What Is It?
In its simplest form PS is brittle. Its mechanical properties are dramatically improved by blending with polybutadiene, but with a loss of optical transparency. High impact PS (10% polybutadiene) is much stronger even at low temperatures (meaning strength down to −12 C).
PS comes in three guises: as the simple material (“general purpose ps”); as the high impact variant, blended with polybutadiene; and as polystyrene foam, the most familiar and cheapest of all polymer foams. All are FDA approved for use as food containers and packaging.
General purpose PS is easy to mold. Its extreme clarity, ability to be colored, and high refractive index give it a glass-like sparkle, but it is brittle and cracks easily (think of cd cases). It is used when optical attractiveness and low cost are sought, and the mechanical loading is light: cosmetic compacts, transparent but disposable glasses, cassettes of all kinds. Medium and high impact polystyrenes trade their optical for their mechanical properties.
Medium impact PS, translucent, appears in electrical switch gears and circuit breakers, coat hangers, and combs.
High impact PS – a blend of PPO and PS – is opaque, but is tough and copes better with low temperatures than most plastics; it is found in interiors of refrigerators and freezers, and in food trays such as those for margarine and yogurt.
PS can be foamed to a very low density (roughly one-third of all polystyrene in foamed). These foams have low thermal conduction and are cheap, and so are used for house insulation, jackets for water boilers, insulation for disposable cups. They crush at loads that do not cause injury to delicate objects (such as tv sets or to the human body), making them good for packaging.
Disposable cups; light fittings; toys; pens; models; in expanded form – packaging, thermal insulation, and ceiling tiles; tv cabinets; wall tiles; disposable dishes; furniture; molded parts and containers; cd covers, disposable glass, razors, hot drink cups.
The flammability of ps foam, and the use of CFCs as blowing agents in the foaming process was, at one time, a cause for concern. New flame retardants allow ps foams to meet current fire safety standards, and CFC blowing agents have been replaced by pentane, CO , or HFCS which do not have a damaging effect on the ozone layer. PS can be recycled. The monomer, styrene, is irritating to the eyes and throat, but none survives in the polymer.
Polystyrene, PS, is — like PE and PP — a member of the polyolefin family of moldable thermoplastics. In place of one of the H-atoms of the polyethylene it has a -benzene ring. This makes for a lumpy molecule which does not crystallize, and the resulting material is transparent with a high refractive index. The benzene ring absorbs UV light, exploited in the PS screening of flourescent lights, but this also causes the polymer to discolor in sunlight.