Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 286 of 331

testing TESTING COATINGS FOR ADHESION TO SUBSTRATE BY ROBERT D. ATHEY ATHEY TECHNOLOGIES, EL CERRITO, CALIF. The objective of painting is to make the coating adhere to the substrate. Except in rare cases like the "strippable" coatings, adhesion should be quite good. This promotes resistance to chipping on impact or flaking on bending. Strictly speaking, the binder material within the coating formulation has two functions. It has to adhere the whole film to the substrate (an interface adhesion), and it has to adhere the pigment particles together or encapsulate them so they do not fall out of the film (cohesion). If the substrate has previously been coat- ed (e.g., with a primer), the interface bond is particularly important. Testing for adhesion is fraught with complications because the physical failure may be cohesive within the coating film or the substrate or maybe an actual adhesive failure in the interface, whether the interface is with the basic metal substrate or with a primer (or other previously applied) coating. It is not enough to say it fails; how it fails must also be recorded. TENSILE NORMAL TO INTERFACE Many attempts to characterize adhesion use a direct "pull-off" technique. Some of the ASTM standards are listed in Table I. The most commonly used is the tape adhesion test (nominally for steel), although the substrate may af- fect the test result. In one instance, an expert witness in a lawsuit used the test but failed to record the failure as substrate-cohesion derived. Because adhesion to the substrate was at issue, the substrate cohesion failure invalidated this expert's advice. In another instance, a plastic weather shield for an on-deck naval computer de- vice had three coats of paint: an epoxy chromate primer, an acrylic silver "con- ductive" coating and a polyurethane top coat. At a scratch, the paint came off on the tape, with silver on both the tape and the weather shield. Because this was a cohesive failure within the thermoplastic filling between the two thermoset sandwich layers, it was recommended to change the specification. The quicker solution was to find a weaker tape that did not pull off the coating. The tape must be clearly and unequivocally agreed on by buyer and seller. The epoxy-cemented lugs (ASTM D 4541) work quite well when the epoxy adhesive wets and adheres well to the coating. It has been used to show the strength of flame sprayed zinc onto plastic computer cabinetry. Although the lugs are relatively inexpensive, some testers save them for reuse after sanding off the adhesive and coating from the flat adherent surface. This is not rec- ommended because it changes the geometry of the lug (at best making it short- er; at worst making the interface angled so that the next test is not a tensile but a shear test.) There are other adhesion tests reported in the literature, although they do not test coating as applied to the substrate. Some of these involve the use of ten- sile test machines with paint applied to the substrate with a textile strip em- bedded in the paint (the tensile machine pulls substrate and cloth) or have 285

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