WO2011128400A1

WO2011128400A1 - Formulation of alkali - soluble adhesive with high resistance in water

Info

Publication number: WO2011128400A1
Application number: PCT/EP2011/055892
Authority: WO
Inventors: Tito Zanetta; Fabio Chiozza
Original assignee: Vinavil S.P.A.
Priority date: 2010-04-16
Filing date: 2011-04-14
Publication date: 2011-10-20
Also published as: ITMI20100651A1; IT1399502B1

Abstract

Disclosed is an adhesive formulation consisting of a copolymer in dispersion modified by inserting a vinyl ester-unsaturated carboxylic acid copolymer into the dispersion.

Description

FORMULATION OF ALKALI - SOLUBLE ADHESIVE WITH HIGH

RESISTANCE IN WATER

The present invention relates to polymer materials which, when suitably formulated, give the adhesive excellent performances such as very high water resistance even under frosty conditions, and easy removability by alkaline washing.

BACKGROUND OF THE INVENTION

Alkali-soluble adhesives are widely used in the field of labels for glass, plastic and metal containers.

The properties required of the final adhesive are good adhesion to all substrates, high stability and initial setting to allow the use of high speed machines, excellent water resistance under different climatic conditions, good transparency, resistance to yellowing under operating conditions, and easy removability in an alkaline wash.

In general, these adhesives are formulated with natural polymers such as starches, dextrins and caseins in an aqueous carrier, and present limitations on use which affect their performance, and above all prevent them from being used on all substrates and with all application technologies.

Starch-based adhesives give good adherence, but a low initial setting rate which can limit their use in high speed machines. They are also difficult to remove completely by alkaline washing.

Adhesives based on dextrin allow a higher setting rate and easy detachment of labels in washing units. These adhesives are sensitive to condensation water, so under particular conditions, which are frequent in the life-cycle of drinks containers, the label may become detached.

The natural adhesives most commonly used in this field are based on casein. In view of their particular rheological characteristics, they are used to label cold bottles, adhere well even to recycled bottles, and are not very sensitive to condensation water. Said adhesives are particularly indicated for polar substrates such as glass, but do not guarantee the same performance for less polar surfaces such as polyethylene terephthalate (PET). Moreover, in the presence of large amounts of exudation water, the casein tends to break down and produce unpleasant odors, and yellowing appears, which prevents their use for transparent labels. Moreover the price of casein, which is subject to rapid fluctuations, is leading the bottling industry to use alternatives containing no casein, or only small amounts thereof.

For these reasons, there has been a trend in recent years towards synthetic aqueous adhesives, mainly based on polyacrylates or semisynthetics, and still retaining percentages of starches or casein.

Numerous formulations have been patented for this application.

For example, patent U.S. 4,462,838, assigned to Henkel, claims a composition based on starches modified with aqueous acrylic and vinyl polymers which would be particularly effective for application to glass with high-speed machines.

U.S. patent 6,590,019, assigned to the National Starch and Chemical Investment Holding Company Corporation, discloses a semisynthetic adhesive with excellent resistance to cold water and removability by alkaline washing which consists of polymers of various types (acrylic, vinyl, vinyl acrylic and vinylpyrrolidone) modified with a polymer based on styrene/maleic anhydride and with rosin.

Patent U.S. 7,361 ,713, assigned to Air Products, claims acrylic polymers containing acid functions with the addition of polyamide crosslinking agents to improve the resistance of the adhesive to yellowing, especially for gluing transparent labels.

WO 2009/047046 Al , assigned to Henkel AG & Co. KGAA, claims adhesives for labels consisting of vinyl esters C2-C6, unsaturated acids C3-C9 and optionally, other non-polar and polar monomers containing different functional groups, modified by adding at the polymerization stage polyunsaturated monomers such as TPGDA (tripropylene glycol diacrylate) which allow excellent control of the viscosity of the adhesive, which is specific and controllable for each application. The basic formulation exemplified relates to a system containing polyvinyl alcohol, vinyl acetate and crotonic acid as unsaturated carboxylic acid.

However, none of the patents cited has a formulation that meets all the requirements set out above.

DESCRIPTION OF THE INVENTION

It has now been found that the problems of the prior art can be overcome by using a polymer a) containing specific functions which give the adhesive excellent characteristics of adhesion of the labels to the substrate. Said characteristics are also enhanced under extreme conditions of frost and humidity close to saturation level if said polymer is modified by adding a vinyl ester/unsaturated carboxylic acid copolymer b) solubilized in the dispersion under alkaline conditions.

Polymer a) can be produced according to common emulsion polymerization techniques, starting from a wide range of monomers.

Copolymer b) can be produced according to the same techniques and then mixed in the desired ratios with a), although it is particularly convenient to produce b) by polymerization in aqueous suspension, obtaining a solid which can then be easily added and dissolved in the adhesive at its final formulation stage.

The resulting adhesive in solution is free of natural components such as starches and casein, and overcomes the critical factors mentioned in terms of adhesion, stability, easy removability, resistance to frost under conditions of high humidity and resistance to yellowing.

The invention relates in particular to adhesive compositions consisting of:

a) an alkali-soluble polymer in aqueous dispersion and

b) a vinyl ester-unsaturated carboxylic acid copolymer dissolved in the aqueous carrier of the dispersion in the weight concentration of 0.5% to 12% of the dry polymer.

Polymer a) in dispersion is preferably acrylic, vinyl-acrylic or vinyl-acrylic-maleic.

Said polymer can be obtained by polymerizing a mixture consisting of:

10 to 90 parts by weight of one or more comonomers of the group of alkyl-(meta) aery late s having 1 to I O C atoms in the alkyl group; 0 to 50 parts by weight of a vinyl ester of an alkyl acid containing 2 to 16 C atoms in the alkyl chain;

- 0 to 40 parts by weight of a dicarboxylic acid diester wherein each alkyl group of the diester contains 4 to 12 C atoms;

5 to 40 parts by weight of an unsaturated carboxylic acid containing 3 to 5 C atoms.

Vinyl ester-unsaturated carboxylic acid adhesive copolymer b) is obtainable by polymerizing vinyl esters with unsaturated acids and esters having 3 to 18 carbon atoms, preferably a vinyl acetate-crotonic acid copolymer.

The invention also relates to a process for the preparation of adhesive compositions which comprises the addition of copolymer b) to the aqueous dispersion of alkali-soluble copolymer a) and subsequent dissolution of the formulation in an alkaline medium by adding an organic or ammonia base in a pH range of between 8 and 10.

Component a) consists of an aqueous dispersion of an alkali-soluble copolymer which can be prepared by emulsion or radical suspension polymerization of a mixture of acrylic, methacrylic, vinyl and maleic comonomers. The copolymer has a Tg of between -50°C and +50°C, its aqueous dispersion having a solid content of between 10 and 60% by weight, with a particle-size distribution of between 50 and 700 nm.

The formulation can also contain anionic and non-ionic surfactants and transfer agents.

The invention also provides a kit for the formulation of adhesive compositions comprising an aqueous dispersion of copolymer a) and copolymer b) in solid form ready to be added to the aqueous dispersion at the time of use. The kit can include containers, batching units and devices suitable for preparation of the adhesive formulation shortly before use. Typically, the operator will take up from a container an amount of copolymer b) in solid form, typically in pearls, to be added to dispersion a) under suitable temperature and stirring conditions. The amount of copolymer b) to be added will depend on the characteristics of the labelling unit, and may always be determined by the operator, possibly on the basis of specific instructions, which could be included in the kit.

DETAILED DESCRIPTION OF THE INVENTION

Polymer in dispersion a)

The alkali-soluble polymer dispersion according to the present invention contains 10% to 90% by weight (preferably 65% to 85%) of the total monomers of alkyl (metha)acrylates having 1 to 10 C atoms in the alkyl chain. Of said monomers, those preferably used, including in mixtures, are butylacrylate, 2-ethylhexyl acrylate, ethyl acrylate, methyl methacrylate and butyl methacrylate.

The second monomeric component belongs to the group of alkyl acid vinyl esters containing 2 to 16 C atoms, and can be present in amounts ranging between 0% and 50% (by weight of the total monomers), preferably between 0% and 15%. The monomers in this group include, for example, vinyl acetate, vinyl propionate, vinyl butyrate and vinyl versatate.

The third component of the monomer mixture consists of one or more diesters of unsaturated dicarboxylic acids wherein each alkyl group of the diester contains 4 to 8 C atoms: this component can be present in amounts ranging between 0% and 40% (by weight of the total monomers), preferably between 0% and 5%. Those preferably used are dibutyl maleate and dioctyl male ate.

The fourth component of the copolymer is an unsaturated carboxylic acid containing 3 to 5 carbon atoms, which forms up to 40% by weight of the monomer mixture, and preferably between 15% and 23%. Unsaturated carboxylic acids include, for example, acrylic acid, methacrylic acid, itaconic acid and crotonic acid.

The proportion among the monomers is defined in such a way that the polymer has a Tg of between -50°C and + 50°C, preferably between +30 and +40°C.

The dispersion to which the invention relates is prepared by emulsion polymerization in jacket-cooled reactors equipped with variable-speed stirrer and condenser, in the presence of radical initiators such as persulfates or peroxides. The amount of initiators is established on the basis of the reaction conditions and the desired molecular weight, and ranges between 0.015% and 1% by weight of the total monomers, preferably between 0.05% and 0.2%. The preferred initiator is Sodium Persulfate. Typically, 50%-80% of the initiator is added to the initial reactor charge, while the remainder is fed in, together with the monomers, in a time ranging between 2 and 5 hours.

The stabilizer system used during polymerization contains a combination of different anionic and non-ionic surfactants, in amounts of up to 10% by weight of the total monomers, preferably between 3% and 5%. Anionic surfactants include, for example, sodium or ammonium salts of alkyl ether sulfates, alkylaryl sulfonates, alkyl ether phosphates and alkane sulfonates. Non-ionic surfactants include ethoxylated fatty alcohols and alkyl phenol ethoxylates.

The dispersions according to the invention are produced with a solid content of between 30% and 70%, preferably between 50% and 60%. The reaction is conducted at temperatures of between 55°C and 95°C, preferably between 75°C and 85°C, with an initial charge containing up to 40% of the monomers and the remainder being fed in after the initiation in 2-5 hours. Further amounts of initiators (thermal or redox) are added at the final stages of the reaction to bring the conversion to values very close to 100%. During the reaction the pH is maintained at between 2 and 4 (preferably between 3 and 4) with the use of buffers such as sodium acetate and sodium bicarbonate, in amounts of up to 0.5% by weight of the total monomers.

Chain transfer agents such as N-dodecylmercaptan (n-DDM), 2-ethylhexylthioglycolate or 3-mercaptopropionic acid, added separately or mixed with the monomers, are useful to regulate the molecular weight of the polymer and the viscosity of the alkaline solution. The transfer agents are added in amounts ranging between 0.005% and 2% by weight of the total monomers, preferably between 0.1% and 1%. Amounts exceeding 2% cause a deterioration in the cohesive properties of the final adhesive, and low viscosity of the alkaline solution.

Copolymer b)

Polymer b) in pearl form according to the present invention contains between 50% and 98% by weight (preferably 85% to 98%) of the total alkyl acid vinyl ester monomers containing 2 to 16 C atoms. The monomers in this group include, for example, vinyl acetate, vinyl propionate, vinyl butyrate and vinyl versatate.

The second component of the copolymer is an unsaturated carboxylic acid containing 3 to 5 carbon atoms, which forms up to 30% by weight of the monomer mixture, and preferably between 4% and 10%. Unsaturated carboxylic acids include, for example, acrylic acid, methacrylic acid, itaconic acid and crotonic acid.

The proportion between the monomers is defined in such a way that the polymer has a Tg of between +10°C and +90°C, preferably between +35°C and +55°C.

Copolymer b) is prepared by polymerization in suspension in jacket-cooled reactors equipped with variable-speed stirrer and condenser, in the presence of radical initiators such as organo-soluble peroxides or azo compounds. The amount of initiators is established on the basis of the reaction conditions and the desired molecular weight, and ranges between 0.015% and 2% by weight of the total monomers, preferably between 0.05% and 1 %. The preferred initiator is dibenzoyl peroxide. Typically, the initiator is added to the initial reactor charge.

The dispersing system used is based on protective colloids such as polyvinyl alcohol, polyvinylpyrrolidone and cellulose hydroxy ethers, of various molecular weights and in amounts ranging between 0.01% and 2%. Polyvinylpyrrolidone is preferably used, added to the initial reactor load.

The reaction is conducted at temperatures of between 40°C and 100°C, preferably at the boiling point of the mass, with an initial charge containing up to 70% of the monomers and the remainder fed in after the initiation in 2-6 hours. The polymer, obtained in the form of pearls in suspension, is washed, filtered and dried in an airstream.

Copolymer b) is characterised by a K value of between 20 and 120, preferably between 25 and 50. Chain transfer agents can be useful to regulate the molecular weight of the copolymer, preferably propionaldehyde or isopropyl alcohol (IPA), added separately or mixed with the monomers. The transfer agents are added in amounts ranging between 0% and 5% by weight of the total monomers, preferably between 0% and 1.5%.

Adhesive compositions

The compositions according to the invention are prepared by diluting polymer in dispersion a) with water to a solid content of between 10% and 40%. The polymer in pearl form b) is added to dispersion a), thus diluted, in amounts of between 0.5% and 12%, and the mixture is kept under vigorous stirring to dissolve polymer b) and keep it suspended in dispersion a). A stabilising agent consisting of a mixture of esters of adipic, glutaric and succinic acid, having a boiling point between 140 and 190°C, is then added in amounts of between 0.1% and 10%. The mixture is then alkalinized to a pH of between 7 and 9 by adding an organic base. The formulation thus obtained has a Brookfield VT viscosity (20 rpm, 23°C) of between 15000 and 50000 mPa*s.

Alternatively, an aqueous solution of an organic base of polymer b) in pearl form is prepared at a concentration of between 10% and 40%, keeping the system under vigorous stirring for the time required for it to dissolve; the solution of polymer b) is added, in amounts of between 5% and 20%, to dispersion a), diluted as described above, and the mixture is kept under stirring to homogenize the solution of polymer b) in dispersion a). The stabilising agent is then added and the mixture is alkalinized as described above, to a pH of between 7 and 9 and a Brookfield RVT viscosity (20 rpm, 23°C) of between 25000 and 45000 mPa*s.

EVALUATION METHODS

The performance of the adhesive was evaluated with the following tests:

1. IWR: ice water resistance; 2. evaluation of wet tack development with the FIPAGO test;

3. label removal test.

1. IWR: ice water resistance.

An adhesive film 30 μηι thick is applied to label paper and immediately affixed to the standard glass bottle. After 3 days' conditioning at 23°C and 50% H, the labelled bottle is immersed in a water and ice bath suitably maintained in balance with temperature regulation. The condition of the adhesive is periodically monitored by rotating the bottle in the bath. The maximum duration of the immersion is 24 h, and the test is passed if the label still adheres to the bottle after that time. When the test is passed, the specimen is extracted and the label peeled off manually to check the strength of the adhesive and thefiber tear. The test result is expressed as the resistance time, expressed in hours, with a score of 0 to 5 which expresses the strength of the adhesive and the percentage of fibre tear.

2. Evaluation of wet tack development with the FIPAGO test

The evaluation is performed according to standard UNI 101 15 : 1993; the adhesive is spread on a standardized paper backing (FIPAGO 2006) at the rate of 50 g/m² and affixed to another specimen of the same paper. The work performed to detach the specimen is assessed, taking the measurements at increasing closed times, maintaining the open time constant at 2 sec to determine the development of wet tack over time.

3. Test of removal under alkaline conditions.

The labelled bottle is immersed in an alkaline solution (1 % NaOH by weight) at the temperature of 70-75 °C. After 5 min immersion the bottle is rotated in the solution, and the label is observed to see if it separates. The test is positive if, after 5 minutes' immersion, the label easily peels off the bottle; if it does not, the rotation is repeated every 2 minutes and the result is expressed as the time necessary to remove the label.

COMPARATIVE EXAMPLE 1

414 g of water and 48 g of an anionic surfactant ( hodapex LA40S) is loaded into a 3-litre reactor equipped with anchor stirrer or gate, condenser, thermocouple and external jacket, connected to a thermostatic bath. A mixture of monomers containing 360 g of ethyl acrylate, 230 g of methyl methacrylate, 240 g of butyl acrylate and 170 g of methacrylic acid is added to 400 g of water containing 52 g of Rhodafac NS200 S25E, 40g of Rhodapex LA40S and 10 g of n-DDM, under stirring, to form a pre-emulsion.

The initial charge in the reactor is heated to 81 °C, then 70 g of pre- emulsion is added and the reaction is initiated with 5 g of sodium persulfate dissolved in 20 g of water. The remainder of the pre-emulsion is then fed into the reactor together with a solution of 1.29 g sodium persulfate in 60 g of water over 3 hours. The temperature is maintained at 80-83 °C throughout the reaction. At the end of the feed time the product is maintained at the maximum temperature for 120 minutes, and then cooled to ambient temperature.

The dispersion has a solid content of 51.5%, Brookfield RVT viscosity

50 mPa*s (20 rpm, spindle 1), coagula content at 44 microns 0.02%, pH = 3.2. The dispersion is diluted with water to 26.2% and alkalinized to a pH of between 8.0 and 8.5 by adding a 30% w/w ammonia solution. The formulation thus obtained presents a Brookfield VT viscosity (20 rpm, 23°C) of between 15000 and 20000 mPa*s.

COMPARATIVE EXAMPLE 2

This is an adhesive present on the market which is considered as a benchmark for the application of labels to various types of containers. It guarantees high machinability and high, constant production speed; the adhesive mainly consists of casein and an alkali-soluble acrylic dispersion. It covers a highly varied range of applications and requirements, such as:

ice water resistance;

reusability of container;

difficult fixing of polyethylene;

use of glass and metal containers.

The sample was evaluated according to the procedures described above, and the data obtained were then compared with those of the adhesive reported in comparative example 1 and the adhesive forming the subject of the invention (example 1 and example 2).

PREPARATION OF ADHESION PROMOTER B)

0.5 g of PVP K90 (polyvinylpyrrolidone) dissolved in 20 g of water and 20 g of dibenzoyl peroxide (cone. 75%) are loaded into a 3-litre reactor equipped with anchor stirrer or gate, condenser, thermocouple and external jacket connected to a thermostatic bath, followed, under stirring, by 300 g of vinyl acetate, 15 g of crotonic acid dissolved in 285 g of water and 20 g of isopropyl alcohol. The reactor charge is heated to 66°C, and these conditions are maintained until an abundant reflux takes place. Cooling with the jacket is then commenced; after approx. 20 min, 650 g of AVM is fed in over 4 h, and after 30 min, 35 g of crotonic acid dissolved in 665 g of water is fed in over 4h 10 min. At the end of feeding the mixture is left at the maximum temperature for lh 30 min, followed by stripping of the residual monomer and cooling. The polymer in pearl form thus obtained is then washed with water and stove-dried at 25°C under vacuum for a few hours.

The polymer obtained presents the following characteristics: K=30+/-2; Brookfield VT viscosity (20 rpm, 23 °C) at 40% in ethyl acetate: approx. 300 mPa*s; residual humidity <0.5%; Tg 48.5°C; acidity no. 36 mg KOH/g.

Example 1. Preparation of an alkali-soluble formulation for labels from the copolymers disclosed in comparative example 1 , with the addition of an adhesion promoter.

This particular type of product is obtained by dissolving pearls in the aqueous carrier of the acrylic dispersion under alkaline conditions according to the following procedure.

The polymer in dispersion a) is diluted with water until a dry residue of 26.2% is obtained. The polymer in pearl form b) is added to dispersion a), thus diluted, in the amount of 1.9%, and the mixture is maintained under vigorous stirring to keep polymer b) suspended in dispersion a). A formulation stabilising agent (a mixture of diisobutyl adipate 25%-40%, diisobutyl glutarate 42%-60% and diisobutyl succinate 12%-23%) is then added in the amount of 3%. The mixture is then alkalinized to a pH of between 8.0 and 8.5 by adding a 30% w/w ammonia solution. The formulation thus obtained has a Brookfield RVT viscosity (20 rpm, 23°C) of between 25000 and 45000 mPa*s.

Example 2. Preparation of an alkali-soluble formulation for labels from the copolymers disclosed in comparative example 1 , with the addition of an adhesion promoter.

This particular type of product is obtained by dissolving the pearls in an aqueous solution of an organic base, subsequently added to the dispersion according to the following procedure:

A 23% aqueous solution of polymer b) is prepared, proceeding as follows.

The pearls are dispersed in the amount of water, keeping the system under vigorous stirring to maintain polymer b) in suspension, and the mixture is alkalinized by adding a 30% w/w ammonia solution until the pH value is between 8 and 10. The system is heated to a temperature of between 40°C and 50°C and kept under vigorous stirring for approx. 2h 30 min or the time required for complete dissolution of the pearls, ensuring that the pH value remains in the interval specified above.

8.7% of the solution of polymer b) is then added to dispersion a) diluted to 28%, and the mixture is kept under stirring to homogenize the solution of polymer b) in dispersion a). The stabilising agent is then added in the amount of 3% by weight, and the mixture is alkalinized to a pH of between 8.0 and 8.5 by adding a 30% w/w ammonia solution. The formulation thus obtained presents a Brookfield RVT viscosity (20 rpm, 23°C) of between 25000 and 45000 mPa*s.

The results obtained from the evaluation of wet tack development using the FIPAGO test are shown in Graph 1 , which demonstrates that the development of wet tack at low closed times is comparable for all the cases considered, but at high closed times is worse in the case of comparative example 2. Basically, the adhesives described in the invention show greater setting of the commercial product referred to in comparative example 2 at long gluing times, exceeding 7 sec.

The adhesives in examples 1 and 2 were subjected to the ice water resistance and removal under alkaline conditions tests by comparison with comparative examples 1 and 2, with the results shown in the Table. Table: Results of ice water resistance and label removal tests

As illustrated in the Table, comparative example 2 presents ice water resistance and wet tack development equivalent to example 2, but poor label removal in alkaline solution (35 min vs < 5 min for example 2).

In particular, example 1 presents fairly good ice water resistance with less fibre tear than example 2. Comparative example 1 did not pass the test.

Both example 1 and example 2 passed the test of removal under alkaline conditions, because the label separates from the bottle within 5 min of immersion, whereas comparative example 2 presents long removal times.

Claims

1. An adhesive composition consisting of:

a) an alkali-soluble polymer in aqueous dispersion and

b) a vinyl ester-unsaturated carboxylic acid copolymer dissolved in the aqueous carrier of the dispersion in a weight concentration from 0.5% to 12% of the dry polymer.

2. A composition as claimed in claim 1 wherein polymer a) in dispersion is of the acrylic, vinyl acrylic or vinyl-acrylic-maleic type.

3. A composition as claimed in claim 2 wherein polymer a) is obtainable by polymerization of a mixture consisting of:

10 to 90 parts by weight of one or more comonomers of the alkyl-(metha)acrylates group having 1-10 C atoms in the alkyl group;

- 0 to 50 parts by weight of a vinyl ester of an alkyl acid containing

2- 16 C atoms in the alkyl chain;

0 to 40 parts by weight of a dicarboxylic acid diester wherein each alkyl group of the diester contains 4-12 C atoms;

5 to 40 parts by weight of an unsaturated carboxylic acid containing 3-5 C atoms.

4. A composition as claimed in claim 3 wherein alkyl-(metha)acrylates are selected from butylacrylate, 2-ethylhexyl acrylate, ethyl acrylate, methyl methacrylate, butyl methacrylate or mixtures thereof.

5. A composition as claimed in claim 3 or 4 wherein the vinyl ester of an alkyl acid is selected from vinyl acetate, vinyl propionate, vinyl butyrate and vinyl versatate.

6. A composition as claimed in one or more of claims 2 to 5 wherein the unsaturated dicarboxylic acid diester is dibutyl maleate or dioctyl maleate.

7. A composition as claimed in one or more of claims 2 to 6 wherein the unsaturated carboxylic acid is selected from acrylic acid, methacrylic acid, itaconic acid, crotonic acid or mixtures thereof.

8. A composition as claimed in one or more of claims 2 to 7 wherein the vinyl ester-unsaturated carboxylic acid adhesive copolymer b) is obtainable by polymerization of vinyl esters with unsaturated esters and acids having 3-18 carbon atoms.

9. A composition as claimed in claim 8 wherein copolymer b) is a vinyl acetate-crotonic acid copolymer.

10. A process for the preparation of the compositions claimed in claims 1-9 which comprises the addition of copolymer b) to the aqueous dispersion of the alkali-soluble polymer a) and the subsequent dissolution of the product in an alkaline medium by addition of an organic base or ammonia in a pH range of 8 to 10.

1 1. A kit for the formulation of the compositions claimed in claims 1 -9 comprising an aqueous dispersion of polymer a) and copolymer b) in solid form ready for addition to the aqueous dispersion at the time of use.