GB1563236A

GB1563236A - Copolymer latices and their use in paper coating compositions having pick resistance

Info

Publication number: GB1563236A
Application number: GB41606/76A
Authority: GB
Original assignee: Doverstrand Ltd
Current assignee: Temple Fields 515 Ltd
Priority date: 1976-10-06
Filing date: 1976-10-06
Publication date: 1980-03-19
Also published as: FI68855C; DE2744436A1; ES472215A1; SE7710921L; FR2367085B1; JPS5381590A; AU2910077A; ES462920A1; FR2367085A1; FI68855B; FI772925A; NO773392L

Description

(54) COPOLYMER LATICES AND THEIR USE IN PAPER COATING COMPOSITIONS HAVING IMPROVED PICK RESISTANCE (71) We, DOVERSTRAND LIMITED, a British Company, of Temple Fields, Harlow, Essex, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to a copolymer latex and to a composition comprising the copolymer latex as one of its-ingredients for use in coating paper, board and similar substrates, especially those comprising natural cellulose fibres.

In this Specification, for convenience, reference will be made to paper as the substrate to be coated and reference will also be made to paper coating and paper coating compositions. It is to be understood that, in the context of the present Specification, such references include not only pauper, but also board, and similar substrates, especially those comprising natural cellulose fibres.

The use of high polymer latices as binders for pigments in paper coating compositions is well known, a particularly useful class of high polymer latices being that comprising copolymers of styrene and butadiene. Such copolymers usually contain additional copolymerisable monomer units selected from, for example ethylenically unsaturated carboxylic acids such as acrylic, methacrylic, fumaric or itaconic acids, acrylonitrile, esters or amides of acrylic or methacrylic acid, and other monomers.

High polymer latices of this type are frequently preferred over other high polymeric latices for use as paper coating binders because they provide an acceptable standard of performance in respect of numerous properties of importance in paper coatings, both as regards the coating processes used to coat the substrate and in the quality of the resulting coated product. Properties of importance in the coating process include chemical stability, which enables a variety of pigments, electrolytes, dispersing agents, thickeners, and the like to be mixed with the latex without an undue increase in viscosity during admixture or in subsequent storage, and mechanical stability, which permits the latex to be transported and pumped, and for the pigment containing coating compositions to be applied to a variety of paper, board and similar substrates at the high coating speeds typical of present day paper coating technology, without destabilization as evidenced for example by the formation of coagulum or build-up of undesirable residues on coating machinerv.

Properties of importance as regards the coated substrate include high pick strength, both dry and wet, a desirable balance of ink receptivity and ink hold out, and, if so required, the possibility of the provision of relatively high gloss on the surface of the coated substrate, as for example by calendering.

By "pick resistance" is meant the ability of the coated substrate to withstand the application, during the printing processes, of viscous printing inks to the surface of the coated substrate, using high speed printing machines. Inadequate pick resistance is manifested by disruption of the coating during the printing process.

Although adequate pick resistance can frequently be obtained by the use as pigment binders of styrene/butadiene copolymer latices of the prior art, this has generally been possible only by the use of relatively high proportions of such latices in paper coating compositions, resulting in a high raw material cost for such coatings.

It would be desirable to provide latices giving significantly higher dry and wet pick resistance than those of the prior art, either to enable faster printing speeds to be employed, or to make possible a reduction in the proportion of latex used as binder in relation to the quantity of pigment in coating compositions, for obvious reasons of economy.

According to one aspect of the present invention there is provided a latex of a copolymer, comprising, per 100 total parts by weight, from 35 to 65 parts by weight of copolymerised styrene units, from 30 to 60 parts by weight of copolymerised 1,3butadiene units, from 2 to 25 parts by weight of copolymerised acrylonitrile or methacrylonitrile units, from 0.25 to 10 parts by weight of copolymerised units of a hydroxyalkyl monomer (as hereinafter defined), and copolymerised units of a copolymerisable unsaturated carboxylic acid in an amount of up to 5 parts by weight.

The term "hydroxyalkyl monomer" is used herein to mean an ethylenically unsaturated monomer which is an ester of acrylic or methacrylic acid capable of copolymerisation with styrene and/or butadiene and containing a group of the general formula: -CR1R2OH where R1 and R2 may each be a hydrogen atom or a C, to C12 alkyl group. Examples of such hydroxy-containing monomers include hydroxyethyl acrylate, hydroxyethyl methacrylate, hydroxypropyl acrylate, hydroxypropyl methacrylate.

Preferably the copolymer of the present latex comprises, per 100 total parts by weight, from 40 to 60 parts by weight of copolymerised styrene units, from 35 to 55 parts by weight of copolymerised 1 ,3-butadiene units, from 4 to 15 parts by weight of copolymerised acrylonitrile or methacrylonitrile units, from 1 to 5 parts by weight of copolymerised hydroxyalkyl monomer units, and from 1 to 3 parts by weight of copolymerised unsaturated copolymerisable carboxylic acid units.

It is envisaged that, in addition to the monomers heretofore indicated, up to 20% of other copolymerisable monomers may be included in the copolymer, such copolymerisable monomers being herein defined as esters of acrylic, methacrylic, fumaric, maleic, and itaconic acids, substituted styrenes such as alphamethyl styrene and vinyl toluene.

The copolymerisable carboxylic acids which may be utilised in the present latex include acrylic, methacrylic, fumaric, and itaconic acids, acrylic acid being generally preferred.

An essential feature of the present latex is the use of a hydroxyalkyl monomer (as herein defined) in conjunction with either acrylonitrile or methacrylonitrile in the preparation of the copolymer which is preferably prepared by conventional emulsion polymerisation processes. It has been observed that in some cases small degrees of improvement in the pick resistance of paper coating compositions can be achieved by the use as binders of carboxylated styrene/butadiene copolymer latices containing copolymerised units of either a hydroxyalkyl monomer or acrylonitrile, but such improvements, where they exist at all, are much smaller than the improvement in pick resistance achievable by the use of the present latices.

Furthermore paper coating compositions based on carboxylated styrene/butadiene/acrylonitrile copolymer latices not containing hydroxyalkyl monomers, generally show an especially high degree of ink absorbency which, while useful in certain contexts, is not generally desirable.

The present copolymer latices make possible the formulation of paper coating compositions having not only significantly improved pick resistance but also having ink absorbency characteristics which may be varied between fairly wide limits depending upon the exact proportions of copolymerised units of hydroxyalkyl monomer and of acrylonitrile or methacrylonitrile. It will be understood that the ink absorbency may also be regulated by other adjustable parameters of the paper coating composition including the average particle diameter of the latex and the proportion of copolymerised units of butadiene.

The present copolymer latices may be prepared by conventional emulsion polymerisation procedures, and such latices may contain in addition to the monomers, other conventional ingredients such as surfactants, polymerisation initiators, molecular weight modifiers, dispersants, buffers, defoamers, and the like.

The appropriate polymerisation procedures and ingredients will be familiar to those experienced in the practice of emulsion polymerisation.

According to another aspect of the present invention there is provided a composition for coating paper, board and similar substrates, comprising an aqueous dispersion of one or more pigments and a latex of the present invention as a binder for the pigment(s).

In general, the present coating compositions will contain a copolymer latex of the type described above as a binder for a dispersion of pigments such as for example clay or Satin White and may additionally contain dispersing agents such as tetrasodium pyrophosphate, water-soluble colloids and/or thickening agents such as sodium polyacrylate, sodium carboxymethyl cellulose, alkali-soluble carboxylated acrylic latices, defoamers, pH buffers, thermoreactive resins such as melamine-formaldehyde condensates, and other materials familiar to those engaged in the practice of paper coating. Furthermore the present high polymer latices need not necessarily be the sole pigment binder present in the paper coating formulation and additional binders may be included, as for example high polymer latices of the prior art, starch, casein, polyvinyl alcohol, and the like. However, in order to derive the full technical or economic benefits of the present latices, the' proportion of such additional binders should be small and preferably they should be omitted.

The invention will now be further illustrated by the following Examples: Examples 1 to 5.

A series of latices were prepared by emulsion polymerisation having monomer compositions as set out in Table 1.

These latices were used as binders in pigment coating compositions using the following formulation.

Parts by weight (dry solids) "Dinkie" A Clay* 100.00 Tetrasodium pyrophosphate 0.25 Sodium polyacrylate 0.10 Sodium hydroxide 0.10 Sodium carboxymethyl cellulose 0.50 Latex 7.00 Water to total solids content of 50.0% Ammonia to pH of 8.5 "Dinkie" is a registered Trade Mark.

The resulting coating compositions were hand coated by Meyer Rod onto an off-machine air-knife coating grade of base paper having a Cobb value of approximately 25 to a coating weight of 12 g/m.2, air dried for 3 hours, and drying was completed by placing in contact with a heated drum at I 100 C. for 2 minutes.

The coated papers were then conditioned at 230C. and 50% relative humidity before super-calendering through 5 nips at a bowl temperature of 60"C.

Test results are given in Table 1.

TABLE 1.

Latex No.

Monomers 1 2 3 4 5 Butadiene 47 47 47 47 47 Styrene 51 44 42 42 40 Acrylic acid 2 2 2 2 2 Acrylonitrile 0 5 5 7 7 2 hydroxy ethylacrylate 0 2 4 2 4 I.G.T. Pick resistance 58 80 83 77 79 Wet Pick resistance 60 81 80 75 76 The improvement in both dry and wet pick resistance of latices 2 to 5, which contain acrylonitrile and hydroxyethyl acrylate, in comparison with latex 1, is evident.

Examples 6 to 19.

A series of latices were prepared by emulsion polymerisation having monomer compositions as set out in Table 2. These latices were used as binders in pigment coating compositions using the following coating formulation.

Parts by weight (dry solids) "Dinkie" A Clay 100.00 Tetrasodium pyrophosphate 0.25 Sodium polyacrylate 0.10 Sodium Hydroxide 0.10 Sodium carboxymethyl cellulose 0.50 Latex 8.00 Ammonia to pH of 9 Water to total solids content of 50% These coatings were applied by wire-wound rod at 12 Wm.2 to a moderatelysized wood-free coating base paper (sold under the registered Trade Marks (Reed Aerocote"), air dried for 3 hours at ambient temperature, and drying was completed on a print glazer at 1 100C. for 2 minutes. The coated papers were then calendered between rollers at 700 C. through four nips at 500 pounds per linear inch, and were finally conditioned at 230C. and 50% relative humidity.

The dry pick resistance of the coatings was then measured on an I.G.T.

printability tester Type AC2 using IPI Tack Graded Ink No. 5. and the results are given in the final column of Table 2.

In Table 2 latices Nos. 6 to 11 inclusive and 17 are latices not of the present invention and are included for the purposes of comparison. Latices Nos. 12 to 16, 18 and 19 represent latices made according to the present invention. It will be clearly observed that latices Nos. 12 to 19 inclusive exhibit considerably higher pick resistance than the comparative latices Nos. 6 to 11 inclusive.

The latex No. 17 has a composition typical of self-crosslinking polymer latices, and such latices are frequently found to possess insufficient stability during preparation or subsequent processing into paper coating binders to be suitable for commercial use.

TABLE 2

Latex Acrylic Hydroxyalkyl IGT Pick No. Butadiene Styrene Nitrile 1 Acid Monomer Resistance 6 45 53 0 2 0 100 7 49 47 0 2 2 HEMA 97 8 49 47 0 2 2 HEA 102 9 47 49 0 2 2 MoAMd 103 10 47 47 0 2 4 HEA 106 11 47 41 10 2 0 104 12 47 42 7 2 2 HEXA 135 13 47 37 10 2 4 HEA 133 14 47 39 10 2 2 HEMA 122 15 47 42 5 2 4 HEMA 135 16 47 44 5 MAN 2 2 HEA 113 17 47 39 10 2 2 MoAMd 113 18 47 44 5 2 2 HPMA 111 19 47 44 5 2 2 HPA 108 In Table 2 the column headed "Nitrile" indicates the level of acrylonitrile except for Latex No. 16 in which methacrylonitrile (MAN) was used.

Abbreviations in Table 2 are as follows: HEA 2-hydroxyethyl acrylate HPA 2-hydroxypropyl acrylate HEMA 2hydroxyethyl methacrylate HPMA 2-hydroxypropyl methacrylate Moated N-Methylolacrylamide MAN Methacrylonitrile

Claims

WHAT WE CLAIM IS:

1. A latex of a copolymer, comprising, per 100 total parts by weight, from 35 to 65 parts by weight of copolymerised styrene units, from 30 to 60 parts by weight of copolymerised 1,3-butadiene units, from 2 to 25 parts by weight of copolymerised acrylonitrile or methacrylonitrile units, from 0.25 to 10 parts by weight of copolymerised units of hydroxyalkyl monomer (as hereinbefore defined), and copolymerised units of a copolymerisable unsaturated carboxylic acid in an amount of up to 5 parts by weight.

2. A latex as claimed in claim 1, wherein the proportion of copolymerised styrene units is from 40 to 60 parts by weight.

3. A latex as claimed in claim 1 or 2, wherein the proportion of copolymerised 1,3-butadiene units is from 35 to 55 parts by weight.

4. A latex as claimed in any one of claims 1 to 3, wherein the proportion of copolymerised acrylonitrile or methacrylonitrile units is from 4 to 15 parts by weight.

5. A latex as claimed in any one of claims 1 to 4, wherein the proportion of copolymerised hydroxyalkyl monomer units is from 1 to 5 parts by weight.

6. A latex as claimed in any one of claims 1 to 5, wherein the proportion of copolymerised unsaturated carboxylic acid units is from 1 to 3 parts by weight.

7. A latex as claimed in any one of claims 1 to 6, wherein the copolymer comprises up to 20 parts by weight of one or more other copolymerisable monomer(s) (as hereinbefore defined).

8. A latex as claimed in any one of claims 1 to 7, wherein the unsaturated carboxylic acid is selected from acrylic, methacrylic, fumaric and itaconic acids.

9. A latex as claimed in any one of claims 1 to 8, wherein the latex has been prepared by emulsion polymerisation.

10. A latex of a copolymer substantially as hereinbefore described in any one of Examples 2 to 5, 12 to 16, 18 and 19 of the foregoing Examples.

11. A composition for coating paper, board and similar substrates, comprising an aqueous dispersion of one or more pigments and a latex as claimed in any preceding claim as a binder for the pigment(s).

12. A composition as claimed in claim 11, wherein the composition additionally comprises one or more of dispersing agents, water-soluble colloids, thickening agents, defoamers, pH buffers and thermoreactive resins.

13. A composition as claimed in claim 11 or 12, wherein the pigment is clay or Satin White.

14. A composition as claimed in any one of claims 11 to 13, wherein the composition includes a minor proportion of a conventional binder.

15. A composition for coating paper, board and similar substrates substantially as hereinbefore described in any one of Examples 2 to 5, 12 to 16, 18 and 19 of the foregoing Examples.

16. Paper or board coated with the composition claimed in any one of claims 11 to 15.