Butyl rubber, also known as polyisobutylene is a synthetic rubber or elastomer. It was first developed in the 1940’s by German chemists and commercialised in 1943.
The typical composition of butyl rubber is approximately 98% polyisobutylene, with the balance being isoprene. The isoprene units contain a double bond that provides a site for crosslinking during vulcanisation and are located randomly in the polymer chain.
Halogenated butyl rubbers such as chlorinated (chlorobutyl) and brominated (bromobutyl) were an axtension of butyl rubber developed in the 1950’s and 60’s. Compared to butyl rubber they have higher curing rates and can be co-vulcanised with other rubbers.
It has a structure similar to polyetyhylene, except that each second carbon atom in the polymer chain is bonded to two methyl (CH3) groups. It is derived from the monomer isobutylene thus:
Figure 1. Structure of isobutylene and polyisobutylene or butyl rubber.
Polymerisation and Vulcanisation
The polymer is formed by a process called cationic vinyl polymerisation and is highly exothermic. It involved the use of an initiator or cation, which attracts a pair of electrons from the carbon-carbon double bond, thus forming a single bond with the initiator. One of the carbons, previously double bonded is now positively charged and will react with another monomer, similarly to the initiator. The process is repeated the polymer is formed.
The polymerisation reaction is usually carried out at temperatures in the range –100°C to control the reaction rate. At higher temperatures, the reaction proceeds too fast to control.