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Nano Finishes for Uv Protection in Textiles

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[1] 1. Introduction: The electromagnetic spectrum consists of radiations of various wavelengths and frequencies. Of these, Ultraviolet (UV) radiations have wavelengths falling in the range of 212-400 nm. Being very energetic, UV can break chemical bonds, making molecules unusually reactive or ionizing them. Sunburn, for example, is caused by the disruptive effects of UV radiation on skin cells, which is the main cause of skin cancer, if the radiation irreparably damages the complex DNA molecules in the cells (UV radiation is a proven mutagen). The Sun emits a large amount of UV radiation, most of which is absorbed by the atmosphere's ozone layer before reaching the surface [12]. However, the world is witnessing a very rapid depletion of atmospheric ozone, leading to many adverse effects. One of these is an alarming increase in the incidence of skin cancer worldwide. Because ozone is a very effective UV-absorber, each one percent decrease in ozone concentration is predicted to increase the rate of skin cancer by 2-5%. It is estimated by the United States Environmental Protection Agency that ozone depletion will lead to between three and fifteen million new cases of skin cancer in the United States by the year 2075. Other reasons for the skin cancer epidemic can be traced to lifestyle changes such as excessive exposure to sunlight during leisure activities. In the case of agricultural and other outdoor workers, exposure to the sun is an occupational hazard as they have no choice about the duration of their exposure to the sun [1]. Each year over 1000 Australians die from skin cancer while two-thirds of the Australian population develop some form of skin cancer at some point in their lives. Other countries with a high melanoma rate are Canada, New Zealand and Norway. The American Cancer Society estimates that more than one million new skin cancer cases are diagnosed each year in the United States. As frightening as the one million count is, an even more devastating statistic is that an estimated 54,200 of those will be diagnosed with melanoma, the deadliest form of skin cancer. In the light of such devastating circumstances, methods for UV protection must be swiftly discovered and implemented. Two examples of UV protection methods are sunscreen lotions and protective clothing. Using sunscreen should be an important part of a comprehensive sun protection. To take full advantage of sunscreens, you have to be able to select a sunscreen that is both effective and a good fit for your particular situation (skin type, lifestyle, esthetic needs and so forth). On the other hand, clothing is one of the fundamental needs of human beings. Therefore, inducing UV protection in textiles would be much more practical and beneficial. In the current article, inducing UV protection in textiles by means of nanofinishes would be focused upon.
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Page 1: Nano Finishes for Uv Protection in Textiles

[1] 

 

1. Introduction:

The electromagnetic spectrum consists of radiations of various wavelengths and frequencies. Of these, Ultraviolet (UV) radiations have wavelengths falling in the range of 212-400 nm. Being very energetic, UV can break chemical bonds, making molecules unusually reactive or ionizing them. Sunburn, for example, is caused by the disruptive effects of UV radiation on skin cells, which is the main cause of skin cancer, if the radiation irreparably damages the complex DNA molecules in the cells (UV radiation is a proven mutagen). The Sun emits a large amount of UV radiation, most of which is absorbed by the atmosphere's ozone layer before reaching the surface [12]. However, the world is witnessing a very rapid depletion of atmospheric ozone, leading to many adverse effects.

One of these is an alarming increase in the incidence of skin cancer worldwide. Because ozone is a very effective UV-absorber, each one percent decrease in ozone concentration is predicted to increase the rate of skin cancer by 2-5%. It is estimated by the United States Environmental Protection Agency that ozone depletion will lead to between three and fifteen million new cases of skin cancer in the United States by the year 2075. Other reasons for the skin cancer epidemic can be traced to lifestyle changes such as excessive exposure to sunlight during leisure activities. In the case of agricultural and other outdoor workers, exposure to the sun is an occupational hazard as they have no choice about the duration of their exposure to the sun [1]. Each year over 1000 Australians die from skin cancer while two-thirds of the Australian population develop some form of skin cancer at some point in their lives. Other countries with a high melanoma rate are Canada, New Zealand and Norway. The American Cancer Society estimates that more than one million new skin cancer cases are diagnosed each year in the United States. As frightening as the one million count is, an even more devastating statistic is that an estimated 54,200 of those will be diagnosed with melanoma, the deadliest form of skin cancer. In the light of such devastating circumstances, methods for UV protection must be swiftly discovered and implemented. Two examples of UV protection methods are sunscreen lotions and protective clothing. Using sunscreen should be an important part of a comprehensive sun protection. To take full advantage of sunscreens, you have to be able to select a sunscreen that is both effective and a good fit for your particular situation (skin type, lifestyle, esthetic needs and so forth). On the other hand, clothing is one of the fundamental needs of human beings. Therefore, inducing UV protection in textiles would be much more practical and beneficial. In the current article, inducing UV protection in textiles by means of nanofinishes would be focused upon.

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2. UV Radiation:

The ultraviolet radiation band consists of three regions: UVA (320 to 400 nm), UVB (290 to 320 nm), and UVC (200 to 290 nm). UVC is totally absorbed by the atmosphere and does not reach the earth. UVA causes little visible reaction on skin but has been shown to decrease the immunological response of skin cells. UVB is most responsible for the development of skin cancers.

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3. Evaluation of UV protection: To assess the amount of UV protection offered by a particular textile material, the terms Ultraviolet Protection Factor (UPF) and Solar Protection Factor (SPF) must be defined. The tests can be carried out either in vivo or in vitro. UPF is evaluated in vitro while SPF is evaluated in vivo.

3.1. Ultraviolet Protection Factor: The term UPF has been widely adopted by the textile and clothing industry worldwide to denote the protective ability of a textile based on instrumental measurements and is defined in Australian/New Zealand standard AS/NZS 4399:1996. UPF is the ratio of the average effective ultraviolet radiation (UVR) irradiance calculated for unprotected skin to the average effective UVR irradiance calculated for skin protected by the test fabric.

Eq. (1)

where Eλ = erythemal spectral effectiveness, Sλ = solar spectral irradiance in W/m2/nm, Tλ = spectral transmittance of fabric, Δ λ = the bandwidth in nm, and λ = the wavelength in nm.

3.2. Solar Protection Factor: The simplest way of in vivo testing is by attaching rectangular pieces of fabric to the back of a human subject and determining the minimum erythemal dose (MED) of the unprotected and protected skin. MED is defined as the minimum quantity of radiant energy required to produce first detectable reddening of the skin, 22 +/- 2 hours after exposure. MED for unprotected skin is determined first using incremental UVB doses. Subsequently, MED for protected skin is determined by a series of incremental and decremental UVB doses centered at the estimated SPF of the given fabric as estimated from UPF values determined in vitro. The dose that results in a minimal erythema extending to the borders of irradiation is then used to calculate SPF as shown.

The higher the SPF value, the better the fabric's ability to protect against sunburn.

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4. UV protective textiles: Textile material comprises of fabrics. These fabrics can absorb, reflect or scatter incident radiation. Textiles can offer UV protection using various mechanisms, which can be seen in figure 1. However, there are a number of factors that contribute to a fabric's ability to block UVR. These factors include fiber chemistry; fabric construction, porosity, thickness and weight; moisture content and wet-processing history of the fabric such as dye concentration, fluorescent whitening agents, UV-absorbers and other finishing chemicals that may have been applied to the textile material.

Fig.1. Schematic representation of a textile as a barrier to UV radiation [2]

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Examples of the methods in which a textile material can be made UV protective are application of colors, application of UV absorbers and application of nanofinishes. The dyes for coloring a

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textile can affect the UV protectiveness of a fabric, depending on the position and intensity of the UV absorption bands of the dyes and their concentrations. Generally, dark colors provide better UV protection due to increased UV absorption. However, particular hue dyes can vary considerably in the degree of UV protectiveness because of individual transmission and absorption characteristics. To improve UV protection, UV absorbers have been added using different techniques. UV absorbers are colorless compounds that absorb in the wavelength range of 290 to 400 nm. Fabrics can be made opaque to UV radiation with a sufficient level of UV absorber impregnation, and the corresponding UPFs approached the theoretically predicted levels. The other method mentioned above is the use of nanofinishes. Inorganic metal compounds like the oxides of zinc, titanium, cerium and zirconium have been used effectively as coatings on textiles for UV protection. In the following sections, the application and science behind these nanofinishes would be described in greater detail.

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5. Nanofinishes for UV Protection in Textiles:

Nanotechnology can be defined as the study of controlling matter on an atomic and molecular scale involving structures sized between 1 to 100 nm in at least one dimension. Nanoparticles of Zinc oxide (ZnO), titanium dioxide (TiO2) etc. have been used in UV protection of textiles. These inorganic materials absorb the UV radiation, thus blocking the radiation from reaching the wearer’s skin. ZnO and TiO2 are used extensively in sunscreen lotions too. However, conventional sized particles when used in sunscreens lead to an unappealing white color on the skin. This is due to agglomeration of the particles. On the contrary, nanoparticles have high surface-to-volume ratio and can adhere well to the fibers of the fabric and yield a transparent appearance. Due to their high surface area and high surface energy, nanoparticles are bound to the surface of the fibres by van der Waals forces, which lead to reasonable wash fastness. As wash fastness is an important criterion for fabrics, this is another advantage in using nanoparticles. ZnO and TiO2 are non-toxic, compatible with human skin, chemically stable under both high temperature and UV radiation and are easily available [3,4]. These properties make them attractive choices for UV protection. In their work, Karthivelu et al. [4] synthesized ZnO nanoparticles and applied them on cotton/polyester fabrics using a binder resin and a padding mangle. They were then evaluated for their UV protection. They found that applying ZnO nanoparticles on cotton and polyester fabrics increased the UV absorption over the entire investigated spectrum. The UPF values were calculated for UVA and UVB radiation using equation 1. The values reflect the higher UV protection obtained after loading the fabric with ZnO nanoparticles. Woven fabrics showed better UV protection when compared to knitted fabrics. Polyester/cotton blend fabric showed better UV protection than pure cotton fabrics due to the better UV absorption characteristics of polyester. The use of padding mangle led to a few ZnO particles penetrating the surface of the fibres. These particles did not contribute to the overall UV protection. The researchers also pointed out since only one face of the fabric is exposed to UV radiation, methods like spraying with compressed air or spray gun could be used for coating the fabrics. In their study, Mao et al. [5] researched the in-situ growth of ZnO on SiO2 sol coated cotton fabrics via hydrothermal method using relatively higher reagents concentrations. In this case, hexamethyelenetetramine and zincnitrate were used as the reagents. The cotton fabric was then treated in hot water to obtain 1D needle-shaped nano ZnO crystallites. The mechanism of ZnO nanoparticle formation is given as follows:

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Figure 2 shows the development of ZnO nanorods on the fabric surface as evidenced by SEM.

Fig 2. SEM images of the surface of cotton fabric (a) before treatment, (b) after soaking in the SiO2 solution, (c) after chemical deposition of ZnO, and (d) after hot water treatment at 100 °C for 2.5 h. [5] As seen in the image Fig. 2(b), SiO2 sol treatment increased surface area for the growth of the ZnO nanoparticles. Fig. 2(c) shows that before boiling water treatment, the particle morphology and size of ZnO were not uniform on the cotton surface. Two main morphologies of the ZnO particleswere sphere and rod. The particles were large with a big variety in size. The sphere-shaped ZnO particles had diameters in the range of 200 nm to 800 nm. The rodshaped ZnO particles had diameters in the range of 50 to 200 nm and lengths in the range of 1 to 1.5 μm.

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After treated with boiling water for 2.5 h, the morphology of the ZnO crystallites was changed to needle shape (Fig. 2(d)). The needle-shaped ZnO crystallites had more uniform diameter and length and evenly covered the surface of the cotton fibers. The diameters of the needle-shaped ZnO nanorods were in the range of about 20 to 30 nm and the lengths were in the range of about 0.5 to 2 μm. The UV/Vis transmission spectra of the cotton fabric before and after treatment are depicted in figure 3. It can be seen that there is no positive effect of SiO2 coating on the fabric. Boiling water treatment further enhanced the UV protection, especially in the UVB region (250-315 nm).

Fig. 3. UV/Vis transmission spectra of the cotton fabric (a) before treatment, (b) after soaking in the SiO2 solution, (c) after chemical deposition of ZnO, and (d) after hotwater treatment at 100 °C for 2.5 h. The UPF data of ZnO nanorod covered cotton fabric with different boiling water recrystallization treatment time are in figure 4(a). Without boiling water treatment, the UPF value of ZnO covered cotton fabric was about 10. This indicated that the fabric did not have UV protection activity (UPF < 15 is nonrateable), though there was a decent amount of Zn content on the surface of fabric. This is due to the agglomeration and clustering of ZnO particles. The UPF value of ZnO covered cotton fabric increased when increasing boiling water treatment time as in figure 4(a).

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After the sample was treated in boiling water for 1.5 h, its UPF value was 57.3, which is considered as excellent UV blocking property.

Fig. 4. UPF values of ZnO covered cotton fabric (a) after boiling water treatment for different times, and (b) treated at different water temperatures for 3 h. It was also found that even after 20 washes, the fabric possessed good UV blocking property as shown in figure 5.

Fig. 5. UV/Vis transmission spectra of nano ZnO coated cotton fabric (a) before treatment, (b) after soaking in the SiO2 solution, chemical deposition of ZnO, and boiling water treatment for 3 h, and (c) after 20 washes.

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J. F. Lima et al. [6] investigated ZnO:CeO2-based nanopowders with low catalytic activity as UV absorbers. Even though CeO2 has excellent UV absorption characteristics, it has not been used much in sunscreens and textile coating applications due to its low photocatalytic activity. Low photocatalytic activity would mean that organic materials would degrade. The use of cerium oxide together with zinc oxide should reduce the catalytic and photocatalytic activities, i.e., the evolution of oxygen molecules, thus preventing degradation of organic materials. Fine ZnO:CeO2-based particles with very small size exhibit unique UV absorbing ability, high stability at high temperatures, high hardness, and low activity as catalyst. Lee [7] tried to develop UV-protective textiles based on electrospun zinc oxide nanocomposite fibers. Layered fabric systems with electrospun zinc oxide nanocomposite fiber webs were fabricated at various concentrations of zinc oxide in a range of web area densities. The effects of zinc oxide concentration and web area density on the UV-protective properties of layered fabric systems were examined. In figure 6, SEM micrographs of electrospun fibers and layered fabric system with electrospun zinc oxide nanocomposite fibers are shown. ZnO nanoparticles are clearly seen in Fig. 6(b). At the optimal condition, ZnO nanocomposite fibers were electrospun directly onto a polypropylene nonwoven substrate to form a layered fabric system. Fig 6(c) illustrates the cross-sectional view of a layered fabric system in which a very thin layer of nanocomposite fiber web was deposited onto a conventional nonwoven substrate. The figure clearly shows the relative thickness of an electrospun nanofiber web layer compared to a conventional spunbond nonwoven. Electrospun nanofiber webs are extremely thin, light-weight, and mechanically flexible; thus desired functionalities could be imparted without significant increases in weight or thickness by incorporating functional materials into nanofibrous structures.

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Fig. 6.SEM micrographs of (a) electrospun polyurethane nanofiber web, (b) electrospun polyurethane/zinc oxide nanocomposite fiber web and the cross-sectional view of a nanocomposite fiber (inset), and (c) cross-sectional view of a layered fabric system. Blocking percentages for UVA and UVB radiation were calculated alongwith the UPF values. The values are shown in table 1.

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Table 1. Blocking percentages for UV-A and UV-B radiation, and UPF values of layered fabric systems and control fabric In both UV-A and UV-B regions, the UV blocking ability of layered fabric systems increased with increasing ZnO concentrations of the nanocomposite fiber web. It also increased with the increasing electrospun web area density of the ZnO nanocomposite fiber web. However, the air and moisture vapor transmission reduced with increase in web area denisy, thus adversely affecting the comfort of the fabric. In another study, Mihailovic et al. [8] tried to functionalize polyester fabric with alginates and TiO2 nanoparticles. In today’s world, people are looking for multifunctionality in every area. In this case, the researchers tried to combine both antimicrobial property as well as UV blocking property in the fabric. Alginates possess an abundance of carboxylic groups which may provide additional binding sites for TiO2 nanoparticles. The UPF values were calculated and are presented in table 2. Figure 7 shows the UV transmission spectra of polyester fabric before and after treatment at various stages.

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Table 2 UPF values of PES fabrics loaded of with TiO2 nanoparticles

Fig 7. Transmission spectra of PES fabrics loaded of with TiO2 nanoparticles

UPF value of 43 and corresponding UPF rating of 40 categorize the polyester fabric with an excellent UV protection. The TiO2 nanoparticle deposition onto the fabrics led to a rise of UPF values to the level corresponding to UPF rating of 50+, which assigns the maximum UV protection. UPF value of the TiO2 fabric decreased by 27.7% after five washing cycles and consequently UPF rating dropped from 50+ to 50. The washing of ALG + TiO2 fabric induced slight decrease in UV transmission intensity, but still its UPF value is by 7% higher compared to TiO2 fabric that was not washed. These results imply good laundering durability of the ALG +

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TiO2 fabric. Good laundering durability can be attributed to enhanced binding efficiency of TiO2 nanoparticles to the polyester fabric modified by alginate. In other studies, cotton wound dressings were coated with chitosan loaded with ZnO and TiO2 nanoparticles. Chitosan imparts antimicrobial property while UV blocking property is imparted by the zinc oxide and titanium dioxide nanoparticles. These fabrics were further tested for their wash fastness, tear strength etc. In another study, Katangur et al. [9] formulated particle-embedded acrylic coatings that are transparent to visible light but absorb UV radiation. The UV absorption behavior of nano- and micron size particles was studied and explained. Thick coatings of 10 µm and 20 µm were applied to Kevlar fabrics and the mechanical behavior of the fabrics before and after UV exposure was studied to evaluate the performance of coatings. Figures 8 and 9 show the UV absorbance spectra of different sized nanoparticles and tear strength variation in TiO2 nanoparticle coated Kevlar fabric.

Fig. 8. Absorption spectra from various size TiO2 particles.

Going from 50 mm particles to 50 nm particles provides 100 times the surface area per unit mass. Experimental results confirm that smaller, or nanosized, particles absorb more UV and scatter less visible radiation.

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Fig. 9. Kevlar fabric tear strength vs. time of UV exposure.

Furthermore, Zhang et al. [10] studied the photostability of wool doped with photocatalytic titanium dioxide nanoparticles. They conducted yellowness and photo-induced chemiluminescence (PICL) measurements and showed that nanocrystalline TiO2 could effectively reduce the rate of photoyellowing by inhibiting free radical generation in doped wool, and that a higher concentration of TiO2 contributed to a lower rate of photooxidation and reduced photoyellowing. Hence nanocrystalline TiO2 acts primarily as a UV absorber on wool in dry conditions and not as a photocatalyst. Zohdy et al. [11] subjected cotton, polyester and their blend fabrics to gamma irradiation after coating them with ZnO nanoparticles for surface curing. They also used Alum as a coating material. Alum too imparted some UV blocking ability. Figure 10 shows the micrographs of various fabrics before and after treatment.

SEM micrographs of (a) uncoated polyester fabrics (b) ZnO coated polyester.

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SEM micrographs of (a) untreated Cotton fabrics (b) Alum/ZnO coated fabric

SEM micrographs of (a) untreated Blend fabrics (b) Alum coated fabric

Fig. 10

The variation in UV blocking property was also recorded as a function of concentration of UV absorber ingredients. It was found that the UPF increased with increase in the concentration of the UV absorber.

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6. Conclusion:

Presented here were some representative examples on the effectiveness of inorganic metal oxide nanoparticles as UV blockers in textile applications. Needless to say, there is much more research going on to study and understand these further. From the literature surveryed, it was concluded that nanosized metal oxides, especially zinc oxide and titanium dioxide, provide good UV protection in textiles. They can be applied on the fabric using different techniques. They adhere to the surface of the fabric and have good wash fastness. Also they are attractive in terms of aesthetics as they form transparent coatings on the fabric. Decrease in the size of the particles leads to an increase in the UV absorption capability due to the greater surface-to-volume ratio. Furthermore, they can be used with other functional components to produce multifunctional fabrics.


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