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The Case of Applying Waterborne Acrylic Paints in Cooler Weather

By James D. Machen
KTA-Tator, Inc.

A small construction company was the general contractor for the construction of a new outdoor parking facility for a major overnight and longer-term parking company.

The roof canopy over the approximately 300,000-square-foot parking area was comprised of main structural support members with roof joists supporting a corrugated metal roof deck. The painted structural steel and steel roof joists were the subjects of this investigation. The non-ferrous metal corrugated roof deck was not painted.

The structural support steel and the roof bar joists were fabricated and shop primed by two separate suppliers. The shop fabricators applied a shop coat of oxide primer. Product data sheets for the primer indicated that these coatings were alkyd primers. Both data sheets recommended an applied dry film thickness of approximately 1 mil.

Sometime after fabrication and shop priming, the main structural members and bar joists were delivered and erected on site, and field finish painting was performed by a separate field painting contractor.

There were no formal painting specifications concerning the shop or field painting requirements for the structural steel and bar joints; however, it was reported that the manufacturer of the field finish paint recommended applying a waterborne acrylic paint product. As a result the field painting contractor spray applied one coat of a waterborne acrylic coating as the finish coat over the shop-primed steel. The product was described as an interior waterborne acrylic latex dry fall that was to be applied when the air, surface, and material temperatures were between 50°F minimum and 110°F maximum. It was also recommended that the surface temperature be at least 5°F above the dew point, and the maximum relative humidity be 75%. The product was recommended to be applied at a dry film thickness of 3 to 4.5 mils.

Spray application of the waterborne finish coating occurred in a timeframe between mid-October and early December, with touch-up continuing until mid-December. During the painting operations, the corrugated roof decking was not in place. Roof decking placement followed closely behind the finish painting operations. Painting was performed outside, unprotected, and exposed to the typical ambient conditions encountered in Pennsylvania during the mid to late fall season. A review of weather records indicated that during the October painting period, the average temperature ranged from 42°F to 59°F. Included in this average were overnight temperatures that fell as low as 35°F and daytime temperatures that rose as high as 68°F. On approximately 2 of the 13 days in October when paint was applied, records indicated that the air temperature never rose higher than 50°F. During the month of November, the average temperature ranged from 39°F to 52°F, with individual lows to 32°F and individual highs up to 68°F. During the December painting days, the average temperature ranged from 41°F to 52°F, with individuals lows of 27°F and individual highs up to 70°F. Records indicated that on approximately 17 of the 30 days in November, the temperature never rose any higher than 50°F. In addition, on 7 of the first 14 days in December, the temperature never rose higher than 50°F.

Typical mid and late fall days in the area are comprised of a mixture of cool and warm days where steel surface temperatures can rapidly cool in the evening. As the steel surfaces reach the dew point of the surrounding air, the moisture in the air (dew) condenses on the surface. In fact, considerable moisture condensation was observed on the underside of the corrugated roof panels during the field investigation.

Sometime after completion of the field finish painting, peeling finish paint was noted on the structural steel and bar joists and on the asphalt parking lot surface.

Because of the large size of this parking area and the cost associated with the inability to use parking spaces due to falling paint, an independent third party opinion as to the cause of this coating failure was desired, and KTA was contacted to perform an inspection.

Field Investigation

KTA visited the parking facility approximately one month after completion in January. All of the finished painted structural steel and roof joists were visually inspected from the ground and with an extension ladder.

The field investigation involved a thorough visual examination, magnetic dry film thickness measurements, adhesion testing, and sample procurement.

Visual Examination

Extensive peeling of the white topcoat from the underlying primer was occurring across the surface of the main support steel and the bar joists. On any given surface, the estimated rate of visible disbonding ranged from as low as 10% to greater than 50% of the surface area. While peeling was randomly distributed throughout the structure, the worst areas (50% or greater peeling) occurred on the steel near the rear of the lot where the roof deck had not yet been installed.

Peeling most often appeared to begin with a series of spider web-like cracks in the white coating film; the disbonding followed that cracking pattern. In many instances, the underlying primer was visible between the cracks in the white film. Sometimes peeling was closely associated with wrinkles and wrinkle-shaped blisters in the white paint film.

The asphalt parking lot surface was often covered with disbonding white paint chips. Close examination of these chips from the ground found that some chips were brittle and some chips were soft and appeared to be dissolving. In fact, paint chips fractured into small flakes upon handling.

Moisture condensation had accumulated on the bottom side of the corrugated roof panels throughout the parking area. This moisture was dripping onto the painted surfaces below.

Dry Film Thickness

Total system magnetic dry film thickness (DFT) measurements (primer plus white topcoat) were taken at numerous locations across the main structural steel and bar joists. The results of measurements taken in failing and non-failing areas were as follows:

Average dry film thickness measurements were also taken on the bare primer areas. The average primer reading was 3 to 4 mils in thickness.

Adhesion

Adhesion was evaluated in accordance with ASTM D3359, Method A (X-cut), along with subjective probing of the paint film with a razor knife. The tests at each location were performed in duplicate. In all instances, adhesion of the white topcoat, whether on the main structural steel or bar joists, was rated as poor (ASTM rating of 0A to 1A). The test process detached the coating system between the white finish coat and the primer layer.

Substrate Examination

In general, all structural steel surfaces were covered with a layer of intact mill scale; however, on some areas examined, light surface rusting was also visible.

Laboratory Investigation

The laboratory investigation consisted of visual and microscopic observations, and infrared spectroscopy.

Visual and Microscopic Observations

A Nikon Model SMZ-10 stereo zoom microscope with magnification to 40X was used to examine the paint chip samples. In general, the coating chips averaged 6.0 to 8.0 mils in thickness, and the front of the chips were glossy and covered by large nodules and shallow craters.

Infrared Spectroscopy

Infrared spectroscopy of sample paint chips was performed with a Mattson Galaxy Model 3020 fourier transform infrared spectrometer. The analysis confirmed that the shop primer was indeed an alkyd and the finish coat was a waterborne acrylic product. Field sample spectra of the waterborne acrylic, when compared to laboratory control samples, showed no differences.

Results of Field and Laboratory Investigations

The field investigation and laboratory analysis produced information that can be used to explain the cause of this premature coating failure at the parking facility. This information is discussed below.

Infrared Spectroscopy

Infrared spectroscopic analysis of primer samples identified the primers as alkyds and the white finish coat as an acrylic. The infrared spectra produced by failing and non-failing field samples of the white topcoat were compared with laboratory control samples, no dissimilarities were found. This finding indicates that an alkyd primer and acrylic topcoat were, in fact, used. In addition, the spectra found that no changes in the chemical composition of the white finish coat had occurred. This indicated that chemical degradation of the white finish had not played a role in the failure. Furthermore, there was no intercoat contamination detected on the primer surfaces on the backside of the disbonded white topcoat. Therefore, intercoat contamination was also ruled out as a cause for the failure.

Dry Film Thickness

Primer Coat

The manufacturer's product data for the alkyd oxide primers recommended that the primers be applied at a dry film thickness of 1 mil. Field measurements found that the actual primer thickness ranged from 2 to 4 mils. Even though a few thickness measurements were slightly higher than recommended, such deviations with alkyd primers are normal, expected, and not a cause for concern. Furthermore, upon reviewing primer dry film thickness data as it related to peeling and non-peeling areas, there was no correlation. The white acrylic peeled from the primer regardless of its thickness.

Waterborne Acrylic Finish Coat

The product data sheet recommended applying the waterborne acrylic finish coat at a dry film thickness of 3 to 4.5 mils. Actual field measurements found that the applied dry film thickness ranged from 3 to 7 mils, with an average thickness of approximately 4 mils. Even though a few thickness measurements recorded were as high as 7 mils, such deviations when spray applying waterborne acrylic products to structural steel and roof joists of this type common. Furthermore, as with the primer coat, there was no relationship between the film thickness of the acrylic and whether or not it disbonded. It was found that a 3-mil thick film of acrylic was just as likely to peel from the primer as was a 7-mil layer.

Adhesion

In all cases, field testing of the white topcoat remaining on the primed steel indicated that the adhesion was poor.

Conclusions

The results of the field and laboratory analysis, along with the known ambient conditions under which the waterborne acrylic was applied, indicates that this product was inappropriate for use on this project. Waterborne acrylic coatings achieve their cure by a process known as coalescence. Coalescence is defined as the formation of a coating film by the evaporation of water from waterborne coating. The coalescence process is very temperature sensitive and generally needs a temperature (air, surface, and material) of at least 50°F to properly occur. The resin in a waterborne acrylic coating is present in the can as small, spherical particles dispersed in water. They also contain small quantities of special solvents known as coalescing or latent solvents. The primary solvent, water, carries the acrylic paint particles to the surface and then evaporates. The coalescing solvent's responsibility is to remain in the paint film long enough to superficially soften or melt the outside of individual particles and allow them to flow together into a homogenous paint film. At temperatures above 50°F, the resin particles are relatively soft and able to easily deform. Unfortunately, at cooler temperatures (i.e., below 50°F), the resin is to stiff to deform, so the particles do not flow together to form a homogeneous film. Furthermore, curing proceeds at a slower rate when temperatures are on the cooler end of the manufacturer's application parameters. When the complete drying or curing of a coating does not occur, the applied paint film does not develop proper film integrity, as it remains sensitive for longer periods. As a result, the coating may produce a film with negative physical attributes such as weak cohesive strength, marginal adhesive qualities, porosity, and/or other problems, such as diminished weathering resistance.

The waterborne acrylic product data sheet listed 50°F as the minimum air, surface, and material temperature at which waterborne acrylic was to be applied. It was readily apparent from reviewing the local weather information that numerous days during the application period had weather conditions (low temperature, moisture condensation, and humidity) that were capable of adversely affecting coalescence of the paint film.

Moisture condensation and high humidity, along with the cooler air temperatures that were encountered, either stopped curing altogether or delayed curing and consequently prolonged the time period that the paint film was temperature and moisture sensitive. When the uncured paint film was exposed to the stresses of moisture (condensation from dew and high humidity) entering the film, along with daytime warming and evening cooling of the ambient air and steel surfaces, the resulting cracking, peeling, and disbonding occurred in the weakened paint film. Therefore, considering the timing of the coating work, it was quite apparent that this waterborne product should not have been used on this project. In fact, any product which cures by coalescence would have been at high risk for problems when applied under the conditions encountered while painting this project.

Recommendations

Because the existing coatings were poorly adhered, high pressure water washing at 4,000 to 5,000 psi using a 0° oscillating nozzle on the pressure washer spray wand was used to effectively remove the vast majority of the coating. Additionally, it was discovered that by using heated water, the waterborne acrylic coating was softened and even easier to remove. During the removal process, it was also found that some small areas of the white acrylic had better and adhesion and remained adhered to the primer after pressure washing. With this in mind, if any white coating remained after initial pressure washing, it was removed by increasing dwell times and the angle of attack with the 4,000 to 5,000 psi heated pressurized water. When weather conditions were favorable, an appropriate product designed for exterior exposure resistance, such as an alkyd finish, was applied. Alkyd products dry and cure by a different mechanism that is not as temperature sensitive as the coalescence curing process. Therefore, if cool weather (i.e., temperatures down as low as 40°F) was anticipated during application and curing, an exterior alkyd coating was a much safer choice.

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