JP7609710B2 - Water-containing rubber composition and method for producing rubber molded article - Google Patents

Description

The present invention relates to a water-containing rubber composition, and more specifically to a water-containing rubber composition that is used as a raw material in the manufacture of fiber-reinforced rubber molded articles, particularly in the form of a master batch, and a method for manufacturing rubber molded articles using the same.

In the past, in order to improve the mechanical properties of rubber molded bodies, it has been considered to reinforce them by blending short fibers such as cellulose fibers, nylon fibers, polyester fibers, and aramid fibers. Among these, short fibers made of aramid fibers are widely used due to their excellent mechanical properties, fatigue resistance, heat resistance, and chemical properties.

However, because the surface of aramid short fibers is relatively inactive, they have low adhesion to rubber and are poorly dispersible when mixed into rubber, meaning that the inherent excellent properties of aramid short fibers cannot be fully demonstrated. Furthermore, even if aramid short fibers are added to rubber, they are not distributed completely uniformly during use, so wear progresses from areas where there are no aramid short fibers, making it impossible to obtain sufficient wear resistance.

One way to improve the adhesive strength between the reinforcing short fibers and the rubber is to reduce the fiber diameter of the reinforcing short fibers or to use pulp with fibril on the surface of the reinforcing short fibers, thereby increasing the contact area between the reinforcing short fibers and the rubber. However, the larger the contact area, the more difficult it becomes to disperse the reinforcing fibers evenly in the rubber.

Among these, Patent Document 1 proposes a granular elastomer composition for use as a masterbatch material in which aramid pulp having fibrils on the fiber surface is mixed in advance with a liquid elastomer. In aramid pulp, the fibril portion is finely divided, but the stem portion is thick and the fibrils can be cut off from the stem, so the adhesive strength with rubber is still insufficient, and further improvement in rubber reinforcement properties is expected.

Recently, however, techniques have been proposed for nano-sizing aramid fibers. For example, Non-Patent Documents 1 and 2 propose methods for obtaining nano-sized aramid fibers, i.e., aramid nanofibers, by treating aramid fibers in a dimethyl sulfoxide solvent containing potassium hydroxide, or by spraying alkali-treated aramid fibers with high-pressure water. By using these aramid nanofibers to reinforce rubber, it is expected that the adhesive strength with the rubber will be improved, and the rubber elastic modulus and abrasion resistance will be greatly improved.

Patent Document 2 proposes a method for producing a short fiber-containing rubber master batch, in which an aqueous dispersion of cellulose or short aramid fibers with an average fiber diameter of less than 0.5 μm (500 nm) is mixed with rubber latex by stirring, and the mixture is sprayed and dried in an atmosphere of shock waves generated by pulse combustion. However, since aramid fibers exhibit very strong hydrogen bonds, even after mixing with rubber latex, if the moisture is completely removed, the aramid nanofibers that are partially in contact with each other will aggregate in the master batch. Furthermore, once the aramid nanofibers have aggregated, they are difficult to redisperse in the subsequent process, making it difficult to obtain a good reinforcing effect.

Non-Patent Documents 3 and 4 propose a method of adding epichlorohydrin to a potassium hydroxide-added dimethyl sulfoxide solvent containing aramid nanofibers, obtaining an aqueous dispersion of aramid nanofibers modified with epichlorohydrin, mixing the aqueous dispersion with a latex (aqueous dispersion) mixture of carboxylated nitrile rubber and styrene butadiene rubber or a latex of styrene butadiene rubber alone, removing water, and adding a vulcanizing agent to obtain a rubber composition. In this method, the surface of the aramid nanofibers is modified with epichlorohydrin, which makes it easier to prevent the aramid nanofibers from aggregating together, but the process of performing epichlorohydrin treatment in a solvent requires a long treatment time and epichlorohydrin remaining in the solvent makes it difficult to recover a high-purity solvent, so there remain industrial challenges.

Japanese Unexamined Patent Publication No. 137940/1983 Patent No. 3998692

ACS Nano 2011, 5, 6945. RSC Adv. 2014, 4, 40377. Composites Part B: Engineering 2019,166,196-203 Materials Chemistry and Physics 2019,238,121926

The object of the present invention is to provide a water-containing rubber composition that can be used in the form of a masterbatch in the production of rubber and can impart high rubber elasticity and abrasion resistance to the resulting rubber.

That is, the present invention is a moisture-containing rubber composition containing aramid nanofibers, rubber, and water, the aramid nanofibers having an average fiber diameter of less than 500 nm, the weight ratio of the aramid nanofibers to the rubber in the moisture-containing rubber composition being 1:99 to 35:65, and the weight ratio of the sum of the aramid nanofibers and the rubber to the water being 98:2 to 1:99, and the moisture-containing rubber composition being used as a master batch material for producing a rubber composition.

The present invention provides a water-containing rubber composition that can be used in the form of a masterbatch in rubber production and can impart high rubber elasticity and abrasion resistance to the resulting rubber.

The present invention will be described in detail below.

[Aramid nanofiber]
In the present invention, the aramid nanofibers are aromatic polyamide fibers having an average fiber diameter of 500 nm or less.

Specific examples of aromatic polyamide fibers include polyparaphenylene terephthalamide fiber, which is a para-type aromatic polyamide fiber (e.g., Twaron (registered trademark) manufactured by Teijin Aramid B.V.), copolyparaphenylene 3,4'-oxydiphenylene terephthalamide fiber, which is a copolymer type aromatic polyamide fiber (e.g., Technora (registered trademark) manufactured by Teijin Limited), and polymetaphenylene isophthalamide fiber, which is a meta-type aromatic polyamide fiber (e.g., Conex (registered trademark) manufactured by Teijin Limited).

As the aramid nanofiber, among aromatic polyamide fibers, para-aromatic polyamide fibers having high strength and elastic modulus are preferable.
The aramid nanofibers can be produced by a known method using the aromatic polyamide fibers described above, for example, a method of stirring and treating the aromatic polyamide fibers in a polar solvent such as dimethyl sulfoxide containing a strong base such as potassium hydroxide, a method of treating the aromatic polyamide fibers under high shear force such as by spraying high-pressure water, or a method of treating the aromatic polyamide fibers by electrospinning.

Among these, the method of stirring aromatic polyamide fibers in a polar solvent such as dimethyl sulfoxide containing a strong base such as potassium hydroxide is preferred because it produces aramid nanofibers with relatively little variation in fiber diameter.

The average fiber diameter of the aramid nanofiber is 500 nm or less, preferably 100 nm or less. If the average fiber diameter exceeds 500 nm, it is difficult to realize the high specific surface area that is a characteristic of nanofibers, and sufficient adhesion to the rubber cannot be obtained, and there are many areas where the aramid nanofibers are not present, so the elastic modulus and abrasion resistance of the rubber cannot be sufficiently improved, which is undesirable. The lower limit of the average fiber diameter is, for example, 1 nm. This is because it is difficult to manufacture fibers with an average fiber diameter of less than 1 nm.

The aspect ratio of the aramid nanofibers, expressed as fiber length/fiber diameter, is preferably 10 to 10,000, and more preferably 50 to 5,000. If the aspect ratio is less than 10, sufficient reinforcing properties cannot be obtained, which is undesirable. This is presumably because each individual aramid nanofiber tends to come out of the rubber. On the other hand, if the aspect ratio exceeds 10,000, entanglement between the aramid nanofibers tends to occur, making it difficult to obtain a uniform dispersion, which is undesirable.

[Rubber]
The rubber contained in the water-containing rubber composition of the present invention is a polymeric material having elasticity. As the rubber, unvulcanized or uncrosslinked rubber is used, and preferably, natural rubber (NR), styrene butadiene rubber (SBR), nitrile rubber (NBR), polyisoprene rubber (IR), butadiene rubber (BR), butyl rubber (IIR), ethylene propylene rubber (EPM), chloroprene rubber (CR), acrylic rubber (ACM), silicone rubber (Q), and fluororubber (FKM) are used before vulcanization. A plurality of types of rubber may be included. Styrene butadiene rubber is particularly preferred as the rubber.

[Ratio of aramid nanofiber to rubber component]
In the water-containing rubber composition of the present invention, the weight ratio of aramid nanofiber to rubber is 1:99 to 35:65. If the amount of aramid nanofiber is less than 1% by weight, sufficient reinforcement properties cannot be obtained. On the other hand, if the amount of aramid nanofiber is more than 35% by weight, the amount of rubber between the single fibers of the aramid nanofiber is insufficient, the distance between the aramid nanofibers is shortened, and aggregation due to hydrogen bonding or a large amount of entanglement occurs, which is not preferable.

[Moisture content in water-containing rubber composition]
In the water-containing rubber composition of the present invention, the weight ratio of the total of the aramid nanofibers and the rubber to the water is 98:2 to 1:99, preferably 50:50 to 1:99. When water is present in the rubber composition within this weight ratio range, water is present between one aramid nanofiber and another aramid nanofiber, and it is possible to suppress the aramid nanofibers from re-aggregating due to hydrogen bonding. On the other hand, when the weight ratio of the total of the aramid nanofibers and the rubber to the water exceeds 1:99 and the water-containing rubber composition contains a large amount of water, most of the water is water, which increases the industrial logistics cost and makes it difficult to remove the water during use.

The water-containing rubber composition of the present invention may further contain additives that are commonly used in general rubber, such as reinforcing agents (fillers) such as carbon black and silica, various oils, antioxidants, and plasticizers. These additives can be used by kneading them using a general method.

[Method for producing water-containing rubber composition]
The water-containing rubber composition of the present invention can be produced by mixing an aramid nanofiber water dispersion and a rubber water dispersion in a solids weight ratio of aramid nanofiber to rubber of 1:99 to 35:65 to obtain a rubber mixture, and then removing a portion of the water.

A water dispersion of aramid nanofibers can be produced by gradually adding water to an organic solvent in which aramid nanofibers have been dissolved or dispersed, thereby replacing the organic solvent with water. A water dispersion of rubber can be produced by gradually adding water to an organic solvent in which rubber has been dissolved or dispersed, thereby replacing the organic solvent with water. A water dispersion of rubber that is commercially available as rubber latex can also be used.

[Master batch]
The water-containing rubber composition of the present invention is used for producing a rubber molded article, and is compounded in the form of a masterbatch.

The amount of aramid nanofibers contained in the rubber molded body by using the water-containing rubber composition of the present invention in the form of a master batch is preferably 0.1 to 30% by weight, more preferably 0.5 to 20% by weight, based on the total weight of the rubber in the rubber molded body. When the rubber molded body contains aramid nanofibers in this range, the elastic modulus and abrasion resistance of the final rubber molded body can be improved. If the content is less than 0.1% by weight, the reinforcing effect of the aramid nanofibers cannot be sufficiently obtained, which is not preferable. On the other hand, if the content exceeds 30% by weight, it becomes difficult to uniformly disperse the aramid nanofibers, and a reinforcing effect commensurate with the amount added cannot be obtained, which is not preferable.

[Method for producing rubber molded body]
The present invention further provides a method for producing a rubber molded article, comprising: a first step of mixing an aramid nanofiber water dispersion and a rubber water dispersion in a solids weight ratio of aramid nanofiber to rubber of 1:99 to 35:65, and further removing a portion of the water to obtain a water-containing rubber composition; a second step of mixing the water-containing rubber composition obtained in the first step with solid rubber, and further removing the water to obtain a solid rubber composition; a third step of mixing the solid rubber composition obtained in the second step with a vulcanizing agent and/or a crosslinking agent to obtain an unvulcanized rubber composition and/or an uncrosslinked rubber composition; and a fourth step of vulcanizing and/or crosslinking the unvulcanized and/or uncrosslinked rubber composition obtained in the third step to obtain a rubber molded article.

The first step is to mix an aramid nanofiber water dispersion with a rubber water dispersion in a solids weight ratio of aramid nanofiber to rubber of 1:99 to 35:65, and then remove some of the water to obtain a water-containing rubber composition. By removing some of the water, the weight ratio of the total of the aramid nanofiber and rubber to water in the obtained water-containing rubber composition is set to 98:2 to 1:99.

The rubber contained in the rubber water dispersion is a polymeric material having elasticity. As this rubber, unvulcanized or uncrosslinked rubber is used, and preferably, natural rubber (NR), styrene butadiene rubber (SBR), nitrile rubber (NBR), polyisoprene rubber (IR), butadiene rubber (BR), butyl rubber (IIR), ethylene propylene rubber (EPM), chloroprene rubber (CR), acrylic rubber (ACM), silicone rubber (Q), or fluororubber (FKM) before vulcanization is used. Multiple types of rubber may be included. Styrene butadiene rubber is particularly preferred as the rubber.

The second step is a step of mixing the water-containing rubber composition obtained in the first step with solid rubber, and then removing the water to obtain a solid rubber composition. The solid rubber used here is pre-vulcanized or pre-crosslinked, and pre-vulcanized natural rubber (NR), styrene butadiene rubber (SBR), nitrile rubber (NBR), polyisoprene rubber (IR), butadiene rubber (BR), butyl rubber (IIR), ethylene propylene rubber (EPM), chloroprene rubber (CR), acrylic rubber (ACM), silicone rubber (Q), and fluororubber (FKM) can be used. Multiple types of solid rubber may be used.

It is preferable that this solid rubber is a rubber that has a high affinity to the rubber contained in the water-containing rubber composition of the present invention. For this reason, it is preferable that the solid rubber exhibits a Hansen solubility parameter close to that of the rubber in the water-containing rubber composition of the present invention. Note that "close" here means that the difference in Hansen solubility parameter is 2 or less, preferably 1 or less.

The water-containing rubber composition and the solid rubber can be mixed using a kneading machine such as a pressure kneader or a Banbury mixer, which are generally used for kneading rubber. In these kneading machines, the kneading temperature is set to 100°C or higher and lower than 180°C, and water is removed during kneading to obtain a solid rubber composition.

The third step is a step of mixing the solid rubber composition obtained in the second step with a vulcanizing agent and/or a crosslinking agent to obtain an unvulcanized and/or uncrosslinked rubber composition. This mixing is preferably performed in a state where the water-containing rubber composition has been cooled to a temperature at which vulcanization and/or crosslinking does not proceed.

The fourth step is to vulcanize and/or crosslink the unvulcanized and/or uncrosslinked rubber composition obtained in the third step to form a rubber molded article.

The present invention will be described in more detail below with reference to the following examples. The evaluation methods used in the examples are as follows:

(1) Average fiber diameter The rubber component of the unvulcanized and/or uncrosslinked rubber composition was dissolved in toluene to obtain a toluene solution containing aramid nanofibers. The toluene solution was spread thinly with a glass rod and dried in a dryer at 70° C. to obtain a sheet-like sample made of aramid nanofibers.

The structure of the samples was observed using a scanning electron microscope (JEOL Ltd., plate: JSM-6330F). From an image observed at a magnification setting of 50,000x, an image region measuring 1,800 nm to 2,000 nm horizontally and 1,200 nm to 1,500 nm vertically was selected, and this image region was further divided vertically and horizontally into four to define a total of 16 grid regions. One sample present within each grid region was selected and the fiber diameter was measured on the image. The fiber diameters of a total of 16 samples were measured, and the average value of these measurements was taken as the average fiber diameter.

(2) Stress at 5% elongation and 10% elongation of vulcanized rubber sheet Samples were cut out of the vulcanized rubber sheet in the orientation direction of the aramid nanofibers, and tensile tests were carried out using a dumbbell-shaped No. 3 specimen according to JIS K6251 to measure the stress at 5% elongation and 10% elongation. The higher these stresses, the better the elastic modulus.

(3) Abrasion resistance of vulcanized rubber sheet The surface of the vulcanized rubber sheet was rubbed with sandpaper of No. 320 to evaluate the abrasion resistance. Specifically, using a friction and abrasion tester manufactured by Orientec Co., Ltd., the vulcanized rubber sheet was rotated and abraded at a speed of 100 rpm under a load of 0.19 MPa so that the cross section of the reinforcing fiber in the reinforcing fiber orientation direction of the vulcanized rubber sheet was in contact with the sandpaper, and the amount of abrasion after 10 minutes was calculated from the change in weight. The smaller the amount of abrasion, the better the abrasion resistance.

Example 1
<First step (preparation of aramid nanofiber aqueous dispersion)>
202 g of Twaron (registered trademark) pulp (product number: Type 1094) manufactured by Teijin Aramid Co., Ltd. (solid content weight: 50 g, moisture content weight: 152 g), 64 g of additional water, 25 g of potassium hydroxide, and 1,950 g of dimethyl sulfoxide (DMSO) were placed in a 2 L plastic container and mixed at room temperature to prepare a mixed liquid, which was then kept in a dryer set at 70° C. for 3 hours.

After being held at 70°C for 3 hours, the color of the mixture changed from yellow to reddish brown, and no pulp-like material was observed in the mixture. At this point, the mixture was returned to room temperature, and it was visually confirmed that the viscosity had increased compared to immediately after mixing.

A sufficient amount of water was then added to this mixture to coagulate the aramid fibers derived from Twaron (registered trademark) pulp, and the mixture of DMSO, potassium hydroxide, and water was removed while continuing to add more water to obtain an agglomerate of the aramid fibers. The agglomerate was passed through a wet grinder to obtain an aramid nanofiber water dispersion. The obtained aramid nanofiber water dispersion was an aqueous dispersion containing 0.45% by weight of aramid nanofiber.

<First step (preparation of rubber-water dispersion)>
Nipol LX112 (manufactured by Nippon Zeon Co., Ltd., containing 40.5% by weight of styrene-butadiene rubber) was diluted 10 times with water to obtain a rubber water dispersion containing 4.05% by weight of styrene-butadiene rubber.

<First step (preparation of water-containing rubber composition)>
The aramid nanofiber water dispersion and the rubber water dispersion were mixed to obtain a mixture in which the weight ratio of the aramid nanofiber to the styrene-butadiene rubber was 10:90 and the weight ratio of the total of the aramid nanofiber and the styrene-butadiene rubber to water was 2.25:97.75.

This mixture was dried until the weight ratio of the total of the aramid nanofibers and the styrene-butadiene rubber to water became 10:90, to obtain a water-containing rubber composition. The average fiber diameter of the aramid nanofibers in the obtained water-containing rubber composition was 30 nm.

<Second step>
The water-containing rubber composition obtained above was mixed into the unvulcanized rubber compound described below so that the amount of aramid nanofiber mixed was 1% by weight per total weight of the rubber components (natural rubber and styrene-butadiene rubber) contained in the rubber water dispersion and the unvulcanized rubber compound, and the water contained in the rubber mixture was evaporated while kneading for 60 minutes at 140°C in an MS pressure kneader (DS3-10MHHS, manufactured by Moriyama Seisakusho Co., Ltd.), thereby obtaining a solid rubber composition. The obtained solid rubber composition was then left to stand overnight and cooled.

(Composition of unvulcanized rubber compound)
Natural rubber: 60 parts by weight Styrene butadiene rubber: 40 parts by weight Coumarone: 10 parts by weight Zinc oxide: 5 parts by weight Stearic acid: 1 part by weight Antioxidant: 1 part by weight Carbon black: 50 parts by weight

<Third process>
Sulfur and N-cyclohexyl-2-benzothiazolylsulfenamide as a vulcanization accelerator were added in an amount of 2% by weight each based on the total weight of the rubber components (natural rubber and styrene-butadiene rubber) contained in the solid rubber composition to prepare an unvulcanized rubber composition.

<Fourth step>
The unvulcanized rubber composition was sheeted to a thickness of about 1 mm using an open roll (9-inch test roll manufactured by Kansai Roll Co., Ltd.) to obtain a vulcanized rubber composition sheet. Two vulcanized rubber composition sheets were laminated and press-vulcanized at 150° C. for 30 minutes under 1 MPa to obtain a 2 mm-thick vulcanized rubber sheet.
The components of the water-containing rubber compositions and the physical properties of the vulcanized rubber sheets are shown in Table 1.

Example 2
In the first step (preparation of a water-containing rubber composition) in Example 1, the mixture was dried until the weight ratio of the total of the aramid nanofibers and the styrene-butadiene rubber to water became 30:70, thereby obtaining a water-containing rubber composition.
The average fiber diameter of the aramid nanofibers in the obtained water-containing rubber composition was 30 nm, the same as in Example 1. Using the obtained rubber composition, a vulcanized rubber sheet was produced in the same manner as in Example 1. The evaluation results are shown in Table 1.

Example 3
An aramid fiber water dispersion containing 1.01% by weight of aramid nanofiber was prepared in the same manner as in Example 1, except that the amount of water added during preparation of the aramid nanofiber water dispersion was changed.
The same rubber water dispersion as in Example 1 was mixed with the above-mentioned aramid nanofiber water dispersion to obtain a mixture in which the weight ratio of the aramid nanofiber to the styrene-butadiene rubber was 20:80 and the weight ratio of the total of the aramid nanofiber and the styrene-butadiene rubber to the water was 2.25:97.75.

This mixture was dried until the weight ratio of the total of the aramid nanofibers and the styrene-butadiene rubber to water became 10:90, thereby obtaining a water-containing rubber composition. The average fiber diameter of the aramid nanofibers in the obtained water-containing rubber composition was 30 nm.
The obtained water-containing rubber composition was used to prepare a vulcanized rubber sheet in the same manner as in Example 1. The evaluation results are shown in Table 1.

Comparative Example 1
In the first step (preparation of rubber-water dispersion), a water-containing rubber composition having a rubber component to water ratio of 10:90 was obtained using only a latex containing styrene-butadiene rubber (Nipol LX112, active ingredient 40.5% by weight, manufactured by Zeon Corporation) without using aramid nanofibers.
The obtained water-containing rubber composition was used to prepare a vulcanized rubber sheet in the same manner as in Example 1. The evaluation results are shown in Table 1.

Comparative Example 2
In the first step (preparation of an aqueous dispersion of aramid nanofibers), aramid nanofibers were not used. Instead, aramid pulp (Twaron (registered trademark) pulp (product number: Type 1094) manufactured by Teijin Aramid Co., Ltd.) was diluted with water to prepare an aqueous dispersion containing 0.45% by weight of aramid pulp, which was used as the aqueous dispersion.

Except for this, a water-containing rubber composition was obtained in the same manner as in Example 1. The average fiber diameter of the stem portion of the aramid pulp in this water-containing rubber composition was 12 μm (12,000 nm) when observed at 500 times with a scanning electron microscope.
The obtained water-containing rubber composition was used to prepare a vulcanized rubber sheet in the same manner as in Example 1. The evaluation results are shown in Table 1.

Comparative Example 3
A rubber composition was obtained in the same manner as in Example 1, except that the water dispersion containing the aramid nanofibers and the latex of the styrene-butadiene rubber were mixed, and then the water was completely dried to obtain a water-containing rubber composition in which the ratio of the total amount of the aramid nanofibers and the rubber component to the water due to the equilibrium water content due to water absorption after drying was 99:1.

The average fiber diameter of the aramid nanofibers in the obtained rubber composition was 30 nm, the same as in Example 1, but it was observed using a scanning electron microscope that single fibers of the aramid nanofibers having an average fiber diameter of 30 nm were partially aggregated together.
The obtained water-containing rubber composition was used to prepare a vulcanized rubber sheet in the same manner as in Example 1. The evaluation results are shown in Table 1.

As shown in Table 1, the vulcanized rubber sheets made from the moisture-containing rubber compositions containing aramid nanofibers obtained in Examples 1 to 3 showed a higher improvement in rubber elasticity modulus and abrasion resistance than the vulcanized rubber sheet made from the moisture-containing rubber composition not containing aramid nanofibers in Comparative Example 1 and the vulcanized rubber sheet made from the moisture-containing rubber composition containing conventional aramid pulp in Comparative Example 2.

On the other hand, the vulcanized rubber sheet made from the water-containing rubber composition of Comparative Example 3, in which the water content of the water-containing rubber composition was insufficient and aggregation of the aramid nanofiber single fibers was observed, showed a smaller improvement in rubber elasticity and abrasion resistance than the vulcanized rubber sheet made from the water-containing rubber composition containing conventional aramid pulp of Comparative Example 2.

The water-containing rubber composition of the present invention can be used for tires, transmission belts, rubber hoses, rubber rolls, anti-vibration rubber, etc.

Claims (2)

A method for producing a rubber molded product, comprising: a first step of mixing an aramid nanofiber water dispersion and a rubber water dispersion in a solids weight ratio of aramid nanofiber to rubber of 1:99 to 35:65, and further removing a part of the water to obtain a water-containing rubber composition (provided that in the water-containing rubber composition, the weight ratio of the total of the aramid nanofibers and the rubber to the water is within a range of 10:90 to 30:70); a second step of mixing the water-containing rubber composition obtained in the first step with a solid rubber, and further removing the water to obtain a solid rubber composition; a third step of mixing the solid rubber composition obtained in the second step with a vulcanizing agent and/or a crosslinking agent to obtain an unvulcanized rubber composition and/or an uncrosslinked rubber composition; and a fourth step of vulcanizing and/or crosslinking the unvulcanized and/or uncrosslinked rubber composition obtained in the third step to obtain a rubber molded product. The method for producing a rubber molded article according to claim 1, wherein the rubber in the rubber-water dispersion is styrene-butadiene rubber.