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Preparation and evaluation of an eco-friendly, reactive, and phytic acid-based flame retardant for wool
Reactive and Functional Polymers 134 (2019) 58–66
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Preparation and evaluation of an eco-friendly, reactive, and phytic acidbased flame retardant for wool ⁎
Xian-Wei Chenga,b, Jin-Ping Guana, Paul Kiekensb, Xu-Hong Yanga, , Ren-Cheng Tanga, a b
T
⁎
National Engineering Laboratory for Modern Silk, College of Textile and Clothing Engineering, Soochow University, 199 Renai Road, Suzhou 215123, China Center for Textile Science and Engineering, Department of Materials, Textiles and Chemical Engineering, Ghent University, 907 Technologiepark, Ghent 9052, Belgium
A R T I C LE I N FO
A B S T R A C T
Keywords: Wool Phytic acid Flame retardant Functional modification Durability
Bio-derived phytic acid exhibits great potential to improve the flame retardancy of textile materials, but it has poor washing durability. In order to address this problem, an efficient, reactive, and phosphorus-containing flame retardant (FR) HPPHBTCA was synthesized using phytic acid, pentaerythritol and 1,2,3,4-butanetetracarboxylic acid, and the chemical structure of HPPHBTCA was characterized. HPPHBTCA was applied to develop FR functional wool fabric, and its FR efficiency and washing durability were evaluated. The wool fabric treated with 0.14 mol/L HPPHBTCA had self-extinguishing performance even after 20 washing cycles during the vertical burning test, presenting good FR ability and resistance to washing. The catalytic char-forming effect of HPPHBTCA contributed to the enhanced FR and smoke suppression properties of wool fabric, and the ester bonds formed between HPPHBTCA and wool fiber resulted in the good washing durability. The HPPHBTCA treatment had a negligible effect on the whiteness, tensile strength and handle of wool fabric. This study offers a novel route to prepare the eco-friendly and durable FR agent using natural and phosphorus-containing compound.
1. Introduction Green and sustainable flame retardant (FR) products are drawing a growing attention in textile modifications due to the environmental pollution of textile industry. Although the cost-effective halogenated FR agents had a broad application in the functional modification of textiles and other polymeric materials in the past, they provoke health and environmental concerns due to the generation of toxic gases during combustion as well as the bioaccumulation and persistence in organisms and environment, resulting in the prohibition of use in textile industry [1]. The Zirpro FR system, being the most commercially available FR approach for protein fabrics, causes the metal pollution issue of water environment [2]. In addition, the phosphorus-based Pyrovatex CP and Proban, which are widely used for conferring durable FR property to cellulose fabrics, may induce damage to human health and environment due to the gradual release of formaldehyde in use or storage [3,4]. Renewable and biodegradable FR products provide great potential in addressing the aforementioned problems owing to their availability and good environmental benefits. In this context, a series of proteinbased polymers such as deoxyribonucleic acid, whey proteins, caseins, and hydrophobins have found their potential applications in the FR
⁎
treatment of textile materials (cotton, polyester or their blends) [5–8]. In addition, the application of other FR agents from natural resources to the finishing of textiles has also caused great attention [9,10]. For instance, FR function can be imparted to wool and wool/silk blend by phytic acid (PA) [11,12], to cotton by chitosan/PA combination and tannic acid [13,14], and to silk by condensed tannin [15]. The green, renewable and phosphorus-rich characteristics make PA an attractive natural product in the filed of FR modification. PA contains 6 phosphate groups and 12 hydroxyl groups in very close proximity, enabling it to interact with almost all kinds of positively charged compounds and metal ions [13]. Therefore, PA has high selectivity to be applied alone or together with other agents to provide FR functionality. Because of the strong ability to form complexes with polycations, PA was employed as an interesting candidate for layer by layer assembly to develop intumescent FR coatings with nitrogen- and silicon-containing compounds for textile materials [13,16–18]. Based on the same capacity, PA was used together with positively charged nitrogen-containing compounds to design polyelectrolyte complexes for fabricating FR polymers [19,20]. These phosphorus- and nitrogen/silicon-containing systems could effectively improve the flame retardancy of target materials thanks to the formation of highly expanded and charred structures.
Corresponding authors. E-mail addresses: [email protected] (X.-H. Yang), [email protected] (R.-C. Tang).
https://doi.org/10.1016/j.reactfunctpolym.2018.11.006 Received 10 September 2018; Received in revised form 6 November 2018; Accepted 8 November 2018 Available online 09 November 2018 1381-5148/ © 2018 Published by Elsevier B.V.
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Scheme 1. Synthesis route of HPPHBTCA.
Scheme 2. Cross-linking reaction between wool fiber and HPPHBTCA.
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2. Experimental 2.1. Materials Wool fabric (plain weave, 125 g/m2) was supplied by Shanghai Textile Industry Institute of Technical Supervision, China. PA (70% aqueous solution) was provided by Chengdu Ai Keda Chemical Technology Co. Ltd., China. PER, BTCA and dimethyl sulfoxide-d6 (DMSO‑d6) were purchased from Sinopharm Chemical Reagent Co. Ltd., China. The detergent for wool textiles was obtained from Shanghai Zhengzhang Laundering and Dyeing Co. Ltd., China. 2.2. Synthesis of HPPHBTCA PER (2.72 g, 0.02 mol) was mixed with PA (6.60 g, 0.007 mol) in a three-necked flask (250 mL). The mixture was maintained at 130 °C in the stirring condition. After 2 h, the viscous yellow liquid (hexapentaerythritol phytate ester) was obtained. Then BTCA (4.68 g, 0.02 mol) was added into the flask, and the heating of the mixture was continued at 130 °C. The mixture became more and more viscous with increasing reaction time, and turned out to be an elastic, yellow and transparent solid after 45 min, demonstrating the rapid cross-linking reaction between the components. The crude product was dried using a freeze-drying machine, and then purified using ethanol. The synthesis route of HPPHBTCA is presented in Scheme 1.
Fig. 1. FT-IR spectra of PA and HPPHBTCA.
Our previous work involved the application of PA to develop functional wool and wool/silk blend fabrics [11,12]. The as-prepared textiles exhibited remarkably enhanced thermal stability and flame retardancy. Although the electrostatic interaction took place between the protonated amino groups of protein fiber and the ionized phosphate groups of PA, the achieved flame retardancy was not durable due to the water solubility of PA and the lack of covalent cross-linking between PA and protein fiber. The challenging issue regarding to the durability of PA on textiles remains unsolved, which will impede the industrial application of PA in some extreme environments. Therefore, improving the washing durability of PA on textiles has become a necessary issue. The phosphate groups of PA make it possible to introduce polyhydric alcohols like pentaerythritol (PER) on PA by esterification reaction [21,22]. Furthermore, the free active hydroxyl structures of PER endow the formed phosphate ester with a great potential to be crosslinked onto polycarboxylic acid [23]. In the present study, PA was used together with PER and 1,2,3,4-butanetetracarboxylic acid (BTCA) to synthesize a reactive and phosphorus-containing FR agent, namely hexapentaerythritol phytate hexabutane tetracarboxylic acid ester (HPPHBTCA), which was expected to be grafted on wool fiber by the formation of ester covalent bonds during the pad-dry-cure finishing process because the esterification reaction between HPPHBTCA and wool fiber would take place at high curing temperatures [24,25]. The aim of this work was to develop durable FR wool textiles using the derivative from natural compounds. In this regard, the molecule structure of HPPHBTCA was characterized. The thermal stability and FR performance as well as the FR mechanism of the treated wool fabrics were discussed. At last, the impact of the FR treatment on the physical properties of wool fabric was evaluated.
Fig. 2.
13
C (a) and
31
2.3. Fabric treatment procedure For easy application, the viscous HPPHBTCA liquid obtained at 30 min was used as the FR agent and dissolved in distilled water. Two HPPHBTCA solutions whose total volume was 100 and 50 mL, corresponding to 0.07 and 0.14 mol/L, respectively, were used in the fabric treatment. The fabric samples were firstly dipped in the HPPHBTCA solutions at room temperature with a liquor ratio of 25:1. After 10 min, the samples were squeezed using a two-roll laboratory padder with two dips and two nips, giving a wet pickup of 100 ± 5%. Afterwards, the padded samples were dried and cured at 80 and 160 °C, respectively for 3 min. The cross-linking reaction between wool fiber and HPPHBTCA during the curing process is presented in Scheme 2. The obtained fabrics were washed thoroughly in distilled water, and then dried at 60 °C for 30 min. The weight gain of the modified wool fabric was determined using Eq. (1):
Weight gain (%) = 100 × (W1 − W0)/ W0
(1)
where W0 and W1 denote the weight of the fabrics before and after modification, respectively. Specifically, Wool-1 and Wool-2 in the Results and Discussion section denote the fabrics treated with 0.07 and 0.14 mol/L HPPHBTCA, respectively.
P (b) NMR spectra of PA and HPPHBTCA. 60
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2.4. Characterizations The Fourier transform infrared (FT-IR) analysis was conducted on the Nicolet 5700 FT-IR spectrometer (Thermo Fisher Scientific Inc., USA) using KBr pellets. The 13C and 31P liquid state nuclear magnetic resonance (NMR) spectra were recorded on the Bruker Avance III 400 MHz spectrometer (Bruker BioSpin GmbH, Germany) using DMSO‑d6 as a deuterated solvent. The surface morphologies of the wool fibers and chars from the vertical burning test were observed by the Hitachi-S4800 field-emission SEM (Hitachi High Technologies America, Inc., USA). The vertical burning and limiting oxygen index (LOI) tests were carried out according to GB/T 5455–2014 and GB/T 5454–1997, respectively; the burning behavior of the fabrics based on the vertical burning test was rated according to GB/T 17591–2006. The P content of the burned and unburned wool samples was detected on the inductively coupled plasma optical emission spectrometer (ICP-OES) according to our previously described method [11]. The smoke production capacity of the wool fabrics was evaluated on the FTT0064 NBS smoke density test chamber (Fire Testing Technology Ltd., UK) according to ISO 5659.2; each specimen with a dimension of 75 × 75 mm2 (two sample layers) was exposed horizontally to an external heat flux of 25 kW/m2 in the flameless combustion mode. The thermogravimetry (TG) curves of the wool fabrics were recorded on the Diamond TG/DTA SII thermal analyzer (Perkin-Elmer, USA). The washing of the treated fabrics was conducted at 40 °C according to our previously described method [12]; each wash was continued for 30 min. The mechanical properties such as tensile strength, bending length and flexural rigidity of the wool fabrics were measured according to our previously described method [11,26]; each test was repeated at least 5 times to ensure data accuracy. The Hunter whiteness index of the fabrics was measured on the HunterLab UltraScan PRO reflectance spectrophotometer according to our previously described method [12].
Fig. 3. FT-IR spectra of the treated and untreated wool fabrics.
3. Results and discussion 3.1. Characterizations of HPPHBTCA 3.1.1. FT-IR of HPPHBTCA FT-IR analysis was conducted to detect the functional groups of HPPHBTCA. The FT-IR spectra of PA and HPPHBTCA are presented in Fig. 1. For the spectrum of PA, the bands at around 3490 and 1641 cm−1 are ascribed to the OH stretching vibration of PA and the hydration of water molecules, respectively. The bands at 1164 and 989 cm−1 are associated with the stretching vibration of P]O, OePeC and PeO groups [27,28]. Some additional absorption bands were observed for HPPHBTCA as compared to PA. The significant difference between 1164 and 989 cm−1 is ascribed to the stretching vibration of CeO group in HPPHBTCA. The new bands at 1465 and 1384 cm−1 are assigned to the CeH and CH2 bending vibration of PER and BTCA in the product [29]. In addition, the new strong band at 1724 cm−1 corresponds to the stretching vibration of ester carbonyl bonds formed between PER and BTCA [24]. 3.1.2. NMR of HPPHBTCA The possible molecular structure of HPPHBTCA was further determined by liquid state 13C and 31P NMR spectra. As shown in Fig. 2, the septet adsorption band at 39.63 ppm in the 13C NMR spectra belongs to the CH3 groups of DMSO‑d6 [30]. The signal peaks at 75.62, 73.14, 72.89 and 72.23 ppm in the 13C NMR spectrum of PA are ascribed to the CH groups (C1~C6) of inositol ring [31]. Compared with the spectrum of PA, the spectrum of HPPHBTCA showed the new signals at 61.40 and 60.45 ppm which should be attributed to the CH2 groups (C7~C18), and the signal at 45.29 ppm corresponding to C19, C20 and C21 carbon atoms of PER in HPPHBTCA [32]. Moreover, the new signals at 174.12 and 173.36 ppm are ascribed to the carbonyl carbons
Fig. 4. SEM micrographs of the treated and untreated wool fabrics.
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Fig. 5. Weight gain (a), char length (b), LOI (c) and P content (d) of the treated wool fabrics after washing.
3.3. Flammability and smoke generation of the treated wool
of carboxylic acid (C28~C33) and ester moieties (C22~C27), respectively. The signals at 42.24 and 32.22 ppm are assigned to the CH2 (C34~C38) and CH (C39~C45) groups of BTCA in HPPHBTCA, respectively [33,34]. In addition, the significant chemical peak of 31P position shifting from −0.36 to −0.69 ppm should be ascribed to the formation of phosphate ester covalent bonds [35]. These features explicitly confirm the successful cross-linking modification of PER and BTCA to PA.
3.3.1. Flammability The FR efficiency of the treated wool was determined using the vertical burning and LOI tests. The untreated wool fabric got ignited easily during the vertical burning, and then the flame quickly burned the entire sample, remaining only a small quantity of char residue. The untreated wool sample obtained the highest char length of 30 cm, revealing its poor flame retardancy. However, the modification with PA and HPPHBTCA dramatically altered the burning behavior of wool. The treated wool substrates inflated immediately when ignited, displaying excellent char-forming ability. Interestingly, the expanded char was able to withstand the open fire, thus the flames on the treated wool selfextinguished before the ignition source was removed. As a result, the char length of the treated wool samples decreased obviously. Both PA and HPPHBTCA improved the FR performance of wool fabric by an evident charring mechanism. As shown in Fig. 5a, the wool fabrics treated with 0.14 mol/L PA (Wool-2-PA), 0.07 (Wool-1) and 0.14 mol/L (Wool-2) HPPHBTCA obtained a weight gain of 5.1%, 11.6% and 17.2%, respectively. As shown in Fig. 5b, all the treated wool fabrics exhibited a low char length of shorter than 12 cm, presenting improved flame retardancy. Correspondingly, the wool fabrics treated with 0.07 and 0.14 mol/L HPPHBTCA had a higher LOI of 28.5% and 30.3%, respectively compared with 23.6% of the untreated wool (Fig. 5c). As shown in Fig. 5b, by increasing the washing cycles, the treated wool displayed a gradual increase in char length, meaning a reduction trend of the FR ability. Furthermore, the char length of the PA treated wool fabric showed much higher increase than that of the HPPHBTCA treated wool fabric. The vertical burning photos (Fig. 6) of the wool fabrics treated with 0.14 mol/L PA (Wool-2-PA) and 0.14 mol/L HPPHBTCA (Wool-2) after washing gave the visual comparison. After 5 washing cycles, the wool fabric treated with 0.14 mol/L PA could not achieve B1 classification where the char length is shorter than 15 cm according to GB/T 17591–2006, displaying poor washing durability. However, the wool fabric treated with 0.14 mol/L HPPHBTCA was still self-extinguishing during the vertical burning and achieved the B1 classification after 20 washing cycles. Even for the wool fabric treated
3.2. FT-IR and morphology of the treated wool The possible reactions between HPPHBTCA and wool fiber were studied by FT-IR. As shown in Fig. 3, compared with the spectrum of the untreated wool, the spectra of the HPPHBTCA treated wool fabrics showed several new absorption bands. The P]O and OePeC absorption bands occurred at 1168 and 1060 cm−1, respectively in the spectra of treated wool [27,28]. The new band at 1724 cm−1 is ascribed to the stretching vibration of ester carbonyl groups [24], revealing the formation of ester covalent bonds between the carboxyl groups of HPPHBTCA and the hydroxyl groups of wool fiber. Besides, the amide groups formed between the carboxyl groups of HPPHBTCA and amino groups of wool fiber may also contribute to the cross-linking effect [25]. However, the formed amide groups can not be distinguished from the original peptide absorption in the FT-IR spectra. These results above are consistent with the reaction in Scheme 2, which shows the cross-linking reaction between HPPHBTCA and wool fiber. Furthermore, although PA contains 12 hydroxyl groups attaching to the phosphate groups, only parts of them can react with PER owing to the stereo-hindrance effect. Thus the residual phosphate groups of HPPHBTCA can combine with wool fiber through electrostatic interaction. The surface morphology of the wool fabrics was observed by SEM. The SEM micrographs shown in Fig. 4 confirm the attachment of cuticle layer on the surface of wool fibers, and also suggest the clean surface of the untreated wool fiber. For the treated wool fiber, a thin coating was found to homogeneously distribute around the fiber surface and deposit in the interstices between fibers. It seems that the ester cross-linking and electrostatic interactions between HPPHBTCA and wool fiber, which were illustrated by the FT-IR analysis, are strong enough to allow the deposition of HPPHBTCA on the wool fiber surface. 62
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Fig. 6. Vertical burning photos of the wool fabrics treated with 0.14 mol/L PA (upper) and HPPHBTCA (down) after different cycles of washing.
is much higher than 24.9% of the PA-treated wool (Wool-2-PA). The repeated washing test convincingly demonstrates that HPPHBTCA is an effective and durable FR agent for wool. The P content of the wool fabrics treated with 0.14 mol/L PA and HPPHBTCA after repeated washing was evaluated by ICP-OES. As shown in Fig. 5d, by increasing the washing cycles, the P content of the
with 0.07 mol/L HPPHBTCA, it still met the B1 classification standard after 10 washings, and met the B2 classification standard after 20 washing cycles, showing good washing durability. As depicted in Fig. 5c, the LOI of the treated fabrics had a reduction trend with repeated washing cycles. In particular, after 20 washing cycles, the wool treated with HPPHBTCA (Wool-2) still had a high LOI of 28.1%, which 63
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Fig. 7. Ds curves of wool fabrics obtained by smoke density test. Table 1 Smoke density parameters of wool fabrics. Sample
Time (s)
Dsmax
Control Wool-1 Wool-2
571 495 489
93.7 50.4 41.3
treated wool showed the similar reduction trend like LOI, revealing that the decreased FR performance discussed above is attributed to a decrease in P content. Moreover, the P content of the PA treated wool showed greater reduction compared with that of the HPPHBTCA treated wool. It is believed that the ionic bonding between PA and wool fiber is not resistant to washing. However, the cross-linking effect formed between the carboxyl groups of HPPHBTCA and the hydroxyl and amino groups of wool fiber is strong enough to withstand the robust washing, thus the wool treated with HPPHBTCA exhibits good durability to washing. These results above reveal the successful application of the reactive phosphorus-containing HPPHBTCA to wool fabric, and the treated wool fabrics possess desirable FR property and washing durability.
3.3.2. Smoke generation performance As shown in Fig. 7 and Table 1, the untreated wool produced heavy smoke, and had a high maximum specific optical density (Dsmax) of 93.7. However, the Wool-1 and Wool-2 samples displayed greatly suppressed smoke release in the whole combustion process, and had a low Dsmax value of 50.4 and 41.3, respectively. The significant suppression in smoke release is due to the ability of the FR agent to catalyze the conversion of organic matters to efficient char, and thus reduce the flame intensity. Additionally, the treated wool fabrics exhibited a shorter time to reach Dsmax compared with the untreated fabric. Anyway, the high efficiency of HPPHBTCA in reducing the smoke
Fig. 9. SEM micrographs of the char residues of wool fabrics after the vertical burning.
Fig. 8. TG curves of wool fabrics in nitrogen (a) and air (b). 64
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summarized as follows. Upon exposed to a flame or a heat flux, HPPHBTCA decomposes at first and releases phosphoric acids which can catalyze the formation of thermal stable char and intumescent structures rich in phosphorus. These changes enable the heat and oxygen transfer between gas and condensed phases to be impeded. Thus the further burning of underlying materials is blocked, and thereby the flame retardancy and thermal stability of wool textiles are improved. As a conclusion, HPPHBTCA can enhance the FR performance of wool by a condensed-phase action.
Table 2 Whiteness index, tensile strength, bending length and flexural rigidity of the wool fabrics. Sample
Whiteness index
Tensile strength (N)
Bending length (mm)
Flexural rigidity (mN cm)
Control Wool-1 Wool-2
79.8 ± 0.1 73.8 ± 0.1 69.8 ± 0.2
245.6 ± 1.0 256.6 ± 3.5 253.3 ± 3.8
16.4 ± 0.4 17.3 ± 0.3 17.8 ± 0.2
0.910 ± 0.063 1.042 ± 0.055 1.138 ± 0.037
generation of wool lowers the fire risk, and allows people to have more possibility of survival from a fire.
3.6. Physical properties of the treated wool The whiteness, tensile strength and handle of the wool fabrics before and after treatment were assessed. As shown in Table 2, wool fabrics showed little decrease in whiteness after treatment because of the faint yellow color of HPPHBTCA solution. The breakage of the untreated wool fiber follows the slippage mechanism of molecule chains [38]. For the treated wool fabrics, HPPHBTCA acting as a crosslinking agent among wool molecules reinforces the connection of fibers, causing more difficult slippage of molecule chains. Therefore, the treated fabrics exhibited little increase in tensile strength. In addition, the FR treatment adversely affected the softness of wool fabric as indicated by the slightly increased bending length and flexural rigidity, which is acceptable in the wool textile wet processing. As a conclusion, the FR treatment has little impact on the physical properties of wool.
3.4. TG analysis of the treated wool The thermal degradation and stability of the wool fabrics were assessed in nitrogen and air. As shown in Fig. 8a, the untreated wool experienced a two-step thermal degradation in nitrogen. The initial low weight loss below 150 °C was attributed to the loss of the physically bound water. The main and fast weight loss occurred in the temperature range of 220 to 425 °C, involving the decomposition of the peptide chain and production of combustible gases [36,37]. As shown in Fig. 8b, in air atmosphere, wool underwent a more complicated degradation process. Below 450 °C, the weight loss of wool took place in a similar way to the degradation of wool in nitrogen. However, at higher temperature, a new decomposition step related to the further oxidation of the remaining residues occurred in the presence of air. Thus, the untreated wool exhibited a char residue of 20.2% at 700 °C in nitrogen, and a lower char residue of 2.7% in air. As shown in Fig. 8, the HPPHBTCA treated wool showed the similar pyrolysis steps like the untreated wool but exhibited higher thermal degradation temperature in both nitrogen and air. The difference of weight loss between the treated and untreated wool appeared at around 350 °C which was in the second pyrolysis region, and it became more obvious with increasing degradation temperature. In other words, the treated wool had lower weight loss than the untreated one. This finding suggests that HPPHBTCA can change the thermal degradation pathway of wool, and at the same time favor the formation of a protective and quite stable char. As a result, the treated wool exhibited a remarkably increased char residue at the end of the degradation process. The Wool1 and Wool-2 samples yielded a residual char of 17.5% and 23.8%, respectively at 700 °C in air. Such high char residue of the treated wool demonstrates that HPPHBTCA can enhance the flame retardancy of wool by a condensation phase mechanism.
4. Conclusions In this study, an eco-friendly and reactive FR HPPHBTCA for the FR treatment of wool fabric was successfully synthesized using PA, PER and BTCA. The carboxyl groups of HPPHBTCA could react with the hydroxyl and amino groups of wool to form ester and amide bonds, contributing to the appreciable washing durability of the treated wool. The wool fabric treated with 0.14 mol/L HPPHBTCA was able to selfextinguish in the vertical flammability test and such FR ability was maintained after 20 washing cycles. Compared with the untreated wool, the treated fabrics exhibited significantly reduced smoke generation. The TG and P content analyses suggested that HPPHBTCA acted in the condensed phase to confer FR ability to wool. In addition, the physical properties of wool fabrics had small changes after the FR treatment. The limitation regarding to the poor washing durability of PA on wool fabric was overcome by the facile chemical modification of PA. The present study indicates that the sustainable and reactive phosphorus-containing FR has tremendous potential in practical application.
3.5. Morphology and P content of the wool char residues
Acknowledgements
Fig. 9 shows the SEM micrographs of the burnt residues of wool fabrics obtained from the vertical burning test. The untreated wool lost the original fibrous structure after burning and displayed a fragile and thin char as a result of the complete burning and insufficient char formation; some voids were formed due to the inflate action of the generated volatiles during burning. The treated wool fabrics maintained the original texture after burning, demonstrating that they have high thermal stability and good flame retardancy. In addition, globular structures were formed on the char residues of Wool-1 sample at localized spaces, and Wool-2 sample showed more expanded globular structures at enlarged spaces on the char residue in comparison with Wool-1 sample. Furthermore, the char residues exhibited much higher P content than the treated fabrics. For example, the unburned and burned Wool-2 samples had a P content of 16.23 and 39.66 mg/g, respectively, illustrating the mechanism of FR action of HPPHBTCA in condensed phase. On the basis of the thermal analyses and SEM observations of the char residues, the FR mechanism of the treated wool can be
This work was supported by Jiangsu Provincial Key Research and Development Program of China [BE2015066], and Postgraduate Research and Practice Innovation Program of Jiangsu Province [KYCX17_1986]. References [1] L.S. Birnbaum, D.F. Staskal, Brominated flame retardants: cause for concern? Environ. Health Perspect. 112 (2004) 9–17. [2] A.R. Horrocks, Overview of traditional flame-retardant solutions (including coating and back-coating technologies), in: J. Alongi, A.R. Horrocks, F. Carosio, G. Malucelli (Eds.), Update on Flame Retardant Textiles: State of the Art, Environmental Issues and Innovative Solutions, First Ed., Smithers Rapra Technology Ltd., Shawbury, 2013, pp. 123–178. [3] A.R. Horrocks, Flame retardant challenges for textiles and fibers: new chemistry versus innovatory solutions, Polym. Degrad. Stab. 96 (2011) 377–392. [4] Z. Yang, B. Fei, X. Wang, J.H. Xin, A novel halogen-free and formaldehyde-free flame retardant for cotton fabrics, Fire Mater. 36 (2012) 31–39. [5] J. Alongi, R.A. Carletto, A. Di Blasio, F. Carosio, F. Bosco, G. Malucelli, DNA: a novel, green, natural flame retardant and suppressant for cotton, J. Mater. Chem. A 1 (2013) 4779–4785.
65
Reactive and Functional Polymers 134 (2019) 58–66
X.-W. Cheng et al.
(1985) 213–228. [22] Y. Jia, Y. Lu, G. Zhang, Y. Liang, F. Zhang, Facile synthesis of an eco-friendly nitrogen–phosphorus ammonium salt to enhance the durability and flame retardancy of cotton, J. Mater. Chem. A 5 (2017) 9970–9981. [23] X. Huang, J. Guo, X. Gong, S. Li, Y. Gong, Q. An, F. Guan, Preparation and characterization of pentaerythritol/butane tetracarboxylic acid/polyethylene glycol crosslinking copolymers as solid-solid phase change materials, J. Macromol. Sci. A 53 (2016) 500–506. [24] C.Q. Yang, X. Wang, Formation of cyclic anhydride intermediates and esterification of cotton cellulose by multifunctional carboxylic acids: an infrared spectroscopy study, Text. Res. J. 66 (1996) 595–603. [25] S.H. Hsieh, Z.K. Huang, Z.Z. Huang, Z.S. Tseng, Antimicrobial and physical properties of woolen fabrics cured with citric acid and chitosan, J. Appl. Polym. Sci. 94 (2004) 1999–2007. [26] X.W. Cheng, C.X. Liang, J.P. Guan, X.H. Yang, R.C. Tang, Flame retardant and hydrophobic properties of novel sol-gel derived phytic acid/silica hybrid organicinorganic coatings for silk fabric, Appl. Surf. Sci. 427 (2018) 69–80. [27] X. Guo, Y. Miao, P. Ye, Y. Wen, H. Yang, Multi-walled carbon nanotubes in aqueous phytic acid for enhancing biosensor, Mater. Res. Express 1 (2014) 025403. [28] G. Jiang, J. Qiao, F. Hong, Application of phosphoric acid and phytic acid-doped bacterial cellulose as novel proton-conducting membranes to PEMFC, Int. J. Hydrogen. Energ. 37 (2012) 9182–9192. [29] Z. Wang, P. Lv, Y. Hu, K. Hu, Thermal degradation study of intumescent flame retardants by TG and FTIR: Melamine phosphate and its mixture with pentaerythritol, J. Anal. Appl. Pyrolysis 86 (2009) 207–214. [30] H.E. Gottlieb, V. Kotlyar, A. Nudelman, NMR chemical shifts of common laboratory solvents as trace impurities, J. Organomet. Chem. 62 (1997) 7512–7515. [31] Y. Feng, Y. Zhou, D. Li, S. He, F. Zhang, G. Zhang, A plant-based reactive ammonium phytate for use as a flame-retardant for cotton fabric, Carbohydr. Polym. 175 (2017) 636–644. [32] F. Liu, A.M. Orendt, D.W. Alderman, D.M. Grant, Carbon-13 chemical shift tensors in pentaerythritol, J. Am. Chem. Soc. 119 (1997) 8981–8984. [33] H. Kono, S. Fujita, Biodegradable superabsorbent hydrogels derived from cellulose by esterification crosslinking with 1, 2, 3, 4-butanetetracarboxylic dianhydride, Carbohydr. Polym. 87 (2012) 2582–2588. [34] H. Kono, T. Nakamura, Polymerization of β-cyclodextrin with 1, 2, 3, 4-butanetetracarboxylic dianhydride: Synthesis, structural characterization, and bisphenol a adsorption capacity, React. Funct. Polym. 73 (2013) 1096–1102. [35] A. Bebot-Brigaud, C. Dange, N. Fauconnier, C. Gérard, 31P NMR, potentiometric and spectrophotometric studies of phytic acid ionization and complexation properties toward Co2+, Ni2+, Cu2+, Zn2+ and Cd2+, J. Inorg. Biochem. 75 (1999) 71–78. [36] A.R. Horrocks, P.J. Davies, Char formation in flame-retarded wool fibers. Part 1. Effect of intumescent on thermogravimetric behavior, Fire. Mater. 24 (2000) 151–157. [37] C.M. Tian, Z. Li, H.Z. Guo, J.Z. Xu, Study on the thermal degradation of flame retardant wools, J. Fire Sci. 21 (2003) 155–162. [38] H. Liu, W. Yu, Study of the structure transformation of wool fibers with Raman spectroscopy, J. Appl. Polym. Sci. 103 (2007) 1–7.
[6] F. Bosco, R.A. Carletto, J. Alongi, L. Marmo, A. Di Blasio, G. Malucelli, Thermal stability and flame resistance of cotton fabrics treated with whey proteins, Carbohydr. Polym. 94 (2013) 372–377. [7] J. Alongi, R.A. Carletto, F. Bosco, F. Carosio, A. Di Blasio, F. Cuttica, V. Antonucci, M. Giordano, G. Malucelli, Caseins and hydrophobins as novel green flame retardants for cotton fabrics, Polym. Degrad. Stab. 99 (2014) 111–117. [8] G. Malucelli, F. Bosco, J. Alongi, F. Carosio, A. Di Blasio, C. Mollea, C. Mollea, F. Cuttica, A. Casale, Biomacromolecules as novel green flame retardant systems for textiles: an overview, RSC Adv. 4 (2014) 46024–46039. [9] L. Costes, F. Laoutid, S. Brohez, P. Dubois, Bio-based flame retardants: when nature meets fire protection, Mater. Sci. Eng. R 117 (2017) 1–25. [10] S. Basak, S.W. Ali, Sustainable fire retardancy of textiles using bio-macromolecules, Polym. Degrad. Stab. 133 (2016) 47–64. [11] X.W. Cheng, J.P. Guan, G. Chen, X.H. Yang, R.C. Tang, Adsorption and flame retardant properties of bio-based phytic acid on wool fabric, Polymers 8 (2016) 122. [12] X. Zhang, X.-Y. Zhou, X.-W. Cheng, R.-C. Tang, Phytic acid as an eco-friendly flame retardant for silk/wool blend: a comparative study with fluorotitanate and fluorozirconate, J. Clean. Prod. 198 (2018) 1044–1052. [13] G. Laufer, C. Kirkland, A.B. Morgan, J.C. Grunlan, Intumescent multilayer nanocoating, made with renewable polyelectrolytes, for flame-retardant cotton, Biomacromolecules 13 (2012) 2843–2848. [14] S. Nam, B.D. Condon, Z. Xia, R. Nagarajan, D.J. Hinchliffe, C.A. Madison, Intumescent flame-retardant cotton produced by tannic acid and sodium hydroxide, J. Anal. Appl. Pyrolysis 126 (2017) 239–246. [15] T.T. Yang, J.P. Guan, R.C. Tang, G. Chen, Condensed tannin from Dioscorea cirrhosa tuber as an eco-friendly and durable flame retardant for silk textile, Ind. Crop. Prod. 115 (2018) 16–25. [16] C.K. Kundu, W. Wang, S. Zhou, X. Wang, H. Sheng, Y. Pan, L. Song, Y. Hu, A green approach to constructing multilayered nanocoating for flame retardant treatment of polyamide 66 fabric from chitosan and sodium alginate, Carbohydr. Polym. 166 (2017) 131–138. [17] X. Wang, M.Q. Romero, X.Q. Zhang, R. Wang, D.Y. Wang, Intumescent multilayer hybrid coating for flame retardant cotton fabrics based on layer-by-layer assembly and sol–gel process, RSC Adv. 5 (2015) 10647–10655. [18] Z.F. Li, C.J. Zhang, L. Cui, P. Zhu, C. Yan, Y. Liu, Fire retardant and thermal degradation properties of cotton fabrics based on APTES and sodium phytate through layer-by-layer assembly, J. Anal. Appl. Pyrolysis 123 (2017) 216–223. [19] T. Zhang, H. Yan, L. Shen, Z. Fang, X. Zhang, J. Wang, B. Zhang, Chitosan/phytic acid polyelectrolyte complex: a green and renewable intumescent flame retardant system for ethylene-vinyl acetate copolymer, Ind. Eng. Chem. Res. 53 (2014) 19199–19207. [20] T. Zhang, H. Yan, L. Shen, Z. Fang, X. Zhang, J. Wang, B. Zhang, A phosphorus-, nitrogen-and carbon-containing polyelectrolyte complex: preparation, characterization and its flame retardant performance on polypropylene, RSC Adv. 4 (2014) 48285–48292. [21] G. Camino, L. Costa, L. Trossarelli, F. Costanzi, A. Pagliari, Study of the mechanism of intumescence in fire retardant polymers: Part VI-Mechanism of ester formation in ammonium polyphosphate-pentaerythritol mixtures, Polym. Degrad. Stab. 12
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Reactive and Functional Polymers journal homepage: www.elsevier.com/locate/react
Preparation and evaluation of an eco-friendly, reactive, and phytic acidbased flame retardant for wool ⁎
Xian-Wei Chenga,b, Jin-Ping Guana, Paul Kiekensb, Xu-Hong Yanga, , Ren-Cheng Tanga, a b
T
⁎
National Engineering Laboratory for Modern Silk, College of Textile and Clothing Engineering, Soochow University, 199 Renai Road, Suzhou 215123, China Center for Textile Science and Engineering, Department of Materials, Textiles and Chemical Engineering, Ghent University, 907 Technologiepark, Ghent 9052, Belgium
A R T I C LE I N FO
A B S T R A C T
Keywords: Wool Phytic acid Flame retardant Functional modification Durability
Bio-derived phytic acid exhibits great potential to improve the flame retardancy of textile materials, but it has poor washing durability. In order to address this problem, an efficient, reactive, and phosphorus-containing flame retardant (FR) HPPHBTCA was synthesized using phytic acid, pentaerythritol and 1,2,3,4-butanetetracarboxylic acid, and the chemical structure of HPPHBTCA was characterized. HPPHBTCA was applied to develop FR functional wool fabric, and its FR efficiency and washing durability were evaluated. The wool fabric treated with 0.14 mol/L HPPHBTCA had self-extinguishing performance even after 20 washing cycles during the vertical burning test, presenting good FR ability and resistance to washing. The catalytic char-forming effect of HPPHBTCA contributed to the enhanced FR and smoke suppression properties of wool fabric, and the ester bonds formed between HPPHBTCA and wool fiber resulted in the good washing durability. The HPPHBTCA treatment had a negligible effect on the whiteness, tensile strength and handle of wool fabric. This study offers a novel route to prepare the eco-friendly and durable FR agent using natural and phosphorus-containing compound.
1. Introduction Green and sustainable flame retardant (FR) products are drawing a growing attention in textile modifications due to the environmental pollution of textile industry. Although the cost-effective halogenated FR agents had a broad application in the functional modification of textiles and other polymeric materials in the past, they provoke health and environmental concerns due to the generation of toxic gases during combustion as well as the bioaccumulation and persistence in organisms and environment, resulting in the prohibition of use in textile industry [1]. The Zirpro FR system, being the most commercially available FR approach for protein fabrics, causes the metal pollution issue of water environment [2]. In addition, the phosphorus-based Pyrovatex CP and Proban, which are widely used for conferring durable FR property to cellulose fabrics, may induce damage to human health and environment due to the gradual release of formaldehyde in use or storage [3,4]. Renewable and biodegradable FR products provide great potential in addressing the aforementioned problems owing to their availability and good environmental benefits. In this context, a series of proteinbased polymers such as deoxyribonucleic acid, whey proteins, caseins, and hydrophobins have found their potential applications in the FR
⁎
treatment of textile materials (cotton, polyester or their blends) [5–8]. In addition, the application of other FR agents from natural resources to the finishing of textiles has also caused great attention [9,10]. For instance, FR function can be imparted to wool and wool/silk blend by phytic acid (PA) [11,12], to cotton by chitosan/PA combination and tannic acid [13,14], and to silk by condensed tannin [15]. The green, renewable and phosphorus-rich characteristics make PA an attractive natural product in the filed of FR modification. PA contains 6 phosphate groups and 12 hydroxyl groups in very close proximity, enabling it to interact with almost all kinds of positively charged compounds and metal ions [13]. Therefore, PA has high selectivity to be applied alone or together with other agents to provide FR functionality. Because of the strong ability to form complexes with polycations, PA was employed as an interesting candidate for layer by layer assembly to develop intumescent FR coatings with nitrogen- and silicon-containing compounds for textile materials [13,16–18]. Based on the same capacity, PA was used together with positively charged nitrogen-containing compounds to design polyelectrolyte complexes for fabricating FR polymers [19,20]. These phosphorus- and nitrogen/silicon-containing systems could effectively improve the flame retardancy of target materials thanks to the formation of highly expanded and charred structures.
Corresponding authors. E-mail addresses: [email protected] (X.-H. Yang), [email protected] (R.-C. Tang).
https://doi.org/10.1016/j.reactfunctpolym.2018.11.006 Received 10 September 2018; Received in revised form 6 November 2018; Accepted 8 November 2018 Available online 09 November 2018 1381-5148/ © 2018 Published by Elsevier B.V.
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Scheme 1. Synthesis route of HPPHBTCA.
Scheme 2. Cross-linking reaction between wool fiber and HPPHBTCA.
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2. Experimental 2.1. Materials Wool fabric (plain weave, 125 g/m2) was supplied by Shanghai Textile Industry Institute of Technical Supervision, China. PA (70% aqueous solution) was provided by Chengdu Ai Keda Chemical Technology Co. Ltd., China. PER, BTCA and dimethyl sulfoxide-d6 (DMSO‑d6) were purchased from Sinopharm Chemical Reagent Co. Ltd., China. The detergent for wool textiles was obtained from Shanghai Zhengzhang Laundering and Dyeing Co. Ltd., China. 2.2. Synthesis of HPPHBTCA PER (2.72 g, 0.02 mol) was mixed with PA (6.60 g, 0.007 mol) in a three-necked flask (250 mL). The mixture was maintained at 130 °C in the stirring condition. After 2 h, the viscous yellow liquid (hexapentaerythritol phytate ester) was obtained. Then BTCA (4.68 g, 0.02 mol) was added into the flask, and the heating of the mixture was continued at 130 °C. The mixture became more and more viscous with increasing reaction time, and turned out to be an elastic, yellow and transparent solid after 45 min, demonstrating the rapid cross-linking reaction between the components. The crude product was dried using a freeze-drying machine, and then purified using ethanol. The synthesis route of HPPHBTCA is presented in Scheme 1.
Fig. 1. FT-IR spectra of PA and HPPHBTCA.
Our previous work involved the application of PA to develop functional wool and wool/silk blend fabrics [11,12]. The as-prepared textiles exhibited remarkably enhanced thermal stability and flame retardancy. Although the electrostatic interaction took place between the protonated amino groups of protein fiber and the ionized phosphate groups of PA, the achieved flame retardancy was not durable due to the water solubility of PA and the lack of covalent cross-linking between PA and protein fiber. The challenging issue regarding to the durability of PA on textiles remains unsolved, which will impede the industrial application of PA in some extreme environments. Therefore, improving the washing durability of PA on textiles has become a necessary issue. The phosphate groups of PA make it possible to introduce polyhydric alcohols like pentaerythritol (PER) on PA by esterification reaction [21,22]. Furthermore, the free active hydroxyl structures of PER endow the formed phosphate ester with a great potential to be crosslinked onto polycarboxylic acid [23]. In the present study, PA was used together with PER and 1,2,3,4-butanetetracarboxylic acid (BTCA) to synthesize a reactive and phosphorus-containing FR agent, namely hexapentaerythritol phytate hexabutane tetracarboxylic acid ester (HPPHBTCA), which was expected to be grafted on wool fiber by the formation of ester covalent bonds during the pad-dry-cure finishing process because the esterification reaction between HPPHBTCA and wool fiber would take place at high curing temperatures [24,25]. The aim of this work was to develop durable FR wool textiles using the derivative from natural compounds. In this regard, the molecule structure of HPPHBTCA was characterized. The thermal stability and FR performance as well as the FR mechanism of the treated wool fabrics were discussed. At last, the impact of the FR treatment on the physical properties of wool fabric was evaluated.
Fig. 2.
13
C (a) and
31
2.3. Fabric treatment procedure For easy application, the viscous HPPHBTCA liquid obtained at 30 min was used as the FR agent and dissolved in distilled water. Two HPPHBTCA solutions whose total volume was 100 and 50 mL, corresponding to 0.07 and 0.14 mol/L, respectively, were used in the fabric treatment. The fabric samples were firstly dipped in the HPPHBTCA solutions at room temperature with a liquor ratio of 25:1. After 10 min, the samples were squeezed using a two-roll laboratory padder with two dips and two nips, giving a wet pickup of 100 ± 5%. Afterwards, the padded samples were dried and cured at 80 and 160 °C, respectively for 3 min. The cross-linking reaction between wool fiber and HPPHBTCA during the curing process is presented in Scheme 2. The obtained fabrics were washed thoroughly in distilled water, and then dried at 60 °C for 30 min. The weight gain of the modified wool fabric was determined using Eq. (1):
Weight gain (%) = 100 × (W1 − W0)/ W0
(1)
where W0 and W1 denote the weight of the fabrics before and after modification, respectively. Specifically, Wool-1 and Wool-2 in the Results and Discussion section denote the fabrics treated with 0.07 and 0.14 mol/L HPPHBTCA, respectively.
P (b) NMR spectra of PA and HPPHBTCA. 60
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2.4. Characterizations The Fourier transform infrared (FT-IR) analysis was conducted on the Nicolet 5700 FT-IR spectrometer (Thermo Fisher Scientific Inc., USA) using KBr pellets. The 13C and 31P liquid state nuclear magnetic resonance (NMR) spectra were recorded on the Bruker Avance III 400 MHz spectrometer (Bruker BioSpin GmbH, Germany) using DMSO‑d6 as a deuterated solvent. The surface morphologies of the wool fibers and chars from the vertical burning test were observed by the Hitachi-S4800 field-emission SEM (Hitachi High Technologies America, Inc., USA). The vertical burning and limiting oxygen index (LOI) tests were carried out according to GB/T 5455–2014 and GB/T 5454–1997, respectively; the burning behavior of the fabrics based on the vertical burning test was rated according to GB/T 17591–2006. The P content of the burned and unburned wool samples was detected on the inductively coupled plasma optical emission spectrometer (ICP-OES) according to our previously described method [11]. The smoke production capacity of the wool fabrics was evaluated on the FTT0064 NBS smoke density test chamber (Fire Testing Technology Ltd., UK) according to ISO 5659.2; each specimen with a dimension of 75 × 75 mm2 (two sample layers) was exposed horizontally to an external heat flux of 25 kW/m2 in the flameless combustion mode. The thermogravimetry (TG) curves of the wool fabrics were recorded on the Diamond TG/DTA SII thermal analyzer (Perkin-Elmer, USA). The washing of the treated fabrics was conducted at 40 °C according to our previously described method [12]; each wash was continued for 30 min. The mechanical properties such as tensile strength, bending length and flexural rigidity of the wool fabrics were measured according to our previously described method [11,26]; each test was repeated at least 5 times to ensure data accuracy. The Hunter whiteness index of the fabrics was measured on the HunterLab UltraScan PRO reflectance spectrophotometer according to our previously described method [12].
Fig. 3. FT-IR spectra of the treated and untreated wool fabrics.
3. Results and discussion 3.1. Characterizations of HPPHBTCA 3.1.1. FT-IR of HPPHBTCA FT-IR analysis was conducted to detect the functional groups of HPPHBTCA. The FT-IR spectra of PA and HPPHBTCA are presented in Fig. 1. For the spectrum of PA, the bands at around 3490 and 1641 cm−1 are ascribed to the OH stretching vibration of PA and the hydration of water molecules, respectively. The bands at 1164 and 989 cm−1 are associated with the stretching vibration of P]O, OePeC and PeO groups [27,28]. Some additional absorption bands were observed for HPPHBTCA as compared to PA. The significant difference between 1164 and 989 cm−1 is ascribed to the stretching vibration of CeO group in HPPHBTCA. The new bands at 1465 and 1384 cm−1 are assigned to the CeH and CH2 bending vibration of PER and BTCA in the product [29]. In addition, the new strong band at 1724 cm−1 corresponds to the stretching vibration of ester carbonyl bonds formed between PER and BTCA [24]. 3.1.2. NMR of HPPHBTCA The possible molecular structure of HPPHBTCA was further determined by liquid state 13C and 31P NMR spectra. As shown in Fig. 2, the septet adsorption band at 39.63 ppm in the 13C NMR spectra belongs to the CH3 groups of DMSO‑d6 [30]. The signal peaks at 75.62, 73.14, 72.89 and 72.23 ppm in the 13C NMR spectrum of PA are ascribed to the CH groups (C1~C6) of inositol ring [31]. Compared with the spectrum of PA, the spectrum of HPPHBTCA showed the new signals at 61.40 and 60.45 ppm which should be attributed to the CH2 groups (C7~C18), and the signal at 45.29 ppm corresponding to C19, C20 and C21 carbon atoms of PER in HPPHBTCA [32]. Moreover, the new signals at 174.12 and 173.36 ppm are ascribed to the carbonyl carbons
Fig. 4. SEM micrographs of the treated and untreated wool fabrics.
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Fig. 5. Weight gain (a), char length (b), LOI (c) and P content (d) of the treated wool fabrics after washing.
3.3. Flammability and smoke generation of the treated wool
of carboxylic acid (C28~C33) and ester moieties (C22~C27), respectively. The signals at 42.24 and 32.22 ppm are assigned to the CH2 (C34~C38) and CH (C39~C45) groups of BTCA in HPPHBTCA, respectively [33,34]. In addition, the significant chemical peak of 31P position shifting from −0.36 to −0.69 ppm should be ascribed to the formation of phosphate ester covalent bonds [35]. These features explicitly confirm the successful cross-linking modification of PER and BTCA to PA.
3.3.1. Flammability The FR efficiency of the treated wool was determined using the vertical burning and LOI tests. The untreated wool fabric got ignited easily during the vertical burning, and then the flame quickly burned the entire sample, remaining only a small quantity of char residue. The untreated wool sample obtained the highest char length of 30 cm, revealing its poor flame retardancy. However, the modification with PA and HPPHBTCA dramatically altered the burning behavior of wool. The treated wool substrates inflated immediately when ignited, displaying excellent char-forming ability. Interestingly, the expanded char was able to withstand the open fire, thus the flames on the treated wool selfextinguished before the ignition source was removed. As a result, the char length of the treated wool samples decreased obviously. Both PA and HPPHBTCA improved the FR performance of wool fabric by an evident charring mechanism. As shown in Fig. 5a, the wool fabrics treated with 0.14 mol/L PA (Wool-2-PA), 0.07 (Wool-1) and 0.14 mol/L (Wool-2) HPPHBTCA obtained a weight gain of 5.1%, 11.6% and 17.2%, respectively. As shown in Fig. 5b, all the treated wool fabrics exhibited a low char length of shorter than 12 cm, presenting improved flame retardancy. Correspondingly, the wool fabrics treated with 0.07 and 0.14 mol/L HPPHBTCA had a higher LOI of 28.5% and 30.3%, respectively compared with 23.6% of the untreated wool (Fig. 5c). As shown in Fig. 5b, by increasing the washing cycles, the treated wool displayed a gradual increase in char length, meaning a reduction trend of the FR ability. Furthermore, the char length of the PA treated wool fabric showed much higher increase than that of the HPPHBTCA treated wool fabric. The vertical burning photos (Fig. 6) of the wool fabrics treated with 0.14 mol/L PA (Wool-2-PA) and 0.14 mol/L HPPHBTCA (Wool-2) after washing gave the visual comparison. After 5 washing cycles, the wool fabric treated with 0.14 mol/L PA could not achieve B1 classification where the char length is shorter than 15 cm according to GB/T 17591–2006, displaying poor washing durability. However, the wool fabric treated with 0.14 mol/L HPPHBTCA was still self-extinguishing during the vertical burning and achieved the B1 classification after 20 washing cycles. Even for the wool fabric treated
3.2. FT-IR and morphology of the treated wool The possible reactions between HPPHBTCA and wool fiber were studied by FT-IR. As shown in Fig. 3, compared with the spectrum of the untreated wool, the spectra of the HPPHBTCA treated wool fabrics showed several new absorption bands. The P]O and OePeC absorption bands occurred at 1168 and 1060 cm−1, respectively in the spectra of treated wool [27,28]. The new band at 1724 cm−1 is ascribed to the stretching vibration of ester carbonyl groups [24], revealing the formation of ester covalent bonds between the carboxyl groups of HPPHBTCA and the hydroxyl groups of wool fiber. Besides, the amide groups formed between the carboxyl groups of HPPHBTCA and amino groups of wool fiber may also contribute to the cross-linking effect [25]. However, the formed amide groups can not be distinguished from the original peptide absorption in the FT-IR spectra. These results above are consistent with the reaction in Scheme 2, which shows the cross-linking reaction between HPPHBTCA and wool fiber. Furthermore, although PA contains 12 hydroxyl groups attaching to the phosphate groups, only parts of them can react with PER owing to the stereo-hindrance effect. Thus the residual phosphate groups of HPPHBTCA can combine with wool fiber through electrostatic interaction. The surface morphology of the wool fabrics was observed by SEM. The SEM micrographs shown in Fig. 4 confirm the attachment of cuticle layer on the surface of wool fibers, and also suggest the clean surface of the untreated wool fiber. For the treated wool fiber, a thin coating was found to homogeneously distribute around the fiber surface and deposit in the interstices between fibers. It seems that the ester cross-linking and electrostatic interactions between HPPHBTCA and wool fiber, which were illustrated by the FT-IR analysis, are strong enough to allow the deposition of HPPHBTCA on the wool fiber surface. 62
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Fig. 6. Vertical burning photos of the wool fabrics treated with 0.14 mol/L PA (upper) and HPPHBTCA (down) after different cycles of washing.
is much higher than 24.9% of the PA-treated wool (Wool-2-PA). The repeated washing test convincingly demonstrates that HPPHBTCA is an effective and durable FR agent for wool. The P content of the wool fabrics treated with 0.14 mol/L PA and HPPHBTCA after repeated washing was evaluated by ICP-OES. As shown in Fig. 5d, by increasing the washing cycles, the P content of the
with 0.07 mol/L HPPHBTCA, it still met the B1 classification standard after 10 washings, and met the B2 classification standard after 20 washing cycles, showing good washing durability. As depicted in Fig. 5c, the LOI of the treated fabrics had a reduction trend with repeated washing cycles. In particular, after 20 washing cycles, the wool treated with HPPHBTCA (Wool-2) still had a high LOI of 28.1%, which 63
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Fig. 7. Ds curves of wool fabrics obtained by smoke density test. Table 1 Smoke density parameters of wool fabrics. Sample
Time (s)
Dsmax
Control Wool-1 Wool-2
571 495 489
93.7 50.4 41.3
treated wool showed the similar reduction trend like LOI, revealing that the decreased FR performance discussed above is attributed to a decrease in P content. Moreover, the P content of the PA treated wool showed greater reduction compared with that of the HPPHBTCA treated wool. It is believed that the ionic bonding between PA and wool fiber is not resistant to washing. However, the cross-linking effect formed between the carboxyl groups of HPPHBTCA and the hydroxyl and amino groups of wool fiber is strong enough to withstand the robust washing, thus the wool treated with HPPHBTCA exhibits good durability to washing. These results above reveal the successful application of the reactive phosphorus-containing HPPHBTCA to wool fabric, and the treated wool fabrics possess desirable FR property and washing durability.
3.3.2. Smoke generation performance As shown in Fig. 7 and Table 1, the untreated wool produced heavy smoke, and had a high maximum specific optical density (Dsmax) of 93.7. However, the Wool-1 and Wool-2 samples displayed greatly suppressed smoke release in the whole combustion process, and had a low Dsmax value of 50.4 and 41.3, respectively. The significant suppression in smoke release is due to the ability of the FR agent to catalyze the conversion of organic matters to efficient char, and thus reduce the flame intensity. Additionally, the treated wool fabrics exhibited a shorter time to reach Dsmax compared with the untreated fabric. Anyway, the high efficiency of HPPHBTCA in reducing the smoke
Fig. 9. SEM micrographs of the char residues of wool fabrics after the vertical burning.
Fig. 8. TG curves of wool fabrics in nitrogen (a) and air (b). 64
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summarized as follows. Upon exposed to a flame or a heat flux, HPPHBTCA decomposes at first and releases phosphoric acids which can catalyze the formation of thermal stable char and intumescent structures rich in phosphorus. These changes enable the heat and oxygen transfer between gas and condensed phases to be impeded. Thus the further burning of underlying materials is blocked, and thereby the flame retardancy and thermal stability of wool textiles are improved. As a conclusion, HPPHBTCA can enhance the FR performance of wool by a condensed-phase action.
Table 2 Whiteness index, tensile strength, bending length and flexural rigidity of the wool fabrics. Sample
Whiteness index
Tensile strength (N)
Bending length (mm)
Flexural rigidity (mN cm)
Control Wool-1 Wool-2
79.8 ± 0.1 73.8 ± 0.1 69.8 ± 0.2
245.6 ± 1.0 256.6 ± 3.5 253.3 ± 3.8
16.4 ± 0.4 17.3 ± 0.3 17.8 ± 0.2
0.910 ± 0.063 1.042 ± 0.055 1.138 ± 0.037
generation of wool lowers the fire risk, and allows people to have more possibility of survival from a fire.
3.6. Physical properties of the treated wool The whiteness, tensile strength and handle of the wool fabrics before and after treatment were assessed. As shown in Table 2, wool fabrics showed little decrease in whiteness after treatment because of the faint yellow color of HPPHBTCA solution. The breakage of the untreated wool fiber follows the slippage mechanism of molecule chains [38]. For the treated wool fabrics, HPPHBTCA acting as a crosslinking agent among wool molecules reinforces the connection of fibers, causing more difficult slippage of molecule chains. Therefore, the treated fabrics exhibited little increase in tensile strength. In addition, the FR treatment adversely affected the softness of wool fabric as indicated by the slightly increased bending length and flexural rigidity, which is acceptable in the wool textile wet processing. As a conclusion, the FR treatment has little impact on the physical properties of wool.
3.4. TG analysis of the treated wool The thermal degradation and stability of the wool fabrics were assessed in nitrogen and air. As shown in Fig. 8a, the untreated wool experienced a two-step thermal degradation in nitrogen. The initial low weight loss below 150 °C was attributed to the loss of the physically bound water. The main and fast weight loss occurred in the temperature range of 220 to 425 °C, involving the decomposition of the peptide chain and production of combustible gases [36,37]. As shown in Fig. 8b, in air atmosphere, wool underwent a more complicated degradation process. Below 450 °C, the weight loss of wool took place in a similar way to the degradation of wool in nitrogen. However, at higher temperature, a new decomposition step related to the further oxidation of the remaining residues occurred in the presence of air. Thus, the untreated wool exhibited a char residue of 20.2% at 700 °C in nitrogen, and a lower char residue of 2.7% in air. As shown in Fig. 8, the HPPHBTCA treated wool showed the similar pyrolysis steps like the untreated wool but exhibited higher thermal degradation temperature in both nitrogen and air. The difference of weight loss between the treated and untreated wool appeared at around 350 °C which was in the second pyrolysis region, and it became more obvious with increasing degradation temperature. In other words, the treated wool had lower weight loss than the untreated one. This finding suggests that HPPHBTCA can change the thermal degradation pathway of wool, and at the same time favor the formation of a protective and quite stable char. As a result, the treated wool exhibited a remarkably increased char residue at the end of the degradation process. The Wool1 and Wool-2 samples yielded a residual char of 17.5% and 23.8%, respectively at 700 °C in air. Such high char residue of the treated wool demonstrates that HPPHBTCA can enhance the flame retardancy of wool by a condensation phase mechanism.
4. Conclusions In this study, an eco-friendly and reactive FR HPPHBTCA for the FR treatment of wool fabric was successfully synthesized using PA, PER and BTCA. The carboxyl groups of HPPHBTCA could react with the hydroxyl and amino groups of wool to form ester and amide bonds, contributing to the appreciable washing durability of the treated wool. The wool fabric treated with 0.14 mol/L HPPHBTCA was able to selfextinguish in the vertical flammability test and such FR ability was maintained after 20 washing cycles. Compared with the untreated wool, the treated fabrics exhibited significantly reduced smoke generation. The TG and P content analyses suggested that HPPHBTCA acted in the condensed phase to confer FR ability to wool. In addition, the physical properties of wool fabrics had small changes after the FR treatment. The limitation regarding to the poor washing durability of PA on wool fabric was overcome by the facile chemical modification of PA. The present study indicates that the sustainable and reactive phosphorus-containing FR has tremendous potential in practical application.
3.5. Morphology and P content of the wool char residues
Acknowledgements
Fig. 9 shows the SEM micrographs of the burnt residues of wool fabrics obtained from the vertical burning test. The untreated wool lost the original fibrous structure after burning and displayed a fragile and thin char as a result of the complete burning and insufficient char formation; some voids were formed due to the inflate action of the generated volatiles during burning. The treated wool fabrics maintained the original texture after burning, demonstrating that they have high thermal stability and good flame retardancy. In addition, globular structures were formed on the char residues of Wool-1 sample at localized spaces, and Wool-2 sample showed more expanded globular structures at enlarged spaces on the char residue in comparison with Wool-1 sample. Furthermore, the char residues exhibited much higher P content than the treated fabrics. For example, the unburned and burned Wool-2 samples had a P content of 16.23 and 39.66 mg/g, respectively, illustrating the mechanism of FR action of HPPHBTCA in condensed phase. On the basis of the thermal analyses and SEM observations of the char residues, the FR mechanism of the treated wool can be
This work was supported by Jiangsu Provincial Key Research and Development Program of China [BE2015066], and Postgraduate Research and Practice Innovation Program of Jiangsu Province [KYCX17_1986]. References [1] L.S. Birnbaum, D.F. Staskal, Brominated flame retardants: cause for concern? Environ. Health Perspect. 112 (2004) 9–17. [2] A.R. Horrocks, Overview of traditional flame-retardant solutions (including coating and back-coating technologies), in: J. Alongi, A.R. Horrocks, F. Carosio, G. Malucelli (Eds.), Update on Flame Retardant Textiles: State of the Art, Environmental Issues and Innovative Solutions, First Ed., Smithers Rapra Technology Ltd., Shawbury, 2013, pp. 123–178. [3] A.R. Horrocks, Flame retardant challenges for textiles and fibers: new chemistry versus innovatory solutions, Polym. Degrad. Stab. 96 (2011) 377–392. [4] Z. Yang, B. Fei, X. Wang, J.H. Xin, A novel halogen-free and formaldehyde-free flame retardant for cotton fabrics, Fire Mater. 36 (2012) 31–39. [5] J. Alongi, R.A. Carletto, A. Di Blasio, F. Carosio, F. Bosco, G. Malucelli, DNA: a novel, green, natural flame retardant and suppressant for cotton, J. Mater. Chem. A 1 (2013) 4779–4785.
65
Reactive and Functional Polymers 134 (2019) 58–66
X.-W. Cheng et al.
(1985) 213–228. [22] Y. Jia, Y. Lu, G. Zhang, Y. Liang, F. Zhang, Facile synthesis of an eco-friendly nitrogen–phosphorus ammonium salt to enhance the durability and flame retardancy of cotton, J. Mater. Chem. A 5 (2017) 9970–9981. [23] X. Huang, J. Guo, X. Gong, S. Li, Y. Gong, Q. An, F. Guan, Preparation and characterization of pentaerythritol/butane tetracarboxylic acid/polyethylene glycol crosslinking copolymers as solid-solid phase change materials, J. Macromol. Sci. A 53 (2016) 500–506. [24] C.Q. Yang, X. Wang, Formation of cyclic anhydride intermediates and esterification of cotton cellulose by multifunctional carboxylic acids: an infrared spectroscopy study, Text. Res. J. 66 (1996) 595–603. [25] S.H. Hsieh, Z.K. Huang, Z.Z. Huang, Z.S. Tseng, Antimicrobial and physical properties of woolen fabrics cured with citric acid and chitosan, J. Appl. Polym. Sci. 94 (2004) 1999–2007. [26] X.W. Cheng, C.X. Liang, J.P. Guan, X.H. Yang, R.C. Tang, Flame retardant and hydrophobic properties of novel sol-gel derived phytic acid/silica hybrid organicinorganic coatings for silk fabric, Appl. Surf. Sci. 427 (2018) 69–80. [27] X. Guo, Y. Miao, P. Ye, Y. Wen, H. Yang, Multi-walled carbon nanotubes in aqueous phytic acid for enhancing biosensor, Mater. Res. Express 1 (2014) 025403. [28] G. Jiang, J. Qiao, F. Hong, Application of phosphoric acid and phytic acid-doped bacterial cellulose as novel proton-conducting membranes to PEMFC, Int. J. Hydrogen. Energ. 37 (2012) 9182–9192. [29] Z. Wang, P. Lv, Y. Hu, K. Hu, Thermal degradation study of intumescent flame retardants by TG and FTIR: Melamine phosphate and its mixture with pentaerythritol, J. Anal. Appl. Pyrolysis 86 (2009) 207–214. [30] H.E. Gottlieb, V. Kotlyar, A. Nudelman, NMR chemical shifts of common laboratory solvents as trace impurities, J. Organomet. Chem. 62 (1997) 7512–7515. [31] Y. Feng, Y. Zhou, D. Li, S. He, F. Zhang, G. Zhang, A plant-based reactive ammonium phytate for use as a flame-retardant for cotton fabric, Carbohydr. Polym. 175 (2017) 636–644. [32] F. Liu, A.M. Orendt, D.W. Alderman, D.M. Grant, Carbon-13 chemical shift tensors in pentaerythritol, J. Am. Chem. Soc. 119 (1997) 8981–8984. [33] H. Kono, S. Fujita, Biodegradable superabsorbent hydrogels derived from cellulose by esterification crosslinking with 1, 2, 3, 4-butanetetracarboxylic dianhydride, Carbohydr. Polym. 87 (2012) 2582–2588. [34] H. Kono, T. Nakamura, Polymerization of β-cyclodextrin with 1, 2, 3, 4-butanetetracarboxylic dianhydride: Synthesis, structural characterization, and bisphenol a adsorption capacity, React. Funct. Polym. 73 (2013) 1096–1102. [35] A. Bebot-Brigaud, C. Dange, N. Fauconnier, C. Gérard, 31P NMR, potentiometric and spectrophotometric studies of phytic acid ionization and complexation properties toward Co2+, Ni2+, Cu2+, Zn2+ and Cd2+, J. Inorg. Biochem. 75 (1999) 71–78. [36] A.R. Horrocks, P.J. Davies, Char formation in flame-retarded wool fibers. Part 1. Effect of intumescent on thermogravimetric behavior, Fire. Mater. 24 (2000) 151–157. [37] C.M. Tian, Z. Li, H.Z. Guo, J.Z. Xu, Study on the thermal degradation of flame retardant wools, J. Fire Sci. 21 (2003) 155–162. [38] H. Liu, W. Yu, Study of the structure transformation of wool fibers with Raman spectroscopy, J. Appl. Polym. Sci. 103 (2007) 1–7.
[6] F. Bosco, R.A. Carletto, J. Alongi, L. Marmo, A. Di Blasio, G. Malucelli, Thermal stability and flame resistance of cotton fabrics treated with whey proteins, Carbohydr. Polym. 94 (2013) 372–377. [7] J. Alongi, R.A. Carletto, F. Bosco, F. Carosio, A. Di Blasio, F. Cuttica, V. Antonucci, M. Giordano, G. Malucelli, Caseins and hydrophobins as novel green flame retardants for cotton fabrics, Polym. Degrad. Stab. 99 (2014) 111–117. [8] G. Malucelli, F. Bosco, J. Alongi, F. Carosio, A. Di Blasio, C. Mollea, C. Mollea, F. Cuttica, A. Casale, Biomacromolecules as novel green flame retardant systems for textiles: an overview, RSC Adv. 4 (2014) 46024–46039. [9] L. Costes, F. Laoutid, S. Brohez, P. Dubois, Bio-based flame retardants: when nature meets fire protection, Mater. Sci. Eng. R 117 (2017) 1–25. [10] S. Basak, S.W. Ali, Sustainable fire retardancy of textiles using bio-macromolecules, Polym. Degrad. Stab. 133 (2016) 47–64. [11] X.W. Cheng, J.P. Guan, G. Chen, X.H. Yang, R.C. Tang, Adsorption and flame retardant properties of bio-based phytic acid on wool fabric, Polymers 8 (2016) 122. [12] X. Zhang, X.-Y. Zhou, X.-W. Cheng, R.-C. Tang, Phytic acid as an eco-friendly flame retardant for silk/wool blend: a comparative study with fluorotitanate and fluorozirconate, J. Clean. Prod. 198 (2018) 1044–1052. [13] G. Laufer, C. Kirkland, A.B. Morgan, J.C. Grunlan, Intumescent multilayer nanocoating, made with renewable polyelectrolytes, for flame-retardant cotton, Biomacromolecules 13 (2012) 2843–2848. [14] S. Nam, B.D. Condon, Z. Xia, R. Nagarajan, D.J. Hinchliffe, C.A. Madison, Intumescent flame-retardant cotton produced by tannic acid and sodium hydroxide, J. Anal. Appl. Pyrolysis 126 (2017) 239–246. [15] T.T. Yang, J.P. Guan, R.C. Tang, G. Chen, Condensed tannin from Dioscorea cirrhosa tuber as an eco-friendly and durable flame retardant for silk textile, Ind. Crop. Prod. 115 (2018) 16–25. [16] C.K. Kundu, W. Wang, S. Zhou, X. Wang, H. Sheng, Y. Pan, L. Song, Y. Hu, A green approach to constructing multilayered nanocoating for flame retardant treatment of polyamide 66 fabric from chitosan and sodium alginate, Carbohydr. Polym. 166 (2017) 131–138. [17] X. Wang, M.Q. Romero, X.Q. Zhang, R. Wang, D.Y. Wang, Intumescent multilayer hybrid coating for flame retardant cotton fabrics based on layer-by-layer assembly and sol–gel process, RSC Adv. 5 (2015) 10647–10655. [18] Z.F. Li, C.J. Zhang, L. Cui, P. Zhu, C. Yan, Y. Liu, Fire retardant and thermal degradation properties of cotton fabrics based on APTES and sodium phytate through layer-by-layer assembly, J. Anal. Appl. Pyrolysis 123 (2017) 216–223. [19] T. Zhang, H. Yan, L. Shen, Z. Fang, X. Zhang, J. Wang, B. Zhang, Chitosan/phytic acid polyelectrolyte complex: a green and renewable intumescent flame retardant system for ethylene-vinyl acetate copolymer, Ind. Eng. Chem. Res. 53 (2014) 19199–19207. [20] T. Zhang, H. Yan, L. Shen, Z. Fang, X. Zhang, J. Wang, B. Zhang, A phosphorus-, nitrogen-and carbon-containing polyelectrolyte complex: preparation, characterization and its flame retardant performance on polypropylene, RSC Adv. 4 (2014) 48285–48292. [21] G. Camino, L. Costa, L. Trossarelli, F. Costanzi, A. Pagliari, Study of the mechanism of intumescence in fire retardant polymers: Part VI-Mechanism of ester formation in ammonium polyphosphate-pentaerythritol mixtures, Polym. Degrad. Stab. 12
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