Siby Varghese

Influence of interfacial adhesion on the mechanical properties and fracture behaviour of short sisal fibre reinforced polymer composites

Abstract The mechanical properties of sisal fibre composites (randomly oriented) of several thermoset resin matrices, [polyester, epoxy, phenol-formaldehyde] and a thermoplastic matrix [low density polyethylene (LDPE)] were evaluated with respect to fibre length and fibre loading. All the composites showed a general trend of increase in properties with fibre loading. However, the optimum length of the fibre required to obtain an increase in properties varied with the type of matrix. Among polyester, epoxy and phenol-formaldehyde composites of sisal fibre, a phenolic type resin performed as a better matrix than epoxy and polyester resins with respect to tensile and flexural properties due to the high interfacial bonding in phenolic composites. However, as compared to thermoset resin composites, sisal fibre-LDPE composites showed a better reinforcing effect due to the high matrix ductility and high strength/modulus ratio of sisal as compared to that of LDPE matrix.

Studies on the effect of blend ratio and crosslinking system on thermal, X-ray and dynamic mechanical properties of blends of natural rubber and ethylene-vinyl acetate copolymer

Abstract Differential scanning calorimetry (d.s.c.), X-ray, dynamic mechanical and microscopy studies were carried out using a series of blends of natural rubber (NR) and ethylene-vinyl acetate copolymer (EVA) in the crosslinked and uncrosslinked states. The crosslinking system (sulfur, peroxide and mixed) and blend ratio do not influence significantly the melting temperature (Tm) of EVA. The decrease in percentage crystallinity of EVA with NR content is related to the incomplete crystallization, as indicated by the lower heat of fusion (ΔH). The glass transition temperature (Tg) of NR in the uncrosslinked blend did not change substantially and this indicated incompatibility. However, in the case of crosslinked systems, depending on the type of crosslinking agents used, the Tg of the NR phase in the blend was shifted to higher temperatures. This has been explained with reference to the predominant crosslinking of the NR phase. From X-ray diffraction patterns, the interplanar distances (d values) were calculated. With the addition of NR there was a tendency for the d values to increase in all systems. The crystallinity was measured by X-ray and the results were in line with those of d.s.c. measurements. The dynamic mechanical properties such as tan δ, storage modulus and loss modulus of the blends were evaluated. The existence of separate Tg values in dynamic mechanical thermal analyser studies indicates that the blends are incompatible. The morphology of the uncrosslinked blends has been studied by scanning electron microscopy.

Effect of Adhesion on the Equilibrium Swelling of Short Sisal Fiber Reinforced Natural Rubber Composites

Abstract Solvent swelling of natural rubber composites, containing both untreated and acetylated short sisal fiber, has been investigated in a series of normal alkanes such as pentane, hexane, heptane and octane. The restriction on elastomer swelling exerted by sisal fiber as well as the anisotropy of swelling of the composite have been confirmed by this study. The results showed that increased fiber content and the addition of bonding agent reduced the swelling considerably. It has been demonstrated that with improved adhesion between short fiber and rubber, the factor, (VI−VF)/VI, decreases, where VI and VF are the volume fraction of rubber in dry and swollen samples, respectively.

Sorption and diffusion of aromatic hydrocarbons through filled natural rubber

Abstract The sorption and diffusion of four aromatic hydrocarbons through vulcanized natural rubber filled with high-abrasion furnace black (HAF) and that filled with silica (Ultrasil VN3) at the same loading have been investigated in the temperature range of 28–70°C by conventional weight-gain experiments. The diffusion results have been analysed in terms of the Fickian diffusion equation. The transport behaviour in both types of samples was found to deviate from the regular Fickian trend, which has been explained in terms of the slow relaxation of rubber chains caused by the presence of fillers. The experimental diffusion data have been compared with the theoretical predictions. The variation of diffusion coefficient with concentration of the solvents has been discussed. From the sorption data, the enthalpy and entropy of sorption and the rubber-solvent interaction parameter have been computed.

Short sisal fibre reinforced natural rubber composites: high-energy radiation, thermal and ozone degradation

Abstract The effects of acetylation and bonding agent on thermal ageing, γ-radiation and ozone resistance of short sisal fibre reinforced natural rubber composites have been evaluated. At high fibre loadings the composites showed better resistance to ageing, especially in the presence of bonding agent. Acetylation of fibre and fibre orientation were also found to reduce the extent of degradation. The dosage of γ-radiation was found to influence the extent of the retention/degradation process.

Effect of Aluminum Powder on Filled Natural Rubber Composites

Abstract The effect of aluminum powder on the properties of natural rubber composites containing high abrasion furnace black (HAF), general purpose furnace black (GPF), acetylene black, china clay and precipitated silica was studied. In all cases the total filler content including aluminum powder is fixed at 40 parts per hundred rubber (phr). Gradual replacement of these fillers by aluminum powder showed a marked increase in thermal conductivity. This increased thermal conductivity of aluminum powder filled composites decreased the vulcanization time and led to uniform curing throughout the material, especially for thick articles. Mechanical properties like tensile strength, tear strength, rebound resilience, heat build-up, hardness, compression set, etc., are comparatively better for these composites. Aluminum powder incorporated vulcanizates also showed good resistance against thermal aging and oxidative degradation. Thus, use of aluminum powder in rubber compounds can lead to energy saving in the vulca...

Effect of adhesion on the equilibrium swelling of natural rubber-aluminium powder composites

Equilibrium swelling of natural rubber composites containing aluminium powder has been investigated in a series of aromatic hydrocarbon solvents, such as benzene, toluene, xylene, and mesitylene. These composites were vulcanized by four vulcanizing systems, viz. conventional, efficient, dicumyl peroxide, and a mixture, consisting of sulfur and dicumyl peroxide. In each system, the effect of aluminium powder with and without bonding agent was studied. The results showed that addition of bonding agent reduced the swelling considerably, and its effect is more pronounced in the conventional system due to increased adhesion. The dependence of the diffusion coefficient on the crosslinking system and the solvent–polymer interaction parameter were calculated from diffusion data. These results are also indicative of the improved adhesion with hexa-resorcinol–silica bonding system in these composites.

Compatibilization of SBR/NBR Blends Using Chemically Modified Styrene Butadiene Rubber

Abstract The effect of dichlorocarbene modified styrene butadiene rubber (DCSBR) as a compatibilizer in blends of acrylonitrile butadiene rubber and styrene butadiene rubber (SBR/NBR) has been studied. The cure characteristics, physical properties, crosslink density, FTIR studies and low temperature transitions of the blends were determined as a function of chlorine content of DCSBR. Compatibilizing efficiency depended on the chlorine content of DCSBR and composition of blend constituents. Effective compatibilization was achieved when chlorine content of compatibilizer was 25% and SBR content of blend was either 50% or lower. FTIR studies, glass transition behavior measured by DSC and DMA showed that an appreciable extent of molecular level miscibility has been achieved in SBR/NBR blends by using DCSBR as a compatibilizer. Improvement in mechanical properties such as tensile strength, tear strength, resilience, hardness and compression set were achieved both when DCSBR was added and chlorine content of DCSBR increased up to 25%. The resistance of the vulcanizate towards air and oil aging improved with compatibilization. The change in technological properties correlated with crosslink density of the blends assessed from swelling studies and stress-strain data.

Viscoelastic Properties of Silica-Filled Natural Rubber and Ethylene-Vinyl Acetate Copolymer Blend

Abstract The viscoelastic properties such as storage modulus (E′), loss modulus (E”) and damping (tan δ) of unfilled and silica-filled blends of natural rubber (NR) and ethylene-vinyl acetate copolymer (EVA) have been evaluated. The blends were crosslinked by a mixed system consisting of accelerated sulfur and peroxide. The tan δ values of unfilled blends showed two transitions corresponding to EVA and NR phases. This indicated that the blends are immiscible. The damping curves of EVA and EVA-rich blends possess three distinct regions: a rubbery plateau, a melt transition, and a viscous plateau. It is seen that the addition of silica decreased the tan δ values of the samples substantially except that of NR, where a slight increase in tan δ values can be observed at high temperatures. The sharp decrease in tan δ in filled EVA and EVA-rich blends is associated with the decrease in crystallinity of the EVA phase due to the presence of filler. Addition of silica filler increased the storage and loss modulus o...

Effect of bonding agents on natural rubber–aluminium powder composites

The effects of various bonding agents, namely the hexamethylene tetramine-resorcinol system (HR), bis[3-(triethoxysilyl)propyl]tetrasulphide (Si-69), cobalt naphthenate (CoN), and toluene diisocyanate (TDI), on the mechanical properties of aluminium powder-filled natural rubber composites have been investigated. Studies were done both as a function of the bonding agent by keeping the loading of aluminium powder constant and as a function of the aluminium powder loading along with the bonding agents. Shore A hardness, rebound resilience, heat build-up, etc., are increased by the use of bonding agents, due to the improved adhesion between the natural rubber and aluminium powder. The addition of a bonding agent minimizes the DIN abrasion loss and percentage of compression set. The tensile properties can also be improved by adding bonding agents. Of the various bonding agents used in this study, the silane coupling agent (Si-69) and the hexamethylene tetramine-resorcinol (HR) system were found to be better for natural rubber-aluminium powder composites.