Multimodal polypropylenes: The close interplay between catalysts, processes and polymer design

Abstract

Multimodality offers the possibility to tailor the continuous phase (matrix) and/or the dispersed phase of monophasic and/or heterophasic polypropylenes (PP) to get unique property profiles. This review proposes a holistic view of multimodalities in PP combining an overview of multistage processes—as prerequisite to manufacture such materials, a deep dive in the catalyst requirements—as complementary design and key-process elements, and a discussion of polymer architectures—as enabler to produce differentiated grades. Hence the main processes allowing making multimodal PPs in at least two (consecutive) independent reactor zones are discussed. Their specificities are highlighted; ways to overcome their limitations presented. Benefits of bimodal or even trimodal homopolymers to broaden the standard monomodal product envelop in terms of stiffness, melt strength and processability are shown. The enlargement of the softening range for multimodal random copolymers while maintaining good thermal stability or enrich the (high molecular) phase with comonomer to retard critical failure under for example fatigue condition are demonstrated. The advantages of bimodal rubbers to optimise both primary (impact, stiffness, softness) and secondary properties (e.g. surface aesthetics, organoleptics, transparency) are emphasised. Original concepts—and their benefits—like combining a homopolymer fraction and a random fraction in the continuous phase are presented. Heterophasic PP/EPR (ethylene–propylene rubber) with a trimodal matrix or a trimodal discrete phase is discussed. A specific focus is put on the complex set of catalyst requirements needed to make PP in a multistage process—whether or not it contains a high amount of dispersed (rubbery) phase. Being spherical, exhibiting a very high activity and (stereo)selectivity over a wide polymerisation time, having the targeted porosity, having a broad hydrogen response, allowing a qualitative and quantitative incorporation of comonomers are a few of the key attributes of a desired catalyst system.