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Re: mouthpiece patina, additional thoughts

Author: L. Omar Henderson
Date: 2006-06-08 13:37

(Disclaimer – I chemically treat mouthpieces to restore black color and have analyzed several examples of rubber structure in “classic” mouthpieces for clients)
Interesting thread and thanks to the mouthpiece makers who have great insight into mouthpiece form and function. The patina or color change seen in older mouthpieces is indeed due to colored sulphur compounds that are the result of interactions with various chemicals in the air and are not easily chemically reversed to native sulphur or chemically changed to black sulphur compounds.. Oxygen per se is not the operative chemistry but oxidation due to ozone accelerates the unbinding (breaking of chemical crosslinks) of polymer chains at the surface of the mouthpiece which is also accelerated by UV radiation and frees molecular sulphur and other impurities in the vulcanized rubber. This unbinding, leading to what we call oxidation, does soften the rubber because it reverses the vulcanization process. Actually, the colored sulphur compounds help protect the surface from further degradation because they are chemically very stable and block further chemical reactions whereas free sulphur is rather chemically active forming sulfides, sulfates, etc..

It all depends on the raw rubber stock, formulation of rubber, sulphur and other catalysts, and vulcanization process parameters – heat, pressure, time, and subsequent “curing” steps as to whether a particular mouthpiece is more or less prone to the effects of oxidation. Old Chedville mouthpieces show a significant amount of variability in their molecular crosslinking patterns, sulphur and impurity content, and susceptibility to oxidation depending on when and where they were produced. The process of vulcanization was also variable from time to time in the same factory and between batches of mouthpieces and also dependant on the machinery available at the time (as detailed by an extensive review of available factory records, texts, and personal communication with factory workers). Aging of vulcanized rubber also leads to a certain degree of unbinding in vulcanized rubber since the catalytic vulcanization process is more or less random and influenced by various impurities in the stock rubber used. Modern spectral analysis techniques can accurately define the molecular and cross polymer chain bonding patterns leading to a “signature” for a particular piece of vulcanized rubber. These techniques have been developed primarily in the tire industry as quality control procedures. The more unstable crosslinks will break more quickly than more stable crosslinking patterns and may lead to an overall aging redefinition of the bonding patterns. For the most part the worst oxidation and discoloration occur in less than 1/10 mm surface depth but can extend deeper depending on the particular rubber mouthpiece.
L. Omar Henderson

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