Q: FROM MENTEE Q: FROM MENTEE YUNJI K. IN IL
Hello, when I was cooking a sudden idea came to me, and I was curious about the subject plastic.
When I put plastic containers, most of the ones from the store melted. Some of the plastic containers like glad and other brand didn't. That is why I realized that those were different kinds of plastic. So, I was wondering, what is this key "ingredient" that enables the object to stay exactly like it was before and not undergo its physical change. I know this could sound a bit silly, but I
really want to know.
Thank you.

July 3, 2002
A: FROM FROM MENTOR DANELL OLIVER-COLLINS. To read Danell's bio click here.
You came to the right place for this questions. This is right up my alley.
And this is not a silly question it is this type of curiosity which got me
involved in plastics engineering.
There are many different types of plastic out there; approximately 20
classifications of plastic. Which are then seperated into two categories,
thermoplastic and thermoset. What you encountered are thermoplastic type
plastics. These plastics are more commonly used and melt at lower
temperatures. And by lower temperatures I mean less than 500°F.
I'll address your temperature question specifically or else I may get
carried away. The key "ingredient" you are looking for is actually the
molecular structure of the plastic. The manufacturing process and number
of carbon, oxygen, hydrogen, nitrogen,chloride, etc atoms in the molecular
chain determine the temperature at which the plastic will melt. For
instance, I believe the plastics you were working with are polyethylenes.
Its configuration is -(CH(subscript: 2)-CH(subscript: 2))(subscript: n)- a
very basic configuration. Carbon and hydrogen and the simple chain
configuration make this plastic very easy to work with and have a
relatively low melt temperatures. The reason why many companies use
polyethylene for their products is because of its low cost and ease to
manufacture. Companies that produce similar products like the plastic
containers don't use the exact same polyethylene. There are probably 1000
different types of polyethylene's on the market today. Each differing by
something so minor that it doesn't change the molecular configuration, but
it does change the physical properties such as one polyethylene may have a
melt temperature of 150°F and another may have a melt temperature of 156°F.
Plus we must consider the way the plastic containers were manufactured.
The temperature at which they were formed and how they cooled before
packaging. For the plastic container to deform it must go above its melt
temperature.
I'll compare polyethylene to polyvinyl chloride just to clarify my point.
Polyvinyl chloride (most commonly used as water pipes) has a configuration
of -(CHCl-CH(subscript: 2))(subscript: n)- just by throwing that chloride
in there I've changed the molecular structure and physical properties
dramatically. You can really see this when you compare your plastic
containers to PVC water pipes.
This explanation just scratches the surface of the wonderful world of
plastic. If you would like more information please feel free to let me
know!
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