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carbon nanotube composite coating
Composites: Part B 45 (2013) 282–289
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UV radiation induced flame retardant cellulose fiber by using polyvinylphosphonic acid/carbon nanotube composite coating Mazeyar Parvinzadeh Gashti a,⇑, Arash Almasian b a b
Department of Textile, Islamic Azad University, Shahre Rey Branch, Tehran, Iran Department of Environmental Research, Institute for Color Science and Technology, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 1 February 2012 Received in revised form 10 May 2012 Accepted 16 July 2012 Available online 16 August 2012 Keywords: A. Fabrics/textiles B. Thermal properties D. Chemical analysis
a b s t r a c t Carbon nanotubes (CNTs) were stabilized on a cotton surface using vinylphosphonic acid monomer as a crosslinking agent and benzophenone as a catalyst. The influence of CNTs and polyvinylphosphonic on the thermal properties and flammability of the cellulose fiber was investigated using Fourier transform infrared spectrophotometer (FTIR), differential scanning calorimeter (DSC), thermo-gravimetric analyzer (TGA), horizontal flammability test (HFT), scanning electron microscope (SEM) and energy dispersive X-ray spectrometer (EDS). The possible interactions between CNTs, polyvinylphosphonic acid and cellulose at the surface were elucidated by the FTIR spectroscopy. The results indicated that polyvinylphosphonic/ CNT nanocomposite improves the thermal stability and decreases the flammability of the substrate. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Cotton is the most used textile fiber in the world with unique combination of properties, including high strength, durability, softness, good dyeability and biodegradability, and for many centuries it has found use in the textile production [1,2]. Besides many beneficial properties, cotton shows various disadvantages such as shrinking in the wash, high wrinkling, low resistance to acids, color loss by bleeding, damaging by sunlight and mildew, retaining water after wetting and high flammability [3,4]. Cellulose fibers such as cotton and viscose are highly combustible and continue to burn after removal from a flame. In this case, their behavior against heat and flame should be concerned in safety and protective aspects of textile products used in public spaces such as schools, theatres or special event venues. Most efforts in the field of flame-retardant (FR) finishing were made on modifying the burning behavior of cotton fibers [5–7]. To do this, a great number of approaches have been carried out on cotton including, non-durable treatments (ammonium phosphates and mixtures with other salts, ammonium polyphosphates, guanidine phosphates, organophosphorus oxyanion salt, organic nitrogencontaining compounds, organic N- and P-containing compounds, and combinations of previous materials with borax) [8–12], semi-durable treatments (halo-organic-antimony III oxide, intumescent based back-coatings and inorganic sol–gel coatings) [13–18], and durable finishes (tetrakis hydroxymethyl phosphonium salt, N,N0 -dimethylol dialkyl phosphonopropionamides and ⇑ Corresponding author. Tel.: +98 (0)9123137115; fax: +98 (0)21 22593135. E-mail address: [email protected] (M. Parvinzadeh Gashti). 1359-8368/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compositesb.2012.07.052
derivatives, and phosphorus-rich oligomeric alcohol-terminated methylphosphonatephosphate) [19–21]. Ultraviolet-curable flame-retardant monomers have recently been of special interest to textile chemists in textile finishes [22–29]. The most widely studied nano-fibrous materials with respect to polymer flame retardancy are carbon nanotubes (CNTs) [30,31]. However, lack of compatibility between these particles and textiles has limited their application in the textile industry [32,33]. As far as we know, there is no report introducing UV curable flame retardants to stabilize CNTs on cotton fibers. In this study, for the first time, we utilize vinylphosphonic acid monomers as a bridge between cellulose and CNTs. According to our method, the direct utilization and stabilization of CNTs resulted in achievement of high efficient flame retardant finishing of cotton fabrics.
2. Experimental 2.1. Materials The bleached 100% cotton fabric with the weight 1688.55 g/m2, width 140 cm was used from Yazd Baft Co., Iran. Multiwall carbon nanotubes (MWCNTs) from the Iranian Research Institute of petroleum Industry was used with 5% of carbon impurities. Its length wall was about 10 lm and the average outer diameter was 10–30 nm. Vinylphosphonic acid with the density of 1.37 g/cm3 and more than 97% purity was provided from Archimica, Germany. Sulfuric acid, nitric acid, benzophenone and sodium carbonate were supplied by Merck Chemical Co., Germany. Nonionic detergent from Shirley Development Limited was used for washing.
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2.2. Preparation of colloidal dispersions of MWCNTs and their coating on cotton Stabilization of MWCNTs on cotton was produced by a fivestep method. First, 220 mg of MWCNTs was dispersed in 180cc sulfuric acid and 60cc nitric acid, and the mixture was further treated with an ultrasonic machine at 40 °C for 5 h [34]. Second, the mixture was poured in cold water, centrifuged and washed by successive agitations/centrifugations with deionized water for several times. Third, 220 mg vinylphosphonic acid and 110 mg benzophenone were added into the MWCNTs dispersion with vigorously stirring at 30 °C for 2 h. Four pieces of cotton fabrics were then dipped in a bath containing 4 mol benzophenone and deionized water (molar ratio of 1:10) at 30 °C for 2 h. Finally, all the cotton samples were treated with functionalized MWCNTs and cross-linking of the fabrics was conducted by UV irradiation (Germicidal UV lamp from Keosan Enterprise Co. Ltd.: 15 W/ 0.3 A, UV-C, kmax = 250 nm) at ambient temperature for 30, 60, 90 and 120 min. The treated fibers were fully dried at 70 °C in an oven. The coated fabrics were then washed separately at 40 °C for 15 min using 2 g/L sodium carbonate and 1 g/L nonionic detergent. The samples were washed separately at 50 °C for 10 min using 1% nonionic detergent. It was found useful for removing residual MWCNTs, polyvinylphosphonic acid and benzophenone. 2.3. Evaluation of samples using Fourier transform infrared spectroscope (FTIR)
Fig. 1. Flame chamber used in this study with horizontal position for samples.
where L is the burned length in millimeters (mm) and T is the time in seconds (s) [26,30]. 2.8. Evaluation of morphology by microscopy The surface of the fibers was investigated using a Scanning Electron Microscopy (SEM) XL30, Philips. The surface of samples was first coated with a thin layer of gold (10 nm) by physical vapor deposition using a sputter coater (SCDOOS, BAL-TEC). The presence of phosphorous element on the fiber surface was also determined by energy dispersive X-ray microanalysis (EDX) attached to the SEM. 3. Results and discussion
The chemical compositions of the fabrics were examined by the FTIR spectroscopy [Bomem-MB 100 Series (Hartmann and Broun)rsqb]. 2.4. The degree of grafting The percentage of grafting was determined as follows:
Grafting yield ð%Þ ¼ ðW g W 0 Þ=W 0 100
ð1Þ
where W0 and Wg are the weights of the fabric samples before and after grafting, respectively. 2.5. Calorimetric analysis A differential scanning calorimetry of the samples was carried out using a Perkin Elmer pyris 6 DSC model integrated with an IBM personal computer. The samples were heated from 30 up to 450 °C at a rate of 5 °C/min in a nitrogen atmosphere. 2.6. Thermogravimetric analysis The thermal degradation properties of the samples were performed on a TGA-PL thermoanalyzer from UK. In each case a 5 mg sample was examined under an N2 at a heating rate of 5 °C/ min from room temperature to 650 °C. 2.7. Determination of flammability Flammability of samples was evaluated in accordance with ASTM D 635. Five samples from each fabric were cut with dimensions of 10 2 cm. Specimens were placed in the flammability chamber in the horizontal position (Fig. 1). After testing, the time and extent of burning area were measured and the rate of burning (V) was calculated in millimeters per second (mm/s) for each specimen by the following formula:
V ¼ L=T
ð2Þ
3.1. Structural information by FTIR spectra The infrared spectra of the untreated and polyvinylphosphonic acid cross-linked cotton as well as the sample coated with polyvinylphosphonic acid/MWCNT composite under UV irradiation are shown in Fig. 2. The intense CH2 asymmetric and symmetric stretchings, CH2 in plane bending, intermolecular OAH stretching, OAH out of plane bending and vibrations relating with adsorbed H2O molecules in cellulose chains appear at 2906, 1457, 3424 and 447 cm1 respectively. The bands appearing at 1647 and 1028 cm1 represent bending and stretching motions of the adsorbed water molecules and CAO bonds in the cotton chains. Ether bonds for cotton were characterized by CAOAC symmetric and asymmetric stretching vibrations at 1028 and 1162 cm1 (Fig. 2a) [35,36]. After cross-linking of cotton with vinylphosphonic acid (Fig. 2b), the intensity of the bands at 3843 and 3753 cm1 were decreased. These changes are associated with decrement in interand intra-chain hydrogen bonds due to coating of monomer on the cotton fiber surface by the polymerization process. The weak peaks at 1275 and 1034 cm1 exhibited by the monomer treated surface are due to the absorption bands of the P@O and PAOAC stretching vibrations which are characteristics of the cross-linked phosphonic acid groups, respectively. A broader peak was observed at 3436 cm1 for vinylphosphonic acid cross-linked fiber due to the acidic OH stretching vibrations of the free phosphonic acids [36,37]. As it can be seen in Fig. 2c, small peaks appeared at 1550 and 2364 cm1 assign C@C bonds in MWCNTs and phosphine (PAH) stretching groups in vinylphosphonic acid, respectively. As the intensity of phosphine groups are very weak, they may be due to impurities in vinylphosphonic acid used or degradation after UV irradiation [38]. A band appeared at 1744 and 1800 cm1 can be expressed as carbonyl anhydrides on the surface of the modified MWNTs which confirms successful incorporation of them in the cross-linking process of fibers [39].
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Fig. 2. FTIR spectra of samples: (a) untreated cotton, (b) cotton cross-linked with polyvinylphosphonic acid under UV irradiation for 120 min and (c) cotton cross-linked with polyvinylphosphonic acid/MWCNT composite under UV irradiation for 120 min.
Fig. 3. The possible mechanism for stabilization of MWCNTs on cellulose using vinylphosphonic acid bridge.
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Fig. 4. The effect of UV irradiation time on photografting.
Fig. 3 shows the possible mechanism for stabilization of MWCNTs on cellulose using vinylphosphonic acid bridge. 3.2. Cross-linking rate of monomer and MWCNTs The effect of UV irradiation time on the degree of photografting of vinylphosphonic acid and MWCNTs was investigated and result is shown in Fig. 4. The maximum irradiation time was 120 min. It can be observed that grafting yield of vinylphosphonic acid reached its maximal value with increasing the irradiation time to 90 min and it became relatively constant for 120 min. This may suggest some homopolymer chains have attached on the surface of cellulose fiber by coupling reaction which limited the grafting yield [40,41]. Degree of grafting can reach up to 6.5% when grafting time is 120 min. It is suggested that irradiation time controls the diffusion of monomer, the higher is the irradiation time to 90 min, the more polymerization chains can be formed. This may be due to the consumption of benzophenone and monomers, and the increasing coverage of the cotton surface by grafting chains. The results illustrated in Fig. 4 also shows that, for both monomer and MWCNTs used, the grafting yields initially increased with increasing irradiation time and more MWCNTs are incorporated in grafting. From this data, we concluded that the samples grafted with vinylphosphonic acid showed lower yield of grafting as compared with those coated with vinylphosphonic acid/MWCNTs composite [42,43]. In water, cellulose carries negative charges, which together with the negative charges of the modified MWCNTs causes an electrostatic repulsion. To improve the performance of MWCNTs on cotton, vinylphosphonic acid was used in the presence of benzophenone as a catalyst. Vinylphosphonic acid is a flame retardant monomer which makes a cross-linking with the cellulose chain [29,39]. MWCNTs are attached to cellulose by intermediate grafting of monomer with hydroxyl groups, forming a covalent bond, to complete the ether linkage [19,20,34]. In other words, the electrostatic repulsion between MWCNTs and cotton is limited, which causes a better interplay of covalent bonds, electrostatic and van der Waals forces leading to a greater affinity of MWCNTs to cotton [44]. 3.3. Determination of thermal properties and flammability Fig. 5 illustrates measured DSC curves of the untreated and polyvinylphosphonic acid cross-linked cotton as well as the samples coated with polyvinylphosphonic acid/MWCNT composite under UV irradiation. Two peaks at 110 and 354 °C were generated in the curve for the cotton fiber due to the glass transition and melting occurring in the crystalline regions of the cellulose chains [45].
Fig. 5. DSC curves for the samples: (a) untreated cotton, (b) cotton cross-linked with polyvinylphosphonic acid after UV irradiation for 120 min, (c) cotton crosslinked with polyvinylphosphonic acid/MWCNTs composite after UV irradiation for 30 min, (d) cotton cross-linked with polyvinylphosphonic acid/MWCNTs composite after UV irradiation for 60 min, (e) cotton cross-linked with polyvinylphosphonic acid/MWCNTs composite after UV irradiation for 90 min and (f) cotton cross-linked with polyvinylphosphonic acid/MWCNTs composite after UV irradiation for 120 min.
After cross-linking of the cotton with polyvinylphosphonic acid, the glass transition (Tg) and melting temperatures (Tm) were increased. There was an increase in Tm of cellulose fiber after incorporation of MWCNTs in nanocomposite from 354 to 357 °C. It can be due to excellent thermal conductivity of the MWCNTs that influence the heat transition toward cellulose molecular chains and the absorbed heat for chain movements [46,47]. On the other hand, the interaction between cotton surface and the MWCNTs was fairly strong which effectively restricted the motions of the molecular segment in cellulose [48]. This result is incidentally consistent with our result obtained from FTIR spectra. Fig. 6 summarizes the thermal degradation of the untreated cotton and those of the samples cross-linked with MWCNTs and polyvinylphosphonic acid. As it can be seen from the figure, the TGA
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Fig. 6. Thermal degradation of (A) untreated cotton, (B) cotton cross-linked with polyvinylphosphonic acid after UV irradiation for 120 min, (C) cotton cross-linked with polyvinylphosphonic acid/MWCNTs composite after UV irradiation for 30 min, (D) cotton cross-linked with polyvinylphosphonic acid/MWCNTs composite after UV irradiation for 60 min, (E) cotton cross-linked with polyvinylphosphonic acid/MWCNTs composite after UV irradiation for 90 min and (F) cotton cross-linked with polyvinylphosphonic acid/MWCNTs composite after UV irradiation for 120 min.
curves of cotton consist of three regions of 1, 2 and 3 as initial, main, and char decomposition regions. In the first stage, the changes of the thermal properties and the weight loss of fibers are due to some physical damages occurring mostly in the amorphous region of the cellulose [49]. The main thermal stage occurs in the second region, where the weight loss is significant. It is stated by different researchers that glucose together with all kinds of combustible gases are generated in this region [50–52]. They found that thermal degradation in this region takes place in the crystalline region of cellulose fibers. The production of char occurs at the third region at higher temperatures of 400 °C. This process continues by dehydrating and charring reactions, releasing water and carbon dioxide and increasing the carbon and charred residues [49,50]. The sample cross-linked with polyvinylphosphonic acid showed higher degradation compared with the untreated cotton. This was due to flame retardant properties of polyvinylphosphonic acid which provided a cotton textile with higher thermal stability [29]. Incorporation of MWCNTs in coating resulted to improve the thermal stability. This depended on duration of UV irradiation process and there was much improvement in thermal stability of cotton with an extension of UV irradiation. This improvement of thermal properties is attributed to the high heat resistance, the heat insulation effect and the mass transport barrier toward cellulose molecular chains exerted by the MWCNTs themselves which are a measure of flame retardancy [30]. On the other hand, the formation of MWCNTs-bonded macroradicals on the cotton surface effectively improved the thermal stability of cotton, which is also consistent with our result obtained from FTIR spectra [31,44]. Table 1 shows the summary of the length, time and rate of burning for the untreated and cross-linked cotton textiles. The images for the untreated and cross-linked cotton fabrics after the flammability test are also given in Fig. 7. The longer burning length and higher burning rate indicated the greater the flammability. The untreated cotton fiber ignites easily after exposing to flame. However from Table 1, it is clear that the burning length for the sample cross-linked with polyvinylphosphonic acid was decreased and the burning rate was lower than that of the untreated fabric. Any increase in time of UV irradiation caused more decrease in the flammability and the burning rate. It can be seen that the time needed
for burning of the same length for the cotton cross-linked samples was longer compared with the untreated cotton. Results obtained also indicated that the amount of monomer grafted on the cotton surface is highly related with duration of UV irradiation. It can be suggested that polyvinylphosphonic acid cross-linked on the cotton surface decomposes before cotton and generates phosphoric acid as a protective layer [45]. This may lead to phosphorylation of cellulose at C-6 hydroxyl group of the anhydroglucose units. The process reduces the production of levoglucosan as the fuel available for flame propagation and increases the amount of char formation [49]. This result is confirmed by FTIR spectra. Table 1 shows that the burning behavior for the samples crosslinked with polyvinylphosphonic acid/MWCNTs decreased and the flame speed was slower than that without MWCNTs. Besides, an increase in duration of UV irradiation caused a greater decrease in flammability. The excellent flame retardant properties of the samples cross-linked with polyvinylphosphonic acid/MWCNTs could be attributed to the presence of MWCNTs which causes a synergistic enhancement in the efficiency of polyvinylphosphonic acid on the cotton. Some researchers suggested that the flame-retardant effect found in the polymer–CNT composites arises from the formation of char layers, which are due to the uniform dispersion of CNTs within the composites [30]. Others claimed that the MWCNTs in the composite act as radical scavengers to delay the thermal degradation and thus improve the thermal stability [31]. Here we suggest two different factors for the flame resistance of cotton after crosslinking with polyvinylphosphonic acid/MWCNTs: Firstly, the network generated at the surface of fiber works as a shield and results in a significant reduction in heat release rate, decreasing the fiber degradation rate. Secondly, the presence of MWCNTs increases the thermal conductivity of coating. As a result, the time to ignition and the peak heat release rate of polyvinylphosphonic acid/MWCNTs network increase due to presence of MWCNTs. 3.4. Microscopic characterization The SEM images of the untreated cotton fiber and the sample cross-linked with polyvinylphosphonic acid/MWCNTs nanocomposite coating are shown in Fig. 8a–c. It can be seen that the
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M. Parvinzadeh Gashti, A. Almasian / Composites: Part B 45 (2013) 282–289 Table 1 Burning rate for untreated cotton and those cross-linked with polyvinylphosphonic acid/MWCNTs under UV irradiation. Sample
Duration of UV irradiation (min)
Burning length (mm)
Ignition time (s)
Burning rate (mm/s)
Untreated cotton Cotton cross-linked with polyvinylphosphonic acid
– 60 120 30 60 90 120
80 73 67 45 40 25 18
12.83 12.94 14.93 17.16 17.95 18.68 19.12
6.23 5.64 4.49 2.62 2.23 1.34 0.94
Cotton cross-linked with polyvinylphosphonic acid/MWCNT composite
Fig. 7. Cotton fabrics after the flammability test (a) untreated cotton, (b) cotton cross-linked with polyvinylphosphonic acid after UV irradiation for 60 min, (c) cotton crosslinked with polyvinylphosphonic acid after UV irradiation for 120 min, (d) cotton cross-linked with polyvinylphosphonic acid/MWCNTs composite after UV irradiation for 30 min, (e) cotton cross-linked with polyvinylphosphonic acid/MWCNTs composite after UV irradiation for 60 min, (f) cotton cross-linked with polyvinylphosphonic acid/ MWCNTs composite after UV irradiation for 90 min and (g) cotton cross-linked with polyvinylphosphonic acid/MWCNTs composite after UV irradiation for 120 min.
Fig. 8. SEM images of (a) untreated cotton fiber (7500), (b) cotton cross-linked with polyvinylphosphonic acid/MWCNTs composite after UV irradiation for 60 min (7500) and (c) cotton cross-linked with polyvinylphosphonic acid/MWCNTs composite after UV irradiation for 120 min (7500).
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Fig. 9. Energy dispersive X-ray analysis of (a) cotton cross-linked with polyvinylphosphonic acid/MWCNTs composite after UV irradiation for 60 min and (b) cotton crosslinked with polyvinylphosphonic acid/MWCNTs composite after UV irradiation for 120 min.
untreated fiber has fibrils with a relatively smooth surface. The surface of the untreated fiber has no deposition of MWCNTs and the cross-linking agent. The high magnification SEM images of the cotton fiber cross-linked with polyvinylphosphonic acid/MWCNTs show a formation of the aggregated MWCNTs on the surface of the cotton. In general, the ability of nano-particles to aggregate on the surface of textiles and polymers is defined by several factors such as size, mobility, end-group functionalities, relative composition and molecular architecture [35–37, 53–58]. As mentioned previously, the FTIR spectra of samples illustrated that the hydroxyl group of cotton cross-linked with polyvinylphosphonic acid to form ACAOACA linkages and PAO bonds. It seems that these interactions are strong enough to allow the deposition of MWCNTs on the cellulose surface. We have confirmed this phenomenon in cases where particles were incorporated into multilayer organic coatings [59–64]. Fig. 9a,b shows the presence of chemical elements on the cotton surface after cross-linking with polyvinylphosphonic acid/MWCNT composite under UV irradiation. In these patterns, Au peaks clearly show that gold is successfully coated on surfaces of all fibers. EDX analysis of cross-linked textiles illustrated more efficient interactions between cotton surface and polyvinylphosphonic acid/ MWCNT composite leading to the presence of P at these sample´s surface. This finding is also further supported by FTIR and crosslinking rate tests.
4. Conclusion MWCNTs and a flame retardant cross-linking agent were used to fabricate a flame retardant coating on the cotton through UV irradiation. The stabilization of CNTs in the composite coating was achieved by a reaction of polyvinylphosphonic acid and cellulose chains. This reaction process resulted in the attachment of MWCNTs to the surfaces of the cellulose fibers. The FTIR spectra showed the cross-linking reaction between the hydroxyl groups of cellulose and vinylphosphonic acid monomer to form linkages in the presence of CNTs under UV light. The results obtained from TGA and HFT tests demonstrated an improvement of the thermal properties and flammability of the coated samples. This can be as a result of a high heat resistance, a heat insulation effect and the mass transport barrier of CNTs embedded in the coating. This textile composite coating is therefore very promising for civil applications as an effective light-weight flame retardant material.
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[44] Yuyang L, Xiaowen W, Kaihong Q, Xin JH. Functionalization of cotton with carbon nanotubes. J Mater Chem 2008;18:3454–60. [45] Yao F, Wu Q, Lei Y, Guo W, Xu Y. Thermal decomposition kinetics of natural fibers: activation energy with dynamic thermogravimetric analysis. Polym Degrad Stab 2008;93:90–8. [46] Hossein Rahimi M, Parvinzadeh M, Yousefpour Navid M, Ahmadi S. Thermal characterization and flammability of polyester fiber coated with nonionic and cationic softeners. J Surfact Deterg 2011;14:595–603. [47] Parvinzadeh Gashti M, Hossein Rahimi M, Yousefpour Navid M. Coating of macroemulsion and microemulsion silicones on poly(ethylene terephthalate) fibers: evaluation of the thermal properties and flammability. J Appl Polym Sci 2012;125:1430–8. [48] Zhu P, Sui S, Wang B, Sun K, Sun G. A study of pyrolysis and pyrolysis products of flame-retardant cotton fabrics by DSC, TGA, and PY–GC–MS. J Anal Appl Pyrolsis 2004;71:645–55. [49] Tsafack MJ, Levalois-Grützmacher J. Flame retardancy of cotton textiles by plasma-induced graft-polymerization (PIGP). Surf Coat Technol 2006;201: 2599–610. [50] Parvinzadeh M, Assefipour R, Kiumarsi A. Biohydrolysis of nylon 6,6 fiber with different proteolytic enzymes. Polym Degrad Stabil 2009;94:1197–205. [51] Parvinzadeh M, Ebrahimi I. Influence of atmospheric-air plasma on the coating of a nonionic lubricating agent on polyester fiber. Radiat Eff Def Solids 2011;166:408–16. [52] Parvinzadeh Gashti M, Eslami S. Structural, optical and electromagnetic properties of aluminum–clay nanocomposites. Superlattice Microst 2012; 51:135–48. [53] Abidi N, Cabrales L, Hequet E. Thermogravimetric analysis of developing cotton fibers. Thermochim Acta 2010;498:27–32. [54] Parvinzadeh M, Hajiraissi R. Macro- and microemulsion silicone softeners on polyester fibers: evaluation of different physical properties. J Surfact Deterg 2008;11:269–73. [55] Parvinzadeh M, Memari N, Shaver M, Katozian B, Ahmadi S, Ziadi I. Influence of ultrasonic waves on the processing of cotton with cationic softener. J Surfact Deterg 2010;10:219–23. [56] Parvinzadeh M, Moradian S, Rashidi A, Yazdanshenas ME. Surface characterization of polyethylene terephthalate/silica nanocomposites. Appl Surf Sci 2010;256:2792–802. [57] Parvinzadeh M, Moradian S, Rashidi A, Yazdanshenas ME. Effect of addition of modified nanoclays on surface properties of the resultant polyethylene terephthalate/clay nanocomposites. Polym Plast Technol Eng 2010;49:1–11. [58] Parvinzadeh M, Ebrahimi I. Atmospheric air-plasma treatment of polyester fiber to improve the performance of nanoemulsion silicone. Appl Surf Sci 2011;257:4062–8. [59] Parvinzadeh M, Eslami S. Optical and electromagnetic characteristics of clay– iron oxide nanocomposites. Res Chem Intermed 2011;37:771–84. [60] Alimohammadi F, Parvinzadeh Gashti M, Shamei A. A novel method for coating of carbon nanotube on cellulose fiber using 1,2,3,4-butanetetracarboxylic acid as a cross-linking agent. Prog Org Coat 2012;74:470–8. [61] Parvinzadeh Gashti M, Almasian A. Preparation of electromagnetic reflective wool using nano-ZrO2/citric acid as inorganic/organic hybrid coating. Sensor Actuat A-Phys 2012;187:1–9. [62] Alimohammadi F, Parvinzadeh Gashti M, Shamei A, Kiumarsi A. Deposition of silver nanoparticles on carbon nanotube by chemical reduction method: Evaluation of surface, thermal and optical properties. Superlattice Microst 2012;52:50–62. [63] Naveau E, Detrembleur C, Jerome C, Alexandre M. Patenting Activity in Manufacturing Organoclays for Nanocomposite Applications. Recent Pat. Mater. Sci. 2009;2:43–9. [64] Fu G, Liu H, Gong W. Study on Preparation of Composite Materials of Montmorillonite. IJMSCI 2011;1:18–22.
Contents lists available at SciVerse ScienceDirect
Composites: Part B journal homepage: www.elsevier.com/locate/compositesb
UV radiation induced flame retardant cellulose fiber by using polyvinylphosphonic acid/carbon nanotube composite coating Mazeyar Parvinzadeh Gashti a,⇑, Arash Almasian b a b
Department of Textile, Islamic Azad University, Shahre Rey Branch, Tehran, Iran Department of Environmental Research, Institute for Color Science and Technology, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 1 February 2012 Received in revised form 10 May 2012 Accepted 16 July 2012 Available online 16 August 2012 Keywords: A. Fabrics/textiles B. Thermal properties D. Chemical analysis
a b s t r a c t Carbon nanotubes (CNTs) were stabilized on a cotton surface using vinylphosphonic acid monomer as a crosslinking agent and benzophenone as a catalyst. The influence of CNTs and polyvinylphosphonic on the thermal properties and flammability of the cellulose fiber was investigated using Fourier transform infrared spectrophotometer (FTIR), differential scanning calorimeter (DSC), thermo-gravimetric analyzer (TGA), horizontal flammability test (HFT), scanning electron microscope (SEM) and energy dispersive X-ray spectrometer (EDS). The possible interactions between CNTs, polyvinylphosphonic acid and cellulose at the surface were elucidated by the FTIR spectroscopy. The results indicated that polyvinylphosphonic/ CNT nanocomposite improves the thermal stability and decreases the flammability of the substrate. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Cotton is the most used textile fiber in the world with unique combination of properties, including high strength, durability, softness, good dyeability and biodegradability, and for many centuries it has found use in the textile production [1,2]. Besides many beneficial properties, cotton shows various disadvantages such as shrinking in the wash, high wrinkling, low resistance to acids, color loss by bleeding, damaging by sunlight and mildew, retaining water after wetting and high flammability [3,4]. Cellulose fibers such as cotton and viscose are highly combustible and continue to burn after removal from a flame. In this case, their behavior against heat and flame should be concerned in safety and protective aspects of textile products used in public spaces such as schools, theatres or special event venues. Most efforts in the field of flame-retardant (FR) finishing were made on modifying the burning behavior of cotton fibers [5–7]. To do this, a great number of approaches have been carried out on cotton including, non-durable treatments (ammonium phosphates and mixtures with other salts, ammonium polyphosphates, guanidine phosphates, organophosphorus oxyanion salt, organic nitrogencontaining compounds, organic N- and P-containing compounds, and combinations of previous materials with borax) [8–12], semi-durable treatments (halo-organic-antimony III oxide, intumescent based back-coatings and inorganic sol–gel coatings) [13–18], and durable finishes (tetrakis hydroxymethyl phosphonium salt, N,N0 -dimethylol dialkyl phosphonopropionamides and ⇑ Corresponding author. Tel.: +98 (0)9123137115; fax: +98 (0)21 22593135. E-mail address: [email protected] (M. Parvinzadeh Gashti). 1359-8368/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compositesb.2012.07.052
derivatives, and phosphorus-rich oligomeric alcohol-terminated methylphosphonatephosphate) [19–21]. Ultraviolet-curable flame-retardant monomers have recently been of special interest to textile chemists in textile finishes [22–29]. The most widely studied nano-fibrous materials with respect to polymer flame retardancy are carbon nanotubes (CNTs) [30,31]. However, lack of compatibility between these particles and textiles has limited their application in the textile industry [32,33]. As far as we know, there is no report introducing UV curable flame retardants to stabilize CNTs on cotton fibers. In this study, for the first time, we utilize vinylphosphonic acid monomers as a bridge between cellulose and CNTs. According to our method, the direct utilization and stabilization of CNTs resulted in achievement of high efficient flame retardant finishing of cotton fabrics.
2. Experimental 2.1. Materials The bleached 100% cotton fabric with the weight 1688.55 g/m2, width 140 cm was used from Yazd Baft Co., Iran. Multiwall carbon nanotubes (MWCNTs) from the Iranian Research Institute of petroleum Industry was used with 5% of carbon impurities. Its length wall was about 10 lm and the average outer diameter was 10–30 nm. Vinylphosphonic acid with the density of 1.37 g/cm3 and more than 97% purity was provided from Archimica, Germany. Sulfuric acid, nitric acid, benzophenone and sodium carbonate were supplied by Merck Chemical Co., Germany. Nonionic detergent from Shirley Development Limited was used for washing.
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2.2. Preparation of colloidal dispersions of MWCNTs and their coating on cotton Stabilization of MWCNTs on cotton was produced by a fivestep method. First, 220 mg of MWCNTs was dispersed in 180cc sulfuric acid and 60cc nitric acid, and the mixture was further treated with an ultrasonic machine at 40 °C for 5 h [34]. Second, the mixture was poured in cold water, centrifuged and washed by successive agitations/centrifugations with deionized water for several times. Third, 220 mg vinylphosphonic acid and 110 mg benzophenone were added into the MWCNTs dispersion with vigorously stirring at 30 °C for 2 h. Four pieces of cotton fabrics were then dipped in a bath containing 4 mol benzophenone and deionized water (molar ratio of 1:10) at 30 °C for 2 h. Finally, all the cotton samples were treated with functionalized MWCNTs and cross-linking of the fabrics was conducted by UV irradiation (Germicidal UV lamp from Keosan Enterprise Co. Ltd.: 15 W/ 0.3 A, UV-C, kmax = 250 nm) at ambient temperature for 30, 60, 90 and 120 min. The treated fibers were fully dried at 70 °C in an oven. The coated fabrics were then washed separately at 40 °C for 15 min using 2 g/L sodium carbonate and 1 g/L nonionic detergent. The samples were washed separately at 50 °C for 10 min using 1% nonionic detergent. It was found useful for removing residual MWCNTs, polyvinylphosphonic acid and benzophenone. 2.3. Evaluation of samples using Fourier transform infrared spectroscope (FTIR)
Fig. 1. Flame chamber used in this study with horizontal position for samples.
where L is the burned length in millimeters (mm) and T is the time in seconds (s) [26,30]. 2.8. Evaluation of morphology by microscopy The surface of the fibers was investigated using a Scanning Electron Microscopy (SEM) XL30, Philips. The surface of samples was first coated with a thin layer of gold (10 nm) by physical vapor deposition using a sputter coater (SCDOOS, BAL-TEC). The presence of phosphorous element on the fiber surface was also determined by energy dispersive X-ray microanalysis (EDX) attached to the SEM. 3. Results and discussion
The chemical compositions of the fabrics were examined by the FTIR spectroscopy [Bomem-MB 100 Series (Hartmann and Broun)rsqb]. 2.4. The degree of grafting The percentage of grafting was determined as follows:
Grafting yield ð%Þ ¼ ðW g W 0 Þ=W 0 100
ð1Þ
where W0 and Wg are the weights of the fabric samples before and after grafting, respectively. 2.5. Calorimetric analysis A differential scanning calorimetry of the samples was carried out using a Perkin Elmer pyris 6 DSC model integrated with an IBM personal computer. The samples were heated from 30 up to 450 °C at a rate of 5 °C/min in a nitrogen atmosphere. 2.6. Thermogravimetric analysis The thermal degradation properties of the samples were performed on a TGA-PL thermoanalyzer from UK. In each case a 5 mg sample was examined under an N2 at a heating rate of 5 °C/ min from room temperature to 650 °C. 2.7. Determination of flammability Flammability of samples was evaluated in accordance with ASTM D 635. Five samples from each fabric were cut with dimensions of 10 2 cm. Specimens were placed in the flammability chamber in the horizontal position (Fig. 1). After testing, the time and extent of burning area were measured and the rate of burning (V) was calculated in millimeters per second (mm/s) for each specimen by the following formula:
V ¼ L=T
ð2Þ
3.1. Structural information by FTIR spectra The infrared spectra of the untreated and polyvinylphosphonic acid cross-linked cotton as well as the sample coated with polyvinylphosphonic acid/MWCNT composite under UV irradiation are shown in Fig. 2. The intense CH2 asymmetric and symmetric stretchings, CH2 in plane bending, intermolecular OAH stretching, OAH out of plane bending and vibrations relating with adsorbed H2O molecules in cellulose chains appear at 2906, 1457, 3424 and 447 cm1 respectively. The bands appearing at 1647 and 1028 cm1 represent bending and stretching motions of the adsorbed water molecules and CAO bonds in the cotton chains. Ether bonds for cotton were characterized by CAOAC symmetric and asymmetric stretching vibrations at 1028 and 1162 cm1 (Fig. 2a) [35,36]. After cross-linking of cotton with vinylphosphonic acid (Fig. 2b), the intensity of the bands at 3843 and 3753 cm1 were decreased. These changes are associated with decrement in interand intra-chain hydrogen bonds due to coating of monomer on the cotton fiber surface by the polymerization process. The weak peaks at 1275 and 1034 cm1 exhibited by the monomer treated surface are due to the absorption bands of the P@O and PAOAC stretching vibrations which are characteristics of the cross-linked phosphonic acid groups, respectively. A broader peak was observed at 3436 cm1 for vinylphosphonic acid cross-linked fiber due to the acidic OH stretching vibrations of the free phosphonic acids [36,37]. As it can be seen in Fig. 2c, small peaks appeared at 1550 and 2364 cm1 assign C@C bonds in MWCNTs and phosphine (PAH) stretching groups in vinylphosphonic acid, respectively. As the intensity of phosphine groups are very weak, they may be due to impurities in vinylphosphonic acid used or degradation after UV irradiation [38]. A band appeared at 1744 and 1800 cm1 can be expressed as carbonyl anhydrides on the surface of the modified MWNTs which confirms successful incorporation of them in the cross-linking process of fibers [39].
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Fig. 2. FTIR spectra of samples: (a) untreated cotton, (b) cotton cross-linked with polyvinylphosphonic acid under UV irradiation for 120 min and (c) cotton cross-linked with polyvinylphosphonic acid/MWCNT composite under UV irradiation for 120 min.
Fig. 3. The possible mechanism for stabilization of MWCNTs on cellulose using vinylphosphonic acid bridge.
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Fig. 4. The effect of UV irradiation time on photografting.
Fig. 3 shows the possible mechanism for stabilization of MWCNTs on cellulose using vinylphosphonic acid bridge. 3.2. Cross-linking rate of monomer and MWCNTs The effect of UV irradiation time on the degree of photografting of vinylphosphonic acid and MWCNTs was investigated and result is shown in Fig. 4. The maximum irradiation time was 120 min. It can be observed that grafting yield of vinylphosphonic acid reached its maximal value with increasing the irradiation time to 90 min and it became relatively constant for 120 min. This may suggest some homopolymer chains have attached on the surface of cellulose fiber by coupling reaction which limited the grafting yield [40,41]. Degree of grafting can reach up to 6.5% when grafting time is 120 min. It is suggested that irradiation time controls the diffusion of monomer, the higher is the irradiation time to 90 min, the more polymerization chains can be formed. This may be due to the consumption of benzophenone and monomers, and the increasing coverage of the cotton surface by grafting chains. The results illustrated in Fig. 4 also shows that, for both monomer and MWCNTs used, the grafting yields initially increased with increasing irradiation time and more MWCNTs are incorporated in grafting. From this data, we concluded that the samples grafted with vinylphosphonic acid showed lower yield of grafting as compared with those coated with vinylphosphonic acid/MWCNTs composite [42,43]. In water, cellulose carries negative charges, which together with the negative charges of the modified MWCNTs causes an electrostatic repulsion. To improve the performance of MWCNTs on cotton, vinylphosphonic acid was used in the presence of benzophenone as a catalyst. Vinylphosphonic acid is a flame retardant monomer which makes a cross-linking with the cellulose chain [29,39]. MWCNTs are attached to cellulose by intermediate grafting of monomer with hydroxyl groups, forming a covalent bond, to complete the ether linkage [19,20,34]. In other words, the electrostatic repulsion between MWCNTs and cotton is limited, which causes a better interplay of covalent bonds, electrostatic and van der Waals forces leading to a greater affinity of MWCNTs to cotton [44]. 3.3. Determination of thermal properties and flammability Fig. 5 illustrates measured DSC curves of the untreated and polyvinylphosphonic acid cross-linked cotton as well as the samples coated with polyvinylphosphonic acid/MWCNT composite under UV irradiation. Two peaks at 110 and 354 °C were generated in the curve for the cotton fiber due to the glass transition and melting occurring in the crystalline regions of the cellulose chains [45].
Fig. 5. DSC curves for the samples: (a) untreated cotton, (b) cotton cross-linked with polyvinylphosphonic acid after UV irradiation for 120 min, (c) cotton crosslinked with polyvinylphosphonic acid/MWCNTs composite after UV irradiation for 30 min, (d) cotton cross-linked with polyvinylphosphonic acid/MWCNTs composite after UV irradiation for 60 min, (e) cotton cross-linked with polyvinylphosphonic acid/MWCNTs composite after UV irradiation for 90 min and (f) cotton cross-linked with polyvinylphosphonic acid/MWCNTs composite after UV irradiation for 120 min.
After cross-linking of the cotton with polyvinylphosphonic acid, the glass transition (Tg) and melting temperatures (Tm) were increased. There was an increase in Tm of cellulose fiber after incorporation of MWCNTs in nanocomposite from 354 to 357 °C. It can be due to excellent thermal conductivity of the MWCNTs that influence the heat transition toward cellulose molecular chains and the absorbed heat for chain movements [46,47]. On the other hand, the interaction between cotton surface and the MWCNTs was fairly strong which effectively restricted the motions of the molecular segment in cellulose [48]. This result is incidentally consistent with our result obtained from FTIR spectra. Fig. 6 summarizes the thermal degradation of the untreated cotton and those of the samples cross-linked with MWCNTs and polyvinylphosphonic acid. As it can be seen from the figure, the TGA
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Fig. 6. Thermal degradation of (A) untreated cotton, (B) cotton cross-linked with polyvinylphosphonic acid after UV irradiation for 120 min, (C) cotton cross-linked with polyvinylphosphonic acid/MWCNTs composite after UV irradiation for 30 min, (D) cotton cross-linked with polyvinylphosphonic acid/MWCNTs composite after UV irradiation for 60 min, (E) cotton cross-linked with polyvinylphosphonic acid/MWCNTs composite after UV irradiation for 90 min and (F) cotton cross-linked with polyvinylphosphonic acid/MWCNTs composite after UV irradiation for 120 min.
curves of cotton consist of three regions of 1, 2 and 3 as initial, main, and char decomposition regions. In the first stage, the changes of the thermal properties and the weight loss of fibers are due to some physical damages occurring mostly in the amorphous region of the cellulose [49]. The main thermal stage occurs in the second region, where the weight loss is significant. It is stated by different researchers that glucose together with all kinds of combustible gases are generated in this region [50–52]. They found that thermal degradation in this region takes place in the crystalline region of cellulose fibers. The production of char occurs at the third region at higher temperatures of 400 °C. This process continues by dehydrating and charring reactions, releasing water and carbon dioxide and increasing the carbon and charred residues [49,50]. The sample cross-linked with polyvinylphosphonic acid showed higher degradation compared with the untreated cotton. This was due to flame retardant properties of polyvinylphosphonic acid which provided a cotton textile with higher thermal stability [29]. Incorporation of MWCNTs in coating resulted to improve the thermal stability. This depended on duration of UV irradiation process and there was much improvement in thermal stability of cotton with an extension of UV irradiation. This improvement of thermal properties is attributed to the high heat resistance, the heat insulation effect and the mass transport barrier toward cellulose molecular chains exerted by the MWCNTs themselves which are a measure of flame retardancy [30]. On the other hand, the formation of MWCNTs-bonded macroradicals on the cotton surface effectively improved the thermal stability of cotton, which is also consistent with our result obtained from FTIR spectra [31,44]. Table 1 shows the summary of the length, time and rate of burning for the untreated and cross-linked cotton textiles. The images for the untreated and cross-linked cotton fabrics after the flammability test are also given in Fig. 7. The longer burning length and higher burning rate indicated the greater the flammability. The untreated cotton fiber ignites easily after exposing to flame. However from Table 1, it is clear that the burning length for the sample cross-linked with polyvinylphosphonic acid was decreased and the burning rate was lower than that of the untreated fabric. Any increase in time of UV irradiation caused more decrease in the flammability and the burning rate. It can be seen that the time needed
for burning of the same length for the cotton cross-linked samples was longer compared with the untreated cotton. Results obtained also indicated that the amount of monomer grafted on the cotton surface is highly related with duration of UV irradiation. It can be suggested that polyvinylphosphonic acid cross-linked on the cotton surface decomposes before cotton and generates phosphoric acid as a protective layer [45]. This may lead to phosphorylation of cellulose at C-6 hydroxyl group of the anhydroglucose units. The process reduces the production of levoglucosan as the fuel available for flame propagation and increases the amount of char formation [49]. This result is confirmed by FTIR spectra. Table 1 shows that the burning behavior for the samples crosslinked with polyvinylphosphonic acid/MWCNTs decreased and the flame speed was slower than that without MWCNTs. Besides, an increase in duration of UV irradiation caused a greater decrease in flammability. The excellent flame retardant properties of the samples cross-linked with polyvinylphosphonic acid/MWCNTs could be attributed to the presence of MWCNTs which causes a synergistic enhancement in the efficiency of polyvinylphosphonic acid on the cotton. Some researchers suggested that the flame-retardant effect found in the polymer–CNT composites arises from the formation of char layers, which are due to the uniform dispersion of CNTs within the composites [30]. Others claimed that the MWCNTs in the composite act as radical scavengers to delay the thermal degradation and thus improve the thermal stability [31]. Here we suggest two different factors for the flame resistance of cotton after crosslinking with polyvinylphosphonic acid/MWCNTs: Firstly, the network generated at the surface of fiber works as a shield and results in a significant reduction in heat release rate, decreasing the fiber degradation rate. Secondly, the presence of MWCNTs increases the thermal conductivity of coating. As a result, the time to ignition and the peak heat release rate of polyvinylphosphonic acid/MWCNTs network increase due to presence of MWCNTs. 3.4. Microscopic characterization The SEM images of the untreated cotton fiber and the sample cross-linked with polyvinylphosphonic acid/MWCNTs nanocomposite coating are shown in Fig. 8a–c. It can be seen that the
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Duration of UV irradiation (min)
Burning length (mm)
Ignition time (s)
Burning rate (mm/s)
Untreated cotton Cotton cross-linked with polyvinylphosphonic acid
– 60 120 30 60 90 120
80 73 67 45 40 25 18
12.83 12.94 14.93 17.16 17.95 18.68 19.12
6.23 5.64 4.49 2.62 2.23 1.34 0.94
Cotton cross-linked with polyvinylphosphonic acid/MWCNT composite
Fig. 7. Cotton fabrics after the flammability test (a) untreated cotton, (b) cotton cross-linked with polyvinylphosphonic acid after UV irradiation for 60 min, (c) cotton crosslinked with polyvinylphosphonic acid after UV irradiation for 120 min, (d) cotton cross-linked with polyvinylphosphonic acid/MWCNTs composite after UV irradiation for 30 min, (e) cotton cross-linked with polyvinylphosphonic acid/MWCNTs composite after UV irradiation for 60 min, (f) cotton cross-linked with polyvinylphosphonic acid/ MWCNTs composite after UV irradiation for 90 min and (g) cotton cross-linked with polyvinylphosphonic acid/MWCNTs composite after UV irradiation for 120 min.
Fig. 8. SEM images of (a) untreated cotton fiber (7500), (b) cotton cross-linked with polyvinylphosphonic acid/MWCNTs composite after UV irradiation for 60 min (7500) and (c) cotton cross-linked with polyvinylphosphonic acid/MWCNTs composite after UV irradiation for 120 min (7500).
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Fig. 9. Energy dispersive X-ray analysis of (a) cotton cross-linked with polyvinylphosphonic acid/MWCNTs composite after UV irradiation for 60 min and (b) cotton crosslinked with polyvinylphosphonic acid/MWCNTs composite after UV irradiation for 120 min.
untreated fiber has fibrils with a relatively smooth surface. The surface of the untreated fiber has no deposition of MWCNTs and the cross-linking agent. The high magnification SEM images of the cotton fiber cross-linked with polyvinylphosphonic acid/MWCNTs show a formation of the aggregated MWCNTs on the surface of the cotton. In general, the ability of nano-particles to aggregate on the surface of textiles and polymers is defined by several factors such as size, mobility, end-group functionalities, relative composition and molecular architecture [35–37, 53–58]. As mentioned previously, the FTIR spectra of samples illustrated that the hydroxyl group of cotton cross-linked with polyvinylphosphonic acid to form ACAOACA linkages and PAO bonds. It seems that these interactions are strong enough to allow the deposition of MWCNTs on the cellulose surface. We have confirmed this phenomenon in cases where particles were incorporated into multilayer organic coatings [59–64]. Fig. 9a,b shows the presence of chemical elements on the cotton surface after cross-linking with polyvinylphosphonic acid/MWCNT composite under UV irradiation. In these patterns, Au peaks clearly show that gold is successfully coated on surfaces of all fibers. EDX analysis of cross-linked textiles illustrated more efficient interactions between cotton surface and polyvinylphosphonic acid/ MWCNT composite leading to the presence of P at these sample´s surface. This finding is also further supported by FTIR and crosslinking rate tests.
4. Conclusion MWCNTs and a flame retardant cross-linking agent were used to fabricate a flame retardant coating on the cotton through UV irradiation. The stabilization of CNTs in the composite coating was achieved by a reaction of polyvinylphosphonic acid and cellulose chains. This reaction process resulted in the attachment of MWCNTs to the surfaces of the cellulose fibers. The FTIR spectra showed the cross-linking reaction between the hydroxyl groups of cellulose and vinylphosphonic acid monomer to form linkages in the presence of CNTs under UV light. The results obtained from TGA and HFT tests demonstrated an improvement of the thermal properties and flammability of the coated samples. This can be as a result of a high heat resistance, a heat insulation effect and the mass transport barrier of CNTs embedded in the coating. This textile composite coating is therefore very promising for civil applications as an effective light-weight flame retardant material.
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