JPH05261811A - Manufacture of inflation film - Google Patents

Abstract

PURPOSE:To establish a method of film with a stability of bubble and a high productivity in the case of manufacturing a film of low melt-viscosity LLDPE through air-cooling inflation. CONSTITUTION:The manufacture of a long-neck type film is as follows: a linear low density polyethylene with a density of 0.91-0.94g/cm<3> is used, wherein the lip gaps are made 2.5-10.0mm, the die temperature is kept at 170-210 deg.C, a frost line 3 is allowed to be 4-20 times of the die 1 diameter and the expansion ratio is made 2-5 times.

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 blown film of a linear low density polyethylene (hereinafter referred to as LLDPE) having excellent transparency with high productivity.

[0002]

2. Description of the Related Art There have been many proposals for producing an inflation film by an air-cooling method, and the shape of a resin bubble for forming a film by these proposals is Japanese Patent Publication No.
2180, it is roughly divided into three or four types (FIGS. 2 to 4).

Factors that determine this shape include air cooling capacity, film take-up speed, molten resin temperature, etc., and medium density polyethylene (hereinafter referred to as MDPE),
High-density polyethylene (henceforth HDPE), etc.
In the case of linear polyethylene having a high density, film forming by a so-called long-neck type bubble shown in FIG. 1 is mostly adopted, and a large amount is supplied to a field such as a shopping bag as a high-strength balance film.

On the other hand, the molecular structure of LLDPE is higher than that of high-pressure process low-density polyethylene (hereinafter referred to as LDPE).
Even though it is similar to PE, it has a low density, and because it does not contain an ultra-high molecular weight component, its melt viscosity is much smaller than that of HDPE for balance films, so its rheology is H.
Unlike DPE, it is easy to cause skin roughness due to melt fracture and the stability of the bubble is poor, and it is difficult to make the bubble into a shape such as a long neck type. Usually, the bubble shape of a type or c type Molded (Japanese Patent Laid-Open No. 57-34920).

However, the biggest problem in the production of an inflation film due to the shape of the bubble is that it is difficult to take off at high speed while keeping the transparency of the film high, and the productivity is limited, and the bubble rapidly expands. Since there is no part, it is impossible to increase the strength.

[0006]

DISCLOSURE OF THE INVENTION The present invention provides LLDPE in the production of a film by an air-cooled inflation method.
Eliminates the instability of bubbles due to the low melt viscosity of
The goal was to stabilize the bubble and establish a highly productive LLDPE film manufacturing method.

[0007]

The present invention is a linear copolymer having a density of 0.91 to 0.94 g / cm ³ , which is a copolymer of ethylene and an α-olefin having 3 to 10 carbon atoms. Low density polyethylene with lip gap of 2.5-1
A method for producing an inflation film, comprising extruding from a die having a diameter of 0.0 mm at 170 ° C. to 210 ° C., expanding the expansion ratio to 2 to 5 while keeping the frost line distance 4 to 20 times the diameter of the die, and taking over. Through the development, we were able to achieve the above objectives.

The LLDPE which can be used in the present invention uses a transition metal catalyst in a high pressure method or a low pressure method and has 3 to 10 carbon atoms.
Is a copolymer of α-olefin and ethylene and has a density of about 0.91 to 0.94 g / cm ³ . Generally, the density varies depending on the content of α-olefin, but when butene-1 is used as a comonomer, it has 10 carbon atoms.
If the number of side chains is 20 to 25, then about 0.91,
If the number of side chains is 3 to 5, it is said to be 0.94, which is within this range. Examples of the α-olefin in this case include propylene, butene-1, hexene-1, 4-methylpentene-1, octene-1 and the like. The density is 0.
When it exceeds 94 g / cm ³ , the transparency of the film is deteriorated, which is not preferable.

JIS of LLDPE which is the object of the present invention
The melt flow rate (hereinafter referred to as MFR) measured by the method of K 7210, Table 1, Condition 4 is not limited, but is applicable to about 0.3 to 5.0 g / 10 min. 0.4-3.0g / 10th place LLD
It is preferably applied to PE.

The molecular weight distribution of this LLDPE (M _W /
It is generally represented by M _N. ) Is smaller than HDPE, which is commercially available for balance film, and is usually LLDP.
If it is about 2.0 to 10.0, which is commercially available as E, it can be used.

It should be noted that, for this LLDPE, 1000-
Density 0.91 obtained by polymerizing in the presence of a free radical generator such as peroxide under a high pressure of 3500 Kg / cm ^2.
~0.93g / cm ³ about, MFR is 0.3~5.0g
/ 10 min high pressure low density polyethylene (hereinafter LDPE
Say. 50% by weight or less of the total weight may be blended.

Blending of LDPE in excess of this proportion should be avoided as it will result in a marked reduction in impact strength of the product film. The effect of lowering the pressure at the tip of the extruder can be clearly observed in LLDPE containing 5% or more of this LDPE.

The raw material LLDPE used in the present invention is, of course, an additive such as an antioxidant, an ultraviolet absorber, a slip agent, an antiblocking agent, and a colorant, which are blended with polyethylene which is a usual film raw material. Alternatively, an antistatic agent or the like may be blended.

The device used for producing the blown film of the present invention has a die lip gap of 2.
In addition to the large size of 5 to 10 mm, either upward or downward may be used, and it is almost HDP from the extruder to the take-up machine.
The same inflation device as E can be used.
In the production of LLDPE film, it is a new discovery that the transparency can be increased by expanding the lip gap with a long neck type bubble, and the transparency can be ensured even in the case of molding in a high frost line. This is because the high frost line gradually cools the molten resin as compared with the short frost line, so it was thought that the transparency would be poor, but it was surprising that the effect could be almost ignored.

During operation, the extrusion temperature is lowered so that the bubble shape becomes the long neck shape shown in FIG.
It must be adjusted by adjusting the temperature and flow rate of the cooling air at the die exit, increasing the take-up speed, and so on.

The preferable extrusion temperature of the resin varies depending on the resin, but it should be set to 170 to 210 ° C., which is slightly lower than the normal temperature of 220 ° C., and in particular, the bubble shape is a type shown in FIG. 2 or c type shown in FIG. It is better to set the temperature as low as possible so long as melt fracture does not occur.

If a slight melt fracture occurs, this problem can be avoided by increasing the lip gap.

Since LLDPE is a resin having a property of being extremely susceptible to melt fracture, the lip gap is much larger than that of general HDPE, 2.5-10 mm,
It is necessary to make it preferably 4 to 8 mm. If this lip gap is less than 2.5 mm, the transparency will decrease,
If it is larger than 10 mm, it becomes difficult to adjust the uneven thickness of the film. Also, when skin roughness due to melt fracture is observed, it is avoided by increasing the lip gap. If it is still unavoidable, take measures such as raising the resin temperature.

Also in LLDPE, HDPE or MD
Similar to PE, the expansion ratio (apparent, bubble diameter / die diameter) affects the impact strength of the film. When it is small or large, the strength balance is lost and the vertical tearing property becomes stronger, or the ring tears. It usually occurs within 2 to 5, preferably within 2.8 to 3.8.

When the bubble shape is a long neck type, the bubble diameter is preferably set to be smaller than the die diameter once as shown in FIG. 1 and becomes a minimum (hereinafter referred to as a neck point) and then expands rapidly. Shape. By making the diameter of the neck point smaller than the diameter of the die in this way, the substantial expansion ratio can be increased, and the properties such as impact strength of the manufactured film can be improved.

Therefore, even when films having the same diameter are manufactured using the same size die, a higher substantial expansion ratio can be obtained by reducing the diameter of the neck point, and in general theory. It means that a film with high impact strength can be produced.

However, if the diameter of the neck point is reduced to less than 0.7 of the die diameter, bubbles are fluttered and the stability is impaired. Therefore, it is usually 0.75 to 0.95, which is a product quality and productivity. A well-balanced operation is possible.

The frost line (line for crystallizing the molten resin) is a distance from the die surface, which is 4 to 20 times, preferably 5 to 12 times, more preferably 7 to 10 times the diameter of the die.
Adjust the cooling air so that it is between 12 times. If the length is shorter than 4 times, it is necessary to strongly apply the cooling air, the bubbles are shaken, the operation becomes unstable, and the thickness of the film varies. On the other hand, making it longer than 20 times will reduce the impact strength, so it should be avoided.

When operated under such conditions, the stability of bubbles is increased even for long-neck type bubbles, and high-speed take-up becomes possible, and it is stable even at 40 m / min or more, which is faster than the conventional LLDPE inflation speed. Operation is now possible.

[0025]

The melt viscosity of LLDPE is lower than that of HDPE, and it has not been adopted to form a bubble with a long neck in the film production by the ordinary inflation method. Therefore, the stability of the bubble is not good, and the fluctuation of the bubble and the unevenness of the film thickness are caused when the high-speed molding is performed, so that it has not been adopted as an industrial production method.

According to the present invention, the extrusion temperature is set as low as possible, and the die lip gap is 2.5 to 10 mm.
We succeeded in making the shape of the bubble a long neck by keeping it large, and by rapidly expanding it stabilizes the bubble even at high-speed molding (take-up speed is faster than before) and has good transparency. In addition, we were able to establish a manufacturing method for a film that had sufficient strength and had little thickness unevenness.

[0027]

The present invention will be described in more detail with reference to Examples and Comparative Examples. The impact strength is JIS K-721.
1. Haze gloss and image clarity are JIS K-710
5 was measured.

(Example 1) Density is 0.922 g / cm
³ , an ethylene-butene-1 copolymer (LLDPE) having an MFR of 0.80 g / 10 min was used, and the extruder bore diameter was 50.
mmφ, die diameter 80mmφ, die lip gap 3.0mm, using extrusion machine at 190 ℃, film thickness 30μm, folding width 400mm, take-up speed 30m / min, bubble is long neck type. The diameter was smaller than the diameter, and molding was performed at a frost line distance of 457 mm. The results are shown in Table 1.

(Example 2) LLDPE used in Example 1
Of 80% by weight and MFR of 2.50 g / 10 min LDPE
20% by weight of the composition and a lip gap of 5.0
mm and a folding width of 330 mm, a film was obtained under the same conditions as in Example 1. The results are shown in Table 1.

(Comparative Example 1) The same resin composition as in Example 2 was used except that the lip gap of the die was 1.0 mm and the folding width of the film was 400 mm, and a film of the same size was obtained under the same molding conditions. It was The results are shown in Table 1.

Comparative Example 2 A film was formed under the same conditions as in Comparative Example 1 except that the die lip gap was 3.0 mm and the frost line distance was 2400 mm. The results are shown in Table 1.

(Comparative Example 3) Dip lip gap 3.
0mm, expansion ratio 1.59, take-up speed 25m / min
A film was obtained under the same conditions as in Comparative Example 1 except that In this case, the bubbles were unstable and the operation was difficult, and there was a large variation in the film thickness. Table 1 shows the properties of the film obtained in a relatively stable manner.

(Comparative Example 4) Take-up speed 40 m / min
However, since the pressure at the tip of the extruder became high and felt dangerous, it was not possible to collect a sample under stable operating conditions.

[0034]

[Table 1]

[0035]

EFFECT OF THE INVENTION LLDPE is LDPE even if it has a low density.
It is known that it has a higher strength than H.P. and is superior in transparency (haze and clarity) to HDPE.
The present invention has developed a method for producing an inflation film that takes advantage of this characteristic.

According to the method of the present invention, LL having a low melt viscosity
Bubbles are stable even with DPE, and the strength of the resulting film in the vertical and horizontal directions is balanced. Especially, since it is a long-neck type, the vertical and horizontal balance is good, and the clarity is excellent. It will be a useful material for use.

[Brief description of drawings]

FIG. 1 is a cross-sectional view when a method of the present invention is carried out.

FIG. 2 is a sectional view of a bubble shape according to a conventional method.

FIG. 3 is a sectional view of a bubble shape according to a conventional method.

FIG. 4 is a cross-sectional view of a bubble shape according to a conventional method.

[Explanation of symbols]

 1 Die 2 Air ring 3 Frost line

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Bamba Takeshi Kawasaki-ku, Kawasaki-ku, Kanagawa 3-2 Chidori-cho, Showa Denko KK Kawasaki Resin Research Laboratory (72) Inventor Masashi Chide 3-2 Showa Denko KK Kawasaki Resin Research Laboratory

Claims (3)

[Claims] 1. A copolymer of ethylene and an α-olefin having 3 to 10 carbon atoms and having a density of 0.91 to 0.9.
4 g / cm ³ linear low density polyethylene from a die with a lip gap of 2.5-10.0 mm 1
A method for producing an inflation film, which comprises extruding at 70 ° C. to 210 ° C., expanding the expansion ratio to 2 to 5 while keeping the frost line distance 4 to 20 times the die diameter, and then taking over. 2. A linear low-density polyethylene is a linear low-density polyethylene containing 50% by weight or less of high-pressure low-density polyethylene, and has an MFR of 0.3 to 5.0 g /
The method for producing an inflation film according to claim 1, which is 10 minutes. 3. The method for producing an inflation film according to claim 1, wherein the bubble shape is a so-called long-neck type, and the minimum diameter of the cut is smaller than the die diameter.