Degradation caused by sunlight results in a decrease in the physical properties of textiles, particularly the tensile and tear strength. The useful life of dyed textile products exposed to sunlight is often limited, due to rapid colour fading. Although this colour fading only affects the appearance of the textile, it is nonetheless extremely significant because it can lead to fabrics being discarded and replaced before their physical properties have undergone any significant deterioration (Sun et al., 2009). In any polymer, the corrosion process caused by oxygen in the atom state is accelerated by UV radiation, resulting in weight loss and deterioration (Huang, 2005).
106 Understanding and improving the durability of textiles
6.2.1 The effect of solar radiation on natural textiles
Wool fibres
A complex photochemical reaction occurs when wool is exposed to UV and longer wavelength radiation (380–475 nm), particularly in the presence of moisture. The result of such radiation is a loss of fibre mechanical strength and colour changes (Zhang et al., 2008).
In terms of mechanical strength, wool fibres remain stable over 12 hours of UV irradiation. After that, and up to approximately 48 hours of irradiation time, a linear relationship between the irradiation time and the decrease in fibre strength is observed (Zimmerman and Hocker, 1996). In loomstate fabrics, wool fibres undergo an 8% to 20% decrease in tensile strength after 40 to 120 hours of UV irradiation (Treigiene and Musnickas, 2003).
As a result of UV irradiation, wool first undergoes bleaching (photobleaching) and then yellows when exposed to a strong source (photoyellowing). There is evidence that UV absorbers can lessen this damage (Fan, 2005). Wool’s susceptibil- ity to photoyellowing is attributed to its inherent absorption in the UV-B (280–315 nm) and UV-A (315–400 nm) regions, mainly due to the presence of the aromatic amino acid residues tryptophan, tyrosine and phenylalanine, and natural yellow chromophores (Nicholls and Pailthorpe, 1976). Photoyellowing occurs ten times faster in humid conditions than in dry conditions (Timar-Balazsy and Eastop, 1998).
Because the yellow chromophores are present in greater numbers in the cuticle scales of wool fibres, photoyellowing occurs much more quickly in the cuticle layer than in the cortex layer, which contains cortical fibrils responsible for the mechanical strength of the fibre. One of the roles of the wool cuticle may therefore be to protect these cortical fibrils from free radical oxidation during exposure to the UV wavelengths present in sunlight.
Cotton fibres
Pure native cellulose absorbs UV radiation strongly between 200 and 300 nm, but only very weakly up to 400 nm. The fact that cellulose absorbs mainly in the far ultraviolet region can be explained by the strong chemical bonds throughout its polymer. Once electromagnetic radiation has been absorbed, a free radical photo- chemical reaction starts in the cellulose. The most common photochemical damage to cellulose occurs through photo-oxidation. This process occurs simultaneously by two main routes: (i) oxidation of the hydroxyl side groups, which results in changes in the colour, polarity, solubility and water absorption–desorption prop- erties of the cellulose; and (ii) rupture of the glycosidic ether bonds between cellulose units, which causes a decrease in the degree of polymerization, thereby changing the solubility, mechanical and other properties of the cellulose. Photo- oxidized cellulose is usually rigid and brittle, and its resistance to mechanical treatments is limited (Timar-Balazsy and Eastop, 1998).
Effects of light exposure on textile durability 107 El-Taieb and Shakour (2003) investigated the effect of solar irradiation on 100% cotton fabric and mixed synthetic fabric (65% polyester and 35% cotton) in a variety of urban and industrial districts in Cairo over a period of 15 months. The tensile strength of the cotton fabric and the mixed synthetic fabric decreased by different amounts as a function of exposure time in both warp and weft directions.
The highest deterioration in tensile strength, 86.2%, was observed for the cotton fabric that had been exposed in the industrial district. The lowest loss of tensile strength of cotton was 58 %, detected in the fabric that had been exposed in a purely residential area with low population density. The loss in tensile strength of mixed synthetic textile fabrics ranged from 54% to 73%. The difference between the tensile strength of the two fabric types increased with exposure time, especially during the summer, as the intensity of solar radiation increased. After 15 months of exposure to direct solar radiation, the cotton fabric lost about 25% of its tensile strength, and the mixed synthetic fabric lost about 19.5% of its tensile strength. The percentage loss of elongation (elasticity) of cotton ranged from 72% to 89% and of mixed synthetic fabrics ranged from 67.2% to 88.2% (El-Taieb and Shakour, 2003).
Linen fabrics
Abdel-Karrem (2005) showed that after exposure to UV rays for 200 hours, linen textiles became darker and showed some losses in both tensile strength (approxi- mately 15%) and elongation (approximately 10 %). X-ray results have shown that after irradiation, the crystallite size of the linen fibres slightly decreases in the longitudinal dimensions of the linen fabrics, while the size of their lateral dimen- sions and the crystallinity index remain almost unchanged (Abdel-Karrem, 2005).
Silk fibre
Of all natural fibres, silk is the most sensitive to electromagnetic radiation, and undergoes photodegradation in both dry and wet conditions.
Radiation with wavelengths of 220 to 370 nm causes photoyellowing and photodegradation of silk: irradiation by visible light results in fading. As with wool, the susceptibility of silk to photodeterioration is due to the presence of tryptophan, tyrosine and phenylalanine amino acid residues in its amorphous regions. These absorb ultraviolet radiation (250–300 nm), and the tryptophan and tyrosine residues undergo photo-oxidation. During oxidation, the residues turn into various chromophatic groups, causing the material to develop a yellow, brown, grey or light-pink colour, and leading to rigid and mechanically weakened silk after 30 hours of UV irradiation (Shubhra et al., 2011).
Saravanan (2007) found that mulberry silk undergoes greater photodegradation than muga silk. The photodeterioration of silk is also determined by its pH: the maximum resistance to radiation has been found to be at pH 10, and this decreases
108 Understanding and improving the durability of textiles
rapidly above pH 11 and below pH 3. Silk is less resistant to radiation in the common neutral region (pH 6–8) than when it is more acid or alkaline (Timar- Balazsy and Eastop, 1998).