Abstract
The performance of a composite is not only dependent on the properties of the constituent elements, the fibre, the size and the matrix, but also on the interface between them. Good interfacial bonding is essential for the effective transfer of stress from the matrix to the fibre. Two methods of improving the interfacial adhesion in E-glass reinforced thermoplastics have been studied, namely by coating the fibre with a size specific to the matrix, or by using an in-situ reactive processing technique to activate the matrix itself. The size modification and the matrix modification systems were evaluated in E-glass reinforced thermoplastics with matrices of polyamide 6.6 and polypropylene, respectively. The elements which constitute the composite, namely the fibre, the size and the matrix, were characterized both individually and when combined in the composite. Both polymers presented spherulitic structures, however, the nature of these crystallites was shown to be dependent on both the processing conditions and the chemical modification, for the polypropylene. The physical properties of the unreinforced matrices were examined by tensile testing and by viscoelastic analysis, and were shown to be structure-related. The characterization of the size coating on the fibres proved difficult due to the small dimensions of the fibre, the very thin size layer and the insulating nature of the specimens which resulted in charging with electron microscopy techniques. However, by employing a wide range of experimental methods the polymer of the size and the fibre/size interface were successfully charcterized. The behaviour of the size was found to be dependent on the type of matrix reactive polymer added to the silane coupling agent. The composite materials, of both systems, were characterized structurally, by employing image analysis to examine the fibre length and orientation distributions, and transmission optical microscopy to observe the matrix. The fibre length and orientation distributions were found to be dependent on the processing conditions employed, but independent of the interfacial modification, of both systems. The mechanical properties of the composites were measured by tensile testing and viscoelastic analysis. Three methods were developed to evaluate the strength of the fibre/matrix bonding in the injection moulded composites: Viscoelastic analysis, with a torsion pendulum was shown to be sufficiently sensitive to detect variations resulting from the type of size applied to the fibres, or the addition of small quantities of modifying agents to the matrix polymer. The composite was characterized, in its bulk form, by comparing the damping properties of the material. An adhesion efficiency model was developed to rank the composites with respect to their interfacial bond strength. This method was found to be an effective tool for comparing the different adhesion enhancement systems. The third method employed was the microdrop test which provided a direct measurement of the interfacial shear stress in each system.