Good that the other pc brought up Bayer. Read on!
SELF-HEALING AUTOMOTIVE COATING Scratches on a car put a damper on the driver’s pleasure and reduce gloss. Bayer researchers have developed a new coating that repairs small blemishes itself and is also especially environment-friendly.
Every car owner wants his or her vehicle to have a glossy, shiny surface. But the shine fades a little with each new scratch. Such minimal abrasions occur during washing, for example, whether the car is washed by hand or in the car wash. All it takes are tiny particles of dirt on the sponge or brush and, after a few short years, the once glossy car looks as if it had been scrubbed with steel wool. The problem could soon be a thing of the past, however. Dr. Markus Mechtel and the research team at Bayer MaterialScience Coatings have now developed coating raw materials for the formulation of clearcoats that can heal these “wounds”: small scratches simply disappear.
The magic formula: “2K-PUR Clear Coat, plasticized and highly crosslinked”, an extremely resistant and easy-flowing polyurethane mixture comprising the polyisocyanate Desmodur® and the polyol Desmophen®. When the PU coating is slightly heated – and all it takes is sunshine on a summer day – small blemishes on the car’s sheet metal simply flow back together.
Less solvent thanks to new PU components in the coating
But the new Bayer coating doesn’t just have amazing “self-healing powers”, it is also environmentally friendly. The components used in the PU coating are significantly less viscous than those typically used before. This means that the automakers do not have to use as much solvent, which in turn benefits the environment.
The scratch resistance of a coating is almost entirely a function of the topmost transparent layer, which is only a few hundredths of a millimeter thick and protects the layers below. The demands placed on this outer skin are enormous. Not only should it be scratch-resistant, it also has to withstand aggressive chemical compounds such as salt water, tree sap and bird droppings – and all of this at extreme temperatures ranging from plus 70 to minus 30 degrees Celsius while subjected to UV radiation and high humidity. Conventional coatings fail in that it is difficult to combine chemical resistance and high scratch resistance in one coating. Many coating raw materials in the Bayer testing laboratory fail early on in testing for this same reason when subjected to the laboratory car wash and weathering unit.
Wash brushes bearing quartz sand rotate above coated sheet metal and 6,000 Watt xenon lamps simulate extremely intense sunshine at up to 100 percent humidity – all in the presence of an aggressive acid mixture. Because the Bayer researchers carefully modified the surface coating’s “inner values”, the new PU coating literally came through this torture shining.
Scratches simply disappear in just a few minutes
“The self-healing ability of the coating is attributable to the density of its molecules and its glass transition temperature,” says Thomas Klimmasch, head of the automotive coatings lab at Bayer MaterialScience. The glass transition temperature is a transition point below which the network of coating molecules hardens like glass and can no longer move. At higher temperatures, the network of polymer chains is elastic and can
flex in response to mechanical loads.The lower the glass transition temperature, the better the flow behavior and thus the self-healing ability of the coating. With highly flexible coatings, scratches appear as fine depressions that flow together in just a few minutes – more or less right before the eyes of the observer. A low glass transition temperature also has its disadvantages: both its resistance to chemicals and its polishability decrease. The coating becomes too soft to buff.
“A glass transition temperature of between 50 and 60 degrees Celsius has proven to be ideal,” says Mechtel.
With some coatings, mechanical loads such as those that occur due to brush impacts in a car wash result in irreversible surface damage in the form of fine brittle fractures. Such scratches do not heal themselves even at high temperatures. To make the coating even more resistant, the researchers increased the network density, tightening the molecular network. “This normally makes the material harder and more brittle,” explains Mechtel.
The first scratches only occur at higher loads but brittle fractures occur much, much sooner. The researchers’ goal was therefore to increase the density of the network while keeping the glass transition temperature constant. They made very specific changes to the composition of the raw material components. The result: the coating is better protected against not only plastic deformation but also brittle fracture.
What does this mean for car owners?
Fewer scratches occur in the car wash and those that do are small, self-healing blemishes. “With longer heat phases, residual gloss can recover from 70 to 80 percent back up to 90 percent,” says Mechtel. The researchers are now working on making the scratches disappear in cold weather and not just during the summer.
Coating in motion
The scratch resistance of coatings is tested using the “nanoscratch” method. A very fine metal tip is drawn with increasing pressure over the surface. At very high pressure, the transition occurs between plastic deformation and brittle fracture. This transition is then visualized using optical or atomic force microscopy.
Plastic deformation
Deformation scratches are no problem for the new coating. At high temperatures, sunshine causes the coating to flow into the scratch, largely returning the surface to its original smooth state.
Source: Bayer Research