Charoen Nakason

The grafting of maleic anhydride onto natural rubber

Abstract Graft copolymers of natural rubber and maleic anhydride (NR-g-MA) or maleated natural rubber (MNR) were synthesized in a toluene solution. Benzoyl peroxide (BPO) was used to initiate the free radical graft copolymerization. Influence of monomer and initiator concentrations together with effect of the reaction temperature and reaction time were investigated. Quantities of the grafted anhydride (MA) were determined by titration of the carboxylic acid functions derived from the anhydride functions. Estimation of the grafted MA level was also performed using IR absorbance ratio of the peaks at 1780–1784 cm−1 plus 1854 cm−1 to 835 cm−1. We found that quantities of the grafted MA on NR molecules increase with increasing monomer and initiator concentrations. The increase in reaction time and reaction temperature also causes increasing levels of the grafted MA. We also found that the glass transition temperature of the MNR was higher than that of the pure NR. Furthermore, the Tg values increase with increasing the monomer concentration used in the graft copolymerization.

Rheological properties of maleated natural rubber and natural rubber blends

Two types of natural rubber (ADS and STR 5L) were used to prepare maleated natural rubber (MNR). Melt rheological properties of MNRs (i.e. MNR-ADS and MNR-STR) and their blends with the same type of natural rubber were quantified. We found that the Mooney viscosities of MNR-ADS/ADS blends were higher than those of MNR-STR/STR 5L blends. The log additive rule was applied to the Mooney viscosity data. Positive deviation blends (PDB) were observed. A power law model was also applied to the shear flow data at a wide range of shear rates. The pseudoplastic (shear-thinning) behavior for all sets of MNR/NR blends were observed with the power law index, n, lower than 1. According to the log additive rule, plots of the consistency index against MNR levels in the blends also indicated positive deviation blends. Shear viscosity data of the blends also confirmed the shear-thinning behavior and the positive deviation blends. From the rheological property point of view, we therefore concluded that the MNR/NR blends were compatible.

Thermoplastic elastomers-based natural rubber and thermoplastic polyurethane blends

Thermoplastic elastomers based on the blends of thermoplastic polyurethane (TPU) and natural rubber were prepared by a simple blend technique. The influence of the two different types of natural rubber (i.e., unmodified natural rubber (NR) and epoxidized natural rubber (ENR)) on properties of the blends was investigated. The main aim of this study was to improve heat resistance and damping properties, and also to prepare the TPU material with low hardness by blending with various amounts of natural rubber. It was found that the TPU/ENR blends exhibited superior modulus, hardness, shear viscosity, stress relaxation behavior and heat-resistant properties compared to the blends with TPU and unmodified NR. This was attributed to higher chemical interaction between the polar functional groups of ENR and TPU by improving the interfacial adhesion. It was also found that the ENR/TPU blends exhibited finer grain morphology than the blends with unmodified NR. Furthermore, lower tension set, damping factor (Tan δ) and hardness, but higher degradation temperature, were observed in natural rubber/TPU blends compared to pure TPU. This proves the formation of TPU material with high heat resistance, low hardness and better damping properties. However, the blends with higher proportion of natural rubber exhibited lower tensile strength and elongation at break.

Rheological and curing properties of reactive blending products of epoxidised natural rubber and cassava starch

Abstract Epoxidised natural rubber (ENR) has been prepared and used as a blending ingredient together with a compatibiliser for blending of natural rubber (air dry sheet, ADS) and cassava starch. Mooney viscosities of the blends were quantified at 100°C and rheological properties in terms of shear stress and shear viscosity were plotted against shear rates. The results showed that pure ENR gave a lower Mooney viscosity, shear stress, and shear viscosity than blends with cassava starch. Mooney viscosity, shear stress, and shear viscosity for the blends increased with increasing levels of starch. At the same level of cassava starch blended, the highest values of these quantities were observed for the blends with ENR. The blend of ADS with ENR as a compatibiliser showed lower values than those of ENR itself, but higher than those of ADS with the starch. The results are described in terms of the level of chemical interaction between polar groups in ENR and in cassava starch. Curing behaviour for compounds of ENR, ADS, and ADS with ENR as a compatibiliser were studied. The results found that ENR exhibited a long delay (∼ 10 min) before the vulcanisation took place compared with 1 min for ADS compounds. In the curing curve for ENR, an equilibrium value at maximum torque was not found indicating that the stiffness of the ENR compounds still increased with increasing testing time until 60 min. Stiffness of the ENR compounds also increased with increasing levels of cassava starch.

Electrical, dielectric, and dynamic mechanical properties of conductive carbon black/epoxidized natural rubber composites:

Electrically conductive epoxidized natural rubber filled with conductive carbon black was prepared. The AC conductivity (σAC), dielectric constant (ɛ′), loss factor (tan δ*) and dynamic mechanical properties of the composites were studied. It was found that the epoxide groups in epoxidized natural rubber molecules positively contributed to AC conductivity, dielectric constant, and tan δ* of the composites. Especially, the composite with epoxidized natural rubber containing 50% mol epoxide (epoxidized natural rubber–50) showed better electrical and dynamic mechanical properties than the composites with epoxidized natural rubber–25 or NR. The effects of conductive carbon black loading level on electrical conductivity and dielectric constant of the epoxidized natural rubber–50/conductive carbon black composites was also studied. The percolation threshold was found at very low content of conductive carbon black at volume fraction of 0.07 with the critical exponent value 2.04. Furthermore, the glass transition temperatures of epoxidized natural rubber–50/conductive carbon black composites were higher than those of epoxidized natural rubber–25/conductive carbon black or NR/conductive carbon black composites, and increased with conductive carbon black content.

Novel Interpenetrating Polymer Networks Based on Natural Rubber/Poly(vinyl alcohol)

Fully interpenetrating polymer networks (IPN) based on natural rubber (NR) and poly(vinyl alcohol) (PVA) were prepared by using glutaraldehyde as the common cross-linking agent. The mechanical performance of this system has been studied in detail. Stress-stain behavior, tensile strength, elongation at break and youngs modulus was determined. Effects of weight fraction on the physical properties of NR/PVA IPNs were also studied along with the thermal degradation behavior using thermogravimetric analysis. A remarkable improvement was observed in the mechanical strength and the thermal stability of the blend by the formation of IPN. The activation energy of degradation was analyzed using the Horowitz-Metzger equation. The solvent resistance properties of these samples were investigated by the equilibrium swelling method using benzene as a solvent. The impact of weight fraction of PVA on the solvent transport of the system was also analyzed. A reduction in swelling was exhibited with the addition of PVA to natural rubber by the formation of IPN. The water absorbability of the blend decreases by the addition of glutaraldehyde.

Influence of Filler from a Renewable Resource and Silane Coupling Agent on the Properties of Epoxidized Natural Rubber Vulcanizates

Rice husk ash (RHA) was used as a reinforcing filler in epoxidized natural rubber (ENR) with various loading levels (0, 10, 20, and 30 phr), and silica filled ENR was also studied for comparison. The effects of RHA content on cure characteristics, mechanical properties, dynamic mechanical properties, and thermoelastic behavior of the filled ENR composites were investigated. It was found that the incorporation of RHA significantly affected the cure characteristics and mechanical properties. That is, the incorporation of RHA caused faster curing reactions and increased Young’s modulus and tensile strength relative to the unfilled compound. This might be attributed to the metal oxide impurities in RHA that enhance the crosslinking reactions, thus increasing the crosslink density. Further improvements in the curing behavior and the mechanical properties of the filled composites were achieved by in situ silanization with bis(triethoxysilylpropyl) tetrasulfide (Si69). It was found that the rubber-filler interactions reinforced the composites. This was indicated by the decreased damping characteristic and the other changes in the mechanical properties. Furthermore, the ENR composites with Si69 had improved filler dispersion. Temperature scanning stress relaxation (TSSR) results suggest that the metal oxide impurities in RHA promote degradation of the polymer network at elevated temperatures.

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.

Influence of Reaction Volume on the Properties of Natural Rubber-g-Methyl Methacrylate

Graft copolymer of natural rubber and poly(methyl methacrylate) was prepared using CHP/TEPA redox initiators at 50°C and a reaction time of 3 h. Various reaction volumes (i.e., 0.5, 100, and 200 L) were used to prepare the graft copolymer which was then characterized by Fourier transform infrared spectrophotometer and proton nuclear magnetic resonance spectrophotometer (1H-NMR) techniques. It was found that conversion of monomer to polymer and grafting efficiency slightly decreased with increasing reaction volumes. Quantity of grafted poly(methyl methacrylate) was calculated based on the integrated peak areas of the 1H-NMR spectra and quantitative analysis by extraction method. It was found that both techniques gave similar level of the grafted poly(methyl methacrylate) onto the natural rubber backbone. Furthermore, Mooney viscosities, glass transition temperature (Tg) and degradation temperature (Td) of the natural rubber and poly(methyl methacrylate) were slightly decreased with increasing the reaction volumes.

Effect of Modified Natural Rubber and Functionalization of Carbon Nanotubes on Properties of Natural Rubber Composites

The intention of this work was to prepare natural rubber filled with carbon nanotubes (CNT). Various types of natural rubber, including unmodified natural rubber and epoxidized natural rubber with 25 mol% epoxy groups (i.e., ENR-25) were exploited. The state of CNT dispersion in the unmodified natural rubber matrix was improved by the modification of CNT surface with silane coupling agent (Si-69). The modification was carried out by in-situ method, i.e., the silane was directly added during the preparation process of the composites in the internal mixer. The interaction between CNT surface and rubber molecules, with and without silane modification, was characterized by ATR-FTIR measurements. The mechanical properties and crosslink densities were investigated, the electrical conductivity was characterized to determine the percolation threshold.