Tuesday, April 21, 2009

The Double-Edged Sword of UV Light

With a little bit of luck, I might working on a UV-curing project soon, something I really get giddy about. I hope that I never lose the awe of seeing a useless low viscosity mix of chemicals turn into a hard polymer after just a few seconds exposure to UV light. Skip all those thermal/emulsion/suspension/bulk/ polymerizations with the long induction and gelation times. Pump those electrons into an excited state with those absorbed photons and let the good times roll. UV curing is instant gratification.

And yet UV light is also an endless source of degradation. All those excited electrons can go the other way and start chewing up the polymer's backbone, leading to discoloration (to all colors of the rainbow - my favorite being the "pinking" that occurs in white PVC), chalking and eventually failure. Despite the branching reactions that often occur (leading to an ever increasing degradation rate), the reaction occurs painfully slowly.

In both cases, the reactions start with the photon being absorbed and exciting an electron to a reactive state. It's just a question of which way the electron will fall. I have the good fortune in my current job to work both sides of the slope.

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