Durable flame-retardant finishing of cotton fabric via constructing multiple crosslinked layers

doi:10.21203/rs.3.rs-4898470/v1

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Durable flame-retardant finishing of cotton fabric via constructing multiple crosslinked layers

https://doi.org/10.21203/rs.3.rs-4898470/v1

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To improve the flame-retardant performance of cotton fabric, multiple crosslinked layers were constructed on cotton fiber surfaces through polymerization of m-phenylenediamine (MPD)/tetrakis hydroxymethyl phosphonium chloride (THPC) and trimesoyl chloride (TMC). The resulting morphology, general properties, flame retardancy, and durability were characterized and analyzed. Cotton fabric alternately finished with MPD-TMC and THPC-TMC displayed greater changes in surface morphology than that finished with single crosslinked products. The former material had the highest weight gain percentage and possessed significantly higher limiting oxygen index (LOI) value and weaker heat release, accompanied by formation of a more robust carbonaceous layer during micro combustion calorimetry. After alternating finishing with MPD-TMC and THPC-TMC, the cotton fabric exhibited increased flexural rigidity and decreased air permeability, with the change influenced by the number of finishing cycles. Increased finishing cycles also resulted in an increased LOI, up to 33.5% after five cycles. According to micro combustion calorimetry results, heat release was suppressed more effectively as long as the number of finishing cycles reached three, such that finishing with 3 cycles was optimal in this study. Moreover, a fabric with 3 cycles of alternating finishing almost kept its LOI value after washing in disturbed water for 24 h, exhibiting good durability for flame retardancy, which would be beneficial in real applications.

Cotton fabric

Flame-retardant finishing

Crosslinked layer

Polymerization

Durability

As the largest reserves of natural fibers, cotton fibers are widely used in various textiles, exhibiting excellent renewability and biodegradability, compared with synthetic fibers. However, fabric made of cotton fiber can easily ignite and vigorously burn when exposed to an irradiative heat flux or a flame, due to high flammability of cotton fiber. These shortcomings seriously limit its application in many fields. In recent years, much effort has been applied for solving these problems, with flame-retardant finishing the most direct and effective means for improving flame resistance of cotton fabric (Chavali et al. 2020; Sun et al. 2021).

Cotton fabric undergoes intense pyrolysis reactions during combustion, generating a large amount of heat and many flammable substances. Various flame retardants designed to limit the propagation of a flame or even to prevent ignition of the cotton fabrics have been developed and employed by both academics and the industrial world (Chavali et al. 2020; Liu et al. 2021; Sun et al. 2021; Wang et al. 2021a). As described in the literature, upon the application of a flame or exposure to an irradiative heat flux, these flame retardants might exploit different mechanisms, such as char formation (Ling et al. 2023), intumescence (Alongi et al. 2015; Le Bras et al. 1999), reactions in the gas phase (Camino and Costa 1988), cooling (heat sink mechanism;Mark et al. 1975), and dilution. Among these mechanisms, the formation of char (a stable carbonaceous residue on the surface) provides a barrier to inhibit gaseous products from diffusing to the flame and protects cotton fabric from oxygen and heat, not only endowing cotton fabric with flame resistance but also presenting excellent smoke suppression (Wu et al. 2024).

Traditional flame retardants are halogen-based (mainly brominated and chlorinated) products, but some of them have been recently banned by many countries and regions, because of their high toxicity for human beings and animals (Van der Veen and De Boer 2012; Zaikov and Lomakin 2002). Instead, research has addressed the design of halogen-free flame retardants, including nitrogen (Coralli et al. 2023), phosphorus (Salmeia et al. 2016), silicon (Branda et al. 2022), and their composite systems (Chen et al. 2015; Kang et al. 2022; Li et al. 2024; Wang et al. 2021b). Typically, nitrogen-based products can produce abundant noncombustible gases during decomposition, that can dilute the concentration of combustible gases and promote the formation of a carbon layer (Wang 2024). Therefore, nitrogen-based flame retardants have always been the core of flame-retardant system construction.

It is well known that polyisophthaloyl metaphenylene diamine (PMIA) fiber has excellent heat resistance and flame retardancy, because of its stable chemical structure and nitrogenous molecules (Liang et al. 2020; Song et al. 2023a). It has a limiting oxygen index (LOI) value greater than 28% and self-extinguishing character. Above 370°C, it releases a small amount of CO₂, CO, and N₂ during decomposition processes. At a high temperature of 400°C, the fibers carbonize to form an insulating layer which can block external heat moving into the interior and have an effective flame-retardant effect. Inspired by this, a crosslinked aromatic polyamide layer, whose chemical structure is similar to PMIA, can be constructed on the surfaces of cotton fibers via polymerization of m-phenylenediamine (MPD) and trimesoyl chloride (TMC). The difference is only that PMIA is chain-like, whereas crosslinked aromatic polyamide is network-like. Different from small molecule or chain polymer flame retardants, crosslinked flame retardants also provide more durable flame-retardant effect. This polymerization reaction is more commonly used to prepare thin-film composite membranes for reverse osmosis or nanofiltration (Habib and Weinman 2021; Yang et al. 2020), but its use in flame-retardant finishing of textiles has not been proposed.

To investigate the effect of crosslinked aromatic polyamide layer on flame resistance of cotton fabric, the above-mentioned polymerization reactions were used for flame-retardant finishing of cotton fabric. The morphology, general properties, and flame retardancy of finished cotton fabric were characterized and analyzed. Furthermore, the phosphorus-containing crosslinked layer, made by polymerizing tetrakis hydroxymethyl phosphonium chloride (THPC) and TMC, was used to form multiple layers along with the crosslinked aromatic polyamide layer. During combustion, phosphorus-based flame retardants can generate acidic products which can promote cellulose dehydration and carbonization to form a protective carbon layer on fabric surfaces (Song et al. 2023b; Wang et al. 2020), such that it can cause synergistic effects (Zhang et al. 2021) with nitrogen-containing crosslinked polyamide to counteract the weaknesses of the flame-retardant effects of a single crosslinked layer. This study offered a new solution for effectively improving the flame-retardant durability of cotton fabric and provided ideas for promoting the development and industrial application of flame-retardant finishing technology for textiles.

2.1. Materials

Cotton fabric (plain weave) was commercially purchased. MPD (99%) and TMC (98%) were bought from Shanghai Macklin Biochemical Technology Co., Ltd. (Shanghai, China). THPC aqueous solution with 80 wt% was provided by Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). N-hexane and NaOH (analytical pure) were obtained from Tianjin Fuyu Fine Chemical Co., Ltd. (Tianjin, China). Distilled water was used in experiments.

2.2 Flame-retardant finishing of cotton fabric

Before flame-retardant finishing, cotton fabric was pretreated by immersion into 2-wt% NaOH aqueous solution at 80°C for 2 h. Then, the treated fabric was thoroughly washed by distilled water and dried at 70°C, named as COT.

Three monomer solutions, 5-wt% MPD aqueous solution, 5-wt% THPC aqueous solution, and 5g/L TMC solution in n-hexane, were prepared for polymerization reactions. Notably, the wanted THPC aqueous solution was obtained by accurately diluting the original THPC aqueous solution with distilled water. In a polymerization reaction, the cotton fabric was first soaked in MPD solution or THPC solution at 25°C for 5 min, excess solution squeezed out via a roller, and then the cotton fabric dried at 70°C. The polymerization reaction occurred by soaking the MPD or THPC treated cotton fabric in TMC solution at 25°C for 5 min, with the excess TMC rinsed out using pure n-hexane. After thoroughly drying at 60°C, MPD-TMC or THPC-TMC finished cotton fabric was obtained (Fig. 1).

According to the type and times of the above polymerization reactions, various finished cotton fabrics were prepared. The cotton fabric, after repeated finishing by MPD-TMC polymerization, was named PMT-n, where n represented the number of finishing cycles. Similarly, cotton fabric finished many times with THPC-TMC polymerization were also obtained and named PHT-n. Furthermore, The two kinds of polymerization reactions were combined together to finish cotton fabric, namely cotton fabric finished by MPD-TMC, followed by finishing with THPC-TMC, with repeated processes obtaining finished cotton fabric named PMT/PHT-n, where n represents number of MPD-TMC/THPC-TMC processing cycles. The flame-retardant finishing process of cotton fabric was enhanced according to a schematic diagram (Fig. 2), by which multiple crosslinked layers covering the fibers was built.

2.3 Characterization

The surface morphologies of the cotton fabric before and after flame-retardant finishing were observed using a digital triocular biological microscope (CX31, Olympus Co., Ltd., Beijing, China).

Fabric thicknesses were obtained using a portable thickness gauge and the planar density determined by weighing fixed-area samples. Weight gain percentage was obtained by calculating the ratio of added weight from finishing to weight of original fabric. These tests were conducted in triplicate times for each sample to obtain an average value.

Flexural rigidity was evaluated by the bending length through a cantilever beam method (Fan et al. 2021). Two-centimeter wide strips along the warp direction were used as test specimens and their warp bending length calculated according to Eq. 1 as

$$\:\text{C}=\text{L}\sqrt[3]{\frac{\text{cos}(\theta\:/2)}{8\text{tan}\theta\:}}$$

where C is the bending length (cm), L the cantilever extension length (cm), and θ the angle between the line from the fixed point to free end and horizontal plane. Triplicate tests were carried out for each sample to obtain an average value.

The air permeability of all samples was tested using a fully automatic permeability instrument (YG461E-III, Ningbo Textile Instrument Factory, Ningbo, China), with differential pressure across the samples of 100 Pa. Five tests were executed for each sample to obtain an average value.

The thermal behavior of samples was analyzed with a synchronous thermal analyzer (SDT Q600, TA, Newcastle, USA) at a heating rate of 10°C/min from 30 to 800°C under a N₂ atmosphere.

The LOI value of samples was tested using a limiting oxygen index meter (CO1, Motis Fire Technology Co., Ltd., Jiangsu, China), in which each sample was 12 cm×5 cm in size and three tests performed for each sample to obtain an average value.

Combustion behaviors of samples under increasing temperature conditions were tested using a micro combustion calorimeter (FAA-PCFC, Concept Equipment Co., Ltd, Rustington, UK) according to the ISO 5660-1:2002 standard, which required sample quality of 5 mg. The flow rates of N₂ and O₂ in the pyrolysis chamber were 80 and 20 mL/min, respectively, and the temperature from room temperature to 760°C with a heating rate of 1°C/s.

3.1 Effects of different finishing systems

Different crosslinked systems were used to finish cotton fabric and the surface morphologies of cotton fabric before and after finishing examined (Fig. 3). The textures of pretreated cotton fabric made up of warp and weft yarns were clear and there existed significant spaces between yarns in the fabric, with the fibers thin and with smooth surfaces companied with no attachments (Fig. 3a). After finishing with single DMP-TMC or THPC-TMC treatments, the yarns become thicker and spaces smaller, because of the gaps between fibers in yarns were filled with flame retardants. In addition, little change was seen in the surface color of PHT-6, while PMT-6 showed a slightly deep surface color, because the change in surface color was caused by partial oxidation of DMP-TMC and the discoloration degree depended on the quantities of attached DMP-TMC. With regard to PMT/PHT-3, its texture became obscure from the flame retardant covering and the space between yarns not clear due to flame retardant filling. Significantly, the surface appeared yellow and fibers became very thick, which indicated that larger quantities of flame retardants attached to the fabric.

To further investigate the effects of different finishing systems on the properties of cotton fabrics, the general properties were tested (Table 1). The thickness and planar density of the cotton fabrics were seen to be increased after finishing, which was because of the attachment of flame retardant. Different finishing methods led to different weight gains, with the weight gain percentage of PMD/PHT-3 the largest, reaching 28.1%, far greater than other finished fabrics, which provided it with the greatest thickness and planar density. There existed strong interactions between MPD and THPC, such that they mutually promoted adsorption during the construction of alternating multilayer structures, thus creating extremely clear weight gains (Fig. 3d). Combining and analyzing weight gains and changes in planar density, PMD/PHT-3 was found to have the greater area shrinkage than other finished fabrics due to significant yarn expansion caused by high amounts of flame retardant attachment. This change in shape was bound to have an impact on fabric flexibility and permeability.

Bending length measured using the cantilever beam method reflected flexural behavior, an important feature in evaluating fabric wearability and style. Cotton fabric before finishing was seen to possess low bending length and exhibited cloth softness (Table 1). After finishing with single DMP-TMC or THPC-TMC, fabric showed little change in bending length, while PMD/PHT-3 possessed clearly larger bending length, which was related to the amount of flame retardant attachment. Yarn expansion and area shrinkage caused by more attachments endowed the fabric with stiffness to a larger extent. Air permeability of cotton fabric before and after finishing showed that raw cotton fabric had higher air permeability (616.7 mm/s) than all finished fabrics due to the clear space between yarns (Fig. 3). With fibers covered with crosslinked layers, yarns expanded and crowded out spaces, such that there were clear retardations of air penetration. In particular, relatively lower air permeability (303.9 mm/s) was detected for PMD/PHT-3, because of the largest morphologic changes (Fig. 3d). Overall, air permeability of fabrics after finishing was acceptable although there was a certain reduction.

Table 1

General characterization of cotton fabric and fabrics finished with different crosslinked systems
Sample name	Thickness (mm)	Planar density (g/m²)	Weight gain percentage (%)	Bending length (cm)	Air permeability (mm/s)
COT	0.50 ± 0.01	114.1 ± 4.3	—	1.1 ± 0.1	616.7 ± 43.7
PMT-6	0.54 ± 0.02	152.3 ± 6.2	7.9 ± 1.0	1.3 ± 0.2	461.2 ± 10.2
PHT-6	0.51 ± 0.02	148.2 ± 4.7	2.9 ± 0.6	1.2 ± 0.1	403.1 ± 29.6
PMD/PHT-3	0.57 ± 0.02	182.5 ± 6.1	28.1 ± 3.5	2.6 ± 0.2	303.9 ± 13.3

Before considering the effects of finishing processes on the flame-retardant behavior of cotton fabric, it was worth looking at the thermal behavior of various fabrics as assessed by thermogravimetric (TG) analyses (Fig. 4). Cotton thermal degradation in N₂ involved two main degradation steps. The first takes place around 100°C and was attributed to the evaporation of the residual water. The second degradation step, which occurs around 369°C, was ascribed to decomposition of the polymer chains, with the residual weight only 10.4% of the original. After finishing with crosslinked products, the residual weight increased, reaching 15.8, 13.7, and 23.7% for PMT-6, PHT-6, and PMT/PHT-3, respectively. The reason for this was that solid or gas covering layers derived from flame retardants protected cotton fabric through gas or condensed phase flame retardant mechanisms, which also reduced the maximum weight loss rates more or less. With respect to PMT-6, it had the maximum weight loss temperature similar to those of cotton fabric, but those of PHT-6 and PMT/PHT-3 were lower than cotton fabric. This was attributed to THPC-TMC decomposition, which generated phosphoric acid or metaphosphate that could catalyze cotton cellulose dehydration and carbonization. According to the low-temperature carbonization of cotton cellulose and clearly increased residue char of finished fabrics, PMT/PHT-3 exhibited the better thermal stability, as a joint result of the two kinds of compounds in alternating multilayer structures, which was beneficial for improving flame retardancy.

The flame retardancy of cotton fabric before and after flame-retardant finishing was evaluated by LOI, with the LOI value of unfinished cotton fabric at 17.5%, as a flammable material (Fig. 5). Compared with COT, the LOI values of PMT-6 and PHT-6 increased to 19.5% and 19%, respectively, but the flame-retardant effect was relatively limited, which was related to the single flame retardant composition and small attachment amounts. For PMT/PHT-3, its LOI value significantly increased to 28.5% and was not ignited in an air atmosphere, reaching the LOI standard of flame-retardant cotton fabric (≥ 26%). In addition, pure cotton fabric burned thoroughly without char formation, according to the ash after LOI testing, while finished fabric burned to form char to various degrees, because the degradation products of flame retardants prompted dehydration and carbonization of cotton fabric to form a protective carbon layer of heat insulation and oxygen isolation. With regard to PMT/PHT-3, its shape was basically maintained even with charring after LOI testing, thus exhibiting better flame-retardant properties than other samples. This was ascribed to the high amount of flame retardants and their synergistic effects in alternating multilayer structures.

The combustion behavior of cotton fabric before and after finishing was measured by micro combustion calorimetry. Heat release rate (HRR) curves from various samples were collected to determine peak HRR (PKHRR), temperature at peak HRR (TPHRR), and total heat release (THR; Fig. 6 and Table 2). HRR curves different fabrics treated by different finishing systems showed that PKHRR of finished cotton fabrics was lower than that of COT (307.2 W/g), especially PMT/PHT-3, which had a significantly lower PKHRR (65.0 W/g) than other samples, presenting excellent suppression of heat release. The TPHRR of PMT-6 was close to that of COT (377.5°C), but that of PHT-6 and PMT/PHT-3 were all much lower than COT. These results indicated that THPC-TMC not only inhibited heat release, but promoted dehydration and carbonization of cotton at low temperatures, which was consistent with the above TG analytical results. There existed two peaks for the HRR curve of PMT/PHT-3, because large amounts of flame retardants presented easier decomposition than cotton cellulose and the heat release of cotton fabric was suppressed into a slower process. In addition, PMT/PHT-3 also displayed much lower THR and reduced fire hazards, owing to effective suppression of heat release.

Table 2

Results from micro combustion calorimetry tests for cotton fabric and fabrics finished with different crosslinked systems
Sample name	PKHRR (W/g)	TPHRR (°C)	THR (kJ/g)
COT	307.2	377.5	15.8
PMT-6	215.6	372.5	10.4
PHT-6	241.0	318.5	10.1
PMD/PHT-3	65.0	332.5	2.2

3.2 Effect of different finishing cycles

According to the above results, the combination of MPD-TMC and THPC-TMC produced better flame-retardant effects than a single flame retardant. The construction of alternating multilayer structures with two crosslinked products was optimal for improving flame retardancy. Meanwhile, flame-retardant properties were largely determined by the amount of flame retardant attachment, which could be adjusted by the number of finishing cycles. Thus, the effects of MPD-TMC/THPC-TMC finishing cycles on various properties of cotton fabric were investigated. Fabric thickness and planar density were found to have a roughly increasing tendency with increased finishing cycles (Table 3). This was easily explained as more finishing cycles resulted into more crosslinked layers, thus increasing the amount of flame retardant attachment that corresponded to the weight gain percentage. More flame retardant attached to the fibers occupied interfibrous spaces and increased the thickness values measured under a set loading pressure.

The finishing cycles also impacted on flexural rigidity and permeability of the finished fabrics. The bending length was observed to have an increasing trend with increased finishing cycles and the trend similar to the variation in fabric thickness (Table 3). The covering of more crosslinked layers enlarged fiber diameters and enhanced fabric tightness, such that the finished cotton fabric obtained larger flexural rigidity. The air permeability of these finished fabrics was correlated reciprocally with increased finishing cycles and decreased from 418.2 mm/s for PMD/PHT-1 to 219.3 mm/s for PMD/PHT-5. Yarn expansion and space compression retarded air penetration, with fabric tightness positively associated with amount of flame retardant attachment. Therefore, air permeability was adjusted by altering finishing cycles. Furthermore, with the stiffness and air permeability meeting certain application requirements, the flame-retardant performance for these finished fabrics appeared more significant.

Table 3

General characterization of cotton fabrics with different MPD-TMC/THPC-TMC finishing cycles
Sample name	Thickness (mm)	Planar density (g/m²)	Weight gain percentage (%)	Bending length (cm)	Air permeability (mm/s)
PMD/PHT-1	0.52 ± 0.01	152.9 ± 6.5	4.2 ± 0.5	1.2 ± 0.1	418.2 ± 20.3
PMD/PHT-2	0.54 ± 0.02	170.4 ± 5.4	16.5 ± 2.4	1.7 ± 0.2	377.3 ± 21.4
PMD/PHT-3	0.57 ± 0.02	182.5 ± 6.1	28.1 ± 3.5	2.6 ± 0.2	303.9 ± 13.3
PMD/PHT-4	0.59 ± 0.01	194.9 ± 7.8	41.2 ± 2.8	2.6 ± 0.2	264.4 ± 16.7
PMD/PHT-5	0.62 ± 0.02	203.8 ± 6.7	50.8 ± 5.9	2.9 ± 0.2	219.3 ± 17.1

The main objective of this study was to endow cotton fabric with good flame-retardant performance that could also be adjusted by altering MPD-TMC/THPC-TMC finishing cycles. With increased finishing cycles, the LOI values of finished cotton fabrics exhibited a predictable increasing trend (Fig. 7). Cotton fabrics finished less than 3 cycles were noticed to have unimpressive flame retardancy, with LOI values lower than 26% and higher LOI values obtained by further increasing the finishing cycles. With regard to PMT/PHT-5, its LOI value reached 33.5% higher than the flame-retardant LOI standard. From the appearance after LOI testing, all finished cotton fabrics burned to form char and their shape maintained despite the charring. This was because the fabric was dehydrated and carbonized under the action of flame retardants, to form a protective carbon layer which effectively suppressed the combustion. In addition, with increased finishing cycles, the char became stiffer, indicating that more crosslinked layers were conducive to the formation of more robust carbonaceous protective layers.

The combustion behavior of finished cotton fabrics was clearly influenced by finishing cycles, according to the micro combustion calorimetry results (Fig. 8 and Table 4). With MPD-TMC/THPC-TMC, increased finishing cycles, the PKHRR decreased from 74.5 W/g of PMD/PHT-1 to 39.6 W/g of PMD/PHT-5, exhibiting incremental suppression of heat release. In TPHRR, the value decreased from 338.1°C to 307.7°C, because of above-mentioned low-temperature carbonization. It was also noticed that there existed only one peak for HRR curves of PMT/PHT-1 and PMT/PHT-2, but two peaks for that of fabrics with more finishing cycles, which was due to easier decomposition of flame retardants than that of cotton cellulose. When the amount of flame retardants reached a certain level due to more crosslinked layers, a more robust carbonaceous layer was formed and the combustion of cotton cellulose restricted by flame retardants transformed into an independent and slower heat release after decomposition of flame retardants. In addition, finished cotton fabric with three or more finishing cycles displayed significantly lower THR than when finishing cycles were less than 3, at which heat release was suppressed more effectively.

According to the comprehensive consideration of LOI and micro combustion calorimetry results flame retardancy of finished fabric would have significant improvement as long as the number of finishing cycles reached 3. Therefore, finishing with 3 cycles was optimal, in which flame retardancy was satisfactory and the finishing process acceptable.

Table 4

Results from micro combustion calorimetry tests for cotton fabrics with different MPD-TMC/THPC-TMC finishing cycles
Sample name	PKHRR (W/g)	TPHRR (°C)	THR (kJ/g)
PMD/PHT-1	74.5	338.1	4.7
PMD/PHT-2	71.5	335.1	4.1
PMD/PHT-3	65.0	332.5	2.2
PMD/PHT-4	52.8	322.1	2.6
PMD/PHT-5	39.6	307.7	1.8

3.3 Durability of flame retardancy

Generally speaking, washing could weaken the functionality of cotton fabric obtained from post-treatment. Thus, to evaluate the durability of flame retardancy, PMT/PHT-3, possessing well good overall performance, was selected and used in washing experiments, with COT without finishing used as reference. The LOI values were measured when the fabrics were submerged in disturbed water for various times and the variation of LOI reflected the durability of flame retardancy to a certain extent. COT had a stable LOI value because there existed no flame retardant on it (Fig. 9). The LOI value of PMT/PHT-3 decreased slightly in the initial stage of washing, but remained nearly constant in follow-up washings. The crosslinked layers on finished fabric suffered damage from washing disturbance and some unreacted monomers easily washed out, such that there was decreased flame-retardant effect after initial washing. Although the flame retardancy of finished fabric decreased somewhat after washing, its LOI value still remained 26.6% when washed for 24 h, which was much higher than that of COT and attributed to the crosslinking of the flame retardants on the fabric. Flame retardant monomers took place in crosslinking reactions with each other or cotton cellulose to enhance the covering layer robustness and adhesion fastness. To sum up, the construction of alternating crosslinked layers not only endowed cotton fabric excellent flame retardancy, but solved the durability problems of cotton fabric finished by flame retardants.

The flame-retardant performance of cotton fabric was effectively improved by the construction of alternating crosslinked layers. First, PMT/PHT-3 showed greater change in surface morphology than PMT-6 and PHT-6, because of its highest weight gain percentage. Moreover, PMT/PHT-3 had significantly higher LOI values and lower heat release and produced a more robust carbonaceous layer, evaluated through LOI testing and micro combustion calorimetry. Based on the synergistic action of two kinds of flame retardant, alternating finishing with MPD-TMC and THPC-TMC was selected for further study. The finished cotton fabric exhibited increased flexural rigidity and decreased air permeability, with these changes influenced by finishing cycles. Increased number of finishing cycles also resulted in increased LOI values, up to 33.5% for PMT/PHT-5. According to micro combustion calorimetry results, heat release was suppressed more effectively as long as the number of finishing cycles reached 3, such that finishing with 3 cycles was optimal in this study. Through washing experiments, PMT/PHT-3 almost keep its LOI value after washing in disturbed water for 24 h, exhibiting good durability of flame retardancy, which would be conducive for practical applications.

Conflicts of interest

The authors declare that they have no conflict of interest.

Compliance with ethical standards

Not applicable.

Funding

The authors gratefully acknowledge the financial support from the National Key Research and Development Program of China (Grant No. 2017YFB0309100).

Author Contribution

Methodology: Z. Fan, R. Liu; Formal analysis and investigation: Z. Fan, Y. Xue, H. Liu; Writing - original draft preparation: Z. Fan; Writing - review and editing: Y. Yu; Funding acquisition: R. Liu.

Availability of data

Not applicable.

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No competing interests reported.

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Durable flame-retardant finishing of cotton fabric via constructing multiple crosslinked layers

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Abstract

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1. Introduction

2. Experimental section

2.1. Materials

2.2 Flame-retardant finishing of cotton fabric

2.3 Characterization

3. Results and discussion

3.1 Effects of different finishing systems

3.2 Effect of different finishing cycles

3.3 Durability of flame retardancy

4. Conclusions

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Conflicts of interest

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References

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Version 1