PS (Polystyrene)

Overview

Readily recognizable due to its range of everyday uses such as CD cases, pens, food packaging, and packaging foam. PS faces competition from paper, glass, wood, PP, PVC, and ABS. The three main types are: general purpose (crystal) PS, high impact PS, and polystyrene foam.

Strengths

Cheap. Low mold shrinkage allowing for precision molding, high dimensional stability. Water-clear and glossy. Relatively stiff and hard for a commodity thermoplastic. Minimal water absorption. Very good electrical and dielectric properties (though poor tracking resistance). Excellent resistance to gamma radiation. Copes well with low temperatures. Good insulator. Useful crush behavior of foamed material.

Limitations

General purpose grades tend to be brittle and susceptible to fracture (also very low elongation at fracture). Very poor wear resistance and high friction, very susceptible to stress cracking, very poor fatigue resistance. Medium and high impact grades are opaque. Negligible mechanical properties above ~80 °C (175 °F). Generally poor chemical resistance and stress crack/craze resistance (though good in water, brine, dilute NaOH, and non-oxidizing acids). Poor UV resistance (UV stabilization possible but has a relatively small effect). Burns easily.

Designation

Polyvinyl Benzene, Poly(1-phenylethylene)

Typical uses

General purpose PS is typically used for short-lived, non-technical applications. Packing chips, toys, light diffusers, beakers, cutlery, general household appliances, video/audio cassette cases, electronic housings, refrigerator liners.

Compositional summary

(CH(C6H5)-CH2)n
Material family	Plastic (thermoplastic, amorphous)
Base material	PS (Polystyrene)
CAS number	9003-53-6

Effect of composition

Fillers and reinforcements are rarely used with pure PS as they have little effect on hardness, brittleness, and stress crack resistance. High impact polystyrene contains butadiene, increasing toughness, suffering increased moisture absorption and reduced electrical properties. PS is used in a very wide range of copolymers such as ABS, MABS, ASA, SAN, and SB. alpha-methylstyrene comonomer gives a higher heat distortion point. Methylmethacrylate gives high transparency. Vinyl carbazole gives a higher heat distortion point but is toxic. AN gives higher stiffness and toughness but increases water absorption, decreases electrical properties and turns the copolymer yellowish. Acrylic acid ester-elastomer, butadiene rubber, isoprene rubber, and EPDM-rubber are impact modifiers causing lower heat distortion temperature, stiffness, and rigidity with increased weatherability. Thermoplastic PUR-elastomer gives increased impact and wear resistance. Syndiotactic PS has a higher melting point, heat distortion temperature, and is stiffer, being considered a high-temperature resistant plastic.

Processing properties

First commercial production

1937

Forming

Excellent for injection molding. Low mold temperatures and high injection speeds cause frozen in stresses and orientation. Impact modifiers increase melt viscosity. Very easy to extrude, rotational mold, blow-mold, vacuum-form, and structural foam-mold, unless glass fibers are present. Glass fibers make these unsuitable.

Machining

Poor machinability. High impact grades less likely to splinter. Low softening temperatures require that the material is cooled during machining.

Joining

Very suitable for bonding, particularly with solvents, e.g. toluene or cyclohexanone. Very suitable for welding by friction (though can be difficult due to britillity), hot gas, hot plate, and ultrasonic (radio freq. not possible). Suitable for laser welding. Low elongation makes snap-fitting difficult except when filled.