Butyl rubber is a copolymer of isobutylene and isoprene. Simple copolymers are known as “regular” or “clear” butyl rubber, or simply “butyl.” When butyl rubbers are modified after polymerization with the halogens bromine or chlorine, a second family, known as halobutyl rubber (or halobutyls), results.
The principal characteristic common to both butyl and halobutyls is low permeability to gases and moisture. Butyl rubbers are, thus, used primarily in applications where retention of gases or moisture is important. Approximately 86% of the worldwide usage of butyl rubbers is in tires or inner tubes. Other applications in which the barrier characteristics of butyl rubbers find use include membranes for lining tanks and reservoirs, pharmaceutical closures, sealants, and electronic components encapsulation.
The secondary characteristic of high energy absorption properties leads to use in automotive engine and body mounts and other vibration-damping applications, as well as in chewing gum.
As is the case with most synthetic rubbers, butyl rubbers are mixed with other ingredients by the rubber processor (e.g., tire company) to produce compounds having good physical strength and resistance to atmospheric degradation. Vulcanization is accomplished in the normal manner using sulphur and accelerators. Due to the low unsaturation of the butyl polymer chain, the number of sites where crosslinks can be formed is limited. Butyl rubbers, therefore, vulcanize much more slowly than most synthetic rubbers and do not blend with, or stick to, other rubbers, as these preferentially absorb the curatives.
This is not a disadvantage in components made entirely out of butyl rubber, such as inner tubes. However, in a tubeless tire (as used on the majority of passenger cars), and on many trucks and buses in industrialized countries, it is necessary to stick an impermeable layer of rubber, or “liner,” to the inside of the tire in order to retain air.
By reacting butyl rubber with chlorine or bromine, chlorobutyl or bromobutyl rubbers, collectively known as “halobutyls,” are obtained.
The process involves dissolving butyl rubber in a solvent and adding elemental bromine or chlorine to the cement thus formed. The resulting rubber is highly reactive, and special precautions must be taken to ensure its stability in the finishing processes. The enhanced reactivity permits a vulcanization rate equivalent to that of other synthetic rubbers. Blends with other polymers (e.g., natural rubber) become possible, and compounds based on halobutyl polymers adhere fully to other rubbers in such applications as the tire inner liner.
Though bromobutyl was first produced many years ago, it was only in the early 1970s that the above-mentioned stability problem was overcome and a viable commercial product was introduced. Chlorobutyl is somewhat less reactive than bromobutyl and, unlike bromobutyl, usually needs to be blended with natural rubber (NR) to ensure sufficient adhesion to the tire carcass when used in tire inner liners. In the past, the use of NR reduced compound costs per Kg, but with a significant sacrifice in impermeability. At current prices, it is doubtful if the use of NR can be justified, except where suitable equipment for the processing of 100% bromobutyl compounds is not available. One hundred percent bromobutyl liners can be made thinner for the same impermeability, or show better air retention at the same thickness. For these (and other) reasons, bromobutyl is generally the preferred liner material for high-quality tires.
As halogenation is an additional step requiring further capital-intensive investment, Halobutyls cost more to produce than regular butyl, and sell for higher prices. They are thus not used in inner tubes, except in specialized cases.