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Why are gutta-percha and balata harder and more leathery than natural rubber?

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

Gutta-percha and balata are harder and more leathery than natural rubber due to their distinct molecular structure. While natural rubber consists almost exclusively of the cis-1,4 isomer of polyisoprene, gutta-percha and balata are composed primarily of the trans-1,4 isomer.1

The trans-1,4 polymer is highly crystalline, which imparts the tough, hard, and leathery physical properties characteristic of these materials. In contrast, the cis-1,4 configuration found in natural rubber creates a resilient, elastic elastomer that is capable of significant stretching. Because of this structural difference, gutta-percha and balata do not possess the same elasticity as natural rubber, making them better suited for applications like golf ball covers, machine belting, and cable sheathing.123