The ultimate properties of amorphous rubbers at temperatures above Tg are considered in terms of stress-strain curves to rupture measured at different strain rates and temperatures. The consideration indicates that a specimen held at a fixed elongation should break eventually, provided the elongation exceeds a critical value. This expected behavior was found by studying an SBR rubber. For samples maintained at different elongations for up to seven days, both the time to break and the stress at break were measured at eight temperatures from 1.7° to 60°C. For comparison, the ultimate properties were also measured at different strain rates and temperatures. The comparison indicates that for a given ultimate elongation and stress at break, the time to break under conditions of constant elongation is less than under conditions of constant strain rate.
A study was made previously of the temperature and strain rate dependence of the stress at break (tensile strength) and the ultimate elongation of an unfilled SBR rubber. In that study, stress-strain curves to the point of rupture were measured with an Instron tensile tester on ring type specimens at 14 temperatures between −67.8° and 93.3° C, and at 11 strain rates between 0.158×10−3 and 0.158 sec−1 at most temperatures. The tensile strength was found to increase with both increasing strain rate and decreasing temperature. At all temperatures above −34.4° C, the ultimate elongation was likewise found to increase with increasing strain rate and decreasing temperature but at lower temperatures the opposite dependence on rate was observed; at −34.4° C, the ultimate elongation passed through a maximum with increasing rate.
The homopolymer and butadiene copolymers of 1,1‐dihydroperfluorobutyl acrylate form a new class of vulcanizable elastomers with interesting and useful properties, of which solvent resistance is the most outstanding. The homopolymer (poly‐FBA), vulcanized by means of a polyfunctional amine in the presence of a reinforcing pigment, appears most promising because it offers the following unusual combination of desirable properties: (1) resistance to hydrocarbon solvents; (2) resistance to a variety of lubricants, hydraulic fluids, and similar high boiling liquids at temperatures up to 400°F.; (3) resistance to oxidation by ozone and fuming nitric acid; (4) fair stability and physical properties in air at elevated temperatures. It was shown that many of these properties depend upon compounding recipes and that further improvements can be anticipated.
Stress relaxation measurements on SBR were carried out at temperatures from −5 to +60° C and at initial strains of up to 550%. The effects of strain and time were found to be factorable, so that the isochronal stress-strain curve may be written as a modified Hooke's law with a time-dependent modulus: S=E(t)eƒ(α), where ƒ(α) is an appropriate function of the strain. By defining a strain-reduced stress S*=S/∫(α), i.e., a strain-reduced modulus E*(t)=E(t)ƒ(α), it can be shown that Ferry's method of reduced variables may be extended to large deformations. An appropriate strain function was obtained from the empirical Martin-Roth-Stiehler equation as ƒ(α)=α−2 exp A(α−α−1) with A=0.40. Although it cannot yet be certain that A is truly a constant and the same for all elastomers, this equation has the advantage of being valid right out to the breaking strain.
Stress relaxation measurements on SBR were carried out at temperatures from −5 to +60°C and at initial strains of up to 550%. The effects of strain and time were found to be factorable, so that the isochronal stress-strain curve may be written as a modified Hooke's law with a time dependent modulus: S = E(t)ef(α), where f(α) is an appropriate function of the strain. By defining a strain-reduced stress S* = S / f(α), i.e., a strain-reduced modulus E*(t) = E(t)f(α), it can be shown that Ferry's method of reduced variables may be extended to large deformations. An appropriate strain function was obtained from the empirical Martin-Roth-Stiehler equation [Trans. Inst. Rubber Ind. 32, 189 (1956)] as f(α) = α−2 expA (α−α−1) with A =0.40. Although it cannot yet be certain that A is truly a constant and the same for all elastomers, this equation has the advantage of being valid right out to the breaking strain.
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