Article
Advanced Phosphorus–Protein Hybrid Coatings for Fire Safety of Cotton Fabrics, Developed Through the Layer-by-Layer Assembly Technique Xuqi Yang 1 , Xiaolu Li 2 , Wenwen Guo 3 , Abbas Mohammadi 4 , Marjan Enetezar Shabestari 5 , Rui Li 1 , Shuyi Zhang 1 and Ehsan Naderi Kalali 1, * 1
2 3 4 5 * Academic Editor: Gianluca Tondi Received: 3 January 2025 Revised: 25 February 2025
Department of Safety Engineering, Faculty of Geoscience and Engineering, Southwest Jiaotong University, 111 2nd Ring Rd North Section 1, Jinniu District, Chengdu 610032, China College of Materials and Chemistry, China Jiliang University, Hangzhou 310018, China Key Laboratory of Eco-Textiles, Ministry of Education, College of Textile Science and Engineering, Jiangnan University, 1800 Lihu Avenue, Wuxi 214122, China Department of Chemistry, University of Isfahan, Isfahan 81746-73441, Iran School of Fire and Safety Engineering, Zigong 643000, China Correspondence: ehsan@swjtu.edu.cn
Abstract: An advanced, eco-friendly, and fully bio-based flame retardant (FR) system has been created and applied to the cellulose structure of the cotton fabric through a layer-bylayer coating method. This study examines the flame-retardant mechanism of protein-based and phosphorus-containing coatings to improve fire resistance. During combustion, the phosphate groups (−PO4 2− ) in phosphorus containing flame retardant layers interact with the amino groups (–NH2 ) of protein, forming ester bonds, which results in the generation of a crosslinked network between the amino groups and the phosphate groups. This structure greatly enhances the thermal stability of the residual char, hence improving fire resistance. Cone calorimeter and flammability tests show significant improvements in fire safety, including lower peak heat release rates, reduced smoke production, and higher char residue, all contributing to better flame-retardant performance. pHRR, THR, and TSP of the flame-retarded cotton fabric demonstrated 25, 54, and 72% reduction, respectively. These findings suggest that LbL-assembled protein–phosphorus-based coatings provide a promising, sustainable solution for creating efficient flame-retardant materials.
Accepted: 4 March 2025 Published: 31 March 2025 Citation: Yang, X.; Li, X.; Guo, W.; Keywords: bio-based flame retardant; layer-by-layer (LbL) assembly coating process; eco-friendly
Mohammadi, A.; Enetezar Shabestari, M.; Li, R.; Zhang, S.; Naderi Kalali, E. Advanced Phosphorus–Protein Hybrid Coatings for Fire Safety of Cotton Fabrics, Developed Through the Layer-by-Layer Assembly Technique. Polymers 2025, 17, 945. https://doi.org/10.3390/ polym17070945 Copyright: © 2025 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/ licenses/by/4.0/).
1. Introduction Among various natural textiles, cotton fabric, made from biodegradable cellulose, offers numerous benefits, including superior hygroscopicity, softness, comfort, and breathability. Due to its distinctive characteristics, cotton fabric is extensively utilized in various domains such as apparel, furniture, military uniforms, home decor, and industrial applications [1–3]. Despite its many advantages, cotton fabric’s primary shortcoming is its flammability. This limitation restricts its suitability for high-performance uses which necessitate fire-resistance. Cotton is composed of chain segments of carbohydrate, making it highly flammable. Upon ignition, it undergoes significant degradation, producing highly combustible volatiles that lead to rapid fire spread and substantial smoke release, thereby heightening the risk of fatalities and severe societal damage [4]. Consequently, there is an urgent need to enhance the flame retardancy of cotton fabric to comply with mandatory
flammability standards. Thus, modifying cotton fabric to impart superior flame-retardant properties is essential. Halogen-containing flame retardants are acknowledged as the most efficacious compounds for enhancing the fire resistance of cotton fabrics [5,6]. Nonetheless, their combustion is accompanied by the release of toxic fumes, including carcinogenic dioxins, which pose severe threats to human health and contribute substantially to environmental pollution. Consequently, numerous countries have decided to prohibit the use of halogenated flame retardants [7,8]. Instead, flame retardants containing nitrogen, phosphorus, silicon, and boron are extensively employed for this purpose [9]. Among these, phosphorus-based flame retardants are particularly noted for their efficacy and low toxicity when applied to the cotton fabrics, as conducted by Nguyen et al.’s investigation [10]. During the combustion process, phosphorus-containing flame retardants (FRs) [8] generate nonvolatile phosphorus-based acids. These acids can esterify and dehydrate decomposed cellulose, leading to the formation of a coherent char residue. This char acts as a barrier, impeding the transfer of heat and oxygen between the condensed and gaseous phases, thereby facilitating the cessation of combustion [11]. Commercialized flame retardants, such as Pyrovatex CP® and PROBAN® , incorporate reactive phosphorus or N-CH2 OH groups synthesized with formaldehyde. Consequently, treated cotton fabrics tend to emit formaldehyde during their use [12]. From an environmental protection and ecological outlook, there is a growing emphasis on substituting harmful, toxic, and halogen-containing flame retardants with environmentally safe alternatives [13]. To this end, various techniques have been employed to develop environmental-friendly flame-retardant coatings for the cotton fabric. On the other hand, there are several methods of applying the flame retardant materials to the cotton fabrics including sol–gel processes [14], layer-by-layer (LbL) assembly [15,16], plasma treatment [17], and polyelectrolyte deposition methods [18]. Due to its convenient processing and the abundance of its components, layerby-layer (LbL) assembly process widely utilized with various materials [19], such as chitosan/ammonium polyphosphate (APP) [20,21], chitosan/montmorillonite [22], chitosan/phytic acid (PA) [23,24], and protein/PA [25,26]. Utilizing the LbL assembly technique, Liu et al. [27] integrated 3-aminopropyl triethoxysilane, sodium phytate, and chitosan to create a nano-coating that capable cotton fabrics with self-extinguishing properties at a coating load of about 32 wt%. Ammonium polyphosphate-derived intumescing flame retardants have garnered significant attention due to their low toxicity and high efficiency. They are frequently utilized in the preparation of layer-by-layer assembly coatings. In a particular study, Fang et al. [28] treated cotton fabric with chitosan and APP using the LbL assembly method. The results indicated that, by increasing the number of bilayers to 20 or more, significantly reduced the heat release rate to approximately one-fifth of that of untreated cotton fabric. Regardless of the main advantages of the LbL assembly technique, such as high customizability and simplicity, it involves a multistep adsorption process that requires specialized equipment and long-term operations, thus hindering large-scale production. Therefore, LbL assembly technique operation steps needs to be minimized [29]. As mentioned before, natural fiber fabrics are highly flammable. Thus, the development of high-performance flame retardants or advanced flame-retardant technologies is crucial to ensure the safety and reliability of natural polymer-based composites. Layer-by-layer (LbL) assembly offers a promising alternative to traditional additive flame retardants due to its high flame-retardant efficiency, environmental acceptability, and minimal impact on the intrinsic properties of polymers [30]. LbL assembly is versatile, cost-effective, and applicable to various materials, including polyelectrolytes, nanoparticles, and biomolecules. It has been utilized for applications such as gas barriers, antimicrobial coatings, biosensing, charge storage, antireflection, and drug delivery [31]. Recently, it has been applied to
design flame retardant coating as well. The LbL method provides several advantages over traditional flame retardant techniques. It constructs flame-retardant multilayer films on the substrate’s surface, directly interfering with combustion and avoiding the challenges of incorporating flame retardants into the substrate, which can adversely affect its mechanical properties [32,33]. Additionally, LbL assembly allows for the fabrication of multilayer films with controllable thickness, composition, and function using simple, versatile, and mild experimental conditions, such as room temperature, atmospheric pressure, and low concentrations of assembly materials (below 1 wt%), making it a cost-effective route for fabricating coatings [34,35]. Consequently, in Qiu et al.’s investigation [31], flame retardant coatings fabricated through the straightforward and eco-friendly layer-by-layer assembly method are particularly significant, as they enhance the flame retardant properties of polymers without altering their intrinsic characteristics. It is worth noting that the LbL assembly method has been effectively utilized to construct thermally insulating and fire-shielding coatings composed of inorganic nanoparticles or hybrid organic–inorganic systems [36,37]. Since its initial application, significant advancements have been made, resulting in enhanced coating efficiency and, at times, unmatched properties. For instance, in the context of cotton, initial systems struggled to preserve fabric structure post-flammability tests, whereas current systems can now achieve self-extinguishing capabilities while maintaining the integrity of most of the fabric [38]. The range of reagents and substrates has expanded to include various nanoparticles and ecofriendly polyelectrolytes, which have been applied to fabrics, foams, and thin films [39–41]. Moreover, in-depth studies on deposition parameters have provided a better understanding of the relationship between coating morphology and final properties [32,42]. This work represents an advanced approach focused on the novel employment of a ecofriendly and bio-based layer-by-layer coatings to fabricate a feasible and efficient structure capable of enhancing the fire safety of cotton fabrics for the indoor use. In the present project, phytic and pea protein was used as positive and negative charge coating with the aim of enhancing the fireproof properties of the cotton fabric in air (characterized by flammability and combustion tests, thermo-stability analysis, and spectroscopy, respectively. More particularly, the pea protein exhibits an intumescent-like and char-forming agent system possessing excellent synergy, in which phytic acid is able of formation phosphoric acid at elevated temperatures, thus promoting the char formation. Moreover, pea protein can generate water vapor during the dehydration in the presence of phosphoric acid, and its synergism with phytic acid favors the production of effective residual char that can significantly enhance the resistance of the cotton fabric during combustion significantly. Finally, SEM, FTIR, and Raman spectroscopy were conducted in order to characterize the morphological structure of the flame-retardant coating before and after the combustion, and the performance of the flame-retarded cotton fabrics were considered. For this purpose, thermogravimetric and cone calorimetry analysis were tested to analyze the thermolysis and fire-retardant specifications of the flame-retardant treated cotton fabrics. Finally, the mechanical behaviors were characterized by tensile tests.
2. Experiment 2.1. Materials Pure cotton fabrics (100%, 220 g/m2 ) was supplied by the Shaoxing Manheng Textiles Company (Shaoxing, China), and used as the substrate. Phytic acid (PA, 50 wt% aqueous solution) were purchased from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). Pea protein was purchased from Shanghai Haiwanyile Biotechnology Co., Ltd (Shanghai, China). All reagents were used to prepare 6 wt% phytic acid solution and 1.5 wt% pea protein solution for layer-by-layer deposition using deionized water.
solution) were purchased from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). Pea protein was purchased from Shanghai Haiwanyile Biotechnology Co., Ltd (Shanghai, China). All reagents were used to prepare 6 wt% phytic acid solution and 1.5 4 of 17 wt% pea protein solution for layer-by-layer deposition using deionized water. 2.2. 2.2. Preparation Preparation of of Flame-Retardant Flame-Retardant Solution Solution ◦ C deionized Pea Pea protein protein was was dissolved dissolved in in 60 60 °C deionized (DI) (DI) water water and and adjusted adjusted the the pH pH value value to 9 using the 1 M HCl solution which was stirred for 1 h to prepare 1.5 wt% pea protein to 9 using the 1 M HCl solution which was stirred for 1 h to prepare 1.5 wt% pea protein solution. wt% PA PA solution solutionwas wasprepared preparedby bydiluting dilutingthe theconcentrated concentrated solution in solution. A A 66 wt% PAPA solution in DI DI water, and its pH was adjusted to 4 using the 1 M NaOH solution. water, and its pH was adjusted to 4 using the 1 M NaOH solution. 2.3. Treatment Treatment of Pure Cotton Fabrics with LBL Flame Retardant 2.3. Before the the LBL LBL deposition, deposition, the the cotton cotton fabric fabric was was washed washed with with DI DI water water and and dried dried to to Before remove the impurities. is remove impurities. The The process processof ofpreparing preparingcotton cottonfabric fabricflame flameretardant retardantcomposite composite prepared byby alternatively adsorbing thethe positive and negative polyelectrolyte according to is prepared alternatively adsorbing positive and negative polyelectrolyte according LBL technology, asas shown ininFigure charge to LBL technology, shown Figure1.1.Cotton Cottonfibers fibersusually usually containing containing negative charge due to to the presence of carboxyl and hydroxyl groups. Therefore, Therefore, cotton cotton fabric fabric is is firstly firstly due immersed in the positive pea immersed pea protein proteinsolution solutionfor for55min, min,then thenwashed washedwith withdeionized deionizedwater wa◦ andand dried at 80 C °C forfor 30 30 min. After that, thethe cotton fabric was subsequently immersed in ter dried at 80 min. After that, cotton fabric was subsequently immersed thethe negative PAPA solution forfor 5 min, then washed with DI water andand dried at 80 C in in negative solution 5 min, then washed with DI water dried at ◦80 °Cthe in oven for 30 to complete the 1the bi-layer (BL). (BL). The above process was repeated to obtain the oven formin 30 min to complete 1 bi-layer The above process was repeated to 3, 6, 9, 3, and 12and BL pea protein/PA flame flame retardant coating on cotton fabrics whichwhich were obtain 6, 9, 12 BL pea protein/PA retardant coating on cotton fabrics named by 3BL, were named by 6BL, 3BL, 9BL, 6BL, and 9BL,12BL. and 12BL.
Figure 1. 1. Schematic Schematic flame-retardant flame-retardant mechanism mechanism of of the the FR FR coating. coating. Left: Left: the the chemical chemical structure structure of of the the Figure flame retardant coating; right: illustration of the network generated during the combustion between flame retardant coating; right: illustration of the network generated during the combustion between the phytic acid and pea protein. the phytic acid and pea protein.
2.4. Characterizations 2.4. Characterizations The surface morphology and elemental distribution on cotton fabrics and residual surface morphology elemental distribution on cotton and residual charsThe were analyzed using a and scanning electron microscope (SEM)fabrics (JSM 7800F Prime, chars were analyzed using a scanning electron microscope (SEM) (JSM 7800F Prime, OXFROD X-Max 80, Toyama, Japan) at an accelerating voltage of 5 kV, equipped with an OXFROD X-Max 80,X-ray Toyama, Japan) at (EDX). an accelerating voltage of 5 kV, equipped with an energy-dispersive spectrometer To improve electrical conductivity, before energy-dispersive X-ray spectrometer (EDX).under To improve conductivity, before microscopy, sputter-coating with chromium vacuumelectrical was applied to the samples. microscopy, sputter-coating with chromium under vacuum was applied to the samples. Fourier-transform infrared (FTIR) spectrums were recorded with a Spectrum 100-T infrared (FTIR) spectrums were recorded with a Spectrum 100-T FTIRFourier-transform spectrometer (Thermo Fisher Nicolet iS50, Waltham, MA, USA) across a wavenumber − 1 FTIR spectrometer (Thermo Fisher Nicolet iS50, Waltham, MA, USA) across range of 4000 to 500 cm , averaging 16 scans per spectrum at a resolution of 4 cm−a1 . The FTIR spectrometer utilized an attenuated total reflectance method to characterize raw chemicals and cotton fabrics prior and after the coating process. Thermal stability of the cotton fabric was evaluated using a TGA (Netzsch TG 209 F1, Selb, Germany) instrument with a heating rate of 10 ◦ C/min under nitrogen atmosphere. To ensure accuracy, each specimen was examined twice. The theoretical results were computed
using a linear formula of the values from pristine cotton, PA, and pea protein, based on the following equation: Wth ( T )FR = v · Wexp ( T )PC + x · Wexp ( T )PA + y · Wexp ( T ) PE , v + x + y = 1 (1)
The experimental TG readings of non-treated cotton, PA and pea protein are denoted as Wexp ( T ) PC , Wexp ( T )PA , and Wexp ( T ) Pea , respectively. The weight percentages of pristine cotton, PA, and pea protein are represented by v, x, and y, respectively. For the above calculation, the weight-gain of each bilayer was approximately 16.5 g/m2 and used to calculate the weight of bilayers for each individual specimen. Thermogravimetric analysis was performed at a heating rate of 10 degrees Celsius per minute from 35 ◦ C to 800 ◦ C under a nitrogen atmosphere. About 10 mg of sample was used for this purpose. Limiting oxygen index (LOI) was determined using a limiting oxygen indexer instrument (HC-2C, Nanjing Shangyuan Analytical Instrument Co., Ltd. Nanjing, China) according to the GB/T 5454-1997 [43] standard regulation, with sample dimensions of 150 mm by 58 mm. The vertical burning analysis was performed employing a vertical burning instrument with sample dimensions of 300 mm by 78 mm, following the GB/T 5455-2014 regulations [44]. A laser-equipped Raman spectrometer (Thermo Scientific DXR, Waltham, MA, USA) was employed to record spectra in the range of 500 to 2000 cm−1 with a wavelength excitation of 532.17 nm. Flammability tests were performed using a Cone calorimeter (FTT, Derby, UK) according to the ISO 5660-1 standard [45], with a heat-flux of 25 kW/m2 and sample dimensions of 100 mm by 100 mm by 1 mm. Mechanical properties of the cotton fabrics were evaluated using an Instron electronic universal testing machine (6025/5800R) following ASTM D-5035-11 standard [46], with sample dimensions of 100 mm by 25 mm. A 1 kN cross-head and a tensile momentum of 300 mm/min were used to determine the tensile properties. The weight gain of the flame-retardant coating on the cotton was calculated using the following calculation format: Weight gain% = 100 × (W2 − W1 )/W1
where W1 is the weight of the initial cotton and W2 is the weight of the treated cotton. The weight gain of the flame retardant coating summarized in Table 1. Each value is the average of 3 readings per each sample. Table 1. Weight gain percentage.
Number of BL Pure 3BL 6BL 9BL 12BL Weight gain (%) 0 29.3 32.2 40.7 47.9
3. Results and Discussion 3.1. Structural and Morphological Characterizations The formation of the flame-retardant (FR) coating on cotton fabrics was examined using scanning electron microscopy (SEM). SEM images revealed that the applied FR one bilayer coating forms a uniform thin film, (as shown in Figure 2b), surrounding the cotton fibers.
Polymers 2025, 17, x FOR PEER REVIEW Polymers 2025, 17, 945 6 of 17 bilayercoating coatingforms formsa auniform uniformthin thin film, shown Figure 2b), surrounding cotton 6 of 17 bilayer film, (as(as shown inin Figure 2b), surrounding thethe cotton fibers. fibers.
Figure cotton fibers’ cross section SEM prior (a) and after (b)after coating and thethe microThe cotton fibers’ cross section SEM prior (a) and after (b) coating process, and microFigure2.2. 2.The The cotton fibers’ cross section SEM prior (a) and (b) process, coating process, and the graphs ofofthe flame deposited onon thethe silicon micrographs of theection cross section of the LbLretardants flame retardants deposited onsilicon thewafer. silicon graphsofofthe thecross cross section theLbL LbL flame retardants deposited wafer.wafer.
Figure shows that the untreated fabric contains relatively low phosphorus element Figure fabric relatively low phosphorus element Figure333shows showsthat thathe theuntreated untreated fabricontains contains relatively low phosphorus element while after treatment, the fabric exhibited an outstanding phosphorus distribution, while after treatment, the treated exhibited an outstanding phosphorus distribuwhile after treatment, the treated fabric exhibited an outstanding phosphorus distribusuggesting successful modification. The The elemental spectra additionally validate the higher tion, successful modification. elemental spectra additionally validate thethe tion,suggesting suggesting successful modification. The elemental spectra additionally validate higher phosphorus content of the treated over the untreated sample indicative of its in-inphosphorus content of the treated over the untreated sample indicative of its increased higher phosphorus content of the treated over the untreated sample indicative of its creased potential. fire-retardant potential. creasedfire-retardant fire-retardant potential.
Figure 3. The upper part of the image shows scanning electron microscopy (SEM) micrographs of Figure 3. The Theupper upperpart partofofthe theimage imageshows showsscanning scanningelectron electron microscopy (SEM) micrographs Figure 3. microscopy (SEM) micrographs of both untreated and and treated cotton fabrics and elemental distribution mapping. The lower part deof both untreated treated cotton fabrics and elemental distribution mapping. The lower both untreated and treated cotton fabrics and elemental distribution mapping. The lower partpart depicts elemental distribution mapping of carbon (C) and (P), with (a) showing the depicts elemental distribution mapping of carbon (C)phosphorus and phosphorus (P),panel with panel (a) showing picts elemental distribution mapping of carbon (C) and phosphorus (P), with panel (a) showing the untreated cotton fabricfabric and panel (b) showing the treated cottoncotton fabric.fabric. the untreated cotton and panel (b) showing the treated untreated cotton fabric and panel (b) showing the treated cotton fabric.
TheFourier Fourier Transform Infrared (FTIR) spectra (Figure 4) presented in thecomimage The Transform Infrared (FTIR) spectra (Figure 4) presented in the image The Fourier Transform Infrared (FTIR) spectra (Figure 4) presented in the image comcompare the characteristic absorption of cotton, pure cotton, pea protein, acid, and pare the characteristic absorption bandsbands of pure pea protein, phyticphytic acid, and the pareflame-retardant-coated the characteristic cotton absorption bands pure cotton, pea protein, phytic acid,(black and the the cotton 12 of bilayers (12BL). The pure cotton spectrum (black flame-retardant-coated withwith 12 bilayers (12BL). The pure cotton spectrum flame-retardant-coated cotton with 12 bilayers (12BL). The pure cotton spectrum (black 1,−1 , line) 3340 cm⁻ line)shows showstypical typicalcellulose cellulosepeaks, peaks,including includinga abroad broadabsorption absorptionband bandaround around 3340 cm 1 line) shows typical cellulose peaks, including a broad absorption band around 3340 cm⁻ − 1 1 which vibrations, and a peak near 2900 cm⁻cm attributed to , whichcorresponds correspondstotoO-H O-Hstretching stretching vibrations, and a peak near 2900 attributed 1 attributed to which corresponds to O-H stretching vibrations, a1370 peak near 2900 − 1 and −1 are 1and 1 are C-H stretching. Additionally, the peaks at 1420atcm⁻ and cm⁻ associated with Cto C-H stretching. Additionally, the peaks 1420 cm 1370 cmcm⁻ associated 1 1 C-H stretching. Additionally, the peaks at 1420 cm⁻ and 1370 cm⁻ are associated with C− 1 1 to the C-O tretching Hwith bending in cellulose, while the while band near 1060 cm⁻ C-H bending in cellulose, the band near corresponds 1060 cm corresponds to the C-O 1 corresponds to the C-O stretching H bending in cellulose, while the band near 1060 cm⁻ instretching cellulose.inIncellulose. the spectrum ofspectrum phytic acid the characteristic P=O stretching In the of (green phytic line), acid (green line), the characteristic P=O in cellulose. In the spectrum of phytic acid (green line), the characteristic P=O stretching 1 1 − 1 − 1 band appears around 1250around cm⁻ , and at 900 stretching, stretching band appears 1250the cmpeak , and thecm⁻ peakisatindicative 900 cm ofis P-O indicative of P-O 1 is indicative of P-O stretching, band appears around the 1250 cm⁻1, and the peak at 900 cm⁻flame confirming the presence of phosphorus-based flame retardants. The pea protein stretching, confirming presence of phosphorus-based retardants. Thespectrum pea protein 1 and 1, which confirming the presence of phosphorus-based retardants. pea1540 protein −The 1 , which (blue line) shows prominent peaks aroundpeaks 1650 flame cm⁻ 1540cm cm⁻ correspond to spectrum (blue line) shows prominent around 1650 and cm−spectrum 1 and 1540 cm⁻1, which correspond to (blue line) shows prominent peaks around 1650 cm⁻ correspond to the amide I and amide II bands, respectively, indicating protein structures. SEM micrographs and elemental mapping of carbon (C) and phosphorus (P) for untreated (a) and treated (b) cotton fabrics are shown in the image.
due to the coating. For instance, the P=O stretching at 1250 cm⁻1 and the amide I and II bands from the protein indicate the successful deposition of the flame-retardant bilayers on the cotton fabric. This shift and reduction in intensity signify effective interaction 7 ofbe17 tween the cotton fabric and the flame-retardant layers.
Figure Figure4. 4. Representative RepresentativeFTIR FTIRspectra spectraof of pure purecotton cotton fabric, fabric, and and flame flame retardant retardantcoated coated(12BL) (12BL)in inthe the presence of pea protein and phytic acid. presence of pea protein and phytic acid.
Sample Phytic acid (PA) Pea Protein (PE) Pure Cotton (PC) 3BL 6BL 9BL 12BL
The 12BL spectrum (red line) reflects the integration of the pea protein and phytic acid 3.2. Thermal Stability with the cotton. The peaks from both phytic acid and pea protein can be seen, with the Generation of the char from decomposed products are critical factors influencing the characteristic peaks of cellulose still present, but slightly shifted or reduced in intensity due flame-retardant properties of polymer-based substances. Therefore, it is crucial to study to the coating. For instance, the P=O stretching at 1250 cm−1 and the amide I and II bands the thermal resistivity of polymer composites. The thermal gravimetric analysis (TGA) from the protein indicate the successful deposition of the flame-retardant bilayers on the data for pristine cotton and its air-free flame-retardant (FR) coatings are outlined in Figure cotton fabric. This shift and reduction in intensity signify effective interaction between the 5 and Table 2. The parameter T5% denotes the temperature at which 5% of the mass is lost cotton fabric and the flame-retardant layers. and serves as an indicator of thermal stability. PristineStability cotton exhibits a primary degradation step occurring between 240 °C and 450 3.2. Thermal °C, with a T5% value of 80 °C.from Afterdecomposed the TGA, there was minimal ash factors residueinfluencing left for pristine Generation of the char products are critical the cotton. In contrast, the 3-bilayer composite, which includes the flame-retardant coating, flame-retardant properties of polymer-based substances. Therefore, it is crucial to study the shows a lower T 5% compared to pristine cotton. This founding attributed to the lower dethermal resistivity of polymer composites. The thermal gravimetric analysis (TGA) data composition temperature of the flame-retardant used coating compositions. increasing infor pristine cotton and its air-free (FR) coatingsHowever, are outlined in Figurethe 5 and corporation of phytic acid pea protein in the 6BL, and higher Table 2. The parameter T5%and denotes the temperature at 9BL, which 5%12BL of theresulted mass isin lost and T 5% values compared to the 3BL composite. Despite these increases, the T5% values of the serves as an indicator of thermal stability. bilayer compounds remain lower than those of pristine cotton. Moreover, the amount of T5% values 9BL demonstrates that it is in optimum point and to prove this result the Table 2. TGAin results. 12BL demonstrated decreases. Residual Yield @ 800 ◦ C Residual Yield @ 800 ◦ C (Under N2 ), wt% (Under Air), wt% T (◦ C) 5%
Figure 5. Thermogravimetric analysis of pure and flame retardant coated cotton cotton fabrics. cotton exhibits a primary degradation step occurring between 240 ◦ C and TablePristine 2. TGA results. Sample Phytic acid (PA) Pea Protein (PE) Pure Cotton (PC) 3BL 6BL 9BL 12BL
450 ◦ C, with a T5% value of 80 ◦ C. After the TGA, there was minimal ash residue left for Residual Yield @ 800 °C (Under pristine cotton. In Yield contrast, the°C 3-bilayer Residual @ 800 (Under composite, N2), wt% which includes the flame-retardant Air), wt% T5% (°C) coating, shows a lower T5% compared to pristine cotton. This founding attributed to the Calculated Experimental lower decomposition temperature of the used coating compositions. However, increasing 155 -61.7 5.0 the incorporation of phytic acid and pea protein in the 6BL, 9BL, and 12BL resulted in 79 -10.3 5.0 higher T5% values compared to the 3BL composite. Despite these increases, the T5% values 80 -2.1 3.78 of the bilayer compounds remain lower than those of pristine cotton. Moreover, the amount 72 10.1 22.4 18.3 of T5% values in 9BL demonstrates that it is in optimum point and to prove this result the 88 13.9 25.3 20.7 12BL demonstrated decreases. 111 16.9 34.7 28.4 This reduced19.5 thermal stability is attributed 95 28.8to the lower thermal stability 23.6 of pea protein and phytic acid, which accelerates the decomposition process within the FR coatings. The addition of thethermal number of bilayers significantly the residual char yield of This reduced stability is attributed to theenhanced lower thermal stability of pea pro◦ C, the char residual yields for 3BL, 6BL, 9BL, and 12BL were the composites. At 800 tein and phytic acid, which accelerates the decomposition process within the FR coatings. 22.4%, 25.3%, of 34.7%, and 28.8%, respectively, which are substantially higher than 2.1% The addition the number of bilayers significantly enhanced the residual char the yield of residual yield of untreated cotton. To validate the effectiveness of the FR coating, theoretical the composites. At 800 °C, the char residual yields for 3BL, 6BL, 9BL, and 12BL were 22.4%, residual34.7%, yieldsand were calculated (Table 2). In allare FRC composites,higher the experimental residual 25.3%, 28.8%, respectively, which substantially than the 2.1% residyields exceeded the calculated values, indicating synergistic effects between the cotton ual yield of untreated cotton. To validate the effectiveness of the FR coating, theoretical fabric, phytic protein, and 2). number of layered-by-layered coatings that were residual yieldsacid, were pea calculated (Table In all FRC composites, the experimental residual previously mentioned. yields exceeded the calculated values, indicating synergistic effects between the cotton
fabric, phytic acid, pea protein, and number of layered-by-layered coatings that were pre3.3. Flammability viously mentioned. UL-94 and LOI analysis are widely used techniques to evaluate the effectiveness of flame-retardant 3.3. Flammabilitycoatings on treated cotton fabrics [38,39]. As indicated in Table 3, the Limiting Oxygen Index (LOI) of the untreated cotton fabric was 19%, demonstrating a severe UL-94 and LOI analysis are widely used techniques to evaluate the effectiveness of fire hazard. In contrast, the LOI value of 3BL reached 23%, representing a 26% increment flame-retardant coatings on treated cotton fabrics [38,39]. As indicated in Table 3, the Limcompared to the untreated cotton fabric. This indicates that the coating transformed the iting Oxygen Index (LOI) of the untreated cotton fabric was 19%, demonstrating a severe fabric from highly flammable to highly flame-retardant, only at three bilayers. fire hazard. In contrast, the LOI value of 3BL reached 23%, representing a 26% increment compared to the untreated cotton fabric. This indicates that the coating transformed the fabric from highly flammable to highly flame-retardant, only at three bilayers. Moreover, according to the digital illustration in Figure 6, the pure cotton fabric immediately ignited upon exposure to fire, with flames rapidly spreading to the top of the sample, resulting in complete destruction. Remarkably, when pristine cotton fabrics were treated with three bilayers, they exhibited an improved ability to form char, resulting in a stable but crumbly residual char; however, it did not pass the UL-94 vertical burning test.
Increasing the bilayers of progressively enhanced the fabric’s resistance to ignition, increased the LOI value, and resulted in formation an intact residual char. Among these, 6BL showed significant difficulty to be ignited, easily secured the vertical burning tests, 9 of 17 and achieved an LOI value higher than 3BL. Notably, this pattern is consistent between 9BL and 6BL, with 9BL exhibiting superior performance. However, as previously mentioned, 9BL represents the optimal configuration. Figure 6 further demonstrates that inTable 3. Flammability assessment outcomes for both untreated and treated textiles. creasing the bilayer to 12BL results in a decline in performance. Sample LOI (%) Damage (mm) wasAfter Flame Time (s) From Figure 6, it is evident that less amountLength cotton fabric damaged after ignition, and the morphological structural of the char residue remained indicates that Pristine Cotton 17 Burn out/damaged char intact. This38 3BL 23 (PA) and pea protein 220 (PE) effectively hindered 15 flame propthe combination of phytic acid 6BL 3 agation, even at low weight26 percentages [31,32]. 75 9BL 29 62 1 12BL 26 85 3 Table 3. Flammability assessment outcomes for both untreated and treated textiles. Sample LOI (%) Damage Length (mm) After Flame Time (s) Moreover, according to the digital illustration in Figure 6, the pure cotton fabric Pristine Cotton 17 Burn out/damaged char 38 immediately ignited upon exposure to fire, with flames rapidly spreading to the top of the 3BL 23 220 15 sample, resulting in complete destruction. Remarkably, when pristine cotton fabrics were 6BL 26 75 3 treated with three bilayers, they exhibited an improved ability to form char, resulting in a 9BL 29 62 1 stable but crumbly residual char; however, it did not pass the UL-94 vertical burning test. 12BL 26 85 3
Figure 6. 6. Digital Digital images images depicting depicting untreated untreated and and fire-retardant-treated fire-retardant-treatedcotton cotton fabrics fabrics following following the the Figure vertical verticalburning burninganalysis. analysis.
Increasing the bilayers 3.4. Cone Calorimetry Test of progressively enhanced the fabric’s resistance to ignition, increased the LOI value, and resulted in formation an intact residual char. Among these, The fire-resistance properties of untreated cotton and coated cotton fabrics treated 6BL showed significant difficulty to be ignited, easily secured the vertical burning tests, with different bilayers (3BL, 6BL, 9BL, and 12BL) of a specific solution were further examand achieved an LOI value higher than 3BL. Notably, this pattern is consistent between 9BL ined using cone calorimetry. The results, depicted in Figure 7 and summarized in Table 4, and 6BL, with 9BL exhibiting superior performance. However, as previously mentioned, include heat release rate (HRR), total heat release rate (THR), and mass-loss data of both 9BL represents the optimal configuration. Figure 6 further demonstrates that increasing the untreated cotton and flame-retardant-coated cotton fabric specimens. It is noteworthy that bilayer to 12BL results in a decline in performance. all pHRR values of the flame-retardant cotton samples were significantly lower compared From Figure 6, it is evident that less amount cotton fabric was damaged after ignition, to the untreated cotton control (250 kW/m2). The pHRR values exhibited a gradual reducand the morphological structural of the char residue remained intact. This indicates tion as the loading of the flame-retardant coating increased. Notably, 9BL demonstrates that the combination of phytic acid (PA) and pea protein (PE) effectively hindered flame the lowest pHRR value of 192 kW/m2, which is 30% lower than that of untreated cotton. propagation, even at low weight percentages [31,32]. The THRs of the flame-retardant-coated cotton fabrics are significantly lower than those of untreated cotton, reduced from 3.89 to 3.55, 3.64, and 1.82 MJ/m2, respectively, indicat3.4. Cone Calorimetry Test ing an effective suppression of total heat release. The decreased THR suggests that a The fire-resistance properties of untreated cotton and coated cotton fabrics treated with different bilayers (3BL, 6BL, 9BL, and 12BL) of a specific solution were further examined using cone calorimetry. The results, depicted in Figure 7 and summarized in Table 4, include heat release rate (HRR), total heat release rate (THR), and mass-loss data of both untreated cotton and flame-retardant-coated cotton fabric specimens. It is noteworthy that all pHRR
values of the flame-retardant cotton samples were significantly lower compared to the untreated cotton control (250 kW/m2 ). The pHRR values exhibited a gradual reduction as the loading of the flame-retardant coating increased. Notably, 9BL demonstrates the lowest pHRR value of 192 kW/m2 , which is 30% lower than that of untreated cotton. The THRs of Polymers 2025, 17, x FOR PEER REVIEW 10 of 17 the flame-retardant-coated cotton fabrics are significantly lower than those of untreated cotton, reduced from 3.89 to 3.55, 3.64, and 1.82 MJ/m2 , respectively, indicating an effective suppression of total heat release. compounds The decreased THR suggests that a greater amount of greater amount of carbonaceous remained in the condensed phase, potencarbonaceous compounds remained in the condensed phase, potentially attributed to the tially attributed to the strong synergistic effect of the bilayers’ number. This effect leads to strong synergistic effectofofcombustible the bilayers’organic number. This effect a reduced conversion of a reduced conversion volatiles intoleads fuel. to The inclusion of low percombustible organic volatiles into fuel. The inclusion of low percentage of phytic acid and centage of phytic acid and pea protein as the flame-retardant system leads in a substantial pea proteininasformation the flame-retardant leads in aproduction, substantial increment in formation of increment of residual system char, less smoke and more fire safety. Forresidual char, less smoke production, and more fire safety. Formation of residual char and mation of residual char and THR were in agreement with the findings rom thermogravTHR were in agreement the7 findings from thermogravimetric analysis (TGA). Figure imetric analysis (TGA). with Figure shows that the flame-retardant coated cotton fabrics ig-7 shows that the flame-retardant coated cotton fabrics ignited slightly quicker than untreated nited slightly quicker than untreated cotton because of the rapid decomposition of the cotton because ofcoating. the rapid decomposition coating. cotton The residual flame-retardant The residual char of of the the flame-retardant flame-retardant-coated fabricschar are of the flame-retardant-coated cotton fabrics are significantly higher compared to untreated significantly higher compared to untreated cotton (Table 4). The smoke emission paramcotton (Table 4).the The smoke emission rate parameters, including smoke production rate eters, including smoke production (TSR), total smoke the production (TSP), CO pro(TSR), total smoke production (TSP), CO production, and CO/CO proportion are also duction, and CO/CO2 proportion are also summarized in Table 4. The2TSR and TSP results summarized in Table 4. Thesignificant TSR and TSP resultsinofcomparison the 9BL sample of the 9BL sample showed reduction with showed those of significant untreated reduction in comparison with those of untreated and treated cotton fabric. and treated cotton fabric.
Figure of of thethe pure andand flame-retardant treated cotton fabrics collected Figure 7. 7. (a) (a)HRR HRRand and(b) (b)THR THRfigures figures pure flame-retardant treated cotton fabrics colfrom cone test. test. lectedthe from thecalorimetry cone calorimetry Table Table4. 4. The The results results for for both both pure pure and and treated treated cotton cotton fabrics fabrics from from cone cone colometry colometry test. test. Sample Sample Pure Cotton Cotton Pure 3BL 3BL 6BL 6BL 9BL 9BL 12BL 12BL
pHRR THR CO THR FIGRA FIGRA Residual Residual TSR TSR TSPTSP MARHE MARHE CO pHRR 2) 2) 2 ·s) 2 /m2 ) 2) 2) (kW/m (MJ/m (kW/m Mass (%) (m (m (kW/m (kg/kg) 2 2 2 2 2 2 2 (MJ/m ) (kW/m ⋅s) Mass (%) (m /m ) (m ) (kW/m ) (kg/kg) (kW/m ) 250250 3.893.89 6.256.25 2.6 2.6 42.5 42.5 0.600.60 55.5 0.200 55.5 0.200 230230 3.553.55 5.855.85 35.2 35.2 68.8 68.8 0.580.58 60.2 0.044 60.2 0.044 241 3.64 6.42 38.6 67.1 0.62 66.2 0.033 241 3.64 6.42 38.6 67.1 0.62 66.2 0.033 192 1.82 5.02 41.8 13.0 0.17 50.9 0.034 192 1.82 5.02 41.8 13.0 0.17 50.9 0.034 255 3.20 6.80 37.4 44.3 0.49 59.5 0.045
CO/CO 22 CO/CO 0.0128 0.0128 0.0354 0.0354 0.0304 0.0304 0.0270 0.0270 0.0326 0.0326
Raman spectroscopy is a widely used and effective technique for evaluating the degree Raman spectroscopy is a widely used and effective technique for evaluating the deof graphitization in residual carbon char, which is closely related to to thethe flame-retardant gree of graphitization in residual carbon char, which is closely related flame-retardproperties. In this study, Raman spectroscopy analyze the the ant properties. In this study, Raman spectroscopy(Figure (Figure8)8)was wasemployed employed to to analyze char residues of both untreated cotton fabric and the flame-retardant-coated samples. The char residues of both untreated cotton fabric and the flame-retardant-coated samples. The residual char obtained obtainedafter afterthe thecone conetests tests were utilized characterization. The residual char were utilized forfor thisthis characterization. The Ra−1 and Raman spectra of the char residues exhibited couple of absorption peaks at 1360 cm man spectra of the char residues exhibited couple of absorption peaks at 1360 cm⁻1 and 1568 cm⁻1, known as the D and G bands, respectively. The ratio of the integrated intensity of these two bands (ID/IG) was used to determine the graphitization degree of the residual chars [40]. The D peak (disorder band) indicates the presence of disorder in the carbon planar structure due to defects or functional groups. The G peak (graphite band) arises
fewer defects at this processing stage. In general, highly graphitized carbon materials have better thermal stability, meaning they retain their structure at high temperatures. In flame retardant applications, the formation of a stable char layer is crucial, as it shields 11 of 17 the material from further combustion. Such materials can form more stable, less flammable char layers. The 9BL sample, with the highest ID/IG ratio (0.82), shows the most disorder, but still maintains good flame retardant properties. The higher D peak in 9BL may aid in better 1568 cm−1 , known as the D and G bands, respectively. The ratio of the integrated intensity char formation during combustion. For instance, in some studies, alongside an increase in the of these two bands (ID /IG ) was used to determine the graphitization degree of the residual ID/IG ratio, a significant increase in the LOI values is observed, suggesting improved perforchars [40]. The D peak (disorder band) indicates the presence of disorder in the carbon mance due to its structural defects. These defects could promote cross-linking reactions, formplanar structure due to defects or functional groups. The G peak (graphite band) arises ing a protective char layer that insulates the material, reduces heat release, and lowers flamfrom the E2g mode of graphite, and involves vibrations of sp2 -bonded carbon atoms in a mability. The strong char formed in 9BL could result from both structural features and flame 2D-hexagonal lattice. retardants, creating a more cohesive heat barrier [47–49].
Figure Figure8.8.Raman Ramanspectra spectraanalysis analysisofofthe theresidual residualchar charofofpristine pristinecotton. cotton.
The base asample hasunderstanding the lowest ID /Iof ratio, showing less disorder compared to the G the To gain thorough flame-retardant mechanism, we conducted layer-by-layer flame-retardant-treated samples. As the number of layers increases, the SEM and EDX spectroscopy characterization of the residual chars on the flame-retardant IDcoated /IG ratios risefabric. from 0.78 to 0.82 for 3BL to 9BL, indicating more disorder cotton or defect density. cotton Figure 9 presents micrographs of both untreated fabric and For the 12BL sample, a slight decrease in the I /I ratio to 0.79 suggests stabilization or D tests G flame-retardant-coated specimens after the cone at two different amplifications. The fewer defects atrevealed this processing In general, carbonprotective materials have observations that in stage. the case of 3BL, highly a densegraphitized and continuous layer better thermal stability, meaning they retain their structure at high temperatures. uniformly covered the surface of the cotton fibers. In contrast, the char residue of the unIn flame applications, formation of aunstable. stable char layer is crucial, as it treated cottonretardant fabric appeared fluffy,the fragile, thin, and shields the material from further combustion. Such materials can form more stable, less flammable char layers. The 9BL sample, with the highest ID /IG ratio (0.82), shows the most disorder, but still maintains good flame retardant properties. The higher D peak in 9BL may aid in better char formation during combustion. For instance, in some studies, alongside an increase in the ID /IG ratio, a significant increase in the LOI values is observed, suggesting improved performance due to its structural defects. These defects could promote crosslinking reactions, forming a protective char layer that insulates the material, reduces heat release, and lowers flammability. The strong char formed in 9BL could result from both structural features and flame retardants, creating a more cohesive heat barrier [47–49]. To gain a thorough understanding of the flame-retardant mechanism, we conducted SEM and EDX spectroscopy characterization of the residual chars on the flame-retardant coated cotton fabric. Figure 9 presents micrographs of both untreated cotton fabric and flame-retardant-coated specimens after the cone tests at two different amplifications. The observations revealed that in the case of 3BL, a dense and continuous protective layer uniformly covered the surface of the cotton fibers. In contrast, the char residue of the untreated cotton fabric appeared fluffy, fragile, thin, and unstable.
Polymers Polymers2025, 2025,17, 17,945 x FOR PEER REVIEW 12 of of 17 12
Figure 9. SEM micrographs and digital images of the residual chars for pure cotton, 3BL, 6BL, 9BL, and 12BL after the cone calorimetry test.
Additionally, the weft–warp structure of the FR-coated samples remained intact, similar to the specimen prior the cone test. The EDX spectroscopy results showed a uniform dispersion of phosphorus (P) element within the char layer (Figure 10), indicating the effective action of the flame-retardant coating in promoting condensation. It is hypothesized that phosphoric acids derived from phytic could catalyze cotton fabric bi- 3BL, 6BL, 9BL, Figure9.9.SEM SEMmicrographs micrographs and digitalacid images of the the residualthe chars Figure and digital images of residual chars for pure cotton, layers. The combination of these observations that the formed char layer acts as and12BL 12BLafter after thecone cone calorimetrysuggests test. and the calorimetry test. an efficient insulation coating, preventing oxygen and heat transferal to the substrate. the of samples intact, The reaction ofAdditionally, phytic acid and pea protein structure during combustion was considered in Additionally, the weft–warp weft–warp structure of the the FR-coated FR-coated samples remained similar totothe prior thethe cone test. The EDX spectroscopy results showed a uniform order to propose the flame-retardant mechanism of the coatings. The reactions are among similar thespecimen specimen prior cone test. The EDX spectroscopy results showed a unidispersion of phosphorus (P) element within the char layer (Figure 10), indicating the dispersion of phosphorus (P)a element within the char layer (Figure the pea proteinform and the phytic acid, particularly crosslinking network between amino 10), indicating effective action of theoflame-retardant coating in promoting condensation. It is hypothesized the effective action the flame-retardant coating promoting condensation. It is hypothgroups in pea protein and phosphate groups from phytic acid as in a dominant reaction (Figthat phosphoric acids derived from phytic acid could catalyze the cotton fabric bilayers. esizedoccurs that phosphoric acids derived from phytic acid could biure 1). The reaction by phosphorylation of the amino groups (–NHcatalyze 2) of thethe peacotton fabricThe combination of these observations suggests between that the formed layer acts an efficient layers. The combination of these observations suggests that char the formed char acts as protein, and this esterification can result in crosslinking phosphate groups (– aslayer preventing and heat transferal to transferal the substrate. an efficient insulation coating, preventing oxygen and heat to the substrate. PO42−) of phyticinsulation acid and coating, the amine groups inoxygen the protein. The reaction of phytic acid and pea protein during combustion was considered in order to propose the flame-retardant mechanism of the coatings. The reactions are among the pea protein and the phytic acid, particularly a crosslinking network between amino groups in pea protein and phosphate groups from phytic acid as a dominant reaction (Figure 1). The reaction occurs by phosphorylation of the amino groups (–NH2) of the pea protein, and this esterification can result in crosslinking between phosphate groups (– PO42−) of phytic acid and the amine groups in the protein.
Figure 10. Phosphorus mapping (EDAX) of the residual ofresidual the purechars cotton (a), cotton (PC) (a), Figure elemental 10. Phosphorus elemental mapping (EDAX)chars of the of(PC) the pure and 9BL sampleand (b). 9BL sample (b).
reaction of phytic acid and peacoatings, protein during combustion was considered in To explain theThe flame-retardant mechanism of the the combustion interacorder to propose the flame-retardant mechanism of the coatings. The reactions are among tions between phytic acid and pea protein were considered. A key reaction between the thephytic pea protein and theaphytic acid, particularly a crosslinking between amino pea protein and acid involves crosslinking network formed between network the amino groups in pea protein and phosphate groups from phytic acid as a dominant reaction Figure 10. elemental (EDAX) of the residual chars ofgroups the pure(–NH cotton2 )(PC) (a), (Figure 1).Phosphorus The reaction occursmapping by phosphorylation of the amino of the andprotein, 9BL sample pea and(b). this esterification can result in crosslinking between phosphate groups (–PO4 2− ) of phytic acid and the amine groups in the protein. Toexplain explainthe theflame-retardant flame-retardant mechanism of the coatings, combustion interacTo mechanism of the coatings, the the combustion interactions tions between phytic acid and pea protein were considered. A key reaction between the between phytic acid and pea protein were considered. A key reaction between the pea pea protein and phytic acid involves a crosslinking network between thegroups amino protein and phytic acid involves a crosslinking network formedformed between the amino
groups in the pea protein and the phosphate groups from the phytic acid (Figure 11). This in the peaoccurs protein and thethe phosphate groups from theamino phyticgroups acid (Figure reaction reaction through phosphorylation of the (–NH11). 2) inThis the pea prooccurs through the phosphorylation of the amino groups (–NH ) in the pea protein, and 2 tein, and this esterification leads to crosslinking between the phosphate groups (–PO 42−) 2 − this esterification leads to crosslinking between the phosphate groups (–PO ) from phytic 4 from phytic acid and the amine groups in the protein. This type of crosslinking enhances acid and the stability amine groups the improving protein. This of crosslinking enhances thermal the thermal of the in char, firetype retardancy performance [50].the According stability of the char,shown improving fire retardancy performance [50]. cm⁻ According the FTIR 1, whichto to the FTIR results in Figure 11, the amide I band at 1650 represents −1 , which represents the physical results shown in Figure 11, the amide I band at 1650 cm the physical mixture, disappears due to the interaction with the protein’s carbonyl group. mixture, disappears to the interaction with the protein’s carbonyl group. Additionally, 1, which Additionally, some due peaks below 1000 cm⁻ correspond to the individual finger−1 , which correspond to the individual fingerprints of phytic some peaks below 1000 cm prints of phytic acid and pea protein, merged or changed due to new chemical interacacid tions.and pea protein, merged or changed due to new chemical interactions.
Figure Figure 11. 11. Representative Representative FTIR FTIR spectra spectra of of the the pure pure FR FR materials materials used used in in this work, and their physical and chemical chemical mixture. mixture. −1 , associated with the P–O and On the other other hand, hand,peaks peaksaround around1000–1200 1000–1200 cm 1, associated On the cm⁻ with the P–O and P=O P=O vibrations, shifted or intensified, indicating an interaction between the phosphate vibrations, shifted or intensified, indicating an interaction between the phosphate groups groups and the protein side-chains. Additionally, sharper or more distinct peaks in the and the protein side-chains. Additionally, sharper or more distinct peaks in the 3000–3500 −1 region, related to hydrogen bonding, revealed the formation of stronger 3000–3500 cm 1 cm⁻ region, related to hydrogen bonding, revealed the formation of stronger hydrogen hydrogen bonds in themixture. chemicalAs mixture. result, theinteractions chemical interactions between bonds in the chemical a result,As thea chemical between phytic acid phytic acid and pea protein (as seen in the FTIR spectra) lead to a uniform chemical and pea protein (as seen in the FTIR spectra) lead to a uniform chemical blend with strong blend withbonds, strong hydrogen bonds, or ionic phosphate–protein and hydrogen covalent or ionic covalent phosphate–protein connections, andconnections, amide involveamide involvement. It can be concluded that these hydrogen bonds help stabilize the ment. It can be concluded that these hydrogen bonds help stabilize the structure at lower structure at lower The catalyze phosphate catalyze theand formation char temperatures. The temperatures. phosphate groups thegroups formation of char promoteofcrossand promote crosslinking, while the protein’s carbon and nitrogen components contribute linking, while the protein’s carbon and nitrogen components contribute to carbonization, to carbonization, forming acarbonaceous nitrogen-doped carbonaceous network. After combustion, forming a nitrogen-doped network. After combustion, the resulting char the resulting char forms a 3D-network composed of phosphorus-rich and nitrogen-doped forms a 3D-network composed of phosphorus-rich and nitrogen-doped carbon structures carbon (Figureis1).thermally This network thermally stable, mechanically strong, and (Figurestructures 1). This network stable,ismechanically strong, and chemically homochemically homogeneous, reflecting the synergistic interactions seen in the FTIR spectra of geneous, reflecting the synergistic interactions seen in the FTIR spectra of the chemical the chemical mixture [51,52]. mixture [51,52].
3.5. Mechanical Properties 3.5. Mechanical Properties The tensile test results summarized in Figure 12 and Table 5, compared the mechanical The tensile test results summarized in Figure 12 and Table 5, compared the mechanbehavior of PC fabric with flame-retardant-coated cotton fabrics with 3, 6, 9, and 12 bilayers ical behavior of PC fabric with flame-retardant-coated cotton fabrics with 3, 6, 9, and 12 of flame retardant coatings. As illustrated, the stress–strain curves indicate that pure bilayers of flame retardant coatings. As illustrated, the stress–strain curves indicate that cotton fabric (PC) reaches an optimal tensile strength of around 32 MPa with a strain of pure cotton fabric (PC) reaches an optimal tensile strength of around 32 MPa with a strain approximately 21%, reflecting better mechanical integrity. In contrast, the flame-retardantof approximately 21%, reflecting better mechanical integrity. In contrast, the flame-retardcoated fabrics show a decrement in stress and strain as the bilayer count increases. For ant-coated fabrics show a decrement in stress and strain as the bilayer count increases. For example, the 3BL fabric shows a similar trend to pure cotton, but with reduced peak example, the 3BL fabric shows a similar trend to pure cotton, but with reduced peak strength and stretch. As the number of bilayers increases, especially with 6BL, 9BL, and
strength and stretch. As the number of bilayers increases, especially with 6BL, 9BL, and 12BL stress at atfailure failuredemonstrates demonstrateseven evenmore more drops, with 12BL fabric 12BLfabrics, fabrics, the the stress drops, with thethe 12BL fabric disdisplaying the weakest mechanical performance. This suggests that adding more flame playing the weakest mechanical performance. This suggests that adding more flame reretardant bilayers compromises mechanical integrity, possibly due to increased brittleness tardant bilayers compromises mechanical integrity, possibly due to increased brittleness or orstiffness stiffnessfrom fromthe thecoating coatingmaterials. materials.However, However,the the12BL 12BLfabric fabricstill stillachieves achievesstress stresslevels levels above 22 MPa, retaining a substantial portion of its strength. above 22 MPa, retaining a substantial portion of its strength.
Figure Figure12. 12.Stress–strain Stress–straindiagrams diagramsofofpure purecotton cotton(PC) (PC)and andflame-retardant-treated flame-retardant-treatedcotton cottonfabrics. fabrics. Table Table5.5.Tensile Tensiletest testresults resultsofofthe thepure pureand andflame flameretarded retardedcotton cottonfabrics. fabrics.
Sample Sample Pure PureCotton Cotton 3BL 3BL 6BL 6BL 9BL 9BL 12BL 12BL Elongation Elongation at at Break Break (%) (%) 21.6 21.6 19.4 19.4 19.0 19.0 17.5 17.5 16.1 16.1 StressStress (MPa)(MPa) 32.1 32.1 26.6 26.6 23.6 23.6 25.1 25.1 22.4 22.4
4.4.Conclusions Conclusions In Inthis thisstudy, study,we wetried triedtotodemonstrate demonstratethe thesuccessful successfuldevelopment developmentof ofaasustainable, sustainable, highly forfor cotton fabrics using thethe layer-by-layer assembly highlyeffective effectiveflame flameretardant retardantcoating coating cotton fabrics using layer-by-layer assemtechnique. By incorporating bio-based materials such asaspea bly technique. By incorporating bio-based materials such peaprotein proteinand andphytic phyticacid, acid, this thiscoating coatingenhances enhancesthe theflame flameresistance resistanceof ofcotton cottonfabrics fabricswhile whilemaintaining maintainingaabalance balance between betweenmechanical mechanicalintegrity integrityand andfire fireprotection. protection.The Theflame flameretardant retardantcoating coatingexhibited exhibited great greatdecrement decrementon onthe theflammability flammabilityof ofthe thecotton cottonfabric fabricduring duringthe thevertical verticalburning burningtest, test, while showed 70% increment to the LOI values of the 9BL samples compare to the pure while showed 70% increment to the LOI values of the 9BL samples compare to the pure cotton results revealed a 30% reduction in the heat heat release cottonfabric. fabric.The Thecone conecalorimeter calorimeter results revealed a 30% reduction in peak the peak rerate (pHRR), 46% reduction in the total heat release rate (THR), and 30% reduction in the lease rate (pHRR), 46% reduction in the total heat release rate (THR), and 30% reduction total smoke (TSP) for(TSP) the 9-bilayer (9BL) coated fabric compared to the untreated in the total production smoke production for the 9-bilayer (9BL) coated fabric compared to the fabric, while the residual char increased significantly. Thermogravimetric analysis (TGA) untreated fabric, while the residual char increased significantly. Thermogravimetric analfurther showed a substantial with the 9BLwith fabric 34.7% chara ysis (TGA) further showed aimprovement, substantial improvement, theexhibiting 9BL fabrica exhibiting ◦ C, compared to just 2.1% for pure cotton. While the mechanical properties yield at 800 34.7% char yield at 800 °C, compared to just 2.1% for pure cotton. While the mechanical of the fabrics gradually the increasing number of bilayers, the 9-bilayer properties of the fabrics decrease graduallywith decrease with the increasing number of bilayers, the 9configuration offers an optimal trade-off, delivering substantial flame retardancy without bilayer configuration offers an optimal trade-off, delivering substantial flame retardancy severely strength. strength. These findings highlighthighlight the potential of eco-friendly without compromising severely compromising These findings the potential of eco- Polymers 2025, 17, 945
flame retardant coatings to enhance the safety of cotton fabrics, opening new possibilities for their application in industries where fire protection is critical.