JP2004291489A - Method for kneading and granulating polyethylene for film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a polyethylene which is excellent in molding stability and uniformity of thickness of a film when high speed molding is performed in inflation molding, and is excellent in a balance among those properties and film strength, and to provide an inflation film of the polyethylene. <P>SOLUTION: A method for manufacturing a polyethylene particle is characterized by that the polyethylene is brought into contact with a mixed gas containing 0.1-21 volume% oxygen while it is agitated in a mixer provided with an agitating device, and the mixture is fed into an extruder to perform kneading and granulation. The polyethylene is characterized by that it is a polyethylene resin composition comprising the polyethylene having (i) a density of 0.940-0.970 g/cm<SP>3</SP>, (ii) a melt flow rate (MFR) of 0.01-0.5 g/10 min, and (iii) a molecular weight distribution (Mw/Mn) measured by GPC of 15-80, and its fluidity index (FI) and melt flow rate (MFR) has a relation of FI≥1,500×MFR, and it is extrusion-molded by the above described method. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【００２６】
【発明の効果】
本発明によれば、従来の高密度ポリエチレンフィルムよりも、厚みが均一でバラつきが小さく、しかもフィルム強度とのバランスに優れた、ポリエチレンフィルムを得ることができる。従って、レジ袋、規格袋、ファッションバッグ、小物包装袋、各種部品包装用、建材用等への使用に際して、薄肉化が可能である。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for producing a polyolefin for inflation molding, particularly to a polyethylene for inflation molding, a method for producing the same, and an inflation film. More specifically, the present invention relates to a method for producing polyethylene, which is excellent in molding stability and uniformity of film thickness during high-speed molding in inflation molding and excellent in balance with film strength, and relates to a polyethylene and an inflation film.
[0002]
[Prior art]
There are various methods for producing a polyethylene film, and in particular, an inflation molding method is widely used. In general, a spiral die is used in the inflation molding method.However, in order to improve productivity, when high-speed molding is performed, the flow rate of the polyethylene melted at the die exit due to the effect of the spiral causes a difference in the produced film. The thickness becomes uneven. This phenomenon is prominent in thin films. If the film thickness is non-uniform, defective bag making, poor printing and trouble in the transport process occur. High-temperature molding and narrowing of the die lip interval as a measure to make the wall thickness uneven from the molding processing surface have little effect, and if a rotary die is adopted, the cost increases due to equipment enhancement.
[0003]
On the other hand, in order to improve the fluidity of the molten polyethylene composition, a wide molecular weight distribution can be considered as an improvement measure from the raw material side. However, in this method, a gel derived from an ultrahigh molecular weight component is easily generated. A method of crosslinking polyethylene with oxygen or a radical generator has also been proposed. As a method for cross-linking polyethylene, JP-A-8-208988 describes a method in which a radical generator is added and kneading and granulation is performed. However, excessive dispersion reaction partially occurs due to poor dispersion of the radical generator. In addition, problems such as generation of fish eyes and gel and instability of the extrusion amount occur. Also, a granulation method in which an oxygen mixed gas is directly injected into a hopper or an extruder of an extruder at the time of kneading and granulating (JP-A-8-90633, JP-A-11-71427, and JP-A-11-254506) is disclosed. However, in these methods, since the oxygen-mixed gas is injected while the polyethylene before granulation is stationary, the contact with the gas is insufficient, the effect of uniformizing the film thickness is small, and fisheye or gel is not used. Is more likely to occur.
[0004]
[Patent Document 1]
JP-A-8-90633 [Patent Document 2]
JP-A-11-71427 [Patent Document 3]
JP-A-11-254506
[Problems to be solved by the present invention]
SUMMARY OF THE INVENTION An object of the present invention is to provide a method for producing polyethylene and a polyethylene and an inflation film capable of further uniformizing the film thickness while maintaining a balance with the film strength.
[0006]
[Means for solving the problem]
As a result of intensive studies, the present inventors have found that the above-mentioned problems can be solved by bringing the oxygen mixed gas and the polyethylene into sufficient contact while stirring with a mixing mixer before kneading and granulating with an extruder. That is, the present invention is characterized in that, in a mixer equipped with a stirrer, the polyethylene is brought into contact with a mixed gas containing 0.1 to 21% by volume of oxygen while stirring the polyethylene, supplied to an extruder and kneaded and granulated. This is a method for producing particles.
Also, the polyethylene resin composition extruded by this method,
(I) a density of 0.940 to 0.970 g / cm ³ ,
(Ii) a melt flow rate (MFR) of 0.01 to 0.5 g / 10 minutes,
(Iii) The molecular weight distribution represented by Mw / Mn (Mw = weight average molecular weight, Mn = number average molecular weight) is 15 to 80, and its fluidity index (FI: [1 / sec]) and melt flow The rate (MFR: [g / 10 min]) is FI ≧ 1500 × MFR
It is desirable to have the relationship of
[0007]
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The method for kneading and granulating polyethylene according to the present invention, the polyethylene extruded by the granulating method and the film made of the polyethylene will be specifically described below.
[0008]
Granulation method of polyethylene
Before kneading and granulating polyethylene, which is usually in a powder state, with an extruder, an oxygen mixed gas adjusted to an oxygen concentration of 0.1 to 21% by volume was injected into a mixing mixer and sufficiently contacted with the oxygen mixed gas while stirring. The powder is supplied to an extruder and kneaded and granulated. The mixing mixer may be dedicated to the oxygen mixed gas, but is desirably used also as a mixer for mixing various additives, from the viewpoint of suppressing the enlargement of the equipment and the cost of adding.
[0009]
This mixed gas is a mixed gas of an inert gas and oxygen, and a gas mixture of nitrogen and air is particularly preferable. The oxygen concentration of the mixed gas is 0.1 to 21% by volume, preferably 0.3 to 8% by volume.
The powder of polyethylene mixed with various additives is sent to a hopper of an extruder by a mixing mixer, and is kneaded and granulated by the extruder.
[0010]
Density polyethylene according to the polyethylene <br/> present invention is in the range of 0.940 to 0.970 g / cm ^3, it is particularly preferably from 0.945~0.965g / cm ^3. When the density of polyethylene is within this range, a polyethylene film having appropriate rigidity can be obtained.
The melt flow rate (MFR) of this polyethylene is in the range of 0.01 to 0.5 g / 10 minutes, particularly preferably in the range of 0.01 to 0.1 g / 10 minutes. When the melt flow rate (MFR) of the polyethylene composition is in this range, film formation is easy, and a polyethylene film having excellent impact strength and tear strength can be obtained.
[0011]
The molecular weight distribution (Mw / Mn: Mw = weight average molecular weight, Mn = number average molecular weight) of the polyethylene measured by GPC is preferably from 15 to 80, particularly preferably from 20 to 70. When the molecular weight distribution (Mw / Mn) of the polyethylene measured by GPC is within this range, the film can be easily formed and a polyethylene film having excellent impact strength can be obtained. Further, the ethylene-based resin composition according to the present invention satisfies the relationship between the fluidity index (FI: 1 / sec) and the melt flow rate (MFR: g / 10 minutes), where FI ≧ 1500 × MFR. In particular, it is preferable that the relationship represented by FI ≧ 2000 × MFR is satisfied. The fluidity index (FI: 1 / sec) is defined as the shear rate at which the sample is extruded from the capillary at 190 ° C. and the shear stress reaches 2.4 × 10 ⁶ dyne / cm ² . In the present invention, a capillary flow tester manufactured by Toyo Seiki Seisaku-sho, Ltd. was used, and the resin temperature was measured at 190 ° C. and the shear stress range was about 5 × 10 ^{4 to} 3 × 10 ⁶ dyne / cm ^2. Is 2.4 × 10 ⁶ dyne / cm ² . The measurement is performed by changing the nozzle diameter as follows according to the melt flow rate (MFR) of the sample to be measured.
1.0mm when 10 ≧ MFR> 3
2.0mm when 3 ≧ MFR> 0.8
When 0.8 ≧ MFR 3.0 mm
[0012]
The polyethylene used in the present invention may be a high-density polyethylene having the above-mentioned density, melt flow rate and molecular weight distribution measured by GPC, and FI. Not only ethylene homopolymer, but also ethylene and a small amount of α- An olefin, for example, α-olefin such as propylene, 1-butene, 3-methyl-1-butene, 1-hexene, 3-methyl-1-pentene, 4-methyl-1-pentene or the like of 3 mol% or less is copolymerized. Also, an ethylene / α-olefin copolymer can be used.
[0013]
Polyethylene film The polyethylene film according to the present invention is desirably produced from the above-mentioned polyethylene by an inflation molding method.
Film formation by the inflation molding method is performed by extruding the above-described polyethylene through an annular slit die and expanding the same by a predetermined airflow.
The resin temperature at the time of extruding polyethylene is preferably set to 180 to 250 ° C. The height of the whitening point from the die surface is desirably 8 to 15 times the die diameter. The expansion ratio is preferably 1.5 to 6 times. Further, it is desirable that the polyethylene film according to the present invention has a thickness of 5 to 60 μm, preferably 6 to 50 μm.
[0014]
【Example】
Hereinafter, the present invention will be described with reference to examples, but the present invention is not limited to these examples.
The thickness, impact strength, Elmendorf tear strength and fisheye judgment of the polyethylene films in Examples and Comparative Examples were determined according to the following test methods.
(1) Film thickness The film thickness was measured at intervals of 15 mm using a film thickness continuous measurement device K-306A and K-310C manufactured by Anritsu Corporation, and the variation was obtained by calculation. That is, the average value and the standard deviation value of the film thickness were measured with these devices, and (2 × standard deviation value) / (average value of the film thickness) was defined as the variation of the film thickness.
[0015]
(2) Dart Impact Strength Dart impact strength was determined by performing an impact test according to JIS K 7124.
[0016]
(3) Elmendorf tear strength Elmendorf tear strength was determined by conducting a tear test according to JIS K 7128.
The components used in Examples and Comparative Examples are as follows.
[0017]
(4) A sample of 20 cm × 1 m was sampled from a 30 μm film formed by a fish eye inflation molding method, and the number of fish eyes large (0.2 mm <) and small (0.2 mm ≧) was measured by a fish eye counter SD500 manufactured by Kowa Co., Ltd. Was counted and evaluated in three steps according to the following equation.
N = (large + (small / 10)) / film weight [pieces / g]
○… N ≦ 3
△… 3 <N ≦ 30
× ... N <30
[0018]
(5) Molecular weight distribution The molecular weight distribution (Mw / Mn) was measured as follows using GPC-150C manufactured by Millipore. The separation column was TSK GNH HT, the column size was 72 mm in diameter and 600 mm in length, the column temperature was 140 ° C., and the mobile phase was o-dichlorobenzene [manufactured by Wako Pure Chemical Industries, Ltd.] and antioxidant. 0.025% by weight of BHT [manufactured by Takeda Pharmaceutical Co., Ltd.] was used as an agent, moved at 1.0 ml / min, the sample concentration was set to 0.1% by weight, the sample injection amount was set to 500 μl, and the differential detector was used. A refractometer was used. For standard polystyrene, molecular weights of Mw <1000 and Mw> 4 × 10 ⁶ were manufactured by Tosoh Corporation, and those of 1000 <Mw <4 × 10 ⁶ were manufactured by Pressure Chemical Company.
[0019]
Embodiment 1
[polyethylene]
A high-density polyethylene powder having a density of 0.952 g / cm ³ , an MFR of 0.04 g / 10 min, an Mw / Mn of 33, and an FI of 155 s ^-1 is provided with an additive mixing mixer (Henschel mixer) at the front stage of the hopper. Using the extruder, kneading and granulation were performed under the following granulation conditions. The polyethylene powder was stirred while injecting a mixed gas of oxygen / nitrogen into the mixing mixer, and was supplied to a hopper that had been purged with nitrogen after a predetermined residence time.
[0020]
[Granulation conditions]
Stirring speed of mixing mixer: 500 rpm
Oxygen concentration of mixed gas injected into mixing mixer: 0.5 [% by volume]
Flow rate of the mixed gas injected into the mixing mixer: 20 Nm ³ / h
Retention time of polyethylene in mixing mixer: 60 seconds Retention time of polyethylene in hopper: 30 minutes Extruder: single screw, screw diameter 300 mmφ, L / D = 30
Resin temperature: 250 ° C
Additives: 500 ppm phenolic heat stabilizer, 500 ppm phosphorus heat stabilizer, 1500 ppm hydrochloric acid absorbent
The kneaded and granulated polyethylene particles were formed into a film by an air-cooled inflation method under the following molding conditions to produce a film having a smooth surface, a thickness of 20 μm, and a folding width of 450 mm. The films thus obtained were evaluated for thickness variation, Elmendorf tear strength, dart impact strength, and fish eye by the above-described methods. Table 1 shows the results. A film having a uniform thickness, a small variation and a small fish eye was obtained.
[0021]
[Molding condition]
Molding machine: 65 mmΦ inflation molding machine made by Placo Die diameter: 100 mmΦ
Molding temperature: cylinder 200 ° C, die 200 ° C
Take-off speed: 80m / min Whitening point height: 110cm
Expansion ratio: 2.9 times
Embodiment 2
The same operation as in Example 1 was performed except that the oxygen concentration of the mixed gas supplied to the mixing mixer was 5.0% by volume. Table 1 shows the results.
[0023]
[Comparative Example 1]
The operation was performed in the same manner as in Example 1 except that neither the mixing mixer nor the hopper supplied the oxygen mixed gas. Table 1 shows the results. The obtained film had a non-uniform thickness as compared with Example 1.
[0024]
[Comparative Example 2]
The same operation as in Example 1 was performed except that the mixed oxygen gas was not supplied to the mixing mixer, but 5.0% by volume of the mixed oxygen gas was injected only into the hopper. Table 1 shows the results. The deterioration of fish eyes is conspicuous as compared with Example 2.
[0025]
[Table 1]

[0026]
【The invention's effect】
According to the present invention, it is possible to obtain a polyethylene film having a uniform thickness, a small variation, and an excellent balance with the film strength as compared with a conventional high-density polyethylene film. Therefore, it can be made thinner when used in shopping bags, standard bags, fashion bags, accessory packaging bags, various component packaging, building materials, and the like.

Claims (3)

A method for producing polyethylene particles, comprising: contacting a polyethylene with a mixed gas containing 0.1 to 21% by volume of oxygen while stirring the polyethylene in a mixer equipped with a stirrer, supplying the mixed gas to an extruder, and kneading and granulating the polyethylene particles. . The polyethylene has (i) a density of 0.940 to 0.970 g / cm ³ ,
(Ii) a melt flow rate (MFR) of 0.01 to 0.5 g / 10 minutes,
(Iii) A polyethylene resin composition comprising polyethylene having a molecular weight distribution (Mw / Mn: Mw = weight average molecular weight, Mn = number average molecular weight) of 15 to 80 measured by GPC, and a fluidity index (FI: [ 1 / sec] and melt flow rate (MFR: [g / 10 min]) are FI ≧ 1500 × MFR
2. A polyethylene resin composition which is extruded by the method according to claim 1. A film using the polyethylene according to claim 2.