Heat Shrink Tubing

How does heat shrink tubing work?
- Cynthia (age 17)
uiuc, Champaign

The previous answer here was that when the monomers in the plastic link up to make polymers, the plastic volume decreases. If that’s true at all, and it may well be completely false, it accounts for very little of the heat-shrink effect.
Heat-shrink tubing can shrink more than a factor of two in diameter, thickening its walls while hardly changing its length. That’s a special effect, much more dramatic than any volume changes. According to Wikipedia, the key is to forcibly stretch out some narrow plastic tube. It gets stuck in the stretched shape, even though it still has some cross-links that are trying to pull it back to the smaller diameter. When it’s heated up it loosens up enough to slide back to its most stable shape, which is near the original narrow diameter.

Mike W.

(published on 10/22/2007)

I had always wondered how heat-shrink tubing worked, so I was glad to see an answer posted on your site. However, your response provokes yet another question. You said that "Heat shrink tubing is made up of plastic that is only partially polymerized... You can think of it as if the plastic is only part-way formed before exposed to the heat." If this is so, then why is it not a goopy mess like all those semi-gel plastic toys--sticky worms, etc.--which are also partially polymerized? I’ve always thought of "partially polymerized" as being synonymous with "not completely solidified." Yet shrinkable tubing is clearly quite stable in its unshrunk form. How is this possible?
- Andy Baird (age 50)
Princeton, NJ, US

Your question is very good. The old answer you saw was wrong. The partially polymerized glop you mention is indeed quite a bit different from any heat-shrink tubing I’ve ever seen.

Mike W.

(published on 10/22/2007)

HI, On Wikipedia and this site, there is a preconceived notion that heat shrink tubing is made of monomers and partially polymerized material. Then by heating the tube, it continues its polymerization. I believe this statement to be completely false. I run a website called BuyHeatShrink.com http://www.buyheatshrink.com. I pride myself in not only selling products, but educating my customers. I am interested in an open discussion on this subject. If I"m wrong, I"ll happily correct the information on my site. Back to the subject of incorrect information: If the material is partially polymerized or un-polymerized, how is it that a heat gun operating at a few hundred degrees fahrenheit can polymerize the material after 30 seconds exposure, but an extruder (used to make the tubing) can"t polymerize the material at hotter temperatures for sometimes 8-10 minutes? The following information was retreived from http://www.buyheatshrink.com How is Shrink tubing manufactured? Basically, a special thermoplastic compound (i.e. plastic pellets) is extruded and made into a tube. The tube is then exposed to high intensity radiation, and expanded to twice its original diameter. This tubing is supplied in what"s called the "expanded" state. In some cases the expansion may be greater than 2:1 . Such heat shrink tubes are supplied in an "overexpanded" state. Common types include 3:1 heat shrink , 4:1 heat shrink , and even 6:1 heat shrink. Adhesive lined heat shrink (also known as dual wall heat shrink tubing) is also readily available. When the heat shrink tube is supplied in an expanded state, it has what is called "memory". This means that when it is exposed to high heat (commonly referred to as the shrink temperature or activation temperature), it will relax back to its original diameter. This is called the recovered state.
- Marc (age 38)
NJ

Marc- Thanks. But if you read carefully, you’ll see that we had already rejected and replaced the incorrect explanation about polymerization. The main difference between what you write and what we have above is the point about irradiation. Do you know what role that plays?  Is it applied in the original state to create cross-links, providing memory of the state?
Mike W.

(published on 02/04/2008)

In the 90's I worked at the Thermofit facility at the Raychem campus in Menlo Park and as an IT support contractor I had access to the entire facility to provide equipment support. There is no doubt about the use of raditation in the process. There were two reactors in the facility and badging and protective clothing was required to access that area.
- Ron (age )
Danville, Ca, USA

Thanks for the info.

Mike W.

(published on 09/22/2009)

Is it possible to return the tubing to its expanded state? How?
- Joy (age 28)
Cavite, Philippines

I doubt that there's any practical way to reverse this process fully, since the whole system is far from equilibrium. It's possible, however, that if the tubing were expanded out while hot (say using a tapered rod) then cooled in the expanded state, that one could get it to shrink again on reheating (after the rod was removed). Somebody who knows more about the details might know how well this works.

Mike W.

(published on 09/30/2010)

Im not a chemist by any means my backround is farming. Trying to find a solution to a problem i had i cast plastic blocks using 2 types of thermoset plastics. Attempting to get a sturdy nonconductive mount for a convyor. Was playing arround with a scrap peice and found it reacted much the way heat shrink did after crushing a 1 inch by 1 inch block a 200 ton press to about 1/8 inch thick and then applying heat(just below the melting point) it would return to very neer its origional shape. I was able to repeat this process multiple times. The 2 plastics (and i cant remember the names) physical properties were, long coiled chemical chains much like springs or slinkeys with high melting point, and the second was a very fluid with very short chemical chains and a low melting point. Mixed 45-1 respectivly. Could this farmer have stumbled upon heat shrink? I havent yet found a good answer as to just what heat shrink is or its reaction to heat. I have a very mechanical mind and am currious about the mechanics at a molecular leval.
- Ben (age 26)
Hemlock,MI.USA

This is a really interesting result. It sure sounds like you made something with the same basic physics as heat shrink material. Here's our best guess of what's happening.

The crushed plastic is obviously in a very different shape than how it started. Apparently there are a lot of weak bonds keeping it in the new shape. When you heat it up, these bonds can shake loose, letting the plastic take on another shape. Since it looks a lot like the starting shape, that indicates that a network of strong bonds was left from the initial shape, and these relax back to their most comfortable position once the weaker bonds are loosened.

Very cool!


Mike W.

(published on 11/30/2012)

Here's how heat-shrink tubing works. You extrude a tube of a crosslinkable, semi-crystalline polymer whose crystals melt a bit above the service temperature. A mixture of an ethylene elastomer and HDPE is usual. You crosslink the polymer tube, for instance using irradiation. Then you heat the tube and stretch the it to the larger diameter. While stretched, cool it down to freeze it into the new shape using the crystals to keep it in the larger diameter. Then, when you're ready, use a hair dryer or heat gun to melt the crystals and allow the crosslinked polymer molecules to pull it back to the size it had before your stretched it. Try this with a crisp or sweet packet, or the wrap from a soft drink bottle. Put a stick in the packet, and >>carefullycare: the label will be hot afterwards- do not touch it until it has cooled down again
- Tim Clayfield (age 48)
Switzerland

Thanks for this informed description.. It fits well with our other information.

Mike W.

(published on 06/10/2013)