Three-layer polyethylene (3LPE) coatings are widely used as mainline coatings for buried oil and gas pipelines. Liquid epoxy is a common Field-Joint-Coating (FJC) solution due to its ease of application. Unfortunately, field applied liquid epoxy on 3LPE results in poor bonding between the liquid epoxy and the high-density polyethylene (HDPE) outer layer. This is because the non-polar HDPE has no functional groups that can interact/react with the polar epoxy. One current method for increasing the surface energy of HDPE is flame treatment, which can be inconsistent in the field. Alternative methods such as corona discharge and plasma are impractical in the field according to many operator companies.
Chemical oxidation via six different solutions is used herein to oxidize the HDPE surface. The effect of treatment time and temperature has been studied. ATR-FTIR and XPS tests clearly show that polar functional groups such as carbonyl and hydroxyl groups are created on the surface after treatment. The Dyne number of the HDPE increased from 38 dyne/cm to 70 dyne/cm. Pull-off strength according to ASTM D4541 shows the adhesion of HDPE to liquid epoxy increased to more than 2200 psi, significantly more than the highest reported result in the literature after flame treatment, which is 1100 psi. The result of adhesion tests after 28-days immersion in hot water shows that chemical treatment has no adverse effect on HDPE in the long-term.
Different treatment times and temperatures were studied to optimize the procedure for field application. Currently, the focus is to transform the best solution into a ready-to-use paste.
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