Butyl rubber structure

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. 

TPEs from blends of rubber and plastics constitute an important category of TPEs. These can be prepared either by the melt mixing of plastics and rubbers in an internal mixer or by solvent casting from a suitable solvent. The commonly used plastics and rubbers include polypropylene (PP), polyethylene (PE), polystyrene (PS), nylon, ethylene propylene diene monomer rubber (EPDM), natural rubber (NR), butyl rubber, nitrile rubber, etc. TPEs from blends of rubbers and plastics have certain typical advantages over the other TPEs. In this case, the required properties can easily be achieved by the proper selection of rubbers and plastics and by the proper change in their ratios. The overall performance of the resultant TPEs can be improved by changing the phase structure and crystallinity of plastics and also by the proper incorporation of suitable fillers, crosslinkers, and interfacial agents. 

Isoprene (2-methyl 1,3-butadiene) is the second most important conjugated diolefin after butadiene. Most isoprene production is used for the manufacture of cis-polyisoprene, which has a similar structure to natural rubber. It is also used as a copolymer in butyl rubber formulations. 

Problem 31.6 Draw the structure of an alternating segment of butyl rubber, a copolymer of iso-prene (2-methyl-],3-butadiene) and isobutylene (2-methylpropene) prepared using a cationic initiator. 

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). 

Within the database there may be many chemicals for which butyl rubber gloves provide good protection and we could reasonably anticipate that at least some structural similarities would exist among these chemicals that would help us rationalize the choice of butyl rubber. 

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. 

Isophthaloyl chlorides, 19 715 Isophytol, 24 502, 550 Isopolytungstate compounds structures of, 25 383-384 Iso prefix, 13 594-595 Isoprene, 24 501 Alfrey-Price parameters, 7 617t block copolymer synthesis, 7 647t butyl rubber polymers, 4 433 commercial block copolymers, 7 648t glass transition and melting... 

Butyl rubber (HR) is widely used for inner tubes and as a sealant. It is produced using the cationic polymerization with the copolymerization of isobutylene in the presence of a small amount (10%) of isoprene. Thus, the random copolymer chain contains a low concentration of widely spaced isolated double bonds, from the isoprene, that are later cross-linked when the butyl rubber is cured. A representation is shown in structure 5.20 where the number of units derived from isobutylene units greatly outnumbers the number of units derived from the isoprene monomer. The steric requirements of the isobutylene-derived units cause the chains to remain apart giving it a low stress to strain value and a low Tg. 

Halogenated butyl rubbers have particularly advantageous adhesion behavior, flexural strength, service life and impermeability to air and water (40). The specific structure of the halogenated butyl rubber depends on the conditions of halogenation. 

F 2. FloRy, P. J. ESects of molecular structure on physical properties of butyl rubber. Ind. Engng. Chem. 38, 417 (1946). 

Figure 11.15 Presumed structure of the branch point of butyl rubber...

Figure 11 General scheme for butyl rubber halogenation and observed structures...

As shown in Table 1, poly(isobutylene-co- -pinenes) containing up to 10 mole% -pinene are rubbery materials Tg< —53 ). In view of the structural similarity between our copolymer and butyl rubber, we decided to use conventional butyl rubber cure conditions (3) to prepare our vulcanizates (see Experimental). Figure 2 shows the curing rate of two copolymers containing 3 and 10 mole% fi-pi-nene units respectively, as determined by a Monsanto Rheometer at 160 C (320 F). As expected, the curing rate of the cr lymer containing 10 mole% unsaturation is several times hi r than that of the material with 3 mole% unsaturation level. 

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