A facile strategy to achieve efficient flame-retardant cotton fabric with durable and restorable fire resistance

Highlights

• Durable and restored fireproof cotton fabrics were prepared via a facile strategy.

• Prominent fire resistance and durability to 50 LCs with LOI of 32.7% was achieved.

• The reduction in washing durability can be recovered by feasible acetic acid soaking.

• Improved fire resistance was due to graphitized carbon layers and C/N heterocycles.

Abstract

Novel, durably flame-retardant cotton fabrics with fascinatingly restorable fire resistance were prepared by simple grafting of phosphate and melamine, which contributed to a phosphorous-nitrogen (P/N) synergism. During the preparation process, phosphate was grafted with the hydroxyl groups of cellulose to form P-O-C bonds; subsequently, melamine reacted with the aldehyde groups of cellulose generated from phosphorylation to form P/N modified fabrics (P/N-Cotton). The P/N synergistic effect endowed cotton fabric with prominent fire resistance and durability. The Limit oxygen index (LOI) was as high as 51.1 ± 0.3% and was maintained at 32.7 ± 0.8% after 50 laundering cycles (LCs). The total heat release (THR) and peak values of heat release rate (pHRR) of P/N-Cotton were significantly reduced by 70.11% and 64.19%, respectively. Amazingly, the modified cotton fabrics exhibited restorable fire resistance once immersed into the acetic acid solution due to the ion exchange between Na⁺ from detergent and H⁺ from acetic acid. The LOI of P-Cotton and P/N-Cotton after 50 LCs can be restored to 32.4 ± 0.7% and 35.7 ± 0.7% from 20.8 ± 0.7% and 32.7 ± 0.8%, respectively. The analysis of gas-phase products and residual char demonstrated that the incorporation of phosphate and melamine facilitated cellulose fibers to convert to dense and stable graphitized carbon layers and carbon-nitrogen heterocycles, effectively protecting the fabric from decomposing to flammable gases.

Introduction

Cotton fabric has been widely employed in the fields of clothing industry and home textiles due to its excellent renewability, air permeability, flexibility, skin-friendliness, and comfort. However, its flammability is a major hidden danger in applications, which causes fire to spread quickly in a blaze because of its extremely Low limit oxygen index (LOI) of 18%. Commercially, the fire resistance of cotton fabrics is acquired by incorporating flame retardants onto their surface using special organophosphorus finishing treatments, such as Proban and Pyrovatex CP, which can undergo more than 50 laundering cycles (LCs) derived from the stable chemical bond. However, the release of formaldehyde is caused by the break of the ether bond between methylene ether and cellulose molecule when washing [1], [2]. Considering the ecologically sustainable development, the widely used flame retardants that will release formaldehyde for cotton fabrics will fade out because of their deleterious effects on mankind and ecological environments. Thus, it is urgent to develop a formaldehyde-free process for fire resistance of cotton fabrics with durability.

Recently, many efforts have been made to improve the flame retardancy of cotton fabrics through various surface treatment methods such as spray, sol-gel reaction, layer-by-layer (LbL) self-assembly method and chemical grafting based on silicon (Si), nitrogen (N), phosphorous (P) flame retardant additives [3], [4], [5], [6]. Guo et al. [7] introduced alkylammonium functional silsesquioxane/phytic acid and TiO₂@polydimethylsiloxane into cotton fabrics via a two-step spraying method, the optimized coated sample showed self-extinguishing behavior and enhanced LOI of 29%, but only withstanding 5 LCs. Rosace et al. [8] treated cotton fabrics with an organophosphorus/nitrogen system based on two-stage procedures of impregnation and curing reactions, the after-flame (AF) time of the treated cotton fabrics decreased to 5 s from 23 s, but increased sharply after washing. To endow cotton fabric with more efficient flame retardancy and durability, the chemical grafting method may be considered to be an efficient approach. Jia et al. [9] grafted urea and phosphoric acid onto cotton, the LOI was high to 41.0% and maintained at 28.4% after 50 LCs. Xu et al. [10] treated cotton with a casein-based flame retardant, the LOI maintained at 26.7% after 40 LCs. In addition, the casein-based flame retardant displayed excellent dilution and suppression effect in the gaseous phase. Although advanced progress has been gained in flame retardant cotton fabric, all of these cases could not maintain LOI value over 30% after 50 LCs due to the formation of sodium salt during home laundering because of the hydrolysis and ion exchange of the ester bond (P-O-C) [11]. Thus, a challenge regarding permanent washing durability remains unsolved. Furthermore, to our knowledge, most of efforts have been focused on improving flame retardancy and durability, but not how to recover the flame retardancy after washing.

In this work, we proposed a novel fireproof cotton fabric with enhanced washing durability but also surprisingly restorable flame retardancy through a green and facile strategy from NH₄H₂PO₄ and melamine grafting. Firstly, phosphate was grafted onto the surface of cellulose fiber of cotton fabric, accompanied by the generation of carbonyl groups (C = O) on cellulose surface due to the oxidization of free hydroxyl group during phosphorylation process. Subsequently, melamine was introduced into cellulose surface by Schiff base reaction between amino groups of melamine and the generated carbonyl groups. Consequently, the resultant samples displayed outstanding fire resistance and washing durability. The LOI value after 50 LCs still remained at 32.7%, revealing permanent flame retardancy. Interestingly, the reduction in laundering durability could be recovered via a feasible acetic acid soaking method. The mechanism of the efficient, durable and restorable flame retardancy of cotton fabric was also discussed in detail.