This question is based on the assumption that most materials expand when heated. Why does shrink wrap do the opposite?
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Heat shrink plastics will shrink in 1 or 2 dimensions, but expand in the 3rd (thickness). That is, the volume of the plastic material is still greater when warm than when cold, but it’s shape changes significantly.
Shrink wrap is a bit like putting a stretched rubber band in a cold freezer. The rubber band will freeze in this stretched condition and stay there until it warms up enough for the molecules to more easily change shape. Shrink wrap just operates at higher temperatures — staying “frozen” at room temperature, and taking a higher temperature to allow the molecules to move.
The polymers are fabricated into an elongated state and when heat is applied, they return to their preferred coiled up state, which causes them to shrink.
Imagine a bunch of strips of paper, all in parallel to each other, and all of the same length, let’s say 12 inches. If you crumple them all up, the result would be a much shorter length. That’s what the heat is doing to the material the plastic is made of.
It doesn’t shrink, it just changes shape. The volume of the material remains the same after “shrinking”. It’s just a bit thicker.
If you put a water drop on a waxed surface, the drop forms a bead. If you try to spread out that bead and freeze it, you get a flat sheet of ice. If you melt it again, the molecules’ mutual attraction causes the sheet of ice to contract from the edges to form a bead of liquid water again, because this is the lowest energy state with the lowest internal stresses. The same principle applies to plastic – when you heat it, it “relaxes” into a more stable shape, typically conforming to whatever it’s wrapped around as it contracts and gets thicker.
Shrink wrap will actually expand when heated, but only as much as other plastic products do, which isn’t much.
The shrinking isn’t from thermal contraction it’s from the polymer chains pulling together.
From something like this
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to this
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Which is why once it’s shrunk it is thicker.
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