Separation and Reuse of Multilayer Food Packaging in Cellulose Reinforced Polyethylene Composites

Abstract

The recycling of multilayer packaging is hindered by the problematic separation of the packaging components. Among the numerous multilayer packaging, the ubiquitous 5-layer polyethylene/cardboard/polyethylene/aluminum/polyethylene one is one of the most difficult to recycle. This work examines how the synthetic polymer, cellulose and aluminum components can first be efficiently separated and secondly, how the recovered cellulose and polyethylene can be mixed to produce value-added composites. Size reduction and hydro-mechanical treatment of the packaging enabled the separation of cellulose from the PE and dual PE/aluminum layer. The delamination of the PE from the aluminum required a chemical treatment in an organic acid followed by separation by flotation/sedimentation. Instead of reusing the cellulose directly into a composite, the fibers were fractionated in order to produce fermentable sugars for a bio-refining operation and to recover value-added side-products known as microcrystalline cellulose (MCC). Thus, through the reaction with concentrated sulfuric acid, the recovered cellulose was decrystallized and partially hydrolyzed. It was then precipitated in a non-protic polar anti-solvent, and hydrolyzed in dilute sulfuric acid solution to generate MCC. This MCC was incorporated at levels up to 50 wt% into the recovered polyethylene using a twin-screw extrusion mixing process and then injection-molded. The elastic modulus and tensile strength increased linearly with the MCC content. A polyethylene grafted with maleic anhydride was added as a coupling agent between MCC and polyethylene during the compounding stage to further improve the composite properties. The composites were more homogeneous and the modulus and tensile strength were increased by around 20 % in presence of the coupling agent. Micrographs on rupture surfaces corroborated the improvement of cellulose wettability and interfacial adhesion in presence of the coupling agent.