Isoprene-isobutylene rubbers

 

Isoprene-isobutylene rubbers (IIR) are special rubbers. Their characteristic properties are small gas and steam permeability and also very good resistance to oxidation, ozonization and thermal degradation and against various chemicals.

They are produced by cationic copolymerization of isobutylene and isoprene in chloromethane solution in the presence of AlCl3 as catalyst and water or HCl as co-catalyst at temperature under 90°C. The copolymerization proceeds also at this temperature with very high rates. The produced copolymer has high molecular weight and it precipitates from the reaction environment (precipitation copolymerization) in the form of hard non agglomerated particles (temperature of polymerization is lower than IIR Tg). Isobutylene and isoprene copolymerization in hexane and in the presence of Al(alkyl)2Cl as catalyst is homogeneous and can be performed at temperatures -40 to -50°C.

 

Structural units of IIR rubbers

Molecular weights of IIR rubbers are between 150 000 450 000 g.mol-1 and they have quite broad distribution, Mw/Mn≈ 2.5-5. Isobutylene structure units are arrangement in copolymer chains mainly by the way head-to tail. Content of isoprene is low (0.5 - 6 % mol) and his structural units are statistically arrangement in copolymer macromolecules in form of trans-1.4 units. In commercial types IIR to one isoprene structural unit appears approximately 40 structural units of isobutylene. Isoprene does not affect the IIR properties, double bonds in his structural units are used mainly for cross-linking.

IIR rubbers have very low content of unsaturated bonds which during the vulcanization expire. That is why they and their vulcanizates are resistant to ageing. Low rate of sulfur vulcanization is proportional to low content of unsaturated bonds. IIR hardly co-vulcanize with other unsaturated rubbers (NR, BR, SBR), the rate of cross-linking is increasing by increasing of double bond content, however at the same time the properties of vulcanizates are deteriorated. In the sulfur cross-linking systems fast (thiurams) or very fast accelerators (dithiocarbamates) and also in combinations with thiazoles (MBT) or systems based on sulfur donors are preferred. For vulcanization of IIR the quinone dioxime in combination with oxidizing agent are used (lead oxides, MBTS) and phenolformaldehyde resins in combination with chlorine containing substances (SnCl4, FeCl3, chlorinated polymers) or their halogenated derivates. In the presence of these cross-linking systems can be obtained vulcanizates resistant to thermal degradation. The use of peroxides is not suitable for cross-linking because they can cause degradation of the rubbers´ macromolecules.

 IIR degradation in presence of peroxides

Halogenated IIR is more suitable for co-vulcanization with diene rubbers. Its properties are very similar to IIR rubbers but their sulfur cross-linking is faster and vulcanizates with higher level of cross-links are achieved. They can be vulcanized by multifunctional amines, too.

Developmental types of rubbers which are isobutylene based are star-branched IIR which can be processed easier and also brominates copolymers of isobutylene and p-methyl styrene which are resistant to ozone and high temperatures and they are reactive at co-vulcanization with rubbers for general use.

 

Summary

  • special rubbers
  • characteristic properties are small gas and steam permeability as well as very good resistance to oxidation, ozonization and thermal degradation and against various chemicals
  • typcally produced by cationic copolymerization of isobutylene and isoprene in chloromethane solution
  • sulfur vulcanization is predominantly used but the rate of vulcanization is low due to low content of unsaturated bonds
  • IIR hardly co-vulcanize with other unsaturated rubbers (NR, BR, SBR)
  • halogenated IIR is more suitable for co-vulcanization with diene rubbers for its properties are very similar to IIR rubbers but their sulfur cross-linking is faster and vulcanizates with higher level of cross-links are achieved
  • halogenated IIR is produced by means of post polymerization modification of the IIR

 

 
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