UV Resistance and Warranties - Read the small print

In the last post we discussed UV light and how it can cause Polycarbonate sheet to weather. This weathering damages the Polycarbonate causing it to loose impact strength, yellow and have a lower light transmission. Manufacturers can protect Polycarbonate sheet from the effects of weathering by blocking some or all of the UV light; this protection can be achieved in a number of ways and we will discuss some of them in a future blog post.

How successfully sheet manufacturers protect their sheet against the effects of UV can often be determined from the limited warranties that the manufacturers provide to their customers. As with all legal documents it pays to read the small print in these warranties before deciding which product to purchase. Often the marketing claims in the brochures are not directly related to the details in the limited warranties. In this blog we shall examine in detail the warranties of two manufacturers of weather resistant Polycarbonate sheet; by looking at these examples, hopefully it will give you an idea of the items to look for when deciding on which Polycarbonate sheet to buy for applications requiring weather resistance.

It is also worth noting, that for most of the warranties to be honored by the manufacturers, a warranty agreement must be signed at the time of purchase and proof of purchase must be retained. Manufacturers often rely on the fact that only a small percentage of customers complete a warranty form or even know that they exist. When buying a product with a warranty, make sure that you complete the paperwork at the time of purchase and store the necessary documentation in a safe place.

Our first example is a Polycarbonate sheet with marketing documents claiming that the product “offers long-term weatherability and is backed by a 15 year performance warranty.” From this statement a customer might infer that the product will be replaced if it weathers within 15 years. When we inspect the standard warranty we find:

- If the sheet breaks within 15 years it will be replaced.

- If the yellow index of the material increases by more than 5 in a five-year period it will be replaced.

- If the yellow index of the material increases by more than 10 in a ten-year period it will be replaced.

- If the yellow index of the material increases by more than 15 in a ten to fifteen year period the material will be replaced in accordance with the table below.

- If the light transmission of the material reduces by more than 5% in a five-year period it will be replaced.

- If the light transmission is reduced by more than 7% in a ten-year period it will be replaced.

- If the light transmission is reduced by more than 10% in a ten to fifteen year period the material will be replaced in accordance with the table below.

Month 121 Customer must pay 67% of original purchase price

Month 150 Customer must pay 83% of original purchase price

Month 170 Customer must pay 94% of original purchase price

A sheet would normally be considered as badly weathered if the yellow index increase by 15 and if the light transmission is reduce by 10%. If this type of damage occurs after only 10 years and one day and the sheet would only be replaced if the customer paid 67% of the original purchase price, any customer that inferred that the sheet would be replaced free of charge for fifteen years if it weathers would not be correct.

Our second example is a Polycarbonate sheet that has marketing documents claiming: “A ten-year limited warranty backs the material against excessive yellowing, loss of light transmission and breakage.” When we read the standard warranty we find:

- The material will be replaced if it breaks from impact with only hand thrown objects within seven years.

- If the yellow index of the material increases by more than 6 in a seven-year period it will be replaced.

- If the yellow index of the material increases by more than 10 in a seven to ten-year period it will be replaced in accordance with the table below.

- If the light transmission is reduced by more than 6% in a seven-year period it will be replaced.

- If the light transmission is reduced by more than 6% in a seven to ten-year period the material will be replaced in accordance with the table below.

Year 7 Customer must pay 0% of original purchase price

Year 8 Customer must pay 55% of original purchase price

Year 9 Customer must pay 70% of original purchase price

Year 10 Customer must pay 85% of original purchase price

Again, these conditions are very different than an assumption that the material will be replaced free of charge for any weathering damage for a ten year period. The sheet will only be replaced for free for seven years and then only if it weathers more than the amount quoted in the warranty.

Reading these warranties can be very useful in deciding which material to use. If for example, preventing loss in light transmission of more than 6% for ten years is an important parameter then the second material may be the better option. However, it would still be best to talk to the manufacturer about the application and see if they could extend the full replacement cost warranty out to the ten years.

Another example is breakage; if 15 years against breakage is required, the warranty covering the first material is a much better option.

Warranties are a much better method of evaluating how well a manufacturer believes their material performs than marketing brochures. Time taken understanding the warranty and filling in the paperwork can ensure that material is correctly specified and that you are protected if the material does not perform to the expected level.

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.