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Introduction
Butyl Rubber is produced by co-polymerising isobutylene with small amounts of
isoprene. Feed stocks containing 1.5 to 4.5 percent isoprene are generally used
to make the current commercial grades. The polymerisation reaction is unique and
proceeds rapidly at very low temperatures. The outstanding performance of these
isobutylene-isoprene co-polymers leads to their use in many different
applications requiring their unique combination of properties.
History
The history of this rubber’s development began in the early 1870s when low
molecular weight homopolymers of isobutylene were produced at room temperature.
During the years that followed higher molecular weight polyisobutylenes were
produced. Although they had many rubber-like properties, their completely
saturated chemical structure made vulcanization impossible by normal methods.
In 1937, Sparks & Thomas at the Exxon Research & Engineering company were
successful in introducing unsaturation by incorporating a few isoprene units
into the co-polymer by making in vulcanizable. The vulcanizates and outstanding
resistance to heat, ozone, chemical attach am tearing.
Major Suppliers
The major suppliers of this speciality elastomer in the U.S. are Exxon Chemical
Company and Cities Service Company, Columbian Division with their respective
trade names ExxonButyl. In Canada Government owned polysar Ltd. has the
manufacturing capacity. In the International scene butyl Production facility
includes plants in England, France and Japan operated by affiliated of Exxon
Chemical company and in Belgium by polysar Belgium, an affiliate of polysar
Canada.
Manufacturing
The commercial production of Butyl Rubber is carried out by co-polymerising
small amounts of isoprene with isobutylene catalysed by Aluminium Chloride
dissolved in Methyl Chloride. The reaction is unique and the extremely rapid
polymerisation is conducted at a low temperature. The purity of isobutylene is
important to the acquisition of high Molecular Weight.
is applied to ensure good catalyst dispersion. Since the reaction is exothermic,
colling is important and this is done by vapourising liquid ethylene in the
ractor jacket. The polymer formed, flows into a tank of hot water where the
unreacted monomerrs are recovered. The slurry is then stabilised by adding sinc
stearate to prevent agglomeration. A small quantity of Antioxidant is also added
to the polymer. The slurry after passing over a screen and filter, and through a
tunnel drier, is extruded and hot-milled (to remove traces of moisture)
Commerical Grades
Two parameters are generally specified to characterise the stanadard
polymers. They are Monnerr Viscosity, which is related to the moleculer weight
of the polymer and the degree of unsaturation ( the mole % isoprene in the
polymeric chain).
Applications
(1) Unsaturation 0.6 - 1.2 Mole (%)
For applications requiring maximum resistance to ozone, weather, flexing and
chemicals. Areas of usage include irrigation tubing tank linings, roof
coverings, cable insulation, etc.
Unsaturation 1.5 - 2.0 Mole (%)
Grades with unsaturation in this range and mooney viscosity between 50 - 60 are
the standard butyl grades which are used in the largest volume. The higher level
of unsaturation gives a faster cure rate and high molecular weight allows high
filler loading and a greater compounding lattitude - Applications include,
tubes, bladders, adhesive a variety of automobile parts like engine mounts,
bumbers, etc.
Compounding (General)
Apart from the selection of polymer which is based on the properties explained
above, one must also consider the effects of fillers, plasticizen and cure
systems to achieve the optimum required performance expected.
In the case of filllers, carbon black gives a good reinforcement with an
increase in tensile strength and hardness. The choice and type depends on the
end property required. But beyond an optimum concentration increasing amount of
filler generally reduce tensile strength. Normally as in any case, smaller
particle size black generally produce compound with higher T.S., Hardness,
Modulus, etc. High structured blacks are used to imporve extrusion smoothness.
The smaller particle black compounds are somewhat difficult to process because
of greater heat buildup during milling extruding.
Mineral fillers also enhance reinforcement but to a lesser effect than carbon
black. Hard clay and silica yield good tensile strength and are mildly
reinforcing. But they tend to produce stiffer stocks. Hot mixing imporves
flexibility and processing characteristics.
Plasticizers / Process AIDS
Since Butyl Rubber is a highly saturated material the recommended plasticizers
are also highly saturated and of relatively low polarity. Hence paraffine
process oil, which has a predominantly saturated back bone is the most
compatible of all the process oils.
Highly unsaturated plasticizer should be avoided as they have the tendency to
bleed out ans sometimes to retard the rate of cure. Dosage of the process oils
depends on the end properties and type of process intended, for example,
frictioning/calendering of extruding, etc.
Antioxidants / Antiozonants
Antioxidants / Antiozonants are not required as the elastomer is inheraently
resistant to Oxygen/Ozone. But in articles which are subjected to severe
exposure it may be advantageous to incorporate either an antiozonant or micro
wax. (2.0 - 3.0 phr will suffice)
Cure Systems
Because of the low level of unsaturation in Butyl Rubber, there are relatively
few locations at which cross-linking takes place. For this reason, fast
accelerators like thiurams or dithio carbamates are generally used alongwith
Sulphur to cure butyl compound. To activate organic accelerators, butyl
compounds normally require Sinc Oxide at the 5 Phr level. Fatty acid is not
required for the vulvanization of butyl compounds. It is only used as a
processing aid.
But for outstanding heat resistance, there is different king of cure system is
available for butyl rubber. This is called ‘Resin Cure’ which will be elaborated
in the specific compounding section.
Processing
Butyl elastomer is easily processed using conventional equipment/mechinery. The
low level of unsaturation of these polymers contributes to their resistance to
breakdown during processing. This allows one to perform operation such as heat
treatment or high temperature mixing to alter the vulcanizate properties of the
compound. High allows one to perform operations such as heat treatment or high
temperature mixing to alter the vulcanizate properties of the compound. High
temperature processing often improves vulcanizate properties.
Butyl Rubber is ideal for heat treatment because of its inherent thermal
stability which prevents polymer degradation. This technique can be brought
about by the use of a chemical agent also. This treatment will enhance polymer
to filler bounding and better filler dispersion. This can be carried out in the
normal Bandury Mixer. Usually reinforcing fillers are added early in the mix and
to ensure good dispersion, the batch should be brought up to the temperature
before the addition of plasticizer, etc. When the compound is not heat treated,
sulphur and accelerators can be added during the usual Bandury Mix.
Compounding for specific properties
Gas Impermeability
The Poly Isobutylene portion of the Butyl molecule provides a high degree of
impermeability to butyl, which leads this polymer to an exculsive use in inner
tubes. The CH3 group clustering around the back bone packs the molecule tightly
to prevent any seepage of air.
Other gases like Helium, Hydrogen, Nitrogen and Carbon dioxide are also well
retained by butyl. Hence this rubber finds extensive usage in inner tubes, inner
liners, Air bellows, pneumatic springs, etc.
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