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Why does natural rubber become soft and sticky at warm temperatures?

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Natural rubber becomes soft and sticky at warm temperatures because, in its raw state, it behaves essentially as a viscous liquid. Without modification, the long polymer chains that make up natural rubber are only held together by physical entanglements, which allow the molecules to slide past one another when heated, causing the material to flow and lose its structural integrity.1

To overcome this, natural rubber is typically subjected to vulcanization, a process discovered by Charles Goodyear in 1839. During vulcanization, sulfur is added to the rubber and heated, creating chemical cross-links between the polymer chains. These covalent bonds tie the molecules together into a permanent, three-dimensional network, which prevents the material from flowing or becoming sticky even when exposed to high temperatures.213