The application of recycled polypropylene in active closures

Abstract

Polymers have a long history of development and their application widely supports the normal operation of industry and society. Yet, the environmental consequences of using virgin plastics demand new sustainable solutions. In this quest, being capable of predicting the mechanical behavior of post-consumer recycled plastics is essential to support the adoption of these materials in engineering applications. This literature review focuses on recycled polypropylene and its mechanical viability when used in closures, a common part in packaging industry. Recycled polypropylene’s performance is found to be dependent on crystallinity, which in turn is dependent on the length of the polymer chains, isotacticity and other co-polymerization segments. This document also reviews constitutive models applicable to predict the behavior of closures via the the finite element method. A suitable thermo-viscoplastic model is selected, setting the stage for the finite element analyses to be conducted during the dissertation work. The application of recycled polypropylene is challenging due to its low mechanical properties. In the thesis, we demonstrate the possible aspects which decide the properties of post consumer recycled polypropylene (PCR-PP), including molecular weight, tacticity, interface structure and degradation. After discussing the material properties, the focus is to testify the application of PCR-PP in closures for particular functionality. For active closures, the weakest structure is the hinge, at the middle of lid. To verify if the material is applicable to active closures. Finite element analysis are conducted to help develop new products in a more efficient way, and also reduce the plastic waste. In the thesis, the explicit dynamic method and the static general method are applied with different boundary conditions in the model to simulate the differentiation of closures including manufacturing conditions. The explicit dynamic simulation involves higher computational cost, and due to the existence of inertial effect, the observed reaction force is significantly higher than it should be when the mass scaling is over a certain region. Considering an implicit static analysis was found to be proper. Once the loading condition was simplified and by carefully choosing an propriate thermo-visco-plastic constitutive model, the finite element analysis can predict the snap through behavior of the closure. The snapthrough behavior also shows that the equivalent plastic strain at the hinge middle is higher than the maximum tensile strain of polypropylene(QCP-300P) under room temperature. However, when subjected to 69 degree Celsius, the closure is expected to deform without failing. This result demonstrate the necessity of bending the closures right after injection molding, which fit with the experimental result.