Author: ClaV
Date: 2015-07-28 16:56
Silversorcerer wrote:
> Are phenols and formaldehyde more carcinogenic than
> acrylonitrile? I just wondered? In fact it appears that all
> three of the constituents that make up ABS are carcinogenic. I
> am sure that contributes to the superior mechanical properties
> of ABS, if there are any.
>
Good questions!
The key to this is that bakelite is a thermoset, again.
In bakelite, by a nature of the polymerization process, it is difficult to completely react monomers. Some monomers and the oligomers (just few monomer molecules reacting) remain. That is corroborated by phenol smell upon scratching/cutting.
In a modern polymerization process, the reacting monomers can be handled efficiently, if reacted fast and completely (the case of ABS and other good plastics). No acrylonitrile remain in ABS. Even upon heating acrylonitrile is not the main decomposition product. It is worth to mention that good carbon fibers can be produced by pyrolysis of poly(acrylonitrile) fibers (common acrylic fibers that if not by knowledge but by all previous experience are safe to handle/wear).
Now to mechanical properties and thermosets again.
In ABS, long linear chains are formed - regular and strong. ABS is smartly engineered with monomers imparting rigidity (styrene) and elasticity (butadiene and acrylonitirile imparts good chemical stability - a bit simplified, but relevant picture. To give a relevant example, simple poly(ethylene), effectively a hydrocarbon, if produced in very long chains (UHMWPE) becomes stronger than Kevlar. So the length of polymer chain (the number of monomer molecules combined in a chain) is the key to superior mechanical properties, which works well in the case of ABS.
In contrast, bakelite polymerization process is a mess, and there is no uniformity, which creates a lot of weak links and imposes brittleness. In some properties bakelite is closer to ceramics than polymers.
> It is interesting that one chooses to characterize a number of
> differences in material properties as "inferior". Whether these
> properties make something inferior depends on the use. Given
> the high demand for bakelite at the time, it must have been the
> superior material for many of the uses it served. Without
> characterizing any material as inferior, we could review the
> differences that might be valuable.
>
> 1) Weather neutral.
> 2) dimensional stability
> 3) Durability constant over a wide temperature range.
> 4) consistently machinable to high tolerances
> 5) resistance to chemical damage, chemically neutral.
Did you have a chance to look through the file with bakelite properties that I referred to for the mechanical properties?
1) I am not sure what is it exactly; does it effectively refer to chemical stability at common conditions of use (weather can be different)?
2) See below
3) Bakelite is quite good for thermal stability. Is is crucial for clarinets?
4) If bakelite chirps, how can it be easily machined to high tolerances?
5) Chemically stability of bakelite is relatively poor, bakelite is susceptible to acids
> You keep talking about the problems of shrinkage during the
> molding process
Yes, during typical polymerization, there is a substantial shrinkage.
> It seems though that the the only material that is
> superior is one that can be accurately molded to specifications
> as opposed to machined to specifications.
Very true, to my knowledge and understanding and that is great that we had a chance to discuss it. Molded plastic clarinets are easy to make at a moderate cost, as well as moderate tolerances. Post-processing can be made to bring plastic clarinets into professional grade, but at appreciable cost.
>
> There are machining marks on all of the bakelite parts I have
> seen. Are ABS clarinets molded completely as a finished
> product? Does this create an instrument that is as consistent
> in dimensions as those that are the result of machining to
> dimensional tolerances?
A good question. The shrinkage is there. With stronger and uniform an ABS, it is easy to factor it out in molding design. Again, I do believe that the best practice would be to machine precisely the bore and the toneholes after molding. There micron tolerances (at least ca. 10 microns) become important. Molded instruments would be difficult to make to such high tolerances.
> A more favorable history of bakelite, which shows it being
> sawn, machined in every conceivable way and polished is given
> here:
> http://www.elvenkrafte.com/bakelite%20presentation.htm
>
> Obviously the problem with bakelite chirping could be very
> distracting during quiet passages in compositions. Be careful
> not to chirp your bakelite clarinet.
That is nice sentimental story. Importantly, do they give machining tolerances?
Or by this logic, is anything cuttable and polishable good for clarinets?
I do believe in good craftsmen - people can make great things. But in some cases, it is much more difficult and not cost-effective.
The USRR bakelite clarinet that I have is molded, there is chirping with the use, the bell is cracked. Does bakelite make a good material for clarinet? No!
Can decent clarinets be made out of bakelite, if careful and patient - perhaps.
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