Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
1996-06-21
2001-03-20
Michl, Paul R. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
active
06204349
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to pipes made of polyethylene resins, excellent in rigidity, impact resistance and long-term durability under stress (environmental stress cracking resistance, internal pressure creep resistance).
2. Background Art
Polyethylene pipes are light in weight, easy to handle, and are non-corrosive. In addition, their rigidity is so high that they can be laid under the ground, and their flexibility is also so high that they can follow a movement of ground. Thanks to these advantageous characteristics, the amount of polyethylene pipes used is rapidly increasing in recent years.
Polyethylene pipes are required to have the following properties:
(1) the above-described characteristics;
(2) impact resistance sufficient to endure impacts given at the time when and after they are set; and
(3) excellent long-term durability under gas or water pressure (specifically, environmental stress cracking resistance and internal pressure creep resistance). The conventional polyethylene pipes already posess the above properties (1) and (2). With respect to the long-term durability (3), the conventional pipes can 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 severer 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.
The environmental stress cracking resistance of the conventional polyethylene materials for pipes, as evaluated by the method described below, have been found to be at most 20 hours.
In order to improve the environmental stress cracking resistance of a polyethylene, 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.
An object of the present invention is to overcome the aforementioned drawbacks in the prior art and provide pipes made of polyethylene resins, which have improved environmental stress cracking resistance and thus have improved long-term durability with the rigidity and impact resistance maintained high and which can be produced with good moldability at high productivity.
SUMMARY OF THE INVENTION
It has now been found that the above object can be attained by the use of a specific linear polyethylene resin having specific physical properties.
Thus, the present invention provides a pipe made of a polyethylene resin which is a linear polyethylene having the following physical properties (1) to (4):
(1) a melt flow rate of 0.02 to 0.2 g/10 min,
(2) a flow ratio of 50 or more,
(3) a density of 0.945 to 0.960 g/cm
3
, and
(4) a relaxation parameter H, represented by the following equation (I), of 2.00×10
−8
dyn/cm
2
or less:
H
=
-
E
(
10
3
)
Log
E
(
10
3
)
-
Log
E
(
10
0
)
Log
10
3
-
Log
10
0
(
I
)
wherein E(&tgr;) represents a relaxation modulus at time &tgr;.
The polyethylene pipes of the present invention have high rigidity and impact resistance and enhanced long-term durability (environmental stress cracking resistance and internal pressure creep resistance).
DETAILED DESCRIPTION OF THE INVENTION
The linear polyethylene for use in the present invention is preferably produced by the use of a Ziegler-Natta catalyst. A catalyst system composed of (a) a solid catalytic component containing at least Mg, Ti and halogen, and (b) an organoaluminum compound is suitable as the Ziegler-Natta catalyst. Examples of such a catalyst system include those described in Japanese Laid-Open Patent Publications Nos. 119980/1974, 58189/1974, 142689/1975 and 61406/1981. These catalysts are advantageous with respect to polymerization activity and coloring over other catalysts such as titanium trichloride-alkyl aluminum catalysts and titanium tetrachloride-trialkoxy vanadyl-alkyl aluminum catalysts. When a catalyst other than the Ziegler-Natta catalysts is used for producing a linear polyethylene, it is difficult to control the copolymerizability of monomers used and the molecular weight distribution of the resulting polyethylene; the intended environmental stress cracking resistance can be obtained with difficulty. Among the above-described Ziegler-Natta catalysts which can be suitably used in the present invention, preferred one is a catalyst system as described in Japanese Laid-Open Patent Publication No. 61406/1981, etc., consisting of (a) a reaction product of an oxygen-containing organic Mg compound, an oxygen-containing organic Ti compound and an organoaluminum halide, and (b) an organoaluminum compound. This catalyst system has particularly high activity, can easily control the molecular weight distribution of the resulting polyethylene, and hardly produce fish eyes in the resulting polyethylene.
A compound represented by the following general formula:
Mg(OR
1
)
m
X
1
2 −m
wherein R
1
represents an alkyl, aryl or cycloalkyl group, X
1
represents a halogen atom, and m is 1 or 2, may be used as the oxygen-containing organic Mg compound for preparing the reaction product. Specific examples of such a compound include magnesium diethoxide, magnesium dimethoxide, magnesium diphenoxide, magnesium monoethoxychloride, magnesium monophenoxychloride, magnesium monoethoxybromide and magnesium monoethoxyiodide. Of these, magnesium diethoxide is particularly preferred.
A compound represented by the following general formula:
Ti(OR
2
)
n
X
2
4−n
wherein R
2
represents an alkyl, aryl or cycloalkyl group, X
2
represents a halogen atom, and n is 1 to 4, may be used as the oxygen-containing organic Ti compound. Specific examples of such a compound include tetraethoxytitanium, tetra-n-butoxytitanium, diethoxydichlorotitanium, di-n-butoxydichlorotitanium, triethoxymonochlorotitanium, tri-n-butoxymonochlorotitanium, ethoxytrichlorotitanium, n-butoxytrichlorotitanium and methoxytribromotitanium. Of these, tri-n-butoxymonochlorotitanium is particularly preferred.
A compound represented by the following general formula:
AlR
3
p
X
3
3−p
wherein R
3
represents an alkyl, aryl or cycloalkyl group, X
3
represents a halogen atom, and p is a number in the range of 0<p<3,may be used as the organoaluminum halide. Specific examples of such a compound include ethyl aluminum dichloride, ethyl aluminum sesquichloride, diethyl aluminum monochloride and n-propyl aluminum dichloride. Of these, ethyl aluminum sesquichloride is particularly preferred.
The above-described compounds are reacted in the following manner:
First of all, an oxygen-containing organic Mg compound and an oxygen-containing organic Ti compound are mixed with each other, and the mixture is heated to 100-160° C. to obtain a homogeneous liquid. When the mixture cannot be readily made into a homogenous liquid, it is preferable to allow an alcohol to exist in the mixture. Examples of the alcohol include ethyl alcohol, n-butyl alcohol and n-octyl alcohol.
Subsequently, an inert hydrocarbon solvent is added to the liquid to obtain an inert hydrocarbon solution. To this inert hydrocarbon s
Kashiwagi Yasuhiro
Mizuno Yasuhisa
Shinohara Yoshinao
Suzaki Miki
Tanaka Eiji
Michl Paul R.
Mitsubishi Chemical Corporation
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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