Since the electric drive is relatively heavy, BMW uses lightweight construction for the body. Yet the innovative materials in the outer skin have another purpose: a modern look. Both the plastic panels and the carbon-finish roof are coated with Wörwag products.
“In particular, the roof of the i3 puts high requirements on the paint,” explains Dr. Markus Schmidtchen, head of development of functional paint systems. Unlike other carbon components, it is composed not only of brandnew fibers, but also of recycled scraps. There are economic as well as environmental benefits to using recycled materials. Production is thus less complicated than when new fiber mats are used—saving both time and money.
To maximize stability and rigidity, liquid epoxy resin is injected into the heated unfinished roof part, which is pressed into a mold. The fabricated part is then cured, ground smooth, and a bonding agent is applied. This bonding agent is a transparent hydro-primer made up of two components. It ensures that the clear coat applied at the end of the process has optimum durability. Soot is added to the primer for a black-tinted look. “The dark shade is purely for the appearance,” explains Schmidtchen. “It is the clear coat that has the protective function.
Protecting car skins
Since ultraviolet rays corrode the plastic resin, BMW uses a clear coat with an especially high sun protection factor on the carbon roof of the i3. Like a sunscreen—which protects human skin against the sun’s damaging UV rays for at least a certain amount of time—the protective coat, especially designed for CFRP applications, prevents the roof from getting “sunburn.” But this sunscreen lasts as long as the car does!
Two components achieve this. First, radical interceptors keep the coat from weathering, i.e. they make it resistant to heat, moisture, and UV rays. Radicals are usually highly reactive particles that are formed in chemical processes and would damage the coat. Second, absorbers intercept UV rays. To maximize the protective effect, the clear coat is applied in a thickness of 100 micrometers (μm). Normally, the coat thickness of a clear coat on plastic is only 30 to 40 μm, and 40 to 50 μm on metal bodies.
“The roof of the BMW i3 puts high requirements on the paint. A special clear coat protects the carbon finish from UV rays.” Dr. Markus Schmidtchen
In contrast to traditional vehicle construction, during production of the BMW i3 the entire bodywork is not protected against corrosion, coated, and dried in multiple steps.
Instead, the roof, bumpers, front, rear and side panels are coated individually, which is more economical. As a result, when paint is applied to the carbon roof in the flat-bed automatic paint sprayers, there is very little overspray—and thus, very little waste.
All in all, it takes only half the energy and 70 percent less water to manufacture an i3 in comparison with a conventional vehicle. The Leipzig plant has four of its own wind generators to handle the electricity requirements. The joint venture SGL ACF (Automotive Carbon Fibers), which manufactures the carbon fibers for the CFRP body in Moses Lake, Washington State, obtains all of its operating current from hydraulic energy.
Last but certainly not least, the integrated concept for the i3 includes the fact that BMW not only offers its customers renewable electricity but a Europe-wide charging infrastructure as well.
Dr. Markus Schmidtchen
has been in charge of the development of functional paint systems at Wörwag since 2008. As the name suggests, the department optimizes paint properties such as UV protection and weathering resistance. Schmidtchen studied chemistry and earned his doctorate at the Research Institute for Pigments and Paints in Stuttgart, now the Fraunhofer Institute for Manufacturing Engineering and Automation (IPA).
Text: Thorsten Schönfeld
Illustration: Nils Krämer
Photos: BMW AG, Boris Schmalenberger