Depth-resolved infrared microscopy and UV-VIS spectroscopy analysis of an artificially degraded polyester-urethane clearcoat Koen N.S. Adema a, b , Hesam Makki a, b , Elias A.J.F. Peters a , Jozua Laven a , Leendert G.J. van der Ven a , Rolf A.T.M. van Benthem a , Gijsbertus de With a, * a Eindhoven University of Technology, Laboratoryof Materials and Interface Chemistry, Department of Chemical Engineering and Chemistry, P.O. Box 513, 5600 MB Eindhoven, The Netherlands b Dutch Polymer Institute (DPI), P.O. Box 902, 5600 AX Eindhoven, The Netherlands article info Article history: Received 7 July 2014 Received in revised form 11 September 2014 Accepted 7 October 2014 Available online 17 October 2014 Keywords: Photodegradation Polyester-urethane UV-VIS spectroscopy FTIR-ATR microscopy abstract Polyester-urethane resins are important candidates for high performance, outdoor coating applications. Despite their relevance, quantitative information regarding the photodegradation of these materials is scarcely available. In the present study, a model polyester-urethane clearcoat without additives is arti- ficially degraded and the changes in optical properties and chemical composition are studied by UV-VIS spectroscopy and FTIR-ATR microscopy, respectively. The change of the optical properties throughout the coating thickness is quantified and interpreted using a newly developed optical model. Chemical changes are quantified in a depth-resolved manner by combining FTIR-ATR microscopy with optical profilometry in order to visualise the time evolution of compositional gradients during photodegradation by accurate assignment of the correct depth position to all recorded ATR spectra. The rate of change for the con- centration of several chemical entities in the model polyester-urethane was found to become constant after the initial stage of weathering. The loss of urethane crosslinks in the coating occurs faster and to a much larger extent as compared to ester bond scission. Results from the optical and the chemical characterisation are combined to propose a kinetic model for ester bond photolysis that provides an estimate of the quantum efficiency of this process. © 2014 Elsevier Ltd. All rights reserved. 1. Introduction Polymer coatings are used for a wide range of purposes, although industrial applications are still mainly focussing on the classical roles of protection and decoration. The protective function is especially critical for a clearcoat that forms the separating layer between the sensitive components of a typical multi-layered decorative coating and the environment. For outdoor coating ap- plications, such as in the automotive or aerospace industry, the clearcoat has to resist severe stresses in terms of UV-radiation combined with temperature, humidity, (atmospheric) precipita- tion and mechanical wear [1,2]. Stability against photodegradation, or weathering, is an essential characteristic that a successful clearcoat must possess for application outdoors. Polyester-based resins are good candidates for exterior applications because they combine good mechanical properties with outdoor durability. Especially polyesters based on isophthalic acid (IPA) are known for their stability against weathering [3,4]. Although IPA-containing polyester resins are very relevant for the coatings industry, there is only a limited number of studies on their photodegradation. Other polymers are often preferred in weathering research because of the complicated photo-oxidation chemistry of the polyester and its relatively high stability against photodegradation, which means that even accelerated weathering experiments are often very time-consuming. Weathering of a polyester in a coating adds an extra layer of complexity due to the fact that such coatings typically degrade in depth in a spatially inhomogeneous fashion, meaning that depth-resolved characteri- sation is required for a thorough understanding of the changes inside the coating during photodegradation. In this article, we perform such type of analysis on an artificially degraded polyester- urethane clearcoat. We use a model coating system based on hydroxy-functional poly(neopentylisophthalate) (PNI), crosslinked with the isocyanurate trimer of hexamethyldiisocyanate (HDT) (Scheme 1) and study two of its basic characteristics that change during weathering: optical properties and chemical composition. * Corresponding author. E-mail address: G.deWith@tue.nl (G. de With). Contents lists available at ScienceDirect Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab http://dx.doi.org/10.1016/j.polymdegradstab.2014.10.004 0141-3910/© 2014 Elsevier Ltd. All rights reserved. Polymer Degradation and Stability 110 (2014) 422e434