JP7480744B2 - Pulp fiber-containing pre-sheet - Google Patents

Description

The present invention relates to a pre-sheet containing pulp fibers.

2. Description of the Related Art Fiber-reinforced composite materials, which are made of reinforcing fibers such as glass fibers or pulp fibers and a matrix resin, are used in general industrial components, sports and leisure goods, aircraft bodies, automobile bodies, building materials, and the like.
Known fiber-reinforced composite materials include fiber-reinforced plastics (FRP), which are made by impregnating a long-fiber cloth with a thermosetting resin and curing it, and fiber-reinforced thermoplastics (FRTP), which have chopped short reinforcing fibers dispersed in a thermoplastic resin.
As a method for producing a fiber-reinforced thermoplastic plastic, a method is known in which a pre-sheet for producing a fiber-reinforced thermoplastic plastic containing reinforcing fibers and thermoplastic fibers is produced in advance and this pre-sheet for producing a fiber-reinforced thermoplastic plastic is molded (see Patent Document 1).

JP 2014-047244 A

However, the method described in Patent Document 1 results in insufficient mechanical properties of the fiber-reinforced thermoplastic plastic, and is also insufficient in terms of environmental consideration. Therefore, an object of the present invention is to provide a pre-sheet for suitably producing an environmentally friendly fiber-reinforced thermoplastic plastic having excellent mechanical properties. Another object of the present invention is to provide an environmentally friendly fiber-reinforced thermoplastic plastic having excellent mechanical properties.

As a result of intensive research, the inventors discovered that the above-mentioned problem can be solved by using pulp fibers as reinforcing fibers, providing a layer containing fine fibrous cellulose, and further setting the content ratio of the fine fibrous cellulose within a specific range, and thus completed the present invention.
The present invention includes the following aspects.
[1] A pulp fiber-containing pre-sheet having a fine fibrous cellulose-containing layer formed on at least a pulp fiber-containing layer,
the pulp fiber-containing layer comprises pulp fibers and a thermoplastic resin;
the fine fibrous cellulose-containing layer is an outermost layer,
The content ratio of fine fibrous cellulose in the pulp fiber-containing pre-sheet is 0.5% by mass or more,
The content ratio of fine fibrous cellulose in the fine fibrous cellulose-containing layer is 10% by mass or more.
Pulp fiber-containing pre-sheet.
[2] The pre-sheet according to [1], wherein the content ratio of the pulp fibers in the pulp fiber-containing pre-sheet is 40 to 50 mass %.
[3] The pre-sheet according to [1] or [2], wherein the content ratio of the fine fibrous cellulose in the pre-sheet is 1.0 to 5.0 mass %.
[4] The pre-sheet according to any one of [1] to [3], wherein the content ratio of the resin in the fine fibrous cellulose-containing layer is 70% by mass or less relative to 100% by mass of the total cellulose.
[5] The pre-sheet according to any one of [1] to [4], wherein the content ratio of the fine fibrous cellulose in the fine fibrous cellulose-containing layer is 10 to 95 mass %.
[6] The pre-sheet according to any one of [1] to [5], wherein the pulp fiber-containing layer is composed of a pulp fiber layer and a thermoplastic resin layer.
[7] A pulp fiber-containing molding obtained by molding a single body or a laminate of the pre-sheet according to any one of [1] to [6].
[8] The pulp fiber-containing molding according to [7], which is used as an automobile part, an electric device part, a molded product for an electronic device part, or a precision device part.

The pre-sheet of this embodiment is environmentally friendly and can provide a fiber-reinforced thermoplastic plastic with excellent mechanical properties. The molded article of this embodiment is environmentally friendly and has excellent mechanical properties.

1 is a schematic diagram showing a web forming device used in a manufacturing method for producing a pulp fiber-containing pre-sheet of the present embodiment. FIG. 2 is a graph showing the relationship between the amount of NaOH dropped onto a slurry containing fibrous cellulose having phosphorus oxoacid groups and pH. FIG. 3 is a graph showing the relationship between the amount of NaOH dropped onto a slurry containing fibrous cellulose having a carboxy group and the pH.

1. Pre-sheet of the present embodiment
The pre-sheet of this embodiment is a pulp fiber-containing pre-sheet having a fine fibrous cellulose-containing layer formed on at least a pulp fiber-containing layer, the pulp fiber-containing layer containing pulp fibers and a thermoplastic resin, the fine fibrous cellulose-containing layer being the outermost layer, the content ratio of fine fibrous cellulose in the pulp fiber-containing pre-sheet being 0.5% by mass or more, and the content ratio of fine fibrous cellulose in the fine fibrous cellulose-containing layer being 10% by mass or more. According to the pre-sheet of this embodiment, a molded product having excellent mechanical properties can be provided by being subjected to molding processing.

<1-1. Layer structure of pre-sheet>
The layer configuration of the pre-sheet of this embodiment includes a configuration in which a pulp fiber-containing layer and a fine fibrous cellulose-containing layer are laminated. The layer configuration of the pulp fiber-containing layer includes a configuration in which a thermoplastic resin layer is laminated on at least one surface of a laminate of a plurality of pulp fiber layers. A thermoplastic resin layer may be disposed between the plurality of pulp fiber layers. The layer configuration of the pulp fiber-containing layer of this embodiment also includes a configuration in which a pulp fiber layer is laminated on at least one surface of a laminate of a plurality of thermoplastic resin layers. A pulp fiber layer may be disposed between the plurality of thermoplastic resin layers.

Thus, the pulp fiber-containing layer in the pre-sheet of this embodiment is a laminate consisting of one single layer or two or more layers. The number of layers and the composition of the layers included in the pulp fiber-containing layer can be set in various ways depending on the layer thickness of each layer that constitutes the pulp fiber-containing layer relative to the target thickness of the pulp fiber-containing layer, the desired physical properties of the pre-sheet and molded body, etc. The number of layers included in the pulp fiber-containing layer of this embodiment can be, for example, one, two, three, four, five, or more.

For example, the following pulp fiber-containing layer configurations, provided for purposes of illustration and not limitation, are within the scope of the present invention.
(1) A structure consisting of a pulp fiber layer only.
(2) A structure in which a thermoplastic resin layer and a pulp fiber layer are laminated.
(3) A configuration in which a thermoplastic resin layer, a pulp fiber layer, and a thermoplastic resin layer are laminated in this order.
(4) A configuration in which a thermoplastic resin layer, a pulp fiber layer, and a pulp fiber layer are laminated in this order.
(5) A configuration in which a thermoplastic resin layer, a thermoplastic resin layer, and a pulp fiber layer are laminated in this order.
(6) A configuration in which a pulp fiber layer, a thermoplastic resin layer, and a pulp fiber layer are laminated in this order.
(7) Any combination of the configurations shown in (2) to (6) above.
(8) A configuration in which one or more thermoplastic resin layers or pulp fiber layers are laminated on the surface or between the configurations shown in (2) to (7) above.

Among these, possible configurations of (7) will be explained with examples. For example, when the configuration of (7) is a combination of (2) and (2), such configurations include a configuration in which a thermoplastic resin layer, a pulp fiber layer, a thermoplastic resin layer, and a pulp fiber layer are laminated in this order, a configuration in which a thermoplastic resin layer, a pulp fiber layer, a pulp fiber layer, and a thermoplastic resin layer are laminated in this order, and a configuration in which a pulp fiber layer, a thermoplastic resin layer, a thermoplastic resin layer, and a pulp fiber layer are laminated in this order. In this way, when combining several configurations, configurations in which the laminates are laminated in the same orientation (back and front facing each other) as well as laminated with fronts adjacent to each other and backs adjacent to each other are included.

In addition, possible configurations of (8) will be explained with examples. For example, when the configuration of (8) is based on a combination of (3) and (4), such a configuration may include a configuration in which a thermoplastic resin layer, a pulp fiber layer, a thermoplastic resin layer, a thermoplastic resin layer, a thermoplastic resin layer, a pulp fiber layer, and a pulp fiber layer are laminated in this order. This is an example in which a thermoplastic resin layer is further added between (3) and (4). That is, for example, when one thermoplastic resin layer is thin, the thickness and strength of the obtained pre-sheet can be improved by stacking multiple thermoplastic resin layers in this way.

When multiple pulp fiber layers are included in one pre-sheet and pulp fiber-containing layer, each pulp fiber layer may be the same or different. Also, when multiple thermoplastic resin layers are included in one pre-sheet and pulp fiber-containing layer, each thermoplastic resin layer may be the same or different.

The outermost layer of the pulp fiber-containing layer is preferably a thermoplastic resin layer. When the outermost layer of the pulp fiber-containing layer is a thermoplastic resin layer, it has excellent adhesion to the fine fibrous cellulose-containing layer described below, and as a result, the pre-sheet and molded article of this embodiment have excellent mechanical properties.

Other layers may be placed between the layers to reinforce the structure.

<1-2. Pulp fiber-containing layer>
The pre-sheet of the present embodiment has a pulp fiber-containing layer. The pulp fiber-containing layer may be a single layer or multiple layers.
When the pulp fiber-containing layer of this embodiment is composed of a pulp fiber layer, the pulp fiber-containing layer of this embodiment is preferably formed by hot pressing one or more pulp fiber sheets, or by hot pressing one or more pulp fiber sheets and one or more thermoplastic resin sheets.
The pulp fiber-containing layer of the present embodiment includes pulp fibers and a thermoplastic resin. The pulp fibers are contained in the pulp fiber layer, and the thermoplastic resin is contained in the pulp fiber layer and/or the thermoplastic resin layer.
In this specification, the terms pulp fiber-containing layer and pulp fiber layer are used with different meanings, and the pulp fiber-containing layer is a higher-level concept that includes one or more pulp fiber layers and/or thermoplastic resin layers.

<1-3. Pulp fiber sheet and pulp fiber layer>
The pulp fiber sheet is subjected to a molding process to produce a molded body, thereby becoming a pulp fiber layer. The pulp fiber layer can become a part or the whole of the pulp fiber-containing layer. Unlike the fine fibrous cellulose-containing layer, the pulp fiber layer is a layer that does not intentionally contain fine fibrous cellulose and does not use fine fibrous cellulose.
The pulp fiber sheet is a sheet-like material containing pulp fibers and, if necessary, a thermoplastic resin. The pulp fiber sheet can be, for example, a nonwoven fabric, and can be formed by, for example, an airlaid method, a wet papermaking method, a spunlace method, a needle punch method, or the like.
Pulp fibers applicable to the pulp fiber sheet of the present embodiment are not particularly limited in terms of their manufacturing method and type. For example, they may be chemical pulps such as hardwood and/or softwood kraft pulp, mechanical pulps such as SGP, RGP, BCTMP, and CTMP, waste paper pulps such as deinked pulp, and non-wood pulps such as kenaf, jute, bagasse, bamboo, straw, and hemp. Chlorine-free pulps such as ECF pulp and TCF pulp may also be used.

Among the above pulp fibers, kraft pulp fiber, particularly softwood kraft pulp fiber (NBKP), is preferably used because it can improve the mechanical properties of the molded body obtained from the pre-sheet.

The pulp fibers may be in the form of, for example, chopped fibers. In particular, when the pulp fiber sheet of the present embodiment is formed by an airlaid method, the pulp fibers may be in the form of, for example, defibrated chopped fibers.
The average fiber length (Lm) of the pulp fibers used in the pulp fiber sheet of the present embodiment is preferably 1 mm or more and 100 mm or less, more preferably 2 mm or more and 80 mm or less, and even more preferably 2 mm or more and 60 m or less. When the average fiber length is within the above range, the production of the web is easy and a molded product with high strength can be obtained.
Here, chopped fiber refers to short fibers obtained by cutting a fiber bundle. A fiber bundle is a bundle of several hundred to several thousand reinforcing fibers bundled together with a binder such as water or resin. Defibrated chopped fiber refers to a large number of fibers obtained by defibrating chopped fiber.

As an example of chopped fiber that can be used in the present invention, chopped pulp fibers with an average fiber diameter of 4 to 10 μm and an average fiber length of 3 to 13 mm manufactured by Toho Tenax Co., Ltd. are known.

(Thermoplastic resin)
The pulp fiber sheet may contain a thermoplastic resin. The thermoplastic resin has a property of softening and becoming plastic when heated to an appropriate temperature, and solidifying when cooled. In this embodiment, the thermoplastic resin is a matrix resin in the molded body (fiber-reinforced thermoplastic plastic).

The pulp fiber sheet of this embodiment preferably contains a thermoplastic resin as a constituent component. The thermoplastic resin applicable to the pulp fiber sheet of this embodiment may be fibrous or particulate. From the viewpoint of increasing the strength of the molded product obtained from the pre-sheet, it is preferable that the thermoplastic resin is fibrous.

Examples of thermoplastic resins include polypropylene (PP), polyethylene terephthalate (PET), polyamides such as nylon 6, nylon 11, nylon 12, nylon 66, nylon 610, nylon 612, nylon 6T, nylon 9T, and nylon M5T, polycarbonate, polylactic acid, and polyetherimide (PEI). Two or more types of thermoplastic resins may be used in combination.

The thermoplastic resins of the pulp fiber layer and the thermoplastic resin layer that make up each layer in one press sheet may be the same or different. From the viewpoint of interlayer adhesion of the press molded body, it is preferable that they are the same. In addition, by blending different thermoplastic resins in the thermoplastic resin layer and the pulp fiber layer, the physical properties of each layer of the press sheet can be changed as desired. This makes it possible to impart functions to the molded product, for example, by making the physical properties of the surface and the interior different.

When the thermoplastic resin is fibrous, the fineness of the thermoplastic fiber is preferably 1 dtex to 120 dtex, and more preferably 1 dtex to 85 dtex. The average fiber length of the thermoplastic fiber is preferably 1 to 50 mm, more preferably 1 to 40 mm, and even more preferably 2 to 30 mm. When the fineness and average fiber length of the thermoplastic fiber are within the above ranges, it is easy to form a pulp fiber layer, and it is easy to obtain a uniform binding force and dispersion state.

When the thermoplastic resin is particulate, the average particle size of the thermoplastic particles is preferably 1 to 1,000 μm, and more preferably 10 to 800 μm.

When the thermoplastic resin is in pellet form, each side preferably has a length of 0.1 to 5 mm, and more preferably 0.5 to 5 mm.

(Heat-fusible resin)
The pulp fiber sheet may contain a heat-fusible resin. For example, when the pulp fiber sheet is formed by an airlaid method without using any water, that is, when the pulp fiber sheet is produced by a so-called TDS method (Totally Dry System), the pulp fiber sheet preferably contains a heat-fusible resin for thermal bonding that bonds the components together by heat. Since the TDS method does not use any water, the pulp fiber sheet can be formed without impairing the functions of the components.
The heat-fusible resin in the present invention, which may be blended as necessary, is a binder resin that can melt during pre-sheet preparation and bind the pulp fibers and the thermoplastic resin depending on the relationship between its melting point and the heat treatment temperature in the pre-sheet preparation step. The heat-fusible resin can also function as a matrix resin in a molded body (fiber-reinforced thermoplastic plastic). In this manner, in this embodiment, both the thermoplastic resin and the heat-fusible resin can be the matrix resin in the fiber-reinforced thermoplastic plastic, but hereinafter, when the term "matrix resin" is used simply in this specification, it refers to the thermoplastic resin.

The heat-sealable resin may be in a fibrous or particulate form. When the pre-sheet is manufactured by a wet method, the heat-sealable resin may be in the form of particles dispersed in a liquid.

Examples of heat-sealable resins include polyethylene (PE), polypropylene, low-melting polyethylene terephthalate, low-melting polyamide, low-melting polylactic acid, polybutylene succinate, acrylic, urethane, styrene-butadiene copolymer, polyvinyl alcohol (PVA), etc. Two or more types of heat-sealable resins may be used in combination.

When the heat-fusible resin is fibrous, the fineness of the heat-fusible fiber is preferably 1 dtex to 120 dtex, and more preferably 1 dtex to 85 dtex. The average fiber length of the heat-fusible fiber is preferably 1 to 100 mm, more preferably 1 to 60 mm, and even more preferably 2 to 30 mm. When the fineness and average fiber length of the heat-fusible fiber are within the above ranges, the formation of a pulp fiber layer is easy, and a uniform binding force and dispersion state are easily obtained.

When the heat-fusible resin is in particulate form, the average particle size of the heat-fusible particles is preferably 10 to 1,000 μm, and more preferably 20 to 800 μm. When dispersed in an aqueous solution, the average particle size of the heat-fusible particles is preferably 10 nm to 100 μm.

(Composite of heat-sealable resin and thermoplastic resin)
The above-mentioned thermoplastic resin and heat-fusible resin may be combined to form a composite.

Examples of composites of thermoplastic resin and heat-sealing resin include core-sheath fibers having a core made of thermoplastic resin and a sheath made of heat-sealing resin, side-by-side fibers in which one half of a cross section perpendicular to the longitudinal direction is made of heat-sealing resin and the other half is made of thermoplastic resin, and core-shell particles having a core made of thermoplastic resin and a shell made of heat-sealing resin. Among these, core-sheath fibers are preferably used because different types of resins can be easily composited.

An example of a core-sheath fiber is a PP/PE composite core-sheath fiber that has a core made of polypropylene fiber (melting point 160°C) and a sheath made of polyethylene (melting point 130°C) formed around the core.

Other core-sheath fibers include, for example, PET/low melting point PET composite core-sheath fibers, high density polyethylene/low density polyethylene composite core-sheath fibers, polyethylene/low melting point PET composite core-sheath fibers, polyamide/low melting point polyamide composite core-sheath fibers, polylactic acid/low melting point polylactic acid composite core-sheath fibers, polylactic acid/polybutylene succinate composite core-sheath fibers, etc.

These composites can provide heat fusion properties due to the presence of the heat fusion resin exposed to the outside, and can function as a binder. Therefore, these composites can be blended as a heat fusion resin in this embodiment. In addition, since these composites contain a thermoplastic resin, they can impart strength to a molded body obtained from a pre-sheet produced containing this.

In this embodiment, it is preferable to use a combination of pulp fibers and core-sheath fibers in the pulp fiber sheet. In this case, the pulp fiber layer contains a composite of pulp fibers, heat-fusible resin, and thermoplastic resin. The mass ratio of the pulp fibers to the core-sheath fibers is preferably 50:50 to 90:10, and more preferably 60:40 to 80:20.

The basis weight of the pulp fiber sheet in this embodiment is preferably 50 to 300 g/ ^m2 , and more preferably 150 to 200 g/ ^m2 .

(Method of manufacturing pre-sheet using airlaid method)
Next, a preferred method for producing the pulp fiber sheet of the present embodiment using an airlaid method will be described in detail. The preferred method for producing the pulp fiber sheet includes a fiber defibration step, a mixing step, and a web formation step. Each step will be described below.

(Fibrillation process)
The fiberization process is a process in which the chopped fibers are fiberized by air flow and/or mechanical shear to obtain fiberized chopped fibers.

In the method of defibrating chopped fibers using an air flow, an air flow is formed using a blower or the like, chopped fibers are fed into the air flow, and the chopped fibers are defibrated by the stirring effect of the air flow. Defibrating using an air flow can prevent pulp fibers from breaking and shortening. In particular, pulp fibers are brittle and easily broken by mechanical shearing forces, but defibrating using an air flow can prevent breakage.

As a defibrating method, defibrating with a swirling air flow is preferred. A defibrating method using a swirling air flow can defibrate the chopped fibers sufficiently. Therefore, when forming an airlaid web by the airlaid method, the dispersion of the defibrated chopped fibers can be further improved.

As an example of a defibrating method using a swirling air flow, there is a method in which chopped fibers are put into a blower and defibrated by the blower. Another example is a method in which air is sent into a cylindrical container in the circumferential direction by a blower to form a swirling flow, and chopped fibers are supplied into the swirling flow and stirred to defibrate. As a method using air flow and mechanical shear in combination, there is a method in which a baffle plate is provided inside the blower, so that the chopped fibers hit the baffle plate and are defibrated by the air flow and mechanical shear. Examples of methods for defibrating with a mechanical shear include a method in which fiber bundles are compressed and spread with a roll, and a method in which they are carded with a roller equipped with needles.

The air flow speed is selected appropriately depending on the amount of chopped fiber, but is typically within the range of 10 to 150 m/sec.

(Mixing process)
The mixing step is a step in which the defibrated chopped fibers are mixed with a heat-fusible resin, a thermoplastic resin, etc. to obtain a web raw material. Auxiliaries such as a flame retardant, which are added as necessary, can be added in this mixing step.

During mixing, it is preferable to stir the defibrated chopped fibers, thermoplastic resin, and heat-fusible resin in order to improve the dispersion of the defibrated chopped fibers. However, in order to prevent breakage of the defibrated chopped fibers, it is preferable to use stirring using air flow rather than stirring using mechanical shear force.

The mixing process may be performed after the defibration process or simultaneously with the defibration process. If the mixing process is performed simultaneously with the defibration process, the air flow during the defibration process is used to mix the defibrated chopped fibers with the heat-fusible resin, etc.

(Web forming process)
The web forming process is a process for forming a web from a web raw material. In this embodiment, an airlaid method is adopted to obtain an airlaid web. Here, the airlaid method is a method for forming a web by depositing fibers randomly in a three-dimensional manner using an air flow.

In the web forming process in this embodiment, for example, a web forming device 1 shown in FIG. 1 is used. This web forming device 1 includes a conveyor 10, an air-permeable endless belt 20, a fiber mixture supplying means 30, a first carrier sheet supplying means 40, a second carrier sheet supplying means 50, and a suction box 60.

The conveyor 10 is composed of a plurality of rollers 11. The air-permeable endless belt 20 is attached to the conveyor 10 and rotates. The fiber mixture supplying means 30 supplies the fiber mixture together with an air flow to the air-permeable endless belt 20. The first carrier sheet supplying means 40 supplies a first carrier sheet 41 toward the air-permeable endless belt 20. The second carrier sheet supplying means 50 supplies a second carrier sheet 51 toward the first carrier sheet 41 that has passed through the air-permeable endless belt 20. The suction box 60 sucks the air-permeable endless belt 20 from its inside.

In the web forming device 1, the fiber mixture supply means 30 is installed above the air-permeable endless belt 20, the first carrier sheet supply means 40 is installed upstream of the air-permeable endless belt 20, and the second carrier sheet supply means 50 is installed downstream of the air-permeable endless belt 20.

In the web forming process using the web forming device 1, the rollers 11 are rotated in the same direction to drive the conveyor 10 and rotate the air-permeable endless belt 20. In addition, the first carrier sheet 41 is fed from the first carrier sheet supply means 40 so as to contact the top of the air-permeable endless belt 20.

Next, while sucking the air-permeable endless belt 20 with the suction box 60, the fiber mixture is lowered from the fiber mixture supply means 30 together with the air flow, and the fiber mixture falls and is deposited on the first carrier sheet 41 on the air-permeable endless belt 20. This forms the air-laid web A.

Next, a second carrier sheet 51 is supplied onto the airlaid web A from a second carrier sheet supplying means 50 to obtain an airlaid web-containing laminated sheet.

The first carrier sheet 41 and the second carrier sheet 51 function as a transport means for transporting the airlaid web in the airlaid method.

(Binding process)
When the method further includes a bonding step, the bonding method is selected from a chemical bonding method, a thermal bonding method, and a multi-bonding method. Any method may be selected, but the thermal bonding method is often used from the viewpoint of the bonding strength with the reinforcing fibers. The bonding step using the thermal bonding method is a step in which the airlaid web is heat-treated to bond the defibrated chopped fibers together with a heat-fusible resin.

Examples of heat treatments for air-laid webs include hot air treatments and infrared irradiation treatments, with hot air treatment being preferred in terms of the low cost of the equipment required.

As the hot air treatment, a method of heat treating the air-laid web by contacting it with a through-air dryer equipped with a rotating drum having air permeability on the circumferential surface (hot air circulating rotary drum method),
Examples of the heat treatment method include a method in which the airlaid web is passed through a box-type dryer and hot air is passed through the airlaid web (hot air circulating conveyor oven method).

<1-4. Thermoplastic resin sheet and thermoplastic resin layer>
The thermoplastic sheet is subjected to a molding process to produce a molded article, thereby becoming a thermoplastic resin layer. The thermoplastic resin layer can become a part of the pulp fiber-containing layer.

The thermoplastic resin sheet may be a nonwoven fabric, and may be formed, for example, by an airlaid method, a wet papermaking method, a spunlace method, a needle punch method, or the like. Specifically, for example, the thermoplastic resin sheet may be formed by forming reinforcing fibers into a sheet shape using a carding machine and then entangling the fibers by needle punching. The thermoplastic resin sheet is preferably a spunbond nonwoven fabric.

The thermoplastic resin sheet may be used as a carrier sheet for an airlaid web when a pulp fiber sheet is formed by an airlaid method. According to this method, a pre-sheet can be produced in one step (one pass). In addition, a pre-sheet (and a pulp fiber-containing layer) having high interlayer strength between the pulp fiber sheet and the thermoplastic resin sheet can be produced.
By carrying out the heat treatment at a temperature equal to or higher than the melting point of the heat-fusible resin, it is possible to promote interlayer adhesion between the layers in the pulp fiber-containing layer and bonding between the components within the layer.

When fibers are used in the thermoplastic resin sheet of this embodiment, the average fiber length (Ls) of the fibers is preferably 5 mm or more and 100 mm or less, more preferably 5 mm or more and 80 mm or less, and even more preferably 5 mm or more and 60 m or less. When the average fiber length is within the above range, the web can be easily manufactured and a molded product with high strength can be obtained.

The thermoplastic resin sheet of this embodiment distributes the thermoplastic resin evenly in each layer of the pre-sheet, making it possible to uniformize the strength of the pre-sheet in the thickness direction.

The thermoplastic resin applicable to the thermoplastic resin sheet of this embodiment may be in the form of fibers, particles, or pellets. In order to increase the strength of the molded product obtained from the pre-sheet, it is preferable that the thermoplastic resin be in the form of fibers.

The thermoplastic resin of the thermoplastic resin sheet can be the same as the thermoplastic resin that can be used in the pulp fiber sheet of this embodiment described above. The thermoplastic resin of the thermoplastic resin sheet is preferably polyolefin, more preferably polypropylene. The thermoplastic resin sheet is preferably polyolefin nonwoven fabric, more preferably polypropylene nonwoven fabric, and even more preferably polypropylene spunbond nonwoven fabric. Commercially available polyolefin nonwoven fabric can be used.

The thermoplastic resin sheet in this embodiment has a basis weight of preferably 10 to 100 g/ ^m2 , and more preferably 20 to 50 g/ ^m2 .

<1-5. Fine fibrous cellulose sheet and fine fibrous cellulose-containing layer>
The pre-sheet of the present embodiment has a fine fibrous cellulose-containing layer. The fine fibrous cellulose-containing layer may be a single layer or multiple layers.
The fine fibrous cellulose-containing layer of this embodiment is present at least as the outermost layer of the pre-sheet of this embodiment. The fine fibrous cellulose-containing layer of this embodiment may be present not only as the outermost layer of the pre-sheet of this embodiment but also as the innermost layer of the pre-sheet of this embodiment. It is preferable that both the outermost layer and the innermost layer of the pre-sheet of this embodiment are fine fibrous cellulose-containing layers. The fine fibrous cellulose-containing layers of the outermost layer and the innermost layer may be the same or different.

The fine fibrous cellulose-containing layer of this embodiment is preferably formed by hot pressing a fine fibrous cellulose sheet. Hereinafter, the fine fibrous cellulose sheet is also simply referred to as the sheet of this embodiment.

The sheet of this embodiment is preferably a sheet containing first cellulose fibers having a fiber width of 10 μm or more and second cellulose fibers having a fiber width of 1000 nm or less. In this specification, cellulose fibers having a fiber width of 1000 nm or less are sometimes referred to as fine fibrous cellulose.

The sheet of this embodiment has good adhesion to the resin. Specifically, after a resin layer is laminated on the sheet of this embodiment and heat pressed (e.g., at a press pressure of 0.5 MPa or more), the adhesion between the resin layer and the sheet is good, no peeling occurs between the layers, and the laminated structure is maintained.

The basis weight of the sheet of this embodiment is preferably 100/m ² or more, more preferably 120 g/m ² or more, and even more preferably 150 g/m ² or more. The basis weight of the sheet of this embodiment is preferably 300/m ² or less, more preferably 250 g/m ² or less, and even more preferably 200 g/m ² or less. The basis weight of the sheet can be calculated, for example, in accordance with JIS P 8124:2011. The basis weight of the sheet of this embodiment can be appropriately adjusted depending on the performance required for the molded product, and in applications where a thin molded product or a thin sheet is required, the basis weight of the sheet of this embodiment may be less than 100/m ² or may be 40 g/m ² or less.

The thickness of the sheet in this embodiment is preferably 5 μm or more, more preferably 10 μm or more, even more preferably 20 μm or more, and particularly preferably 30 μm or more. The upper limit of the sheet thickness is not particularly limited, but can be, for example, 1000 μm. The sheet thickness is measured, for example, in accordance with JIS P 8118:2014.

The density of the sheet of this embodiment is preferably 0.1 g/cm ³ or more, more preferably 0.2 g/cm ³ or more, even more preferably 0.3 g/cm ³ or more, even more preferably 0.50 g/cm ³ or more, even more preferably 0.60 g/cm ³ or more, and particularly preferably 0.65 g/cm ³ or more. The upper limit of the density of the sheet is not particularly limited, but is preferably, for example, 5.0 g/cm ³ or less. Here, the density of the sheet is calculated from the basis weight measured in accordance with JIS P 8124:2011 and the thickness of the sheet measured in accordance with JIS P 8118:2014. The density of the sheet may be appropriately adjusted, for example, by performing a calendar treatment, depending on the performance required for the molded body or sheet.

The sheet of this embodiment may be a single-layer sheet or a multi-layer sheet. The sheet of this embodiment is preferably a single-layer sheet. Specifically, it is preferable that both the first cellulose fibers having a fiber width of 10 μm or more and the second cellulose fibers having a fiber width of 1000 nm or less are randomly present in the single-layer sheet. The sheet of this embodiment may be a multi-layer sheet in which two or more single-layer sheets containing both the first cellulose fibers having a fiber width of 10 μm or more and the second cellulose fibers having a fiber width of 1000 nm or less are laminated. For example, the sheet of this embodiment may be a multi-layer sheet having a first layer containing both the first cellulose fibers having a fiber width of 10 μm or more and the second cellulose fibers having a fiber width of 1000 nm or less, and a second layer containing both the first cellulose fibers having a fiber width of 10 μm or more and the second cellulose fibers having a fiber width of 1000 nm or less. In this case, the first layer and the second layer may be the same layer or different layers. The thickness and basis weight of the first layer and the second layer may be the same or different. In other words, the sheet of this embodiment relates to a multi-layer sheet (e.g., a two-layer sheet) in which two or more single-layer sheets are laminated.

(First Cellulosic Fiber)
The sheet of this embodiment does not need to include the first cellulose fibers, but the sheet of this embodiment preferably includes the first cellulose fibers. Here, the first cellulose fibers are preferably cellulose fibers having a fiber width of 10 μm or more. The fiber width of the first cellulose fibers may be 10 μm or more, but is preferably 15 μm or more, more preferably 20 μm or more, and even more preferably 25 μm or more. The fiber width of the first cellulose fibers is preferably 100 μm or less, more preferably 80 μm or less, even more preferably 60 μm or less, and particularly preferably 40 μm or less. In this specification, the first cellulose fibers are also referred to as coarse cellulose fibers or pulp.

Details regarding the first cellulose when the sheet of this embodiment contains the first cellulose are provided below.

The fiber width of the first cellulose fiber can be measured using a Kajaani fiber length measuring instrument (FS-200 model) manufactured by Kajaani Automation. Here, the fiber width of the first cellulose fiber refers to the fiber width of the trunk fiber of the cellulose fiber. For example, when the cellulose fiber is a fibrillated cellulose fiber, the fiber width of the first cellulose fiber refers to the fiber width of the trunk fiber that constitutes the main axis, not the fiber width of the fibrillated and branched fibers.

Pulp is preferably used as the fiber raw material of the first cellulose fiber. Examples of pulp include wood pulp, non-wood pulp, and deinked pulp. Examples of wood pulp include, but are not limited to, chemical pulp such as hardwood kraft pulp (LBKP), softwood kraft pulp (NBKP), sulfite pulp (SP), dissolving pulp (DP), soda pulp (AP), unbleached kraft pulp (UKP), and oxygen bleached kraft pulp (OKP), semi-chemical pulp such as semi-chemical pulp (SCP) and chemi-ground wood pulp (CGP), and mechanical pulp such as groundwood pulp (GP) and thermomechanical pulp (TMP, BCTMP). Examples of non-wood pulp include, but are not limited to, cotton-based pulp such as cotton linters and cotton lint, and non-wood-based pulp such as hemp, straw, and bagasse. Examples of deinked pulp include, but are not limited to, deinked pulp made from waste paper. As the first cellulose fiber, one of the above types may be used alone, or two or more types may be mixed and used.

The first cellulose fiber preferably has an ionic substituent. The ionic substituent may include, for example, either one or both of an anionic group and a cationic group. The anionic group is preferably at least one selected from, for example, a phosphorus oxo acid group or a substituent derived from a phosphorus oxo acid group (sometimes simply referred to as a phosphorus oxo acid group), a carboxy group or a substituent derived from a carboxy group (sometimes simply referred to as a carboxy group), and a sulfone group or a substituent derived from a sulfone group (sometimes simply referred to as a sulfone group), more preferably at least one selected from a phosphorus oxo acid group and a carboxy group, and particularly preferably a phosphorus oxo acid group. By the first cellulose fiber having the above-mentioned ionic substituent, for example, it is possible to suppress the occurrence of twisting in the sheet during the sheet manufacturing process.

The phosphorus oxoacid group or a substituent derived from the phosphorus oxoacid group is, for example, a substituent represented by the following formula (1). The phosphorus oxoacid group is, for example, a divalent functional group obtained by removing a hydroxy group from phosphoric acid. Specifically, it is a group represented by -PO ₃ H _2. The substituent derived from the phosphorus oxoacid group includes a salt of the phosphorus oxoacid group, a phosphorus oxoacid ester group, and the like. The substituent derived from the phosphorus oxoacid group may be contained in the cellulose fiber as a group in which a phosphoric acid group is condensed (for example, a pyrophosphate group). The phosphorus oxoacid group may be, for example, a phosphorous acid group (phosphonic acid group), and the substituent derived from the phosphorus oxoacid group may be a salt of the phosphorous acid group, a phosphorous ester group, or the like.

式（１）中、ａ、ｂおよびｎは自然数である（ただし、ａ＝ｂ×ｍである）。α^１，α^２，・・・，α^ｎおよびα’のうちａ個がＯ^－であり、残りはＲ，ＯＲのいずれかである。なお、各α^ｎおよびα’の全てがＯ^－であっても構わない。Ｒは、各々、水素原子、飽和－直鎖状炭化水素基、飽和－分岐鎖状炭化水素基、飽和－環状炭化水素基、不飽和－直鎖状炭化水素基、不飽和－分岐鎖状炭化水素基、不飽和－環状炭化水素基、芳香族基、またはこれらの誘導基である。また、ｎは１であることが好ましい。

In formula (1), a, b, and n are natural numbers (wherein a=b×m). Of α ¹ , α ² , ..., α ⁿ , and α', a are ^O- , and the rest are either R or OR. All of the α ⁿ and α' may be ^O- . Each R is a hydrogen atom, a saturated linear hydrocarbon group, a saturated branched hydrocarbon group, a saturated cyclic hydrocarbon group, an unsaturated linear hydrocarbon group, an unsaturated branched hydrocarbon group, an unsaturated cyclic hydrocarbon group, an aromatic group, or a group derived therefrom. Also, n is preferably 1.

Examples of saturated linear hydrocarbon groups include, but are not limited to, methyl, ethyl, n-propyl, or n-butyl groups. Examples of saturated branched hydrocarbon groups include, but are not limited to, i-propyl or t-butyl groups. Examples of saturated cyclic hydrocarbon groups include, but are not limited to, cyclopentyl or cyclohexyl groups. Examples of unsaturated linear hydrocarbon groups include, but are not limited to, vinyl or allyl groups. Examples of unsaturated branched hydrocarbon groups include, but are not limited to, i-propenyl or 3-butenyl groups. Examples of unsaturated cyclic hydrocarbon groups include, but are not limited to, cyclopentenyl or cyclohexenyl groups. Examples of aromatic groups include, but are not limited to, phenyl or naphthyl groups.

In addition, the derivative group in R may be, but is not limited to, a functional group in which at least one of functional groups such as a carboxy group, a hydroxy group, or an amino group is added to or substituted for the main chain or side chain of the various hydrocarbon groups described above. In addition, the number of carbon atoms constituting the main chain of R is not particularly limited, but is preferably 20 or less, and more preferably 10 or less. By setting the number of carbon atoms constituting the main chain of R within the above range, the molecular weight of the phosphorus oxoacid group can be set to an appropriate range, which facilitates penetration into the fiber raw material and increases the yield of fine cellulose fibers.

β ^b+ is a monovalent or higher cation made of an organic or inorganic substance. Examples of the monovalent or higher cation made of an organic substance include aliphatic ammonium or aromatic ammonium, and examples of the monovalent or higher cation made of an inorganic substance include, but are not limited to, ions of alkali metals such as sodium, potassium, or lithium, cations of divalent metals such as calcium or magnesium, or hydrogen ions. These can be used alone or in combination of two or more. Examples of the monovalent or higher cation made of an organic or inorganic substance include, but are not limited to, sodium or potassium ions, which are less likely to yellow when a fiber raw material containing β is heated and are easy to use industrially. Note that β ^b+ may be an organic onium ion, and in this case, it is particularly preferable that it is an organic ammonium ion.

The amount of ionic substituent introduced into the first cellulose fiber is preferably 0.3 mmol/g or more per 1 g (mass) of cellulose fiber, more preferably 0.4 mmol/g or more, even more preferably 0.5 mmol/g or more, even more preferably 0.7 mmol/g or more, and particularly preferably 1.0 mmol/g or more. The amount of ionic substituent introduced into the first cellulose fiber is preferably 5.20 mmol/g or less per 1 g (mass) of cellulose fiber, more preferably 3.65 mmol/g or less, and even more preferably 3.00 mmol/g or less. Here, the unit mmol/g indicates, for example, the amount of substituent per 1 g of mass of the first cellulose fiber when the counter ion of the anionic group is a hydrogen ion (H+). By setting the amount of ionic substituent introduced within the above range, the occurrence of twisting in the sheet during the sheet manufacturing process can be more effectively suppressed.

The amount of ionic substituents introduced into the first cellulose fiber can be measured, for example, by neutralization titration. In the measurement by neutralization titration, an alkali such as an aqueous sodium hydroxide solution is added to the obtained slurry containing the first cellulose fiber, and the amount introduced is measured by determining the change in pH.

Figure 2 is a graph showing the relationship between the amount of NaOH dripped onto a slurry containing cellulose fibers having phosphorus oxo acid groups and the pH. The amount of phosphorus oxo acid groups introduced into the cellulose fibers is measured, for example, as follows.

First, a slurry containing cellulose fibers is treated with a strongly acidic ion exchange resin. Note that, for the first cellulose fibers, a defibration treatment similar to the defibration treatment step described below is performed on the measurement subject prior to the treatment with the strongly acidic ion exchange resin.
Next, the change in pH is observed while adding the aqueous sodium hydroxide solution, and a titration curve as shown in the upper part of Figure 2 is obtained. In the titration curve shown in the upper part of Figure 2, the measured pH is plotted against the amount of alkali added, and in the titration curve shown in the lower part of Figure 2, the increment (differential value) (1/mmol) of pH against the amount of alkali added is plotted. In this neutralization titration, two points are confirmed in the curve plotting the measured pH against the amount of alkali added, where the increment (differential value of pH against the amount of alkali added) is maximum. Of these, the first maximum point of the increment obtained after starting to add the alkali is called the first endpoint, and the second maximum point of the increment obtained next is called the second endpoint. The amount of alkali required from the start of titration to the first end point is equal to the amount of the first dissociated acid of the cellulose fiber contained in the slurry used for titration, the amount of alkali required from the first end point to the second end point is equal to the amount of the second dissociated acid of the cellulose fiber contained in the slurry used for titration, and the amount of alkali required from the start of titration to the second end point is equal to the total amount of dissociated acid of the cellulose fiber contained in the slurry used for titration. The value obtained by dividing the amount of alkali required from the start of titration to the first end point by the solid content (g) in the slurry to be titrated is the amount of phosphorus oxo acid group introduced (mmol/g). Note that when simply referring to the amount of phosphorus oxo acid group introduced (or the amount of phosphorus oxo acid group), it refers to the amount of the first dissociated acid.

In FIG. 2, the region from the start of titration to the first end point is called the first region, and the region from the first end point to the second end point is called the second region. For example, when the phosphorus oxoacid group is a phosphate group and this phosphate group undergoes condensation, the amount of weak acid groups in the phosphorus oxoacid group (also referred to as the second dissociated acid amount in this specification) appears to decrease, and the amount of alkali required in the second region becomes smaller than the amount of alkali required in the first region. On the other hand, the amount of strong acid groups in the phosphorus oxoacid group (also referred to as the first dissociated acid amount in this specification) is equal to the amount of phosphorus atoms regardless of the presence or absence of condensation. In addition, when the phosphorus oxoacid group is a phosphorous acid group, the weak acid groups no longer exist in the phosphorus oxoacid group, so the amount of alkali required in the second region becomes smaller, or the amount of alkali required in the second region may become zero. In this case, there is only one point on the titration curve where the pH increment is maximum.

The amount of phosphorus oxoacid groups introduced (mmol/g) above indicates the amount of phosphorus oxoacid groups in acid-type cellulose fibers (hereinafter referred to as the amount of phosphorus oxoacid groups (acid type)), since the denominator indicates the mass of the acid-type cellulose fibers. On the other hand, when the counter ion of the phosphorus oxoacid group is replaced with an arbitrary cation C so as to be the charge equivalent, the amount of phosphorus oxoacid groups in the cellulose fibers with the cation C as the counter ion (hereinafter referred to as the amount of phosphorus oxoacid groups (C type)) can be obtained by converting the denominator to the mass of the cellulose fibers when the cation C is the counter ion.

That is, it is calculated according to the following formula.
Amount of phosphorus oxoacid group (C type)=Amount of phosphorus oxoacid group (acid type)/{1+(W-1)×A/1000}
A [mmol/g]: total amount of anions derived from phosphorus oxoacid groups in cellulose fibers (total amount of dissociated acids from phosphorus oxoacid groups)
W: Formula weight per valence of cation C (for example, Na is 23, Al is 9).

Figure 3 is a graph showing the relationship between the amount of NaOH dropped onto a dispersion containing cellulose fibers having carboxy groups as ionic substituents and the pH. The amount of carboxy groups introduced into the cellulose fibers is measured, for example, as follows.

First, a dispersion liquid containing cellulose fibers is treated with a strongly acidic ion exchange resin. For the first cellulose fibers, a defibration process similar to the defibration process described below is performed on the measurement target prior to treatment with the strongly acidic ion exchange resin.

Next, the change in pH is observed while adding the aqueous sodium hydroxide solution, and a titration curve as shown in the upper part of Figure 3 is obtained. In the titration curve shown in the upper part of Figure 3, the measured pH is plotted against the amount of added alkali, and in the titration curve shown in the lower part of Figure 3, the increment (differential value) (1/mmol) of pH against the amount of added alkali is plotted. In this neutralization titration, one point where the increment (differential value of pH against the amount of added alkali) is maximum is confirmed in the curve plotting the measured pH against the amount of added alkali, and this maximum point is called the first end point. Here, the region from the start of titration to the first end point in Figure 3 is called the first region. The amount of alkali required in the first region is equal to the amount of carboxyl groups in the dispersion liquid used for titration. The amount of alkali required in the first region of the titration curve (mmol) is divided by the solid content (g) in the dispersion liquid containing the cellulose fiber to be titrated to calculate the amount of carboxyl groups introduced (mmol/g).

The above-mentioned amount of carboxyl groups introduced (mmol/g) indicates the amount of carboxyl groups possessed by acid-type cellulose fibers (hereinafter referred to as the amount of carboxyl groups (acid type)) since the denominator is the mass of the acid-type cellulose fibers. On the other hand, when the counter ion of the carboxyl group is replaced with an arbitrary cation C so as to be the charge equivalent, the amount of carboxyl groups possessed by the cellulose fibers in which the cation C is the counter ion (hereinafter referred to as the amount of carboxyl groups (C type)) can be obtained by converting the denominator to the mass of the cellulose fibers in which the cation C is the counter ion. That is, it is calculated by the following formula.
Carboxy group amount (C type)=Carboxy group amount (acid type)/{1+(W-1)×
(Amount of carboxyl group (acid type))/1000}
W: Formula weight per valence of cation C (for example, Na is 23, Al is 9)

When measuring the amount of ionic substituents by titration, if too much sodium hydroxide solution is added or if the titration interval is too short, the amount of ionic substituents may be lower than it should be and an accurate value may not be obtained. For example, an appropriate amount of sodium hydroxide and titration interval is preferably 10 to 50 μL of 0.1 N sodium hydroxide solution added every 5 to 30 seconds. In addition, in order to eliminate the effects of carbon dioxide dissolved in the cellulose fiber-containing slurry, it is preferable to measure while blowing an inert gas such as nitrogen gas into the slurry from 15 minutes before the start of titration until the end of titration.

When the first cellulose fiber and the second cellulose fiber are mixed, they are separated and collected by centrifugation, and the amount of phosphorus oxoacid groups introduced is measured by the method described above. Centrifugation is performed by adjusting the solid concentration of the cellulose dispersion containing the mixed first cellulose fiber and the second cellulose fiber to 0.2% by mass, and using a refrigerated high-speed centrifuge (Kokusan Co., Ltd., H-2000B) at 12,000 G for 10 minutes. The precipitated solids are then collected as the first cellulose fiber, and the supernatant liquid is collected as the second cellulose fiber.

The water retention of the first cellulose fiber is preferably 220% or more, more preferably 230% or more, even more preferably 240% or more, even more preferably 250% or more, and particularly preferably 280% or more. The water retention of the first cellulose fiber is preferably 600% or less, more preferably 500% or less, and even more preferably 400% or less. The water retention of the first cellulose fiber is a value measured in accordance with J. TAPPI-26. The water retention of the first cellulose fiber is the water retention of the first cellulose fiber before being made into a sheet, but the water retention of the first cellulose fiber after being made into a sheet may satisfy the above range.

The content of the first cellulose fiber is preferably 10% by mass or more, more preferably 20% by mass or more, even more preferably 30% by mass or more, more preferably more than 40% by mass, 41% by mass or more, 45% by mass or more, 50% by mass or more, even more preferably more than 50% by mass or more or 55% by mass or more, even more preferably 60% by mass or more, and particularly preferably 70% by mass or more. The content of the first cellulose fiber is preferably 99% by mass or less, more preferably 95% by mass or less, even more preferably 90% by mass or less, even more preferably 85% by mass or less, and particularly preferably 80% by mass or less, based on the total mass of the cellulose fiber contained in the sheet of this embodiment. The content of the first cellulose fiber may be 0% by mass based on the total mass of the cellulose fiber contained in the sheet of this embodiment.

<First cellulose fiber manufacturing process>
<Phosphorus Oxoacid Group Introduction Step>
When the first cellulose fiber has a phosphorus oxo acid group as an ionic substituent, the manufacturing process of the first cellulose fiber includes a phosphorus oxo acid group introduction step, which is a step of reacting at least one compound (hereinafter also referred to as "compound A") selected from compounds capable of introducing a phosphorus oxo acid group by reacting with a hydroxyl group possessed by a fiber raw material containing cellulose, with the fiber raw material containing cellulose. This step results in the production of a fiber into which a phosphorus oxo acid group has been introduced.

In the phosphorus oxoacid group introduction step according to this embodiment, the reaction of the cellulose-containing fiber raw material with compound A may be carried out in the presence of at least one selected from urea and its derivatives (hereinafter also referred to as "compound B"). On the other hand, the reaction of the cellulose-containing fiber raw material with compound A may be carried out in the absence of compound B.

One example of a method for allowing compound A to act on a fiber raw material in the presence of compound B is to mix compound A and compound B with a fiber raw material in a dry, wet, or slurry state. Of these, it is preferable to use a fiber raw material in a dry or wet state, and it is particularly preferable to use a fiber raw material in a dry state, because the reaction is highly uniform. The form of the fiber raw material is not particularly limited, but it is preferable to use a cotton-like or thin sheet-like form. Compound A and compound B can be added to the fiber raw material in a powder form, a solution form dissolved in a solvent, or a melted state by heating to a melting point or higher. Of these, it is preferable to add them in a solution form dissolved in a solvent, especially in an aqueous solution form, because the reaction is highly uniform. Compound A and compound B may be added to the fiber raw material simultaneously, separately, or as a mixture. The method of adding compound A and compound B is not particularly limited, but when compound A and compound B are in a solution form, the fiber raw material may be immersed in the solution to absorb the liquid and then removed, or the solution may be dripped onto the fiber raw material. Alternatively, the required amounts of compound A and compound B may be added to the fiber raw material, or excess amounts of compound A and compound B may be added to the fiber raw material, and then the excess compound A and compound B may be removed by squeezing or filtration.

Compound A used in this embodiment may be any compound that has a phosphorus atom and can form an ester bond with cellulose, and may include, but is not limited to, phosphoric acid or its salt, phosphorous acid or its salt, dehydrated condensed phosphoric acid or its salt, and anhydrous phosphoric acid (diphosphorus pentoxide). Phosphoric acid may be of various purities, for example, 100% phosphoric acid (orthophosphoric acid) or 85% phosphoric acid. Phosphorous acid may be 99% phosphorous acid (phosphonic acid). Dehydrated condensed phosphoric acid is phosphoric acid in which two or more molecules are condensed by a dehydration reaction, and examples of such include pyrophosphoric acid and polyphosphoric acid. Phosphates, phosphites, and dehydrated condensed phosphates include lithium salts, sodium salts, potassium salts, and ammonium salts of phosphoric acid, phosphorous acid, or dehydrated condensed phosphoric acid, and these may be neutralized to various degrees. Among these, from the viewpoints of high efficiency of introduction of phosphate groups, easier improvement of defibration efficiency in the defibration step described below, low cost, and ease of industrial application, phosphoric acid, sodium salt of phosphoric acid, potassium salt of phosphoric acid, ammonium salt of phosphoric acid, phosphorous acid, sodium salt of phosphorous acid, potassium salt of phosphorous acid, and ammonium salt of phosphorous acid are preferred, and phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, ammonium dihydrogen phosphate, or phosphorous acid and sodium phosphite are more preferred.

The amount of compound A added to the fiber raw material is not particularly limited, but for example, when the amount of compound A added is converted to the amount of phosphorus atoms, the amount of phosphorus atoms added to the fiber raw material (bone dry mass) is preferably 0.5% by mass or more and 100% by mass or less, more preferably 1% by mass or more and 50% by mass or less, and even more preferably 2% by mass or more and 30% by mass or less. By setting the amount of phosphorus atoms added to the fiber raw material within the above range, the yield of the first cellulose fiber can be further improved. On the other hand, by setting the amount of phosphorus atoms added to the fiber raw material to the above upper limit or less, a balance can be achieved between the effect of improving the yield and costs.

As described above, compound B used in this embodiment is at least one selected from urea and its derivatives. Examples of compound B include urea, biuret, 1-phenylurea, 1-benzylurea, 1-methylurea, and 1-ethylurea. From the viewpoint of improving the uniformity of the reaction, it is preferable to use compound B as an aqueous solution. Furthermore, from the viewpoint of further improving the uniformity of the reaction, it is preferable to use an aqueous solution in which both compound A and compound B are dissolved.

The amount of compound B added relative to the fiber raw material (bone dry mass) is not particularly limited, but is preferably from 1% by mass to 500% by mass, more preferably from 10% by mass to 400% by mass, and even more preferably from 100% by mass to 350% by mass.

In the reaction of a fiber raw material containing cellulose with compound A, in addition to compound B, for example, amides or amines may be included in the reaction system. Examples of amides include formamide, dimethylformamide, acetamide, and dimethylacetamide. Examples of amines include methylamine, ethylamine, trimethylamine, triethylamine, monoethanolamine, diethanolamine, triethanolamine, pyridine, ethylenediamine, and hexamethylenediamine. Among these, triethylamine in particular is known to act as a good reaction catalyst.

In the phosphorus oxo acid group introduction step, it is preferable to add or mix compound A or the like to the fiber raw material, and then heat-treat the fiber raw material. It is preferable to select a temperature for the heat treatment that can efficiently introduce phosphorus oxo acid groups while suppressing thermal decomposition and hydrolysis of the fiber. The heat treatment temperature is preferably, for example, 50°C to 300°C, more preferably 100°C to 250°C, and even more preferably 130°C to 200°C. In addition, various devices having heat media can be used for the heat treatment, such as a stirring dryer, a rotary dryer, a disk dryer, a roll-type heater, a plate-type heater, a fluidized bed dryer, a band-type dryer, a filtration dryer, a vibration fluidized dryer, an airflow dryer, a reduced pressure dryer, an infrared heater, a far-infrared heater, a microwave heater, and a high-frequency dryer.

In the heat treatment according to this embodiment, for example, a method of adding compound A to a thin sheet-like fiber raw material by a method such as impregnation and then heating the material, or a method of heating the fiber raw material and compound A while kneading or stirring the fiber raw material with a kneader or the like can be adopted. This makes it possible to suppress unevenness in the concentration of compound A in the fiber raw material and introduce phosphorus oxoacid groups more uniformly onto the surface of the cellulose fibers contained in the fiber raw material. This is thought to be due to the fact that when water molecules move to the surface of the fiber raw material as it dries, the dissolved compound A is attracted to the water molecules by surface tension and is prevented from moving to the surface of the fiber raw material in the same way (i.e., causing unevenness in the concentration of compound A).

In addition, the heating device used for the heat treatment is preferably a device that can constantly discharge, from the system, for example, moisture held by the slurry and moisture generated in association with the dehydration condensation (phosphate esterification) reaction between compound A and hydroxyl groups contained in cellulose or the like in the fiber raw material. An example of such a heating device is an oven using a blowing air system. By constantly discharging moisture from within the system, it is possible to suppress the hydrolysis reaction of phosphate ester bonds, which is the reverse reaction of phosphate esterification, and also to suppress acid hydrolysis of the sugar chains in the fibers. This makes it possible to obtain a first cellulose fiber with a high axial ratio.

The heat treatment time is preferably from 1 second to 300 minutes after the moisture has been substantially removed from the fiber raw material, more preferably from 1 second to 1000 seconds, and even more preferably from 10 seconds to 800 seconds. In this embodiment, the amount of phosphorus oxoacid groups introduced can be kept within a preferred range by setting the heating temperature and heating time within an appropriate range.

The phosphorus oxo acid group introduction process needs to be carried out at least once, but can also be repeated two or more times. By carrying out the phosphorus oxo acid group introduction process two or more times, a large number of phosphorus oxo acid groups can be introduced into the fiber raw material.

<Carboxy Group Introduction Step>
When the first cellulose fiber has a carboxy group as an ionic substituent, the process for producing the first cellulose fiber includes a carboxy group introduction step, which is carried out by subjecting a fiber raw material containing cellulose to an oxidation treatment such as ozone oxidation, oxidation by the Fenton method, or TEMPO oxidation treatment, or treating the fiber raw material with a compound having a carboxylic acid-derived group or a derivative thereof, or an acid anhydride of a compound having a carboxylic acid-derived group or a derivative thereof.

Examples of compounds having a group derived from a carboxylic acid include, but are not limited to, dicarboxylic acid compounds such as maleic acid, succinic acid, phthalic acid, fumaric acid, glutaric acid, adipic acid, and itaconic acid, and tricarboxylic acid compounds such as citric acid and aconitic acid. Examples of derivatives of compounds having a group derived from a carboxylic acid include, but are not limited to, imidized products of acid anhydrides of compounds having a carboxy group, and derivatives of acid anhydrides of compounds having a carboxy group. Examples of imidized products of acid anhydrides of compounds having a carboxy group include, but are not limited to, imidized products of dicarboxylic acid compounds such as maleimide, succinimide, and phthalimide.

The acid anhydrides of compounds having a group derived from carboxylic acid include, but are not limited to, acid anhydrides of dicarboxylic acid compounds such as maleic anhydride, succinic anhydride, phthalic anhydride, glutaric anhydride, adipic anhydride, and itaconic anhydride. In addition, the derivatives of acid anhydrides of compounds having a group derived from carboxylic acid include, but are not limited to, acid anhydrides of compounds having carboxy groups such as dimethylmaleic anhydride, diethylmaleic anhydride, and diphenylmaleic anhydride in which at least some of the hydrogen atoms are substituted with a substituent such as an alkyl group or a phenyl group.

When the TEMPO oxidation treatment is performed in the carboxyl group introduction step, it is preferable to perform the treatment under conditions of pH 6 or more and 8 or less. Such a treatment is also called neutral TEMPO oxidation treatment. The neutral TEMPO oxidation treatment can be performed, for example, by adding pulp as a fiber raw material, a nitroxy radical such as TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) as a catalyst, and sodium hypochlorite as a sacrificial reagent to a sodium phosphate buffer solution (pH = 6.8). By further allowing sodium chlorite to coexist, aldehydes generated in the oxidation process can be efficiently oxidized to carboxyl groups. The TEMPO oxidation treatment may also be performed under conditions of pH 10 or more and 11 or less. Such a treatment is also called alkaline TEMPO oxidation treatment. The alkaline TEMPO oxidation treatment can be performed, for example, by adding a nitroxy radical such as TEMPO as a catalyst, sodium bromide as a cocatalyst, and sodium hypochlorite as an oxidizing agent to pulp as a fiber raw material.

The amount of carboxyl groups introduced into the first cellulose fiber varies depending on the type of the substituent. For example, when the carboxyl groups are introduced by TEMPO oxidation, the amount is preferably 0.10 mmol/g or more per 1 g (mass) of cellulose fiber, more preferably 0.20 mmol/g or more, even more preferably 0.40 mmol/g or more, and particularly preferably 0.60 mmol/g or more. The amount of carboxyl groups introduced into the first cellulose fiber is preferably 3.65 mmol/g or less, more preferably 3.00 mmol/g or less, even more preferably 2.50 mmol/g or less, even more preferably 2.00 mmol/g or less, even more preferably 1.50 mmol/g or less, and particularly preferably 1.00 mmol/g or less. In addition, when the substituent is a carboxymethyl group, the amount may be 5.8 mmol/g or less per 1 g (mass) of cellulose fiber. By setting the amount of carboxyl groups introduced within the above range, it is possible to easily refine the fiber raw material and increase the stability of the first cellulose fiber.

<Cleaning process>
In the first method for producing cellulose fibers in this embodiment, a washing step can be carried out on the ionic substituent-introduced fibers as necessary. The washing step is carried out by washing the ionic substituent-introduced fibers with water or an organic solvent, for example. The washing step may be carried out after each step described below, and the number of washing steps carried out in each washing step is not particularly limited.

<Alkaline treatment step>
In the first cellulose fiber manufacturing method of the present embodiment, the ionic substituent-introduced fiber after washing may be subjected to an alkali treatment as necessary. In this case, it is preferable to dilute the ionic substituent-introduced fiber after washing with 10 L of ion-exchanged water, and then add 1N alkaline solution little by little while stirring to adjust the pH to 12 to 13. The alkaline compound contained in the alkaline solution is not particularly limited, and may be an inorganic alkaline compound or an organic alkaline compound. In this embodiment, it is preferable to use, for example, sodium hydroxide or potassium hydroxide as the alkaline compound because of its high versatility. In addition, the solvent contained in the alkaline solution may be either water or an organic solvent. Among them, the solvent contained in the alkaline solution is preferably a polar solvent containing water or a polar organic solvent exemplified by alcohol, and more preferably an aqueous solvent containing at least water. As the alkaline solution, for example, an aqueous sodium hydroxide solution or an aqueous potassium hydroxide solution is preferable because of its high versatility.

The temperature of the alkaline solution in the alkaline treatment step is not particularly limited, but is preferably 5°C to 80°C, and more preferably 10°C to 60°C. The immersion time of the ionic substituent-introduced fiber in the alkaline solution in the alkaline treatment step is not particularly limited, but is preferably 5 minutes to 30 minutes, and more preferably 10 minutes to 20 minutes. The amount of the alkaline solution used in the alkaline treatment is not particularly limited, but is preferably 100% by mass to 100,000% by mass, and more preferably 1000% by mass to 10,000% by mass, based on the absolute dry mass of the ionic substituent-introduced fiber. The above-mentioned washing step may be further carried out after the alkaline treatment step.

(Second Cellulosic Fibers)
The sheet of the present embodiment includes second cellulose fibers (fine fibrous cellulose) having a fiber width of 1000 nm or less. The fiber width of the second cellulose fibers can be measured, for example, by observation with an electron microscope. The second cellulose fibers are, for example, monofilament cellulose.

The fiber width of the second cellulose fiber may be 1000 nm or less, preferably 500 nm or less, more preferably 300 nm or less, even more preferably 200 nm or less, even more preferably 100 nm or less, even more preferably 50 nm or less, even more preferably 20 nm or less, and particularly preferably 10 nm or less. Since the sheet of this embodiment contains fine fibrous cellulose with such a small fiber width, it has excellent mechanical properties when formed into a molded body. By using the second cellulose fiber in combination with the first cellulose fiber (coarse cellulose fiber), it is possible to effectively improve adhesion with the resin. In addition, when fine fibrous cellulose with a small fiber width is used in combination with the first cellulose fiber (coarse cellulose fiber) having an ionic substituent, it is possible to suppress the occurrence of twisting in the sheet during the sheet manufacturing process. In addition, when fine fibrous cellulose with a small fiber width is used in combination with the first cellulose fiber (coarse cellulose fiber) having an ionic substituent, the transparency of the sheet is improved.

The fiber width of the second cellulose fibers is measured, for example, using an electron microscope as follows. First, an aqueous suspension of the second cellulose fibers with a concentration of 0.05% by mass or more and 0.1% by mass or less is prepared, and the suspension is cast on a hydrophilized carbon film-coated grid to prepare a sample for TEM observation. When wide fibers are included, an SEM image of the surface cast on glass may be observed. Next, observation is performed using an electron microscope image at a magnification of 1000x, 5000x, 10000x, or 50000x depending on the width of the fiber to be observed. However, the sample, observation conditions, and magnification are adjusted to satisfy the following conditions.
(1) A straight line X is drawn at an arbitrary location within the observed image, and 20 or more fibers intersect with the straight line X.
(2) Within the same image, a straight line Y is drawn that intersects the straight line perpendicularly, and 20 or more fibers intersect the straight line Y.
For the observation images that satisfy the above conditions, the widths of the fibers that intersect with the lines X and Y are visually read. In this manner, three or more sets of observation images of at least the surface portions that do not overlap each other are obtained. Next, for each image, the widths of the fibers that intersect with the lines X and Y are read.

The fiber length of the second cellulose fibers is not particularly limited, but is preferably, for example, 0.1 μm or more and 1000 μm or less, more preferably 0.1 μm or more and 800 μm or less, and even more preferably 0.1 μm or more and 600 μm or less. By setting the fiber length within the above range, destruction of the crystalline regions of the second cellulose fibers can be suppressed. It is also possible to set the slurry viscosity of the second cellulose fibers to an appropriate range. The fiber length of the second cellulose fibers can be determined, for example, by image analysis using TEM, SEM, or AFM.

The second cellulose fibers preferably have a type I crystal structure. Here, the fact that the second cellulose fibers have a type I crystal structure can be identified from a diffraction profile obtained from a wide-angle X-ray diffraction photograph using CuKα (λ=1.5418 Å) monochromatized with graphite. Specifically, it can be identified from the presence of typical peaks at two positions, near 2θ=14° to 17° and near 2θ=22° to 23°.

The proportion of type I crystal structure in the second cellulose fiber is, for example, preferably 30% or more, more preferably 40% or more, and even more preferably 50% or more. This is expected to provide even better performance in terms of heat resistance and low linear thermal expansion. The degree of crystallinity is determined by measuring the X-ray diffraction profile and using the pattern in a conventional manner (Seagal et al., Textile Research Journal, Vol. 29, p. 786, 1959).

The axial ratio (fiber length/fiber width) of the second cellulose fibers is not particularly limited, but is preferably, for example, 20 to 10,000, and more preferably 50 to 1,000. By setting the axial ratio to the above lower limit or more, it is easy to form a sheet containing the second cellulose fibers. By setting the axial ratio to the above upper limit or less, it is preferable in that, for example, when the second cellulose fibers are treated as an aqueous dispersion, handling such as dilution is easier.

The second cellulose fiber in this embodiment has, for example, both crystalline and non-crystalline regions. In particular, the second cellulose fiber having both crystalline and non-crystalline regions and a high axial ratio is realized by the manufacturing method of fine fibrous cellulose described below.

In this embodiment, the second cellulose fiber preferably has an ionic substituent. The ionic substituent may include, for example, either one or both of an anionic group and a cationic group. The anionic group is preferably at least one selected from, for example, a phosphorus oxo acid group or a substituent derived from a phosphorus oxo acid group (sometimes simply referred to as a phosphorus oxo acid group), a carboxy group or a substituent derived from a carboxy group (sometimes simply referred to as a carboxy group), and a sulfone group or a substituent derived from a sulfone group (sometimes simply referred to as a sulfone group), more preferably at least one selected from a phosphorus oxo acid group and a carboxy group, and particularly preferably a phosphorus oxo acid group. The phosphorus oxo acid group is the same as the phosphorus oxo acid group that the first cellulose fiber may have. When both the first cellulose fiber and the second cellulose fiber have an ionic substituent, the ionic substituent of the first cellulose fiber and the ionic substituent of the second cellulose fiber may be the same or different, but are preferably the same.

The amount of ionic substituent introduced into the second cellulose fiber is, for example, preferably 0.1 mmol/g or more per 1 g (mass) of the second cellulose fiber, more preferably 0.3 mmol/g or more, even more preferably 0.5 mmol/g or more, even more preferably 0.7 mmol/g or more, and particularly preferably 1.0 mmol/g or more. The amount of ionic substituent introduced into the second cellulose fiber is preferably 5.20 mmol/g or less per 1 g (mass) of the cellulose fiber, more preferably 3.65 mmol/g or less, and even more preferably 3.00 mmol/g or less. Here, the unit mmol/g indicates, for example, the amount of substituent per 1 g of the mass of the second cellulose fiber when the counter ion of the anionic group is a hydrogen ion (H ⁺ ).

The amount of ionic substituent introduced into the second cellulose fibers can be measured, for example, by a neutralization titration method. In the measurement by the neutralization titration method, an alkali such as an aqueous sodium hydroxide solution is added to a slurry containing the obtained second cellulose fibers, and the amount introduced is measured by determining a change in pH. A specific method for measuring the amount of ionic substituent introduced is the same as the method for measuring the amount of ionic substituent introduced into the first cellulose fibers. Note that, when measuring the amount of ionic substituent introduced into the first cellulose fibers, a defibration treatment is performed before the treatment with a strongly acidic ion exchange resin, but when measuring the amount of ionic substituent introduced into the second cellulose fibers, it is not necessary to perform a defibration treatment before the treatment with a strongly acidic ion exchange resin.

The content of the second cellulose fiber is preferably 1% by mass or more, more preferably 5% by mass or more, even more preferably 10% by mass or more, even more preferably 15% by mass or more, and particularly preferably 20% by mass or more, based on the total mass of the cellulose fiber contained in the sheet of this embodiment. The content of the second cellulose fiber is preferably 90% by mass or less, more preferably 80% by mass or less, even more preferably 70% by mass or less, even more preferably 50% by mass or less, even more preferably 40% by mass or less, and particularly preferably 30% by mass or less, based on the total mass of the cellulose fiber contained in the sheet of this embodiment. The content of the second cellulose fiber may be 100% by mass, based on the total mass of the cellulose fiber contained in the sheet of this embodiment.

<Production process of fine fibrous cellulose>
<Fiber raw materials>
Fine fibrous cellulose is produced from a fiber raw material containing cellulose. The fiber raw material containing cellulose is not particularly limited, but it is preferable to use pulp because it is easily available and inexpensive. Examples of pulp include wood pulp, non-wood pulp, and deinked pulp. Examples of wood pulp include, but are not particularly limited to, chemical pulp such as hardwood kraft pulp (LBKP), softwood kraft pulp (NBKP), sulfite pulp (SP), dissolving pulp (DP), soda pulp (AP), unbleached kraft pulp (UKP) and oxygen bleached kraft pulp (OKP), semi-chemical pulp such as semi-chemical pulp (SCP) and chemi-ground wood pulp (CGP), mechanical pulp such as groundwood pulp (GP) and thermomechanical pulp (TMP, BCTMP), etc. Examples of non-wood pulp include, but are not particularly limited to, cotton-based pulp such as cotton linters and cotton lint, and non-wood-based pulp such as hemp, wheat straw, and bagasse. The deinked pulp is not particularly limited, but may be, for example, deinked pulp made from waste paper. The pulp of this embodiment may be one of the above-mentioned types alone or a mixture of two or more types.

Among the above pulps, for example, wood pulp and deinked pulp are preferred from the viewpoint of ease of availability. Furthermore, among wood pulps, for example, chemical pulp is more preferred from the viewpoint of having a high cellulose ratio and a high yield of fine fibrous cellulose during defibration treatment, and of obtaining long-fiber fine fibrous cellulose with a large axial ratio due to little decomposition of cellulose in the pulp, and kraft pulp and sulfite pulp are even more preferred.

As fiber raw materials containing cellulose, for example, cellulose contained in sea squirts and bacterial cellulose produced by acetic acid bacteria can be used. Also, instead of fiber raw materials containing cellulose, fibers formed from linear nitrogen-containing polysaccharide polymers such as chitin and chitosan can be used.

<Phosphorus Oxoacid Group Introduction Step>
When the fine fibrous cellulose has phosphorus oxoacid groups, the process for producing the fine fibrous cellulose includes a phosphorus oxoacid group introduction step, which is the same as the phosphorus oxoacid group introduction step in the process for producing the first cellulose fiber.

<Carboxy Group Introduction Step>
When the fine fibrous cellulose has a carboxy group, the process for producing the fine fibrous cellulose includes a carboxy group introduction step, which is the same as the carboxy group introduction step in the process for producing the first cellulose fiber.

<Cleaning process>
In the method for producing fine fibrous cellulose in this embodiment, the fibers into which phosphorus oxoacid groups have been introduced may be subjected to a washing step as necessary. The washing step is the same as the washing step in the first cellulose fiber production process.

<Alkaline treatment step>
When producing fine fibrous cellulose, an alkali treatment may be carried out on the fiber raw material between the ionic substituent introduction step and the defibration step described below. The alkali treatment method is the same as the alkali treatment method in the first cellulose fiber production step. After the alkali treatment step, the above-mentioned washing step may be further carried out.

<Defibrillation treatment>
By defibrating the fibers in the defibration process, fine fibrous cellulose can be obtained. In the defibration process, for example, a defibration treatment device can be used. The defibration treatment device is not particularly limited, but for example, a high-speed defibrator, a grinder (stone mill type grinder), a high-pressure homogenizer, an ultra-high-pressure homogenizer, a high-pressure collision type grinder, a ball mill, a bead mill, a disk type refiner, a conical refiner, a biaxial kneader, a vibration mill, a homomixer under high-speed rotation, an ultrasonic disperser, or a beater can be used. Among the above defibration treatment devices, it is more preferable to use a high-speed defibrator, a high-pressure homogenizer, or an ultra-high-pressure homogenizer, which are less affected by the grinding media and have less risk of contamination.

In the defibration process, it is preferable to dilute the fibers with a dispersion medium to make them into a slurry state. As the dispersion medium, one or more selected from water and organic solvents such as polar organic solvents can be used. The polar organic solvent is not particularly limited, but is preferably, for example, alcohols, polyhydric alcohols, ketones, ethers, esters, aprotic polar solvents, etc. Examples of alcohols include, for example, methanol, ethanol, isopropanol, n-butanol, isobutyl alcohol, etc. Examples of polyhydric alcohols include, for example, ethylene glycol, propylene glycol, glycerin, etc. Examples of ketones include, for example, acetone, methyl ethyl ketone (MEK), etc. Examples of ethers include, for example, diethyl ether, tetrahydrofuran, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol mono n-butyl ether, propylene glycol monomethyl ether, etc. Examples of esters include, for example, ethyl acetate, butyl acetate, etc. Aprotic polar solvents include dimethylsulfoxide (DMSO), dimethylformamide (DMF), dimethylacetamide (DMAc), and N-methyl-2-pyrrolidinone (NMP).

The solids concentration of the fine fibrous cellulose during the defibration process can be set as appropriate. In addition, the slurry obtained by dispersing the fibers in a dispersion medium may contain solids other than the fibers having ionic substituents introduced therein, such as urea with hydrogen bonding properties.

(ratio)
The mass ratio of the first cellulose fibers to the second cellulose fibers (first cellulose fibers: second cellulose fibers) is preferably 30:70 to 90:10, more preferably 40:60 to 90:10, even more preferably 60:40 to 90:10, and particularly preferably 70:30 to 90:10. The mass ratio of the first cellulose fibers to the second cellulose fibers may be first cellulose fibers: second cellulose fibers = 0:100. Here, the first cellulose fibers in the sheet can be observed, for example, with a scanning electron microscope (Hitachi High-Technologies Corporation, S-3600N). The second cellulose fibers can be observed, for example, with a high-resolution field emission scanning electron microscope (Hitachi, Ltd., S-5200). The mass ratio may be calculated from the volume ratio of each fiber by such observation. However, the mixing ratio of each cellulose fiber in the sheet manufacturing process described below is equal to the ratio of the first cellulose fiber to the second cellulose fiber in the sheet.

(Other fibers)
The sheet of this embodiment may contain other cellulose fibers in addition to the first cellulose fibers and the second cellulose fibers. Examples of the other cellulose fibers include highly beaten pulp obtained by beating the first cellulose fibers to a fiber width of more than 1 μm and less than 10 μm. Here, the fiber width of the other fibers refers to the fiber width of the stem fibers of the cellulose fibers. For example, when the other fibers are fibrillated cellulose fibers, the fiber width of the other fibers refers to the fiber width of the stem fibers, not the fiber width of the fibrillated and branched fibers.

Beating of other cellulose fibers can be carried out, for example, using a defibration processing device. There are no particular limitations on the defibration processing device. Examples include high-speed defibrators, grinders (stone mill type grinders), high-pressure homogenizers, ultra-high-pressure homogenizers, Clearmix, high-pressure collision type grinders, ball mills, bead mills, disk type refiners, and conical refiners. In addition, wet grinding devices such as twin-shaft kneaders, vibration mills, homomixers under high-speed rotation, ultrasonic dispersers, beaters, etc. can be used as appropriate.

(Water-soluble polymers/low molecular weight compounds)
The sheet of the present embodiment may further contain a water-soluble polymer. Examples of water-soluble polymers include synthetic water-soluble polymers exemplified by carboxyvinyl polymers, polyvinyl alcohol (PVA), alkyl methacrylate-acrylic acid copolymers, polyvinylpyrrolidone, sodium polyacrylate, polyethylene glycol, diethylene glycol, triethylene glycol, polyethylene oxide, propylene glycol, dipropylene glycol, polypropylene glycol, isoprene glycol, hexylene glycol, 1,3-butylene glycol, and polyacrylamide; thickening polysaccharides exemplified by xanthan gum, guar gum, tamarind gum, carrageenan, locust bean gum, quince seed, alginic acid, pullulan, carrageenan, and pectin; cellulose derivatives exemplified by carboxymethylcellulose, methylcellulose, and hydroxyethylcellulose; starches exemplified by cationic starch, raw starch, oxidized starch, etherified starch, esterified starch, and amylose; glycerins exemplified by polyglycerin; hyaluronic acid, and metal salts of hyaluronic acid. Of these, the water-soluble polymer is preferably polyvinyl alcohol.

The sheet of this embodiment may contain a hydrophilic low molecular weight compound instead of the water-soluble polymer. Examples of hydrophilic low molecular weight compounds include, but are not limited to, glycerin, diglycerin, erythritol, xylitol, sorbitol, galactitol, and mannitol.

The content of the water-soluble polymer or hydrophilic low molecular weight compound in the sheet is preferably 0.1 parts by mass or more, more preferably 0.5 parts by mass or more, even more preferably 1.0 parts by mass or more, and particularly preferably 5.0 parts by mass or more, relative to 100 parts by mass of the total cellulose fibers. The content of the water-soluble polymer is preferably 100 parts by mass or less, more preferably 50 parts by mass or less, even more preferably 30 parts by mass or less, and particularly preferably 20 parts by mass or less, relative to 100 parts by mass of the total cellulose fibers. By setting the content of the water-soluble polymer or hydrophilic low molecular weight compound within the above range, the strength of the sheet can be more effectively improved.

(Paper strength agent)
The sheet of the present embodiment preferably further contains a paper strength enhancing agent. This makes it possible to further improve the strength of the sheet. Examples of the paper strength enhancing agent include dry strength agents and wet strength agents. Examples of the dry strength agents include cationic starch, polyacrylamide (PAM), carboxymethyl cellulose (CMC), acrylic resin, etc. Examples of the wet strength agents include polyamide epihalohydrin, urea, melamine, thermally crosslinked polyacrylamide, etc. Among them, the sheet of the present embodiment preferably contains polyamine polyamide epihalohydrin.

Polyamine polyamide epihalohydrin is a cationic thermosetting resin obtained by synthesizing polyamide polyamine by heat condensation of an aliphatic dibasic carboxylic acid or its derivative with a polyalkylene polyamine, and then reacting the polyamide polyamine with epihalohydrin. Since polyamine polyamide epihalohydrin is an aqueous resin, polyamine polyamide epihalohydrin can also be added as an aqueous solution to the sheet-forming slurry.

Examples of polyamine polyamide epihalohydrins include polyamine polyamide epichlorohydrin, polyamine polyamide epibromohydrin, and polyamine polyamide epiiodohydrin.

The content of the paper strength agent in the sheet is preferably 0.05 parts by mass or more, more preferably 0.1 parts by mass or more, even more preferably 0.5 parts by mass or more, and particularly preferably 2.0 parts by mass or more, per 100 parts by mass of cellulose fiber. The content of the paper strength agent is preferably 100 parts by mass or less, more preferably 50 parts by mass or less, even more preferably 30 parts by mass or less, even more preferably 15 parts by mass or less, and particularly preferably 7.0 parts by mass or less, per 100 parts by mass of cellulose fiber. By setting the content of the paper strength agent within the above range, the strength of the sheet can be improved more effectively.

The sheet of this embodiment preferably contains both polyvinyl alcohol and polyamine polyamide epihalohydrin. By containing both of these, the adhesion between the fine fibrous cellulose-containing layer and the pulp fiber-containing layer in the pre-sheet of this embodiment is excellent. As a result, the pre-sheet and molded article of this embodiment have excellent mechanical properties.

(Optional ingredients)
The sheet of the present embodiment may contain optional components other than the above-mentioned components, such as preservatives, antifoaming agents, lubricants, UV absorbers, dyes, pigments, stabilizers, surfactants, sizing agents, coagulants, retention improvers, bulking agents, drainage improvers, pH adjusters, fluorescent whitening agents, pitch control agents, slime control agents, antifoaming agents, water retention agents, and dispersants.

The content of the above optional components contained in the sheet of this embodiment is preferably 50% by mass or less, more preferably 40% by mass or less, even more preferably 30% by mass or less, even more preferably 20% by mass or less, even more preferably 10% by mass or less, and particularly preferably 5% by mass or less, relative to the total mass of the sheet.

(Resin Layer)
A resin layer may be provided on the surface of the sheet of the present embodiment that is in contact with the pulp fiber-containing layer. The resin layer is preferably a resin layer formed by coating (coated resin layer).

The resin layer is a layer containing a modified polyolefin resin, and is preferably a layer containing the modified polyolefin resin as a main component. Here, the main component refers to a component contained in an amount of 50% by mass or more relative to the total mass of the resin layer. The content of the modified polyolefin resin is preferably 60% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, and particularly preferably 90% by mass or more relative to the total mass of the resin layer. The content of the modified polyolefin resin may be 100% by mass.
The modified polyolefin resin is obtained by modifying a polyolefin resin. Examples of the modification method of the polyolefin resin include acid modification, chlorination, and acrylic modification. Among them, the modified polyolefin resin is preferably an acid-modified polyolefin resin. In this case, the acid-modified component is preferably an unsaturated carboxylic acid component. The unsaturated carboxylic acid component is a component derived from an unsaturated carboxylic acid or its anhydride, and examples of the unsaturated carboxylic acid component include acrylic acid, methacrylic acid, maleic acid, maleic anhydride, itaconic acid, itaconic anhydride, fumaric acid, and crotonic acid. Among them, the unsaturated carboxylic acid component is preferably at least one selected from acrylic acid, methacrylic acid, maleic acid, and maleic anhydride, and particularly preferably at least one selected from maleic acid and maleic anhydride.

The olefin components constituting the modified polyolefin resin include alkenes having 2 to 6 carbon atoms, such as ethylene, propylene, isobutylene, 1-butene, 1-pentene, and 1-hexene. The modified polyolefin resin may be a copolymer having two or more of the above olefin components. The modified polyolefin resin may also contain other copolymer components, such as vinyl acetate and norbornenes, in addition to the above olefin components.

Among these, the modified polyolefin resin is preferably a modified polypropylene resin, more preferably an acid-modified polypropylene resin, and even more preferably a maleic oxidized polypropylene resin or a maleic anhydride polypropylene resin.

It is also preferable that the modified polyolefin resin is a chlorinated polyolefin resin. In this case, the chlorine content is preferably 5% by mass or more, and more preferably 10% by mass or more, based on the total mass of the chlorinated polyolefin resin. Also, it is preferable that the chlorine content is 50% by mass or less, based on the total mass of the chlorinated polyolefin resin.

The modified polyolefin resin is particularly preferably an acid-modified chlorinated polyolefin resin. In particular, the modified polyolefin resin is preferably a maleic anhydride-chlorinated polyolefin resin or a maleic anhydride-chlorinated polyolefin resin. By using an acid-modified chlorinated polyolefin resin, a laminate sheet with superior transparency can be obtained.

The main chain of the modified polyolefin resin preferably has a graft chain containing a carboxy group at the end. In this specification, the graft chain is a linking group that is bonded to the main chain of the modified polyolefin resin and a group having a carboxy group at the end of the linking group. The linking group that constitutes the graft chain is preferably an alkylene group having 1 to 10 carbon atoms. That is, in a preferred embodiment of the present invention, the modified polyolefin resin is a graft polymer in which the main chain skeleton is a polyolefin and -R-COOH groups (R represents an alkylene group having 1 to 10 carbon atoms) are grafted to the main chain skeleton.

The modified polyolefin resin may be a water-based modified polyolefin resin, but is preferably an organic solvent-based modified polyolefin resin. When the modified polyolefin resin is an organic solvent-based modified polyolefin resin, the resin composition forming the resin layer containing the modified polyolefin resin contains an organic solvent. Examples of the organic solvent used in this case include aliphatic organic solvents, alcohol-based organic solvents, ketone-based organic solvents, ester-based organic solvents, ether-based organic solvents, aromatic organic solvents, and cyclic alkane-based organic solvents. Among them, the organic solvent is preferably an aromatic organic solvent or a cyclic alkane-based organic solvent. Examples of the aromatic organic solvent include benzene, toluene, and xylene. Examples of the cyclic alkane-based organic solvent include cyclohexane, cyclopentane, cyclooctane, and methylcyclohexane. When a resin layer is formed from a resin composition containing the organic solvent as described above, a small amount of the organic solvent remains in the resin layer without volatilizing. For this reason, the resin layer may contain an aromatic organic solvent or a cyclic alkane-based organic solvent.

As the modified polyolefin resin, a commercially available product may be used. Examples of commercially available products include Hardlen TD-15B manufactured by Toyobo Co., Ltd., Hardlen F-2MB manufactured by Toyobo Co., Ltd., and Arrowbase SB-1230N manufactured by Unitika Ltd. The resin layer may further contain an adhesion aid in addition to the modified polyolefin resin.

(Method of manufacturing sheet)
The method for producing a sheet of this embodiment includes a step of forming a sheet from a slurry containing second cellulose fibers having a fiber width of 1000 nm or less and, if necessary, first cellulose fibers having a fiber width of 10 μm or more. Hereinafter, the method for producing a sheet of this embodiment will be described on the premise that the first cellulose fibers are used.

When the sheet of this embodiment is a multi-layer sheet having a first layer containing both first cellulose fibers having a fiber width of 10 μm or more and second cellulose fibers having a fiber width of 1000 nm or less, and a second layer containing both first cellulose fibers having a fiber width of 10 μm or more and second cellulose fibers having a fiber width of 1000 nm or less, a step may be provided in which the first layer (or second layer) is formed from a slurry containing the first cellulose fibers having a fiber width of 10 μm or more and the second cellulose fibers having a fiber width of 1000 nm or less, and then a slurry containing the first cellulose fibers having a fiber width of 10 μm or more and the second cellulose fibers having a fiber width of 1000 nm or less is applied onto the first layer to form the second layer (or the first layer). Alternatively, a first layer and a second layer may be formed from a slurry containing first cellulose fibers having a fiber width of 10 μm or more and second cellulose fibers having a fiber width of 1000 nm or less, and then these layers may be stacked together to form a multilayer sheet having the first and second layers.

The solids concentration in the slurry in which the first cellulose fibers and the second cellulose fibers are dispersed is preferably 10% by mass or less, and more preferably 5% by mass or less. In addition, the solids concentration in the slurry is preferably 0.01% by mass or more.

Here, the water retention of the first cellulose fiber is preferably 220% or more, more preferably 230% or more, and even more preferably 240% or more. The water retention of the first cellulose fiber is preferably 600% or less. The water retention of the first cellulose fiber is a value measured in accordance with J. TAPPI-26. In the sheet manufacturing process of this embodiment, by using a first cellulose fiber with a water retention of 220% or more, a slurry in which the fibers are uniformly dispersed can be obtained. Furthermore, by using a first cellulose fiber with a water retention of 220% or more, aggregation of the cellulose fibers can be suppressed even in a slurry with a high cellulose fiber concentration.

<Papermaking process>
When making a paper from a slurry containing the first cellulose fibers and the second cellulose fibers, the slurry is made into paper using a papermaking machine. The papermaking machine used in the papermaking process is not particularly limited, but may be, for example, a continuous papermaking machine such as a fourdrinier type, a cylinder type, or a tilt type, or a multi-layer papermaking machine that combines these. In the papermaking process, a known papermaking method such as handmaking may be used.

The papermaking process is carried out by filtering and dehydrating the slurry with a wire to obtain a sheet in a wet paper state, and then pressing and drying the sheet. The filter cloth used to filter and dehydrate the slurry is not particularly limited, but it is more preferable that, for example, fibrous cellulose does not pass through and the filtration speed does not become too slow. Such filter cloth is not particularly limited, but for example, a sheet, fabric, or porous membrane made of an organic polymer is preferable. The organic polymer is not particularly limited, but for example, a non-cellulose organic polymer such as polyethylene terephthalate, polyethylene, polypropylene, polytetrafluoroethylene (PTFE), etc. are preferable. In this embodiment, for example, a porous membrane of polytetrafluoroethylene with a pore size of 0.1 μm to 20 μm, and a woven fabric of polyethylene terephthalate or polyethylene with a pore size of 0.1 μm to 20 μm are examples.

In the sheeting process, a method for producing a sheet from the slurry can be carried out, for example, by using a production device equipped with a water squeezing section in which a slurry containing cellulose fibers is discharged onto the top surface of an endless belt and the dispersion medium is squeezed out of the discharged slurry to produce a web, and a drying section in which the web is dried to produce a sheet. An endless belt is disposed between the water squeezing section and the drying section, and the web produced in the water squeezing section is transported to the drying section while still placed on the endless belt.

The dehydration method used in the papermaking process is not particularly limited, but examples include dehydration methods commonly used in paper manufacturing. Among these, a method in which dehydration is performed using a fourdrinier, cylinder, or inclined wire, and then further dehydration using a roll press, is preferred. Furthermore, the drying method used in the papermaking process is not particularly limited, but examples include methods used in paper manufacturing. Among these, drying methods using a cylinder dryer, Yankee dryer, hot air dryer, near-infrared heater, infrared heater, etc. are more preferred.

After such a papermaking process, one side of the obtained sheet may be subjected to a calendaring process. Alternatively, one side may be rewetted with a rewetting liquid and subjected to a rewet casting process in which the sheet is pressed against a casting drum.

<Coating process>
In the step of coating a substrate with a slurry containing the first and second cellulose fibers (coating step), for example, a slurry (coating liquid) containing fibrous cellulose is coated on the substrate, and the coated slurry is dried to form a sheet, which can be peeled off from the substrate to obtain a sheet. Furthermore, by using a coating device and a long substrate, the sheet can be produced continuously.

The material of the substrate used in the coating process is not particularly limited, but a substrate with high wettability to the slurry is better in that it can suppress shrinkage of the sheet during drying, but it is preferable to select a substrate from which the sheet formed after drying can be easily peeled off. Among these, resin films or plates or metal films or plates are preferred, but there are no particular limitations. For example, resin films or plates such as polypropylene, acrylic, polyethylene terephthalate, vinyl chloride, polystyrene, polycarbonate, and polyvinylidene chloride, metal films or plates such as aluminum, zinc, copper, and iron plates, and those whose surfaces have been oxidized, stainless steel films or plates, and brass films or plates, etc. can be used.

In the coating process, if the viscosity of the slurry is low and it spreads on the substrate, a damming frame may be fixed on the substrate to obtain a sheet of a desired thickness and basis weight. The damming frame is not particularly limited, but it is preferable to select one that allows the edge of the sheet to be easily peeled off after drying. From this perspective, a molded resin plate or metal plate is more preferable. In this embodiment, for example, resin plates such as polypropylene plates, acrylic plates, polyethylene terephthalate plates, vinyl chloride plates, polystyrene plates, polycarbonate plates, and polyvinylidene chloride plates, metal plates such as aluminum plates, zinc plates, copper plates, and iron plates, and plates whose surfaces have been oxidized, stainless steel plates, brass plates, and the like can be used.

The coating machine used to apply the slurry to the substrate is not particularly limited, but examples that can be used include a roll coater, gravure coater, die coater, curtain coater, and air doctor coater. Die coaters, curtain coaters, and spray coaters are particularly preferred because they can produce a more uniform sheet thickness.

The slurry temperature and the ambient temperature when applying the slurry to the substrate are not particularly limited, but are preferably, for example, 5°C to 80°C, more preferably 10°C to 60°C, even more preferably 15°C to 50°C, and particularly preferably 20°C to 40°C. If the application temperature is equal to or higher than the lower limit, the slurry can be applied more easily. If the application temperature is equal to or lower than the upper limit, the evaporation of the dispersion medium during application can be suppressed.

In the coating step, the slurry is preferably applied to the substrate so that the finished sheet has a basis weight of preferably 5 g/m ² to 500 g/m ² , more preferably 10 g/m 2 to 300 g/m ² . In addition, the coating step can also produce a thin sheet having a basis weight of, for example, 30 g/m ² or less.

As described above, the coating process includes a process of drying the slurry applied to the substrate. The process of drying the slurry is not particularly limited, but may be performed, for example, by a non-contact drying method, a method of drying while restraining the sheet, or a combination of these.

Non-contact drying methods are not particularly limited, but include, for example, a method of drying by heating with hot air, infrared rays, far-infrared rays, or near-infrared rays (heat drying method), or a method of drying by creating a vacuum (vacuum drying method). Heat drying and vacuum drying may be combined, but heat drying is usually used. Drying with infrared rays, far-infrared rays, or near-infrared rays can be performed using, for example, an infrared device, a far-infrared device, or a near-infrared device, but is not particularly limited.

The heating temperature in the heat drying method is not particularly limited, but is preferably from 20°C to 150°C, and more preferably from 25°C to 105°C. If the heating temperature is equal to or higher than the lower limit, the dispersion medium can be quickly evaporated. Furthermore, if the heating temperature is equal to or lower than the upper limit, the cost required for heating can be reduced and discoloration of the cellulose fibers due to heat can be suppressed.

<1-6. Preliminary sheet>
The outermost layer of the pre-sheet of the present embodiment is a layer containing fine fibrous cellulose, which provides excellent mechanical properties when formed into a molded article.

In this embodiment, the ratio of the content of fine fibrous cellulose in the pre-sheet is 0.5% by mass or more. In addition, the ratio of the content of fine fibrous cellulose in the fine fibrous cellulose-containing layer is 10% by mass or more. Therefore, when the molded body is formed, excellent mechanical properties are obtained.

The content ratio of the fine fibrous cellulose in the pre-sheet of this embodiment is preferably 1.0% by mass or more. The content ratio of the fine fibrous cellulose in the pre-sheet of this embodiment is preferably less than 10% by mass, more preferably 8.0% by mass or less, and even more preferably 5.0% by mass or less.

The content ratio of fine fibrous cellulose in the fine fibrous cellulose-containing layer of this embodiment is preferably 15% by mass or more, and more preferably 20% by mass or more. The content ratio of fine fibrous cellulose in the fine fibrous cellulose-containing layer of this embodiment is preferably 40% by mass or less, more preferably 30% by mass or less, and even more preferably 25% by mass or less.

The ratio of the pulp fiber content in the pre-sheet of this embodiment is preferably 20% by mass or more, more preferably 30% by mass or more, and even more preferably 40% by mass or more. The ratio of the pulp fiber content in the pre-sheet of this embodiment is preferably 70% by mass or less, more preferably 60% by mass or less, and even more preferably 50% by mass or less.

The total ratio of (1) the pulp fiber content in the pulp fiber layer and (2) the cellulose content in the fine fibrous cellulose-containing layer in the pre-sheet of this embodiment is preferably more than 50% by mass, and more preferably 51% by mass or more. When the ratio is more than 50% by mass, the pre-sheet and molded body of this embodiment are more environmentally friendly and can contribute to biodegradability after disposal and reduced load on incinerators during incineration.

<2. Molded body>
The molded article of the present embodiment can be obtained by heating and then pressing the pre-sheet of the present embodiment. The heating temperature is preferably a temperature that exceeds the melting point of the thermoplastic resin. The pressure condition is preferably 1.5 to 10 MPa.

The molded article of this embodiment can be suitably used for automobile parts, electrical equipment parts, molded products for electronic equipment parts, or precision equipment parts.

The present invention will be specifically explained below with reference to examples and comparative examples. However, the present invention is not limited to the embodiments of the examples.

Example 1
<Preparation of a fine fibrous cellulose-containing sheet including a resin layer>
[Preparation of phosphorylated pulp]
As the raw material pulp, softwood kraft pulp (solid content 93% by mass, basis weight 208 g/ ^m2 sheet form, Canadian standard freeness (CSF) measured according to JIS P 8121-2:2012 after disintegration is 700 ml) manufactured by Oji Paper Co., Ltd. was used. This raw material pulp was subjected to phosphorylation treatment as follows. First, a mixed aqueous solution of ammonium dihydrogen phosphate and urea was added to 100 parts by mass (bone dry mass) of the raw material pulp to adjust the composition to 45 parts by mass of ammonium dihydrogen phosphate, 120 parts by mass of urea, and 150 parts by mass of water, thereby obtaining a chemical-impregnated pulp. Next, the obtained chemical-impregnated pulp was heated in a hot air dryer at 165°C for 200 seconds to introduce phosphoric acid groups into the cellulose in the pulp, thereby obtaining a phosphorylated pulp.

[Cleaning process]
The phosphorylated pulp was then washed. The washing was carried out by repeatedly pouring 10 L of ion-exchanged water into 100 g (absolute dry weight) of phosphorylated pulp to obtain a pulp dispersion, stirring the pulp to uniformly disperse the pulp, and then filtering and dehydrating the pulp. The washing was completed when the electrical conductivity of the filtrate reached 100 μS/cm or less.

[Neutralization treatment]
Next, the washed phosphorylated pulp was subjected to a neutralization treatment as follows: First, the washed phosphorylated pulp was diluted with 10 L of ion-exchanged water, and then a 1N aqueous sodium hydroxide solution was gradually added while stirring to obtain a phosphorylated pulp slurry having a pH of 12 to 13. Next, the phosphorylated pulp slurry was dehydrated to obtain a phosphorylated pulp that had been subjected to a neutralization treatment.

[Cleaning process]
Next, the phosphorylated pulp after the neutralization treatment was subjected to the above-mentioned washing treatment. The phosphorylated pulp thus obtained was subjected to infrared absorption spectrum measurement using FT-IR. As a result, absorption due to phosphoric acid groups was observed near 1230 cm ^-1 , and it was confirmed that phosphoric acid groups were added to the pulp. In addition, when the phosphorylated pulp thus obtained was subjected to analysis using an X-ray diffraction device, typical peaks were confirmed at two positions near 2θ = 14 ° or more and 17 ° or less and near 2θ = 22 ° or more and 23 ° or less, and it was confirmed that the cellulose had type I crystals. The amount of phosphoric acid groups (amount of first dissociated acid) measured by the measurement method described later was 1.45 mmol / g. In addition, the fiber width measured by the measurement method described later was 30 μm. Ion exchange water was added to the obtained phosphorylated pulp to obtain a first cellulose fiber dispersion containing first cellulose fibers with a solid content concentration of 2 mass %.

[Miniaturization]
The first cellulose fiber dispersion obtained by the above method was treated twice with a wet pulverization device (Starburst, manufactured by Sugino Machine Corp.) at a pressure of 200 MPa to obtain a second cellulose fiber dispersion containing second cellulose fibers. X-ray diffraction confirmed that the second cellulose fibers maintained cellulose I type crystals. The amount of phosphate groups (amount of first dissociated acid) measured by the measurement method described below was 1.45 mmol/g. In addition, the fiber width measured by the measurement method described below was 3 to 5 nm.

[Sheeting]
The first cellulose fiber dispersion, the second cellulose fiber dispersion, a polyvinyl alcohol solution (manufactured by Nippon Synthetic Chemical Industry Co., Ltd., Gohsenex Z-200), and a polyamine polyamide-epichlorohydrin solution (manufactured by Arakawa Chemical Industry Co., Ltd., Arafix 255) were mixed together to obtain a coating liquid 1 so that the first cellulose fiber was 75 parts by mass, the second cellulose fiber was 25 parts by mass, polyvinyl alcohol (PVA) was 10 parts by mass, and polyamine polyamide-epichlorohydrin (PAE) was 5 parts by mass. The solid content concentration of the coating liquid 1 was adjusted to 0.5% by mass. Next, the coating liquid 1 was weighed so that the basis weight of the obtained sheet (layer composed of the solid content of the coating liquid) was 180 g/m ² , and the coating liquid 1 was applied to a commercially available acrylic plate and dried in a thermostatic dryer at 50 ° C. A metal frame (with inner dimensions of 180 mm x 180 mm and a height of 5 cm) was placed on the acrylic plate to provide a predetermined basis weight. The dried sheet was then peeled off from the acrylic plate to obtain a sheet containing the first cellulose fibers and the second cellulose fibers. The first cellulose fibers had a fiber width of 29 μm, a water retention of 371%, and a fiber width of 3 to 5 nm.

A maleic anhydride modified polypropylene resin solution (Hardlen NS-2000, manufactured by Toyobo Co., Ltd.: 15% by weight of polypropylene resin component, 75% by weight of methylcyclohexane, 10% by weight of butyl acetate) was applied with a bar coater to the surface of the fine fibrous cellulose-containing sheet opposite the surface that had been peeled off from the acrylic plate, to obtain a coating layer. The fine fibrous cellulose-containing sheet on which the coating layer had been formed was then heated at 100°C for 1 hour to harden the coating layer, thereby converting the coating layer into a resin layer. This resulted in a fine fibrous cellulose-containing sheet including a resin layer. The thickness of the resin layer was 5 μm.

<Preparation of pulp fiber sheet>
The NBKP was defibrated using a swirling jet airflow defibrator to obtain defibrated chopped fibers. The processing air speed in the defibrator was 45 m/min, and the turbulent flow was created by a baffle installed in the device.

Then, the resulting defibrated chopped fiber was uniformly mixed with a core-sheath type heat-fusible composite fiber (PET/PE composite core-sheath fiber, core melting point 260°C, sheath melting point 100°C, fineness 2.2 dtex, fiber length 5 mm) in a ratio (mass ratio) of 73/27 using air flow to obtain a fiber mixture.

Next, an airlaid web was formed from the fiber mixture using the web forming apparatus 1 shown in Fig. 1. Specifically, a first carrier sheet 41 was fed by a first carrier sheet supplying means 40 onto an air permeable endless belt 20 mounted on a conveyor 10 and running. In Example 1, a reinforcing fiber sheet (basis weight 100 g/ ^m2 ) produced by forming defibrated chopped fibers into a sheet using a carding machine and then entangling the sheet with a needle punch was used as the first carrier sheet 41. The "basis weight" was measured according to "Paper and paperboard -- Method of measurement of basis weight" described in JIS P8124:2011.

While the air-permeable endless belt 20 was being sucked by the suction box 60, the fiber mixture was dropped and deposited on the first carrier sheet 41 together with the air flow from the fiber mixture supplying means 30. At that time, the fiber mixture was supplied so that the basis weight of the air-laid web portion was 180 g/ ^m2 . Here, the air-laid web portion was the portion that was to become the pulp fiber layer of the pre-sheet.

Next, a second carrier sheet 51 was laminated on the fiber mixture deposit on the first carrier sheet 41 by a second carrier sheet supplying means 50 to obtain an airlaid web-containing laminated sheet. In Example 1, a reinforcing fiber sheet (basis weight 100 g/ ^m2 ) produced by forming defibrated chopped fibers into a sheet using a carding machine and then entangling the sheet with a needle punch was used as the second carrier sheet 51. That is, in Example 1, the same reinforcing fiber sheet was used for the first carrier sheet 41 and the second carrier sheet 51.

The obtained air-laid web-containing laminated sheet was passed through a box-type dryer with a hot air circulation conveyor oven system and subjected to a hot air treatment at a temperature of 140° C. Thereafter, the first carrier sheet and the second carrier sheet were peeled off to obtain a pulp fiber sheet having a basis weight of 180 g/ ^m2 .

<Preparation of Molded Product of the Present Embodiment>
First, a spunbond nonwoven fabric (basis weight 40 g/ ^m2) composed of polypropylene (PP) fibers was prepared. Next, the spunbond nonwoven fabric (SB) was cut into 20 cm x 20 cm pieces, thereby obtaining six cut pieces. Next, the pulp fiber sheet (PF) was cut into 20 cm x 20 cm pieces, thereby obtaining five cut pieces. The SB and CF will become a thermoplastic resin layer and a pulp fiber layer, respectively, after the heat press treatment described later.

Next, 6 sheets of SB and 5 sheets of PF were alternately stacked so that the SB was both surface layers (both end surface layers) to produce an 11-layer laminate. Furthermore, a fine fibrous cellulose-containing sheet having a resin layer was stacked on the SB so that the main surface of the SB, which is the outermost layer (top layer) of the laminate, was in contact with the main surface of the resin layer. This resulted in a laminate structure consisting of fine fibrous cellulose-containing sheet A/resin layer A/11-layer laminate/resin layer B/fine fibrous cellulose-containing sheet B. The fine fibrous cellulose-containing sheet A constitutes the surface layer, and the fine fibrous cellulose-containing sheet B constitutes the back layer. Next, the laminate structure was placed in a stainless steel mold having an opening of 20 cm x 20 cm and a depth of 2 mm. Next, the mold was set in a heat press machine, and the laminate structure was treated with hot air at a temperature of 140 ° C. to obtain a pulp fiber-containing pre-sheet of Example 1. The layer structure of the pulp fiber-containing pre-sheet of Example 1 is fine fibrous cellulose-containing layer A/resin layer A/pulp fiber-containing layer/resin layer B/fine fibrous cellulose-containing layer B. The pulp fiber-containing pre-sheet of Example 1 was pre-pressed at a temperature of 180° C. and a pressure of 1.5 MPa for 10 minutes, and then pressed at 5 MPa for 15 minutes. It was then cooled at 5 MPa to obtain the pulp fiber-containing molded body of Example 1. The layer structure of the pulp fiber-containing molded body of Example 1 is fine fibrous cellulose-containing layer A/resin layer A/pulp fiber-containing layer/resin layer B/fine fibrous cellulose-containing layer B. However, the resin layer A and the resin layer B are thin films that cannot be recognized as layers. The pulp fiber-containing layer is composed of six thermoplastic resin layers and five pulp fiber layers.

(Example 2, Comparative Example 1)
The pre-sheets and pulp fiber-containing molded bodies of Example 2 and Comparative Example 1 were obtained in the same manner as Example 1, except that the basis weight of the fine fibrous cellulose-containing sheet, the basis weight of the pulp fiber sheet, the mass ratio of each component, etc. were changed to the values and types listed in Table 1.

<Evaluation>
The flexural modulus, flexural strength, impact resistance and impact force of the obtained molded article were measured by the following methods. The measurement results are shown in the table below.

(Method of measuring flexural modulus and flexural strength of molded products)
The obtained molded product was cut into a width of 15 mm and a length of 100 mm using a diamond cutter to prepare a test piece. After measuring the thickness of the test piece, a three-point bending test was performed at a speed of 5 mm/min and a support distance of 80 mm according to the "Bending test method for carbon fiber reinforced plastics" described in JIS K7074 to measure the bending modulus and bending strength. The bending modulus and bending strength values were used as indicators for judging the rigidity of the molded product. The larger the value, the higher the rigidity and the better the molded product was evaluated to be.

(Method of measuring impact resistance of molded products)
The obtained molded product was cut into a width of 10 mm and a length of 100 mm using a diamond cutter to prepare a test piece. The thickness of the test piece was measured, and then the impact resistance was measured by the "Charpy impact test for plastics" described in JIS K7111-1. The larger the value, the better the impact resistance and the stronger the impact resistance.
(Puncture impact force)
The puncture impact force was measured using a puncture impact force tester in accordance with JIS K7211-2.

1 Web forming device 30 Fiber mixture supply means 41 First carrier sheet 51 Second carrier sheet A Air laid web

Claims (8)

A pulp fiber-containing pre-sheet having a fine fibrous cellulose-containing layer formed on at least a pulp fiber-containing layer,
the pulp fiber-containing layer comprises pulp fibers and a thermoplastic resin;
the fine fibrous cellulose-containing layer is an outermost layer,
The content ratio of fine fibrous cellulose in the pulp fiber-containing pre-sheet is 0.5% by mass or more,
The content ratio of fine fibrous cellulose in the fine fibrous cellulose-containing layer is 10% by mass or more.
Pulp fiber-containing pre-sheet. The pre-sheet according to claim 1, wherein the content of the pulp fibers in the pulp fiber-containing pre-sheet is 40 to 50 mass %. The pre-sheet according to claim 1 or 2, wherein the content of fine fibrous cellulose in the pre-sheet is 1.0 to 5.0% by mass. The pre-sheet according to any one of claims 1 to 3, wherein the content ratio of the resin in the fine fibrous cellulose-containing layer is 70% by mass or less relative to 100% by mass of the total cellulose. The pre-sheet according to any one of claims 1 to 4, wherein the content ratio of fine fibrous cellulose in the fine fibrous cellulose-containing layer is 10 to 95 mass %. The pre-sheet according to any one of claims 1 to 5, wherein the pulp fiber-containing layer is composed of a pulp fiber layer and a thermoplastic resin layer. A pulp fiber-containing molded body obtained by molding a single or laminated pre-sheet according to any one of claims 1 to 6. The pulp fiber-containing molded article according to claim 7, which is for use as an automobile part, an electrical equipment part, a molded product for an electronic equipment part, or a precision equipment part.

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