Bimodal polyolefin

In addition, in recent years the products themselves have become much more complex and therefore more difficult to process. One example is the bimodal polyolefins, which have excellent mechanical properties and processability at the same time. These properties are achieved by the manufacture of extremely broad to multimodal molecular weight distributions in appropriately equipped reactors [1]. 

The new concept for processing of bimodal polyolefins was first presented in 2004 at the Kunststoffmesse in Dtisseldorf. The system, known as the ZSK-NT, combines two corotating, twin-screw kneaders in a cascade arrangement. Figure 15.1 shows the setup of the two ZSKs operating as ZSK-NT. ... 

Heidemeyer, P., Pfeiffer,., Special Requirements on Compounding Technology for Bimodal Polyolefines and their Industrial Application, Macromol. Symp. , 181, (2002) pp. 167-176... 

P. Heidemeyer P., Pfeiffer H. J. Special requirements on compounding technology for bimodal polyolefines and their industrial application Macro-molecular Symposia Volume 181, Issue 1, May 2002, 167-176... 

Today, large polyolefin extruders are built as ZSK MEGAcompounders, mainly as 320, 350, and 380 Me sizes. The reason for this is clear from Fig. 14.8 which shows the same relationship as Figure 14.7, but specifically for polyolefin applications, here with the ZSK Me limit curve for maximum throughput. Normally, the torque limit is only reached with LLDPE and FfDPE if pressure is also built up by the ZSK and the powder from the polymerization reactor is coarse enough to prevent intake problems. With PP or melt-charged LDPE or two-stage machines for bimodal polyethylene, the torque required is well below the Me limit curve. 

Amorphous silica-alumina (Si02-AI2O3) has been also tested for the catalytic cracking of polyolefins [36, 37, 43]. This acid solid is featured by having a broad distribution of pore sizes, which is determined by the synthesis procedure. Moreover, the occurrence of a bimodal pore size distribntion (e.g. meso-macroporous) is usually present. The aluminium... 

If we exclude the case of the Italian talc, all the other distribution functions are bimodal and present a main peak at low energy around 17 kj/mole characteristic for the basal surface and a very weak peak at the highest energy at 27 kj/mole corresponding to lateral surfaces. It demonstrates that industrial talcs having low specific surface area exhibit mainly basal surfaces and that the difference in behaviour, as a specific filler for polyolefines origin certainly from other more macroscopic factors such as shape and size of... 

A combination of different reactors is frequently used to produce polymers with special molecular and morphological characteristics. For example, polyolefins with a bimodal MWD that combine good mechanical properties (provided by the high molecular weight mode) with a good processing behavior (low melt viscosity provided by the low molecular weight mode) are produced in a combination of a loop reactor and a fluidized... 

The relationships between Pr and cj) have been derived for suspensions of monodispersed hard spheres in Newtonian liquids. However, most real systems are polydispersed in size and do not necessarily consist of spherical particles. It has been found that here also Simha s Eq. 7.24, Mooney s Eq. 7.28, or Krieger-Dougherty s Eq. 7.8 are useful, provided that the intrinsic viscosity and the maximum packing volume fraction are defined as functions of particle shape and size polydispersity. For example, by allowing with composition, it was possible to describe the Pr versus cj) variation for bimodal suspensions (Chang and Powell 1994). Similarly, after values of [q] and (j)i were experimentally determined, Eq. 7.24 provided good description for the versus cj) dependence of several multiphase systems, e.g., PVC emulsions and plastisols, mica-reinforced polyolefins, and sealant formulations (Utracki 1988, 1989). 

Knuuttila H, Lehtinen A, Salminen H (2000) Metallocene catalyst technology in a bimodal polymerization process. In Scheirs J, Kaminsky W (eds) Metallocene-based polyolefins. Wiley, Hoboken, pp 365-378... 

Neuutn scattering is often perftrmed as a function cf particle concentration to determine specific interactions between components. Blends cf metallocene synthesized polyolefins (32) and efi ds cf solvent on dendrima- size (33) have been studied using SANS. Interface widths between two polymer blend components in the melt have been studied by neuu-on reflectivity (34). Dynamic studies have been undertaken on bimodal melts (35) using neutron spin echo techniques. Some of these recent developments will be reported in ttie following chrqiters. 

Figure 8.25 shows the chemical composition distribution obtained with Eq. (28) of a polyolefin made with a catalyst having three distinct active-site types, whose chain length distribution has already been presented in Figure 8.24. Notice the characteristic bimodal distribution of ethylene/a-olefin copolymers made with het-...

FIG. 56 These molecular weight profiles show Exxon s new developmental iPPs made with dual metallocene catalysts. The bimodal copol5mer may be the first true bimodal made in one reactor the broad MWD homopolymer may be the first pp with control of multitacticity. (From Exxon paper at Polyolefins X.)... 

In addition to long-chain branching binary metallocene systems are also used to constiurt bimodal MWDs for polyolefins that have betta processability than narrow distribution materi-als. The Iowa molecular weight chains reduce melt viscosities and thus make the materials easia to be processed. Another development is the tandem catalysis for LLDPEs. The current industrial practice involves two steps. Ethylene monomas are first oligomerized into a-olefms such as butene, hexene, and oaene. The a-olefins are incorporated into polyethylene chains... 

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