Dear Michael Bay, please check the following documents. My Regards

https://doi.org/10.1002/polc.5070230124

https://doi.org/10.1002/masy.19991430110

https://www.researchgate.net/publication/227727106_Degradation_mechanism_of_polyether-urethane_EstaneR_induced_by_high-energy_radiation_II_Oxidation_effects

https://www.researchgate.net/publication/230236978_Photooxidative_degradation_of_polyether-based_polymers

https://doi.org/10.1016/j.polymdegradstab.2007.01.034

https://doi.org/10.1002/(SICI)1522-9505(19981201)261-262:13.0.CO;2-O

https://link.springer.com/article/10.1023%2FA%3A1021148205366

https://doi.org/10.1016/j.polymdegradstab.2011.10.020

Dear Michael Bay, please check the following documents. My Regards

https://doi.org/10.1002/polc.5070230124

https://doi.org/10.1002/masy.19991430110

https://www.researchgate.net/publication/227727106_Degradation_mechanism_of_polyether-urethane_EstaneR_induced_by_high-energy_radiation_II_Oxidation_effects

https://www.researchgate.net/publication/230236978_Photooxidative_degradation_of_polyether-based_polymers

https://doi.org/10.1016/j.polymdegradstab.2007.01.034

https://doi.org/10.1002/(SICI)1522-9505(19981201)261-262:13.0.CO;2-O

https://link.springer.com/article/10.1023%2FA%3A1021148205366

https://doi.org/10.1016/j.polymdegradstab.2011.10.020

Please have at this:

Article Photodestruction and photooxidative destruction of polyurethanes

Dear Michael,

I think you are going to see the onset of photodegradation as early as the technique's sensitivity you are using allows you to. It is not about when starts the photodegradation, but how you are going to measure it.

From my previous experience, mechanical essays "take the picture" of the photodegradation way before the infrared spectroscopy takes (to stay in the classics). If you are more resourceful and want to go into the very beginning, an XPS can help you better so you can start to see the first reactions of the oxygen with the produced radicals or the double bonds or crosslinking being formed. If you want to go a step before, a good EPR can give you some insights into the formation of the radicals.

Going into the specifics:

"I am curious on the minimum intensity and time to initiate the photo oxidation process. Will 200 mW do it?"

As said before, depends on the technique's sensitivity and on having enough time to gather enough chemical groups for the technique's sensitivity. If you want a straight answer, even if it is not practical, the time to begin the photodegradation would be the time for the radical formation because of the absorption of the UV light, and the time to begin the photooxidation would be added by the time of the reaction between this radical and oxygen. These answers can be answered by a theoretical chemist who works with orbitals and reactivity.

"Will it begin at 1 W but only after 2 minutes?"

Impossible to know without testing and it depends on your technique's sensitivity.

"Does any amount of UV light initiate it regardless of intensity?"

Given enough time, yes!

"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?"

Yes and yes.

"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?"

It is a bit more complicated than that because energy from the light is not readily available once hits the surface, it must be absorbed by the "chromophore groups" before starting any photochemical reactions. At this point, photodegradation gets tricky because this work that chromophores do might be done by impurities. The classic example is polyethylene and polypropylene, which theoretically do not photodegrade under the Sunlight because the UV there is not absorbed by their chemical structure, but it is absorbed by the chemical impurities and the synthesis defects, both unavoidable so far. So, even if a theoretical chemist helps you, there are more practical considerations regarding polymer degradation to be taken.

I would like to highlight that even IUPAC, when defined polymer degradation, didn't take into consideration chemical thresholds, but practical aspects. Take a look at the definition: https://goldbook.iupac.org/terms/view/PT07144