Investigation of factors impacting the in-service degradation of aerospace coatings
Introduction
The exterior decorative coating is one of the most visible elements of a commercial airplane, projecting a corporate image and differentiating among airline operators. The current generation of commercial aerospace polyurethane topcoats have an expected service life of approximately 3–5 years, limited primarily by color shift and loss of gloss [1]. In order to reduce the environmental impact and maintenance costs of stripping and repainting aircraft, the goal for the next generation of exterior aerospace coating systems is a service life of 8–10 years. Part of the approach for achieving this goal is to replace conventional monocoat systems with basecoat/clearcoat systems, where the pigment is incorporated into the basecoat to provide color and hide, while the clearcoat provides gloss, weathering protection, and chemical resistance. This technology transition follows the lead of the automotive industry, which implemented basecoat/clearcoat systems almost 30 years ago. While service experience with the newest aerospace basecoat/clearcoat formulations is currently too limited to draw conclusions regarding long-term durability [2], data from fleet surveys (Fig. 1) and reports in the literature [3] indicate that first generation clearcoats (applied over monocoats, but not a “basecoat/clearcoat” system) do improve gloss retention.
A significant challenge in the effort to identify and qualify highly durable coating formulations is that conventional weathering test methods are not necessarily predictive of in-service performance. For example, monocoat colors of the same paint system (such as blue and white) that perform differently in SAE J2527 [4] can show equivalent performance in color shift and gloss loss in a service environment (Fig. 2). More challenging is that colors and paint systems that perform well in accelerated testing can fail prematurely in service [5]. As a result, it's difficult to accurately identify in the laboratory those coating systems which will be highly durable in a service environment. The goal of the current work was thus twofold: (1) to compare the performance of a monocoat system with and without a first generation clearcoat in accelerated weathering tests; and (2) to explore variations in accelerated weathering test methodologies that would produce results aligning with operational experience (and expectations) for clearcoat systems.
Reconciling the differences between the predictions of accelerated weathering tests and in-service performance requires, as a start, a better understanding of the impact of key environmental stressors on the degradation mechanisms of aerospace coatings. During the life of an aircraft, the exterior will be subjected to mechanical stresses and a variety of environmental stressors at both ground and cruising altitudes. These include solar radiation, moisture, temperatures ranging from −50 °C to 80 °C due to environmental exposure, localized temperatures over 120 °C due to hot air exhausts, particulate matter, humidity variations, and atmospheric pollutants such as sulphuric acid aerosol from volcanic eruptions. A set of weathering cycles has been designed which incorporates these stressors and allows their impacts to be systematically investigated. White polyurethane-based commercial topcoat, with and without polyurethane clearcoat, was exposed to different weathering cycles over the course of six months. Samples were removed at intervals, changes in gloss, color, hardness and surface roughness were measured, and chemical changes were monitored using FTIR spectroscopy.
Section snippets
Materials
The aliphatic polyurethane-based coatings used in this work are commercially available high solids aerospace coatings complying with Boeing Product Standard BMS10-72 “Exterior Decorative Paint System”. They are high gloss coatings, required to have an initial 60° gloss value above 90 and a 20° gloss value above 75. Panel Set 1 was a white monocoat system while Panel Set 2 was the same white monocoat with clearcoat applied on top. Each coating system included an epoxy-based primer and was
Gloss loss
Fig. 3(a) and (b) shows gloss as a function of radiant exposure for Panel Set 1 (no clearcoat) and Panel Set 2 (with clearcoat) for the three weathering cycles. Only Panel Set 1 exposed to the high temperature Cycle C described in Table 1 showed any reduction in gloss, and not until a radiant exposure of about 6000 kJ/m2 (at 340 nm) was reached. Gloss loss is reported to arise from chemical degradation of the polyurethane eroding the coating surface which then exposes the inorganic pigments to
Conclusions
The goals of the current work were to compare the performance in accelerated weathering testing of an aerospace decorative polyurethane monocoat system with and without clearcoat and to explore variations in test methodologies that could produce results aligning with operational experience for the coatings of interest. The coatings were subjected to QUV accelerated weathering protocols with varying temperature and UV irradiance, plus the application of stressors which attempted to mimic
Acknowledgements
The authors would like to thank Dara Ung, Randy Jahren, Elden Altizer, Jason Bolles, and Mike Andrews of Boeing Research & Technology for providing thermal treatment and in-service data, and The Boeing Company for providing funding to support this work.
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