Engineering and characterisation of the interface in flax fibre/polypropylene composite materials. Part I. Development and investigation of surface treatments

Natural fibres have long been used as cost-cutting fillers in the plastics industry. Nowadays, they are considered to be a potential replacement of glass fibres for use in composite materials. However, although natural fibres have many advantages, the most important being their low cost and low density, they are not totally free of problems. A serious problem of natural fibres is their strong polar character, which creates many problems of incompatibility with most thermoplastic matrices (especially polyolefins). Surface treatments, although having a negative impact on economics, are potentially able to overcome the problem of incompatibility. The present study focuses on the development, optimisation and characterisation of two such treatments; acetylation and stearation. The two treatments were applied on two grades of flax fibres (green and dew retted flax), the results are discussed in terms of process variables, such as temperature, time of treatment, recycling of reactants, etc. Three characterisation techniques were applied on the treated and untreated fibres; X-ray diffraction, scanning electron microscopy, and inverse gas chromatography. It was found that both treatments result in a removal of non-crystalline constituents of the fibres, and alter the characteristics of the surface topography. It was also found that both treatments change the fibre surface free energy, with acetylation increasing it and stearation decreasing it.