Characterization of post-use polyethylene and polypropylene recyclate blends for pipe applications


Papers # 2018 Las-Vegas

Despite recent efforts in promoting the utilization of recycled polymeric materials for more diverse and more advanced applications, there are still significant knowledge gaps with regards to the applicability of certain recyclate types for specific “re-use” purposes. When it comes to pipe applications, for instance, a considerable body of work exists on recycled polyvinylchloride (rPVC), however, systematic investigations of mechanically recycled polyethylene (rPE) and polypropylene (rPP) for structural performance applications such as pipes are still rare. Especially studies of the resistance against slow crack growth, which is a highly relevant failure mechanism of pressurized plastic pipes, are lacking for rPE and rPP. This is particularly true for blends of rPE and rPP, which are of high practical relevance not at least due to cost advantages over mono-fraction recyclates.

In the present work two rPE and two rPP grades originating from post-use sources such as bottle caps, extrusion blow molded objects and injection molded items were analyzed together with six PE-PP recyclate blends produced from these four base recyclates. The recyclate blends differed in the rPE-to-rPP ratio and the amount of calcium carbonate additive used in the recyclate compounding and blending step. 

Composition and molecular characteristics of both post-use base recyclates and recyclate blends were investigated using Fourier-transform infrared (FTIR) spectroscopy, differential scanning calorimetry (DSC) and thermo-gravimetric analysis (TGA). A subsequent mechanical characterization based on concepts of linear-elastic fracture mechanics was performed using compact-type (CT) specimens to determine the crack growth resistance of the various recyclate materials under cyclic loads. 

Small amounts of legacy substances were detected in both rPE and rPP. In terms of crack growth resistance, clear rankings of the materials were derived, with the recyclate blends being inferior to the neat rPE or rPP materials.

Related keywords : Polyethylene polypropylene blend, mechanical recycling, post-consumer, legacy substances, crack growth resistance.
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