75RR wrote: ↑Thu Jul 15, 2021 7:34 am
This is what Wiki says: ...
McMaster-Carr has three different types advertising resistance to: [impact], [impact, UV], [impact, UV, scratch]. These probably correspond to the three cases mentioned on Wikipedia: UV stabilizer added to the bulk material; additional UV-resistant coating (maybe the 15-50 um thickness
Hobbyst46 mentioned in #138); coextrusion of some thicker surface layer that additionally gives scratch resistance.
tpruuden wrote: ↑Wed Jul 14, 2021 10:35 pm
I believe the UV protection acts as a waveplate (not sure, how much exactly the polarization is shifted). They say, that it is a co-extruded layer...
I only bought the basic type but there was already very strong birefringence in the 1/8" material, 30X greater than the birefringence in the 7/32" material (
#137), and enough to bias the central dark band way off the edge of the bar with reasonable bending. That gives some evidence that strong birefringence can come from the base material itself (depending on manufacturing process maybe?) without any extra coatings. Also, polycarbonate is specifically chosen for its strong photoelasticity, does not seem surprising that it would also be prone to strong inherent birefringence depending on manufacturing process. (Either due to internal stress, molecular alignment without stress, or both -- still not sure I understand that relationship mentioned in
#135.)
LouiseScot wrote: ↑Thu Jul 15, 2021 9:52 am
I think this UV coating business is all a red herring!
Seems likely, but would be interesting to compare. I already added UV/scratch-resistant sheets to my cart during the earlier discussion but have not needed to order anything else from McMaster yet.