Why does pvc heat shrink film shrink when heated?

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pvc heat shrink film shrinks after heating, breaking the law of physics “thermal expansion and contraction”, why does this phenomenon occur?

pvc heat shrink film will appear after heating the phenomenon of shrinkage, chasing its cause, most people do not quite understand, just as a heat shrink film performance to answer, today, Lianzhong packaging small to tell you the principle of shrink film shrinkage.

In fact, heat shrink film shrinkage after heating is a physical phenomenon that needs to be explained through certain physical knowledge, which simply means that the thermal movement of molecules causes the film to contract. The macromolecules of ordinary plastic film are in their natural curled state and do not generally shrink when heated.

The case of shrink film is different, as it undergoes a two-step process of film formation, with the raw materials melting and stretching the film at a defined temperature and under the action of an external force, with the macromolecules unable to slip between each other and not in their natural state of being frozen.

At room temperature, the plastic macromolecules are poorly mobile and therefore cannot rely on molecular movement to eliminate this unnatural state, which is why heat shrinkable films do not shrink at room temperature.

However, this is only a superficial phenomenon, and the seemingly stable heat-shrinkable film already has unstable elements lurking inside. When the heat shrink film is reheated to a certain temperature, its frozen macromolecules start to move again, trying to return from their unnatural state to their natural state, resulting in shrinkage.

In fact, the phenomenon of shrinkage after heating is essentially the same as the relaxation and rebound of a rubber band, and the shrink film will return to its original state when the temperature is lowered.