Evolving Requirements for Accelerated Testing of Paints and Coatings
By Cynthia Challener, CoatingsTech Contributing Writer
The performance of paints and coatings in their intended purposes must be proven before any new product reaches the market. For paints and coatings designed to protect assets in exterior environments, accelerated testing is one means by which formulators assess the specific performance properties relevant to different end-use applications.
In addition to transportation-related (vehicles, trains, airplanes, ships, bridges, marinas, and other infrastructure) and exterior architectural coatings, long-term performance under harsh conditions is required for coatings used in the oil and gas, petrochemical, wastewater industries, among others.
As coating technologies advanced and performance requirements and expectations changed, accelerated testing systems are also evolving to more adequately assess real-life behavior of applied films.
Traditional accelerated tests involve evaluation of the corrosion or weathering capability of coatings through exposure to aggressive chemical solutions and/or light sources that mimic solar energy. As coating technologies advanced and performance requirements and expectations changed, accelerated testing systems are also evolving to more adequately assess real-life behavior of applied films.
Traditional accelerated laboratory test methods were largely established to meet the needs of the automotive and architectural coating sectors. The oldest methods are the ASTM B117 salt fog test for evaluation of corrosion performance and ASTM D4587 for weathering assessment based on UV exposure coupled with condensation. ASTM D5894 combines the two, providing for cyclic salt fog/UV testing.
For the automotive industry in particular, SAE J2334 and GMW14872 are widely used accelerated corrosion tests. In these tests, the instruments are specially designed boxes into which coated panels are placed where they are exposed to harsh chemicals and UV light sources.
In simple salt fog cabinets, panels are held at a constant temperature and continuously sprayed with a salt solution at a constant rate. More complex tests may include steps such as introducing different gases and humidity, temperature, and drying cycles. The density and concentration of the salt solutions and environmental conditions are measured and controlled.
For weathering tests, UVA, UVB, xenon arc, or carbon arc bulbs can be used depending on the goals of the tests. Xenon arc bulbs create light of greater intensity, while carbon arc bulbs provide a broader wavelength range than UVA and UVB bulbs, which generate the wavelengths of the solar spectrum that are most damaging to polymeric binders in coating systems.
These laboratory test methods are generally conducted internally by large established paint companies. Smaller firms and startup entities will typically rely on a contract testing lab because they lack in-house resources.
Third-party testing is also employed by large formulators, for a variety of reasons, according to Robert B. Leggat, consulting and laboratory services manager for KTA-Tator. “Confirming internal results via third-party verification gives new product developers more confidence to make the further investment necessary to scale up from the benchtop to commercial production,” he explains.
In addition, independent testing of a company’s products with competitor coatings can eliminate any issues resulting from inherent or perceived bias. A few third-party testing companies also provide accelerated natural exposure testing services. These facilities are located in places such as Arizona and Florida and use mirrors to keep sunlight directed at test panels as the sun moves across the sky.
The requirements for accelerated testing vary according to the intended application of the coating, however, and established test methods aren’t always suitable, according to Leggat. Architectural coatings applied to wood are primarily concerned with polymer degradation from weathering, but those applied to metal will have both weathering and corrosion performance requirements.
The location where coatings will be used may also impact testing requirements—coatings used in northern and southern climates will experience very different conditions, for instance. Coatings on vehicles driven in northern climates may need to resist road salt exposure, while those along the coasts must resist damage by ocean salt and sand. City and rural environments can be quite different as well.
In the petrochemical industry, the concern is corrosion under insulation, and UV exposure is not an issue. Coatings used to protect assets in the oil and gas industry must resist exposure to hydrogen sulfide at high temperatures and pressures. Chalking isn’t an issue for these coating systems. As exposure environments vary, there is no “one-size-fits-all” option when it comes to accelerated testing, and exposures must be matched to the environment and material in question. In the case of hydrogen sulfide testing at high temperatures and pressures, traditional cabinets cannot reproduce these conditions, so testing for these coatings is done in an autoclave.
As accelerated test methods become more complex, it also becomes more challenging to identify and isolate factors that contributed to the failure of a coating, Leggat notes. On the flip side, some tests have become so specific that they are only directly relevant to a particular environment. SAE J2334, for instance, was developed by steel and automotive companies to simulate the most aggressive sites for automotive corrosion.
Tests may also not replicate the actual mode of failure. If an accelerated test causes blistering or yellowing, but the failure observed in actual use is chalking, then the stressors causing the actual failure have not been identified. This issue is common with salt fog testing, according to Leggat. “Coating failures typically do not match the appearance of actual corroded products because the continuously wet conditions in the salt fog test do not induce the same corrosion morphologies observed in a service environment with wet and dry periods,” he says.
As a result, the best use of laboratory accelerated tests is for comparative testing of existing and modified formulations or for competitor products. “Laboratory testing using standardized methods provides well-defined conditions that afford repeatable results. For a product with known laboratory and in-service performance histories, it is possible to predict the in-service performance of a modified formula in which an ingredient has been replaced for availability, cost, regulatory, or other reasons based on the accelerated testing results,” Leggat explains.
The real desire, however, is to have laboratory accelerated testing methods that provide results that correlate with actual in-service performance. Currently the only way to determine if the results accurately predict performance is to determine how a coating performs in service over time and then look back to see how the two compare. Formulators would also like to see accelerated test methods that can be conducted more quickly while providing accurate predictive information, according to Leggat. “Some of the current tests require exposure for 5,000 hours, which is still a long time in the context of the shortened development cycles facing formulators today,” he says.
Other limitations of accelerating testing methods in use today include the level of subjectivity involved in reporting results and the lack of real-time data generation. “Often the outcomes of these accelerated tests are reported on a scale based on visual observation of, for instance, the level of blistering or spot versus general rusting. These general results are supplemented with more specific details, such as the size, distribution, and number of blisters, but in essence these results are semiqualitative and somewhat subjective,” Leggat notes.
The lack of real-time monitoring of test conditions complicated this issue. For instance, in the salt fog test, the amount of accumulated fog or mist is only determined on an intermittent basis. In several cyclic tests, there are only periodic mass-loss measurements of bare steel coupons; thus, there is no real-time control of the test conditions, making it difficult to fully ensure consistency during the test and repeatability from one test to another.
Evolving coating technologies also create challenges for accelerated testing. “The polymer and additive chemistries used today are very different than those in use when the standard accelerated test methods were introduced,” says Leggat. “Many coatings today are water-based rather than solventborne. High-solids, UV-cured, and powder coatings are gaining market share as well. Toxic chromate corrosion inhibitors have been replaced with phosphates and molybdates, which themselves are receiving scrutiny.
But some test methods are designed to evaluate performance based on certain chemistries and assumptions about how coatings provide protection and therefore cannot accurately measure the performance of current formulations. Leggat notes a prime example: zinc-rich primers, which are widely used to protect steel assets.
“These coatings perform miserably in salt fog tests because the zinc is rapidly consumed in the wet conditions,” he explains. Current tests also have difficulty evaluating the performance of water-based coatings containing latex spheres and thin industrial coating systems that are just a few microns thick. “As coating technologies evolve, accelerated testing methods need to be modified in order to keep pace so they are designed to be able to evaluate performance based on the protection mechanisms involved,” he says.
Expectations for coating performance are also changing and uncovering limitations with current accelerated test methods. In the wastewater industry, for instance, aging infrastructure combined with changing regulations and increasing urbanization are creating new challenges for coating protection. Leggat also points to bridge coatings as one example of how expectations of coating performance are creating difficulties. In the past, bridge coatings were expected to last 20 years. Today, some specifications require more than 40 years of performance. With the 5,000-hour corrosion test in use today, no difference can be detected between coatings designed to last 20 and 40 years.
Fortunately, advances in digital technologies have helped address some of these issues. Better software that leverages big data analytics and artificial intelligence (AI) is having the biggest impact now that processing speeds and access to these advanced capabilities have increased. “Testing has moved from more simplistic assessments based on threshold values—this is or is not corrosion—to recognition of a range of features that help differentiate true corrosion from blistering resulting from a scribe or from a surface feature that is showing through the coating,” Leggat says.
Meanwhile, sensors embedded in test panels below the coating surface are providing large quantities of real-time data for the substrate. “Rather than base our determinations about corrosion on the appearance of the coating and the assumption that changes in appearance reflect performance properties, we can use data at the substrate and in the coating—changes in capacitance, water uptake and other parameters—to determine the initiation of corrosion and its mechanism. In many applications, the appearance of the coating doesn’t matter as long as it continues to provide the required level of protection,” he says. In addition, there is growing use of spectroscopic techniques (e.g., electrochemical impedance spectroscopy) to evaluate the barrier properties of coatings as accelerated testing proceeds.
Instrument manufacturers are also finding ways to help simplify and reduce the cost of more involved accelerated testing protocols. For example, one recent development is the combination of accelerated corrosion and weathering testing into a single cabinet so panels don’t need to be moved between two cabinets for prohesion testing (ASTM D5894). They are also offering cabinets that can perform exposure tests using multiple types of gases.
“Reducing the number of cabinets needed for different tests reduces their footprint and frees up space. Combined cabinets are also less expensive than the cost of buying the individual cabinets. The benefit to the formulator is greater flexibility and the opportunity to perform more tests on the same budget,” Leggat says.
Combined effects testing is another approach used to generate more predictive results for accelerated testing. For example, Leggat points to the incorporation of a fatigue machine inside a salt for chamber to evaluate how corrosion impacts fatigue strength and other mechanical properties. Such a custom-designed system—the Accelerated Combined-Effects Simulation test chamber—has been installed at the Air Force Research Laboratory at Wright-Patterson Airforce base, enabling researchers to recreate the broad range of conditions under which military assets operate, including UV radiation, temperature, humidity, and various gaseous environments.
There are also larger quasimanual cabinets the size of a room or even a building in which a car, tank or aircraft can fit, and the entire object can be sprayed to study corrosion of complete assets. Some of these tests include operational events to further stress the systems to identify corrosion and reliability issues that might not otherwise be observed by accelerated testing or operational testing alone.
One challenge that remains despite all of these advances is the need to identify the appropriate tests to perform for a given coating formulation and application. “There are many different test methods—MIL, ISO, ASTM—and not all are required for every coating, and a certain level of awareness about the tests and test conditions is needed in order to identify the right tests for a given product,” Leggat says.
Third-party testing labs and consultants can play a role here, particularly for smaller companies with new products that will compete head-to-head with coatings already on the market. “Conducting all the tests listed on the technical data sheet for an existing paint may be unnecessarily costly. We can recommend which tests are most important to conduct first, because if the candidate coating doesn’t pass those, there won’t be a need to do any further evaluations,” says Leggat.