Shipowners and managers have long recognised the benefits of premium tank coatings, but how best to choose the right option?
More than any other marine coating, tank coatings are expected to perform under the most demanding circumstances. Exposed to different corrosive cargoes, mechanical stress and aggressive cleaning, tank coatings must not only be durable, they must be flexible enough to facilitate quick turn-around times between cargoes, without the risk of contamination.
But how to you compare the performance of different tank coatings that are exposed to such harsh and variable conditions? While a laboratory test cannot replace real-world conditions, comparative analysis can be useful in establishing measurable performance metrics. In 2020, Jotun, a provider of tank coatings, opened a new state-of-the-art Research and Development Centre in Sandefjord, Norway. The new facility has enabled the company to explore different ways to test and develop specialised marine coatings, from primers to anti-foulings, topcoats to tank coatings.
According to one of Jotun’s chief chemists, Gard Reian, owners and ship managers are increasingly focussed on the link between tank coatings and long-term profitability. “In today’s increasingly data-centric industry, owners and managers are seeking more evidence-based tools to evaluate the performance of tank coatings over time,” he says. “Our challenge was to select metrics that mimic real-world conditions, establish a solid testing methodology, and experiment with different products in a controlled setting. After all, what can be measured can be managed.”
Mr Reian says that in the case of tank coatings, owners are looking for solutions that both extend periods between dry dockings and enable rapid changeover times between cargoes. “It is well known that cargoes featuring smaller molecules, such as methanol or ethylene dichloride, cause some swelling of organic coatings,” he says. “It is important that the coating can handle this continuous swelling/de-swelling without cracking prematurely. Based on this, it was of interest to test some of the most well-known tank coatings and try to find out which of these can ensure the flexibility to carry different cargoes.”
Jotun’s comparative analysis was based on Dynamic Mechanical Analysis (DMA), a broad term that encompasses several ways of measuring the mechanical properties of polymers. When working with thermoset coatings like those used to protect tanks, Jotun focussed on two testing methods: creep recovery and tan delta analysis.
For coatings researchers, creep recovery is one of the most fundamental tests of material behavior and is directly applicable to product performance. “The creep recovery experiment allows us to examine a material’s response to a constant load and its behavior after the removal of that load,” says Mr Reian. “How much strain can a coating endure and how fast does it recover to its original condition? We can measure this on a DMA instrument, an experiment that allows us to plot how much a coating will be strained as a function of time.”
This metric is important, because the higher the strain value, the more flexible the coating will be. “The high crosslink density needed to achieve good chemical resistance in a tank coating will lead to an inherently ‘brittle’ coating,” he says. “This brittleness will get worse as the coating ages, meaning the coating will not last as long, resulting in more frequent dry dockings.”
Tangent of Delta analysis (or tan delta), is used to quantify the way in which a material absorbs and disperses energy. “Put simply, tan delta measures the relationship between elastic and viscous properties of the polymer,” he says. “A coating with a high tan delta will generally have a high crosslink density and therefore be more resilient towards aggressive chemicals.”
Mr Reian says that when applied to polymers, tan delta analysis is dynamic, so his team applied an oscillating force and ran the experiment over a wide temperature range. “At the onset, the coating starts to lose strength and becomes more susceptible to chemical attack and failure,” he says. “The higher the curing temperature of a thermoset coating, the higher will the resulting tan delta be at temperatures up to the offset tan delta value.”
“Creep recovery is one of the most fundamental tests of material behavior”
Jotun selected five premium tank coatings for the test (including Jotun’s Tankguard Flexline, or Coating D), each based on different binder technologies. A single coat of each tank coating was applied using a draw-down applicator. It was applied directly to mylar polyester for easy removal of free coating film. The coatings were applied with dry film thicknesses, as specified by their respective technical data sheets. All coatings were cured at 23 °C for 14 days. To study the effect of aging, one set of each coating was given an additional hot cure at 60 °C for 16 hours.
For the creep recovery test, a sample of free film was placed under a given force (5 MPa) for a given period (10 minutes). The experiment was run at a constant temperature of 23°C. For the tan delta analysis, samples of free film were tested dynamically from -20 °C to 180 °C, with a fixed displacement of 5 µm and a temperature increase of 4 °C / minute.
Mr Reian says the results of both tests were illuminating. “In the creep recovery test, it was apparent that all coatings become less flexible after a hot cure, but we detected significant differences between the various coatings, ranging from a modest 17% (Coating D) reduction in flexibility to a high of 62% for another product,” he says. “This provides a good measure of how effectively a coating resists absorption, which is a critical factor in ensuring cargo flexibility.”
The tan delta analysis was also revealing. By applying a hot cure and increasing the molecular movements in the coating, the test triggered further reactions between unreacted groups, resulting in a higher crosslink density. The film becomes less susceptible to chemical absorption after a hot cure, but it also becomes less flexible. “As expected, the test showed that the peak tan delta temperature increased for all coatings, but results varied depending on which binder technologies were used,” he says. “We saw that products using novalac epoxy tended to perform better than products based on alternate or hybrid binder technologies.”
Mr Reian says that based on the creep recovery and tan delta tests, which provided valuable information on the differences in mechanical properties of each coating, Coating D emerged as the best choice for the customer looking for full-on flexibility for their cargo vessels. “The difference between the tank coatings tested were subtle, but when you consider that even small differences in flexibility and durability can have a big impact on performance, the right choice can help owners save money over time,” he says.
Mr Reian acknowledges that the test was commissioned by Jotun to help the organisation better understand tank coatings and how its products compare with those produced by competitors, but he is confident that the methodology used for the comparative analysis is sound. “Obviously, we are pleased that our product scored well, but for owners seeking more evidence-based tools to evaluate tank coatings, we invite them to review our findings themselves,” he says.