Dixit Guleria

Multi-material multi-layer flexible packaging is extensively employed 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, recycling this type of packaging presents significant challenges as they are composed of multiple layers of different plastics (e.g., polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polyamide (PA)) and non-polymeric materials (e.g., aluminium foil, metallized films), which are laminated together. The limitations in recycling mixed-material structures, results in most of these packages being disposed of through landfilling or incineration. This practice impedes recycling efforts and undermines the principles of circular economy.

Mono-material multi-layer flexible packaging systems are emerging as a promising alternative to improve recycling rates. While multiple variations are often needed to maintain functionality, using a single chemical composition allows for better recycling outcomes. 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, the outer layers of flexible packaging use non-PE polymers (e.g, PET, PA) for stiffness, and barrier properties. However, advancements in polyethylene film orientation techniques, specifically uniaxial orientation or machine direction orientation (MDO) process have demonstrated significant potential for enhancing the mechanical and barrier performance of PE films. This advancement offers the opportunity to replace non-PE materials in outer layers with machine direction oriented polyethylene (MDO-PE) films, thereby supporting the development of recyclable mono-material PE-based flexible packaging.

During the MDO process, the precursor polymer film is stretched in the machine direction at a certain rate, and at a certain orientation temperature which depends on the polymer’s chemical nature and is typically close to the melting point of the polymer. As a result, significant improvements are observed in the film's physical properties, including modulus, stiffness, tensile strength, as well as barrier properties.

This thesis investigates the influence of key PE resin characteristics (e.g., density, average molecular weight, branching, branching distribution, molecular weight distribution) and MDO process parameters (e.g., draw ratio) on the targeted mechanical, barrier and optical properties of MDO-PE films. A comprehensive understanding of structure-property relationships is developed through extensive microstructural characterizations and morphological analysis. Additionally, the mechanical recyclability of virgin PE films and the incorporation of post-consumer recyclate (PCR) material with virgin materials for subsequent MDO operation is investigated for high performance applications.

The design challenges associated with achieving efficient mechanical recycling of conventional flexible packaging are outlined in Chapter 1, with an examination of current strategies and technological adaptations aimed at addressing these challenges to support the circular economy for such systems. PE-based mono-material flexible packaging is highlighted as a promising solution to these challenges. Furthermore, the potential of the MDO or uniaxial orientation process in enhancing the performance of PE films for the PE-based mono-material flexible packaging is explored.

As mentioned earlier, conventional multilayer flexible packages typically use some non-PE material (e.g. PET or PA) to impart stiffness and gas barrier properties against gases like oxygen. PE can reach a certain modulus by increasing resin density, but this is typically not sufficient. Stiffness of a material is a function of its modulus and therefore, enhancement in modulus of PE film enhances stiffness. Sufficient polymer film stiffness is required for various packaging operations such as printing and lamination, and for the efficient running of packaging machine lines. Low-stiffness films might sag while running on packaging lines. Existing research recognizes the critical role played by MDO process parameters such as orientation temperature, draw ratio, stretch rate, and annealing time on final mechanical properties attained by MDO films. However, only a few studies have systematically investigated the role of resin parameters such as density, branching, branching distribution (BD), average molecular weight, and polydispersity on final mechanical properties attained by the MDO-PE films.

The role of principal resin parameters of PE, i.e., dominantly resin density and short-chain branching (SCB) distribution coupled with molecular weight, in lab-scale MDO film stretching at the fixed MDO process parameters of orientation temperature, stretch rate, annealing time, and process draw ratio was investigated in Chapter 2. Five distinct PE resins and blends were processed in a lab-scale setup to produce compression molded base sheets and further MDO-PE films. Uniaxial stretching or MDO operation notably enhanced the tensile modulus of MDO-PE films along the machine direction, particularly in higher density blends to as much as about 350%, comparable to conventionally used polymers. Challenges related in stretching extremely high-density base sheet led to breakage. Certain resin compositions exhibited unique molecular architecture, facilitating enhanced tensile modulus and axial stiffness. The study further addressed the structural evolutions and changes in surface morphology of the PE base sheets after going through MDO operation and how they relate to the final overall tensile properties of the MDO films. MDO-PE films consistently demonstrated fibrillated and oriented microstructure aligned parallel to the stretch direction.

In Chapter 2, compression molded base sheets which are usually mechanically isotropic in nature were used as a starting material to produce lab-scale MDO-PE films. The study in Chapter 2 presented critical insights of structural evolutions in MDO process through a lab-scale setup. However, at industrial scale, MDO process is performed on precursor cast or blown films, which are in general mechanically anisotropic. Blown film extrusion ranks among the foremost polymer processing methodologies, with an annual processing capacity of billions of kilograms of polymer, predominantly polyethylene. Integrating the MDO process sequentially with film blowing presents the potential to enhance the packaging materials characteristics and streamline the shift towards mono-material packaging, thereby positively impacting recyclability and polymer circularity. Therefore, for subsequent studies, lab-scale setup was scaled up to pilot-scale setup to produce MDO-PE films. In this setup, blown films produce by pilot-scale blown film extrusion line were used as a precursor starting materials to perform MDO operation on a pilot-scale MDO setup.

The influence of MDO on the blown film with pre-existing random orientation has not been yet explored extensively. After gaining insights on impact of resin density and SCB distribution in Chapter 2, focus was to investigate the impact of other key resin molecular properties and MDO process parameter of draw ratio. Hence, the influence of PE molecular properties such as number-average molecular weight (Mn), weight-average molecular weight (Mw), polydispersity and comonomer content, as well as the effects of the key MDO process parameter of draw ratio, on the structural evolution, morphology and mechanical properties of MDO-PE films intended for use as outer layers in mono-material flexible packaging design was investigated in Chapter 3. Five PE grades and their blends were processed into blown films, and selected films were subjected to pilot-scale MDO at varying draw ratios. The findings reveal that higher fraction of low-molecular-weight chains results in a higher natural draw ratio, leading to the higher modulus in the MDO films. Furthermore, extremely higher MDO draw ratios lead to increased fibrillation, microstructural orientation and enhanced crystallinity, which further significantly enhances the modulus of MDO films (up to 13 times of the starting blown film), comparable to those of conventionally used polymers in the outer layers of conventional multilayer flexible packaging.

One of the important aspects of the work performed in this thesis was to experimentally validate the mechanical recyclability of mono-materials fractions. Re-extrusion is a widely used method for recycling mono-material fractions. Re-extrusion involves shredding the material (blown films in this study) into small flakes after sorting and cleaning which is then used as an input material in an extruder to re-pelletize the material. The re-pelletized resins can be further used to make new materials (blown films and subsequent MDO films in this study). Blending recycled and virgin polyethylene stands as a recognized method for modifying the properties of recycled material. While numerous studies have investigated the recycling and degradation of polyethylene during processing, particularly those used in packaging applications, research on blown films containing recycled polyethylene is limited. Thus, the mechanical recyclability of virgin PE blown films and the incorporation of post-consumer recyclate (PCR) for in-line MDO-PE film applications, with a focus on targeted mechanical properties of modulus and stiffness for the outer layer of mono-material PE-based flexible packaging design was investigated in Chapter 4. Experiments reveal that MDO-PE films from reprocessed material (after 5 reprocessing cycles) retained 77% of their virgin MDO film tensile modulus, despite exhibiting onset of slight molecular degradation. Moreover, incorporating 5% PCR into PE blends yielded tensile properties comparable to those of virgin MDO films, demonstrating the feasibility of these blends for high-performance MDO-PE films in mono-material PE-based flexible packaging applications.

After investigating key mechanical properties, the focus was to investigate vital barrier properties (oxygen barrier, functional barrier) and optical properties of MDO-PE films, intended to be used as outer layers in mono-material PE based flexible packaging. A good barrier against oxygen is required to prevent the spoilage of food products inside the package, and for prolonging the shelf life of the food product. A major issue with PE is its high oxygen permeability. However, it has been demonstrated in the previous studies that oxygen as well as moisture barrier of PE films can be enhanced by MDO operation. The optical properties of polymers, including gloss, transparency, clarity, haze, color, surface appearance, and refractive index, are closely associated with the quality and visual performance of plastic products. MDO operation significantly impacts the optical properties of films due to evolutions in microstructure and surface morphology. Thus, the effects of MDO on the barrier and optical properties of PE films for applications in PE-based mono-material flexible packaging was investigated in Chapter 5. The MDO process in general enhanced oxygen and functional barrier properties by increasing crystallinity and microstructural orientation under specific parameters, with results varying based on resin density and processing conditions. Despite these improvements, the oxygen barrier performance of MDO-PE films remained inferior to conventional high oxygen barrier performance polymers like PET and PA, highlighting the need for additional barrier layers for advanced applications which must be further compatible with mechanical recycling. MDO also influenced optical properties, increasing haze due to changes in crystalline microstructure and surface morphology, while gloss initially decreased and subsequently increased at higher draw ratios.

The key learnings and conclusions from experimental work in Chapters 2,3,4 and 5 of this thesis are summarised in Chapter 6. Further, the future research directions to advance the design of PE-based mono-material flexible packaging are outlined. Possible key challenges in large-scale adaptation of MDO-PE films to sustain the circular economy of flexible packaging systems are explored. Lastly, concluding thoughts for the performed investigations in entire thesis are presented.