Ftir spectroscopy has been used to follow the weathering chemistry of a HALS and UVA free polyester/urethane and a HALS and UVA free acrylic/melamine clearcoat cured on silicon wafers and subjected to exposure in Florida, Arizona, and in a number of accelerated weathering tests. All accelerated tests except EMMAQUA-NTW, ozone filtered xenon arc, and 3M Proprietary exposures distort the polyester/urethane’s weathering chemistry relative to its Florida and Arizona exposure weathering chemistry. Distorted weathering chemistry was attributed primarily to spectral power distribution mismatch between artificial radiation and sunlight. Experiments were initiated with an ozone filtered xenon arc radiation source whose spectral power distribution matches sunlight with great fidelity in the 290–400 nm region.

The long-term weathering behavior of two UV cure clearcoat systems, one a monocure, the other a dual cure system, was studied by infrared spectroscopy, UV spectroscopy, Raman spectroscopy, dynamic mechanical thermal analysis (DMTA), and fracture energy measurements. The photooxidation rate and consumption of residual acrylate double bonds during weathering was highly dependant on the presence of hindered amine light stabilizers (HALS) for both systems. Ultraviolet light absorbers (UVA) had little effect on the photooxidation rate or rate of consumption of acrylate double bonds during weathering. Changes in fracture energy (brittleness) of the clearcoats mirrored the changes found in the consumption of residual acrylate double bonds. DMTA analysis of both clearcoats indicated no increase in crosslink density as weathering progressed, suggesting the acrylate double bonds were not reacting to form additional crosslinks.

Ultraviolet light absorber additives (UVAs) are added to the basecoat and clearcoat layers of modern, multi-layer, automotive paint systems in order to protect the clearcoat and screen the underlying layers from harmful UV light. Unfortunately, these UVAs are subject to photodegradation resulting in diminished UV protection as a paint system weathers. This paper describes an ultraviolet micro-spectroscopic technique that has been developed to measure ultraviolet light absorber content, location, and longevity in the clearcoat layer of complete, intact, automotive, paint systems. Three automotive paint systems containing different UVAs were examined initially and after exposure in Florida. UVA content and distribution were determined in each of the paint systems with weathering and the longevity was determined. This work demonstrates an accurate, unambiguous technique for quantifying UVA loss in coatings.

A variety of spectroscopy and sample preparation techniques are described that allow chemical composition changes to be followed for isolated clearcoat samples, clearcoats in complete paint systems, and all coating layers in complete paint systems as a function of exposure. The analysis techniques provide photooxidation rate, ultraviolet light absorber effectiveness and longevity, and hindered amine light stabilizer effectiveness and longevity information that can be used to dramatically reduce the possibility of introducing inferior clearcoat/basecoat paint systems in service.

ESR is used to quantify the steady-state concentration of HALS-based nitroxyl radicals and the concentration of nitroxyl radicals produced when HALS and its inhibition cycle products are oxidized with peracid for four clearcoat/basecoat paint systems differing in HALS type, HALS concentration, clearcoat chemistry, and outdoor exposure conditions. Clearcoat bulk and surface analysis results are compared. Step-wise analysis is used to determine the distribution of HALS and its inhibition cycle products in two paint systems as a function of exposure. Micro-UV spectroscopy is used to determine the distribution of ultraviolet light absorber additive in the same paint systems. Taken together, the results indicate that the concentration behavior of HALS and UVA additives during the early stages of weather exposure can provide direct insight into the long-term weathering performance of clearcoat/basecoat paint systems.

We have developed 1D and 2D electron spin resonance imaging (ESRI) in order to deduce the spatial variation of radical properties in polymers exposed to UV radiation in the presence of oxygen, or thermally treated. The accelerated degradation of poly (acrylonitrile-butadiene-styrene) (ABS) containing Tinuvin 770 as the hindered amine stabilizer (HAS) was studied in a weathering chamber equipped with a Xe source that mimics the spectral range of sunlight. The HAS-derived nitroxides detected by ESR occupy two sites and were assigned to radicals located in domains differing in their monomer composition. The spatial distribution of the radical intensity obtained by 1D ESRI indicated that nitroxide radicals are produced initially only on the irradiated side, but the intensity of radicals on the opposite side increases with irradiation time. By contrast, the spatial distribution of nitroxides produced during thermal degradation at 600C is spatially homogeneous. Via 2D ESRI it was possible to visualize the spatial variation of the ESR spectra and to deduce the relative intensity of the nitroxides in the two distinct sites along the selected spatial coordinate. The spectral profiling made possible by ESRI provided spatial details that can be used to deduce morphology-sensitive chemical processes along the sample depth.

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 1636cm−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.

Transmission UV spectroscopy measurements of clearcoat ul traviol et light absorber (UVA) disposition, electronspin resonance (ESR) spectroscopy measurements of clearcoat and basecoat Active hindered amine light stabilizer (HALS) disposition, and transmission Fourier transform infrared (FTIR) measurements of photooxidation have been carried out on 5 µm thickslioes of clearcoat/basecoat/primer/e-coatpaint systems on steel panels as a function of outdoor exposure. These analysis results are combined with clearcoat fracture energy measurements to assess the possibility that a clearcoat/basecoat paint system will resistcatastrophic cracking/peeling failure at long times. Taken together, all results indicate that these nontraditional paint weathering performance metrics should be added to the existing repertoire of paint weathering performancemetrics to ensure that inferior clearcoat/basecoat automotive paint systems are not introduced into service.

The chemical stress relaxation behavior of two acrylic/melamine clearcoats has been investigated to determine the effect of hindered amine light stabilizers (HALS) and ultraviolet light absorbers (UVA) on the rates of photooxidation induced crosslink/chain scission and crosslink formation. The addition of a HALS to one of the clearcoats, Mel′-A, slows the rate of crosslink/chain scission and in high enough concentrations stops the formation of crosslinks. In the other clearcoat, Mel′-N, the addition of small amounts of either HALS or UVA slows the rate of photooxidation. The overall changes in crosslink density are the same for Mel′-A containing small amounts of HALS and for Mel′-A containing moderate amounts of both HALS and UVA, indicating that the same changes in crosslink density can be achieved by different degradation pathways. These findings indicate that small changes in clearcoat stabilization packages can have dramatic effects on long-term weathering performance of acrylic/melamine coatings.

Solid-state cross-polarization magic angle spinning 15N nuclear magnetic resonance spectroscopy has been used to follow the chemistry of a 15N-enriched hindered amine light stabilizer and its corresponding nitroxide in an acrylic/melamine and an acrylic/urethane coating system, as a function of ultraviolet light exposure. The two coating systems are based on the same acrylate copolymer. Samples were photolyzed in a QUV accelerated weathering tester equipped with FS-340 UV-A fluorescent bulbs. The tester was operated in the “light only” exposure mode at an air temperature of 40°C and a dew point of 25°C. Spectra reveal the formation of HALS based amino ether derivatives and hydroxylamine. No evidence for the accumulation of HALS decomposition products or non-reactive HALS products was found. Direct evidence that, the amino ether derivatives formed in the two coatings system are distinctly different, was found. One amino ether derivative was unambiguously identified as a methyl adduct. The formation of methyl radicals during the photolysis of both coatings was demonstrated.

Two analytical techniques based on electron spin resonance spectroscopy have been developed to estimate the longevity of hindered amine light stabilizer (HALS) additives in clearcoat/basecoat paint systems at long exposure times. The first technique is based on determination of the steady-state concentration of nitroxyl radical in weathered clearcoats. The second one is based on determination of the concentration of clearcoat HALS transformations products that can be oxidized to nitroxyl radical by p-nitroperbenzoic acid in methylene chloride at 20°C. The two techniques provide complimentary information. Both techniques are used to estimate HALS longevity in four weathered clearcoat/basecoat/primer/electrocoat automotive paint systems.

Time-of-flight secondary ion mass spectroscopy has been used to image the 18O-labeled products formed in all coating layers when automotive paint systems are exposed to ultraviolet light and heat in an 18O2 atmosphere. The 18O− ion intensity maps observed for model paint systems match the expected effects of ultraviolet light absorber additives, hindered amine light stabilizer additives, and pigments on photooxidation. The 18O− ion intensity maps observed for paint systems weathered in Florida for 3 years and then subsequently exposed to ultraviolet light for 1000 h in a 25% 18O2 in dry nitrogen atmosphere correctly anticipate their long-term, >8 years, Florida exposure behavior. ©

ADVERTISEMENT RETURN TO ISSUEPREVCommunication to the...Communication to the EditorNEXTElectron Spin Resonance Imaging of Degradation and Stabilization Processes: Behavior of a Hindered Amine Stabilizer in UV-Exposed Poly(acrylonitrile−butadiene−styrene) PolymersMikhail V. Motyakin, John L. Gerlock, and Shulamith SchlickView Author Information Department of Chemistry, University of Detroit Mercy, Detroit, Michigan 48219-0900, and Mail Drop 3083/SRL, Ford Research Laboratory, Ford Motor Company, P.O. Box 2053, Dearborn, Michigan 48121-2053 Cite this: Macromolecules 1999, 32, 16, 5463–5467Publication Date (Web):July 20, 1999Publication History Received23 March 1999Published online20 July 1999Published inissue 1 August 1999https://pubs.acs.org/doi/10.1021/ma9904363https://doi.org/10.1021/ma9904363rapid-communicationACS PublicationsCopyright © 1999 American Chemical SocietyRequest reuse permissionsArticle Views269Altmetric-Citations30LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-Alertsclose SUBJECTS:Degradation,Electron paramagnetic resonance spectroscopy,Irradiation,Magnetic properties,Polymers Get e-Alerts

The durability of automotive paint systems continues to be a great concern to both auto companies and their coating suppliers. Recent advances in assessing the durability of coatings by measuring weathering-induced chemical composition changes have greatly increased our ability to discern superior from inferior coatings. However, different coatings will likely tolerate different amounts of weathering-induced chemical composition changes while still maintaining their mechanical integrity. Thus, a means of linking chemical composition changes to changes in relevant mechanical properties would be highly desirable. The fracture energy, the amount of mechanical energy required to propagate a crack in a material, is a sensitive measure of the brittleness of a material and is relevant to a number of potential failure mechanisms in automotive paint systems. The fracture energy of clearcoats can vary widely depending on the formulation of the clearcoat (initial chemical composition and additive package) and on the amount of weathering. Weathering embrittles most coatings. Weathering-induced changes in the fracture energy are related to chemical composition changes occurring in the clearcoat. Because the brittlest materials will not crack without an applied stress, the stress distribution in complete paint systems as a function of weathering must also be known to accurately anticipate mechanical failures. Measuring thermoelastic constants of individual layers allows for computation of the stresses in complete paint systems. Stresses tend to increase with weathering. The presence of flaws in the clearcoat changes the stress distribution dramatically. Coupled with fracture energy measurements, the stress measurements provide additional insight into paint system failure mechanisms.