D. K. Tripathy

Effect of compatibiliser, curing sequence and ageing on the thermal stability of silicone rubber, EPDM rubber and their blends

Abstract The thermal stability of the vulcanisates of silicone rubber, EPDM, their blends and aged specimens of these systems has been studied by non-isothermal thermogravimetry. The activation energy for the degradation process suggests that a different route is followed for the degradation of EPDM after ageing. The effect of a silane-grafted EPR and also the effect of a programmed two-stage curing process on thermal stability has also been studied. Both are found to contribute moderately to the thermal stability of the blends.

Effect of aluminium silicate filler on morphology and physical properties of closed cell microcellular ethylene–octene copolymer

The effect of aluminium silicate filler on the morphology of the microcellular ethylene–octene copolymer (Engage) has been studied from SEM photomicrographs with variation of blowing agent and silicate filler loading in comparison to the unfilled vulcanizates. The average cell size, maximum cell size and cell density varies with variation of blowing agent and filler loading. Physical properties like relative density, hardness, tensile strength, elongation at break, modulus, tear strength decreases with blowing agent concentration. However tensile strength, modulus (100%), tear energy and hardness varies linearly with the density of the filled vulcanizates. The elastic nature of closed cells reduces the hysteresis loss compared to solid compounds. Set properties improve with blowing agent concentration. It is observed that stress relaxation behavior is independent of blowing agent loading i,e., density of closed cell microcellular vulcanizates. Theoretically flaw sizes are found to be about 2.57 times larger than maximum cell sizes observed from SEM photomicrographs.

The effect of carbon black concentration on the dynamic mechanical properties of bromobutyl rubber

The effect of carbon black concentration on the dynamic mechanical properties of bromobutyl rubber vulcanizates has been studied over a wide range of temperature (− 150 to + 250 °C), frequency (3.5 to 110 Hz) and dynamic strain amplitude (0.07 to 5%). The influence of carbon black concentration on the glass-rubber transition has also been investigated with respect to the isochronal variation in dynamic properties. The influence of carbon black concentration consists mainly of the change in the levels of the moduli values in the glassy and rubbery state. In the glassy region; the increase in stiffness with carbon black loading may be fully explained by the hydrodynamic effect of the carbon black particles embedded in the polymer continuum. With increased carbon black concentration the glass-rubber transition temperature (tan δ peak temperature) does not show a shift in its location but peak shoulder broadening and decrease in peak height are observed. At a particular temperature, the effect of carbon black concentration on dynamic properties of bromobutyl rubber is dependent on the combined effects of applied strain amplitude and frequency. With increase in filler concentration the thermal stability of the vulcanizate increases.

Blends of poly[ethylene(vinyl acetate)] and polychloroprene: Studies on capillary and dynamic flows

Rheological properties of the blends of poly[ethylene(vinylacetate)] (EVAc; vinylacetate content 28%) and polychloroprene (CR) have been measured through capillary and dynamic uniaxial elongational flows. Capillary flow indicates their shear thinning behavior. The decrease in the out of phase viscosities with increasing frequency is in accordance with the power law equation, whereas dynamic elongational viscosities follow nonlinear relationship in log-log plot with an initial increase at 11 Hz, followed by a very sharp drop. With an increase in temperature, the viscosity for capillary flow of all blends goes down due to their positive activation energy of flow but for dynamic elongational flow of EVAc blended with CR, viscosity increases, except for 30/70 blend and pure CR, in which case the dynamic elongational viscosity decreases with an increase in temperature. This abnormal behavior in dynamic elongational viscosity is due to the process of melting and recrystallization of EVAc at low heating rate (1°C/min) beyond the melting temperature. Capillary viscosities of all blends show positive deviation from the log additive values of pure polymers. But in the case of dynamic elongational flow, all blends show positive deviation at frequencies of 3.5 and 35 Hz and at higher temperatures (80–120°C).

Influence of large static deformations on the dynamic mechanical properties of bromobutyl rubber vulcanizates: Part I. Effect of carbon black loading

Abstract The dynamic mechanical properties of bromobutyl rubber vulcanizates loaded with varying proportions of highly reinforcing carbon black (N-220) have been studied over a wide range of static extensions, using a viscoelastometer Rheovibron DDV-III-EP. The static strain and frequency range were 0–300% and 3·5–110 Hz, respectively. The dynamic strains superimposed on the static deformation were varied between 0·025% and 5% DSA at the lowest value of λ (deformation ratio). Studies on the dynamic Mullins effect (stress-softening) and dynamic relaxation have also been carried out.

Dynamic mechanical properties of epoxidised natural rubber vulcanisates: Effect of curing system and ageing

Abstract The dynamic mechanical properties of ENR-50 have been evaluated using various curing systems for varying periods of ageing. Conventional, conventional with Na 2 CO 3 , semi-efficient vulcanisation (SEV), peroxide and dibasic acid curing systems were used. Conventional systems with and without Na 2 CO 3 were found to cause a high degree of hardening furanisation with an obvious increase in the modulus values. A second T g peak was observed after 2–6 days of air ageing. The SEV system was also found to affect the ageing characteristics and hence the dynamic mechanical properties. Even after six days of ageing, peroxide and dibasic acid cured vulcanisates did not show much change in dynamical mechanical properties. It could be inferred that sulphur crosslinking in ENR is very liable to oxidative ageing and the dynamic mechanical properties are hence considerably affected.

Morphology and ageing behaviour of silicone-EPDM blends

The silicone-EPDM system has been studied with blend ratios (silicons:EPDM) of 50∶50, 40∶60 and 30∶70. The first set of blends was prepared by roll-mixing of the elastomers and curative whereas in the second set, DCP-cured powder of EPDM, prepared in a Brabender plasticorder, was mixed with silicone rubber and DCP in a roll mill. A comparison of blends show that the second set exhibits inferior properties as compared to the first set. Specimens were also aged at 150 and 175 °C for various periods. The physical properties of the aged specimens show a reversal in trend, i.e. the second set of specimens shows better physical properties on ageing. The strengthening of the silicone phase by use of a different curative (Perkadox 14/40) in the 50∶50 blend of the second set shows a beneficial action on ageing. Morphology studies with SEM confirm the two-phase structure for both sets of blends. In both cases phase separation occurs with ageing, but the continuous phase for the second set of blends maintains its continuity for a higher period than the corresponding specimens of the first set.

The influence of curing systems on the properties of bromobutyl rubber: Part 2—Effect of concentration of curing resin on the dynamic mechanical properties

Abstract Measurements of isochronal dynamic mechanical properties of cured, uncured and thermally aged rubber vulcanizates have been carried out using a Rheovibron DDV-III-EP elasticoviscosity meter as a function of temperature between −150°C and +250°C. The effects of curing resin (phenol/formaldehyde[resole] resin; SP-1045) concentration and time of heat ageing on different relaxation transition characteristics and dynamic mechanical properties have been discussed. It has been observed that the molecular mobility is not affected, but only the sensitivity of E (in-phase modulus) in the dispersion region is affected by resin content and time of ageing. With increase in resin content in the vulcanizate, on ageing, the crosslinking reaction becomes more predominant over degradation of polymer chains and scission of crosslinks.

Strain dependent dynamic mechanical properties of silica filled ethylene vinyl acetate rubber

The dynamic mechanical properties of silica filled ethylene vinyl acetate (EVA) rubber have been studied at room temperature (25°C) at different frequencies (3.5 to 110 Hz) and varying dynamic strain amplitudes (0.07 to 5%). From the dynamic analysis it was found that the trend of variation of in-phase modulus, out-of-phase modulus, and tan δ with dynamic strain for lightly filled EVA vulcanizates is similar to that of the gum EVA vulcanizate. The changes in low strain dynamic properties can be ascribed to the mobility of amorphous chains. For highly filled vulcanizates, the nature of the variation of dynamic mechanical properties follows the pattern of filled conventional rub ber systems. For the systems with intermediate filler loading, the characteris tics follow a transition between those exhibited by the gum and the highly filled systems. The increase of high and low strain moduli with volume fraction of filler could be partly due to the hydrodynamic effect of filler dispersed in the rubber matrix and partly due to the enhanced adsorptive interaction between silica and EVA rubber.

Wear, tear and tensile failure of silica-filled ethylene vinyl acetate rubber

Abstract The physical properties of ethylene vinyl acetate rubber filled with various loadings of precipitated silica (0–22 vol.%) were studied. The results were correlated with scanning electron microscopy of the tensile, tear and wear fracture surfaces. Plausible mechanisms operating in tensile, tear and abrasion failures are discussed.