Multimodal Polymers with Supported Catalysts

Over the last 60 years the ability to reduce olefinic refinery gases or liquids into a metastable solid in a controlled manner has created the colossal business of polyolefin materials. Their continued success is thanks to a deep understanding of how to meet and predict a customer’s needs in terms of a price/performance package and translate that back through the chain of knowledge (Fig. 1.1). This demand has led to constant evolutions within all areas of the business, punctuated by more than its fair share of revolutionary breakthroughs in the areas of catalyst, polymerization process, and polymer processing technology.

This chapter aims to discuss some important technical aspects of the commercial support related to each catalyst technology toward the production of different polyolefin grades. It approaches several industrial factors that are defined by the fine tuning between support and catalyst preparation, such as the catalyst fragmentation, the control of the final particle morphology and the morphological replication phenomena, the heat and mass transfer limitations during the polymerization reaction, and the uses of prepolymerization stage. The understanding of these parameters and the limits of the polymerization processes are the first step for designing a suitable support for a heterogeneous catalyst. Aspects such as chemical composition of the support, its surface characteristics, morphology (surface area, particle size, particle size distribution, and porosity), mechanical strength, and other characteristics of relevance for the two most common supports in the polyolefin industry—MgCl₂ and SiO₂—are discussed as well.

In processes that rely on the use of heterogeneous catalysis as the major means of production, it should be quite obvious that understanding how the catalyst particles evolve will play an important role in many aspects related to quality and reactor performance. At the risk of oversimplifying things, the principal roles of the heterogeneous catalyst particles used in olefin polymerisation can be seen as being (1) to carry the active sites upon which the polymer is formed; and (2) to provide a structure for creating “solid” particles that can be easily transported, recovered and processed. It is therefore important for us to understand how the process used to make the polymer impacts the particle and the active sites (and vice versa!). From the schema in Fig. 3.1, where these concepts are applied to a heterogeneously catalysed olefin polymerisation process, it can be seen that one needs to consider many different length scales, from the reactor which has volumes on the order of several tens of cubic metres, to the catalyst and polymer particles with characteristic diameters on the order of 10⁻⁶ to 10⁻³ m, and finally the active sites with characteristic sizes on the order of Ångströms. The figure also suggests that in many ways one can consider the catalyst and polymer particles as being at the heart of a polymerisation process. This is of course not to over-simplify the technological challenges of correctly operating the reactors, nor to assume that we have totally mastered the behaviour of the active sites either. However, as we shall see below, the very fact that we are using heterogeneous catalysts implies that mass transfer limitations can eventually limit the concentrations of active species at the active sites, or that the quality of the polymer (sticky/hard, brittle/flexible) can have a major impact on reactor behaviour. For these, and many other related reasons it is therefore of importance to understand what happens to the particles injected into the reactor during the polymerisation.

Polymer reaction engineering is a theoretical and technical framework for understanding and describing polymerization processes and polymer microstructure. The goal is to establish a virtual pathway from polymerization recipe and process to polymer properties. All phenomena that affect the process and the polymer properties, including reaction kinetics, thermodynamic and reactor performance considerations should be taken into account. The key to this quest is understanding polymerization kinetics and the effect of reactor residence time on polymer microstructure.

Polyolefins are the most widely used plastics today due to their low production cost and wide range of applications (packaging and other disposables, building and construction, agriculture, appliances, transportation, electrics and electronics, furniture, communication, automotive industry, etc.). It is well acknowledged that the degree of technological and scientific sophistication in relation to the catalytic polyolefin manufacturing has no equal among other synthetic polymer production processes. Presently, the total polyolefin world-production exceeds 130 million tons per year covering around 45% of the total plastic production (about 1.5 times the steel consumption in volume).

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.

Polyethylene is one of the most widely used polymers, and it can be found in various industrial applications. The annual production and consumption of polyethylene is currently higher than 100 million tons worldwide, or about 40% of the consumption of all thermoplastic materials [1].