JP2750610B2 - Polypropylene multilayer shrink film - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial application) The polypropylene-based multilayer shrink film of the present invention wraps a product roughly and shrinks it by a heating device (shrinkage tunnel) to perform tight packaging. It is used for heat shrink wrapping. And cup foods, lactic acid bacteria drinks, paper pack drinks, frozen and chilled foods,
It is used for heat shrink wrapping of pharmaceuticals, aerosol cans, or daily necessities such as stationery and toys, and exhibits an excellent effect particularly on accumulation wrapping and tamper proof packaging.

(Prior Art) As shrink films used in the field of heat shrink packaging, there are various films made of polyethylene resin, polypropylene resin, polyvinyl chloride resin or the like, and many are commercially available. Above all, a shrink film made of a polypropylene resin is widely and generally used due to its inherent excellent properties such as transparency, gloss, moisture-proof property, and pollution-free property. But,
Shrink film made of polypropylene resin cannot generate heat shrinkage unless heated to a considerably high temperature compared to shrink film made of polyvinyl chloride resin, and the heat shrink wrapping temperature range is narrow, making it suitable for heat shrink wrapping. Is not always good.

As a solution to such a disadvantage, a shrink film in which both outermost layers are made of a polypropylene-based resin and the core layer is mainly made of a linear bottom density polyethylene resin is disclosed in
166049. Also, a shrink film using a special linear bottom density polyethylene resin having a very low density is disclosed in JP-A-63-214446, and a core layer is formed of an ethylene-vinyl acetate copolymer (hereinafter, ethylene-vinyl acetate copolymer). Shrink films mainly comprising a polymer are referred to as EVA) are disclosed in JP-A-54-155282 and JP-A-57-165254.

However, among these, those using EVA for the core layer,
In addition to having an acetic acid odor, various problems occurred in forming a multilayer unstretched raw film by coextrusion.

(Problems to be Solved by the Present Invention) The present invention provides a shrink film using a polypropylene resin, which has a wide heat shrink wrapping temperature range, has excellent heat shrinkability at low temperatures, and has a good packaging finish. Easy to be
In addition, the present invention aims to provide a film which has high fusing seal strength and tear propagation strength, is odorless, and has good film forming properties and stretchability by coextrusion.

That is, it is intended to obtain a polypropylene-based multilayer shrink film that also has the above-mentioned properties without impairing the properties such as transparency and glossiness inherent to the polypropylene-based resin, and further, moisture-proof properties and non-polluting properties. It is.

(Means for Solving the Problems) In the present invention, in order to obtain a polypropylene-based multilayer shrink film having the above-mentioned various properties, both outermost layers are made of a polypropylene-based resin, and the core layer is made of a copolymer of ethylene and methyl methacrylate. (Hereinafter, a copolymer of ethylene and methyl methacrylate is referred to as EMMA). That is, both outermost layers have a melting point of 135 to 150 ° C. and a melt flow index is made of a crystalline polypropylene resin having a melting point of 1 to 7, and the core layer has a melting point of 90 to 102 ° C. and the melt flow index is used for both outermost layers. With a polypropylene-based multilayer shrink film obtained by stretching a co-extruded multilayer unstretched raw material consisting of a copolymer of ethylene and methyl methacrylate having a melt flow index of 1/3 to 2 times the melt flow index of the crystalline polypropylene resin is there. And preferably, the crystalline polypropylene resin used for both outermost layers is an ethylene-propylene copolymer,
Alternatively, it is an ethylene-propylene-butene copolymer.

 Hereinafter, the contents of the present invention will be described in detail.

First, as the crystalline polypropylene resin used for both outermost layers, the melting point is in the range of 135 to 150 ° C., and the MI is 1 to 7;
Are within the range. As such a crystalline polypropylene resin, those obtained by copolymerizing propylene with about 2 to 4% by weight of ethylene and those obtained by copolymerizing propylene with about 3 to 8% by weight of ethylene and butene are preferably used.

Next, EMMA used for the core layer is a random copolymer resin obtained by high-pressure radical polymerization of ethylene and methyl methacrylate, and MI is 1/3 to 2 times the MI of the crystalline polypropylene resin used for both outermost layers. Are within the range.
The resin is a flexible resin due to the crystal inhibitory effect of the side chain of methyl methacrylate, and moreover, EVA
It has better thermal stability and is not a resin that generates acid even when thermally decomposed like EVA.

If the melting point of the crystalline polypropylene resin used for both outermost layers exceeds 150 ° C, stretching at low temperatures becomes difficult, and it will be stretched at high temperatures. Of course, when such a resin is used, the heat shrinkability at a high temperature is deteriorated, so that the suitability for heat shrink wrapping is reduced. On the other hand, when the temperature is lower than 135 ° C., heat resistance is inferior, melt whitening or the like easily occurs in a shrinking tunnel, and the suitability for heat shrink wrapping also decreases. In addition, the slidability (hot-slip property) of the package immediately after leaving the shrink tunnel is poor.

Furthermore, when the MI of the crystalline polypropylene resin used for both outermost layers is less than 1, the stretching stress increases,
On the other hand, if it exceeds 7, local stretching occurs at the time of stretching, and both have poor stretchability.

Next, when the melting point of EMMA used in the core layer exceeds 102 ° C., the stretchability is reduced, and stretching at a low temperature becomes impossible, and the temperature range of the heat shrink wrapping becomes narrow. In addition, the interlayer adhesion strength is weakened, and the strength of the fusing seal is lowered. Further, when the melting point is less than 90 ° C., not only is the film-forming property of the multilayer unstretched raw material by co-extrusion inferior, but also it is difficult to exert the heat treatment effect for eliminating the natural shrinkage of the obtained shrink film. If the film is left as it is, the film may be deformed or the smoothness may be deteriorated.

Further, if the MI of the EMMA is less than 1/3 of the MI of the crystalline polypropylene resin used for both outermost layers, the co-extruded multilayer unstretched raw material has poor formation and poor transparency. If the number exceeds twice, a part of the surface of the co-extruded unstretched multilayer raw material has intermittent irregularities in the flow direction of the resin, and it was not possible to co-extrude a good multilayer unstretched raw material.

The melting point in the present invention is a value measured by a differential scanning calorimetry for both the crystalline polypropylene resin and EMMA.

MI is a value measured at a temperature of 230 ° C. and a load of 2160 g for a crystalline polypropylene resin, and at a temperature of 190 ° C. and a load of 2160 g for EMMA in accordance with the method A of JIS-K-7210.

Regarding the thickness constitution of each layer of the polypropylene-based multilayer shrink film of the present invention, the thickness ratio of the core layer is preferably in the range of 30 to 70% with respect to the entire thickness. When the thickness ratio of the core layer is less than 30%, the heat shrinkability at low temperature is inferior, it is difficult to widen the heat shrink wrapping temperature range, and the tear propagation strength, which is important for the suitability of the packaging machine, is also weakened. Also 70
%, The stretchability is inferior, and the heat resistance is also inferior, and the suitability for heat shrink wrapping is reduced.

Further, the total thickness is suitably in the range of 8 to 100 µ, more preferably 10 to 50 µ.

It is to be noted that the addition of another resin or additive to each layer or the provision of a new layer is not hindered at all without departing from the scope of the present invention.

Next, as a method for producing the polypropylene-based multilayer shrink film of the present invention, first, a multilayer unstretched raw material is co-extruded from a multilayer die using a plurality of extruders. Then, after cooling and solidifying the multilayer unstretched raw material, the multilayer unstretched raw material is reheated to a stretchable temperature, and at least 3.
After stretching 0 times or more, it is cooled. As the stretching method,
Although it is possible to use a tenter method or an inflation method, it is preferable to stretch by the inflation method because it is easy to make the heat shrinkage characteristics in the longitudinal direction and the transverse direction very similar. For that purpose, it is necessary to obtain a multilayer unstretched raw material as a multilayer tubular sheet using a multilayer circular die. When the resulting multilayer biaxially stretched film is left to stand naturally, it is deformed due to the natural shrinkage of the film itself, or the surface condition is deteriorated. It is more preferable to reduce the amount of natural shrinkage by performing heat treatment so as not to lower the amount of the resin.

Further, as the heat shrink wrapping method using the polypropylene-based multilayer shrink film of the present invention, a heat shrink wrapping line used for a conventional shrink film made of a polypropylene resin can be used as it is. And, compared with the conventional shrink film, a good heat-shrinkable package can be obtained in a wide temperature range from a low temperature to a high temperature, so that it is not necessary to set the temperature in the shrink tunnel to severe. A good heat-shrinkable package can be easily obtained. Further, since the tear propagation strength is high, there is no breakage of the bag from the air vent hole at the time of automatic packaging, and further, since the fusing seal strength is high, there is no occurrence of breakage from the fusing seal portion.

(Effect of the Invention) The polypropylene-based multilayer shrink film of the present invention is excellent in low-temperature heat shrinkability and has a wide heat shrink wrapping temperature range, so that a favorable heat shrink wrap can be easily obtained.
Further, since the tear propagation strength and the fusing seal strength are strong, there are few troubles in the heat shrink wrapping. Further, the obtained heat-shrinkable package does not have an acetic acid odor unlike a shrink film using EVA as a core layer. Furthermore, the polypropylene-based multilayer shrink film of the present invention is excellent in productivity because not only is it easy to form a co-extruded film of a multilayer unstretched raw material, but also it is easy to stretch.

In addition, the product of the present invention is provided with everything that impairs the properties of the polypropylene resin such as transparency, gloss, moisture-proof property, and pollution-free property.

(Example) Hereinafter, an example and a comparative example are shown and the present invention will be described more specifically.

Example 1 A tube-shaped 3 having a crystalline polypropylene resin (ethylene-propylene-butene copolymer) having a melting point of 138 ° C. and MI 1.5 as both outermost layers and a core layer of EMMA having a melting point of 95 ° C. and MI 0.8. A layer unstretched raw material was prototyped with three extruders and a three-layer circular die for coextrusion, but could easily be formed into a film. And the overall thickness of the obtained multilayer unstretched raw material is about
At 240μ, the thickness ratio of each layer was 1: 2: 1 from one outer layer. In the coextrusion film formation, the film was rapidly cooled by a conventional water cooling system immediately after coextrusion.

This multilayer unstretched raw material was stretched 4.2 times in both the longitudinal and transverse directions by the conventional inflation method, but it can be easily stretched at a lower temperature than the conventional polypropylene resin, and a multilayer biaxially stretched film is obtained. I was able to do it. The multilayer biaxially stretched film was heat-set while being relaxed to obtain a polypropylene-based multilayer shrink film having a total thickness of about 15 μm.

Using the obtained polypropylene-based multilayer shrink film, heat shrink wrapping of sake (1.8 l) in a paper container was performed. As a result, even if the shrink tunnel temperature set to 160 ° C is changed up and down by about 15 ° C, there is no such thing as inferior tension or wrinkles in the corners, and transparency and gloss. Excellent good finish was obtained. or,
The obtained heat-shrinkable package did not break the bag from the fusing seal portion or the air vent hole in general handling.

Example 2 A crystalline polypropylene resin (ethylene-propylene copolymer) having a melting point of 145 ° C. and an MI of 6.0 was used as both outermost layers.
A tubular three-layer unstretched raw material having a core layer of EMMA of 0 ° C. and MI 4.5 was formed in the same manner as in Example 1. The total thickness of the obtained multilayer unstretched raw material was about 310 μm, and the thickness ratio of each layer was 1: 4: 1 from one outer layer.

This multilayer unstretched raw material is biaxially stretched by a conventional inflation method as in Example 1, and then thermally fixed to easily obtain a polypropylene-based multilayer shrink film having a total thickness of about 20μ. Was.

Using the obtained polypropylene-based multilayer shrink film, three notebooks were heat-shrink wrapped. As a result, the shrink tunnel temperature set at 170 ° C is raised and lowered by 20 ° C.
Even when the degree was changed, neither tightness was inferior nor wrinkles were generated at the corners, and a good finish excellent in transparency and gloss was obtained. In addition, even if the obtained package is dropped, the bag does not break from the seal portion, and even if the packages are rubbed with each other, the bag may break from the air vent hole. Was.

Comparative Examples 1 and 2 In Comparative Example 1, both outermost layers were made of a crystalline polypropylene resin of MI 9.0, and the core layer was used as the core layer of Example 2.
In Comparative Example 2, a multilayer unstretched raw material was used in Comparative Example 2 in which both outermost layers were made of a crystalline polypropylene resin having an MI of 0.5 and the core layer was an EMMA of MI 0.8 used in the core layer of Example 1. A film was formed in the same manner as in Example 1. The melting points of the crystalline polypropylene resins and EMMA were in accordance with the requirements of the present invention. The overall thickness of the obtained multilayer unstretched raw material and the thickness ratio of each layer were the same as in Example 1.

Using this multilayer unstretched raw material, biaxial stretching was attempted by a conventional inflation method as in Example 1. However, in Comparative Example 1, local stretching was terrible, and not only did the thickness deviation accuracy deteriorate, but also the stretching stability was poor. Further, in Comparative Example 2, the stretching stress was strong, and the stretched bubble was easily ruptured.

Comparative Examples 3 and 4 In Comparative Example 3, both outermost layers were made of a crystalline polypropylene resin having an MI of 5.5, and the core layer used in Example 1 had an MI of 0.8.
In Comparative Example 4, both outermost layers were made of a crystalline polypropylene resin having an MI of 1.5, and the core layer was made of EMMA with an MI of 4.5 used in Example 2. An attempt was made by the same method as in Example 1. The melting points of the crystalline polypropylene resins and EMMA were in accordance with the requirements of the present invention. Also, the overall thickness of the multilayer unstretched raw material,
The thickness composition ratio of each layer was the same as in Example 1.

However, all of them failed to obtain a good multilayer unstretched raw material. That is, in Comparative Example 3, the texture of the co-extruded unstretched raw material was poor, and the transparency was poor. Also, in Comparative Example 4, although a minimal portion, continuous irregularities were generated along the flow direction of the resin at a few places on the surface of the unstretched multilayer coextruded from the multilayer circular die. It was unsuitable as a raw material.

Comparative Example 5 The same outermost layer was made of the same crystalline polypropylene resin as in Example 1, and the core layer was made of EVA having a melting point of 97 ° C. and an MI of 1.0. Prototype was made. However, the suitable extrusion temperature range is very narrow, and it has been difficult to obtain a good multilayer unstretched raw material.

Further, the shrink film obtained by stretching using the unstretched raw material in the same manner as in Example 1 not only produces an acetic acid odor in the fusing seal at the time of heat shrink wrapping, but also obtains it. The heat shrink package also has an acetic acid odor,
It was not suitable for food packaging.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code FI B29K 105: 02 B29L 9:00

Claims (2)

(57) [Claims] (1) Both outermost layers are made of a crystalline polypropylene resin having a melting point of 135 to 150 ° C. and a melt flow index of 1 to 7, and a core layer has a melting point of 90 to 102 ° C. and a melt flow index of both outermost layers. Copolymer extruded multilayer unstretched raw material consisting of a copolymer of ethylene and methyl methacrylate having 1/3 to 2 times the melt flow index of the crystalline polypropylene resin used. Shrink film. 2. The method according to claim 1, wherein the crystalline polypropylene resin used for both outermost layers comprises an ethylene-propylene copolymer or an ethylene-propylene-butene copolymer. 2. The polypropylene-based multilayer shrink film according to claim 1.