Evaluating sag resistance with a multinotched applicator: correlation with surface flow measurements and practical recommendations

Sag is a coating defect that results from excessive, gravity-driven flow after deposition. Accordingly, characterizing resistance to sag is critically important. In this paper, sag resistance predicted using a multinotched applicator test is compared with results obtained using an in situ particle tracking technique that measures surface velocity. Four commercial latex paints dried on substrates inclined at three angles were investigated. The results are used to provide insight into the strengths and limitations of using a multinotched applicator to evaluate sag resistance. For coatings dried on vertical surfaces (90°), the suggested condition for the multinotched applicator, sag lengths found by particle tracking show differences between paints that the multinotched applicator ranked as identical. At smaller angles (e.g., 10°), the resolution of the multinotched applicator test is greatly enhanced owing to a reduction in the shear stress difference between adjacent coated lines. Based on these results, specific recommendations are made for successfully employing a multinotched applicator to evaluate sag resistance based on user-specific goals.