Elastomers temperature effects

Finally, it is instructive to compare the temperature effect on the tensile strength of the SBS and SIS block polymers. As noted previously (Figure 6) the tensile strength of an elastomer vulcanizate can be related to the difference between the test temperature and the Tg of the elastomer, in accordance with the viscoelastic theory of tensile strength. Since the Tg values for polyisoprene ( — 65°C) and polybutadiene ( —95°C) differ... 

Temperature effects are important, not only in high temperature applications, but also during accelerated aging of a material for laboratory study and subsequent evaluation of in-service life-time. Solubility of water In Neoprene Increases greatly with temperature as does permeation rate. The relative permeation rates of 3.5% saltwater and deionized water were shown to depend on elastomer composition. 

Hilado, C. J., Huttlinger, P. A. Integration of temperature effects in toxicity of off-gases from plastics. J. Elastomers and Plastics, 13, 108 (1981)... 

Rate and Temperature Effects. Like adhesive tack, autohesion of elastomers is strongly dependent on test rate and temperature. Furthermore, as shown in Figure 13 for the T-peel autohesion of a cold emulsion SBR, relative autohesion Pr (for a given time and pressure of contact) is not unique, but it too depends markedly on test conditions (104). 

The technique also measures the modulus (stiffness) and damping (energy dissipation) properties of materials as they are deformed under periodic stress. Such measurements provide quantitative and qualitative information about the performance of the materials. The technique can be used to evaluate elastomers, viscous thermoset liquids, composite coatings, and adhesives, and materials that exhibit time, frequency, and temperature effects or mechanical properties because of their viscoelastic behaviour. 

Temperature Elastomer/post-elastic stiffiiess, Increase for low temperatures effect depends on exposure duration elastomer becomes brittle below glass transition temperature (compound dependent, —55 °C for natural rubber) ... 

Details are given of the competing chemical reactions involved in the peroxide vulcanisation of elastomers. The effect of various factors including the type of polymer, type and concentration of peroxide, cure temperature, and the presence of additives such as co-agents and antidegradants are discussed. 100 refs. 

Zigzag chain formations can straighten out under stress, thus leading to larger elongations in elastomers. The temperature effects are also exhibited in the viscoelastic behavior of such polymers. 

Elastomeric Modified Adhesives. The major characteristic of the resins discussed above is that after cure, or after polymerization, they are extremely brittie. Thus, the utility of unmodified common resins as stmctural adhesives would be very limited. Eor highly cross-linked resin systems to be usehil stmctural adhesives, they have to be modified to ensure fracture resistance. Modification can be effected by the addition of an elastomer which is soluble within the cross-linked resin. Modification of a cross-linked resin in this fashion generally decreases the glass-transition temperature but increases the resin dexibiUty, and thus increases the fracture resistance of the cured adhesive. Recendy, stmctural adhesives have been modified by elastomers which are soluble within the uncured stmctural adhesive, but then phase separate during the cure to form a two-phase system. The matrix properties are mosdy retained the glass-transition temperature is only moderately affected by the presence of the elastomer, yet the fracture resistance is substantially improved. 

Butadiene—Methacrylic Acid Ionomers. Carboxyl groups can readily be introduced into butadiene elastomers by copolymerization, and the effects of partial neutralization have been reported (63—66). The ionized polymers exhibit some degree of fluidity at elevated temperatures, but are not thermoplastic elastomers, and are very deficient in key elastomer properties such as compression set resistance. 

Nitrile Rubber. Nitrile mbbers are made by the emulsion copolymerization of acrylonitrile (9—50%) and butadiene (6) and designated NBR. The ratio of acrylonitrile (ACN) to butadiene has a direct effect on the properties on the nature of the polymers. As the ACN content increases, the oil resistance of the polymer increases (7). As the butadiene content increases, the low temperature properties of the polymer are improved (see Elastomers, SYNTHETIC-NITRILE RUBBER). 

Physical Factors. Unsatuiated elastomers must be stretched for ozone cracking to occur. Elongations of 3—5% are generally sufficient. Crack growth studies (10—18) have shown that some minimum force, called the critical stress, rather than a minimum elongation is required for cracking to occur. Critical stress values are neady the same for most unsaturated mbbers. However, polychloroprene has a higher critical stress value than other diene mbbers, consistent with its better ozone resistance. It has been found that temperature, plasticization, and ozone concentration have httie effect on critical stress values. 

In Chapters 3 and 11 reference was made to thermoplastic elastomers of the triblock type. The most well known consist of a block of butadiene units joined at each end to a block of styrene units. At room temperature the styrene blocks congregate into glassy domains which act effectively to link the butadiene segments into a rubbery network. Above the Tg of the polystyrene these domains disappear and the polymer begins to flow like a thermoplastic. Because of the relatively low Tg of the short polystyrene blocks such rubbers have very limited heat resistance. Whilst in principle it may be possible to use end-blocks with a higher Tg an alternative approach is to use a block copolymer in which one of the blocks is capable of crystallisation and with a well above room temperature. Using what may be considered to be an extension of the chemical technology of poly(ethylene terephthalate) this approach has led to the availability of thermoplastic polyester elastomers (Hytrel—Du Pont Amitel—Akzo). 

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