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The Case of the Coating That Wouldn't Stick to Stucco
By William D. Corbett, KTA-Tator, Inc.
The management of a West Coast condominium complex contacted KTA concerning a catastrophic coating failure on the exterior portions of several condominium units. A flexible, waterproof coating applied six months earlier had developed large blisters and was peeling and disbonding. Because of the unsightly appearance and concern over the protective capacity of the coating in this condition, KTA was engaged to launch a field and laboratory investigation to determine the cause of the failure and make recommendations for remedial action.
Background information is extremely helpful when conducting a coating failure analysis, including the time of year that painting was performed, weather conditions, governing specifications, etc. Unfortunately, in this case, any background information available was sketchy, however, the following information was revealed. The exterior of the condominium units were finished with a stucco surface followed by the application of polyvinyl acetate-acrylic material. The coating was in service approximately six months before the failure was noticed. Based on this background information, the following field investigation was conducted.
Field Investigation
The field investigation consisted of observing the pattern and extent of failure, determining the moisture content of the stucco, conducting chemical testing, and conducting subjective knife probing to assess the adhesion characteristics of the coating.
The coating in the failing areas consisted of a tough, rubbery outer layer and a soft "mushy" under layer, generally observed to be very moist and sticky. Predominant sources of water entry were noted at cracks in the mortar joints between blocking, at ledges and balcony edges and at the base of the structures (foundation). The surfaces beneath the coating were apparently wetted by capillary action moisture permeation. A few areas did exhibit pinholes in the coating. While this is certainly a source for moisture, areas free of pinholing were also catastrophically failing. In the "worst" areas, actual pouches of moisture-saturated, soft coating were present.
The moisture content of the stucco was determined using a Delmhorst Moisture Meter adapted with 3/4" pin electrodes driven into the stucco. Readings varied from wet to dry, however, the failing sites usually read wet, while non-failing areas read dry.
In addition to moisture content, field chemical tests were performed in order to better assess the mode of failure. A small sample of the stucco was powdered and added to an equal volume of distilled water. The measured electrical resistance was low, and the chloride concentration extremely high. This indicated the presence of considerable electrolyte - a substantial part being chlorides. The pH was 11 (very alkaline; neutral is 6-7).
Finally, subjective knife probing revealed that the weakly bonded area generally extended 3" to 6" beyond the visibly failed area.
Review of the Field Investigation
- The failing coating consisted of a tough rubbery outer layer and a soft, mushy under layer.
- The worst areas correspond with sources of water entry.
- The stucco beneath the failing areas was wet, while non-failing areas were relatively dry.
- The electrical resistance of the stucco beneath failing coating was low; the chloride content high; and the pH alkaline (11).
With the field investigation complete, samples were returned to the KTA laboratory for analysis such that the mode of failure could be pinpointed.
Laboratory Testing and Failure Cause
Laboratory testing consisted of a microscopic evaluation of the failing and non-failing coating, and infrared spectroscopic analysis of the tough outer layer and the soft under layer of the failing coating.
After extensive field and laboratory testing, the failure mechanism was determined to be saponification of the binder portion of the vinyl-acrylic coating directly adjacent to the stucco surface.
Saponification is another term used to define alkaline hydrolysis, wherein the action of water and alkalinity serves to break an ester down to a carboxylic acid and an alcohol. If the pH remains alkaline, the carboxylic acid will be present in the form of a carboxylic acid salt (i.e., a carboxylate anion combined with a metal cation, such as sodium or calcium). The result is the destruction and weakening of the film at the substrate interface.
The presence of the alkaline water under the film caused the saponification of the acrylic coating. (Alkalinity alone without the water will not cause the problem.) Saponification will cause a variety of changes in the physical characteristics of the film as well. First, the acrylic film becomes rigid. Second, exposure to the alkaline solution from the stucco forms a hydrophilic layer of low molecular weight calcium soap beneath the film which attracts additional water and causes the saponification to spread. Finally, the chemical "anchor" is removed from the film which results in diminished adhesion.
The saponification failure mechanism is further supported by the location of the failures. That is, disbonding was generally observed only in areas exposed to water permeation through unscheduled openings in the structure. The bulk of the surface which was sealed by the coating and not exposed to water was soundly adherent with no signs of deterioration.
This water permeation of the stucco substrate at ledges, mortar joints, edges of balconies, and areas near the ground which were wetted by capillary action, caused the alkaline salts in the stucco to be leeched into the water, resulting in the saponification.
The PVA coating is rubbery and, after the bond is destroyed, the elastic coating expands to contain water that enters between it and the stucco. The appearance of the failed surface depends on: the amount of water; entrance source of the water; and/or the localized bond of the coating to the stucco.
Remedial Work
KTA recommended that prior to recoating, the poorly adherent material be removed and that the sources of moisture leakage be repaired. Further, it was recommended that a material compatible with the base coat and resistant to alkaline attack be selected and applied in accordance with the manufacturer's instructions. Proper surface preparation and application of a coating material which will resist the alkaline environment should provide long-term protection of the exterior of the condominium units.
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