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UV Stability

UV-stability of polymers

It is well known that plastics in general and polyolefins in case of synthetic turf are sensitive to degradation when exposed to sunlight. This degradation is called photo-degradation. Photo-degradation is a process that generally results in breaking down the polymer chains in the polymer/product. Unless stabilizers are added to the polymer this circular chain reaction accelerates. Ultimately photo-degradation causes cracking, chalking, color changes and loss in physical properties.

In particular, the non-visible UV radiation in sunlight is responsible for photo-degradations of polymers. The UV radiation is characterized by its short wavelengths and makes up less than 5% of sunlight. The solar UV radiation spectrum is divided into three categories UVA, UVB and UVC. The UV radiation that reaches the earth’s surface is for more than 98% UVA, because of absorption in the atmosphere's ozone layer of UVB and UVC. Although UVC is completely absorbed, a small part of UVB reaches the earth’s surface. This small portion of UVB radiation is compared to UVA very aggressive towards photo-degradation, because of its very short wavelengths.

In order to test the UV stability of polymers outdoor exposure to natural weathering is a very realistic, but a very time consuming method. Several weathering test methods have been developed using artificial light sources to provide accelerated test procedures. These test methods are used to predict the UV stability of a product over a number of years. In general there are three types of artificial weathering light sources available for accelerated weathering tests: UVA, UVB and Xenon. The relationship between the spectrum of these light sources and sunlight can be found in the next table.

Light sourceRealism compared to sunlight
UVAReasonable match of solar spectrum in UV region, but not for the long wavelengths (visible and infrared)
UVBShorter wavelengths compared to UVA.  The spectrum of this light source also contains short wavelengths that are not present in solar radiation on the earth’s surface. These wavelengths are very aggressive towards polymers.
XenonXenon lights provide the best simulation of sunlight. If proper applied the radiation can match sunlight closely.

 



Xenon-Arc Exposure of Plastics Intended for Indoor Application (ASTM D4459-99)

 PURPOSE

To determine what changes, if any, occur to plastics after exposure in a window glass-filtered xenon–arc device.  This method is intended to simulate the effects produced by exposure to solar radiation through glass.

 SUMMARY

Plastic test bars are placed in metal clamps inside the Xenon Arc instrument.  The instrument is operated in the continuous light mode without any water spray.  The irradiance is controlled to 0.3 + 0.02 W/m2 at 340 nm.  The black panel temperature is controlled to 55 + 2°C, and the relative humidity is controlled to 55 + 5%.  The filter combination on the Xenon Arc lamp is a borosilicate inner filter and soda-lime glass outer filter.  The aging is typically performed for a total of 300 hr, with color measurement being taken at 100, 200, and 300 hr.

Color Measurements

 Color measurements are made on plastic parts using a colorimeter with an integrated-sphere geometry.  Typically, a D65 illuminant and 10° observer angle are used.  Results are typically reported using either a Hunter Lab scale (L, a, b, YI, and DE) or CIE scale (L*, a*, b*, YI*, and DE*).