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Light Stability Characteristics
All polyethylenes are susceptible to degradation upon long-term exposure to sunlight.
This deterioration is brought about by chemical changes which occur in the polyethylene as a result of exposure
to the ultraviolet (UV) portion of light. Degradation results in the polyethylene becoming embrittled, reduc-ing
the impact resistance and elongational properties of the part. Degradation from UV light can be effectively inhibited
by the addition of UV stabilizers, which protect the polyethylene through preferentially absorbing, transferring
or reflecting UV energy.
The UV life of a part is dependent upon UV additive level and type as well as part thickness
and design, pigment type, level and effectiveness of dispersion, processing conditions and the geographic location
where the molded part is used (see Figure 3). It is important when comparing resin UV performance to ensure that
the testing has been done on a consistent basis. In Figure 1, accelerated weathering data is presented. Generally,
2,000 hours corresponds to 1 year in Florida and 1,400 hours to 1 year in Southern Canada. Often terms like "UV-
8" are used. UV- 8 means the material can withstand 8,000 hours in a Xenon Ci- 65 weatherometer. UV- 2 or
UV- 4 would mean 2,000 or 4,000 hours respectively. Hence, UV- 8 corresponds to approximately 4 years of continuous
outdoor exposure in Florida.
It is important to understand which weatherometer, i. e. Carbon Arc or Xenon, was used,
as well as the details of how the weatherometer was run.ASTM D- 2565 is the recognized standard.Testing can be
performed using actual outdoor weathering exposure, such as Florida and Arizona, to confirm this data. Note Figure
1 uses the industry standard criteria of when the sample has reached less than 50% of its original break elongation
to determine the end of the test. In most cases useful life of the part extends beyond this point. All samples
in Figure 1 are nonpig-mented as supplied by Exxon Chemical. The UV performance test data can be found on our data
sheets for each specific grade.
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Figure 2. Comparison of Rotational Molding UV Performances
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