Friday, July 23, 2010

Acrylic Acids and Acrylic Esters

Acrylic and methacylic acids can be prepared in a number of ways, one important commercial method being by the hydrolysis of acrylonitrile or methacrylonitrile. The Acids polymerize readily, but do not form stable thermoplastic polymers because they crosslink on heating; the resulting polymers, being water-soluble, are sometimes used as thickeners in aqueous adhesives.

Acrylic esters
The important esters are methyl acrylate, ethyl acrylate, isobutyl acrylate, methyl methacrylate and ethyl methacrylate. They are not normally made from the corresponding acids but from a variety of other starting materials, one of which is usually hydrogen cyanide or its sodium salt. The methacrylates are harder and tougher than the acrylates; all are soluble in a wide range of solvents; chloroform, ethylene dichloride, toluene or mixtures of toluene and acetone being generally preferred for adhesive solutions. The polymers are also soluble in the monomers which are fairly volatile liquids, adhesives can therefore be made by dissolving the polymer in the monomer, and this type of liquid adhesive, wherein the solvent can be polymerized, is important in certain applications, and especially in bonding polymethylmethacrylate sheet. Initial adhesion takes place largely through the tackiness of the dissolved polymer, but if a catalyst such as an organic peroxide is added, the monomer subsequently polymerises. In one patent specification a formulated adhesive is claimed that bonds glass to plastic surfaces, giving “adhesive strength that is higher than the cohesive strength of the glass itself. A practical example of bonding to glass is that in which an acrylic molding is glued to a television tube as a safeguard in case of implosion.

An adhesive consisting of a solution of methacrylic acid and polymethyl methacrylate in methyl methacrylate monomer, with highly active catalysts, is said to cure rapidly at room temperature to give high joint strength on aluminium. A similar adhesive, said to stick well to metal and glass, is similar but incorporates acrylonitrile and styrene together with a catalyst.

A polymerizable acrylate adhesive will bond cyclised rubber to polymethyl methacrylate sheet giving, after fairly prolonged heat curing, a strength of about 900 psi at room temperature.