Creep, Recovery, and Permanent Set for GR-S and Hevea

Abstract

A study was made of creep, recovery, and permanent set for Hevea and GR-S over a 1000-hr. period and a range of elongations at 35°C. Creep for GR-S is greater than for Hevea. The stress for GR-S is considered to be supported to a greater extent by a relatively unstable secondary bond structure. This is consistent with the large creep values for GR-S at low elongations and the large ratio of primary or recoverable creep to permanent set. Hevea, in contrast, shows low creep at both high and low elongations and maximum creep at intermediate elongations for which the structure is heterogeneous, consisting of amorphous and crystalline phases. A procedure was worked out for determining the contribution of permanent set to the observed creep. The creep curves were concave to the strain axis when plotted against logarithmic time. After correction was made for permanent set, they were found to be approximately linear, thus extending the range of application of the Tobolsky-Eyring theory of creep. At the start of the creep test the flow appears to be due largely to the reversible yielding of relatively weak bonds which can reform under the action of the elastic network when the stress is removed. For longer periods of time, or for higher elongations, the flow involves more deep seated changes in structure. Larger units of structure are displaced or stronger bonds broken with resultant permanent molecular displacements upon removal of the stress. For Hevea at high elongations all of the flow was accounted for by permanent set.