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On the flameproof treatment of ramie fabrics using a spray-assisted layer-by-layer technique
Polymer Degradation and Stability 121 (2015) 11e17
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Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
On the flameproof treatment of ramie fabrics using a spray-assisted layer-by-layer technique Li Zhao a, Hongqiang Yan b, Zhengping Fang a, b, *, Jing Wang c, Hao Wang c a
MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China b Lab of Polymer Materials and Engineering, Ningbo Institute of Technology, Zhejiang University, Ningbo 315100, China c Centre of Excellence in Engineered Fiber Composites, University of Southern Queensland, Toowoomba, Queensland 4350, Australia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 May 2015 Received in revised form 28 July 2015 Accepted 9 August 2015 Available online 11 August 2015
Flame-retardant coatings were applied to ramie fabric. These consisted of oppositely-charged polyelectrolyte polyethyleneimine (PEI) and ammonium polyphosphate (APP). Application was made using, both a spray-assisted layer-by-layer (LBL) technique and by the conventional dipping LBL method. Thermogravimetric analysis showed that all the coated fabrics left two to three times as much residual char as did uncoated ones. Use of the spraying method was able to achieve the similar thermal stabilization at fewer bilayer number in coating that compared with the dipping method. Additionally, spraying also exhibited a more obvious reduction in both heat release capacity and peak heat release rate in a microscale combustion calorimeter test and cone calorimetry. The cloth acquired the same selfextinguishing property during the vertical flame test and resulted in a more compact and intact char residues. These results demonstrate that the spray-assisted LBL technique represents a relatively efficient and practical alternative to the conventional dipping LBL technique for imparting flame-retardant behavior to ramie fabric. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Ramie fabric Spray Layer-by-layer Flame retardancy
1. Introduction The fiber of the ramie plant has long history of being woven into fabric. With properties of renewability, biodegradability, low cost, low density and excellent mechanical strength, it has great potential for applications in automotive, aerospace, construction, and military areas as reinforcement in composites, besides its traditional use in home textile and furnishing industries [1,2]. However, the intrinsic drawbacks of natural cellulosic fabrics, such as low limiting oxygen index (LOI) and combustion temperature, make it highly flammable and impose great restrictions on their fields of application. Thus, flame retardant modification for natural fiber fabrics has attracted increasing attentions from both academe and industry. Until now, a number of treatments, such as dyeing and finishing [3,4], surface coating [5,6], graft copolymerization [7e11], solegel [12e14] and LBL assembly, have been developed to enhance the
* Corresponding author. MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Zhejiang University, Hangzhou 310027, China. E-mail address: [email protected] (Z. Fang). http://dx.doi.org/10.1016/j.polymdegradstab.2015.08.007 0141-3910/© 2015 Elsevier Ltd. All rights reserved.
thermal stability and flame retardancy of fabrics. Among them, LBL assembly represents an innovative approach suitable for various substrates (fabrics, ceramics, plastics, metals, etc.). It generally consists of alternately immersing the substrates into oppositely charged polyelectrolyte solutions or suspensions, which results in the construction of multiple positively and negatively charged layers on the surface of the substrates. The initial attempt to introduce this method to the textile flame retardant field was carried out by Grunlan and co-workers [15]. Multilayer flameretardant architectures consisting of polyethylenimine (PEI) coupled with laponite nano-platelets were produced on cotton fabrics. However, the FR improvement of treated fabrics was less than satisfactory because the only physical barrier effect produced by limited laponite nano-platelets. Alternatively to laponite, different negative-charged inorganic materials such as SiO2 [16], clay [17], carbon nanofiber [18], POSS [19] and MWCNT-NH2 [20] paired with PEI have been developed and successfully applied to fiber fabrics and polyurethane foam. In order to further improve the FR performance, the LBL architectures contains completely organic coatings, namely, intumescent LBL coatings (such as PAA/PSP, chitosan/PSP, chitosan/APP, PAA/APP, etc.) [19,21e24] have been developed on plant fabrics. More specifically, the chitosan/APP
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could act as a typical intumescent system, since chitosan represents a carbon source and foaming agent as well, whereas APP in situ generates phosphoric acid at high temperatures, favoring the char formation of both chitosan and cellulose. Additionally, intumescent multilayer nano-coatings made only with renewable biomass polyelectrolytes have been applied to cotton quite recently with remarkable decrease of both peak heat release rate (PHRR) and total heat release (THR) [25,26]. When referring to natural fabrics, the intumescent systems were found to be the most promising flame retardant strategy. All the above-mentioned LBL coatings were implemented by means of the dipping method. In contrast, the spraying method could represent a more promising approach for its high efficiency and easy implementation in large industrial scale, as comprehensively reviewed by Schaaf and co-workers [27]. The results have shown that the spraying method could obtain the same homogeneity and consistency level of the depositing coatings, resulting in the desirable properties on substrate materials. Until very recently, horizontal spraying has been developed and successfully applied to impart flame retardant properties to fabrics. This was found to be relatively adaptive for both cotton and polyester textiles, with satisfactory results from flammability and combustion tests [28,29]. In this work, a simple coating system of polyethyleneimine (PEI) paired with commercially available ammonium polyphosphate (APP) was employed, using both dipping and spraying methods. The fire performance as well as the depositing process, morphology and thermal stability of the both spraying-coated and dippingcoated ramie fabrics were evaluated and compared. The results demonstrated the superiority of the spraying technique. 2. Materials and methods 2.1. Chemicals and substrates Plain ramie fabrics, purchased from Jiangxi Jingzhu Ramie Textile Co., Ltd. (China), were cut into pieces and washed with detergent in deionized water several times, followed by drying under vacuum at 60 C for 2 h. Polyethylenimine (PEI 50 wt% aqueous solution, Mw ¼ 70,000) was purchased from Aladdin. Ammonium polyphosphate (APP, JLSAPP 104MF, P% ¼ 28.0e30.0 wt%) was obtained from Hangzhou JLS Flame Retardants Chemical Co., Ltd. (China). Sodium hydroxide (NaOH, 96.0%) was purchased from Sinopharm Chemical Reagent Co., Ltd. (China). Concentrated hydrogen chloride (HCl, 36.5e38%) was obtained from Hangzhou Chemical Reagent Co., Ltd. (China).
The APP aqueous solution with the predetermined 0.9 wt% concentration was prepared using conventional acidebase titrations, and the PEI solution was diluted to the same concentration. Both polyelectrolyte solutions were adjusted to pH 9 using 1.0 mol/L NaOH or HCl solution. Deionized water with a resistance of 18 MU was used in all experiments. 2.2. Layer-by-layer spraying method Ramie fabrics, horizontally fixed on brandreths (shown in Fig. 1), were alternately sprayed with the positively and negatively charged polyelectrolyte solutions in order to construct the multiple flame-retardant bilayer coatings on the surface. It should be noted that spray was applied to both sides of the fabrics. More specifically, spray nozzles were 2 mm in inner diameter, and the spray gun was kept at a working distance of 55 cm from the fabric surface. All the specimens were simply marked as (PEI/APP)n, where n represents the number of complete assembly cycle of the PEI and APP bilayer (BL). The ramie fabrics were naturally air dried after each spray. Finally, the treated ramie fabrics were dried under vacuum at 60 C for 2 h and stored in a desiccator for the next tests. 2.3. Layer-by-layer dipping method For comparison, we also carried out the traditional LBL dipping treatment to fabrics. The ramie fabrics were alternately immersed into PEI (þ) and APP () polyelectrolyte solutions to generate flame retardant multilayers. The first dip into PEI solution was for 5 min and the subsequent dips were for 2 min. Each dip was followed by rinsing with DI water, wrung out to expel liquid among fabrics and wind drying at 50 C for 5 min. After the desired number of bilayers had been deposited, ramie fabrics were dried at 60 C in an oven for 2 h before testing as same as the spraying method. 2.4. Characterization The multiple layers of the LBL treated fabrics were checked using a Nicolet 6700 attenuated total reflection Fourier transform infrared (ATR-FTIR) spectrometer in the frequency region of 4000e400 cm1 at a 4 cm1 resolution with 32 scans. The crystal used in ATR-FTIR is metal germanium (Ge) and the penetration depth was supposed to be 2e3 mm. Field-emission scanning electron microscopy (FE-SEM, Hitachi S-4800, operated at 3 kV) was employed to study the surface morphology of the ramie fabrics before and after the flame-retardant treatment, as well as the char
Fig. 1. Preparation of PEI/APP coated ramie fabrics using LBL spraying technique.
L. Zhao et al. / Polymer Degradation and Stability 121 (2015) 11e17
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Fig. 2. ATR-FTIR spectra of the uncoated and coated ramie fabrics using (a) spraying method and (b) dipping method.
residues after vertical flame test. Prior to the analysis, fabric pieces (5 mm 5 mm) were fixed by conductive adhesive tapes and goldsputtered for 80 s to increase the conductivity. Thermogravimetric analysis (TGA) was conducted on a NETZSCH TG 209 F1 analyzer to study the thermal properties under air or nitrogen atmosphere. The specimens (8 ± 0.5 mg) were first kept at 100 C for 10 min to remove the moisture and then heated up to 600 C at a heating rate of 20 C/min. Microscale combustibility experiments were carried out on a MCC-2 microscale combustion calorimeter (Govmark) with samples weighing 5 ± 0.5 mg in triplicate. The combustor was heating up to 650 C at 1 C/s heating rate and the oxygen/nitrogen flow rate was set at 20/80 mL/ mL. Cone calorimetry (model FTT0242-Standard Cone, Fire Testing Technology Limited Ltd. UK) was employed to investigate the combustion behavior of square samples (100 100 mm2; 3 overlapped layers of fabrics) under 35 kW/m2 irradiative heat flux, following the procedure reported in literature [30]. Time to ignition (TTI), heat release rate and corresponding peak (PHRR), total heat release (THR) and final residue were evaluated. Vertical flame tests (VFT) were carried out in reference to ASTM D 6413 with a horizontal and vertical flammability cabinet (model CZF-3, Nanjing Jiangning Analytical Instrument Factory, Nanjing, China). The test was repeated 3 times for each sample, measuring burning time, burning rate, and the final residue.
3. Results and discussion 3.1. Characterization of the (PEI/APP)n coating The ATR-FTIR spectra of the uncoated and coated fabrics are shown in Fig. 2, the surface chemical structure of fabrics was determined to monitor the LBL growth process. For the both spraying and dipping methods, the coated fabrics showed strong new absorption bands at near 1252 and 862 cm1, which could be ascribed to P]O and PeOeP vibration peaks in APP, respectively [22]. With the increase of the bilayer number, the intensity of the above two characteristic peaks increased accordingly. Furthermore, it was clearly seen that the coating build-up of the spray-coated samples were much more obvious in comparison with the dipcoated one under the same transverse and longitudinal axis dimension, which, to some extent, reflects the superiority of spraying method. SEM observation was performed to assess the morphology of the fabrics before and after the LBL spraying. The surface of the uncoated ramie fabrics was smooth, with clear gaps between fibers (Fig. 3e). As the assembly layers increased gradually, the gaps between the fibers were finally filled up. For example, the surfaces of spray-coated 15BL and 20BL, as well as dip-coated 20BL samples were almost completely covered, well evidenced in Fig. 3c, d, h. When compared the 5BL (Fig. 3a, f) and 10BL (Fig. 3b, g) samples,
Fig. 3. Top view SEM images of spray-coated fabrics: (a) (PEI/APP)5, (b) (PEI/APP)10, (c) (PEI/APP)15, (d) (PEI/APP)20; (e) uncoated ramie fabrics and dip-coated fabrics: (f) (PEI/APP)5, (g) (PEI/APP)10 and (h) (PEI/APP)20.
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the LBL spraying method allows the formation of a more uniform and compact coating on ramie fabrics in comparison with the dipping method.
Table 1 Thermal properties of the ramie fabrics before and after modification under nitrogen and air conditions. Add T5% ( C) on % N2 Air
Sample
3.2. Thermal properties To better assess the effect of the coating on thermal stability of the ramie fabrics, TGA and DTG curves under N2 atmosphere of the uncoated and spraying-coated samples were obtained, as shown in Fig. 4. And the detailed thermal data are listed in Table 1. For all of the coated fabrics, the 5 wt% weight loss temperature (T5%) and maximum weight loss temperature (Tmax1) shifted to lower temperatures compared with the uncoated one. This was mostly because the earlier degradation of APP in the coating [31]. However, the char residues at 600 C were much higher than that of the control sample, which suggested the coatings could effectively improve the thermal stability and promote the char formation [32,33]. It was also significant to note that the spraying-coated 5BL specimen almost reached the same effect as in the dipping-coated 15BL sample, the 5BL spraying-coated specimen showed a 20 wt% char residue increase at the coating weight of 12 wt%; while the 15BL dipping-coated showed 25 wt% char residue increase at the 19 wt% coating weight. That is to say, the application of LBLspraying method can dramatically reduce the bilayer cycles, which takes less time and brings more convenience to the whole LBL assembly process. The representative TGA and DTG curves under air atmosphere (Fig. 5) had very different features compared with those under nitrogen condition. The uncoated sample exhibited two distinct steps of thermal decomposition in air with the maximum weight loss temperatures (Tmax1, Tmax2) being 342 C and 443 C, respectively. With the introduction of PEI/APP multilayers, the T5% and Tmax1 moved toward lower temperatures with the increase of bilayer number. The second degradation peak diminished gradually after each spraying and almost disappeared after 10BL coatings. According to the literature [34], the second degradation peak could be mainly attributed to the thermal oxidative decomposition of the char and the following release of CO2 and CO. Additionally, the stable intumescent char formed in the first degradation step was reasonably thick and compact, which effectively prevented oxygen and heat from penetrating inside and made a significant improvement to the fabrics in the second thermal decomposition process. Furthermore, the char residues at 600 C for all the coated fabrics were notably higher than those of the uncoated one. It was also noted from the N2, fewer bilayers of spraying coating could achieve the similar thermal stability as in the dipping coating.
Tmax2 ( C)
Char yield (wt%) at 600 C
N2
Air
N2
Air
N2
325 315 361
342
e
443
20
8
12 30 39 56
295 296 288 286
295 295 291 191
326 317 319 311
322 327 317 316
e e e e
491 e e e
40 43 47 48
24 24 31 35
5BL 4 10BL 8 15BL 19 20BL 39
302 298 298 293
299 296 293 290
327 315 314 312
323 312 309 311
e e e e
483 500 e e
29 38 45 47
8 15 27 35
Ramie
0
Spray-coated 5BL 10BL 15BL 20BL Dip-coated
Tmax1 ( C)
Air
3.3. Flammability and combustion properties MCC is a conventional technique to evaluate the flammability properties of polymer materials by simulating the hightemperature pyrolysis. In this paper MCC was applied to investigate the key fire parameters of the ramie fabrics. The collected data are listed in Table 2, including heat release capacity (HRC, J/g K), peak heat release rate (PHRR, W/g), temperature at maximum heat release rate (TPHRR) and total heat release (THR, kJ/g). To better understand the difference between the two LBL assembly methods, HRR-Temperature curves of the ramie fabrics are showed in Fig. 6. HRC, PHRR and THR decreased significantly in corresponding to the increase of bilayers for both spray-coated and dip-coated samples, which clearly indicated that multilayers of PEI and APP were indeed an effective protective layer, strongly hindering the release of cellulose pyrolysis products and leading to a gradual reduction in heat release rate. Consistent with the TG data, the residues for the spraycoated ramie fabrics increased substantially 22e44 wt% compared with 9 wt% in the uncoated sample. The residues in the spraycoated samples were slightly higher than those in the dip-coated samples. Additionally, the spraying method showed more obvious reduction in HRC and PHRR, while the dipping method seemed have more advantages in THR at high bilayers since the spraying method exhibited indistinctive variation for all the coated fabrics. Pursuing this research, cone calorimetry tests have been performed in order to assess the reaction and efficiency of the prepared coatings upon exposure to a heat flux. Table 3 collects cone calorimetry data of untreated and treated ramie fabrics. As is clearly observable, all the treated fabrics allow decreasing PHRR, THR and increasing char residue, but they do not significantly change TTI. The best results have been reached in the 15BL spray-coated
Fig. 4. Representative TGA curves (a) and DTG curves (b) for the uncoated and spray-coated ramie fabrics with 5, 10, 15 and 20 bilayers under nitrogen atmosphere.
L. Zhao et al. / Polymer Degradation and Stability 121 (2015) 11e17
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Fig. 5. Representative TGA curves (a) and DTG curves (b) for the uncoated and spray-coated ramie fabrics with 5, 10, 15 and 20 bilayers under air atmosphere.
Table 2 Microscale combustion calorimetry results for the ramie fabrics before and after modification. Sample
Add on (%)
Ramie
HRC (J/g k)
PHRR (W/g)
280 ± 0
0
TPHRR ( C)
274.8 ± 1.8
Char yield (%) at 600 C
THR (kJ/g)
376.8 ± 0.7
12.1 ± 1.1
9±1
Spray-coated
5BL 10BL 15BL 20BL
12 30 39 56
145 134 81 72
± ± ± ±
2 6 1 2
140.3 127.6 79.4 70.6
± ± ± ±
1.3 5.6 0.8 2.3
330.1 327.9 321.4 319.7
± ± ± ±
0.8 2.3 3.3 2.5
5.6 5.2 4.5 4.8
± ± ± ±
0.4 0.7 0.2 0.8
22 26 33 44
± ± ± ±
2 3 1 4
Dip-coated
5BL 10BL 15BL 20BL
4 8 19 39
170 145 110 97
± ± ± ±
3 5 2 3
166.0 140.6 107.1 95.3
± ± ± ±
2.4 1.0 1.8 0.5
342.1 323.1 319.2 318.2
± ± ± ±
0.0 1.1 0.8 0.4
8.6 5.1 4.0 3.8
± ± ± ±
0.2 0.1 0.3 0.3
16 31 34 39
± ± ± ±
1 1 0 1
Fig. 6. Representative heat release rate curves of the uncoated and coated ramie fabrics using (a) spraying method and (b) dipping method.
Table 3 Combustion data obtained by cone calorimetry and vertical flame test. Sample
Cone
Ramie
VFT
TTI (s)
PHRR (kW/m2)
THR (MJ/m2)
Residue (%)
Burning rate (cm/s)
Char length (cm)
18 ± 1
280 ± 14
6.0 ± 0.1
e
6.0
30
Spray-coated
5BL 10BL 15BL 20BL
18 20 21 25
± ± ± ±
2 2 1 2
171 147 117 128
± ± ± ±
12 9 4 15
4.0 3.5 1.7 2.4
± ± ± ±
0.2 0.1 0.1 0.1
16.2 26.5 38.1 38.0
± ± ± ±
2.1 1.9 0.4 0.8
5.6 3.6 2.0 2.0
30 26 12 9
Dip-coated
5BL 10BL 15BL 20BL
18 16 15 17
± ± ± ±
2 1 1 2
214 185 159 160
± ± ± ±
5 1 12 7
4.9 4.9 4.3 4.5
± ± ± ±
0.3 0.2 0.2 0.5
3.0 5.2 9.6 12.5
± ± ± ±
0.7 0.3 0.5 0.3
6.0 3.9 3.0 2.2
30 30 18 10
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L. Zhao et al. / Polymer Degradation and Stability 121 (2015) 11e17
Fig. 7. Digital images of vertical flame tests for fabrics treated with both spraying and dipping methods.
Fig. 8. Top view SEM images (first row: low magnification; second row: high magnification) of uncoated ramie fabrics (a, f) and spray-coated fabrics: (PEI/APP)5 (b, g), (PEI/APP)10 (c, h), (PEI/APP)15 (d, i) and (PEI/APP)20 (e, j) after vertical flame test.
fabrics: indeed, in this case, the coating is thicker and more homogeneous even without the largest add-on weight and is responsible for a significant increase of the final residue (þ16.2%) and a remarkable reduction of PHRR (58.2%) and THR (71.7%). Thus, from an overall consideration, lower THR, PHRR and higher residue have been found for the spraying method compared with the dipping technique. To further evaluate the flame-retardant properties, all the ramie fabrics were exposed to vertical flame tests. Digital images of 5 s after ignition, 2 s after removing the flame as well as the collected char residues are shown in Fig. 7. The flammability data are collected in Table 3 as well. Without any flame-retardant treatment, the ramie fabrics suffered general conflagration with a vigorous flame that spread rapidly to the top of the specimen. In contrast, the fabrics coated only by 5BL PEI/APP burnt mildly at a slower pace, almost preserved original woven structure of the fabrics with a less char length and burning time. Moreover, it was seen that further enhancement for the selfextinguishing ability was achieved with the increase of spraying cycles verified by the fact that the 15BL, 20BL spraying and 20BL dipping samples instantly extinguished after removing the flame. That is to say, the controlled density and thickness for the intumescent coating contributed significantly to the self-extinguishing properties during combustion. Although both the spraying and dipping techniques have achieved the same FR effect with 20 bilayers, the spray-coated ramie fabrics showed better results at fewer bilayers (Fig. 7). Hence, the LBL-spraying method tended to gain a more compact flame retardant coating with same number of
cycles when compared with the dipping method, indicating its higher efficiency. 3.4. Analysis of the collected char residues The surface morphologies of all the spray-coated and uncoated fabrics after VFT were studied using FE-SEM. Fig. 8 presents both the low- and high-magnification SEM images of the representative char residues. As for the uncoated fabrics (Fig. 8a and f), a diameter shrinkage was observed in both warp and weft direction, leaving an obviously loose and slender char residue. In contrast, the 5BL sample preserved the integrity of the original texture of the ramie fabrics, with apparently intumescent-like bubbles on the ablation surface (Fig. 8b and g). Furthermore, it was noted that the residual char after VFT became more compact and consistent as the spraying cycles was increased from 5 to 20. Although the LBL-dipping method could also achieve the same flame retardant effect, the spraying method represented a more efficient and practical approach. 4. Conclusions In this work, intumescent (PEI/APP)n coatings have been deposited on the external surface of ramie fabrics, using both LBLspraying and LBL-dipping methods. It can be concluded that the spraying method is more effective and practical in comparison with the traditional LBL-dipping technique. TGA, MCC as well as cone results all revealed that the spraying method could achieve the
L. Zhao et al. / Polymer Degradation and Stability 121 (2015) 11e17
similar thermal stabilization and flame-retardant effect in fewer coating compared with the LBL dipping method. Additionally, the spraying method was able to obtain the same self-extinguishing property during the VFT with a more compact and intact char residue. Based on the above, it is possible to draw a conclusion that the LBL spraying method could be an attractive alternative to the dipping method in an industrial scale, by virtue of its efficiency and feasibility. Acknowledgments This work was financially supported by the National Basic Research Program of China (No. 2010CB631105) and Ningbo Natural Science Foundation of China (No. 2015A610028). We also want to thank Prof. John Billingsley for his generous help in the modification of the manuscript. References [1] R. Kozłowski, M. Władyka-Przybylak, Flammability and fire resistance of composites reinforced by natural fibers, Polym. Adv. Technol. 19 (2008) 446e453. [2] J.T. Kim, A.N. Netravali, Mechanical, thermal, and interfacial properties of green composites with ramie fiber and soy resins, J. Agric. Food Chem. 58 (2010) 5400e5407. [3] C.Q. Yang, W. Wu, Combination of a hydroxy-functional organophosphorus oligomer and a multifunctional carboxylic acid as a flame retardant finishing system for cotton: part I. The chemical reactions, Fire Mater 27 (2003) 223e237. [4] Z. Yang, X. Wang, D. Lei, B. Fei, J.H. Xin, A durable flame retardant for cellulosic fabrics, Polym. Degrad. Stab. 97 (2012) 2467e2472. [5] A.R. Horrocks, P.J. Davies, B.K. Kandola, A. Alderson, The potential for volatile phosphorus-containing flame retardants in textile back-coatings, J. Fire. Sci. 25 (2007) 523e539. [6] J. Sunissa, S. Nattinee, S. Chanida, S. Mantana, Influence of hydrophobic substance on enhancing washing durability of water soluble flame-retardant coating, Appl. Surf. Sci. 275 (2013) 239e243. [7] K. Opwis, A. Wego, T. Bahners, E. Schollmeyer, Permanent flame retardant finishing of textile materials by a photochemical immobilization of vinyl phosphonic acid, Polym. Degrad. Stab. 96 (2011) 393e395. [8] H. Yuan, W. Xing, P. Zhang, L. Song, Y. Hu, Functionalization of cotton with UVcured flame retardant coatings, Ind. Eng. Chem. Res. 51 (2012) 5394e5401. [9] H. Yuan, W. Xing, P. Zhang, Functionalization of cotton with UV-cureed flame retardant costings, Ind. Eng. Chem. Res. 51 (2012) 5394e5401. [10] S. Shahidi, Novel method for ultraviolet protection and flame retardancy of cotton fabrics by low-temperature plasma, Cellulose 21 (2014) 757e768. [11] B. Paosawatyanyong, P. Jermsuitjarit, W. Bhanthumnavin, Graft copolymerization coating of methacryloyloxyethyl diphenyl phosphate flame retardant onto silk surface 10 (2014) 1585e1590. [12] J. Alongi, M. Ciobanu, G. Malucelli, Novel flame retardant finishing systems for cotton fabrics based on phosphorus-containing compounds and silica derived from solegel processes, Carbohydr. Polym. 85 (2011) 599e608. [13] J. Alongi, G. Malucelli, State of the art and perspectives on solegel derived hybrid architectures for flame retardancy of textiles, J. Mater. Chem. 22 (2012) 21805e21809. [14] J. Vasilijevic, B. Tomsic, I. Jerman, Novel multifunctional water- and oilrepellent antibacterial, and flame-retardant cellulose fibres created by the
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sol-gel process, Cellulose 21 (2014) 2611e2623. [15] Y. Li, J. Schulz, J. Grunlan, Polyelectrolyte/nanosilicate thin film assemblies influence of pH on growth, mechanical behavior, and flammability, ACS Appl. Mater. Interfaces 1 (2009) 2338e2347. [16] G. Laufer, F. Carosio, R. Martinez, G. Camino, J. Grunlan, Growth and fire resistance of colloidal silica-polyelectrolyte thin film assemblies, J. Colloid Interface Sci. 356 (2011) 69e77. [17] Y. Li, J. Schulz, S. Mannen, C. Dellhom, J. Grunlan, Flame retardant behavior of polyelectrolyte/clay thin film assemblies on cotton fabrics, ACS Nano 4 (2010) 3325e3337. [18] Y. Kim, R. Davis, A. Cain, J. Grunlan, Development of layer-by-layer assembled carbon nanofiber-filled coatings to reduce polyurethane foam flammability, Polymer 52 (2011) 2847e2855. [19] Y. Li, J. Schulz, A. Morgan, S. Chang, Y. Yang, J. Grunlan, Intumescent allpolymer multilayer nanocoating capable of extinguishing flame on fabric, Adv. Mater 23 (2011) 3926e3931. [20] T. Zhang, H. Yan, M. Peng, L. Wang, Z. Fang, Construction of flame retardant nanocoating on ramie fabric via layer-by-layer assembly of carbon nanotube and ammonium polyphosphate, Nanoscale 5 (2013) 3013. [21] T. Guin, M. Krecker, A. Milhorn, J. Grunlan, Maintaining hand and improving fire resistance of cotton fabric through ultrasonication rinsing of multilayer nanocoating, Cellulose 21 (2014) 3023e3030. [22] F. Carosio, J. Alongi, G. Malucelli, Layer-by-layer ammonium polyphosphatebased coatings for flame retardancy of polyester-cotton blends, Carbohydr. Polym. Sci. 88 (2012) 1460e1469. [23] J. Alongi, F. Carosio, G. Malucelli, Influence of ammonium polyphosphate-/ poly(acrylic acid)-based layer by layer architectures on the char formation in cotton, polyester and their blends, Polym. Degrad. Stab. 97 (2012) 1644e1653. [24] J. Alongi, F. Carosio, G. Malucelli, Influence of ammonium polyphosphate-/ poly(acrylic acid)-based layer by layer architectures on the char formation in cotton, polyester and their blends, Polym. Degrad. Stab. 98 (2013) 1626e1637. [25] G. Laufer, C. Kirkland, A. Morgan, J. Grunlan, Intumescent multilayer nanocoating, made with renewable polyelectrolytes, for flame-retardant cotton, Biomacromolecules 13 (2012) 2843e2848. [26] H. Pan, L. Song, L. Ma, Y. Pang, Y. Hu, Layer-by-layer assembled thin films based on fully biobased polysaccharides: chitosan and phosphorylated cellulose for flame-retardant cotton fabric, Cellulose 21 (2014) 2995e3006. [27] P. Schaaf, J.C. Voegel, L. Jierry, F. Boulmedais, Spray-assisted polyelectrolyte multilayer buildup: from step-by-step to single-step polyelectrolyte film constructions, Adv. Mater 24 (2012) 1001. [28] J. Alongi, F. Carosio, A. Frache, G. Malucelli, Layer by layer coatings assembled through dipping, vertical or horizontal spray for cotton flame retardancy, Carbohydr. Polym. 92 (2013) 114. [29] F. Carosio, F. Cuttica, J. Alongi, Flame retardancy of polyester fabrics treated by spray-assisted layer-by-layer silica architectures, Ind. Eng. Chem. Res. 52 (2013) 9544e9550. [30] J. Tata, F. Carosio, J. Alongi, A. Frache, Optimization of the procedure to burn textile fabrics by cone calorimeter: part I. Combustion behavior of polyester, Fire Mater 35 (2011) 397e409. [31] P.J. Davies, A.R. Horrocks, A. Alderson, The sensitisation of thermal decomposition of ammonium polyphosphate by selected metal ions and their potential for improved cotton fabric flame retardancy, Polym. Degrad. Stab. 88 (2005) 114e122. [32] Z. Yang, X. Wang, D. Lei, B. Fei, J. Xin, A durable flame retardant for cellulosic fabrics, Polym. Degrad. Stab. 97 (2012) 2467e2472. [33] H. Ma, L. Tong, Z. Xu, Z. Fang, Y. Jin, F. Lu, A novel intumescent flame retardant: synthesis and application in ABS copolymer, Polym. Degrad. Stab. 92 (2007) 720e726. [34] Y. Sekiguchi, F. Shafizadeh, The effect of inorganic additives on the formation, composition, and combustion of cellulosic char, J. Appl. Polym. Sci. 29 (4) (1984) 1267e1286.
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Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
On the flameproof treatment of ramie fabrics using a spray-assisted layer-by-layer technique Li Zhao a, Hongqiang Yan b, Zhengping Fang a, b, *, Jing Wang c, Hao Wang c a
MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China b Lab of Polymer Materials and Engineering, Ningbo Institute of Technology, Zhejiang University, Ningbo 315100, China c Centre of Excellence in Engineered Fiber Composites, University of Southern Queensland, Toowoomba, Queensland 4350, Australia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 May 2015 Received in revised form 28 July 2015 Accepted 9 August 2015 Available online 11 August 2015
Flame-retardant coatings were applied to ramie fabric. These consisted of oppositely-charged polyelectrolyte polyethyleneimine (PEI) and ammonium polyphosphate (APP). Application was made using, both a spray-assisted layer-by-layer (LBL) technique and by the conventional dipping LBL method. Thermogravimetric analysis showed that all the coated fabrics left two to three times as much residual char as did uncoated ones. Use of the spraying method was able to achieve the similar thermal stabilization at fewer bilayer number in coating that compared with the dipping method. Additionally, spraying also exhibited a more obvious reduction in both heat release capacity and peak heat release rate in a microscale combustion calorimeter test and cone calorimetry. The cloth acquired the same selfextinguishing property during the vertical flame test and resulted in a more compact and intact char residues. These results demonstrate that the spray-assisted LBL technique represents a relatively efficient and practical alternative to the conventional dipping LBL technique for imparting flame-retardant behavior to ramie fabric. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Ramie fabric Spray Layer-by-layer Flame retardancy
1. Introduction The fiber of the ramie plant has long history of being woven into fabric. With properties of renewability, biodegradability, low cost, low density and excellent mechanical strength, it has great potential for applications in automotive, aerospace, construction, and military areas as reinforcement in composites, besides its traditional use in home textile and furnishing industries [1,2]. However, the intrinsic drawbacks of natural cellulosic fabrics, such as low limiting oxygen index (LOI) and combustion temperature, make it highly flammable and impose great restrictions on their fields of application. Thus, flame retardant modification for natural fiber fabrics has attracted increasing attentions from both academe and industry. Until now, a number of treatments, such as dyeing and finishing [3,4], surface coating [5,6], graft copolymerization [7e11], solegel [12e14] and LBL assembly, have been developed to enhance the
* Corresponding author. MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Zhejiang University, Hangzhou 310027, China. E-mail address: [email protected] (Z. Fang). http://dx.doi.org/10.1016/j.polymdegradstab.2015.08.007 0141-3910/© 2015 Elsevier Ltd. All rights reserved.
thermal stability and flame retardancy of fabrics. Among them, LBL assembly represents an innovative approach suitable for various substrates (fabrics, ceramics, plastics, metals, etc.). It generally consists of alternately immersing the substrates into oppositely charged polyelectrolyte solutions or suspensions, which results in the construction of multiple positively and negatively charged layers on the surface of the substrates. The initial attempt to introduce this method to the textile flame retardant field was carried out by Grunlan and co-workers [15]. Multilayer flameretardant architectures consisting of polyethylenimine (PEI) coupled with laponite nano-platelets were produced on cotton fabrics. However, the FR improvement of treated fabrics was less than satisfactory because the only physical barrier effect produced by limited laponite nano-platelets. Alternatively to laponite, different negative-charged inorganic materials such as SiO2 [16], clay [17], carbon nanofiber [18], POSS [19] and MWCNT-NH2 [20] paired with PEI have been developed and successfully applied to fiber fabrics and polyurethane foam. In order to further improve the FR performance, the LBL architectures contains completely organic coatings, namely, intumescent LBL coatings (such as PAA/PSP, chitosan/PSP, chitosan/APP, PAA/APP, etc.) [19,21e24] have been developed on plant fabrics. More specifically, the chitosan/APP
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could act as a typical intumescent system, since chitosan represents a carbon source and foaming agent as well, whereas APP in situ generates phosphoric acid at high temperatures, favoring the char formation of both chitosan and cellulose. Additionally, intumescent multilayer nano-coatings made only with renewable biomass polyelectrolytes have been applied to cotton quite recently with remarkable decrease of both peak heat release rate (PHRR) and total heat release (THR) [25,26]. When referring to natural fabrics, the intumescent systems were found to be the most promising flame retardant strategy. All the above-mentioned LBL coatings were implemented by means of the dipping method. In contrast, the spraying method could represent a more promising approach for its high efficiency and easy implementation in large industrial scale, as comprehensively reviewed by Schaaf and co-workers [27]. The results have shown that the spraying method could obtain the same homogeneity and consistency level of the depositing coatings, resulting in the desirable properties on substrate materials. Until very recently, horizontal spraying has been developed and successfully applied to impart flame retardant properties to fabrics. This was found to be relatively adaptive for both cotton and polyester textiles, with satisfactory results from flammability and combustion tests [28,29]. In this work, a simple coating system of polyethyleneimine (PEI) paired with commercially available ammonium polyphosphate (APP) was employed, using both dipping and spraying methods. The fire performance as well as the depositing process, morphology and thermal stability of the both spraying-coated and dippingcoated ramie fabrics were evaluated and compared. The results demonstrated the superiority of the spraying technique. 2. Materials and methods 2.1. Chemicals and substrates Plain ramie fabrics, purchased from Jiangxi Jingzhu Ramie Textile Co., Ltd. (China), were cut into pieces and washed with detergent in deionized water several times, followed by drying under vacuum at 60 C for 2 h. Polyethylenimine (PEI 50 wt% aqueous solution, Mw ¼ 70,000) was purchased from Aladdin. Ammonium polyphosphate (APP, JLSAPP 104MF, P% ¼ 28.0e30.0 wt%) was obtained from Hangzhou JLS Flame Retardants Chemical Co., Ltd. (China). Sodium hydroxide (NaOH, 96.0%) was purchased from Sinopharm Chemical Reagent Co., Ltd. (China). Concentrated hydrogen chloride (HCl, 36.5e38%) was obtained from Hangzhou Chemical Reagent Co., Ltd. (China).
The APP aqueous solution with the predetermined 0.9 wt% concentration was prepared using conventional acidebase titrations, and the PEI solution was diluted to the same concentration. Both polyelectrolyte solutions were adjusted to pH 9 using 1.0 mol/L NaOH or HCl solution. Deionized water with a resistance of 18 MU was used in all experiments. 2.2. Layer-by-layer spraying method Ramie fabrics, horizontally fixed on brandreths (shown in Fig. 1), were alternately sprayed with the positively and negatively charged polyelectrolyte solutions in order to construct the multiple flame-retardant bilayer coatings on the surface. It should be noted that spray was applied to both sides of the fabrics. More specifically, spray nozzles were 2 mm in inner diameter, and the spray gun was kept at a working distance of 55 cm from the fabric surface. All the specimens were simply marked as (PEI/APP)n, where n represents the number of complete assembly cycle of the PEI and APP bilayer (BL). The ramie fabrics were naturally air dried after each spray. Finally, the treated ramie fabrics were dried under vacuum at 60 C for 2 h and stored in a desiccator for the next tests. 2.3. Layer-by-layer dipping method For comparison, we also carried out the traditional LBL dipping treatment to fabrics. The ramie fabrics were alternately immersed into PEI (þ) and APP () polyelectrolyte solutions to generate flame retardant multilayers. The first dip into PEI solution was for 5 min and the subsequent dips were for 2 min. Each dip was followed by rinsing with DI water, wrung out to expel liquid among fabrics and wind drying at 50 C for 5 min. After the desired number of bilayers had been deposited, ramie fabrics were dried at 60 C in an oven for 2 h before testing as same as the spraying method. 2.4. Characterization The multiple layers of the LBL treated fabrics were checked using a Nicolet 6700 attenuated total reflection Fourier transform infrared (ATR-FTIR) spectrometer in the frequency region of 4000e400 cm1 at a 4 cm1 resolution with 32 scans. The crystal used in ATR-FTIR is metal germanium (Ge) and the penetration depth was supposed to be 2e3 mm. Field-emission scanning electron microscopy (FE-SEM, Hitachi S-4800, operated at 3 kV) was employed to study the surface morphology of the ramie fabrics before and after the flame-retardant treatment, as well as the char
Fig. 1. Preparation of PEI/APP coated ramie fabrics using LBL spraying technique.
L. Zhao et al. / Polymer Degradation and Stability 121 (2015) 11e17
13
Fig. 2. ATR-FTIR spectra of the uncoated and coated ramie fabrics using (a) spraying method and (b) dipping method.
residues after vertical flame test. Prior to the analysis, fabric pieces (5 mm 5 mm) were fixed by conductive adhesive tapes and goldsputtered for 80 s to increase the conductivity. Thermogravimetric analysis (TGA) was conducted on a NETZSCH TG 209 F1 analyzer to study the thermal properties under air or nitrogen atmosphere. The specimens (8 ± 0.5 mg) were first kept at 100 C for 10 min to remove the moisture and then heated up to 600 C at a heating rate of 20 C/min. Microscale combustibility experiments were carried out on a MCC-2 microscale combustion calorimeter (Govmark) with samples weighing 5 ± 0.5 mg in triplicate. The combustor was heating up to 650 C at 1 C/s heating rate and the oxygen/nitrogen flow rate was set at 20/80 mL/ mL. Cone calorimetry (model FTT0242-Standard Cone, Fire Testing Technology Limited Ltd. UK) was employed to investigate the combustion behavior of square samples (100 100 mm2; 3 overlapped layers of fabrics) under 35 kW/m2 irradiative heat flux, following the procedure reported in literature [30]. Time to ignition (TTI), heat release rate and corresponding peak (PHRR), total heat release (THR) and final residue were evaluated. Vertical flame tests (VFT) were carried out in reference to ASTM D 6413 with a horizontal and vertical flammability cabinet (model CZF-3, Nanjing Jiangning Analytical Instrument Factory, Nanjing, China). The test was repeated 3 times for each sample, measuring burning time, burning rate, and the final residue.
3. Results and discussion 3.1. Characterization of the (PEI/APP)n coating The ATR-FTIR spectra of the uncoated and coated fabrics are shown in Fig. 2, the surface chemical structure of fabrics was determined to monitor the LBL growth process. For the both spraying and dipping methods, the coated fabrics showed strong new absorption bands at near 1252 and 862 cm1, which could be ascribed to P]O and PeOeP vibration peaks in APP, respectively [22]. With the increase of the bilayer number, the intensity of the above two characteristic peaks increased accordingly. Furthermore, it was clearly seen that the coating build-up of the spray-coated samples were much more obvious in comparison with the dipcoated one under the same transverse and longitudinal axis dimension, which, to some extent, reflects the superiority of spraying method. SEM observation was performed to assess the morphology of the fabrics before and after the LBL spraying. The surface of the uncoated ramie fabrics was smooth, with clear gaps between fibers (Fig. 3e). As the assembly layers increased gradually, the gaps between the fibers were finally filled up. For example, the surfaces of spray-coated 15BL and 20BL, as well as dip-coated 20BL samples were almost completely covered, well evidenced in Fig. 3c, d, h. When compared the 5BL (Fig. 3a, f) and 10BL (Fig. 3b, g) samples,
Fig. 3. Top view SEM images of spray-coated fabrics: (a) (PEI/APP)5, (b) (PEI/APP)10, (c) (PEI/APP)15, (d) (PEI/APP)20; (e) uncoated ramie fabrics and dip-coated fabrics: (f) (PEI/APP)5, (g) (PEI/APP)10 and (h) (PEI/APP)20.
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the LBL spraying method allows the formation of a more uniform and compact coating on ramie fabrics in comparison with the dipping method.
Table 1 Thermal properties of the ramie fabrics before and after modification under nitrogen and air conditions. Add T5% ( C) on % N2 Air
Sample
3.2. Thermal properties To better assess the effect of the coating on thermal stability of the ramie fabrics, TGA and DTG curves under N2 atmosphere of the uncoated and spraying-coated samples were obtained, as shown in Fig. 4. And the detailed thermal data are listed in Table 1. For all of the coated fabrics, the 5 wt% weight loss temperature (T5%) and maximum weight loss temperature (Tmax1) shifted to lower temperatures compared with the uncoated one. This was mostly because the earlier degradation of APP in the coating [31]. However, the char residues at 600 C were much higher than that of the control sample, which suggested the coatings could effectively improve the thermal stability and promote the char formation [32,33]. It was also significant to note that the spraying-coated 5BL specimen almost reached the same effect as in the dipping-coated 15BL sample, the 5BL spraying-coated specimen showed a 20 wt% char residue increase at the coating weight of 12 wt%; while the 15BL dipping-coated showed 25 wt% char residue increase at the 19 wt% coating weight. That is to say, the application of LBLspraying method can dramatically reduce the bilayer cycles, which takes less time and brings more convenience to the whole LBL assembly process. The representative TGA and DTG curves under air atmosphere (Fig. 5) had very different features compared with those under nitrogen condition. The uncoated sample exhibited two distinct steps of thermal decomposition in air with the maximum weight loss temperatures (Tmax1, Tmax2) being 342 C and 443 C, respectively. With the introduction of PEI/APP multilayers, the T5% and Tmax1 moved toward lower temperatures with the increase of bilayer number. The second degradation peak diminished gradually after each spraying and almost disappeared after 10BL coatings. According to the literature [34], the second degradation peak could be mainly attributed to the thermal oxidative decomposition of the char and the following release of CO2 and CO. Additionally, the stable intumescent char formed in the first degradation step was reasonably thick and compact, which effectively prevented oxygen and heat from penetrating inside and made a significant improvement to the fabrics in the second thermal decomposition process. Furthermore, the char residues at 600 C for all the coated fabrics were notably higher than those of the uncoated one. It was also noted from the N2, fewer bilayers of spraying coating could achieve the similar thermal stability as in the dipping coating.
Tmax2 ( C)
Char yield (wt%) at 600 C
N2
Air
N2
Air
N2
325 315 361
342
e
443
20
8
12 30 39 56
295 296 288 286
295 295 291 191
326 317 319 311
322 327 317 316
e e e e
491 e e e
40 43 47 48
24 24 31 35
5BL 4 10BL 8 15BL 19 20BL 39
302 298 298 293
299 296 293 290
327 315 314 312
323 312 309 311
e e e e
483 500 e e
29 38 45 47
8 15 27 35
Ramie
0
Spray-coated 5BL 10BL 15BL 20BL Dip-coated
Tmax1 ( C)
Air
3.3. Flammability and combustion properties MCC is a conventional technique to evaluate the flammability properties of polymer materials by simulating the hightemperature pyrolysis. In this paper MCC was applied to investigate the key fire parameters of the ramie fabrics. The collected data are listed in Table 2, including heat release capacity (HRC, J/g K), peak heat release rate (PHRR, W/g), temperature at maximum heat release rate (TPHRR) and total heat release (THR, kJ/g). To better understand the difference between the two LBL assembly methods, HRR-Temperature curves of the ramie fabrics are showed in Fig. 6. HRC, PHRR and THR decreased significantly in corresponding to the increase of bilayers for both spray-coated and dip-coated samples, which clearly indicated that multilayers of PEI and APP were indeed an effective protective layer, strongly hindering the release of cellulose pyrolysis products and leading to a gradual reduction in heat release rate. Consistent with the TG data, the residues for the spraycoated ramie fabrics increased substantially 22e44 wt% compared with 9 wt% in the uncoated sample. The residues in the spraycoated samples were slightly higher than those in the dip-coated samples. Additionally, the spraying method showed more obvious reduction in HRC and PHRR, while the dipping method seemed have more advantages in THR at high bilayers since the spraying method exhibited indistinctive variation for all the coated fabrics. Pursuing this research, cone calorimetry tests have been performed in order to assess the reaction and efficiency of the prepared coatings upon exposure to a heat flux. Table 3 collects cone calorimetry data of untreated and treated ramie fabrics. As is clearly observable, all the treated fabrics allow decreasing PHRR, THR and increasing char residue, but they do not significantly change TTI. The best results have been reached in the 15BL spray-coated
Fig. 4. Representative TGA curves (a) and DTG curves (b) for the uncoated and spray-coated ramie fabrics with 5, 10, 15 and 20 bilayers under nitrogen atmosphere.
L. Zhao et al. / Polymer Degradation and Stability 121 (2015) 11e17
15
Fig. 5. Representative TGA curves (a) and DTG curves (b) for the uncoated and spray-coated ramie fabrics with 5, 10, 15 and 20 bilayers under air atmosphere.
Table 2 Microscale combustion calorimetry results for the ramie fabrics before and after modification. Sample
Add on (%)
Ramie
HRC (J/g k)
PHRR (W/g)
280 ± 0
0
TPHRR ( C)
274.8 ± 1.8
Char yield (%) at 600 C
THR (kJ/g)
376.8 ± 0.7
12.1 ± 1.1
9±1
Spray-coated
5BL 10BL 15BL 20BL
12 30 39 56
145 134 81 72
± ± ± ±
2 6 1 2
140.3 127.6 79.4 70.6
± ± ± ±
1.3 5.6 0.8 2.3
330.1 327.9 321.4 319.7
± ± ± ±
0.8 2.3 3.3 2.5
5.6 5.2 4.5 4.8
± ± ± ±
0.4 0.7 0.2 0.8
22 26 33 44
± ± ± ±
2 3 1 4
Dip-coated
5BL 10BL 15BL 20BL
4 8 19 39
170 145 110 97
± ± ± ±
3 5 2 3
166.0 140.6 107.1 95.3
± ± ± ±
2.4 1.0 1.8 0.5
342.1 323.1 319.2 318.2
± ± ± ±
0.0 1.1 0.8 0.4
8.6 5.1 4.0 3.8
± ± ± ±
0.2 0.1 0.3 0.3
16 31 34 39
± ± ± ±
1 1 0 1
Fig. 6. Representative heat release rate curves of the uncoated and coated ramie fabrics using (a) spraying method and (b) dipping method.
Table 3 Combustion data obtained by cone calorimetry and vertical flame test. Sample
Cone
Ramie
VFT
TTI (s)
PHRR (kW/m2)
THR (MJ/m2)
Residue (%)
Burning rate (cm/s)
Char length (cm)
18 ± 1
280 ± 14
6.0 ± 0.1
e
6.0
30
Spray-coated
5BL 10BL 15BL 20BL
18 20 21 25
± ± ± ±
2 2 1 2
171 147 117 128
± ± ± ±
12 9 4 15
4.0 3.5 1.7 2.4
± ± ± ±
0.2 0.1 0.1 0.1
16.2 26.5 38.1 38.0
± ± ± ±
2.1 1.9 0.4 0.8
5.6 3.6 2.0 2.0
30 26 12 9
Dip-coated
5BL 10BL 15BL 20BL
18 16 15 17
± ± ± ±
2 1 1 2
214 185 159 160
± ± ± ±
5 1 12 7
4.9 4.9 4.3 4.5
± ± ± ±
0.3 0.2 0.2 0.5
3.0 5.2 9.6 12.5
± ± ± ±
0.7 0.3 0.5 0.3
6.0 3.9 3.0 2.2
30 30 18 10
16
L. Zhao et al. / Polymer Degradation and Stability 121 (2015) 11e17
Fig. 7. Digital images of vertical flame tests for fabrics treated with both spraying and dipping methods.
Fig. 8. Top view SEM images (first row: low magnification; second row: high magnification) of uncoated ramie fabrics (a, f) and spray-coated fabrics: (PEI/APP)5 (b, g), (PEI/APP)10 (c, h), (PEI/APP)15 (d, i) and (PEI/APP)20 (e, j) after vertical flame test.
fabrics: indeed, in this case, the coating is thicker and more homogeneous even without the largest add-on weight and is responsible for a significant increase of the final residue (þ16.2%) and a remarkable reduction of PHRR (58.2%) and THR (71.7%). Thus, from an overall consideration, lower THR, PHRR and higher residue have been found for the spraying method compared with the dipping technique. To further evaluate the flame-retardant properties, all the ramie fabrics were exposed to vertical flame tests. Digital images of 5 s after ignition, 2 s after removing the flame as well as the collected char residues are shown in Fig. 7. The flammability data are collected in Table 3 as well. Without any flame-retardant treatment, the ramie fabrics suffered general conflagration with a vigorous flame that spread rapidly to the top of the specimen. In contrast, the fabrics coated only by 5BL PEI/APP burnt mildly at a slower pace, almost preserved original woven structure of the fabrics with a less char length and burning time. Moreover, it was seen that further enhancement for the selfextinguishing ability was achieved with the increase of spraying cycles verified by the fact that the 15BL, 20BL spraying and 20BL dipping samples instantly extinguished after removing the flame. That is to say, the controlled density and thickness for the intumescent coating contributed significantly to the self-extinguishing properties during combustion. Although both the spraying and dipping techniques have achieved the same FR effect with 20 bilayers, the spray-coated ramie fabrics showed better results at fewer bilayers (Fig. 7). Hence, the LBL-spraying method tended to gain a more compact flame retardant coating with same number of
cycles when compared with the dipping method, indicating its higher efficiency. 3.4. Analysis of the collected char residues The surface morphologies of all the spray-coated and uncoated fabrics after VFT were studied using FE-SEM. Fig. 8 presents both the low- and high-magnification SEM images of the representative char residues. As for the uncoated fabrics (Fig. 8a and f), a diameter shrinkage was observed in both warp and weft direction, leaving an obviously loose and slender char residue. In contrast, the 5BL sample preserved the integrity of the original texture of the ramie fabrics, with apparently intumescent-like bubbles on the ablation surface (Fig. 8b and g). Furthermore, it was noted that the residual char after VFT became more compact and consistent as the spraying cycles was increased from 5 to 20. Although the LBL-dipping method could also achieve the same flame retardant effect, the spraying method represented a more efficient and practical approach. 4. Conclusions In this work, intumescent (PEI/APP)n coatings have been deposited on the external surface of ramie fabrics, using both LBLspraying and LBL-dipping methods. It can be concluded that the spraying method is more effective and practical in comparison with the traditional LBL-dipping technique. TGA, MCC as well as cone results all revealed that the spraying method could achieve the
L. Zhao et al. / Polymer Degradation and Stability 121 (2015) 11e17
similar thermal stabilization and flame-retardant effect in fewer coating compared with the LBL dipping method. Additionally, the spraying method was able to obtain the same self-extinguishing property during the VFT with a more compact and intact char residue. Based on the above, it is possible to draw a conclusion that the LBL spraying method could be an attractive alternative to the dipping method in an industrial scale, by virtue of its efficiency and feasibility. Acknowledgments This work was financially supported by the National Basic Research Program of China (No. 2010CB631105) and Ningbo Natural Science Foundation of China (No. 2015A610028). We also want to thank Prof. John Billingsley for his generous help in the modification of the manuscript. References [1] R. Kozłowski, M. Władyka-Przybylak, Flammability and fire resistance of composites reinforced by natural fibers, Polym. Adv. Technol. 19 (2008) 446e453. [2] J.T. Kim, A.N. Netravali, Mechanical, thermal, and interfacial properties of green composites with ramie fiber and soy resins, J. Agric. Food Chem. 58 (2010) 5400e5407. [3] C.Q. Yang, W. Wu, Combination of a hydroxy-functional organophosphorus oligomer and a multifunctional carboxylic acid as a flame retardant finishing system for cotton: part I. The chemical reactions, Fire Mater 27 (2003) 223e237. [4] Z. Yang, X. Wang, D. Lei, B. Fei, J.H. Xin, A durable flame retardant for cellulosic fabrics, Polym. Degrad. Stab. 97 (2012) 2467e2472. [5] A.R. Horrocks, P.J. Davies, B.K. Kandola, A. Alderson, The potential for volatile phosphorus-containing flame retardants in textile back-coatings, J. Fire. Sci. 25 (2007) 523e539. [6] J. Sunissa, S. Nattinee, S. Chanida, S. Mantana, Influence of hydrophobic substance on enhancing washing durability of water soluble flame-retardant coating, Appl. Surf. Sci. 275 (2013) 239e243. [7] K. Opwis, A. Wego, T. Bahners, E. Schollmeyer, Permanent flame retardant finishing of textile materials by a photochemical immobilization of vinyl phosphonic acid, Polym. Degrad. Stab. 96 (2011) 393e395. [8] H. Yuan, W. Xing, P. Zhang, L. Song, Y. Hu, Functionalization of cotton with UVcured flame retardant coatings, Ind. Eng. Chem. Res. 51 (2012) 5394e5401. [9] H. Yuan, W. Xing, P. Zhang, Functionalization of cotton with UV-cureed flame retardant costings, Ind. Eng. Chem. Res. 51 (2012) 5394e5401. [10] S. Shahidi, Novel method for ultraviolet protection and flame retardancy of cotton fabrics by low-temperature plasma, Cellulose 21 (2014) 757e768. [11] B. Paosawatyanyong, P. Jermsuitjarit, W. Bhanthumnavin, Graft copolymerization coating of methacryloyloxyethyl diphenyl phosphate flame retardant onto silk surface 10 (2014) 1585e1590. [12] J. Alongi, M. Ciobanu, G. Malucelli, Novel flame retardant finishing systems for cotton fabrics based on phosphorus-containing compounds and silica derived from solegel processes, Carbohydr. Polym. 85 (2011) 599e608. [13] J. Alongi, G. Malucelli, State of the art and perspectives on solegel derived hybrid architectures for flame retardancy of textiles, J. Mater. Chem. 22 (2012) 21805e21809. [14] J. Vasilijevic, B. Tomsic, I. Jerman, Novel multifunctional water- and oilrepellent antibacterial, and flame-retardant cellulose fibres created by the
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