JPH05293886A - Method for producing transparent polyethylene film - Google Patents

Abstract

(57) [Summary] [Object] To provide a method for producing a polyethylene film having excellent strength, rigidity, transparency, and longitudinal / horizontal strength balance. [Structure] From a polyethylene composition containing a high-density polyethylene having a small molecular weight distribution (weight average molecular weight / number average molecular weight), linear low-density polyethylene, and low-density polyethylene, it is rapidly cooled at a high blow ratio by a multistage air-cooling inflation method. To produce a polyethylene film.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a polyethylene film, and more particularly to a method for producing a polyethylene film which is excellent in strength, rigidity, transparency and longitudinal / lateral strength balance.

[0002]

2. Description of the Related Art Films made of polyethylene are inexpensive and have excellent transparency, gloss, heat sealability, etc., and are therefore used for various packaging. As this polyethylene film, a low-density polyethylene or a linear low-density polyethylene that is usually formed by a water-cooled inflation method, an air-cooled inflation method, a T-die method, or the like is used.

However, a film made of low-density polyethylene or linear low-density polyethylene has a problem that it is not suitable for an automatic wrapping machine because it is flexible and it is difficult to wrap at high speed.

Therefore, for the purpose of imparting mechanical strength and rigidity, a film obtained from a composition obtained by blending linear low-density polyethylene with high-density polyethylene has been proposed (JP-A-58). -59242, JP-A-59-7
No. 4139). Also, a film is produced from a single piece of high-density polyethylene.

However, a polyethylene film obtained by a normal air-cooled inflation method film-forming from a high-density polyethylene alone or a composition obtained by blending a high-density polyethylene with a linear low-density polyethylene is excellent in mechanical strength and rigidity. However, because of its low transparency, there is a problem that its intended use is limited for packaging.

In order to solve the above problems,
Although the film is rapidly cooled by the water-cooled inflation method or the T-die method, the transparency of the film is improved, but the film is likely to curl, and the physical properties of the film are directional. There is a problem of easy tearing in the MD direction. In addition to the above-mentioned method, a T-die method, an inflation method, or the like is used to form a raw film and then stretch it. However, such a film has a weak tear propagation resistance and increases the number of manufacturing processes. However, it is also disadvantageous in terms of cost.

Therefore, it is an object of the present invention to provide a method for producing a polyethylene film which is excellent in strength, rigidity, transparency, strength / width strength balance and the like.

[0008]

As a result of earnest research in view of the above problems, the present inventors have found that the molecular weight distribution (weight average molecular weight /
A high density polyethylene having a small number average molecular weight), a linear low density polyethylene, and a polyethylene composition containing a low density polyethylene, a film obtained by quenching at a high blow ratio by a multi-stage air cooling inflation method has strength, The present invention has been accomplished by discovering that it has excellent rigidity, transparency, and strength balance in the vertical / horizontal directions.

That is, the method for producing a polyethylene film of the present invention comprises: a high density polyethylene having a weight average molecular weight / number average molecular weight of 6.5 or less of 30 to 60% by weight; a low density polyethylene of 10 to 30% by weight; Polyethylene 20-60
A polyethylene composition containing 100 wt% of the polyethylene film is formed by an air-cooled inflation method, and the first cooling ring provided at the root of the bubble of the polyethylene film and the bubble portion downstream of the expansion part are provided. The bubble is cooled by at least one second cooling ring.

The present invention is described in detail below. The polyethylene film produced in the present invention comprises high density polyethylene having a specific molecular weight distribution, low density polyethylene, and linear low density polyethylene.

The high density polyethylene (HDPE) used in the present invention is 0.940 g / cm ³ or more, preferably 0.950-
It has a density of 0.965 g / cm ^{3 and} a molecular weight distribution (weight average molecular weight / number average molecular weight) of 6.5 or less.
When the molecular weight distribution exceeds 6.5, the transparency of the obtained film is lowered. The preferable molecular weight distribution value is 2.0 to 6.0. High density polyethylene (HDPE) is usually 4.0 x
Weight average molecular weight of 10 ⁴ to 30 × 10 ⁵ and 8.5 × 10 ^{3 to} 4.0
With a number average molecular weight of × 10 ⁴ , preferably the weight average molecular weight is 1.5 × 10 ^{5 to} 2.0 × 10 ⁵ , the number average molecular weight is
It is 2.5 × 10 ^{4 to} 4.0 × 10 ⁴ .

The value of the molecular weight distribution indicates the weight average molecular weight / number average molecular weight. The small value means that the difference between the weight average molecular weight and the number average molecular weight is small, that is, the molecular weight. It means that many molecules exist near the peak in the distribution curve.

The above high density polyethylene (HDPE) is preferably 0.01 to 10.0 g / 10 min, particularly preferably 0.1 to 2.0 g.
It has a melt index of 10 minutes (190 ℃, 2.16kg load).

Low density polyethylene (LDPE) has a density of 0.910 to 0.935 g / cm ³ , preferably 0.920 to 0.925 g.
/ cm ³ branched polyethylene. The low density polyethylene (LDPE) is preferably 0.1 to 15.0 g / 10 minutes,
Particularly preferably, it has a melt index (190 ° C., 2.16 kg load) of 0.5 to 3.0 g / 10 min.

Further, linear low density polyethylene (LLDPE)
Is ethylene having 4 or more carbon atoms, preferably 6 or more α-
It is a short-chain branched polyethylene obtained by copolymerizing about 15% by weight or less of an olefin and has a density of 0.915 to 0.935 g / cm ³ , preferably 0.920 to 0.930 g / cm ³ .

The linear low density polyethylene (LLDPE) is
Preferably 0.1 to 15.0 g / 10 minutes, particularly preferably 0.1 to
It has a melt index of 2.5 g / 10 min (190 ° C, 2.16 kg load).

The blending ratio of each component as described above is 30 to 60% by weight of high-density polyethylene, preferably 30 to 50% by weight, and 10 to 30% by weight of low-density polyethylene, preferably 10 to 20% by weight. %, And linear low density polyethylene is 20
-60% by weight, preferably 30-50% by weight.

If the high-density polyethylene is less than 30% by weight,
The polyethylene film obtained has insufficient mechanical strength, and when it exceeds 60% by weight, transparency and processability are deteriorated.

When the low density polyethylene is less than 10% by weight,
The transparency and processability (stability during film formation) are insufficient, and a uniform film cannot be obtained. Further, if it exceeds 30% by weight, mechanical strength and rigidity are lowered.

When the linear low density polyethylene is less than 20% by weight, the transparency is insufficient, and when it exceeds 60% by weight, the mechanical strength and rigidity are deteriorated.

The above composition has a density of the whole composition.
It is preferably 0.930 to 0.955 g / cm ³ , particularly 0.940.
It is preferably ˜0.950 g / cm ³ . By setting the density of the entire composition to be somewhat high in this way, the strength and rigidity of the film can be improved.

Further, in the present invention, an inorganic filler, a heat stabilizer, an ultraviolet absorber, an antistatic agent, an antioxidant, a coloring agent and the like can be appropriately added to the above resin component.

In the present invention, a polyethylene film is produced from the above polyethylene composition by the following air-cooled inflation method.

First, the high density polyethylene (HDPE),
Knead low density polyethylene (LDPE), linear low density polyethylene (LLDPE) and desired additives. Kneading is 15
It is preferable to carry out at a resin temperature of 0 to 250 ° C., particularly 170 to 250 ° C.
It is preferred to carry out at a resin temperature of 200 ° C. Kneading temperature is 150
If it is less than ℃, it will be difficult to sufficiently melt and knead, and the extrusion amount will be unstable. If it is higher than 250 ℃,
It is not preferable because the heat deterioration of the resin proceeds.

The resin temperature immediately after being extruded from the inflation die is preferably 260 ° C. or lower. If the temperature of the bubble immediately after die extrusion exceeds 260 ° C, the bubble cannot be cooled to a predetermined temperature by the cooling ring.

The bubbles extruded from the die are stretched not only in the MD direction but also in the TD direction while being cooled by the air cooling inflation method. This state is schematically shown in FIG.

In FIG. 1, the air-cooled inflation device is a die 1, a first cooling ring 2 provided immediately above the die 1, and a first cooling ring 2 provided downstream (upper in the figure) of the first cooling ring 2. Two cooling rings 3 are provided.

In order to manufacture the polyethylene film of the present invention by the air-cooled inflation method using the apparatus having the above construction, first, the molten polyethylene composition is extruded from the annular orifice 11 of the die 1 to form the bubbles 4. Immediately after being extruded, the bubble 4 has a small diameter due to its low melt tension, and forms a so-called neck portion 41. In the neck portion 41, the bubble 4 is stretched mainly in the MD direction. Next, the bubble 4 expands rapidly to a predetermined bubble diameter. In the expansion section 42, the bubble 4 is stretched not only in the MD direction but also in the TD direction. There is a frost line 43 near the upper end of the expansion section 42, where the polyethylene composition is cooled and solidified. The second cooling ring 3 provided above the frost line 43 further cools the bubbles.

In the air-cooled inflation method, the temperature of each part of the bubble 4 is preferably controlled as follows. (a) The temperature immediately after extrusion from the die 1 is 270 ° C or lower. (b) Cool the neck 41 to below 190 ° C. (c) Frost line 43 cooled to below 130 ° C. (d) Cooled to 90 ° C or less by the second cooling ring 3.

The condition (a) is as described above, but the condition (b) is that the neck portion 41 must be cooled to 190 ° C. or less to sufficiently expand the TD in the next expanding portion 42. Cannot be achieved. That is, 19 at the neck 41
If it is not cooled to 0 ° C. or less, the expanded portion 42 does not have a sufficient melt tension, and stretching in the MD direction mainly occurs. On the other hand, if the neck portion 41 is cooled too much, it becomes difficult to form the expanded portion 42 (that is, the expansion of the bubble 4 in the TD direction). Therefore, the lower limit of the bubble temperature in the neck portion 41 is set to about 160 ° C. preferable.

The blow-up ratio is preferably from 2 to 5, and particularly preferably from 3 to 4, in order to obtain a film having a good strength / width strength balance.

With respect to the condition (c), cold drawing of the bubble 4 can be achieved by lowering the bubble temperature in the frost line 43 to 130 ° C. or lower.

Regarding the condition (d), by cooling the bubble 4 above the frost line 43 to 120 ° C. or less, particularly 90 ° C. or less, the crystal size is made fine and the formation of uniform thin bubbles is stabilized. be able to. If the temperature of the bubbles above the frost line 43 is kept higher than 120 ° C. without providing the second cooling ring 3, crystallization will proceed and a transparent film will not be obtained. In addition, the bubbles become unstable and film formation becomes difficult.

The relationship of the positions where the above cooling rings are installed is provided as shown in FIG. 1. These positions are as follows.
It is adjusted by the die diameter, neck height, take-up speed, bubble diameter, frost line position, etc. In particular, the second cooling ring 3 is preferably arranged near the frost line portion.
In addition, in this embodiment, the number of the second cooling ring is one, but two or more may be provided.

In the above method, it is preferable to use humidified air as the cooling air to be injected into the bubbles 4 from the first and second cooling rings 2 and 3 from the viewpoint of cooling efficiency. Humidified air is air that has been humidified and cooled with cold water and contains substantially saturated water, and has a cooling effect of about 5 ° C. more than simple cooling air.

The polyethylene film thus obtained has a haze of 2 to 10% and is excellent in transparency, has a 1% tensile elastic modulus of 4000 to 6500 kgf / cm ² , and a tensile breaking strength of 300 to 500 kgf / cm. ² , the vertical / horizontal strength balance (MD / TD) is 1.5 or less, and the mechanical strength is also good.
The thickness can be appropriately set according to the conditions of the air-cooled inflation method, but is usually 10 to 100 μm.
Is.

[0037]

In the present invention, a polyethylene composition containing high-density polyethylene having a small weight-average molecular weight / number-average molecular weight, linear low-density polyethylene, and low-density polyethylene is used at a high blow ratio by a multistage air-cooling inflation method. Since the polyethylene film is formed by rapid cooling, the obtained film has excellent transparency and good mechanical strength.

Although the reason why such an effect is obtained is not necessarily clear, the high-density polyethylene having a small weight average molecular weight / number average molecular weight is excellent in strength and transparency. And a composition of linear low density polyethylene and low density polyethylene,
This is considered to be due to the combined effect of rapid cooling at a high blow ratio and film formation by the multi-stage air cooling inflation method.

[0039]

The present invention will be described in more detail by the following examples. In each of the examples and comparative examples, the following raw material resins were used. [1] High-density polyethylene HDPE: [Density 0.956 g / cm ³ , MI (190 ° C, 2.16 kg
Load) 0.34 g / 10 minutes, Mw / Mn = 5.48] HDPE: [density 0.955 g / cm ³ , MI (190 ° C, 2.16 kg)
Load) 0.68 g / 10 min, Mw / Mn = 5.96] HDPE: [Density 0.955 g / cm ³ , MI (190 ° C, 2.16 kg)
Load) 0.26 g / 10 min, Mw / Mn = 7.97] HDPE: [density 0.959 g / cm ³ , MI (190 ° C, 2.16 kg)
Load) 0.90 g / 10 min, Mw / Mn = 7.53] [2] Low density polyethylene LDPE: [density 0.928 g / cm ³ , MI (190 ° C, 2.16 kg load) 2.50 g / 10 min] [3] linear Low-density polyethylene LLDPE: [Density 0.931 g / cm ³ , MI (190 ° C, 2.16 kg
Load) 0.92g / 10 minutes)

Examples 1 to 4 and Comparative Examples Comparative Examples 1 to 3 High density polyethylene (HDPE to), low density polyethylene (LDPE), and linear low density polyethylene (LLDPE) in the ratio shown in Table 1. Mixed and twin-screw extruder at 200 ℃
Was melt-kneaded and pelletized. This was formed into a film having a thickness of 25 μm by an air cooling inflation method using an apparatus as shown in FIG.

The film forming conditions by the air-cooled inflation method were as follows. Position of first cooling ring Position of bubble extrusion Position of second cooling ring Frost line Parting speed 23.0m /
Min. Blow-up ratio 3.4 Resin extrusion temperature 270 ℃ Bubble temperature at frost line 130 ℃ Bubble temperature at second cooling ring 90 ℃

The tensile breaking strength, 1% tensile modulus, internal haze and haze of the obtained polyethylene film were measured. The results are shown in Table 2.

Table 1 Composition (parts by weight) Example 1 Example 2 Example 3 Example 4 HDPE 50-50 50 HDPE-50-LDPE 10 10 20 30 LLDPE 40 40 30 20

Table 1 (continued) Composition (parts by weight) Comparative Example 1 Comparative Example 2 Comparative Example 3 HDPE--50 HDPE ---- HDPE 50--HDPE-50--LDPE 10 10 50 LLDPE 40 40--

Table 2 Physical property Example 1 Example 2 Example 3 Example 4 Tensile breaking strength ⁽¹⁾ MD 456 438 424 388 TD 356 405 359 3081% tensile modulus ⁽²⁾ MD 4995 5460 5157 5069 TD 5674 6457 6207 6277Haze ⁽³⁾ Of which 2.7 3.3 2.5 2.7 All 6.5 7.9 9.3 9.5

Table 2 (continued) Physical property Comparative Example 1 Comparative example 2 Comparative Example 3 Tensile breaking strength ⁽¹⁾ MD 399 404 367 TD 372 347 2481% tensile modulus ⁽²⁾ MD 5375 5520 5083 TD 6524 6767 6764Haze ⁽³⁾ Of which 4.2 4.5 2.2 All 31.1 27.1 11.4

Comparative Examples 4 and 5 The resin composition of Example 1 was used as a raw material resin, and the resin temperature was set to 260.
A film having a thickness of 25 μm was formed by a blown water cooling inflation method at a temperature of 30 ° C. and a water temperature of about 30 ° C. (Comparative Example 4). Also,
The resin composition of Example 1 was used as the raw material resin, and the resin temperature was 260 ° C.
Then, it was melt extruded from a T-die onto a chill roll (about 30 ° C.) and cooled to form a film having a thickness of 25 μm (Comparative Example 5). The physical properties of each obtained polyethylene film were measured. The results are shown in Table 3.

[0048]

(1) Tensile breaking strength: measured according to ASTM D882 (unit: kgf / cm ² ). (2) 1% tensile modulus: measured according to ASTM D882 (unit: kgf / cm ² ). (3) Haze: Measured according to ASTM D1003 (in%) for the one coated with silicone oil (inside) and the film itself (all).

As is clear from Tables 2 and 3, the polyethylene films of Examples 1 to 4 have a smaller haze value than the films of Comparative Examples 1 to 4, and also have strength, rigidity and longitudinal / lateral directions. The strength balance of No. 1 also had a good value. Further, although the haze value was slightly larger than the films of the water-cooled inflation method and the T-die method of Comparative Examples 4 and 5, the strength, rigidity, and longitudinal / horizontal strength balance were excellent.

[0051]

As described in detail above, in the present invention, a polyethylene composition containing high density polyethylene having a small weight average molecular weight / small number average molecular weight, linear low density polyethylene, and low density polyethylene, Since the polyethylene film is produced by the multi-stage air-cooled inflation method, the obtained polyethylene film has excellent transparency and longitudinal / horizontal strength balance as well as good mechanical strength and rigidity.

The polyethylene film obtained by the method of the present invention as described above is suitable for various wrapping, particularly for wrapping by an automatic wrapping machine.

[Brief description of drawings]

FIG. 1 is a schematic view showing an air-cooled inflation method to which the method for producing a polyethylene film of the present invention can be applied.

[Explanation of symbols]

 1 ... Die 2 ... First Cooling Ring 3 ... Second Cooling Ring 4 ... Bubble 41 ... Neck 42 ... Expansion 43 ... Frost Line

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Internal reference number FI technical display area B29L 7:00 4F

Claims (1)

[Claims] 1. A high density polyethylene having a weight average molecular weight / number average molecular weight of 6.5 or less of 30 to 60% by weight, a low density polyethylene of 10 to 30% by weight, and a linear low density polyethylene of 20 to 60.
A polyethylene composition containing 100 wt% of the polyethylene film is formed by an air-cooled inflation method, and the first cooling ring provided at the root of the bubble of the polyethylene film and the bubble portion downstream of the expansion part are provided. A method for producing a polyethylene film, characterized in that the bubble is cooled by at least one second cooling ring.