JP7637401B2 - Functional sheet and method for producing the same - Google Patents

Description

The present invention relates to a functional sheet and a method for manufacturing the functional sheet. More specifically, the present invention relates to a functional sheet that can be used for important documents, etc., and a method for manufacturing such a functional sheet.

For important documents such as official documents and internal company documents, it is recommended to use a writing instrument (such as a ballpoint pen) with ink that cannot be erased with an eraser. Furthermore, when correcting handwriting, erasing with erasing fluid or erasing tape is not permitted, and a procedure such as crossing out the mark with a double strikethrough and then stamping it with a correction seal is required. This creates the problem that correcting important documents is time-consuming and laborious.

Incidentally, ink has been developed that allows handwriting to be easily thermally erased by rubbing and can maintain this state at room temperature (for example, Patent Document 1).

Patent Document 1 discloses a writing implement that uses thermochromic ink, and describes how a friction body provided at the rear of the barrel of the writing implement can rub the handwriting with the friction body to generate heat, which causes the thermochromic ink to thermally change color, thereby erasing the handwriting. This ink cannot be erased with an eraser at room temperature, as with normal ink. For this reason, if the writing implement of Patent Document 1 is used, handwriting (meaning letters, pictures, etc. written on paper, etc.) can be easily erased by simply applying a certain amount of frictional heat when corrections are required, and the erased handwriting can be written on repeatedly, so incorrect handwriting can be easily erased and corrected despite the use of ink.

JP 2006-063238 A

However, the ink used in the writing implement of Patent Document 1 can be thermally erased at any time regardless of the passage of time by applying heat of a predetermined temperature or higher to the handwriting. Therefore, although the writing implement of Patent Document 1 has excellent practicality, the reality is that it cannot be used for important documents.
Therefore, there is a need for stationery that allows easy correction when necessary when writing with ink, but does not allow handwriting to be easily erased over time.

In view of the above circumstances, the present invention aims to provide a functional sheet and a manufacturing method thereof, in which ink applied to the surface can be easily erased and corrected, but which becomes difficult to erase after a certain period of time has passed.

The functional sheet of the present invention is a sheet member capable of controlling the permeation of liquid, and is characterized in that it has a sheet surface layer located on a substrate surface layer, and the sheet surface layer has a flow path capable of controlling the permeation of liquid from the surface of the sheet surface layer inward.
The method for producing the functional sheet of the present invention is a method for producing the functional sheet of the present invention, and is characterized in that the dispersion is applied onto the surface of the substrate, and then the substrate is dried by a heat press method or a rotary dryer method.

According to the functional sheet of the present invention, since the surface layer of the sheet is provided with flow paths capable of controlling the permeation of liquid, it is possible to control the erasing time of liquid applied to the surface of the functional sheet.
According to the method for producing a functional sheet of the present invention, it is possible to produce a functional sheet capable of controlling the erasing time of a liquid applied to the surface of the functional sheet.

1A is a schematic explanatory diagram of a functional sheet 1 of this embodiment, in which (A) is a schematic plan view, (B) is a schematic cutaway plan view of the functional sheet 1 with a portion of the sheet surface layer 3 peeled off, and (C) is a schematic cross-sectional view of (A). 2A to 2C are schematic diagrams showing an example of a method for producing the functional sheet 1 of the present embodiment. 1A to 1C are diagrams showing experimental results (photographs of the sheet surface and cross section of functional sheet 1). FIG. 13 is a diagram showing the experimental results (surface roughness of functional sheet 1). FIG. 13 is a diagram showing the experimental results (ink penetration speed in functional sheet). FIG. 13 is a diagram showing the experimental results (ink penetration speed in functional sheet). FIG. 13 is a diagram showing the experimental results (ink penetration speed in functional sheet). 13 is a diagram showing experimental results (ink transfer method). FIG. 13 is a diagram showing the experimental results (ink penetration speed in functional sheet). FIG. 13 is a diagram showing experimental results (outline of the ink erasing device). FIG. 13 is a graph showing the experimental results (ink remaining rate). FIG. 13 is a diagram showing the experimental results (ink remaining state in the comparative example). FIG. 13 is a graph showing experimental results (relationship between ink remaining rate and elapsed time). FIG. 13 is a diagram showing the experimental results (surface roughness of the uneven functional sheet). FIG. 13 is a diagram showing the experimental results (ink remaining on the uneven functional sheet). FIG. 13 is a diagram showing the experimental results (surface roughness of the uneven functional sheet). FIG. 1 is a diagram showing experimental results (coating amount and film thickness). 11A to 11C are diagrams showing experimental results (surface and cross-sectional observation results of a functional sheet). FIG. 13 is a graph showing the experimental results (ink remaining rate). FIG. 13 is a diagram showing experimental results (functional sheet using PVA). 13A and 13B are diagrams showing experimental results (surface and cross-sectional observation results of a functional sheet using PVA). 13A and 13B are diagrams showing experimental results (evaluation results of liquid permeation control).

Next, an embodiment of the present invention will be described with reference to the drawings.
First, the functional sheet 1 of the present embodiment will be described, and then a method for manufacturing the functional sheet 1 will be described.

<Functional Sheet 1 of the Present Embodiment>
The functional sheet 1 of this embodiment (hereinafter referred to as functional sheet 1) is characterized in that the penetration time of a liquid applied to the sheet surface (i.e., the surface 3s of the sheet surface layer 3) can be controlled by providing a sheet surface layer 3 on the substrate surface 2s of the substrate 2 as a surface layer. In particular, the functional sheet 1 is characterized in that the erasure time of handwriting or the like based on the liquid can be controlled. For example, in the case where the liquid is ink described below, it is possible to impart an ink erasure function for a certain period of time by controlling the penetration of the solvent contained in the ink.

In this specification, the term "liquid" refers to a liquid containing dyes, pigments, etc., that is used to write pictures or letters on the surface of a sheet, transfer them using a stamp, print them, etc. Examples include ink, which will be described later. In the following explanation, ink is used as a representative liquid. Note that "applying ink" means "applying liquid."

"Applying ink" to the sheet surface of functional sheet 1 is a concept that includes the formation of handwritten marks on the surface of functional sheet 1 by writing, transfers such as letters and pictures transferred using a stamp or the like, and printed matter such as letters and pictures printed by printing.
In this specification, writing refers to making handwritten marks on the surface of functional sheet 1 using a writing instrument, and handwritten marks refer to letters, pictures, etc. made on the surface of functional sheet 1 using ink.
In this specification, erasing refers to adsorbing or peeling off handwriting on the surface of the functional sheet 1 with an erasing member such as an eraser.

As described above, the functional sheet 1 has the substrate 2 and the sheet surface layer 3. As described later, the functional sheet 1 may be a sheet-like member having the same structure as the sheet surface layer 3. In other words, the functional sheet 1 may be a member consisting of only the sheet surface layer 3 formed in a sheet shape.
In the following, a configuration in which a functional sheet 1 has a base material 2 and a sheet surface layer 3 provided on the base material 2 will be described as a representative example.

As shown in FIG. 1, the functional sheet 1 has a substrate 2 and a sheet surface layer 3 provided on the substrate 2, and is a sheet that allows adjustment of the adhesion time of the ink applied to the surface 3s of the sheet surface layer 3 (i.e., the sheet surface).

(Substrate 2)
The substrate 2 is not particularly limited as long as it is a material capable of providing the sheet surface layer 3. For example, various materials such as a film, a sheet, a plate, etc. can be adopted. The internal structure is also not particularly limited, and may be, for example, a structure through which the liquid described below can permeate, a structure which is difficult for the liquid to permeate, or a structure through which the liquid cannot permeate.

In the following, a description will be given of a member in a sheet state in which the substrate 2 is provided with a substrate surface layer 2a having a substrate surface 2s on the surface side.
The substrate surface 2s of the substrate 2 is a surface disposed opposite to the rear surface of the sheet surface layer 3 of the substrate 2. The substrate surface layer 2a has a structure that allows ink to permeate from the substrate surface 2s toward the inside of the substrate 2 when ink is applied to the substrate surface 2s.
That is, the substrate 2 has at least a substrate surface layer 2a as a surface layer in which a large number of voids 2h having openings are formed on a substrate surface 2s. Of course, the voids 2h may be formed so as to communicate between the front and back surfaces of the substrate 2.

The substrate surface layer 2a of the substrate 2 has the function of allowing ink to penetrate into the substrate 2, and is formed so that the ink penetration power is greater than the liquid penetration power of the sheet surface layer 3 described below.

As long as the substrate surface layer 2a has the above-mentioned function, the size of the voids 2h of the substrate 2 is not particularly limited. For example, the voids 2h formed in the substrate surface layer 2a can be formed so that their average pore size is on the order of microns, which is larger than the average pore size (e.g., on the order of nanometers) of the flow paths 3h formed in the surface 3s of the sheet surface layer 3 described later.
If the size of the gap 2h in the substrate surface layer 2a is formed to the size described above, when the ink applied to the surface 3s of the sheet surface layer 3 reaches the substrate surface 2s, the ink can penetrate into the gap 2h through the opening of the gap 2h formed in the substrate surface 2s and move toward the inside of the substrate 2.

The permeability of the substrate surface layer 2a of the substrate 2 to allow a liquid to permeate therethrough can be measured by the method described in the examples below.
In addition, the average pore size of the voids 2h in the substrate surface layer 2a of the substrate 2 may be determined by randomly selecting openings of the voids 2h formed on the surface 2s of the substrate surface layer 2a, measuring the longest diameter of each opening, and averaging the obtained values.

As a substrate 2 having the above-mentioned functions, for example, a material having porosity (porous material) can be used as the substrate 2. Such porous materials can be made of, for example, resin or stone, or even paper. Paper materials are aggregates formed by intertwining fibers, and have the above-mentioned structure, so they are defined as porous materials in this specification.

Examples of paper-based porous members include, of course, paper nonwoven fabric, but also filter paper, Japanese paper, copy paper, coated paper, fine paper, medium-quality paper, craft paper, recycled paper, wrapping paper, paperboard, cardboard base paper, laminated paper, etc. Examples of resin-based members include sponge-like sheets and nonwoven fabric made of PET fibers, and examples of stone-based members include sheets made from finely crushed lime, etc.

When a paper substrate 2 of the functional sheet 1 is used, the substrate 2 is composed of the fibers F as described above, but the fiber width and fiber length are not particularly limited.
The fibers F constituting the base material 2 are thicker in width and longer in length than the fine fibers ff of the sheet surface layer 3 described later. For example, fibers having a width of about 1 μm to 50 μm and a length of about 0.1 mm to 10 mm are used.

The average thickness of the base material 2 is not particularly limited. For example, it can be formed to be 50 μm or more. In particular, if the average thickness of the base material 2 is formed to be 60 μm to 120 μm, it is possible to impart flexibility, thereby improving handleability.

The average thickness of the substrate 2 can be measured by the method described in the examples below.

(Seat surface layer 3)
The sheet surface layer 3 is a layer formed by laminating on the substrate 2 so that one surface is located outside and the other surface is located on the substrate surface 2s side of the substrate 2. That is, it is a surface layer laminated on the substrate 2 so that the surface 3s of the sheet surface layer 3 is located on the surface of the functional sheet 1. The sheet surface layer 3 has a plurality of flow paths 3h that can allow the ink to permeate from the surface 3s of the sheet surface layer 3 to the substrate 2 while retaining the ink on the surface 3s of the sheet surface layer 3 in a state where ink is applied to the surface 3s of the sheet surface layer 3. Specifically, the sheet surface layer 3 has a dense structure (hereinafter simply referred to as a dense structure) and has a function of permeating the ink (specifically, the solvent in the ink) into the inside by guiding the retained ink (specifically, the solvent in the ink) into the plurality of flow paths 3h (also referred to as ink penetration characteristics).

The flow paths 3h of the sheet surface layer 3 are formed to communicate with the surface 3s of the sheet surface layer 3 and the back surface (the surface facing the substrate 2) located on the opposite side. As shown in FIG. 1, the surface 3s of the sheet surface layer 3 is formed so that the openings of the flow paths 3h are located thereon. In other words, the sheet surface layer 3 is formed so that when ink is applied to the surface 3s of the sheet surface layer 3 (i.e., the sheet surface of the functional sheet 1), the ink can be guided from the openings of the flow paths 3h formed on the sheet surface 3s into the flow paths 3h, and moved, i.e., permeated, into the sheet surface layer 3.

When ink is applied to the surface 3s of the sheet surface layer 3, the ink is formed to have a slower penetration rate into the sheet surface layer 3 than the penetration rate of the ink into the substrate 2 (referred to as the liquid penetration power of the substrate 2). In other words, the sheet surface layer 3 is formed to have a lower liquid penetration power than the substrate surface layer 2a of the substrate 2.
The permeation speed of the ink can be adjusted by the flow paths 3h of the sheet surface layer 3. For example, the flow paths 3h of the sheet surface layer 3 are formed so that the average pore size is smaller than the average pore size of the voids 2h of the substrate 2. More specifically, when the average pore size of the voids 2h inside the substrate 2 is on the order of microns, the size of the flow paths 3h (i.e., the average pore size) is formed to be fine on the order of nanometers, so that the liquid permeation power of the sheet surface layer 3 can be made lower than the liquid permeation power of the substrate surface layer 2a of the substrate 2.

The ink permeability of the sheet surface layer 3 can be measured by the method described in the examples below.
The average pore size of the flow paths 3h can be determined by randomly selecting openings of the flow paths 3h formed on the surface 3s of the sheet surface layer 3, measuring the longest diameter of each opening, and averaging the obtained values.

The sheet surface layer 3 is not particularly limited as long as it has a dense structure as described above and is provided with a plurality of flow paths 3h having the above-mentioned functions.
For example, the sheet surface layer 3 may have a structure in which a plurality of fine fibers ff are contained in the sheet surface layer 3, a structure in which a plurality of bubble-like voids are present, or a structure that combines both. Below, the sheet surface layer 3 having the above-mentioned structure will be described, but it goes without saying that the sheet surface layer 3 is not limited to such a structure as long as it can exhibit the above-mentioned functions.

First, the structure of the sheet surface layer 3 containing a plurality of fine fibers ff will be described.
The fine fibers ff contained in the sheet surface layer 3 refer to fibers having an average fiber diameter of 3 nm to 1 μm and an average fiber length of 1 mm or less. In particular, fibers having an average fiber diameter of 3 nm to 100 nm are called nanofibers nf.
1, when the substrate 2 contains fibers F, the fine fibers ff of the sheet surface layer 3 of the functional sheet 1 are finer than the fibers F of the substrate 2. In other words, the sheet surface layer 3 is a layer formed to have a denser structure than the substrate 2 by containing fine fibers ff finer than the substrate 2. For example, when the substrate 2 is made of paper (e.g., copy paper), the sheet surface layer 3 is composed of fine fibers ff that are thinner and shorter than the fibers F (average fiber diameter 5 μm to 50 μm, average fiber length 0.5 mm to 10 mm) that compose the substrate 2.

The sheet surface layer 3 is formed by stacking a number of fine fibers ff as described above in close contact with each other. For example, as shown in FIG. 1, the sheet surface layer 3 is formed to have a denser structure than the fiber F structure of the base material 2 made of paper (e.g. copy paper). Naturally, as shown in FIG. 1, the surface 3s of the sheet surface layer 3 is also formed to have a denser structure than the surface 2s of the base material 2. The dense structure of the surface 3s of the sheet surface layer 3 can be evaluated by the surface roughness described in the examples below.

A mesh-like gap is formed inside the sheet surface layer 3. The mesh-like gap is formed by stacking a plurality of fine fibers ff so that the gaps formed between adjacent fine fibers ff are connected to each other. Specifically, as shown in FIG. 1, the sheet surface layer 3 has a plurality of surface gaps and back gaps formed between adjacent fine fibers ff on the surface 3s and back surface (the surface facing the substrate surface 2s of the substrate 2). Then, inside the sheet surface layer 3, an internal gap is formed in which adjacent gaps in the vertical direction (from the surface 3s to the substrate surface 2s direction) communicate with each other. Many of the multiple internal gaps formed inside the sheet surface layer 3 are formed to communicate with the surface gaps and back gaps located on the surface 3s of the sheet surface layer 3. In other words, the sheet surface layer 3 is formed to have a plurality of mesh-like communicating gaps that communicate with the front and back of the sheet surface layer 3 by containing a plurality of fine fibers ff.
The communicating gap corresponds to the flow path 3h, and the surface gap formed in the sheet surface layer 3 of the communicating gap corresponds to the opening of the flow path 3h. In other words, the sheet surface layer 3 contains a plurality of fine fibers ff, and thus has an opening on the surface 3s of the sheet surface layer 3, and has a structure including a plurality of flow paths 3h extending from the opening toward the other surface (back surface side) of the sheet surface layer 3.

In particular, when nanofibers nf are used as the fine fibers ff, the structure of the sheet surface layer 3 can be made denser. In this case, the structure of the surface 3s of the sheet surface layer 3 can be made denser, so that the ink barrier properties against the ink applied to the surface 3s of the sheet surface layer 3 can be improved. In other words, when nanofibers nf are used as the fine fibers ff, the retention time of the ink applied to the surface 3s of the sheet surface layer 3 can be improved. This makes it easier to control the ink erasing function.
The average fiber diameter and average fiber length of the fine fibers ff can be measured by the method described in the examples described later.

Next, the structure of the flow passage 3h of the sheet surface layer 3 having a plurality of bubble-like voids will be described.
The sheet surface layer 3 has a structure in which a plurality of bubble-like voids are formed inside, and adjacent voids are connected to each other to form a network of continuous voids. The sheet surface layer 3 having such a structure can be formed as follows, but is not limited to such a method. For details of the formation method, see the method described in the Examples.

For example, first, a resin is mixed with a solvent to prepare a liquid in which the resin is dissolved in the solvent (a sheet surface layer forming liquid to be described later). This liquid is applied onto the substrate surface 2s of the substrate 2 to form a layer corresponding to the sheet surface layer 3.
Then, the mixture is dried at a predetermined temperature. The drying temperature is preferably adjusted so that the solvent easily boils in the layer corresponding to the sheet surface layer 3. For example, the drying temperature is adjusted to be equal to or higher than the boiling point of the solvent in the liquid. Then, in the drying process, the solvent evaporates and bubbles are formed in the layer corresponding to the sheet surface layer 3. At this time, the layer has fluidity, and adjacent bubbles grow while connecting with each other to form continuous bubbles. This phenomenon occurs at every point in the layer, and a plurality of continuous bubbles are formed that connect the surface layer of the layer and the substrate surface 2s. Moreover, adjacent continuous bubbles are formed in a complex mesh shape so as to connect with each other. Furthermore, the bubbles generated in the layer reach the surface of the layer (the surface corresponding to the surface 3s of the sheet surface layer 3) and are released from the surface as gas to the outside.
After the drying step, the substrate 2 (substrate 2 having a layer corresponding to the sheet surface layer 3 applied onto the substrate surface 2s) is allowed to cool, whereby a functional sheet 1 having a sheet surface layer 3 formed by solidifying the resin in the layer corresponding to the sheet surface layer 3 on the substrate surface 2s of the substrate 2 can be formed. In the sheet surface layer 3, a network of voids is formed based on the network of open air bubbles as described above. Moreover, fine openings communicating with the network of voids are formed on the surface 3s of the sheet surface layer 3. The fine openings on the surface 3s of the sheet surface layer 3 are based on openings formed when air bubbles generated within the layer during the drying step are released to the outside.
The mesh-like voids in the sheet surface layer 3 correspond to the flow paths 3h, and the minute openings formed in the sheet surface layer 3 correspond to the openings of the flow paths 3h.

By preparing as described above, the sheet surface layer 3 can be formed of a resin material and has a structure having a plurality of flow paths 3h formed in a mesh shape with openings on the surface 3s inside. Since the flow paths 3h are formed by bubbles formed by evaporation of the solvent, the flow path diameter can be formed to be small. In addition, the size of the bubbles can be changed depending on the type of solvent and the drying temperature. For example, when the sheet surface layer 3 is formed using a resin, it is possible to make the flow paths 3h of the sheet surface layer 3 smaller than when fine fibers ff are used, so that the sheet surface layer 3 can be formed to have a denser structure.
Moreover, since the sheet surface layer 3 is formed from a liquid resin, the surface 3s of the sheet surface layer 3 can be formed more smoothly than when it is formed from fine fibers ff. This makes it possible to further improve the ink barrier property of the sheet surface layer 3 against ink.

The flow passage 3h in the sheet surface layer 3 has a flow passage wall formed of the above-mentioned resin. Since the flow passage 3h is formed by connecting a plurality of bubbles or continuous bubbles as described above, the cross-sectional area of the flow passage 3h is formed to change along the flow passage direction. Specifically, the inner diameter of the flow passage is formed to narrow and widen.
In this case, when the solvent contained in the ink applied to the surface 3s of the sheet surface layer 3 enters the flow path 3h, the solvent can be retained in a portion (liquid retention portion) with a large cross-sectional area located near the surface 3s, which makes it easier to control the ink erasing function.

The resin to be mixed with the liquid is not particularly limited as long as it can be dissolved in a solvent and has the property of being solidified after a drying process. For example, polyvinyl alcohol (hereinafter sometimes referred to as PVA) and vinyl acetate can be formed into a film after drying and have a certain degree of flexibility, so they are preferable. In particular, from the viewpoint of handling, it is preferable to use polyvinyl alcohol that is soluble in water.
The solvent to be mixed with the liquid is not particularly limited as long as it can dissolve the resin to be mixed. For example, various compounds such as water, alcohols, and acetone can be used as the solvent. These compounds may be used alone or in combination of two or more.

The sheet surface layer 3 may have a structure containing the above-mentioned multiple fine fibers ff and resin. The sheet surface layer 3 having such a structure can be formed, for example, by using a liquid containing multiple fine fibers ff and resin in a solvent. The obtained sheet surface layer 3 can form complex flow paths 3h within the sheet surface layer 3. Specifically, the sheet surface layer 3 can be formed to have a structure having a complex network-like flow path 3h, including multiple interconnected gaps, mesh-like voids based on continuous air bubbles, and voids in which the interconnected gaps adjacent to these voids are connected to each other.

The average thickness of the sheet surface layer 3 is not particularly limited.
For example, the sheet surface layer 3 can be formed so that its average thickness is 1 μm to 30 μm.
If the average thickness of the sheet surface layer 3 is made smaller than 1 μm, the distance from the substrate 2 becomes short, and the time during which the ink can be erased tends to become too short. On the other hand, if the average thickness of the sheet surface layer 3 is made larger than 30 μm, the distance from the substrate 2 becomes long, and the time required for fixing the ink tends to become too long. For this reason, in order to allow the ink applied to the surface 3s of the sheet surface layer 3 (i.e., the surface of the functional sheet 1) to properly penetrate toward the substrate 2, it is preferable to form the average thickness of the sheet surface layer 3 so as to be within the above range.

The average thickness of the sheet surface layer 3 can be measured by the method described in the examples below.

As described above, the functional sheet 1 has a plurality of fine flow paths 3h and a sheet surface layer 3 having a dense structure. For example, the functional sheet 1 can be formed to have a structure including a base material 2 and a sheet surface layer 3 provided on a base material surface 2s, which is the surface of the base material surface layer 2a of the base material 2.
By forming the sheet surface layer 3 of the functional sheet 1 into a dense structure as described above, it is possible to exhibit a barrier property against ink (when the liquid is ink, an ink barrier property) when ink (liquid) is applied to the sheet surface (i.e., the surface 3s of the sheet surface layer 3) of the functional sheet 1. In other words, it is possible to suppress the ink applied to the sheet surface of the functional sheet 1 from rapidly penetrating into the functional sheet 1 and to retain the ink for a certain period of time.
For example, when handwriting is made on ordinary paper using ink, the solvent contained in the ink rapidly penetrates into the paper immediately after writing, and the ink adheres to the paper surface. In other words, the written characters are formed as handwriting immediately after writing, and the adhered ink cannot be easily erased. In contrast, the functional sheet 1 can retain the ink on the surface of the functional sheet 1 to a certain extent immediately after writing due to the sheet surface layer 3 having a dense structure, and therefore can prevent the ink from adhering immediately after writing.

Moreover, the sheet surface layer 3 of the functional sheet 1 has a mesh-like structure of multiple fine flow paths 3h with openings on the surface 3s (corresponding to the surface of the functional sheet 1). Therefore, the solvent contained in the ink held on the surface 3h of the sheet surface layer 3 can be guided into the flow paths 3h from the openings formed on the surface 3s. In other words, when ink is applied to the surface (surface 3s of the sheet surface layer 3), the dense structure of the sheet surface layer 3 as described above allows the ink to be temporarily held on the surface 3s of the functional sheet 1, and the solvent contained in the ink in this state can penetrate from the surface to the inside more slowly than with ordinary paper.

The structure of the sheet surface layer 3, i.e., the dense structure having multiple fine flow paths 3h, can be formed by using multiple fine fibers ff, etc., as described above.

In addition, the substrate 2 of the functional sheet 1 has a structure having a plurality of voids 2h for allowing liquid to penetrate into the substrate surface layer 2a, similar to the flow paths 3h of the sheet surface layer 3. On the other hand, the liquid penetration power of both (the flow paths 3h of the sheet surface layer 3 and the voids 2h of the substrate 2) is formed so that the liquid penetration power of the sheet surface layer 3 is lower than the liquid penetration power of the substrate surface layer 2a of the substrate 2. For example, when general paper (e.g., notebook paper, copy paper, etc.) is used as the substrate 2, the substrate 2 is formed so that the penetration speed of the ink (specifically, the penetration speed of the solvent in the ink) when the ink is directly applied to the substrate surface 2s of the substrate 2 is faster than the penetration speed of the ink in the sheet surface layer 3.
For this reason, when ink is applied to the surface of the functional sheet 1, the ink solvent that has permeated into the sheet surface layer 3 reaches the contact surface between the sheet surface layer 3 and the substrate 2 and is guided from the flow path 3h of the sheet surface layer 3 into the voids 2h of the substrate 2. At this time, the permeation speed of the ink solvent moving into the voids 2h of the substrate 2 is faster than when moving through the flow path 3h. As a result, the permeation speed of the ink solvent in the sheet surface layer 3 can be increased with the increase in the permeation speed of the ink solvent permeating into the substrate 2. In other words, by configuring the functional sheet 1 to include the substrate 2, it becomes easier to fix the ink applied to the surface of the functional sheet 1.
In other words, by configuring the functional sheet 1 to include a substrate 2, it is possible to more appropriately control the adhesion speed of the ink applied to the surface of the functional sheet 1 (surface 3s of the sheet surface layer 3).

Therefore, when ink is applied to the sheet surface of the functional sheet 1 (specifically, the surface 3s of the sheet surface layer 3), the dense structure of the sheet surface layer 3 exerts an ink barrier property against the ink, causing the ink to remain on the sheet surface, while the ink (specifically, the solvent contained in the ink) can be slowly permeated toward the substrate 2 through the multiple fine flow paths 3h formed in the sheet surface layer 3.

Generally, ink contains a fixing agent and a solvent in addition to pigments and dyes, as described below. Therefore, when ink is applied to the sheet surface of the functional sheet 1, the solvent in the ink can be caused to move from the sheet surface of the functional sheet 1 through the flow paths 3h in the sheet surface layer 3 and penetrate toward the substrate 2, as described above. Meanwhile, the openings of these minute flow paths 3h are formed to be equal to or smaller than the size of the pigments, etc.

As a result, the pigments in the ink are held so as to remain near the sheet surface of the functional sheet 1 to which it is applied, or near the surface 3s of the sheet surface layer 3. Then, the solvent concentration in the ink applied to the sheet surface of the functional sheet 1 decreases as the solvent penetrates into the sheet surface layer 3. As the solvent decreases, the concentration of the fixing agent in the ink on the surface of the sheet 1 increases. When the concentration of the fixing agent in the applied ink reaches a certain level, the pigments or dyes in the ink are fixed to the sheet surface of the functional sheet 1 (i.e., the surface 3s of the sheet surface layer 3) or near the inner side of the surface 3s of the sheet surface layer 3.

In other words, the functional sheet 1 is a component that can suppress the ink from adhering immediately after the ink is applied to the surface (surface 3s of the sheet surface layer 3), while at the same time allowing the ink to adhere after a predetermined time has elapsed, thereby exhibiting functions that are in a trade-off relationship that could not be expected to coexist with conventional technology.
Specifically, the functional sheet 1 has a sheet surface layer 3 having a dense structure and fine flow paths 3h provided on a substrate 2, and by controlling the penetration time of the solvent in the ink applied to the sheet surface into the sheet surface layer 3 and/or the substrate 2, it is possible to adjust the time it takes for the ink to harden.

Therefore, by using functional sheet 1, it is possible to control the adhesion time of ink applied to the sheet surface, and therefore the time over which the ink can be erased. In other words, by using functional sheet 1, ink applied to the sheet surface can be erased for a specified time, and once the specified time has elapsed, the ink can be fixed to the sheet surface, making it difficult or impossible to erase. In other words, functional sheet 1 is a sheet on which handwriting, etc. cannot ultimately be erased, but handwriting, etc. can be corrected from immediately after writing, etc., for a specified period of time.

In this way, for example, when paper is used as the material for the substrate 2 of the functional sheet 1 and cellulose nanofibers are used as the fine fibers ff of the sheet surface layer 3, it is possible to make the functional sheet 1 difficult to distinguish from ordinary paper when viewed. However, as described above, when handwriting is formed by writing with ink on the sheet surface (surface 3s of the sheet surface layer 3) of such a functional sheet 1, it is possible to make it possible to make the handwriting erasable and correctable for a certain period of time immediately after writing, but to make it impossible to erase or correct the handwriting after a certain period of time.
Specifically, when the functional sheet 1 is formed from a paper material, handwriting etc. using ink can be easily erased or corrected with a commercially available eraser from immediately after writing until a specified time, just as if it were written with a pencil or the like, and the functional sheet 1 of this embodiment can be used for various documents such as official documents, internal company documents, hospital charts, experimental records, and other important documents or documents that are recommended for storage in a state in which handwriting etc. cannot be easily erased with an eraser or frictional heat, etc.

In addition, in the specification, "adhesion" refers to a state in which ink or the like applied to the surface of the functional sheet 1 (surface 3s of the sheet surface layer 3) cannot be removed due to frictional forces caused by an erasing member such as an eraser.
Furthermore, as described above, the functional sheet 1 can be used for a variety of purposes, such as resumes, application forms for job hunting, contracts, and application forms and documents that require signatures, as long as it is necessary to make it impossible to erase or correct the handwriting in the end, without being limited to the above uses.

(Liquid retention rate (%) by functional sheet 1)
As described above, the functional sheet 1 has a sheet surface layer 3 with a dense structure and multiple fine flow paths 3h, so that the ink applied to the surface can be erased for a specified period of time, and can then be made unable to be erased after the specified period of time.
The function of such a functional sheet 1 can be evaluated by the retention rate (%) of a predetermined liquid. A detailed measurement method can be used according to the method described in the examples below.

For example, the functional sheet 1 can be evaluated based on the retention rate (%) of butyl alcohol after a certain time has elapsed when butyl alcohol is applied to the surface (surface 3s of the sheet surface layer 3). Moreover, the evaluation value obtained is far superior to that of ordinary paper (e.g., copy paper, etc.). In other words, the functional sheet 1 can retain liquid applied to its surface for a longer period of time than ordinary paper.

Specifically, when 10.0 mg of butyl alcohol is applied to the surface of ordinary copy paper, the retention rate (%) of butyl alcohol after 2 minutes (at a given temperature and humidity) is about 80% by mass, whereas the functional sheet 1 has a retention rate of 90% or more by mass. After 4 minutes, the retention rate of copy paper is about 50% by mass, whereas the functional sheet 1 has a retention rate of 70% or more by mass. Moreover, after 6 minutes, the retention rate of copy paper is about 30% while the functional sheet 1 has a retention rate of 50% or more by mass, and after 8 minutes, the retention rate of copy paper is about 10% while the functional sheet 1 has a retention rate of 40% or more by mass. Furthermore, after 10 minutes, the retention rate of copy paper is almost 0%, whereas the liquid retention rate of functional sheet 1 can maintain a retention rate of 30% or more by mass.

(Ink)
As described above, the ink is a liquid that contains at least a solvent, a colorant, and a fixing agent, and examples of the ink include commonly used water-based inks and oil-based inks.
The solvent has a function of dispersing or dissolving the colorant and fixing agent, and examples of the solvent include organic solvents such as ketones, alcohols, and ethyl acetate. The colorant imparts color to the ink, and examples of the solvent include pigments such as carbon black and organic pigments, and dyes such as anionic azo dyes, phthalocyanine dyes, and azo metal complex dyes. The fixing agent has a function of fixing the colorant to the sheet surface of the functional sheet 1, and examples of the fixing agent include synthetic resins, natural resins, and derivatives thereof.
In addition to the above-mentioned solvents, the ink may contain generally used auxiliary agents, pH adjusters, preservatives, and the like.

(Surface structure of functional sheet 1)
The surface 3s of the sheet surface layer 3, which corresponds to the surface of the functional sheet 1, is desirably formed to be smooth in order to improve the erasability of the ink on the surface 3s. If the surface 3s of the sheet surface layer 3 has too deep recesses, the ink applied to the surface 3s will easily penetrate into the deep recesses. When an eraser or the like is used, if the rubber or the like cannot be brought into contact with the inside of the deep recesses, the ink that has penetrated into the deep recesses will be difficult to contact with the rubber or the like. In this case, there is a possibility that some ink will remain unerased. For this reason, it is desirably that the sheet surface, i.e., the surface 3s of the sheet surface layer 3, is in a state in which it is easy for an eraser or the like used to erase the ink applied to the surface 3s to come into contact with the eraser or the like.

(Surface roughness)
The smoothness, i.e., surface roughness, of the surface 3s of the sheet surface layer 3 may be appropriately adjusted depending on the material of the eraser or the like used.
For example, when using a general plastic eraser, the surface 3s of the sheet surface layer 3 can be formed so that the surface roughness is 10 μm or less. Specifically, the lower limit is preferably 0.1 μm or more, more preferably 0.5 μm or more, and even more preferably 1.2 μm or more. The upper limit is preferably 10 μm or less, more preferably 9 μm or less, and even more preferably 8 μm or less. If the surface 3s of the sheet surface layer 3 of the functional sheet 1 is too smooth, the ball at the tip of the pen tends to slip when writing with a ballpoint pen, making it difficult for the ball to rotate. Therefore, the surface roughness of the surface of the functional sheet 1 (i.e., the surface 3s of the sheet surface layer 3) is preferably 0.1 μm to 10 μm, more preferably 0.5 μm to 9 μm, and even more preferably 1.2 μm to 8 μm.
Furthermore, by making the surface 3s of the sheet surface layer 3 have a smooth structure as described above, even if ink penetrates into the recesses, it can be easily erased with an eraser or the like.

The surface roughness of the surface 3s of the sheet surface layer 3 can be measured in accordance with JIS P 8119, which is the method described in the examples below, and is the average value obtained.

In addition, the materials for the substrate 2 and the sheet surface layer 3 of the functional sheet 1 can be appropriately selected from the materials described above, but in terms of ease of handling, it is preferable to select materials that have affinity with each other.
For example, when paper is used as the material for the base material 2, if cellulose nanofibers are used as the fine fibers ff of the sheet surface layer 3, the cellulose fibers F constituting the base material 2 and the cellulose nanofibers of the sheet surface layer 3 will be strongly bonded by hydrogen bonds, thereby improving the connectivity between the two.

In the above example, the functional sheet 1 has been described as being in contact with the substrate surface 2s of the substrate 2 and the sheet surface layer 3, but the functional sheet 1 may also have a structure (multilayer structure) having a connecting layer between the two that has the function of allowing liquid to penetrate.
The material of this connecting layer is not particularly limited as long as it has a flow path through which a liquid flows, connecting the front and back sides (i.e., the surface on the back side of the sheet surface layer 3 and the surface on the substrate surface layer 2s side) in the same manner as the sheet surface layer 3. For example, it may be a material containing the above-mentioned porous member or the same fibers as the sheet surface layer 3.

When the connecting layer is made of fibers, for example, if the fine fibers ff of the sheet surface layer 3 are cellulose nanofibers and the fibers F of the substrate 2 are cellulose fibers, cellulose nanofibers, which have a high affinity for both, can be used as the fibers of the connecting layer.
In this case, the connecting layer and the sheet surface layer 3 can be firmly connected by hydrogen bonds between the cellulose nanofibers, and the connecting layer and the substrate 2 can firmly connect the cellulose nanofibers and the cellulose fibers by hydrogen bonds. In other words, by making the connecting layer contain a material that has a high affinity for both (the sheet surface layer 3 and the substrate 2), the layers of the functional sheet 1 can be firmly connected without using an adhesive.

The method for forming the connection layer is not particularly limited.
For example, the sheet surface layer 3 and the substrate 2 may be molded separately, and then the two may be connected with a connecting layer to form the functional sheet 1, or the connecting layer and the sheet surface layer 3 may be laminated by being applied in sequence onto the substrate surface 2s of the substrate 2. When a connecting layer is provided between the substrate 2 and the sheet surface layer 3, even if the concentration and thickness of the fine fibers ff contained in the sheet surface layer 3 are constant, the flexibility of the moldability of the functional sheet 1 can be improved by appropriately adjusting the thickness of the connecting layer and the composition contained therein.

In addition, in the above example, a case was described in which the ink applied to the sheet surface (surface 3s of the sheet surface layer 3) of the functional sheet 1 penetrates from the sheet surface layer 3 to the substrate 2, but the structure may be such that the ink does not necessarily penetrate into the substrate 2.
Specifically, the functional sheet 1 has the above-mentioned configuration, and the structure of the substrate 2 is not particularly limited as long as the substrate 2 can exhibit the above-mentioned functions (ink barrier properties and ink penetration properties) against the ink when the ink is applied thereto. In other words, the substrate 2 of the functional sheet 1 is not particularly limited as long as the substrate 2 can exhibit the function of being able to erase the ink applied to the surface of the functional sheet 1 for a predetermined period of time and becoming unable to be erased after the predetermined period of time by the sheet surface layer 3 of the functional sheet 1 having the above-mentioned structure.
More specifically, even if the substrate 2 is in a state in which the solvent contained in the ink applied to the sheet surface of the functional sheet 1 does not penetrate into the substrate 2 or most of it remains on and/or inside the sheet surface layer 3, as long as the sheet surface layer 3 has the above-mentioned structure, the effects of the functional sheet 1 can be properly exhibited.

For example, in the case of low-viscosity ink, the ink (specifically, the solvent contained in the ink) applied to the sheet surface of the functional sheet 1 permeates from the surface 3s toward the substrate 2, but a phenomenon may occur in which the ink stagnates in the sheet surface layer 3 before reaching the substrate 2. A similar phenomenon may also occur, for example, when the amount of ink applied is small. Furthermore, a similar phenomenon may also occur in relation to the film thickness of the sheet surface layer 3, for example.

(Method of producing cellulose nanofiber)
The method for producing the cellulose nanofibers used in the sheet surface layer 3 and the connecting layer described above is not particularly limited.
For example, the raw pulp fibers may be finely divided (defibrated) by mechanical treatment, or may be finely divided by chemical treatment such as enzyme treatment, acid treatment, etc. As mechanized treatment, for example, a low-pressure homogenizer, a high-pressure homogenizer, a grinder, a cutter mill, a jet mill, a water jet method (WJ method), a single-screw extruder, a twin-screw extruder, an ultrasonic agitator, or the like may be used to convert the pulp fibers into cellulose nanofibers.
The fibers that are the raw material for cellulose nanofibers are not particularly limited as long as they are fibers whose main component is cellulose, such as pulp fibers, wood flour, plant residues, and papermaking residues.

(Ink erasure evaluation method)
The method for evaluating the erasability of ink applied to the surface 3s of the sheet surface layer 3, which is the sheet surface of the functional sheet 1 of this embodiment, is determined as follows by a method conforming to JIS 6050 (2008).

The ink mark on the surface of functional sheet 1 is erased by moving an erasing member such as an eraser back and forth a predetermined number of times at a predetermined moving speed while applying a predetermined load to the member, and the retention rate of the mark is then calculated using the following formula (1) to evaluate the sheet surface.
The remaining rate of the handwriting is calculated by comparing the areas of the remaining ink portions using the image software described in the Examples below. The image processing conditions are set as described in the Examples below, with a selection width and RGB thresholds, and the remaining rate is calculated by comparing the number of pixels of the binarized images.

Residual rate (%) = (number of ink pixels after erasure / number of ink pixels before erasure) x 100 (1)

In the above example, the functional sheet 1 is configured to include the substrate 2, so that the penetration of the ink can be appropriately controlled. However, as described above, the functional sheet 1 does not necessarily have to include the substrate 2. That is, the functional sheet 1 may be a sheet-like member having a structure equivalent to that of the sheet surface layer 3, as described above. Specifically, the functional sheet 1 may be a sheet-like member having a structure equivalent to that of the sheet surface layer 3 without including the substrate 2. For example, if the functional sheet 1 is formed into a sheet-like member having a structure equivalent to that of the sheet surface layer 3 using a sheet surface layer forming liquid for forming the sheet surface layer 3 described later, the functional sheet 1 having a structure equivalent to that of the sheet surface layer 3 can be formed. This functional sheet 1 can exert the same function as the sheet surface layer 3 provided on the substrate surface 2s of the substrate 2 described above.
For this reason, this functional sheet 1 can be attached to an existing component (a component equivalent to the substrate 2), and can therefore impart the same function as described above to any component (the ability to erase ink marks, etc. for a certain period of time).
In other words, if the functional sheet 1 is formed as a sheet-like member having the same structure as the sheet surface layer 3, the degree of freedom in handling can be improved.
It goes without saying that the above-mentioned existing components can be anything, such as a notebook, a board, a desk, or a wall, and are not limited to the above-mentioned components.
The method for providing the functional sheet 1 to an existing member is not particularly limited. For example, the two may be connected to each other by the above-mentioned connecting layer, or may be connected to each other by a removable adhesive or the like.

<Method of manufacturing functional sheet 1>
Next, a description will be given of an example of a method for manufacturing the functional sheet 1. Specifically, a description will be given of an example of a method for manufacturing the functional sheet 1 having a configuration including a sheet surface layer 3 and a substrate 2.

In the following, a functional sheet 1 having a structure in which a sheet surface layer 3 is in contact with a substrate surface 2s of a substrate 2 will be described as a representative example.
The following describes a case where easily available copy paper is used as the substrate 2 of the functional sheet 1. The following also describes a case where the sheet surface layer 3 has a structure containing fine fibers ff, and the fine fibers ff are cellulose nanofibers that have a high affinity for the substrate 2, which is copy paper.

The manufacturing method of the functional sheet 1 is generally as follows: a liquid containing cellulose nanofibers (hereinafter referred to as a sheet surface layer forming liquid) is applied to the substrate surface 2s of the substrate 2 to form the sheet surface layer 3, and the sheet in this state is dried at a predetermined drying temperature.

The "sheet surface layer forming liquid" corresponds to the dispersion liquid in the claims.

This sheet surface layer forming liquid is a liquid in which cellulose nanofibers are added to water so that the solids concentration is a predetermined concentration. The solids concentration of the cellulose nanofibers can be adjusted appropriately depending on the method for producing the functional sheet, and is not particularly limited.

For example, when the production method uses the coating method or pressing method described below, cellulose nanofibers are mixed into a sheet surface layer forming liquid and the solid content concentration of the cellulose nanofibers is adjusted to 1 mass % or more.
In these methods, if the solid content concentration of the cellulose nanofiber is lower than 1 mass%, the fluidity of the cellulose nanofiber increases, making it difficult to form a surface layer film when forming the sheet surface layer. Also, since the amount of water absorbed by the substrate 2 increases, the sheet shrinks more when drying the functional sheet 1, making it more likely to wrinkle. Furthermore, the amount of energy consumed for drying increases. On the other hand, if the solid content concentration of the cellulose nanofiber is higher than 10 mass%, the fluidity of the cellulose nanofiber decreases, making it difficult to form a uniform sheet surface layer film.

Therefore, when producing functional sheet 1 by the above method, from the viewpoint of forming a uniform sheet surface film, the content of cellulose nanofibers in the sheet surface forming liquid is preferably such that the solids concentration is 1% by mass or more and 10% by mass or less, and more preferably such that the solids concentration is 2% by mass or more and 8% by mass or less.

The solid content concentration of the cellulose nanofiber is calculated by the following formula (2): CNF in the formula means cellulose nanofiber.

(Solid concentration of CNF) = (Dry weight of CNF) / Liquid weight of CNF) × 100 (2)

(Method of forming sheet surface layer 3)
The method for forming the sheet surface layer 3 on the substrate surface 2s of the substrate 2 is not particularly limited as long as the sheet surface layer 3 can be formed on the substrate surface 2s.
For example, the sheet surface layer 3 may be formed by a method in which a sheet surface layer forming liquid is applied onto the substrate surface 2s of the substrate 2 using a bar coater or the like (coating method), and then the sheet surface layer 3 is subjected to a drying step; the sheet surface layer 3 may be formed by a method in which a sheet surface layer forming liquid is applied onto the substrate surface 2s of the substrate 2, and then the sheet surface layer 3 is formed by a pressing method, and then the sheet surface layer 3 is subjected to a drying step; or the sheet surface layer 3 may be formed by a method in which a sheet surface layer forming liquid is applied onto the substrate surface 2s of the substrate 2, and then the shaping of the sheet surface layer 3 and the drying step are performed simultaneously by a heat pressing method.

(Coating method)
First, a case where the sheet surface layer 3 is formed on the substrate surface 2s of the substrate 2 by a coating method will be described.

In the coating method, first, a sheet surface layer forming liquid is prepared by dispersing cellulose nanofibers in water so that the solid content concentration is a predetermined concentration (e.g., 4% by mass). Next, the prepared sheet surface layer forming liquid is supplied to a coating device such as a bar coater, and a coating film for the sheet surface layer 3 is formed on the substrate surface 2s of the substrate 2 so that the film thickness is a predetermined thickness (coating film forming process). Since the sheet in this state contains moisture, the functional sheet 1 of this embodiment can be manufactured by supplying the sheet to a drying process in which the sheet is dried at a predetermined temperature (e.g., 120°C). The dryer in the drying process is not particularly limited as long as it can dry the sheet.

As the coating method, in addition to the above-mentioned methods, for example, the following industrially usable methods may be adopted.
For example, the sheet surface layer forming liquid prepared as above is applied to another substrate (for example, Pet nonwoven paper, or a flat plate member described later) using a bar coater or the like to form a coating film. Next, a laminate is formed by laminating a substrate 2 (for example, copy paper) onto the coating film (corresponding to the object to be dried described later). That is, this laminate (object to be dried) is arranged in the order of the other substrate (flat plate member), coating film, and substrate 2 from one side to the other side so that the coating film is sandwiched between the other substrate (flat plate member) and the substrate surface 2s of the substrate 2. Next, this laminate (object to be dried) is supplied to a drying process in which it is dried at a predetermined temperature (for example, 120 ° C). In the drying process, a method can be adopted in which a sheet supplied from a supply port is dried with an internal heating unit using a high-temperature rotary dryer (for example, a rotary dryer manufactured by Kumagai Riki Kogyo Co., Ltd. described in the examples) whose inside can be set to a predetermined temperature, and the dried sheet is discharged from a discharge port.

The sheet surface layer 3 may be formed as a single layer as in the above example, but may also have a multi-layer structure. For example, sheet surface layer forming liquids with different solid content concentrations of cellulose nanofibers may be applied in sequence. In this case, it may be possible to more finely adjust the surface roughness of the sheet surface and the penetration power of the applied ink, compared to applying the ink in a single step.

When forming the functional sheet 1, which is a sheet-like member having the same structure as the sheet surface layer 3, by a coating method, a coating is formed on a flat plate member described below using a bar coater or the like. The functional sheet 1 having the above structure can be produced by supplying the coating to a drying process without laminating a substrate 2 (e.g., copy paper) on top of the coating.

(Press method)
When the sheet surface layer 3 is formed on the substrate surface 2s of the substrate 2 by the heat press method, a sheet surface layer forming liquid prepared in the same manner is applied to one surface of a flat plate member.
The flat plate member is formed so that one surface thereof is a smooth surface (smooth surface). Since the surface roughness of the smooth surface of the flat plate member greatly affects the surface roughness of the functional sheet 1 of this embodiment (i.e., the surface roughness of the surface 3s of the sheet surface layer 3), it is preferable to adopt a flat plate member formed so as to have the same surface roughness as the surface 3s of the sheet surface layer 3 described above. For example, a flat sheet or flat film made of a resin such as polyethylene or polypropylene can be adopted as the flat plate member.
Next, the substrate 2 is arranged so that the sheet surface layer forming liquid applied to the smooth surface of the flat plate member is sandwiched between the flat plate member and the substrate 2. At this time, the substrate 2 is arranged so that the substrate surface 2s of the substrate 2 is parallel to the smooth surface of the flat plate member. The substrate, the sheet surface layer forming liquid, and the substrate 2 arranged in this order are then sandwiched between a pair of draining sheets to prepare an object to be dried. In other words, the object to be dried is arranged in the order of draining sheet, flat plate member, sheet surface layer forming liquid, substrate 2, and draining sheet (see FIG. 2(A)).
Next, the prepared material to be dried is fed to the pressure section of a heat press machine. This pressure section is designed to apply uniform pressure to the material to be dried and to apply a desired temperature to the material to be dried.

As described above, by supplying the material to be dried to the pressure section of the heat press machine, the sheet surface layer forming liquid can be uniformly applied to the substrate surface 2s of the substrate 2. In other words, by using the heat press method, the coating film formation for the sheet surface layer 3 on the substrate surface 2s of the substrate 2 (coating film forming step) and the drying step can be performed simultaneously, which provides the advantage of excellent productivity.
Then, when the drying is completed, the workpiece is removed from the heat press, and the draining sheet and the flat plate member are removed to produce the functional sheet 1 of this embodiment (see FIG. 2(D)). At this time, since the coating surface for the sheet surface layer 3 is formed in contact with the smooth surface of the flat plate member, there is also an advantage that the surface 3s of the sheet surface layer 3 can be formed into a smooth surface.

In this specification, coating film formation refers to forming a film for the sheet surface layer 3 on the substrate surface 2s of the substrate 2, and includes the concept of forming a coating film by spreading the sheet surface layer forming liquid with a bar coater or the like, and forming a film by applying pressure to the sheet surface layer forming liquid applied to the substrate surface 2s of the substrate 2.

The drainage sheet for the material to be dried is not particularly limited as long as it is a sheet-like material that has water permeability and can remove wastewater when pressurized. For example, the drainage sheet can be made of wire mesh, wire sheet, plastic wire, a sheet with many through holes (such as a punched sheet), nonwoven fabric, blanket, etc.

The material to be dried may also have a water-absorbent sheet disposed on the other side of the liquid-draining sheet that is not in contact with the flat plate member or base material 2 (see FIG. 2(B)). By disposing a water-absorbent sheet between the liquid-draining sheet and the pressure unit, the water discharged during pressure application can be more appropriately collected. There are no particular limitations on the water-absorbent sheet, so long as it is capable of absorbing water. For example, the water-supply sheet can be made of filter paper, nonwoven fabric, blanket, felt, etc.

Furthermore, if a pressure-following heat press is used so that the same pressure is always applied to the material to be dried when pressure is applied, it becomes easier to form the sheet surface layer 3 with the desired thickness.

The pressing method is a method that does not have a function equivalent to the drying step in the heat pressing method, and in this method, the prepared material to be dried is supplied to the drying step in the same manner as in the coating method to produce the functional sheet 1 of this embodiment.

(Drying temperature)
The drying temperature in the drying step (including the drying temperature in the heat pressing method) may be any temperature that allows the moisture in the formed coating film for the sheet surface layer 3 to evaporate appropriately.
For example, the drying temperature can be 50° C. or higher, more preferably 80° C. or higher. Furthermore, if the drying temperature is set higher than the boiling point of water (for example, 100° C. or higher), water vapor can be generated in the coating film for the sheet surface layer 3. The water vapor generated in the coating film moves toward the coating film surface, and at that time, a water vapor migration path is formed in the coating film. This water vapor migration path becomes the flow path 3h of the sheet surface layer 3 of the functional sheet 1 of this embodiment.

On the other hand, even if the drying temperature is lowered below 100°C and water vapor is not generated in the coating film, flow paths 3h can be formed in the sheet surface layer 3 of the functional sheet 1 of this embodiment. For example, flow paths 3h can be formed between the fibers by increasing the fiber diameter of the cellulose nanofibers in the sheet surface layer forming liquid (for example, an average fiber diameter of several tens of nm to about 100 nm) or by mixing the cellulose nanofibers with fine fibers ff having an average fiber diameter of 1 μm or less that is thicker than the cellulose nanofibers. In this case, the drying temperature can be made lower than the boiling point of water, which is the solvent of the sheet surface layer forming liquid, making it easier to form a smooth surface.

Therefore, the drying temperature in the drying process may be set to 100°C or higher when water vapor is generated in the coating film for the sheet surface layer 3 as described above, whereas the drying temperature may be set to lower than 100°C when the temperature is adjusted based on the fiber diameter of the fine fibers ff in the sheet surface layer forming liquid, and may be appropriately adjusted depending on the method for forming the flow paths 3h of the sheet surface layer 3 of the functional sheet 1 of this embodiment.
When the former method is employed, the temperature is preferably 100° C. or higher, more preferably 120° C. or higher. The upper limit is not particularly limited, but from the viewpoint of operability, the temperature is preferably 200° C. or lower, more preferably 150° C. or lower. When the latter method is employed, the temperature is preferably lower than 100° C., and from the viewpoint of operability, the temperature is preferably 80° C. or lower and 50° C. or higher.

In the above example, the functional sheet 1 is described as having a substrate 2 that has a function of allowing liquid to permeate therethrough, but as described above, the substrate 2 may have a structure that allows liquid to penetrate less easily or does not allow liquid to penetrate therethrough. In other words, as long as the sheet surface layer 3 of the functional sheet 1 has the above structure and has the same function as described above (the function of adjusting the ink adhesion speed), the material and structure are not particularly limited. In this case, the degree of freedom of the functional sheet 1 can be improved.
Various materials can be used as the substrate 2. For example, coated paper having a film-like material coated on its surface, a resin material, or a glass material can be used as the substrate 2.

In the above example, the flow paths 3h of the sheet surface layer 3 of the functional sheet 1 are formed to communicate with the front and back of the sheet surface layer 3, but the flow paths 3h may have no opening on the substrate 2 side. In other words, the sheet surface layer 3 may have a structure in which, when ink is applied to the surface 3s (sheet surface), the ink enters the flow paths 3h but does not penetrate to the substrate 2.
If the sheet surface layer 3 has a flow path 3h having such a structure, the ink applied to the sheet surface (i.e., the surface 3s of the sheet surface layer 3) can penetrate into the flow path 3h, thereby achieving the same effect as described above.

In the above example, a case where fine fibers ff are mixed into the sheet surface layer forming liquid is described, but as described above, resin may be mixed in place of or together with the fine fibers ff.

The resin is not particularly limited as long as it can form the flow paths 3h when the sheet surface layer 3 is formed. For example, the resin may be PVA or the like as described above. In particular, from the viewpoint of ease of handling, water-soluble resins such as PVA are preferable because they can use water as a solvent and have affinity with the cellulose nanofibers used as the fine fibers ff.
The resin can be mixed in the sheet surface layer forming liquid so that the solid content concentration becomes a predetermined concentration.

For example, when the above-mentioned coating method or pressing method is used, a resin is mixed into the sheet surface layer forming liquid, and the solid content concentration of the resin is adjusted to 1% by mass or more.
In these methods, when the solid content of the resin is lower than 1% by mass, the fluidity becomes high, making it difficult to form a surface layer film when forming the sheet surface layer. In addition, the amount of water absorbed by the substrate 2 increases, so that the sheet shrinks more when drying the functional sheet 1, making it more likely to wrinkle. Furthermore, the amount of energy consumed for drying also increases. On the other hand, when the solid content of the resin is higher than 10% by mass, the fluidity becomes low, making it difficult to form a uniform sheet surface layer.

Therefore, when producing functional sheet 1 by the above method, from the viewpoint of forming a uniform sheet surface film, the resin content of the sheet surface forming liquid is preferably 1% by mass or more and 15% by mass or less in solids concentration, more preferably 1% by mass or more and 10% by mass or less in solids concentration, and even more preferably 2% by mass or more and 8% by mass or less in solids concentration.

The present invention will be described in more detail below with reference to examples. Note that the present invention is not limited to the following examples.

<Experiment 1>
(Preparation of functional sheet)
Copy paper (Multipaper Super White J 701-697, manufactured by Askul Corporation) was used as the substrate for the functional sheet.
The sheet surface layer forming liquid for forming the sheet surface layer of the functional sheet was prepared by dispersing cellulose nanofibers in water to a predetermined solids concentration (hereinafter, the prepared sheet surface layer forming liquid is referred to as a CNF dispersion).
The cellulose nanofibers were prepared using a product manufactured by Sugino Machine Ltd. (model number: AMa).

The average fiber width and average fiber length of the obtained cellulose nanofibers were measured using a scanning probe microscope (manufactured by Shimadzu Corporation, model number: SPM-9700, simply referred to as SPM).
SPM measurement method: Nanocellulose was prepared with pure water to a solid content concentration of 0.001 to 0.005% by mass, and spin-coated onto a silica substrate coated with PEI (polyethyleneimine). 20 nanocellulose pieces were randomly selected from the spin-coated silica substrate and observed using an SPM.
The cellulose nanofibers had an average fiber width of 10 nm to 50 nm and an average fiber length of 100 nm to several μm.

The obtained cellulose nanofibers were used to prepare a CNF dispersion using the above formula (2) so that the solids concentration was 2% by mass.

The prepared CNF dispersion was applied onto the surface of the substrate to form a sheet surface layer.
The sheet surface layer was formed by the following heat pressing method.
First, 15 mL of CNF dispersion was applied onto a polypropylene sheet (hereinafter referred to as PP film). Copy paper (simply indicated as paper in FIG. 2) serving as a substrate was placed on the applied CNF dispersion. Then, the laminate of the PP film, CNF dispersion, and copy paper was sandwiched between a pair of metal wire sheets (mesh 300) (simply indicated as wire sheets in FIG. 2). Furthermore, a water-absorbing sheet was placed on the outside of each of the metal wire sheets. As shown in FIG. 2(B), a laminate in which the water-absorbing sheet, metal wire sheet, copy paper, CNF dispersion, PP film, metal wire sheet, and water-absorbing sheet were laminated from the upper side in this order was subjected to a heat press machine (manufactured by AS ONE Corporation, model number: AH-2003C) (see FIG. 2(C)), and a functional sheet was produced (see FIG. 2(D)).

The absorbent sheet used in the experiment corresponds to the "absorbent sheet" in this embodiment, the metal wire sheet corresponds to the "drainage sheet" in this embodiment, the PP film corresponds to the "flat member" in this embodiment, the CNF dispersion corresponds to the "sheet surface layer forming liquid" in this embodiment, and the copy paper corresponds to the "substrate" in this embodiment.

The functional sheet was removed from the heat press after the temperature had sufficiently dropped in order to prevent the functional sheet from curling when the heat press was released.

The conditions for the heat press were as follows:
Heating temperature: 120°C, heating time: 20 minutes, pressure: 2 MPa
The pressing pressure was adjusted to a constant pressure since it decreased over time.

The surface and cross section of the prepared functional sheet (hereinafter also referred to as CNF-coated paper) were observed using a low-vacuum scanning electron microscope (SEM) (JSM-it300LA, manufactured by JEOL Ltd.) The surface and cross section of the base copy paper were also observed.
The surface observation conditions were as follows:
The deposition was carried out by depositing platinum to a film thickness of 18 nm.
The acceleration voltage was set to 10 to 15 kV, and the vacuum level was set to HighVac. mode.
To prepare the cross section, a cross section polisher (manufactured by JEOL Ltd.) was used, with the voltage set to 4.5 kV, the Ar gas flow rate set to 4.9 mL/min, and the processing time set to 2 hours, and cross section processing was performed while cooling with liquid nitrogen.

FIG. 3 shows the observation results of each surface and each cross section.
As shown in Figure 3, fibers were clearly visible on the surface of the copy paper, and unevenness of various sizes was confirmed. On the other hand, no large holes like those seen on copy paper were confirmed on the surface of the functional sheet. This is because the surface of the functional sheet is covered with a dense fiber structure of cellulose nanofibers, which covers the surface of the base paper (copy paper).
As shown in FIG. 3, the thickness of the sheet surface layer of the functional sheet was about 9 μm.

The surface roughness of the functional sheet (surface roughness) was measured using a confocal laser microscope (LSM5 PASCAL MAT, manufactured by Carl Zeiss).
The laser irradiation area was 400 ^μm2 .
4, the surface roughness of the functional sheet was 1 μm to 2 μm. In other words, it was confirmed that the surface roughness of the functional sheet produced by this production method was 3 μm or less.
The surface roughness of the PP film used in the heat pressing method was 0.115 μm.

(Evaluating the ink erasing function of functional sheets)
Next, the ink erasing function of the functional sheets thus prepared was evaluated.

(Evaluation using a contact angle measuring instrument)
First, in the experiment, a contact angle measuring device (KES-FB4-A, manufactured by Kato Tech Co., Ltd.) was used to confirm that the sheet surface layer of the functional sheet controlled the penetration of ink.
In the experiment, 5 mg of ink was dropped onto the surface of the functional sheet (the surface of the sheet outer layer), and the state of the ink was observed after 1 second, 5 seconds, and 30 seconds using a contact angle measuring device.
As a comparative example, copy paper was used as the substrate.
The ink used was "Jet Stream" oil-based ballpoint ink manufactured by Mitsubishi Pencil Co., Ltd.

Figure 5 shows the state when ink is dropped onto a functional sheet and copy paper. From the top, it shows the state after 1 second, 5 seconds, and 30 seconds. It was confirmed that the height of the liquid level changes over time in both the functional sheet and copy paper cases.
In the case of the functional sheet, the thickness was 106 μm, 88 μm, and 70 μm after 1 second, 5 seconds, and 30 seconds, respectively. On the other hand, in the case of the copy paper, the thickness was 88 μm, 70 μm, and 52 μm after each number of seconds. The wetting spread also changed over time. In the case of the functional sheet, the thickness was 0.43 cm, 0.44 cm, and 0.48 cm after 1 second, 5 seconds, and 30 seconds, respectively. In the case of the copy paper, the thickness was 0.43 cm, 0.44 cm, and 0.52 cm after each number of seconds.

Here, the change in the liquid level indicates not only the degree to which the substrate absorbs the ink, but also how the ink spreads over the substrate.
Therefore, as shown in Figure 6, the extent of ink spreading 5 seconds after the ink was dropped was 0.44 cm for both the functional sheet and the copy paper, so we decided to examine the ink absorption by focusing on the state after 5 seconds. Five seconds after the ink was dropped, the height of the ink surface was 88 μm for the functional sheet and 70 μm for the copy paper. In other words, the copy paper had a lower ink surface by 18 μm.
Even though the ink spread is the same, the ink level is lower on the copy paper, which indicates that the copy paper absorbs more ink than the functional sheet. In other words, the functional sheet absorbs ink more slowly than the copy paper. This phenomenon was observed even one second after the ink was dropped.
From these experimental results, it was confirmed that the main factor affecting ink penetration into functional sheets is the presence or absence of a sheet surface layer. In other words, it was confirmed that ink penetration can be suppressed by providing a sheet surface layer on the surface of copy paper.

(Effect of sheet surface on ink solidification)
Next, the influence of the sheet surface layer of the functional sheet on the solidification of the ink applied to the surface of the functional sheet (the surface of the sheet surface layer) was confirmed.

In this experiment, it was confirmed that the ink penetrates into the sheet surface layer of the functional sheet from the viewpoint of the amount of evaporation of the ink applied to the surface of the functional sheet (the surface of the sheet surface layer). It was also confirmed that this penetration power is smaller than that of the copy paper of the comparative example. In other words, this experiment confirmed that the penetration power of the ink applied to the surface of the functional sheet can be reduced by providing a sheet surface layer.

In the experiment, a certain amount of ink was dropped onto a functional sheet using a syringe of a contact angle measuring instrument, and the amount of ink that evaporated from the functional sheet was confirmed.
As a comparative example, a glass plate (slide glass: manufactured by Toshin Rikoh Corporation) and copy paper (manufactured by Askul Corporation, Multipaper Super White) were used.
50 mg of ink was dropped onto each surface (the surface of the sheet surface layer in the case of functional sheets). The weight change after dropping was measured using an analytical balance (GR300, manufactured by A&G). The weight change was calculated by measuring the weight of the functional sheets and other materials used in the experiment in advance, and subtracting the weight of the ink and its change. This experiment was easily affected by temperature and humidity, so it was carried out under conditions of a humidity-controlled room (23°C, humidity 50%).

The upper graph in FIG. 7 is a graph showing the change in ink weight over time for 5 hours after the ink was dropped.
The lower diagram in FIG. 7 shows the state 5 hours after the ink was dropped.
The largest ink surface area was copy paper, followed by glass plate and functional sheet. Comparing the glass plate and functional sheet, the glass plate showed a tendency to wet and spread more. In other words, the glass plate had a larger surface area exposed to air, but the functional sheet took longer to dry than the upper diagram in Figure 7. The item that dried the fastest was copy paper. Conversely, the item that dried the slowest was the glass plate, and the functional sheet dried a little faster than the glass plate.
From the experimental results, it was thought that the copy paper had many voids inside, as is clear from the SEM observation results (Figure 3), and therefore the ink tended to penetrate into the inside and evaporate into the atmosphere faster than the other samples. On the other hand, since there was no penetration into the glass plate, the ink only evaporates to the upper side (into the atmosphere), which is thought to be why the weight change was small.
It was confirmed that, with functional sheets, upward evaporation (evaporation into the atmosphere) occurs in the same way as with glass plates and copy paper, but the downward penetration force (i.e. the penetration force of ink from the surface of the sheet toward the substrate) is smaller than that of copy paper.

Next, the solidification time of the ink applied to the surface of the functional sheet (the surface of the sheet outer layer) was evaluated.

In the experiment, the drying speed of ink on the surface of a functional sheet or the like was evaluated with reference to the JIS ink drying test (K 5701-1) (see FIG. 8).
First, ink was applied to the surface of the functional sheet (surface of the sheet surface layer) using a bar coater (PI-1210, manufactured by Tester Sangyo Co., Ltd.) to adjust the coating thickness to 75 μm. The coating speed was set to 10 mm/sec. The solidification time of the applied ink was confirmed using a rubber stamp. Similarly, a glass plate and copy paper were used as comparative examples.
In the experiment, a rubber stamp was pressed against the applied ink with a load of 250 g, and then this rubber stamp was transferred to another copy paper with a load of 250 g, as shown in Figure 8 (B). This procedure was repeated at regular intervals in a humidity-controlled room, and the time until the ink could no longer be transferred to the copy paper was measured.
The results are shown in Figure 9. As shown in Figure 9, it took the longest time for the glass plate to dry, taking 150 minutes. The fastest drying time was for copy paper (60 minutes). On the other hand, it took 90 minutes for the functional sheet. From these experimental results, it was confirmed that the functional sheet, by providing a sheet surface layer, can delay the ink solidification time compared to copy paper.

(Handwriting erasure test)
It was confirmed that the functional sheet allows handwriting formed with ink on the sheet surface (surface of the sheet outer layer) to be erased for a certain period of time, but becomes difficult to erase after the certain period of time has passed.

In the experiment, we created an ink erasing device that can mechanically evaluate the ink erasing function quantitatively.
This ink erasing device was designed to allow the erasing speed, erasing pressure, and eraser installation angle to be adjusted as desired so that the conditions for erasing with the eraser could be fixed (see Figure 10). The outline of the ink erasing device is as follows.

(Erasing speed)
As shown in Fig. 10, the ink erasing device has a parallel movement type incubator installed under the eraser, and the eraser speed can be adjusted by adjusting the rotation speed (rpm) of the incubator. The parallel movement speed of the eraser was adjusted to 150 cm/min by setting the rotation speed of the incubator to 25 rpm. This experiment was conducted in accordance with the test for measuring the erasing performance of erasers specified in JIS 6050:2008.
The number of reciprocating movements was 10.
(Erasing pressure)
As shown in FIG. 10, a rod-shaped member was provided so as to be perpendicular to the axial direction of the support column of the eraser, and a weight (250 g) was attached to this rod-shaped member to serve as the load of the eraser.
(Eraser installation angle)
As shown in FIG. 10, a stick-shaped eraser (Tombow, MONO) was used and fixed at an angle of 45° to the surface of the functional sheet.

The ink eraser creates a straight line of writing on the surface of copy paper using a pencil (manufactured by Mitsubishi Pencil Co., Ltd., hardness HB), and it has been confirmed that the ink eraser can erase more than 98% of this writing in just two passes.

The evaluation of the erasure of handwriting was performed as follows.
Using the image processing software Win-Roof 2013, the area of the part where graphite was attached before and after the eraser was moved back and forth was compared, and the remaining rate of graphite in the pencil was calculated.
The image processing conditions were a selection width of 700 μm, and RGB thresholds were set in the range of 0 to 150, and the remaining rate was calculated by comparing the number of pixels in the binarized images. The remaining rate was calculated based on the above formula (1).

Next, five lines were drawn on the surface of the functional sheet (surface of the sheet outer layer) with an arbitrary force using a Jet Stream made by Mitsubishi Pencil Co., Ltd. Next, the number of pixels in the ink area was measured using image processing software Win-Roof. Then, using the ink erasing device prepared, erasing tests were performed immediately after writing, and 1 hour, 3 hours, 6 hours, and 24 hours later. The number of pixels in the ink area after erasing was determined by image processing, and the ink remaining rate was calculated. The ink remaining rate was calculated by averaging the value of the five lines.
As a comparative example, the ink remaining rate was calculated in the same manner using copy paper.

Figure 11 shows photos of the functional sheet before and after the erasure test and the ink remaining rate after erasure. Figure 12 shows photos of the copy paper before and after the erasure test. Figure 13 shows a graph of the relationship between the two (ink remaining rate (%) and elapsed time).
As shown in Figures 12 and 13, about 80% of the ink remained on the copy paper of the comparative example even immediately after writing. On the other hand, as shown in Figures 11 and 13, only 2% of the ink remained on the functional sheet immediately after writing. In other words, it was confirmed that 98% of the ink could be erased immediately after writing. It was also confirmed that only 40% or less of the ink remained (i.e., about 60% of the ink could be erased) for up to about 10 hours, while 61% of the ink could remain after 24 hours. In other words, it was confirmed that with the prepared functional sheet, it was possible to erase the ink so that the ink remaining rate was about 40% or less for up to about 10 hours after writing, while it was confirmed that after 24 hours, the ink was fixed and difficult to erase.

The mechanism by which handwriting can be erased for a certain period of time immediately after writing on a functional sheet, but cannot be erased after the certain period of time has passed, is believed to be due to the structure of the functional sheet, as described below.

The cause of this was thought to be the difference in ink behavior due to the difference in structure between the functional sheet and copy paper.
As shown in FIG. 3, the copy paper is made of laminated pulp fibers, and the surface of the paper is uneven due to the laminated state of the fibers, and it can be seen that there are recesses in the recesses and gaps between the fibers. The cross-sectional observation also clearly shows that there are many gaps between the fibers. On the other hand, in the case of the functional sheet, no fibers were observed on the surface at the same magnification, and a dense structure of cellulose nanofibers was observed. The cross-sectional observation also confirmed that the surface layer of the functional sheet has a sheet surface layer made of cellulose nanofibers, and that it exists on the surface of the substrate with voids, with a dense structure of about 9 μm in thickness. The following was inferred by comparing the difference in structure between the functional sheet and the copy paper with the ink erasing results.

First, the phenomenon that occurred immediately after writing was inferred as follows.
In the case of copy paper, it is believed that the ink transferred by writing penetrates into the recesses on the paper surface and the gaps between the fibers. Furthermore, before the solvent in the ink evaporates, the viscosity of the vehicle (fixing agent) is low, i.e., it has high fluidity, so it is believed that the pigment or dye penetrates from the paper surface to the inside of the paper layer along with the fixing agent due to capillary action. Due to this ink behavior, even if you try to erase the ink that has penetrated into the paper layer with an eraser, the paper's recesses prevent the eraser from coming into contact with the ink that has penetrated into the paper layer, so it is believed that the ink cannot be erased.
On the other hand, in the case of the functional sheet, as shown by the results of the roughness measurement (surface roughness) of the sheet surface (surface of the sheet outer layer) being approximately 3 μm or less, it is possible to bring the eraser into contact with the written ink because the sheet surface is smooth, making it easier to erase the handwriting.

Next, the phenomenon that occurs when a certain amount of time has passed after writing is inferred as follows.
While no significant difference was observed in the appearance of the copy paper over time when compared to that immediately after writing, in the case of the functional sheet, as shown in Figures 11 and 13, it can be confirmed that the amount of ink that has adhered is clearly different immediately after writing and 24 hours later. It can also be confirmed that the amount of adhered ink increases as time passes.

The ink used in the experiment is mainly composed of polyvinyl butyral, a fixing agent, 3-methoxy-3-methylbutanol, a solvent, and carbon black and ethyl violet, colorants. When the ink solidifies, the solvent is reduced and the fixing agent is fixed. In other words, in this experiment, some of the solvent, 3-methoxy-3-methylbutanol, permeates from the surface of the functional sheet (the surface of the sheet outer layer) toward the substrate over time, and some evaporates from the part of the ink exposed to the air. In other words, it is thought that as the solvent in the ink applied to the surface of the functional sheet decreases, the concentration of polyvinyl butyral, a fixing agent in the ink, increases, causing the ink to fix near the surface of the functional sheet (i.e. the surface of the sheet outer layer and its interior).

In the ink erasing test with an eraser, it is believed that over time the strength of the bond between the ink and the functional sheet reverses between the physical force caused by the friction of the eraser and the intermolecular force acting between the ink and the benzene phthalate plasticizer, a component of the eraser. In other words, the viscosity of the ink is low immediately after writing because the concentration of solvents in the ink is high. However, over time the amount of solvents in the ink decreases as described above, and the concentration of polyvinyl butyral, the fixing agent, increases.

Then, it was thought that because the viscosity of the ink increases with an increase in the concentration of the fixing agent in the ink, the binding strength between the functional sheet and the ink increases over time compared to immediately after writing, making the ink more difficult to erase with an eraser. In other words, it was thought that the binding strength between the functional sheet and the fixing agent in the ink becomes stronger than the two actions that occur when erasing ink with an eraser, and as a result, the amount of ink that adheres increases over time, improving the retention rate.

(Relationship between surface smoothness of functional sheets and ink erasability)
Next, the effect of the average smoothness of the surface of the functional sheet (the surface of the sheet outer layer) on erasing ink with an eraser was examined.
In the experiment, a concave-convex functional sheet was produced by forming concaves and convexes on the surface of the functional sheet as follows.
This embossed functional sheet was produced in the same manner as the production of the functional sheet described above, except that a PET nonwoven fabric was used instead of the PP film.

The surface of the embossed functional sheet was observed with a confocal laser microscope in the same manner as in the observation of the surface of the functional sheet described above, and the surface roughness was confirmed in FIG.
As shown in Figure 14, irregularities of about 10 μm were observed on the surface of the irregular functional sheet. When A to E shown in the graph of surface irregularities shown in the lower diagram of Figure 14 were compared with A' to E' of the two-dimensional image in the upper diagram of Figure 14, they matched, respectively, and it was confirmed that the shape of the fibers of the PET nonwoven fabric was transferred to the surface of the irregular functional sheet. In other words, in the graph in the lower diagram of Figure 14, the depressions are caused by the fibers of the PET nonwoven fabric being pressed, and the protrusions are caused by the gaps formed between the fibers of the PET nonwoven fabric.

Next, a straight line was written on the surface of the prepared uneven functional sheet using ink in the same manner as for the functional sheet described above, and the erasure of the written mark was evaluated using the ink erasing device prepared. The evaluation method was the same as for the functional sheet described above.

The experimental results are shown in FIG. 15 and FIG.
Fig. 15 is a photograph showing the remaining area of ink when the ink was erased immediately after writing. Fig. 16 is a photograph showing the surface shape of the remaining area of ink in Fig. 15 measured using a confocal laser microscope.
As shown in Figure 16, a depression of about 10 μm was confirmed in the area where the ink remained on the surface of the uneven functional sheet. It is believed that the ink was in a position that could not be reached by the eraser due to the depression, and therefore the ink could not be erased with the eraser even immediately after writing.

<Experiment 2>
In experiment 1, a functional sheet (CNF-coated paper) was produced using the heat press method. When a handwriting erasure test was conducted on this functional sheet using a ballpoint pen, 98% of the ink was erased immediately after writing, and the ink remaining rate increased over time.
Therefore, in Experiment 2, it was confirmed that a functional sheet can be produced using a coating method that improves productivity.

In Experiment 2, an automatic coating device capable of continuous production (Tester Sangyo, PI-1210) and a Baker-type applicator (Tester Sangyo, SA-201) were used.
First, a CNF dispersion liquid with a solid content concentration of 5% (corresponding to the "sheet surface layer forming liquid" in this embodiment) was coated on a PEN film (Q51 by AS ONE Corporation, corresponding to the "flat member" in this embodiment).
The cellulose nanofibers (hereinafter referred to as CNF) used were prepared using cellulose nanofibers manufactured by Sugino Machine Ltd. (model number: BiNFi-s FMa).
Coating conditions: The coating thickness was set to 4 mil (101.6 μm) and 8 mil (203.2 μm), and the coating was formed at a speed of 50 mm/sec. The mil unit of the coating thickness is the unit of coating clearance set in the Baker applicator, and 4 mil corresponds to a clearance of 101.6 μm and 8 mil corresponds to a clearance of 203.2 μm.
Next, copy paper (Multipaper Super White, manufactured by Askul Corporation, which corresponds to the "substrate" in this embodiment, hereinafter sometimes simply referred to as PPC) was placed on the surface of the coating film (which corresponds to the sheet surface layer of the functional sheet) formed on the surface of the PEN film.
This three-layer laminate consisting of the PEN film, coating film, and copy paper (corresponding to the material to be dried in this embodiment) was passed through a rotary dryer (manufactured by Kumagai Riki Kogyo Co., Ltd.) heated to 80°C for approximately 7 minutes to dry it, producing a functional sheet.

The thickness of the functional sheet (also called CNF coated paper) was measured according to JIS 8118, and the thickness of the sheet surface layer (coating film) of the functional sheet was calculated. In addition, the mass of the functional sheet and PPC were measured, and the coating amount of the CNF dispersion was calculated.

A cross-section polisher (HITACHI, IM5000 CTC) was used to prepare a cross section of the functional sheet.
The cross-section was processed under the following processing conditions: voltage 2.5 kV, gas flow rate 0.14 cm ³ /min, processing time 2.5 hours.
Surface and cross-sectional observations of the functional sheet and PPC were performed using a tabletop scanning electron microscope (Phenom-World, G2 pro-HR).
Observation conditions: Acceleration voltage 10 kV

FIG. 17 shows the coating amount and the thickness (film thickness) of the sheet surface layer of functional sheets produced with coating film thicknesses of 4 mil and 8 mil.
Graph (A) in FIG. 17 shows the calculated coating weight, and graph (B) shows the calculated film thickness of the sheet surface layer.
By increasing the coating thickness from 4 mil to 8 mil, the coating weight increased from 4.6 g/ ^m2 to 10.5 g/ ^m2 and the coating thickness increased from 5.4 μm to 10.1 μm, respectively, and the surface layer of the functional sheet was almost doubled in value, in line with the coating thickness.
From these results, it was confirmed that the film thickness and coating amount of the surface layer of the functional sheet can be controlled by the coating film thickness.

FIG. 18 shows the results of observation of the surfaces and cross sections of the functional sheet and PPC.
Figures 18(a) and (b) are a surface image and a cross-sectional image of the PPC.
Figure 18 (c) and (d) are surface and cross-sectional images of functional sheet A (CNF coated paper) produced with a coating thickness of 4 mil.
Figure 18 (e) and (f) are surface and cross-sectional images of functional sheet B (CNF coated paper) produced with a coating thickness of 8 mil.

Surface observation revealed that pulp fibers and fillers were observed in the PPC, whereas no pulp fibers were observed in the functional sheet (CNF-coated paper), suggesting that the PPC surface was covered by the CNF contained in the sheet surface layer.
From the cross-sectional observation, it was observed that a CNF coating layer was formed on the surface layer of the functional sheet (CNF coated paper). In addition, the thickness of the surface layer (also called the CNF layer) of the functional sheet (CNF coated paper) was 3.9 μm for a coating thickness of 4 mils (functional sheet A) and 6.8 μm for a coating thickness of 8 mils (functional sheet B). This result differed from the measured thickness of the surface layer of the sheet by a paper thickness gauge by about 1.5 to 3 μm. However, it was confirmed that the thickness of the surface of the functional sheet followed the coating thickness.
From the above results, it was possible to produce a functional sheet using conventional coating technology.

(Handwriting erasure test)
In the experiment, the handwriting of the ballpoint pen was evaluated using the same ink erasing device as in Experiment 1.
(1) Writing Speed Using the same ballpoint pen as in Experiment 1, the writing speed was adjusted to 150 cm/min to write a straight line.
(2) Writing Pressure A 500 g weight was attached to the ballpoint pen to keep the writing pressure constant.
(3) Ballpoint Pen Installation Angle The ballpoint pen was fixed at a 90 degree angle to the paper.
(4) Erasing of Handwriting An erasing test was carried out under the same conditions as in Experiment 1 using the same ink erasing device.

(Results of handwriting erasure test)
FIG. 19 shows the results of a handwriting erasure test on functional sheets (CNF coated paper) made with coating thicknesses of 4 mil and 8 mil.
Fig. 19 shows optical microscope images of the results of erasing ballpoint pen marks. In Fig. 19, (A, a) shows the result immediately after writing, (B, b) shows the result after 2 hours, and (C, c) shows the result after 48 hours.
As shown in FIG. 19, immediately after writing (A, a), handwriting could be erased on both functional sheets made with coating thicknesses of 4 mil and 8 mil, but it was confirmed that ink gradually remained over time after 2 hours (B, b) and 48 hours (C, c).

From the above results, we were able to create a functional sheet (CNF coated paper) using a coating method that allows for continuous production. Furthermore, on this functional sheet (CNF coated paper), characters written with a ballpoint pen could be easily erased with an eraser immediately after writing, and the ink retention rate increased over time.

<Experiment 3>
In experiment 3, it was confirmed that a functional sheet having a sheet surface layer formed using a polymer material (resin) instead of CNF exhibited effects similar to those in experiments 1 and 2.
In the experiment, polyvinyl alcohol (hereinafter simply referred to as PVA) was used as an example of a polymer material.

(Preparation of PVA aqueous solution)
Granular PVA (Poval N-300, manufactured by Kuraray Co., Ltd.) was gradually added to warm water at 80° C. with stirring to prepare a PVA aqueous solution with a solids concentration of 10% (corresponding to the “sheet surface layer forming liquid” in this embodiment).

In the experiment, the same automatic coating device and Baker-type applicator as in Experiment 2 were used to coat the PVA aqueous solution onto the same PEN film as in Experiment 2.
Next, copy paper was placed on the surface of a coating film (corresponding to the sheet surface layer of the functional sheet) made of an aqueous PVA solution formed on the surface of the PEN film, in the same manner as in Experiment 2, to prepare a three-layer laminate (corresponding to the material to be dried in this embodiment).
This laminate was passed through a rotary dryer heated to 80° C. for about 7 minutes to dry, thereby producing a functional sheet.

The thickness of the prepared functional sheet (also called PVA coated paper) was measured in accordance with JIS 8118, and the thickness of the sheet surface layer (coating film) of the functional sheet was calculated.
The cross section of the functional sheet was prepared using the same device and under the same conditions as in Experiment 2. The surface and cross section were observed using the same device and under the same conditions as in Experiment 2.

FIG. 20 shows the thickness (film thickness) of the sheet surface layer of functional sheets produced with coating film thicknesses of 4 mil and 8 mil.
By increasing the coating thickness from 4 mil to 8 mil, the film thickness increased from 2.6 μm to 4.8 μm, and the sheet surface layer of the functional sheet had a thickness nearly double that of the coating thickness.

FIG. 21 shows the results of observing the surface and cross section of the functional sheet.
21A and 21A are surface and cross-sectional images of a functional sheet using PVA produced with a coating thickness of 4 mil.
FIG. 21B and FIG. 21B are surface and cross-sectional images of a functional sheet using PVA produced with a coating thickness of 8 mil.

Surface observation showed that the coating surface was smooth. In addition, cross-sectional observation showed that although there was some variation in thickness of the surface layer of the functional sheet (PVA coating), it was possible to form a PVA coating layer.

(Handwriting erasure test)
The handwriting erasure test was performed using the same ballpoint pen as in Experiment 1 and the same ink erasing device as in Experiment 1.
Functional sheets made with coating thicknesses of 4 mil and 8 mil were written on with a ballpoint pen and then subjected to an erasure test. The results showed that the handwriting could be erased immediately after writing, and the ink retention rate increased over time.

<Experiment 4>
In experiment 4, the liquid permeation control function of the sheet surface layer of the functional sheet was evaluated.
In the experiment, t-butyl alcohol (manufactured by Wako Pure Chemical Industries, Ltd.: 028-03386) was used as the evaluation liquid.

10.0 mg of t-butyl alcohol was dropped onto the surface of each test piece (in the case of a functional sheet, the surface of the sheet outer layer) in one place so as to form an approximately circular shape.
For each test piece, copy paper (Askul Corporation, Multipaper Super White, basis weight 68 g/ ^m2 , 210 mm x 297 mm), the functional sheet prepared in experiment 2 (CNF dispersion, coating thickness 4 mil (101.6 μm), 210 mm x 297 mm), and the functional sheet prepared in experiment 3 (PVA aqueous solution, coating thickness 4 mil (101.6 μm), 210 mm x 297 mm) were used.
The weight change after dropping was measured using an analytical balance (GR300 manufactured by A&G).
The change in weight of t-butyl alcohol in each test piece was calculated by subtracting the weight of each test piece before the addition of t-butyl alcohol, which was measured in advance, from the weight of each test piece at the time of mass measurement.
The retention rate (%) of t-butyl alcohol after a predetermined time had elapsed was calculated by dividing the mass of t-butyl alcohol at each measurement time by the added mass, and expressing the result as a percentage.
Since this experiment is easily affected by temperature and humidity, it was carried out in a humidity-controlled room (23° C., humidity 50%).

The experimental results are shown in FIG.
FIG. 22 shows the time series of the retention rate (%) of t-butyl alcohol for 30 minutes after t-butyl alcohol was dropped onto each test piece.
22, in the functional sheets of Experiments 2 and 3, when 10 mg of t-butyl alcohol was added, the retention rate after 2 minutes at a temperature of 23 and humidity of 50% was 92% by mass, and after 4 minutes the retention rate was 80% by mass or more. Furthermore, the retention rate after 6 minutes was 70% or more, and even after 10 minutes the retention rate was 50% or more.
On the other hand, in the case of copy paper, the retention rate after 4 minutes was about 50% by mass, after 8 minutes it was about 10%, and after 10 minutes it was almost zero.

The experimental results show that the retention rate of butyl alcohol varies depending on the material of the surface layer of the functional sheet. In other words, it was confirmed that the penetration of liquid can be more appropriately controlled by adjusting the material of the surface layer of the functional sheet. Therefore, it has become clear that by using the functional sheet of the present invention, it is possible to appropriately control the solidification speed of ink from ballpoint pens, markers, etc., and to control the ink erasing function and ink remaining rate.

The functional sheet of the present invention is suitable for use on various documents where it is recommended that handwriting cannot be erased in the end, such as official documents, company documents, hospital charts, and experiment records, as well as important documents and papers such as resumes, application forms for job hunting, contracts, and applications and papers that require a signature.

1 Functional sheet 2 Substrate 2s Substrate surface 3 Sheet surface layer 3s Surface of sheet surface layer

Claims (23)

A sheet member capable of controlling adhesion of ink applied to a sheet surface ,
The substrate has a substrate and a sheet-like sheet surface layer provided on a substrate surface of the substrate,
The sheet surface layer is
The sheet has a flow path capable of controlling the permeation of a solvent contained in the ink from the surface of the sheet surface layer toward the base material , and has an open-cell structure in which bubble-like voids are connected,
When 10.0 mg of t-butyl alcohol is applied to the surface of the sheet surface layer, the retention rate of t-butyl alcohol at a temperature of 23° C. and a humidity of 50% satisfies any one of the following conditions (1) to (5):
A functional sheet characterized by the above.
(1) The retention rate of t-butyl alcohol after 2 minutes is 90% or more and 92% or less by mass.
(2) The retention rate of t-butyl alcohol after 4 minutes is 70% or more and 83% or less by mass.
(3) The retention rate of t-butyl alcohol after 6 minutes is 50% or more and 72% or less by mass.
(4) The retention rate of t-butyl alcohol after 8 minutes is 40% or more and 62% or less by mass.
(5) The retention rate of t-butyl alcohol after 10 minutes is 30% or more and 54% or less by mass. The surface of the sheet surface layer is
The surface roughness calculated using a confocal laser microscope is 0.1 μm or more and less than 10 μm.
2. The functional sheet according to claim 1. 3. The functional sheet according to claim 1, wherein the thickness of the surface layer of the sheet is 1 μm or more and 30 μm or less. The substrate is
4. The functional sheet according to claim 1 , wherein the substrate surface layer having the substrate surface has a function of allowing a liquid to permeate inward from the substrate surface. 5. The functional sheet according to claim 1 , wherein the substrate is made of a porous material. 6. The functional sheet according to claim 1 , wherein the substrate is made of paper. The sheet surface layer is
7. The functional sheet according to claim 1 , wherein the liquid permeability of the sheet surface layer is lower than the liquid permeability of the substrate surface layer. 8. The functional sheet according to claim 1, further comprising a connecting layer between the surface sheet and the base material. The sheet surface layer contains fine fibers,
The fine fibers are
9. The functional sheet according to claim 1, wherein the average fiber width is 3 nm to 1 μm and the average fiber length is 100 nm to 1 mm. 10. The functional sheet according to claim 9, wherein the fine fibers are cellulose nanofibers. 11. The functional sheet according to claim 1, wherein the open cell structure is made of a resin. The functional sheet according to claim 11, wherein the resin is polyvinyl alcohol. A method for producing the functional sheet according to any one of claims 1 to 12 ,
A drying step of applying the dispersion onto a surface of a substrate and drying the applied dispersion into a layer, wherein in the drying step,
The substrate coated with the dispersion is dried at 80° C. to 150° C. by a heat press method. Before carrying out the drying step,
the dispersion liquid is applied onto a flat surface of a flat plate member having a smooth surface, the flat plate member and the substrate are disposed so as to sandwich the applied dispersion liquid between the flat plate member and a substrate surface of the substrate, and drainage sheets are disposed on the back surface of the flat plate member and the back surface of the substrate, respectively, to prepare an object to be dried, and the object to be dried is supplied to the drying step;
In the drying step,
14. The method for producing a functional sheet according to claim 13, wherein the material to be dried is supplied to a pressure section of a heat press machine heated to a predetermined temperature, and the material to be dried is dried while being pressed. 15. The method for producing a functional sheet according to claim 13 , wherein the dispersion contains fine fibers and/or a resin. The method for producing a functional sheet according to claim 15 , wherein the fine fibers are mixed so that the solid content concentration is 1% by mass to 10% by mass. The method for producing a functional sheet according to claim 15 , wherein the resin is mixed so that the solid content concentration is 1% by mass to 15% by mass. A method for producing the functional sheet according to any one of claims 1 to 12 ,
A drying step of applying the dispersion onto a surface of a substrate by a coating method and drying the dispersion;
In the drying step,
A method for producing a functional sheet, comprising drying at 80°C to 150°C by a rotary drying method. In the step of applying the dispersion,
The dispersion liquid is applied in the form of a film on a flat surface of a flat plate member having a smooth surface, and then the substrate is arranged so that the coating film is sandwiched between the flat plate member and a surface of the substrate to prepare an object to be dried, and the object to be dried is supplied to the drying step;
In the drying step,
20. The method for producing a functional sheet according to claim 18 , wherein the material to be dried is supplied into a rotary dryer heated to a predetermined temperature, and the material is dried while being rotated. 20. The method for producing a functional sheet according to claim 18 , wherein the dispersion contains fine fibers and/or a resin. The method for producing a functional sheet according to claim 20 , characterized in that the fine fibers are mixed so that the solid content concentration is 1% by mass to 10% by mass. The method for producing a functional sheet according to claim 20 , wherein the resin is mixed so that the solid content concentration is 1% by mass to 15% by mass. A sheet member capable of controlling the permeation of liquid,
A functional sheet having the same structure as the sheet surface layer of the functional sheet according to any one of claims 1 to 12 .

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