WO1995027826A1

WO1995027826A1 - Pre-pasted wallcoverings and their production

Info

Publication number: WO1995027826A1
Application number: PCT/GB1995/000858
Authority: WO
Inventors: Adrian Swinburn Allen; Malcolm Skinner
Original assignee: Allied Colloids Limited
Priority date: 1994-04-11
Filing date: 1995-04-11
Publication date: 1995-10-19
Also published as: GB9407143D0; AU2220795A

Abstract

A pre-pasted wallcovering has a pre-paste coating which is a dried residue of an intimate mixture of potentially adhesive polymeric particles and a dried residue of a solution of water-soluble polyelectrolyte. Such a coating can be made by applying an aqueous coating composition of the polymeric particles dispersed in an aqueous polyelectrolyte solution which has a viscosity depressing concentration, and drying the coated substrate, whereby upon rewetting the viscosity of the coating is higher than the viscosity of the coating before the drying.

Description

Pre-pasted Wallcoverings and their Production

This invention relates to the provision of the pre- paste coating in pre-pasted wallcovering.

The pre-paste coating is a dried coating which, when wetted by application of water (usually by immersion in water) becomes adhesive and, in particular, provides rheology and adhesion properties suitable for wall coverings.

The traditional way of making such coatings is by dusting adhesive particles onto the substrate after wetting it with water or an aqueous adhesive. However the handling of the dust, and its possible entrain ent in the atmosphere, causes some problems.

A successful way of making pre-pasted wallcoverings involves providing a reverse phase emulsion potentially adhesive polymer particles emulsified in a hydrocarbon, coating this reverse phase emulsion onto the sheet substrate, and evaporating the hydrocarbon to leave a dry discontinuous pre-paste coating of potentially adhesive polymeric particles. Various detailed ways of forming and applying the reverse phase emulsion have been proposed, but they all involve the evaporation of the hydrocarbon. Accordingly all these techniques necessitate handling and evaporating and recovering the hydrocarbon, and this can be inconvenient.

The use of an aqueous composition (having an aqueous continuous phase) would be desirable as a way of eliminating the use of the hydrocarbon. However the pre- paste coating must include sufficient polymer to provide adequate adhesion and depositing this amount of polymer from an aqueous composition has, prior to the invention, been difficult to achieve. If the coating composition contains sufficient polymer to give an adequate coating amount, the composition is practice would be much too viscous to apply and would not give an adequate discontinuous coating. If the concentration of polymer is sufficiently low that the aqueous composition can be conveniently applied by conventional coating machinery, the amount of polymer deposited on the substrate is inevitably too low to be useful unless an unacceptably large volume of composition is applied, and this involves processing problems.

Aqueous compositions of water soluble or water swellable polymer particles are described in U.S.4,380,600 and EP 170,394 wherein certain polymers, optionally in combination with monomeric electrolyte, are dissolved in the continuous phase of a dispersion of soluble or swellable polymer. This dissolved polymer inhibits the swelling or dissolution of the soluble or swellable polymer and accordingly dispersed soluble or swellable polymer particles are held in a substantially unswollen, non- adhesive, state in this environment.

We have described in PCT/GB94/01469 (not published at the priority date of this application) a process of making a prepasted wallcovering comprising coating an aqueous polymer composition containing electrolyte on to a substrate and drying the coating. In this process the polymer is preferably an LCST polymer (low critical solution temperature polymer) . It is stated that the electrolyte preferably has a monovalent cation and an anion which is generally sulphate but can be chloride, carbonate or nitrate.

Processes are known in which the pre-paste coating is made by sequential coating of a relatively non-adhesive polymeric material that is activatable to render it adhesive and an activator for the polymer in separate coatings. For instance there may be sequential coatings of a neutral or acidic emulsion of alkali swellable or soluble polymer and of an alkali, whereby there is little or no activation of the polymer during initial coating and drying but, upon immersion in water, the alkali dissolves and activates the polymer. However this multi-stage coating application necessitates extra coating and drying apparatus and imposes undesired restrictions on materials that can be used for the coating and for the substrate, for instance because of the risk of discolouration of some substrates by some alkalis. It has also been proposed to print deposits of adhesive. This can solve some of the problems but it incurs the disadvantage of maintaining complex printing apparatus in accurate use, despite the adhesive properties of the material that is being printed. Accordingly there is a need to provide a simple way of forming a pre-paste coating that does not incur these various disadvantages.

A pre-pasted wallcovering according to the invention comprises a sheet substrate carrying on one side a dry discontinuous pre-paste coating which is an intimate mixture of potentially adhesive polymeric particles and the dry residue of a solution of water soluble polyelectrolyte, wherein the amounts of the polyelectrolyte and polymeric particles are such that upon wetting the dried coating with water a viscous, wet, prepaste coating is formed.

Thus in the invention, a blend is provided of potentially adhesive polymer particles and water soluble polyelectrolyte, initially as a solution and then as a dried coating, which is such that the viscosity can increase when sufficient water is applied to the coating. The normal method of wetting a dried prepaste coating involves the application of excess water to the dried coating, for instance by immersion, and some leaching of the polyelectrolyte will occur into the wetting water. Because the viscosity of the rewetted coating is higher than the viscosity of the initial coating composition, this allows the composition to be applied at a convenient coatable viscosity and rheology at one water content, but to become much more viscous, so as to provide suitable pre- paste properties, when the coating is thoroughly wetted prior to use as a pre-paste. For instance if the dried pre-paste coating is sprayed with a small amount of water it may form a coating of relatively low viscosity, but when immersed in water it forms a coating having sufficiently high viscosity and other properties that it will serve as a wet pre-paste coating. Although substantially all the polyelectrolyte which is dissolved in the initial coating composition will remain accessible (as dried residue) to the pre-paste coating, in some processes sufficient of the polyelectrolyte may soak into the body of the sheet material that it may not be readily accessible upon wetting. Accordingly, in some instances the performance of the dried coating on wetting may be somewhat different from the performance of the initial coating composition. In particular, polyelectrolyte may be sufficiently remote from the polymer that the pre-paste may have suitable adhesive properties even without leach of polyelectrolyte from the pre-paste.

Another reason why the viscosity of the rewetted coating is higher than the viscosity of the initial aqueous coating can be because the soluble polyelectrolyte attracts water more strongly than the particulate polymer in the aqueous coating composition and in the coating. Accordingly, when the amount of water is kept relatively low in the coating composition, for instance as described in EP 170,394, the water preferentially remains in the aqueous continuous phase and the swelling and dissolution of the particulate polymer is minimised. However when the dried coating is wetted prior to use, for instance by immersion, the dried coating is inevitably contacted with a large excess of water with the result that sufficient water is present in the coating to swell, and optionally dissolve, the particulate polymer.

The invention therefore includes wallcoverings made by coating the substrate with a coatable aqueous polymeric composition which is a dispersion of potentially adhesive polymer particles in an aqueous polyelectrolyte solution which has a viscosity-depressing concentration of electrolyte, and drying the coated substrate. In particular, it includes such wallcoverings in which, upon immersing the dried coating in water, polyelectrolyte is leached from the composition and the viscosity of the resultant aqueous coating is higher than the viscosity of the coating before the drying of the coated substrate. Thus, the amounts of polyelectrolyte and polymer particles can be such that the viscosity of an aqueous coating of the dissolved polyelectrolyte and the particles can be increased by leaching electrolyte from the coating with water to provide a wet, prepaste, coating.

The potentially adhesive polymer particles generally have a size such that at least 90% by weight are above 0.02μm and generally above O.lμ , and often of 0.5μm. At least 90% by weight are preferably below lOOμm and generally below 50μm. Often at least 90% by weight is below 30μm, generally below lOμm, preferably below 5μm.

The potentially adhesive polymeric particles are generally formed by polymerisation of monoethylenically unsaturated monomer or monomer blend. The polymer in the particles is usually ionic and so the monomer or monomer blend usually includes ionic monomer.

Suitable ethylenically unsaturated ionic monomers are anionic monomers such as acrylic acid or methacrylic acid and sulphonic anionic monomers such as acrylamide omethylpropane sulphonic acid. In use, anionic monomers are generally present in the form of sodium salt or other water soluble salt, but in some instances the polymer is initially synthesised in the free acid form.

Suitable cationic monomers include diallyl dialkyl quaternary monomers such as diallyl dimethyl ammonium chloride (DADMAC) and dialkylamino alkyl (meth)-aerylate or -methacrylate, which can initially be supplied in free-base form but in the pre-paste is generally in the form of an acid addition or quaternary ammonium salt. In some instances, it is desirable that a mixture of potentially adhesive anionic particles and potentially adhesive cationic particles should be used. When the polymeric particles are formed of a copolymer, the co-monomer is generally non-ionic and, depending upon the method of synthesis, may be soluble or insoluble. The preferred soluble non-ionic monomer is acrylamide. Suitable insoluble non-ionic monomers include alkyl acrylates, styrene, vinyl acetate, and acrylonitrile. The preferred insoluble monomers are alkyl acrylates and methacrylates wherein the alkyl group is generally methyl or ethyl. The potentially adhesive polymeric particles will be soluble or swellable in water at substantially all pH values if they are made from a water-soluble monomer blend (for instance sodium acrylate and acrylamide) . If the particles are made from a water-insoluble monomer blend then they may only be soluble or swellable under alkaline conditions (if the blend includes potentially anionic monomer) or under acid conditions (if the blend includes potentially cationic monomer) .

The particles may be wholly soluble in the water, aqueous acid or aqueous alkali, or they may be insoluble but swellable. Insolubility may be due to the inclusion of a sufficient amount (eg. 20 to 80% by weight) of a water- insoluble monomer in the monomer blend. Preferably, however, the polymer particles are cross-linked so as to render them water-swellable but water-insoluble.

Cross-linking may be through pendant groups in the polymer, for instance by reaction of epoxides or polyvalent metal compounds with carboxylic pendant groups, but preferably the cross-linking is by incorporation of a polyethylenically unsaturated cross-linking agent in the monomer or monomer blend from which the particles are made.

Methylenebisacrylamide is a typical cross-linking agent which can be used but many others are known and suitable.

The aqueous polyelectrolyte solution must contain polymeric electrolyte but if desired it can also include monomeric electrolyte for instance an inorganic electrolyte such as sodium chloride. At least 50% by weight of the electrolyte is normally polymeric electrolyte and preferably at least 80% and usually at least 100% of the electrolyte is polymeric electrolyte.

The use of polymeric electrolyte is useful not only because of its viscosity-depressing effect in the initial coating composition and its ability to allow a higher viscosity upon rewetting, for instance as a result of some leaching of the polyelectrolyte from the coating, but also because it may itself contribute to film forming properties in the final adhesive coating. Its molecular weight is generally in the range 1000 to 100000, measured by gel permeation chromatography although in some instances it can be higher, for instance up to 500,000 or even a million or more. The polyelectrolyte generally should have as high a charge density as possible and so generally is a homopolymer of ionic groups although if desired it can be copolymerised with acrylamide or other non-ionic monomer, generally in an amount of below 50% and usually below 20% by weight of the polyelectrolyte.

The polyelectrolyte should normally be co-ionic with the ionic particulate polymer. When the particulate polymer is anionic, the polyelectrolyte is preferably a homopolymer or copolymer of sodium acrylate or other anionic monomer. Other anionic polyelectrolytes that may be used include copolymers of sodium acrylate and sodium acrylamidomethyl propane sulphonate, and sodium acrylate with sodium allyl sulphonate. Copolymers of such monomer or monomer blend with, for instance 5-80%, acrylamide can be used.

When, as is preferred, the particulate polymer is cationic, the polyelectrolyte is preferably cationic, preferably polyDADMAC or a copolymer of DADMAC with 0-50% acrylamide. Other low molecular weight cationic polymers include polyethyleneimine and polymers of dialkylaminoalkyl (meth) -acrylate or -acrylamide with 0-50% acrylamide. The amount of polyelectrolyte (optionally with monomeric electrolyte) must be sufficient to suppress swelling or dissolving of the polymer particles in the coating composition and is generally at least 5% by weight of the aqueous composition. It should be sufficiently low that activation occurs quickly upon rewetting, for instance as a result of washing some polyelectrolyte from the coating. Accordingly the amount of polyelectrolyte is normally below 40%. The concentration of polymer is generally in the range 10 to 30%. Although minor amounts of various additives can be included, generally the composition consists of polyelectrolyte, polymer and water. In particular the composition can be free of hydrocarbons or other organic solvents.

When the pre-paste coating is examined microscopically it will be seen to comprise the discrete particles of potentially adhesive polymer and the dried residue of the polyelectrolyte. The coating composition of potentially adhesive soluble or swellable polymer particles in a solution of polyelectrolyte can be formed by several different methods, and each of these methods constitute a further aspect of the invention. In one method, the potentially adhesive polymer is made by polymerisation to form a solid phase (powder or solid gel) , and a dispersion is formed of this solid phase in the aqueous electrolyte. For instance potentially adhesive solid grade polymer can be formed and then dispersed into the electrolyte. Generally the solid grade polymer is made by bulk gel polymerisation or reverse phase bead polymerisation and the solid form is then milled into the polyelectrolyte solution so as to form an emulsion of the potentially adhesive polymer in the polyelectrolyte. Such a technique is described, for the manufacture of other products, in EP-A-170394 and reference should be made to that for a description of suitable techniques and materials. The extent of milling dictates the particle size, which is normally above 20μm but, with additional milling, it can be reduced to a smaller particle size, for instance below lOμ . In a second method, a water-soluble monomer or monomer blend including polymerisation initiator is dissolved in the aqueous electrolyte and subjected to polymerisation conditions, whereby precipitation polymerisation occurs to form the desired coating composition. The particle size is generally in the range 0.5 to lOμm or 20μm.

In a third method, a water-insoluble monomer blend is dissolved in aqueous polyelectrolyte at a pH at which the monomer blend is insoluble, precipitation polymerisation is caused to occur, and the pH of the composition is then adjusted to a value at which the polymer particles are potentially swellable or soluble. Thus a potentially anionic blend of monomers (for instance 30-70% acrylic acid and 70-30% methyl acrylate) may be dissolved into an aqueous solution of polyacrylic acid together with initiator to result in precipitation polymerisation, and then alkali can be added to convert the polyacrylic acid to sodium polyacrylate and the precipitated polymer to a copolymer of sodium acrylate and methyl acrylate. Similar effects can be achieve with other monomers and polymeric acids, or with a cationic monomer blend in an aqueous solution of the free-base form of a polyelectrolyte, in which event acid can be added to render the polymer ionic.

In a fourth method, an oil-in-water emulsion polymer is formed in conventional manner from a water-insoluble monomer blend (for instance by copolymerisation of methacrylic acid and ethyl acrylate or of a free-base monomer and ethyl acrylate) , and the oil-in-water emulsion can then be added to aqueous polyelectrolyte which then or subsequently has its pH adjusted to a value at which the particles of the polymer emulsion (which are normally below 5μm in size) will become swellable or soluble. Suitable proportions of polymer and polyelectrolyte, and suitable combinations of materials, may be as described in EP-A-170394.

Suitable coating weights are 1 to lOg/m . The substrate can be any paper or other substrate suitable as a wallcovering substrate.

The following are some examples. Example 1

Dispersion of a partially insoluble solid grade polymer in an aqueous polymer base.

40g of a 70:30 (w/w) aerylamide:sodium acrylate partially insoluble, cross-linked solid grade polymer (molecular weight above 1,000,000 and particle size 90%<150μm) was dispersed in 200g of 40% aqueous sodium polyacrylate solution (mol wt<100,000) using a Silverson mixer, to ensure that a uniform dispersion was obtained.

The mixture showed no settlement over a 1 week period. It could be coated on paper and dried. When the coating was evaluated as a pre-paste showed good adhesion properties. Example 2

Example 1 was repeated except the aqueous polymer solution was a 40% solution of an 80:20 sodium acrylate:sodium AMPS copolymer, (mol wt<100,000). The product was stable to settlement over a 1 week period and when coated and evaluated as a pre-paste on paper gave good adhesion properties. Example 3

Preparation of partially insoluble polymer in an aqueous polymer base.

Into 90g of an 80:20 sodium acrylate:sodium AMPS copolymer solution (40%, mol wt<100,000) was added 48g of a monomer solution comprising a 75:25 mix of ACM:NaAc (50% solids, pH 7.0). 62g water was added to adjust the overall non-aqueous content of the mixture to 30%, followed by the additions of methylene bisacryla ide as cross-linker and 2,2' azobis-(2-amidinopropane)dihydrochloride as polymerisation initiator. The mixture was stirred at 60^βC for 6 hours while polymerisation occurred.

The product obtained was a suspension of fine particles in the aqueous polymer base and was coatable as a prepaste coating. Example 4

Dispersion of an alkali swellable emulsion polymer in an aqueous polymer base.

Into lOOg of a 30% polysodium acrylate solution was added lOOg a 30% w/w emulsion of methacrylic acid:ethylacrylate (25:75) copolymer emulsion in water. Aqueous sodium hydroxide was added to give a final product at neutral pH which can be applied to paper as a pre-paste. Example 5 Preparation of an alkali swellable emulsion polymer in an aqueous polymer base.

To lOOg of 0.15% acrylic acid solution polymer (mol wt<5,000,000) was added 85g followed by a mixture of 3.0g acrylic acid and 12.0g methyl acrylate. O.Olg methylene bisacrylamide and 0.02g 2,^,2 azo- bis(2-amidinopropane) dihydrochloride were added and the mixture heated at 70°C for 6 hours.

The product obtained was neutralised with caustic to give a product that could be applied to paper as a pre- paste.

Example 6

Into 90g of 40% polyDADMAC solution (molecular weight

<1,000,000 preferably <100,000) was added 62g of water and

48g of a cationic monomer solution consisting of a 75:25 mix of acrylamide:dimethylaminoethylacrylate quaternised with Nacl (50% solids pH 4.0).

0.02g methylene bisacrylamide and 0.02g 2' ,2 azo-bis (2-amidinopropane) dihydrochloride were added and the mixture heated at 70°C for 6 hours. The final product at 30% was a fine dispersion (2.5μm) of polymer particles suspended in polyDADMAC solutions. The product showed no settlement over several weeks and could be coated on to paper for use as a pre-paste.

Claims

1. A prepasted wallcovering comprising a sheet substrate carrying on one side a dry discontinuous prepaste coating which comprises potentially adhesive polymeric particles, characterised in that the coating is an intimate mixture of potentially adhesive polymeric particles and the dry residue of a solution of water soluble polyelectrolyte, wherein the amounts of the polyelectrolyte and polymeric particles are such that upon wetting the dried coating with water a viscous, wet, pre-paste coating is formed.

2. A wallcovering according to claim 1 in which the potentially adhesive polymer particles have a size at least 90% by weight below lOμm.

3. A wallcovering according to claim 1 in which the potentially adhesive polymer particles are formed of monoethylenically unsaturated monomer or monomer blend including ionic monomer.

4. A wallcovering according to any preceding claim in which the potentially adhesive polymeric particles are formed by polymerisation of monoethylenically unsaturated monomer or monomer blend including cross linking agent in an amount such that the particles are cross linked and water swellable but water insoluble.

5. A wallcovering according to any preceding claim in which the particulate polymer is co-ionic with the polyelectrolyte.

6. A wallcovering according to any preceding claim which has been made by coating the substate with a coatable aqueous polymeric composition which is a dispersion of potentially adhesive polymeric particles in an aqueous polyelectrolyte solution which has a viscosity-depressing concentration of electrolyte, and drying the coated substrate.

7. A wallcovering according to claim 6 in which the amount of polyelectrolyte was 5 to 40% by weight of the aqueous composition.

8. A wallcovering according to claim 6 or claim 7 in which the dispersion of potentially adhesive polymer particles was formed by dispersing solid grade polymer into the aqueous polyelectrolyte solution.

9. A wallcovering according to claim 6 or claim 7 in which the dispersion of polymer particles was formed by dissolving a water soluble monomer or monomer blend including polymerisation initiator in the aqueous polyelectrolyte and the blend was subjected to precipitation polymerisation.

10. A wallcovering according to claim 6 or claim 7 in which the dispersion of polymeric particles was formed by dissolving a water insoluble monomer blend in an aqueous polyelectrolyte at a pH at which the blend is insoluble, precipitation polymerisation was caused to occur, and the pH of the composition was then adjusted to a value at which the polymer particles are swellable or soluble.

11. A wallcovering according to claim 6 or claim 7 in which the dispersion of polymeric particles was formed by subjecting a water insoluble monomer blend to oil-in-water emulsion polymerisation to form an emulsion, and this emulsion was incorporated into aqueous polyelectrolyte at a pH at which the polymer emulsion particles are swellable or soluble.

12. A wallcovering according to any of claims 6 to 11 in which, upon immersing the dried coating in water, polyelectrolyte is leached from the composition whereby the viscosity of the resultant aqueous coating is higher than the viscosity of the coating before the drying of the coated substrate.