The present work was carried out to study the properties of both natural rubber composites reinforced by short polyaramid fibres and natural rubber laminates reinforced by filament polyaramid. Initial work was concentrated to obtain a suitable direct bonding agent and curing system for high modulus nylon 6,6 which was used as the control fibre. Two different types of bonding and curing systems were established as useful for bonding polyaramid to rubbers namely resorcinol-hexamethylene tetraminesilica (HRH) and sulphur, and a blocked diisocyanate in conjunction with a diurethane-crosslinker. Each system was found to offer a different type of adhesion, the HRH formed physical bonds and the NCO/urethane chemical links between fibre and rubber. When the proportions of these bonding agents were optimised in the NR rubber compound vulcanizate properties were significantly improved. Investigation established that for polyaramid fibres the optimum aspect ratio of approximately 1250 was found to give maximum composite reinforcement as judged by the tensile properties of the composite. The experimentally found optimum of 4 parts of blocked diisocyanate was used for extensive investigations of composites properties with respect to fibre orientation and content. Further studies on composites with polyaramid short fibres demonstrated the necessity to optimize the proportions of blocked diisocyanate in the composite to achieve maximum strength properties, e. g. unoptimised with 30% fibre gave a tensile strength of 26.7 MPa whereas optimised, with 30% fibre, gave a tensile strength of 34.4 MPa. Post treatment of composites and laminates by heating or with high energy radiation, using a Co60 source, improved the quality of adhesion still further by the formation of more chemical links between polyaramid and rubber though some degradation of rubber strength simultaneously took place. The bonding agent, diisocyanate, was found unsuitable for use in a sulphur curing system, and was considered to form complexes with the accelerator DCBS and sulphur, to react with zinc salts, and further to act as a crosslinker for rubber. It was therefore found necessary to vulcanize the rubber with a diurethane crosslinker which did not show cure interference with the diisocyanate and the latter then functions primarily as a bonding agent though a minor proportion is shown to simultaneous crosslink the rubber. The Resorcinol-hexamethylene tetramine-silica system. was found to contribute a type of physical adhesion, possibly that of hydrogen bonding, between polyaramid and rubber since this resin formed insitu during vulcanization of rubber was found to crosslink the rubber; reaction mechanisms for both these bonding systems are postulated.

A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science. Johannesburg, 2014. / This study concentrated on two main thrusts: 1) the optimal synthesis and characterisation of various micro- and nanosized carbon materials and 2) a comparative investigation of the capabilities of these carbonaceous materials to reinforce a locally available styrene butadiene rubber (SBR), which was commonly used to make car tyres. In the former case, a range of carbon materials including nitrogen doped/undoped carbon nanotubes as well as carbon microspheres (CMSs) were successfully synthesized by two different techniques (i.e. chemical vapour deposition (CVD) and hydrothermal synthesis). These were then fully characterised by numerous techniques which included: TEM, TGA, FTIR, PXRD, laser Raman spectroscopy, Zeta potential measurements and BET surface area analysis. In the latter case, these micro and nanocarbon materials were systematically added to SBR at various loadings (ranging from 0.125–0.500% (m/m)). Here the tensile strengths of the resultant composites, loaded with these various micro and nanocarbon materials, were measured for comparison to establish which (if any) was the best reinforcement material for SBR. Results obtained from the tensile strength measurements of the variously loaded SBR composites, showed that irrespective of the method of synthesis (i.e. CVD or hydrothermal synthesis) carbon microspheres (undoped, doped, functionalised or unfuntionalised) performed more poorly as fillers than carbon nanotubes. Furthermore the results obtained, from the various characterisation techniques mentioned previously, indicated that the lower performance of these microspheres as fillers may have been due to their: size, shape and low surface areas. By contrast when the tensile strengths of SBR reinforced with either CNTs or NCNTs were measured, the former outperformed the latter as fillers. It was speculated, based upon the data obtained, that NCNTs were poorer fillers than CNTs due to their higher negative surface charges, larger diameters and lower crystallinity. Hence this study has shown that low loadings (i.e. 0.250 % (m/m)) of the correctly matched type of carbonaceous material can significantly enhance the tensile strength and Young’s modulus of a locally available styrene butadiene rubber.

No description available.

In response to increased demand for fuel efficiency and sustainability, companies are seeking advancements in tire technologies to reduce fuel consumption and greenhouse gas emissions. There are several factors that affect fuel efficiency in tires. The tire tread is of most interest and relevance today, as it is responsible for the grip and wear issues, thereby requiring the highest performance. Silica is added as a reinforcing filler to the tread compounds to achieve a lower rolling resistance, while maintaining high grip and wear resistance. Due to the polarity difference between hydrophilic silica and hydrophobic rubber, industries mostly use silane agents to aid the dispersion. Such dispersion methods involve surface modification of silica in situ but provide very little control over the dispersion states achieved. Polymer grafted nanoparticles have recently attracted attention due to their ability to control and optimize dispersion. Tuning parameters like grafting density and ratio of matrix to graft chain length results in various dispersion states, which in turn have a direct correlation with reinforcement. However, most of their applications have been restricted to plastics, especially in the melt state. In this dissertation, we extended the use of grafted nanoparticles to polyisoprene rubber composites, where the grafted polymer and elastomer matrix don’t have the same chemical microstructure. We explored wide range of morphologies, ranging from well dispersed, connected networks, strings, sheets and small clusters. After studying the relative importance of entropic vs enthalpic effects on the self-assembly of nanoparticles, we observed how the morphologies evolved on crosslinking. The different morphologies studied were independent of the state of crosslinking and the chemical composition of the matrix, with minor changes occurring in samples where the nanoparticles were well dispersed. We next investigated the mechanical properties of the composites in two critical areas: linear regime and nonlinear regime. In each area, we performed systematic studies on the different morphologies in order to isolate the morphology that shows the optimal properties under different conditions. We demonstrated that nanoparticle dispersion states play a very important role in mechanical properties and are sensitive to the state of the polymer. While the connected network morphology shows the maximum reinforcement in the melt state, it is the aggregated sheets morphology at a lower grafting density that shows the highest reinforcement in the crosslinked state. This morphology at intermediate grafting density also shows the highest strain dependence in the Payne effect measurements. To understand the reinforcement mechanism in the linear and nonlinear measurements, we attempted to explore the underlying dynamics through NMR and XPCS. The evaluation crosslink density of the rubber matrix using NMR suggests that we do not have a fully developed immobilized layer and there is no variation in crosslink density in the polymer phase across the composites. This eliminates the popular theory that immobilized polymer layer in the vicinity of nanoparticles is responsible for reinforcement in rubber composites. Our research shows that filler network plays a crucial role in determining the dynamic properties of filled elastomers. At the intermediate grafting density region where sheets morphology form, the particles are aggregated with particle-particle contacts while having enough graft chains to entangle and crosslink. We believe that the crosslinked grafts connect the different sheets in the composite and aid in an improved stress transfer. This causes the filler network to percolate at much lower NP loadings. This lower percolation threshold for sheets morphologies explains the high reinforcement in the linear regime and large modulus drop in Payne effect measurements at lower nanoparticle loadings as compared to the other morphologies. Furthermore, the macroscale mechanical properties and results of XPCS derived microscopic dynamics align well. We observed that when the particles are dispersed they show faster dynamics as compared to the aggregated morphologies above the percolation threshold. At lower nanoparticle loadings, all the morphologies show similar dynamics, emphasizing the importance of percolation. In summary, we have used a combination of different techniques to understand the underlying mechanism of rubber reinforcement. We have identified which dispersion states affect reinforcement in the linear and nonlinear regime under different conditions. We combined macroscale mechanical testing with nanoscale dynamic measurements to draw a holistic picture on the dependence of reinforcement on nanoparticle dispersion states.

L'objectif de ces travaux était de mettre au point une nouvelle voie de modification de la silice comme renfort d'élastomères silicones. La polymérisation par ouverture de cyclosiloxanes anionique (POCA) amorcée directement à partir de la silice dispersée en phase aqueuse a été réalisée. Les caractéristiques des silices utilisées dans cette étude ainsi que leur comportement en dispersion aqueuse ont tout d'abord été étudiés. La silice affiche des groupements silanolates à la surface de la silice pour des pH supérieurs au point de charge nulle. A pH 7, ces groupements silanolates en surface sont capables d'amorcer une polymérisation des cyclosiloxanes, et pas en suspension aqueuse. L'influence du contre-ion et de sa concentration sur le greffage obtenu s'est avérée primordiale, en complément des propriétés adsorbantes de la silice via les liaisons siloxanes. Les silices obtenues par ce nouveau procédé ont fait l'objet d'analyses poussées par thermogravimétrie, fragmentation-CPG, 29Si RMN simple impulsion et pyrolyse CPG-SM dans le but de décrire précisément la conformation du greffage. Le procédé a été transféré à plus grande échelle pour permettre la production d'importantes quantités de silice qui ont ensuite été incorporées dans une matrice silicone. Des limites de densités de greffage au-delà et en deçà desquelles la silice est mal dispersée dans le matériau ont été mises en évidence par traitement d'image. Les propriétés des élastomères ont été évaluées à partir de tests de dureté, de traction uniaxiale, de traction cyclique et de résistance à la déchirure dans le but d'étudier l'influence du greffage (conformation et groupements vinylés) sur le renforcement. / This work deals with a new way of modifying silica surface in a view to reinforce silicone elastomers. The surface-initiated ring-opening-polymerization (Si-ROP) of cyclosiloxanes was performed directly from the surface of silica dispersed in water. The characteristics of silica used in this study and their behavior in aqueous dispersion were first studied. For pH higher than the Point of Zero Charge, silica presents silanolate groups at its surface that are able to initiate the ROP of cyclosiloxanes from the surface, and not in aqueous suspension. The influence of the counter-cation and its concentration proved to be essential, in addition to silica's adsorbing properties. Modified silicas obtained by this new process were deeply analyzed by thermogravimetric analyses (TGA), fragmentation-GC, simple impulsion 29Si RMN and pyrolysis GC-MS in order to describe precisely the grafting conformation. The polymerization process was then scaled up to produce higher quantities of modified silica, which were incorporated in a model silicone formulation. Highest and lowest grafting densities tended to poor silica dispersions, as shown by image treatment. Hardness tests, uniaxial tensile and cyclic tests and tear resistance tests were performed in order to evaluate the influence of the grafting (conformation and vinylated groups) on silicone elastomers properties.