Butyl Rubber
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.
Composition
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.
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.
Key Properties
Key Properties of butyl rubber include:
· Air tight and gas impermeable, (a unique property
of butyl rubbers)
· Flexibility
· Good weathering resistance
· Resistant to ozone
· Good vibration damper
· Biocompatible
Applications
Typical applications include:
· Liners for tubeless tyres
· Inner tubes
· Inner tubes for footballs, basketballs etc
· Stoppers for medicine bottles and pharmaceuticals
· In sealants and adhesives
· O-rings
· Joint replacements (biomedical)
· Chewing gum
· Tank and pond liners
· Speaker surrounds
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