In 1839, by inserting sulphur atoms as bridges between polymer chains, Charles Goodyear transformed Natural Rubber from an interesting curiosity into a superb engineering material that has had a huge impact on society. It took another 100 years before sulphur links began to appear in thermoplastics. However, natural polymers such as proteins and DNA rely on such sulphur links.
Today sulphur, in the form of sulphide and sulphone links, enhances properties of high temperature stability, chemical resistance and fire performance in thermoplastics such as polyphenylene sulphide (PPS) and polyethersulphone (PES).
PPS, essentially replaces the oxygen links in polyethylene oxide (PPO) with sulphur links in the polymer backbone. It offers excellent dimensional stability at high temperature (particularly when filled), broad chemical and solvent resistance and good fire performance. However, the melt temperature is high (285oC). Also there are some other moulding issues to be aware of. Namely, the low melt viscosity makes it eminently suitable for thin sections.
Unfilled PPS can be used as high performance fibres, for technical textile applications, including filters, flame resistant clothing and advanced composites for the aerospace industry.
Moulding grades of PPS are generally filled with glass fibre and mineral filler to improve stiffness at high temperature. Unlike most other thermoplastics, PPS can accept filler loadings up to 65% to produce very stiff, heat resistant mouldings. My students mistook these for metal castings, even down to the metallic ‘ring’, until the more discerning realised that the mouldings, although heavier than other thermoplastics, were significantly lighter than most metals.
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