Introduction
Plant-based fibres offer both ecological and economic advantages over glass or synthetic fibres due to their biodegradability, low cost, non-abrasiveness, safe fibre handling, low energy consumption, high specific properties, relatively low density and diversity (Bagheri et al.
2018; Bledzki and Gassan
1999; Ghasemi et al.
2017; Mohanty et al.
2006; Wambua et al.
2003). As a result, the wood plastic composites (WPC) market has grown rapidly, especially in synthetic fibre replacement applications in the automotive sector (Markets & Markets
2020; Marsh
2003; Suddell
2008; Wambua et al.
2003). In the making of WPC, plant fibres are usually modified to facilitate compatibility with thermoplastics or thermosets. One of the most common methods of fibre modification is alkali treatment, commonly known as alkalization or mercerization (Farsi
2010; Liu et al.
2004; Mwaikambo and Ansell
2002). Knowledge of wood fibre structural modifications is necessary to inform and explain the observable properties such as mechanical and rheological properties of related wood-fibre products. The current study therefore seeks to contribute to the field of WPC by investigating alkalization factors and their effects on wood flour properties.
Wood flour (WF) alkalization involves the use of sodium hydroxide (NaOH) to modify the fibre structure. The process involves the removal of WF-waxy materials, oils, proteins, amorphous lignin and hemicelluloses (Bledzki and Gassan
1999; Mwaikambo and Ansell
2002). This results in fibre with altered physico-chemical properties such as crystallinity, thermal stability, surface properties and lignocellulosic content. Research has shown that WF alkalization results in a high degree of crystalline cellulose as well as rough fibre surfaces. The increase in the crystallinity index has been shown to result from the reduction of the amorphous phase and or the increased order of cellulose crystallites (Aboul-Fadl et al.
1985; Mwaikambo and Ansell
2002). Alkalization results in cellulose I transition to cellulose II type, where the cellulose polymer chains are shortened and more ordered, thereby increasing the crystallinity index (Bledzki and Gassan
1999; Mwaikambo and Ansell
2002). However, the crystallinity index then declines at high alkali concentrations due to cell wall damage. This results in a less thermally stable amorphous cellulose, with reduced crystallite length (Aboul-Fadl et al.
1985; Bisanda and Ansell
1991; Nguyen et al.
1981). On the other hand, changes in the lignin structure and content were shown to result in a decrease in the peak degradation temperature (
Tpeak) of WF (Bisanda and Ansell
1991; Chen et al.
2014; Nguyen et al.
1981; Ohtani et al.
2001). Moreover, due to its wide range of thermal degradation temperatures, the degradation of lignin also influences the degradation of hemicelluloses (Tserki et al.
2005). Lignin degradation leaves the hemicelluloses exposed to subsequent degradation (Chen et al.
2014; Ohtani et al.
2001). Hemicellulose has been shown to degrade in the range of 180–350 °C, lignin in the range 250–500 °C, while cellulose in the range 275–350 °C (Kim et al.
2006). WF thermal stability has also been reported to be influenced by extractives removal (Chen et al.
2014; Poletto et al.
2010). Extractives comprise a vast array of materials that have relatively low thermal stability such as fatty acid esters (of fatty alcohols, terpene alcohols, and sterols), fats (i.e. fatty acid glycerol esters) and phenolics (i.e. stilbenes, tannins, lignans and flavonoids) (Yang and Lu
2021). As such, removal of the thermally less stable extractives generally results in an improvement in the WF thermal stability (Chen et al.
2014; Poletto et al.
2010).
Inarguably, studies have made clear the role of alkali treatment of plant fibres; however, most studies have looked more into the effects of varying alkali concentration at specific temperatures, or
vice versa. This study aims to contribute towards existing literature by investigating the individual and (possible) combined effects of alkalization conditions of time, temperature and alkali concentration following a mixture design. In this study, alkalization is conducted through a central composite design (CCD) where temperature, time and alkali concentration are varied while morphology, thermal stability and crystallinity are used to monitor the alkalization process. Implementation of a CCD will allow for probing into alkalization effect at mid- and extreme points of experimental factors, thereby giving a clear understanding of factorial effects on response variables. Morphological imaging will be done through scanning electron microscopy (SEM), thermal stability tests through thermogravimetric analyses (TGA) and the degree of crystallinity using wide-angle X-ray diffraction. Alkalization was conducted on waste meranti wood flour (
Shorea spp.), obtained from a local manufacturing furniture shop. Meranti WF was chosen for our project as it was readily available as waste, at no cost, and has been identified to exhibit good mechanical properties (Database
2021) thereby possessing the potential of beneficial reinforcement properties as a filler in WPC. In addition, compared to many hardwoods, meranti wood has a relatively lower extractives content compared to other hardwoods (Database
2021) and thus severe treatment conditions to remove extractives may not be necessary.
Conclusion
Alkalization results in WF physico-chemical changes that, if not monitored accurately, may result in serious loss of essential WF properties. Careful optimization of alkalization can result in WF with improved thermal stability, crystallinity and surface features without compromising the WF structural integrity. According to the experimental domain, alkalization of meranti WF resulted in thermal stabilities lower than untreated WF. However, good thermal stability of alkali treated WF was found at mild conditions (e.g. 50 °C/30 min/5%), while severe treatment conditions lowered WF thermal stability (e.g. at 38 °C/96 min/15%). No model could be established to predict and explain alkalization through multiple linear regression analyses due to the co-dependence of treatment factors. It was revealed in the investigation that temperature, alkali concentration and time exhibit individual as well as combined effects on WF treatment. As such, an increase in temperature or alkali concentration under mild conditions (e.g. 30 min/5% or 30 min/25 °C, respectively) increased thermal stability, whereas increases in temperature or alkali concentration under harsher conditions (e.g. 96 min/15% or 90 min/50 °C, respectively) resulted in reduced thermal stability. Furthermore, high alkali concentrations and longer treatment durations show a combined effect and seem to be detrimental to WF thermal stability. Based on the findings from the investigation, it therefore can be recommended that alkalization of meranti WF be done under mild conditions to improve WF thermal stability and surface features without compromising the WF chemical and structural integrity. Also, due to the use and generation of high temperatures (⁓180–260 °C) during internal mixing, extrusion or injection moulding, the use of alkalized WF with compromised thermal stability would likely result in exacerbated degradation of the WF during composite processing and, consequently, poor mechanical properties of composites. Therefore, careful optimization of WF alkalization processes is imperative prior composite processing.
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