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AssigneeBorealis Technology Oy
InventorsMats Bäckman, Eeva-Leena Heino, Solveig Johansson, Jussi Laurell, Arja Lehtinen, Jarmo Lindroos, Marja Ora, Elisabeth Ribarits, Teija Sihvonen, Jari {umlaut over (Aa)}rilä
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A multimodal polymer composition for pipes is disclosed as well as a pipes made thereof. The polymer is a multimodal polyethylene with a density of 0.930-0.965 g/cm3, and a viscosity at a shear stress of 747 Pa (η747 Pa) of at least 650 Pa.s, said multimodal polyethylene comprising a low molecular weight (LMW) ethylene homopolymer fraction and a high molecular weight (HMW) ethylene copolymer fraction, said HMW fraction having a weight ratio of the LMW fraction to the HMW fraction of (35-55):(65-45). Preferably, the multimodal polyethylene has a viscosity at a shear stress of 2.7 kPa (η2.7 kPa) of 260-450 kPa.s; and a shear thinning index (SHI) defined as the ratio of the viscosities at shear stresses of 2.7 and 210 kPa, respectively, of SHI27/210=50-150, and a storage modulus (G′) at a loss modulus (G″) of 5 kPa, of G′5 kPa≧3000 Pa. The pipe is made of the multimodal polymer composition and withstands a stress of 8.0 MPa gauge during 50 years at 20° C. (MRS8.0). Preferably, the pipe has a rapid crack propagation (RCP) S4-value, determined according to ISO 13477:1997(E), of −5° C. or lower and a slow crack propagation resistance, determined according to ISO 13479:1997, of at least 500 hrs at 4.6 MPa/80° C. The polymer composition affords good non-sagging properties to pipe made thereof.
A multimodal polymer composition for pipes, wherein said multimodal polymer composition is a multimodal polyethylene with a density of 0.930-0.965 g/cm3, and a viscosity at a shear stress of 747 Pa (η1747 Pa) of at least 650 kPa.s, said multimodal polyethylene comprising a low weight average molecular weight (LMW) ethylene homopolymer fraction and a high weight average molecular weight (HMW) ethylene copolymer fraction, said HMW fraction having a weight ratio of the LMW fraction to the HMW fraction of (35-55):(65-45).
A pipe characterised in that it is a pressure pipe comprising the multimodal polymer composition according to any one of the preceding claims, which pipe withstands a pressure of 8.0 MPa gauge during 50 years at 20° C. (MRS8.0).
A process for producing a bimodal polyethylene composition, which process comprises catalytically polymerising ethylene sequentially in a prepolymerisation stage, a loop-reactor stage and a gas phase reactor stage, and in which:
Dependencies are inferred from the claim text.