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June 17 - June 21, 2019

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 Zenith Czora of Master Builders Solutions on July 16, 2019:
 The additional coating to be applied over the existing coating should have at least the same permeability rating or slightly higher to allow moisture to escape but does not allow bulk liquid to penetrate. Adhesion and permeance properties of the existing coating should be checked before applying the additional coating.

From Warren Brand of Chicago Corrosion Group on June 25, 2019:
Great question - and permeability is a poorly understood concept. The guiding principal here is Fick's Law of Diffusion (Google it if you're interested.), which basically says that if you double the thickness of a semi-permeable membrane (which all coatings and paints are), then permeation drops by half (There are a variety of caveats to this, the most important of which is how much moisture is being held in the membrane,). However, a good friend who is a physicist at NIST explained it this way. Imagine placing screen doors one on top of the other. The more level of doors, the more "tortuous" a path liquid (or in our case, moisture vapor) must take. MIC materials do the same thing - the MIC particles lay "flat" within the plane of the paint (regardless of whether the paint is on a vertical or horizontal surface) creating a hole bunch of plates which make the path through the paint (for moisture vapor) more tortuous. Hmmmm. Sounds like it's time for a blog.....

From Laurie Mackay of Blueocean Group on June 16, 2019:
Interesting, slightly ambiguous question: Let's have an insight on pigmentation. Barrier (MIO, Glassflake, Mica), Sacrificial (Zn) and Nodal (TiO2 colour). Each brings a different role to play in the coating film, either in the primer, intermediate or topcoat. If the pigment being used was barrier type, then I would suggest a review of the PVC/CPVC to understand how effective the reduction of permeation will actually be using ASTM E96 & ASTM D1653. Permeability: In simple terms, I understand the phenomena to be: Permits the passage of water vapour or oxygen/hydrogen gas. The rate of permeability depends on film thickness and pigmentation. Protective coatings with barrier pigments slow down the passage and diffusion of moisture and vapour. Organic coatings can transmit water, electrolyte and oxygen readily enough to permit the corrosion cycle to initiate. The coatings industry also records failure issues through osmosis and the cold-wall effect (contributory factors). It's not all negative, though. Many coatings are designed to permit equilibrium of vapours to mitigate blisters on porous substrates, e.g., wood & concrete. Maintenance coating projects must permit an aggressive (hot) solvent to soften/etch the previous topcoat and achieve adhesion. That's enough from me.

From Aajjay Sunkay of ASSETREIFURB ENGINEERRS on June 16, 2019:
When you add an additional coat at the time of original painting, permeability is a function of how the cross-linking of the polymer takes place during cure and how tightly the structure is formed by resin polymer after polymerization. Also, permeability  depends on which type of filler you are using in coating formulation: it's inertness, size, shape, and aspect ratio. Also, it is important that the loading of fillers into the coating, which decides alignment of these fillers after application, results in creation of a tortuous path and lengthy path. Hence,  even if you add the addtional coat, it may or may not increase the resistance to permeability. It is most of the time inherent characteristics of the coating rathe than adding an additional coat to the original painting. The old coating needs to be treated as a new surface and must be well prepared with sweep blasting to have a profile. Then adding a new maintenance coat increases the resistance to permeability of old coat. 

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

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