Heterophasic copolymers of isotactic polypropylene: development of methods for improving toughness without losing mechanical strength

2 months ago

Angelo Giordano, Claudio De Rosa

Heterophasic copolymers of isotactic polypropylene: development of methods for improving toughness without losing mechanical strength

A study of the structure and physical properties of heterophasic copolymers (HECOs) of isotactic polypropylene (iPP) is reported. Nowadays iPP is extensively used in a wide range of sectors, including electronics, automotive, fibers and filaments, and food packaging [1,2]. Notwithstanding all the major aspects that make iPP one of the most widely used and synthesized polymers, the biggest drawback of this material relies on its brittleness and low toughness, especially at low temperatures [2]. One of the employed strategies to increase the toughness consists of incorporating a rubbery phase, generally an ethylene-propylene random copolymer (EPR), directly in the polymerization reactor [3-6] by sequentially synthesizing the homopolymer and the EPR copolymer in multiple reactors process [5], resulting in the production of heterophasic copolymers (HECOs) of iPP [3-6]. These novel materials have made it possible to further extend the range of applications of iPP [7]. The incorporation of an elastomeric phase in the thermoplastic iPP matrix results in a marked drop in stiffness and mechanical strength with an increase in impact resistance (toughness). The main objective of this work is to identify and tune the ideal composition of HECO materials that can reconcile the apparent conflict between toughness and mechanical strength. HECOs are known for their significant molecular complexity because of the synthetic process and the catalyst involved in their production. Indeed, since HECOs are obtained by using heterogeneous multi-site Ziegler-Natta catalysts, they are characterized by a complex heterogeneous composition,[8] especially regarding the embedded rubbery phase, that can affect the final properties. Moreover, the properties of HECOs also depend on the low compatibility between the crystalline iPP thermoplastic matrix and the rubbery EPR phase. In this work, strategies for improving the compatibility between the different components of HECOs have been studied by adding compatibilizers of different molecular structures and architectures. Block copolymers containing crystalline blocks of isotactic polypropylene linked either to crystalline polyethylene or LLDPE blocks or to amorphous blocks of propylene-ethylene copolymers have been used as additives. Random propylene-ethylene (C3C2) copolymers with low ethylene concentrations have also been blended with HECO samples. Remarkable results, in terms of enhanced ductility and toughness, have been obtained with the use of monodisperse iPP-b-PE and iPP-b-LLDPE block copolymers and with random C3C2 copolymers with low ethylene concentrations. These studies have revealed that the addition of iPP-b-PE block copolymers, characterized by a long PE block, and random C3C2 copolymers of low ethylene concentrations, gives a remarkable improvement in ductility and toughness of HECOs. The beneficial effect was generally found in HECO systems with high ethylene concentrations of the rubbery phase. These findings demonstrate the potential for adjusting the mechanical properties of these commonly used materials through the targeted additives incorporation.

References

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  8. W. Liu, J. Zhang, M. Hong, P. Li, Y. Xue, Q. Chen, X. Ji Polymer 2020, 188, 122146.
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