Polystyrene

PROPERTIES

Polystyrene (PS) is a clear, amorphous, nonpolar commodity thermoplastic that is easy to process and that can be easily converted into a large number of semi-finished products like foams, films, and sheets. It is one of the largest volume commodity plastic, comprising approximately seven percent of the total thermoplastic market1. PS is a very good electrical insulator, has excellent optical clarity due to the lack of crystallinity, and has good chemical resistance to diluted acids and bases. It is also easy to fabricate into a large number of finished goods since it is a viscous liquid above its glass transition temperature (Tg) that can be easily molded. However, polystyrene has several limitations. It is attacked by hydrocarbon solvents, has poor oxygen and UV resistance, and is rather brittle, i.e. it has poor impact strength due to the stiffness of the polymer backbone. Furthermore, its upper temperature limit for continual use is rather low due to the lack of crystallinity and its low glass transition temperature of about Tg = 373 K (100°C). Below its Tg, it has medium to high tensile strength (35 - 55 MPa) but low impact strength (15 - 20 J/m). Despite all these weaknesses, styrene polymers are very attractive large-volume commodity plastics.

Some of its weaknesses can be overcome by copolymerization with other monomers. For example, polystyrene can be copolymerized with methyl methacrylate. The copolymer poly(styrene-co-methyl methacrylate) (PSMMA) has higher clarity and improved chemical and UV stability.

One of the most important styrene copolymers is poly(styrene-co-acrylonitrile) (PSAN). It has much improved chemical resistance, better heat stability, and improved mechanical properties. However, these copolymers often yield yellow products.

Probably of equal importance are poly(styrene-co-butadiene) (SBR, SBS) and poly(styrene-co-acrylonitrile-co-butadiene) (ABS). Both copolymers have very high stress and impact resistance and ABS has higher tensile strength than pure PS.

To increase the heat resistance, styrene is sometimes copolymerized with small amounts of maleic anhydride or it is copolymerized with this monomer to an alternating structure. The copolymer poly(styrene-co-maleic anhydride) (PSMA) has a higher Tg than pure polystyrene (400 - 430 K), improved heat resistance and high dimensional stability.

Many styrene derivatives have been synthesized on a laboratory scale and some have been extensively investigated. However, no other styrene polymer has become a large-volume commodity thermoplastic. Among those that are commercially produced are α-methylstyrene, o-, m-, and p-methylstyrene, methoxystyrene, chlorostyrene, divinylbenzene and p-divinylbenzene. The later is used as a cross-linking agent in a large number of different polymer materials.

Polystyrene is a not biodegradable plastic and resistant to photolysis. It is a major contributor to the debris in the ocean. Although recycable, polystyrene is not recycled in many parts of the world. The biggest problem is expandable polystyrene (EPS); due to its low density, it takes up a relative large amount of space in landfills.

In recent years, the (food) packaging industry has developed alternative insulating plastics for thermal applications, like Versalite which is an expanded polypropylene (PP) that can be recycled right along with other PP products in the general recycle stream. We expect other lower-cost and lower-density resins to gain market share in traditional large volume applications of expandable polystyrene.