Originally Posted by
Broctoon
I've never repaired bike tubes using rubber cement, only the vulcanizing solvent. When people call this stuff "glue" or "cement" they're not being technically accurate. How it actually works is by chemically softening the tube, so the patch will become welded to it, so to speak.
It is analogous to welding vs. brazing metal. In welding, we're actually melting the base metal. The molten metals from both pieces mix, and when they cool, they've become one. With brazing, only the filler material melts. The base metal gets hot enough to bond with the filler, but does not melt. (Incidentally, the technical difference between soldering and brazing is the temperature involved, which is dictated by the kind of filler metal you want to use, which in turn is generally dictated by the joint strength needed.) That's enough for today's lecture.
Anyway, the reason you wait a couple minutes for vulcanizing solvent to dry is because it's not acting like glue to form a bond. It's dissolving the outer layer of the rubber, which combines with the soft rubber of the patch to be very strong.
That's not how vulcanizing works. The vulcanizing fluid isn't softening the tube. You don't let the solvent in the vulcanizing fluid evaporate so that it will dissolve the outer layer of the rubber. You let the solvent evaporate because it interferes with the chemical process.
The vulcanizing fluid contains a accelerator/promoter in small quantities that acts on another compound that the patch is coated in. The two chemicals combine and start forming new chemical linkages between the tube and the patch through the dry vulcanizing fluid. They aren't "melted" together but are chemically bonded. The process will continue until the promotor and the active compound are consumed...a process that takes some time.
For many years I've been using Park Tools patch kits because that's what my LBS carries and I've been happy with them. I always carry a tube and the patch kit and when I have the rare (knock on wood) flat I usually use the tube and then repair the damaged one at home.
Today I went to buy a new kit and my LBS was closed so I went to REI and bought a few. When I got home a noticed that the Park Tools glue was labeled "Self Vulcanizing Fluid" while the Novara kit contained "Rubber Cement".
Two questions:
1. Are the two glues the same? Self Vulcanizing Fluid sounds fancy but the Park Tool kit is actually cheaper.
2. Is all rubber cement the same? (Don't laugh!) If rubber cement is all the same, I'll buy a big $1.00 bottle
from Staples or X-Mart for home use and save the small tubes for road emergencies.
Thank you.
I would suspect that the two fluids are different. Rubber cement is a suspension of rubber particles in a solvent. For it to work properly, you should spread it on the two different surfaces, let the solvent evaporate and then put the two surfaces together. It sounds the same and is, in fact, the same process for patching a tube.
The difference, however, is what happens after the tube/glue and the patch contact. When using just rubber cement, all you are getting is the bond between the two tacky surfaces. The glue remains mostly unchanged. If you were to use a solvent on the glue and analyze it, it would look like the glue you put on in the first place. It's a bit like sticking two pieces of tape together. The adhesive is still the same adhesive on both sides of the patch/tube.
When you use a vulcanizing fluid, the process starts chemical reactions which form a stronger bond. Although it doesn't "melt" the tube or patch, it is closer to the welding analogy that Broctoon uses. The new bonds become part of the tube. A well cured patch should tear the tube if you try to remove it. A rubber cement patch probably won't.
I am of the opinion that a large reason that patch jobs fail for so many is because they are doing two things wrong. The main one is not letting the vulcanizing fluid dry long enough. I tell people while teaching them how to patch tubes that you can't wait too long. I've forgotten patch jobs for up to 3 weeks and had the patch bond perfectly. (This is why the vulcanizing fluid isn't "softening" the rubber of the tube.) Many people try to get the patch to stick to wet vulcanizing fluid and that just doesn't work.
The other reason that so many fail when patching tubes is because they use the wrong materials. They don't understand the chemistry and vulcanizing fluid looks a lot like rubber cement. Just because they look alike doesn't mean they work the same way.