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I know how to change a puncture, like most I've being do it since forever now. However I still don't know how it works. I know the 'glue' isn't glue in the sense that we would normally use it so how does it work? How dry does it need to be - do you need to leave it only 10 seconds or is 10minutes also ok?
Any chemists in?
The patch kinda melts into the rubber of the tube so it becomes like one piece of rubber, like a weld almost on a chemical level.
That's the extent of my in depth scientific understanding.
Leave for 5-10 minutes then stick patch on.
5 minutes on Google leads to this of Reddit:
Chemist here - natural rubber is a polymer (long chain-like molecules). Vulcanizing adds cross-links (through disulfide bonds) to the rubber, basically turning the strands of rubber molecules into a net, greatly increasing strength. Bike tubes are vulcanized rubber, but the outer surfaces are treated such that all those cross-linking sulfur groups aren't reaching out and trying to grab anything. You put on some vulcanizing fluid (henceforth "glue") and a few disulfide bonds in the tube get broken and re-formed with bonds to the polymers in the glue. Once the glue dries (there's a bit of solvent that has to evaporate) the inner side of the glue spot is chemically bound to the tire. The outer side is left with a bunch of free sulfur groups waiting to grab onto some other sulfur groups. Then you peel that piece of foil off the orange side of the tire patch (which exposes the free sulfur groups left on the patch) and press it to the glue spot - you've now made millions of chemical bonds between the patch and the glue spot. It's not really glued, though - the patch-"glue"-tire system is now one single molecule all chemically bound together.
The chemical bond holding things together is why:
The tube has to be clean and dry - the sulfur groups reaching out for something to grab onto will grab dirt, water, and other gunk instead of the patch.
You can't use duct tape or regular glue - these are sticky substances that don't vulcanize the rubber together. Rubber cement may hold a patch in place but it is NOT the same stuff.
Glueless patches kinda suck - the vulcanizing fluid in the little tubes works better at making bonds with the punctured bike tube.
You can make patches out of old tubes - at its most basic you're vulcanizing two pieces of rubber together, so two pieces of bike tube will stick to each other.
Used to be far too eager to apply patch to glue, now at least 3mins, far higher success rate.
Pretty much what beej said really.
Adhesives chemically bind two surfaces. The reason you wait before applying the patch is to allow the solvent to evaporate. It's only in the glue to help application/spreading. Once it's gone you can stick the patch. I probably only wait a few seconds. Apply the glue, prepare the patch (remove foil etc.) then stick it on.
As with all adhesion situations surface prep is important. Clean around the hole. Roughen if you can and don't handle the patch too much. Firm pressure to ensure a good contact over the entire surface of the patch. Give it time to react once on. Wait a decent amount of time before inflating. If at home I normally give it an hour.
I've never found old tubes to work that well. When you get the patches with the red and black side, the red side is a softer rubber better suited to sticking to the tube.
I'm a paint chemist, which is pretty much a glue chemist.
If you happen to have the glue handy: Does it improve the success rate with clueless patches?
Don't forget to grate the chalk over the excess 'vulcanised' area.
@beej - thanks, that was exactly what I was looking for. I did have a good hunt around but I forgot what a great source of info. reddit is