Figures courtesy of the author.

The use of infrared radiation (IR) thermal imaging is not something new to the industry and has shown a wide variety of uses in law enforcement, safety, military, medical, construction and sporting areas. Following the development of IR and thermal imaging (TI), the first commercial IR-TI cameras came into use in the 1960s from the combined efforts of Texas Instruments Inc., Hughes Aircraft Company and Honeywell, employing heat given off by an object to help locate said object^1-3. IR-TI works based on emissivity, which is different from night vision, which amplifies or intensifies available light to locate and see objects. Night vision is characterized by the “green glow” in the object area (Fig. 1).

Fig. 1: Infrared thermal image (left) and night vision (right). 

In firefighting applications, IR-TI can provide critical information to size up a fire incident, track fire growth and locate victims, other first responders and egress routes. Over the past several years, the National Institute of Standards and Technology (NIST) Fire Research Division has been developing a suite of performance test methods for inclusion in a national consensus-based standard on thermal imaging cameras used by first responders. The performance metrics are directly related to the environment in which the images are used and the tasks typically performed⁴. The same is true for law enforcement observation work.

A more focused use for this technology would in the construction industry to monitor the insulative properties of structures, heat loss and possible water damage. For instance, Figure 2 shows an IR-TI of a home during the winter, showing potential heat-loss areas. The red-to-white color is the area with the most heat, with blue being the cooler spots. This is the thermal-imaging or Ironbow palette in IR-TI and as one can see, quite a bit of heat loss occurs at window and door areas.

Fig. 2: IR-TI images of a home during the winter showing potential heat-loss areas.

In general construction work, IR-TI has recently shown promise for evaluation of concrete support pillars for strength, which allows for detection of damaged areas in the pillars prior to concrete structural failure occurring⁵. This article will focus on practical applications related to protective coating work.

Use in Coating Application

A technical paper was presented at the SSPC 2018 conference on the subject of determining the dry-film thickness (DFT) of applied coating systems⁶. While this was not IR-technology specific, the use of certain filters did provide for visual evaluation of the applied DFT of the coatings. So, let’s take this one step further.

What if IR-TI could be used to evaluate and monitor the actual application of coatings? This is an interesting concept that has some practical potential. Because IR-TI works by detecting heat emanating from the item observed, the applied coating system must be of thermoset technology — thermoset coatings are typically exothermic in nature, meaning heat is given off during the reaction and curing stage.

A common thermoset coating system used in protective coatings work consists of two-part, epoxy-based materials. It is well known that two-part epoxy systems are exothermic. If the two components are mixed and then allowed to set in the container for an extended period of time, there will be excessive heat generated, melting a container if that container is made of plastic.

Figure 3 is an IR-TI of spray application of a two-part epoxy system, showing that the spray is hotter (yellow to red color) than the surrounding area (blue to green color), but rapidly cools to the substrate temperature.

Fig. 3: IR-TI images of a two-part, spray-applied epoxy coating.

In order to demonstrate practical use of the IR-TI during coating application work, it would be best if the applied thermoset coating system would be conducive to thick-film application with a fast reactivity and significant exotherm. The type of coating systems that fall into this category are the fast-set, plural-component polyurea and polyurea/polyurethane hybrid systems⁷. These systems are applied at temperatures and with exotherms exceeding 200 F.

IR-TI in Polyurea Coating Application

An aromatic, fast-set polyurea system was being applied for support and protection of various automotive parts during shipment. The substrate itself is harsh on the parts, so a protective layer of polyurea is applied to cushion and protect against damage during transport.

Figure 4 shows the coated part. Normal visual inspection shows the applied polyurea system to be uniform in application and aesthetically pleasing. Wet-film-thickness (WFT) measurements were not possible during the actual application as the gel time of this polyurea system was about three or four seconds. Gel time for systems is the time that it takes to change from the liquid, fluid state to a solid membrane. Also of note is the temperature of the applied coating within two minutes of application when this photo was taken. Spray application was performed at about 150 F (65 C), so there is cooling after application.

Fig. 4: Application of fast-set polyurea to dunnage pieces.

However, when the same coated dunnage pieces are evaluated using IR-TI, a different observation can be noted. In Figure 5, both a white-heat output (white and gray) is used as well as the typical Ironbow color IR-TI palettes. One can easily see that there are some lines of delineation on the panel signifying heat emissivity variability in the applied coating system. This can be directly related to variable applied film thickness of the fast-set, thermoset coating technology.

Fig. 5: Coating thickness evaluations of two different panels sprayed with fast-set polyurea. The image on the left shows white heat output with Ironbow (in color) on the right. In the photo on the left, the white areas are hotter and at a higher DFT. In the Ironbow photo, the yellow is cooler as compared to the red. The yellow is the lower DFT and the red is the higher.

In this situation, it was specified that 20 mils (0.5 mm) of the polyurea system was to be applied, and in this case, the applicator used a single-pass, one-coat technique, which is not necessarily bad practice for paints and coatings in thin-film application. From visual inspection and shown in Figure 4, the applied thickness appeared to be uniform and the correct amount of material had been used per the surface area coated.

Again, the thickness variability was due to the application technique and not the material being used (in this case a fast-set polyurea system). And, even though the thickness averaged out over the panel surface, the thinner areas could lead to poor performance and part failure. Hence, application training and following industry standards of application are important⁸.

This type of high-solids coating system should be applied in a multi-coat, crisscross pattern. Spray pattern should overlap at least 50 percent and not match “edge-to-edge” of the spray pattern. This is known standard industry spray technique. And while the “W” technique of application may be employed in roller application of paints and coatings, this is not good practice for spray-applied coating systems. Figure 6 shows IR-TI of both correct and improper spray technique. Visually, the surface of the applied coating was acceptable, but thickness uniformity was not consistent in the improper, W pattern.

Fig. 6: IR-TI of proper (top) and improper (bottom)spray-application technique.

One of the more common applications for polyurea and polyurea/polyurethane hybrid spray coating systems is application over a geotextile fabric, primarily used in secondary containment application areas. Direct, simple non-destructive DFT testing is not as easy as if the coating were applied to a solid substrate and meeting required DFT conformance^{9, 10}. It has been shown that ultrasonic DFT testing can be performed, but applicators should be versed in the use of this evaluation technique¹¹.

IR-TI can be used to monitor application consistency, rather than relying on just visual observation. To illustrate this, many uses of the polyurea/geotextile are in the form of in-house, factory-applied polyurea to the geotextile. This helps to ensure consistency and quality in the produced composite panels. Figures 7 and 8 illustrate this work.

Fig. 7: Photograph of polyurea/geotextile panel production.

Fig. 8: The slight wrinkling of the same polyurea/geotextile fabric can be a common occurrence.

In Figure 8, the application of the polyurea systems appears normal. The slight wrinkling of the fabric can be a common occurrence in this type of work and will depend upon the type of fabric used. When viewing the IR-TI in this figure, the thermal signature seems consistent and uniform overall. While both pictures are of the same geotextile coating work, the one on the right is further progressing in the spray processing. The slight wrinkling does have an effect on heat reflectance, which should be accounted for especially in outdoor evaluation work.

An important aspect of the fast-set, plural-component polyurea systems is that high processing temperature is required to achieve proper mix and resulting polymer properties¹². This heat also lowers the mix viscosity and allows the material to flow better on the substrate; i.e., proper substrate wet-out. If the proper temperature is not achieved in application, problems may arise related to substrate adhesion or surface aesthetics of the applied polyurea coating system.

Once the coating system has been applied and cured, IR-TI can be used to evaluate the coated surface for voids or blistering of the coating from the substrate. In Figure 9, the blistered area is shown as a light blue color above and to the right of the window area.

Fig. 9: An area of blistering in an applied coating is detected by the IR-TI image on the right.

Conclusion

The use of IR-TI allows one to see what the human eye cannot. In coating application, what may appear by visual observation to be uniform and consistent may be plagued with defects and inconsistencies. Once fast-set, plural-component coatings are applied and allowed to set and cure, DFT measurements can be taken and may require repair to certain areas.

While there can be correlation to applied DFT, at present, this is a tool to monitor application technique as opposed to replacing actual DFT measurements. IR-TI can also be used in video capture as a teaching and training tool for proper application techniques to help ensure consistent application DFT of fast-set, plural-component coating systems. While not absolute, it is a start to real-time monitoring of application, with additional work in progress.

About the Author

Dudley Primeaux is the owner and operator of Primeaux Associates LLC, formed in 2001, providing a variety of consulting, product development, training and inspection services related to the coatings industry. He was the former director of education and development at VersaFlex Incorporated in Kansas City, Kansas. Primeaux has Bachelor and Master of Science degrees in chemistry from Lamar University in Beaumont, Texas, was employed by Texaco Chemical Company and Huntsman Corporation, was a former partner in EnviroChem Technologies, LLC, and former CSO of Inspar Robotic Technologies, Inc.

He is active in NACE and SSPC and holds SSPC-PCS, CCI Concrete Coatings Inspector and Level-II certifications. Primeaux serves as chairman of the SSPC C.1.9 Polyurea Committee. He is a JPCL Top Thinker, recipient of the 2012 Wayne Kraus Technical Award, recipient of the 2013 SSPC Coating Education Award for the new plural component course, and recipient of a JPCL Editor’s Award in 2015 and the SSPC Outstanding Publication Award in 2016.

Primeaux is named inventor of 31 U.S. patents and eight European patents, and an accomplished author of over 50 technical publications relating to polyurea elastomeric coating and lining technology, performance testing and inspection.

References

1. Thermal Imaging Guidebook for Industrial Applications, FLIR and Infrared Training Center (ITC), FLIR Systems AB, 2011.
2. Bagavathiappan, S. et al., “Infrared Thermal Imaging for Detection of Peripheral Vascular Disorders,” Journal of Medical Physics, Volume 34(1), Jan – Mar 2009, pages 43 – 47.
3. Goswami, Priyanka, “Thermal Imaging and its Applications,” Technology, Linkedin Slideshare, December 12, 2014, pages 1 – 31.
4. NFPA 1801, “Standard on Thermal Imagers for the Fire Service,” National Fire Protection Association,  Quincy, Massachusetts, 2018.
5. DuBose, Ben, “Thermal Imaging Used for Nondestructive Testing of Concrete Pillars,” Materials Performance, NACE International, April 29, 2018.
6. Yajko, Mike et al., “Visual Determination of Film Thickness via Real-Time Enhanced Digital Imaging,” SSPC 2018, New Orleans Convention Center, New Orleans, Louisiana, January 14 – 18, 2018, pages 1 – 8.
7. SSPC-Paint 45, “Two-Component, Thick-Film Polyurea and Polyurea/Polyurethane Hybrid Coatings, Performance-Based,” Coating Standard No. 45, SSPC: The Society for Protective Coatings, Pittsburgh, Pa., December 16, 2013.
8. SSPC-PA 14, “Application of Thick Film Polyurea and Polyurethane Coatings to Concrete and Steel Using Plural Component Equipment,” Coating Application Standard No. 14, SSPC: The Society for Protective Coatings, Pittsburgh, Pa, September 25, 2012.
9. SSPC-PA 2, “Procedure for Determining Conformance to Dry Coating Thickness Requirements,” Coating Application Standard No. 2, SSPC: The Society for Protective Coatings, Pittsburgh, Pa, May 1, 2012.
10. SSPC-PA 9, “Measurement of Dry Coating Thickness on Cementicious Substrates Using Ultrasonic Gages,” Paint Application Specification No. 9, SSPC: The Society for Protective Coatings, Pittsburgh, Pa, February 1, 2008.
11. Primeaux II, D. J. and K. Bower, “Evaluation of Applied Film Thickness for Polyurea Thick-Film Elastomeric Coating/Lining Systems,” PACE 2009; The Power of Paint & Coatings, New Orleans Convention Center, New Orleans, La, February 15 – 18, 2009.
12. Primeaux II, D. J., “Application of 100% Solids, Plural-Component Aliphatic Polyurea Spray Elastomer Systems,” SSPC 2000, Nashville, Tenn., November 2000, pp. 95 – 102.