Ebonite

Ebonite is a brand name for very hard rubber first obtained by Charles Goodyear by vulcanizing natural rubber for prolonged periods. For vulcanizing natural rubber he received patent number 3633 from the United States Patent Office on June 15, 1844. Besides natural rubber, Ebonite contains about 25–80% sulfur and linseed oil. Its name comes from its intended use as an artificial substitute for ebony wood.

The material is known generically as hard rubber and has formerly been called vulcanite, although that name now refers to the mineral vulcanite.

Charles Goodyear's brother Nelson Goodyear experimented with the chemistry of ebonite composites. In 1851 he used zinc oxide as a filler.

The sulfur percentage and the applied temperatures and duration during vulcanizing are the main variables that determine the technical properties of the hard rubber polysulfide elastomer. The occurring reaction is basically addition of sulfur at the double bonds, forming intramolecular ring structures, so a large portion of the sulfur is highly cross-linked in the form of intramolecular addition. High sulfur content up to 40% may be used for greatest resistance to swelling and minimal dielectric loss. The strongest mechanical properties and greatest heat resistance is obtained with sulfur contents around 35%. Best impact strength is obtained with somewhat lower sulfur contents around 30%. The rigidity of hard rubber at room temperature is attributed to the van der Waals forces between the intramolecular sulfur atoms. Raising the temperature gradually increases the molecular vibrations that overcome the van der Waals forces making it elastic. Hard rubber has a content mixture dependent density around 1.1 to 1.2. When reheated hard rubber exhibits shape-memory effect and can be fairly easy reshaped within certain limits. Depending on the sulfur percentage hard rubber has a thermoplastic transition or softening temperature of 70 to 80 °C (158 to 176 °F).

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