JPS62201229A - Heat shrinkable film - Google Patents

Abstract

PURPOSE:To obtain a heat shrinkable film being reduced in thickness irregularity, enhanced in orientation efficiency and excellent in heat shrinkability at low temp., by specifying an ethylenic polymer. CONSTITUTION:An ethylenic polymer composition, which contains one or more kind of a linear copolymer of ethylene and alpha-olefin and of which the heat absorbing area equal to or less than temp. 10 deg.C lower than m.p. (heat absorbing main peak) in a melting curve of said copolymer of copolymer mixture due to a differential scanning calorimeter (hereinafter referred to as DSC) occupies 55% or more of a total heat absorbing area, is subjected to melt extrusion and solidified under cooling to obtain a film which is, in turn, biaxially stretched by 2 times or more to form a heat shrinkable film of which the area shrinking ratio of 20% or more at 90 deg.C and the thickness irregularity is 20% or less. As alpha-olefin, there are butene-1 and pentene-1 etc. and the pref. content of alpha- olefin is 0.5-10mol%. If the condition such that the heat absorbing area equal to or less than temp. 10 deg.C lower than m.p. in the melting curve of the resin to be used occupies 55% or more of the total heat absorbing area is not satisfied, no uniformity of stretching is obtained and, if stretching temp. is raised in order to obtain uniformity, the lasting stability of stretching is damaged.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to shrink packaging materials, and more specifically, the present invention relates to shrink packaging materials, and more specifically, it is made from a composition mainly containing a specific ethylene-α-olefin copolymer, and This relates to plastic packaging films with excellent properties and small thickness irregularities.

(Prior Art) Conventionally known heat-shrinkable films include polyvinyl chloride, biaxially stretched polypropylene films, and polyethylene films. Among these, polyethylene film is manufactured by the so-called inflation method, in which a tube extruded from a round die is blown up using gas pressure. Due to its characteristics, it is widely used for shrink wrapping applications.

(Problems to be solved by the invention) Polyethylene heat-shrinkable films manufactured by the inflation method do not have effective molecular orientation due to the manufacturing method, so the film has low strength and elongation, resulting in high shrinkage unless the temperature is close to the melting point. The performance characteristics are not necessarily satisfactory.

In order to improve these defects, so-called irradiated polyethylene film is produced by irradiating a polyethylene film with ionizing radiation to cause intermolecular crosslinking, and then heating and stretching the film.
Although there are improvements such as increased strength and decreased elongation due to the orientation effect of stretching, it still has drawbacks such as poor heat sealability, inability to collect waste, and inevitable cost increases due to radiation treatment.

The present inventors conducted various studies on polyethylene-based shrink films from the raw material and technical aspects, and improved the heat-shrinkability by stretching and orientation while taking advantage of the excellent properties of polyethylene films such as heat-sealability and impact resistance. We have discovered a heat-shrinkable polyethylene film and its manufacturing method. (Japanese Patent Publication No. 57-36.142) The film obtained by this invention exhibits the shrinkage rate and shrinkage stress necessary for shrink wrapping even below the υ melting point, which is different from the so-called blown film.
However, although it has good shrink-wrapping properties that tightly adhere to the packaged object, the stability of the stretched tube and the uniformity of stretching are not necessarily satisfactory, and it has the disadvantage of large thickness unevenness. It was difficult to achieve a sufficient stretching orientation effect, and therefore the shrinkage properties at low temperatures remained problematic.

(Means for Solving the Problems) As a result of intensive studies aimed at increasing the stretching orientation effect and improving the stability and uniformity of stretching, the present inventors have identified an ethylene-based polymer, thereby inventing the present invention. reached.

That is, the present invention includes one or more linear copolymers of ethylene and α-olefin, and the melting point (endothermic main peak temperature) of the copolymer or copolymer mixture measured by DSC is ) is obtained by melt-extruding an ethylene polymer composition whose endothermic area at a temperature below 10°C is 55 inches or more of the total endothermic area, and stretching the film obtained by cooling and solidifying the film by at least twice the length and width. It is a heat-shrinkable film having an area shrinkage rate of 20% or more and thickness unevenness of 20% or less at 90°C.

The main resin of the ethylene polymer composition used in the present invention is an ethylene-α-olefin linear copolymer, which may be used alone or in a mixture of 211 or more.

α-olefins copolymerized with ethylene include butene-1, pentene-1, hexene-1, heptene-1, octene-1,4 methylpentene-1, decene-1, undecene-1, dodecene-1, etc. be. These copolymerizations can be carried out 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. 50-32270.49-35345, Japanese Patent Application Laid-open No. 55-78004, 55-86804, Japanese Unexamined Patent Publication No. 5
4-154488, etc. can be used. The content of α-olefin is preferably in the range of 0.5 to 10 mol.

Further, high-pressure polyethylene, ethylene-vinyl acetate copolymer ionomer, ethylene-propylene copolymer, etc. can be mixed and used within a range that does not impede the purpose of the present invention.

Also, lubricant, anti-blocking agent, antistatic agent.

It goes without saying that additives such as antifogging agents are used as appropriate to provide their respective effective effects.

The resin used in the present invention is the above-mentioned polyethylene polymer composition, and the endothermic area of the melting curve measured by a differential scanning calorimeter is 55% of the total endothermic area at a temperature 10°C lower than the melting point (endothermic main peak). Must be at least 100%. If this condition is not met, it is not easy to obtain uniformity of stretching, and if the stretching temperature is increased to obtain uniformity, the sustained stability of stretching is impaired, and the stretching orientation effect is less likely to occur. Shrinkage performance at low temperatures becomes insufficient. Products that meet this condition are not foolproof as they can be stretched uniformly and stably at a temperature below the melting point of 10°C without being limited to a narrow temperature gradient range, and have a large stretching orienting effect at 90°C. A highly practical heat-shrinkable film having an area shrinkage rate of 20% or more and a thickness unevenness of 20% or less can be obtained.

For measurements using the differential scanning calorimeter described here, samples 6 to
8 mg was sealed in Aluminum Yang and heated to 190 ml under a nitrogen stream.
℃ and held at this temperature for 1 hour, then about 10℃
After cooling to room temperature at / min, heating rate is 10℃/
A melting curve obtained at min and a sensitivity of 25 μ/sec is used.

The raw film for stretching used in the present invention can be manufactured and stretched by known methods, but the following describes the process of forming a tubular film.
This will be explained in detail by taking the case of stretching as an example.

First, the above-mentioned specific range of resin is heated and melted, kneaded, extruded into a tube shape, and cooled and solidified to obtain a raw fabric.

The obtained tubular original fabric is fed to a tubular stretching apparatus such as that shown in FIG. Simultaneous biaxial orientation is achieved by expansion and stretching at a low temperature. The stretching ratio does not have to be the same in the vertical and horizontal directions, but in order to obtain good physical properties such as strength, the stretching ratio should be 2 in both the vertical and horizontal directions.
It is at least twice as large, preferably at least 2.5 times.

The film taken out from the stretching device can be annealed if necessary.

The present invention will be specifically described below based on Examples.

The measurement items shown in the examples were determined by the following method.

1. Area shrinkage rate A film cut into a square with a length and width of 10 crn was immersed in a glycerin bath at a predetermined temperature for 10 seconds, and the shrinkage rate was calculated using the following formula.

Area shrinkage rate = 100-AXB However, A and B are the vertical and horizontal lengths after immersion, and the unit is m.
) is shown.

2. Thickness uneven contact type electronic micrometer (manufactured by Anritsu Electric Co., Ltd.)
Maximum value (Tmax) for the chart in the tube circumferential direction measured at a full scale of 8 μm using a 6C model)
The minimum value (Tmin) and average value (T) were determined and calculated using the following formula.

1. However, T is the arithmetic mean value of the values read from chart positions corresponding to 10 m intervals on the measurement film.

Example 1 An ethylene-butene-1 copolymer with a density at 25°C of 0.9221/cJ, a melt index of O, ['/10 min, and a main peak temperature of 126°C in the melting curve by DSC, j5116 Ethylene polymer tl-17 whose endothermic area below ℃ was 63.8% of the total endothermic area
The mixture is melted and kneaded at 0 to 230°C, extruded through an annular die kept at 230°C, and cooled by sliding on the outer surface of a cylindrical mandrel in which cooling water is circulated. A tubular unstretched film having a thickness of about 66% and 250 μm was obtained. This unstretched film was used as a raw material, and it was introduced into a biaxial stretching apparatus shown in FIG.
The film was stretched 4 times in length and width at 05°C. The stretched film was annealed with hot air at 75° C. for 10 seconds in a tube-shaped annealing device, then cooled to room temperature, folded, and rolled up.

The stability of the stretched tube was good, with no vertical movement of the stretching point or swinging of the tube, and no uneven stretching conditions such as necking were observed.

The obtained stretched film had a thickness of 16 μm, an area shrinkage rate of 900 of 31.5%, and a thickness unevenness of 15%. A pizza pie having a diameter of 15 crn and a thickness of about 1 crn was prepackaged using this film and passed through a shrink tunnel in which hot air at 110° C. was blown for about 3 seconds. The packaging was in a good condition with the cracks tightly attached, and no change was observed in the packaged items.

Example 2 An ethylene-octene-1 copolymer with a density of 0.917117 cm' at 25°C and a melt index of 2.3F710 minutes, whose main peak temperature was 121°C in the melting curve by DSC and 111°C. Stretching was carried out in the same manner as in Example 1, except that an ethylene polymer having the following endothermic area of 57% of the total endothermic area was used, and the stretching temperature was 90 to 100°C.

There was no vertical movement of the stretching point or swinging of the tube, and the stability of the stretched tube was good, and no uneven stretching was observed.

The obtained stretched film had an average thickness of 16 μm and a thickness unevenness of 8 μm.
The area shrinkage rate at 90° C. was 27%. Using this film, a j? I.J.
Five lactic acid bacteria drinks filled in styrene containers were collected and prepackaged, and passed through a hot air shrink tunnel at 100° C. for 3 seconds.

Good shrink packaging was obtained despite the short shrinkage time.

Example 3: Density at 25°C: 0.925117cm3, melt index: 1. ON/1 (70% by weight of ethylene-4-methylpentene-1 copolymer and a density of 0 at 25°C)
．． 92317cm' Melt index 0.8.9/10
Regarding the melting curve by DSC of a melt mixture with 30% by weight of ethylene-butene-1 copolymer, the main peak temperature is 124°C, 1), the endothermic area below 114°C is 58.8 of the total endothermic area % of the ethylene polymer composition was melt-kneaded at 170 to 230°C, extruded through an annular die maintained at 230°C, and the outer surface of the ethylene polymer composition was melt-kneaded at 170 to 230°C and extruded through an annular die maintained at 230°C. The film was cooled by passing through a water tank and taken out to obtain an unstretched film having a diameter of about 66 mm and a thickness of 320 μm. This unstretched film is introduced into a biaxial stretching device at 95 to 105°C, 4.3 times vertically and 3 times horizontally.
After stretching 8 times, it was annealed at 75° C. for 10 seconds, folded, and rolled up.

The stability of the stretched tube was good, with no vertical movement of the stretching point or swinging of the tube, and no uneven stretching such as necking or longitudinal cracking was observed.

The average thickness of the obtained film was 20μ, and the thickness unevenness was 10%.
The area shrinkage rate at 90°C was 25.8%. Shrink wrapping was performed using this film under the same conditions as in Example 1. The packaging had good adhesion, and no change was observed in the packaged items.

Comparative Example 1 Density at 25°C: 0.918,! i'/cm3 Melt index: 1.0.9/10 minutes The melting curve of an ethylene-butene-1 copolymer by DSC has a main peak temperature of 120°C, and the endothermic area below 110°C is the total endothermic area. Example 1
The film was formed, stretched, and annealed using the same method and conditions as .

At such a low stretching temperature, the stretched tube swayed, and a necking phenomenon was also observed in the stretched portion. When the stretching temperature was lowered to increase the stability of the drawn tube, necking was severe and cine uniformity increased. When the stretching temperature was raised to 103°C and 108°C to alleviate necking, the stability deteriorated due to severe up-and-down movement and shaking of the stretching tube.

The stretched film obtained at a stretching temperature of 103 to 108°C had an average thickness of 16 μm, a thickness unevenness of 27 cm, and an area shrinkage rate of 15.4% at 90°C.

This film had poor flatness and was not suitable for continuous packaging using an automatic packaging machine. Further, even in the shrinking process under the same conditions as in Example 1, there were many wrinkles and irregularities, and the packaging was not in a good condition. Good packaging conditions were obtained by increasing the temperature of the hot air and increasing the time it took to pass through the shrink tunnel, but under these conditions some of the items to be packaged melted and deformed, resulting in a significant decrease in product value. became.

Comparative Example 2: Density at 25°C: 0.921, melt index: 0.
751! /10 min ethylene-hexene-1 copolymer 6
0% by weight and 40% by weight of an ethylene-octene-1 copolymer with a density of 0.920 melt index 1.0 g/10 min at 25°C, with a main peak temperature of 127°C for the melting curve by DSC. lfi 117
An ethylene polymer composition having an endothermic area of 50.4% of the total endothermic area below .degree. C. was film-formed and stretched and annealed under the same method conditions as in Example 3.

The necking phenomenon in the stretched portion was remarkable, and when the stretching temperature was raised to alleviate necking, the stretched tube was so shaken that a stable stretched state could not be obtained.

The stretched film obtained at a stretching temperature of 105 to 112°C had an average thickness of 16 μm, thickness unevenness of 23 cm, and area shrinkage rate at 90° C. of 16.4 cm.

(Function and Effect) Among linear copolymers of ethylene and α-olefin, or mixtures thereof, as the raw material resin, in the melting curve of the raw material measured by differential scanning calorimeter, the melting point (endothermic main peak) is 10 By using a material in which the endothermic area of the part below the low temperature is 55% or more of the total endothermic area, it can be stably stretched at a relatively low temperature below the melting point, and as a result, the thickness unevenness is small. It is possible to stably produce a shrinkable film with improved orientation efficiency and excellent heat shrinkage rate at low temperatures.

[Brief explanation of drawings]

FIG. 1 is an explanatory sectional view of equipment used in an embodiment of the present invention.

Claims (1)

[Claims] 1. A differential scanning calorimeter (hereinafter abbreviated as DSC) of a linear copolymer of ethylene and α-olefin, including one or more linear copolymers of this copolymer or a copolymer mixture. Regarding the melting curve measured by
Area shrinkage rate at 90°C obtained by stretching a film obtained by melt-extruding an ethylene polymer composition whose endothermic area below a low temperature is 55% or more of the total endothermic area, cooling and solidifying it, and stretching it twice or more in both length and width. is 20% or more, and the thickness unevenness is 20% or less. 2. The heat shrinkage according to claim 1, wherein the α-olefin is one or more selected from the group of 1-butene, 1-pentene, 1-hexene, 1-octene, and 1-4-methylpentene. sex film.