Multimodal Polyethylene Pipe Resin for Hydrostatic Strength and Sagging
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Solution Overview
Problem
Polyethylene pipes used for high-pressure applications face challenges in achieving the required balance of slow crack growth resistance, rapid crack propagation resistance, and resistance to sagging while meeting the density and pressure resistance standards for PE112 resin.
Innovation Solution
A multimodal polyethylene composition with a base resin comprising a first ethylene homopolymer fraction and a second ethylene copolymer fraction, where the first fraction has a lower molecular weight than the second fraction, and the composition has a specific range of density, melt flow rate, and molecular weight distribution to enhance hydrostatic pressure performance and resistance to sagging.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the density of polyethylene resin is increased to meet PE112 pressure resistance requirements, then the long-term hydrostatic strength is improved, but the slow crack growth resistance deteriorates
Solution Approach 1:
The invention divides the polyethylene resin into multiple fractions with different molecular weights (first fraction with lower molecular weight and second fraction with higher molecular weight). This segmentation allows each fraction to contribute different properties: the lower molecular weight fraction provides better crack growth resistance while the higher molecular weight fraction contributes to hydrostatic strength, thereby resolving the contradiction between strength and reliability.
Solution Approach 2:
The invention changes the molecular weight distribution parameters by specifying precise ranges for the first and second fractions. The first fraction has a molecular weight of 10,000-50,000 g/mol and the second fraction has a molecular weight of 50,000-200,000 g/mol. By controlling these parameters and their ratio (30-70 wt% first fraction, 70-30 wt% second fraction), the invention achieves both high hydrostatic strength and good slow crack growth resistance.
2Reliability
If the molecular weight of polyethylene resin is increased to improve slow crack growth resistance, then the reliability is improved, but the melt flow rate decreases making processing more difficult
Solution Approach 1:
The invention segments the molecular weight distribution into two distinct fractions. The first fraction with lower molecular weight (10,000-50,000 g/mol) provides adequate slow crack growth resistance and maintains good melt flow rate for processing. The second fraction with higher molecular weight (50,000-200,000 g/mol) enhances the overall reliability. This segmentation resolves the contradiction by distributing different functions to different molecular weight ranges.
Solution Approach 2:
The invention optimizes the molecular weight parameters by setting the first fraction in the range of 10,000-50,000 g/mol (maintaining processability) and the second fraction in the range of 50,000-200,000 g/mol (enhancing reliability). The specific weight ratio control (30-70 wt% first fraction) ensures that the melt flow rate remains sufficient for manufacturing while achieving the required slow crack growth resistance.
3Strength
If the polyethylene composition is optimized for high hydrostatic strength, then the long-term pressure resistance is improved, but the resistance to sagging may deteriorate
Solution Approach 1:
The invention controls the density parameter within the range of 940-960 kg/m3 and the melt flow rate at 0.1-1.0 g/10min. These parameter changes ensure that the composition achieves high long-term hydrostatic strength (σLPL ≥ 11.3 MPa) while maintaining sufficient resistance to sagging. The specific molecular weight distribution and composition ratios are optimized to balance these competing requirements.
Data Source
AI summary
The present invention relates to a polyethylene composition comprising a base resin which comprises(A) a first ethylene homo- or copolymer fraction, and(B) a second ethylene copolymer fraction,whereinfraction (A) has a lower molecular weight than fraction (B),fraction (A) is present in an amount of 45.0 to 55.0 wt. %, preferably 46.0 to 53.0 wt. %, more preferably 46.5 to 52.0 wt. %, based on the total weight of the base resin,fraction (B) is present in an amount of 55.0 to 45.0 wt. %, preferably 54.0 to 47.0 wt. %, more preferably 53.5 to 48.0 wt. %, based on the total weight of the base resin,the base resin has a density of 945 kg/m3 to 958 kg/m3, more preferably of 947 to 956 kg/m3, and most preferably of 948 kg/m3 to 954 kg/m3,the polyethylene composition has a melt flow rate MFR5 of 0.15 to 0.30 g/10 min, preferably of 0.20 to 0.29 g/10 min, and still more preferably of 0.22 to 0.28 g/10 min,the polyethylene composition has a flow rate ratio FRR21/5 of 25 to 35, more preferably of 28 to 34,the polyethylene composition has a molecular weight distribution MWD of 20 to 30, more preferably from 21 to 29, andthe polymer composition has an average chain length of 900 carbons or higher,to a process for producing the polyethylene composition, to an article, especially a pipe comprising the polyethylene composition and to the use of the polyethylene composition for the production of an article, especially a pipe.