High-impact polystyrene solvent resistance

A wide variety of well-known polymers are currently rubber modified, always with the intent of improving the toughness of the material either at ambient temperature, or, often, at sub-ambient. Most well known is high impact polystyrene (HIPS) which, in terms of composition, is polystyrene containing 5-10% polybutadiene rubber. ABS (acrylonitrile/butadiene/styrene) is similar to HIPS except the glassy polymer is the more heat and solvent resistant poly(styrene-co-acrylonitrile). Poly(vinyl chloride) (PVC), polypropylene (PP), epoxy resins, and nylons are all available in rubber-... 

Of the economical plastics mentioned above, GP polystyrene is becoming less popular, as it is one of the most brittle plastics unless it is impact modified. GP or (GPPS) polystyrene has good clarity, is highly permeable (compared with most other plastics) to moisture and gases, has poor heat and solvent resistance. It crazes then disintegrates in contact with isopropyl myristate (which is used in some pharmaceutical formulations). It is, however, an excellent material to mould (low shrinkage)... 

Union Carbide (34) and in particular Dow adopted the continuous mass polymerization process. Credit goes to Dow (35) for improving the old BASF process in such a way that good quality impact-resistant polystyrenes became accessible. The result was that impact-resistant polystyrene outstripped unmodified crystal polystyrene. Today, some 60% of polystyrene is of the impact-resistant type. The technical improvement involved numerous details it was necessary to learn how to handle highly viscous polymer melts, how to construct reactors for optimum removal of the reaction heat, how to remove residual monomer and solvents, and how to convey and meter melts and mix them with auxiliaries (antioxidants, antistatics, mold-release agents and colorants). All this was necessary to obtain not only an efficiently operating process but also uniform quality products differentiated to meet the requirements of various fields of application. In the meantime this process has attained technical maturity over the years it has been modified a number of times (Shell in 1966 (36), BASF in 1968 (37), Granada Plastics in 1970 (38) and Monsanto in 1975 (39)) but the basic concept has been retained. 

Synthetic rubbers, discussed in Section 15.16, are often copolymers chemical repeat units that are employed in some of these rubbers are shown in Table 14.5. Styrene-butadiene rubber (SBR) is a common random copolymer from which automobile tires are made. Nitrile rubber (NBR) is another random copolymer composed of acrylonitrile and butadiene. It is also highly elastic and, in addition, resistant to swelling in organic solvents gasohne hoses are made of NBR. Impact-modified polystyrene is a block copolymer that consists of alternating blocks of styrene and butadiene. The rubbery isoprene blocks act to slow cracks propagating through the material. 

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