When doing a science fair project it is best to do a lot of research into the literature that is available before locking down on a topic for the project. You may want to refine you project so it is not too broad.
In the past, this was a task best done in the library, but now you have a great resource with the Internet.
One resource that I never quote from in a report, but gives me good direction is WIKIPEDIA.
It gives me a good overview and at the end gives me lists of other sources.
For instance, the WIKI on plastic
https://en.wikipedia.org/wiki/Plasticsays in the first paragraph
A plastic material is any of a wide range of synthetic or semi-synthetic organic solids that are malleable. Plastics are typically organic polymers of high molecular mass, but they often contain other substances. They are usually synthetic, most commonly derived from petrochemicals, but many are partially natural.[2]
Which answers some of your questions in the post you just made.
WIKI is patrolled by dedicated members as well as the chemical society, which reduces the chance of trolling vandalism and quackery.
There are also several search engines on the internet that can give you links that will add to your knowledge. This forum many times suggests using GOOGLE. You have to be a savvy user to select the appropriate search question to get your best results. Also, you need to back up whatever you find on a search to insure you have accurate results and books at the library can help you with verification.
For instance I did a GOOGLE on
plastics and petroleum
and got lots of hits.
Now it is a process of sorting through the links and selecting what might be an initial resource.
After doing you initial search of the information you can "define you problem precisely", which is one of the first steps in asking a science question.
Here on this forum we teach you how to find your answer and give hints when you may be getting the wrong answer.
If you have not done so already, please read the forum rules.
Click on the link near the top center of the forum page.
Forum Rules: Read This Before Posting.
After doing some research, please post the answers to your questions and we will make suggestions.
After all this is your science project.