Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 266 of 331

testing THE EFFECTS OF SOLVENT ENTRAPMENT BY RICARDO J. ZEDNIK AND RON JOSEPH (1944-2011), EXPONENT FAILURE ANALYSIS ASSOCIATES, MENLO PARK, CALIF. There is a common perception that a thicker coating will necessarily provide better protection to the substrate than a thinner coating. Specifications nor- mally call for a minimum dry film thickness (DFT), but often may not mention a maximum. Practically, one of the considerations limiting the maximum coat- ing thickness is often cost. In applications where a functional coating is in- tended to provide critical protection to prevent corrosion, abrasion, or chemical attack, it may be tempting to significantly exceed the minimum specified re- quirements in the spirit of "better too much than too little." In this article, we discuss why this common misconception can lead to cata- strophic failure of a coating system, including cracking and delamination, which can leave the underlying substrate exposed and vulnerable. Paint manufacturers normally formulate their products with a blend of two or more solvents so that they evaporate gradually and uniformly. The most volatile solvents evaporate shortly after application. Less volatile solvents are designed to remain in the film until the coating has had an opportunity to prop- erly wet the surface. When the coating has the recommended thickness and the application conditions are appropriate (relative humidity, ventilation, temperature, etc.), the solvents fully evaporate as the coating solidifies. This results in a solid, high-quality, and coherent coating that has no voids and fully protects the un- derlying substrate, as illustrated in Figure 1. If indicated by the manufacturer, ad- ditional coats can be applied to build toward the desired total dry film thickness. In order to build a thick protective coating system, specifications often call for the application of two coats of the topcoat. The second coat helps to isolate the topcoat from the substrate and reduces the possibility that the environment (oxygen, moisture and chemicals) can diffuse to the substrate. Another reason for the second coat is that is often covers existing pin holes and voids in the first top- coat. Painters, however, are often tempted to apply the two coats wet-on-wet or, worse, still apply one thicker coat. This cannot always be blamed on the painters, since they are often pushed by supervisors or managers who want to get the job out of the spray booth quickly to meet production schedules. The downside is that when the wet film thickness is excessive, not all solvents are able to evaporate. This is because an impermeable layer of dry paint forms on the surface while the bulk of the paint is still wet; solvents are, therefore, prevented from efficiently leaving the surface, as shown in Figure 2(a). Instead, these trapped solvents form bubbles that substantially weaken the coating, often along the substrate interface, as shown in Figure 2(b). Although the solvents may eventually escape after the paint has fully dried, the bubbles are permanent, as shown in Figure 2(c). These voids result in a porous, non-coherent coating that directly compromises the integrity of the paint system. Pores significantly reduce the structural strength of the coating and form an easy pathway for fracture. Mechanical stress can lead to a delamination crack that propagates between voids, causing the entire coating to fail; the exposed substrate is now vulnerable to deterioration, including corrosion, abrasion, and chemical attack. 265

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