Butyl rubbers elastomer selection

Plastics, such as PE, PP, polystyrene (PS), polyester, and nylon, etc., and elastomers such as natural rubber, EPDM, butyl rubber, NR, and styrene butadiene rubber (SBR), etc., are usually used as blend components in making thermoplastic elastomers. Such blends have certain advantages over the other type of TPEs. The desired properties are achieved by suitable elasto-mers/plastic selection and their proportion in the blend. 

In the literature, there are several reports that examine the role of conventional fillers like carbon black on the autohesive tack (uncured adhesion between a similar pair of elastomers) [225]. It has been shown that the incorporation of carbon black at very high concentration (>30 phr) can increase the autohesive tack of natural and butyl rubber [225]. Very recently, for the first time, Kumar et al. [164] reported the effect of NA nanoclay (at relatively very low concentration) on the autohesive tack of BIMS rubber by a 180° peel test. XRD and AFM show intercalated morphology of nanoclay in the BIMS rubber matrix. However, the autohesive tack strength dramatically increases with nanoclay concentration up to 8 phr, beyond which it apparently reaches a plateau at 16 phr of nanoclay concentration (see Fig. 36). For example, the tack strength of 16 phr of nanoclay-loaded sample is nearly 158% higher than the tack strength of neat BIMS rubber. The force versus, distance curves from the peel tests for selected samples are shown in Fig. 37. 

Figure 10-18 presents the GC/MS analysis of a rubber formulation for an elastomeric insulator for devices to replace those that had been characterized after field-aging. In this example, a high quality silicone rubber was selected for inherent resistance to oxidation and UV damage, plus favorable dielectric properties. Figure 10-18 presents the GC/MS analysis of this elastomer, confirming pure silicone rubber and Tinuvin P, a UV absorbing polymer stabilizer. To reduce cost, many suppliers provide silicone rubber components that are often combined with butyl or nitrile rubber to reduce cost. 

The above criteria were employed to select several commercially supplied Class PS elastomers for laboratory screening by employing selected tests taken from National Bureau of Standards NBSIR 77-1437(j4) and ANSI/ASTM D-3667-78 specifications for "Rubber Seals Used in Flat-Plate Solar Collectors". Four silicone, three EPDM, two fluorocarbon, three epichlorohydrin, one ethylene-acrylic, one polyacrylic, one chlorosulfonated polyethylene, one bromobutyl and two butyl rubbers were studied in these screening tests. These materials are identified in Table I and those compositions which were revealed by their manufacturers are shown in Table II. Undoubtedly some materials which should have been included were omitted however, we hope that this sampling will provide an indication of the applicability of a wide range of materials for use as sealants in thermal solar collectors. 

The elastomers considered in this section have been selected considering the most commonly used in rubber base adhesives natural rubber butyl rubber and polyisobutylenes styrene-butadiene rubber nitrile rubber polychloroprene rubber (neoprene). Typical properties of these rubbers are shown in Table 2. 

COMPOUNDING OF UNMODIFIED BUTYL RUBBER 3.1. Selecting the Elastomer... 

Where there is a need for very rapid cures coupled with optimum resistance to heat, weather and ozone, or for maximum adhesion to highly unsaturated elastomers, the halogenated butyl rubbers should be considered. Table 3 lists examples of appropriate selections of elastomers by unsaturation level, for a variety of applications. 

Butyl rubber is capable of very much lower water absorption rates than other common elastomers, partly because its structure restricts thermal motion (which is essential for diffusion), but mainly because it contains virtually no electrolytic impurities. This latter reason is, of course, a factor in the selection of butyl rubber for electrical insulation. 

A comparative analysis on thermal stability of two elastomers, butyl mbber and ethylene-propylene rubber, is presented in Figs. 30a - c. The CL investigations on these polymers [94J2] emphasize the differences that exist between them regarding the susceptibility upon oxidation. These results allow the proper selection of materials for particular applications. 

Many modifications of the elastomers above have been developed chiefly by copolymerization with various synthetic resins. These modified elastomers furnish the engineer with a wide selection of materials suitable for special uses. An example of a relatively new hydrocarbon rubber is a copolymer of ethylene and propylene, EPR. This rubber has outstanding resistance to ozone weathering. It has lower tear resistence and higher air permeability than Buna S (SBR), but can be lined with butyl to correct the latter property. 

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