Mechanical properties of natural rubber vulcanizates in finite deformation

Abstract

Abstract Mechanical properties of four kinds of natural rubber vulcanizates differing in vulcanization conditions, and consequently in degree of crosslinking (having values of the Mooney‐Rivlin constant C 1 ranging from 0.68 to 1.98) were observed under orthogonal biaxial stretching in a range of strain invariants I i from 3.4 to 9.0 (extension ratios λ i from 0.7 to 3.0). The results obtained were analyzed by two methods. One method employed the Valanis‐Landel postulate that the strain‐energy function W (λ 1 , λ 2 , λ 3 ) is a separable symmetric function of the principal extension ratios, i.e., W (λ 1 ,λ 2 ,λ 3 ) = w (λ 1 ) + w (λ 2 ) + w (λ 3 ); the other utilized the contour plots of ∂ W ( I 1 , I 2 )/∂ I 1 and ∂ W ( I 1 , I 2 )/∂ I 2 surface within the ( I 1 , I 2 ) domain. The postulate for W was examined in detail with good agreement with experimental results. The dependences of ∂ W ( I 1 , I 2 )/∂ I 1 and ∂ W ( I 1 , I 2 )/∂ I 2 surfaces on the degree of crosslinking and temperature were further investigated, with the following conclusions. The surfaces have fairly steep slopes for the region of relatively small deformation (i.e., I 1 < 5) and become flat with increasing I i for all the test specimens. The slope becomes less steep with decreasing degree of crosslinking. The values of ∂ W /∂ I 1 increase linearly and the ∂ W ( I 1 , I 2 )/∂ I 2 surface becomes flat, both with increasing temperature: i.e., the temperature dependence of ∂ W /∂ I 1 further depends on I i . The ∂ W ( I 1 , I 2 )/∂ I 2 surface has a maximum near 40°C.