Regardless of the dyes used, all fabrics used can be subject to deterioration under
118 Understanding and improving the durability of textiles
direct daylight or reflected UV light (Fianu et al., 2005). Indeed, photo- degradation is observed in almost all polymer materials upon prolonged exposure to sunlight, thereby restricting their application for outdoor use; however, this degradation can be minimized by using UV stabilizers (Hustvedt and Crews, 2005). The UV resistance of specific textile materials is influenced by the type of dye or pigment, the absorptive groups presented in the dyestuff, colour depth after dyeing, and the uniformity and additives present in the finishes (Saravanan, 2007).
The lightfastness of a dyed fibre usually increases with the dye concentration, mainly due to the increase in the average size of the sub-microscopic particles that the dye forms in the fibre (Cristea and Vilarem, 2006). Mordant-dyed fabrics show greater fastness properties than those with no mordanting.
The ability of dyed polymers to withstand prolonged exposure to sunlight without the dyestuff fading or undergoing physical deterioration is largely deter- mined by the photochemical characteristics of the dye itself (Oda, 2005). The light-induced fading of dyes in the presence of air usually caused by oxidation, and such photo-oxidation reactions may involve oxygen free radicals, singlet oxygen or superoxide ion. These autooxidation reactions are generally considered to occur on exposure to UV radiation and are retarded by the addition of UV absorbers or antioxidants (Oda, 2001). Shamey and Sinha (2003) described several methods available for the photostabilization of different polymers. These stabilisers can be classified into three major groups according to their mechanism of polymer photo- stabilization: ultraviolet absorbers, antioxidants and excited state quenchers (Koussoulou, 1999).
6.6.1 UV absorbers
The amount of UV radiation absorbed by a polymer upon natural weathering can be reduced substantially by using additives that compete with the photosensitive chromophores of the polymer substrate for the absorption of the incident photons.
UV absorbers are often used to conserve the material properties of polymers or textiles (Tsatsaroni and Eleftheriadis, 2004), for example, to improve the light fastness of textile coloration, particularly for products that demand very high levels of lightfastness, such as polyester or nylon (Lee et al., 2001). The protection can be realized either by the embedding of UV absorbers into the polymers, or by coating the polymers with UV absorbing materials.
UV absorbers are organic or inorganic colourless compounds with very strong absorption in the UV range of 290–400 nm which, when incorporated into textile fibres, convert electronic excitation energy into thermal energy (Holme, 2003).
Effective UV absorbers provide long-term stabilization against UV radiation without being themselves destroyed by exposure (Koussoulou, 1999), and they dissipate the absorbed energy to avoid degradation or loss in colour (Gantz and Sumner, 1957). Several studies have investigated the usefulness of UV absorbers
Effects of light exposure on textile durability 119 in reducing colour alteration when used in two different ways: the direct applica- tion of UV absorbers to fibres, and the use of UV filtering materials over the light sources (Woeppel and Crews, 1990).
A variety of organic UV absorbers are available, with some proving more suitable for synthetic fibres and others more suitable for use with natural fibres. For synthetic fibres, phenyl salicylates, benzophenones, benzotriazoles, cyanoacrylates and phenyltriazine are used, while for natural fibres the most common absorbers are benzotriazole derivatives and oxalic acid dianilide derivatives, with sulpho- nated benzotriazole or sulphonated benzophenone often used for wool and silk products in particular (Mallik and Arora, 2003; Holme, 2003). Substituted benzophenones are the most effective compounds that have been found to date (Cristea and Vilarem, 2006), with products such as benzotriazole, hydro benzophenone and phenyltriazine primarily used for coatings and padding proc- esses in order to achieve broad protection against UV rays (Rupp et al., 2001;
Schmidt-Przewozna and Kowalinski, 2008).
Inorganic UV absorbers are preferable to organic UV absorbers as they are non- toxic and chemically stable when exposed to both high temperatures and UV.
Inorganic UV absorbers are usually certain semiconductor oxides such TiO2, ZnO, SiO2 and Al2O3 (Christensen et al., 1999; Lipp-Symonowicz et al., 2006; Joshi and Reddy, 2005).
An increase in performance can be achieved by combining the two types of UV absorbers (Mahltig et al., 2005a; Erdem et al., 2010): frequently used organic UV absorbers absorb only UV light of specific wavelengths due to the typical absorp- tion bands of molecular systems. Coatings containing only organic absorbers do not provide complete protection against UV light; currently inorganic UV absorb- ers such as TiO2 and ZnO are increasingly being used. These compounds can be applied as pigments in a binder or can be deposited as a pure oxide layer from the liquid or gaseous phase.
The main limitations of UV absorbers are that they cannot be applied in a single bath along with other finishing agents. From a practical point of view, therefore, UV absorbers containing reactive groups are particularly interesting (Czajkowski et al., 2006).
6.6.2 Antioxidants
Antioxidants are of special interest because they are probably the most effective light stabilizers (Crews and Clark, 1990). Antioxidants are organic compounds that are added to oxidizable organic materials to retard autooxidation and, in general, to prolong the useful life of the substrates. Relatively few chemical classes are effective as antioxidants. Those in common use today are hindered phenols, secondary aromatic amines, certain sulphide esters, trivalent phosphorous com- pounds, hindered amines, metal dithiocarbamates and metal dithiophosphates (Cristea and Vilarem, 2006).
120 Understanding and improving the durability of textiles
Antioxidants act in three different ways, depending on their composition (Koussoulou, 1999):
(i) They compete with the polymer in reacting with peroxide radicals, and in doing so prevent the photooxidation of the polymer caused by the reaction with peroxide radicals.
(ii) They trap alkyl and peroxide radicals without leaving the polymer to go through the propagation step of the photodegradation.
(iii) They prevent the fragmentation of hydroperoxide groups in the photoexcited polymer during photodegradation reactions by decomposing the peroxides in a different way.
Antioxidants are usually added to textile products as polymer additives rather than as topical finishes. When applied in combination with UV absorbers or other antioxidants, they can reduce fading and strength losses far more effectively than either product applied alone (Saravanan, 2007).
6.6.3 Excited state quenchers
The final category of light stabilizers, the excited state quenchers, act on the photoexcited molecules of the polymers themselves. The excited chromophores responsible for photooxidation can transfer their energy to an adequate accepter or quencher, before chemical bonds are broken and the radical initiated reaction proceeds. If the excitation energy of the irradiated polymer molecules can be transferred to the additive before any photochemical reactions occur, the photostabilization of the polymer will be successful. The purpose of an excited state quencher is to receive the excitation energy of the polymer molecules and dissipate it harmlessly as heat. The commonly used light stabilizers of this kind are organic complexes of transition metals such as Ni, Fe and Cr. (Oda and Kitao, 2008; Moura et al., 1997)
6.6.4 Other stabilizers and new techniques of improving light stability
Derivatives of 2,2,6,6-tetramethylpiperidine are called hindered amine light stabi- lizers (HALs). HALs protect polymers chemically rather than physically, and their effectiveness is dependent on optimum dispersal in the binding agents. HALs are extremely efficient at preventing the light-induced degradation of most polymers (Moura et al., 1997; Gijsman et al., 1993).
The UV protection properties of a textile may be increased by the introduction of appropriate residues to the molecule of a reactive dye. The coloration of cellulose fabrics with the use of specially tailored dyes could eliminate the necessity of using additional UV absorber auxiliaries, and could therefore decrease the number of chemicals used in the dyeing process (Czajkowski and Paluszkiewicz, 2008).
Effects of light exposure on textile durability 121 Mondal and Hu (2007) have presented a novel approach to the development of excellent protection from UV radiation in cotton fabrics by means of water vapour permeable coatings containing multiwall carbon nanotubes, which are stable and strongly UV absorbing.
Nanotechnology can also be used to improve light stability. Nanoparticles are commonly applied to textiles by coating, which can be carried out by spraying, transfer printing, washing, rinsing and padding, often with no impact on the texture or comfort of the fabric.
Additional improvements can be achieved by co-embedding dyes and organic UV absorbers into the same nanosol coating (Díaz-Flores et al., 2000; Mahltig et al., 2004). Nanosols containing dyes or pigments can be used to prepare coloured textile coatings (Mahltig et al., 2005b), while optimized UV protective coatings with the full absorption of virtually all UV light can be realized by the sol–gel technique, by embedding both inorganic and organic UV absorbers into one nanosol coating (Mahltig et al., 2005a). This embedding of UV absorbers into sol–
gel coatings can improve the fastness properties of dyes used in textiles. The fastness properties of dyed textiles can be improved by sol–gel coatings, or else an uncoloured sol–gel treated fabric can be subsequently dyed to achieve improved fastness properties (Mahltig and Textor, 2006; Trepte and Bửttcher, 2000). The main point of interest for practical applications is which method has most success, the embedding of UV absorbers into a sol–gel coating, the addition of a sol–gel coating to a previously dyed fabric, or a sol–gel pretreatment (Mahltig et al., 2004;
Díaz-Flores et al., 2000).