What’s the chemistry? Puncture exposes something that the sealant reacts with? In which case with a big hole (e.g. GCN’s nail hammering experiment) how does the middle of the hole get sealed?

Yours Mystified.
Fenlander

I should probably know this but I didn’t, so I did some research.
It seems that the active ingredient that does the sealing is the latex. In the sealant this is a liquid and it turns to a solid to seal a hole.
Actually, the liquid is a dispersion of small particles of solid latex mixed (not dissolved, it’s a bit different) with water. As the water evaporates, the small particles coalesce (join together) and form a solid lump.

A small amount of ammonia (which is alkaline, and thus prevents the liquid from becoming acidic) is usually added to the liquid to stop it solidifying too easily, I expect this affects the bonding in the liquid dispersion and makes all the tiny solid latex particles repel each other, which stops them sticking together into clumps prematurely.

And when you get a puncture, you’re relying on the small particles of latex being forced together by the bottleneck and coagulating to form a solid plug. Possibly this is accelerated by the drop in pressure from inside to outside the tyre. I expect you need a lump big enough for other bits to stick to it, until the entire hole is covered.

I should add that this is different to how paint works. They use a solvent (‘VOC’) to dissolve a solid (pigment) and make a liquid paint. As the solvent evaporates (that’s why paints smell strongly) the solid comes out of solution and is a solid again. Paint = liquid. Stans = dispersion of solid latex in liquid.
HTH.

And when you get a puncture, you’re relying on the small particles of latex being forced together by the bottleneck and coagulating to form a solid plug. Possibly this is accelerated by the drop in pressure from inside to outside the tyre. I expect you need a lump big enough for other bits to stick to it, until the entire hole is covered.

From my reading and experience that is the general gist of it. Which is also why over time it dries out but leaves a coating inside the tyre – some brand new tyres need a couple of additions as the first one is basically providing the air tight seal in the sidewall (Yes Conti we are looking at you)

Excellent thank you. Great to have an idea of how it works.

Slight hijack. I’ve always wondered why in bike tyres you seem to have to top up regularly (say every 3 months), but not in car tyres. Is there a difference in the approach somewhere? The top up faff puts me off tubeless.

Slight hijack. I’ve always wondered why in bike tyres you seem to have to top up regularly (say every 3 months), but not in car tyres. Is there a difference in the approach somewhere? The top up faff puts me off tubeless.

Do car tyres have sealant in them? Mine don’t.

You never see anyone at Kwik Fit doing the Stans Dance, so I reckon not.

So how do car tubeless work, just thick rubber?

/edit, ah yes would seem so…

https://en.wikipedia.org/wiki/Tubeless_tire

And when you get a puncture, you’re relying on the small particles of latex being forced together by the bottleneck and coagulating to form a solid plug. Possibly this is accelerated by the drop in pressure from inside to outside the tyre. I expect you need a lump big enough for other bits to stick to it, until the entire hole is covered.

Yes, and this is why it sometimes (joyfully) sprays out of the hole for while before there’s enough latex to plug the puncture.

UST bike tyres work like car tyres – airtight without sealant. But as bike tyres puncture so easily almost everyone adds sealant anyway.

Yes, and this is why it sometimes (joyfully) sprays out of the hole for while before there’s enough latex to plug the puncture.

Also why if you get a puncture that doesn’t seal quickly, then getting it hole down (so the liquid sealant can run to it) and also hammering some air in there asap can get it to fix.

The first bit makes sure there’s plenty of latex trying to get out of the hole, the second makes it all try to get out quickly. It’s a bit like trying to get a load of people through a door all at once, if they all queue up and file out nicely then eventually they’ll all leave, if everyone runs at it quickly two people will arrive at once, get themselves wedged in the doorway, and everyone else pushing from behind will just wedge them in further.

Reminds me of the queue for the ski lift at Mayrhofen 😉

I top up when I change or when it comes off, it’s also very easy as you can just remove the valve core, add more and replace the core, tyre will inflate as it’s not lost the bead.
Car tyres don’t have sealant but are considerably thicker and go flat if you remove the nail etc from them, my last ardent I removed had a few small sealant plugs in it that I’d never even registered so well worth it not to ruin a day out.

Reminds me of the queue for the ski lift at Mayrhofen

Reminds me of a queue anywhere in Europe.

There, I’ve finally done it, found a positive benefit of Brexit, we can bring back proper queuing.

Of I remember correctly theres slightly more to it than just latex particles in suspension.

Latex forms a shear thickening liquid (like custard, you can run on it but if you stop you sink). But unlike custard it doesnt go back to liquid again as the protein strings which were all dispersed are forced together, line up then spring back and get tangled, forming a solid mass.