Considerations for Characterizing Moisture Effects in Coatings Weathering Studies

Henry K. Hardcastle and William L. Meeks
Atlas Material Testing Technology LLC

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This paper investigates the effects of weathering factors on moisture absorption of coatings and presents results from several projects including real-time and accelerated weathering studies of coatings moisture absorption. Moisture variable characteristics observed in outdoor exposure environments are reviewed. Data is presented from experiments performed in end-use and accelerated weathering environments to characterize moisture characteristics on coatings. The information from natural outdoor environment characterizations and DOEs indicate new approaches for characterizing coatings weathering with regard to moisture effects. Considerations and new approaches for performing coating weathering studies are discussed.

1. Introduction and Background

Modern outdoor coatings in end-use react in varying degrees to environmental factors (variables). Sunlight, temperature, and moisture often play critical roles in degradation of coatings in end-use. Consequently, researchers include these factors in weathering test method development. The coatings industry recently increased focus regarding the effects of moisture on coatings degradation. The effects of moisture and its interactions with other weathering factors on coatings degradation represent critical considerations for some coating systems end-use performance.

For example, a recent ASTM G90 weathering investigation using accelerated weathering test apparatus included nine variables in a fractional factorial design of experiment (DOE)1. This DOE included temperature, irradiance, and moisture variables as well as several pre-exposure treatments. Results of this experiment clearly showed application of a nighttime warm water soak had a significant and important effect on the visual appearance (20° gloss) of the automotive paint system exposed. Additionally, PAS FTIR analysis also indicated the nighttime warm water soak effected the chemistry of the paint system. This experiment showed moisture played a key roll in critical performance degradation and that the 40°C nighttime soaking enhanced the rate of photo-oxidation in the exposures.

Another publication further detailed aspects of moisture effects on automotive coatings degradation2. This paper concluded that of the two most common accelerated weathering tests for automotive coatings, SAE J1960 and ASTM G90, neither meet the minimum water saturation time criterion. The paper also hypothesized the importance of sufficiently saturating coating systems in artificial accelerated weathering tests and highlighted the notion of scaling moisture absorption to ultraviolet (UV) dose. The paper showed limitations of current accelerated test methods as well as the difficulties for developing new test methods with regard to properly controlled moisture variables.

A third paper presented results from outdoor real-time weathering DOE exposures in subtropical Florida and desert Arizona3. This simple full factorial 23 DOE dramatically showed the effect of moisture application on visual appearance (20° gloss) of automotive coatings over long term exposure (>6 years). This weathering DOE compared the effect of moisture application to the effects of exposure angle and exposure location as well as highlighted the moisture factor’s powerful effect in end-use style exposures.

These investigations indicate that moisture plays a critical role in the weathering degradation and suggest a need for better understanding considerations regarding properly applying moisture variables in weathering test method development. Proper application of moisture variables appears important for both simulation and acceleration in laboratory and outdoor weathering test methods. The Misovski et al. paper hypothesized shortfalls of current test methods with regard to applying moisture and, more specifically, that the current accelerated weathering protocols do not allow for sufficient time during their nominal wet cycles for paint systems to become saturated.

The observation that current weathering methods do not apply moisture sufficiently to properly simulate end-use conditions or behavior leads to the question, How much absorbed moisture is appropriate for these test methods? This paper investigates considerations for answering this question from three contexts: (1) material characteristic considerations, (2) end-use environment characteristic considerations, and (3) weathering test method characteristic considerations, as diagrammatically illustrated in figure 1.

Figure 1. Interplay of Considerations for Moisture Effects in Coatings Weathering.
Considerations

2. Material Considerations

Different materials absorb moisture differently. Consideration of a material’s moisture absorption characteristics appears important for understanding its outdoor end-use behavior as well as developing appropriate accelerated weathering test methods. By simply measuring the moisture absorption of materials, many considerations important for weathering test methods become apparent. The material considerations phase of this investigation included several small and simple experiments to observe: (1) the importance of coating system difference on absorption, (2) the effect of temperature, and (3) the effect of orientation for applying water spray on moisture absorption.

Clearly, an important aspect of moisture characterization includes the method of measurement. Throughout this paper, investigations utilize a simple mass uptake measurement. In summary, this simple technique utilized a Sartorius semi-microbalance (model R 160 P) with a read out sensitivity of 2x10-5 g to weigh 101.6 x 152.4 mm nominal aluminum painted panels. Further details of this measurement can be found elsewhere2. Prior to the investigations, any back side coating of the specimens was removed and the back side of the aluminum substrate was allowed to oxidize for several days in warm, high humidity conditions until the oxide layer had redeveloped. The simple measurement technique of observing mass of moisture uptake (moisture absorption) during moisture application allows considerable insight into different materials absorption characteristics.

2.1. Material Moisture Absorption in Immersion

Spray (and rain), immersion, condensation, and other factors effect coating system moisture content in weathering test methods and end-use environments. Immersion represents a simple technique that can be used to characterize the effect some variables have on coatings absorption. In this investigation, the mass uptake method measured the effect of water immersion confirming two phenomena important to moisture application in weathering studies: (1) absorption characteristics of different coatings and (2) the effect of immersion water temperature. This type of simple measurement also appears to yield useful information for characterizing end-use behavior as well as for test method development.

2.1.1. Different Coatings

In order to observe the effect of coating type on moisture absorption in immersion, three different types of commercially available coatings on aluminum were obtained. A yellow automotive basecoat-acrylic melamine clearcoat was compared with a white acrylic water base coil coating and a brown polyvinylchloride (PVC) coil coating. The three coating systems displayed a variety of different chemistries and physical characteristics. The specimens equilibrated in an uncontrolled office environment for several days. After equilibration, the microbalance recorded initial weights of each specimen. Warm tap water (38±1°C) filled an insulated immersion vessel (previous investigations showed no difference between tap and de-ionized water in this part of the investigation). At time “0:00” all specimens were immersed into the water. At regularly timed intervals, specimens were removed from immersion and thoroughly wiped dry using paper towels so that no visible moisture remained on any surface of the specimen. The immersion time and specimen weight was recorded for all three coating types.

Figure 2 shows the results obtained from the different coating immersion trials. Two general observations become apparent from figure 2. First, the different coating systems appeared to show very different moisture absorption rates. For example, the white acrylic water base coil coating system appeared to absorb moisture much more rapidly than the yellow automotive basecoat-acrylic melamine clearcoat system. Second, the different coating systems appeared to achieve different levels of moisture absorption (saturation) at different times. The white acrylic water base coil coating system appeared to achieve a plateau in moisture absorption more rapidly than the automotive coating or PVC coil coating systems.

Figure 2. Moisture Absorption of Three Different Coatings in 38
Absorption of coatings

2.1.2. Different Temperatures

In order to observe the effect of temperature on moisture absorption in immersion, three different temperatures of immersion water were used to immerse the yellow automotive basecoat-acrylic melamine clearcoat system. The coating system equilibrated for several days in an office environment. Initial weights were obtained and specimens were then immersed in 21°, 27°, and 38°±1°C temperature tap water. At regularly timed intervals, specimens were removed from immersion and thoroughly wiped dry using paper towels so that no visible moisture remained on the specimens’ surfaces. Specimens were then immediately weighed and the time from immersion and the change in weights were recorded within each of the immersion temperatures.

Figure 3 shows the results from the different temperature immersion trials on the yellow automotive basecoat-acrylic melamine clearcoat. Two general observations become apparent from figure 3. First, the different temperature immersions showed different water absorption rates. Second, specimens immersed in the 38°C water absorbed significantly more moisture than in 21°C water by the same point in time.

Figure 3. Moisture Absorption of Auto Coating in Three Immersion Temperatures.
Absorption of auto coatings

2.1.3. Theoretical Considerations

The simple moisture absorption trials in the previous two sections illustrate principles of diffusion elucidated by Fick’s laws (1855) and the Einstein relation (1905). Fick’s first law predicts steady state diffusion and is summarized by the expression;

F = -D * (?C / ?x)

Where F is the rate of transfer, C is the concentration of diffusing substance, x is a space co-ordinate normal to the section, and D is a diffusion coefficient4.

In the context of this paper, it is important to consider that real world weathering is rarely in steady state (as observed in the end-use section of this paper). Fick’s second law describes continually changing states of diffusion when the concentration within the diffusion volume changes with time (as is more typically the case with moisture in weathering) and is summarized by the expression;

?C / ?t = D * (?2C / ?x2)

The above is shown only to highlight the importance of D (the diffusion coefficient) in moisture diffusion into weathering materials. Additional applications must consider multi-dimensional and non-Fickian behavior near Tg.

The Einstein relation5 further develops D in relation to Brownian motion by the expression;

D = BkT

Where D is again the diffusion coefficient, B is a mobility coefficient, k is Boltzmann’s constant and T is the absolute temperature. Three important considerations of these theoretical explanations include: (1) the behavior observed in this paper is consistent with accepted explanations, (2) that different moisture-material systems have different rates of moisture diffusion, and (3) the temperature is a critical variable effecting moisture absorption in weathering. Barrer (1937) showed the effect of temperature on the diffusivity of gasses with the relationship6;

D = D0 exp (-Ed/RT)

Where Ed is the activation energy. This form appears Arrhenius-like and indicates that that the diffusion coefficient at different temperatures may be well predicted by an Arrhenius-like function.

2.2. Material Absorption in Spray

Most weathering studies and test methods apply moisture to materials in the form of spray or condensation rather than immersion. Sprayed moisture application can occur with the specimen in a horizontal position while the moisture stream falls vertically on the specimen as illustrated in figure 4. Spray moisture application can also occur with the specimen in a vertical position while the moisture stream sprays horizontally on the specimen. This investigation compared spray induced moisture absorption on horizontally and vertically oriented specimens and the effect of air temperature using a 22 full factorial DOE. The DOE used the same yellow automotive basecoat-acrylic melamine clearcoat system from the previous immersion trials.

Figure 4. Orientations for Water Spray.
Orientations for water

This DOE recorded specimens moisture absorption after 15 minutes of water spray for each trial. The single point-in-time measurement indicated the propensity of the material system to absorb moisture under varying conditions of specimen orientation and environment temperature.

In order to compare moisture absorption at different angles with the effect of air temperature, a simple apparatus was constructed which fixed the relative position of a spray nozzle 22 cm distant from the material specimen. A pivot on the apparatus allowed orientations shown in figure 4. The two settings for the angles in this DOE were to orient the specimens horizontally and vertically.

In order to compare moisture absorption at the different spray angles to moisture absorption at different temperatures, trials were conducted with the laboratory air temperature at 30°C (high) and 23°C (low) nominal settings. The possibility of variations in ambient conditions during the trials (experimental error due to air conditioning cycling) prompted four replicate runs of the experiment on four different days.

Table 1 shows the results and analysis of this DOE. Figure 5 shows the main effects graph. The mechanics of DOE analysis is fairly standardized and widely published and it will be left to other texts to outline standard analysis procedures. The Barrentine reference fully documents the analytical procedures used in this study7.

Table 1. Results and Analysis for Angle
Results and analysis for angle
Figure 5. Main Effects Graph for 2x2 DOE.
Main effects graph

Two general observations become apparent from the results of this DOE. First, there was no significant difference in moisture absorption between a horizontally applied water spray and a vertically applied water spray. There was no significant difference in moisture absorption after 15 minutes for either orientation in figure 4. Spray applied to a horizontally oriented specimen resulted in about the same moisture absorption as moisture spray applied to a vertically oriented specimen. Second, there was a significant increase in the amount of moisture absorbed after 15 minutes of spray in the 30°C ambient temperature compared to the 23°C ambient temperature condition in this experiment.

3. End-Use Environment Considerations

Different end-use environments exhibit different moisture characteristics. Consideration of the moisture characteristics of a material's end-use environment appears important for understanding a specific material’s end-use behavior as well as for developing appropriate accelerated test methods. Many considerations important for weathering test methods become apparent by simply measuring the moisture absorption of materials in their end-use environment. This phase of the investigation included observations of material absorption characteristics in the end-use environment. These included the importance of coating system differences on absorption in the end-use environment and the effects of the outdoor diurnal cycle.

Clearly, an important aspect of end-use environment characterization includes specifying which end-use environment. This paper presents observations from the southern Florida subtropical environment which has long been used in the weathering industry as a reference weathering environment. Some researchers use material degradation functions observed in southern Florida as the standard for good artificial–accelerated weathering methodologies. This investigation included observations at Atlas Material Testing LLC’s South Florida Test Service exposure site located at 25° 52’ North latitude, 80° 27’ West longitude. As a counter point, the investigation also included observations from the central Arizona desert environment at Atlas’ DSET Laboratories located at 33° 54’ North latitude, 112° 81’ West longitude.

3.1. Outdoor Southern Florida Environment Exposures

A number of different environmental variables effect moisture available for material moisture absorption outdoors. In southern Florida, moderate temperatures, frequent precipitation, high relative humidity, and night condensation interact to provide plentiful moisture available for moisture absorption. Consideration of the effects of these environmental variables on material-specific moisture absorption represents an important objective for test method development.

Almost every night, moisture condensed from the surrounding air mass immerses exposed specimens under millimeters of liquid water in southern Florida. Figure 6 shows the extent of one night’s condensation on a commercially available yellow automotive basecoat-acrylic melamine clearcoat system just at sunrise on a horizontal southern Florida exposure during July 12, 2007. Time-of-wetness measurements indicate the amount of time liquid moisture wets exposed surfaces. Figure 7 shows an example of time-of-wetness records for southern Florida. Relative humidity, time-of-wetness, and condensation co-vary with other diurnal variables including temperature and interact to provide moisture for material absorption. The amount of moisture a particular specimen absorbs, however, depends also on a variety of material specific variables as shown in the previous section of this paper.

Figure 6. Florida Night Condensation.
Florida night condensation
Figure 7. Average Time-of-Wetness Florida and Arizona Near Horizontal.
Average time of wetness

In-situ measurements of materials exposed in end-use environments represents one of the best ways to consider the moisture absorption of specific materials in end-use exposure. In order to characterize moisture absorption in southern Florida, this investigation observed multiple samples of three different coating systems on exposure throughout a diurnal cycle in southern Florida. The same materials used in the material considerations section of this investigation were placed on exposure. First, specimens equilibrated in an uncontrolled office environment (approximately 23°C, 60% relative humidity) for several days. Initial weights of each specimen were then obtained using the same microbalance and weighing method used in previous sections of this investigation. Specimens were then placed outdoors on an unbacked, horizontal exposure during July 11 and 12, 2007. At regularly timed intervals, specimens were measured for absorbed moisture using the same method and scale as used in the material considerations phase of this investigation.

Figure 8 shows the diurnal moisture absorption-desorption cycle observed for the yellow automotive basecoat-acrylic melamine clearcoat system. Figure 9 shows the cycle observed for the white acrylic water base coil coating. Figure 10 shows the cycle observed for the brown PVC coil coating. This data represents a best estimate characterization for how these coatings absorb and desorb moisture during the diurnal cycle in the southern Florida outdoor exposure environment.

Figure 8. Diurnal Moisture Cycle of Auto Coating in Southern Florida.
diurnal moisture cycle
Figure 9. Diurnal Moisture Cycle of Acrylic Coating in Southern Florida.
cycle of acrylic coating
Figure 10. Diurnal Moisture Cycle of PVC Coating in Southern Florida.
cycle of pvc coating

Several general observations become apparent from the results shown in figures 8-10. First, the amount of moisture absorbed at different points of the cycle appears to be material specific. Second, the materials desorbed moisture during periods of solar irradiance and higher temperatures. Additionally, coatings absorbed moisture more gradually after sunset and desorbed it more rapidly after sunrise. Third, the results show no obvious “step” in the curve when visible moisture condensed onto specimens. A rate change was not observed as the moisture transitioned from relative humidity to liquid wetness.

The total quantity of moisture that moved into and out of the coating systems during this cycle represents one of the most important observations of this investigation. By characterizing the amplitude of this cycle in southern Florida for these specimen sets, future characterizations of weathering test methods can utilize the same specimen sets to directly compare artificial-accelerated test method moisture absorption observations back to the benchmark characterizations obtained in the southern Florida cycle.

3.2. Outdoor Central Arizona Environment Exposures

Environmental variable conditions in central Arizona differ significantly from southern Florida. In central Arizona exposures, higher summer and cooler winter temperatures, infrequent precipitation, low relative humidity and condensation reduce coatings moisture absorption compared to southern Florida exposures. Consideration of the effects of central Arizona environmental variables on material specific moisture absorption (as compared to the southern Florida environment) highlights the need to consider different end-use exposure environmental conditions on moisture effects.

Almost every night, moisture does not condense on exposed specimens in central Arizona. Figure 7 compares time-of-wetness in central Arizona to southern Florida. The majority of wet time coincides with the winter rainy season and summer “monsoon” season in central Arizona. It appears that while night condensation in southern Florida results in significant amounts of wet time, precipitation may relate more closely to wet time in central Arizona.

In order to observe moisture absorption in central Arizona, multiple samples of the three different coating systems previously characterized in southern Florida were observed on exposure throughout a diurnal exposure in central Arizona. As in the previous characterizations, specimens equilibrated in an uncontrolled office environment for several days. Initial weights of each specimen were then obtained using the same microbalance and weighing method used in previous phases of this investigation. Specimens were placed outdoors on unbacked, horizontal exposure. At regularly timed intervals, specimens were measured for absorbed moisture.

Figure 11 shows the diurnal moisture absorption-desorption cycle for the yellow automotive basecoat-acrylic melamine clearcoat system. Figure 12 shows the cycle observed for the white acrylic water base coil coat system. Figure 13 shows the cycle observed for the brown PVC coil coat system. This data represents a best estimate characterization for how these coatings absorb and desorb moisture during the diurnal cycle in the central Arizona outdoor exposure environment.

Figure 11. Diurnal Moisture Cycle of Auto Coating in Central Arizona.
cycle of auto coating
Figure 12. Diurnal Moisture Cycle of Acrylic Coating in Central Arizona.
cycle of acrylic coating
Figure 13. Diurnal Moisture Cycle of PVC Coating in Central Arizona.
cycle of pvc coating

Several general observations become apparent by comparing the results shown in figures 11-13 to the previously obtained data. First, the amplitudes of the central Arizona moisture absorption cycles appears significantly less than for the southern Florida cycles for all three of the coating systems in this investigation. Second, the amount of moisture absorbed at different points of the cycle appears to be material specific.

4. Weathering Test Method Considerations

Different weathering test methods result in different rates of moisture absorption for specific materials. Important considerations for designing wetting cycles into weathering test methods must account for material specific characteristics as well as characteristics specific to the end-use environment.

Simply measuring the moisture absorption of materials in weathering test methods, and comparing the moisture absorbed in the test method to results obtained in end-use characterizations reveals many considerations important for weathering test methods. This phase of the investigation characterized some basic material absorption behavior in several weathering test methods including the importance of coating system differences and comparison of test methods to outdoor diurnal cycle observations.

This paper presents observations of moisture absorption from three weathering test methods; an outdoor spray rack exposure in Arizona, ASTM G90 cycle 1, and ASTM G90 cycle 38. This part of the investigation also used the materials and weigh up methods introduced in previous sections of this study.

4.1. Spray Rack, June in Central Arizona

A previous weathering DOE investigation found significant and important effects of moisture sprays on appearance of automotive coating systems3. Those trials exposed specimens similar to the yellow automotive basecoat-acrylic melamine clearcoat observed in this paper. This phase of the investigation characterized the moisture absorption of similar specimens exposed on the spray rack method used in that study.

The spray rack test method oriented specimens facing south at a 45° angle from horizontal. A plywood rack exposed the backed specimens to a light rain-like de-ionized water spray during solar radiation. A spray nozzle applied high purity de-ionized water to wet the specimens. A single spray event lasted approximately one minute and occurred each hour during the daytime.

The same yellow automotive basecoat-acrylic melamine clearcoat specimens characterized previously were place on the rack and allowed to equilibrate without spray for several hours under the summer sun. Initial specimen weights were then obtained for each specimen using the same microbalance used previously in this investigation. The water spray apparatus was then turned on and applied the one-minute duration spray at a 1-hour frequency during the afternoon of June 14, 2007.

At regular intervals during the exposure, specimens were removed from the rack. All visible moisture was removed from the specimen by wiping thoroughly with paper towels. Specimens were then re-weighed and the absorbed moisture was calculated.

Figure 14 shows the results of this characterization for three spray cycles. Several general observations become apparent from the results in figure 14. First, the specimens appeared to rapidly absorb moisture during the spray period to a level much less than observed in the natural outdoor cycle characterized in southern Florida. Second, soon after the spray was turned off the specimens desorbed moisture and the weights appeared to level off between 30 and 45 minutes after the spray.

Figure 14. Moisture Cycles of Auto Coating on Spray Rack in Arizona.
cycle of auto coating

4.2. ASTM G90 Cycle 1 Daytime

ASTM G90 utilizes a fresnel-type system to accelerate effects of solar exposure on materials in a natural accelerated test method. ASTM G90 – 05 details apparatus design and operation. ASTM G90 outlines three typical water spray cycles. Cycle 1 prescribes an 8-minute water spray every hour during the day and three 8-minute water sprays at night. This phase of the investigation characterized the moisture absorption of specimens exposed to the ASTM G90 cycle 1 during the daytime.

The same yellow automotive basecoat-acrylic melamine clearcoat specimens characterized previously were placed in the G90 target area and allowed to equilibrate under full sun for several hours. Initial specimen weights were then obtained for each specimen using the same microbalance used in previous phases of this investigation.

The water spray system of the device was then activated which applied the 8-minute duration spray at an hourly frequency during the afternoon of July 5, 2007. At regular intervals during the exposure, specimens were removed from the target area. Any visible moisture was removed from the specimen by wiping thoroughly with a paper towel. Specimens were then re-weighed and the absorbed moisture was calculated.

Figure 15 shows the results of this characterization for three cycles. Several general observations become apparent from the results shown in figure 15. First, the specimens appeared to rapidly absorb moisture during the spray period to a level much less than observed in the natural diurnal cycle characterized in southern Florida. Second, soon after turning off the water spray, specimens desorbed moisture and the weights appeared to level off between 30 and 45 minutes after the spray.

Figure 15. Moisture Cycle of Auto Coating on ASTM G90 Cycle 1.
cycle of auto coating

4.3. ASTM G90 Cycle 3 Nighttime

An alternative to “daytime” wetting in the G90 test method includes a nighttime wetting option. ASTM G90 cycle 3 prescribes four 3-minute sprays every hour during the night (from 7 p.m. to 5 a.m.) with no water spray during daytime. This phase of the investigation characterized the moisture absorption of specific specimens exposed to the ASTM G90 cycle 3 during the nighttime.

Once again, the same yellow automotive basecoat-acrylic melamine clearcoat characterized previously were placed in the G90 target area and allowed to equilibrate under full sun for several hours. Initial weights were then obtained for each specimen using the same microbalance used in previous phases of this investigation. The water spray system was activated which applied the four 3-minute sprays every hour from 7 p.m. to 5 a.m. during the night of August 2 and morning of August 3, 2007.
At regular intervals during the night, specimens were removed from the target area and all visible moisture was removed with a paper towel. Specimens were re-weighed and the absorbed moisture was calculated. Figure 16 shows the results of this characterization. This data represents a best estimate characterization for how this coating absorbs and desorbs moisture during the diurnal cycle in the G90- cycle 3 exposure environment.

Figure 16. Moisture Cycle of Auto Coating on ASTM G90 Cycle 3.
moisture cycle of auto coating

Several general observations become apparent by comparing the results in figure 16 to the previously obtained data. First, the moisture absorption of the yellow auto basecoat-acrylic melamine clearcoat system on the G90 cycle 3 appears very different than the moisture absorption on the G90 cycle 1 (e.g. compare figure 16 to figure 15). Second, the moisture absorption of the yellow auto basecoat-acrylic melamine clearcoat system on the G90 cycle 3 appears reasonably similar to the moisture absorption observed in the southern Florida characterization obtained in this investigation (e.g. compare figure 16 to figure 8).

5. Discussion

Previous sections characterized absorption of moisture in specific coating systems during immersion and spray in the laboratory, in end-use conditions in southern Florida and central Arizona, and in three accelerated weathering test methods.

Moisture absorption behavior alone does not characterize weathering of a coating system. Simply maximizing moisture absorption variables for one coating type in a weathering test method may not result in simulation of a southern Florida degradation function for different coatings. Different coating systems appear to fail in different ways. For example one type of catastrophic failure observed in automotive coating end-use condition may involve separation of clearcoat from basecoat or basecoat from primer. A different type failure observed in automotive coating end-use may involve rapid loss of gloss from the exposed surface of the clearcoat. Developing a test method with variables set to optimize the propensity for the separation failures may not reveal useful information regarding surface gloss loss. The relationship between moisture absorption and different failure mechanisms is still not well understood and requires considerable research by the coatings industry. There may be many other links in the chain connecting simple coating moisture absorption and saturation to the variety of weathering induced coating failures observed in end-use. This paper and similar ongoing efforts only address considerations for controlling the moisture absorption, not the effects of the moisture manifested within coating systems.

Additionally, using a single method approach to control the moisture variable will result in different moisture amounts in different coating systems as shown by the empirical results (figure 2) as well as the theoretical explanations presented by Fick and Einstein. This consideration brings into question the “One-Size-Fits-All” approach popular in standardization of test methods today. Optimizing moisture variable settings in a standard accelerated weathering test method for one coating system does not necessarily optimize the moisture variable settings for a different system. Given the parameters in the Fick and Einstein relationships governing moisture diffusion, one wonders if manufacturing variations within a single coating type may also have significant effects on moisture absorption and constitute “different coating systems” in this context. It is also unclear how diffusion coefficients change as a function of weathering – moisture may diffuse into a coating system at a different rate after several years of weathering than it did initially on exposure.

Within the considerations outlined in this paper, assertions that the two most common accelerated weathering tests for automotive coatings, SAE J1960 Jun89 and ASTM G90, currently do not meet the minimum water saturation time criterion, naturally lead to the question, For which coating system? This logical consideration may also extend into the context of proper scaling of moisture with UV dose in accelerated weathering test methods. Adjusting moisture cycle variables (spray cycle duration, for example) for one type of coating system may properly scale saturating events to UV dose for that type of coating system while resulting in improper scaling for a different coating system with different moisture absorption characteristics. A more prudent approach may be to build flexibility into weathering test methodology and then customize variable settings as appropriate for different coating system moisture absorption characteristics.

These considerations become evident when comparing the moisture absorption behavior in southern Florida to the G90 cycles. The current G90 cycle 1 does not provide southern Florida-like saturating conditions for the automotive coating system in this investigation as shown by comparing figure 8 with figure 15. The current G90 cycle 3 does appear to be in reasonable agreement with southern Florida-like saturating conditions for the automotive coating system in this investigation as shown by comparing figure 8 with figure 16. Changing the moisture controlling variables of the G90 cycle 1 test method (in order to achieved proper scaling with UV dose) may allow this yellow automotive basecoat-acrylic melamine clearcoat system to achieve the saturation levels observed in southern Florida, however, the changed settings may not optimize the wetting events for the white acrylic water base coil coating system for southern Florida degradation simulation. A more scientific approach may be to develop a flexible method and apparatus and then customize the moisture absorption controlling variables of one method for the specific moisture absorption characteristics of the yellow automotive basecoat-acrylic melamine clearcoat and customize the variables in a second method for the white acrylic water base coil coating.

6. Conclusions

The weighing method used in this investigation appears to be a simple and very useful method to characterize moisture absorption of coatings in end-use as well as different weathering test methods. The most important findings can be summarized as follows:

  • Different coating systems absorb moisture differently.
  • The effect of water temperature on the moisture absorption of the yellow automotive basecoat-acrylic melamine clearcoat system in immersion appeared highly significant in this study.
  • The effect of orientation angle (horizontal vs. vertical) on the moisture absorption of the yellow automotive basecoat-acrylic melamine clearcoat system under water spray did not appear significant while the effect of ambient temperature appeared highly significant in this study.
  • Different weathering exposure environments resulted in different moisture absorption for the coating systems in this study.
  • The observations and foregoing conclusions of this study appeared generally consistent with Fick and Einstein's explanations for diffusion behavior.
  • The current G90 cycle 1 moisture absorption behavior of the yellow auto basecoat-acrylic melamine clearcoat system observed in this study did not appear similar to the behavior observed outdoors in the southern Florida diurnal cycle.
  • The current G90 cycle 3 moisture absorption behavior of the yellow auto basecoat-acrylic melamine clearcoat system observed in this study appeared similar to the behavior observed outdoors in the southern Florida diurnal cycle.
  • Weathering test methods and devices need to incorporate flexible moisture absorption variable control to optimize moisture absorption of different coating systems in order to simulate and predict performance in different end-use environments.

Acknowledgement

The authors would like to acknowledge and thank Tony Misovski of Ford Research for developing and teaching the weighing method used in this investigation without which, this paper (and future efforts) would not be possible.

References 

  1. Hardcastle, Henry K., "Characterizing the Effect of Weathering Variables Using Accelerated Fractional Factorial Experiments," in Natural and Artificial Ageing of Polymers, Thomas Reichert, ed. Pinsfal, Germany, Gesellschaft Fur Umweltsimulation, 2004.
  2. Misovski, T. Nichols, M.E. and H.K. Hardcastle, "The Influence of Water on the Weathering of Automotive Paint Systems," currently in proceedings publication, 4th International Symposium on Service Life Prediction, National Institute for Standards and Technology, Key Largo, FL. December, 2006.
  3. Hardcastle, Henry K., "Effects of Moisture, Location, and Angle on Automotive Paint System Appearance During Natural Weathering," currently in proceedings publication, 3rd European Weathering Symposium, Gesellschaft Fur Umweltsimulation, Krakow, Poland, 2007.
  4. Crank, J. and G.S. Park, "Measurement Methods," in Diffusion in Polymers, J. Crank and G.S. Park, ed. New York, Academic Press, 1968, pp.1-2.
  5. Ghez, R., A Primer of Diffusion Problems, New York, John Wiley & Sons, 1988, pp.86-87.
  6. Crank, J. and G.S. Park, "Measurement Methods," in Diffusion in Polymers, J. Crank and G.S. Park, ed. New York, Academic Press, 1968, pp.46.
  7. Barrentine, L.B., An Introduction to Design of Experiments - A Simplified Approach, Milwaukee, Wisconsin, ASQ Quality Press, 1999.
  8. ASTM G90-05 "Practice for Performing Accelerated Outdoor Weathering of Non-metallic Materials Using Concentrated Natural Sunlight," 2005 Annual Book of ASTM Standards, vol. 14.02, West Conshohocken, PA, American Society for Testing and Materials, 2005.