Novel and eco-friendly flame-retardant cotton fabrics with lignosulfonate and chitosan through LbL: Flame retardancy, smoke suppression and flame-retardant mechanism

Highlights

• Lignosulfonate and chitosan were chosen to act as flame retardant.

• CS/LS can form effective intumescent flame-retardant system.

• The addition of CS can reduce the afterglow time of LS/cotton.

• CS/LS/cotton presents well smoke suppression performance.

Abstract

Bio-based materials have been noticed for the continuous environmental pollution and resource shortage. In this paper, lignosulfonate (LS) and chitosan (CS) were selected to be flame-retardants. In the vertical flame test (VFT), LS/cotton-17.1 wt%, CS/LS/cotton-17.0 wt%, and CS/LS/cotton-25.2 wt% obtained lower afterflame times than that of uncoated cotton fabrics, while LS/cotton-17.1 wt% had serious afterglow time of 70 s. However, the afterglow time of CS/LS/cotton-25.2 wt% was 11 s, which decreased owing to the increased amount of CS. LS/cotton-17.1 wt%, CS/LS/cotton-17.0 wt%, and CS/LS/cotton-25.2 wt% presented limiting oxygen index values of 24.7%, 25.0%, and 26.0%, respectively. The scanning electron microscope images of char residues after the VFT showed that CS and LS formed an intumescent flame-retardant system, and cotton fibers remained intact structure. Meanwhile, the results of thermogravimetric analysis suggested that flame-retardant cotton fabrics can generate stable char residues. In N₂ atmosphere, CS/LS/cotton-25.2 wt% generated 27.1% stable char residues. Moreover, the addition of CS/LS decreased the heat release rate and total heat release values and this system performed well in smoke suppression. The flame-retardant mechanism of the system might belong to gas-phased because more non-combustible products were generated in the thermal degradation process.

Introduction

The fire caused by flammable materials in our daily lives can bring us serious property loss and casualties, and cotton fabrics belong to a flammable material [[1], [2], [3], [4]]. Cotton fabrics have been utilized in different fields as well as the application of them to expand the fire risk [[5], [6], [7]]. Therefore, it is necessary to conduct flame retardant treatment on cotton fabrics [8,9]. This has been demonstrated in several studies that positive results have been reported in halogenated containing flame retardants and they have been widely applied in the market [10,11]. However, halogenated flame retardants might generate harmful gases during combustion, release hazardous substances like formaldehyde into indoor air which might influence the health of people, and be recorded in animals and fishes with a high concentration [[12], [13], [14]]. Accordingly, to reduce the damages raised by halogenated containing flame retardants and the waste of resources, the studies and utilization of eco-friendly flame retardants seem to be a feasible choice [15,16]. Based on the advantages of biodegradability, clean and large reserves, bio-based materials have obtained widespread notice as flame retardants for cotton fabrics [17], such as lignosulfonate (LS) [[18], [19], [20]], chitosan (CS) [[21], [22], [23], [24]], DNA [25,26], starch [27,28], phytic acid [24,[29], [30], [31]] and protein [29,32,33] et al.

Layer-by-Layer assembly technology (LbL) is a common surface modification method for forming flame retardant coatings on cotton fabrics [34,35]. This technique was first proposed by Kirkland [36] and Iler [37] in 1965–1966, and they proposed a new approach to surface treatment. Since LbL has attracted wide attention for its simple, efficient, and inexpensive procedures [34], it has been applied to produce flame-retardant cotton fabrics. Grunlan et al. [38] have utilized LbL technology with branched polyethylenimine and Laponite clay platelets for the flame-retardant modification of cotton fabrics, which was the first application of LbL in the field of flame retardation. Polyamidoamines are one of synthetic bio-inspired polymers which have been applied in the flame-retardant properties of cotton fabrics, and presented excellent effects in recent researches [4,[39], [40], [41]]. Li et al. [42] produced coated samples with APTES and sodium phytate by LbL, and the sample with 15BL can extinguish. Zhang et al. [43] also produced flame-retardant cotton fabrics through LbL and the flame retardancy of the 8BL sample is significantly enhanced. It should be pointed out that cotton fabrics need to obtain multiple layers to have good flame-retardant performance in most of the papers, which is not conducive to the manufacture and adhibition of coated samples. The flame-retardant samples with fewer layers were prepared in this work.

Lignin is a renewable resource that is widely found in all plants except mosses and fungi, and it is the second richest organic substance in the world except cellulose [44,45]. And LS which belongs to the commercial lignin is one of the by-products during the sulfite pulping process; however, there is only 1% of the LS production which has been used [46]. Waste LS has been utilized in different fields, including flame retardants. LS can act as carbon and acid source in flame-retardant treatments because it contains aromatic structure as well as sulfonyl, carboxyl, and phenolic hydroxyl groups [20,47]. LS might decrease the combustion rate and promote the formation of char residues, resulting in the formation of more stable carbon residues [48,49]. Pan et al. [50] has prepared successfully flame-retardant flexible polyurethane foam (FPUF) using LS and CS through LbL technology. FRUF-8BL presents a 42% reduction in peak heat release rate (PHRR) and obtains 5.4 wt% char residues in the thermogravimetric analysis (TGA), indicating that the fire resistance and thermal stability of FRUF have been improved. Khalid Safi et al. [51] chosen CS, LS, and boric acid for the flame-retardant preparation of cotton through LbL, and the flame-retardant cotton fabrics present great antibacterial activity, flame retardancy, and antioxidant activity. However, the mechanism of the flame-retardant system and the smoke suppression function of CS were barely mentioned.

In this paper, LS and CS have been chosen to treat cotton fabrics through LbL technology. Limiting oxygen index (LOI), scanning electron microscope coupled with energy dispersive spectrometer (SEM-EDS), microscale combustion calorimetry (MCC), vertical flame test (VFT), cone calorimeter test (CCT), thermogravimetric analysis along with Fourier transform infrared analysis (TG-FTIR) was utilized to test samples. This study attempts to expand the application of bio-based flame retardants such as LS and CS in cotton fabrics and simplify the preparation process of flame-retardant cotton fabrics.