Carbon fiber reinforced thermosets (CFRT) show a beneficial combination of a low density and a high strength and stiffness. However they are prone to crack initiation and propagation. To increase the material’s resistance to cracking, different strategies can be followed. One possibility is the toughening of a polymer intrinsically, e.g. by adjusting the functionality of the resin to reduce the cross-link density. Another way, without affecting the chemical base structure of the material is the introduction of second phase fillers, such as nanoparticles, into the resin. Thereby, different fracture mechanisms can be activated to increase the toughness of the polymer.
Nano-scaled core-shell rubber particles (CSR), which are highly concentrated and pre-dispersed in a carrier resin (masterbatch) or self-assembling macromolecules, so called block-copolymers (BCP), can be used to toughen polymers according the latter principle. CSR and BCP offer easy dispersion processes, an even particle distribution throughout the composite (no filtration) and a constant glass transition temperature.
Within this intrinsic research project, core-shell-rubber particles and self-assembling block-copolymers were introduced into a thermosetting matrix to increase toughness and ductility of a CFRT. The interlaminar fracture toughness GIc of the pristine and modified composites was examined to assess the influence of both types of modifiers (Figure 1). Toughening of the matrix by low concentrations of BCP and CSR (≤ 3 vol.-%) increases the interlaminar fracture toughness by more than 50%. A further increase of the modifier concentration nearly linearly improves GIc. Combining the modifiers in a hybrid modification (BCP/CSR) in the resin another, yet less pronounced improvement of the interlaminar fracture toughness prevails. Generally, increasing the modifier concentration in the resin, more energy dispersive mechanisms can be activated during crack propagation, enabling the matrix to absorb a higher amount of deformation energy. Contradictory, the energy absorption capacity of the matrix is hindered by the dense packing of the reinforcing fibers, limiting the evolution of a plastic deformation zone. Figure 2 shows the fracture surface of a modified CFRT. Feature A indicates a good fiber matrix bonding, since matrix residues are present on the fiber surface, whereas feature B illustrates a high degree of ductility of the matrix. The spherical features are created by the deformation and cavitation of modifier particles.
In this project, it was possible to drastically improve the interlaminar fracture toughness of a CFRT without increasing the processing efforts by the application of self-assembling macromolecules and core-shell-rubber particles. Already small concentrations can significantly increase the resistance to crack propagation of a thermosetting matrix, without affecting the glass transition temperature of the fiber composite.
Dipl.-Ing. Andreas Klingler
Institut für Verbundwerkstoffe GmbH
Telephone: +49 (0) 631/2017 414