Tuesday, July 13, 2010

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.

Polystyrene Copolymers
More valuable as adhesive are the copolymers of styrene and butadiene. Butadiene is a reactive gaseous monomer in plentiful supply from oil and natural gas sources. It homopolymerises to form rubbery polymers; it copolymerises with a number of monomers including styrene, and in doing so imparts a rubbery property to the styrene according to the quantity incorporated. Thus the normal brittleness of polystyrene is reduced, even to the point of flexibility in some grades of synthetic rubber by co-polymerising with butadiene. These copolymers are useful adhesives, especially in the form of aqueous dispersion or lattices containing about 50% copolymer. At a monomer ratio of about 50:50 they are general technique is to allow the emulsion to break, creating a tacky surface on each adherent, before assembling. If the ratio of styrene to butadiene is of the order of 60:40 and higher, the copolymer is non tacky and forms the type used as a hot melt adhesives.

Styrene readily forms copolymers with a large number of other monomer including such lesser known ones as methyl vinyl ketone and methylene dimethacrylate, and undoubtedly many or these copolymers have useful properties, but their manufacture for this use alone not be worthwhile.