PSI - Issue 42

Jessica Hinczica et al. / Procedia Structural Integrity 42 (2022) 139–146 Hinczica et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction and motivation Recycling of plastics has already gained increasing attention in many sectors, including science, industry and commerce, due to the economic and technological importance of the materials, as well as the environmental and social challenges associated with the circular economy of plastic products. Therefore, recycling is the most favourable option for end-of-life waste management treatment according to the European Waste Framework Directive (2008/98/EC), especially as it fosters the implementation of a circular economy (Rajendran et al. 2013; Reichel et al. 2016; Stahel 2016). In Europe, next to polyethylene (PE), the most used material with a market share of 20 % is polypropylene (PP) (PlasticsEurope 2020). Therefore, PP is of important concern for recycling activities, which can contribute to improving the environmental footprint and optimising the manufacturing process (Mannheim und Simenfalvi 2020). With the largest contribution of 40 %, polymers are mostly applied in the packaging sector, followed by building & construction 20 % and automotive 10 %. In the latter two sectors, due to relatively long design lifetimes of 50 to 100 years, and high safety requirements in human transportation, respectively, not only short-term material properties are important. Especially the resistance against slow crack growth (SCG) is a crucial parameter for a reliable lifetime prediction as well as a prevention against premature structural failures (PlasticsEurope 2018). The success of a recycling process depends on many factors among the value chain: from waste collection to sorting, processing, cleaning, and conditioning before the recycled material can be used in the manufacturing of the same or another new product, open-loop recycling (Aumnate et al. 2019). Currently, only 13 % of the collected post-consumer waste is incorporated into new products (PlasticsEurope 2020). From a re-processing point of view, especially the rheological and mechanical properties of the material are decisive for determining the usability of the material in the manufacturing process and in terms of suitability for the field of application (Aumnate et al. 2019). With 46 %, most of the recyclates are used in new products with their application in the building & construction sector, which requires long structural lifetimes. The resistance against SCG is a critical property in this context as it describes the resilience of the material against the initiation and the growth of quasi-brittle cracks at application relevant loading conditions. The German Federal Association for Secondary Raw Materials and Waste Management (BVSE) emphasises that recycling and research should not focus only on ultimate solutions with 100 % content of recycled material but rather determine useful and practicable blends of recycled and virgin polymers that are more suitable for a wider range of industrial applications (Probst 2020). During the lifetime and reprocessing, recycled material undergoes degradation, which is manifested by a decrease in molecular length (Aurrekoetxea et al. 2001; Spicker et al. 2019; Guerrica-Echevarría et al. 1996). Also, previous works have shown that the addition of recyclates to virgin materials has only little influence on the elastic modulus and tensile strength (Gustavo Barbosa et al. 2017; Aurrekoetxea et al. 2001). In addition, organic and inorganic contaminants as well as additives added by suppliers are assumed to influence the various properties of recyclates (Jmal et al. 2018). With the current commercial sorting methods, it is not possible to achieve 100 % pure PP recyclates. For polyolefins, organic contaminants of PE in PP recyclates are crucial for SCG performance, as PE and PP are not miscible (Messiha et al. 2020). There are many other influences which affect the rheological and mechanical properties of recyclates and virgin/recyclate blends. In this paper, only the rheological influence by measuring the melt flow rate (MFR) and zero viscosity as well as the influence of inorganic impurities on the mechanical properties and especially on the SCG behaviour will be discussed. 2. Experimental 2.1. Materials In order to investigate which parameters react sensitively to the influence of PP-r on PP-v, a virgin copolymer PP-v1 with a MFR of 0.3 g/10min (230 °C/2.16 kg), which is used for extruded non-pressure pipes and two recyclates, PP-r1 for injection moulding with a MFR of 11 g/10min (230 °C/2.16 kg) and an extrusion grade PP-r2 with a MFR of 3.5 g/10min (230 °C/2.16 kg) were blended and analysed. For this purpose, two material series with different blend ratios were produced by compounding, according to Table 1.