Enhancing the performance properties of ester-cross-linked cotton fabrics using Al 2 O 3 -NPs

Abstract

In this research, use of aluminum oxide nano-particles (Al₂O₃-NPs) as a catalyst and co-catalyst in the wrinkle-resistant finishing of cotton fabric with 1,2,3,4-butanetetracarboxylic acid (BTCA), was investigated. For this, cotton fabrics were cross-linked with BTCA catalyzed by Al₂O₃-NPs or sodium hypophosphite (SHP) in the presence of Al₂O₃-NPs as the co-catalyst. Surface morphology and chemical composition of the treated fabrics beside the fabric properties, such as wrinkle resistance, flame retardancy, yellowing, and air permeability, were also evaluated. Scanning electron microscope and energy-dispersive X-ray microanalysis proved the presence of Al₂O₃-NPs on the fabric structure. The cross-linking of the fabrics with BTCA catalyzed by Al₂O₃-NPs or SHP was identified by Fourier transform infrared spectroscopy. The fabric test results indicated that the Al₂O₃-NPs co-catalyst could act as a multifunctional finishing material. The flame retardant property of the fabric, besides wrinkle resistance, was improved. It was also concluded that Al₂O₃-NPs could enhance the finishing performance and minimize the side effect of SHP.

Keywords

textile care/wrinkling cotton performance properties multifunctional finishes ester-cross-linking aluminum oxide nano-particle co-catalyst

Apparel products made from natural cotton fibers have a large share of the textile market due to their excellent properties, such as breathability, high moisture capacity, softness, hypoallergenic, and anti-static properties.¹ However, cotton fabrics have some disadvantages, such as easy wrinkling and shrinkage in practical applications.² It has been believed that chain slippage under moist conditions is responsible for wrinkling. Therefore, cross-linking adjacent cellulose chains could be logically a way of improving the crease recovery of the cotton fabrics. Cellulose cross-linkers can be divided into two groups: those that form three-dimensional polymers by self-polymerization reactions, as well as cross-linking reactions with cellulose, and those that form covalent bonds by reacting with hydroxyl group of cellulose in such a way that a cross-linked structure emerges.^3–5

The cross-linkers are also known as easy care, wrinkle-resistant, crease recovery, or durable press finishing agents. Recently, many extensive researches have been made to use formaldehyde-free polycarboxylic acids to replace the traditional formaldehyde-based cross-linkers.⁶ Among the various effective polycarboxylic acids, 1,2,3,4-butanetetracarboxylic acid (BTCA) is the most effective ester-type formaldehyde-free cross-linking agent for cotton fabrics with respect to the level of wrinkle-resistant properties, speed of curing, and laundering durability.^2,3 The esterification mechanism of cotton cellulose with BTCA is realized in two steps. In the first step, the formation of a cyclic anhydride intermediate by the dehydration of two adjacent carboxylic acid groups is carried out. In the second step, the ester-cross-links are formed by the reaction between the anhydrides and the hydroxyl groups of the cellulose macromolecules.^2–6 The formation of cyclic anhydrides without a catalyst at lower temperatures is slow. On the other hand, usage of a catalyst may accelerate both the formation of anhydride and its reaction with cellulose. Therefore, alkaline metal salts of phosphorus-containing mineral acids, such as sodium hypophosphite, disodium phosphite, or monosodium phosphate, have been used as catalysts to accelerate the reaction at a high-temperature curing process.^2,7 Among these catalysts, SHP is the most effective catalyst for the wrinkle-resistant treatment with a BTCA cross-linker. However, the use of SHP as a catalyst has several disadvantages, such as (i) mechanical strength loss, (ii) high cost, and (iii) tendency to cause shade changes in fabrics dyed with sulfur or reactive dyes due to the reducing action of SHP on azo groups or sulfur-containing substituent.⁸ In additionally, phosphine, which occurs during the decomposition of the SHP, can cause the creation of toxic and flammable gas during condensation and consume the oxygen in rivers and lakes.^8–10

These cases also limit the using of BTCA as a cross-linking agent in the textile industry. Therefore, using nano-size salt, metal, and metal oxides, such as MgCl₂, Ag, TiO₂, SiO₂, and ZrO₂ as the acting catalyst or co-catalyst in the treatment with polycarboxylic acids has recently been investigated to minimize the side effects of phosphorous-based catalysts. Besides minimizing the side effects of the SHP catalyst by using nano-size metal oxides as the catalyst in cross-linking of the cotton with polycarboxylic acid agents, multi-functional properties could be given to the fabrics.^6,11,12

A literature survey indicated that there is no investigation about the characteristics of aluminum oxide nano-particles (Al₂O₃-NPs) acting as a catalyst or co-catalyst in the wrinkle-resistant finishing treatment of cotton fabrics with BTCA. In our study, it was aimed to investigate the characteristics of Al₂O₃-NPs as a catalyst or co-catalyst in a BTCA wrinkle-resistant finishing system. Therefore, the probability of using Al₂O₃-NPs as catalyst to replace SHP and co-catalyst for reducing the concentration of SHP compound was studied. According to the findings in the literature, Al₂O₃-NPs provide better strength properties, air permeability, flame retardant, and ultraviolet (UV) protection properties for the fabrics.¹³ For this reason, we also studied the possibility of manufacturing functional cotton fabrics by means of Al₂O₃-NPs application through one bath pad-dry-cure application process. As a result, in the present study, various features of the treated fabrics, including surface morphology, flame retardancy, tensile strength, and air permeability, together with wrinkle resistance, were also evaluated and thoroughly discussed.

Experimental details

Materials

In this study; desized, scoured, and bleached plain weave (100% cotton) fabrics with a weight of 130 g/m² (28 yarns per cm in weft direction and 55 yarns per cm in warp direction) supplied from Anteks (Turkey) were used. The cross-linking agent was BTCA (with the purity of 99%, Sigma Aldrich). Analytical grade SHP (with a purity of 99%, Sigma Aldrich) was used to catalyze the esterification reaction between BTCA and cellulose. Al₂O₃-NPs (Sigma Aldrich, Al₂O₃, <50 nm powder) was also used as a catalyst/co-catalyst.

Cross-linking of the fabrics with BTCA

Aqueous solutions of BTCA containing different concentrations of catalysts were prepared at room temperature by stirring at 10,000 rpm for half an hour with a homogenizer. The cotton fabric samples were immersed in the prepared solutions for 5 minutes and then passed through squeeze rollers at the conditions of 2 bar pressure and 2 m/min speed by using foulard to reach an average wet pick up of 90%. Padded fabrics were pre-dried and then cured at the conditions given in Table 1 to complete cross-linking reaction of BTCA on cotton. In the study, SHP was used as a catalyst. Al₂O₃-NPs were used as catalyst to replace SHP and co-catalyst for reducing the concentration of SHP compound. For this, the concentration of SHP was reduced from 4% to zero when Al₂O₃-NPs were added to the solution. The amount of materials added in solution is given in Table 1. The cross-linked cotton fabric samples were pre-conditioned at 20 ± 1℃ and 65 ± 2% relative humidity for 24 hours prior to testing and analysis.

Table 1.

The cross-linking treatment conditions of the fabric samples

	Concentration of chemical agent (w/v)
Samples	BTCA (%)	SHP (%)	Nano Al₂O₃ (%)	Padding conditions	Drying and curing conditions
CO-BTCA	8	0	0
CO-SHP-2	8	2	0
CO-SHP	8	4	0
CO-SHP-A-0.5	8	3.5	0.5	2 bar pressure	Drying at 85℃ for 3 minutes
CO-SHP-A-1	8	3	1	2 m/min	&
CO-SHP-A-2	8	2	2		Curing at 180℃ for 3 minutes
CO-SHP-A-3.5	8	0.5	3.5
CO-A-4	8	0	4

BTCA: 1,2,3,4-butanetetracarboxylic acid; SHP: Sodium Hypophosphite.

Scanning electron microscope and energy-dispersive X-ray analysis

The surface morphology of untreated and treated cotton fabric samples was examined by the Philips XL-30 S FEG Scanning Electron Microscope with an accelerating voltage of 4 kV and a current of 10 µA at a high magnification power up to 5000×. The number of Al₂O₃-NPs on the fabric structure was also determined by energy-dispersive X-ray (EDX) microanalysis attached to the scanning electron microscope (SEM) instrument.

Fourier transform infrared spectroscopy analysis

The Perkin Elmer Spectrum BX of the Fourier Transform Infrared Spectrophotometer was used to investigate chemical compositions of cotton fabric samples cross-linked with BTCA. The spectroscopic analysis of treated and untreated cotton fabric samples were examined by KBr technique at 4 cm⁻¹ resolution with 2 cm⁻¹ intervals and 16 scan numbers. The same amount of fabric was cut into small pieces, and mixed in KBr to prepare the KBr disk. The scanning range was between 4000 and 400 cm⁻¹ during Fourier transform infrared (FT-IR) analysis.

Wrinkle-resistance test

Wrinkle-resistant performance of the untreated and treated cotton fabric samples was determined according to the test method of TS 390 EN 22313/April 1996 Standard. Specimens were prepared in 40 mm × 10 mm swatches and 1000 ± 5 g of weight was loaded on the folded specimens. The recorded vertical angle guidelines were aligned and the wrinkle recovery angles (WRAs) were measured at 0.5, 30, and 60 minutes after removal of the weight. Measurements were repeated five times in both the warp and weft directions and the mean value of the WRA of each sample was calculated.

Fabric flammability test

Flammability of all of the fabric samples was measured in accordance with ASTM D1230-94 (“Standard Test Method for Flammability of Apparel Textiles”, reapproved in 2001) by using a 45° flammability tester BV AFC Auto test instrument. After finishing the test, the time required for the combustion of the test length of the fabric sample was recorded as the burning time. Finally, the results of the average of five ignition and burning time measurements were calculated to determine the flammability classifications.

Char yield calculation

The evaluation of char yield is an appropriate factor for studying the influence of the flame retardant function. For this reason, the weight of each sample before and after the flammability test was measured and then the char yield was calculated according to Equation (1):

Charyield = (W_{2} / W_{1}) \times 100

(1)

Here, W₁ and W₂ represent the weight of the sample before and after burning, respectively.^14–16

Washing of the treated fabrics

To investigate durability of the finishing treatment on the fabric against repeated washing, treated fabrics were washed in a Gyrowash machine for 30 min at 40℃, in accordance with TS EN ISO 105 C 06/A15 at 40℃ for 30 minutes by using ECE standard detergent without optical brightener. Samples washed were rinsed by tap water and then they were dried by hanging.

Fabric whiteness and yellowness

The changes in the values of whiteness and yellowness of the fabric samples were measured by using a spectrophotometer of X-Rite 938 in accordance with ASTM E313-05. The changes in the value of whiteness and yellowness were measured by taking the untreated fabric as reference samples. Color studies are quantified by the CIELAB with a three-axis system.¹⁷ This study was focused on the L value, whiteness, and yellowness index to determine of the yellowing of the treated fabrics. The result was the average of the five measurements taken for each fabric sample.

Air permeability test

Air permeability of the fabrics was measured according to the test method TS 13934-1/1999 EN ISO 9237/1995 Standard using TexTest Instruments FX 3300 Air Permeability Tester III at 100 Pa pressure. The test result was the average of the 10 measurements taken for each fabric sample.

Tensile strength test

The untreated and treated cotton fabric samples were also tested according to EN ISO 13 934-1/1999 Standard using the Lloyd LR5K Plus electronic tensile strength machine to investigate the tensile strength of the fabrics. Five samples of both of warp and weft direction were tested for each fabric and the average values were calculated as warp and weft tensile strength of the samples.

Evaluation of test results statistically

Evaluation of the test results was made using PASW Statistics 18.0 for Windows statistical software. Analyses of variance (ANOVAs) were applied to determine the statistical importance of the variations of the test results. To deduce whether the parameters were significant or not, p values were examined. If the value is greater than 0.05 (p > 0.05), the difference will not be important and should be ignored.

Results and discussion

SEM analysis results

Figure 1(a) presents the SEM image of the untreated fabric specimen. According to the SEM image, the cotton sample has smooth fiber surface and normal spiral structure of the cotton fibers can be clearly defined.

Figure 1.

Scanning electron microscope images and energy-dispersive X-ray graphics of untreated (a) and cross-linked cotton samples with 8% 1,2,3,4-butanetetracarboxylic acid (BTCA), (b) CO-SHP: catalyzed by 4% SHP , (c) CO-SHP-A-0.5: catalyzed by 3.5% SHP and 0.5% aluminum oxide nano-particles (Al₂O₃-NPs), (d) CO-SHP-A-1: catalyzed by 3% SHP and 1% Al₂O₃-NPs, (e) CO-SHP-A-2: catalyzed by 2% SHP and 2% Al₂O₃-NPs, (f) CO-SHP-A-3.5: catalyzed by 0.5% SHP and 3.5% Al₂O₃-NPs, and (g) CO-A-4: catalyzed by only 4% Al₂O₃-NPs.

Figure 1(b) illustrates the SEM image of the cotton sample cross-linked with 8% BTCA catalyzed by SHP. The image given shows that the surface of the fibers has the deposition of the BTCA cross-linking agent. The morphology of fibers changed after undergoing treatment with 8% BTCA catalyzed by 4% SHP at low pH value, that is, pH 1.8. The morphology of the fabrics treated with 8% BTCA catalyzed by (c) 3.5% SHP and 0.5% Al₂O₃-NPs, (d) 3% SHP and 1% Al₂O₃-NPs, (e) 2% SHP and 2% Al₂O₃-NPs, (f) 0.5 % SHP and 3.5% Al₂O₃-NPs, and (g) 4% Al₂O₃-NPs were also examined with SEM images, which are given in Figures 1(c)–(g), separately. According to the SEM images, there was a morphological change on fiber surfaces and the large number of Al₂O₃-NPs deposited on the surface of the fabrics. SEM images also showed that the Al₂O₃-NPs were uniformly deposited on the fiber surface or interstices between the fibers. However, it was also observed that agglomeration of these particles was to a certain extent due to the surface attraction between nano-size particles. Agglomeration of nano-particles on the fabric surface increased with the high concentration of co-catalyst Al₂O₃-NPs added in BTCA solution.

The EDX spectrum has been used to analyze the absorbed particles that exist on the surface of treated cotton.^18,19 The results of the EDX analysis of the specimens were reported based on the atomic percentages (%A) of the detected elements. The EDX spectrums and the atomic percentage of different elements detected in the untreated and cross-linked cotton fabrics are given in Figure 1 and Table 2, respectively. Through the obtained results, it was confirmed that Al₂O₃-NPs were absorbed on the surface of the treated fabrics. The atomic percentage of the Al element on the fabric increased as the concentration of Al₂O₃-NPs in the BTCA the solution increased from 0.5% to 3.5%. The maximum Al element percentage was measured for the fabric treated with BTCA catalyzed by SHP and 3.5% Al₂O₃-NPs. However, the Al atomic percentage decreased for the samples treated with BTCA in the presence of 4% Al₂O₃-NPs catalyst. The change in atomic percentages of the Na and P was almost proportional to the concentration of SHP catalyst.

Table 2.

The atomic percentage of different elements determined by energy-dispersive X-ray analysis

Samples	C (%)	O (%)	Al (%)	Na (%)	P (%)
Untreated	71.30	28.70	–	–	–
CO-SHP	66.23	32.36	–	0.98	0.43
CO-SHP-A-0.5	62.34	35.55	0.27	1.40	0.44
CO-SHP-A-1	63.63	34.53	0.48	1.11	0.26
CO-SHP-A-2	65.85	31.32	1.46	1.06	0.30
CO-SHP-A-3.5	63.17	34.20	2.54	1.05	0.22
CO-A-4	68.67	30.27	1.06	–	–

FT-IR analysis results

The FT-IR spectroscopy was used to study the formation of ester-cross-links between the carboxylic acid group of BTCA and the hydroxyl group of cellulose. The IR spectra of the cotton fabrics untreated and treated with BTCA is shown in Figure 2. A broad peak centered at 3348 cm⁻¹ corresponding to O-H stretching was observed in the spectrum of untreated and treated cotton samples. Also observed were a broad peak at 2900 cm⁻¹ for C-H stretching and peaks at the 1000–1300 cm^–1 region for -C-O stretching. A peak seen around 1640 cm⁻¹ is due to the adsorbed water molecules. FT-IR spectra of treated cotton fabrics generates a new absorption band at 1722 cm^–1 that is believed to be the carbonyl peak of the ester group formed from the reaction between the carboxylic acid group of BTCA and the hydroxyl group of cellulose.^18,20,21 This band was present at IR spectra of all treated cotton fabrics, although it was not seen in untreated cotton spectra. Based on these results it was concluded that BTCA cross-linked to the cotton cellulose by esterification reaction in the presence of SHP with and without Al₂O₃-NPs.

Figure 2.

Fourier transform infrared spectra of the cotton samples untreated (a) and cross-linked with 8% 1,2,3,4-butanetetracarboxylic acid, (b) CO-SHP: catalyzed by 4% SHP, (c) CO-SHP-A-0.5: catalyzed by 3.5% SHP and 0.5% aluminum oxide nano-particles (Al₂O₃-NPs), (d) CO-SHP-A-2: catalyzed by 2% SHP and 2% Al₂O₃-NPs, and (e) CO-A-4: catalyzed by only 4% Al₂O₃-NPs.

Wrinkle-resistant test results

The average WRAs through the warp and weft directions of cotton samples are shown in Table 3. The results indicated that there was an increase in the WRA of the cotton samples treated with BTCA catalyzed by SHP in comparison to the untreated cotton samples. It might be that the BTCA catalyzed by SHP was able to cross-link the hydroxyl groups of the cellulosic macromolecules.^4,22,23 FT-IR results showed that BTCA molecules were able to cross-link the hydroxyl groups of the cellulosic macromolecules effectively in terms of the formation of ester bonds between the cyclic anhydride ring of BTCA and the hydroxyl group of cellulose in the presence of SHP (Figure 2). This finding was consistent with the literature that SHP is an effective catalyst for esterification reactions of cotton cellulose in the presence of BTCA.^24–26 In this study, the effect of Al₂O₃-NPs as co-catalyst and catalyst on the wrinkle resistance of cotton fabric was also investigated. As shown in Table 3, WRA values of the fabrics treated with constant concentrations of BTCA in the presence of co-catalyst Al₂O₃-NPs increased at first. On the other hand, WRA values decreased when the Al₂O₃-NPs concentration was higher than 2%. This reduction in WRA values could be attributed to the agglomeration of Al₂O₃-NPs in the solution bath and also inhibition of the action of the cross-linking agent on the fabric at higher concentration of Al₂O₃-NPs. This finding agreed with SEM images showing agglomeration of Al₂O₃-NPs on the fabric surface. Therefore, it was concluded that the fabrics cross-linked by BTCA using SHP and co-catalyst Al₂O₃-NPs at sufficiently low concentration had higher recover angles than those of fabrics cross-linked by BTCA using the SHP catalyst. Based on Table 3, it was also concluded that the WRA value of the fabric cross-linked using the Al₂O₃-NPs catalyst was higher than that of untreated cotton fabric, but lower than that of the fabrics cross-linked using the SHP catalyst. Therefore, it was concluded that cross-linking of BTCA to cellulose in the presence of Al₂O₃-NPs catalyst decreased. According to the EDX analysis results, the Al element percentage measured by EDX decreased for the fabrics cross-linked with BTCA catalyzed by the Al₂O₃-NPs catalyst. So, it was also concluded that cross-linked BTCA to the cellulose binds the nano particles on the fabric structure.

Table 3.

The wrinkle recovery angle (WRA) values, flame retardant test results, and surface color characteristics of the fabrics

Samples	WRA			Flame retardant test results			Color characteristics
Samples	Warp (°)	Weft (°)	(W+F) (°)^a	Average time to ignition (s)	Average burning time (s)	ΔL^b	The mean whiteness index^b	The mean yellowness index^b
Untreated	5 1	6 0.63	11	3 0.13	10 0.01	– –	– –	– –
CO-SHP	128.2 3.43	144.2 2.71	272.4	4 0.28	23.63 0.03	–6.03 0.02	−9.2 0.12	+2.85 0.03
CO-SHP-A-0.5	144.2 2.72	156 4.4	300.2	4 0.34	24.37 0.21	–5.69 0.38	−8.43 0.25	+0.55 0.01
CO-SHP-A-1	153.6 4.45	171 1.54	324.6	4 0.2	25.45 0.28	–5.43 0.45	−8.33 0.16	+0.25 0.03
CO-SHP-A-2	153 5.97	172.4 1.55	325.4	4 0.11	26.2 0.13	–5.09 0.02	−8.02 0.21	+0.15 0.01
CO-SHP-A-3.5	52.5 2.45	66.4 1.55	118.9	4 0.24	26.5 0.10	–4.95 0.02	+9.28 0.32	+0.10 0.01
CO-A-4	11.6 1.85	25.2 1.54	36.8	4 0.38	29.27 0.09	–4.08 0.18	+15.42 0.38	−3.18 0.05

The sum of the WRA in the warp (W) and weft (F) directions.

These values were measured by referencing the untreated samples.

Note: The mean values of the test results were given in the first link as the standard deviation values were given in the second link.

The increasing at WRA value and FT-IR results indicated that the esterification reactions of cellulose with polycarboxcylic acids can be occur effectively using the catalytic effect of Al₂O₃-NPs. There is no detailed information about the characteristics and catalytic effect of Al₂O₃-NPs in a BTCA wrinkle-resistant finishing system. The catalytic processes of Al₂O₃-NPs could involve several numbers of possible reactions. The esterification reaction of cellulose with polycarboxcylic acids can occur effectively in the presence of Lewis or Bronsted acids due to the inclination of oxygen in the carbonyl group to attract protons.^27,28 It is also known that the surface of Al₂O₃-NPs has a positive charge in the BTCA solution because of the acidic pH, which was below the point of zero charge (vary from 7 to 10, depending on the type of alumina). This positive charge may act as a Lewis acid catalyst and activate the carbonyl group of acid toward the addition of hydroxyl group of cotton fibers. Finally, it leads to an ester bond formation between the hydroxyl groups and carbonyl groups.²⁶ Another possible effect of Al₂O₃-NPs on WRA in the wrinkle-resistant treatment was that the Al₂O₃-NPs having spherical morphology could fill the amorphous region of the cellulose. Therefore, increasing of the WRA can be attributed to restricting the molecular movement of cellulose due to the nano-particles inside the fiber.

Flame retardant test results

Polycarboxylic acids, such as BTCA, citric acid, malic acid, and succinic acid, in the presence of phosphorus-based salt SHP were used as the nontraditional flame retardants for cotton fleece, carpet, and cotton/polyester blend fleece in the literature.^29–32 Moreover, the use of nano-size metal oxide particles as fire retardant additives in polymeric materials seems to be very promising.^33–35 Therefore in this study, flame retardant properties of the cross-linked cotton samples with BTCA catalyzed by SHP and/or Al₂O₃-NPs were also investigated. To determine flame retardant properties of the fabrics, a flame was applied to the fabrics for 3 seconds. Once the burning starts in the fabric, the burning time was recorded and the fabric was classified according to the burning time. Ignition and burning times of the untreated and treated cotton samples cross-linked with BTCA catalyzed by SHP with and without Al₂O₃-NPs are presented in Table 3. According to the test results, the untreated cotton fabrics and treated cotton samples had different ignition times. As all of the cross-linked cotton fabrics were ignited at 4 seconds, the untreated cotton samples were ignited at 3 seconds. Also, the burning time and burning characteristics of all fabrics showed different trends. The untreated samples were burned at 10 seconds completely. The burning times of the cotton samples cross-linked with BTCA at the same concentration increased and the results changed depending on catalyst type and amount. The cotton fabric cross-linked with BTCA using the SHP catalyst was burned at 23.63 seconds, while the cotton fabric treated with BTCA catalyzed by Al₂O₃-NPs was burned at 29.27 seconds. The increasing of burning time means that the fabrics treated with BTCA burned more slowly.

Polycarboxylic acids bonded to cellulose with ester-cross-links are effective in reducing the flammability of the cotton.²⁹ The flame retardancy test results also showed that fabric flammability decreased and when the Al₂O₃-NP concentrations increased together with the reduction in SHP concentrations. This case may result from Al₂O₃-NPs placed in the structure of the fabrics. It was known that the metal oxide nano particles improve the thermal stability and flammability properties of the polymeric materials. The metal oxide nano-particles restrict the mobility of polymer chains and increase heat transfer inside the materials, which limits the ablation of the surface, the migration of gas bubbles and the release of combustible volatiles.^34,35 The flammability results of untreated and treated cotton fabrics were classified according to the ASTM D1230-94 standard. As seen in Table 3, all studied fabrics were classified as “Class 1”, indicating that these samples have a burning time of more than 7 seconds or surface flash without base burn (regardless of burning time).

In order to illustrate the burning characteristics of the fabrics visually, photos of the untreated and treated fabrics after the flame retardant test are given in Figures 3(a)–(g). The char yield of untreated and treated samples was also calculated and is graphed in Figure 3(h) to evaluate the flame retardant effect of the different applications. According to Figure 3, untreated samples completely burned and char yields value was calculated as 46%. The increase in char yield implied that the cross-linking with BTCA were effective in reducing flammability of treated fabrics. On the other hand, the char yield value of the samples cross-linked with BTCA in the presence of co-catalyst also increased as the concentration of Al₂O₃-NPs increased from 0.5% to 3.5%. The maximum char yield value was calculated for the fabric treated with BTCA catalyzed by 4% Al₂O₃-NPs. These findings were compatible with the results of the flame retardant test and the photos given in Figure 3.

Figure 3.

Char yield values (h) and the photographs of the samples for the flammability test of untreated (a) and cross-linked with 8% 1,2,3,4-butanetetracarboxylic acid, (b) CO-SHP: catalyzed by 4% SHP, (c) CO-SHP-A-0.5: catalyzed by 3.5% SHP and 0.5% nano-Al₂O₃, (d) CO-SHP-A-1: catalyzed by 3% SHP and 1% nano-Al₂O₃, (e) CO-SHP-A-2: catalyzed by 2% SHP and 2% nano-Al₂O₃, (f) CO-SHP-A-3.5, and (g) CO-A-4: catalyzed by only 4% nano-Al₂O₃.

Laundering test

The durability of the treatment to repeated home laundering was evaluated by performing 1 and 10 washing cycles at 40℃ for 30 min. Fabrics CO-SHP, CO-SHP-A-2, and CO-A-4 were washed and their flame retardant test and WRA measurements were repeated. The test and measurement results of the washed fabrics are given in Table 4. All samples showed a different decrease of WRA after repeated washing. Sample CO-SHP showed the most obvious decrease during washing and sample CO-A-4 showed the lowest decrease. The WRA values decreased to 169.66, 212, and 29.67 for CO-SHP, CO-SHP-A-2, and CO-A-4 samples after one washing, respectively. The decrease of WRA increased with the increase of washing cycles. The WRA values decreased to 76, 167 and 25 for CO-SHP, CO-SHP-A-2 and CO-A-4 after 10 washing cycles, respectively. According to the flame retardant test results of all washed samples, the mean ignition time decreased from 4 to 3 seconds. However, the average burning times of all of the samples, even after 10 washing cycles, were higher than that of the untreated sample. By the end of 10 washing cycles, the WRA and the average burning time of CO-SHP and CO-SHP-A-2 fabrics decreased, but still showed sufficiently high WRA and flame retardant properties compared to the untreated fabric. These results showed that the cross-linking process with BTCA catalyzed by Al₂O₃ NPs co-catalyst also had a good durability after the washing cycles.

Table 4.

Laundering test results after one and 10 washings

Samples	WRA after one washing			WRA after 10 washing cycles			Flame retardant test results after one washing		Flame retardant test results after 10 washing cycles
Samples	Warp (°)	Weft (°)	(W+F) (°)^a	Warp (°)	Weft (°)	(W+F) (°)^a	Average time to ignition (s)	Average burning time (s)	Average time to ignition (s)	Average burning time (s)
CO-SHP	90.33 0.32	79.33 1.29	169.66	32.67 3.1	44.33 1.12	76	3 0.01	20.67 0.09	3 0.1	16.3 0.06
CO-SHP-A-2	99 3.01	113 2.71	212	75 0.2	92 0.13	167	3 0.13	22.79 0.21	3 0.03	18.45 0.16
CO-A-4	12 0.01	17.67 0.13	29.67	16 0.10	9 0.15	25	3 0.17	25.35 0.01	3 0.21	11.05 0.01

The sum of the wrinkle recovery angle in the warp (W) and weft (F) directions.

Note: The mean values of the test results were given in the first link as the standard deviation values were given in the second link.

Whiteness and yellowness values of the fabrics

In this study, lightness, whiteness, and yellowness indexes of the fabrics were measured to determine the effect of BTCA cross-linking catalyzed by SHP and/or Al₂O₃-NP co-catalyst on the yellowness of the cotton. Color studies were quantified by the CIELAB with a three-axis system, that is, lightness (L*) from 0% (black) to 100% (white); yellowness* from blue (−b) to yellow (+b).¹⁷

The whiteness and yellowness indexes of the untreated fabric were taken as a reference and the color changes after wrinkle treatment were evaluated according to that of the untreated fabric. The results of the treated fabric are given in Table 3. It was determined that the yellowness index value of the fabric cross-linked with BTCA catalyzed by SHP was +2.85, which means that the yellowness of the fabrics increased after treatment. However, the yellowness index values decreased and whiteness index values increased when Al₂O₃-NPs were used as the co-catalyst. The maximum whiteness and minimum yellowness values were obtained for the fabric treated with BTCA catalyzed by the Al₂O₃-NP catalyst. The lightness values (ΔL) coincided with the results of whiteness and yellowness measurements. These findings also agreed with the literature that the yellowness of the cotton fabrics increased with cross-linking by BTCA in the presence of SHP catalyst due to the high-temperature curing effects on cellulose polymer chains or formation of unsaturated compounds, and the high acidity of the cross-linking agent, that is, pH 1.8.^4,6,7 The presence of white Al₂O₃-NPs attached on the fabric surface might minimize the problem of fabric yellowing because whiter fabrics were obtained with the higher concentration of Al₂O₃-NPs. In addition, decreasing of acidity of the solution with increasing concentration of Al₂O₃-NPs (i.e. pH 2.6–3) could be another reason for the reduction in the yellowness index.

Air permeability test results

The cross-linked cotton samples with BTCA catalyzed by SHP with and without Al₂O₃-NPs were tested to investigate the effect of cross-linking and Al₂O₃-NPs on the air permeability property of the fabrics. The air permeability test results of the fabrics are given in Table 5. Untreated fabrics have higher air permeability compared to the cross-linked cotton samples with BTCA catalyzed by SHP and Al₂O₃-NPs. The air permeability of the cross-linked fabric decreased because the cross-linking between fibers reduced the inter-fiber and inter-yarn spaces. The reduction in the air permeability of the fabrics was also proportional to the amount of Al₂O₃-NPs used as the co-catalyst. As the number of Al₂O₃-NPs in the co-catalyst increased, not only did the cross-linking of cellulose with BTCA increase, but also the number of Al₂O₃-NPs placed in the space between yarns and fibers physically increased. Al₂O₃-NPs could easily fill up the amorphous region of cellulose due to small particle size. All of these effects lead to a lower air permeability effect on the cotton fabrics.

Table 5.

Air permeability values and analysis of variance results of the fabrics

Samples code	The number of the sample	Air permeability values(l/m²/s)
		Subset for alpha = 0.05
		1	2	3	4
Tukey HSD a
CO-A-4	10	64.0600
CO-SHP-A-3.5	10	67.3600	67.3600
CO-SHP-A-2	10		72.4000
CO-SHP-A-1	10			98.9000
CO-SHP-A-0.5	10			101.1600
CO-SHP	10			105.8000
Untreated	10				160.1000
Sig.	10	.879	.501	.152	1.000

Means for groups in homogeneous subsets are displayed.

Uses harmonic mean sample size = 10.000.

The air permeability of the fabric samples was evaluated by ANOVA in order to demonstrate the importance of each variable. According to the statistical analysis results given in Table 5, air permeability of the cross-linked cotton samples with BTCA catalyzed by SHP without and with the Al₂O₃ co-catalyst was considerably lower than untreated fabric at the 0.05 level. Furthermore, the statistical difference between the air permeability values of the treated fabrics becomes important as the Al₂O₃-NPs concentration increases from 1% to 4%.

Tensile strength test results

As seen from literature, the strength loss of the cotton fabrics cross-linked with a polycarboxylic acid occurs.³⁶ Cross-linking between cellulose molecules causes stiffening of the cellulosic macromolecular network and fiber embrittlement, thus reducing the mechanical strength of the treated cotton.³⁷ In addition to this, cross-linking treatment carried out at low pH value has a severe effect on the reduction in the mechanical strength of treated cotton. In this part, the tensile strength of cotton fabric cross-linked with BTCA catalyzed by SHP with and without nano-Al₂O₃ co-catalyst at different concentrations was investigated. Tensile strength test results of the fabrics are presented in Table 6. It can be inferred from Table 6 that tensile strength of the cotton fabrics cross-linked with BTCA catalyzed by SHP significantly decreased at a statistically important level. The loss in tensile strength was a result of the cross-linking of cellulose effectively and high acidity (i.e. pH 1.8) of BTCA solutions with SHP.

Table 6.

Tensile strength values and analysis of variance results of the fabrics

Samples code	The number of the sample	Tensile strength values (N)
		Subset for alpha = 0.05
		1	2	3	4	5
Warp direction (Tukey HSD^a)
CO-SHP	5	326.4800
CO-SHP-A-0.5	5	355.3960
CO-SHP-A-1	5		424.6500
CO-SHP-A-2	5			514.6160
CO-SHP-A-3.5	5			518.0260
CO-A-4	5				555.8500
Untreated	5					643.8700
Sig.		1.000	1.000	1.000	.909	1.000
Weft direction (Tukey HSD^a)
CO-SHP	5	119.1260
CO-SHP-A-0.5	5	126.5740
CO-SHP-A-1	5		152.2580
CO-SHP-A-2	5		156.6160
CO-SHP-A-3.5	5			183.0000
CO-A-4	5				207.6740
Untreated	5					496.7430
Sig.		.587	.992	1.000	1.000	1.000

Means for groups in homogeneous subsets are displayed. ^aUses harmonic mean sample size= 5.000.

On the other hand, nano metal oxide particles could minimize the side effects of the cross-linking process by SHP. It can be inferred from Table 6, adding Al₂O₃-NPs as catalyst/co-catalyst in solution reduced tensile strength loss of the fabrics cross-linked with BTCA. Decreasing the acidity value (i.e. pH 2.6–3) with increasing the concentration of Al₂O₃-NPs could be the reason for the increase.

According to the findings obtained in this study, it was concluded that the most suitable catalyst concentration should be 2% SHP in the presence of 2% Al₂O₃-NPs to improve wrinkle resistance, tensile strength, and flame retardant properties of the cotton fabric. To prove the catalytic/co-catalytic effect of Al₂O₃-NPs, the sample cross-linked by BTCA without a catalyst (coded as CO-BTCA) and the sample (coded as CO-SHP-2) cross-linked without co-catalyst using the same amount of SHP catalyst that of the samples coded as CO-SHP-A-2 were prepared and WRA and tensile properties were tested. The WRA and tensile strength test results of these fabrics are presented in Table 7. It can be inferred from Table 7 that the WRA value of the fabric cross-linked using the Al₂O₃-NP co-catalyst was higher than that of untreated cotton fabric and that of the fabrics cross-linked using only SHP catalyst at different concentrations. It is concluded that BTCA without a catalyst can cross-link the cellulose, according to the increasing WRA and decreasing tensile strength results of the sample coded as CO-BTCA given in Table 7. In addition to this, the reducing tensile strength losses of the cotton fabrics cross-linked with BTCA catalyzed by only the SHP catalyst at different concentrations were higher than that of cotton fabric cross-linked with BTCA catalyzed by SHP with nano-Al₂O₃ co-catalyst. The increasing WRA value and reducing tensile strength loss results were the evidence of the co-catalytic effect of Al₂O₃-NPs.

Table 7.

The wrinkle recovery angle (WRA) values and tensile strength values of the selected fabrics to determine the co-catalytic effect of aluminum oxide nano-particles

Samples	WRA			Tensile strength values (N)
Samples	Warp (°)	Weft (°)	(W+F) (°)^a	Warp direction	Weft direction
Untreated	5 1	6 0.63	11	643.8700	496.7430
CO-BTCA	78.67 1.03	102.67 2.01	181.34	568.0890	396.4935
CO-SHP	128.2 3.43	144.2 2.71	272.4	326.4800	119.1260
CO-SHP-2	103.33 2.41	128.67 0.83	103.33 2.41	403.673	152.0789
CO-SHP-A-2	153 5.97	172.4 1.55	325.4	514.6160	156.6160

The sum of the WRA in the warp (W) and weft (F) directions.

Note: The mean values of the test results were given in the first link as the standard deviation values were given in the second link.

Conclusion

This study aimed to assess the characteristics of Al₂O₃-NPs used as the catalyst and co-catalyst in the cross-linking process of the cotton fabrics with BTCA. For this aim, cotton fabrics were treated with BTCA solutions containing SHP with and without Al₂O₃-NPs through one bath pad-dry-cure method, and surface morphology, wrinkle resistance, flame retardancy, yellowness, tensile strength, and air permeability of the fabrics were studied. It was found that the surfaces of fibers changed as they were subjected to treatment with BTCA solution containing 4% SHP catalyst. There was a high deposition of Al₂O₃-NPs on the surface of the fabrics cross-linked using SHP with Al₂O₃-NPs, for which the particles were uniformly deposited on the cotton fiber surface or interstices between the fibers by the padding process with a certain extent of agglomeration of these particles due to the surface attraction between small nano-particles. Agglomeration of nano-particles on the fabric surface increased as the concentration of co-catalyst Al₂O₃-NPs added in BTCA solution increased. According to the FT-IR results, cross-linking of cotton fabrics by BTCA catalyzed by SHP with and without the Al₂O₃-NP co-catalyst was carried out. The cross-linking with BTCA in the presence of SHP with Al₂O₃-NPs that acted as the co-catalyst could enhance the wrinkle resistance of cotton fabrics at statistically significant level. The Al₂O₃-NPs used as a co-catalyst also acted as a multifunctional finishing agent to improve the flame retardant property of the fabrics. According to the findings obtained in this study, it was concluded that the most suitable catalyst concentration should be 2% SHP in the presence of 2% Al₂O₃-NPs to improve both wrinkle-resistance and flame retardant properties of the cotton fabric.

It was also concluded that treatment with BTCA catalyzed by SHP causes a decrease in tensile strength, whiteness, and air permeability of the fabrics. On the other hand, the addition of Al₂O₃-NPs co-catalyst in cross-linking the process of the cotton fabrics with BTCA catalyzed by SHP caused reduction of the tensile strength loss and increase of the whiteness of the fabrics. Considering the cost of the finishing treatment, the cost of finishing treatment per 100 mL solution decreased from 1.076 euro to 1.05075 euro when the amount of the Al₂O₃-NPs co-catalyst increased from 0% to 3.5%.

Footnotes

Funding

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

Acknowledgment

The authors are pleased to acknowledge Prof. Dr Halil Turgut Şahin for color measurements.

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