Bio-Based Flame Retardant Superhydrophobic Coatings by Phytic Acid/Polyethyleneimine Layer-by-Layer Assembly on Nylon/Cotton Blend Fabrics

Abstract

The inherent flammability and hydrophilicity of nylon/cotton (NC) blend fabrics limit their practical applications. Traditional hydrophobic treatments often involve fluorinated compounds or nanomaterials, which raise environmental concerns and exhibit poor durability. To address these issues, this study developed a sustainable multifunctional finishing strategy. Initially, the nylon/cotton blended fabric was pretreated with 3-glycidyloxypropyltrimethoxy silane (GPTMS). An intumescent flame retardant coating based on bio-derived phytic acid (PA) and polyethyleneimine (PEI) was constructed on NC fabrics via a layer-by-layer (LBL) self-assembly process. Subsequently, polydimethylsiloxane (PDMS) was grafted to reduce surface energy, imparting synergistic flame retardancy and superhydrophobicity. The treated fabric (C-3) showed excellent flame retardant and self-extinguishing behavior, with no afterflame or afterglow during vertical burning and a char length of only 35 mm. Thermogravimetric analysis revealed a residual char rate of 43.9%, far exceeding that of untreated fabric (8.6%). After PDMS modification, the fabric reached a water contact angle of 157.8°, indicating superior superhydrophobic and self-cleaning properties. Durability tests showed that the fabric maintained its flame retardancy (no afterflame or afterglow) and superhydrophobicity (WCA > 150°) after 360 cm of abrasion and five laundering cycles. This fluorine-free, nanoparticle-free, and environmentally friendly approach offers a promising route for developing multifunctional NC fabrics for applications in firefighting clothing and self-cleaning textiles.

1. Introduction

Nylon and cotton fibers, as two important textile materials, are widely used in various fields such as apparel, home textiles, and technical textiles due to their outstanding properties. Nylon is renowned for its high strength, abrasion resistance, and good elasticity [1], while cotton is favored for its moisture absorption, breathability, softness, comfort, and natural environmental friendliness [2]. Nylon/cotton blended fabrics combine the advantages of both fibers, maintaining the comfort of cotton while enhancing the fabric’s strength and abrasion resistance. As a result, they have had promising applications in areas such as interior decoration, outdoor sportswear, and industrial applications in recent years [3]. However, traditional nylon/cotton blend fabrics still have functional limitations, with flammability and fire risks restricting their widespread use in many fields [4,5,6]. Therefore, improving the flame retardancy of nylon/cotton blend fabrics is crucial [7,8].

At present, surface modification has become one of the most convenient and practical strategies for flame retardant modification of textiles, as it can largely maintain the properties of bulk substrates [9]. Surface modification techniques include layer-by-layer assembly (LBL) [10], spraying methods [11], sol–gel methods [12], fabric immersion [13], and chemical grafting methods [14]. Among them, the LBL method is favored due to its simple operation, mild conditions, and controllable number of layers [15]. Currently, the most commonly used flame retardants for textiles are halogenated and phosphorus-based. Although halogenated flame retardants are highly effective, they release toxic gases during combustion [16], which limits their use. In response to the green sustainability initiative, there is an urgent need to develop halogen-free, environmentally friendly flame retardants [17,18,19,20]. Phosphorus-based flame retardants are widely used due to their high efficiency and low toxicity [21]. Phytic acid (PA), as a natural phosphorus-rich compound, has attracted attention in textile flame retardant research [22,23]. During thermal decomposition, phytic acid (PA) generates phosphoric acid derivatives that serve as acid sources to catalyze the dehydration and carbonization of the fabric, thereby suppressing flame propagation [24]. Polyethyleneimine (PEI) is a water-soluble polymer with high chemical reactivity. It can react with hydroxyl groups in cellulose and exhibits flame retardant effects in both the gas phase and condensed phase, making it widely applicable in flame retardant systems [25]. Olga Zilke et al. [26] deposited bilayers of phytic acid and polyvinylamine on cotton fabrics via a layer-by-layer assembly technique. The treated fabrics exhibited excellent flame retardant properties, with no afterflame or afterglow time. Furthermore, the lower carbon monoxide, formaldehyde, methanol, and acrolein release, shown by TGA-FTIR, proves that the flame retardant coatings also reduce the toxicity of the pyrolysis gases. Zheng et al. [27] improved the flame retardant of cotton fabrics by depositing a bio-based flame retardant coating consisting of laccase and PA via a layer-by-layer self-assembly. The modified cotton fabric displayed a self-extinguishing phenomenon upon removing the fire during the vertical burning tests. The limiting oxygen index (LOI) value of the modified fabric was largely elevated to 43%, and the final carbon residue content reached 39%. Cheng et al. [28] prepared flame retardant coating containing phytic acid (PA), chitosan (CH), and biochar (BC) via the layer-by-layer assembly. The peak heat release rate (pkHRR) and total heat release rate (THR) of PA/CH/BC-coated cotton fabric were significantly reduced compared with those of untreated cotton fabric, with the LOI value increasing from 18.6% to 64.1%. This indicates that the LBL method can effectively integrate various functional materials to construct a uniform flame retardant coating on the fabric surface [29]. Compared to traditional flame retardants, PA is more environmentally and human-friendly and can promote dehydration and carbonization reactions at high temperatures, enhancing flame retardant performance [30,31,32].

Although the flammability of the fabric has been addressed, its hydrophilicity and susceptibility to contamination remain significant drawbacks [33]. These issues not only affect comfort but also increase cleaning difficulty and cost, thus impacting the service life and applicability of nylon/cotton blended fabrics [34,35]. Therefore, developing multifunctional fabrics with both flame retardant and superhydrophobic properties is of great significance. Gao et al. [36] improved the superhydrophobicity, flame retardancy, and conductivity of cotton fabric by spraying octadecylamine modified carboxylated carbon nanotubes (CNT-ODA)/polydimethylsiloxane (PDMS) and then depositing polyethyleneimine modified halloysite nanotubes (P-HNTs) and phytic acid (PA) using the LBL method. The obtained multifunctional cotton fabric exhibited superhydrophobicity (WCA = 162°) and excellent surface stability. Li et al. [37] prepared flame retardant, antibacterial, and superhydrophobic PET fabric using coating technology. The multifunctional PET fabric exhibited an LOI value of 29.0%, with a water contact angle (WCA) greater than 150°, and showed excellent self-cleaning and antifouling properties. However, fluorine-based modifiers pose environmental and health risks [38], while nanoparticles (such as carbon nanotubes, TiO₂, and SiO₂) are limited in their application due to poor adhesion and insufficient durability. Therefore, developing nylon/cotton fabrics modified with fluorine-free, nanoparticle-free, and environmentally friendly agents, which combine flame retardancy and superhydrophobicity, holds significant research value and application potential, and this will greatly fulfill the requirements for broader applications [39,40].

This study constructs a composite flame retardant system of phytic acid (PA) and polyethyleneimine (PEI) based on layer-by-layer assembly (LBL) technology. Initially, 3-glycidyloxypropyltrimethoxy silane (GPTMS) was employed as a covalent coupling agent between the phytic acid/polyethyleneimine (PA/PEI) flame retardant coating and the cotton cellulose (NC) fabric. GPTMS grafts onto the cotton cellulose surface via Si–O–C covalent bonds, thereby significantly enhancing the wash durability of the functionalized NC fabric. Through electrostatic adsorption and hydrogen bonding interactions, nanometer-scale assembly is achieved on the surface of NC fabric, followed by surface modification with polydimethylsiloxane (PDMS) to obtain multifunctional composite textiles. The flame retardancy and superhydrophobic properties of the fabric before and after modification were evaluated using a vertical burning tester and contact angle measurement system, confirming the feasibility of the composite modification treatment. Using multidimensional characterization techniques, including field emission scanning electron microscopy (FE-SEM), Fourier-transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and a series of durability tests, the effects of the treatment process on the substrate’s microstructure, chemical composition, structure, thermal stability, and service durability were thoroughly investigated. This study provides a theoretical foundation and technical reference for the development of high-end textiles, such as intelligent protective clothing and self-cleaning materials, by employing a fluorine-free and nanoparticle-free environmentally friendly fabrication approach.

2. Materials and Methods

2.1. Materials

Nylon/cotton (NC) blend fabric (40/60, 150 g/m²) was purchased from Dongguan Jisheng Textile Co., Ltd. (Dongguan, China); 3-glycidoxypropyltrimethoxy silane (97%, GPTMS), phytic acid (50%, PA), and polyethyleneimine (M.W. 10,000, PEI) were obtained from Macklin Chemicals Co., Ltd. (Shanghai, China); polydimethylsiloxane and curing agent (Sylgard184, PDMS) were purchased from Dow Corning Silicone Co., Ltd. (Dongguan, China).

2.2. Preparation of Flame Retardant Superhydrophobic NC Fabrics

2.2.1. Pretreatment of NC Fabrics

First, the NC fabrics were cut into 300 × 90 mm strips. Then, the NC fabric strips were ultrasonicated in anhydrous ethanol for 30 min to remove the grease attached to the NC fabrics, then dried in an oven at 80 °C for 1 h. Subsequently, the fabrics were ultrasonically cleaned in deionized water for 30 min to eliminate water-soluble contaminants. The wet fabrics were then dried in an oven at 80 °C for 1 h. The dried fabrics were immersed in 3-glycidoxypropyltrimethoxy silane (GPTMS) solution (pH 4–5, adjusted by 1 mol/L HCl, concentration of 2.5 wt%) with a material to liquor ratio of 1:40 at 65 °C for 4 h. During this process, GPTMS underwent hydrolysis and condensation to form covalent bonds with the fabric via siloxane linkages, thereby enhancing wash durability [41]. Finally, the treated fabrics were dried at 80 °Cand stored for further use.

2.2.2. Preparation of Flame Retardant Fabrics

Preparation of PA and PEI Solutions

A certain amount of polyethyleneimine (PEI) was weighed and dissolved in distilled water to obtain a 6.0 wt% PEI solution. Appropriate amounts of phytic acid (PA) were placed into a 500 mL beaker and diluted with distilled water to prepare PA solutions with mass fractions of 5.0 wt%, 5.5 wt%, 6.0 wt%, and 6.5 wt%, respectively.

Preparation of Flame Retardant NC Fabrics

To balance the flame retardancy and wearability of the NC fabric, a flame retardant coating with a concentration gradient structure is constructed by regulating the layer-by-layer (LBL) assembly process. The NC fabric was first immersed in a 6.0 wt% PEI solution for 15 min, followed by drying at 80 °C. It was then immersed in PA solutions of varying concentrations for 15 min and again dried at 80 °C. This process completed one layer of PA/PEI assembly. The same procedure was repeated four times for each sample. After completing four cycles of layer-by-layer (LBL) assembly, the fabrics were rinsed with deionized water for 2 min and dried at 80 °C. During the LBL assembly process, the concentration of PEI was kept constant, while the concentrations of PA varied as shown in

Table 1. Concentrations of PA/PEI solutions for layer-by-layer (LBL) self-assembly.

Sample	PEI (wt%)	PA (wt%)
C-0	--	--
C-1	6.0%	5.0%
C-2	6.0%	5.5%
C-3	6.0%	6.0%
C-4	6.0%	6.5%

. The fabrics treated with different PA concentrations were labeled as C-1, C-2, C-3, and C-4, respectively, while the untreated NC fabric was labeled as C-0.

2.2.3. Preparation of Flame Retardant Superhydrophobic Fabrics

The flame retardant fabric obtained by the layer-by-layer (LBL) assembly method was selected, and the best-performing sample in terms of flame retardancy was subjected to hydrophobic modification. The detailed preparation process is as follows: 2 g PDMS prepolymer and 0.20 g curing agent (weight ratio: 10:1) were dissolved in 50 mL ethyl acetate. Then, the flame retardant NC fabrics were soaked in the solution for 10 min. Finally, the samples were cured and dried in an oven at 100 °C for 15 min and labeled as P@C-3. Figure 1 shows the detailed preparation process.

Meanwhile, to investigate the individual flame retardant effects of PA and PEI on the fabric, the optimal concentration of PA/PEI with the best flame retardant performance is selected for separate treatment. The NC fabric is immersed in each solution for 15 min, then removed and dried at 80 °C. This process is repeated four times. Subsequently, the samples are modified with PDMS to achieve superhydrophobicity and are designated as C(PA) and C(PEI), respectively. As a control, the NC fabric is treated only with PDMS and labeled as C(PDMS).

2.3. Characterizations

Scanning electron microscopy (SEM, SU8600, Hitachi High-Technologies, Shanghai, China) was employed to analyze the surface morphology of the NC fabrics before and after flame retardant treatment and combustion. The samples were mounted on the test platform using black conductive adhesive tape and sputter-coated with gold for 60 s before observation.

Fourier-transform infrared spectroscopy (FTIR, VERTEX 70 RAMI, BRUKER OPTICS, Beijing, China) was employed to characterize the chemical structures of the fabrics before and after treatment. The samples were individually mixed with solid potassium bromide (KBr) at a mass ratio of 1:100, pressed into pellets using the KBr pellet method, and analyzed over a spectral range of 4000–400 cm⁻¹.

A contact angle goniometer (SCI4000, Beijing Global Hengda Technology Co., Ltd., Beijing, China) was used to measure the water contact angle of NC fabrics before and after superhydrophobic treatment to evaluate their wettability. A 5 µL droplet of ultrapure water was placed on the surface of the treated samples, and the contact angle was recorded.

The add-on of coated NC fabrics was calculated as follows:

$$addon\left(\%\right)={\displaystyle \frac{W-W_0}{W_0}}\times 100\%$$

(1)

where W₀ is the weight of the uncoated NC fabric and W is the weight of NC fabrics after different treatments.

Thermogravimetric analysis (TGA) was conducted using a thermogravimetric/differential thermal analyzer (HITACHI STA7300, Hitachi, Tokyo, Japan) to assess the thermal stability of the treated fabrics. A 4 mg sample was weighed and placed in a crucible, then heated from room temperature to 800 °C at a rate of 10 °C/min under a nitrogen atmosphere with a flow rate of 10 mL/min.

The vertical flammability test was conducted using a UL-94 horizontal and vertical burning tester (Dongguan Zhongnuo Quality Inspection Instrument and Equipment Co., Ltd., Dongguan, China) in accordance with GB/T 5455—2014, “Textiles—Burning behavior—Determination of damage length, afterglow time, and afterflame time in the vertical direction.” Fabric samples measured 300 × 90 mm, and the flame height was maintained at 20 ± 2 mm.

The softness of the NC blended fabric samples before and after flame retardant treatment is characterized using the ring method (IS 7016 11). Fabric samples measuring 160 mm × 40 mm are folded end-to-end into a ring shape, placed on a horizontal surface, and photographed. The test results indicate that a lower ring height corresponds to better fabric softness.

Durability was evaluated via a washing resistance test following the AATCC 61-2006 standard [42]. The treated samples were washed using AATCC standard detergent in a magnetic stirrer setup, with each washing cycle lasting 30 min. After each cycle, the fabrics were rinsed with deionized water and dried in an oven at 80 °C. This procedure was repeated for five cycles. The superhydrophobicity and flame retardant performance after washing were measured to assess durability.

3. Results

3.1. Surface Morphology Analysis

The surface morphologies of the untreated fabric (C-0), the flame retardant treated NC fabric (C-3), and the flame retardant superhydrophobic treated NC fabric (P@C-3) were observed by SEM, and relevant images are shown in Figure 2. The surface of the untreated fabric exhibited a relatively smooth and flat morphology with clearly defined fiber structures (Figure 2a). After treatment with PA and PEI, a continuous and uniform coating formed on the fiber surface through electrostatic interactions between the positively charged PEI and negatively charged PA (Figure 2b). Following further modification with PDMS, the P@C-3 fabric (Figure 2c) maintained an intact fibrous morphology without significant damage to the underlying PA/PEI coating. Additionally, the presence of a smooth silicone layer on the surface confirmed that the dual-functional flame retardant and superhydrophobic coating was successfully constructed via the layer-by-layer assembly process. The PDMS film was stably deposited on the surface of the fabric fibers.

Furthermore, elemental mapping was employed to further investigate the surface elemental distribution of the fabric after flame retardant and superhydrophobic treatments. As shown in Figure 3, elements P and Si were distinctly detected on the surface of the P@C-3 sample. Moreover, elements including Si, P, N, O, and C were uniformly distributed across the fabric surface without noticeable aggregation, confirming that the flame retardant and superhydrophobic coatings were evenly deposited on the treated NC fabric.

3.2. FTIR Analysis

Fourier-transform infrared (FTIR) spectroscopy was used to investigate the chemical structure variations of the NC fabric (C-0), the flame retardant treated NC fabric (C-3), and the flame retardant superhydrophobic treated NC fabric (P@C-3). As shown in Figure 4, the broad peak at 3297 cm⁻¹ observed for the C-0 fabric is attributed to the stretching vibration of N–H bonds in nylon. The maximum absorption at 3082 cm⁻¹ corresponds to the amide II band, which arises from the overlapping bending and stretching vibrations of N–H and C–N bonds [43], respectively. Additionally, the peak at 1640 cm⁻¹ corresponds to the C=O stretching vibration of the amide groups in nylon.

Compared to C-0, the spectra of C-3 and P@C-3 exhibit a characteristic N-H stretching vibration peak at 3378 cm⁻¹, confirming the successful incorporation of PEI [44]. Additionally, the presence of a P=O stretching vibration peak at 1295 cm⁻¹ indicates the successful introduction of PA [45], which interacts with the hydroxyl groups on the fabric surface through hydrogen bonding or esterification. PEI, as a polymer containing abundant -NH₂ and -NH- functional groups, interacts electrostatically with the negatively charged PA molecules. Due to the opposite charges of PA and PEI, electrostatic attraction facilitates their alternate deposition, forming a layer-by-layer assembled structure. Moreover, hydrogen bonding between the -OH groups of PA and the -NH₂ groups of PEI further strengthens the interlayer interactions.

Furthermore, after GPTMS treatment, the FTIR spectrum of the C-3 fabric exhibits characteristic peaks at 1100 cm⁻¹ and 1041 cm⁻¹, corresponding to Si-O-C and Si-O stretching vibrations, respectively [46]. These peaks confirm the hydrolysis and condensation reactions between GPTMS and the fabric, forming covalent Si-O-C bonds that enhance the washing durability of the coating. In the FTIR spectrum of the P@C-3 fabric, the intensified absorption peaks at 1100 cm⁻¹ and 1041 cm⁻¹, compared to those in C-3, are primarily attributed to enhanced Si-O-C and Si-O stretching vibrations from the PDMS structure. The vibration peak observed at 941 cm⁻¹ is likely the result of overlapping characteristic bands associated with unreacted vinyl and Si–H bonds in PDMS, indicating that the presence of amino groups partially inhibits the crosslinking process, which further confirms the successful deposition of PDMS on the fabric surface. These results collectively demonstrate that the flame retardant and superhydrophobic coating was successfully constructed on the surface of the fabric.

3.3. Thermal Stability Property Analysis

The thermal stability of the NC fabric samples, including the untreated fabric (C-0), the flame retardant treated fabrics (C-1 to C-4), the flame retardant superhydrophobic fabric (P@C-3), and the individually treated fabrics, was analyzed by thermogravimetric analysis (TG) and derivative thermogravimetry (DTG). The corresponding TG and DTG curves under a nitrogen atmosphere are presented in Figure 5a–d, respectively, with relevant data summarized in

Table 2. Thermal gravimetric analysis data of different samples under N₂ atmosphere.

Sample	T-10%/°C	T-50%/°C	T-max/%	Vmax (%/min)	Residue at 800 °C (wt%)
C-0	348	390	378	1.269	8.6
C-1	278	407	329	0.608	37.9
C-2	274	456	328	0.283	41.1
C-3	282	458	327	0.356	43.9
C-4	281	445	328	0.412	39.9
P@C-3	277	425	324	0.452	35.1
C(PA)	356	405	382	1.008	15.2
C(PEI)	307	404	435	0.828	10.3

T_-10%, T_-50%, and T_-max correspond to temperatures with mass loss rates of 10% and 50% and a maximum mass loss rate, respectively.

. According to Figure 5a,b, the C-0 fabric exhibited slight weight loss below 200 °C, primarily due to the evaporation of absorbed moisture. The major thermal degradation occurred between 321 °C and 456 °C, with a final char residue of only 8.6% remaining at 800 °C. The DTG curve indicates that the C-0 fabric reached its maximum weight loss rate at approximately 376 °C.

After PA/PEI treatment, the thermal degradation curves of fabrics C-1 to C-4 exhibited a trend similar to that of the untreated fabric C-0. Meanwhile, with increasing PA concentration, the flame retardant efficiency improved. When the PA concentration reached 6%, the char residue was maximized. However, further increasing the PA concentration to 6.5% resulted in a reduced char residue in C-4 compared to C-3, indicating saturation of the flame retardant loading. Taking C-3 as a representative example, moisture evaporation occurred in the range of 88–212 °C due to heat exposure. Between 212 °C and 484 °C, thermal decomposition of the PA/PEI coating generated small molecules and gaseous products, resulting in mass loss [47]; this stage facilitated the formation of an expanded char layer. In the temperature range of 484–800 °C, the PA/PEI coating further promoted char formation and foaming, contributing to the overall thermal stability of the system.

Although the char residue of the flame retardant treated samples (C-1 to C-4) significantly increases at 800 °C, with the C-3 sample exhibiting the highest char yield of 43.9%, both their initial decomposition temperature and the temperature corresponding to the maximum weight loss rate are lower than those of the untreated fabric (C-0). This phenomenon is primarily attributed to the early thermal decomposition of PA and PEI at lower temperatures [48]. Nevertheless, the compact char layer formed by the PA/PEI flame retardant system effectively suppresses the generation of volatile and flammable degradation products at elevated temperatures, thereby markedly enhancing the flame retardant performance of the fabric.

The thermal stability of fabrics treated with PA alone, PEI alone, and the composite-treated sample P@C-3 is presented in Figure 5c,d. The char yield of the PA-treated fabric reaches 15.2%, which is higher than that of the PEI-treated counterpart (10.3%). This indicates that PA, acting as an acid source, thermally decomposes to produce phosphoric or polyphosphoric acid, which catalyzes the dehydration of the NC fabric and facilitates the formation of an intumescent char layer, thereby providing an effective barrier protection [22]. Moreover, the P@C-3 sample achieves a char yield of 35.1% at 800 °C, which is significantly higher than that of the untreated C-0 and the single-component treated samples, demonstrating that P@C-3 maintains a stable char layer at elevated temperatures. The results demonstrate that the combination of nitrogen-rich PEI and phosphorus-containing PA produces a pronounced phosphorus–nitrogen (P–N) synergistic effect, which effectively inhibits the thermal degradation of NC fabrics at elevated temperatures. The resulting intumescent flame retardant system exhibits both gas-phase and condensed-phase flame retardant mechanisms, thereby significantly enhancing the overall flame retardancy of the fabric.

3.4. Flame Retardancy

The flame retardancy of the modified NC fabrics was evaluated by vertical flame testing, and the obtained experimental data are presented in

Table 3. Vertical flame test results of untreated and functionally treated fabrics.

Sample	Add-On (wt%)	Vertical Burning Test
		Afterflame Time (s)	Afterglow Time (s)	Damaged Length (mm)
C-0	--	26	9	300 ± 0
C-1	22.6	24	11	300 ± 0
C-2	25.3	0	0	55 ± 2
C-3	27.1	0	0	35 ± 2
C-4	29.4	0	0	99 ± 3
P@C-3	31.6	0	0	78 ± 1
C(PA)	15.3	20	7	300 ± 0
C(PEI)	18.6	18	9	300 ± 0
C(PDMS)	5.3	23	12	300 ± 0

. The burning behaviors of the untreated and modified NC fabrics during the vertical flame test are shown in Figure 6. The untreated NC fabric (C-0) ignited within 3 s and burned rapidly with intense combustion lasting approximately 30 s, leaving no residual char. In sharp contrast, the sample C-3, treated with PA and PEI, did not ignite during the entire 30 s test period. A residual char length of only 35 mm was observed after burning, indicating excellent flame retardant performance, with a weight gain of 27.1% at this stage.

The P@C-3 fabric, which underwent additional superhydrophobic modification, also retained good flame retardant performance during the 30 s testing period, exhibiting no afterflame or afterglow, with a char length of 78 mm. The results of the vertical burning tests confirm that the flame retardant treatment with PA and PEI significantly enhanced the flame resistance of NC fabrics, and the subsequent PDMS treatment did not compromise this flame retardant effect. To evaluate the individual effects of PA, PEI, and PDMS on flame retardancy, vertical flame tests were conducted on the C(PA), C(PEI), and C(PDMS) fabrics, as shown in Figure 7. It is observed that fabrics treated with PA, PEI, or PDMS alone exhibit poor flame retardant performance, as they are easily ignited and rapidly consumed by flame, leaving only minimal residual char.

Meanwhile, the surface morphology of the combustion residues was examined via SEM. As shown in Figure 8a,b, the combustion residues of the flame retardant and superhydrophobic treated fabrics retained an intact fibrous structure. Prominent expanded bubbles and a dense char layer were observed on the fiber surfaces, attributed to the thermal decomposition of PEI assembled on the fabric surface during combustion, releasing inert gases that formed a flame retardant protective layer. Additionally, the thermal degradation of PA produced phosphoric acid, which promoted char formation and effectively insulated the fabric from heat. The PA/PEI coating exhibited an intumescent flame retardant effect to the NC fabric, significantly enhancing its flame resistance [25]. These results demonstrate the synergistic flame retardant performance of the PA/PEI composite system. Although the combustion of PDMS generated stable SiO₂ that partially interfered with the formation of the char layer, it did not impair the flame retardant performance of the sample P@C-3, which still exhibited self-extinguishing behavior after flame removal.

3.5. Analysis of Fabric Softness

Softness tests were conducted on the fabric samples, and the corresponding ring heights of the untreated NC fabric (C-0) and the flame retardant superhydrophobic fabric (P@C-3) were measured to be 0.9 cm and 1.0 cm, respectively, as shown in Figure 9a,b. These results indicate that the P@C-3 fabric retained good softness and flexibility after functional treatment. Although the weight of P@C-3 increased by 31.6% compared to C-0, it still exhibited excellent softness [49]. This demonstrates that the softness of P@C-3 was largely preserved, ensuring its physical usability was not compromised.

3.6. Superhydrophobicity and Self-Cleaning Performance

As shown in Figure 10, the hydrophobicity of the treated NC fabrics was evaluated by measuring the static water contact angle (WCA). The untreated fabric (C-0) and the flame retardant treated fabric (C-3) exhibited contact angles of 0° and 110.4°, respectively, indicating the absence of superhydrophobicity. In contrast, the P@C-3 fabric treated with PDMS exhibited a high contact angle of 157.8°. The water droplet retained a nearly spherical shape for over 30 s, demonstrating that the P@C-3 fabric possesses excellent superhydrophobic characteristics. This is attributed to the deposition of PDMS, which reduces the surface energy of the NC fabric and consequently imparts low-adhesion superhydrophobic properties to the fabric [49].

As shown in Figure 11a, the untreated C-0 fabric was rapidly wetted upon immersion in water, whereas the P@C-3 fabric remained unwetted and exhibited a silver mirror effect on its surface, illustrated in Figure 11b. This phenomenon is attributed to the presence of a large amount of air trapped within the microscale rough structures on the surface of the P@C-3 fabric, resulting in the formation of a solid–air–water three-phase interface. Air acts as an effective hydrophobic medium. When light passes from a medium with a higher refractive index (water) into one with a lower refractive index (air), and the incident angle exceeds a critical value, total internal reflection occurs, eliminating the refracted light and reflecting all incident light. This is a distinctive optical characteristic of superhydrophobic surfaces [50,51]. Following superhydrophobic modification, the P@C-3 fabric demonstrated excellent water repellency against a wide range of liquids, including acidic (pH = 1), alkaline (pH = 14) solutions, milk, coffee, and tea. As shown in Figure 11c, droplets of these liquids remained intact on the fabric surface without spreading or staining, clearly confirming the successful superhydrophobic treatment of the NC fabric.

Furthermore, the water adhesion properties of the superhydrophobic fabric surface were evaluated via a self-cleaning test. We observe from Figure 11d that dry soil was applied to the surfaces of both the untreated C-0 and the superhydrophobic P@C-3 fabrics, followed by rinsing with water. The hydrophilic C-0 fabric retained the dirt–water mixture, becoming visibly soiled, whereas the P@C-3 fabric allowed the dirt to roll off along with the water droplets, demonstrating a distinct self-cleaning effect due to its superhydrophobic nature. Additionally, an antifouling test was performed on both C-0 and P@C-3 fabrics. As we can see in Figure 11e, after immersion in muddy water, the C-0 fabric was visibly contaminated in the soaked area, while the P@C-3 fabric remained clean and retained its pristine appearance. These results confirm that the P@C-3 fabric exhibits excellent superhydrophobicity along with superior self-cleaning and antifouling capabilities.

3.7. Durability Test

To assess the durability of the flame retardant superhydrophobic NC fabric, the P@C-3 samples were subjected to both abrasion and washing tests. As shown in Figure 12a, the abrasion durability of the P@C-3 fabric was evaluated using 800-grit sandpaper under a 200 g load in a cyclic abrasion test. The initial water contact angle (WCA) of the pristine P@C-3 fabric was 157.8°, which slightly decreased to 151.3° after 18 abrasion cycles (360 cm). Despite the abrasion, the fabric surface retained its superhydrophobicity. These results indicate that the low surface energy PDMS coating exhibits good mechanical robustness under frictional stress.

It can be seen in Figure 12b that after five standard washing cycles, the WCA of the P@C-3 fabric remained at 150.4°, indicating strong adhesion of the PDMS coating to the fabric and excellent wash resistance. The superhydrophobic property of the fabric surface was still pronounced after washing. Furthermore, vertical burning tests performed after abrasion and washing tests confirmed that the P@C-3 fabric still maintained its flame retardant property, with no evidence of afterflame or afterglow (Figure 13a,b). This performance can be attributed, in part, to the hydrolysis and condensation of GPTMS, which formed covalent bonds with active hydroxyl groups on the fabric surface, thereby enhancing the wash durability of the coating [52]. On the other hand, the superhydrophobic coating served as a protective barrier, preserving the integrity of the flame retardant layer and minimizing its loss during washing. Overall, the experimental results demonstrate that the P@C-3 fabric retains excellent superhydrophobicity and flame retardant properties even after mechanical abrasion and repeated washing.

4. Conclusions

In this study, a simple layer-by-layer (LBL) self-assembly method was employed to fabricate flame retardant NC fabrics through electrostatic interactions and hydrogen bonding between PA and PEI. The subsequent grafting of PDMS effectively lowered the surface energy of the fabric, resulting in the successful preparation of flame retardant superhydrophobic NC fabrics. The PA/PEI-modified fabric (C-3) exhibited excellent flame retardancy and self-extinguishing behavior in vertical burning tests, with a char length of only 35 mm. Thermogravimetric analysis (TGA) further confirmed a significant increase in char residue to 43.9%, compared to only 8.6% for the untreated fabric. After PDMS modification, the fabric achieved a water contact angle (WCA) of 157.8°, demonstrating excellent superhydrophobicity and self-cleaning capability. Even after 360 cm of sandpaper abrasion and five cycles of standard washing, the modified fabric retained its flame retardant properties, and the WCA remained above 150°, indicating durable surface functionality. This environmentally friendly fabrication strategy successfully achieves dual flame retardant and superhydrophobic functionality in NC fabrics. The developed material presents great potential for applications in firefighting apparel, decorative fabrics, flame resistant packaging, and self-cleaning textiles. Furthermore, this work offers an innovative solution with both scientific significance and industrial feasibility for the low-carbon transformation of traditional textile materials. The resulting material holds great promise for practical use in firefighter uniforms, fireproof thermal insulation curtains, and outdoor functional self-cleaning textiles.