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.

Acrylic Resin Adhesive

The acrylic resins may be considered as belonging to the vinyl family. Compounds with the prefix “acryl” may be divided into two classes: the first comprising those derived from acrylic acid, and the second, acrylonitrile and acrylamide. The term acrylic or acrylate is conventionally reserved for compounds of the former class and polymers derived from them. Acrylonitrile, which is vinyl cyanide, and the acrylamide are so called because they are respectively the precursor and amide of acrylic acid, but neither of these monomers or their polymers are conventionally designated acrylics. The acrylic of acrylate monomers have the general formula.

CH₂=CCH₃COOCH₂

And the most important members of this class are acrylic acid and methacrylic acid (R=H or CH₂ and R’=H) and their esters with the lower aliphate alcohols (R’=alkyl)

Polystyrene

Styrene is a vinyl monomer usually made by catalytic dehydrogenation of ethyl benzene. It polymerises readily to produce polymers with average molecular weights between about 50,000 and 200,000. As an adhesive it is used in solution or as an aqueous dispersion. In the latter form (another example of emulsion polymerization) its use is more restricted than in the former. In both forms it is of course necessary to limit applications to those where at least one of the adherents is porous, and one important example is the sticking of polystyrene tiles to a plaster wall.

Surprisingly, polystyrene sticks well to wood, but not very well to metal nor to plastic materials except polystyrene itself. For bonding the latter, a low molecular weight styrene polymer plus a peroxide catalyst is an excellent adhesive capable of polymerising in the glueline.

Polystyrene does not by itself appear to be much favoured as a hot melt adhesive. Its flow temperature is reasonably low, being of the order of 120-130oC (a value that can be reduced considerably by additives) but it is a highly flammable and somewhat brittle polymer by itself.

Polyvinyl Alkyl Ethers

Polyvinyl alkyl ethers (some of which are elastomers although not generally classified as synthetic rubbers) are further examples of vinyl polymers having valuable adhesive properties. The monomers are prepared from acetylene and an alcohol in the presence of an alkali alcoholate. Polyvinyl ethyl ether, polyvinyl n-buthyl ether and polyvinyl isobutyl ether all range from extremely tacky semi solid to elastomeric solids, and therefore polymer blends can be made having properties that are appropriate to a wide range of adhesive applications. Polyvinyl methyl ether is a highly viscose liquid or a semi solid. The largest use of the polyvinyl ethers is in pressure sensitive adhesives. Polyvinyl stearyl ether is a waxy solid that is sometimes used as a modifier in adhesive compositions.