Article pubs.acs.org/IECR
Poly(acrylic acid)/Clay Thin Films Assembled by Layer-by-Layer Deposition for Improving the Flame Retardancy Properties of Cotton Guobo Huang,*,† Huading Liang,† Xu Wang,‡ and Jianrong Gao‡ †
School of Pharmaceutical and Chemical Engineering, Taizhou University, Linhai, Zhejiang 317000, P. R. China State Key Laboratory Breeding Base of Green Chemistry Synthesis Technology, Zhejiang University of Technology, Hangzhou, Zhejiang 310014, P. R. China
‡
S Supporting Information *
ABSTRACT: A flame-retardant poly(acrylic acid) (FR-PAA) was prepared by copolymerization of N-(2-(5,5-dimethyl-1,3,2dioxaphosphinyl-2-ylamino)-ethylacetamide-2-propenyl acid (DPEPA) and acrylic acid and used to fabricate various FR-PAA/ montmorillonite (MMT) thin films via layer-by-layer (LbL) deposition as a flame-retardant coating system for cotton fabric. Thermogravimetric analysis (TGA) indicated that treatment of FR-PAA/MMT thin films improved the thermal stability of cotton fabric. Cone calorimeter testing showed that the cotton fabrics coated with the FR-PAA/MMT thin films had less flammability with lower peak heat release rate (PHRR), lower total heat release (THR), and lower average mass loss rate (AMLR). In addition, scanning electronic microscopy (SEM) demonstrated that the surface of the fabrics coated with FR-PAA/ MMT films after combustion was covered by a layer of continuous and compact char. polyhedral oligomeric silsesquioxanes.27 Carosio et al. have already shown that this strategy allows enhancing the thermal stability and flame retardancy of polyester using different organic and inorganic components.28−30 Furthermore, novel LbL coatings resulting from the assembly of ammonium polyphosphate, chitosan, and silica have been very recently proposed for polyester−cotton blends.31 The treated fabrics have evidenced a significant improvement of their thermal stability, together with an increased time to ignition, a lowering of the heat release rate, as well as a significant decrease in smoke release rate and production of carbon monoxide. Although hybrid organic−inorganic coatings prepared via LbL assembly, as a flame-retardant system for cotton fabrics, have been developed in recent years, it is still a critical challenge in the nanocomposites fabrication combining intumescent flame retardant with clay via LbL assembly as a flame retardant system for the cotton textile. In this paper, flame-retardant poly(crylic acid) (FR-PAA) is prepared by free radical polymerization of acrylic acid (AA) with N-2-(5,5-dimethyl-1,3,2-dioxaphosphinyl2-ylamino)-ethylacetamide-2-propenyl acid (DPEPA). Subsequently, clay platelets are deposited with FR-PAA to generate FR-PAA/MMT thin film. This thin film is applied to modify cotton fabrics. The thermal stability and flame-retardant properties of the fabrics coated with FR-PAA/MMT thin film are investigated by thermogravimetric analysis (TGA) and vertical flame and cone calorimeter tests. It is concluded that the combination of FR-PAA and MMT via LbL assembly can improve the flame retardancy of cotton fabrics, thus promoting development of a new kind of flame-retardant textiles.
1. INTRODUCTION Cotton is one of the most important natural textile fibers used to produce clothing, furnishing materials, household goods, and many other products, but it is also highly flammable for its low limiting oxygen index and combustion temperature.1 Once cotton fabric is ignited, it burns rapidly and the flame spreads quickly, potentially causing fatal burns in a short time. Hence, reducing the flammability of cotton fabrics is becoming imperative, in particular for those flame-retardancy properties demanding materials. Many methods, such as pad−dry−cure,2 chemical modification,3,4 graft polymerization by ionized radiation,5 plasma,6 and sol−gel7,8 treatment, have been used to impart flame retardancy properties to cotton. Nevertheless, such techniques present some disadvantages owing to their complex procedures and deleterious effects on the mechanical properties of the treated cotton fabrics.3,5 Thus far, many types of flame retardants, including halogen derivatives9 and boron-containing10 and phosphorus-based compounds,11−13 have been used to treat cotton fabrics without considering their effects on the environment. Among the available methods, the layer-by-layer (LbL) assembly technique is a relatively novel process to confer flame retardancy and thermal stability to textiles and is associated to a minimized environmental impact. The advantages of the LbL assembly technique include simplicity, universality, and thickness control at the nanoscale level, compared with the other traditional coating methods for cotton fabrics.14,15 Moreover, LbL assembly can be used to combine a wide variety of species including nanoparticles,16−18 nanosheets,19−21 and nanowires22,23 with polymers, thus merging the properties of each type of material. Recently, the LbL assembly technique has shown encouraging results for enhancing the textile flame retardancy. Grunlan et al. demonstrated that LbL can enhance the flame retardancy of cotton fabrics, as well, using a branched polyethylenimine coupled to laponite,24 sodium cloisite,25 silica nanoparticles,26 or © 2012 American Chemical Society
Received: Revised: Accepted: Published: 12299
March 27, 2012 August 23, 2012 September 4, 2012 September 4, 2012 dx.doi.org/10.1021/ie300820k | Ind. Eng. Chem. Res. 2012, 51, 12299−12309
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Article
Figure 1. Schematic illustration for the synthesis of an acrylic acid and a DPEPA copolymer (FR-PAA).
Figure 2. Scheme of the LbL film deposition. Steps 1 and 3 represent adsorption of PAA and MMT nanolayers; steps 2 and 4 are rinsing and drying steps.
2. EXPERIMENTAL SECTION 2.1. Polyelectrolytes Solutions. 2,2-Dimethyl-1,3-propanediol phosphoryl chloride (DPPC) and N1-(5,5-dimethyl1,3,2-dioxaphosphinyl-2-yl)ethane-1,2-diamine (DPEA) were prepared according to the published procedure.32 N-2-(5,5Dimethyl-1,3,2-dioxaphosphinyl-2-ylamino)-ethylacetamide-2propenyl acid (DPEPA) was prepared as follows: DPEA (0.1 mol), maleic anhydride (0.1 mol), and 30 mL of dried trichloromethane were mixed in a glass flask for 5 h at 40 °C. The mixture was then filtered and washed by ethyl ether to give the product as a white solid (see Supporting Information). A FR-PAA was prepared using the procedure shown in Figure 1. PAA modified with 10 and 20 wt % DPEPA is denoted by PAD1 and PAD2. Two different compositions (90/10 and 80/20 by wt %) of AA and DPEPA were taken in a 250 mL flask equipped with 0.5 wt % potassium persulfate as the initiator.
Total solution concentration was 10 wt %. Solutions were initially purged with nitrogen for 30 min and maintained at 70 ± 0.5 °C in a water bath for 4 h. Copolymers were precipitated twice in ethanol when the reaction completed and dried at 60 °C until a constant weight was obtained. pH values of unadjusted PAA, PAD1, and PAD2 solution are 3.3, 3.5, and 3.6, respectively, but were adjusted to 4 and 7 by adding 1 M sodium carbonate solutions. Molecular weights of PAD1 and PAD2 were determined with gel permeation chromatography (GPC). Pristine montmorillonite (MMT) was mined at An’ji, Zhejiang, China, and supplied by Anji Yu Hong Clay Chemical Co. Ltd. (Zhejiang, China). Amino-functionalized montmorillonite (APTMS-MMT) was prepared by grafting of 20 wt % 3aminopropyltrimethoxysilane (APTMS) on the surface of MMT according to the procedures published previously.33 APTMS12300
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Table 1. Characteristics of the Prepared Copolymers comonomer sample
AA (M2), g (mol)
DPEPA (M1), g (mmol)
yield (%)
f1 DPEPAa (mol %, feed)
F1 DPEPAb (mol %, obsd)
Mnc ( × 10−3)
Mwc ( × 10−3)
viscosityd (cp)
PAA PAD1 PAD2
10.0 (0.139) 9.0 (0.125) 8.0 (0.111)
0 (0) 1.0 (3.27) 2.0 (6.54)
72 63 61
0 2.62 5.88
0 2.44 5.37
89 83 78
174 161 153
6.4 8.7 10.2
a
Relative amount of DPEPA monomer in the feed in mol %. f1 + f 2 = 1. bObserved relative amount of DPEPA monomer in the copolymer in mol % measured by 1H NMR. F1 + F2 = 1. cMolecular weights determined by size exclusion chromatography with respect to polystyrene standard. d Viscosity measurements were carried out on a low-shear apparatus; polymer concentration is 1.0 wt %; temperature is 30 ± 0.3 °C; shear rate is 1.0 s−1.
flammability properties such as heat release rate (HRR), peak of heat release (PHRR), total heat release (THR), time to ignition (TTI), average specific extinction area (ASEA), and average mass loss rate (AMLR) were simultaneously measured. According to the ISO 5660 standard, the fabrics were cut to a dimension size of 100 × 100 mm. Ten pieces of fabrics were wrapped with aluminum foil and then put in a box with the same dimension in the horizontal orientation. For each sample, the experiments were repeated three times in order to ensure reproducible and significant data. Experimental error was evaluated as standard deviation (σ). Before the measurements, all samples for TGA and vertical flame and cone calorimeter tests were placed in a vacuum oven for 2 h at 80 °C and then at 25 °C until the beginning of the test to have a relative humidity of 0%. Scanning electron microscopy (SEM) measurements for the chars were performed with a Hitachi S-4800(II) SEM with an acceleration voltage of 15 kV. Energy-dispersive X-ray (EDX) measurements were conducted on a Noran Vantage-ESI EDX micro analyzer equipped in the SEM. The Elmendorf tearing test, which uses a falling pendulum to determine the amount of force required to tear the fabric according to ASTM D 1424, was used to evaluate tear strength. Tensile strength of the fabric was measured according to ASTM D 5035-95. Tensile strength retention and tear strength retention were calculated by dividing the tensile strength and tear strength of the coated fabric by that of the control fabric, respectively.
MMT was dispersed in the diluent HCl solution to obtain 0.5 wt % cationic mixtures with ∼6.7 of the pH value level. 2.2. Layer-by-Layer Deposition. The FR-PAA/MMT multilayer films were grown on a planar glass substrate. The glass surface was cleaned for 2 h with a concentrated sulfuric acid solution containing potassium dichromate before it was immediately dipped into the FR-PAA solution (0.5 wt %) for 30 s. After being dried with a stream of filtered air, the glass substrate was immersed in a dispersion of APTMS-MMT (0.5 wt %) for 30 s to allow MMT adsorption to take place. Then the substrate was rinsed with deionized water and dried with a stream of filtered air. FR-PAA/MMT multilayer films were grown by repeating the above procedure a few times with a glass substrate coated with a 40-bilayer-LbL film. The deposition steps and schematic structure of the assembled film are sketched in Figure 2. 2.3. Characterization and Measurement. FT-IR spectra of control DPEPA and FR-PAA dispersed in potassium bromide discs were recorded with a Nicolet (model 5700 FT-IR) spectrophotometer, scanning range 400−4000 cm−1. 1H NMR and 13C NMR spectra were recorded with a Bruker Avance III (500 MHz) spectrometer in CDCl3 using tetramethylsilane (TMS) as internal standard. High-resolution mass spectrometry (HRMS) was performed with a Therm LCQ TM Deca XP plus mass spectrometer coupled to a Waters 2695 liquid chromatograph. The assembly thickness on the silicon wafer was measured with a M-2000DI discrete wavelength ellipsometer (Microphotonics, J. A.Woollam Co.). The HeNe laser (632.8 nm) was set at an incidence angle of 65°. A WA 10005 electron microbalance from Shanghai Fangrui Instrument Co. Ltd., measurement range of 0.01−1 g, was used to measure the mass increments of 0.01 m2 of the control fabric. Atomic force microscopy (AFM) images were performed in a dry state on a commercial instrument of AIST-NT Co. SmartSPM AFM, and the measurement was operated in tapping mode using silicon cantilevers with a force constant of 40 N/m. Thermogravimetic analysis (TGA) was carried out on a Q600SDT thermogravimetric analyzer. Sample weights were the range of 12−15 mg. All TGA samples were measured from 30 to 600 °C at a heating rate of 10 °C/min under a continuous air flow. Flame-retardancy properties were studied using two complementary tests in order to assess both flammability and combustion behavior of the materials under study. Vertical flame testing was carried out by applying a 2 mm methane flame once for 10 s to specimens 200 × 16 mm2; the after-flame and after-glow times and final residue were evaluated. This test aims to mimic the procedure described in ISO 15025 standard, commonly employed for protective garments. The combustion behavior of uncoated and coated fabrics was evaluated by a cone calorimeter (Fire Testing Technology, FTT) with a heat flux of 35 kW/m2. The
3. RESULTS AND DISCUSSION 3.1. Synthesis. An AA and a DPEPA copolymer (FR-PAA) was prepared by free radical polymerization in aqueous medium and characterized by FT-IR and 1H NMR (see Supporting Information) with yields varying from 61% to 72%. Before measurements, these copolymers were precipitated from acetone/acetic ether in order to remove unreacted AA and then washed by ethanol/trichloromethane twice to remove traces of unreacted phosphorus−nitrogen-containing compound DPEPA and any DPEPA homopolymer. As shown in Table 1, f1 is the relative amount of DPEPA monomer in the feed in mol %. F1 is the relative amount of DPEPA monomer in the copolymer in mol % measured by 1H NMR,34 and the F1 value is obtained by calculating the integral proportion of the methyl hydrogen (at 1.04 ppm) from DPEPA and the methine hydrogen (at 1.53 ppm) from PAA. In all cases, the DPEPA monomer content (F1) in the copolymer is less than that in the feed (f1), which indicates that this polymerization gives incomplete conversion of monomers. However, the slight difference between F1 and f1 for the same copolymer (PAD1 or PAD2) suggests that some DPEPA homopolymer could form during polymerization. However, any DPEPA homopolymer produced would have been removed by the ethanol/trichloromethane solvent wash 12301
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252 °C, Td2 = 398 °C) and has about 15.7 wt % residues at 600 °C.36 Compared with pure PAA, Td1 of PAD1 and PAD2 decreases while Td2 of the copolymers increases. Meanwhile, Td1 of PAD2 is 10 °C lower than that of PAA, while Td2 of PAD2 is 28 °C higher than that of PAA. The residue of PAD2 at 600 °C is ∼7 wt % higher than that of PAA. The reduction of Td1 of the copolymer is attributed to the first degradation of phosphorus− nitrogen-containing compound DPEPA. When the temperature is increased, the carbonaceous layers generated from DPEPA delay the second thermal degradation (Td2) of the copolymers. These TGA results indicate that the phosphorus−nitrogencontaining compound DPEPA has a positive effect on the thermal stability of PAA, especially at the high-temperature range. 3.2. Growth and Structure of Thin Film. The influence of the pH value of the polyelectrolytes on growth of the thin films was evaluated by ellipsometry. Six different thin film recipes, PAA, PAD1, and PAD2 (pH 4 or pH 7) with MMT at 0.5 wt %, were used to prepare the films whose growth is shown in Figure 4. All these systems grow linearly as a function of PAA/MMT,
during workup because of its high solubility in the above solvent. This would lead not only to lower copolymer yields but also to lower mol % of DPEPA incorporated into the copolymer as indicated by the F1 values in Table 1. In addition, the viscosity of FR-PAA solution increases relative to PAA solution for the monomer DPEPA, improving the hydrophobicity of the copolymer. The TG and dTG curves of PAA and FR-PAA (PAD1 and PAD2) under continuous air flow are displayed in Figure 3. The
Figure 4. Film thickness as a function of the number of bilayers deposited for a series of LbL assemblies made with different unadjusted and pH 7 PAA and P(AA-co-DPEPA) solution and the MMT mixture.
PAD1/MMT, and PAD2/MMT bilayers (BL) deposited. Film thicknesses show a similar change for the films made with the same pH polyelectrolytes solution. Differences observed between high- and low-pH systems are ascribed to the different degrees of charge density of the weak polyelectrolytes (PAA, PAD1, and PAD2). When polyelectrolytes are highly charged, the polyelectrolyte chains adopt a flat conformation due to selfrepulsion of like charges along its backbone, whereas at low charge density, the polyelectrolytes have a more coiled and bulky conformation due to intrachain H bonding. Similar results were shown in a recent study of branched polyethylenimine/MMT LbL films.24,25 In addition, the film thicknesses increase with increasing DPEPA content of FR-PAA, which is attributed to improvement of the FR-PAA hydrophobicity. In order to better understand this growth process, an electron microbalance was used to measure the weight added by coated cotton fabric associated with deposition of each individual layer. Figure 5 shows the mass of the coated cotton fabrics for the six different recipes described above. The weight of 1 m2 of the control fabric is ∼50 g, so the weight added to the fabric by
Figure 3. TG and dTG curves for PAA, PAD1, and PAD2.
Table 2. Data of TGA for PAA, PAD1, and PAD2 sample
Td1a (°C)
Td2a (°C)
residue at 600 °C (%)
PAA PAD1 PAD2
252 244 242
398 413 426
15.7 19.2 22.5
a
First and second thermal degradation temperature is denoted Td1 and Td2, respectively.
corresponding TGA data are presented in Table 2. The thermal degradation temperature (Td) is characterized by the maximum weight loss rate in thermogravimetry.35 Two weight-loss regions are found for these samples. The first and second thermal degradation temperature is denoted Td1 and Td2, respectively. Pure PAA decomposes at a temperature of 200−500 °C (Td1 = 12302
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higher charge density they have stronger electrostatic adsorption with positively charged MMT and lie flatter on the charged substrate due to intrachain self-repulsion than in their low charge density state, which results in thinner, flatter layer deposition and less clay platelets deposition due to the relatively smooth surface. FR-PAA (PAD1 or PAD2) chains are more coiled and entangled than PAA for improvement of the FR-PAA hydrophobicity. This results in thicker, rougher layer deposition that would conceivably allow for more clay platelets to deposit due to the greater surface area of this relatively coarse surface.25 Tapping mode AFM was used to characterize the surface of PAD2/MMT thin films (see Supporting Information). Clay platelets are well dispersed in the PAD2 matrix and become oriented parallel to the surface to maximize the attractive energy.37 The height of clay platelets in the first deposited cycle, ∼6.8 nm, is slightly larger than that of single clay platelet (about ∼2.1 nm), which is attributed to the MMT stacked structure made up of three clay layers. However, as the number of BL is increased to 10, the contours of clay platelets become blurred, and the height has no measurable value due to the entanglement of polymer chains. As shown in Figure 6a, the optical image of a 40 BL PAD2 pH 7/MMT film deposited on the cleaned glass substrate shows that the films have a high level of flatness and homogeneity and the transparency decreases compared with the glass substrate. Figure 6b and 6c shows SEM images of the surface of 10 BL and 40 BL PAD2 pH 7/MMT films. Most of separate and flat clay platelets are dispersed uniformly on the surface of the film. The amount of MMT particles on the surface of 40 BL PAD2 pH 7/MMT films is more than that of 10 BL PAD2 pH 7/MMT films for the increase of deposited cycle. In addition, the image of the cross section (Figure 6d) shows that a 40 BL PAD2 pH 7/MMT film has the thickness of ∼500 nm. Its surface topography is
Figure 5. Mass of film-coated cotton fabrics as a function of individually deposited clay and polymer layers for PAA, PAD1, and PAD2 (pH 4 or 7)/MMT systems.
coating is 0.8−5.9 wt %. There is much difference observed in the mass per layer of the film-coated cotton fabrics made with the polyelectrolytes (PAA, PAD1, and PAD2) at pH 4 and the same concentrations of MMT suspensions (0.5 wt %); however, the films made with polyelectrolytes at pH 7 show a slight difference in mass after ∼15 BL coating. PAA, PAD1, and PAD2 at pH 7 deposits less in each layer than these polyelectrolytes at pH 4, while PAD2 deposits more in each layer than PAD1 at the same pH value. In all six recipes, the film thickness and weight addition for coated cotton fabric are influenced by the degrees of charge density and DPEPA content of polyelectrolytes under the same concentration of MMT suspensions. As mentioned above, when the polyelectrolytes (PAA, PAD1, and PAD2) at pH 7 have a
Figure 6. Optical images (a) of a 1 in × 3 in microscope glass slide coated with 40 BL PAD2 pH 7/MMT films. SEM images of the surface of 10 BL (b) and 40 BL (c) PAD2 pH 7/MMT films. (d) SEM images of the cross-section of 40 BL PAD2 pH 7/MMT films. 12303
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Figure 7. SEM images of cotton fabrics coated with LbL films: (a and b) control cotton fabric; cotton fabrics coated with 5 BL (c and d), and 20 BL (e and f) PAD2 pH 7/MMT films.
involves two competitive pathways which yield aliphatic char and volatile products; in the second step (400−600 °C), some aliphatic char converts to aromatic, yielding CO and CO2 as a consequence of simultaneous carbonization and char oxidation. In the present work, two decomposition steps are observed for uncoated and coated fabrics between 250 and 550 °C. Cotton fabric mainly decomposes at 270−380 (Td1 =322 °C) and 420− 520 °C (Td2 =466 °C) and leaves negligible residue (∼0.5 wt %) at above 500 °C. Td1 and Td2 of 5BL (PAA/MMT)-coated fabric display a slight increase compared with the control fabric, and the residue from 5BL (PAA/MMT)-coated fabric is ∼3 wt % heavier than that from the control fabric at above 500 °C. When increasing the number of BL from 5 to 20, Td1 and Td2 of 20BL (PAA/MMT)-coated fabric are 6 and 10 °C higher than that of the control fabric, respectively, and the residue from 20BL (PAA/MMT)-coated fabric increases obviously compared with the control fabric. These behaviors are ascribed to the protective role of the PAA/MMT coating on cotton fibres and also indicates that a sufficient number of BL improves effectively the thermal properties of the cotton textile. For the 20 BL (PAD2/MMT)coated fabric, Td1 and Td2 are 9 and 11 °C higher than those of the
homogeneous and uniform and shows architecture with indications of stratification. Figure 7a, 7c, and 7e shows low-magnification SEM images of the control fabric, 5 BL and 20BL (PAD2 pH 7/MMT)-coated fabric. The dimensions of the weave structure in uncoated and coated fabrics are identical, which means that the LbL coating process does not alter the fabric dimensions. Figure 7 b shows that the fiber surface in the control fabric is clean and smooth. The fiber surface become coarser after a 5 BL PAD2 pH 7/MMT coating as shown in Figure 7 d. With increasing deposited cycles to 20, the fibers of the coated fabric are covered with more continuous and thick films compared with 5 BL (PAD2 pH 7/ MMT)-coated fabric (Figure 7f). 3.3. Thermal Properties. In order to know the combination effect of FR-PAA and MMT via LbL assembly, the thermal properties of cotton fabrics coated with polyelectrolytes pH 7/ MMT films were investigated by TGA. Figure 8 shows TG and dTG curves for each of three coating recipes at 5 (Figure 8a and 8b) and 20 BL (Figure 8c and 8d). Corresponding TGA data are presented in Table 3. Usually cotton mainly decomposes through two steps under an air atmosphere:7,38 the first (300−400 °C) 12304
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Figure 8. TG and dTG curves for cotton fabrics coated with 5 BL (a, b) and 20 BL (c, d) of polyelectrolytes pH 7/MMT films at a heating rate of 10 °C/ min in air.
better than that of PAA and MMT in terms of delaying the thermal degradation of the textile and enhancing char formation. During the coated fabrics decomposition in air the phosphoric acid generated from the PAD2/MMT coating on heating probably reacts with MMT to form silicoaluminophosphatecontaining layers,39,40 which inhibits effectively production of volatile products and promotes forming of carbonaceous multilamellar structure produced from cellulose by auto-crosslinking, thus resulting in the increase of Td and yield char. 3.4. Flammability. Figure 9 shows heat release rate curves of the cotton fabrics coated with polyelectrolytes pH 7/MMT films at 35 kW/m2. The cone calorimetry data are shown in Table 4. Cotton fabric is a kind of easily flammable cellulose material, and its peak heat release rate (PHRR) reaches a value ≈ 144 kW/m2. In comparison to the control fabric, the PHRR of the 5 BL and 20 BL (PAA/MMT)-coated fabrics is reduced by about 14% and 24%, respectively, even though total heat release (THR), average specific extinction area (ASEA), and average mass loss rate (AMLR) remained almost the same. The time to ignition (TTI) of the 20 BL (PAA/MMT)-coated fabric is 6 s longer than that of the control fabric. For the fabrics coated with FR-PAA/MMT films, both the PHRR AHRR and the AMLR are reduced obviously with addition of DPEPA. Compared with the control fabric, PHRR and THR are reduced by 47% and 18% for the 20BL (PAD2/MMT)-coated fabric; moreover, the TTI of the
Table 3. Data of TGA for Various Samples residue (%)
sample cotton 5BL (PAA/MMT)coated fabric 20BL (PAA/MMT)coated fabric 5BL (PAD1/MMT)coated fabric 20BL (PAD1/ MMT)-coated fabric 5BL (PAD2/MMT)coated fabric 20BL (PAD2/ MMT)-coated fabric
addona (%)
Td1b (°C)
Td2b (°C)
0 0.79
322 323
466 469
25.5 26.1
0.5 3.8
0.1 2.9
2.91
338
476
27.3
7.9
5.9
0.86
325
472
26.4
5.2
3.6
3.34
342
481
29.3
9.9
7.8
1.03
327
474
26.6
6.4
4.9
3.59
347
487
31.8
11.8
8.9
400 °C 500 °C 600 °C
a Weight added by coating fabric. bTemperature at the maximum weight loss rate.
20 BL (PAA/MMT)-coated fabric, respectively, and the final residue shows a 50% increament compared with the 20 BL (PAA/MMT)-coated fabric. These results indicate that the combination of FR-PAA and MMT via LbL assembly is much 12305
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polyethylenimine/MMT films as a flame-retardant system for the cotton fabric.25 It further comfirms that combination of FR-PAA and MMT has a positive effect on the improvement of flame retardancy. 3.5. Morphology of the Chars. The micromorphologies of the final chars of the control and coated cotton fabrics after cone calorimeter testing were examined by SEM. Figure 11a shows that the ashes of the control fabric no longer preserve the fabric structure and shape of the original fibers. Some loose and separated fiber strands remain on the char surface after combustion. Figure 11b shows that the relatively intact fabric structure is still maintained in the residue of the 20BL (PAA/ MMT)-coated fabric, which is attributed to an effective barrier of MMT platelets on the cotton fibers.26 However, the residue of the 20BL (PAD2/MMT)-coated fabric shows a different morphology. It is clearly observed that the continuous and compact swollen charred layers, jammed with a number of clay platelets, are covered on the fabric surface, which indicates that the phosphorus−nitrogen-containing compound DPEPA plays a very important role which leads to formation of a swollen and compact char during combustion. Figure 11d shows the results of EDX analysis for residues of the coated cotton fabrics after cone calorimeter testing (see Supporting Information). Around 1.3 wt % phosphorus and 20.8 wt % carbon still remain in the char of the 20BL (PAD2/MMT)-coated fabric, indicating the excellent carbonization ability of DPEPA. The carbon content of the char for the 20BL (PAD2/MMT)-coated fabric is ∼5% higher than that for the 20BL (PAA/MMT)-coated fabric, which indicates that combination of FR-PAA and MMT improves the charforming ability of the coated fabric. 3.6. Physical Properties. Li et al. compared the yarn numbers of uncoated fabric with 5 BL- or 20 BL-coated fabrics with polyethylenimine/MMT film assemblies and found that there was a slight difference of the yarn numbers in the warp or filling direction between uncoated and coated fabrics.25 It is believed that the coating of polyelectrolyte and clay platelets on the fabric does not significantly alter its comparison of physical properties of the treated fabric to control materials. The tear strength and tensile strength of the 5 BL- or 20 BL-coated fabrics are shown in Table 5. With increasing number of BL from 5 to 20 for the same sample, both tear and tensile strength in the warp direction improves while the strength in the filling direction decreases. For 20 BL-coated fabrics, when the DPEPA content of FR-PAA increases, the tear strength in the warp direction is in the range from 12.8 (103% retention) to 13.7 N (107% retention) and tensile strength in the warp direction is in the range from 344.6 (107% retention) to 357.5 N (111% retention), respectively, while the tear and tensile strength in the filling direction shows a slight decrease. The data presented here
Figure 9. Heat release rate curves for the control fabric and 5 BL- and 20 BL-coated cotton fabrics.
20BL (PAD2/MMT)-coated fabric is 17 s longer than that of the control fabric. Meanwhile, the values of PHRR, THR, and AMLR of the 20BL (PAD2/MMT)-coated fabric are lower than those of 20BL (PAA/MMT)- and (PAD1/MMT)-coated fabrics. This indicates that introduction of DPEPA for PAA improves the flame-retardant properties of the coated fabrics, and this effect is enhanced by increasing the DPEPA content of FR-PAA. In addition, a 20 BL coating reduces much more obviously the flammability (including PHRR, THR, TTI, etc.) of cotton fabric compared with a 5 BL coating, which indicates that a sufficient number of BL provides a more effective flame retardancy for the textile. Figure 10 shows digital photos for the residues of control and coated cotton fabrics after cone calorimeter testing. The digital photo shows that the control fabric is almost burnt out, and a trace amount of white discrete ashes are formed at the end of the cone calorimetry testing. For the 20BL (PAA/MMT)-coated fabric, a uniform and continuous char with the fabric structure is clearly obsearved (see Supporting Information). The residue of the 20BL (PAD2/MMT)-coated fabric is heavier and darker than that of the 20BL (PAA/MMT)-coated fabric. Vertical flame test was also used to measure the flame retardancy of the textile in this paper. During vertical flame testing it is found that the flame of the control fabric sample is brighter and more vigorous compared with the coated fabrics, the after-glow times for the coated fabrics are 7−16 s less than that for the uncoated fabric, and the final residues for the coated fabrics are more than that for the uncoated fabric; however, the control and coated fabrics show similar after-flame time (see Supporting Information). Similar results were obtained for Table 4. Cone Calorimetry Data for Various Samplesa
a
sample
TTI ± σ (s)
PHRR ± σ (kW/m2)
THR ± σ (MJ/m2)
ASEA ± σ (m2/kg)
AMLR ± σ (g/s)
cotton 5BL (PAA/MMT)-coated fabric 20BL (PAA/MMT)-coated fabric 5BL (PAD1/MMT)-coated fabric 20BL (PAD1/MMT)-coated fabric 5BL (PAD2/MMT)-coated fabric 20BL (PAD2/MMT)-coated fabric
42 ± 3 44 ± 2 48 ± 3 47 ± 3 53 ± 3 48 ± 2 59 ± 4
144 ± 6 124 ± 6 109 ± 4 116 ± 5 85 ± 5 105 ± 4 77 ± 2
25.1 ± 0.5 23.4 ± 0.4 22.4 ± 0.4 22.8 ± 0.3 22.1 ± 0.3 22.1 ± 0.4 20.7 ± 0.3
22.6 ± 0.7 22.1 ± 0.8 21.3 ± 0.6 22.6 ± 0.6 23.4 ± 0.6 23.3 ± 0.8 25.7 ± 0.7
0.057 ± 0.004 0.053 ± 0.003 0.045 ± 0.003 0.046 ± 0.004 0.037 ± 0.002 0.044 ± 0.003 0.032 ± 0.003
TTI: Time to ignition; PHRR: peak release rate; THR: total heat release; ASEA: average specific extinction area ; AMLR: average mass loss rate. 12306
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Figure 10. Digital photos of the residues after cone calorimeter testing: residues of the control fabric (a), 20BL (PAA/MMT)-coated fabric (b), and 20BL (PAD2/MMT)-coated fabric (c).
Figure 11. SEM images of the char after cone calorimeter testing: chars of the control fabric (a), 20BL (PAA/MMT)-coated fabric (b), and 20BL (PAD2/MMT)-coated fabric (c); EDX curves of the chars after cone calorimeter testing (d).
Table 5. Physical properties of uncoated and coated fabric tear strength (N) sample
warp
filling
cotton 5BL (PAA/MMT)-coated fabric 20BL (PAA/MMT)-coated fabric 5BL (PAD1/MMT)-coated fabric 20BL (PAD1/MMT)-coated fabric 5BL (PAD2/MMT)-coated fabric 20BL (PAD2/MMT)-coated fabric
12.4 ± 0.6 12.5 ± 0.6 12.8 ± 0.5 12.7 ± 0.5 13.1 ± 0.7 12.8 ± 0.6 13.3 ± 0.8
11.3 ± 0.4 11.1 ± 0.3 10.9 ± 0.3 11.0 ± 0.4 10.7 ± 0.3 10.7 ± 0.2 10.4 ± 0.3
tear strength retention (%) warp 101 ± 5 103 ± 4 102 ± 5 106 ± 6 103 ± 6 107 ± 7
indicate that the coating of FR-PAA and clay platelets on the fabric via LbL assembly causes almost no fabric strength loss.
tensile strength (N)
tensile strength retention (%)
filling
warp
filling
warp
filling
98 ± 4 96 ± 4 97 ± 3 95 ± 3 95 ± 2 92 ± 2
321.4 ± 12.9 335.7 ± 13.4 344.6 ± 13.2 337.8 ± 14.1 352.4 ± 13.5 339.1 ± 12.2 357.5 ± 13.8
317.5 ± 11.4 321.6 ± 11.6 319.4 ± 10.9 315.6 ± 11.7 312.4 ± 10.2 307.4 ± 9.4 302.6 ± 10.5
104 ± 4 107 ± 4 105 ± 5 110 ± 5 106 ± 6 112 ± 7
103 ± 4 101 ± 4 99 ± 3 96 ± 3 97 ± 3 95 ± 2
Films assembled with PAA, PAD1, and PAD2 (pH 4 or 7) and 0.5 wt % MMT suspensions showed linear growth as a function of the number of BL deposited. A higher FR-PAA pH resulted in much thinner films due to higher charge density, while a higher DPEPA content of FR-PAA resulted in thicker films for the hydrophobicity of polyelectrolytes. Ttreatment of FR-PAA/ MMT films increased the thermal degradation temperature of
4. CONCLUSIONS A FR-PAA, prepared by copolymerization of AA and DPEPA, was used to prepare various FR-PAA/MMT thin films via LbL assembly as a flame-retardant coating system for cotton fabric. 12307
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cotton fabric. Cone calorimeter experiments showed that the flammability (including TTI, PHRR, THR, AMLR, etc.) was reduced for the coated fabrics. Compared with the control fabric, PHRR and THR were reduced by 47% and 18% for the 20BL (PAD2/MMT)-coated fabric, respectively. Meanwhile, the physical properties of the coated fabrics exhibited almost no deterioration compared with the control fabric. SEM images showed that the fabric structure was still maintained and the fabric was covered with a continuous and compact swollen charred layer after combustion for the 20BL (PAD2/MMT)coated fabric. In brief, the combination of FR-PAA and MMT via LbL assembly improved the flame retardancy of cotton fabric.
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ASSOCIATED CONTENT
S Supporting Information *
FT-IR, 1H NMR, 13C NMR and mass-spectrometry data for DPEPA, FT-IR and 1H NMR spectra of DPEPA, PAA, PAD1, and PAD2, AFM height and phase surface images of 10 BL pH 7 PAD2/MMT films, SEM images of the char of the coated fabric, table of vertical flame testing data, table of EDX analysis of the residues. This material 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]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Support from the Zhejiang Provincial Natural Science Foundation of China (Y4110026) and the Key Innovation Team of Science & Technology in Zhejiang Province (2012R10018-02) is gratefully acknowledged.
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REFERENCES
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