EP0721530A1

EP0721530A1 - Paper coating pigment

Info

Publication number: EP0721530A1
Application number: EP94928446A
Authority: EP
Inventors: Patrick Arthur Charles Gane; Philip Martin Mcgenity; Janet Susan Preston
Original assignee: ECC International Ltd
Current assignee: Imerys Minerals Ltd
Priority date: 1993-10-01
Filing date: 1994-09-30
Publication date: 1996-07-17
Also published as: BR9407681A; FI962512A0; JPH09504057A; KR960705109A; GB9320233D0; FI962512A; WO1995009948A1; CZ93796A3; AU7787394A

Abstract

There is disclosed the use, in a paper-making process, of a paper coating pigment the surfaces of which have been modified with a treating agent having a hydrophobic portion to confer hydrophobic or enhanced hydrophobic character on the pigment surfaces, for the purpose of reducing the coefficient of friction of a web of coated paper prepared therefrom. There is also disclosed a paper coating composition comprising an aqueous suspension of (a) an adhesive, (b) a paper coating pigment which comprises a particulate, inorganic material which has been modified by treatment with a treating agent, prior to incorporation in the paper coating composition, having a hydrophobic portion to confer hydrophobic or enhanced hydrophobic character on the pigment surfaces; and (c) a dispersing agent for the modified particles of inorganic material.

Description

PAPER COATING PIGMENT

This invention relates to pigments for paper coating compositions. Herein, the expression "paper coating composition" and related expressions such as "paper coating pigment" are to be construed broadly to cover compositions, etc. for use in the coating of all fibrous base materials, including, for instance, board. Paper coating compositions generally comprise one or more pigments and an adhesive material in suspension in an aqueous medium, and optionally other additives such as rheology modifiers. The most commonly used pigments are generally natural or synthetic inorganic materials of white colour and of fine particle size, generally such that at least about 40% by weight of the particles have an equivalent spherical diameter (esd) which is smaller than 2μm. The equivalent spherical diameter is as determined by sedimentation. Examples of pigments which are commonly used in paper coating compositions are kaolin or china clay, natural or precipitated calcium carbonate, satin white, calcium sulphate and talc.

It has been discovered in the paper coating industry that when large reels of coated paper are wound or unwound at high speed, frequent breakages occur in the web of coated paper. One cause of this is believed to be the relatively high coefficient of friction of the coated surface of certain types of coated paper which inhibits free movement of one turn of a reel of paper with respect to an adjacent turn. In order to overcome this problem, some operators include in the paper coating compositions which they use, from 10% to 40% of the total weight of the dry pigment in the composition of talc. This reduces the coefficient of friction of the surface of the coated paper prepared using the composition and gives it a "slippery" feel. Talc of the quality required is. however, expensive and, in some cases the slippery surface of the paper can cause problems in printing processes in which the paper is used.

WO 93/09289 concerns a paper coating composition for preparing a coated paper for use in a gravure printing process, which paper coating composition contains a particulate, inorganic paper coating pigment which has been modified by treatment with a treating agent which renders the pigment surfaces hydrophobic or enhances their hydrophobicity. The use of the coating composition in accordance with this reference is stated to make it possible to prepare a coated paper which gives improved gravure printing results, and especially improved gravure print quality, or reduced number of missing dots per unit area of printed image, and print gloss. In the "background" section of the reference, it is mentioned that talc has a naturally hydrophobic surface and that the surface of paper coated with a composition containing it has a reduced coefficient of friction. However, it has previously been thought that the "slipperiness" of talc has been due to the ease with which it can be delaminated, rather than to the hydrophobic nature of its surface (see, for example, "Pigments for Paper", Ed. by R.W. Hagemeyer (Chapter 10 by G.J. Gill) published by TAPPI, Atlanta, Georgia, USA, 1984).

EP-A-0026091 describes a method of making fillers for papermaking hydrophobic by coating the filler particles with wax, preferably paraffin wax for the purpose of sizing. The technique of sizing aims to prevent excessive penetration of water into paper during coating, writing or printing with water- containing inks.

According to a first aspect of the present invention, there is provided the use, in a paper making process, of a paper coating pigment the surfaces of which have been modified with a treating agent having a hydrophobic portion to confer hydrophobic or enhanced hydrophobic character on the pigment surfaces, said use being for the purpose of reducing the coefficient of friction of a web of coated paper prepared therefrom. According to a second aspect of the present invention, there is provided a paper coating composition, for use in reducing the coefficient of friction of a paper web produced from the coated paper, comprising an aqueous suspension of

(a) an adhesive;

(b) a paper coating pigment which comprises a particulate, inorganic material which has been modified by treatment with a treating agent, prior to incorporation in the paper coating composition, wherein the treating agent employed to treat the particles of inorganic material has a non-polar hydrophobic portion comprising at least one hydrocarbon group having a chain length of from 8 to 30 carbon atoms and a polar portion which is capable of binding with sites on the surface of the pigment particles; and

(c) a dispersing agent for the modified pigment. According to a third aspect of the present invention there is provided a paper coating composition which comprises an aqueous suspension of

(a) an adhesive;

(b) a paper coating pigment which comprises (i) a first particulate, inorganic material which has been modified by treatment with a treating agent, prior to incorporation in the paper coating composition, wherein the treating agent employed to treat the particles of inorganic material has a non-polar hydrophobic portion comprising at least one hydrocarbon group having a chain length of from 8 to 30 carbon atoms and a polar portion which is capable of binding with sites on the surface of the pigment particles and (ii) a second,

SUBSTITUTE5HEET(RULE26) un odified, particulate, inorganic material; and

(c) a dispersing agent blend which comprises (i) a first dispersing agent suitable for use with said modified first, particulate inorganic material and ( ii ) a second dispersing agent suitable for use with said second, unmodified, particulate inorganic material.

Preferably, the paper coating composition comprises a pigment at least 15% by weight of which consists of the modified particulate, inorganic material.

A paper web made from a coated paper produced from the paper coating compositions of or by the process of the present invention may be reeled or unreeled at high speed with diminished risk of breakage. Although it is known from WO 93/09289 that treatment, with treating agents of the type described above, of a pigment in a paper coating composition will render the surfaces of the pigment hydrophobic and will enhance certain of the gravure printing properties of a coated paper prepared from the composition, we have now discovered that, surprisingly, the treated pigment also reduces the coefficient of friction of the surface of coated paper prepared from a composition containing the treated pigment. The paper coating compositions of the present invention will usually comprise a pigment of which at least 15% by weight is constituted by a particulate inorganic material which has been modified by treatment with the treating agent, the remainder, if any, being a particulate inorganic material which has not been so modified. More preferably, at least 20%, and most preferably, at least 40% by weight of the pigment is constituted by a particulate inorganic material which has been modified by treatment with the treating agent. In some embodiments of the present invention the treated inorganic material will be the sole pigment

SUBSTITUTESHEET(RULE28) used, but it is generally preferable that not more than 85%, and more preferably not more than 80%, by weight of the pigment is constituted by particulate, inorganic material which has been modified by treatment with the treating agent. Advantageously, when there is used a mixture of a modified first, particulate, inorganic material and a second, unmodified particulate inorganic material, the modified inorganic material has a coarser particle size distribution than the unmodified inorganic material, i.e. the modified inorganic material has a smaller percentage of particles with an equivalent spherical diameter below 2μm.

The particulate, inorganic material which is treated in accordance with the invention may be, for example, kaolin or china clay, natural or precipitated calcium carbonate, satin white, calcium sulphate or talc. The particulate, inorganic material preferably has a high aspect ratio (as measured by the method described in British Patent Specif cation No. 2274337); and if kaolin or china clay is used then a material having an aspect ratio of at least 15 is preferably used.

From the point of view of the gloss of the resultant coating, it is preferred that the particle size distribution of the modified inorganic material is one in which at least 30% of particles have an equivalent spherical diameter less than 2μm, but a very fine pigment, for instance one having more than 95% of particles having an equivalent spherical diameter less than 2μm, is less preferred as such a pigment would normally be expensive and might give inferior gravure printing properties at low coat weights. Preferably, there is used a particulate, inorganic material which has a particle size distribution such that not more than 70% by weight consists of particles having an equivalent particle size less than 2μm. The untreated inorganic material may be a conventional natural or synthetic inorganic material of white colour and fine particle size.

The treating agent used to treat the particulate, inorganic material preferably has a non-polar portion, comprising at least one hydrocarbon group having a chain length of from 8 to 30 carbon atoms whereby the particles of inorganic material are modified such that they are provided with a plurality of hydrocarbon groups each having a chain length of from 8 to 30 carbon atoms, and preferably also has at least one polar group capable of binding the treating agent to the surfaces of the untreated inorganic material.

The polar portion which is capable of binding with sites on the inorganic particle surfaces may bind either directly or indirectly, for example with an intermediate material which binds the site on the particle surface with the polar portion of the treating agent. The treating agent may be capable of binding with the site on the inorganic particles either in aqueous suspension or in a .dry mix of the material. The precise nature of the polar portion of the treating agent will usually be determined empirically. However, in some cases, it may be possible to infer an appropriate surface treatment agent from a knowledge of the surface chemistry of the material.

Preferably, the quantity of the treating agent used will be from 0.05% to 5.0% by weight, based on the weight of the inorganic material being treated. Most preferably, the amount of the treating agent used will be from 0.1% to 2.0% by weight, based on the weight of the inorganic material being treated.

Some materials used as pigments in paper coating compositions, typically natural or synthetic silicates, and especially kaolin, have surfaces which have a number of acidic sites. In accordance with the present invention these materials are preferably treated with a primary, secondary or tertiary amine which has at least one hydrocarbon group having a chain length from 8 to 30 carbon atoms, but a quaternary ammonium compound, having at least one hydrocarbon group with a chain length from 8 to 30 carbon atoms, may also be used to treat natural or synthetic silicates and aluminosilicates. Examples of suitable amines and quaternary ammonium compounds include primary octadecylamine, primary hydrogenated tallow amine, trimethyl hydrogenated tallow ammonium chloride and dimethyl di(hydrogenated tallow) ammonium chloride, and mixtures of two or more thereof.

On the other hand, calcium carbonate, which is another material commonly used as a paper coating pigment, is preferably treated with a treating agent such as a saturated or unsaturated fatty acid having at least one hydrocarbon group of chain length from 8 to 30 carbon atoms. Such fatty acids include stearic acid, palmitic acid and oleic acid, and mixtures of two or more thereof.

In a paper coating composition in accordance with this invention the adhesive is preferably a latex, which may be, for example, a styrene-butadiene latex or an acrylic latex, and which may, but need not, be of the alkali-swelling type. The quantity of the latex used is preferably in the range from 3% to 6% by weight of latex solids, based on the dry weight of the surface treated pigment. The latex is normally used in the form of an aqueous emulsion containing about 50% by weight of latex solids.

When using a dispersing agent blend, the preferred first dispersing agents for use with the modified inorganic material when the latter is a calcium carbonate, are a group of linear alcohol ethoxylates represented by the general chemical formula: RO-(CH₂-CH₂-0)_nH, where R is an alkyl group having from 8 to 24 carbon atoms and n is in the range from 1 to 20, with compounds having from 10 to 30 carbon atoms being most preferred; and for use with the modified inorganic material when the latter is an aluminosilicate or a silicate, such as kaolin, are a group of alkyl sulphates or alkyl sulphonates wherein the alkyl group has a chain length of from 8 to 20 carbon atoms, with water-soluble salts of dodecyl sulphate being most preferred.

The preferred second dispersing agents are selected from, for example, a water-soluble salt of a polyphosphoric acid, a water-soluble salt of a polysilicic acid, a water-soluble salt of a poly(acrylic acid) or a poly(methacrylic acid).

The preferred blend for use when the modified inorganic material is a modified calcium carbonate pigment is a blend of a linear alcohol ethoxylate with a water-soluble polyacrylate salt. Presently preferred when the modified inorganic material is a kaolin clay is a blend of an alkyl sulphate or alkyl sulphonate (as a major, e.g. 70%, component) with a water-soluble polyacrylate salt. The dispersing agent is selected to give optimum performance with a particular surface treated pigment.

The quantity of dispersing agent used is preferably in the range of from 0.05% to 2.5% by weight, based on the weight of the dry pigment.

SUBSTITUTESHEET(RULE26, The invention will now be illustrated by the following Examples.

EXAMPLE 1 Six paper coating compositions were prepared all according to the following recipe:-

Ingredient Parts by weight

Pigment 100

Latex adhesive 5

Sodium hydroxide to pH 8.5 Water to 61% wt. solids

The latex adhesive was a 50% by weight aqueous suspension of particles of a styrene-butadiene copolymer of the type commonly used as an adhesive in preparing coated paper for printing processes. The amount of the latex adhesive used is given above as parts by weight of solid latex on solid pigment.

In each composition a different pigment was used as described below:-

Pigment A was a kaolin clay having a platey particle shape (with an aspect ratio of 60) and a particle size distribution such that 57.6% by weight consisted of particles having an equivalent spherical diameter smaller than 2μm, 37.0 % by weight consisted of particles having an equivalent spherical diameter smaller than lμm and 11.1% by weight consisted of particles having an equivalent spherical diameter smaller than 0.25μm. Pigment A was suspended in water containing 0.3% by weight, based on the weight of the dry pigment, of a sodium polyacrylate dispersing agent having a number average molecular weight of about 6500, to form a suspension containing 62% by weight of dry pigment solids.

Pigment B consisted of a mixture of 70% by weight of Pigment A in the form of a suspension prepared as described above and 30% by weight of a talc in the form of a commercially prepared suspension containing 65% by weight of dry talc (with an aspect ratio of 22), the talc having a particle size distribution such that 40% by weight consisted of particles having an equivalent spherical diameter smaller than 2μm.

Pigment C was used in the form of a mixture containing sufficient of the suspension of Pigment A described above to provide 50% by weight of dry pigment A, the other 50% by weight being provided by a suspension containing 65% by weight of a dry surface treated kaolin clay. This kaolin clay, constituting the other 50% by weight of the mixture, had a particle size distribution such that 80% by weight consisted of particles having an equivalent spherical diameter smaller than 2μm, 65% by weight consisted of particles having an equivalent spherical diameter smaller than lμm and 15% by weight consisted of particles having an equivalent spherical diameter smaller than 0.25μm, and had an aspect ratio of 25. The kaolin was surface treated by adding to the kaolin in a high-speed mixer 1% by weight, based on the weight of dry kaolin, of a primary alkylamine in which the alkyl component was derived from hydrogenated tallow. The kaolin and the primary alkylamine were mixed together at 90°C for 5 minutes to form a substantially uniform coating of the primary alkylamine on the kaolin particles. The surface treated kaolin clay was suspended in water containing 1.8% by weight, based on the weight of dry surface treated clay, of a sodium dodecyl sulphate dispersing agent, to form a suspension containing 65% by weight of the surface treated clay for mixture with the suspension of Pigment A.

Pigment D consisted entirely of the 65% by weight dry surface treated clay suspension prepared as described for Pigment C above. Pigment E was the untreated paper coating grade kaolin clay which was used to prepare the alkylamine coated clay as described under Pigment C. This clay was suspended in water containing 0.3% by weight of the 5 same sodium polyacrylate dispersing agent used for Pigment A, to form a suspension containing 68% by weight of the dry clay.

Pigment F was used in the form of a mixture of a suspension containing 58% by weight of dry surface

10 treated platey kaolin clay which was prepared by treating Pigment A with 1% by weight, based on the weight of dry kaolin, of the same primary alkylamine and using the same procedure, as were described for Pigment C, and a suspension of Pigment E as described

15 above. The two suspensions were combined in proportions such as to provide 50% by weight of the dry surface treated clay and 50% by weight of the dry untreated clay.

In each case the paper coating composition was

-*> 20. coated on to a base paper of weight 38.4g.m ^**** using a laboratory paper coating machine of the type described in British Patent Specification No. 2225261 at a paper speed of 400m.min and a blade angle of 45°. For each composition several different runs were performed at

25 different blade pressures to give a range of different coat weights between about 6g.m ^***• and about lOg.m ^*•**. After drying, each sample of coated paper was supercalendered by 10 passes through the nip of a laboratory supercalender at a speed of 36m.min , a

30 temperature of 65"C and a pressure of lOOOpsi (6.89MPa).

The coefficient of friction was measured for each sample of coated paper and was plotted graphically against the coat weight. For each composition the

35 coefficient of friction corresponding to a coat weight of 8g.m was found by interpolation. The coefficient of friction was measured by means of a Monsanto tenso eter by attaching a sample of coated paper to the highly polished surface of a weighted stainless steel block which is drawn across a second sample of the coated paper which is attached to a glass bed plate which in turn is securely located on the bed plate of the tensometer. The stainless steel block is drawn across the surface of the second coated paper sample by means of a flexible steel wire which passes around a substantially frictionless pulley and is connected at its further end to the cross head of the tensometer. The dimensions of the block are 60mm X 45mm X 6mm and the weight of the block and the wire together is 128 grammes. The block is also loaded with a 1kg weight placed on top of the block. The cross head ascends at a constant rate of 25mm.min^" and the loaded block is drawn across the surface of the paper sample at the same speed. A load cell mounted in the cross head gives a continuous read-out of the tension in the wire in Newtons. A second digital read-out displays the distance travelled by the block. A reading of the tension in the wire is taken for every interval of 5mm traversed by the block. A total of twelve readings is obtained in this way for every sample of coated paper and an average tension is deter¬ mined. From this average value the coefficient of friction of the surface of the coated paper is calculated by means of the formula μ = F/Mg where μ is the coefficient of friction F is the tension in the wire M is the total load on the coated paper (1.128Kg), and g is the acceleration due to gravity (9.81m.s^-2)

The results obtained are set forth in the following table.

Pigment Coefficient of Friction

A (comparative) 0.31

B (comparative) 0.25 C (invention) 0.28

D (invention) 0.23

E (comparative) 0.31

F (invention) 0.26

These results show that when the pigment consists totally of kaolin clay which has been surface treated with a primary alkylamine the coefficient of friction of the coated paper is reduced to below that obtained with a pigment 30% by weight of which is constituted by talc. When the pigment consists of 50% by weight of surface treated kaolin clay and 50% by weight of un¬ treated kaolin clay, a lower coefficient of friction is obtained if the surface treated kaolin clay has a relatively coarse particle size distribution.

EXAMPLE 2 Two paper coating compositions were prepared according to the following recipe:-

Ingredient Parts by weight

Pigment 100

Latex adhesive 12 Sodium hydroxide to pH 8.5

Water to 68% wt. solids

The latex adhesive was of the same type as that used in Example 1. The pigments used in the two compositions were, respectively, Pigment G and Pigment H. Pigment G was a ground natural chalk having a particle size distribution such that 85% by weight consisted of particles having an equivalent spherical diameter smaller than 2μm, 51% by weight consisted of particles having an equivalent spherical diameter smaller than lμm and 6% by weight consisted of particles having an equivalent spherical diameter smaller than 0.25μm. The material had an aspect ratio of 4). Pigment H was a surface treated pigment prepared by mixing natural chalk which had been ground to the same particle size distribution and had the same aspect ratio as Pigment G, with 1% by weight, based on the weight of dry chalk, of stearic acid. Pigment G was suspended in water containing 0.5% by weight, based on the weight of dry calcium carbonate, of a sodium polyacrylate dispersing agent having a number average molecular weight of 3,200, and Pigment H was suspended in water containing 1% by weight, based on the weight of dry chalk, of a linear alcohol ethoxylate dispersing agent, and 0.2% by weight, based on the weight of dry chalk, of the same polyacrylate dispersing agent as was used for Pigment G. In each case the paper coating composition was coated on to a precoated woodfree base paper of weight 95g.m ^■* using a laboratory paper coating machine of the same type as was used in Example 1 at a paper speed of 400m.min^-1 and a blade angle of 45°. For each composition several different runs were performed at different blade pressures to give a range of different coat weights between about 6g.m~² and about lOg.m^" . After drying, each sample of coated paper was supercalendered using the procedure described in Example 1.

The coefficient of friction was measured for each sample of coated paper by the procedure described in Example 1 and was plotted graphically against the coat weight. For each composition the coefficient of friction corresponding to a coat weight of 8g.m was found by interpolation.

The results are set forth in Table 2 below:-

Table 2

Pigment Coefficient of Friction

G (comparative) 0.37 H (invention) 0.27

EXAMPLE 3 Two paper coating compositions were prepared according to the following recipe:-

Ingredient Parts by weight Pigment 100

Latex adhesive 5

Sodium hydroxide to pH 8.5

Water to 60% wt. solids

The latex adhesive was of the same type as that used in Example 1.

The pigments used in the two compositions were mixtures comprising two or more of the following component pigments:-

Pigment I was a kaolin clay having a particle size distribution such that 65% by weight consisted of particles having an equivalent spherical diameter smaller than 2μm, 43% by weight consisted of particles having an equivalent spherical diameter smaller than lμm and 13% by weight consisted of particles having an equivalent spherical diameter smaller than 0.25μm, and with an aspect ratio of 57.

Pigment J was a paper coating grade talc having a particle size distribution such that 40% by weight consisted of particles having an equivalent spherical diameter smaller than 2μm, and with an aspect ratio of 22.

Pigment K was a surface treated kaolin prepared by adding to the kaolin in a high speed mixer 1% by weight, based on the weight of dry kaolin, of a primary alkylamine in which the alkyl component was derived from hydrogenated tallow. The kaolin and the primary alkylamine were mixed together at 90°C for 5 minutes to form a substantially uniform coating of the primary alkylamine on the kaolin particles. The kaolin, prior to the surface treatment, had a particle size distribution such that 80% by weight consisted of particles having an equivalent spherical diameter smaller than 2μm, 65% by weight consisted of particles having an equivalent spherical diameter smaller than lμm and 15% by weight consisted of particles having an equivalent spherical diameter smaller than 0.25μm, and had an aspect ratio of 25. Pigment L was a surface treated kaolin prepared by surface treating in exactly the same manner as described for Pigment K a kaolin having a particle size distribution such that 62% by weight consisted of particles having an equivalent spherical diameter smaller than 2μm, 42% by weight consisted of particles having an equivalent spherical diameter smaller than lμm and 11% by weight consisted of particles having an equivalent spherical diameter smaller than 0.25μm, and having an aspect ratio of 55. Pigment M was an untreated kaolin having a particle size distribution such that 79% by weight consisted of particle having an equivalent spherical diameter smaller than 2μm, 64% by weight consisted of particles having an equivalent spherical diameter smaller than lμm and 20% by weight consisted of particles having an equivalent spherical diameter smaller than 0.25μm, and having an aspect ratio of 25.

Each component pigment was separately suspended in water. The treated kaolin pigments were dispersed with the aid of 1.8% by weight, based on the weight of dry surface treated kaolin, of the same sodium docecyl sulphate dispersing agent as was used in Example 1. The untreated kaolin pigments were dispersed with the aid of 0.3% by weight of the same polyacrylate dispersing agent as was used for pigment G. The suspensions of the component pigments were mixed in proportions such as to give the proportions by weight of the dry component pigments shown in Table 3 below.

In each case the paper coating composition was coated on to a base paper of weight 34g.m ^**-^* using a pilot-scale paper coating machine having a roll applicator and a doctor blade set at an angle of 45° at a paper speed of 1400m.min^-1. For each composition several different runs were performed at different blade pressures to give a range of different coat weights between about 8g.m~² and about 12g.m~ . After drying, each sample of coated paper was supercalendered using the procedure described in Example 1.

The coefficient of friction was measured for each sample of coated paper by the procedure described in Example 1 and was plotted graphically against the coat weight. For each composition the coefficient of friction corresponding to a coat weight of lOg.m^-**** was found by interpolation.

The results are set forth in Table 3 below:- Table 3

Pigment Coefficient of Friction

70% 1/30% J (comparative) 0.26

20% K/30% L/50% M (invention) 0.26

In addition to the ingredients shown in the recipe given above, the comparative composition contained 0.15

SUBSTITUTESHEET(RULE28) parts by weight, and the composition of the invention 0.05 parts by weight, of a water soluble viscosifying polymer.

These results show that, in accordance with the invention, it is possible to avoid the necessity of using talc in a paper coating composition without suffering an undesirable increase in the coefficient of friction of the coated paper.

EXAMPLE 4 Two paper coating compositions were prepared according to the following recipe:-

Ingredient Parts by weight

Pigment 100

Latex adhesive 5 Sodium hydroxide to pH 8.5

Water to 60% wt. solids

The latex adhesive was a 50% by weight aqueous suspension of particles of an alkali-swelling acrylic latex. The pigments used in the two compositions were, respectively, Pigment K and Pigment N (which was untreated kaolin of the same particle size distribution as that from which Pigment K was prepared) .

Pigment K was suspended in water with the aid of 1.8% by weight, based on the weight of dry surface treated kaolin, of the same sodium dodecyl sulphate as was used in Example 1. Pigment N was dispersed with the aid of 0.3% by weight of the same polyacrylate dispersing agent as was used for Pigment G. In each case the paper coating composition was

_ coated on to a base paper of weight 39g.m ^***• using a laboratory paper coating machine of the same type as was described in Example 1 at a paper speed of 400m.min^-1 and a blade angle of 45°. For each composi-

SUBSTITUTESHEET(RULE 2β) tion several different runs were performed at different blade pressures to give a range of different coat

_ _9 weights between about 6g.m ^**-^* and about lOg.m ^■***. After drying, each sample of coated paper was supercalendered using the procedure described in Example 1.

The results are set forth in Table 4 belo :-

Table 4

Pigment Coefficient of Friction Pigment N (comparative) 0.34

Pigment K (invention) 0.24

EXAMPLE 5 Three paper coating compositions were prepared according to the following recipe:-

Ingredient Parts by weight

Pigment 100

Latex adhesive 5

Sodium hydroxide to pH 8.5 Water to 52-54% wt. solids

In addition the compositions contained small amounts (up to 0.15 parts by weight) of the same viscosifying polymer as was used in Example 3.

The latex adhesive was of the same type as that used in Example 1.

The pigments used in the compositions comprised Pigment K and/or Pigment 0 which was an untreated kaolin having a particle size distribution such that

SUBSTITUTESHEET(RULE28) 58% by weight consisted of particles having an equivalent spherical diameter smaller than 2μm, 37% by weight consisted of particles having an equivalent spherical diameter smaller than lμm and 11% by weight consisted of particles having an equivalent spherical diameter smaller than 0.25μm, and having an aspect ratio of 60. The pigments were used either alone or in a mixture in the proportions by weight shown in Table 5 below. Each component pigment was separately suspended in water, The treated kaolin pigment was dispersed with the aid of 1.8% by weight, based on the weight of dry surface treated kaolin, of the same sodium dodecyl sulphate as was used in Example 1. The untreated kaolin pigment was dispersed with the aid of 0.3% by weight of the same polyacrylate dispersing agent as was used for Pigment G.

In each case the paper coating composition was

_9 coated on to a base paper of weight 33g.m -^**• using a pilot-scale paper coating machine having a roll applicator and a doctor blade set at an angle of 45° at a paper speed of 1300m.min^-1. For each composition several different runs were performed at different blade pressures to give a range of different coat

_9 ₉ weights between about 6g.m ^*•*• and about lOg.m . After drying, each sample of coated paper was supercalendered using the procedure described in Example 1.

The coefficient of friction was measured for each sample of coated paper by the procedure described in Example 1 and was plotted graphically against the coat weight. For each composition the coefficient of friction corresponding to a coat weight of 8g.m^~2 was found by interpolation.

The results are set forth in Table 5 below:-

SUBSTITUTESHEET(RULE28) Table 5

Pigment Coefficient of Friction

100% 0 (comparative) 0.36

70% 0/30% K (invention) 0.32

100% K (invention) 0.24

EXAMPLE 6 Two paper coating compositions were prepared according to the following recipe:-

Ingredient Parts by weight Pigment 100

Latex adhesive 5

Sodium hydroxide to pH 8.5

Water to 59-60% wt. solids

The latex adhesive was of the same type as that used in Example 1.

The pigments used in the compositions comprised, respectively, Pigment N and a mixture consisting of 50% by weight of Pigment N and 50% by weight of Pigment K. Each component pigment was separately suspended in water, The treated kaolin pigment was dispersed with the aid of 1.8% by weight, based on the weight of dry surface treated kaolin, of the same sodium dodecyl sulphate as was used in Example 1. The untreated kaolin pigment was dispersed with the aid of 0.3% by weight of the same polyacrylate dispersing agent as was used for Pigment G.

In each case the paper coating composition was

_2 coated on to a base paper of weight 37g.m ^***• using a pilot-scale paper coating machine having a roll applicator and a doctor blade set at an angle of 45° at a paper speed of 1400m.min . For each composition several different runs were performed at different

SUBSTITUTESHEET(RULE28) blade pressures to give a range of different coat weights between about 6g.m and about lOg.m . After drying, each sample of coated paper was supercalendered using the procedure described in Example 1. The coefficient of friction was measured for each sample of coated paper by the procedure described in

Example 1 and was plotted graphically against the coat weight. For each composition the coefficient of

_2 friction corresponding to a coat weight of 8g.m was found by interpolation.

The results are set forth in Table 6 below:-

Table 6

Pigment Coefficient of Friction 100% N ( comparative ) 0.29

50% N/50% K ( invention ) 0. 26

Claims

CLAIMS :

1. The use, in a paper making process, of a paper coating pigment the surfaces of which have been modified with a treating agent having a hydrophobic portion to confer hydrophobic or enhanced hydrophobic character on the pigment surfaces, said use being for the purpose of reducing the coefficient of friction of a web of coated paper prepared therefrom.

2. A paper coating composition, for reducing the coefficient of friction of a paper web produced from the coated paper, comprising an aqueous suspension of:

(a) an adhesive;

(b) a paper coating pigment which comprises a particulate, inorganic material which has been modified by treatment with a treating agent, prior to incorporation in the paper coating composition, wherein the treating agent employed to treat the particles of inorganic material has a non-polar hydrophobic portion comprising at least one hydrocarbon group having a chain length of from 8 to 30 carbon atoms and a polar portion which is capable of binding with sites on the surface of the particles of inorganic material; and

(c) a dispersing agent for the modified particles of inorganic material.

3. A paper coating composition, for reducing the coefficient of friction of a paper web produced from the coated paper, which comprises an aqueous suspension of:

(a) an adhesive; (b) a paper coating pigment which comprises (i) a first particulate, inorganic material which has been modified by treatment with a treating agent, prior to incorporation in the paper coating composition, wherein the treating agent employed to treat the particles of inorganic material has a non-polar hydrophobic portion comprising at least one hydrocarbon group having a chain length of from 8 to 30 carbon atoms and a polar portion which is capable of binding with sites on the surface of the particles of inorganic material and (ii) a second, unmodified, particulate, inorganic material; and

(c) a dispersing agent blend which comprises (i) a first dispersing agent suitable for use with said modified first, particulate inorganic material and (ii) a second dispersing agent suitable for use with said second, unmodified, particulate inorganic material.

4. A paper coating composition as claimed in claim 2 or 3, wherein the composition comprises a paper coating pigment at least 15% by weight of which consists of the modified particulate inorganic material.

5. A paper coating composition as claimed in claim 3, wherein the composition comprises a paper coating pigment of which from 40% to 80% by weight consists of the first, modified, particulate inorganic material.

6. A paper coating composition as claimed in claim 3 or 5, wherein the modified, particulate, inorganic material has a coarser particle size distribution than the unmodified, particulate inorganic material.

7. A paper coating composition as claimed in claim 6, wherein the modified particulate inorganic material contains less than 70% by weight of particles having an equivalent spherical diameter smaller than 2μm.

8. A paper coating composition as claimed in claim 2, 3, 4, 5, 6 or 7, wherein the inorganic particulate material which has been modified is a kaolin (or china clay), a natural or precipitated calcium carbonate, satin white, calcium sulphate or talc.

9. A paper coating composition as claimed in claim 2, 3, 4, 5, 6 or 7, wherein the quantity of the treating agent used is from 0.05% to 5.0% by weight, based on the weight of the particulate inorganic material being treated.

10. A paper coating composition as claimed in claim 9, wherein the quantity of the treating agent used is from 0.1% to 2.0% by weight, based on the weight of the particulate inorganic material being treated.

11. A paper coating composition according to claim 2, 3, 4, 5 or 6, wherein the inorganic material being treated is a natural or synthetic silicate having surfaces which have a number of acidic sites and wherein the treatment agent is a primary, secondary or tertiary amine which has at least one hydrocarbon group having a chain length from 8 to 30 carbon atoms or a quaternary ammonium compound, having at least one hydrocarbon group with a chain length from 8 to 30 carbon atoms.

12. A paper coating composition according to claim 2, 3, 4, 5 or 6, wherein the inorganic material being treated is a natural or synthetic calcium carbonate and wherein the treatment agent is a saturated or unsaturated fatty acid having at least one hydrocarbon group of chain length from 8 to 30 carbon atoms.

13. A paper coating composition according to claim 11, wherein the dispersing agent for the modified particulate inorganic material consists predominantly of a water-soluble salt of an alkyl sulphate or an alkyl sulphonate in which the alkyl group has a chain length of from 8 to 20 carbon atoms.

14. A paper coating composition according to claim 13, wherein the alkyl sulphate or alkyl sulphonate dispersing agent is blended with another dispersing agent, for the unmodified particulate inorganic material, which is chosen from a water- soluble salt of a polyphosphoric acid, a water-soluble salt of a polysilicic acid, a water-soluble salt of a poly(acrylic acid) or a poly(methacrylic acid).

15. A paper coating composition according to claim 12, wherein the dispersing agent for the modified particulate inorganic material consists predominantly of a linear alcohol ethoxylate which can be represented by the general chemical formula: RO-(CH₂-CH₂-0)_nH, where R is an alkyl group having from 8 to 24 carbon atoms and n is in the range from 1 to 20.

16. The use in a paper making process of a paper coating composition as claimed in any one of claims 2 to 15, said use being for the purpose of reducing the coefficient of friction of the resultant coated paper web.