Tetsuji Kawazura

Preparation and characterization of natural rubber dispersed in nano-matrix

Abstract Preparation of a model nano-matrix-dispersed polymer was investigated in terms of graft-copolymerization of deproteinized natural rubber latex with styrene, using tert -butyl hydroperoxide/tetraethylenepentamine as an initiator. The products were characterized by 1 H-NMR spectroscopy and size-exclusion-chromatography after ozonolysis. The grafting efficiency of styrene was found to be more than 90% under the best condition of the graft-copolymerization. The morphology of the film specimens, prepared from graft-copolymers, was observed by transmission electron microscopy after staining the films with OsO 4 . Natural rubber particle of about 0.5 μm in diameter was dispersed in polystyrene matrix of about 15 nm in thickness.

Thermoreversible crosslinking rubber using supramolecular hydrogen bonding networks

Thermoreversible crosslinking rubber (TRC-IR) was easily synthesized by modification of isoprene rubber (IR) with maleic anhydride followed by the addition of 3-amino-1,2,4-triazole (ATA), in solid phase. The mechanical properties of the resulting rubber were more similar to the sulfur-vulcanized rubber than general thermoplastic elastomers (ex. SEBS). Although the tensile strength and elongation at break were lower than those of a corresponding sulfur-vulcanized rubber, the moduli were as high as those of sulfur-cured rubber. Re-molding of TRC-IR could he repeated more than 10 times without significantly changing its mechanical properties. Differential scanning calorimetry (DSC) and infrared analyses revealed that the superior mechanical properties and good recyclability are attributable to the strong hydrogen bonding. The TRC-IR showed an endothermic transition peak at around 185 °C on the DSC chart, indicating cleavage of the hydrogen bonding. Infrared analyses also revealed that the absorption peaks of carboxylic acid were shifted to a lower region by the strong hydrogen bonding. The thermoreversible crosslinking system was also applied to EPM, EBM (ethylene-butene rubber), and IIR. These rubbers also showed superior mechanical properties as well as excellent recyclability.

Morphology and Crystallization Behavior of Lightly Crosslinked Natural Rubber in Blend

Isothermal crystallization of natural rubber (NR) dispersed in styrene-butadiene rubber (SBR) was made at -25 °C to investigate effects of both gel fraction of the rubber and morphology of the blend on the crystallization. NR, thus used, was lightly crosslinked model compound (model-NR), which was cured with dicumylperoxide at 160 °C after mastication. The model-NR was mechanically mixed with a large amount of SBR to form droplets of the rubber, a size of which was dependent upon both gel content and crosslink density of the gel fraction. The crystallization of the model-NR in the droplets was quite slow, corresponding to the level reported in the previous work. 16 A rate of crystallization and Avrami exponent were dependent upon the size of the droplets, but not on the gel content and the crosslink density of the model-NR. The suppression in the crystallization was attributed to the homogeneous nucleation occurring in the droplets. This finding was proved, using rubbers obtained from two clones of Hevea brasiliensis, i.e. RRIM600 and RRIM2025, respectively.

Morphology dependence of crystallization of natural rubber in blends

Crystallization of natural rubber (NR) was investigated in different morphology for NR/styrene butadiene rubber (SBR) blend and NR/polystyrene-(b)-polyisoprene (SI)/polystyrene (PS) blend. A purified NR (PC-TE) was prepared from pale crape via transesterification. In the blends, PC-TE formed various morphologies; that is, matrix phase, island phase and continuous phase with a nano-scale, respectively, in dependence upon the ratio of the rubbers. The crystallization rate of the blends was also significantly associated with the morphology of the rubbers.