Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
2001-04-10
2002-08-27
Nutter, Nathan M. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C138S177000, C138SDIG007, C166S242100
Reexamination Certificate
active
06441096
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a multimodal polymer composition for pipes and a pipe prepared thereof.
BACKGROUND OF THE INVENTION
Nowadays, pipes of polymer material are frequently used for various purposes, such as fluid transport, i.e. transport of liquid or gas, e.g. water or natural gas, during which the fluid can be pressurised. Moreover, the transported fluid may have varying temperatures, usually within the temperature range from about 0° C. to about 50° C. Such pressure pipes are preferably made of polyolefin plastic, usually unimodal ethylene plastic such as medium density polyethylene (MDPE; density: 0.930-0.942 g/cm
3
) and high density polyethylene (HDPE; density: 0.942-0.965 g/cm
3
). By the expression “pressure pipe” herein is meant a pipe which, when used, is subjected to a positive pressure, i.e. the pressure inside the pipe is higher than the pressure outside the pipe.
Polymer pipes are generally manufactured by extrusion, or, to a smaller extent, by injection moulding. A conventional plant for extrusion of polymer pipes comprises an extruder, a die-head, a calibrating device, cooling equipment, a pulling device, and a device for cutting or for coiling-up the pipe.
The manufacture of PE materials to be used in pressure pipes is discussed in an article by Scheirs et al [Scheirs, Böhm, Boot and Leevers: PE100 Resins for Pipe Applications, TRIP Vol. 4, No 12 (1996) pp. 408-415]. The authors discuss the production technology and properties of PE100 pipe materials. They point out the importance of proper comonomer distribution and molecular weight distribution to optimize the slow crack growth and rapid crack propagation.
The European patent application EP 739937 A2 discloses a pipe having improved properties. The pipe is made of a bimodal PE resin, and it has a specified stress cracking resistance, impact strength and stiffness. The publication discloses that preferably the material should have an MFR
s
not higher than 0.35 g/10 min.
The properties of conventional polymer pipes are sufficient for many purposes, although enhanced properties may be desired, for instance in applications requiring high pressure resistance, i.e. pipes that are subjected to an internal fluid pressure for a long and/or short period of time. As examples of properties which it is desirable to improve may be mentioned the processability, the impact strength, the modulus of elasticity, the rapid crack propagation resistance, the slow crack growth resistance, and the design stress rating of the pipe.
A problem when manufacturing large diameter pipes, particularly from multimodal, such as bimodal, polymer material, is that it is difficult to maintain uniform dimensions all over the pipe. This is due to gravity flow of the polymer melt, causing it to flow from the upper part of the pipe to the lower part (often called “sagging”). Thus, the wall thickness at the upper part of the pipe becomes smaller than at the lower part of the pipe. The sagging problem is particularly pronounced for thick-walled large diameter pipes.
The above sagging problem has been discussed in the German patent application DE 19604196 A1. It discloses a process to manufacture a large-bore, thick walled pipe of polyethylene. The pipe is extruded through a ring formed die and cooled on both inner and outer surfaces. This double sided cooling is said to eliminate the deformation of the pipe due to gravity-induced flow of the melt emerging from the die.
The sagging problem has also been discussed in an article by D. N. Githuku and A. J. Giacomin, “Elimination of Sag in Plastic Pipe Extrusion”, Intern. Polymer Processing VII (1992) 2, 140-143. The conventional way to reduce sag is by manually adjusting the die eccentricity which typically requires three or four tries at start-up to get an acceptable thickness profile. The article proposes a new way to reduce sag, namely by rotating the pipe during cooling.
A mathematical mode of cooling and solidification, coupled with gravity induced flow during the cooling of extruded plastic pipes is set up and solved by the finite element method in an article by J. F. T. Pittman, G. P. Whitman, S. Beech, and D. Gwynn, “Cooling and Wall Thickness Uniformity in Plastic Pipe Manufacture”, Intern. Polymer Processing IX (1994) 2, 130-140. Melt rheology and determination of melt flow properties at the very low stress levels that are relevant at sag is also discussed.
SUMMARY OF THE INVENTION
It has now been discovered that the above sagging problem can be overcome by preparing the pipe from a specific, well defined type of multimodal polyethylene. More particularly, the multimodal polyethylene should have a medium to high density, a high viscosity at low shear stress, a carefully selected ratio between its low molecular weight fraction and high molecular weight fraction, and include a comonomer in its high molecular weight fraction only. Preferably, the multimodal polyethylene should have a specific molecular weight and a well defined molecular weight distribution.
Thus, the present invention provides a multimodal polymer composition for pipes, characterised in that it is a multimodal polyethylene with a density of 0.930-0.965 g/cm
3
, and a viscosity at a constant shear stress of 747 Pa (&eegr;
747
Pa) 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)
It is much preferred that the multimodal polyethylene has a viscosity at a shear stress of 2.7 kPa (&eegr;
2.7
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 kPa and 210 kPa, respectively, of SHI
2.7/210
=50-150, and a storage modulus (G′) at a loss modulus (G″) of 5 kPa, of G′
5 kPa
≧3000 Pa. Preferably densities in the range 0.937-0.942 g/cm
3
are used for smaller diameter MD pressure pipes while higher densities of 0.943-0.955 g/cm
3
are used for larger diameter HD pressure pipes.
The present invention also provides a pipe comprising said multimodal polymer composition, which pipe withstands a hoop 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.
Other distinguishing features and advantages of the invention will appear from the following specification and the appended claims.
DETAILED DESCRIPTION OF THE INVENTION
As stated above, the pressure pipe composition of the present invention is made from a specific multimodal polyethylene. This is in contrast to prior art polyethylene pipes which usually are made of unimodal polyethylene or bimodal polyethylene which does not have the specific molecular weight distribution and composition of the multimodal polyethylene of the present invention.
The “modality” of a polymer refers to the form of its molecular weight distribution curve, i.e. the appearance of the graph of the polymer weight fraction as function of its molecular weight. If the polymer is produced in a sequential step process, utilizing reactors coupled in series and using different conditions in each reactor, the different fractions produced in the different reactors will each have their own molecular weight distribution. When the molecular weight distribution curves from these fractions are superimposed into the molecular weight distribution curve for the total resulting polymer product, that curve will show two or more maxima or at least be distinctly broadened in comparison with the curves for the individual fractions. Such a polymer product, produced in two or more serial steps, is called bimodal or multimodal depending on the number of steps. In the following all polymers thus produced in two or more seq
Bäckman Mats
Heino Eeva-Leena
Johansson Solveig
Laurell Jussi
Lehtinen Arja
Borealis Technology Oy
Nutter Nathan M.
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