Tailor‐made controlled rheology polypropylenes from metallocene and Ziegler–Natta resins

Abstract

Production of controlled rheology polypropylenes (CRPPs) is practiced industrially by modifying existing commodity Ziegler-Natta resins through peroxide-induced β-scission reactions, resulting in materials with controlled rheological properties and accompanying narrower molecular weight distributions (MWDs). In this work, this methodology was studied using both metallocene-based polypropylenes (mPPs) and Ziegler-Natta-based polypropylenes (ZN-PPs). Numerical simulations based on a previously proposed kinetic model indicated that the nature of the starting resin has a significant effect on the control of MWD polydispersity index (PDI) and weightaverage molecular weight (M W ) of the resulting CRPP. Based on these observations, experiments were carried out to demonstrate the feasibility of producing CRPP with targeted molecular and rheological characteristics. Commercial mPP and ZN-PP resins were selected to produce CRPP with similar M W or melt flow rates (MFRs) but varying PDIs. The rheological properties and MWDs of these materials were evaluated through oscillatory shear and gel permeation chromatography (GPC) measurements and their extrusion behavior was briefly studied and assessed with respect to these properties.

Tacticity Changes during Controlled Degradation of Polypropylene

“…Controlled degradation of polypropylene (CPP) is an industrial process wherein polypropylene (PP) with a higher molecular weight (MW) is converted to a lower-MW PP with better processing properties. ,− , The extruder is employed as a chemical reactor for the PP chains in the melt state to achieve mixing at molecular scales with small amounts of peroxides dissociating into initiator radicals responsible for degradation reactions. CPP leading to a shift toward lower MW has been studied by many researchers, both by experiments and modeling. − ,− Previously, our group has specifically studied the problem of mixing on a microscale in CPP by developing a model based on Ottino’s micromixing concept . According to this concept, mixing takes place by the thinning and folding of alternating liquid layers of different composition.…”

Chemical modification of poly(1‐butene) resins through reactive processing

“…These trends are similar to those observed in the production of CRPP from metallocene PP commodity resins. [37,42] In this figure, the lines represent predictions from a kinetic degradation model previously developed for the peroxide modification of PP for the production of CRPP. [7,9] It can be observed, that even without any modification, this kinetic model can predict reasonably well the effect of peroxide concentration on Mw and PDI.…”

Time/Molecular Weight Superposition to Describe the Behavior of Controlled-Rheology Polypropylenes

“…At the end of the polymerization reactor, polypropylene (PP) resins generally have a high molecular weight and a wide molecular weight distribution, which gives them high viscosity and elasticity values. To make them easier to be processed in extrusion or injection molding, they are chemically modified with peroxides, which break the polymer chains by scission and lead to so-called controlled-rheology polypropylenes (CR-PPs) [1][2][3][4][5][6][7][8]. In addition to the characterization of the kinetic reactions and the development of kinetic models [9,10], the rheological behavior of these CR-PPs has been studied by many authors.…”

“…Later on, the viscoelastic properties of the same samples were quantified using an integral constitutive equation of the K-BKZ type [12]. Tzoganakis [13] also proposed a comparison of the viscoelastic behavior between linear and branched CR-PPs, while Nie and Tzoganakis [8] compared CR-PPs prepared from metallocene and Ziegler-Natta resins. Carrot et al [14] used the molecular weight distribution (MWD) data of a series of CR-PPs to predict their linear viscoelastic behavior.…”