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What is the principle of shrinkage of heat shrinkable film when heated?

What is the principle of shrinkage of heat shrinkable film when heated? I will share with you today.The macromolecules of ordinary plastic films are in a state of natural curling and generally do not shrink when heated. The situation of shrink film is different. Its film formation has gone through two steps. After the raw material is melted, the film is stretched at a specified temperature and under the action of external force. The macromolecules cannot slip with each other, and the unnatural state is frozen. .At room temperature, the mobility of plastic macromolecules is poor, so this unnatural state cannot be eliminated by molecular motion, which is why the heat-shrinkable film does not shrink at room temperature. However, this is only a superficial phenomenon, and this seemingly stable thermal shrinkage has hidden unstable factors inside.When the heat-shrinkable film is reheated to a certain temperature, the frozen macromolecules will start to move again, trying to recover from the unnatural state to the natural state, resulting in shrinkage. This phenomenon is essentially the same as when a stretched rubber band is relaxed and then springs back.

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