Why doesn’t silicone melt in the oven? Because it doesn’t have a specific melting point.
Most of us will not put rubber or plastic dishes in our oven because they will obviously melt. Natural rubber has a melting point of 180°C. The melting point of plastic depends on the type of plastic. PET (polyethylene terephthalate) melts at 255°C and HDPE (high-density polyethylene) melts at 130°C.
What happens when plastic melts? Plastics are made up of polymers, which are chains of carbon atoms. The polymers join together to make polymer chains. When they are cool, the polymers form stable structures. When they are heated, the heat energy makes the atoms in the chains start to vibrate more. The chains become less stable and start to move. At this point the plastic becomes soft and can’t hold its shape anymore. The point where a material becomes soft and pliable is called the “glass transition phase”.
Silicone rubber is made from silicone, which is a synthetic material. It contains polymers in the same way as rubber and plastic, but the polymers are made of chains of silicon and oxygen atoms in that order, rather than the carbon and hydrogen in plastic. Silicone is often confused with silicon, but they are not the same thing. Silicon is the element that silicone is produced from.
To make silicone, you first need to find silica. Quartz sand is a great source of silica and when it is heated to 1800°C it leaves silicon. The silicon is ground into a powder and mixed with a chemical called methyl chloride. This produces methyl chlorosilane, which is made up of several different compounds. To make silicone, the manufacturers need to take out a chemical called dimethyldichlorosilane. All of the compounds in the methyl chlorosilane have different boiling temperatures, so they remove the dimethyldichlorosilane by heating the mixture precisely. The manufacturers then mix the dimethyldichlorosilane with water, which breaks it into hydrochloric acid and disilanol. The hydrochloric acid reacts with the disilanol making polydimethylsiloxane, which is mixed with a polymerization initiator and, voila, you have silicone.
The produced silicone is a very sticky gel or liquid. It has to be cured before we can use it as an object. There are many ways to do this, but the most common seems to be by using peroxide. The peroxide forms carbon bonds between the polymer chains in the silicone rubber and makes a very stable shape.
So, why doesn’t silicone rubber melt in the oven? Pure silicone and silicone rubber are two different things. Silicone rubber is an elastomer, which means it has weaker forces that regular silicone, allowing to be more flexible. Pure silicone has a very strong atomic structure. It is made of long chains of silicon and oxygen that are attracted to each other very strongly. For a material to melt, the atoms need to have enough energy that they vibrate too much for their bonds to other atoms to hold them. Silicon and oxygen are attracted to each other so strongly, that you need to heat them to 1414°C to give them enough energy to break their bonds. The bonds in silicone rubber are not as strong as those in pure silicone because it is elastic and silicone rubber objects are usually only good up to about 300°C. However, they don’t melt at temperatures higher than this. What happens is the polymers lose their elasticity and the material becomes hard and brittle. If you keep heating it, it will combust at about 450°C. It won’t melt.
Silicon was isolated by Jons Jackob Berzelius, a Swedish chemist in 1823. Henry Sainte-Claire Deville managed to make crystalline silicone in 1854. J. F. Hyde managed to commercialize the process in 1930. Silicone rubber was invented in 1940 by Frederich Stanley Kipping. He thought it was too sticky and couldn’t see a use for it. In the 1940s, scientists and engineers at Corning Glass were looking for insulating materials for electric motors and generators. They had insulators, but they couldn’t find a rubber sealant that could survive the temperatures. They realized that silicone rubber would do the job and its first use was discovered. They quickly saw that it had a whole range of uses and they started to develop it commercially.
As a side note, James Wright, a chemical engineer working for General Electric invented silly putty in 1943. America couldn’t get natural rubber because of the war and he was one of the chemists looking for alternatives. He mixed boric acid with silicone oil and ended up with what would become silly putty. And this is what I learned today.
Sources
https://www.sciencedirect.com/science/article/pii/S2542504820300245
https://en.wikipedia.org/wiki/Plastic
https://firesafetysupport.com/what-temperature-does-rubber-melt/
https://education.seattlepi.com/hot-water-melt-plastic-3626.html
https://en.wikipedia.org/wiki/Silicone_rubber
https://en.wikipedia.org/wiki/Silicone
https://silicone.co.uk/news/temperatures-can-silicone-rubber-withstand/