Effect of Epoxy Chain Extender and Multiple Processing on Poly-(R)-3-Hydroxybutyrate’s Properties

The semi-crystalline poly-(R)-3-hydroxybutyrate (PHB) is a biobased and biodegradable polymer. This makes it a promising alternative to polypropylene (PP), especially for packaging applications. PHB has excellent barrier properties to O2, CO2, and H2O, but is susceptible to degradation from heat and hydrolysis. The epoxy chain extender Joncryl® was added to PHB in a simulated recycling process to reverse the degradation due to processing. The effects of the chain extender and the degradation due to processing were investigated with thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), small strain oscillatory plate-plate rheometry, tensile tests, and notched Charpy impact tests. With the addition of the chain extender, a decrease in peak crystallization temperature and tensile modulus, and an increase in zero-shear viscosity and elongation at break were observed. For each additional processing step the zero-shear viscosity, the elongation at break, and the notched impact strength decreased, while the tensile modulus increased. The effect of the thermal load during processing on the material properties is significantly higher compared to the effect of the addition of the chain extender. Therefore, the practical application of the investigated chain extender alone in a multi-stage recycling process seems limited. This is due to the low processing temperature of PHB, which seems to limit the full potential of Joncryl® due to the slow reaction speed at this temperature.