Suwaluk Wisunthorn

Comparative study of the mesostructure of natural and synthetic polyisoprene by size exclusion chromatography-multi-angle light scattering and asymmetrical flow field flow fractionation-multi-angle light scattering

This paper presents results from the first analyses of the mesostructure of natural rubber (NR) by asymmetrical flow field flow fractionation (AF4). The results are compared with those obtained by size exclusion chromatography (SEC) in terms of average molar masses, radius of gyration and insoluble part (or gel quantity). Comparable results were obtained for the sample not containing gel. Conversely, for samples with gel, significant differences were found due to the presence of microaggregates. Contrary to SEC, AF4 fractionation enables partial fractionation of polyisoprene chains and microaggregates in a single run without preliminary treatment. The results presented here also highlight the special structure (very compact spheres) of microaggregates in NR compared to chemical crosslinked microaggregates in synthetic polyisoprene. The advantages and drawbacks of both techniques for analysing NR samples are also discussed.

Biodegradability and Thermal Properties of Novel Natural Rubber/Linear Low Density Polyethylene/Thermoplastic Starch Ternary Blends

This work aimed to prepare biodegradable thermoplastic elastomers based on NR/LLDPE/TPS ternary simple blends to achieve some exclusive properties, i.e., good biodegradability in terms of water absorption and weight loss after burial, together with reasonable mechanical and thermal properties. A comparative study on biodegradability and other related properties of NR/LLDPE binary and NR/LLDPE/TPS ternary blends was performed. It was found that increasing the TPS proportion decreased storage modulus and complex viscosity. In addition, the size of dispersed TPS domains in the NR/LLDPE co-continuous matrix increased with TPS proportion, while the mechanical properties in terms of 100% moduli, tensile strength, elongation at break, and hardness decreased. This might be attributed to decreased interfacial adhesion with increasing size of TPS domains. Furthermore, increasing the TPS loading in the blend reduced the temperatures for 5 or 50% mass loss (T5 or T50) and the degradation temperature (Td). However, the biodegradability improved, in terms of increased water absorption and weight loss after burial in soil, with the loading level of TPS.

Effect of Rubber Clone on Fatty Acid Composition and Properties of Air Dried Sheet

Properties of air dried sheet (ADS) made from four different clones of Hevea brasiliensis (BPM24, RRIM600, PR255 and PB235) were investigated. BPM24, RRIM600 and PR255 rubbers show a bimodal molar mass distribution (MMD) while PB235 rubber has quasi-unimodal molar mass distribution. It was found that ADS from rubber clone with bimodal MMD (BPM24, RRIM600 and PR255) displayed lower original plasticity (P0) but higher plasticity retention index (PRI). Those rubber samples with bimodal MMD show higher accelerated storage hardening test (ASHT) values than ADS from PB235. Within bimodal rubbers, ADS from PR255 displayed lower PRI and P than the 2 other clones, i.e. RRIM600 and BPM24. ADS made from BPM24 and RRIM600 presented similar fatty acid composition with 3 major fatty acids namely linoleic acid (C18:2), stearic acid (C18:0) and oleic acid (C18:1). In contrast, major fatty acids of ADS made from PR255 were linoleic acid (C18:2) and furan fatty acid. Moreover, ADS from PB235 showed the highest amount of furan fatty acid which confirmed the results of Liengprayoon et al. [1]. Relationship between mesostructure, fatty acids composition and ADS properties are discussed.

Mesostructure Evolution during Storage of Rubber Films from Washed Cream Latex and Skim Latex

Fresh field latex (FL) of Hevea brasiliensis was centrifuged to separate the major compartments: (i) the cream, (ii) the skim plus the C-serum, and (iii) the bottom fraction. Cream was dispersed in distilled water to prepare cream washed latex or CL sample. Skim was collected together with C-serum to prepare the skim latex or SKL sample. Then, films were made from FL, CL and SKL. The mesostructure (macromolecular structure and aggregates or gel) of the natural rubber films was studied by size exclusion chromatography coupled to a multiangular light scattering detector (SEC-MALS). The mesostructure was analyzed on fresh films and on films stored in the laboratory for three months (slow structuring). This study showed that FL and CL films had bimodal elution profiles of long and short polyisoprene chains as revealed by the two peaks on the chromatograms at low and high elution volumes, respectively. On the contrary, SKL films exhibited unimodal elution profiles at low elution volume and a highly abnormal molar masses elution profiles. After 3 months storage, the mesostructure of FL evolved dramatically especially the number-average molar masses (Mn), which increased by 60%. The mesostructure of CL films evolved at a lesser extent and, the mesostructure of SKL films exhibited no significant change. These results indicated (i) the skim rubber particles are not prone to slow structuring during storage in presence of serum and (ii) some constituents either in the serum or/and in the bottom fraction could still be present with cream and be involved in the slow structuring of cream particles latex.