Investigation of factors impacting the in-service degradation of aerospace coatings

Abstract

The impact of in-service environmental stressors on the durability of exterior decorative aerospace coating systems was investigated using accelerated weathering for a high-gloss polyurethane-based monocoat with and without clearcoat. Color, gloss, surface roughness, hardness, and chemical composition changes were studied by varying UV irradiance, temperature, thermal extremes, particulate matter, and acid environment while using constant moisture condensation conditions. The use of a clearcoat was found to enhance the resistance to gloss loss regardless of the stressors applied; however, the clearcoat system also produced a larger increase in hardness under all experimental conditions and a larger color shift for all stressors except for the particulate matter and particulate matter combined with acid. A correlation between color shift and chemical degradation was established by monitoring changes in amide and carbonyl functional groups as a function of UV irradiance, temperature, and thermal extremes. The particulate matter, with or without acid was found not to affect chemical degradation, but produced large color shifts for both coating systems and some loss of gloss at high radiant exposures for the clearcoat system. For the accelerated tests studied here, only the highest UV irradiance and temperature level, with or without additional stressors, produced changes in the clearcoat relative to the monocoat system without clearcoat that correlate with in-service performance observations.

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.