ABSTRACT

Repair and replacement of exterior coating systems that no longer meet aesthetic or protective requirements generate a significant volume of environmentally hazardous waste, which includes the coating material combined with solvents and/or media used to remove the coatings, as well as the waste materials generated in surface preparation and reapplication of the coating system. There are strong economic and environmental drivers to extend the service life of aerospace coatings. However, development, selection, and use of the most durable coatings systems have often been limited by the ability to predict service performance in accelerated tests. Current accelerated test methods do not adequately employ the chemical, thermal, mechanical, or radiative stressors that produce relevant damage mechanisms in coated structures that can be used for accurate quantification of coating performance and service life. Test methodologies are being developed that employ combined environmental and mechanical loading modes to overcome this issue. The mechanisms and kinetics of damage progression are quantified continuously throughout a test using in situ measurements of coating system properties and substrate corrosion. Mechanical test fixtures and simulated structural components are being used to apply stresses to coating systems in accelerated atmospheric test chambers. The combined mechanical and environmental tests are expected to produce failure modes not achieved using traditional atmospheric test chambers. An overview is given of the test methods, in situ measurement systems, coating characterization, and combined effects atmospheric exposure testing.

INTRODUCTION

A significant volume of environmentally hazardous waste is generated during repair and replacement of many coating systems such as those applied to the exterior of aircraft. Hazardous air pollutants (HAPs) and volatile organic compounds (VOCs) are needed, often in large quantities, for most remediation activities. The waste streams are associated with not only the chemical strippers and dusts generated during removal but also with substrate pretreatment chemicals that may contain hexavalent chromium, a known carcinogen. The amount of physical blast media and chemical stripper needed for stripping a single aircraft can be on the order of tens-of-thousands of pounds and several hundred gallons, respectively.

Current standardized coating testing methodologies have not been suitable for accurately predicting the lifetime and performance of conventional and advanced coating systems on aircraft structures. During service, aircraft coating systems are subject to a combination of dynamic chemical, thermal, mechanical, and radiative stressors. The failure modes driven by the combined influence of all of these stressors are often different than the failure modes induced in standardized test methods. This is because standard methods do not account for mechanical stress and may only consider a single variable at a time. In some cases, components of a coating stack-up are qualified individually and not at the system level where incompatible material properties developed during service conditions lead to cracks and checks in the coatings. The result of the lack of understanding of causes of in-service failures and the inability to replicate them afforded by current test methods is the qualification of sub- optimal material systems that require shortened repair/replacement cycles. New representative coating evaluation protocols are needed to realistically assess the performance of complex coating material combinations, there-by enabling more accurate service life predictions, optimized maintenance intervals, accelerated development of high performance coatings, and ultimately significant reduction in hazardous waste generation.

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