Characterization of Hevea Brasiliensis Rubber from Virgin Trees: A Possible Role of Cis-Polyisoprene in Unexploited Tree

Abstract

The role of rubber in Hevea brasiliensis was analyzed based on the structural analysis of rubber in rubber trees as latex. The rubber obtained from a mature-tree which has never tapped before, so-called “virgin mature-tree” (Vir-NR), contained the gel fraction higher than 80%, while that from a regularly tapped mature-tree (Reg-NR) was less than 3%. The gel showed almost the same structure as a crosslinked rubber prepared from fresh latex in the presence of peroxide, with the molecular weight between crosslinks (Mc) of 3×103. This value is extremely low comparing with that of 7×105 observed for the soft-gel in rubber from high-ammonia latex and Pale crepe. The sol fraction from Vir-NR was an oxidative degraded product containing aldehyde and epoxide groups. Aging properties of Vir-NR are much worse than those of Reg-NR. This indicates that rubber accepts free-radicals to form C-C crosslinking and partly oxidative scission of main-chains during storage in laticiferous tubes of Hevea trees. This suggests that a possible role of rubber is a scavenger of free-radicals in latex, due to lack of the naturally occurring antioxidants. The consecutive tappings of latex from the first opening mature-tree resulted in the formation of rubber similar to that obtained from Reg-NR, after six days. This modification indicates that the new regulation mechanisms in latex biosynthesis, which is induced by exploitation, can avoid the rubber chain crosslinkings and increase their elongations.

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

“…After 10 weeks of tapping, the MG rates were still around 20–30%, depending on the clone. Unlike seedlings, it took around 10 months of tapping to reach MG rates (1–5%) comparable to those of trees regularly tapped for several years 9, 11…”

“…Sakdapipanich et al9 showed that the macrogel upon tree opening (first tapping at time = 0) was not modified by transesterification, unlike the macrogel present in rubber from regularly tapped trees. The latter became soluble after transesterification.…”

“…The latter became soluble after transesterification. This macrogel upon tree opening was due to covalent crosslinking of polyisoprene macromolecules within the rubber particles 9. Such crosslinking reactions are catalyzed by toxic oxygen species (O 2 · − , OH · , etc.)…”

“…The quantity of gel decreased during successive tappings, becoming constant. Sakdapipanich et al,9 who did not study the microgel, showed that NR from virgin mature trees (10–16 years old), tapped for the first time, contained as much as 80% gel (or macrogel). The amount of the macrogel fell from 80 to about 3% after 6 days of tapping (tapping every day).…”

Molecular Structure of Natural Rubber and Its Characteristics Based on Recent Evidence

“…In contrast, the residual solvent-soluble fraction (the 'sol fraction') is of low molecular weight and formed by oxidative degradation. Both the 1 H-and 13 C-NMR spectra of the sol fraction of the rubber from virgin tree show prominent signals due to epoxide and hydroperoxide, as well as aldehyde (Tangpakdee & Tanaka, 1998b;Sakdapipanich et al, 1999a). It is remarkable that the 13 C-NMR signals due to methyl protons of trans-isoprene units in the initiating terminal were not detected in the sol fraction of virgin tree.…”