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Laboratory and test-site testing of moisture-cured urethanes on steel in salt-rich environment.
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Laboratory and test-site testing of moisture-cured urethanes on steel in salt-rich environment.
  • Publication/ Report Number:
  • Resource Type:
  • Geographical Coverage:
  • Edition:
    Final report; Oct. 1,1996-Sept. 30, 1998.
  • NTL Classification:
    NTL-HIGHWAY/ROAD TRANSPORTATION-Pavement Management and Performance ; NTL-HIGHWAY/ROAD TRANSPORTATION-Materials ; NTL-HIGHWAY/ROAD TRANSPORTATION-Construction and Maintenance ;
  • Abstract:
    Three 3-coat moisture-cured (MC) urethane commercial products formulated for protecting new steel (SSPC-SP 10) and power

    tool-cleaned steel (SSPC-SP 3) surfaces against corrosion were evaluated; the total coating film thickness was about 75

    microns. Zinc-rich MC-urethane primers were used for SSPC-SP IO steel surfaces whereas the primers for SSPC-SP 3

    surfaces contained zero or a small amount of zinc: the same midcoats and topcoats were used for both steel surfaces. Sealers

    with film thickness of 25 microns were also studied for any potential effect on coating performance of the coating systems for

    power tool-cleaned surface. The volatile-organic-compound content of all the coating materials was below 340 g/L. The

    Federal Highway Administration (FHWA)-developed cyclic testing method was conducted to compare the performance of these

    coating systems. The test included freeze, ultra-violet light/condensation, and salt-fog/dry-air cycles. An aggressive outdoor

    marine exposure at Sea Isle City, New Jersey, was also performed for all the coating systems for comparison. A.number of

    physical and chemical properties of the MC-urethanes were examined to study their effect on coating performance. All of the

    coating systems maintained their total film thickness, topcoat hardness, and adhesion strength throughout the 4,000-h test

    period. The three commercial products were found to contain different chemical compositions and performed differently. All the

    coating systems had an adhesion strength about 10.5 MPa. No surface failures were observed on any of the test panels after

    the 4,000-h laboratory test; however, all of them developed creepages at an intentional scribe (undercutting). The laboratory test

    results in conjunction with the chemical analysis results suggested that pigment particle size distribution in the primers played a

    more important role than other factors in the formation of creepage at the scribe. A proper pigment particle gradation improved

    the coating performance. A chloride concentration of 20 micrograms per square of centimeter on the steel surfaces was found

    to be significant to reduce the coating performance at the scribe of SSPC-SP 10 steel surfaces; however, the chloride effect was

    found to be minimal for SSPC-SP 3 steel surfaces. Interestingly, all the scribe creepages increased linearly with the laboratory

    test time up to 4,000 h. Furthermore, the addition of a sealer to the primedmidcoafftopcoat systems unexpectedly impaired the

    performance at the scribe on SSPC-SP 3 steel surfaces when compared with those systems without a sealer. On the other

    hand, a coating system with sealer/primer/topcoat without a midcoat developed severe undetfilm corrosion probably due to

    insufficient film thickness.

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