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   IndiaRubberDirectory.com > Rubber Engineering > Butyl Rubber

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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