Intrinsic Defects and the Failure Properties of cis-1,4-Polyisoprenes

Abstract

Inherent flaw sizes were determined from fatigue lifetimes, and from the crack length dependence of the strain energy to break, for four cis-1,4-polyisoprenes compounded to have the same crosslink density and low strain hysteresis. Both techniques indicated that the flaws intrinsic to guayule rubber (GR), and to a lesser extent conventional natural rubber, are larger than those found in deproteinized NR. This result may not be surprising; however, the failure properties of the elastomers, expected to depend on flaw size, were surprising. The guayule rubber and a natural rubber of relatively low purity (SMR-10) had the highest tensile strengths, tear strengths, breaking energies, and fatigue lifetimes, while DPNR exhibited the worst failure properties. Such an inverse correlation between flaw size and failure performance is due to the dominant effect of strain-induced crystallization. GR and SMR-10 have the highest propensity for strain-induced crystallization, while DPNR is the least strain-crystallizable. Interestingly, all rubbers exhibited the same isotropic crystallization behavior.

The Fatigue Life of Hevea and Guayule Rubbers

“…DPNR exhibits the worst fatigue properties. These results are consistent with an earlier comparison of these rubbers at a fixed crosslink density (single value of modulus = 1 MPa), 25 in which the failure performance was in the order GR > RSS > DPNR. This rank ordering parallels the strain-crystallizability of the rubbers, 35 indicating that the non-polymeric contaminants nucleate crystallization.…”

“…We have shown that GR and NR of relatively low purity have better failure properties, at least as unfilled (gum) networks at a single crosslink density, than purer grades of natural rubbers. 25 Such a result is due to the enhancement of strain-crystallization by the non-polymeric ingredients of the material. 35 In the present work, we extend the comparison to rubbers having a range of crosslink densities, and also examine the effect of carbon black on the relative performance.…”

“…Using the former method (since the present work is concerned with fatigue lifetimes), c 0 was deduced for the rubbers herein, 25 and these results are given in Table II. There is no expectation that crosslinking will change the size of the inherent flaw, although it can affect an elastomer's response to flaws.…”

“…24 Previously, we showed that those grades having more non-rubber contaminants are associated with larger intrinsic flaws; however, when compared at equal modulus, the less pure grades exhibited the best failure properties. 25 This is contradictory, since flaws concentrate stress, thus promoting fracture and fatigue. However, these same non-rubber contaminants nucleate crystallization.…”

“…51,52 A quadratic dependence was found for the present materials. 25 Integration of Equation (3) gives an expression for the fatigue life for uniaxially strained rubber 45,46 …”