AOC, Zoltek, Astar develop novel CF-SMC for high-performance automotive parts on an industrial scale

Under the UK government-funded research project, TUCANA, AOC AG (Schaffhausen, Switzerland), with Astar (Biscay, Spain), has developed a new sheet molding compound (SMC) based on Daron polyurethane hybrid technology, that enables the production of chopped carbon fiber molded parts on an industrial scale with the mechanical performance of epoxy resin CF-SMC, and the manufacturing ease of unsaturated polyester resin (UPR) and vinyl ester resin (VER) SMC. Together, the CF-SMC supports the development of structural automotive parts with low density, E-coat capability and low emissions, while maintaining the design flexibility typical for composites. It will also be used in combination with Zoltek’s (St. Louis, Mo., U.S.) lower cost split-tow fiber.

“In recent years, novel SMC materials based on carbon fiber have become commercially available and are now applied at full industrial scale to produce ultralight structural parts that outperform their equivalents in aluminum and steel,” explains Ron Verleg, senior R&D scientist at AOC. “Several thermosetting resin systems can be used with the SMC process, with each one having its specific advantages and disadvantages.”

UPRs are the workhorse resin for SMC applications, AOC notes, offering good mechanical properties, accepting high filler loadings (lowering the compound’s cost) and flow well in the mold cavity. Yet, when used with carbon fibers, the incomplete wetting and poor level of adhesion of UPR onto the carbon fiber surface result in molded parts with low mechanical properties.

Alternately, VERs are mainly used to achieve higher mechanical properties in the carbon fiber molded part, although thickening VERs to the required level for SMC molding is a challenge, and viscosity tends to be too high to fully impregnate the fine carbon fiber filaments, especially when higher fiber volume fractions are required.

Project TUCANA expects to deliver this vision by enabling cost-effective, scalable carbon fiber composite solutions.

Further, epoxy resins (EPRs) have also been fine-tuned to enable high mechanical properties to be achieved in SMC parts. However, it has been challenging to run this process in a cost-competitive way in high-volume applications, AOC says. The main drawback of EPR SMC systems is a difficult impregnation, maturation and molding process that requires several time-consuming temperature steps.

To solve these issues, AOC developed its Daron SMC technology, which offers benefits such as prolonged storage time for the compound (up to six months at room temperature), and optimized flow in compression molding (complete filling of the mold cavity, including inserts and ribs), resulting in a tensile modulus of 43 GPa, and tensile strength over 300 MPa. Daron SMC also features a styrene-scavenging technology that results in an optimal radical polymerization, leading to extremely low volatile organic emissions (far below the 100 μg/g threshold set for interior applications). 

“Because of the low viscous nature of Daron resins, the fine filament bundles of the carbon fiber can be impregnated extremely well up to high volume fractions,” says Verleg. “Furthermore, the Daron SMC technology leads to an ideal physical and chemical interaction between the cured resin matrix and the carbon fiber.”

To help eliminate the cost-prohibitive nature of CF-SMC in the composites industry, carbon fiber manufacturer Zoltek has also developed a lower cost 50K split-tow carbon fiber that can be opened up during the SMC compounding process, while providing small tow (roughly 3K) carbon fiber performance. 

Both of these technologies — Zoltek’s split-tow fiber technology and AOC’s Daron SMC technology — have been developed under the TUCANA project, led by Jaguar Land Rover (Whitley, U.K.), which is bringing together a consortium of academic and industry partners with the aim of delivering stiffer and lighter vehicle structures to boost the performance of electrified vehicles (EVs). Project TUCANA expects to deliver this vision by enabling cost-effective, scalable carbon fiber composite solutions, including the CF-SMC, which so far has complied with all the project’s specifications to date, including mechanical strength and moldability; CF-SMC panels run through the production line paint shop have also proven that SMC based on the Daron resin system does not show any delamination when processed with defined molding parameters.

In addition to its use for the production of structural interior automotive parts, “further potential applications of this new high-performance CF-SMC include dynamically loaded parts such as engine sub frames and steering knuckles,” concludes Luuk Groenewoud, strategic projects manager at AOC. “This makes the material system a highly desirable solution for future high-volume production series in automotive applications.”