Plastic and nonmetallic article shaping or treating: processes – Direct application of fluid pressure differential to... – Including application of internal fluid pressure to hollow...
Reexamination Certificate
2000-06-14
2003-02-25
McDowell, Suzanne E. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Direct application of fluid pressure differential to...
Including application of internal fluid pressure to hollow...
Reexamination Certificate
active
06524520
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method for manufacturing a hollow molded article having a layer of ethylene/&agr;-olefin copolymer and containers obtained by such method. More specifically, it relates to a method for manufacturing a hollow molded article having improved extrudability and mechanical strength and containers obtained by such method.
2. Description of the Background
Containers manufactured by the blow-molding method are used for various applications. For example, such containers comprising a single layer are used as food, kerosene and other containers; such containers comprising multiple layers with an ethylene/vinyl alcohol copolymer or polyamide resin are used as soft blow-molded containers such as mayonnaise and ketchup containers to keep the flavor of food; such containers in the form of blow-molded tubes are used as wasabi Japanese horseradish paste or mustard paste containers; and such containers in combination with corrugated board boxes, steel cans, etc. are used as back-in-boxes, drum-inner-bags, etc. Polyethylene is widely used for the raw material of these containers, and particularly a linear low density polyethylene (L-LDPE) is mainly used in the areas requiring heat resistance and mechanical strength.
However, the linear low density polyethylene tends to cause its parison to suffer a drawdown at the time of extrusion because of its low melt tension. On account of this, the linear low density polyethylene is not only difficult to mold but also hollow molded articles manufactured therefrom show a deviation in their wall thickness, thereby causing a decline in their mechanical strength. If a formulation in which a high-pressure low density polyethylene is blended to the linear low density polyethylene is adopted to improve such problem, the melt tension will increase, but the strength, for example, drop impact strength of a hollow molded article thereof may decline in some cases because the high-pressure low density polyethylene itself does not always have high mechanical strength.
Further, conventional linear low density polyethylene resins, having been manufactured by use of a titanium polymerization catalyst, have a broad molecular weight distribution and consequently contain a low molecular weight matter intrinsically. Because of this, there is a concern that such low molecular weight matter may be dissolved into the contents of hollow molded articles made of such conventional linear low density polyethylene resins. In this respect, new polyethylene resins manufactured by use of a metallocene polymerization catalyst have the characteristic of having a relatively narrow molecular weight distribution and composition or component distribution, and there is little concern that such polyethylene resins will cause the problem of any low molecular weight matter dissolving into the contents of hollow molded articles made thereof. However, it is said that with even such polyethylene resin yet having low melt tension, the manufacture of hollow molded articles is difficult similarly.
SUMMARY OF THE INVENTION
Accordingly it is an object of the invention to provide a method for manufacturing a hollow molded article showing an improved drawdown characteristic of a parison.
Another object of the invention is to provide a method for manufacturing a hollow molded article showing high mechanical strength and a small amount of extraction of its material into the contents of the hollow molded article.
Further object of the invention is to provide a container obtained by the method.
Other and further objects, features and advantages of the invention will appear more fully from the following description.
The present invention is related to a method for manufacturing a hollow molded article, in which at a time of blow-molding a parison having at least one ethylene/&agr;-olefin copolymer layer by extruding it through an extruder die:
(1) the copolymer meets following physical properties (a) to (e):
(a) a density (d) is 0.880 to 0.970 (g/cm
3
);
(b) a melt flow rate (MFR) is 0.01 to 10 (g/10 minutes);
(c) a relationship between a melt tension (MT (g)) at 190° C. and the melt flow rate (MFR (g/10 minutes)) is:
MT>
2.2
×MFR
−0.84
;
(d) a relationship between a decane soluble matter content (W (wt %)) at 23° C. and the density (d) is:
W<
80×exp{−100(
d−
0.88)}+0.1;
and
(e) a relationship between a temperature (Tm (° C.)) at a position of the highest peak of an endothermic curve as measured by use of a differential scanning calorimeter (DSC) and the density (d) is:
Tm<
400×
d−
248,
and
(2) a resin pressure P (kg/cm
2
) at a time of extruding the parison has following relations with a resin temperature T (° C.), an extrusion rate K (kg/hour), an extruder die lip cross-section area A (cm
2
) and the melt flow rate MFR (g/10 minutes) of the resin.
150
33
{K
/(
MFR
)
1.5
/(
T−
120)/
A}
0.2
≦P≦
500×
{k
/(
MFR
)
1.5
/(
T−
120)/
A}
0.2
.
The present invention is also related to a container manufactured by the method, and it may be either single-layer container or multi-layer container.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The hollow molded article manufacturing method of the invention relates to a method for manufacturing a hollow molded article having an ethylene/&agr;-olefin copolymer layer. Next is given a specific explanation of each constituent of the manufacturing method and containers obtained by the method.
Ethylene/&agr;-olefin Copolymer
The material of such hollow molded article is a random copolymer of an ethylene and an &agr;-olefin. As such &agr;-olefin, an &agr;-olefin having 3 to 20 carbon atoms is preferable. Specific examples of the &agr;-olefin include propylene, 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, 1-octene, 1-decene and 1-dodecene.
The ethylene content of the copolymer is normally 94 to 99 mol %, preferably 96 to 98 mol %, and the comonomer &agr;-olefin content is normally 1 to 6 mol %, preferably 2 to 4 mol %. The ethylene and &agr;-olefin contents can be measured by use of
13
C-NMR.
Specifically, the composition of the copolymer can be determined normally by measuring
13
C-NMR spectrum of a sample obtained by dissolving approximately 200 mg of the copolymer uniformly in 1 ml of hexachlorobutadiene in a test tube 10 mm&phgr; under the conditions of the test temperature of 120° C., test frequency of 25.05 MHz, spectrum width of 1500 Hz, pulse repeating time of 4.2 seconds and pulse width of 6 &mgr;sec.
The density of the copolymer is in the range of 0.880 to 0.970, preferably 0.890 to 0.940 (g/cm
3
). If the density is in this range, it is desirable that a hollow molded article having excellent low-temperature impact resistance, rigidity and chemical resistance may be obtained. Here, the density is measured at 23±0.1° C. in accordance with Method D of JIS K-7112.
Moreover, the value of the MFR of the copolymer as measured at 190° C. and under a load of 2.16 kg in accordance with ASTM D-1238 is 0.01 to 10, preferably 0.05 to 9 (g/10 minutes). It is preferable that the MFR should be in this range because a hollow molded article having a satisfactory appearance may be obtained at high molding speed.
In addition, it is desirable that the copolymer should show the following relationship between melt tension (MT (g)) at 190° C. and melt flow rate (MFR (g/10 minutes)):
MT>
2.2×
MFR
−084
,
preferably,
MT>
4.0
×MFR
−0.84
.
If the melt tension is in this range, the drawdown characteristic of the parison is improved, and the mechanical strength of the hollow molded article is increased.
Here, the melt tension is determined by measuring the stress of a melted polymer when it is stretched at a constant rate. Specifically, resin pellets are fed to a melt tension tester, available from Toyo Seiki Seisakusho, and the melt tension is measured under the conditions of resin temperature: 190° C., extrusion rate: 15 mm/min., take-up speed: 10 to 20 m/
Iwamasa Kenji
Tsuji Yoichiro
Birch & Stewart Kolasch & Birch, LLP
McDowell Suzanne E.
Mitsui Chemicals Inc.
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