Extensional Rheology and Extrusion Blow Molding of Idealized High‐Density Polyethylene/Polypropylene Blends as Model Post‐Consumer Resins

ABSTRACT The increasing use of post‐consumer recycled (PCR) polyolefins in extrusion blow molding is limited by polypropylene (PP) contamination in high‐density polyethylene (HDPE), which can adversely affect processing and product performance. In this study, simulated PCR blends of 90/10 HDPE/PP with and without an olefin block copolymer (OBC) compatibilizer were compounded and extrusion blow molded into bottles using a stereolithography‐fabricated mold. Oscillatory shear and extensional rheology were used to characterize material deformation. Although shear behavior was similar, PP exhibited the lowest extensional viscosity and greatest parison sag, while HDPE and both blends exhibited strain hardening. The OBC increased T‐peel strength approximately 18‐fold and tensile strain at break approximately 5‐fold and 7‐fold in the transverse and machine directions, respectively. Compatibilized bottles maintained structural integrity during top‐load compression, whereas neat HDPE, PP, and the uncompatibilized blend exhibited premature neck cracking. Wall‐thickness measurements showed thinner PP bottle bodies and bottoms. Differential scanning calorimetry indicated processing‐induced changes in crystalline morphology. Water vapor permeability was lowest for the uncompatibilized blend, while the compatibilized blend showed permeability comparable to neat HDPE. Overall, OBC compatibilization improves the performance and durability of HDPE/PP blends for extrusion blow molding applications.