CompositesWorld
Issue link: https://cw.epubxp.com/i/576646
CompositesWorld.com 73 NEWS N E W S N S N E W S E N W S W Painting Carbon Fiber Body Panels carbon fber body panels are among the remaining "low-hanging fruit" in terms of efective impact, yet ever out of reach for a variety of logistical, marketing, cost and technical reasons. Many people in the composites industry point to the Chevy Corvette hood as proof that carbon fber composites are a viable, lightweight alternative to steel and aluminum in a commercial — albeit high-end — high-performance vehicle, capable of producing a Class A fnish. But the Corvette hood is a relatively limited-run part by typical automotive measures (~40,000 units/yr) made by a technologically advanced process that entails multiple layers of prepreg, vacuum bagging and autoclave cure or a rapid out-of- autoclave method, yet still requires post-mold fnishing. For carbon fber to have a chance at competing for high-volume body-panel applications, it must be made by a short-cycle process capable of automation within tight tolerances to minimize post processing. Te obvious candidate process is resin transfer molding (RTM). In anticipation of the new CAFE requirements, BASF Coatings (Münster, Germany and Southfeld, MI, US) conducted a study evaluating the paintability of carbon fber test samples produced via RTM, comprising 48K tow carbon fber fabric and epoxy resin. Don Campbell, BASF's group leader, coating solutions, notes the company has a history of developing composites-specifc paint systems for Class A automotive parts. In 2001, BASF was given a PACE award for its DynaSeal "dual cure" primer designed to seal porosity defects in SMC. Tese porosity defects usually arise from the protrusion of random fbers through the surface of the panel, leading to "pop" defects in the top coat if not properly sealed. In one respect, the challenge in generating a Class A surface on a carbon fber panel is similar to that encountered with glass-rein- forced panels: Carbon, like glass, has texture that must be covered or flled before a clearcoat is applied. Unlike chopped glass-flled composites, such as SMC, the most weight-efcient carbon fber compos- ites are manufactured from contin- uous fber, which can be woven, knitted or braided. Carbon fber also has virtually no coefcient of thermal expansion (CTE), while glass has a fairly signifcant CTE. Both of these factors, Campbell and his team learned, have consequences when attempting to generate a painted Class A fnish on a carbon fber composite part molded via RTM. Campbell's team selected a set of primers and low-bake clearcoats that are specially formulated to fow and fll surface texture. After application and cure of the primer, one set of panels was sanded and another set was not. Both sets were then clearcoated and oven-baked at 100°C. When the panels were removed from the oven, both sets were, initially, texture- and defect-free. However, as the panels cooled to room temperature, texture appeared in the surfaces of panels in both groups. Te unsanded panels showed surface grooves as wide as 2 mm; the sanded panels had smaller grooves, about 0.25 mm wide. FIG. 2 The diferential expansion of the substrate arises as a result of the resin-rich domains of epoxy laying between non-homogeneously distributed carbon fber bundles at the substrate surface. The carbon exhibits little to no thermal expan- sion, whereas epoxy, with a CTE of 15 to 100 ppm/°C, diferentially expands and contracts with changing temperature. Source | BASF clearcoat at room temperature substrate Primer sanding Clearcoat baking clearcoat system after cooling baked primer on CF substrate Why Sanding is Inefective system at bake temperature primer c l e a r c o a t a t r o o m t e m p e r a t u r e s u b s t r a t e P r i m e r s a n d i n g C l e a r c o a t b a k i n g c l e a r c o a t s y s t e m a f t e r c o o l i n g b a k e d p r i m e r o n C F s u b s t r a t e s y s t e m a t b a k e t e m p e r a t u r e p r i m e r FIG. 3 Sanding of the primer did not eliminate texture in the clearcoat after panels cooled to room temperature. The diagrams here explain the phenomenon: The baked, primed substrate was removed from the oven and sanded smooth at room temperature (top sequence), but the substrate, with its characteristic textured surface, remained beneath the primer. As the clearcoat was applied and ramped up to bake temperature (middle images), the substrate expanded, causing ridges in the solidifed primer layer. However, the still liquid clearcoat fowed to a smooth level over the ridges until it cured and set at the bake temperature. As the panel cooled again (bottom image), the substrate shrank, forming ridges, the ridges in the primer disappeared, but adhesion between the primer and clearcoat caused grooves in the clearcoat. Source | BASF