JP7319875B2 - NATURAL RUBBER COMPOSITION, MANUFACTURING METHOD THEREOF, AND VIBRATION-ISOLATING MEMBER - Google Patents

Description

TECHNICAL FIELD The present invention relates to a natural rubber composition suitable for manufacturing anti-vibration members used in vehicles, buildings and the like, and a method for manufacturing the same.

2. Description of the Related Art In a vehicle such as an automobile, a vibration isolating member is arranged at a portion where vibration and noise are generated to reduce transmission of vibration and diffusion of noise. This type of anti-vibration member is required to have strength to support a heavy object such as an engine, and anti-vibration performance to absorb and suppress vibration. A dynamic magnification is one of the characteristics that indicate the anti-vibration performance. The dynamic ratio is the ratio of the dynamic spring constant to the static spring constant (dynamic spring constant (Kd)/static spring constant (Ks)), and the smaller the dynamic ratio, the higher the vibration damping performance.

Natural rubber is mainly used as a material for the vibration-isolating member because of its high tensile strength and low heat generation due to vibration. The natural rubber is blended with particulate reinforcing materials such as carbon black and silica to increase strength. The greater the amount of reinforcing material, the greater the hardness and strength (static spring constant) of the anti-vibration member. However, there is a limit to the effect, and the weight of the anti-vibration member increases, increasing the burden on the environment. In addition, as the amount of the reinforcing material added increases, the friction of the reinforcing material due to vibration increases, resulting in an increase in energy loss and an increase in the dynamic magnification.

In recent years, attempts have been made to reduce the environmental load and to reduce the weight of products by using biomass that exists in nature. For example, a technique is known in which cellulose fibers are used as reinforcing materials for rubber materials such as natural rubber. Since cellulose fibers tend to entangle and aggregate, it is difficult to uniformly disperse them in a rubber material. In this regard, Patent Literatures 1 and 2 disclose rubber compositions improved so that cellulose fibers are uniformly dispersed.

JP 2013-018918 A JP 2013-166914 A JP 2016-040362 A Japanese Patent Application Laid-Open No. 2008-189718

However, even if aggregation of cellulose fibers is suppressed, the aspect ratio (ratio of fiber length to fiber diameter) of cellulose fibers is large. There is a problem that it is difficult to improve the reinforcing property.

For example, Patent Document 3 discloses a rubber composition having a rubber component and mixed powder containing silica and microfibril cellulose. The mixed powder containing silica and microfibril cellulose is produced by mixing silica and cellulose fibers in a solvent (paragraphs [0049] and [0073] of the same document), and silica and cellulose fibers are Not chemically bonded. According to the mixed powder containing silica and microfibril cellulose, the intrusion of silica into the cellulose fibers may suppress aggregation of the cellulose fibers. Not reached.

The present invention has been made in view of such circumstances, and provides a natural rubber composition from which a rubber member having a small environmental load, light weight and excellent reinforcing properties can be produced, and a method for producing the same. is the subject. Another object of the present invention is to provide a lightweight, high-strength, and excellent vibration-isolating member using the natural rubber composition.

(1) In order to solve the above problems, the natural rubber composition of the present invention is characterized by having a natural rubber component and a composite filler formed by chemically bonding a hydrophilic polymer and a metal oxide. do.

(2) The vibration-damping member of the present invention is characterized by being manufactured using the natural rubber composition of the present invention having the constitution of (1) above.

(3) The method for producing a natural rubber composition of the present invention comprises stirring an aqueous solution containing a compound represented by the following chemical formula (A) or (B) and a hydrophilic polymer at a pH of 7 or more and 10 or less. Then, a composite filler manufacturing step of manufacturing a composite filler in which a metal oxide produced by hydrolysis of the compound and the hydrophilic polymer are chemically bonded, and the composite filler is added to natural rubber latex. and a drying step of drying the dispersion.
_{AnMmOx ( A} )
A _n M _m (OR) _x (B)
[A is an alkali metal element, an alkaline earth metal element, or a Group 3 element, M is a transition metal element or metalloid element, n is an integer, and m and x are natural numbers]

(1) The natural rubber composition of the present invention has a composite filler. Therefore, a product having high strength (a crosslinked product of a natural rubber composition) can be produced without blending a petroleum-derived reinforcing material such as carbon black. In addition, since it uses natural rubber produced from rubber tree sap and does not require the use of petroleum-derived reinforcing materials, it has a low environmental impact and is useful as a sustainable material.

A composite filler is formed by chemically bonding a hydrophilic polymer and a metal oxide. Hydrophilic polymers have a high affinity for water, and are therefore excellent in dispersibility in aqueous dispersion media such as natural rubber latex. Therefore, a natural rubber composition can be produced without using a solvent, and environmental load can be reduced. Composite fillers have both organic and inorganic characteristics and can achieve desired physical properties even in relatively small amounts. Therefore, it is possible to reduce the weight of the product manufactured from the natural rubber composition of the present invention and to reduce the dynamic magnification. For example, even when a fibrous material is used as the hydrophilic polymer, it is difficult to agglomerate because the metal oxide is chemically bonded. In addition, fibrous materials have the drawback of being inferior in reinforcing properties in the compression direction due to their large aspect ratios. Directional strength can also be increased.

(2) As explained in (1) above, the cross-linked product obtained by cross-linking the natural rubber composition of the present invention can achieve both desired strength and low dynamic modulus due to the reinforcing effect of the composite filler. Therefore, the vibration-isolating member of the present invention has excellent vibration-isolating performance. Further, according to the natural rubber composition of the present invention, there is no need to use a petroleum-derived reinforcing material such as carbon black, and the content of the filler can be reduced. can be realized.

(3) In the composite filler manufacturing step of the method for manufacturing the natural rubber composition of the present invention, the compound represented by the chemical formula (A) or (B) undergoes hydrolysis and polycondensation reactions via sol and gel states. The hydrophilic polymer and the metal oxide are combined (chemically bonded) by using the process of forming a metal oxide. By utilizing the sol-gel reaction, various kinds of metal oxides can be composited with hydrophilic polymers. At this time, by adjusting the pH of the aqueous solution to 7 or more and 10 or less, the hydrolysis of the compound is promoted and the metal oxide is easily precipitated. In the method for producing a natural rubber composition of the present invention, a composite filler is produced in advance and added to natural rubber latex to prepare a dispersion. By carrying out the production of the composite filler in a separate process from the preparation of the dispersion, the formation of a composite between the hydrophilic polymer and the metal oxide can be sufficiently advanced. The use of the composite filler produced in this manner improves the tensile properties of the product, leading to improved durability. In addition, since the non-rubber components such as proteins and lipids originally contained in the natural rubber latex serve as a dispersant, aggregation of the composite filler in the dispersion preparation process can be suppressed.

Incidentally, Patent Document 4 describes, as a method for producing a rubber composition, a step (A) of obtaining a rubber masterbatch by acid-coagulating a mixture of synthetic rubber latex and water glass, followed by drying. It is In addition, paragraph [0031] of the same document describes that animal polysaccharides such as chitosan and chitin derivatives may be mixed in step (A). However, the rubber composition described in Patent Document 4 is intended to improve the grip performance and wear resistance of the tire, reducing environmental load, strength in the direction of compression, anti-vibration performance, etc. is not considered. For this reason, Patent Document 4 does not include the concept of reacting water glass with animal polysaccharides to combine silica with animal polysaccharides, nor does it describe a process for combining them. Since the rubber composition described in Patent Document 4 is a composition of synthetic rubber, it does not contribute to reduction of environmental load. Synthetic rubber latex, unlike natural rubber latex, does not have proteins that act as dispersants. Therefore, animal polysaccharides are added only as dispersants.

1 is a scanning electron microscope (SEM) photograph of Composite Filler A used in Example 1. FIG.

Hereinafter, the natural rubber composition, the method for producing the same, and the vibration-damping member of the present invention will be described in detail.

<Natural rubber composition>
The natural rubber composition of the present invention has a natural rubber component and a composite filler in which a hydrophilic polymer and a metal oxide are chemically bonded.

[Natural rubber component]
The natural rubber component is a component obtained by drying natural rubber latex, and may contain not only rubber components but also non-rubber components such as proteins and lipids originally contained in natural rubber. As the natural rubber latex, field latex collected by tapping or latex treated by adding ammonia thereto (high ammonia latex) may be used.

[Composite filler]
A composite filler is formed by chemically bonding a hydrophilic polymer and a metal oxide. The hydrophilic polymer may be any polymer that can be dispersed in an aqueous dispersion medium such as natural rubber latex and can chemically bond with a metal oxide. Although the form of chemical bonding is not particularly limited, for example, the hydrophilic polymer contains one or more selected from hydroxyl groups, carboxyl groups, amino groups, and amide groups because it is capable of forming hydrogen bonds with hydroxyl groups of metal oxides. It is desirable to have In addition, from the perspective of reducing environmental impact and using sustainable materials, it is possible to use biomass (materials derived from living organisms) obtained from animals and plants instead of materials derived from fossil fuels such as petroleum. desirable. In this case, the hydrophilic polymer may be selected from proteins and polysaccharides. Proteins include collagen, keratin, elastin, fibroin, and the like. Polysaccharides include cellulose, chitin, chitosan, pectin, and the like. Among them, cellulose is preferred because it can be obtained easily and relatively inexpensively from plants such as wood.

The shape of the cellulose is not particularly limited, and may be particulate, fibrous, or the like, but fibrous is preferred from the viewpoint of enhancing reinforcing properties. Among them, cellulose nanofiber, which is obtained by miniaturizing plant fibers to nanometer order, is suitable as a reinforcing material for natural rubber because it has both lightness and strength. Conventionally, when cellulose fibers such as cellulose nanofibers are blended alone, there has been a problem that the fibers are easily entangled and aggregated. Moreover, when cellulose fibers are blended, due to their large aspect ratio, the effect of improving the reinforcing property in the stretching direction can be obtained, but there is also the problem that the effect of improving the reinforcing property in the compression direction cannot be expected. In this regard, in the natural rubber composition of the present invention, when cellulose fibers are used as the hydrophilic polymer, a composite filler in which metal oxides are chemically bonded to cellulose fibers is used instead of using cellulose fibers alone. As a result, aggregation of the cellulose fibers can be suppressed, and the reinforcing property can be improved not only in the extension direction but also in the compression direction.

Examples of metal oxides include silica particles, titanium oxide particles, zinc oxide particles, zirconia particles, selenium oxide particles, cobalt oxide particles, and the like. The metal oxide is preferably produced by hydrolysis of a compound represented by the following chemical formula (A) or (B), which will be described in detail later in the process for producing a natural rubber composition. A hydrophilic polymer and a metal oxide can be easily combined by utilizing the process of hydrolysis and polycondensation reaction of the compound to form a metal oxide via a sol and gel state.
_{AnMmOx ( A} )
A _n M _m (OR) _x (B)
[A is an alkali metal element, an alkaline earth metal element, or a Group 3 element, M is a transition metal element or metalloid element, n is an integer, and m and x are natural numbers]
One of the compounds represented by the chemical formula (A) is sodium silicate. Sodium silicate is also called water glass, can be used as an aqueous dispersion medium, and is inexpensive and readily available. By the way _, as defined in JIS K 1408-1966, water glass _has the general formula "Na ₂ O.nSiO _{2.xH 2} O (where n is a molar ratio)”. It is desirable that the metal oxide is silica particles produced by hydrolysis of sodium silicate.

If the amount of metal oxide in the composite filler is too small, the effect of suppressing aggregation of the hydrophilic polymer cannot be obtained, and the effect of improving the reinforcing property in the direction of compression is small. Therefore, in the composite filler, the mass ratio of the metal oxide to the hydrophilic polymer is desirably 0.1 times or more, more preferably 0.5 times or more. On the other hand, if the amount of metal oxide is too large, the metal oxides bind to each other, become large, and surround the hydrophilic polymer. Therefore, in the composite filler, the mass ratio of the metal oxide to the hydrophilic polymer is desirably 3 times or less and 2 times or less.

Composite fillers have a high reinforcing effect even in small amounts compared to conventional reinforcing materials such as carbon black. Therefore, it is possible to reduce the weight of the natural rubber composition and its crosslinked product. The content of the composite filler in the natural rubber composition of the present invention is preferably 1 part by mass or more, preferably 3 parts by mass or more per 100 parts by mass of the natural rubber component, from the viewpoint of improving reinforcing properties. From the viewpoint of reducing the weight, it is preferably 10 parts by mass or less, more preferably 8 parts by mass or less.

<Method for producing natural rubber composition>
(1) The method for producing a natural rubber composition of the present invention comprises a composite filler production step, a dispersion liquid preparation step, and a drying step. Hereinafter, this manufacturing method is referred to as the first manufacturing method.

[Composite filler manufacturing process]
In this step, an aqueous solution containing a compound represented by the following chemical formula (A) or (B) and a hydrophilic polymer is stirred at a pH of 7 or more and 10 or less, and the compound is hydrolyzed. This is a step of manufacturing a composite filler in which the metal oxide and the hydrophilic polymer are chemically bonded.
_{AnMmOx ( A} )
A _n M _m (OR) _x (B)
[A is an alkali metal element, an alkaline earth metal element, or a Group 3 element, M is a transition metal element or metalloid element, n is an integer, and m and x are natural numbers]
Group 3 elements to which the sol-gel method can be applied among the elements constituting the compound of the chemical formula (A) or (B) include scandium, yttrium, and lanthanides. Metalloid elements include boron, silicon, germanium, arsenic, antimony, and tellurium. Compounds represented by the chemical formula (A) include sodium silicate (Na ₂ SiO ₃ ), lithium silicate (Li ₂ O.nSiO ₂ ), potassium silicate (K ₂ O.nSiO ₂ .xH ₂ O), silicon beryllium oxide ( _Be2O4Si ), magnesium silicate _{( xMgO.ySiO2.nH2O} ), calcium _silicate ( _Ca2O4Si ) _, lithium titanate ( _Li2TiO3 ), sodium titanate ₍ Na _2Ti3O7 ), potassium titanate ( _K2Ti4O9 ) _{, magnesium} titanate ( _MgTiO3 ), calcium titanate ( _CaTiO3 ), _strontium titanate ( _{SrTiO3 )} , barium titanate ( _BaTiO3 ) , aluminum titanate ( _Al2TiO5 ), _lead titanate ( _PbTiO3 ), _cesium titanate ( _{Cs2Ti4O9 ) ,} lanthanum titanate ( _La2Ti2O7 ), lithium aluminate ( _{LiAlO2 )} , sodium aluminate (NaAlO ₂ ), potassium aluminate (KAlO ₂ ), beryllium aluminate (Al ₆ BeO ₁₀ ), magnesium aluminate (MgAl ₂ O ₄ ), calcium aluminate (Al ₂ CaO ₄ ), strontium aluminate (SrAl ₂ O ₄ ), barium aluminate (BaAl ₂ O ₄ ), yttrium aluminate (Y ₃ Al ₅ O ₁₂ ), zinc aluminate (ZnAl ₂ O ₄ ), lanthanum aluminate (LaAlO ₃ ), lithium cobaltate (LiCoO ₂ ), sodium cobaltate (Na _x CoO ₂ ), and the like.

In the chemical formula (B), R constituting the alkoxy group ((OR) _x ) is an optionally substituted alkyl group. The types of alkoxy groups may be the same or partially different. Alkyl groups include methyl group, ethyl group, n-propyl group, isopropyl group, n-butyl group, isobutyl group, t-butyl group, n-pentyl group, isopentyl group, neopentyl group, n-hexyl group and isohexyl group. , n-heptyl group, n-octyl group, n-nonyl group, isononyl group, n-decyl group, n-dodecyl group, n-octadecyl group and the like. Among them, an alkyl group having 1 to 6 carbon atoms is desirable. Examples of substituents include halogen atoms such as a fluorine atom, a chlorine atom, a bromine atom and an iodine atom, as well as an alkoxy group and a (meth)acryloxy group. The alkoxy group as a substituent is preferably an alkoxy group having 1 to 6 carbon atoms, such as methyloxy, ethyloxy, propyloxy, isopropyloxy, n-butyloxy, isobutyloxy and t-butyloxy. be done.

The hydrophilic polymer is as described above, and may be selected from proteins such as collagen, keratin, elastin and fibroin, and polysaccharides such as cellulose, chitin, chitosan and pectin.

The mixing ratio of the compound represented by the chemical formula (A) or (B) and the hydrophilic polymer may be appropriately determined in consideration of the amount of metal oxide produced by hydrolysis of the compound. Considering the effect of suppressing aggregation of the hydrophilic polymer and the effect of improving the reinforcing property in the compression direction of the crosslinked product, the mass ratio of the compound of the chemical formula (A) or (B) to the hydrophilic polymer is 0.1 times or more, More preferably, it is 0.5 times or more, 3 times or less, and more preferably 2 times or less.

The reason is that the pH of the aqueous solution promotes the hydrolysis of the compound of the chemical formula (A) or (B), facilitates the precipitation of the metal oxide, and promotes the complex reaction between the hydrophilic polymer and the metal oxide. to 7 or more and 10 or less. Acids such as hydrochloric acid, nitric acid, and sulfuric acid may be used to adjust the pH. Stirring of the aqueous solution is preferably carried out at 10° C. or less, more preferably in an ice bath, for the purpose of reducing the particle size of the metal oxide (making the metal oxide particles finer). The stirring time is desirably 24 hours or longer.

[Dispersion preparation step]
This step is a step of adding the manufactured composite filler to natural rubber latex to prepare a dispersion.

As described above, field latex obtained by tapping or high ammonia latex may be used as the natural rubber latex. The solid content concentration (% by mass of dry rubber) of natural rubber latex is not particularly limited. If the solid content concentration is too low, the resulting natural rubber composition will be less, which is not economical. On the other hand, if the solid content concentration is too high, the rubber component in the natural latex becomes unstable, and the rubber particles tend to agglomerate. For example, it is desirable to set the solid content concentration to 10% by mass or more and 60% by mass or less.

As will be described later, when the dispersion is sprayed onto the substrate and dried in a relatively short time, the dispersion may contain a cross-linking agent, a vulcanization accelerator, a processing aid, and the like. A surfactant may also be added to enhance their dispersibility. By preliminarily blending a cross-linking agent or the like, the subsequent kneading process becomes unnecessary, and the manufacturing process can be simplified and the cost can be reduced. Examples of vulcanization accelerators include thiazole-based, sulfenamide-based, thiuram-based, dithiocarbamate-based, and guanidine-based accelerators. Processing aids include zinc oxide, stearic acid, and the like. Apart from this, in order to impart desired physical properties to the crosslinked product, an antiaging agent, a plasticizer, a coloring agent, and the like may be blended as appropriate. It should be noted that the natural rubber composition of the present invention does not exclude a mode containing conventional reinforcing materials such as carbon black and silica.

[Drying process]
This step is a step of drying the prepared dispersion. A drying method is not particularly limited. For example, the dispersion may be heated in an oven to dry it. Considering the suppression of deterioration of the rubber component due to heat, suppression of precipitation of the composite filler, improvement of productivity, and the like, a shorter drying time is desirable. For example, when the solid content of the natural rubber latex is 10% by mass or more, the drying speed of the dispersion is preferably 1200 mL/hour or more per 1 L of the dispersion. This drying rate means that the amount of water that evaporates is 1200 mL or more per hour when drying 1 L of the dispersion. The higher the drying speed, the shorter the drying time, the more the deterioration of the rubber component and the precipitation of the composite filler due to heat can be suppressed, and the productivity is also improved. The drying speed can be adjusted by various methods such as drying in an air stream, coating on the substrate, or drying by spraying.

From the viewpoint of increasing the drying rate, it is desirable to dry the dispersion by coating or spraying it on a heated substrate. These methods allow drying in a relatively short time. Among them, the method of spraying the dispersion liquid is desirable. The sprayed dispersion adheres to the substrate surface in dots. For this reason, the specific surface area is increased compared to the case where it is applied to the base material surface, and it is easy to dry.

The shape of the substrate is not particularly limited. For example, a rotating member such as a drum may be used as the substrate. In this case, the dispersion liquid is sprayed onto the heated endless annular surface of the rotating member (for example, the outer peripheral surface of the drum), and the coating liquid is dried while rotating the endless annular surface. Then, the obtained solid matter (natural rubber composition) may be peeled off from the endless annular surface in order. By doing so, it becomes possible to automate a series of steps of spraying the dispersion, drying, and peeling off the natural rubber composition, thereby significantly improving productivity. An example of a drying apparatus using a drum is a solid natural rubber production apparatus described in Japanese Patent No. 5745830.

The temperature of the substrate surface is desirably in the range of 120°C or higher and 200°C or lower. If the temperature of the substrate surface is too low, the dispersion cannot be sufficiently dried within a practical drying time. Conversely, if the temperature of the base material surface is too high, the adhered dispersion liquid is excessively heated, which may deteriorate the rubber component.

(2) The method for producing the natural rubber composition of the present invention is not limited to the above first production method. In the above-described first manufacturing method, the pre-manufactured composite filler was added to the natural rubber latex, but the composite filler may be generated in the natural rubber latex instead of being pre-manufactured.

The second method for producing the natural rubber composition of the present invention is to directly add the hydrophilic polymer, which is the raw material of the composite filler, and the compound represented by the above chemical formula (A) or (B) to the natural rubber latex. , stirring to prepare a dispersion, and drying the prepared dispersion. In the second production method, the compounding reaction between the hydrophilic polymer and the metal oxide proceeds in natural rubber latex. At this time, it is not always necessary to adjust the pH of the dispersion. It may be made easier to facilitate the complexing reaction between the hydrophilic polymer and the metal oxide.

<Anti-vibration material>
The vibration-damping member of the present invention is manufactured using the natural rubber composition of the present invention. Specifically, first, the natural rubber composition of the present invention is blended with a cross-linking agent, a vulcanization accelerator, a processing aid, etc., and if necessary, an anti-aging agent, a plasticizer, a colorant, etc. are added. The agents are blended and kneaded using a Banbury mixer, a kneader, a roll, or the like. After that, the kneaded product is maintained at a temperature of 140 to 180° C. for 5 to 30 minutes for cross-linking. The anti-vibration member of the present invention includes engine mounts, transmission mounts, body mounts, cab mounts, member mounts, differential mounts, connecting rods, torque rods, strut bar cushions, center bearing supports, torsional dampers, Suitable for steering rubber couplings, tension rod bushes, bounce stoppers, FF engine roll stoppers, muffler hangers, stabilizer ring rods, radiator supports, control arm bushes, suspension arm bushes, etc.

EXAMPLES Next, the present invention will be described more specifically with reference to examples.

<Production of composite filler A (composite filler production process)>
First, sodium silicate anhydrous (Na ₂ SiO ₃ , manufactured by Nacalai Tesque Co., Ltd.) was added to an aqueous dispersion of cellulose nanofibers (“Binfis (registered trademark) MBa” manufactured by Sugino Machine Co., Ltd.), and cellulose nanofibers were added. and sodium silicate at a mass ratio of 1:1 and stirred. Next, hydrochloric acid (manufactured by Nacalai Tesque Co., Ltd., concentration 35%) was added to this aqueous solution to adjust the pH of the aqueous solution to 7, and the mixture was stirred in an ice bath for 24 hours. The resulting slurry was then wrapped in a dialysis film and washed with water. Thus, a slurry-like composite filler A was produced in which silica particles produced by hydrolysis of sodium silicate and cellulose nanofibers were hydrogen-bonded.

FIG. 1 shows a scanning electron microscope (SEM) photograph of the manufactured composite filler A (magnification: 30,000 times). As shown in FIG. 1, it was confirmed that granular silica particles were bound to cellulose nanofibers.

<Production of composite filler B (composite filler production process)>
Slurry was prepared in the same manner as composite filler A, except that when adding anhydrous sodium silicate to the aqueous dispersion of cellulose nanofibers, the mass ratio of cellulose nanofibers and sodium silicate was changed to 1:0.11. Composite filler B having a shape was produced.

<Production of natural rubber composition>
(1) Example 1
As natural rubber latex, latex obtained by adding 0.5% by mass of ammonia to field latex collected by tapping from Hevea brasiliensis was used (the same applies to the following examples and comparative examples). The solid content of natural rubber latex is 40% by mass. For Example 1, a natural rubber composition was produced by the first production method described above.

First, composite filler A was added to natural rubber latex and stirred to prepare a dispersion (dispersion preparation step). The amount of the composite filler A added was 5 parts by mass with respect to 100 parts by mass of the solid content (natural rubber component) of the natural rubber latex. Next, the prepared dispersion was dried by spraying it on the outer peripheral surface of a rotating drum with a spray nozzle (drying step). The rotation speed of the drum is 1 rpm (one rotation per minute), and the outer peripheral surface of the drum is heated to 150°C. The inner diameter of the spray nozzle was 2 mm, and the spray flow rate was 100 ml/min. The droplets of the sprayed dispersion adhere to the outer peripheral surface of the drum in the form of dots, and while drying as the drum rotates, they bind to each other and solidify into a sheet. Then, when the drum made 3/4 rotation (drying time: 45 seconds), the sheet-like natural rubber composition was peeled off from the outer peripheral surface of the drum. In this way, the natural rubber composition of Example 1 was produced through a series of steps of spraying the dispersion, drying, and peeling the sheet. The drying speed of the dispersion liquid in the drum drying is 1600 mL/hour per 1 L of the dispersion liquid.

(2) Example 2
A natural rubber composition of Example 2 was produced in the same manner as the natural rubber composition of Example 1, except that composite filler B was added instead of composite filler A.

(3) Example 3
For Example 3, a natural rubber composition was produced by the second production method described above. First, an aqueous dispersion of cellulose nanofibers (same as above) and anhydrous sodium silicate (same as above) were directly added to natural rubber latex and stirred to prepare a dispersion. The mass ratio of cellulose nanofibers and sodium silicate was 1:1. The total amount of cellulose nanofibers and sodium silicate added was 5 parts by mass in terms of solid content with respect to 100 parts by mass of the solid content (natural rubber component) of the natural rubber latex. In the dispersion liquid, a composite filler was produced in which silica particles produced by hydrolysis of sodium silicate and cellulose nanofibers were hydrogen-bonded. Next, the dispersion was dried by spraying it on the outer peripheral surface of a rotating drum. The drying conditions are the same as in Example 1. Thus, the natural rubber composition of Example 3 was produced.

(4) Comparative Example 1
A natural rubber composition of Comparative Example 1 was produced in the same manner as the natural rubber composition of Example 1, except that 20 parts by mass of carbon black was added instead of the composite filler A.

(5) Comparative Example 2
Natural rubber latex alone was sprayed onto the outer peripheral surface of a rotating drum and dried to produce a natural rubber composition of Comparative Example 2. The drying conditions are the same as in Example 1.

(6) Comparative Example 3
A natural rubber composition of Comparative Example 3 was prepared in the same manner as the natural rubber composition of Example 1, except that an aqueous dispersion of cellulose nanofibers (same as above) was added in terms of solid content of 5 parts by mass instead of composite filler A. manufactured things.

<Production of crosslinked product>
To 100 parts by mass of each of the natural rubber compositions of Examples and Comparative Examples produced, 5 parts by mass of two types of zinc oxide (manufactured by Sakai Chemical Industry Co., Ltd.) and stearic acid as a processing aid (manufactured by Kao Corporation) were added. LUNAC (registered trademark) S30”) 1 part by mass, vulcanization accelerator CBS (N-cyclohexyl-2-benzothiazolesulfenamide, Sanshin Chemical Industry Co., Ltd. “Sancellar (registered trademark) CZ-G”) 1 part by mass and 2.5 parts by mass of sulfur (“Sulfax T-10” manufactured by Tsurumi Chemical Industry Co., Ltd.) were added and kneaded using a roll. This kneaded product was crosslinked by maintaining it at 160° C. for 30 minutes. Then, a test piece was produced from each crosslinked product for each of the following evaluation items, and its properties were evaluated.

Raw materials and evaluation results of the crosslinked products produced from the natural rubber compositions of Examples 1 to 3 and Comparative Examples 1 to 3 (hereinafter referred to as "crosslinked products of Example 1", etc.) are summarized in Table 1. show. The evaluation results are shown as relative values when the crosslinked product of Comparative Example 1 is taken as 100.

<Evaluation of Crosslinked Product>
[Vibration characteristics]
A test piece was prepared based on JIS K 6394:2007, and the vibration characteristics of the crosslinked product in the compression direction and extension direction were evaluated according to the test method specified in JIS K 6385:2012.

(1) Vibration characteristics in the direction of compression First, a cylindrical rubber piece (50 mm in diameter, 25 mm in height) is cut out from the crosslinked material, and disc-shaped metal fittings (60 mm in diameter, 6 mm in thickness) are attached to the two bottom surfaces of the rubber piece. A test piece was prepared by bonding one by one. After applying a load from the axial direction of the test piece and compressing it by 7 mm, it was unloaded and restored. After that, the load was applied again to compress it by 7 mm. At that time, a load-deflection curve was created in the process of applying the load, and from the load-deflection curve, the load values when the deflection was 1.5 mm and 3.5 mm were read to calculate the spring constant Ks. In addition, while the test piece was axially compressed by 2.5 mm, a constant displacement harmonic compression vibration with an amplitude of 0.05 mm centering on the compressed position was applied at a frequency of 100 Hz. Then, the storage spring constant (Kd ₁₀₀ ) at 100 Hz was calculated by the non-resonant method specified in JIS K 6385:2012 (citing JIS K 6394). The dynamic magnification was obtained by dividing the storage spring constant _Kd100 by the spring constant Ks ( _Kd100 /Ks).

(2) Vibration Characteristics in Extension Direction First, a strip-shaped rubber piece (20 mm long, 5 mm wide, 1 mm thick) was cut out from the crosslinked product to obtain a test piece. Then, both ends of the test piece in the longitudinal direction were gripped with chucks (10 mm between chucks), and the test piece was stretched by ₁ mm at a frequency of 1 Hz. Next, the test piece was stretched by 0.02 mm at a frequency of 100 Hz, and the stress at that time was taken as the storage elastic constant E'100 at a frequency of ₁₀₀ Hz. Then, the storage elastic constant _E'100 at a frequency of 100 Hz was divided by the storage elastic constant _E'1 at a frequency of 1 Hz ( _E'100 / _E'1 ).

(3) Results As shown in Table 1, when the spring constants Ks of the crosslinked products of Examples 1 to 3 were compared, they were equal to or higher than the crosslinked product of Comparative Example 1. In other words, it was possible to maintain or increase the strength in the compression direction by blending the composite filler. In the cross-linked product of Example 2, the ratio of sodium silicate to cellulose nanofibers was small in the production of the composite filler, so it is considered that the effect of improving the strength by the silica particles was small. In addition, the dynamic magnification of the crosslinked products of Examples 1 to 3 is smaller than that of the crosslinked product of Comparative Example 1, indicating that the anti-vibration performance is improved. On the other hand, in the crosslinked product of Comparative Example 3, in which cellulose nanofibers were blended alone, the spring constant Ks was smaller than that of the crosslinked product of Comparative Example 1, and strength improvement in the compression direction was not observed. Also, the decrease in dynamic magnification was small, and no improvement in vibration damping performance in the direction of compression was observed.

Comparing the fresh elastic constant _E'1 at a frequency of 1 Hz, the crosslinked products of Examples 1 to 3 were equal to or higher than the crosslinked product of Comparative Example 1. In other words, by blending the composite filler, it was possible to maintain or increase the strength in the direction of elongation. The crosslinked product of Example 3 was formed with a composite filler in natural rubber latex rather than using a prefabricated composite filler. For this reason, compared with the case of pre-production, the formation of the composite filler is less likely to proceed, and the amount of bonding between the isoprene chains in the latex and the composite filler is _less . was not large. Also, the "E' ₁₀₀ /E' ₁ " values of the crosslinked products of Examples 1 to 3 are smaller than that of the crosslinked product of Comparative Example 1, indicating that the anti-vibration performance is improved.

As described above, according to the crosslinked products of Examples 1 to 3 having a composite filler, compared to the crosslinked product of Comparative Example 1 having conventional carbon black, while maintaining the strength and vibration characteristics in the extension direction, It was confirmed that the strength and vibration characteristics were improved.

[Tensile properties]
A dumbbell-shaped No. 7 test piece was prepared based on JIS K 6251: 2017, a tensile test was performed, and 100% modulus (M ₁₀₀ ), tensile strength at break (T _b ), and elongation at break ( _Eb ) was measured.

As shown in Table 1, the 100% modulus, tensile strength at break, and elongation at break of the crosslinked products of Examples 1 and 2 were all greater than those of the crosslinked product of Comparative Example 1. In other words, the compounding of the composite filler improved the tensile properties and increased the durability. On the other hand, the 100% modulus of Example 3 was smaller than that of the crosslinked product of Comparative Example 1. The crosslinked product of Example 3 was formed with a composite filler in natural rubber latex rather than using a prefabricated composite filler. For this reason, it is considered that the formation of the composite filler was less likely to proceed than in the case of pre-manufacturing.

[density]
The density of the crosslinked product was measured according to JIS K 6268:1998, Method A. As shown in Table 1, the densities of the crosslinked products of Examples 1 to 3 were lower than that of the crosslinked product of Comparative Example 1. Since the composite filler can provide desired physical properties even in a small amount, it is possible to reduce the density, in other words, reduce the weight, compared to the crosslinked product of Comparative Example 1 having conventional carbon black.

INDUSTRIAL APPLICABILITY According to the natural rubber composition of the present invention, it is possible to produce a vibration-isolating member that is lightweight and has excellent strength and vibration-isolating performance while reducing environmental load.

Claims (14)

having a natural rubber component and a composite filler formed by chemically bonding a hydrophilic polymer and a metal oxide,
A natural rubber composition , wherein the metal oxide is produced by a sol-gel reaction . 2. The natural rubber composition according to claim 1, wherein said hydrophilic polymer has at least one selected from hydroxyl groups, carboxyl groups, amino groups and amide groups. 3. The natural rubber composition according to claim 1, wherein said hydrophilic polymer is one or more selected from proteins and polysaccharides. 4. The natural rubber composition according to any one of claims 1 to 3, wherein said hydrophilic polymer comprises cellulose. 5. The natural rubber composition according to any one of claims 1 to 4, wherein said hydrophilic polymer is fibrous. 6. The natural rubber composition according to any one of claims 1 to 5, wherein said metal oxide is at least one selected from silica particles, titanium oxide particles, zinc oxide particles , zirconia particles and cobalt oxide particles. 7. The metal oxide according to any one of claims 1 to 6, which is produced by a sol-gel reaction using a compound represented by the following chemical formula (A) or (B) as a starting material. Natural rubber composition.
A. _n M. _m O. _x ... (A)
A. _n M. _m (OR) _x ... (B)
[A is an alkali metal element, an alkaline earth metal element, or a Group 3 element, M is a transition metal element or metalloid element, R is an alkyl group, n is an integer, and m and x are natural numbers] 8. The natural rubber composition according to any one of claims 1 to 7, wherein said metal oxide is silica particles produced by a sol-gel reaction of sodium silicate. The natural rubber composition according to any one of claims 1 to 8 , wherein the content of said composite filler is 1 part by mass or more and 10 parts by mass or less per 100 parts by mass of said natural rubber component. 10. The natural rubber composition according to any one of claims 1 to 9 , wherein in said composite filler, the mass ratio of said metal oxide to said hydrophilic polymer is 0.1 times or more and 3 times or less. A vibration-damping member manufactured using the natural rubber composition according to any one of claims 1 to 10 . An aqueous solution containing a compound represented by the following chemical formula (A) or (B) and a hydrophilic polymer is stirred at a pH of 7 or more and 10 or less, and metal oxidation produced by a sol-gel reaction of the compound. A composite filler manufacturing process for manufacturing a composite filler in which a substance and the hydrophilic polymer are chemically bonded;
a dispersion preparation step of adding the composite filler to natural rubber latex to prepare a dispersion;
a drying step of drying the dispersion;
A method for producing a natural rubber composition, comprising:
_{AnMmOx ( A} )
A _n M _m (OR) _x (B)
[A is an alkali metal element, an alkaline earth metal element, or a Group 3 element, M is a transition metal element or metalloid element, R is an alkyl group, n is an integer, and m and x are natural numbers] 13. The method for producing a natural rubber composition according to claim 12, wherein the drying step is performed by applying or spraying the dispersion onto a heated substrate. The solid content of the natural rubber latex is 10% by mass or more,
14. The method for producing a natural rubber composition according to claim 12 or 13 , wherein the drying speed of the dispersion in the drying step is 1200 mL/hour or more per liter of the dispersion.