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Name: Hauke
Status: student
Grade: 6-8
Location: AZ
Country: USA
Date: Spring 2012

What does a PET(E) plastic water bottle become after it is exposed to ultraviolet light after a long period of time and how would it benefit society?

Hi Hauke,

PET (polyethylene terephalate) used in water bottles, pop bottles etc, mainly does not break down when exposed to UV. PET is not greatly affected by UV light. All that happens is that the surface layer a few molecules thick tends to develop a "cloudy" appearance after extended UV exposure. This process takes months of sunlight exposure before being noticeable. The inner layers are mainly unaffected. After a very long exposure, some brittleness can be expected.

I am unclear what you mean regarding your question of what benefit this is to society. Are you thinking that exposure of PET to UV light may be a way to break down or eliminate waste PET? In fact, this takes far too long to be a useful process.

Regards, Bob Wilson


Interesting question. PET itself, is very stable to UV light and has a high UV transmittance. Therefore, very little happens to the PET bottle with long exposure to UV light. On the other hand, because of its high transmittance of UV light, contents placed in a PET bottle that are sensitive to UV light can be greatly affected. PET plastic that is targeted for packaging UV sensitive materials can be tinted and/or have UV adsorbing additives in them to try to minimize the effect on the contents. Since a PET water bottle contains a material that is not UV sensitive (water), these bottles are generally clear and do not have UV adsorbing additives.

Now how is this beneficial to society? One way this property of a PET water bottle is being used to benefit society is by using the bottle for SODIS (solar water disinfection). SODIS is practiced in rural and undeveloped areas that don’t have a safe drinking water supply. SODIS is carried out by placing contaminated water in a bottle, capping it and exposing it to sunlight for ~6 hrs. The UV light that easily passes through the PET kills the pathogens in the water, making it safe to drink. Here is a link to more information on SODIS.

Thanks for the challenge! Hope this helps Steve Ryba

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