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 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. As a result of having a maximum sulfur content up to 40%, it may be used to resist swelling and minimize dielectric loss. The strongest mechanical properties and greatest heat resistance is obtained with sulfur contents around 35% while the highest impact strength can be obtained with a lower sulfur content of 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 easily reshaped within certain limits. Depending on the sulfur percentage hard rubber has a thermoplastic transition or softening temperature of .
