Weathering and surface defects.
In addition to pits, defects caused by weathering include water spotting, acid etch, and blisters. Water spots, which usually are rings rather than spots, are caused by residues from rain water or hard water droplets from car washing. The residues settle down into the coating and hydrated carbonates hydrogen bond to the surface. Acid etch is a direct result of weathering that includes acid precipitation (rain, dew, or fog), often compounded by pollution fallout. Hydrolysis plus photo-oxidation leads to chemical bond breaking, embrittlement, and erosion, resulting in low gloss irregular spots rather than rings. Etch spots harm appearance, but also can threaten protection because stresses can concentrate at them and cause cracking. There have been many cases where cracking in the field was associated with etch spots. Ironically, as gloss decreases with weathering, water rings and etch spots become less noticeable.
Blisters are dome-like defects that occur on weathering or other exposure to water or moisture vapor. Salts in or under the coating and residual hydrophilic solvents or surfactants in the coating attract water and cause it to be absorbed. The coating (baked or air-dry) acts as a semi-permeable membrane, takes in water, and then forms blisters to relieve the pressure that builds up.
The temperature and humidity cycling involved in weathering tends to increase stress at the same time that degradation processes are making the coating less resistant to stress. For example, fresh coatings usually are very resistant to cracking under normal use stresses, but the weathering process reduces tensile strength, elongation and fracture resistance at the same time that internal stress is increasing. Stress may be released in the form of cracks. The increase in stress also may lead to a loss of adhesion to a substrate or undercoat. This may be localized or occur in great swatches. Adhesion also may be lost due to exudation during weathering of material from a substrate (for example, plasticizer or mold release migrating from a plastic) or a lower coating layer.
Although not a point defect, scratch and abrasion are more likely to occur as a coating weathers. This is due to surface erosion and increasing brittleness that lowers resistance to sharp objects, abrasives, car wash brushes, etc. A substantial decrease in scratch resistance can occur after only a few months and continue to drop for years. Until recently, auto companies only required testing of fresh panels, but they have begun to realize that good initial scratch and abrasion resistance may not mean good resistance after a few months. They now ask for data from weathered panels as well.
Resistance to stones and other kinds of impact depends on the mechanical properties of coatings layers and the adhesion between them. Degradation and embrittlement with exposure can lead to more surface damage from stone impacts, but may or may not make the chip rating worse. It may well change the locus of failure to one of the upper layers. Stresses in the coating probably affect chips, but little is known about the relationship. Certainly it would seem logical that a highly stressed coating would be more likely to respond to being struck by a stone by cracking or chipping.
"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 Norristoum Rd., Blue Bell, PA 19422; or email firstname.lastname@example.org.
By Clifford K. Schoff
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|Title Annotation:||Coatings Clinic|
|Author:||Schoff, Clifford K.|
|Date:||Sep 1, 2007|
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