Antibacterial Finishing of Vat Dyed Cotton Fabrics with a Reactive N -Halamine

Abstract

To obtain antibacterial properties for colored cotton fabrics, vat dyes were chosen to dye cotton because they can avoid serious discoloration during chlorination. In this study, we synthesized a reactive N-halamine precursor, 4-(4-(2,2,6,6-tetramethyl-4-piperidinol)-6-chloro-1,3,5-triazinylamino)-benzenesulfonate (BTMPT), and coated it on colored cotton fabrics that were dyed with three different vat dyes. The optimum pH for chlorination of the treated cotton fabrics was investigated. Chlorination at pH 11 achieved a small color difference and greater than 0.2% of active chlorine loading. When challenged, the chlorinated fabrics inactivated all inoculated Staphylococcus aureus (Gram-positive) and Escherichia coli (Gram-negative) within 5 min. The treated cotton fabric had greater breaking strength than those treated with the traditional process, as well as good washing stability.

Introduction

Cotton fabric is one of the most widely-used fabrics worldwide. However, it is susceptible to microorganism contamination, especially in hot and humid environments, which directly or indirectly affect the performance of the fabric. ¹ It can provide a vehicle for the direct or indirect transmission of some microbes, which can have a negative effect on human health. Therefore, the application of an antibacterial finishing for cotton fabrics has drawn much attention from researchers.^2–5 Antibacterial agents for textiles mainly include organic,^6–8 inorganic,^9,10 and natural antibacterial agents.^11,12 In these applications, commercial antimicrobial agents have their limitations. N-Halamine antimicrobial agents contain the structure of N-X (where X is a halogen), and can be grouped into amines, amides, and imides. They have high antimicrobial activities, resulting in a long and stable bactericidal effect. Fast inactivation rate and reproducible antibacterial abilities give them advantages over other antibacterial agents.^13–16 During the bactericidal process, the oxidative halogen atom is consumed, and the N-X converts into N-H, thus losing antibacterial activity. However, the N-H can be oxidized back to N-X again by chlorination to restore the bactericidal activity.^17–20

N-Halamines have been extensively studied.^21–24 However, in various applications, there are few studies on the N-halamine coating of dyed cotton fabrics. ²⁵ This is because cotton fabrics coated with N-halamine precursors need to be chlorinated with a sodium hypochlorite solution for antibacterial activity. Most cotton fabrics dyed with reactive dyes have poor chlorine bleaching fastness, since they are not resistant to chlorination.^26,27 Vat dyes, however, have excellent comprehensive fastness, including chlorine bleaching fastness. The vat dyeing of cotton fabric relies on intermolecular forces between dye and fabric, and the reactive hydroxyl groups on the cotton fabric makes further functional finishing possible.^28–30 Generally speaking, fabrics dyed with vat dyes have good washing performance, and do not cause a serious color fading problem after chlorination. ³¹ After antibacterial finishing, cotton fabrics dyed with vat dyes can gain antibacterial properties, while maintaining the original color performance.

In this study, cotton fabrics were dyed with three vat dyes, and the dyed cotton fabrics were coated with a synthesized, reactive N-halamine precursor, 4-(4-(2,2,6,6-tetramethyl-4-piperidinol)-6-chloro-1,3,5-triazinylamino)-benzenesulfonate (BTMPT), by using a reactive dye dyeing process. After chlorination, the coated dyed cotton fabrics acquired antibacterial activity (Fig. 1). BTMPT has a similar structure to triazine reactive dyes, which can react with cellulosic fibers and form covalent bonds. Through a reactive dyeing processes under medium temperature and chlorination, colored cotton fabrics obtained antibacterial activity.

Fig. 1

Biocidal and regeneration mechanism of N-halamine.

Experimental

Materials

Bleached cotton fabrics were provided from Zhejiang Guangdong Textile Dyeing Garment Co. Ltd. Cyanuric chloride (99%) was obtained from J & K Chemical Co. Ltd. 2,2,6,6-Tet-ramethyl-4-piperidinol was purchased from Jiaxing Sicheng Chemical Co. Ltd. C.I. Vat Brown BR, C.I. Vat Olive, and C.I. Vat Yellow 1 were provided from Yorkshire Chemical Co. Ltd. Other chemicals were purchased from Sinopharm Chemical Reagent Co. Ltd. and used without further purification.

Instruments

An Avance III 400 MHz digital nuclear magnetic resonance (NMR) spectrometer (Bruker AXS GmbH) was used to characterize the synthesized product (BTMPT). Fourier transform infrared (FTIR) spectra of cotton and coated cotton fabrics were recorded on a Nicolet Nexus spectrometer (Nicolet Instrument Corp.) using the attenuated total reflectance (ATR) method. Surface morphologies of uncoated and coated cotton fabrics were investigated using a SU1510 spectrometer (Hitachi). The color parameters of dyed cotton were measured on a Datacolor SF 600 spectrophotometer (Datacolor).

Preparation of BTMPT-Coated Dyed Cotton Fabrics

The cotton fabrics were dyed with three vat dyes (Vat Brown BR, Vat Olive, and Vat Yellow 1, Fig. 2a–c) through the leuco-dyeing process. Dyeing solution (vat dye 2% owf, NaOH (15 g/L), and Na₂S₂O₄ (20 g/L) was prepared and heated to 60 °C for 10 min, followed by adding the cotton fabric. After 40 min, the cotton fabric was removed, evenly squeezed, and placed in the air for 10 min. The dyed cotton fabric was finished with water washing, soap washing using Na CO (2 g/L) and soap fakes (Sinopharm Chemical Reagent Co. Ltd., 2 g/L) with a liquor ratio of 1:30 at 95 °C for 10 min), and drying.

Fig. 2

Structures of (a) Vat Brown BR, (b) Vat Olive, (c) Vat Yellow 1, and (d) BTMPT

BTMPT (Fig. 2d) was synthesized and coated onto the dyed cotton fabrics according to the literature procedure. ³²

Chlorination and Titration

The pH of diluted commercial sodium hypochlorite solutions (2 g/L active chlorine) were adjusted to 7, 9, and 11 by dilute sulfuric acid solution, and the BTMPT-coated dyed cotton fabrics were immersed in these solutions for 3 h at ambient temperature. Then, the chlorinated cotton samples were washed thoroughly with distilled water and dried at 45 °C for 1 h to remove free chlorine residues on the surface.

The chlorine loadings on fabric samples were determined by iodometric titration. The samples were immersed in KI solution, and a few drops of 1% starch solution were added as indictors. After thorough soaking, the mixture was titrated with 0.001 N sodium thiosulfate solution. The titration ended when the color of the solution turned to colorless. The concentrations of active chlorine on the cotton samples were calculated according to Eq. 1.

$$\left[{\text{Cl}}^{+}\right]\%=\left(35.45\text{NV}\right)/\left(2\text{W}\right)\times 100$$

Eq. 1

[Cl⁺] % is the wt% of oxidative chlorine on the sample, N and V are the normality (equiv/L) and volume (L) of the titrant sodium thiosulfate, respectively, and W is the weight of the cotton sample (g).

Color Measurement

The color parameters (K/S value and color difference) of the colored cotton fabrics were measured on the Datacolor SF 600.

The color differences (ΔE) of the dyed fabrics before and after chlorination were calculated according to Eq. 2.

$$\Delta E=\sqrt{{\left(\Delta L\right)}^2+{\left(\Delta a\right)}^2+{\left(\Delta b\right)}^2}$$

Eq. 2

ΔE indicates the color difference of the fabric, and ΔL, Δa, and Δb represent the difference of L, a, and b values of the cotton fabric before and after chlorination, respectively.

Antimicrobial Test

According to AATCC TM100-2004, ³³ control and unchlorinated/chlorinated coated fabric samples were challenged with S. aureus (ATCC 6538) and E. coli O157:H7 (ATCC 43895) using a sandwich test method. ³⁴ The fabrics (0.4 g) were cut into 2.54 × 2.54 cm samples. Bacteria were suspended in pH 7, 100 mM phosphate buffer, and 25 μL of the bacterial suspension were added to the middle of two pieces of the coated cotton samples. After 1, 5, 10, and 30 min contact time, the samples were quenched with 5 mL of 0.02 N sodium thiosulfate solution to remove all the oxidative chlorine. Then, the solution was diluted serially by 100 mM, pH 7 phosphate buffer and placed on trypticase soy agar plates. The plates were incubated at 37 °C for 24 h, and bacterial colonies were recorded for antimicrobial activity analysis. The control sample was tested with the same method mentioned above at the contact time of 30 min.

Washing Stability Test

The durability and stability of BTMPT and chlorine on the coated cotton samples against standard washing cycles were evaluated according to AATCC TM61-2010. ³⁵ Stainless steel canisters containing 0.15% AATCC detergent water solution (150 mL) and 50 stainless steel balls were placed in a Launder-Ometer (SDL Atlas) and rotated at 42 rpm and 49 °C. The coated cotton samples (2.54 × 5.08 cm) were subjected to 5, 10, 25, and 50 washing cycles. Each washing cycle of 45 min, was equivalent to five machine washings. All samples were washed with distilled water and then dried at ambient temperature. The chlorine loadings of half the samples were determined directly, and the other half of the samples were chlorinated again, and the chlorine loads were measured. The chlorine loadings on the samples were determined by the titration method discussed previously.

Breaking Strength Test

An electronic fabric strength tester, YG(B)026D-250 (Darong Textile Instrument Co. Ltd.), was used to evaluate the breaking strength of untreated cotton, dyed cotton, and unchlorinated and chlorinated dyed cotton fabrics according to the GB/T3923-2013 method. The measurement was carried out at ambient temperature. Tree replicates were prepared for each sample (5 × 20 cm), and the average value was recorded for analysis.

Results and Discussion

Characterization of BTMPT-Coated Dyed Cotton Fabrics

The FTIR-ATR spectra of Vat Brown BR-dyed cotton fabrics that were uncoated, those coated with BTMPT before chlorination, and those coated with BTMPT after chlorination are shown in Figs. 3a–c. respectively. New absorption peaks on the unchlorinated treated fabric appeared at 2846 and 1560 cm^–1, which were attributed to methyl and benzene ring telescopic vibrations from BTMPT, providing evidence that BTMPT was coated onto the cotton fabrics. After chlorination, the above two peaks were shifted to 2852 and 1564 cm^–1 respectively, due to conversion of N-H to N-Cl. The affinity of oxidative chlorine for the electron cloud density of the adjacent methyl and benzene rings led to the absorption peak shift to higher values.^32,37

Fig. 3

FTIR-ATR spectra of (a) dyed cotton, (b) dyed cotton-BTMPT, and (c) dyed cotton-BTMPT-Cl.

Fig. 4a–c shows the surface morphologies of uncoated and coated cotton fabrics. The surface of uncoated cotton was smooth, and the surface of the Vat Brown BR-dyed cotton fabric had some impurities. The surface of the Vat Brown BR-dyed cotton fabric coated with BTMPT became rough due to the bonding of BTMPT on the cotton surface.

Fig. 4

SEM micrographs of (a) cotton, (b) dyed cotton, and (c) dyed cotton-BTMPT.

Effect of Chlorination pH on Chlorine Content and Color

In this study, Vat Brown BR, Vat Olive, and Vat Yellow 1, having good color fastness to chlorine bleaching, were used to dye cotton fabrics. Sodium hypochlorite solutions at various pH values were prepared to chlorinate the BTMPT-coated dyed cotton fabric. The color changes and chlorine loadings of the dyed cotton fabrics before and after chlorination were then investigated. With the decrease of pH of chlorination, the chlorine loadings on BTMPT-coated dyed cotton fabrics increased gradually (Fig. 5). When the pH of chlorination reached 11, the chlorine loadings on Vat Brown BR-dyed cotton fabric were 0.21%, the K/S value decreased from 5.2 to 4.8, and the ΔE value (2.2) was the least. The chlorine loadings met the antibacterial activity requirements, and the original color of the dyed fabric was well preserved. When the pH of chlorination was 7, chlorine loadings on the fabric reached 0.31%, the K/S value decreased to 4.2, and the ΔE value (4.1) was the greatest. The cotton fabrics dyed with Vat Olive and Vat Yellow 1 and chlorinated at pH 11 showed higher ΔE values of 4.3 and 7.4, respectively. The chlorine loadings were 0.21% and 0.24%, respectively. When the chlorination pH was decreased to 7, the chlorine loadings on two antibacterial dyed cotton fabrics both reached 0.29%, the K/S of the two fabrics decreased from 6.5 to 4.1 and 5.7 respectively, and the ΔE values increased to 17.6 and 11.2, respectively.

Fig. 5

Effect of chlorination pH on the (a) chlorine loading, (b) K/S value, and (c) ΔE values.

Antimicrobial Properties

The N-halamine antibacterial precursor BTMPT is water-soluble owing to its sulfonate group. Bacteria and other microorganisms can be easily adsorbed on the surface of BTMPT-coated fabric, which increases the contact probability between bacteria and BTMPT to achieve bacterial reduction. ³⁸ The chlorine loadings on the fabric reflect its antibacterial property; higher active chlorine loadings resulted in better antibacterial activity.

Antibacterial activities of the Vat Brown BR-dyed cotton fabrics coated with BTMPT after chlorination with a chlorine load of 0.28% are shown in Table I. Fabrics were challenged with S. aureus and E. coli O157:H7 at 1.07 × 10⁶ CFU/sample and 1.47 × 10⁶ CFU/sample, respectively. The chlorinated BTMPT-coated dyed cotton showed excellent antibacterial activity and inactivated 100% of S. aureus and E. coli O157:H7 within 5 min. The bacterial reduction of dyed cotton fabrics may due to the adhesion of the bacteria and mild antibacterial property of anthraquinone-based vat dyes.^39,40

Table I

Antibacterial Properties of Vat Brown BR Dyed Cotton Fabrics Coated with BTMPT

Samples	Contact Time (min)	S. aureus Reduction a		E. coli O157: H7 Reduction b
		Average (%)	Average (log)	Average (%)	Average (log)
Untreated Cotton	30	64.31	0.48	66.73	0.48
Dyed Cotton	30	91.73	1.08	82.22	0.75
Dyed Cotton-BTMPT	30	90.61	1.03	91.34	1.06
Dyed Cotton-BTMPT-Cl Cl+% = 0.28	1	99.96	3.36	91.80	1.09
	5	100	6.03	100	6.17
	10	100	6.03	100	6.17
	30	100	6.03	100	6.17

a

Inoculum was 1.07 × 10⁶ CFU/sample.

b

Inoculum was 1.47 × 10⁶ CFU/sample.

Washing Stability

Antibacterial fabrics coated with N-halamine might lose chlorines (including recoverable and unrecoverable losses) during washing cycles. The degree of the loss depends on bonds between antibacterial agents and fibers, and the strength of the binding force is determined by the structure and the molecular weight of the antimicrobial agent. ⁴¹ The washing stabilities of chlorine in Vat Brown BR-dyed cotton fabric coated with BTMPT before and after chlorination are shown in Fig. 6.

Fig. 6

The washing stabilities of chlorine in dyed antibacterial cotton fabrics.

The chlorine loadings gradually decreased with the increase of washing cycles. The chlorine loadings on cotton fabric after 50 washing cycles remained 0.16%, which is sufficient to inactivate bacteria. ³² The loss of chlorine loadings can be recovered to at least 78.6% of initial content after re-chlorination.

Breaking Strength

The breaking strengths of the Vat Brown BR-dyed cotton fabrics before and after coating with BTMPT were tested and the results are shown in Fig. 7. The strength of cotton fabrics decreased after dyeing and antibacterial finishing, and was directly affected by process temperature, acidity, and alkalinity, among other factors. The effect of dyeing on the strength of cotton fabric was greater than that of antimicrobial finishing, most likely attributed to a greater effect on the fluidity of fibers in and between molecules when dyed. ⁴² The strength of the cotton fabric in warp and weft directions were 665 N and 304 N and decreased to 614 N and 288 N after dyeing, respectively. After coating with BTMPT, the strength of the dyed cotton fabric was further decreased slightly both in warp and weft to 599 N and 284 N, respectively. The strength reduction was small compared with untreated cotton, and the retention rates of cotton fabric in warp and weft were 90% and 93.4%, respectively. After chlorination, the strength of the antibacterial cotton fabric reduced further to a small degree, and the retention rates in warp and weft were 87.8% and 79.6%, respectively.

Fig. 7

Breaking strengths of the dyed cotton fabrics coated with BTMPT.

Conclusion

Colored cotton fabrics with excellent antibacterial properties were obtained by Vat Brown BR dyeing followed by coating with the N-halamine precursor BTMPT. The low pH value of chlorination could cause serious color fading, especially for the Vat Olive and the Vat Yellow 1-dyed cottons. Based on the Vat Brown BR-dyed fabric antibacterial activity results, it can be assumed that the other two dyed and treated fabrics prepared in this study obtained a sufficient chlorine load to inactivate bacteria at pH 11 as well.

The Vat Brown BR-dyed antibacterial cotton achieved more than 0.2% of chlorine loadings as well as a small color change of 2.2. The chlorinated BTMPT-coated Vat Brown BR-dyed cotton inactivated 100% of S. aureus and E. coli O157:H7 within 5 min, showing excellent antibacterial activity. Machine washing led to the loss of chlorine loadings on the Vat Brown BR-dyed and coated fabrics. About half of the active chlorines remained after 50 washing cycles, and over 78% of the original chlorine content remained after re-chlorination. Dyeing and antibacterial finishing caused the decrease of cotton fabric strength, and the loss of strength was less under mild conditions than that for the traditional processes.