Wood fibre alkalization effect on the thermal stability of meranti wood flour: a modification of the conventional method

Alkalization of plant or wood fibre (WF) is the most widely used method of chemical modification to improve reinforcement in thermoplastic composites. This process involves the complete or partial removal of extractives and or modification of lignocellulosic material. While research has shown that removal of the less thermally stable extractives results in an improvement in fibre thermal stability, in the current work it has been shown through single-factor analyses, Fourier transform infrared microscopy, scanning electron microscopy, thermogravimetric analyses and wide angle X-ray diffraction that meranti WF thermal stability is largely influenced by the holistic changes in the WF structure, which itself is affected by alkalization factors. After implementing stepwise regression on a central composite design, no empirical model could be established to explain or predict thermal stability due to interaction of treatment factors. As a result, single-factor analyses of temperature, time and alkali concentration were conducted. Single-factor analyses showed that different combinations of time, temperature and alkali concentration through a central composite design result in WF with different thermal stabilities, lignocellulosic content, crystallinities, crystallite sizes, extractives content and morphology. Alkali-treated meranti WF showed lower thermal stability compared to the untreated WF. Mild treatment conditions (e.g. 50 °C/30 min/5%) were seen to result in the most thermally stable WF. Increasing temperature, treatment duration and alkali concentration increased thermal stabilities except at harsh conditions (e.g. 50 °C/90 min/15%). A combination of high alkali concentration and long treatment times showed a combined detrimental effect on WF thermal stability. Changes in the lignocellulosic structure, crystallinity, crystallite sizes and surface features explain the observed changes in thermal stabilities.