Evolution in the natural rubber native structure and plasticity retention index from the first tapping of clonal trees

This study was carried out with five rubber clones planted in Côte d'Ivoire from the first tapping up to the 18th month of tree tapping (1 tapping/4 days). Changes in the natural rubber native mesostructure (macromolecular structure, macrogel, and microgel) of films prepared from fresh field latex were monitored. At the same time, the evolution of the thermooxidation sensitivity of raw rubber samples [grade 10 technically specified rubber (TSR10)] was also monitored with the plasticity retention index (PRI). The substantial initial macrogel rate (70–86%, depending on the clone) fell during the first 18 months of tree tapping to reach a few percent. However, during the same period, the initially low microgel rate (5–15%) increased and then remain stabilized around 55% rubber. The macromolecular structure [weight‐average molecular weight (Mw) and molar mass distribution (MMD)] also changed after tree opening. Mw increased and stabilized after 7.5 months of tapping. The bimodal MMD primarily involved short chains (molar mass < 400 kg/mol) at the opening of the trees. The TSR10 samples, prepared with latex from virgin trees, showed high PRIs and, therefore, low sensitivity to thermooxidation. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 903–909, 2005