A facile and effective flame-retardant coating for cotton fabric with α-aminodiphosphonate siloxane
Introduction
Owing to increasing environmental threats, textiles as one of the most important protective equipment should be rendered with various functions needed in different surroundings to protect people and fortune [[1], [2], [3]]. As the final step and important process for manufacturing textiles, finishing is the main method to provide properties and performances for textiles. Various surface finishing technologies, such as layer by layer (LBL) assembly [4,5], chemical grafting [6,7] and sol-gel method et al. [[8], [9], [10]], have been used to prepare functional textiles with antibacterial [11], flame retardant [12], water repellent [13], UV protective [14] or self-cleaning properties [15].
Among these methods, sol-gel technology has been one of the most important promising technologies in the textile industry due to its simple process, more efficient, mild reaction conditions, low cost and environmentally friendly [7]. Through simple hydrolysis and condensation of alkoxide precursors, a hybrid organic-inorganic coating can be deposited on the surface of textiles. After drying and curing under the appropriate conditions, they can form hydrogen and covalent bonds with fibers to increase the adhesion of the functional coating to the textiles [8,16].
In recent years, functional coating prepared by sol-gel technology has been exploited to impart flame retardancy to textiles using tetraethyl orthosilicate (TEOS) [17,18], tetramethyl orthosilicate (TMOS) [19], 3-glycidoxypropyltriethoxysilane (GPTS) and phosphorus/nitrogen-containing siloxane compounds as precursors [20]. The effect of silica (SiO2) layer made by TEOS on the flame retardancy of viscose fabrics was firstly investigated by Hribernik and coworkers. The results indicate that silica coating can increase the decomposition temperature of viscose fibers and significantly hinder volatiles generated. However, this pure SiO2 coating cannot bring ideal flame retardancy to viscose fabrics [21]. To solve this problem, some phosphorus, nitrogen, and/or boron-rich compounds have been chosen or synthesized and doped with/without silicon-based precursors to prepare efficient flame-retardant textiles. For example, (3-trimethoxysilylpropyl)diethylenetriamine and phenylphosphonic acid were combined to construct flame-retardant coating for cotton, PET and blended fabrics by Kappes and coworkers [22]. By introducing phosphorus/nitrogen-rich component, the flame retardancy was significantly improved and self-extinguishing behavior can be obtained. Zhang et al. prepared different kinds of boron-doped silica sols involving hydrolysis of TEOS precursor to improve flame retardancy and thermal stability of the wool fabric. The results showed that good smoke suppression property can be observed for the NH4HB4O7 doped silica sol coated fabric, and tensile strength and air permeation can be greatly maintained [23]. Furthermore, some phosphorous/nitrogen-containing siloxane compounds were synthesized and employed as an efficient flame retardant for textiles. A halogen-free phosphorus-silicone flame retardant (DOPO-VTS) was synthesized by Simončič and coworkers. The Si-DOPO coating on cotton fibers can increase char-forming ability and decrease heat release, but cannot prevent flaming combustion [24,25]. Hybrid phosphorus-doped silica architectures with different layers were deposited onto the surface of cotton fabrics using diethylphosphatoethyltriethoxysilane (DPTS) as the precursor. This coating can act as a protective part on the degradation of cotton fiber, hindering the formation of volatile species, and promoting the formation of char [26].
To further improve the flame retardancy of textiles, some phosphorus-nitrogen-containing compounds have been synthesized and can render textiles with good fire protection because of synergism effect existed. Phosphoramidates and α-aminophosphonates are two important kinds of phosphorus-nitrogen-containing compounds used in flame retardant filed. The reaction of an amine with a suitable phosphoryl halide species has been widely used to prepare phosphoramidates, but phosphoryl halides are hazardous and expensive. This problem can be solved by Atherton-Todd reaction, which is a facile and friendly method to prepare phosphoramidates. In our previous works, some kinds of phosphoramidates siloxane compounds have been synthesized by Atherton-Todd reaction and applied to prepare flame-retardant cotton fabrics. The results presented that perfect flame retardancy with high limited oxygen index (LOI, >30%) and self-extinguishing behavior can be obtained by promoting char formation, decreasing heat and flammable volatiles release, and increasing inflammable gas release [27]. The most widely used synthetic route to α-aminophosphonates is the Kabachnik-Fields reaction [28], which includes three components of carbonyl compound, diakylphosphite and amine. The possible reaction route is that a Schiff base between carbonyl compound and the primary amine forms firstly and immediately reacts with P–H bonds in diakylphosphite.
In this research, a kind of α-aminodiphosphonate siloxane compound (TTPBD) has been synthesized by Kabachnik-field reaction using 3-aminopropyltriethoxysilane as amine, paraformaldehyde as carbonyl compound, and dimethyl phosphate as diakylphosphite. The chemical structure of TTPBD was characterized by FTIR and NMR. This compound contains siloxane group, which can be deposited onto cellulose fabrics with covalent connect through sol-gel technology. Meanwhile, TTPBD can render cotton fabrics with good synergistic flame retardancy with the existence of silicon, phosphorus and nitrogen. The surface morphology and coating composition of TTPBD-coated cotton samples were determined by FT-IR, XPS and SEM-EDX. The flame retardant property of cotton fabrics was measured by LOI, vertical flammability test. The flame-retardant mechanism, including gaseous and condensed products, heat release and smoke production during thermal decomposition and combustion, was clarified by TG, TG-IR, SEM-EDX and cone calorimetry test. Besides, the tensile strength, breaking elongation, whiteness, bending properties, and washing durability were investigated.
Section snippets
Materials
Bleached cotton fabrics (14.75 tex × 14.75 tex, 122 g/m2) were obtained from Qingdao Fenghuang Dyeing & Printing Co., Ltd., China. 3-aminopropylmethyldimethoxysilane was provided by J&K Scientific Co., Ltd, China. Paraformaldehyde and dimethyl phosphate were purchased from Macklin Biochemical Co., Ltd, Shanghai, China. Tetrahydrofuran and sodium hydroxide (NaOH) were purchased from Sinopharm Chemical Reagent Co., Ltd, China. All reagents and medicines were used without further purification.
Synthesis of TTPBD
FTIR-ATR spectra and XPS
FTIR-ATR was used to investigate the characteristic functional group and difference of cotton samples before and after coating. FTIR-ATR spectra of control cotton and TTPBD-coated cotton samples are presented in Fig. 2a. Compared with untreated cotton samples, TTPBD-coated cotton samples showed three new characteristic absorption peaks: 1225 cm−1 (stretching vibration of PO), 845 cm−1 and 774 cm−1 (symmetric stretching vibration of O–P–O) [29,30]. The appearance of these new characteristic
Conclusions
A reactive α-aminodiphosphonate siloxane (TTPBD) for cellulose fabrics was successfully synthesized and deposited onto cotton fabrics through sol-gel technology to render them with flame retardancy. After TTPBD coating, the limiting oxygen index (LOI) of cotton fabric can reach to 27.5%, and it can pass the vertical flammability test with no afterflame/aftergrow appearance and 10.7 cm of char length. Through SEM-EDX, TG, TG-IR and cone calorimetry test, it can be concluded that TTPBD coating on
Credit author statement
Denghui Xu: Investigation, Data collection and analysis, Writing-original draft, Writing-review & editing; Zhiying Gao, Bing Xu, Hao Ren, Xingshun Zhao, Yanan Zhang: Methodology, Investigation, Data curation; Shijie Wang: Validation, Data curation; Zhiming Jiang: Idea, Supervision, Funding acquisition, Writing-reviewing & Editing; Ping Zhu: Instrumentation, Resources, Writing-review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors would like to thank the National Natural Science Foundation of China (51703101).
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2022, Progress in Organic CoatingsCitation Excerpt :In addition, this treatment has a potential for practical applications in real life as the FR coating only caused a minor decrease in tensile strength and elongation at break (Table 7, Entry 10). A similar reactive siloxane 81 was prepared by employing Kabachnik–Fields reaction in which (3-aminopropyl)triethoxysilane was used as the starting material (Scheme 43) [117], yet the synthesis involves the use of formaldehyde here. Then, the cotton fabric was immersed in the solution containing the compound whose pH was adjusted to 12 with the addition of NaOH, followed by padding and curing.