- Jul 23, 2008
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The plastic bottle was empty, closed and left in the car with very high outside temperature. Returned back after 10 hours or so and the bottle had shrunk significantly. So what's the explanation?
I put an empty plastic water bottle in my car in the sun for 10 hours....
- Thread starter nublikescake
- Start date
Plastic shrinks when exposed to high temperatures. It's the same concept that makes shrinkwrap work.
Air escaped the cap as it it expanded. Then when it cooled the volume of air was less than before.
- Jul 23, 2008
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That sounds like the most plausible explanation to me so far. Thanks for all the responses guys!
I should add for clarification that we're talking about 5 gallon polycarbonate bottles and I think these ones don't shrink like the cheaper quality ones.
its simple really
Pure physics. Heated air expands, becomes lighter and rises. Cooler air is dense and falls. Also the plastic becomes softer (more maleable) this causes the tightness on the cap to loosen up a bit and some of the air inside begins to escape the bottle very slowly and also softer more maleable plastic causes the bottles to deform a bit.
When hot air expands it will not have anywhere to go inside a closed bottle so it will start creating a lot of pressure and the air will look for a way out so either it will slowly escape through the lid (or even pop off the lid completely on some bottles, depending on the type of lid). Once enough air has escaped the remaining air will be already be expanded and since it cant expand any more it will no longer keep on escaping the bottle to relieve the pressure. however once such air cools down it will contract and will not be enough to fill the bottle completely so it creates a vaccum which basically sucks the plastic inwards shrinking the bottle.
Pure physics. Heated air expands, becomes lighter and rises. Cooler air is dense and falls. Also the plastic becomes softer (more maleable) this causes the tightness on the cap to loosen up a bit and some of the air inside begins to escape the bottle very slowly and also softer more maleable plastic causes the bottles to deform a bit.
When hot air expands it will not have anywhere to go inside a closed bottle so it will start creating a lot of pressure and the air will look for a way out so either it will slowly escape through the lid (or even pop off the lid completely on some bottles, depending on the type of lid). Once enough air has escaped the remaining air will be already be expanded and since it cant expand any more it will no longer keep on escaping the bottle to relieve the pressure. however once such air cools down it will contract and will not be enough to fill the bottle completely so it creates a vaccum which basically sucks the plastic inwards shrinking the bottle.
You dolts. Have you not heard of in-car chupacabras? They often strike empty plastic bottles and reduce their sizes when the owner is not looking so when they return, they are entertained by people coming up with highly scientific reasons as to why this had happened.
The cause is a combination of the type of polymer and the manufacturing technology for bottles. Many polymers retain a "memory effect". Usually to make an object from the polymer they heat it until it is soft and deformable - that is, above the Glass Transition Temperature. Then they re-form it into something and cool it while it is held in this form, so it stays that way. But if it is re-warmed back to above the Glass Transition Temperature, it may try to return toward its original shape, although it will never actually get all the way back. The actual temperatures are important. If the polymer is heated hot enough the first time, all the intermolecular bonding is destroyed and new bonds formed as it cools, so the "memory effect" can be removed. But that temperature is higher than what is needed just to soften it and allow it to be re-shaped, so almost nobody bothers to do that as the original item is formed.
In the case of the water bottle, the manufacturing process is called "blow molding". The mold is a hollow shape with some cooling on the outside. The machine basically sticks a spout into the top if the mold, squirts out a blob of molten polymer and then blows it up with air, just like a balloon inside the mold. The process forces the polymer blob to expand to exactly cover the inside surface of the mold. As it does that, the polymer loses heat to the mold itself, causing it to cool down below the Glass Transition Temperature and "harden". Then they split the mold in half (it's made in two parts), pull out the bottle, cut off the excess plastic near the top where the plastic and air were injected, and Voila! If that bottle is heated enough later, it will try to return toward original form, which was a blob of polymer - that is, it will shrink down, with the walls thickening up, but it will never get all the way back to a simple blob.
Some plastic items, especially heavier more expensive items with larger openings, are made instead with injection molding. In this case the mold usually is in two or more pieces and the entire mold is filled with molten polymer - there is no air-filled pace inside the item while the molding is happening. Typically these things are done at higher temperatures than blow molding because they want the polymer to be REALLY soft so it flows quickly and fills the mold completely. That means the polymer is hot enough to break down the original intermolecular bonds and destroy most of the "memory" in it. Besides, for items made this way the polymer used often is very different from the polycarbonate used for blow-molded bottles, so the polymer may have very little memory effect to begin with. For these items, heating up beyond the Glass Transition Temperature will allow it to deform and collapse according to the effect of gravity on a soft semi-fluid plastic, but it won't simply shrink back toward its original form.
In the case of the water bottle, the manufacturing process is called "blow molding". The mold is a hollow shape with some cooling on the outside. The machine basically sticks a spout into the top if the mold, squirts out a blob of molten polymer and then blows it up with air, just like a balloon inside the mold. The process forces the polymer blob to expand to exactly cover the inside surface of the mold. As it does that, the polymer loses heat to the mold itself, causing it to cool down below the Glass Transition Temperature and "harden". Then they split the mold in half (it's made in two parts), pull out the bottle, cut off the excess plastic near the top where the plastic and air were injected, and Voila! If that bottle is heated enough later, it will try to return toward original form, which was a blob of polymer - that is, it will shrink down, with the walls thickening up, but it will never get all the way back to a simple blob.
Some plastic items, especially heavier more expensive items with larger openings, are made instead with injection molding. In this case the mold usually is in two or more pieces and the entire mold is filled with molten polymer - there is no air-filled pace inside the item while the molding is happening. Typically these things are done at higher temperatures than blow molding because they want the polymer to be REALLY soft so it flows quickly and fills the mold completely. That means the polymer is hot enough to break down the original intermolecular bonds and destroy most of the "memory" in it. Besides, for items made this way the polymer used often is very different from the polycarbonate used for blow-molded bottles, so the polymer may have very little memory effect to begin with. For these items, heating up beyond the Glass Transition Temperature will allow it to deform and collapse according to the effect of gravity on a soft semi-fluid plastic, but it won't simply shrink back toward its original form.
Jeff7
Lifer
- Jan 4, 2001
- 41,596
- 19
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Polyethylene, polypropylene, polycarbonate, polyvinyl acetate, polystyrene, and lots and lots of others.
Polymer = many mers. Each plastic molecule is made up of long chains of mers, individual groupings of atoms, which are repeated over and over along the chain.
Also, read what Paperdoc posted.
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