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CL BLOG Continued from page 43
Annual recoating of facilities
facilities. The original coat is not removed but painted over. If I have a good applied coating and then I make successive recoatings over the original coating, assuming the surface prepara- tion for the application of the new coating is good, then can the coating disbond from the metallic substrate because of the increase in thickness?
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what is sometimes referred to as "trip- ping." This is caused by the styrene [WN\MVQVO \PM ÅZ[\ KWI\
Overcoating an existing coating can cause the coating beneath to disbond, resulting in separation at either the substrate/metal interface
or between existing coats such as separation between a topcoat and the prime coat. Stresses induced into the old coating system cause this damage by shrinking the new material as it dries and cures. In any case, performing an adhesion
test of the existing coating before any overcoat work is a good idea. Often, a crosshatch and tape pull test is adequate. 6)+- 1V\MZVI\QWVIT [\IVLIZL[ LMÅVM this procedure, and there is a test kit available. For severe applications such as immersion service or where a very high ÅTU Y]ITQ\a Q[ ZMY]QZML ILPM[QWV X]TT tests are recommended.
Depending upon the type of coat- ing, it can be "made" to decrease adhesion. Polyester, reapplied over itself too soon, can result in
It sounds as though you are deal- ing with an alkyd system. All coatings tend to lose adhesion with age. Alkyds tend to lose suf-
We have a mainte- nance program that includes annual re- painting of existing
pending upon the cosolvents employed or thinners used. Similar effects can be obtained with water-based systems, but tripping is not evidenced. Prepolymers "let go." Epoxy and polyurethane (amines)
can form carbamates on the surface and lead to delamination of subsequent coats. Long-term immersion of coatings results in reductions in bond strength when com- pared to original bond strength.
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If the adhesion is less than 100%, increased thickness will improve the likelihood of delamination, notwithstanding the fact that some systems have cosolvents that will N]Z\PMZ QUXIK\ \PM QV\MOZQ\a WN \PM ÅZ[\ coat. Bond strength determination in straightforward recoating situations can determine whether the original coating is "sound" or needs total removal.
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LQNNMZMVKM QV \PMZUIT KWMNÅKQMV\ M`XIV- sion (CE). Epoxies, for example, have a CE that can be four to 10 times that of steel. Even in moderate climates, the thermal cycle subjected to numerous overcoats will eventually exceed the elon- gation limits of the coating and cracking will appear in the coating. Consult the manufacturers of the coat-
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ings that you have in place as a starting point. If overcoating has gone on for years, examining the existing coating is a good place to start. Finally, intercoat delamination is a very likely scenario if surface preparation and ÅTU \PQKSVM[[ NWZ XZM^QW][ KWI\[ WN XIQV\ were not specified with consideration given to the stresses that would be induced during cure of subsequent coats of paint in future years.
Continued on page 47 March 2012 MATERIALS PERFORMANCE 45
All coatings have a limit beyond which overcoating is normally impractical. One dominant phys- ical limitation is caused by the
Apparent adhesion is what you ZMNMZ \W