polymers

Article Protective Bleaching of Hair in a Neutral Ethanol–Water System

Liangjun Xia 1,2,† ID , Chunhua Zhang 1,†, Wenfang Xu 1, Kundi Zhu 1, Aming Wang 1, Ye Tian 3, Yunli Wang 1,4,5,* and Weilin Xu 1,*

1 State Key Laboratory of New Materials and Advanced Processing Technologies, Wuhan Textile University, Wuhan 430200, ; [email protected] (L.X.); [email protected] (C.Z.); [email protected] (W.X.); [email protected] (K.Z.); [email protected] (A.W.) 2 Institute for Frontier Materials, Deakin University, Geelong, Victoria 3216, Australia 3 Guangdong Esquel Co., Ltd., Esquel Group, Foshan 528500, China; [email protected] 4 College of Chemistry and Chemical Engineering, Wuhan Textile University, Wuhan 430200, China 5 Hubei Biomass and Eco-Dyeing & Finishing Key Laboratory, Wuhan Textile University, Wuhan 430200, China * Correspondence: [email protected] (Y.W.); [email protected] (W.X.); Tel./Fax: +86-27-5936-7690 (Y.W. & W.X.) † These authors contributed equally to this work.

 Received: 5 June 2018; Accepted: 30 June 2018; Published: 3 July 2018 

Abstract: As conventional bleaching under alkaline conditions is chemically damaging to protein fibers, a three-stage protective bleaching process in neutral ethanol–water mixtures was proposed for camel hair using mordanting with ferrous salts, oxidative bleaching with hydrogen peroxide, and reductive bleaching with sodium hydrosulfite. The aim of this work was to improve the whiteness degree of camel hair without substantial tenacity loss. In addition, the roles of ethanol during the bleaching treatment were also examined by characterizing the fibers using scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared (FTIR) spectroscopy, and X-ray diffraction. The whiteness degree and mechanical properties of camel hair bleached in the neutral ethanol–water system were significantly superior to those of fibers bleached by a conventional method. SEM images showed no visible cracks on the scales of fibers bleached in the ethanol–water system, whereas large grooves were observed on fibers bleached in aqueous solution. TEM images confirmed the positive influence of ethanol on the mordanting process, and FTIR spectra suggested that ethanol reduced the breakage of hydrogen bonds in the fibers during the oxidative bleaching process. These findings indicate the potential of this protective bleaching method for application to a broad range of other natural protein fibers.

Keywords: camel hair; natural ; bleaching; ethanol–water mixture; whiteness; mechanical properties

1. Introduction As an important class of specialty natural fiber, camel hair has distinctive characteristics, such as luster, softness, warmth, and natural colour [1–3]. Owing to its exceptional temperature-regulating properties, camel hair is an ideal material for apparel applications. Therefore, the demand for such rare animal fibers may increase as their use in some consumer items, such as high-grade fabrics, makes them more attractive [4]. Despite its small quantitative contribution, the significance of camel hair in the apparel and textile industry should not be underestimated. Camel hair is normally found in various shades of brown or gray; however, high levels of whiteness are essential for apparel fibers [5]. To achieve white or pastel colours, these fibers must be bleached to selectively decolorize the natural

Polymers 2018, 10, 730; doi:10.3390/polym10070730 www.mdpi.com/journal/polymers Polymers 2018, 10, 730 2 of 18 pigment before dyeing [6,7]. Hence, investigations of protein fiber bleaching are of practical as well as academic interest. Several methods are known for protein fiber bleaching, and alkaline hydrogen peroxide has become one of the most widely used bleaching agents for protein fibers, such as , cashmere, hair, and camel hair. The typical process involves pretreatment with a mordant followed by bleaching with hydrogen peroxide under alkaline conditions [6]. Chen et al. [8] studied the influence of chlorination followed by mordanting with Fe2+ and bleaching with hydrogen peroxide on the properties of bleached karakul wool. This modification process was found to improve the felting propensity of karakul wool slightly while remarkably increasing the whiteness. In another study, Mortazavi et al. [9] suggested that the use of Cu2+ as a catalyst in the mordanting process before peroxide bleaching enhances the optical properties of karakul wool fibers. This finding was shown more systematically by Yan et al. [10], who explored the relationship between the use of hydrogen peroxide and the properties of protein fibers, showing that the handling and mechanical properties of yak hair improved after bleaching at an optimal concentration of hydrogen peroxide. Interestingly, Arifoglu et al. [11] observed that a high concentration of urea (>100 g/L) plays a significant role in promoting the whiteness of wool and reducing the bleaching time, whereas a lower concentration of urea has no significant effect. In their work on bleaching cashmere, Nakajima et al. [12] suggested that adding small amounts of sodium bisulfite to the rinse bath could effectively promote the whiteness of Mongolian cashmere. However, as a wet process of the textile industry, bleaching is chemically damaging to protein fibers, especially under alkaline conditions, which can result in considerable loss of breaking tenacity [13–15]. The breaking tenacity has an important influence on the finished product rate. ‘Green’ solvents are defined as solvents that have minimal environmental influence arising from their use in chemical production [16]. Hence, to address this issue, alcohols, such as ethanol, have been taken into account. They are nontoxic, environment-friendly, widely available, have low boiling points, and are easily recycled [17–20]. Additionally, owing to its low surface tension, low viscosity, low molecular weight, and strong permeability, the application of ethanol in textile dyeing has received considerable attention [21–23]. Nevertheless, in the past few decades, very little scientific research has been focused on investigating the application of ethanol to bleaching protein fibers, including camel hair. In this work, to obtain maximum whiteness with minimal tenacity damage, we proposed an approach for bleaching camel hair under neutral conditions with the addition of ethanol. Our new method is based on the oxidation-reduction bleaching method and consists of three stages: mordanting with ferrous salts, oxidative bleaching with hydrogen peroxide in mixtures of ethanol–water, and reductive bleaching with sodium hydrosulfite in the aqueous solution. This study was undertaken with the aim of determining a method for improving the whiteness degree of camel hair without substantial tenacity loss.

2. Materials and Methods

2.1. Materials Scoured camel hair with a mean diameter of 22.22 µm and an average length of 43.62 mm was kindly supplied by Henan Riyifengda Hair Products Co. Ltd., China. Ferrous sulfate heptahydrate (FeSO4·7H2O) was used as the mordant. Hydrogen peroxide (H2O2) was a 30% (w/w) aqueous solution. Sodium hydrosulfite (Na2S2O4), absolute ethanol, and all other chemicals were of analytical grade and purchased from Sinopharm Chemical Reagent Co. Ltd., Shanghai, China.

2.2. Bleaching The bleaching process for camel hair mainly consisted of three stages: mordanting with FeSO4·7H2O, oxidative bleaching with H2O2, and reductive bleaching with Na2S2O4. All trials Polymers 2018, 10, 730 3 of 18

Polymers 2018, 10, x FOR PEER REVIEW 3 of 18 werePolymers carried 2018 out, 10 in, x anFOR XH-KG55B PEER REVIEW (Foshan Automation Equipment Co. Ltd., Foshan, China) laboratory3 of 18 dyeing apparatus at a fiber-to-liquor ratio of 1:100, and cold deionized water was used for washing dyeing apparatus at a fiber-to-liquor ratio of 1:100, and cold deionized water was used for washing fibersdyeing throughout apparatus the experiments.at a fiber-to-liquor A more ratio detailed of 1:100, descriptionand cold deionized of each water of the was bleaching used for stages washing follows. fibers throughout the experiments. A more detailed description of each of the bleaching stages fibers throughout the experiments. A more detailed description of each of the bleaching stages follows. 2.2.1.follows. Mordanting

2.2.1. Mordanting The2.2.1. composition Mordanting of the ethanol–water mordant bath was 2 g/L of FeSO4·7H2O in 40% deionized water andThe 60% composition absolute ethanol of the ethanol by volume.–water Camelmordant hair bath fibers was 2 (1.0 g/L g)of wereFeSO4 introduced·7H2O in 40% into deionized the bath at ◦ The composition of the ethanol–water mordant bath was 2 g/L of FeSO4·7H2O in 40% deionized 55 Cwater and mordantingand 60% absolute was ethanol carried by out volume. at this Camel temperature hair fibers for (1.0 30 g) min. were After introduced mordanting, into the thebath fibers water and 60% absolute ethanol by volume. Camel hair fibers (1.0 g) were introduced into the bath wereat rinsed 55 °C atand room mordanting temperature was carried for at out least at this 10 min temperature and then for wrung 30 min. to After 100% morda waternting, content. the fibers at 55 °C and mordanting was carried out at this temperature for 30 min. After mordanting, the fibers were rinsed at room temperature for at least 10 min and then wrung to 100% water content. were rinsed at room temperature for at least 10 min and then wrung to 100% water content. 2.2.2. Oxidative Bleaching 2.2.2. Oxidative Bleaching Oxidative2.2.2. Oxidative bleaching Bleaching was performed in a mixed solution containing 20 g/L of hydrogen peroxide Oxidative bleaching was performed in a mixed solution containing 20 g/L of hydrogen◦ peroxide in 100% absoluteOxidative ethanol. bleaching The was camel performed hair fibersin a mixed were solution immersed containing into the 20 bathg/L of at hydrogen 65 C and peroxide treated for in 100% absolute ethanol. The camel hair fibers were immersed into the bath at 65 °C and treated for 1 h followedin 100% absolute by rinsing ethanol. for at Th leaste camel 10 min.hair fibers Subsequently, were immersed the rinsed into the fibers bath at were 65 °C squeezed and treated to for remove 1 h followed by rinsing for at least 10 min. Subsequently, the rinsed fibers were squeezed to remove excess1 h water followed before by rinsing being placedfor at least into 10 the min. reductive Subsequently, bleaching the rinsed bath. fibers were squeezed to remove excess water before being placed into the reductive bleaching bath. excess water before being placed into the reductive bleaching bath. 2.2.3. Reductive Bleaching 2.2.3. Reductive Bleaching 2.2.3. Reductive Bleaching ◦ ReductiveReductive bleaching bleaching was was carried carried out out at at 6060 °CC for 20 min min in in a abath bath containing containing 15 g/L 15 g/Lof sodium of sodium Reductive bleaching was carried out at 60 °C for 20 min in a bath containing 15 g/L of sodium hydrosulfitehydrosulfite in 100% in 100% water. water. The The fibers fibers were were washed washed and then then dried dried in in air. air. hydrosulfite in 100% water. The fibers were washed and then dried in air. BleachingBleaching of camel of camel hair hair in the in the water water system system was was carried carried out out under under the the same same conditions,conditions, except that Bleaching of camel hair in the water system was carried out under the same conditions, except that only aqueous solutions were used. The experimental procedures and flow chart for bleaching of onlythat aqueous only aqueous solutions solutions were used. wereThe used. experimental The experimental procedures procedures and and flow flow chart chart for for bleaching bleaching of of camel camel hair in ethanol–water and aqueous systems are shown in Figures 1 and 2, respectively. The hair incamel ethanol–water hair in ethanol and–water aqueous and aqueous systems systems are shown are shown in Figures in Figure1 ands 21, and respectively. 2, respectively. The optimalThe optimal conventional bleaching was performed in the aqueous solutions with the addition of desired conventionaloptimal conventional bleaching wasbleaching performed was perform in theed aqueous in the aqueous solutions solutions with with the additionthe addition of of desired desired alkali alkali and chemical auxiliaries. and chemicalalkali and auxiliaries.chemical auxiliaries.

Figure 1. Simulation of experimental procedures. FigureFigure 1. 1.Simulation Simulation of experimental procedures. procedures.

Figure 2. Flow chart for bleaching of camel hair in the ethanol–water and water systems. FigureFigure 2. Flow 2. Flow chart chart for for bleaching bleachingof of camelcamel hair in in the the ethanol ethanol–water–water and and water water systems. systems.

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2.3. Characterization

2.3.1. Weight Loss To evaluate the weight loss after bleaching, the treated sample was weighed and the weight loss percentage of the bleached fibers was evaluated using following equation [24]:

Weight loss (%) = (m1 − m2)/m1 × 100% (1) where m1 and m2 are the weight of fibers before and after bleaching treatment, respectively. Before weighing, all samples were dried in an oven at 60 ◦C until reaching a constant weight.

2.3.2. Degree of Whiteness The whiteness of all the samples was measured using a whiteness meter (WSB-II, Wenzhou Instruments and Apparatus Co. Ltd., Wenzhou, China). To compare the bleaching treatment in the ethanol–water and water systems, the percentage increase in whiteness was calculated using the following equation [25]:

Whiteness increase (%) = (w1 − w2)/w2 × 100% (2) where w1 and w2 are the whiteness of the samples after and before bleaching, respectively.

2.3.3. Mechanical Properties The mechanical properties of the bleached samples were determined using a Favimat-Airobot 2 system (Textechno H. Stein, Monchengladbach, Germany) at a crosshead speed of 10 mm/min and a gauge length of 20 mm according to ASTM 76. All samples were equilibrated under standard conditions (20 ◦C, 65% relative humidity (RH)) for at least 48 h before testing. Each sample was tested 100 times and the average values are presented in this paper.

2.3.4. Swelling Ratio To examine the effect of ethanol on the swelling properties of camel hair, a series of trials were performed. A single fiber was used in each swelling experiment, and its diameter was measured before and after swelling. Each swelling bath was composed of 2 g/L FeSO4·7H2O in different mixtures of ethanol and water (ethanol:water = 0:100, 60:40, and 80:20 (v/v)). Then, the single fiber was immersed into the bath at 55 ◦C and allowed to swell for 30 min. Each single fiber was positioned between two thin glass plates. A standard microscope combined with image analysis software was employed to measure the change in fiber diameter during swelling, and the swelling ratio (Rs) was calculated by the following equation [26]: Rs(%) = Da/Db × 100% (3) where Da and Db are the average diameter of a fiber after and before swelling, respectively.

2.3.5. Contact Angles A drop shape analyzer (DSA25, Krüss, Hamburg, Germany) equipped with a video measuring system with a fast and high-resolution USB3.0 camera was used to measure contact angles. To alleviate the effect of droplet deformation on the contact angle resulting from the force of gravity, we chose a drop volume of 2 µL, as this volume maintained its spherical form on the glass slide. The contact angle was measured 1 s after release of the droplet [27], and the contact angles were calculated using the supplied software (Drop shape Analysis, DSA Version 1.92.1.1, Hamburg, Germany). Polymers 2018, 10, 730 5 of 18

2.3.6. Scanning Electron Microscopy The surface morphology of the samples was inspected using scanning electron microscopy (SEM, JSM-IT300, JEOL, Tokyo, Japan) after gold–palladium coating at a voltage of 10 kV with a magnification of 100–3000×. Polymers 2018, 10, x FOR PEER REVIEW 5 of 18 2.3.7. Transmission Electron Microscopy 2.3.7. Transmission Electron Microscopy The cross-sectional morphologies of the camel hair sample were examined by transmission electron microscopyThe cross-sectional (TEM, morphologies JEOL-2100F, JEOL,of the camelTokyo, hair Japan) sample fitted were with examined an energy by transmissiondispersive X-ray spectroscopyelectron microscopy (EDS) detector. (TEM, JEOL-2100F, JEOL, Tokyo, Japan) fitted with an energy dispersive X-ray spectroscopy (EDS) detector. 2.3.8. Fourier Transform Infrared Spectroscopy 2.3.8. Fourier Transform Infrared Spectroscopy Fourier transform infrared (FTIR) spectra were recorded using a Vertex 70 spectrometer (Bruker, Fourier transform infrared (FTIR) spectra were recorded using a Vertex 70 spectrometer (Bruker, Karlsruhe, Germany) via reflection-absorption spectroscopy. The data were collected over 128 scans in Karlsruhe, Germany) −via1 reflection-absorption spectroscopy.−1 The data were collected over 128 scans the rangein the of range 500–4000 of 500 cm–4000with cm−1 awith resolution a resolution of 4 cmof 4 cm. Samples−1. Samples were were dried dried in ain vacuum a vacuum before before testing. testing. 2.3.9. X-ray Diffraction X-ray2.3.9. X- diffractionray Diffraction (XRD) data was obtained using wide-angle XRD analysis (X’Pert PRO, PANalyticalX-ray B.V., diffraction Almelo, The (XRD Netherlands)) data was obtained using Cu using Kα radiation wide-angle at 40 XRD kV analysisand 40 mA. (X’P Samplesert PRO, were scannedPANalytical from 5 toB.V., 70◦ Almelo(2θ) with, The a stepNetherland size ofs) 0.02 using◦. Cu Kα radiation at 40 kV and 40 mA. Samples were scanned from 5 to 70° (2θ) with a step size of 0.02°. 3. Results and Discussion 3. Results and Discussion 3.1. Degree of Whiteness and Weight Loss 3.1. Degree of Whiteness and Weight Loss During the bleaching process, the colour of camel hair changes and the underlying whiteness is exposed.During To examine the bleaching the effect process, of ethanol the colour on theof camel bleaching hair changes treatment, and the photographs underlying whiteness were taken is and exposed. To examine the effect of ethanol on the bleaching treatment, photographs were taken and the whiteness and weight loss values of the bleached camel hair were determined as summarized in the whiteness and weight loss values of the bleached camel hair were determined as summarized in Figure3 and Table1. Figure 3 and Table 1.

Figure 3. Photographs of control sample (a) and samples bleached in the ethanol–water (b) and water Figure 3. Photographs of control sample (a) and samples bleached in the ethanol–water (b) and water systems (c). systems (c). Table 1. Degree of whiteness and weight loss of camel hair bleached in the ethanol–water and water Tablesystems. 1. Degree of whiteness and weight loss of camel hair bleached in the ethanol–water and water systems. Degree of Whiteness Increase Weight Loss Camel Hair Whiteness (%) (%) Camel Hair Degree of Whiteness Whiteness Increase (%) Weight Loss (%) Control samples 12.3 ± 0.2 - - Control samples 12.3 ± 0.2 - - Samples bleached in the ethanol– Samples bleached in the ethanol–water system32.4 32.4 ±± 1.5 163.1 163.1 ±± 0.30.3 10.1 10.1 ± 0.2± 0.2 Samples bleachedwater in the system water system 19.0 ± 0.8 54.1 ± 0.2 9.1 ± 0.1 Samples bleached in the water 19.0 ± 0.8 54.1 ± 0.2 9.1 ± 0.1 system

As can be seen from Table 1, the degree of whiteness of bleached camel hair was greater than that of the control fibers in all cases. These results indicate that the bleaching treatment in the ethanol–

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As can be seen from Table1, the degree of whiteness of bleached camel hair was greater than that of the control fibers in all cases. These results indicate that the bleaching treatment in the ethanol–water system was more effective than that in the water system in terms of whiteness and weight loss values. The whiteness of camel hair bleached in aqueous solutions increased from 12.3 to 19.0, whereas that bleached in the ethanol–water system showed a dramatic improvement in whiteness, increasing from 12.3 to 32.4. This difference in whiteness could be ascribed to the deposition of excessive amounts of ferrous ions on the fibers during mordanting in aqueous solution [6]. Additionally, compared with the slight decrease in weight observed for the fibers bleached in the water system, more evident decreases of weight were observed when ethanol was present during the mordanting and oxidative bleaching processes. This increased weight loss may be due to the removal of more pigments and impurities from camel hair during the ethanol-assisted bleaching treatment [28]. Furthermore, the effect of ethanol was evaluated according to the whiteness increase calculated using Equation (2). The obtained whiteness increase values indicate that a specific concentration of ethanol may be important for promoting an effective bleaching process. After bleaching, a nearly 70% higher degree of whiteness was achieved in the presence of ethanol than without the addition of ethanol, indicating that more melanin is removed from the fibers when bleaching occurs in the ethanol–water mixture. In view of the findings of Laxer and Whewell [29], we consider that under the mordanting conditions in the ethanol–water mixture, it may be that only melanin reacts with ferrous ions, whereas keratin does not absorb ferrous ions from the solution.

3.2. Mechanical Properties In order to study the impact of bleaching treatment on the mechanical properties of camel hair, the breaking tenacity, minimum and maximum breaking tenacity, and breaking elongation of different samples were determined as presented in Table2.

Table 2. Mechanical properties of camel hair bleached in the water and ethanol–water systems.

Breaking Tenacity Minimum and Maximum of Camel Hair Breaking Elongation (%) (cN/dtex) Breaking Tenacity (cN/dtex) Control sample 1.65 ± 0.19 1.13/2.55 36.18 ± 4.03 Sample bleached in the ethanol–water system 1.24 ± 0.32 0.70/2.12 30.53 ± 11.12 Sample bleached in the water system 0.84 ± 0.22 0.48/1.24 16.74 ± 3.92

It is apparent that camel hair can be chemically attacked by the mordanting and bleaching agents in both the water and ethanol–water systems. The overall results clearly confirm that the bleaching in the water system caused excessive damage to the bleached camel hair, while the damage incurred by the fibers during the bleaching treatment was significantly reduced by adding ethanol to the baths (Table2). In particular, the breaking tenacity and breaking elongation values resulting from bleaching in aqueous solution were dramatically lower than those resulting from bleaching in the ethanol–water mixture. The breaking tenacity and elongation at break of the camel hair bleached in the water system decreased from 1.65 to 0.84 cN/dtex and from 36.18% to 16.74%, respectively, indicating that severe damage occurs under these conditions. By contrast, the breaking tenacity and elongation at break of the fibers bleached in the presence of ethanol only exhibited a slight decrease, from 1.65 to 1.24 cN/dtex and from 36.18% to 30.53%, respectively. Additionally, the camel hair bleached in the ethanol–water mixture had a much higher breaking energy and modulus than the fibers bleached in aqueous solution. The tenacity-elongation curves of camel hair before and after bleaching treatment are shown in Figure4. Compared with the fibers bleached in the water system, the camel hair bleached in the ethanol–water system exhibits a higher tenacity and breaking elongation. According to the investigation of Xiao et al. [30], tensile curves exhibit three distinct phases: a linear Hookean region at strains less than 3%, a yield region at strains from 3% to 25%–30%, and a post-yield region at strains beyond 30%. However, for the camel hair bleached in aqueous solution, the increased stiffness in the post-yield tensile region disappeared. Polymers 2018, 10, 730 7 of 18 Polymers 2018, 10, x FOR PEER REVIEW 7 of 18

Polymers 2018, 10, x FOR PEER REVIEW 7 of 18

Figure 4. Tenacity-elongation curves of camel hair before and after bleaching treatment. Figure 4. Tenacity-elongation curves of camel hair before and after bleaching treatment.

TEM imagesFigure 4.of Tenacity transverse-elonga fibertion sectionscurves of showcamel hairthe beforemorphology and after of bleaching camel hair treatment. under different magnifications (Figure 5). The centrally located medulla can be clearly observed, which is considered TEM images of transverse fiber sections show the morphology of camel hair under different to provide thermoregulatory properties to camel hair. At high magnification, the cuticle, cell magnificationsTEM images (Figure of5). transverse The centrally fiber locatedsections medullashow the can morphology be clearly of observed, camel hair which under is different considered membranemagnifications complex, (Figure and 5). macrofibril The centrally structures located aremedulla well- preserved.can be clearly Moreover, observed, melanin which isgranules considered are to provide thermoregulatory properties to camel hair. At high magnification, the cuticle, cell membrane alsoto provideobserved thermoregulatory to be spread irregularly properties within to the camel fiber. hair. At high magnification, the cuticle, cell complex,membrane and macrofibril complex, and structures macrofibril are structures well-preserved. are well Moreover,-preserved. melanin Moreover, granules melanin are granules also observed are to bealso spread observed irregularly to be spread within irregularly the fiber. within the fiber.

Figure 5. Transmission electron micrographs (TEM) of camel hair. (a) A cross section showing the centrally located medulla. (b) A highly magnified image showing its arrangement in an irregular form withFigure the 5. cuticle, Transmission matrix, macrofibrilelectron micrographs, and melanin (TEM) granule. of camel hair. (a) A cross section showing the Figure 5. Transmission electron micrographs (TEM) of camel hair. (a) A cross section showing the centrally located medulla. (b) A highly magnified image showing its arrangement in an irregular form centrallyItwith is well located the -cuticle,known medulla. matrix, that the ( bmacrofibril) tensile A highly properties, and magnified melanin of imagecamel granule. showinghair are determined its arrangement by the in fibrils an irregular and matrix form with[30]. Therefore, the cuticle, we matrix, consider macrofibril, that the anddisappearance melanin granule. of the post-yield tensile region in the camel hair bleachedIt is wellin the-known water thatsystem the tensilemay be properties related to of serious camel hairdamage are determinedof the fibrils by and the matrixfibrils and [31] matrix. This would lead to the reduction of disulfide bonds and interfaces between the fibrils and matrix, as the It[30] is. well-knownTherefore, we thatconsider the that tensile the disappearance properties of of camel the post hair-yield are tensile determined region in bythe thecamel fibrils hair and fibrils are assumed to be connected mechanically to the matrix via a number of covalent linkages at matrixbleached [30]. Therefore, in the water we system consider may that be therelated disappearance to serious damage of the post-yieldof the fibrils tensile and matrix region [31] in. the This camel variouswould leadintervals to the along reduction the axis of ofdisulfide the fibrils bonds [32,33] and. interfaces between the fibrils and matrix, as the hair bleached in the water system may be related to serious damage of the fibrils and matrix [31]. fibrilsFurthermore, are assumed water to be is connected thought to mechanically have a considerable to the matrix effect viaon thea number matrix ofproteins covalent in linkagesthe fibers, at Thiswhich wouldvarious may leadintervals contribute to the along reduction to the reducing axis of of the the disulfide interactions fibrils [32,33] bonds between. and protein interfaces chain betweens [34–36]. theFurther, fibrils in protein and matrix, as thefibers, fibrilsFurthermore, both are the assumed –NH water and to beis – C=Othought connected groups to have mechanicallyof amides a considerable are toable the effect to matrix form on thehydrogen via matrix a number proteinsbond of interactions covalent in the fibers, linkages in at variouspolypeptidewhich intervals may contributechains, along both to the reducingas axisinter of- orthe the intra interactions fibrils- chain [32 bonds, 33between]. [37] protein. As the chain intrinsics [34 –stability36]. Further, of the in α protein-helix, andFurthermore,fibers, even both the the fiber, water –NH results isand thought from–C=O intramolecular groups to have of aamides considerable hydrogen are able bonds effectto form [38] on , hydrogenthe attack matrix bondof proteins hydrogen interactions in bonds the in fibers, whichbypolypeptide may hydrogen contribute peroxidechains, to both reducingmay as reduce inter the- theor interactions intra mechanical- chain bondsbetweenproperties [37] protein. ofAs camel the chainsintr hair.insic [34 stability–36]. Further, of the α- inhelix, protein fibers,and both even the the –NH fiber, and results –C=O from groups intramolecular of amides hydrogen are able bonds to form [38], the hydrogen attack of bond hydrogen interactions bonds in by hydrogen peroxide may reduce the mechanical properties of camel hair. polypeptide chains, both as inter- or intra- chain bonds [37]. As the intrinsic stability of the α-helix, and even the fiber, results from intramolecular hydrogen bonds [38], the attack of hydrogen bonds by hydrogen peroxide may reduce the mechanical properties of camel hair. Polymers 2018, 10, 730 8 of 18

Polymers 2018, 10, x FOR PEER REVIEW 8 of 18 This suggestion was confirmed by FTIR analysis, which showed that considerably more hydrogen bonds wereThis suggestionbroken in the was fibers confirmed bleached by in FTIR the water analysis, system which than showed in the ethanol-assistedthat considerably bleached more hydrogen bonds were broken in the fibers bleached in the water system than in the ethanol-assisted camelPolymers hair. Finally,2018, 10, x aFOR study PEER byREVIEW Feughelman [37] suggested that the presence of cysteine in the8 of protein18 fibersbleached is mainly camel responsible hair. Finally, for a the study high by stability Feughelman of the [37] fibers suggested during that degradation. the presence of cysteine in the proteinThis fibers suggestion is mainly was responsible confirmed for by the FTIR high analy stabilitysis, which of the showed fibers during that considerably degradation. more 3.3. SEMhydrogen bonds were broken in the fibers bleached in the water system than in the ethanol-assisted 3.3bleached. SEM camel hair. Finally, a study by Feughelman [37] suggested that the presence of cysteine in SEM was employed to observe the surface morphology of camel hair. Figure6 shows SEM the protein fibers is mainly responsible for the high stability of the fibers during degradation. imagesSEM of the was control employed sample to as observe well as the camel surface hair morphology bleached in theof camel ethanol–water hair. Figure and 6 water shows systems. SEM images of the control sample as well as camel hair bleached in the ethanol–water and water systems. In the3.3 SEM. SEM image of the fibers bleached in the ethanol–water system (Figure6b), no cracks are visible In the SEM image of the fibers bleached in the ethanol–water system (Figure 6b), no cracks are visible on the scales of the fibers, meaning that the impact of degradation could be controlled with the on the SEM scales was of theemployed fibers, to meaning observe that the surface the impact morphology of degradation of camel could hair. beFigure controlled 6 shows with SEM the addition of ethanol. In contrast, the fibers bleached in aqueous solution (Figure6c) obviously suffered additionimages of of ethanol. the control In contrast, sample as the well fibers as camel bleached hair bleachedin aqueous in thesolution ethanol (Figure–water 6c) and obviously water systems. suffered considerable damage. This serious damage may result from the fact that hydrogen peroxide has considerableIn the SEM damimageage. of theThis fibers serious bleached damage in the may ethanol result–water from syst theem fact (Figure that hydrogen 6b), no cracks peroxide are visible has an anoxidative oxidativeon the scaleseffect effect ofon on thethe the fibers,surface surface meaning scales scales of that of camel camel the impacthair. hair. During During of degradation oxidation, oxidation, could disulfide disulfide be controlled linkages linkages withof cystine, of the cystine, peptidepeptideaddition linkages, linkages, of ethanol. and and hydrogen Inhydrogen contrast, bonds bondsthe fibers maymay bleached be attacked in aqueous by by hydrogen hydrogen solution peroxide(Figure peroxide, 6c), resulting obviously resulting in suffered inbreakage breakage considerable damage. This serious damage may result from the fact that hydrogen peroxide has an ofof these these interactions interactions [39 ].[39] These. These results results confirm confirm that that bleaching bleaching in the in presence the presence of ethanol of ethanol under under neutral oxidative effect on the surface scales of camel hair. During oxidation, disulfide linkages of cystine, conditionsneutral conditions makes camel makes hair camel less accessiblehair less accessible to the proteolytic to the proteolytic attack, which attack, is which in accord is in with accord the with results peptide linkages, and hydrogen bonds may be attacked by hydrogen peroxide, resulting in breakage forthe the results mechanical for the propertiesmechanical of properties camel hair. of camel hair. of these interactions [39]. These results confirm that bleaching in the presence of ethanol under neutral conditions makes camel hair less accessible to the proteolytic attack, which is in accord with the results for the mechanical properties of camel hair.

Figure 6. SEM images of camel hair: (a) control; (b) bleached in the ethanol–water system; (c) bleached Figure 6. SEM images of camel hair: (a) control; (b) bleached in the ethanol–water system; (c) bleached in the water system. in the water system. Figure 6. SEM images of camel hair: (a) control; (b) bleached in the ethanol–water system; (c) bleached 3.4. FTIRin the water system. 3.4. FTIR 3.4.FTIR FTIR spectra of camel hair before and after bleaching treatment are shown in Figure 7. Similar FTIR spectra of camel hair before and after bleaching treatment are shown in Figure7. Similar absorption bands are observed in the spectra of the three samples at around 3271 cm−1 (O-H and N- absorptionFTIR bands spectra are observedof camel hair in the before spectra and ofafter the bleaching three samples treatment at around are shown 3271 in cm Figure−1 (O-H 7. Similar and N-H), H), 2922 cm−1 (-CH2), 1626 cm−1 (Amide I), 1516 cm−1 (Amide II), and 1232 cm−1 (Amide III). These are absorption−1 bands are observed−1 in the spectra of the−1 three samples at around 3271−1 cm−1 (O-H and N- 2922the cm typical(-CH absorption2), 1626 cmpeaks (Amideof protein I), 1516fibers cm according(Amide to previous II), and 1232 investigations cm (Amide [39,40] III).. No These new are H), 2922 cm−1 (-CH2), 1626 cm−1 (Amide I), 1516 cm−1 (Amide II), and 1232 cm−1 (Amide III). These are thechemical typical groups absorption or free peaks residues of protein were formed fibers by according the bleaching to previous treatment. investigations However, the [39 peak,40]. around No new the typical absorption peaks of protein fibers according to previous investigations [39,40]. No new chemical groups−1 or free residues were formed by the bleaching treatment. However, the peak around 3270chemical cm for groups the unbleached or free residues fibers were is sharper formed bythan the that bleaching of the treatment.fibers bleached However, in both the peakthe water around and 3270 cm−1 for the unbleached fibers is sharper than that of the fibers bleached in both the water and ethanol3270 –cmwater−1 for systems. the unbleached fibers is sharper than that of the fibers bleached in both the water and ethanol–waterethanol–water systems. systems.

Figure 7. FTIR spectra of camel hair before and after bleaching treatment. FigureFigure 7. FTIR7. FTIR spectra spectra of of camel camel hairhair before and and after after bleaching bleaching treatment. treatment.

Polymers 2018, 10, x FOR PEER REVIEW 9 of 18 Polymers 2018, 10, 730 9 of 18

The main structural units in camel hair are successive α-helix turns, which are the largest class of proteinThe main secondary structural structures units in [41] camel. According hair are to successive the workα of-helix Hameed turns, and which Guo are [42] the, in largest the secondary class of proteinstructure secondary of proteins, structures N-H groups [41]. Accordingin the α-helix to theare workgenerally of Hameed hydrogen and bonded Guo [42 with], in C=O the secondarygroups of structurethe amino of acids. proteins, Hence, N-H the groups peak at in 3270 the αcm-helix−1 can are be generally attributed hydrogen to this hydrogen bonded bonding with C=O interaction. groups of theA greater amino acids.reduction Hence, in the the intensity peak at 3270 of this cm −peak1 can was be attributed observed tofor this the hydrogen fibers bleached bonding in interaction. the water Asystem greater than reduction those in bleached the intensity in the of this ethanol peak– waswater observed system, for indicating the fibers bleachedthat the in breakage the water of system these thanintramolecular those bleached hydrogen in the ethanol–water bonds during system, the indicating bleaching that treatment the breakage in aqueous of these intramolecular solution was hydrogencomparatively bonds severe. during the bleaching treatment in aqueous solution was comparatively severe. Similarly, thethe peakspeaks aroundaround 1626,1626, 1516,1516, andand 12321232 cmcm−−1 are sharper for the fibers fibers bleached in the ethanol–waterethanol–water system than for the fibersfibers bleached in the water system, which could be attributed to lessless damagedamage toto somesome amideamide groupsgroups duringduring thethe bleachingbleaching treatmenttreatment [[39]39].. TheThe SEMSEM imagesimages showshow thatthat thethe cuticle ofof camel hair bleached in the aqueous solution was seriously damaged. These findings findings are consistent with the changes observedobserved inin thethe mechanicalmechanical propertiesproperties ofof camelcamel hair.hair. Typically, in the range from from 1300 1300 to to 1000 1000 cm cm−1−, 1spectra, spectra of ofsuch such fibers fibers are arecharacterized characterized by the by thepresence presence of medium of medium-to-high-to-high intensity intensity bands bands attributed attributed to various to various sulfur-containing sulfur-containing chemical chemical groups groups[40]. Cross [40-].linkages Cross-linkages in α-keratin in αfibers-keratin are formed fibers are by formed–S-S– groups by –S-S– that groupscontribute that to contribute the physical to and the physicalmechanical and properties, mechanical as properties, well as the asstructural well as thestability structural, of camel stability, hair of[43] camel. Accordingly, hair [43]. the Accordingly, dramatic thedecrease dramatic in the decrease mechanical in the properties mechanical and properties the destruction and the of destruction the fibers bleached of the fibers in the bleached conventional in the conventionalwater system watercould systemalso be couldascribed also to be breakage ascribed of to these breakage disulfide of these bonds disulfide [44,45] bonds. [44,45].

3.5. XRD Figure8 8 displays displays thethe XRDXRD patternspatterns ofof thethe camelcamel hairhair samples.samples. AllAll thethe samplessamples showshow thethe typicaltypical diffraction pattern of α-keratin-keratin with a prominent 2θ 2θ peak around 1010°◦ and a broad peak around 2222°,◦, corresponding toto crystallinecrystalline spacingsspacings ofof 9.829.82 andand 4.39 Å,Å , respectively [[46]46].. Compared with the control sample, the intensity of these peaks was decreased after the bleaching treatment.treatment. This change may be attributed to the breakage of hydrogen bonds and covalent interactions during the bleaching bleaching process, process, leadingleading to the destruction destruction of of some some crystals crystals and and amorphous amorphous regions regions [40,47] [40,47.]. As As the the peak peak around around 10° 10 is◦ ischaracteristic characteristic of ofthe the hydrated hydrated crystalline crystalline structure structure of of camel camel hair, hair, the the decreased decreased intensity suggested thatthat thethe α-helix-helix crystal structure was weakened by the bleaching treatment.treatment. Moreover, it is possible thatthat thethe marginsmargins ofof thetheα α-helical-helical crystalline crystalline phase phase were were disordered disordered by by hydrogen hydrogen peroxide, peroxide, resulting resulting in reducedin reduced peak peak intensities. intensities.

Figure 8. X-rayX-ray diffraction intensity curves of camel hair samples.

3.6. Effects of Ethanol on Mordanting 3.6. Effects of Ethanol on Mordanting To examine the effect of ethanol on the mordanting process, a comparison was made between To examine the effect of ethanol on the mordanting process, a comparison was made between the the camel hair samples mordanted in the water and ethanol–water systems. Camel hair was camel hair samples mordanted in the water and ethanol–water systems. Camel hair was mordanted mordanted in a FeSO4 solution at a concentration of 2 g/L in water (100%) or ethanol–water (60%/40%, in a FeSO4 solution at a concentration of 2 g/L in water (100%) or ethanol–water (60%/40%, v/v) for v/v) for 30 min at 55 °C for 30 min. Subsequently, the rinsed and dried samples were examined by 30 min at 55 ◦C for 30 min. Subsequently, the rinsed and dried samples were examined by TEM. TEM.

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Polymers 2018, 10, x FOR PEER REVIEW 10 of 18 Figure9 presents TEM images of the cross sections of unmordanted (a–c) and camel hair mordantedPolymersFigure 2018 in, the109, x presents FOR ethanol–water PEER TEM REVIEW images (d–f) of and the cross water sections (g–i) systems of unmordanted under different (a–c) and magnifications. camel10 of hair 18 A comparisonmordanted of in Figure the ethanol9a,d,g– revealswater (d that–f) andthe cell water membrane (g–i) systems complex under of different the control magnifications. sample undergoes A comparisonFigure 9of presentsFigure 9a,d TEM,g reveals images that of the the cell cross membrane sections complex of unmordanted of the control (a–c) sample and camelundergoes hair some kind of modification in the samples mordanted in both the water and ethanol–water systems, mordantedsome kind of in modification the ethanol– inwater the samples (d–f) and mordant water (ged– i)in systemsboth the underwater and different ethanol magnifications.–water systems, A probably owing to the swelling of camel hair during the mordanting process with ferrous ions [48,49]. comparisonprobably owing of Figure to the 9a,d swelling,g reveals of camel that hairthe cell during membrane the mordanting complex processof the control with ferrous sample ions undergoes [48,49]. At a highersomeAt a higher kind magnification of magnification modification (Figure ( Figurein the9b,e,h), samples9b,e,h) spherical, spherical mordant melaninmelanined in both granules granules the water are are andapparent apparent ethanol in the– inwater thecortical systems, cortical cells. cells. probably owing to the swelling of camel hair during the mordanting process with ferrous ions [48,49]. At a higher magnification (Figure 9b,e,h), spherical melanin granules are apparent in the cortical cells.

Figure 9. TEM images of camel hair under different magnifications. (a–c): control sample; (d–f): Figure 9. TEM images of camel hair under different magnifications. (a–c): control sample; (d–f): sample sample mordanted in the ethanol–water system; (g–i): sample mordanted in the water system. mordantedFigure in9. TEM the ethanol–water images of camel system; hair under (g–i): different sample mordanted magnifications. in the (a–c water): control system. sample; (d–f): sampleIt is common mordanted knowledge in the ethanol that the–water chelating system; activity (g–i): sample of melanin mordanted for ferrous in the waterions is system. relatively strong It[50] is. common Hence, to knowledge further investigate that the the chelating influence of activity ethanol of on melanin the adsorption for ferrous of ferrous ions isions relatively on It is common knowledge that the chelating activity of melanin for ferrous ions is relatively strong strongmelanin [50]. Hence, granules, to elemental further investigate mappings of the the influence melanin granules of ethanol mordanted on the adsorption in the ethanol of– ferrouswater and ions on [50]. Hence, to further investigate the influence of ethanol on the adsorption of ferrous ions on melaninwater granules, systems were elemental obtained mappings using the of TEM the- melaninEDS technique granules as shown mordanted in Figure in 10. the ethanol–water and melanin granules, elemental mappings of the melanin granules mordanted in the ethanol–water and waterwater systems systems were were obtained obtained using using the the TEM-EDS TEM-EDS technique as as shown shown in inFigure Figure 10. 10.

Figure 10. TEM images and EDS elemental mappings. (a) TEM images of a melanin granule

mordanted in the ethanol–water system; (b) EDS elemental mapping of Fe in a melanin granule (a); FigureFigure(c)10. TEMTEM 10. images TEM images of images a andmelanin EDSand granule elemental EDS elemental mordanted mappings. mappings in the (a water) TEM. (a) system; images TEM images(d of) EDS a melanin of elemental a melanin granule mapping granule mordanted of mordanted in the ethanol–water system; (b) EDS elemental mapping of Fe in a melanin granule (a); in theFe ethanol–water in a melanin granule system; (c ();b ()e EDS) EDS elemental line scan profiles mapping of Fe of recorded Fe in a melanin along the granule line show (a);n (inc) ( TEMa,c). images (c) TEM images of a melanin granule mordanted in the water system; (d) EDS elemental mapping of of a melanin granule mordanted in the water system; (d) EDS elemental mapping of Fe in a melanin Fe in a melanin granule (c); (e) EDS line scan profiles of Fe recorded along the line shown in (a,c). granule (c); (e) EDS line scan profiles of Fe recorded along the line shown in (a,c).

Polymers 2018, 10, 730 11 of 18

The density of an element is indicated by the relative brightness, and the intensity of colour indicates its distribution in the melanin granules [51,52]. It is clearly observed that the distribution of Fe in the melaninPolymers 2018 granule, 10, x FOR PEER mordanted REVIEW in the ethanol–water system (Figure 10b) is more11 uniformof 18 than that mordantedThe in density the water of an element system is (Figure indicated 10 byd). the This relative was brightness, further and confirmed the intensity by theof colour EDS line scan profiles (Figureindicates 10 itse), distribution which manifested in the melanin the granules existence [51,52] and. It is relatively clearly observed uniform that the distribution distribution of of Fe in the melanin granuleFe in themordanted melanin granule in themordanted ethanol–water in the ethano system.l–water Thissystem is ( mainlyFigure 10b) due is more to the uniform strong than permeability that mordanted in the water system (Figure 10d). This was further confirmed by the EDS line scan of ethanol into camel hair. This even distribution may lead to greater catalytic decomposition of profiles (Figure 10e), which manifested the existence and relatively uniform distribution of Fe in the hydrogenmelanin peroxide granule in the mordanted subsequent in the bleaching ethanol–water process, system. resulting This is in mainly the higher due to degree the strong of whiteness for bleachedpermeability camel hair. of ethanol into camel hair. This even distribution may lead to greater catalytic Then,decomposition a series of trials of hydrogen were carried peroxide out in the to examinesubsequent the bleaching effect ofprocess, ethanol resulting on the in swellingthe higher properties degree of whiteness for bleached camel hair. of camel hair. Wetted length measurements were performed before and after swelling in the water and Then, a series of trials were carried out to examine the effect of ethanol on the swelling properties ethanol–waterof camel mixtures hair. Wetted using length the measurements optical method were described performed inbefore Section and after 2.3.4 swelling. Three in swelling the water tests were conductedand on ethanol single–water fibers mixtures in the waterusing the and optical ethanol–water method described mixtures in Section and 2.3.4. 10 Three measurements swelling tests were taken to obtain thewere diameters conducted on before single andfibers after in the thewater swelling and ethanol process–water asmixtures shown and in 10 Figure measurements 11. were taken to obtain the diameters before and after the swelling process as shown in Figure 11.

Figure 11. Optical measurements of camel hair diameter before and after swelling in the water and Figure 11.ethanolOptical–water measurements systems: (a) Ethanol:water of camel = hair 0:100 diameter (v/v), before before swelling; and (b) Ethanol:water after swelling = 0:100 in ( thev/v),water and ethanol–waterafter swelling; systems: (c) Ethanol:water (a) Ethanol:water = 60:40 ( =v/v 0:100), before (v /swelling;v), before (d) Ethanol:water swelling; ( b= )60:40 Ethanol:water (v/v), after = 0:100 (v/v), afterswelling; swelling; (e) Ethanol:water (c) Ethanol:water = 80:20 (v/v), = before 60:40 swelling; (v/v), (f) before Ethanol:water swelling; = 80:20 ( (dv)/v), Ethanol:water after swelling. = 60:40 (v/v), after swelling; (e) Ethanol:water = 80:20 (v/v), before swelling; (f) Ethanol:water = 80:20 (v/v), Table 3 exhibits the average diameters of camel hair before (Db) and after (Da) swelling as well after swelling.as the average swelling ratio (Rs). The presence of ethanol does not dramatically modify the morphology of camel hair, as the surface morphologies of the fibers are similar, but does affect the swelling properties. The swelling of camel hair in the water system and its dispersion were lower Table3 exhibits the average diameters of camel hair before ( Db) and after (Da) swelling as well as the average swelling ratio (Rs). The presence of ethanol does not dramatically modify the morphology of camel hair, as the surface morphologies of the fibers are similar, but does affect the swelling properties. The swelling of camel hair in the water system and its dispersion were lower than those in different ethanol–water systems, meaning that camel hair was more susceptible to the sorption of ethanol than water, and therefore absorbed more ethanol than water. Polymers 2018, 10, 730 12 of 18

Table 3. PolymersAverage 2018, 10 values, x FOR PEER of diameters REVIEW and swelling ratios for camel hair obtained by the optical12 of method. 18

than Systemthose in different ethanol Swelling–water Bath systems, meaning Db (µ thatm) camel hair D awas(µm) more susceptible Rs to the sorption of ethanol than2 water, g/L FeSO and· 7HthereforeO absorbed more ethanol than water. Water 4 2 21.29 ± 0.34 21.62 ± 0.55 1.02 ± 0.03 Ethanol:water = 0:100 (v/v) Table 3. Average values of diameters and swelling ratios for camel hair obtained by the optical 2 g/L FeSO ·7H O Ethanol–watermethod. 4 2 23.15 ± 1.21 24.95 ± 1.34 1.14 ± 0.08 Ethanol:water = 60:40 (v/v) 2 g/L FeSO ·7H O Ethanol–waterSystem Swelling4 2 Bath 23.70 ±D1.41b (μm) 27.88D±a 1.38(μm) 1.20 R±s0.02 Ethanol:water = 80:20 (v/v) 2 g/L FeSO4·7H2O Water 21.29 ± 0.34 21.62 ± 0.55 1.02 ± 0.03 Ethanol:water = 0:100 (v/v) 2 g/L FeSO4·7H2O The enhancedEthanol–water swelling of fibers in the presence of23.15 ethanol ± 1.21may 24.95 bedue ± 1.34 to the1.14 following ± 0.08 reasons. Firstly, as the ratio of ethanolEthanol:water in the mordanting = 60:40 (v/ bathv) increases, the sites in the fiber form more bonds 2 g/L FeSO4·7H2O Ethanol–water 23.70 ± 1.41 27.88 ± 1.38 1.20 ± 0.02 with ethanol; thus, the volumeEthanol:water of ethanol = 80 absorbed:20 (v/v) is greater than that of water. Secondly, the fiber, especially α-keratin, forms stronger hydrogen bonds with ethanol than with water; hence, increasing The enhanced swelling of fibers in the presence of ethanol may be due to the following reasons. the impetusFirstly, for swellingas the ratio ofof ethanol the fiber in the with mordanting ethanol. bath Thirdly, increases, the the nonpolar sites in the groups fiber form in moreα-keratin bonds are more likely to formwith hydrophobicethanol; thus, the bonds volume in of aqueous ethanol absorbed solution, is greater leading than to that decreased of water. absorption Secondly, the of fiber, water [37,53]. Furthermore,especially whenα-keratin, examining forms stronger swelling, hydrogen especially bonds with the ethanol effective than with wetting water; properties hence, increasing of a fiber when dipped intothe a impetus solution, for swelling it is essential of the fiber to determinewith ethanol. contactThirdly, the angle nonpolar values, groups as thein α- wettingkeratin are performance more of likely to form hydrophobic bonds in aqueous solution, leading to decreased absorption of water the fibers plays[37,53]. an important role in wet processing. Wettability investigations normally involve the measurement ofFurthermore, contact angles when examining as the initial swelling, data, especially indicating the the effective wetting wetting performance properties of when a fiber a solid and liquid interact.when Thedipped contact into a angle solution, is defined it is essential as the to angle determine formed contact by angle the intersection values, as the of wetting the liquid–solid interface [54performance,55]. In this of the experiment, fibers plays the an importantcontact angle role in between wet processing. a glass Wettability slide and investigations a liquid was used to normally involve the measurement of contact angles as the initial data, indicating the wetting simulate theperformance contact anglewhen a betweensolid and liquid a camel interact. hair The fiber contact and angle a liquid. is defined as the angle formed by the Figureintersection 12 presents of the the liquid contact–solid interface angles [54,55] formed. In this by experime differentnt, liquidthe contact drops angle on between smooth a glass glass slides. The liquidslide drop and composed a liquid was ofused water to simulate and mordantthe contact hadangle abetween contact a camel angle hair of fiber 51.0 and◦, whereas a liquid. the contact angle of the liquidFigure drop12 presents composed the contact of mordantangles formed in 90%by different ethanol liquid and drops 10% on water smooth by glass volume slides. was only ◦ The liquid drop composed of water and mordant had a contact angle of 51.0°, whereas the contact 23.1 , indicatingangle of the that liquid the drop surface composed tension of mordant of the in droplet 90% ethanol was and reduced 10% water with by volume the addition was only of ethanol. It is important23.1°, indicating to note that that the the surface decrease tension inof the surface droplet tension was reduced may with lead the addition to an increase of ethanol. in It is the affinity between theimportant solution to note and that the the fibers decrease [56 ].in Thus,surface duringtension may the lead mordanting to an increase process, in the affinity the wetting between properties of camel hairthe solution in ethanol–water and the fibers mixtures [56]. Thus, isduring greater the mordanting than that inprocess, aqueous the wetting solution properties owing of to camel the reduction hair in ethanol–water mixtures is greater than that in aqueous solution owing to the reduction in in angle contactangle contact [54]. [54] As. As a a result, result, ferrous ferrous ions ions are more are morelikely to likely be absorbed to be in absorbed the ethanol– inwater the system ethanol–water system thanthan in in the the water water system. system. It It is is known known that that ferrous ferrous ions ionshave ahave positive a positive effect on effect the catalytic on the catalytic decompositiondecomposition of hydrogen of hydrogen peroxide peroxide [57 [57–59–59].]. In In the the classical classical Fenton Fenton-type-type system, system, the reaction the of reaction of ferrous ionsferrous with ions hydrogen with hydrogen peroxide peroxide is is used used toto generategenerate hydroxyl hydroxyl radicals, radicals, which then which carry then out carry out chemical oxidation of melanin in camel hair. chemical oxidation of melanin in camel hair.

Figure 12. Illustration of contact angles formed by liquid drops on smooth glass slides: (a) Ethanol: Figure 12. Illustration of contact angles formed by liquid drops on smooth glass slides: (a) Ethanol: water = 0:100 (v/v), 2 g/LFeSO4·7H2O; (b) Ethanol: water = 60:40 (v/v), 2 g/L FeSO4·7H2O. water = 0:100 (v/v), 2 g/LFeSO4·7H2O; (b) Ethanol: water = 60:40 (v/v), 2 g/L FeSO4·7H2O.

Moreover, the mordant used in this study is soluble in water but insoluble in ethanol; hence, the large amount of ethanol acts as a filling agent in the mordanting bath. Consequently, the concentration of Fe(II) ions in the ethanol–water mixture was higher than that in the aqueous solution, which may have had a positive influence on ferrous ion uptake by the camel hair. Polymers 2018, 10, 730 13 of 18

Polymers 2018, 10, x FOR PEER REVIEW 13 of 18 On the other hand, the absence of water has been shown to prevent oxidation of Fe(II) to Fe(III) [60]. Moreover, the mordant used in this study is soluble in water but insoluble in ethanol; hence, the Accordingly, the concentration of Fe(III) in aqueous solution may be somewhat higher than that in large amount of ethanol acts as a filling agent in the mordanting bath. Consequently, the the ethanol–waterconcentration mixture. of Fe(II) Although ions in the absorption ethanol–water of mixture Fe(III) maywas higher result than in a that similar in the mordanting aqueous effect as Fe(II), thesolution, use ofwhich Fe(III) may ishave not had as a selective,positive influenc leadinge on ferrous to heavy ion uptake damage by the of camel the fiberhair. proteins during the bleaching procedureOn the other hand, [61]. the Therefore, absence of in water the has water been system, shown to camelprevent hair oxidation is more of Fe(II) likely to Fe(III) to be attacked, [60]. Accordingly, the concentration of Fe(III) in aqueous solution may be somewhat higher than that which agrees with the observed changes in the mechanical properties of bleached camel hair. in the ethanol–water mixture. Although absorption of Fe(III) may result in a similar mordanting effect Consequently,as Fe(II), the based use of onFe(III) these is not findings, as selective, the leading basic to principles heavy damage of the of the mordanting fiber proteins process during the in the water and ethanol–waterbleaching procedure systems [61] are. outlinedTherefore, inin Figurethe water 13 system,a,b, respectively. camel hair is Inmore these likely two to be systems, attacked, camel hair can be swollenwhich agrees in the with mordanting the observed process.changes in the During mechanical the processproperties ofof bleached swelling camel and hair. diffusion, ferrous Consequently, based on these findings, the basic principles of the mordanting process in the ions are permeated into the swollen camel hair. Then, ferrous ions can be combined with melanin water and ethanol–water systems are outlined in Figure 13a,b, respectively. In these two systems, granules incamel camel hair hair. can be However, swollen in the mordanting camel hair process. mordanted During inthe the process ethanol–water of swelling and system diffusion, has a higher swelling ratioferrous compared ions are permeated with that into in the the swollen water camel system. hair. Additionally,Then, ferrous ions due can to be the combined addition with of ethanol, more ferrousmelanin ions granules are permeated in camel hair. into However, camel hair the camel and hair then mordanted combined in the with ethanol melanin–water granules. system has Therefore, a higher swelling ratio compared with that in the water system. Additionally, due to the addition of the presence of ethanol can benefit the mordanting process greatly, resulting in the absorption of Fe(II) ethanol, more ferrous ions are permeated into camel hair and then combined with melanin granules. being moreTherefore, selective the by presence the melanin of ethanol in camel can benefit hair. the mordanting process greatly, resulting in the absorption of Fe(II) being more selective by the melanin in camel hair.

(a)

(b)

Figure 13. Principle of mordanting in (a) the water system and (b) the ethanol–water system. Figure 13. Principle of mordanting in (a) the water system and (b) the ethanol–water system. 3.7. Effects of Ethanol on Oxidative Bleaching 3.7. Effects of EthanolTo investigate on Oxidative the impact Bleaching of ethanol on oxidative bleaching of camel hair, a series of trials were carried out. Initially, all the samples were immersed in a bath of 2 g/L FeSO4·7H2O in 60% ethanol To investigate the impact of ethanol on oxidative bleaching of camel hair, a series of trials were and 40% water by volume for 30 min at 55 °C. After this mordanting process, the fibers were rinsed carried out.at room Initially, temperature all the for samples at least 10 were min and immersed then wrung in to a 100% bath water of 2 g/Lcontent. FeSO Oxidative4·7H2 bleachingO in 60% ethanol and 40% waterwas carried by volume out in a formixed 30 solution min at of 55 20◦ C.g/L Afterof hydrogen this mordanting peroxide in different process, ratios the of fibersethanol were and rinsed at room temperaturewater for 1 for h at at 65 least °C. Subsequently, 10 min and thenthe samples wrung were to 100%rinsed waterfor at least content. 10 min, Oxidative dried, and bleachingthen was examined using SEM. carried out in a mixed solution of 20 g/L of hydrogen peroxide in different ratios of ethanol and water The surface characteristics of camel hair bleached using different concentrations of ethanol ◦ for 1 h at 65duringC. the Subsequently, oxidative bleaching the samples process are were shown rinsed in Figure for 14. at It least is well 10-known min, that dried, the morphology and then examined using SEM.of protein fibers, such as wool and camel hair, is characterized by the scales, which greatly contribute The surfaceto protecting characteristics the protein fibers of camel from damage hair bleached and affect using other essential different properties, concentrations such as luster of ethanol and during shrinkage [62]. The SEM images demonstrate that the scales on the fibers bleached with the solution the oxidative bleaching process are shown in Figure 14. It is well-known that the morphology of protein fibers, such as wool and camel hair, is characterized by the scales, which greatly contribute to protecting the protein fibers from damage and affect other essential properties, such as luster and shrinkage [62]. The SEM images demonstrate that the scales on the fibers bleached with the solution composed of 100% ethanol were clear and arranged compactly around each fiber. As the concentration of ethanol in the oxidative bleaching solution was reduced, significant degradation of camel hair was observed. In other words, the cuticle of camel hair was damaged dramatically as the volume of water increased during the oxidative bleaching process. Polymers 2018, 10, x FOR PEER REVIEW 14 of 18 Polymers 2018, 10, x FOR PEER REVIEW 14 of 18 composed of 100% ethanol were clear and arranged compactly around each fiber. As the composedconcentration of 100%of ethanol ethanol in the were oxidative clear bleaching and arranged solution was compactly reduced, around significant each degradation fiber. As of the Polymersconcentration2018camel, 10 hair, 730 was of ethanolobserved. in In the other oxidative words, bleaching the cuticle solution of camel washair reduced,was damaged significant dramatically degradation as the of14 of 18 camelvolume hair of was water observed. increased In during other words,the oxidative the cuticle bleaching of camel process. hair was damaged dramatically as the volume of water increased during the oxidative bleaching process.

Figure 14. SEM images of bleached camel hair with different contents of ethanol during oxidative Figure 14.bleaching:SEM images(a) Ethanol:water of bleached = 100:0 camel (v/v); hair(b) Ethanol:water with different = 80:20 contents (v/v); (c of) Ethanol:water ethanol during = 60:40 oxidative bleaching:Figure(v/v); ( a14.(d)) Ethanol:water Ethanol:waterSEM images of= 40:60 =bleached 100:0 (v/v ();v camel(/ev) );Ethanol:water ( bhair) Ethanol:water with different= 20:80 (v =/contentsv); 80:20 (f) Ethanol:water ( vof/ vethanol); (c) Ethanol:water =during 0:100 ( voxidative/v). = 60:40 (v/v);bleaching: (d) Ethanol:water (a) Ethanol:water = 40:60 = (v 100:0/v); (e(v)/v Ethanol:water); (b) Ethanol:water = 20:80 = 80:20 (v/v );(v/ (vf);) Ethanol:water(c) Ethanol:water = 0:100= 60:40 (v /v). (v/Tov); (studyd) Ethanol:water the influence = 40:60 of ethanol (v/v); ( econtent) Ethanol:water on mechanical = 20:80 (propertiesv/v); (f) Ethanol:water of camel hair, = 0:100 the (variationv/v). of the breaking tenacity of bleached camel hair with the ethanol content is graphically represented in To study the influence of ethanol content on mechanical properties of camel hair, the variation FigureTo study 15. As the one influence can see, ofa steady ethanol increase content in on breaking mechanical tenacity properties was observed of camel with hair, an increasingthe variation of theof breaking volumethe breaking ratio tenacity oftenacity ethanol of of bleached in bleached oxidative camelcamel bleaching hair withsolution. with the the ethanolThis ethanol is incontent accord content is with graphically is the graphically results represented from represented the in in FigureFigureSEM 15 15.images. As As one onein Figure can can see, se 14.e, a Consequently,a steadysteady increaseincrease the mechanical in in breaking breaking properties tenacity tenacity ofwas camel was observed observedhair can with be withprotected an increasing an increasing in volumevolumethe ratio presence ratio of ethanol of of ethanol ethanol. in oxidative in oxidative bleaching bleaching solution. solution. This This is is in in accord accord with with the the results results fromfrom thethe SEM imagesSEM in images Figure in 14 Figure. Consequently, 14. Consequently, the mechanical the mechanical properties properties of camelof camel hair hair can can be be protectedprotected in in the presencethe presence of ethanol. of ethanol.

Figure 15. Effects of ethanol content in oxidative bleaching solution on breaking tenacity of camel hair.

Pham et al. [63] reported that contact between hydrogen peroxide and iron-containing minerals FigureFigure 15. Effects 15. Effects of ethanol of ethanol content content in oxidativein oxidative bleaching bleaching solution solution on on breaking breaking tenacity tenacity ofof camelcamel hair. (Fe2+) may result in the formation of highly reactive and damaging hydroxyl radicals (·OH). This was hair. shown more clearly by Smith et al. [64], who concluded that iron-containing minerals are more likely to activate Fenton reactions, leading to the catalytic decomposition of H2O2. During this reaction, PhamPham et al. et [ 63al.] [63] reported reported that that contact contact between between hydrogen peroxide peroxide and and iron iron-containing-containing minerals minerals 2+ hydrogen peroxide could form reactive oxygen species, such as hydroxyl radicals (·OH), perhydroxyl (Fe )(Fe may2+) may result result in thein the formation formation of of highly highly reactivereactive and damaging damaging hydroxyl hydroxyl radicals radicals (·OH). (·OH). This Thiswas was radicals (HO2·), and superoxide anions (O2·−). According to the work of Dunford [65], the mechanism shownshown more more clearly clearly by Smithby Smith et et al. al. [64 [64]], who, who concluded concluded that that iron iron-containing-containing minerals minerals are more are more likely likely of the reaction between Fe2+ and H2O2 has been widely assumed to be as follows. to activateto activate Fenton Fenton reactions, reactions, leading leading to to the the catalytic catalytic decompositiondecomposition of of H H2O22O. During2. During this thisreaction, reaction, hydrogenhydrogen peroxide peroxide could could form form reactive reactive oxygen oxygen species, species, such as as hydroxyl hydroxyl radicals radicals (·OH), (·OH), perhydroxyl perhydroxyl radicals (HO2·), and superoxide anions (O2−·−). According to the work of Dunford [65], the mechanism radicals (HO2·), and superoxide anions (O2· ). According to the work of Dunford [65], the mechanism of the reaction between2+ Fe2+ and H2O2 has been widely assumed to be as follows. of the reaction between Fe and H2O2 has been widely assumed to be as follows.

2+ 3+ − Fe + H2O2 → Fe + ·OH + OH (4)

Fe2+ + ·OH → Fe3+ + OH− (5)

·OH + H2O2 → HO2· + H2O (6) 2+ 3+ − HO2· + Fe → Fe + HO2 (7) Polymers 2018, 10, 730 15 of 18

− 3+ 2+ O2· + Fe → Fe + O2 (8) In our work, there are several possible reasons for the observation of less damage and an enhanced degree of whiteness when ethanol–water mixtures are used during the bleaching process. Firstly, ethanol may accelerate the initial oxidation of ferrous ions during the oxidative bleaching process [66,67]. The effect of ethanol may be interpreted based on the mechanism given below. . CH3CH2OH + ·OH + HO2· → CH3CHOH + H2O + H2O2 (9) .  .  CH3CHOH + O2 → CH3C O2 HOH (10)  .  CH3C O2 HOH + HO2· → CH3C(O2H)HOH + O2 (11)

According to Equations (5), (7), and (9), ethanol molecules will compete with ferrous ions (Fe2+) for hydroxyl radicals (·OH) and perhydroxyl radicals (HO2·). This would lead to a corresponding reduction in the number and reactivity of the radicals produced. Thus, according to the SEM observations in Figure 14, attack of the fibers by radicals can be decreased to some extent by increasing the concentration of ethanol. Meanwhile, as ferrous ions (Fe2+) are preferentially absorbed by the melanin pigments, a strong interaction is formed between them during the mordanting process; thus, the structure of the melanin polymer can be effectively disrupted by the oxidation of ferrous ions. Meanwhile, ferrous ions are more firmly bound to melanin than to keratin. Accordingly, although the reactions shown in Equations (9)–(11) seem to be minor reactions compared with the radical reactions (Equations (4)–(8)), they only occur in melanin, not in keratin, which may also lead to decreased fiber damage. Moreover, ethanol may directly participate in the free radical reaction induced by ferrous iron through Fenton-type reactions, forming hydroxyethyl free radicals as an intermediate from hydroxyl radicals (·OH) and perhydroxyl radicals (HO2·)[68,69], thereby causing more damage to melanin as reflected by the observed degree of whiteness (Table1).

4. Conclusions To protect camel hair and enhance its whiteness degree during bleaching, the fibers were mordanted with ferrous sulfate heptahydrate and then subjected to oxidative bleaching with hydrogen peroxide in a neutral ethanol–water system. This process significantly improved the whiteness degree of the fibers without substantial tenacity loss compared with those of the fibers bleached in aqueous solution. Swelling is crucial for effective mordanting, and increased swelling was observed in the presence of ethanol, likely owing to good permeation of ethanol into the fibers. SEM images showed that ethanol could prevent the fibers from suffering serious damage during the bleaching treatment. Increased modification of the scales was observed when a higher ratio of water was applied during the oxidative bleaching process, whereas treatment in ethanol under neutral conditions made the camel hair less accessible to proteolytic attack. Moreover, changes in the FTIR absorption band corresponding to hydrogen bonding in the camel hair fibers suggested that breakage of these intramolecular hydrogen bonds during the bleaching treatment in aqueous solution was relatively more severe than that during treatment in the ethanol–water system. These findings indicate that the impact of chemical bleaching on the whiteness degree and mechanical properties of camel hair can be well-controlled by the addition of ethanol during the mordanting and oxidative bleaching processes. Further work in this direction is being performed to investigate the effect of this approach on the desirable properties of other natural protein fibers.

Author Contributions: W.X. conceived of the main idea and supervised the project. Y.W. designed the experiments. L.X. performed the experiments and wrote the manuscript with support from C.Z. Y.T. provided significant guidance on the bleaching mechanism. W.X., K.Z., and A.W. provided critical feedback and helped shape the research, analysis, and manuscript. Funding: Distinguished Young Scientists (grant number 51325306); Natural Science Foundation of Hubei Province of China (grant number 2014CFB753). Polymers 2018, 10, 730 16 of 18

Acknowledgments: This work was supported by the China National Funds for Distinguished Young Scientists (grant number 51325306) and the Natural Science Foundation of Hubei Province of China (grant number 2014CFB753). Conflicts of Interest: The authors declare no conflict of interest.

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