KTA-TATOR,
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The Case of the Vinyl That Wouldn't Adhere
By Eric Kline, KTA-Tator, Inc.
Recently, KTA-Tator, Inc. was contacted by a general contractor concerning a paint failure on a new bridge. Much of the steel for the bridge had been shop blast cleaned and prime coated with an inorganic zinc rich primer, then field coated with two coats of a vinyl topcoat. Field topcoating efforts had been undertaken in late May, and within a few weeks large patches of vinyl coating had delaminated from the zinc rich primer.
When the KTA Consultant visited the jobsite, he discovered many areas where the vinyl topcoat had disbonded in large patches (exceeding one square foot). In addition to these areas of spontaneous failure, many other locations were found where the vinyl, while not spontaneously failing, had very poor adhesion.
In every case, the failure appeared to be a clean adhesive break between the vinyl and the underlying zinc rich primer. The zinc rich primer itself had excellent adhesion to the steel, and appeared to be fully cured when subjected to a solvent rub test.
When conducting any failure analysis, one of the most important procedures is to look for patterns to the failure. Unfortunately in this case, there seemed to be no pattern. Failure was noted on piers close to the embankment, and also on spans in the middle of the bridge. Failure occured on all sides of the bridge and even beneath the structure in sheltered areas. There were even a few small plates which had been spot blast cleaned in the field, then field primed and topcoated. Adhesion on these plates was also very poor, indicating that the problem was not related to the priming procedure used in the shop. In all, twenty areas were evaluated for adhesion, and eighteen failed.
The KTA Consultant collected many samples of delaminated paint chips, along with scrapings of the underlying zinc rich primer. Scrapings were also taken from the two nonfailing areas. In addition an unopened five gallon container of vinyl. topcoat which was left over from the painting operations was obtained.
While many dry film thickness measurements were taken on-site, additional thickness measurements were obtained microscopically in the laboratory. Neither the field or laboratory measurements indicated thickness to be a problem. Although there were some areas where the vinyl was almost twice as thick as specified, there were many failed areas where the coating thickness was within specification. Likewise, the field thickness measurements revealed that the zinc rich primer was applied at approximately 2.0 mils, in accordance with specification.
Microscopic observations did not detect the presence of any contamination on the backside of the disbonded chips. Furthermore, when infrared spectroscopy was used to compare the failing paint to both the nonfailing paint and the field wet sample retain, no differences could be seen, further ruling out the possibility of contamination or adulteration. contamination also seemed an unlikely possibility, based on the widespread nature of the failure, and the fact that it occured on both the shop and field primed steel.
Infrared spectroscopic analysis on the scrapings of zinc rich primer taken from beneath failed areas was also performed. These spectra were compared to a spectrum of a fresh sample of inorganic zinc obtained from the manufacturer and applied by KTA to steel test panels. once again, no differences were detected, ruling out the possibility of contamination or the substitution of an unspecified coating material.
However, the KTA laboratory was able to duplicate the field observed failure. Primed test panels were topcoated with the unused field vinyl and allowed to air dry for approximately two weeks. The adhesion of the coating was evaluated, and was found to be very poor, with the vinyl topcoat delaminating cleanly from the zinc rich primer. Therefore, although the analytical testing to date had not revealed any reason for the failure, it now appeared certain that the problem was somehow related to a product deficiency, rather than an application or contamination problem.
There are several different types of vinyl coatings on the market. Perhaps the simplist and most common type is a copolymer of vinyl chloride and vinyl acetate, sometimes modified with a small amount of vinyl alcohol. Paints based on this type of resin are used quite often as intermediate coats or topcoats in a multicoat vinyl system, and adhere quite well to other coats of vinyl. Their adhesion to steel or other metallic substrates, however, can be poor.
A second type of vinyl resin is based on vinyl chloride and vinyl acetate, with approximately 1% of maleic acid. The incorporation of a small amount of maleic acid is necessary to provide good adhesion to metallic substrates. This type of resin is used quite often in vinyl primers.
Based on the field observations and laboratory results, it appears that the vinyl coating used on the bridge may have been based on the first type of vinyl resin, rather than the maleic acid modified version necessary for application over metallic substrates. Although infrared spectroscopy was performed on the various paint samples, it is not capable of detecting small amounts (1%) of maleic acid. Accordingly, a different test method was used.
The test used for determining the presence of maleic acid in a vinyl resin is an acid-base titration, a very tedious andc omplex procedure. This test method was performed on the wet sample of vinyl coating, and no maleic acid was detected.
The cause of the coating failure seemed apparent, however one last step in confirming the failure was carried-out. A sample of a maleic acid modified vinyl was purchased from a different manufacturer and applied to zinc-rich primed panels. After air drying for two weeks, excellent innercoat adhesion was found.
Repairing the problem in the field entailed removal of all poorly adherent topcoat material via high pressure water blasting and the subsequent application of a maleic acid modified vinyl primer system.
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