Science Buddies: "Ask an Expert"

This summer, I started working on my science fair project for this coming year and I was wondering if I could ask for some advice.

The experiment that I would like to do (and has been approved by my science teacher) is to test the scope of photodegradation on plastic grocery bags based on the size and color of the plastic bag and the type of water (saltwater vs. fresh water) in which the bag is located.

What I am thinking of doing is cutting up a few different size squares of a few colors of plastic grocery bags (that I would obtain online from a supplier) and placing them in buckets/trays of water (ocean and tap water). I would place the buckets under ultraviolet lamps to simulate an extended period of sunlight for a few months. If I can think of a way to get the buckets/trays to shift around continuously as well, I would like to do that too to simulate wave action. I would weigh and measure and make other physical observations about the squares of plastic before and after they have been in the water.

My biggest concern is whether my experiment will be meaningful and yield any useful data. I know that photodegradation is a very slow process and I don’t have all that long to do the experiment. I have to finish the experiment phase in December to meet the timeframe for science fair set by my teacher.

Do you think my experiment would yield any change in data or would nothing happen to the plastic due to the extremely long time it takes plastic to degrade?

Do you have any suggestions on ways to better simulate exposure to the sun? In your experience, are there any UV lamps that are better for this purpose that are available for purchase at a reasonable price? I also haven't been able to find any specific data that shows compares UV light exposure to sunlight (ex how many days of continuous UV light exposure equals how many years of sunlight exposure). Do you have any suggestions on where I can find that data?

I also know that I have to use protective glasses and gloves when I am around UV light. Any suggestions as to the best way to shield the UV light so it only focuses on my samples?

I saw in one of research papers that they dried plastics in an oven for an hour prior to obtaining a dry weight. Is the oven drying necessary to get a more accurate reading of the weight? Or could I just let the plastic pieces air-dry out and then weigh them?

How precise of a scale do you think I need in order to take the incremental measurements of weight changes in the plastic? I know the more precise the better, but what level of precision do you think would be required to measure the small changes in the plastic? Is that type of scale commercially available at a fairly reasonable price? If precise scales are not that costly, then my parents might buy one for this process. I just don’t know what level of precision is required and how commercially available they are.

If this type of scale is not commercially available at a reasonable price, I am hoping to find a lab in San Diego that is willing to let me use a scale. Do you have any contacts with a lab in San Diego that I might contact to get access to a scale?

Do you have a recommendation for the type of container to hold the water and plastic? I was thinking of using plastic buckets or glass or metal baking pans to hold the plastic pieces. I guess the plastic in the bucket could degrade too. I am just trying to find something commercially available as a container. If I can construct some kind of vibrating apparatus with a motor, I am not sure which type of container would be easiest to attach.

Thank you for any help or suggestions you might have! Justin

Hi,

Welcome to Science Buddies! This sounds like a great project and you have some really good questions.

When doing a science project, the best place to find answers is to look in the scientific literature and then find a method that is suitable for your research objective. It sounds like you have already been reading some literature, but here is some additional information that I found:

Here is an article where the authors used 5 x 40 cm pieces of plastic bags weighing 300 mg and incubated them while shaking at 24 degrees Centigrade. The pieces of plastic were weighed every 7 days to the closest milligram for 49 days and the pieces were dried with a fan before weighing. This looks like a good procedure if you have access to a scientific balance with an accuracy of 1 milligram.

http://plasticsoupfoundation.org/wp-con ... urtles.pdf

Here is a comprehensive review article that describes several methods for testing degradation of plastic bags. This article has lots of references in the bibliography that you might want to look up. The background information would be useful for your research paper as well.

http://igem.molgenrug.nl/iGEM2010/brain ... dation.pdf

There is a standard test method for measuring the degradation of plastic bags that has been published by the American Society for Testing and Materials (ASTM). You should try to contact this organization and ask if you could obtain a copy of the test method without having to pay for it. The science fair judges would be impressed if you used the standard ASTM method, but you can use any method as long as you describe exactly what you did.

http://www.astm.org/Standards/D7475.htm

Your idea for an experimental design sounds like a good one to me. Since you want to compare seawater and fresh water, you would want to maintain all conditions identical, except the source of the water. The controlled conditions for the experiment would include the temperature, UV light exposure, movement of sample. Your experiment would yield data that would be meaningful if your experiment is completely controlled and if you include at least duplicate samples and very accurate and sensitive

This experiment is a little complicated, so it would be good to set up a pilot experiment as quickly as possible to verify feasibility and then set up a definitive experiment. Your experiment time should be at least 49 days, but if the ambient temperature is less than 22 degrees Centigrade, you would probably have to extend the time. Since you are in San Diego, I don’t think temperature will be a problem, but you should probably plan to measure the temperature during the experiment. It will get a little cooler, so an experiment done in October/November will go slower than an August/September on.

The type of scale that you need with a precision of 1 mg is very expensive and it would probably be good if you had access to a research laboratory with this equipment. You might look for pricing of a used scale from a local company that resells scientific equipment. I don’t know of anyone in San Diego, but you should call any local colleges and private laboratories close to you and explain what you need. I think that measuring the results over time (every 7 days or so), would yield excellent data for analysis, so you need access to a balance that you can use for an hour or two every week.

The first paper I cited above used fans to dry the plastic strips before drying and this seems like an excellent method. It seems like using an oven could easily expose the plastic to temperatures that would melt the plastic (uncontrolled parameter). While you are looking for used scientific equipment, also ask about a vacuum oven. It seems like using a small vacuum oven at a low temperature (30 degrees Centigrade) for an hour or two would be a really good method for this step. You should probably do a pilot experiment and determine a standard method for you to use for this important step. To validate your method, you would weigh the dried strips after a period of drying and then continue the drying and weighing until the weight is constant (no water left to evaporate).

Plastic buckets would probably be better than metal ones. The metal containers would leach metal ions that could affect the growth of the microorganisms that are degrading the plastic bags. Glass would be the best material as it is inert, but it is breakable and much more expensive. Whatever container you use, make sure you use identical containers so that this is another controlled parameter in your experiment. You don’t need really large containers if you will be starting with plastic strips that weight 1-2 grams.

You have time to do 2 x 49 day experiments before your project is due, so I would recommend starting as quickly as possible so you will have time to analyze your results. You will learn a lot from the first trial and probably improve the procedure, so your first test should be more of a small test run before the larger definitive experiment. If you run results in triplicate for the second experiment, you will have good data for statistical analysis.

Instead of using an artificial UV light, can you should consider using natural sunlight to simulate actual environmental conditions? There is plenty of great sunshine in San Diego that would work perfectly if you can work outdoors. If you did use UV lights, you would need to set up the experiment in a closed room, and turn off the UV lights before entering the room. Otherwise, the conditions would be unsafe.

I think I have answered all of your questions, but let me know if you think of anything else.


Donna Hardy

Hi Donna,

This is Justin's mom. He is at a couple scout camps over the next few weeks and so he may not be able to respond personally for a little bit. However, on his behalf, I wanted to thank you very much for the insightful and extremely useful suggestions. I will print this out for him and it will definitely give him a lot to work with. I am sure he will be back on to thank you and ask you any questions that he might have.

Thanks again!

Hi,

You are welcome! Thanks for your message. If you can, please print out the review article as well. The review article includes information on the chemistry of plastic and a description with references of all of the methods used to measure degradation of plastics. Justin is going to measure the degradation by weighing the plastic, but he could also include a visual observation, and he should be familiar with other methods that are used. While he as camp. maybe he would have time to read and understand as much of the review article as possible. And, he should be thinking about what controls he will be using. He can post additional questions on this topic when he returns from camp.

Donna Hardy