Giordano, Angelo (2023) Innovative materials based on isotactic polypropylene: solving the conflit between strenght and toughness in heterophasic copolymers of polypropylene. [Tesi di dottorato]

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Abstract

In this thesis work, an extensive study of the physical properties, composition, and microstructure of high-impact heterophasic copolymers (HECOs) of isotactic polypropylene (iPP) was carried out to investigate the mechanisms responsible for the high toughness and low mechanical strength in iPP-based materials. HECOs are produced by a synthetic process, using heterogeneous Ziegler-Natta catalysts, which have the advantage of finely dispersing a rubbery ethylene-propylene copolymer (EPR) in a crystalline thermoplastic iPP matrix. This process produces materials with an extreme molecular complexity in which, in addition to the thermoplastic iPP matrix, there may be completely amorphous or low-crystallinity copolymer chains. The different microstructure and composition of macromolecules constituting the fractions of HECOs have a strong influence on their morphology and final properties. An in-depth study of the heterogeneity of these systems could allow identifying the molecular rules of each component and their mixture in the toughening and strengthening effect to obtain materials with tailored levels of damage tolerance for specific applications. Firstly, to explore the effect of the rubbery phase on the iPP matrix, tensile tests have been performed revealing that the introduction of the rubbery phase leads to significant changes in the mechanical behavior of iPP. Overall, the mechanical parameters depend on the intrinsic composition of HECOs such as the EPR content and its ethylene concentration. However, these two variables are not the unique factors that influence the properties of HECOs. In fact, it was found that the different nature of the elastomeric phase affects the morphology and degree of dispersion of EPR particles within the iPP matrix. The higher the ethylene concentration of the elastomeric phase the larger the size and distance of the elastomeric domains, resulting in less efficient dispersion and in a lack of mechanical properties. Therefore, the mechanical behavior exhibited by HECO materials is actually to be attributed to the morphology-composition combination. The compositional heterogeneity of HECOs was studied after a detailed fractionation process which allowed obtaining a series of fractions characterized by different compositions, microstructures, crystallinities, and molecular weights. The analysis revealed that HECOs are characterized by high crystalline iPP-based fractions and amorphous or poorly crystalline PE-based fractions with crystallinity and melting temperatures depending on the length of propylene and/or ethylene sequences. The study of the mechanical properties showed that the highly crystalline fractions of the polypropylene matrix have a behavior typical of brittle and very stiff materials while fractions exhibiting weak crystallinity as polyethylene are very ductile and exhibit strain-hardening phenomenon at high strains. Therefore, the improvement of the mechanical properties of the iPP matrix depends on the dispersion of the EPR phase but also on the amount of ductile fractions that compose the C3C2 copolymer of the EPR phase. Since the elastomeric EPR phase is incompatible with the iPP matrix due to the presence of macromolecules rich in ethylene sequences that are long enough to crystallize and since the final properties of HECOs strongly depend on the degree of dispersion of the phase-separated particles of the EPR phase, the possible improvement of the dispersion of the EPR phase has been studied by using different types of compatibilizers. The study carried out during this thesis work evidenced that the addition of iPP-b-PE block copolymers and random propylene-ethylene copolymers provides excellent benefits to the final properties of HECO systems. This result could be the turning key for further improving the toughness of HECO materials.

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