The flammability of cotton is a significant concern for technical applications, prompting ongoing research into solutions to mitigate this risk. Traditional flame-retardant methods utilizing acid-based approaches are complex and can negatively affect the mechanical properties of textiles. To address these challenges, this study focuses on developing a liquid bio-based flame retardant (LBF) utilizing a lignin-silica-based liquid (LSL) extracted from rice husk (RH) and 9, 10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO). A one-pot dip-coating technique is utilized to treat cotton with the LSL, and the flammability and mechanical properties of the treated cotton are subsequently tested. Fourier-transform infrared spectroscopy (FTIR) confirms the covalent bond formation of the LSL with DOPO and the hydrogen bond formation of the LBF with cotton. Scanning electron microscopy (SEM) confirms the uniformity of the coating. The experimental results demonstrate that the treated cotton exhibits self-extinguishing behavior during a vertical burning test (VBT), with a 78% reduction in peak heat release and a 65% reduction in total heat released during pyrolysis combustion flow calorimetry. Surprisingly, the treatment also improves the tensile behavior of the cotton by 21.7% and thermal stability by producing a protective char layer that accounted for 36.9% of the final residue. This study provides a promising approach for improving the flame resistance and mechanical strength of cotton for technical applications.
Research Article
Effect of Synthesized Lignin-Based Flame Retardant Liquid on the Flame Retardancy and Mechanical Properties of Cotton Textiles
https://doi.org/10.21203/rs.3.rs-3335885/v1
This work is licensed under a CC BY 4.0 License
Journal Publication
published 01 Jun, 2024
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Rice husk
lignin
cotton
coating
mechanical behavior
flame retardancy
sustainability
Cotton fabric is one of the most popular natural textiles used in a variety of ways in daily life and is a major element in the global textile market. The discovery of cotton remnants in Nubia and Mohenjo-Daro from approximately 2700 BCE supports the theory that cotton was cultivated at locations that already had successful agricultural, spinning, and weaving skills (Lee and Fang 2015). In addition to being commonly used in apparel and indoor applications, cotton can also be used to produce high-performance products for energy harvesting, sensing, electromagnetic interference shielding (Wang and Memon 2020), wound healing (Liu et al. 2020), and natural fiber-reinforced polymer composites (Radoor et al. 2022). Despite its various uses, the high flammability of cotton poses a risk to goods and human life. Approximately 20% of global fire incidents are induced by the burning of textiles. Hundreds of millions of dollars are lost and numerous individuals are killed or injured each year. Therefore, it is desirable to endow textile materials with flame-retardant characteristics to address these difficulties (Jiang et al. 2019).
Ongoing attempts are being made to enhance the flame retardancy of cotton, which has led the market and accounted for more than a 39% share of the global textile revenue in 2021. Therefore, various technologies have been utilized to decrease the flammability of cotton, such as dip-coating (Li et al. 2019), layer-by-layer coating (Shi et al. 2022), graft polymerization (Duan et al. 2022), plasma treatment (Lazar et al. 2020), chemical vapor deposition (Hilt et al. 2014), UV-curable coatings (Zhang et al. 2021b), radiation grafting (Verma and Kaur 2012), and microwave treatment (Yu et al. 2021). Among these techniques, the one-pot dip-coating fiber treatment approach is simple, affordable, controlled, and more effective for uniformly applying flame retardants to fabrics (Maksym et al. 2022).
As the world grapples with environmental concerns and the need for sustainable materials, biomass has emerged as a promising alternative owing to its abundant availability and eco-friendly nature. Biomass is derived from organic matter and has gained significant attention in recent years owing to its versatile applications in various industries. In particular, biomass has achieved remarkable developments in the field of flame retardancy, offering novel solutions for fire safety in diverse sectors (Costes et al. 2017). Sustainable flame-retardant compounds, such as proteins, deoxyribonucleic acid (Shah et al. 2017), chitosan (Prabhakar et al. 2019, 2022), urea (Luo et al. 2022), starch (Lu et al. 2022), alginate (Pan et al. 2016), phytic acid (Li et al. 2022), tannic acid (Wang et al. 2022), vanillin (Sharma and Ali 2022), eggshells (Ahmed et al. 2021), and clay (Prabhakar et al. 2015) are being utilized owing to their thermal stability and flame-resistant properties.
Among the vast array of biomass sources, rice husk (RH) is a valuable resource owing to its abundant availability and unique composition. RH makes up roughly 20% of the weight of the whole grain and is a common byproduct of the rice milling industry. RH is a major agricultural waste product generated in rice processing countries. However, RH represents a significant threat to the environment owing to the emission of carbon dioxide in places where it is dumped, as well as the lingering smoke and methane produced by open-air burning. RH can be used in commercial businesses or science as a substitute solution for rational waste management because it is readily available and inexpensive (Satbaev et al. 2021). RH consists of approximately 22% lignin and 20% ash, the majority of which is silica (Salanti et al. 2010). Studies have shown that RH is thermally stable and flame-resistant when employed as a filler in polymeric materials. This is because the silica present in the RH enhances the flame-retardant properties through the formation of thermal shielding and diffusion barriers during burning (Zhao et al. 2009; Kavitha et al. 2021).
Lignin is a component of lignocellulosic biomass that acts as a natural adhesive protecting cellulose from water and enzymes. Consequently, plants are more resistant to pathogens and biomass degradation. Lignin is a byproduct of paper and ethanol manufacturing processes and is the second most abundant biomass after cellulose. However, less than 2% of produced lignin is used in industry for purposes such as additives, dispersants, binders, and surfactants (Bajwa et al. 2019). Lignin is used to synthesize bio-based flame retardants for various materials, including cotton, owing to its highly phenolic structure and char-forming capability upon heating (Mandlekar et al. 2018). Shukla et al. treated cotton textiles with different concentrations of sodium lignin sulfonate (SLS). They found that cotton with 30% SLS was endowed with good flame retardancy, thermal stability, and UV protection properties (Shukla et al. 2019). Recently, Petkovska et al. coated cotton with environmentally friendly magnesium lignosulfonate, chitosan, and monoammonium phosphate using a layer-by-layer technique. The proposed combination showed high efficiency in reducing the flammability of cotton, along with enhanced UV blocking and antimicrobial properties (Petkovska et al. 2022).
The process of endowing cotton fabrics with flame retardancy often results in the challenge of maintaining the original mechanical properties. The application of flame retardants can lead to a more than 40% decrement in tensile strength (Xu et al. 2019b). Furthermore, some studies do not contain any information regarding mechanical properties. A decrease in the tensile strength can affect the durability and performance of cotton, which may make it unsuitable for certain applications. Therefore, finding ways to mitigate this limitation and improve the mechanical properties of flame-retardant-treated cotton fabric is an ongoing challenge.
Considering the above issues, this study extracted lignin and a silica-rich liquid (LSL) from waste RH and combined them with 9, 10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO). The obtained liquid bio-based flame retardant (LBF) was applied to a cotton surface using a facile one-pot dip-coating technique to enhance flame retardancy and thermal stability. The deposition of the LBF and cotton morphology were analyzed using Fourier-transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) coupled with an energy-dispersive X-ray spectrometer (EDS). Thermal properties were investigated using thermogravimetric analysis (TGA), and flame retardancy was investigated using a vertical burning test (VBT), cone calorimetry, and pyrolysis combustion flow calorimetry (PCFC). A flame-retardant mechanism was proposed based on the results of the char FTIR, SEM coupled with EDS (SEM-EDS), and TGA coupled with FTIR (TG-FTIR). In addition, a comprehensive study of the mechanical properties was conducted. To the best of our knowledge, lignin- and silica-rich substrates extracted from RH were used for the first time to strengthen the flame retardancy of cotton.
2.1. Materials
Rice husk was harvested from the suburbs of Changwon in South Korea. NaOH (98%) was purchased from Samchun Pure Chemical Co., Ltd., Korea. 2/1 twill-weaved cotton with an areal density of 180 g/m2 was purchased from Doufabric Co., Ltd., South Korea. Lignin and triethylamine (≥ 99.5%) were provided by Sigma-Aldrich, USA. 9, 10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (> 97%) was provided by Tokyo Chemical Industry Co. Ltd., Japan.
2.2. Extraction of LSL
After washing and drying, 50 g. of rice husk was placed into 350 ml of 8% NaOH aquatic solution and mixed with a magnetic stirrer for 1 hour under 800 rpm at 30°C. After that, the substrate was autoclaved (Portable Autoclave YX-24LM, EORAL, China) at 126°C for 4 h. Resulting dark mixture was filtrated through 100 µm sieved to remove solid materials.
2.3. Preparation of Lignin-Based Flame Retardant
To determine the optimum ratio of DOPO and LSL, 5, 10, and 15 g of DOPO were added into 100 ml of lignin-rich liquor and mixed with a magnetic stirrer under 60°C for 8 h. Consequently, 0.4 ml of triethylamine was added dropwise, and stirring continued for 2 more hours. The LBFs were named LD-5, LD-10, and LD-15 for 5, 10, and 15 g of DOPO respectively.
2.4. Fabrication of Flame-Retardant Cotton Fabric
To eliminate contaminants cotton fabrics were soaked in 2% NaOH for an hour, extensively washed to pH 7, and dried. Afterward, cotton was dipped into FR solutions for 30 min, squeezed till 200% wet weight gain, and dried at 20°C followed by drying at 40°C drying oven till stable mass. Cotton treated with LSL, LD-5, LD-10, and LD-15 were named CLSL, CLD-5, CLD-10, CLD-15 respectively. The fabrication method of flame-retardant cotton is shown in Fig. 1(a)
2.5. Characterizations and Testing
2.5.1. FTIR
Infrared spectra were recorded using an FTIR spectrometer from Jasco International Co. Ltd., Japan. The spectra were collected over a wavenumber range of 400 to 4000 cm− 1, with a resolution of 4 cm− 1. Each spectrum was collected from 32 scans.
2.5.2. SEM-EDS
The surface morphology of the untreated and treated cotton as well as the char remnants from the VBT was examined using scanning electron microscopy (TESCAN, LYRA3XM, Czech Republic). To evaluate the distribution of the treatment along the cotton and to examine the chemical composition of the fabrics and char residues, an energy-dispersive X-ray spectrometer (D8 Discover, Bruker Corp., USA) coupled with the SEM apparatus was employed. An automated fine coater was used to sputter-coat gold onto all of the samples (JEOL JFC-1600).
2.5.3. Thermogravimetric Analysis
A thermogravimetric analyzer (STA 6000, PerkinElmer, USA) was utilized to examine the thermal degradation behavior of cotton samples (9–11 mg) in the temperature range of 30–700°C at a heating rate of 10°C/min under nitrogen and air environments.
2.5.4. Pyrolysis Combustion Flow Calorimetry
A pyrolysis combustion flow calorimeter (Federal Aviation Administration, Fire Testing Technology) was used as per ASTM D7903 to determine the flame retardancy of the cotton and coatings. The furnace temperature was 900°C, the oxygen concentration was 20% (v/v), and the heating rate was adjusted to 1°C/min.
2.5.5. Vertical Burning Test
The fire test was performed in accordance with ASTM D6413 standard in a UL 94 test chamber (Fire Testing Technology, UK). 38-mm fire source was placed 19 mm below the middle of the sample (300 × 76 mm2) and immediately removed after 12 s. Afterflame, afterglow, and char length were measured accordingly.
2.5.6. Cone Calorimetry
Combustion properties were assessed using a cone calorimeter (Fire Testing Technology ltd., United Kingdom) according to ASTM E1354. The samples (100 × 100 mm2) were arranged horizontally and their bottoms and sides were covered with aluminum foil while being exposed to an external heat flux of 35 kW/m2. To avoid sample curvation, a wire grid was used.
2.5.7. TG-FTIR
The TG-FTIR was performed using a Discovery TGA 550 (TA Instruments, USA) coupled with a Nicolet iS50 FTIR Spectrometer (Thermo Fisher Scientific, USA) via a transfer pipe. Subsequently, 9 mg of cotton fabric samples were heated in a nitrogen or air environment from 50 to 700°C at a rate of 10°C/min.
2.5.8. Tensile Test
The tensile properties of the cotton fabrics (ASTM D5035) were checked using UTM RB 301 UNITECH-T R&B Inc., South Korea. The five specimens of the size 150 × 35 mm2 in warp and weft directions have been tasted under 50 mm/min of crosshead speed. The gauge length was equal to 75 mm.
3.1. Analysis of LBF
3.1.1. FTIR
The FTIR spectra of the LSL, DOPO, and LBF (LD-5) between 400 and 4000 cm-1 are shown in Figure 2(a). In the FTIR spectrum of the LSL, typical characteristic peaks were observed at 3435, 2930, 1622, 1498, 1317, 1038, and 896 cm-1 that were attributed to the stretching of the hydroxyl group in the phenolic and aliphatic structures, C-H stretching in the aromatic methoxy groups, water, C-H deformation vibration of the aromatic ring, O-H vibration of the phenolic group, C-O-C stretching, and β-glucosidic bonds between sugars, respectively (Boeriu et al. 2004; Moghaddam et al. 2017). The peaks at 812 and 627 cm-1 were assigned to the stretching vibrations of the Si-O-Si and Si-O bands, respectively, confirming the presence of silica in the LSL (Xue et al. 2020). Similarly, DOPO showed peaks at 2390, 1277, and 753 cm-1 that were attributed to the P-H stretching vibration, stretching vibration of P=O, and P-O-Ph stretching vibrations, respectively (Wang et al. 2019).
The spectrum of the LBF exhibited strong evidence of the introduction of phosphate groups onto the LSL, as indicated by the prominent peaks attributed to the stretching vibrations of P=O and P-O-Ph at 1277 and 753 cm-1, respectively. Additionally, the emergence of a new peak corresponding to the P-C group (Zhang et al. 2018) at 1471 cm-1, combined with the absence of a P-H DOPO peak at 2390 cm-1 and decreased absorption intensity of the O-H group strongly supports the presence of covalent bonding between the LSL and DOPO, as shown in Figure 1(b).
3.1.2. TGA
The thermal properties were examined in a nitrogen atmosphere, and the thermograms are shown in Figure 2(b) with the corresponding numerical data listed in Table 1. The LSL degraded gradually from approximately 150°C to 700°C in three degradation stages. The weight loss was attributed to moisture evaporation at temperatures lower than 150°C. The low-molecular-weight lignin fraction disintegrated at approximately 285°C. The third decomposition step was caused by the cleavage of the inter-unit linkages of higher-molecular-weight lignin and the evaporation of monomeric phenols (Jiao et al. 2019). The LSL demonstrated better thermal stability at temperatures higher than 340°C when compared to commercially available lignin (Lcom). This can be attributed to the higher amount of high-molecular-weight lignin and silica present in the LSL. The residual char weight (at 700°C) and temperature at 30% of weight loss (T30%) of the LSL were higher than those of Lcom, as listed in Table 1. In contrast, DOPO showed rapid degradation between 243°C and 422°C owing to its decomposition to dibenzofuran and diphenyl with the generation of PO and PO2 radicals (Wang et al. 2021).
The LBF demonstrated a different degradation pattern than those of the LSL and DOPO. All the LBFs showed higher thermal stability compared to constituents during degradation at temperatures between 230°C and 370°C. The LBF achieved an enhanced temperature at 10% mass loss (T10%) in comparison with those of LSL and DOPO, as shown in Table 1. The phosphoric acid released from the DOPO in this temperature range initiated the formation of intumescent char. Furthermore, the continued release of phosphoric acid after 370°C accelerated the degradation of the lignin. This resulted in decreased thermal stability at elevated temperatures and an excessive amount of acid that may be the reason for decreased thermal stability. The maximum degradation of the LD-5, LD-10, and LD-15 samples occurred at 459°C, 475°C, and 479°C and the weights of final residues were 59.6%, 55.4%, and 51.1%, respectively. In summary, the LBF enhanced the thermal stability of cotton by producing intumescent char. In addition, the phosphorous present in the degradation products acted as a char-forming promoter that enhanced the thermal stability of the LBF.
Table 1. TGA and PCFC Data of the Lcom, LSL, DOPO, and LBF.
3.1.3. PCFC
The flame-retardant efficiency of the LBF was examined using PCFC. The heat release rate (HRR) is shown in Figure 2(c) and important corresponding data are presented in Table 1. DOPO exhibited the highest peak heat release rate (pHRR)of 408 W/g at 322°C and 30.7 kJ/g of the total heat release (THR). DOPO mainly decomposed into dibenzofuran, diphenyl, PO, and PO2 radicals with the formation of phosphorous-containing char (Wang et al. 2021). The pHRR and THR of the LSL were 21 W/g and 2.9 kJ/g, respectively. Owing to the presence of silica, the pHRR of the LSL was 40.5 W/g lower and occurred at 101°C higher when compared to the Lcom, indicating better flame retardancy.
The LBFs showed completely different flammability behaviors compared to those of LSL and DOPO. The combustions of the LBFs started at lower temperatures than that of the LSL. This can be explained by the formation of phosphoric acid generated from DOPO that decreased the decomposition temperature of the LBF. The LD-5 sample achieved a pHRR and THR of 61.6 W/g and 9.8 kJ/g, respectively, whereas the other samples showed comparatively higher flammability. The intumescence of the char was observed after the test, and the phenomena were more pronounced for the LD-10 and LD-15 samples (Figure S1). The excessive amount of gases released from DOPO decreased the char thickness. Subsequently, the combustible materials passed through the cracks that formed on the char surfaces. The pHRR values of the LD-10 and LD-15 samples were both 76.2 W/g, whereas the THR values were 11.7 and 14.4 kJ/g, respectively. DOPO promoted the intumescence of the char and contributed to the thermal shielding effect of the LBF. Similarly, the LBF can enhance the flame retardancy of cotton by blocking heat transfer to the underlying material.
3.2. Characterization of Cotton Fabrics
3.2.1. FTIR and Morphology of the Cotton Fabrics
FTIR spectroscopy was used to characterize the chemical structures of the untreated and LBF-treated cotton fabrics. The FTIR spectra of the untreated cotton, CLSL, and LBF-treated samples (CLD-5) are shown in Figure 2(d). The bands in the spectrum of the untreated cotton at 3330, 2900, 1636, 1430, 1317, and 1033 cm-1 were attributed to the O-H, C-H, water, H-C-H, C-O, and combined C-O and O-H groups, respectively. For the CLSL, a new peak centered at 845 cm-1 corresponded to the Si-O-Si stretching vibration of the LSL, indicating the deposition of LSL on the cotton surface.
The spectrum of the LBF-treated sample exhibited new peaks at 1137 and 753 cm-1 belonging to the P-O-C aromatic stretching vibration (Zhu et al. 2014; Wang et al. 2019; Ao et al. 2020), whereas the peak at 1267 cm-1 was attributed to P=O (Castellano et al. 2019). Additionally, the -OH stretching vibration shifted from 3330 to 3326 cm-1, indicating a hydrogen bond formation between the hydroxyl groups of the LSL or LBF with similar groups of cellulose molecules in the cotton fabric (Zhang et al. 2021a; Song et al. 2022). The results of the FTIR analysis indicated the formation of hydrogen bonds between the LBF and cotton that can increase the flame retardancy of the fabric.
The surface morphologies and chemical compositions of the fabrics were examined using SEM-EDS to analyze the influence of the treatments on cotton. The SEM images of untreated cotton and the CLSL, CLD-5, CLD-10, and CLD-15 samples are shown in Figure 3. Their corresponding X-ray spectra are shown in Figure S2. The surface of the untreated cotton appeared smooth and clean, with distinct gaps between the fibrils. After treatment, the cotton fibrils were covered with the substrate, and the gap size between the fibrils decreased compared to that of untreated cotton. Additionally, the coating adhered more securely to the surface of the cotton with increasing DOPO concentration, and the roughness of the single fibers and coating uniformity increased.
EDS analysis was conducted to determine the chemical composition of the cotton fabrics. Untreated cotton consisted of 53.27% carbon and 46.73% oxygen. The EDS spectrum of the CLSL exhibited the presence of silicon and sodium, indicating that the LSL was attached to the cotton surface. Meanwhile, the CLD-5, CLD-10, and CLD-15 samples contained 1.82%, 2.68%, and 2.8% phosphorous, respectively. The silicon concentration marginally dropped as the DOPO concentration increased. Furthermore, EDS elemental mapping (Figure S3) showed a homogeneous distribution of the LBF constituents throughout the cotton surface, implying the successful deposition of the LBF and equal flame retardancy throughout the fabric.
3.3. Flammability
3.3.1. UL-94 Vertical Burning Test
A VBT was performed to assess the impact of the LBF on the flame retardancy of the cotton fabrics. Digital images of the samples taken during the test are shown in Figure 4 and the data collected from the experiment are presented in Table 2. The videos recorded during the VBT are provided in the Supporting Information. The untreated cotton exhibited a rapid fire spread and burned out, leaving a minor amount of residue. The afterflame and afterglow durations were measured at 17 and 30 s, respectively. The CLSL fabric experienced no afterflame. However, the length of the damaged material was 251 mm after smoldering for 347 s. The CLD-5 and CLD-10 samples also immediately extinguished after the removal of the fire source, and the afterglow phenomena lasted for 33 and 17 s, respectively. Additionally, smoldering only appeared on the charred part of the fabric without spreading throughout the unburned material. The CLD-15 sample burned for 3 s and sustained afterglow for 5 s, resulting in the longest damaged area of 85 mm among the LBF-treated cottons. The lengths of the burned areas for the CLD-5 and CLD-10 samples were 63 and 75 mm, respectively. A distinct intumescence phenomenon for the LBF-treated cotton occurred after removing the burner. The VBT results indicated that the LBF treatment significantly enhanced the flame retardancy of cotton fabrics compared to untreated cotton. The LBF-treated samples demonstrated intumescence effects and reduced afterflame and afterglow durations. These findings highlighted the potential of using an LBF as an effective flame retardant for cotton production.
Table 2. Corresponding Data for TGA and VBT.
3.3.2. PCFC
PCFC was performed to further understand the flammability of the LBF-treated cotton. The HRR of the treated cotton and corresponding data are provided in Figure S4(a) and Table S1, respectively. Untreated cotton showed a rapid single HRR with a pHRR of 242 W/g at 380°C. This was owing to the formation of flammable compounds that contributed to its flammability, such as aldehydes, ketones, and ethers (Maksym et al. 2022). Conversely, treated samples combusted in two stages. The initial pHRR for the CLSL, CLD-5, CLD-10, and CLD-15 samples occurred at temperatures of 321°C, 302°C, 306°C, and 317°C with corresponding values of 53.3, 53, 59.6, and 60.9 W/g, respectively. The first combustion peak occurred because of the enhanced charring ability of the treatments, as discussed in Section 3.2.2.1. Subsequently, a second peak emerged at 444°C for CLSL, with a corresponding value of 18.6 W/g. In the case of the LBF-treated fabrics, the second pHRRs appeared at 414°C, 414°C, and 425°C with values of 33.2, 44.6, and 49.8W/g for the CLD-5, CLD-10, and CLD-15 samples, respectively. At this stage, the flammable gases released from the uncombusted material trapped inside the char were liberated through microcracks (Prabhakar et al. 2022). The THR of untreated cotton was the highest and equaled 15.6 kJ/g, whereas those of the CLSL, CLD-5, CLD-10, and CLD-15 samples decreased by 65.4%, 53.2%, 39.7%, and 34%, respectively. These findings demonstrate the effectiveness of the LBF treatment in altering the flammability behavior of cotton fabrics. Additionally, the two-stage combustion pattern observed in the LBF-treated samples and reduced pHRR and THR values compared to the untreated cotton indicate improved flame retardancy and enhanced charring ability.
3.3.3. Cone Calorimetry
Table 3. Results of Cone Calorimetry Tests for Untreated Cotton, CLSL, and CLD-5.
A cone calorimeter test was performed on the untreated cotton, CLSL, and CLD-5 samples to examine their combustion behaviors. The HRR, THR, total smoke production (TSP), CO production (COP), and CO2 production (CO2P) curves are presented in Figure 5. The corresponding data, including time to ignition (TTI), pHRR, maximum average rate of heat emission (MARHE), and CO2/CO ratio are presented in Table 3. The untreated cotton, CLSL, and CLD-5 samples exhibited pHRRs of 239, 162, and 202 kW/m2 and THRs of 20.9, 6.1, and 11.9 MJ/m2, respectively. The 23 s delay in TTI for the CLSL and decreased pHRR and THR compared to untreated cotton confirmed the formation of char residue during heating, which can hinder cotton from burning and reduce combustion heat (Mandlekar et al. 2018). The CLS-5 sample ignited earlier than untreated cotton due to phosphoric acid produced from the LBF during combustion that catalyzed the degradation of the cellulose. Phosphorous char was formed at this stage, lowering the flammability of the cotton by converting flammable materials into non-combustible char (Li et al. 2022). Furthermore, the production of COP was higher for the treated samples, as demonstrated in Figure 5(d). This resulted from the aromatic structure of lignin, which can accelerate the carbonization reaction of cotton and create a protective char layer that prevents the contact of degradation products with oxygen, minimizing further CO combustion to generate CO2(Li et al. 2019). The evidence supporting this observation is further strengthened by the substantial decrease in the CO2/CO ratio. In the case of the CLSL, the ratio decreased from an initial value of 36.1 (as seen in the untreated cotton) to 15.7. Similarly, the ratio decreased from 36.1 to 25.4 for the CLD-5 sample. The combustion of the CLD-5 sample produced the highest quantity of smoke, as shown in Figure 5(c). This occurred due to the PO∙ and PO2∙radicals released from the phosphorous in the LBF reacting with the H∙ and OH∙ radicals, generating a much higher amount of non-flammable volatile products during the burning. The increased amount of residue after the test exhibited a flame-retardant effect in the condensed phase. The high char yield demonstrated that the flammable substrates remained in the condensed phase during burning, thereby reducing their flammability. Overall, the findings of this study highlight the improved fire performance and reduced flammability of the LBF-treated cotton sample compared with those of untreated cotton. The formation of a protective char layer and decreased CO2/CO ratio contributed to the potential of these treatments as effective flame retardants for cotton materials in various applications.
3.4. Flame-Retardant Mechanism
The pyrolysis products of untreated and treated cotton fabrics were analyzed using TG-FTIR to investigate the flame-retardant mechanism in the gas phase, as shown in Figures S5(a)-(c). The peak at 3593 cm-1 was attributed to the stretching vibration of the -OH group of the water, whereas the peaks at 2975, 2912, and 2815 cm-1 were ascribed to the absorption of the aliphatic C-H bond of the hydrocarbons. The peaks at 2358, 2184, 1724, and 1056 cm-1 (1100 cm-1 for CLSL and CLD-5) corresponded to the absorption of CO2, stretching vibration of CO, vibrations of C=O of carbonyl compounds, and C-O-C of ethers, respectively (Chen et al. 2020; Wang et al. 2023). The peak signals for untreated cotton intensified when the temperature reached 345°C. The maximum amounts of pyrolysis gas for the CLSL and CLD-5 samples were released at 275°C and 270°C, respectively. These findings further confirmed the catalytic influence of the LSL and LBF on cellulose and were consistent with the outcomes obtained from the TGA analysis. The disappearance of the characteristic peaks of aliphatic hydrocarbons (2975 and 2815 cm-1) and significant reduction in the intensities of peaks related to hydrocarbons (2912 cm-1), carbonyl compounds, and ethers are shown in Figures S5(b) and (c). Specifically, a detailed representation of the intensities of the characteristic peaks associated with flammable hydrocarbons, carbonyl compounds, and ethers, along with the peaks corresponding to non-flammable CO2, are shown in Figure 6(a) (Xu et al. 2019a). The CLSL and CLD-5 samples clearly released fewer flammable gases, whereas the production of CO2 was enhanced. These results suggest that phosphoric acid increased the carbonization of cellulose during combustion rather than decomposing and producing flammable volatiles (Wang et al. 2018; Xu et al. 2019b).
FTIR analysis was performed to understand the chemical structure of the residue, as shown in Figure S5(d). The peaks describing the molecular structure of cellulose disintegrated after burning owing to thermal-oxidative depolymerization. The peaks at 1588 and 1018 cm-1 can be ascribed to the C=C stretching vibration and stretching of the C-O band of the aromatic structure of carbonaceous char (Shukla et al. 2019). The char residues of both CLSL and CLD-5 samples exhibited peaks at 1430 cm-1 (belonging to the Na-O band) and 880 cm-1 (attributed to Si-O-Si stretching vibrations)(Huisken et al. 1999). This is the reason for the enhanced thermal stability of the char at elevated temperatures. Meanwhile, the char of the CLD-5 sample showed the presence of P=O and P-O-P groups at 1215, 1018, and 750 cm-1, which strongly indicated the generation of stable phosphorous char (Prabhakar and Song 2020).
The residues of the untreated and treated cotton fabrics after the VBT were studied using SEM to further investigate the flame-retardant mechanism. The morphologies of the untreated cotton, CLSL, and CLD-5 chars are shown in Figure 6S. The residue of the untreated cotton was weak and brittle owing to the complete combustion of cellulose and insufficient char formation. The SEM images of the CLSL sample showed that the single cotton fibers preserved their structure. In addition, intumescence could be observed on and outside of the fiber surface. However, the char was not strong enough to stop the cotton from smoldering, according to the VBT. In contrast, the surface of the CLD-5 sample was compact and continuous, without holes or cracks. The formation of a protective char barrier effectively hindered heat transfer to the unburned material, significantly decreasing the afterglow.
EDS was conducted to investigate the elemental compositions of the CLSL and CLD-5 sample residues. The CLSL surface retained carbon, oxygen, sodium, and silica after burning, whereas the LCD-5 sample showed the presence of phosphorus in addition to these elements, as shown in Figure 6(b). It was concluded that the presence of phosphorus played a crucial role in imparting excellent flame-retardant properties to cotton. This was achieved by creating a highly efficient phosphorus-char protective layer that resulted in enhanced flame retardancy.
According to the TG-IR, SEM-EDS, and FTIR results, a possible flame-retardant mechanism of LBF-treated cotton can be explained as the phosphorous-containing groups of the LBF generating phosphoric acid before any effects from cellulose and LSL. Furthermore, the accelerated degradation of the LSL and cellulose contributed to the formation of phosphate esters that catalyzed the carbonization of the cellulose and LSL, thereby decreasing the formation of flammable gases. Meanwhile, the released non-flammable water vapor and CO2 swelled the ester, providing intumescence to the char and greatly reducing heat and mass transfer. Moreover, a ceramic-metallic barrier could be generated from silicone- and sodium-containing compounds, greatly improving the strength of the char and preventing further material oxidation of the underlying material by diminishing heat transfer (Nam et al. 2017; Yan et al. 2017)
3.5. Tensile Properties
The effect of the treatment on the tensile properties of cotton textiles was studied by estimating the tensile strength, strain at break, and tensile modulus in the warp and weft directions. The test results are presented in Figures 7(a) and (b) and Table S2. The warp tensile strength and strain at the breaking point of the cotton fabric are shown in Figure 7(a). The tensile strengths of the untreated cotton, CLSL, CLD-5, CLD-10, and CLD-15 samples were 32.4, 34.5, 31.7, 34.7, and 32.9 MPa, respectively. Notably, all treated samples showed an improvement in the strain at break. In the weft direction, only the CLD-15 sample showed a 3.4% reduction in tensile strength, whereas the CLSL, CLD-5, and CLD-10 samples exhibited improvements of 18.6%, 9.7%, and 31.7%, respectively. The strain at break slightly improved for the CLSL and CLD-10 samples, whereas the CLD-5 and CLD-15 samples exhibited minor reductions. A thin layer of LSL or LBF covering the cotton improved the stress-bearing capacity by evenly distributing the load in the principal direction and transferring it from the principal to the transverse direction. SEM images of the cotton fabric after the tensile test confirmed this hypothesis because they revealed damage to the flame-retardant coating layer, as shown in Figure S7.
In contrast, the tensile modulus of untreated cotton in the warp direction was 0.523 GPa, and the reductions in the CLSL, CLD-5, CLD-10, and CLD-15 samples were 23.5%, 27.7%, 27.9%, and 29.8%, respectively. Meanwhile, tensile moduli in the weft direction for the CLSL, CLD-5, CLD-10, and CLD-15 samples decreased compared to untreated cotton (0.403 GPa) by 13.9%, 7.4%, 10.7%, and 15.9%, respectively. The reductions in the moduli can be attributed to the crystallinity reduction caused by amorphous lignin (Ma et al. 2015). Overall, the LBF achieved a positive effect on the tensile strength and elongation at break of the cotton fabric.
3.7 Thermal Properties of Cotton Fabrics
The TG curves of untreated and treated cotton under a nitrogen atmosphere are shown in Figure 8(a) and the corresponding data are listed in Table 2. All samples exhibited weight loss at temperatures between 30°C and 150°C due to water evaporation. Untreated cotton exhibited rapid mass loss with the maximum loss rate (TRmax) at 383°C, which was associated with the depolymerization of cellulose to produce combustible gases, volatile liquids, and solid residue (Xu et al. 2019b). The thermal degradation of the treated fabrics was initiated at lower temperatures compared with the untreated cotton. Thus, the TRmax of the CLSL, CLD-5, CLD-10, and CLD-15 samples were 286°C, 289°C, 288°C, and 322°C respectively. This phenomenon occurred because low-molecular fractions of lignin and phosphoric acid from DOPO promoted dehydration and carbonization before the pyrolysis of cellulose, which enhanced char formation from combustible materials to improve thermal stability and flame retardancy(Liu et al. 2017; Shukla et al. 2019). This explanation is corroborated by the lower weight-loss rate of the treated cotton fabrics during differential thermal analysis (DTG), as shown in Figure S4(b). Furthermore, the T10% for treated cotton slightly fluctuated at approximately 255°C, while the temperature at 60% mass loss (T60%) for the untreated cotton, CLD-5, CLD-10, and CLD-15 samples were observed at 389°C, 665°C, 553°C, 450°C, and 401°C, respectively. This indicates enhanced thermal stability at elevated temperatures. The amounts of residue at 700°C for the untreated cotton, CLSL, CLD-5, CLD-10, and CLD-15 samples were 6.7%, 38.4%, 36.9%, 31.9%, and 27.8%, respectively. Incorporating DOPO resulted in a reduction in the T60% and weight of the final residue. This may have been due to the immoderate amount of gases liberated from the samples containing more DOPO, which could reduce the strength of the char. The second reason for this is the reduction in the thermally stable silica (as confirmed by EDS) in the sample with a higher DOPO concentration, which could also lead to a decline in the thermal stability at elevated temperatures.
TGA was performed in air to study the thermal decomposition of the cotton fabrics in an oxidizing environment. The thermo-oxidative behaviors of the cotton samples are shown in Figure 8(b). The initial mass loss at approximately 150°C was associated with moisture evaporation. Untreated cotton degraded between 320°C and 378 °C, producing CO, CO2, and CH4 as volatile products and approximately 8.5% of oxidized char. Furthermore, the char degraded between 377°C and 508°C, resulting in a 1.8% ash retention. The T10% for treated cotton samples was approximately 88°C lower compared to that of untreated cotton. The DTG results showed that the temperature of the first maximum rate of thermal oxidation (TRmax1) for the CLD-5 sample occurred at 261°C, as shown in Figure S4(c). This was the lowest compared with other treated cotton samples. The TRmax1 for the CLSL, CLD-10, and CLD-15 samples occurred at 298°C, 281°C, and 289°C, respectively. The thermal behavior of the treated samples between 235°C and 338°C displayed a notable mass loss attributed to the thermal-oxidative degradation of cellulose, which was accelerated by the phosphoric acid released from the LBF and/or lignin. However, the subsequent thermal behaviors of the samples exhibited distinct characteristics. Upon reaching 338°C, the CLSL sample demonstrated a gradual loss in mass that continued until it reached 381°C. Beyond this point, a rapid mass loss occurred again, with the second maximum rate of thermal oxidation (TRmax2) at 411°C. A relatively low mass loss was observed between 420°C and 665°C, followed by a weight drop of 3.1%. However, the LBF-treated cotton fabrics exhibited better thermal stability than that for the CLSL. Accordingly, the TRmax2 for the CLD-5, CLD-10, and CLD-15 samples occurred at 649°C, 545°C, and 461°C, respectively. The TRmax2 was higher for the LBF-treated cotton than that for the CLSL owing to the formation of thermally stable char that protected the underlying material from thermo-oxidative degradation. Moreover, the third degradation stage for the LBF-treated cotton fabrics vanished. TRmax2 for the CLD-10 and CLD-15 samples occurred 104°C and 188°C earlier than that for the CLD-5 sample, respectively. This may be owing to the excessive amount of gases released from the DOPO that were present in the CLD-10 and CLD-15 samples, resulting in a decrease in the temperature at which degradation of the previously formed char occurred. In other words, the char formed in these samples was less stable than those formed in the other treated cotton samples, leading to its degradation at lower temperatures.
A new biomass-derived (RH-extracted LSL) flame retardant was successfully synthesized and applied to cotton fabrics via a one-pot dip-coating process. FTIR and SEM-EDS analyses confirmed the successful deposition of the LBF onto cotton fabrics. Flammability assessments demonstrated a remarkable enhancement in flame resistance properties, with self-extinguishing capabilities and reductions in pHRR and THR. Char analysis indicated that the LBF facilitated the formation of a dense char layer on the cotton surface that hindered mass and heat transfer.TG-FTIR analysis demonstrated a significant reduction in flammable gas liberation. Additionally, the LBF improved the tensile strength and elongation at break of cotton. The application of the LBF resulted in a substantial improvement in the thermal stability of treated cotton, with the formation of a 36.9% char residue at 700˚C. The outcomes of this study are of utmost importance as they provide a more sustainable and effective flame-retardant solution for various industries, including textiles, home furnishings, and protective clothing. Overall, this study offers valuable contributions to academia and industry by providing a promising direction for future research and practical applications in the field of flame retardancy.
RH – rice husk; LSL – lignin and silica-rich liquid; LBF – liquid bio-based flame retardant; LD-5, 10, 15 – LBF synthesized by adding 5, 10, and 15 g of DOPO respectively; CLSL – cotton treated with LSL; CLD-5, 10, 15 – cotton treated with LD-5, 10, 15 respectively; Lcom – commercially available lignin.
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
Maksym Li: Conceptualization, Writing – original draft, Writing. Prabhakar M.N.: Methodology, Validation, Data curation, Supervision. Jung-IL Song: Supervision.
ACKNOWLEDGMENT
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) [2018R1A6A1A03024509, 2023R1A2C1006234 and 2022R1I1A1A01069282].
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No competing interests reported.
