Enhancement of Thermal Stability, Flame Retardancy, and

Article pubs.acs.org/IECR

Enhancement of Thermal Stability, Flame Retardancy, and Antimicrobial Properties of Cotton Fabrics Functionalized by Inorganic Nanocomposites V. Rajendran,*,† N. R. Dhineshbabu,† R. Rajesh Kanna,† and Karan V. I. S. Kaler†,‡ †

Centre for Nano Science and Technology, K.S. Rangasamy College of Technology, Tiruchengode 637215, Tamil Nadu, India Department of Electrical and Computer Engineering, Schulich School of Engineering, University of Calgary, Calgary, Alberta, Canada

‡

ABSTRACT: In this investigation, various metal oxide (ZrO2, MgO, TiO2) nanoparticles were prepared using the hot-air spray pyrolysis method. The prepared nanoparticles were characterized using the X-ray diffraction (XRD) technique, scanning electron microscopy (SEM), and transmission electron microscopy. The colloidal silica (SiO2) sol was prepared using the sol−gel method, mixed with various metal oxides (ZrO2, MgO, TiO2). Uncoated cotton fabrics were separately impregnated with the prepared nano(composite) sols followed by the pad-dry-cure method. The structural analysis of the coated and uncoated fabrics was performed using XRD. The surface morphology of the coated and uncoated cotton fabrics was analyzed using SEM. The elemental analysis using energy-dispersive spectroscopy confirmed the presence of nanoparticles along with cellulose on the surface of the fabric. The thermal stability and flame retardancy properties and residue of the coated and uncoated fabrics were studied. The coated cotton fabrics showed better antibacterial activity against Staphylococcus aureus and Escherichia coli. The biocompatibility performance of the coated fabrics was in the order TiO2/SiO2 > MgO/SiO2 > SiO2 > ZrO2/SiO2.

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INTRODUCTION Scientific advancements and technological development play a vital role in the innovative application of materials and processes in almost all the commercial market domains, such as cosmetics, electronics, and textiles.1−3 Among these, textile materials and products are of particular importance as they find daily utility by a vast number of users. In recent years, the textile industry has increasingly focused on the development of high-quality, economically viable, environmentally friendly materials and fabrics and related products for the global consumer market. In the area of traditional textiles, cotton fabrics are considered to be of particular importance due to their unique and desirable physicochemical and functional properties.2−5 Recently, the application of nanotechnologies has benefited the textile industry by enabling the creation of new fabrics with enhanced and multifunctional material properties.4−7 Instead of nanotechnology, a new approach of nanomaterials has attracted the researchers and worldwide finishers of textile materials.4,5 For example, nanoparticles of metal (Ag, Au, and graphene) as well as metal oxides (such as TiO2, MgO, SiO2, CuO, ZrO2and ZnO) have been used to functionalize textile fabrics such as cotton using various approaches.7−21 The incorporation of nanoparticles in such fabric materials improves their functional features such as UV protection, antibacterial activity, flame retardancy, thermal stability, and physicochemical properties.9−13 The incorporation of nanoparticles in fabrics may be readily achieved using a sol−gel preparation method followed by the pad-dry-cure method.13 Here, silica sol (nanosol) is used as a binder to coat nanoparticles on the surface of the fabric materials.14 The silica sol-based nanometal oxide (TiO2, ZnO) composites have better functional properties, suitable for fabrics, as previously reported.14,17,18,21 Furthermore, our previous studies clearly show that the composite (ZrO2/SiO2) can be used as a fire retardant for cotton fabrics.22 © XXXX American Chemical Society

To the best of our knowledge, no detailed investigation has been carried on the utility of inorganic (metal oxide) composite coatings on the fabrics. In this work, a number of different metal oxide nanoparticles (ZrO2, MgO, and TiO2) were produced by the hot-air spray pyrolysis method. The obtained nanoparticles were separately mixed with silica sol and subsequently used to coat cotton fabrics using the pad-dry-cure method in order to investigate their multifunctional properties.

Figure 1. Indigenous spray pyrolyzer setup.

Received: July 2, 2014 Revised: November 23, 2014 Accepted: November 25, 2014

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dx.doi.org/10.1021/ie502584m | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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Figure 2. XRD pattern of (a) ZrO2, (b) TiO2, and (c) MgO nanoparticles.

Table 1. Physical Parameters of Different Nanoparticles from XRD pattern

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from TEM image

s. no.

nanoparticles

JCPDS file no.:

crystallite size (nm)

phase structure

morphology

size (nm)

1 2 3 4

ZrO2 MgO TiO2 SiO2

81−1314 89−7746 21−1276

31 22 41

monoclinic cubic rutile

spherical needle like spherical spherical

50−200 50−200 50−150 50−100

was continuously flowed to the deblocking unit with the help of a feed pump. In this step, the hot compressed air (20 kg cm−2) was introduced into the reaction chamber, and the precursor was sprayed into the chamber with the help of a two-fluid nozzle assembly in the deblocking unit. In the spray pyrolysis method, the reaction occurs in the solution droplets, while the solvent evaporates. Atomization was regulated by controlling the pressure of the compressed air. The sprayed and atomized nanoparticles of Zr(NO3)2 were decomposed at 600 °C to obtain ZrO2 nanoparticles. After the completion of one full cycle (90 min) of the process, ZrO2 nanoparticles were collected from the cyclone. A similar procedure was adapted for the preparation of MgO and TiO2 nanoparticles from the precursors of Mg(NO3)2 and Ti(OH)2, respectively. In brief, 50 mL of 0.10 M aqueous Ti(SO3)2 solution was prepared by the sol−gel method, and the same amount of sodium hydroxide was dropwise added to the former aqueous solution under vigorous stirring conditions.

EXPERIMENTAL SECTION Materials. Bleached and mercerized cotton fabrics (116 ends/84 picks) with a density of 138.84 g m−2 were used as the substrate. Tetraethyl orthosilicate (TEOS; 99%), sodium hydroxide (NaOH; 99.9%), ammonia (25%), ethanol (EtOH; 99.9%), polyethylene glycol (PEG 400; 99.5%), zirconyl nitrate (Zr(NO3)2; 99%), and magnesium nitrate (Mg(NO3)2; 99%), purchased from Merck, India, and deionized water were used without any further purification. Titanium sulfate (Ti(SO3)2) was synthesized from rutile sand.23 Nanoparticle Preparations and Characterization. An automated pilot-plant hot-air spray pyrolysis setup (Figure 1) was designed24 and used for the preparation of different metal oxide (MgO, ZrO2, and TiO2) nanoparticles. Initially, Zr(NO3)2 powder (5 g) was dissolved in 500 mL of distilled water, and the mixture solution was stirred in a magnetic stirrer (rotating speed: 25 rpm). The prepared solution was used as a precursor, which B



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Figure 3. SEM-EDS images of ZrO2, TiO2, and MgO nanoparticles.

This process was continued for a further 45 min until a precipitate was formed. The obtained precipitate of titanium hydroxide was used as the precursor for spray pyrolysis. The collected nanoparticles were used to analyze their chemical and structural features using conventional techniques, as described next. The structural examination of the nanoparticles was carried out with the help of an XRD spectrometer (X’Pert PRO; PANalytical, Almelo, The Netherlands) with Cu Kα as a radiation source (λ = 0.1560 nm) at a scanning rate of 0.02° and a scan angle 2θ ranging from 20° to 80°. From such measurements, the average

crystallite size of the prepared nanoparticles was determined using the Scherrer formula.25 The surface morphology of the nanoparticles was analyzed by scanning electron microscopy (SEM), whereas the elemental composition of the nanoparticle sample was determined using energy-dispersive X-ray spectroscopy coupled with SEM (SEM-EDS; JEM-200; JEOL, USA) at an accelerating voltage of 20 kV. The individual morphology of the grains was determined by high-resolution transmission electron microscopy (JEM-2100F; JEOL) coupled with a selected area electron diffraction (SAED) pattern operating at 100 kV. C



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For TEM imaging, 0.01 mL of each of the prepared samples was dispersed in ethanol, and the suspension was sonicated in an ultrasonic bath. The obtained suspension was drop-cast on a carbon-coated copper grid and dried before its examination. Sol Preparation and Characterization. The silica hydrosol was prepared by the sol−gel process. In this process, a 250-mL beaker was filled with 180 mL of ethanol, to which 6 mL of TEOS was added dropwise under continuous stirring. After stirring for 30 min, 2 mL of nonionic surfactant of PEG was added to the solution under vigorous stirring followed by the addition of 4 mL of NH4OH to obtain a suspension of silica sol, referred to as Si. To the prepared composite sol of the various nanoparticles (ZrO2, MgO, and TiO2) we added 1 g of the respective particles into the prepared sol separately under vigorous stirring for 30 min at room temperature. The prepared sol−nanoparticle mixer was sonicated for 15 min to form a composite sol. The obtained homogeneous composite sols are referred to as ZrS, MgS, and TiS. The morphology of the prepared silica and composite sol was analyzed using TEM imaging. Alkali Treatment, Coating of Fabric with Nanoparticles, and Characterization. The rinsed cotton fabric (15 × 15 cm) was treated twice with 5 wt % NaOH solution at room temperature, washed in hot (60 °C) deionized water, and then dried at 50 °C for 5 min. The above-treated fabric samples were immersed in a homogeneous solution containing Si, ZrS, MgS, and TiS hybrid sol separately and coated using a pad-drycure method. During the coating process, the fabric was padded at a speed of 5 rpm and under a pressure of 4 kg cm−2 for 15 min at room temperature.22 The padded fabrics were air-dried and cured at 80 °C for 5 min in an oven. Hereafter, the uncoated fabric, fabric coated with Si, and the hybrid (ZrS, MgS, and TiS)coated fabrics are referred to as UC, SiC, ZrSC, MgSC, and TiSC fabrics, respectively. The weight of the coated fabrics, namely SiC, ZrSC, MgSC, and TiSC fabrics, was 178.64, 192.23, 193.54, and 192.80 g m−1, respectively. The weight ratio of composite nanoparticle-coated fabrics was more than that of SiC fabric, which was due to the addition of nanoparticles (MgO, ZrO2, and TiO2) in silica sol. However, the thickness of the UC, SiC, ZrSC, MgSC, and TiSC fabrics increased to 0.21, 0.23, 0.24, 0.24, and 0.24 mm, respectively. The variation in the thickness may be because of the addition of nanoparticles on the surface of the fabric. The structural property of UC and coated fabric samples was analyzed by XRD pattern. The chemical structures of coated and UC fabric samples (each 1 cm2) were analyzed by Fourier transform infrared spectroscopy equipped with attenuated total reflection (FTIR-ATR; spectrum 100; Perkin-Elemer; USA). The morphological characterization of coated and UC fabrics was carried using SEM-EDS. Fabric samples (1 cm2) were mounted on a sample holder using double-sided conductive carbon adhesive tape. For SEM imaging, gold was sputtered on the fabric samples before examination. The water contact angle (WCA) was measured using a static contact angle meter (AST Products Inc., Japan) using distilled water (5 μL) placed on the fabric surface with a micropipette. The thermal behavior of the fabric samples was studied by thermogravimetric analysis (TGA). Using alumina pans containing approximately 10 mg of each sample, we tested the sample in atmospheric air at a heating rate of 20 °C min−1 and a temperature from 30 to 550 °C. The flame retardancy tests of the UC and coated fabric samples (150 mm × 70 mm) were carried out using an inclined flame tester (Auto Flame I; Premier, India) in accordance with the ASTM D1230-97 Standard Test Method.

Figure 4. TEM images of ZrO2, TiO2, and MgO nanoparticles.

Photographs of the uncoated and coated samples during the inclined flame test were obtained with a camera (NV3; Canon). The residue of the burned fabric was examined through SEM analysis. The antimicrobial properties of the coated and uncoated fabrics were tested against the common clinical pathogens such as Gram-positive Staphylococcus aureus (ATCC 6538P) and Gram-negative Escherichia coli (ATCC 9677) using the Kirby− Bauer disk-diffusion method. The obtained bacterial strains were maintained overnight, and the culture of each pathogen was inoculated on nutrient agar (HiMedia, India) medium and sterilized at 37 °C for 24 h. The sterile Mueller−Hinton agar plates were prepared and swabbed with fresh inoculums of E. coli and S. aureus. The pathogen-inoculated fabrics (UC, SiC, and MgSC) were placed in the inoculated plate kept in an incubator at 37 °C for 24 h. After incubation, the zone of inhibition around the fabrics was measured using a transparent ruler (millimeter scale). Further, quantitative antimicrobial activity in all fabrics was analyzed in accordance with AATCC test 100−207 (AATCC 2007). Freshly prepared nutrient broth was transferred to a 10-mL fresh sterile test tube, and then fresh strains of E. coli and S. aureus were inoculated for 3 h at 37 °C. During inoculation, the culture was diluted with a dilute nutrient broth (1:20 dilution in water) to obtain a bacterial suspension. For each bacterial strain, eight pieces of UC fabric samples and two pieces of each coated fabric were inoculated with the bacterial suspension. The suspension was added dropwise to each fabric sample in order to facilitate absorption. Subsequently, the inoculated fabric samples were placed in an incubator at 37 °C for 24 h. The number of colonies formed on the plates after incubation was measured to D



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Figure 5. TEM images of colloidal particles with inside of SAED pattern (a) SiO2, (b) ZrO2/SiO2, (c) TiO2/SiO2 and (d) MgO/SiO2 sol.

the EDS pattern shows the presence of the elements (O, Mg, Ti, and Zr). The bulk internal morphology of all the nanoparticles is shown in Figure 4, whereas their respective shape and the sizes are given in Table 1. The concentric ring pattern of the SAED pattern confirms the crystalline nature of the nanoparticles. TEM images of the prepared solution of Si, ZrS, MgS, and TiS are shown in Figure 5a−d. They show spherical morphology of the individual silica nanoparticles, which are aggregated to form chain-like structures with an average dimension of about 100 nm. The TiO2, MgO, and ZrO2 nanoparticles are incorporated into silica nanoparticles, as shown in Figure 5b−d, which was because of the van der Waals attraction force involved in the composite nanoparticles.26 The SAED pattern confirms the amorphous and polycrystalline nature of SiO2 and composite (ZrS, MgS, and TiS) nanoparticles, respectively. Surface Properties of Coated and Uncoated Fabrics. The surface morphologies of the cotton fabrics coated with Si, ZrS, MgS, and TiS nanocomposites and UC fabrics are shown in the SEM images for direct comparison. Figure 6 shows the typical top view of the coated and UC fabric samples at a lower magnification. From such images, it is evident that the treatment of the fabrics with ZrO2, SiO2, MgO, and TiO2 nanoparticles results in accumulation of these particles on the fabric surface. Furthermore, the interaction between the nanoparticles and the fabric surface results in the formation of an extended network structure,

determine the bacterial growth on the fabric samples by inoculum cell density (CFU ml−1). Glutaraldehyde Fixation. Sample fixative solution, 2.5% (v/v) glutaraldehyde in phosphate buffer (pH 7.2), was freshly prepared and stored in a brown bottle at 4 °C until use. Hot airdried samples were kept in the screw cap scintillation vials with 5 mL of fixative solution. The sample vials were incubated at 4 °C for 8 h. The dehydration process for all of the samples was carried out with ethanol at different concentrations (25%, 35%, 45%, 50%, 75%, 85%, 95%, and absolute alcohol) at frequent intervals (2 min). The samples were stored in 100% alcohol until SEM processing.

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RESULT AND DISCUSSION Characterization of Nanoparticles and Colloidal Sol. The XRD patterns of TiO2, MgO, and ZrO2 nanoparticles are shown in Figure 2. As anticipated, the locations of the 2θ peaks completely differ for every sample. The diffraction peaks of each sample were correlated with their own standards (JCPDS file), and the average value of each sample crystallite size and the phase structures is summarized in Table 1. Figure 3 shows the surface morphology of the TiO2, MgO, and ZrO2 metal oxide nanoparticles. The morphologies of TiO2 and ZrO2 nanoparticles are compact and spherical; however, the MgO nanoparticles show a flower-shaped morphology. Furthermore, as shown in Figure 3, E



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asymmetric glucose ring, and C−O stretching for cellulose on the cotton fabrics.28−30 In addition, the peak observed at 795 cm−1 shows the stretching and bending modes of Si−O−Si in the SiC fabric. The complex oxides (TiS, ZrS, MgS) which are formed at the interfaces on the fabric surface are due to the existence of Ti−O−Si (948 cm−1),18 Zr−O−Si (680 cm−1),22 and Mg−O−Si (642 cm−1) vibrations as observed through IR spectra, respectively. The FTIR results confirm that SiO2, TiO2/SiO2, ZrO2/ SiO2, and MgO/SiO2 hybrids, along with cellulose backbones, exist on the cotton fabric substrate. The surface wettability test of coated and UC fabrics is measured through static contact angle measurements as shown in Figure 9. From Figure 9a, it can be seen that the UC fabric is instantly wetted on the fabric surface by the water droplet due to the high hydrophilic nature (angle 0°), which is attributed to the hydroxyl group and frequent holes in its weave structure. As shown in Figure 9b, the WCA is nearly 10° for the silica coated fabric, which may be due to the existence of alkoxy and hydroxyl groups in the SiC fabric.31 The same contact angle is revealed in all nanocomposite coated (ZrSC, TiSC, and MgSC) fabrics. The above observations show that the nanocomposite coated fabric samples exhibit the wetting property of hydrophilic cotton fabrics which is similar to UC fabric. Thermal and Fire Resistance Property of the Coated and Uncoated Fabrics. The degradation temperature of the coated and uncoated fabrics was studied by TGA. The TGA curves of the all samples are analyzed, and the results are plotted

as confirmed by the micrographic images. EDS measurement were carried out to confirm the elemental composition of the coated cotton fabrics, and the results are summarized in Figure 6. The EDS analysis shows that the Si element is present in all the coated fabrics along with C, O, Zr, Ti, and Mg. Interestingly, silica nanoparticles deposited on the surface of the fabrics are combined with TiO2, MgO, and ZrO2 in the respective samples. Structural, Functional, and Wettability Analysis of Cotton Fabrics. The structural analyses of UC, SiC, MgSC, ZrSC, and TiSC fabrics are shown in Figure 7. It is evident that all the samples show diffraction peaks at 6.52°, 22.82°, and 34.58° corresponding, respectively, to (101), (002), and (040) diffraction peaks of cellulous fibers (JCPDS file no. 03-0226).27 It is noteworthy that the diffraction peak of cotton fiber is similar to that of SiC-coated fabric because of the amorphous nature of silica;22 however, nanoparticles are also present in the coated fabrics, as is evident in Figure 7. The nanoparticles in these cases are adhered to the surface of fabric material. These results indicate that the nanoparticles are assembled and localized on the inner structure of the cotton fabrics due to the interfacial forces acting between the nanoparticles and the cotton fabric. FTIR-ATR spectra of UC, SiC, ZrSC, TiSC, and MgSC fabrics are shown in Figure 8. In all coated and UC fabrics, the absorption peaks observed at 3400, 2875, 1650, 1450, 1200, 1130, and 1080 cm−1 are ascribed due to, respectively, −OH stretching, −CH stretching, H−O−H bending, −CH bending, C−O−C

Figure 6. continued F



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Figure 6. SEM-EDS images of (a) UC, (b) SiC, (c) ZrSC, (d) TiSC, and (e) MgSC fabric samples.

(carbonization),33 as shown in Figure 10b. The metal oxide composites (ZrS, TiS, and MgS) resulted in a significant reduction in the onset temperature when compared to silica and UC cotton fabrics (Tonset5%: 293, 284, and 299, respectively, vs 319 for silica-coated and 315 °C for UC fabrics). The first and second decomposition temperatures (Tmax1 and Tmax2) of all coated fabrics are shown in Figure 10, and the results are summarized in Table 2, which shows the degradation of cellulose species leading to the formation of a high quantity of residue. Furthermore, the formation of residue at (Tmax) 400 °C corresponds to the weight % of all fabric samples, as shown in Table 2. In particular, ZrS-coated fabric induces the formation of

in Figure 10a−e. For cellulose fabrics, three stages are involved in the degradation mechanism. In the first stage (300−400 °C), it shows two pathways that result in char formation (aliphatic) and volatilization. In the second stage (400−600 °C), the decomposition is observed to occur resulting in the formation of residue from aliphatic to aromatic char, due to the carbonization process. During the last (third) stage (600−800 and above 800 °C), some residues are completely decomposed/oxidized, which results in the combustion of carbonaceous species,32 and the UC fabrics are completely degraded, as shown in Figure 10a. The thermal stability properties of cotton fabrics are modified by the synergistic effect of silica, which is evident in final residue G



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Figure 7. XRD pattern of all coated and uncoated fabric samples.

Figure 9. Contact angle of UC and SiC fabric samples.

stability of the coated fabric can be significantly enhanced, resulting in superior fire resistance performance. To identify the flammability properties of UC and nanocompositecoated fabrics, we subjected the samples to an inclined flame test.34 The images of the collected residue of the test samples are shown in Figure 11 for comparison. Exposure of UC fabric to a direct flame results in the flame spreading over the edge of the fabric until the fabric is completely consumed. Furthermore, there is distinct lack of residue in the UC fabric when the cellulose material is completely burned (Figure 11a). In contrast, when exposed to the same flame, the coated fabrics are consumed at a much slower rate and the entire fabric sample is engulfed in flame and is not limited to the sample edge. In addition, there is a presence of residue in the fragments of the coated fabrics. For all the coated fabrics used in this study, both the burning time and the presence of final residue are increased compared to the UC

Figure 8. FTIR spectra of all coated and uncoated fabric samples.

large amounts of char amount when compared to SiC-, TiS-, and MgS-coated fabrics. This observation suggests that thermal H



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Figure 10. TGA measurements of all coated and uncoated fabric samples.

and char formation of nanocomposite (ZrSC, TiSC, and MgSC)coated fabrics are 19.4, 18.9, and 18.4 s, respectively, versus SiCcoated fabric (13.6 s) and UC fabric. A detailed examination of the residue of the combustion fabric sample was carried using an SEM after the flame test, as shown in Figure 11b. For the uncoated cotton fabrics, the combustion of the fabric results in a clear degradation of the fiber weave structure and incompletely burned residue (see Figure 11b(i)) due to organic degradation of the fabric.32−34 In contrast, the residue obtained from the SiC-coated fabric (Figure 11b(ii)) shows a preserved weave structure of the fabric. Remarkably, the residue of nanocomposite (ZrS, TiS, and MgS)-coated fabrics is also fully intact in the weave structure when compared with that of SiCcoated fabric, as shown in Figure 11b(iii−v). It is evident from

Table 2. TGA Data for Coated and Uncoated Fabrics s. no.

samples

Tonset5% (°C)

Tmax1 (°C)

Tmax2 (°C)

residue at 400 °C (%)

1 2 3 4 5

UC SiC ZrSC MgSC TiSC

281 224 235 222 239

341 236 288 282 274

589 541 537 562 549

38 40 52 45 44

fabric. The average burning time of the all fabric samples is measured during the flammability test. The char lengths of ZrSCcoated fabrics that passed the inclined flame test are MgSC > SiC > ZrSC > UC

CONCLUSION Various metal oxide nanoparticles were successfully prepared by the hot-air spray pyrolysis method. A composite mixture of nanoparticles and sol was prepared by the sol−gel process and used to impregnate cotton fabrics. The deposition of the nanoparticles on the cotton fabrics was confirmed by SEM and EDS analyses. The thermal stability and flame retardancy potential of uncoated cotton fabrics was significantly enhanced K



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Figure 14. Antibacterial activity of before and after wash.

by incorporation of metal oxide nanoparticles. Moreover, better performance of nanocomposite-coated fabrics was observed in both thermal and flame retardancy tests compared to nanoparticle (SiO2)-coated fabrics. Moreover, the intrinsic antimicrobial property is disintegration of nanoparticles (TiO2, MgO) through the SiO2 nanoparticle surface, which leads to effective antimicrobial properties of the coated cotton fabric relative to SiO2 and ZrO2/SiO2 coated fabrics. The functionalized cotton fabrics are eco-friendly for use in industrial and biomedical textiles.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +91-4288-274741-4. Mobile: +91-9994130303. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support (ERIPR/ER/ 0905103/M/01/1279) from Defence Research Development Organisation (DRDO), New Delhi, for this project. The authors thank S. Aravindh, Department of Microbiology, K.S. Rangasamy College of Arts and Science, for providing the necessary facility to carry out this study. L



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Enhancement of Thermal Stability, Flame Retardancy, and

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