Carbon SMC: High Performance Delivered at Industrial Scale25 October 2021
Combining a unique Daron® resin from AOC with carbon fibers in SMC enables the reliable manufacture of components with superior mechanical strength, low density, E-coat capability and low emissions, while maintaining the design flexibility typical for composites. Luuk Groenewoud and Ron Verleg of AOC explain how the new CF-SMC has supported the development of structural automotive parts for the UK government funded research project TUCANA.
The Sheet molding compound (SMC) process has proven to be one of the most versatile production methods for composite components. It combines low waste production and high-volume capability with design freedom and integration of functions. For these reasons, SMC is broadly used for high volume manufacturing of strong, durable and lightweight composite components, with applications in the transportation, electrical, building and construction, and consumer markets.
Making SMC parts with Carbon 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 ultra-light 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, as SMC compounds based on UPR result in good mechanical properties, can accept 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 CF surface result in molded parts with low mechanical properties.
Vinyl ester resins (VERs) are mainly used to achieve higher mechanical properties in the CF molded part, which result from the increased mechanical performance of the cured VER matrix. However, the thickening of VERs to the required level for SMC molding is a challenge. Furthermore, viscosities of VERs are usually too high to be able to fully impregnate the fine CF filaments, especially when higher fiber volume fractions are required.
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. The main drawback of EPR SMC systems is a difficult impregnation, maturation and molding process that requires several (time consuming) temperature steps.
Perfect compatibility with carbon fibers
“To solve these issues AOC has developed a unique SMC technology that enables chopped CF molded parts with the mechanical performance of Epoxy Resin CF-SMC to be manufactured with the ease of UPR and VER SMC”, adds Luuk Groenewoud, Strategic Projects Manager at AOC. “This breakthrough technology is based on AOC’s Daron® polyurethane hybrid technology.”
The Daron® SMC technology provides unique benefits, including prolonged storage time for the compound (up to 6 months at room temperature), and perfect flow in compression molding (complete filling of the mold cavity, including inserts and ribs). The Daron® SMC technology also features a styrene-scavenging technology that results in an optimal radical polymerization, leading to extremely low volatile organic emissions.
“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 Ron Verleg. “Furthermore, the Daron® SMC technology leads to an ideal physical and chemical interaction between the cured resin matrix and the carbon fiber.”
These advantages, combined with excellent flow behavior during molding, mean that the resulting mechanical properties of the parts are very high (Tensile Modulus at 43 GPa, Tensile Strength over 300 MPa).
So far the growth in the industry of CF-SMC technology has been slow, often because the system cost was prohibitive. Carbon fiber manufacturer Zoltek has 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) CF performance. When opening up the 50K fiber into 3K bundles the strength properties increase, clearly beyond those of a 12K fiber.
The TUCANA project (details through this link), led by Jaguar Land Rover, brings together a consortium of world-leading academic and industry partners with the aim of delivering stiffer and lighter vehicle structures to boost the performance of electrified vehicles. Project TUCANA will deliver this vision by enabling cost effective, scalable carbon fiber composite solutions. As part of the project, Astar is using Zoltek’s split-tow fiber technology in combination with the AOC Daron SMC technology to produce a CF-SMC that has complied with all the project’s specifications to date, including mechanical strength and mold-ability.
In addition to mechanical performance and cost, another pre-requisite of high-volume automotive manufacturing is the ability of the composite part to withstand the painting process, including the E-coat step. The latter process is executed at relatively high temperatures that can reach up to 200°C for approximately 30 minutes. In the TUCANA project, panels have been run through the production line paint shop and it has been proven that SMC based on the Daron® resin system does not show any delamination when processed with defined molding parameters.
Furthermore, Daron CF-SMC technology includes a unique styrene scavenging feature that results in the absence of styrene emissions and a very low total VOC value (far below the 100 µg/g threshold set for interior applications), while the smell was rated 3 in VDA 270 emissions testing. The Daron CF-SMC can therefore be used for the production of interior parts.
High performance delivered at industrial scale
By combining forces along the value chain, Zoltek, AOC and Astar have managed to produce CF-SMC cost-effectively on an industrial scale. The combination of Zoltek’s split-tow fiber, the opening up of the fiber by Astar, and the use of Daron® SMC resin from AOC has resulted in a CF-SMC that can be used in the engineering and production of structural interior automotive parts as investigated by the UK government funded research project TUCANA, combining benefits of great mechanical properties, processing robustness in larger series, cost efficiency, extremely low part emissions, and design freedom.
“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. “This makes the material system a highly desirable solution for future high volume production series in Automotive applications.”