What Happens When You Bend a Plastic Cup?

In summary, a group of physics friends observed that when they bent a plastic cup, a white patch would form and the plastic would no longer be transparent in that area. They were unable to explain this at a molecular level and asked for an explanation. One theory proposed was that through friction, heat, and pressure, cross-linkages formed in the polymer chains of the plastic, altering its structure. However, someone else suggested that it could be a stress pattern from stretching molecular bonds, which can also cause a prismatic effect. This is similar to creating a polarizer by stretching Saran wrap in one direction. Therefore, the phenomenon observed with the plastic cup could be due to polarization along the directions of stress.
  • #1
Mephisto
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I was drinking from a plastic cup today with some physics friends, and we noticed that when we bent the cup, a white patch of plastic would form there, and the plastic would not be transparent at that point anymore. We were baffled by this observation, and none of us could really explain what happened at the molecular level. Does anyone know what happens there?
my best theory was that through friction, heat and pressure, cross-linkages formed in the polymer chains of the poly-ethelene, or whatever it is, basically altering the structure of the material... but I'm not sure how right that is... does that seem at all plausible?
Does anyone happen to know the answer?
 
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  • #2
That sounds reasonable, but I'm not sure about the friction and heat contributions. You can very slowly bend a piece of PET, to minimize those factors, and the result will be the same. It seems to me that it's a stress pattern from stretching some of the molecular bonds. In my experience, you'll also notice that you get a prismatic effect in the area, which indicates that the refractive index of the plastic is messed up.
 
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  • #3
You can make a simple polarizer by stretching Saran wrap in one direction. That's the basic phenomenon I believe acted upon your cup - polarization along the directions of stress.
 

What is plastic made of?

Plastic is a synthetic material made from a combination of natural and man-made substances such as cellulose, coal, natural gas, salt, and crude oil.

Why do plastic cups bend?

Plastic cups bend because they are made from a type of plastic called thermoplastic, which becomes soft and pliable when heated.

What happens when you bend a plastic cup?

When you bend a plastic cup, the plastic molecules are rearranged and the material becomes more flexible. This allows the cup to bend without breaking.

Will a plastic cup break if you bend it too much?

Yes, if a plastic cup is bent too much, the material will eventually reach its breaking point and snap. This is due to the plastic molecules becoming too stretched and unable to hold their shape.

Can a bent plastic cup be fixed?

It is difficult to fix a bent plastic cup, as the plastic molecules have been permanently rearranged. However, you can try using heat to soften the plastic and reshape it back to its original form.

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