Polyurethane hybrid resin makes carbon fibre SMC more attractive for cars
|Andy Pye looks at a development in carbon-fibre reinforced resins which will make composites more attractive for automotive manufacture.|
|Over the last 40 years or so, plastics have revolutionised the way that cars are made. While a car from the 1950s had almost no plastic, the typical automobile today has more than 120kg of plastic. Plastics now make up approximately 50% of the volume of a new vehicle, but less than 10% of its weight.|
Overall, the consumption of polymers in the automotive sector is expected to continue to grow. The transition toward a circular economy will require the automotive industry to rethink the ways that vehicles and their materials are designed, constructed, used, and then re-used.
Traditionally, carmakers were reluctant to reduce the weight of the car, unless cost-savings could be made as well. But this is changing, as the move to net zero pushes up the priority of saving further weight. Electric vehicles are limited by range, and so the more the weight of the car can be reduced, the further it can travel without the need to recharge.
The application of polymer components within engine transmission will become more common, as high temperature performance is not a requirement for electric battery components, the fuel systems or other required ICE parts. The entire structure of the battery pack offers opportunities for polymers and composites.
The sheet moulding 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.
But its growth of CF-SMC technology in the automotive industry has been slow, often because the system cost was prohibitive.
“In recent years, novel SMC materials based on carbon fibre have become commercially available and are now applied at full industrial scale to produce ultra-light structural parts that outperform their equivalents in aluminium 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 mould cavity. Yet, when used with carbon fibres, the incomplete wetting and poor level of adhesion of UPR onto the CF surface result in moulded parts with low mechanical properties.
Vinyl ester resins (VERs) are mainly used to achieve higher mechanical properties in the CF moulded part, which result from the increased mechanical performance of the cured VER matrix. However, the thickening of VERs to the required level for SMC moulding is a challenge. Furthermore, viscosities of VERs are usually too high to be able to fully impregnate the fine CF filaments, especially when higher fibre 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 moulding process that requires several (time consuming) temperature steps.
“AOC has developed unique SMC technology that enables chopped CF moulded parts with the mechanical performance of Epoxy Resin CF-SMC to be manufactured with the ease of UPR and VER SMC”, says 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 moulding (complete filling of the mould 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 fibre 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 fibre.”
These advantages, combined with excellent flow behaviour during moulding, mean that the resulting mechanical properties of the parts are very high (Tensile Modulus at 43GPa, Tensile Strength over 300MPa).
Combining Daron with carbon fibres in SMC enables reliable manufacture of components with mechanical strength, low density, E-coat capability and low emissions, while maintaining the design flexibility typical for composites. In a the UK government funded research project called TUCANA, this new CF-SMC has enabled the development of structural automotive parts
Led by Jaguar Land Rover, TUCANA 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, whilst enabling cost effective, scalable carbon fibre composites to be manufactured.
As part of the project, Astar is using Zoltek’s split-tow fibre 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 mouldability.
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 moulding 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 100µg/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.