Guida, Adriano (2024) A microstructural insight in polyethylene based bioriented mono-materials: from fundamental to processing. [Tesi di dottorato]

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Abstract

The primary objective of this PhD project is to generate new knowledge on the structural transformations of polyethylene (PE) during biaxial stretching, focusing on the relationship between chain microstructure, crystallization behaviour, processing conditions and final properties of biaxially oriented PE (BOPE) films. This study aims to investigate how different PE grades, including HDPE, LLDPE and their blends, perform during the stretching process, aiming to optimize their use as mono-materials for next-generation packaging applications. The innovative aspects of the thesis lie in providing insights into the structural mechanisms at play during the biaxial orientation process, offering potential breakthroughs in expanding PE application as a sustainable alternative to multilayer films. The work is structured to build a systematic and interdisciplinary chain of knowledge, linking microstructural characteristics with processing conditions and the resulting properties of BOPE. To achieve this goal, three universities and three industrial partners have collaborated synergistically, under the coordination of the DPI. Indeed, the PhD project brings together the Soft Condensed Matter Lab from University of Naples Federico II (which this PhD candidate belongs to), the Zernike Institute for Advanced Materials from University of Groningen for structure and properties investigation, and the Department of Industrial Engineering from University of Salerno for rheology and modeling part. The ex-situ comprehensive characterization of the selected PE grades is coupled with in-situ structural analysis performed using synchrotrons within EU facilities (ESRF in Grenoble, ALBA in Barcelona) with a new device for lab scale biaxial stretching suitably developed by the engineering group from Salerno. The commercial PE grades investigated have been supplied by Borealis, DOW and SABIC, which have been helpful in providing an industrial perspective during the development of the thesis. Chapter 2 is dedicated to the description of all the experimental procedures and techniques employed in this work. The starting basic characterization of the samples, presented in Chapter 3, includes microstructural analysis to check molecular mass and distribution, branch type and branching concentration and corresponding distribution. Tensile tests have been carried out on compression molded films of the samples for a mechanical characterization of the PE grades. In addition, the characterization of thermal properties, crystallization behaviour and thermal fractionation are illustrated. In Chapter 4 phenomena induced by temperature examined through the application of a thermal protocol are discussed. They have been investigated using DSC, coupling the results with structural (WAXS, SAXS) and morphological (POM, SEM) analysis. In Chapter 5, are reported the results achieved by performing in situ experiments aimed at gaining a better understanding of the phenomena induced by temperature and the thermal protocol adopted for ex-situ analysis in conditions emulating the stages preceding tentering. Further in-situ analysis aimed at understanding the complex structuring process that occur during biaxial stretching in the semi-solid state of cast sheets of selected grades produced at Karo Brukner facility, using the biaxial stretching device, are illustrated in Chapter 6. In situ experiments are performed by collecting 2D-WAXS and 2D-SAXS data while heating/cooling/stretching the PE samples. The results will be laid out in detail and discussed in depth in each Chapter, allowing for a comprehensive interpretation of the findings. In Chapter 7, the main conclusion of this work will be illustrated.

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