Klarinet Archive - Posting 000114.txt from 2010/09

From: Jennifer Jones <helen.jennifer@-----.com>
Subj: [kl] Vulcanized rubber mpc deterioration - chemistry
Date: Tue, 07 Sep 2010 05:35:50 -0400

I could believe that some sulfur is freed up as the outer portion of
the mouthpiece deteriorates. Ozone, nitrogen oxides and sulfur
dioxide probably attack the mouthpiece surface particularly badly when
it is exposed to heat.

Ozone probably pulls sulfur out of the mouthpiece and breaks double
bonds in the rubber.

Sulfur dioxide might leave sulfur in the mouthpiece; rubber bands
harden with time when exposed to air; someone told me that vulcanized
rubber was like rubber left out to harden in the air. This seems
unlikely to me because the sulfur in sulfur dioxide is already
oxidized, and given our oxidizing atmosphere, it seems that sulfur in
SO2 is probably pretty stable and that it wouldn't go to the more
reduced state of crosslinking polymerized isoprene(rubber) units. I
don't think it would be entropically favored either because SO2
molecules are much smaller than the isoprenoids (the reaction results
in a fewer number of molecules).

The rotting process probably does not occur by crosslinking with
sulfur at all (even though sulfur is a group 6A with oxygen, we really
don't have much S2 in our atmosphere).

Hence, the rotting process of rubber probably involves ozone more than
sulfur dioxide. Reaction of ozone with isoprenoids would definitely
be entropically favored, it would result in a larger number of
molecules, until the concentration of that larger number of molecules
becomes prohibitive to the forward progression of that reaction...
That seems unlikely; more likely an insulating layer of rotten rubber
would keep inner portions from reacting with the ozone. The outer
layer of rotted rubber probably has mercaptans in it. Perhaps some
disulfide forms while oxygen forms epoxides; oxygen and sulfur
probably have a higher affinity for carbon than eachother. Sulfur
probably has a higher affinity for oxygen than itself. Oxygen has a
higher affinity for carbon than sulfur. Oxygen also has a higher
affinity for carbon than sulfur. Thus, the oxygen will react more
readily with carbon than sulfur and you may have a side reaction of
sulfur bonding with itself. Sulfur might act as a good leaving group
in a reaction of a mercaptan or sulfur cross-linked rubber with
oxygen... I am reaching the limits of my chemistry. I am figuring
based upon the electronegativity of these atoms: oxygen is more
electronegative than sulfur, which is more electronegative than
carbon. The more electrons something has, the more it wants to bond
with something with fewer electrons (These are electrons in the outer
shell/orbital of the atom and mostly applies to the atoms earlier in
the periodic table.) Atoms that are more electronegative tend to form
more polar bonds; carbon-oxygen bonds are more polar than
carbon-sulfur bonds; polar bonds are more easily broken. So maybe
sulfur would not be a good leaving group, even though oxygen would
want the electrons of carbon more than sulfur. Probably the adjacent
carbons would be the leaving groups, especially since sulfur may also
be pulling electrons away from the carbon it is bonded to; hence, the
breaking of the isoprenoid chains of rubber (rotting). So there are
probably more mercaptans (long chain, more than perhaps
methylmercaptan) than disulfide in the rotting rubber mouthpiece.

The mouthpiece I am talking about was from the 70s. By the time I got
it in the early 90s, it was already discolored and smelled of sulfur.
Something that I mostly didn't notice, except on occasion. I probably
accommodated to the smell and I definitely didn't sniff it on purpose,
since it was never an appealing smell. I don't think the mouthpiece
had seen much sunlight. It spent a long time in a dark case. I would
expect sunlight to make the rubber oxidize faster, but with enough
time, it does it anyway.

What I want to know is what on earth makes rotting mouthpieces green
or tan? It is certainly not melanin that makes our skin darken in the
sun (as mentioned in the post comparing ligature shadow on the mp).
Is it a difference in composition of the rubber or a matter of
diffraction caused by little particulates that form on the surface
(like cracking in the outside of a rubber band). Rubber bands don't
really change color when they decay; instead, they darken. Maybe the
vulcanized rubber is photobleaching?

-Jennifer

The archives are good for words that aren't common in the archives
(e.g. speaker bushes). Hence, they are most easily used by
experienced knowledgeable people who can tell when a word is rare.
Archives are also easily used by people who know them and know how to
search for what they are looking for. However, they are an excellent
resource to have available to anyone willing to go through the posts.

http://www.answers.com/topic/mercaptan
http://en.wikipedia.org/wiki/Epoxide
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