Abstract
Chemically treated layered silicates (clays) can be combined with normal polymer matrix materials to form a nanocomposite in which clay layers are distributed throughout the material. Previous researchers have shown that these high aspect ratio clays can alter the properties of a range of thermoplastic and thermosetting polymers by a number of mechanisms; improving mechanical and thermal properties and reducing permeability. This study involves the investigation of these novel materials to assess their potential applicability within the civil engineering industry and to assess in which areas and situations they might be used. Extensive research was conducted into the processing required for these materials to achieve sufficient organoclay exfoliation with a range of matrix materials. The subsequent nanocomposite materials were assessed using a range of characterisation techniques including XRD, TEM, SEM, OM, TGA, DSC and FTIR spectroscopy. Organoclay morphology was found to be highly dependent on the type of surfactant and curing agent used and resulted in a variety of different types of nanocomposite being formed. A variety of new manufacturing techniques were developed to generate void free and dimensionally consistent pure polymer and fibre composite specimens that allowed the frequently subtle property variations due to organoclay to be detected. A range of mechanical, thermal and durability properties were investigated to assess the differences that organoclay can generate when incorporated in the pure polymer and in a glass or carbon fibre composite. Mechanical testing of the pure polymer revealed small increases in tensile, flexural and compressive properties in glassy polymers; whereas in elastomeric polymers the properties can be improved by a factor of 3 due to the high relative properties of organoclay compared to the polymer. When incorporated in a fibre composite the organoclay offered little improvement when in a glass fibre composite but was able to increase the properties of a carbon fibre composite. It is thought that this increase does not occur due to increased mechanical properties of the polymer commensurate with the law of mixtures theory but due to changes in the fibre-matrix interphase. The permeability of nanocomposites when exposed to water was not improved, although the solvent permeability of some matrix materials was significantly reduced. Although a high degree of nanoscale exfoliation had been achieved, with highly separated clay platelets, the macroscale dispersion was not sufficient to result in reductions in Fickian water uptake via a tortuous path mechanism. Whereas, the reduction in solvent permeability was thought to arise from changes in the rate of polymer relaxations due to polymer chain mobility being constrained by organoclay and altering the rate of Case II uptake. The mechanical durability of pure polymer and glass fibre nanocomposites and the thermal durability of pure polymer nanocomposites were investigated. Little improvement was observed in the long-term durability properties of these materials after prolonged environmental conditioning as a result of organoclay. The influence that organoclay has on polymer chain constraint was investigated by DSC and DRS to assess which combinations of materials develop significant changes to the polymer network. It was found that the same nanocomposite formulations that resulted in reduced solvent uptake also resulted in increased thermal and reduced dielectric properties. Due to the requirement for high quality processing and the need to control cure cycle the implementation of nanocomposites would only be feasible within a premanufactured product and could not be used onsite with confidence until new and improved materials or processing methods are developed. Reductions in permeability would have to be improved to a level beyond that observed in this investigation and to a level witnessed in a only a few cases involving epoxy nanocomposites to warrant the additional expense of incorporating and processing organoclay It cannot currently be guaranteed that this level of permeability improvement would be established due to the limited number of cases in which this has been achieved. Therefore, the present state of the art does not allow sufficient improvements to be attained and the development of superior organoclays capable of becoming exfoliated with relative ease, or methods of processing that are proven to be highly effective, cost efficient, reproducible and rapid would be required before this technology could be applied to civil engineering materials. However, the future potential of nanocomposite materials remain significant and their application in civil engineering composites will offer significant advantages as the technology develops to allow economical processing and increased property advantages.