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

Heat shrinkable film is used for the sales and transportation of various products, and its main function is to stabilize, cover and protect the product. The shrink film must have high puncture resistance, good shrinkage and a certain shrinkage stress. During the shrinking process, the film cannot produce holes. Since shrink film is often used outdoors, it is necessary to add UV anti-ultraviolet agent. What is the principle of heat shrinkable film shrinking when heated? The macromolecules of ordinary plastic film 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, plastic macromolecules have poor mobility, so molecular motion cannot be used to eliminate this unnatural state. This is why the heat-shrinkable film does not shrink at room temperature. However, this is only a superficial phenomenon, and unstable factors are hidden inside this seemingly stable heat shrinkable film. 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 a stretched rubber band that rebounds after being relaxed. Heat shrinkable film manufacturers talk about the misunderstanding of packaging use, and the introduction of heat shrinkable film packaging