Published: Jan 2012
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THE TESTING OF COATINGS ALWAYS HAS A RELAlationship to coating failures, either directly or indirectly. Testing before failure helps to ascertain serviceability, establish the best formulation variables, maintain quality assurance of coating products during manufacture, develop storage and application parameters, ensure specification viability, and, in essence, prevent early failure. Testing after failure assists in determining cause, hopefully prevents future coating failures of a similar nature, and aids in placing liability. Failure modes run the full gamut. Coating failure can originate from within, as with deficiencies in a coating's formulation or its manufacturer. Anomalies in the coating's film-forming chemistry from competing atmospheric side reactions are very common with some urethanes and epoxies. Failures can originate external to the coating material from improper specification, inappropriate or inadequate surface preparation, and application deficiencies. When a failure occurs and is investigated, the coatings practitioner must maintain an open mind. Often there are multiple causes impacting the failure. The analysis of coating failures should follow a systematic program based on the “scientific method.” Briefly this translates to (1) investigate, (2) hypothesize, (3) test, and (4) conclude or reconsider. Test programs after failure are often a part of the failure analysis process and have the purpose of establishing viability of a proposed failure mechanism. Designing the test and evaluation program requires knowledge of the total process from specification to paint manufacturing to paint application. In part, it requires knowledge of the laws of physics and chemistry that are controlling what is happening at interfaces. The coating system contains many types of interfaces including the substrate/coating interface, emulsion resin particle/carrier surfaces, inter-coat interfaces, and the interfaces between the pigments and binder in the coating formulation. The chemistry and physics of solutions controls apparent viscosity and can impact osmotic failure modes. Polymer curing and weathering reactions, corrosion, and dimensional stability after cure are just a few of the other sources of coating failures often reported. As a preliminary to designing test programs to determine failure modes, the investigator must appreciate not only the processes of surface preparations, cleaning, and pretreatments, but coating application techniques as well. He must be knowledgeable about the different types of coatings and their typical degradation and stabilization mechanisms.
Mills, George D.
President, George Mills and Associates International, Inc., Foley, Alabama