JP7168115B1 - polypropylene heat-shrinkable film - Google Patents

Abstract

A heat-shrinkable polypropylene film having a specific gravity of 0.95 or less, excellent low-temperature shrinkability, low natural shrinkage, and excellent perforation opening properties is provided. A film satisfying the following (1) to (7). (1) Hot water heat shrinkage in the width direction when immersed in 90° C. hot water for 10 seconds is 40% or more and 80% or less. (2) Hot water heat shrinkage in the longitudinal direction when immersed in 90° C. hot water for 10 seconds is 1% or more and 12% or less. (3) Specific gravity is 0.87 or more and 0.95 or less. (4) Perpendicular tear strength in the longitudinal direction after 10% contraction in the width direction in hot water at 80°C is 100 mN/mm or more and 330 mN/mm or less. (5) The difference Ny-Nx between the width direction refractive index and the longitudinal direction refractive index is 0.005 or more and 0.028 or less. (6) The natural shrinkage in the width direction after aging for 672 hours in an atmosphere of 40°C and 65% RH is 0.1% or more and 3.0% or less. (7) Natural shrinkage in the longitudinal direction after aging for 672 hours in an atmosphere of 40°C and 65% RH is 0.1% or more and 1.0% or less. [Selection figure] None

Description

TECHNICAL FIELD The present invention relates to a polypropylene-based heat-shrinkable film suitable for a heat-shrinkable label, a label using the film, and a package using the label.

Heat-shrinkable films are widely used mainly as labels for preventing the contents of glass bottles and plastic bottles from being soiled or tampered with, and for protecting packaged goods and products. A particularly large amount of use is for labeling PET bottles for beverages. In general, when attaching a heat-shrinkable film to a PET bottle as a label, the label-covered PET bottle is conveyed to a steam-filled tunnel on a line with a conveyor, and steam is blown in the tunnel to make the film 70 degrees. It is shrunk by heating up to about 100°C and attached. Therefore, stretched films of polyester, polystyrene, and polyvinyl chloride, which are excellent in heat shrinkability at low temperatures, are widely used.

On the other hand, beverage PET bottles are attracting attention as a renewable resource. PET bottles are collected after use and recycled as resin for molding, but labels must be separated from bottles because they use a different plastic from PET bottles and are printed. is.

In other words, if the label is mixed in, the quality of the recycled PET bottle will deteriorate, so it is necessary to remove the label from the bottle and recycle it. At present, it is necessary for collectors to peel these off, increasing the burden of recycling. This is because polystyrene, polyester, and polyvinyl chloride have a small difference in specific gravity from PET bottles, making it difficult to float and sink in water. Floating/sinking separation based on the difference in specific gravity is industrially very easy. In particular, while PET bottles have a specific gravity of about 1.3 or more, labels with a specific gravity of 1.0 or less can be separated by specific gravity using water. If such labels are used, even if labels are mixed in when collecting PET bottles, they can be separated very efficiently. It is also possible to improve the quality of the resin.

Under these circumstances, polypropylene has a low specific gravity of about 0.9 and can be separated by floating with water compared to PET. had a problem of insufficient contractility. In addition, compared to polyester and polystyrene, post-shrinkage (natural shrinkage) at room temperature is greater, which causes changes in film dimensions, and deformation of roll-wound film causes print misalignment and wrinkles during secondary processing. There was a problem that trouble was likely to occur.

Various proposals have been made so far for the purpose of improving this shrinkability.
For example, in Patent Document 1, a petroleum resin or a cyclic polyolefin resin having a high glass transition temperature is added to improve low-temperature shrinkability and reduce natural shrinkage. However, since the resin with a high glass transition temperature is added, the shrinkage rate at 90 ° C. is still insufficient, and the shrink finish is inferior. reduce efficiency. Also, since polypropylene is softer than polyester or polystyrene, it has a strong stickiness when tearing along the perforations. Furthermore, since the above-mentioned film is a film stretched only in the width direction, it is difficult to tear along the perforations in the longitudinal direction, which is the direction perpendicular to the main shrinkage direction. was there.

In addition, Patent Document 2 proposes a film laminated with an amorphous resin having a high glass transition temperature such as a cyclic olefin resin, or controlling the molecular weight distribution of polypropylene to be small for the purpose of reducing the natural shrinkage rate. However, these films had insufficient shrinkage at low temperatures (90° C.) relative to the shrinkage required for beverage labels. Moreover, as described above, the cyclic olefin resin has a problem that its specific gravity is large.

In Patent Document 3, the addition of petroleum resin or polyethylene enables stretching at low temperatures and improves low-temperature shrinkability. However, the addition of petroleum resin not only increases the specific gravity and reduces the efficiency of floating-sink separation, but also the film obtained by stretching at low temperature has a large post-shrinkage (natural shrinkage) at room temperature, and the film dimension changes. In addition, problems such as printing misalignment and wrinkles are likely to occur during secondary processing due to deformation of the film in a rolled state.

On the other hand, the inventors have found that the perforation opening performance in the longitudinal direction is improved by stretching not only in the width direction but also in the longitudinal direction. The shrinkage rate also increases, and when the label is shrunk and attached, it also shrinks in the direction perpendicular to the direction of shrinkage, resulting in increased distortion of the label (so-called vertical shrinkage). Furthermore, there is also the problem that stretching increases the natural shrinkage in the longitudinal direction, and the film in a roll state tends to be deformed or slack due to tight winding during storage.

Japanese Patent No. 4380330 Japanese Patent No. 4574462 Japanese Patent No. 5690103

An object of the present invention is to provide a heat-shrinkable polypropylene film having a specific gravity of 0.95 or less, excellent low-temperature shrinkage, low natural shrinkage, and excellent perforation opening properties. there is

As a result of intensive studies, the inventors of the present invention have found that the above problems can be solved by means shown below, and have completed the present invention.
That is, the present invention consists of the following configurations.
1. A heat-shrinkable polypropylene film which is a stretched film made of a polypropylene resin composition and which satisfies the following (1) to (7).
(1) The film has a hot water heat shrinkage of 40% or more and 80% or less in the width direction of the film when the film is immersed in hot water of 90°C for 10 seconds.
(2) The film has a hot water heat shrinkage rate of 1% or more and 12% or less in the longitudinal direction of the film when the film is immersed in hot water of 90°C for 10 seconds.
(3) The specific gravity of the film is 0.87 or more and 0.95 or less.
(4) The perpendicular tear strength in the longitudinal direction per unit thickness after being shrunk by 10% in hot water at 80°C is 100 mN/mm or more and 330 mN/mm or less.
(5) The difference Ny-Nx between the refractive index (Ny) in the width direction and the refractive index (Nx) in the longitudinal direction is 0.005 or more and 0.028 or less.
(6) The heat-shrinkable polypropylene film has a natural shrinkage rate of 0.1% or more and 3.0% or less in the width direction after aging for 672 hours in an atmosphere of 40°C and 65% RH.
(7) The heat-shrinkable polypropylene film has a natural shrinkage rate of 0.1% or more and 1.0% or less in the longitudinal direction after aging for 672 hours in an atmosphere of 40°C and 65% RH.
2. The value obtained by dividing the maximum tensile stress in the range of 0 to 10% elongation in a tensile test in the longitudinal direction by the tensile stress at 30% elongation is 0.5 or more and 1.9 or less 1 . 2. The heat-shrinkable polypropylene film described in 3. 1. The haze of the film is 1% or more and 12% or less. or 2. The heat-shrinkable polypropylene film according to .
4. It has one or more melting peak temperatures in the ranges of 70° C. to 100° C. and in the range of over 100° C. to 170° C. or less, as measured by a differential scanning calorimeter.1. ~3. The heat-shrinkable polypropylene film according to any one of .
5. Non-reverse heat flow measured with a temperature modulated scanning calorimeter has a peak in the range of 20° C. or higher and 60° C. or lower, and the endothermic amount of the peak is 0.3 J/g or higher and 0.95 J/g or lower. 1. ~ 4. The heat-shrinkable polypropylene film according to any one of .
6. 1. The refractive index Ny in the width direction is 1.490 or more and 1.530 or less. ~ 5. The heat-shrinkable polypropylene film according to any one of .
7. 1. The heat-shrinkable polypropylene film has a thickness of 12 μm or more and 100 μm or less. ~6. The heat-shrinkable polypropylene film according to any one of .
8. 1. The polypropylene-based resin composition constituting the heat-shrinkable polypropylene-based film comprises ethylene or 1-butene as a copolymerization component. ~7. The heat-shrinkable polypropylene film according to any one of .
9. 1 above. ~8. A label comprising the heat-shrinkable polypropylene film according to any one of 1 above as a base material and provided with perforations or a pair of notches.
10. 1 above. ~8. A package comprising a label made of the polypropylene-based heat-shrinkable film according to any one of 1 and 2, and having the label on at least a part of the circumference of an object to be packaged.
11. 9. above. 2. A package comprising the label according to 1. on at least a part of the circumference of an object to be packaged.

The polypropylene-based heat-shrinkable film of the present invention has a high shrinkage rate at 90°C, and not only can it be used as a beverage label with a good finish without wrinkles or insufficient shrinkage, but it also has a perforated openability. Furthermore, since the natural shrinkage rate is small both in the width direction and the longitudinal direction, the dimensional change and the occurrence of wrinkles are small even after the film has been stored for a long time, and it is excellent in practical use. In addition, since the specific gravity is small, it is possible to efficiently separate the floating and sinking from the PET bottle, so that it is excellent in recyclability.

FIG. 2 is an explanatory drawing showing the shape of a test piece in the measurement of right-angle tear strength (the unit of length of each portion of the test piece in the drawing is mm).

The polypropylene heat-shrinkable film obtained by stretching the polypropylene resin composition used in the present invention has a melting point (melting peak temperature) measured by a differential scanning calorimeter (DSC) of 70°C to 100°C and 100°C to 100°C. It is preferable to have at least one or more in the range of more than °C and 170 °C or less. Heat-shrinkable film for PET bottle labels generally shrinks when heated by steam at temperatures between 70°C and 100°C. The crystal melts and develops shrinkage properties. On the other hand, having a melting point of 100° C. or higher and 170° C. is preferable because heat resistance can be improved and fusion and blocking during heating can be suppressed. It is more preferable to have melting points in the ranges of 75° C. to 97° C. and 110° C. to 165° C., and particularly preferably in the ranges of 80° C. to 95° C. and 115° C. to 160° C., respectively.

As the polypropylene resin used in the present invention, not only propylene homopolymer, but also copolymers of propylene and other α-olefins, polypropylene with low stereoregularity, and the like can be suitably used. As the α-olefin copolymerization component used in the copolymer of propylene and other α-olefins, α-olefins having 2 to 8 carbon atoms, such as ethylene and 1-butene, are particularly preferred. -hexene, 1,4-methyl-1-pentene and the like can also be used. The copolymer is preferably a random or block copolymer obtained by polymerizing propylene with one or more of the above-exemplified α-olefins. The steric structure of these polymers is not particularly limited, and may be isotactic, atactic, syndiotactic, or a mixture of these structures. Further, the resin composition obtained from the propylene homopolymer or propylene-α-olefin copolymer used has a molecular weight of 0.85 or more and 0.95 or less, preferably 0.86 or more and 0.93 or less, and particularly preferably 0.87. It is desirable that it is more than or equal to 0.92 or less. A polypropylene-based resin having a specific gravity of 0.85 or less is generally very low in crystallinity, and is therefore not preferable because it is difficult to obtain the required shrinkage ratio even when a stretched film is formed from the resin. Moreover, when the specific gravity is 0.95 or more, the specific gravity of the film obtained after the film formation becomes larger than 0.95, which is not preferable.
As described above, in the present invention, the cyclic olefin resin is preferably not contained because it increases the specific gravity and reduces the low-temperature shrinkage. Also, petroleum resin (sometimes referred to as hydrocarbon resin) and its hydrogenated resin increase the natural shrinkage rate by increasing the specific gravity as described above, so it is preferable not to include them.

The specific gravity of the heat-shrinkable polypropylene film obtained by stretching the unstretched sheet obtained from the polypropylene resin composition should be 0.87 or more and 0.95 or less. Further, the specific gravity is more preferably 0.88 or more and 0.94 or less, and particularly preferably 0.88 or more and 0.93 or less. If a raw material having a specific gravity of 0.87 or less is used, it is not preferable because the crystallinity is lowered and the shrinkage rate is lowered and the heat resistance is lowered. Further, if the specific gravity is more than 0.95, the heat-shrinkable film is not preferable because the efficiency of floating and sinking separation may decrease when printing is performed when the heat-shrinkable film is used as a label.

The melting point of the polypropylene resin can be controlled by adjusting the type and amount of the α-olefin. In order to obtain a polypropylene resin composition having different melting peaks in the above two temperature ranges, It is preferable to use a mixture of copolymers having different melting points in an appropriate ratio, or a block copolymer of α-olefin. When polypropylene resins having different melting points are mixed, the weight ratio of the resin with the lower melting point is preferably 30 wt % or more and 85 wt % or less, more preferably 35 wt % or more and 80 wt % or less. The total copolymerization ratio of α-olefins contained in the polypropylene resin composition is preferably 8 mol % or more and 24 mol % or less, more preferably 10 mol % or more and 22 mol % or less.

Moreover, the melt flow rate (MFR) of the polypropylene resin composition at 230°C is 0.1 to 30 g/10 min. is preferably within the range of 0.5 to 20 g/10 min. is more preferably within the range of 1.0 to 15 g/10 min. is particularly preferred. MFR is 0.1g/min. If it is less than that, the pressure during extrusion becomes high and streaks tend to occur from the die, which is not preferable. On the other hand, if it is more than 30 g/10 min, the ejection becomes unstable, the stress during stretching decreases, resulting in increased thickness unevenness, and the viscosity decreases, which is not preferable because it tends to break during stretching. The melt flow rate can be measured by a method conforming to JIS-K-7210.

In the present invention, various additives and fillers such as heat stabilizers, antioxidants, light stabilizers, antistatic agents, lubricants, nucleating agents, Addition of flame retardants, pigments, dyes, calcium carbonate, barium sulfate, magnesium hydroxide, mica, talc, clay, zinc oxide, magnesium oxide, aluminum oxide, antibacterial agents, antifog agents, additives that impart natural degradability, etc. It is possible to In particular, organic lubricants such as fatty acid amides, fatty acid alkylamines, fatty acid alkylamine esters, and fatty acid monoglycerol esters are typical from the standpoint of providing ease of handling, lubricity, antistatic properties, and heat-blocking properties. It is preferable to add an anti-blocking agent typified by a surfactant, silica, and PMMA. When a surfactant is added, the upper limit is set at 20,000 ppm and adjusted appropriately so that the required antistatic properties can be obtained. Moreover, when adding an antiblocking agent, it adjusts in the range of 300 ppm or more and 10000 ppm or less. If the amount of these added exceeds the above upper limit, the haze is lowered and the transparency is impaired, which is not preferable. Furthermore, other thermoplastic resins, thermoplastic elastomers, rubbers, etc. may be blended within limits that do not impair the properties of the film of the present invention.

The film of the present invention has the same type of polypropylene-based resin layer and other resin layers on the surface as long as the film does not impair its properties, such as polyethylene, saponified ethylene-vinyl acetate copolymer, and polyvinyl alcohol. Layers may be laminated.

The film of the present invention can be surface-treated, if necessary, to the extent that the properties of the present invention are not impaired. Examples of surface treatment methods include corona discharge treatment, plasma treatment, flame treatment, acid treatment, and the like, and are not particularly limited. It is preferable to perform corona discharge treatment, plasma treatment, and flame treatment, which can be continuously treated and can be easily performed before the winding process in the production process of the film, and corona discharge treatment is a means for improving surface wetting tension. is particularly preferred.

The polypropylene-based heat-shrinkable film of the present invention can be produced as a single layer, but it may also have a laminated structure comprising multiple layers such as a skin layer and a core layer, or more layers. A multilayer co-extrusion method or the like can be used as a method for lamination. In the case of a laminated structure of two or more layers, each layer is preferably composed of a polypropylene-based resin composition.

Furthermore, the thickness of the heat-shrinkable polypropylene film of the present invention is preferably in the range of 12 μm or more and 100 μm or less. Furthermore, the thickness of the film is more preferably in the range of 15 µm or more and 80 µm or less. If the thickness of the film is 12 μm or less, the feeling of stiffness is lowered, and wrinkles are likely to occur when the film is attached as a label, which is not preferable. Moreover, if the thickness is 100 μm or more, the raw material cost of the film increases, which is not preferable.

The package of the present invention is formed by covering at least part of the outer periphery of an object to be packaged with a label obtained from the heat-shrinkable polyester film of the present invention, preferably having perforations or notches, and heat shrinking the label. It is what is done. Examples of packaging objects include PET bottles for beverages, various bottles, cans, plastic containers for sweets and lunch boxes, paper boxes, and the like. In general, when a label obtained from a heat-shrinkable polypropylene film is heat-shrunk to cover an object to be packaged, the label is heat-shrunk by about 5 to 40% and adhered to the package. . In addition, the label to be covered on the packaging object may be printed or may not be printed.

As a method for producing a label, when a rectangular film is rolled up and the edges are overlapped, an organic solvent is applied to the inside of the edges of one side and glued, or the edges are overlapped and heat-sealed. A method of heat-sealing using heat sealing such as fusion sealing, a method of heat-sealing using an ultrasonic seal, or the like can be used. In addition, hot melt adhesives, UV curing adhesives, low molecular weight polyolefins, solvent-based adhesives using chloroprene, acid-modified polyolefins, chlorinated polyolefins, etc., water-based adhesives using phenolic resins, polyvinyl acetate, etc., epoxies Various adhesives such as chemically reactive adhesives using resins, urethane resins, silicon resins, etc. may be applied to the ends of the film for bonding, and these various methods can be used according to the film processing method. can. When using an organic solvent or a liquid adhesive that can be applied to various types of film to bond the ends together, apply the solvent or adhesive slightly inward from the edge of one side of the rolled film. However, it is also possible to use a method in which the film is immediately rolled, the ends are overlapped and adhered, and the tube-shaped body is cut to form a label.

Examples of organic solvents for bonding the polypropylene film of the present invention to a label include aliphatic hydrocarbons such as normal hexane, normal heptane, cyclohexane, and methylcyclohexane, and aromatic hydrocarbons such as toluene, xylene, and trimethylbenzene. , methylene chloride, halogenated hydrocarbons such as chloroform, phenols such as phenol, furans such as tetrahydrofuran, etc., or mixed solvents thereof can be used. It is preferable because the adhesive strength can be expressed.

The polypropylene-based heat-shrinkable film of the present invention was treated in hot water at 90°C for 10 seconds under no load. It is necessary that the heat shrinkage (that is, the hot water heat shrinkage at 90° C.) is 40% or more and 80% or less.
Thermal shrinkage = {(length before shrinkage - length after shrinkage) / length before shrinkage} x 100 (%) Formula 1

If the hot water heat shrinkage in the width direction at 90° C. is less than 40%, the amount of shrinkage is so small that wrinkles and sagging occur in the label after heat shrinkage, which is not preferable. Although there is no particular upper limit for the hot water heat shrinkage rate, the technically achievable upper limit of the present invention is 80%. The lower limit of the hot water heat shrinkage in the width direction at 90°C is preferably 42% or more, more preferably 45% or more, and particularly preferably 50% or more.

In addition, the heat-shrinkable polypropylene film of the present invention has a hot-water heat shrinkage rate of 1% or more and 12% or less in the longitudinal direction when treated for 10 seconds in warm water at 90° C. under no load. is required. On the other hand, if the longitudinal hot water heat shrinkage ratio at 90° C. exceeds 13%, when the label is used, vertical sink marks are large and distortion is likely to occur, which is not preferable. The hot water heat shrinkage in the longitudinal direction is preferably close to 0%, but the technical lower limit of the present invention is 1%. Moreover, the range of shrinkage in the longitudinal direction is more preferably 1% or more and 11% or less, and particularly preferably 1% or more and 10% or less.

The heat-shrinkable polypropylene film of the present invention preferably has a haze of 1% or more and 12% or less. If the haze is 12% or more, the transparency is low and the appearance is not suitable, which is not preferable. Further, the technical lower limit of haze in the present invention is up to 1%, and practically, even the lower limit of 2% is sufficient. Regarding the upper limit of haze, it is more preferably 11% or less, particularly preferably 10% or less. The haze varies depending on the amount of additives such as lubricants, and the use of polypropylene obtained by copolymerizing the above polypropylene resin composition with an α-olefin can suppress and reduce the generation of spherulites. Haze is measured by a method conforming to JIS K7136.

The heat-shrinkable polypropylene film of the present invention must have a natural shrinkage rate (dimensional change rate) of 3.0% or less in the width direction after aging for 672 hours in an atmosphere of 40° C. and 65% RH. It is preferably 2.5% or less, more preferably 2.0% or less. By setting the natural shrinkage rate at 40°C and 65% RH to 3.0% or less, it is possible to prevent dimensional changes over time when stored at room temperature or in a warehouse in summer, and when stored in a roll state. It is possible to reduce troubles during secondary processing such as wrinkles due to shrinkage over time and printing misalignment. If the natural shrinkage rate in the width direction is large. The width of the film that can be used during storage decreases, which worsens the yield during processing.In addition, in the roll state, both ends of the film are not gripped in the width direction, so natural shrinkage during storage causes the film to roll. It is not preferable because it tends to cause buckling deformation inside and wavy wrinkles. Further, the lower limit of the natural shrinkage rate is preferably closer to 0%, but in the case of the present invention, 0.1% is the technical lower limit.

The heat-shrinkable polypropylene film of the present invention must have a natural shrinkage rate (dimensional change rate) of 1.0% or less in the longitudinal direction after aging for 672 hours in an atmosphere of 40° C. and 65% RH. It is preferably 0.8% or less, more preferably 0.6% or less. By setting the natural shrinkage rate at 40°C and 65% RH to 1.0% or less, wrinkles and sag caused by tight winding due to natural shrinkage will occur when stored in a roll at room temperature or in a warehouse in summer. , can reduce the occurrence of roll deformation. Further, the lower limit of the natural shrinkage rate is preferably closer to 0%, but in the case of the present invention, 0.1% is the technical lower limit.

The polypropylene-based heat-shrinkable film of the present invention has an endothermic amount of 0.30 J/g or more in non-reverse heat flow observed at 20° C. or more and 60° C. or less measured by a temperature modulated differential scanning calorimeter (temperature modulated DSC). It is preferably 0.95 J/g or less. If the heat absorption amount is 0.95 J/g or less, the natural shrinkage rate can be reduced to 3.0% or less. A smaller amount of heat absorption is preferable because the natural shrinkage rate can be further reduced, but the technical lower limit of the present invention is 0.30 J/g. Further, the upper limit is more preferably 0.90 J/g or less, particularly preferably 0.85 J/g or less. Details of this endothermic component are described below.

The present inventors have investigated a method for reducing the natural shrinkage over time of a heat-shrinkable polypropylene film obtained by stretching at a low temperature. It was found that the heat of fusion and the heat of fusion were not correlated with the natural shrinkage rate. In addition, since spontaneous shrinkage occurs irreversibly over time at room temperature, which is considerably lower than the melting point of the resin, we thought that the irreversible thermal properties seen below room temperature due to amorphous molecular chains are related.

In order to investigate this irreversible change, the present inventors analyzed the non-reverse heat flow of the polypropylene-based heat-shrinkable film studied by temperature-modulated DSC. As a result, it was confirmed that there is an endothermic peak in the temperature range of 20° C. to 60° C., which is lower than the crystalline melting point, and that the natural shrinkage rate and the endothermic amount derived from this endothermic peak show a positive correlation. That is, the inventors have found that the natural shrinkage rate can be controlled by controlling the amount of heat absorbed in the range of 20°C to 60°C, and have completed the present invention. A method for controlling the amount of heat absorbed will be described later in the description of the film forming method.

The polypropylene-based heat-shrinkable film of the present invention preferably has a refractive index (Ny) in the width direction measured by an Abbe refractometer of 1.490 or more and 1.530 or less, and 1.500 or more and 1.520 or less. is more preferable. If Ny is less than 1.490, the orientation hardly progresses and the required hot water shrinkage cannot be obtained, which is not preferable. Further, when Ny is 1.530 or more, the orientation is high, and the oriented crystallization increases high-melting-point crystals, resulting in a decrease in hot-water shrinkage in the width direction, which is not preferable.

In the polypropylene-based heat-shrinkable film of the present invention, the value (Ny-Nx) obtained by subtracting the refractive index (Nx) in the longitudinal direction from the refractive index (Ny) in the width direction measured by an Abbe refractometer is 0.005 or more. It must be 0.028 or less. Also, it is preferably 0.006 or more and 0.026 or less, more preferably 0.007 or more and 0.023 or less. More preferably, the upper limit is 0.015 or less. By stretching the film so that the refractive indices in the longitudinal direction and the width direction are balanced, it is possible to achieve both the required hot water shrinkage in the longitudinal direction and the width direction and the perforation opening performance in the longitudinal direction.

The polypropylene-based heat-shrinkable film of the present invention has the maximum value of tensile stress (F10 max) at an elongation rate of 0 to 10% and the tensile stress at an elongation rate of 30% (F30 ) ratio (F10max/F30) is preferably 0.5 or more and 1.9 or less. If the tensile stress in the elongation range of 0 to 10% is large and the tensile stress at the elongation rate of 30% is small, the film is torn along the perforations rather than tearing along the perforations. The film is cut during stretching and tearing, and the tear tends to propagate in the direction that deviates from the perforation. That is, if the tensile stress ratio is greater than 1.9, the perforation unsealability deteriorates, which is not preferable. If the tensile stress ratio is less than 0.5, the film tends to tear due to impact such as dropping during transportation when used as a packaging label, which is not preferable. The above tensile stress ratio is more preferably 0.6 or more and 1.8 or less, and further preferably 0.7 or more and 1.7 or less.

The heat-shrinkable polypropylene film of the present invention must have a transverse tear strength of 100 mN/mm or more and 330 N/mm or less in the longitudinal direction when the film is shrunk 10% in hot water at 80°C in the width direction. If the right-angle tear strength is less than 100 mN/mm, the label may be easily torn due to impact such as dropping during transportation. If the tear strength exceeds 330 N/mm, the cuttability (ease of tearing) at the initial stage of tearing becomes poor, which is not preferable. The lower limit of the right-angle tear strength is preferably 120 N/mm or more, more preferably 140 N/mm or more, and particularly preferably 150 N/mm or more. Moreover, the upper limit of the perpendicular tear strength is preferably 315 N/mm or less, more preferably 300 N/mm or less, and particularly preferably 280 N/mm or less.

As a result of intensive studies, the present inventors have found a method of achieving both perforation opening performance and reduction of shrinkage in the longitudinal direction by stretching in the longitudinal direction and heat-treating under predetermined conditions. Furthermore, in the width direction, it is controlled to produce a crystal component with a low melting point range of 70° C. or more and 100° C. or less, and by stretching in the width direction at a low temperature from this state, it has a low melting point crystal oriented in the stretching direction. It has been found that a high shrinkage rate can be obtained by forming a structure. Furthermore, a method for reducing the natural shrinkage of the film after stretching by the above method was also investigated, and by applying relaxation treatment (relax) to the stretched film under appropriate conditions, the shrinkage obtained by stretching We found a way to reduce the natural shrinkage rate without impairing the Based on these findings, the inventors have arrived at the present invention. A specific method is described below.

When the raw material resin is melt-extruded, it is preferable to dry the polypropylene-based resin raw material using a dryer such as a hopper dryer or a paddle dryer, or a vacuum dryer. After the polyolefin resin raw material is dried in this manner, it is melted at a temperature of 200 to 300° C. and extruded into a film using an extruder. Any existing method such as a T-die method, a tubular method, or the like can be employed for such extrusion.

Then, an unstretched film can be obtained by rapidly cooling the extruded sheet-shaped molten resin to 30° C. or lower. As a method for quenching the molten resin, a method of casting the molten resin from a die onto a rotating drum controlled at a low temperature by means of a cooling chiller or the like, followed by rapid solidification to obtain a substantially non-oriented resin sheet is preferably used. can be adopted.

After obtaining the unstretched sheet, the unstretched sheet preheated by the heating rolls is stretched in the longitudinal direction by utilizing the speed difference of the rolls. Stretching is preferably carried out in two stages. The temperature of the stretching rolls is preferably 50° C. or more and 100° C. or less, and the temperature of the first stretching rolls is preferably 10° C. or more higher than the temperature of the second stretching rolls. By stretching the film in two stages with such a temperature pattern, the stress received by the film in the stretching process can be reduced, the increase in shrinkage in the longitudinal direction can be reduced, and the perforation opening property can be improved. In addition, by increasing the temperature in the first stage and decreasing the temperature in the second stage, the stretching stress at the yield point can be decreased and the stress in the latter half of the stretching can be increased. By reducing the ratio, it becomes easier to improve perforation unsealability. On the other hand, if the roll temperature is lower than 50°C, the film slips and the stretching becomes difficult to stabilize, which is not preferable. On the other hand, if the temperature is higher than 100° C., the film tends to be fused to the heating roll, and troubles such as the film winding around the roll tend to occur, which is not preferable. A more preferable roll temperature range is 55° C. or higher and 90° C. or lower.

In addition, the draw ratio in the longitudinal direction is in the range of 1.1 times to 1.8 times in the first stage and 1.2 times to 3.0 times in the second stage, and the stretch ratio in the second stage is the same as that in the first stage. It is preferable to adjust the draw ratio to be higher than the draw ratio of . By setting the stretching ratio of the second stage higher than that of the first stage, it is easy to reduce thickness unevenness in the longitudinal direction. Further, the draw ratio in the first stage is more preferably 1.15 times or more and 1.6 times or less, and more preferably 1.2 times or more and 1.5 times or less. The draw ratio in the second stage is preferably 1.3 times or more and 2.7 times or less, more preferably 1.5 times or more and 2.3 times or less. Moreover, it is preferable to adjust the total draw ratio of the first stage and the second stage to be 1.2 times or more and 5.0 times or less. If the total draw ratio is more than 4.5 times, it becomes difficult to reduce the shrinkage rate in the longitudinal direction and the natural shrinkage, and the shrink finish and the appearance during roll storage tend to deteriorate, which is not preferable. On the other hand, if it is less than 1.2 times, it is not preferable because the perforated openability tends to deteriorate. The total draw ratio is more preferably 1.3 times or more and 4.2 times or less, and further preferably 1.4 times or more and 4.0 times or less.

For the purpose of reducing the hot water shrinkage and natural shrinkage in the longitudinal direction, it is preferable to perform relaxation treatment in the longitudinal direction (MD relax). The MD relax process is a method of shrinking the film by reducing the speed between the rolls while heating the film on the transport rolls with hot air or an infrared heater, a method of shrinking the clip by narrowing the pitch while heating in the tenter, and a drive type. It is possible to preferably use a method of reducing the speed of the clips in a tenter having a clip of 100 mm to shrink the clips. Moreover, the MD relaxation treatment can be performed either before stretching in the width direction or after stretching in the width direction. The relaxation rate is preferably adjusted appropriately within the range of 3% or more and 50% or less, more preferably 5% or more and 40% or less. If the relaxation rate is 3% or less, the natural shrinkage rate in the longitudinal direction increases, which is not preferable. On the other hand, if the relaxation rate is 50% or more, the ease of opening the perforations is deteriorated, and the heating time required for the relaxation treatment is lengthened, resulting in an increase in equipment cost, which is not preferable.

Preferred conditions for the stretching in the width direction and the steps before and after the stretching are described in detail below.
(1) Heat treatment before stretching in the width direction In the production of the film of the present invention, heat treatment is performed at a temperature of 100° C. or more and 140° C. or less for a time of 3.0 seconds or more and 50.0 seconds or less before stretching in the width direction. preferably. The heat treatment can be performed by providing a line for performing heat treatment using hot air or a ceramic heater while the film is being transported, or before stretching in the width direction with both ends in the width direction held by clips in a tenter. can also be implemented. By performing such a heat treatment, a crystalline component having a melting point in a low temperature range of 70° C. or higher and 100° C. or lower is selectively generated, so that the low melting point crystalline component can be left even after stretching in the width direction. It becomes possible to obtain a heat-shrinkable film with a high shrinkage rate. This heat treatment also has the effect of reducing the hot water shrinkage rate and the natural shrinkage rate in the longitudinal direction.

If the temperature of the intermediate heat treatment exceeds 140° C., a crystal component having a high melting point of 100° C. or higher is likely to be formed, and the shrinkage ratio after stretching in the width direction decreases, which is not preferable. On the other hand, if the heat treatment temperature is lower than 100°C, it becomes difficult to reduce the shrinkage in the longitudinal direction, and the formation of low-melting crystalline components due to the heat treatment does not proceed, so the shrinkage during stretching in the width direction decreases. It is not preferable because it becomes easy to Moreover, the lower limit of the heat treatment temperature is preferably 105° C. or higher, and more preferably 110° C. or higher. On the other hand, the upper limit of the heat treatment temperature is preferably 135° C. or lower, more preferably 130° C. or lower. On the other hand, it is preferable to appropriately adjust the heat treatment time within the range of 1.0 seconds or more and 50.0 seconds or less according to the raw material composition.

(2) Cooling after Heat Treatment and Stretching in the Width Direction It is preferable to cool the film to 70° C. or less before stretching it in the width direction after the heat treatment described above. By performing cooling in this way, the low-melting crystal component can be left. After such cooling, it is preferable to stretch the film in the width direction so as to obtain a magnification of 2.5 times or more and 10.0 times or less in a tenter with both ends in the width direction held by clips. Moreover, the upper limit of the lateral stretching ratio is more preferably 9.0 times or less, and particularly preferably 8.0 times or less.

Furthermore, stretching can be performed in two or more stages. That is, after the film is stretched once in the width direction inside the tenter, a zone in which no stretching is performed is provided, and then the film is stretched again in the width direction inside the tenter. The draw ratio in the first stage is preferably 1.5 times or more and 7 times or less, more preferably 2 times or more and 5.5 times or less. The draw ratio in the second stage is preferably 1.5 times or more and 5 times or less, more preferably 1.8 times or more and 3.5 times or less. Further, the magnification of each step is adjusted within the above range so that the total magnification is 2.5 times or more and 10.0 times or less. By performing the stretching separately by such a method, breakage during stretching can be reduced, and an increase in the natural shrinkage rate due to stretching can be easily reduced.

Further, the stretching temperature is preferably in the range of 50°C or higher and 100°C or lower. If the stretching temperature is lower than 50°C, fine voids are generated during stretching, resulting in an increase in haze or breakage. Not only is the shrinkage rate in the width direction lowered, but also thickness unevenness in the width direction is increased, which is not preferable.
The lower limit of the temperature for stretching in the width direction is more preferably 55°C or higher, and particularly preferably 60°C or higher. Moreover, the upper limit of the temperature for lateral stretching is more preferably 90° C. or lower, and particularly preferably 85° C. or lower.

(3) Relaxation processing in the width direction (TD relaxation processing)
In addition, in the production of the film by the transverse stretching method of the present invention, as described above, after stretching in the width direction, the both ends in the width direction are held with clips in the tenter, and the temperature is 60 ° C. or more and 100 ° C. or less. Then, within a time of 0.5 seconds or more and 20.0 seconds or less, the tenter is narrowed in the width direction so that the relaxation rate obtained by Equation 2 is in the range of 13% or more and 40% of the length in the width direction. It is necessary to relax and contract (TD relax).
TD relaxation rate = {(length before relaxation - length after relaxation) / length before relaxation} x 100 (%) Equation 2
If the temperature at which the relaxation treatment is performed is lower than 60°C, the film will not shrink sufficiently upon relaxation and wrinkles will occur, which is not preferable. On the other hand, if the temperature is higher than 100° C., the reduction in shrinkage rate becomes large, and the shrinkage becomes insufficient when used as a heat-shrinkable film, which is not preferable. A more preferable temperature for the relaxation treatment is 65° C. or higher and 95° C. or lower, and particularly preferably 70° C. or higher and 90° C. or lower.
Also, if the length in the width direction narrowed by the relaxation treatment is less than 13% of the length before the treatment, the natural shrinkage rate increases, which is not preferable. On the other hand, if it is more than 40%, the 90°C shrinkage rate is lowered and the productivity is lowered, which is not preferable. It is desirable that the relaxation treatment be performed in a range of more preferably 15% or more and 35% or less, particularly preferably 17% or more and 33% or less.

By performing such a relaxation treatment, the final film retains the low-melting-point crystalline component and maintains the necessary shrinkage rate as a heat-shrinkable film, while reducing the amount of heat absorbed in the above-described non-reverse heat flow, resulting in natural shrinkage. rate can be reduced. Moreover, when the relaxation treatment is performed by dividing the stretching in the width direction into two or more stages, it is also possible to perform the relaxation treatment separately after each stretching. In the relaxation treatment, in addition to narrowing the tenter in the width direction, the clips that hold the ends of the film are released while the film is heated near the exit of the tenter, and the film is wound while shrinking in the width direction. It is also possible to implement

The present invention will be described in more detail below with reference to Examples, but the present invention is not limited to the aspects of the Examples, and can be modified as appropriate without departing from the scope of the present invention. be. Tables 1 and 2 show raw materials used in Examples and Comparative Examples, raw material formulations used in Examples and Comparative Examples, and film production conditions (stretching/heat treatment conditions, etc.). The resin raw materials PP1 to PP4 in Table 1 are as follows.
・ Resin raw material PP1: polypropylene-butene random copolymer (manufactured by Mitsui Chemicals, Inc., trade name “Tafmer XM7070”, MFR = 7.0 g / 10 minutes, melting point 77 ° C., specific gravity 0.88)
・ Resin raw material PP2: polypropylene-butene random copolymer (manufactured by Mitsui Chemicals, Inc., trade name “Tafmer XM7080”, MFR = 7.0 g / 10 minutes, melting point 83 ° C., specific gravity 0.88)
・ Resin raw material PP3: polypropylene-ethylene random copolymer (trade name “Novatec EG6D” manufactured by Japan Polypropylene Corporation, MFR = 1.9 g / 10 minutes, melting point 145 ° C., specific gravity 0.90)
・ Resin raw material PP4: Polypropylene (trade name “FS2011DG3” manufactured by Sumitomo Chemical Co., Ltd. MFR = 2.5 g / 10 minutes, melting point 159 ° C., specific gravity 0.91)
In addition, MFR described the value in 230 degreeC.
Moreover, the evaluation method of the film is as follows.

[Tm (melting point)]
Using a differential scanning calorimeter (model: DSC60) manufactured by Shimadzu Corporation, 5 mg of the film was sampled in an aluminum pan and heated from -40 ° C. to 200 ° C. at a heating rate of 10 ° C./min. The heat flow (unit: W/g) was measured at a sampling interval of 0.5 seconds, and the heat flow was plotted on the vertical axis (heat absorption in the negative direction) and the temperature (unit: °C) on the horizontal axis to form a heat flow curve. got
A regression line was constructed with values in the range of 55-60° C. of the heat flow curve. Regarding heat flow, when the difference obtained by subtracting the value on the heat flow curve from the value on the regression line is obtained at each temperature from 60 ° C to 175 ° C in order from the low temperature side, the difference exceeds 0.001 W / g for the first time. The temperature was taken as the melting initiation temperature.
Similarly, a regression line was created with values of 175° C. to 180° C. of the heat flow curve. The temperature at which the difference exceeds 0.001 W/g for the first time when the difference obtained by subtracting the value on the heat flow curve from the value on the regression line is obtained in order from the high temperature side for heat flow from 175 ° C to 60 ° C. was taken as the melting end temperature.
A straight line passing through points on the heat flow curve at this melting start temperature and melting end temperature was used as a baseline. For the heat flow at each temperature from 70° C. to 170° C., the difference was obtained by subtracting the value on the heat flow curve from the value on the baseline. Among the values of the difference at each temperature, the point where the value is greater than 0.02 W / g and is the maximum value with respect to temperature is taken as the melting peak of the heat flow curve, and the temperature of the melting peak is taken as the melting point. .

[Amount of heat absorbed in non-reverse heat flow of temperature-modulated DSC]
Using a temperature-modulated differential scanning calorimeter (TM DSC) "DSC250" (manufactured by TA instruments), 5 mg of the film was weighed in a T-ZERO pan, and the average heating rate from -40°C to 200°C was measured in heat-only mode. The temperature was raised at 2°C/min and the modulation cycle was 40 seconds, and the non-reverse heat flow was measured at a sampling interval of 0.1 second. , a non-reverse heat flow curve with temperature (unit: °C) on the horizontal axis was obtained.
A regression line was constructed from values between 18 and 20° C. for the non-reverse heat flow curve. Regarding the non-reverse heat flow, the difference obtained by subtracting the value on the non-reverse heat flow curve from the value on the regression line is obtained in order from the low temperature side in the range of 20 ° C to 60 ° C, and the value is 0.0005 W / g for the first time. The endothermic start temperature was defined as the temperature at which the temperature was equal to or above.
Similarly, a regression line was created from the values of the non-reverse heat flow curve from 60°C to 62°C. When the difference obtained by subtracting the value on the non-reverse heat flow curve from the value on the regression line is obtained sequentially from 60°C to 20°C, the temperature at which the difference value becomes 0.0005 W / g or more for the first time is the endothermic end temperature. and
Using the straight line that connects the points on the non-reverse heat flow curve at the endothermic start temperature and the endothermic end temperature as the baseline, the area of the portion surrounded by the non-reverse heat flow curve and the baseline is calculated, and the peak endothermic amount (unit: : J/g).

[Identification of α-olefin component]
The types of α-olefins such as ethylene and butene in the propylene-α-olefin copolymer are determined by the method described on pages 615-617 of Polymer Analysis Handbook (published by Kinokuniya Shoten, 1995), 13C NMR spectrum. determined by law. It is also possible to determine by IR spectroscopy according to the method described in the section "(i) Random Copolymer" on page 256 of the same book.

[Thermal shrinkage rate (hot water heat shrinkage rate)]
Cut the film into a square of 10 cm × 10 cm so that it is parallel to the roll unwinding direction (hereinafter, for the cut film, the roll unwinding direction before cutting is the longitudinal direction, and the direction perpendicular to the unwinding direction on the film surface is width direction), immersed in warm water at a predetermined temperature ± 0.5 ° C. for 10 seconds in an unloaded state to thermally shrink, then immersed in water at 23 ° C. ± 0.5 ° C. for 10 seconds, removed from water The lengthwise and widthwise dimensions of the film were measured by pulling out the film, and the thermal shrinkage rate was obtained according to the above formula (1).

[Natural shrinkage]
Cut out the film in a square shape of 210 mm × 210 mm so that the film is parallel to the roll unwinding direction, draw a marked line of 200 mm in each of the longitudinal direction and the width direction of the cut sample, 40 ° C., 65% RH atmosphere Aging was performed by leaving it for 672 hours at room temperature. The natural shrinkage rate was calculated by the following formula 3 for each of the longitudinal direction and the width direction.
Natural shrinkage ratio = {(marked line length before aging - marked line length after aging) / width direction marked line length before aging} x 100 (%) Equation 3

[Haze]
Measured according to JIS K 7136 in an atmosphere of 23° C. using a haze meter (manufactured by Nippon Denshoku Industries Co., Ltd., 300A). In addition, the measurement was performed twice and the average value was obtained.

[Tensile test]
According to JIS-K-7127, the film is sampled to a width of 15 mm and a length of 100 mm to obtain a test piece, and a universal tensile tester (manufactured by Shimadzu Corporation, Autograph) is used to measure both ends of the test piece (film longitudinal direction). Grasped by adjusting the distance between chucks to 20 mm, a tensile test was performed under the conditions of a tensile speed of 200 mm / min. ) was calculated.

[Right angle tear strength]
After the film is shrunk by 10% in the width direction in hot water adjusted to 80° C., it is sampled as a test piece of a predetermined size according to JIS-K-7128. After that, both ends of the test piece were gripped with a universal tensile tester (manufactured by Shimadzu Corporation, Autograph), and the strength at tensile breakage in the direction perpendicular to the main contraction direction of the label was measured under the conditions of a tensile speed of 200 mm / min. measurement. Then, the right angle tear strength per unit thickness is calculated using the following formula 4.
Right-angle tear strength = strength at tensile break/thickness Formula 4

[specific gravity]
Based on JIS K7112, the specific gravity of the film was calculated from the ratio of the density of the film measured by the density gradient tube method to the density of water at a temperature of 23°C.

[Refractive index]
Using Atago's "Abbe refractometer 4T type", each sample film before shrinking was left in an atmosphere of 23 ± 2 ° C. and 65 ± 5% RH for 2 hours or more, and then measured in the longitudinal direction and the width direction. Refractive indices (Nx, Ny) were measured.

[Shrinkage finish]
[Label shrinkage distortion]
Both ends of a heat-shrinkable film with a lattice pattern at 10mm intervals are overlapped by 5mm and thermally fused with an impulse sealer “HAKKO311” (manufactured by Hakko Co., Ltd.) to create a cylindrical label (main shrinkage direction of the heat-shrinkable film). in the circumferential direction). A 500 ml PET bottle (body diameter 67 mm, neck minimum diameter 25 mm, product name “Jurokucha”) was covered with a label, and a Fuji Astec Inc. steam tunnel (model: SH-1500-L) with a zone temperature of 85 ° C. ) for 10.0 seconds, the label was heat shrunk and attached to the bottle. When attaching the label, the neck portion was adjusted so that the portion with a diameter of 43 mm was positioned at one end of the label. As an evaluation of the finish after shrinkage, the magnitude of deviation from the horizontal plane of the grid pattern in the 360-degree direction of the mounted label body, excluding the grid including the fused portion, is measured as strain, and the maximum value of strain is obtained. rice field. Evaluation was performed according to the following criteria.
◎: Maximum strain less than 1.0 mm ○: Maximum strain 1.0 mm or more and less than 2.0 mm ×: Maximum strain 2.0 mm or more

[Label wrinkles]
A label was attached to a PET bottle under the same conditions as the conditions for shrinkage distortion of the label described above, and the wrinkle generation state was evaluated according to the following criteria.
A: The number of wrinkles having a size of 2 mm or more is 0.
○: The number of wrinkles having a size of 2 mm or more is 1 or more and 2 or less.
x: The number of wrinkles having a size of 2 mm or more is 3 or more.

[Perforation openability]
A label preliminarily perforated in a direction perpendicular to the main shrinkage direction was attached to a PET bottle under the same conditions as the conditions for shrinkage distortion of the label described above. However, the perforations were formed by inserting holes with a length of 1 mm at intervals of 1 mm, and two perforations were provided in the vertical direction (height direction) of the label over a width of 22 mm and a length of 170 mm. After that, the bottle was filled with 500 ml of water and refrigerated at 5°C. The number of bottles in which the label could not be removed from the bottle due to tearing of the label was counted, and the perforation opening failure rate (%) for all 50 samples was calculated. If the perforation opening defect rate is 20% or less, it is practically acceptable.

[Evaluation of roll appearance after aging]
(Appearance change after storage at 40°C for 672 hours)
The film is slit and wound into a roll having a width of 300 mm and a length of 300 m, and the roll is placed in a temperature-controlled room adjusted to an atmosphere of 40 ° C. and 65 ± 10% RH, stored for 672 hours, removed from the temperature-controlled room, and rolled. The state of wrinkles when the film was unwound for 3 m was evaluated according to the following criteria.
A: The number of wrinkles with a width of 1 cm or more is 0.
○: The number of wrinkles of 1 cm or more is 1 or more and 2 or less.
x: The number of wrinkles of 1 cm or more is 3 or more.

[Float/sink separation adequacy evaluation]
Cut the film into a square of 100 mm x 100 mm, put 2 L of water in a cylindrical container with a bottom diameter of 130 mm, adjust the temperature to 23 ° C ± 2 ° C, and then sink the film to the bottom, being careful not to adhere bubbles. It was visually evaluated according to the following criteria whether the film floats out of water.
○: The film floats up to the surface of the water
×: The film does not float to the surface of the water

[Example 1]
500 ppm of SiO ₂ (Sylysia 266 manufactured by Fuji Silysia Co., Ltd.) was added as a lubricant to a dry blend of polypropylene resins PP1 and PP3 at a weight ratio of 60/40 from an extruder, melt extruded at 230°C, and cooled to 20°C. It was then wrapped around a rotating metal roll and quenched to obtain an unstretched film (polypropylene resin sheet) having a thickness of 370 µm. At this time, the take-up speed of the unstretched film (rotational speed of the metal roll) was about 5.0 m/min. Met.

After that, the obtained unstretched film is guided to a longitudinal stretching machine in which a plurality of roll groups are continuously arranged, and after preheating on preheating rolls to a film temperature of 65°C, the surface temperature is set to 75°C. Between the stretching rolls, the roll speed on the high speed side was set to 6.5 m / min, and the first stage was stretched by 1.3 times. It was set to 0 m/min and stretched 2.0 times, for a total of 2.6 times of stretching. After that, the film was conveyed by a roll set to a surface temperature of 60 ° C., the roll was decelerated to 11.0 m / min while blowing hot air set to 95 ° C., and a 15% relaxation treatment was performed in the longitudinal direction, resulting in a thickness of 168 μm. A longitudinally uniaxially stretched film was obtained.

The resulting longitudinally uniaxially stretched film is heat-treated with hot air in a heat treatment zone provided on a conveying line in which a plurality of roll groups are continuously arranged, until the film temperature reaches 125 ° C., then air-cooled to 60 ° C. It was led to a tenter in which a stretching zone and a relaxation treatment zone were continuously provided.

After preheating the film to a temperature of 60° C., the film is stretched in the transverse direction at 60° C. at 3.0 times in the first stage, and further stretched at 60° C. at 2.2 times in the second stage. and a 20% relaxation treatment at 75° C. for 7 seconds in a relaxation treatment zone to obtain a biaxially stretched film having a thickness of 30 μm.

After the film has been laterally stretched as described above, the ends are slit and wound onto a paper tube to form a biaxially stretched film (polypropylene heat-shrinkable film) having a width of 500 mm and a thickness of 30 μm. A film roll was obtained. Then, the properties of the obtained film were evaluated by the methods described above. Table 2 shows the evaluation results.

[Example 2]
The same procedure as in Example 1 was performed except that the discharge rate was adjusted to make the thickness of the unstretched sheet 439 μm, and the stretching ratio in the first stage in the longitudinal direction was changed to 1.5 times to make the total stretching ratio 3.0 times. A biaxially stretched film having a thickness of 30 μm was obtained by forming a film according to the method. Table 2 shows the evaluation results.

[Example 3]
The same procedure as in Example 1 was performed except that the discharge rate was adjusted to set the thickness of the unstretched sheet to 521 μm, and the stretching ratio in the first stage in the longitudinal direction was changed to 1.8 times to make the total stretching ratio 3.6 times. A biaxially stretched film having a thickness of 30 μm was obtained by forming a film according to the method. Table 2 shows the evaluation results.

[Example 4]
The thickness of the unstretched sheet was adjusted to 252 μm by adjusting the discharge amount, and the stretching ratio in the first stage in the longitudinal direction was changed to 1.2 times, and the stretching ratio in the second stage was changed to 1.3 times, so that the total stretching ratio was 1.3. A biaxially stretched film having a thickness of 30 μm was obtained in the same manner as in Example 1, except that the tension was increased 6 times and the relaxation rate in the longitudinal direction was changed to 5%. Table 2 shows the evaluation results.

[Example 5]
Film formation was carried out in the same manner as in Example 1, except that the thickness of the unstretched sheet was adjusted to 353 μm by adjusting the discharge rate and the relaxation rate in the longitudinal direction was changed to 20% to obtain a biaxially stretched film with a thickness of 30 μm. rice field. Table 2 shows the evaluation results.

[Example 6]
Film formation was carried out in the same manner as in Example 1 except that the thickness of the unstretched sheet was adjusted to 393 μm by adjusting the discharge rate and the relaxation rate in the longitudinal direction was changed to 10% to obtain a biaxially stretched film with a thickness of 30 μm. rice field. Table 2 shows the evaluation results.

[Example 7]
A biaxially stretched film with a thickness of 30 μm was obtained by the same method as in Example 1, except that the polypropylene resin used was changed to a dry blend of PP1 and PP4 at a weight ratio of 60/40. . Table 2 shows the evaluation results.

[Example 8]
A biaxially stretched film with a thickness of 30 μm was obtained by the same method as in Example 1 except that the polypropylene resin used was changed to a dry blend of PP2 and PP3 at a weight ratio of 60/40. . Table 2 shows the evaluation results.

[Example 9]
A biaxially stretched film having a thickness of 30 μm was obtained by the same method as in Example 1, except that the polypropylene resin used was changed to a dry blend of PP1 and PP3 at a weight ratio of 70/30. . Table 2 shows the evaluation results.

[Example 10]
An unstretched film was formed in the same manner as in Example 1, and after stretching in the longitudinal direction, the obtained longitudinally uniaxially stretched film was heat-treated on a conveying line in which a plurality of roll groups were continuously arranged. In the zone, heat treatment was performed with hot air until the film temperature reached 125°C. After that, the film was air-cooled to 60° C. and led to a tenter provided with a lateral stretching zone and a relaxation treatment zone continuously. After preheating the film to a temperature of 60° C., the film is stretched in the transverse direction at 60° C. at 3.0 times in the first stage, and further stretched at 60° C. at 2.2 times in the second stage. and 15% relaxation treatment in the width direction at 75 ° C. for 7 seconds in the relaxation treatment zone and 15% relaxation treatment in the longitudinal direction by narrowing the distance between clips to biaxially stretch the film to a thickness of 30 μm. got the film. Table 2 shows the evaluation results.

[Example 11]
A biaxially stretched film having a thickness of 30 μm was obtained by forming a film in the same manner as in Example 1, except that the temperature of the relaxation treatment after stretching in the width direction was changed to 80°C. Table 2 shows the evaluation results.

[Example 12]
A biaxially stretched film having a thickness of 30 μm was obtained by the same method as in Example 1, except that the temperature of the relaxation treatment after the width direction stretching was changed to 85°C. Table 2 shows the evaluation results.

[Comparative Example 1]
An unstretched sheet having a thickness of 168 μm was formed under the same conditions as in Example 1, but only the discharge rate was changed. In the zone, the film was heat-treated with hot air until the film temperature reached 125°C, then air-cooled to 60°C, and led to a tenter in which a transverse stretching zone and a relaxation treatment zone were continuously provided. Thereafter, the film is preheated to a temperature of 60°C, then stretched in the transverse direction at 60°C at 3.0 times in the first stage, and further at 60°C at 2.2 times in the second stage. and a relaxation treatment of 15% at 75° C. for 7 seconds in a relaxation treatment zone to obtain a transversely uniaxially stretched film having a thickness of 30 μm. Table 2 shows the evaluation results. Since the film was not stretched in the longitudinal direction, the film had a high defect rate when opened at the perforations.

[Comparative Example 2]
A biaxially stretched film having a thickness of 30 μm was obtained by forming a film in the same manner as in Example 1, except that the temperature of the heat treatment performed before stretching in the width direction was changed to 95°C. Table 2 shows the evaluation results. Since the hot water shrinkage rate in the longitudinal direction was high, the film was greatly distorted due to vertical shrinkage at the time of shrinkage finish, and the film had an inferior appearance at the time of shrinkage finish.

[Comparative Example 3]
Under the same conditions as in Example 1, an unstretched sheet having a thickness of 437 μm was formed by changing only the discharge rate, and the film was formed by the same method as in Example 1, except that the relaxation treatment in the longitudinal direction was not performed. A biaxially stretched film having a thickness of 30 μm was obtained. Table 2 shows the evaluation results. Due to the high natural shrinkage in the longitudinal direction, wrinkles were generated when the stored film roll was unwound after a lapse of time, and the film was inferior in practical use.
[Comparative Example 4]
Under the same conditions as in Example 1, an unstretched sheet having a thickness of 437 μm was formed by changing only the discharge rate, and film formation was performed in the same manner as in Example 1, except that the relaxation treatment in the width direction was not performed. A biaxially stretched film having a thickness of 30 μm was obtained. Table 2 shows the evaluation results. Since the natural shrinkage rate in the width direction was high, wrinkles were generated in the width direction when the stored film roll was unwound after the passage of time, and the film was inferior in practical use.

As is clear from Table 2, the films obtained in Examples 1 to 12 all have high shrinkability in the width direction and low shrinkage in the orthogonal longitudinal direction, so that good results are obtained even in shrink finishing. Met. The perforation openability was also good. Further, the relaxation treatment reduced the natural shrinkage rate, hardly caused the occurrence of wrinkles after aging, and was sufficient for practical use as a heat-shrinkable film. Since only polypropylene-based resin is used, the specific gravity is very small, making it suitable for floating/sinking separation from PET bottles. On the other hand, the films obtained in Comparative Examples 1 to 4, as described in Table 2 and the sections of Comparative Examples above, did not have properties sufficient for practical use as a heat-shrinkable film.

Since the polypropylene-based heat-shrinkable film of the present invention has excellent shrinkage properties and perforation opening properties as described above, it is suitably used for packaging various articles including labels for PET bottles. be able to. In addition, since the specific gravity is less than 1.0, it is suitable for floating and sinking separation from PET bottles, and can contribute to the efficiency of recycling PET bottles.

Claims (11)

A heat-shrinkable polypropylene film which is a stretched film made of a polypropylene resin composition and which satisfies the following (1) to (7).
(1) The film has a hot water heat shrinkage of 40% or more and 80% or less in the width direction of the film when the film is immersed in hot water of 90°C for 10 seconds.
(2) The film has a hot water heat shrinkage rate of 1% or more and 12% or less in the longitudinal direction of the film when the film is immersed in hot water of 90°C for 10 seconds.
(3) The specific gravity of the film is 0.87 or more and 0.95 or less.
(4) The perpendicular tear strength in the longitudinal direction per unit thickness after being shrunk by 10% in hot water at 80°C is 100 mN/mm or more and 330 mN/mm or less.
(5) The difference Ny-Nx between the refractive index (Ny) in the width direction and the refractive index (Nx) in the longitudinal direction is 0.005 or more and 0.028 or less.
(6) The heat-shrinkable polypropylene film has a natural shrinkage rate of 0.1% or more and 3.0% or less in the width direction after aging for 672 hours in an atmosphere of 40°C and 65% RH.
(7) The heat-shrinkable polypropylene film has a natural shrinkage rate of 0.1% or more and 1.0% or less in the longitudinal direction after aging for 672 hours in an atmosphere of 40°C and 65% RH. A claim characterized in that the value obtained by dividing the maximum tensile stress at an elongation of 0 to 10% by the tensile stress at an elongation of 30% in a tensile test in the longitudinal direction is 0.5 or more and 1.9 or less. Item 1. The heat-shrinkable polypropylene film according to Item 1 2. The heat-shrinkable polypropylene film according to claim 1, wherein the film has a haze of 1% or more and 12% or less. The polypropylene-based heat according to claim 1, which has one or more melting peak temperatures measured by a differential scanning calorimeter in the range of 70 ° C. to 100 ° C. and 100 ° C. to 170 ° C. or less. shrink film Non-reverse heat flow measured with a temperature modulated scanning calorimeter has a peak in the range of 20° C. or higher and 60° C. or lower, and the endothermic amount of the peak is 0.3 J/g or higher and 0.95 J/g or lower. The polypropylene-based heat-shrinkable film according to claim 1. 2. The heat-shrinkable polypropylene film according to claim 1, wherein the refractive index Ny in the width direction is 1.490 or more and 1.530 or less. 2. The heat-shrinkable polypropylene film according to claim 1, wherein the heat-shrinkable polypropylene film has a thickness of 12 [mu]m or more and 100 [mu]m or less. 2. The heat-shrinkable polypropylene film according to claim 1, wherein the polypropylene-based resin composition constituting the heat-shrinkable polypropylene film contains ethylene or 1-butene as a copolymer component. A label comprising the heat-shrinkable polypropylene film according to any one of claims 1 to 8 as a base material and provided with perforations or a pair of notches. 9. A packaging body comprising a ring label made of the heat-shrinkable polypropylene film according to any one of claims 1 to 8 on at least a part of the circumference of an object to be packaged. A package comprising the label according to claim 9 on at least part of the circumference of an object to be packaged.

Images