Polyethylene Pipe Resin Density and Melt Flow Control
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Solution Overview
Problem
Polyethylene compositions for pipe applications face challenges in achieving a balance of properties such as processability, slow crack growth resistance, rapid crack propagation resistance, and homogeneity, particularly due to the dependency of crystallinity and density, which affects their performance in withstanding hoop stresses and internal pressures.
Innovation Solution
A polyethylene composition with a base resin having a density of 952.0 to 957.0 kg/m³, a melt flow rate of 0.12 to 0.21 g/10min, and a polydispersity index of 4.9 to 9.0 Pa⁻¹, produced using a multistage process involving a silica-supported Ziegler Natta catalyst, which enhances molecular weight distribution and homogeneity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If polyethylene compositions are designed with high density to meet PE100 requirements, then strength and pressure resistance are improved, but processability and homogeneity deteriorate
Solution Approach 1:
The patent applies parameter changes by precisely controlling the density of the base resin to be greater than 952.0 kg/m³, along with controlling molecular weight distribution and polydispersity index. This specific parameter range optimizes both the strength for PE125 classification and the processability during manufacturing.
Solution Approach 2:
The patent uses a composite approach by combining a base resin with specific properties (density >952.0 kg/m³) with carefully controlled molecular weight distribution. This creates a composite material structure that achieves both high strength and good processability.
2Strength
If polyethylene compositions are designed with high density to meet PE100 requirements, then strength and pressure resistance are improved, but homogeneity deteriorates
Solution Approach 1:
The patent applies parameter changes by precisely controlling the density of the base resin to be greater than 952.0 kg/m³, along with controlling molecular weight distribution and polydispersity index. This specific parameter range optimizes both the strength for PE125 classification and the processability during manufacturing.
3Productivity
If conventional Ziegler-Natta catalysts are used to produce multimodal resins, then production capability is maintained, but the balance of properties (processability, crack resistance, homogeneity) is insufficient
Solution Approach 1:
The patent changes the catalyst parameters by specifying a silica-supported Ziegler-Natta catalyst with particular composition ranges (Al: 1.30-1.65 mol/kg silica, Mg: 1.25-1.61 mol/kg silica, Ti: 0.70-0.90 mol/kg silica) and particle size (7-15 μm). This enables production of resin with density >952.0 kg/m³ and high polydispersity index, achieving both productivity and superior property balance.
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 composition achieves improved balance of properties, including enhanced slow crack growth resistance, rapid crack propagation resistance, and homogeneity, allowing pipes to withstand higher hoop stresses and internal pressures, thus meeting the requirements for PE125 classification.
Implementation Method 1
polymerizing ethylene in the presence of (i) a silica supported Ziegler Natta catalyst
Implementation Method 2
polymerizing ethylene in the presence of (i) a silica supported Ziegler Natta catalyst having a molar composition
Data Source
Figure 1
AI summary
The present invention is concerned with a polyethylene composition comprising a base resin having a density of more than 952.0 kg/m3 and equal to or less than 957.0 kg/m3, wherein the composition has a melt flow rate MFR5 of 0.12 to 0.21 g/10min and the polyethylene composition has a polydispersity index PI within the range of higher than 4.9 Pa-1 and equal to or less than 9.0 Pa-1. Furthermore, the present invention concerns a polyethylene composition obtainable by a multistage process, the multistage process comprising polymerizing ethylene in the presence of a silica supported Ziegler Natta catalyst in a reactor cascade comprising a first loop reactor, a second loop reactor and a gas phase reactor for obtaining the base resin and then extruding the base resin together with stabilizers and carbon black into the polyethylene composition. Furthermore, the present invention relates to an article, such as a pipe, made from the polyethylene composition and the use of the polyethylene composition for the production of such an article.