Uniaxial Orientation of Polyethylene Films for Recyclable Flexible Packaging
Dixit Guleria has successfully defended his PhD thesis at the Department of Chemical Engineering and Chemistry on May 21st.
Multi-material multi-layer flexible packaging is extensively used in the food and pharmaceutical industries due to its outstanding mechanical and barrier properties, as well as user-friendly characteristics such as lightweight design and enhanced portability. However, the inclusion of multiple polymeric (mainly plastics) and non-polymeric materials with different chemical compositions in producing these packages significantly hinders their mechanical recyclability.
PE (polyethylene) based mono-material flexible packaging is emerging as a promising alternative to improve mechanical recycling rates. PE with its different types (low-density, linear low-density and high-density) is widely used in several applications due to its versatility in structural and functional properties. Combining these PE types in packaging can offer both functionality and recyclability. Traditionally, outer layers of flexible packaging use non-PE materials for stiffness and barrier properties, however advancements in PE film orientation techniques, specifically machine direction orientation (MDO) enhances its mechanical and barrier properties, supporting the development of recyclable mono-material PE based flexible packaging.
The work in this thesis focused on studying the impact of key PE resin parameters and MDO process parameters on targeted properties (e.g. stiffness, oxygen barrier) of MDO-PE films and providing comprehensive understanding of structure-property relationships through extensive microstructural characterizations and morphological analysis. Additionally, the mechanical re-processability of base PE films and the integration of post-consumer recyclate (PCR) into base films to produce MDO-PE films were investigated to experimentally validate the mechanical recyclability and performance.
The experimental research demonstrated that careful selection of PE resin parameters, such as resin density, molecular weight, short chain branching distribution, and polydispersity, along with optimization of MDO processing conditions, including draw ratio and orientation temperature, can significantly enhance the performance of MDO-PE films. These improvements make them suitable as outer layers in mono-material PE based sustainable flexible packaging. Enhanced crystallinity, high microstructural orientation, and improved mechanical properties such as increased stiffness and strength were observed, bringing the performance of MDO-PE films closer to that of traditionally used high-modulus materials in outer layers like PET (polyethylene terephthalate) and PA (polyamide). Low levels of PCR content were successfully incorporated without compromising the mechanical properties of the films, underscoring their potential for mechanical recyclability. Additionally, modest improvements in oxygen barrier and optical properties were observed, further highlighting the promise of MDO-PE films in advancing sustainable mono-material PE based flexible packaging.
This advancement in design has the potential for improved mechanical recyclability of flexible packaging, thereby supporting plastic circularity.