Since the industrial revolution, rapid economic and social development has always been associated with a huge demand for resources, in which plastics have flourished during this period due to their advantages of low quality, durability and high cost competitiveness but over-dependence on petrochemical resources, while resource is often the basis for the long-term sustainable development of human societies, posing a serious environmental challenge, and in the process of dealing with the environmental and resource barriers, and a number of new production and use strategies have been attempted, such as the development of renewable resources and the search for environmentally friendly, low-cost materials [1–7]. The fact that the versatility of plastics has led to a huge increase in their utilisation, from 5 million tonnes globally in the 1950s to over 330 million tonnes today, is evidence enough that the plastics industry is a significant contributor to environmental pollution and the trend today is to find a way to reduce the number of plastics used [8–9]. Recently, wood plastic materials (natural fiber reinforced polymers) have attracted the attention of researchers due to their superior biodegradability, easy availability, low cost, low density, corrosion resistance, higher specific strength and higher modulus of elasticity compared to glass fibres than existing materials [10–18]. However, traditional wood-plastic composites (WPC) make less use of recyclable waste resources such as wood felling and barrel residue (WFBR), wood processing residue (WPR), bamboo processing residue, etc., which is not in line with the reality of the current shortage of forest resources [10, 19]. Therefore, it is necessary to find a new filler to replace the increasing shortage of wood in order to alleviate this trend and at the same time create high economic value [11, 20–23].
Rice husk Charcoal (RHC) is a waste material produced after the production of rice husk electricity, it has a very high amorphous silica content (95%) and after burning the waste material is usually landfilled which can cause some serious environmental problems, at this time its reuse as a filler in composite materials can solve the problem of the large accumulation of electricity waste RHC due to the growth of the population and the increased demand for rice and electricity, as it is not only cheap but also solves the problem of environmental pollution caused by landfills [24–26]. Polypropylene (PP) is one of the most consumed varieties of plastics, a polymer made from the polymerization of propylene monomer, usually divided into homopolymer polypropylene, block copolymer polypropylene and random copolymer polypropylene to aggregate state structure makes it has good electrical properties and insulation properties as well as almost no change in performance due to moisture, but PP as a petroleum-based plastic when used in large quantities to cause environmental pollution, has a low modulus, poor heat resistance and easy ageing which limits its further application [27, 28]. Therefore, the preparation of PP/rice husk charcoal composites not only broadens the application area of PP, but also gives a new value to waste rice husk charcoal.
The interface between filler and resin matrix is one of the main issues affecting the overall performance of composites which is one of the major challenges to broaden their practical applications [29–31]. The presence of many hydroxyl groups on the surface of inorganic fillers makes them highly hygroscopic, resulting in poor compatibility with hydrophobic matrices while hydrogen bonds formed by the hydroxyl groups on the surface of the fillers can prevent their uniform distribution in the matrix, causing agglomeration and reducing the performance of the composite [32, 33]. Typically, the filler surface is pretreated or a third component is introduced between the two phases as a compatibilizer to improve interfacial bonding which is classified into two categories depending on the filler surface treatment method: physical and chemical methods [34–38]. In this issue, a lot of previous studies have been done, Mehdi Chougan et al [39], pretreated wheat straw using hot water and water vapor as raw material, and the results showed that the tensile strength, elongation at break, elastic modulus, and toughness of wheat straw bio based composite materials treated with hot water and steam synergistic pretreatment increased by 166%, 18%, 68%, and 285%, respectively, compared to the untreated ones. Li Chao et al. [40] utilized the silane coupling agent KH550 to modify the surface of Coal Gangue (CG)-Polyethylene (PE) composites. They conducted a comprehensive analysis and explanation of the interface formation mechanism and its impact on the composites' properties. The study found that the coupling agent facilitated covalent bonding between PE and CG, prevented particle agglomeration, and improved dispersion stability. The introduction of KH550 contributed to non-uniform nucleation, enhanced the crystallinity of the composites, and improved interfacial bonding with the resin, resulting in improved overall compatibility. Mechanical tests demonstrated a significant increase in tensile and flexural strength at the optimal KH550 content. Overall, this study presents a strategic approach for optimizing CG-PE composites, showcasing their potential application in the automotive industry.
Silane coupling agents are organosilicon compounds that contain both non-hydrolysable and hydrolysable groups in the molecule which are recognized as one of the effective surface treatment agents because of their ability to chemically combine (couple) at the interface of inorganic and organic materials to enhance the interfacial bonding force and strengthen the physicochemical properties of composite materials [35, 38, 40–42].
The incorporation of RHC as a filler material into composites is an attractive option from an environmental and economic point of view and offers a viable option for the secondary use of agricultural waste. In this paper, a silane coupling agent (KH-560) was used to modify the surface of RHC to improve the interfacial compatibility of the two phases, and a series of RHC-filled PP-reinforced composites were prepared using a melt-blending technique and the effects of RHC content on the thermomechanical, crystalline and rheological properties of PP/RHC composites were investigated. The innovation of this study lies in the preparation of high value-added composites from thermal scrap and the secondary use of the scrap. In the same research area, there are few reports on the thermomechanical as well as rheological properties of RHC-filled PP composites in China and abroad. Therefore, this paper is dedicated to investigating the factors affecting the thermodynamic as well as rheological properties of PP/RHC composites in order to improve the overall performance of the material and thus broaden its application directions.