Automaker BMW stepped outside the box when it recently adhesive bonded carbon fiber/epoxy roofs to 1,500 limited-edition BMW M3 CSL sedans. The automaker wanted to enhance the car's performance by shedding as much weight as possible — other composite components include the doors, front skirt, trunk lid and rear bumper support. The 13 lb/6 kg carbon roof not only reduces vehicle weight but also lowers its center of gravity, improving stability and handling. Yet, a bonded solution meant that the composite/adhesive combination had to possess stiffness, strength, torsional rigidity, crash impact resistance and vibration performance equal to a welded steel roof. Manufacturing goals would include ease of application, adequate open time and a cure time of less than two hours.
BMW conducted extensive adhesive validation studies, including dynamic and static torsion tests, lap shear tests, dynamic mechanical analysis, peel tests, aging/climatic stress tests and an actual crash test, using different steel paints, surface prep methods and curing times. While phenolics, epoxies and acrylates were considered, the company selected PLIOGRIP, a two-part polyurethane manufactured by Ashland Specialty Polymers and Adhesives (Dublin, Ohio), which meets the structural performance requirements for bonds between dissimilar materials at service temperatures of 80°C/176°F (the highest the roof will see in hot, sunny weather) yet has sufficient elasticity (~50 percent elongation) to handle vibration loads.
At installation, the M3 is taken off the production line, workers manually clean the roof with alcohol and water, roughen the mating surfaces, and then hand-place a continuous bead of adhesive onto the car's steel roof opening. An assembly jig assists technicians in lowering the roof into place (see photo). Pressure is applied with clamps to ensure a tight bond, and "squeeze-out" of excess adhesive is visually confirmed. After a 100-minute room-temperature cure, excess adhesive is trimmed and the M3 goes back on line. Ashland's European technical manager Hartwig Lohse says PLIOGRIP's even cure speed, relatively low exotherm and balance of strength and elasticity were key: "While epoxies may have higher modulus of shear, the polyurethane has adequate shear to maintain vehicle rigidity — the advantage of higher elasticity is by far the predominant factor."