JPS6174819A - Manufacture of polyethylene oriented film - Google Patents

Abstract

PURPOSE:To offer a polyethylene oriented film which is transparent and superior in moisture resistance, by a method wherein a polyethylene resin sheet constituted by cross-linking so as to deteriorate inwardly a degree of cross-linking in the direction of a thickness of a molded article is rolled and then oriented. CONSTITUTION:As for cross-linking of a sheetlike or tubular raw fabric made of polyethylene resin, it is performed from both sides so as to deteriorate inwardly a degree of the cross-linking in the direction of a thickness of raw fabric. As for the degree of the cross-linking, it is desirable that a gel content ratio whose degree of the cross-linking is lowest is less than 0-5%, the gel content ratios of cross-linked surface layers each of both the sides are more than 5% and especially within a range of 20-70% in constitution of the cross-linking of the raw fabric. This polyethylene resin sheet is rolled and then oriented.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a stretched polyethylene film (specifically, the degree of crosslinking is
Rolling a polyethylene resin bottom shape with a drop in the middle direction,
Next, the present invention relates to a method for producing a stretched polyethylene film.

Conventional technology Polyethylene has traditionally been used in many applications, but
Films made from high-density polyethylene generally have poor transparency and are rarely used for applications requiring display effects. As a method for improving the transparency of a polyethylene film, for example, a method is known in which a film of polyethylene or the like is uniformly cross-linked and stretched using radiation (Japanese Patent Publication No. 18893/1983).

However, although the cross-linked stretched polyethylene film obtained by this method has improved transparency and strength, it does not have sufficient moisture resistance. It is necessary to provide a transparent resin layer. This impairs the transparency and packaging properties of the film, and also increases the manufacturing cost.

An object of the present invention is to provide a method for producing a stretched polyethylene film that is transparent and has excellent moisture resistance.

Means for Solving the Problems The gist of the present invention is that the degree of crosslinking in the thickness direction of the molded product is
This is a method for producing a stretched polyethylene film, which comprises rolling a polyethylene resin sheet that has been crosslinked so as to decrease in the middle direction, and then stretching it.

In a preferred embodiment of the present invention, the rolling and stretching described above include
This is carried out by rolling in the machine direction (MD), preferably 1.5 times or more, and further stretching in the transverse direction (TD), preferably 3 times or more, at a temperature below the melting point of the resin. In a particularly preferred embodiment, it is made of polyethylene resin and has moisture permeability (
y) is a numerical value that satisfies the following formula, preferably <t'f
A stretched polyethylene film having a haze of 5% or less is obtained.

A≦17 x −”2 [However, y is moisture permeability (97 m” / 24 hours),
is the thickness of the stretched film (μ)] Next, the manufacturing method of the present invention will be explained in detail.

The polyethylene resin in the present invention includes high-density polyethylene, medium-density polyethylene, and low-density polyethylene.
, polyethylene such as linear low density polyethylene, or ethylene and propylene with an ethylene content of 5% by weight or more, 1-buty/, 1-pentene, 1-hex/, 4-
with α-olefins such as methyl-1-pe/tene and 1-octene, or vinyl monomers such as vinyl acetate, (meth)acrylic acid, (meth)acrypa acid ester, acrylamide, acrylonitrile, styrene, and vinyl chloride. Examples include copolymers, and these polyethylene resins may be used alone or in a mixture of two or more. In particular, these polyethylene resins have a density of CL9351/
υ1 or more, preferably 11950? Melt flow index (from J, measured with K676Q at temperature 190℃ and load IK 2.16'Kg, hereinafter M
) or (LO!M'/10 minutes or more, preferably α
Crystalline polyethylene or ethylene copolymer having a molecular weight of 5 to 201F710 is preferred. In addition, known additives such as antioxidants, ultraviolet absorbers, anti-blocking agents, lubricants, neutralizing agents, pigments, dyes, etc. can be added to these polyethylene resins as necessary.

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 notebook-shaped straddle tube-shaped original fabric. Bath-melt extrusion The bottom shape is extruded through a commonly used T-die to make a flat fabric, extruded from an annular die to make a tube-shaped fabric, and cut open a tube-shaped fabric to make a 7-ton-shaped fabric. Any method may be used, such as a method of making a roll, or cutting both sides of a tube-shaped raw fabric to make two sheet-shaped raw fabrics. In this case, the thickness of each raw film may be such that it can be crosslinked from both sides in the direction of the thickness of the raw film so that the degree of crosslinking decreases in the middle direction, and the thickness of the film after stretching is It depends on the situation, but usually 1”C21
0-2000 pm, preferably 400-1000 pm
A range of m is desirable from the viewpoint of handling and formation of the above-mentioned crosslinking.

In the present invention, it is necessary to crosslink the sheet-like or tube-like raw fabric made of polyethylene resin from both sides so that the degree of crosslinking decreases toward the inside in the thickness direction of the raw fabric.

The degree of crosslinking is expressed by the gel fraction, but in order to achieve the purpose of the present invention, in the crosslinked structure of the above raw fabric,
It is preferable that the gel fraction at the lowest degree of crosslinking is 0 to less than 5%, and the gel fraction of each crosslinked surface layer on both sides is 5% or more, particularly in the range of 20 to 70%. In particular, the gel fraction with the lowest degree of crosslinking is O
It is preferable that the crosslinked layer/uncrosslinked layer/crosslinked layer be s-bottomed in the thickness direction of the original fabric.In this case, the groove bottom ratio of each layer is 1:α1 to 10′. It is desirable that the degree of crosslinking of each crosslinked layer on both sides be the same.

If the above-mentioned crosslinking is not carried out so that the degree of crosslinking decreases in the direction of the thickness of the original fabric, especially if the gel fraction of the degree of crosslinking is more than 5%, rolling at a low temperature is required. becomes difficult. Furthermore, although the stretching process is uniform and the transparency is improved, a film with improved moisture resistance, which is the main objective of the present invention, cannot be obtained. Further, when the degree of crosslinking of each crosslinked surface layer on both sides is less than 5%, rolling is possible, but the stretching process in the TD direction is not uniformly performed, and the transparency and moisture resistance of the film are not improved. -10,000, gel fraction is 7
If it exceeds 0%, the film is likely to break during stretching and cannot be stretched smoothly. Furthermore, if cross-linking is performed uniformly throughout the entire thickness of the original fabric, rolling at low temperatures will be difficult, and although stretching will be performed uniformly and transparency will be improved, moisture resistance will not be improved. . - 10,000, if the transparency is not sufficiently improved by crosslinking only on one side in the thickness direction of the original fabric, and if crosslinking is performed around the entire circumference so that the degree of crosslinking decreases from -1 in the thickness direction of the original fabric, However, the moisture resistance of the resulting film is not sufficiently improved, and both are unfavorable.

In addition, the above gel fraction 2 shows the insoluble portion obtained by extracting the sample with boiling p-xolene.

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 irradiation time of the ij X-ray 1 is 5 to 50 megarads, preferably t'115 to 50 megarads. And it is sufficient. Further, the irradiation may be carried out simultaneously on the front and back of the original paper notebook or on the inside and outside of the original paper tube, or separately on the front and back or on the 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 when the original polyethylene resin is extruded and melted or after extrusion, cooling and assimilation. Further, the ability to transmit the electron beam can be adjusted by adjusting the applied voltage depending on the thickness of the original fabric, or by masking with a shielding material such as a film, sheet, or plate.

Next, to give an example of adjusting the amount of electron beam irradiation, for example, if the thickness of the original fabric to be irradiated is 500 μm,
25 sheets of thin film are stacked closely together and
A test piece with a thickness of 00 μm was irradiated with the same amount of electron beams from both sides in the thickness direction, resulting in a crosslinked test piece with a thickness of 20 μm.
By separating the specimen into 25 films and measuring the degree of crosslinking in each film, it is possible to determine the distribution 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 a mass, the transparency of the resulting film is not sufficiently improved.

In addition, as a method of crosslinking by multilayer coextrusion of polyethylene resin blended with a crosslinking agent, for example, a polyethylene resin blended with a crosslinking agent such as an organic peroxide,
For notebook-like raw fabrics, the outer layers are on both sides in the thickness direction, and for tube-shaped raw fabrics, they are the inner and outer layers in the thickness direction. One example is a method in which a material containing an oxide is supplied to a multilayer coextruder so as to form an intermediate layer in the thickness direction of the original fabric, and then crosslinked and coextruded at a temperature equal to or higher than the melting point of the resin. Among the above-mentioned crosslinked molded products, the tube-shaped original fabric is used by cutting it into a sheet shape by incising or cutting it.

Rolling was carried out by rolling the crosslinked raw sheet notebook at a temperature below the melting point of the resin by 1.5 times or more in the machine direction (MD). Desirably, at a temperature range of room temperature to 135°C, preferably 70 to 150°C, 1.5 times or more in the MD direction using a rolling roll,
Preferably it is rolled 2 to 6 times. When the rolling temperature exceeds the melting point, the moisture resistance of the film obtained is insufficiently improved;
- If the rolling ratio is less than 1.5 times, the effect of improving moisture resistance is insufficient.

In addition, for stretching, the rolled notebook is stretched in the transverse direction (TD) by at least 3 times, preferably 4 to 8 times, at a temperature from the softening point to the melting point of the resin. Preferably rs, y
The film is stretched in the TD direction at a temperature of 100 to 150°C, preferably 5 times or more, preferably 4 times or more, using a tenter. The temperature of tensile stretching is
If it is below the softening point, the resin will not be sufficiently softened and uniform and stable stretching will not be possible, and if it exceeds the melting point, the resin will melt excessively and stable stretching will not be possible, and the resulting film will not have improved moisture resistance. Not enough.

Furthermore, if the stretching ratio is less than 3 times, uniform stretching cannot be achieved and a film with excellent transparency cannot be obtained.

In addition, in order for the obtained stretched film to have heat shrinkability, when used as a base film for composite packaging, heat setting is performed at a temperature below the melting point of the stretched film, for example, 110 to 135°C, and the MD direction is It is desirable that the heat shrinkage rate is 1.5% or less, preferably 1.0% or less.

Effects of the Invention As described above, according to the method of the present invention, a stretched polyethylene film that is transparent and has excellent moisture resistance, which could not be obtained by conventional methods, can be obtained. In particular, compared to the conventional method of stretching the original film by 9 tensions, it is possible to roll the MD direction at a lower temperature and lower magnification, so it is possible to produce stretched films with uniform and anisotropic properties, such as T
A product with good cuttability in the D direction can be obtained. In addition, the low-temperature rolling process saves energy and is economical to manufacture.

The stretched film according to the present invention has the above-mentioned excellent properties and can be used for various purposes, but is particularly useful as a transparent packaging base material that requires moisture proofing, and as a base film for other adhesive tapes. It can be used for etc.

EXAMPLES Next, the present invention will be explained in more detail with reference to examples. In addition, the test method in the present invention is the following 9.

(1) Haze: A day TM D1003 (2)
Moisture permeability: J Engineering SZ 0208B method (temperature 40°C, relative humidity 90%) (3) Gel fraction: ASTM D2765 A method Example 1 High density polyethylene (density a 958 t/an",
A sheet material having a thickness of 0.45 mm was formed using a T-die extrusion sheet forming machine.

Using an electron beam irradiation device (manufactured by GSI), this sheet-like material was exposed to 165KV on each front and back under a nitrogen gas atmosphere.
A 20 megaland electron beam was irradiated under the condition of 45 mA. In order to know the crosslinking degree of the irradiated surface of this crosslinked sheet and/or the inside of the sheet in the thickness direction,
The thickness of HDPK is 30 μm, which is the same as 21 μm thin films.
A test piece with a thickness rl and 45 mm was obtained by stacking a single film of 45 mm, and the crosslinking degree of each thin film was examined by irradiating it with an electron beam under the same conditions.The degree of crosslinking of the thin film on both sides of the irradiated surface was The gel fraction was 50 cm, and the gel fraction was 0 at the lowest layer a inside the thickness direction. In addition, the tank bottom ratio of the thickness of the crosslinked layer and the uncrosslinked layer is: crosslinked layer: uncrosslinked layer: crosslinked layer = 1 = 1:
It was 1.

This crosslinked sheet was rolled 2 times in the MD direction at a temperature of 100°C.
．． Rolled 5 times, then stretched 6 times in the TD direction with a tenter stretching machine at a temperature of 130°C to obtain biaxial stretching H to a thickness of 30 μm.
DPI! An i-film was obtained. The characteristics of this film are shown below.
Shown in 1.

Note that when 1 d of this film was magnified 100 times with a stereomicroscope and the film surface was picked up with sharp tweezers, the crosslinked layer on the surface was gently peeled off, but the middle layer of the uncrosslinked layer was fibrillated.

The same was true for the opposite side of the film.

Examples 2 to 6 Adjustment of the thickness of the original fabric and the degree of crosslinking in the thickness direction of the original fabric using polyethylene resins with different densities and MIs,
Each stretched film was obtained in the same manner as in Example 1 except that rolling and stretching were performed under the conditions shown in Table 1. The characteristics of each film are also listed in Table 1.

Comparative Example 1 In Example 1, the applied voltage of the electron beam irradiation device was increased to increase the electron beam penetration ability, and the gel fraction of the original fabric 7-t was 55%.
Using a raw sheet with uniform crosslinking in the thickness direction, roll rolling and tenter stretching were performed under the conditions shown in Table 1 to obtain a stretched film. Full table of characteristics of this film-1
Also listed.

Comparative Example 2 In Example 1, crosslinking by electron beam was performed from one side of the original sheet in the thickness direction, and the degree of crosslinking on the irradiated side and non-irradiated side of one original sheet was 50L% and 0% gel fraction, respectively. and
The ratio of the crosslinked layer and uncrosslinked layer in the thickness direction of the original fabric is each 1.
:19 was used, and roll rolling and tenter stretching were performed under the conditions shown in Table 1 to obtain a stretched film. The characteristics of this film are shown in Table 1 ((also listed).

Comparative Example 3 Density α960f/cn? , MI5P/10 min HDPI
A raw crosslinked sheet similar to that in Example 1 was obtained using the following.

This sheet was stretched 2.5 times by tension stretching between rolls in the MD force direction at 130'c, and then stretched 6 times by tenter stretching. The obtained film was non-uniform and could not be put to practical use. As described above, in tensile stretching, if the stretching ratio is less than 3 times, a uniform stretched film cannot be obtained.

Claims (1)

[Claims] A method for producing a stretched polyethylene film, which comprises rolling and then stretching a polyethylene resin sheet that has been crosslinked so that the degree of crosslinking decreases in the thickness direction of the molded product.