The alteration in lifetime relevant material characteristics, such as slow crack growth (SCG) resistance, of a virgin PE-HD pipe grade (PE80) with successively increasing amounts of a virgin PP block copolymer (PP-B) was studied in frame of this research. For this purpose, Cyclic Crack Round Bar (CRB) tests were performed, demonstrating the high negative impact of an already small amount of polymeric impurity, which leads to a significant reduction of the SCG resistance. Already 5% mixing content evokes a dramatic decrease of SCG resistances of up to 40 and 70 % for PE-HD and PP-B, respectively. As SCG is generally accepted to be a critical failure mechanism for plastics used in long-term applications, such as pipes, this effect must be carefully considered in all discussions about the use of nonvirgin polymers.

Thermoplastic materials have established a reputation for long-term reliability in low-pressure gas and water distribution pipe systems. However, occasional Slow Crack Growth (SCG) and Rapid Crack Propagation (RCP) failures still occur. SCG may initiate only a small leak, but it has the potential to trigger RCP, which is much rarer but more catastrophic and destructive. RCP can create a long, straight or meandering axial crack path at speeds of up to hundreds of meters per second. It is driven by internal (residual) and external (pressure) loads and resisted by molecular and morphological characteristics of the polymer. The safe installation and operation of a pipe throughout its service lifetime therefore requires knowledge of its resistance to RCP, particularly when using new materials. In this context, the RCP resistance of five different polyamide (PA) 12 grades was investigated using the ISO 13477 Small-Scale Steady State (S4) test. Since these grades differed not only in molecular weight but also in their use of additives (impact modifiers and pigments), structure-property relationships could be deduced from S4 test results. A new method is proposed for correlating these results more efficiently to evaluate each grade using the crack arrest lengths from individual S4 test specimens.

The influence of micro‐structural changes within polyamide 12 (PA12) grades on their resistance against crack initiation and subsequent Slow Crack Growth (SCG) was examined. Possible micro‐deformation mechanisms and processes guiding slow crack extensions in PA12 are discussed. Special focus was put on the formation of plastic zones as predecessors to microscopic cracks prior to crack initiation via crack freezing analysis. Simultaneously, variations in the pure SCG resistance after a recorded craze‐crack transition were determined via cyclic Cracked Round Bar (CRB) tests. In that context, a homologous series of PA12 grades of increasing molecular weight (M_W), as well as a systematically developed series of PA12 grades incorporating a pigment and/or an impact modifier, were selected for this study. Results show a good correlation between the crack initiation resistance and the disentanglement resistance of PA12 chains as well as the size of the plastic zone. In that context, an increasing average M_W, leads to increasing disentanglement resistances due to hydrogen bond effects. An impact modification promotes the development of notably larger plastic zones. Contrarily, colored grades exhibit a reduced plastic zone size, rendering a lower amount of dissipated energy before physical crack initiation. Pure SCG resistance follows a similar trend and is improved by high average M_W as well as the use of impact modifier. Crack growth acceleration, however, is assumed to occur alongside the relatively weak interfaces between polymer matrix and pigments.

Driven by global sustainability goals, the use of recycled polymers is increasing, but their inconsistent quality poses a challenge for reliable use in technical applications. In this study, ten batches of an industrially used post‐consumer polypropylene (PP) recyclate were examined over 15 months, all ordered with identical specifications for melt flow rate (MFR = 7–13 g/10 min) and density (0.97 ± 0.03 g/cm ³ ). Despite fulfilling these specifications, considerable differences were found in rheological, thermal and mechanical properties. Filler content varied between 2% and 24%, strongly affecting mechanical performance. While Young's modulus and yield stress varied slightly, elongation at break differed greatly (10%–100%). Particularly large differences were found in the crack resistance. At the same stress level, the number of cycles until failure ranged between 3000 and 6.52 million cycles. These deviations are attributed to variations in molecular structure, filler content and polymeric and non‐polymeric impurities. The findings highlight that, despite fulfilling the target specifications for MFR, considerable compositional differences can significantly affect the material performance. Standard parameters are therefore insufficient to assess the suitability of recycled PP for technical applications. Comprehensive characterization, narrower specification limits and improved sorting are essential for the reliable and sustainable use of recyclates in long‐term applications.

Zusammenfassung: Die Qualität von Polyethylen (PE-)-Rezyklaten ist entscheidend für deren Einsatz in neuen Anwendungen. Der Eintrag gealterter PE-Produkte in den Recyclingstrom kann diese Qualität jedoch mindern. Um den Bedarf an Neumaterial oder zusätzlichen Stabilisatoren zur Kompensation dieser Qualitätsminderung zu reduzieren, ist eine zuverlässige Identifikation und Aussortierung stark gealterter Materialien erforderlich, um die Qualität von PE-Rezyklaten nachhaltig zu verbessern. In dieser Studie wurde das Potenzial der Alterungs- bzw. Degradationserkennung von PE mittels Nahinfrarot (NIR)-Hyperspektraltechnik untersucht. Hierfür wurden PE-Proben künstlich durch UV-Bestrahlung gealtert. Die Degradation wurde durch mechanische Prüfungen erfasst, wobei die Bruchdehnung als Kriterium zur Einteilung in die gewählten Degradationsklassen diente. Die NIR-Hyperspektraldaten wurden an einem NIR-Labor-Setup aufgenommen. Die Trennbarkeit wurde anhand von Klassifikationsmodellen bewertet, wobei sowohl multivariate Datenanalyse (Partial Least Squares Discriminant Analysis) als auch ein Verfahren des maschinellen Lernens (Support Vector Machine) eingesetzt und anschließend miteinander verglichen wurden. Die Ergebnisse zeigen, dass unter Verwendung von maschinellem Lernen hochgradig und weniger gealterte PE-Proben mit hoher Genauigkeit getrennt werden können. Damit eröffnet sich die Möglichkeit, bereits etablierte NIR-Sortiersysteme nicht nur zur Identifizierung von Polymertypen, sondern auch gezielt zur Degradationserkennung einzusetzen, was ein wichtiger Schritt hin zu qualitativ hochwertigeren Rezyklaten sowie Ressourcenschonung ist.

To evaluate the influence of extrusion on material performance, three processing methods, single extrusion (1x Ex), double extrusion (2x Ex), and double screw compounding (C), were investigated for virgin polypropylene (vPP), recycled PP (rPP), and a blend containing 10% rPP (rPP10). Calculated thermal stresses and shear during processing revealed that 1x Ex imposed the lowest load, 2x Ex doubled thermal and shear stress, and C introduced more than 16.8 times higher shear. Rheological measurements revealed a slight viscosity increase in vPP after 1x Ex, due to branching, followed by a decrease under higher stresses. In contrast, rPP exhibits viscosity reduction independent of processing method. Melt flow rate increased with processing stresses for vPP and rPP10 samples, while the rPP samples remained unaffected. Standard thermal, FTIR, and mechanical tests detected no significant differences, but oxidation onset temperature indicated stabilizer consumption. Crack resistance was highly sensitive, while vPP remained unaffected, rPP already exhibited a tenfold decrease in crack resistance after 1x Ex, quantified by the number of cycles until failure. In rPP10, vPP compensated rPP degradation, but higher stresses reduced crack resistance. The results highlight that conventional tests may overlook early degradation, whereas rheological analysis can indicate material performance, but requires careful interpretation.