Effect of Synthesized Chitosan Flame Retardant On Flammability, Thermal, And Mechanical Properties of Vinyl Ester/Bamboo Nonwoven Fiber Composites

In this study, chitosan-based bio-ame retardant additive (referred to as NCS) was prepared by altering the chitosan (CS) chemically with silica (S) via ion interchange reaction and studied the effect on ame retardant, thermal and mechanical properties of vinyl ester/bamboo ber (VE/BF) composites manufactured by the vacuum assisted resin transfer molding (VARTM) process. Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis used to characterize the NCS. The spectral results revealed a new peak at 1560 cm -1 corresponding to NH 3+ – - O Si, bring up the interactive bond between CS and S. SEM, and XRD showed the diverse morphology (coarse surface), and signicant decrement in the intensity of diffraction patterns respectively support further the formation of NCS. The heat release rate of NCS decreased signicantly by 76%, and residual char increased by 47% compared with chitosan. The ame retardant and thermal behavior of NCS-VE/BF composites were examined by UL-94 standards, micro and cone calorimeter and thermogravimetric analysis. The results showed a delay in burning time in UL-94, enhanced LOI % and decrement of peak heat release rate and total heat release rate compared to pure composites by 32, 14, and 18%, respectively. The residual char increased by 47%. The mechanical properties also improved satisfactorily. Overall, the synthesized NCS could be suitable for the fabrication of sustainable ame-retardant natural ber composite without deterioration of mechanical properties that are suitable for substructural parts in engineering applications.


Introduction
Natural bers (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 re and are thermally weak. However, although NFs have su cient mechanical strength and are competitive with synthetic bers, such as carbon and glass bers, they cannot control the ammability properties of polymer composites as reinforcements (Elsabbagh et al. 2018). The incorporation of ame retardant (FR) additives are the most common approach for improving the ame 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 re 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 ame resistance of NF composites without signi cantly 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 e ciency for polymers and need to be functionalized effectively by chemical modi cation. Moreover, the selection of a suitable biobased material for ame retardancy and chemical modi cation is critical. A few biomaterials are available as waste that can enhance ame 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 modi ed by grafting copolymerization and compounding due to the chemical structure of an amino polysaccharide. The multi hydroxyl and amidogen groups present in chitosan promote ame 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/ ax fabric (TPS/FF) system and effectively improved its thermal and ame retardancy properties to reach those of commercial FRs with satisfactory improvements in its mechanical properties  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 ber 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 ame 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 ber (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 ammability, 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 ame retardancy and mechanical strength.

Preparation of NCS
First, the required amount of chitosan akes (10 w/v) was dispersed in 2% acetic acid solution with ultrapure water under 500 rpm stirring conditions for 4 h at 60°C to obtain a homogenous chitosan solution. Subsequently, added the required amount of silicon dioxide solution (2.5 w/v) to the chitosan solution (equivalent to the molar ratios of SiO 2 in accordance with chitosan), and the stirring was continued at 500 rpm for an h at 80°C. The end of the reaction was determined by the formation of a yellowish-brown thick precipitate, and subsequently, cooled to room temperature and subjected to vacuum ltration. The obtained product was thoroughly washed with deionized water, dried overnight (~ 6 hrs.) in an oven at 70°C, and the obtained akes nely ground into a powder using a ball mill.

Manufacture of NCS-VE/BF composites
The VARTM process was used to manufacture the NCS-VE/BF composites, as discussed by Chalapathi et al. (2020). Six bamboo nonwoven fabrics (200 × 200 × 0.39 mm) and 0.4-MPa vacuum pressure were used in the VARTM process. The infused wet composites were subjected to a pre-curing process at room temperature for 5 h, followed by post-curing at 60°C for 3 h in a composite curing oven (Fleta 5, LK LAB Co., Ltd, Republic of Korea). The overall preparation of NCS and the manufacturing process of the NCS-VE/BF composites are demonstrated comprehensively by schematic illustration, as depicted in Scheme 1.

NCS characterization
The details of the characterization instruments adopted for analyzing the fabricated NCS compound are as follows: a scanning electron microscope with an ion sputter coater (Model: Emcrafts cube 2, EMCRAFTS Co., Korea) was used for microanalysis. A particle size analyzer (Model: Malvern Mastersizer) was used for particle size analysis. FTIR spectrometer (FT-IR-6300, United Kingdom) using the KBr disk method was employed for spectral analysis. X-ray diffractometer (Model: Bruker, D8 Discover) with Niltered Cu Ka radiation at a speed of 50°C/min, current of 30 mA, and range of 0°-50° was used for XRD analysis. A particle size analyzer (Model: Mastersizer 2000, Malvern Panalytical Ltd., United Kingdom) was used to calculate the mean particle size and obtain the particle size distribution.

XRD analysis
The XRD patterns of NCS and the corresponding raw materials chitosan and silicon dioxide are shown in Fig. 2

Microanalysis
Micro images of NCS and the corresponding raw materials chitosan and silicon dioxide additives are shown in Fig. 2(B). Silicon dioxide particles showed smooth metallic surface morphology without any voids. The particles varied the size, majorly two types of particle sizes are observed in the micrograph, i.e., 2-10 µm and ~ 10-20 µm. Chitosan exhibited a rough and continuous morphology without voids. However, in NCS, the micrograph showed an entirely different morphology than CS, and S compound con rms the reaction between them. The shape of the particles seems slightly in between the trend of CS and S, but the course surface with continuous morphology without voids could be observed. In addition, 2-5 µm sized particles are stick to the surfaces of larger-sized particles and form lumps in between them. The NCS particles are also of two types of sizes, similar to silicon dioxide particles. In conclusion, the morphology of NCS showed a clear difference in morphology than CS and S might help to understand further and support the aforementioned FTIR and XRD analysis.

Particle size analysis
The particle size distribution of the NCS additive is shown in Fig. 2(C). Two average particle sizes of NCS were approximately 100 and 1000 µm, with volume percentages of 2 and 24, respectively. The mechano ball mill process interestingly produced a precise NCS particle size of about 1258 µm at 22% volume.

Elemental analysis
The composition of chitosan and the NCS compound elements include carbon, hydrogen, nitrogen, and sulfur, tabulated in Fig. 2(D). From the table, chitosan has the main elements of C, H, and N with 42%, 6.88%, and 8.17% content, respectively. Besides, the synthesized NCS compound showed similar to chitosan elements, i.e., C, H, and N; however, a signi cant reduction could be observed in the values, 22.33%, 5.12%, and 3.38%, respectively. In addition, 0.46% of silicon element can be seen in the table representing silicon moiety obtained by the chemical reaction with SiO 2 . Hence, the elemental analysis further strengthens the spectral and diffraction analysis to form the NCS compound.

Thermal analysis
The thermograms of chitosan and NCS additive are shown in Fig. 3(A). Chitosan exhibited a two-step degradation process in the thermogram; the initial step at < 100°C was due to the exclusion of the absorbed water. The prominent pyrolytic degradation of the chitosan molecule was observed at 350°C and continued to 700°C owing to dehydration of saccharide rings and decomposition of the acetylate and deacetylate units. The multihydroxyl group present in chitosan formed a dense char of almost 30% at 700°C. Besides, the NCS compound showed a very different trend in thermograms, and the thermal degradation progressed as a single-step process. Initial decomposition temperatures at around 100°C maintained constant weight without degradation, and the weight loss occurred after 100°C, and the degradation proceeded with a marginal decrement in the weight loss. The maximum degradation occurred above 300°C with weight loss of around 20%; in chitosan, the % was almost 60, which clearly refers to the signi cant slowdown of the thermal degradation process due to the synergistic effect of the silicon and chitosan. It may be understood, in the thermal degradation process, the silicon moiety present in the NCS protects the material from thermal degradation. In addition, NCS produced nearly 56% char residue at 700°C showing substantial thermal stability. The chitosan and silica moieties present in NCS probably produce a carbonaceous and silica-based char residue.
The oxidative thermal degradation of NCS is shown in Fig. 3(A 1 ). As can be observed from the gure, the thermal oxidation of NCS occurs in two steps in the 30-400°C temperature range. Almost 20% of weight loss occurred at initial degradation temperature > 240°C without decline so drastic. After that, a drastic thermal degradation occurred, directly resembling chitosan in the nitrogen atmosphere, which refers to the degradation of saccharide rings. However, after 400°C, the slowdown of thermal degradation could be seen; almost 15% of weight loss occurred until 580°C. Interestingly even at elevated temperatures from 580 to 700°C around the thermal degradation stopped, no further weight loss occurred and formed almost 37% char residue showed signi cant thermal stability under air atmosphere.

HRR analysis
The HRR peaks of chitosan and the NCS additive are shown in Fig. 3(B). From the gure, it can be observed that both CS and NCS produced two pHRR peaks during the combustion analysis. Chitosan burned rapidly during microcalorimetry analysis refers to the sensitivity with the re and produced the highest pHRR with a value of 61.9 kW/g at 321°C. The second pHRR peak was also generated with the value of 18 kW/g at 538°C due to thermal stresses. Remarkably, NCS showed changed ammability behavior than CS. The synergistic effect of CS and SiO 2 effectively decreased by showing the pHRR value 14.9 kW/g, which refers to the severe decrement of pHRR about 75% compared with CS with an equal amount of ame retardant ]. Moreover, the delayed temperature of about 10°C could be observed for the generated pHRR peak, further supporting the ammability property of the NCS compound. Crack formation due to thermal stress also happened in NCS due to dense char, although it is almost 90 C later than CS. Overall, a signi cant improvement is achieved in the ammability property of the NCS compound owing to the production of the dense char from the synergistic effect of the silicon dioxide and chitosan moieties through the condensed phase.  (Fig. 3B). The combined spectral peaks of VE and BF in the VE/BF composite persists in NCS-VE/BF composites. In fact, several apparent changes occurred in the spectrum of VE/BF composites after the addition of NCS. For instance, the disappearance of spectral peaks at 910 and 1644 cm − 1 , the spectral peak at approximately 3400 cm − 1 corresponding to the -OH groups slightly shifted to higher wavenumbers with the percentage increase in NCS and the formation of a new peak at 797 cm − 1 . Hence, these spectral alternations might be evidence for the interaction effect of NCS on the VE/BF composites.

XRD analysis
The XRD patterns of the VE/BF and NCS-VE/BF composites are shown in Fig. 4(B). The VE/BF composite shows a broad peak at approximately 19.9° with an intensity of 6215 counts. It suggests the non-crystallinity of the VE/BF composite with an amorphous structure. Whereas, the addition of the NCS additive signi cantly in uences the structural behavior of the VE/BF composites. It can be observed that sharp XRD peaks appear at 20.8, 26.6 (predominate), 36.5, 39.4, and 42.4° for the 2 wt.% NCS-loaded VE/BF composite, which gradually becomes predominant with increasing percentage of NCS. These sharp peaks exhibit the induced effect of the NCS crystalline structure and the intensity of the peaks could be clearly observed in the gure at 6% NCS-VE/BF composite spectrum. The appearance of the NCS crystalline peaks also indicates that the powder particles are distributed homogeneously throughout the VE/BF composite. Overall, the XRD results further strengthen the spectral analysis for the interaction of NCS with VE/BF composites.

Microanalysis
The surface and tensile fracture surfaces of the VE/BF and NCS-VE/BF composites were examined by microanalysis, and the micrographs are shown in Fig. 5. The surface SEM images clear present the differences in the structural morphology of the VE/BF and NCS-VE/BF composites. The VE/BF composites are almost smooth, with some impurities are found to be present everywhere on the surface while cutting the specimen. In comparison, the surface of the 2 wt.% NCS-loaded VE/BF composites seem coarse, and some of NCS particles covered by the VE resin are observed on the surface. This rough behavior gradually predominates, and even the number of particles on the surface increases with increasing wt.% of NCS in VE/BF composites. Furthermore, the EDX and EDX mapping support the microanalysis by exhibiting additives and their distribution, as shown in Fig. 5. From the EDX graphs, the presence of 'silicon element in the NCS inclusion and its increment with increasing percentage can be observed, absent in those of the VE/BF composites. These observations strongly support the EDX mapping results shown in the gure.

Thermal analysis
The TGA curves of the VE/BF and NCS-VE/BF composites under pyrolytic and oxidative atmospheres are shown in Fig. 6 and important data is tabulated in Table 1. In the pyrolytic atmosphere, the trends of the thermograms are almost similar, with marginal differences in the initial degradation process. An approximately 1% weight loss occurs below 150°C, corresponding to the degradation of the sample by the evaporation of free water. The major degradation process under the effect of constituents (zeolites) present in the bamboo bers, a slight degradation step observed at approximately 350-400°C, more predominant in the case of the VE/BF composites and marginal for the NCS-VE/BF composites. At the end of the maximum degradation process, the char's mass residue at 700°C for the VE/BF and 2, 4, and 6 wt.% NCS-VE/BF composites are 4.7, 6.8, 8.2, and 9.0% corresponding. Therefore, the introduction of NCS can accelerate the decomposition of the VE/BF composites by forming about 47% char residue compared with pure VE/BF that can suppress the release of combustible gases.
In the oxidative atmosphere, the trends of the thermograms are almost similar, and no apparent changes are observed between the VE/BF and NCS-VE/BF composites. However, the VE/BF composites (~ 350°C) start major degradation process slightly earlier than that of the NCS-VE/BF composites (~ 359°C). This behavior of the VE/BF composites progressively continues at the end of the maximum degradation. In addition, the inclusion of NCS shows an impact, even in small quantities, on maintaining a marginal dominance throughout the degradation process. The VE/BF composites completely combust without leaving any char residue at 700°C. However, in the case of a high percentage of NCS loading, the VE/BF composites are not combustible and left behind at approximately 1.5 wt.% stable residues indicating the enhancing thermal stability of the composites even under an oxidative atmosphere.  Table 2. The weak ammability properties of both VE and BFs induce rapid burning of the VE/BF composites with a burn time of 1.53 min. However, the burning time meaningfully increased with the incorporation of NCS into VE/BF composites. Addition of 2 wt.% NCS delays the burn time to 2.32 min, an approximately 34% delay compared to that of the VE/BF composites. This trend gradually increases for 4 and 6 wt.% NCS addition, with delays of 2.35 and 2.54 min, respectively, mainly due to the formation of the char residue, particularly the carbonaceous silicate char, which slow down the propagation of the re and protected the underlying material (Shen et al. 2014). In addition, the inclusion of NCS into the composites effectively improved the modulus of the composites as discussed in Sect. 3.2.6. The high stiffness leads to heat resistance, which is one of the factors in the re triangle, and hence, results in a low burning rate (Atta ur Rehman et al. 2018). Besides, the ammability ratings of NCS-VE/BF composites obtained by vertical burn test and LOI results are tabulated in Table 2. The incorporation of NCS could not make the VE/BF composites reach VB levels; all the composites burnt completely. However, as shown in (supplementary information) SI. Figure 1, the digital images during the vertical burn test exhibited the rate of burning time decreased after incorporation of NCS than VE/BF composites. In addition, the oxygen percentage decently increased with the increasing amount of NCS in VE/BF composites. The O 2 % reasonably increased for 2, 4, and 6% of NCS in VE/BF composites by 28, 31, and 33%, respectively.

Micro and cone calorimetry analysis
The microcalorimetry analysis was carried out to obtain the HRR and THR data for further understand the ammability behavior of the NCS-VE/BF composites. The HRR and THR data of the VE/BF and various NCS-loaded VE/BF composites are shown in Fig. 6(B) and tabulated in Table 1. It can be observed from the gure that the attained data are in agreement with the HBT results. The pHRRs of the VE/BF and 2-6% NCS-VE/BF composites are 422.7, 381.5, 373.6, and 361.9 W/g, respectively. The inclusion of NCS into the VE/BF composites signi cantly suppressed the pHRR by approximately 14%, and the reduction gradually continues with an increasing percentage of NCS. Correspondingly, the THR data follow the pHRR trend, showing values of 25.5, 24.7, 23.3, and 20.7 kj/g for the VE/BF and 2-6% NCS-VE/BF composites. The inclusion of NCS lowers the THR by approximately 18%. In the same way, the re growth index (FIGRA) ( Table 2) indicated the growth pro le respectively reduced by 28, 31, and 33% with the addition of 2, 4, and 6% of NCS to VE/BF composites. Besides, as shown in SI Fig. 2, the CO 2 , CO and SPR of the NCS-VE/BF composites also reduced slightly around 20%. The reduction in the smoke production rate responsible for the condensed phase could be attributed to the control exerted on the VE/BF composites by the inclusion of NCS. (Song et al. 2014). Hence, the UL-94 test, LOI, micro and cone calorimeter results concluded that, the NCS additive can serve as a ame retardancy for natural ber polymer composites by a condensed phase arising from carbonaceous char. The plausible ame retardant mechanism of NCS-VE/FF composite during the combustion process as shown in Scheme 2.

Char analysis
The SEM images of the residual char surfaces of VE/BF, 2%NCS-VE/BF, 4%NCS-VE/BF, and 6%NCS-VE/BF composites after horizontal burn test are shown in Fig. 7. It can be observed that the VE/BF composites showed a thin and loose akes kind of brittle scarred char produced through quick evaporation of the condensed phase. As a result, the intensity of the re propagation increased by allowing the two factors in the re triangle, oxygen and heat. However, the introduction of NCS into VE/BF composites exhibited the dense and continual char formation, which may not allow ammable gases and resist the propagation of the combustion. The gradual improvement of the compact char formation could be observed with higher loadings of NCS additive. In addition, the SEM-EDX analysis presented the major elements present in the char residue of VE/BF and NCS-VE/BF composites. The VE/BF exhibited the major elements C and O refers the carbonaceous char. However, inclusion of NCS into VE/BF composites showed the Si element in the char along with C and O. The % of Si and O increased and decreased the % of C in the char residue in 4 and 6 wt.% of NCS-VE/BF composites which means the crystalline silica was presented in the char residue.
3.2.6. Mechanical performance 3.2.6.1. Tensile Properties An evident increase in the tensile properties of the VE/BF composite was observed as a result of the NCS addition. The tensile test results are presented in Fig. 8 and Table 3 ). However, in this study, the NCS particles enhance the interfacial bonding of the BFs with VE, and hence, increase the tensile strength of the composites. This increase in the tensile strength supports -OH interactions between the NCS particles and VE/BF composites, as revealed in the FTIR analysis. These results also verify the ne dispersion of the NCS particles in the composites, as evident from the SEM images of the composite surfaces. The deposition of the NCS particles on the BF surfaces enhances the interfacial bonding with the VE matrix, which results in a reduced number of ber pull-outs during the tensile fracture, as is evident from the SEM fractographs in Fig. 5. The decrease in the ber pull-out spots in the NCS-lled VE/BF composites indicates enhanced interfacial bonding between the bers and the matrix. The friction at the interfaces between the bers and the matrix in the presence of NCS facilitates the transfer of load from the matrix to the bers, thereby causing a dominant ber breakout failure instead of ber pull-outs.
The higher stiffness of the NCS particles compared to those of BFs and VE also shows its effect in the form of the higher tensile modulus of the NCS-lled VE/BF composites. The results indicate that the tensile modulus of the VE/BF composite increases after the incorporation of the NCS particles. The tensile moduli of the composites are in the order of 6% NCS-VE/BF (2.38 GPa) > 4% NCS-VE/BF (1.76 GPa) > 2% NCS-VE/BF (1.68 GPa) > VE/BF (1.57 GPa). The increase in the tensile modulus is as before attributed to the enhanced interfacial bonding between the bers and the matrix in the presence of NCS particles. In the NCS-lled VE/BF composites, the load is largely transferred to the BFs, which has a higher tensile modulus than that of the VE matrix, causing the tensile moduli of these composites to increase. A similar trend is reported in the literature regarding the tensile moduli of other particulate-lled NF composite materials (Chee et al. 2021;Beigloo et al. 2020).

Flexural Properties
An increase in the exural properties was observed after incorporating NCS particles into the VE/BF composite. The results of the three-point bending test are summarized in Fig. 8 and Table 3. The maximum exural strength is observed after incorporating 4% NCS particles into the VE/BF composite. The exural modulus of the composites also increases after loading the NCS particles in the VE/BF composite. The 4 wt% NCS-incorporated VE/BF composite displays the highest exural modulus, which is 2.33 times that of the basic VE/BF composite. The exural modulus of the composites follow the order 4% NCS-VE/BF (18.01 GPa) > 2% NCS-VE/BF (14.68 GPa) > VE/BF (7.7 GPa) > 6% NCS-VE/BF (2.2 GPa). The increase in the exural modulus of the VE/BF composite after the incorporation of NCS is attributed to the compatibility and enhanced interfacial bonding. The higher exural and tensile moduli also ensure higher thermal stability of the VE/BF composites in the presence of NCS particles.

Impact Properties
The ability of the VE/BF composites to absorb energy before impact failure was observed to increase after the incorporation of NCS particles. The results of the Izod impact test are summarized in Fig. 8 and Table 3. The VE/BF composite with 4 wt% NCS particles absorbs the highest amount of energy in the impact test, which is 53% of that of the VE/BF composite. The composites absorb the impact energy in the following order: 4% NCS-VE/BF (0.0314 J/mm) > 2% NCS-VE/BF (0.0306 J/mm) > 6% NCS-VE/BF (0.026 J/mm) > VE/BF (0.0205 J/mm). The impact properties of the composites decrease after NCS loading exceeds 4 wt%. The 6% NCS-VE/BF composite absorbs lower impact energy than the other NCS-lled VE/BF composites; may be due to the agglomeration of particles, which causes stress concentration, hence, rapid failure. The rigidity of the NCS particles as compared to the VE resin also enhances the impact properties of the VE/BF composites. A similar trend regarding the enhancement in the impact properties of NF-based polymer composites caused by FR particles is reported in the literature (Raghul et al. 2020;Bozkurt et al. 2017).

Fracture morphology
No noticeable changes were observed among the composites in the fracture micrographs, except for the resin's coarse surface responsible for the ller addition and the nonwoven fabric fracture behavior. The interfacial bonding between the bers and the matrix was marginally weak, and few ber pullouts were observed in the VE/BF composites. However, the ller-loaded resin improved the interfacial bonding in the composites and tightly adhered to the bers. Thus, the ber breakage is preponderated over the ber and matrix interfacial failure described in the FTIR and mechanical section.

Conclusions
In this study, chitosan was successfully modi ed to a superior FR and thermally stable form (NCS) by a simple chemical method. Characterization techniques such as SEM, elemental analysis, FTIR, XRD, TGA, and microcalorimetry proved the chemical formation of NCS and its thermal and FR properties. NCS-VE/BF composites showed satisfactory ame retardancy and thermal stability compared to VE/BF composites: delay burn time of approximately 40%, enhanced LOI about 33% and HRR decrease of roughly 14%, and TGA data showed 9 wt.% at 700°C. Interestingly, incorporating the NCS additive signi cantly improved the tensile, exural, and impact properties of the VE/BF composites. Furthermore, SEM analysis demonstrated a homogeneous distribution of the additives, which was the main factor responsible for changing the properties of the composites. In summary, the above results con rmed that NCS could be used as an FR in NFs composites with improved mechanical strength and thermal stability.   Figure 1