Sheet Molding Compound (SMC) has high market relevance in the market of glass-fiber-reinforced polymer composites (GFRPC), as 20 % of glass fibers produced in Europe are handled in SMC process. Normally, SMC consists of a highly filled thermoset resin system, mostly based on unsaturated polyester resin, glass fibers, application specific additives, and filler materials, see Figure 1. SMC is, based on its mechanical properties, predestinated for the use in semi-structural parts or hang-on parts – e. g. for automotive applications. The scope of application can be diversified by a variation of the specific additives and the directly linked change of the composition of the semi-finished product. The properties of the material itself can be changed, from e.g. electrical applications for cabinets to parts with an equal thermal expansion coefficient as steel. Furthermore, surface quality can be optimized to Class-A, which provides opportunities towards optical applications in automotive sector. Further material advantages are the lower price for the semi-finished product as well as the possibility of cost-efficient large-scale processing in a parallel regulated compression molding process.
In a classical SMC, based on UP resin systems, approximately 40 % of the total weight of the SMC semi-finished product consist of mineral filler materials, see Figure 1. These fillers can be divided into functional filler materials (e.g. aluminum hydroxide Al(OH)3 as flame retardant) and non-functional filler materials (e.g. calcium carbonate (CaCO3)). The density of this conventional filler materials is between 2.4 g/cm³ (Al(OH)3) and 2.71 g/cm³ (CaCO3). Due to the density of the filler materials, the density of the SMC semi-finished product is between 1.7 and 1.9 g/cm³.
Within the research project BioSMC (funded by ZIM, funding reference: ZF4052322EB8) Lorenz Kunststofftechnik GmbH (Lorenz) and Institut für Verbundwerkstoffe (IVW) develop an SMC semi-finished product with high bio-based content. A replacement of oil-based and mineral components of classical SMC formulations with bio-based and renewable components is investigated. Furthermore, the semi-finished products developed in this project should be able to be processed with conventional SMC plant technology and should have consistent mechanical characteristics. In a first step, mineral components of classical SMC formulations were replaced by bio-based and renewable components. These fillers are, for example, by-products from food industry, such as those ones produced during oil extraction, and do not compete with the food industry. Based on extensive basic investigations regarding density, particle size distribution, dispersibility and influence of the fillers on the viscosity of the resin paste, sunflower seed shell flour was finally selected as potential substitute filler for further investigations. This filler shows a density of 0.95 ± 0.08 g/cm³.
The use of bio-based and renewable fillers in SMC semi-finished products leads to a significant increase in the resin paste viscosity, see Figure 2, and requires a modification of SMC formulation. Otherwise, a reliable impregnation of the reinforcing fibers on a conventional (laboratory) SMC plant is not possible, see Figure 3.
By replacing conventional mineral fillers with bio-based fillers, a reduction of the density of the semi-finished product from 1.89 g/cm³ to 1.45 g/cm³ was enabled, which is a reduction of 22 %. Mechanical values could be set to an almost constant level, regardless of the content of bio-based fillers, see Figure 4. Due to the low density of the SMC semi-finished product, which is less than 1.5 g/cm³, the BioSMC semi-finished products can be considered as potential competitors to low density (LD) SMC materials, which are filled with hollow glass spheres and – as a result - are very brittle and expensive.
In cooperation with the Institute for Biotechnologie und Wirkstoffforschung (IBWF), extensive test series regarding the influence of bio-based fillers on moisture absorption behavior and mould formation were conducted. The test series showed no negative influence on the bio-based filler in SMC .
In the further course of the project, the replacement of conventional unsaturated polyester resin by bio-based resin and the replacement of glass fibers by natural fibers will be investigated.
The good mechanical results show that the use of bio-based fillers in SMC is a realistic option. The use of former by-products from the food and forestry industries as raw materials in new technical materials of the plastics industry enables ecologically sensible upcycling possibilities. For series production, further investigations regarding fire behavior and paintability have to be carried out.
Dr.-Ing. (FH) Florian Gortner
Institut für Verbundwerkstoffe GmbH
Telefon: +49 631 2017 439