Recent Advances in Structural Characterization of Natural Rubber

Abstract

Structural characterization of rubber from Hevea brasiliensis (NR) has been carried out to elucidate the biosynthesis mechanism of rubber molecule as well as to find the relationship between physical properties characteristic of NR and its structure. Recent advances of structural studies have provided a series of new information on the structure of long-chain branching based on the result of selective decomposition of branch-points by chemical and enzymatic treatments as well as physical treatment such as polar solvent treatments and washing of NR latex by centrifugation in the presence of a surfactant. The measurement of the resulting rubber with NMR, FTIR, SEC and dilute solution viscosity provided confirming evidence that the initiating terminal (ω-terminal) with an unidentified functional group and phosphate terminal (α-terminal) form branch-points by hydrogen bond, ionic bond or micelle formation of phospholipids linked to both terminal groups. Based on these results, the origin of green properties characteristic of NR has been explained and a new mechanism of storage hardening has been proposed. The purification method by saponification of NR latex developed for the structural characterization has been applied to produce purified NR latex free from Type I allergic reaction. Instantaneous coagulation of saponified latex by the use of a flocculant and formic acid has provided solid saponified NR having good green and cured rubber properties.

Characterisation of macrogel composition from industrial natural rubber samples: Influence of proteins on the macrogel crosslink density

“…Interestingly, the crosslink density of a macrogel exponentially increases with its protein concentration (R 2 = 0.967). Several studies have already suggested that proteins are partly responsible for the presence of macrogel in NR samples [14,21]. Our results showed that proteins play a major role in the structuring of a macrogel, and seem to be involved in the majority of crosslinks.…”

“…Our results showed that proteins play a major role in the structuring of a macrogel, and seem to be involved in the majority of crosslinks. The influence of esterified lipids on the presence of macrogel has also been reported [21,22]. In this paper, we have shown that extractable lipids amounted to less than 0.5% (w/w) for macrogels with low protein concentrations and could be neglected compared to proteins.…”

“…However, it is worth noting that the method given by Liengprayoon et al [35] can only be used to study extractable lipids. Non-extractable esters [17] might partly be responsible for the associative structure of NR, as suggested by Tanaka and coworkers [21,22]. FT-IR characterization of the two macrogels showed the presence of non-extractable esters ( Figure 6).…”

“…For example, macrogel can be eliminated by transesterification or deproteinization of NR. Consequently, Tanaka and cowork-ers [21,22] suggested the presence of esterified compounds, such as phospholipids, covalently bonded to the !-terminal group of the macromolecule. The authors also suggested physical interactions between proteins and the "-terminal group of the polymer [21].…”

“…Consequently, Tanaka and cowork-ers [21,22] suggested the presence of esterified compounds, such as phospholipids, covalently bonded to the !-terminal group of the macromolecule. The authors also suggested physical interactions between proteins and the "-terminal group of the polymer [21]. Nevertheless, the role of each nonisoprene compound, as well as the nature of the terminal groups of the cis-1,4-polyisoprene chains of NR, is still unclear.…”