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?

Better Living Through Radiochemistry

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.

A Series of Tubes

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.

Adding Value

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.

Shrink Tube Kits from various vendors
Left to right: Sparkfun // Digikey // Adafruit

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.

 

74 thoughts on “Heat Shrink Tubing and the Chemistry Behind Its Magic

      1. Oh, it still is expensive but it’s worth every penny when you need it. If you actually need rated equipment (i.e. marine, mining, general heavy industry), Raychem is durable as hell against the elements (UV, soil abrasion, whatever), offers strain relief in the terminators themselves, etc. You’re already paying a 4 bucks a foot for that 6 AWG/3 UL listed trailing cable, a few more bucks to terminate stuff properly is a no-brainer.

        They even offer awesome fiber terminators that splice on like magic to the Corning SMF-28 ULL cables, which you’re already paying an arm and a leg for – when you tally up the the zoning permits, the trenching, the peering, the labor, a few hundred dollars on a terminator is chump change to guarantee .25db insertion loss (usually around ~.18).

        1. the brand name stuff i have run across most often have been 3M stuff, that said we have been supplied a wide range of weird heatshrink, some was even conductive, for use on the copper grounding circuit in steel structures, a little resistance means a lot when everything around is a conductor.

          the tube that comes with a heat actived glue inside is especially nice for securing connectors.

    1. If you buy the little hobby shop “kits” it’s expensive. Buying in bulk is dirt cheap. On ebay I can get 25-50 feet for only 20 bucks. Comes in a nice storage box too.

      1. That’s more expensive than signal wire like Cat5e. 500ft for $50, you can usually find 100ft for $10 plus connectors.

        I assure you 50ft for $20 is not “dirt cheap”. Luckily you don’t need very much shrink tubing for most projects. Although for rewiring my motorcycle/dirtbike, I’ve chosen to run the tubing the entire length of the wire bundles for all exposed sections.

    2. Exactly, it’s dirt cheap everywhere I have been able to buy it. I am guessing the writer is in The Ukraine or another war torn country where rebels have to smuggle it in?

  1. HOT TIP! Use a thin layer of hot melt glue on your connection. Slip shrink over glue and it;s best shrunk with a heat gun. Benefits? You make yur own self-encapsulating shrink. 1) You.re now waterproof. 2) You’re strain relieved too. Stranded wire flexes some, but at the point it is soldered, it becomes solid wire which tolerates little flexing before breaking. Harbor Freight sells a shrink asst. black, 4 ft. long, largest is 1 inch dia 6 pcs. about $6 on sale. The 1 inch slips over most connectors

    1. I used the hot melt glue trick when installing a Neutrik powerCON connector on the bumper of my diesel truck for powering the block heater. Worked great. The back of the connector was exposed to the wheel well, so I used several nested layers of heat shrink, each layer being a different diameter. In between layers I’d apply hot melt glue. I ended up with something I felt was very robust and would likely be as close to waterproof as I could get. There’s an element of satisfaction associated with heating the heat shrink and seeing the smallest amount of hot melt glue ooze out… you know it’s sealed well.

      1. Save the hot glue. Use the right tool for the job. Marine grade heat shrink, mentioned in the article, is lined with hot melt like glue. It is designed from the start to create waterproof connections. You usually get a couple of pieces with a well pump to connect the wires that will be underwater for years. (220vac @ 60A)

        Costs more but JFW every time. I use it in all the coax that goes up the tower it is buried.

        1. Until I learned all that stuff (and the marine version may not have even existed then), I’d pack the tube with silicon seal before I shrunk it – would squeeze a bit out but the connections (intended for buried yard-light wiring in very damp soil) are going strong after decades of use. My dad was a machinists’ mate in the navy in WWII and he said they used to pack all ship’s connectors with grease – if water can’t get in, corrosion didn’t happen. Works beautifully on boat trailer lights etc.

          1. In Australia, we aren’t allowed to use silicone caulking to seal underground or underwater electrical joints.
            The silicone absorbs a bit of the water and it’s only a matter of time before it goes pop.

            We used to use barrel crimps to join the wire, some glue heatshrink to seal the crimp joints and then pot the whole thing in epoxy. You can buy kits that have all the stuff to do a join

        2. I used to do a lot of tower work, installing VHF communications antennas. We never used heat shrink tubing for any of the outdoor connections (though we never buried coax either). It’s always self-fusing rubber (or now, silicone) tape, overwrapped with plain but good vinyl tape for sun protection. Here’s a video that shows how it’s done: https://youtu.be/vwjEnyFSJEY?t=14
          We usually used more layers than that though. Tape is cheap compared to getting to the site and up the tower.

          1. I second the self fusing tape with vinyl over-wrap for big stuff. I used a ton of it installing a massive 480V delta transformer and making connections to the branch circuits for welding machines.

            I do like the double wall heat shrink for low voltage wiring, haven’t seen it fail yet.

          2. Agreed. I used to work alongside electricians hooking up 5000hp motors fed by 13.8KV feeds (steel mill water pump station), self-amalgamating tape covered by friction tape (vinyl),

            They would use BOXES of the stuff for a single hookup.

          3. Yep, we use the same for ‘quick’ repairs on deep underwater (10k+ ft) instruments. 2-3 layers of vulcanizing/self-fusing tape, scotchcote between each layer, with extra-thick vinyl tape for abrasion protection. It’s messy but works wonders.

            Proper long-term fixes or permanent splices are potted and molded in polyurethane.

        3. Of course you should use the stuff if you have it available. But more often I haven’t when I do some repair at home on something like Saturday afternoon or Sunday. So i happily used the hot glue. It is more effort than using a proper glue lined tubing, but it’s worth it when it enables me to finish the job.

      2. Hot “glue” Plastic is not a glue and has excellent peal-ability. Epoxy or rtv silicone are much better. That hot snot is fitting for dried flowers and 3D arts and crafts sessions.

        1. In the name of whatever you call holy, don’t ever use ordinary RTV silicone on electrical connections. The normal smelly hardware store stuff is almost always the acetic acid cure variety, and causes corrosion on bare wire, especially fine wire and bare metal on PCBs. If you must pot, use a 2-part silicone. If you absolutely must use the RTV stuff, use the “low odor” type, which is a methanol cure, and let it set completely (days) before hermetically encapsulating it in something like heatshrink.

          1. You can buy tubes of silicone sealer used to make fish tanks and that is non-acidic otherwise it would kill the fish!

            There are two types of hot melt “glue” on the markets. One is actually just plastic and has no glue at all. The other has glue mixed into the plastic and works quite well for some things. The ‘plastic only’ is useless as you can very easily just break it off what you stick it to. The only place the glue-less sticks would be useful is where it enters cavities that will not allow the cooled of glue to escape.

            The two have a slightly different shade and probably a slightly different color as well.

        2. plain hot glue sticks did the trick for me for potting guitar pickups. It’s not as messy as wax, and it’s not too hard to peel it apart with a heatgun (and gloves) if you’re unhappy with it.

    2. I’ve encased small pcbs this way to help them hold up in factory environments. Yes, servicing them isn’t possible but the protection keeps the need for service to almost nothing and they’re cheap enough to throw away

    3. Or, if you’re really lazy, you can cut up little strips of hot-glue, and just shove ’em in the tubing with the wire…
      It’s *great* when you’ve got some wires which need a bit of spacing… e.g. heat-shrinking a housing-less micro USB connector after soldering wires onto its tabs, shove little slivers of hot-melt glue between each wire, adds some strain-relief. Also helps the heatshrink to remain stuck to the housing-less connector. Oversize it a bit, then cut down with a razor-knife (the hot-melt-“plastic” is better, in this case, than the hot-melt-glue).

    1. Solder sleeves! Those used to be mindbogglingly expensive, and maybe the ones with pedigree papers still are, but I’ve picked up a few assortments for cheap. The solder alloy in the expensive ones is probably better, but if I pre-flux the wires just a teensy bit, the cheap ones work fine.

      They are great for attaching drain wires to coax shields, among other fiddly tasks.

    2. When I was in the Air Force we used a version of this to terminate a coax to a Cannon plug. It had two leads, one for the center and one for the shield.We used a hot air gun that used compressed air. The were tricky to install, and you knew you were in for a treat when you had to replace one.

    3. Never seen those before, but I used and loved something similar back in my mechanic days: crimp connectors with a slug of solder in the middle and heat shrink on the outside. Crimp like any other crimp connector, then heat and the solder flows and the tubing shrinks and life’s good.

  2. Decent H/S tubing is wonderous stuff where used correctly. Use a flame-rated polyolefin where insulating > 40V or >4A or >24VA. Cheap H/S can be a fire or shock hazard, and outgasses some rather nasty toxic stuff when stressed, and will degrade rapidly if exposed to UV/sun/etc.

  3. I buy those cheap bulk packs on ebay with all different sizes the only problem is though I always get to the last few larger sizes and don’t have a use for them so I started using them for cable management it works really well. cool article btw.

  4. “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.”

    Skip to 3:30, it shows him using it to melt heat shrink…

  5. Smooth move Hackaday. Now everyone knows it’s atomic! Heatshrink will be banned in the UK and California. Ironic, considering everyone in Silicon Valley drives past Raychem sooner ot later.

    1. Some versions can be heated and stretched, but you have to apply force evenly, and using tools with no sharp edges of points.

      At my work we have done so a few times, when a supplier forgot to install the HS tubing before crimping fittings into place.
      Definitely not the preferred method of getting HS tubing onto hoses, and a bit fiddly. Typically have one or two split when being stretched, for each that is successful.

    2. Even better… if you’ve got a tube that’s slightly too small for your needs, or you need more than a 2:1 ratio, and have a pair of needlenose pliers and some patience, you can stretch most heatshrink quite a bit further than its unshrunk size. Don’t need to apply heat to do-so. Though, of course, overstretching causes rippage.

      1. hah, I guess I just reworded Josh’s statement ;)
        Right, need a huge shrinkage-ratio to slip over an already-connected connector. Always wondered about those sheets, seems like they’d be perfect for damaged outer insulation on vacuum cords, etc.

  6. In other words it’s not heat shrink tube it’s heat return to original size tube. ;)

    Kroy makes fancy label printers that can thermal print on various types of tape, and onto heat shrink tube. Some years ago, I found a Kroy handheld printer (built in keyboard and an RS232 connection) for $10 at a thrift store. This being BSP (Before SmartPhones) I went home to look it up online and see how much the tape cartridges were. $20 each, no matter what contents – or who was selling them. $20 from Kroy and ALL vendors selling them. Can you say Price Fixing? “Eff that!” I said and didn’t go back to buy it.

  7. The best RTV going for general purpose electronics use is Dow 3145. As to heat shrink devices, nothing beats the Raychem AA400. It operates by heating compressed air in a pencil handset and allows very precise shrinking without melting everything in the near vicinity. Both are pricey, 3145 at about $30 a tube and a used AA400 north of $250 on ebay.

  8. back in the 80’s, the gun shop I worked in had a gas leak, after a LOT of hassling the gas company they finally fixed it.
    it turned out to be a gas line to what were the old stables and their gas lights!

    they dug up the footpath, uncovered the 1800’s era gas main, cleaned it up, wrapped it in a heat shrink sheet, then used a shower rose sized propane burner to shrink it.

    the “stuff” had the 3M logo all over it, I’d used heaps of heat shrink tubing but had never seen it in sheets like that.

    1. I have seen similar when the utility company fixed/spliced power cables which are buried in the street. Sometimes they even have preformed heat shrink elements to terminate a cable into 4 single conductors, something like 240mm² cross section each. And always a big propane burner for shrinking.
      I myself think I use the lighter more often than a hot air gun for this, just because of convenience.

    2. Marinas heatshrink boats the same way – well, minus digging up the footpath, that is. The stuff comes on rolls and we used the same kind of propane burner/flamethrower to shrink it.

      1. You can heatshrink a boat!? Didn’t know that! Reminds me of that artist bloke who wrapped an entire island in plastic. If he’d have brought in a squad with flamethrowers it would’ve saved on cleaning it while the owner was away.

  9. I did hear that one recognized technique is to knot small bundles of strands together, then shrink over that.
    Makes a mechanically strong connection without soldering which is handy if combined with the glue method, also IIRC NASA specifies that method because it distributes the connection over several locations so that the wire can’t short out against adjacent wires.
    A related method I often use on my repaired laptop mains adaptors is to solder the ground pin in 3 locations (wiith 1/3 of the strands) and double heatshrink over the centre contact, then heatshrink or Polymorph over the outer.
    Never had one fail yet however it does not guard against lead chafing at the adaptor end.
    One thing to watch out for though is that some plastics (notably brown hot melt glue) if heated over years can go conductive.

    1. I’ve never heard that, and I’ve never found heatshrink tubing that can shrink tightly enough for what I would consider to be reliable enough longterm.

      Use the proper crimp, then cover with heatshrink.

  10. Nuclear grade Raychem. I have used about a million US Dollars worth since 1980. The fracking HP’s would throw away any yellow bag left unattended, so I bet they disposed of far more that that. Also we used heat guns because you can’t have an open flame without a fire watch at a nuclear power plant in the US. Covering wire with a thick Kapton coating with Raychem heat shrink and then covering all that with Nuclear grade Raychem with all the special shims and such. So much fun.

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