In a multiple choice pre-test I used for short courses for industry, some candidates were surprised to find that polystyrene had been around as a commercial material longer than polyethylene, polypropylene and nylon. Today polystyrene still commands 10 percent of the thermoplastics market despite some limiting properties, such as modest impact resistance, poor solvent resistance, yellowing on exposure to sunlight and burning readily with copious smoke. Despite these serious disadvantages, polystyrene had little opposition in the early days and soon became the material of choice for moulders, who liked its ease of moulding, low mould shrinkage, transparency, rigidity and gloss finish. It is hard to believe now that, in the 1940s, polystyrene was used to manufacture ashtrays and even harder to believe that customers bought this highly inflammable product.
Leaving aside the sub-sector of expanded polystyrene, manufacturers addressed some of the shortcomings of the solid, general-purpose polystyrene grades (GPPS). The heat distortion temperature (HDT) of GPPS falls tantalising close to the significant 100oC mark but manufacturers soon worked out that increasing the molecular weight (chain length) and reducing polymerisation residues could squeeze a few more degrees, critical for hot-fill, food packaging applications. Even more impressive ‘high heat’ grades were achieved by incorporating alpha methyl styrene as a comonomer.
Impact resistance was significantly increased through incorporation of small rubbery particles to give high impact polystyrene (HIPS) (toughend polystyrene sounds a bit less boastful) but transparency, rigidity, electrical properties and water resistance had to be sacrificed. Styrene butadiene copolymers (SB resins) provided toughness without losing transparency, ideal for blister packaging. Styrene-acrylonitrile copolymers (SAN) offer a good combination of transparency, toughness, HDT and chemical resistance. More recently copolymers of styrene and maleic anhydride (SMA) yield grades with better HDT, the most significant being the glass fibre filled grades, with HDT around 120oC, designed to compete with engineering thermoplastics.
Is there more life to come in the field of styrenic polymers?