My bicycle was simply sitting at rest indoors when it exploded. A slit about ~6mm long was in the tube. I thought it might be too big to patch, but I happen to have some quit big patches to try. I cleaned the area w/denatured alcohol then roughened the surface w/the sanding tool. I covered the whole patch footprint area with rubber cement. Around 90 sec. later applied the patch and lightly clamped it down. About 5 min later: installed the tube and inflated to 4.5 bar.
Couple min later it exploded again. As the image shows, it blew exactly along the line of the original slit. AFAICT, I could not have applied this patch better. That is, the air went through the patch instead of around it.
Why did this happen? The patch is thicker and harder than the tube. So if the adhesive does its job well, then I would expect the patch to be stronger than the rest of the tube. The elasticity is lower in the patch, so I suppose that must be the problem.
So I have to wonder: would it be more or less effective to cut previously damaged spare tubes to use for patching, instead of a patch? I wonder if larger holes need patches with more elasticity.
Rubber cement vs. contact glue
Patch kits include a tiny tube of “rubber cement”. The instructions say to wait 3 min before applying the patch. That’s similar to contact glue instructions. The only difference is contact glue instructions direct us to glue BOTH surfaces then press them together a few min later, whereas the repair kits never say to put glue on the patch (why is that?).
Are the two glues chemically different? I ask because it may not make sense for me to buy another patch kit when I happen to have a very big tube of contact glue for shoe repair.
It doesn’t look like the patch bonded to the tire. I’m guessing a bubble formed between the patch and the edges of the hole. This led to a catastrophic failure.
The rubber cement used in tires is used because it’s basically rubber suspended in a solvent. When the solvent evaporates it leaves the rubber behind which bonds the two sides together to make them one structure. I don’t think the same failure will happen with rubber cement.
Edit: a word
To be clear, I used the rubber cement that came with the patch kit. I have not tried using contact glue (which may¹ even be the same thing). But these tiny tubes that come with the patch kits are more costly per unit than contact glue. Now that I am almost out of rubber cement, I’m considering using contact glue for the next patches.
¹ I cross-posted to !Chemistry@mander.xyz in case a chemist has the answer.
Ah, in that case I’m not sure. Might be like the other commenter said and the patches are either too old or too large. If you have an old tube maybe try the contact cement first on one of them to make sure it works before trying it on the one that matters?