Cross-linked Polyethylene Pipe Molecular Weight Distribution
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Solution Overview
Problem
Existing polyethylene pipe manufacturing processes face challenges in achieving good cross-linking ability and processability, particularly in screw extrusion, due to the use of polymers with high melt flow rates, which result in poor dimensional stability and increased crosslinking agent consumption.
Innovation Solution
The development of a cross-linked polyethylene pipe using an ethylene polymer with a density of at least 948 kg/m3, produced by single-site catalysis, with specific shear thinning indices and molecular weight distribution, allowing for improved processability and cross-linkability through irradiation rather than peroxide crosslinking.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If polymers with low melt flow rate (high molecular weight) are used to improve crosslinking response and reduce crosslinking agent consumption, then crosslinking efficiency is improved, but processability deteriorates with reduced line speed at extrusion
Solution Approach 1:
The patent changes the molecular weight distribution parameters of the polyethylene polymer, specifically using a bimodal distribution with a narrow low molecular weight fraction (Mw 10,000-50,000) and a broad high molecular weight fraction (Mw 50,000-500,000). This parameter optimization allows the polymer to exhibit both good processability (higher MFR) and good crosslinking ability simultaneously, resolving the contradiction between extrusion speed and crosslinking efficiency.
2Productivity
If polymers with high melt flow rate are used to improve processability and extrudability, then line speed at extrusion is improved, but melt strength deteriorates causing dimensional instability and parison collapse
Solution Approach 1:
The patent creates a composite molecular weight distribution within the polyethylene polymer, combining two distinct molecular weight fractions (low and high). The low molecular weight fraction provides good flow and processability, while the high molecular weight fraction provides melt strength and dimensional stability. This composite structure at the molecular level resolves the contradiction between extrudability and dimensional stability.
3Ease of manufacture
If higher MFR resins are used to achieve better extrudability, then processability is improved, but crosslinkability deteriorates requiring larger amount of crosslinking agent or stronger irradiation dose
Solution Approach 1:
The patent optimizes the molecular weight distribution parameters by incorporating a low molecular weight fraction (Mw 10,000-50,000) that has higher reactivity toward crosslinking agents and irradiation. This low molecular weight fraction acts as a crosslinking promoter, ensuring good crosslinkability even when the overall polymer has high MFR for good extrudability, thus resolving the contradiction between ease of manufacture and crosslinkability.
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 resulting pipes exhibit enhanced pressure test performance at high temperatures and pressures, maintaining stability and reducing the need for crosslinking agents, while ensuring dimensional integrity and long-term pressure resistance.
Implementation Method 1
crosslinking it by irradiating the pipe
Data Source
AI summary
A cross-linked polyethylene pipe comprising an ethylene polymer with a density of at least 948 kg/m3 obtained by polymerization with a single-site catalyst and having a shear thinning index SHI2.7/210 of less than 10; and wherein said pipe has a pressure test at 4.8 MPa and 95° C. of at least 500 h and at 12.4 MPa and 20° C. of at least 500 h.