The Rubber Hydrocarbon in Freshly Tapped Hevea Latex

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Abstract
The rubber hydrocarbon in latex as it leaves the tree is already a high polymer, with a broad distribution of molecular weight ranging from several millions to below 100,000, with the major part of the hydrocarbon in the higher molecular weight ranges. Changes in the hydrocarbon subsequent to its leaving the tree tend to be degradative, but may also involve cross-linking reactions leading to changes in plasticity. Structural changes in rubber attributed to restricted cross-linking have a greater influence on plasticity than on solution viscosity. A gel component (probably microgel) present in the freshly tapped hydrocarbon is of considerable importance in determining the hardness of raw rubber, but greatly complicates the interpretation of viscometric data, of which evaluation by the equation: [η]=5.02×10−4 M0.67 only appears to be valid for gel-free rubber fractions. Plasticity and intrinsic viscosity cannot, therefore, be generally correlated, although the plasticity is influenced by the spread of the molecular-weight distribution into lower molecular weight regions. The presence of a high proportion of microgel in latex of untapped trees and of branches remote from the tapping panel of trees in regular tapping, and the presence of combined oxygen in the low-molecular fractions of rubber hydrocarbon obtained from both normal and microgel latexes may be of interest in connection with questions of biosynthesis and metabolism of the rubber hydrocarbon of Hevea brasiliensis.

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