Before rubber is ready for use in everyday products like garden hoses and rubber duckies, it must be cured. The curing process involves adding plasticizers to the natural rubber that will make it soft and pliable. Over time, the chemicals used to soften rubber will wear and your rubber products will lose softness, strength and pliability.
Natural rubber doesn't have much elasticity. When temperatures drop, the rubber becomes brittle; when they rise, natural rubber tends toward degradation. By manipulating the chemical make-up of natural rubber, these properties can be improved drastically. Natural rubber is made up of long polymer chains that can slide away from each other, causing instability. Through chemical manipulation, these long polymer chains can be cross-linked. This makes a stronger compound whose molecules will not slide.
Curing & Vulcanization
When you cure natural rubber, you are cross-linking the polymer molecules. The length of the cross-links determines the characteristics of your final rubber product. Since the 1800s, the process of curing rubber for use in various industries has sparked many developments in curing agents. The first chemical used to cure rubber was sulfur. The act of curing rubber into usable materials is called vulcanization. Vulcanization makes natural rubber less sticky and much more durable by adding curing agents. Over time, though, the curing agents begin to degrade and your rubber will lose strength and softness.
Sulfur was the first curing agent used to soften natural rubber and it is still used today. Sulfur can't be used to soften synthetic rubbers, though. Synthetic rubbers, like styrene-butadiene used to make car tires, require peroxide-based curing systems. While sulfur is a powerful chemical, in order to keep up with advancing technologies, sulfur alone is not enough to produce quality rubber products anymore.
A total curing package must be made using several chemical additives, with sulfur as the main ingredient. The additives are necessary to increase stability in the final product. Common additives include the accelerators zinc oxide and stearic acid and anti-degradants that will slow down the degradation potential of the rubber when it is exposed to heat.