Polymers butyl rubber

The use of liquid and supercritical CO2 as an alternative, environmentally friendly or benign solvent should be mentioned here. De Simone reported the first examples of carbocationic polymerization in hquid and supercritical CO2. Vinyl ethers and oxetane containing fluorinated side chains yielded low molecular weight products (77). Kennedy reported the carbocationic polymerization of isobutylene in supercritical and liquid CO2 (78), also yielding low molecular weight polymer. Butyl rubber was reportedly produced in liquid C02/hexane mixture (79). However, it seems that the initial excitement surroimding liquid and supercritical CO2 as alternative solvents for polymerization has disappeared. 

Suitable polymers butyl rubber, chlorobutyl rubber, EPDM, SBR ... 

Recommended for polymers butyl rubber, CSPE, EPDM, epichlorohy-drin, hydrogenated nitrile rubber, hydrogenated rubber, NBR, PAC, TPE ... 

Butyl mbber, a copolymer of isobutjiene with 0.5—2.5% isoprene to make vulcanization possible, is the most important commercial polymer made by cationic polymerization (see Elastomers, synthetic-butyl rubber). The polymerization is initiated by water in conjunction with AlCl and carried out at low temperature (—90 to —100° C) to prevent chain transfer that limits the molecular weight (1). Another important commercial appHcation of cationic polymerization is the manufacture of polybutenes, low molecular weight copolymers of isobutylene and a smaller amount of other butenes (1) used in adhesives, sealants, lubricants, viscosity improvers, etc. 

Butyl rubber and other isobutylene polymers of technological importance iaclude various homopolymers and isobutylene copolymers containing unsaturation achieved by copolymerization with isoprene. Bromination or chlorination of the unsaturated site is practiced commercially, and other modifications are beiag iavestigated. 

Table 2. Polymer—Solvent Interaction Parameters for Polyisobutylene and Butyl Rubber...

Until the mid-1950s the only polyolefins (polyalkenes) of commercial importance were polyethylene, polyisobutylene and isobutylene-isoprene copolymers (butyl rubber). Attempts to produce polymers from other olefins had, at best, resulted only in the preparation of low molecular weight material of no apparent commercial value. 

In addition there is the possibility that other olefins may generate polymers with low Tg s which show little or no crystallinity at room temperature and are therefore potentially elastomeric. One commercial example is butyl rubber (designated HR), a copolymer of isobutene with a small amount of isoprene. 

Whilst polyisobutene is a non-rubbery polymer exhibiting high cold flow (see Section 11.3), the copolymer containing about 2% isoprene can be vulcanised with a powerful accelerated sulphur system to give moderately rubbery polymers. The copolymers were first developed in 1940 by Esso and are known as butyl rubbers and designated as HR. As they are almost saturated they have many properties broadly similar to the EPDM terpolymers. They do, however, have two properties that should be particularly noted ... 

The low unsaturation requires powerful curing systems whilst the hydrocarbon nature of the polymer causes bonding problems. To overcome these problems chlorinated and brominated butyl rubbers (CIIR and BUR) have been introduced and have found use in the tyre industry. 

A somewhat different approach to the production of thermoplastic polyolefin rubbers has been adopted by Allied Chemical with their ET polymers. With these materials butyl rubber is grafted on to polyethylene chains using a phenolic material such as brominated hydroxymethyl phenol. The initial grades of these polymers, which were introduced commercially towards the end of the 1970s, had polyethylene butyl rubber ratios of 50 50 and 75 25. Both low-density and high-density polyethylene-based varieties were produced. 

Butyl rubber (BR) and polyisobutylene (PIB) are widely used in adhesives as primary elastomeric binders and as tackifiers and modifiers. The main difference between these polymers is that butyl is a copolymer of isobutylene with a minor amount of isoprene (which introduces unsaturation due to carbon-carbon double bonds), while polyisobutylene is a homopolymer. 

Polyisobutylene has a similar chemical backbone to butyl rubber, but does not contain double carbon-carbon bonds (only terminal unsaturation). Many of its characteristics are similar to butyl rubber (ageing and chemical resistance, low water absorption, low permeability). The polymers of the isobutylene family have very little tendency to crystallize. Their strength is reached by cross-linking instead of crystallization. The amorphous structure of these polymers is responsible for their flexibility, permanent tack and resistance to shock. Because the glass transition temperature is low (about —60°C), flexibility is maintained even at temperatures well below ambient temperature. 

Most rubbers used in adhesives are not resistant to oxidation. Because the degree of unsaturation present in the polymer backbone of natural rubber, styrene-butadiene rubber, nitrile rubber and polychloroprene rubber, they can easily react with oxygen. Butyl rubber, however, possesses small degree of unsaturation and is quite resistant to oxidation. The effects of oxidation in rubber base adhesives after some years of service life can be assessed using FTIR spectroscopy. The ratio of the intensities of the absorption bands at 1740 cm" (carbonyl group) and at 2900 cm" (carbon-hydrogen bonds) significantly increases when the elastomer has been oxidized [50]. 

Heat resistance. It depends on the content of PIB in the butyl rubber polymer the higher the PIB content, the lower the heat resistance and the higher the tack. 

Process systems handling polymers and resins (e.g., butyl rubber or ethylene-propylene diene monomer rubbers) are often subject to plugging at dead-end locations such as PR valve inlets. In extreme cases, complete blockage of inlet piping and valve nozzle can result. This problem can be eliminated by the application of a flush-seated PR valve, in which dead-end areas are eliminated by placing the valve disc flush with the vessel wall, in the flow pattern of the contents. 

It is possible to distinguish between SBR and butyl rubber (BR), NR and isoprene rubber (IR) in a vulcan-izate by enthalpy determination. In plastic-elastomer blends, the existence of high Tg and low Tg components eases the problems of experimental differentiation by different types of thermal methods. For a compatible blend, even though the component polymers have different Tg values, sometimes a single Tg is observed, which may be verified with the help of the following equation ... 

Butyl ruhher vulcanizates have tensile strengths up to 2,000 psi, and are characterized hy low permeahility to air and a high resistance to many chemicals and to oxidation. These properties make it a suitable rubber for the production of tire inner tubes and inner liners of tubeless tires. The major use of butyl rubber is for inner tubes. Other uses include wire and cable insulation, steam hoses, mechanical goods, and adhesives. Chlorinated butyl is a low molecular weight polymer used as an adhesive and a sealant. 

The standard polymers used for rubber linings consist of materials that are cross-linkable macromolecules which, on mixing with suitable reactants that form strong chemical bonds, change from a soft deformable substance into an elastic material. These polymers include natural rubber and its corresponding synthetic, c/s-polyisoprene, styrene-butadiene rubber, polychloroprene, butyl rubber, halogenated butyl rubbers, acrylonitrile-... 

Butyl rubber, halogenated butyl rubber linings and ethylene propylene rubber linings have excellent chemical resistance at temperatures up to 120°C. There are cases where butyl rubber linings have been subjected to temperatures down to —65°C without deterioration. Linings based on these polymers do not exhibit good abrasion resistance, which can only be rated as fair. 

Methylpropene can be made to continue the process to yield high polymers—cationic polymerisation—but most simple alkenes will go no further than di- or tri-meric structures. The main alkene monomers used on the large scale are 2-methyIpropene (— butyl rubber ), and vinyl ethers, ROCH=CH2 (— adhesives). Cationic polymerisation is often initiated by Lewis acid catalysts, e.g. BF3, plus a source of initial protons, the co-catalyst, e.g. traces of HzO etc. polymerisation occurs readily at low temperatures and is usually very rapid. Many more alkenes are polymerised by a radical induced pathway, however (p. 320). 

Ionic Polymerization. Ionic polymerizations, especially cationic polymerizations, are not as well understood as radical polymerizations because of experimental difficulties involved in their study. The nature of the reaction media is not always clear since heterogeneous initiators are often involved. Further, it is much more difficult to obtain reproducible data because ionic polymerizations proceed at very fast rates and are highly sensitive to small concentrations of impurities and adventitious materials. Butyl rubber, a polymer of isobutene and isoprene, is produced commercially by cationic polymerization. Anionic polymerization is used for various polymerizations of 1,3-butadiene and isoprene. 

Chemical reactions are used to modify existing polymers, often for specialty applications. Although of considerable importance for plastics, very few polymer reactions (aside from crosslinking) are important for elastomers. Chlorination and bromination of Butyl rubber to the extent of about one halogen atom per isoprene unit yields elastomers which are more easily crosslinked than Butyl rubber. Substitution occurs with rearrangement to yield an allylic halide structure... 

Experiments have been made in which uncross-linkable polymer rubbers have been added to a similar rubber that is subsequently cross-linked (199). As an example, polyisobutylene was added to butyl rubber before it was cross-linked. The polyisobutylene molecules were not attached to the network structure, so they could be extracted by a solvent. As expected, the polyisobutylene greatly increased the creep compliance over that of the pure butyl rubber. 

Polymerised isobutylene, a non-vulcanisable, rubber-like polymer. See Butyl Rubber. Polyisoprene... 

Rubber-like materials now superseding the traditional mastics and putties used in the building industry. Such sealants (also termed mastics) are based on butyl rubber, liquid polysulphides, silicone rubbers, polybutylene, nitrile rubbers and plasticised vinyl polymers. SEBS... 

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