JPH10119211A - Heat-shrinkable film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a heat-shrinkable polyoleifn-based film in which while holding the characteristics such as low fusing sealing temperature characteristics of a conventional polyethylene film and also while utilizing characteristics shch as transparency and shock strength, heat-shrinkable stress is lower and heat-shrinkable temperature range is broader. SOLUTION: In the heat-shrinkable film, melt flow ratio and molecular weight ratio respectively satisfy relation shown in the following formulas: I10 /I2 >=5.63 (wherein, I10 shows melt index per 10 minutes in 10kg load at 190 deg.C and I2 shows melt index per 10 minutes in 2.16kg load at 190 deg.C), Mw /Mn <=(I10 /I2 )-4.63 (wherein, Mw shows weight average molecular weight and Mn shows number average molecular weight). The heat-shrinkable film has a layer of polyolefin holding long-chain branching.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat-shrinkable film having a polyolefin layer having a specific property and a long-chain branched structure. More specifically, the present invention relates to a heat-shrinkable film having a polyolefin layer having a long-chain branched structure having specific melt fluidity and molecular weight distribution and having excellent heat-shrinkage stress and other suitable packaging properties.

[0002]

2. Description of the Related Art At present, there is a heat shrink wrapping technique using a heat shrinkable film as one field of a wrapping technique, which is used for wrapping products in various fields. The heat shrink wrapping here means that the entire packaged object is first covered with a heat shrinkable film,
A fusing seal is made between the opposing films to form a bag-shaped loose package, and then the film is shrunk by external heating,
It refers to a tension-packed state, and is characterized in that the packaging operation is easy, the package is suitable for batch packaging, and a package with excellent appearance is obtained.

[0003] In this heat shrink wrapping, the main condition required for the heat shrinkable film is that the film shrinks at a heating temperature lower than the melting point of the film (for example, to avoid melting and tearing of the film and overheating damage of the packaged object, etc.). Wide heat shrink temperature range (because the thermal properties of the film itself are wide, easy to adjust the heat shrink processing machine temperature, etc.), the heat shrink stress of the film is the deformation of the package Strength or less (because if the heat shrinkage stress of the film is too high, the packaged body will be deformed and the commercial value will be reduced), and the mechanical properties of the film fusing seal after heat shrinkage are high (film fusing) If the sealing strength is low, the tensile stress will concentrate after heat shrink wrapping and the bag will break easily.)
After heat shrink wrapping, do not leave incompletely shrinkable parts in the film fusing seal part (leaving the incompletely shrinkable parts will result in poor appearance of the package and reduce the commercial value). In addition to high transparency and high gloss, the film is odorless at the time of film fusing sealing (for improving commercial value, environment or food hygiene, etc.).

Conventionally, as a heat-shrinkable film, a film obtained by biaxially stretching a film mainly made of polyethylene, polypropylene, or polyvinyl chloride is most commonly used. In the case of a biaxially stretched polyvinyl chloride film, although the transparency and glossiness of the film after heat shrinkage are excellent, a normal heat fusing sealer cannot be used, and special equipment such as a high frequency fusing sealer is required. , High heat shrinkage stress, and the mechanical strength of the fusing seal does not have toughness as in the case of polyethylene and polypropylene, so it is easy to break, tear propagation phenomenon is observed, and bad smell is generated at the time of fusing sealing. In some cases, this is not a preferable packaging material. The biaxially stretched polypropylene film uses a high stereoregularity polypropylene resin, and is excellent in transparency and glossiness of the film after heat shrinkage, so that the appearance of the package is good and the product value is high. Generally, the heat shrink temperature is high, the heat shrink temperature range is narrow, so the range of packaging processing conditions is narrow, the heat fusing seal strength of the film is weak, and the heat shrink stress is high, so deformation and destruction of the packaged object also occur. It is easy to occur, heat shrinkage unevenness during heat shrinkage or heat shrinkage incomplete portion (also referred to as incomplete shrinkage portion, which is particularly likely to occur at the end and corner of the fusing seal, and is generally called dog ear because of its shape. , Called dogear.)
Disadvantages such as easy occurrence of cracks. The biaxially stretched polyethylene film has various characteristics depending on the type of polyethylene, but generally has a low fusing seal temperature, is excellent in such points as a high strength of the fusing seal portion, but has excellent blocking resistance, transparency, Impact strength, tear strength, etc. are inferior to those of polyvinyl chloride or polypropylene, and the heat shrinkage stress is relatively high, the heat shrinkage temperature width is narrow, and the melt is easily broken by heating for heat shrinkage. In addition, pulling due to uneven shrinkage, pock-like irregularities often occur,
There are also many problems, such as dog ears tending to occur frequently in the film fusing seal after heat shrink packaging.

[0005] The above problems in the case of polyethylene are largely solved mainly by the progress and improvement of the polymerization catalyst.
Moreover, diversified heat-shrinkable films to which new excellent properties have been imparted are emerging. That is, from low-density polyethylene (LDPE) by a high-pressure method, to medium- and high-pressure high-density polyethylene (HDPE) using a Ziegler catalyst or a Phillips catalyst, and linear low-density polyethylene (LLDP).
E), and furthermore, it is well known that it has been developed and diversified to linear low density polyethylene (LLDPE) using a metallocene catalyst. Since a single-site catalyst with a uniform active site is used, the molecular weight distribution of the polymer is narrow, and the ethylene and comonomer α
-Since the polymerization with the olefin is carried out uniformly, the monomer component ratio distribution between molecules is narrow,
A heat-shrinkable multilayer film obtained by co-extrusion molding with another type of resin has been proposed, in which the blocking property, low-temperature sealing property, transparency, impact strength, and tear strength of the film are greatly improved (Japanese Patent Application Laid-Open No. Hei 7-314624). 7-309962
However, the temperature range suitable for heat shrinkage in heat shrink wrapping is still narrow, and the occurrence of uneven shrinkage and dog ear during the heat shrink wrapping has not been improved and is still unsolved.

[0006]

SUMMARY OF THE INVENTION The present invention relates to a heat-shrinkable film made of a linear low-density polyethylene using a metallocene catalyst in the above-mentioned various heat-shrinkable packaging materials. While maintaining the high strength properties of the fusing seal part during heat shrinkage, and paying attention to its excellent properties in terms of blocking properties, low-temperature sealing properties, transparency, impact strength, and tear strength, while utilizing these properties, An object of the present invention is to propose a heat-shrinkable polyolefin-based film having a lower heat shrinkage stress, less occurrence of dog ears, and a wider heat shrink temperature range.

[0007]

Means for Solving the Problems The present inventors have proposed a method for reducing the heat shrinkage stress and expanding the heat shrinkage temperature range in a heat shrink wrapping using a heat shrinkable film made of linear low density polyethylene by using a metallocene catalyst. After extensive research,
The present inventors have found that a film having a polyolefin layer having a long-chain branched structure having both specific melt fluidity and molecular weight distribution is suitable, and completed the present invention.

That is, the gist of the present invention is as follows. (1) Melt flow ratio and molecular weight ratio are (1) I ₁₀ / I ₂ ≧ 5.63 (where I ₁₀ is a load index of 10 kg, temperature 190 ° C., melt index per 10 minutes, I ₂ is a load index of 2. 16k
g, temperature 190 ° C., melt index per 10 minutes. (2) Mw / Mn ≦ (I ₁₀ / I ₂ ) −4.63 (where Mw represents a weight average molecular weight and Mn represents a number average molecular weight, respectively) Heat-shrinkable film having a layer of (Second) The polyolefin having a long chain branch is ethylene-
2. The heat-shrinkable film according to the above item 1, which is a 1-octene copolymer. (3) Melt flow ratio and density are (1) 5.63 ≦ I ₁₀ / I ₂ ≦ 15 (2) 0.86 ≦ d ≦ 0.94 (where d indicates density (g / cm ³ )) A heat-shrinkable film that satisfies the following relationship and has a polyolefin layer having a long-chain branch described in the first or second aspect. (4) A heat having a three-layer structure in which one of the polyolefin layer having a long-chain branch and the propylene-based polymer layer according to any one of the first to third aspects is an intermediate layer, and the other is both outer layers. Shrinkable film.

Hereinafter, the contents of the present invention will be described in detail. The polyolefin according to the present invention is defined as I ₁₀ / I ₂ ≧ 5.63, preferably 5.63 ≦ I ₁₀ / I ₂ ≦ 15, more preferably 5.63 ≦ I ₁₀ / I ₂ ≦ 8. .5. Here, the melt flow ratio of I ₁₀ / I ₂ is a ratio of two melt flow values measured under different fluidity measurement conditions, and is a measure of the molecular weight distribution from the viewpoint of fluidity. If I ₁₀ / I ₂ is less than 5.63, the moldability is poor. Specifically, there is a problem in that melt fracture surface properties occur under high shear stress.

The polyolefin according to the present invention has Mw
/ Mn ≦ (I ₁₀ / I ₂ ) −4.63, but 1.5
≦ [Mw / Mn ≦ (I ₁₀ / I ₂ ) −4.63] ≦ 3 is more preferable. Here, the ratio of Mw / Mn is a ratio of the molecular weight based on different measuring methods, that is, the weight average molecular weight and the number average molecular weight, and serves as a standard of the molecular weight distribution. Mw / Mn
If> (I ₁₀ / I ₂ ) −4.63, the optical characteristics of the film are inferior. Specifically, there is a problem in that glossiness is low and haze is high.

Further, the density of the polyolefin according to the present invention is
It is preferably 0.86 g / cm ³ or more and 0.94 g / cm ³ or less, more preferably 0.86 g / cm ³ or more.
0.91 g / cm ³ or less, more preferably 0.86 g / cm 3
³ or more and 0.89 g / cm ³ or less. 0.86g density
If it is less than / cm ³ , it is not preferable because the film surface becomes sticky. ^{On the other hand} , if it exceeds 0.94 g / cm ³ , it is not preferable because moldability is poor.

The term "polyolefin having a long-chain branch" as defined in the present invention means that the backbone of the polymer has 1 carbon atom.
Per 000 are substituted by 0.01 to 3 long-branched chains ", but this number is preferably 0.01
, And more preferably 0.05 to 1. As the polyolefin having a long chain branch, ethylene and an α-olefin copolymer are preferable, and as the α-olefin, 1-butene, 1-hexene, 2-methyl-1
-Pentene and 1-octene are preferred, and 1-octene is particularly preferably used. By using the above-mentioned α-olefin as a comonomer and forming a long-chain branch, the values of I ₁₀ / I ₂ and Mw / Mn can be specified in desired ranges, and a wide heat shrinkage can be obtained as described later. Temperature range can be given, and the heat shrinkage stress can be reduced.

The "heat-shrinkable film using a polyolefin having a long-chain branch" defined in the present invention is obtained by producing a flat or tubular film by a known production method and processing method using the above-mentioned specific polyolefin. A method of simultaneous or sequential biaxial stretching by a tenter method, an inflation method or a tubular method is most preferable. In order to impart high heat shrinkability, the stretching speed is preferably as high as possible, the stretching temperature is preferably as low as possible, and quenching after the stretching is preferably employed. However, to adjust the degree of heat shrinkage,
After the stretching process, an annealing process may be appropriately performed.

[0014] The polyolefin resin according to the present invention includes an antiblocking agent, a lubricant, an antistatic agent, a nucleating agent, a stretching orientation agent, an antioxidant, a heat stabilizer, a light stabilizer which is generally known as a molding additive. Stabilizers, flame retardants, pigments, fillers, compatibilizers and the like can be added as appropriate.

The heat-shrinkable film according to the present invention can be used in the above-mentioned specific polyolefin resin film in accordance with demands for improvement in mechanical strength, fusing seal strength, packaging suitability, etc., as long as the heat-shrinkable property is not impaired. Although it can be used as a composite film in which a stretched or non-stretched synthetic resin layer is laminated, a polypropylene-based resin or a composite film with the film is particularly preferable in terms of heat resistance. The composite film with the polypropylene-based resin or a film thereof,
One having a three-layer structure in which the heat-shrinkable polyolefin layer according to the present invention is an intermediate layer, or a three-layer structure in which the heat-shrinkable polyolefin layer is used as both outer layers, and a polypropylene-based resin or a film thereof is arranged as the intermediate layer. Is particularly preferred in terms of heat resistance during heat shrink treatment, low heat shrinkage stress, and expansion of the heat shrink treatment temperature range.

The other synthetic resins include low density polyethylene, linear low density polyethylene, high density polyethylene, ethylene-vinyl acetate copolymer, ethylene-methyl methacrylate copolymer, Ionomers such as ethylene-methyl acrylate copolymers and metal salts of ethylene-acrylic acid copolymers,
Petroleum resins and the like can be mentioned. The other synthetic resin to be laminated may be a stretched film, or may be compounded into the polyolefin-based heat-shrinkable film according to the present invention as an unstretched film. After lamination, a stretching treatment may be performed at the same time to perform a heat shrinkability imparting treatment. Further, these films having a multilayer structure are produced by laminating a plurality of films by an ordinary method, and also by a method of melting and laminating the above various resins on the surface of the polyolefin film according to the present invention.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to embodiments. As an intermediate layer forming resin of the composite film (Example 1) 3-layer structure, I ₁₀ / I ₂ is 9.8, Mw / Mn of 2.2, a density of 0.902 g / cm ³ Ethylene-1- and octene copolymer 10 parts by weight, an ethylene content of 2.3 wt% ethylene - a resin composition comprising a propylene copolymer 90 parts by weight, as the both outer layers forming resin, I ₁₀ / I ₂ is 9 .8, M
Using an ethylene-1-octene copolymer having a w / Mn of 2.2 and a density of 0.902 g / cm ³ ,
It is supplied to three extruders connected to a circular die set at 240 ° C. for layer coextrusion molding, and the layer thickness ratio is 1 (outer layer): 2 (intermediate layer): 1 (outer layer). The resulting sheet was extruded and quenched by a water cooling method to form a raw tubular sheet for stretching. The tubular sheet is then heated to a furnace temperature of 400.
℃ 5 times in both the vertical and horizontal directions by tubular stretching while heating by passing through a tubular far-infrared furnace at
Immediately by an air cooling method, a heat-shrinkable biaxially stretched film having a thickness of 12 μm was obtained. The maximum heat shrinkage stress of the obtained film was a low value of 16 kg / cm ² . The difference between the temperatures at which the shrinkage rate shows 5% and 30%, respectively (hereinafter,
(Referred to as shrinkage temperature width) is as wide as 39 ° C., and it was found that desired shrinkage characteristics can be easily obtained. Next, a heat-shrinkable film of 20 cm (width) x 40 cm (length) x 5 cm (thick)
Was packed in a cardboard box (paper thickness 1 mm) and subjected to a heat shrink wrap test through a shrink wrapping machine set at a shrink tunnel temperature of 150 ° C. A good packaging condition was obtained, and no dog ears, which are usually generated at the end of the fusing seal portion or the fusing seal portion furthest from the packaged object, were observed. Reduce the shrink tunnel temperature by 2 more than the above temperature.
A similar shrink wrap test was performed at 0-30 ° C.
Of the film surface, there is no melting tearing and whitening phenomenon often seen in the portion not in contact with the packaged object, and no air vent hole provided in the fusing seal portion or no bag breaking phenomenon at the sealing portion. Was.

Example 2 A propylene-ethylene-butene copolymer having an MI of 8, a density of 0.85 / cm ³ and a Vicat softening point of 94 ° C. was used as a resin for forming an intermediate layer in Example 1, A heat-shrinkable biaxially stretched film having a thickness of 10 μm was obtained under the same conditions as in Example 1 except that the layer thickness ratio was changed to 35 (outer layer): 30 (intermediate layer): 35 (outer layer).
Next, various characteristics were measured under the same conditions as in Example 1. The maximum heat shrinkage stress of the biaxially stretched film is 20 kg / cm ²
It was found that the shrinkage temperature range was as wide as 44 ° C., and that desired shrinkage characteristics could be easily obtained. Further, when a heat shrinkage packaging test was performed, no deformation was observed in the cardboard box, a smooth packaging state in which the entire surface of the film was tensioned was obtained, and no dog ear was observed in the fusing seal portion. In the shrink wrapping test with the shrink tunnel temperature further increased, no melting and tearing and no whitening were observed in the non-contact portion of the film surface with the packaged object, and there was no bag breaking at the fusing seal. Did not.

Example 3 As a resin for forming an intermediate layer of a composite film having a three-layer structure, I ₁₀ / I ₂ is 7.9, Mw / Mn.
Is 2.0 and a density of 0.870 g / cm ³ is used as a resin for forming both outer layers.
920 g / cm ³ of ethylene-1-octene copolymer was used and supplied to three extruders connected to a T-die set at 230 ° C. for three-layer coextrusion molding. The thickness ratio is 1 (outer layer): 2 (middle layer): 1 (outer layer)
Then, a raw sheet for stretching was formed. Next, the sheet was stretched at a stretching temperature of 105 using a table tenter.
The film was simultaneously stretched in the machine direction and the transverse direction at ℃ to obtain a heat-shrinkable film having a thickness of 12 μm. Next, various characteristics were measured under the same conditions as in Example 1. The maximum heat shrinkage stress of the biaxially stretched film was a low value of 21 kg / cm ² , and the shrinkage temperature range was as wide as 25 ° C., indicating that desired shrinkage characteristics could be easily obtained. Further, when a heat shrinkage packaging test was performed, no deformation was observed in the cardboard box, a smooth packaging state in which the entire surface of the film was tensioned was obtained, and no dog ear was observed in the fusing seal portion. In the shrink wrapping test with the shrink tunnel temperature further increased, no melting and tearing and no whitening were observed in the non-contact portion of the film surface with the packaged object, and there was no bag breaking at the fusing seal. Did not.

(Example 4) As the resin for forming both outer layers in Example 3, I ₁₀ / I ₂ was 9.8, Mw / Mn was 2.2,
A layer thickness ratio was 1 (outer layer): 1.5 (intermediate layer): 1 (outer layer) in the same manner as in Example 3 except that an ethylene-1-octene copolymer having a density of 0.902 g / cm ³ was used. ) Was formed. The film was stretched in the same manner as in Example 3 except that the stretching temperature was 97 ° C., to obtain a heat-shrinkable film having a thickness of 15 μm. Next, various characteristics were measured under the same conditions as in Example 1 except that the shrink tunnel temperature in the heat shrink packaging test was set to 135 ° C. The maximum heat shrinkage stress of the biaxially stretched film was a low value of 17 kg / cm ² , and the shrinkage temperature range was as wide as 26 ° C., indicating that desired shrinkage characteristics could be easily obtained. Further, when a heat shrinkage packaging test was performed, no deformation was observed in the cardboard box, a smooth packaging state in which the entire surface of the film was tensioned was obtained, and no dog ear was observed in the fusing seal portion. In the shrink wrapping test with the shrink tunnel temperature further increased, no melting and tearing and no whitening were observed in the non-contact portion of the film surface with the packaged object, and there was no bag breaking at the fusing seal. Did not.

(Example 5) As a resin for forming an intermediate layer in Example 3, I ₁₀ / I ₂ is 11, Mw / Mn is 2.3,
A layer thickness ratio of 1 (outer layer): 3 (intermediate layer): 1 (outer layer) was obtained in the same manner as in Example 3 except that an ethylene-1-octene copolymer having a density of 0.895 g / cm ³ was used. A raw sheet for stretching was formed. The film was stretched in the same manner as in Example 3 except that the stretching temperature was 96 ° C., to obtain a heat-shrinkable film having a thickness of 20 μm. Next, various characteristics were measured under the same conditions as in Example 1 except that the shrink tunnel temperature in the heat shrink packaging test was set to 135 ° C. The maximum heat shrinkage stress of the biaxially stretched film was a low value of 16 kg / cm ² , and the shrinkage temperature range was as wide as 23 ° C., indicating that desired shrinkage characteristics could be easily obtained. Further, when a heat shrinkage packaging test was performed, no deformation was observed in the cardboard box, a smooth packaging state in which the entire surface of the film was tensioned was obtained, and no dog ear was observed in the fusing seal portion. In the shrink wrap test with the shrink tunnel temperature even higher, the film surface melts and breaks at the non-contact portion with the packaged object,
No whitening phenomenon was observed, and no bag breaking phenomenon occurred at the fusing seal portion.

Comparative Example 1 The resin for forming the intermediate layer and the resin for forming both outer layers in Example 1 had an ethylene content of
A heat-shrinkable biaxially stretched film having a thickness of 12 μm was obtained under the same conditions as in Example 1 except that 2.3% by weight of the ethylene-propylene copolymer was used. Various properties of the obtained film were measured in the same manner as in Example 1. The maximum heat shrinkage stress was an extremely high value of 40 kg / cm ² , and the shrinkage temperature width was found to be extremely narrow at 19 ° C. Next, a heat shrink wrapping test was performed at 150 ° C., 155 ° C., and 160 ° C., respectively, in the same manner as in Example 1, and shrink wrapping with heat shrinkage was possible at 160 ° C. for the first time. However, a large dog ear was generated at the end of the fusing seal portion, wrinkles were observed on a part of the film surface, and the finished packaging was not good. When the same heat shrink wrapping test was performed with the shrink tunnel temperature further raised from 5 ° C. to 10 ° C. than 160 ° C., a bag breaking phenomenon was observed at the fusing seal portion and the air vent hole portion.

Comparative Example 2 As the resin for forming the intermediate layer in Comparative Example 1, I ₁₀ / I ₂ was 5.60 and Mw / Mn was 2.60.
A biaxially stretched film having a thickness of 12 μm was obtained under the same conditions as in Example 1 except that an ethylene-1-butene copolymer having a density of 0.880 g / cm ³ was used. The maximum heat shrinkage stress of the obtained film was a very high value of 43 kg / cm ² , and the shrinkage temperature range was found to be very narrow at 15 ° C. Next, a shrink wrap test was performed in the same manner as in Example 1 except that the shrink tunnel temperature was set to 155 ° C., but shrink wrap was possible, but a large dog ear was generated at the end of the fusing seal portion. In addition, pock-like irregularities were observed on a part of the film surface, and the finished packaging was not good. When a similar shrink packaging test was performed by further increasing the shrink tunnel temperature by 5 to 10 ° C., a bag break phenomenon was observed at the fusing seal portion and the air vent hole portion.

Comparative Example 3 As a tree for forming an intermediate layer in Comparative Example 1, I ₁₀ / I ₂ was 5.50, Mw / Mn was 2.3,
Except for using an ethylene-1-butene copolymer having a density of 0.912 g / cm ³ , a thickness of 1 was obtained under the same conditions as in Example 1.
A 2 μm biaxially stretched film was obtained. The maximum heat shrinkage stress of the obtained film was a very high value of 42 kg / cm ² , and the shrinkage temperature range was found to be very narrow at 12 ° C.
Further, Example 1 except that the shrink tunnel temperature was set to 155 ° C.
When the shrink wrap test was performed in the same manner as above, shrink wrap could be performed, but a large dog ear was generated at the corner where the fusing seal crossed, and the film was pulled on the film near the corner of the packaged object. Wrinkles were observed and the finished packaging was not good. Further reduce the shrink tunnel temperature by 5 to 1
When the same shrink packaging test was performed at 0 ° C higher,
A bag breaking phenomenon was observed in the fusing seal portion and the air vent hole portion.

[0025]

According to the present invention, a polyolefin having a specific melt flow ratio (I ₁₀ / I ₂ ) and a specific Mw / Mn and having a long chain branch is used as a single-layer film, or as an intermediate layer or both outer layers in a three-layer structure. It was found that the multilayer film having the multilayer structure was excellent in low heat shrinkage stress, wide heat shrinkage temperature range, high strength of the fusing seal portion, that is, so-called dog ear was not generated.

Claims (4)

[Claims] The melt flow ratio and the molecular weight ratio are as follows: (1) I ₁₀ / I ₂ ≧ 5.63 (where I ₁₀ is a load index of 10 kg, temperature 190 ° C., melt index per 10 minutes, I ₂ is a load index of 2 .16k
g, temperature 190 ° C., melt index per 10 minutes. (2) Mw / Mn ≦ (I ₁₀ / I ₂ ) −4.63 (where Mw represents a weight average molecular weight and Mn represents a number average molecular weight, respectively) Heat-shrinkable film having a layer of 2. The heat-shrinkable film according to claim 1, wherein the polyolefin having a long chain branch is an ethylene-1-octene copolymer. 3. The melt flow ratio and the density are as follows: (1) 5.63 ≦ I ₁₀ / I ₂ ≦ 15 (2) 0.86 ≦ d ≦ 0.94 (where d is the density (g / cm ³ )) A heat-shrinkable film which satisfies the following relationship and has a layer of a polyolefin having long-chain branching according to claim 1 or 2. 4. A three-layer structure in which one of the polyolefin layer having a long chain branch and the propylene-based polymer layer according to any one of claims 1 to 3 is an intermediate layer, and the other is both outer layers. Heat shrinkable film.