Occasionally we get questions about the effects of sunlight exposure on PVC Pipe and Fittings. We have put together this information to help answer some of those questions.
The sun emits visible and non-visible radiation. One component of non-visible radiation is ultraviolet (UV) light. Like many other substances, PVC plastics are affected by exposure to UV radiation.
The exposure of PVC to the UV component of sunlight causes PVC molecules in the first .001" to .002" of exposed surface to become permanently converted to a complex structure typified by polyene formations. The result is a brownish discoloration, often called "UV discoloration", "UV degradation", or more commonly "sunburned". UV discoloration does not occur where PVC is not exposed to sunlight, and ceases when exposure ends.
The discoloration process is time-dependent, and can be slowed with the addition of UV absorbers in the PVC compound. The most common additive used for this purpose is titanium dioxide, which also functions as a pigment.
Research has been done to determine the effects of long-term (two year) sunlight exposure on PVC pipe (see "UNITR-5: The Effects of Ultraviolet Aging on PVC Pipe", by the Uni-Bell PVC Pipe Association.) Other than visible discoloration, the following summarizes the results on pipe physical properties:
||Effect of Sunlight Exposure
|Modulus of Elasticity
Research indicates that the sole performance characteristic adversely affected by sunlight exposure to pipe is impact resistance (the ability of the pipe to withstand a rapid external force or violent contact). UV discoloration does not effect the pressure capacity of the pipe, or the pipe stiffness (external load capacity).
The degree to which impact strength is affected by sunlight depends upon a number of variables, including: time of exposure, climatic conditions, diameter of pipe, wall thickness of pipe, and type of PVC compound used. The thicker the pipe wall, the more unnoticeable the effect becomes. Care may be required when handling thin wall pipe to avoid impact or breakage, but once the pipe is installed however, there will be no effect on the pipe's pressure capacity, or external load capacity.
For permanent above-ground PVC Pipe and Fitting installations, it is recommended that the pipe and fittings be protected from sunlight exposure. This can be accomplished by wrapping the pipe and fittings with an opaque material, or more commonly, by painting it. If pipe or fittings are to be painted, a water-based latex paint for exterior use is recommended.