JPH0521930B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD The present invention relates to a novel method for producing an ethylene resin multilayer crosslinked film. More specifically, the present invention uses an ethylene-based resin that can improve various physical properties of the film, such as heat sealability, tear strength, and gloss, depending on the purpose by controlling the degree of crosslinking of each layer in the multilayer film. The present invention relates to a method for producing a multilayer crosslinked film as a material. Conventional technology Conventionally, ethylene resin films have often been crosslinked for the purpose of increasing transparency, heat resistance, tensile strength, etc. Methods for this crosslinking treatment include, for example, electron beam irradiation. is known (Special Publication No. 37-18893). However, although the electron beam irradiation crosslinked film obtained by this method has superior strength, transparency, and heat shrinkage properties compared to ordinary non-crosslinked films, it is However, it has the disadvantage of poor heat-sealability and reduced efficiency in packaging operations. In order to improve such poor heat-sealing properties, the film is multilayered and, for example, a relatively large amount of antioxidant or condensed polycyclic aromatic compound is used as a crosslinking inhibitor, or a crosslinking accelerator is used. A method of controlling the degree of crosslinking using polyfunctional monomers has been proposed (Japanese Patent Application Laid-Open No. 1973-
Publication No. 12167, Japanese Patent Application Publication No. 18228-1983, Japanese Patent Application Publication No. 1982-18228
-95568). All of these methods use the same main layer to increase stretchability and provide good mechanical properties as a crosslinked film, and a surface layer in a low or non-crosslinked state to provide good heat sealability. It is intended to be obtained under a standard electron beam irradiation dose, and the effect of controlling the degree of crosslinking is exhibited to some extent. However, when a film containing the above-mentioned crosslinking inhibitor or accelerator is used for food packaging, etc., its range of use is inevitably limited from the viewpoint of safety and hygiene. On the other hand, once packaging films reach the distribution channel, they often come into contact with food in various forms, so as a producer, it is important not only to use packaging films for food packaging but also for general packaging films. Although it is desirable to pay close attention to the sanitary safety of the film, it is extremely difficult to carry out this strictly. Therefore, there is a need for an effective method for controlling crosslinking that is not subject to food hygiene or food safety restrictions, but such a method has not been realized so far. Therefore, for example, it is possible to obtain a crosslinked laminated film with improved heat sealability by lowering the crosslinking degree of the surface layer, or conversely, to obtain a crosslinked laminated film with excellent gloss and chemical resistance by increasing the crosslinking degree of the surface layer. At present, a packaging film that fully utilizes the ability to adjust the degree of crosslinking has not yet been released on the market. By the way, when considering the factors that crosslinking affects the physical properties of a film, the gel fraction of the film, which is a measure of the degree of crosslinking, can be mentioned. Figure 1 shows
This is a graph showing the relationship between the gel fraction, heat sealing strength, and shrinkage stress of this film.The horizontal axis shows the gel fraction, and the vertical axis shows the heat sealing strength and shrinkage stress.As is clear from this graph, The lower the gel fraction is, the higher the heat sealing strength is, and the higher the gel fraction is, the higher the shrinkage stress is. Therefore, a single film cannot satisfy both properties at the same time. In order to improve heat sealability while maintaining the original mechanical properties of a crosslinked film, crosslinking control such as suppressing the gel fraction of the surface layer of the film is necessary, for example. Figure 2 shows the crosslinking efficiency of resins with different melting points, and is a graph showing the relationship between the electron beam absorbed dose and the gel fraction, with the horizontal axis representing the electron beam absorbed dose and the vertical axis representing the gel fraction. rate. As is clear from FIG. 2, when the same absorbed dose is applied to ethylene resins, the lower the melting point (crystallinity) of the resin, the higher the irradiation efficiency tends to be. On the other hand, when attempting to improve the heat-sealability of a film by making it multilayered, it is common to use a heat-sealing layer with a low melting point and a heat-resistant layer with a high melting point. However, when an electron beam is irradiated on a multi-layer structure consisting of a heat-sealable layer made of ethylene-vinyl acetate copolymer and a heat-resistant layer made of high-density polyethylene, the heat-sealed layer gels, as is clear from Figure 2. The fraction is higher than that of the heat-resistant layer, making it impossible to obtain good heat-sealing properties. Therefore, in a crosslinked ethylene resin film, the heat sealability cannot be improved simply by layering resins with different melting points, and the degree of crosslinking must be controlled. Problems to be Solved by the Invention Under these circumstances, the present invention aims to improve various physical properties such as heat sealability, tear strength, and gloss according to the purpose by controlling the degree of crosslinking of each layer in a multilayer film. The purpose of this invention is to provide a method for producing a multilayer crosslinked ethylene resin film that can be improved and can be safely used for various types of packaging, including food packaging. Means for Solving the Problems As a result of extensive research to achieve the above object, the present inventors have discovered that at least two types of ethylene resin layers containing polyhydric alcohol fatty acid esters at different concentrations are laminated. However, the inventors have discovered that this object can be achieved by irradiating this laminated film with ionizing radiation to cause crosslinking to different degrees of crosslinking, and have completed the present invention based on this knowledge. That is, the present invention involves laminating at least two types of ethylene resin layers containing polyhydric alcohol fatty acid esters at different concentrations, and then irradiating the thus obtained laminated film with ionizing radiation. The present invention provides a method for producing a crosslinked film, characterized in that each layer is crosslinked until a required degree of crosslinking is obtained. The present invention will be explained in detail below. Examples of the ethylene resin used in the method of the present invention include high density polyethylene, medium density polyethylene, low density polyethylene, linear low density polyethylene, very low density polyethylene, and ethylene-vinyl acetate copolymer. Also,
The various polyethylenes mentioned above may be a homopolymer of ethylene, or a random or block copolymer of ethylene and other α-olefins. Examples of the other α-olefins include propylene, butene-1,4-methylpentene-
1, hexene-1, octene-1, decene-1,
Examples include dodecene-1. These ethylene resins may be used alone or in combination of two or more. In the method of the present invention, polyhydric alcohol fatty acid esters used to control the degree of crosslinking include, for example, polyhydric alcohols such as glycerin, polyglycerin, sorbitan, sorbitol, lauric acid, myristic acid, palmitic acid, stearic acid, Examples include one type or a mixture of two or more types selected from ester compounds with fatty acids such as oleic acid. The amount of these polyhydric alcohol fatty acid esters added varies depending on the type of polymer, the structure of the laminated film, the amount of irradiation, etc., but in the layer that suppresses the degree of crosslinking, it is usually 1.0 to 10.0 based on the weight of the resin.
Selected within the range of weight %. If this amount is less than 1.0% by weight, the effect of crosslinking control will not be sufficiently exhibited;
If it exceeds 10.0% by weight, there will be a tendency for transparency to be impaired due to bleeding to the surface and for film forming properties to be reduced. On the other hand, in the layer that does not suppress the degree of crosslinking, the amount added is usually selected within the range of 5.0% by weight or less, but it is 1/1/2 of the layer in which the degree of crosslinking is to be suppressed.
It is desirable to keep it to 2 or less. There are no particular limitations on the method of multilayering in the present invention, and any method may be selected from the methods commonly used in the production of multilayer films, such as coextrusion, lamination, coating, etc. Coextrusion is particularly preferred. In addition, the layer structure of the layer that suppresses the degree of crosslinking and the layer that maintains the degree of crosslinking may be two or more layers, but in order to obtain good heat sealability in all packaging forms, it is necessary to suppress the degree of crosslinking. Three or more layers, including two surface layers and an inner layer that maintains the degree of crosslinking, are preferred. Regarding the ratio of the thickness of each layer, for example, in the case of a three-layer film, the ratio of surface layer/inner layer/surface layer is usually 1/
Selected from 1/1 to 1/20/1. In the present invention, the crosslinking treatment is performed by irradiation with ionizing radiation. As this ionizing radiation, for example, α rays, β rays, γ rays, neutron beams, accelerated electron beams, etc. are used, and the absorbed dose is usually 1 to 1.
Selected within the range of 20 Mrad. The type of ionizing radiation and the absorbed dose are appropriately selected depending on the type of polymer, the thickness of the film, the layer structure, etc. The multilayer film crosslinked in this manner may be used as it is without being stretched, or may be stretched and oriented as necessary. For example, in order to obtain a film with heat shrinkability, it is preferable to carry out stretching or orientation. At this time, there is no particular restriction on the method of stretching treatment, and depending on the required heat shrinkability, it is possible to follow a commonly used method. It is sufficient to stretch the film by about 2 to 10 times. In addition, in the present invention, when producing a multilayer film, within the scope of not impairing the purpose of the present invention,
Various additives normally used in plastic films, such as antifogging agents, heat stabilizers, antioxidants,
If desired, the film may contain an ultraviolet absorber, an antiblocking agent, a slip agent, an antistatic agent, and the like. Effects of the Invention The polyhydric alcohol fatty acid ester used in the present invention has sufficient ability to suppress crosslinking, and the degree of crosslinking can be finely controlled by changing the type and amount added. Therefore, according to the method of the present invention, the degree of crosslinking of each layer constituting a multilayer film can be freely adjusted, and various physical properties such as heat sealability, tear strength, and gloss of the film can be improved depending on the purpose. . For example, to improve heat sealability, it is sufficient to reduce the degree of cross-linking on the sealing surface, to improve the gloss of the film, it is sufficient to relatively increase the degree of cross-linking of the surface layer, and to improve tear strength. In order to achieve this, a low crosslinked or non-crosslinked layer may be provided. Furthermore, the polyhydric alcohol fatty acid ester used in the present invention is listed in the Polyolefin Hygiene Council's Positive List and is safe from a food hygiene perspective. It can be safely used in a variety of packaging, including . Also, regarding the multilayer structure, depending on the purpose,
It is possible to increase the functionality of the film from 2 layers to 3 layers to 5 layers. Examples Next, the present invention will be explained in more detail with reference to Examples, but the present invention is not limited to these Examples in any way. In addition, each characteristic was calculated|required as follows. (1) Gel fraction A sample is extracted with boiling p-xylene, and the percentage of insoluble portion is expressed using the following formula, and is used as a measure of the degree of crosslinking. Gel fraction (wt%) = Sample weight after extraction / Sample weight before extraction x 100 (2) Heat seal strength At 130℃ with Sentinel heat sealer,
This is the peel strength value measured using a tensile tester using a 15 mm wide test piece that was sealed under 0.5 sec conditions. (3) Shrinkage stress Measured according to ASTM-D-2838 method. (4) Absorbed dose expressed in rad. 1 rad means that 100 erg of energy is absorbed per gram of material. (5) Haze Measured by ASTM-D-1003 method. (6) Tear impact strength Measured according to JIS-P-8116 method. (7) Free shrinkage rate Measured by ASTM-D-2732 method. (8) Bottom seal temperature range Film made of commercially available polypropylene (10
cm x 20 cm), a 4-layer film layer and a 2-layer film layer were formed at the bottom of the tray, and the film was heat-sealed by pressing with a force of 2 g/cm ² for 2 seconds. The bottom seal temperature range was defined as the range from the temperature at which the film does not peel off when the edge of the film is pulled gently to the temperature at which the film shrinks and the seal peels off or a hole is formed. (9) Gloss Measured by ASTM-D-2457-70 method. Further, as the ethylene resin, the following was used. EVA-a; ethylene vinyl acetate copolymer,
VAc=10wt%, MI=1.0 EVA-b; VAc=15wt%, MI=1.0 LDPE-a; low density polyethylene, density=0.92
g/cm ³ , MI=0.4, mp=106℃ LDPEb; Density=9.919g/cm ³ , MI=2.0, mp=
105℃ HDPE-a; High density polyethylene, density = 0.95
g/cm ³ , MI = 1.0, mp = 132°C LLDPE-a; linear low-density polyethylene, density = 0.93 g/cm ³ , MI = 2.1, Mitsui Petrochemicals Ltd. Urtozex 3022 LLDPE-b; density = 0.93 g/cm ³ , MI=2.0, Dowrex 2032 manufactured by Dow Chemical Company Reference example 1 EVA-a was extruded using a single extruder at a die temperature of 200°C.
A tubular film with a thickness of 600μ was molded. This tube-shaped film was flattened and irradiated with 0, 1.8, 2.2, and 2.7 Mrad using an electron beam irradiation device (manufactured by Nisshin High Voltage Co., Ltd.) at 500 KV.
Crosslinked. Gel fraction is 0%, respectively.
They were 10wt%, 20wt%, and 30wt%. Next, the tube was heated to a film temperature of 90℃ for the uncrosslinked tube and 130℃ for the crosslinked tube, at a film temperature of 7 times in the MD direction and TD.
A 15 μm film was obtained by stretching 5.7 times in the direction. FIG. 1 shows the relationship between the gel fraction of the film, heat seal strength, and shrinkage stress. As can be seen from FIG. 1, the lower the gel fraction, the better the heat sealing strength, and the higher the gel fraction, the better the shrinkage stress. Reference Example 2 HDPE-a, LDPE-a and EVA-a were each produced in a single screw extruder at a die temperature of 200 to 240°C.
It was extruded into a 400 μm tubular film. This tubular film was flattened and crosslinked by irradiation of 3 to 20 Mrad at 500 KV using an electron beam irradiation device (manufactured by Nissin High Voltage Co., Ltd.). Figure 2 shows the relationship between absorbed dose and gel fraction. As is clear from FIG. 2, when the same absorbed dose is applied to ethylene-based resins, the lower the melting point (crystallinity) of the resin, the higher the irradiation efficiency tends to be. Reference example 3 LDPE-a 100 parts by weight and various additives shown in Table 1
2.0 parts by weight were kneaded using a kneader at 180°C for 10 minutes, and then kneaded using a single screw extruder at a die temperature of 200°C.
A tube-shaped film with a thickness of 400 μm was formed.
This tube-shaped film was flattened and cross-linked by irradiating it with 5.5 Mrad at 500 KV using an electron beam irradiation device. Table 1 shows the results of measuring the gel fraction along with the samples without additives. Further, the same procedure as above was carried out using 4.0 parts by weight and 6.0 parts by weight of diglycerin monooleate, respectively, and the relationship between the amount of diglycerin monooleate added and the gel content is shown in FIG.

[Table] As is clear from Table 1 and Figure 3, the polyhydric alcohol fatty acid ester used in the present invention has sufficient crosslinking inhibiting ability, and by changing its type and amount added, Fine control of the degree of crosslinking is possible. Example 1 96 parts by weight of EVA-b, 2.0 parts by weight of glycerin monooleate as a crosslinking regulator, and 2.0 parts by weight of diglycerin monooleate were heated at 160°C using a kneader.
For the surface layer, mix the mixture for 10 minutes with
LLDPE-a was used for the inner layer and was extruded into a tube with a thickness of 400 μm from an annular die for two types and three layers using a single screw extruder. The thickness ratio of surface layer/inner layer/surface layer was 1/8/1. This tube-shaped film was flattened and exposed to electron beam irradiation equipment (manufactured by Nissin High Voltage Co., Ltd.) under the conditions of 500 KV and 21 mA.
Crosslinking treatment was performed by irradiating with 7.5 Mrad. Next, this tube was heated through an infrared heating furnace and stretched 6.5 times in the MD direction and 6 times in the TD direction at a film temperature of 150° C. to obtain a crosslinked film with a thickness of 10 μm. This was designated as sample No. 1. The total gel fraction of this film was 32 wt%. Furthermore, since the gel fraction of the inner layer-only sample with the surface layer removed was 30 wt%, the gel fraction of the surface layer is estimated to be around 40 wt%. As a comparative example, a single-layer film was obtained using LLDPE-a as a raw material and irradiated and stretched under the same conditions as Sample No. 1. This was designated as sample No. 2. The gel fraction is
It was 30wt%. Table 2 shows the physical properties of these films.
Sample No. 1 has the same properties as sample No. 2 in terms of shrinkage characteristics (free shrinkage rate, shrinkage stress) because the inner layer, which makes up the main part of the film, maintains a highly cross-linked structure. On the other hand, sample No. 1 suppresses the degree of crosslinking of the surface layer to a gel fraction of 80 to 40%, resulting in a bottom seal range three times higher and tear strength 1.5 times higher than sample No. 2. It's clearly an improvement.

[Table] Example 2 The resin of the surface layer and inner layer, the type and amount of crosslinking inhibitor added, the thickness ratio of each layer, the electron beam absorption dose, and the stretching temperature and magnification in Example 1 were changed as shown in Table 3. Other than that, a film was formed in the same manner as in Example 1. The average gel fraction of the film is 20~
It was in the range of 40wt%. The seal strength of the obtained film is also listed in Table 3. Samples Nos. 3 to 5 are laminated with different types of polymers, and Samples No. 6 and 7 are laminated with the same type of polymer, but both have good heat seal strength by suppressing the degree of crosslinking on the surface. has been realized. On the other hand, Samples Nos. 8 and 9 were the same as Samples Nos. 3 and 6, respectively, except that no crosslinking inhibitor was added, but they had poor heat sealability.

【table】 [Brief explanation of the drawing]

Figure 1 is a graph showing an example of the relationship between gel fraction, heat seal strength, and shrinkage stress in crosslinking of ethylene-vinyl acetate copolymer films, and Figure 2 is a graph showing the relationship between film gel fraction, heat seal strength, and shrinkage stress in crosslinking of ethylene-vinyl acetate copolymer films. FIG. 3 is a graph showing an example of the relationship between dose and gel fraction. FIG. 3 is a graph showing an example of the relationship between the amount of crosslinking regulator added and gel fraction in the crosslinking treatment of low density polyethylene film.

Claims (1)

[Claims] 1 At least two types of ethylene resin layers containing polyhydric alcohol fatty acid esters at different concentrations are laminated, and then the thus obtained laminated film is irradiated with ionizing radiation to achieve the required crosslinking for each layer. A method for producing a crosslinked film, which comprises crosslinking until a certain degree of strength is obtained.