Ebonite

[5] The sulfur percentage and the applied temperatures and duration of vulcanizing are the main variables that determine the technical properties of the hard rubber polysulfide elastomer.

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.

The sulfates condense water from the air, forming a hydrophilic film with favorable wettability characteristics on the surface.

[citation needed] Hard rubber was used in the cases of automobile batteries for years, thus establishing black as their traditional colour even long after stronger modern plastics like polypropylene were substituted.

It was used for decades in hair combs made by Ace, now part of Newell Rubbermaid, although the current models are known to be produced solely with plastics.