As automakers increase their use of lightweight aluminum and carbon fiber to improve fuel economy, the process of treating and painting surfaces is changing. In Part 1, in TMV’s spring issue, PPG Industries discussed new pre-treatment steps needed to increase the use of aluminum in vehicles. In Part 2, BASF discusses the challenges of coating carbon fiber.
Costly, half the weight of steel yet just as strong, carbon fiber has increasingly become a material of choice for automakers.
Formable into complex shapes, the high-tech material gets used in body panels and interior components on race cars, high-end sports cars, and electric vehicles. Performance cars from Chevrolet’s Corvette Stingray to the Lamborghini Aventador use carbon fiber extensively to boost the vehicles’ power-to-weight ratios.
From engineers to designers to environmental regulators, carbon fiber holds the answer to many of the biggest challenges facing automakers. However, costs are prohibitively high, and companies can’t use decades-old methods of applying paint to the material.
“A number of automakers have approached the paint community, and their desire is to utilize the process used today to paint the vehicle, but to incorporate all of these composites,” says Paul Lamberty, technical manager at BASF Coatings Solutions North America. “That’s the challenge. Today, nobody is successful at meeting that challenge.”
Carbon fiber and body in white
Carbon fiber is closer to a fabric than it is to the metal panels automakers are accustomed to painting. Imagine how many coats of paint it would take to make a sweater as smooth as a piece of sheet metal, and you get an idea of the scope of the challenge.
BMW avoids painting carbon fiber panels with i3
With its i3 compact electric car, German automaker BMW wanted to use colored carbon fiber body panels to slash weight while still offering protection against crashes. Instead of solving the challenges of painting the panels, BMW found an alternative process.
The i3 uses an aluminum frame, upon which workers affix a carbon fiber shell structure. Instead of treating, priming, sanding, and painting those surfaces, BMW attaches thin, pre-painted thermoplastic panels on top of the carbon fiber, creating a smooth, colored surface.
“Water consumption is reduced by 70% because the process does not involve priming, painting, and drying of the complete body, as with conventional models,” a BMW official says. “Instead, the bumpers, and front, rear, and side parts of the BMW i3 can simply be painted individually, which conserves resources.”
Also, avoiding the body dip process cuts about 22 lb of weight out of each vehicle, according to BMW officials. With electric vehicles, weight determines how far the cars can go on a single charge, so shaving pounds means adding miles. At a little bit more than 2,600 lb, the i3 is about 1,000 lb lighter than the steel-bodied electric Ford Focus.
What automakers want is “body in white” – a phrase that goes back to the earliest days of the auto industry when finished vehicle shells were built and painted separate facilities from the assembly plants.
In most auto plants today, semi-finished cars move into the paint shop with most of their exterior metal panels welded together. Manufacturers dip the partially assembled vehicle bodies into pretreatment and electrocoating baths before sending them on to the paint booths where robots apply thin layers of primer and paint, giving vehicles the smooth finish that customers have come to expect. Paint shop pros call these Class A surfaces.
The body-in-white process doesn’t work for carbon fiber.
“Current carbon fiber substrates are still very rough and inhomogeneous. They tend to be porous with pits and voids as well as with fibers resting on the surface,” Lamberty says. “Some of these fibers are not fully covered with resin and act as micro wicks. All of these substrate defects cause imperfections in the paint coating.”
Manual, expensive painting processes
Even if the panel was smooth, carbon fiber provides extra challenges. Most automotive paint shops use electrostatic systems that run a mild electrical current through the body panel, creating an electromagnetic charge. That charge attracts the misted paint molecules, creating even coverage coats and less overspray than gravity driven processes. With carbon fiber, Lamberty says, the material isn’t uniformly conductive, so electrostatic systems can lead to paint clumping on parts of panels that carry more charge.
“You can end up with different thicknesses of coating. It causes a very rough and unacceptable appearance,” he explains. Lamberty says coatings companies and carbon fiber producers are exploring new techniques to lessen such challenges, such as the use of conductive primers and extra sanding steps.
The bigger problem, though, is that automakers now must take significant portions of the vehicle painting process off the assembly line when they use carbon fiber. Manufacturers have spent decades and billions of dollars to make the painting process as fast and flawless as possible.
Lamberty adds, “The way they’re painting carbon fiber now is really not production feasible. It’s very costly.”
For now, the main solution is elbow grease. Automakers take the carbon fiber panels off the assembly line and, much like aftermarket car painters, apply primer, sand the surface, and then apply another layer of primer. They repeat the process until they get a panel as smooth as a piece of sheet metal, and then they paint it.
Between the roughness of the surface and the conductivity challenges, carbon fiber is not only more expensive than steel, it requires repetitive manual finishing, adding manpower costs to the equation. That kind of hand work is possible with low-volume, expensive sports cars, but it’s too time consuming and costly for plants producing 100,000 or more vehicles per year.
Electrocoating and multi-material panels
Problems in preparing carbon fiber start before panels get to the paint booths. Automakers electrocoat (e-coat) vehicles with thin deposits of zinc and other materials to protect against rust and corrosion. The curing processes used in e-coating can be too hot for the composite body panels, causing surface distortions that would require more sand, prime, and paint steps.
In some cases, the carbon fiber itself can stand up to the higher heats, but other composites used with it cannot.
Lamberty explains, “There are various types of carbon fiber composite and reinforced materials. Not every type of carbon fiber reinforced plastic (CFRP) is resistant enough at high temperatures. For example, carbon fiber reinforced polyurethane starts to lose stiffness at high temperatures and will sag, warp, and distort.”
Lightweight materials and fuel economy
Despite manufacturing challenges, interest in carbon fiber continues to build. With federal fuel economy regulations mandating an average 54.5mpg by 2025, all major automakers have said slashing vehicle weight will be a priority for more than a decade. A study published in 2012 by the Massachusetts Institute of Technology’s Sloan Automotive Laboratory found that producers would need to cut average vehicle weights by 27% and add more hybrid and electric vehicles to their fleets to comply with new standards.
For example, the 3,215 lb Toyota Camry is the best-selling car in the country. A 27% weight loss would cut 868 lb. Without extensive use of carbon fiber composites, that may not be possible. Ford late this year will launch its new 2015 F-150 pickup, a vehicle that swapped steel body panels for aluminum, slashed its engine displacement by 23%, and reduced frame weight by replacing standard steel rails with lower-weight, higher-strength steel alloys. The total F-150 diet saved Ford about 700 lb.
“They’re going to have to use either carbon fiber or something similar to it, whatever they invent,” says Paul Lamberty, technical manager at BASF Coatings Solutions North America.
However, even carbon fiber body panels will be too heavy on their own.
“They’re going to be using mixed substrates on the vehicle forward,” Lamberty says. “They’ll use carbon fiber wherever they need the structural aspects, then they’ll use other lightweight substrates elsewhere.”
Carbon and composite materials are already corrosion resistant, so today’s panels often skip e-coating. But, getting back to the body-in-white concept, Lamberty says automakers want to send entire vehicle bodies through all processes, regardless of the main material used.
“There’s a good reason for this. Even with carbon fiber and composites, they still use metal,” Lamberty says.
Metal tabs and inserts can be built into carbon fiber panels in the molding process, creating anchors to attach panels to frames, for example, or to add metal hinges for door placements.
“With any exposed metal part, they want that corrosion protection, so they’re going to want everything e-coated,” Lamberty notes.
Looking for solutions
Lowering the temperature of the e-coat process could allow lightweight CFRP and other reinforced composites to go through baths and ovens without warping. Lamberty says BASF and others in the automotive paint world are working on new chemistries that offer metal parts the needed corrosion resistance while operating coolly enough not to harm composites.
A colder bath could solve one problem yet create others. Some of the adhesive bonding techniques used in auto body construction rely on high-temperature treatments to fully cure their connections. With steel, that can happen in e-coat or in the bake-drying steps that follow painting in most plants. As with e-coating temperatures, paint baking can warp and distort some composites. So, if automakers turn to lower-temperatures and less oven time, some adhesives would need to be changed as well.
Once companies solve temperature issues, the challenges of surface roughness, void spots, and stray fibers wicking away primer and paint would still remain. In addition, carbon fiber is far too expensive for widespread use. Estimates vary, but a 2009 study from Ford put the price premium at more than 10-times steel prices. A 2011 report from the Rocky Mountain Institute (RMI) environmental group shows about a three-times cost premium (RMI showed a similar price difference to Ford’s study, but its report accounted for cost savings from simplified manufacturing processes).
Lamberty says many different groups – coating producers, material specialists, automakers, mold companies – are working to develop the technologies needed to get carbon fiber cost effective and suitable for traditional automotive assembly.
“We’re all working together on this one. We are looking at in-mold coatings to apply with the formation of the part with carbon fiber,” Lamberty says. “We are also formulating the coatings to minimize the effects of the wicking and porosity of the substrate. It’s definitely going to be a marriage of technologies – not just the substrate layers but the paint chemistry that goes into each layer.”
About the author: Robert Schoenberger is the editor of TMV and can be reached at firstname.lastname@example.org or 330.523.5381.