Long-term weathering behavior of UV-curable clearcoats: Depth profiling of photooxidation, UVA, and HALS distributions

Abstract

The long-term weathering performance of two UV-curable clearcoat systems was studied using in-plane microtomy in combination with infrared spectroscopy, UV spectroscopy, and ESR spectroscopy. Oxygen transport characteristics were also studied using the half-time method. The photooxidation versus depth profile was highly dependant on the presence of hindered amine light stabilizers (HALS) for both coating systems. Ultraviolet light absorbers (UVA) had little effect on the photooxidation profile. A photooxidation gradient was formed in both clearcoats due to a reduction in oxygen solubility when compared to standard thermoset clearcoats. This gradient was only seen in formulations not containing HALS.A utomotive clearcoats continue to evolve due to a variety of driving forces that include environmental regulations, customer performance demands, and cost pressures. High-solids solventborne 1K and 2K clearcoats are currently the industry standards in North America. However, super high-solids, powder, waterborne, and slurry clearcoats either have been recently introduced or are in advanced development. Each of these clearcoats offers reduced VOCs in comparison to conventional thermoset clearcoats, but all come with performance or cost compromises as well.Another class of clearcoats being developed, but not yet commercial for automobile bodies, is UV-curable clearcoats. UV-curable clearcoats typically crosslink via free radical polymerization upon exposure to UV radiation. These coating systems are typically mixtures of oligomers, monomers, solvents, one or more photoinitiator (PI), and other additives for weatherability enhancement, including ultraviolet light absorbers (UVA) and hindered amine light stabilizers (HALS). 1 UV-curable coatings are well established in such industries as telecommunications, wood finishing, and flooring manufacturing, but have only recently been seriously considered by the automotive industry for body paint applications. These coatings are of major interest for a variety of reasons, the most widely discussed of which is the potential for outstanding scratch resistance. This improved scratch resistance is mainly due to the extremely high crosslink density that can be achieved by UV-curable coatings. 2,3 In addition, UV-curable clearcoats have the potential to offer lower VOCs, improved process efficiency, shorter curing times, and lower energy consumption. These potential process advantages must be weighed against issues associated with reparability, adhesion, yellowing, and curing shadowed areas on highly contoured objects. One solution that addresses the shadowing issue is to utilize both thermal and UV curing to process the clearcoat. These systems, known as dual cure systems, allow the partial thermal cure of shadowed regions while giving the properties of full UVcure systems in unshadowed regions.Because UV-curable coatings have not been used extensively in outdoor environments, their long-term weathering behavior is largely unknown. The research to date, mainly by Decker, 4-7 Valet, 8...

Model-based analysis of photoinitiated coating degradation under artificial exposure conditions

“…In addition, the uncertainty of the diffusion parameters and the relative humidity employed during the experiments have not been reported by Colin et al 27 The value used for the oxygen solubility could also be inaccurate. Seubert et al 57 found that oxygen solubility in UV-curable urethane acrylate clearcoats was dependent on the curing conditions. So, it can be concluded that the value used for the diffusion coefficient is presently the right order of magnitude, but somewhat low compared with expectations.…”

“…Schulz 1 states that numerous studies have demonstrated that oxygen diffusion in transparent polymer films is the rate-determining step and refers to experiments in pure oxygen, as opposed to air, clearly showing an effect of oxygen concentration in the gas phase on the rate of degradation. Seubert et al, 57 on the other hand, mention that the accepted belief has been that the availability of oxygen was never a rate limiting process in the photooxidation of binders, but then also show that for UV-curable urethane acrylate clearcoats the oxidation can indeed be oxygen diffusion-limited.…”

Enhancing weathering durability of pre-protected and unprotected wood by using bark extracts as natural UV absorbers in waterborne acrylic coating

“…6 In some studies, the natural durability and service life of materials were increased by adding different UV absorbers (UVA), light stabilizers (HALS), colorants, etc., to resin-based coatings. [7][8][9] Some additives used as UV absorbers have provided the wood with high durability due to their high capability of UV absorption in outdoor conditions. 10 In recent years, there have been many studies on evaluating bark for preservation of wood.…”

Direct detection of additives and degradation products from polymers by liquid extraction surface analysis employing chip‐based nanospray mass spectrometry

“…The detection and characterisation of HALS within polymer‐based surface coatings have received some interest of late, as the performance and chemical activity of HALS in situ can have dramatic effects on plastic and coating performance over the lifetime of the material . Many different analytical techniques have been used for the direct detection of HALS from polymer‐based materials, the most notable being ambient mass spectrometry (MS) techniques such as atmospheric pressure – matrix assisted laser desorption ionisation (AP‐MALDI) and desorption electrospray ionisation (DESI) .…”