Bimodal Polyethylene Pipe Resin for Heat and Stress Crack Resistance
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Solution Overview
Problem
Polyethylene pipes used for transporting water and gas face challenges in maintaining mechanical strength and stress crack resistance at elevated temperatures, leading to the need for thicker pipes and increased material consumption, while achieving both high Minimum Required Strength (MRS) and good mechanical properties is difficult due to the inverse relationship between hydrostatic pressure testing (HPT) and notched pipe testing (NPT) results.
Innovation Solution
A polyethylene composition comprising a blend of two ethylene polymers with specific molecular weight distribution and density ratios, produced using a Ziegler-Natta catalyst in a multistage process, to achieve balanced performance in both HPT and NPT, thereby meeting PE100 or PE100RC requirements.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If polyethylene pipes are designed to withstand high temperatures and pressures, then mechanical strength is improved, but material consumption increases due to thicker pipe walls
Solution Approach 1:
The patent changes the molecular parameters of the polyethylene by creating a bimodal distribution with specific weight average molecular weights (200,000-500,000 for first polymer, 800,000-1,500,000 for second polymer) and controlling comonomer content (0.1-0.7 mol-%). This allows achieving high MRS at 20°C without increasing pipe thickness
Solution Approach 2:
The patent creates a composite polyethylene system by blending two distinct ethylene polymers with different molecular weights and properties. The first polymer provides baseline strength while the second high molecular weight polymer enhances long-term strength and stress crack resistance, achieving balanced performance
2Strength
If polyethylene composition is optimized for high Minimum Required Strength (MRS), then short term pressure resistance is improved, but long term stress crack resistance deteriorates
Solution Approach 1:
The patent segments the polyethylene properties by dividing the molecular weight distribution into two distinct populations: first ethylene polymer (lower MW: 200,000-500,000) for short-term strength and HPT performance, and second ethylene polymer (higher MW: 800,000-1,500,000) for long-term strength and stress crack resistance. This segmentation allows independent optimization of each property
Solution Approach 2:
The patent carefully controls the comonomer content parameter (0.1-0.7 mol-% of C3-C8 α-olefin) and the ratio between the two polymers (30-70 wt.% each) to achieve the optimal balance between short-term pressure resistance and long-term stress crack resistance
3Reliability
If comonomer content in polymer is increased to improve notched pipe test results, then stress crack resistance is improved, but hydrostatic pressure test results deteriorate
Solution Approach 1:
The patent optimizes the comonomer content parameter to a narrow range (0.1-0.7 mol-%) and controls the density (945-955 kg/m³) to achieve balanced performance in both notched pipe tests and hydrostatic pressure tests, avoiding the inverse relationship that plagues conventional single-mode polymers
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 ensures high hydrostatic pressure resistance and stress crack resistance, allowing for thinner pipes that meet PE100 or PE100RC standards, with improved performance in both short-term and long-term stress crack resistance.
Implementation Method 1
The base resin (B) is obtained in the presence of a Ziegler-Natta catalyst (ZN) obtained by a process comprising the steps of reacting a support, at least the surface of which comprises a magnesium halide compound
Data Source
Figure 1~2
AI summary
The present invention is directed to base resin (B) comprising a polyethylene composition (PC), said polyethylene composition (PC) comprising a first ethylene polymer (B1) and a second ethylene polymer (B2) having a higher weight molecular weight than the first ethylene polymer (B1). Further, the present invention is directed to a pipe comprising said base resin (B).