JPH10217328A - Manufacture of inflation film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for inflation forming a film of superior transparency composed of a propylene polymer stably at high speed. SOLUTION: In a method for forming a propylene polymer, preferably a propylene polymer having the heat of fusion of 50J/g or lower measured by a differential scanning calorimeter is formed by the air inflation process, cooling air of -30-10 deg.C sprayed on the outer surface side of a cylindrical molten material of a propylene extruded from an annular die and taken off as a film at the blow-up ratio of 3-10. MI of the prepylene polymer measured at 230 deg.C is preferably 0.01-50g/10 minutes, more preferably 0.1-30g/10 minutes, and furthermore preferably 1-20g/10 minutes.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a method for producing a blown film. More particularly, the present invention relates to a method for producing an inflation film, which is capable of producing a propylene-based polymer film having excellent transparency at high speed with stability using an air-cooled inflation molding method.

[0002]

2. Description of the Related Art Hitherto, films produced by the inflation method have been widely used for packaging and agricultural purposes. Above all, in the fields of food packaging films such as wrap films and stretch films, protective films, masking films, etc., films have recently been produced by air-cooled inflation using polyolefins instead of polyvinyl chloride, which has been widely used until now. It has become Here, the cooling air usually used for air cooling in the above air cooling inflation method has a temperature of 2 ° C.
Generally, the spraying amount is about 10 m ³ / h to 20 m ³ / h per 0 kg to 50 ° C. and 1 kg / h of the resin discharge amount.

[0003] As a raw material, a propylene-based soft resin is suitable because flexibility and heat resistance are required. However, it is difficult to obtain a highly transparent film due to a low crystallization rate of a propylene-based polymer, and it is difficult to stabilize an inflation bubble due to a low melt tension. Further, it is known that a propylene-based polymer causes so-called “tension thinning” in which elongational viscosity decreases with an increase in processing speed. Such a property promotes uneven thickness and vibration of the bubble when inflation of the bubble, and causes a decrease in stability.

[0004] For this reason, a method of stabilizing inflation bubbles with a two-stage outlet for the external cooling air has been used (Japanese Patent Laid-Open No. 5-169529).
And methods of performing internal cooling have been performed. Further, a method of heating only the vicinity of the exit of the die in order to improve transparency (Japanese Patent Application Laid-Open No. 8-39666) has been proposed.

[0005]

By using the above method, it is possible to form a linear low-density polyethylene or a high-pressure low-density polyethylene with high transparency and high speed. However, since the propylene-based polymer tends to have a large crystal size, it has been still extremely difficult to obtain a highly transparent film by the air-cooled inflation method.

[0006] As means for making the cooling more rapid,
It is conceivable as an effective method to increase the expansion ratio of the inflation bubble to rapidly reduce the thickness of the tube and rapidly increase the cooling efficiency of air cooling. However, since the propylene polymer has a low melt tension due to a low crystallization rate and exhibits the above-mentioned “tension thinning” property, it is difficult to keep the expansion coefficient high and stabilize the film formation. Was.

On the other hand, as a method of inflation molding a propylene-based polymer, a water-cooled inflation method, which is a method of bringing cooling water into contact with extruded bubbles to instantaneously cool and solidify, is widely used. However, in the water-cooling method, gradual cooling by air is hardly performed up to such a point to maintain the molten state until it contacts the cooling water,
The flow direction (hereinafter abbreviated as MD) and the width direction of the film (hereinafter abbreviated as TD) under conditions close to the isothermal condition.
Bubble is likely to be unstable because of spreading, and high-speed film formation cannot be performed. Further, in the case of a thin film or a soft film, there is a problem that wrinkles are easily generated in the film due to the weight of the cooling water.

In view of the above background, an object of the present invention is to provide a method for stably and rapidly producing a film made of a propylene polymer having excellent transparency by an air-cooled inflation method.

[0009]

Means for Solving the Problems The present inventors have intensively studied a method for producing a propylene-based film free from the above-mentioned disadvantages, and as a result, by performing rapid air cooling under specific conditions,
The inventors have found that the above problems can be solved, and have completed the present invention.

That is, the present invention relates to a method of forming a propylene polymer into a film by an air-cooled inflation method, wherein a temperature of -30 to 10 ° C. is applied to the outer surface of a cylindrical melt of the propylene polymer extruded from an annular die. A method for producing an inflation film, comprising blowing cooling air onto the film at an expansion ratio of 3 to 10.

A major cause of difficulty in producing a propylene-based film having excellent transparency by the air-cooled inflation method is light scattering by spherulites grown by slow cooling. Therefore, by suppressing the crystallinity and the spherulite size, the transparency is improved. For that purpose, it is necessary to rapidly cool around the melting point of the bubble. On the other hand, it is effective to increase the expansion ratio of the inflation bubble to rapidly reduce the thickness of the tube and rapidly increase the cooling efficiency of air cooling as a means to make the cooling more rapid in film production by the air-cooled inflation method. is there. However, since the propylene-based polymer has a low melt tension due to a low crystallization rate and exhibits the “tension thinning” property,
In order to stabilize the film formation while keeping the expansion coefficient high, it is necessary to further lower the temperature of the cooling air to -30 to 10C. The present invention has been completed based on the above findings, by using a combination of such requirements, using a propylene-based polymer by air-cooled inflation method,
It is possible to stably produce a film having excellent transparency.

In the present invention, the propylene-based polymer is
It is a homopolymer of propylene or a copolymer with other monomers copolymerizable with propylene. Other copolymerizable monomers include ethylene, 1-butene, and 3-methyl-
Α having 4 to 12 carbon atoms, such as 1-butene, 4-methyl-1-pentene, 1-hexene, 1-octene, and 1-decene
-Olefins and diene such as divinylbenzene, 1,4-cyclohexadiene, dicyclopentadiene, cyclooctadiene, and ethylidene norbornene. Of these, ethylene is preferable.

The method for producing these propylene-based polymers is not particularly limited, and is generally a Ziegler-Natta catalyst using a combination of a so-called titanium-containing solid transition metal portion and an organic component, particularly a transition metal component. Is a solid component or titanium trichloride containing titanium, magnesium and halogen as essential components and an electron donating compound as an optional component, using a catalyst in which an organometallic component is an organoaluminum compound, using slurry polymerization, gas phase polymerization, and bulk polymerization. ,
It can be obtained by one-stage or multi-stage polymerization using a solution polymerization or a combination thereof. In addition, these propylene-based polymers may be random polymers or block copolymers, but preferred are random copolymers. In addition, two or more of these propylene polymers may be used.

The above-mentioned propylene polymer preferably has a heat of fusion of 50 J / g or less, more preferably 30 J / g or less, as measured by a differential scanning calorimeter. In that case, the spherulite size can be reduced to the wavelength of visible light or less, the difference in the refractive index between the crystalline part and the amorphous part of the propylene-based polymer becomes smaller, and the transparency tends to be the best.
When a propylene-based polymer having such a heat of fusion is used in a mixture with another propylene-based polymer, the content is preferably 50% by weight or more.

Further, the propylene-based polymer preferably satisfies the following conditions with respect to the content of MI and other copolymerizable monomers.

I) MI M of the propylene polymer of the present invention measured at 230 ° C.
I is preferably 0.01 to 50 g / 10 min, more preferably 0.1 to 30 g / 10 min, and still more preferably 1 to 10 g / 10 min.
20 g / 10 min. When MI is in the above range,
In particular, a film excellent in film appearance without surface roughness and excellent in film strength can be obtained.

Ii) Content of other copolymerizable monomer When the propylene-based polymer of the present invention is a copolymer of propylene with another monomer copolymerizable with propylene, another monomer is used. The body content is preferably up to 40% by weight, more preferably up to 30% by weight. Within such a range of the content of the other monomer, it is possible to obtain a film having particularly small troubles such as partial elongation at the time of unwinding the film.

In the present invention, the propylene-based polymer is melt-kneaded by an extruder or the like, supplied to an annular die, and extruded from the die into an annular shape. If the resin temperature at this time is too high or too low, it tends to be difficult to stably form a film having high transparency. From that point of view, the molten resin temperature at the exit of the die is 20 to 15 with respect to the melting point of the resin measured by a differential scanning calorimeter.
A temperature 0 ° C. higher is particularly preferred. When the temperature is lower than the above-mentioned temperature, the surface of the film tends to be rough due to stick-slip or the like, and the transparency near the surface tends to decrease. If the resin temperature is higher than the above range, the melt tension is low, and it tends to be difficult to form a stable film. In addition, the above-mentioned molten resin temperature is
When a mixture of two or more propylene polymers is used, the melting point may be set higher than the melting point.

In the present invention, the cylindrical melt of the extruded propylene polymer is taken into a film, and the outer surface of the film is blown with cooling air at -30 to 10 ° C. Perform in 3-10. At this time, as the expansion of the cylindrical melt, a known inflation method in which air is fed into the cylindrical melt and expanded by the air pressure is used without any limitation. In order to take up the cylindrical melt at a large expansion ratio as described above, it is general to carry out the extrusion and the take-up direction from the annular die upward.

When the expansion ratio is lower than the above range, the film is not easily abruptly thinned, so that the transparency tends to be reduced, and the formed film is oriented in the MD direction, and the strength is liable to be reduced. On the other hand, when the expansion ratio exceeds the above range, the inclination of the bubble due to the eccentricity of expansion is likely to occur, and it is difficult to stably form a film at a high speed, and also the thickness of the manufactured film in the TD direction. The error is likely to be large and the quality of the film is likely to be inferior. The MD direction described in the present invention is a direction in which a propylene-based polymer is extruded from a circular die. In addition, regarding the film surface direction, MD
The direction perpendicular to the direction is called the TD direction.

In order to stabilize the film formation, it is essential that the cooling efficiency is higher than that of the conventional air-cooled inflation method. As a result of various studies, the temperature of the cooling air blown to the outer surface side of the cylindrical melt to be expanded as described above is -30 to 10 ° C, more preferably 0 to 0 ° C.
It was found that controlling the temperature to 5 ° C. was very effective.
If the above temperature exceeds the above, a sufficient temperature difference between the thick and thin portions of the bubble cannot be obtained, so that the stability of film formation, especially the stability of film formation at high speed, is inferior. In addition, it is difficult to increase the bubble expansion ratio to 3 times or more, and it is difficult to obtain a film having good MD / TD balance of transparency and mechanical properties. On the other hand, it is technically difficult to stably obtain a large amount of air having a temperature lower than -30 ° C.

In the present invention, the β region of the bubble is rapidly cooled by blowing the cooling air at the above temperature. Here, the β region of the bubble is a region from the point at which the extruded cylindrical melt begins to expand rapidly in the lateral direction due to the internal pressure in the bubble to the frost line where crystallization occurs. In the present invention, by rapidly cooling the β region of the bubble in this way, the propylene-based polymer becomes apparently “tension thickening”, and the film formation can be stabilized while maintaining a high expansion coefficient. . The specific spraying position may be appropriately set between the extrusion position and the frost line. However, in order to better cool the β region, the spraying position is 4/4 of the extrusion position side between the extrusion position and the frost line. 5 or less, preferably 7/1
It is preferred to be within 0. In order to enhance the cooling effect, it is preferable to provide a plurality of locations, particularly at a plurality of locations near the extrusion position and beyond.

The amount of spray is preferably an amount of air that can be sufficiently cooled and does not vibrate due to the wind speed. The total amount per 1 kg / h of resin discharge is 5 to 50 m.
³ / h, more preferably 10 to 20 m ³ / h.

In the present invention, such blowing of the cooling air is generally performed by blowing the air having the above-mentioned temperature from a cooling ring provided outside the cylindrical melt to be taken. Cooling of the cooling air can be performed by, for example, heat exchange of a usual compressor. The distance between the outer surface of the cylindrical melt and the air outlet of the cooling ring is preferably 5 to 50 mm.

Further, the draft ratio is preferably from 5 to 200. When the draft ratio is in this range, MD /
A film having a good balance of TD can be formed stably.

The thickness of the blown film obtained by the method of the present invention is preferably 5 to 200 μm, more preferably 7 to 100 μm, and more preferably 1 to 100 μm.
More preferably, it is 0 to 50 μm. When the film is in the above range, it is easy to form an accurate film, transparency is easily obtained, and the cost is advantageous.

The film of the present invention is not limited to a single-layer film, and may be a multilayer film obtained by laminating another resin layer on at least one of the film surfaces if necessary as long as the object of the present invention is not impaired. It may be manufactured as. Therefore, in such a case, the present invention includes a mode in which cooling air is blown to the outer surface side of the multilayer cylindrical melt containing the propylene-based polymer layer. The propylene-based polymer layer preferably accounts for 50% or more of the total film. In particular, in order to further improve the optical properties such as transparency and transparency, the high-pressure low-density polyethylene, ethylene-α-olefin random copolymer, ethylene-vinyl acetate copolymer, ethylene- Ethyl acrylate, ethylene-methyl methacrylate,
A resin layer composed of a single or a mixture of a plurality of ethylene polymers represented by ethylene-ethyl methacrylate may be laminated.

In the present invention, additives suitable for imparting certain properties to the film to be produced can be added. For example, antioxidants, weathering agents such as ultraviolet absorbers, antiblocking agents, flame retardants, antifoggants, adhesives,
Examples include coloring agents, matting agents, antistatic agents, oxygen and carbon dioxide gas absorbents, gas adsorbents, freshness preserving agents, enzymes, deodorants, and fragrances. In addition, other components may be added as needed as long as the features of the present invention are not impaired.

[0029]

As described above, an inflation film formed by forming a propylene-based polymer under specific conditions is not only excellent in transparency but also advantageous in cost because it can be easily formed at high speed. .

Therefore, the film formed by the method of the present invention is most suitable for applications requiring transparency by utilizing the above excellent properties and quality. For example, various packaging films such as for food and medical use,
Tray, container, lid, etc. that require heat sealing, wrap film, wrap stretch film, stretch film, shrink film, blister pack film, stand pouch film, integrated packaging film,
Examples include distribution packaging films, heavy packaging films, agricultural films, adhesive tapes, surface protection films, and the like.

[0031]

The present invention will be described in more detail with reference to the following Examples, which by no means limit the scope of the present invention.

(1) Measurement method Heat of fusion, melting point Melt at 230 ° C. for 5 minutes by heat flux differential scanning calorimetry using DSC200 manufactured by Seiko Instruments Inc., and cool at a temperature lowering rate of 10 ° C./min. The measurement was performed under the condition of a heating rate of 10 ° C./min.

The density was measured according to JIS-K7112.

The measurement was carried out according to MI JIS-K7210.

The expansion ratio was calculated by the following equation when the tube folding diameter 1 and the die diameter D when passing through the take-off roll were used.

Expansion ratio = (2 × l) / (π × D) Haze Measured according to JIS-K7105.

Maximum take-up speed The take-up speed is increased from 30 m / min in increments of 5 m / min. When the maximum thickness of the film exceeds three times the minimum thickness due to the deformation of bubbles, the immediately preceding speed is taken up. Speed.

(2) Resin used

[0039]

[Table 1]

Examples 1 to 9 Comparative Examples 1 to 4 Using the resins shown in Table 2, using an upward air-cooled blown film production apparatus equipped with a spiral die having a die diameter of 100 mm and a lip clearance of 1.3 mm, the results are shown in Table 2. Films were produced under the conditions shown. The outer cooling rings are installed at two positions near the extrusion position of the propylene-based polymer and near the center between the extrusion position and the frost line, and a cooling amount of the total amount shown in Table 2 from these two cooling rings is provided. Air was blown out. The distance between the outer surface of the cylindrical melt and the air outlet of the cooling ring was 15 mm. The thickness of the obtained film was 20 μm.

Table 3 shows the maximum take-off speed and the haze of the obtained film in each production.

[0042]

[Table 2]

[0043]

[Table 3]

Claims (2)

[Claims] 1. A method for forming a propylene polymer into a film by an air-cooled inflation method, wherein cooling air of -30 to 10 ° C. is provided on an outer surface side of a cylindrical melt of the propylene polymer extruded from an annular die. And blowing the film at an expansion ratio of 3 to 10 to produce a blown film. 2. The method according to claim 1, wherein the propylene-based polymer has a heat of fusion of 50 J / g or less as measured by a differential scanning calorimeter.