The world is currently grappling with challenges of global warming, environmental pollution, and energy crisis due to overdependence on non-renewable petroleum-based raw materials [1]. This has necessitated the need for eco-friendly and sustainable materials for diverse engineering applications such as aerospace, automobile, sports, marine, and packaging, among others. Natural filler-based polymer materials are currently used in these areas due to their lightweight, high corrosion resistance, nonbiodegradability, sustainability, nontoxicity, and low-cost attributes as compared with synthetic counterparts [2], [3]. As such, reinforcements from natural sources have, in the recent past, attracted increased research attention as an environmentally friendly and inexpensive substitute for synthetic reinforcements in composite development. Unlike synthetic materials, natural materials offer the benefits of lightweight, low cost, renewability, biodegradability, high abundance, renewability, high specific stiffness, and nontoxicity [4], [5].
In recent years, natural lignocellulosic materials, such as hemp, bamboo, flax, sisal, etc., have become the most significant reinforcing materials in polymer matrices. The use of these wood-based raw materials involves cutting plant fibres, contributing to the triple planetary crisis: loss of biodiversity, pollution, and climate change [6], [7]. With increasing areas of application of polymer composites, the consumption of wood-based raw materials is expected to increase. Therefore, substitutions for wood-based raw materials are inevitably needed. Recently, there has been a growing research interest in agricultural waste (agro-waste) as a substitute for wood-based raw materials. Of the various agro-wastes (such as bagasse, barley, wheat, rice, sorghum, etc.), coconut shell presents interesting properties in biodegradable polymer composites due to its good thermal stability [8].
Coconut is an important global crop, particularly in Southeast Asia: Indonesia, the Philippines, and Malaysia. The cultivation of this plant has spread to tropical regions of the world, such as South America and Africa. The coconut shell is a protective layer of the coconut fruit that comprises endocarp, endosperm, and mesocarp layers [9]. In 2017, Nigeria produced 267,520 metric tons of coconut fruits, translating to more than 40,000 metric tons of coconut shell agro-waste. However, more than half of the coconut shell waste generated in third-world countries, such as Nigeria, is used for open-burning charcoal production as well as dumped in landfills, causing air and land pollution, respectively, due to the lack of technological capacity for sustainable and eco-friendly utilization of these wastes [10]. The pollution caused by improper coconut waste management has become a global issue that needs a concerted approach. In fact, several studies have investigated the possibility of using coconut shell agro-waste for charcoal production [11], sustainable biofuel production [10], [12], [13], concrete production [14], and particle filler in eco-composite materials [8], [15]–[18]. Research on the use of coconut shell waste could provide solutions to the global energy crisis and environmental issues besides boosting the agricultural economy [19], [20].
It is well documented that natural cellulosic materials have hydrophilic lignocellulosic molecules and low thermal stability, rendering them incompatible with polymeric matrix [21]. Therefore, surface modification is a significant step to enhance fibre/matrix compatibility, hence interfacial bonding strength. With the increased potential of natural material-based composites in diverse industrial applications, surface modification of natural fibers and particles is becoming a major research area. Generally, silane, alkali, peroxide, bleaching, acetylation, and benzoylation treatments of natural fibers have been widely investigated so far [22]–[26]. According to studies, alkali and bleaching treatments have been reported as the most effective and popular surface modification techniques, resulting in enhanced natural fibre/matrix interfacial compatibility as well as microstructural modification of natural fibres [27], [28]. For instance, a comparative study on the impact of the alkali and bleaching treatments of natural cellulosic fibres from Juncus Effesus L plant revealed that bleach-treated fibres exhibited enhanced properties and relatively high cellulosic content due to significant removal of amorphous constituents, mainly hemicellulose and lignin [29]. Similarly, characterization results of Bauhinia Variegata natural fibre subjected to alkali and bleaching treatments reported improved tensile strength and Young's Modulus as well as reduced hydrophilic nature of the fibre with chemical modification and enhanced natural fibre/matrix interfacial bonding strength [24].
Although numerous studies have been dedicated to the application of coconut shell agro-waste and surface treatment of natural materials, the impact of various chemical surface treatments on surface morphology, thermal stability, and chemical characteristics of coconut shell powder/filler has seldom been investigated and reported. Actually, subjecting natural materials to different chemical surface treatments results in different fibre properties [30]–[32]. Therefore, in this paper, a research attempt is made to enhance the thermal stability, morphological, and chemical properties of coconut shell powder by different surface treatments. The current study investigated the impact of alkali treatment, bleaching treatment, and combined NaOH-bleaching treatment on the morphological/structural, chemical, and thermal characteristics of coconut shell-derived cellulosic powder, and evaluated the most effective surface treatment method to eliminate amorphous components, thus improving the fibre properties.