Exposure to UV and degradation of coatings

It has been suggested that the decrease in the S parameter on exposure time is a result of the decrease in the defect (free volume) size or concentration due to irradiation [30]. This is a result of a net increase in the cross-linking density of the samples as a result of photodegradation. It has been demonstrated that there is a direct relationship between the decrease in the S parameter (loss of free volume) and an increase in the cross-linking density due to degradation for polyurethane coatings [32], We can postulate 

This 11/2 value is a characteristic parameter representing the durability of the coating with respect to weathering or photodegradation [30]. 

---