A series of particulate reinforced natural rubber composites are prepared using both model (glass sphere) and commercial (carbon black and precipitated silica) reinforcing filler materials having a range of surface activities. Small strain reinforcement and viscoelastic behaviour of the model (glass sphere-filled) microcomposites are found to be well described by hydrodynamics and temperature-insensitive stiffening mechanisms such as strain amplification and elastomer occlusion. This means that the energy applied to the model materials during small strain deformations is entirely stored and dissipated within the elastomer phase. For carbon black-filled natural rubbers such mechanisms are no longer found to completely describe the levels of reinforcement and viscoelastic behaviour. This is the case particularly for high surface area carbon blacks of small aggregate size. For carbon blacks, additional mechanisms of reinforcement are identified and associated with the formation of a filler network and with effects at the polymer-filler interface. For all the compounds considered in this study, no direct experimental evidence is found for the formation of significant volumes of interphase polymer exhibiting retarded chain dynamics near the filler surface. The observation of a secondary dissipation process in rubbery region small strain dynamic mechanical and creep measurements of carbon black-natural rubber compounds where the polymer filler interaction is particularly poor (where the carbon black surface is graphitised) indicates that there may be a significant slippage of polymer chain segments at the filler-elastomer interface. There is some limited evidence from small strain creep testing to indicate that this process also occurs in commercial carbon black-filled compounds but to a much reduced extent. To the best of the author's knowledge this is the first time that such processes have been observed in carbon black filled elastomers.

This study aims to understand the behaviour of a novel elastomer where hollow microspheres are used as a particulate filler. The behaviour of elastomers filled with rigid particles, is fairly well understood, where the stiffness increases as the amount of filler material is increased, alternatively, foamed elastomers which are usually produced with either closed cells or open cells, have been shown to become softer as the volume of the voids are increased. When traditional foam materials are compressed they exhibit non-linear behaviour in three distinct phases, the cell walls firstly bend, then they buckle and this is followed by densification. To understand the overall physical behaviour of the material, tensile tests of the elastomer material were conducted using unfilled materials and filled with a range of the hollow sphere filler volume fractions. Compression tests were also conducted on small cylinders, again using unfilled and filled rubbers with a range of filler volume fractions. The physical tests showed that increasing the filler volume fraction increased the reinforcing effect at low strains with an associated increase in stiffness, but the material then became increasingly less stiff at higher strains. To understand the behaviour of the bulk material, the mechanical behaviour of single hollow spheres under strain were investigated, both as a standalone material and then also embedded in an elastomer. To examine the mechanical behaviour of a single hollow plastic sphere a single microsphere was compressed using nano-indentation, the tests were then replicated at a larger scale using model table tennis balls. FEA software was used to model the behaviour of both types of hollow sphere, as well as a wide range of other spheres to better understand their buckling behaviour, to help predict the behaviour of microspheres with different ratios of wall thickness to diameters. To examine the behaviour of hollow spheres in a rubber matrix, simple cylindrical unit cells were made with a single hollow plastic sphere embedded within them. These model cylinders were produced with a translucent elastomer containing a single table tennis ball. Their behaviour in compression and in tension and that of a single hollow plastic sphere embedded in an elastomer was also modelled using FEA software, the effects of debonding and buckling were determined for small and large strains and were used to examine the more complex behaviour of the filled composite. A comparison between the measured behaviour and the various models indicates that the bulk behaviour of the microsphere filled elastomers is primarily determined by a progressive dewetting process of the rubber away from the microsphere in tension and by buckling phenomena of the hollow spheres in compression.