Mechanism of photo stabilization of perfluoropolyether coatings by hindered amine stabilisers

Perfluoropolyethers show promising properties for coil coating application and a suitable layer is obtained by incorporating them in a crosslinked pol yurethane, network. However, photo-oxidative stability of urethane linkages is intrinsically low and an appropriate stabiliser must be added to the re actants prior to curing. The N-methyl HAS bis(1,2,2,6,6-pentamethyl-4-piper idyl) sebacate (Tinuvin765) has been selected for this model study. The che mical structure of the network strongly depends on the curing conditions: c oil coating conditions (280 degreesC/1 min in air) lead to a polyurethane n etwork also containing some urea-bridges and unreacted isocyanate. HAS furt her modifies the network increasing the urea-bridges at the expenses of the unreacted isocyanate. This kind of network is shown to be intrinsically mo re resistant to the photo-oxidation than pure polyurethane. In curing condi tions the N-methyl HAS is 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. (C) 2001 Elsevier Science L td. All rights reserved.