Bio-based polymers are becoming increasingly important to reduce the dependence on fossil raw materials. This includes hybrid materials, such as fiber-reinforced composites or multi-layer composites, made from bio-based polymers. By combining materials with different physical properties, bio-based hybrid materials have both technological and ecological advantages (saving of fossil resources; reduction of CO2-emissions) compared to conventional petrochemical materials. So far, mainly stiff epoxy resins have been considered in order to obtain high-performance materials. However, the potential of flexible epoxy resins for bio-based hybrid materials has not been investigated yet. Therefore, this work focused on the development, characterization and optimization of bio-based hybrid materials by using flexible epoxy resins. This includes both flexible fiber-reinforced composites (flexible materials with high mechanical properties for use as technical textiles) and multi-layer composites with increased fracture toughness. A special focus was on achieving a high bio-based carbon content and on avoiding hazardous chemicals.
For this purpose, a bio-based epoxy resin based on plant oils and sebacic acid, which is flexible at room temperature, was developed. Therefore, various industrially available epoxidized plant oils with different epoxy equivalent weights and grades were used. The best mechanical properties (tensile strength of 1,3 MPa at an elongation at break of 27%) were obtained with a medical grade epoxidized linseed oil using an experimentally optimized mixing ratio (carboxyl/epoxy group in a molar mixing ratio of R=0,69), which was determined by differential scanning calorimetry and infrared spectroscopy. These mechanical properties were found to be independent of the epoxy equivalent weight within the range of industrial variations. In addition, the bio based epoxy resin was biodegradable under controlled composting conditions (32% in 52 days).
Knitted fabrics were used as the reinforcement for the flexible fiber-reinforced composites. Tensile tests were performed and monitored using digital image correlation to gain knowledge about the deformation behavior of these flexible fiber-reinforced composites. This was done using an interlock structure and a commercial petrochemical resin system to establish a solid foundation of knowledge. In addition, the effect of the fiber material on the tensile properties of the knitted fabrics and the fiber-reinforced composites was determined. Based on these results, rayon knitted fabrics were selected as the reinforcement for the bio-based epoxy resin. In the next step, the effect of three different knitted structures on the tensile, flexural, tear propagation and puncture impact properties was investigated. The best mechanical performance (highest tensile strength, tear strength and absorbed energy in puncture impact tests) was achieved with the interlock structure. However, the flexural modulus was in the middle range. Overall, the developed fiber-reinforced composites exhibit particularly high flexibility (low flexural modulus ranging from 21 to 207 MPa), which makes them promising for use as technical textiles.
The multi-layer composite aimed to exhibit an increased fracture toughness compared to a stiff epoxy resin by using the flexible, bio-based epoxy resin as a crack arrestor. For this purpose, a stiff-flexible-stiff layered structure was realized, with the bio-based epoxy resin forming the flexible middle layer. For the stiff outer layers, citric acid was used as hardener for the epoxidized linseed oil, resulting in a stiff epoxy resin after curing. The fracture toughness was determined using monotonic 3-point bending tests on single-edge notched bending specimens. The fracture toughness of the stiff epoxy resin increased by a factor of 13 with a middle layer thickness of 0,1 mm. However, the stiffness decreased by 44%. Increasing the middle layer thickness to 1,3 mm resulted in a further increase in fracture toughness (total factor of 24), but also reduced the stiffness by 67%.
In conclusion, this work has demonstrated the applicability and potential of bio based, flexible epoxy resins in hybrid materials such as fiber-reinforced composites and multi-layer composites. To further expand the advantages compared to conventional epoxy resins in the future, it is particularly important to reduce the curing time, and thus the energy required during production.