Improved Polypropylene Thermoformability through Polyethylene Layering

Alex Jordan, Laryssa Meyer, Kyungtae Kim, Bongjoon Lee, Frank S. Bates, Chris Macosko

Research output: Contribution to journalArticlepeer-review

1 Scopus citations


Due to its low cost, stiffness, and recyclability, isotactic polypropylene (iPP) is an excellent candidate for packaging applications. However, iPP is notoriously difficult to thermoform due to its low melt strength. The addition of just 10 thin layers of high-molecular-weight, linear low-density polyethylene (LLDPE) into iPP sheets by coextrusion significantly increased extensional viscosity and reduced sag. Both LLDPE and iPP were metallocene-catalyzed with excellent adhesion as measured in our previous work. We performed a series of hot tensile tests and sheet sag measurements to determine the properties of the iPP sheet and the multilayer sheet between 130 and 180 °C. To evaluate the thermoformability of these multilayer sheets, truncated conical cups were positive vacuum formed at different temperatures and heating times, and the crush strength was measured. Cups that released easily from the mold with good shape retention and a crush strength within 80% of the maximum value were used to define a temperature−time thermoformability window. We estimated the maximum stress that occurred during the thermoforming process to be 5 MPa. Layer thicknesses before and after thermoforming were used to estimate an average strain of 0.78. The thin LLDPE layers decreased the yield stress below 5 MPa. This enabled thermoforming at sheet temperatures as low as 150 °C. The immiscible LLDPE interfaces increased extensional viscosity, which decreased sag in the multilayer sheets compared to iPP. This broadened the thermoforming range to temperatures as high as 180 °C and allowed longer heating times. These highly thermoformable, layered sheets may be recycled as iPP since they contain only 8% of LLDPE.

Original languageEnglish (US)
Pages (from-to)34134-34142
Number of pages9
JournalACS Applied Materials and Interfaces
Issue number29
StatePublished - 2022

Bibliographical note

Funding Information:
The authors thank Dr. Hanseung Lee for his assistance with SEM imaging. They also thank Dr. Olivier Lhost and Dr. Yves Trolez of TotalEnergies for their helpful discussions on the thermoforming process. This research was supported by a grant from TotalEnergies with partial support by the Industrial Partnership for Research in Interfacial & Materials Engineering (IPRIME). Polymers were graciously provided by TotalEnergies and ExxonMobil Corporation. Parts of this work were carried out in the Characterization Facility, University of Minnesota, a member of the NSF-funded Materials Research Facilities Network ( ) via the MRSEC program. The Hitachi SU8320 cryoSEM and cryospecimen preparation system were provided by NSF MRI DMR-1229263.

Publisher Copyright:
© 2022 American Chemical Society.


  • multilayer coextrusion
  • polymer mechanics
  • polyolefins
  • process analysis
  • thermoforming


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