Light-stabilization of polymeric materials by grafted UV-cured coatings

Abstract

The weathering resistance of organic materials has been substantially increased by protecting their surface with a photocured coating containing both a UV absorber (UVA) and a radical scavenger (HALS). A kinetic study by real‐time IR spectroscopy has shown that HALS have no effect on the cure rate, whereas UV absorbers slow down the cure process, due to their radiation inner filter effect. 3D analysis of the depth of cure profile revealed an incomplete cure at the coating/substrate interface, leading to adhesion failure. To prevent this detrimental effect, the UV‐cured coating was photochemically grafted onto the substrate. The polymer material was first coated with a thin layer of a benzophenone solution in a diacrylate monomer. Polymer radicals, generated by hydrogen abstraction from the substrate by the excited benzophenone molecules, effectively initiate the polymerization of the acrylate functions, thus ensuring a chemical bonding between the coating and the substrate. The grafting reaction was characterized by ATR spectroscopy analysis and by surface energy measurements. Excellent adhesion was achieved by applying to the treated substrate a photocurable acrylate coating, containing the light stabilizers, because of the copolymerization reaction taking place with the unreacted acrylate double bonds of the base coat, upon UV exposure. The efficiency of this on‐line stabilization process has been demonstrated on poly(vinyl chloride) that was made eight times more resistant to accelerated weathering. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. A Polym. Chem. 36: 2571–2580, 1998

Search for Unified Action Mechanism of Hindered Amine Light Stabilizers

“…HALS and benzotriazole type UVA do not interact directly, but apparently work synergistically, because of independent involvement in different light stabilization processes, while the antioxidant activity of HALS protects useless consumption of UVA 38) . HALS tend to be consumed slowly in the presence of benzotriazole type UVA, so the UVA continues to be effective much longer, although HALS do not affect any photolysis of UVA 39) . That is, HALS in combination with UVA is preferred because of apparent stabilization of UVA against photolysis, resulting in synergism.…”

Kinetic Study and New Applications of UV Radiation Curing

“…The weathering resistance of various organic materials used in outdoor applications (plastics, wood, painted metal) has been greatly improved by means of UV-cured polyurethane-acrylate coatings containing light stabilizers. [98][99][100][101] The main role of UV absorbers (hydroxyphenylbenzotriazoles or hydroxyphenyltriazines) is to screen the harmful UV-A and UV-B radiation of sunlight to avoid photodegradation of the coated organic substrate. HALS radical scavengers are also needed to increase the light stability of the coating, as well as the lifetime of the UV absorber.…”

Dual‐Curing of Waterborne Urethane‐Acrylate Coatings by UV and Thermal Processing

“…This surprising result was explained by considering that the polymerization will only proceed in an oxygen‐depleted medium, where nitroxyl radicals can no more be formed by oxidation of the HALS stabilizer. Indeed, in the presence of the stable nitroxyl radical 2,2,6,6‐tetramethylpiperidine‐ N ‐oxyl ([TEMPO] = 0.1 wt.‐%), the photopolymerization was completely inhibited 30. Consequently, the slowing down effect of Tinuvin 292 observed on the acrylate loss upon heating of a dual‐cure system (Figure 9) strongly suggests that these functional groups are disappearing by an oxidation process, rather than by polymerization.…”