JPH07276495A - Polyethylene resin for molding inflation film and production of inflation film - Google Patents

Abstract

PURPOSE:To provide a polyethylene resin for molding an inflation film capable of reducing the thickness deviation of the film even in the high speed molding of an air cooling inflation film and not generating various troubles in a secondary processing process. CONSTITUTION:A polyethylene resin for molding an inflation film is characterized by that density is 0.935-below 0.970g/cm<3>, a melt flow rate is 0.01-below 5.0g/10min and a strain curing parameter is 0.20-below 5.0.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a blown film molding polyethylene resin which has a good precision of uneven thickness when molding an air-cooled blown film and which can be molded at high speed.

[0002]

2. Description of the Related Art Polyethylene resin is extruded in a molten state from a tubular die into a tubular shape, which is cooled and solidified while being expanded by internal pressure and continuously wound to form an air-cooled inflation film, which requires high-speed molding to ensure high productivity. Has been done. However, if a high extrusion rate is used for high-speed molding, it is necessary to increase the cooling air volume for cooling and solidifying the molten bubbles, which makes the molten bubbles unstable and locally causes the molten bubbles to be flattened. It becomes difficult to adjust the uneven thickness. At this time, it is common to use a bubble stabilizer in order to prevent the fluctuation of the molten bubble generated by increasing the cooling air and to reduce the thickness deviation of the formed film (for example, Japanese Patent Publication No. 2180). However, in order to improve productivity, a high extrusion rate is required.When a high extrusion rate is used, the shear rate of the resin at the die outlet increases, so the spiral flow of resin inside the die remains unchanged from the die outlet. The so-called spiral mark that appears in the flow of water is generated. At this time, at the die slip outlet, the resin flow rate deviation becomes large between the portion where the spiral mark is generated and the other portion,
It was difficult to eliminate the thickness deviation of the film even when the bubble stabilizer was used for molding. For this reason, wrinkles and talumi are generated in the raw material of the film, and there are various problems such as printability and slitability in the secondary processing step of the film or a decrease in bag-making speed during film bag-making and heat-sealing failure. Was there.

In particular, the high-density polyethylene resin is sufficiently stretched and deformed with respect to the viscosity of a thin film portion that has been sufficiently stretched and deformed when air-cooled inflation film molding is performed, as compared with low-density polyethylene having long chain branches. Since the ratio of the viscosity of the received thin film portion is not large, the thin portion of the molten bubble is selectively stretched, and the uneven thickness accuracy is extremely poor. In order to disperse the thickness deviation of the film over time in order to solve this problem, a method of using a rotary die that rotates the inflation film molding die body itself at a constant cycle is also adopted, but the essence of the above problem However, there is a problem in that it is not a solution and the cost for modifying the existing equipment is high.

The internal cooling method developed for speeding up the air-cooled inflation molding is intended to improve the cooling efficiency of the molten bubble by the internal circulating air of the molten bubble to realize stable high-speed molding. The thickness deviation of the film was not always small.

In order to suppress the generation of resin spiral marks at the die outlet, it is effective to narrow the lip gap of the die. However, in air-cooled inflation film molding of high density polyethylene resin, the lip gap of the die is Usually, it is formed with a thickness of about 0.8 to 1.5 mm, but by making the lip gap of the die narrower, melt fracture of the resin easily occurs, and the transparency of the formed film is significantly reduced. was there.

[0006]

DISCLOSURE OF THE INVENTION The present invention provides a polyethylene resin for inflation film molding which can reduce the thickness deviation of the film even in high-speed molding of an air-cooled inflation film and does not cause various troubles in the secondary processing step. Another object of the present invention is to provide a method for producing an inflation film having excellent film thickness deviation.

[0007]

[Means for Solving the Problems] The above-mentioned problems are solved when the density is 0.
935 g / cm ³ or more 0.970 g / cm less than ^3, a melt flow rate of 0.01 g / 10 minutes or more 5.0 g / 1
It is solved by a high-density polyethylene resin for inflation film molding having a strain hardening parameter of 0.20 or more and less than 5.0 and a method for producing an inflation film using the polyethylene resin for inflation film molding of less than 0 minutes.

The polyethylene resin used in the present invention is a homopolymer of ethylene, or ethylene and C ₃ -C.
It is a copolymer with ₁₀ α-olefins. Examples of the α-olefin include propylene, butene-1, and hexene-1, and a plurality of α-olefins may be used. The polyethylene resin used in the present invention includes those produced by using a so-called Ziegler catalyst or Phillips catalyst and partially modified by a treatment such as electron beam crosslinking or crosslinking by addition of peroxide.

[0009] The density of the polyethylene resin used in the present invention is required to be less than 0.935 g / cm ³ or more 0.970 g / cm ^3, less than 0.937 g / cm ³ or more 0.965 g / cm ³ is Preferably 0.938 g /
less cm ³ or more 0.960 g / cm ³ is particularly preferred.
A resin having a density of less than 0.935 g / cm ³ makes the film less elastic, while it is difficult to produce a polyethylene resin having a density of 0.970 g / cm ³ or more.

On the other hand, the melt flow rate (JIS K72
It is measured at 10 and is hereinafter referred to as MFR. ) Is 0.01 g /
It is necessary to be 10 minutes or more and less than 5.0 g / 10 minutes, preferably 0.02 g / 10 minutes or more and less than 3.0 g / 10 minutes, and 0.03 g / 10 minutes or more and less than 2.0 g / 10 minutes. Particularly preferred. When air-cooled inflation molding is performed using a polyethylene resin with an MFR of less than 0.01 g / 10 minutes, a high quality film is obtained when the extrusion rate is high for high-speed molding and the resin melt fracture at the die exit becomes noticeable. Absent. In addition, since the polyethylene resin having an MFR of 5.0 g / 10 minutes or more has a small melt tension, the fluctuation of the molten bubble due to the amount of cooling air is large and high-speed molding is difficult.

The molecular weight distribution (M _W / M _N measured by gel permeation chromatography is used as an index of the molecular weight distribution. The larger this value is, the wider the molecular weight distribution is). And 5
It is preferably 60 or more and less than 60, particularly preferably 10 or more and less than 50. When M _W / M _N is less than 2, the moldability of the film is significantly reduced, and when M _W / M _N is 70 or more, the heat-sealing strength of the molded film is significantly reduced.

When the calorific value in the melting behavior of the resin is measured using a differential scanning calorimeter (hereinafter referred to as DSC), peaks appear at at least one place, and the maximum melting point among them is generally 120 ° C. or higher and lower than 140 ° C. And twelve
5 ° C or higher and lower than 135 ° C is preferable, 127 ° C or higher and 133 ° C
It is particularly preferable that the temperature is lower than ° C. When the maximum melting point is less than 120 ° C, the rigidity of the formed film becomes weak, and the maximum melting point is 140 ° C.
If it is above, the film becomes brittle and the tear strength is lowered.

The strain hardening parameter of the polyethylene used in the present invention must be 0.20 or more and less than 5.0, preferably 0.30 or more and less than 1.0, and 0.4
It is particularly preferably 0 or more and less than 0.80. When the strain hardening parameter is less than 0.20, the thickness deviation of the air-cooled inflation-molded film becomes large and the secondary processing becomes difficult, so that a high quality film cannot be obtained. When a resin having a strain hardening parameter of 5.0 or more is subjected to inflation by air cooling, a large amount of gel is generated in the film, and a high quality film cannot be obtained. When the strain hardening parameter is 0.20 or more, the thick portion of the molten bubble is selectively stretched by the internal pressure of the bubble, and the thin portion of the melt bubble has a high elongation viscosity due to the strain hardening phenomenon, so that it is not stretched much by the internal pressure. The thickness deviation of the formed film is reduced.

Here, the definition of the strain hardening parameter will be described. First, the elongational viscosity necessary for obtaining the strain hardening parameter is measured. The polyethylene resin formed into a strand with a uniform diameter was held in a constant temperature bath at 150 ° C for 10 minutes, then stretched from both ends of the strand at a constant strain rate, and the tension and the diameter of the strand at that time were determined with time. The elongational viscosity at each strain amount is calculated by.

FIG. 1 shows an example of measurement of elongational viscosity at each strain amount. The viscosity curve obtained in this measurement has a linear part 1 and a non-linear part 2. FIG. 2 shows the measured data illustrated in FIG. 1 with a strain rate of 0.03 / sec and a strain rate of 0.05.
This is an example in the case of measuring at two strain rates of 1 / second. The viscosity curve measured at a strain rate of 0.05 / sec shows the non-linear portion 3 earlier than the measurement start, as compared with the viscosity curve measured at a strain rate of 0.03 / sec. Figure 3
Is a linear portion of the viscosity curve when measured at a strain rate of 0.03 / sec, and a non-linear portion of the viscosity curve when measured at a strain rate of 0.05 / sec. Strain rate 0.0 at the same time from the start of measurement
The ratio of the viscosity at the time of measurement at a strain rate of 0.05 / sec to the viscosity at the time of measurement at 3 / sec was measured over time, and the strain rate of 0.
It is a graph of the strain amount at the time of measurement at 05 / sec, and a linear relationship is established. The slope of this graph is the strain hardening parameter.

The strain hardening parameter becomes large due to the presence of a molecular component having a long relaxation time in the resin. Therefore,
As a means for obtaining a polyethylene resin having a strain hardening parameter of 0.20 or more, introduction of a molecular component having a long relaxation time by addition of peroxide, electron beam crosslinking, addition of ultra-high molecular weight polyethylene resin or crosslinked polyethylene, etc. Is mentioned. Conventional high-density polyethylene has a wide molecular weight distribution produced by, for example, two-step polymerization and contains a molecular component with a long relaxation time to some extent.
It was insufficient to set the strain hardening parameter to 0.20 or more. In addition, almost no attempts have been made to add peroxide or to crosslink with an electron beam.

To mold an inflation film using the polyethylene resin of the present invention, a molding temperature of 1 is used.
It is extruded in a tubular shape from a tubular extrusion die in a molten state at 60 to 260 ° C., cooled and solidified while expanding at an internal pressure in a blow-up ratio range of 1.1 to 6.0, and continuously wound at a molding speed of 60 m / min or more. Good. If the molding temperature is lower than 160 ° C., the resin is not sufficiently melted, which makes it difficult to mold. If the molding temperature exceeds 260 ° C., the resin deteriorates and a high quality film cannot be obtained.

The blow-up ratio is the ratio of the maximum bubble diameter to the diameter of the die. Even if the blow-up ratio is less than 1.1 or the blow-up ratio exceeds 6.0, the vertical and horizontal strengths are A well-balanced, high-quality film cannot be obtained.

One of the features of the polyethylene resin of the present invention is that it can be molded at high speed. Conventional film thickness 5-20
The molding speed when molding a 0 μm film is 5 to 60 m.
Since the polyethylene resin of the present invention can be molded at this molding speed, it can also be molded at a molding speed of 60 m / min or more. The high-speed molding referred to in the present invention means molding in which the film forming speed is 60 m / min or more.

That is, the take-up speed is generally 5 m / min or more and less than 150 m / min, and 40 m / min or more and 130 m.
/ Min is preferable, and 60 m / min or more and less than 120 m / min is particularly preferable. If the take-up speed is less than 5 m / min, the film becomes extremely weak and the take-up speed is 150 m.
If it is higher than 1 minute, the TD strength of the film is significantly reduced.

When an inflation film is formed by using the polyethylene resin of the present invention, an inflation film having a significantly excellent film thickness deviation can be obtained. The uneven thickness of a film refers to the ease of reducing the thickness deviation of the film, and is defined as the difference between the maximum value and the minimum value of the film thickness when the thickness deviation of the film is as small as possible. The uneven thickness of a film is often used as an index of the formability of the film.

Conventionally, when the amount of cooling air is increased in order to increase the extrusion rate for the purpose of high production rate, the bubbles locally become flat. Further, when the extrusion amount was increased, the spiral mark of the resin became remarkable, and the thickness deviation of the film increased. The thickness deviation of this film is adjusted by adjusting the die slip interval with an adjustment bolt, but this method can only make adjustments in the symmetrical direction of the die slip, and when forming at a high extrusion rate, a film with a small thickness deviation can be used. It was difficult to get. When the polyethylene resin of the present invention is used, the thickness deviation of the film can be remarkably reduced by using it together with the method of adjusting the die slip interval with an adjusting bolt, and even if the cooling air is increased, the bubbles are locally flattened. It never happens.

[0023]

EXAMPLES The present invention will be described in more detail with reference to Examples and Comparative Examples. The impact strength of the film was determined according to JIS P8134 by the impact strength when punching out the center part of the test piece in which a 1-inch hemisphere impact ball was clamped. Further, the thickness deviation of the film was determined by the difference between the maximum value and the minimum value of the thickness of the formed film which was continuously measured in the circumferential direction. Moldability (bubble stability)
The evaluation was shown by the following criteria. A: Bubbles are stable for a long time, and a good film is obtained. ◯: Stable molding is possible although slight movement occurs in the bubble. Δ: Bubbles fluctuate up and down, and film width fluctuates. X: Bubbles are not stable and stable molding is impossible.

(Example 1) Density is 0.945 g / cm
^3. An air-cooled inflation film of high-density polyethylene (referred to as polyethylene A) having a MFR of 0.05 g / 10 minutes and a strain hardening parameter of 0.35 and using hexene-1 as a comonomer was formed. Inflation film is molded with a screw diameter of 75 mm
Bubbles were extruded from a die having a die slip diameter of 120 mmφ provided in a φ extruder, and a film having a blow-up ratio of 3.0 and a thickness of 30 μm was taken at a take-up speed of 30 m / min, 60 m / min and 80 m / min. The results are 80 as shown in Table 1.
There was no problem in molding at m / min, the thickness deviation of the obtained film was as good as 3 μm, and the impact resistance strength (film impact) of the film was 540 kg · cm / mm.

Example 2 An inflation film was molded in the same manner as in Example 1 except that polyethylene A was cross-linked with peroxide and a strain hardening parameter of 0.60 (polyethylene B) was used. The density of polyethylene B was 0.942 g / cm ³ , and the MFR was 0.04 g / 10 minutes. The results are as shown in Table 1.
There was no problem in molding at 80 m / min, the thickness deviation of the obtained film was as good as 2 μm, and the impact resistance strength (film impact) of the film was 520 kg · cm / mm.

(Example 3) Density is 0.943 g / cm
³ , MFR 0.1g / 10 minutes, strain hardening parameter 0.40, high-density polyethylene (referred to as polyethylene C) air-cooled inflation film using butene-1 as a comonomer was molded to obtain moldability and molding. The thickness deviation of the film was measured. Inflation film is formed by extruding bubbles from a die with a die slip diameter of 120 mmφ provided in an extruder having a screw diameter of 75 mmφ, and a film having a thickness of 30 μm at a blow-up ratio of 3.0 and a take-up speed of 30 m / min, 60 m / min and 80 m.
Per minute. The results are shown in Table 1. The moldability of the film was good. Also, the thickness deviation of the film is as good as 2 μm, and the impact strength of the film (film impact)
Was 440 kg · cm / mm.

(Example 4) Polyethylene C was cross-linked with an electron beam and had a density of 0.940 g / cm ³ and MFR.
Is 0.08 g / 10 minutes, strain hardening parameter is 0.65
An air-cooled blown film was molded using the medium density polyethylene (referred to as polyethylene D) under the same conditions as in Example 3. The thickness deviation of the film was as small as 1.5 μm, which was excellent, and the impact resistance strength (film impact) of the film was 410 kg · cm / mm.

(Comparative Example 1) Density 0.945 g / cm
³ , air-cooled inflation under the same conditions as in Example 1, using high-density polyethylene (referred to as polyethylene E) having a MFR of 0.05 g / 10 min, a strain hardening parameter of 0.15, and hexene-1 as a comonomer. The film was formed. As shown in Table 1, the moldability of the film was poor, the difference between the maximum value and the minimum value of the film was as large as 6 μm, and the impact strength (film impact) of the film was 590 kg · cm / mm.

[0029]

[Table 1]

[0030]

By using the polyethylene resin for forming an inflation film of the present invention, it is possible to significantly reduce the thickness deviation of the formed film in the formation of an air-cooled inflation film, and to form a high-quality inflation film at a high speed. It has become possible.

[Brief description of drawings]

FIG. 1 is an example of measurement of change with time of extensional viscosity of a polyethylene resin stretched at a constant strain rate.

FIG. 2: Strain rate 0.03 / sec and strain rate 0.05 /
It is an example of measuring the change over time in the extensional viscosity of a polyethylene resin stretched in seconds.

FIG. 3 is an explanatory diagram of a method for obtaining a strain hardening parameter.

[Explanation of symbols]

1 linear part of extensional viscosity curve 2 non-linear part of extensional viscosity curve 3 non-linear part of extensional viscosity curve measured at a strain rate of 0.05 / sec 4 measurement time range of strain hardening parameters

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Toshio Taka 3-2, Chidori-cho, Kawasaki-ku, Kawasaki-shi, Kanagawa Akira Kawasaki Plastics Research Laboratory

Claims (3)

[Claims] 1. The density is 0.935 g / cm ³ or more and 0.9.
Less than 70 g / cm ³ , melt flow rate 0.01 g
A polyethylene resin for blown film molding having a strain hardening parameter of 0.20 or more and less than 5.0 for 10 minutes or more and less than 5.0 g / 10 minutes. 2. A molecular weight distribution (M _W / M _N ) of 2 or more 70
And the maximum melting point by differential scanning calorimeter is 120 ° C.
The polyethylene resin for blown film molding according to claim 1, which has a temperature of 140 ° C. or higher. 3. The polyethylene resin according to claim 1 or 2 is extruded into a tubular form from a tubular extrusion die in a molten state at a molding temperature of 160 to 260 ° C., and a blow-up ratio of 1.1 to.
A method for producing an inflation film, comprising cooling and solidifying while expanding at an internal pressure in a range of 6.0, and continuously winding the film at a molding speed of 60 m / min or more.