In the automotive industry, especially in lightweight construction for hybrids and electric vehicles, there is growing demand for components that are not just light, but also exhibit high stiffness and strength. Carbon-fiber reinforced plastics, usually simply called carbon, are often used in place of steel or aluminum in these cases. But these high-performance materials are expensive to manufacture and often cannot be produced in the high unit volumes required. Sheet-molding compounds (SMCs) have been used for decades to make plastic components in large volumes, but until now have not been suitable for comparable lightweight construction purposes. One of the keys to lighter construction is the plastic. The epoxy resins typically used in carbon to give high strength and stiffness are hard to apply in SMCs and have therefore not been used in the automotive market. But now, with Vestalite S, Evonik has succeeded in developing an epoxy hardener that eliminates these issues in compounds. Together with Lorenz Kunststofftechnik, based on the proven SMC recipe of this thermoset manufacturer, a material has been developed that meets all requirements for lightweight construction, including flame resistance. It also exhibits improved mechanical properties and is readily workable. Evonik and Lorenz hope to identify further potential uses for this material.
Conventional SMCs made with polyester resins typically have mechanical properties like a bending strength of 200 MPa, flexural modulus of elasticity of 10,000 MPa and an impact resistance of 90 kJ/m2 at a density between 1.7 g/cm³ and 1.85 g/cm³.They are suitable for many uses in the electrical and automotive industries, for example interior paneling and wheel covers. However, in vehicle construction there is a growing need for low-density, and thereby low-mass, materials with better mechanical properties, such as bending strength of > 350 MPa, flexural modulus of elasticity > 18,500 MPa and impact resistance of > 150 kJ/m2.
Peter Ooms, Sales Director at Lorenz Kunststofftechnik says:
“SMCs with epoxy instead of polyester resins can deliver these numbers. But up until now these thermosets were difficult to work with and the mold material was hard to shape, which greatly restricted design freedom. Glass-fiber reinforced epoxy SMCs were impractical, and therefore never really reached market maturity for large series.”
For automotive applications requiring very light components, carbon is often used instead, but it is expensive. So Lorenz has a clear focus on glass fiber.
Ooms says :
“Glass-fiber reinforced epoxy SMCs have a similar strength to carbon fiber, but are 50 percent less expensive to make.”
Furthermore, Lorenz has an established process for recycling glass-fiber reinforced SMC material, which is an important consideration given the sustainability requirements in the automotive industry.
New development without the former weaknesses of epoxy SMCs
A new glass-fiber reinforced epoxy SMC without the earlier material’s weaknesses could meet this demand, and Evonik’s newly developed hardener makes it possible.
Dr. Leif Ickert, who is in charge of Marketing Composites and Adhesives in the Crosslinkers division at Evonik Resource Efficiency, explains :
“Vestalite S is a diamine-based epoxy hardener that in conjunction with epoxy resins creates an SMC that is simple and quick to work with.”
Compounds with Vestalite S evince a high storage stability of the SMC mold material before hardening, while still permitting fast hardening in the manufacturing process, within three minutes.
Ickert adds :
“What’s more, the flow and deformation properties of semifinished pieces in pressing are better, which enables higher component quality.”
SMCs with Vestalite S have no styrene emissions and only low VOC emissions, making them suitable for automotive interior components as well.
Evonik and the thermoset experts at Lorenz have been collaborating since 2018 on the development of this SMC.
Ickert explains :
“With this collaboration, plus supplemental analyses and case studies, such as in the European research project Alliance, we want to show that our hardener can generate the desired properties in epoxy SMCs. We contribute expertise in epoxy hardeners while our partner Lorenz has the capabilities in SMC compound semifinished components. Our joint venture VESTARO GmbH adds automotive technology and technology consulting to the mix.”
“Our goal was to create a compound that makes the best possible use of the potential of epoxy SMCs – in other words a compound that has all the properties a material needs for lightweight automotive and other components. We have succeeded in this.”
Six years ago the Lower Saxony-based company had already brought out SMC 0208, a much more flame- and corrosion-resistant semifinished material than other standard SMCs, which is used in applications like construction and shaped parts.
Ooms explains :
“We used our basic SMC formulation for the epoxy SMC, but changed it a little. We replaced UP resin with epoxy resin and also switched out some other components. Vestalite S is naturally a key ingredient.”
Low density and ideal mechanical properties
The result is a combination of epoxy, glass-fiber reinforcement and fillers that has a density between 1.5 and 1.7 g/cm³ and demonstrates good flow. It does not stick to molds. The mechanical properties are as follows: bending strength of > 350 MPa, flexural modulus of elasticity > 18,500 MPa and impact resistance of > 150 kJ/m2. Carbon has bending strength and impact resistance averaging 320 MPa and 55 kJ/m2 respectively. Due to its basic formulation, the new semifinished material also has the general advantages of SMCs, including very good flow properties.
“Unlike other plastics, there are almost no combustible petroleum derivatives in it. Its oxygen index of > 65 percent makes it twice as flame-resistant as normal plastics considered very flame-resistant.”
Even when an open flame is applied, it goes out on its own within a short time, and has no tendency to deform or drip, even under high heat. The compound retains high impact resistance at temperatures of -30 °C, so there is no risk of brittleness or breakage.
These properties make epoxy SMCs ideal for use in battery housings in electric and hybrid vehicles, for example. Up until now these have been made of steel or aluminum, since they need to be very strong to hold the heavy batteries, resist kinetic energy in collisions, and also present no additional risk in case of fire. This new semifinished material meets these requirements and offers plenty of design freedom for a high degree of component and functional integration, which results in lower costs, lower weight, and higher safety.
Ooms explains :
“Like with other SMCs, the specific mix of characteristics is not set in stone. That means that the exact fire and mechanical properties can be adjusted to the maximum or other levels, depending on the application.”
Technologies: Compression Moulding