Coatings World - February 2009

From Figure 8 it can be seen that post curing the coating in hot water or hot oil gives a softer/less brittle film than curing the coating in hot air at the same temperature. Indeed, the storage modulus is more than double up to 50°C. This confirms the hypothesis that both water and oil is taken up to by the coating, to some extent, working as a softening agent. The danger of this, however, is that depending on what chemicals are transported, this higher flexibility might only be temporary. If one were to load a hot water cured tank with for example hydroxide slurry (which is very hygroscopic in nature) all the remaining water could possibly be drawn out of the coating leaving behind a highly stressed coating full of “gaps”. This could in turn leave to cracking and, consequently, corrosion of the underlying steel structure.

CONCLUSION

A novolac epoxy-based tank coating was analyzed using dynamic mechanical analysis resulting in a greater understanding of the coating system and how it is affected by varying post curing time, temperature and medium. As expected, Tg was found to increase with post curing time and/or post curing temperature. A suffciently good Tg value (~100°C) was achieved by curing at 60°C for two days or 80°C for one day, regardless of cure medium. Tg was also found to be nearly independent of initial cure temperature ( 5°C - 40°C) as long as the coating was

post cured at an elevated temperature.

Good correlations were found between mechanical properties of the coating and real life testing. The post-cured film was denser and its glass transition occurred at a higher temperature, which resulted in a more protective coating that could withstand higher working temperatures and more corrosive chemicals.

Research into the effect of different post curing media showed that post curing in either hot water or hot oil resulted in a more flexible coating with a higher glass transition temperature compared to the coating post cured in hot air. This heightened flexibility might however only be temporary and so this needs to be studied further. CW

Acknowledgements

First of all, I would like to thank Vivian Farstad for introducing me to DMA as a valuable method for deciding coating properties. Secondly, I could not have written this article had it not been for the encouragement from my manager, Ståle Nordlien.

Bibliography

[1] Menard, Kevin P. Dynamic Mechanical Analysis: A practical introduction; 1^sted., CRC Press LLC, Florida 1999. [2] Hill, Loren W. J. Coat. Tech., 64, 1992, 29-40. [ 3] Rinker, E. B., Ashour, S.S. and Sandall, O.C. Ind. Eng. Chem. Res., 39, 2000, 4346-4356.