Heat Shrink Tubing And The Chemistry Behind Its Magic

There's a lot to be said in favor of getting kids involved in hacking as young as possible, but there is one thing about working in electronics that I believe is best left as a mystery until at least the teenage years — hide the shrink tube. Teach them to breadboard, have them learn resistor color codes and Ohm's Law, and even teach them to solder. But don't you dare let them near the heat shrink tubing. Foolishly reveal that magical stuff to kids, and if there's a heat source anywhere nearby I guarantee they’ll blow through your entire stock of the expensive stuff the minute you turn your back. Ask me how I know.

I jest, but only partly. There really is something fun about applying heat shrink tubing, and there's no denying how satisfying a termination can be when it's hermetically sealed inside that little piece of inexplicably expensive tubing. But how does the stuff even work in the first place?

Like a lot of things in electronics today, heat shrink tubing was a product of the Cold War era. In the mid-1950s, Paul Cook, a chemical engineer with experience in the radiation treatment of polymers, started a company to develop commercial applications for radiochemistry, the aptly named Raychem Corporation.

One of Cook's key innovations was in the field of cross-linking polymers. Recall that polymers are just long chains of small subunits. In the case of plastics, most subunits are small organic monomers; vinyl is polymerized into polyvinyl chloride, or urethane becomes polyurethane. These chains can be many hundreds or thousands of monomers in length, and the number and orientation of the chains determine in large part the properties of the material. But polymer chains can also bind across their length, or cross-link, forming networks of chains and resulting in different properties for the material.

Cross-linking can be accomplished by many means: heat, the addition of chemical cross-linking compounds, or change in pressure or pH. Radiation can also be used to form cross-links, and this is where Cook's expertise came to play. He knew that cross-linking certain plastics with radiation could change their thermal properties and induce a memory in the plastic. The cross-linked plastic could then be heated past its previous melting point, stretched, and cooled. Crystals would form to lock in the expanded shape, but when later heated, the crystals would melt, releasing the energy stored in the cross-links and returning the plastic to its pre-stretched dimensions.

Processes obviously vary by manufacturer, but most modern heat shrink tubing is created basically the same way. Plastic pellets are heated and extruded into a tube with the diameter and wall thickness of the desired final shrunk dimensions. Cross-linking by irradiation occurs after extrusion, while chemical cross-linking occurs during the plastic formulation and extrusion phase. Which type of radiation is used depends on the plastic, and are generally trade secrets. PVC, polyethylene, polyamides, and others can all be cross-linked by electron beam processing, while other polymers need an alpha or gamma source, or even UV light or RF radiation.

The cross-linked tube is then stretched, usually by air pressure, to the desired pre-shrunk dimension. A lot of tubing is expanded to twice its original diameter, in which case it is referred to as "2:1" tubing. The expanded tubing is cooled, locking in the crystalline structure until heated again at application.

Aside from the properties of the plastic itself and its shrinking characteristics, manufacturers have added a number of specialized treatments to heat shrink tubing over the years. Colorants are often added to allow end users to color code connections, although clear tubing has applications where inspection of what's inside the finished connection is important. UV blocking compounds are added to tubing intended for outdoor applications. Sometimes an adhesive lining is co-extruded with the main tubing, often a heat-activated one. When the tubing is reheated, the adhesive lining melts as the tubing shrinks, forming a watertight seal. The same approach can be used to create a conductive lining, either with conductive polymers or actual solder. Manufacturers are now even custom printing heat shrink tubing so that users can identify connections.

Application processes differ by tubing specifications, of course, but applying heat is a pretty basic process. Personally, I prefer a good quality heat gun, but in a pinch a hair dryer can be used. I tend to avoid open flames like matches and lighters because I always seem to scorch the tubing or melt the insulation on the wires. I’ve also had limited luck using soldering irons, but [W2AEW] recently reviewed a butane-powered soldering iron with a nozzle attachment that I bet would make a dandy cordless heat gun for heat shrink applications.