Polyurethane elastomers have demonstrated unusually excellent physical and mechanical properties, but these advantages are considerably diminished at elevated temperatures.

The peroxide prevulcanization of NR latex using a range of commercially-available organic peroxides and an inorganic peroxide (potassium peroxydisulphate), in both activated and non-activated systems, has been investigated. A range of reducing agents and compounds that are known to promote peroxide-initiated emulsion polymerization and peroxide curing of po1yesters have been evaluated as promoters for the peroxide prevulcanization of NR latex. A few reactive peroxyesters have been found to be effective as prevulcanizing agents at temperatures in the range 80 °C-lOO °C. the effectiveness of the prevulcanization systems was characterized by the rate and efficiency of crosslinking achieved by these systems. Fructose-activated peroxyester and fructoseactivated hydroperoxide systems were found to effect prevulcanization at temperatures in the range 50 °C-80 °C. There is no clear correlation between the structure/reactivity of peroxyesters and the effectiveness of fructose-activated prevulcanization systems. The relative reactivity of the alkoxy radicals generated by the commercial hydro peroxides PQ~tly exp\a~~s the differences in the effectiveness of various fructose-activated hydroperoxide prevulcanization systems. The prevulcanization kinetics for the fructose-activated t-butyl peroxyisobutyrate (tBPIB) system have been investigated. The overal rate of tBPIB decomposition in NR latex, in both non-activated and fructose-activated systems was found to be first-order reaction with respect to tBPIB concentration. However, investigation of initial rate of tBPIB decomposition in NR latex indicates that the initial rate of tBPIB decomposition in NR latex is half order with respect to initial tBPIB concentration. This is probably a consequence of induced decomposition of tBPIB by certain non-rubber substances, and_termination by recombination of radicals derived from tBPIB. But, the reason for the difference in the reaction order with respect to tBPIB concentration, at the initial stage of the reaction and during the run is not clear. The prevulcanization kinetics also exhibit a number of other peculiar characteristics. Thus at temperatures greater than 70°C, and using a high fructose concentration, the rate coefficient for crosslink formation tends to be greater than that for peroxide decomposition. This is probably attributed to the differences in the temperature-coefficients of the various competing reactions during peroxide prevulcanization of NR latex. The instantaneous crosslinking efficiency was found to increase linearly with prevulcanization time. At temperatures greater than 70°C, the instantaneous crosslinking efficiency can attain values greater than 50%, indicating the involvement of alkyl radicals as well as the alkoxy radicals in the crosslinking reaction. The experimental activation energies for peroxide decomposition and crosslink formation were found to decrease to apparently constant values with increasing fructose/ peroxide concentration ratio. The rate of tBPIB decomposition was found to be significantly determined by activation free energy and not just activation energy for the decomposition. The factors which influence the physical properties of films from peroxide-prevulcanized NR latex have been investigated. The crosslink concentration was found to be the most important factor in determining the physical properties of films from peroxide-prevulcanized NR latex. Factors that account for the differences in the physical properties of films from peroxide- and sulphur-prevulcanized latices, and peroxide gum NR vulcanizates have been discussed. Attempts to improve the ageing properties of films from peroxideprevulcanized NR latex indicate that a preventive antioxidant is an essential component for an effective antioxidant system for these films.

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The modes of action of a diamine salt of fatty acid with a general structure of [RNHz+(CHz)JNH3+(R'COO-)z], referred to as a multifunctional additive (MFA) in carbon black and silica filled natural rubber (NR) compound have been studied. The mechanical properties have been enhanced by incorporating the MFA. Optimum mechanical properties for the natural rubber filled with carbon black compound are obtained at an MFA level which is estimated to give complete monolayer coverage of the carbon black surface. Studies on silica filled NR show a much higher optimum MFA level than for carbon black filled compounds, which is attributed to a different orientation of the MFA molecules at the silica-rubber interface. The MFA molecules may stand on the filler surface with their polar sites attached to its surface, giving rise to a much higher monolayer value. Filler dispersion, measured by computer-aided image analysis and scanning electron microscopy (SEM), showed that MFA gives improved dispersion at equivalent mixing time. The effect of the MFA on crosslink density of carbon black filled NR compounds was found to be small, in contrast to silica filled NR compounds, where the increase in crosslink density enhanced properties significantly. Techniques to examine the properties of compounds with and without MFA at equivalent levels of filler dispersion have been developed. Results show that, in addition to the improvement in filler dispersion, increased energy dissipation at the rubber-MFA-filler interfaces improves tear and tensile strength. A mechanism of adsorption to explain the mode of action of the MFA at the elastomer/filler interface has also been proposed.

The ability of metals and alloys to form and retain protective oxide scales is crucial to their stability at elevated temperatures for extended times. Hence the identification of factors that promote or limit the integrity of oxides on high temperature materials has been the subject of intensive investigations. In the present study the mechanical properties of this chromium-rich scale on 304 stainless steel foil has been investigated in relation to the deformation rates in the substrate. It was shown that heavy cold working (up to 90%) delays the onset of breakaway oxidation and results in a very adherent scale. The cracking behaviour of the scale was found to be strain rate and temperature dependent under slow strain rate conditions when the substrate deforms by creep. No strain rate dependence was observed over the temperature range 700-900°C when faster strain rates (> 10⁻⁵ sec⁻¹) were applied. The transition between these two responses was found to vary only slightly with temperature between 5.0x10⁻⁵ sec⁻¹ and 7.8x10⁻⁵ sec⁻¹, increasing as the temperature is raised. A new method has been described for determining the fracture behaviour of oxide scale by estimating the composite defect size. From a knowledge of the onset of scale cracking, determined in situ using the acoustic emission technique, it was possible to correlate the measured intercrack spacing with the fracture toughness from which the tensile properties of the scale can be evaluated.

Natural rubber from the para rubber tree Hevea brasiliensis is one of world’s most important natural resources. Despite its use in the manufacture of a wide range of essential items the mechanisms by which natural rubber is synthesised is poorly understood. Natural rubber is a long chain cis-polyisoprene, composed of units of isopentenyl pyrophosphate (IPP) which is contained within rubber particles. Rubber particles consist of a hydrophobic polyisoprene interior surrounded by a monolayer membrane. Due to the insoluble nature of rubber the only place that polymerisation could occur is on this membrane by a hypothetical membrane bound rubber transferase. Whether this is a single enzyme or complex is currently unknown. Cis-prenyltransferases (CPTs) are a group of enzymes responsible for the polymerisation of cis-polyisoprene. The first plant CPTs were identified in Arabidopsis which paved the way for the identification of two Hevea CPTs, HRT1 and HRT2. HRT2 is able catalyse the formation of long chain polyisoprene products in the presence of rubber particles and is the focus of this thesis. Hevea takes at least 4 years to reach maturity. Its long life cycle coupled with the difficulty of genetic transformation meant that direct study on Hevea was not feasible for this project. Instead transient expression in N. benthamiana as well as some preliminary work in L. sativa was used to characterise rubber associated proteins. HRT2 was found to be a cytosolic protein and in theory incapable of polymerising natural rubber on the surface of rubber particles. The scope of the project was therefore widened to include possible interactors that could act to bring it to a membrane. These interactors were identified and cloned based on existing literature and included small rubber particle protein (SRPP), rubber elongation factor (REF), rubber biosynthesis stimulator protein (RBSP) and a newly characterised cis-prenyltransferase like (CPTL) protein, HevNogo. Whilst HRT2 on its own was unable to associate with a membrane, expression with HevNogo induced its subsequent localisation to the plasma membrane. This interaction initially took place on the endoplasmic reticulum. The HRT2/HevNogo complex may be part of a rubber transferase complex, however it is likely that additional components are required.