Modified polyester resins for natural fibre composites

Unsaturated polyester resin, which is commonly used as the matrix in synthetic glass fibre composites, has been modified to make it more compatible with cellulose fibres. Long kenaf fibres were alkalized with a 6% NaOH solution, combined with four different polyester resin formulations, A, B, C and D, and hot-pressed to form natural fibre composites. Polyester resin A was a conventional unsaturated polyester resin in styrene monomer, Crystic 2-406PA. The molecular structure of polyester B was based on polyester A, modified to make it more polar in nature to better react with the surface of natural fibres, and this modification resulted in the best mechanical properties. The flexural modulus and flexural strength of polyester B composites gave the highest values and the unmodified polyester resin (polyester A) gave the lowest. The opposite trend was obtained for the impact test as expected. Dynamic mechanical analysis (DMA) showed that polyester B composites gave the highest storage modulus (E′) values and the lowest tan δ values. Scanning electron microscope (SEM) micrographs of impact fracture surfaces performed on the polyester B composites clearly demonstrated better interfacial adhesion between fibre and matrix. A moisture absorption test also showed that polyester B composites gave the most superior bonding and adhesion of all the other polyester–kenaf composites.