The Extensional Rheology of Biodegradable Polymers Determines Their Propensity to Be Film Blown

Non-degradable plastic pollution, particularly from single-use packaging, poses a growing threat to the environment. To address this, biodegradable alternatives have gained attention. However, processing these biodegradable polymers into thin films for packaging applications remains a challenge due to their poor film blowing behaviour, which is neither well understood nor documented. This study presents a novel conceptual framework to help understand the underlying reasons for this poor performance. Through standardised methods, the systematic assessment of the extensional rheology behaviour of various biodegradable polymers relative to low-density polyethylene (LDPE) was characterised over a range of temperatures and strain rates, with the degree of strain hardening being recorded for each. The extensional properties were then correlated with observations from pilot-scale film blowing experiments. The findings reveal that partial crosslinking (e.g. partially cross-linked polycaprolactone) facilitates significant strain hardening behaviour across a wide temperature range and allows for film blowing. Conversely, polymers that only display strain hardening at temperatures slightly above their melting point (e.g. poly(butylene adipate-co-terephthalate) and poly(butylene succinate)) are more challenging to film blow. Additionally, polymers lacking strain hardening behaviour (e.g. poly(3-hydroxybutyrate-co-3-hydroxyvalerate), poly(3-hydroxybutyrate-co-3-hydroxyhexanoate), poly(lactic acid), and linear polycaprolactone) present difficulties in film blowing due to excessive sagging and low melt strength, resulting in bubble instability and collapse. This research sheds light on the rheological properties that impact the processability of biodegradable polymers for practical applications, providing valuable insights for the development of sustainable plastic alternatives.