JPH07308970A - Polyethylene synthetic paper - Google Patents

Abstract

PURPOSE:To form synthetic sheet having excellent orientation, tensile strength, white concealability by so crosslinking a sheetlike molded form containing a predetermined density or more of polyethylene resin and inorganic fine particle powder of a predetermined particle size from the surface toward a center as to reduce a crosslinking degree, and then orienting it. CONSTITUTION:5-100 pts.wt. of inorganic fine powder having a mean particle size of 0.1-10mum of calcium carbonate is contained in 100 pts.wt. of polyethylene resin having a density of 0.935g/cm<3> or more, and blended. It is melted and kneaded by a kneader to a uniform composition, melted and extruded by an extrusion molding machine, then cooled to be solidified to a raw fabric sheet having a thickness of several times as large as a desired synthetic sheet. Then, after the sheet is crosslinked with a gradient so that a crosslinking degree is reduced from the surface toward a center, the sheet is uniaxially or biaxially oriented, and the synthetic paper of desired thickness is molded. Accordingly, the paper has excellent orientation, white concealability, printability and tensile strength.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a polyethylene-based synthetic paper. More specifically, it has writability, printing characteristics, legibility (whiteness / opacity) of written matter or printed matter, strength, foldability, etc., and can be suitably used in fields such as books, maps, and packaging materials. It relates to polyethylene-based synthetic paper.

[0002]

2. Description of the Related Art As a base material for synthetic paper, polypropylene, polyester, polystyrene, polyvinyl chloride, etc. which have been stretched are used. Specific examples include polypropylene or polyester containing an inorganic fine powder (JP-A-48-34235, JP-A-49-12815),
A white layer coated on polypropylene, polyester, polystyrene, or polyvinyl chloride (JP-A-49-1)
2815 publication) and the like. Especially for polypropylene components,
Synthetic paper obtained by stretching a raw sheet containing inorganic fine powder has good whiteness, excellent tensile strength, a large number of fine voids (voids) on the surface, and excellent printability. Widely used.

However, the synthetic paper containing polypropylene as a main component is inferior in foldability (dead hold property) and is not suitable for a packaging material used by folding. Therefore, in order to improve this, polypropylene is mixed with talc and rutile type titanium dioxide, and the mixture is kneaded, extruded from a T die, and brought into contact with a cooling roll with an air knife to form a sheet of synthetic paper (JP-A-48- 96637 publication),
Synthetic paper in which 5 to 50% of a foldability improving agent such as polystyrene is mixed with polypropylene, formed into a sheet, and then stretched, and polypropylene to which inorganic fine powder is added is stuck to one or both sides (Japanese Patent Publication No. 54-17789). However, there is a problem that the manufacturing process is complicated and the cost is high.

[0004]

High-density polyethylene (HDPE) is excellent in dead-holding property, and attempts have been made to produce it by inflation processing into synthetic paper. However, the thickness accuracy decreases due to uneven thickness. Therefore, it is not suitable for high precision printing. It should be noted that it is not possible to use synthetic paper as a synthetic paper because HDPE causes film rupture when stretched similarly to polypropylene and the like, and polyethylene synthetic paper has not been realized so far. Therefore, an object of the present invention is to solve the problem at the time of stretching processing of a polyethylene resin and to provide a polyethylene synthetic paper excellent in dead hold property.

[0005]

Means for Solving the Problems As a result of diligent studies on synthetic paper containing a polyethylene resin as a main component, the present inventors have found that a relatively large amount of inorganic material is used in order to enhance whiteness and opacity (hereinafter referred to as white hiding property). About a raw sheet made by adding fine powder to polyethylene, cross-linking with a gradient so that the degree of cross-linking is high in the surface layer and becomes low inside the thickness direction, stretching process becomes possible and the characteristics as synthetic paper are obtained. They have found that they are sufficiently satisfied, and have completed the present invention.

According to the present invention, the following synthetic paper is provided. 1) Polyethylene resin with a density of 0.935 g / cm ³ or more 10
5 parts by weight of inorganic fine powder having 0 parts by weight and an average particle size of 0.1 to 10 μm
After cross-linking a sheet-shaped molding containing 100 parts by weight so that the degree of cross-linking decreases from the surface toward the center,
Polyethylene-based synthetic paper uniaxially or biaxially stretched. 2) Inorganic fine powder is calcium carbonate, talc, mica,
2. The polyethylene-based synthetic paper according to 1, which is at least one selected from the group consisting of silica and magnesium oxide. 3) The polyethylene-based synthetic paper as described in 2 above, wherein the inorganic fine powder is surface-treated with a silane coupling agent, a titanate coupling agent, or a fatty acid. 4) The polyethylene-based synthetic paper as described in 1 above, wherein crosslinking is performed by electron beam irradiation. 5) The polyethylene synthetic paper as described in 1 above, which contains 20 to 70 parts by weight of inorganic fine powder with respect to 100 parts by weight of the polyethylene resin. 6) The polyethylene-based synthetic paper as described in 1 above, which is formed by laminating three or more layers. 7) The polyethylene-based synthetic paper as described in 6 above, wherein the content of the inorganic fine powder in the surface layer is higher than the content of the inorganic fine powder in the inner layer.

[0007]

DETAILED DESCRIPTION OF THE INVENTION The polyethylene-based synthetic paper of the present invention will be described in detail below. The polyethylene resin used in the synthetic paper of the present invention is crystalline polyethylene produced by the medium- and low-pressure method and has a density of 0.935 g / cm ³ or more, preferably
Melt index (MI) of 0.950 g / cm ³ or more
It is a high density polyethylene (HDPE) of 0.05 g / 10 min or more, preferably 0.5 to 20 g / 10 min. In addition, the HDPE
, Other polyolefins such as low density polyethylene (LDPE) and linear low density polyethylene (LL)
It is also possible to use a mixture of DPE) up to 30% by weight. Additives such as antioxidants, ultraviolet absorbers, antistatic agents, pigments and dyes that are usually used can be appropriately added to these polyethylene resins.

In the present invention, a fine powdery inorganic compound (inorganic fine powder) is added to the synthetic paper in order to impart a white hiding property. Examples of the inorganic compound include metal compounds such as magnesium oxide, calcium oxide, aluminum oxide (alumina), diatom dioxide (silica), metal oxides such as titanium dioxide, magnesium hydroxide, calcium hydroxide, aluminum hydroxide, orthotitanic acid. Metal hydroxides such as, metal carbonates such as potassium carbonate, sodium hydrogen carbonate, sodium carbonate, magnesium carbonate, calcium carbonate, metal sulfates such as magnesium sulfate, calcium sulfate, barium sulfate and aluminum sulfate, sodium metasilicate, anhydrous In addition to silicates such as sodium silicate, lithium metasilicate, anhydrous lithium silicate, potassium metasilicate, anhydrous potassium silicate, magnesium silicate, magnesium metasilicate, calcium silicate, and aluminosilicate, zeolite, ma Ca, talc, bentonite, hydrotalcite and the like. Among these inorganic fine powders, calcium carbonate, talc, mica, silica, magnesium oxide and the like are preferable, and calcium carbonate is particularly preferable. These inorganic fine powders may be natural products or synthetic products, and may be used in combination of two or more kinds.

The average particle size of the inorganic fine powder is 0.1-1.
0 μm, preferably 0.5-5 μm, more preferably 0.8
The one having a thickness of 3 μm is used. If the average particle size exceeds 10 μm, the surface becomes rough, which hinders the subsequent printing process and deteriorates the printability. On the other hand, if it is less than 0.1 μm, void formation due to stretching becomes difficult to occur, and good physical properties cannot be obtained.

The blending amount of the inorganic fine powder is 5 to 100 parts by weight, preferably 25 to 70 parts by weight, based on 100 parts by weight of the polyethylene resin. When the content of the inorganic fine powder is less than 5 parts by weight, the generation of voids is small, so the writability and printability are poor, and the white hiding property is also reduced. On the other hand, when it exceeds 100 parts by weight, the melt tension is lowered, and the sheet is broken during stretching, which makes it difficult to process it into a paper form.

In the present invention, it is preferable to use the inorganic fine powder treated with the surface treating agent in order to suppress the detachment (drainage) of the inorganic fine powder especially during the stretching process. Surface treatment agents include vinyltriethoxysilane, vinyltris (β
-Methoxyethoxy) silane, γ-aminopropyltriethoxysilane and other silane coupling agents, isopropyl trioctanoyl titanate, isopropyl isostearoyl diacrylic titanate, dicumylphenyloxyacetate titanate and other titanate coupling agents, stearic acid, olein Acids, fatty acids such as palmitic acid, and metal salts thereof can be used.

Next, a method for producing the polyethylene-based synthetic paper according to the present invention will be described. After dry blending the above-mentioned polyethylene resin, inorganic fine powder and other additives, a conventionally known method, that is, a batch type kneader, a Banbury mixer, a brabender, a kneading roll,
A uniform composition is obtained by melt-kneading using a kneading machine such as a single-screw extruder or a twin-screw extruder.The composition is supplied to an extruder and melt-extruded, followed by cooling and solidification to obtain a desired synthetic paper. A raw sheet having a thickness of 5 to 20 times the thickness is obtained. Next, the raw sheet is subjected to crosslinking with a gradient so that the degree of crosslinking decreases from the surface toward the center, and then uniaxially or biaxially stretched to obtain a synthetic paper having a desired thickness.

For forming the raw sheet, various conventionally known methods can be adopted. For example, the kneaded product can be extruded from a T-die to form a smooth original sheet. If a twin-screw extruder equipped with a T-die is used, kneading of polyethylene resin and inorganic fine powder and sheet forming can be continuously performed in one step.

The thickness of the original sheet may be such that it can be crosslinked so as to have a gradient in the degree of crosslinking in the thickness direction from both surfaces toward the center, the stretching ratio and the thickness of the synthetic paper after stretching. Determined by It is usually in the range of 210 to 2000 μm, preferably 400 to 1000 μm.

In the present invention, it is preferable to use an original sheet having a multi-layer structure because the gradient of cross-linking degree, whiteness hiding property and dead hold property can be easily adjusted.
That is, in order to avoid the influence of the decrease of the melt tension due to the addition of the inorganic fine powder on the stretchability, printability as the surface layer, a layer with a high inorganic fine powder content for improving the white hiding property etc. is used, and the stretching of the surface layer as the inner layer. It is preferable to use a layer having a low inorganic fine powder content that compensates for the deterioration of the properties.

For example, as the surface layer, inorganic fine powders 20 to 5 are used.
0 parts by weight (100 parts by weight of polyethylene resin, the same applies hereinafter), preferably 30 to 40 parts by weight, containing 1
A three-layer structure using two layers of 0 to 25 μm, preferably 10 to 15 μm, and having a thickness of 20 to 40 μm, preferably 25 to 30 μm, containing 30 parts by weight or less of inorganic fine powder as an inner layer. , But the number of layers is 3
The number of layers is not limited to four, and a structure of four or more layers is also possible.

Such a multi-layered original sheet can be produced by a method of laminating a sheet of each layer by a calendar method, a laminating method or the like, or a method of co-extruding the kneading composition of each layer. it can.

In the cross-linking of the single-layer or multi-layer original sheet, it is necessary to cross-link from both surface sides so that the degree of cross-linking decreases toward the center in the thickness direction. The degree of crosslinking is generally represented by a gel fraction. In the present invention, when a sample is extracted with boiling p-xylene and the weight% of the insoluble portion is represented as a gel fraction, the crosslinking gradient of the original sheet is It is preferable that the gel fraction with the lowest degree of crosslinking is 0 to less than 5%, and the gel fraction of the surface layer with the highest degree of crosslinking on both sides is 5% or more, particularly 20 to 70%. In the case of a raw sheet having a three-layer structure, it is preferable that it is composed of a minimum cross-linking degree layer having a gel fraction of 0% in the central layer and cross-linking layers on both sides. Ratio of uncrosslinked layer: Crosslinked layers on both sides = 1: 0.1 to 1
The range of 0 is desirable. Further, it is preferable that the crosslinked layers on both sides have the same degree of crosslinking.

The cross-linking is not carried out so that the cross-linking degree decreases in the middle direction in the thickness direction of the raw sheet, and particularly when the gel fraction of the lowest cross-linking degree layer exceeds 5%, the stretching process is carried out. Stretching tension becomes large and it becomes difficult to stretch. The degree of cross-linking of the cross-linked surface layers on both surfaces is 2 in terms of gel fraction.
If it is less than 0%, the stretching process is not performed uniformly, while
When the gel fraction exceeds 70%, the film is broken during the stretching process and smooth stretching cannot be performed. Furthermore, it is not preferable that only one side in the thickness direction of the raw fabric is cross-linked, or if the degree of cross-linking on both sides is significantly different, the film is likely to break during stretching.

As a method for performing crosslinking with such a crosslinking degree gradient, for example, α rays from both sides of the original sheet,
Ionizing radiation such as β-ray (electron beam) and γ-ray is used, and a method of irradiating with an electron beam is preferably used because of its easy handling.

The electron beam irradiation conditions vary depending on the thickness of the raw material, the type of resin, the molecular weight, and the molecular weight distribution, but the electron beam irradiation amount is usually 20 to 500 kGy, preferably 50.
~ 200 kGy. Irradiation may be performed on the front and back sides of the original material simultaneously, on the front and back sides separately, or on the front and back sides several times. The raw material upon irradiation may be in a melted state after extrusion or in a cooled and solidified state. The irradiation dose is preferably the same on both sides. Further, the electron beam irradiation amount, which is closely related to the gradient of the degree of crosslinking, can be adjusted by adjusting the applied voltage with respect to the original thickness, masking with a shield plate, or the like.

In the present invention, it is possible to know the relationship between the electron beam irradiation dose and the crosslinking degree gradient (distribution state) of the original sheet by using a large number of thin test pieces. That is, for example, when the thickness of the material to be irradiated is 500 μm, 20 μm
Tightly overlapping 25 thin films with a thickness of 5 to form a test piece with a thickness of approximately 500 μm, and irradiating the same amount of electron beams from both sides in the thickness direction to the cross-linked test piece with a thickness of 20 μm.
It is possible to know the distribution state of the crosslinking degree in the thickness direction of the test piece by separating it into 25 films of m and measuring the crosslinking degree of each. Similarly, in the case of a multilayer sheet, the distribution state of the degree of crosslinking can be known by closely superimposing corresponding thin films.

The electron beam irradiation can be carried out in the presence of air, but it is preferably carried out in an atmosphere of an inert gas such as nitrogen, argon or helium.

In the present invention, the crosslinked raw fabric sheet is then heated and stretched uniaxially or biaxially at a predetermined magnification by a usual roll method, tenter method or tubular method. Biaxial stretching may be simultaneous or sequential stretching.

The stretching temperature is generally below the melting point of the polyethylene resin, preferably in the range from the softening point to the melting point of the resin, specifically 70 to 135 ° C., preferably 100.
~ 130 ° C. If the stretching temperature is lower than the softening point, the resin is not sufficiently softened and uniform and stable stretching cannot be performed. On the other hand, if the melting point is exceeded, the strength cannot be improved by stretching.

The stretching ratio is preferably 3 times or more, and more preferably 4 times or more in one direction or both in the longitudinal and transverse directions. If the stretching ratio is less than 3 times, uniform stretching is difficult and a film having a uniform thickness cannot be obtained.

Since the stretched synthetic paper has heat shrinkability, it is aged at a temperature below its melting point, for example, 110 to 140 ° C., so that the heat shrinkage is 1.5% or less, preferably
It is preferably set to 1.0% or less.

[0028]

EXAMPLES The present invention will be specifically described with reference to Examples and Comparative Examples, but the present invention is not limited to the following examples. The raw materials and additives used in the following examples are as follows. Polyethylene: (1) HDPE-1: Density 0.953 g / cm ³ , MI 0.8 g / 10
Min, (2) HDPE-2: Density 0.959 g / cm ³ , MI5.0
g / 10 minutes, (3) HDPE-3: Density 0.953 g / cm ³ , MI
1.2 g / 10 minutes. Polypropylene: (1) PP: density 0.91 g / cm ³ , MI ³ g / 10 minutes. Additives: (1) Calcium carbonate: particle size 1.5 μm, treated with magnesium stearate, (2) Antioxidant: n-octadecyl-3- (4′-hydroxy-3 ′, 5′-di-t -Butylphenyl) propionate, (3) Neutralizer: calcium stearate.

Example 1 HDPE-1 (100 parts by weight), calcium carbonate (45
Parts by weight), an antioxidant (0.05 parts by weight), and a neutralizing agent (0.05 parts by weight) are blended in a Henschel mixer, and then the mixture is melt-kneaded in a twin-screw extruder set at 220 ° C. to pelletize the outer layer. Used as raw material.

After HDPE-1 (100 parts by weight), antioxidant (0.05 parts by weight), and neutralizing agent (0.05 parts by weight) were blended in a Henschel mixer, this mixture was mixed with 220 parts.
It was melt-kneaded by a twin-screw extruder set to ℃ and pelletized to obtain a raw material for the inner layer.

From each of the above pellets, a three-layer multi-manifold type T-die extrusion sheet molding machine was used to extrude at an extrusion temperature of 220 ° C. to a thickness of 600 μm (outer layer / inner layer /
Outer layer = 150 μm / 300 μm / 150 μm) to obtain a raw sheet.

This raw sheet was irradiated with an electron beam of 100 kGy on each of the front and back under an atmosphere of nitrogen gas under the condition of 250 KeV-4 mA to crosslink the raw sheet.

From this crosslinked original sheet, 10 × 10 c
A sample of m was cut out and heated to 130 ° C., and sequentially drawn by a batch drawing machine (manufactured by Toyo Seiki Co., Ltd.) in the longitudinal direction 1.2 times and in the transverse direction 6 times to obtain a synthetic paper having a thickness of 75 μm.

Example 2 A synthetic paper was obtained in the same manner as in Example 1 except that 9 parts by weight of calcium carbonate was blended in the raw material for the inner layer.

Example 3 A synthetic paper was obtained in the same manner as in Example 1 except that HDPE-2 was used in place of HDPE-1 as the raw material for the outer layer.

Example 4 HDP as the raw material for the outer layer and the raw material for the inner layer in Example 1
The same operation as in Example 1 was carried out except that HDPE-2 was used instead of E-1, to obtain a synthetic paper.

Example 5 Using the raw material for the outer layer and the raw material for the inner layer used in Example 1, a three-layer multi-manifold type T-die extrusion sheet molding machine was used to extrude at an extrusion temperature of 220 ° C. to a thickness of 80.
0 μm (outer layer / inner layer / outer layer = 200 μm / 400 μm /
A raw sheet of 200 μm) was obtained. On this original sheet,
250 KeV under nitrogen gas atmosphere using electron beam irradiation device
Under the condition of -4.5 mA, 110 kGy electron beam was irradiated to each of the front and back to crosslink. 1 from this crosslinked raw sheet
A 0 × 10 cm sample was cut out and heated to 130 ° C., and then sequentially stretched by a batch stretching machine (manufactured by Toyo Seiki Co., Ltd.) in the longitudinal direction 3 times and in the transverse direction 5 times to obtain a synthetic paper having a thickness of 50 μm.

Example 6 HDPE-3 (100 parts by weight), calcium carbonate (40
Parts by weight), an antioxidant (0.05 parts by weight), and a neutralizing agent (0.05 parts by weight) were blended in a Henschel mixer, and this mixture was melt-kneaded and pelletized by a twin-screw extruder set at 220 ° C. . The pellets were extruded using a T-die extrusion sheet molding machine at an extrusion temperature of 220 ° C. to obtain a single-layer sheet raw material having a thickness of 600 μm.

This raw sheet was irradiated with an electron beam of 100 kGy on both sides under the condition of 250 KeV-4.5 mA in a nitrogen gas atmosphere using an electron beam irradiation apparatus to crosslink. A sample of 10 × 10 cm was cut out from this crosslinked raw sheet and heated to 130 ° C., and then sequentially stretched 1.2 times in the longitudinal direction and 6 times in the lateral direction by a batch stretching machine (manufactured by Toyo Seiki Co., Ltd.) to synthesize a thickness of 75 μm. Got the paper.

Comparative Example 1 Using the raw material for the inner layer of Example 1, the pellets were extruded using a T-die extrusion sheet molding machine at an extrusion temperature of 220 ° C.,
A 600 μm thick single-layer sheet stock was obtained. The raw sheet was irradiated and stretched in the same manner as in Example 1 to obtain a synthetic paper.

Comparative Example 2 The same operation as in Example 1 was carried out except that the electron beam was not irradiated, but the film was broken during stretching.

Table 1 shows the constitutions of the raw sheet layers, the stretching conditions, the film thickness after stretching and the like in each of the above Examples and Comparative Examples.

[0043]

[Table 1]

Regarding the raw sheets produced in Examples 1 to 6 and Comparative Examples 1 to 2, the degree of crosslinking on the irradiation surface and the state of crosslinking inside the sheet in the thickness direction have a thin film having a corresponding composition and a thickness of 20 μm. 30 sheets are stacked and thickness is 600μm
The thin film was irradiated with an electron beam under the same conditions to measure the gel fraction as the degree of crosslinking of each thin film and the composition ratio of the thickness of the crosslinked layer and the uncrosslinked layer. The gel fraction was measured according to ASTM D2765 (method A). As a result, the maximum degree of crosslinking of the thin films on both sides of the irradiated surface in each example was 50%, and the lowest degree of crosslinking inside the thickness direction was 0.
%, And the compositional ratio of the thickness of the crosslinked layer and the thickness of the uncrosslinked layer was as follows: crosslinked layer: uncrosslinked layer: crosslinked in the raw sheets of Examples 1 to 4 and Comparative Examples 1 and 2. Layer = 1: 1.5:
1, the raw sheet of Example 5 had a crosslinked layer: uncrosslinked layer: crosslinked layer = 1: 2: 1.

The synthetic paper obtained in each example was evaluated for stretchability, white hiding power, printability and dead hold property, and tensile strengths in the machine direction (MD) and the transverse direction (TD) were measured. The results are shown in Table 2. The evaluation method and measurement method are as follows.

1) Stretchability: The state in a batch stretching machine was evaluated. ∘: good stretchability, ◯: slightly inferior stretchability, changes such as slowing the stretching speed are necessary, x: film cannot be stretched and ruptures. 2) Whiteness hiding property: Synthetic paper was placed on a white paper on which characters or figures were drawn, and the whiteness and the appearance of characters were evaluated. ◎ ... white opaque, ○ ... slightly translucent, ×
… Transparent. 3) Printability: An arbitrary pattern was drawn on the surface of the synthetic paper with a commercially available water-based pen, and immediately wiped off with a dry gauze to evaluate the residual condition of the pattern. ○: Ink remained, ×: No ink remained. 4) Dead hold property: Fold the sample in two on a glass plate, put the glass plate on it, and further 500gf
After applying a load by applying the weight, the load was immediately removed and the bending condition was visually evaluated. ○: No return after bending, ×: Return by 90 ° or more after bending. 5) Tensile strength: Measured according to ASTM D882.

[0047]

[Table 2] Example No. Stretchability White color printability DH property * Tensile strength (kg / cm ² ) Concealment property (MD × TD) Example 1 ◎ ○ ○ ○ 630 × 1620 Example 2 ◎ ◎ ○ ○ 620 × 1580 Example 3 ◎ ○ ○ ○ 580 × 1210 Example 4 ◎ ○ ○ ○ 400 × 1050 Example 5 ○ ◎ ○ ○ 840 × 1330 Example 6 ○ ◎ ○ ○ 350 × 980 Comparative Example 1 ◎ × × ○ 900 × 1940 Comparative Example 2 × − − − − * DH property: Dead hold property

The synthetic paper (Comparative Example 1) stretched after cross-linking a single layer of HDPE by electron beam irradiation is excellent in stretchability and tensile strength, but inferior in white hiding property, printability and dead hold property. There is. Further, when a multilayer sheet in which the outer layer is HDPE containing inorganic fine powder and the inner layer is HDPE is stretched without cross-linking, a film breaks during stretching (Comparative Example 2). On the other hand, the synthetic paper of the present invention (Examples 1 to 6)
It can be seen that is excellent in stretchability, whiteness hiding property, printability, dead hold property, and tensile strength.

[0049]

As described above, the polyethylene-based synthetic paper of the present invention has a white hiding property, printability, tensile strength, etc.
It is also excellent in dead hold (foldability), books,
It can be used for a wide range of purposes such as catalogs, maps, packaging materials, posters used outdoors, waterproof notebooks, etc.

Claims (7)

[Claims] 1. A sheet-shaped molded article containing 100 parts by weight of a polyethylene resin having a density of 0.935 g / cm ³ or more and 5 to 100 parts by weight of an inorganic fine powder having an average particle size of 0.1 to 10 μm,
Polyethylene-based synthetic paper obtained by uniaxially or biaxially stretching after crosslinking so that the degree of crosslinking decreases from the surface toward the center. 2. The polyethylene-based synthetic paper according to claim 1, wherein the inorganic fine powder is at least one selected from the group consisting of calcium carbonate, talc, mica, silica and magnesium oxide. 3. The inorganic fine powder is a silane coupling agent,
The polyethylene-based synthetic paper according to claim 2, which is surface-treated with a titanate coupling agent or a fatty acid. 4. The polyethylene-based synthetic paper according to claim 1, wherein the crosslinking is carried out by electron beam irradiation. 5. The synthetic polyethylene paper according to claim 1, which contains 20 to 70 parts by weight of inorganic fine powder with respect to 100 parts by weight of the polyethylene resin. 6. The polyethylene-based synthetic paper according to claim 1, which is formed by laminating three or more layers. 7. The polyethylene-based synthetic paper according to claim 6, wherein the content of the inorganic fine powder in the surface layer is higher than the content of the inorganic fine powder in the inner layer.