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
2002-08-16
2004-09-07
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...
Reexamination Certificate
active
06787608
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a high density polyethylene composition with a bimodal molecular weight distribution and articles made therefrom, especially high topload blow moldings and high temperature or high pressure, long duration pipes.
BACKGROUND OF THE INVENTION
Polyethylene pipes are light in weight, easy to handle, and are non-corrosive. In addition, their rigidity is relatively high that they can be laid under the ground, and their flexibility is also relatively high that they can follow a movement of ground. Due to these advantageous characteristics, the amount of polyethylene pipes used is rapidly increasing in recent years.
In addition to the above desirable characteristics, polyethylene pipes should have (1) impact resistance sufficient to endure impacts given at the time when and after they are set; and (2) excellent long-term durability under gas or water pressure (specifically, environmental stress cracking resistance and internal pressure creep resistance).
With respect to the long-term durability, conventional pipes may meet the ISO standard, i.e. 50-year durability at normal temperatures under an internal pressure, expressed in terms of circumferential stress, of approximately 8 MPa. However, the conventional polyethylene pipes are still insufficient in the long-term durability for use under more severe conditions, such as main pipes for gases or running water which have a large diameter and undergo high internal pressure. For this reason, they are presently used only for branch pipes and the like, having a small diameter.
The long-term durability of a polyethylene pipe is considered to be determined by the environmental stress cracking resistance, that is the resistance to cracking which is caused when an internal pressure applied to the pipe acts as a tensile stress in the circumferential direction on the pipe over a long period of time. Therefore, in order to improve the long-term durability of polyethylene pipes, it is necessary to improve the environmental (tensile) stress cracking resistance.
For plastic pipe applications, circumferential (hoop) stress performance as set forth in ISO 9080 and ISO 1167 is an important requirement. These procedures describe the long-term creep rupture behavior of plastic materials by an extrapolation methodology wherein the hydrostatic strength of pipe materials over 50 years at 20° C. are predicted. Typically, for long term predictive performance testing, candidate pipe materials are placed at various stresses and the lifetime at a given temperature is determined. For extrapolations to 50 years at 20° C., testing is also performed at higher temperatures. The measured lifetime curves at each temperature typically consists of either a high stress, lower lifetime ductile failure mode or a lower stress, longer lifetime brittle failure mode. The ductile failure mode is referred to as Stage I failure and conversely the brittle failure mode is referred to as Stage II failure.
First and second generation polyethylene pipes for water and gas distribution have minimum required strength (MRS) ratings for respective hoop stresses of 6.3 and 8 MPa and are known as PE63 and PE80, respectively. Third generation polyethylene pipes, which are known as PE100 pipes, conform to a MRS rating of 10. The MRS rating is based on the above ISO procedures wherein a MRS rating of 10 specifies that pipes made from the polyethylene materials must withstand 10 MPa at 20° C. for 50 years.
Another important pipe or durable material performance requirement is resistance to rapid crack propagation (RCP). The RCP of a pipe material is typically measured by testing extruded pipe in accordance with ISO 13477 (the so-called ‘S4’ test). But the S4 test is not susceptible to small scale evaluation and as such various small scale tests have been introduced in the plastic pipe industry. Small scale testing includes the inverted Charpy test and the Plane High-Speed Double Torsion test, as well as ranking tests such as a critical strain energy release rate test or G
c
measurement on compression molded materials. Also, the lower the ductile to brittle transition temperature, T
db
, of a material, the better is its RCP resistance.
In order to improve the environmental stress cracking resistance of a polyethylene composition, it is known to increase the molecular weight or to decrease the density of the polyethylene. However, when the molecular weight is increased, the fluidity of the polyethylene is lowered, so that the molding properties such as pipe-extrusion properties and injection moldability are impaired. When the density is decreased, the rigidity of the polyethylene is unfavorably lowered.
Although numerous pipe compositions have been known and used, there continues to exist a need for improved durable materials, especially for transmission and distribution pipe service for gases and water. Preferably, the materials should exhibit improved durability and/or higher temperature service lives. In particular, there is still a need for high density polyethylene durable materials with better resistance to slow crack propagation and/or rapid crack propagation.
SUMMARY OF THE INVENTION
We have discovered a bimodal high density polyethylene composition that exhibits improved durability. The new composition comprises at least a low-molecular-weight (LMW) ethylene homopolymer component having a molecular weight distribution, MWD
L
, of less than about 8 and a homogeneous, high-molecular-weight (HMW) ethylene interpolymer component. The composition is characterized as having a bimodal molecular weight distribution and a ductile-brittle transition temperature, T
db
, of less than −20° C. Preferably, the overall M
w
/M
n
(indicative of the molecular weight distribution or MWD) of the novel composition is relatively narrow, and the M
w
/M
n
of the LMW component is relatively narrow, or the MWD for both the LMW component and the HMW component is also relatively narrow, or the MWD of the each component is relatively narrow and completely distinct from one another. In some embodiments, the HMW component is characterized by a “reverse comonomer distribution.”
REFERENCES:
patent: 3456044 (1969-07-01), Pahlke
patent: 3485706 (1969-12-01), Evans
patent: 3645992 (1972-02-01), Elston
patent: 4322027 (1982-03-01), Reba
patent: 4340563 (1982-07-01), Appel et al.
patent: 4352849 (1982-10-01), Mueller
patent: 4413110 (1983-11-01), Kavesh et al.
patent: 4597920 (1986-07-01), Golike
patent: 4663220 (1987-05-01), Wisneski et al.
patent: 4668566 (1987-05-01), Braun
patent: 4798081 (1989-01-01), Hazlitt et al.
patent: 4820557 (1989-04-01), Warren
patent: 4837084 (1989-06-01), Warren
patent: 4865902 (1989-09-01), Golike et al.
patent: 4927708 (1990-05-01), Herran et al.
patent: 4937299 (1990-06-01), Ewen et al.
patent: 4952451 (1990-08-01), Mueller
patent: 4963419 (1990-10-01), Lustig et al.
patent: 5008204 (1991-04-01), Stehling
patent: 5059481 (1991-10-01), Lustig et al.
patent: 5089321 (1992-02-01), Chum et al.
patent: 5206075 (1993-04-01), Hodgson, Jr.
patent: 5218071 (1993-06-01), Tsutsui et al.
patent: 5241031 (1993-08-01), Mehta
patent: 5252017 (1993-10-01), Hodel
patent: 5272236 (1993-12-01), Lai et al.
patent: 5278272 (1994-01-01), Lai et al.
patent: 5290498 (1994-03-01), Shiraki et al.
patent: 5292845 (1994-03-01), Kawasaki et al.
patent: 5417561 (1995-05-01), Shiraki et al.
patent: 5665800 (1997-09-01), Lai et al.
patent: 5683767 (1997-11-01), Shiraki et al.
patent: 5834393 (1998-11-01), Jacobsen et al.
patent: 5908679 (1999-06-01), Berthold et al.
patent: 6043180 (2000-03-01), Jacobsen et al.
patent: 6054544 (2000-04-01), Finlayson et al.
patent: 6114483 (2000-09-01), Coughlin et al.
patent: 6194520 (2001-02-01), Cheruvu et al.
patent: 6218472 (2001-04-01), Debras et al.
patent: 6221982 (2001-04-01), Debras et al.
patent: 6346575 (2002-02-01), Debras et al.
patent: 6441096 (2002-08-01), Backman et al.
patent: 6545093 (2003-04-01), de Lange et al.
patent: 2002/0045711 (2002-04-01), Backman et al.
patent: WO 00/22040 (2000-04-01), None
U.S. patent ap
Dixit Ravi S.
Gemoets Frederick E. L.
Jivraj Noorallah
Nicasy Ruddy
Schouterden Patrick J.
Dow Global Technologies Inc.
Nutter Nathan M.
LandOfFree
Bimodal polyethylene composition and articles made therefrom does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Bimodal polyethylene composition and articles made therefrom, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Bimodal polyethylene composition and articles made therefrom will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3255944