Storage temperature history includes the temperature fluctuations experienced during shipment, further complicating an accurate knowledge of conditions to which the product has been exposed prior to use. Because one might not always know the temperature during the entire storage period, there may be times when it is necessary to determine the degree of deterioration from thermal aging to establish whether a powder coating can still be used for its original intended purpose. The difficulty in this determination is in selecting tests that are both indicative of the potential problems induced by the thermal aging while being economical to perform. This can be a challenge, especially for small and medium-sized businesses.

One of the tests often considered the most definitive and informative is differential scanning calorimetry (DSC). Although DSC gives the most insight into the chemical changes in the powder, it requires expensive specialized equipment. In this article we will discuss, in addition to DSC, some simpler laboratory tests, some of which are recommended by the Powder Coating Institute to evaluate product aging.¹

To evaluate the effectiveness of several commonly used tests for thermal aging, we tested 13 powders that had been exposed during warehouse storage to elevated ambient temperatures (approximately 90°F, or 32°C) for less than one week. The degree of aging was evaluated using DSC, and the results were compared with those of less sophisticated laboratory “bench” tests. Additionally, retained samples of the same powder lots, properly stored, were also evaluated as controls and the results of the aged and properly stored powders were compared.

DSC

DSC is a method that very accurately measures the quantity of heat that is either absorbed or released by a material as a function of temperature and time. As a material is heated at a predetermined rate, or exposed to a constant temperature over time, the heat flow into or out of the material relative to a baseline standard allows detection of physical and chemical changes. This permits determination of phase transition temperatures (e.g., melting point, glass transition, crystallization), as well as curing rates and energies released (e.g., during cross-linking).

Although the exact heating procedure can vary, the following DSC sequence was used in this case:

1. Start at 25°C
2. Ramp to 100°C at 10 °C/min (1st heat)
3. Quench to 25°C
4. Ramp to 250°C at 10 °C/min (2nd heat)
5. Quench to 25°C
6. Ramp to 250°C at 10 °C/min (3rd heat)

Figure 1 schematically shows how the powder coating product’s structure changes when heated from room temperature: (a) for virgin powder and (b) for damaged powder, to the melting temperature (c), and above the curing temperature (d).

Figure 2 shows sample DSC curves obtained from the same powder when stored under manufacturer-recommended conditions (virgin and undamaged) and at an elevated temperature (over-aged and damaged).

During the first heating, the powder was elevated to 100°C, which is above the melting temperature but below the curing temperature (Fig. 2, red curves). This corresponds to the structure depicted in Figure 1(c). The undamaged powder melted at approximately 56°C and required 6.9 J/g of energy. The aged powder had a higher melting point of 61°C and required 8.1 J/g to melt. The higher melting temperature and energy indicate that some agglomeration or cross-linking had already taken place during the improper storage of the powder at an elevated temperature.

When the powder reached 100°C, it was cooled down to 25°C and the second heating cycle commenced. During the second heating (Fig. 2, blue curve), the powder was heated above the curing temperature until it was fully cured, shown in Figure 1(d). Curing started at approximately 126°C for both the original and the aged powders. The bell-shaped curve in this cycle shows that powders give off heat during the curing process.

During the curing process, the polymer chains cross-link (i.e., they bond to each other), which gives off energy; this curing peak is, therefore, a measure of the degree of cross-linking that takes place during the curing process. A damaged powder may give off less energy (i.e., smaller peak size) than an undamaged product because some cross-linking may have already occurred during storage. The curing onset temperature is the temperature at which the polymer in the powder begins to significantly cross-link. The glass transition temperature, Tg, occurs where the solid, brittle powder starts to soften.

During the third heating, the fully cured and finished powder coating was tested; it maintains the solid and hard structure depicted in Figure 1(d). During this cycle, the powder should already be completely cured and the only parameter measured is the T_g, the temperature at which the powder coating begins to soften—it is one indicator of the integrity and quality of the final coating.

We considered a powder sample to be over-aged (damaged) if one or more measured parameters exceeded the respective control by at least 10%.

Water Bath

The water bath procedure to evaluate sintering, as described in Powder Coating Institute (PCI) Recommended Procedure #1 for “Assessing the Storage Stability of Coating Powders,” was followed to evaluate the powder sintering (agglomeration) properties. Sintering is undesirable because it inhibits the free-flowing characteristics of the powder particles during application. Agglomerated powder can affect the appearance of the cured coating and can cause clogging of the powder feed system. For example, Figure 3 (left) shows one of the 13 tested powders that had partially sintered, whereas the photograph on the right shows the virgin, unsintered sample of the same batch of powder.

The following water bath test protocol was used (as shown in Fig. 4):

1. A glass test tube was filled 2⁄3 with powder. A 100-g weight was gently placed on top of the powder and the test tube sealed using a rubber stopper.
2. The test tube was immersed to the level of powder in a water bath at 45°C (~113°F) for 24 hours.
3. Upon cooling back to room-temperature, the powder was removed from the test tube and poured into a small plastic Ziploc bag.

A 125-g weight was lightly rolled over the powder in the Ziploc bag to break up weakly agglomerated particles (which would be unlikely to have a detrimental effect on the material). The powder was examined for remaining agglomeration using a stereomicroscope.

In this test, both the aged and the control samples were sintered due to the elevated temperature (45°C) of the water bath. We considered a sample to have failed if it sintered more than the respective control.

Inclined Plate Flow

PCI Procedure #7 for “Inclined Plate Flow” was followed to evaluate the powder flow properties during the curing cycle. To some extent the appearance of the cured powder is dependent on its flow characteristics. For instance, a high-gloss powder must be able to flow well to allow the molten powder to level into a smooth film.
The following test protocol was used:

1. A pellet mold press was employed at ambient temperature to produce three 1-g pellets of 1 cm in diameter for each powder. The manual hydraulic press was operated by pushing down on a lever. The pressure exerted on the pellet is dependent on the operator, but the test method does not specify that a specific pressure be used.
2. A steel Q-panel, 4" × 12", was preheated in a convection oven at 148°C. Three powder pellets were placed on the horizontal surface of the panel. After 30 seconds the pellets had started to melt and adhere to the panel. The Q-panel was then inclined to 65° while remaining in the oven to allow the powder to melt and flow. After 15 minutes, the panel was removed, laid flat, and allowed to cool to room temperature.
3. The length of flow for each pellet was measured and the average reported.

The purpose of this test is to determine to what extent an unused powder coating has already started to agglomerate, gel, and cure, even while it is still in the powder state. A powder that flows too short a distance during this test indicates that agglomeration, gelling, or curing has already started while the powder was in storage. For instance, in Figure 5 the aged and original black powders flowed the same distance, indicating that this powder passed. On the other hand, in Figure 5 the aged gray powder flowed only a short distance compared with the control. This indicates that it may have been damaged during storage.

According to PCI #7, this “useful indicator” test “is recommended as an ‘in house’ tool only” since “oven drafts…pill variations significantly affect results making interlab reproducibility somewhat difficult to correlate.” Given the purported lack of reproducibility and the suggestion by PCI that it be used simply as an indicator test, we only considered the results different if there was at least a 25% average difference in the flow length between the control and the powder lot under test.

Gel Time

The gel time is the length of time required for a solid dry powder to transform into a viscous gel-like state at a given temperature.

The subsequent protocol was followed (Fig. 6):

1. 1 g of powder was placed in the hemispherical indentation of a heated gel-time stage (crucible) that was maintained at 195°C.
2. The powder melted within one or two seconds after it was poured into the hot crucible. The powder was continuously stirred using a thin wooden rod and, as expected, its viscosity dropped significantly as it melted. As soon as the powder approached its gel time (ranging from several seconds to several minutes) the viscosity began to significantly increase. This phenomenon was monitored by periodically removing the stick from the molten powder in the crucible. Prior to gelling, sticky molten powder strings formed between the melt and the bottom of the rod. However, as soon as the powder gelled, the molten powder lost its stickiness and was no longer able to support a long string between the rod and bottom of the crucible. The gel time was measured from the moment the powder was poured into the crucible until the instant it is no longer able to support a string.
3. The total time (in seconds) for the powder to gel was recorded.

Some powders gelled within seconds after being poured into the crucible, whereas others remained molten for more than 2 minutes. Despite the fact that the same person conducted most of the tests, repeatability was poor.

These tests showed that high-gloss powders exhibited long gel times (some more than 15 minutes), whereas heavily textured powders gelled within 6–20 seconds. These observations were easy to understand since one wants a high-gloss coating to have sufficient time to flow into a smooth film. A textured coating should gel before it can flow out.

Due to the subjectivity and poor reproducibility of this procedure, the test was repeated at least in triplicate. The gel times were then averaged. Various factors contributed to the poor reproducibility. For example, the stirring speed varied because it was performed manually; frequency of lifting the stirrer to determine if the gel time had been reached; and disparities in the temperature of the melt due to variations in lifting and stirring also affected the results.

In light of this poor reproducibility, we again required a relatively generous margin of 25% average difference in gel time to conclude that there was a significant difference between the control and material under test. One would expect powders that had been thermally damaged to exhibit shorter gel times than their respective control samples.

Test Panels (Visual Appearance)

From the powder coater’s perspective, visual appearance is one of the most important criteria by which to judge quality. If the physical and chemical properties of a sample and control are the same, but they clearly differ in color, texture, or gloss, then product cannot be used for its originally intended purpose, at least if aesthetics are a critical factor. Therefore, if there are marked differences in appearance between the control and the test samples, then the product cannot be sold as originally intended.

To evaluate the appearance of the applied product, three 3" × 6" steel Q-panels were sprayed with powder using an industry-standard electrostatic corona gun. The panels were cured in a convection oven according to the manufacturer’s specifications. The test panels were visually inspected under different lighting conditions and angles and compared side-by-side to the control

Gloss Measurements

Gloss is a quantifiable measure of how well a surface reflects light at various angles. For the purposes of these measurements we used a NovoGloss Statistical Glossmeter 20º/60º/85º. For high-gloss finishes a 20° angle is used, whereas for low-gloss an 85° angle is preferred. Intermediate-gloss finishes are measured at 60°. The gloss of a test powder sample was considered acceptable if the average gloss was within 25% of the control sample average gloss.

Conclusions

As seen in the table, the only product that was essentially identical to the control was the Brown-Matte Epoxy powder. This powder passed all tests and was, therefore, considered acceptable. The other 12 powders each failed at least two of the six tests. Because each set of powders came from a single batch, these differences were attributed to the dissimilar storage conditions, consistent with an elevated temperature.

We found that the most reliable lab bench test for evaluating if there was thermal degradation during storage was the visual inspection of spray-out panels. The visual appearance test agreed 10 out of 13 times (77%) with the DSC results; in every conflicting case, the visual appearance test was more stringent that the DSC. By contrast, all the other bench tests disagreed with the DSC results six or more times out of the 13 tests.

References

1. The Powder Coating Institute. Powder Coating: The Complete Finisher’s Handbook. 3rd Ed., 2004.

About the Authors

Dr. Ricardo J. Zednik is a materials scientist and a consultant at Exponent Failure Analysis Associates in Menlo Park, Calif. He has worked in the energy and electronics industries and has extensive experience in thin-film materials.

Ron Joseph, organic coatings editor of Metal Finishing, is a full-time paint and coating consultant at Exponent Failure Analysis Associates in Menlo Park, Calif. He has worked in the paints and coatings industry for more than 38 years.