GB2086917A - Wet Adhesion Promoters and Aqueous Coating Compositions Containing Them - Google Patents

Abstract

A wet adhesion promoter for an aqueous film-forming surface coating composition is the product obtained by mixing substantially equimolar proportions of (a) one or more esters of acrylic or methacrylic acid with a hydroxy alkanol containing from 2 to 6 carbon atoms and (b) a derivative of urea or ethylene urea containing an N- hydroxymethyl group. The hydroxy alkanol is preferably 2-hydroxyethanol and the derivative of urea or ethylene urea is preferably N- hydroxymethyl ethylene urea, the product having the likely formula. <IMAGE> An aqueous film-forming surface coating composition is prepared by the emulsion polymerisation of an ethylenically unsaturated monomer in the presence of a minor proportion of the wet adhesion promoter. This composition may be mixed with an aqueous copolymer emulsion prepared without the wet adhesion promoter, to produce a further aqueous film-forming surface coating composition which also exhibits wet adhesion properties.

Description

SPECIFICATION Wet Adhesion Promoters and Aqueous Coating Compositions Containing Them This invention relates to wet adhesion promoters and to aqueous surface coating compositions containing them, and is particularly concerned with such compositions which are film-forming at ambient temperature and which comprise aqueous emulsion copolymers formed by aqueous emulsion polymerisation of one or more ethylenically unsaturated monomers.

It has been well known for over a decade that the adhesion to certain substrates of integrated surface coatings, deposited by absorption and evaporation of water at ordinary ambient temperatures from compositions which comprise film-forming copolymers formed by emulsion polymerisation, is greatly enhanced if the polymerisation recipe includes minor proportions of an ethylenically unsaturated substance containing certain polar groups.

The substrates referred to include in particular those surfaces which have previously been painted with oleoresinous paints, especially paints formulated with alkyd resins, and wooden and fibre-board surfaces, whether previously painted or unpainted. Such wooden and fibreboard surfaces are particularly prone to small-scale movements in response to changes in ambient temperature and humidity.

Damage to surface coatings can occur under such influences as cleaning of the surface of the coating with water by rubbing, exposure to a steamy atmosphere such as that of a bathroom or kitchen, or mere outdoor exposure to natural climatic conditions. Weakening of adhesion of the coating to the substrate is manifested by such effects as flaking, blistering and cracking of the coating, the result of the failure ranging from catastrophic to unsightly. Due to such weakening of adhesion, a surface coating composition may not be suitable for marketing for use under a wide range of practical conditions.Therefore it will be seen that the enhanced adhesion exhibited by the surface coatings deposited from surface coating compositions containing the above-mentioned adhesion-enhancing substances is most desirable when the coating is subjected to humid conditions, or when the surface is actually moist or wet.

The improvement in wet adhesion properties which is conferred upon the coating by the presence, in an emulsion polymerisation recipe, of as little as 2% by weight of the total weight of monomers, or even less than 2% by weight, of a suitable ethylenically unsaturated substance containing a polar group can be remarkable. No plausible scientific explanation has yet been advanced which will explain satisfactorily the reasons for the magnitude of the effect, or that it can be common to a variety of surfaces of diverse chemical and physical character, as diverse as wood and alkyd resin paint on metal.Further, the presence of only certain classes of polar groups imparts a high degree of efficacy to the adhesion promoting substance, yet the structure of these polar groups can be quite diverse chemically, and the presence of a common structural configuration which will explain the specificity of the effect is difficult to recognise. It follows that it is difficult to identify new adhesion promoting substances by theoretical prediction rather than by numerous practical trials.

In our experience, an effective wet adhesion promoting substance must containan ethylenically unsaturated group and must be present during the emulsion polymerisation stage. It seems therefore that at least a proportion of the substance must be bound by copolymerisation into the copolymer macromolecules in order to be effective. This hypothesis is supported by the finding that the addition of such an ethylenically unsaturated substance to a polymeric emulsion after the emulsion polymerisation step does not confer wet adhesion properties.Further, treatment in aqueous emulsion of a copolymer containing certain reactive groups, which does not exhibit wet adhesion properties, with a substance co-reactive with those groups, can be used to produce a copolymer with improved wet adhesion properties, when the result of the reaction is the introduction into the copolymer of structures which impart wet adhesion properties.

For convenience, these wet adhesion-promoting ethylenically unsaturated substances are designated hereinafter as "wet adhesion promoters".

The patent literature is replete with disclosures of wet adhesion promoters which are effective to a greater or lesser degree towards promoting the desired effect, and lengthy classifications of allegedly effective substances have been provided. A selection from the lists of those substances which are sufficiently effective wet adhesion promoters becomes very limited in practice, for the following reasons: (1) Economics A wet adhesion promoter is usually considerably more expensive than any of the principal monomers, and only a minor proportion, say about 2% by weight, should be required in order to produce an effective result.

The raw materials required for the preparation of the wet adhesion promoter should therefore be reasonably cheap and readily available, and the preparative reaction should be reasonably simple and result in good yields.

(2) Toxicology The substance should be physiologically innocuous enough to present no unacceptable handling difficulties, either during the emulsion polymerisation step or during subsequent stages leading to the finished surface coating composition. Use of the composition itself during application to the substrate must not be liable to create immediate or latent toxicological responses. In this respect, the use of hydrazine and aziridine derivatives requires particular caution.

(3) Odour Air drying surface coating compositions are frequently used indoors under conditions which provoke complaints of the odour of residual volatile substances. In this regard, the presence of trace proportions of allyl alcohol is very objectionable, this substance being formed by the decomposition during emulsion polymerisation of allylic wet adhesion promoters.

(4) Polymerisation Kinetics The conversion rate during emulsion polymerisation and the final conversion should not be depressed for as wide a range as possible of combinations of monomers useful for preparing aqueous surface coating compositions.

(5) Colloidal Properties The colloidal stability of the emulsion during polymerisation should not be impaired in such a way as to aggravate tendencies towards formation of coagulum, floc and oversize particles. The presence of the wet adhesion promoter should be neutral with respect to the versatility of formulation by recipes directed towards attaining specific colloidal properties-e.g. mean particle size, particle size distribution, viscosity, surface tension, chemical stability, mechanical and shear stability, freeze-thaw stability and shelf iife--appropriate to the desirable properties of the surface coating composition.

(6) Physical Properties Other desirable physical properties of the copolymer or of films of the surface coating composition should not be unduly impaired as a consequence of the inclusion in the coating composition of the wet adhesion promoter for the purpose of optimising the wet adhesion properties of the resulting coating. Relevant properties are minimum film formation temperature, hardness, surface tack, flexibility, mar resistance, resistance to ageing, flammability, toughness, gloss, corrosion of the substrate etc.

(7) Colour A film of the copolymer or of the surface coating composition should not become yellowed or otherwise discoloured, and should not be altered on ageing under conditions of use due to the presence of the wet adhesion promoter.

We have now discovered that highly efficient wet adhesion promoters, having advantageous properties with respect to the considerations discussed above, are available as the products of a relatively simple reaction from relatively inexpensive and readily commercially available starting materials.

Accordingly, one aspect of the present invention broadly provides a wet adhesion promoter, wherein the promoter is the product obtained by mixing substantially equimolar proportions of (a) one or more esters of acrylic or methacrylic acid with a hydroxy alkanol containing from 2 to 6 carbon atoms and (b) a derivative of urea or ethylene urea containing an N-hydroxymethyl group.

Suitable esters include the acrylates and methacrylates of 2-hydroxyethanol and 2 hydroxypropanol, the 2-hydroxyethanol esters being preferred.

Suitable derivatives of urea and ethylene urea include N-hydroxymethyl ethylene urea, N,N' bis(hydroxymethyl)ethylene urea; N-hydroxymethyl urea and N-methyl-N-hydroxymethyl ethylene urea, N-hydroxymethyl ethylene urea being preferred.

The derivatives of urea and ethylene urea, constituting the derivatives (b) above, may be prepared; by the addition of the appropriate quantity, in terms of molar proportions, of formaldehyde to an aqueous solution of urea, ethylene urea, or a monoalkyl substituted urea or ethylene urea, such as N methyl ethylene urea. A base such as sodium hydroxide is added to adjust the pH of the mixture to a value in the range of 7 to 10, preferably 7.5 to 8.5. The reaction mixture is allowed to stand for a minimum of 4 hours at ordinary ambient temperatures, for example, about 1 5 hours, in order to allow the reaction to come to equilibrium.

This reaction product is then mixed with an appropriate quantity, in molar proportions, of one or more esters (a) above. The latter ester may conveniently be dissolved in water, or in a mixture of organic solvent and water. The pH of the mixture of ester (a) and derivative (b) is reduced to a value in the range of 2 to 5, preferably 2.5 to 3.5. An acid may be used to reduce the pH, preferably an organic carboxylic acid which will not impart undesirable properties to the final surface coating. To this end it is convenient to use an ethylenically unsaturated acid which subsequently is rendered unobjectionable by copolymerisation, especially acrylic acid. In any event, the acid may be added at any time during or after mixing the ester (a) and the derivative (b). Conveniently, the acid may be already mixed with the solution of the ester (a).

We believe that a reaction takes place involving condensation between the ester (a) and the derivative (b) with the elimination of a molecule of water and the formation of a substance exemplified by the structural formula:

A substance of this formula would be formed by the mixture of 2-hydroxyethyl acrylate and Nhydroxymethyl ethylene urea in substantially equimolar proportions.

The molar ratio of ester (a) to derivative (b) should be at least 0.9. If an excess of ester (a) is employed, the surplus is simply consumed by copolymerisation.

However, we do not intend to suggest that a substance having such a structure is necessarily formed and is responsible for promoting wet adhesion of a coating. It is possible that the reaction product contains an equilibrium mixture of substances, only one of which has a structure corresponding to that exemplified. In any case it is not necessary to isolate the substance or substances before use, and the reaction mixture may be employed in an emulsion polymerisation stage without further treatment, or after such simple further treatment as pH adjustment, dilution with water or other inert diluent, mixture with other monomers etc. The simplicity of the method of preparation of the wet adhesion promoter is an advantage of the present invention.

Accordingly another aspect of the present invention provides a method for the preparation of an aqueous copolymer emulsion suitable for use as a film-forming aqueous coating composition, wherein at least one ethylenically unsaturated monomer is subjected to emulsion polymerisation in the presence of a minor proportion of the wet adhesion promoter described above.

The invention also provides an aqueous surface coating composition comprising the copolymer emulsion prepared by the method just indicated.

Surface coatings deposited from aqueous coating compositions comprising such an aqueous copolymer emulsion exhibit excellent wet adhesion to substrates, especially to surfaces previously painted with alkyd gloss paints, even when the copolymer emulsion contains a notional proportion of as little as 2% by weight of the wet adhesion promoter.

Furthermore, the present aqueous surface coating compositions may comprise a mixture of two or more copolymer emulsions, only one of which has been prepared by aqueous emulsion polymerisation in the presence of the wet adhesion promoter. It has been found that air-dried surface coatings deposited by such compositions nevertheless still exhibit wet adhesion properties ranging from excellent to adequate, provided that the emulsion containing the wet adhesion promoter has been prepared by a recipe which ensures maximum development of wet adhesion properties, for example, in the form of a small particle emulsion. It may even be possible to utilise as little as 20% by weight, or even less, of emulsion copolymer containing the wet adhesion promoter, based on the total weight of emulsion polymer or copolymer present in the composition.

This finding extends the practical scope of the invention. For example, it is possible to prepare a copolymer emulsion containing a proportion of wet adhesion promoter of 2% of the weight of copolymer, the properties of the emulsion having been adjusted through the polymerisation recipe in order to maximise wet adhesion properties. This emulsion may then be used for blending with other copolymer emulsions, the properties of which have been optimised for particular surface coating properties, but which would not otherwise exhibit good wet adhesion properties.

It is to be appreciated that the blending does not invariably give wet adhesion performances as good as those obtained when all the copolymer contains the wet adhesion promoter, but the result may often be good enough for practical purposes and the blending procedure is convenient for many large-scale operations.

No restriction need be placed on the method of use of the wet adhesion promoter in the emulsion polymerisation stage, which is conventional. As indicated earlier, the polymerisation recipe and the details of the procedure are adjusted in order to achieve the properties required of the final surface coating composition, including wet adhesion properties.

The proportion of wet adhesion promoter in the aqueous coating composition which is useful in achieving the desired result is preferably not in excess of 0.04 moles of wet adhesion promoter of structure equivalent to that shown in the formula above for every 100 g. by weight of combined monomer, or, more preferably, a quantity not in excess of 0.025 moles.

In general the ethylenically unsaturated monomers suitable for forming the aqueous copolymer emulsions can be divided into two types, the first of which comprises those monomers, principally neutral monomers, which are conveniently polymerised to form the major part of the dispersed polymer resin, and the second of which comprises those monomers which may be used optionally in conjunction with one or more of the first type to obtain some especially desirable resin property.

Monomers which fall into the first of these categories include vinyl esters of carboxylic acids, e.g.

vinyl acetate, vinyl propionate, vinyl pivalate and the vinyl esters of mixed tertiary C10 carboxylic acid; esters of acrylic or methacrylic acids, e.g. ethyl acrylate, butyl acrylate, 2-ethyl hexyl acrylate, methyl methacrylate and ethoxyethyl methacrylate; vinyl aromatic compounds, e.g. styrene, vinyl toluene and chlorostyrene; conjugated dienes, e.g. butadiene and isoprene; vinyl arid vinylidene halides; acrylonitrile and methacrylonitrile and ethylene and higher alpha-olefins e.g. but-1-ene, isobutene and dec-1 -ene.

Monomers in the second category include the mono and diesters of maleic, fumaric and itaconic acids, e.g. dibutyl maleate, monobutyl maleate, diethyl fumarate, dimethyl itaconate and monobutyl itaconate; allyl compounds, e.g. allyl acetate; esters of crotonic acid, e.g. methyl crotonate; and alpha methyl styrene. Also included in this group are potentially reactive monomers including unsaturated carboxylic acids, e.g. acrylic, methacrylic, maleic, fumaric, itaconic, crotonic and cinnamic acids; acrylamide, methacrylamide and their N substituted derivatives, e.g. N-methylolacrylamide N-butoxymethacrylamide; hydroxyalkyl esters of polymerisable unsaturated acids; and glycidyl esters of polymerisable unsaturated acids.

It will be appreciated, however, that the specific composition of the polymer particles in the latex is not critical.

The polymer latices used in the surface coating compositions of the present invention will in general be prepared in the presence of stabilisers which may include high molecular weight colloids and ionic and nonionic surfactants used singly or in combination. Typical high molecular weight colloids suitable for this purpose are hydroxyethyl cellulose polyvinyl alcohol and alkali soluble acrylic copolymers. The ionic surfactants, which may be either ionic or cationic include such materials as sodium lauryl sulphate, sodium di-2-ethyl hexyl sulphosuccinate or cetyl trimethyl ammonium bromide.

Typical nonionic surfactants are condensates of ethylene oxide with alkyl phenols or fatty alcohols.

The copolymer emulsions may be prepared by methods known per se. A free radical source is used as a polymerisation initiator; these catalysts are well known and any such catalyst may be used in the present process. This initiator may be used as a single compound or as a mixture and can be watersoluble or oil-soluble. Examples of suitable catalysts are ammonium peroxydisulphate, potassium peroxydiphosphate, benzoylperoxide and azobisisobutyronitrile. A catalyst activator e.g. sodium formaldehyde sulphoxylate, may also be used and chain-transfer agents, e.g. carbon tetrachloride or ndodecyl mercaptan may also be present.

The polymerisation is carried out by dispersing the reactants in water in the presence of the stabilisers to form an emulsion and the process of polymerisation may be batchwise, continuous or semi-hatch.

In addition to pigments, which may be any one or more of those pigments conventionally used in the art the coating compositions of the present invention may also include such materials as extenders, thickeners, dispersing agents, wetting agents, defoamers, solvents and plasticisers and any other additives which may be relevant to the application for which the coating is intended.

The invention will now be illustrated by the following Examples.

Example 1 An aqueous solution of a wet adhesion promoter, useful for preparing a composition according to the invention, was prepared as follows To a solution of ethylene urea (8.6 g, 0.1 moles) in water (12.9 g) was added a 20% w/w aqueous solution of sodium hydroxide (0.03 ml) followed by a 38% w/w aqueous solution of formaldehyde (7.9 g, 0.1 mole). The solution was allowed to stand at ambient temperature overnight.

This solution was then added to a solution of hydroxypropyl acrylate (13 g, 0.1 mole) and acrylic acid (0.5 ml) in water (20 g). The final pH of the mixture was 4. The solution was allowed to stand for 3 days at room temperature before use.

An emulsion polymer was then made using the following recipe Initial Aqueous Phase: Hard water 218 9 "Synperonic" NP12* 2.5 g *"Synperonic" NP 12 is a registered Trade Mark for a non-ionic surfactant formed by condensation of nonyl phenol with an average of 12 moles of ethylene oxide. It is supplied by l.C.I. Ltd.

Monomers: "Dowfax" 2A1 (45%)+ 3.25 g Methyl Methacrylate 115 9 Butyl Acrylate 125 g Catalyst Solution: Ammonium Persulphate 0.6 g Hard water 20 g "Dowfax" 2A1 (45%) 4.25 g Delayed Solution: Adhesion Promoter solution as described above 10 g Final A ddftion: Aqueous ammonia solution (d=0.910) 0.3 g The initial aqueous phase was heated to 700 C. and 1 5% of the mixed monomers and 1 5% of the adhesion promoter solution were added with stirring, followed by 20% of the catalyst solution.The temperature was raised to 900 C., held at this temperature for 15 minutes and then addition of the remainder of the monomers, catalyst solution and adhesion promotor solution was started, the additions being completed after 2 hours. The mixture was stirred throughout whilst maintaining the temperature at 900 C. After a further 30 minutes, the mixture was cooled and the ammonia added.

There was obtained an emulsion polymer having a non-volatile content of 50.3%, a pH of 6.3 and a particle size of 0.15 microns.

Example 2 (Comparative) In this example, the emulsion was made as in Example 1 except that the adhesion promotor was omitted. This example serves as a control.

Example 3 The emulsions made as the previous two examples were blended together in the ratio 20% emulsion containing adhesion promoter (Example 1) to 80% emulsion containing no adhesion promoter (Example 2). This example demonstrates how the adhesion of an emulsion containing no copolymerised wet adhesion promoter can be markedly improved by the addition of a minor proportion of an emulsion containing an adhesion promoter described in the invention.

+"Dowfax" 2A1 is a registered Trade Mark for an anionic surfactant, which is the disodium salt of dodecyldiphenyl ether disulphonic acid, supplied by Dow Chemical Company.

Adhesion Results for Paints The emulsions described in the Examples were each made up into gloss paints using the following recipe: parts by weight "Revacryl" 451(1) (40%) 8.69 Water 8.69 were mixed then Aqueous ammonia solution (d=0.89) 0.80 was added followed by "Tamol" SG1 (2) 0.35 Propylene glycol 2.90 "Hercules" 1512M3 0.64 "Triton" GR5MI4) 60% 0.23 Tioxide RDH215) 57.92 These ingredients were then ground at high speed to give a good dispersion.The following ingredients were mixed and added: Water 5.30 Propylene glycol 9.18 Butyl dioxitol 5.30 Under slow stirring the following ingredients were then added in order parts by weight Polymer Emulsion (Examples 1, 2 or 3) at 50% non-volatile content 148.4 "Revacryl" 450(6) 4.9 Water 12.3 The pH of the paint was adjusted to 8-8.5 with aqueous ammonia solution (d=0.890).

The ingredients in the formulation identified by registered Trade Mark and numeral superscript are described as follows; (1) A dispersion of an alkali-soluble dispersant and gloss improver (Harlow Chemical Co. Ltd.) (2) Dispersant (Rohm s Haas) (3) Antifoam (Hercules Powder Co.) (4) Sulphosuccinate surfactant (Rohm 8 Haas) (5) Titanium tioxide (B.T.P. Ltd.) (6) A dispersion of an alkali-soluble thickener and flow improver (Harlow Chemical Co. Ltd.) Paint panels were then prepared by applying a coat of black alkyd gloss paint to a ground glass plate, allowing the paint to air dry and then stoving the panels overnight at 1 500C. The test emulsion paint was applied using a 100 micron applicator and the paint films were allowed to dry overnight at ambient temperature. Four cuts were then made across the width of the panels to give spacings of -", 111 " and 1 " and the panels were soaked in water for one hour. The panels were placed on an R.E.L. scrub tester and scrubbed in a direction perpendicular to the direction of the cuts using brushes weighted to 900 g. After 5,000 cycles the panels were examined for damage.

State ofPanel Emulsion after 5000 cycles Example 1- (example of the invention) Ver little damage Example 2- Control, no adhesion promoter Complete removal of paint film Example 3- Blend of adhesion Slight removal of promoted and non-adhesion paint film at " promoted emulsion (example width. Remainder of the invention) of film intact.

Claims (14)

Claims

1. A wet adhesion promoter, wherein the promoter is the product obtained by mixing substantially equimolar proportions of (a) one or more esters of acrylic or methacrylic acid with a hydroxy alkanol containing from 2 to 6 carbon atoms and (b) a derivative of urea or ethylene urea containing an N-hydroxymethyl group.

2. A wet adhesion promoter, according to Claim 1, wherein said hydro,xy alkanol is 2 hydroxyethanol or 2-hydroxypropanol.

3. A wet adhesion promoter according to Claim 1 or 2, wherein said derivative of urea or ethylene urea is N-hydroxymethyl ethylene urea, N,N'-bis(hydroxymethyl)ethylene urea, N-hydroxymethyl urea or N-methyl-N'-hydroxymethyl ethylene urea.

4. A wet adhesion promoter according to any preceding claim, wherein said ester and said derivative of urea or ethylene urea are mixed in water or a mixture of organic solvent and water, and wherein the pH of the resulting mixture is adjusted to a value of 2 to 5.

5. A wet adhesion promoter according to Claim 4, wherein the pH is adjusted to a value of 2.5 to 3.5.

6. A wet adhesion promoter according to Claim 4 or 5, wherein an organic carboxylic acid is used to adjust the pH.

7. A wet adhesion promoter according to Claim 6, wherein the acid is an ethylenically unsaturated acid.

8. A wet adhesion promoter as hereinbefore described with reference to the foregoing Examples.

9. A method for the preparation of an aqueous copolymer emulsion, suitable for use as an aqueous film-forming surface coating composition, wherein at least one ethylenically unsaturated monomer is subjected to emulsion polymerisation in the presence of a minor proportion of the wet adhesion promoter claimed in any preceding claim.

10. A method for the preparation of an aqueous copolymer emulsion according to Claim 9, wherein the proportion of the wet adhesion promoter used is such that the final copolymer comprises up to 0.04 moles of wet adhesion promoter per 100 g. of copolymer.

11. A method for the preparation of an aqueous copolymer emulsion according to Claim 10, wherein the final copolymer comprises up to 0.025 moles of wet adhesion promoter per 100 g. of copolymer

12. A method for the preparation of an aqueous copolymer emulsion according to any of Claims 9 to 11, wherein said polymerisation proceeds in the presence of stabilisers and/or polymerisation initiator catalysts.

13. A method for the preparation of an aqueous copolymer emulsion suitable for use as a filmforming aqueous surface coating composition, substantially as hereinbefore described with reference to the foregoing Examples.

14. An aqueous film-forming surface coating composition comprising the aqueous copolymer emulsion prepared by the method claimed in any of Claims 9 to 13.

1 An aqueous film-forming surface coating composition comprising a mixture of the aqueous copolymer emulsion prepared by the method claimed in any of Claims 9 to 13 with an aqueous copolymer emulsion prepared without the wet adhesion promoter.

1 6. An aqueous film-forming surface coating composition according to Claim 1 4 or 1 5, comprising additives selected from pigments, extenders, thickeners, dispersing agents, wetting agents, defoamers, solvents and plasticisers.

1 7. An aqueous film-forming surface coating composition substantially as hereinbefore described with reference to the foregoing Examples.