I've heard from others that air is what causes sealant to seal. People have said they've read that "somewhere" but I never seem to be able to find those articles.

The reason I think air can't be the sole factor is that there is air inside the tires—in mountain bike tires there is a lot of it. Why wouldn't it just coagulate there? And if you say, well, over time it dries out, that's true—but a puncture doesn't take months to seal.

On top of that, I've accidentally left a bottle of orange sealant with the cap off literally for months, and there was only the barest trace of "rubbery" sealant on the threads of the cap neck.

I imagine this must have something to do with either

  • the cold produced by the expanding gas coming out of a puncture, or
  • the low pressure caused by the expanding gas moving rapidly (see Bernoulli's Principle)
  • some other unexplained factor.

The reason I want to know is because if I knew the principle behind sealant sealing I might be able to figure out a way to prevent undetectable slow leaks, which I've been getting off and on since I switched to tubeless on my road bike.

  • 3
    This is a good question, I was wondering the same for a while. Commented Mar 8, 2019 at 3:16
  • I may be totally wrong, but I guess that it might be primarily the high viscosity that keeps the sealant from escaping quickly. Also, the sealants I used when I still used them (before I switched to puncture resistant tires, which was an excellent riddance) contained both tiny grains of rubber and thin strands of fiber. Both small enough to just flow through a big hole, but for a small hole they would get trapped in/before the hole as the liquid slowly leaked out, obstructing the liquid's path. As far as I remember, the seals would never become perfect, always keeping leaking extremely slowly. Commented Mar 8, 2019 at 3:58
  • 3
    Sealants are based on Latex. I'm no chemist, so im just quoting Wikipedia: "Latex is a stable dispersion (emulsion) of polymer microparticles in an aqueous medium." "Synthetic latexes are used in coatings (e.g. latex paint) and glues because they solidify by coalescence of the polymer particles as the water evaporates, and therefore can form films without releasing potentially toxic organic solvents in the environment." The details on how it drys so fast would be a good question for Chemistry Stack Exchange.
    – airace3
    Commented Mar 8, 2019 at 6:53
  • 1
    Re the slow leaks - try laying your bike on its side and slowly spin the wheel. Could be the sealant isn't over the microholes, which could be on the bead or even on the rim valley/nipple holes.
    – Criggie
    Commented Mar 8, 2019 at 8:00
  • 1
    @Criggie The bigger the bucket/tub/whatever, the more of the wheel you can submerge, making leak searching easier and faster. Because you have to submerge as much as you can, knock off any bubbles, then wait for new bubbles to keep forming at the same location. That can be a real pain if you can only effectively submerge 1/20th or less of the rim/tire joint at any one time. Not sure I'd want to actually submerge the hub, though. Commented Mar 8, 2019 at 22:15

1 Answer 1


Here's a photo of sealant failing inside a tyre. These are the latex molecules lumping up together rather than staying in suspension in the carrier liquid which is glycol-based but varies by brand.

These have the glorious name of "boogers" and are a sign the sealant is not going to work very well. This tyre should be cleaned out completely and a new serve of sealant added.

enter image description here

This is what should happen at the hole when a tyre punctures.

The carrier is either a water-based glycol/propylene with cellulose and thickener agents like aramid/ceramic fibres, or non-water based latex mix which doesn't need extra thickeners but does have a limited effective life. Latex also has some level of Ammonia which can contribute to corrosion. Either product will distribute around the inside of the tyre while you ride. This leaves a thin layer waiting just inside the area where the puncture forms.

The lighter-weight carrier fluids exit the hole along with the escaping air. This leaves a visible wet spot on the outside.

From http://www.utahmountainbiking.com/fix/sealant-failures.htm

As the carrier flows through the hole with the air, the heavier/larger suspended particles get stuck and form a blockage, which slows the air leakage and eventually seals. In theory, the plug will flex and more air will leak, but the sealant will do the same, continuously packing into the weak spots and stopping the leak.

This only works if the sealant is liquid. The carrier is volatile and will bleed out through your tyre over time anyway, leaving a dried mess in the tyre. If your bike is not ridden much, the sealant will pool at the bottom and that's not a lot of use plus it will unbalance the tyre.

Many sealants can be re-energised by adding more fresh product, but mixing brands is an unknown because the chemistry will vary between brands. Best to dismount the tyre and do a complete clean before changing brands. Some riders recommend changing tyres every 12 months regardless of wear.

  • 2
    Please critique this answer - I personally don't ride tubeless cos my rims are all too old, and I don't like the mess seen on other people's tubeless bikes.
    – Criggie
    Commented Mar 8, 2019 at 22:47
  • 1
    Thanks. This makes sense. I ride around 10,000 miles (16,000 km) a year, so my tires don't last long enough to get "boogers" in them. ^_^
    – Robusto
    Commented Mar 9, 2019 at 15:00

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