Method for producing cellulose nanofiber-containing wet masterbatch

Abstract

To provide a method for producing a cellulose nanofiber-containing wet masterbatch through a wet masterbatch process, in which aggregation of cellulose nanofibers in a dehydration step is prevented, thereby producing a cellulose nanofiber-containing wet masterbatch having superior dispersibility of cellulose nanofibers and superior properties including rubber reinforcing performance and processing suitability.SOLUTION: The present invention provides a method for producing a cellulose nanofiber-containing wet masterbatch by coagulating a mixture of a cellulose nanofiber water dispersion and rubber latex to obtain a rubber / cellulose nanofiber composite, which is then dehydrated and dried. The dehydration is carried out while pressing and heating the rubber / cellulose nanofiber composite in the thickness direction on the outer peripheral surface of a drum.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for producing a wet masterbatch containing rubber latex and cellulose nanofibers. [Background technology]

[0002] Conventionally, the wet masterbatch method has been used to improve the dispersibility of rubber reinforcing fillers such as carbon black and silica (see Patent Document 1). The wet masterbatch method involves mixing a dispersion, in which a rubber reinforcing material is previously dispersed in a dispersion medium, with rubber latex in a liquid phase, and then adding a coagulant such as acid to obtain a coagulated product, which is then dehydrated and dried to produce a rubber masterbatch. Compared to the dry masterbatch method, in which a filler and rubber are mixed in a solid phase, the wet masterbatch method provides superior filler dispersibility and produces a rubber composition with excellent physical properties such as rubber reinforcement and processability.

[0003] In recent years, there has been an increasing number of attempts to use cellulose nanofibers, a lightweight and strong biomass material, as a reinforcing material for rubber. Cellulose nanofibers can be produced by biosynthesis and other preparation methods, but in general, pulp is mechanically defibrated in water to obtain an aqueous dispersion (referred to as an "aqueous dispersion" in this invention). In some cases, chemical treatment may be performed before mechanical defibration.

[0004] When using the dry masterbatch method to obtain rubber / cellulose nanofiber composites, the cellulose nanofibers must be separated from the cellulose nanofiber aqueous dispersion, dried, and extracted. In this case, the cellulose nanofibers aggregate tightly together due to hydrogen bonds formed through the hydroxyl groups of the cellulose molecules, making them extremely difficult to disaggregate. To prevent this, measures such as protecting the hydroxyl groups and hydrophobizing treatments are used.

[0005] Meanwhile, the wet masterbatch method, which allows the use of aqueous cellulose nanofiber dispersions as they are in rubber composites, has been attracting attention. In this method, an aqueous cellulose nanofiber dispersion is mixed with rubber latex in the liquid phase and coagulated to obtain a rubber / cellulose nanofiber composite. The presence of rubber molecules between the cellulose nanofibers can suppress self-aggregation of the cellulose nanofibers during drying, and this method is expected to have the great advantage of improving the dispersibility of the cellulose nanofibers (see Patent Documents 2 and 3). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-75900 [Patent Document 2] Japanese Patent Application Publication No. 2019-163414 [Patent Document 3] Japanese Patent Publication No. 2020-55962 Summary of the Invention [Problem to be solved by the invention]

[0007] However, it was found that when high shear forces are applied to the rubber / cellulose nanofiber composite during the dehydration process of the wet masterbatch method, aggregation occurs between the hydrophobic rubber and the hydrophilic cellulose nanofibers, which do not mix well with each other, reducing the original performance of the cellulose nanofibers and resulting in a wet masterbatch with reduced rubber reinforcement properties.

[0008] For example, the dehydration process in the mass production process of the wet masterbatch method typically uses a single-screw or multi-screw extruder to apply shear force and squeeze the material. However, when rubber / cellulose nanofiber composites are dehydrated using these extruders, the cellulose nanofibers aggregate, reducing their rubber reinforcement properties. If the cellulose nanofibers are naturally dehydrated by air drying or other methods without applying shear force, this problem of aggregation does not occur, but dehydration takes a long time, making it unsuitable from the perspective of industrial productivity.

[0009] An object of the present invention is to provide a method for producing a cellulose nanofiber-added wet masterbatch by a wet masterbatch method, which prevents aggregation of cellulose nanofibers during the dehydration process, thereby producing a cellulose nanofiber-added wet masterbatch that has excellent dispersibility of cellulose nanofibers and excellent physical properties such as rubber reinforcement and processability. [Means for solving the problem]

[0010] As a result of extensive research conducted by the inventors to solve the above problems, it was found that by using a device that does not apply shear force to the rubber / cellulose nanofiber composite, such as a drum dryer, in the dehydration process of the wet masterbatch method, aggregation of the cellulose nanofibers can be suppressed and the reinforcing properties of the rubber will not be impaired.

[0011] In other words, the present invention is a method for producing a cellulose nanofiber-added wet masterbatch by dehydrating and drying a rubber / cellulose nanofiber composite obtained by coagulating a mixed liquid of a cellulose nanofiber aqueous dispersion and rubber latex, and the dehydration is carried out while pressing and heating the rubber / cellulose nanofiber composite in the thickness direction on the outer peripheral surface of a drum. In one embodiment of the present invention, the temperature of the drum is preferably adjusted to 130°C to 160°C. In one embodiment of the present invention, it is preferable to use a double-drum type drum dryer as the drum. In one embodiment of the present invention, the drum gap of the double-drum type drum dryer is preferably adjusted to 0.35 mm to 0.85 mm.

[0012] As will be described later, inorganic fillers such as carbon black and silica may be blended into the mixture of the cellulose nanofiber aqueous dispersion and rubber latex. [Effects of the Invention]

[0013] According to the method for producing a cellulose nanofiber-added wet masterbatch of the present invention, it is possible to produce a cellulose nanofiber-added wet masterbatch that has excellent dehydration efficiency and high mass productivity, and that has excellent physical properties such as rubber reinforcement and processability, without reducing the rubber reinforcement effect of the cellulose nanofibers. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a schematic side view illustrating an example of a dehydration method using a double-drum type drum dryer. [Figure 2] FIG. 1 is a schematic side view illustrating an example of a dehydration method using a twin-drum drum dryer. [Figure 3] FIG. 1 is a schematic side view illustrating an example of a dehydration method using a single-drum type drum dryer. [Figure 4] FIG. 1 is a schematic side view illustrating an example of a dehydration method using a single-drum type drum dryer. [Figure 5] FIG. 1 is a schematic side view illustrating an example of a dehydration method using a single-drum type drum dryer. [Figure 6] FIG. 1 is a schematic side view illustrating an example of a dehydration method using a single-drum type drum dryer. [Figure 7] FIG. 1 is a schematic side view illustrating an example of a dehydration method using a single-drum type drum dryer. [Figure 8] FIG. 1 is a schematic side view illustrating an example of a dehydration method using a single-drum type drum dryer. [Figure 9] 1 is a transmission polarizing microscope photograph showing the results of morphological observation of the rubber compositions obtained in Reference Example 1 and Comparative Reference Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0015] The present invention will be described in detail below, but the explanation of the constituent elements described below is a representative example of an embodiment of the present invention, and the present invention is not limited to these contents.

[0016] [Method of manufacturing cellulose nanofiber-added wet masterbatch] The method for producing a cellulose nanofiber-added wet masterbatch of the present invention is a method for producing a cellulose nanofiber-added wet masterbatch by dehydrating and drying a rubber / cellulose nanofiber composite obtained by coagulating a mixed liquid of a cellulose nanofiber aqueous dispersion and rubber latex, and is characterized in that the dehydration is carried out while pressing and heating the rubber / cellulose nanofiber composite in the thickness direction on the outer peripheral surface of a drum.

[0017] [Cellulose nanofiber aqueous dispersion] The cellulose nanofibers used in the present invention are made from cellulose fibers prepared from various natural plant fibers, and are prepared by dispersing the raw material in water and subjecting it to chemical or mechanical treatment, thereby adjusting the average fiber diameter to approximately less than 1000 nm. A method for chemically treating such raw materials includes, for example, adding 2,2,6,6-tetramethylpiperidine-1-oxyl radical as a catalyst to cellulose fibers dispersed in water, adjusting the pH to 10, adding an aqueous sodium hypochlorite solution and stirring, then filtering / washing, and further diluting with water. As a method for mechanical treatment, for example, cellulose fibers dispersed in water are ground by a millstone.

[0018] In the present invention, the "average fiber diameter" refers to a value obtained by randomly extracting 10 cellulose nanofibers from a scanning electron microscope (SEM) image, measuring the minor axis, and calculating the arithmetic mean.

[0019] In the present invention, the solids concentration (cellulose nanofiber concentration) of the cellulose nanofiber aqueous dispersion used when mixing with rubber latex to prepare a mixed liquid is preferably adjusted to 0.4% to 1.0% by mass, particularly 0.5% to 0.7% by mass. If the solids concentration exceeds the upper limit, the fluidity may be low, which may cause problems during liquid phase mixing. On the other hand, if the solids concentration is below the lower limit, the production efficiency of the composite will decrease.

[0020] From the viewpoint of the size and dehydration efficiency of the rubber / cellulose nanofiber composite obtained by coagulating a mixture of an aqueous cellulose nanofiber dispersion and rubber latex, the aqueous cellulose nanofiber dispersion is preferably mixed with rubber latex so that the amount of cellulose nanofiber is 1 to 40 parts by mass, and more preferably 5 to 30 parts by mass, per 100 parts by mass of the solid content of the rubber latex.

[0021] [Rubber latex] As the rubber latex, one or more of natural rubber latex and synthetic rubber latex can be used. Natural rubber latex is a natural product of plants, and is preferably a natural rubber / water system in which the dispersion medium is water. Regarding natural rubber latex, concentrated latex, fresh latex known as field latex, etc. can be used without distinction. Examples of synthetic rubber latex include those produced by emulsion polymerization of styrene-butadiene rubber, polybutadiene rubber, polyisoprene rubber, nitrile rubber, and chloroprene rubber.

[0022] From the viewpoints of productivity, fluidity of the latex, and ensuring mechanical stability, the solid content of the rubber latex is preferably 10% by mass to 60% by mass, particularly 20% by mass to 30% by mass, in terms of natural rubber latex and styrene-butadiene rubber. Here, the solid content of the rubber latex corresponds to the rubber component.

[0023] [Filling material] In the present invention, from the viewpoint of improving the rubber reinforcement properties of the resulting cellulose nanofiber-added wet masterbatch, inorganic fillers such as carbon black and silica can be blended as the filler.

[0024] Examples of carbon black that can be used include carbon blacks typically used in the rubber industry, such as SAF, ISAF, HAF, FEF, and GPF, as well as conductive carbon blacks such as acetylene black and ketjen black. The carbon black may be granulated carbon black, which is granulated in the typical rubber industry in consideration of its handleability, or ungranulated carbon black.

[0025] As the silica, for example, wet silica or dry silica can be used, and among them, it is preferable to use wet silica containing hydrated silicic acid as the main component.

[0026] Inorganic fillers such as carbon black and silica are preferably mixed as an aqueous dispersion with the cellulose nanofiber aqueous dispersion and rubber latex. Alternatively, these inorganic fillers may be mixed in advance with either the cellulose nanofiber aqueous dispersion or the rubber latex, and then mixed with the other.

[0027] When inorganic fillers such as carbon black and silica are added, the amount added is adjusted according to the purpose.

[0028] <Coagulant> In the present invention, a mixture of an aqueous cellulose nanofiber dispersion and rubber latex, optionally containing the above-mentioned filler, is coagulated and preferably further filtered to obtain a rubber / cellulose nanofiber composite. The coagulant added in this process can be an acid such as formic acid or sulfuric acid, or a salt such as sodium chloride or calcium chloride, which is commonly used to coagulate rubber latex.

[0029] <Flocculant> In the present invention, a flocculant may be added to control the coagulation state (size of coagulated aggregate particles) of the resulting rubber / cellulose nanofiber composite. Examples of the flocculant that can be used include cationic polymer flocculants.

[0030] <Rubber / Cellulose Nanofiber Composite> A mixture of an aqueous dispersion of cellulose nanofibers, optionally containing a filler, and rubber latex is coagulated, and preferably further filtered to obtain a rubber / cellulose nanofiber composite to be subjected to the following dehydration. The water content of the resulting composite is preferably adjusted so that the composite can be efficiently adhered to the outer peripheral surface of a drum dryer.

[0031] In the present invention, it is an essential requirement that the rubber / cellulose nanofiber composite be dehydrated while being pressed and heated in the thickness direction on the outer peripheral surface of a drum (specifically, in the thickness direction of the layer of the rubber / cellulose nanofiber composite attached to the outer peripheral surface of the drum). By simultaneously pressing and heating in the thickness direction, it is possible to prevent aggregation of the cellulose nanofibers, and when the obtained cellulose nanofiber-added wet masterbatch is used to form a rubber composition, the rubber reinforcing properties are not impaired and an excellent rubber reinforcing effect can be achieved. The water content of the rubber / cellulose nanofiber composite after dehydration is preferably 50% by mass or less, more preferably 30% by mass or less, and even more preferably 10% by mass or less.

[0032] To improve the dehydration efficiency of the rubber / cellulose nanofiber composite, the drum temperature is preferably 110°C or higher, more preferably 130°C or higher, and even more preferably 150°C or higher. On the other hand, when taking into consideration the thermal degradation of the rubber component of the rubber / cellulose nanofiber composite, the drum temperature is preferably adjusted to 160°C or lower. Here, the temperature of the drum refers to the temperature of the surface (outer periphery) of the drum.

[0033] As the drum, using a double-drum type drum dryer described below is advantageous in that it can quickly dry the rubber / cellulose nanofiber composite and prevent the cellulose nanofibers from agglomerating. In this case, from the viewpoint of dewatering efficiency, the drum gap of the drum dryer (d in FIG. 1 shown later) is preferably set to 0.35 mm to 0.85 mm, more preferably 0.35 mm to 0.50 mm.

[0034] <Drum dryer> In the present invention, dehydration of the rubber / cellulose nanofiber composite is carried out using a drum, preferably a drum dryer. Drum dryers are often used to dry slurries, but in the present invention, they are used to dehydrate the rubber / cellulose nanofiber composite.

[0035] Drum dryers include double-drum type, twin-drum type, single-drum type, etc. In the present invention, any type of drum dryer may be used, but it is preferable to use a double-drum type drum dryer, which can simultaneously apply pressure and heat using a pair of drums.

[0036] FIG. 1 is a schematic side view showing an example of a dehydration method using a double-drum type drum dryer. This double-drum drum dryer has a pair of opposing drums 1, 1. The gap d between the drums 1, 1 can be adjusted by adjusting the distance between their axial centers. The drums 1, 1 are provided with heaters (not shown) for regulating the temperature of their peripheral surfaces. The drum on the left side of FIG. 1 rotates clockwise, and the drum on the right side rotates counterclockwise.

[0037] The rubber / cellulose nanofiber composite is supplied to the space 2 above the drums 1, 1. As a result, a liquid pool of the rubber / cellulose nanofiber composite L is formed in this space 2. This liquid pool of the rubber / cellulose nanofiber composite L is pressed in the gap between the rotating drums 1, 1 as described above, and a liquid film F of a predetermined thickness is formed on the outer circumferential surfaces of the drums 1, 1. This liquid film F is heated by the heat received from the outer circumferential surfaces of the drums 1, 1, and becomes a dehydrated material S. The dehydrated material S adhering to the outer circumferential surfaces of the drums 1, 1 is scraped off by a doctor blade 3 and stored in a container 4. The dehydrated material S thus dehydrated is sent to the next drying process.

[0038] In the present invention, a twin-drum type or single-drum type drum dryer other than the double-drum type may also be used. FIG. 2 is a schematic side view showing an example of a twin-drum drum dryer. Similar to the double-drum drum dryer of FIG. 1, this twin-drum drum dryer supplies a liquid pool of the rubber / cellulose nanofiber composite L to the upper space 2 between a pair of drums 1,1. The drums 1,1 rotate in the opposite direction to that in FIG. 1. That is, the drum on the left side of FIG. 2 rotates counterclockwise, and the drum on the right side rotates clockwise. As a result, a liquid film F is formed on the outer peripheral surface of the drums 1,1. This liquid film F is heated and pressed to form a dehydrated material S, which is then scraped off by a doctor blade 3 and stored in a container 4.

[0039] 3 to 8 are schematic side views showing examples of single-drum type drum dryers. These single-drum type drum dryers are equipped with one drum 1. In the single-drum type drum dryers shown in FIGS. 3 to 8, a pressure plate (not shown) is provided to press the liquid film F formed on the drum surface, thereby enabling dehydration by heating and pressing.

[0040] In the single-drum drum dryer shown in Figure 3, the rubber / cellulose nanofiber composite L in the container 5 is thrown up by the impeller 6 onto the outer surface of the drum 1 and adheres to it, forming a liquid film F. This liquid film F becomes the dehydrated material S, which is scraped off by the doctor blade 3 and stored in the container 4.

[0041] In the single-drum drum dryer shown in Figure 4, the lower part of the drum 1 is immersed in the rubber / cellulose nanofiber composite L in the tray-shaped container 7. As the drum 1 rotates, a liquid film F is formed on the outer surface of the drum 1. This liquid film F becomes the dehydrated material S, which is scraped off by a doctor blade 3 and stored in a container 4.

[0042] In the single-drum drum dryer of Figure 5, a coating roller 9 is disposed facing the lower part of the drum 1. The coating roller 9 is disposed so that its axis is parallel to the axis of the drum 1. A predetermined gap is provided between the outer circumferential surface of the coating roller 9 and the outer circumferential surface of the drum 1. The lower part of the coating roller 9 is immersed in the rubber / cellulose nanofiber composite L in the tray-shaped container 8. By rotating the drum 1 clockwise and the coating roller 9 counterclockwise, a liquid film F is formed on the outer surface of the drum 1. This liquid film F becomes the dehydrated material S, which is scraped off by the doctor blade 3 and stored in the container 4.

[0043] In the single-drum drum dryer of Figure 6, a coating roller 9 is disposed facing the upper part of the drum 1. The coating roller 9 is disposed so that its axis is parallel to the axis of the drum 1. A predetermined gap is provided between the outer circumferential surface of the coating roller 9 and the outer circumferential surface of the drum 1. A rubber / cellulose nanofiber composite L is supplied to the space between the coating roller 9 and the outer peripheral surface of the drum 1. By rotating the drum 1 counterclockwise in FIG. 6, a liquid film F is formed on the outer peripheral surface of the drum 1. This liquid film F becomes a dehydrated material S, which is scraped off by a doctor blade 3 and stored in a container 4.

[0044] In the single-drum drum dryer of Figure 7, a coating roller 9 is disposed facing the side of the drum 1. The coating roller 9 is disposed so that its axis is parallel to the axis of the drum 1 and is at approximately the same height as the axis of the drum 1. A predetermined gap is provided between the outer circumferential surface of the coating roller 9 and the outer circumferential surface of the drum 1. A rubber / cellulose nanofiber composite L is supplied to the space between the coating roller 9 and the outer peripheral surface of the drum 1. By rotating the drum 1 counterclockwise in FIG. 7, a liquid film F is formed on the outer peripheral surface of the drum 1. This liquid film F becomes a dehydrated material S, which is scraped off by a doctor blade 3 and stored in a container 4.

[0045] In the single-drum drum dryer of Figure 8, multiple coating rollers 9 (three in this embodiment) are arranged at different installation heights so as to face the top and sides of the drum 1. Each coating roller 9 is arranged so that its axis is parallel to the axis of the drum 1. A predetermined gap is left between the outer circumferential surface of the coating roller 9 and the outer circumferential surface of the drum 1. A rubber / cellulose nanofiber composite L is supplied from a feed pipe 10 to the space between the coating roller 9 and the outer peripheral surface of the drum 1. By rotating the drum 1 counterclockwise in FIG. 8, a liquid film F is formed on the outer peripheral surface of the drum 1. This liquid film F becomes a dehydrated material S, which is scraped off by a doctor blade 3 and stored in a container 4.

[0046] <Drying> As the final step in producing a cellulose nanofiber-added wet masterbatch, a drying step is usually carried out to remove water, which is the dispersion medium. In the present invention, the drying method is not limited, and any known method can be used, for example, a hot air dryer, a vacuum dryer, a drum dryer, a band dryer, etc. In the case of hot air drying, the drying temperature is preferably set to 60°C to 100°C from the viewpoints of drying efficiency and preventing thermal degradation of the rubber. There are no restrictions on the moisture content in the cellulose nanofiber-added wet masterbatch produced by removing the dispersion medium through drying, but it is generally preferred that it be 2.0 mass % or less. [Example]

[0047] The present invention will be described in more detail below using examples, but the present invention is not limited to the following examples as long as it does not depart from the gist of the invention.

[0048] [raw materials] The various raw materials used in the following Examples, Comparative Examples, Reference Examples and Comparative Reference Examples are as follows:

[0049] <Cellulose nanofiber aqueous dispersion> Nippon Paper Industries Co., Ltd.: Cellulose nanofiber aqueous dispersion (solids: 1.2% by mass, viscosity: 4600 mPa s, pH: 7.6, transparency: 39.6%) (Note: Viscosity, pH, and transparency were measured at a solids content of 1.0% by mass.)

[0050] <Natural rubber latex> Resitex Co., Ltd.: Natural rubber latex (solid content: 60% by mass, high ammonia type)

[0051] [Reference example 1] The cellulose nanofiber aqueous dispersion was diluted with water to a solids concentration of 0.6% by mass and stirred at room temperature for 10 minutes using a Waring blender (LBC-15, manufactured by Osaka Chemical Co., Ltd., rotation speed 16,300 rpm) to obtain a test cellulose nanofiber aqueous dispersion. The test cellulose nanofiber aqueous dispersion prepared above was added to natural rubber latex so that the cellulose nanofibers were 20 parts by mass per 100 parts by mass of the solids of the natural rubber latex, and the mixture was stirred for 2 minutes to obtain a mixture of the cellulose nanofiber aqueous dispersion and rubber latex.

[0052] To the mixture obtained above, a polymer flocculant, polyamine (Poly-8 manufactured by Tosoh Corporation) was added to a solids concentration of 0.005% by mass and mixed for 2 minutes. To this, an aqueous formic acid solution (manufactured by Asahi Chemical Industry Co., Ltd., solids content: 10% by mass) was added, and the pH of the mixed slurry solution was adjusted to 3.0 to cause acid coagulation. The resulting coagulation was filtered using a mesh (material: polyethylene, mesh size: 0.18 mm) to obtain a rubber / cellulose nanofiber composite.

[0053] The rubber / cellulose nanofiber composite (moisture content: 75% by mass) obtained by the above procedure was naturally dehydrated by air drying, and then dried under reduced pressure at 60°C to obtain a cellulose nanofiber-added wet masterbatch with a moisture content of 1.5% by mass or less.

[0054] To the obtained wet masterbatch, stearic acid, sulfur, zinc oxide, vulcanization accelerator, and antioxidant were added in the amounts shown in Table 1, and the mixture was kneaded with an open roll to obtain a rubber composition.

[0055] [Comparative Reference Example 1] A cellulose nanofiber-added wet masterbatch was produced in the same manner as in Example 1, except that the filtered rubber / cellulose nanofiber composite was dehydrated using a single-screw screw press extruder (V-01, manufactured by Suehiro EPM Co., Ltd.), and a rubber composition was obtained in the same manner.

[0056] [Table 1]

[0057] [evaluation] The rubber compositions obtained in Reference Example 1 and Comparative Reference Example 1 were each vulcanized at 145°C for 30 minutes to produce vulcanized rubber, and the morphology of the cellulose nanofibers in the rubber was observed using the method described below.

[0058] <Morphological observation> The morphology of cellulose nanofibers in the rubber was observed by cutting 5 μm sections of the vulcanized rubber and observing them using a transmission polarizing microscope. The results are shown in Figure 9(A) and (B). Aggregates of cellulose nanofibers were observed in the rubber in Figure 9(B) of Comparative Reference Example 1. As a result of using a screw press extruder that applies shear force to compress the rubber during the dehydration process, aggregation occurred between the hydrophobic rubber and the hydrophilic cellulose nanofibers, which do not mix well with each other. On the other hand, no cellulose nanofiber aggregates were observed in Figure 9(A) of Comparative Reference Example 1, which was naturally dehydrated by air drying. This allows the inherent reinforcing properties of cellulose nanofibers to be exerted, resulting in a rubber composition with excellent hardness and tensile strength. These results show that if the cellulose nanofibers are naturally dehydrated by air drying, they will not aggregate together, but if they are squeezed using an extruder to apply high shear force to improve dehydration efficiency, the cellulose nanofibers will aggregate together and the intended rubber reinforcement effect will not be obtained.

[0059] [Example 1] Using the same procedure as in Reference Example 1, a mixed liquid of an aqueous dispersion of cellulose nanofibers and rubber latex was obtained. Separately, carbon black (Mitsubishi Chemical Corporation: DIABLACK #20, nitrogen adsorption specific surface area: 45 m 2 / g) was dispersed in water to a solids content of 8% by mass, and the dispersion was stirred at room temperature for 10 minutes using a homogenizer (T25, manufactured by IKA Japan, rotation speed: 4000 rpm) to produce a carbon black slurry. This was then added to the above-prepared mixed liquid of cellulose nanofibers and natural rubber latex so that the carbon black was 10 parts by mass per 100 parts by mass of the solids content of the rubber latex. This was used as a mixed slurry solution of the respective raw materials.

[0060] To the resulting mixed slurry solution of each raw material, polyamine (Poly-8 manufactured by Tosoh Corporation) was added as a polymer flocculant to a solid content of 0.005% by mass, and the mixture was mixed for 2 minutes. To this was added an aqueous formic acid solution (manufactured by Asahi Chemical Industry Co., Ltd., solid content: 10% by mass), and the pH of the mixed slurry solution was adjusted to 3.0 to cause acid coagulation. The resulting coagulation was filtered using a mesh (material: polyethylene, mesh size: 0.18 mm) to obtain a rubber / cellulose nanofiber composite.

[0061] The filtered rubber / cellulose nanofiber composite (moisture content: 75% by mass) obtained by the above procedure was dehydrated using a double drum dryer (manufactured by Nippon Dryer Co., Ltd., model: DD-32x12) at a drum temperature of 157°C, a rotation speed of 0.50 rpm, and a gap between the drums of 0.50 mm, while the rubber / cellulose nanofiber composite was pressed and heated in the thickness direction on the outer surface of the drum, as shown in Figure 1. After dehydration with the drum dryer and before hot air drying, the moisture content of the cellulose nanofiber-added wet masterbatch was 27% by mass. Next, the mixture was further dried under reduced pressure at 60°C to obtain a cellulose nanofiber-added wet masterbatch having a moisture content of 1.5% by mass or less.

[0062] To the obtained wet masterbatch, stearic acid, sulfur, zinc oxide, vulcanization accelerator, and antioxidant were added in the amounts shown in Table 2, and the mixture was kneaded with an open roll to obtain a rubber composition.

[0063] [Example 2] Except for changing the drum temperature of the double drum dryer to 126°C, a cellulose nanofiber-added wet masterbatch was produced and a rubber composition was obtained in the same manner as in Example 1. Note that the moisture content of the cellulose nanofiber-added wet masterbatch after dehydration and before hot air drying was 52% by mass.

[0064] [Example 3] Except for changing the gap between the drums of the double drum dryer to 0.35 mm, a cellulose nanofiber-added wet masterbatch was produced and a rubber composition was obtained in the same manner as in Example 1. Note that the moisture content of the cellulose nanofiber-added wet masterbatch after dehydration and before hot air drying was 20% by mass.

[0065] [Comparative Example 1] A cellulose nanofiber-added wet masterbatch was produced in the same manner as in Example 1, except that the coagulated product after filtration was dehydrated in a single-screw press extruder (V-01, manufactured by Suehiro EPM Co., Ltd.) instead of a double drum dryer, and a rubber composition was obtained in the same manner. Note that the moisture content of the cellulose nanofiber-added wet masterbatch after dehydration and before hot air drying was 30% by mass.

[0066] Comparative Example 2 In Comparative Example 1, a cellulose nanofiber-added wet masterbatch was produced in the same manner as in Comparative Example 1, except that dehydration was repeated three times using a single-screw screw press extruder (V-01), and a rubber composition was obtained in the same manner.

[0067] Comparative Example 3 A cellulose nanofiber-added wet masterbatch was produced in the same manner as in Example 1, except that the coagulated material after filtration was dehydrated by natural dehydration through air drying instead of using a double drum dryer, and a rubber composition was obtained in the same manner.

[0068] [Physical property evaluation] The rubber compositions obtained in Examples 1 to 3 and Comparative Examples 1 to 3 were each vulcanized at 145°C for 30 minutes to produce vulcanized rubbers, and the hardness and tensile stress were measured using the methods described below. The results are shown in Table 2.

[0069] <Hardness> The hardness was measured in accordance with JIS K7215. The measurement results were expressed as an index, with the hardness value of Example 1 being set at 100. The larger the value, the higher and more favorable the rubber hardness is.

[0070] <Tensile stress> The tensile stress was measured by a tensile test in accordance with JIS K6251 (2017). The measurement results are expressed as an index, with the value of the tensile stress at a predetermined elongation in Example 1 being set to 100. The larger the value, the higher and more favorable the tensile stress at a predetermined elongation.

[0071] [Table 2]

[0072] Table 2 shows that when a drum dryer is used as the dehydration method as in Examples 1 to 3, the physical properties (hardness, tensile stress) of the resulting rubber composition are no different from those when air drying is used as the dehydration method in Comparative Example 3. Furthermore, these results show that aggregation of cellulose nanofibers is suppressed during the dehydration process. On the other hand, when a single-screw press extruder (V-01) was used as the dehydration method as in Comparative Examples 1 and 2, the physical properties (hardness, tensile stress) of the resulting rubber composition were lower than those of Examples 1 to 3, indicating that the rubber reinforcement properties were reduced due to the aggregation of cellulose nanofibers. [Explanation of symbols]

[0073] 1 drum 3 Doctor Blade 4 containers L Rubber / cellulose nanofiber composite F liquid film S Dehydrated product

Claims

1. A method for producing a cellulose nanofiber-added wet masterbatch by dehydrating and drying a rubber / cellulose nanofiber composite obtained by coagulating a mixed liquid of an aqueous cellulose nanofiber dispersion and rubber latex, wherein the dehydration is carried out while pressing and heating the rubber / cellulose nanofiber composite in the thickness direction on the outer peripheral surface of a drum.

2. The method for producing a cellulose nanofiber-added wet masterbatch according to claim 1, wherein the temperature of the drum is adjusted to 130°C to 160°C.

3. The method for producing a cellulose nanofiber-added wet masterbatch according to claim 1 or 2, wherein a double-drum drum dryer is used as the drum.

4. The method for producing a cellulose nanofiber-added wet masterbatch according to claim 3, wherein the drum gap of the double-drum drum dryer is 0.35 mm to 0.85 mm.

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