Multimodal Polyethylene Resin Balancing Pipe Strength and Sagging
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Solution Overview
Problem
Polyethylene pipes used for pressure applications face challenges in meeting the requirements for PE112 classification due to the trade-off between density, slow crack growth resistance, and rapid crack propagation resistance, as increased density compromises slow crack growth resistance.
Innovation Solution
A multimodal polyethylene composition with a specific combination of ethylene homo- and copolymer fractions, optimized molecular weight distribution, and additives to achieve a balance of high hydrostatic pressure performance, stress crack resistance, and sagging resistance, including a base resin with a density of 945 to 958 kg/m³, a melt flow rate of 0.15 to 0.30 g/10min, and an average chain length of 900 carbons or higher.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the density of polyethylene resin is increased to improve pressure resistance, then the long term hydrostatic strength is improved, but the slow crack growth resistance decreases
Solution Approach 1:
The invention changes the molecular weight distribution parameters of the polyethylene resin, specifically using a multimodal distribution with a high molecular weight fraction (Mw > 100,000) and a low molecular weight fraction (Mw < 10,000). This parameter change allows achieving PE112 classification with density between 945-958 kg/m³ while maintaining slow crack growth resistance, resolving the contradiction between density-induced strength improvement and crack resistance degradation
Solution Approach 2:
The invention creates a composite molecular structure within the polyethylene resin by combining fractions with different molecular weights. The high molecular weight fraction provides slow crack growth resistance while the low molecular weight fraction contributes to pressure resistance and processability. This composite approach at the molecular level enables simultaneous achievement of strength and reliability without the trade-off present in conventional single-mode resins
2Strength
If the density of polyethylene resin is increased to meet PE112 requirements, then the hydrostatic pressure performance is improved, but the rapid crack propagation resistance decreases
Solution Approach 1:
The invention modifies the molecular weight distribution parameters by incorporating a high molecular weight fraction with Mw > 100,000 that contains long chain branches. This parameter change enables the resin to achieve PE112 hydrostatic pressure performance (σLPL ≥ 11.3 MPa) while maintaining rapid crack propagation resistance, as the long chain branches provide energy dissipation mechanisms during crack propagation
3Reliability
If the molecular weight is increased to improve crack resistance, then the slow and rapid crack propagation resistance are improved, but the processability and sagging resistance worsen
Solution Approach 1:
The invention segments the molecular weight distribution into distinct fractions: a high molecular weight fraction (Mw > 100,000) for crack resistance and a low molecular weight fraction (Mw < 10,000) for processability. This segmentation allows each fraction to perform its specialized function without compromising the other, enabling both improved reliability and maintained ease of manufacture
Solution Approach 2:
The invention changes the molecular weight distribution parameters to create a bimodal or multimodal distribution with specific weight fractions. The low molecular weight fraction (Mw < 10,000) with appropriate weight percentage improves processability and sagging resistance during extrusion and installation, while the high molecular weight fraction maintains crack propagation resistance
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, wherein fraction (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/10min, preferably of 0.20 to 0.29 g/10min, and still more preferably of 0.22 to 0.28 g/10min, 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, and the 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.