 1, n = z = m = 0, Yi, Y₂ and X are absent; Ri, R₂ and R₃, or Rj, R₂, R₃, R₄ and R₅ groups are collectively selected such that the molecule has an overall solubility of at least 0.001 g/L; Ri is a hydroxymethyl (HOCH₂) group, R₂ and R₃, or R₂, R₃ and R₄ are independently selected from hydrogen, optionally substituted linear or branched alkyl groups, or optionally substituted aryl groups, the optional substitution referring to the presence of substituents selected from ether, amino, hydroxy, ester, thioether, amide, carbonyl, carboxyl, and carboxylate moieties; wherein when X is present, X is an inorganic or organic anion, and the value of m is < 5; the total charge of yn = zm; Y₁ is a hydroxymethyl group (CH₂OH) and t is zero or 1; R₁, R₂ and R₃, or R₁, R₂, R₃, R₄ and Y₂ are independently selected from hydrogen, a Lewis acid such as boron trifluoride (BF₃), optionally substituted linear or branched alkyl groups, or optionally substituted aryl groups, the optional substitution referring to the presence of substituents selected from ether, amino, hydroxy, ester, thioether, amide, carbonyl, carboxyl, and carboxylate moieties.

18. A paper or paperboard according to claim 17, treated with said filler, said compound (A), and a formaldehyde-reducing agent such as sodium hydrosulflte or sodium borohydride, or a formaldehyde-reacting agent such as sodium bisulfite.

19. A paper or paperboard according to claim 17 or 18, wherein said paper or paperboard is coated with a coating composition containing said filler and said compound (A), or containing said filler, said compound (A) and a formaldehyde-reducing agent such as sodium hydrosulflte or sodium borohydride, or a formaldehyde-reacting agent such as sodium bisulfite.

20. A paper or paperboard according to claim 17 or 18, wherein a filler composition containing said filler and said compound (A), or containing said filler, said compound (A) and a formaldehyde-reducing agent such as sodium hydrosulfite or sodium borohydride, or a formaldehyde-reacting agent such as sodium bisulfite, is incorporated in said paper or paperboard.

21. A paper or paperboard according to any one of claims 17 to 20, which is lignin- free.

22. A paper or paperboard according to any one of claims 17 to 20, which is lignin- containing.

23. A paper or paperboard according to any one of claims 17 to 22, wherein said compound (A) is tris(hydroxymethyl)phosphine.

24. A paper or paperboard according to any one of claims 17 to 22, wherein said compound (A) is tetrakis(hydroxymethyl)phosphonium sulphate or chloride.

25. In a method of manufacturing paper or paperboard wherein filler is present in the paper or paperboard, the improvement wherein said filler has been pre-treated with a compound (A):

Z [X] .¹m- (A)

wherein t is zero or 1 ; when t = 0, R₄R₅PY₂ is absent and R₃ is bonded to the P of the R₁R₂PY₁ group; R₅ is absent, an alkylene group (CH₂)_S (s = 1 to 12) interrupted by 0 to 6 oxygen (O) atoms or secondary amino (NR') groups, and/or substituted by a zero to 2s number of a hydroxyl, alkyl, aryl, thio, thioether, amino, ester, amide, carboxyl and/or carboxylate groups, or a phenylene group substituted by a zero to 4 number of a hydroxyl, alkyl, aryl, thio, thioether, amino, ester, amide, carboxyl, carboxylate, and/or sulfonate groups; or preferably R₅ is an alkylene group (CH₂)s (s = 1 to 4) where the carbon chain is optionally interrupted by one or two oxygen (O) atom(s); wherein when y = 1, n = z = m = 0, Yi, Y₂ and X are absent; R₁, R₂ and R₃, or R₁, R₂, R₃, R₄ and R₅ groups are collectively selected such that the molecule has an overall solubility of at least 0.001 g/L; R₁ is a hydroxymethyl (HOCH₂) group, R₂ and R₃, or R₂, R₃ and R₄ are independently selected from hydrogen, optionally substituted linear or branched alkyl groups, or optionally substituted aryl groups, the optional substitution referring to the presence of substituents selected from ether, amino, hydroxy, ester, thioether, amide, carbonyl, carboxyl, and carboxylate moieties; wherein when X is present, X is an inorganic or organic anion, and the value of m is < 5; the total charge of yn = zm; Y₁ is a hydroxymethyl group (CH₂OH) and t is zero or 1; R₁, R₂ and R₃, or R₁, R₂, R₃, R₄ and Y₂ are independently selected from hydrogen, a Lewis acid such as boron trifluoride (BF₃), optionally substituted linear or branched alkyl groups, or optionally substituted aryl groups, the optional substitution referring to the presence of substituents selected from ether, amino, hydroxy, ester, thioether, amide, carbonyl, carboxyl, and carboxylate moieties.

26. The method according to claim 25, wherein said filler has been pre-treated with said compound (A), and a formaldehyde-reducing agent such as sodium hydrosulfite or sodium borohydride, or a formaldehyde-reacting agent such as sodium bisulfite.

27. The method according to claim 25 or 26, wherein said filler is incorporated in a pulp from which the paper or paperboard is formed.

28. The method according to claim 25 or 26, wherein said filler is coated on said paper or paperboard.

29. A method according to any one of claims 25 to 28, wherein said compound (A) is tris(hydroxymethyl)phosphine.

30. A method according to any one of claims 25 to 28, wherein said compound (A) is tetrakis(hydroxymethyl)phosphonium sulphate or chloride.

31. A method according to any one of claims 25 to 30, wherein said paper or paperboard is lignin-free.

32. A method according to any one of claims 25 to 30, wherein said paper or paperboard is lignin-containing.

33. A paper or paperboard treated with a filler that has been pre-treated by a compound (A):

z [X] i^:in- (A)

wherein t is zero or 1 ; when t = 0, R₄R₅PY₂ is absent and R₃ is bonded to the P of the R]R₂PY₁ group; R₅ is absent, an alkylene group (CH₂)_S (s = 1 to 12) interrupted by 0 to 6 oxygen (O) atoms or secondary amino (NR') groups, and/or substituted by a zero to 2s number of a hydroxyl, alkyl, aryl, thio, thioether, amino, ester, amide, carboxyl and/or carboxylate groups, or a phenylene group substituted by a zero to 4 number of a hydroxyl, alkyl, aryl, thio, thioether, amino, ester, amide, carboxyl, carboxylate, and/or sulfonate groups; or preferably R₅ is an alkylene group (CH₂)s (s = 1 to 4) where the carbon chain is optionally interrupted by one or two oxygen (O) atom(s); wherein when y = l, n = z = m = 0, Y₁, Y₂ and X are absent; R₁, R₂ and R₃, or R₁, R₂, R₃, R₄ and R₅ groups are collectively selected such that the molecule has an overall solubility of at least 0.001 g/L; Rj is a hydroxymethyl (HOCH₂) group, R₂ and R₃, or R₂, R₃ and R₄ are independently selected from hydrogen, optionally substituted linear or branched alkyl groups, or optionally substituted aryl groups, the optional substitution referring to the presence of substituents selected from ether, amino, hydroxy, ester, thioether, amide, carbonyl, carboxyl, and carboxylate moieties; wherein when X is present, X is an inorganic or organic anion, and the value of m is < 5; the total charge of yn = zm; Y₁ is a hydroxymethyl group (CH₂OH) and t is zero or 1; Ri, R₂ and R₃, or R₁, R₂, R₃, R₄ and Y₂ are independently selected from hydrogen, a Lewis acid such as boron trifluoride (BF₃), optionally substituted linear or branched alkyl groups, or optionally substituted aryl groups, the optional substitution referring to the presence of substituents selected from ether, amino, hydroxy, ester, thioether, amide, carbonyl, carboxyl, and carboxylate moieties.

34. A paper or paperboard according to claim 33, wherein said filler has been pre- treated by the compound (A) and a formaldehyde-reducing agent such as sodium hydrosulfite or sodium borohydride, or a formaldehyde-reacting agent such as sodium bisulfite.

35. A paper or paperboard according to claim 33 or 34, wherein said paper or paperboard is coated with a coating composition containing said filler pre-treated by said compound (A) or said filler pre-treated by said compound (A) and a formaldehyde- reducing agent such as sodium hydrosulfite or sodium borohydride, or a formaldehyde- reacting agent such as sodium bisulfite.

36. A paper or paperboard according to claim 33, 34 or 35, wherein the pre-treated filler is incorporated in said paper or paperboard.

37. A paper or paperboard according to any one of claims 33 to 36, which is lignin- free.

38. A paper or paperboard according to any one of claims 33 to 36, which is lignin- containing.

39. A paper or paperboard according to any one of claims 33 to 38, wherein said compound (A) is tris(hydroxymethyl)phosphine.

40. A paper or paperboard according to any one of claims 33 to 38, wherein said compound (A) is tetrakis(hydroxymethyl)phosphonium sulphate or chloride.