JP7527122B2 - Rubber molded body and its manufacturing method - Google Patents

Description

The present invention relates to a rubber molded article obtained by crosslinking a rubber composition and a method for producing the same.

Most rubber products are manufactured by crosslinking and molding a rubber composition in which a crosslinking agent, filler, etc. are mixed with solid raw rubber. Sulfur crosslinking, which uses sulfur as a crosslinking agent, is the mainstream method for crosslinking rubber, and in order to advance crosslinking at a practical level, vulcanization accelerators and zinc oxide are also added in addition to sulfur. In order to mix and disperse the crosslinking agent and other ingredients in the raw rubber, kneading is required while applying high shear force at high temperatures using a Banbury mixer, kneader, etc. If the kneading is insufficient and the mixed materials are not dispersed properly, the physical properties of the rubber product will be affected. Among them, zinc oxide is known to be difficult to disperse due to its high coagulation tendency. For example, Patent Document 1 describes a method for improving the dispersibility of zinc oxide, in which a slurry solution containing fillers such as zinc oxide and carbon black is dispersed using a high shear mixer, then mixed with a rubber solution, coagulated, and dried. Patent Document 2 describes a method for mixing zinc oxide into a silica dispersion, adding rubber latex, and coagulating and drying.

JP 2006-213791 A JP 2011-256373 A JP 2014-105296 A JP 2004-27012 A

The objective of both of the methods described in Patent Documents 1 and 2 is to increase the dispersibility of zinc oxide (zinc white) so that it can function as an auxiliary accelerator for sulfur crosslinking. To solve this problem, zinc oxide is dispersed in a liquid beforehand and then mixed with rubber latex. However, the resulting latex is coagulated and dried to form a rubber composition, to which sulfur and other ingredients must be added, and then kneaded, just as in the past.

As mentioned above, when a rubber composition is kneaded, high shear forces are applied at high temperatures, which can cause the molecular chains of the rubber polymer to break and the shape of the rubber particles to become distorted, resulting in a deterioration of physical properties. For example, one of the characteristics of natural rubber is its large molecular weight, but if the polymer chains are broken by kneading, this advantage may not be fully realized. In addition, when the latex is coagulated with an acid or base, the shape of the rubber particles may become distorted.

The above-mentioned Patent Document 3 describes a method for manufacturing a rubber composition for tire base treads, which uses modified natural rubber with a reduced phosphorus content, thereby eliminating the need for mastication, which was previously performed to improve processability. However, according to the method described in Patent Document 3, natural rubber is saponified and alkali-treated, and then washed to modify it. When natural rubber is saponified and alkali-treated, the amount of protein contained in the natural rubber is reduced, which reduces the inherent functions of the natural rubber and affects the physical properties of rubber products manufactured from it. Since protein is water-soluble, it is also washed out by subsequent washing. Furthermore, according to the method described in Patent Document 3, mastication is not required, but kneading after adding sulfur or the like is still required, as in the past.

On the other hand, Patent Document 4 describes a dipping method in which a product mold is dipped into rubber latex containing a crosslinking agent and a water-soluble amino acid and then pulled out. With the dipping method, crosslinking can be achieved without kneading to obtain a product. However, the dipping method can only be applied to the manufacture of thin products such as gloves, and since the crosslinking agent and other substances tend to aggregate in the rubber latex, it is difficult to control the crosslinking and physical properties.

The present invention was made in consideration of these circumstances, and aims to provide a rubber molded article with excellent physical properties by utilizing the properties of rubber, and a method for producing the same.

(1) In order to solve the above problems, the rubber molded body of the present invention is a rubber molded body obtained by crosslinking a rubber composition containing a rubber polymer, zinc oxide, a vulcanization accelerator, and a sulfur-based vulcanizing agent, and is characterized in that, when a measurement range of 1 μm in length on a cross section of the rubber molded body is line-analyzed by an energy dispersive X-ray analyzer, the proportion of the area in which the zinc intensity ratio is 0.2 or less when the maximum intensity value is 1 is 20% or more and 95% or less of the measurement range.

(2) The method for producing a rubber molded article of the present invention is a suitable example of the method for producing a rubber molded article of the present invention having the configuration of (1) above, and is characterized by having a raw latex production process for producing a raw latex containing rubber latex, zinc oxide, a vulcanization accelerator, and a sulfur-based vulcanizing agent, a drying process for obtaining a rubber composition by spraying the raw latex onto a heated substrate surface and drying it, and a crosslinking process for crosslinking the rubber composition.

(1) The rubber molded article of the present invention is a molded article obtained by sulfur crosslinking a rubber composition. In the rubber molded article of the present invention, the presence of a portion where the zinc intensity ratio is 0.2 or less as a result of line analysis using an energy dispersive X-ray analyzer (hereinafter sometimes referred to as an "EDX device") means that the amount of zinc differs between the inside and outside of the rubber particle. This can also be said to include a portion where the crosslinked structure does not contain zinc oxide.

In general, sulfur crosslinking proceeds through the steps of (a) the step in which zinc oxide, an accelerating aid, coordinates with a vulcanization accelerator as a zinc cation to create a precursor, and (b) the step in which the precursor connects sulfur to a rubber polymer. Since these two steps (a) and (b) occur at almost the same place, it is believed that the presence of both sulfur and zinc oxide is essential for crosslinking. Therefore, a method has been adopted that improves the dispersibility of zinc oxide and actively contributes zinc oxide to crosslinking. In contrast, the inventors have found that crosslinking may proceed without the contribution of zinc oxide, and that in the part where crosslinking is performed without the contribution of zinc oxide, the crosslinked structure is rather strengthened, and the performance of the rubber molded body as a whole is improved. It is presumed that in the part where crosslinking is performed without the contribution of zinc oxide, the vulcanization accelerator ionizes without zinc, connecting the sulfur and the rubber polymer, and crosslinking proceeds.

The rubber molded article of the present invention has a portion in which zinc oxide is not included in the crosslinked structure, in other words, has a portion that is crosslinked without the contribution of zinc oxide, and therefore, as shown in the examples below, physical properties such as compression set are improved. In addition, since the dispersibility of zinc oxide is unlikely to affect the physical properties, kneading in the manufacturing process can be eliminated, or if it is performed, it can be simplified by shortening the time or lowering the degree of dispersion. This allows for simplification of the manufacturing process and cost reduction. In addition, since there is less breakage of the molecular chains of the rubber polymer and less deformation of the rubber particles due to kneading, the inherent properties of the rubber can be exhibited, leading to improved physical properties.

When a measurement range of 1 μm in length in the cross section of the rubber molded body of the present invention is line-analyzed by an EDX device, the zinc intensity ratio is 0.2 or less in a proportion of 20% to 95% of the measurement range. Here, the zinc intensity ratio is the ratio of the zinc intensity of the remaining part when the maximum value of the intensity of the characteristic X-ray (Kα ray having 8.63 keV energy) is set to 1. Hereinafter, a method for calculating the proportion of the part with a zinc intensity ratio of 0.2 or less in line analysis using an EDX device will be described. FIG. 1A shows a schematic cross-sectional view of the rubber molded body of the present invention. FIG. 1B shows a schematic diagram of the zinc distribution (roughness and density of concentration) in the rubber molded body of the present invention. FIG. 1C shows a schematic diagram of the zinc intensity obtained by line analysis using an EDX device.
(I) As shown in FIG. 1A, a cross section of a rubber molding is observed by a scanning electron microscope, and within a field of view of 5 μm×5 μm, a measurement range A-A′ with a length of 1 μm is arbitrarily determined, excluding the zinc oxide agglomerates.
(II) Using an EDX device, excitation is performed at an accelerating voltage of 200 kV, and the intensity of the emitted characteristic X-rays is subjected to line analysis. Line analysis is performed on the measurement range A-A', and the zinc distribution (roughness or density of concentration) is obtained from the distribution of the intensity of the zinc characteristic X-rays (Kα rays with an energy of 8.63 keV) as shown in Figure 1C. Since the concentration is proportional to the intensity, the zinc distribution (roughness or density of concentration) can be obtained from the intensity of the characteristic X-rays. The length of the horizontal axis of the obtained zinc distribution (the length between A-A') is defined as a.
(III) When the maximum intensity of the characteristic X-rays of zinc (Kα rays with energy of 8.63 keV) is taken as 1, an upper limit line is drawn at the point where the ratio (intensity ratio) of the intensity of zinc to the other parts is 0.2, and the total length of the horizontal axis of the parts where the intensity of zinc is below the upper limit line (b1 + b2 + b3) is calculated.
(IV) The proportion of the length of the horizontal axis between A-A' where the zinc intensity ratio is 0.2 or less is calculated using the formula "(b1+b2+b3)/a×100", and this value is the "proportion of the portion where the zinc intensity ratio is 0.2 or less".

(2) According to the method for producing a rubber molded body of the present invention, a raw latex is produced by blending a sulfur-based vulcanizing agent necessary for sulfur crosslinking with rubber latex, and then the raw latex is sprayed and dried. When the raw latex is sprayed, the specific surface area is larger and the latex dries more easily than when the raw latex is applied (solid coating). This shortens the drying time and suppresses deterioration of the rubber latex due to heat. In addition, in the raw latex, sulfur penetrates into the interior of the rubber particles, but zinc oxide does not penetrate easily. If the raw latex is quickly dried in this state and then crosslinked, it is believed that normal crosslinking with the contribution of zinc oxide will proceed near the surface of the rubber particles, and crosslinking without the contribution of zinc oxide will proceed inside the rubber particles. As a result, the rubber molded body of the present invention is produced, in which the crosslinking form differs between the inside and outside of the rubber particles.

According to the method for producing a rubber molded body of the present invention, the conventional kneading process is no longer necessary, and the production process can be simplified and costs can be reduced. For example, the obtained rubber composition can be directly melted and injection molded. If the process from the state of rubber latex to the blending of sulfur-based vulcanizing agents, drying, crosslinking and molding is carried out continuously, production efficiency can be improved. In addition, since kneading that applies high temperature and high shear force to the rubber composition is not performed, the molecular chain of the rubber polymer is less likely to be cut and the shape of the rubber particles is less likely to be distorted. Furthermore, since coagulation with an acid or base, which has been conventionally performed, is not required, the shape of the rubber particles is less likely to be distorted. If treatment with an acid or base is not performed, the ion balance does not change, so zinc oxide is likely to remain near the surface of the rubber particles. Thus, according to the method for producing a rubber molded body of the present invention, high-quality rubber products of various shapes can be produced by taking advantage of the properties of rubber.

1 is a schematic cross-sectional view of a rubber molded article of the present invention. FIG. 2 is a schematic diagram of zinc distribution (density/roughness of concentration) in the rubber molded article of the present invention. FIG. 1 is a schematic diagram of zinc intensity obtained by line analysis using an EDX device. 2 is a schematic diagram showing one embodiment of a drying step in the method for producing a rubber molded article of the present invention. FIG. 1 is an STEM image of a rubber molding of Example 1. 1 is a SEM image of the rubber molding. 1 is a sulfur mapping image of the rubber molding. 13 is a zinc mapping image of the rubber molding. This is the intensity of sulfur obtained by line analysis using an EDX device. This is the zinc intensity obtained by line analysis using an EDX device.

Next, an embodiment of the rubber molded body and its manufacturing method of the present invention will be described. Note that the rubber molded body and its manufacturing method of the present invention are not limited to the following embodiments, and can be embodied in various forms with modifications and improvements that can be made by those skilled in the art, as long as they do not deviate from the gist of the present invention.

<Rubber Molded Body>
The rubber molded article of the present invention is obtained by crosslinking a rubber composition having a rubber polymer, zinc oxide, a vulcanization accelerator, and a sulfur-based vulcanizing agent. The rubber polymer may be natural rubber or synthetic rubber. Examples of synthetic rubber include chloroprene, styrene-butadiene rubber, butadiene rubber, and nitrile rubber. The rubber composition may be a compound of solid materials or a liquid composition produced by drying. In the case of a liquid composition, a natural or synthetic rubber latex, a rubber solution in which a rubber polymer is dissolved or dispersed in a solvent, or the like may be used. From the viewpoint of being able to mix raw materials such as the rubber polymer and zinc oxide without kneading, the rubber composition is desirably a dried product obtained by drying a raw material latex having a latex-like rubber polymer, zinc oxide, a vulcanization accelerator, and a sulfur-based vulcanizing agent.

From the viewpoint of realizing a rubber molded body having a portion in which zinc oxide is not included in the crosslinked structure by partially allowing crosslinking to proceed without the contribution of zinc oxide, it is desirable for the zinc oxide to be present near the surface of the rubber polymer particles in the rubber composition. Also, from the viewpoint of promoting the progress of crosslinking, the content of zinc oxide is preferably 0.5 parts by mass or more, more preferably 2 parts by mass or more, per 100 parts by mass of rubber polymer. On the other hand, from the viewpoint of suppressing deterioration of physical properties and suppressing the generation of agglomerates, the content of zinc oxide is preferably 20 parts by mass or less, more preferably 10 parts by mass or less, per 100 parts by mass of rubber polymer.

Examples of vulcanization accelerators include thiazoles, sulfenamides, thiurams, dithiocarbamate salts, and guanidines. Examples of sulfur-based vulcanizing agents include powdered sulfur, precipitated sulfur, colloidal sulfur, surface-treated sulfur, insoluble sulfur, and sulfur compounds. Examples of sulfur compounds include sulfur chloride, sulfur disulfide, morpholine disulfide, alkylphenol disulfide, and polymer polysulfides. For example, when powdered sulfur is used, the content is set to 0.3 parts by mass or more, more preferably 0.7 parts by mass or more, per 100 parts by mass of rubber polymer, from the viewpoint of sufficiently progressing crosslinking. On the other hand, if the sulfur content is high, the number of crosslinking points increases and the crosslinking density increases, but the crosslinking points are easily broken by heat. For this reason, if the sulfur content is high, the heat resistance of the rubber molded body may decrease. Therefore, the content of powdered sulfur is set to 5 parts by mass or less, more preferably 3 parts by mass or less.

To impart desired physical properties to the rubber molded product, the rubber molded product (rubber composition) may contain fillers such as carbon black and silica. It may also contain antioxidants, plasticizers, colorants, etc.

The rubber molded article of the present invention has a novel crosslinked structure in that it has a portion that does not contain zinc oxide. As described above, in line analysis using an EDX device, the proportion of the portion where the zinc intensity ratio is 0.2 or less is 20% or more, and more preferably 30% or more. Conversely, this proportion is 95% or less, and more preferably 80% or less.

<Method of Manufacturing Rubber Molded Article>
The method for producing the rubber molded article of the present invention is not particularly limited. A rubber composition containing a solid material may be crosslinked, or a liquid composition in which raw materials such as zinc oxide are dispersed in a rubber latex or a rubber solution may be dried to produce a rubber composition. In the latter case, the drying method is not particularly limited, and may include oven drying, sun drying, spraying, etc. In either case, it is preferable that the zinc oxide is present near the surface of the rubber particles in the rubber composition. As a suitable production method, the method for producing the rubber molded article of the present invention is described below. The method for producing the rubber molded article of the present invention includes a raw material latex production step, a drying step, and a crosslinking step.

[Raw material latex manufacturing process]
This process is a process for producing a raw material latex containing rubber latex, zinc oxide, a vulcanization accelerator, and a sulfur-based vulcanizing agent.

The rubber component of the rubber latex may be natural or synthetic rubber. Examples of natural rubber latex include field latex collected by tapping, or latex treated with ammonia (high ammonia latex). Examples of synthetic rubber latex include latexes of chloroprene, styrene-butadiene rubber, butadiene rubber, and nitrile rubber. The rubber component concentration of the rubber latex (mass % of rubber polymer, the same applies below) is not particularly limited. If the rubber component concentration is too low, the amount of rubber composition obtained is small, which is not economical. On the other hand, if the rubber component concentration is too high, the rubber component in the latex becomes unstable and the rubber particles tend to aggregate. For example, in the case of natural rubber, it is desirable to set the rubber component concentration to 10 mass % or more and 60 mass % or less.

Conventionally, solid natural rubber is produced by coagulating natural rubber latex with acid, forming the coagulated product into a sheet while squeezing out the water, and then rinsing with water. In addition to rubber components, natural rubber contains non-rubber components such as proteins and lipids, but water-soluble components such as proteins flow out together with the water when stretching the product with a roll to form a sheet or when rinsing with water. This has led to a problem that the inherent functions of natural rubber are not fully exhibited. In this regard, the production method of the present invention allows natural rubber latex to be solidified as it is. In other words, there is no need for a process of coagulating the natural rubber latex, squeezing out the water from the coagulated product, and rinsing with water before the raw material latex production process. This makes it possible to suppress the outflow of water-soluble non-rubber components. As a result, it is possible to produce rubber molded products with excellent physical properties such as strength by utilizing the crosslinking-promoting effect and reinforcing effect of the proteins originally contained in natural rubber.

The zinc oxide, vulcanization accelerator, and sulfur-based vulcanizing agent are as described above in the rubber molded body of the present invention. The raw latex may contain fillers such as carbon black and silica, antioxidants, plasticizers, colorants, etc. Furthermore, in order to improve the dispersibility of the raw materials, it is desirable for the raw latex to contain a surfactant. The surfactant is not particularly limited and may be selected from anionic surfactants, cationic surfactants, amphoteric surfactants, and nonionic surfactants. In order to fully exert the effect of improving the dispersibility of the raw materials, the content of the surfactant is preferably 0.1 parts by mass or more, more preferably 0.3 parts by mass or more, per 100 parts by mass of the rubber polymer in the latex. On the other hand, since a high content of the surfactant leads to a decrease in strength after crosslinking, the content of the surfactant is preferably 2 parts by mass or less, more preferably 1 part by mass or less, per 100 parts by mass of the rubber polymer.

The method for producing the raw material latex is not particularly limited. For example, it may be produced by a step of producing a crosslinking raw material dispersion in which zinc oxide, a vulcanization accelerator, and a sulfur-based vulcanizing agent are dispersed in water, and a step of adding the crosslinking raw material dispersion to the rubber latex. Instead of directly adding a crosslinking raw material such as a sulfur-based vulcanizing agent to the rubber latex, the crosslinking raw material can be highly dispersed in the rubber latex by adding a crosslinking raw material dispersion that has been previously dispersed in water to the rubber latex. The crosslinking raw material dispersion may be produced by mixing water and the crosslinking raw material using a ball mill, homogenizer, ultrasonic disperser, or the like. If a surfactant is to be added, it may be added at this stage. After the produced crosslinking raw material dispersion is added to the rubber latex, it may be appropriately stirred using a stirrer or the like.

[Drying process]
In this step, the raw material latex obtained in the previous step is sprayed onto a heated substrate surface, followed by drying to obtain a rubber composition.

The method of spraying the raw latex is not particularly limited, but for example, when spraying with a sprayer, the nozzle diameter is too small, so the nozzle may become clogged or the rubber latex and the crosslinking raw material may easily separate. For this reason, it is desirable to spray the raw latex so that the raw latex adheres to the substrate surface in dots, in other words, in droplets of a predetermined size. This makes it difficult for the nozzle to become clogged, and allows the rubber latex to be dried while maintaining a state in which the crosslinking raw material is uniformly dispersed. In addition, scattering of the raw latex is suppressed, and the yield is improved. For example, the inner diameter of the nozzle used for spraying is preferably 0.5 mm or more and 1 mm or more from the viewpoint of suppressing clogging of the raw latex. In addition, it is preferable for the inner diameter to be 5 mm or less and 3 mm or less from the viewpoint of increasing the drying efficiency by adhering the raw latex in dots.

The shape of the substrate onto which the raw latex is sprayed is not particularly limited. For example, a rotating member such as a drum may be used as the substrate. In this case, the raw latex is sprayed onto the heated endless annular surface of the rotating member (for example, the outer peripheral surface of a drum), the endless annular surface is rotated while drying, and the resulting rubber composition is peeled off in sequence. This makes it possible to automate the series of steps of spraying the raw latex, drying, and peeling off the rubber composition, and significantly improves productivity.

The temperature of the substrate surface should be such that the sprayed raw latex can be dried in a practical time. Considering the thermal degradation of the raw latex, it is desirable that the drying time be 0.1 minutes or more and 5 minutes or less. On the other hand, if the temperature of the substrate surface is too high, the attached raw latex may be overheated and may be thermally deteriorated. Taking these factors into consideration, it is desirable that the temperature of the substrate surface be in the range of 120°C or more and 200°C or less.

As one embodiment of this process, a drying method using a drying device equipped with a rotating drum will be described below. It goes without saying that this process is not limited to the drying method described below. Figure 2 shows a schematic diagram of the drying process of this embodiment. Figure 2 corresponds to a cross-sectional view of the drying device as seen from the side.

As shown in FIG. 2, the drying device 1 includes a drum 10, a rotating shaft 11, a heating device 20, a spray nozzle 30, a scraper 40, and a discharge hopper 50. The drum 10 is made of steel and has a hollow cylindrical shape that is long from the front to the back (left and right) of the page. The rotating shaft 11 is fixed to the center of both the left and right walls of the drum 10. Therefore, as shown by the white arrow in FIG. 2, the drum 10 can rotate clockwise around the rotating shaft 11.

The heating device 20 is disposed close to the outer peripheral surface of the upper front side of the drum 10. The heating device 20 extends in the left-right direction with approximately the same length as the drum 10. The heating device 20 is equipped with a coil (not shown) connected to an AC power source. When electricity is applied to the coil, high-density eddy currents are generated on the outer peripheral surface of the upper front side of the drum 10. The eddy currents generate Joule heat, which heats the outer peripheral surface of the upper front side of the drum 10. In this way, the heating device 20 heats the outer peripheral surface of the drum 10.

The spray nozzle 30 is disposed above the heating device 20 and close to the outer peripheral surface of the drum 10. The spray nozzle 30 can move back and forth along the length of the drum 10 (the left-right direction intersecting the rotation direction of the drum 10). Raw latex is supplied to the spray nozzle 30 via piping (not shown). Droplets G1 of the raw latex are sprayed from the spray nozzle 30, and the droplets G1 adhere to the outer peripheral surface of the drum 10 in the form of dots. A coating film is formed on the outer peripheral surface of the drum 10 by spraying the droplets G1 while moving the spray nozzle 30 back and forth along the length of the drum 10.

The scraper 40 is positioned close to the outer peripheral surface of the drum 10 near the front center. The scraper 40 is made of steel and has a thin plate shape that extends in the length direction of the drum 10. The lower end of the scraper 40 is in elastic contact with the outer peripheral surface of the drum 10.

The discharge hopper 50 is made of steel and has a bottomed, angular cylindrical shape that is tapered downward. The discharge hopper 50 is located below the front side of the drum 10.

The rotation direction of the drum 10 is defined based on the starting point A where the droplets G1 adhere to the drum 10. That is, the clockwise direction from the starting point A in FIG. 2 is defined as the "rotation direction". First, the drum 10 is rotated clockwise (front → up → back → down → back again) around the rotating shaft 11. The rotation speed is 1 rpm (one rotation per minute). Next, the coil of the heating device 20 is energized to heat the outer peripheral surface of the upper front side of the drum 10. By rotating the drum 10 in this state, the entire outer peripheral surface of the drum 10 is heated. Then, when the outer peripheral surface of the drum 10 reaches 130°C, the spray nozzle 30 is repeatedly moved back and forth in the left and right directions to spray droplets G1 of the raw latex from the spray nozzle 30 onto the outer peripheral surface of the drum 10. The droplets G1 adhere to the outer peripheral surface of the drum 10 in the form of dots. The droplets G1 adhere to the outer peripheral surface of the drum 10 in an overlapping manner, forming a coating film on the outer peripheral surface of the drum 10. The droplets G1 dry and bond together as the drum 10 rotates, forming a sheet G2 (rubber composition).

When the drum 10 has rotated about 3/4 of a turn, the sheet G2 is peeled off from the outer circumferential surface of the drum 10 by the scraper 40. The peeled sheet G2 falls under its own weight and is stored in the discharge hopper 50. The outer circumferential surface of the drum 10 is then heated again to a predetermined temperature by the heating device 20. In this manner, a series of steps is repeated: spraying of the droplets G1 of the raw latex → drying → peeling of the sheet G2. The thickness of the obtained sheet G2 is about 0.1 to 1 mm. In addition, because the heating time is short, just under 1 minute, the sheet G2 is not easily deteriorated by heat.

When the rubber composition obtained in this process is used, there is no need to knead the rubber composition before the next crosslinking process. In other words, according to the method for producing a rubber molded article of the present invention, the kneading process of the rubber composition can be omitted. However, this does not exclude the possibility of adding additional kneading to improve the dispersibility of fillers such as carbon black and silica that are difficult to disperse in water.

[Crosslinking process]
This step is a step of crosslinking the rubber composition obtained in the previous step. The rubber composition may be crosslinked after being molded into a desired shape, or may be crosslinked simultaneously with molding by compression molding, injection molding, etc. For example, in the case of injection molding, the dried rubber composition may be melted once and then rapidly filled into a mold.

The crosslinking method may be appropriately selected taking into consideration the type of rubber and the desired physical properties. For example, the rubber composition may be crosslinked by holding it at a specified temperature for a specified time. Alternatively, the rubber composition may be crosslinked by irradiating it with an electron beam. When crosslinking is performed by heating, it is preferable to perform the crosslinking at a temperature of 120°C or higher. Conversely, from the viewpoint of suppressing deterioration of the rubber composition due to heating, the upper limit of the crosslinking temperature should be set to 180°C. The heating time should be about 1 to 30 minutes, taking into consideration the thermal deterioration of the rubber composition.

Next, the present invention will be explained in more detail with reference to examples.

<Production of rubber molded body by non-kneading method>
A raw material latex was produced in the compounding ratio shown in Table 1 below, and then dried to obtain a rubber composition, which was then crosslinked without kneading to produce a rubber molded article. The compounding ratio of the raw materials shown in Table 1 is in parts by mass relative to 100 parts by mass of the rubber polymer (solid content of natural rubber latex).

First, powdered sulfur (Tsurumi Chemical Industry Co., Ltd.'s "Sulfax (registered trademark) T-10"), two types of zinc oxide (Sakai Chemical Industry Co., Ltd.'s "Sakai Chemical Industry Co., Ltd."), a vulcanization accelerator (Sansera (registered trademark) CZ-G" by Sanshin Chemical Industry Co., Ltd.), and a surfactant (Kao Corporation's "Demol (registered trademark) NL") were added to pure water and mixed in a ball mill to produce a crosslinked raw material dispersion. At this time, carbon black was also added in Example 3, and silica was also added in Example 4 to produce a crosslinked raw material dispersion. Next, the natural rubber latex was diluted so that the rubber component concentration was 30 mass%. As the natural rubber latex, field latex extracted by tapping from Hevea brasiliensis was used to which 0.5 mass% ammonia was added. Then, the crosslinked raw material dispersion was added to the diluted natural rubber latex, and the mixture was stirred using a stirrer to produce raw material latex (raw material latex production process).

The raw latex was then sprayed onto the outer periphery of the rotating drum with a spray nozzle and dried (see Figure 2 above). The outer periphery of the drum was heated to about 130°C, and the drum was rotated at 1 rpm. The inner diameter of the spray nozzle was 2 mm, and the spray flow rate was 100 ml/min. The sprayed droplets of raw latex adhered to the outer periphery of the drum in the form of dots, and were solidified into a sheet by bonding together. When the drum had rotated about 3/4 times (drying time 45 seconds), the sheet-like rubber composition was peeled off from the outer periphery of the drum (drying process).

The obtained rubber composition was crosslinked under conditions of 150°C x 20 minutes to produce a rubber molded body (crosslinking step). The rubber molded bodies produced in this manner are referred to as the rubber molded bodies of Examples 1 to 4. The rubber molded bodies of Examples 1 to 4 are included in the concept of the rubber molded body of the present invention.

<Production of rubber molded body by kneading process>
A rubber molded body was produced by a conventional method involving kneading. First, the natural rubber latex (rubber component concentration 30% by mass) used in the non-kneading method was sprayed onto the outer peripheral surface of a rotating drum with a spray nozzle and dried in the same manner as in the drying process in the non-kneading method to produce a rubber sheet. Next, powdered sulfur (same as above), two types of zinc oxide (same as above), a vulcanization accelerator (same as above), and a surfactant (same as above) were added to the produced rubber sheet, and the mixture was kneaded using a roll to produce a rubber composition. At this time, carbon black was also added to Comparative Example 3, and silica was also added to Comparative Example 4 to produce a rubber composition. The compounding ratio of each raw material (parts by mass relative to 100 parts by mass of rubber polymer) is shown in Table 1 below. Then, the produced rubber composition was crosslinked under the conditions of 150°C x 20 minutes to produce a rubber molded body. The rubber molded bodies obtained in this manner are referred to as rubber molded bodies of Comparative Examples 1 to 4.

<Microscopic Observation and EDX Analysis of Rubber Molded Body>
The cross section of the produced rubber molded product was observed with a STEM (scanning transmission electron microscope: JEM-2800 manufactured by JEOL Ltd. The acceleration voltage for STEM observation was 200 kV. As an example, FIG. 3 shows a STEM image of the rubber molded product of Example 1. The image in FIG. 3 is a bright field image with a magnification of 100,000 times. In addition, in order to confirm the distribution of sulfur and zinc in the rubber molded product, the cross section of the rubber molded product was observed with a scanning electron microscope (S-3400N manufactured by Hitachi High-Technologies Corporation). Elemental mapping was performed using an EDX device (EX250 manufactured by Horiba, Ltd.). As an example, FIG. 4 shows an SEM image of the rubber molded body of Example 1. FIG. 5 shows a sulfur mapping image of the same rubber molded body. FIG. 6 shows a zinc mapping image of the same rubber molded body. As shown in FIGS. 5 and 6, sulfur penetrates into the interior of the natural rubber particles and is present almost uniformly. In contrast, zinc is present in large amounts near the surfaces of the natural rubber particles and is almost absent inside.

Furthermore, a line of 1 μm in length in a 5 μm × 5 μm field of view observed by a scanning electron microscope was used as the measurement range, and line analysis was performed using an EDX device (zinc is Kα radiation with 8.63 keV energy, sulfur is Kα radiation with 2.31 keV energy). In the above-mentioned Figures 4 to 6, the lines of the measurement range are shown surrounded by a square. The line analysis was performed starting from the lower end of the line. Figure 7 shows the sulfur intensity obtained by line analysis. Figure 8 shows the zinc intensity obtained by line analysis. Table 1 shows the zinc intensity ratio and the proportion of the part where the zinc intensity ratio is 0.2 or less in each rubber molded body, together with the compounding ratio of the raw material. The "zinc intensity ratio in the rubber molded body" shown in Table 1 is the value obtained by dividing the minimum value of the zinc intensity by the maximum value (minimum value/maximum value).

As shown in Figure 7, in the rubber molding of Example 1, the variation in sulfur intensity depending on the measurement position was small. From this result and the sulfur mapping image in Figure 5 above, it can be seen that sulfur contributes to the formation of cross-linked structures regardless of whether it is inside or outside the natural rubber particle. In contrast, as shown in Figure 8, the zinc intensity varied greatly depending on the measurement position, and as shown in Table 1, the proportion of areas where the zinc intensity ratio was 0.2 or less accounted for 45% of the measurement range. The zinc intensity ratio differed by a maximum of 10 times. From this result and the zinc mapping image in Figure 6 above, it can be seen that zinc hardly contributes to the formation of cross-linked structures inside the natural rubber particle.

In the rubber molded bodies of Examples 2 to 4, the zinc strength also varied greatly depending on the measurement position, with the proportion of areas where the zinc strength ratio was 0.2 or less accounting for more than 40% of the measurement range. The zinc strength ratio differed by a maximum of 10 times in Examples 3 and 4, and a maximum of 25 times in Example 2. On the other hand, in the rubber molded bodies of Comparative Examples 1 to 4, which were manufactured using a kneading method, there was no significant difference in zinc strength depending on the measurement position, and there were no areas where the zinc strength ratio was 0.2 or less.

<Physical Properties of Rubber Molded Product>
First, the scorch time of the rubber composition before crosslinking was measured for each of the manufacturing methods with and without kneading. Next, the 100% elongation tensile stress (100% modulus), elongation at break, and compression set of the manufactured rubber molded body were measured. The measurement results of scorch time and compression set that had a significant effect are summarized in Table 1 above. The measurement method is as follows.

[Scorch Time]
The scorch time (T ₉₀ ) of the rubber composition was measured in accordance with the Mooney scorch test specified in JIS K 6300-1: 2013. For the measurement, a Mooney viscometer manufactured by Ueshima Seisakusho Co., Ltd. was used.

[100% modulus, elongation at break]
A dumbbell-shaped No. 5 test piece as defined in JIS K 6251:2017 was prepared from the rubber molded body, and a tensile test as defined in the same JIS was carried out to calculate the 100% modulus and the elongation at break.

[Compression set]
A large test piece as specified in JIS K 6262:2013 was prepared from the rubber molded body, and a compression device as specified in the same JIS was used to apply strain to the test piece at a temperature of 85°C for 72 hours, and the compression set was calculated.

As shown in Table 1, when comparing rubber compositions with the same composition, the scorch time of the rubber compositions was shorter in Examples 1 to 4, which were produced using the non-kneading method. In other words, crosslinking proceeded faster in Examples 1 to 4. The compression set was also smaller in Examples 1 to 4, which were produced using the non-kneading method.

The rubber molded article of the present invention has a novel crosslinked structure and excellent physical properties such as compression set, making it useful as a variety of rubber products. The method for producing a rubber molded article of the present invention makes it possible to efficiently produce high-quality rubber products without kneading. The method for producing a rubber molded article of the present invention is useful as a method for producing industrially mass-produced rubber products, especially rubber products using natural rubber.

1: Drying device, 10: Drum, 11: Rotating shaft, 20: Heating device, 30: Spray nozzle, 40: Scraper, 50: Discharge hopper, G1: Droplet, G2: Sheet

Claims (7)

A rubber molded article obtained by crosslinking a rubber composition having a rubber polymer, zinc oxide, a vulcanization accelerator, and a sulfur-based vulcanizing agent,
When a measurement range of any 1 μm length extending linearly from the outside of a rubber particle toward the center in a cross section of the rubber molded product is subjected to line analysis using an energy dispersive X-ray analyzer, the ratio of the portion in which the zinc intensity ratio is 0.2 or less when the maximum intensity value is 1 is 20% or more and 95% or less of the measurement range,
A rubber molded article characterized in that sulfur is uniformly present inside the rubber particles . The rubber molded article according to claim 1, wherein the rubber composition is a dried raw material latex containing the rubber polymer in a latex form, the zinc oxide, the vulcanization accelerator, and the sulfur-based vulcanizing agent. The rubber molded article according to claim 1 or 2, wherein the zinc oxide is present near the surface of the rubber polymer particles in the rubber composition. The rubber molded article according to any one of claims 1 to 3, wherein the zinc oxide content is 0.5 parts by mass or more and 20 parts by mass or less per 100 parts by mass of the rubber polymer. The rubber molded article according to any one of claims 1 to 4, wherein the rubber composition further comprises a filler. A method for producing the rubber molded article according to claim 1, comprising the steps of:
A raw latex production process for producing a raw latex having rubber latex, zinc oxide, a vulcanization accelerator, and a sulfur-based vulcanizing agent;
a drying step of spraying the raw material latex onto a surface of a substrate heated to a temperature of 120° C. or more and 200° C. or less using a nozzle having an inner diameter of 0.5 mm or more and 5 mm or less, and drying the raw material latex by keeping it with the heat of the substrate for 0.1 minute or more and 5 minutes or less to obtain a rubber composition;
a crosslinking step of crosslinking the rubber composition by holding the rubber composition at a temperature of 120° C. or more and 180° C. or less for 1 minute or more and 30 minutes or less ;
The method for producing a rubber molded article comprising the steps of: The method for producing a rubber molded article according to claim 6, wherein the zinc oxide is present near the surface of the rubber particles in the rubber composition.

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