The industry guys that sell paints—for boat hulls, façades, or interiors—like to call them ‘coatings’. Should they simply be considered ‘paint’, or could it be reconsidered? Well they are very durable, highly certified and rather expensive, so let’s play along and call them coatings. They do coat, after all, and don’t have much to do with canvas, linseed oil and fan brushes.
Then, several years ago, the first truly active paints arrived on the market. They don’t only give a surface a new colour or keep the water out, but also act at the micro—or even nano—scale to kill bacteria, absorb harmful pollutants, control air temperature or reduce friction. And because they weren’t (yet) available in a local hardware store, maybe these are worthy of being called coatings.
These active coatings open up the typical aesthetic-, water resistant-, anti-corrosive properties of the coating family to an entirely new area of performance and application. The first active coating that we encountered was thermo-active, valuable for maintaining comfort inside and reducing heat absorption from outside buildings. At the time, the project consortium worked with the company that invented this coating to add a titanium dioxide component to the coating to improve interior air quality.
Titanium dioxide is a photocatalytic agent, and was, until now, activated with exposure to a significant amount of UV light. There are titanium dioxide coatings on the market that function with sunlight, but our team’s goal was to trigger the same activation with the typical light levels of an artificially lit space. Not only did we achieve this, but in quite a marketable way.
Titanium dioxide coatings can be simply brushed or sprayed on. The next step technologically is powder coating, which is electrostatically applied, then cured. A common industrial powder is PVDF (polyvinylidene difluoride), not be be confused with the more complex PVD (physical vapour deposition) coating process. The most common type of PVD coating is magnetron sputter vacuum deposition, or simply ‘sputter’ coating.
This is, in fact, a plasma process during which a material is transformed from a solid phase to a plasma phase and then back to a thin film condensed phase. The PVD coating process is most commonly used to create thin films with specific electronic, optical, chemical and mechanical characteristics. And while electronic and optical properties are very interesting, we’ll focus on chemical and mechanical attributes.
Let’s return to titanium dioxide to understand the difference in properties between chemically active surfaces, known in scientific terms as functional interfaces, and physically active surfaces, or structured interfaces. Titanium dioxide is active as a crystalline particle at the nanoscale; at the micro and macro scales, it is agglomerates to a non-structured powder.
Imagine that the functional crystalline particles are coating the precision structure of an already applied coating, as if you were carefully painting a woven basket. This way, it is possible to control the distribution of active particles while also increasing the active surface area.
The coating of a superyacht hull may represent up to 5% of the budget and is more a system than simply a coating. What differentiates this coating process from other industries, façades for instance, is the lack of a controlled coating environment, as the application usually takes place in the yard.
The system requires an infrastructure (scaffolding, dust removal, humidity control, et al) and phasing (fairing, color, clear coat, maintenance, et al) – the complexity of which can have a significant impact on planning. Given this expense and complexity, the coating system of a yacht should do more than simply conserve metals and give the boat an aesthetic touch. It should be an active, multifunctional, environmentally responsible skin.
For instance, structured anti-adhesive coatings can replace toxic tributyltin (TBT) and copper based antifouling coatings. The benefits for maintenance and long-term durability of anti-fouling, anti-adhesive active coatings has the greatest value for both the crew and owner. Taking up the explorer yacht trend, an anti-icing coating could be valuable for boats looking to sail the Northwest Passage or explore Antarctica. Fraunhofer IGB, who developed the aforementioned titanium dioxide coating, has developed an anti-icing plasma coating for airplane wings and wind turbine blades.
There is also environmental value: a coating in the engine room and adjacent mechanical spaces can reduce carbon monoxide and other toxic gases. With available titanium dioxide products, one can simply use stronger UV light when mechanical rooms are not occupied to remove pollutants. Additional membranes and coatings combining functional and structured surfaces could increase filtration quality for engine exhaust, further lowering the footprint of superyachts.
Active-interface researchers like Fraunhofer IGB are developing the technologies described above and beyond, including aerodynamic and anti-radar coatings. Biocompatible and bio-based materials, and drug release systems for medical devices or packaging, as well as, sustainable coatings top their to-do list. At the same time, awareness of lifecycle and eco-toxicity are of primary importance during the design of a material.
Clean water solutions are a primary application of anti-fouling coatings where they reduce surface build-up of minerals and bacteria. Microorganism-based sensing coatings change color in the presence of toxic substances. Researchers' more speculative thoughts address multifunctional, switchable bio-materials, extending the application of bio-processes from functional coatings into structured coatings and mechanically active and self-healing surfaces.
Application of these nearly invisible technologies is exciting. Normally, the nanoscale is more or less out of our reach, and sometimes we can think at the microscale. But we mostly design at the macro- or mesoscales, so coatings, as on the hull of a boat, are engaged at the functional and aesthetic levels. Now, we need to delve deeper into the design of coatings.
For instance, increased use of biocompatible materials is making recycling and circularity more achievable, where coatings are often the barrier to sustainable processes. Bio-compatibility is also important to the next generation of modified-release drugs and body-augmentation implants. Increased public and private surveillance textile coatings will make us invisible to cameras, motion sensors and radar. Surfaces can be structured or textured to make them more comfortable –on a desktop, for example.
The efficacy of chemically active coatings often benefits from increased surface area. The surfaces can be designed to be super textured at the meso and macro scales. High surface-to-volume geometries like tetrahedrons or Menger sponges are not coherent at a spatial scale, but can create high surface areas and wonderful textures at the micro and meso scales.
As with combining structured and functional interfaces, the next generation of additive manufacturing will combine with active coatings to provide the efficiency and precision for creating new high-surface-area materials. The next 5-10 years will see a proliferation of high-surface-area and active coatings in the marketplace – and perhaps even in our local hardware stores – from which superyacht early-adopters would stand to greatly benefit.
Thank you to Michaela Müller of the Fraunhofer IGB for her input on the science of active coatings and a look into their future. I would also like to thank Joost Mertens of Vripack for his background and insight into the superyacht industry. Please follow their links to learn more about them.
Check back soon for the next instalment on 'Aerogel'.
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