G. Unnikrishnan

Investigation on interfacial adhesion of short sisal/coir hybrid fibre reinforced natural rubber composites by restricted equilibrium swelling technique

Interfacial adhesion of sisal/coir hybrid fibre reinforced natural rubber (NR) composites has been characterized by restricted equilibrium swelling technique with special reference to the effects of fibre loading, orientation and bonding agent. The swelling parameters of NR composites with and without bonding agent were evaluated with three aromatic solvents, namely; benzene, toluene and xylene, by a sorption gravimetric method. As fibre content and penetrant size increase, the solvent uptake has been found to decrease due to the increased hindrance and good fibre–rubber interaction. The bonding agent added mixes showed enhanced restriction to swelling and it is seen that the ratio of change in volume fraction of rubber before and after swelling to the volume fraction of rubber before swelling (V 0 – V r/V 0) is lower for bonding agent added composites, when compared to an unbonded one. The anisotropic swelling studies were carried out to analyse the extent of fibre alignment and fibre–matrix interaction. In strongly bonded composites, the swelling has been mainly observed to take place in the thickness direction, as attested by optical photographs. The rubber–fibre interaction has also been examined by Lorenz–Parks and Kraus equations.

Dielectric properties of short sisal/coir hybrid fibre reinforced natural rubber composites

The dielectric properties, such as dielectric constant, volume resistivity and dielectric loss factor, of sisal/coir hybrid fibre reinforced natural rubber composites have been studied as a function of fibre loading, fibre ratio, frequency, chemical modification of fibres and the presence of a bonding agent. The dielectric constant values have been found to be higher for fibre filled systems than pure natural rubber. This has been attributed to the polarization exerted by the incorporation of fibres into the matrix. Dielectric constant values were observed to be decreased with increase in frequency due to the decreased interfacial and orientation polarization at higher frequencies. Whereas dielectric constant increases with fibre loading because of the increment in number of polar groups after the addition of hydrophilic lignocellulosic fibres. The volume resistivity of the composites was found to be decreased with fibre loading and a percolation threshold has been obtained at 15.6% volume of fibres. Fibre treatment, such as alkali, acetylation, benzoylation, peroxide and permanganate, were carried out to improve the adhesion between fibres and matrix. The dielectric constant values were lower for systems consisting of fibres subjected to chemical treatments due to the increased hydrophobicity of fibres. The addition of a two-component dry bonding agent consisting of hexamethylene tetramine and resorcinol, used for the improvement of interfacial adhesion between the matrix and fibres, reduced the dielectric constant of the composites. When the weight percentage of sisal fibre was increased in the total fibre content of the hybrid composites, the dielectric constant was found to increase. The added fibres and different chemical treatments for them increased the dielectric dissipation factor. A dielectric relaxation has been observed at a frequency of 5 MHz.

Mechanical Properties of Sisal/Coir Hybrid Fibre Reinforced Natural Rubber

Natural rubber was reinforced with various concentrations of sisal and coir fibres. Green strength measurements were carried out to determine the fibre orientation. The rubber–fibre interfacial bond was improved by a resorcinol-hexamethylene tetramine dry bonding system. The composites were vulcanized at 150°C, and the curing characteristics of the hybrid system were studied. The stress- strain behaviour, tensile strength, modulus, elongation at break, tear strength in longitudinal and transverse fibre orientations, Shore-A hardness and abrasion resistance were determined by standard ASTM procedures. The tensile strength, tear strength and tensile modulus increased with the addition of fibres up to 30 phr and then decreased at still higher fibre loadings. The hardness, density and abrasion resistance increased with the addition of fibres whereas the elongation at break decreased. The fracture surfaces of the composites were examined by scanning electron microscopy (SEM). The tensile properties were considered using parallel and Hirsch models.

Cellulose nano-particles from Pandanus: viscometric and crystallographic studies

Pandanus utilis a plant abundant throughout Kerala, India has been used as a source to isolate nano-particles. The fibres were extracted from Pandanus plant by biological natural retting. Extracted fibres were dried, ground and treated with 5 % NaOH followed by bleaching using 5 wt % hypochlorite solution. Bleached cellulose was hydrolysed using sulphuric acid. After centrifugation, suspension was freeze dried. Effect of Acid concentration, temperature and time of hydrolysis on the isolation of cellulose nanoparticles was studied. FESEM images of cellulose showed the presence of spherical nano-particles. Dynamic light scattering revealed homogeneous dispersion of nano-particles. Degree of polymerisation of cellulose was determined viscometrically using cupriethylene-diamine as solvent. Removal of lignin and hemicelluloses was confirmed from FTIR spectra. X-Ray diffractograms of powdered fibre, bleached cellulose and nano-cellulose were compared. Using peak height method and peak de-convolution method, crystallinity indices were determined. Using Scherrer equation crystallite size was calculated and it further confirmed that particles are of nanometre size.

Thermal behavior of chemically treated and untreated sisal fiber reinforced composites fabricated by resin transfer molding

Using thermogravimetric analysis (TGA), the thermal behavior of sisal fibers and sisal/polyester composites, fabricated by resin transfer molding (RTM), has been followed. Chemical treatments have been found to increase the thermal stability, which has been attributed to the resultant physical and chemical changes. Scanning electron microscopy (SEM) and infrared (FT-IR) studies were also performed to study the structural changes and morphology in the sisal fiber during the treatment. The kinetic studies of thermal degradation of untreated and treated sisal fibers have been performed using Broido method. In the composites, as the fiber content increases, the thermal stability of the matrix decreases. The treated fiber reinforced composites have been found to be thermally more stable than the untreated derivatives. The increased thermal stability and reduced moisture behavior of treated composites have been correlated with fiber/matrix adhesion.

Viscoelastic and thermal properties of eco-friendly composites fabricated by resin transfer molding

This article reports the result of the effect of banana fiber on the dynamic mechanical and thermal properties of polyester composites. Composites were prepared using resin transfer molding by varying the fiber content (20, 30, 40, and 50 vol.%). Studies revealed that increase in the fiber content will increase the storage modulus (E′) and the maximum is given by the composites having a fiber loading of 40 vol.% at all temperature ranges. The peak height of loss modulus (E″) and damping curves (tan δ) were lowered with respect to the fiber content. Glass transition temperature (Tg) calculated from the E″ and tan δ shifted toward high temperature with the addition of fiber. An attempt has been made to correlate the experimental and theoretical values of storage modulus and tan δ. Thermogravimetric analysis showed that incorporation of fiber to the matrix will affect the thermal stability of the composites.

Sorption of aromatic hydrocarbons into styrene butadiene rubber/poly(ethylene-co-vinyl acetate) blends

Benzene, toluene and xylene sorption by styrene-butadiene rubber/poly (ethylene-co-vinyl acetate) (SBR/EVA) blends in the temperature range of 26-56 °C have been investigated. The blends were vulcanised by three different systems viz. sulphur, dicumyl peroxide and a mixed system of sulphur and peroxide. The effects of various parameters such as blend composition, vulcanising systems, crosslink density, penetrant molecule size, and temperature on the sorption parameters were studied. Blends with higher EVA contents, vulcanised by the peroxide system, were found to exhibit the lowest interaction with the solvents. The mechanism of sorption deviated regularly from the normal Fickian behaviour, found for conventional elastomers, with increase in EVA content. The molecular mass between crosslinks, kinetic rate constant, transport coefficients and related thermodynamic parameters have been computed to complement the experimental observations. The experimental results have been compared with different theoretical predictions.

Separation of n-hexane/acetone mixtures by pervaporation using natural rubber membranes

Natural rubber (NR) membranes crosslinked by four different systems viz., conventional (CV), efficient (EV), dicumyl peroxide (DCP), and a mixture consisting of sulfur and peroxide (mixed), were employed for the separation of n-hexane/acetone mixtures of different compositions. The membranes exhibited preferential permeation toward n-hexane because of the closer solubility parameter values. The selectivity of the membranes depended on the nature and distribution of crosslinks between the macromolecular chains of the membrane but was independent on the thickness of the membrane. The effects of feed composition, cure time of the membranes, and the molecular size of the permeate on the permeation flux and selectivity were investigated.

Analysis of Physical and Solvent Transport Behavior of Poly(Ethylene-co-Vinyl Acetate)/Silica Composites

This article deals with the effect of nano silica on the cure, mechanical, solvent transport, dielectric, and thermal behavior of poly(ethylene-co-vinyl acetate) (EVA). X-ray diffraction is used to analyze the composite morphology. The cure time and scorch time of the composite was found to be decreasing with filler content. The mechanical properties of the composites such as tensile strength, hardness, and abrasion resistance increased with filler incorporation. The mechanical properties of the composites have been compared with some of the other EVA composites. The solvent resistance of the composite has also been tremendously improved with filler loading. The overall thermal stability of the composite has been found to be improved with filler loading.

Sorption and Mechanical Properties of Nitrile Rubber Compatibilized Natural Rubber-Poly (Ethylene-Co-Vinyl Acetate) Blends

Acrylonitrile butadiene rubber (NBR) has been used as a compatibilizer for natural rubber—poly (ethylene-co-vinyl acetate) (NR—EVA) blends, vulcanized by sulfur. A significant reduction in solvent uptake is observed for compatibilized samples. This has been explained in terms of the increased interfacial adhesion between NR and EVA components of the blends in the presence of the compatibilizer. The threshold compatibilizer loading has been found to be 6 phr. The observed sorption features have been complemented by the evaluation of the surface morphology and mechanical properties of the blends. The kinetics of transport process through the compatibilized blends has also been discussed.