Butyl rubber: A unique polymer with a diverse cure chemistry
The article in RFP Rubber Fibres Plastics 02 2021 will explain the chemical constitution of Butyl Rubber (IIR), the resulting chemical properties and describe some of the unique cure properties of this class of polymers and will highlight the versatility of brominated butyl (BIIR).
K. Kulbaba, D. Adkinson, J. Zhou (Arlanxeo)
Butyl Rubber (IIR) is a unique polymer with high permeation resistance, high damping and excellent thermal and chemical stability. The relatively low levels of unsaturation in these polymers provide limited sites for curing. Halogenation allows for improved cure compatibility with other rubber types with high diene content by increasing both the vulcanization rate and state of cure.
Butyl Rubber (IIR) is a copolymer of isobutylene and isoprene obtained via a cationically initiated polymerization reaction at temperatures below -90 °C. The resulting copolymer has a limited number of unsaturated sites along the polymer backbone for rubber curing or for further functionalization reactions.
Due to the isobutylene units along the backbone with bulky methyl groups, IIR adopts a densely packed structure resulting in excellent permeation resistance to various gases and moisture. Additionally, the bulky methyl groups of the isobutylene units hinder the polymer chain mobility, especially in respect of chain torsion which enables absorbed energy to be transformed into polymer chain motions, resulting in effective damping of low-frequency vibrations. Due to the low level of unsaturation along the polymer backbone, IIR exhibits excellent resistance to aging, high temperatures, acids, bases, ozone, and other chemicals.
Butyl rubber is well suited for a variety of applications which require excellent permeation resistance to both gas and moisture, high damping of low-frequency vibrations, and resistance to aging, high temperatures, acids, bases, ozone, and other chemicals. Halogenation allows for improved cure compatibility with other rubber types with high diene content by increasing both the vulcanization rate and state of cure. While both CIIR and BIIR will respond to a wide range of curatives and cure systems (accelerated sulphur cure systems, phenolic resins), BIIR is more reactive and versatile in its response. BIIR can be cured with sulphur alone (i.e. without zinc oxide or other accelerators present) and exhibits greater curing efficiency when zinc oxide is used as the sole curative. Peroxides with addition of a suitable co-agent or zinc oxide alone can be used to produce vulcanizates having superior resistance to ozone, and very low compression set at elevated temperatures. The unique balance of low permeation rates, low temperature flexibility, high damping characteristics and cure versatility of halobutyl rubbers offers potential for a wide range of applications.
K. Kulbaba, D. Adkinson, J. Zhou, RFP Rubber Fibres Plastics, 02 2021, 96-101.