The trend to use recyclates as a substitute or supplement for virgin material has become increasingly important in recent years. So far, there are little to no regulations for the quality of a recyclate and no parameters that are required to be fulfilled. The quality of a recyclate depends on the composition of waste and sorting accuracy, which corresponds to the purity of the recyclate. For manufacturers, consistent recyclate quality is of high importance to ensure consistent production processes and required properties of the final products over time.
In Europe, most of the recycled material is processed into new products in the construction industry (PlasticsEurope 2020). Recyclates and blends of recyclates with virgin material are already applied for various parts in the plastic pipe industry (Kunststoffrohrverband e.V. - Fachverband der Kunststoffrohr-Industrie 2022). Currently available recycled polypropylene (PP) types do not fulfill the requirements for long service lifetime due to their lower quality. However, a small amount of recycled material added to virgin PP grades can already lead to a reduction in service lifetime (Utracki and Wilkie 2014).

For this purpose, a broad variation of properties of a PP post-consumer recyclate (PCR) over time were investigated. Ten batches of a PCR, available on the European market, were ordered from one supplier over a 15-month period. Thermal, mechanical, rheological and thermomechanical properties were characterized by means of various standard test methods. Since recyclates are used in long-term applications, the lifetime under cyclic load was characterized by means of a cyclic Cracked Round Bar (CRB) test according to ISO 18489. Additionally, fracture surfaces were examined by light microscopy and Scanning Electron Microscopy (SEM) to evaluate variations in failure mechanisms. Another focus was on the application of ONR CEN/TS 14541-2, a standard for recommendations on the properties of thermoplastic recyclates for pipes and fittings. It was investigated whether already existing standards provide valid guidelines for the processing of recycled materials in the pipe sector.
High variations were observed in various properties of PP recyclates, especially for results of thermogravimetric analysis (TGA), cyclic cracked round bar (CRB) test and melt flow rate (MFR). The obtained results show that MFR of the ten batches varies between 6 g/10 min and 12 g/10 min and the amount of inorganic ingredients varies strongly. Six batches have a comparable amount of inorganic components of about 10 %, while one batch nearly has three times the proportion inorganic ingredients. For batches with a high amount of inorganic ingredients notch sensitivity increases. In the conducted CRB tests, all batches showed lower crack resistance under cyclic loading compared to a conventional virgin extrusion grade. Especially, the scattering of results is significantly higher. Differences in MFR and amount of inorganic ingredients are also visible in results of the CRB test. Batches with low MFR lead to a better performance, while a high amount of inorganic ingredients reduces resistance against cyclic load. In addition, it was investigated whether batch variations in the CRB test can be compensated by blending batches with virgin material. It was found, that an addition of 25 % recyclate drastically reduces the resistance against cyclic load of a specific virgin material. However, the results show that mixing virgin material with 25% recycled material cannot compensate for the batch variations of the recycled material. The addition of 25% recyclate to a given virgin material drastically reduced the lifetime under cyclic load.
The CRB test has proven to be a suitable method to rank recyclates in terms of lifetime relevant properties but regulations regarding standard ONR CEN/TS 14541-2 should be adapted. For other common standard test methods, the allowable property intervals should be reduced to guarantee good quality and sufficient material properties during processing and subsequent product application.