City Research Online

Abstract

Plastisols constitute a large proportion of the coated steel strip market. A plastisol comprises a dispersion of polymer, generally poly(vinyl chloride) produced by aqueous emulsion or suspension polymerisation processes, that is subsequently dried and redispersed in an essentially non-volatile diluent plasticiser such as di(2-ethylhexyl) phthalate. Once applied, the plasticiser swells the polymer on heating so forming a tough and flexible coating. For environmental reasons, there is interest within industry in acrylic plastisols. However, acrylic plastisols gel prematurely at room temperature. The stability of vinyl plastisols is generally thought to be due to the presence of microcrystalline domains present in poly(vinyl chloride), and not present in amorphous acrylic polymers, retarding the swelling action of the plasticiser.

Improved plastisol stability was achieved using acrylic polymers of uniform particle size prepared by non-aqueous dispersion and aqueous emulsion polymerisation techniques with thermally reversible cross-links analogous to the microcrystalline domains present in polyfvinyl chloride). Thermally reversible cross-links were provided by hydrogen bonding and metal ion crosslinking. Novel procedures were developed for the non-aqueous dispersion polymerisation of mixtures of polar monomers and the preparation of polymers cross-linked by metal ions. Further improvements in acrylic plastisol stability were seen on application of the principle of osmotic deswelling. Plastisol stability was measured viscometrically.

Plastisols based on copolymers of methyl methacrylate and methacrylic acid or 2- hydroxyethyl methacrylate, and butylbenzyl phthalate showed improved stability compared to those based on poly(methyl methacrylate). This was attributed to hydrogen bonding. Polymers were prepared with high levels of methacrylic acid and 2- hydroxyethyl methacrylate, reaching 10% and 15% by weight respectively. Methyl methacrylate - methacrylic acid copolymers were also prepared with a range of particle sizes. It was observed that the stability of plastisols prepared from these polymers improved with increasing particle size. This was related to the surface to volume ratio of the polymer particles. Concentrating the methacrylic acid component into the surface region of a methyl methacrylate - methacrylic acid copolymer, using a variable composition seed and feed method of preparation, also rendered the polymer less susceptible to swelling by plasticiser. Improved stability was also seen with plastisols based on carboxylic acid containing polymers which had been neutralised with organometallic compounds such as diethyl zinc compared to unneutralised controls.

A theory for osmotic limitation of swelling of crosslinked polymers by strong solvents was presented. The mechanism relies on an osmotic force promoted by the presence of a low molecular weight polymer dissolved exclusively in the solvent phase tending to deswell the cross-linked polymer. The applicability of this theory to the problem of stabilising acrylic plastisols was tested. Plastisol stability was improved on addition of low molecular weight poly(butyl acrylate) and poly(methyl methacrylate) to the plasticiser phase. Experimental evidence suggested that there was some penetration of poly(butyl acrylate) into a 95:5 (wt) methyl methacrylate - methacrylic acid copolymer. Better exclusion of poly(butyl acrylate) from the cross-linked polymer could be achieved, and hence the osmotic deswelling increased, by increasing the molecular weight of the linear polymer.
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Publication Type:	Thesis (Doctoral)
Subjects:	Q Science > QD Chemistry
Departments:	Doctoral Theses

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