Manufacturing method of paper substrate and paper substrate

Abstract

To provide a paper substrate having a waterproofness and heat resistance.SOLUTION: A paper substrate includes pulp fibers, the pulp fibers are coated with a coating layer that includes a metal oxide or silicon oxide (SiOxCyHz), and a surface layer of the coating layer comprises an alkyl group.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a paper substrate and a method for manufacturing the paper substrate. [Background technology]

[0002] Paper containers are made from renewable natural resources and are more biodegradable than plastic or polystyrene foam containers, making them widely used as environmentally friendly containers. Paper containers tend to be strongly influenced by the properties of the paper substrate, and paper substrates made of untreated pulp fibers do not provide water resistance or gas barrier properties. For this reason, paper containers made from composite paper substrates laminated with waterproof resins or gas barrier layers are widely used. For example, waterproofing and gas barrier properties have been imparted by laminating metal foil such as aluminum to the paper substrate or by laminating resins such as polyethylene or polyethylene terephthalate to the paper substrate. For example, Patent Document 1 discloses a paper container made from a paperboard substrate to which an inorganic compound vapor-deposited film, a thermoplastic resin layer, or a polyethylene resin layer has been laminated. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2000-95233 Summary of the Invention [Problem to be solved by the invention]

[0004] However, paper containers in which resins such as polyethylene or polyethylene terephthalate are laminated onto a paper base material have a heat resistance temperature of only about 200°C, and such paper containers could not be used for high-temperature cooking using ovens, etc.

[0005] The present invention has been made in view of the above problems, and has an object to provide a paper substrate that is waterproof and heat resistant. [Means for solving the problem]

[0006] The paper base material for achieving the above object is characterized in that it contains pulp fibers, the pulp fibers are coated with a coating layer containing metal oxide or silicon oxide, and the surface layer of the coating layer is an alkyl group. [Effects of the Invention]

[0007] By providing the paper substrate in this manner, it is possible to provide a paper substrate that is waterproof and heat resistant. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a schematic diagram of a paper substrate 10. [Figure 2] 1 is a schematic diagram of fibers 11 that make up a paper substrate 10. FIG. [Figure 3] This is an example of a film forming device that coats fibers with a coating layer. [Figure 4] FIG. 2 is a schematic diagram illustrating a film forming process. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[0010] Figure 1 is a cross-sectional view of paper 100 according to this embodiment. Figure 2(a) is an enlarged view of a paper substrate 10 included in the paper 100 of Figure 1. Figure 2(b) is a cross-sectional view of the paper substrate 10. The paper substrate 10 is composed of pulp fibers 11, a coating layer 12 provided so as to cover the pulp fibers 11, and a surface layer 13 having alkyl groups provided on the surface of the coating layer 12.

[0011] Pulp fibers 11 are made by extracting plant fibers, dispersing them in water, and intertwining them into a thin, flat layer using the water as a medium. They are generally obtained by pulping hardwood fibers and coniferous tracheids. Paper 100 can be considered a sheet made by intertwining the pulp fibers 11. Chemically, the pulp fibers 11 are primarily composed of polysaccharides, most of which are cellulose. Such paper 100 can be used to make paper containers, etc., and any type of paper can be used depending on the container shape, desired rigidity, etc. Examples of well-known types of paper that can be used include fine paper, construction paper, art paper, coated paper, pure white roll paper, kraft paper, water-resistant label paper, cob base paper, cardboard, ivory paper, and paperboard such as manila cardboard, milk carton base paper, cup base paper, synthetic paper, and clay-coated paper.

[0012] The coating layer 12 functions as a moisture barrier layer that prevents moisture from penetrating the pulp fibers 11 and is made of an inorganic compound such as a metal oxide or silicon oxide (SiOxCyHz; y / x<1.6). Examples of metal oxide films include Al2O3, SiO2, TiO2, Nb2O5, Ta2O5, ZrO2, MgO, and ZnO. Silicon oxides, as used here, do not include silicone. This is because silicon oxides other than silicone (SiOxCyHz; y / x<1.6) have better moisture barrier properties than silicone, which has a high carbon content. The coating layer 12 may be a single layer, or it may be composed of multiple layers of inorganic compounds. From the viewpoint of water resistance, the thickness of the coating layer 12 is preferably 5 nm to 500 nm.

[0013] The surface layer 13, which is formed on the outermost surface of the coating layer 12, has an alkyl group and is chemically bonded to the coating layer 12. Examples of alkyl groups include -CH3, -C2H5, and -C3H8. By providing the surface layer 13 with an alkyl group, the surface layer 13 can achieve a water-repellent contact angle of 90 degrees or more and 180 degrees or less. The contact angle is one of the indicators used to evaluate wettability and is a numerical representation of the degree of swelling (height of the liquid) of a droplet formed when a liquid is dropped onto a solid. The surface layer 13, which is made of an alkyl group, has a thickness of 0.1 nm or more and 1.0 nm or less.

[0014] In this way, the surface of the pulp fibers 11 constituting the paper 100 is coated with a coating layer 12, which serves as a moisture barrier layer that prevents moisture from penetrating the pulp fibers 11. Furthermore, by forming the surface layer 13 of the coating layer 12 as a layer containing alkyl groups, water repellency can be imparted. In other words, by endowing the pulp fibers 11 with water-repellent and moisture-permeation-preventing properties, the paper 100 using the pulp fibers 11 can be made waterproof. By using such paper 100 for paper cobs, etc., it is possible to prevent liquid leakage even when water is left inside for a long period of time. Furthermore, because the paper 100 of the present invention does not use metal foil or resins such as polyethylene or polyethylene terephthalate, it can be used in a microwave oven and is also heat-resistant enough to be used for high-temperature cooking in an oven.

[0015] Next, a method for providing a coating layer 12 and a surface layer 13 having an alkyl group so as to cover the pulp fiber 11 will be described. Such coating layer 12 and surface layer 13 can be provided using atomic layer deposition (ALD). The ALD method, as disclosed in, for example, Patent Document (JP-A-2009-525406), is a method for forming a thin film in atomic layer units by repeatedly alternately introducing and exhausting two or more types of source gases, thereby causing a reaction between the source materials adsorbed on the surface to be film-formed.

[0016] 3 is a schematic diagram of an atomic layer deposition apparatus used to form a thin film layer by the ALD method. The atomic layer deposition apparatus is equipped with a film formation chamber 1 that maintains a vacuum state inside, and an exhaust system 2 consisting of a vacuum pump and the like that evacuates the film formation chamber 1. The apparatus is also equipped with a source gas supply unit 3 and an oxidation gas supply unit 4 that supply a source gas and an oxidation gas to the film formation chamber 1. The source gas or a mixed gas of the source gas and a rare gas is introduced through a source gas inlet 5, and the oxidation gas or a mixed gas of the oxidizing gas and a rare gas is introduced through an oxidation gas inlet 6.

[0017] The raw material gas supply unit 3 and the oxidizing gas supply unit 4 are each provided with a gas valve that switches the gas supply on and off and a mass flow controller (not shown) that controls the gas flow rate, allowing the supply of raw material gas and oxidizing gas to be switched on and off. The film formation chamber 1 has a stage 7 on which paper 100 made of pulp fibers 11 without a coating layer 12 and a surface layer 13 is placed, and heaters 8 that can heat paper 100 are provided around the stage.

[0018] Next, a method for forming a moisture barrier layer using the atomic layer deposition apparatus shown in FIG. 3 will be described using FIG. 4. Four film formation steps are shown in FIGS. 4(a) to 4(d). This embodiment will be described using limited gas species, film materials, etc., but the scope of the present invention is not limited to this. First, the film formation chamber 1 is opened in advance, and the paper 100 prepared is placed on a stage inside the film formation chamber 1, and the inside of the film formation chamber 1 is evacuated. At the same time, the paper 100 is heated by a heater.

[0019] When forming a coating layer 12 made of an Al2O3 film as a moisture barrier layer, first the pressure inside the film formation chamber 1 is set to about 10 Pa and the pulp fibers 11 are set to about 200°C. Once the temperature reaches about 200°C, the gas valve of the raw material gas supply unit 3 is opened and raw material gas, TMA molecules (trimethylaluminum ((CH3)3Al)) and inert gas argon, are introduced into the film formation chamber 1 through the raw material gas inlet 5. The TMA molecules react with the hydroxyl groups (-OH groups) terminating on the surface of the pulp fibers 11, causing chemical adsorption to the surface (Figure 4(a)).

[0020] After the TMA molecules are chemically adsorbed onto the surface of the pulp fiber 11, the gas valve of the raw material gas supply unit 3 is closed and unnecessary raw material gas is purged. At this time, the surface of the pulp fiber 11 is terminated with methyl groups (-CH3 groups) because the TMA molecules have been adsorbed (Fig. 4(b)).

[0021] Next, the gas valve of the oxidizing gas supply unit 4 is opened, and HO, an oxidizing gas, is supplied into the film formation chamber 1 from the oxidizing gas inlet 6, and the terminal methyl groups (-CH groups) react with HO to form AlO (Fig. 4(c)). After that, the gas valve of the oxidizing gas supply unit 4 is closed, and unnecessary oxidizing gas is purged (Fig. 4(d)).

[0022] 4, four cycles of (a) introduction of source gas, (b) purging, (c) introduction of reactive gas, and (d) purging are repeated until a coating layer 12 of the desired thickness is formed, and film formation is finally completed with step (b). Completing film formation with the step shown in FIG. 4(b) allows the surface layer 13 of the coating layer 12 to be made of methyl groups (-CH3 groups).

[0023] When using Al2O3 deposited by the ALD method as the coating layer 12 serving as a moisture barrier layer, the thickness of the coating layer 12 is preferably set to 5 nm or more and 500 nm or less. A thin film thickness results in an insufficient moisture barrier, while a thick film may cause film cracking due to film stress. When depositing Al2O3 films using the ALD method, the deposition rate is approximately 1 Å per cycle. To achieve a physical thickness of 10 nm, approximately 100 cycles of the steps shown in Figures 4(a) to 4(d) are required. Furthermore, the ALD method allows for the formation of 1 Å atomic layers one by one, making it easy to control the film thickness. Furthermore, because films deposited by the ALD method have excellent step coverage, they can be formed without the influence of oblique shadowing, even on pulp fibers with complex geometries, resulting in a dense, defect-free film covering more than 90% of the surface of the pulp fibers 11. This means that a film with extremely high moisture barrier properties (water resistance) can be formed.

[0024] As described above, according to this embodiment, by coating pulp fibers 11 with coating layer 12 and surface layer 13 having alkyl groups, paper 100 can be provided using paper base material 10 that is waterproof and heat resistant.

[0025] Next, we will explain the mechanism of liquid penetration into paper. The mechanism of liquid penetration can be divided into two phenomena: (1) diffusion penetration into the fibers and inter-fiber bonds, and (2) capillary penetration into the voids between fibers.

[0026] These penetrations cause the fibers to increase in thickness, loosen the bonds between the fibers, and the interaction between the two causes an increase in the volume of voids between the fibers, weakening the bonds between the fibers, reducing the strength of the paper and allowing liquids to pass through.

[0027] The coating layer 12 has moisture barrier properties and serves to prevent the penetration of phenomenon (1) into the pulp fibers. In addition, the surface layer 13, which has alkyl groups on the outermost surface, has a contact angle with water greater than 90° and is water-repellent, preventing the penetration of phenomenon (2) into the pulp fibers. The phenomenon in which a liquid penetrates into a small space regardless of gravity or the direction of movement is called capillary action. The surface tension of the liquid applies a force in the direction that the liquid tries to shrink, while at the same time, an adhesive force acts between the liquid and the surface (wetting). In the case of a surface that is easy to wet, the liquid is attracted to the surface and penetrates. Penetration can be prevented by making the surface water-repellent (difficult to wet).

[0028] Generally, if we consider a model in which an infinite number of capillaries are bundled together and placed on the surface of water, and the difference in height between the liquid inside and outside the capillary is h (the higher side inside the tube is positive), it is known to be expressed as follows.

[0029] Difference in liquid level inside and outside the capillary tube: h (the higher side inside the tube is positive)

[0030]

number

[0031] Surface tension of the liquid: T Contact angle between the liquid and the pipe material: θ Difference in liquid level inside and outside the pipe: h (the higher level inside the pipe is positive) Liquid density: ρ ·Gravitational acceleration: g

[0032] The smaller the contact angle, the smaller h becomes, and the less liquid penetrates. When h<0, that is, when the contact angle between the liquid and the pipe material is greater than 90°, water penetration can be suppressed.

[0033] Similar dry film deposition methods to ALD include physical vapor deposition (PVD) and chemical vapor deposition (CVD). When using PVD, thin films typically grow in a columnar or island-like pattern. This can lead to grain boundaries and defects such as pinholes, which are inseparable from the projection effect even when the film is thickened. This makes it difficult to improve gas barrier properties in proportion to the film thickness, making it difficult to achieve high moisture barrier properties. Furthermore, thicker films can cause internal stress in the thin film to become significant, leading to cracks and other problems, which can actually lead to a decrease in gas barrier properties.

[0034] Film deposition using CVD methods, such as plasma CVD, which uses an electric field applied at a high frequency of 13.56 MHz, is expected to exhibit high moisture barrier properties due to the low frequency of film defects in the film, but has the disadvantage of a slow film deposition speed.Even if film deposition conditions such as pressure are changed to increase the film deposition speed, or the applied frequency is changed by using the substrate as the cathode, film defects such as grain boundaries occur, and the film density does not increase, making it difficult to achieve high moisture barrier properties.

[0035] The ALD method forms atomic layers one by one at a rate of about 1 Å / cycle, making it easy to control film thickness and providing excellent step coverage. Therefore, even with complex pulp fiber shapes, a dense, defect-free film can be obtained that covers more than 90% of the pulp fiber surface without any oblique shadowing effect, making it possible to form a film with extremely high moisture barrier properties. In other words, the ALD method is preferable for forming the paper substrate 10 of this embodiment.

[0036] In this embodiment, an example has been described in which a covering layer 12 and a surface layer 13 are provided on paper 100 manufactured by intertwining pulp fibers 11, but paper 100 may also be manufactured by intertwining pulp fibers 11 after providing the covering layer 12 and the surface layer 13 on pulp fibers 11.

[0037] In addition, the coating process may be performed on only one side, instead of both sides, i.e., the coating layer 12 and the surface layer 13 may be provided on only one side of the paper 100. Alternatively, the coating layer 12 may be provided on both sides of the paper 100, and the surface layer 13 may be provided on only one side.

[0038] <Example> Next, the present invention will be described in detail based on examples. The following evaluation methods (1) to (3) were used.

[0039] (1) Contact angle evaluation One drop (2 μl) of distilled water was dropped onto the sample surface, and the contact angle was measured one second later using a contact angle measuring device (DropMaster 300, manufactured by Kyowa Interface Science Co., Ltd.).

[0040] (2) Evaluation of Cobb water absorbency The Cobb water absorbency was measured by the Cobb method in accordance with JIS P 8140. That is, 100 ml of distilled water was brought into contact with the surface of the coating layer, and the weight of water absorbed per unit area was measured after 120 seconds.

[0041] (3) Waterproofing evaluation To evaluate waterproofness, a paper container measuring 10 cm long x 10 cm wide x 5 cm deep was prepared. The surface of the coating layer was positioned on the inner surface of the paper container. 0.5 L of water was poured into the paper container, which was then heated to 250°C in an oven. After the paper container returned to room temperature, the state of water seepage and the shape of the box were visually observed, and waterproofness was evaluated based on the following criteria. ○: Water has not seeped onto the outside of the container or onto the inside of the container above the water level, and the shape of the box is maintained. △: Water has not seeped onto the outside of the container, and the shape of the box is maintained (however, water has seeped into the inside of the container). ×: Water seeps out onto the outside of the container, and the box shape cannot be maintained.

[0042] Example 1 The paper 100 produced in this example will be described using the apparatus shown in Figure 3. The pulp fibers 11 were made of high-quality paper with a basis weight of 209.5 g / m and a thickness of 250 μm. An Al2O3 film was formed on the pulp fibers 11 by atomic layer deposition. TMA and HO were used as the raw material gas and reactive gas, respectively. The pulp fibers 11 were heated to 200 °C, and the four steps shown in Figure 4 ((1) introduction of raw material gas, (2) purging, (3) introduction of reactive gas, and (4) purging) were repeated 10 times to form a film with a thickness of approximately 10 nm, with the final step (2) completing the film formation. The thickness, elemental composition, and surface coverage of the coating layer 12 formed on the pulp fiber surface of the pulp fiber 11 were determined by cutting out the pulp fiber 11 on which the coating layer 12 was formed, and observing the cross section with a transmission electron microscope (TEM). In addition, elemental analysis of the film was performed by energy dispersive X-ray fluorescence spectroscopy (EDX) at specific locations within the field of view.

[0043] When the film thickness was evaluated using contrast on a TEM photograph, it was confirmed that the film was approximately 10 nm thick, covering more than 90% of the pulp fiber surface, and that the film thickness of the layer with C on the outermost surface was approximately 0.8 nm. The film thickness value contains some error due to the unclear thin film interface. The contact angle of the surface with water was also measured and found to be 91.9°. The Cobb water absorbency was also 0.2 g / m2. Regarding waterproofing, water did not seep onto the outside of the container or onto the inside of the container above the water line, and the shape of the box was maintained.

[0044] Example 2 Using the same high-quality paper as in Example 1, an SiO2 film was formed on pulp fibers 11 by atomic layer deposition. TEOS (Si(OC2H5)4) and H2O were used as the raw material gas and reactive gas, respectively. The pulp fibers 11 were heated to 200°C, and the four steps shown in Figure 4, (1) introduction of raw material gas, (2) purging, (3) introduction of reactive gas, and (4) purging, were repeated 10 times to form a film with a thickness of approximately 10 nm, with film formation finally completed at step (2).

[0045] The composition of the SiO2 film produced in Example 2 was analyzed by photoelectron spectroscopy (XPS: X-ray photoelectron spectroscopy), and found to be SiOxCyHz, y / x=0.2. The analysis conditions were as follows: ·Method: X-ray photoelectron spectroscopy (XPS) ·Equipment: Quantera SXM (ULVAC PHI) ·X-ray: Al Kα monochromatic X-ray (1487eV) / 25W / 15kV / 100μmφ Neutralization: Use of an electron gun and an argon ion gun Detection: Survey spectrum (binding energy BE = 0 to 1150 eV / Pass energy PE=280keV) Narrow spectrum (C1s, O1s / PE=112keV)

[0046] The contact angle of the surface with water was measured to be 90.1°. The Cobb water absorbency was 0.4 g / m2. Regarding waterproofing, water did not seep onto the outside of the container or onto the inside of the container above the water line, and the shape of the box was maintained. However, water did seep into the inside of the container.

[0047] (Comparative Example 1) In Comparative Example 1, the same high-quality paper as in Example 1 was used, and an Al2O3 film was formed by atomic layer deposition. The difference from Example 1 is that the four steps shown in Figure 4, (1) introduction of source gas, (2) purging, (3) introduction of reactive gas, and (4) purging, were repeated 10 times to form a film with a thickness of approximately 10 nm, and film formation was completed at (4). In other words, film formation was completed without alkyl groups being provided on the surface layer of the coating layer 12.

[0048] The contact angle of the surface with water was measured to be 72.9°. The Cobb water absorbency was 12.9 g / m2. Regarding waterproofing, water did not seep onto the outside of the container or onto the inside of the container above the water line, and the shape of the box was maintained. However, water did seep into the inside of the container.

[0049] (Comparative Example 2) In Comparative Example 2, an Al2O3 film was formed by reactive DC sputtering using the same high-quality paper as in Example 1. The conditions for forming the Al2O3 film were as follows: using an Al target, film formation power: 2 kW, sputtering pressure: 0.4 Pa, process gas Ar: 300 sccm, reactive gas O2: 60 sccm, and film thickness 10 nm.

[0050] The contact angle of the surface with water was measured to be 62.6°. The Cobb water absorbency was 15.4 g / m2. Regarding waterproofing, water was seeping out onto the outside of the container, and the box was no longer able to maintain its shape.

[0051] (Comparative Example 3) Comparative Example 3 is a commercially available silicone paper with a basis weight of 41 g / m² and a paper thickness of 60 μm. The composition of the silicone paper was analyzed by photoelectron spectroscopy in the same manner as in Example 2, and found to have a y / x of SiOxCyHz of 2.5. The contact angle of the surface with water was measured and found to be 96.3°. The Cobb water absorbency was also found to be 1.0 g / m². Regarding waterproofing, water did not seep onto the outer surface of the container or onto the inner surface of the container above the water level, and the shape of the box was maintained. However, water did seep into the inner surface of the container.

[0052] Comparative Example 4 Comparative Example 4 is a paper cob with a basis weight of 190 g / m² and a thickness of 230 μm, in which polyethylene resin is laminated to pulp fibers. The contact angle of the surface with water was measured to be 92.5°. The Cob water absorbency was 0.1 g / m². Regarding waterproofing, water seeped into the outer surface of the container, and the box shape could not be maintained.

[0053] The papers obtained in each of the Examples and Comparative Examples were evaluated for contact angle, Cobb water absorbency, and waterproofness, and the results are shown in Table 1.

[0054] [Table 1]

[0055] (Evaluation of Examples and Comparative Examples) In Examples 1 and 2, papers with a contact angle of 90° or more, low Cobb water absorbency, and high waterproofing were obtained. In contrast, Comparative Example 1 had a low contact angle and high Cobb water absorbency, but water seeped into the inner surface of the container in terms of waterproofing. This is thought to be because, while diffusion penetration into the fibers and interfiber bonds was suppressed, capillary penetration into the interfiber voids was not suppressed. Comparative Example 2 had a low contact angle and high Cobb water absorbency, and water seeped into the outer surface of the container in terms of waterproofing. This is thought to be because the surface coverage of the sputtered film on the pulp fibers was low and the contact angle was large, so diffusion penetration into the fibers and interfiber bonds and capillary penetration into the interfiber voids were not suppressed. Comparative Example 3 had a high contact angle, but the Cobb water absorbency was somewhat high, and water seeped into the inner surface of the container in terms of waterproofing. This is because silicone contains more CH3 groups than silicone in Example 2, preventing capillary penetration into the voids between fibers. This is thought to be because its diffusion and penetration into the fibers and interfiber bonds was inferior to silicon oxides other than silicone (SiOxCyHz; y / x<1.6). Comparative Example 4 had a contact angle of 90° or more and a low Cobb water absorbency, but water seeped out to the outer surface of the container in terms of waterproofing. This is thought to be because the moisture barrier properties were high before heating, but the polyethylene resin deteriorated after heating to 250°C.

[0056] <Summary of the embodiment> The disclosure of the present specification includes at least the following configurations.

[0057] (Item 1) A paper substrate comprising pulp fibers, The pulp fibers are coated with a coating layer containing a metal oxide or a silicon oxide (SiOxCyHz), The paper substrate is characterized in that the surface layer of the coating layer is an alkyl group.

[0058] (Item 2) Item 2. The paper substrate according to item 1, wherein the silicon oxide (SiOxCyHz) satisfies y / x<1.6.

[0059] (Item 3) 3. The paper base material according to item 1 or 2, wherein the surface coverage of the pulp fibers with the coating layer is 90% or more.

[0060] (Item 4) 4. The paper substrate according to any one of items 1 to 3, wherein the coating layer has a thickness of 5 nm to 500 nm.

[0061] (Item 5) 5. The paper substrate according to any one of items 1 to 4, wherein the water contact angle of the surface layer is 90 degrees or more and 180 degrees or less.

[0062] (Item 6) 6. The paper substrate according to any one of items 1 to 5, wherein the coating layer contains any one of Al2O3, SiO2, TiO2, Nb2O5, Ta2O5, ZrO2, MgO, and ZnO.

[0063] (Item 7) 7. The paper substrate according to any one of items 1 to 6, wherein the coating layer is provided on only one side of the pulp fibers.

[0064] (Item 8) providing pulp fibers; forming a coating layer containing a metal oxide or a silicon oxide (SiOxCyHz) on the surface of the pulp fiber; The method for producing a paper substrate, wherein the film-forming step includes a step of converting the surface layer of the coating layer into an alkyl group.

[0065] (Item 9) 9. The method of claim 8, wherein the film forming step is performed using atomic layer deposition.

Claims

1. A paper substrate comprising pulp fibers, The pulp fibers are coated with a coating layer containing a metal oxide or a silicon oxide (SiOxCyHz), The paper substrate is characterized in that the surface layer of the coating layer is an alkyl group.

2. The paper substrate according to claim 1, characterized in that the silicon oxide (SiOxCyHz) satisfies y / x<1.

6.

3. The paper base material according to claim 1, wherein the surface coverage of the pulp fibers with the coating layer is 90% or more.

4. The paper substrate according to claim 1 , wherein the coating layer has a thickness of 5 nm to 500 nm.

5. 2. The paper substrate according to claim 1, wherein the surface layer has a contact angle with water of 90 degrees or more and 180 degrees or less.

6. The coating layer is Al ２ O ３ , SiO ２ , TiO ２ , Nb ２ O ５ , Ta ２ O ５ , ZrO ２ 2. The paper substrate according to claim 1, further comprising any one of MgO and ZnO.

7. The paper base material according to claim 1 , wherein the coating layer is provided on only one side of the pulp fibers.

8. providing pulp fibers; forming a coating layer containing a metal oxide or a silicon oxide (SiOxCyHz) on the surface of the pulp fiber; The method for producing a paper substrate, wherein the film-forming step includes a step of converting the surface layer of the coating layer into an alkyl group.

9. The method of claim 8 , wherein the film forming step is performed using atomic layer deposition.

Images