Photo degradation of plastics

By Paul DornathChemistry 226 H Spring 2007

What are plastics made out of?

Plastics are large molecules called polymers composed of repeated segments called monomers with carbon backbones.

Sometimes plastics are made with other chemicals that may form organic molecules when plastics degrade.

http://images.google.com/imgres?imgurl=http://www.tvo.org/iqm/plastic/graphics/animationthumb.jpg&imgrefurl=http://www.tvo.org/iqm/plastic/animations.html&h=62&w=84&sz=3&hl=en&start=35&tbnid=Je84Ag9AS9BbfM:&tbnh=56&tbnw=76&prev=/images%3Fq%3Dplastic%2Bmonomer%26start%3D21%26gbv%3D2%26ndsp%3D21%26svnum%3D10%26hl%3Den%26sa%3DN

Solar Radiation

43 % of all the solar radiation is in the inferred spectrum, greater than 700 nm (This does little to plastics other than heating them).

52 % is visible light 700 – 400 nm

The last 5% is ultraviolet 400 to 290 nm

Global UV plot (dark green is the highest)

What exactly counts as UV anyway?

The lower boundary in Earth’s atmosphere solar UV spectrum is caused by ozone shielding

UV radiation can be classified as near, far or extreme UV but it is also possible to classify UV radiation in terms of UVA, UVB and UVC

UVA, UVB, UVC

Plastics tend to break down when exposed to UVA or higher radiation. (340 nm for the CH3-CH3 bond)

UV is ionizing radiation

The bonds between the atoms in many plastics have dissociation energies that are very similar to the quantum energy present in UV radiation.

Plastic degradation

UV energy absorbed by plastics can excite photons, which then create free radicals.

While many pure plastics cannot absorb UV radiation, the presence of catalyst residues and other impurities will often act as free radical receptors, and degradation occurs.

PET Plastic bottle experiment

When I went to India I noticed that they sell all their soda and water bottles outside in the sun!

Other studies have shown that onions mutate when grown in water from PET bottles left in the sun for many weeks.

My Questions

What happens to PET bottles when they’re left in the sun for ten days?

What chemicals are leaching into the water from the plastic?

PET plastic

Polyethylene Terephthalate (PET) plastic is a polymer made from ethylene glycol and terephthalic acid.

PET is the also known as #1 plastic

Project goals

One: Simulate the previous testing conditions but expose bottles for only ten days. Expose bottles to UV radiation and 18º-30º Celsius

temperatures for 12 hours a day for ten days Continuously expose bottles to 40º C in the dark Leave control bottles in the dark at room

temperature (18º plus or minus 2º) Two: Use mass spectrometry to find the

concentration of terephthalic acid in each sample

Goal #1: Building an ultraviolet incubator

Used 500W Halogen light to simulate the sun

Installed an air conditioning system to maintain temperature.

GC-MS testing A Gas

chromatogram- mass spectrometer can be used to find the relative abundance of chemicals in a solution.

Molecules produce specific peaks at a certain time.

Molecules in the tested water

PET

Benzoic acidEthyl Benzene 4-methyl Benzoic acid

91 Dalton Peak 105 Dalton Peak 91 and 119 Dalton Peak

Mass Spec charts

35 day UV testControl bottle

Mass Spec charts

Ten day dark testTen day UV test

Comparing test resultsAbundance of Analogous Products of PET Structure

0

50

100

150

200

250

300

350

Control Ten Day Dark 40° Ten Day UV 18°-30° 35 Day UV 18°-30°

Samples

Ab

un

dan

ce

Abundance of 91

Abundance of 105

Conclusion

PET bottles can degrade in the sun and in the heat. This could be a health issue and more tests should be run under these conditions to analyze the safety of using PET as a food storage material.

Possible future tests

Test with Fruit flies by adding the test water to the media

Grow onions with water from five week UV test

Perform a growth curve using yeast in treated water.

Create a standard curve for how much leaching occurs over time

Questions?