Method for producing composite material, and composite material

Abstract

To provide a method for producing a composite material and a composite material in which water absorption property can be reduced without hydrophobing a nano-cellulose fiber.SOLUTION: A method for producing a composite material includes: a step of mixing rubber latex containing a rubber component, cellulose nanofiber aqueous dispersion containing cellulose nanofiber in which aqueous solvent contains a carboxy group, and unsaturated carboxylic acid salt containing at least any one of aluminum ion, zinc ion and iron ion to obtain a mixture; a step of heating the mixture and reducing moisture content to obtain a first mixture; a step (S40) of cleaning the first mixture and reducing sodium ion in the first mixture to obtain a second mixture; a step (S50) of heating the second mixture and reducing moisture content to obtain a third mixture; and a step (S60) of kneading the third mixture to obtain a composite material. The cellulose nanofiber includes a substance derived from oxidized cellulose oxidized by using N-oxyl compound.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for producing a composite material containing cellulose nanofibers, and the composite material. [Background technology]

[0002] In recent years, plant-derived cellulose nanofibers, which have a low environmental impact as part of the Sustainable Development Goals (SDGs), have been attracting attention. In this context, rubber composite materials using cellulose nanofibers as fiber reinforcement have also been proposed (Patent Document 1).

[0003] However, cellulose nanofibers are generally available commercially as aqueous dispersions, which must be mixed with a rubber component dispersion (latex) and then dried to obtain a solid rubber composite. During this drying process, the cellulose nanofibers bond and aggregate, leaving numerous agglomerates in the rubber composite. These agglomerates are known to affect basic physical properties of rubber products, such as tensile strength and elongation at break.

[0004] Therefore, a method for producing a composite material containing cellulose nanofibers that reduces agglomerates of cellulose nanofibers and is easy to apply to rubber products has been proposed (Patent Document 2).

[0005] According to the composite material manufacturing method of Patent Document 2, a composite material with few cellulose nanofiber agglomerates can be produced, and by using the composite material as a masterbatch for rubber products, the cellulose nanofibers can be easily applied to rubber products.

[0006] However, even composites with few cellulose nanofiber aggregates tended to exhibit reduced physical properties upon water absorption, which is thought to be due to the release of moisture from the interaction between the hydrophilic groups of the cellulose nanofibers and the rubber molecules.

[0007] On the other hand, a method for producing a composite material has also been proposed in which cellulose nanofibers are hydrophobized by converting the hydrophilic hydroxyl groups into hydrophobic groups, thereby improving compatibility with matrices such as synthetic resins and forming composites (e.g., Non-Patent Document 1). [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-95611 [Patent Document 2] International Publication No. 2023 / 218814 [Non-patent literature]

[0009] [Non-Patent Document 1] Daisuke Kuroki, "Development Status of Modified Cellulose Nanofibers," Journal of the Imaging Society of Japan, Vol. 55, No. 3, 2016: 369-374 Summary of the Invention [Problem to be solved by the invention]

[0010] However, if cellulose nanofibers are made hydrophobic, the cellulose nanofibers tend to aggregate with each other, making defibration difficult, and the physical properties of the composite material deteriorate.

[0011] Therefore, the present invention provides a method for producing a composite material that can reduce water absorption without hydrophobizing cellulose nanofibers, and the composite material. [Means for solving the problem]

[0012] The present invention has been made to solve at least some of the above-mentioned problems, and can be realized as the following aspects or application examples.

[0013] [1] One aspect of the method for producing a composite material according to the present invention is to A step of mixing a rubber latex containing a rubber component, a cellulose nanofiber aqueous dispersion containing cellulose nanofibers containing carboxyl groups in an aqueous solvent, and an unsaturated carboxylate salt containing at least any one of aluminum ions, zinc ions, and iron ions to obtain a mixed liquid; heating the mixture to reduce the water content to obtain a first mixture; washing the first mixture to reduce sodium ions in the first mixture to obtain a second mixture; heating the second mixture to reduce the water content to obtain a third mixture; kneading the third mixture to obtain a composite material; Including, The cellulose nanofibers are characterized by including those derived from oxidized cellulose that has been oxidized using an N-oxyl compound.

[0014] [2] In one embodiment of the method for producing the composite material, The rubber latex may be natural rubber latex containing 0.01% or more of ammonia.

[0015] [3] In one embodiment of the method for producing the composite material, In the step of obtaining the first mixture, the cellulose nanofibers are contained in an amount of 10 parts by mass or more and 30 parts by mass or less relative to 100 parts by mass of the rubber component, The cellulose nanofibers have an average fiber diameter of 2 nm or more and 8 nm or less, an average fiber length of 200 nm or more and 800 nm or less, and a carboxy group content of 1.2 mmol / g to 1.7 mmol / g, The mixed liquid may contain the unsaturated carboxylate in an amount of 0.6 to 7.0 times the equivalent of the carboxyl groups contained in the cellulose nanofibers.

[0016] [4] In one embodiment of the method for producing the composite material, In the step of obtaining the third mixture, the second mixture can be heated so that the moisture content of the third mixture becomes 3% or less.

[0017] [5] In one embodiment of the method for producing the composite material, The composite material may have a water absorption rate of 10% or less in a water absorption test in which the composite material is immersed in water at 70°C for 48 hours.

[0018] [6] In one embodiment of the method for producing the composite material, The composite material may have a change rate of −25% to +30% in the 20% modulus value in a tensile test based on JIS K6251 after the water absorption test relative to the 20% modulus value before the water absorption test.

[0019] [7] One aspect of the composite material according to the present invention is A composite material comprising a rubber component, cellulose nanofibers containing a carboxy group in an aqueous solvent, and an unsaturated carboxylate salt containing any one of aluminum ions, zinc ions, and iron ions, The composite material has a water absorption rate of 10% or less in a water absorption test in which it is immersed in water at 70°C for 48 hours. It is characterized by the fact that [Effects of the Invention]

[0020] According to one aspect of the method for producing a composite material and the composite material of the present invention, the composite material has few agglomerates of cellulose nanofibers, and yet the deterioration of physical properties due to water absorption is minimal. [Brief explanation of the drawings]

[0021] [Figure 1] 1 is a flowchart of a method for producing a composite material according to one embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0022] Preferred embodiments of the present invention will be described in detail below with reference to the drawings. Note that the embodiments described below do not unduly limit the content of the present invention as set forth in the claims. Furthermore, not all of the configurations described below are necessarily essential constituent elements of the present invention. Note that "to" indicates that the upper and lower limits are included unless otherwise specified.

[0023] A. Composite materials The composite material according to this embodiment includes a rubber component, cellulose nanofibers containing a carboxy group, and an unsaturated carboxylate salt containing at least any one of aluminum ions, zinc ions, and iron ions.

[0024] The composite material may be used as a masterbatch for blending cellulose nanofibers into rubber products. The composite material is moisture-free or low-moisture due to the drying process in the step of obtaining the first mixture described below. Normally, the cellulose nanofibers tend to aggregate due to hydrogen bonding. However, this aggregation is suppressed by the rubber component and the unsaturated carboxylate salt containing at least aluminum ions, zinc ions, and iron ions. Therefore, the composite material does not contain cellulose agglomerates that can be identified, at least visually (visual observation of scanning electron microscope images). Visually identifiable cellulose agglomerates can often be identified if they have a maximum width of 10 μm or more, and can be reliably identified if they have a maximum width of 50 μm or more.

[0025] Furthermore, when the cellulose nanofibers in a composite material are mixed as a masterbatch with an additional rubber component, they are easily defibrated and dispersed by the rubber component and at least one of aluminum ions, zinc ions, and iron ions. The additional rubber component preferably does not contain water. Therefore, the composite material has fewer cellulose nanofiber agglomerates, resulting in excellent reinforcement for rubber products. The composite material may also be used as a masterbatch for rubber products and diluted with the rubber component. The quality of a composite material is determined by the absence of agglomerates, at least by visual observation. However, simply visually observing a portion of the composite material to identify agglomerates is insufficient because it is not possible to determine the overall condition. Therefore, it is preferable to determine the defibration and dispersion of cellulose nanofibers through physical property testing of the composite material. The physical properties of a composite material can be determined by a significant improvement in the 50% modulus (σ50) measured by a tensile test due to the incorporation of cellulose nanofibers. Improvements in tensile strength (Tb) can also be used as a criterion for evaluating a composite material. Improving elongation at break (Eb) is also an important factor, but since the elongation at break (Eb) decreases as the cellulose nanofiber content increases, it is preferable to change the evaluation criteria according to the increase rate of 50% modulus. Furthermore, rubber products diluted using the composite material as a masterbatch have improved elongation at break (Eb) compared to the composite material, so there are no problems when using the rubber product. The overall evaluation criteria in the examples described below can be used as criteria for determining whether a composite material is good or bad.

[0026] The content of cellulose nanofiber in the composite material is 100 parts by mass of rubber component. The content of cellulose nanofibers is preferably 10 parts by mass or more and less than 50 parts by mass, for example, 10 parts by mass or more and less than 30 parts by mass. To improve physical properties, the content of cellulose nanofibers is preferably 10 parts by mass or more, more preferably 20 parts by mass or more. Furthermore, if the content of cellulose nanofibers is 50 parts by mass or more, processing becomes difficult, if it is less than 50 parts by mass it is processable, and if it is 30 parts by mass or less it becomes easier to process.

[0027] The composite material can contain at least one of aluminum ions, zinc ions, and iron ions in an amount of 0.6 to 7.0 times equivalent relative to the carboxyl groups contained in the cellulose nanofibers, and preferably 0.7 to 5.0 times equivalent.

[0028] The composite material is less likely to absorb water due to the reduced sodium ions, as described below. The composite material's less likely to absorb water can be prevented from deteriorating in physical properties in a humid environment. For example, the composite material has a water absorption rate of 10% or less in a water absorption test in which the composite material is immersed in water at 70°C for 48 hours. Furthermore, the composite material preferably has a water absorption rate of 5% or less in the same water absorption test.

[0029] The composite material shows little deterioration in physical properties before and after water absorption. This is presumably because the hydrophobic interaction between the cellulose nanofibers and rubber molecules in the composite material is maintained even after water absorption. For example, the rate of change in the 20% modulus value measured in a tensile test based on JIS K6251 after the water absorption test relative to the 20% modulus value measured before the water absorption test preferably does not exceed −25%, and is preferably between −25% and +30%, in terms of the stability of physical properties. For example, the rate of change in the 50% modulus value measured in a tensile test based on JIS K6251 after the water absorption test relative to the 50% modulus value measured before the water absorption test preferably does not exceed −35%, and is preferably within ±35%, in terms of the stability of physical properties. For example, the rate of change in the tensile strength value measured in a tensile test based on JIS K6251 after the water absorption test relative to the tensile strength value measured before the water absorption test preferably does not exceed −45%, and is preferably within ±45%, in terms of the stability of physical properties. For the composite material, for example, the rate of change in the breaking elongation value in a tensile test based on JIS K6251 after the water absorption test relative to the breaking elongation value before the water absorption test preferably does not exceed −10%, and is preferably 0% to +20% in terms of stability of physical properties.

[0030] B. Raw materials Next, each raw material used in the production of the composite material according to this embodiment will be described.

[0031] B-1. Cellulose nanofiber Cellulose nanofibers contain carboxyl groups. The presence of carboxyl groups in cellulose nanofibers facilitates defibration during the defibration process from cellulose raw materials due to the repulsive action of the anionic carboxyl groups. The carboxyl group content of cellulose nanofibers can be 1.2 mmol / g to 1.7 mmol / g. If the carboxyl group content of cellulose nanofibers is 1.2 mmol / g or less, the cellulose nanofibers are likely to be insufficiently defibrated. If the carboxyl group content of cellulose nanofibers is greater than 1.7 mmol / g, damage from TEMPO oxidation can reduce the molecular weight of the cellulose nanofibers, or some of the cellulose nanofibers can become water-soluble polysaccharide components that separate from the fiber components and dissolve during washing with water, resulting in a reduced yield. The carboxyl group content can be measured by preparing a 0.5 to 1% by mass slurry from a cellulose nanofiber sample whose dry mass has been precisely weighed. The pH is adjusted to approximately 2.5 with 0.1 M aqueous hydrochloric acid, and then 0.05 M aqueous sodium hydroxide solution is added dropwise to measure the electrical conductivity. Measurements are continued until the pH reaches approximately 11. The carboxyl group content is determined using the following formula from the amount of sodium hydroxide (V) consumed in the neutralization step of a weak acid where the change in electrical conductivity is gradual.

[0032] Carboxylic group content (mmol / g) = V (ml) × 0.05 / dry mass of cellulose sample (g) The cellulose nanofibers preferably have an average fiber diameter of 2 nm to 8 nm, more preferably 2 nm to 6 nm. The average fiber length is preferably 200 nm to 800 nm, more preferably 300 nm to 600 nm. Having the average fiber diameter and average fiber length within these ranges is preferable because they provide a reinforcing effect on rubber products, improving, for example, rigidity, tensile strength, and elongation at break. The average fiber diameter and average fiber length are the arithmetic mean values ​​of the fiber diameter and fiber length obtained from observation of each fiber using a field emission scanning electron microscope (FE-SEM).

[0033] Cellulose nanofibers are commercially available as aqueous dispersions. The cellulose nanofiber aqueous dispersion can have a cellulose nanofiber solids content of 0.01% to 5% by mass, and preferably 0.1% to 2% by mass. If the cellulose nanofiber solids content in the aqueous dispersion is less than 0.01% by mass, the drying step described below will take a long time. If it exceeds 5% by mass, the inorganic acid polyvalent metal salt and unsaturated carboxylate salt described below cannot be uniformly treated, and agglomerates of cellulose nanofibers are likely to form.

[0034] The raw material for cellulose nanofibers may be derived from plant materials such as wood, or may be derived from non-plant materials such as animal materials such as sea squirts, or microorganisms such as bacteria. Methods for producing cellulose nanofibers using plant raw materials include, for example, chemical treatment of the raw material to make it easier to defibrate, followed by physical treatment using mechanical shear force to defibrate the raw material, or physical defibration of the raw material using known mechanical high shear force methods such as high-pressure homogenizer method, grinder milling method, freeze-pulverization method, high-shear kneading method, and ball mill milling method.

[0035] The method for oxidizing the cellulose raw material is not particularly limited, but one example is a method in which the cellulose raw material is oxidized in water using an oxidizing agent in the presence of a substance selected from the group consisting of N-oxyl compounds, bromides, iodides, or mixtures thereof. According to this method, the primary hydroxyl group at the C6 position of the glucopyranose ring on the cellulose surface is selectively oxidized to generate a group selected from the group consisting of an aldehyde group, a carboxy group, and a carboxylate group. The concentration of the cellulose raw material during the reaction is not particularly limited, but is preferably 5% by mass or less.

[0036] An N-oxyl compound refers to a compound that can generate a nitroxyl radical. An example of a nitroxyl radical is 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO). Any compound that promotes the target oxidation reaction can be used as the N-oxyl compound.

[0037] The amount of the N-oxyl compound used is not particularly limited as long as it is a catalytic amount capable of oxidizing the raw cellulose. For example, the amount is preferably 0.01 mmol or more, more preferably 0.02 mmol or more, per 1 g of bone-dry cellulose. The upper limit is preferably 10 mmol or less, more preferably 1 mmol or less, and even more preferably 0.5 mmol or less. Therefore, the amount of the N-oxyl compound used is preferably 0.01 mmol to 10 mmol, more preferably 0.01 mmol to 1 mmol, and even more preferably 0.02 mmol to 0.5 mmol, per 1 g of bone-dry cellulose.

[0038] Bromides are compounds containing bromine, such as alkali metal bromides that can dissociate and ionize in water, such as sodium bromide. Iodides are compounds containing iodine, such as alkali metal iodides. The amount of bromide or iodide used is: The amount may be selected within a range that can promote the oxidation reaction. The total amount of bromide and iodide is preferably 0.1 mmol or more, more preferably 0.5 mmol or more, per 1 g of bone-dry cellulose. The upper limit is preferably 100 mmol or less, more preferably 10 mmol or less, and even more preferably 5 mmol or less. Therefore, the total amount of bromide and iodide is preferably 0.1 to 100 mmol, more preferably 0.1 to 10 mmol, and even more preferably 0.5 to 5 mmol, per 1 g of bone-dry cellulose.

[0039] The oxidizing agent is not particularly limited, but examples thereof include halogens, hypohalous acids, perhalogen acids, their salts, halogen oxides, and peroxides. Among these, hypohalous acids or their salts can be used because they are inexpensive and have a low environmental impact. Hypochlorous acid or its salts are more preferred, and sodium hypochlorite is even more preferred. The amount of oxidizing agent used is preferably 0.5 mmol or more, more preferably 1 mmol or more, and even more preferably 3 mmol or more, per 1 g of bone-dry cellulose. The upper limit is preferably 500 mmol or less, more preferably 50 mmol or less, and even more preferably 25 mmol or less. Therefore, the amount of oxidizing agent used is preferably 0.5 mmol to 500 mmol, more preferably 0.5 mmol to 50 mmol, more preferably 1 mmol to 25 mmol, and most preferably 3 to 10 mmol, per 1 g of bone-dry cellulose. When an N-oxyl compound is used, the amount of oxidizing agent used is preferably 1 mol or more per 1 mol of N-oxyl compound. The upper limit is preferably 40 mol. Therefore, the amount of the oxidizing agent used is preferably 1 mmol to 40 mol per 1 mol of the N-oxyl compound.

[0040] Conditions such as pH and temperature during the oxidation reaction are not particularly limited. Generally, the oxidation reaction proceeds efficiently even under relatively mild conditions. The reaction temperature is preferably 4°C or higher, more preferably 15°C or higher. The upper limit is preferably 40°C or lower, more preferably 30°C or lower. Therefore, the temperature is preferably 4°C to 40°C, and may be approximately 15°C to 30°C, i.e., room temperature. The pH of the reaction solution is preferably 8 or higher, more preferably 10 or higher. The upper limit is preferably 12 or lower, more preferably 11 or lower. Therefore, the pH of the reaction solution is preferably 8 to 12, more preferably approximately 10 to 11. Usually, as the oxidation reaction proceeds, carboxyl groups are generated in the cellulose, and the pH of the reaction solution tends to decrease. Therefore, to efficiently proceed with the oxidation reaction, it is preferable to add an alkaline solution such as an aqueous sodium hydroxide solution to maintain the pH of the reaction solution within the above range. Water is preferred as the reaction medium during the oxidation because of its ease of handling and the low occurrence of side reactions.

[0041] The reaction time for oxidation can be set appropriately depending on the degree of progress of the oxidation, and is usually 0.5 hours or more. The upper limit is usually 6 hours or less, preferably 4 hours or less. Therefore, the reaction time for oxidation is usually 0.5 hours to 6 hours, for example, about 0.5 hours to 4 hours.

[0042] The oxidation may be carried out in two or more separate reaction stages. For example, the oxidized cellulose obtained by filtration after the completion of the first reaction stage can be oxidized again under the same or different reaction conditions, thereby enabling efficient oxidation without reaction inhibition by sodium chloride produced as a by-product in the first reaction stage.

[0043] Another example of a carboxylation (oxidation) method is a method of oxidation by ozone treatment. This oxidation reaction oxidizes at least the hydroxyl groups at positions 2 and 6 of the glucopyranose ring that constitutes cellulose, and decomposes the cellulose chain. Ozone treatment is usually carried out by contacting the cellulose raw material with a gas containing ozone. The ozone concentration in the gas is 50 g / m 3 The upper limit is 250 g / m 3 Preferably, it is 220 g / m or less. 3 Therefore, the ozone concentration in the gas is preferably 50 g / m or less. 3 ~250g / m 3 It is preferable that the thickness is 50 g / m 3 ~220 g / m 3The amount of ozone added is preferably 0.1 parts by mass or more, and more preferably 5% by mass or more, relative to 100% by mass of the solids content of the cellulose raw material. The upper limit is usually 30% by mass or less. Therefore, the amount of ozone added is preferably 0.1 to 30% by mass, and more preferably 5 to 30% by mass, relative to 100% by mass of the solids content of the cellulose raw material. The ozone treatment temperature is usually 0°C or higher, and preferably 20°C or higher. The upper limit is usually 50°C or less. Therefore, the ozone treatment temperature is preferably 0 to 50°C, and more preferably 20 to 50°C. The ozone treatment time is usually 1 minute or more, and preferably 30 minutes or more. The upper limit is usually 360 minutes or less. Therefore, the ozone treatment time is usually about 1 to 360 minutes, and preferably about 30 to 360 minutes. When the ozone treatment conditions are within the above-mentioned ranges, excessive oxidation and decomposition of cellulose can be prevented, resulting in a good yield of oxidized cellulose.

[0044] The product obtained after the ozone treatment may be further subjected to a post-oxidation treatment using an oxidizing agent. The oxidizing agent used in the post-oxidation treatment is not particularly limited, but examples thereof include chlorine compounds such as chlorine dioxide and sodium chlorite; oxygen, hydrogen peroxide, persulfuric acid, and peracetic acid. Examples of methods for the post-oxidation treatment include dissolving these oxidizing agents in water or a polar organic solvent such as alcohol to prepare an oxidizing agent solution, and immersing the cellulose raw material in the oxidizing agent solution.

[0045] The amount of carboxyl groups, carboxylate groups, and aldehyde groups contained in cellulose nanofibers can be adjusted by controlling the oxidation conditions, such as the amount of oxidizing agent added and the reaction time.

[0046] B-2. Rubber component The rubber component of the composite material is provided as a rubber latex containing the rubber component. The rubber component is preferably natural rubber (NR), carboxy-modified nitrile rubber (X-NBR), or carboxystyrene butadiene rubber (X-SBR). The rubber latex preferably contains ammonia. By including ammonia in the rubber latex, sodium ions contained in the first mixture can be substituted with ammonium ions in the step of obtaining a second mixture, which will be described later. The ammonia content in the rubber latex is 0.01% or more, preferably 0.1% or more and 0.7% or less. The rubber latex may be natural rubber latex containing 0.01% or more, preferably 0.1% or more and 0.7% or less of ammonia. The weight-average molecular weight of the rubber component is preferably 50,000 to 3,000,000, and more preferably 100,000 to 2,000,000. In the present invention, the "weight average molecular weight" refers to a value measured in terms of standard polystyrene by gel permeation chromatography (GPC) using tetrahydrofuran as a solvent.

[0047] Carboxy-modified nitrile rubber (carboxy-modified acrylonitrile butadiene rubber) or carboxy-modified styrene butadiene rubber is a modified diene rubber that has been functionalized with carboxy groups. The carboxy groups in carboxy-modified nitrile rubber or carboxy-modified styrene butadiene rubber enhance its affinity with cellulose nanofibers that contain phosphorus oxoacid groups, and the carboxy groups bond with the cellulose nanofibers during kneading, thereby enhancing their defibration ability. It is also believed that the defibration ability of natural rubber is enhanced by the interaction between the free radicals generated by mastication and the cellulose nanofibers.

[0048] The composite material may also contain known fillers that are compounded in rubber compositions, such as carbon black, silica, clay, aluminum hydroxide, calcium carbonate, mica, talc, aluminum hydroxide, aluminum oxide, titanium oxide, and sulfuric acid. Examples include barium and lecithin. The composite material may contain a crosslinking agent. When the composite material contains a crosslinking agent, the crosslinked body and / or rubber product of the present embodiment can be suitably obtained.

[0049] B-3. ​​Unsaturated carboxylic acid salts Unsaturated carboxylates containing at least aluminum ions, zinc ions, or iron ions are used in the production of composite materials. Therefore, the unsaturated carboxylates are water-soluble. Examples of unsaturated carboxylates include (meth)acrylic acid, maleic acid (anhydride), fumaric acid, itaconic acid (anhydride), crotonic acid, and salts thereof. By mixing the water-soluble unsaturated carboxylates with an aqueous cellulose nanofiber dispersion and latex, the carboxyl and carbonyl groups form hydrogen bonds with the carbonyl or hydroxyl groups of the cellulose nanofibers, thereby forming pseudo-crosslinks between the cellulose nanofibers. Metals that form these salts include aluminum, zinc, and iron. The unsaturated carboxylate can be at least one selected from aluminum acrylate, zinc acrylate, iron acrylate, aluminum methacrylate, zinc methacrylate, and iron methacrylate. When forming the unsaturated carboxylate compound, one or more of the above metals can be used to form a metal salt compound. The content of the unsaturated carboxylate will be explained in "C. Manufacturing method of composite material." The unsaturated carboxylate is preferably water-soluble so that it can be mixed with rubber latex. If the unsaturated carboxylate is not water-soluble, it may be dissolved in water using a cationic surfactant such as dodecylmethylammonium chloride.

[0050] C. Composite Material Manufacturing Methods Next, a method for producing a composite material will be described. Fig. 1 is a flowchart of a method for producing a composite material according to one embodiment. The method for producing a composite material can use the raw materials described in B above. The method for producing a composite material can produce the composite material described in A above.

[0051] 1, the method for producing a composite material includes a step of obtaining a mixed solution (S10), a step of obtaining a first mixture (S30), a step of obtaining a second mixture (S40), a step of obtaining a third mixture (S50), and a step of obtaining a composite material (S60). In this embodiment, an example will be described in which a kneading step (S20) is further included after S10.

[0052] S10: The step of obtaining a mixed solution is a step of obtaining a mixed solution by mixing a rubber latex containing a rubber component, a cellulose nanofiber aqueous dispersion containing cellulose nanofibers containing carboxyl groups in an aqueous solvent, and an unsaturated carboxylate salt containing at least aluminum ions, zinc ions, or iron ions. The cellulose nanofibers include those derived from oxidized cellulose oxidized using an N-oxyl compound. The rubber latex, the cellulose nanofiber aqueous dispersion, and the unsaturated carboxylate salt are all provided in an aqueous solvent and can be mixed using a known mixer. If the unsaturated carboxylate salt is not water-soluble, it may be dissolved in water using a surfactant and then mixed with the rubber latex. Examples of the mixer that can be used include a magnetic stirrer and a propeller mixer.

[0053] In the step (S10) of obtaining a mixed solution, the amount of cellulose nanofibers can be 10 parts by mass or more and less than 50 parts by mass, and preferably 10 parts by mass or more and 30 parts by mass or less, per 100 parts by mass of the rubber component. When the amount of cellulose nanofibers in the step of obtaining a mixed solution is 10 parts by mass or more per 100 parts by mass of the rubber component, composite materials can be provided to the market as masterbatches containing high concentrations of cellulose nanofibers, and the amount is more preferably 15 parts by mass or more, and even more preferably 20 parts by mass or more. Furthermore, if the amount is less than 50 parts by mass, stirring and kneading are possible although it takes a long time to work, and in consideration of workability, it is more preferable that the amount is 30 parts by mass or less. The rubber component is not limited as long as it can be used as a rubber latex, but natural rubber, carboxy-modified nitrile rubber, and carboxy-modified styrene-butadiene rubber are preferred.

[0054] The amount of unsaturated carboxylate can be adjusted appropriately depending on the salt type and the purpose of the blend. The mixed solution is blended to contain an unsaturated carboxylate containing at least aluminum ions, zinc ions, or iron ions in an amount between 0.6 and 7.0 times the equivalent of the carboxy groups contained in the cellulose nanofibers. While 1 equivalent is theoretically appropriate, blending amounts less than 1 equivalent can be used depending on the type of latex and the required properties of the rubber product. Furthermore, blending amounts greater than 1 equivalent can be used due to uneven distribution of additives or incompatibility of local structures. As mentioned above, an amount of 0.6 or more equivalents is preferred because it inhibits the aggregation of cellulose nanofibers through the development of a pseudo-crosslinked structure. An amount of 7.0 or less equivalents is preferred because the impact of the aggregation of excess unsaturated carboxylate on the rubber product is relatively minimal. Furthermore, the amount of aluminum ions, zinc ions, and iron ions is more preferably 0.7 to 5.0 times the equivalent of the carboxy groups of the cellulose nanofibers, from the viewpoint of inhibiting the aggregation of cellulose nanofibers and excess material.

[0055] Here, the amounts of aluminum ions, zinc ions, and iron ions can be calculated according to the molar amount of carboxy groups contained in the cellulose nanofibers. For example, if the total amount of anions derived from the carboxy groups of the cellulose nanofibers used in S10 is 1.5 mmol / g, the amount of unsaturated carboxylate can be calculated so that it is 0.6 to 7.0 equivalents relative to that molar amount.

[0056] For example, if the total amount of anions derived from the carboxyl groups of the cellulose nanofiber is 1.5 mmol / g, then 20 g (20 parts by mass) of cellulose nanofiber will be 30 mmol, and to this, for example, 3.5 g of zinc methacrylate will be blended in an amount equivalent to 1, or 7.0 g of zinc methacrylate will be blended in an amount equivalent to 2.

[0057] S20: Kneading the Mixed Liquid is a step of kneading the mixed liquid obtained in S10. In the kneading step, the viscosity of the mixed liquid at 25°C measured using a vibration viscometer in accordance with JIS Z8803 (2011) is preferably 100 mPa·s to 1000 mPa·s. A tuning fork vibration rheometer, for example, can be used as the vibration viscometer. If the viscosity of the mixed liquid is within the above range, kneading can be performed by utilizing the restoring force due to the elasticity of the mixed liquid. The region in which the mixed liquid exhibits elasticity is determined by a stress-strain curve obtained using a dynamic viscoelasticity test, and is a range in which the elastic modulus is constant regardless of stress and in which stress and strain are proportional. Such an elastic region can also be estimated from the viscosity of the mixed liquid. Kneading can be performed using a device capable of applying a certain degree of shear force to the mixed liquid, such as a planetary mixer, a three-roll mill, or a two-roll mill. S10 can be performed until the viscosity of the mixed liquid becomes nonlinear (i.e., resembles a non-Newtonian fluid).

[0058] S30: The step of obtaining a first mixture is a step of heating the mixed liquid after the kneading step of S20 to reduce the water content and obtain a first mixture. In the step of obtaining a first mixture, the mixed liquid may be heated to evaporate the water content. The mixed liquid is heated, for example, in an oven at 40 to 60°C for 1 to 24 hours. The first mixture preferably has a water content of 15% or less. Reducing the water content of the first mixture improves the workability in the next step. The water content can be measured using a heat-drying moisture meter. Alternatively, the water content may be calculated from the blending amounts by measuring the mass before and after drying. For the drying in S30, any known method can be used as long as it can efficiently remove the aqueous solvent from the mixed liquid, and for example, degassing while heating may be used. In addition, although the water content of the mixed liquid is reduced in this embodiment, this is not limiting. Washing in S40 may also be used. If purification is possible, the mixed liquid obtained in S20 may be used as the first mixture as it is.

[0059] S40: The step of obtaining a second mixture involves washing the first mixture obtained in S30 to reduce the sodium ions in the first mixture to obtain a second mixture. The step of obtaining a second mixture is an ion exchange process. The step of obtaining a second mixture can also be performed by placing the first mixture in water. Since the sodium ions in the first mixture will flow out into the water, it is preferable to use running water. In the washing step, other components may be added to the water as long as the sodium ions can migrate from the first mixture into the running water. As the sodium ions in the first mixture decrease, the water absorption capacity of the composite material decreases. The sodium ions in the first mixture are replaced by ammonium ions contained in, for example, rubber latex. The reduction in sodium in the second mixture can be confirmed by X-ray fluorescence analysis.

[0060] S50: The step of obtaining a third mixture involves heating the second mixture to reduce the moisture content and obtain a third mixture. In the step of obtaining a third mixture, the moisture content in the second mixture is evaporated and reduced by heating. The second mixture is heated, for example, in an oven at 40°C to 160°C for 1 hour to 24 hours. If the moisture content of the third mixture is 3% or less, kneading in S60 is possible, which is preferable. The moisture content can be measured by the method described above.

[0061] S60: The process for obtaining a composite material involves kneading the third mixture to obtain a composite material. The third mixture contains numerous aggregates of cellulose nanofibers due to the reduction in water content caused by S50. It is presumed that the presence of the rubber component, aluminum ions, and unsaturated carboxylate salts in the mixture inhibits bonding between the cellulose nanofibers. Kneading the mixture in this state by applying high shear force is preferable because it improves the dispersion of the cellulose nanofibers in the rubber component. Kneading in S60 can be carried out by compressing the third mixture and then using the elastic recovery of the rubber component to significantly move the cellulose nanofibers.

[0062] The process for obtaining the S60 composite material is not particularly limited, but for example, an open roll method, an internal kneading method, or a multi-screw extrusion kneading method can be used, of which the open roll method is preferred.

[0063] When kneading by the open roll method, the third mixture is wound around one of the rolls and kneaded to appropriately sever the molecular chains of the rubber component and generate free radicals. Because these free radicals easily bond with cellulose nanofibers, it is preferable to include this step in the method for producing a composite material. It is preferable that the method for producing a composite material of the present invention further includes a thin-squeezing step after this kneading.

[0064] The roll gap in the thinning step is preferably more than 0 mm and not more than 1 mm, more preferably more than 0 mm and not more than 0.5 mm, and even more preferably 0.1 mm or more and not more than 0.5 mm.

[0065] The roll surface temperature in the thin-threading step is preferably 0°C or higher and 50°C or lower, and more preferably 5°C or higher and 30°C or lower.

[0066] The number of times of treatment in the thin-threading step is preferably from 1 to 10 times, and more preferably from 1 to 5 times.

[0067] The surface speed ratio of the two rolls in the thinning step is preferably 1.05 or more and 3.00 or less, more preferably 1.05 or more and 2.00 or less, even more preferably 1.05 or more and 1.5 or less, and particularly preferably 1.05 or more and 1.20 or less. stomach.

[0068] By setting the roll gap, roll surface temperature, number of treatments, and roll surface speed ratio in the thin-passing step as described above, the temperature of the composite material itself can be adjusted to 0°C or higher and 70°C or lower, more preferably 0°C or higher and 50°C or lower, and even more preferably 5°C or higher and 30°C or lower. By adjusting the temperature within this range and kneading, the elasticity of the rubber component can be utilized to defibrate the cellulose nanofibers, and the defibrated cellulose nanofibers can be dispersed in the composite material.

[0069] The composite material may be removed from the kneader as a masterbatch (intermediate), and then re-introduced into the kneader, after which additional rubber components may be added and kneaded. In this case, it is preferable from the viewpoint of processability that the additional rubber components be solid components that do not contain moisture. Antioxidants, crosslinking agents, etc. may be added to the composite material and further kneaded.

[0070] Uncrosslinked composite materials can be further vulcanized to form crosslinked materials. Despite containing cellulose nanofibers, vulcanized composite materials exhibit low water absorption. For example, a water absorption test in which the composite material is immersed in water at 70°C for 48 hours shows a water absorption rate of 10% or less. The physical properties of vulcanized composite materials change little before and after water absorption. This is presumably because the hydrophobic interaction between the cellulose nanofibers and rubber molecules is maintained even after water absorption. Furthermore, it is presumed that ammonia volatilizes during vulcanization, causing carboxyl groups, phosphorus oxoacid groups, and substituents derived from phosphorus oxoacid groups to become hydrophobic. Therefore, the cellulose nanofibers have a high affinity with rubber molecules, and their physical properties are less likely to deteriorate even after water absorption.

[0071] The composite material does not contain any visible agglomerates of cellulose nanofibers. Such agglomerates have a maximum width of 10 μm or more. The composite material obtained in this manner may be used as a masterbatch. Because the composite material does not contain agglomerates that are likely to become fracture initiation points, crosslinked bodies and / or rubber products with excellent mechanical properties can be produced.

[0072] As used in this specification and claims, "at least one of A, B, and C" or "one or more of A, B, and C," in connection with a listing of one or more elements, is intended to include A only, B only, C only, the pair of A and B, the pair of A and C, the pair of B and C, the pair of A, B, and C, etc.

[0073] In the present invention, some configurations may be omitted or various embodiments and modifications may be combined within the scope of the features and effects described in this application.

[0074] The present invention includes configurations that are substantially the same as those described in the embodiments (configurations with the same functions, methods, and results, or configurations with the same purpose and effects). The present invention also includes configurations that replace non-essential parts of the configurations described in the embodiments. The present invention also includes configurations that achieve the same effects or purposes as the configurations described in the embodiments. The present invention also includes configurations that add publicly known technology to the configurations described in the embodiments. [Example]

[0075] Examples of the present invention will be described below, but the present invention is not limited to these.

[0076] (Production of cellulose nanofibers) 5.00 g (bone-dry) of bleached unbeaten kraft pulp (brightness 85%) derived from softwood was mixed with 39 mg of TEMPO (Sigma-Aldrich) (0.05 mmol per 1 g of bone-dry cellulose) and 514 mg of sodium bromide (1.0 mmol per 1 g of bone-dry cellulose). The mixture was added to 500 ml of an aqueous solution containing TEMPO-oxidized cellulose (1) and stirred until the pulp was uniformly dispersed. Sodium hypochlorite aqueous solution was added to the reaction system to adjust the sodium hypochlorite concentration to 6.0 mmol / g, initiating the oxidation reaction. The pH of the system decreased during the reaction, but was gradually adjusted to pH 10 by adding 3 M sodium hydroxide aqueous solution. The reaction was terminated when the sodium hypochlorite was consumed and the pH no longer changed. The reaction mixture was acidified with hydrochloric acid, filtered through a glass filter, and thoroughly washed with water to obtain oxidized pulp (carboxylated cellulose). The pulp yield was 90%, the oxidation reaction took 90 minutes, and the carboxyl group content was 1.6 mmol / g. The mixture was adjusted to 1.0% (w / v) with water and subjected to three passes in an ultra-high-pressure homogenizer (20 °C, 150 MPa) to obtain a dispersion of TEMPO-oxidized cellulose nanofibers (TOCN). The TOCN had an average fiber diameter of 3 nm and an average fiber length of 550 nm.

[0077] (1) Sample preparation (1-1) Examples 1 to 6 Rubber latex and a 40% zinc methacrylate solution were added to a cellulose nanofiber aqueous dispersion (solvent: water) with a TOCN concentration of 1% (can be diluted to 0.4% to 0.7%) shown in Tables 1 and 2, and mixed using a Tornado mixer (PM-204, manufactured by AS ONE Corporation) to obtain a mixture (S10). The mixture was then further kneaded using a planetary mixer (planetary mixer, manufactured by THINKY Corporation) (S20). The zinc methacrylate solution was prepared by stirring at room temperature with an aqueous solution of surfactant dissolved in pure water.

[0078] Next, the mixed liquid was poured into a tray and dried in an oven at 40°C for about 24 hours to reduce the amount of aqueous solvent, thereby obtaining a first mixture (S30). The moisture content of the first mixture obtained by drying was 15% or less for all first mixtures.

[0079] Next, the first mixture was washed in running water at 80°C for 24 hours to obtain a second mixture with reduced sodium (S40).

[0080] Next, the second mixture was dried in an oven at 40°C for 24 hours to reduce the amount of the aqueous solvent, thereby obtaining a third mixture (S50). The moisture content of the third mixture was 3% or less.

[0081] Next, the third mixture was wrapped around an open roll (two rolls, Yasuda Seiki Co., Ltd., 191-TH test mixing roll) and kneaded, and then the third mixture was thin-spun (roll temperature: room temperature, roll gap: 0.7 mm, roll speed ratio: 26 / 30) to obtain a composite material (S60). In Examples 2, 3, 8, and 9, the third mixture was kneaded to obtain a masterbatch, and the third mixture was added to the diluted natural rubber (solid) that had been wrapped around an open roll and kneaded, and further kneaded and thin-spun to obtain a composite material with an adjusted cellulose nanofiber content.

[0082] Furthermore, the composite material was wound around the open roll again, and the antioxidant and crosslinking agent were added and mixed. The separated sheet was pressure-molded at 165°C for 20 minutes to obtain a sheet-shaped crosslinked sample of each example having a thickness of 1 mm.

[0083] where: "NR (Latex)": Natural rubber latex, manufactured by Regitex, ULACOL, total solids 61%, ammonia 0.1%, "NR (Diluted Rubber)": Natural rubber, RSS#3, "TOCN": TEMPO-oxidized cellulose nanofiber, manufactured by Nippon Paper Industries Co., Ltd., TOCN Cellenpia, carboxyl group amount 1.6 mmol / g, average fiber diameter 3 nm, average fiber length 550 nm, "MAZn": Zinc methacrylate (manufactured by Asada Chemical Industry Co., Ltd., product name M-CP, purity 96.9%) CAS No. 13189-00-9), "Surfactant": dodecyltrimethylammonium chloride, Although not listed in the table, each example contained 2 phr of peroxide (Percumyl D, 98% or higher purity, manufactured by Nippon Oil & Fats Co., Ltd.), 3 phr of zinc oxide (99.5% purity, manufactured by Seido Chemical Co., Ltd.), 1.5 phr of antioxidant MB (secondary antioxidant, manufactured by Kawaguchi Chemical Co., Ltd.), and 1.5 phr of antioxidant RD (primary antioxidant (amines)) as crosslinking agents.

[0084] (1-2) Comparative Examples 1 to 5 Crosslinked samples of Comparative Examples 1 to 5 were obtained with the formulations shown in Table 3. In Comparative Example 1, a crosslinked sample was obtained by carrying out the same process as in Example 1, except that the unsaturated carboxylate and surfactant were not added and the ion exchange treatment (S40) was not performed. In Comparative Example 2, a crosslinked sample was obtained by carrying out the same process as in Example 1, but that the unsaturated carboxylate and surfactant were not added. In Comparative Example 3, a crosslinked sample was obtained by carrying out the same process as in Example 1, except that the ion exchange treatment (S40) was not performed. In Comparative Example 4, a crosslinked sample was obtained by carrying out the same process as in Example 1, but with sodium methacrylate added instead of zinc methacrylate and surfactant. In Comparative Example 5, a crosslinked sample was obtained by carrying out the same process as in Example 1, but with zinc chloride added instead of zinc methacrylate and surfactant.

[0085] where: "MANa": sodium methacrylate (manufactured by Asada Chemical Industry Co., Ltd., product name S-MA, purity 99.9%, CAS No. 2-1027 (5536-61-8)), "ZnCl2": Zinc chloride (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., zinc chloride, purity 99.9%, CAS No. 7646-85-7) Formulated with:

[0086] (1-3) Comparative Example 6 and Examples 7 to 9 Crosslinked samples of Comparative Example 6 and Examples 7 to 9 were obtained using the formulations shown in Table 4. In Comparative Example 6, aluminum acrylate was added instead of zinc methacrylate and a surfactant, and a crosslinked sample was obtained by carrying out the same treatment as in Example 1, except that the ion exchange treatment (S40) was not carried out. In Example 7, aluminum acrylate was added instead of zinc methacrylate and a surfactant, and a crosslinked sample was obtained by carrying out the same treatment as in Example 1. In Examples 8 and 9, crosslinked samples were obtained by adding aluminum acrylate instead of zinc methacrylate and a surfactant, and a crosslinked sample was obtained by carrying out the same treatment as in Example 2.

[0087] where: "AAAl": Aluminum acrylate (manufactured by Asada Chemical Industry Co., Ltd., product name AAL7, purity 23%) Formulated with:

[0088] (2) Evaluation method and evaluation (2-1) Water absorption evaluation Crosslinked samples of the examples and comparative examples (test pieces punched into a dumbbell shape (dumbbell No. 6) as defined in JIS K 6251:2017) were immersed in water at 70°C for 48 hours, and the water absorption (%) was measured from the change in mass of the sample before and after the test, and the results are shown in Tables 1 to 4. The mass (g) and water absorption (%) are also shown in Tables 1 to 4, with the samples before the test referred to as pre-water absorption samples and the samples after the test referred to as post-water absorption samples.

[0089] (2-2) Tensile test For each sheet-shaped cross-linked sample, the JIS Test specimen punched into a dumbbell shape (dumbbell No. 6) as specified in K 6251:2017 Using a tensile testing machine (Shimadzu Corporation, Autograph AG-X), Tensile tests were conducted in accordance with K6251:2017 at 23±2°C with a gauge length of 20 mm and a tensile speed of 500 mm / min. Table 1 shows the measured values ​​for the 20% modulus (σ20 (MPa)), 50% modulus (σ50 (MPa)), tensile strength (Tb (MPa)), and elongation at break (Eb (%)). The percent change in the 20% modulus (σ20 (MPa)), 50% modulus (σ50 (MPa)), tensile strength (Tb (MPa)), and elongation at break (Eb (%)) of the specimen after water absorption compared to the specimen before water absorption was calculated and is shown in Tables 1 to 4.

[0090] [Table 1]

[0091] [Table 2]

[0092] [Table 3]

[0093] [Table 4]

[0094] (2-3) Coagulation evaluation The fracture surfaces of each crosslinked sample after the tensile test were visually inspected using a scanning electron microscope to confirm the presence or absence of agglomerates. Agglomerates are areas where the cellulose nanofibers are insufficiently dispersed, and visible agglomerates have a maximum width of 10 μm or more.

[0095] In the crosslinked samples of Examples 1 to 9, no aggregates of 10 μm or more were visually observed.

[0096] (2-4) Overall rating The crosslinked samples of Examples 1 to 9 had lower water absorption rates than the crosslinked samples of Comparative Examples 1 to 6. The crosslinked samples of Examples 1 to 3, 8, and 9 had water absorption rates of 5% or less, and the crosslinked samples of Examples 4 to 7 had water absorption rates of 5% to 10%. The crosslinked samples of Comparative Examples 1 to 6 had water absorption rates of 16.2% or more.

[0097] The crosslinked samples of Examples 1 to 5 and 7 to 9 had a change rate of 20% modulus (σ20) of less than ±15%. The crosslinked sample of Example 6 had a change rate of 20% modulus (σ20) of less than ±25%. The crosslinked samples of Examples 1 to 3 and 6 to 9 had a change rate of 50% modulus (σ50) of less than ±20%. The crosslinked samples of Examples 4 and 5 had a change rate of 50% modulus (σ50) of less than ±26%. The crosslinked samples of Examples 1 to 3, 8 and 9 had a change rate of tensile strength (Tb) of less than ±20% and a change rate of elongation at break (Eb) of less than ±10%. The crosslinked samples of Examples 4 to 7 had a change rate of tensile strength (Tb) of less than ±41% and a change rate of elongation at break (Eb) of less than ±19%.

Claims

1. A step of mixing a rubber latex containing a rubber component, a cellulose nanofiber aqueous dispersion containing cellulose nanofibers containing carboxyl groups in an aqueous solvent, and an unsaturated carboxylate salt containing at least any one of aluminum ions, zinc ions, and iron ions to obtain a mixed liquid; heating the mixture to reduce the water content to obtain a first mixture; washing the first mixture to reduce sodium ions in the first mixture to obtain a second mixture; heating the second mixture to reduce the water content to obtain a third mixture; kneading the third mixture to obtain a composite material; Including, The method for producing a composite material includes cellulose nanofibers derived from oxidized cellulose oxidized using an N-oxyl compound.

2. The method for producing a composite material according to claim 1 , wherein the rubber latex is natural rubber latex containing 0.01% or more of ammonia.

3. In the step of obtaining the mixed solution, the cellulose nanofibers are contained in an amount of 10 parts by mass or more and 30 parts by mass or less per 100 parts by mass of the rubber component, The cellulose nanofibers have an average fiber diameter of 2 nm or more and 8 nm or less, an average fiber length of 200 nm or more and 800 nm or less, and a carboxy group content of 1.2 mmol / g to 1.7 mmol / g; The method for producing a composite material according to claim 1 or 2, wherein the mixed solution contains the unsaturated carboxylate in an amount of 0.6 to 7.0 times the equivalent of the carboxyl groups contained in the cellulose nanofibers.

4. 3. The method for producing a composite material according to claim 1, wherein in the step of obtaining the third mixture, the second mixture is heated so that the moisture content of the third mixture is 3% or less.

5. 3. The method for producing a composite material according to claim 1, wherein the composite material has a water absorption rate of 10% or less in a water absorption test in which the composite material is immersed in water at 70°C for 48 hours.

6. 6. The method for producing a composite material according to claim 5, wherein a rate of change in a 20% modulus value of the composite material, measured in a tensile test based on JIS K6251 after the water absorption test, relative to a 20% modulus value before the water absorption test, is −25% to +30%.

7. A composite material comprising a rubber component, cellulose nanofibers containing a carboxy group in an aqueous solvent, and an unsaturated carboxylate salt containing any one of aluminum ions, zinc ions, and iron ions, A composite material having a water absorption rate of 10% or less in a water absorption test in which the composite material is immersed in water at 70°C for 48 hours.

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