Measurement of adhesion properties between topcoat paint and metallized/galvanized steel with surface energy measurement equipment.
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Measurement of adhesion properties between topcoat paint and metallized/galvanized steel with surface energy measurement equipment.

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Measurement of adhesion properties between topcoat paint and metallized/galvanized steel with surface energy measurement equipment.
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      The objectives of this research project are: (1) Compare the adhesion properties of NEPCOAT-approved topcoat paint over metallized or galvanized steel. Use “surface-energy” measuring technique to characterize the wetting properties of the liquid paint on the profiled zinc surfaces. Explore correlation between the adhesive strength and the liquid paint wetting properties. As control the adhesion properties of topcoat paint over zinc primer painted steel substrates will also be measured. (2) Investigate various factors affecting the adhesion of topcoat paint over galvanizing. (3) Report and recommend practices that produce the best adhesion of NEPCOAT-approved topcoat paints over metallized and particularly galvanized steel surfaces. We prepared four different types of test panels coated with five different commercial paint systems. The paint systems include four systems adapted from the NEPCOAT list of intermediate and top paints qualified for bare steel, and one system of epoxy sealer for metallized surface. Four types of substrates were used for fabricating the test panels: (1) galvanized steel with mechanical grinding to produce rough surface, (2) galvanized steel with blast profiling to produce rough surface, (3) galvanized steel stored indoors for two weeks before blast profiling and painting, and (4) metallized steel with inherent roughness due to the thermal spray process. We recorded, as a function of time, the contact angle of droplets of freshly prepared liquid paints on the replicas of the substrate used for spray painting. The cured test panels were subject to pull-off strength tests according to the ASTM D4541 standard, and the X-cut tape tests according to Method A of ASTM D3359 standard. Images of the pull-off test break surfaces were photographed and examined. We analyzed the correlation between the pull-off strengths and the contact angles. The correlation provided insight on the relative adhesive strengths of the different paint-substrate pairs. We concluded that (1) the NEPCOAT paints could be used for galvanized and metallized steel to obtain comparable adhesion performance as that of the zinc-rich organic primer coated steel, (2) although the NEPCOAT intermediate paint on the metallized surface has adequate pull-off strength to pass the inspection, it is highly recommended that the state DOT specification of the use of sealant is strictly followed, (3) although the exposure to atmosphere after galvanizing is commonly recognized as a problem for paint adhesion, we found that a time delay of two weeks between galvanizing and profiling/painting is permissible if the galvanized steel is stored in the normal indoor dry atmosphere, (4) we think a refined quantitative correlation between pull-off strength and contact angle could be useful for optimizing the paint-tosubstrate match.
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