The Effects of Ultraviolet Light on Polymeric Materials
UV light contributes to the development of skin cancer, the aging of plastics, and the discolouration of dyes and artwork. Typical property changes in a material include reduced ductility (increased brittleness), chalking, colour changes, a reduction in toughness, and cracking. Recently, polymers have been developed that are designed to degrade under light, and are used in products like Biodegradable? plastic bags.
The effect of UV light on a polymeric material depends on the flux (power) of the light, its wavelength, and the chemical structure of the material. Ultraviolet light interacts primarily with a structure’s pi electrons, meaning that double bonds and aromatic groups in a structure interact most strongly with it. For example, nylon 6 absorbs UV light in its amide bonds:
Structure of nylon 6
Polymers containing no pi electron clouds, e.g. polyethylene, are relatively unaffected by UV light.
The effect of UV light on a material can be determined using a number of analytical methods; typically:-
• Chemical structure analyses by employing UVand infrared spectroscopy and/or NMR (nuclear magnetic resonance);
• Surface analyses by scanning electron microscopy (SEM),
• Detection of free radicals by EPR (Electron Paramagnetic Resonance);
• measurement of molecular weight by viscosity measurements or end group analysis; or by
• changes in mechanical properties.
A major factor for UV resistance is the bulk or thickness of the material: the thicker the material, the more UV resistant it is because less UV will penetrate through to the centre of the material.
The UV resistance of materials may be determined by outdoor or laboratory testing methods. Outdoor testing is more expensive and time consuming than laboratory testing, but it remains the most appropriate by which other methods are compared. Laboratory test methods are not easily correlated with outdoor exposures because the wavelengths of the light sources do not always match those of sunlight.
Sunlight intensity varies considerably according to location, time of day, season, and atmospheric conditions. In the UK the light flux reaching sea level is greatest in July at noon. However, the overall flux is far less than that in, for example, a desert at the Equator, or elsewhere at high altitude. Thus, the testing of a material’s resistance to UV degradation must be carefully considered according to location of use. Common testing sites are Arizona, Florida and Japan. These areas have high ambient temperatures and levels of ultraviolet radiation.