An eco-friendly NP flame retardant for durable flame-retardant treatment of cotton fabric
Graphical abstract
Introduction
Cotton fiber is a natural biological macromolecule, which is mainly composed of cellulose [1]. Cotton fiber is breathable, moisture-absorbing, soft, biodegradable, and easy to dye and process [2], giving it broad application potential in various fields, such as apparel, packaging, medical, military, etc. [3]; however, cotton fibers are extremely flammable and pose severe fire hazards, which greatly limits their applications [4]. Therefore, it is necessary to endow cotton fibers with flame retardancy to meet the needs of different applications.
Flame retardants are mainly used to treat natural or synthetic polymeric materials to inhibit the spread of flames to prevent loss of life and property [5]. Halogen-based flame retardants are widely used for cotton fabrics because of their excellent flame retardancy, they can prevent the combustion of cotton fabrics through a gas-phase mechanism [6]. Nevertheless, halogen-based flame retardants produce harmful substances during combustion [7], [8], [9], causing many European countries to prohibit their use [10]. Phosphorus-based flame retardants have gradually become an excellent substitute for halogen-based flame retardants due to their high efficiency, low toxicity, and environmental friendliness [11]. These compounds mainly function through a condensed-phase mechanism. During thermal decomposition, phosphorus-containing flame retardants release phosphoric acid and polymetaphosphoric acid to promote the dehydration of cellulose into char, thus inhibiting the spread of fire [12]; however, phosphorus-based flame retardants also have disadvantages such as high dosages, high costs, and low durability [13].
The methods for developing flame-retardant cotton fabrics main include layer by layer (LBL) assembly, sol-gel technology, UV curing technique and grafting modification [14]. The advantages of LBL assembly are mild reaction conditions and low loss of mechanical properties [15]. However, there are also some obvious shortcomings, such as complex preparation process, long time consuming, poor durable flame retardancy and poor handle and air permeability [16]. Sol-gel technology can achieve a uniform distribution of flame retardant on cotton fabric, and the loss of mechanical properties of cotton fibers after flame retardant treatment is small, but the sol-gel technology has a long cycle, which is not conducive to large-scale industrial production [17]. UV curing technique has low equipment requirements and is conducive to mass processing, but the flame-retardant raw materials are expensive [18].
Grafting modification has been widely studied in recent years because of its advantages such as short time consumption, good flame retardant effect, and little influence on handle and air permeability of cotton fabric [19]. Proban and Pyrovatex® are industrial phosphorus-containing flame retardants [20]. The P-CH2-OH active group in the flame retardant molecule reacts with the -OH group of cellulose to form a C-O-C covalent bond, which endows the cotton fabric with excellent durable flame retardancy [21]. Liu et al. oxidized the hydroxyl groups in cellulose to aldehydes, which then reacted with amino compounds to form CN bonds. Then, diethyl phosphite was grafted onto cellulose by an addition reaction. After 10 mild soaping treatments, the LOI value of the finished cotton fabric was 27.4%, and the flame retardancy was well maintained [22]. Zhang et al. synthesized a phosphorus-containing flame retardant, DPG, which was then grafted onto cotton fabric using urea and dicyandiamide as catalysts. The DPG-treated cotton fabric met the flame retardant standard after 20 laundering cycles [13].
The flame-retardant finishing of cotton fabrics has the following two problems: (1) The flame retardant releases harmful substances such as hydrogen halides or formaldehyde during combustion process, which pose health and environment hazards [17], [23]; (2) the durable flame retardancy of flame-retardant cotton fabrics cannot meet the requirements of commercial use [24]; therefore, it is necessary to develop a halogen-free, formaldehyde-free, efficient, and durable flame retardant for cotton fabric.
Recently, various eco-friendly durable flame retardants were successfully synthesized, such as phosphorus-containing compounds derived from sol-gel processes [25], (N, N-dimethyloctadecyl phosphate acrylamide) NDOPA [26], non-halogenated organophosphorus flame retardant by UV photoinitiated thiol-ene click chemistry [27], etc. However, their preparation process is complex and the raw materials are expensive, which makes them unsuitable for industrial mass production. In our previous work, flame retardants AASMP containing –P=O(O-NH4+)2 reactive groups were successfully synthesized [28]. AASMP-treated cotton fabrics will not release formaldehyde during use, but formaldehyde was used in the synthesis process. Meglumine contains multiple active hydroxyl groups, and it can be applied to many areas through chemical modification. In this paper, the flame retardant, ASMPEA, was synthesized by chemical modification of meglumine. No harmful substances (such as halogens or formaldehyde) were generated during the synthesis process, and the ASMPEA-treated cotton fiber obtained good durable flame retardancy. In addition, the synthesis of ASMPEA is simple and mild.
Section snippets
Materials
Cotton fabric (100%, 128.61 g/m2) was obtained from Chaotianmen Market in Chongqing, China. Phosphoric acid (85%) and meglumine were purchased from Macklin Biochemical Co., Ltd. (Shanghai, China). Urea was obtained from Yuanye Biological Technology Co., Ltd. (Shanghai, China). Dicyandiamide was obtained from Chengdu Kelong Chemical Reagent Co. Ltd. (Chengdu, China). Ethanol was purchased from Chongqing Chuandong Chemical Co., Ltd. (Chongqing, China). Sodium hydroxide and hydrogen peroxide were
FTIR
The FTIR spectra of ASMPEA, C0, and FRC-30 are shown in Fig. 1. In the infrared spectra of ASMPEA, the peak near 3335.2 cm-1 corresponded to the vibration absorption of NH4+ groups [24]. The peaks at 1650.0 cm-1, 1406.8 cm-1, and 1287.6 cm-1 were due to the presence of NH, CN, and PO groups [29], [30]. In addition, FRC-30 and C0 displayed broad absorption bands at 3389.1 cm-1 and 2900.4 cm-1 corresponding to the stretching vibrations of –OH and CH bonds, respectively [31]; however, compared
Conclusions
An efficient and durable, eco-friendly PN flame retardant ASMPEA was synthesized by a one-pot method. SEM, XRD, and EDS results demonstrated that the morphology and crystal structure of the treated cotton were unchanged and that ASMPEA infiltrated the inner space of the cotton fiber. In the vertical flammability tests, the treated cotton fabric self-extinguished once the fire was removed and produced char, whereas the control cotton completely burned. The TG results revealed that the
Funding
No funding.
Ethics approval
Compliance with ethical standards.
Consent for publication
The manuscript has been approved by all authors for publication.
Availability of data and material
The data is availability.
CRediT authorship contribution statement
Ying Liao: Writing-original draft, Writing-review & editing.
Yu Chen: Writing-review & editing.
Caiyan Wan: Supervision.
Guangxian Zhang: Resource.
Fengxiu Zhang: Resources.
Declaration of competing interest
No conflict of interest appears for the manuscript submission.
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