Sunday, December 13, 2009

UV light and Polycarbonate

We are very familiar with the visible light spectrum. This spectrum ranges from Violet light with wavelengths of 380-450 nanometers to Red light with wavelengths of 620-750 nanometers. We can see this spectrum as a rainbow when visible light is split into its component colors. Below the visible spectrum there is UV light and above the visible spectrum there is Infrared light.

In this article we will be discussing the UV range of the spectrum and its importance for Polycarbonate. The UV spectrum is classified into three broad groups:

UVA Wavelengths of 320-400 nanometers

UVB Wavelengths of 280-320 nanometers

UVC Wavelengths of 100-280 nanometers

You may be familiar with these terms from Sun tan lotion or Sunglasses.

Planck’s constant is an important number when finding the relationship between a Photon of light and its wavelength. The relation is known as the Planck relation and is expressed as:

Energy of a Photon = speed of light x Planck’s constant / Wavelength of light.

From this relation, we can see that higher wavelengths of light have lower energy. Therefore visible light has less energy than UVA light. UVA light also has less energy than UVB light. It is this energy that is destructive and it is why we need to protect our skin, eyes and Polycarbonate from the effects of UV light.

It is not just the energy of different wavelengths that is important, but it is also the quantity of UV light exposure that is important.

In the upper atmosphere light consists of 1.3% UVB and 6.7% UVA giving a total of 8.0%. The atmosphere is reasonably good at absorbing the UV radiation, particularly UVB, so by the time light reaches sea level, it consists of 0.3% UVB and 5.7% of UVA giving a total of 6.0%.

The absorbtion of UV light varies with latitude, at higher latitudes more UVB is absorbed. This latitude affect is why skin protection is particuarly important near the equator.

Also absorbtion of UV varies significantly with altitude, as there is much less atmosphere for light to pass through. This altitude affect is one of the reasons why skin protection is necessary while skiing (in addition to reflection by snow). At a 20 degree solar elevation, UVB increases by about 20% for every 1000 meters increase in altitude and UVA increases by about 12%.

Polycarbonate becomes damaged by UV wavelengths below 300 nanometers and is particularly vulnerable to wavelengths in the range of 280-290 nanometers. The UV light below 300 nanometers starts to cause micro-cracks in the surface, over time it weakens the strength of the Polycarbonate and causes the material to turn yellow. Increasing the amount of UV exposure, particularly to the higher energy UVB will increase the rate of degradation of the Polycarbonate. Also thermal cycling and rain can help to increase the rate of degradation caused by UV exposure.

Because the amount of UV light and the energy of UV light are important factors in the weathering process, we can see how the location of the Polycarbonate installation is likely to affect the amount of degradation due to weathering.

Polycarbonate sheet installed on a mountain near the equator is likely to degrade much quicker than sheet installed at sea level in Alaska. This effect is due to the fact that the amount of UV light in the 280-290 nanometer range is significantly lower at locations at sea level and higher latitudes. Location is an important factor in specifying what type of UV protection to add to sheet as the ratio of UVA to UVB light can vary significantly. Asking your Polycarbonate sheet manufacturer to design a UV protection package for your location is important for preventing damage due to weathering.

In a future article we will discuss how Polycarbonate sheet manufacturers can protect Polycarbonate sheet against weathering and what to look for in manufacturer's warranties concerning UV resistance.

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