3 - Engineering the fibre-matrix interface in natural-fibre composites

Abstract

The interface between the reinforcing agent and the matrix plays a pivotal role in determining the mechanical properties of composite materials. A strong interface creates a material that displays exemplary strength and stiffness but is very brittle in nature with easy crack propagation through the matrix and fibre. A weaker interface reduces the efficiency of stress transfer from the matrix to the fibre and consequently the strength and stiffness are not as high, but in contrast, toughness is increased. Natural-fibre composites are distinct from artificial fibres. The main problem in the case of natural-fibre composites is that natural fibres are highly heterogeneous materials both physically and chemically. This chapter discusses the interface in natural-fibre composites and also highlights the peculiarities of these systems. It discusses the methods of interfacial modification and experimental techniques for interfacial bond assessment in terms of mechanical behaviour. The main focus is on the use of model composites and micromechanical testing. Natural fibres consist mainly of cellulose, which is a natural polymer rich in hydroxyl groups. The remaining components of natural fibres are also molecules rich in hydroxyl, carboxyl and other functional groups. Consequently, the fibres are usually strong polar materials and exhibit significant hydrophilicity. In contrast, most polymer matrices tend to be apolar and mostly hydrophobic. As a result, there are significant problems of compatibility between the fibre and the matrix, leading to poor dispersion, a weak interface and ultimately inferior quality composites. Such compatibility problems have to be tackled with the use of appropriate methods to improve adhesion between the fibre and the matrix. There are essentially two different possible general strategies to achieve this goal, the first being to alter the fibre surface and the second being to modify the matrix properties.