Method for kneading natural rubber and rubber composition

Abstract

To provide a method for kneading natural rubber, which can reduce a viscosity of natural rubber and achieve an excellent processing property while suppressing deterioration of a physical property during mastication of natural rubber.SOLUTION: There is provided a method for kneading natural rubber, comprising: a step of masticating an unvulcanized natural rubber. The mastication of the unvulcanized natural rubber is carried out in an atmosphere having an oxygen concentration of 10% or less.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a method for kneading natural rubber and a rubber composition. [Background technology]

[0002] Generally, natural rubber (NR) is produced in tropical countries such as Thailand, Malaysia, and Indonesia. Natural rubber is widely used in large quantities in the rubber and tire industries due to its excellent physical properties. Natural rubber latex has a low Mooney viscosity of 60-70 immediately after production, but after harvest, the Mooney viscosity rises to nearly 90-100 during the several months it is stored and transported (this is called storage hardening).

[0003] The cause of storage hardening of natural rubber is believed to be that heterogeneous bonds (aldehyde groups, etc.) in the isoprene chain react with proteins and amino acids in the natural rubber, crosslinking and causing gelation (an increase in the amount of gel), and gelation in natural rubber deteriorates its processability. In addition, in general, a large molecular weight is preferable for natural rubber in terms of its physical properties, and a decrease in molecular weight has a negative effect on the physical properties of natural rubber. This molecular weight and amount of polymer gel are greatly affected by the manufacturing conditions of the natural rubber after extraction and the storage conditions thereafter.

[0004] For this reason, when rubber materials such as tires are manufactured using natural rubber, mastication is usually performed to reduce the Mooney viscosity of natural rubber (NR). However, during this mastication process, viscosity reduction occurs simultaneously through the chemical reaction of oxygen during the kneading process and through physical forces, which not only destroys the polymer gel mainly composed of peptide bonds in NR, but also simultaneously cuts the NR main chain, resulting in a deterioration of the physical properties of natural rubber, such as its fracture resistance and crack resistance.

[0005] In order to solve the problems during the mastication of such natural rubber, for example, Patent Document 1 discloses a technique of adding an antioxidant during the mastication of natural rubber. According to the technique of Patent Document 1, it has become possible to suppress the cleavage of the NR main chain during mastication, relatively reduce the polymer gel, and maintain a high molecular weight.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0007] However, recently, rubber compositions applied to tires are required to have higher levels of processability, fracture resistance, crack resistance, etc. Along with this, the development of a kneading technique for natural rubber that can achieve better processability while suppressing a decrease in physical properties has also been desired.

[0008] Therefore, an object of the present invention is to provide a kneading method for natural rubber that can suppress a decrease in physical properties during the mastication of natural rubber, reduce the viscosity of natural rubber, and achieve excellent processability. Another object of the present invention is to provide a rubber composition excellent in processability, fracture resistance, and crack resistance.

Means for Solving the Problems

[0009] As a result of investigations to solve the above problems, the inventor has found that in the kneading process of natural rubber, viscosity reduction due to chemical reaction of oxygen and viscosity reduction due to physical force are carried out simultaneously. Since the viscosity reduction by oxygen cuts the main chain of the polymer, the molecular weight of natural rubber becomes small, leading to deterioration of physical properties. Then, as a result of further intensive research, in the kneading process of unvulcanized natural rubber, the oxygen concentration is reduced compared to the conventional kneading process (specifically, in an atmosphere with an oxygen concentration of 10% or less), and by performing viscosity reduction mainly by physical force, the above-mentioned cleavage of the polymer main chain can be further suppressed. Therefore, it has been found that the molecular weight of natural rubber can be maintained high, and the viscosity can be reduced while suppressing deterioration of physical properties, and the present invention has been completed.

[0010] The gist configuration of the present invention for solving the above problems is as follows. (1) A method for kneading natural rubber, including a kneading step of kneading unvulcanized natural rubber, characterized in that the kneading of the unvulcanized natural rubber is carried out in an atmosphere with an oxygen concentration of 10% or less. The method for kneading natural rubber having the above configuration can reduce the viscosity of natural rubber while suppressing deterioration of physical properties during kneading of natural rubber, and can achieve excellent processability.

[0011] (2) The method for kneading natural rubber according to (1), characterized in that in the kneading step, an inert gas is sealed in the kneader. The method for kneading natural rubber having the above configuration can more reliably suppress deterioration of physical properties during kneading of natural rubber, reduce the viscosity of natural rubber, and achieve excellent processability.

[0012] (3) The method for kneading natural rubber according to (2), characterized in that the inert gas is nitrogen. The method for kneading natural rubber having the above configuration can more reliably suppress deterioration of physical properties during kneading of natural rubber, reduce the viscosity of natural rubber, and achieve excellent processability.

[0013] (4) In the mastication step, the kneading method of natural rubber according to any one of (1) to (3), characterized in that a processing aid is further added. The kneading method of natural rubber having the above configuration can achieve better processability.

[0014] (5) In the mastication step, the kneading method of natural rubber according to any one of (1) to (4), characterized in that an antioxidant is further added. The kneading method of natural rubber having the above configuration can more reliably suppress the deterioration of physical properties during the mastication of natural rubber, reduce the viscosity of natural rubber, and achieve excellent processability.

[0015] (6) A rubber composition, characterized in that it contains natural rubber obtained by the kneading method according to any one of (1) to (5) as a rubber component. The rubber composition having the above configuration is excellent in processability, fracture resistance, and crack resistance.

Effects of the Invention

[0016] According to the present invention, it is possible to provide a kneading method of natural rubber that can reduce the viscosity of natural rubber while suppressing the deterioration of physical properties during the mastication of natural rubber and achieve excellent processability. Further, according to the present invention, it is possible to provide a rubber composition excellent in processability, fracture resistance, and crack resistance.

Embodiments for Carrying Out the Invention

[0017] Hereinafter, an embodiment of the kneading method of natural rubber and the rubber composition of the present invention will be specifically illustrated and described. Note that the compounds described in this specification may be partially or entirely derived from fossil resources, may be derived from biological resources such as plant resources, or may be derived from recycled resources such as used tires. Further, it may be derived from a mixture of any two or more of fossil resources, biological resources, and recycled resources. In this specification, the biological resources (biomass resources) refer to carbon-neutral organic resources derived from living organisms, including, for example, those stored in the form of starch, cellulose, etc., the bodies of animals that grow by eating plants, and products obtained by processing plants and animals, excluding fossil resources (such as petroleum, coal, natural gas, etc.). These biological resources may be edible or inedible, but it is preferable that they do not compete with food and are inedible from the perspective of effective resource utilization.

[0018] Also, in this specification, the recycled resources (recyclable resources) refer to resources obtained by recycling (recycling) products that have been used once, collected without being used, or discarded. For example, recycled resources include resources obtained by recycling used rubber products such as used tires.

[0019] <Mixing Method of Natural Rubber> The mixing method of natural rubber of the present invention is a mixing method of natural rubber that includes a kneading step of kneading unvulcanized natural rubber. And the present invention is characterized in that the kneading of the unvulcanized natural rubber is carried out in an atmosphere with an oxygen concentration of 10% or less.

[0020] In the kneading step, by reducing the oxygen concentration compared to the conventional kneading step (setting the atmosphere with an oxygen concentration of 10% or less) and performing viscosity reduction mainly with physical force, it is possible to suppress the cleavage of the polymer main chain that occurs during the kneading of unvulcanized natural rubber and suppress the decrease in the molecular weight of natural rubber. As a result, even when the viscosity of the unvulcanized natural rubber is reduced by kneading and the processability is improved, the physical properties (such as fracture resistance characteristics and crack resistance characteristics) can be maintained well.

[0021] (Kneading Step) The mixing method of natural rubber of the present invention includes a kneading step of kneading unvulcanized natural rubber. The mastication of the unvulcanized natural rubber refers to the process of kneading only the unvulcanized natural rubber or the unvulcanized natural rubber and some compounding agents using a kneader or the like prior to the kneading step of the rubber composition. By performing the mastication step, the viscosity (Mooney viscosity) of the unvulcanized natural rubber can be reduced, so that the processability and workability of the unvulcanized natural rubber can be improved.

[0022] Note that the unvulcanized natural rubber is not particularly limited and can be appropriately selected according to the required performance and conditions. For example, there are two types according to the grading in the international quality packaging standard (commonly known as the Green Book) of various grades of natural rubber, ribbed smoked sheet (RSS type) and technically graded rubber (TSR type). Among these, the TSR type is roughly divided into two types, the wet process and the dry process. The common process is from the collection of natural rubber latex to the production of cup lump (putting formic acid in advance in the cup of latex and solidifying it naturally, the one tapped by the collecting farmer, etc.). This cup lump, USS / RSS, etc. are mixed and coagulated by the wet process or the dry process and transferred to a bucket, dried by a dryer, and then press-molded into a block and packed.

[0023] In addition, the kneader used for the mastication is not particularly limited as long as it is a commonly used type of machine. Also, the kneading temperature during kneading is not particularly limited. For example, under the temperature condition of 100 to 160 °C, the kneading time can be about 1 to 3 minutes.

[0024] And, as described above, in the kneading method of the natural rubber of the present invention, the mastication of the unvulcanized natural rubber is performed in an atmosphere with an oxygen concentration of 10% or less. In the mastication of the unvulcanized natural rubber, the viscosity reduction due to the chemical reaction of oxygen and the viscosity reduction due to physical force are performed simultaneously. Among these, regarding the chemical reaction with oxygen, since the polymer main chain of natural rubber is cleaved, when the oxygen concentration exceeds 10%, polymer cleavage cannot be sufficiently suppressed, leading to a deterioration in the physical properties of natural rubber. From the same perspective, the mastication of the unvulcanized natural rubber is preferably carried out in an atmosphere of 5% or less, more preferably in an atmosphere of 3% or less. Also, from the perspective of manufacturing efficiency and the like, since it is difficult to set the oxygen concentration to 0%, the oxygen concentration is preferably 1% or more.

[0025] In addition, there are no particular limitations on the method of setting the oxygen concentration during the mastication of the unvulcanized natural rubber to 10% or less, and it can be appropriately selected according to the manufacturing equipment and the like. For example, from the point that the oxygen concentration can be reduced efficiently and simply, it is preferable to enclose an inert gas in the kneader in the mastication step. There are no particular limitations on the method of enclosing the inert gas, and it can be appropriately selected according to the use of the kneader.

[0026] Also, the inert gas only needs to have low reactivity and not promote the oxidation of the rubber, and it can be appropriately selected while considering the manufacturing cost and the purification cost. For example, nitrogen or rare gas etc. can be mentioned. From the perspective of easy availability and cost, it is preferably nitrogen or argon, and more preferably nitrogen.

[0027] As described above, in the mastication step, only the unvulcanized natural rubber can be kneaded, or several compounding agents can be added to the unvulcanized natural rubber and kneaded. For example, from the perspective of further reducing the Mooney viscosity of the unvulcanized natural rubber, it is preferable to add a processing aid in the mastication step.

[0028] Examples of the processing aid include known processing aids such as peptizers. Particularly from the perspective of further reducing the Mooney viscosity, it is preferable to use a peptizer. Also, the compounding amount of the processing aid can be appropriately selected according to the required performance. From the viewpoint of more surely reducing the Mooney viscosity, it is preferably 0.01 part by mass or more with respect to 100 parts by mass (dry mass) of the unvulcanized natural rubber. Further, from the viewpoint of suppressing the deterioration of the fracture properties of natural rubber, the compounding amount of the processing aid is preferably 2 parts by mass or less, more preferably 1 part by mass or less, and still more preferably 0.1 part by mass or less with respect to 100 parts by mass (dry mass) of the unvulcanized natural rubber.

[0029] Also, in the mastication step, from the viewpoint of further enhancing the physical properties such as the fracture resistance and crack resistance of natural rubber, it is preferable to add an antioxidant in addition to the unvulcanized natural rubber. By adding the antioxidant, the cleavage of the polymer chains during the mastication of the unvulcanized natural rubber described above can be suppressed, and the decrease in the molecular weight of natural rubber can be further suppressed.

[0030] Here, the antioxidant is not particularly limited, and examples thereof include antioxidants commonly used in the rubber industry, such as naphthylamine-based, p-phenylenediamine-based, hydroquinone derivatives, bis, tris, polyphenol-based, diphenylamine-based, quinoline-based, monophenol-based, thiobisphenol-based, and hindered phenol-based antioxidants. Among these, p-phenylenediamine-based antioxidants are preferable in that they can further exhibit the above-described suitable effects. These antioxidants are generally added as a part of rubber compounding agents after mastication, but in the present invention, adding them in the mastication step is meaningful and can exhibit the effects of the present invention.

[0031] Examples of the p-phenylenediamine-based antioxidant include N,N'-diphenyl-p-phenylenediamine, N-isopropyl-N'-phenyl-p-phenylenediamine, N,N'-di-2-naphthyl-p-phenylenediamine, N-cyclohexyl-N'-phenyl-p-phenylenediamine, N-phenyl-N'-(3-methacryloyloxy-2-hydroxypropyl)-p-phenylenediamine, N,N'-bis(1-methylheptyl)-p-phenylenediamine, N,N'-bis(1,4-dimethylpentyl)-p-phenylenediamine, N,N'-bis(1-ethyl-3-methylpentyl)-p-phenylenediamine, N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine, etc. Among these, N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine and N-isopropyl-N'-phenyl-p-phenylenediamine are most preferred in terms of further effects and cost.

[0032] These antioxidants can be used alone or in combination of two or more. They are used in the range of 0.2 to 4 parts by mass, preferably 0.5 to 3 parts by mass, based on 100 parts by mass of the unvulcanized natural rubber as the dry mass. When the addition amount of the above antioxidant is 0.2 parts by mass or more, the physical properties such as the fracture resistance and crack resistance of the natural rubber can be further improved. When it exceeds 4 parts by mass, the effect is almost saturated.

[0033] In the kneading method of the natural rubber of the present invention, by passing through the preliminary kneading step, it is possible to suppress the cleavage of the polymer main chain and the decrease in the molecular weight of the natural rubber compared with the conventional process.

[0034] <Rubber composition> The rubber composition of the present invention contains natural rubber (hereinafter sometimes referred to as "natural rubber of the present embodiment") obtained by the kneading method of the natural rubber of the present invention described above as a rubber component. As a result, while suppressing the deterioration of the physical properties of the rubber component, excellent processability is obtained, and thus the processability, fracture resistance, and crack resistance of the rubber composition can be improved.

[0035] Regarding the rubber component, in addition to the natural rubber of the present embodiment, various synthetic rubbers can be contained. Examples of the synthetic rubber include diene-based synthetic rubbers such as butadiene rubber (BR), isoprene rubber (IR), styrene-butadiene rubber (SBR), styrene-isoprene-butadiene rubber (SIBR), chloroprene rubber (CR), acrylonitrile-butadiene rubber (NBR), and non-diene-based synthetic rubbers such as ethylene-propylene-diene rubber (EPDM), ethylene-propylene rubber (EPM), and butyl rubber (IIR).

[0036] In addition, regarding the natural rubber of the present embodiment, it can be used as it is, or can be made into a modified natural rubber or a natural rubber purified to a high purity by a centrifugation process or the like.

[0037] Note that the rubber composition of the present invention can contain a filler and various additives other than the rubber component. Examples of the filler include carbon black, silica, and other inorganic fillers. The carbon black is not particularly limited and can be appropriately selected according to the purpose. For example, those of FEF, SRF, HAF, ISAF, and SAF grades can be used. In addition, the content of carbon black in the rubber composition is not particularly limited and can be appropriately selected according to the purpose. For example, it may be 10 to 100 parts by mass with respect to 100 parts by mass of the rubber component. The silica is not particularly limited and can be used according to the application, such as general grade silica and special silica surface-treated with a silane coupling agent or the like. For example, wet silica can be used as the silica. In addition, the content of silica in the rubber composition is not particularly limited and can be appropriately selected according to the purpose. For example, it may be 10 to 200 parts by mass with respect to 100 parts by mass of the rubber component. Examples of the various additives include additives such as resins, anti-aging agents, vulcanizing agents, vulcanization accelerators, silane coupling agents, softeners, tackifiers, dispersants, crosslinking aids, stearic acid, colorants, antistatic agents, lubricants, etc., as well as various known compounding chemicals commonly used in the rubber industry. Commercially available products can be used for these.

[0038] Moreover, the method for producing the rubber composition of the present invention is not particularly limited. For example, a filler and various additives appropriately selected as necessary are compounded with the rubber component, followed by kneading, heat treatment, extrusion, etc., to produce the rubber composition. Also, by vulcanizing the obtained rubber composition, a vulcanized rubber can be obtained.

[0039] The conditions for the kneading are not particularly limited, and various conditions such as the charging volume of the kneading apparatus, the rotational speed of the rotor, the ram pressure, etc., and the kneading temperature, kneading time, type of kneading apparatus, etc., can be appropriately selected according to the purpose. Examples of the kneading apparatus usually include a Banbury mixer, an internal mixer, a kneader, rolls, etc., used for kneading rubber compositions. Also, the kneading can be carried out in an atmosphere with an oxygen concentration of 10% or less, similar to the mastication step of the unvulcanized natural rubber described above.

[0040] Regarding the conditions for the heat treatment, there are no particular limitations, and various conditions such as the heat treatment temperature, heat treatment time, heat treatment apparatus, etc., can be appropriately selected according to the purpose. Examples of the heat treatment apparatus usually include heat treatment roll machines used for heat treatment of rubber compositions. Regarding the conditions for the extrusion, there are no particular limitations, and various conditions such as the extrusion time, extrusion speed, extrusion apparatus, extrusion temperature, etc., can be appropriately selected according to the purpose. Examples of the extrusion apparatus usually include extruders used for extrusion of rubber compositions. The extrusion temperature can be determined appropriately.

[0041] There are also no particular restrictions on the apparatus, method, conditions, etc. for performing the vulcanization, and they can be appropriately selected according to the purpose. Examples of the apparatus for performing vulcanization generally include a molding vulcanizer using a mold used for vulcanizing a rubber composition. As the vulcanization conditions, the temperature is, for example, about 100 to 190°C.

[0042] In addition, the rubber composition of the present invention can be used for various applications. Applications include tires, automotive parts (automotive seats, automotive batteries (such as lithium-ion batteries), weather strips, hose tubes, vibration-proof rubbers, cables, sealing materials, etc.), conveyor belts, crawlers, vibration-proof rubbers, hoses, resin pipes, sound-absorbing materials, bedding, precision parts for office equipment (OA rollers), bicycle frames, golf balls, tennis rackets, golf shafts, resin additives, filters, adhesives, pressure-sensitive adhesives, inks, medical devices (medical tubes, backs, micro needles, rubber sleeves, artificial organs, caps, packings, syringe gaskets, medicine plugs, prosthetic feet, prosthetic limbs), cosmetics (UV powders, puffs, containers, waxes, shampoos, conditioners), detergents, building materials (floor materials, seismic isolation rubbers, base isolation rubbers, building films, sound-absorbing materials, waterproof sheets, heat-insulating materials, joint materials, sealing materials), packaging materials, liquid crystal materials, organic EL materials, organic semiconductor materials, electronic materials, electronic devices, communication devices, aircraft parts, mechanical parts, electronic parts, agricultural materials, electric wires, cables, fibers (wearable substrates), daily necessities (toothbrushes, shoe soles, glasses, dummy baits, binoculars, toys, dust masks, garden hoses), robot parts, optical parts, road materials (asphalt, guardrails, poles, signs), protective gear (shoes, bulletproof vests), exterior parts of electrical equipment, exterior parts of OA equipment, soles, sealing materials, etc.

Examples

[0043] Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to the following examples at all.

[0044] Using the natural rubbers of Reference Example 1 shown in Table 1 and Reference Example 2 shown in Table 2, mastication was carried out under the conditions described in each Example and Comparative Example of Tables 1 and 2. For the obtained samples of unvulcanized natural rubber, the weight average molecular weight and Mooney viscosity were measured as follows.

[0045] <Measurement method> (1) Weight average molecular weight (Mw) The weight average molecular weight (Mw) of the polymer component in the rubber composition was determined by gel permeation chromatography (GPC) measurement under the following conditions. The weight average molecular weight (Mw) is in terms of polystyrene. · Column: Manufactured by Tosoh Corporation: TSKgel GMHXL · Eluent: Tetrahydrofuran · Flow rate: 1 mL / min · Temperature: 40 °C · Detector: RI Note that, based on each reference example, the higher the weight average molecular weight, the better the physical properties of the natural rubber.

[0046] (2) Mooney viscosity For each unvulcanized natural rubber, the Mooney viscosity was measured in accordance with "Mooney viscosity, Mooney scorch time" of JIS K 6300-1 (2001). Note that, for the measurement results of the measured Mooney viscosity, the smaller the numerical value, the smaller the unvulcanized viscosity, indicating better processability.

[0047]

Table 1

Table 2

[0048] In addition, separately from the unvulcanized natural rubbers described in Tables 1 and 2, an unvulcanized rubber composition having the weight average molecular weight described in Table 3 was prepared, and the evaluation of the burst resistance property (TB) and crack resistance property was carried out. The weight average molecular weight was measured by the same method as described above. The polymer component of the unvulcanized rubber composition described in Table 3 is natural rubber, and in addition to the polymer component, fillers such as carbon black and processing aids are included, but they do not affect the test results.

[0049] <Performance Evaluation> (1) Fracture Resistance Property (TB) Vulcanized rubber was prepared from the unvulcanized natural rubber of each reference example under vulcanization conditions of 145 °C for 45 minutes. Each vulcanized rubber was processed into dumbbell-shaped No. 3 test pieces, and the breaking stress (TB) was measured using a tensile testing apparatus (Shimadzu Corporation). Specifically, the test piece was pulled at a speed of 500 mm / min at 25 °C, and the value obtained by dividing the tensile force recorded when the test piece was cut by the cross-sectional area of the test piece before the test was calculated. The measurement results of the breaking stress (TB) are shown in Table 3, and the larger the numerical value, the better the tensile strength.

[0050] (2) Crack Resistance Property Evaluation Vulcanized rubber was prepared from the unvulcanized natural rubber of each reference example under vulcanization conditions of 145 °C for 45 minutes. Each vulcanized rubber was processed into JIS No. 3 test pieces. A 0.5-mm crack was made in the center of the test piece in the longitudinal direction of the test piece. Using a tensile testing apparatus (manufactured by Shimadzu Corporation), under the condition of an ambient temperature of 80 °C, repeated stress was applied in the strain range of 0% to 70% in the longitudinal direction of the test piece, and the number of times until the test piece was cut was measured. Taking the measurement result of Reference Example 1 as 100, each result was expressed as an index, and the results are shown in Table 3. The larger the numerical value, the better the crack resistance property.

[0051]

Table 3

[0052] From the results in Table 3, it was found that the unvulcanized natural rubber with a high weight-average molecular weight is excellent in fracture resistance property and crack resistance property. Also, from the results in Table 1 and Table 2, it can be seen that each sample of the examples has a higher weight-average molecular weight than each sample of the comparative examples, that is, it is excellent in fracture resistance property and crack resistance property, and also excellent in processability.

Industrial Applicability

[0053] According to the present invention, it is possible to provide a kneading method for natural rubber that can reduce the viscosity of natural rubber while suppressing a decrease in physical properties during mastication of natural rubber, and can achieve excellent processability. Further, according to the present invention, it is possible to provide a rubber composition excellent in processability, fracture resistance characteristics, and crack resistance characteristics.

Claims

1. A kneading method for natural rubber, comprising a kneading step of kneading unvulcanized natural rubber, wherein the kneading of the unvulcanized natural rubber is carried out in an atmosphere with an oxygen concentration of 10% or less. A kneading method for natural rubber.

2. The kneading method for natural rubber according to claim 1, wherein in the kneading step, an inert gas is enclosed in the kneader.

3. The kneading method for natural rubber according to claim 2, wherein the inert gas is nitrogen.

4. The kneading method for natural rubber according to claim 1 or 2, wherein in the kneading step, a processing aid is further added.

5. The kneading method for natural rubber according to claim 1 or 2, wherein in the kneading step, an anti-aging agent is further added.

6. A rubber composition, characterized by containing natural rubber obtained by the kneading method according to claim 1 or 2 as a rubber component.

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