What is anti-UV finishing of textiles and what are the common UV absorbers?

Abstract: The mechanism of ozone layer destruction by HFCs and the damage to human body are reviewed from the influence of fiber type and structure of fabric on the anti-UV performance of fabric. The deficiencies of current fabric anti-UV finishing are comprehensively analyzed, and nano-UV resistance is proposed as the research direction of anti-UV finishing.

Keywords: ozone, fabric, anti-UV, finishing agent

1 Introduction

Since the discovery of ultraviolet light by the German physicist Ritter in 1801, people have known and researched about ultraviolet light for more than two centuries. Ultraviolet radiation from the sun, an electromagnetic ray, the International Commission on Illumination C.I.E will be distinguished into three different bands of the ultraviolet spectrum, according to the ultraviolet wavelength can be divided into: near ultraviolet LNA (315-400nm), far ultraviolet UVB (280-315nm) and ultra-short ultraviolet LNC (100-280). The electromagnetic waves in sunlight with wavelengths below 300nm are almost absorbed by the ozone in the atmosphere and hardly reach the ground. Therefore, most of them reach the ground are near and far ultraviolet. Moderate exposure to UV radiation is beneficial to the body, helping the body absorb calcium, strengthening the body's non-specific immune function and body defense, and promoting the secretion function of certain hormones in the body. However, excessive exposure to ultraviolet radiation can cause discomfort and even many diseases. Numerous animal experiments have shown that UV-B radiation can damage the cornea and lens, causing cloudy eyes. It has also been shown that UV radiation can disrupt the body's immune system (e.g., causing melanin production), suppress immunity, and worsen or increase diseases such as tuberculosis and herpes simplex.

    Excessive UV radiation can cause serious damage to the skin in particular. The shorter the wavelength of ultraviolet light, the more harmful it is to human skin. Short-wave UVB can pass through the dermis, while medium-wave can enter the dermis. Excessive UV light can damage the DNA in skin cells causing sun spots and hyperpigmentation, and if the DNA damage in epidermal cells increases beyond its repair capacity, it can cause skin cancer.

In recent years, as people's demand for a higher standard of living grows, a large amount of fluorine and chlorine compounds have been emitted into the atmosphere for comfortable enjoyment, causing increasingly serious damage to the ozone layer. The amount of ultraviolet radiation reaching the ground has increased, and the damage to the human body has become more and more serious. Therefore the human body protection from ultraviolet radiation research has been paid attention to, and clothing is the most effective protection tool for the human body, so the anti-UV finishing of textiles is necessary to prevent UV damage to the skin while maintaining the original performance of the fabric has become one of the important issues in the textile dyeing and finishing industry.   

2 fabric anti-ultraviolet finishing

2.1 Factors affecting the transmission of ultraviolet light through the fabric

    Without anti-UV finishing of the fabric itself has a certain anti-UV effect, its anti-UV effect with the type of fabric, morphological structure, structural organization and thickness. It is generally believed that the molecular structure contains benzene ring, aromatic amino acid fibers (such as: polyester, wool, silk, etc.), the following 300nm UV light has a strong absorption, UV transmission rate is low; form is smooth cylindrical fiber on the light reflection rate is high; fabric organization interwoven point less, floating long longer structure (such as satin organization), anti-UV radiation effect is better; thicker fabric UV transmission rate is less less.

    Cotton fiber structure does not contain benzene ring, and has a longitudinal twist not smooth form, ultraviolet light is most likely to pass through the cotton fabric, so the cotton fabric anti-UV finishing is particularly important.

2.2 Classification of anti-UV finishing agents

    The research of anti-UV finishing agent has attracted people's attention since early, according to the anti-UV mechanism can be divided into: reflective anti-UV finishing agent, absorbing anti-UV finishing agent and nano-type anti-UV finishing agent.

2.2.1 Reflective anti-UV finishing agent

    Reflective anti-UV finishing agent has no absorption effect on UV, but only relies on the reflection of light to reduce the transmission rate of UV. Also known as UV shielding agent. These shielding agents are non-toxic, tasteless, non-irritating, thermally stable, non-decomposable and non-volatile, mostly metals, metal oxides and salts, typically Ti02, Zn0, Al02, kaolin, talc, carbon black, iron oxide, lead oxide and CaC03, etc. The reflectivity of UV light at 310-400nm can be as high as 95%.

    Although UV shielding agent has unique advantages, but by its finishing fabric breathability, feel and washability is poor, and fabric color, fastness and whiteness has decreased, some even make the human body allergic reaction. Therefore, this type of finishing agent is mostly used for sunshades, tents, etc., and rarely used in clothing fabrics.

2.2.2 Absorbent anti-UV finishing agent

    Absorbent anti-UV finishing agent, also known as UV absorbers, can strongly and selectively absorb high-energy UV, to other lower-energy forms (such as: longer wavelengths of light or heat) released, so as to avoid UV damage to human skin, while the absorbers themselves are not damaged by UV. The absorber principle is generally considered to be the transfer of intramolecular protons: the hydroxyl group in the structure and the N or O atoms in the nearby structure form an intramolecular six-membered ring containing hydrogen bonds, and the six-membered ring is opened by the absorption of energy from UV irradiation, accompanied by the conversion of the enol and ketone structure, which converts harmful energy into harmless light waves or heat energy release, and the six-membered ring is closed again to restore.

As a fabric UV absorber should have the following conditions.

(1) safe and non-toxic, no allergic reaction to human body, and cannot be a threat to human health.

(2) Meet the requirements of environmental protection.

(3) Good resistance to commonly used solvents and washing resistance.

(4) No coloring after absorbing ultraviolet light.

(5) does not affect or less affect the physical properties of the fabric such as whiteness, fastness, strength and hand feel and fabric style.

(6) There is a certain degree of stability.

    Common UV absorbers are.

① benzophenone chemical compound class.

    Is the earliest use of ultraviolet absorbers, the carbonyl group in the molecules of such compounds and hydroxyl groups form intramolecular hydrogen bonds, forming a chelate ring structure. In the absorption of ultraviolet light, the internal hydrogen bonding oscillation, the stable chelate ring open, the absorbed energy will be released in the form of heat, in addition, the carbonyl group in the molecule will be absorbed by the UV energy excitation, the generation of enol-type structure of the reciprocal isomerism, which also consumes part of the energy.

    Such compounds include: 2-hydroxy-5-chlorobenzophenone, 2-4 dihydroxybenzophenone, etc. These compounds have multiple hydroxyl groups and have good adsorption capacity for fibers that can absorb UV light from 280 to 400 nm, mainly used in polypropylene, polyester, cellulose and other fibers. However, it is almost no absorption of ultraviolet light below 280nm, sometimes easy to yellow, in addition to the price is relatively expensive so little use in the market.

② Salicylate esters.

    Salicylate UV absorbers also have intrinsic hydrogen bonds in the molecule, the beginning of the UV absorption capacity is low, and the absorption range is narrow (less than 340nm), but after a certain period of irradiation, absorption gradually increased. The reason is that molecular rearrangement occurs under UV irradiation to form a benzophenone structure with high UV absorption capacity, which strengthens its UV absorption capacity. Rearrangement of the generated dihydroxybenzophenone and its derivatives can absorb part of the visible light and appear yellow. And lead to yellowing of the fabric after finishing. In addition, these absorbers have a low melting point, easy to sublimate, low absorption coefficient, so the use of less.

③ Benzotriazoles.

    Benzotriazole UV absorbers are similar to benzophenones: benzotriazole UV absorption effect is better than benzophenone UV absorbers, can absorb 300-400nm light, and does not absorb light above 400nm wavelength, so it does not yellow, and has the advantages of volatility resistance, oil resistance. The structure of benzotriazole class absorbers and disperse dyes are very close, so the scope of application is limited, can be processed to polyester using high temperature and high pressure method, but to be applied to nylon, wool, silk and cotton fabrics, need to be connected to the appropriate number of sulfonic acid groups in the molecule.

④ Triazines.

    Triazine and triazole UV absorbers both contain N, relying on N, H formation of intramolecular hydrogen bonding and enol-type and ketone-type structure conversion to effectively absorb UV light, with a high absorption capacity of 280 ~ 380nm UV light. The absorption effect of triazine absorbers is proportional to the number of hydroxyl groups, so the absorption capacity is stronger than the benzotriazole class, the disadvantage is that the compatibility with polymers is poor, and the fabric is easy to color after finishing.

⑤ Organonickel type.

    Organonickel polymers as UV absorbers are different in mechanism of action from the above four types of absorbers, often classified as bursting agents (also known as de-activators, extinction agents, laser state bursting or energy bursting agents).

    Organonickel absorbers have a low absorption capacity, when the organonickel polymer molecules receive ultraviolet light energy is excited into the excited state, when the ultraviolet light is lost, and then from the excited state back to the ground state, the ultraviolet energy into a low-energy spectrum emitted. This reduces the transmission rate of UV light. Organonickel absorbers can be used with some fiber fabrics under certain conditions can form chelate complexes, but often have color, there are limitations to the use.

2.2.3 Nano-type anti-UV finishing agents

    The development of nanomaterials provides a new way for UV shielding agents. Compared with the general UV shielding agent, nano UV shielding agent has a large specific surface area, high surface energy, easy to combine with the material, small particle size, lower diffuse reflectivity of visible light, higher transparency, and less impact on the style of the fabric being finished.

    The absorption mechanism of nano-materials on UV is different from the above-mentioned types of organic absorbers. Take nano-Ti02 as an example: the electronic structure of Ti02 is a full valence band and an empty conduction band, the band gap energy is 3.OeV ~ 3.2eV, and its forbidden band width is generally below 3.OeV. After Ti02 absorbs UV photons with energy greater than or equal to its forbidden band width, the electrons in the valence band will be excited to the conduction band, thus producing highly reactive free moving electrons (e-optics) in the valence band and conduction band respectively. The photogenerated electrons (e-) and holes (h+) in the valence band and conduction band, respectively.

    The electron-hole pairs generated by the absorption of UV light in Ti02 nanoparticles are recombined during various redox reactions to release energy in the form of heat or fluorescence, and on the other hand, they can be dissociated into free holes and free electrons that migrate freely in the lattice to the lattice surface or other reaction sites and are immediately captured by the surface groups. Usually titanium dioxide will surface water activation to produce surface hydroxyl groups to capture free holes, forming hydroxyl radicals, while the free radicals will absorb state oxygen to produce superoxide radicals, which will kill the surrounding bacteria and viruses. Therefore, the fabric after finishing by nano material not only has the ability of anti-UV, but also has the effect of sterilization.

  The fabric after finishing not only has excellent UV resistance, but also has good washability and does not affect the breathability, handfeel and other properties of the fabric. Li Hong et al. used nano Zn0 on linen fabric anti-UV finishing research, the same achieved better results.

3 Different anti-UV finishing agent research comparison

    In summary, the research of anti-UV finishing agent has gone through the process of shielding agent, absorption and development with nano-materials, the shielding or absorption of UV range from small to large, and achieved better results, but there are still some shortcomings. Although the traditional UV shielding agent has a good anti-UV effect in some aspects, such as Si02 on the wavelength of 400nm below the UV reflectivity of up to 95%, but by their treatment of the physical properties of the fabric and fabric feel by serious damage to the quality of the fabric decline, poor washability.

    UV absorbers have a certain anti-UV effect, stable quality, the physical properties and style of the fabric after finishing is maintained, but there is also a narrow range of absorption, finishing fabric yellowing, limited application and other defects. In order to improve the performance of UV absorbers, some scholars have modified the existing absorbers to improve the application performance. For example, Shujuan Yu [20] used polyethylene glycol, polyethylene glycol monomethyl ether, and carboxymethyl chitosan water-soluble polymers to modify benzophenones and benzotriazole absorbers, which effectively improved the water solubility of benzophenones and benzotriazole absorbers. Zhang Huifang et al. used the homemade multi-component anti-UV finishing agent UV-5 and UV-6, and explored a set of finishing process through experiments to improve the scope of application of anti-UV finishing agent and achieved a better anti-UV effect.

    In addition to the existing absorption modification, nano anti-UV research has gradually attracted attention in recent years. Due to the special physical structure of nano-materials makes it have better anti-UV effect, and the finishing day has no adverse effect on fabric color and light, hydrophilicity and hand feel. And Ti02 and Zn0 nanomaterials when its absorber with the same dose conditions, the absorption capacity in the UV region is strong, the absorption peak is higher. The smaller the particle size, the larger the shielding area of light, generally between 30 and 100 nm when they have the best shielding effect on ultraviolet. It can be said that the development of nano-UV absorbers is one of the main directions of anti-UV finishing of fabrics. At present, the main problem to be solved in the nano-UV finishing is how to prevent the agglomeration of nanomaterials, the combination of finishing agent and fiber and reduce the strength loss of the fabric.

4 Conclusion

    At present, the anti-UV finishing of fabrics has achieved certain results. However, the current finishing agent in the use of varying degrees of some shortcomings, after finishing the fabric has anti-UV performance, but may lead to finishing fabric strength, whiteness and breathability have a certain degree of decline, affecting the comfort of use. So in maintaining the original style of the fabric at the same time, improve the anti-UV performance of the fabric finishing agent research is still an important research direction of today's textile dyeing and finishing, and the development of nano-UV absorbers is one of the main research directions of anti-UV finishing of fabrics.