Internal stress.

Although internal stress is not itself a defect, it often is a factor in defects such as cracking, lifting, curling, chipping, and peeling. Coatings are exposed to many external stresses such as impacts, vibration, abrasion, and scratches, but stresses also can occur within coatings. Internal stress develops when a coating tries to contract because of crosslinking, cooling, or a drop in humidity and is constrained from doing so by the substrate to which it is attached. Figure 1 shows cracking and curling due to internal stress, and loss of adhesion also due to stress and poor surface preparation.

Crosslinking causes stress in a coating because of shrinkage. Overbakes and multiple bakes (as with high bake repairs in an auto factory) cause additional stress and repairs give thicker films, which are more likely to crack. However, most stress comes from temperature and humidity changes (due to thermal and humidity expansion/contraction mismatches between layers and between the coating and the substrate). If the stress exceeds the tensile strength of the coating, the coating will crack. If the stress exceeds the adhesive strength between coating and substrate, the coating will lift. Defects such as pinholes, pops, and acid etch can act as stress concentrators and contribute to failure. Weathering and mechanical cycling due to thermal expansion and contraction can reduce tensile strength and adhesion, making a coating less resistant to internal stresses.

[FIGURE 1 OMITTED]

A case history involving an architectural paint may help in the understanding of internal stress and how it occurs. The paint was a new hard, tough exterior alkyd designed to be highly mildew resistant (for use in the Southeastern U.S.). It was introduced in late summer. Much of it was applied in the autumn, usually over multiple coats of old paint. The winter was very cold and the spring had warm days and unusually cold nights. In the spring, large strips of paint lifted and peeled, often down to the wood. Many of the strips rolled up like window shades and numerous sheets of peeled paint were available for testing. We carried out investigations on the specimens with a variety of techniques, including microscopy, hardness tests, and thermal mechanical analysis (TMA) for softening temperatures and indentation hardness/creep. We concluded that the root cause was high internal stress due to application of a hard topcoat over softer undercoats along with heat/cold cycling that caused expansion and contraction of the wood and the coatings. Stress was relieved on warm days, but built back up again when the temperature dropped at night. In addition, the alkyd coating continued to cure, became harder, and froze in more and more stress as time went on. The paint had to be reformulated to be more flexible and have less internal stress. The only internal stress test that we had at the time was a simple one involving the application of paint to one side of long Mylar[R] (PET) strips and hanging them from the ceiling. We watched the strips roll up and down with changes in temperature and humidity.

More quantitative means for measuring internal stress are available. Probably the best is the CoRI Stressmeter (made under license by Elcometer, see www.elcometer.com). The Stressmeter measures the extent of deflection of a coated steel shim that rests horizontally on two knife edges. The apparatus is in an enclosure so that temperature and relative humidity can be controlled. Internal stress also can be measured by the deflection of a coated metal strip (like the bimetallic strip used in a thermostat) held at one end as in ASTM D 6991, "Measurement of Internal Stress in Organic Coatings by Cantilever (Beam) Method." See papers by D. Perera and coworkers for detailed information on both methods and results from them, particularly the Stressmeter.

"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 [e-mail].