Polypropylene Post-Consumer Recyclate Compounds for Thermoforming Packaging Applications

Abstract

Polypropylene (PP) plastic packaging waste consists of a variety of different plastic packaging products with a great span in rheological and mechanical behavior. Therefore, the resulting post-consumer recyclates usually show melt mass-flow rates (MFR) in the region of injection molding grades and intermediate mechanical properties. High-quality packaging applications demand a distinct property profile that is met by tailor-made PP grades and cannot be met by recyclates with intermediate performance. One such application with high market volume is high-stiffness thermoforming trays. The aim of this research was to blend intermediate-performance recyclates with a virgin PP grade to obtain compounds that fulfill the rheological and mechanical demands of this application. Three commercially available PP post-consumer recyclates were acquired and compounded with different blending ratios with a high stiffness, low MFR virgin PP grade. As the pure recyclates show different rheological properties, the blending ratios had to be adapted for each of them to fit into the MFR range of 2–4 g/10 min which is desirable for thermoforming applications. The resulting PP recyclate compounds show a distinct correlation of recyclate content with rheological and mechanical performance. However, the resulting property profile was directly dependent on the performance of the originally used recyclate. The best-performing recyclate could be used in a blending ratio of 65 m% recyclate content while adhering to both property limits, the MFR of 2–4 g/10 min and the lower bound tensile stiffness of 1500 MPa.

Strategies for improving product properties of thermoformed multilayer cups with contents of post‐consumer polypropylene recyclates

“…Only a few virgin PPs with a higher stiffness are commercially available for packaging applications 36 . The tensile moduli of the four recyclates are significantly lower than those of the virgin PPs ranging from 1040 to 1240 MPa, which is in good agreement with further literature focusing on rPP 10,27 . The yield stress is also significantly lower.…”

“…All recyclates achieve higher values, which can be correlated with a certain toughness of the material due to PE and PP copolymer incorporation. The values of the recyclates range from 4.3 to 6.5 kJ/m 2 , which are common values for commercial PP recyclates 8,27 . The unnotched samples revealed a reverse trend.…”

“…However, by mixing PP recyclates with virgin PP, the required property window was met for certain mixing ratios. 26,27 The objective of this work is to evaluate the applicability of certain strategies to improve product properties of thermoformed cups, which fulfill the design from recycling approach and are thus partly made with recyclate. Four commercially available post-consumer PP recyclates were characterized at material level regarding their thermal and mechanical properties and were compared to standard virgin PP for packaging applications to determine the applicability for thermoforming applications.…”

“…The material properties in short‐term tests showed properties as to be expected in virgin material, but these materials showed clear deficits in the long‐term tests such as environmental stress cracking resistance (ESCR). However, by mixing PP recyclates with virgin PP, the required property window was met for certain mixing ratios 26,27 …”

Simultaneous Modification of Properties Relevant to the Processing and Application of Virgin and Post-Consumer Polypropylene

“…Two and three types with similar MFR in the range of pipe (<1 g/10 min) and extrusion grades (8 g/10 min), respectively, were mixed with up to four pre-selected homopolymer grades (8–25 g/10 min) in 20% increments between 0% and 100% via compounding. Other property modifiers such as fillers and compatibilizers were neglected in this study as these components would not be compliant with packaging applications in a circular economy [ 33 ]. Since the MFR is an often-used parameter for materials and it is relevant for the processing method, MFR values were determined for each blend and the applicability of mixing rules (i.e., linear, Arrhenius, and Cragoe) was tested.…”