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Conventional multi-material multi-layer flexible packaging offers excellent properties. However, it has recycling challenges, necessitating a shift to mono-material multi-layer flexible packaging for example all-polyethylene (PE) packaging which can be tailored through various synthesis and processing methods for different layers. In this work, we study how the key molecular properties (number-average molecular weight (M n), weight-average molecular weight (M w ), molecular weight distribution (MWD), comonomer content (short chain branching)) and machine direction orientation (MDO) process draw ratio (MDX) influence the final morphology and mechanical properties of MDO-PE films which are intended as the outer layer of mono-material all-polyethylene multi-layer flexible packaging. Five PE grades and various blends were extruded and blown into films. Selected blown films were machine direction oriented to obtain the final MDO-PE films. Furthermore, one selected PE blown film was processed at different MDO process draw ratios while keeping other process parameters constant. From the molecular properties point of view, the higher molecular weight fractions provide a higher possibility for uniform stretching whereas lower molecular weight fractions provide a higher natural draw ratio and therefore higher modulus and stiffness enhancement. Further, the results show that increasing MDO process draw ratio leads to more fibrillation and increased crystallinity. Consequently, the tensile modulus and stiffness at the higher draw ratios increase as well and are comparable to conventionally used polymers in outer layers of multilayer flexible packaging. Thus, this work demonstrates that MDO-PE films with enhanced modulus can provide sufficient stiffness for the design of outer layer of mono-material multi-layer all-PE packaging which presents higher potential for mechanical recyclability. [ABSTRACT FROM AUTHOR]

This research explores the potential of PE-based mono-material flexible packaging as a sustainable alternative to traditional designs, emphasizing its efficient mechanical recyclability. Typically, non-PE materials are used in the outer layers of multilayer flexible packaging to ensure adequate stiffness and barrier properties. The stiffness of PE films can be significantly improved through the machine direction orientation (MDO) process. Our study investigates the influence of key polyethylene (PE) resin parameters, specifically, resin density and short-chain branching (SCB) distribution, with indications of molecular weight on lab-scale MDO film stretching and its subsequent effects on mechanical properties. We processed 5 distinct PE resins and blends in a lab-scale setup to produce compression molded base sheets and further MDO-PE films, characterizing them using shear rheology, GPC, DSC, and iCCD analyses. Tensile testing provided insights into the mechanical characteristics, while X-ray scattering (SAXS and WAXS) and AFM studies analysed structural evolution and morphology. Uniaxial stretching notably enhanced the tensile modulus of MDO-PE films along the machine direction, particularly in higher density blends, comparable to conventionally used polymers. Challenges related to extremely high-density base sheets led to localized stretching and breakage. Certain resin compositions exhibited unique molecular architecture, facilitating enhanced tensile modulus and axial stiffness. Our study offers insights into the microstructural changes and surface morphology of MDO-PE films, underscoring the potential use of stiffness-enhanced MDO-PE films as outer layers in PE-based flexible packaging designs. • MDO stretching boosts tensile modulus along machine direction in PE films. • High-density resin base sheets showed increased microstructure fibrillation because of higher resistance to tensile load. • Tensile modulus improved up to 3.5 times, especially on stretching high-density resins base sheets. • Low-density resin's distinct molecular architecture enhances modulus and stiffness while facilitating stretching ease. [ABSTRACT FROM AUTHOR]