bloke wrote: ↑Sun Dec 17, 2023 8:01 am
Carbon fiber comes unglued.
That's like saying brass comes unsoldered. Which it does, if 1.) you wait long enough, and 2.) it wasn't soldered very well in the first place.
Gluing plastic requires using the correct glue, and for carbon-composites that means good-quality epoxy. That includes the binder, not just the adhesive at the joints.
Now, as to what the difference is between "plastic" and "carbon fiber". The original meaning of "plastic", which is still relevant in metallurgy, is a material behavior where strain under load is not released linearly when the load is removed. It's the alternative to "elastic", which means that strain under load is linear with stress and when the stress is removed the strain remains linear with stress. (Strain is how much a material moves under stress, and stress is the load divided by the effective area of the material cross section at right angles to the load.) This is the opposite of what people think it means--they think a rubber band is elastic, when they have a lot of plastic properties. Steel springs are highly elastic, and when you compress one it returns efficiently (meaning, without producing heat) to its original shape. If you stress steel enough, it becomes plastic--usually noticed by staying deformed after the stress is removed. In the plastic region, steel gets hot as it is worked and eventually fails from fatigue. Brass has similar properties, but with much lower stress required to move it from elastic to plastic.
When most people use the word plastic, they really mean synthetic polymers--long-chain hydrocarbons that link together to make a solid material. Some polymers melt when hot, and some just burn when hot. The latter have to be formed before the hydrocarbons cure (think: Bakelite; see below for epoxy). Most plastics people see these days are thermoplastic--they get more plastic (melt) as they get hotter. PVC, polyethylene, polyester, polycarbonate (Lexan), polypropylene--all these are thermoplastic.
Epoxy is a polymer (polyepoxide) that is a thermoset--it does not melt with heat but burns first. (It will release its adhesion long before it burns, however.) Epoxy is heavily cross-linked--the polymer chains tie to each other in all directions tightly. That makes it much stronger than most plastics used in consumer goods. But it's also brittle, which is why it works best with reinforcement.
The main problem with plastic is that it is...plastic. When stressed, it deforms, and long-term stress usually results in permanent deformation called creep. And the most rigid polymers like epoxy are rather brittle. This is where reinforcement comes in. The traditional reinforcement for plastic is glass fibers, which are usually embedded in a plastic resin made from polyester. Polyester is pretty weak as plastics go, but it's strong enough if the structure is appropriately designed. Glass-reinforced polyester is, of course, called fiberglass. Note that the polyester resin used in most fiberglass includes a hardener that imparts some thermosetting properties making it less susceptible to melting.
Carbon fibers are about 25% stronger than glass fibers, but the density of carbon fiber material is about two-thirds of glass of the same dimensions. So, these two attributes combined mean that carbon fibers have a strength-to-weight ration about twice that of glass fibers.
Some plastics can be dissolved using a solvent, and can therefore be welded together using a solvent. These joints are as strong as any weld in a metal. PVC is an example, and when used in plumbing is joined using solvent adhesives. ABS is another example. Waxy plastics like polyethylene don't solvent-weld easily, but all thermoplastic materials can be welded just like metal by melting them together, often with some filler material. I've welded polyethylene tanks using filler rods made from cut-up similar tanks and a hot-air plastic welder or even a big soldering iron. But thermosets like epoxy can't be welded either using heat or solvents, so the only way to join them is using epoxy as a glue. Here's where design comes in--the surface of the carbon fiber needs to have enough tooth to mechanically interlock with the epoxy used to glue the parts together. And carbon composites bond well to metals using epoxy, too--at least as well as low-temperature solder. Low-temperature solder joints have a lot of overlap because of that, and so should epoxy joints.
A carbon-fiber tuba can therefore be lighter than a fiberglass tuba of the same strength, and if the joints are properly designed, it will resist abuse and age better than brass for dent resistance and at least as good for adhesion.
My Martin TB-31 fiberglass tuba is close to the same size as a new-style King 2341. Here it is next to my Martin EBB-534, which is a rip-off of the 2341:
Sound-wise, it's a big, round, mellow sound with a lot of warmth. The previous owner of mine was Leonard Jung, and nobody ever complained about his ability to be musical with it. He used this tuba precisely because it was light and he was old and had difficulty with heavier instruments. I think it was Matt Walters who found it for him way back when Matt was working for Dave Fedderly. Note the tuning stick, which is mostly wood and operates the main slide. That was Mr. Jung's tool for dealing with only three valves.
The Martin weighs maybe 9 or 10 pounds.
But Martin (incorrectly) used the polyster resin fiberglass binder as the adhesive for the braces that attach the fiberglass outer branches to each other and to the brass valve body. Those have all let go. When I rebuild it, I'll use epoxy to stick it back together. But the ferrules, which have a lot of overlap in the joints are solidly connected still, 55-60 years after it was made. The bell edge is also damaged, but I think that can all be stabilized with epoxy.
Making this a four-valve tuba probably wouldn't be any more difficult than sticking a valve body from a 2341 or clone onto it, but there might be intended consequences playing-wise. The dog-leg is pretty straight, and a fourth valve could probably be grafted in, even if using the Art Hovey trick of using a rotary valve with a linkage and paddle under the pinky.
Rick "but tain't nobody making
these any more" Denney
and keep the faith! 
