Mixed Catalyst System for Bimodal Polyethylene Production
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Solution Overview
Problem
Current mixed or co-supported catalyst systems for producing bimodal polyethylene do not effectively achieve reversed comonomer incorporation and lack the use of polymerization catalysts with phosphinimide or ketimide ligands, which are necessary to enhance the properties of polyethylene resins.
Innovation Solution
A mixed catalyst system comprising a supported group 6 metal-based polymerization catalyst and a supported group 4 metal-based polymerization catalyst, where the latter includes an organometallic complex with phosphinimide or ketimide ligands, is used to produce polyethylene with a broad or multimodal molecular weight distribution and reversed comonomer incorporation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If conventional mixed or co-supported catalyst systems are used, then polyethylene with broad molecular weight distribution is produced, but reversed comonomer incorporation is not achieved
Solution Approach 1:
The catalyst system is segmented into two distinct supported catalysts with different hydrogen responses. The first catalyst (low hydrogen response) produces high molecular weight fractions with high comonomer incorporation, while the second catalyst (high hydrogen response) produces low molecular weight fractions with low comonomer incorporation, achieving the desired reversed comonomer distribution that segmented catalyst functions cannot achieve with a single catalyst system
Solution Approach 2:
Different regions of the polymer product have different comonomer incorporation levels tailored to specific molecular weight ranges. The high molecular weight fraction exhibits high comonomer incorporation for improved ESCR, while the low molecular weight fraction exhibits low comonomer incorporation for good processability, creating local quality variations within the polymer structure
2Productivity
If chromium catalysts are combined with group 4 transition metal catalysts, then catalytic activity is enhanced, but catalyst deactivation occurs
Solution Approach 1:
The catalyst system segments the chromium and group 4 transition metal catalysts into separate supported catalyst entities rather than combining them into a single catalyst. This segmentation prevents direct interaction and mutual deactivation between the catalyst components while still allowing both to function simultaneously in the polymerization process
Solution Approach 2:
The support material acts as an intermediary carrier for both catalyst components. Each catalyst is supported on its own support particles, and the support serves as a mediator that allows both catalysts to be present in the reactor without direct contact that would cause deactivation, while still enabling both to contribute to polymerization
3Stability of the object's composition
If co-supported catalysts are used, then homogeneous product particles are obtained, but device complexity increases
Solution Approach 1:
The support particles serve as intermediaries that carry the catalyst components. By using separately supported catalysts on their own support particles, the system achieves a balance between complexity and performance, avoiding the need for complex co-supported structures while still producing homogeneous product particles through the mediating role of the supports
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The catalyst system achieves a bimodal polyethylene with higher comonomer incorporation in the higher molecular weight fraction, improving the mechanical properties and processing characteristics of the resin, making it suitable for various applications such as film, pipe, and geomembranes.
Implementation Method 1
a first catalyst component comprising a chromium catalyst supported on an inorganic oxide and a second catalyst component comprising an organometallic catalyst supported on an inorganic oxide
Data Source
Figure 1~2
Figure 3
AI summary
Polyethylene is made by (co)polymerizing ethylene in a gas-phase reactor using a mixed catalyst system comprising a chromium catalyst supported on silica and a Group 4 transition metal catalyst, separately supported on silica. The Group 4 transition metal catalyst is defined by the formula shown, wherein M is a Group 4 metal, Pl is a phosphinimide or ketimide ligand (shown), L is a monoanionic ligand which is a cyclopentadienyl or a bulky heteroatom type ligand, m is 1 or 2, n is 0 or 1, and p is an integer. The mixed catalyst system gives access to polyethylene having a broad or bimodal molecular weight distribution. In the copolymerization of ethylene, reversed or partially reversed comonomer distribution is achieved: the supported Group 4 component provides polymer segments having higher molecular weight and also higher comonomer incorporation than polymer segments produced at the supported chromium sites.(Formulae I,II,III).