Reducing photoyellowing of wool fabrics with silica coated ZnO nanoparticles

Abstract

Though ZnO nanoparticles (NPs) are an excellent UV absorber, their photocatalytic activity greatly limits the application areas of these particles. Under sunlight exposure, ZnO NPs used as a UV absorber can accelerate the wool yellowing process by generating free radicals. To reduce this photocatalysis effect, a physical barrier has been fabricated by coating the ZnO NPs with a silica layer (ZnO@SiO₂), hence providing good UV-shielding with low photocatalytic activity. The structure and optical properties of ZnO and ZnO@SiO₂ NPs were characterized by transmission electron microscope (TEM) and UV–Vis spectrum. The photocatalytic activity of ZnO and ZnO@SiO₂ NPs was evaluated by photo-degradation of Rhodamine B. The ZnO and ZnO@SiO₂ NPs were applied to knitted wool fabrics using the dip coating method. The treated wool fabrics were characterized by a scanning electron microscope (SEM) and the photoyellowing level of treated fabrics after exposure under simulated sunlight was evaluated by a Datacolor Spectraflash spectrophotometer. The ZnO@SiO₂ NPs demonstrated excellent protection of wool against photoyellowing.

Keywords

wool ZnO nanoparticles silica coating photoyellowing photocatalysis UV absorber

Wool fiber is well known as a superior natural textile due to its resilience and softness; however the photodegradation of wool caused by exposure to sunlight, particularly from UV light,^1–4 makes the fiber weak and yellow.

The UV light found in sunlight spans a range of 100 nm to 400 nm, and consists of three parts: UVA (∼315–400 nm), UVB (∼280–315 nm) and UVC (∼100–280 nm).⁵ Due to atmospheric absorption, particularly blocking by the ozone layer, only UVA and a limited amount (less than 3%) of UVB reach the earth’s surface. In wool, there is a significant amount of natural chromophores, including those UV- and Vis-absorbing ones. The UV-absorbing chromophores, such as tryptophan, tyrosine and phenyl alanine, are present in the form of aromatic amino acid residues and can absorb radiation in the range of 250–320 nm; while absorption of the Vis-absorbing chromophores spans a range from near UV to visible region (∼350–500 nm), which leads to the creamy color of natural undyed wool. The mechanism of wool photoyellowing is complex, including both chemical and physical properties, apparently linked to the absorption of the previously mentioned UVA and UVB of various natural chromophores in wool fiber. The electronic energy level of chromophores in wool fiber increases as light is absorbed, which is capable of elevating the whole molecule to a state of higher energy, an excited state.⁵ This process is called photoexcitation. It is also the first step in a photochemical reaction which is easily created⁶ along with the alteration of chemical bonds in molecules of wool fiber. The reactions involved in color changes of wool under exposure to sunlight are highly complicated. Previous studies have revealed that photochemical effects of sunlight on wool have two consequences: the long wavelength component bleaches the wool, whereas the short wavelength ray, in the absence of long ones, yellows wool.^1,7,8

Photoyellowing is a significant problem for the wool industry. Commercial bleaching and fluorescent whitening agent (FWA) finishing processes are widely employed to improve the whiteness of wool, but the rate of photoyellowing of bleached^9,10 and FWA-treated^11,12 wool fabrics is much faster than non-treated ones, especially when wet. The most widely used protective method to neutralize this destructive attack from UV light is the application of UV absorbers.^13–15 Applying UV absorbers to wool provides protection by absorbing the harmful UV radiation and dissipating the energy in some innocuous form, such as heat.¹⁶ Although the UV absorbers have been successfully applied to many textiles to neutralize the destructive attack of UV light, such as photofading of dyes;¹⁷ for preventing wool yellowing, there are still many challenges to improve the effectiveness of both organic and inorganic UV absorbers.

For organic UV absorbers, a common characteristic in chemical structure is the large conjugate system (aryl ring), which gives the substance ability to dissipate the absorbed UV energy via intra-molecular tautomerization between ground state and excited state. The most successful of these UV absorber structures is known as ‘Cibafast W’ which has been commercialized by CSIRO since 1990.¹⁸ It however, is effective against phototendering rather than photoyellowing.⁴ Its application to bleached wool is not very successful due to the decrease in whiteness as it absorbs blue light; while it can also not be used on FWA-treated wool as it blocks excitation wavelengths of FWA, like most UVA absorbers.¹⁹ Unlike organic UV absorbers, inorganic UV absorbers, such as ZnO, TiO₂ and SiO₂, show nontoxicity, chemical stability at high temperature, and permanent stability under UV exposure.²⁰ Development of nano-science and -technology provides new approaches to better apply these inorganic UV absorbers.^19,21–25 As semi-conductor oxides, the photocatalytic activity of nano-scaled ZnO and TiO₂ cannot be ignored. Wool is so highly sensitive and delicate that the process of photoyellowing can be greatly accelerated by reactive radicals (ROS) generated by the photosensitized inorganic UV absorbers through photocatalysis.^20,26 In addition, the poor fastness is another drawback of inorganic UV absorbers.

It is crucial that these inorganic UV absorbers do not have the photocatalysis effect while shielding UV radiation. Based on our previous work,²⁸ surface modification by silica coating has been proven effective in reducing the photocatalytic activity of ZnO nanoparticles (NPs). Hence an investigation of ZnO NPs with a silica layer (ZnO@SiO₂₎ on reducing photoyellowing of wool fabrics under artificial sunlight has been explored in this study. Compared to the uncoated ZnO NPs, the photocatalytic activity can be reduced due to the blocking effect of solid silica shells, which can obviously enhance the UV protecting effect of ZnO core particles. Another feature is that, the outermost silica layers can be activated by surface modification, so that the fastness of materials can be enhanced during the application. Preliminary investigations showed ZnO@SiO₂ NPs indeed reduced the photoyellowing of wool.

Experimental details

Material

A 100% wool fabric (undyed, double jersey knitted interlock of 245 g/m², made from 19.5 µm of Australian merino wool top which had been chlorine/Hercosett treated to machine washable (Woolmark machine wash) standards) was obtained from AIM Sports Pty Ltd. (Australia). Zinc acetate dehydrate (Zn(Ac)₂•2H₂O) was purchased from AJAX Chemicals Ltd. (Australia). Sodium hydroxide (NaOH 98%), polyvinylpirrolidone (PVP, Mw = 1,300,000), tetraethylorthosilicate (TEOS), and aqueous ammonium hydroxide (29.3 wt.% in water) were obtained from Sigma-Aldrich (Australia). All the other chemicals were of analytical reagent grade and used as received.

Preparation of core-shell ZnO@SiO₂ NPs

ZnO NPs were prepared following the method described by Wang et al.²⁹ In a typical run, 0.27 g of Zn(Ac)₂•2H₂O and 0.5 g of PVP were dissolved in 50 mL of ethanol and refluxed at 60℃ for 0.5 h under constant stirring. Then, the reaction solution was allowed to cool down to room temperature. A NaOH solution was prepared separately by dissolving 0.09 g of NaOH in 50 mL of ethanol at room temperature in an ultrasonic bath. The NaOH solution was dropwise added into the reaction solution under constant stirring to form PVP-capped ZnO NPs. The solution mixture was then stirred continuously at 60℃ for up to 3 h. The PVP-capped ZnO NPs were flocculated from ethanol by the addition of hexane (30 mL of hexane/10 mL of reaction mixture) and subsequently separated by centrifugation at 6000 rpm for 10 min. Then the ZnO NPs were washed with ethanol/hexane mixture three times.

The Stöber method was employed to form a silica layer on ZnO NPs at room temperature using the typical process as follows: 1.64 mL solution mixture was purified and then redispersed in 6.57 mL ethanol solution containing 4.2% ammonium hydroxide (29.3 wt.%). This dispersion was subsequently added to by 0.066 mL TEOS ethanol solution (10 wt.%) under magnetic stirring. The surface charge of newborn ZnO NPs is slightly negative, which could absorb NH₃ċH₂O. In the Stöber method coating process, ammonia was added in the reaction system first and mixed well, then TEOS was added. Because the strong hydrogen bonds could form between ammonia and the hydroxyl groups of silica, the silica layer could be easily coated onto the surface of ZnO. This reaction system was kept stirring for 3 h to obtain ZnO@SiO₂ NPs. Then the nanoparticles were separated by centrifugation at 6000 rpm for 5 min, and washed by ethanol three times.

Fabric treatment

ZnO and ZnO@SiO₂ ethanol dispersions were prepared before fabric treatment, where the content of ZnO NPs was maintained the same (0.3 g/L). Both of the dispersions were respectively applied on fabrics (2.5 cm × 5 cm, zigzag sewn) at various o.w.f. values: 1%, 5%, 11%, 15% and 19% by a dip coating method. Treated samples were dried in darkness at room temperature overnight.

Characterization

A transmission electron microscope (JEOL-2100) was used to observe the morphologies of particles. The samples were first diluted with ethanol, then deposited onto carbon-coated copper grids and air dried before examination.

The UV–Vis absorption spectra of ZnO NPs before and after silica coating were obtained from a Varian Cary 3E UV–Vis spectrophotometer. The photocatalytic performance of ZnO NPs before and after silica coating was evaluated by the photodegradation of Rhodamine B under simulated sunlight. The experiment was carried out as follows: the obtained particles were added into Rhodamine B aqueous solution, and the concentration of ZnO was kept at 0.2 g/L. The whole solution was stirred constantly for 1 h in the dark to achieve adsorption–desorption of Rhodamine B molecules on the surface of catalyst before illumination. Subsequently, the suspension was irradiated with simulated sunlight using an Atlas Suntest CPS1 instrument equipped with a 1500 W air cooled xenon arc lamp (light range ∼300–800 nm wavelengths) and a filter (coated quartz dish). The dye concentration was indicated by the maximum absorbance at 554 nm of dye solution. Thus, the change of dye concentration under UV irradiation could be used to compare the photocatalytic activity of ZnO NPs before and after silica coating. At given intervals, 4 mL of the suspension was extracted and then centrifuged at 6000 rpm for 10 min to separate the nanoparticles from the supernatant. UV–Vis absorption spectra of the supernatant were measured with a Varian Cary 3E UV–Vis spectrophotometer.

The surfaces of the fabrics after treatment were investigated by a scanning electron microscope (ZEISS Supra 55 SEM VP). Elemental analysis of the fabric surface was completed using energy dispersive X-ray spectroscopy (EDX, Oxford Instruments, UK) with an accelerating voltage of 15 kV and work distance of 8 mm. Six randomly selected points in each sample were chosen for analysis. The capability of the treated fabrics and quartz slide coated ZnO film to block UV rays was measured for UV protection factor (UPF, Fangyuan Instruments, China). Five randomly selected areas in each sample were chosen for analysis.

Assessment of photoyellowing of wool

A quartz slide coated with mixtures of the ZnO NPs dispersion and acrylic polymer was prepared by a dip coating method. Multiple coatings were applied until the UV absorbance of the film was in the required range (the UV absorbance is between ∼2.8–3.0).^20,21 The UV absorbance and transparency of the films on quartz slides were determined with the Varian Cary 3E UV–Vis spectrophotometer.

Treated wool fabrics and an untreated wool fabric under a quartz slide coated film were exposed for 168 h in Atlas Suntest CPS1 instrument equipped with 1500 W air cooled xenon arc lamp (light range ∼300–800 nm wavelengths). An untreated sample was used as a blank control. The temperature inside the box was 35℃, at a dose of 300 W/m². The level of photoyellowing for wool fabrics during the aging process was evaluated by the changes of whiteness and yellowness values before and after artificial sunlight irradiation. The more severe the yellowing of the fabrics, the higher the value of yellowness and the lower the value of whiteness. The value was determined with a Datacolor SF 600 Plus-CT Spectraflash spectrophotometer.

Results and discussion

Morphology of ZnO@SiO₂ NPs

Figure 1(a) and (b) show TEM micrographs of ZnO NPs and ZnO@SiO₂ NPs respectively. The ZnO NPs were nearly spherical in morphology, and the average particle size was around 5 nm. The morphology of these ZnO@SiO₂ NPs has a core-shell structure with a belt type distribution. The silica shells could be observed as a grey network structure and ZnO core particles as small dark dots. The thickness of the silica shells is about ∼2–5 nm.

Figure 1.

TEM images of (a) ZnO NPs and (b) ZnO@SiO₂ NPs.

Optical properties of ZnO@SiO₂ NPs

The UV–Vis absorbance spectrum and photocatalytic activity of ZnO NPs and ZnO@SiO₂ NPs are presented in Figure 2.

Figure 2.

(a) UV–Vis absorbance spectrum and (b) photodegradation of Rhodamine B by ZnO NPs and ZnO@SiO₂ NPs.

Figure 2(a) shows that both ZnO and ZnO@SiO₂ ethanol dispersions have similar UV–Vis absorption curves; however the absorbance intensity of ZnO around 350 nm is decreased by ∼19% after silica coating. In addition, a slight increase of absorbance is observed between 370 and 520 nm after the silica coating, which means that the transparency is decreased. This is due to the scattering effect of silica shells, where the light does not penetrate the silica-layer preventing absorption by the ZnO core particles. This decrease of UV absorption could also arise from the loss of ZnO NPs, due to the etching of ammonia. As the coating time is extended, the transparency reduces, which can be attributed to the increased thickness of silica shells. As small semiconductor quantum dots, there is an empirical relationship between the average diameter (D) of ZnO NPs and the absorption shoulder (λ_1/2) of the absorbance spectrum.³⁰ The particle size can be calculated based on equation (1):³⁰

1240 / λ_{1 / 2} = 3.301 + 294.0 / D^{2} + 1.09 / D

(1)

Thus, the particle size is estimated to be around 4.92 nm, which is consistent with the result of TEM.

Figure 2(b) shows the photocatalytic degradation of Rhodamine B with ZnO NPs and ZnO@SiO₂ NPs of equal amounts of ZnO. A/A₀ is the ratio of the absorption of the dye at any time to the initial absorption, which represents the residual dye in the solution during UV irradiation. It can be seen that ZnO NPs show much greater photocatalytic activity than ZnO@SiO₂ NPs. Approximately 96% of the dye is degraded in the presence of uncoated ZnO NPs after 120 min of irradiation, while the catalytic system of ZnO@SiO₂ is about 48%. Therefore, the outmost silica-layer can markedly shield the photocatalytic activity of ZnO core particles.

Surface analyses of wool fabrics

Figure 3(a) to (c) show the SEM images of untreated woolen fibers, ZnO and ZnO@SiO₂ NPs treated woolen fibers respectively. Figure 3(e) and (d) are the surface of ZnO and ZnO@SiO₂ NPs treated woolen fibers. Two EDX curves confirm the presence of ZnO and ZnO@SiO₂ NPs on the surface of fabrics respectively. Although the size of the ZnO and ZnO@SiO₂ NPs is so small that it is not clearly visible by SEM, it still can be seen from Figure 3(a) to (c) that untreated wool fiber has a smoother surface than the ZnO and ZnO@SiO₂ NPs treated fibers. It also can be seen from Figure 3(b) and (c) that the ZnO@SiO₂ NPs treated fibers show rougher and more irregular surface than ZnO NPs treated fibers, due to the larger particle size. However, the ZnO@SiO₂ coating demonstrates better continuity than a ZnO layer in magnification (Figure 3(d) and (e)). This is because there are plenty of –OH groups covering the silica shell surface in ZnO@SiO₂ NPs, which leads to stronger attractive interactions and forces between the particles as well as the particles and fibers.

Figure 3.

SEM images of wool fibers with (a) no treatment, (b) 15% o.w.f. ZnO NPs treatment, (c) 15% o.w.f. ZnO@SiO₂ NPs treatment, and magnification of the fiber surface coating with (d) 15% o.w.f. ZnO NPs and (e) 15% o.w.f. ZnO@SiO₂ NPs. The insets are the EDX plots of the fibers after treatment.

The surface change of the wool fabrics before and after treatment could also be investigated by looking at the UPF values of the fabrics, which is shown in Figure 4. It can be seen that both ZnO and ZnO@SiO₂ NPs coating increase the UPF values of wool fabrics, and the increments for two groups are both enhanced almost linearly with the increase in the ZnO and ZnO@SiO₂ NPs dosage. Therefore, the UPF values can be controllable according to the amount of ZnO NPs. For a given amount of ZnO, the UPF values of the specimens from two groups are quite similar. It demonstrates that the UV shielding ability of ZnO NPs is not weakened after silica coating, although the UV absorption of ZnO NPs is reduced after silica coating (Figure 4). This is likely due to the scattering effect of silica shell, which could protect the wool fabrics against irradiation to a certain extent.

Figure 4.

UPF values of ZnO and ZnO@SiO₂ NPs treated wool fabrics as well as quartz slide coated ZnO film.

Effects of ZnO and ZnO@ SiO₂ NPs on wool fabrics against photoyellowing

The whiteness and yellowness values of the wool fabrics treated with various dosages for different type of nanoparticles, as well as an untreated sample under a quartz slide coated ZnO film as a function of time are shown in Figure 5. The absorbance intensity of the quartz slide coated ZnO film at 350 nm was 3.03, which is equivalent to a UV absorption >99.8%.^20,21 The quartz slide coated ZnO film was used to absorb the UV component of sunlight; at the same time the quartz slide provided a physical barrier to obstruct the direct contact between ZnO NPs and wool fabric. The difference between the whiteness/yellowness of the fabrics at any given time and initially is represented by W–W₀ and Y–Y₀ respectively, which indicates the extent of discolouration during UV irradiation.

Figure 5.

Effects of ZnO and ZnO@SiO₂ NPs on photoyellowing of knitted wool fabrics.

The whiteness of untreated wool fabric decreased severely after UV exposure, while the whiteness of the fabric exposed under a quartz slide coated with ZnO film improved remarkably. The treatment of ZnO@SiO₂ NPs indeed retarded the yellowing process of wool fabrics, and the protection is enhanced along with the increase of the amount of ZnO@SiO₂ NPs. Conversely, the protection of ZnO NPs is not so effective at either too low or too high NP usage. After 168 h of irradiation, the decrease in whiteness value of untreated sample is 20.72; whilst the increase in whiteness value of the fabric exposed under a quartz slide coated with ZnO film is 45.26. Protection to a certain extent is observed under 5% o.w.f. ZnO treatment; however, greater than 5% o.w.f. ZnO resulted in much severer yellowing than the untreated sample (a decrease of 28.31). The optimum protective result is observed with the 19% o.w.f. ZnO@SiO₂ NPs, where the decrease in whiteness is 4.72.

Simulated sunlight contains both UV and blue light wavelengths. Wool yellows under sunlight exposure while yellow wool also bleaches.³¹ Both processes happen simultaneously during exposure. For wool fiber, the maximum yellowing occurs at wavelengths shorter than 331 nm.¹ Between ∼331–398 nm, both yellowing and bleaching occur simultaneously, where yellowing starts slightly earlier than bleaching.³² The most effective wavelengths of photobleaching range from 400 nm to 450 nm.⁸ There are three main factors which affect the color changes in wool fabrics treated by ZnO and ZnO@SiO₂ NPs: photoyellowing and photobleaching from solar irradiation, as well as the photocatalytic effect from ZnO NPs. The photoyellowing and photocatalytic effect make fabrics yellow; however photobleaching whitens them. Therefore, the discolouration of wool under irradiation is a combined result of three effects.

Figure 5 indicates that the rate of photobleaching of the fabric exposed under a quartz slide was quite fast at first and then gradually slowed down to zero. In the first 24 h, the whiteness improved to 31.97, whilst the increase was 7.05 over the next 48 h irradiation. The critical value of whiteness appeared after 120 h exposure, which hardly changed despite the prolonged irradiation time. It is known that ZnO film works as a filter to remove the UV component and keep visibility, which could also be confirmed by the change in UPF values. Due to the physical isolation of the quartz slide, the photocatalytic effect of ZnO film is entirely blocked.²⁰ Therefore longer wavelength irradiation causes the photobleaching of the fabric. These results confirm that, the UV component of the artificial sunlight is the major cause of photoyellowing, while the short-wavelength visible range causes photobleaching. In addition, the extent and rate of photobleaching is influenced by the initial color of the wool. Owing to the fact that the base color of undyed wool is creamy, its propensity to photobleach is greater than for the bleached wool.⁸

Figure 5 also shows that ZnO NPs are not effective in preventing wool fabrics from photoyellowing. It is worth mentioning that, in the group of ZnO NPs treated samples, the yellowing extent was slow from 1% o.w.f. to 5% o.w.f., but then suddenly accelerated from 5% o.w.f. to 19% o.w.f. This trend is different from the ZnO@SiO₂ NPs group, in which the photodegradation was gradually retarded as the nanoparticles concentration increased. These unpredictable behaviors could be related to the changes in optical properties of ZnO NPs before and after silica coating as indicated in Figure 2. Both ZnO and ZnO@SiO₂ NPs showed a strong absorption of UV irradiation below 370 nm, but little blocking of the wavelengths longer than 370 nm which can cause photobleaching of wool. At the same time, ZnO NPs showed much stronger photocatalytic activity than the ZnO@SiO₂ NPs. For low-level usages of ZnO NPs the amount of UV absorbed is not enough to block the harmful UV irradiation that results in photoyellowing. As the usage increases, the UV shielding could be improved, while the photocatalysis effect could also be noticeably enhanced. Therefore, the accelerated yellowing processes both under low-level and high-level ZnO NPs usages result from two distinct main causes, which are photoyellowing from UV irradiation and yellowing from free radicals, respectively. In addition, the results in Figure 5 indicate that ZnO@SiO₂ NPs indeed show much better protection than the ZnO NPs at the same usages, which also confirms that the yellowing at high-level of ZnO NPs treatments is caused by the photocatalytic effect of ZnO.

Slight photobleaching was observed after the first 24 h irradiation in blank control and a higher amount of ZnO@SiO₂ NPs treated fabrics, especially at 19% o.w.f. ZnO@SiO₂ NPs dosage, but not in ZnO NPs treated samples. Because the silica shell could shield the photocatalysis of ZnO core particles to a certain extent; the yellowing caused by the photocatalysis effect is greatly suppressed. Similar to the quartz slide coated ZnO film, high-level usage of ZnO@SiO₂ NPs works as a UV filter which could lead to effective bleaching. However, compared with the complete blocking from the quartz slide, the photocatalytic activity of ZnO@SiO₂ NPs is still more noticeable than with the ZnO film.

Conclusions

Core-shell structured ZnO@SiO₂ NPs have been fabricated and coated onto knitted wool fabrics as an UV absorber. After 168 h of simulated solar irradiation, these nanoparticles have been found to greatly retard the photoyellowing rate of wool fabrics. This protection is from the high UV shielding ability and low photocatalytic activity of ZnO@SiO₂ NPs, which is due to the interdiction of coated silica shell. The optimum UV protective result was observed at 19% o.w.f. ZnO@SiO₂ NPs.

Footnotes

Funding

This research was supported by Deakin University’s PhD Scholarship and the National Natural Science Foundation of China (grant number NSFC 51302197).

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Reducing photoyellowing of wool fabrics with silica coated ZnO nanoparticles

Abstract

Keywords

Experimental details

Material

Preparation of core-shell ZnO@SiO2 NPs

Fabric treatment

Characterization

Assessment of photoyellowing of wool

Results and discussion

Morphology of ZnO@SiO2 NPs

Optical properties of ZnO@SiO2 NPs

Surface analyses of wool fabrics

Effects of ZnO and ZnO@ SiO2 NPs on wool fabrics against photoyellowing

Conclusions

Footnotes

Funding

References

Preparation of core-shell ZnO@SiO₂ NPs

Morphology of ZnO@SiO₂ NPs

Optical properties of ZnO@SiO₂ NPs

Effects of ZnO and ZnO@ SiO₂ NPs on wool fabrics against photoyellowing