The flammability of natural fibers is their main limitation in high-temperature engineering applications, a contemporary issue in the present green engineering sector. Therefore, the current investigation focused on developing fire-resistant natural fibers through surface coating with hybrid biochemicals using the Taguchi approach (L9 orthogonal array). The low-cost and eco-friendly chemicals, chitosan (CTS), sodium bicarbonate (SBC), and ammonium polyphosphate (APP), were utilized by dissolving 3 wt.% and 6 wt.% of each in an aqueous medium and coating the mixture on bamboo nonwoven fibers (BNF). The surface chemistry of the modified BNF was investigated by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and scanning electron microscopy (SEM). The effect of the hybrid coating on the flammability and thermal stability of the BNF was studied. The experimental results indicate that the synergistic effect of the chemicals through the intumescent mechanism effectively improved the flame resistance of BNFs. The results of the vertical burning test of the coated fibers indicated their exceptional self-flame-extinguishment, reaching the V0 level, with excellent thermal stability, and almost 40 wt.% residue at 700°C. Overall, the hybrid coat could enhance the flame-resistant properties of natural fibers, thereby overcoming the limitations of natural fibers and making them suitable for use as sub-structural components in engineering applications as reinforcements.
Research Article
Hybrid Eco-friendly Coating on Bamboo Fabric through Taguchi Formulations for Flame and Thermal Resistance
https://doi.org/10.21203/rs.3.rs-1976669/v1
This work is licensed under a CC BY 4.0 License
Journal Publication
published 16 May, 2023
You are reading this latest preprint version
Natural fibers
taguchi formulations
surface treatments
eco-friendly materials
flame resistance
thermal stability
Natural fibers (NFs) have attracted significant attention as a potential replacement for synthetic fibers in the present engineering sector because of their biocompatibility, biodegradability, renewability, affordability, eco-friendliness, and abundance (Zhang 2020). They are lightweight, with good vibration stability and sound absorption, which has drawn the attention of the transportation and acoustic engineering industry. NFs can significantly suppress greenhouse emissions and carbon footprint of polymer composites when used as reinforcements (Hermann 2011). They help to reduce the utilization of petroleum-based polymers and alleviate demands on the recycling process because they are environmentally degradable, thereby reducing plastic pollution (Zhu 2022). In addition, NFs do not produce poisonous gases during the recycling process and incineration.
Therefore, the utilization of NFs in all engineering fields, even for high-end applications, is essential. However, the disadvantages of NFs must be considered. Extensive studies have investigated the hydrophilic properties of NFs, their interfacial interactions with hydrophobic polymers, water absorption, susceptibility to damage by moths and other insects, durability, tendency to degrade at high temperatures, and flame resistance (Väisänen 2017). The mechanical properties are of particular concern; NFs are not suitable for structural applications where high strength is needed. NFs are best suited for sub-structural components, especially in the construction, automobile, marine, and aerospace industries. However, substructural components are mostly involved in fire initiation and propagation during fire accidents (Shokouhi 2019). Therefore, for the effective utilization of NFs in the engineering sector, their flame resistance has to be enhanced.
Several approaches for implementing flame resistance in NFs have been reported in the literature (Prabhakar 2015). Currently, common flame-retardant treatments are widely used in industry (Sanchez-Olivares 2019). Several flame retardants have been reported, such as halogens (Hahladakis 2018), phosphorus (Prabhakar 2022), nitrogen (Li, 2020) silicon (Branda, 2016), carbon (Araby 2021), metals (Chaitanya 2020), and borates (Dogan 2021). However, during experimentation, these retardants create a toxic environment and result in fire accidents, restricting the use to a few flame-retardants, such as halogenated additives. Eco- and bio-based-flame retardants such as ammonium polyphosphates, magnesium hydroxide, zinc borate, chitosan, oyster shell powder, and vermiculate are also in use. Nevertheless, obtaining a good liquid formulation with the available flame retardants for smooth application to natural fibers is challenging, which makes the treatment process difficult, tedious, and expensive.
Chitosan (CTS) is the second most abundant amino saccharide and non-toxic polymer; it is usually regarded as a biowaste. Its multiple hydroxyl and amino groups induce flame retardancy and serve as carbonizing agents in an intumescent system (Prabhakar 2018). CTS can easily dissolve in a slightly acidic aqueous medium to form a solution. Meanwhile, sodium bicarbonate (SBC), known as baking soda, is an eco-friendly inorganic compound widely used for cooking, cleaning, and deodorizing purposes. SBC shows flame retardancy by absorbing heat during decomposition via an endothermic process and liberating carbon dioxide and water. The ionic nature of SBC decomposition allows it to dissolve in water to produce sodium and bicarbonate ions (Farooq 2018). Ammonium polyphosphate (APP) is another environmentally-friendly inorganic compound that has been extensively used as a phosphate fertilizer. It is the most widely used intumescent flame-retardant additive and works in solid and gas phases. The commercial form of APP (APP-I) has a higher solubility in water; therefore, it can be used in flame-retardant coatings (Liu 2010).
Surface modification with chemical treatments has been developed as an effective approach to improve surface chemistry; in NFs, it can be used to make the surface more hydrophobic and improve the thermal and flame-retardant properties. However, a single flame-retardant treatment may not be effective and desired; therefore, mixing two or three suitable flame-retardants at an optimum ratio, called a hybrid flame-retardant (HFR) treatment, can be considered a viable solution. Several studies have focused on the HFR approach for flax, cotton, silk, etc., using a combination of dopamine-iron phosphonate, nitrogen-modified silane hybrid-phytic acid, sol-gel, etc. (Makym 2022; Bo-Wen Liu 2021; Cheng 2018). However, to the best of the authors’ knowledge, there has been no significant research on the eco-friendly hybrid flame retardant treatment of natural fibers.
Therefore, the present research mainly focused on developing an eco-friendly hybrid flame retardant treatment by utilizing solutions of chitosan, sodium bicarbonate, and ammonium polyphosphate on bamboo non-woven fabrics via Taguchi formulations. SEM, FTIR, TGA-DTG, the UL-94 vertical flame test, and microcalorimetry were used to study the treated fiber surface morphology and thermal and flame retardancy, respectively. The obtained results demonstrated a significant enhancement in the thermal stability and flame retardancy of the bamboo fibers. Hence, it is anticipated that the eco-hybrid flame-retardant treatment could effectively enhance the flame-resistance properties of both bamboo and natural fibers, similar to non-bio-flame-retardant treatments.
2.1. Materials
Specialized bamboo non-woven fabric manufactured using an eco-friendly process of natural fermentation was obtained from Sungchang Industries, Korea. The flame retardant, chitosan (CTS) (MW ~ 600,000, viscosity 50–800 mPa-s, degree of deacetylation 80–95%), was obtained from Kwang Jin Chemical Co. Ltd., Korea. Sodium bicarbonate was obtained from Samchun Chemical Co., Korea, and ammonium polyphosphate (APP) (average degree of polymerization n > 1000) was supplied by Yee Young Cerachem Ltd., Korea.
2.2. Preparation of FR treatment solutions
Basic solutions of the three flame-retardants were prepared using the following dissolution procedure. Different solution concentrations were prepared according to the Taguchi formulations, such as 0 wt. %, 3 wt. %, and 6 wt. % as discussed in the next section. Chitosan (CTS) solution was prepared by dissolving the required amount of chitosan flakes in pre-prepared 2% acetic acid at a stirring speed of approximately 500 rpm. Stirring was continued for 12 h at a constant temperature of 60°C to obtain a homogeneous chitosan solution. Ammonium polyphosphate (APP) solution was prepared by adding the required amount to a three-necked round-bottom flask containing 100 mL of hot (80°C) deionized water equipped with a stirrer (600 rpm), nitrogen gas, and a thermometer placed over a heat bath. During this process, the ammonium polyphosphate particles were completely dissolved in water, resulting in a homogeneous ammonium phosphate solution. A sodium bicarbonate (SBC) solution was prepared by adding the required amount of SBC to deionized water and stirring at 500 rpm for 1 h at room temperature.
2.3. Fabric treatment process
The hybrid treatment of the BNF process is illustrated in Fig. 1. Before the hybrid treatment, the bamboo fabrics (300 mm × 250 mm × 0.26 mm, abbreviated as BF) were thoroughly washed with DI water to remove any impurities and dried for 2 h in a 60°C oven. The dried fabrics were then immersed for 30 min in a CTS solution, and the material was scrunched up and then left to dry completely at room temperature to retain the treated chemical on the fibers. Then, the CTS-treated fabric was dipped in the NBC solution for 30 min and dried in air. A similar procedure was followed with the APP solution to achieve tri-layer deposition on the fabric. The treated BNF were dried for approximately 12 h at 60°C before testing.
2.4. Taguchi design of experiments and hybrid surface treatment
A common statistical technique, Taguchi design of experiments (DOE), was used to design the formulations of the three eco-flame retardants with the help of the commercially available statistical analysis software Minitab 17. The three eco-flame retardants (CTS, SBC, APP) were considered as three factors, and their levels (the concentrations of each factor such as 0 wt.%, 3wt.%, and 6 wt.%) were input into the software. The Taguchi DOE minimized the number of experiments from 27 to 9 based on the L9 orthogonal array, as tabulated in Table 1, and treatments were carried out accordingly on the BF.
Table 1
L9 orthogonal array of Taguchi DOE
2.5. Characterization
The surface morphology of the treated BF was observed using scanning electron microscopy (SEM) (Emcrafts Cube II, Korea) at an acceleration voltage of 20 kV. The samples were sputter-coated with gold using an Ion Sputter Coater G20 (GSEM, Korea) instrument under an argon flow. The chemical interaction study of the treated BF was performed using a Fourier transform infrared spectrometer (Model: FTIR-6300, JASCO International Co., Ltd., Japan) in the wavenumber range 4000–400 cm− 1, and the attenuated total reflectance technique was used with 32 scans for each case at a resolution of 4 cm− 1. The crystal structure of the treated BF was studied using an X-ray diffractometer (Bruker, D8 Discover) with Ni-filtered Cu Kα radiation. The patterns were recorded in the 10–50° range at a rate of 5°/min and a current of 30 mA. The thermal stability of the treated BF was evaluated using a thermogravimetric analyzer (TGA Q600/SDT, TA Instruments, USA) at a heating rate of 10°C/min under both inert (N2) and air atmospheres with temperatures ranging from 30 to 700°C. The burning behavior of the treated BF was tested using the vertical burn test in the FTT0082 UL94 chamber (Model:11313) (Fire Testing Technology, UK), and the burn rate and level were calculated as per UL94 test standards [24]. The flammability behavior of BF was verified using a microcalorimeter (pyrolysis combustion flow calorimeter) (Fire Testing Technology, UK), as per the ASTM D 7309 standard. The furnace temperature was 900°C, the flow rate was 100 cm3/min, and the oxygen concentration was 20% (v/v).
3.1. Surface morphology
The effects of the hybrid treatment on the surface morphology of the fibers were analyzed through SEM coupled with mapping; representative images of pure and coated BNF are shown in Fig. 2 and SI-Fig. 1. Pure bamboo fibers (L1) had a comparatively smooth surface with clean grooves. However, the hybrid-treated fibers (L2–L9) showed a coarse surface texture with a thin layer of material covering the fibers; the surface grooves were filled with the chemical treatments. The hybrid combination of SBC and APP (L2 and L3) resulted in a rough morphology and increased the roughness significantly. In the case of the CTS-SBC hybrid treatment (L6 and L8), the fiber surface exhibited roughness at lower percentages, but a higher percentage of the surface morphology was almost unchanged. The CTS-APP hybrid treatment (L4 and L7) had a lower percentage of the morphology of the fiber, but at higher percentages, the fiber surface became predominantly coarse. However, the hybrid combination of the three flame retardants at both concentrations (L5 and L9) left the fiber morphology almost unchanged, although the grooves were filled with the coating chemicals.
In the EDS mapping of pure BNF, the only elements appearing were C and O, representing lignocellulosic fibers without any inorganic impurities. However, the hybrid surface coating significantly changed the fiber surface composition, showing the elements of the respective coating chemicals. In addition, most elements appeared to be based on the top-layer chemicals. In the case of the combination of SBC and APP coating (L3), P, N, and Na are detected in addition to C and O and are uniformly distributed within the coated chemical. Similarly, the combination of CTS and SBC coating (L6) exhibited a higher percentage of Na in addition to C and O owing to the top layer of SBC. L7 showed C, O, N, P, and Na, corresponding to CTS and APP, and the predominant phosphorus content reflects the top layer being APP. In the hybrid combination (CTS-SBC-APP), the major elements of the chemicals C, O, N, Na, and P were detected. Overall, the mapping images of the coated fibers showed uniform dispersions of the respective major elements, which further demonstrated the complete attachment of CTS, SBC, and APP thin coatings to the bamboo nonwoven fabric.
3.2. Chemical interaction
The chemical interaction of the hybrid treatment with bamboo fibers was analyzed using spectral analysis, and the obtained spectra are shown in Fig. 4(A). The pure bamboo fibers showed spectral peaks at 3442, 2923, 1740, 1429, 1624, 1075, 775, and 690 cm− 1 corresponding to –OH, the C–H carbonyl peak (hemi-cellulose), hemicellulose, C–O–C stretching, cellulose, and glycosidic linkages, respectively (Afrin 2012). Significant changes in the spectral and peak intensities could be observed in all hybrid chemically-treated fibers, and the combination of chemical compounds clearly shows interaction with the fibers as presented in Fig. 3. For the treated fibers, the spectral peaks at 3442 cm− 1, 775 cm− 1, and 690 cm− 1 almost disappeared. The peak intensity at 2923 cm− 1 increased, the hemicellulose peak at 1740 cm− 1 disappeared, and the peak at 1075 cm− 1 shifted to lower wavenumbers. However, the peak at 1429 cm− 1 exhibited no significant change after the hybrid treatment, clearly demonstrating the presence of cellulose. In the case of the APP combinations (L2, L3, L4, L5, L7, L9), the hybrid-treated fibers showed the characteristic peaks of APP, which are the δN-H and γN-H peaks of NH4+ at 3225 and 1430 cm− 1 respectively. Additionally, peaks appear at 1258 and 890 cm− 1, corresponding to γP = O and P-OH, respectively (Prabhakar 2017). Similarly, in the CTS combinations (L4, L5, L6, L7, L8, and L9), the hybrid-treated fibers showed characteristic peaks of chitosan at 3408 and 1635 cm− 1 representing the combination of –OH and N–H groups and N–H (amid-II), respectively (Prabhakar 2018). Overall, the obtained peaks clearly indicate that the hybrid treatment effectively changed the surface chemistry of the bamboo fibers.
3.3. Crystalline structure
The surface chemistry of the hybrid-treated bamboo fibers was further confirmed by X-ray diffraction analysis, and the obtained patterns of the pure and treated bamboo fibers are shown in Fig. 4(B). The pure bamboo fiber exhibited a high-intensity sharp peak at 26.5°, representing the semi-crystalline nature of the fiber, its reflections corresponding to the 002 crystallographic plane of the cellulose-I lattice. There were also broad peaks at 25° and 35°, corresponding to the amorphous parts of the lignocellulosic fiber, which are hemicellulose and lignin [23]. For the treated fibers, the amorphous peaks disappeared, and sharp crystalline peaks appeared, based on the combination of the hybrid treatment. The APP-containing hybrid-treated fibers (L2, L3, L4, and L7) exhibited sharp peaks at 23.7°, 29.1°, 33.8°, and 37.8° owing to the crystalline nature of APP. With the combination of SBC and APP (L2 and L3), the intensity increased significantly due to the crystalline nature of both APP and SBC (Asim 2020). The amorphous nature of CTS suppressed the crystalline peaks of APP (L4 and L7). The hybrid combinations L5 and L9 both showed similar diffraction patterns, exhibiting broad peaks due to the amorphous characteristics. Overall, the diffraction patterns support both the morphological and spectral analysis data, indicating that the hybrid chemicals were successfully loaded onto the surface of the bamboo fibers.
3.4. Thermal properties analysis of treated fiber
3.4.1. Thermogravimetric analysis
TG analysis was performed to estimate the impact of the hybrid chemical treatment on the thermal properties of the bamboo fibers. Figure 5 and Table 2 display the weight loss vs. temperature plots of the pure and surface-treated BNF under inert (N2) and oxidative (O2) atmospheres. The thermograms of the samples under an inert atmosphere, as shown in Fig. 3(A), have two thermal degradation steps at approximately > 100°C and < 209°C, representing the onset and maximum degradation points, respectively. All the fibers followed a similar trend. An initial degradation of approximately 10 wt. % weight loss occurred around 100°C due to the elimination of loosely-bonded water and volatile compounds. Pure BF (L1) showed a different maximum thermal degradation than the treated fibers. It occurred at approximately 300°C owing to the typical degradation behavior of natural fibers containing high amounts of multi-hydroxyl groups and lignin (Prabhakar 2019). At a higher temperature of approximately 700°C, the fiber burned almost completely, leaving 6.3 wt. % of char residue, demonstrating the heat sensitivity of the natural fiber. All the treated fibers exhibited a similar trend in the degradation process. They showed onset degradation at 100°C and major degradation (Tmax) at 210°C by the elimination (~ 10 wt. %) of volatile compounds (moisture) and the decomposition of the major constituents present in the natural fibers as well as the respective hybrid treatment chemicals. At Tmax, the natural fibers decompose gradually by the thermal degradation of hemicellulose (200–230°C), followed by lignin (230–340°C) and cellulose (350°C), which can be observed in the pure BF thermogram. However, APP-containing hybrid treatments significantly suppressed Tmax, lowering it by ~ 100°C and started degradation at lower temperatures because the chemical combination of APP in the hybrid treatment released phosphoric acid that could support thermal degradation along with heat, thereby increasing the impact of the degradation double. Similarly, the SBC-containing hybrid treatments also initiated earlier degradation because of the effective removal of loosely bonded natural fiber constituents, especially hemicellulose. Notably, the treated fibers L3, L4, L5, L7, and L9 showed approximately 40% residual char, and L2, L6, and L8 fibers showed around 28% char at 700°C, representing excellent thermal stability. This positive phenomenon was seen for APP because the released phosphoric acid reacts with the possible hydroxyl groups of the bamboo fibers and forms esters, thereby resulting in the formation of a thermally insulating char and an increase in the % of APP. CTS can act as a synergistic supporting agent for producing a dense carbonaceous char (Prabhakar 2018); it effectively supports the intumescent flame retardancy of APP. Overall, the three chosen chemical combinations worked synergistically to improve the thermal stability of bamboo fibers.
The oxidative atmospheres and thermograms of the pure and treated fibers show a two-step thermal degradation process, as shown in Fig. 5(B), at approximately > 100°C and < 206°C, representing the onset and maximum degradation points, respectively. The trend of all thermograms was similar to that of the inert atmosphere at the onset and maximum temperatures. However, at higher temperatures, the char residue of pure BF was oxidized with oxygen from the air atmosphere, resulting in the formation of CO and CO2 and reaching zero residue at 650°C by gradual weight loss. Notably, even under an air atmosphere, the treated fibers contained a higher proportion of char residue at 700°C, as observed in the figure. The L3 and L7 combination treatments produced the highest amounts of char at 31.3 and 29.9 wt. % respectively, reflecting their excellent thermal stabilities, matching the results of the nitrogen atmosphere thermograms. The higher percentage of APP combined with SBC and CTS worked effectively via an intumescent mechanism, producing a dense char. This finding is consistent with our previous research findings on APP and APP-CTS combinations on flax fiber (Prabhakar 2019; Prabhakar 2020) [26, 27]. L4, L5, L6, and L8 showed the next highest residue percentages of 22.6, 23.8, 28.5%, and 28.4%, respectively, due to the action of APP and the carbon agent CTS. The L2 sample showed the lowest percentage of char residue, which might be due to the predominant APP content and lower carbon content; the result is similar to that seen in the inert atmosphere, giving a lower char residue than other combinations. The L5 samples show better thermal stability than the L9 samples, which may also be the reason for the scarcity of the carbonization agent and APP supremacy; thus, a higher amount of APP in the combination releases more acid, which strongly affects the organic compounds, thereby affecting the esterification process and eventually influencing the thermal stability (Prabhakar 2020). Overall, the combination of SBC, APP, and CTS worked effectively to improve the thermal stability of the bamboo fibers, and the combination of 6% APP with 6% SBC and 6% CTS showed the best thermal stability.
Table 2
Thermogravimetric analysis results of pure and treated BNF
3.4.2. Differential scanning calorimetry
DSC was performed to understand the chemical alterations between the treatment chemicals and fiber constituents when exposed to heat and to further support the TGA results. The DSC curves of the pure and treated bamboo fibers are shown in Fig. 6 (SI-Fig. 2), and the key numerical data are presented in Table 2 (SI-Table 1). There were two broad endothermic peaks and two exothermic peaks observed at approximately 72–98°C and 224–261°C, and 160–200°C and 280–390°C, respectively. The initial endothermic peaks correspond to the heat of vaporization of the water absorbed by the fibers. The treated fabrics showed peaks at a lower temperature range (72–78°C) and with broader peaks than pure BF (98°C) because the treatment increased the length–diameter ratio of the fiber during surface treatment, which facilitated the evaporation of surface moisture/trapped water from the fibers, as well as volatile materials present in the coated chemicals, and the dehydration of the saccharide rings (Hung 2021). The pure fibers, and samples L6 and L8, did not show any endo- or exothermic peaks in the region of 150–240°C, suggesting that the fibers are stable between these temperatures. The second endothermic peak occurred at approximately 220–260°C, and APP evolved ammonia and water and formed crosslinked polyphosphoric acid. The exothermic peak at approximately 250°C for the pure fibers corresponds mainly to hemicellulose degradation. In the treated fibers, the weakly bonded constituents of CTS, APP, and SBC were also degraded in addition to the hemicellulose. The second exothermic peak appeared at approximately 280–392°C, and the pure fiber showed a peak at 327°C, corresponding to the degradation of lignin. Chitosan degraded at ~ 300°C, related to the decomposition of the amine units (GlcN). Similarly, APP showed a peak at 350°C, at which temperature the polyphosphoric acid may evaporate and dehydrate into P4O10.
3.5. Burning behavior
The UL-94 vertical burning test is an elementary tool used to evaluate the flame retardancy of materials. The vertical burning test results of pure and surface-treated BNF are presented in Table 3. The digital pictures captured during the test are provided in the supplementary section SI-Fig. 3. The exposure of pure BNF to flame caused the fabric to immediately catch fire; the flame extended vigorously upward for 6 s, burning up the BNF completely within 18 s. Similarly, the L9 fabric quickly caught fire, the fire propagated rapidly for 10 s, and the fabric scorched completely within 16 s. In the case of L6 and L8, the material also burned completely but took longer, with durations of approximately 418 and 387 s, respectively. In contrast, the flames on the L2, L3, L4, L5, and L7 fabrics self-extinguished once the flame source was removed, extinguishing at 9, 8, 9, 9, and 12 s, respectively, after ignition and self-extinguished after the flame source was removed. The self-extinguishing behavior of the chemically treated fabrics followed the order: L3(8s) > L2(9s) = L4(9s) = L5(9s) > L7(12s). The black char on these fabrics extended only 4.5 cm, and the 1 cm reddish patch seen on the fabrics shows that the fire could damage only a small portion of fabric; the rest of the fabric remained undamaged. Therefore, the combination CST-APP-SBC treatment contributed to the flame resistance of the bamboo fabric by forming a combination of phosphoric and carbonaceous shielding char layer.
Table 3
Vertical burn test and microcalorimeter test results of the pure and treated BNF.
|
Vertical burn test |
Sample |
L1 |
L2 |
L3 |
L4 |
L5 |
L6 |
L7 |
L8 |
L9 |
|
Burning time [sec] |
18 |
9 |
8 |
9 |
9 |
418 |
12 |
387 |
16 |
|
|
Burnt length (mm) |
300 |
56 |
50 |
74 |
58 |
300 |
72 |
300 |
75 |
|
|
UL-94 Rank |
- |
V0 |
V0 |
V0 |
V0 |
- |
V0 |
- |
V0 |
|
|
Micro calorimeter |
Time [sec] |
320.5 |
270.0 |
253.0 |
260.5 |
234.4 |
260.5 |
248.5 |
271.0 |
292.5 |
|
Temperature [°C] |
354.9 |
253.1 |
244.6 |
253.0 |
244.1 |
270.9 |
243.6 |
291.9 |
282.4 |
|
|
Peak HRR [W/g] |
307.28 |
41.64 |
22.97 |
31.82 |
17.79 |
37.71 |
19.23 |
48.05 |
22.68 |
3.6. Flammability behavior
The flammability of the treated BNF was further examined using a microcalorimeter test to support and strengthen the outcomes obtained from the burning test results. Plots of the heat release rate (HRR) peaks of pure and treated BNF are shown in Fig. 7. It can be clearly seen that pure BNF burns quickly after ignition, and a sharp HRR peak appears (at 320 s) with a peak heat release rate (pHRR) as high as 307 kJ/g. Treatment with BNF greatly suppressed the pHRR by retarding combustion. The HRR data of L2, L3, L4, L5, L6, L7, L8, and L9 are 41, 22, 31, 17, 37, 19, 48, and 22, respectively. It can be observed the pHRR values decrease by at least 84.3%, At 3.7 kJ/g, the pHRR of treated BNF is 94.2% lower than that of pure BNF. However, as shown in SI-Fig. 4 the combustion of the treated BNF started at lower temperatures, at 8% minimum, 26% maximum, and 20.4% minimum, 31.2% maximum, respectively. The temperature drop also agrees with previous results reported by Prabhakar (2022), where APP-treated flax fabrics with 5 wt. % APP exhibited a 296.3°C reduction in the temperature at which pHRR occurred (from 376.7°C for pure flax fibers), with a 49% reduction in pHRR. In the present study, the temperature reduction was slightly greater, approximately 28%; however, the hybrid treatment enhanced the flame retardancy by suppressing pHRR by at least 84%. These findings provide evidence that the combination of CNS-APP-SBC is effective in improving the flame retardancy of BNF and further supporting the flame retardancy mechanism.
3.7. Char morphology
The char residue of the treated BNF obtained from vertical flame testing was examined using scanning electron microscopy coupled with EDS. Representative images are shown in Fig. 8. The SEM images in the figure show that discontinuous fine char flakes with a completely collapsed fiber structure could be observed for pure BNF, indicating the heat sensitivity of the lignocellulosic fibers and supported by EDS showing only the elements C and O. However, the burned areas of the hybrid-coated bamboo fabrics maintained the non-woven fabric structure and integrity. For the L3 fibers, a foamy surface formed due to the phosphoric acid generated during the combustion of APP that effectively reacted with carbonaceous compounds such as bamboo fibers and sodium bicarbonate, and the respective elements P (phosphorus), Na (sodium), and C and O, were detected by EDS. Similarly, for L7, the APP coating on the top layers clearly shows a foam structure on the fibers. EDS shows the corresponding elements P, Na, C, and O. In the case of L6, the char appears to have a flake structure formed by the Na coating on the fibers by obtaining a higher flame retardancy. EDS also supports this by showing the element Na, which is present at a higher percentage than L3 ( in which SBC is found in the lower layer) owing to the upper layer of SBC. In the case of the hybrid coating L9, the fiber structure was not disrupted, and a smooth surface morphology with a coating of char ash could be observed on the fibers; the EDS supported this finding, showing the elements P, Na, C, and O that indicated the presence of flame-resistant elements even after combustion. In addition, the P contents in the L3, L7, and L9 char layers were considerably higher than that of the coating before the combustion test. Hence, it can be concluded that the hybrid coating achieved effective flame retardancy through an intumescent mechanism via both condensed and gas phases.
3.8. Signal/Noise (S/N) ratio analysis
The S/N ratio analysis is a statistical method based on the Taguchi technique. This is an essential tool for understanding the impact of the individual chemicals used in the hybrid coating on the flammability behavior of bamboo nonwoven fibers. The experimental values of the vertical burning test, the microcalorimeter HRR values, and their corresponding S/N ratios are tabulated in SI-Table 2. The plot of the mean S/N ratios of factors vs. levels is shown in SI Fig. 5. By utilizing the obtained results, the influence of each factor on flammability can be estimated separately. Because of the S/N ratio, an option with a lower burning time data has better flame retardancy. The S/N ratio analysis results are as follows. The dotted circle in the figure shows the mean value of the S/N ratio, and the individual values of each factor at different levels are plotted. The S/N ratio values for each factor and level were compared to the mean values. As a result of the analysis using Minitab, it was possible to determine that a formulation containing 3% chitosan, 0% sodium bicarbonate, and 6% APP was optimal, having the highest impact on burning time. Table 3 shows the S/N ratio response for the burning time. The optimum value of each factor level was that with the highest S/N ratio. The rank of each factor shows its importance in terms of its influence on properties. Compared to chitosan and sodium bicarbonate, the difference in the S/N ratio graph of APP is significant, indicating that the APP flame-retardant treatment has a large effect on the results. Therefore, APP (rank 1) had the most significant influence on the burning time.
3.9. Contour plots for burning time and pHRR
Contour plots examined the relationship between the response variable and the two control variables by viewing the discrete contours of the predicted response variables. Fig. SI Fig. 5 shows contour plots explaining the relationship between the process parameters concerning the burning time and peak-HRR, respectively. From SI-Fig. 5a, a high level of SBC and a medium level of CST improved the burning time. SI-Fig. 5b shows that a medium level of APP and a high level of CST led to a high burning time. Finally, SI-Fig. 5c shows that a medium level of APP and a high level of SBC led to the generation of a high burning time. Similarly, SI-Fig. 5(d–f) shows the combined relationship between the process parameters and peak HRR of the composites. Peak HRR was obtained at medium to high CST, SBC, and APP levels. To attain a low peak-HRR, the selected process parameters in the range 3–6% are feasible. The scientific reasons for the aforementioned observations are explained in correlation with the experimental results in the previous sections.
3.10. Flame retardant mechanism
The possible flame-retardant mechanism is shown in Fig. 9, based on the experimental results and with the support of published research. As mentioned earlier, four combinations focused on this mechanism, which was explored using the subsequent batches.
Batch 1 (SBC-APP): The burning process of the coated fabric begins with the release of ammonia, water, phosphoric acid, and carbon dioxide (Huang 2021). In later stages, the released acid reacts with cellulose and carbonate compounds via an esterification reaction through dehydration, resulting in the formation of a densely crosslinked char with a P-O-P or P-O-C structure, as shown in FTIR (SI Fig. 3) with P, Na, O, and C (Deng 2014).
Batch 2 (CTS-APP): During the initial burning stages, the coated fibers release ammonia, water, and phosphoric acid, and a synergistic effect can be seen between APP, CTS, and cellulose fibers that results in a phosphorus-based carbonaceous char with a P-O-P or P-O-C structure, as shown in FTIR (SI Fig. 3) with P, O, and C (Deng 2014).
Batch 3 (CTS-SBC): The elimination of water and carbon dioxide occurs in the initial combustion stages. Later, ammonia is released, and a crosslinked hexatomic ring structure is formed by the process of self-condensation with the decomposed polyhydric alcohols, thereby creating a carbonaceous char with a C-O-C structure, as shown in FTIR (SI Fig. 3) with only C and O detected (char EDS) (Maddalena 2018).
Batch 4 (CTS-SBC-APP): The hybrid-coated fiber combustion starts with the removal of water, ammonia, nitrogen, and carbon dioxide. In later stages, the dense char formed consisted of C-O-C, P-O-P, and P-C-P, as shown in FTIR (SI Fig. 3) with the elements P, Na, O, and C detected (char EDS).
Overall, the hybrid chemical coating worked synergistically on the bamboo fabric to achieve superior flame retardancy, irrespective of the chemical combination. However, the APP combination was the most effective and followed an intumescent flame retardant mechanism. The sodium bicarbonate and chitosan combination contributed to flame retardancy by releasing high amounts of moisture and N2 at earlier combustion stages. The formation of char consisted of metal and dense carbon.
A superior flame-retardant bamboo fabric was fabricated by an eco-friendly hybrid coating consisting of CTS, NBC, and APP with Taguchi formulations via a dipping system. The morphological, spectral, and diffraction analyses explained the surface chemistry of the coated fabrics resulting from the uniform surface coating with respective elements, chemical bonding interactions, and crystallinity, respectively. Most of the formulations (L2 to L9) coated onto the bamboo fabrics were able to self-extinguish with 50 mm burnt length from the vertical combustion results, while the pHRR value was suppressed by up to 94.2%. Moreover, the high char residue with 40% remaining from the TGA results at 700°C was supported by DSC data, demonstrating the excellent thermal stability of the fabrics. The flame-retardant mechanism of the bamboo fabric occurred through the intumescence of the condensed phase. The dense char further supported the char morphology with a solid structure containing phosphors and sodium elements. Owing to its promising flame retardancy and thermal stability, the fabricated bamboo fabric can be used as a potential reinforcement for flame-retardant composites. The hybrid coating system provides an economical and facile approach for enhancing flame retardancy in natural fibers.
Ethics approval and consent to participate
Not applicable
Availability of data and materials
The authors declare that there are no available data and materials
Competing interest
The authors declare that they have no competing interest
Funding
National Research Foundation of Republic of Korea
Authors’ contribution
M.N. Prabhakar: Conceptualization, Writing – original draft, Writing – review & editing. Yu Ruiwen: Methodology, Validation. Dong Woo Lee: Data curation. Song Jung-IL: Supervision.
Acknowledgment
This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Science Education (2018R1A6A1A03024509 and 2021R1A2B5B03002355).
- Afrin T, Tsuzuki T, Wang X (2012) UV absorption property of bamboo. J Text Inst 103(4):394-399. https://doi.org/10.1080/00405000.2011.580543
- Araby S, Philips B, Meng Q, Ma J, Laoui T, Wang CH (2021) Recent advances in carbon-based nanomaterials for flame retardant polymers and composites. Compos Part B: Eng 212:108675. https://doi.org/10.1016/j.compositesb.2021.108675
- Asim M, Paridah MT, Chandrasekar M, Shahroze MR, Jawaid M, Nasir M, Siakeng R (2020) Thermal stability of natural fibers and their polymer composites. Iran Polym J 29:625–648. https://doi.org/10.1007/s13726-020-00824-6
- Benyahia A, Merrouche A, El Z, Rahmouni A, Rokbi M, Serge W, Kouadri Z (2014) Study of the alkali treatment effect on the mechanical behavior of the composite unsaturated polyester-Alfa fibers. Mech Ind 15:69–73. https://doi.org/10.1051/meca/2013082
- Bo-Wen Liu, Hai-Bo Zhao, Yu-Zhong Wang (2021) Advanced flame retardant methods for polymer materials. Adv Mater 2107905. https://doi.org/10.1002/adma.202107905.
- Branda F, Malucelli G, Durante M, Piccolo A, Mazzei P, Costantini A, Silvestri B, Pennetta M, Bifulco A (2016) Silica Treatments: A Fire Retardant Strategy for Hemp Fabric/Epoxy Composites. Polymers 22:8(8) 313. https://doi.org/10.3390/polym8080313
- Chaitanya S, Ravi Kumar C, Jung Il S (2020) Microwave-synthesized Mg+2 doped jute fibers and their application as a reinforcement in biocomposites. Compos Part B: Eng 197 108154. https://doi.org/10.1016/j.compositesb.2020.108154
- Cheng XW, Liang CX, Guan JP, Yang XH, Tang RC (2018) Flame retardant and hydrophobic properties of novel sol-gel derived phytic acid/silica hybrid organic-inorganic coatings for silk fabric. Appl Surf Sci 427(A) 69-80. https://doi.org/10.1016/j.apsusc.2017.08.021
- Deng CL, Du SL, Zhao J, Shen QZ, Deng C, Wang YZ (2014) An intumescent flame retardant polypropylene system with simultaneously improved flame retardancy and water resistance. Polym Degrad Stab 108:97-107. https://doi.org/10.1016/j.polymdegradstab.2014.06.008
- Dogan M, Dilem Dogan S, Atabek Savas L, Ozcelik G, Tayfun U (2021) Flame retardant effect of boron compounds in polymeric materials, Compos Part B: Eng. 222:109088. https://doi.org/10.1016/j.compositesb.2021.109088
- Farooq M, Sipponen M.H, Seppälä A, Österberg M (2018) Eco friendly flame retardant cellulose nano fibril aerogels by incorporating sodium bicarbonate. ACS Appl Mater Interfaces 10(32):27407-27415. doi: 10.1021/acsami.8b04376
- Hahladakis JN, Velis CA, Weber R, Iacovidou E, Purnell P (2018) An overview of chemical additives present in plastics: Migration, release, fate and environmental impact during their use, disposal and recycling. J Hazard Mater 344:179-199. https://doi.org/10.1016/j.jhazmat.2017.10.014
- Hermann BG, Debeer L, De Wilde B, Blok K, Patel MK (2011) To Compost or Not to Compost: Carbon and Energy Footprints of Biodegradable Materials’ Waste Treatment. Polym Degrad Stab 96(6):1159–1171. https://doi.org/10.1016/j.polymdegradstab.2010.12.026
- Huang Z, Runm Jin Wu B, Ma N, Chang Tsai TJF (2021) High efficiency ammonium polyphosphate intumescent encapsulated polypropylene flame retardant. J Appl Polym Sci 138:50413. https://doi.org/10.1002/app.50413
- Li N, Ming J, Yuan R, Fan S, Liu L, Li F, Wang X, Yu J, Wu D (2020) Novel eco-friendly flame retardants based on nitrogen-silicone Schiff base and application in cellulose. ACS Sustain Chem Eng 8(1) 290-301. https://doi.org/10.1021/acssuschemeng.9b05338
- Liu G.S, Chen WY, Yu JG, (2010) A Novel Process to Prepare Ammonium Polyphosphate with Crystalline Form II and its Comparison with Melamine Polyphosphate. Ind Eng Chem Res 49:12148– 12155. https://doi.org/10.1021/ie1014102
- Maddalena L, Carosio F, Gomez J, Saracco G, Fina (2018) A Layer-by-layer assembly of efficient flame retardant coatings based on high aspect ratio graphene oxide and chitosan capable of preventing ignition of PU foam. Polym Degrad Stab 152:1-9. https://doi.org/10.1016/j.polymdegradstab.2018.03.013
- Maksym L, Prabhakar MN, Jung-il S (2022) Preparation of a novel bio-flame-retardant liquid for flame retardancy of natural fibers and their composites, J Ind Text doi:10.1177/15280837221079273.
- Prabhakar MN, Atta Ur Rehaman Shah, Jung-Il Song (2015) A Review on the Flammability and Flame Retardant Properties of Natural Fibers and Polymer Matrix Based Composites. Compos Res 28(2):29–39. https://doi.org/10.7234/composres.2015.28.2.029
- Prabhakar MN, Naga Kumar C, Lee Dong Woo, Song Jung-IL (2022) Hybrid approach to improve the flame-retardant and thermal properties of sustainable biocomposites used in outdoor engineering applications. Compos Part A Appl Sci Manuf 152:106674. https://doi.org/10.1016/j.compositesa.2021.106674
- Prabhakar MN, Jung-il S (2018) Fabrication and characterisation of starch/chitosan/flax fabric green flame-retardant composites. Int J Biol Macromol 119:1335-1343. https://doi.org/10.1016/j.ijbiomac.2018.07.006
- Prabhakar MN, Atta ur Rehman Shah, Jung-il S (2017) Improved flame-retardant and tensile properties of thermoplastic starch/flax fabric green composites. Carbohydr Polym 168:201-211. https://doi.org/10.1016/j.carbpol.2017.03.036
- Prabhakar MN, Raghavendra GM, Vijaykumar BVD, Kalpesh Patil, Jongchul Seo, Jung-il S (2019) Synthesis of a novel compound based on chitosan and ammonium polyphosphate for flame retardancy applications. Cellulose 26:8801–8812. https://doi.org/10.1007/s10570-019-02671-y
- Prabhakar M.N, Jung-il S (2020) Influence of chitosan-centered additives on flammable properties of vinyl ester matrix composites. Cellulose 27:8087–8103. https://doi.org/10.1007/s10570-020-03313-4
- Sanchez-Olivares G, Rabe S, Pérez-Chávez R, Calderas F, Schartel B (2019) Industrial-waste agave fibres in flame-retarded thermoplastic starch biocomposites. Compos Part B: Eng. 177:107370. https://doi.org/10.1016/j.compositesb.2019.107370
- Shokouhi M, Nasiriani K, Khankeh H, Fallahzadeh H, Khorasani-Zavareh D (2019) Exploring barriers and challenges in protecting residential fire-related injuries: a qualitative study. J Inj Violence Res 11(1):81-92. doi.10.5249/jivr.v11i1.1059.
- Väisänen T, Das O, Tomppo L (2017) A review on new bio-based constituents for natural fiber-polymer composites. J Clean Prod 149:582-596. https://doi.org/10.1016/j.jclepro.2017.02.132
- Wang Y, Cheng Y, Yu M, Li Y, Cao J, Zheng L, Yi H (2017) Methane explosion suppression characteristics based on the NaHCO3/red-mud composite powders with core-shell structure. J Hazard Mater 335:84-91. https://doi.org/10.1016/j.jhazmat.2017.04.031
- Xue Y, Hu W, Hu X (2020) 7 - Thermal analysis of natural fibers, Editor(s): Michael Jaffe, Joseph D. Menczel, In The Textile Institute Book Series, Thermal Analysis of Textiles and Fibers, Woodhead Publishing, Sawston, pp. 105-132.
- Zhang Z, Cai S, Li Y, Wang Z, Long Y, Yu T, Shen Y (2020) High performances of plant fiber reinforced composites—A new insight from hierarchical microstructures, Compos Sci Technol 194:108151. https://doi.org/10.1016/j.compscitech.2020.108151
- Zhu Z, Liu W, Ye S, Batista L (2022) Packaging design for the circular economy: A systematic review. Sustain Prod Consum 32 817-832. https://doi.org/10.1016/j.spc.2022.06.005
No competing interests reported.