Smooth coatings surfaces have traditionally been considered to be best for showing color and for functions like aerodynamics and anti-adhesion. Now results in R&D projects are demonstrating that rough, structured surfaces may be in many circumstances a better option.
Many of these surfaces are already present in nature. So the objective of researchers and developers of coatings has been to mimic biology.
Universities and research institutes and producers of coatings and coatings materials in Europe are beginning to commercialize the outcomes of years of R&D into the biomimetics or mimicking of biological surface structures. The development phase of a lot of the research has been accelerated by advances in nanotechnology, which have enabled natural processes to be reproduced with the help of nanoparticles and materials.
As a result smooth and completely flat coating surfaces may no longer be valued quite so highly as before in some sectors of the market.
However a major challenge facing the developers of the new coatings is ensuring that the creation of one property does not sacrifice another, such as improved aerodynamics diminishing the visual appeal of a vehicle.
“We are at a stage with some innovation projects in which we know the concept behind a new coating structure works because it provides a new function,” said an R&D director at one research-oriented European coatings company. “But since it lowers the overall performance of the product we’re having to find another category of coating to apply it to.”
One of the latest research initiatives in the development of coating structures in Europe is a Swiss-backed project led by Clariant, a major producer of coatings materials, and by Zurich University of Applied Sciences (ZHAW). It also involves the research foundation Gebert Ruef Stiftung, the Swiss Federal Office of Energy (SFOE), and the Hamburg-based RETC Renewable Energy Technology Centre.
Clariant will be providing its expertise in polymer production and development to help commercialize R&D work by ZHAW’s School of Engineering, which has been working on surface polymers including those with hydrophobic and oleochemical properties with anti-adhesion and easy-to-clean functions.
ZHAW’s researchers have been investigating the antifreeze proteins (AFPs) possessed by plants, insects and fish and other animals to ensure their survival in the Artic and other areas with subzero temperatures. Similar R&D is being done by other European research establishments such as the Fraunhofer Institute and Bochum University in Germany but they have focused on developing coatings and other anti-freeze products incorporating AFPs.
The objective at ZHAW has been to make polymer surface structures mimicking those on the proteins which curb the growth of ice crystals within plants and animals so that they can live in freezing temperatures.
Clariant is hoping that the project with ZHAW will lead to a “breakthrough in anti-freeze technology” enabling polymer-based coatings to be used to reduce the formation of ice on equipment like the rotors of wind turbines. Another aim is the creation of hydrophobic anti-freeze coatings that prevent flowing water freezing so that windscreens and aircraft windows can stay ice-free for extended periods.
“Ice causes major operational problems and costs to businesses and utilities but also to private individuals,” said Achim Stankowiak, Clariant’s head of application engineering and aviation. “The potential for coatings that can prevent or slow down ice formation is therefore enormous.”
In the marine sector, a recent completed European Union-funded R&D project on nanostructured surfaces for controlling biofouling concluded that rough heterogeneous nanoscale surfaces are more effective at stopping adhesion by fouling organisms than smooth homogeneous ones. Nanostructured surfaces are also potentially a substitute for biocides in combating fouling, according to a summary of the results of the five-year scheme called AMBIO.
“Structure cannot be separated from the chemical composition of anti-fouling coatings particularly since both together have an important influence on surface energy,” said Parnia Navabpour, a scientist at UK-based Teer Coatings Ltd., one of 31 industrial and academic partners in AMBIO.
Nonetheless, discovering coating structures for ships hulls which both inhibit the adhesion of microorganisms and retain energy efficiency and fuel consumption levels by leaving the hydrodynamics of the vessel unimpaired could be a big problem.
An R&D team at Fraunhofer’s Manufacturing Engineering and Applied Materials Research unit (IFAM) at Bremen, Germany, won one of the institute’s main research awards this year by developing a system for applying a coating imitating the scales on sharkskin, which significantly raises drag resistance in air and water.
One of the difficulties to be overcome by the researchers was to find a method for applying on an industrial scale the coating on complex three-dimensional surfaces. “Our solution consisted of not applying the paint directly but instead through a stencil,” said Manfred Peschka, a scientist on the project.
With the help of nanoparticles, the coating withstands on aircraft extreme temperatures, intensive UV radiation and high speeds. On large container vessels it reduces friction with water by five percent. But the researchers are still looking for an anti-fouling solution for ships. One option they are investigating is structuring the paint in such a way that the fouling organism cannot gain a firm grip on the surface.
Developers of structured coatings also see the prospect of using them for improving the aerodynamics of automobiles. Lower drag levels are already being achieved with rough coating surfaces on racing cars and exclusive sports models.
“In devizing super-efficient aerodynamics in cars we can learn a lot from the animal world from creatures like fish which don’t have smooth skins,” Frank Stephenson, styling director at McLaren Automotive, a UK manufacturer of sports cars, said at a recent design meeting in London. “The future with cars could lie with different coating surfaces than at present.”
In the mass market for automobiles, however, the big problem is that structured coating surfaces with higher drag resistance seem at the moment unlikely to have the same optical appeal as the smooth surfaces which car owners now take for granted.
“Structured coatings can be applied to aircraft without this difficulty,” said Lothar Schaefer, a research coordinator at Fraunhofer. “With cars the visual appearance of these coatings may not be what the customers want, even though with nanomaterials the structural patterns will not be apparent to the eye.”
Perhaps the recent introduction of matt clearcoats with rough surfaces to give a muted silky finish to certain exclusive automobiles in Europe will lead to opportunities for combining attractive optical effects with improved aerodynamics.