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July 7 - July 14, 2017

What is the effect on the permeability of a coating when you add additional coats, either at the time of the original painting or as a maintenance activity?


Selected Answers

From Michael Quaranta of OPERATIONS 40 on July 14, 2017:
Now for some certified numbers Two Samples - PERMEANCE WVT ASTM E96 0.451 / 0.300 WVT ASTM E96 2.58 x 10-8 1.72 x 10-8 PERMABILITY WVT ASTM E96 0.0126 0.198 WVT ASTM E96 1.83 x 10-11 1.74 x10-11 How do these numbers rate for a water-based epoxy concrete coating? Samples were 0.028 and 0.040 thick.

From Bill Slama of International Paint/Ceilcote Products on July 13, 2017:
There is a math error in my previous submission. For one coat at .05 perms and one coat at .01 perms, the combined lining would have permeance of 0.00833 perms, NOT 0.0333 perms.

From Bill Slama of International Paint/Ceilcote Products on July 12, 2017:
Permeability is, of course, a property of a material regardless of thickness. The ability of a given film (thickness) to allow permeation is "permeance," which is the material permeability (typically in perm inches) divided by the layer thickness. Permeance is an important property for coatings and linings that will be immersed in aqueous solutions at elevated temperatures. Permeability, as the term is used related to coatings and linings, refers to the ability of water to pass through an organic film, not to be confused with "porosity."        For a given system, the rate of water transmission is proportional to the difference in the partial pressure of water on each side of the film. Where multiple layers are used, the combined permeance is equal to 1 divided by the sum of the individual reciprocal permeance values of each layer of the total coating/lining system. So if the lining base coat has a permeance of .05 perms and the top coat has a permeance of .01 perms, the combined lining would have a permeance of .0333 perms.

From john schultz of o-gee paint co on July 12, 2017:
The ability of moisture to permeate any individual coating will not change. However, the system as a whole will become less permeable with an increasing number of coats. Pinholes of individual coatings will also be bridged over, further reducing the ability for moisture to migrate through the film(s).

From Zenith Czora of Parex Davco on July 12, 2017:
It is dependent on the permeance of the additional coating. The additional coating should have a similar perm rating or permeability to the previous coating. Permeability is a desirable property to allow water vapor to escape from the substrate, but if the permeability is too high, carbon dioxide can diffuse from the atmosphere onto the substrate resulting in carbonation. The rate of CO2 diffusion into substrate mainly depends upon the thickness of the film and partly depends upon how compact is the film. The ideal dry film thickness of the coating that can also provide anti- carbonation properties is  around 300 um dft (about 12 mils).

From Trevor Neale of Blastco on July 11, 2017:
Further coats will not affect the permeability of the coating, but it may delay moisture penetration through to the substrate.

From Bryant Chandler of Greenman-Pedersen, Inc. on July 10, 2017:
The permeability is decreased with additional layers of coating. The amount of decrease will depend on the thickness added, the type of crosslinking or cure and the type of fillers.

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Tagged categories: Asia Pacific; Coatings Technology; EMEA (Europe, Middle East and Africa); Latin America; North America; Permeability


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