Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 267 of 331

(a) Figure 3 gives an example (b) Figure 1. (a) Properly applied coating of the recommended thickness allows all the solvents to evaporate. (b) This results in a solid, coherent coating with no voids that effectively pro- tects the substrate. of a severe case of solvent en- trapment that has led to a de- lamination failure of a single thick coat from an impervi- ous substrate. In Figure 3(a) one can see that voids are ab- sent near the smooth surface of the coating. Prior to the failure, a visual inspection of the coating's surface would have erroneously concluded that the coating was perfect- ly adequate. However, the de- lamination crack has propa- gated through multiple voids and resulted in complete fail- ure of the coating system. As shown in Figure 3(b), the coating was severely weakened by the numerous voids, which permeate throughout the un- derside of the coating. Solvent entrapment is often easily visible under a stereomicroscope because smooth rounded voids are produced. In fact, the surfaces of the voids appear to smooth and shiny, as can be seen in Figure 3. The voids could also be air bubbles that were entrapped when the coating was mixed or applied. To differentiate between entrapped solvent and air bub- bles, one can send a sample of the coating for analysis by Thermal Gravimetric Analysis (TGA) and / or Gas Chromatography Mass Spectroscopy (GC-MS). Even coatings that have been aged in the field for several years might still con- tain some entrapped solvents. TGA measures the weight loss of a coating as its temperature is increased. Provided this test is conducted with a control sample that is fully cured it is of- ten possible to determine if solvents are still present in the aged coating; however, it does not identify the solvents. On the other hand, GC-MS captures the solvents as they evaporate from the heated coating to both identify and quantify each sol- vent as it passes through the instrument. Accuracy of these measurements depends on several factors, including sample size. Coating dry film thickness is not the only variable that needs to be carefully controlled: the environmental condition under which a particular product is applied is also important. Manufacturers typically specify these conditions, which include temperature and relative humidity. Nevertheless, applicators may seek ways to accelerate the drying process by blowing air across the surface or ap- plying heat, particularly when multiple coats are required. Unfortunately, mod- ifying the application environment to achieve this "acceleration" often comes at a price: for example, air movement or high temperature will increase the rate of solvent evaporation. If solvents evaporate too quickly from the coating surface, once again, an impermeable dry "skin" will form. This thin layer will prevent further solvent evaporation; solvent entrapment caused by this mechanism will 266

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