TESTING OF COATINGS FOR MOISTURE VAPOR TRANSMISSION

摘要

The protective coatings are our main emphasis, and many coatings are intended to protect something from water or moisture. These are termed moisture barrier coatings, and proof of their moisture barrier properties can be made by assessing moisture damage (such as corrosion, which is complicated and suspect), or by a more rigorous moisture vapor transmission measurement. First let us examine how moisture may migrate through a film. Three migration methods are used to explain the differences of scale perceived in barrier film assessments. We distinguish among them by the form of the water going through and the rate and amount that passes through. The largest scale of water transmission is the macro scale, where liquid water finds crevices, cracks, or pinholes to pass through unobstructed. The next smaller scale is the micro scale, where the water, as liquid, dissolves in the film, and migrates through to evaporate, if allowed, from the other side. The smallest scale of migration is the molecular scale, where vapor molecules of water find their way through the "free volume" of the film. Remember that polymer molecules are a random array of long strings, like a plate of spaghetti, and the free volume is the empty space between those strings (where the spaghetti sauce would go). Moisture vapor transmission is important in many protective systems. Soggy potato chips are a result of vapor transfer of water through the bag. In finishing, corrosion and even blister formation occurs under films. Humidity changes can stop and start the corrosive action of filiform corrosion on clear coated steel. One can actually see the Liesgang Effect of concentration gradients as the rusting line pushes its way forward under the film. However, the main point was that the moisture vapor permeating from the outside air had to feed the growing corrosion front enough water to swell the coating/steel corrosion interface front.