Question: We are bonding steel to etched PTFE with an epoxy adhesive for outdoor applications. The bonds are quite strong, but appear to discolor and lose strength in the field. The PTFE side is exposed to sunlight. What do you think is the problem?
Answer: The probable reason for your problem is the ultraviolet (UV) transparency of the plastic and the fact that epoxy bonds are not very UV resistant. Even epoxy coatings are not recommended for outdoor applications because they discolor under UV. While PTFE is very resistant to degradation by UV light and may appear translucent, it does transmit UV light, which is almost certainly degrading your epoxy adhesive.