JPS6054334B2 - Method for producing heat-shrinkable cross-linked polyethylene resin film - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field] The present invention relates to a method for producing a crosslinked and stretched film having excellent heat shrinkability, particularly uniaxial shrinkability in the transverse direction, using a polyethylene resin as a base material.

[Problems with conventional technology] Conventionally, biaxially stretched biaxially shrinkable films are generally known as cross-linked stretched films based on polyethylene resin, and are used for shrink wrapping films, etc. has been done.

In addition, a crosslinked film that is uniaxially shrinkable in the machine direction (lengthwise direction of the film) and stretched in the machine direction has recently been developed and is beginning to be used as a film for integrated shrink packaging in the form of a sleeve.

However, the only uniaxially shrinkable film in the transverse direction (width direction of the film) is vinyl chloride resin film.
Although it is used as a film for cap seals and shrink labels, a polyethylene resin film with excellent uniaxial shrinkability in the transverse direction has not yet been developed.

A crosslinked and stretched heat-shrinkable polyethylene resin film and its manufacturing method are disclosed in Japanese Patent Publication No. 37-188
Publication No. 93, Japanese Patent Publication No. 48-27904, Japanese Unexamined Patent Publication No. 5
One disclosed in Japanese Patent No. 1-59973 and the like is known.

In Japanese Patent Publication No. 37-18893, polyethylene is extruded into a film, this film is stretched, the film is irradiated with at least 2 Mrep, heated, and biaxially formed by hot stretching. A method of making a film that is oriented in a direction is disclosed.

The heat shrinkage rate of the film obtained by this method is 96
℃, 35-40% in the vertical direction, 30-50% in the horizontal direction (
Examples 1 to 4), and had biaxial shrinkability in the longitudinal and transverse directions, and no films with excellent uniaxial shrinkability in the transverse direction were obtained. Also, in Japanese Patent Publication No. 48-27904, a film consisting of a low-density ethylene polymer and a high-density ethylene polymer was disclosed in 90-11.
A film obtained by heating to 5° C., stretching at least 5 times in the longitudinal and transverse directions, and irradiation crosslinking in the stretched state, and a method for producing the same are disclosed.

The heat shrinkage rate of the film obtained by this method is 10
19-23% in the vertical direction and 31-34% in the horizontal direction at 0°C (
Examples 1 to 4), the heat shrinkage rate is small, and a film with excellent uniaxial shrinkability in the transverse direction has not been obtained. Further, JP-A-51-59973 discloses a method for producing heat-shrinkable films and tapes, which is characterized by stretching a synthetic resin thin film and subjecting it to a post-crosslinking treatment.

The film obtained by this method has a heat shrinkage rate of 13 in the machine direction.
~15% and 12% in the transverse direction (Examples 2 and 3), and it is also stated that the heat shrinkage rate is reduced by crosslinking after stretching, but on page 3 of the specification of the same publication. As shown in FIG. 2, the heat shrinkage rate of the film uniaxially stretched at a stretching ratio of 1.7 times is about 70%, but 5 Mrad. 1
0 Mrad. ．． It is stated that when irradiated with 15 Mrad each, the shrinkage rate is significantly reduced to about 20% and about 5%, respectively. That is, even by these methods, a film having excellent heat shrinkability only in the lateral direction has not been obtained. In addition, it is generally difficult to recover and reuse crosslinked polyethylene resin films, and therefore, it is desirable to employ a stretching film forming method that can minimize the film loss rate.

The inflation biaxial stretching method is a desirable method in that there is no film loss (trim loss) after film formation. Establishing biaxial stretching, in which the stretching ratio in the longitudinal direction (direction) is significantly larger than that in the machine direction (take-up direction), as inflation stretching, in which stretching in the biaxial directions proceeds simultaneously, is aimed at cross-linked polyethylene resins. In the technical field of It has been considered difficult to stabilize the bubble. [Object of the present invention] The present invention was made in view of the above-mentioned current situation, and its first purpose is to achieve a film with a heat shrinkage rate of 20% in the vertical direction, which is practically desirable as a film for cap seals and shrink labels.
Hereinafter, it is an object of the present invention to provide a manufacturing method for completing a crosslinked polyethylene resin film, a film having excellent transverse uniaxial shrinkability with a heat shrinkage rate of 50% or more in the transverse direction (circumferential direction). The object of the above is to provide a manufacturing method that achieves the first object by extrusion-inflation biaxial stretching, which is excellent in production characteristics such as ease of use and operability of the device.

[Summary of the invention]

The above object of the present invention is to provide a manufacturing method of the present invention, namely:

A heat-shrinkable, cross-linked polyethylene resin film with a gel fraction of 50 to 8% and a film heat shrinkage rate significantly larger in the transverse direction than in the longitudinal direction. To obtain this, first, a crosslinked polyethylene resin tubular fabric having a gel fraction of 30 to 75% by weight is created by an extrusion method using a circular die, and then this fabric is heated to 90 to 150°C. The film was then subjected to inflation biaxial stretching at a stretching ratio of 2.5 times or less in the longitudinal direction and 3 times or more in the transverse direction, and then irradiated with radiation to reduce the gel fraction value to 50.
The longitudinal heat shrinkage rate was adjusted to 8% to 8%.
% or less, and a method for producing a heat-shrinkable crosslinked polyethylene film, the method comprising obtaining a film having a transverse heat shrinkage rate of 50% or more. This can be easily achieved by adopting

[Description of the article targeted by the manufacturing method of the present invention] The polyethylene resin used as the base material in the present invention has a density of 0.915 to 0.935 y/C at 25°C! 11 A low density ethylene resin preferably having a melt index of 0.920 to 0.930 y/al and a melt index of 0.3 to 30 f/10 min at 190°C, preferably 0.5 to 1.5 y/10 min, In addition, the density is 0.940-0.980f/Clll, preferably 0.950-0.970y/All, and the melt index is 0.3-3.0y/1 powder, preferably 0.5
Ethylene homopolymer typified by high-density polyethylene resin with a powder of ~1.5y/1, and a powder whose melt index is 0.3-3y/1, preferably 0.5-1.5y, whose main component is ethylene It is a mixture of ethylene and one or more polyethylene resins such as vinyl acetate, propylene, and copolymers with α-olefins such as 1-butene and 1-pentene.

Furthermore, additives commonly used in the above polyethylene resin, such as heat stabilizers, ultraviolet absorbers, lubricants, antiblocking agents, antistatic agents, antifogging agents, fillers, pigments,
A dye or the like may be added. In the above, when the melt index is less than 0.3, the melt viscosity of the resin is high, and when manufacturing a tube-shaped raw fabric by melt extrusion with an extruder,
Manufacturing problems such as increased load on the extruder are likely to occur; on the other hand, if the melt index is large, exceeding 3,
As the melt viscosity of the resin decreases, the so-called drawdown phenomenon of the resin is likely to occur when melt extruding the resin from an extruder to produce the original fabric, or when heating and stretching the original fabric, so the so-called drawdown phenomenon of the resin tends to occur. It is better to be careful when setting conditions.

As a target film of the present invention, the gel fraction of the film is 50 to 8% by weight, preferably 55 to 75% by weight.

Gel fraction: Approximately 200 mg of sample was boiled in 500 cc of xylene.
The film is immersed in a film for 10 hours, the insoluble part is dried, and the value is the weight of the insoluble part expressed as a percentage of the weight before treatment, and indicates the degree of crosslinking of the film. This gel fraction will be described in detail in Examples below, but is a value determined to provide uniaxial contractility in the transverse direction. Further, the target film of the present invention has a heat shrinkage rate of 20% or less in the machine direction, preferably 15% or less.
% or less, 50% or more in the lateral direction, preferably 60% or more.

The longitudinal direction refers to the length (take-up) direction of the film, and the transverse direction refers to the width direction (circumference) of the film. Also, to measure the heat shrinkage rate, mark lines of 10 cm each in length and width were attached to the film. The sample was left in an air vent set at a predetermined temperature, and the film was shrunk to its original length (10 cm).
This is the value expressed as a percentage of the length that has shrunk compared to the actual length. The temperature at which the thermal shrinkage rate is shown varies depending on the type of polyethylene resin used, the softening point of the resin, and the crystal melting point, but in the film made of the polyethylene resin used in the present invention,
At about 130℃. Since the maximum thermal shrinkage rate is shown at different temperatures, in the present invention, the shrinkage rate at a temperature of 130°C (average value of n = 10)
is expressed as the heat shrinkage rate. Also, this heat shrinkage rate is the same in the lateral direction as that of the vinyl chloride resin used as the cap seal or shrink label film mentioned above! The characteristic value of the axially shrinkable film, that is, the heat shrinkage rate, is 10 to 20% in the longitudinal direction.
, is set based on a value of 50 to 65% in the horizontal direction. As described above, the target film of the present invention is a film obtained from polyethylene resin under specific manufacturing conditions described below, and has excellent uniaxial shrinkability in the transverse direction, and is an unprecedented novel film.

[Contents of the manufacturing method of the present invention and its significance]

Hereinafter, a manufacturing method for obtaining a novel and useful film, which is the object of the present invention, will be explained.

The polyethylene 7 resin used as the base material of the film of the present invention is fed to a commonly used extruder (if it is made of one type of resin, it is fed as is), and if it is made of two or more types of resin, it is fed to a commonly used extruder. (Supplied by dry blending a fixed amount of each resin using a drum blender, pump blender, etc., or by melting and mixing dry blended resins)
The material is extruded into a tube shape from an annular die, cooled and solidified to produce a raw material (original fabric).

At this time, the cooling solidification method includes cooling with an air ring,
Cooling with a water-cooling ring, cooling with a chill roll, etc. is used, but the method of cooling the tube-shaped raw fabric using a water-cooling ring and bringing water at a temperature of 5 to 20°C directly into contact with the raw fabric to rapidly solidify it is a method using air. Compared to the air-cooling method using a cylinder, the surface of the cooled and solidified raw fabric is smoother and more transparent, and the processing stability is improved when manufacturing thick raw fabric. In this respect, it is a particularly useful method in carrying out the present invention. High energy ionizing radiation such as electron beam, γ ray, β ray, etc.
The original fabric is cross-linked by rad irradiation. As for the radiation to be irradiated, it is most practical to use an electron beam in terms of operability and workability of the equipment, and also in terms of heat generation during irradiation.
For example, a method of repeating irradiation at 2 to 5 Mrad/time in several batches is preferred, and crosslinking is carried out to a gel fraction in the range of 30 to 75% by weight. As a guideline for the irradiation dose when employing radiation crosslinking, if the irradiation dose to the original fabric is less than 8 Mrad, the degree of crosslinking of the resin, that is, the gel fraction, will be too small, and the stability and uniformity when stretching the original fabric will be reduced. is insufficient.

Furthermore, when the film after stretching is re-irradiated with radiation, as employed in the Examples described later, the heat shrinkage rate of the resulting film is reduced, and the film has excellent uniaxial shrinkability in the transverse direction, which is the objective of the present invention. is not obtained. Furthermore, if the irradiation dose to the original fabric exceeds 20 Mrad, the gel fraction will increase, making it difficult to increase the stretching ratio when stretching the original fabric.

Furthermore, when the stretched film is re-irradiated with radiation as employed in the examples described later, the resulting film has a large longitudinal heat shrinkage rate, and does not have the transverse uniaxial shrinkage that is the objective of the present invention. It becomes impossible to obtain a good film. As described above, the degree of crosslinking (gel fraction) of the original fabric, as well as the stability and uniformity of stretching, makes it easy to set the stretching conditions particularly for inflation biaxial stretching, which is the object of the present invention.

The amount of radiation irradiation is determined by the type of resin used and the desired thermal shrinkage rate, but it is necessary to make the gel fraction of the original fabric 30 to 75% by weight, and furthermore 50 to 7% by weight. Irradiation dose is preferred. Further, this crosslinking may be carried out by chemical crosslinking using a conventionally known crosslinking agent, ultraviolet crosslinking, or the like. Next, the original fabric is heated with a heated gas, liquid, infrared heater, or the like, and stretched at a predetermined stretching ratio by an ordinary inflation method to obtain a film. As for the method of heating the original fabric, a method using an infrared heater is recommended due to the operability of the device and
Desirable in terms of workability. The heating temperature also depends on the resin used.
It can be determined as appropriate from the point of view of the stretching ratio, stability and uniformity of stretching, etc., but it is approximately 10 to
The temperature range is from 20°C lower to higher, ie 90-150°C, preferably 110-140°C. If the heating temperature is lower than 90'C, it is difficult for the resin to completely melt and soften, resulting in problems such as insufficient stability and uniformity of stretching and difficulty in increasing the stretching ratio.
At temperatures exceeding 50°C, the resin melts excessively, resulting in a so-called drawdown phenomenon, resulting in insufficient stretching stability and uniformity. In addition, the stretching ratio is mainly determined by the desired longitudinal and transverse heat shrinkage rates,
It is 2.5 times or less in the vertical direction and 3 times or more in the horizontal direction.

When the stretching ratio in the machine direction exceeds 2.5 times, the heat shrinkage rate in the machine direction of the obtained film is large, and even if the film is recrosslinked after stretching, the degree of decrease in the heat shrinkage rate in the machine direction is small. A film with a desired heat shrinkage rate of 20% or less in the machine direction cannot be obtained, and on the other hand, at a stretching ratio of less than 3 times in the transverse direction, the resulting film has a small heat shrinkage rate in the transverse direction, and furthermore, it is difficult to regenerate after stretching. The reduction in heat shrinkage due to crosslinking is too large, making it impossible to obtain a film having a heat shrinkage of 50% or more in the transverse direction, which is the object of the present invention. In the stretching process by the inflation method, a stretching ratio of 1.5 to 2 times in the longitudinal direction and a range of 3.5 to 5 times in the transverse direction is suitable for the stability and uniformity of stretching and the heat shrinkage rate. In order to stabilize the inflation biaxial stretching valve, it is preferable to maintain the stretching ratio in the transverse direction at a value of about 2 to 25 times the stretching ratio in the longitudinal direction. most desirable. The film of the present invention is then obtained by re-crosslinking the stretched film, that is, generally by re-irradiating it with radiation. This re-irradiation of the film is performed in order to adjust the heat shrinkage rate of the film in the longitudinal and lateral directions to the desired heat shrinkage rate, and is a major feature of the manufacturing method of the present invention. This is a method to reduce the amount to 20% or less. The degree of crosslinking is adjusted so that the gel fraction of the final film is 50 to 8% by weight, but the amount of re-irradiation in generally used re-irradiation is determined by the gel fraction of the original film before stretching and the stretching process. Although it differs depending on the stretching ratio in the longitudinal and transverse directions (particularly the stretching ratio in the longitudinal direction), in general, the larger the stretching ratio in the longitudinal direction, the larger the re-irradiation amount. In the method of the present invention, 8
~20 Mrad1 and even 10~15r! 4 rad is preferred. When the re-irradiation amount is less than 8r1!4 rad, the degree of decrease in the longitudinal heat shrinkage rate is small, and even if re-irradiation is performed, the heat shrinkage rate exceeds 20%.On the other hand, when the re-irradiation amount is more than 20 Mrad, the longitudinal Even if the heat shrinkage rate is 20% or less, the heat shrinkage rate in the transverse direction is less than 5, making it difficult to obtain a film with excellent uniaxial shrinkability in the transverse direction, which is the object of the present invention. It is disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 51-59973 that the heat shrinkage rate of the film is reduced by irradiating such a stretched film with radiation. Since the irradiation is applied to the stretched film from a raw material that has not been irradiated, the degree of reduction in heat shrinkage is extremely large. I can't get it at all.

Furthermore, Japanese Patent Publication No. 48-27904 discloses a method of irradiating a film with a stretching ratio of 5 times or more in the longitudinal and lateral directions. It is stated that the thermal shrinkage rates in the longitudinal and lateral directions are almost unchanged. Even after stretching a conventionally known uncrosslinked original fabric and irradiating it with radiation, it was not possible to obtain a film with excellent transverse uniaxial shrinkage, which is the object of the present invention. l Regarding the method of the present invention, the degree of reduction in heat shrinkage in the longitudinal and transverse directions exhibits unique behavior due to recrosslinking, as will be described in the Examples below. It is thought that the following three conditions are intricately intertwined: the stretching ratio, the stretching ratio during stretching of the original fabric (particularly the stretching ratio in the machine direction), and the amount of recrosslinking after stretching.

Although the details of these mechanisms of action are not clear, the following actions are considered. In general, the stretching orientation when polyethylene resin etc. is stretched is when each molecular chain is simply flow-oriented in the stretching direction, that is, when each molecular chain is oriented due to a simple shift between each molecular chain In other words, it is possible that the carbon-carbon chains within each molecular chain are stretched and oriented in the initial stage.

Normally, it is thought that the above-mentioned inter- and intra-molecular chain orientations coexist, but if the stretching ratio is small or the stretching temperature is high, the former flow orientation between each molecule may occur. Conversely, when the stretching ratio is large, or when the stretching temperature is appropriate (this is the so-called stretching start temperature, which varies depending on the resin), it is thought that orientation mainly occurs within each molecular chain. In any of these stretching orientations,
It has the property of shrinking and returning to its pre-stretching state by heating again after stretching and orientation. In the stretching method of the present invention, since the stretching ratio in the longitudinal direction is set smaller than that in the transverse direction, it is thought that the orientation of each molecular chain in the longitudinal direction is mainly the fluid orientation between each molecular chain. In addition, it is thought that the orientation in the lateral direction is mainly within each molecular chain.

Generally, the higher the crystallinity of the resin, the lower the crosslinking efficiency.
For example, the G value of low density polyethylene (a value indicating ease of crosslinking; the higher the value, the easier it is to crosslink.

) is said to be 3 to 6, and 1 to 3 for high density polyethylene. From the above, in the film of the present invention, in the longitudinal direction where the flow orientation between each molecular chain is the main one, the behavior is largely amorphous region-like, whereas the stretching orientation within each molecular chain is In the main transverse direction, the crystalline region-like behavior is large, and therefore the crosslinking efficiency in response to irradiation is different in the longitudinal and transverse directions, and the longitudinal direction is more selectively crosslinked than the transverse direction. Therefore, it is thought that the degree of decrease in heat shrinkage rate due to recrosslinking after stretching is greater in the longitudinal direction than in the transverse direction.

In addition, in the method of the present invention in which a raw fabric cross-linked within a certain range is stretched, the raw fabric has a three-dimensional structure cross-linked between or within molecular chains. Because the fluidity decreases, it is thought that the flow orientation between molecular chains is less than when using a non-crosslinked original fabric, and the degree of decrease in heat shrinkage rate due to post-stretching irradiation becomes smaller. It is considered that a large decrease in thermal shrinkage rate due to irradiation as disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 51-59973 does not occur.

As described in detail above, the method of the present invention crosslinks the original fabric in a predetermined range before stretching, sets the stretching ratio in the longitudinal and lateral directions during stretching to a predetermined range, and This is a completely new manufacturing method that can only be achieved by combining steps in which the resulting film is irradiated in a predetermined range.

The re-irradiated film is then processed using the commonly used methods.
It is slit appropriately and wound up. The thickness of the film is appropriately selected depending on the field of use of the film, but in the inflation method using a tube-shaped original film, the thickness is 5 to 1.
00μ films can be suitably produced.

Examples and comparative examples of the present invention will be described below.

Example 1 65% by weight of resin A and 35% by weight of resin C listed in Table 1 were dry blended, and a diameter of 45T! r! N. ．． L/D=2
It is extruded downward from an annular die of 5 diameter with 2 slits provided at the tip of an extruder with 8 screws at a maximum cylinder temperature of 260 °C, a die part temperature of 250 °C, and a screw rotation speed of 80 r'Pm. , about 20C7 from the die tip! Using a water-cooled ring with an inner diameter of 657 wt that has a slit for water to come out, the ring was cooled and solidified at the position , and was taken out at a take-up speed of 4.5 rrL/min, with a folding width of 100 wt.
-FOltl A tube-shaped original fabric with a thickness of 300 μm was prepared.

Using an electron beam accelerator (manufactured by Nissin High Voltage Co., Ltd., EPS5OO) with an accelerating voltage of 500 KV and a maximum current density of accelerating tube of 25 rr1A, each tube-shaped original fabric was heated at 6 Mrad.
110 Mrad and 120 Mrad were irradiated. Next, the tubular irradiated fabric was fed into a heating tube equipped with an annular infrared heater at a speed of 1.6771,/min, and the tubular irradiated fabric was heated to 120 to 130°C to double the longitudinal stretching ratio. At the same time, air was introduced into the tube to inflate it to a horizontal stretching ratio of 4 times, and then it was uniformly cooled by air from the outer periphery using an air ring. ) A tube-shaped film was prepared by removing slits from both ends to obtain a film having a width of 30C3, which was peeled into two pieces and wound into a roll. The obtained films were then subjected to 5IS-4rad, .
Reirradiation was performed at 10 Mrad, 115 Mrad, and 120 Mrad.

Gel fraction of the obtained film, heat shrinkage rate at 130°C,
And the gel fraction of the irradiated original fabric before stretching was measured.

The results are shown in Table 2 (the notes column in the table shows actual examples, and the ratio shows comparative examples).
The figure shows the change in the thermal shrinkage rate in the lateral direction. In the figure, 1 and ■ indicate the case of 6 Mrad, ■ and V indicate the case of 10 Mrad, and ■ and ■ indicate the case of 20 Mrad, and the subscripts of each point in the figure, for example, 1 and 1'' indicate the second The film No. 1 in the table is shown, and the same applies below.As is clear from these results, by re-irradiating the film after stretching,
It can be seen that the heat shrinkage rate decreases and that the effect of re-irradiation on the heat shrinkage rate varies depending on the irradiation dose of the original fabric before stretching.

That is, sample No. 1 with a raw gel fraction of 2% by saliva volume. l
~NO. In No. 5, the heat shrinkage rate of the film before re-irradiation was 47% in the longitudinal direction and 72% in the transverse direction, but in No. 5 after re-irradiation at 5 Mrad. Even in the case of No. 2, the heat shrinkage rate decreased significantly in both the vertical and horizontal directions, by 24% in the vertical direction and 45% in the horizontal direction, and the film that was the object of the present invention could not be obtained. On the other hand, sample No. 1 obtained from the original fabric gel fraction 5% heel weight. 6~NO. For the 1O film, the re-irradiation dose is 10, 1\20 Mrad.
O. 8.No. 9.No. The film of IO had a heat shrinkage rate of 20% or less in the longitudinal direction and a heat shrinkage rate of 50% or more in the transverse direction, and the film that is the object of the present invention was obtained. In addition, sample No. 1 obtained from the original gel fraction of 7%. ll～N
O. For 15 films, the re-irradiation dose is 15 Mrad12
0 Mrad, NO. l4, NO. Film No. 15 was the film that is the object of the present invention.

Although the amount of re-irradiation to the film increases),
Although the Genohi fraction shows a near S'-constant value, it is thought that this is because crosslinking and molecular chain scission occur due to irradiation, and an equilibrium state is reached, so to speak. Example 2 Among the original fabrics obtained by the method of Example 1, the original fabric with a gel fraction of 5% by weight was used, and the stretching ratio in the longitudinal and transverse directions was set to 1.
212. Stretched at the same time as Example 1 at 2.214.4 times and 2.816.5 times, and then 5 Mr.
A film was obtained by re-irradiation with ad110 Mrad and 115 Mrad.

Table 3 shows the heat shrinkage rate of the obtained film. (The notes in the table indicate examples, and the ratios indicate comparative examples.) As is clear from Table 3, the heat shrinkage rate changes depending on the stretching ratio and the amount of re-irradiation.

That is, in the films of NO16 to NO19, since the transverse stretching ratio was as small as 2.5 times, the transverse heat shrinkage did not reach 50% upon re-irradiation. Also NO. 2
In films 1 of No. 4 to No. 27, the longitudinal stretching ratio was as large as 2.8 times, so that even after re-irradiation, the longitudinal heat shrinkage did not fall below 20%.

On the other hand, NO. Film No. 23 has a heat shrinkage rate of 15 in the longitudinal direction.
% and 55% in the transverse direction, and the film satisfied the purpose of the present invention. Example 3 In Example 1, the resin composition was changed as shown in Table 4, a tube-shaped raw fabric was created, and 20 Mrad was applied to the tube-shaped raw fabric.
irradiated, stretching magnification: 1.6 f8 in the vertical direction, 3.2 times in the horizontal direction,
Stretching temperature NO. 28 is 110℃, NO. 29~NO. 3
4 was stretched at 130-140°C and then re-irradiated at 15 Mrad.

Table 4 shows the heat shrinkage rate of the obtained film. As mentioned above, the film obtained by the present invention is obtained only under specific manufacturing conditions as shown in FIGS.
It is obtained by a manufacturing method in which the stretched film is further irradiated with radiation. It is a film with excellent uniaxial shrinkability in the transverse direction, and has the following characteristics. (a) The crosslinked original fabric is stretched so that the gel fraction is 30 to 75% by weight, at a stretching ratio of 2.5 times or less in the machine direction and at least 3 times in the transverse direction, and then exposed to radiation. Re-crosslinking (8~20Mr
Since the film is obtained by the new manufacturing method of the present invention, which involves ad re-irradiation, the film has a heat shrinkage rate of 20% or less in the longitudinal direction, a shrinkage rate of 50% or more in the transverse direction, and has excellent uniaxial shrinkability in the transverse direction. It is a resin film.

(b) It is a film that has been stretched and oriented after crosslinking, and has excellent mechanical strength, heat resistance, optical properties, etc.

(c) It is a film based on polyethylene resin, and is excellent in terms of food hygiene, waste disposal, etc.

A film that satisfies (a) to (c) above is extremely useful, for example, as a film for cap seals or shrink labels, and outperforms conventional PVC films.

The present invention is a manufacturing method that makes it possible to complete such a highly useful film for the first time using cross-linked polyethylene resin, and uses an inflation method that is advantageous in terms of equipment and work efficiency. In terms of its manufacturing method, it can be said to be an excellent invention that will play a major role in industry.

[Brief explanation of drawings]

Figure 1 shows the film obtained in Example 1. The longitudinal heat shrinkage rate at 130°C is shown.

Claims (1)

[Claims] 1 Gel fraction based on polyethylene resin is 50~
In order to obtain a heat-shrinkable cross-linked polyethylene resin film with a heat shrinkage rate of 80% by weight, which has a significantly larger value in the transverse direction than in the longitudinal direction, first, an extrusion method using an annular die was used. A polyethylene resin tubular crosslinked raw fabric with a gel fraction of 30 to 75% by weight is created, and then this raw fabric is heated to 90 to 150°C and stretched 2.5 times or less in the machine direction and 3 times or more in the transverse direction. The gel fraction value of the film is adjusted to a range of 50 to 80% by weight by inflation biaxial stretching at a magnification, and then the gel fraction value of the film is adjusted to a range of 50 to 80% by weight by radiation irradiation, so that the heat shrinkage rate in the longitudinal direction is 20% or less, and the heat shrinkage rate in the transverse direction is A method for producing a heat-shrinkable crosslinked polyethylene resin film, characterized by obtaining a film having a shrinkage rate of 50% or more.