Fall 2002 Vol. 4, No. 2

Background

Since beta-hydroxyalkylamide (ß-HAA) cure chemistry was introduced to the powder coating industry in the early 1990s, this technology has experienced significant growth in the exterior durable market. In 2001, one-third of all powder coatings produced worldwide for outdoor applications were based on ß-HAA crosslinking chemistry and the performance of these systems is well established. Typical end uses include finishes for architectural facades, fencing, aluminum window profiles, full-body automotive primers, outdoor electrical casings and exterior light fixtures.

The applications described above all require excellent resistance to corrosive environments. Water absorption and water vapor permeation are two important factors when assessing the corrosion resistance of a coating. Since ß-HAA based powder coatings are commonly used for applications that require long-term warranties on gloss retention and chalking, it was important to characterize the permeation properties of these systems. An independent study was initiated by EMS Primid to evaluate the moisture absorption of polyester/ß-HAA powder coatings. The study was carried out by the Research Institute for Pigments and Coatings in Stuttgart, Germany. A polyester/TGIC formulation was selected as a control formulation.

ß-Hydroxyalkylamide cure chemistry

The workhorse ß-HAA hardener is Primid® XL-552. This curing agent is a tetra-functional molecule, having four available hydroxyl sites. The hydroxyl groups are in beta-positions relative to the nitrogen atoms and react under the influence of temperature with the carboxyl sites of the polyester. The ß-HAA cure reaction begins at approximately 300ºF. The result is a highly crosslinked, three-dimensional network of ester linkages. The ester bonds formed during the crosslinking reaction produce films with excellent mechanical properties and outstanding UV and corrosion resistance.

Water absorption and water vapor permeation testing conditions and results

The coatings evaluated for this study were prepared in the EMS-Primid laboratory in Domat-Ems, Switzerland. The polyester/ Primid® XL-552 and polyester/TGIC binders were evaluated using 40% TiO₂-filled systems. To exclude any influence of the substrate on moisture absorption or permeation, the coating samples were submitted to the Research Institute as free, cured films. The films were prepared at thicknesses between 2.4 to 3.2 mils.

Tables 1 and 2 compare the water absorption between 40% TiO2-filled Primid® XL-552 and TGIC-cured systems. The coatings underwent cure cycles of 10 and 30 minutes at 360°F and 20 and 60 minutes at 330°F. No differences were detected in the percent moisture absorbed between these two systems. The specimens were tested at a temperature of 73°F and 50% relative humidity.

In addition to water absorption, water vapor permeation was also measured in terms of water vapor transmission rate, permeation coefficient and diffusion resistance number.

The water vapor transmission rate refers to the steady flow of water vapor through a unit area under specific conditions of temperature and humidity. The permeation coefficient describes the amount of the water vapor diffused through a permeable barrier, in this case the cured films. The diffusion resistance number is the ratio of the diffusion stream coefficient of air and a relevant substance and compares the diffusion resistance of the test material (i.e., water vapor) to air when measured at the same temperature and flow rate.

40% TiO₂-filled polyester/Primid® XL-552 and polyester/TGIC coatings were cured at 10 and 30 minutes at 360°F and 20 and 60 minutes at 330°F. Following testing at a temperature of 73°F and 50% relative humidity, no differences were noted between the permeation behavior of the Primid® XL-552 and TGIC crosslinked films as shown in Tables 3 and 4.

Table 5 shows the influence of test temperature on the water vapor permeation. Testing was performed at 73°F and 104°F. As expected, moisture permeation was higher at the elevated temperature. However, once again, no differences were measured between the two binder types.

Under the test conditions used, the water absorption and water vapor permeability characteristics of polyester/Primid® XL-552 and polyester/TGIC powder coatings were essentially the same. The results for transparent coating were not included in this article to avoid redundancy. However, nonpigmented systems followed the same trend as for the 40% TiO₂-filled systems. The results of this study verify what has been experienced for over a decade of industrial usage. The barrier properties of polyester/Primid® XL-552 systems result in coatings with outstanding resistance to the most severe UV and corrosive environments.