POLYMER COMPOSITION FOR PIPES

摘要

1. A multimodal polymer composition for production of pipes, comprising multimodal polyethylene obtained by polymerization with use of Ziegler-Natta type catalyst or metallocene catalyst in several polymerization reactors with a density of 0.930-0.965 g/cm, and a viscosity at a shear stress of 747 Pa (eta747Pa) of at least 650 kPa.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) and with lower limit of molecular weight is 3500. 2. A composition as claimed in claim 1, wherein the multimodal polyethylene has a viscosity at a shear stress of 2.7 kPa (eta2.7 kPa) in range 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 SHI2.7/210=50-150, and a storage modulus (G') at a loss modulus (G'') of 5 kPa, of G'5 kPa3000 Pa. 3. A composition as claimed in claim 1 or 2, wherein the multimodal polymer is a bimodal polyethylene produced by (co) polymerization in at least two steps. 4. A composition as claimed in any one of claims 1-3, wherein the ethylene copolymer of the HMW fraction is a copolymer of ethylene and a comonomer selected from the group consisting of 1-butene, 1-hexene, 4-methyl-1-pentene, and 1-octene. 5. A composition as claimed in any one of claims 1-4, wherein the amount of comonomer is 0.1-2.0 mol % of the multimodal polymer. 6. A composition according to any of claims 1-5, having a weight ratio of the LMW fraction to the HMW fraction of (43-51):(57-49).7. A composition as claimed in any one of claims 1-6, wherein the multimodal polymer has an MFR5 of 0.1-1.0 g/10 min. 8. A composition as claimed in any one of claims 1-7, wherein the polymer is obtained by slurry polymerization in presence of procatalist and cocatalist in a loop reactor of a LMW ethylene homopolymer fraction, followed by gas-phase polymerization of a HMW ethylene copolymer fraction. 9. A composition as claimed in claim 8, wherein the slurry polymerization is preceded by a pre-polymerization step. 10. A composition as claimed in claim 9, wherein the polymer is obtained by pre-polymerization in a loop reactor, followed by slurry polymerization in a loop reactor of a LMW ethylene homopolymer fraction, and gas-phase polymerization of a HMW ethylene copolymer fraction. 11. A composition as claimed in any one of claims 8-10, wherein polymerization procatalyst and cocatalist are added to the first polymerization reactor only. 12. A composition as claimed in claim 11, wherein the polymerization catalyst is a Ziegler-Natta type catalyst. 13. A pipe characterized 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 degree C. (MRS8.0). 14. A pipe as claimed in claim 13, wherein the pipe is a pressure pipe withstanding a pressure of 10 MPa gauge during 50 years at 20 degree C. (MRS10.0). 15. A pipe as claimed in claim 13 or 14, wherein the pipe has a rapid crack propagation (RCP) S4-value, determined according to ISO 13477:1997(E), of -5 degree C. or lower. 16. A pipe as claimed in claim 15, wherein the pipe has a rapid crack propagation (RCP) S4-value, determined according to ISO 13477:1997(E), of -7 deg. C. or lower. 17. A pipe as claimed in any one of claims 13-16, wherein the pipe has a slow crack propagation resistance, determined according to ISO 13479:1997, of at least 500 hrs at 4.6 MPa/80 degree C. 18. A method of producing of bimodal polyethylene composition as claimed in claim 1, providing for catalytic polymerization of ethylene successively on pre-polymerization step, on pre-polymerization in a loop reactor, and on the step of gas-phase polymerization, wherein - the whole catalyst, applicable in this method, together with ethylene is brought up on pre-polymerization step, on which slurry polymerization is held in a loop reactor with the following obtaining of ethylene homopolymer fraction, which is from 1 to 5% (mass) of final bimodal polyethylene product; - slurry catalyst/polymer is passed from pre-polymerization step to mentioned polymerization in a loop reactor, where slurry polymerization is further held at the temperature from 92 degree C to 98 degree C in presence of hydrogen and ethylene at molarity ratio of H2/ethylene in the range from 200 to 800m/km, and - slurry catalyst/polymer is brought up from mentioned polymerization in a loop reactor step where polymerization is held at the temperature range from 75 degree C to 90 degree C with the addition of co-monomer in presence of hydrogen and ethylene at molarity ratio of H2/ethylene, not exceeding 50 m/km; - at the same time, mass ratio of quantity of ethylene homopolymer fraction to quantity of ethylene copolymer in final bimodal polyethylene product is (43-51) : (57-49), mentioned product being containing from 0.1 to 2.0% (mol.) of said copolymer. 19. A method according to claim 18, wherein said polymerization catalyst is obtained by method comprising the following steps: - interaction of carrier, containing silicon oxide and magnesium halogenide, with aluminium halogenide, described by formula RnAlX3-n(where n is 1 or 2, X represents halo, and R represents C1-C10-alkyl); - joining up of mentioned product of interaction with the composition, containing magnesium, bonded with alkyl and with alkoxide, and described by snap formula R'nMg(OR')2-n (R' represents C1-C20-alkyl, and n is in range from 0.01 to 1.99); - addition to product of said unification of halogenide quadrivalent titanium. 20. A method according to claim 18 or 19, wherein obtained bimodal polyethylene product is mixed with technical carbon as stuff or dye and extruded fashioning of pipe.