JP7699352B2 - Cosmetic sheets - Google Patents

Description

The present invention relates to a cosmetic sheet containing cellulose nanofibers.

Gel-like sheets containing polyhydric alcohols to improve moisturizing properties are known as cosmetic sheets. As a gelling agent for such gel-like sheets, a new material called cellulose nanofiber (hereinafter sometimes abbreviated as "CNF"), which is made by breaking down the fibers that make up plant-derived cellulose materials, has been attracting attention.

For example, there is a sheet-like material that contains CNF, polyvalent metal ions, and polyhydric alcohol (see, for example, Patent Document 1). The sheet-like material in Patent Document 1 is said to have high moisture retention and can be used for a long period of time.

There is also a sheet-like material containing CNF and water (see, for example, Patent Document 2). Cosmetics that have a transparent appearance are preferred by consumers, but if the fiber diameter of the CNF is large or if the CNFs aggregate together, the light scattering by the CNF increases, and the transparency of the sheet-like material decreases. Therefore, in the sheet-like material of Patent Document 2, the fiber diameter of the CNF is set to 1000 nm or less in order to improve the transparency of the sheet.

By the way, to produce CNF, the raw material cellulose must be defibrated into small pieces. Even if you try to defibrate cellulose as is, the fibers are bound together by strong hydrogen bonds, so defibration requires a huge amount of energy and it is difficult to obtain CNF of the appropriate size.

In Patent Documents 1 and 2, carboxyl groups or phosphorous groups are introduced into cellulose to electrically repel the cellulose fibers, making defibration easier, and the resulting CNF is made into a sheet-like material.

JP 2018-2668 A Patent No. 6575700 specification

CNF containing carboxyl or phosphite groups can be easily refined to improve transparency, but because it is made by modifying cellulose to give it ionic properties, it is prone to discoloration over time when used in cosmetics, and there is also a risk of its physical properties changing when it coexists with various additives.

Once carboxyl or phosphite groups have been introduced into CNF, it is difficult to remove the functional groups and return it to a nonionic form. The sheet-like material in Patent Document 1 contains CNF with carboxyl groups, and the sheet-like material in Patent Document 2 contains CNF with phosphite groups, and neither is suitable for use as a cosmetic material.

The present invention was made in consideration of the above problems, and aims to provide a cosmetic sheet that is excellent in water resistance, transparency, and skin protection effects such as trapping harmful substances, and is suitable for use on the face, etc.

The configuration of the cosmetic sheet according to the present invention for solving the above problems is characterized by:
The composition contains nonionic cellulose nanofibers having an average fiber diameter of 2 to 30 nm, a polyhydric alcohol, and water.

In the cosmetic sheet of this configuration, cellulose nanofibers with an average fiber diameter of 2 to 30 nm are uniformly dispersed throughout the sheet, suppressing light scattering and providing high light transmittance, improving the sense of transparency required when used on the face, etc. In addition, non-ionic cellulose nanofibers are chemically stable, and when used as a cosmetic sheet, they have high water resistance, are less likely to discolor over time, and do not react with various additives even when coexisting with them, making them an ideal product for use on the face, etc.

In the cosmetic sheet according to the present invention,
The polyhydric alcohol is preferably at least one selected from the group consisting of 1,3-butylene glycol, glycerin, pentylene glycol, propylene glycol, and dipropylene glycol.

The cosmetic sheet of this configuration has a suitable flexibility and light transmittance because the polyhydric alcohol is at least one selected from the group consisting of 1,3-butylene glycol, glycerin, pentylene glycol, propylene glycol, and dipropylene glycol.

In the cosmetic sheet according to the present invention,
The content of the cellulose nanofiber is 3.5 to 25% by mass,
The content of the polyhydric alcohol is 50 to 71.5% by mass,
the mass ratio (A:B) of the content (A) of the cellulose nanofiber to the content (B) of the polyhydric alcohol is 1:2 to 1:20;
It is preferable that the moisture content is 25% or less.

The cosmetic sheet of this configuration has a cellulose nanofiber content of 3.5 to 25 mass%, a polyhydric alcohol content of 50 to 71.5 mass%, the mass ratio (A:B) is 1:2 to 1:20, and the water content is 25% or less, resulting in excellent light transmittance, strength, and flexibility, making it an ideal product for cosmetics to be used on the face, etc.

In the cosmetic sheet according to the present invention,
The thickness is preferably 100 μm or more and 2000 μm or less.

The cosmetic sheet of this configuration has a thickness of 100 μm or more and 2000 μm or less, resulting in a flexible, easy-to-use product while maintaining the strength of the film.

In the cosmetic sheet according to the present invention,
It is preferable that the total light transmittance when the thickness is 100 μm is 92% or more.

The cosmetic sheet of this configuration has high light transmittance, which improves the sense of transparency required when used on the face, etc.

The cosmetic sheet of the present invention will be described in detail below. However, the present invention is not intended to be limited to the embodiments and examples described below.

[Cosmetic sheet]
The cosmetic sheet of the present invention contains CNF, a polyhydric alcohol, and water.

<CNF>
CNF is a main material that serves as a gelling agent in the cosmetic sheet of the present invention. Nonionic CNF with an average fiber diameter of 2 to 30 nm is used. Since the fiber diameter of microfibrils, which are the structural units of natural cellulose, is 2 to 3 nm in higher plants, in order to make the average fiber diameter of CNF less than 2 nm, it is necessary to use a great deal of energy to make it finer than that, which increases the production cost. On the other hand, if the average fiber diameter of CNF is made larger than 30 nm, light scattering increases and transparency may decrease. If the transparency of the cosmetic sheet decreases, there is a risk that dullness or the like will occur and the appearance will be impaired when it is applied to cosmetics used on the face, etc. The crystalline type of cellulose may be either type I or type II, but type I is stable in a solvent such as water and can impart better shape retention when incorporated into cosmetics, etc. Nonionic CNF can be produced by suspending natural cellulose in water and finely pulverizing it by physical means. In addition, since the cellulose material is easily converted into a nanofiber state by electrostatic repulsion between fibers by introducing ionic functional groups through chemical modification rather than defibrating the cellulose material as it is, it is preferable to first chemically modify the cellulose material and then regenerate the material by removing the introduced functional groups. An example of such chemically modified cellulose is xanthated cellulose, which is obtained by adding carbon disulfide to alkali-treated cellulose to introduce xanthate groups (-OCSS ^- M ⁺ ).

Xanthated cellulose can be easily converted back to hydroxyl groups by a regeneration process such as acid treatment or heat treatment to remove the xanthate groups. By undergoing a process such as regeneration of xanthated cellulose, the amount of impurities in the CNF can be reduced. It is desirable that all of the xanthate groups introduced by chemical modification have been converted back to hydroxyl groups in the CNF used in the cosmetic sheet of the present invention, but some xanthate groups may remain as long as it does not interfere with the production or use of the cosmetic sheet. The content of xanthate groups in xanthated cellulose is evaluated by the average degree of xanthate substitution, which is the average number of hydroxyl groups substituted with xanthate groups per glucose unit of cellulose. In the cosmetic sheet of the present invention, the CNF preferably has an average degree of xanthate substitution of 0.01 or less, more preferably 0.005 or less. If the average degree of xanthate substitution is 0.01 or less, it exhibits the same reactivity as unmodified CNF and can be suitably used as nonionic CNF in cosmetics for use on the face, etc.

The CNF content in the cosmetic sheet is preferably 3.5 to 25% by mass. If the CNF content in the cosmetic sheet is within the above range, a cosmetic sheet with sufficient strength and excellent flexibility can be produced.

<Polyhydric alcohol>
Examples of polyhydric alcohols include diols having two or more hydroxyl groups in the molecule, such as 1,3-butylene glycol, pentylene glycol, propylene glycol, and dipropylene glycol, and glycerin, etc. These polyhydric alcohols may be used alone or in a mixture of two or more kinds.

The content of polyhydric alcohol in the cosmetic sheet is preferably 50 to 71.5% by mass. If the content of polyhydric alcohol in the cosmetic sheet is within the above range, the CNF can be uniformly dispersed in the sheet, and a cosmetic sheet with excellent moisture retention and flexibility can be obtained. In addition, the mass ratio (A:B) of the CNF content (A) to the polyhydric alcohol content (B) in the cosmetic sheet is preferably in the range of 1:2 to 1:20 by mass. If the mass ratio (A:B) in the cosmetic sheet is within the above range, the cosmetic sheet will have excellent light transmittance, strength, and flexibility, making it suitable for cosmetics to be used on the face, etc.

[Thickness of cosmetic sheet]
A cosmetic sheet is required to have a strength that does not break when it follows the movement of the skin. The thickness of the cosmetic sheet of the present invention is preferably 100 μm or more. If the thickness is 100 μm or more, the product can flexibly follow the movement of the skin while improving the strength of the film, resulting in an easy-to-use product. The upper limit of the thickness of the cosmetic sheet is preferably 2000 μm or less. If the thickness exceeds 2000 μm, it is difficult to follow the movement of the skin, adhesion is reduced, and it becomes difficult to apply the sheet to the skin.

[Light transmittance]
Cosmetic sheets are desired to have high light transmittance in order to obtain excellent transparency. The cosmetic sheet of the present invention preferably has a total light transmittance of 92% or more when the thickness is set to 100 μm. If the total light transmittance is 92% or more, the sheet has appropriate light transmittance and can improve the transparency required when used on the face, etc. The total light transmittance of the cosmetic sheet can be measured, for example, using a haze meter (HM-150N, manufactured by Murakami Color Materials Research Institute Co., Ltd.).

[Production of CNF]
CNF derived from xanthated cellulose, which is suitable as the CNF used in the cosmetic sheet of the present invention, can be produced by sequentially carrying out the following steps (I) to (V).

(I) Alkali treatment of cellulose materials Examples of cellulose materials include wood pulp such as kraft pulp and sulfite pulp, biomass-derived materials such as wood flour and rice straw, paper-derived materials such as waste paper, filter paper, and paper powder, and cellulose processed products that retain crystallinity such as powdered cellulose and microcrystalline cellulose of micrometer size. However, the cellulose materials are not limited to these examples. Among these cellulose materials, it is preferable to use wood pulp because it is easy to obtain and inexpensive.

In the alkali treatment of cellulose materials, the cellulose materials are treated with an aqueous solution of an alkali metal hydroxide such as sodium hydroxide or potassium hydroxide to obtain alkali cellulose. The concentration of the aqueous solution of an alkali metal hydroxide is preferably 4% by mass or more. If the concentration of the aqueous solution of an alkali metal hydroxide is less than 4% by mass, the mercerization of cellulose does not proceed sufficiently, and the amount of by-products generated during the subsequent xanthate conversion becomes significant, resulting in a low yield.

(II) Xanthate treatment In the xanthate treatment, carbon disulfide (CS ₂ ) is reacted with alkali cellulose to convert the (-O ^- M ⁺ ) group into a (-OCSS ^- M ⁺ ) group to obtain xanthate cellulose. The content of xanthate groups in xanthate cellulose is evaluated by the average degree of xanthate substitution. The average degree of xanthate substitution can be determined using the Breedee method. The procedure of the Breedee method is to weigh 1.5 g of xanthate cellulose as a solid content and add 40 mL of saturated ammonium chloride solution (5°C). After mixing well with a glass rod, filter and thoroughly wash with saturated ammonium chloride solution. Next, 50 mL of 0.5 mol/L sodium hydroxide solution (5°C) is added and stirred, and then neutralized with 1.5 mol/L acetic acid. Then, 250 mL of ion-exchanged water is added and thoroughly stirred, and 10 mL of 1.5 mol/L acetic acid and 10 mL of 0.05 mol/L iodine solution are added. This solution is titrated with 0.05 mol/L sodium thiosulfate solution using 1% by mass starch aqueous solution as an indicator. Using the titration amount of sodium thiosulfate in the above procedure and the amount of cellulose in the sample, the following formula (1):
Average degree of xanthate substitution = (0.05 × 10 × 2 - 0.05 × sodium thiosulfate titration (mL)) / {1000 × (amount of cellulose in sample (g) / 162.1)} ... (1)
The average degree of xanthate substitution is calculated by the above formula. The cellulose content in the xanthated cellulose is measured as follows. First, the xanthated cellulose is dispersed in water, and hydrochloric acid is added to perform a regeneration treatment. Next, the cellulose after the regeneration treatment is filtered, thoroughly washed, and then completely dried to measure the mass of only the cellulose, and the cellulose content in the xanthated cellulose is calculated.

When producing CNF for use in the cosmetic sheet of the present invention, it is preferable that the average degree of xanthate substitution in the xanthate treatment is 0.1 or more and 0.4 or less. If the average degree of xanthate substitution is less than 0.1, the subsequent defibration treatment may not be performed sufficiently. If the average degree of xanthate substitution exceeds 0.4, the hydrophilicity becomes too high and there is a risk of dissolving during the defibration treatment.

(III) Defibration treatment The defibration treatment is preferably carried out after dispersing the xanthate cellulose in water. A general method can be used as the defibration treatment method. For example, a method of defibrating using a rotary homogenizer, a bead mill, an ultrasonic disperser, a high-pressure homogenizer, a disc refiner, etc. can be mentioned.

Cellulose materials require a large amount of energy to defibrate as is, but with xanthate cellulose, the electrostatic repulsion between the fibers due to the xanthate group improves dispersibility, so the energy required for defibration is extremely small and it can be defibrated under relatively mild conditions.

(IV) Regeneration Treatment Regenerated CNF can be obtained by regenerating the xanthated CNF. In this regeneration treatment, the xanthate group (-OCSS ^- M ⁺ ) is eliminated and converted to a hydroxyl group (-OH), thereby regenerating the xanthated cellulose into cellulose.

As an example of a regeneration treatment, a method of treatment using an acid can be used. The acid can easily promote a reaction that removes the xanthate group and converts it to a hydroxyl group. Examples of the acid used here include mineral acids and organic acids, and in particular, mineral acids such as hydrochloric acid, sulfuric acid, and nitric acid are preferred.

As another regeneration method, the xanthate CNF can be heated to dissociate carbon disulfide from the xanthate CNF molecules and regenerate it into cellulose, thereby obtaining CNF.

(V) Redispersion treatment The CNF after the regeneration treatment has the xanthate groups removed compared to the xanthated CNF before regeneration, and is in a state where some of the fibers are aggregated due to hydrogen bonds and entanglement between the fibers. Therefore, the aqueous dispersion of the CNF after the regeneration treatment is subjected to a dispersion treatment again to obtain a CNF dispersion. Hereinafter, this dispersion treatment performed after the regeneration treatment is referred to as "redispersion treatment". For the redispersion treatment, general devices and methods used for dispersion treatment such as a rotary homogenizer, a high-pressure homogenizer, and an ultrasonic disperser can be used.

As described above, by carrying out steps (I) to (V) in order, the average fiber diameter of the obtained CNF derived from xanthated cellulose can be adjusted to 2 nm or more and 30 nm or less. The average fiber diameter of the CNF is measured by the following procedure. Ion-exchanged water is added to the obtained CNF to prepare an aqueous dispersion with a solid content concentration of 0.1% by mass, and the dispersion is centrifuged (12,000 G, 10 minutes) using a centrifuge (Avanti J-251, manufactured by Beckman Coulter) to settle the undefibrated material. The supernatant is further diluted with ion-exchanged water, applied to a support film, stained with uranyl acetate, and dried on the support film to obtain a dried specimen. The specimen is observed using a transmission electron microscope (TEM: JEM-1400, manufactured by JEOL Ltd.) at an accelerating voltage of 120 kV, and 50 nanofibers are selected from the 50,000x image, and the fiber diameters of each are measured to obtain the average value, which is the average fiber diameter.

[Production of cosmetic sheets]
The cosmetic sheet of the present invention is produced by sequentially carrying out the steps of (i) adding a polyhydric alcohol to an aqueous dispersion of nonionic CNF having an average fiber diameter of 2 to 30 nm to prepare a gel composition, (ii) forming the gel composition into a sheet, and (iii) drying the sheet-like gel composition. In the step (ii), the method of forming the gel composition into a sheet can be, for example, a method of putting the gel composition into a container capable of retaining the shape of the gel composition, or a method of coating or casting the gel composition on a smooth substrate. In the step (iii), the method of drying the sheet-like gel composition can be, but is not limited to, air drying, heat drying, vacuum drying, or a combination of these.

[Production Example 1 of Nonionic CNF]
Bleached kraft pulp (NBKP) of softwood was weighed out so that the pulp solid content was 100 g, and 2500 g of 8.5 mass % sodium hydroxide aqueous solution was added and stirred at room temperature for 3 hours to carry out an alkali treatment. The pulp after this alkali treatment was subjected to solid-liquid separation to obtain a dehydrated alkali cellulose.

The dehydrated alkali cellulose prepared above was weighed to have a pulp solid content of 100 g, and 35 g of carbon disulfide (35% by mass of the pulp solid content) was added to proceed with the sulfurization reaction to perform a xanthate treatment, thereby obtaining xanthated cellulose.

The xanthated cellulose was weighed out so that the pulp solid content was 10 g, dispersed by adding ion-exchanged water, and subjected to solid-liquid separation and thoroughly washed with ion-exchanged water. After washing, all of the xanthated cellulose was collected, and ion-exchanged water was added to make 2 kg of an aqueous suspension with a cellulose concentration of 0.5% by mass. This aqueous suspension was defibrated by passing it three times using a high-pressure homogenizer at a pressure of 80 MPa to obtain xanthated CNF. The average degree of xanthate substitution of the obtained xanthated CNF was 0.263, the fiber diameter was 3.0 to 7.4 nm, and the average fiber diameter was 6.1 nm.

33 mL of 1 mol/L sulfuric acid was added to 1.5 kg of the aqueous suspension of xanthated CNF obtained by the above procedure (cellulose concentration 0.5% by mass) and a regeneration process was carried out. After completion of the process, the mixture was neutralized to pH 7 with 1 mol/L sodium hydroxide solution to obtain an aqueous suspension of regenerated CNF. When the average degree of xanthate substitution was measured, it was found to be less than the lower limit of measurement, 0.001, confirming that the xanthate groups had almost completely been released by the acid treatment and had returned to hydroxyl groups.

The aqueous suspension of regenerated CNF obtained above was thoroughly washed by adding ion-exchanged water while being centrifuged and dehydrated using a centrifugal dehydrator. After washing, all of the regenerated CNF was collected, and ion-exchanged water was added to make 1 kg of an aqueous dispersion with a CNF solid content concentration of 1.0 mass %, which was then redispersed at a pressure of 80 MPa using a high-pressure homogenizer to obtain non-ionic CNF, which is Production Example 1. After the redispersion process, the average fiber diameter of the redispersed regenerated CNF was calculated to be 3.0 to 7.4 nm, with the average fiber diameter being 6.0 nm.

[Production Example 2 of Nonionic CNF]
The redispersion of the regenerated CNF (average fiber diameter: 6.0 nm) obtained in Production Example 1 above was diluted to a solid content concentration of 0.5% by mass, centrifuged (75,000 G, 10 minutes) using a centrifuge (Avanti J-251, manufactured by Beckman Coulter), the centrifugal supernatant was collected, and concentrated to a solid content concentration of 1.0% by mass using an evaporator. The average fiber diameter of the regenerated CNF in the centrifugal supernatant was calculated to be 2.0 to 3.1 nm, and the average fiber diameter was 2.6 nm.

[Production Example 3 of Nonionic CNF]
In the same manner as above, except that the aqueous suspension of xanthated cellulose of Production Example 1 was passed through the high-pressure homogenizer twice and defibrated, the average degree of xanthate substitution was 0.264, the fiber diameter was 2.0 to 51.4 nm, and the average fiber diameter was 28.1 nm. Xanthated CNF was obtained.

This xanthated CNF was treated in the same manner as in Production Example 1 to obtain a redispersion of nonionic CNF with fiber diameters of 2.0 to 50.7 nm and an average fiber diameter of 28.0 nm.

<Preparation and Evaluation of Cosmetic Sheet>
Cosmetic sheets of the present invention (Examples 1 to 4) were prepared, and the hardness, shape retention, and water resistance were measured. For comparison, cosmetic sheets outside the scope of the present invention (Comparative Examples 1 to 3) were prepared, and similar measurements and evaluations were performed.

Example 1
Ion-exchanged water and 1,3-butylene glycol (BG) as a polyhydric alcohol were added to the aqueous dispersion of CNF (average fiber diameter 6.0 nm) of Production Example 1, and the final concentrations were adjusted to 0.5% by mass for the CNF content and 5% by mass for the 1,3-butylene glycol content. The mixture was then stirred at 3,000 rpm for 5 minutes using a homomixer (Homomixer MARKII 2.5 type, manufactured by Primix Corporation) to obtain a gel composition. This gel composition was transferred to a plastic petri dish so that the cellulose amount was 28.5 g/ ^m2 , and the moisture was evaporated and dried for 15 hours in a 50°C air dryer, obtaining a cosmetic sheet of Example 1 having a CNF content of 6.8 mass%, a 1,3-butylene glycol content of 68.2 mass%, a mass ratio (A:B) of the CNF content (A) to the 1,3-butylene glycol content (B) of 1:10, and a moisture content of 25%.

Example 2
In Example 1, the CNF in the gel composition was changed to the CNF (average fiber diameter 2.6 nm) of Production Example 2. Otherwise, the same method as in Example 1 was used to obtain a cosmetic sheet of Example 2 having a CNF content of 6.8 mass%, a 1,3-butylene glycol content of 68.2 mass%, a mass ratio (A:B) of the CNF content (A) to the 1,3-butylene glycol content (B) of 1:10, and a moisture content of 25%.

Example 3
In Example 1, the CNF in the gel composition was changed to the CNF (average fiber diameter 28.0 nm) of Production Example 3. Otherwise, the same method as in Example 1 was used to obtain a cosmetic sheet of Example 3 having a CNF content of 6.8 mass%, a 1,3-butylene glycol content of 68.2 mass%, a mass ratio (A:B) of the CNF content (A) to the 1,3-butylene glycol content (B) of 1:10, and a moisture content of 25%.

Example 4
In Example 1, the content of 1,3-butylene glycol in the gel composition was adjusted to 10 mass%. Otherwise, a cosmetic sheet of Example 4 was obtained in the same manner as in Example 1, in which the CNF content was 3.5 mass%, the 1,3-butylene glycol content was 71.5 mass%, the mass ratio (A:B) of the CNF content (A) to the 1,3-butylene glycol content (B) was 1:20, and the moisture content was 25%.

Comparative Example 1
In Example 1, the gel composition was adjusted so as not to contain 1,3-butylene glycol and the CNF content was 0.5 mass%. A cosmetic sheet of Comparative Example 1 having a moisture content of 25% was obtained in the same manner as in Example 1.

Comparative Example 2
Xanthan gum (KELTROL CG-T, manufactured by Sansho Co., Ltd.) was used instead of CNF in Example 1. Otherwise, a cosmetic sheet for Comparative Example 2 was obtained in the same manner as in Example 1, in which the xanthan gum content was 6.8% by mass, the 1,3-butylene glycol content was 68.2% by mass, the mass ratio (A:B) of the xanthan gum content (A) to the 1,3-butylene glycol content (B) was 1:10, and the moisture content was 25%.

Comparative Example 3
Ion-exchanged water and 1,3-butylene glycol (BG) as a polyhydric alcohol were added to an aqueous dispersion of mechanically defibrated CNF (manufactured by Sugino Machine Ltd.: BiNFi-sWMa-10002, fiber diameter 24.1 to 53.2 nm, average fiber diameter 31 nm) to adjust the final concentration of CNF to 0.5 mass% and the final concentration of 1,3-butylene glycol to 5 mass%, and then the mixture was stirred at 3,000 rpm for 5 minutes using a homomixer (Homomixer MARKII 2.5 type, manufactured by Primix Corporation) to obtain a gel-like composition. This gel composition was transferred to a plastic petri dish so that the cellulose amount was 28.5 g/ ^m2 , and the moisture was evaporated and dried for 15 hours in a 50°C air dryer, obtaining a cosmetic sheet for Comparative Example 3 having a CNF content of 6.8 mass%, a 1,3-butylene glycol content of 68.2 mass%, a mass ratio (A:B) of the CNF content (A) to the 1,3-butylene glycol content (B) of 1:10, and a moisture content of 25%.

<Hardness>
The hardness of the cosmetic sheet was measured using a rubber/plastic hardness tester (Durometer GS-701N, manufactured by TECLOCK). If the measured value by the rubber/plastic hardness tester is less than 20, the cosmetic sheet is too soft and difficult to handle. If the measured value is 20 or more and less than 40, the cosmetic sheet is neither too soft nor too hard and is easy to handle. If the measured value is 40 or more, the cosmetic sheet is easy to handle but too hard to adhere to the skin.

<Shape retention>
The cosmetic sheet was evaluated for its shape retention by observing the change in shape when pinched with fingers and the presence or absence of restoring ability to bending deformation. The cosmetic sheet was deemed to have restoring ability to bending deformation if it did not break when bent 180 degrees. The evaluation criteria for shape retention were as follows:
◯: The sheet shape is maintained even when pinched with fingers, and it has recovery properties against bending deformation.
Δ: The sheet shape can be maintained even when pinched with fingers, but there is no recovery from bending deformation.
×: The gel is viscous or fluid, and cannot maintain its sheet shape when pinched with fingers.

<Water resistance>
The mass of the cosmetic sheet was measured, and then it was immersed in ion-exchanged water and left to stand at 50° C. for 20 hours. After 20 hours of immersion, it was observed whether the cosmetic sheet was dissolved. If it was not dissolved, the cosmetic sheet was taken out and the mass was measured, and the mass was calculated based on the following formula (2):
Mass change rate (%) = (mass after immersion (g) / mass before immersion (g)) × 100 (2)
The mass change rate was calculated by the following.

<Thickness>
The thickness of the cosmetic sheet was measured by observing the cross section of the sheet cut with a cutter under a digital microscope (VHX-100, manufactured by KEYENCE).

The manufacturing conditions, hardness, shape retention, water resistance, and thickness of the cosmetic sheets of Examples 1 to 4 and Comparative Examples 1 to 3 are shown in Table 1.

In all of the cosmetic sheets of Examples 1 to 4, after the gel composition dried, the 1,3-butylene glycol did not separate and formed a uniform coating, and the sheets had appropriate hardness (softness) and shape retention, with values measured using a rubber/plastic hardness tester ranging from 20 to less than 40. Furthermore, the cosmetic sheets of Examples 1 to 4 did not dissolve even after 20 hours of immersion in ion-exchanged water, confirming that they had high water resistance. The weight change rate after 20 hours of immersion was 100 to 129%.

On the other hand, the cosmetic sheet of Comparative Example 1, which did not contain polyhydric alcohol, formed an uneven coating after drying, was hard, had no recovery to bending deformation, and the coating cracked. The cosmetic sheet of Comparative Example 2, which did not contain CNF, separated 1,3-butylene glycol after drying of the gel composition. It was also confirmed that the cosmetic sheet of Comparative Example 2 was dissolved when immersed in ion-exchanged water and did not have water resistance. The cosmetic sheet of Comparative Example 3, which used mechanically defibrated CNF, had good water resistance and shape retention, but lacked softness and did not form a uniform coating after drying.

Next, cosmetic sheets of the present invention (Examples 5 to 11) were prepared and their light transmittance was evaluated. For comparison, cosmetic sheets outside the scope of the present invention (Comparative Examples 4 and 5) were also prepared and similar evaluations were carried out.

[Examples 5-1, 5-2, and 5-3]
Ion-exchanged water and 1,3-butylene glycol (BG) as a polyhydric alcohol were added to the aqueous dispersion of CNF (average fiber diameter 6.0 nm) of Production Example 1, and the final concentrations were adjusted to 0.5% by mass for the CNF content and 1,3-butylene glycol content of 1%, 5%, and 10% by mass for the 1,3-butylene glycol content, respectively. Each was then stirred at 3,000 rpm for 5 minutes using a homomixer (Homomixer MARKII 2.5 type, manufactured by Primix Corporation) to obtain three types of gel compositions. Each gel composition had a cellulose content of 28.5 g/m The cosmetic sheet of Example ^5-1 , which has a CNF content of 25 mass%, a 1,3-butylene glycol content of 50 mass%, a mass ratio (A:B) of the CNF content (A) to the 1,3-butylene glycol content (B) of 1:2, and a moisture content of 25%, a CNF content of 6.8 mass%, and a 1,3-butylene glycol content of 68.2 mass%, was transferred to a plastic petri dish and dried for 15 hours in a 50°C air blower to evaporate the moisture. %, the mass ratio (A:B) of the CNF content (A) to the 1,3-butylene glycol content (B) was 1:10, and the moisture content was 25%, and the cosmetic sheet of Example 5-3 was obtained, the CNF content was 3.6 mass%, the 1,3-butylene glycol content was 71.5 mass%, the mass ratio (A:B) of the CNF content (A) to the 1,3-butylene glycol content (B) was 1:20, and the moisture content was 25%.

Example 6
Ion-exchanged water and propylene glycol (PG) as a polyhydric alcohol were added to the aqueous dispersion of CNF (average fiber diameter 6.0 nm) of Production Example 1, and the final concentration was adjusted to 0.5% by mass for CNF and 10% by mass for propylene glycol, and then the mixture was stirred at 3,000 rpm for 5 minutes using a homomixer (Homomixer MARKII 2.5 type, manufactured by Primix Corporation) to obtain a gel composition. The gel composition was transferred to a plastic petri dish so that the cellulose content was 28.5 g/ ^m2 , and the moisture was evaporated and dried for 15 hours in a 50°C air dryer to obtain a cosmetic sheet of Example 6 having a CNF content of 3.6% by mass, a propylene glycol content of 71.5% by mass, a mass ratio (A:B) of the CNF content (A) to the propylene glycol content (B) of 1:20, and a moisture content of 25%.

Example 7
As the polyhydric alcohol, pentylene glycol (PeG) was used instead of propylene glycol. A cosmetic sheet of Example 7 was obtained in the same manner as in Example 6, except that the CNF content was 3.6% by mass, the pentylene glycol content was 71.5% by mass, the mass ratio (A:B) of the CNF content (A) to the pentylene glycol content (B) was 1:20, and the moisture content was 25%.

Example 8
As the polyhydric alcohol, glycerin (GLY) was used instead of propylene glycol, and otherwise the same method as in Example 6 was used to obtain a cosmetic sheet of Example 8 having a CNF content of 3.6 mass%, a glycerin content of 71.5 mass%, a mass ratio (A:B) of the CNF content (A) to the glycerin content (B) of 1:20, and a moisture content of 25%.

Example 9
A 1:1 mixture of 1,3-butylene glycol (BG) and glycerin (GLY) was used as the polyhydric alcohol instead of propylene glycol. A cosmetic sheet of Example 9 was obtained in the same manner as in Example 6 except for the above, in which the CNF content was 3.6% by mass, the mixture content of 1,3-butylene glycol and glycerin was 71.5% by mass, the mass ratio (A:B) of the CNF content (A) to the mixture content (B) of 1,3-butylene glycol and glycerin was 1:20, and the moisture content was 25%.

Example 10
In Example 5-3, the CNF in the gel composition was changed to the CNF (average fiber diameter 2.6 nm) of Production Example 2. Otherwise, the same method as in Example 5-3 was used to obtain a cosmetic sheet of Example 10 having a CNF content of 3.6 mass%, a 1,3-butylene glycol content of 71.5 mass%, a mass ratio (A:B) of the CNF content (A) to the 1,3-butylene glycol content (B) of 1:20, and a moisture content of 25%.

Example 11
In Example 5-3, the CNF in the gel composition was changed to the CNF (average fiber diameter 28.0 nm) of Production Example 3. Otherwise, a cosmetic sheet of Example 11 was obtained in the same manner as in Example 5-3, in which the CNF content was 3.6% by mass, the 1,3-butylene glycol content was 71.5% by mass, the mass ratio (A:B) of the CNF content (A) to the 1,3-butylene glycol content (B) was 1:20, and the moisture content was 25%.

Comparative Example 4
Ion exchanged water and 1,3-butylene glycol (BG) as a polyhydric alcohol were added to an aqueous dispersion of mechanically defibrated CNF (BiNFi-sWMa-10002, average fiber diameter 31 nm, manufactured by Sugino Machine Ltd.) to adjust the final concentration to 0.5% by mass for CNF and 5% by mass for 1,3-butylene glycol. The mixture was then stirred at 3,000 rpm for 5 minutes using a homomixer (Homomixer MARKII 2.5, manufactured by Primix Corporation) to obtain a gel-like composition. This gel composition was transferred to a plastic petri dish so that the cellulose amount was 28.5 g/ ^m2 , and the moisture was evaporated and dried for 15 hours in a 50°C air dryer, obtaining a cosmetic sheet of Comparative Example 4 having a CNF content of 6.8 mass%, a 1,3-butylene glycol content of 68.2 mass%, a mass ratio (A:B) of the CNF content (A) to the 1,3-butylene glycol content (B) of 1:10, and a moisture content of 25%.

Comparative Example 5
In Example 6, ethanol (EtOH), a monohydric alcohol, was used instead of propylene glycol, a polyhydric alcohol, and the rest of the process was the same as in Example 6 to obtain a cosmetic sheet of Comparative Example 5, which had a CNF content of 3.6 mass%, an ethanol content of 71.5 mass%, a mass ratio (A:B) of the CNF content (A) to the ethanol content (B) of 1:20, and a moisture content of 25%.

<Light transparency>
The light transmittance of the cosmetic sheets of the present invention (Examples 5 to 11) was measured by sandwiching the cosmetic sheets between glass plates and irradiating them with visible light of all wavelengths using a haze meter (HM-150N, Murakami Color Materials Research Institute Co., Ltd.) to determine the ratio of total transmitted light (total light transmittance). The total light transmittance was also measured in the same manner for cosmetic sheets outside the scope of the present invention (Comparative Examples 4 and 5).

<Thickness>
The thickness of the cosmetic sheet was measured by observing the cross section of the sheet cut with a cutter under a digital microscope (VHX-100, manufactured by KEYENCE).

The total light transmittance and thickness of the cosmetic sheets of Examples 5 to 11 and Comparative Examples 4 and 5 are shown in Table 2. In Table 2, the cosmetic sheets of Examples 5-1, 5-2, and 5-3 are collectively shown as Example 5. Table 2 also shows the total light transmittance and thickness of the cosmetic sheets with a polyhydric alcohol content of 0% by mass.

As shown in Table 2, in the cosmetic sheets of Examples 5 to 11, when the content of polyhydric alcohol in the cosmetic sheets was 50% by mass or more, the thickness was 100 μm or more and the total light transmittance was 92% or more. In addition, in the cosmetic sheets of Examples 5-1, 5-2, and 5-3, the thickness of the film increased as the content of 1,3-butylene glycol increased, but the light transmittance tended to improve. On the other hand, in the cosmetic sheet of Comparative Example 4, the thickness was similar to that of Examples 5-2 and 6, but the total light transmittance was significantly reduced. In the cosmetic sheet of Comparative Example 5, the coating was nonuniform, similar to the cosmetic sheet with a polyhydric alcohol content of 0% by mass, and the sheet itself was hard and had no restoring ability to bending deformation.

<Harmful substance trapping effect>
A cosmetic sheet (Example 12) of the present invention was prepared, and a permeation test of cigarette smoke as a harmful substance was carried out. An aldehyde compound contained in cigarette smoke was used as an index of permeation. In the permeation test, the prepared cosmetic sheet was sandwiched between an aluminum plate with holes and a silicone packing, and the plate sandwiching the cosmetic sheet was placed on top of a well of a 24-well plate containing 1 mL of pure water. The 24-well plate was placed in an airtight container into which smoke from two cigarettes was introduced, and the aldehyde compound in the pure water in the well after standing for 1 hour was detected. The aldehyde compound was reacted with 25 μM NBD-hydrazine in the presence of 0.5% by mass trifluoroacetic acid in the dark for 30 minutes, and the fluorescence intensity (Ex: 360 nm/Em: 450 nm) was measured using a microplate reader (Spectra Max Gemini, manufactured by Molecular Devices). The ratio of this fluorescence intensity to the blank corresponds to the amount of aldehyde compound adsorbed onto the cosmetic sheet, and the transmittance of the aldehyde compound that permeated the cosmetic sheet was calculated from this and used as an index of the harmful substance trapping effect.

[Examples 12-1, 12-2, and 12-3]
Ion-exchanged water and 1,3-butylene glycol (BG) as a polyhydric alcohol were added to an aqueous dispersion of CNF (average fiber diameter 6.0 nm) of Production Example 1, and the final concentrations were adjusted to 0.5% by mass for the CNF content and 1,3-butylene glycol content of 1% by mass, 5% by mass, and 10% by mass, respectively. Each was then stirred at 3,000 rpm for 5 minutes using a homomixer (Homomixer MARK II 2.5 type, manufactured by Primix Corporation) to obtain three types of gel compositions. 1 mL of each gel composition was dropped onto a membrane filter (10 μm JH, manufactured by Merck Millipore), and the water was evaporated and dried for 15 hours in a 50° C. air blower dryer to obtain a cosmetic sheet of Example 12-1 having a CNF content of 25% by mass, a 1,3-butylene glycol content of 50% by mass, a mass ratio (A:B) of the CNF content (A) to the 1,3-butylene glycol content (B) of 1:2, and a water content of 25%, a CNF content of 6.8% by mass, and a 1,3-butylene glycol content of 50% by mass. The cosmetic sheet of Example 12-2 had a CNF content of 68.2% by mass, a mass ratio (A:B) of the CNF content (A) to the 1,3-butylene glycol content (B) of 1:10, and a moisture content of 25%, and the cosmetic sheet of Example 12-3 had a CNF content of 3.6% by mass, a 1,3-butylene glycol content of 71.5% by mass, a mass ratio (A:B) of the CNF content (A) to the 1,3-butylene glycol content (B) of 1:20, and a moisture content of 25%.

Table 3 shows the transmittance of aldehyde compounds for the cosmetic sheets of Examples 12-1, 12-2, and 12-3. In Table 3, the transmittance is expressed as a percentage, with "no sheet" (where no sheet is formed on the membrane filter) being taken as 100%. For comparison, the transmittance of aldehyde compounds for a cosmetic sheet with a polyhydric alcohol content of 0% by mass is also shown.

As shown in Table 3, in the cosmetic sheets of Examples 12-1, 12-2, and 12-3, when the content of 1,3-butylene glycol in the cosmetic sheets was 50% by mass or more, the transmittance of aldehyde compounds was 66% or less. In particular, in the cosmetic sheet of Example 12-3, in which the content of 1,3-butylene glycol was 71.5% by mass, the transmittance of aldehyde compounds was 30% or less, confirming that the cosmetic sheet has an excellent effect of trapping harmful substances.

The cosmetic sheet of the present invention has excellent water resistance, transparency, and skin protection effects by trapping harmful substances, and can be used for cosmetic applications on the face, etc., and medical applications such as covering wounds, etc.

Claims (3)

The present invention comprises nonionic cellulose nanofibers having an average fiber diameter of 2 to 30 nm, a polyhydric alcohol, and water,
The thickness is 100 μm or more and 2000 μm or less,
The cosmetic sheet has a total light transmittance of 92% or more when the thickness is set to 100 μm . The cosmetic sheet according to claim 1, wherein the polyhydric alcohol is at least one selected from the group consisting of 1,3-butylene glycol, glycerin, pentylene glycol, propylene glycol, and dipropylene glycol. The content of the cellulose nanofiber is 3.5 to 25% by mass,
The content of the polyhydric alcohol is 50 to 71.5% by mass,
the mass ratio (A:B) of the content (A) of the cellulose nanofiber to the content (B) of the polyhydric alcohol is 1:2 to 1:20;
3. The cosmetic sheet according to claim 1, which has a moisture content of 25% or less.