Method of producing inflation film and said film

Details Method of producing inflation film and said film Method of producing inflation film and said film

1998-10-19

2001-08-14

Wu, David W. (Department: 1713)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Polymers from only ethylenic monomers or processes of...

C526S160000, C526S943000, C525S240000, C428S516000

Reexamination Certificate

active

06274691

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of producing an inflation film, which comprises processing a specific ethylene polymer or a resin composition containing said ethylene polymer at low temperature, an inflation film, and a multi-layer inflation film.
2. Description of the Related Art
Ethylene polymers have widely been used as a packaging material. To these ethylene polymers, for example, mechanical properties such as rigidity, impact strength and the like are required. As a method of improving the strength of a packaging film obtained by an inflation method, for example, a technique of processing at the temperature lower than that of a normal inflation processing by using a specific die gap is disclosed (Japanese Patent Kokoku Publication Nos. 6-9863 and 6-9864).
However, this technique is not preferred because processing at low melting temperature accompanies an increase in viscosity of the resin and an increase in loading of a processing machine and electricity consumption arises. To obtain good processability by lowering the melt density of the resin, there is disclosed that the molecular weight of the resin is lowered to improve the fluidity. In case of a High-pressure low-density polyethylene and a straight-chain low-density polyethylene, disclosed in the publications, the strength of the resulting film is drastically lowered by a decrease in molecular weight. Therefore, it is hardly to say that the mechanical strength reaches a satisfactory level.
It has recently been known that an ethylene polymer wherein the weight proportion of the cold xylene-soluble portion is small, for example, an ethylene polymer, which is produced by using a catalyst containing a transition metal compound having a group with a cyclopentadiene type anion skeleton, i.e. metallocene polymerization catalyst, exhibits excellent mechanical properties. However, such an ethylene polymer produced by using the metallocene polymerization catalyst has a problem such as fish eye at the time of film molding due to poor thermal stability because a torque at the time of processing is high to easily cause shear and heat generation.
On the other hand, high quality such as low odor, low taste or the like has been required to a food packaging material in which the ethylene polymer is widely used.
However, it is hardly to say that a film obtained by molding the conventional ethylene polymer according to a conventional processing method has odor and taste in a satisfactory level. It is generally considered that taste transferred to foods and odor of the packaging film produced by using the ethylene polymer are generated by oxidation of the molten ethylene polymer in an air and thermally decomposition of said polymer when the film is molded by the inflation molding method. One reason of odor and deterioration of taste includes additives to be added to the ethylene polymer, such as antioxidants, lubricants and the like. These additives were added for the purpose of preventing fish eye and blocking of the film, which are caused in the film at the time of processing. In a conventional technique, the addition of these additives was essential.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an inflation film, which is superior in low odor, low taste transfer and mechanical properties, and a method of producing the same.
The present inventors have studied intensively about the method of producing an inflation film, which comprises processing an ethylene polymer or a resin composition containing the ethylene polymer. As a result, they have found that the object of the present invention is accomplished by processing a specific ethylene polymer or a resin composition containing said ethylene polymer in a die gap at a melting temperature lower than 170° C. Thus, the present invention has been accomplished.
That is, the present invention provides a method of producing an inflation film, which comprises processing an ethylene polymer or a resin composition containing said ethylene polymer in a die gap at a melting temperature of not more than 170° C., wherein said ethylene polymer is produced by an ionic polymerization method and said ethylene polymer has a density of 0.89 g/cm
3
to 0.96 g/cm
3
, a melting viscosity &eegr;*[170] at 170° C. under a shear rate of 100 rad/sec of 0.001×10
4
to 3.5×10
4
poise and a weight proportion a (% by weight) of a cold xylene-soluble portion satisfying the following formula (1):
a<
4.8×10
4
×(0.95−
d
)
3
+10
6
×(0.95−
d
)
4
+0.5  formula (1)
(wherein a is a weight proportion (% by weight) of a cold xylene-soluble portion of the ethylene polymer and d is a density (g/cm
3
) of an ethylene polymer).
The present invention also provides an inflation film obtained by the above method.
The present invention also provides a multi-layer inflation film comprising at least one layer of the inflation film obtained by the above method.
The present invention will be described in detail below.
PREFERRED EMBODIMENTS OF THE INVENTION
The ethylene polymer used in the present invention is an ethylene polymer produced by an ionic polymerization method, wherein said ethylene polymer has a density of 0.89 to 0.96 g/cm
3
, a melting viscosity &eegr;*[170] at 170° C. under a shear rate of 100 rad/sec of 0.001×10
4
to 3.5×10
4
poise and a weight proportion a (% by weight) of a cold xylene-soluble portion satisfying the following formula (1):
a<
4.8×10
4
×(0.95−
d
)
3
+10
6
×(0.95−
d
)
4
+0.5  formula (1)
(wherein a is a weight proportion (% by weight) of a cold xylene-soluble portion of the ethylene polymer and d is a density (g/cm
3
) of an ethylene polymer).
The ethylene polymer used in the present invention is a homopolymer of ethylene or a copolymer of ethylene and at least one &agr;-olefin having 3 to 20 carbon atoms or diolefin having 4 to 20 carbon atoms, which is produced by an ionic polymerization method. Examples of the copolymer of ethylene include ethylene-&agr;-olefin copolymer, ethylene-diolefin copolymer and ethylene-&agr;-olefin-dioelfin copolymer.
Examples of the copolymerization component used in the present invention include propylene, butene-1, pentene-1, hexene-1, heptene-1, octene-1, nonene-1, decene-1, hexadecene-1, eicosene-1, 4-methyl-pentene-1, 4-methyl-hexene-1, vinylcyclohexane, vinylcyclohexene, styrene, norbornene, butadiene, 1,5-hexadiene, isoprene and the like.
The content of the copolymerization component is preferably from 0.05 to 50% by weight.
The ionic polymerization method in the present invention is a so-called polymerization method including coordination polymerization and cationic polymerization, and radical polymerization is excluded.
Accordingly, regarding the ethylene polymer in the invention, a low-density polyethylene (High-pressure LDPE) and an ethylene-vinyl acetate copolymer produced by a high-pressure radical polymerization method are excluded. The High-pressure LDPE produced by the high-pressure radical polymerization method is not preferred because the strength is low even if the production method of the present invention is applied.
As the ionic polymerization method, coordination polymerization is preferred. Examples of the metal component contained in the catalyst used in the coordination polymerization include zirconium, hafnium, vanadium, chrome, nickel, palladium or transition metals of lanthanum series.
The ethylene polymer used in the present invention is preferably an ethylene polymer produced in the presence of a catalyst containing said transition metal compound, particularly a catalyst containing a transition metal having a group with a cyclopentadiene type anion skeleton.
Said transition metal compound is a so-called metallocene compound and is normally represented by the general formula ML
i
X
n-i
(wherein M is a transition metal atom of Group IV or lanthanum series of the Periodic Tables of the elements; L is a gr

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