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Specialty Chemistry Forums => Materials and Nanochemistry forum => Topic started by: jkmiller on November 24, 2020, 02:59:33 PM

Title: Thermosetting properties of Silicone rubber
Post by: jkmiller on November 24, 2020, 02:59:33 PM
I know that silicone is not a thermoplastic and will not become more fluid with more heat. Correct me if I am wrong, but I believe this is due to the crosslinks between the individual polymer strands being too thermodynamically stable, and the polymer backbone would rather decompose than break the crosslinks. My questions are:

A. Which bonds are breaking under thermal decomposition?
B. Is heat alone enough to further crosslink an already cured silicone?
C. Is further crosslinking (like in question B) what makes silicones brittle when subjected to intense heat?
D. would sulfur crosslinks instead of silica derived crosslinks or organic crosslinks break under high temperatures and cause the silicone to melt instead of decompose (thermoset to thermoplastic)? - I found a couple of older patents for sulfur cured silicone, but they are not as clear on what is bonded to what on the polymer, so I couldn't figure out which bond would break if subjected to high temperatures, although I know that S-C and S-Si are fairly weak compared to Si-O and C-C bonds. Here is one of the patents for reference:

I am quite new to the field of polymer physics and materials science (I am a biochemistry major), but I hope at least some of my knowledge is heading in the right direction. Thank you for your *delete me*
Title: Re: Thermosetting properties of Silicone rubber
Post by: wildfyr on November 25, 2020, 10:45:01 AM
Hi there,

Silicone is generally crosslinked via one of two broad methods. Both involve the use of vinyl groups on the backbone. One method uses some kind of radical initiator like an organic peroxide and the vinyl groups will polymerize. The other is the usage of vinyl groups off the backbone of one polymer, and simple hydrogens directly on the silicon (aka silanes). In the presence of certain Pt catalysts like chloroplatinic acid, a reaction called hydrosilation creates alkyl linkages. In both cases these are very stable. I think you would find your direct answer if you google "thermolysis of silicone" but I think Si-C is the weak link.

B. Yes, up to a certain point.

C. Depends on the silicone, but a byproduct of silicone degradation is the rather brittle compound of silica also. As well as oxidation reactions, again depending on what your alkyl or aryl sidechains are.

D. Certainly alternate crosslinks could create a silicone that thermolysis to a liquid instead of making garbage, however I think "vulcanization" is a poor route. Rubber tires as crosslinked this way, and they certainly don't melt with heat. Thiols to disulfides are encouraged by the presence of heat and oxygen.