What is the thermal stability of wool?

The compound 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTCA) is an eco-friendly water treatment agent possessing flame-retardant phosphorus element and multi-carboxylic acid groups in its molecular structure. In the present work, PBTCA is employed as a finishing agent to improve the flame retardancy of the wool fabrics by the pad-dry-cure technique. The treated wool (10.2% weight gain) by 100 g/L of PBTCA showed an increased flame retardancy with a limiting oxygen index value (LOI) of 44% with a minimum char length of 40 mm. Importantly, the treated wool can self-extinguish after 30 washing cycles. The PBTCA-treated wool exhibited better stability with obviously increased char residue of 39.7% and 28.7% at 600 °C, while only 25.9% and 13.2% were measured for the control wool in nitrogen and air atmosphere, respectively. In addition, the high thermal stability of the treated wool with astonishing char-forming ability is confirmed by the SEM images of the wool after the isothermal heating treatment at different temperatures. Finally, a two-stage flame-retarding mechanism of enhanced crosslinking and char formability of PBTCA-treated wool is proposed and analyzed by infrared spectroscopy (TG-FTIR) and thermal (DSC and TGA) results of the pyrolytic volatiles of the treated wool.

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In the present work, we employed the PBTCA on the wool fabrics to investigate the flame retardancy and thermal behavior of the treated fabrics. The study is carried out by considering the weight loss analysis during heating of the wool treated by PBTCA in the counterparts of other phosphorus-containing chemicals. A slight weight loss is observed in the wool treated by PBTCA during the first decomposition process, while comparatively more weight loss is found in the other flame retardants. To get in depth into the flame-retardant mechanism and physical properties of the wool, the treated wool fabrics are examined using the thermo-analytical method, isothermal heat treatment, and infrared analysis.

Based on the above-mentioned findings, an environmental-friendly and bio-degradable phosphorus-containing polycarboxylic acid (2-phosphonobutane-1,2,4-tricarboxylic acid, PBTCA) [ 29 ] aroused our interest. The chemical structure of PBTCA is shown in . As presented, this compound is composed of the phosphorus element, which was regarded as having flame-retarding property, and multi-carboxyl groups, which has been found to be able to crosslink with hydroxyl (-OH) groups in cellulose [ 30 , 31 ] and -OH and/or amino (-NH 2 ) groups in keratin chains in the wool [ 19 , 28 ]. Initially, PBTCA has been widely explored as an effective scale inhibitor, dispersant, and corrosion inhibitor due to its excellent chelating property [ 29 , 32 , 33 ]. Moreover, PBTCA has been examined as an anti-wrinkle agent for cotton fabrics as it possesses the structural character of polycarboxylic acids, forming ester crosslinking with -OH groups in the cellulose [ 34 , 35 ].

Furthermore, a bio-based product of high phosphorus content, phytic acid (PA), has been investigated as well to confer flame resistance in the wool [ 6 , 19 , 20 ]. Additionally, a few organic compounds containing sulfur, boron, and silicon were tested as flame retardants for wool [ 21 , 22 , 23 , 24 ]. Some nano-based treatments have also been carried out to achieve flame-retardant wool [ 19 , 25 ]. Despite the excellent flame retardancy, most of these approaches are not reliable under the repeated washing due to the lack of covalent bonds between the flame retardant and wool keratin polypeptide. In this direction, various additives have been explored to improve water durability [ 19 , 21 , 26 , 27 ]. Among them, the introduction of polycarboxylic acids as the crosslinking agent, for example, 1,2,3,4-butanetetracarboxylic acid (BTCA), is verified as an effective organic compound to build the covalent connection between the flame retardant and wool [ 19 , 28 ].

In the past decades, various flame retardants have been developed to improve the flame retardancy of wool fabrics [ 2 , 5 , 6 , 7 ]. The halogen-based compounds including chloro- and bromo- derivatives had been investigated and coated to develop high efficiency and durable wool [ 2 ]. Nowadays, the halogen-containing flame retardants are rarely used due to releasing out of toxic dioxins during the burning process [ 2 , 8 ]. From the s, the mordanting treatment of wool using zirconium/titanium complexes and their modified compounds has been investigated extensively [ 9 , 10 ]. Based on these findings, the famous &#;Zirpro&#; process was developed for making a flame-retardant wool with excellent washing durability. However, the effluents produced during the process have been criticized for the heavy metal pollution [ 11 , 12 ], and the treated wool turned yellow and lost its flame resistance if exposed to light or washed in alkaline solution [ 13 ]. The phosphorus-based compounds also received significant attention in the exploitation of various flame retardant polymers including wool fabrics for their low toxicity and high efficiency [ 5 ]. Notably, the two durable flame-retardant systems, namely Proban and Pyrovatex CP, which were designed and commercially used for cotton fabrics over the decades, were attempted on the wool fabrics. Although imparting a high degree of flame retardancy, the two systems were finally overlooked for releasing the toxic formaldehyde during the preparation of the flame-retardant fabrics and end-use process [ 2 , 14 , 15 , 16 ]. A vinyl phosphate was also applied on the wool by the graft method, demonstrating high flame retardancy with LOI above 35, but the tensile strength of the grafted wool was reduced sufficiently and needed to be improved [ 12 ]. Recently, the enhancement of flame resistance of the wool fibers has been focusing on the usage of sustainable flame retardants. The plant-based bio-molecules, for instance, banana pseudostem sap (BPS) and green coconut shell extract (CSE), have been investigated for enhancing flame retardancy in the protein wool owing to the content of phosphate, phosphite, nitrogen, silicates, and some metallic salts, endowing the self-extinguishable property and imparting coloration in the fabric as well [ 13 , 17 , 18 ].

As one of the most essential natural protein fibers, wool is widely used in apparel, interior textiles, and industrial applications due to its superior aesthetic qualities along with excellent breathability, warmth, elasticity, and especially, the inherent low flammability. In woolen fabrics, the high amount of nitrogen, sulfur, and moisture contents are responsible for the intrinsic flame retardancy with a limiting oxygen index (LOI) of &#;25% [ 1 , 2 ]. However, the wool fabrics, which are not of very dense construction and heavy, still cannot pass the vertical burning test [ 3 , 4 ]. Therefore, it is necessary to enhance the flame retardancy of the wool.

The DSC testing was conducted using control wool and the PBTCA-treated wool via a differential scanning calorimeter (DSC , PerkinElmer, Walsham, MA, USA). The amount of the sample, nearly 6 mg, was placed in the crucible cell under the nitrogen gas flow at a rate of 50 mL/min. The heating rate was kept at 15 °C/min, and the observation was carried out in the temperature range 50&#;300 °C.

To observe the effect of PBTCA on the thermal stability of wool fabrics before and after the finishing treatment, the scanning electron microscope (SEM) images of the samples after the isothermal heating treatment as per the previous report [ 38 ] were captured. The fabric samples of the size of 50 mm × 50 mm were heated in the muffle roaster for 10 min at 200 °C, 300 °C, 400 °C, 500 °C, and 600 °C, respectively. Afterward, the fabrics were observed using SEM (VEGA 3, TESCAN, Czechoslovakia) at an accelerating voltage of 10 kV.

The combustion behavior of the wool samples was measured according to the ISO standard with a cone calorimeter device (FTT007, Fire Testing Technology, East Grinstead, UK). The sample was prepared in the size of 100 mm × 100 mm. Three pieces of a sample were wrapped in aluminum foil and burned at a heat flux of 35 kW/m 2 . The assessment values such as the time to ignition (TTI), the heat release rate (HRR), the total heat release (THR), and other related data were measured.

To assess the resistance against the water of the treated wool fabrics, the treated samples were impregnated in the aqueous solution containing 0.15 wt % of AATCC 193 standard detergent at 45 °C for a specific period. The washing duration of 45 min was called 5-cycles washing. Thus, the fabric was taken out after 5, 10, 15, 20, and 25 washing cycles, and subsequently, the fabric was rinsed with distilled water and dried in the oven at 80 °C for 1 h. The flame retardancy of the washed wool samples was assessed with LOI.

The pad-dry-cure method was used for the flame-retardant treatment of wool fabrics. The pretreated wool fabric was first immersed in the aqueous PBTCA solution of various concentrations (20 g/L, 40 g/L, 60 g/L, 80 g/L, and 100 g/L at pH of 1.43~0.93) for 10 min at room temperature. Then, the padded process using a two-roll laboratory padding machine was carried out. The wet pick-up from the fabric was 100 ± 5% after two dips and two nips. Afterward, the padded fabric was dried in an oven at 80 °C for 3 min and subsequently cured at 170 °C for 3 min. At the end of the treatment, the wool fabric was rinsed with distilled water and dried at room temperature. The samples finished with the PBTCA of different concentrations were termed as W20, W40, W60, W80, and W100, respectively.

Basically, the wool fiber is covered by one or two layers of the cuticle, and the cuticle is surrounded by a fatty acid layer of 18-methyleicosanoic acid (18-MEA). The layer of 18-MEA is covalently bonded to the epicuticle through thioester bonds, which are together attached with the scaly surface, causing a water-repellent surface and a barrier to chemical treatment [ 36 ]. As demonstrated in the previous research, the partial removal of the scale layer by oxidation can assist the chemical diffusion into the fiber interior [ 37 ]. Therefore, in this work, the pretreatment of the wool was carried out to promote the diffusion of the finishing agent. Firstly, the raw wool fiber was treated in a solution containing NaClO (0.2% active chlorine, pH = 3) for 15 min at 25 °C. Thereafter, the chlorinated wool passed through the solution containing 0.1 wt % Na 2 CO 3 and 0.05 wt % Na 2 SO 3 at 40 °C and pH 9.0 for 40 min. So, the pretreated wool was used for further experimental steps as control wool.

The woven wool fabric (147 g/m 2 , 102S/2 &#; 54S) was supplied by Shandong Ruyi Wool Textile Garment Group Co. Ltd., Jining, China. The sodium hydroxide (NaOH) was purchased from Xilong Science Co. Ltd., Shantou, China. The anhydrous sodium carbonate (Na 2 CO 3 ), sodium hypochlorite (NaClO), sulfuric acid (H 2 SO 4 ), and anhydrous sodium sulfite (Na 2 SO 3 ) were purchased from Sino-pharm Chemical Reagent Co. Ltd., Shanghai, China. All the reagents were used as the reagent grade without any further purification process. The compound, 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTCA, 50% (w/w) aqueous solution), was purchased from Taihe Water Treatment Technologies CO. Ltd., Zaozhuang, China.

3. Results and Discussion

3.1. Flame Retardancy and Durability

In the present work, the vertical burning and LOI tests were carried out to determine the flame retardancy and the durability of PBTCA-treated wool samples. shows the weight gain, LOI, and VBT results of the wool treated with different concentrations of PBTCA. The control wool fabric showed an LOI of 24% and burned out completely during the vertical flammability tests, demonstrating a full char length of 300 mm. In comparison with the control wool, the wool fabrics treated with PBTCA show significantly higher LOI values and lower char length. With the increasing concentrations of PBTCA, the treated wool fabric acquires weight gain and shows improved flame-retardancy behavior, which is indicated by the increased LOI and decreased char length. The fabric finished with 100 g/L of PBTCA displayed an LOI of 44.6% and a decreased char length of 40 mm, respectively. Moreover, as compared to the control wool, the burning part of the PBTCA finished fabrics inflated obviously, showing the outstanding char-forming ability. The inflation of the charred wool is presented in the following CCT tests. These results indicated that the finishing of PBTCA can significantly improve flame retardancy in the treated wool.

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The water durability of the PBTCA-treated fabrics was examined, and the results are illustrated in a,b. With the increasing washing cycles, the treated wool fabrics show a reduced flame retardancy, which was verified by the increased char length and decreased LOI value. Specifically, even after 30 washing cycles, the treated wool fabrics still can sustain and self-extinguish in the burning process, showing a char length of 205 mm and LOI of 25.6. These findings indicated that the PBTCA-treated wool possessed good resistance to the laundry. According to the chemical structure of PBTCA and wool, some reactions may occur between the carboxyl groups of PBTCA and the -OH and/or -NH2 groups of the wool fabrics, as illustrated in c. It is well verified by the obtained ATR-FTIR spectra of the treated wool, as shown in d. In addition, the strong ionic bonds between the negative phosphate groups or carboxylate groups in PBTCA and the positive -NH3+ groups in wool may be attributed to the chemical reactions [19].

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As shown in d, the peaks that appeared at cm&#;1 and cm&#;1 in the samples of control wool, W20, W60, and Wl00 are the characteristic bands of amide I (C=O stretching) and amide II (N-H stretching) for wool fibers [27,39,40], respectively. In comparison with the spectrum of the control wool, the spectra of the treated samples displayed new peaks at cm&#;1 and cm&#;1, which are attributed to the vibration of P=O and PO2&#; bonds from the phosphate groups in PBTCA [41]. A shoulder peak appeared at cm&#;1, which is ascribed to the stretching vibration of the ester carbonyl bonds and is possibly formed between the carboxylic acid in the PBTCA molecule and hydroxyl groups of serine, threonine, and tyrosine in wool fabrics, indicating the successful crosslinking reaction of PBTCA onto the wool fiber. As for the absorption of the amide through the carboxyl groups in PBTCA and amino groups in wool, it cannot be distinguished due to its overlapping by the original peptide adsorption from the wool.

3.2. Combustion Properties

The cone calorimetry test was employed to further investigate the flame retardancy of the treated wool under the heat flow. The THR and HRR of the control wool and PBTCA-treated (100 g/L) wool are shown in . The obtained results such as the time to ignition (TTI), peak of heat release rate (PHRR), the average effective heat of combustion (Aver-EHC), the ratio of carbon dioxide (CO2) to carbon monoxide (CO), and the burnt residue after the test are presented in .

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Table 1

SamplesTTI (s)PHRR (kW/m2)Aver-EHCCO2/COResidues (%)Control.919.513.11.4W.014.817.426.8Open in a separate window

During the test, the PBTCA-treated wool underwent a less intensive burning behavior compared to the control wool after being ignited for a longer time. Burning of the treated wool fabric developed slowly and terminated in a comparatively longer time, releasing less heat. Specifically, as shown in a,b and , the peak of the PHRR and THR of the PBTCA-treated fabric is 188.0 kW/m2 and 6.8 MJ/m2, respectively, which are much less than those values of the control wool (305.9 kW/m2 and 10.8 MJ/m2). In addition, the treated wool fabric turned into a significantly expanded intumescent char residue as compared to the control wool after the cone calorimetry test, as shown in e,f. This type of char residue was beneficial to hinder the transfer of the heat and gaseous compounds between the outer and inner parts of the char covering the substrate, retarding the flame and suppressing the smoke production [18,39]. After the cone test, the remained char residue of 26.8% and 1.4% were measured for the treated wool fabric and the control wool, respectively, indicating that involving PBTCA on the wool can promote the char formability of the wool fabrics. The results can be further proved by the following TG analysis. Furthermore, as indicated in c,d, the treated wool released a low CO2P and high COP amount. The treated wool fabric also showed lower Aver-EHC and higher CO2/CO than those values of the control wool, as illustrated in . The low Aver-EHC represented a flame inhibition effect induced by PBTCA on the wool during the combustion. The high CO2/CO suggested insufficient combustion and indicates that the introduction of this flame-retarding agent can hinder the combustion process in the treated wool [42], which is supposed to be attributed to the protection produced by the intumescent char layer during the combustion process.

The above-mentioned results of LOI, char length, and the cone calorimetry tests indicated that the treatment with PBTCA can enhance the flame retardancy of the wool fabrics by impeding the combustion and imparting the char formation reactions.

3.3. Analysis of Thermal Properties

The thermal and thermal-oxidative stability of the PBTCA-treated wool fabrics were assessed by the TG analysis and compared to that of control wool. The obtained TG and DTG data of the control wool and the treated wool in nitrogen and air atmosphere are plotted in and Figure S1. The related thermal decomposition data are listed in . The comparison of the actual TG curves of the PBTCA-treated wool and the calculated curves (i.e., the curves calculated by the linear combination of the weight loss of wool and PBTCA) are displayed in .

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Table 2

SamplesT10% a (°C)T50% b (°C)Residue at 700 °C (%)N2Control.2W.8W.3W.7AirControl.36W.3W.6W.1Open in a separate window

In the nitrogen, as can be seen from a and Figure S1a, the wool fabrics underwent a degradation process of two main stages. The first stage takes place around 100 °C and corresponds to the desorption of the physically absorbed water. The second stage occurs above nearly 200 °C, where the major and sudden weight loss is observed. It is ascribed to breakage of the hydrogen bond in the peptide helical structure and to the change of the solid to liquid state of wool along with the cleavage of the disulfide bonds, causing the release of several volatile species, as demonstrated in the previous reports [6,43,44]. The PBTCA-treated wool exhibited higher thermal degradation temperature in the counterpart of the control wool, as indicated by the data listed in and the values of T10% shown in a. In addition, the introduction of PBTCA significantly enhanced the thermal stability of the treated wool, yielding more char residue. All the PBTCA-treated wool samples of W20, W60, and W100 exhibited higher residue char; for example, the value of the residue obtained from W100 is 39.7% at 600 °C in nitrogen, while the control wool just left a residue of 25.9%. It is quite clear that the residue difference between the treated wool and the control wool is larger than the value caused by the weight gain of the PBTCA itself, as indicated in a. The actual TG curve of the PBTCA-treated wool indicated a much higher residue than the calculated one in the whole decomposition process. The calculated TG curve is based on a hypothesis of no chemical reaction occurring between the wool and the PBTCA. These results revealed that the degradation of PBTCA upon heating interferes with the degradation process of wool by favoring the char formation.

In the air, there are three stages during the pyrolysis process, as can be seen in b and Figure S1b. The degradation curves follow the same general trend as that in nitrogen until 440 °C. The samples underwent a rapid decomposition above 440 °C in the air, which is related to the further oxidation of the remaining residue. The flame-retardant samples with high char residue exhibit the higher antioxidant ability of the formed char. Specifically, all the PBTCA-treated wool samples displayed a slower weight loss and higher residue in comparison with the untreated fabric, as shown in and b. For example, 28.7% of the residue was retained for the W100 sample at 600 °C, while only 13.2% was left for the untreated wool. Furthermore, with the increasing temperature, weight loss in the control wool increased dramatically; almost nothing was left at 700 °C. These findings were further verified by the &#;Difference&#; curve between the actual and calculated curves, as indicated in b. The introduction of PBTCA significantly promotes the thermal stability of the formed char, which is even more obvious than its performance on the treated wool in nitrogen, as shown in a. Thus, the significantly enhanced antioxidant ability of the formed char residue may be attributed to the enhanced crosslinking mechanism including the possible formation of P-N and P-O bonds for the introduction of phosphorus [43].

3.4. Isothermal Heat Treatment

To further observe the effect of the PBTCA on the thermal stability of the treated wool, the control and treated fabrics were heated in the muffle roaster at 200 °C, 300 °C, 400 °C, 500 °C, and 600 °C for 10 min, and subsequently, the surface morphology was observed by SEM, as shown in . During the isothermal heating, both the control and PBTCA-treated wool carbonized after being heated at 300 °C for 10 min, and their surface appearance is much similar to that before and after treatment at 200 °C, as shown in Figure S2. As shown in , the PBTCA-treated wool displayed better thermal stability, as there is no obvious change in the surface after being heated at the higher temperature (b1, b2, and b3) while the control fiber showed the swelling sign after being heated at 400 °C (a2) as the fiber became plumper than that at 300 °C (a1). Furthermore, a little amount of the control wool fibers broke and melted after being heated at 500 °C, and there was almost nothing left except for some porous ash for the control wool, as shown in (a4). On the other hand, the PBTCA-treated wool after being heated at 600 °C for 10 min retained the good texture of the fabric and seemed to be covered by a thick coating, as shown in (b4). It may be the poly-phosphorus acid layer formed from the pyrolysis of PBTCA and/or the melting and charring of the wool, which generate a compact char to retard the flame in the condensed phase and are helpful to restrain the smoke production.

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Additionally, a smoke density test was performed to confirm this analysis, and the results are illustrated in Figure S3. In the whole burning process, much less smoke is generated from the PBTCA-treated wool than that from the control wool. The outstanding smoke-restraining property is ascribed to the formation of an intumescent char layer on the surface of the fabrics.

All the above-mentioned changes in the PBTCA-treated wool as compared with the control wool are well consistent with the results obtained from the TG test. The decomposition temperature shifted to a higher temperature after treatment with PBTCA, and the treated wool demonstrated a better char-forming ability. Therefore, the thermal stability of the PBTCA-treated fabric was greatly improved.

3.5. DSC

Interestingly, the wool treated with PBTCA exhibited a slightly different decomposition trend below about 300 °C, as indicated by the curves in the dash frame in a, in comparison with the effect of other phosphorus-containing compounds reported previously such as phosphoric acid [45], phytic acid [6,20], and ammonium phosphate compounds [39,43]. Specifically, the PBTCA-treated wool displayed an obviously lower and delayed weight loss. Similar results were also observed in the air atmosphere. We speculate that this phenomenon is caused by the enhanced crosslinking (upon heating) between peptide chains for the involved PBTCA, producing isopeptide or ester bonds, restraining the nature of the α-helix structure, changing phase from solid to liquid along with the rupture of disulfide bonds, and consequently, the thermal stability of the wool was improved [46]. In order to confirm this speculation, a DSC test was carried out for the control and PBTCA-treated wool, and the results are shown in .

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DSC is a powerful tool to investigate changes in the structure and the chemical damages in various keratins, modified keratins, and model keratin substances [47]. In the corresponding DSC curves, one or two endothermic peaks are observed in the temperature range 230&#;255 °C. Generally, wool is regarded as the composite of a microfibril&#;matrix structure, where a helical microfibril is embedded in an amorphous matrix of proteins associated with the microfibril. The first of the two peaks (peaks filled with blue color at 234 °C and 247 °C) for wool keratin is confirmed as a microfibrillar peak and assigned to an irreversible helix unfolding superimposed of the many decomposition reactions [47,48].

As per the previously reported literature, the temperature required for this denaturation process was found to be strongly affected by the crosslinking density in the matrix phase [49,50]. From , the helix peak temperature increased from 234 to 247 °C, which supports the fact that the considerable stabilizing effects were produced possibly by enhanced crosslinking due to PBTCA rather than the disulfide crosslinking exclusively. A new endothermic peak (filled with green color) in the curve of the PBTCA-treated wool was observed at about 210 °C, which may be attributed to the reaction heat needed for the crosslinking between PBTCA and PBTCA and NH2 and/or OH groups of wool molecules. It is consistent with the esterification and acylation temperatures, as illustrated in c. These observations support the explanation of the thermal stability of the PBTCA-treated wool, which can be ascribed to the enhanced crosslinking.

3.6. TG-FTIR

To investigate the flame-retardant mechanism, TG-FTIR was used to monitor the volatile pyrolysis products during the thermal degradation process in the nitrogen atmosphere. As shown in , the 3D TG-FTIR images show the overall IR absorption of the pyrolytic volatiles produced from the control and PBTCA-treated fabric. FTIR spectra at four specific temperatures, namely 220 °C, 260 °C, 295 °C, and 310 °C, are presented in c,d. The four temperatures were peaked from the range in the area of gray color in a to verify and highlight the different decomposition processes due to PBTCA.

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By comparing the 3D image and spectra of the control wool to that of the treated, it can be seen that similar volatile species were released from the two wool samples during the decomposition process. The bands at the range of &#; cm&#;1 correspond to the hydrocarbon volatiles [51]. The peaks at cm&#;1 and cm&#;1 are attributed to CO2, the peaks that appeared at cm&#;1 and cm&#;1 are ascribed to the absorption of CO, the peak at cm&#;1 is ascribed to the volatility of H2S, the peaks at cm&#;1 and cm&#;1 are attributed to the SO2, and the peaks at 964 cm&#;1 and 930 cm&#;1 are attributed to the NH3. It can be concluded that significantly fewer amounts of volatiles were produced from the PBTCA-treated wool (W100), as indicated in b,d, especially in higher temperatures. A much more later decomposition took place for the PBTCA-treated sample, as almost no volatile was monitored until at 220 °C except for small CO2 absorption bands at around cm&#;1 ( b). In contrast, there were significantly more amounts of volatiles observed at wavenumbers around cm&#;1, cm&#;1, cm&#;1, and cm&#;1 in a, which are attributed to the release of H2O, CO2, H2S, SO2, and NH3. These findings also support the results from the DSC tests. The enhanced crosslinking of the isopeptide or ester bonds between peptide chains is caused by the introduction of PBTCA, reducing the decarboxylation and the amino groups breaking off from the peptide chains. Furthermore, the peaks assigned to sulfur-containing chemicals in the range of &#; cm&#;1 decreased dramatically for the PBTCA-treated wool, as displayed in b.

3.7. Analysis of the Flame-Retardancy Mechanism

All the demonstrated results indicate that the incorporation of PBTCA can hinder the pyrolysis of the wool protein. Based on the above-mentioned analysis, the underlying mechanisms are proposed, as shown in . In the low-temperature range, the introduction of the PBTCA can facilitate more isopeptide and/or ester bonds upon heating between the peptide chains, thereby improving the thermal stability of the treated wool. In addition, the enhanced crosslinking also restrains the release out of ammonia and CO2 caused by the side amide and carboxyl groups breaking off, showing less volatiles and delay decomposition for the PBTCA-treated wool. At high temperature (possibly above 300 °C as indicated in and ), the decomposition product of PBTCA can form a layer of polyphosphoric acid or diffuses into the wool, and the catalytic forming of a stable char layer occurred, suppressing the decomposition process of the treated wool and retarding it in the condensed phase, which is also confirmed by the EDS test results of PBTCA-treated wool after and before VBT, as shown in Figure S4. By comparing with the wool fabric before burning, the content of C and P showed an obvious increment, indicating an excellent carbonization ability of P in the condensed phase.

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Wool fibre is a popular fibre for the manufacture of apparel and floor coverings, but it does not have adequate thermal stability, antistatic, UV resistance, and antibacterial properties that are required for some applications, such as outerwear and hospital gowns. In this work, a wool fabric was treated with para-aminobenzenesulphonic acid (ABSA) by the oxidative polymerisation method and its effect on the thermal stability, UV radiation resistance, electrical conductivity and antibacterial properties of the treated fabric was systematically evaluated. It was found that the ABSA treatment had synergistic effects on the various functional properties of the treated fabric. The ABSA treatment not only made the fabric antibacterial but also enhanced its UV radiation absorption capability, surface hydrophobicity, electro-conductivity, tensile strength, and thermal stability. The maximum degradation temperature of the wool fibre increased from 339.5 °C to 349.6 °C and the UV-B transmission through the fabric at 290 nm reduced to 1.5%. The surface hydrophobicity of the treated fabric samples also improved as the surface contact angle of the fabric increased from 119.5° for the untreated to 131.7° for the fabric treated with 4% ABSA. The surface electrical resistance decreased from × 10 9 to 484 × 10 9 Ohm cm &#;1 , and the treated fabric also showed excellent antibacterial activity against Staphylococcus aureus and Klebsiella pneumoniae. The developed treatment could be used in the textile industry as an energy-efficient process for the multi-functionalisation of wool and other polyamide fibres.

The target of this work is to develop an energy-efficient treatment that makes wool fabric multifunctional. In this work, for the first time, we are reporting the application of ABSA for the improvement of various physical and chemical properties of wool fabric. The effect of ABSA treatment on thermal stability, mechanical properties, antibacterial, antistatic, and UV radiation resistance of wool fabric were systematically investigated and reported.

Recently the synthesis of sulphonated polyaniline, a self-doped electroconductive polymer, has attracted the attention as an alternative to polyaniline, 20&#;22 but the poor reactivity of some sulphonic acid derivatives of aniline containing an electron attractive sulfonic acid group made the synthesis of high molecular weight polymer difficult. 23 Therefore, the synthesis of sulphonated polyaniline is carried out either by sulphonation of polyaniline or by copolymerisation with sulphonic acid derivatives of aniline. 20,24&#;26 In situ polymerisation of various sulphonated derivatives of aniline at mild conditions without using any oxidising agent produces only dimers as the presence of the amino group in the ortho position and methoxy group in the benzene ring inhibits their polymerisation. 27 The oxidative polymerisation of 2-methoxyaniline-5-sulphonic acid successfully produced a mixture of low (M n = ) and high molecular weight (M n = ) polymers. However, they showed poor electrical conductivity (9 × 10 &#;3 S cm &#;1 ) compared to the doped-polyaniline, 26,28 but that could be enough for antistatic applications.

The generation of static electricity in apparel can have serious effects as the static charge can cause fire and damage to electronic equipment. Wool fibre-made apparel is mostly used as outerwear and can encounter electronic equipment, and the static charge formed in the apparel may damage them. Polyaniline is one of the most explored polymers for making textiles electro-conductive and antistatic. 13,14 However, its monomer, aniline, is not only highly toxic with an LD 50 (rat) value of 100 mg kg &#;1 but also a possible carcinogen. 15 Moreover, aniline is soluble in water only at highly acidic conditions but at that conditions, it cannot be absorbed into wool fibre as both aniline and wool fibre are cationic. However, its sulphonated derivatives, such as ABSA with an LD 50 (rat) value of 12 300 mg kg &#;1 could be an eco-friendly alternative. 16 ABSA can readily be absorbed into the wool fibres by electrostatic attractions and also can be polymerised at ambient conditions like aniline. 15,17 It has been reported that polyaniline treated fabrics showed very good antibacterial activity, 18,19 but no published literature until present reported the application of ABSA to any textile fibre as an antibacterial agent.

Antibacterial treatments are frequently used for the treatment of textile fabrics to protect the wearer from harmful bacteria and fungi. Quaternary ammonium compounds, 8 chitosan, 9 polyhexamethylene biguanide, 10 ε-poly-l-lysine, 11 and silver nanoparticles, 4 have been investigated to make wool fabric antibacterial. Wool fabrics treated with quaternary ammonium compounds not only provide antibacterial activity but also enhance their antistatic properties. However, some of these compounds, such as triclosan (which is a chlorinated bisphenol), are known to produce antibiotic resistance. 12 These treatments provide protection against only selective bacterium class and therefore, there is a need to develop new antibacterial treatments that do not produce antibiotic resistance.

To make wool fabric UV protective, a range of methods including the treatment with silver and cupric oxide nanoparticles, 3,4 honeysuckle extract, 5 β-cyclodextrin, 6 and UV absorbers derived from hydroxyphenyl benzotriazole, 7 have been investigated. UV radiation absorbers and nanoparticles provide some levels of protection to the wearer from detrimental solar irradiation but some of these compounds and nanoparticles may have harmful effects on human health.

Because of their exceptional warmth, fire-retardancy, stain-resistance, and odour control properties, wool fibres are used for the manufacture of apparel and interior textiles. 1,2 However, wool fibre has poor UV protection, antibacterial, and antistatic properties. To enhance these properties, wool fabrics are treated with various organic compounds and polymers by multi-stage treatments, which increase the treatment cost and time. Technical textiles (such as workwear), as well as outdoor textiles, need to have multifunctional properties, such as antistatic, UV protective, and antibacterial properties. Multi-stage treatments are usually used to introduce these properties. However, if they can be introduced by a single treatment, it may reduce the fibre damage, production cost, and time.

The antibacterial activity of wool fabrics treated with various concentrations of ABSA was performed according to the AATCC Test Method 147- (Assessment of Textile Materials: Parallel Streak Method) against Staphylococcus aureus and Klebsiella pneumoniae. In this method, the bacterial culture was prepared using a nutrient broth of 5 g l &#;1 peptone and 3 g l &#;1 beef extracts with the pH set at 6.8 ± 0.1. Five streaks of 100 times diluted bacteria were placed on the sterilised agar gel in a Petri dish by using an inoculating loop and a specimen was gently placed in intimate contact with the agar surface. The Petri dishes were incubated for 48 h at 37 ± 2 °C and the growth of bacteria along the side and underneath of the test specimen was observed.

The contact angle was measured in a dynamic mode by using a KSV Contact Angle Measurement Apparatus (Model: CAM 100, KSV Instruments, Helsinki, Finland). For each treatment, the contact angle was measured at 10 positions of top and bottom surfaces of the fabric and the average contact angle was reported. The first measurement was taken immediately after placing the droplet of water on the test fabric surface and then at 30 s interval until 120 s. The surface morphologies of the fabric were examined by a Hitachi scanning electron microscope (Model: TM Plus, Hitachi Corporation, Japan) at an accelerated voltage of 15 kV without any conductive coating. The elemental analysis of C, O, N, and S was carried out by an energy dispersive X-ray (EDX) using the same scanning electron microscope that was used for the surface morphology study equipped with Quantax 75 energy dispersive X-ray attachment. The change in chemical characteristics of the surface of the treated wool fabrics was assessed by the Fourier transform infrared (FTIR) spectroscopy. The FTIR spectra of the fabric surface were recorded by a spectrometer (Model: System , PerkinElmer Corporation, USA) equipped with an attenuated total reflectance (ATR) attachment at a resolution of 4 cm &#;1 in the range from 650 to cm &#;1 by using a ZnSe crystal and 64 scans were signal-averaged. The untreated and various ABSA-treated wool fabrics were preconditioned at the standard atmospheric conditions (20 ± 2 °C and 50 ± 2% relative humidity) for 48 h and the surface resistance measurement was carried out at that standard atmospheric conditions at an applied voltage of 100 V by a surface/volume resistance meter with a concentric ring probe (Model 152-1, Trek, Inc., Lockport, USA). At least 10 measurements were taken for each treated sample at various positions of the fabric and the averages are reported here.

The thermal stability of ABSA-treated and untreated wool fabrics was assessed by thermo-gravimetric (TG) analysis. 12 mg of treated and untreated wool fabric samples were heated by a Shimadzu Thermogravimetric Analyser (Model: TGA-50H, Shimadzu Corporation, Japan) from room temperature to 600 °C at a heating rate of 10 °C min &#;1 under the nitrogen atmosphere. The tensile properties were measured at 20 ± 2 °C and 65 ± 2% relative humidity (RH) according to ASTM Test Method D-06: Standard Test Method for Breaking Force and Elongation of Textile Fabrics (Strip Method) by using an Instron Universal Tensile Testing Machine (Model , Instron Testing Systems, Inc., Norwood, USA) by extending a 25.4 × 152.4 mm sample at 50 mm min &#;1 using a gauge length of 100 mm. The samples were pre-conditioned at the above-mentioned conditions for 3 days and at least 10 samples were tested for each treatment.

The treatment of wool fabric with ABSA was carried out in an Ahiba laboratory dyeing machine (Model , Datacolor International, Switzerland) using materials to liquor ratio of 1 : 30. Wool fabric samples were placed in the treatment bath prepared with the required quantity of ABSA, 1 g l &#;1 Albegal FFA (antifoaming agent), and 0.25 g l &#;1 Sandozin MRN. A wool fabric sample wrapped on a perforated stainless-steel carrier was introduced into the bath. The pH of the bath was set at 3.0 ± 0.1 and the temperature of the bath was then slowly increased to 25 °C at 1 °C min &#;1 , held for 15 min, PPS added and held for 120 min. After the completion of the polymerisation treatment, the liquor drained, and the treated samples were washed with 1 g l &#;1 Sandoclean PC at 50 °C for 15 min. They were then rinsed with cold water several times and dried at 60 °C for 30 min.

Results and discussion

Polymerisation of ABSA and the interaction with wool fibre

The possible mechanism of polymerisation of ABSA and the interaction of the polymerisation product with wool fibre are shown in . The oxidative polymerisation of ABSA possibly did not produce a high molecular weight polymer rather produced dimers of ABSA due to the steric hindrance induced by the sulfonic acid group at the C4 position of ABSA,23 which can act as an ionic crosslinking agent. The sulphonate groups of dimers of ABSA formed ionic crosslinking with the two protonised amino groups of two macromolecular chains of wool fibre as shown in .

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Electrical conductivity and the durability of the treatment to washing

The electrical conductivity was measured by measuring the surface resistance, the higher the surface resistance the lower the electrical conductivity. shows the surface electrical resistance of wool fabric treated with various concentrations of ABSA treated at pH 3 for 60 minutes. The untreated wool fabric showed quite high electrical resistance, × 109 Ohm cm&#;1. The treatment with ABSA had a positive effect on the reduction of surface resistance of wool fabrics. Even at 1% on the weight of fibre (owf) of ABSA dosage, the surface resistance was reduced to 690.4 × 109 Ohm cm&#;1. Further increase in the ABSA concentration to 4% owf reduced the surface resistance to 532.7 × 109 Ohm cm&#;1.

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shows the effect of treatment pH on the surface resistance of the ABSA-treated fabrics. The surface resistance increased with the increase in the treatment pH. The lowest surface resistance was shown by the fabric treated at pH 3. The surface resistance is inversely proportional to the applied dosage of ABSA. The increase of pH reduces the absorption of ABSA by wool. Wool fibres absorb ABSA in acidic conditions as the isoelectric point of wool fibre in an aqueous medium is at pH 4.5 (i.e. above this pH point wool fibre is anionic), and the decrease in pH below 5 increases the cationic charge of wool fibre. Therefore, the absorption of ABSA by wool was the highest at pH 3 and the fabric treated at pH 3 showed the lowest surface charge. shows the effect of treatment time on the wool fabric treated at pH 3 with 4% ABSA. The increase in treated time decreased the surface resistance as the increase in treatment time increased the absorption of ABSA into the wool fibre. The sodium dodecylbenzene sulphonate and cetyltrimethylammonium bromide mixed surfactant-doped polyaniline nanofibres showed electrical conductivity as high as 0.102 S cm&#;1.29 It was reported that ABSA as a dopant at the ABSA to aniline ratio of 3&#;:&#;7 produced a polyaniline film that had an electrical conductivity of 2.4 × 10&#;3 S cm&#;1 compared to 100% polyaniline film, which exhibited the conductivity of 3.3 × 10&#;8 S cm&#;1. However, the increase of the ABSA ratio to 7&#;:&#;3, the electrical conductivity of polyaniline decreased to 5.7 × 10&#;3 S cm&#;1. The separation of the low molecular weight components of poly(2-methoxyaniline-5-sulphonic acid) from the high molecular weight components increased the electrical conductivity of the produced film to 9 × 10&#;3 S cm&#;1.28 The wool fabric treated with 4% ABSA in this work exhibited quite poor electrical conductivity, which is consistent with the results reported for the sulphonated polyaniline films and nylon and polyester fabric treated with polyaniline.24,30 However, this level of conductivity is enough for the application as an antistatic fabric. As the optimum pH and treatment time are 3 and 120 min respectively, further experiments were carried out at those conditions.

The durability of the treatment to washing was measured by measuring the surface resistance of the fabric treated with 4% owf ABSA at pH 3 for 30 min after 5, 10, 15 and 20 cycles of washing in a Wascator. Fig. S1 (ESI&#;) shows the surface resistance of fabric after various cycles of washing. The unwashed fabric showed the surface resistance of 480.4 × 109 Ohm cm&#;1. Hardly any change of the surface resistance observed for the fabric washed for 5 cycles of washing. Even after 20 washing only a marginal increase in the surface resistance was observed (495.3 × 109 Ohm cm&#;1), which shows that the treatment is quite durable to washing.

Thermal stability

shows the thermogravimetric curves of untreated and ABSA-treated wool fabric at 4% owf. It is evident that the thermal stability of wool fabric considerably improved as the temperature of the maximum degradation moved from a lower temperature to a higher temperature. The untreated wool fabric showed a mass loss at three stages: from room temperature to 110 °C, 241 to 457 °C and 457 to 600 °C. The maximum mass loss occurred at 241 to 457 °C and it was &#;53%. The next highest mass loss (almost 9%) occurred at room temperature to 105 °C, which is mainly due to the loss of absorbed moisture. The wool fabric treated with 4% owf ABSA had a mass loss at two stages: 247 to 505 °C and 505 to 600 °C. At room temperature to 247 °C, very negligible mass loss was observed (&#;1.5%) compared to 9% mass loss observed for the control wool, which suggests that the ABSA treatment considerably reduced the moisture content of wool fibre.

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In the first stage, the maximum mass loss (&#;73.5%) occurred due to the loss of volatiles formed by the thermal degradation of wool keratin and ABSA. The maximum degradation temperature of the control wool was at 339.5 °C, which moved to 349.6 °C for the wool fabric treated with 4% owf ABSA. The maximum mass loss occurred at the second stage (for the ABSA treated wool at the first-stage) due to the loss of volatiles formed by the thermal degradation of wool keratin with the elimination of hydrogen sulphide gas due to the destruction of disulphide bonds of wool, which is thought to be the primary cause of mass loss at this stage.31,32 However, it can be seen the char yield decreased from 29.1% for the control wool fabric to 26.7% for the wool fabric treated with ABSA, which suggest that the degradation of ABSA possibly continued with an increase in the temperature or the degradation products of ABSA induced further degradation of wool keratin. Possibly, the increase in sulphur content in the ABSA-treated wool increased its thermal stability as sulphur species slowed down the thermal degradation of wool by destroying the radicals generated during the combustion of wool fibre.31

Surface hydrophobicity

shows the contact angle of the untreated and treated wool fibre surface and the optical micrographs of the water droplets on the surface of the fabric are shown in Fig. S2 (ESI&#;). The treatment of wool fabric with ABSA made the fabric strongly hydrophobic. The control wool fabric showed some level of hydrophobicity as the contact angle at 0 s was 119.5°, which slowly decreased to 115.9° after 120 s, which is consistent with the previously published data.33,34 The surface of the untreated wool fabric is hydrophobic because of the presence of 18-methyl eicosanoic acid (18-MEA), which is bonded to the fibre surface through thioester bonds. The contact angle, especially at 120 s, increased with an increase in the ABSA concentrations. The highest contact angle was shown by the surface of wool fabric treated with 4% owf ABSA, and the contact angle increased to 131.7°, which was stable at least for 120 s. The blocking of water-loving amino groups of wool by crosslinking with ABSA polymer enhanced the hydrophobicity of wool fabric.

ABSA dosage (% owf)Average contact angle (°) at0 s30 s60 s90 s120 s.........................7Open in a separate window

UV transmission through the wool fabric

shows the UV light transmission through the untreated and treated fabrics. UV radiation levels are divided into three zones, UV-A (320&#;400 nm), UV-B (290&#;320 nm) and UV-C (200&#;290 nm). Of the solar UV radiation reaches the earth, 6% in the UV-B region and the rest 94% in the UV-A region but UV-B is times more damaging compared to the UV-A. Therefore, the fabric's ability to block UV-B is the determining factor to protect the wearer from the damaging effects of sun exposure. Therefore, for apparels, UV transmission at 290 nm is measured.35 For the untreated control fabric, the UV transmission at 290 nm is 3.86%. Generally, a fabric showing less than 2% of UV-B transmission is treated as a good UV protective fabric. The increase in ABSA concentration progressively decreased the UV light transmission through the fabric, and at 4% ABSA concentration, the UV light transmission reduced to 1.56% at 290 nm. The results suggest that the treatment of wool fabric with ABSA considerably increased its UV protection capability. The UV-vis spectrum of ABSA measured by others shows that ABSA and polyaniline both have an absorption peak at 248 nm suggesting that it may absorb UV radiation.36,37 Therefore, it is not unusual that ABSA dimer treated wool fabric showed good UV radiation absorption capacity.

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Antibacterial properties

shows the antibacterial performance of wool fabric treated with various concentrations of ABSA against Staphylococcus aureus (Gram-positive) and Klebsiella pneumoniae (Gram-negative) bacteria. The untreated fabric did not show any antibacterial activity and no zone of inhibition as spontaneous growth of bacteria was observed directly under the specimens for both types of bacteria (not shown). In the case of 1% ABSA, scanty growth of bacteria directly under the test specimens was observed and no zone of inhibition was observed (not shown). The increase of concentration of ABSA suppressed the growth of both types of bacteria directly under the test specimens but the samples did not show any zone of inhibition even at the highest concentration investigated (4% owf). Therefore 2% owf should be the lowest concentration of ABSA consistent with the non-leaching type antibacterial treatments. The results suggest that no leaching of ABSA took place into agar gel medium and ABSA is quite effective as an antibacterial agent.

ABSA conc. (%)Antibacterial performance against Staphylococcus aureus Klebsiella pneumoniae 2 3 4 Open in a separate window

Hejchman et al. reported that N-(3,5-dichloro-2-hydroxybenzylidene)-4-aminobenzenesulphonic acid showed strong antibacterial activity against Staphylococcus aureus.38 Antibacterial activity of poly(sodium 4-styrene sulphonate) against Neisseria gonorrhoeae, and Chlamydia trachomatis is already known.39 Yadav et al. found that 4-[1-(substituted aryl/alkyl carbonyl)-benzoimidazol-2-yl]-benzenesulphonic acids showed excellent antibacterial activity.40 Linear alkyl benzene sulphonic acid also reported to have antibacterial activity against some aquatic bacteria.41 Therefore, it is not surprising that ABSA-treated fabric showed good antibacterial activity. ABSA polymer probably induced changes in the pHin and also caused hyperpolarisation of the cell membrane of bacteria resulting in the death of bacteria.42

Tensile properties

The mechanical properties were assessed to observe the effect of ABSA addition to wool on its tensile strength, breaking tenacity and elongation at break. The addition of ABSA to wool only showed a marginal effect on the tensile strength of wool fibre as shown in . The tensile strengths in the warp and weft directions shown by the untreated fabric were 11.1 ± 0.2 and 9.6 ± 0.3 MPa respectively. The tensile strength in both directions slightly increased with an increase in the applied dosage of ABSA as the tensile strength increased from 11.1 ± 0.2 and 9.6 ± 0.3 MPa to 12.0 ± 0.2 and 10.3 ± 0.3 MPa respectively for the fabric treated with 3% owf of ABSA. Further increase in the concentration did not further increase the tensile strength. On the other hand, the breaking tenacity, elongation at peak and elongation at break of the fabric slightly decreased with an increase in the applied dosage of ABSA. It appears that anionic sulphonate groups of ABSA dimers formed ionic crosslinking with the cationic amino groups of wool fibre, which increased the tensile strength of the fabric but decreased the molecular chain movement in wool fibre resulting in a decrease in elongation at peak and elongation at break. It is known that ionic crosslinking and polymeric coating increases the tensile strength of the coated fabric but reduces the extensibility.43,44 Therefore, ionic crosslinking probably caused the increase in the tensile strength of the ABSA-treated fabric.

ABSA dosage (% owf)Warp-directionWeft-directionTensile strength (MPa)Elongation at peak (%)Breaking force (N)Elongation at break (%)Tensile strength (MPa)Elongation at peak (%)Breaking force (N)Elongation at break (%)011.1 ± 0.255.4 ± 1. ± 458.8 ± 1.19.6 ± 0.352.7 ± 2. ± 167.0 ± 0..6 ± 0.352.9 ± ± 256.2 ± 0.910.0 ± 0.250.1 ± 1. ± 367.8 ± 0..9 ± 0.350.9 ± ± 359.3 ± 1.210.2 ± 0.249.8 ± 2. ± 368.2 ± 1..0 ± 0.250.2 ± ± 257.9 ± 0.810.3 ± 0.350.0 ± 1. ± 267.3 ± 0..0 ± 0.250.7 ± ± 459.7 ± 2.110.3 ± 0.353.0 ± 2. ± 269.0 ± 0.8Open in a separate window

Elemental analysis

The EDX spectra of wool fabric treated with various concentrations of ABSA are shown in Fig. S3 (ESI&#;) and the chemical compositions are provided in Table S1 (ESI&#;). The treatment with ABSA had a considerable effect on the elemental compositions of wool fibre, especially on the sulphur content of the fibre. The composition of C, O, N, and S of untreated wool fibre was 52.68, 25.92, 19.79 and 1.61% respectively as shown in Table S1 (ESI&#;). The C and O content decreased, and the S content increased with an increase in the concentration of ABSA. The S content increased from 1.61% for the control fabric to 4.55% for the fabric treated with 4% owf ABSA. As ABSA contains sulphur, suggests the presence of ABSA in the treated wool fibres. The elemental mappings of wool fabric treated with various concentrations of ABSA are shown in , which shows that all the elements including the S element are uniformly distributed in the treated fibres suggesting that ABSA was uniformly absorbed into the wool fibre.

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Surface morphologies

SEM scanning was carried out to observe the surface coverage of wool fibres by poly(ABSA) at different concentrations of ABSA monomer. shows the SEM images of wool fabrics treated with 1, 2, 3, and 4% ABSA at 25 °C. The wool fabric surface treated with 1% ABSA still exhibited the typical scaly structure of the wool fibre with some uniform but small size deposition of ABSA on the fibre surface. The surface coverage of wool fibre gradually increased with an increase in the concentration of ABSA. The fabric treated with 2% ABSA showed more coverage of fibre surface by ABSA compared to the wool fabric sample treated with 1% ABSA. The fabric treated with 4% ABSA not only shows the coverage of single fibre with ABSA but also shows the binding of fibres by the formed ABSA polymer. The formation of the film suggests that ABSA was polymerised to poly(ABSA) by oxidative polymerisation. However, the small increase in tensile strength of the treated wool fabric (only 8.2%) suggests that the polymerisation only produced low molecular weight polymer and the formed ABSA films could be quite weak. Therefore, the increase in ABSA concentration only marginally increased the tensile strength of the fabric.

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ATR-FTIR

The ATR-FTIR spectra of untreated and wool fabrics treated with various concentrations of ABSA are shown in . The spectrum of wool fibre shows typical IR bands of wool fibre, such as wool keratin-related IR bands of amide III, amide II and amide I peak at , , , and cm&#;1, respectively.45 The peak at cm&#;1 could be attributed to the C&#;N stretching and also to the N&#;H in-plane bending vibrations (amide II). The broad IR band at cm&#;1 could be attributed to the hydroxyl groups of wool fibre. The band at cm&#;1 could be attributed to the Bunte-salt groups of wool fibre. Conversely, the wool fabric treated with ABSA shows new bands at 668, 800, and cm&#;1 and they could be attributed to the benzene ring, C&#;S bond, and sulphonate groups of ABSA respectively. The spectra of ABSA-treated wool fabrics also show new IR bands at cm&#;1, which is due to the C&#;N stretching vibration of ABSA. The intensity of IR bands of sulphonate groups at cm&#;1 increased with an increase in the concentration of ABSA. The presence of sulphonate groups (&#;SO3) in the spectra of wool fabric treated with various concentrations of ABSA suggests the presence of ABSA in the treated fabric and the decrease in the intensity of amide peaks suggest the formation of crosslinking of ABSA with the amino groups of wool fibre.

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