Butyl Rubbers

Isobutylene-Isoprene Rubber (IIR), Chlorobutyl Rubber (CIIR), Bromobutyl Rubber (BIIR)

Butyl rubbers are prepared by copolymerizing small amounts of isoprene with isobutylene. Isoprene units are placed randomly in the isobutylene chain in trans-1,4 form. Adjusting the polymerization temperature and the proportion of monomers can vary the composition of the polymer. A typical butyl rubber contains 0.5 ... 3 mole percent isoprene. 

The properties of butyl rubbers depend on the length of the molecule chains and the saturation degree. When the amount of double bonds is low, rubber has good oxygen and ozone resistance. A greater amount of double bonds accelerates the vulcanization process and increases the amount of cross-links.

Isobutylene and isoprene units. 

The properties of butyl rubbers can be improved by adding 1 ... 2 weight percent of halogens and by forming chlorobutyl (CIIR) and bromobutyl (BIIR) rubbers. Halogens are mostly joined to the double-bonded carbon without the methyl group in the isoprene unit. The addition of the halogens increases chain flexibility and enhances cure compatibility in blends with other diene rubbers.

The butyl rubber can be cured with sulphur, but it needs accelerator. Dioxime compounds together with an oxidizing agent can also be used. In that case, cross-links stand heat better than sulphur bonds. CIIR and BIIR have more reactive points in the cross-linking if a curing agent (sulphur or metal oxides) has been used in curing. Peroxides cannot be used, because they may break down the elastomer chains.

Advantages of butyl rubbers:

  • stabile in long-term-use and at high temperatures
  • low gas permeability
  • good ozone resistance
  • good weather resistance
  • elasticity in wide temperature range -73...100°C
  • low water absorption
  • resistant to oxidizing agents, vegetable and animal fats and polar solvents
  • heat stability

Disadvantages:

  • poor wear resistance
  • not resistant to hydrocarbon solvent and oil
  • relatively low elasticity  

The properties of halogenated butyl rubbers are similar to those of basic butyl rubber. However, they have lower gas permeability and better thermal, ozone, weather and chemical resistance. Halogenated butyl rubbers are used in applications that require rubber with a high vulcanization rate.

Applications:

  • inner tyres of cars and bicycles
  • steam hoses
  • coatings of fabrics and cables
  • base element of chewing gum
  • waterproof films
  • gutter gasket
  • inner tubes
  • pharmaceutical closures and membranes
  • vibration isolation

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