Post by Bob La Londe I thought it was exposure to sunlight/UV that caused most of the
sidewall degradation ... or are those aftermarket tire covers (in
particular for RV's and campers) just another scam ?
Yeah I don't know for sure. UV is certainly capable of damaging a lot
of things. I do know CO2 is an issue with some rubbers. I was told
by... well somebody... that tires were among them.
Nobody likes to be wrong, so I did some look ups.
CO2 is said to contribute to the breakdown of rubber on several sites.
Some say "distressed" CO2 and others just generically say CO2. O3 also
contributes to the break down of rubbers and is more reactive. Then I
looked at concentrations per a few other references. They say CO2 is
present at ground level from 300 to 900 PPM (million) where as O3 is
typically present at 20-30 PPB (billion).
As to whether the difference in available molecules makes a real
difference in which has more net affect I do not know, but the numbers
do make you think.
I recall now where I first ran across the reference to CO2 and its
reactivity with rubbers. I don't recall exactly who it was (could have
been Bob Sterne), but it was in regards to tuning, building, and
repairing airguns. Admittedly air can be quite distressed in a spring
piston gun generating enough sudden compression to detonate oils or in a
PCP gun where air can be stored at pressures as high as 4500PSI. Over
300 bar for the metric crowd.
I'm not saying I was right and you were wrong. Not at all. I could
very well be wrong still. My "expertise" with material science is
limited to rote memory and blue collar experience. I'm just stating it
might not be as cut and dried as as it seems. I would argue in full on
flat Earther fashion... "Nothing is ever totally settled science." LOL.
--
Bob La Londe
CNC Molds N Stuff
From Wiki
iny traces of ozone in the air will attack double bonds in rubber
chains, with natural rubber, polybutadiene, styrene-butadiene rubber
and nitrile rubber being most sensitive to degradation.[1] Every
repeat unit in the first three materials has a double bond, so every
unit can be degraded by ozone. Nitrile rubber is a copolymer of
butadiene and acrylonitrile units, but the proportion of acrylonitrile
is usually lower than butadiene, so attack occurs. Butyl rubber is
more resistant but still has a small number of double bonds in its
chains, so attack is possible. Exposed surfaces are attacked first,
the density of cracks varying with ozone gas concentration. The higher
the concentration, the greater the number of cracks formed.
Ozone-resistant elastomers include EPDM, fluoroelastomers like Viton
and polychloroprene rubbers like Neoprene. Attack is less likely
because double bonds form a very small proportion of the chains, and
with the latter, the chlorination reduces the electron density in the
double bonds, therefore lowering their propensity to react with ozone.
Silicone rubber, Hypalon and polyurethanes are also ozone-resistant.
Form of cracking
Macrophotograph of ozone cracking in NBR (Nitrile Butadiene Rubber)
diaphragm seal
Ozone cracks form in products under tension, but the critical strain
is very small. The cracks are always oriented at right angles to the
strain axis, so will form around the circumference in a rubber tube
bent over. Such cracks are very dangerous when they occur in fuel
pipes because the cracks will grow from the outside exposed surfaces
into the bore of the pipe, so fuel leakage and fire may follow. Seals
are also susceptible to attack, such as diaphragm seals in air lines.
Such seals are often critical for the operation of pneumatic controls,
and if a crack penetrates the seal, all functions of the system can be
lost. Nitrile rubber seals are commonly used in pneumatic systems
because of its oil resistance. However, if ozone gas is present,
cracking will occur in the seals unless preventative measures are
taken. Ozone attack will occur at the most sensitive zones in a seal,
especially sharp corners where the strain is greatest when the seal is
flexing in use. The corners represent stress concentrations, so the
tension is at a maximum when the diaphragm of the seal is bent under
air pressure.
The reaction occurring between double bonds and ozone is known as
ozonolysis when one molecule of the gas reacts with the double bond:
A generalized scheme of ozonolysis
The immediate result is formation of an ozonide, which then decomposes
rapidly so that the double bond is cleaved. This is the critical step
in chain breakage when polymers are attacked. The strength of polymers
depends on the chain molecular weight or degree of polymerization, the
higher the chain length, the greater the mechanical strength (such as
tensile strength). By cleaving the chain, the molecular weight drops
rapidly and there comes a point when it has little strength
whatsoever, and a crack forms. Further attack occurs in the freshly
exposed crack surfaces and the crack grows steadily until it completes
a circuit and the product separates or fails. In the case of a seal or
a tube, failure occurs when the wall of the device is penetrated.
The carbonyl end groups which are formed are usually aldehydes or
ketones, which can oxidise further to carboxylic acids. The net result
is a high concentration of elemental oxygen on the crack surfaces,
which can be detected using energy-dispersive X-ray spectroscopy in
the environmental SEM, or ESEM. The spectrum at left shows the high
oxygen peak compared with a constant sulfur peak.