Thermal Shock and Coating Integrity

Posted on Wednesday, July 8, 2020


By Kevin Biller

According to Corrosionpedia, thermal shock is “a variation in temperature which causes tension in a material. It frequently causes breakage in the material and is most common in brittle materials such as ceramics.” We’re not talking about ceramics here, but industrial grade powder coatings. Regardless, maintaining coating integrity over significant temperature variations is a requirement for many products that are finished with powder coatings, especially in extremely cold environments.

Materials engineers specifying coatings for their parts are tasked with anticipating a myriad of conditions to which their products will be exposed. Some requirements are pretty obvious – resistance to food stains for refrigerators and stoves, or sunlight for window frames and building facades. Other properties may pertain to fabrication demands such as adhesion to metal during fastening processes or flexibility if post-forming of parts occurs. However, specifying coating performance for extreme weather conditions is a challenge, especially when considering exposure to sub-zero temperatures and the potential for rapidly changing temperatures in service.

This Technology Interchange installment will address the issue of powder coating performance in the extremely cold climates experienced in northern and Arctic regions, as well as consumer and commercial freezers. Grab your parka, a wool hat, and a hot cup of joe and let’s look at the issues.

Ductile-Brittle Behavior in Polymers
The majority of powder coatings are based on binders that are comprised of reactive polymers and curing agents, whereas thermoplastic powders do not use curing agents and therefore rely intrinsically on the polymer itself for performance. Regardless, all polymers undergo a physical transformation below a certain temperature which increases rigidity, making the coating more brittle. Embrittlement can cause a coating to lose adhesion to the substrate and reduce impact and chip resistance. It is within a formulator’s purview to ensure that a powder coating can withstand the rigors of severe low service temperatures and still maintain required performance.

The physical property of a material that governs its behavior at or below freezing conditions is the ductile/brittle transition temperature (DBTT). DBTT is the minimum temperature in which a given material has the ability to absorb a specific amount of energy without fracturing. DBTT is usually not a specific temperature, but rather a temperature spread with a 10 degree Celsius (50 degree Fahrenheit) range.

For you science nerds, the standard to measure DBTT is ISO 6603-2, Multi-axial instrumented impact. It specifies a test method for the determination of puncture impact properties of rigid polymers, using instruments for measuring force and deflection. It is applicable if a force-deflection or force-time diagram, recorded at nominally constant striker velocity, is necessary for detailed characterization of the impact behavior. ISO 6603-1 can be used if it is sufficient to characterize the impact behavior of materials by a threshold value of impact-failure energy based on many test specimens.

From a practical sense, powder coatings developed to withstand extremely cold temperatures have to maintain flexibility, impact resistance, and adhesion at temperatures as low as -30 degrees Celsius (-22 degrees Fahrenheit). Let’s look at a few examples.

Automotive Coatings
OEM automotive coatings need to perform in the balmy climate of the Florida Keys as well as the frigid cold of Fairbanks, AK. Accordingly, automotive paint engineers have developed specifications that encompass the wide range of service temperatures in which their products have to perform. Many of these coatings are part of a multi-layer system of various coating technologies including galvanizing, electrocoat, solvent-borne and water-based liquid coatings, as well as powders.

Powder coatings are commonly used as a primer surfacer on auto bodies and as a clear topcoat on alloy wheels. Primer surfacers are intermediate coats that are applied to electrocoat primers and subsequently topcoated by liquid basecoats and clear topcoats. The function of a primer surfacer is to provide a smooth surface for the basecoat/topcoat, as well as a chip-resistant barrier to protect the underlying metal auto body.

Achieving good intercoat adhesion between coating layers is a challenge, however, as the additional requirement of performance at sub-zero temperatures makes the goal significantly more difficult. Automotive companies require the coating system to pass the rigorous “Gravelometer” test (ASTM D-3170/SAE J400) which is performed at -29 degrees Celsius (-20 degrees Fahrenheit). Europeans describe the test per ISO 20567-1. Determination of stone chip resistance of coatings. This test involves propelling a pint of standardized road gravel (about 300 stones) in a 70 psi air stream at a coated test panel at sub-zero temperatures. 

The damage incurred is assessed as the density and number of stone chip failures. Automakers have their own testing protocols; for example, GM’s GMW14700 and Volkswagen-Audi’s PV 1213 test methods. Criteria varies by automaker, but a minimum density of small defects is considered acceptable.

One of the largest automotive end uses for powder coatings is the clear topcoat for alloy wheels, both clear single coats and, more commonly, a multi-coat metallic basecoat followed by a clear powder topcoat. Polyester-based powders tend to serve the entry and lower level wheel market, whereas acrylics dominate the application to wheels destined for the luxury car market.

As you might expect, the coating systems applied to alloy wheels must look great while also performing under extreme environmental conditions. Adhesion, corrosion resistance, UV durability, and resistance to cleaners and polishing compounds are obvious requirements. In addition, these durable coatings must withstand the North Dakota steam cleaner test protocol (my words). This test is embodied in Ford test method FLTM BI 107-05 and GM’s GMW15919 method. Coated wheel specimens are scribed, then immersed in 38 degrees Celsius (100 degrees Fahrenheit) water for three to four hours. Next, they’re conditioned for another three hours in a -29 degrees Celsius (-20 degrees Fahrenheit) freezer. Frozen wheel specimens are thenwithdrawn from the freezer and within 30 seconds blasted for an additional 30 seconds with a steady stream of steam. Coatings cannot delaminate or blush due to humidity pick up. Coated specimens are allowed to recover for 24 hours to eliminate blush.

Another rather complicated test is the GMW14124 Automotive Environmental Cycles protocol. This requirement involves an exhaustive series of thermal cycling as depicted in Table 3. Scribed wheel specimens should exhibit no change of the surface appearance and scribe creep back should be less than or equal to 2.0 mm after 10 cycles.

In spite of these extremely rigorous requirements, powder coatings have been successfully used to coat alloy wheels since they grew in popularity back in the 1980s.

Functional Powder Coatings for Pipelines
Powder coatings have been used to coat oil and natural gas pipelines since the 1960s. One of the most prominent projects involved construction of the 800-mile Trans-Alaskan Pipeline System, which traverses from Prudhoe Bay in the Arctic to the Valdez marine terminal on the southern coast of Alaska. This 48-inch diameter pipe was coated with fusion-bonded epoxy powder and has stood the test of time. Besides needing to cure within seconds and withstand the constant exposure to crude oil and sediments, this durable coating had to be extremely flexible to withstand post-forming processes such as “grooving” to afford connecting with other pipe.

Canadian Standards Association’s Z245.20.21 specification for external fusion-bonded epoxy coating for steel pipe is considered the industry standard for powder coatings used on gas and oil pipelines. The flexibility of a candidate coating is evaluated two different ways. Test 12.11 requires the powder coating to be applied to a 25 mm x 200 mm x 6.4 mm (1 in. x 8 in. x 0.25 in.) thick steel bar at a minimum 300 microns (around 12 mils). The coated specimen is chilled at -30 degrees Celsius (-22 degrees Fahrenheit) for a minimum of one hour. The coated piece is then placed in a hydraulic mandrel and deflected (bent) until the coating fractures. This bending process is to take no longer than ten seconds. A passing value is determined by an equation that takes into account the diameter of the pipe to be coated.

Impact resistance is also a requirement for pipeline powder coatings. Test 12.13 involves applying 1.5 joules (or 3.0 joules for abrasion resistant requirements) of force to a powder that has been applied at the same thickness as the bend test on the same size steel bar. The coated specimen is conditioned at -30 degrees Celsius (-22 degrees Fahrenheit) for a minimum of one hour. The force is applied by a 1.0 kg falling weight through a 15.4 mm (ca. 5/8 in.) ball bearing. The impact is repeated three times on the same coated bar within 30 seconds of removal from the freezer. The impacts must be at least 50 mm apart. The coating is then evaluated for holidays, a.k.a. fissures.

Throughout the history of powder coated pipelines, epoxy-based powders have been the gold standard for performance for their fast cure, incredible toughness, corrosion resistance, and the ability to maintain film integrity over a wide temperature range. Newer generations of fusion-bonded epoxy powders have been developed to perform at the other end of the temperature spectrum to accommodate hotter materials transported through pipelines.

Appliance Coatings
Powder coatings are commonly used for consumer and commercial refrigeration components. Wire racks and baskets are finished with powders that must meet food contact regulations by the Food and Drug Administration (CFR 175.300) and the National Science Foundation (NSF-51). In addition to food contact requirements, these coatings must adequately cover weldments and sharp edges. And, of course, overall performance is expected at temperatures as low as -18 degrees Celsius (0 degrees Fahrenheit). Epoxy-based coatings were originally used to meet these requirements; however, eventually hybrid powders (epoxy-polyester) replaced them for better economics with comparable performance. Thermoplastic powder coatings, including PVC (polyvinyl chloride) and nylon types, are used on some freezer racks as well.

When dealing with extreme environmental conditions such as sub-zero service temperatures, powder coatings can exceed the performance required. Whether it is a pristine glossy clearcoat for a luxury automobile or 20 mils of a down and dirty functional coating for pipelines that will be buried in gravel trenches, powder coatings can chill with the best of them.

Kevin Biller is technical editor of Powder Coated Tough and president of The Powder Coating Research Group.