CoatingsPro Magazine

JUL 2013

CoatingsPro offers an in-depth look at coatings based on case studies, successful business operation, new products, industry news, and the safe and profitable use of coatings and equipment.

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there is a family of cadmium pigments that is referred to as mercadmium, which contains elevated levels of mercur y and cadmium. Painting an Aboveground PVC Water Main A client of ours will be suspending a polyvinyl chloride (PVC) water main beneath a concrete bridge and wishes to have the pipe painted for ultraviolet (UV) protection. We don't believe that UV degradation on the PVC will be a problem because the project is in Vancouver, British Columbia, where the climate is mild and the UV is not too intense. The pipe will be sheltered from rain and sunlight most of the time; however, there may be instances when it does receive exposure to sunlight and windblown rain. We plan on having the new pipe thoroughly cleaned to remove any oil, grease, dirt, etc., followed by light sanding with 350- to 400-grit sandpaper to abrade the surface. For coating, there appears to be a number of systems including the following: • System No. 1 — bonding primer: water-based emulsion top-coated with one or two coats of exteriorgrade latex • System No. 2 — bonding primer: water-based emulsion top-coated with one or two coats of an exterior alkyd • System No. 3 — one or two coats of polyurethane (PUR) The environment is not severe, so I favor the first system because of my concern for possible attack and longterm degradation of the PVC by solvents in Systems 2 and 3 or possible leaching of solvents through the pipe wall into the potable water over a period of time. Does anyone have experience painting aboveground PVC pipe for potable water service and what coatings were used? I have used System No. 1 with ver y good resu lts. You may develop a mildew problem, but I don't see that as a serious concern. Nu merou s te s t s have b e en conducted that have concluded Q: A: A: Unfortunately, without diagnosing the failure, trying different systems may leave you open to the same problems again. that UV exposure reduces the impact strength of PVC by approximately one third of its original value. There has not been a lot of research into the appropriate paint system; however, the plastic pipe industry typically recommends a waterbased material rather than something like PUR. Our company specifies acrylic latex for all exposed PVC and chlorinated PVC pipe. Wet Adhesion Test on Organic Finishes We are checking wet adhesion of an organic coating system that consists of an epoxy primer with a poly uret ha ne topcoat over a zinc phosphate G-90 hot dip ga lva nized steel substrate. The G-90 designates a coating weight for the zinc layer that is 0.0008-inch (0.02 mm) thick. After full cure, the coating passes the dr y adhesion test. The wetness test that I am running consists of immersing the part in water for 24 hours and damping the moisture off after that period until dry so the tape will stick, and using a multi-blade tool to scribe a cross-hatch into the coating down to the substrate. We put the tape on and the coating consistently fails the adhesion test. Is this the proper means by which wet adhesion should be checked? Why do we pass dry adhesion and fail wet? The G-90 zinc finish is chrome rinse-free and water-quench free. We run it through a zinc phosphate line and want to avoid hand scuffing parts to gain a mechanical tooth on the surface. The condition of the G-90 is bright with a medium spangle. I believe the zinc phosphate is the culprit — provided the mixing of the coating is correct, the environmental requirements are met, the coating thick- Q: A: nesses are right, and the proper cure/ recoat times are followed. I suspect that a plain detergent wash, followed by a freshwater rinse to a neutral pH, drying, and then coating will yield better adhesion than what you are getting. Nothing will be as good as abrasion. I think the zinc phosphate does not do well in submersion service. How thick are the individual coatings? You should try and get some numbers on adhesive strength. Many state they get good adhesion to galvanizing but not in ser vice under stress. If you immerse the sample in water, t here may be suf f icient time for t he z i nc pho s ph ate i nter f ac e , or zi nc phosphate itsel f, to absorb enough water to reduce the strength below critical. Un f o r t u n a t e l y, w i t h o u t diagnosing the failure, trying different systems may leave you open to the same problems again. Any effort to use a different system from the one you have already used, assuming it has been recommended and previously assessed as suitable, is unfair for that supplier/ manufacturer and may result in the same problem with the next supplier/ manufacturer. I suggest you go to the current supplier/manufacturer and have them offer advice on the problem. Also, let them know you are having it thirdparty reviewed. O u r c o mp a ny s u c c e s s f u l l y ut i lizes polya mide epox y on many ga lvanized immersed surfaces quite regularly. I f ind better success in coating ga lvanized surfaces when the dry film thicknesses are kept lower vs. higher. For instance, I like to keep the total system thicknesses less than 6 dr y mi ls (152 µ m) — achieved in multiple coats. CP A: A: A: July 2013 g www.coatingspromag.com 19

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