Blister Test for Measurements of Coating Adhesion and Degradation

Adhesion is one of the key parameters that affect the protective capability of organic coatings. Nevertheless, the adhesion measurement techniques commonly used are inaccurate poorly quantify the adhesion strength value, making the study of adhesion difficult.  In addition, there is a lack of knowledge of the role of pretreatments in improving coating adhesion.

The effects of surface topography and non-chromate conversion coatings (NCCs) on adhesion of non-pigmented epoxy, polyvinyl butyral (PVB), and Aviox CF on AA2024-T3 substrates were studied using the blister test.  This approach uses a substrate with a through-hole that is bridged by a coating on the top surface.  The hole was pressurized using D.I. water or 0.5 M NaCl at constant pressure or constant infusion rate. The adhesion strength can be assessed using the fracture energy calculated from the pressure, height, and radius of the blister. The NCCs Alodine 5200 and Alodine T5900 were applied prior to coating of the substrate.

The results show that the non-pigmented epoxy primer provides very good adhesion; it often fails cohesively before delaminating. None of the epoxy-coated samples treated with NCC showed delamination during constant pressure experiments using D.I. water or in 0.5 M NaCl. Therefore, the effect of NCC could not be assessed when using non-pigmented epoxy. A degrease and deoxidize treatment greatly improved adhesion of the epoxy and PVB compared to the as-received condition. In addition, roughness increased adhesive strength of the PVB. Both epoxy and PVB experienced plastic deformation during the test, so that the adhesion strength assessment was only qualitative. Aviox CF allows successful blister formation without plastic deformation. Testing of Aviox CF coated samples using D.I. water as pressurizing fluid suggests that adhesion strength followed the following trend: No Pretreatment ≈  Alodine 5200 > Alodine T5900.