Comparisons of Weathering Performance of Two Automotive Refinish Coatings: A Case Study H. Yari, M. Mohseni, B. Ramezanzadeh Department of Polymer Engineering, Amirkabir University of Technology, Tehran, Iran Received 9 April 2008; accepted 2 September 2008 DOI 10.1002/app.29341 Published online 5 December 2008 in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: The aim of this study was to compare the weathering performance of two commonly refinish coat- ings used in a domestic car manufacturer based on poly- urethane (PU) and acid-catalyzed alkyd-melamine resins. The replacement of the alkyd-melamine refinish with the PU system was shown to largely lower the reported fading behavior of the monocoat system. For better comparison between the refinish coatings, an original equipment man- ufacturers’ (OEM) topcoat, cured at production line, was also used. Different characterization methods including DSC, atomic force microscopy, thermo gravimetric analy- sis, DMTA, and Fourier transform infrared spectroscopy were performed to elucidate the changes observed in the optical, mechanical, and weathering performance of stud- ied coatings. It was found that the main reason for the in- ferior behavior of the acid-catalyzed coatings compared with that of the PU and OEM was due to its low crosslink- ing density. The poor weathering resistance of the former coatings was attributed to its incomplete curing reaction and therefore, to its immatured film formation. V V C 2008 Wiley Periodicals, Inc. J Appl Polym Sci 111: 2946–2956, 2009 Key words: refinish coatings; polyurethane; alkyd/ melamine; accelerated weathering; thermal analysis INTRODUCTION The general purposes of coating application in auto- motive industries are as follows: (1) to protect the car against environmental conditions such as humid- ity, sunlight, and pollutants in the air 1 and (2) to impart desirable esthetic appearance. To accomplish these functions, the coatings themselves must remain intact for a long time in a harsh environment. Photo and hydrolytic degradations are two common proc- esses occurred, resulting in changes in chemical structure of an automotive coating. 2–5 These chemi- cal alterations may greatly influence the physical, 6 mechanical, 7–9 and electrochemical 10 properties of the coating. Therefore, protection upon weathering degrada- tion seems to be indispensable. In recent decades, UV absorbers and hindered amine light stabilizers have been used for photo stabilization of coatings. 11 These usually absorb the harmful part of the solar spectrum or may deactivate free radicals to reduce photo oxidation rate. 12,13 To lower hydrolytic degra- dation, several approaches such as use of coatings with highly chemical stable linkages upon water uptake have been adopted. Improved surface hydro- phobic and greater crosslinking density are the two scenarios by which water permeability can be dimin- ished. 14 Air drying and refinish coatings, due to being cured at lowered temperatures, are more sus- ceptible to permeate water leading to accelerated coating degradation. Because of the mechanical damages (e.g., scratch and impact), which may often occur during the assembling of a car, remediation of the affected area by the aid of low temperature cured refinish coat- ings seems inevitable, mainly because of the heat sensitivity of the plastic parts used. From the view- point of weathering resistance, these coatings must be as comparable as the original equipment manu- facturers’ (OEM) coatings introduced in the produc- tion line. Incomparable weathering resistance of refinish coating and OEM may well lead to local defects such as gloss and/or hue variation. Two types of refinish coatings being currently used are based on two-pack polyurethane (PU) and acid-catalyzed alkyd/melamine resins. The use of an acid catalyst is to lower the curing temperature of the OEM coatings. 15 General mechanism of curing process for alkyd melamine coating in the presence of an acid catalyst is shown as a two step (A and B) in Figure 1. The present work was aimed to attribute a reported gloss and/or hue changes occurred on dif- ferent parts of a domestic car brand to the chemis- tries used as the refinish coating. Two different coating systems were an acid catalyzed alkyd/mela- mine and PU. Various analytical techniques Journal of Applied Polymer Science, Vol. 111, 2946–2956 (2009) V V C 2008 Wiley Periodicals, Inc. Correspondence to: M. Mohseni (mmohseni@cic.aut.ac.ir).