JP2021030501A - Stretch packaging film - Google Patents

Abstract

To provide a stretch packaging film that has good packaging machine suitability, elastic recovery property, adhesiveness, cutting-property, folding-property, etc., that hardly causes tearing during packaging, and that has a transparent property and excellent appearance.SOLUTION: There is provided a stretch packaging film with at least three layers of two surface layers and an intermediate layer, in which the two surface layers contain a polyethylene resin (A) as a main component, and the intermediate layer contains a resin composition (Z) composed of a polypropylene resin (B) having a weight average molecular weight (Mw) of 300,000 or more and an ethylene unit content of 8 mass% or more and a polypropylene resin (C) having an ethylene unit content of less than 8 mass% as a main component.SELECTED DRAWING: None

Description

The present invention relates to a stretch packaging film. More specifically, the present invention relates to a stretch wrapping film preferably used for food packaging, particularly a stretch wrapping film containing no chlorine or polyvinyl chloride plasticizer.

Conventionally, polyvinyl chloride-based films have been mainly used as films for stretch packaging for so-called pre-packaging, which are used by placing fruits and vegetables, meat, fish and shellfish, prepared foods, etc. on a lightweight tray and overlapping them. Polyvinyl chloride-based film has good packaging efficiency, and the packaging suitability is preferable, such as a beautiful packaging finish. In addition, it has elastic recovery that returns to its original state when the film after packaging is pressed with a finger to deform it. Excellent quality advantage recognized by both sellers and consumers that the product value does not decrease, such as excellent bottom folding stability, and film peeling is unlikely to occur during transportation display. To have. However, in recent years, with respect to polyvinyl chloride films, problems such as hydrogen chloride gas generated during incineration and elution of a large amount of plasticizer have been regarded as problems.

For this reason, various materials alternative to polyvinyl chloride films have been studied, and in particular, various stretch packaging films using polyolefin resins have been proposed. Among them, ethylene-vinyl acetate copolymer is used as the main component for the front and back layers, and the intermediate layer is used for the reasons such as good surface characteristics as a stretch film, transparency, moderate heat resistance, flexibility in material design, and economy. In addition, studies on stretch packaging films having a two-kind, three-layer structure using various polypropylene-based resins as main components are being actively conducted.

For example, a stretch for food packaging having a mixed resin layer containing three specific components including a polypropylene resin as a main component, and having a storage elastic modulus and loss tangent within a specific range by dynamic viscoelasticity measurement. A film has been proposed (Patent Document 1).

Further, in the mixed resin layer containing two kinds of polypropylene-based resins as main components, the crystallization curve shape, the crystallization peak temperature, the amount of heat of crystallization, etc. in the differential scanning calorimeter are defined for each polypropylene-based resin, and two kinds are defined. A film for stretch wrapping in which the relationship between polypropylene-based resins is defined is proposed (Patent Documents 2 and 3).

Japanese Unexamined Patent Publication No. 9-165491 Japanese Unexamined Patent Publication No. 2011-68844 Japanese Unexamined Patent Publication No. 2012-162074

The two-kind, three-layer stretch wrapping film that uses various polypropylene-based resins as the main component in the intermediate layer as described above has suitability for various wrapping machines (cutting property, packaging wrinkles, bottom folding stability, tearing property), etc. Is a good film, but even if two types of polypropylene-based resins having a crystallization curve shape, a crystallization peak temperature, a crystallization calorific value, etc. specified in Patent Documents 2 and 3 are selected, in some cases, Problems such as coloring of the formed film and tearing of the film during film packaging were sometimes observed.

In view of the above-mentioned problems of the prior art, the present invention is a film for stretch packaging having a two-kind, three-layer structure in which a polypropylene-based resin is used as a main component in the intermediate layer, and is suitable for a packaging machine (cutability, packaging wrinkles, etc.). It is an object of the present invention to provide a stretch packaging film having a transparent and excellent appearance, which has good bottom folding stability, tearability, etc., and is less likely to be torn during packaging.

As a result of diligent studies, the present inventors have found that the combination of the weight average molecular weight and the ethylene unit content of the polypropylene resin used for the intermediate layer is important for improving defects such as film coloring and film tearing during packaging. We investigated that it was a factor and completed the present invention.

That is, the gist of the present invention is as follows.

[1] A stretch wrapping film having at least three layers of both surface layers and an intermediate layer. Both surface layers contain a polyethylene resin (A) as a main component, and the intermediate layer has a weight average molecular weight (Mw). ) Is 300,000 or more and the ethylene unit content is 8% by mass or more, and the polypropylene resin (B) has an ethylene unit content of less than 8% by mass (C). A film for stretch packaging containing Z) as a main component.

[2] The stretch according to [1], wherein the molecular weight distribution (Mw / Mn) calculated from the weight average molecular weight (Mw) and the number average molecular weight (Mn) of the polypropylene-based resin (B) is 4.5 or more. Packaging film.

[3] The stretch packaging film according to [1] or [2], wherein the polypropylene-based resin (C) has a weight average molecular weight (Mw) of 300,000 or more.

[4] The mass ratio of the resin composition (Z) when the total mass of the polypropylene-based resin (B) and the polypropylene-based resin (C) is 100% by mass is (B) / (C) =. The stretch packaging film according to any one of [1] to [3], which is 15 to 60% by mass / 85 to 40% by mass.

[5] When the intermediate layer further contains the following resin (D) and the total of the resin composition (Z) and the resin (D) is 100% by mass, the content of the resin (D) is 1. The stretch packaging film according to any one of [1] to [4], which is ~ 40% by mass.
Resin (D): At least one resin selected from petroleum resin, terpene resin, kumaron-inden resin, rosin-based resin, or hydrogenated derivatives thereof.

[6] When the intermediate layer further contains the following resin (E) and the total of the resin composition (Z) and the resin (E) is 100% by mass, the content of the resin (E) is 1 to 1. The stretch packaging film according to any one of [1] to [5], which is 40% by mass.
Resin (E): At least one resin selected from a copolymer of a vinyl aromatic compound and a conjugated diene, or a hydrogenated derivative thereof.

The stretch wrapping film of the present invention is a stretch wrapping film having good wrapping machine suitability, elastic recovery, adhesiveness, cutability, foldability, etc., which is less likely to tear during packaging, and has a transparent and excellent appearance. is there. The stretch wrapping film of the present invention has good suitability for various wrapping machines (cutting property, wrapping wrinkles, bottom folding stability, tearing property) when an automatic wrapping machine is used, and can be used for packaging such as food. It can be suitably used as a non-chlorine-based stretch wrapping film having a transparent and excellent appearance when it is displayed and displayed neatly.

Hereinafter, the stretch packaging film of the present invention as an example of the embodiment of the present invention will be described. However, the scope of the present invention is not limited to the embodiments described below.

In the present specification, the "main component" refers to a component that occupies the largest mass ratio in the constituent composition, and the ratio is preferably 50% by mass or more, more preferably 55% by mass or more, and 60% by mass. % Or more is more preferable. Further, when "X to Y" (X and Y are arbitrary numbers) is described, it means "X or more and Y or less", and "preferably larger than X" and "preferably smaller than Y". It includes the meaning of.

Further, the "monomer component" refers to a repeating unit derived from a monomer which is a raw material for producing a resin and incorporated into the resin, and is also referred to as a "structural unit" or a "unit".
The "ethylene unit" refers to a repeating unit derived from ethylene, which is a raw material for producing a resin, and incorporated into the resin. The same applies to the "vinyl acetate unit" and the "propylene unit".

The stretch packaging film of the present invention (hereinafter, also referred to as "film of the present invention") has at least three layers of both surface layers and an intermediate layer, that is, at least three of one surface layer / intermediate layer / the other surface layer. A laminated film having layers, both surface layers containing a polyethylene resin (A) as a main component, and the intermediate layer having a weight average molecular weight (Mw) of 300,000 or more and an ethylene unit content of 8. It is characterized by containing, as a main component, a resin composition (Z) of a polypropylene-based resin (B) having a mass% or more and a polypropylene-based resin (C) having an ethylene unit content of less than 8% by mass.
In the present invention, the "intermediate layer" means any one of the layers sandwiched between the two surface layers. Therefore, when the film of the present invention has three layers, it is as described above, but when the film has four or more layers, the "intermediate layer" means the M layer described later. Further, when there are five or more layers, two or more "intermediate layers" may be provided.

In the present invention, the "stretch packaging film" has a meaning that broadly includes a packaging film having extensibility and self-adhesiveness. Typically, a packaging film for pre-packaging in which fruits and vegetables, meat, fish and shellfish, prepared foods, etc. are placed on a lightweight tray and overlapped, a packaging film that overlaps to fix the cargo when carrying the cargo, and the like can be mentioned. Can be done.

In addition, "sheet" generally refers to a product that is thin by definition in JIS, and its thickness is generally small and flat for its length and width, and "film" generally refers to its length and width. A thin flat product whose thickness is extremely small and whose maximum thickness is arbitrarily limited, and which is usually supplied in the form of a roll (Japanese Industrial Standard JIS K6900). For example, in terms of thickness, in a narrow sense, a film having a thickness of 100 μm or more may be referred to as a sheet, and a film having a thickness of less than 100 μm may be referred to as a film. However, the boundary between the sheet and the film is not clear, and in the present invention, it is not necessary to distinguish between the two in terms of wording. Therefore, even if the term "film" is used, the term "sheet" is included, and even if the term "sheet" is used, the term "sheet" is used. It shall include "film".

[Both surface layers of film]
Both surface layers of the film of the present invention need to contain the polyethylene resin (A) as a main component. By containing the polyethylene resin (A) as the main component in both surface layers of the film, it is possible to improve heat sealability (bottom sealability), self-adhesiveness, moldability when inflation, etc., especially at low temperatures. ..

<Polyethylene resin (A)>
In the present invention, the polyethylene-based resin (A) is a resin containing ethylene as a main monomer component. The main monomer component refers to a monomer component that occupies 50% by mass or more and 100% by mass or less in the resin. Therefore, the polyethylene-based resin (A) may be an ethylene homopolymer, or may be a copolymer containing ethylene as a main monomer component and containing other monomer components. Examples of copolymers include ethylene / propylene copolymer, ethylene / (1-butene) copolymer, ethylene / (1-pentene) copolymer, ethylene / (1-hexene) copolymer, ethylene. -Ethylene- (α-olefin) copolymers such as (4-methyl-1-pentene) copolymer, ethylene- (1-heptene) copolymer, ethylene- (1-octene) copolymer, and ethylene -Vinyl acetate copolymer, ethylene- (meth) acrylic acid copolymer, ethylene- (meth) acrylic acid ester copolymer, ethylene-glycidyl acrylate copolymer, ethylene-vinyl alcohol copolymer, Examples thereof include ethylene / ethylene glycol copolymer, ethylene / maleic anhydride copolymer, ethylene / styrene copolymer, ethylene / diene copolymer, ethylene / cyclic olefin copolymer and the like. Here, "(meth) acrylic acid" represents one or both of "acrylic acid" and "methacrylic acid".
Further, the polyethylene-based resin (A) may be an ionomer containing metal ions in a resin containing ethylene as a main monomer component. Further, it may be a multidimensional copolymer containing two or more kinds of monomer components other than ethylene, such as an ethylene / propylene / (1-butene) copolymer.

Among these, an ethylene homopolymer, an ethylene / (α-olefin) copolymer, and an ethylene / vinyl acetate copolymer are preferable, and an ethylene / vinyl acetate copolymer is particularly preferable.

When the polyethylene-based resin (A) is an ethylene-vinyl acetate copolymer, the vinyl acetate unit content is preferably 5 to 25% by mass. When the vinyl acetate unit content is 5% by mass or more, the obtained film is soft, and not only can the flexibility and elastic recovery be maintained, but also surface adhesiveness can be imparted, which is preferable, 25% by mass. The following is preferable because the surface adhesiveness is not too strong and the unwinding property and appearance can be maintained well.

The polyethylene-based resin (A) may be linear or branched. The method for producing the polyethylene-based resin (A) is not particularly limited, and a known polymerization method using a known catalyst for olefin polymerization, for example, a multisite catalyst typified by a Ziegler-Natta type catalyst or a metallocene-based catalyst. Examples thereof include a polymerization method using a single-site catalyst typified by.

The polyethylene-based resin (A) preferably has a melting point of 70 to 130 ° C., more preferably 80 to 120 ° C. When the melting point of the polyethylene resin (A) is 70 to 130 ° C., the tensile strength and dimensional stability of the film can be improved, which is preferable.
Here, the melting point is determined by using a differential scanning calorimeter (DSC) to raise the temperature of about 10 mg of the resin from −40 ° C. to 200 ° C. at a heating rate of 10 ° C./min, hold the resin at 200 ° C. for 1 minute, and then cool the resin at a cooling rate of 10. It is the crystal melting peak temperature (Tm) (° C.) obtained from the thermogram measured when the temperature was lowered to −40 ° C. at ° C./min and the temperature was raised to 200 ° C. at a heating rate of 10 ° C./min again. Further, the melting point of the resin, which will be described later, is also a value when measured in the same manner.

The polyethylene-based resin (A) preferably has a melt flow rate (MFR) of 0.1 to 20 g / 10 minutes, more preferably 0.5 to 10 g / 10 minutes. When the MFR is 0.1 g / 10 minutes or more, the extrudability of the film can be sufficiently maintained, which is preferable. Further, when the MFR is 20 g / 10 minutes or less, the film-forming stability of the film can be maintained, and uneven thickness and variation in mechanical strength can be suppressed, which is preferable.
Here, MFR is a value measured in accordance with JIS K7210, and the measurement conditions are 190 ° C. and a 2.16 kg load.

In the present invention, the polyethylene-based resin (A) contained in both surface layers may be any one as long as it is a resin containing ethylene as a main monomer component, and may be one kind, a copolymerized monomer component, its composition, and physical characteristics. There may be two or more kinds of different things such as.

<Other resins>
Both surface layers of the film of the present invention may contain a polyethylene-based resin (A) as a main component, and may contain other resins other than the polyethylene-based resin (A).
In this case, examples of the other resin that may be contained in both surface layers include one or more of polypropylene-based resin, resin (D) described later, resin (E) described later, and the like, but polyethylene. In order to sufficiently obtain the above-mentioned effect by containing the based resin (A), when both surface layers of the film of the present invention contain a resin other than the polyethylene resin (A), the content thereof is the polyethylene resin (A). ) And other resins in a total of 100% by mass, preferably 30% by mass or less, particularly preferably 0 to 15% by mass.

[Intermediate layer of film]
The intermediate layer of the film of the present invention contains a polypropylene resin (B) having a weight average molecular weight (Mw) of 300,000 or more and an ethylene unit content of 8% by mass or more, and an ethylene unit content of less than 8% by mass. It is necessary to contain the resin composition (Z) with the polypropylene-based resin (C) as a main component. The intermediate layer preferably contains the resin composition (Z) as a main component, and further contains the resin (D) and / or the resin (E) described later.
The ethylene unit content and the propylene unit content of the polypropylene-based resin (B) and the polypropylene-based resin (C) are measured by the method described in the section of Examples described later.

<Polypropylene resin (B)>
In the present invention, the polypropylene-based resin (B) is a resin containing propylene as a main monomer component and having an ethylene unit content of 8% by mass or more. The main monomer component refers to a monomer component that accounts for 50% by mass or more and 92% by mass or less in the resin.
Therefore, the polypropylene-based resin (B) has a propylene component and an ethylene component as essential monomer components, and the propylene unit content in the polypropylene-based resin (B) is 50% by mass or more and 92% by mass or less. The ethylene unit content is 8% by mass or more and 50% by mass or less.

The ethylene unit content of the polypropylene-based resin (B) is 8% by mass or more, preferably 10% by mass or more, and more preferably 15% by mass or more. When the ethylene unit content of the polypropylene resin (B) is 8% by mass or more, the flexibility of the film is improved, elastic recovery such as wrinkle suppression when packaged is imparted, and adhesion such as bottom folding stability is imparted. Gender can be imparted. On the other hand, from the viewpoint of maintaining the rigidity of the film, the ethylene unit content is preferably 50% by mass or less, preferably 40% by mass or less, and particularly preferably 30% by mass or less.

The polypropylene-based resin (B) may have a monomer component other than propylene and ethylene as long as it satisfies the above-mentioned propylene unit content and ethylene unit content. Examples of monomer components other than propylene and ethylene include α-olefins such as 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, 1-heptene and 1-octene, and vinyl acetate, ( Examples thereof include (meth) acrylic acid, (meth) acrylic acid ester, glycidyl (meth) acrylate, vinyl alcohol, ethylene glycol, maleic anhydride, styrene, diene, and cyclic olefin. Further, it may be a multiple copolymer containing two or more kinds of the above-mentioned monomer components. When the polypropylene-based resin (B) has a monomer component other than propylene and ethylene, 1-butene is preferable as the monomer component. Further, two or more of these resins may be used in combination.

The propylene unit content of the polypropylene-based resin (B) is particularly preferably 60 to 90% by mass from the viewpoint of maintaining the rigidity of the film.

The polypropylene-based resin (B) may be a block copolymer, a random copolymer, or a graft copolymer as long as it satisfies the above-mentioned propylene unit content and ethylene unit content. It may be coalesced.

It is most important in the present invention that the polypropylene-based resin (B) has a weight average molecular weight (Mw) of 300,000 or more. When the weight average molecular weight (Mw) of the polypropylene-based resin (B) is 300,000 or more, it is possible to suppress coloring of the film so that it becomes yellowish. In addition, it is possible to suppress a problem that the film is torn during packaging with a packaging machine. Furthermore, it is possible to impart appropriate slipperiness to the film.
On the other hand, when the weight average molecular weight (Mw) of the polypropylene-based resin (B) is less than 300,000, it is easily colored and the film has poor slipperiness, so that it is not preferable as a stretch packaging film for which suitability for a packaging machine is required. In particular, when the weight average molecular weight (Mw) of the polypropylene-based resin (B) is less than 300,000, it has been confirmed that the film tends to tear easily when packaging with a packaging machine.

That is, according to the study by the present inventors, in the polypropylene-based resin (B), the crystallization peak temperature (Tc), the crystal melting peak temperature (Tm), and the crystal melting peak temperature (Tm) calculated from the values measured by the differential scanning calorimeter (DSC), and , Even if the thermal properties (numerical values) such as the amount of heat of crystallization (ΔHc) and the amount of heat of crystal fusion (ΔHm) calculated from these peak areas are about the same, the weight average molecular weight (Mw) of the polypropylene-based resin (B) ) Is 300,000 or more, good packaging machine suitability is shown, and when the weight average molecular weight (Mw) is less than 300,000, it is clear that defects such as film tearing are observed, and the polypropylene resin Regarding (B), it is an important index to improve the suitability of the packaging machine by setting the weight average molecular weight (Mw) in a specific range rather than the thermal properties such as the crystallization curve shape, the crystallization peak temperature, and the calorific value of crystallization. It was confirmed that.
When two or more polypropylene-based resins (B) having an ethylene unit content of 8% by mass or more are used in combination, even if the weight average molecular weight (Mw) of one of the resins is less than 300,000, they are used in combination. When the weight average molecular weight (Mw) of the polypropylene-based resin (B) of the entire resin is 300,000 or more, it corresponds to the polypropylene-based resin (B) in the present invention. The same applies to the molecular weight distribution (Mw / Mn) described later.

The weight average molecular weight (Mw) of the polypropylene-based resin (B) may be 300,000 or more, but is preferably 330,000 or more, more preferably 360,000 or more, from the viewpoint of improving the suitability of the packaging machine. is there. The upper limit of the weight average molecular weight (Mw) of the polypropylene-based resin (B) is not particularly limited, but is preferably 1.5 million or less, particularly 1,000,000 or less, from the viewpoint of extrusion moldability.

Further, the molecular weight distribution (Mw / Mn) calculated from the weight average molecular weight (Mw) and the number average molecular weight (Mn) of the polypropylene-based resin (B) is preferably 4.5 or more. When the molecular weight distribution (Mw / Mn) is 4.5 or more, the molding processability of the film is improved, tearing of the film can be suppressed, and appropriate slipperiness can be imparted to the film, which is preferable. The molecular weight distribution (Mw / Mn) of the polypropylene-based resin (B) is more preferably 5.0 or more, further preferably 5.5 or more. On the other hand, when this molecular weight distribution (Mw / Mn) is too large, a component having an extremely low molecular weight or a component having an extremely high molecular weight may be included. Therefore, the molecular weight distribution (Mw / Mn) may be 20 or less, particularly 15 or less, because there is a concern that a component having an extremely low molecular weight bleeds and a component having an extremely high molecular weight becomes an unmelted product at the time of molding. preferable.

The weight average molecular weight (Mw) and number average molecular weight (Mn) of the polypropylene-based resin (B) and the polypropylene-based resin (C) described later are polystyrene-equivalent values obtained by gel permeation chromatography (GPC), and are detailed. Is a value measured by the method described in the section of Examples described later.

The polypropylene-based resin (B) is observed in the DSC measurement described in the section of Examples described later when the temperature is kept at 200 ° C. for 1 minute and then lowered to −40 ° C. at a cooling rate of 10 ° C./min. The crystallization peak temperature (Tc) is preferably 70 ° C. or higher.
When the crystallization peak temperature (Tc) of the polypropylene-based resin (B) is 70 ° C. or higher, the compatibility with the polypropylene-based resin (C) described later is enhanced, and whitening is unlikely to occur during stretch packaging, which is preferable. The crystallization peak temperature (Tc) of the polypropylene-based resin (B) is more preferably 80 ° C. or higher. The upper limit of the crystallization peak temperature (Tc) of the polypropylene-based resin (B) is not particularly limited, but is preferably 150 ° C. or lower.

The polypropylene-based resin (B) is crystallized from the crystallization peak area observed when the temperature is lowered to −40 ° C. at a cooling rate of 10 ° C./min after holding at 200 ° C. for 1 minute in DSC measurement. The calorific value (ΔHc) is preferably 35 J / g or less.
When the calorific value (ΔHc) of the polypropylene resin (B) is 35 J / g or less, it is preferable because it is easy to secure the flexibility of the film and the suitability for the packaging machine. The calorific value (ΔHc) of the polypropylene resin (B) is more preferably 30 J / g or less, and further preferably 25 J / g or less. The lower limit of the calorific value (ΔHc) for crystallization of the polypropylene resin (B) is not particularly limited, but is preferably 1 J / g or more from the viewpoint of preventing blocking.

As described above, the melting point of the polypropylene-based resin (B) is 200 ° C. by raising the temperature of about 10 mg of the resin from −40 ° C. to 200 ° C. at a heating rate of 10 ° C./min using a differential scanning calorimeter (DSC). Obtained from the thermogram measured when the temperature was lowered to -40 ° C at a cooling rate of 10 ° C./min and then raised to 200 ° C. (re-heated) at a heating rate of 10 ° C./min again. The crystal melting peak temperature (Tm) (° C.).
The crystal melting peak temperature (Tm) of the polypropylene-based resin (B) is preferably 110 ° C. or higher, more preferably 120 ° C. or higher. When the crystal melting peak temperature (Tm) of the polypropylene-based resin (B) is 110 ° C. or higher, it is preferable because it can impart appropriate rigidity and heat resistance to the film. The upper limit of the crystal melting peak temperature (Tm) of the polypropylene-based resin (B) is not particularly limited, but is preferably 160 ° C. or lower.

Further, the polypropylene-based resin (B) has a crystal melting heat amount (ΔHm) of 35 J / g or less calculated from the crystal melting peak area observed when the temperature is re-heated as described above in the DSC measurement. Is preferable.
When the heat of crystal melting (ΔHm) of the polypropylene-based resin (B) is 35 J / g or less, it is preferable because it is easy to secure the flexibility of the film and the suitability for the packaging machine. The heat of crystal fusion (ΔHm) of the polypropylene-based resin (B) is more preferably 30 J / g or less, and further preferably 25 J / g or less. The lower limit of the amount of heat of crystal fusion (ΔHm) of the polypropylene resin (B) is not particularly limited, but is preferably 1 J / g or more from the viewpoint of preventing blocking.

The method for producing the polypropylene-based resin (B) is not particularly limited, and a known polymerization method using a known catalyst for olefin polymerization, for example, a multisite catalyst typified by a Ziegler-Natta type catalyst or a metallocene catalyst. Examples thereof include a slurry polymerization method, a solution polymerization method, a massive polymerization method, a vapor phase polymerization method, and a reactor type polymerization method using a single-site catalyst typified by.

In the present invention, only one type of polypropylene-based resin (B) may be used, or two or more types having different ethylene unit contents, monomer component compositions, physical properties, etc. may be used. When two or more kinds of polypropylene-based resins (B) are used, the total is the mass of the polypropylene-based resin (B), and the mass ratio in the resin composition (Z) described later is calculated.

<Polypropylene resin (C)>
In the present invention, the polypropylene-based resin (C) is a resin containing propylene as a main monomer component and having an ethylene unit content of less than 8% by mass. The main monomer component refers to a monomer component that occupies 50% by mass or more and 100% by mass or less in the resin.
Therefore, the polypropylene-based resin (C) may be a propylene homopolymer or a copolymer containing propylene as a main monomer component and containing other monomers. However, when an ethylene component is contained, it is important that the ethylene unit content is less than 8% by mass.

The ethylene unit content of the polypropylene-based resin (C) is less than 8% by mass, preferably 6% by mass or less, and more preferably 4% by mass or less. When the ethylene unit content of the polypropylene resin (C) is less than 8% by mass, it is possible to impart appropriate rigidity and heat resistance to the film, and the film can be cut and folded into a tray when packaged. Can be improved.

The polypropylene-based resin (C) may have a monomer component other than propylene and ethylene as long as it satisfies the above-mentioned propylene unit content and ethylene unit content. Examples of monomer components other than propylene and ethylene include α-olefins such as 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, 1-heptene and 1-octene, and vinyl acetate, ( Examples thereof include (meth) acrylic acid, (meth) acrylic acid ester, glycidyl (meth) acrylate, vinyl alcohol, ethylene glycol, maleic anhydride, styrene, diene, and cyclic olefin. Further, it may be a multiple copolymer containing two or more kinds of the above-mentioned monomer components. When the polypropylene-based resin (C) has a monomer component other than propylene and ethylene, 1-butene is preferable as the monomer component.

The propylene unit content of the polypropylene-based resin (C) is preferably 92 to 100% by mass from the viewpoint of maintaining the rigidity of the film.

The polypropylene-based resin (C) may be a block copolymer, a random copolymer, or a graft copolymer as long as it satisfies the above-mentioned propylene unit content and ethylene unit content. It may be coalesced.

The polypropylene-based resin (C) preferably has a weight average molecular weight (Mw) of 300,000 or more from the viewpoint of suppressing film tearing during packaging and imparting appropriate slipperiness. From such a viewpoint, the weight average molecular weight (Mw) of the polypropylene-based resin (C) is more preferably 330,000 or more, and particularly preferably 360,000 or more. The upper limit of the weight average molecular weight (Mw) of the polypropylene-based resin (C) is not particularly limited, but is preferably 1.5 million or less, particularly 1,000,000 or less, from the viewpoint of extrusion moldability.

Further, the molecular weight distribution (Mw / Mn) calculated from the weight average molecular weight (Mw) and the number average molecular weight (Mn) of the polypropylene-based resin (C) is preferably 4.5 or more. When the molecular weight distribution (Mw / Mn) is 4.5 or more, the molding processability of the film is improved, tearing of the film can be suppressed, and appropriate slipperiness can be imparted to the film, which is preferable. The molecular weight distribution (Mw / Mn) of the polypropylene-based resin (C) is more preferably 5.0 or more, further preferably 5.5 or more. On the other hand, when this molecular weight distribution (Mw / Mn) is too large, a component having an extremely low molecular weight or a component having an extremely high molecular weight may be included. Therefore, the molecular weight distribution (Mw / Mn) is 20 or less, particularly 15 or less, because there is a concern that a component having an extremely low molecular weight may bleed or a component having an extremely high molecular weight may become an unmelted product at the time of molding. Is preferable.

The polypropylene-based resin (C) is observed in the DSC measurement described in the section of Examples described later when the temperature is kept at 200 ° C. for 1 minute and then lowered to −40 ° C. at a cooling rate of 10 ° C./min. The crystallization peak temperature (Tc) is preferably 70 ° C. or higher.
When the crystallization peak temperature (Tc) of the polypropylene-based resin (C) is 70 ° C. or higher, the compatibility with the polypropylene-based resin (B) described above is enhanced, and whitening is unlikely to occur during stretch packaging, which is preferable. The crystallization peak temperature (Tc) of the polypropylene-based resin (C) is more preferably 80 ° C. or higher, and even more preferably 90 ° C. or higher. The upper limit of the crystallization peak temperature (Tc) of the polypropylene-based resin (C) is not particularly limited, but is preferably 150 ° C. or lower.

The polypropylene-based resin (C) has crystallization calculated from the crystallization peak area observed when the temperature is lowered to −40 ° C. at a cooling rate of 10 ° C./min after holding at 200 ° C. for 1 minute in DSC measurement. The calorific value (ΔHc) is preferably 35 J / g or more. When the calorific value (ΔHc) of the polypropylene resin (C) is 35 J / g or more, it is possible to impart appropriate rigidity and heat resistance to the film, and the film can be cut when packaged and folded into a tray. It is preferable because the property can be improved. The calorific value (ΔHc) of the polypropylene resin (C) for crystallization is more preferably 40 J / g or more, and further preferably 45 J / g or more. The upper limit of the amount of heat of crystallization (ΔHc) of the polypropylene-based resin (C) is not particularly limited, but is preferably 100 J / g or less from the viewpoint of suppressing film whitening during stretching.

As described above, the melting point of the polypropylene-based resin (C) is 200 ° C. by raising the temperature of about 10 mg of the resin from −40 ° C. to 200 ° C. at a heating rate of 10 ° C./min using a differential scanning calorimeter (DSC). Obtained from the thermogram measured when the temperature was lowered to -40 ° C at a cooling rate of 10 ° C./min and then raised to 200 ° C. (re-heated) at a heating rate of 10 ° C./min again. The crystal melting peak temperature (Tm) (° C.).
The crystal melting peak temperature (Tm) of the polypropylene-based resin (C) is preferably 110 ° C. or higher, more preferably 120 ° C. or higher. When the crystal melting peak temperature (Tm) of the polypropylene-based resin (C) is 110 ° C. or higher, it is preferable because it can impart appropriate rigidity and heat resistance to the film. The upper limit of the crystal melting peak temperature (Tm) of the polypropylene-based resin (C) is not particularly limited, but is preferably 160 ° C. or lower.

Further, the polypropylene-based resin (C) has a crystal melting heat amount (ΔHm) of 35 J / g or more calculated from the crystal melting peak area observed when the temperature is re-heated as described above in the DSC measurement. Is preferable.
When the heat of crystal melting (ΔHm) of the polypropylene resin (C) is 35 J / g or more, it is possible to impart appropriate rigidity and heat resistance to the film, and the film can be cut when packaged and folded into a tray. It is preferable because the property can be improved. The heat of crystal fusion (ΔHm) of the polypropylene-based resin (C) is more preferably 40 J / g or more, and further preferably 45 J / g or more. The upper limit of the amount of heat of crystal melting (ΔHm) of the polypropylene-based resin (C) is not particularly limited, but is preferably 100 J / g or less from the viewpoint of suppressing film whitening during stretching.

The method for producing the polypropylene-based resin (C) is not particularly limited, and is a known polymerization method using a known catalyst for olefin polymerization, for example, a multisite catalyst represented by a Ziegler-Natta type catalyst or a metallocene catalyst. Examples thereof include a slurry polymerization method, a solution polymerization method, a massive polymerization method, and a gas phase polymerization method using a single-site catalyst typified by.

In the present invention, only one type of polypropylene-based resin (C) may be used, or two or more types having different monomer component compositions, physical properties, etc. may be used. When two or more kinds of polypropylene-based resins (C) are used, the total is the mass of the polypropylene-based resin (C), and the mass ratio in the resin composition (Z) described later is calculated.

<Resin composition (Z)>
The intermediate layer of the film of the present invention is composed mainly of a resin composition (Z) of a polypropylene-based resin (B) and a polypropylene-based resin (C).

The resin composition (Z) is a mixture of a polypropylene resin (B) and a polypropylene resin (C), and the total mass of the polypropylene resin (B) and the polypropylene resin (C) is 100% by mass. The mass ratio is polypropylene resin (B) / polypropylene resin (C) (in the present invention, this ratio is referred to as “(B) / (C)”) = 15 to 60% by mass. It is preferably / 85-40% by mass.
Since the proportion of the polypropylene-based resin (B) in 100% by mass of the mixed polypropylene-based resin of the resin composition (Z) is 15 to 60% by mass, the flexibility of the film required for the stretch packaging film and the packaging In addition to imparting elastic recovery such as suppressing wrinkles when the film is used, it is possible to impart adhesiveness such as bottom folding stability. Further, since the proportion of the polypropylene-based resin (C) in 100% by mass of the mixed polypropylene-based resin of the resin composition (Z) is 85 to 40% by mass, the appropriate rigidity of the film required for the stretch packaging film is appropriate. It is possible to impart heat resistance, and it is possible to improve the cuttability of the film when it is packaged and the foldability into the tray.
The mass ratio of the polypropylene-based resin (B) to the polypropylene-based resin (C) is preferably (B) / (C) = 18 to 57% by mass / 82 to 43% by mass, and (B) / (C). = 21 to 54% by mass / 79 to 46% by mass is more preferable.

<Resin (D) / Resin (E)>
(Resin (D))
The intermediate layer of the film of the present invention preferably contains the following resin (D) in addition to the resin composition (Z).
Resin (D): At least one resin selected from petroleum resin, terpene resin, kumaron-inden resin, rosin-based resin, or hydrogenated derivatives thereof.

Here, the petroleum resin includes an alicyclic petroleum resin obtained from cyclopentadiene or a dimer thereof, an aromatic petroleum resin obtained from a C9 component, and a copolymerized petroleum resin of an alicyclic and aromatic petroleum resin. , Or their hydrogenated derivatives and the like.
Examples of the terpene resin include terpene resins obtained from β-pinene, terpene-phenol resins, and hydrogenated derivatives thereof.
As the kumaron-indene resin, a thermoplastic synthetic resin obtained by purifying a 160-180 ° C. fraction of tar and polymerizing kumaron having 8 carbon atoms and indene having 9 carbon atoms as main monomers, or hydrogenated derivatives thereof, etc. Can be mentioned.
Examples of the rosin-based resin include rosin resins such as gum rosin and wood rosin, esterified rosin resins modified with glycerin and pentaerythritol, and hydrogenated derivatives thereof.

As the resin (D) as described above, it is preferable to use a hydrogenated derivative from the viewpoints of color tone, thermal stability, and compatibility. The resin (D) has various softening temperatures mainly depending on the molecular weight, but the softening temperature is preferably 100 to 150 ° C, more preferably 110 to 140 ° C.

Specifically, as the resin (D), the product names of Mitsui Chemicals Co., Ltd. "Hilets" and "Petrodin", the product name of Arakawa Chemical Co., Ltd. "Arcon", and the product name of Yasuhara Chemical Co., Ltd. "Clearlon" , Idemitsu Petrochemical Co., Ltd.'s trade name "Imarb", Eastman Chemical Co., Ltd.'s trade name "Escolets", and other commercially available products can be used.

Only one kind of these resins (D) may be used, or two or more kinds thereof may be mixed and used.

When the intermediate layer contains the resin (D), the content of the resin (D) is such that the content of the resin (D) is 100% by mass when the total of the resin composition (Z) and the resin (D) is 100% by mass. It is preferably 1 to 40% by mass.
When the content of the resin (D) is 1% by mass or more, the waist, cutability, and bottom folding stability of the film can be imparted. The content of the resin (D) is preferably 5% by mass or more, more preferably 10% by mass or more. Further, if the content of the resin (D) is 40% by mass or less, the low temperature suitability required for the stretch film is not easily impaired as the glass transition temperature rises, and the film is blocked by bleeding of low molecular weight substances. Hateful. The content of the resin (D) is preferably 35% by mass or less, more preferably 30% by mass or less.

From the preferable content ratios of the polypropylene-based resin (B) and the polypropylene-based resin (C) described above, the total mass of the polypropylene-based resin (B), the polypropylene-based resin (C), and the resin (D) was set to 100% by mass. In the case, the mass ratio of the polypropylene-based resin (B), the polypropylene-based resin (C), and the resin (D) is (B) / (C) / (D) = 10 to 50% by mass / 75 to 30% by mass / 1. It is preferably ~ 40% by mass.

(Resin (E))
The intermediate layer of the film of the present invention preferably contains the following resin (E) in addition to the resin composition (Z).
Resin (E): At least one resin selected from a copolymer of a vinyl aromatic compound and a conjugated diene, or a hydrogenated derivative thereof.

In the copolymer of the vinyl-based aromatic compound and the conjugated diene, as the vinyl-based aromatic compound, for example, styrene homologues such as styrene and α-methylstyrene can also be used. Further, examples of the conjugated diene include 1,3-butadiene, isoprene, 1,3-pentadiene and the like, and these can be used alone or in combination of two or more. Further, as the third component, a small amount of a component other than the vinyl aromatic compound and the conjugated diene may be contained. However, when a double bond mainly composed of a vinyl bond in the conjugated diene portion remains, the thermal stability and weather resistance are extremely poor. To improve this, 80% or more, or even 95% of the double bond. It is preferable to use the one to which hydrogen is added as described above.

Further, the ratio of the vinyl aromatic compound component and the conjugated diene component contained in such a copolymer is preferably 3 to 40% by mass / 97 to 60% by mass in terms of weight ratio. Here, when the vinyl-based aromatic compound component in the copolymer composition is 3% by mass or more, it is preferable that the rigidity of the film for stretch packaging is not excessively lowered, and on the other hand, it is 40% by mass or less. The effect of imparting flexibility to the film and reducing the tensile elastic modulus is sufficient without the rigidity being too high.

The melt flow rate (MFR) of the resin (E) is preferably 0.1 to 20 g / 10 minutes, more preferably 0.2 to 10 g / 10 minutes. When the MFR is 0.1 g / 10 minutes or more, the extrudability of the film can be sufficiently maintained, which is preferable. Further, when the MFR is 20 g / 10 minutes or less, the film-forming stability of the film can be maintained, and uneven thickness and variation in mechanical strength can be suppressed, which is preferable.
Here, MFR is a value measured in accordance with JIS K7210, and the measurement conditions are 190 ° C. and a 2.16 kg load.

As the resin (E), specifically, the styrene-butadiene block copolymer elastomer is manufactured by Asahi Kasei Co., Ltd. under the trade name "Tuffprene", and the hydrogenated derivative of the styrene-butadiene block copolymer is manufactured by Asahi Kasei Co., Ltd. "Tough Tech", as a hydrogenated derivative of styrene-isoprene block copolymer, trade name "Septon" manufactured by Kuraray Co., Ltd., and as a styrene-vinylisoprene block copolymer elastomer, trade name "Hybler" manufactured by Kuraray Co., Ltd. Commercially available products such as can be used.

Only one kind of these resins (E) may be used, or two or more kinds thereof may be mixed and used.

When the intermediate layer contains the resin (E), the content of the resin (E) is 1 to 40% by mass when the total of the resin composition (Z) and the resin (E) is 100% by mass. Is preferable.
When the content of the resin (E) is 1% by mass or more, the waist, cutability, and bottom folding stability of the film can be imparted. The content of the resin (E) is preferably 5% by mass or more, more preferably 10% by mass or more. Further, if the content of the resin (E) is 40% by mass or less, the low temperature suitability required for the stretch film is not easily impaired as the glass transition temperature rises, and the film is blocked by bleeding of low molecular weight substances. Hateful. The content of the resin (E) is preferably 35% by mass or less, more preferably 30% by mass or less.

From the preferable content ratios of the polypropylene-based resin (B) and the polypropylene-based resin (C) described above, the total mass of the polypropylene-based resin (B), the polypropylene-based resin (C), and the resin (E) was set to 100% by mass. In the case, the mass ratio of the polypropylene-based resin (B), the polypropylene-based resin (C), and the resin (E) is (B) / (C) / (E) = 10 to 50% by mass / 75 to 30% by mass / 1. It is preferably ~ 40% by mass.

(Resin (D) and Resin (E))
The intermediate layer of the film of the present invention may contain only one of the above resin (D) and the resin (E), or may contain both, but the film has a waist and cut property. From the viewpoint of imparting bottom folding stability, it is preferable to contain both the resin (D) and the resin (E).
In this case, assuming that the total of the resin composition (Z), the resin (D), and the resin (E) is 100% by mass, the resin composition (Z) is 55 to 98% by mass, and the resin (D) and the resin (E). ) And 2 to 45% by mass in total.
Further, when the total mass of the polypropylene-based resin (B), the polypropylene-based resin (C), the resin (D), and the resin (E) is 100% by mass, the polypropylene-based resin (B), the polypropylene-based resin (C), The mass ratio of the resin (D) and the resin (E) is (B) / (C) / (D) / (E) = 5 to 40% by mass / 65 to 25% by mass / 1 to 30% by mass / 1 to 1. It is preferably 30% by mass.

<Other resins>
In addition to the resin composition (Z), the resin (D), and the resin (E), the intermediate layer of the film of the present invention may contain a resin other than these resins, and the other resins are described above. Examples thereof include polyethylene-based resin (A).

When the intermediate layer of the film of the present invention contains the polyethylene-based resin (A), it may be the same polyethylene-based resin as the polyethylene-based resin (A) constituting both surface layers, or it may be a different polyethylene-based resin. However, it is preferably the same polyethylene-based resin. If the polyethylene-based resin (A) contained in the intermediate layer and the ethylene-based resin (A) constituting both surface layers are the same polyethylene-based resin, the adhesion between the intermediate layer and both surface layers can be improved. In addition to being able to enhance the mechanical properties of the entire film, for example, trimming loss that occurs when both ends of a film-formed film are cut and trimmed can be prepared by adding it as a constituent raw material for the intermediate layer. It is possible to eliminate waste and reduce material costs.

When the polyethylene-based resin (A) is contained in the intermediate layer of the film of the present invention, the content of the polyethylene-based resin (A) is usually 1 to 40% by mass. When the content of the polyethylene resin (A) is 1% by mass or more, the low temperature suitability required for the stretch film is sufficient. The content of the polyethylene-based resin (A) is preferably 5% by mass or more, more preferably 10% by mass or more. Further, when the content of the polyethylene-based resin (A) is 40% by mass or less, the bottom folding stability and heat resistance are sufficient. The content of the polyethylene-based resin (A) is preferably 35% by mass or less, more preferably 30% by mass or less.

[Additive]
The following additives may be appropriately added to both surface layers and / or intermediate layers of the film of the present invention in order to impart performance such as antifogging property, antistatic property, slipperiness, and adhesiveness. it can.

Here, as various additives, for example, a fat which is a compound of a fatty acid alcohol having a carbon number of usually 1 to 12, preferably 1 to 6, and a fatty acid having a carbon number of usually 0 to 22, preferably 12 to 18. Examples include group alcohol fatty acid esters. Specifically, monoglycerin oleate, polyglycerin oleate, glycerin trilysinolate, glycerin acetyl lysinolate, polyglycerin stearate, glycerin laurate, polyglycerin laurate, methylacetyl lysinolate, ethyl acetyl lysinolate, butyl acetyl lysine. Nolate, propylene glycol oleate, propylene glycol laurate, pentaerythritol oleate, polyethylene glycol oleate, polypropylene glycol oleate, sorbitan oleate, sorbitan laurate, polyethylene glycol sorbitan oleate, polyethylene glycol sorbitan laurate, etc .; , Polyethylene glycol, polypropylene glycol, etc.); Paraffin-based oil, etc. can be mentioned. These can be used in combination of two or more.

The blending amount of these additives is usually 0.1 to 12 parts by mass, preferably 1 to 8 parts by mass with respect to 100 parts by mass of the resin composition constituting both the surface layer and the intermediate layer.

[Film stacking structure]
The film of the present invention mainly contains a resin composition (Z) containing a polyethylene-based resin (A) as a main component in both surface layers and a polypropylene-based resin (B) and a polypropylene-based resin (C) in an intermediate layer. A laminated film composed of at least three layers contained as a component may be used, and an adhesive layer may be used as necessary for the purpose of improving mechanical properties and interlayer adhesiveness without exceeding the gist of the present invention. , Other layers such as a gas barrier layer (hereinafter, may be abbreviated as P layer) may be introduced as appropriate. Here, the surface layer (hereinafter, may be abbreviated as S layer) is a layer forming both surfaces, but further has a layer having the same composition as the layer forming the surface layer inside. It doesn't matter. Further, the intermediate layer (hereinafter, may be abbreviated as M layer) may have at least one layer between the two surface layers, and may have two or more layers.

The laminated structure of the film of the present invention includes, for example, a three-layer structure composed of (S layer) / (M layer) / (S layer), (S layer) / (P layer) / (M layer) / (S layer). 4-layer structure consisting of (S layer) / (P layer) / (M layer) / (P layer) / (S layer), (S layer) / (M layer) / (P layer) / (M layer) A five-layer structure consisting of / (S layer) and the like can be typically mentioned. In this case, the resin composition and thickness ratio of each layer may be the same or different.

In the film of the present invention, the ratio of the thickness of the surface layer is preferably 10 to 65% when the total thickness of the film is 100%. When the thickness ratio of the surface layer is within the above range, stable film formation stability can be obtained, and suitable surface adhesiveness can be imparted to the stretch film. Here, the thickness ratio of the surface layer refers to the total thickness ratio of the surface layers on both surfaces of the laminated film.

Further, when considering stable film forming processability, surface adhesiveness, various physical properties as a film for stretch packaging, and economy, the thickness ratio of the surface layer is 20 when the total thickness of the film is 100%. More preferably, it is ~ 60%.

The thickness (total thickness) of the film of the present invention is in the range used as a normal stretch packaging film, that is, usually about 8 to 30 μm, preferably about 9 to 20 μm.

[Film manufacturing method]
The film of the present invention may be a non-stretched film or a stretched film. As will be described later, when the film of the present invention is produced by inflation molding, it is usually a stretched film. The film of the present invention can be produced, for example, by melt-extruding a material from an extruder and molding it into a film by inflation molding or T-die molding.

In the case of a laminated film, it is preferable to coextrude with a multilayer die using a plurality of extruders. Practically, it is preferable to melt-extrude the material resin from the annular die for inflation molding, and the blow-up ratio (tube diameter / die diameter) at that time is usually 3 or more, preferably 4 to 7. At that time, the cooling method may be either a method of cooling from the outer surface of the tube or a method of cooling from both the outer surface and the inner surface of the tube.

Further, the obtained film is heated below the crystallization temperature of the resin and stretched 1.2 to 5 times in the vertical direction of the film by utilizing the speed difference between the nip rolls, or 1.2 in the vertical and horizontal directions of the film. It may be biaxially stretched to about 5 times. As a result, the cut property can be improved and the heat shrinkage property can be imparted.

[Physical characteristics of film]
The storage elastic modulus of the film of the present invention is 100 MPa to 1 GPa as the storage elastic modulus (E') at 20 ° C. measured at a vibration frequency of 10 Hz and a strain of 0.1% by the dynamic viscoelasticity measurement method described in JIS K7244. It is preferably in the range. When the film is used as a soft film, the value of elastic modulus near room temperature is an index. When the storage elastic modulus (E') at 20 ° C. is 100 MPa or more, problems such as poor workability due to the films sticking to each other at room temperature are unlikely to occur. On the other hand, when the storage elastic modulus (E') at 20 ° C. is 1 GPa or less, the film is not too hard and stretches moderately, which is advantageous in soft film applications.

^{The flexibility of the film of the present invention is usually 40 to 140 kgf / cm 2} as a 100% elongation tensile stress in the film width direction measured by performing a tensile test at a temperature of 20 ° C. and a tensile speed of 200 mm / min according to JIS Z1702. , Preferably 50 to 130 kgf / cm ² .

Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited by the following examples.

In the following, various measured values and evaluations of the film and its materials were carried out as follows. Here, the flow direction of the film from the extruder is referred to as a vertical direction, and the direction perpendicular to the flow direction is referred to as a horizontal direction or a width direction.

In this example, when selecting the polypropylene-based resin (B), select a commercially available polypropylene-based resin that is considered to have a different ethylene unit content and weight average molecular weight (corresponding to MFR), and perform the following measurements. The candidate resin was selected by performing.

[Measurement of physical properties of propylene resin]
(1) Weight average molecular weight, molecular weight distribution The polypropylene-based resin used in Examples and Comparative Examples was used in a high-temperature GPC system (Viscotek triple detector HT-GPC model SG system) manufactured by Malvern Instruments. The weight average molecular weight (Mw), the number average molecular weight (Mn), and the molecular weight distribution (Mw / Mn) were calculated by measuring temperature: 140 ° C., detector RI (reference flow method), and polystyrene-equivalent molecular weight. These values are summarized in Table 1.

(2) Ethylene unit content The polypropylene-based resin used in Examples and Comparative Examples was ^{subjected to 13} C-MMR spectrum measurement using a nuclear magnetic resonance apparatus, and the spectral intensity of methyl carbon derived from propylene units and the spectral intensity derived from ethylene units. The mass ratio of propylene unit and ethylene unit was calculated from the intensity ratio of the methyl carbon spectrum of. These values are summarized in Table 1.

(3) DSC measurement As the polypropylene resin used in Examples and Comparative Examples, DSC-7 manufactured by PerkinElmer Co., Ltd. was used, and about 10 mg of the resin was heated at a heating rate of 10 ° C./min according to JIS K7121 and JIS K7122. The temperature was raised from 40 ° C. to 200 ° C., held at 200 ° C. for 1 minute, then lowered to −40 ° C. at a cooling rate of 10 ° C./min, and again raised to 200 ° C. at a heating rate of 10 ° C./min.
At this time, the amount of heat of crystallization (ΔHc) was calculated from the crystallization peak temperature (Tc) obtained in the temperature lowering process and the crystallization peak area. Further, the crystal melting peak temperature (Tm) obtained in the reheating process and the amount of heat of crystal melting (ΔHm) were calculated from the crystal melting peak. These values are summarized in Table 1.

[Evaluation of film]
(4) Flexibility According to JIS Z1702, a tensile test was conducted at a temperature of 20 ° C. and a tensile speed of 200 mm / min, and 100% elongation tensile stress (kgf / cm ^{2) in the width direction of the films obtained in Examples and Comparative Examples.} ) Was measured, and the flexibility of the obtained film was evaluated according to the following criteria. Here, "⊚" or "◯" was evaluated as having good film flexibility.
⊚: Tensile stress value (kgf / cm ² ) is 50 or more and less than 80 ○: Tensile stress value (kgf / cm ² ) is 40 or more and less than 50, or 80 or more and less than 90 Δ: Tensile stress value (kgf / cm ² ) is 20 or more and less than 40, or 90 or more and less than 140 ×: Tensile stress value (kgf / cm ² ) is less than 20 or 140 or more

(5) Storage elastic modulus According to the dynamic viscoelasticity measurement method described in JIS K7244, the storage elastic modulus at 20 ° C. in the width direction of the films obtained in Examples and Comparative Examples at a vibration frequency of 10 Hz and a strain of 0.1%. (E') was measured.

(6) Packaging machine suitability Using the films (width 350 mm) obtained in Examples and Comparative Examples, a expanded polystyrene tray (length 300 mm, width 134 mm) was used by an automatic weighing and packaging machine (“WM-AI” manufactured by Ishida Co., Ltd.). , Height 22 mm) was packaged, and the cutability, packaging wrinkles, bottom folding stability, tearability, and tearing were evaluated according to the criteria shown in Table 2 below, and the suitability for the packaging machine was determined.

(7) Coloring of Film Roll The films obtained in Examples and Comparative Examples were wound around a paper tube, and a film roll was collected. At that time, the film rolls and those on which coloring was observed on the end faces of the film rolls were marked with "x" (colored), and those not particularly colored were marked with "○" (without coloring). did.

[Materials used other than propylene resin]
In the following Examples and Comparative Examples, the following materials were used as the materials used other than the propylene-based resin.

<Polyethylene resin (A)>
Polyethylene resin (A-1): Ethylene-vinyl acetate copolymer (vinyl acetate unit content: 15% by mass, melting point: 90 ° C., MFR (JIS K7210, temperature 190 ° C., load 2.16 kgf): 2.0 g / 10 minutes)

<Resin (D)>
Hydrogenated petroleum resin (D-1): ("Arcon P125" manufactured by Arakawa Chemical Industry Co., Ltd., softening temperature: 125 ° C.)

<Resin (E)>
Styrene-based elastomer (E-1): Hydrogenated derivative of block copolymer of styrene and conjugated diene (MFR (JIS K7210, temperature 190 ° C., load 2.16 kgf) 0.5 g / 10 minutes, styrene unit content 12 mass%)

<Additives>
Anti-fog agent: diglycerin oleate

[Example 1]
As a molding material for both surface layers, 97% by mass of a polyethylene resin (A-1) and 3% by mass of an antifogging agent (diglycerin oleate) were melt-kneaded.
As the resin composition (Z) of the intermediate layer, polypropylene-based resin (B-1), polypropylene-based resin (C-1), hydrogenated petroleum resin (D-1), and styrene-based elastomer (E-1) were used. B-1) 20% by mass, (C-1) 40% by mass, (D-1) 25% by mass, and (E-1) 15% by mass were melt-kneaded.
These melt-kneaded resins were coextruded and inflation-molded at an annular three-layer die at 190 ° C. and a blow-up ratio of 5.0 to obtain a three-layer film having a total thickness of 10 μm (2 μm / 6 μm / 2 μm). The obtained film was evaluated in the above-mentioned (4) to (7). The results are shown in Table 3.

[Example 2]
As shown in Table 3, the polypropylene-based resin (B-1) used in Example 1 was added to 32.5% by mass, the polypropylene-based resin (C-1) was added to 32.5% by mass, and the styrene-based elastomer (E). A three-layer film having a total thickness of 10 μm (2 μm / 6 μm / 2 μm) was obtained by the same method as in Example 1 except that -1) was changed to 10% by mass. The obtained film was evaluated in the above-mentioned (4) to (7). The results are shown in Table 3.

[Example 3]
As shown in Table 3, the polypropylene-based resin (B-1) used in Example 1 was changed to the polypropylene-based resin (B-2), and the polypropylene-based resin (B-2) was 16% by mass and the polypropylene-based resin. By the same method as in Example 1 except that (C-1) was changed to 48% by mass, the hydrogenated petroleum resin (D-1) was changed to 20% by mass, and the styrene elastomer (E-1) was changed to 16% by mass. A three-layer film having a total thickness of 10 μm (2 μm / 6 μm / 2 μm) was obtained. The obtained film was evaluated in the above-mentioned (4) to (7). The results are shown in Table 3.

[Comparative Example 1]
A 10 μm (2 μm / 6 μm / 2 μm) three-layer film was obtained by the same method as in Example 1 except that the polypropylene resin (b-1) was used instead of the polypropylene resin (B-1). The obtained film was evaluated in the above-mentioned (4) to (7). The results are shown in Table 3.

From the results in Table 3, it can be confirmed that Examples 1 to 3 can obtain good results in all of the packaging machine suitability, flexibility, and appearance (coloring of the film roll).
On the other hand, in Comparative Example 1, since the weight average molecular weight (Mw) of the polypropylene-based resin (b-1) used is out of the range specified by the present invention, the obtained film is liable to be torn during packaging. , I was not satisfied with the film for stretch packaging. In addition, the slipperiness of the film was also poor, and poor handling was felt. Furthermore, when the film was formed into a film roll, the film roll was colored yellowish as compared with other examples, and the appearance was also insufficient.

Here, when the polypropylene-based resin (B-1) used in Example 1 and the polypropylene-based resin (b-1) used in Comparative Example 1 are compared, (B-1) and (b-) are shown in Table 1. It can be seen that all of 1) have the same ethylene unit content and the same thermal properties such as crystallization peak temperature (Tc) and crystallization heat quantity (ΔHc). However, there was a large difference in the packaging machine suitability of the film obtained in Example 1 and the film obtained in Comparative Example 1.
From this result, it can be seen that the weight average molecular weight (Mw) of the polypropylene-based resin (B) defined by the present invention is an important factor for improving the suitability for the packaging machine.

As described above, the present invention has been described in relation to the most practical and preferable embodiments at present, but the present invention is not limited to the embodiments disclosed in the present specification. It can be changed as appropriate within the scope of the claims and the gist of the invention that can be read from the entire specification, or within the range not contrary to the idea, and it is understood that the film accompanied by such a change is also included in the technical scope of the present invention. It must be.

The film of the present invention has good suitability for various wrapping machines (cutting property, wrapping wrinkles, bottom folding stability, tearing property) when an automatic wrapping machine is used, and displays and displays foods and other objects to be packaged neatly. Therefore, it can be suitably used as a non-chlorine-based stretch packaging film having a transparent and excellent appearance.

Claims (6)

A stretch wrapping film having at least three layers of both surface layers and an intermediate layer. Both surface layers contain a polyethylene resin (A) as a main component, and the intermediate layer has a weight average molecular weight (Mw) of 300. A resin composition (Z) of a polypropylene-based resin (B) having an ethylene unit content of 000 or more and an ethylene unit content of 8% by mass or more and a polypropylene-based resin (C) having an ethylene unit content of less than 8% by mass. A film for stretch packaging, which is included as the main component. The stretch packaging film according to claim 1, wherein the molecular weight distribution (Mw / Mn) calculated from the weight average molecular weight (Mw) and the number average molecular weight (Mn) of the polypropylene-based resin (B) is 4.5 or more. .. The stretch packaging film according to claim 1 or 2, wherein the polypropylene-based resin (C) has a weight average molecular weight (Mw) of 300,000 or more. The mass ratio of the resin composition (Z) when the total mass of the polypropylene-based resin (B) and the polypropylene-based resin (C) is 100% by mass is (B) / (C) = 15 to 60. The stretch packaging film according to any one of claims 1 to 3, which is by mass% / 85-40% by mass. When the intermediate layer further contains the following resin (D) and the total of the resin composition (Z) and the resin (D) is 100% by mass, the content of the resin (D) is 1 to 40% by mass. The stretch packaging film according to any one of claims 1 to 4, which is%.
Resin (D): At least one resin selected from petroleum resin, terpene resin, kumaron-inden resin, rosin-based resin, or hydrogenated derivatives thereof. When the intermediate layer further contains the following resin (E) and the total of the resin composition (Z) and the resin (E) is 100% by mass, the content of the resin (E) is 1 to 40% by mass. The stretch packaging film according to any one of claims 1 to 5.
Resin (E): At least one resin selected from a copolymer of a vinyl aromatic compound and a conjugated diene, or a hydrogenated derivative thereof.