Naked Science Forum
General Science => General Science => Topic started by: scottyuk on 06/08/2006 13:32:58
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Does any body know why a crisp packet shrinks, but holds its shape when you put it in the oven?
I assume it's something to do with the heat affecting the thermoplastic polymers which make up the crisp packet? If, so what is exactly happening. Is this the same process as shrink wrapping?
To compare i put one in the microwave and it shrunk, but didn't really hold its shape. Is this due to uneven heating?
Any help much appreciated!
MIke
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Welcome Mike,
I don't know !!...something to do with what you have mentioned but the anti-nefarious chums who regularly deposit their academic goodness here will soon be filling your question with answering goodness and crisp packet love !
have you tried that experiment (as mentioned in one of the podcasts) where you take one of those silver coated packets of crisps, eat the crisps, clean the packet..split it open...whack it into microwave hell for ten seconds and then enjoy your own little light show !! ?
By the way....ti's quiet at the weekends so grace us with some patience....and don't be a stranger eh ?...you are welcome here.
Men are the same as women, just inside out !
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What? Is this my Neilsy??? Welcome back, Missed you much!!! Kisses!!!
"Lo" Loretta
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Thanks Neil,
At present I'm actually doing a small project about the science of the kitchen, so yes I have certainly enjoyed putting various objects in the microwave, with varying degrees of success. The problem is that i'm a physicist and therefore don't really know the science of thermoplastic polymers! I was hoping somebody more chemistry minded could help out!
cheers
Mike
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Your help will come Mike....Weekends are ALWAYS quiet here.
I do wish you continued success in your studies....sounds like great fun.
Men are the same as women, just inside out !
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Can't really answer this particular query, since I don't know about the plastics involved, and in any case this seems to be an irreversible reaction that does not revert to its original shape when the material is cooled.
Nonetheless, as something that may or may not shed some light on this, I remember seeing something many years ago when rubber was heated and cooled, and while most materials expand when heated, and shrink when cooled, the rubber was doing the exact reverse. The explanation of this went something like this: rubber is a long chain polymer, and when the polymer was cold, the chains relaxed into a long straight chain; but as the rubber was heater, it the agitation of the chains caused the chains to fold up, and so shrink in length.
One common issue with polymers when they are heated is the may cross link between the chains – this too could cause changes in structure – it would certainly make the material more brittle.
George
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Plastics are made up of very long molecules - think of them as spaghetti. The plastic sheet that makes up the packet was made by stretching out a lump of plastic till it is very thin. This means that the molecules are all stretched out fairly taut.
When they are cold the molecules are relatively still and can't move past one another. When you heat them up a bit they all start wriggling which means they can wriggle past one another. If you stretch out a piece of rope and then wiggle one end it will go all wiggley and essentially get shorter but wider, the plastic molecules do exactly the same thing making the plastic film thicker but smaller.
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http://en.wikipedia.org/wiki/Shrinky_Dinks