(Carl Fogel) may have said:If I follow what you're suggesting, transparency isn't involved?
Transparency to visible light and ransparency to UV are not necessarily the same thing; indeed, many
visible-transparent glasses are UV-opaque. More often than not, the two characteristics vary. In the
case of the cable ties, the UV-penetration-damage problem is exacerbated both by the fact that the
opacity of the black colorant is generally not high, and the fact that cable ties involve long-chain
polymers for which the loss of integrity of the molecules at the surface can apparently have an
exaggerated effect on the strength of the item. Cable ties, it should be remembered, have
thicknesses on the order of 1mm for the most common, ranging up to 2.5mm for the largest I've used.
It only requires a relatively small amount of long-term UV penetration for the material to be
heavily compromised.
Quoted message said:So does anyone know what's actually going wrong when carbon-fiber deteriorates under sunlight,
assuming that it does, as Simon's original post suggested, go milky and crack?
When the milkiness appears, that indicated resin degradation; it will generally start at the
surface, but in my experience (with similar materials in other fields) by the time the milkiness is
visible in a part that is under 6mm thick, it's too late to forestall structural failure. In the
instances I've seen, once the resin began to decompose, it continued, whether the UV exposure
continued or not.
Quoted message said:It sounds as if many carbon-fiber products have either anti-UV stuff mixed in, or else a clear
anti-UV coating, but I'm back to wondering what actually happens when the carbon-fiber
degrades--how does it go beyond the equivalent of a tan?
If the UV protection is in the form of a coating, then wherever the coating is compromised by
friction or other degradation, the resin below will be exposed; depending upon the UV-transparency
of the resin, the rate and depth of damage could be anywhere from annoying to hazardous in a year's
continuous exterior exposure in a climate with significant environmental UV. (There are occasionally
some small advantages to living in the smog capital of the US; we have *many* reasonably cloudless
days when the UV level is low enough that sunscreen is not needed. This does not ameliorate the
other effects to any useful extent, however.)
If the UV protection is in the form of a UV blocking agent in the resin itself, then unless that
agent is leachable or was not employed in the correct concentration, the damage should be limited to
the surface of the resin.
In my opinion, all of these concerns are things whose levels of hazard will, for most carbon-fiber
bike components, only be possible to establish by experience. Those who wish to have the light
weight bits will just have to be aware that what they are buying *may* have a problem in this area,
and they have only the manufacturer's assurances to rely upon for durability prediction.
--
My email address is antispammed; pull WEEDS if replying via e-mail. Yes, I have a killfile. If I
don't respond to something, it's also possible that I'm busy.
