Polyethylene is sensitive to UV radiations, less than polypropylene. Polyolefin-based pipes need stabilization to combat UV light and heat.
Pipes are made by extrusion processing of polyolefin. Fittings are made by injection-molding process. Pipes used in irrigation of greenhouses and field watering are exposed to extreme outdoor weather. Since then pipes are meant to draw lifetime of 2-3 years or more, therefore, they need to have appropriate stabilization.
The HDPE, LDPE, and LLDPE can be stabilized by UV absorbers of type benzophenone or benzotriazole and also combination of HALS and UV-absorbers. Usually pipes used in irrigation are pigmented with white titanium dioxide or carbon black. Titanium dioxide may be used up to an additional level of 0.5 wt%. Each of the HALS and UV absorber may be added up to 0.05 wt% [50-53].
Polypropylene in comparison with polyethylene is more sensitive to UV light. Stabilizers including benzotriazoles together with HALS can be used to provide
UV resistance required for long-term use of pipes. However, large differences in UV stability are found on changing unpigmented to white, yellow, red, and blue pigmented samples. In combination with a UV absorber (benzophenone type), titanium dioxide has a slightly negative effect, whereas chrome yellow and cadmium red show a slightly positive effect .
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