JPH0418347A - Polyethylenic heat-shrinkable laminated film - Google Patents

Abstract

PURPOSE:To reduce thickness irregularity and to improve sealing properties at the time of high speed packing by respectively using a resin having high heat of fusion in the innermost and outermost layers and using at least one layer based on an ethylene/alpha-olefin linear copolymer having a specific crystal m.p. as an intermediate layer. CONSTITUTION:At least one layer based on linear low density polyethylene characterised by that density is 0.870 - 0.930 g/cm<3>, a melt index is 0.1 - 10 g/10 min, the main peak temp. Tma of a fusion curve after quenching is 118+ or -5 deg.C and the temp. difference between main peak temp. Tmb after gradual cooling and Tma is 3 deg.C or more is contained as an intermediate layer and layers based on linear low density polyethylene (B) characterized by that a melt index is 0.1 - 10g/10 min, the total heat of fusion in a fusion curve after gradual cooling is 110 mJ/mg or more and the heat of fusion at main peak temp. or higher is 15 mJ/mg or more are contained as the innermost and outer most layers and the thickness of the intermediate layer to all of layers is 60% or more and the thickness of each of the innermost and outermost layers is set to 1 mum or more.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a polyethylene heat-shrinkable laminated film, more specifically a polyethylene-based heat-shrinkable laminated film that is made of a specific ethylene copolymer and has small thickness unevenness and is suitable for packaging machines. Concerning an excellent heat-shrinkable laminated packaging film.

(Prior Art) Stretched films of polyvinyl chloride, polypropylene, polyethylene, and the like are conventionally known as heat-shrinkable films. Among these, polyethylene heat-shrinkable films have attracted attention for their heat-sealability, low cost, and excellent impact resistance, and are expected to be used in many fields.

On the other hand, the packaging speed of packaging machines has increased significantly in recent years.
There is a need to supply packaging materials suitable for these new packaging machines.

(The problem that the invention aims to solve) In recent years, the speed of technology has increased significantly, so
It has been pointed out that the melt-cut sealability is not necessarily satisfactory, and there is a desire for a film that has excellent low-temperature shrinkability, small thickness unevenness, and excellent melt-cut sealability.

When using polyethylene heat-shrinkable film as a packaging material in automatic packaging machines (bellow packaging machines, half-folding automatic packaging machines), the packaging speed of packaging machines has increased significantly in recent years, resulting in heat seal failure (partially If the film is not sealed (not sealed) or if the film has poor flatness, wrinkles tend to form during film running, which can cause not only running problems but also poor heat sealing.

When packaging with an automatic packaging machine, heat sealing is generally performed using a heat knife, but sealing failure occurs when the resin adheres to the cutting edge of the heat knife during sealing using a heat knife, resulting in the product not being cut. This means that holes may occur, or in extreme cases, there may be no seal at all. In addition, one of the causes of poor sealing is the sticking of the film to the base of the heat knife receiver.

Furthermore, if the film has large thickness unevenness or poor flatness, wrinkles are likely to form during film running, and pinholes are likely to occur when the wrinkled portions are sealed by melting, resulting in poor sealing.

(Means for Solving the Problems) The present invention solves the above-mentioned problems and provides a heat-shrinkable film with small thickness unevenness and excellent fusing sealability on automatic packaging machines. After careful consideration of the structure and various raw material resins, we decided to use resins with a high heat of fusion at temperatures above the melting point for the innermost and outermost layers, respectively.
By using at least one layer mainly composed of a linear copolymer of ethylene and α-olefin having a specific crystal melting point in the intermediate layer, thickness unevenness can be reduced and sealing suitability during high-speed packaging can be improved. This is what led to the present invention.

That is, in the present invention, the intermediate layer has a density of 0.870 to 0.930.
g/cm3, melt index 0.1-10g/1
0 minutes, and in the melting point measurement using a differential scanning calorimeter (hereinafter abbreviated as DSC), after holding at 190°C for 30 minutes, the temperature was lowered to 20°C at a cooling rate of 100°C/min, and then at a heating rate of 10°C/min. Ta
+a, To+a is in the range of 118±5℃,
Furthermore, after holding at 190°C for 30 minutes, the temperature is lowered to 20°C at a cooling rate of 10°C/min, and then the temperature is raised at a heating rate of 10°C/min. ! (hereinafter referred to as the melting curve after slow cooling) when the main peak temperature is Tmb, Tll1
At least one layer made of a composition mainly composed of linear low density polyethylene (A) with a temperature difference of 3° C. or more between b and Tma, and a melt index of 0.1 to 10 g/
10 minutes, and the total heat of fusion in the melting curve after slowing 4 is 110 mJ/■ or more, and the heat of fusion at a temperature equal to or higher than the main peak temperature is 15 mJ/ntg or more. Linear low-density polyethylene (B) A laminated film comprising layers each consisting of a composition as a main component as the innermost layer and the outermost layer and stretched in at least one axis, wherein the thickness of the intermediate layer is 60% or more of the total layer, and the innermost layer and the outermost layer are Each thickness is 1μ or more, thickness unevenness is less than 10%, 9
The gist of the present invention is a polyethylene heat-shrinkable laminated film characterized by an area shrinkage rate of 20% or more at 0°C. The above DSC measurement method is for samples 8 to 10i.
+8 is sealed in an aluminum pan and carried out in a nitrogen stream.

In the present invention, the linear low density polyethylene (A) used as the main component of at least one intermediate layer includes ethylene, propylene, butene-1, pentene-1, etc.
, hexene-1, heptene-1,4-methylpentene-
1. Copolymer with 1 or more α-olefins having 3 to 20 carbon atoms, preferably 4 to 8 carbon atoms, containing 1 octene, 1 decene, 1 undecene, and 1 dodecene. can be given.

Among these resins, the main components are at least 50% by weight of ethylene and two types of α-olefins, and the total degree of branching is 2. Particularly preferred are those which are terpolymer copolymers with OCH3/100C or more, or those whose main components are ethylene and an α-olefin having 4 to 8 carbon atoms and whose degree of branching is 2.5CH3/100C or more. More preferably, a copolymer of ethylene and 1-butene, a terpolymer of ethylene and 4-methylpentene-1 and 1-butene, and a terpolymer of ethylene and 1-octene and 1-butene. is even more suitable.

Moreover, a linear copolymer of ethylene and α-olefin (A)
A material having a density of 0.870 to 0.930 g/cw+3 and a melt index of 0.1 to 10 g/10 minutes is used, and more preferably a density of O-910 to
0.925g/cm3, melt index 0.2~
A material having a characteristic value of 5.0 g/10 minutes is used.

If the density is less than 0.870 g/cm3, the effect of improving the stiffness of the film will be small, and if it exceeds 0.930 g/cm3, the low-temperature shrinkability will be insufficient, which is not preferable. If the melt index is less than 0.1 g/10 minutes, it is undesirable because the motor load during melt extrusion will increase and processing suitability will deteriorate, and if it exceeds 10 g/10 minutes, the stability in the stretching process will be insufficient. Therefore, it is not desirable.

As the linear low-density polyethylene (A), one whose melting curve after quenching has an endothermic area in the Ta+a±5° C. range of 15% or more of the total endothermic area is more preferably used.

It is necessary that the thickness of the intermediate layer is 60% or more of the total thickness. When the thickness of the intermediate layer is less than 60%, the thickness unevenness of the stretched film becomes large. Further, it is necessary to select the thickness of the innermost layer and the outermost layer after stretching to be at least 1 μm or more each, and if the thickness is less than 1 μm, excellent heat-sealability cannot be exhibited.

The intermediate layer is made of the above linear low density polyethylene resin (A).
The linear copolymer (B) of ethylene and α-olefin used as the main component in the innermost layer and the outermost layer, respectively, may be composed of multiple layers consisting of resins of different grades belonging to , the total heat of fusion in the melting curve after slow cooling is 110 mJ/11
1 g or more, and has a heat of fusion of 15 mJ/lag or more at a temperature equal to or higher than the main peak temperature, and a melt index of 0.1 to 10 g/10 minutes, more preferably 0.
Those having characteristic values of 2 g to 5.0 g/10 minutes are used.

If the melt index is less than 0.1 g/l for 0 minutes, it is unfavorable in terms of a decrease in processability and a decrease in transparency due to roughening of the film surface, and if it exceeds 10 g/10 minutes, the heat-sealing strength decreases and stretching It also has a negative effect on workability.

The above linear copolymer of ethylene and α-olefin (B
) The α-olefin copolymerized with ethylene is not particularly limited, and includes, for example, those having 4 to 12 carbon atoms,
Butene-1, Pentene-1, Hexene-1, Heptene-1
1, octene-1,4-methylpentene-1, decene-
1, undecene-1, dodecene-1, etc.
α-olefins having 4 to 8 carbon atoms are more preferably used.

The above-mentioned linear low-density polyethylenes (A) and (B) can be easily obtained by a low-medium pressure method using a so-called Ziegler-Natsuta type catalyst, and their production methods are described in Japanese Patent Publication No. 32270/1983, JP-A-49-35345, JP-A-55-78004, JP-A-55-8
This can be achieved by techniques disclosed in Japanese Patent Application Laid-open No. 6804, Japanese Patent Application Laid-open No. 154488/1983, and the like.

Furthermore, the resin compositions used in each layer may be used alone or in a mixture of two or more, and furthermore, high-pressure polyethylene, ethylene-vinyl acetate, etc. Polymers, ionomers, and polyolefin resins such as ethylene-propylene copolymers can be used in combination.

In addition, the laminated film of the present invention has the above-mentioned middle layer and innermost layer,
In addition to the outermost layer, there is one or 2N intermediate layer made of a polyolefin resin other than the linear low-density polyethylene resin (A) and (B), within a range that satisfies the thickness conditions of each layer.
It may contain more than that. Polyolefin resins used for such an intermediate layer include linear low-density polyethylene resins other than the linear low-density polyethylene resins (A) and (B), high-pressure polyethylene, ethylene-propylene copolymers, and other polyolefin resins. Examples include resins, and one or more types thereof may be appropriately selected and used within a range that does not impede the purpose of the present invention.

In addition, additives such as lubricants, anti-blocking agents, antistatic agents, and anti-fogging agents may be used as appropriate to provide their respective effective effects, especially in the innermost layer.
Useful for the outermost layer.

Next, a method for manufacturing the film of the present invention will be described. The method for producing an unstretched film used to produce the stretched film of the present invention using the above-mentioned resin and the method for producing a stretched film by stretching this unstretched film can be carried out by known methods, but the following methods may be used. This will be specifically explained by taking as an example the case of forming and stretching a three-layer laminated tubular film.

First, the linear copolymer of ethylene and α-olefin (
A) is melt-kneaded using three extruders so that the intermediate layer and the linear copolymer of ethylene and α-olefin (B) are the inner and outer layers, and the mixture is coextruded into a ring shape from a three-layer annular die and stretched. A tubular unstretched film is produced by rapidly cooling and solidifying the film.

The obtained tubular unstretched film is fed to a tubular stretching apparatus such as that shown in FIG. Simultaneous biaxial orientation is caused by expansion stretching by applying gas pressure inside the tube at low temperature. The stretching ratio does not necessarily have to be the same lengthwise and crosswise, but in order to obtain excellent physical properties such as strength and shrinkage, it should be at least 2 times, preferably 2.5 times or more, and more preferably 3 times in both directions. It is preferable to stretch the film to a length higher than that.

The film taken out from the stretching device can be annealed if desired, and this annealing can suppress natural shrinkage during storage.

(Examples) The present invention will be specifically explained below with reference to Examples, but the present invention is not limited to these Examples.

In addition, each physical property measurement shown in this example was carried out by the following method.

■Weigh a sample with a heat of fusion of 8 to 10IIIg and then seal it in an aluminum pan.
Using a differential calorimeter, the temperature was raised from room temperature to 190°C in 30 m1/s+in of desired air, and maintained at this temperature for 30 minutes.
It is then cooled to room temperature at about 10° C./va + n.

Thereafter, the heat of fusion is calculated from the area of the endothermic peak using a melting curve obtained at a temperature increase rate of 10° C./win.

(2) Area Shrinkage Rate A film cut into a square of 10.0 cis in length and width was immersed in a glycerin bath at 90° C. for 10 seconds, taken out, and rapidly cooled in water. The length and width in both length and width were measured and calculated using the following formula.

Area shrinkage rate = 100-AXB However, A and B are the vertical and horizontal lengths after quenching (unit: 1
) is shown.

■The maximum value (T
1lax), the minimum value (Twin), and the average value (T), and calculated using the following formula.

1. However, T is the arithmetic mean value of the values read from the chart corresponding to 101W11W1 intervals of the measurement film.

■Thickness of each layer The thickness of each layer in the lamination was measured by observing the cross section of the film with a microscope.

■Haze Compliant with ASTM-D 1003.

■Packaging machine aptitude (a) Tokiwa Kogyo Co., Ltd.! ! Automatic packaging machine (model PW-R
2. Using a film with a width of 380 m in the pillow packaging machine,
Cup noodles were packaged at a speed of 100 pieces/minute. The sealed portion after shrink-wrapping was observed, and those with no strings or pinholes of 1 m or more in the sealed portion were judged to be good.

Using this packaging machine, 100 pieces were continuously packaged, and the quality of the products was measured. Incidentally, the margin of film dimensions during preliminary packaging before shrinkage was 10% for both length and width.

(b) Half-folding automatic packaging machine manufactured by Kyowa Denki Co., Ltd. (model AT-5)
00), a half-folded film with a width of 400- is supplied, and the length is 23.
100 bento boxes (200 g) of size 5 (?ll, width 15.5 cm, height 5.6 (!II+)) were continuously packaged at a speed of 25 pieces/1 minute, and the yield rate was measured.

The standards for non-defective products were the same as for pillow packaging. In addition, the margin for preliminary packaging was set at 13% for both length and width.

Example 1 A linear low-density polyethylene resin made of a ternary copolymer of ethylene having the properties shown in Table 1 and 4-methylpentene-1 and butene-1 as comonomers was used as an intermediate layer, and the same Ethylene and 4- with the properties shown in 1.
Three extruders (for the middle layer,
A three-layer annular product was melt-kneaded at 170°C to 240°C (for the innermost layer and for the outermost layer), and maintained at 240°C by setting the extrusion amount from each extruder assuming the thickness ratio shown in Table 1. It was coextruded downward from the die.

The formed three-layered tube is cooled by sliding it on the outer surface of a cylindrical cooling mandrel in which cooling water is circulated on the inside, while the outer side is passed through a water tank, and then taken out.
An unstretched film with a length of 5 m and a thickness of 320 μm was obtained. The thickness of each layer was adjusted by adjusting the screw rotation speed and take-up speed of the extruder. This tubular unstretched film was introduced into the tubular biaxial stretching apparatus shown in FIG. 1, and stretched four times in length and width at 95 to 105°C to obtain a laminated biaxially stretched film. Next, this stretched film was treated with hot air at 75° C. for 10 seconds in a tube annealing device, cooled to room temperature, folded and rolled up.

The stability during stretching was good, with no vertical movement of the stretching point or swinging of the stretching tube, and no uneven stretching conditions such as necking were observed. The obtained stretched film had the thickness structure shown in Table 1, had excellent transparency and low-temperature shrinkability, and had small thickness unevenness.

As shown in Table 3.4, continuous packaging evaluation of cup ramen and bento boxes was performed using a pillow packaging machine and a half-folding automatic packaging machine, and it was found that there were few seal defects and good packaging suitability over a wide temperature range. It had a

Example 2.3 A heat-shrinkable laminated film was produced in the same manner as in Example 1 using the resin composition shown in Table 1. The film was excellent in transparency and low-temperature shrinkability, had small thickness irregularities, and had excellent packaging suitability.

Example 4 A heat-shrinkable laminated film was produced in the same manner as in Example 1 using the resin composition shown in Table 1. Although 5000 ppm of stearic acid monoglyceride was added as an antifogging agent to the inner and outer layers, the film had excellent transparency and low-temperature shrinkability, had small thickness irregularities, and was excellent in packaging suitability.

Example 5 In Example 1, the extrusion device was replaced with a device having a four-layer annular die connected to four extruders, and a high-pressure low-density polyethylene resin (density 0.922) was used between the middle layer and the innermost layer.
g/Cm3, Ml is 0.5 g/]O min), and the thickness ratio of each layer of the innermost layer, second intermediate layer, intermediate layer, and outermost layer is 15. 10.60.
A four-layer heat-shrinkable laminated film was produced in the same manner as in Example 1 except that the resin used and the stretching conditions were set to 15. The thickness unevenness of the obtained film was 9.5%, the area shrinkage rate was 25.0%, and the haze was 3.0%, indicating that the film had excellent thickness unevenness, low-temperature shrinkability, and transparency, and was excellent in packaging suitability. Ta.

Comparative Example 1 As shown in Table 1, a linear low-density polyethylene resin having a melting point after quenching of 126°C and no melting point in the range of 11B±5°C was used for the intermediate layer, and Example 1 was used for the inner and outer layers. Using the same linear low-density polyethylene resin as that used for the inner and outer layers of Example 1, it was extruded under the same conditions as in Example 1, cooled, and taken off to produce a laminated unstretched film material with a diameter of about 75-1 and a thickness of 320 μm. I got the opposite. The thickness of each layer was adjusted by adjusting the screw rotation speed and take-up speed of the extruder.

Next, in the same manner as in Example 1, 9
It was stretched 4 times in length and width at a temperature of 5 to 105°C.

A heat-shrinkable laminated film with poor stability during stretching, large fluctuations, and large thickness unevenness was obtained.

This tubular unstretched film was introduced into the tubular biaxial stretching apparatus shown in FIG. 1, and stretched four times in length and width at 95 to 105°C to obtain a laminated two-wheel stretched film. Next, this stretched film was passed through a tube annealing device for 7
After being treated with hot air at 5° C. for 10 seconds, it was cooled to room temperature, folded and rolled up.

The resulting heat-shrinkable film was run on a packaging machine to evaluate its suitability for packaging, and although resin adhesion to the fused seal bar and threading were slightly improved, the sealing performance was insufficient as wrinkles were likely to occur. Met.

Comparative Example 2.3 The intermediate layer shown in Table 1 used the same linear low-density polyethylene resin as in Example 1, and the inner and outer layers used linear low-density polyethylene resin with a total heat of fusion of 110 mJ/IIg or less, compared to Comparative Example 2. For low density polyethylene resin, comparative example 3, the total heat of fusion was 110 m
A heat-shrinkable laminated film was obtained in the same manner as in Example 1 using a linear low-density polyethylene resin whose heat of fusion at temperatures above the melting point was 15 mJ/11 g or above, but not below J/w+g. . The resulting heat-shrinkable film was transparent, had low-temperature heat-shrinkability, and had little thickness unevenness;
In a package test using a packaging machine, resin adhesion to the heat knife and film adhesion to the heat knife holder were observed, and there was also a hole about 3 mm long in the seal area, resulting in very poor sealing performance. It was stable.

Comparative Example 4 The resin used for the intermediate layer of Example 3 was used for the intermediate layer, and the resin used for the inner and outer layers of Example was used for the inner and outer layers.

A heat-shrinkable laminated film was obtained in the same manner as in Example 1, except that the thickness of the intermediate layer was 50% of the total thickness. The film had poor stability during the stretching process and had large thickness unevenness.

Transparency and low-temperature heat shrinkability were excellent, but in a packaging test using a packaging machine, no resin was observed to adhere to the heat knife, nor did the film stick to the base of the heat knife holder, but the flatness was poor and wrinkles were observed. The sealing performance was insufficient because it was easy for the particles to enter.

Comparative Example 5 A heat-shrinkable laminated film was obtained in which both the intermediate layer and the inner and outer layers had the same resin composition as in Example 1. The thickness of the inner and outer layers of the heat-shrinkable film was set to be very thin, less than 1 μm.

Although the heat-shrinkable laminated film had excellent transparency and low-temperature heat-shrinkability, and had small thickness unevenness, a packaging test using a packaging machine showed that molten resin adhered to the heat knife, and the sealing performance was insufficient. Ta.

[Hereinafter referred to as "mill"] (Effects of the Invention) When producing a heat-shrinkable film, the present invention uses at least one layer mainly composed of a linear low-density polyethylene resin having a specific melting point structure as an intermediate layer. By adopting a laminated structure using linear low-density polyethylene resin with a specific heat of fusion as a layer, the tube has good stretching stability, has small thickness unevenness, has excellent low-temperature shrinkability, and has good heat sealability. In addition, the present invention provides a heat-shrinkable material that has low adhesion to a heat knife during heat sealing and is therefore highly suitable for high-speed packaging.

[Brief explanation of drawings]

FIG. 1 is a sectional view for explaining a tubular biaxial stretching apparatus used in Examples. A...Stretching device

Claims (4)

[Claims] (1) In polyethylene heat-shrinkable laminated film,
As an intermediate layer, the density is 0.870-0.930g/cm^
3. The melt index is 0.1 to 10 g/10 minutes, and in the melting point measurement using a differential scanning calorimeter (hereinafter abbreviated as DSC), after holding at 190°C for 30 minutes, the temperature was lowered to 20°C at a cooling rate of 100°C/min. and then increase the temperature to 10
When the main peak temperature of the melting curve (hereinafter referred to as the melting curve after rapid cooling) obtained when the temperature is raised at a rate of °C/min is Tma, Tma is in the range of 118 ± 5 °C, and the temperature is maintained at 190 °C for 30 minutes. When the main peak temperature of the melting curve (hereinafter referred to as the melting curve after slow cooling) obtained when the temperature is lowered to 20 °C at a post-cooling rate of 10 °C/min and then heated at a temperature increase rate of 10 °C/min (hereinafter referred to as the melting curve after slow cooling) is Tmb. , at least one layer consisting of a composition mainly composed of linear low density polyethylene (A) with a temperature difference between Tmb and Tma of 3 ° C. or more, and a melt index of 0.1 to 10 g/10 minutes, And the total heat of fusion in the melting curve after slow cooling is 110 mJ/mg
The above-mentioned structure includes, as the innermost layer and the outermost layer, layers each composed of a composition mainly composed of linear low-density polyethylene (B) having a heat of fusion of 15 mJ/mg or more at a temperature equal to or higher than the main peak temperature, and at least one axial A laminated film stretched in the direction, in which the thickness of the intermediate layer is 60% or more of all the layers, the thickness of the innermost layer and the outermost layer is 1 μm or more each, the thickness unevenness is 10% or less, and the area shrinkage rate at 90 ° C. 20% or more of polyethylene heat-shrinkable laminated film. (2) The heat-shrinkable polyethylene according to claim 1, characterized in that in the melting curve of the linear low-density polyethylene (A) after quenching, the endothermic area in the range of Tma±5°C is 15% or more with respect to the total endothermic area. laminated film. (3) Linear low density polyethylene (A) has at least 50%
The polyethylene heat-shrinkable laminate according to claim 1, characterized in that it is a terpolymer containing ethylene and two types of α-olefins in an amount of at least 2% by weight and a total degree of branching of at least 2.0CH_3/100C. film. (4) Linear low-density polyethylene (A) contains ethylene and an α-olefin having 4 to 8 carbon atoms as main components, and has a degree of branching of 2.
．． The polyethylene heat-shrinkable laminated film according to claim 1, characterized in that the polyethylene heat-shrinkable laminated film has a hardness of 5CH_3/100C or more.