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Heat shrink tubing is a type of plastic tubing whose diameter shrinks when it is heated. The materials used to make heat shrink tubing vary, but include complex polymers and similar materials. Heat shrink tubing is most commonly used in electrical applications in order to protect, insulate, or repair wires, terminals, and other types of electrical connections. It extends the life of these items and ensures their good performance.
Depending on the exact type of material used, there are two main ways that heat shrink tubing can work. Tubing can be specially treated during the manufacturing process to shrink when heated. This type of tubing is said to be expansion-based. When it is manufactured, it is heated to near its melting point and stretched to expand its diameter. It is then rapidly cooled in order to help it keep its shape. When heated later in consumer use, the tubing will shrink back down to the size it was before it was stretched in the first place.
There is some suggestion that heat shrink tubing can be made through a process that involves a change in the chemical structure of the material itself. In this type of tubing, many monomers — single molecules of a certain compound — will group together into chains when heated. This process is called polymerization. As the monomers become more closely joined together, the material effectively shrinks. The difference in size before and after polymerization is not dramatic, however, so it seems likely that nearly all heat shrink tubing is at least in part expansion-based.
Every kind of heat shrink tubing on the market has its own shrink ratio, meaning the degree to which it will shrink. Tubing with a two-to-one shrink ratio, for example, will shrink to half its original size. One that has a six-to-one shrink ration will shrink to one-sixth its original size. The needed shrink ratio will depend on the specific application the tubing is used for. Heat shrink tubing has numerous common uses, such as insulating outdoor wires and cables against the elements, grouping multiple wires together, and color-coding wires for easy identification.
The Raychem Corporation was the original developer of heat shrink tubing in the 1950s. While all plastics shrink when heated, the company developed a way to treat plastics to make them shrink, but not melt or degrade, even in very high temperatures. They discovered that when certain polymers are exposed to radiation, they develop stronger internal chemical bonds. This process became known as cross-linking, and it gives the tubing the ability to shrink down to its original dimensions, as well as giving it very high strength and durability.