SULFUR VULCANIZATION OF LOW- VERSUS HIGH-UNSATURATED RUBBERS (IIR AND EPDM VERSUS NR AND BR): PART II—NETWORK STRUCTURE AND TENSILE PROPERTIES
Sulfur vulcanization is the most common crosslinking technology for unsaturated rubbers. To enhance our generic understanding of the structure–property relationships for sulfur-vulcanized rubber networks, we have studied two low-unsaturated rubbers, IIR and EPDM, and two high-unsaturated rubbers, NR and BR, at varying levels of sulfur curatives. In the first part of this series, the rheometer torque maximum and the compression set as a function of the temperature were discussed. In this second part, the effects of the level of the rubber unsaturation, the density of the trapped entanglements, and the chemical crosslink density on the network structure and tensile properties are discussed quantitatively. Our results reveal that the networks of the vulcanized BR and EPDM consist mainly of trapped entanglements. For EPDM and IIR, all unsaturation can be fully converted to sulfur crosslinks. The tensile strength at break (TS) of vulcanized EPDM and BR is independent of the permanent network density, which is the sum of the chemical crosslink density and the trapped entanglements. The TS shows a pronounced maximum versus the permanent crosslink density for vulcanized IIR and NR, due to the absence of the reinforcing effect of strain-induced crystallization (SIC) at low crosslink densities and the suppression of SIC at high crosslink densities. The elongation at break decreases with increasing network density, following a power-law relation. Mooney–Rivlin analysis of the stress–strain curves confirms our findings of the network structure as obtained from rheometry. However, an unexpected, curved course of the second Mooney–Rivlin parameter as a function of the varying sulfur content is observed for the EPDM samples, indicating that vulcanized EPDM has a different, entanglement-dominated network structure in contrast to IIR, NR, and BR.ABSTRACT
Property matrix comparing the ability for strain-induced crystallization and the degree of unsaturation of IIR, EPDM, NR, and BR polymers.
Illustration of network formation. The chemical crosslink density increases with curatives content, and the transient entanglements are converted to trapped entanglements. The permanent network density is the sum of the chemical crosslink density and the trapped entanglement density, and approaches the total network density at high curatives content.
DMTA test results on uncrosslinked rubbers. (a) Storage modulus, G′, (solid) and loss modulus, G″, (dashed) versus temperature. The dots denote GN at minimum tan(δ). (b) Loss factor, tan(δ), versus temperature.
Tensile test results for uncrosslinked rubbers (green strength). (a) Stress–strain curves and (b) true stress versus strain.
Correlation between tensile strength (TS) at break measured at 23°C and entanglement density calculated at 23°C for uncrosslinked rubbers. The line is a fit forced through the origin. The error bars denote the 2σ standard deviations obtained from multiple TS tests.
MDR rheometer curves at 160°C for IIR, EPDM, NR, and BR compounds with 1.5 phr sulfur. Shown is the elastic contribution to the torque, S′, versus time.
Rheometer network density, νrheo, of IIR, EPDM, NR, and IIR vulcanizates versus elemental sulfur content in units phr (bottom x axis) and available sulfur content in mol/kg (top x axis). Solid curves and dashed lines are fits to guide the eye.
Tensile test results of sulfur-vulcanized (a) BR, (b) NR, (c) EPDM, and (d) IIR at varying elemental sulfur content (in phr). Note the different scales on the x and y axes of the stress–strain curves when comparing the different types of rubber.
Tensile strength versus elongation at break of IIR, EPDM, NR, and BR vulcanized at different sulfur levels. For NR and IIR, a maximum in tensile strength is observed with data fitted with Weibull curves. For EPDM and BR, linear fits are shown.
(a) Tensile strength and (b) elongation at break (logarithmic scale) as a function of rheometer network density at 23°C for IIR, EPDM, NR, and BR vulcanized at different sulfur levels.
(a) Experimental stress–strain curve (blue) measured on an IIR test specimen vulcanized with 1.5 phr sulfur. The solid black curve is the result of the Mooney–Rivlin analysis. (b) Corresponding Mooney–Rivlin plot. Vertical dashed lines mark the linear-fit range that was used to obtain the Mooney–Rivlin parameters.
(a) First, C1, and (b) second, C2, Mooney–Rivlin parameters for IIR, EPDM, NR, and BR, vulcanized with varying content of sulfur curatives. The error bars show the 2σ standard deviations obtained from multiple TS tests. The dotted lines denote the x axis.
Ratio of Mooney–Rivlin parameters, C2/C1, versus available sulfur content. The error bars show the 2σ standard deviations.
(a) Comparison of permanent network densities obtained from Mooney–Rivlin analyses of stress–strain curves measured at 23°C with network densities from rheometry calculated at 23°C for sulfur-vulcanized IIR, EPDM, NR, and BR. The dotted line denotes the x axis. (b) Total network densities obtained from Mooney–Rivlin analyses measured at 23°C versus rheometer network density calculated at 23°C. The dashed lines mark the bisectors.
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