NITROGEN- AND PHOSPHORUS-CONTAINING FIRE-RESISTANT MATERIALS BASED ON POLYACRYLONITRILE

АННОТАЦИЯ

Данная работа посвящена получению и изучению физико-химических свойств азот- и фосфорсодержащих материалов на основе полиакрилонитрильного волокна.

Структуру полученных образцов охарактеризовали с помощью инфракрасной спектроскопии с преобразованием Фурье (FTIR). Термическую стабильность и огнестойкость оценивали с помощью термогравиметрического анализа и изучения кислородного индекса полученных образцов.

ABSTRACT

In this work is devoted to obtaining nitrogen- and phosphorus-containing materials based on polyacrylonitrile fiber and studying their physicochemical properties.

The structure of the resulting samples was characterized by Fourier transform infrared spectroscopy (FTIR). Thermal stability and fire resistance were evaluated using thermogravimetric analysis and the study of the oxygen index of the obtained samples.

Ключевые слова: полиакрилонитрил, мочевина, модификация, огнестойкий, лимитирующий кислородный индикатор, ортофосфатная кислота.

Keywords: polyacrylonitrile, urea, modification, flame retardant, limiting oxygen index, phosphoric acid.

INTRODUCTION

PAN fiber is widely used for textile products [1], membrane materials and construction products because of the excellent fluffy handle, elasticity, warmth retention, sunlight resistance and radiation resistance. However, low limiting oxygen index (LOI) of PAN (17–18%), severe melt-dripping and large amount of smoke released during its combustion increase the fire hazards which induce the need to improve the flame retardancy of PAN [2]. In addition, the released toxic gases in the combustion process seriously restricts its wide application in textile industry and other fields. Generally, blending [3], copolymerization [4], finishing [5] and chemical modification [2] are the common techniques to obtain flame retardant PAN fibers or fabrics. The chemical modification is a simple and easy route to prepare flame retardant PAN fiber or fabric that will not produce toxic gas and smoke. Grafting is one of the most important modification methods for synthetic polymers. Special physical and chemical properties of matrix polymers can be obtained by grafting some functional groups onto polymer chains. There are few reports on the use of the grafting method to prepare flame retardant PAN via the chemical modification of the monomer units of macromolecular chains. The utilization of a combination of hydrolysis of pendant groups and chemical modification of PAN homopolymer to prepare flame retardant PAN has not been reported. In recent years, chemical modification of PAN has attracted great attention. Generally, PAN fibers are chemically treated with hydrazine hydrate, hydroxylamine or other amination reagent to impart flame retardant properties. Ren et al. [3] prepared the fire retardant PAN fiber through the reaction of hydrazine hydrate with PAN fiber. The appearance of new C=N bond indicated that hydrazine hydrate was really reacted with PAN fiber, and the flame retardant performance of PAN fiber increased greatly with the prolonged modification time. Similarly, Kang et al. [6] employed blending solution of sodium hydroxideand hydrazine hydrate to prepare PAN fibers with flame retardancy, good moisture absorption and mechanical properties. Therefore, environmentally friendly and efficient phosphorous-containing flame retardants have become the current research focus [7].

PAN fiber modified with urea is mainly used as ion exchange material, however, the flame retardant study of PAN fiber or fabric modified with urea has rarely been reported to the best of our knowledge. In this work, we exploited a flame retardant PAN fabric by the amination and phosphorylation. Firstly, PAN fabric was reacted with urea to form amine groups functional PAN fabric. Then, amined PAN was reacted with phosphoric acid to prepare flame retardant PAN fabric. Phosphorus is known as an efficient flame retardant element widely used for various materials. In this paper, amination of PAN fabric followed by phosphorylation was performed to prepare flame retardant PAN fabric. The modified PAN fabric possessed durable flame retardant performance, remarkable thermal stability and char yield. The flame retardant mechanism and washing durability were investigated in detail.

EXPERIMENTAL SECTION

Amination of PAN fabric

The amined PAN fabric was prepared via the reaction between nitrile group
(–C≡N) of PAN and urea. The reaction was carried out in a liquid urea in a closed stainless steel container at 160 ^oC for 4 h. The treated PAN fabric was taken out, rinsed with deionized water several times to remove the remained urea, and dried in an oven at 50 ^oC for 24 h to constant weight.

Phosphorylation of amined PAN

Phosphorus acid (PA) was diluted to the concentration of 5 wt%. After  that, amined PAN was impregnated into the phosphorus acid, reaction was carried out at room temperature for 12 h. The phosphorylated amined PAN (P-A-PAN) was taken out, washed with deionized water and dried at 50 ^oC for 12 h to constant weight.

RESULTS AND DISCUSSION

FTIR analysis

The FTIR spectra of original PAN and FR-PAN fabrics are shown in Figure 1. and Figure 2.

Figure 1. FTIR spectra of PAN fabric

 Figure 2. FTIR spectra of flame retardant PAN fabric

The FTIR spectra of the control sample is presented in Figure 1. For the blank fabric, the adsorption peaks of stretching vibrations at 2241 cm^-1 (C≡N), 1729 cm^-1 (C=O of ester group), and 1450 cm^-1 (C-H bending in CH2) are clearly observed, which are the characteristic peaks of the copolymerized PAN fibers of methyl acrylate and itaconic acid.

Compared with the original PAN sample, some typical peaks also appeared in the FR-PAN sample (Figure 2). For the spectra of FR-PAN, the peak at 2241 cm^-1 almost disappeared, which indicated that the reaction between urea and CN groups was successfully happened. The emerging peak at 1639 cm^-1 could be attributed to the carboxyl group (COOH). It suggested that as the hydrolysis of PAN continued, more and more COOH generated. Moreover, a broad band at 2980–3600 cm^-1 appeared in FR-PAN spectra, which was mainly due to the presence of hydroxyl (OH) and NH₂. However, as far as FR-PAN was concerned, another three new peaks appeared in the spectrum. The band at 1656 cm^-1 with a larger intensity was attributed to COOH. The band at 1246 cm^-1 was assigned to P=O, P–O and P–O–C, while the peak at 923 cm^-1 was due to the presence of P–OH bond. These results confirmed that amined-PAN had successfully reacted with phosphoric acid and formed a phosphorylated PAN fabric, that is, flame retardant PAN fabric.

Thermogravimetric analysis

The thermogravimetric (TG) analysis curves and data of the original PAN (a), and FR-PAN (b) samples are shown in Figures 3 and 4 as well as Table 1. Typically, the thermal mass loss of FR-PAN can be divided into three stages. The first stage ranged from 83 ^oC to 272 ^oC, corresponding to the adsorbed moisture evaporation and the decomposition of small molecules of the sample, which caused a mass loss of about 5 wt%. The second stage started from 272 ^oC to 450 ^oC following a large number weight loss due to dehydrogenation, the release of HCN, and molecular chain degradation during the cyclization reaction of nitrile group [8]. The last stage (450–800 ^oC) was mainly oxidation stage of the residue with less mass loss [9]. The mass loss of FR-PAN at 800 ^oC was 46%, which was 9% less than that of original PAN at 800 ^oC with 55%. Furthermore, after a critical temperature (ca. 350 ^oC), the decomposition rate of FR-PAN was obviously less than of original PAN. The results show the obvious increase of the residual carbon of flame retardant modified PAN.

Figure 3. TG curves of original PAN and FR-PAN

The phosphoric acid, formed by the thermal decomposition of the phosphate group in the side chain of FR-PAN and dehydration from the phosphate group, and the metaphosphoric acid formed by the polymerization can promote the cyclization of C≡N to form carbon layer. Therefore, a compact expanded carbon layer was formed on the surface of polymer matrix. As a result, the carbon residue amount in FR-PAN was higher than that in original PAN. Moreover, the char residue was increased from 44.9 wt% for the blank fabric to 54.05 wt% for the FR-PAN fabric, indicating excellent char-forming ability and thermal stability. In addition, the maximum mass loss rates of FR-PAN fiber are lower than that of pure PAN fiber.

Figure 4 illustrates the differential thermogravimetric (DTG) curves of the original PAN and FR-PAN samples. The original PAN had only a peak, indicating a major decomposition stage. Compared with original PAN, FR-PAN had multiple peaks, suggesting that its decomposition stage was complex. The weight loss curve of FR-PAN was the fastest in the range of 400-450 ^oC, and slower in the range of 250-330 ^oC. The thermal decomposition of FR-PAN was earlier and wider than that of original PAN.

Figure 4. DTG curves of original PAN and FR-PAN

During the burning process, flame retardant unit bearing P element would transform to phosphoric acid and metaphosphatic acid, which then promoted the dehydration and carbonization of PAN and formed the char residue. It was more favorable for the inhibition of the transfer of heat and combustible gases.

Table 1.

 Thermogravimetric analysis data of original PAN and FR-PAN.

LOI analysis of original PAN and FR-PAN fabrics

The LOI value of original PAN and FR-PAN fabrics before and after washing was presented in Table 2. As the LOI value of PAN fiber is only 18%, PAN fiber belongs to a highly flammable fiber. The LOI value of FR-PAN rose up to 30.4% after the chemical modification with urea and phosphoric acid, which increased by 12.4% compared to that of PAN. However, it was inflammable in horizontal flame with an excellent flame retardancy and thermal stability. The phosphorylation of amined PAN had generated lots of phosphorus-containing acidic groups, which could promote cyclization and carbonization of PAN. In order to verify the durability of the flame retardant PAN fabric, the fabric was washed. The LOI value of FR-PAN fabric was 29.2%, 28.7%, 28,6 and 28.5% after 5, 10, 15 and 20 washing cycles, respectively. The washed fabrics were all incombustible and less smoke release. This demonstrated that FR-PAN had good washing durability, attributing to the phosphorus and nitrogen-containing components linked to PAN fabric by chemical bonding. That is to say, the flame retardant components could not be easily washed away during washing. However, after 20 washing cycles, the LOI value dropped down to 28.5%. This result implied that long-time severe hydraulic shear during washing could cause damage to the flame retardant PAN fabric to some extent, resulting the LOI value decrease.

Table 2.

The limiting oxygen index values of different fabrics before and after washing

Conclusions

In conclusion, a novel flame retardant method based on chemical modification for polyacrylonitrile fabric was developed. PAN fabric was successfully modified with urea to prepare amined PAN fabric followed by phosphorylation with phosphoric acid to obtain durable FR-PAN fabric. Analytical techniques including Fourier transform infrared spectroscopy was used to measure the structures of original PAN and FR-PAN samples. Thermogravimetric analysis and derivative thermogravimetric analysis were used to analyze their thermal properties.TG-DTG results showed that the introduction of amine group and phosphate could catalyze the cyclization of PAN, resulting in increased char residue (54%) at 800 ^oC. The results proved that the chemical modification was successfully conducted as expected. The LOI value of flame retardant PAN fabric reached to 30.4% and its LOI value still remained at 28.5% after 20 washing cycles. Therefore, nitrogen- and phosphorus-containing flame retardant fabric has excellent thermal stability and durable flame retardant performance.

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