JP3529455B2 - Stretch film for food packaging - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a stretch film used for food packaging, and more particularly to a stretch film made of a chlorine-free material.

[0002]

2. Description of the Related Art Conventionally, a polyvinyl chloride type has been mainly used as a stretch film for so-called prepackaging, in which fruits and vegetables, meat, side dishes, etc. are placed on a lightweight tray and overlapped with the film. This has good packaging efficiency, good packaging finish, and other properties, as well as excellent elastic recovery to return to its original shape even if the film after pressing is pressed with a finger, and also has a good bottom sealing property. This is because it has the quality advantage recognized by both sellers and consumers that the product value does not decrease, such as the film peeling does not occur easily during transportation display.

[0003]

However, in recent years, problems such as hydrogen chloride gas generated during incineration and elution of plasticizer contained in a large amount have been regarded as problems with polyvinyl chloride films. For this reason, various materials have been studied as alternatives to polyvinyl chloride films, and in particular, various stretch films having a structure using a polyolefin resin have been proposed. For example, ethylene-vinyl acetate copolymer (EVA),
Stretch films having a structure such as EVA / polybutene-1 / EVA and EVA / linear ethylene-α-olefin copolymer / EVA have been proposed.

However, it is difficult to satisfy all of the characteristics such as wrapping workability, wrapping finish, elastic recovery, and bottom sealability. Further, a non-vinyl chloride type stretch film having EVA laminated on both surfaces of a styrene-butadiene block copolymer hydrogenated material layer has also been proposed (Japanese Patent Publication No.
Although there is an advantage that the elastic recovery property against deformation is good, it is not yet sufficient in terms of packaging workability, packaging finish, bottom sealing property, and the like.

[0005]

As a result of intensive studies, the present inventors have succeeded in obtaining a non-vinyl chloride type stretch film excellent in the above-mentioned various properties, and the gist thereof is as follows (A) and It has at least one mixed resin layer containing the component (B) as a main component, and has a frequency of 10H by dynamic viscoelasticity measurement.
z, storage elastic modulus (E ′) measured at a temperature of 20 ° C. is 5.0
A stretch film for food packaging is characterized by having a range of × 10 ^{8 to} 5.0 × 10 ⁹ dyn / cm ² and a loss tangent (tan δ) of 0.2 to 0.8.

(A) Copolymer of vinyl aromatic compound and conjugated diene or hydrogenated derivative thereof (B) Propylene polymer

The present invention will be described in detail below. The stretch film of the present invention comprises (A) a copolymer of a vinyl aromatic compound and a conjugated diene or a hydrogenated derivative thereof,
(B) At least one mixed resin layer containing two components of a propylene polymer as a main component is provided, and the film as a whole has specific viscoelastic properties.

Here, the copolymer of the vinyl aromatic compound and the conjugated diene, which is the component (A), generally has the properties of having rubber elasticity, flexibility, resistance to breakage, and good transparency. And the rigidity of vinyl aromatic compounds and the elastomer of conjugated dienes
It is suitable for achieving the object of the present invention by balancing the sex.

However, conventionally, styrene-butadiene block copolymers and styrene-isoprene block copolymers have been known as copolymers of this kind, and some of them are also used for film, but most of them are used. Is not viscoelastic for the present invention. That is, conventionally used copolymers of this type are usually much lower than 0 ° C., usually −
It has a glass transition temperature near 50 ° C., and the loss tangent (tan δ) described below is extremely small at room temperature.

Applicable to the present invention is that it can achieve the viscoelastic properties described below when it is mixed with the component (B). Generally, it is possible to bring the glass transition temperature close to the room temperature. Tan δ in FIG. Specifically, the glass transition temperature is −20 ° C. or higher, preferably 0 to
20 ° C is easy to adapt. As such a copolymer, the weight ratio of the vinyl aromatic compound and the conjugated diene is 4
A block copolymer in the range of 0/60 to 10/90 in which the glass transition temperature of the conjugated diene block is higher than usual is mentioned.

Specifically, as a conjugated diene block,
Some have a block obtained by random copolymerization of styrene or the like with a conjugated diene, or some have a tapered block obtained by copolymerizing styrene with a conjugated diene block having a certain concentration gradient. It is also effective to use isoprene block having a high 3,4-bond ratio as the conjugated diene block. Further, a copolymer of a vinyl aromatic compound and a conjugated diene may be blended with another material having a high glass transition temperature and good compatibility, for example, a hydrogenated petroleum resin. In general, isoprene is preferably used as the conjugated diene.

Further, in the present invention, a random copolymer of a vinyl aromatic compound and a conjugated diene can be used as long as the viscoelastic property is suitable. In that case, the weight ratio of the vinyl aromatic compound and the conjugated diene is generally in the range of 30/70 to 60/40. Furthermore, as these copolymers, hydrogenated conjugated dienes can be particularly preferably used. Hydrogenation suppresses a crosslinking reaction in a molding process such as melt extrusion, and when the component (B) described below is mixed, compatibility with the component (B) is improved and transparency can be maintained. From this point of view, the preferable hydrogenation rate is 50% or more, more preferably 60% or more.

Styrene is a typical vinyl aromatic compound, but o-styrene, p-styrene, α-methylstyrene and the like can also be used. In addition, examples of the conjugated diene include butadiene, isoprene, and 1,3-pentadiene.

However, when it is attempted to secure a predetermined viscoelastic property as the whole film only by the copolymer of the vinyl aromatic compound as the component (A) and the conjugated diene, if the glass transition temperature is low (-20 ° C. or less). , The storage elastic modulus (E ') at 20 ° C becomes low, and it is too soft. Conversely, when the glass transition temperature is high (around 10 ° C), the storage elastic modulus (E ') at 20 ° C falls within the range, but the loss tangent (tan δ).
Is too high, and it is difficult to obtain the physical properties of the present invention as viscoelastic properties.
In addition, sufficient strength cannot be obtained as a stretch film, which is disadvantageous in terms of cost.

Therefore, in the present invention, a propylene polymer as the component (B) is mixed with the component (A). The propylene-based polymer is selected because it adjusts the viscoelastic properties of the composition and does not significantly impair the transparency of the component (A). The propylene-based polymer is a resin containing 70 (mol)% or more of propylene, and is polypropylene (homopolymer), a copolymer of propylene and ethylene or α-olefin having 4 to 12 carbon atoms, or these. The mixture of can be illustrated.
If the propylene content is less than 70%, the crystallinity is too low to achieve the viscoelasticity adjustment and strength up of the film as a whole. In addition, it lacks stability during film formation.

Propylene-based polymers generally have high crystallinity and high strength, and have a relatively high melting point and good heat resistance among the polyolefin polymers, but require a large force during stretching due to their high crystallinity. However, they also show non-uniform elongation and these properties remain in the composition. Therefore, in the present invention, in order to obtain a film having good elongation, it is preferable to use a propylene polymer having a relatively low crystallinity as at least a part of the propylene polymer. As the copolymer in this case, a copolymer obtained by copolymerizing propylene with ethylene or α-olefin having 4 to 12 carbon atoms in an amount of about 3 to 30 mol% is preferable.

The film of the present invention has a storage elastic modulus (E) measured at a frequency of 10 Hz and a temperature of 20 ° C. by dynamic viscoelasticity measurement.
′) Is 5.0 × 10 ⁸ to 5.0 × 10 ⁹ (dyn / cm
² ) and the loss tangent (tan δ) is 0.2
It is in the range of 0.8. E'is 5.0 here
When it is less than × 10 ⁸ dyn / cm ^2, it is not suitable as a stretch film because it is soft and the stress against deformation is too small, resulting in poor workability and no film tension of the packed product. On the other hand, when E'exceeds 5.0 × 10 ⁹ dyn / cm ² , the film becomes hard and difficult to stretch, and the tray is likely to be deformed or crushed.

When tan δ is less than 0.2, the restoring behavior with respect to the elongation of the film is instantaneous, so that the film is restored in a short time before the film is folded into the bottom of the tray, and the film is well conditioned. Wrinkles tend to occur without stretching. Also in the heat-sealed state of the bottom portion, in the case of stretch packaging, it is difficult to perform sufficient fusion bonding due to heat, so that the bottom seal is likely to be peeled off gradually during transportation or display after packaging.

When tan δ exceeds 0.8, the packaging finish is good, but plastic deformation is exhibited, and the tension of the packed products against the external force is too weak, and due to stacking during transportation or display, etc. The film on the top of the tray easily sags, and the product value is likely to decrease. Further, in the case of automatic packaging, problems such as a defective check are likely to occur because it tends to stretch vertically. A particularly preferable range of tan δ is 0.30
It is 0.60.

Since the stretch film may be used at low temperature, it is desirable that the stretch film has excellent low temperature characteristics (especially elongation). For that purpose, the dynamic viscoelasticity was measured at a frequency of 10 Hz and a temperature of 0 ° C. The storage elastic modulus (E ′) of the film is preferably in the range of 1.5 × 10 ¹⁰ dyn / cm ² or less. Since many of vinyl aromatic compound / conjugated diene copolymers suitable for the present invention have a higher glass transition temperature than usual ones, care should be taken to satisfy the above-mentioned properties in order to secure flexibility such as low temperature elongation. It is preferable to adjust the ratio of the diene component of the vinyl aromatic compound / conjugated diene copolymer and the mixing ratio of the component (A) and the component (B).

In order to set E'and tan δ of the film of the present invention within the above range, it is most effective to adjust the mixing ratio of both the components (A) and (B). The copolymer of the vinyl aromatic compound as the component (A) and the conjugated diene, which has been modified appropriately for the present invention, generally has a tan δ in the range of 0.7 to 1.5. On the other hand, the propylene-based polymer as the component (B) generally has a ta of 0.01 to 0.10.
Therefore, in order to have tan δ in the range of 0.2 to 0.8 as a mixture, (A) /
The mixing ratio of (B) may be approximately 70/30 to 40/60.

According to the present invention, the stretch film comprising the mixture layer described above can be obtained, but it can be laminated with another non-vinyl chloride material layer if desired. Other resin layers include, for example, polyolefin polymers and flexible styrene-butadiene elastomers, and when laminated with these, film film stability, blocking resistance, tackiness, and slipperiness are imparted. can do.

Here, as the polyolefin polymer,
Low-density polyethylene, ultra-low-density polyethylene (copolymer of ethylene and α-olefin), ethylene-vinyl acetate copolymer (EVA), ethylene-alkyl acrylate copolymer, ethylene-alkyl methacrylate copolymer, ethylene- Acrylic acid copolymers, ethylene-methacrylic acid copolymers, ionomers such as low-density polyethylene, and propylene-based elastomer materials are suitable.

In practice, for example, EVA can be preferably used, and the EVA has a vinyl acetate content of 5 to 5.
25% by weight, preferably 10 to 20% by weight, a melt flow ratio (MFR) of 0.2 to 2 g / 10 minutes (190
(° C, 2.16 kg load) is preferable in terms of strength, flexibility, film forming workability, and the like. The thickness of the film of the present invention is generally in the range used for ordinary stretch packaging, that is, about 8 to 30 μm, typically 1
It is in the range of about 0 to 20 μm.

The film of the present invention can be obtained 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 advantageous to coextrude with a multilayer die. Practically, it is preferable to melt-extrude the material resin from the annular die and perform inflation molding. At that time, the blow-up ratio (bubble diameter / die diameter) is preferably 4 or more, and particularly preferably in the range of 5 to 7. is there.

Various additives may be added to the film of the present invention in order to impart antifogging properties, antistatic properties, slip properties and the like. For example, a surfactant such as glycerin fatty acid ester, polyglycerin fatty acid ester, sorbitan fatty acid ester, and ethylene oxide adduct can be appropriately added.

[0027]

EXAMPLES The effects of the present invention will be clarified by the following examples. The characteristics and performance of the film were measured and evaluated by the following methods. 1) E ', tanδ Viscoelasticity spectrometer VES- manufactured by Iwamoto Manufacturing Co., Ltd.
Using F3, vibration frequency 10Hz, temperature 20 ° C and 0
It was measured in the transverse direction of the film at ° C.

2) Stretch wrapping suitability Using a stretch film having a width of 350 mm, an automatic wrapping machine (ISHIDA Wmin MK- manufactured by Ishida Koki Co., Ltd.) is used.
II) Expanded polystyrene tray (200 m long)
m, width 130 mm, height 30 mm) were packaged and the items shown in Table 4 were evaluated. Further, using the same film and tray, a packaging test was carried out by a hand packaging machine (Dialapper A-105 manufactured by Mitsubishi Plastics, Inc.).

3) Stability of film formation The stability of bubbles during film formation was evaluated by an inflation film formation equipment.

⊚: extremely stable ○: Stable Δ: Somewhat unstable ×: Film formation is not possible

Example 1 Styrene 20% by weight, 3,4
A hydrogenated derivative of styrene-isoprene-styrene triblock copolymer (SIS) consisting of 80% by weight of polyisoprene with a bonding ratio of 55% (glass transition temperature -19
℃) 70% by weight, softening point 125 ℃ (glass transition temperature 8
30% by weight of hydrogenated petroleum resin (1 ° C.), and further 30% by weight with respect to 70% by weight of the composition of the hydrogenated SIS and the hydrogenated petroleum resin (hereinafter, this composition is referred to as hydrogenated SIS-based composition). Propylene-based copolymer (propylene-ethylene random copolymer, ethylene content 4 mol%, 23
0 ° C. MFR = 6.5 / 10 minutes, hereinafter abbreviated as PP), and 3 parts by weight of diglycerin monooleate as an antifogging agent were melt-kneaded and subjected to inflation molding to obtain a film having a thickness of 15 μm.

The characteristics measured with the hydrogenated SIS alone were as follows: storage modulus E'at 0 ° C. 2.0 × 10 ⁹ dyn / cm ² storage modulus E at 20 ° C. E ′ 2.1 × 10 ⁸ dyn / cm was the loss tangent tan [delta 0.44 glass transition temperature -19 ° C. at ² 20 ° C.. In addition, the characteristics measured only with the hydrogenated SIS-based composition are as follows: storage modulus E ′ at 0 ° C. 1.4 × 10 ¹⁰ dyn / cm ² storage modulus E at 20 ° C. E ′ 6.0 × 10 ⁸ dyn / cm ² Loss tangent tan δ at 20 ° C 1.20 Glass transition temperature 5 ° C.

Example 2 Hydrogenated SI used in Example 1
A film having a thickness of 15 μm was obtained in the same manner as in Example 1 except that the mixing ratio of the S composition and the propylene copolymer (PP) was changed to 60/40. (Example 3) Low crystalline propylene-ethylene-propylene copolymer elastomer as propylene-based copolymer (88 mol% propylene, 12 mol% ethylene, 230 ° C MF)
A film having a thickness of 15 μm was obtained in the same manner as in Example 2 except that R = 1.5 / 10 minutes, and hereinafter PER was used. (Example 4) The hydrogenated SIS composition used in Example 3 and the propylene copolymer (PER) were mixed at a mixing ratio of 50/50.
A film having a thickness of 15 μm was obtained in the same manner as in Example 3 except that

(Example 5) Hydrogenated SI used in Example 1
A mixture composition of S-based composition and propylene-based copolymer (PP) (weight ratio: 70/30) is used as an intermediate layer and has a thickness of 11 μm.
And EVA (vinyl acetate content 15% by weight, 190 ° C
MFR = 2.0 / 10 minutes) A composition prepared by kneading 3 parts by weight of diglycerin monooleate as an antifogging agent to 100 parts by weight of the composition was used as the front and back layers, each having a thickness of 2 μm. , Total thickness 15μm (2/11/2
μm) film was obtained.

Example 6 Hydrogenated SI used in Example 3
A total thickness of 15 μm (2/11) was obtained in the same manner as in Example 5 except that a mixed composition (weight ratio: 60/40) of the S-based composition and the propylene-based copolymer (PER) was used as the intermediate layer.
/ 2 μm) was obtained.

(Comparative Example 1) Hydrogenated SI used in Example 1
A film having a thickness of 15 μm was obtained in the same manner as in Example 1 except that the mixing ratio of the S composition and the propylene copolymer (PP) was changed to 30/70. (Comparative Example 2) The hydrogenated SIS composition used in Example 3 and the propylene copolymer (PER) were mixed at a mixing ratio of 30/70.
A film having a thickness of 15 μm was obtained in the same manner as in Example 3 except that (Comparative Example 3) The hydrogenated SIS composition used in Example 3 and the propylene copolymer (PER) were mixed at a mixing ratio of 80/20.
A film having a thickness of 15 μm was obtained in the same manner as in Example 3 except that

(Comparative Example 4) Hydrogenated derivative of styrene-butadiene-styrene triblock copolymer containing 13% by weight of styrene and 87% by weight of polybutadiene (hydrogenated SBS,
KERATON G1657 manufactured by Ciel Chemical Co., Ltd., glass transition temperature −42 ° C.) 70% by weight, 30% by weight of the same hydrogenated petroleum resin as in Example 1 and 3 parts by weight of diglycerin monooleate as an antifogging agent are melt-kneaded. And thickness 15
A film of μm was obtained. The characteristics measured with this hydrogenated SBS alone are as follows: storage modulus E'at 0 ° C. 4.0 × 10 ⁷ dyn / cm ² storage modulus E ′ at 20 ° C. 3.5 × 10 ⁷ dyn / cm ² 20 ° C. The loss tangent was tan δ 0.05.

Comparative Example 5 A film having a total thickness of 15 μm was obtained in the same manner as in Example 6 except that the thickness of each layer in Example 6 was changed to 6/3/6 (μm).

(Comparative Example 6) Linear ethylene-butene-1
Copolymer (VLDPE, butene-1 content 14% by weight,
Density 0.905) as an intermediate layer of 7 μm, Examples 5, 6
The EVA resin composition used in Step 2 was used as the front and back layers, each of 4 μm
A film having a total thickness of 15 μm (4/7/4 μm) was obtained. Comparative Example 7 A commercially available polyvinyl chloride stretch film (thickness: 15 μm) was evaluated.

Tables 1 and 2 show the results of measurement and evaluation of viscoelastic properties and film-forming stability of these films, and Table 3 shows the results of evaluation of packaging suitability. In Tables 1 and 2, the hydrogenated SIS composition is simply referred to as "SIS system".

[0041]

[Table 1]

[Table 2]

[Table 3]

[Table 4]

The film of Example 1 was prepared by mixing a hydrogenated SIS composition and a propylene polymer to obtain 20
Both the E'and tan δ characteristics at ° C were within the ranges specified in the present invention, indicating good packaging suitability. The films of Examples 2 to 6 also have excellent packaging suitability.

[0043]

EFFECTS OF THE INVENTION According to the stretch film of the present invention, when used in an automatic wrapping machine or the like, the film can be cut / conveyed and lapped without any problems, the bottom sealing property is good, and the film tension is high. A good package can be obtained, and it has a feature that has never been seen as a non-PVC-based stretch film.

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Claims (2)

(57) [Claims] 1. A storage elastic modulus (E ') measured by dynamic viscoelasticity measurement at a frequency of 10 Hz and a temperature of 20 ° C., which has at least one mixed resin layer containing the following components (A) and (B) as main components. Is 5.0 × 10 ⁸ to 5.0 × 10 ⁹ dyn /
A stretch film for food packaging having a cm ² and a loss tangent (tan δ) in the range of 0.2 to 0.8. (A) Copolymer of vinyl aromatic compound and conjugated diene or hydrogenated derivative thereof (B) Propylene polymer 2. A frequency of 10 Hz measured by dynamic viscoelasticity,
The stretch film for food packaging according to claim 1, wherein the storage elastic modulus (E ′) of the film measured at a temperature of 0 ° C. is 1.5 × 10 ¹⁰ dyn / cm ² or less.