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Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results and discussion
  6. Conclusions
  7. References

This study has been carried out to synthesize nano ZnO on wool fabric and also to investigate influences of nano photo reactors on wool fabric characteristics. Zinc acetate has been used as a precursor and the synthesis process has been done in water and water/ethanol media. The treated wool fabrics were heated at 80°C for 10 h to dehydrate Zn(OH)2 obtaining ZnO. The fabric samples were then subjected to daylight for 7 days to examine the influence of nano ZnO photo reactor on the fabric properties. SEM images revealed the embedding of ZnO nanoparticles on the fabrics and X-ray diffraction verified the nanoparticles composition. The Yellowness Index (YI) of the fabrics was measured with Color Eye XTH that has been reduced with increasing pH, Zn(CH3COO)2 concentration, ethanol and heating. The lower water contact angle and time of water absorption confirmed higher hydrophilic properties of the treated fabrics. Interestingly, a higher tensile strength obtained on the wool fabrics proved the interaction of ZnO with protein chains of wool, which was verified through lower alkali solubility of treated fabric with nano ZnO and confirmed more benefits of the in situ synthesis process.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results and discussion
  6. Conclusions
  7. References

Recently, more attention was focused on the utilization of semiconductors, for instance ZnO, TiO2, Fe2O3 and CdS as photo catalysts for the degradation of inorganic and organic materials [1]. The most remarkable advantage of ZnO in comparison with TiO2 is its absorption of a greater fraction of UV spectrum with the related threshold of 425 nm [2]. ZnO as a semiconductor with a large band gap (3.3 eV), big excitation binding energy (60 meV), n-type conductivity, is plentiful in nature and environmentally friendly [3]. Zinc oxide is widely applied in various fields such as UV-blocking, surface acoustic wave filters, photo detectors, gas sensors [4], and purification of either wastewater or drinking water for human use [5] due to its distinguished photo catalytic, optical, electrical, electronic, dermatological and antibacterial properties [6].

UV radiation is responsible for yellowness of wool fabrics [7]. The critical impediment of the wool is the lower photo stability in comparison with other fibers, such as cotton or polyester [8]. Up to now, various finishing approaches have been employed to prevent wool from photo yellowing and to reduce its pace. For example, applying thiourea/formaldehyde on wool is able to reduce the wool photo yellowing; however, it does not have a permanent impact on wool due to its low durability against washing processes [9]. Also, treatment of wool with sulfonated 2-hydroxyl phenyl benzotriazole as UV absorber was developed [10]. Other approaches are on the basis of utilizing UV absorbers, hydroperoxide decomposers, metal chelators and free radical scavengers [9, 10]. Tetrakis hydroxyl methyl phosphonium chloride, N-acetylcysteine, thioglycollic acid and bisulfate are regarded as oxygen scavenger and photo yellowing lessening agent [9, 10].

Two kinds of UV absorbers including organic and inorganic are available in the market. Inorganic UV absorbers such as ZnO, TiO2, SiO2 and Al2O3 are more widely utilized due to possessing thermal stability and nontoxicity as compared to organic ones [6, 11]. Using inorganic materials in the nano scale will increase the absorption of UV rays more than micro scale [12]. It has been demonstrated that mineral absorbers such as nano ZnO and TiO2 can be used for preventing wool fabrics from photo yellowing [13, 11]. Presence of melanin in the wool fiber acting as a light absorber and makes the fabric appear colorful [14]. ZnO can be classified as one of the members in metal oxides group, characterized by photo catalytic and photo-oxidizing capacity against biological and chemical species [15]. This also acts as a powerful antimicrobial agent due to electrostatic interaction between the nanoparticle and microbial cell surface resulting in the cell damage [15], thus nano ZnO has a potential effect for decomposing other organic species such as natural pigments in cream raw wool fibers and other hydrophobic compounds on the wool fabric to produce white and hydrophilic surface. Therefore, ZnO nanoparticles have two effects on the wool fabric: first, they prevent yellowing and second decompose the natural pigments due to their photo catalytic activity.

ZnO has also been used in textiles for various purposes such as UV-blocking and antibacterial efficacy on cotton [16, 17], UV-blocking on wool fabrics [6], self-cleaning properties on cellulosic fibers [18], hydrophobic modification of cotton fabrics [19], antistatic finishing of polyester fabric [20] and stain-removing textiles [21] with different synthesis methods. The most number of former approaches applied on cotton, wool and other textiles were almost similar consisting of two steps, synthesis of nano ZnO and application of the nano ZnO on the fabric surface by immersing the fabric into the ZnO solution [6, 17, 18, 20, 21] or by coating the synthesized nanoparticles on fabric using acrylic binder [22]. Furthermore, a few prior methods for application of ZnO on textiles considered synthesis and application of nano ZnO on textiles in one step, such as in situ ZnO fabrication on cotton fibers using microwave [16]. As-synthesized ZnO precipitate was put into a flask, then ammonia was added dropwise and after it became clear under magnetic stirring, cotton fibers were dipped into the flask and put into the ultrasonic oscillator and then cotton fiber was taken out and put into a microwave oven [16]. Also, in situ synthesis of ZnO nanoparticles on cotton fabric coated with SiO2 through hydrothermal method [23]. Cotton fabric coated with SiO2 was immersed in a solution containing hexamethylenetetramine and zinc nitrate and then the temperature was raised to 80°C to synthesize nano ZnO on cotton fabric [23]. The advantages of in situ synthesis method are higher washing durability and performance of ZnO on fabric [16, 23].

In this work, synthesis and application of ZnO nanoparticles on wool fabric has been carried out in one step. In other words nano ZnO was fabricated on wool fabric in the presence of Zn(CH3COO)2 and NH3, NaOH as precursors in the water and water/ethanol media through simple wet method to obtain hydrophilic wool with special characteristics. To synthesize and apply nano ZnO on wool fabric, alkaline reaction medium was utilized to produce as-synthesized ZnO with positive charge [24] that attach on the wool fabric surface with negative charge [25].

Materials and Methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results and discussion
  6. Conclusions
  7. References

Materials

Chemicals including zinc acetate dehydrate, sodium hydroxide, absolute ethanol, ammonia and citric acid were all purchased from Merck (Germany) and protease obtained from Novozyme (Denmark). The wool fabric with the mass per unit surface of 340 g m−2 was used. The wool fabrics were scoured by nonionic detergent to remove the wax, grease and other impurities from the fabrics and then treated with protease (pH = 5 at 60°C) for 45 min to modify the fabric surface.

In situ synthesis of nano ZnO on wool

Zinc acetate (10%, 20% and 30%) was dissolved in 100 mL water at 30°C under vigorous stirring. The solution was then kept under constant stirring using magnetic stirrer to completely dissolve zinc acetate. The wool fabric was immersed in the solution and remained till the end of the treatment. The temperature was then raised as the ammonia was being added under constant stirring, drop by drop into the solution (pH > 9) until the temperature reached 90°C and then NaOH solution was added dropwise to the solution, with gentle stirring over a period of time to keep the pH above 9. The wool fabric was finally treated at 90°C for 1 h. The samples were named as A (10% zinc acetate), B (20% zinc acetate) and C (30% zinc acetate).

More experiments were performed under the same conditions as C with the pH lower than 9 to examine the effect of pH and named as D. Two other experiments were performed under the same conditions as C in the different media. The first was performed in 50% water/50% ethanol named as E and the second was performed in 25% water/75% ethanol named as F to examine the effect of ethanol. The wool fabrics were cut into two pieces, one piece was put in an oven (80°C), for 10 h, and the other was dried under atmospheric conditions to examine the effect of heating. Both samples were then subjected to daylight for 7 days during summer time in Tehran, Iran.

Characteristics

The ZnO-treated fabrics were observed by SEM, XL30 Philips. A thin layer of gold was coated on the samples under vacuum conditions. The crystal structure of nano ZnO was analyzed by XRD utilizing a Bruker D8 Advance X-rays Diffractometer equipped with a Cu Ka (k = 1.54 A°) source (used voltage 40 kV, current 40 mA).

The presence of Zn(OH)2 was examined through evaluation of weight reduction in samples after heating at 80°C for 10 h. The value of YI of treated fabrics and untreated fabric were measured using Color Eye XTH system. Hydrophilic properties of the samples were assessed by measuring the water contact angle with Krüss K100-SF system and time of water absorption according to BS 4554 standard method.

Tensile strength of the treated fabrics was performed according to BS 2576 standard method. To confirm cross-links created between wool peptide chains by nano ZnO, alkali solubility of wool fabrics was examined according to Harris and Smith approach. The wool fabrics were treated with 100 mL of 0.1 N solution of sodium hydroxide at 65°C for 1 h. The samples were then rinsed in distilled water, and treated with acetic acid for 20 min at 40°C. After drying, weight variation of the samples was reported [26].

Durability of whiteness against washing was finally examined on samples C and F through washing with anionic detergent at 60°C for 20 min and rinsing with distilled water and drying. The washing was repeated five times and then the YI value of the samples was measured.

Results and discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results and discussion
  6. Conclusions
  7. References

SEM images

The SEM images of untreated fabric, samples C, E and F have been illustrated in (Fig. 1a–l). The synthesized ZnO particles through the above methods are close to spherical and rod shape with the mean particle size about 70 nm.

image

Figure 1. SEM images of various samples a: untreated, b, c, d and e: C, f, g and h: E, i, j, k and l: F.

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The nanoparticles are well dispersed on the fiber surface in both samples, though some aggregated nanoparticles can be observed. Nanoparticles because of the small dimension properties, high surface activity and large specific surface area, have a tendency to be agglomerated easily [20]. The reaction in the ethanol and water mixture results in the formation of smaller nanoparticles with respect to the reaction carried out in water [21]. Particles obtained on samples E and F are more monodisperse and separated than the particles obtained on sample C. This indicates greater dispersion efficiency for synthesis method applied on samples E and F with respect to sample C [21]. The mean size of nanoparticles on sample F consists of spherical shape about 75 nm and rod shape about 50 and 90 nm for width and length, respectively. Also the mean size of nanoparticles on sample E is about 85 nm, and on sample C it is about 95 nm.

XRD analysis

Figure 2a,b show the XRD spectrum of the ZnO nanoparticles prepared through samples F (Fig. 2a) and C (Fig. 2b). The diffraction peaks observed at 2θ values of 9.8° and 20.2° are related to diffraction peaks of wool fabric [27]. The spectrum reveals the clear peaks of ZnO in the crystal structure of zincite. All the diffraction peaks are mainly attributed to the diffraction peaks from the hexagonal phase ZnO (JCPDS card file No. 36–1451). The XRD pattern of ZnO for sample C represents five main peaks at 31.54°, 34.13°, 36.7°, 47.4°, 56.36° (2Ө),which can be related to (100), (002), (101), (102) and (110) planes and the XRD pattern of ZnO for sample F also represents 11 primary peaks at 31.54°, 34.13°, 36.7°, 47.4°, 56.36°, 62.82°, 67.67°, 68.85°, 89.2°, 95.14° and 103.7° (2Ө), which can be related to (100), (002), (101), (102), (110), (110), (103), (112), (201), (203), (211) and (105) planes. The XRD pattern of sample F showed more crystalline, sharper and broader planes than those of sample C, which indicated that the amount of ZnO in sample F is more than that in sample C. Edgar and Simpson indicated that absorption of aluminum ions by wool in water/alcohol liquor increased with increase in alcohol ratio in solution [28].

image

Figure 2. XRD spectrums of different nano ZnO synthesized on samples a: F, b: C.

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Weight reduction after heating

There is also a possibility of synthesis of Zn(OH)2 in the final product when ZnO is synthesized with zinc acetate and NaOH [29, 30]. Zn(OH)2 functions as an intermediate product which can be decomposed into ZnO and water by means of heating of samples or increasing NaOH in the solution [31]. The presence of Zn(OH)2 was verified with examination of weight reductions in samples F, C and untreated fabric after heating at 80°C for 10 h, which were 4.12%, 3.6% and 1.7%, respectively. The weight reduction in samples F and C is more than that of untreated fabric indicating that the extra weight reduction in these samples can be related to evaporation of water molecules produced from dehydration of Zn(OH)2 to ZnO. Also the weight reduction in sample F is more than that of sample C as a result of presence of more ZnO and Zn(OH)2 on sample F that was confirmed by XRD patterns.

Yellowness indexes

The YI values of treated fabrics and untreated fabric are shown in Table 1. The best whiteness belongs to the sample with the least YI.

Table 1. Yellowness Indexes (YI) of different heated and non heated samples
SampleYI (heated)YI (non heated)
A35.0136.14
B29.9831.02
C29.2531.01
D32.2032.29
E28.0528.10
F23.8426.20
Untreated35.3233.73

In animal fibers, two kinds of pigments, eumelanin and pheomelanin are responsible for making fibers to appear colored. Pheomelanin natural pigments cause the wool fabrics to get yellow [14]. To produce a white wool fabric these pigments should be demolished. ZnO can be classified as one of the members in metal oxides group, characterized by photocatalytic and photo-oxidizing capacity against biological and chemical species. It uses a multifunctional nanoplatform that bombards hazardous cells from outside by releasing reactive oxygen species (ROS) [32]. It has been shown that releasing ROS during UV illumination plays the most important role in photocatalytic activity [33]. When the samples treated with nano ZnO are subjected to daylight these ROS are produced and they decompose the natural pigments, which whiten the wool fabrics. Photocatalytic activity of ZnO is due to donor states caused by the presence of a great number of defect sites such as oxygen vacancies and interstitial atom of zinc and the states of acceptors that result from zinc vacancies and interstitial atoms of oxygen. During exposure to UV rays, interfacial transfer of electron occurs prevalently between the oxygen vacancies and interstitial atom of zinc [3]. This electron transfer takes place on condition that the energy of incident ray is equal or larger than band gap of zinc oxide (3.3 ev) [30], leading to production of electrons in conduction band of the semiconductor and electron holes in band of valence [34]. The mechanism of photocatalytic activities of nano ZnO can be summarized in Reactions (1)-(12):

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The generated hole (h+) is a strong oxidative while negative electron (e) on the band of conduction possesses a reducing potential. Holes react with molecules of water and produce hydroxyl groups (.OH) and negative electrons react with molecules of oxygen and reduce them into the anions of superoxide radical [11]. The ROS and hydroxyl radicals destroy different compounds. Hydroxyl radicals have the highest activity to demolish bacteria and organic molecules [34]. For example, oxidation mechanism of hydroquinone monomers of melanin with ROS is shown in Scheme 1:

By nucleophilic attack of OOH, melanin ring is opened and decomposed [35]. Increasing ethanol decreases the YI. The reaction in the ethanol and water mixture results in the formation of smaller nanoparticles with respect to the reaction carried out in water. The particle size plays a primary role in determining their adhesion to the fibers [21]. Moreover, higher UV absorbance is obtained when nanoparticles produced from synthesized method on sample F [5] led to the higher photocatalytic activities on sample F [4] containing 75% ethanol and indicates the least YI. ZnO properties depend on synthesis conditions such as reaction temperature and concentration of reactants [15]. Sample F was synthesized in ethanol/water media whereas sample C was synthesized in 100% water. Therefore, the presence of ethanol in synthesis of sample F is the reason for the difference between the YI values of the two samples F and C. The synthesis of ZnO nanoparticles in the presence of ethanol led to the formation of smaller particles than that in pure water. There is an intrinsic relation between particle size (ratio of surface to volume) and surface oxygen vacancies and photocatalytic activity, therefore the smaller particle size (higher specific surface area) lead to the larger surface oxygen vacancy content and higher photocatalytic activity [36, 37]. Surface oxygen vacancies and defects can trap photoinduced electrons throughout the time of photocatalytic reaction leading to a powerful interaction between the trapped electrons and adsorbed O2 and preventing electron hole pairs from recombination [36]. The electron hole pairs with longer life time are apt to produce a larger quantity of photoreactive species [38]. Thus, more ROS can be produced on sample F during daylight irradiation that leads to the decomposition of more natural pigments and creation of whiter wool surface on sample F in comparison with sample C.

The heat treatment of samples decreases the value of YI, during this stage, through conversion of Zn(OH)2 into ZnO [31]. Increasing NaOH is desirable for removing Zn(OH)2 in the product, thus pure ZnO nano particles are elicited by this process [31]. This has clearly affected the YI value (between samples C and D), which has declined in sample C.

The percentage of ethanol in synthesis process was varied for samples E and F which was more for sample F. The difference between YI values of these two samples can be related to the application of more ethanol in sample F than sample E, which led to the formation of smaller particles on sample F with higher photocatalytic activity. This led to decomposing more natural pigments resulting in a whiter wool surface for sample F than sample E as shown in Table 1.

Furthermore, the YI value of untreated fabric after heating increased indicating that chemical structure of wool fibers has changed during this stage [39].

Hydrophilic properties

Results of the water contact angle and time of water droplet absorption are shown in Table 2.

Table 2. Different characteristics of samples
SampleContact angle (˚)Water absorption (s)Maximum load (heated) (N)Maximum load (non heated) (N)
Untreated857200293.60299.83
A8.60308.72302.51
B7.50315.11308.19
C7.20323.21310.73
D8.10324.56310.97
E00329.11313.91
F00332.60315.50

Wax available in raw wool fabrics causes the raw wool fabrics to have a hydrophobic surface, which contains lipids, fatty acids and esters, ethers, alcohols, cholesterols and lanolin [40]. When the samples treated with ZnO are exposed to daylight, ZnO produces some ROS [31], which can decompose organic compounds, acids and alcohols producing a more hydrophilic surface. Also wool structure consists of three main parts of medulla, cortex and cuticle [41]. A cuticle itself is divided into three smaller parts including epicuticle, endocuticle and exocuticle [41, 42]. Exocuticle and epicuticle cause the wool fabric to have a hydrophobic surface. The morphology of the wool surface is responsible for its physicochemical properties [43-45]. On the hydrophilic surface, water droplets cannot maintain their spherical shape [46]. Presence of ZnO on the fabric surface with a photocatalytic activity during light illumination develops a hydrophilic surface, in which holes and electrons play an important role causing redox reactions. The suggested mechanism in Scheme 2 shows that negative electrons take part in the reduction reactions of Zn2+ to Zn1+ generating the anions of oxygen (O2) then the positive holes are responsible for oxidation reactions in which anions of superoxide are altered to molecules of oxygen and then they leave their place and remaining oxygen vacancies can be occupied by water molecules while generating OH groups, improve the hydrophilic properties of the fabric [26, 47-50].

In a similar way, the surface hydroxyl molecules density has an excellent effect on improving the hydrophilicity, producing more hydrogen bonds with molecules of waters [51]. Through light illumination, the contact angles can be declined and water can be absorbed faster on the fabric surface [48]. More hydroxyl radicals will be generated on the surface with higher photocatalytic activity. The high photocatalytic activity means the more photocreated holes produced, and more hydroxyl groups absorbed from water and consequently more hydroxyl radicals produced [52]. More hydroxyl groups result in a higher water absorption rate and a smaller water contact angle [48]. The hydrophilic properties of the samples treated with ZnO improved until the time of water absorption reached zero second.

Moreover, the results of the water contact angle verify this point as the water contact angle of sample F treated in 75% ethanol which has the highest photocatalytic activity indicating the most hydrophilic surface. The differences between water contacts angles of various samples are similar to the YI values as smaller ZnO on the fabric surface leads to the higher hydrophilicity and lower YI. Sample F has a higher photocatalytic activity than sample C with a more hydrophilic surface. Presence of ethanol in synthesis process of ZnO on sample E leads to a higher photocatalytic activity and more hydrophilic surface than sample C. Furthermore, sample C and D were synthesized at pH >9 and <9, respectively, thus the higher pH on sample C led to a more hydrophilic surface for sample C with more ZnO [31].

Tensile strength

Tensile strength of samples treated with nano ZnO was compared with the untreated fabric is shown in Table 2. The synthesis of ZnO nanoparticles on wool fabrics increases the maximum force which is mainly caused by the unexpected behavior of wool fabrics in the presence of aqueous environments (shrinkage), and specifically in alkaline media. This decreases its resistance and provides destructive effects on its structural characteristics [6].

Also presence of ZnO and Zn(OH)2 possibly lead to interaction between protein chains of wool producing bonds between protein chains and increasing tensile strength. These created bonds or cross-links were confirmed with more stable behavior of wool in alkaline solution that is shown in Fig 3. Sample F containing 75% ethanol has the most maximum force. Existence of ethanol in synthesis of ZnO nanoparticles on sample F enhanced the absorption of ZnO on or within sample E and F. This led to the creation of more bonds or cross-links between wool peptide chains and higher maximum force.

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Figure 3. Alkali solubility of different samples 1: raw, 2: A, 3: B, 4: C, 5: D, 6: E, 7: F.

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Alkali solubility

The results of alkali solubility of wool samples are shown in Fig 3. The disulfide linkage breakdown is responsible for destruction of wool structure in alkaline solution or with oxidizing and reducing agents [53, 54]. Replacing disulfide groups with more stable cross-links is desirable, it is better if this modification is carried out without change in wool-desirable properties [55]. In this study, more desirable properties have been produced along with the above modification. The alkali solubility of samples has decreased indicating presence of ZnO on wool structure created cross-links between wool peptide chains. Also, it has been demonstrated that nano TiO2 acts in a similar manner to a cross-linking agent on cotton structure [56]. As photodegradation mechanism of ZnO and TiO2 is similar [57] and they have close band gap energy (3.3, 3.2 eV, respectively) [33, 58] and other similar properties such as UV absorption [13, 11] and antibacterial effect [18, 59], expectation of such a behavior from ZnO on wool structure is not out of question. Sample F has the lowest alkali solubility and the highest maximum force which can be related to more ZnO nanoparticles on the sample and more creation of cross-links. Presence of ethanol in synthesis of ZnO on samples F and E increases the absorption of ZnO on the samples and results in more cross-links in comparison with the other samples. Sample D synthesized at pH < 9 results in less damage to disulfide bonds than sample C indicating more stability in alkaline solution with higher maximum force. The alkali solubility of other samples declined with increase of nano ZnO.

Durability of whiteness

Results of YI of samples C and F after washing process are shown in Table 3. YI has little change indicating permanency of the obtained whiteness on the wool fabric by this method that will not change after washing. Whiteness produced by the nano ZnO can be related to white color of the nano ZnO itself deposited on the wool surface and photocatalytic activity of the nano ZnO after exposure to daylight, which can decompose the colored pigments of wool. Scouring of wool fabric may cause the nano ZnO to be removed from the wool surface and reduce the presence of the nano ZnO on the wool surface, but natural color destroyed by photocatalytic activity is not reversible and causes the wool fabric to have a permanent white surface.

Table 3. Yellowness Indexes (YI) of samples C and F after washing
SampleYI
C29.01
F22.97

Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results and discussion
  6. Conclusions
  7. References

In this article, in situ synthesis of nano ZnO on wool fabric through different conditions including water and ethanol media, pH, Zn(CH3COO)2 concentration and heating treatment on formation of nano ZnO was studied. SEM images showed distribution of nano ZnO on the fabric surface with mean particle size about 70 nm. The XRD results indicate the synthesis of ZnO with hexagonal wurtzite phase. On the basis of these findings, nano ZnO was successfully synthesized on wool fabric and the nano reactor activated under daylight irradiation produced a permanent hydrophilic surface with lower yellowing and higher tensile strength. Moreover, the presence of nano ZnO on the fabric surface prevented the wool fabric from yellowing.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results and discussion
  6. Conclusions
  7. References
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