Continuous exposure to sunlight can cause the destruction of the polymers from which textile fibres are made, mainly as a result of extensive chain scissions, which lead to changes in their chemical and physical properties. The destruction process is largely caused by UV radiation – the region of solar radiation with the highest photon energy. Wavelengths in this region lead to the excitation of electrons in a chemical bond, raising them to a higher level of energy: this is known as a photochemical process (Zemaitaitis, 2001).
The UV radiation that reaches the earth’s surface accounts for only about 6% of the total solar radiation at maximum exposure, and has wavelengths from 290 to 400 nm (Saravanan, 2007). This region of UV radiation deserves particular attention in connection with the photo destruction of textile fibres, because in most fibre polymers, the bond dissociation energies correspond to 290–400 nm wave- lengths (Reinert and Fuso,1997). Further details are given in Table 6.1.
The main cause of outdoor degradation of polymers is the absorption of UV radiation from sunlight, with energies ranging from 300 to 450 kJ/mol (Katangur et al., 2006). Polymers containing double bonds, aromatic, ketone or aldehyde groups all absorb solar UV radiation. Of these, polydienes, cellulose acetates, polyamides, and natural silk have the least resistance to light, while polymers with aromatic groups have better resistance (Zemaitaitis, 2001). Table 6.2 compares the electro- magnetic radiation energy of light of various wavelengths with the dissociation energy of various chemical bonds (bond strength) present in the polymers of fibres.
With regard to their ability to resist photo degradation, the polymers currently in use can be divided into three broad categories (Saxena and Srivastava, 2010):
• highly stable polymers: polytetrafluoroethylene, polymethyl methacrylates;
• moderately stable polymers: polyethyleneterephthalate, polycarbonates,
• poorly stable polymers: polyolefins, polyvinyl chloride, polystyrene, poly- amides, natural silk, cellulose.
Table 6.1 Intensity of global radiation (sum of direct and scattered radiation) at the earth’s surface (summer, vertical incidence) and its classification (Reinert and Fuso,1997)
Region of Wavelength Radiation intensity Mean photon
solar radiation (nm) energy (kJ/mol)
(W/m2) (%)
UV-B 280–320 5 0.5 400
UV-A 320–360 27 2.4 350
360–400 35 3.2 315
Visible 400–800 580 51.6 200
Infrared 800–3000 472 42.1 63
Effects of light exposure on textile durability 113 Table 6.2 Comparison of bond strength with the energy of electromagnetic radiation at various wavelengths (Timar-Balazsy and Eastop, 1998)
Electromagnetic radiation Bond
Type Wavelength Energy Type Strength
(nm) (kJ) (kJ)
UV < 400 > 300 Alkane: CH3–H 427
400–430 472–277 Alcohol: CH3O–H 419
Alcohol: CH3–OH 373
Methyl chloride: CH3–Cl 344 Ether: C2H5O–C2H5 331
Blue 430–490 277–247
Blue–green 490–510 247–235
Green 510–530 235–223
Absorbance by Green–yellow 530–560 223–214 chromophoric systems
Yellow 560–590 214–207
Orange 590–610 207–197
Red 610–700 197–176
Infrared > 700 < 176 Nitrate: C2H5O–NO2 151 Butyl-hydrogen peroxide: 151
C4H9O–OH
Dibutyl peroxide: 147
C4H9O–OC4H9
Alkoxy radical: CH3–CH2O 54
Photo degradation is known to be more prominent on the surface than in the bulk (Allen and Edge, 1992). As fibres have a considerably larger specific surface (surface area per unit volume) in comparison with moulded articles and films, it is to be expected that fibres should be more susceptible to photo degradation (Aslanzadeh and Haghighat Kish, 2005). The ageing behaviour of specific fibres, including photo degradation, is governed primarily by the nature of the fibre, i.e.
the nature of the polymer. Other factors, such as polymer purity, fibre thickness and fibre cross-section also play an important secondary role (Reinert and Fuso, 1997).
Photo degradation occurs due to the activation of the polymer macromolecule via the absorption of a photon of light by the polymer (Saxena and Srivastava, 2010). In some cases, light is absorbed by photo initiators rather than directly by the polymer. In this case, the photo initiators then cleave into free radicals and initiate destruction. For example, pure polythene and polypropylene do not absorb light radiation; however, Zemaitaitis (2001) showed that over a period of some years of exposure to the sun, some products made from these polymers did undergo a loss in mechanical strength and other properties due to impurities or photo initiators that emerged during polymer synthesis.
A key principle of photochemical destruction is the first law of photochemistry, also known as the Grotthuss–Draper law: for light to produce an effect upon matter
114 Understanding and improving the durability of textiles
it must be absorbed. According to this law, only radiation absorbed by a molecule can be effective in producing a photophysical process (e.g. bond dissociation) or a photochemical change in that molecule. UV radiation and light must be absorbed discretely by certain bonds and groups in the molecule. When energies are sufficient to cause the breakage of the bond, a photochemical reaction can take place. Degradation can occur when the amount of energy absorbed exceeds the bond energy of a polymer (Katangur et al., 2006).
When a polymer molecule absorbs electromagnetic radiation, its energy in- creases by an amount equal to the energy of an absorbed photon E: (Saxena and Srivastava, 2010):
E = E2 – E1 = hν [6.1]
where E2 and E1 are the energies of a single molecule in the final (excited) and initial states, respectively; h is Planck’s constant; and ν is the frequency of radiation.
The excited molecule may then lose the absorbed energy by a number of means:
heat; the emission of radiant energy in the form of fluorescence; chemical change within the molecule; the breaking of chemical bonds (photolysis); or the transfer of energy to another atom or molecule (Saxena and Srivastava, 2010).
On absorbing light radiation, fibre polymers may undergo photochemical dissociation; photolysis can result if the absorbed energy is sufficient to cause scission of a bond in the molecule (Timar-Balazsy and Eastop, 1998). If the polymer of the fibre has only strong covalent bonds, photolysis occurs on exposure to UV radiation, but not with other wavelengths.
The presence of heat and moisture accelerates the rate of photo decomposition.
If fibres are exposed to light in an atmosphere containing oxygen and humidity, oxidative destruction (photo-oxidation) may occur. This type of degradation includes processes such as chain scission, cross-linking and secondary oxidative reactions, and occurs by the free radical mechanism (Saxena and Srivastava, 2010). For example, ketones excited by light radiation easily form free radicals, which start destructive chain reactions (Zemaitaitis, 2001):
C C C O
C C O
C C C
~ ~ hv ~ . +. ~ ~ . +CO + . ~
[6.2]
According to Timar-Balazsy and Eastop (1998), the photodegradation process can be divided into primary and secondary steps. In the primary steps of this process, an atom (or a group of atoms) is eliminated and a chemically very active radical remains. This radical forms a peroxide radical with atmospheric oxygen, which, in turn, abstracts a hydrogen atom from a neighbouring molecule. The secondary steps do not require further absorption of electromagnetic radiation but are entirely
Effects of light exposure on textile durability 115 heat-dependent. These reactions involve the products formed in the primary reactions: the peroxide bond has low bond dissociation energy and can be disrupted by atmospheric heat energy at ambient temperatures.
The amount of damage depends not only on the wavelength of the radiation, but also on the amount of radiation to which the material is exposed during the whole period of exposure. It is important to note that some photochemical reactions do not stop in the dark, and that light damage is cumulative.
A wide range of photochemical reactions lead to the formation of various deterioration products in the fibre polymers. If such products contain a chromophoric group, which can be part of a chromophoric system, the colour of the fibre will change. If the covalent bonds in the polymer chain backbone undergo rupture (causing chain scission), a decrease in degree of polymerization occurs, leading to a decrease in mechanical strength (Timar-Balazsy and Eastop, 1998).