Due to the worldwide increase in plastic products and applications and the associated rise in plastic waste, the global perspective on sustainability has intensified over the last decade. A prime example of this is the European Green Deal, which has made the circular economy one of its main priorities. However, the transition to a circular economy has significant implications for waste management and the reuse of materials. New mandatory recycling targets for plastics for example, ensuring that all packaging on the EU market is reusable or recyclable in an economically viable way by 2030 - will therefore lead to a high availability of recyclates in the future. Thus, useful applications must be found for these recyclates. One possibility is the blow molding of bottles and containers for the storage and/or transport of non-food contents, such as cosmetics, household products or chemicals. However, the influence of media in such applications often leads to premature failure due to environmental stress cracking (ESC).
For this reason, the focus of this work was placed on the resistance of different polyethylene (PE) types towards ESC. Eight different PE types, including four high-density polyethylene (HDPE), two low-density polyethylene (LDPE) and two PE recyclates were tested for ESC resistance using different test methods and conditions. A standard test widely used in the industry to determine the ESC of PE was selected for this purpose - the so-called Bell Test (BT, ASTM D1693). In addition, a less common ESC test, the so-called Buckled Plate Test (BPT) according to Chang und Donovan 1989 was used. Both tests were carried out at different temperatures (50 °C and 80 °C) and media influences (air vs. 2 % lauramine oxide solution). As a supplementary fracture mechanics test, a static compact tension (CT) test was carried out to determine the crack propagation speed.
The basic characterization (e.g. density, melt flow rate (MFR), mechanical parameters, etc.) showed that the two recyclates have inorganic impurities in the form of fillers or foreign particles. The flow properties confirmed the processing types of the individual materials specified by the manufacturer. The mechanical properties showed that both the flexural modulus and the flexural stress were strongly dependent on the temperature. However, the values of the HDPEs were generally higher than those of the LDPEs. The recyclates were also at the same level as the HDPEs.
When determining the ESC resistance, the BT and BPT showed that the HDPEs are the more resistant materials compared to the other materials. The recyclates exhibited the lowest ESC resistance in both tests. The ESC resistance deteriorated when the test temperature was increased, with the exception of the LDPEs, which softened to such an extent that a ductile failure pattern developed instead of stress cracking.
The compact tension test also highlights large differences between the materials tested and shows similar trends in crack growth behavior as the ESC tests. Recyclates exhibited the highest crack propagation rate, while HDPEs were the most resistant against crack propagation. No CT tests were carried out for the LDPEs, as they soften too much at 80 °C and measurements as part of linear elastic fracture mechanics are not possible.
The results generated in this master's thesis show that the less complex BT can provide a good overview of the ESC resistance. The disadvantages of this method are that the failure time of the individual test specimens varies greatly, which makes the assessment more difficult. The buckled plate test is considerably more complex in comparison, but produces similar results in terms of ESC resistance as the BT. Different crack kinetics were found in this test compared to the CT test. Furthermore, this work shows that the tested recyclates exhibited significantly poorer ESC resistance compared to virgin material. The proportion of recyclate should therefore be regulated for the desired products and applications so that premature failure, also due to the reduced properties of the given recyclate, can be ruled out.