Poly[methyl methacrylate]

Polymethyl methacrylate (PMMA) is a plastic material formed from polymers of methyl methacrylate, the methyl ester of methacrylic acid. It is a thermoplastic polymer. It is also known by the trade names of Plexiglas, Perspex, Amanite, Lucite, Trespex, Vitroflex, Acrivill, Perclax, Limacryl, Crylux, Oroglas, Setacryl, Altuglas.

It is a thermoplastic substance obtained by polymerization of methyl methacrylate. The monomer polymerizes quite easily even at ordinary temperature and conditions, but industrially the reaction is controlled with chain initiators, such as peroxides, in the complete absence of oxygen. Gases carried out, particularly in bulk polymerization, are removed from the system so that the product is free of gas bubbles given its use as a glass substitute.

The polymethylmethacrylate has a light transmission coefficient of 92%, higher than any other plastic material, refractive index 1.49, density 1.18, breaking load of approx. 600 kg/cm2, water absorption of 0.3%; moreover, the flexural, impact and compression strengths are little affected by temperature in the range -60 °C +80 °C. It has good chemical resistance to oils, alcohols, diluted acids and alkalis, excellent temperature resistance and light stability.

After mass polymerization in an autoclave, a semi-finished product is obtained by casting in a mold, which is then refined by cutting or sawing it with traditional tools. In many applications, however, it is preferred to resort to polymerization directly in the mold using as a medium to be polymerized a syrup formed by partially polymerized polymethyl methacrylate or a solution of the polymer in the monomer.

In particular, to form sheets of different thicknesses, a mold is used consisting of two sheets of glass spaced by an elastic medium, for example a tube that makes the seal at the edges; inside is poured the syrup and then the whole is heated in an oven, practically at atmospheric pressure. When polymerization is over, after cooling, the glass sheets are removed and the polymethylmethacrylate sheet, with perfectly parallel faces, is subjected to controlled annealing to detension the article.

For the forming of tubes or solid rounds, cylindrical moulds are used, rotating in the first case, static in the second. For particular applications, where a higher surface hardness is required, copolymerization with methacrylic anhydride is used, which makes the polymeric structure of the part partially cross-linked. The multidirectional hot stretching of polymethylmethacrylate sheets, up to 100-150% elongation, transforms the product from amorphous to crystalline, with the result of greater stiffness and strength.

Leave a Comment