Fraunhofer-Institut für Angewandte Polymerforschung (IAP)
In the project a contribution to the problem of insufficient adhesion between polylactic acid (PLA) and other bio-based plastics with cellulosic reinforcing fibers was sought. The aim was to improve the mechanical properties of the respective completely bio-based cellulose short fiber reinforced composites via tailored modification of the fiber-matrix interphase. This was achieved using polyvinyl acetate (PVAc) and derivatives thereof, in particular block copolymers of PVAc and polyvinyl alcohol (PVOH) by taking advantage of the excellent adhesive and film-forming properties of those copolymers on the fiber surface. Valuable results were generated with respect to the interface properties and their influence and the mechanical properties such as tensile, bending and impact behavior with special emphasis on the latter.
The following sub-tasks were dealt with in the project. On the one hand, a flexible process was established to continuously and homogeneously coat the cellulose fibers with defined amounts of PVAc and its derivatives from aqueous solutions. On the other hand, a continuous compounding method to incorporate the modified fibers in the matrix materials was set up. This started with the optimization of the process on a small (kneader) scale and proceeded to an up-scaled process on twin screw extruders. After injection molding of the composites, the influence of the interphase modification was studied in detail and structure-property relations were established.
A flexible process was developed for coating short-cut and staple fibers and applied for cellulose fibers and PVAc as well as its derivatives. Moreover, a lab scale (200 g) compounding regime was established for studying the influence of process conditions on the resulting composite structures and properties. Optimum parameters were defined. A detailed screening of numerous PVAc dispersions was carried out and composites with PLA as the thermoplastic matrix were manufactured and tested.
It was shown that PVAc-treated fibers lead to a significant increase in composite impact properties. Notched Charpy impact values (acN) increased by remarkable 65 % while un-notched results (ac) were higher by 25 %. Opposed to conventional impact modifiers this did not lead, practically, to a decrease in tensile strength and modulus. Only strength is reduced by 5 %.
Based on the experience from the screening phase, the best suited system was chosen for scale up both for the fiber treatment and the compounding processes. A throughput of 5 kg/h was realized.
After the scale-up, the positive effect of interphase modification with PVAc and ist derivatives could not be confirmed with respect to increased impact properties. Obviously, the adhesive interactions between coating and cellulose surface were not sufficient to keep the coating on the fiber surface during the shear-intensive compounding process.
In compensating this effect, PVAc dispersions with PHOH as co-component proved most successful.
The latter dispersions were successfully employed in the scaled-up experiments. Structural investigations confirmed the presence of the PVAc/PVOH coating on the fiber surfaces even after compounding and injection molding and, as expected, showed the best mechanical properties.