Recent Advances in the Ultraviolet Protection Finishing of Textiles Najsodobnejši izsledki na področju plemenitenja tekstilij za zaščito pred ultravijoličnim sevanjem

This paper reviews the recent research in the fi eld of application of ultraviolet (UV) protection fi nishes in textile functionalisation. The aim and the UV protecting mechanisms of UV protection fi nishes on textile substrates are described. The standard methods for determination of UV protective properties are presented. Contemporary research directions in the application of environmentally friendly organic UV absorbers and inorganic UV blockers are highlighted and discussed. The bioactive substances as novel organic UV absorbers are exposed. Novel multifunctional properties of textiles including the inorganic UV blockers are described. The ecological issues of the use of UV protection fi nishes are presented.


Introduction
Ultraviolet (UV) protection fi nishes, which are sometimes referred to as UV shielding agents, represent one of the most important groups of chemical fi nishing agents applied to textile materials, with the goal of protecting people and textile materials from the harmful eff ects of UV radiation. Th e energy of UV radiation, which is signifi cantly higher than that of visible light, has the potential to initiate diff erent chemical reactions that may be hazardous to human health and can deteriorate textile fi bres. Although moderate sun exposure has benefi cial health eff ects, overexposure to UV radiation may result in serious harmful health eff ects since both UVA (320-400 nm) and UVB (280-320 nm) rays induce diff erent cellular responses that manifest as pigmentation, sunburn, skin ageing, skin cancer and DNA damage [1][2][3]. Th e disadvantages of longterm exposure of textiles to UV weathering are associated with the cleavage of diff erent chemical bonds by the absorbed UV radiation, which leads to Zala Mavrić, Brigita Tomšič, Barbara Simončič University of Ljubljana, Faculty of Natural Sciences and Engineering, Aškerčeva 12, Ljubljana 1000, Slovenia photochemical degradation of textile fi bres, colour fading, increased crystallinity and other chemical and physical changes [4]. Th erefore, UV protection fi nishes are widely used in the production of functional textiles for sportswear, high-altitude clothing, covering materials, wearable sensors and other technical textiles with high added value. UV protection fi nishes can be divided into two main groups based on their chemical structure: organic and inorganic agents. Th e main task of UV protection fi nishes is to absorb harmful UV radiation and convert this electron excitation energy into harmless heat energy. Th is mechanism of UV protection applies to organic agents, which are able to absorb and quickly transform UV energy into kinetic and heat energy without photodegradation [4,5]. Th ese agents are usually called UV absorbers. Since they are widely used for the prevention of photo-oxidation and the photoaging process in textile fi bres, these agents are occasionally referred to as photostabilising additives [6]. Novel inorganic UV protection fi nishes named UV blockers are available on the market, and they mostly include metal oxides (MOs) in the form of nanoparticles (NPs); however, the UV blocking mechanism is not fully clarifi ed and still under debate [7]. Despite the general assumption that the excellent UV blocking performance of MOs NPs is related to the absorption of UV radiation via the semiconductive properties of these NPs, MOs NPs also refract and/or scatter most UV rays via their high refractive index [8,9]. Moreover, the protective mechanism of inorganic UV blockers is directly dependent of the chemical structure of the particles as well as their size, shape, crystallinity degree and crystal form [10]. However, because UV blockers are main-ly used to protect the skin from UV radiation, their main task when included in clothing is to prevent direct and diff use UV transmittance of UV rays via the textile material ( Figure 1). In addition to UV protective fi nishes applied for sun protective clothing, the UV protective properties of textiles can be signifi cantly improved by designing special textile structures with the appropriate fi bre type, yarn structure, fabric construction with low porosity and high cover factor and by using dyes that absorb radiation in near the UV region [11]. Th e protective properties of clothing can be quantifi ed using in vivo or in vitro methods. In the case of in vivo methods, the sun protection factor (SPF) is determined as the ratio between the minimum erythema dose (MED) on protected skin and unprotected skin [12]: .
In in vitro method, which is usually used for determination of the eff ectiveness of sun protective clothing, the ultraviolet protection factor (UPF) is a measure of the textile protection performance for both UVA and UVB radiation. UPF is calculated from the ratio of the UV radiation transmitted through air and the UV radiation transmitted through the fabric according to the European standard EN 13758 Textiles -Solar UV protective properties [12] as follows: where E λ is the relative erythemal spectral eff ectiveness, S λ is the solar spectral irradiance, T λ is  Th is paper reviews the most contemporary research directions in the application of organic UV absorbers and inorganic UV blockers as well as the ecological aspect of the use of UV protection fi nishes ( Figure 2).

Organic UV absorbers
Organic UV absorbers are uncoloured organic aromatic molecules with conjugated double bonds that absorb UV energy with wavelengths of 290-360 nm, which causes the molecules to reach an excited state; UV energy is then transformed to vibration energy in the UV absorber molecule and heat energy is released to the surrounding environment when the molecules return to the ground state [4,5,11]. According to this protection mechanism, UV absorbers protect textile fi bres from chain fi ssion and cross-linking reactions caused by the photooxidation of polymers, which reduces the weathering rate of textile fi bres. To retain a long-lasting activity, UV absorbers should be stable on exposure to UV radiation [6, 13], and they should return to their original form aft er the reaction. If the molecules of UV absorbers are permanently transformed into their non-absorbing isomers, their UV absorbing properties are destroyed.   [5] the structure of reactive dyes, chlorotriazine, vinylsulfone and N-dihydroxy ethylene reactive groups have been introduced to the UV absorber molecules to provide their high fi xation effi ciency [14,[17][18][19]]. Among reactive UV absorbers, condensation products of aminophenylbenzotriazoles and cyanuric chloride, derivatives of symmetrical triazine 4-(4,6dichloro-1,3,5-triazin-2-yloxy)-2-hydroxyphenyl) (phenyl) methanone and N-dihydroxy ethylene cyanoguanidine were thoroughly investigated ( Figure 4) [14,[17][18][19]. Th ese compounds can be covalently bonded to the hydroxyl groups of cellulose fi bres, which signifi cantly increases their durability. Th e growing use of synthetic organic UV absorbers in recent decades have caused environmental concerns since diff erent toxic degradation products of UV absorbers can bioaccumulate. Th erefore, in recent years, considerable eff ort has been invested in the investigation of UV-absorbing bioactive substances and their introduction into new environmentally friendly production processes of textiles with UV protection properties. Namely, bioactive substances represent biodegradable and never-ending resources of natural fi nishes. Extracts of various plants were studied in this research as UV-absorbing bioactive substances. Th ese include fl avonoids, such as fl avone, fl avonol, luteolin and baicalin, mycrosphorine-like amino acids, tannin, lutein and aloin ( Figure 5) [20-27].
Most plant extracts also exhibit antimicrobial, anticancer and antioxidant properties antiviral activity, which enable them to be used in the development of medical, health and hygiene-related textile materials.
Aloe vera active substances including fl avonoids, aloins, tannins, terpenoids, saponins, anthraquinone derivatives, aloe-emodin-9-anthrone and anthrone-C-glycosidesin have been the subject of many research over the last few years. Aloa vera has several positive characteristics, such as non-cytotoxicity,  . Aft erwards, a homogeneous solution was coated on cotton fabric using the pad-dry-cure method. Th e treated cotton fabric showed good UV protection properties with the UPF value higher than 50 which was attributed to the presence of Aloe vera particles with the absorption maxima in UVB region. A significant amount of Aloe vera/chitosan nanocomposite was found to remain even aft er ten washings which proved the washing durability of the coating. In addition to UV protection, the coated fabric showed excellent water repellent properties as well as antimicrobial activity. Th is highlighted the potential application of Aloe vera/chitosan nanocomposite for multifunctional protective clothing in defence and biomedical fi elds. Carotenoid compounds lutein and lutein esters which represent the main chemical components of marigold (Tagetes erecta) fl owers extract have already been established textile dyeing and protective fi nishing [23,24]. To create UV protection and antioxidant properties with simultaneous coloration on a wool fabric, the extracted solution was applied to un-mordanted and mordanted wool samples by exhaustion procedure [23]. Th e results showed that the marigold extract did not only dye the wool fibres in yellow colour but also appreciably improved the UV absorption properties. Th e presence of iron mordant increased the UV protection due to its UV screening abilities. Th e abstraction of peroxy free radicals by lutein resulted in excellent antioxidant activity that was also enhanced in the presence of mordants.
Baicalin which is a plant extract from Scutellaria baicalensis and belongs to the family of fl avonoids was applied to silk fabric by exhaustion procedure [25]. It was found that the presence of baicalin provided very good UV protection ability for silk fabric due to its strong absorption capability in the UVB region. Unexpectedly, the UV protection eff ect of baicalin was even better than that of the benzotriazole-based UV absorber which was used for comparison due to baicalin is also a good free radical scavenger, the treated silk fabric exhibited a very good antioxidant activity. Another advantage of baicalin is its very pale colour, which has a small impact on the colour of textile material. Due to their various functionalities, such as antioxidant, antimicrobial, UV protective, anti-infl ammatory, anticoagulant, anti-tumour and biodegradable properties, bioactive substances extracted from marine macroalgae (seaweeds), which are highly promising plant-like organisms of brown, red and green colours, already have a widespread use in the production of medical, healthcare, hygiene and protective textiles [27]. As a source of natural UV protection fi nishes with the antioxidant property, fl avonoids, tannins and mycrosphorine-like amino acids are of great importance. Th ey were applied to the cellulose fi bres using the exhaustion or pad-dry procedures.
Since seaweed represents a natural-based cellulosic fi bre, brown algae were dissolved in a solvent containing water and the fi lament yarns were manufactured through a solvent-spinning process. Th e woven and knitted fabrics produced with seaweed fi bres exhibited unique durable multifunctional protective properties of macroalgae.

Inorganic UV blockers
Various semiconducting materials exhibit UV blocking properties, including TiO 2 , ZnO, cerium dioxide (CeO 2 ), zirconium dioxide (ZrO 2 ), magnesium oxide (MgO), aluminium trioxide (Al 2 O 3 ), silicon dioxide (SiO 2 ), copper and copper sulphide (CuS), silver (Ag) as well as graphene-based materials [8, 28-38]. Among these inorganic materials, TiO 2 and ZnO alone as well as in combinations are the most widely investigated as UV blockers [39,40]. Compared with organic UV absorbers, inorganic UV blockers are more biocompatible and much cheaper and have higher thermal and chemical stability, better durability and excellent UV blocking properties over a broad range of UV wavelengths. MOs and CuS are representatives of bang-gap semiconductors and present unique UV protection properties and photocatalytic activity [41,42]. Th ese properties are directly infl uenced by their physicochemical characteristics. In general, NPs are much more eff ective as UV blockers compared with bulk materials because of their large surface area to volume ratio. Furthermore, NPs with a crystal form are characterised by high refractive indices in the UV and visible wavelength ranges, which is why their UV blocking function is attributed to refraction and/or scattering of UV rays. As semiconductors, MOs and CuS are characterised by an electron band structure that includes bands with orbitals and gaps in the UV spectral region with no orbitals [7,43]. Upon the absorption of UV radiation with energy that matches or exceeds the band gap energy, the electrons from the valence band are excited to the conduction band, thus leaving a positively charged hole in the valence band. Th is phenomenon of semiconductors confi rms that the absorption of UV radiation has a signifi cant role in their UV protection mechanism.
Because the photogenerated free electrons and positive holes generate the formation of reactive oxygen species (ROS), such as • OH, • OOH and O 2 •¯, free radicals which initiate different photocatalytic reactions, which enables semiconductors to be used as photocatalytic self-cleaning and antimicrobial agents as well as photocatalysts for degradation of textile dyes in waste water. However, when using semiconductors as UV protection fi nishes, their inherent photocatalytic properties can represent an important disadvantage, since photodegradation of dyes and textile fi bres leads to colour fading and deterioration of the mechanical properties of textile substrates [8]. Compared with MOs and CuS, graphene is classifi ed as a zero band-gap semiconductor due to its special electronic structure. Because of its high absorption capability in the UV region, graphene represents one of the novel materials for the UV protection of textiles that does not lead to harmful photocatalytic degradation [35]. Inorganic UV blockers can be applied in diff erent textile production stages, among which the electrospinning of nanofi bres and fi nishing of fabrics by the sol-gel technique are of great technological importance [44,45]. When applying inorganic UV blockers to textile substrates, researchers have faced several problems associated with the agglomeration of NPs, low adsorption ability of textile fi bres for NPs and low durability of the coating. To resolve these problems, new routes of synthesis of NPs have been introduced to improve the dispersity of NPs, such as the in situ growth of NPs on textile fi bres in the presence of stabilising agents [31, [46][47][48]; and the use of hydrothermal, solvothermal, microwave and sputtering deposition methods [32,45,[49][50][51][52][53][54]. To enhance the adsorption ability of textile fi bres and consequently the washing durability of the coatings, plasma treatments have been used to increase the textile wettability and the surface roughness [54][55][56], polyester fi -bres have been pre-treated with sodium hydroxide to improve their wettability [57,58] and cellulose fi bres have been pre-modifi ed with chloroacetic acid to create new active carboxylic groups [31]. For the production of high added value textile products, the creation of multifunctional properties is of great practical importance. Accordingly, the most contemporary research in the fi eld of UV protection of textile with the use of inorganic UV blockers includes the combination of UV protective properties with antimicrobial, water and oil repellent and easycare properties; water/oil separation capabilities; electrical conductivity; UV aging resistance; thermal stability; and fl ame retardancy. Simultaneous UV blocking properties and antimicrobial and self-cleaning activities were generated for protective clothing, sportswear and medical textiles. To this end, Ag was used in combination with TiO 2 and applied to cellulose fibres by diff erent application processes [50,52,[59][60][61]. Commercial Ag/TiO 2 nanocomposites were applied to cotton fabric during the exhaustion dyeing process to simultaneously load Ag/ TiO 2 and reactive dye on cotton [59]. Th e presence of reactive dyes enhanced the adsorption of Ag/ TiO 2 nanocomposite, resulting in excellent antimicrobial and UV protective properties that were retained aft er multiple domestic washings. Furthermore, the simultaneous colouration and function fi nishing of cotton fabric were performed using Ag colloids of diff erent colours in combination with TiO 2 NPs in an ultrasonic bath [61]. Th e UV blocking activity, antimicrobial effi ciency and selfcleaning property of the cotton fabric treated with the Ag/TiO 2 composite were superior compared with the sample treated with TiO 2 NPs alone. Moreover, the photocatalytic activity of TiO 2 had no negative eff ect on the fabric colour. Durable antimicrobial and UV protective cotton fabric was also created by the deposition of fl owerlike hierarchical TiO 2 micro-NPs via hydrothermal deposition followed by the in situ growth of Ag NPs on the surface of TiO 2 -cotton fabric [50]. Th is new synthesis route of Ag/TiO 2 composite is appropriate for sustainable biomedical applications. A prolonged hydrothermal treatment was also performed for the deposition of Ag doped TiO 2 NPs on cotton fabric [52]. In this novel application method, a small amount of AgNO 3 was added to water during the hydrothermal step, which caused the doping of TiO 2 , thereby imparting strong antimicrobial properties to the treated cotton fabric as well as drastically enhancing its UPF value. Another approach for the preparation of multifunctional fi nishing includes the sol-gel process, which includes the use of Al 2 O 3 /SiO 2 sol modifi ed with Ag/Cu and TiO 2 NPs applied to polyester/cotton fabric by pad-dry-cure procedure [62]. A synergistic eff ect in the interactions between both types of NPs in the coating showed better photocatalytic self-cleaning and protective capabilities against UV radiation as well as bacteria and fungi. Despite the increased roughness of the coating due to the presence of NPs, the coating's resistance to abrasion was not hindered. ZnO/Ag nanocomposites were also used to generate simultaneous UV blocking, self-cleaning and antimicrobial properties of textiles [57,63,64]. Similar to the case of the Ag/TiO 2 nanocomposite [61], simultaneous colouration and functional fi nishing of the cotton fabric was carried out with the use of Ag/ZnO nanocomposite colloids under ultrasound irradiation, and vivid coloured multifunctional cotton fabric samples were prepared [63]. Th e photocatalytic activity of ZnO in the presence of Ag did not cause colour fading of the treated samples irrespective of the colour of the Ag NPs, which was very similar to the results obtained for the Ag/TiO 2 nanocomposite. Th e results also revealed that the presence of ZnO increased the UPF value of the treated cotton sample, which was lower than that provided by TiO 2 at the same concentration. Furthermore, ZnO exhibited signifi cantly higher antibacterial activity compared with TiO 2 at the same concentration. Moreover, the presence of Ag in the nanocomposite enhanced the UPF value and provided the biocidal activity of the fi nish similar to the Ag/TiO 2 nanocomposite. Outstanding durable multifunctional properties of cotton/wool and viscose/wool blended fabrics were obtained by the application of ZnO and Ag NPs in the presence of citric acid (CA), which was used as a crosslinking agent, and succinic acid (SC), which was used as an esterifying agent, and a catalyst via the pad-dry-microwave fi xation procedure [57]. Th e presence of CA and SC increased the content of Ag and ZnO NPs on the fabric samples due to electrostatic attraction, which consequently improved the antibacterial activity and UV blocking properties of the coating in comparison to the fabric treated without the presence of carboxylic acids. A synergistic eff ect of applying Ag/ZnO NPs was observed. Th e multifunctionality of the fabric was also increased by the presence of CA, which represents a non-formaldehyde-containing product for durable press fi nishing because it can signifi cantly increase the wrinkle recover ability and improved the easycare properties of fabrics. Another multifunctionality of high added value textiles is their simultaneous superhydophobicity and UV blocking properties, which are important for sportswear, high-altitude and military clothing, covering materials and water-oil fi lters. Although superhydrophobic UV protected textiles have already been created by the application of a TiO 2 NP dispersion prepared with the use of ultrasonifi cation [49,65], the preparation of coatings with a combination of fl uorinated acrylic copolymers, polyvinyl-, alkyland perfl uoroalkyl-functionalised silsesquioxanes as well as polydimethylsiloxane (PDMS) with diff erent MO NPs still remains the main route for controlling the chemical structure and roughness of the fi bre surface [9, 34, 66-69, 71]. By using a combination of electrospinning and a two-step doctor-blading coating, polyacrylonitrile/ polyurethane nanofi bres with the incorporated TiO 2 NPs were produced and aft erwards coated with 2-hydroxy-4-n-octoxybenzophenone and fl uorinated acrylic copolymer [9]. Th e membrane prepared from the treated nanofi bres presented double UV resistance, water and oil repellency and waterproofbreathable functionality. TiO 2 NPs were also applied to cotton fabric in combination with polyvinylsilsesquioxane, a crosslinking agent with a hydrophobic character, by the twotimes pad-dry-cure procedure [66]. Th e composite coating consisted of large-sized nano-TiO 2 aggregates embedded in the polyvinylsilsesquioxane fi lms by Ti-O-Si covalent bonds, which increased the surface roughness of the hydrophobic cotton surface exhibiting excellent UV blocking property and superhydrophobicity without damaging the mechanical properties. Namely, three-dimensional chemical cross-links among the coating and cellulose significantly increased the fabric tensile strength.
To mimic the superhydrophobic lotus leaf in nature, a ZnO nanorod array with an SiO 2 shell was adhered to cotton fabric and then modifi ed by an alkylsilsesquioxane monolayer [70]. Although the insulating SiO 2 shell eff ectively suppressed the photoactivity of the ZnO nanorods, the alkylsilsesquioxane-modifi ed ZnO/SiO 2 nanorod array provided the cotton fabric with an ultrahigh UPF value and durable superhydrophobic properties, even under prolonged UV radiation. Perfl uoroalkylsilsesquioxane was used for subsequent modifi cation of cotton fabric previously coated by a dense fi lm of CeO 2 [67]. Th e incorporation of CeO 2 particles can cause surface roughness because of the enhanced hydrophobic properties of perfl uoroalkylsilsesquioxane, although it also resulted in a good UV protection property. Two diff erent cost-eff ective and scalable strategies were used to create UV blocking and superhydrophobic coatings using a combination of ZnO and PDMS. Th e fi rst one included the construction of multi-layered PDMS-ZnO-PDMS composite coating on cotton fabric via deposition of adhesive layer of PDMS, which was followed by the subsequent deposition of ZnO NPs to enhance the surface roughness and a second PDMS layer to provide for water repellency [68]. In addition to the multifunctional properties, the treated fabric exhibited superior anti-abrasion and excellent laundering durability and was highly effi cient for repetitive and versatile oil-water separation. Th e second approach included simple dip-coating, dropping, spinning or spraying application procedures of a prepared solution containing PDMS and ZnO NPs to polyester fabric [69]. Th e developed ultra-robust superhydrophobic fabric showed high repellency against water, strong acids, strong alkali and saturated salt solutions. Th e UV resistance of PDMS preserved the super hydrophobicity even aft er a long period of UV illumination. Th e uniform distribution of ZnO in the coating provided a high UV blocking eff ect for the fabric. PDMS was also used in combination with CuS on cotton fabric [34]. In this research, the fl ower-like CuS was fi rst synthesised via the solvothermal method, then dispersed in sol in combination with PDMS and applied to cotton fabric by the dip-cure method. In addition to its excellent UV blocking properties, the fl ower-like CuS, which consisted of self-assembled nanosheets with abundant mesopores between the nanosheets, provided a hierarchical rough surface topography that enhanced the water repellent properties of PDMS. Th e coating exhibited high washing durability and caused a negligible reduction in the mechanical properties of the cotton fabric.
To impart multifunctional UV blocking, selfcleaning, antimicrobial and electrical conductivity properties, graphene, graphene oxide (GO), Ag and antimony (Sb) were used in combination with MOs NPs. Th e GO/TiO 2 nanocomposites were prepared by mixing and sonifi cation of both components in a sol, which was subsequently applied to cotton fabric by the dip-drying method [71]. Th e synergistic UV absorption of TiO 2 and GO provided a good UV blocking property to the fabric. Th e presence of GO in the GO/TiO 2 nanocomposite enhanced the photocatalytic effi ciency of the coating, resulting in the increased photodegradation of the dye stain as well as the antimicrobial activity of the treated fabric. Furthermore, the photoinduced electrons in TiO 2 under UV radiation enhanced the reduction of oxygen functional groups from the GO, thus leading to improved electrical conductivity. To develop UV blocking, electrical conductivity and high thermopower properties on cotton fabric, a ZnO and Sb/Ag/ZnO composite coating was created by the in situ solvothermal growth technique [64]. Th e Ag/ZnO composite imparted an excellent UPF value to the coated fabric, the Sb/Ag/ZnO composite increased the electrical conductivity, and the intergranular crystal structure of Ag/ZnO composite converted the cotton fabric from an insulator into a relatively high thermopower conductor, which is benefi cial for the manufacturing of wearable electronic device material. Graphene/Ag nanocomposites have also been recognised as an eff ective conductive and UV blocking material for polyester surface modifi cation [36]. Th e coating was prepared in two stages that included the application of GO via an immersion-drying procedure and its chemical reduction into graphene nanosheets, which was followed by the in situ synthesis of Ag NPs to the graphene coated polyester fabric. Th e UV protection of the coating was provided by the UV absorption ability of the graphene nanosheets, and it was signifi cantly improved in the presence of small amount of Ag NPs. Both the graphene nanosheets and Ag NPs also formed a conductive network, resulting in excellent electrical conductivity. Th e coated polyester showed excellent mechanical properties, including fl exibility and stretchability, which provided the opportunity to use this material as electrodes in supercapacitors, solar cells and sensors.

Tekstilec, 2018, 61(3), 201-220 Recent Advances in the Ultraviolet Protection Finishing of Textiles
To provide UV resistance and consequently improve the tensile strength of fabric under UV ageing, γ-methacryloxypropyl trimethoxysilane and illite, a natural clay with superior fi lm-forming activity, were used to prevent photocatalytic degradation of textile fi bres due to the inherent photoactivity of TiO 2 [72,73]. A composite polyethylene fabric was prepared by radiation-induced graft polymerisation of γ-methacryloxypropyl trimethoxysilane followed by the subsequent co-hydrolysis of the graft chains with tetrabutyl titanate to create TiO 2 NPs. [72]. In the composed two-layered coating, the inner part consisted of an organically modifi ed solsesquioxane layer to protect the textile fi bres against photodegradation and the outer part consists of TiO 2 NPs to absorb UV radiation. The treated fabric exhibited much higher thermal resistance and signifi cantly better mechanical properties after UV radiation than the untreated fabric. To suppress TiO 2 photocatalytic activity, TiO 2 NPs were loaded onto the surface of illite microlayers in a hydrothermal reaction, and the as-prepared solution was then applied to cotton fabric via ultrasonifi cation and drying [73]. Th e results showed that illite itself refl ected part of UV light and therefore enhanced the UV blocking action of the TiO 2 NPs in the coating. Furthermore, the illite ensured that TiO 2 existed in NP form and increased the refl ection and scattering of UV light. As a result, only small amount of UV light reached the fi bres, thereby maintaining the mechanical properties of the fabric, even aft er long-term UV radiation. New hybrid NPs consisting of nano turbostatic boron nitride and polydopamine shells on a CeO 2 core were synthesized and chemically graft ed to aramid fi bres to improve their UV resistance [74]. Th e double core-shell structure signifi cantly reduced the photocatalytic activity and obviously improved the UV blocking eff ect of CeO 2 . Th e modifi ed fi bres had excellent thermal stability and mechanical properties. Furthermore, the unique properties of graphenebased materials, i.e. UV absorption capabilities without photocatalytic activity and excellent mechanical and chemical properties, enabled their use as a weathering stabiliser of polymeric materials [35]. Namely, the incorporation of GO into polyurethane coating signifi cantly improved the weathering resistance of the coating, and the stabilising performance was much more eff ective compared with that of organic UV absorbers.
To improve and/or impart functional properties to cotton and cotton/polyester fabrics, TiO 2 , ZnO and ZrO 2 NPs were combined with conventional organic fi nishing agents, i.e., modifi ed N-dimethyloldihydroxyethylene urea as an easy-care agent, aminomodifi ed PDMS as a soft ener, oxanilide-based compounds as a reactive UV absorber, nitrogen phosphate as a fl ame retardant agent, fl uorinated acrylic copolymer as a water, oil and stain repellent agent, as well as Ag as an antimicrobial agent, and the textile substrates were applied using the pad-dry-cure procedure [31]. Th e results demonstrated that a synergistic eff ect occurred between the MO NPs and conventional organic agents in the coatings, thus suggesting that these fi nishing formulations can be used for the production of multifunctional textiles with outstanding performance and protective properties.

Ecological concerns associated
with UV protection fi nishes and potential human health eff ects

Organic aromatic compounds
Over the past decade, the application of organic UV absorbers has steadily increased. Th ese products are used in various cosmetic products to provide protection to skin and hair against harmful UV radiation or merely to expand the shelf life of beauty products. An increasing number of materials contain organic UV absorbers, including textiles and plastics [75]. Th erefore, during the life cycle of organic UV absorbers, their entry into the environment is inevitable. Th ese substances have been found in surface waters, wastewater, drinking water, soil, sludge, fi sh and the human body [75]. Most organic UV fi lters based on p-aminobenzoic acid, benzophenone, cinnamate and salicylate have been assessed for multiple hormonal activities as well as for their eff ects on reproduction and fertility [75]. However, in the aquatic environment, predicted no-eff ect concentrations of benzophenone-3 and benzophenone-4 were found to be lower than those previously detected [76], indicating that the ecological risk of these substances remains low. Nevertheless, in ecological risk assessments, the potential biomagnifi cation of organic UV absorbers should not be ignored. Because of their high lipophilicity and biological persistence, these substances have been shown to accumulate in the food chain. Th erefore, in a recent study, the occurrence of eight organic UV fi lters was assessed in fi sh from four Iberian river basins [77], and the accumulation of benzophenone-3, ethylexyl methoxycinnamate and octocrylene was observed. When these chemicals accumulate, biomagnifi cation is of crucial importance since higher levels of observed organic UV absorbers were detected in predator species, which occupy a higher position in the trophic chain. Th e synergistic or antagonistic activities of multimixtures of UV absorbers also need to be considered when estimating their toxicity. Diaz-Cruz et al [75] performed in an in vivo study in which three-component mixtures of 3-benzylidene camphor, benzophenone-1 and benzophenone-2 at diff erent concentrations and relative proportions were assessed for oestrogenic activity, and they showed that generally low concentrations of UV fi lters in the environment may not produce important oestrogenic eff ects on their own but may lead to increased oestrogenic activity of other xenoestrogens in a synergistic manner. In addition to their benefi cial eff ects, UV absorbers infl uence the occurrence of dermatological problems and undesirable oestrogenic and antithyroid eff ects, which are ascribed to the parent compounds as well as their metabolites [78]. Accordingly, in vivo and in vitro studies showed that DNA damage induced by the presence of p-aminobenzoic acid was caused through the photosensitized formation of pyrimidine cyclobutane dimers and nondimer photoproducts as well as the photoaddition of p-aminobenzoic acid to thymine [79]. Importantly, each of these DNA injuries was correlated to a potential skin cancer incidence. However, as pointed out in review article by Godić [80], fear of the possible carcinogenic eff ects of organic UV absorbers on the skin is unjustifi ed since most of these products degrade in the upper layers of the epidermis. Rather, their absorption via food in the gastrointestinal tract and possible carcinogenic eff ects should be of far greater concern. However, despite the potential (eco)toxicity associated with the long-term exposure to organic UV absorbers, their advantages greatly outweigh their potential risks.

Metal and MOs NPs
Rapid development of nanotechnology resulted in a huge production of various metal and MOs NPs, particularly TiO 2 , followed by ZnO and CeO 2 as the second and third largest to the annual volume discharge, respectively [81]. Th ese NPs are daily used in industrial, agriculture, medical and consumer applications. Th erefore, during their life cycle there is a chance of exposure for workers, consumers and the environment. Potential toxicological risks of such exposure cannot be easily predicted or assessed, since NPs has both particulate and molecular identity, which induce diff erent biological or ecological eff ects. Moreover, as pointed out by Pietroiusti [82] there is an urgent need for development of international standard methods for hazard prediction of NPs. Th ree key elements in NPs toxicity screening strategy have been outlined [83]: physiochemical characterization, in vitro assays and in vivo studies. Among physiochemical properties particle size and agglomeration state are one of the critical parameters for toxicity. Namely, it is generally assumed that aggregated nanoparticles are less toxic or biologically active, compared when in their nano-form. Besides particle size distribution, their shape, crystal structure, chemical composition and surface properties are also key parameters, when discussing their potential toxicity [84]. Th e latter can be evaluated by in vitro tests, providing information about in vivo eff ects of NPs. Accordingly, non-cellular tests as well as cell-based systems are taken into consideration, giving information upon biopersistance, free radical generation or activation of humoral systems. Regarding the in vivo testing of NPs toxicity, two tiers of studies were outlined by Oberdöster and his co-workers [83], i.e. tier 1 studies that are relevant to the concerns upon human exposure and tier 2 studies, which provide information for a complete risk assessment of a certain type of NPs. However, as pointed out by Joo et al [81], to resolve these issues fundamental understanding of the transformation processes, fate, transport and assessment of transformed NPs under various environmental conditions is necessary, bearing in mind that environment is a heterogeneous system, whereas impact of co-existing contaminants must be also considered. Accordingly, mobility of NPs, their diffusion and potential toxicity can be strongly aff ected by the presence of natural organic matter (i.e. humic substances and fl uvic acids), whereby disagglomeration takes place [85]. Th e presence of diff erent surfactants which are extensively used for the production of NPs in the environment media is also worrying. Namely, diff erent functional groups present in the structure of surfactants may react with NPs and alter their physiochemical properties. Two mechanisms have been proposed to explain the toxicity of photocatalytically active TiO 2 and ZnO NPs, i.e., the generation of ROS and release of metal ions, which both lead to cellular infl ammation and an oxidative stress response [86]. Th e intensity of both processes depends on the physiochemical properties of the NPs, which is related to their increased specifi c surface area compared with that of micron-sized metal particles. Th erefore, due to ROS formation, the damaging eff ects of TiO 2 and ZnO NPs on diff erent aquatic species and bacteria have been reported [87][88][89][90]. Undoubtedly, the formation of toxic ROS is induced by UV light. However, Adams and his co-workers [91] reported the inhibitory eff ect of TiO 2 and ZnO NPs against gram-positive Bacillus subtilis and gram-negative Escherichia coli under dark conditions, suggesting that another mechanism contributes to the toxicity of these NPs, such as the solubility of oxide NPs and release of metal ions, which strongly infl uences their toxicity. In the case of ZnO NPs, the water chemistry has an important infl uence on the aggregation/dissolution process [92], and the solubility of ZnO NPs can more than double in sea water compared with that of bulk ZnO, thus lead to the increased release of Zn 2+ ions. Th is process is temperature dependent. As observed by Woong [93], the formation of Zn 2+ ions from ZnO NPs decreases as the water temperature increases. Importantly, lowering the water temperature signifi cantly enhances the uptake of natural organic matter and Zn 2+ (as a heavy metal surrogate) to the surface of ZnO NPs [92]. Th erefore, altered ZnO NPs are likely to release surface-adsorbed contaminants at elevated temperatures, thus implementing a "Trojan Horse eff ect" [94]. Accordingly, the direct uptake of TiO 2 and ZnO NPs in a concentration-dependent manner was observed for the model bacterium Salmonella typhimurium, and it demonstrated weak mutagenic and possible carcinogenic potential [84]. Once in the aquatic environment, CeO 2 NPs are highly agglomerative, which keeps the particles' surface area available for interactions with cells or organisms [95]. In this respect, the ecotoxicity of CeO 2 NPs against representatives of diff erent trophic levels, i.e., from primary producers to secondary consumers, was studied. Th e toxic eff ects against algae and amphibian larvae was proven [96; 95; 97], although CeO 2 was not toxic for planktonic crustaceans, midge fl y larvae and zebrafi sh embryos. Compared with TiO 2 and ZnO NPs, the toxicity of CeO 2 likely is not related to the generation of ROS or the direct eff ect of dissolved Ce or CeO 2 NP uptake. Rather, the harmful eff ect of CeO 2 NPs was mainly ascribed to the clustering of the particle aggregates around the algal cells as well as to the tendency of CeO 2 NPs to aggregate and sediment in the case of amphibian larvae [96,97]. Amphibian larvae are fi lter feeders, and their contamination can occur either by skin contact or water fi ltration. Since these sarava organisms are the base of the trophic chain, the entry of the CeO 2 NPs via the trophic route into secondary consumers is extremely alarming, suggesting possible drastic consequences in the case of biomagnifi cation [97]. Th e increased toxicity of CeO 2 NPs was ascribed to their high stability in aquatic environments as a result of possible elevated surface repulsive forces, which also infl uenced their decreased removal from water as well as decreased retention in porous media [98]. During the production and use of metal and metal oxide NPs, human exposure is inevitable. Th ere are various routes for entry of NPs, such as from the occupational environment to the worker's body or from diff erent products into a consumer's body. For functionalized textiles, the most frequent exposure route appears to be the inhalation of dust released from the textile during its use and/or by skin contact. TiO 2 and ZnO NPs are considered harmless when exposed to healthy skin [99,100]. However, mixed-phase TiO 2 (i.e., anatase and rutile) can cause oxidative injury to the skin under sunlight exposure, thus demonstrating more toxic eff ects to cells than pure phase NPs. Namely, induced by UV radiation, mixed-phased TiO 2 NPs caused hierarchical oxidative stress toxicological responses that resulted in human keratinocyte cell death and mouse skin damage. Importantly, without sunlight exposure, mixed-phased TiO 2 NPs did not induce notable toxicity [101]. Upon inhalation, TiO 2 , ZnO and CeO 2 NPs caused an infl ammatory eff ect in human bronchial epithelial cells via oxidative stress [83 102, 103]. In high doses, ZnO NPs also presented a certain level of cytotoxicity and led to reduced lung cell viability or even cell death. Kim and co-workers [104] performed in vitro assessments and compared the toxicity of TiO 2 , ZnO and CeO 2 NPs to human epithelial cells in terms of cell proliferation, cell viability, membrane integrity and oxidative stress. Among the tested NPs, ZnO exhibited the highest cytotoxicity, followed by TiO 2 and CeO 2 , which both showed little adverse eff ects on cell proliferation and cell viability, although in the case of TiO 2 NPs, oxidative stress in a concentration-and time-dependent manner was observed. Nevertheless, when these particles were applied to the fi bres, none of the functionalized textile samples were cytotoxic upon exposure to various human cells, thus demonstrating high biocompatibility [30, 46, 105].

Non-metals
As a representative of a novel class of carbon-based engineered nanomaterials, GO NP production has been constantly increasing and may overtake carbon nanotubes in applications [106]. Despite being a relatively novel material, evidence has demonstrated the cytotoxicity of GO NPs on various types of cells via a number of mechanisms, such as membrane damage, ROS generation, and expression level alterations of several key genes related to apoptosis [107]. Adverse eff ects against non-mammalian were also determined, and toxicity against Caenorhabditis elegans (C. elegans), a representative of roundworms, has primarily been studied [108][109][110][111][112]. Th ese studies showed impaired functions in the intestine, neurons and reproductive organs upon exposure to GO NPs, which were ascribed to a weakened epidermal barrier as a consequence of peroxidase formation in the epidermis. Accordingly, RNA damage was demonstrated, which promoted the susceptibility to GO toxicity and enhanced GO accumulation [106]. Th is fi nding is of particular concern as C. elegans represents an intermediate between unicellular eukaryotes and more complex organisms, such as vertebrates and higher plants, thus showing a possible biomagnifi cation tendency. Th e in vivo biodistribution of GO NPs was studied by Zhang et al [113], who found that aft er their intravenous injection in mice, GO NPs predominantly deposited in the lungs over long time period, and they induced pulmonary oedema and granuloma formation in the lung. However, no noticeable organ damage via infl ammation was observed for surface-modifi ed GO NPs [114], implying that introducing diff erent polymers and functional groups on the surface of graphene decreases its in vivo toxicity, thus improving graphene's biocompatibility.

Conclusion
In this review, the most important research directions in the fi eld of UV protection fi nishing of textiles were presented. In the case of organic UV absorbers, the UV-absorbing bioactive substances have increasingly established as eco-friendly neverending resources of natural fi nishes. Among them, extracts from various plants, such as Marine macroalgae (seaweeds), Marigold (Tagetes erecta), Aloe Vera and Scutellaria baicalensis Georgi have been applied to textile fi bres because of their great UV absorbing potential.
In the case of inorganic UV blockers, the most contemporary research includes the production of textiles with multifunctional UV protection, antimicrobial, water and oil repellent and easy-care properties, electrical conductivity, UV aging resistance, thermal stability and fl ame retardancy. Besides the benefi cial protective eff ect of UV protection fi nishing against harmful UV radiation, the growing use of organic and inorganic UV protection fi nishes over the last decades have caused environmental concerns since diff erent toxic degradation products of the UV absorbers as well as NPs have been found to be bioaccumulated. Th ese environmental issues dictate professional handling with UV protection fi nishes.