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How is rubber made? This is of course different for natural rubber than for synthetic rubber. With natural rubber it starts with the rubber tree. The plantation owners drain a white liquid from the carved bark. That liquid is called latex. In the factory they clean that latex and add some chemicals to it. This produces a solid white substance: raw rubber.
Excipients
We can't do anything with that raw rubber yet. That is why we add fillers and other additives. Which these are depends on the properties that the rubber wants to have. Soot (carbon black) is a commonly used filler. Finally, we add an amount of sulfur. The rubber reacts with it. We call this process vulcanization. It ensures that the rubber gets the desired firmness and flexibility. These are important properties for car tyres, for example.
vulcanizing molecules
What does the sulfur do and what is vulcanization? Consider the molecules of the original rubber. Long polymer molecules. Hardly any connections between those long chains. With the elastomers you saw that a few (not too many) cross-links are needed to make the fabric elastic.
But the long chains are very special in rubber. They can absorb sulfur particles, so that cross-links are created between the chains. With little sulphur, so few cross-links, you get a very flexible, soft rubber. With a lot of sulfur, the rubber becomes firmer and harder. And with a lot of sulfur (actually too much sulfur) you have so many cross-links that you actually have a thermoset. And that is true, you then get a rock-hard substance that we call ebonite.
Synthetic rubber
We make synthetic rubber in a similar way. We do not start with the latex from the tree, but with all kinds of substances from petroleum. We use the molecules thereof (monomers) to make polymers. These are the long chains that make up synthetic rubber. To obtain a synthetic rubber with the desired properties, other substances are added and vulcanization takes place.
Summer tires and winter tires
We use by far the most rubber for making car tires. We know summer and winter tires for cars. In winter, the rubber of the tire is somewhat harder due to the cold. In order to maintain a good grip on the road, we then drive with tires made of softer rubber. At the higher temperature of the summer, these tires would become even softer and therefore wear out much too quickly. Winter tires are therefore made of somewhat softer rubber than summer tyres. The rubber therefore has a slightly different composition.
