6.3.1 Synthetic textiles
Synthetic fibres show differing degrees of sensitivity to light, depending on their chemical composition and stabilization. For example aliphatic and aromatic polyamides are more sensitive, while polyacrylonitrile fibres are less sensitive.
With the exception of some technical fibres, synthetic fibres are usually delustred with titanium dioxide, which causes the breaking of chemical bonds in the polymer molecules, a process known as photolysis, which can be identified by an apparent coarsening of the grains of the delustrant. The delustrant pigments appear larger because they are surrounded by a sphere of a degraded fibre substance with a different refractive index. Matt yarns, which contain the most delustrant, break down significantly faster than bright yarns (i.e. those with greater lustre). It is thought that the titanium dioxide exerts an influence on the photosensitizing degradation process, or, when the yarn is delustred, the light is scattered more internally within the fibre filament. The degree of UV degradation is also deter- mined by the thickness of the filament: the thicker the filament, the better the resistance to UV radiation. This is because thicker filaments allow less radiation to penetrate into the centre of the filament, and the lower specific surface area also reduces the rate of photo-oxidative attack (Thomas and Hridayanathan, 2006;
Fung, 2002). The damage caused by light can usually be detected by fibre-specific reactions and also by non-specific effects such as yellowing, loss of strength and decrease in the average degree of polymerization (Fan, 2005).
Polyamide fibres
Photodegradation, particularly that caused by UV light, remains a common problem with polyamide (nylon) fibres, despite the use of photostabilization processes during fibre production and, in some cases, during finishing as well. As discussed above, the use of delustrants accelerates the photolysis reaction, causing yellowing and loss of strength (Fan, 2005).
Contaminants within and external to the fibre also play a role in accelerating the reaction. The penetration of UV radiation into nylon causes photo-oxidation and results in a decrease in the tensile strength, which can be as much as 100% after 30 days of exposure (Saravanan, 2007). When polyamide fabric was exposed to solar radiation for 180 days, a significant reduction of breaking strength and elongation at break was recorded. A linear relationship between breaking strength and length
Effects of light exposure on textile durability 109 of exposure was observed, while no such linear relationship was found in the case of elongation at break. Multifilament polyamide materials were also found to be more susceptible than the monofilament type, and thinner materials are more susceptible than thicker materials (Thomas and Hridayanathan, 2006; Fung, 2002).
Polyethylene fibres
The density of polyethylene fibre increases with the time of exposure to solar radiation. These results can be explained by a number of factors: an increase in crystallinity; cross-linking reactions, since the material becomes denser due to tighter packing; and the incorporation of oxygen, which increases the weight of the polyethylene. Differential scanning calorimetry (DSC) scans showed a broaden- ing of the melting endotherm peak and the appearance of new peaks on PE-samples degraded by accelerated ageing, which were attributed to changes in crystallite sizes, molecular weight differences (which are brought about by chain breaking) and secondary recrystallization. At late stages of ageing, the polyethylene material became brittle and fragile (Gulmine et al., 2003).
Elastane fibres
As with nylon, elastane fibres are frequently damaged by light, despite the use of UV stabilizers, which can sometimes be partially washed out during dyeing and finishing. This damage is greatest when the light has a high UV component; it affects, for example, textiles used for outdoor sports or bathing. The photolysis resulting from UV irradiation causes discoloration and a loss of strength and elasticity, and sometimes even fibre breakages, and can be accelerated by oils and skin creams as well as sebaceous oils, but not by perspiration. Elastane fibres based on polyethers additionally undergo photo-oxidation (Fan, 2005).
Polypropylene fibres
Polypropylene fibers are highly sensitive to light and oxidation, and hindered amine light stabilizers (HALS) can be used to protect the fibres to some degree (Fan, 2005). The main types of damage that occur as a result of UV irradiation are molecular chain degradation and yellowing during photolysis, accompanied by brittleness and loss of strength. However, loss of strength in polypropylene fibres often does not correlate with melt viscosity or chain length. (Fan, 2005).
Quartz fibres
The strength of quartz fibres decreases over a period of UV exposure. After one hour of exposure to UV, a highly significant decrease in tensile strength can be
110 Understanding and improving the durability of textiles
observed; after three hours, micro-cracks appear on the surface of the fibres, and continue to grow over the course of the exposure; and after 6 hours the tensile strength shows a 75% decrease (Huang, 2005).
Carbon fibres
Carbon fibres show a significant decrease in tensile strength after UV exposure.
UV irradiation causes some grooves on the fibre surface to break away, leaving much wider grooves, which shows that a change in the structure of the fibre has taken place (Huang, 2005).
Glass fibres
After just one hour of UV exposure, glass fibres show a highly significant decrease in strength. Micro-cracks appear on the surface of the fibres, showing that the UV radiation has caused damage (Huang, 2005).
6.3.2 High performance textiles
High performance fibres are known to degrade upon exposure to light, particularly at short wavelengths. Said et al. (2006) investigated the resistance of various commercial high strength fibres to UV radiation. These included Zylon (poly p- phenylene-2,6-benzobisoxazole or PBO), Vectran (aromatic polyester), Kevlar (aramid fibre, aromatic polyamide with structural repeat unit of p-phenylene terephthalamide) and Spectra (fibre with extended chain polyethylene molecules).
The results indicated that exposing high strength fibres in continuous yarn form to UV leads to a significant loss in fibre strength for all except Spectra fibres. For some fibres, these adverse changes in mechanical behaviour occurred during UV exposure of short duration, while for others, they occurred after the maximum 100 days of exposure used in testing.
Kevlar fibres are self-screening, so light stability is dependent on the thickness of the exposed item. Very thin Kevlar 49 fabric, if exposed directly to very high intensity sunlight for an extended period, will lose about half its tensile strength within a few days. If the fabric is thicker, the majority is protected, and strength loss is minimized. Kevlar also shows a levelling effect in loss of strength, suggesting some kind of stabilization with respect to UV exposure.
Vectran showed the highest loss in strength (about 86% reduction) after 144 hours of UV exposure to UV.
At the other end of the scale, UV exposure actually appears to strengthen Spectra fibres rather than degrading them; this is probably due to favourable conditions for cross-linking that may arise as a result of UV exposure (Said et al., 2006).
Effects of light exposure on textile durability 111
Polyaramid fibres
Davis et al. (2010) showed that exposure to simulated UV sunlight at 50 °C and 50% relative humidity causes a significant deterioration in the mechanical proper- ties of polyaramid and polyaramid/polybenzimidazole based fabrics with water repellent coatings. After 13 days of exposure to these conditions, the tear resistance and tensile strength of both fabrics decreased by more than 40%. The fabric containing polybenzimidazole was less impacted by these conditions, and main- tained approximately 20% more of its mechanical properties. The conditions also significantly degraded a water repellent coating on the fabric, but had little impact on the UV light protection of the fabric surface as both fabrics still blocked 94% of UV light after 13 days of exposure. Significant surface decomposition was also observed, along with a change from ductile to brittle failure in the polyaramid fibres (Davis et al., 2010).
Kevlar fibres
UV exposure of short duration does not cause damage to Kevlar fibres. For the first four hours of the UV treatment, there is no significant decrease in tensile strength, but the significance increases from five hours onwards, with micro cracks attested after six hours. Prolonged UV exposure leads to substantial strength losses (Huang, 2005).
Twaron fibres
Twaron fibres are based on poly (p-phenylene terephthalamide) (PPTA); they are sensitive to and can absorb light with wavelengths of 300–450 nm, including UV and some visible parts of the solar light that reaches the earth, leading to a deterioration in the mechanical properties and structure of these fibres. Zhang et al.
(2006) investigated the effects of solar irradiation on Twaron 2000, a kind of para- aramid fibre, similar to Kevlar 129. They concluded that exposure to simulated UV irradiation caused a significant linear decrease in the tenacity, break extension and work to break of the Twaron 2000 PPTA fibre. After a long period of exposure to UV irradiation, the modulus also decreased, and the tensile failure mode changed from fibril splitting to brittle fracture. UV irradiation caused more severe deterio- ration on the surface layer and the crystalline defect or amorphous areas of the fibre: this is a result of chain scission between the amide groups and of end group oxidation, leading to roughening and etching effects on the surface of the fibre and to an obvious shortening of the crystalline correlation length, which are the main structural causes of loss of mechanical strength. The crystalline area remains almost unchanged by UV irradiation; however, the increase in structural imperfec- tions within the defect areas or near the crystalline areas caused by UV irradiation may cause local rearrangement of the crystallite (Zhang et al., 2006).
112 Understanding and improving the durability of textiles