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

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. Presented at the 82nd Annual Meeting of the Federation of Societies for Coatings Technology, on October 27–29, 2004, in Chicago, IL.     

An improved accelerated weathering protocol to anticipate Florida exposure behavior of coatings

A new accelerated weathering protocol has been developed which closely replicates the performance of automotive and aerospace coating systems exposed in South Florida. IR spectroscopy was used to verify that the chemical composition changes that occurred during accelerated weathering in devices with a glass filter that produced a high fidelity reproduction of sunlight’s UV spectrum matched those that occurred during natural weathering. Gravimetric water absorption measurements were used to tune the volume of water absorption during accelerated weathering to match that which occurred during natural weathering in South Florida. The frequency of water exposure was then scaled to the appropriate UV dose. A variety of coating systems were used to verify the correlation between the physical failures observed in the accelerated weathering protocol and natural weathering in South Florida. The new accelerated weathering protocol correctly reproduced gloss loss, delamination, cracking, blistering, and good performance in a variety of diverse coating systems. For automotive basecoat/clearcoat paint systems, the new weathering protocol shows significant acceleration over both Florida and previous accelerated weathering tests. For monocoat aerospace systems, the new weathering protocol showed less acceleration than for automotive coatings, but was still an improvement over previous accelerated tests and was faster than Florida exposure.     

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A simple Raman technique to measure the degree of cure in UV curable coatings

The curing behavior of four UV curable clearcoats was examined using spectra from a confocal Raman microscope. The disappearance of the C=C line near 1636 cm⁻¹ provides the signature for the curing of the samples, but quantification of the degree of cure by standard peak-fitting and baseline subtraction methods does not work well because of sample fluorescence, baseline shifts and overlapping peaks. A smoothed second-derivative processing approach overcomes all of these difficulties and provides a simple, fast and objective quantification procedure. Steep cure gradients through the thickness of the clearcoat, due to screening of the bottom of the clearcoat by both the UVA and photoinitiator, were observed. Cure gradients at the top of the clearcoat due to oxygen inhibition were also observed. The most complete cure throughout the film thickness was obtained with a mixture of standard and red-shifted photoinitiators.     

Finite-element thermal modeling of casting microstructures and defects

Finite-element thermal modeling offers the capability of simulating casting solidification conditions. By combining thermal modeling and inspection, a defect map can be established to enable prediction of casting microstructures and defects. Gamma-prime size, dendrite-arm spacing, equiaxed grain size, shrink, hot tearing, misrun, and other features can all be predicted.     

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