JP7626984B2 - Waterproof paper manufacturing method and waterproof paper - Google Patents

Description

This application relates to a method for imparting water resistance to paper and to water-resistant paper.

Paper weakens when it gets wet. For this reason, various treatments are used to make paper water-resistant. A typical method for making paper water-resistant is to sandwich the paper between plastic sheets. However, against the backdrop of environmental pollution issues such as microplastics in the ocean, the movement away from plastic is accelerating. There is a demand for safe, water-resistant paper with a low environmental impact.

This application was made in light of these circumstances, and aims to provide a waterproof paper that is safe and has a low environmental impact, as well as a method for producing such waterproof paper.

The method for producing waterproof paper of the present application includes an impregnation step in which paper is impregnated with carboxymethyl cellulose to obtain impregnated paper, a coating step in which a dispersion containing cellulose nanofibers and wax is applied to the impregnated paper to obtain composite paper, and a heating step in which the composite paper is heat-treated at a temperature equal to or higher than the melting point of the wax to obtain waterproof paper.

The waterproof paper of the present application comprises impregnated paper comprising paper and carboxymethyl cellulose impregnated into the paper, and a coating layer containing cellulose nanofibers and wax that is coated on at least one side of the impregnated paper, and the contact angle of the surface of the coating layer with water is 100° or more.

This application makes it easy to produce safe, environmentally friendly waterproof paper.

An image of the appearance of the aqueous dispersion of Example 1 (A) and an optical microscope image of emulsified particles in the aqueous dispersion (B) Differential scanning calorimetry chart of soy wax. Electron microscope image of a cross section of raw paper (A) and an electron microscope image of a cross section of the waterproof paper of Example 1 (B). Electron microscope image of the surface of the raw paper (A), an electron microscope image of the surface of the composite paper of Example 1 (B), and an electron microscope image of the surface of the waterproof paper of Example 1 (C). 3 is a graph showing the change over time in the contact angle of water on the surface of the composite paper and waterproof paper of Example 1. 2 is a graph showing the change over time in the contact angle of water on the paper surface at each stage in the manufacturing process of the waterproof paper of Example 1.

The waterproof paper of the embodiment of this application comprises impregnated paper and a coating layer. The impregnated paper comprises paper and carboxymethyl cellulose impregnated into the paper. The paper referred to in this application is a sheet-like material with intertwined fibers. Therefore, the paper referred to in this application includes not only paper made by intertwining plant fibers and then drawing them up, but also paper made by intertwining natural fibers other than plants or chemical fibers without weaving them, such as nonwoven fabric. The paper preferably has intertwined cellulose fibers.

Carboxymethyl cellulose (hereinafter sometimes referred to as "CMC") is a cellulose derivative with a structure in which the H of some of the -OH groups of cellulose is replaced with CH ₂ COOH. A method for impregnating paper with CMC, that is, a method for impregnating the inside of the paper with a liquid substance containing CMC or a salt thereof, evaporating the liquid, and attaching CMC or an insoluble salt of CMC to the inside of the paper, will be described later. At least one side of the impregnated paper is coated with a coating layer. It is preferable that both sides of the impregnated paper are coated with a coating layer. This is because the water resistance of the waterproof paper is improved.

The coating layer contains cellulose nanofibers and wax. Cellulose nanofibers (hereinafter sometimes referred to as "CNF") are fibers made by breaking down cellulose or its derivatives into small pieces with diameters ranging from a few nm to a few tens of nm. Although they are insoluble in water, they disperse in water at low concentrations while retaining their diameters ranging from a few nm to a few tens of nm. Cellulose derivatives have a structure in which the H of some of the -OH groups in cellulose are replaced with substituents.

Waxes are oily substances that are solid at room temperature (e.g., 25°C). Examples of waxes include vegetable waxes such as soy wax, sugarcane wax, and palm wax, animal waxes such as beeswax and glomerata wax, mineral waxes such as montan wax, petroleum waxes such as paraffin wax and microcrystalline wax, and synthetic waxes such as polyethylene wax.

In the waterproof paper of this embodiment, the contact angle of the surface of the coating layer with respect to water is 100° or more. Therefore, the waterproof paper of this embodiment has excellent water resistance. The waterproof paper of this embodiment has a wide range of uses. Examples of uses of the waterproof paper of this embodiment include straws, containers such as tableware or boxes, and nonwoven masks. When the waterproof paper of this embodiment is used as a straw, it is preferable that the air permeability is 0.1 cm ³ /cm ² ·sec or less. This is because liquid can be efficiently sucked. A method for producing waterproof paper in which the contact angle of the surface of the coating layer with respect to water is 100° or more, or waterproof paper in which the air permeability is 0.1 cm ³ /cm ² ·sec or less will be described later. The waterproof paper of this embodiment can realize water-resistant paper products that are required in society, and can contribute widely to society.

The method for producing waterproof paper according to an embodiment of the present application includes an impregnation step, a coating step, and a heating step. In the impregnation step, paper is impregnated with CMC to obtain impregnated paper. The impregnation step may include a contact step. In the contact step, a liquid containing a carboxymethylcellulose salt, such as sodium carboxymethylcellulose, is brought into contact with the paper. More specific contact methods include a method of spraying a liquid containing a carboxymethylcellulose salt on the surface of the paper, or a method of immersing the paper in a liquid containing a carboxymethylcellulose salt. An example of the liquid containing a carboxymethylcellulose salt is an aqueous solution of a carboxymethylcellulose salt.

The paper is then dried, the liquid evaporated, and the carboxymethylcellulose salt adheres to the inside of the paper. By adhering the carboxymethylcellulose salt to the inside, the strength of the paper is improved. However, if paper with the carboxymethylcellulose salt adhered to the inside is immersed in water, some of the carboxymethylcellulose salt will dissolve in the water, which may reduce the strength of the paper.

Therefore, it is preferable that the impregnation process further includes a soaking process after the contact process. In the soaking process, the carboxymethylcellulose salt attached to the inside of the paper is changed to a chemical structure that is less soluble in water. More specifically, the carboxyl group of the carboxymethylcellulose salt is changed from -COOM (M is a metal that constitutes the salt) to -COOH, which is the chemical structure of CMC. When the carboxyl group is -COOH, hydrogen bonds are formed between multiple -COOH, and a physical cross-linking effect occurs between multiple CMCs or within one CMC molecule.

Therefore, the CMC attached to the inside of the paper is almost insoluble in water, and the strength of the impregnated paper can be maintained. In the immersion process, the paper that has been through the contact process is immersed in an acid solution, i.e., acid treatment is performed, thereby converting -COOM to -COOH. Instead of this acid treatment, the paper that has been through the contact process may be immersed in an aqueous solution of a polyvalent metal salt. When an aqueous solution of a polyvalent metal salt is used, multiple ^-COO- and one polyvalent metal ion are bonded by ionic bridges, so that an insoluble salt of CMC is formed inside the paper, and the CMC in the impregnated paper is almost insoluble in water.

If necessary, the liquid containing the carboxymethylcellulose salt may further contain CNF. If a liquid containing a carboxymethylcellulose salt and CNF is used in the impregnation step, the CNF will become a membrane within the paper, and the air permeability of the waterproof paper can be significantly reduced, for example, to 0.1 cm ³ /cm ² ·sec or less. This waterproof paper with low air permeability can be suitably used for straws. The content of CNF in the liquid containing the carboxymethylcellulose salt and CNF is preferably 0.01% by mass to 2% by mass.

In the coating process, a dispersion containing CNF and wax is applied to the impregnated paper to obtain a composite paper. In the coating process, the dispersion may be applied to one side of the impregnated paper, or to both sides of the impregnated paper. The dispersion may be applied to the impregnated paper using a tool such as a bar coater or doctor blade, or by immersing the impregnated paper in the dispersion. The dispersion is obtained by adding CNF to a hydrophilic liquid such as water or alcohol to a concentration of, for example, 0.05% by mass to 2% by mass, stirring the mixture, and then heating the mixture to, for example, 60°C to 100°C while further adding wax heated to, for example, 60°C to 100°C and stirring the mixture.

The resulting dispersion is emulsified. The heated wax turns into droplets in the dispersion medium, and it is believed that the high emulsifying properties of the CNF cause the CNF to adhere to the periphery of the wax droplets, forming emulsified particles in the dispersion medium through the Pickering emulsification effect. The dispersoid is uniformly dispersed even when the dispersion is at room temperature. In other words, whether the wax is liquid or solid, the dispersoid consisting of the wax and the CNF around it is stable. For this reason, the wax can be applied uniformly to the impregnated paper even at room temperature.

The ratio of the mass of the liquid containing CNF to the mass of the wax to be added (mass of the liquid containing CNF/mass of the wax to be added) is preferably 10 to 24. In addition, the mass ratio of the solid content contained in the dispersion, i.e., in this embodiment, the sum of the mass of the CNF and the mass of the wax, is preferably 4% or more and 12% or less. Furthermore, the mass ratio of the CNF contained in the dispersion is preferably 0.1% or more and 0.5% or less. This is because the stability of the emulsified particles in the dispersion medium is improved.

In the heating process, the composite paper is heated at a temperature equal to or higher than the melting point of the wax to obtain waterproof paper. By heating the composite paper at a temperature equal to or higher than the melting point of the wax, for example by drying at a temperature of 60°C to 100°C, the wax melts and forms a water-repellent film on the surface of the composite paper. In this way, according to the method for producing waterproof paper of this embodiment, waterproof paper can be easily produced by simply subjecting the paper to several processing steps.

In this embodiment, it is preferable that the acid solution or polyvalent metal salt, CMC, CNF, and wax are food additives. This is because safety can be guaranteed when the waterproof paper of this embodiment is used for straws, tableware, masks, etc. In addition, because the acid solution or polyvalent metal salt, CMC, CNF, and wax are food additives, the waterproof paper of this embodiment has a low environmental impact and is highly recyclable. Therefore, even if it is necessary to dispose of the waterproof paper, disposal costs can be reduced. Note that food additives are substances used in the food manufacturing process or for the purpose of processing and preserving food.

Example 1: Preparation of waterproof paper
<Impregnation process>
(Contact process)
Sodium carboxymethylcellulose (Happo Shokusan Co., Ltd., CMC) was dissolved in water to prepare a 2% by mass aqueous solution of sodium carboxymethylcellulose. Paper (pure mulberry paper, Nakatomi Washi no Sato, Minobu Town) was immersed in the aqueous solution of sodium carboxymethylcellulose for 1 minute. The paper was removed from the aqueous solution of sodium carboxymethylcellulose and dried at 90°C for 20 minutes.

(Immersion process)
Citric acid (Ken-ei Pharmaceutical Co., Ltd., citric acid (crystal)) was dissolved in water to prepare a 20% by mass citric acid aqueous solution. The citric acid concentration may be 1% by mass to 45% by mass. The paper that had been subjected to the contact process was immersed in this citric acid aqueous solution. The paper removed from the citric acid aqueous solution was dried at 90°C for 30 minutes to obtain impregnated paper. This impregnated paper has limited strength retention because it absorbs water. In order to obtain practical waterproof paper, the paper needs to be treated to repel water.

<Coating process>
Coating paper with wax is one way of treating paper to make it water-repellent. However, the wax needs to be heated while coating, which is problematic in terms of cost and workability. In this application, in order to coat paper with wax at room temperature, emulsified particles of wax were produced using CNF. Wax is solid at room temperature, and becomes liquid when heated above its melting point. However, since wax is composed of oil components, even if the wax is liquid, it does not mix with hydrophilic liquids such as water. Therefore, emulsified particles of wax were produced by utilizing the Pickering emulsification effect of CNF. By emulsifying the wax, it can be stably dispersed in a hydrophilic dispersion medium.

Food additive-compatible CNF (Sugino Machine Co., Ltd., BiNFi-s WFo-10002) was dispersed in water to prepare a 0.5% by mass CNF aqueous dispersion. Food additive-compatible wax soy wax (Supplies for Candles, Soy Wax (Soft Type) Soy Nature Wax C-3) and this CNF aqueous dispersion were heated separately to 80°C, while 1 part by mass of wax and 19 parts by mass of CNF aqueous dispersion were mixed and stirred to obtain an aqueous dispersion containing CNF and wax. Figure 1A shows an external image of this aqueous dispersion at room temperature, and Figure 1B shows an optical microscope image of the emulsified particles. As shown in Figure 1A, in this aqueous dispersion, the emulsified particles were stably dispersed in water. Also, as shown in Figure 1B, it was found that this aqueous dispersion was composed of emulsified particles of about 10 μm. The impregnated paper that had undergone the impregnation process was immersed in this aqueous dispersion. The emulsified particles adhered to the surface of the impregnated paper, resulting in a composite paper.

<Heating process>
Figure 2 shows the results of differential scanning calorimetry of the soy wax used in the coating process. As shown in Figure 2, this soy wax has two endothermic peaks (melting points), with the higher endothermic peak being 57.8°C. Therefore, the soy wax can be melted by heat treatment at 57.8°C or higher. The composite paper that had been through the coating process was then dried at 70°C for 60 minutes. The soy wax in the emulsified particles melted, and a water-repellent film was formed on the surface of the impregnated paper, resulting in a waterproof paper.

Example 2: Preparation of other waterproof papers
The waterproof paper of Example 2 was prepared in the same manner as in Example 1, except that during the contact process of the impregnation step, an aqueous dispersion containing 2 mass% sodium carboxymethylcellulose and 0.01 mass% CNF was used instead of the 2 mass% aqueous solution of sodium carboxymethylcellulose.

〔evaluation〕
FIG. 3A shows an electron microscope image of a cross section of raw paper (hereinafter sometimes referred to as "raw paper"), and FIG. 3B shows an electron microscope image of a cross section of the waterproof paper of Example 1 after the heating process. As shown in FIG. 3A, the raw paper had a shape with gaps where cellulose fibers crossed. In contrast, in the waterproof paper of Example 1, CMC was filled between the cellulose fibers as shown in FIG. 3B. In addition, a water-repellent layer composed of soy wax and CNF was formed on the surface of the waterproof paper of Example 1.

Figure 4A shows an electron microscope image of the surface of the raw paper, Figure 4B shows an electron microscope image of the surface of the composite paper of Example 1, and Figure 4C shows an electron microscope image of the surface of the waterproof paper of Example 1. As shown in Figure 4A, the raw paper had a structure in which cellulose fibers overlapped each other. In contrast, in the composite paper of Example 1, the entire surface of the impregnated paper was covered with emulsified particles, as shown in Figure 4B. Furthermore, in the waterproof paper of Example 1, it was found that the soy wax component in the emulsified particles was melted, as shown in Figure 4C.

Figure 5 shows the change over time in the contact angle of the composite paper of Example 1 and the water-resistant paper of Example 1 on the surface. The contact angle was measured using a contact angle measuring device (Portable Contact Angle Meter PG-X, Matsubo Corporation) (same below). A small drop of water was dropped onto the composite paper or the water-resistant paper, and the change over time in the contact angle was measured. As shown in Figure 5, the contact angle was 72° to 75° for the composite paper, while the contact angle was over 100° for the water-resistant paper. It was found that the water resistance of the paper was improved by the heating process. In addition, the contact angle of the composite paper decreased over time. This is because the composite paper absorbed water. From this result, it was found that it is not enough to simply coat the surface of the impregnated paper with emulsified particles, and it is important for the wax melted in the heating process to uniformly coat the surface of the impregnated paper in order to impart water resistance to the paper.

Figure 6 shows the change over time in the contact angle of the paper surface with water at each stage in the manufacturing process of the waterproof paper of Example 1. That is, tiny water droplets were dropped onto the raw paper, the paper after the contact process of the impregnation process but before the immersion process (hereinafter sometimes referred to as "untreated acid paper of Example 1"), the impregnated paper, and the waterproof paper, and the change in the contact angle over time was measured. As shown in Figure 6, the contact angles were low for all papers other than the waterproof paper, and the contact angles decreased over time. In other words, going through all the steps of the impregnation process, coating process, and heating process had a significant effect on imparting water resistance to the paper.

In addition, the dry weight and the wet weight after immersion in water for 1 minute of each of the four types of paper in Example 1, each 6.5 cm square, were measured, and the water absorption rate (%) was calculated according to the following formula.
(wet weight-dry weight)/dry weight x 100
The water absorption rate of the raw paper was 136.6%, that of the untreated paper was 321.5%, that of the impregnated paper was 63.85%, and that of the waterproof paper was 33.92%. The waterproof paper had a significantly reduced water absorption rate compared to the other papers, demonstrating that it was waterproof.

The tensile strength and air permeability of the paper in the dry and wet states at each stage in the manufacturing process of the waterproof paper of each example were also measured. That is, the tensile strength and air permeability were measured for the raw paper, the unacid-treated paper of Example 1, the paper after the contact process but before the soaking process of the impregnation process of Example 2 (hereinafter sometimes referred to as "unacid-treated paper of Example 2"), the impregnated paper of Example 1, the impregnated paper of Example 2, the waterproof paper of Example 1, and the waterproof paper of Example 2.

The tensile strength was measured using a tensile strength measuring device (Shimadzu Corporation, tabletop precision universal testing machine AUTOGRAPH AG-X 5kN). The dry state refers to a state in which the temperature is 25°C and the relative humidity is 50%. The wet state refers to a state in which the material is immersed in water and excess water is removed using filter paper. The air permeability was measured based on JIS L1096. The results are shown in Table 1.

As shown by the tensile strength in Table 1, it was found that the impregnated paper and waterproof paper of each Example were able to maintain their strength. In addition, the impregnated paper and waterproof paper of Example 2 had low air permeability. From these results, it was found that the air permeability of the impregnated paper and waterproof paper can be significantly reduced by further containing CNF in the liquid containing carboxymethyl cellulose salt used in the impregnation process. In other words, the waterproof paper of Example 2 is suitable for applications that require low air permeability, such as straws.

Claims (9)

an impregnation step of impregnating paper with carboxymethyl cellulose to obtain impregnated paper;
A coating step of coating the impregnated paper with a dispersion containing cellulose nanofibers and wax to obtain a composite paper;
a heating step of heat-treating the composite paper at a temperature equal to or higher than the melting point of the wax to obtain a waterproof paper;
having
The method for producing waterproof paper , wherein the impregnation step comprises a contacting step of contacting the paper with a liquid containing the carboxymethyl cellulose salt . In claim 1 ,
The method for producing waterproof paper, wherein the impregnation step further comprises a soaking step of soaking the paper that has been subjected to the contact step in an acid solution. In claim 1 ,
The method for producing waterproof paper, wherein the impregnation step further comprises an immersion step of immersing the paper that has been subjected to the contact step in an aqueous solution of a polyvalent metal salt. In claim 2 ,
The method for producing waterproof paper, wherein the acid solution, the carboxymethyl cellulose, the cellulose nanofibers, and the wax are food additives. In claim 3 ,
The method for producing waterproof paper, wherein the polyvalent metal salt, the carboxymethyl cellulose, the cellulose nanofibers, and the wax are food additives. In any one of claims 1 to 5,
The method for producing waterproof paper, wherein the liquid containing the carboxymethyl cellulose salt further contains cellulose nanofibers. In any one of claims 1 to 6,
A method for producing waterproof paper, wherein the mass percentage of solids contained in the dispersion is 4% or more and 12% or less, and the mass percentage of cellulose nanofibers contained in the dispersion is 0.1% or more and 0.5% or less. an impregnated paper comprising paper and carboxymethyl cellulose impregnated in the paper;
A coating layer that is coated on at least one side of the impregnated paper and contains cellulose nanofibers and wax;
having
The waterproof paper has a contact angle of 100° or more with respect to the surface of the coating layer. A straw made from the waterproof paper of claim 8, wherein the waterproof paper has an air permeability of 0.1 cm ³ /cm ² ·sec or less.