Glass Fiber Reinforced Epoxy SMC As An Alternative To Carbon

2023 Author : Hannah Pearcy | [email protected] . Last modified: 2023-06-05 00:21

Conventional standard SMCs (Sheet Molded Compounds), which are made with polyester resins, usually have mechanical properties such as a bending strength of 200 MPa, a flexural modulus of 10,000 MPa and an impact strength of 90 kJ / m ² at a density between 1.7 g / cm³ and 1.85 g / cm³. They are suitable for a wide variety of applications in the electrical and automotive industries, including interior linings and hubcaps, for example.

Practical suitability of glass fiber reinforced epoxy SMCs

However, especially in vehicle construction, the need for materials with lower density - and therefore less mass - and improved mechanical properties such as a bending strength of> 350 MPa, a flexural modulus of elasticity of> 18,500 MPa and an impact resistance> 150 kJ / m ^{2 is increasing} . "These values can be achieved with SMCs whose formulation contains epoxy resins instead of polyester resins," explains Peter Ooms, sales manager at Lorenz Kunststofftechnik GmbH. “However, the processing of these thermosets was often problematic and the molding compounds difficult to form, which greatly restricted the freedom of design. Previous glass fiber-reinforced epoxy SMCs were therefore not practical and have therefore hardly made it to the market for the large series.”

Sheet molding compound

Sheet Molding Compound (SMC), English, denotes plate-shaped, dough-like molding compounds made of thermosetting reactive resins and glass fibers for the production of fiber-plastic composites. All necessary components are completely pre-mixed in SMC, ready for processing. Fiber matrix materials that are not supplied as sheets are also called bulk molding compounds .

Usually polyester or vinyl ester resins are used. The reinforcing fibers are cut fibers, more rarely in the form of a mat or fabric, the typical fiber length of which is 25.4 mm (1 inch). Properties and designations of SMC are regulated in EN 14598. A distinction is made between the following types of SMC:

• SMC-LP low profile: system with high surface quality
• SMC-LS low shrink: system with low reaction shrinkage
• C-SMC: System with endless, directional fibers

For applications that require a high degree of lightweight construction, expensive carbon is therefore often used in lightweight automotive construction. In contrast, Lorenz clearly relies on glass fibers. "Glass fiber reinforced epoxy SMCs have comparable strength values to carbon fibers, but are 50% cheaper to manufacture , " says Ooms. In addition, Lorenz has an established process for recycling glass fiber reinforced SMC materials, an important argument in terms of sustainability requirements of the automotive industry.

Special hardener for SMC material

A new glass fiber reinforced epoxy SMC without the previous weaknesses typical of the material, however, could now meet these requirements. The prerequisite for this is a hardener developed by Evonik. "VestaliteS is a diamine-based epoxy hardener , which in combination with epoxy resins results in an easy and quick to process SMC material," explains Dr.-Ing. Leif Ickert, responsible for Marketing Composites and Adhesives, Business Unit Crosslinkers at Evonik Resource Efficiency GmbH.

Compound hardens within 3 min

Compounds with this hardener have a high storage stability of the SMC molding compound in the uncured state and still allow quick curing in the manufacturing process within three minutes. "In addition, the flow and demoulding properties of the semi-finished product are improved during pressing, which enables high component quality," continues Ickert. In addition, SMC with VestaliteS have no styrene and only low VOC emissions and are therefore well suited for interior components in the car.

Evonik has also been working with the duroplast experts from Lorenz Kunststoffe since 2018 to develop a corresponding SMC. "With this type of cooperation as well as additional analyzes and case studies - for example within the framework of the European research project Alliance - we would like to show that our hardener in epoxy-SMC can produce the desired properties," explains Ickert. Evonik brings in the competence of the epoxy hardener, while the competence for the semi-finished product SMC compound comes from the partner Lorenz Kunststofftechnik. A joint venture with Vestaro GmbH also brings in the dimension of vehicle technology and technology consulting.

Seminar tip

The seminar on lightweight materials in construction teaches methods, design principles and construction methods for lightweight structures. The participants get an overview of different lightweight materials, learn about criteria for the selection of materials and learn about the advantages, disadvantages and risks of the different materials.

Modified basic SMC recipe

The aim now was to create a compound that optimally utilizes the potential of epoxy SMC - i.e. has all the properties that make a material ideal for the lightweight construction of automotive and other components. Already six years ago, the Lower Saxony company had developed SMC 0208, a flame and corrosion-resistant semi-finished product that is particularly flame-resistant and corrosion-resistant compared to other standard SMCs . "We also used our basic SMC recipe for the epoxy SMC, but modified it somewhat," explains Ooms. “We replaced the usual UP resin with epoxy resin and also replaced some other components. VestaliteS was naturally added as an essential ingredient.”

The result is a composite material made of epoxy, glass fiber reinforcement and fillers , which has a density between 1.5 g / cm³ and 1.7 g / cm³ and a good flow value and no longer sticks in the tool.

In addition, it has the following mechanical properties:

• a bending strength of> 350 MPa
• a flexural modulus of> 18,500 MPa
• impact strength> 150 kJ / m ²

Compound with adjustable properties

The epoxy SMC is suitable, for example, as a material for the battery housing of electric and hybrid vehicles . So far, this component has been made of steel or aluminum, because it has to be very stable and resilient due to the high weight of the battery, should absorb kinetic forces that arise in the event of an accident and, at the same time, must not pose any additional risk in the event of a fire. The new semi-finished product fulfills these requirements and, due to the high degree of design freedom, allows a high degree of component and functional integration with the effect of reducing costs and weight and increasing safety.

Twice as flame retardant

In contrast, the flexural strength and impact strength of carbon are on average 320 MPa or 55 kJ / m ² . Thanks to its basic recipe, the new semi-finished product also retains the advantages of SMC in general, including very good flow behavior . "In contrast to other plastics, the composition contains hardly any flammable petroleum derivatives," says Ooms.

"With an oxygen index of> 65%, it is twice as flame retardant as conventional plastics, which are considered to be extremely flame-retardant ." Even when exposed to an open flame, it extinguishes itself within a very short time and does not tend to deform or drip even in high heat. In addition, the compound retains high impact strength even at temperatures of -30 ° C, so there is no risk of material brittleness or breakage.