Article pubs.acs.org/IECR
Controlled Formation of Self-Extinguishing Intumescent Coating on Ramie Fabric via Layer-by-Layer Assembly Tao Zhang,†,‡,§ Hongqiang Yan,† Lili Wang,†,‡ and Zhengping Fang*,†,‡ †
Lab of Polymer Materials and Engineering, Ningbo Institute of Technology, Zhejiang University, Ningbo 315100, China MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China § Department of Packaging Engineering, College of Materials and Textiles, Zhejiang Sci-tech University, Hangzhou 310018, China ‡
S Supporting Information *
ABSTRACT: Self-extinguishing multilayer coatings consisting of polyelectrolyte polyethylenimine (PEI) and ammonium polyphosphate (APP) have been constructed by the layer-by-layer assembly technique onto flexible and porous ramie fabric. Attenuated total reflection Fourier transform infrared spectroscopy and energy-dispersive X-ray analysis directly confirmed that PEI and APP were successfully incorporated onto the surface of ramie fabric sequentially. Assessment of the thermal and flammability properties for the coated ramie fabrics showed that the char residue at temperatures ranging from 400 to 600 °C during thermogravimetric analysis (TGA) and the self-extinguishing ability during vertical flame test were significantly enhanced as compared with the pristine sample, which showed strong dependency on the number of deposited layers, especially on the concentration collocation of both polyelectrolytes. This work provided a simple but effective method for the controlled formation of self-extinguishing intumescent coating on ramie fabric and can be applied to other cellulose systems. intumescent flame retardant effect during combustion, suitable swelling agent and carbon source should be simultaneously provided to cooperate with phosphorus containing compounds.20 In this work, we present a simple assembly system of commercially available ammonium polyphosphate (APP) paired with polyethylenimine (PEI), which were alternately adsorbed onto flexible and porous ramie fabric. PEI is one of the most widely used polycations in layer-by-layer assembly, which can easily be carbonized and release gases such as ammonia at high temperature. The objective of the present study was to tune the percentage of coating weight added to the fabric and relative content of phosphorus via appropriate adjustment of concentration collocation for both partners, so as to control the intumescent flame retardant effect and selfextinguishing ability during combustion.
1. INTRODUCTION As a kind of interesting natural cellulosic fiber, ramie shows promising applications as reinforcement of composites in many advanced fields (such as automotive, aerospace, military, and construction industries, etc.) besides home textiles, because of its biodegradability, low price, low density, and excellent mechanical properties.1,2 However, the highly flammable nature always limits its practical applications. Thus, how to enhance thermal stability and flame retardancy of ramie has become one of the main focuses of many endeavors. Up to now, various flame retardants have been introduced to natural cellulosic fibers, and the phosphorus containing intumescent system has been proved to be a promising candidate for halogen-free flame retardant because of high effectiveness, environmental friendliness, and low smoke.3 Several strategies, such as dyeing and finishing,4,5 multistep sol−gel,6,7 and UV-induced graft polymerization,8 have been developed to confer natural cellulosic fibers with phosphorus containing flame retardants. However, those methods always involve multiple chemical or physical steps under special conditions, which increase the difficulty in processing. Recently, surface modification of natural cellulosic fibers by layer-by-layer assembly for improving flame retardancy has attracted considerable interest with respect to both academic research and industrial applications.9−12 This method, based on the regularly alternating physical adsorption of oppositely charged polyelectrolytes, can conveniently control the deposition of a wide variety of species at nanoscale and the resulting properties.13,14 Hence, many functionalized materials including positively charged nanomaterials and polyelectrolytes have been paired with negatively charged phosphorus containing polyelectrolytes to construct flame retardant nanocoating on the surface of fabric.11,12,15−19 However, to reach significant © 2013 American Chemical Society
2. EXPERIMENTAL SECTION 2.1. Materials. Plain ramie fabrics, purchased from Jiangxi Jingzhu Ramie Textile Co., Ltd. (China), were cut into pieces of 300 mm × 76 mm and washed with detergent in deionized water several times, followed by drying under vacuum at 60 °C for 2 h. Quartz slides (10 mm × 20 mm) were treated with freshly prepared boiling piranha solution (H2O2/H2SO4 3:7 v/ v) for 30 min (caution: piranha solution is extremely corrosive), washed thoroughly with deionized water, and then dried with nitrogen prior to use. Polyethylenimine (PEI, branched, Mw = Received: Revised: Accepted: Published: 6138
November 16, 2012 February 24, 2013 April 15, 2013 April 15, 2013 dx.doi.org/10.1021/ie3031554 | Ind. Eng. Chem. Res. 2013, 52, 6138−6146
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Scheme 1. Schematic Illustration of the LBL Assembly Process for Construction of (PEI/APP)n Intumescent Coating on Ramie Fabric
Figure 1. ATR-FTIR spectra (a) of the pristine and (PEI0.4/APP0.9)n treated ramie fabrics in the 1400−700 cm−1 region; top view SEM images of the pristine (b) and treated ramie fabrics with (PEI0.4/APP0.9)n multilayers of 5 (c), 10 (d), and 20 (e) bilayers. The outmost layer is APP layer.
25 000, Mn = 10 000) was acquired from Sigma-Aldrich. Ammonium polyphosphate (APP, JLS-APP104MF, P% = 28.0−30.0 wt %) was used as received 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, 36.5−38%) was obtained from Hangzhou Chemical Reagent Co., Ltd. (China). Deionized water with a resistance of 18 MΩ was used for all the experiments. 2.2. Layer-by-Layer Assembly. For the preparation of polyelectrolyte APP and PEI solution, 1.0 mol/mL NaOH (35 mL) was added to a stirred milky suspension of APP (3.15 g, dry) in deionized water (175 mL) at room temperature, and the obtained solution was stirred for 10 min. This was followed by addition of 1.0 mol/mL HCl (35 mL) to the solution and stirring was continued for another 10 min. After this, the solution was diluted to 0.4 and 0.9 wt % concentrations, respectively, and the pH was adjusted to 9 with 1.0 mol/mL NaOH or HCl solution. The polyelectrolyte PEI solution was, respectively, prepared as 0.4 and 0.9 wt % concentrations using deionized water, and the pH was adjusted to 9 with 1.0 mol/ mL NaOH or HCl solution. As shown in Scheme 1, a typical sample preparation process was as follows: the substrate, a piece of cleaned ramie fabric or quartz slide was first immersed into the PEI solution for 2 min, washed with deionized water twice (each for 1 min), dried with
air, immersed into the APP solution for 2 min, washed with deionized water twice (each for 1 min), and dried with air. This process describes a complete assembly cycle for one bilayer (BL) of PEI and APP, and it was repeated until the desired number (n) of bilayers was obtained. Finally, the treated ramie fabrics were dried under vacuum at 60 °C for 2 h. In this work, we denote briefly the treated ramie fabric that was assembled in PEI solution at a concentration of 0.4 wt % and APP solution at a concentration of 0.9 wt % for 20 bilayers as (PEI0.4/ APP0.9)20. 2.3. Measurements and Characterization. Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectra, in the frequency region of 4000−400 cm−1 at a 4 cm−1 resolution, were recorded by a Nicolet 6700 spectrometer (Thermo-Nicolet) using 32 scans. Atomic force microscopy (AFM) experiments in tapping mode of operation were carried out using a Seiko SPI 3800N station (Seiko Instruments Inc.) in air, and the root-mean-square (RMS) roughness values were obtained from the software. Thermogravimetric analysis (TGA) was conducted under air or nitrogen atmosphere on a NETZSCH TG 209 F1 thermogravimetric analyzer. The samples (8−10 mg) were first kept at 100 °C for 5 min and then heated up to 600 °C at a heating rate of 20 °C min−1. Microscale combustibility experiments were carried out on a Govmark MCC-2 microscale combustion calorimeter (MCC). The specimens (3−5 mg, in triplicate) were first kept at 100 °C 6139
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Figure 2. EDX spectra of the (PEI0.9/APP0.4)n (a, b, c), (PEI0.4/APP0.9)n (e, f, g), and (PEI0.9/APP0.9)n (i, j, k) treated ramie fabrics with 5 (first column), 10 (second column), and 20 (third column) bilayers of intumescent coating; P/C ratio and percentage of coating weight added to the fabric as functions of bilayer number (fourth column) for the (PEI0.9/APP0.4)n (d), (PEI0.4/APP0.9)n (h), and (PEI0.9/APP0.9)n (l) treated ramie fabrics.
for 5 min and then heated up to 600 °C at a heating rate of 1 °C/s. The combustor temperature was set at 900 °C, and the oxygen/nitrogen flow rate was set at 20/80 mL/mL. Vertical flame tests were performed according to ASTM D6413, using a vertical burning tester (CZF-3, Nanjing Jiangning Analytical Instrument Factory, China). The samples (300 mm × 76 mm), held 19 mm over the Bunsen burner, were first exposed to the flame for a period of 12 s and then removed rapidly. The morphologies of the pristine and treated ramie fabrics, along with the char residues after vertical flame tests, were observed using field-emission scanning electron microscopy (FE-SEM, Hitachi S-4800, operated at 3 kV) with an energy-dispersive Xray (EDX) analyzer. Prior to analysis, the samples were goldsputtered for 90 s under a high vacuum to increase the conductivity.
ramie fabrics showed strong absorption bands at approximately 862 and 1252 cm−1, which are, respectively, ascribed to the stretching vibration of PO and P−O−P for APP.15 Furthermore, the absolute values of the absorbance at the two bands increased obviously with the amount of layer-bylayer assembly, indicating that the APP had been successively incorporated into the multilayers.6,15 On the other hand, the corresponding surface morphologies of the coated ramie fabrics were also gradually changed as shown in Figure 1b−e. As the number of deposition cycle increased, the sidewalls of cellulose fibers were successively coated with PEI and APP, and the interspaces between fibers were filled step-by-step. Consequently, a multilayered, thick, and dense intumescent coating was built on the fabric matrix. In order to further monitor the LBL growth, the EDX data of (PEI0.9/APP0.4)n, (PEI0.4/APP0.9)n, and (PEI0.9/APP0.9)n treated ramie fabrics with 5, 10, and 20 bilayers are shown in Figure 2 for a quantitatively comparison. As can be clearly seen, the phosphorus (P) element was detected in all the coated ramie fabrics, indicating the presence of APP in the multilayers. Meanwhile, as the number of deposition cycles increased, the phosphorus/carbon (P/C) and phosphorus/oxygen (P/O) ratio values were substantially increased, revealing reproducible coverage of APP on the sidewall of cellulose fibers,22,23 which
3. RESULTS AND DISCUSSION 3.1. Characterization of Coated Ramie Fabric. In general, surface chemical structures of fabrics can be qualitatively determined by ATR-FTIR spectroscopy.21 The ATR-FTIR spectra of the pristine and treated ramie fabrics with (PEI0.4/APP0.9)n multilayers of 5, 10, and 20 bilayers are shown in Figure 1a as representatives to monitor the efficiency of the LBL growth. It can be clearly seen that all the coated 6140
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Figure 3. Tapping-mode AFM images of (PEI/APP)n multilayer films prepared on quartz slides after 10 bilayers adsorption: (a, d) (PEI0.9/ APP0.4)10, (b, e) (PEI0.4/APP0.9)10, and (c, f) (PEI0.9/APP0.9)10. The outmost layer is APP layer.
2 μm2 scale, the RMS roughness value of (PEI0.9/APP0.4)10 thin film was the lowest at 5.442 nm, and the RMS roughness value of (PEI0.4/APP0.9) 10 thin film was 7.358 nm. Apparently, the higher roughness for (PEI0.4/APP0.9)10 was mostly ascribed to the APP layer. The high concentration of NaCl, formed during the process of polyelectrolyte solution preparation, was capable of making APP molecule adopt coiled configurations in aqueous solution,24,25 resulting in looser adsorption onto PEI layer. Furthermore, when both the PEI and APP were assembled at the same concentration of 0.9 wt %, the PEI chains at pH 9 also adopted coiled configurations and then adsorbed more coiled structures of APP.26 As a consequence, the (PEI0.9/APP0.9)10 thin film showed the highest roughness among the three systems. However, in the case of ramie fabric substrate with hierarchical structure,27 the specific surface area decreased with the number of deposited layers. Hence, the slower growth rate of the (PEI0.9/APP0.9)n system from 10 to 20 bilayers was mostly due to the rapid decrease of specific surface area, because of more thick and dense films filling of the porosity for cellulose fibers. 3.2. Thermal Properties. Figure 4 presents the representative TGA and DTG curves of the pristine and treated ramie fabrics with 20 bilayers of PEI and APP under nitrogen atmosphere, and the detailed data for the samples coated with 5, 10, and 20 bilayers are listed in Table S1, Supporting Information. Compared to the pristine ramie fabric, gradual reductions in 5 wt % weight loss temperature (T5%) and maximum weight loss temperature (Tmax1) were observed as the number of deposition cycles increased during thermal decomposition, which was mostly due to the earlier degradation
was consistent with the ATR-FTIR results. Furthermore, the resulting P/C and P/O ratio values and their dependences on the number of deposited layers can also be easily controlled by tuning the concentration collocation of both polyelectrolytes. As for the (PEI0.9/APP0.4)n system (Figure 2a−d), the P/C ratio values for (PEI0.9/APP0.4)5, (PEI0.9/APP0.4)10, and (PEI0.9/APP0.4)20 were 1.5, 2.7, and 4.2, respectively, which showed linear dependence on the bilayer number. Even at low PEI concentration (0.4 wt %), the (PEI0.4/APP0.9)n system (Figure 2e−h) still showed higher P/C ratio values from 10 to 20 bilayers than those of the (PEI0.9/APP0.4)n system, and the P/C ratio value showed similar exponential dependence on the bilayer number, reflecting the effect of APP concentration on the polyelectrolytes adsorption during the LBL deposition. Simply increasing PEI concentration to 0.9 wt % while keeping APP concentration at 0.9 wt % led to an obvious increase in the P/C ratio value (see Figure 2i−l), suggesting that more APPs were absorbed onto the positively charged PEI layer as compared to the other two systems. However, the P/C ratio value of the (PEI0.9/APP0.9)n system increased almost nonlinearly with the number of bilayers, which exhibited a slow increase from 10 to 20 bilayers. As shown in Figure 2d,h,l, the percentage of coating weight added to the fabric also increased obviously with the amount of layer-by-layer assembly, which showed similar growth behavior to that of EDX data. These observations are interesting and warrant the following discussion. Figure 3 shows the representative tapping mode AFM images for10 bilayers of PEI and APP assembled on quartz slides for different concentration collocations. As can be seen, at the 2 × 6141
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Figure 4. Representative TGA curves (a) and DTG curves (b) for the pristine and treated ramie fabrics with 20 bilayers of intumescent coating under nitrogen atmosphere.
Figure 5. Representative TGA curves (a) and DTG curves (b) for the pristine and treated ramie fabrics with 20 bilayers of intumescent coating under air atmosphere.
of APP.28 However, the char residues at temperatures ranging from 400 to 600 °C for the coated samples were significantly higher than that of the pristine one, indicative of the enhancement of thermal stability and char formation at high temperature provided by the intumescent coating.5,29 For example, the char residues at 600 °C for (PEI0.9/APP0.4)5, (PEI0.9/APP0.4)10, and (PEI0.9/APP0.4)20 showed 14, 20, and 26 wt % increase as compared with that of the pristine one (see Table S1, Supporting Information), respectively, which were remarkably higher than the percentage of coating weight added to the fabric. These results revealed that introducing multilayers of PEI and APP on this flexible and porous substrate allows for a straightforward control of thermal transmission and simultaneous release of cellulose pyrolysis products, which was due to the protective intumescent char layer formed on the surface during thermal degradation. Unlike under nitrogen atmosphere, the pristine ramie fabric decomposition occurred in two distinct steps in air (Figure 5). As listed in Table S1, Supporting Information, the 5 wt % weight loss temperature (T5%) and maximum weight loss temperature (Tmax1, Tmax2) of pristine ramie fabric were 328, 353, and 468 °C, respectively, and the char residue at 600 °C was 6.84%. Once more, introducing multilayers of PEI and APP made the T5% and Tmax1 of coated ramie fabrics shift to lower temperatures with increasing bilayer number. However, the second peak decreased gradually after each deposition cycle and disappeared almost completely after 20 bilayers were built, and the char residues at temperatures ranging from 400 to 600 °C were significantly higher than that of the pristine one. For
instance, the char residues at 600 °C for (PEI0.9/APP0.4)5, (PEI0.9/APP0.4)10, and (PEI0.9/APP0.4)20 showed 2, 7, and 17 wt % increase as compared with that of the pristine one (see Table S1, Supporting Information), respectively, which were substantially higher than the coating weight added to the fabric. According to literature reports, the second decomposition step is mostly ascribed to oxidation degradation of the char layer and the following release of CO and CO2.30 Therefore, the higher thermal oxidation stability in the second stage for coated ramie fabrics was most likely due to the formation of thick and dense stable intumescent chars in the first stage, which can efficiently hinder the oxygen permeability and simultaneous thermal transmission. Besides, simple tuning of concentration collocation for both polyelectrolytes can also effectively control the resulting thermal properties of ramie fabrics. In the case of the (PEI0.4/APP0.9)n system, the char residue under nitrogen or air atmosphere at temperature ranging from 400 to 600 °C was substantially higher than that of the (PEI0.9/APP0.4)n system for the same layer number. Furthermore, as discussed earlier, the P/C ratio value of the (PEI0.9/APP0.9)n system was higher than the other two systems for the same layer number, and the corresponding percentage of coating weight added to the fabric was also the highest as shown in Figure 2 and Table S1, Supporting Information. Consequently, the char residues at 600 °C under nitrogen or air atmosphere for (PEI0.9/APP0.9)5, (PEI0.9/APP0.9)10, and (PEI0.9/APP0.9)20 were substantially higher than those of the other two systems for the same layer number. 6142
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3.3. Flammability Properties. In order to assess the flammability properties of coated ramie fabrics, we first simulated the anaerobic pyrolysis and a subsequent reaction of the volatile pyrolysis products with nitrogen/oxygen (80/20) gas mixture under high temperatures by means of MCC, which is a widely used small-scale tool.8 The representative heat release rates of the pristine and treated ramie fabrics with 20 bilayers of (PEI/APP) are plotted as a function of temperature in Figure 6, and the detailed data for the samples coated with 5,
coating weight among the three systems for the same layer number, had the lowest PHRR and THR values. Consequently, the net increase in char residue for the treated ramie fabrics was remarkably higher than the percentage weight of coating, which showed a similar trend to that of TGA results under nitrogen or air atmosphere. Further assessment of flammability properties for the pristine and coated ramie fabrics was provided by the vertical flame test. The images of 7 and 9 s after ignition, along with the collected char residues are shown in Figure 7 and Figures S1 and S2, Supporting Information, for comparison. For the pristine ramie fabric (Figure 7a,e,i), the flame was very vigorous and spread upward rapidly; the glow disappeared slowly after removing the burner, and the char residue was completely torn apart to form loose fragments. Nevertheless, with increasing number of bilayers, the coated fabric was consumed by the same flame more and more slowly; the glow disappeared rapidly after removing the burner, and the collected char residue was more and more perfect. As a consequence, the char residue for (PEI0.9/APP0.4)20 was very intact and thick (Figure 7b,f,j), although it was wholly covered with the flame during combustion. In contrast, (PEI0.4/APP0.9)20 was self-extinguished at 9 s after ignition, and more than half of the fabric was not burned at all (Figure 7c,g,k). As compared to the other two systems, the (PEI0.9/APP0.9)n system showed increased self-extinguishing ability for the same layer number, remaining relatively intact char residue at last. Consequently, (PEI0.9/ APP0.9)10 was self-extinguished at 7 s after ignition, and about 80% unburned fabric was left (see Figure S2c,f,i, Supporting Information). Moreover, further enhancement for self-extinguishing ability was achieved for (PEI0.9/APP0.9)20, about 90% of which was left and not burned (see Figure 7d,h,l). Hence, these results also further highlighted the importance of the coating weight added to the fabric in controlling the selfextinguishing properties during combustion, which is ascribed to the controlled density and thickness for the intumescent coating. 3.4. Analysis of the Collected Char Residues. Although previous studies used APP as a negatively charged phosphorus containing polyelectrolyte, it is still difficult to construct selfextinguishing coating onto fabric via layer-by-layer assembly.15,16,19 In our study, APP was paired with PEI, and the polyelectrolyte solution concentrations were 0.4 and 0.9 wt %, respectively. By appropriate adjustment of concentration collocation for both partners, the intumescent flame retardant effect and self-extinguishing ability during combustion were easily controlled. To clarify the flame retardant mechanism, the
Figure 6. Representative heat release rate curves for the pristine and treated ramie fabrics with 20 bilayers of PEI and APP.
10, and 20 bilayers are listed in Table S2, Supporting Information. Compared to the pristine ramie fabric, the total heat release (THR) and maximum peak (PHRR) values of treated samples were shown to decrease with increasing number of bilayers, implying the blocked heat transfer process.8,31 For instance, the average PHRR values of (PEI0.9/APP0.4) 5 , (PEI0.9/APP0.4) 10 , and (PEI0.9/ APP0.4)20 were 156.10, 149.90, and 103.70 W g−1, which showed 181.10, 187.30, and 233.50 W g−1 decrease as compared with the pristine one, respectively (see Table S2, Supporting Information). This result clearly demonstrated that multilayers of PEI and APP were indeed an effective protective layer that enabled strong hindering release of cellulose pyrolysis products, leading to a gradual reduction in heat release rate. A further enhancement for flame retardancy was observed for the (PEI0.4/APP0.9)n system, PHRR and THR values of which were substantially lower than those of the (PEI0.9/APP0.4)n system for the same layer number. In addition, the (PEI0.9/ APP0.9)n system, with the highest percentage of intumescent
Figure 7. Images of vertical flame tests for (a, e, i) pristine ramie fabric, (b, f, j) (PEI0.9/APP0.4)20, (c, g, k) (PEI0.4/APP0.9)20, and (d, h, l) (PEI0.9/APP0.9)20, respectively. 6143
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Figure 8. Top view SEM images (first row: before vertical flame test; second row: after vertical flame test) and EDX spectra of char residues after vertical flame test (third row) for (a, e, i) pristine ramie fabric, (b, f, j) (PEI0.9/APP0.4)20, (c, g, k) (PEI0.4/APP0.9)20, and (d, h, l) (PEI0.9/ APP0.9)20, respectively.
highest among the four samples, which was consistent with the EDX data before vertical flame test. Therefore, it was not difficult to understand that the controlled thermal degradation behaviors and self-extinguishing properties were attributed to the controlled intumescent effect of (PEI/APP)n coating during combustion. In this process, PEI also played a critical role, since it can provide good char layer and inert gases at high temperature and cooperate with the released polyphosphoric acid from APP. Hence, this observation carried implications in future assembly system design, where not only the suited oppositely charged polyelectrolytes but also the appropriate adjustment of concentration collocation for both polyelectrolytes needs to be considered.
representative char residues for the pristine and treated ramie fabrics with 20 bilayers before and after vertical flame tests were collected and analyzed by FE-SEM as shown in Figure 8a−h. As for the pristine ramie fabric (Figure 8a,e), the fibers shrunk severely in diameter, leaving an apparently slender and loose char residue. In contrast, the char residue of (PEI0.9/APP0.4)20 (Figure 8b,f) was considerably intact, almost completely retaining the orthogonal plain-weave architecture. Meanwhile, the interspaces between fibers were filled up with intumescent chars, and the surface of ramie fabric was covered with lots of relatively homogeneous and small-sized bubbles. Remarkably, many big bubbles were formed throughout the surface of char residue for (PEI0.4/APP0.9)20 (see Figure 8c,g), demonstrating excellent intumescent effect and fire protection properties.5,11,12 Additionally, (PEI0.9/APP0.9)20, with the highest percentage of coating weight among the three systems for the same layer number, showed the most perfect char residue. As can be clearly seen, the surface of char residue for (PEI0.9/APP0.9)20 (Figure 8d,h) was completely covered with compact and wrinkled intumescent layer, which acted as a strong flame shield for the underlying fabric during combustion. Compared with the previous literature,15,16,19 the intumescent effect is much more effective in our study. Therefore, the resulting (PEI/ APP)n coating shows superior flame retardation. As shown in Figure 8i,j,k,l, the EDX analysis obviously exhibited that P element was still detected in the char residues for coated ramie fabrics, indicating the excellent carbonization ability of APP.32 Meanwhile, the P/C and P/O ratio values were substantially higher than those of samples before the vertical flame test, which was mostly due to the intumescent effect of (PEI/APP)n coating and the subsequent migration to the surface of fabric. Furthermore, the P/C and P/O ratio values of the char residues for (PEI0.9/APP0.9)20 were the
4. CONCLUSIONS In conclusion, this work provided a simple but effective method for the controlled formation of self-extinguishing intumescent coating on ramie fabric via layer-by-layer assembly. As the number of deposition cycles increased, the P/C and P/O ratio values of the coated ramie fabrics were substantially increased, revealing reproducible coverage of APP on the sidewall of cellulose fibers. Significantly, tuning of concentration collocation for both polyelectrolytes also effectively controlled the resulting P/C and P/O ratio values, along with the percentage of coating weight added to the fabric for the same layer number. Studies on thermal properties demonstrated that the char residue of the coated ramie fabric under nitrogen or air atmosphere at temperatures ranging from 400 to 600 °C was remarkably higher than the percentage of coating weight added to the fabric, which showed strong dependency on the number of deposited layers, especially on the concentration collocation of both polyelectrolytes. More interestingly, the PHRR and 6144
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THR values of treated samples during the MCC test, especially the self-extinguishing ability and resulting unburned area during the vertical flame test, were also effectively tailored by varying concentration collocation of both polyelectrolytes. The novelty of this approach to modulate the self-extinguishing ability stems from the control of coating weight added to the fabric and corresponding P/C and P/O ratio values, which therefore, represents a simple and convenient strategy that is broadly applicable to other cellulose systems.
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ASSOCIATED CONTENT
S Supporting Information *
Table of thermal properties for the ramie fabrics before and after modification under nitrogen and air conditions, table of microscale combustion calorimetry results for the ramie fabrics before and after modification, and images of vertical flame tests for treated ramie fabrics with 5 and 10 bilayers of PEI and APP. This information is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel/Fax: +86 574 88130132. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from the National Basic Research Program of China (No. 2010CB631105, 2011CB612307).
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REFERENCES
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