JP7758565B2 - Natural rubber latex, natural rubber, and their manufacturing method - Google Patents

Description

This disclosure relates to natural rubber latex and natural rubber with an adjusted content of non-rubber components, and methods for producing the same.

Natural rubber has excellent properties, such as high tensile strength and low heat generation due to vibration, and is therefore used in a variety of rubber products, including tires, vibration-proof rubber, and belts. In addition to rubber components, natural rubber also contains non-rubber components such as proteins, lipids, and metals, and these non-rubber components are known to affect the processability of rubber compositions and the properties of the resulting rubber products. For this reason, processing methods have been proposed to reduce the non-rubber components that affect these properties, so that the desired properties can be achieved for each rubber product.

For example, Patent Document 1 describes a method for obtaining natural rubber that combines heat aging resistance and low heat buildup by removing proteins from natural rubber latex using mechanical means such as centrifugation to reduce the nitrogen content in the rubber to more than 0.1% by mass but not more than 0.4% by mass. Patent Document 2 describes a method for obtaining natural rubber with low viscosity and easy processing by adding a water-soluble ammonium salt to natural rubber latex to remove metals such as potassium and magnesium as metal salts. Patent Document 3 describes a method for obtaining natural rubber with improved heat aging resistance by adding a chelating agent to natural rubber latex to transfer metal ions to the aqueous phase, thereby reducing the manganese ion concentration in the rubber to 1 ppm by mass or less, the iron ion concentration to 25 ppm by mass or less, and the copper ion concentration to 1 ppm by mass or less. Patent Document 4 describes a method for obtaining natural rubber with a reduced loss tangent tanδ, in which natural rubber latex is saponified with an alkali and then washed to reduce the phosphorus content in the rubber to 200 ppm or less, the total content of sodium and potassium to 350 ppm or less, and the nitrogen content to 0.3 mass% or less.

Japanese Patent Application Laid-Open No. 2004-262973 Japanese Patent Application Laid-Open No. 2007-146114 JP 2013-185095 A JP 2012-180530 A

The components of natural rubber vary greatly due to their natural origin. Research by the inventors has confirmed that variations in the content of non-rubber components result in significant variations in the properties of manufactured rubber products. Furthermore, it has been confirmed that variations in the content of nitrogen, magnesium, and potassium, among other non-rubber components, have a significant impact on the hardness and scorch resistance of natural rubber. Since variations in the hardness and scorch resistance of natural rubber directly affect the properties of rubber products, it is desirable to minimize them. For example, vibration-damping rubber requires strength to support heavy objects such as engines and vibration-damping performance to absorb and suppress vibrations. One characteristic that indicates vibration-damping performance is the dynamic magnification ratio. The dynamic magnification ratio is the ratio of the dynamic spring constant to the static spring constant (dynamic spring constant (Kd) / static spring constant (Ks)). The smaller the dynamic magnification ratio, the higher the vibration-damping performance. Therefore, spring properties such as hardness and static spring constant are important for vibration-damping rubber.

However, even with conventional methods, it is difficult to simultaneously achieve the desired nitrogen, magnesium, and potassium content of non-rubber components. Furthermore, conventional methods have the following problems. The centrifugation method described in Patent Document 1 requires multiple concentration steps and further dilution of the concentrated solids, resulting in complex and costly processes. Furthermore, performing the process under conditions that reduce the magnesium content results in excessively low nitrogen and potassium content. The water-soluble ammonium salt addition method described in Patent Document 2 requires post-treatment removal of the precipitate and centrifugation, which may result in a reduced yield. The chelating agent addition method described in Patent Document 3 adds a chelating agent to natural rubber latex to transfer metal ions in the rubber to the aqueous phase. Formic acid or other suitable acid is then added, followed by coagulation, washing, and drying to obtain solid natural rubber. As a result, the natural rubber latex contains the chelating agent, which may remain in the resulting natural rubber. The saponification process described in Patent Document 4 uses alkalis such as sodium hydroxide and potassium hydroxide, necessitating careful washing of the coagulated rubber to remove residual sodium and potassium.

The present disclosure has been made in light of these circumstances, and aims to provide natural rubber latex, natural rubber, and methods for producing these that can be used to produce rubber products with little variation in properties such as hardness.

(1) In order to solve the above problem, the natural rubber latex of the present disclosure is characterized in that the nitrogen content in the solid content is 0.3% by mass or more and 0.6% by mass or less, the magnesium content is 0% by mass or more and 0.02% by mass or less, and the potassium content is 0.1% by mass or more and 0.3% by mass or less.

(2) The natural rubber disclosed herein is characterized by a nitrogen content of 0.3% by mass or more and 0.6% by mass or less, a magnesium content of 0% by mass or more and 0.02% by mass or less, and a potassium content of 0.1% by mass or more and 0.3% by mass or less.

(3) The method for producing natural rubber latex disclosed herein is a method for producing natural rubber latex having the configuration described in (1) above, characterized by including a treatment step of contacting natural rubber latex with a treating agent capable of capturing at least nitrogen components, magnesium ions, magnesium compounds, potassium ions, and potassium compounds.

(4) The method for producing natural rubber disclosed herein is a method for producing natural rubber as described in (2) above, characterized by comprising a treatment step of contacting natural rubber latex with a treating agent capable of capturing at least nitrogen components, magnesium ions, magnesium compounds, potassium ions, and potassium compounds, and a drying step of drying the treated latex.

(1) In the natural rubber latex of the present disclosure, the content of nitrogen (N), magnesium (Mg), and potassium (K) in the solids is reduced, with each content falling within a specified range. Therefore, the natural rubber latex of the present disclosure produces natural rubber with N, Mg, and K each within a specified range. The resulting natural rubber can then be used to produce rubber products with good properties, such as hardness and scorch resistance, and little variation in properties. As a result, the quality of rubber products can be improved and made uniform.

(2) The natural rubber of the present disclosure has reduced contents of N, Mg, and K, each of which falls within a specified range. Therefore, by using the natural rubber of the present disclosure, it is possible to produce rubber products with good properties such as hardness and scorch resistance, and with little variation in properties. As a result, it is possible to improve and standardize the quality of rubber products.

(3) In the method for producing natural rubber latex disclosed herein, natural rubber latex is brought into contact with a predetermined treating agent, which incorporates the nitrogen, magnesium ions, magnesium compounds, potassium ions, and potassium compounds contained in the latex. This reduces the N, Mg, and K contents in the solids (rubber content) to within a predetermined range. According to the method for producing natural rubber latex disclosed herein, the N, Mg, and K contents in the solids can be reduced and their variability minimized by the simple method of bringing natural rubber latex into contact with a predetermined treating agent. For example, even when a treating agent is added to natural rubber latex, the effects of the treating agent can be minimized by removing the treating agent from the treated latex.

(4) In the disclosed method for producing natural rubber, natural rubber with reduced N, Mg, and K contents and little variation can be easily produced by the simple method of contacting natural rubber latex with a specified treating agent and then drying the treated latex. Furthermore, in the disclosed method for producing natural rubber, there is no need to coagulate the treated latex by adding an acid, for example. For example, even if a treating agent is added to natural rubber latex, the treating agent can be removed from the treated latex to prevent it from remaining in the resulting natural rubber.

The following describes embodiments of the disclosed natural rubber latex, natural rubber, and methods for producing the same. Note that the embodiments are not limited to the following, and various modifications and improvements within the skill of those in the art are possible.

<Natural rubber latex>
In the natural rubber latex of the present disclosure, as will be described later in the manufacturing method, the raw natural rubber latex is contacted with a predetermined treating agent, thereby ensuring that the nitrogen, magnesium, and potassium contents in the solids are within predetermined ranges. Examples of the raw natural rubber latex that can be used include field latex extracted by tapping and latex treated with ammonia (high ammonia latex). The solids (rubber content) concentration of the natural rubber latex is not particularly limited. If the solids concentration is too low, less rubber is obtained after drying, which is uneconomical. Conversely, if the solids concentration is too high, the rubber particles in the latex are more likely to aggregate, resulting in instability. For example, the solids concentration of the natural rubber latex of the present disclosure is desirably 10% by mass or more and 60% by mass or less.

The nitrogen content in the solids of the natural rubber latex of the present disclosure is 0.3% by mass or more and 0.6% by mass or less. The nitrogen content is mainly derived from proteins. Proteins in natural rubber play a role in increasing the crosslinking rate and crosslinking density when natural rubber is sulfur-crosslinked. Therefore, if the nitrogen content in the solids is less than 0.3% by mass, the protein content will be too low, which may reduce the crosslinking rate and crosslinking density and result in reduced tensile strength in the crosslinked product (rubber product). Conversely, if the nitrogen content is greater than 0.6% by mass, the rubber product may be more susceptible to scorching, or may have reduced sag resistance and elongation at break. Furthermore, the dynamic spring constant of the rubber product may increase, which may reduce the vibration-damping performance of the rubber product.

The magnesium content in the solids of the natural rubber latex of the present disclosure is 0% by mass or more and 0.02% by mass or less. Magnesium in the solids can cause a decrease in the crosslinking rate, a decrease in the plasticity residual index (PRI), which is an indicator of aging resistance, and an increase in the gel content that is insoluble in good solvents. Therefore, if the magnesium content in the solids is greater than 0.02% by mass, the static spring constant of the rubber product may decrease, and there is a risk of reduced hardness, vibration-damping performance, and aging resistance. A low magnesium content in the solids is preferable, and 0% by mass is even acceptable. In other words, the solids may not contain magnesium.

The potassium content in the solids of the natural rubber latex of the present disclosure is 0.1% by mass or more and 0.3% by mass or less. The potassium content in the solids affects the crosslinking rate, scorch resistance, etc. If the potassium content in the solids is less than 0.1% by mass, the crosslinking rate will decrease and the vulcanization induction time will be longer. Conversely, if the potassium content is greater than 0.3% by mass, the vulcanization induction time will be shorter.

<Natural rubber>
The natural rubber of the present disclosure is a solid natural rubber obtained by drying the natural rubber latex of the present disclosure. The natural rubber of the present disclosure has a nitrogen content of 0.3% by mass or more and 0.6% by mass or less, a magnesium content of 0% by mass or more and 0.02% by mass or less, and a potassium content of 0.1% by mass or more and 0.3% by mass or less. As will be explained later in the production method, the method for drying the natural rubber latex is not particularly limited. The natural rubber latex may be directly dried without coagulation using an acid.

For example, when producing rubber products using the natural rubber of the present disclosure, a rubber composition can be prepared by blending the natural rubber with a crosslinking agent, vulcanization accelerator, zinc oxide, processing aid, reinforcing material, antioxidant, softener, etc., and then crosslinking the resulting rubber composition. Because the natural rubber of the present disclosure has little variation in properties such as hardness, it is suitable as a material for vehicle parts such as engine mounts and suspension bushings, as well as vibration-proof rubber used in the construction and housing fields.

<Method for producing natural rubber latex>
The method for producing natural rubber latex according to the present disclosure includes a treatment step of contacting natural rubber latex with a treating agent capable of capturing at least nitrogen components, magnesium ions, magnesium compounds, potassium ions, and potassium compounds.

As mentioned above, natural rubber latex, such as field latex or high ammonia latex, can be used as the raw material. The treating agent can be any agent capable of capturing at least nitrogen, magnesium ions, magnesium compounds, potassium ions, and potassium compounds; for example, adsorbents and chelating agents are preferred because they are readily available and easy to handle. When using one or more agents selected from adsorbents and chelating agents, either one of the adsorbents or chelating agents can be used, or both can be used in combination.

Adsorbents include organic ion exchangers, inorganic ion exchangers, synthetic adsorbents, and protein adsorbents. Among these, known organic ion exchangers include ion exchange resins in which ion-exchange functional groups are chemically bonded to polymers, and alginic acid. Examples of ion exchange resins include the "Diaion (registered trademark) series" manufactured by Mitsubishi Chemical Corporation and the "Amberlite (registered trademark) series" sold by Organo Corporation. Examples of inorganic ion exchangers include hydrous metal oxides and their composite oxides, such as zeolites and hydrotalcites. Examples of inorganic ion exchangers include the "IXE (registered trademark) series" manufactured by Toa Gosei Co., Ltd. Synthetic adsorbents are synthetic polymers with a macromechanical network structure that combines a porous structure with a cross-linked structure that is insoluble in solvents. Examples of synthetic adsorbents include spherical particles made of styrene-based resins, acrylic resins, and other resins. Examples of synthetic adsorbents that can be used include the "Diaion (registered trademark) HP series" manufactured by Mitsubishi Chemical Corporation and the "Amberlite (registered trademark) XAD (registered trademark) series" sold by Organo Corporation.

Chelating agents include chelating resins and chelating fibers. Chelating resins are styrene-based resins or the like that have chelating functional groups introduced into them that chelate and adsorb specific ions. Examples of such resins include the "DUOLITE® series" sold by Sumitomo Chemtex Co., Ltd., the "AMBERSEP® series" sold by Organo Corporation, and the "KIRESPEARL® series" manufactured by Chelesto Co., Ltd. Chelating fibers are cellulose fibers or the like that have chelating agents chemically bonded to them. Examples of such fibers include the "CHELESTOCK® series" manufactured by Chelesto Co., Ltd., and the "KANECARON® series" manufactured by Kaneka Corporation. Examples of chelating functional groups and chelating agents include aminopolycarboxylic acids, iminodiacetic acid, and aminomethylphosphate.

There are no particular limitations on the method for contacting natural rubber latex with the treating agent. For example, the treating agent may be added to the natural rubber latex, or the natural rubber latex may be passed through a column packed with the treating agent. The temperature during contact may be room temperature, preferably 10°C or higher and 40°C or lower. In the method of adding a treating agent, it is desirable to maintain the state in which the treating agent has been added for a predetermined period of time to allow the natural rubber latex and the treating agent to react. The maintenance period should be at least 1 hour, or at least 3 hours, and, considering productivity, should be 48 hours or less, 30 hours or less, or even 24 hours or less. During this period, the mixture may be left to stand, or may be shaken, stirred, or the like to promote the reaction between the natural rubber latex and the treating agent.

When adding a treating agent, the amount of treating agent added can be determined appropriately depending on the type of agent so that the trapping effect is achieved. For example, the amount of treating agent added is preferably 20 parts by mass or more, and even 30 parts by mass or more, per 100 parts by mass of the solids content of natural rubber latex. However, if the amount of treating agent added is too large, the trapping effect cannot be expected to improve. For this reason, the amount of treating agent added is preferably 85 parts by mass or less, and even 75 parts by mass or less, per 100 parts by mass of the solids content of natural rubber latex.

A suitable method for producing the natural rubber latex disclosed herein includes a treatment step in which a treating agent is added to natural rubber latex, followed by a treatment agent removal step in which the latex to which the treating agent has been added is filtered to separate the treating agent. By filtering the treating agent, the treating agent can be removed from the natural rubber latex, minimizing its impact on the solids. The filtration method is not particularly limited as long as it can remove the treating agent; for example, a filter such as a mesh (wire screen) can be used. This method reduces the nitrogen, magnesium, and potassium content in the solids and minimizes their variability, without leaving any residual treating agent, through the simple process of adding the treating agent and then filtering it out.

In addition, from the perspective of suppressing aggregation and coagulation of natural rubber latex when it is brought into contact with a treatment agent, a suitable method for producing the natural rubber latex of the present disclosure includes a pH adjustment step in which a pH adjuster is added to the natural rubber latex to adjust the pH to 9 or higher prior to the treatment step. Examples of pH adjusters that can be used include ammonia, diethanolamine, and ammonium hydroxide.

<Natural rubber manufacturing method>
The method for producing natural rubber according to the present disclosure comprises a treatment step of contacting natural rubber latex with a treating agent capable of capturing at least nitrogen, magnesium ions, magnesium compounds, potassium ions, and potassium compounds, and a drying step of drying the treated latex. Since the treatment step is the same as that in the method for producing natural rubber latex according to the present disclosure described above, only the drying step will be described here.

There are no particular limitations on the method for drying the treated natural rubber latex. However, methods that involve adding an acid or the like to coagulate, wash, and dry the latex tend to result in the leaching of useful components contained in the latex. For this reason, it is desirable to dry the natural rubber latex directly from the standpoint of preventing the leaching of useful components and increasing productivity. A preferred method for producing natural rubber according to the present disclosure includes a treatment step in which a treating agent is added to the natural rubber latex, a treatment agent removal step in which the latex to which the treating agent has been added is filtered to separate the treating agent, and a drying step in which the filtrate obtained in the treatment agent removal step is dried. By removing the treating agent before drying the treated natural rubber latex, the natural rubber latex can be dried directly. This method allows for the production of natural rubber with reduced nitrogen, magnesium, and potassium contents and minimal variation, without leaving any remaining treating agent, through the simple process of adding the treating agent, filtering the treating agent, and drying the filtrate.

One method for drying natural rubber latex directly is to heat it in an oven. Considering productivity and the risk of heat-induced degradation of natural rubber, it is desirable to apply or spray the natural rubber latex onto a heated substrate and then dry it. This shortens the drying time, thereby suppressing thermal degradation of natural rubber and improving productivity. Among these methods, spraying the natural rubber latex is particularly desirable. The sprayed natural rubber latex adheres to the substrate surface in a dot pattern. This increases the specific surface area and dries more easily than when the natural rubber latex is applied to the substrate surface. Therefore, the natural rubber latex can be dried in a shorter time.

The shape of the substrate is not particularly limited. For example, a rotating member such as a drum can be used as the substrate. In this case, natural rubber latex is sprayed onto the heated endless annular surface of the rotating member (e.g., the outer peripheral surface of a drum), and the coating liquid is dried while the endless annular surface is rotating. The resulting solid natural rubber is then peeled off in order from the endless annular surface. This makes it possible to automate the series of steps: spraying natural rubber latex → drying → peeling natural rubber. This significantly improves productivity. The substrate surface temperature is preferably in the range of 120°C to 200°C. If the substrate surface temperature is too low, the natural rubber latex cannot be sufficiently dried within a practical drying time. Conversely, if the substrate surface temperature is too high, the attached natural rubber latex may be overheated, resulting in degradation.

The treatment agent removal step is the same as that in the method for producing natural rubber latex of the present disclosure described above. Furthermore, from the perspective of suppressing aggregation and coagulation of the natural rubber latex when it is brought into contact with the treatment agent, the method for producing natural rubber of the present disclosure also preferably includes a pH adjustment step in which a pH adjuster is added to the natural rubber latex to adjust the pH to 9 or higher before the treatment step.

Next, we will explain this disclosure in more detail using examples.

(1) Measurement of N, Mg, and K Contents <Example 1>
Fresh latex produced in Thailand was used as the natural rubber latex. First, the fresh latex was diluted to a solids concentration of 15% by mass, and then aqueous ammonia was added to adjust the pH to 9. Next, 71 parts by mass of chelating fiber ("Chilest Fiber (registered trademark) IRY-L" manufactured by Chelest Co., Ltd.) and 35 parts by mass of a synthetic adsorbent ("Amberlite (registered trademark) XAD (registered trademark) 7HP" sold by Organo Corporation) were added to 100 parts by mass of the solids content of the diluted natural rubber latex, and the mixture was shaken at room temperature for 24 hours. Subsequently, the latex after the reaction was filtered through a mesh to separate the chelating fiber and synthetic adsorbent.

The resulting filtrate was then sprayed onto the outer surface of a rotating drum and dried. The drum rotation speed was approximately 1 rpm (approximately 1 rotation per minute), and the outer surface of the drum had been preheated to approximately 150°C. The sprayed droplets of filtrate dried as the drum rotated, bonding to each other and solidifying into a sheet. When the drum had rotated approximately 3/4 of a turn, the formed sheet was peeled off from the outer surface of the drum. In this manner, solid natural rubber was produced. The produced natural rubber is referred to as the natural rubber of Example 1. The nitrogen, magnesium, and potassium contents of the natural rubber of Example 1 were measured as follows. The nitrogen, magnesium, and potassium contents in the following examples were all measured using the same method.

[nitrogen]
The natural rubber was combusted at 950°C and then subjected to elemental analysis using a PerkinElmer 2400II CHNS/O organic trace elemental analyzer to measure the amount of nitrogen. The mass ratio of nitrogen to the total amount of natural rubber taken as 100% by mass was defined as the nitrogen content.

[magnesium]
Natural rubber was incinerated by a dry incineration method in which it was heated to 550°C in air and burned, and then analyzed to measure the magnesium ion concentration using a polarized Zeeman atomic absorption spectrophotometer "Z-2310" manufactured by Hitachi High-Technologies Corporation. The measured magnesium ion concentration was taken as the magnesium content in the natural rubber.

[potassium]
The natural rubber was analyzed using an ICP emission spectrometer "Optima 4300DV" manufactured by PerkinElmer, Inc. to measure the potassium ion concentration. The measured potassium ion concentration was taken as the potassium content in the natural rubber.

Example 2
A natural rubber was produced in the same manner as in Example 1, except that in the production of the natural rubber of Example 1, no chelating fiber was used and only a synthetic adsorbent was used, and the nitrogen, magnesium, and potassium contents were measured. The amount of synthetic adsorbent added was 35 parts by mass per 100 parts by mass of the solids content of the diluted natural rubber latex. The produced natural rubber is referred to as the natural rubber of Example 2.

<Comparative Example 1>
The fresh latex used as the raw material for the natural rubber of Example 1 was diluted to a solids concentration of 28% by mass, and then natural rubber was produced by spraying the diluted solution onto the outer surface of a rotating drum and drying it in the same manner as in Example 1. The natural rubber produced is referred to as natural rubber of Comparative Example 1. The contents of nitrogen, magnesium, and potassium in the natural rubber of Comparative Example 1 were measured.

<Comparative Example 2>
The fresh latex used as the raw material for the natural rubber in Example 1 was centrifuged at 8,000 rpm for 40 minutes using a centrifuge. An equal amount of pure water was added to the solids after the centrifugation to form a latex, and the same procedure (centrifugation followed by redispersion by adding pure water) was repeated twice to obtain a purified latex. The purified latex obtained was sprayed onto the outer surface of a rotating drum and dried, as in Example 1, to produce natural rubber. The produced natural rubber is referred to as the natural rubber of Comparative Example 2. The nitrogen, magnesium, and potassium contents in the natural rubber of Comparative Example 2 were measured.

<Measurement results>
Table 1 shows the measurement results for the natural rubbers of Examples 1 and 2 and Comparative Examples 1 and 2.
As shown in Table 1, the natural rubbers of Examples 1 and 2 all had lower contents of N, Mg, and K compared to the untreated natural rubber of Comparative Example 1. The N content was in the range of 0.3% by mass to 0.6% by mass, the Mg content was in the range of 0% by mass to 0.02% by mass, and the K content was in the range of 0.1% by mass to 0.3% by mass. In particular, the natural rubber of Example 1, which used a chelating agent and an adsorbent in combination, had a lower K content. The natural rubbers of Examples 1 and 2 are within the concept of natural rubber of the present disclosure. The filtrate obtained during the production process of the natural rubbers of Examples 1 and 2 is also within the concept of natural rubber latex of the present disclosure. In contrast, the natural rubber of Comparative Example 2, which was treated by centrifugation, had excessive nitrogen and potassium removed, resulting in N and K contents of 0.0% by mass, outside the desired ranges.

(2) Evaluation of variations in N, Mg, and K contents, vulcanization induction time, and hardness <Evaluation method>
[Example 1]
Ten pieces of natural rubber were produced by the same process as in Example 1, namely, treatment with chelating fiber and synthetic adsorbent, filtration, and drying. The nitrogen, magnesium, and potassium contents of each natural rubber were measured, and their variations were investigated. Additionally, the vulcanization induction time and hardness, which are indicators of the scorch resistance of natural rubber, were measured as follows, and their variations were investigated. The methods for measuring the vulcanization induction time and hardness in the following examples were all the same.

[Vulcanization induction time (T10)]
A rubber composition was prepared by compounding natural rubber with a crosslinking agent, a vulcanization accelerator, zinc oxide, a processing aid, a reinforcing material, an antioxidant, and a softener, and the vulcanization induction time (T10) of the prepared rubber composition was measured. The vulcanization induction time (T10) was measured using a rotorless rheometer manufactured by Toyo Seiki Seisakusho, Ltd. in accordance with JIS K6300-2. The measurement temperature was 150°C. The vulcanization induction time (T10) corresponds to the vulcanization initiation point in the vulcanization curve, as described in JIS K6300-2:2001.

[Hardness]
A rubber composition was prepared by blending natural rubber with a crosslinking agent, a vulcanization accelerator, zinc oxide, a processing aid, a reinforcing material, an antioxidant, and a softener. The rubber composition was then press-molded at 150°C for 20 minutes to prepare a 2 mm thick rubber sheet. Three of the prepared rubber sheets were stacked to form a test piece, and the Type A durometer hardness of the test piece was measured using a hardness tester ("ASKER P1-A" manufactured by Kobunshi Keiki Co., Ltd.) in accordance with JIS K6253-3:2012. The Type A durometer hardness was measured 15 seconds after the indenter came into contact with the test piece.

[Comparative Example 1]
In the same manner as in the natural rubber of Comparative Example 1, 26 pieces of natural rubber were produced by drying the fresh latex used as the raw material. The contents of nitrogen, magnesium, and potassium, vulcanization induction time, and hardness of each natural rubber were measured, and variations therein were investigated.

[Comparative Example 2]
Seventeen pieces of natural rubber were produced by the same process as in the natural rubber of Comparative Example 2, namely, centrifugal separation, redispersion by adding pure water, and drying. The nitrogen, magnesium, and potassium contents, vulcanization induction time, and hardness of each natural rubber were measured, and the variations in these values were investigated.

<Evaluation results>
Table 2 shows the measurement results for the natural rubbers of Example 1 and Comparative Examples 1 and 2. The number of samples for which the variation was measured varies depending on the object being measured. Therefore, in Table 2, the number of samples measured is indicated by the value n. For example, "n=10" means that the measurement values are for 10 samples.

As shown in Table 2, the natural rubber of Example 1 had smaller lower limits for the N, Mg, and K contents, and smaller variations in the N, Mg, and K contents compared to the untreated natural rubber of Comparative Example 1. For the natural rubber of Example 1, the N content was within the range of 0.3% to 0.6% by mass, the Mg content was within the range of 0% to 0.02% by mass, and the K content was within the range of 0.1% to 0.3% by mass. The natural rubber of Example 1 also had smaller variations in the vulcanization induction time compared to the natural rubber of Comparative Example 1. Similarly, the hardness of the natural rubber of Example 1 also had smaller variations compared to the natural rubber of Comparative Example 1. In contrast, the N and K contents of the centrifuged natural rubber of Comparative Example 2 were too low and outside the desired ranges. Furthermore, although the vulcanization induction time varied only slightly, it was confirmed that the values increased, resulting in a slower vulcanization rate. Although the hardness varied only slightly, it was confirmed that the values decreased.

The natural rubber disclosed herein has little variation in properties such as hardness, making it useful as a material for vehicle parts such as engine mounts and suspension bushings, as well as vibration-damping rubber used in the construction and housing fields.

Claims (4)

A method for producing natural rubber having a nitrogen content of 0.3% by mass or more and 0.6% by mass or less, a magnesium content of 0% by mass or more and 0.02% by mass or less, and a potassium content of 0.1% by mass or more and 0.3% by mass or less, comprising :
a pH adjusting step of adding a pH adjuster to the natural rubber latex to adjust the pH value of the natural rubber latex to 9 or more;
a treating step of contacting the pH-adjusted natural rubber latex with a treating agent capable of capturing at least nitrogen components, magnesium ions, magnesium compounds, potassium ions, and potassium compounds;
a drying step in which the treated latex is dried without coagulating ;
1. A method for producing natural rubber, comprising: The method for producing natural rubber according to claim 1 , wherein the treating agent comprises at least one agent selected from the group consisting of an adsorbent and a chelating agent. the adsorbent is at least one selected from an organic ion exchanger, an inorganic ion exchanger, a synthetic adsorbent, and a protein adsorbent;
3. The method for producing natural rubber according to claim 2 , wherein the chelating agent is at least one selected from the group consisting of a chelating resin and a chelating fiber. the treating step is a step of adding the treating agent to the natural rubber latex whose pH value has been adjusted ,
a treating agent removing step of filtering the latex to which the treating agent has been added to remove the treating agent,
4. The method for producing natural rubber according to claim 1 , wherein the drying step is a step of drying the filtrate obtained in the treating agent removing step without coagulating it .