Flame retardant and hydrophobic coatings on cotton fabrics via sol-gel and self-assembly techniques

Abstract

Nanocoatings consisting of ammonium polyphosphate (APP), sodium montmorillonite (MMT), and vinyltrimethoxysilane (VTMS) were prepared via self-assembly and in situ sol-gel techniques and applied onto cotton fabrics to achieve both flame retardancy and hydrophobicity. The impacts of APP concentration on the hydrophobicity and fire resistance of the coated fabrics were investigated. Attenuated total reflectance Fourier transform infrared (ATR-FTIR) characterization results verified the hydrolysis-condensation reaction of VTMS and the formation of Si-O-Si network structure. X-ray diffraction (XRD) proved the formation of a layered structure based on MMT nanosheets in the coatings. Both vertical flame test (VFT), limiting oxygen index (LOI), thermogravimetric analysis (TGA) and microscale combustion calorimeter (MCC) characterization were conducted to evaluate the flame retardancy, thermostability and heat release behavior of the coated cotton fabrics, respectively. The results suggested that a higher concentration of APP is beneficial for both hydrophobicity and flame retardancy of the coated substrates. Overall, our research provides a facile and very effective approach to prepare flame retardant and hydrophobic multifunctional coating for cotton fabric and other substrates.

Introduction

Cotton is a popular natural resource widely used in the textile industry because of its soft, comfortable, and breathable features, thus ideally used in apparels, draperies, tents, pillowcases, towels, bed sheets, etc. [1] However, cotton fabric is extremely flammable and hydrophilic, which poses challenges in many applications, where heat, humidity, packaging quality, among other factors combine to form fire conducive environments [1].

Unlike synthetic fibers, natural cotton fibers cannot gain flame retardancy by mechanical incorporation of flame retardant additives into their matrices [2], [3]. Surface modification is the main approach to impart flame retardancy to cotton fabric. Methods of surface modification include ultraviolet curing of functional monomers and pre-polymers [4], sol-gel process to form a coating layer directly on cotton [5], [6], and layer-by-layer (LbL) assembly of a thin coating on cotton fabric surface [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]. Two main strategies have been developed to increase the hydrophobicity of cotton fabrics, i.e., grafting of silica nanoparticles [24], [25] and sol-gel process [26], [27], [28]. Many approaches were developed to gain either flame retardancy or hydrophobicity on cotton fabrics, but few can achieve both functions. Cotton fabric comprising both hydrophobic and flame retardant properties is highly desirable for many practical applications.

Sun et al. [29] reported an intumescent flame retardant and self-healing hydrophobic coating applied onto cotton fabrics, involving sequential deposition of a trilayer of branched poly(ethylenimine) (bPEI), ammonium polyphosphate (APP), and fluorinated-decyl polyhedral oligomeric silsesquioxane (F-POSS). However, the high cost of F-POSS restricts the commercialization of this technology. Wang et al. [30] reported a method to fabricate cotton fabrics with hydrophobicity and flame retardancy, consisting of surface modification of cotton fabrics, grafting amino-functionalization of silica nanoparticles, and a final fluorination treatment of the pretreated cotton fabrics. But the complexity of the process makes it difficult to scale up.

Intumescent coatings have been proved to be effective to impart fire resistance to cotton fabrics [10], [12], [14], [15], [17], [23], [31], [32], [33]. Such coatings typically consist of a carbon source, an acid source, and a blowing agent, which help suppress flame propagation through a condensed phase by building thermal insulation layers, thereby protecting substrates from destruction by fire [34]. Sol-gel process is one of the most effective techniques to achieve hydrophobicity through hydrolysis and condensation of silane precursors [24], [35], [36], [37], [38], [39], [40]. The hydrolysis reaction generates silanol groups, and the condensation product of silanol groups is a Si-O-Si network structure. The degree of hydrolysis/condensation determines the level of hydrophobicity [41], [42].

Herein, we aim to develop a facile, scalable, and effective approach to prepare coatings via in situ sol-gel and self-assembly techniques, which can be applied onto cotton fabrics through a single step to achieve both flame retardancy and hydrophobicity.