How vulcanized rubber is made

Raw rubber turns soft and sticky in the heat. Sulfur and a hot mold lock its chains together so it stays springy.

stations: 6
things upstream: 15
raw sources: 7
steps from the ground: 7

The short version

Vulcanized rubber is made by kneading rubber, natural or synthetic, with carbon black, sulfur and other chemicals, shaping the dough in a mold or extruder, and then heating it. Sulfur cross-links tie the long molecules together, so the part keeps its shape and springs back instead of going soft.

Coagulate the latex

Milky latex from rubber trees is mixed with acid so the rubber clumps. The clots are rolled into sheets, washed and dried into the bales that factories buy.

Goes in
Natural rubber latex
Chew the rubber soft

Heavy rollers or an internal mixer break down the tough bales until they are workable. Natural and synthetic rubber are often blended here.

Goes in
Synthetic rubber
Mix in fillers and sulfur

Carbon black, sulfur, accelerators and antioxidants are worked into the rubber. The mix is dispersed so evenly that every cubic centimeter behaves the same.

Goes in
Carbon black Sulfur
Shape the dough

The warm mix is extruded, calendered or pressed into a mold cavity. Not yet vulcanized, it is still soft and could be reshaped.
Heat it to vulcanize

Hot molds or steam heat the part so sulfur bonds form cross-links between the long chains. The rubber turns from sticky dough to a firm, elastic solid that stays flexible.
Trim and test

Excess rubber flash is trimmed off and samples are checked for hardness and stretch. Vulcanized parts cannot be melted and reshaped.

End of the lineVulcanized rubber Next it goes into computer keyboard, vacuum cleaner, electric kettle and 26 more.

Worth repeating

The word vulcanization was suggested by William Brockedon, a friend of Thomas Hancock, after Vulcan, the god associated with heat and sulfur.
In the 1830s Charles Goodyear worked to devise a way to strengthen rubber, because tires of the time went soft and sticky in the heat.