CA1288537C - Polymer latices and paint formulations - Google Patents

Abstract

ABSTRACT
Aqueous polymer emulsions suitable for preparation of high wet adhesion latex paints comprise an emulsified film-forming copolymer of a first, hard monomer such as styrene, a second, soft monomer such as butyl acrylate, a wet adhesion monomer and an acidic copolymerizable dry-adhesion promoter, the copolymer having a minimum film forming temperature of about 25°C. The emulsions are prepared by free radical polymerizations, in latex, by procedures specially adapted to avoid mutual neutralization of the monomers prior to polymerization.

Description

~2~85~37 rrhis invention relates to polymer emulsions, coating compositions and methods for their preparation. More particularly, it relates to latex (water-based) coating compositions, polymers for use in such compositions and their preparation.

~ aints or coatings for household and similar uses are normally either oil-based or water-based. The choice between the use of an oil-based paint or a water-based paint normally depends upon the nature and condition of the substrate to which the paint is to be applied, the appearance which is to be presented by the painted surface, and the environmental conditions to which the painted surface is to be subjected. In general, oil-based paints are capable of providing hard, high gloss, highly water resistant finishes which are readily washable and wipeable for cleaning purposes. They are not, however, suitable for application to porous, damp surfaces.
Water-based paints or latex paints may have a greater range of suitable applications, but to date have not been capable of providing very high adhesion to glossy surfaces under warm moist conditlons. Because of their relative ease of handling, cleaning, diluting, etc., and the reduction in relative amounts of obnoxious and toxic components, latex paints are the first choice of most applicators, for use in applications where they are technically satisfactory. There is, accordingly, a significant practical incentive to extend the range of applications, and generally improve the all-round technical performance of latex paints.

Oil-based paints are commonly solutions or dispersions in a solvent of an appropriately soluble synthetic resin, pigments and other improvers. Commonly the resin is a synthetic oil-modified alkyd resin derived, for example, from glycerol and other alcohol-containing compounds along with phthalic anhydride and saturated and unsaturated acids. Latex paints, on the other hand, are emulsions of film forming resins along with dispersed : ~ - :
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' ~igments, appropriate surfactants, coalescents, stabilizers and the like, to maintain the integrity and stability of the latex prior to use. After application, the water substantially completely evaporates, to leave behind a continuous pigmented resin film. The resins are normally copolymers of a hard monomer (i.e. one whose homopolymer has a minimum film temperature above normal room temperature) and a soft monomer (i.e. one whose homopolymer has a minimum film temperature below normal room temperature).

The minimum film temperature is the lowest temperature at which the dried film appears to be clear and crack-free. It may be measured using an apparatus similar to that described by T.F. Protzman and G.L. Brown in Journal of Applied Polymer Science, Volurne ~, pages 81-85, 1960. An example of such a film-forming copolymer is styrene-butyl acrylate copolymer, where styrene is the hard monomer and butyl acrylate is the soft monomer, as defined above.

One specific end use for which improved latex paints are required is for the application of such coatings over painted surfaces, especially in environments such as kitchens, bathrooms, shower stalls, laundries, etc., where the dried latex coating may be periodically wetted, or at least moistened.
Problems of adhesion of the latex paint to the substrate painted surface are aggravated if the substrate which is to to painted with the latex coating has a glossy or a chalky surface. It has heretofore proved difficult to provide a latex paint with a sufficient degree of wet adhesion, i.e. ability to adhere promptly after application and subsequently under moist conditions, along with a high degree of adhesion under dry conditions.

It is known in the art to emulsify a drying oil and add this emulsion to a latex paint in order to provide additional adhesion of a water-based coating to surfaces that have already ' ' . ', , ~ ' :
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~ ~ r.~ 7 been covered with an alkyd paint. Alternatively, an alkyd resin may itself be emulsified and added to the latex coating, for the same purpose. Both expedients require the add:ition of a drier to cause the eventual air-induced crosslinking oE the drying oil or alkyd. Otherwise, the dried film will be soft, tacky and tend to pick up dirt. Such driers may interfere with the colour acceptance of the water-based paint and the expedients mentioned are, therefore, of limited use.

U.S. Patent 3,194,792 discloses the incorporation into copolymers of certain unsaturated d:icarboxylic acid monoesters of N-(hydroxyalkyl)- or l~-(hydroxyoxaalkyl)--1,3-cyclodiazolidin-2-one. Such copolymers are claimed to have outstanding adhesion to metals, glass and plastics. ~et adhesion to pre-painted substrates is not taught in this case, however.

U.S. Patent 3,356,654 discloses film forming aqueous emulsions of copolymers that exhibit good adhesion to glossy or chalky surfaces, and improved adhesion to surfaces previously coated with oil based paints, as well as improved water resistance. More particularly, the patent shows the use of copolymers of monoethylenically unsaturated materials which contain a hydroxy derivative of certain unsaturated amino acids in combination with other monoethylenic materials providing oxirane functionality. The presence of oxirane functionality is crucial to this teaching.

U.S. Patent 3,652,480 relates to vinyl ester based copolymer emulsions and particularly to those based on vinyl acetate. This patent shows the incorporation of a minor amount of the ammonium salt of an unsaturated carboxylic acid, an alkylene imine to iminate the copolymer, and ammonia as a particular base to maintain the pH of the emulsion in a specified alkaline range.

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~ ~3~537 ~ .S. Patent 3,300,~29 is concerned with blended aqueous dispersions having improved penetration and/or adhesion to various substrates, including powdery or cnalky surfaces and oil paint or alkyd surfaces, and wet adhesion in general. The mixture revealed comprises (1) an aqueous copolymer emulsion (2) a water-soluble ammonium salt of a low molecular weight acid copolymer and (3) dispersing agents. One or both of components (1) and (2) contain polymerized units containing a ureido group. For most purposes, components (1) and (2) are capable of forming films under ambient conditions.

~ .S. Patent 3,369,008 shows the incorporation of N~(cyclic ureidoalkyl) crotonamides in polymers to enhance the adhesion of coatings to many substrates.

European Patent Application 80302150.0 shows sequential emulsion polymerization in an aqueous medium with stress on the incorporation of a copolymerizable acid monomer.

It is an object of the present invention to provide a novel polymer latex.

It is a further object of the present invention to provide a novel process for preparing polymer latex formulations, suitable for preparing paints.

According to the present invention, from one aspect, there is provided a novel aqueous polymer emulsion suitable for making a latex paint formulation which exhibits good wet adhesion properties when applied over a glossy, alkyd painted surface, especially during its early drying stages. The paint formulation also exhibits other desirable attributes needed in a satisfactory paint, such as good hiding power, controlled permeability, colour fastness, ease of application, etc. The emulsion of the present invention includes an aqueous-based medium, and an emulsified, film-forming resin which is a : ' ~ " ' .
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The wet adhesion monomer used in the formulation of the present invention is suitably a nitrogen-containing organic compound having free radical polymerizable vinyl or allylic unsaturation to allow its incorporation into the copolymer structure. The nitrogen functionality may be provided by amine or imine groups or by urea structures or as part of a nitrogen-containing aromatic compound such as vinyl pyridine.
An alkyl ureido compound is preferred. A specific example of a suitable wet adhesion promoting monomer is Sipomer WAM , sold by Alcolac Inc. The mode of action of the wet adhesion promoter monomer is not clearly understood but it is thought perhaps to function by adhering to the pigmented portion of the substrate coating. Only small amounts of wet adhesion promoter need to be included in the formulation, e.g. 0.1-5.0 weight percent and preferably 0.1-1.0 weight percent, based on the total weight of monomers present.

The acidic copolymerizable dry adhesion promoter, in addition to conferring good dry-adhesion on the resultant copolymer film, also acts as an emulsion stabilizer, and serves to prevent coagulation of latex during its preparation by polymerization. Suitably, this monomer is a free radical copolymerizable carboxylic acid, chosen to be copolymerizable at reasonable rates with the monomers comprising the remainder of the copolymer. When styrene is chosen as the hard monomer, as is common, acrylic acid is preferred. It is used in rela-tively small amounts, also 0.1-5.0 percent by weight based upon the total weight of monomers present. The acrylic acid performs well as a dry adhesion monomer, promoting the adhesion of the copolymeric film derived from the latex, to the substrate.
Alternative acid monomers that may be used include acryloxypropionic acid, monomethyl acid itaconate, methacrylic * Trade Mark - . . .
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The hard monomer and the soft monomer used in the latex of the present invention are those commonly used in film forming copolymers for latex paint formulation. The hard monomer is one whose high molecular weight film forming homopolymers have a minimum film temperature of at least 80C. Specific examples of such monomers include styrene, methyl methacrylate, alpha-methylstyrene, lower alkyl substituted styrenes such as vinyl toluene and acrylonitrile. ',tyrene is preferred.

The soft monomer is one whose high molecular weight homopolymers have a minimum film temperature of about 0C or less. Specific examples of such monomers include butyl acrylate, ethyl acrylate, isobutyl acrylate, the various ethyl hexyl acrylates, butadiene, isoprene, lauryl methacrylate, and similar long chain materials, and combinations thereof. Dibutyl maleate may also be used in such combinations. Butyl acrylate is preferred.

A feature of the preferred embodiment of the present invention is the use of an excess of hard monomer over soft monomer in the polymerization recipe so that the minimum film temperature of the latex copolymer is equal to or higher than 25 C. This is in contrast to many conventional latex paint formulations which are based on latices that form films at room temperature.

The general procedures for preparing latex formulations according to the present invention are in accordance with standard emulsion polymerization processes, but with special techniques so that the necessary degree of wet adhesion can be obtained.

'7 Thus the present invention provides a process for preparing a synthetic polymer latex suitable for preparation of high wet adhesion water based coatings which comprises copolymerizing together, in aqueous medium in the presence of at least one appropriate surfactant and an appropriate free radical polymerization system, a hard monomer, a so-Et monomer, a nitrogen-containing copolymerizable wet adhesion monomer and a copolymerizable dry adhesion promoting monomer, said process including the steps of initially preparing a pre-emulsion of the hard monomer, the soft monomer, the dry adhesion promoting monomer and appropriate surfac-tant and optionally a free radical initiator, in water, separately preparing a mix-ture of water and redox polymerization system, adding to said mixture a small quantity of said pre-emulsion and initiating the polymerization therein, adding to the mixture the wet adhesion monomer and continuing polymerization in said mixture, and subsequently adding the remaining quantity of said pre-emulsion to the mixture gradually as polymerization proceeds.

Preferably, the small quantity of pre-emulsion initially added to the mixture constitutes about 0.5-2~ of the total quantity thereof.

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3~3537 ~ Ihe polymerization is initiated with the aid of a free radical initiator, such as an inorganic or organic peroxide, or peroxy catalysts such as persulfates or azo catalysts. In the emulsion polymerizations of this invention, a water soluble initiator is preferred and a redox system is particularly effective. Here the initiator may be an organic peroxide such as tertiary butyl hydroperoxide or an appropriate inorganic compound such as hydrogen peroxide, ammonium persulEate, sodium persulfate or potassium persulfate. The persulfates are preferred, in amounts of about 0.3 percent by weight of the total monomers used. The initiator may be used in conjunction with a reducing agent such as an alkali metal sulfite, bisulfite or metabisulfite, or hydrosulfite or hydrazine. Sodium metabisulfite is preferred, in a weight ratio of about 0.4 parts sodium metabisulfite to 1 part sodium persulfate.

The molecular weight distribution of the polymer formed in the emulsion polymerization may be controlled through use of a chain transfer agent, such as a mercaptan, mercaptoethanol or carbon tetrachloride. However, the preferred embodiment of this invention does not employ such regulators.

A special procedure in the emulsion polymerization relates to the fact that the wet adhesion promoter monomer is basic, whereas the dry adhesion promoting monomer is acidic, so that special steps should preferably be taken to prevent their premature mutual neutralization, which would inhibit their incorporation into the copolymer as independent moieties where each can exert its own beneficial action~ To achieve this, in the preferred process of the present invention, there is adopted a process for preparing a synthetic polymer latex of a hard monomer, a soft or plasticizing monomer, an acidic copolymerizable dry adhesion promoter monomer and a basic wet adhesion monomer, which comprises initially preparing a pre-emulsion of the hard and soft monomers, a peroxidic initiator such as tertiary butyl hydroperoxide, and the acidic : . . . .

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dry ac,hesion promoting rnonomer, and preferably raising the pH of this pre-emulsion to at least 5, e.g. with NH40H. There are prepared three separate aqueous solutions, the first comprising the basic nitrogen-containing wet aclhesion promoter, the second comprising a water soluble free radical initiator such as an inorganic peroxidic initiator, and the third comprising a reducer. A seed emulsion of a small quantity of the aforementioned pre-emulsion, along with appropriate initiator, reducing agent and surfactant in small amounts, is prepared and polymerization is initiated therein. There is gradually added to the seed so formed the rest of the aforesaid pre-emulsion, along with separate additions of initiator, reducer and wet adhesion promoter monomer, i.e. the three water solutions referred to above. In this way, mutual neutralization of the acidic and basic monomers is substantially avoided, and use of excess quantities thereof to allow for such mutual neutralizatiGn is minimized.

~ hus, the polymerization vessel initially contains a very small seeding amount of latex polymer made from the pre-emulsion of monomers and surfactants, along with a quantity of water, and to the vessel are fed three separate reactant streams, namely the remainder of the pre-emulsion, the initiator solution and the reducer solution, e.g. sodium persulphate and sodium metabisulphite. It is also preferred to add all ingredients under an atmosphere of nitrogen, and conduct the polymerization under nitrogen, at a temperature of, for example 60-65C, for a period of 3 1/2-~ hours.

During this preferred polymerization process, the gradual addition of the pre-emulsion to the polymerization vessel finishes first, and then the gradual addition of the initiator/catalyst finishes, and finally the gradual addition of the reducer is finished. By conducting the process in this preferred way, the polymerization reaction kinetics are matched with the rate of addition of reactants to minimize the amounts _g _ ., - ~ ~ .
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~ 3~337 ol- residual monomers in the latex formulation. The resu]tant pro~uct is filtered, to obtain a latex having a 50-55 percent soli~s content, which is normally satisfactory for use in the preparation of paint by addition thereto of water disperslons of pigments containing surfactants in the normal way, without any further treatment. The viscosity and rheology oE the paint may also be adjusted in the usual way, e.g. by addition of thickeners and surfactants or soaps, for better stabilization of the final formulation.

A ~urther special procedure in the preferred process oE
the present invention relates to surfactant selection. In the final latex, there should preferab:Ly be present both an anionic surfactant and a non-ionic surfactant, for best polymerization of the monomer. Such a choice of mixed surfactant, along with proper choice of catalyst and polymerization temperature, ensures the production of a stable latex and the substantially complete absence of residual monomers such as styrene and butyl acrylate from the final latex, the presence of which would make the use of the product obnoxious. Suitable anionic surfactants include dodecyl benzene sulphonate sodium salt and the like.
Suitable non-ionic surfactants include ethoxylated alkyl phenols, where the alkyl group is nonyl, octyl, decyl or the like. In the most preferred procedure, as noted above, a pre-emulsion is formed of the hard and soft monomers, the acidic monomer and an organic hydroperoxide and the full amount of both surfactant types is used in this pre-emulsion. In addition, the initial charge to the kettle, to form a seed polymer, contains a high concentration of sodium dodecyl benzene sulfonate. This is the only step in the process in which dodecyl benzene sulfonate is used. It is intended to produce a very small weight of seed polymer in the form of many particles. If less surfactant than that prescribed is used in the seed phase of the polymerization the final latex may exhibit inadequate shelf and mechanical stability and, surprisingly, may be characterized by an objectionable residual monomer odour.

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~ ~3~37 Specific preferred embodiments of the inven~ion are illustrated in the following specific, non-limiting examples.
In the examples, after the paint forrnulation has been prepared as described below~ it is tested for wet adhesion by four tests, namely a "wet scrub" test, a "wet pull" test, a "tape failure"
test, and a standard water vapour transmission test.

In the wet scrub test, a high gloss alkyd enamel is applied to a Leneta P-121-lON dull plastic panel, using a 3 mil drawdown bar. The alkyd coating is allowed to dry Eor 7 days under ambient conditions. Latex coatings are then applied over the dried glossy alkyd film, again using a 3 mil drawdown bar.
All coatings are allowed to dry for 7 days before testing.

I'he test is run on a Gardner Scrub Machine using a 1 lb. nylon bristle brush with a soap solution. The machine is run at 39 cycles per minute. The brush is soaked in tap water for 1 hour before the test. Ten grams of the scrub medium (detergent + water) are applied to the brush and 5 grams of the medium are applied to the panel in the brush path at the beginning of the test. Results are recorded in terms of complete or partial failures, peeling off of entire strips or breaking into flakes, etc. and the number of cycles when any observation of significance occurs.

In the weighted pull test, the substrate is fir plywood. Plywood panels are primed with one coat of an acrylic latex primer and dried 24 hours. One coat of an alkyd high gloss enamel is applied by brush and let dry 24 hours. This is followed by the application of a second coat of alkyd, which is dried for 7 days prior to use. Then 1-5 grams of test latex paint are applied with five strokes of a 2 inch brush. A piece of cheesecloth is applied. This cloth is 2 inches wide with 4 single layers leaving a 1 inch overhang. The cheesecloth is saturated with an additional 5-8 grams of test paint. The whole is dried at ambient temperatures for 7 days.

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:: , ' : ' . ' - . ' 3S3'7 The actual weighted pull test procedure is as follows:

(a) Make two slits down the test area, 1 inch apart, with a razor blade. Penetrate the alkyd substrate.

(b) Cover the test area with soaking wet cheesecloth for 1 hour.

~ c) Attach test panel to ring stand slanting forward at about 10 from the perpendicular.

(d) Separate 1/2" of test strip from the overhang and attach a slotted 50 gram hanger to the cheesecloth.

(e) Increase weight in 50 gram increments until a rate of separation of 10-20 mm/min. is attained.

(f) Report the amount of weight in grams required to attain the separation and the mode of failure, e.g. cohesive, adhesive or substrate.

In the tape failure test, the crosshatch tape adhesion test is as per CG-SBl-~-71, method 135-8. This involves applying the paint under test to an alkyd painted substrate surface, allowing it to dry, applying pressure sensitive adhesive tape (nScotch~ tape) to it, and peeling off the adhesive tape to see how much paint is removed along with the tape.

The water vapour transmission test is conducted according to a standard ASTM procedure, namely ASTM E96 "Water Vapour Transmission of Materials"r method 3.1.

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35.~7 ~xample 1 A latex was prepared according to the following procedure. A mixture of monomers was made comprising 427.5 grams styrene, 355.5 grams butyl acrylate and 7.95 grams acrylic acid. This is mixture (1). Also, a surfactant-initiator mixture was prepared, (mixture (2)), comprising 274 grams distilled water, 30.0 grams Sipex EST 30 (about 30 percent active), 27.9 grams Siponic OP40 (about 70% active) and 1.35 grams tertiary-butylhydroperoxide (about 70 percent active).
Here, Sipex EST 30 is a product of Alcolac Inc. It is believed to be a surfactant produced by ethyoxylating tridecyl alcohol with about 3-5 moles of ethylene oxide and then converting the terminal hydroxyl to the sodium salt of a chlorosulfonic acid.
Siponic OP40, a product of the same supplier, is the addition product of octylphenol with about 40 moles ethylene oxide.

A pre-emulsion was prepared by slowly adding mixture (1) to mixture (2) with very vigorous stirring. The pH of the pre-emulsion was adjusted to be at least equal to 5.0 by adding 28 percent aqueous ammonia.

An initial charge to a 2 litre reaction kettle was made by dissolving 1.5 grams Siponate DS-10 and 1.20 grams sodium persulfate in 262.2 grams distilled water. This is mixture (3). Here DS-10 is sodium dodecyl benzene sulfonate, product of Alcolac Inc. The contents of the kettle were heated with gentle stirring under a nitrogen purge until their temperature reached 63 +1C.

Sodium metabisulfite, 0.45 grams, was dissolved in mixture (3) in the reaction vessel and the kettle was then charged immediately with 26 grams of the pre-emulsion mentioned above. This reaction, which produces a seed polymer, is allowed to continue for 15 or 20 minutes. Note that a very large concentration of surfactant is used here in the seed preparation 5~37 reaction and that the sodiurn metabisulfite is added only shortly before the seed reaction is begun.

While the seed polymerization is proceeding, the following three solutions are prepared. These are:
- (a) 7.95 grams of Siponic WAM in 11.50 grams distilled water. Siponic WAM is a product oE Alcolac Inc. and is stated to be an allyl ureido compound;
(b) 1.20 grams of sodium persulfate in 41.25 grams distilled water (initiator); and (c) 3.00 grams sodium metabisulfite in 42.0 grams distilled water (reducer).

When the seed polymerization has been completed, the aqueous WAM solution was added to the reaction vessel. Then, addition of the pre-emulsion was started along with separate additions of the initiator and reducer solutions. The total reaction time is 3-3.5 hours with the pre-emulsion addition finishing first and the addition of solution (c) being completed some 10-15 minutes after the rest of the ingredients have all been added. The temperature was maintained at 63 +1C under a nitrogen atmosphere during this reaction.

The temperature was raised to 70C and the reaction mixture was stirred for at least an hour under nitrogen. It was then cooled to 25C and the pH was adjusted to 7.5 using 28 percent aqueous ammonium hydroxide. The emulsion was filtered through a 150 mesh screen. The final concentration was 52 percent solids.

A latex paint, according to the present invention, was made from the following formulation, using the latex described above:

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r,,~3537 SEMI-GLOSS LATEX PAINT

MILL IMP.
BASELBS. GALS. MATERIAL
30.0 3.0 Water 50.0 4.5 Ethylene Glycol 2.0 0.2 Preservative (Cosan PMA30 or equiv.) 10.5 1.0 Disperant I (Tamol 731 or equiv.) 1.0 0.1 DeEoamer (Colloid 643 or equiv.) 260.0 6.3 Pigment (Rutile Titanium Dioxide) 353.5 15.1 LET
DOWN307.0 30.7 Water 98.0 9.5 Opaque Polymer (R&H OP62 or equiv.) 2.5 0.3 Dispersant II (Aminometh~l Propanol) 434.0 41.8 Copolymer Latex (C.Y.W. 8-20) 26.0 2.6 Coalescing Aid (Texanol or equiv.) 2.0 0.2 Nonionic Surfactant (Nonylphenol Ethoxylate)3 3.0 0.3 Defoamer (Colloid 643 or equiv.) 16.0 1.5 Thickener (Alkali Swellable _ Emulsion)4 TOTAL1,242.0102.0 . _ _ 1 Enamel Grade ... NL2020 or equivalent f` 2 Or equivale~t 3 Triton N-57~or equivalent 4 R&H TT-935 or equivalent ~ ~r~cle ~n~rk ..

- ' .' ' ' ~ ~s3~35~7 ~ he paint 50 formed was subjected to the abrasive scrubbing test previously described, and the results thereof compared with those obtained by subjecting to the same test procedure a sample of a paint based on a commercially available latex recommended and promoted on the basis of its wet adhesion properties. This latex had previously been found to be the best commercially available latex in this regard. With the paint formulation prepared according to this example and in accordance with the invention, virtualy none of the experimental paint film had been scrubbed off, after 2,000 scrubs as described. In contrast/ 400 scrubs under the same conditions had removed substantially all of the competitive latex-based paint from the test sample. When the paint prepared according to this example was compared in this abrasive scrubbing test with an oil-based alkyd paint, which is the accepted industry and commercial standard for wet adhesion paints, the paint of this example was founa to be equally as good. The paint formulation of this example was also subjected to the wet pull test described above, and compared in its results on this test with the aforesaid commercial latex based paint. It was found that the paint prepared according to example 1 had four times the adhesion of the competitive commercial paint.

In the tape failure test described above, the paint, according to example 1, compared very favourably with the competitive commercial paint.

The paint of this invention also exhibited exceptionally good early wet adhesion. When the wet scrub test samples had been dried just 24 hours and subjected to this test the coating based on the commercial latex failed after 5 scrubs (i.e. back and forth motion of the brush), while the coating that used the latex of this invention was intact after 800 scrubs.

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', ~ ' 35;~7 As regards the water vapour transmission test conducted using ASTM E96 (bond paper panel coated with 3 mil of wet latex film in a dessiccator), the sample according to the invention gave a value of 1.14 gm per hour per square meter, intermediate between that of a typical alkyd paint (0.27) and a standard commercial latex paint (2.69), indicating a desirable balance of water transmission capability for both exterior and interior paint uses.

Whilst the compositions o~ the present invention are primarily intended for the preparation of latex coating compositions such as paints, for application to hard surfaces, they are not restricted to such uses. They may also be used in sizes or finishes for leathers, textiles, etc., where they may or may not be pigmented.

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Claims (21)

1. An aqueous polymer emulsion suitable for preparation of a high wet adhesion latex coating compositions, and comprising:

an aqueous base medium;

a surfactant emulsifying system;

and an emulsified film-forming copolymer of a first, hard monomer, a second soft monomer, a wet adhesion monomer and an acidic, copolymerizable dry-adhesion promoting monomer, said copolymer having a minimum film-forming temperature of about 25°C.

2. The emulsion of Claim 1 wherein the hard monomer predominates over the soft monomer in the copolymer, on a weight basis.

3. The emulsion of Claim 2 wherein the surfactant emulsifying system comprises an anionic surfactant and a non-ionic surfactant.

4. The emulsion of Claim 3 wherein the wet adhesion monomer is a nitrogen-containing compound having free radical polymerizable carbon-carbon unsaturation..

5. The emulsion of Claim 4 wherein the wet adhesion monomer is an allyl ureido compound or vinyl pyridine.

6. The emulsion of Claim 4 wherein the acid copolymerizable dry-adhesion promoting monomer is a copolymerizable carboxylic acid, which also serves as an emulsion stabilizer.

7. The emulsion of Claim 6 wherein the acid copolymerizable dry-adhesion promoting monomer is acrylic acid.

8. The emulsion of Claim 7 wherein the hard monomer is styrene.

9. The emulsion of Claim 8 wherein the soft monomer is butyl acrylate.

10. The emulsion of claim 1, claim 3 or claim 6, essentially free from unpolymerized residual hard and soft monomers.

11. A latex paint formulation comprising an aqueous emulsion as claimed in claim 1, claim 3 or claim 6, in admixture with a pigment.

12. A process for preparing a synthetic polymer latex suitable for preparation of high wet adhesion water-based coatings which comprises copolymerizing together, in aqueous medium in the presence of at least one appropriate surfactant and an appropriate free radical polymerization system, a hard monomer, a soft monomer, a nitrogen-containing copolymerizable wet adhesion monomer and a copolymerizable dry-adhesion promoting monomer, said process including the steps of initially preparing a pre-emulsion of the hard monomer, the soft monomer, the dry adhesion-promoting monomer and appropriate surfactant and optionally a free radical initiator, in water, separately preparing a mixture of water and redox polymerization system, adding to said mixture a small quantity of said pre-emulsion and initiating polymerization therein, adding to said mixture the wet adhesion monomer and continuing polymerization in said mixture, and subsequently adding the remaining quantity of said pre-emulsion to the mixture gradually as polymerization proceeds.

13. The process of Claim 12 wherein additional quantities of redox polymerization system are added gradually to the mixture as polymerization proceeds.

14. The process of Claim 13 wherein the surfactant emulsifier comprises a non-ionic surfactant and anionic surfactant.

15. The process of Claim 14 wherein the redox polymerization system comprises an initiator and a reducer, said initiator and said reducer being fed to the polymerizable mixture separately from one another and separately from the pre-emulsion.

16. The process of Claim 12 wherein said small quantity of pre-emulsion initially added to the mixture constitutes about 0.5-2.0% of the total quantity thereof.

17. The process of Claim 12 wherein the amount of wet adhesion monomer used in the polymerization is about 0.1-5.0% by weight based upon the total weight of monomers.

18. The process of Claim 17 wherein the wet adhesion monomer is a nitrogen-containing basic compound having free radical polymerizable carbon-carbon unsaturation.

19. The process of Claim 18 wherein the acid copolymerizable dry-adhesion promoting monomer is acrylic acid.

20. The process of Claim 19 wherein the hard monomer is styrene.

21. The process of Claim 20 wherein the soft monomer is butyl acrylate.