Influence of Epoxy Chain Extender and Multi-Stage Compounding Process on the Properties of Poly-(R)-3-hydroxybutyrate

Poly-(R)-3-hydroxybutyrate (PHB) is a semi-crystalline biobased biodegradable polymer, which has the potential of being a future replacement for the fossil-based non-degradable polypropylene (PP), especially as a packaging material. PHB meets high barrier properties to O2, CO2, and H2O, but it is very sensitive to thermo-mechanical and hydrolytic degradation. To improve the ductility of the material and reverse a significant amount of the degradation during processing and recycling, a Joncryl® chain extender was added in a simulated recycling process on a twin-screw extruder. To evaluate the effects of processing and the addition of the chain extender on the properties, thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), small strain oscillatory plate-plate rheometry, tensile tests, and notched impact Charpy tests were carried out. With the addition of Joncryl® a decline in crystallinity and peak crystallization temperature in the DSC measurements was found; for the rheological measurements, an increase in zero shear viscosity was detected; and for the mechanical properties, an increase in elongation at break and a decline in the tangent modulus was observed. Each additional processing step had a vast negative effect on the zero shear viscosity, elongation at break, the notched impact strength, and a positive effect on the tangent modulus. The impact of the processing is more dominant compared to the addition of Joncryl®, and therefore, the usage of Joncryl® is limited in the recycling process of PHB to reverse the thermo-mechanical degradation due to processing. While Joncryl® is a proven chain extender for polyethyleneterephthalate (PET) and polylactic acid (PLA), the significantly lower processing temperature of PHB seemed to limit the full potential of Joncryl® due to the slow reaction speed at this temperature.