16 February 2021 4 8K Report

Hi! I am looking for some specific information in regards to the minimum amount of UV intensity required to begin the photo degradation process in polymers. I understand each polymer is different and impurities, stabilizers, etc. will make answering this question difficult. So, let's assume the prepolymer is a polyether (PPG) based polyurethane extended with TDI and crosslinked (via UV cure and a photo-initiator) through terminal methacrylate groups. No stabilizers.

Then lets say I decide to expose this material to a specific wavelength of UV light, 365 nm, at various intensities such as: 200 mW/cm2, 1 W/cm2, 5 W/cm2, and 15 W/cm2 for 3 minutes each.

I am curious on the minimum intensity and time to initiate the photo oxidation process. Will 200 mW do it? Will it begin at 1 W but only after 2 minutes? Does any amount of UV light initiate it regardless of intensity? I know seeing a color change will help me, but does the free radical / hydrogen abstraction process happen before (or even during) the yellowing process? Is photo oxidation a relatively slow or fast process?

Are there any sort of 'bond energies' I can associate with a given UV intensity to see if it is enough energy to cause bond disassociation?

Any information you can provide would be appreciated!


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