CA1103099A - Water-moistenable adhesive coatings - Google Patents

Abstract

Abstract of the Disclosure The present invention relates to the production of water-moistenable adhesive coatings and to products incorporating such coatings.
A two-stage process for applying such coatings to wallcoverings and the like is known. In this process, a water-insoluble resin copolymer, in which at least one monomer is an acid, is deposited and subsequently neutralized upon the substrate. This process suffers from many disadvant-ages, including the need for excessively high application weights, hazards to equipment and operators, and excessive penetration of the copolymer into the substrate resulting in incomplete neutralization thereof by the subsequently applied alkali. In the present invention, these problems are avoided by neutralizing an acidic copolymer to form a salt thereof before applying such salt to the substrate. The salt may then be applied in various ways, such as from an aqueous or organic suspension or in a dried powderous form.

Description

~3~99 This invention relates to the production of water-moistenable - adhesive coatings and to products incorporating such coatings.
Water-remoistenable adhesive coatings have been made by applying dispersions of materials like starch, various natural gums and cellulose derivatives either from aqueous or organic dispersions or by dusting the dry materials onto a carrier surface which has been previously wetted with water. The use of such materials in these processes is attended by several disadvantages. The dusting process is subject to contamination and safety hazards and is not a preferred method for depositing such coatings, even though it is in fairly wide use, particularly in the manufacture of wall-coverings. The use of either organic or aqueous dispersions requires that all dispersions be well mixed and strongly agitated during application.
- Furthermore, the use of aqueous dispersions of the materials mentioned above involves the handling of media which are relatively viscous and thixotropic. This reflects the need for concentrated dispersions to provide the heavy coating weights which are required for satisfactory adhesion of such adhesive materials.
The water-remoistenable materials mentioned above have the further defects of producing adhesive bonds which have marginal strengths and which are often characterized by insufficient open time, when the adhesive layer is wet, to permit the moving of wallcoverings, for example, into proper reglster positions before the adhesive dries. The water-remoistenable adhesive materialsknown in the industry also usually have low tensile strengths when their shear strength is low, when both properties are referred to the moistened state. These materials further tend to deteriorate during storage when they have been dried after they are first coated onto the surfaces which require such adhesives. This deficiency tends to be particularly severe with adhesives based on starch and starch derivatives.
A further problem is that the adhesive materials which are known in the art and are mentioned above as being in current use are generally sensitive to water, so that inadequate moisture resistance is often obtained ~, :

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U.S. patent No. 3,122,446 (Gold and Marotta) teaches a two-stage process for producing a remoistenable adhesive wherein they deposit a water-insoluble resin copolymer in which at least one monomer in the co-polymer is an acid and then neutralize such a coating in a subsequent, separate opèration by contacting the coating with an alkaline substance in aqueous solution. This process suffers from a number of disadvantages, including the requirement for careful control of the stoichiometry of the initial, acidic coating and the subsequent neutralization step. The two-stage process also entails hazards to equipment and operators from the handling of caustic solutions on coating machines. A further serious dis-advantage is that the copolymer tends to penetrate deeply into the wall-covering substrate and this tends to inhibit its complete neutralization by the subsequently applied alkali. Indeed, analysis has shown that in many dry-strippable wallcoverings formed by the two-stage process, the water remoistenable coating is only about 80 percent neutralized, which is detrimental to the subsequent strippability of the wallcovering. A further ; disadvantage of the two-stage process is the recommended application weights appear to be much greater than necessary and are therefore economi-cally undesirable.
In order to apply high adhesive coating weights of the acid copolymer onto the substrate, Gold and Marotta employ a coating emulsion having a high solids content, preferably at least 25% by weight. By increasing the solids content to this level, the viscosity of the emulsion increases and the tendency for absorption of the copolymer dispersion into the substrate correspondingly decreases. However, there are many advan-tages to using a much lower solids content, which are difficult or impossible to realize by the Gold and Marotta process. Apart from the obvious economic advantage of using smaller amounts of solids in the emulsion, tbere also exist the advantages that low solids content reduces 1~03Q:~

the coating weight and thickness, which eliminates the danger of incomplete drying of the coating as well as improving the handling of the wallcovering, and permits close control of the deposition of adhesive.
A further aspect of the Gold and Marotta process is that, even with the high solids content levels employed, a preliminary coat of a suitable sealant, such as, for example, an aqueous starch dispersion with an alkali, may nevertheless be desirable to prevent undue absorption of the acidic copolymer by highly porous substrates.
It is an object of the present invention to provide a process which is more economical, safer and provides a superior product over that of the conventional two-stage process, as taught by Gold and Marotta.
It is a primary object of the invention to avoid the problems of excessive absorption of the acid copolymers, and/or alkali used in the two-stage process and, thereby, to avoid the necessity for high solids content in the coating emulsion or the application of a sealant to prevent such absorption.
; It is a further object of the invention to eliminate the necessity for neutralization of the acid copolymer upon the wallcovering substrate, thus avoiding the stoichiometry control problems associated therewith.
It is a further object of the invention to avoid the hazards to operators and equipment which arise from the handling of caustic alkali solutions on coating machines.
It is yet a further object of the invention to permit control of the adhesive forces through application of controlled amounts of the neutralized acidic copolymers to the substrate. This is achievable with ; excellent precision because of controllable and limited penetration into the substrate of metered amounts of the exactly neutralized adhesive material.
Other objects and advantages of the invention will hereinafter . .
' 3Q~9 become apparent.
According to the present invention, there is provided a process for coating a substrate with a water-remoistenable dried adhesive which comprises neutralizing an acidic copolymer to give a salt thereof which is water-remoistenable from the dry state and applying said salt to said substrate to form a coating thereon. The substrate may be coated by direct application of the aqueous suspension onto the substrate; by drying of the suspension and dusting of the dried salt in powderous form upon the substrate which has previously been wetted and treated with a tackifier;
or by drying the aqueous suspension and subsequently placing the dried salt i.nto a suspension in an aqueous or organic medium for application to the substrate. Although an organic base may be used as the neutralizing agent, it is preferable that an inorganic base be used, such as sodium hydroxide or another univalent or suitable divalent inorganic base. Preferably, the copolymer is a partially cross-linked copolymer of ethyl acrylate and methacrylic acid. Other suitable copolymer systems are vinyl acetate/
crotonic acid; terpolymers of methyl methacrylate/ethyl acrylate/methacrylic acid; ethyl acrylate/acrylic acid; and terpolymers of vinyl acetate/ethyl acrylate and acrylic acid.
It i9 well known that such polymeric salts tend to thicken the aqueous media to which they are added. This is the basis of the invention ; of Toy and Fordyce, for example, as taught in Canadian patent 623,617.
However, such drastic thickening action as is contemplated by the Toy and Fordyce patent is an unwanted side effect in the application of this invention, even though we take advantage of the higher viscosity of the salts which serves to limit penetration into the substrate. Such thickening is therefore avoided so that the dispersions or solutions used to apply - such adhesives will not be too viscous for easy application with coating machines which are widely used for application of coatings and relatively thin adhesives.

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~103~39 The viscosity of the coating mixture may be controlled by addition of an agent or agents selected from the class consisting of surfactants or water-miscible organic solvents. Preferably, the viscosity control agent is a nonionic surfactant. It is further preferred that the pH of the adhesive dispersion be adjusted to reduce its viscosity by suitable selection of the alkali used in the neutralization, by suitable adjustment with the alkali selected to yield the desired pH, or by back titrating excess alkali ; with an appropriate acid.
A further advantage of the present invention over the two-stage process as taught by Gold and Marotta is that the latter requires a coating emulsion having a high solids content, preferably at least 25% by weight. This restriction upon the two-stage process is necessary to provide sufficient viscosity of the emulsion that the tendency for excessive absorption of the copolymer dispersion into the substrate is avoided. No such restriction is imposed upon the process of the present invention because of the inherently viscous nature of the copolymer salts, as dis-cussed above. Indeed, where the salt is applied as a suspension in an aqueous medium, the solids content of the coating composition need only be about 10%, which provides for reduced coating weight and thickness, reduced danger of incomplete drying of the coating, improved handling of the coated article (particularly in the case of a wallcovering), and closer control over the deposition conditions.
In certain cases where enhanced dry-strippability is required -such as, for example, where the wallcovering is applied to a relatively porous surface - various techniques may be employed. The surface may be previously sized with a suitable size composition, such as a long chain fatty acid or salt tilereof in combination with an appropriate polymeric film former. Alternatively, an inert filler may be added to mixed salts of the neutrali~ed copolymer or suitable release agents may be incor-porated into the adhesive layer.

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If desired, the salt of the acidic polymer may be deposited on a surface by suspending the dry salt in a suitable organic solvent or ; combination of solvents. Preferably, the solvent is methyl ethyl ketone or a mixture of methyl ethyl ketone with other materials with which methyl ethyl ketone will form solutions. More preferably, the solvent is a mixture of methyl ethyl ketone and a lower alcohol, and hydroxypropyl cellulose is added to the solvent mixture, along with the salt of an acidic polymer and other ingredients which may be desirable. The organic solvent may contain an additional polymer which thickens the solution therein of the salt of the acidic polymer and forms a water-sensitive film on drying.
This may be desirable if the dispersion of the polymer salt is too thin for optimum application by a particular coating machine, since the acid copolymer salts used in this invention are not normally thickeners for organic media.
The adhesive may be deposited on a surface by dusting the adhesive onto the surface which has been wetted with water whic.h may contain a ; tackifier such as sodium alginate to hold the dust and the dusted surface is then dried. Preferably, the adhesive is combined in the dry form before dusting with a polymer which is water-sensitive and which will form a film at room temperature. It is further desirable that a release agent is added to the dry powder mixture or to the tackifier layer.
The adhesives of this invention may be activated by moistening with water and the wet bond of these adhesives is so strong that adhered surfaces will remain in firm contact when the adhesive is thoroughly wetted so long as appreciable normal forces are not applied to separate them.
Thus, by the practice of this invention, it is possible to apply water-moistenable adhesive coatings in a single adhesive coating operation to substrates, such as paper, fabrics, laminates, boards or any other surface requiring such adhesive, and to dry the adhesive layer so that the ~3~

coated suxface may then be handled, stored and transported without signi-ficant damage or deterioration of the adhesive coating or coated material and without any special precautions except for the exclusion of liquid water or very humid atmospheres. When the dried, coated surface is moistened with water, the adhesive layer will be activated to form a strong bond to another surface and this bond will be maintained or improved in strength as the remoistened adhesive layer dries in situ between the two surfaces which are to be bonded. Furthermore, the coatings may be readily dried so that the wallcoverings which are so coated on the side opposite to the decorated surface can be stored for long periods in rolls without significant blocking, pick-off or deterioration of the decorated surface or adhesive layer and the adhesive-coated side need only be moistened with ; water before bonding the wallcovering to walls, wallboards or other surfaces.
The novel adhesive will bond strongly to other surfaces while the adhesive layer is wetted with water and will have a relatively long open time so that the wallcovering may be moved or slipped into proper position along the second surface while still maintaining a strong resistance to removal from the second surface in a direction normal to the surfaces of ; the wallcovering and its substrate. That is to say, the mois~ened adhesive will produce a bond between a wallcovering and a second surface with high tensile strength but relatively low shear strength, and that these properties of the wet adhesive layer will be maintained for periods of time which are fairly long compared to the intervals during which it may be desired to adjust the position or register of different wallcovering strips when these strips are first bonded to walls or other surfaces.
By the practice of this invention, it is possible to provide remoistenable adhesive-coated labels, postage stamps, tapes, envelopes, 3~

wall tiles, ceiling tiles, floor coverings and similar objects and to provide a remoistenable adhesive for use in the binding of books, pamphlet~, maga~ines and similar articles.
The novel adhesives are also useful as contact adhesives in that they will hold two surfaces together after initial assembly without application of prolonged pressure. They may also be employed as pressure cements, in which case clamp pressure is applied to hold the surfaces to be bonded together while the water moistenable adhesive dries by loss of water.
The invention will now be described further with reference to the following examples, which are not to be construed as in any way limiting upon the scope of the invention.
In the following examples, which are illustrative of the invention, the parts and percentages are by weight unless noted otherwise.

The following formulation was based on an aqueous emulsion of an acid-containing, cross-linked polymer comprising ethyl acrylate and methacrylic acid in the relative proportions of 60/40.

.;,, polymer (28% solids ln water emulsion) 32 parts ~ 20 water 51 ! defoamer 0.8 anionic wetting agent 0.5 nonionic wetting agent 0.5 fungicide 0.1 sodium hydroxide (10% weight/volume in water) 17 pH 9 The final solids content of this adhesive coating formulation was 10%. It had a viscosity of 9800 centiposes at room temperature, as measured with a number 4 spindle @ 20 r.p.m. in a Brookfield viscometer.

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This viscosity is low enough for coating with wire wound rollers, some rotagravure cylinders and other devices commonly used in the coating of paper and other webs. The viscosity of the mixture was found to decrease somewhat on standing, and this is tentatively attributed to the release of air which had been trapped in the liquid during the mixing process.
The viscosity of this formulation is dependent on the pH to which the acrylic copolymer is taken in the course of the neutralization step, and this dependency may be minor or extreme depending on both the chemical com-position of the polymer/copolymer and the chemical nature of the alkaline titrating agent or an acidic back-titrating agent. For the formulation outlined in Example 1, the viscosity at a pH of 8, determined in the above-mentioned manner with a Brookfield viscometer, was 19,000 centipoises. This viscosity was stable and showed little or no variation over a reasonable period of time. When sufficient NaOH to raise the pH to 9 was added during the neutralization step, the initial viscosity was only 9800 centipoises, and this decreased further to 9000 centipoises after standing five days.
The employment of other titrating agents to adjust the pH of the formulation did not affect the viscosity to the same extent as NaOH did in this example.
For example, sod1um carbonate (Na2CO~) exerted no detectable effect on viscosity even when the pH of Example 1 was raised to 10. It will be noted that these pH levels slightly exceed the point at which the acid polymer is completely neutralized.
The addition of non-ionic surfactants at a concentration of 0.8 parts to the formulation outlined in Example 1 at a pH of 9 resulted in a minor reduction in viscosity from 19,000 to 18,000 centipoises. Similar attempts to control viscosity by the addition of 10 parts diluent (such as propylene glycol, methanol, or methyl ethyl ketone) had marginal success, as may be seen from Examples 2 and 3. The most extensive control of vis-cosity is dependent very strongly on both pH and on the chemical nature of the titrating agent. A subsequent reduction in pH by back-titrating an _g_ , , ' ' ~ . ' -1~03~

alkaline salt mixture with an acid solution exerts a similar effect on - solution viscosity. Depending on the polymer and the acid titrant employed, a reduction in pH can significantly affect viscosity. Large excesses of certain alkaline titrating agents should be avoided so hydrolysis of the ester or other water-sensitive groups which may exist in the acid copolymer does not occur, thereby reducing gel strength in the remoistened adhesive.
The chemical composition of the polymers (or copolymers) has a strong influence on the sensitivity of the resulting solution viscosity to pH and the titrant employed, either for increasing pH during the initial neutralization or decreasing pH during a subsequent back titration. The magnitude of the changes which can occur in a formulation such as outlined in Example 1 vary with the titrants employed, the copolymer compositions and the type and extent of cross-linking. The sodium salt appears to offer the best end-use characteristics at a given viscosity.
~: In terms of ability to reach a very high solution pH during initial formulation, yet produce a neutral product subsequent to drying the polymer salt, ammonium hydroxide (NH40H) can be an important titrant.
Ammonia is driven off during the drying of the high pH solution to produce an essentially neutral, non-remoistenable salt which, like all non-volatile bases such as the alkalis, can be mixed with each other in various propor-tions to produce gel structures of carefully controlled gel, tack, slip, and adhesion.
In all examples cited,the aqueous adhesive mixture was applied to the undecorated side of a wallpaper with an engraved gravure cylinder or with a wire wound roller applicator, known in the coating industry as a Mayer bar. The dried adhesive was applied at coating weights of about

2.0 to 2.5 pounds per ream. Although 90% of the mixture as applied was water, there was no difficulty in drying the wet coating with normal equip-ment and at normal coating speeds, because the wet coating weight per unit area of wallpaper was low. The very high adhesive bond strengths of our adhesive permit operation with relatively dilute aqueous mixtures. The lo--~1~3~

amount of adhesive solids which can be present in a mixture which is fluid - enough to permit easy coating oeprations is still great enough to function extremely well after the water carrier has been driven off. We have invented a process whereby it is possible to obtain water-remoistenable - adhesive coatings characterized by great adhesive strength and low adhesive solids content.
When the dried adhesive layer was remoistened with water, the wall-covering adhered strongly to surfaces, such as walls. Although the wet ad-hesive layer was very tacky and strongly resisted direct normal removal from the wall, the wallcovering could readily be moved laterally along the wall ' to improve its register with corners or sides of strips of wallcovering which had been adhered to the wall previously. This reflects the existence of - a useful slip time. The slip time varies depending on the amount of adhesive which has been applied to the wallcovering, the nature of the paper substrate, and on the nature of the surface to which the wallcovering is being adhered.
When the wall had been sized with a size as hereinafter described and the dried adhesive coating weight was 2.5 pounds per ream of wallpaper, the slip time was greater than five minutes, while the ease of lateral movement of the wallpaper in contact with the wall became progressively less as the remoist-ened adhesive dried. For the above-mentioned adhesive on the wallcovering, similar slip times were obtained on unsized semigloss painted surfaces and slightly shorter slip times on flat painted surfaces. These slip times are more than adequate for wallcovering applications.

The following formulation was based on use of the same acid-con-taining copolymer as in Example 1.
copolymer (28% solids in water~ 33 parts water 51 defoamer 0-5 anionic wetting agent 0-.5 fungicide 0.1 methyl ethyl ketone 10 --1 1-- . ~

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sodium llydroxide (10% in water) as needed to neutralize the acid polymer The final, neutralized water dispersion contained 10% solids. It could be easily applied to the reverse side of a decorated wallpaper, dried readily and formed an adhesive with very good wet and dry adhesion and open time when it was activated by dipping the wallcovering in water.

The following formulation was based on the same copolymer emulsion as was used in the preceeding examples.
copolymer (28% solids in water) 33 parts water 41 defoamer 0.5 anionic wetting agent 0.5 fungicide 0.1 methanol 10 sodium hydroxide (10% in water) 17 This mixture, at a final composition of 10% solids, had a viscosity measured as described above of 12,000 centipoises, and formed an effective wall-covering adhesive with good properties similar to those described for the preceeding formulations. The application viscosity was somewhat higher than that of the adhesive coating dispersion mentioned in the first example.
Also, Example 1 was at a pH of 9 whereas Example 3 was at a pH of approxi-mately 8. This reflects the replacement of the nonionic wetting agent used in Example 1 by methanol in the present example. The coating mixture could easily be applied to the undecorated side of a wallpaper, using a wire wound roller, despite the slightly higher viscosity of the mixture in this example.

An emulsion of the lithium salt of the same polymer as was used in the preceeding examples contained 10% solids and had a pH of about 8.5.
When this emulsion was used as an adhesive, the material dried to form a coating which could be activated by moistening with water. The rewetted coating layer had excellent slip and dried to form a strong bond between a wallpaper and sized wall. The wet tack of the activated lithium salt ., ' .

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adhesive was very similar to that of the sodium salt adhesives described above~

An adhesive formulation was prepared as described in Example 1, except that potassium hydroxide was used in place of sodium hydroxide to neutrali~e the acidic polymer. The properties of the adhesive were subs-tantially the same as those noted above in connection with the formulation described in Example l .
Following Examples 4 and 5, an emulsion containing 10% solids and having a pH of 8.5 was prepared from the magnesium salt of the same polymer used in the preceeding examples. A water remoistenable coating was formed having excellent slip and adhesion characteristics. The re-moistened adhesive layer had a less gelled structure than the previous examples, yet functioned in a similar manner in terms of slip and adhesion.

An emu~sion of a mixed salt of the same polymer as in Example 1, containing 10% solids, was prepared at a pH between 8.5 and 9Ø Mixtures of sodium hydroxide and ammonlum hydroxide were employed to give the desired tack characteristics. As the relative amount of NH40H was increased, the tack of the resulting prepaste decreased accordingly. Whe.n the adhesive was based completely on the ammonium salt of the acidic copolymer and was completely dried on the undecorated side of the wallcovering, the adhesive was not water remoistenable. For the remoistened adhesive, any level of tack could be obtained according to the relative amounts of the sodium and ammonium salts.

A general difficulty in the use of water-borne salts of acidic polymers as adhesives resides in the fact that such systems tend to become extremely viscous at moderate concentrations. It is an advantage not to use .

~` very dilute dispersions since such mixtures require the drying of relatively large volumes of water when the adhesive coating is first applied to the particular substrate. It has been found that aqueous suspensions of salts of acidic polymers with about 10% solids are sufficiently concentrated to permit ready removal of water with normal coating equipment and web speeds.
This is made possible by the fact that a low coating weight around 2 pounds per ream or lower is sufficient to produce high strength bonds between various substrates with our novel adhesives. If extra adhesion is required, dry coating weights of this novel adhesive several orders of magnitude ' 10 greater than those outlined above may be obtained if sufficient drying capacity i9' available. However, such high weights are not usually required for normal purposes, and generally should be avoided for the reasons .
hereinbefore stated.
It is nevertheless undesirable to work with unduly low-concentration ' ~ coating mixtures,since it is economical to coat at the highest possible web ; speeds with given coating and drying equipment. Concentrations of aqueous mixtures substantially below 10% are unlikely to be economical. The solids content of the coating dispersions can be increased to the desired level and the mixture viscosity can be controlled as illustrated in the foregoing examples by adding nonionic surfactants to the solution. Such surfactants ! (e.g., Triton X100) tend to promote air release and accelerate the consequential reduction in viscosity on ageing. Sufficiently high concentrations of ninionic surfactants will function also as release agents, so that mixtures containing such contents of nonionic surfactants enhance the property of dry strippability.
Nonionic surfactants can be added to the emulsion of acidic polymer before neutrali~ation with alkaline substances. Anionic surfactants, however are less suitable, since they are salts of weak acids and strong ~ -14-:"

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- alkalis and serve to partially neutralize the polymer and influence the mixture viscosity. Enhanced strippability may be achieved tbrough the addition of most types of surfactants to the pre-neutralized acid copolymer.
In the foregoing examples, methanol, methyl ethyl ketone and a nonionic surfactant have been used separately as viscosity controllers. The viscosity of a 10% mixture of the sodium salt of the particular acid polymer without any of these additives is about 18,000 centipoises. It is thus possible to effectively control the application viscosity by judicious choice of additives.

This formulation is based on an aqueous emulsion of a cross-linked copolymer based on ethyl acrylate and methacrylic acid in the rela-. tive proportions of 60/40.
water 132.5 defoamer 2.5 anionic wetting agent 0.5 fungicide 0.5 To the foregoing mixture was added:
copolymer (28% solids in water)165 parts nonionic wetting agent 8 methanol 125 sodium hydroxide (10% in water) 85 This mixture, which had a solids content of 1070, had a viscosity of 14,000 centipoises as measured at 20 rpm with a number 4 spindle in a Brookfield viscometer using a solution which had been aged for one day after initial mixing. Its adhesive properties were essentially as noted above in the preceeding examples.
A number of effective nonionic surfactants are suitable in the practice of the invention. This includes the group consisting of alkyl polyoxyalkylene ethanols and propanols and alkyl phenoxy polyoxyalkylene ethanols and propanols. Molecular weights of the useful surface active -- .

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3~ a9 agents for the purposes of the invention may vary over a wide range.
Commercial examples besides Triton X100*include Antarox A400*(a nonyl - phenoxy polyethanoxy ethanol) and Emulphor DDT*and Igepal CA* which are both octyl phenoxy polyethanoxy ethanols wherein the number of polyethanoxy groups approximates 10 and which are similar in composition to Triton X100.
Other examples of suitable nonionic surfactants will be familiar to those skilled in the arts of formulating adhesives and water dispersions of colloidal materials.
When the wallcoverings with appropriate paper substrates had been dried on the wall to which they had been bonded, they could readily ,.
be stripped from the wall by manual force without leaving any significant residue on the wall and without recourse to wetting or steaming, either of which process is normally required for removal of wallcoverings in common current use.
-~ This valuable property of dry strippability was exhibited when ,, the wallcovering had been adhered to a wall which previously had been painted with oil base paint. The dry strippability was, however, decreased when the coated wallcovering was applied to a fresh wallboard surface which had been painted with a latex paint. Such latex paints are normally based on emulsions of vinyl acetate, styrene/butadiene, acrylic, or other water-dispersible polymers. The wet and dry adhesions and slip times were very good over such surfaces, but the long-term adhesion to the substrate was, in some cases, too strong to permit easy and assured stripping of the wallcovering by normal forces under dry conditions. It is possible under these circumstances to leave residues of the wallcovering on the wall or even to damage the wall surface because the adhesive forms a very strong bond, even at the very low coating weights which we have found to be the most economical.
It has been found that all substrates produce dry strippable bonds with products to which our water activatable adhesive has been applied when such substrates have been coated with the following formulation or its *Trademarks :' ,'. ` ~

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variants before the adhesive-coated material is applied. A universal sizing material which is suitable for this application comprises a mixture in water, at about 24~ total solids by weight over volume, of a film forming copolymer emulsion to which as been added about7% by weight on the dry solids of a fatty acid or fatty acid salt, such as stearic acid or sodium stearate, respectively. The copolymer composition is such as to be a film former at room temperature.
Enhanced dry-strippability can also be realised by addition of a release agent in the initial formulation. Such adhesive mixtures do not require the use of a size prior to application of the remoistened adhesive, even in the difficult situations described above. A suitable release agent may be any material which is incompatible with the polymer used as the base for the adhesive of the invention and which is sufficiently mobile at room temperature to bloom to the surface between the two materials which are being bonded. An optimum level of incompatibility is preferable, so that the final bond is not too weak. Suitable release agents which can be incorporated into the adhesive formulation include long-chain fatty acids or salts thereof, such as sodium stearate or a combination of fatty acid salts with commercial anionic detergents. It will be understood in this case that the salts of the fatty acids should be added to the mixture after the acid copolymer has been neutralized so as to ensure no cation exchange reactions occur. Other suitable release agents include emulsions or water dispersions of water-repellant polymers such as silicones and wetting agents - for example, nonionic detergents - in sufficiently high concentra-tions.
Thus, enhanced dry strippability may be accomplished, when needed, by using a size as referred to above, reducing the weight per unit area of adhesive, adding an inert filler to the adhesive layer utilizing mixed salts of the neutralized copolymers, or by incorporating suitable release agents into the adhesive layer.

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The water-remoistenable adhesive described herein can be applied ; to the undecorated side of a wallcovering as a dry dusting powder or from an organic medium as well as from the previously described aqueous medium.
This requires that the neutralized copolymer salt be properly dried to a reasonably uniform particle size of sufficient fineness to permit easy dusting or the formulation of stable dispersions or solutions. To this end, the neutralized solution must be dried at a temperature which is not sufficiently high to cause degradation. Temperature limitations are differ-ent for various copolymer compositions and for different degrees of cross-linking.
The adhesives of this invention may also be coated onto surfaces which require such water-activatable adhesives by first wetting such surfaces with water and then dusting an appropriate dry powder adhesive formulation onto such wetted surfaces and then drying the coating. The initially applied water layer may contain a tackifier/film former to ensure more uniform and reproducible application under operating conditions and to ensure long-term adhesion of the alkali salts of acidic polymers to the surface in question. Water-sensitive film forming polymers and similar materials as exemplified hereinafter may be used. This is essentially the process presently used to an appreciable extent to coat substrates with some of the conventional water-sensitive adhesives such as starch. It is believed that the best binding action is evidenced when the water-sensitive binder has coated the active polymer salt adhesive ingredient before the dry mixture is dusted onto the wet surface. This may be accomplished by mixing the binder polymer with the salt of the acidic polymer in aqueous medium before the salt is dried. Such dry dusting compounds may also optimally contain release agents of a type similar to those mentioned above, to promote strippability of, for example, wallcoverings. In those cases wheFe the film former is sensitive to organic solvents, it is possible to pre-moisten the substrate with an organic solvent of the appropriate ~3~
solvency and volatility to permit rapid yet controlled drying of the - adhesive onto the substrate.
The dry powder copolymer salt adhesive offers definite advantages over conventional powder type systems such as starch. While starches generally require 10 to 20 pounds of adhesive solids per ream, the copolymer salts described herein require only 2 to 2.5 pounds per ream to provide equal or better adhesion for most conventional wallcoverings. This enables faster, cleaner coating operations with less contamination of coating equipment and significantly reduced pick off to the decorated surfaces when packed in conventional rolls. Because of the substantial reduction in the required amount o adhesive employed, there is a proportional reduction in likelihood of a dust explosion during manufacturing. Aesthetically, the lower adhesive weight is more appealing than the more heavily coated starch type powder products and there is less likelihood of powder being dis-lodged during routine handling.
Once the copolymer salt has been dried and rendered a powder of appropriate size, the salt may be re-dissolved or re-suspended in an aqueous or an organic medium to permit controlled application by convention-al coating techniques as outlined previously. Stability o~ such solutions ; 20 is partly dependent on the solvent and surfactant systems employed as well as particle si~e. When re-dissolved in water and formulated as per Examples 1-8 adhesive character is identical to that obtained when the salts are formed in an aqueous medium by direct neutrali~ation of the acidic copolymers. Similarly, once an appropriate dry copolymer salt is obtained, it may be combined with a film former in an appropriate organic solvent system for direct application to a wallcovering or other substrate. The ; advantages of working at low solids content in aqueous media have been mentioned above and these are applicable also to organic media. In the latter case, however, it is necessary to recover solvent which cannot be .:; .

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.~, wasted to the atmosphere as can water in aqueous dispersions. The economic balance in organic application media may then dictate somewhat higher solids levels than would otherwise be optimum in organic applica-tions. We prefer a 10 percent coating solution to ensure uniformly applied low coating weights, which are unique for this adhesive system, but it - may be economically desirable to work at higher concentrations in organic media in order to reduce solvent recovery costs.

The following are examples of adhesives according to the invention in organic liquid-base compositions:

An aqueous emulsion of a cross-linked copolymer based on ethyl acrylate and methacrylic acid in the relative proportions of 60/40 by weight was diluted to about 10% solids by weight over volume and neutral-i~ed with aqueous sodium hydroxide. The resulting dispersion of copolymer salt in water could be dried by spray drying, free~e drying, or by heating in a forced air oven. Forty parts of this copolymer salt was added to 200 parts of the following mixture:
methyl ethyl ketone 200 parts methanol 50 low molecular weight hydroxypropyl cellulose 6 medium molecular weight hydroxypropyl cellulose which was mixed and to which was then added: 2 lecithin 4 nonionic surfactant 6 methyl ethyl ketone 100 methanol 25 ' ~33~

The resulting coating mixture contained 20% solids and had excellent flow on coating machines, good wettability of paper, good wet and dry bond between wallpaper and wallg, good wet slip, and long open time.
It could be drystripped easily from walls as per the preceeding discussion.
The adhesive strength of the remoistened coated layer was still very high when the coating weight was reduced by diluting this composition.

The following mixture was made:
methyl ethyl ketone 250 parts methanol 40 low molecular weight hydroxypropyl cellulose 10 ; and to this mixture was added sufficient dry copolymer salt, as described in Example 9, to bring the total solids content of the final mixture to 38%.
The hydroxypropyl cellulose used in the above formulation was included in part because of its ability to form a film at room temperature.
The dry copolymer salt will generally not adhere uniformly enough to paper, fabric or other surfaces without addition of such a binder when the polymer salt is applied from organic media. The hydroxypropyl cellulose serves as a binder in this case. It also thickens the organic solvent mixture and thus increases the stability of the polymer salt suspension. Methanol is present in this composition to improve the solubility of hydroxypropyl cellulose in methyl ethyl ketone, which is the main solvent. It is necessary also that the film former and suspending agent used in these formulations not repel water, so that the polymer salt' which is the chief adhesive ingredient can be activated when the dried coating is moistened with water. It may further be noted that the binder flmctions to hold the polymer salt on the surface when the adhesive formulation is dry.
The invention further encompasses the use of other auxilliary polymers in place of hydroxypropyl cellulose, with corresponding variations in solvent combinations to serve as organic suspension media. Other .

~3~

possibilities include:

(i) poly ~N-vinyl pyrrolidone) in alcohols ~e.g., methanol, butanol, octanol); amides (e.g., dimethylformamide~; amines ~e.g., ethanol-amine); polyhydroxy compounds ~e.g., ethylene glycol, glycerol, polyethylene glycols); halogen compounds ~e.g. methylene dichloride);
nitroparaffins ~e.g., nitroethane); cyclic ethers ~e.g., tetra-hydrofuran, dioxane); esters ~e.g., ethyl acetate); ketones ~e.g., acetone, methyl ethyl ketone) and polyhalogenated compounds ~e.g., dichlorodifluoromethane). Some of these solvents may be blended with controlled amounts of water and the nature of those liquids whlch are miscible with water will be obvious to those skilled in the production of lacquers and other organic solutions. The ~ 3~9 solubility of the poly ~vinylpyrrolidone) polymer will vary with the water content of the initial polymer, to some extent.
Dried poly (vinylpyrrolidone) films tend to become tacky in highly humid atmospheres. If this tackiness becomes a problem, it can ~- be minimized by incorporating a water-insensitive modifier which is compatible with poly (vinylpyrrolidone). An example is an aryl sulfonamide - formaldehyde resin, at about 10% on the base polymer.
(ii) starches, including modified, converted~ oxidized and dextrinized starches as well as starch derivatives. The solvent mixtures for polymers will vary with the particular material. Amylose starch, for example, was dissolved in mixtures of acetone and dimethyl sulfoxide.
(iii) Polyvinylalcohol in appropriate solvent mixtures, such as combina-tions of water with monohydric alcohols such as methanol, ethanol and isopropanol or formamide or dimethylformamide. Lower molecular weight partially alcoholized polyvinyl alcohols are preferred for the present application. For example, a water-alcohol mixture can be made at room temperature with about 40% isopropanol when the polymer i8 Elvanol 72-60 (du Pont? which is 99 - 100% hydroly~ed and has high molecular weight. About 60% isopropanol can be tolerated in admixture with water when the polymer is Elvanol 50-4Z, which has a slightly lower molecular weight and is 87-89% hydrolyzed.
(iv) Polyallyl alcohol in methanol and mixtures of alcohols.
(v) Polyacrylamide and copolymers of acrylamide in mixtures of water with compounds such as ethylene glycol, glycerol and propylene gly-col.
Poly-acrylamide films tend to be somewhat brittle and may be advantageousely plasticized with polypropylene glycol or ethylene oxide adducts with sorbitan monooleate or long-chain alcohols.
(vi) Polyethylene oxide in chlorinated hydrocarbons, isopropanol, ' .

`: \
3~g While it is not essential that the polymer salt which is used as an adhesive be cross-linked in order to have adhesive character, it is believed that cross-linking offers certain advantages. With cross-linking, the resistance to excessive penetration into the substrate is improved due to the modified gel structure of the cross-linked salt, thereby imparting better slip and adhesion from a lower coat weight of adhesive.
An otherwise miscible polymer may be rendered water-insoluble by incor-porating at least about 0.1 or 0.2 percent by weight of cross-linking agent in the monomer mixture when the initial acidic copolymer is synthesized. Many suitable cross-linking agents are known, including divinyl benzene, diallyl maleate, diallyl phthalate, and in general any copolymeri~able monomer containing at least two non-conjugated vinyl or - allyl groups. It is generally found that 2% is the maximum level of cross-linking agent required for these copolymerizations. Once cross-linked, these polymers have reduced solubility in water and swell on remoistening to yield a gel structure. This cross-linking reduces any tendency for the moistened adhesive to flow or drip when it is used, for example, as an adhesive for wallcoverings. It further tends to extend the open time during which the material which has been coated with an activated adhesive can be slipped laterally while it i9 bonded to another surface. Further advantages may include a reduced tendency for the moistened adhesive to be wiped off the substrate during the moistening of articles such as postage stamps, tapes or labels.

i~, In the foregoing examples, a cross-linked copolymer of ethyl acrylate/methacrylic acid in the ratio 60/40 was employed. In a parallel series of experiments, a non-cross-linked terpolymer of methyl methacrylate/
ethyl acrylate/methacrylic acid in the ratio of 17/50/33 was substituted in otherwise identical formulations. When the non-cross-linked terpolymer was evaluated under equivalent circumstances, several differences in ~ ~Q3~5~g performance criteria were noted. In general, the non-cross-linked acidic polymer salt had a lower solution viscosity at equivalent concentrations and pH levels, tended to have greater flow subsequent to remoistening which sometimes resulted in a tendency for the adhesive to drip off the substrate, had generally poorer wet adhesion to the substrate as manifested in a much greater tendency for the prepaste to be wiped off during wet handling, was more soluble on remoistening, had reduced extent and strength of gel structure, and had similar dry adhesion characteristics.
In general, the amount of acid in such polymer systems is the controlling factor in determining the neutralized solution viscosity.
Higher acid content permits more neutralization sites and thus higher viscosity. The extent of cross-linking greatly influences the gel structure and solubility wherein the higher cross-link density forms firmer gels which are less soluble.
While the non-cross-linked adhesive is suitable for wallcovering applications, the cross-linked type functions more appropriately in this application. The non-cross-linked adhesive is suitable for use in areas where easy removability is of prime concern such as ch~ildren's glue, removable labels, etc.
It will be appreciated from the foregoing that the products and processes of this invention permit the use of low coating weights of adhesive, which result in cost savings and very good slip times, without loss of bond between the coated material and its substrate.
- The foregoing examples are presented purely by way of example.
Various other embodiments and modifications within the spirit of the invention will be apparent to those skilled in the art.

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,

Claims (13)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR
PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for coating a substrate with an adhesive which comprises neutralizing an acidic polymer to give a salt of said polymer, applying said salt to said substrate to form an adhesive coating thereon and, where required, drying the applied coating, said salt being remoistenable and being activatible from its dry state by wetting with water into an adhesive state whereby said salt, after removal of said water, is capable of maintaining said substrate in firm and steady adhesion to a surface after removal of said water.

2. A process for coating a substrate with an adhesive which comprises neutralizing an acidic copolymer formed from two or more different monomers, said copolymer comprising ethyl acrylate and one or more of the group consisting of methacrylic acid, acrylic acid and esters of methacrylic acid and acrylic acid, to give a salt of said copolymer, applying said salt to said substrate to form an adhesive coating thereon and, where required, drying the applied coating, said salt being remoistenable and being activatable from its dry state by wetting with water into an adhesive state whereby said salt, after removal of said water, is capable of maintaining said substrate in firm and steady adhesion to a surface after removal of said water.

3. A process for coating a substrate with an adhesive which comprises neutralizing an acidic copolymer consisting of a partially cross-linked copolymer of ethyl acrylate and methacrylic acid, to give a salt of said copolymer, said salt being formed by the direct reaction in an aqueous solution to produce an aqueous suspension of said salt; applying said salt to said substrate to form an adhesive coating thereon, said salt being remoistenable and being activatable from its dry state by wetting with water into an adhesive state whereby said salt, after removal of said water, is capable of maintaining said substrate in firm and steady adhesion to a surface after removal of water.

4. The process of claim 2, wherein said acidic copolymer consists of ethyl acrylate and methacrylic acid.

5. The process of claim 4 wherein said copolymer is partially cross-linked.

6. The process of any of claims 1, 2 and 3, wherein said salt is formed by direct reaction in an aqueous suspension of said salt.

7. The process of any of claims 1, 2 and 3, wherein said aqueous suspension has a solids content of about 10% by weight and is applied directly to said substrate.

8. The process of any of claims 1, 2 and 3, wherein said aqueous suspension is dried to give said salt in dried form and said salt is applied as a powder to said substrate which has been pre-coated with a tackifier.

9. The process of any of claims 1, 2 and 3, wherein said salt is formed by direct reaction in an aqueous suspension of said salt, and said aqueous suspension is dried to give said salt in dried form and said salt is subsequently re-suspended in an aqueous suspension or in an organic suspension containing a film-former soluble in the organic suspension medium, prior to coating upon said substrate.

10. The process of any of claims 1, 2 and 3, wherein said acid polymer is neutralized with an agent selected from the group consisting of potassium hydroxide, sodium hydroxide, lithium hydroxide, magnesium hydroxide and barium hydroxide.

11. The process of any of claims 1, 2 and 3, wherein said salt is formed by direct reaction in an aqueous suspension of said salt, and said aqueous suspension is dried to give said salt in dried form and said salt is subsequently re-suspended in an aqueous suspension or in an organic suspension containing a film-former soluble in the organic suspension medium, prior to coating upon said substrate, said salt being applied to said substrate as a suspension in methyl ethyl ketone or methyl ethyl ketone wth a lower alcohol.

12. The process of any of claims 1, 2 and 3, wherein said salt is formed by direct reaction in an aqueous suspension of said salt, and said aqueous suspension is dried to give said salt in dried form and said salt is subsequently re-suspended in an aqueous suspension or in an organic suspension containing a film-former soluble in the organic suspension medium, prior to coating upon said substrate, said salt being applied to said substrate as a suspension in methyl ethyl ketone or methyl ethyl ketone with a lower alcohol, and said film-former being hydroxy-propyl cellulose.

13. A process for coating a substrate with an adhesive which comprises neutralizing an acidic copolymer consisting of a partially cross-linked copolymer of ethyl acrylate and methacrylic acid, to give a salt of said copolymer, said salt being formed by the direct reaction in an aqueous solution to produce an aqueous suspension of said salt; said aqueous suspension having a solids content of about 10% by weight; said aqueous suspension being dried to give said salt in dried form prior to the application of said salt to said substrate; applying said salt to said substrate in said dried form or as an aqueous or organic suspension of said dried salt, to form an adhesive coating thereon; said salt formed in said substrate being remoistenable and being activable from its dry state by wetting with water into an adhesive state whereby said salt, after removal of said water, is capable of maintaining said substrate in firm and steady adhesion to a surface after removal of said water.