Why does natural rubber harden in cold conditions?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

truck tires being removed from their molds — rubber Truck tires being removed from their molds.

Natural rubber hardens in cold conditions because it crystallizes slowly when temperatures drop below approximately 5 °C (40 °F).1

In its natural state, rubber is a viscous liquid that can become soft and sticky when warm and hard when cold due to this crystallization process. To overcome these disadvantages, the process of vulcanization is used. By heating rubber with sulfur, cross-links are formed between the polymer chains, creating a loose molecular network. This chemical modification makes the rubber molecules sufficiently irregular to greatly impede crystallization, allowing the material to remain elastic and resilient at both high and low temperatures.1