Engineering structures, such as operating plastic pipes, are often submitted to unexpected influences that may shorten their lifetime. An increasing understanding about the processes that govern these sudden failures has been attained in the last decades. This has led to a remarkable improvement of pipe performances by enhancing the material’s slow crack growth (SCG) resistance (e.g. from PE63 to PE100RC) [1]. Still a great deal of uncertainty is associated with the use of non-virgin grades. This is mainly, because of the unknown effects of impurities that are found in recycled materials. The effects on lifetime relevant properties with regard to contaminants can be divided into three categories [2]: 

i.             polymeric contaminants of a different kind (e.g. PE in PP, etc.)

ii.            polymeric contaminants of the same kind (e.g. PE-LD in PE-HD, etc.)

iii.          non-polymeric contaminants (e.g. inorganic particles, etc)

In that context, effects of impurities were studied in this work by mixing virgin polypropylene (v-PP) grades with actual polypropylene recyclates (r-PP) into different compositions (v-PP/r-PP in %: 100/0, 90/10, 75/25, 50/50 and 0/100). Subsequently, these materials were tested via hydrostatic pressure tests on pipes. A profound dependency of contamination content on final failure time (t_f) could be demonstrated. Additionally, a deeper analysis of fractured pipe samples revealed a clear correlation between the maximum size of incorporated inorganic impurities and t_f. This indicates, that two seemingly identical pipe samples, with regard to content of recycled material, can still have vastly different resulting failure times, based on the size of the introduced critical contaminant (a_max) [3]. Results show, that it is not only necessary to understand the influence of the content and distribution of recyclates on the resulting lifetime of pipes, but more importantly the maximum introduced defect size as well. Consequentially, pipe manufacturers should choose recycled grades carefully, and only after knowing about the feedstock itself, treatment- and mechanical sorting history. 

References

[1]      H. Brömstrup, PE 100 Pipe Systems, Oldenbourg Industrieverlag, Essen, 2012.

[2]      M. Messiha, A. Frank, T. Koch, F. Arbeiter, G. Pinter, Effect of polyethylene and polypropylene cross-contamination on slow crack growth resistance, International Journal of Polymer Analysis and Characterization (2020) 1–18. https://doi.org/10.1080/1023666X.2020.1833143.

[3]      R. Danzer, On the relationship between ceramic strength and the requirements for mechanical design, Journal of the European Ceramic Society 34 (2014) 3435–3460.

https://doi.org/10.1016/j.jeurceramsoc.2014.04.026.

Acknowledgement

The research work of this paper was performed at the Polymer Competence Center Leoben

GmbH (PCCL, Austria) within the framework of the K2 COMET-program of the Federal Ministry for Climate Action, Environment, Energy, Mobility, Innovation and Technology and the Federal Ministry for Digital and Economic Affairs with contributions by DYKA (NL), Fraenkische Rohrwerke Gebr. Kirchner GmbH & Co. KG (GER), Pipelife International GmbH

(AUT), Polypipe Ltd. (UK), Rehau AG & Co. KG (GER), Staatliche Versuchsanstalt – TGM

Kunststoff- und Umwelttechnik (AUT), The European Plastic Pipes and Fittings Association (TEPPFA, BE), Vynova Group (BE), Wavin T&I (NL) as well as the Technical University Wien (AUT) and Montanuniversitaet Leoben (AUT). The PCCL is funded by the Austrian Government and the State Governments of Styria and Upper Austria.

Keywords

circularity of plastics; post-consumer & post-industrial recyclates; slow crack growth; hydrostatic pressure pipe tests