JP2800926B2 - White biaxially oriented polyolefin film and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a white biaxially oriented polyolefin film and a method for producing the same. More specifically, the present invention relates to a white biaxially oriented polyolefin film which is white, has a high optical density, is excellent in hiding properties, and has excellent impact resistance and water vapor barrier properties, and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, as a white biaxially stretched polyolefin film, an inorganic filler such as calcium carbonate or a crystalline and incompatible resin such as polyester is blended and stretched. (Japanese Patent Publication No. 60-37793, Japanese Patent Application Laid-Open No. 61-1557547, Japanese Patent Application Laid-Open No.
No. 57548).

[0003]

However, when a whitening film is obtained by adding an inorganic filler such as calcium carbonate or a crystalline and incompatible resin such as polyester to a polyolefin and a dispersant or a flow initiator to the polyolefin. The volume of the formed void is large, and the moisture absorption of the void-formed product is high, so that production restrictions are required.The whitening film has poor mechanical strength, impact resistance, and water vapor barrier property, and There is a problem that the whiteness decreases over time. Therefore, when the whitening film is used as various printing papers, various labels, various adhesive tapes, various wrapping papers, etc., it has been difficult to sufficiently satisfy the required characteristics.

The present invention solves the problems of the conventional white biaxially oriented polyolefin film, is white, has a high optical density, is excellent in concealing properties, is excellent in impact resistance, is excellent in water vapor barrier properties, and is suitable for various printing papers, various labels, An object of the present invention is to provide a white biaxially stretched polyolefin film suitable as various adhesive tapes, various wrapping papers, and the like.

[0005]

According to the present invention, there is provided a white biaxially stretched polyolefin film according to the present invention comprising 65 to 93% by weight of crystalline polypropylene and 5 to 20% by weight of an amorphous resin having a heat distortion temperature of 120 ° C. or more. % And melting temperature 140 ° C
2 to 15% by weight of the following polyolefin resin,
The film section has a three-layer structure having an unfoamed layer as a central layer and a foamed layer on both sides thereof, and the average thickness ratio of the unfoamed central layer to the film section thickness is 5 to 60%. ,
The optical density is 0.40 or more at a film thickness of 30 μm.

In the white biaxially stretched polyolefin film of the present invention, crystalline polypropylene (hereinafter sometimes abbreviated as PP) has an intrinsic viscosity [η] of 1.
4-3.2 dl / g, preferably 1.6-2.4 dl / g
g, isotactic index (II) is 95% or more, and melt flow index (MFI) is 1.0 to 1
The range of 5 g / 10 minutes is preferable because high mechanical properties and various physical properties such as impact resistance of the film can be obtained, and the uniformity of the optical density becomes good. A small amount of a second component other than propylene, for example, ethylene, butene, hexene, etc., may be randomly copolymerized. Also, known additives such as nucleating agents, antioxidants, heat stabilizers,
Slip agent, antistatic agent, antiblocking agent, filler,
A viscosity modifier, a coloring inhibitor and the like may be contained.

The heat distortion temperature of the amorphous resin of the second component in the present invention (measured according to ASTM-D648, load is 1
8.6 kg / cm ² ) needs to be 120 ° C. or higher, preferably 130 to 180 ° C. When the heat deformation temperature is lower than 120 ° C., void formation by stretching is insufficient, and it becomes difficult to develop concealing properties.

As the amorphous resin, specifically, a polycarbonate resin (hereinafter sometimes abbreviated as PC) having a weight average molecular weight (hereinafter sometimes abbreviated as Mw) of 40000 or less has a good dispersibility in PP. And is excellent in chemical stability. Further, by using such a PC, a film having a three-layer structure having an unfoamed layer as a central layer and foamed layers on both sides thereof can be easily obtained.

The PC is preferably an aromatic polycarbonate, and straight-chain and branched polycarbonates can be used. The PC may be a homopolymer or a copolymer with other monomers. The heat distortion temperature of the PC is 120 ° C. or higher, preferably 130 ° C. or higher. If the heat deformation temperature is lower than 120 ° C., void formation is insufficient, and it becomes difficult to develop concealing properties, which is not preferable. Further, the Mw of the PC is preferably 40000 or less,
If the Mw exceeds 40,000, the dispersibility in the PP deteriorates, the optical density becomes uneven, the concealing property becomes insufficient, and the impact resistance and the water vapor barrier property deteriorate. Further, as a melting property, the melt flow index is 1 to
50 g / 10 min is preferable because the dispersibility in the PP is good and the optical density is high. Also, usually, PC
Is hygroscopic, but in the present invention, when it is mixed with the PP and melt-extruded, voids can be formed without any problem even if it is melt-kneaded without drying, and undried use is preferred from the viewpoint of cost.

[0010] The content of the amorphous resin contained in the film of the present invention must be 5 to 20% by weight,
In particular, the content is desirably 10 to 17% by weight. If the content of the amorphous resin is less than the range of the present invention, the average thickness ratio of the unfoamed center layer obtained from the cross-sectional structure of the film to the cross-sectional thickness of the film exceeds 60%, and the whiteness of the film And the optical density becomes insufficient. If the ratio exceeds this range, the impact resistance and the water vapor barrier property of the film of the present invention are deteriorated.

It is preferable that the amorphous resin has elasticity capable of being deformed (for example, deformed into an elliptical shape) in a stretching step. As the shape of the amorphous resin dispersed in the film of the present invention, the ratio of the major axis to the minor axis is 1.1 or more,
It is preferable because the void shape becomes flat and excellent mechanical properties and impact resistance can be obtained.

A polyolefin resin (hereinafter referred to as L) having a melting temperature of the third component of 140 ° C. or less of the third component contained in the film of the present invention.
-PO is sometimes abbreviated as one or two or more resins selected from ethylene-α-olefin copolymers and ethylene-α, β-unsaturated carboxylic acid ester copolymers. is there. Among them, ethylene-α-olefin copolymers include ethylene-propylene random copolymer (hereinafter abbreviated as EPC), ethylene-α, β-
As the unsaturated carboxylic acid ester copolymer, ethylene-
Acrylic ester copolymer is preferred for obtaining a three-layer structure having an unfoamed layer as a center layer and a foamed layer on both sides thereof, which is a feature of the film of the present invention. In particular, EPC and ethylene-acrylic acid ester copolymer Is more preferred. The melt flow index (MFI) of the EPC and the ethylene-acrylate copolymer is as follows:
A range of 1.0 to 20 g / 10 minutes is preferable because mechanical properties, various physical properties such as impact resistance, and uniformity of optical density are improved.

The melting temperature of the third component L-PO contained in the film of the present invention must be 140 ° C. or lower. When the temperature exceeds 140 ° C., it is difficult to obtain a three-layer structure having an unfoamed layer as a central layer and a foamed layer on both sides thereof, which is a feature of the film of the present invention, and is inferior in film-forming stability,
The impact resistance and water vapor barrier properties of the film also deteriorate.

[0014] The content of L-PO contained in the film of the present invention must be 2 to 15% by weight, and particularly preferably 5 to 12% by weight. When the content of the L-PO is less than 2% by weight, impact resistance and water vapor barrier properties deteriorate. If it exceeds this range, the average thickness ratio of the unfoamed center layer obtained from the cross-sectional structure of the film to the cross-sectional thickness of the film exceeds 60%, and the optical density of the film becomes insufficient.

Since the content of the amorphous resin is 5 to 20% by weight and the content of the L-PO is 2 to 15% by weight, the remaining 65 to 93% by weight is the content of the crystalline polypropylene. .

The thickness of the film of the present invention is not particularly limited, but is preferably 10 to 120 μm, more preferably 20 to 90 μm.

The cross-sectional structure of the film of the present invention has a three-layer structure having an unfoamed layer as a central layer and foamed layers on both sides thereof, and the average thickness of the unfoamed central layer in the film cross-sectional thickness. It is necessary that the length ratio is 5 to 60%. If the cross-sectional structure of the film does not have a three-layer structure in which the entire cross-sectional structure becomes a foamed layer and the non-foamed layer is the center layer and the foamed layers are provided on both sides of the film, the impact resistance and the water vapor barrier property are inferior.
When the average thickness ratio of the unfoamed center layer to the film cross-sectional thickness obtained from the cross-sectional structure is 5% or less, the impact resistance and the water vapor barrier property are poor. Inferior.

The resin composition (mixing ratio) of each layer of the three-layer structure of the film of the present invention is the same. For example, polymers having different types of resins or different resin compositions are coextruded to form a foamed layer / unfoamed layer / foamed layer. Such a multilayer laminated film is not preferable in terms of productivity.

The film of the present invention has a film thickness of 30 μm.
The optical density (OD) at m is 0.40 or more, preferably 0.45 or more. When it is less than 0.40, for example, when used as a packaging film, it is insufficient to conceal the contents to be packaged, and when laminated with other base materials such as paper, the surface of the base material becomes transparent. It is not preferable because it becomes easier.

In the film of the present invention, the sum of the Charpy impact strength in the length direction and the width direction is 15 (k).
g · cm / mm ² ) or more. If it is less than this range, it cannot be put to practical use, for example, when used as a base material of a wrapping paper or an adhesive tape.

The water vapor transmission rate of the film of the present invention is desirably 3.0 (g / m ² · day / 0.1 mm) or less. If it is less than this range, when used for packaging small confectionery or rice confectionery having high hygroscopicity, the long-term storage property is poor.

Further, when printed, the film of the present invention has a glossiness of less than 100%, preferably 90% or less, in order to give a moist and calm color tone without a shiny or shiny feeling on the surface. Desirably. When the glossiness is 100% or more, the printed surface becomes shiny or shiny when printed, making it difficult to see the printed image.

In the white biaxially stretched polyolefin film (layer A) of the present invention, at least one surface thereof is formed of an ethylene-α-olefin copolymer or an ethylene-α, β-unsaturated carboxylic acid ester copolymer. A resin layer (layer B) made of at least one selected resin may be laminated. Among the resins of the layer B, EPC is preferable because of its high heat sealing force and high water vapor barrier property.

The laminated structure of the film of the present invention in which the B layers are laminated is B layer / A layer or B layer / A layer / B layer. The layer thickness of the layer B is preferably 2 to 10 μm.

The film of the present invention can be used by itself for packaging, labeling, adhesive tape, and card. Also, it can be used by laminating it with paper, polyester film, polypropylene film, aluminum foil or the like. In this case, it is preferable to perform a corona discharge treatment on the surface of the layer B in order to improve the adhesiveness. A known method can be used for the corona discharge treatment, but air, carbon dioxide, nitrogen gas, or carbon dioxide / nitrogen gas is preferable as the atmosphere gas at the time of the treatment, and air is particularly preferred in terms of simplicity and economy. preferable.

Next, a method for producing a white biaxially oriented polyolefin film of the present invention will be described. A crystalline PP, an amorphous resin having a heat distortion temperature of 120 ° C. or more, and a melting temperature of 1
A layer resin composed of a mixture of polyolefin resins having a specific range of 40 ° C. or less is supplied to an extruder and melted at a temperature of 270 ° C. or more, preferably 280 ° C. to 300 ° C.
It is extruded into a sheet by a die, and the draft ratio (lip gap of the die / film thickness) is 1.5 or more, preferably 2.
At 0 to 4.0, the sheet is heated to 20 to 100C, preferably 4 to 100C.
It is wound around a drum at a temperature of 0 to 80 ° C., cooled and solidified,
After forming a three-layer structure having a non-foamed layer as a central layer and a foamed layer on both sides thereof, the sheet is subjected to a temperature of 90 ° C to 130 ° C.
After passing through three or more rolls provided with a peripheral speed difference while maintaining the temperature at ° C., and performing multi-stage stretching at a stretching ratio of 1.02 to 2 times in the longitudinal direction (longitudinal direction), further at a temperature of 160 ° C. or less The film is stretched 2 to 6 times in the longitudinal direction, and then vertically stretched at a total of 4 times or more, and immediately cooled to room temperature. The stretching temperature at this time is preferably 140 ° C. or lower, which is the lower limit at which the next transverse stretching property is not deteriorated, because the optical density of the film becomes high. By extending the longitudinal stretching in multiple stages and further stretching, the cross-sectional shape of a three-layered film formed by an unstretched film solidified by extrusion cooling and solidified and having a foam layer on both sides with an unfoamed layer as a central layer is maintained. Meanwhile, the generation rate of small, flat and uniform voids can be increased, and the average thickness ratio of the unfoamed center layer to the cross-sectional thickness of the film is 5 to 60.
%, And a film excellent in impact resistance and water vapor barrier properties can be obtained. Subsequently, the stretched film is guided to a tenter, and a temperature of 170 ° C. or less, preferably 150 to
The film is stretched 5 to 10 times in the width direction (transverse direction) at a temperature of 165 ° C., and then relaxed in the width direction by 2 to 20%.
The film is heat-set at a temperature of 0 to 170 ° C. and wound. Further, the lamination of the layer B is performed by supplying the resin of the layer B to another extruder, and
After melting at a temperature of up to 280 ° C., they are merged in a multilayer molding die in a die so as to have a structure of B layer / A layer or B layer / A layer / B layer, and then formed into a laminated sheet. Further, as another laminating method, after the A-layer resin and the B-layer resin are merged in a short pipe upstream of the die, they are formed into a sheet shape with a T-type die, or in the longitudinal direction in the film forming step. A method is used in which the resin of layer B is extrusion-laminated on the stretched stretched film, the laminated film is guided to a tenter, and stretched in the width direction. Specifically, the resin of the layer B is supplied to an extruder and melted at a temperature of 240 to 260 ° C., and then extruded into a sheet shape by a crow mouth-type die, and the molten sheet is heated to 5 to 50 ° C. A between the cooling roll and the rubber roll at a temperature of
The unstretched sheet of the layer is bonded to a film stretched in the longitudinal direction and pressure-bonded.

The film of the present invention may be added or coated with additives such as an antistatic agent, a weathering agent, an anti-fogging agent, a slipping agent and the like according to the purpose. Further, a known treatment such as a corona discharge treatment may be performed in an air atmosphere, an inert gas atmosphere, or the like for the purpose of surface modification. Further, the film of the present invention can be used by performing embossing, printing, extrusion lamination, laminating with another resin film, paper, cloth or the like according to the purpose.

[Method of measuring characteristics and method of evaluating effects]
The method for measuring the characteristic values and the method for evaluating the effects of the present invention are as follows. (1) Intrinsic Viscosity ([η]) 0.1 g of a sample was completely dissolved in 100 ml of tetralin at 135 ° C., and this solution was measured in a thermostat at 135 ° C. with a viscometer. Determine the viscosity. The unit is dl / g. [Η] = S / 0.1 × (1 + 0.22 × S)

(2) Isotactic index (I
I) Vacuum dry the sample at 130 ° C for 2 hours. From now on weight W
Take (mg) sample, place in Soxhlet extractor and extract with boiling n-heptane for 12 hours. Next, the sample is taken out, sufficiently washed with acetone, vacuum-dried at 130 ° C. for 6 hours, and then the weight W ′ (mg) is measured and determined by the following equation. II (%) = W ′ / W × 100

(3) Film thickness The film thickness was measured using a dial gauge type thickness gauge (JIS-B-7509, measuring element 5 mmφ flat type).

(4) Charpy impact strength A value determined by a Charpy impact tester, and the energy E (kg / cm) required in the direction of cutting the test piece.
Is divided by the sample width (cm), and the calculation method is as follows. E = WR (cos β-cos α) W is the hammer weight (kg), R is the distance (cm) from the rotation center axis of the hammer to the center of gravity, α is the hammer lifting angle, and β is the hammer swing angle after cutting the test piece. . The measurement atmosphere is 25 ° C./65% RH.

(5) Optical density (OD) The optical density (OD) was measured using a densitometer TD-504 manufactured by Macbeth. Assuming that the incident light amount is I ₀ and the transmitted light amount is I, OD = −log (I / I ₀ ). This value was represented by a value converted to a film thickness of 30 μm.

(6) Melting temperature (Tm) and melt crystallization temperature (Tmc) Using a differential scanning calorimeter (DSC-2, manufactured by PerkinElmer), 5 mg of a sample was heated from room temperature to 20 ° C./min. The melting endothermic peak accompanying the melting of the crystal when the temperature is increased at a rate is defined as the melting temperature (Tm). Then 280 ° C
The temperature was raised to the melting holding temperature of
During cooling at a cooling rate of / min, the peak temperature of latent heat accompanying crystallization was defined as the melt crystallization temperature (Tmc).

(7) Weight average molecular weight (Mw) The weight average molecular weight (Mw) was determined under the following measurement conditions. Apparatus: Toso-HLC-A-802 type Column: Toso-GMH6 × 2, G2000H8 Solvent: Tetrahydrofuran Temperature: 23 ° C. Flow rate: 1.5 ml / min

(8) Melt flow index (MF)
I) According to ASTM-D-1238, PP and EPC resins are 230 ° C and 2.16 kg, and ethylene-acrylate copolymer resins are 190 ° C and 2.16 kg.
C was measured at 300 ° C. and 1.2 kg.

(9) Thermal deformation temperature of resin Load 18.6k according to the method of ASTM-D-648.
The temperature measured and measured as g / cm ² was defined as the heat distortion temperature.

(10) Heat sealing force The adhesive layers of the films are superimposed on each other and put between a metal plate and silicon rubber heated at 120 ° C. and a pressure of 1 k
Heat sealing was performed under the conditions of g / cm ² and 1 sec, and the strength at the time of peeling 90 ° at a peeling speed of 300 mm / min was measured.

(11) Delamination of Film Surface A cellophane tape (NO405, manufactured by Nichiban Co., Ltd.) having a width of 18 mm was adhered to the film surface by a length of 50 mm, and the film was manually peeled (speed: about 200 mm / sec). Was determined based on the presence or absence of cleavage of the surface layer portion.

(12) Water Vapor Permeability A value measured under the conditions of 40 ° C. and 90% RH in accordance with JIS-Z-0208 and expressed in g / m ² · day / 0.1 mm.

(13) Observation of Film Cross-Section Structure The film cross-section structure was observed using a field emission scanning electron microscope (FE-SEM). The ratio of the unfoamed layer of the center layer was determined by drawing an average boundary line at the interface between the foamed layer and the unfoamed layer, and calculating the average thickness ratio of the unfoamed center layer to the film cross-sectional thickness.

(14) Observation of Shape of Amorphous Resin in Film A cross-sectional structure of the film was observed using a field emission scanning electron microscope (FE-SEM), and the long and short forms of 100 amorphous resins were observed. The average of the ratio was determined.

(15) Gloss (JIS 6-8741 (Method 2) 60 ° mirror gloss) Using a gloss meter (Model VG107, manufactured by Nippon Denshoku Industries Co., Ltd.)
The gloss at the time of 60 ° reflection was measured and determined.

[0043]

EXAMPLES The present invention will be described based on examples and comparative examples. Examples 1-3, Comparative Examples 1-4 Crystallized PP (Intrinsic viscosity: 1.80 dl / g, MI: 4.
PC (heat deformation temperature: 138 ° C, molecular weight: 22000) as an amorphous resin.
And an ethylene-methyl methacrylic acid copolymer (EMMA) having a melting temperature of 110 ° C. as L-PO, mixed in a ratio shown in Table 1, supplied to an extruder, and melted at a temperature of 290 ° C. The mixture was guided to a mold die, extruded into a sheet, wound around a drum at a draft ratio of 2.0 at a temperature of 60 ° C., and cooled and solidified into a sheet. After maintaining the sheet at 110 ° C. and passing it through four rolls provided with a peripheral speed difference, performing multi-stage stretching at a total stretching ratio of 1.25 times in the longitudinal direction, and further, at a temperature of 120 ° C., between rolls having different peripheral speed differences. , And stretched 4.0 times in the longitudinal direction, and immediately cooled to 40 ° C. Next, the stretched film is guided to a tenter and preheated to a temperature of 165 ° C.
The film was stretched 10 times in the width direction at a temperature of 55 ° C., then subjected to a heat treatment at a temperature of 165 ° C. while giving 5% relaxation in the width direction, and then cooled and wound up. The film properties were as shown in Table 2. The film within the scope of the present invention has a three-layer structure having a non-foamed layer as a central layer in a specific range and a foamed layer on both sides thereof, and has a high optical density, excellent concealing properties, impact resistance,
It also had excellent water vapor barrier properties. Films outside the scope of the present invention were inferior in any of optical density, impact resistance, and water vapor barrier properties, and the films aimed at in the present invention could not be obtained.

Examples 4, 5, Comparative Examples 5, 6, 7 In Example 4, L-PO was an ethylene-methacrylic acid copolymer (EMA) having a melting temperature of 100 ° C.
In Example 5, L-PO was EPC having a melting temperature of 132 ° C. and an ethylene content of 4.5% by weight. In Comparative Example 5, the amorphous resin of Example 1 was polystyrene (PS) (thermal deformation temperature: 78).
° C), and in Comparative Example 6, PC of the amorphous resin of Example 1 was used.
Instead of polybutylene terephthalate (PBT) which is a crystalline polyester resin (thermal deformation temperature: 58 ° C.), in Comparative Example 7, PP having a melting temperature of 162 ° C. was used instead of L-PO of Example 1. A film was produced in exactly the same manner as in Example 1 except that the above procedure was repeated. The properties of the obtained film are as shown in Table 2. The film within the scope of the present invention has a three-layer structure having a non-foamed layer in a specific range as a central layer and foamed layers on both sides thereof, and has a high optical density, excellent concealing properties, impact resistance, and water vapor. Films with excellent barrier properties were out of range, but optical density, impact resistance,
The film was inferior in water vapor barrier property and film forming property, and the film intended in the present invention was not obtained.

Examples 6 and 7 The resin composition of Example 1 (layer A) and the resin of layer B were used in Example 6.
In Example 7, EPC having an ethylene content of 3.7% by weight was used. In Example 7, EMA having a methacrylic acid content of 10% by weight was used.
The extruder was supplied to each of two separate extruders, and the A layer resin was 28
At 0 ° C., the B-layer resin is melted at a temperature of 260 ° C., and is joined in a multi-layer die so as to form a B-layer / A-layer / B-layer. A stretched composite film was manufactured. As shown in Table 2, the film has high optical density, excellent concealing property, excellent impact resistance, excellent water vapor barrier property, no delamination between A layer / B layer, and high heat sealing force. Met.

[0046]

[Table 1]

[0047]

[Table 2]

[0048]

As described above, the white biaxially stretched polyolefin film of the present invention is obtained by successively biaxially stretching a resin obtained by adding a specific amorphous resin and a resin having a specific melting temperature to crystalline PP and mixing them. A film having a three-layer structure having a non-foamed layer as a central layer and foamed layers on both sides thereof, and has the following excellent effects. (1) It is white and excellent in concealing property, and the printed undercoat layer can be omitted, and the print finish is good. (2) It has excellent mechanical strength and impact resistance required for secondary processing, and has good properties for various printing papers and adhesive tape bases. (3) Since the shape of the void is small and flat and uniform, it is excellent in the moisture-proof property required for packaging confectionery and snacks. (4) Since no inorganic particles are added, the film is excellent in film forming properties.

Continued on the front page (51) Int.Cl. ⁶ Identification symbol FI C08J 5/18 CES C08J 5/18 CES C08L 23/12 C08L 23/12 // (C08L 23/12 23:08 69:00) B29K 23: 00 105: 04 B29L 7:00 9:00 C08L 23:10 (58) Field surveyed (Int. Cl. ⁶ , DB name) C08J 9/00-9/42 B32B 1/00-35/00

Claims (5)

(57) [Claims] 1. 65 to 93% by weight of crystalline polypropylene
And 5 to 20% by weight of an amorphous resin having a heat deformation temperature of 120 ° C. or more, and 2 to 15% by weight of a polyolefin resin having a melting temperature of 140 ° C. or less. It has a three-layer structure having a foam layer on both sides, the average thickness ratio of the unfoamed central layer to the film cross-sectional thickness is 5 to 60%, and the optical density at a film thickness of 30 μm is 0.40. A white biaxially oriented polyolefin film characterized by the above. 2. The white biaxially stretched polyolefin film according to claim 1, wherein the amorphous resin having a heat distortion temperature of 120 ° C. or higher is a polycarbonate resin having a weight average molecular weight of 40,000 or less. 3. The polyolefin resin having a melting temperature of 140 ° C. or less is ethylene-α-olefin, ethylene-
3. The white biaxially stretched polyolefin film according to claim 1, which is at least one resin selected from α, β-unsaturated carboxylic acid ester copolymers. 4. An ethylene-α-olefin copolymer, ethylene-α, β on at least one surface of the white biaxially stretched polyolefin film (A layer) according to any one of claims 1 to 3. - at least one resin layer comprising a resin (B layer) white ing by stacking color biaxially oriented polyolefin film selected from unsaturated carboxylic acid ester copolymer. 5. Polyolefin resin 2 having a crystalline polypropylene of 65 to 93% by weight, a heat distortion temperature of 5 to 20% by weight of an amorphous resin having a temperature of 120 ° C. or more, and a melting temperature of 140 ° C. or less.
A resin having a composition of about 15% by weight is melt-molded into a sheet at a draft ratio of 1.5 or more to form a three-layer structure having a non-foamed layer as a central layer and foamed layers on both sides thereof. Is stretched at two or more stages by providing a peripheral speed difference between three or more preheating rolls maintained at 90 ° C. to 130 ° C., and further stretched at a temperature of 160 ° C. or less to a total of four times or more. A method for producing a white biaxially oriented polyolefin film, comprising: stretching in a longitudinal direction at a draw ratio, and then stretching in a transverse direction at a temperature of 170 ° C. or less at a draw ratio of 5 times or more.