Natural fibers (NFs), similar to a composite, with cellulose, hemicellulose, and lignin as reinforcements act as matrices structurally and present unique properties. Hence, NFs play a vital role in reinforcements for composites for engineering applications (Kuranchie et al. 2021). Automobile and aerospace industries have already started using NF composites for structural and substructural parts because of their low density, good mechanical strength, vibration absorption, and ultraviolet ray blocking (Kumar 2020; Lau et al. 2018). The advancements in NFs toward automobile application has been introduced in the driver and front passenger doors and the rear wing of the new 718 Cayman GT4 Clubsport production vehicles of Porsche’s (bioconcept cars). These are formed of an NF composite from economic and energy perspectives (Porsche Newsroom 2019).
The NF cannot withstand fire and are thermally weak. However, although NFs have sufficient mechanical strength and are competitive with synthetic fibers, such as carbon and glass fibers, they cannot control the flammability properties of polymer composites as reinforcements (Elsabbagh et al. 2018). The incorporation of flame retardant (FR) additives are the most common approach for improving the flame retardancy of NF composites and various types of FR additives, including ammonium polyphosphate, magnesium hydroxide, zinc borate, and silicon dioxide, have already been studied (Shukor et al. 2014; Sain et al. 2004; Wu et al. 2020). However, their inclusion predominately interferes with the mechanical properties because of their lack of interfacial chemical bonding with both the matrix and reinforcement, thereby creating stress concentration points. Hence, improving the fire resistance and strength maintenance are challenging tasks for researchers and industries for the usage of NF composites.
Recently, a few research studies have focused on modifying the chemical interaction of FR with either the NF reinforcement or matrix to enhance the flame resistance of NF composites without significantly affecting their mechanical properties (Khalili et al. 2017; Kim et al. 2020; Jeencham et al. 2014). Research is still required in this area due to the demand of the present market and the necessity to achieve desired properties effectively. Bio-FRs can serve this objective, consisting of biomass that provides dense char during combustion; hence they are drawn attention due to green sources and environmentally friendly characteristics compared with existing FRs showing health and environmental hazards, liberating corrosive gasses, etc [Costes et al. 2017]. However, Bio-FRs did not show high efficiency for polymers and need to be functionalized effectively by chemical modification. Moreover, the selection of a suitable bio-based material for flame retardancy and chemical modification is critical. A few biomaterials are available as waste that can enhance flame retardancy, e.g., chitosan, lignin, bone powder, oyster shell powder, and eggshell powder.
Among, chitosan is an exciting and abundant biowaste polymer that can be easily chemically modified by grafting copolymerization and compounding due to the chemical structure of an amino polysaccharide. The multi hydroxyl and amidogen groups present in chitosan promote flame retardancy behavior. In addition, chitosan has been used as a char-forming agent in intumescent systems. Recently, Prabhakar et al. used chitosan in a thermoplastic starch/flax fabric (TPS/FF) system and effectively improved its thermal and flame retardancy properties to reach those of commercial FRs with satisfactory improvements in its mechanical properties (Prabhakar and Song 2018). In their further studies, a novel FR compound produced by effective modification of chitosan with ammonium polyphosphate could significantly enhance the flame retardancy properties of NF composites (Prabhakar et al. 2019; Prabhakar and Song 2020; Shao et al. 2021). Different FRs are used for the modification of chitosan, as established in the literature (Li et al. 2020).
Silicon dioxide is an inorganic material with interesting properties such as low toxicity, good biocompatibility, high versatility, high surface area, uniform porosity, and remarkable chemical and thermal stability (Wang et al. 2020; Huang et al. 2019). In addition, silicon dioxide can act as an effective FR for polymer composites, by condensing the phase and forming an inorganic char, and for wood fiber polymer composites, decreasing the heat release rate (HRR) and total heat release rate (THR) (Pan et al. 2014). Therefore, silicon dioxide gained attention for commercial applications in civil engineering, construction, building, electrical, transportation, aerospace, defense, textile, and cosmetic industries (Hamdani et al. 2009). Moreover, silicon dioxide alone may be able to play a dual role of enhancing the mechanical and flame retardancy properties of NF composites.
Considering the above, this study focused on the preparation of a novel FR additive (referred to as NCS) by a simple chemical approach using chitosan and silicon dioxide. Although the combination of chitosan and silicon dioxide has been studied for other research areas, such as the removal of GO, there is no information available on the synthesis of a chitosan-based silicon dioxide FR additive (NCS). Therefore, this study further investigated its effect on NF polymer (vinyl ester/bamboo fiber (VE/BF)) composites. The chemical features of the synthesized NCS compounds were characterized via scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), thermogravimetric analysis (TGA), and microcalorimetry. The flammability, thermal, and mechanical properties of the NCS additive-incorporated VE/BF composites were examined via horizontal burn test (HBT), cone calorimetry, TGA, XRD, FTIR, and universal testing machine (UTM). This study aim to introduce a chitosan-based FR additive in NF composites to support flame retardancy and mechanical strength.