Synergistically Enhanced Flame Retardancy of A Cotton Fabric Finished by A Silica Sol and A Calcium Hypophosphite- Chitosan Solution Volume 5 - Issue 3

Dan Cheng, Fei You* and Gang Li

• College of Safety Science and Engineering & Institute of Fire Science and Engineering, Nanjing Tech University, China

Received:January 03, 2023   Published: January 13, 2023

*Corresponding author: Fei You, College of Safety Science and Engineering & Institute of Fire Science and Engineering, Nanjing Tech University, China

DOI: 10.32474/LTTFD.2023.05.000213

Abstract PDF

This work aims to improve flame retardant performance of a cotton fabric (COT) by combining a silica (SiO₂) sol with calcium hypophosphite (CaHP) and chitosan (CS) agents. A mixed solution of the latter two and the SiO₂ sol were successively arranged on the cotton fabric surface by a dipping-baking (drying) method. Surface microscopic morphologies of original and treated cotton fabrics were characterized by scanning electron microscopy (SEM) technique. Thermal stabilities and flame retardancy levels of cotton fabrics were evaluated by thermogravimetric (TG), limiting oxygen index (LOI) and vertical flammability (VF) experiments. The results show that CaHP-CS and SiO₂ are successfully sorted to the cotton fabric surface. The char residue rate at 700.0 ℃ of the treated cotton fabric increases 76.4 times by that of original cotton fabric in a TG experiment. It shows a LOI value of 31.3 % (non-flammable grade). It can self-extinguish with a char length of only 7.00 cm (0.23 times by that of original cotton fabric) in a VF experiment. Each component plays both individual and synergistic roles of char formation, heat/oxygen/gaseous product isolation, gaseous product dilution and heat removal in flame retarding the cotton fabric.

Keywords: Cotton Fabric; Flame Retardant; Silica Sol; Synergistic Effect; Sol-gel

Cotton fabrics are favored by the world due to their excellent comfort, hygroscopicity, biodegradability and good dyeing properties [1]. However, cotton fabrics are usually flammable in nature reflected by a LOI value of only 18.0 %. Once ignited, smoldering or/and flaming combustion processes are induced. The flames tend to spread rapidly, a fire without control may occur and cause personnel injuries/deaths and property losses. Therefore, improving the flame retardant properties of cotton fabrics has always been an important research topic [2]. Phosphorus-based flame retardants have been widely used due to their advantages of low toxicity, low dosage, little harm to the environment and less effect on material processing [3]. Among them, calcium phosphate not only contains high phosphorus content, but also processes low cost [4]. Nitrogen-based flame retardants have excellent characteristics such as low smoke, low toxicity and low corrosion. When heated and decomposed, they will produce NH₃ and other non-combustible gases. When combustion occurs, they can play the role of heat absorption, cooling and dilution [5]. Among them, chitosan can be used as both a foaming agent by releasing NH3 and a biomass polysaccharide. It contains many hydrogen bonds in its molecule and promotes dehydration and char formation processes [6]. SiO₂ sol can be used as a non-flammable surface modifier to cover cotton fiber surfaces in the form of a gel coating. Heat, oxygen and gaseous substances can thus be prevented from transferring effectively in a pyrolysis or combustion process [7]. In this sense, it is curious and natural to combine them together and explore whether they have synergistic flame retardant effects on a cotton fabric [8]. In this work, CaHP and CS were made into a mixed solution, then it and a SiO₂ sol were applied to the cotton fabric surface successively via a dipping-baking method. For original and treated cotton fabrics, their surface micro-morphologies were determined by SEM. Their thermal stability and flame retardancy levels were investigated by TG, LOI and VF experiments. The flame retardant mechanisms of the cotton fabric finished by a CaHP-CS solution and a SiO₂ sol-gel system were discussed.

Materials: Cotton fabrics (111 g/m²) were purchased from Nanjing Caimei Textile Co., Ltd. (China). Tetraethylorthosilicate (AR, ≥35.0 %) and chitosan (IR, 80.0-95.0 %) were obtained from Sinopharm Chemical Reagent Co., Ltd. (China). Calcium hypophosphite was provided by Shanghai Aladdin Reagent Co., Ltd. (China). Ethanol (AR, ≥99.6 %) was acquired from Wuxi Yasheng Chemical Co., Ltd. (China). All the reagents were used as received.

Cotton fabric treatment

After washing and drying with deionized water, a cotton fabric was soaked in a CaHP-CS mixed solution and a SiO₂ sol for 30 min and 10 min successively. The cotton fabric was taken out, dried and placed in an oven (PHG-9036A, provided by Shanghai Jinghong Experimental Equipment Co., Ltd., China) to dry at 80 °C for 30 min and cure at 120 °C for 10 min, respectively.

Testing and characterizations

Original and treated cotton fabrics were cut into pieces and sprayed with gold. Surface microscopic morphologies of their bulks and char residues after VF experiments were recorded by a ZEISS Gemini 300 scanning electron microscopy (SEM, Carl Zeiss Co., Ltd., Germany) at 800 magnification and at an accelerating voltage of 10 kV. Their thermal stabilities were investigated under N₂ atmosphere by a TGA/DSC3+ (Mettler-Toledo, USA) apparatus. About 10.0 mg of samples were tested from 30.0 to 700.0 ℃ at a flowing rate of 50 mL/min and a heating rate of 10 ℃/min. Based on GB/T 5454-1997 standard, their LOI values were obtained by operating a JF-3 oxygen index meter (Jiangsu Zhuoheng Measurement & Control Technology Co., Ltd., China) with samples of 150×58 mm². Based on GB/T5455-2014 standard, their VF values were obtained by operating a CZF-5 vertical flammability tester (Jiangsu Zhuoheng Measurement & Control Technology Co., Ltd., China) with samples of 300×89 mm².

Surface morphologies of cotton fabrics

The SEM images of original and the treated cotton fabrics before and after a VF experiment are shown in Figure 1. It can be seen that original cotton fabric is interwoven independently without obvious cracks or adhesion points. The surface of treated cotton fabric is generally rough and covered by a CaHP-CS/SiO₂ gel coating (converted from a sol after baking) with noticeable cracks. After a VF experiment, the fiber surfaces of treated cotton fabric are covered with an obvious and thick char layer induced mainly by degradation and condensation processes of CaHP substance and the SiO₂ gel. Fine bubbles are observed in the char layer since CS substance and the SiO₂ sol-gel system can release NH₃ and H₂O molecules in their degradation and dehydration processes at high temperatures. There are also some cracks in the char layer induced mainly by multiple physical actions like capillary forces, Van der Waals’ forces, surface tensions, diffusivity of the CaHP-CS/SiO₂ composite gel in dehydration and decomposition processes at high temperatures [9].

Figure 1: SEM images of original and treated cotton fabrics.

Thermal stabilities of cotton fabrics

Pyrolysis data of original and treated cotton fabrics, i.e., onset decomposition temperature (T_onset, defined as the temperature with 10 wt% mass loss), endset decomposition temperature (T_endset, defined as the temperature with 95 wt% mass loss), the temperature of maximum mass loss rate (T_max), the mass loss rate at Tmax (R_max) and char residue rate at 700.0 ℃, are summarized in Table 1. According to the Table 1, for original cotton fabric, its R_max is up to 4.61 %/℃, while the char residue rate is only 0.55 %, indicating that original cotton fabric has a low thermal stability. It is easy to be pyrolyzed and burns relatively sufficient once ignited. The sample CaHP-CS/SiO₂ @COT exhibits a very high char residue rate of 42.03 % that is 76.4 times that of original cotton fabric. Its R_max (1.52 %/°C) is lower by 67.03 % than that of original cotton fabric. The entire pyrolysis stage becomes wider than that of original cotton fabric. All these facts fully reflect that thermal stability of the treated cotton fabric is dramatically enhanced. The flame retardant components of CaHP, CS and SiO₂ gel should interact well with each other and play comprehensively excellent synergistic effects. These can also approve the microscopic surface morphologies of cotton fabrics before and after VF experiments as mentioned above.

Flame retardant properties of cotton fabrics

The flame retardant properties of original and treated cotton fabrics are reflected by LOI and VF results as shown in Table 2. WGR is weight gain rate, T1 represents after-flame time, T2 is after-glow time and R is damaged length. It can be seen that the LOI value of the treated cotton fabric increases by 73.9 % as compared with original cotton fabric. In addition, according to the VF experimental data, for original cotton fabric, it burns rapidly (a T1 value of 7.5 s) accompanying with a smoldering process (a T2 value of 15.0 s) after ignition, which is consistent with the char residue rate (0.55 %) of original cotton fabric (Table 1). For CaHP-CS/SiO₂@COT, its T1 and T2 values are 1.2 s and 0 s respectively (Table 2). Therefore, it reaches the B1 rating (char length ≤ 15.0 cm, after-flame time ≤ 5.0 s, after-glow time ≤ 5.0 s) in a VF test [10]. This shows that the SiO₂ gel coating is effectively deposited on the surface of the cotton fabric through hydrogen bonding and physical adsorption. Such a coating can act as a physical barrier to prevent oxygen transmission, heat exchange and oxidative decomposition processes [11]. Moreover, the decomposition process of CaHP generates phosphoric acid and polyphosphoric acid to catalyze cotton fibers into chars. As mentioned above, the decomposition process of CS releases NH₃ and H₂O molecules to dilute gaseous products and cool down heated areas. When they are combined together, a coordinated action of P-N-Si system is induced to behave like an intumescent flame retardant effect. All of these further indicate that each component plays both individual and synergistic roles in flame retarding the cotton fabric.

Table 1: TG data of original and treated cotton fabrics under N₂ atmosphere.

Table 2: LOI and VF experimental data of original and treated cotton fabrics.

This work focuses on the flame retardant performances of a cotton fabric finished by a hybrid system of a CaHP-CS solution and a silica sol. Results show that CaHP-CS/SiO₂ @COT has a non-flammable level. In detail, it has a high char residue rate of 42.03 % (76.4 times that of original cotton fabric) in a TG experiment, a high LOI value of 31.3 %, and a B1 rating in a VF experiment. Phosphoric acid and polyphosphate acid decomposed by CaHP can promote cotton cellulose to dehydrate and form aromatic chars. NH₃ and H₂O molecules decomposed by CS can dilute gaseous products and cool down heated areas. The silica sol can convert to a gel coating with dimensional stability and provide a structural support effect. When such actions are combined, a thick char layer can be produced as a physical barrier; heat exchanges, oxygen and gaseous product transfers can be hindered and isolated; heated areas can be cooled down; gaseous products can be diluted. Therefore, each component can act both individually and synergistically to enhance flame retardancy level of the cotton fabric. In a word, such an attempt is positive to develop a new halogen-free, formaldehyde-free, environmentally friendly flame retardant system for a cotton fabric.

The study is financially supported by the National Natural Science Foundation of China (Grant No. 51376089 and 50906039), the 2019 Key Project of The Natural Science Foundation of the Jiangsu Higher Education Institutions of China (19KJA520007, A Class).

The authors declare that they have no known competing financial interests or personal relationships that could probably influence this work.

1. Wang S, Kong D, Chen H (2022) Construction of a novel B/N/Si synergistic flame retardant    system and its application on cotton fabric. Industrial Crops and Products 178: 114574.
2. Wang S, Liu J, Sun L (2020) Preparation of flame-retardant/dyed cotton fabrics: flame retardancy, dyeing performance and flame retardant/dyed mechanism. Cellulose 27(17): 10425-10440.
3. Ling C, Guo L (2020) Preparation of a flame-retardant coating based on solvent-free synthesis with high efficiency and durability on cotton fabric. Carbohydrate polymers 230: 115648.
4. Savas Atabek L, Tayfun U, Hancer M (2019) The flame‐retardant effect of calcium hypophosphite in various thermoplastic polymers. Fire and Materials 43(3): 294-302.
5. Wang B, Xu YJ, Li P (2020) Flame-retardant polyester/cotton blend with phosphorus/nitrogen/silicon-containing nano-coating by layer-by-layer assembly. Applied Surface Science 509: 145323.
6. Makhlouf G, Abdelkhalik A, Ameen H (2022) Preparation of highly efficient chitosan-based flame retardant coatings with good antibacterial properties for cotton fabrics. Progress in Organic Coatings 163: 10662.
7. Li D, Wang ZH, Zhu YS (2022) Synergistically improved flame retardancy of the cotton fabric finished by silica-coupling agent-zinc borate hybrid sol. Journal of Industrial Textiles 51(5_suppl): 8297S-8322S.
8. Liu J, Dong C, Zhang Z (2020) Durable flame retardant cotton fabrics modified with a novel silicon-phosphorus-nitrogen synergistic flame retardant. Cellulose 27(15): 9027-9043.
9. Zhang XF, Wang ZH, Zhou ST (2022) Enhanced flame retardancy level of a cotton fabric treated by an ammonium pentaborate doped silica-KH570 sol. Journal of Industrial Textiles 52: 1-29.
10. Cheng XW, Tang RC, Guan J P (2020) An eco-friendly and effective flame retardant coating for cotton fabric based on phytic acid doped silica sol approach. Progress in Organic Coatings 141: 105539.
11. Li G, You F, Zhou ST (2022) Preparations, characterizations, thermal and flame retardant properties of cotton fabrics finished by boron-silica sol-gel coatings. Polymer Degradation and Stability 202: 110011.