Comparing Compostable (Starch based) Plastic to Tradition (Petroleum Based) Plastic: An Inquiry – Blog #41

Petroleum-based plastic (top) and Starch-based plastic (bottom) reaction with 5 drips of acid, for 5 minutes.

This week my grade nine applied science designed and performed an inquiry into the difference between compostable and petroleum-based plastic. This idea came to me while shopping at our local “dollar store” where I espied some packages of “compostable plastic bags”. These bags are specifically banned from our municipal composting program and I started to wonder if they really do break down and if so under what conditions. We are currently studying ecology in class so I bought a package of the compostable bags and took them into school. 

Strength test

When I showed the package to the students I had to take a step back because most of them did not know the word or the concept of composting.  So we spent an entire period learning about “compostable” and “ biogradable”. Those who did have prior knowledge shared what they knew, then YouTube and I filled in the blanks.

Now that the students understood that bags that are labeled as “compostable” are expected to break down in nature, I passed out a small piece (about 5 cm x 5 cm) of the two types of plastic to each of them. I encouraged them to feel the plastics and discuss with a partner various properties that could be compared between the two plastics. The students came up with an amazing list of nine properties! The properties they thought could be compared were:

1.      Colour

2.      Texture

3.      Odour

4.      Density

5.      Strength

6.      Stretchability (plasticity)

7.      Reaction in acid

8.      Reaction in water

9.      Flammability

This list of nine properties was the independent variables that we would study during our investigation.

Checking odour by wafting.

Prior to getting into the details of how to test for these properties the class brainstormed what variables would have to be controlled in order for the tests to be fair. They decided the controlled variables should be:

1.      Size of plastic sample

2.      Volume of water (where applicable)

3.      Volume of acid (where applicable)

4.      Temperature of the liquids used

5.      Time (where applicable)

Students were then instructed to work with a partner. Each pair of students was given one of the independent variables to consider. Together the pair of students designed a simple method for testing for their assigned property. They jotted down their design plans and submitted them to me. These pieces of paper became the procedure for the comparison tests. I told the students I would consolidate the procedures that they had designed for each of the nine properties into one list for them to use when performing the inquiry.

Prior to class the next day, I did prepare a table wherein the procedure for each test was listed together with space for the students to record their observations. I also assembled all the apparatus and materials the students could possibly require to complete the lab and placed them in a central location.

The students’ findings to date are that there was very little difference between these two types of plastics, when tested as described above. There remain two steps of my challenge to them:

1.      Tomorrow I will give them the opportunity to design and implement any further tests they wish to use to compare the plastics, ie. boil in acid, boil in water, subject to freezing, etc. It will be up to the students to come up with any test ideas and implement them.

The expression on this studnt's face is typical during a student designed inquiry!

2.      The next day they will have a choice of two activities; either write a letter to one of the plastic manufacturers describing their tests and the results or perform internet research to determine what other tests could have been performed that would have produced significantly different results for the two types of plastics. (ie. microbial activity, extended time, high temperatures)

Due to the high number of English Second Language (ESL) students in my class I am going to give the students the option of working either alone or to pair up in partners of English speakers with ESL students. This written product will be the final evaluation piece for this inquiry.

I would love to hear your ideas for other tests we could do on these two types of plastics, considering the restraints of a typical high school science lab. Please leave your comments and suggestions in the box below.