Softening occurs due to plasticization of the coating by absorbed water. Severe softening and swelling, particularly of the surface layer, may lead to wrinkling. Water spotting is usually due to contaminants in rain or other water that falls on a coating, but spotting is worse and is more likely to be permanent if the coating is softened by water. Blisters occur when there is water-soluble material in or under the coating. If water permeates the coating and dissolves the water-soluble species, it is likely that an osmotic cell will be created that will pull in more water, resulting in a liquid-filled blister. Blushing is a defect in which the surface of a coating becomes white or a lighter color due to absorption of moisture and formation of microvoids. Although haze is any dulling of a coating surface, in some cases, it could also be considered a form of blushing. Moisture may cause only a thin layer of microvoids at or near the surface of a coating. The resulting appearance will not be white, but the gloss will be low. Haze or whitening also may be due to water soluble or dispersible materials such as dispersants, rheology modifiers, and surfactants that come to the surface. Some coatings that adhere very well in the dry state have poor wet adhesion due to water that passes through the film and displaces the coating from the substrate or an intermediate layer. On drying, the coating usually regains its adhesion.
Possible causes of water sensitivity include incomplete cure, retention of amines and hydrophilic solvents, use of pigments with water soluble salts, and the presence of components that are water soluble or swellable. If incomplete cure is the problem, then the bake temperature or bake time may need to be increased or a catalyst may have to be added to achieve sufficient cure. Polar materials such as amines and hydrophilic solvents may be removed by higher or longer bakes or by being replaced by more volatile materials, but the coating may have to be reformulated to remove or reduce water sensitive materials. Water soluble/dispersible dispersants, rheology modifiers, and surfactants are a particular problem because they are needed for properties such as wetting, film formation, color development, and resistance to sag and defects, yet, unlike solvents, cannot be volatilized.
Most tests for water sensitivity involve immersing a coated panel in water or placing it in a humidity cabinet. Cycle tests tend to be more severe than continuous wet-only tests, presumably because wet-dry cycles leach more water-soluble materials or produce more reactions such as corrosion. After liquid water or vapor contact, films may be tested for a variety of properties. Water pick-up can be measured by weighing the panel before and after exposure to water. Softening can be quantified by measuring the change in hardness that occurs. A thermal mechanical analyzer (TMA) or other indentometer is particularly useful for evaluating hardness before and after water contact. Wet adhesion must be tested immediately after water soak. The surface of the panel is patted dry with a cloth or paper towel, but the coating still contains much water. If there is difficulty in identifying the water-soluble materials causing problems, the water used in the tests can be analyzed to see what has been extracted from the coating. It is a good idea to do both immersion and humidity testing because some coatings resist liquid water much better than they do humidity, whereas others tend to respond more to immersion. Resistant to water soak may be due to rapid swelling at the surface that prevents further penetration by water. Humidity, even if it is condensing humidity, often is more completely absorbed by the coating. Tests other than immersion or humidity also can be useful. For example, water sensitivity of interior latex paints often is tested in terms of scrub resistance and that of exterior coatings by their resistance to blistering. Outdoor exposure also can point up water problems, although it is not always possible to separate water effects from other processes.
"Coatings Clinic" is intended to provide a better understanding of the many defects and failures that affect the appearance and performance of coatings. We invite you to send your questions, comments, experiences, and/or photos of coatings defects to Cliff Schoff, c/o "Coatings Clinic," CoatingsTech, 492 Norristown Rd., Blue Bell, PA 19422; or email firstname.lastname@example.org.
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|Title Annotation:||Coatings Clinic|
|Author:||Schoff, Clifford K.|
|Date:||Feb 1, 2007|
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