POLYMER COMPOSITION FOR PIPES

摘要

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 (eta747 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 (eta2.7 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.