"Mechanism of Photostabilization of Perfluoropolyether Coatings by Hindered Amine Stabilisers " Polymer Degradation and Stability

Perfluoropolyethers show promising properties for coil coating application and a suitable layer is obtained by incorporating them in a crosslinked polyurethane network. However, photo-oxidative stability of urethane linkages is intrinsically low and an appropriate stabiliser must be added to the reactants prior to curing. The N-methyl HASbis(1,2,2,6,6-pentamethyl-4-piperidyl) sebacate (Tinuvin765) has been selected for this model study. The chemical structure of the network strongly depends on the curing conditions: coil coating conditions (280 C/1 min in air) lead to a polyurethane network also containing some urea-bridges and unreacted isocyanate. HASfurther modifies the network increasing the urea-bridges at the expenses of the unreacted isocyanate. This kind of network is shown to be intrinsically more resistant to the photo-oxidation than pure polyurethane. In curing conditions the Nmethyl HASis partially transformed into –NH derivatives which are still active antioxidants. During photo-oxidation HAS acts as a radical scavenger but slowly evaporates from the film.