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Name: Bryan Bertoglio
Status: N/A
Age: N/A
Location: N/A
Country: N/A
Date: 1999 


Question:
Hi-

I was just camping last night and watched the plastic bottles thrown into the fire shrink as they always do before catching fire. Why do they do this? I have in the past gotten bottles of various shapes to shrink to form a cylinder w/ diameter equal to that of the neck. Is this the shape of the original bottle "blank" before formed? I also notice the same effect of heat upon the bubble packages many medications use, which will make the bubbled sheet's bubbles shrink until the entire sheet is flat. Why does this happen? Is the plastic constantly experiencing some kind of stress it is too rigid to relieve?

Sincerely,
Bryan Bertoglio



Replies:
Bryan,

I'm not really familiar with the manufacture of plastic bottles, but many polymers will shrink when heated. Polymers are long-chains of bonds (usually between carbon atoms). When a bottle is blown into a mold is it quite likely that these long chains are forced to line up together, the chains are stretched out to their full length. When energy (enough energy) is added to the molecules in the form of heat they "relax" into a more "comforatable" shape for them -- basically a knot. In the process their length has decreased and they have become thicker. If you want an experiment to demonstrate this process, neatly lay several strands of rope out in the back of a pickup truck and then drive fast on a bumpy road (in essence, adding energy to the ropes like heat adds energy to the polymer molecules). I'm sure you can envision other experiments using strings and vibration (energy) that you could do. IFf this is unclear, post again.

Greg


You're right. But the interesting question here is: why plastics? Why don't metal or glass bottles do it? All solid materials have some degree of elasticity, the ability to bounce back from a push or recover their original shape after being bent, twisted, etc. In materials like glass or metal the force restoring the shape comes from the strain on interatomic chemical bonds that the change in shape causes. This force does not change much with temperature. On the other hand, plastics are made of huge molecules that look like very long flexible strings. Ordinarily these molecules are found in the shape of loose tangled up balls (like the yarn after the cat gets done with it). If a piece of plastic is distorted, most of the distortion is accounted for not by strain on chemical bonds but by the straightening and flattening out of the shapes of the molecules (think of pulling the cat's ball of yarn apart until it's longer in one direction). Now at high temperatures there is a lot of jiggling motion going on at the molecular level, and this jiggling will tend to restore the molecules to their original roundish as-tangled- up-as-possible state, just the way giving the ball of yarn back to the cat would. This is the origin of the force that lets plastics bounce back. Notice that unlike the case for metal or glass this force will change with temperature. If I were to stretch the plastic at a sufficiently low temperature (i.e. make a bottle out of it in the way plastic bottles are usually made) there would not be enough jiggling going on at the molecular level to restore the shapes of the molecules and hence the shape of the plastic. But were I then later to raise the temperature sufficiently (by chucking the bottle into the fire) I would restore the jiggling motion and allow the plastic to bounce back.

christopher grayce



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