HALOBUTYL RUBBER POLYMER

Butyl and Halobutyl Rubber. Butyl mbber is made by the polymerization of isobutylene a small amount of isoprene is added to provide sites for curing. It is designated HR because of these monomers. Halogenation of butyl mbber with bromine or chlorine increases the reaction rate for vulcanization and laminates or blends of halobutyl are feasible for production of mbber goods. It is estimated that of the - 100 million kg of butyl (UR) and halobutyl (HIIR) mbber in North America, over 90% is used in tire apphcations. The halogenated polymer is used in the innerliner of tubeless tires. Butyl mbber is used to make innertubes and curing bladders. The two major suppHers of butyl and halobutyl polymers in North America are Exxon and Bayer (see ELASTOLffiRS,SYNTHETIC-BUTYLrubber). 

Elastomer Natural rubber Halobutyl rubber Dimethylpolysiloxane polymer EPDM... 

Several conclusions have to be drawn. The first is related to the obvious gap between the empiricism and even archaism of most of industrial cationic polymerization processes and the level of fundamental science devoted for decades to these reactions. Previous chapters in this volume clearly illustrate the situation. This feature was pointed out in the early book of Kennedy and Marechal [1], and the explanation based on the very favorable price/performances characteristics of the products is still realistic. Nevertheless it is noteworthy that recent improvements or new processes based on more scientific approaches led to a better control of the polymerization, of polymer structure, and to high-performance commercial products which will increasingly occupy the market. This is the case for the recently marketed reactive BF3-based polybutenes with high content of exomethylenic chain ends, for the strongly developing pure monomer hydrocarbon resins ( + 8% in 1994), or for the new benzyl halide-based halobutyl rubber, and it is revealing that these products represent the three families of cationically prepared industrial polymers... 

Figure 5-16. Physical stress relaxation at 25 °C in a crosslinked network made by a zinc oxide cure of a halobutyl rubber. The elastomer has been soaked in oil to speed the relaxation. The line is the fit to the data using the Chasset-Thirion equation with E =0.018 MPa, t= 1.04 s and m = 0.31. The nature of the polymer and the cure makes for a chemically stable structure otherwise, the plot would begin to curve downward at long time.

The manufacture of halobutyl rubbers such as Bromobutyl, Chlorobutyl, and Exxpro [bromopoly(isobutylene-co-/j-methylstyrene)] requires a second chemical reaction the halogenation of the polymer backbone. This can be achieved in two ways, the finished polymer produced in the butyl plant can be dissolved in a hydrocarbon solvent such as hexane or pentane, or a solvent replacement process can be used to dissolve the polymer from the slurry leaving the reactor. A schematic flow diagram of the halogenation process is shown by Figure 2. 

Sandstrom and co-workers utilized the cyclized polyisoprene polymers incorporated into tyre tread compounds to improve traction, tread wear and tear resistance. Cyclized polyisoprene is further based upon the discovery that blends of cyclized polyisoprene polymers with halobutyl rubber and/or NR can be employed as tyre inner liner formulations. 

Halobutyl rubbers can be crosslinked with phenolic resins by the same mechanism postulated for regular butyl rubbers. The usual association with zinc oxide ensures a high state of cure and the vulcanizates have excellent dry heat resistance. However, they are inferior to resin cured regular butyl rubber in resistance to steam and superheated water. Since halogen is present in the polymer itself, another halogen donor is not neeessary in the resin cure of halobutyl rubbers. 

Metal oxides, usually zinc oxide but on occasion lead oxide for improved water resistance, are used as crosslinking agents for halogenated elastomers such as neoprene, halobutyl rubber, and chlorosulfonated polyethylene. The metal oxide abstracts the allylic halogen of adjacent polymer chains to form an oxygen crosslink plus the metal chloride salt. 

The SEM is also used to do X-ray/elemental analysis. This technique is qualitative. X-ray analysis and mapping of the particular elements present is useful for the identification of inorganic fillers and their dispersion in compounds as well as inorganic impurities in gels or on surfaces and curatives, e.g., aluminum, silicon, or sulfur in rubber compounds and Cl and Br in halobutyl blends. Refer to Transmission Electron Microscopy. (Source Cheremisinoff, N.P. Polymer Characterization Laboratory Techniques and Analysis, Noyes Publishers, New Jersey, 1996). 

Thermoplastic vulcanizates (TPVs) are composed of a vulcanized rabber component, such as EPDM, nitrile rubber, and butyl rubber in a thermoplastic olelinic matrix. TPVs have a continuous thermoplastic phase and a discontinuous vulcanized rubber phase. The most common TPV polymer systan is PP/EPDM rubber however, a number of other polymer systems have been commercially developed. These include PP/NBR, PP/butyl and PP/halobutyl, PP/NR, and PP/EVA/EPDM. Producers include Advanced Elastomers Systems (Santoprene, Geolast, and Trefsin). The highly rubberlike properties of TPV have enabled than to perform as engineered thermoplastic rubbers. In numerous application areas they have directly replaced premium-performance thermoset rubber compounds. Prominent among these are dananding automotive applications, electrical insulation and connectors, compression seals, appliance parts, medical devices, and food and beverage contact applications. 

Halogenation is the second major chemical reaction in the production of halobutyl pol5uners. It is usually carried out by adding bromine liquid or chlorine vapor to a solution of rubber in hydrocarbon solvent (hexane or pentane), which is often referred to as cement. The cement must be essentially free of monomers, or low molecular weight toxic species will be formed dining the chlorine or bromine reactions. The halogenation of the butyl backbone is an ionic-substitution reaction in which the halogen is added to the cement stream in a well-mixed reactor (eq. 1). This reaction is unusual for polymers where it s more typical for the bromine to add across the double bond. 

Isobutylene-based polymers exhibit high damping and have imiquely broad damping and shock absorption ranges in both temperature and frequency. Molded rubber parts from butyl and halobutyl find wide applications in automobile suspension bumpers, exhaust hangers, and body moimts. 

Table 2 shows the manufacturing capacity for the world s major butyl and halobutyl polymers manufacturers. In addition to ExxonMobil Chemical and Bayer, during the last 10 years, butyl rubber produced at both Nizhnekamsk and Togliati has been sold throughout the world. Also, in the last 5 years, Sinopec has started to produce butyl polymers at its Yanshan plant in the People s Republic of China. All of the world s butyl plants use the more efficient slurry process except for the Togliati plant, which uses the solution process. 

Hydrotaldtes act in scavenging addic decomposition products of halogenated polymers such as PVC, polychloroprene (CR), chlorosulfonated polyethylene (GSM), chlorinated polyethylene (CPE), epichlorohydrin (ECO), fiuoroelastomers (FKM), and halobutyls (bromobutyl mbber (6IIR), dilorobutyl rubber (CIIR)). 

Exceptional heat resistance and low compression set can be obtained by curing butyl rubbers with dimethylol phenol resins. The curing reaction is very slow even at high temperatures and when activated by halogens. Stannous chloride, and combinations of a halogenated polymer (such as neoprene, halobutyl or brominated resin) with zinc oxide, are the most commonly used halogen-bearing activators. The systems shown in Table 5 are typical. 

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