How to store opened PU sealant cartridges?

Question

I usually have an opened, partially used 300 ml cartridge of polyurethane (PU) sealant around that I slowly use for various repairs. It's the type of cartridges used in a sealant gun.

Now PU cures by reacting with humidity in the air and till now, I could not find a way to store them for longer than 3-4 months without the content starting to harden and going to waste. Ideally I'd like to store them for up to 3 years. That would be longer than the shelf life of even an unopened cartridge. (And if I'd have to remove the sealant from the cartridge for storage, that would also be ok.)

Failure modes: Often the hardening starts in the upper part and slowly progresses downwards. So when poking through the upper part with a nail one might still find soft sealant to use. But sometimes all of the sealant will become more viscous at the same time. It would then not harden completely (yet) but cannot be used with a manual sealant gun anymore. Unopened cartridges seem to harden from humidity entering around the end cap – the piece pushed along once starting to use the sealant gun.

What I tried: I replaced the tip of the cartridge with a screwed on, tight plastic cap for storage. It helps somewhat but is not a reliable or long-term solution, also not even when covering the thread in sealant before. It seems that everything that can transport even small amounts of humidity (and hardened PU sealant does) is sufficient to allow hardening over time. For this reason, methods used for silicone cartridges (woodscrew in the nozzle etc.) don't work for PU sealant: hardened silicone protects the silicone below, but this is not true for PU sealant.

Answer 1

My thoughts go along the line 'seal the cap'.

Whatever you are using, make sure it doesn't react with PU (or in its vapours, in this case).

Of what I know wax is non-reactive with other chemicals and may work well in this case (but I am NOT a chemist, so check that first). Another thing that comes to my mind is some kind of silicone (like the one in hot-glue guns for example), but I have no idea if silicone reacts with PU.

(This is just a suggestion; I've never tried it.)

Answer 2

With silicone sealant cartridges, I've got into the habit of just letting the silicone cure in the nozzle then replacing the nozzle next time I want to use the product. The cured silicone makes a very good seal and the product stays completely usable in the tube. I usually buy the replacement nozzles for around £0.07 each in packs of 10 or 12. Sometimes, I drive a screw into the hardened silicone an cleanly pull it from the nozzle for re-use.

I'll admit I've never tried this with filler/gripper/adhesive tubes, but it might be worth a try.

Answer 3

I store all such compounds upside-down, so that any air which enters is at the bottom of the container and that which is near the nozzle is still liquid. I then unscrew the nozzle, put some saran or other plastic over the top and replace the nozzle or cap to further isolate the contents.

Answer 4

The best moisture barrier is oil. Remove the screw-on cap, cover opening with duct tape to contain the unused contents. Place cartridge vertically into a jar of oil just deep enough to cover the end of the cartridge. Any kind of oil works...olive oil, motor oil etc. The sealant stays like a fresh cartridge, all of it.

Answer 5

You can store opened PU sealant in a fridge or freezer. Freezing does not seem to damage the sealant.

People who tried it report that it works because air humidity is very low in a fridge – and since it's even lower in a freezer, I suspect it works best there. Also, lower temperatures slow down all chemical reactions, so this might be another reason for why and how this works.

Quotes from my source, where I just found out about this technique:

Freezer definitely works. I keep the Sikaflex and 3M 3200 next to the Freezer cold plate which gets down to -25 deg C. I've never had one go hard, but they are never more than 18 months old either. Do expect a little oozing of sealant as the tube warms up though. […]

I've just been using a small tube of Sikaflex that I opened a year ago. It's been kept in the freezer with a thin plastic bag screwed between the tube and the nozzle. I keep all my sealants in the freezer which is one of the excellent tips I have picked up from this forum over the years. […]

I use 20 to 40 600 ml sausages a month and as soon as it's delivered I stick [them] in an old bar fridge in the shed, the stuff at the back is well over 6 months old and still good. Moisture/humidity inside a fridge is very low and it needs moisture to cure, so keeping it refrigerated will help.

Answer 6

I take a wood screw of an appropriate size and insert it into the open end. The sealant that dries is lodged in the thread and comes out with the screw when you want to use the sealant again.

Answer 7

Use PU construction glue instead of sealant for small jobs that won't finish a whole cartridge.

I've read in a forum somewhere, and I can confirm from own experience now, that PU construction glue (such as Sikaflex 252) has less issues with hardening in cartridges during storage, compared to PU sealant (such as Sikaflex 221). The difference is just that PU construction glue is more viscous, stronger after hardening and more expensive. For most purposes, PU construction glue can replace PU sealant without issue.¹ It may be more expensive but is also more versatile as it can be used both as glue and sealant, plus you save the cost of half-empty hardened PU sealant cartridges.

For PU construction glue, any technique that works to protect opened silicone cartridges will also work here. Personally I seal the thread of the nozzle with some PU construction glue before screwing it on, and place a tight-fitting nail or old drill into the open end of the nozzle that can be pulled out to re-open the cartridge.

Update after more testing: This technique does not work that well. The PU glue will not harden through from the nozzle like Sikaflex 221 sealant would, but the whole mass can become more viscous over time (making it difficult to press out). Happened to me after about a year of storage, and when it happens, at least Sikaflex 252 will also take much longer to properly cure once applied (weeks) and parts may not cure properly at all, staying soft and squishy.

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¹ A possible exception is where the PU mass is applied very thick (>7-10 mm maybe). Through-hardening of PU construction glue may then take very long then or not happen at all. Please do your own tests. It will obviously depend on the brand and the circumstances. Also, if this turns out to be an issue, you can try mixing a tiny amount of water into the PU prior to application, as humidity is how liquid PU hardens.

Answer 8

Store the cartridge in a small airtight container together with a drying agent like silica gel. (This is just an idea so far, I'll report how it goes.)

This idea comes from the observation that some commercial PU sealant cartridges in their unopened form come with a double end cap at the very end of the cartridge. One has to remove it before use, and also the drying agent found between it and the moving part. So drying agent is used to prolong the shelf life, and it probably also does so for opened cartridges. Using a small container to enclose less humidity and generous amounts of high-grade drying agent should help.

I also found confirmations of this technique from others around the web:

Another way to stop it going off is to store it in an airtight container containing active silica gel; no moisture, no cure, plus it's ready to use […]

I can second this method. It works very well. I use a large BDS container which holds a part used tube of silicone, one of Sikaflex and an unopened tube of each when running low. All fresh and ready to use. If unused for a long while I may have to clear out the nozzle on the Sikaflex tube. (source)