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Why does natural rubber become sticky in warm conditions?

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

Natural rubber becomes soft and sticky in warm conditions because, in its raw state, it behaves essentially as a viscous liquid.1

At the molecular level, natural rubber consists of long, flexible polymer chains that are naturally entangled, similar to spaghetti. While these entanglements can act as temporary "interlinks" that provide some elasticity, they are not permanent. When exposed to heat, these chains gain enough energy to move more freely, causing the material to lose its structural integrity, flow, and become tacky.12

To address this limitation, the process of vulcanization is used to create a more durable material. By heating raw rubber with sulfur, chemical cross-links are formed between the polymer chains. This creates a permanent molecular network that prevents the chains from sliding past one another, effectively converting the viscous liquid into an elastic solid that maintains its shape and resilience even when warm.134