JPS6176533A - Polyethylene film - Google Patents

Abstract

PURPOSE:To obtain a film having improved heat-sealability without lowering the transparency and moistureproofness of the base film, by radiating electron radiation to a drawn polyethylene film wherein the degree of crosslinking is decreased along the thickness of the film toward the inside. CONSTITUTION:A sheet or tube of a polyethylene resin is irradiated with electron radiation from both sides of the sheet, etc. in nitrogen gas atmosphere at a dose of 5-50 Mrad each, to obtain a sheet wherein the crosslinking degree is decreased along the thickness of the sheet from both surfaces toward inside, the gel fraction of the part having lowest crosslinking degree is 0-5%, the gel fraction of the crosslinked surface layers is >=5%, especially 20-70%, the sheet has a structure of preferably a crosslinked layer/uncrosslinked layer/ crosslinked layer, the ratio of the uncrosslinked layer:each crosslinked layer is 1:0.1-10, and both crosslinked layers has the same degree of crosslinking. The sheet is drawn, and irradiated with electron beam in nitrogen atmosphere at a dose of >=2 Mrad, preferably 2-40 Mrad.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a polyethylene film, and more particularly to a polyethylene film obtained by irradiating a crosslinked polyethylene resin stretched film with electron beams.

Conventional technology Polyethylene has traditionally been used for many purposes.
Among these, films obtained from island-density polyethylene are generally opaque and have poor moisture resistance and surface gloss, so they are rarely used for applications that require display effects. As an improvement method, the present inventors have previously developed a polyethylene film manufacturing method (Japanese Patent Application No. 1982 -47108).

However, although the cross-linked stretched polyethylene film obtained by this method has improved transparency, moisture resistance, and rigidity, when it is heat-sealed at high temperatures, it shrinks near the sealed portion (although it can be heat-sealed).
There was a tendency for this to occur (generally referred to as "1 degree loss"), and there was room for improvement.

Problems to be Solved by the Invention An object of the present invention is to improve the shrinkage in the vicinity of the sealed portion during high-temperature heat sealing without impairing the properties such as transparency and moisture resistance of the stretched polyethylene resin film.

Means for Solving the Problems The gist of the present invention is a polyethylene film obtained by irradiating an electron beam onto a stretched polyethylene resin film in which the degree of crosslinking decreases in the direction of the thickness of the film.

As the polyethylene resin in the present invention.

Polyethylene such as high-density polyethylene, medium-density polyethylene, low-density polyethylene, linear low-density polyethylene, or ethylene with an ethylene content of 50 it% or more, fluoropylene, 1-butene, 1-pentene, 1-hexene, 4-methyl- α of 1-pentene, 1-octene
-Olefins include copolymers with vinyl monomers such as vinyl acetate, (meth)acrylic acid, (meth)acrylic esters, acrylamide, acrylonitrile, styrene, and vinyl chloride; The resin can be used alone or in a mixture of two or more.Among these polyethylene resins, 5% has a density of 0.9.
35g/- or more, preferably r, t Q, 9501/CW
Melt flow index (JIS K67
60 ((measured at a temperature of 190C and a load of 2.46ky,
A crystalline polyethylene or ethylene copolymer having an MI (hereinafter referred to as MI) of 0.05 p710 minutes or more, preferably 0.5 to 20 F/No minutes is preferable. In addition, known additives such as antioxidants, ultraviolet absorbers, antiblocking agents, antistatic agents, lubricants, neutralizing agents, pigments, and dyes can be added to these polyethylene resins as necessary.

In the present invention, the stretched polyethylene resin film serving as the base material is a uniaxially or biaxially stretched film in which the degree of crosslinking decreases in the direction of the thickness of the film.

The polyethylene resin used in the production of the film of the present invention is supplied to a commonly used extruder, and is melt-extruded, cooled, and solidified to form a sheet-like or tube-like raw material.

Melt extrusion molding is a method of extruding from a commonly used T-die to make a flat raw fabric, a method of extruding from an annular die to make a tubular raw fabric, and a method of cutting open a tubular raw fabric to make a sheet-shaped raw fabric. Alternatively, any method may be used, such as cutting both sides of a tube-shaped raw fabric to form two sheet-shaped raw fabrics. In this case, the thickness of each raw film should be such that it can be crosslinked from both sides in the thickness direction of the raw film so that the degree of crosslinking decreases in the middle direction, and it depends on the stretching ratio and the thickness of the film after stretching. Depending on the situation, it is usually ji210~
A range of 2000 pm, preferably 400 to 1000 pm is desirable from the viewpoint of handling and formation of the above-mentioned crosslinking.

In the present invention, the sheet-like or tube-like raw material made of polyethylene toilet fat must be crosslinked from both sides so that the crosslinked layer decreases toward the inside in the thickness direction of the raw material.

The degree of crosslinking vjIi is expressed by the gel fraction, but in order to achieve all the objectives of the present invention, in the crosslinking configuration of the above-mentioned raw fabric, the lowest gel fraction of crosslinking aM is 0 to less than 5%, and both sides of each It is preferable that the gel fraction of the crosslinked surface layer is 5% or more, particularly in the range of 20 to 70%. In particular, it is preferable that the gel fraction with the lowest degree of crosslinking is 0% and that the crosslinked layer/uncrosslinked layer/crosslinked layer is formed in the thickness direction of the original fabric. Each crosslinked layer=1:0. It is desirable that the degree of crosslinking be in the range of i to 10, and it is particularly preferable that the degree of crosslinking of each crosslinked layer on both sides is the same.

If the above-mentioned crosslinking is not performed so that the degree of crosslinking decreases in the direction of the thickness of the original fabric, especially if the gel fraction at the lowest degree of crosslinking exceeds 5%, the stretching process will not be uniform. Although the transparency is improved, a film with improved moisture resistance, which is the main objective of the present invention, cannot be obtained. Further, if the degree of crosslinking of each crosslinked surface layer on both sides is less than 20%, the stretching process will not be uniformly performed and the transparency and moisture resistance of the film will not be improved. - If the gel fraction exceeds 70%, the film tends to break during stretching, making it impossible to stretch smoothly. Furthermore, when cross-linking is performed uniformly across the entire thickness of the original fabric, stretching is performed uniformly and transparency is improved, but moisture resistance is not improved; If only one side is crosslinked, the film will easily break during stretching, and if all layers are crosslinked so that the degree of crosslinking decreases from one side in the thickness direction of the original film, the moisture resistance of the resulting film will not be sufficiently improved. Both are undesirable.

Note that the above gel fraction indicates the insoluble portion obtained by extracting the sample with boiling p-xylene.

Examples of methods for performing such crosslinking include a method of irradiating electron beams from both sides of the original fabric, and a method of multilayer coextrusion of polyethylene resin blended with a crosslinking agent.

The method of irradiating the electron beam varies depending on the thickness of the original fabric, the type of resin, the molecular weight, and the molecular weight distribution, but usually the electron beam irradiation amount is 5 to 50 megarans (Mrad), preferably 15 to 30 megarans. do it. Further, the irradiation may be carried out simultaneously on the front and back sides of the original fabric sheet or on the inside and outside of the original fabric tube, or separately on the front and back sides or inside and outside, or even in several batches. In this case, it is particularly preferable that the irradiation dose to the original fabric is the same on the front and back sides or inside and outside of the original fabric. Further, the irradiation may be carried out while the original polyethylene resin is extruded and melted or extruded and cooled and solidified. Further, the electron beam transmission ability can be adjusted by adjusting the applied voltage depending on the thickness of the original fabric, masking with a shielding plate, etc.

Next, using t-S as an example of adjusting the electron beam irradiation amount, for example, when the thickness of the original fabric to be irradiated is 500 μm, 20
A test piece with a thickness of x-so-o-μm was obtained by closely stacking 25 thin films with a thickness of 25 μm, and this was cross-linked by irradiating the same amount of electron beams from both sides in the thickness direction. By separating the film into 25 films of 20 μm and adding the degree of crosslinking 'It side of each film, it is possible to know the distribution state of the degree of crosslinking in the thickness direction of the test piece. From this result, it is possible to know the relationship between the thickness of the original fabric and the degree of crosslinking depending on the amount of electron beam irradiation.

The above electron beam irradiation is preferably performed in an atmosphere of nitrogen, argon, helium, or other inert gas. Although electron beam irradiation can be performed in the presence of air, the transparency of the resulting film is not sufficiently improved.

Also, as a method of crosslinking, there is a multilayer coextrusion of polyethylene resin blended with a crosslinking agent.

For example, organic peroxide, which is a crosslinking agent blended with polyethylene resin, is used as the outer layer on both sides in the thickness direction for a sheet-like raw fabric, and as the inner and outer layer in the thickness direction for a tube-shaped raw fabric. A material containing no crosslinking material or a material containing an organic peroxide such that the degree of crosslinking is below the above minimum crosslinking degree is fed to a multilayer coextruder as an intermediate layer in the thickness direction of the original fabric, and the material is fed to a multilayer coextruder to form an intermediate layer in the thickness direction of the original fabric. An example of this method is crosslinking coextrusion at a temperature of .

Stretching is carried out by fully heating the crosslinked raw material and stretching it uniaxially or biaxially at a predetermined magnification by a normal roll method, tenter method, tubular method, rolling method, or a combination of these methods to obtain a film. . Biaxial stretching may be simultaneous or sequential.

The stretching temperature is preferably higher than the softening point of the polyethylene resin, particularly in the range from the softening point to the crystal melting point. Specifically 70
~150C1 preferably 70~155C, especially 100~
130C is particularly good. If the stretching temperature is below the softening point, the resin will not be sufficiently softened and uniform and stable stretching cannot be performed. On the other hand, if the temperature exceeds 1,500 degrees Celsius, the oil will melt excessively, making stable stretching impossible, and the resulting film will not be sufficiently improved in moisture resistance.

In addition, the stretching ratio is 5 in one direction or both vertically and horizontally.
It is desirable to carry out at least twice as much, preferably at least four times as much. If the stretching ratio is less than 3 times, uniform stretching is insufficient and it is difficult to obtain a stretched film with excellent transparency.

The method of fully irradiating the stretched polyethylene resin film with electron beams can be carried out in the same manner as the method of irradiating the polyethylene resin raw sheet or raw tube with electron beams. The amount of electron beam irradiation at this time varies depending on the thickness of the stretched film, the molecular weight of the type of resin, the molecular weight distribution, etc., and is usually 2 megalands or more, preferably 2 to 2 megalands.
It is 40 megarads. Further, the atmosphere for irradiation is not particularly limited, but it is desirable to perform the irradiation in an inert gas atmosphere.

In addition to the effects of the invention, the film of the present invention has improved shrinkage due to heat near the heat-sealed portion, and can be heat-sealed at high temperatures.
Heat sealing time can be shortened. Furthermore, the diseal strength can be increased by heat sealing at high temperatures.

Furthermore, there are effects such as increased stability in processing the film, which can reduce the heat shrinkage rate.

The film of the present invention has properties such as transparency, moisture-proofness, and rigidity, and has improved heat-sealing properties without the need for B4, and is used as a base film for packaging, especially for moisture-proof packaging and heat-resistant packaging. It is suitable as Ruru.

Examples Examples of the present invention are shown below. In addition, the test method in the examples is as follows.

(υ Heat-sealing strength U:, @[Heat-sealed parts obtained by pressing the heat-sealed surfaces of the film together at a temperature of Pjiffl for 1 second at a width of 15 m and a pressure of 2 kg/arr' using a heat seal, It was determined by peeling at a peeling speed of 500 m/min.

(2) Heat shrinkage rate: The shrinkage rate was determined when the sample was left at 120t:' for 10 minutes.

(31 Yase: Determined by visual inspection of all heat shrinkage near the heat-sealed part.

(4) Moisture permeability: JIS Z 0208 B method (temperature 4
(measured at 0C1 relative humidity of 90%) Example 1 High density polyethylene (density go, q s 6 i/crt
', Mxo, 51710 minutes, HDPT! )
A sheet material having a thickness of 0.6 mm was formed using a T-tie extrusion sheet forming machine.

Using an electron beam irradiation system @ (manufactured by ESI), this sheet-like original was exposed to 165 kV on each front and back under a nitrogen gas atmosphere.
A 20 megaland electron beam was irradiated under -45+nA conditions. In order to know the degree of cross-linking on the irradiated surface of this cross-linked sheet and inside the sheet in the thickness direction, 30 thin films of 20 μm thick made of the above HDPE were stacked to give a test force of 0.6 strips under the same conditions. When we examined the degree of crosslinking of each thin film by irradiating it with an electron beam, we found that the crosslinking degree of the thin films on both sides of the irradiated surface had a gel fraction of 50%, and the lowest crosslinking inside the thickness direction had a gel fraction of 0%. .

Moreover, the composition ratio of the thickness of the crosslinked layer and the uncrosslinked layer was crosslinked layer: uncrosslinked layer: crosslinked layer = 1:2:1.

This cross-linked sheet was stretched 4 times in the longitudinal direction and 5 times in the transverse direction at a temperature of 130 p'''C to obtain a biaxially stretched HDpE sheet with a thickness of 30 μm.
Got the film. When this film was magnified 100 times with a 1Cnr2f stereomicroscope and the film surface was hung with a sharp bottle set, the surface layer was gently peeled off, but the uncrossed middle layer was fibrillated. The same was true for the opposite side of the film.

The stretched films were each irradiated with electron beams at doses shown in Table 1 in a nitrogen atmosphere.

The heat resistance properties of the obtained Tani film are shown in Table-1.

Examples 3-4 pvpF, (density Q, 95.8 evening/i, yx501/1
0 minutes), a raw sheet with a thickness of 0.4 M was formed in the same manner as in Example 1, and the crosslinked structure of the sheet was electron beam crosslinked with a crosslinked layer/uncrosslinked layer/crosslinked layer = 1:2:1. Stretching was performed to obtain a stretched film with a thickness of 20 μm.

This stretched film was irradiated with electron beams at doses shown in Table 1. The heat resistance properties of each film obtained are also listed in Table-1.

Comparative Example 1.2 In Examples 1 and 3, the heat resistance properties of the biaxially stretched HDPK films that were not subjected to electron beam irradiation treatment are also listed in Table 1.

Comparative Example 3.4 The sheet-like original fabric obtained in Example 1 was irradiated by increasing the applied voltage of the electron beam irradiation device to increase the electron beam penetration ability, and the gel fraction was 55%, and the sheet was A crosslinked sheet was obtained in which the degree of crosslinking in the thickness direction was uniform. This cross-linked sea) f, 15
A biaxially stretched HDPE film with a thickness of 30 μm was obtained by stretching 4 times in the longitudinal direction and 5 times in the transverse direction at 8t:' Heat resistance properties of this film without and with electron beam irradiation are also listed in Table-1.

Claims (1)

[Claims] A polyethylene film made by irradiating a stretched polyethylene resin film with an electron beam in which the degree of crosslinking decreases in the direction of the thickness of the film.