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Originally Posted by trikey
It can also depend on how the lenses are moulded. A small decrease in mould temperature would upset the length of the polymer chains making them more prone to degradation than lenses moulded at the correct temperatures.
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It doesn't affect the polymer chain length, but it certainly impacts on the physical properties of the finished article.
Spotted this:
Mold temperature has perhaps a less obvious but often more profound effect on final properties. In amorphous polymers such as ABS and polycarbonate, higher mold temperatures produce lower levels of molded-in stress and consequently better impact resistance, stress-crack resistance, and fatigue performance.
Quote:
Originally Posted by clf
Would the effect driving through rain, dust smoke even air cause more immediate issues with abrasion or breakdown of the clear coat?
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Of course, in the real world environment, plastics and their coatings are affected by abrasion from dust, etc. Frequent freeze/thaw conditions also pay their part in fatiguing the lenses.
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Perhaps also the uv reflecting of the surface of the polycarbonate to the inside of the clear coat cause issue?
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The UV that degrades polycarbonate headlamp lenses comes from sunlight, which is absorbed by the sacrificial coating. Compounds in the coating react preferentially with UV radiation preventing it getting at the lens itself. The reactive compounds are permanently changed into non-absorbing chemicals. As their activity drops, the underlying polycarbonate will start to degrade. There's little if any UV generated by the headlamp itself, so lenses don't require an internal coating.
I recently repolished my headlamps for the second time after not giving them any protection the first time. They're still off the car (SORN) until the weather warms up a bit. The expensive UV protectant I intend to spray on needs somewhat milder ambient temperatures than we have at present.
Some polishing kits do come with a wipe-on type lens coating which I suspect is next to useless. They will give a very short lived UV mitigation, but you really do need a solid plastic film rich in UV absorbent to do a proper job. Even so, none of this is permanent unless you keep the car in perpetual darkness.
TC