HIGHLY FILLED CROSSLINKABLE EMULSION POLYMER COMPOSITIONS

Abstract: Highiy filled crosslinkable emulsion polymer composi­ tions containing vinyl aromatic monomers and acrylate or methacrylate monomers can be provided with improved reten­ tion of adhesive power in the face of attack by water and im­ proved shelf-life by the presence of zinc ammonium carbonate in the composition. The zinc ammonium carbonate is prefer­ ably in the form of an aqueous solution prepared by reacting an equimolar amount of ammonium carbonate and ammonium hy­ drogen carbonate with zinc oxide and ammonia.

Description: This invention relates to highly filled cross-linkable copolymer emulsion compositions. There are many possible applications where highly filled water based systems would be cheaper and safer to use than materials currently used in such applications, but their employment has been limited by their lack of retention of adhesive power when subject to attack by moisture. Such applications include ceramic tile adhesives, wood fillers, metal fillers, grouts, external coatings, paints, wallpaper, clay coating etc. In most of these applications the ratio of filler to polymer is very high, of the order of 10:1, and attempts to improve the poor adhesion retention of such compositions when applied to substrates subject to attack by moisture have not been particularly successful. In addition, some have a poor shelf-life and tend to gel up on storage after about 1 month. The use of zinc to reduce water-sensitivity in unfilled emulsions such as those used in floor polishes is known (U.K. Specification 1254294). The zinc is introduced into the composition as a water-soluble complex or chelate, for example, zinc carbonate solubilized in ammonium hydroxide. The compositions are formulated so as to be soluble in alkali, and are said to display satisfactory gloss, water resistance, recoatability and removability and it is suggested that the presence of the metal ion renders the coating insoluble in water. Surprisingly we find that the presence of zinc in a highly filled copolymer, conveniently emulsion copolymer composition suitable for use as an adhesive, filling material or coating gives, when applied, a vastly improved retention of adhesive power after attack by water and, before application, a very large increase in shelf-life. The adhesive power of such a composition applied to a substrate which is then immersed in water is several magnitudes greater than for an adhesive composition not containing zinc. It is believed (though the present invention is not based on this theory) that this is due to some interaction occurring between the filler, resin and zinc components. The stability of a composition of the invention as compared with that of a conventional composition is superior, the shelf-life of a composition of the invention being from 6 to 24 months as opposed to about 1 month for a conventional composition. The invention provides an aqueous copolymer emulsion composition which contains (a) above 50% by weight of the total weight of the composition of at least one inorganic filler component, and (b) an aqueous emulsion of a copolymer containing at least one vinyl aromatic monomer, at least one acrylate or methacrylate monomer and from more than 1 up to 10% inclusive by weight of the copolymer of an unsaturated organic acid, for example, (meth) acrylic acid or higher homologue modified by the presence of zinc ammonium carbonate or zinc ammonium hydrogen carbonate or both (such compounds being referred to hereinafter as "zinc ammonium carbonate compounds") in an amount such as to provide from 0.75 to 2 wt. % inclusive of zinc, based on the weight of the aqueous emulsion (b) of copolymer, to permit the formulation of highly filled compositions providing excellent resistance to moisture and good adhesion. The vinyl aromatic monomer in a composition of the invention may, for example, be styrene, vinyl toluene, chlorostyrene or t-butyl styrene and is preferably styrene. The acrylate or methacrylate monomer may, for example, be methyl, ethyl, propyl, butyl, 2-ethyl hexyl and heptyl acrylate or methacrylate. Other unsaturated monomers such as acrylonitrile may be additionally present, preferably in an amount of up to 10% by weight of the total weight of monomers. By "highly filled" we mean composition usually having about 50% to 96% by weight of inorganic filler, more usually above 70%. The unsaturated organic acid may be acrylic, methacrylic or a higher homologue, and it is used in an amount of more than 1% up to 10% by weight based on the copolymer. The zinc ammonium carbonate may take the form of the usual equimolar mixture of zinc ammonium hydrogen carbonate and ammonium carbonate, or other material which functions as zinc ammonium carbonate. The zinc ammonium carbonate compound can be incorporated as an aqueous solution containing 10% by weight of zinc and this solution can be present in the emulsion at levels between 7.5 and 20% by weight based on wet emulsion so as to provide from 0.75 to 2% of zinc based on the weight of the aqueous copolymer emulsion. It is probable that the zinc ammonium carbonate compound acts as a source of ionic crosslinking when the water and ammonia evaporate from the composition, thus reducing its water sensitivity. It also greatly improves the adhesion of the filled composition to a variety of substrates such as mild steel, brick, asbestos cement etc. A suitable zinc ammonium carbonate solution can be made by dissolving zinc oxide in a mixture of ammonium carbonate and ammonia (of specific gravity 0.880) and adjusting the zinc concentration to the desired range with water. The preparation of an aqueous solution of a zinc ammonium carbonate compound and preferred compositions modified thereby will now be described with reference to the following Examples, which include an example of a known composition (Example 3) for comparison. Preparation of Zinc Ammonium Carbonate Dissolve 14g ammonium carbonate (actually an equimolar mixture of ammonium hydrogen carbonate and ammonium carbonate) in 56 g de-mineralized water. Disperse 14 g zinc oxide in the solution and add 15 g ammonia (0.880) with stirring. Continue stirring until a clear solution is obtained and add sufficient demineralised water to obtain a Zn content of 10%. Example 1 A ceramic tile adhesive was made by mixing the following components:- When tested to B.S. 5385 Part 1: 1976: Section A5.8 adhered tiles had a shear strength after 7 days water immersion of 1.0-1.75 MPa. Moreover, when tiles were stuck biscuit to glaze with the above composition and immersed in water the shear strength was still above the 0.4 MPa minimum requirements. Example 2 When a styrene/acrylate copolymer emulsion containing only 1% acrylic acid was used to make a ceramic tile adhesive as described in Example 1 the shear strength of the bond when tested after water immersion was below 0.3 MPa. Example 3 When a ceramic tile adhesive was made as in Example 1 excluding the "zinc ammonium carbonate" the shear strength when tested after water immersion was zero. Example 4 A ceramic tile adhesive was made by mixing the following compositions:- The shelf-life of this adhesive is from 6-12 months. When tested as in Example 1 its shear strength was 0.9 MPa.

Claims: 1. An aqueous copolymer emulsion composition which contains (a) above 50% by weight of the total weight of the composition of at least one inorganic filler component and (b) an aqueous emulsion of a copolymer containing at least one vinyl aromatic monomer, at least one acrylate monomer selected from acrylates and methacrylates and from more than 1 up to 10% inclusive, by weight of the copolymer of an unsaturated organic acid, characterized in that the composition is modified by the presence of (c) a zinc ammonium carbonate compound in an amount such as to provide from 0.75 to 2 wt. % inclusive of zinc, based on the weight of the aqueous emulsion (b) of copolymer. 2. A copolymer emulsion composition according to claim 1, wherein the amount of filler component is above 70% by weight of the composition. 3. A copolymer emulsion composition according to claim 1 or claim 2, wherein the zinc ammonium carbonate compound is present in an aqueous solution. 4. A copolymer emulsion composition according to claim 3, wherein the solution is the product of an equimolar mixture of ammonium carbonate and ammonium hydrogen carbonate reacted with zinc oxide and ammonia. 5. A copolymer emulsion composition according to claim 3 or claim 4, wherein the zinc is added to the composition in the form of an aqueous solution containing 10% by weight of zinc. 6. A copolymer emulsion composition according to any one of the preceding claims, wherein the vinyl aromatic monomer is selected from the group consisting of styrene, vinyl toluene, chlorostyrene and t-butyl styrene. 7. A copolymer emulsion composition according to any one of the preceding claims, wherein the acrylate monomer is selected from the group consisting of methyl, ethyl, propyl, butyl, 2-ethyl hexyl and heptyl acrylates and methacrylates. 8. A copolymer emulsion composition according to any one of the preceding claims, wherein the copolymer additionally includes at least one further unsaturated monomer. 9. A copolymer emulsion composition according to claim 8, wherein the said at least one further unsaturated monomer is present in an amount by weight of the total amount of monomers of up to 10%. 10. A copolymer emulsion composition according to any one of the preceding claims wherein the copolymer is an emulsion copolymer. 11. Use, as a ceramic tile adhesive, of an aqueous copolymer emulsion composition having improved retention of adhesive power in the face of attack by water and increased shelf life, and containing the components (a) and (b) as defined in claim 1, characterized in that the said ceramic tile adhesive is an aqueous copolymer composition according to any one of the preceding claims.

Inventors: PARKER, ROBERT GEORGE
Applicants: SCOTT BADER COMPANY LIMITED
Application year: 1981
Main classification: C08L25
IPC codes: C08K3