Environmental stress cracking resistance (ESCR) of recycled polypropylene (rPP) from yogurt cups

The objective of this thesis was to evaluate the long-term performance of a recycled polypropylene (rPP) derived from a closed-loop recycling stream of yogurt cups. Using fatigue testing in air and oil environment, rPP was compared to a virgin reference material (PP) serving as the standard material for this application. The environmental stress cracking resistance (ESCR) of the tested materials was assessed.
The investigation commenced with the determination of the fundamental mechanical, thermo-analytical, and thermomechanical properties of PP and rPP, as well as of an enhanced recyclate rPP+. These properties were determined by tensile test, Charpy notched impact test, differential scanning calorimetry (DSC), and dynamic-mechanical analysis (DMA). To produce the cracked round bar (CRB) specimens for the ESCR tests, an extrusion-compression molding process was employed to produce the plates, followed by a lathing step to produce the notched specimens. These CRB specimens were then subjected to a series of cyclic tests in both air and oil environment to determine the failure cycles. The fracture surfaces were investigated using optical microscopy.
The basic property analysis revealed deficiency in rPP relative to PP, exhibiting a substantially diminished stiffness of -350 MPa. Consequently, the inferior performance of rPP in the cyclic tests was anticipated. At a stress range of 19 MPa the recyclate rPP only reached 8% and 16% of the cycles until failure of PP in air and oil, respectively. However, when the stress ranges are normalized by their respective tensile moduli, the discrepancy in performance in air becomes very small and in oil no difference can be detected. The overall difference between air and oil is minimal which is expected for semi-crystalline materials. To enhance the recyclates performance and to align the melt flow rates (MFR) with the virgin PP, rPP was blended with a highly stiff heterophasic PP copolymer. This improved recyclate-virgin blend rPP+ was also assessed in cyclic testing in air. A clear improvement of rPP+ compared to rPP was detected, although it still cannot compete with the virgin PP. The stress ranges of all three materials were once again normalized by the tensile moduli that led to overlapping results, which demonstrated the strong dependence of the stiffness on the durability of the materials in cyclic testing.
Due to the recycling initiatives a lot of new materials with unknown long-term properties are entering the market. Although performance differences are visible after basic material testing, the effect of recycling on long-term properties assessed by cyclic testing is even more pronounced.