The UV Resistance of Electrical Polymers For Wire And Cable Applications
By Art Maldonado
Electrical polymers, like the human skin, can be affected by exposure to the Ultraviolet (UV) rays from the sun. The UV rays cause oxidation of the polymers, which is considered the main degradation mechanism of electrical polymers. Degradation from light, or photo-oxidation, is the one of the mechanism by which polymers are damaged. Polymer degradation can also be caused by heat, radiation, and chemical fluid attack. UV light can degrade polymers as much as heat. Degradation of the chemical “linkage” by which the polymer is bonded, also occurs. This linkage is often critical to providing the desired physical properties.
Some polymers are inherently resistant to UV degradation. Examples are Ethylene Propylene Rubber (EPR) and the fluoropolymers, Ethylene Tetrafluoroethylene (ETFE), Fluorinated Ethylene Propylene (FEP), and Perfluoroalcoxy (PFA). However, polymers such as the polyolefins (PO) and polyvinyl chlorides (PVC), are not as resistant to UV light. Antioxidants (AO) and ultraviolet light absorbers are added to the compounds to help prevent UV degradation. AOs and UV absorbers work much like sunscreens on the human skin. They do not completely eliminate the photo-oxidation mechanism, but they slow down the auto-oxidation process.
After exposure to UV light, the polymers links eventually continue to crosslink, increasing the molecular weigh and becoming brittle. Over time, increased tensile strength and decreased elongation occurs. In other types of polymers such as natural rubber and polyisoprene, the reverse occurs. The polymer softens due the chain scission and this result in decreased molecular weight and lower tensile strength values after heat exposure. This degradation mechanism is also known as “reversion”.
Heavy metals such as copper, manganese and iron, can accelerate the oxidation mechanism of polymers. Ionic forms of these metals can catalyze elastomeric oxidative reactions by enhancing the breakdown of peroxides in such a way as to accelerate the oxidation process. In many cases, antioxidants chelate with these metals, rendering them ineffective. In the case of wire insulations, copper ions can migrate into the insulation, causing an accelerated oxidative effect.
When ultraviolet light A, consisting of electromagnetic radiation with wavelengths between 400 and 315 nanometers (nm), and ultraviolet light B, consisting of electromagnetic radiation with wavelengths between 315 and 280 nm, impinge on a polymer surface, hydrogen radicals and polymer radicals (P•) are formed. These radicals set off a sequence of chain reactions, resulting in polymer chain scission and crosslinking. The chart on the right shows the spectrum of light indicating the UV-A and UV-B regions that mostly damage polymers. There are laboratory chambers that simulate the UV-A and UV-B light spectrum. A widely used test for measuring the UV performance of electrical polymers is in UL-1581. The test measures the retention of physical properties such as tensile strength and elongation after UVA and UVB exposure.
