Synthesis of ppy–MgO–CNT nanocomposites for multifunctional applications

Abstract

Cotton is one of the most important raw materials for textile and clothing production. The main drawbacks of cotton fibers are their poor mechanical properties and high flammability. Compared with some synthetic polymer fibers, cotton fabrics treated with modern flame-retardant and reinforcement finishes often cannot meet rigid military specifications. Polypyrrole–magnesium oxide (ppy–MgO) and polypyrrole–magnesium oxide–carbon nanotube (ppy–MgO–CNT) composites were prepared with various weight ratios by in situ chemical polymerization method. 1,2,3,4-Butane tetracarboxylic acid (BTCA) was used as a cross-linking agent in the presence of sodium hypophosphite (SHP). The composite sol was coated on cotton fabric using the pad-dry-cure technique. The coated cotton fabrics were characterized by SEM, EDAX, XRD, UV-DRS and FT-IR analysis, and tested for flame retardant and UPF application. The flame-retardant study showed a maximum char length of 0.3 cm and the char yield was about 49% for the ppy–MgO–CNT composite. For that UPF application, a 30 UPF value was shown for the ppy–MgO–CNT composite. In the case of the antibacterial study, the zone of inhibition was observed for all of the test samples against MRSA and PAO1 bacteria. The zone of inhibition showed as 4.0, 3.0 mm for the ppy–MgO–CNT composite. Hence, the ppy–MgO–CNT composite was found to be efficient.

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1. Introduction

Over the past few decades, the modification of textiles has attracted much attention due to its increasing awareness of human beings toward environmental protection, as well as for a safe, healthy and comfortable life.^1–3 Multifunctional textiles can be fabricated through combined treatments using several materials with a specific property of fibers.⁴ Cellulose is the most abundant biopolymer found on Earth and has many excellent qualities, such as biodegradability, biocompatibility, and eco-friendliness.⁵ Textile industries have been focusing on improving its functional properties, such as fire retardancy, UV protection, self-cleaning and antibacterial properties. High-performance textile materials are greatly appreciated by more discerning and demanding consumer markets. Nanoparticles of metal oxides have been used to incorporate functional properties into textiles. Attempts have been made to use common metal oxides, such as titanium oxide (TiO₂), zinc oxide (ZnO), cupric oxide (CuO) and magnesium oxide (MgO), for providing fabrics with functional properties.⁶ Magnesium oxide (MgO) nanopowder is a non-toxic white powder widely used in industrial applications, such as ceramics, electronics, catalysis, paints and oils. It is one of the most useful refectory materials that improve textile functionalities, such as fire retardancy, UV protection and antibacterial properties, with toxic waste remediation.^7–10 The application of nanotechnologies benefits the textile industry by enabling the creation of new fabrics with enhanced and multifunctional material properties.^11–13 A new approach of nanomaterials has attracted researchers by worldwide finishers of textile materials. For example, nanoparticles of metal (Ag, Au, and graphene) and metal oxides (such as TiO₂, MgO, SiO₂, CuO, ZrO₂ and ZnO) have been used to functionalize textile fabrics by cotton using various approaches. 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.¹⁴ In recent years, the use of a nano titanium dioxide photo-catalyst to cover textiles and the improvement of its surface have expanded due to its ability to absorb ultra-violet irradiation. In addition, the coat of nano TiO₂ particles on fabrics will not affect their breathability and texture.^15–23 The size and surface area of NPs may be responsible for their toxicity, but most of the studies do not reveal the relationship between the physiochemical characteristics of NPs and their toxicity.²⁴ For example, 25 nm anatase and 31 nm anatase/rutile show greater phototoxicity than 142 nm anatase and 214 nm rutile NPs.²⁵ All the sizes and crystal forms (anatase and rutile) of TiO₂ NPs exert toxic (phototoxic) effects on human skin keratinocytes under UVA irradiation in a dose-dependent way. The smaller size nTiO₂ may cause greater cytotoxicity than larger size NPs, and the anatase form may show more phototoxicity than rutile.²⁶ Furthermore, the NPs (rod and sphere) of smaller size show higher toxicity than larger particles. Moreover, the nanorods exhibit more toxicity than spherical particles having the same size and surface area, showing the contribution of shape toward cytotoxicity.²⁷ Several methods can be used to apply finishing materials onto fabrics, such as, spraying, transfer printing, foaming, and padding, among which padding is the most commonly used method. At the same time, various polycarboxylic acids were investigated^28–33 as cross-linkers for the durable press finishing of cotton. The formaldehyde-free formation of ester linkages that improve the tensile and abrasion strength is among the advantages of these compounds.^34–36 Polycarboxylic acids, such as citric acid (CA) and 1,2,3,4-butane tetra carboxylic acid (BTCA), are cotton cross-linking agents.^37–41 These are environment-friendly and can be a suitable substitute for methylals, which contain cross-linkers. Cross-linking of BTCA and CA with cotton fabrics has been proposed to be catalyzed by sodium hypophosphite in acidic conditions. The mechanism of cross-linking is suggested to include cyclic anhydrides as reactive intermediates, which are trapped by hydroxyl groups of fabrics.⁴²

Other oxygen-containing compounds like phosphates or carbonates, as well as noble metal nanoparticles, can be produced in the appropriate flame environment.^43,44 As one of the most significant conducting polymers, ppy has been widely used in electronic devices, biomedical devices, sensors, and other fields for its good conductivity, thermal and photostability in air.⁴⁵ Since ppy also tends to swell and shrink in cycling, many ppy–GO composites have been fabricated to pledge high performance when used for super capacitors.⁴⁶

To the best of our knowledge, no detailed investigation has been carried out so far on the utility of ppy–inorganic (metal oxide) composite coatings on the fabrics flame retardancy grounds. In this work, attempts have been made with different ratios of ppy–MgO composite and ppy–MgO–CNT by in situ chemical polymerization method. The nanocomposites were prepared as sol, and subsequently used to coat cotton fabrics using the pad-dry-cure method for multifunctional applications.

2. Experimental

2.1. Materials and methods

Woven cotton fabric of 80 counts was obtained from South Indian Textile Research Association (SITRA), Coimbatore. The pre-treatment was carried out for the grey fabric by one pot method. The recipe used was 0.4 ml hydrochloric acid (HCl), 39 g sodium carbonate (Na₂CO₃), 1.9 g sodium hydroxide (NaOH) and 1.9 ml hydrogen peroxide (H₂O₂) taken in a 300 ml beaker containing 200 ml of water. Grey cotton fabric (8 cm × 4 cm) was immersed in the bath at 80 °C for 90 min. Magnesium oxide nanoparticles (MgO) (Sigma Aldrich, 97%) with an average size of 50 nm, pyrrole (ppy) (Sigma Aldrich, 99% pure), 1,2,3,4-butane tetracarboxylic acid (BTCA) (Alfa Aesar, 98%), ammonium persulfate (APS) (Alfa Aesar, 98%), single wall carbon nanotube (CNT) (Sigma Aldrich, 97%) with an average size of 25 nm, sodium hypophosphites (SHP) (S.D. Fine, 98%), and polyethylene glycol (Alfa Aesar, 98%) were purchased and used.

2.2. Synthesis of polypyrrole

Pyrrole was purified by double distillation method before use. A volume of 33.6815 ml of pyrrole (0.3 M) was dissolved in 500 ml de-ionized water. After 10 min, APS of 0.1329 g (0.06 M) was prepared in 100 ml de-ionized water and added drop wise. This solution was stirred for 24 h in an ice bath at 0 °C constantly. Excess methanol was poured to regulate the reaction. A black precipitate was obtained. This precipitate was collected and washed with different solvents, such as de-ionized water, methanol and acetone. Then, the sample was dried at 30 °C for 12 h in a vacuum oven.⁴⁷ After that, it was weighed as 32.57 grams.

2.3. Synthesis of ppy–MgO and ppy–MgO–CNT composites by in situ polymerization

Pyrrole of (0.3 M) was dissolved in 500 ml de-ionized water. After 10 min, BTCA (0.1 M) and SHP (1 g) catalyst were added. Stirring was carried out for 30 min at room temperature. A required amount of NaOH was added to bring the pH to neutral. APS (0.06 M) was prepared in 100 ml de-ionized water. MgO (0.25 g) was mixed with the above solution. The solution was sonicated for 30 min. The resulted solution turns milky white. This milky white solution was added dropwise into the above solution, and stirring was carried out for 9 h under a cooling condition at 20 °C. This solution was allowed to settle overnight. A black precipitate was obtained. The precipitate was collected and washed with different solutions, such as de-ionized water, methanol and acetone. Then, the sample left dried at 30 °C for 6 h in a vacuum oven. The procedure has resulted in a ppy–MgO composite at 1 : 1 ratio. This procedure was again extended to prepare ppy–MgO composites at 1 : 2 and 1 : 3 ratios and optimized. In a similar ratio way, CNT at different ratios was added. The samples were designated as ppy–MgO M₁ (1 : 1), ppy–MgO M₂ (1 : 2), ppy–MgO M₃ (1 : 3) and ppy–MgO–CNT M₄ (1 : 3 : 1). The weight gain was as follows: M₁ (0.34 g), M₂ (0.38 g), M₃ (0.40), M₄ (0.45 g). It has been weighted. Cross-linking can be thought of as reducing the number of chain ends, and effectively increasing the molecular weight of polymer system.⁴⁸ In general, cross-linking will increase the strength of the material.⁴⁹ The ppy–MgO composite was confirmed with XRD and FT-IR peak analysis.

2.4. Coating sol preparation

The ppy–MgO or ppy–MgO–CNT composite was prepared by mixing with polyethylene glycol (2 ml). Then, 6 ml deionized water was added into 100 ml absolute ethanol and stirred vigorously at room temperature for 30 min until a homogeneous solution was obtained. Then, 4 ml ammonia was added dropwise into this solution, and was kept under ultrasonic irradiation for 30 min to form the ppy–MgO or ppy–MgO–CNT composite sol. This sol of about (10 g m⁻²) was used for the coating of fabric using the pad-dry-cure method.⁵⁰ The dried cotton fabric was first dipped in ppy–MgO or ppy–MgO–CNT composite sol. The wetted fabric was pulled into the nip between two rotating cylinders in the padding mangle to obtain a uniform coating. This procedure was repeated three times to obtain effective deposition of nanoparticles onto the fabric surface. Afterwards, the padded fabrics were air-dried and cured at 180 °C for 5 min in a hot-air oven. The ppy–MgO or ppy–MgO–CNT nanoparticles were linked to the fabrics with an exchange reaction between the –OH groups of cotton.

3. Results and discussion

3.1. XRD analysis

Fig. 1 illustrates the XRD patterns of the pristine MgO, polypyrrole, M₁, M₂, M₃, and M₄ composite materials. The pristine polypyrrole showed a band (curve b). It has a broad band at lower diffraction angles around 26°, which indicates the amorphous nature. This broadening of the band can be ascribed to the scattering of the bare ppy chains at the interplanar spacing.⁵¹ Pristine MgO shows intense peaks at 42.5°, 62.1°, and 75.5°, indicating the crystalline nature with cubic structure (curve a). The peaks corresponding to the planes (200), (220) and (222) are in good agreement with JCPDS file no. 04-0829.⁵² M₁ and M₂ composites (curve c and d) show an amorphous nature. There is no defined diffraction peak in the composites. When the ratio of the MgO colloidal solution was increased, the peak intensity also increased accordingly. The sharp diffraction peak of MgO implies that MgO is crystalline in nature. It is indicated by the corresponding peaks to the (200), (220) and (222) planes in the (curve e). All of these peaks match very well with the standard MgO of the cubic structure (JCPDS card 04-0829). The crystallite size of ppy–MgO calculated using Scherrer's formula was found to be 18 nm.

L_s = κλ/β_s cos(θ_B)

where κ stands for a geometrical factor that depends on the crystallite apparent radius of gyration from the perspective of reflections with Bragg angle θ_B for X-rays of wavelength λ.

Fig. 1 XRD patterns of the pristine (a) MgO, (b) polypyrrole, (c) M₁, (d) M₂, (e) M₃, and (f) M₄ composite materials.

For instance, crystallites with a cubic shape have κ′ 0.92, while spherical crystallites have κ′ 1.18.

An additional peak is shown as the (002) plane, which indicates the presence of CNT.⁵³ The decrease in the intensities of the MgO peaks in the ppy–MgO–CNT functionalized average size of about 50–75 nm (curve f). It is noted that the ppy–MgO and ppy–MgO–CNT nanocomposites are more crystalline in nature than pristine polypyrrole. These results also indicate that MgO and MgO–CNT nanoparticles are incorporated in the polypyrrole matrix as composites.

Ppy, M₃ and M₄ composites were optimized for further work. The composite sols were coated on cotton fabric. Fig. 2 shows the presence of MgO nanoparticles on the surface of the fabrics. The diffraction peaks observed at 42.5°, 62.1° and 75.5° are correlated with the formation of cubic MgO assigned to the (200), (220), and (222) planes (curve c). Strong, sharp diffraction peaks indicate that the ppy–MgO–CNT was well crystallized. An additional sharp peak is shown at the (002) plane, which indicates the presence of CNT (curve d). The equal ratio of ppy–MgO shows strong peaks. It was observed that the MgO particle interacts on the polypyrrole surface.

Fig. 2 XRD patterns of (a) uncoated, (b) ppy-coated, (c) M₃-coated, and (d) M₄-coated cotton.

3.2. FT-IR analysis

Fig. 3 shows the FTIR-ATR spectra of uncoated, ppy, ppy–MgO and ppy–MgO–CNT coated fabrics. In all coated and uncoated fabrics, the absorption peaks observed at 3300, 2899, 1679, 1426, 1240, 1157, and 1051 cm⁻¹ are ascribed to –OH stretching, –CH stretching, H–O–H bending, –CH bending, C–O–C asymmetric glucose ring, and C–O stretching, respectively, for cellulose on the cotton fabrics.⁵⁴ The ppy spectrum (curve b) shows the characteristic peak attributed to the C–H in-plane deformation vibration at 1051 cm⁻¹, N–C stretching band at 1240 cm⁻¹, and C–C asymmetric stretching vibration at 1427 cm⁻¹.⁵² A peak was observed at 1697 cm⁻¹ for the untreated fabric (curve a). This peak is shifted to 1716 cm⁻¹, which may be due to the cross-linking between cotton and BTCA (curves c and d). There is no structure change, just cross-linking of the cotton fabric and BTCA.⁵⁵ The hetero polar diatomic molecular vibrations of MgO were confirmed by the peak at 680 cm⁻¹.⁵⁶

Fig. 3 FTIR-ATR spectra of (a) uncoated cotton, (b) ppy-coated, (c) M₃-coated, and (d) M₄-coated cotton.

3.3. SEM analysis

Fig. 4 shows the surface analysis of the coated and uncoated fabrics to investigate the morphological change. The MgO nanoparticles, ppy–MgO, ppy–MgO–CNT nanocomposites are grafted on the fabric surface. The uncoated fabric shows a plain surface when compared to the ppy-, M₃- and M₄-coated fabrics. Image (b) shows the pebble-shaped MgO nanoparticles adhering to the fabric surface, whereas needle-like small agglomerated ppy–MgO nanocomposite particles are seen in image (c). The uniform coated ppy–MgO–CNT composite nanoparticles are shown in image (d). The EDAX images indicate the presence of magnesium, oxygen, and carbon in the coated fabric. From the above studies, the elemental wt% of the M₄ fabric (1.10) was found to be decreased when compared to that of the M₃ fabric (1.23) and uncoated fabric. It indicates there is the presence of CNT (71.22) nanoparticles, due to the increase in carbon coating on the M₄ coated fabric. These results confirm the effective coating of the nanoparticles on the surface of the fabric.

Fig. 4 FE-SEM images with EDAX micrographs of the (a) uncoated, (b) ppy-coated, (c) M₃-and (d) M₄-coated cotton fabrics.

3.4. Optical microscopic analysis

Fig. 5 shows the optical microscopic observation that the colorless uncoated fabrics were coated with composites, making them brownish in color. The surface modification of the coated fabric was confirmed by the crosslinking agents, which is present in the coated fabric.

Fig. 5 Optical microscopic images of the uncoated, and M₁–M₄ coated fabrics.

The composite particle in the microscopic images (Fig. 5) of M₁ to M₄ show the presence of ppy–MgO composite in the coated fabric.

3.5. TGA-DTG analysis

The thermogravimetric analyses with TGA (a) and DTG (b) are shown in Fig. 6. The samples were heated up to 600 °C at a heating rate of 10 °C min⁻¹ under a nitrogen flow rate of 20 ml min⁻¹. Image a summarizes the thermal degradation of the uncoated cotton and that of the coated cotton with M₃ and M₄. The TGA curves of cotton consist of three stages as the initial, main, and char decomposition regions. In the first stage, weight loss is due to some physical changes that occurred mostly in the amorphous region of the cellulose. In this stage, the mass loss between 50 °C and 205 °C is due to the loss of water and unreacted substances. In the second stage, the weight loss between 205 °C and 440 °C indicated the decomposition of polymer chains. It is stated by different researchers that glucose together with combustible gases are generated in this stage.⁵⁷ It is reported that thermal degradation in this stage takes place in the crystalline region of the cellulose fibers. The formation of char occurs in the third stage at the higher temperature of 500 °C. This could be due to de-watering and charring reactions, releasing water and carbon dioxide, and increasing charred residues.^58,59 The test sample cross-linked with CNT and BTCA showed superior degradation temperature in the second region. 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 CNT. The DTG curve revels that the burning step is broadened by the presence of ppy. Therefore, the associated heat of combustion is spread in a wider range of temperatures. These findings point out that ppy–MgO–CNT alters the combustion process of cellulose. The residual weight at 385 °C for all of the coated samples is more than double that of the uncoated cotton.

Fig. 6 TGA (a) and DTG (b) curves of the uncoated fabric, ppy–MgO composite-coated fabric, and ppy–MgO–CNT composite-coated fabric.

3.6. Evaluation for flame retardant

The combustion processes of pristine cotton fabric, ppy coated cotton fabric, M₃, and M₄ composite coated fabric were treated. The untreated cotton fabric and the composite-treated cotton samples were hung on a metal supporter, and then ignited with a lighter. The uncoated cotton caught fire immediately. With the progress of the burning, it burns to ashes completely. However, the M₃ and M₄ composite-coated cotton did not burn and just charred on the edge, forming a stiff sinter (transfer of CNTs). These results provide direct evidence of the flame-retardant effect of CNT on cotton textiles. The results of the vertical flammability test are summarized in Table 1. The results depict that the coatings decreased the flammability of the test samples. Noticeably, the BTCA cross-linked with ppy–MgO and ppy–MgO–CNT composite-coated cotton fabrics were effective in reducing flammability. The M₃ and M₄ composite-coated fabrics showed decreased char length and increased char yield. These results confirmed the effect of impregnation of phosphorus on the coated fabric. M₄ shows low char yield compared to other fabrics due to the effective and uniform deposition of the composite. Flame-retardant and super paramagnetic zinc ferrite (ZF) was adopted to decorate boron nitride nanosheet (BNNS) through a typical solvothermal method, so as to afford a ZF-BNNS nanofiller with improved flame-retardant performance. As a result, the density and strength of the carbon layers are increased in association with enhanced insulation shield effect to heat flux, oxygen and combustible pyrolysis products, as well as their suppression to release and transfer during combustion.⁶⁰

Table 1 Vertical flammability test and the results

Test sample	Flammability (45 °C)
	Char length (cm)	Char yield (%)
Untreated cotton	2.0	85
Ppy-coated cotton	1.9	65
MP1-coated cotton	1.0	48
MP2-coated cotton	1.4	47
MP3-coated cotton	0.5	44
MPN-coated cotton	0.3	49

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The optimized composite sol of M₃ and M₄ were applied to military uniforms and designated as MM₃ and MM₄, respectively. The uncoated fabric showed a char length of 7.5 cm and char yield of 72%. The MM₃-coated fabric shows a char length of 3.2 cm and char yield of 75%. The MM₄-coated fabric shows a char length of 2.1 cm and char yield of 87% (Fig. 7).

Fig. 7 Images of the vertical flame testing of the uncoated and composite-coated cotton fabrics (ppy, MM₃ and MM₄).

The size of the specimen should be to 8 cm × 10 cm.

If the specimen has a raised fiber surface, it has to be prepared by brushing.

If the specimen has a flame-retardant finish, then it has to be prepared by dry-cleaning, laundering and drying.

The dried specimen of the fabric is to be fixed on the side of the test area.

Standardized flame is to be applied to the fabric surface near the lower end.

The time required for the flame to proceed up the fabric a distance of 127 mm is measured as showed in Fig. 8.

Fig. 8 (a) Indigenously developed apparatus for flammability test. (b) Fabric sample mounted on the test area. (c) Fabric under flammability test.

Char yield is noted.

Char length is noted.

The burnt sample length is measured with a scale as the char length in cm value. The measurement of the char yield is an appropriate element in determining the flame retardancy.⁶¹ The weight of each sample before and after burning is measured, and the char yield is calculated as [W₂/W₁] × 100, where W₁ and W₂ are the weight of the sample before and after burning.

Table 1 shows that the coatings decreased the flammability of the samples. These results confirmed the impact of phosphorus deposited from SHP. Evidently, BTCA crosslinked with ppy–MgO and ppy–MgO–CNT composite-coated fabrics are effective in reducing flammability of treated fabrics in the presence of SHP. The char length of the samples was measured in the vertical flammability test, and the results are reported in Table 1. The differences in the burning behavior and char length in the vertical flame test between the uncoated sample and composite coated fabrics are the indicators of the flame retardancy and effect. It is noted that the increase of the MgO and CNT ratios has led to a decrease in the fabric flammability. It is interesting to observed that the charred surface of the M₃ and M₄-coated samples was very uniform, indicating that the nanoparticle was uniformly covered in the fabric surface.

3.7. UPF measurement

The ultraviolet protection factor (UPF) measurements were made using a Jasco V-670 spectrophotometer to determine the transmittance of the UV radiation in the conditioned samples, thus evaluating its UV protection properties. The UV-blocking results of uncoated cotton, ppy, M₁, M₂, M₃ and M₄ coated cotton fabrics are shown in Fig. 9(a–f), and the values are given in Table 2. Ultraviolet protection factor (UPF) was computed using Table 2.
where S_λ is the spectral irradiation in the UV region (280–400 nm); E_λ is the relative erythemal spectral effectiveness, T_λ is the spectral transmittance of the fabric; Δ_λ is the increment relating to the wavelength, and λ is the wavelength in nanometer.

Fig. 9 Transmittance versus wavelength for differently treated samples in UV analysis: (a) uncoated cotton, (b) pyrrole coated cotton, (c) M₁ coated, (d) M₂ coated, (e) M₃ coated, (f) M₄ coated.

Table 2 Effect of ratios of ppy–MgO and ppy–MgO–CNT composites coated on cotton for UV protection of uncoated and composite coated cotton fabrics

Composition	UPF value	UV protection
Uncoated cotton	6	Not considerable
Ppy coated cotton	13	Not considerable
M1 coated cotton	16	Good
M2 coated cotton	20	Good
M3 coated cotton	25	Good
M4 coated cotton	30	Good

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In the samples, M₁, M₂, M₃, and M₄, the reflectance values decreased, confirming the presence of UV blocking material (MgO nanoparticles). In coated ppy, the lowest reflectance is observed in the 200–385 nm region. This may be due to the higher possibility of contact between the UV wavelengths without the presence of polycarboxylic acid molecules.⁶²

3.8. Antibacterial activity

Agar diffusion assay. The zone of inhibition was observed for all of the test samples against MRSA and PAO1 bacteria. Uncoated cotton fabric containing DMSO served as the control (c). Among these, the MPN powder exhibited the maximum zone of inhibition (Fig. 10).

Fig. 10 Antibacterial activity (negative control) of © DMSO, (1) M₁, (2) M₂, (3) M₃ and (4) M₄.

The zone of inhibition was observed for all coated test samples against MRSA and PAO1 bacteria, when the uncoated cotton fabric containing DMSO is served as the control (c) (Fig. 11). In the meantime, the zone of inhibition data is given in Table 3.

Table 3 Antibacterial test results of the composite coated fabrics

Test sample	Zone of inhibition (mm)
	MRSA	PAO1
M1 composite	1.0	1.0
M2 composite	2.1	1.5
M3 composite	3.1	2.1
M4 composite	4.0	3.0

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Fig. 11 Antibacterial activity of the negative control (c) DMSO, (1) M₁ coated, (2) M₂ coated, (3) M₃ coated, and (4) M₄ coated cotton.

The results indicate that the coated fabric did not show a good zone of inhibition. The agar diffusion method result reveals that M₄ exhibited the maximum zone of inhibition against the test pathogens.

3.9. Physical properties of the fabrics

Water absorbency of the coated fabric was compared to that of uncoated fabrics. The absorbency was superior for the fabrics coated with M₃ and M₄ composite. The tensile strength of the M₄ coated fabric with catalyst has shown enhanced performance compared to the others (Table 4). This may be due to the intermolecular and intramolecular crosslinking, which reduces the possibility of getting even the stress distribution, causing reduction in the capacity to resist shipment. The tearing strength of sample M₄ has shown improved performance compared to the uncoated one. This may be due to the crosslinking ability of BTCA, hindered by the presence of hydroxyl groups in the structure. The sample coated with the composite showed a moderate increase in the crease recovery angle (CRA) values. The CRA angle of the composite sample M₄ reached up to 140°. This shows that the crosslinking affected by the esterification reaction of the coated cotton has imparted better crease recovery behavior. This may be because BTCA has one more carboxylic acid group, which bonds to the adjacent carbons in their molecular backbones.

Table 4 Physical properties of the fabrics

Specimen	Absorbency (seconds)	Tensile strength (lbf)		Tearing strength (lbf)		CRA (W + F)
		Warp	Weft	Warp	Weft
Uncoated	6	53.5	42.5	2.8	2.2	120
M1	8	56.5	49.4	2.4	2.1	116
M2	9	55.1	48.7	2.5	2.1	128
M3	10	56.5	49.2	2.7	2.2	134
M4	11	64.5	40.2	2.8	2.3	141

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4. Comparative study

Cotton fabrics coated with composites prepared from carboxylic acids and catalysts with metal oxide were evaluated for flame retardancy (Table 5), UPF and antibacterial applications. BTCA with SHP effectively increased the char formation. The nanoparticles deposited on the fabric surface showed stability both before and after washing, which was confirmed through SEM-EDAX analysis. The flammability of the M₄ coated fabric was found to be 0.3 cm better than that of the uncoated fabric. The UPF value of the M₄ coated fabric is 30, which suggests that the ppy–MgO–CNT coated cotton fabrics is a promising multifunctional textile material for military and UPF protection and antibacterial applications (Tables 6 and 7).

Table 5 Flame retardancy

Test fabric	Flame retardant (concentration)	Coating method	Char length (cm)	Ref.
Cotton	Ammonium aluminum sulphate dodecahydrate (0.2 M)	Pad-dry-cure	0.2	63
Cotton	Diethyl-4-methylpiperazin-1-yl phosphoramidate (3.64 ml)	Dip coating	2.3	64
Cotton	BTCA–CA–MF (70 : 70 : 65 g L^−1)	Pad-dry-cure	4.1	65
Wool	ZrOCl2–citric acid–formic acid (7.95 : 9.5 : 8.0%)	Dip coating	0.85	66
Silk	Dimethyl-l-2-(methacryloyloxyethyl)phosphate (20%)	Dip coating	4.5	67
Cotton/polyester	BTCA–uccinic acid–ZnO (0.5 : 0.5 : 0.25%)	Pad-dry-cure	0.30	68
Cotton	Ppy–MgO–CNT (12.5 g : 1.0 g : 0.33 ml : 1.0 g)	pad-dry-cure	0.3	Our work

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Table 6 Ultraviolet protection

Test fabric	UV blocking agent (concentration)	Coating method	UPF	Ref.
Cotton	Copper acetate (0.5 M)	Pad-dry-cure	42.0	69
Cotton	Titania nanosols (45 ml)	Sol–gel process and pad-dry- cure	47.0	70
Cotton	Polyurethane–ZnO (15 : 1 wt%)	Electrospinning	50	71
Cotton	Titania-silica nanosol (6 ml)	Dip-dry-cure	73	72
Cotton	PU–nano-TiO2 (15 g L^−1 : 0.1 wt%)	Layer-by-layer deposition	50	73
Cotton	Chitosan–TiO2 (4% w/v : 1 g)	Pad-dry-cure	50	74
Cotton	TiO2 nanoparticles (3 wt%)	Linking agent anchoring	50	75
Cotton	Needle-shaped ZnO nanorod (0.001 M)	In situ growth	105.1	76
Cotton	CA–SHP–nano-TiO2 (50 : 25 : 5 g L^−1)	pad-dry-cure	135.9	77
Cotton	PU–MWNT (2.5 : 15 wt%)	Pad-dry-cure	421.0	78
Cotton	Chitosan–graphene (2 : 1.0 wt%)	Pad-dry-cure	465.8	73
Cotton	Dumbbell-shaped ZnO crystallites (0.01 M)	Low-temperature growth and hydrothermal	800	75
Wool	CuSO4 (10 ml L^−1)	Dyeing process	65.0	74
Polyester	Silk sericin (5 ml)	Pad-dry-cure	30.9	75
Polyester	Alginates and nano-TiO2 (0.1% : 0.1 M)	Dip-dry-cure	119.8	79
Polyester	Nano structured silver (72.55 wt%)	Magnetron sputtering	302.1	80
Nylon	ZnO–PMMA nanocomposites (1 g : 5 wt%)	Pad-dry-cure	19.4	81
Cotton	Ppy–MgO–CNT (12.5 g : 1.0 g : 0.33 ml : 1.0 g)	Pad-dry-cure	30	Our work

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Table 7 Antibacterial activity

Test fabric	Antibacterial agent (concentration)	Coating method	Zone of inhibition (mm)		Ref.
			S. aureus	E. coli
Cotton	Ag-NPs colloidal solution (100 ppm)	Dip coating	0.5	1.0	82
Cotton	5,5-Dimethylhydantoin (32 g)	Plasma and pad-dry-cure	1.36	0	83
Cotton	ZnO–ureolytic (0.02 M : 1 g L^−1)	Dip coating	1.6	2.0	84
Cotton	Ag–polyvinyl pyrrolidone colloid (50 μg ml^−1 : 240 ppm)	Dip coating	12	11	85
Cotton	Punica granatum L. and Terminalia chebula plant extract (10 g)	Pad-dry-cure	30	28	86
Cotton	Poly-N-vinyl-2-pyrrolidone–ZnO (2% : 20 mg L^−1)	Pad-dry-cure	38	47	87
Polyester	Al2O3–Ag–TiO2 (1 : 1 g)	Pad-dry-cure	12	11	88
Cotton/polyester	Triclosan and Siligen (15 : 3%)	Pad-dry-cure	47	38	89
Cotton	Ppy–MgO–CNT (3 : 0.2 : 0.8 g)	Pad-dry-cure	4.0	3.0	Our work

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Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is financially supported by the Natural Science Foundation of China (NSFC) as a Shandong-NSFC joint project (grant No. U1906222) and the Ministry of Science and Technology, People’s Republic of China as a key R&D project (grant no. 2019YFC1804104). The authors would like to thank Tianjin Bureau of Foreign Experts and Nankai University (Young Foreign Talent Scheme) for funding and facilities to carry out this research work. It was also supported by the 111 program, the Ministry of Education, People’s Republic of China (grant No. T2017002).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1ra07460a

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