Date: Tue, 24 Feb 2004 17:37:35 +0000 From: Grunff <grunffnospam.com> Subject: Re: Preventing/repairing dash cracks
Bob wrote: > I don't know that I accept your hypothesis on the thickness. Sunscreen > for people is not applied in a "thick" layer yet it quite obviously > does the job for people. UV protection _can_ be had in thin layers > of goo. The thickness of the layer is key - while you would easily tolerate a 200um layer on your skin, if you had a layer of anything more than say 20um on your smooth dash it would look like it was covered in grease. In addition, sunscreens work by absorbing UV and decomposing in the process, which is why you need to reapply every few hours. It's this decomposition that provides enough UV absorption. Unless the product is meant to be reapplied to the dash every few hours, it can't work in the same way. > As to glass absorbing UV - I've researched this in-depth by talking > with a professional engineer. Unless the glass is specifically > designed for UV deflection and/or absorption, a significant portion > (most) of the UV passes right through. There are several components of > UV and the nasty ones don't get removed passing through auto glass. It's actually very much the other way round. Deep blue light is ~400nm. Beyond that you're into near UV. Near UV does pass through glass with reasonably low absorption. The cutoff point for ordinary silica glass is around 350-320nm. Anything shorter than that is completely absorbed. These shorter wavelengths contain a lot more energy than the longer wavelengths, and are therefore a lot more damaging to both skin and plastic. They are the ones that matter. -- Grunff (who used to be a chemist and did a lot of spectroscopy during his time in the lab)
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