Organic-inorganic composite, molded article of organic-inorganic composite, and method for producing same

Abstract

To provide an organic-inorganic composite exhibiting excellent mechanical characteristics even when a molded product absorbs water.SOLUTION: The organic-inorganic composite is a composite of a cellulose nanofiber and a calcium phosphate compound, and has a chemical structure in which a part of hydroxy groups contained in the composite is acylated.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to an organic-inorganic composite, a molded product of the organic-inorganic composite, and a method for producing the same. [Background technology]

[0002] Bone is a composite material composed of organic components (collagen, other proteins, cells, etc.) and inorganic components (hydroxyapatite), and is highly tough due to its combination of hardness and flexibility. Inspired by the composition of bone and its excellent mechanical properties, various organic-inorganic composites have been synthesized, and their potential applications as scaffolding materials for bone tissue and artificial bones are being explored.

[0003] For example, Patent Document 1 describes an organic-inorganic composite material that is excellent in biocompatibility, mechanical strength, and flexibility, and that is obtained by combining hydroxyapatite with a partially phosphorylated hydroxy group-containing polymer having a structure in which the hydroxy groups in the polymer have been partially phosphorylated to a degree of phosphorylation of 1 to 20%.

[0004] Patent Document 2 describes an organic-inorganic composite that can retain a considerable amount of its original mechanical strength even in aqueous or high-humidity environments, and a method for producing the same. This document describes an organic-inorganic composite that uses a hydroxyl group-containing polymer as the organic material and a calcium phosphate compound as the inorganic material, and that has a chemical structure in which some of the hydroxyl groups of the hydroxyl group-containing polymer are acylated with aromatic acyl groups or long-chain aliphatic acyl groups. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2017-131300 A [Patent Document 2] Japanese Patent Application Publication No. 2023-47422 Summary of the Invention [Problem to be solved by the invention]

[0006] As mentioned above, research has been conducted to provide organic-inorganic composites that can be used as scaffolding materials for bone tissue, artificial bones, etc., and progress has been made in improving various physical properties such as biocompatibility, mechanical properties, and flexibility.

[0007] Conventional molded organic-inorganic composites exhibit excellent mechanical properties, but suffer from the problem that their mechanical properties deteriorate significantly when immersed in water.

[0008] A primary object of the present disclosure is to provide an organic-inorganic composite that exhibits excellent mechanical properties even when the formed product absorbs water. Another object of the present disclosure is to provide a formed organic-inorganic composite product obtained by forming the organic-inorganic composite, a method for producing the organic-inorganic composite, and a method for producing a formed organic-inorganic composite product. [Means for solving the problem]

[0009] The inventors of the present disclosure conducted extensive research to solve the above-mentioned problems, and as a result, they discovered that by combining cellulose nanofibers as an organic material with a calcium phosphate compound as an inorganic material to form an organic-inorganic composite, and then acylating some of the hydroxy groups contained in the organic-inorganic composite, when the organic-inorganic composite was formed into a molded article, the article exhibited excellent mechanical properties not only before immersion in water but also after immersion in water.

[0010] The present disclosure has been completed based on these findings and through further investigations. That is, the present disclosure provides the inventions of the following aspects.

[0011] Item 1. A composite of cellulose nanofibers and a calcium phosphate compound, An organic-inorganic composite having a chemical structure in which some of the hydroxy groups contained in the composite are acylated. Item 2. The organic-inorganic composite according to Item 1, wherein the calcium phosphate compound comprises hydroxyapatite. Item 3. An organic-inorganic composite according to Item 1 or 2, having a chemical structure in which a portion of the hydroxy groups contained in the composite are acylated with at least one of an aromatic acyl group and an aliphatic acyl group. Item 4. A molded organic-inorganic composite product obtained by molding the organic-inorganic composite according to any one of Items 1 to 3. Item 5. Step 1 of dispersing cellulose nanofibers in an aqueous solution containing sodium hydroxide and sodium phosphate Step 2: Adding a calcium chloride solution to the suspension obtained in step 1 Step 3: recovering a complex of cellulose nanofibers and a calcium phosphate compound from the suspension obtained in step 2 Step 4: Dispersing the composite of cellulose nanofibers and a calcium phosphate compound recovered in step 3 in an organic solvent Step 5: Adding carboxylic acid anhydride or vinyl carboxylate to the organic solvent dispersion obtained in step 4 Step 6: recovering an organic-inorganic composite having a chemical structure in which some of the hydroxy groups contained in the composite of cellulose nanofibers and a calcium phosphate compound obtained in step 5 are acylated. The present invention relates to a method for producing a composite of cellulose nanofibers in which some of the hydroxy groups are acylated and a calcium phosphate compound, the method comprising the steps of: Item 6. A method for molding a composite of the cellulose nanofibers in which some of the hydroxy groups have been acylated according to any one of Items 1 to 4 and a calcium phosphate compound, comprising: The method for molding an organic-inorganic composite includes a step of pressurizing and heating an organic-inorganic composite powder having a chemical structure in which some of the hydroxy groups contained in the composite of the cellulose nanofibers and a calcium phosphate compound are acylated. Item 7. The method for producing a composite according to Item 5, wherein in step 5, the composite of cellulose nanofibers and a calcium phosphate compound is in a fibrous form. Item 8. The method for producing a complex according to Item 5 or 7, wherein the carboxylic acid anhydride is acetic anhydride. [Effects of the Invention]

[0012] According to the present disclosure, it is possible to provide an organic-inorganic composite that exhibits excellent mechanical properties even when the molded article absorbs water. Furthermore, according to the present disclosure, it is possible to provide a molded organic-inorganic composite article obtained by molding the organic-inorganic composite, a method for producing the organic-inorganic composite, and a method for producing a molded organic-inorganic composite article. [Brief explanation of the drawings]

[0013] [Figure 1] 1 is a graph showing the results of thermogravimetric analysis of the composites of Synthesis Example 1 and Examples 1 and 2. [Figure 2] 1 is a graph showing the results of thermogravimetric analysis of the composites of Synthesis Example 2 and Example 3. [Figure 3] 1 is a graph showing the results of powder X-ray diffraction (XRD) crystal structure analysis of the composites of Synthesis Example 1 and Examples 1 and 2. [Figure 4] 1 is a graph showing the results of powder X-ray diffraction (XRD) crystal structure analysis of the composites of Synthesis Example 2 and Example 3. [Figure 5] FIG. 1 is a schematic diagram of the composite of cellulose nanofibers and hydroxyapatite. [Figure 6] 1 is a graph showing the relationship between the decomposition temperature and the reaction temperature for each of the non-acylated conjugates (different ICs) obtained in Example 4. [Figure 7] 1 is a graph showing the relationship between the relative density and strength and the reaction temperature for each of the non-acylated complexes (different IC) obtained in Example 4. [Figure 8] 1 shows IR spectra of each acylated conjugate (different acyl groups) obtained in Example 4. DETAILED DESCRIPTION OF THE INVENTION

[0014] The configurations and combinations thereof in each embodiment are merely examples, and additions, omissions, substitutions, and other modifications of the configurations are possible as appropriate within the scope of the gist of the present disclosure. The present disclosure is not limited to the embodiments.

[0015] In the numerical ranges described in stages in this disclosure, the upper or lower limit value described in a certain numerical range may be replaced with the upper or lower limit value of another numerical range described in stages. Furthermore, separately described upper and lower limits, upper and lower limits, or lower and lower limits may be combined to form a numerical range. Furthermore, in the numerical ranges described in this disclosure, the upper or lower limit value described in a certain numerical range may be replaced with a value shown in the examples.

[0016] [Organic-inorganic composite] The organic-inorganic composite of the present disclosure is a composite of cellulose nanofibers and a calcium phosphate compound. The cellulose nanofibers are organic materials. The calcium phosphate compound is an inorganic material. In the organic-inorganic composite of the present disclosure, the cellulose nanofibers and the calcium phosphate compound are composited together.

[0017] In the present disclosure, cellulose nanofibers are microfibrillated cellulose fibers (microfibrillated cellulose). Microfibrillation is achieved by repeatedly applying strong mechanical shearing forces to raw cellulose fibers. Microfibrillation allows tens of thousands of cellulose nanofibers to be obtained from a single raw fiber. The mechanical shearing forces can be applied, for example, using an ultra-high pressure homogenizer. Known cellulose nanofibers can be used, and commercially available products are also available. These cellulose nanofibers are simply broken down (beaten) by physical action, and no other substituents have been introduced to the hydroxyl groups of the cellulose. Specific examples of such cellulose nanofibers include those available from Daicel Miraize Co., Ltd. under the trade names "Cerish (registered trademark)" and "Nano Cerish (registered trademark)."

[0018] The thickness of cellulose nanofibers is on the nanometer or micrometer level. From the viewpoint of further exerting the effects of the present invention, the average fiber diameter of the microfibrillated cellulose is, for example, 0.01 μm or more and 100 μm or less. From the same viewpoint, the average fiber length of the microfibrillated cellulose is, for example, 1 μm or more and 1000 μm or less.

[0019] A preferred raw material for cellulose nanofibers is plant-based pulp. Typically, cellulose nanofibers are obtained from wood-based pulp. Examples of plant-based pulp include wood-derived kraft pulp such as hardwood bleached kraft pulp and softwood bleached kraft pulp; sulfite pulp; waste paper pulp such as deinked pulp; and mechanical pulp such as ground pulp, pressure groundwood pulp, refiner groundwood pulp, thermomechanical pulp, chemithermomechanical pulp, chemi-mechanical pulp, and chemi-ground pulp. Cellulose nanofibers can also be obtained from non-woody pulp. Examples of non-woody pulp include kenaf, hemp, rice, bagasse, bamboo, and cotton.

[0020] The raw material constituting the cellulose nanofiber may be of only one type or may be of two or more types, and the cellulose nanofiber contained in the organic-inorganic composite of the present disclosure may be of only one type or may be of two or more types.

[0021] Calcium phosphate compounds consist of calcium ions and phosphate ions (PO4 3- ) or diphosphate ion (P2O7 4- ) and hydroxyapatite, one of the components of bone, is a typical example.

[0022] Hydroxyapatite is a calcium 10Although the basic composition is (PO4)6(OH)2, some of the Ca components may be substituted with Sr, Ba, Mg, Fe, Al, Y, La, Na, K, H, etc., some of the PO4 components may be substituted with VO4, BO3, SO4, CO3, SiO4, etc., some of the OH components may be substituted with F, Cl, O, CO3, etc., or some of these components may have defects. Specific examples include carbonate apatite and fluorinated apatite.

[0023] Other examples of calcium phosphate compounds include monocalcium phosphate (MCPM), dicalcium phosphate (DCPD), tricalcium phosphate (α-TCP, β-TCP), tetracalcium phosphate (TTCP), octacalcium phosphate, calcium octaphosphate (OCP), amorphous calcium phosphate (ACP), and other calcium phosphates, as well as hydrates thereof.

[0024] In the present disclosure, the calcium phosphate compound preferably contains hydroxyapatite, but the raw materials constituting the calcium phosphate compound may be only one type (for example, only one type of hydroxyapatite) or may be two or more types (for example, including hydroxyapatite and other calcium phosphate compounds). Furthermore, the calcium phosphate compound contained in the organic-inorganic composite of the present disclosure may be only one type or two or more types.

[0025] The organic-inorganic composite of the present disclosure has a chemical structure in which some of the hydroxyl groups contained in the organic-inorganic composite are acylated. More specifically, it has a chemical structure in which some of the hydroxyl groups of the cellulose nanofibers in the organic-inorganic composite are acylated. The hydroxyl groups (OH groups) on the surface of the cellulose microfibrils (CMFs) that make up the cellulose nanofibers are deprotonated in an alkaline aqueous solution and become negatively charged. For example, as shown in the formula in Figure 5, PO4 3- OH - Ca was added to cellulose nanofibers swollen and highly dispersed in aqueous solution. 2+By adding hydroxyapatite, cellulose nanofibers are nanocomposite-formed. Furthermore, by acylation of this composite with carboxylic acid anhydride, hydrophobicity is imparted to the cellulose microfibrils (CMF) without destroying their fibrous structure.

[0026] In the present disclosure, it is preferable that some of the hydroxy groups contained in the organic-inorganic composite (more specifically, some of the hydroxy groups of the cellulose nanofibers) have a chemical structure in which they are acylated with at least one of an aromatic acyl group and an aliphatic acyl group.

[0027] From the viewpoint of further exerting the effects of the present invention, the aromatic acyl group is preferably a benzoyl group. The benzoyl group may or may not have a substituent on the aromatic ring. Examples of the substituent include a lower alkyl group having about 1 to 6 carbon atoms and a lower alkoxy group having about 1 to 6 carbon atoms. The number and position of the substituent on the aromatic ring are not particularly limited.

[0028] The aliphatic acyl group may be an aliphatic acyl group having about 6 to 20 carbon atoms, from the viewpoint of further exerting the effects of the invention of the present disclosure. When the aliphatic acyl group has 6 or more carbon atoms, the effect of imparting water resistance by acylation to the organic-inorganic composite is suitably enhanced. From the viewpoint of the mechanical properties and water resistance of molded products of the organic-inorganic composite, an aliphatic acyl group having about 10 to 20 carbon atoms is more preferred. Preferred specific examples of the aliphatic acyl group include a lauroyl group, a caproyl group, a capryloyl group, a myristoyl group, a pentadecanoyl group, a palmitoyl group, a heptadecanoyl group, and a stearoyl group. The aliphatic acyl group may be either linear or branched. Furthermore, the aliphatic acyl group may have a substituent, and the number, position, etc. of the substituent are not particularly limited as long as the effects of the invention of the present disclosure are not impaired.

[0029] With regard to the degree of acylation of the hydroxy groups contained in the organic-inorganic composite of the present disclosure, from the viewpoint of further exerting the effects of the present invention, the average degree of substitution per glucose unit is preferably 0.3 or more, more preferably 0.6 or more, even more preferably 1.2 or more, and is preferably 3.0 or less, more preferably 2.9 or less, even more preferably 2.8 or less. An acylation degree of 0.3 or more makes it easier to impart excellent water resistance to the organic-inorganic composite of the present disclosure, and an acylation degree of 3.0 or less makes it easier to impart excellent mechanical properties to the organic-inorganic composite of the present disclosure.

[0030] From the viewpoint of more suitably exerting the effects of the present invention, the inorganic weight fraction (IC) of the acylated organic-inorganic composite of the present disclosure is preferably 40 wt % or more, more preferably 50 wt % or more, and even more preferably 70 wt % or more, and is preferably 90 wt % or less, and more preferably 80 wt % or less. Note that when the organic-inorganic composite is acylated, the proportion of the organic material increases, and therefore the inorganic weight fraction (IC) decreases.

[0031] The inorganic weight fraction (IC) of the organic-inorganic composite can be measured by thermogravimetric analysis (TGA). The inorganic weight fraction IC is calculated using the TGA measurement results according to the following formula. Specific measurements are performed according to the measurement methods described in the Examples. Inorganic weight fraction IC = (weight when heated to 1000°C and held for 10 minutes / weight when heated to 100°C and held for 10 minutes) x 100 (wt%)

[0032] The method for producing the organic-inorganic composite of the present disclosure is not particularly limited as long as the organic-inorganic composite of the present disclosure described above can be produced. The organic-inorganic composite of the present disclosure can be suitably produced, for example, by the method described in the section below titled "Method for producing organic-inorganic composite."

[0033] [Method of manufacturing organic-inorganic composite] The organic-inorganic composite of the present disclosure is a method for producing a composite of cellulose nanofibers and a calcium phosphate compound, which is characterized by including a step of acylating a portion of the hydroxy groups contained in the organic-inorganic composite (acylation step). More specifically, the method includes the steps of preparing a composite of cellulose nanofibers and a calcium phosphate compound, and acylating a portion of the hydroxy groups contained in the composite.

[0034] The composite of cellulose nanofibers and a calcium phosphate compound to be subjected to the acylation step may be prepared, for example, by performing a step of compositeing cellulose nanofibers with a calcium phosphate compound in the method for producing an organic-inorganic composite of the present disclosure. Alternatively, a composite of cellulose nanofibers and a calcium phosphate compound may be obtained separately. The composite of cellulose nanofibers and a calcium phosphate compound (organic-inorganic composite) to be subjected to the acylation step may be in the form of, for example, powder or fiber.

[0035] It is also possible to acylate cellulose nanofibers and then composite them with inorganic materials. However, in this case, the acylation of the hydroxyl groups of the cellulose nanofibers may make composite formation with calcium phosphate compounds difficult. Furthermore, when a composite formed by composites of cellulose nanofibers and calcium phosphate compounds is acylated, it is thought that a portion of the hydroxyl groups located on the surface of the composite is primarily acylated. In contrast, when acylation of cellulose nanofibers is followed by composite formation with calcium phosphate compounds, it is thought that the resulting composite is acylated not only on the surface but also in the interior. Since the water resistance of organic-inorganic composites is thought to be primarily contributed by surface acylation rather than internal acylation, it is thought that it is preferable to acylate a composite formed by composites of cellulose nanofibers and calcium phosphate compounds, which is advantageous for surface acylation.

[0036] Methods for compounding cellulose nanofibers with calcium phosphate compounds include (1) synthesizing calcium phosphate compounds and then mixing them with cellulose nanofibers, and (2) mixing cellulose nanofibers with calcium ions and phosphate ions to grow crystals of calcium phosphate compounds in the presence of cellulose nanofibers, resulting in compounding.

[0037] In the present disclosure, method (2) is particularly effective and preferred, i.e., a method in which cellulose nanofibers are prepared, calcium ions and phosphate ions are added thereto, and crystals of calcium phosphate compounds are grown in the presence of the cellulose nanofibers to form a complex.

[0038] Method (2) above includes adding calcium ions and phosphate ions to a suspension of cellulose nanofibers to cause co-precipitation, and using a semipermeable membrane to slowly diffuse hydroxide ions to promote the crystallization of calcium phosphate compounds.

[0039] Furthermore, when the coprecipitation method of the above-mentioned composite method (2) is used, it can be carried out, for example, as follows. First, a suspension of cellulose nanofibers is prepared. The suspension of cellulose nanofibers is a dispersion of microfibrillated cellulose in a liquid medium such as water. The liquid medium can be water, a hydrophilic organic solvent, or a mixture thereof. Commercially available suspensions of cellulose nanofibers include the aforementioned "Cerish (registered trademark)" and "NanoCerish (registered trademark)" from Daicel Miraize Co., Ltd.

[0040] Next, the suspension of cellulose nanofibers is mixed with phosphate ions and calcium ions, which are the raw materials for calcium phosphate, to co-precipitate an organic-inorganic composite in which the cellulose nanofibers and calcium phosphate are combined.

[0041] As the phosphate ion source, for example, phosphate salts such as Na2HPO4, (NH4)2HPO4, K2HPO4, Li2HPO4, and H3PO4, or phosphoric acid can be used.

[0042] Examples of calcium ion sources that can be used include CaCl2, Ca(NO3)2, CaBr2, CaI2, Ca(OH)2, and Ca(OCOCH3)2.

[0043] As a condition for coprecipitation, the pH is preferably adjusted to, for example, a range of 5 to 10, more preferably a range of 7 to 9. This pH adjustment can be carried out, for example, by adding sodium hydroxide or the like.

[0044] The co-precipitation temperature can be, for example, in the range of 20 to 100° C., and is preferably 40° C. or higher. The higher the co-precipitation temperature, the better the mechanical properties of the resulting organic-inorganic composite and molded product thereof tend to be, but if the temperature is too high, there is a possibility that the organic material may be altered by heat.

[0045] The co-precipitation time may be any time long enough for calcium ions and phosphate ions to co-precipitate in the mixed solution to produce an organic-inorganic composite, and may be, for example, about 1 minute to 24 hours.

[0046] The method for producing an organic-inorganic composite of the present disclosure includes a step of acylating some of the hydroxy groups contained in the composite of cellulose nanofibers and a calcium phosphate compound. This acylation can be achieved by reacting the hydroxy groups contained in the cellulose nanofibers with an appropriate acylating agent.

[0047] Carboxylic acid anhydrides or vinyl carboxylates are suitable as acylating agents. Other acylating agents, such as carboxylic acid chlorides, can also be used. However, when carboxylic acid anhydrides are used, carboxylic acids are generated as by-products of the acylation reaction. The carboxylic acids can be removed from the reaction system by washing with ethyl alcohol and performing solid-liquid separation. In the case of vinyl carboxylates, methyl alcohol is suitable for washing. When vinyl carboxylates are used as acylating agents, the by-product is vinyl alcohol. However, because vinyl alcohol is thermodynamically unstable, it is converted to volatile acetaldehyde. Therefore, the by-product can be easily removed by vacuum drying at a temperature of around 80°C.

[0048] In the method for producing an organic-inorganic composite of the present disclosure, a portion of the hydroxy groups contained in the organic-inorganic composite is acylated with an acyl group. By acylation of the organic-inorganic composite, an acylated organic-inorganic composite is obtained. As the acyl group, the above-mentioned aromatic acyl group and aliphatic acyl group are preferred. Preferred examples of the aromatic acyl group are as described above. The benzoyl group may or may not have a substituent on the aromatic ring. Examples of the substituent include a lower alkyl group having about 1 to 6 carbon atoms and a lower alkoxy group having about 1 to 6 carbon atoms. The number and position of the substituent on the aromatic ring are not particularly limited.

[0049] The aliphatic acyl group is also as described above. From the viewpoint of further exerting the effects of the present invention, examples of the aliphatic acyl group include aliphatic acyl groups having approximately 6 to 20 carbon atoms. When the aliphatic acyl group has 6 or more carbon atoms, the effect of imparting water resistance to the organic-inorganic composite by acylation is suitably enhanced. From the viewpoint of the mechanical properties and water resistance of molded products of the organic-inorganic composite, aliphatic acyl groups having approximately 10 to 20 carbon atoms are more preferred. Preferred specific examples of the aliphatic acyl group include lauroyl, caproyl, capryloyl, myristoyl, pentadecanoyl, palmitoyl, heptadecanoyl, and stearoyl groups. The aliphatic acyl group may be linear or branched. Furthermore, the aliphatic acyl group may have a substituent, and the number, position, etc. of the substituent are not particularly limited as long as the effects of the present invention are not impaired. When the number of carbon atoms in the aliphatic acyl group is small, a high proportion of the hydroxyl groups in the composite of cellulose nanofibers and a calcium phosphate compound are acylated, which is preferable. In this respect, acetic anhydride is preferable.

[0050] The acylation temperature tends to proceed more rapidly at higher temperatures, but if the temperature is too high, the reaction will saturate, so the temperature should be determined taking into consideration energy costs and the thermal impact on raw materials. The acylation temperature is preferably in the range of, for example, 40 to 120°C. When the inorganic weight fraction of the organic-inorganic composite of the present disclosure is high, the acylation reaction tends to proceed more slowly, so it is preferable to set the acylation temperature higher (for example, 80°C or higher). Furthermore, acylation with long-chain aliphatic acyl groups tends to proceed more slowly than acylation with aromatic acyl groups, so it is preferable to set the acylation temperature higher (for example, 80°C or higher).

[0051] [Organic-inorganic composite moldings] The molded organic-inorganic composite of the present disclosure is a molded product of the acylated organic-inorganic composite of the present disclosure (a composite of cellulose nanofibers and a calcium phosphate compound). The acylated organic-inorganic composite of the present disclosure is produced, for example, in the form of a powder or fiber. For example, the powdered organic-inorganic composite is compressed or molded into a desired shape to obtain a molded organic-inorganic composite of the present disclosure. The shape, size, etc. of the molded organic-inorganic composite of the present disclosure can be designed according to the purpose and are not particularly limited.

[0052] The method for molding the acylated organic-inorganic composite of the present disclosure is not particularly limited, but thermocompression molding is preferred. In thermocompression molding, the organic-inorganic composite may be subjected to uniaxial hot press molding (hot press molding) or warm isostatic press molding. Warm isostatic press molding is used to easily improve the mechanical strength and flexibility of the molded product. The molding temperature when performing pressure molding is preferably 80°C or higher and 180°C or lower, more preferably 100°C or higher and 170°C or lower, even more preferably 110°C or higher and 160°C or lower, and particularly preferably 120°C or higher and 140°C or lower. When performing pressure molding at 120°C, the pressure is preferably 50MPa or higher and 1000MPa or lower, preferably 100MPa or higher and 800MPa or lower, and more preferably 200MPa or higher and 500MPa or lower.

[0053] The organic-inorganic composite may be cold isostatically pressed and then hot-pressed. When cold isostatically pressed, the pressing temperature is preferably 5°C or higher and 30°C or lower. When cold isostatically pressed, the pressing pressure is preferably 100 MPa or higher and 300 MPa or lower. The organic-inorganic composite may be preformed before cold isostatically pressed. [Example]

[0054] The present disclosure will be explained more specifically below with reference to examples.

[0055] As the cellulose nanofiber, which is an organic material, CELISH (10% by mass aqueous suspension, product number KY100G) manufactured by Daicel Miraz Corporation was used.

[0056] [Synthesis Example 1] <Production of an organic-inorganic composite with an inorganic weight fraction (IC) of 70% by weight> An organic-inorganic composite of cellulose nanofibers and hydroxyapatite (hereinafter, cellulose nanofibers will be referred to as "CNF," hydroxyapatite as "HAP," and the composite will be referred to as the "CNF-HAP composite") was synthesized using the following procedure. Celish KY100G was weighed to a solid content of 38.6 g into a 1000 mL beaker, and a mixture of 268 mL of 0.2 M NaHPO and 72 mL of 1 M NaOH was added. The mixture was stirred at 600 rpm for 30 minutes at 50 °C. Subsequently, 448 mL of 0.2 M CaCl was added dropwise at a rate of 0.06 mL / s while maintaining the same stirring speed. After the addition, the mixture was aged for 1 hour at the same temperature and stirring speed. The resulting PTS-HAP composite coprecipitate was washed by suction filtration with a large excess of distilled water. The coprecipitate was then dehydrated by adding a large excess (approximately 2000 ml) of acetone and suction filtering, followed by vacuum drying at 80°C for at least 2 hours. It was then ground into a finer powder in a mortar and pestle. All of these powders were mixed together and used as one sample (organic-inorganic composite with a 70% inorganic weight fraction).

[0057] [Synthesis Example 2] <Production of an organic-inorganic composite with a charged inorganic weight fraction (IC) of 50% by weight> An organic-inorganic composite with a charged inorganic weight fraction of 50% by weight was produced in the same manner as in Synthesis Example 1, except that in Synthesis Example 1, 90.0 g of Celish KY100G (solid content concentration 10% by mass) was used, and used as one sample.

[0058] [Example 1] <Preparation of acylated organic-inorganic complex (IC70)> 1.5 g of the CNF-HAP composite powder obtained in Synthesis Example 1, 50 mL of DMF (N,N-dimethylformamide), and 0.5 g of K2CO3 were added to a 150 mL reaction vessel, and 8.53 mL of vinyl benzoate was added. Each was then reacted at 100 °C for 4 hours to benzoylate some of the hydroxy groups contained in the CNF-HAP composite. After the reaction was completed, a large excess of methyl alcohol was added, and after confirming the formation of a precipitate, the mixture was filtered to remove the DMF and unreacted vinyl benzoate. Subsequently, a large excess of distilled water was added, and after confirming that the precipitate had not dissolved, the mixture was filtered to completely remove any remaining K2CO3. A large excess of methyl alcohol was again added to the precipitate remaining on the filter paper, and the mixture was filtered to remove the water contained in the precipitate. The collected precipitate was then vacuum dried at 80 °C for at least 2 hours and then ground into a finer powder in a mortar to obtain the CNF-HAP composite (acylated form) of Example 1.

[0059] Approximately 0.13 g of the powder of the CNF-HAP composite (acylated product) obtained in Example 1 was placed in a rectangular mold with an inner diameter of 4 mm x 13 mm, and uniaxial pressure molding was performed at 120°C and 300 MPa for 5 minutes to produce three rectangular test pieces measuring 4.0 mm x 13.0 mm x (1.5-1.7) mm, and a molded product of the acylated CNF-HAP composite was obtained.

[0060] [Example 2] <Preparation of acylated organic-inorganic complex (IC70)> The CNF-HAP complex (acylated product) of Example 2 was obtained in the same manner as in Example 1, except that the amount of vinyl benzoate used was reduced from 8.53 mL to 1.70 mL.

[0061] Furthermore, using the powder of the CNF-HAP complex (acylated product) obtained in Example 2, a molded product of an acylated CNF-HAP complex was obtained in the same manner as in Example 1.

[0062] [Example 3] <Preparation of acylated organic-inorganic complex (IC50)> The CNF-HAP composite (acylated product) of Example 3 was obtained in the same manner as Example 1, except that the CNF-HAP composite powder obtained in Synthesis Example 2 was used instead of the CNF-HAP composite powder obtained in Synthesis Example 1, and the amount of vinyl benzoate used was changed from 8.53 mL to 17.06 mL.

[0063] Furthermore, using the powder of the CNF-HAP complex (acylated product) obtained in Example 3, a molded product of the acylated CNF-HAP complex was obtained in the same manner as in Example 1.

[0064] [Measurement and evaluation of physical properties] <tga> Thermogravimetric analysis (TGA) measurements and analysis were performed on the composites obtained in the above Synthesis Examples and Examples using a simultaneous differential thermal and gravimetric analyzer (DTG-60) manufactured by Shimadzu Corporation. The results of the thermogravimetric analysis for the composites of Synthesis Example 1 and Examples 1 and 2 are shown in Figure 1. The results of the thermogravimetric analysis for the composites of Synthesis Example 2 and Example 3 are shown in Figure 2. The graphs in Figures 1 and 2 show that the inorganic weight fraction (IC) of the CNF-HAP composites was reduced by acylation.

[0065] The inorganic weight fraction IC of the CNF-HAP composite is calculated using the TGA measurement results and the following formula: IC = (weight when heated to 1000°C and held for 10 minutes / weight when heated to 100°C and held for 10 minutes) x 100 (wt%)

[0066] <xrd> The composites obtained in the above synthesis examples and examples were subjected to powder X-ray diffraction (XRD) using a powder X-ray diffractometer "SmartLab" (Rigaku Corporation) to perform crystal structure analysis. The measurement results are shown in Figures 3 and 4, respectively. From Figures 3 and 4, characteristic peaks of HAP were observed at 26°, 32°, 39°, 46°, 49°, and 53°, indicating that HAP crystals were formed in each composite. Furthermore, from the XRD of each composite of Examples 1-3, it was found that the HAP crystals in these composites were not transformed into other calcium phosphates such as brushite or monetite through acylation.

[0067] <Physical property testing of molded products before immersion in water> A three-point bending test was performed on each of the composite molded products of Synthesis Examples 1 and 2 and Examples 1 to 3 before immersion in water using a fine ceramics bending strength tester "MZ-250" (manufactured by Maruto Co., Ltd.). The distance between supports L was 8 mm, and the crosshead speed for the bending test was 0.5 mm / min. Based on the test results, the three-point bending strength (MPa) and three-point bending strain were calculated using the following formula. 3-point bending strength: σb=(3PL) / (2wt 2 ) Three-point bending strain: εb = (600st) / (L 2 )

[0068] where P is the maximum load (N), L is the distance between supports (mm), w is the width of the test piece (mm), t is the thickness of the test piece (mm), and s is the maximum deflection (mm). The flexural modulus Eb (GPa) of each molded product was also calculated from the slope of the stress-strain curve obtained from the three-point bending test.

[0069] The density, bending strength σb, and elastic modulus Eb of each composite molded product in Synthesis Examples 1 and 2 and Examples 1 to 3 are shown in Tables 1 and 2. For reference, the results of measuring these physical properties for molded products made of ABS resin are also shown in Tables 1 and 2.

[0070] <Physical property testing of molded products after immersion in water> Each composite molded product of Synthesis Examples 1 and 2 and Examples 1 to 3 was immersed in distilled water at room temperature for 24 hours. A three-point bending test was performed on each composite molded product in the water-absorbed state using a fine ceramics bending strength tester "MZ-250" (manufactured by Maruto Co., Ltd.) in the same manner as before immersion in water. The test was performed under the same conditions as before immersion in water.

[0071] The density, bending strength σb, and elastic modulus Eb of each composite molded product of Synthesis Examples 1 and 2 and Examples 1 to 3 after immersion in water are shown in Tables 1 and 2. Note that for each composite molded product of Synthesis Examples 1 and 2, the composite collapsed upon immersion in water, making it impossible to perform physical property tests on the molded product, and therefore the test results are indicated as "-" in Tables 1 and 2. Furthermore, physical property tests on molded products made of ABS resin after immersion were omitted, and therefore the test results are indicated as "-".

[0072] [Table 1]

[0073] [Table 2]

[0074] As shown in Tables 1 and 2, the acylated organic-inorganic composites (composites of cellulose nanofibers and calcium phosphate compounds) of Examples 1 to 3 retain high mechanical properties even after immersion in water. In particular, the composite of Example 3, which has an inorganic polymerization rate IC of about 50% by weight, exhibits higher flexural strength and flexural modulus after immersion in water than before, demonstrating unexpectedly excellent mechanical properties.

[0075] [Example 4] As in Synthesis Example 1, CNF was dispersed in a NaHPO·NaOH aqueous solution at room temperature (25°C), 50, 70, and 90°C. A CaCl aqueous solution was added dropwise to the dispersed solution, as in Synthesis Example 1. The amount of Celish KY100G dispersed was adjusted so that the HAP production weight fraction (inorganic weight fraction ICfeed) was 30, 50, 70, or 96 wt%. The resulting coprecipitate was washed with water and acetone as in Synthesis Example 1 and vacuum dried to obtain a CNF-HAP composite powder. The inorganic weight fraction IC and thermal decomposition temperature in the composite were evaluated by thermogravimetric analysis (TGA), and the mechanical properties after uniaxial pressing were evaluated by a three-point bending test. The results are shown in Figures 6 and 7.

[0076] As shown in Figure 6, the thermal decomposition temperatures of the composites synthesized with ICfeeds of 30, 70, and 96 wt% decreased with increasing conjugation temperature. However, at ICfeed 50 wt%, the thermal decomposition temperature increased with increasing conjugation temperature. Since the thermal decomposition temperature of cellulose decreases with decreasing cellulose crystallinity, it is likely that the CNFs were most regularly oriented with HAP crystal growth at ICfeed 50 wt%. Furthermore, as shown in Figure 7, the most densely molded composite was obtained with ICfeed 30 wt%, which produced the least amount of HAP. However, the composite with the highest flexural strength was obtained at ICfeed 50 wt% and 70°C, which was comparable to the flexural strength of polycarbonate and the modulus of elasticity of glass fiber-reinforced PBT. Adjusting the CNF to HAP ratio can impart both the flexibility and moldability of cellulose nanofibers and the rigidity of hydroxyapatite, thereby significantly improving strength and modulus.

[0077] Next, 1 g of the CNF-HAP composite obtained under conditions of a reaction temperature of 70°C and IC feed of 50 wt% was dispersed in 15 ml of pyridine at 100°C. Carboxylic anhydrides were added and reacted. The carboxylic anhydrides added were 1.75 ml of acetic anhydride for acetylation, 2.37 ml of propionic anhydride for propanoylation, 4.28 ml of hexanoic anhydride for hexanoylation, and 5.49 ml of octanoic anhydride for octanoylation. The reaction was allowed to proceed for 4 hours. The resulting product was washed with ethyl alcohol, vacuum dried, and then powdered. The crystallinity of the CNF in the composite before and after acylation was evaluated by X-ray diffraction (XRD), the progress of acylation was monitored by Fourier transform infrared spectroscopy (FT-IR), and the mechanical properties of the composite after pressure molding were evaluated by three-point bending tests. Furthermore, the mechanical properties of the composites were evaluated by three-point bending tests after immersion in distilled water at room temperature for 2 days, and their water resistance was evaluated.

[0078] XRD results showed that the crystallinity of CNF in the untreated composite, acetylated, propanoylated, hexanoylated, and octanoylated composites was 83, 78, 73, 75, and 70%, respectively.

[0079] As shown in Figure 8, the acylated complexes exhibited peaks of 1730–1743 cm -1 The ester stretching vibration peak was observed at 1028 cm. The COC ether stretching vibration absorption peak (1028 cm) was also observed. -1 When the α-hydroxybenzoate (α-hydroxybenzoate) was normalized to 1, the shorter the alkyl chain, the higher the ester stretching vibration peak. Furthermore, three-point bending tests showed that the flexural strengths of the untreated, acetylated, propanoylated, hexanoylated, and octanoylated composites were 98, 85, 67, 71, and 56 MPa, respectively. Furthermore, the flexural strengths after water immersion were 4, 9, 6, 16, and 19 MPa, respectively, and the flexural strength fractions after water immersion to before water immersion were 4, 11, 9, 23, and 34%, respectively. These results indicate that as the alkyl chain length of the introduced acyl groups decreases, the crystallinity of the CNF in the composite decreases, resulting in decreased mechanical properties, but in return, its water resistance improves. The OH groups of the CNF improve the mechanical properties of the composite by forming interactions between cellulose polymer chains and between cellulose and HAP. The acyl groups weaken these interactions while also reducing the CNF's affinity for water, thereby improving the water resistance of the composite. It was revealed that the hydrophilic / hydrophobic balance of CNFs significantly affects the mechanical properties and water resistance of the composites.< / xrd> < / tga>

Claims

1. A composite of cellulose nanofibers and a calcium phosphate compound, An organic-inorganic composite having a chemical structure in which some of the hydroxy groups contained in the composite are acylated.

2. 2. The organic-inorganic composite according to claim 1, wherein the calcium phosphate compound comprises hydroxyapatite.

3. 3. The organic-inorganic composite according to claim 1, wherein a part of the hydroxyl groups contained in the composite has a chemical structure acylated with at least one of an aromatic acyl group and an aliphatic acyl group.

4. A molded organic-inorganic composite product obtained by molding the organic-inorganic composite according to claim 1 or 2.

5. Step 1: Dispersing cellulose nanofibers in an aqueous solution containing sodium hydroxide and sodium phosphate Step 2: Adding a calcium chloride solution to the suspension obtained in step 1 Step 3: recovering a composite of cellulose nanofibers and a calcium phosphate compound from the suspension obtained in step 2 Step 4: Dispersing the composite of cellulose nanofibers and a calcium phosphate compound recovered in step 3 in an organic solvent Step 5: Adding carboxylic acid anhydride or vinyl carboxylate to the organic solvent dispersion obtained in step 4 Step 6: recovering an organic-inorganic composite having a chemical structure in which some of the hydroxy groups contained in the composite of cellulose nanofibers and a calcium phosphate compound obtained in step 5 are acylated. The present invention relates to a method for producing a composite of cellulose nanofibers in which some of the hydroxy groups are acylated and a calcium phosphate compound, the method comprising the steps of:

6. A method for molding a composite of the cellulose nanofibers of claim 1 in which a portion of the hydroxy groups is acylated and a calcium phosphate compound, comprising: The method for molding an organic-inorganic composite includes a step of pressurizing and heating an organic-inorganic composite powder having a chemical structure in which some of the hydroxy groups contained in the composite of the cellulose nanofibers and a calcium phosphate compound are acylated.

7. The method for producing a composite according to claim 5, wherein in step 5, the composite of cellulose nanofibers and a calcium phosphate compound is in a fibrous form.

8. The method for producing a composite according to claim 5 or 7, wherein the carboxylic acid anhydride is acetic anhydride.

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