Gallic Acid as a Potential Substitution for Phenol in Phenol-Formaldehyde Resin for Biocomposite Matrices

This project explores the production of new resins using gallic acid (GA) as a substitute for phenol in phenol-formaldehyde (PF) resin. A special curing schedule was made tailored to the limitations of the equipment used. Phenol was substituted with GA at various contents until the optimum ratio has been reached, which in this present study is 31% of gallic acid. The effects of co-reaction of GA and phenol with formaldehyde on mechanical, thermal, chemical and morphological properties were investigated. Test specimens were prepared by synthesising the pre-polymer at certain processing conditions and curing it in the autoclave. From the flexural test, addition of GA showed improvement in the flexural modulus and strength. The thermal properties of PF resin and GA resin were investigated by Differential Scanning Calorimetry (DSC), Dynamic Mechanical Analysis (DMA) and Thermogravimetry Analysis (TGA). From the DSC result, no residual exotherm can be seen below 150 ℃ which indicates that the resin is suitable for use in natural fibre composites. Glass transition temperature (Tg) from DMA suggest that substitution of phenol with GA in production of resol resin has increased the glass transition temperature while TGA results showed that increasing GA substitution level in the compound resulted in increasing weight loss at lower temperature, and hence lower thermal stability. Based on Scanning Electron Microscopy (SEM) analysis, the flexural fracture surfaces for PF and GA resin displays no bubbles and voids present in the resin. It shows that the cure cycle proved to be successful in producing bubble-free specimens. In this study, Group Interaction Modelling (GIM) was also used to predict the Tg of the resin, which compares very well with the experimental work. Following the successful production of resin, nettle fibre was added as a reinforcing agent in producing the biocomposites. It shows that up to certain amount of nettle fibres, which is in this study 15 wt% of nettle fibres, the mechanical properties of the resin improved substantially. However, in composite systems, we can see particles and holes where fibres have pulled out upon fracture are clearly visible in the SEM images, which indicates poor interaction between fibre-matrix.