Polyethylene resin, and pipes and pipe joints using same

Details Polyethylene resin, and pipes and pipe joints using same Polyethylene resin, and pipes and pipe joints using same

1997-08-06

2001-02-20

Mullis, Jeffrey C. (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...

C525S319000, C525S324000

Reexamination Certificate

active

06191227

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a polyethylene resin suitable for both extrusion and injection molding and also suitable for providing pipes superior in long-term life, as well as pipes and pipe joints formed using the said resin.
Pipes formed by molding of polyethylene resins are widely in practical use. Recently, such pipes have also come to be used as water pipes and gas pipes. Since these pipes are laid under the ground and are used over an extremely long period after laying, they are required to have a high reliability such that deformation or breakage does not occur over such a long period. This high reliability is represented in terms of, for example, creep resistance which is represented by the time required until breaking of a material applied with a static load and fatigue length which is represented by the time until breaking of a material applied with a load periodically.
Further, since the pipes in question are laid while connecting one to another, it is necessary to use pipe joints. The pipe joints are formed by injection molding, so the resin used for the pipe joints is required to have not only a high fluidity but also a poor creep resistance and a high fatigue life.
Generally, the fluidity of a polyethylene resin can be improved by decreasing its average molecular weight, but a lower average molecular weight has a bad influence on the attainment of a long life.
For obtaining a resin which meets the requirements for pipes, there has been proposed, for example, a method in which ethylene polymers of different molecular weights are mixed together, for example, by two-stage polymerization, melt blending, or dry blending, to broader the molecular weight distribution. For example, however, the resin proposed in Japanese Patent Publication JP63-67811B is inferior in point of service life although it has high rigidity, and the resin proposed in Japanese Patent Laid Open JP8-134285A has high viscosity and thus poor in fluidity at molten state, so is unsatisfactory in point of moldability. Thus, a resin capable of fully satisfying both physical properties and moldability has heretofore been not obtained.
It is an object of the present invention to provide a polyethylene resin superior in all of creep resistance, fatigue life and fluidity and capable of being subjected to injection molding.
It is another object of the invention to provide pipes having the above-mentioned characteristics and pipe joints for use in combination with said pipes.
SUMMARY OF THE INVENTION
Having made intensive studies for achieving the above-mentioned objects, the inventors of the present invention found out that a polyethylene resin whose melt flow rates at different loads satisfy predetermined values and whose dynamic melt viscosity and frequency dispersion satisfy a predetermined relation is superior not only in extrudability and injection-moldability but also in creep resistance and fatigue life and exhibits excellent effects when used in molding for pipes and pipe joints, and as a result, the present invention has been completed.
Firstly, the-present invention resides in a polyethylene resin having a density of 0.915 to 0.955 g·cm
−3
, a melt flow rate of lower than 0.20 dg·min
−1
as measured at 190° C. under a load of 2.16 kgf, a melt flow rate of 17.0 to 70.0 dg·min
−1
as measured at 190° C. under a load of 21.6 kgf, a zero shear viscosity (&eegr;
0
) of 200,000 to 2,000,000 Pa·s, a characteristic time constant (&tgr;
0
) of 50 to 500 s, and a ratio (&tgr;
0
/&eegr;
0
) in the range from 1.0×10
−4
to 4.0×10
−4
, said zero shear viscosity (&eegr;
0
) being obtained by approximating a dynamic melt viscosity (&eegr;*, unit: Pa·s) with equation [1], said dynamic melt viscosity (&eegr;*) being obtained by measurement using a rheometer at 190° C., at a parallel plate spacing of 1.5 mm, at a distortion of 10% to 15% and in the frequency (&ohgr;) range of 100 to 0.01 rad·s
−1
:
η
*
=
η
0
1
+
(
τ
0
·
ω
)
n
[
1
]
Secondly, the present invention resides in a pipe obtained by extrusion of the aforesaid polyethylene resin.
Thirdly, the present invention resides in a pipe joint obtained by injection of the aforesaid polyethylene resin.
Fourthly, the present invention resides in a connected pipe obtained by connecting such extruded pipes as mentioned above with such injection-molded pipe joints as mentioned above.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The polyethylene resin of the present invention has a density in the range of 0.915 to 0.955 g·cm
−3
, preferably 0.935 to 0.955 g·cm
−3
. If the density is lower than 0.915 g·cm
−3
, the resin is too soft for use as a pipe, and a density exceeding 0.955 g·cm
−3
will result in poor creep characteristics and fatigue life.
The polyethylene resin of the present invention is required to have a melt flow rate (hereinafter referred to simply as MFR
2.16
) of lower than 0.20 dg·min
−1
, preferably in the range of 0.02 to 0.20 dg·min
−1
, as measured at 190° C. under a load of 2.16 kgf. If this melt flow rate exceeds 0.20 dg·min
−1
, an insufficient creep resistance will result.
It is also required that the polyethylene resin of the present invention have a melt flow rate (hereinafter referred to as MFR
21.6
) of 17.0 to 70.0 dg·min
−1
, preferably 17.0 to 30.0 dg·min
−1
, as measured at 190° C. under a load of 21.6 kgf. If this melt flow rate is less than 17.0 dg·min
−1
, it is likely that the extrusion of a pipe will become difficult of the productivity will be markedly deteriorated. There also is a fear that the injection molding for a pipe joint may become difficult or the pipe joint after molding may be deformed.
In the polyethylene resin of the present invention, moreover, it is necessary that a dynamic melt viscosity (&eegr;*, unit: Pa·s) and the frequency (&ohgr;, unit; s
−1
) satisfy a predetermined relation, the dynamic melt viscosity being obtained by measurement at 190° C., at a parallel plate spacing of 1.5 mm, at a distortion of 10% to 15%, and in the frequency (&ohgr;) range of 100 to 0.01 s
−1
.
More specifically, when the dynamic melt viscosity and the frequency are fully approximated with the foregoing equation [1], the zero shear viscosity &eegr;
0
is in the range of 200,000 to 2,000,000 Pa·s, preferably 350,000 to 1,000,000 Pa·s, the characteristic time constant (&tgr;
0
) is in the range of 50 to 500 s, preferably 100 to 300 s, and the ratio (&tgr;
0
/&eegr;
0
) is in the range of 1.0×10
−4
to 4.0×10
−4
Pa
−1
.
The &eegr;
0
and &tgr;
0
are obtained on the basis of readings on a rheometer using parallel plates. More specifically, a dynamic melt viscosity (&eegr;*) is measured under the conditions of 190° C., a plate spacing of 1.5 mm, a distortion of 10% to 15% and a frequency range of 100 to 0.01 (unit; rad·s
−1
), and the data obtained is approximated with equation [1]. This approximation affords the zero shear viscosity (&eegr;
0
), characteristic time constant (&tgr;
0
) and parameter (n). Regressive approximation to equation [1] can be calculated by using a suitable computer program of regression available commercially.
The &tgr;
0
is a parameter indicative of a relaxation time and n is a parameter indicative of a shear rate dependence in a high shear rate region.
The equation [1] is an empirical formula generally called “Equation of Cross” and is outlined, for example, in Glenn V. Gordon, Montgomery T. Shaw, “Computer Programs for Rheologists”, Hanser Publishers.
The relation between the dynamic melt viscosity at 190° C. and the frequency can be obtained by using a commercially available device, e.g. RMS-800 rheometer (a product of Rheometrics Co.).
The parameter n in the equation represents stands for a shear rate dependence of melt viscosity in a high shear rate region.
By the phrase “fully approximated” as referred to herein is meant to approximate so that the

Affiliated with

Also associated with

Rating

Polyethylene resin, and pipes and pipe joints using same does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Polyethylene resin, and pipes and pipe joints using same, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Polyethylene resin, and pipes and pipe joints using same will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2611932