Devulcanized rubber, rubber composition and pneumatic tire

Abstract

To provide a devulcanized rubber that exhibits good post-vulcanization physical properties, a rubber composition that contains the devulcanized rubber and has excellent workability, and a pneumatic tire including a rubber part obtained by vulcanizing and molding the rubber composition.SOLUTION: The present invention relates to a devulcanized rubber obtained by mixing a devulcanizing agent having an alkylamide group and a vulcanized rubber. The devulcanizing agent preferably contains an amide compound of an alkylamine having 12 or more carbon atoms. The rubber composition containing the devulcanized rubber has excellent workability, and the devulcanized rubber of the rubber composition has good post-vulcanization physical properties, so that they are suitable as a raw material for a pneumatic tire.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a devulcanized rubber and a rubber composition that exhibit good physical properties after vulcanization, and a pneumatic tire having a rubber portion formed by vulcanizing the rubber composition. [Background technology]

[0002] In recent years, with the rise in environmental awareness, there has been a strong demand for the reuse of waste vulcanized rubber generated from used tires and other rubber products.

[0003] Patent Document 1 listed below describes a method for obtaining a re-vulcanizable devulcanized product, in which the devulcanizing agent is an amine compound, the rubber contains an EPDM polymer, the sulfur-crosslinked rubber is heated to a temperature of 250 to 350°C during devulcanization, and the devulcanization of the sulfur-crosslinked rubber is carried out at a pressure of 5 to 10 MPa. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 4633988 Summary of the Invention [Problem to be solved by the invention]

[0005] As a result of intensive research, the present inventors have found that when the devulcanized product obtained by the above-mentioned conventional technology is compounded into a rubber composition, the vulcanization time and scorch time are significantly shortened, and therefore there is room for further improvement in terms of the processability of the rubber composition. In addition, because the above-mentioned conventional technology is a technology specialized for EPDM-based rubbers, it has been found that there is a limit to the types of rubber that can be used to make the vulcanized rubber that is the raw material for devulcanized rubber.

[0006] The present invention has been made in view of the above-mentioned circumstances, and an object of the present invention is to provide devulcanized rubber that exhibits good physical properties after vulcanization, a rubber composition that contains the devulcanized rubber and has excellent processability, and a pneumatic tire that has a rubber portion formed by vulcanizing and molding the rubber composition.

[0007] The above problems can be solved by the following constitution: The present invention relates to a devulcanized rubber obtained by mixing a devulcanizing agent having an alkylamide group with vulcanized rubber.

[0008] In the above devulcanized rubber, the devulcanizing agent is preferably an amide compound of an alkylamine having 12 or more carbon atoms.

[0009] The present invention also relates to a rubber composition containing the above-described devulcanized rubber.

[0010] Furthermore, the present invention relates to a pneumatic tire having a rubber portion formed by vulcanizing the above-described rubber composition. [Effects of the Invention]

[0011] As mentioned above, conventional techniques use amine compounds as devulcanizing agents. However, because the amino group of the amine compounds is highly basic, rubber compositions containing devulcanized rubber obtained by mixing the devulcanizing agent made of an amine compound with vulcanized rubber tend to have significantly shorter vulcanization times and scorch times.

[0012] The devulcanized rubber according to the present invention is obtained by mixing a devulcanizing agent with vulcanized rubber, and is characterized in that the devulcanizing agent is a compound having an alkylamide group. Compared to an amino group, the density of lone electron pairs on the nitrogen atom is lower in an alkylamide group. Therefore, the alkylamide group has lower basicity than an amino group. Therefore, a rubber composition containing a devulcanized rubber obtained by mixing a devulcanizing agent having an alkylamide group with vulcanized rubber can ensure sufficient vulcanization time and scorch time, resulting in excellent processability. In addition, even when the devulcanized rubber is compounded in a rubber composition, the optimal vulcanization time can be set, resulting in excellent post-vulcanization physical properties of the vulcanized rubber. Therefore, the vulcanized rubber of the rubber composition can be suitably used as the rubber portion of a pneumatic tire. DETAILED DESCRIPTION OF THE INVENTION

[0013] The devulcanized rubber according to the present invention is obtained by mixing a devulcanizing agent having an alkylamide group with vulcanized rubber.

[0014] It is preferable to use a vulcanized rubber of a diene rubber as the raw material for the devulcanized rubber of the present invention. The diene rubber is not particularly limited, and examples thereof include natural rubber (NR), isoprene rubber (IR), butadiene rubber (BR), styrene-butadiene rubber (SBR), acrylonitrile-butadiene rubber (NBR), chloroprene rubber (CR), styrene-isoprene copolymer rubber, butadiene-isoprene copolymer, and styrene-isoprene-butadiene copolymer rubber. These may be used alone or in combination of two or more. The diene rubber is preferably natural rubber, butadiene rubber, and styrene-butadiene rubber, or a blend of two or more of these.

[0015] The devulcanizing agent used as the raw material for the devulcanized rubber of the present invention is a compound having an alkylamide group. Preferably, it is a compound obtained by amidating a long-chain alkylamine compound, and more preferably, it is a compound obtained by amidating an alkylamine compound having 12 or more carbon atoms. Suitable devulcanizing agents include, for example, a compound obtained by amidating hexadecylamine (having 16 carbon atoms), a compound obtained by amidating dodecylamine (having 12 carbon atoms), or a compound obtained by amidating stearylamine (having 18 carbon atoms).

[0016] From the viewpoint of improving the physical properties after vulcanization, the devulcanized rubber according to the present invention is a devulcanized rubber obtained by mixing a devulcanizing agent having an alkylamide group with vulcanized rubber, and it is preferable that the amount of the devulcanizing agent used is 1 to 30 parts by mass when the total amount of the vulcanized rubber is 100 parts by mass.

[0017] The sulfur used as a raw material for vulcanized rubber may be any ordinary rubber sulfur, such as powdered sulfur, precipitated sulfur, insoluble sulfur, highly dispersible sulfur, etc.

[0018] The vulcanized rubber that is the raw material for devulcanized rubber can be a rubber composition that has been vulcanized and contains, in addition to a rubber component and sulfur, compounding agents that are normally used in the rubber industry, such as vulcanization accelerators, carbon black, silica, silane coupling agents, zinc oxide, stearic acid, vulcanization retarders, organic peroxides, antioxidants, softeners such as wax and oil, and processing aids, as needed.

[0019] As the carbon black, any carbon black known to those skilled in the art can be used, such as SAF, ISAF, HAF, FEF, GPF, etc. When the total amount of rubber components in the vulcanized rubber that is the raw material for the devulcanized rubber is taken as 100 parts by mass, the content of carbon black is preferably 30 to 100 parts by mass, and more preferably 30 to 60 parts by mass.

[0020] As the vulcanization accelerator, vulcanization accelerators commonly used for rubber vulcanization, such as sulfenamide-based vulcanization accelerators, thiuram-based vulcanization accelerators, thiazole-based vulcanization accelerators, thiourea-based vulcanization accelerators, guanidine-based vulcanization accelerators and dithiocarbamate-based vulcanization accelerators, may be used alone or in appropriate mixtures.

[0021] As the antiaging agent, antiaging agents commonly used for rubber, such as aromatic amine antiaging agents, amine-ketone antiaging agents, monophenol antiaging agents, bisphenol antiaging agents, polyphenol antiaging agents, dithiocarbamate antiaging agents, and thiourea antiaging agents, may be used alone or in appropriate mixtures.

[0022] The devulcanized rubber according to the present invention is preferably obtained by heating and mixing a vulcanized rubber and a devulcanizing agent while applying shear force, and the devulcanizing agent used is a devulcanizing agent having an alkylamide group. Such a devulcanized rubber can be produced, for example, by heating and mixing a vulcanized rubber and a devulcanizing agent while applying shear force. Examples of methods for applying shear force to the vulcanized rubber and the devulcanizing agent include methods using a rolling mill, Banbury mixer, extruder, gear pump, intermeshing mixer, kneader, or co-kneader commonly used by those skilled in the art to heat and mix the vulcanized rubber and the devulcanizing agent while applying shear force. In the present invention, when applying shear force to the vulcanized rubber and the devulcanizing agent, the rolling mill, Banbury mixer, extruder, gear pump, intermeshing mixer, kneader, and co-kneader may be used alone or in combination. When a roller mill is used, for example, a method known to those skilled in the art may be used in which the devulcanizing agent and vulcanized rubber are mixed and passed through the roller mill so that the devulcanizing agent can sever the sulfur crosslinks in the vulcanized rubber. When a Banbury mixer is used, for example, a method may be used in which the devulcanizing agent and vulcanized rubber are placed in the Banbury mixer and stirred in the Banbury mixer so that the devulcanizing agent can sever the sulfur crosslinks in the vulcanized rubber. The shear pressure when applying shear force may be the same as the shear pressure applied to the material to be kneaded when using a roller mill or Banbury mixer, and may be, for example, about 0.1 to 10 MPa.

[0023] The heating temperature when the vulcanized rubber and devulcanizing agent are heated and mixed is preferably controlled to 100°C or less. By setting the heating temperature to a low level of 100°C or less and effectively applying shear force to the vulcanized rubber and devulcanizing agent, it is possible to produce devulcanized rubber and a rubber composition that exhibit good physical properties after vulcanization. The heating temperature is preferably 50°C or less.

[0024] The resulting devulcanized rubber has selectively cleaved the sulfur crosslinks of the vulcanized rubber to be reused, and therefore, even when mixed with unvulcanized (virgin) rubber components, good post-vulcanization physical properties can be obtained. The rubber composition preferably contains 1 to 30 parts by mass of devulcanized rubber, assuming the total amount of unvulcanized rubber components in the rubber composition is 100 parts by mass. The rubber components, sulfur, vulcanization accelerators, carbon black, silica, silane coupling agents, zinc oxide, stearic acid, vulcanization retarders, organic peroxides, antioxidants, softeners such as wax and oil, processing aids, and the like that can be used to constitute the rubber composition blended with the devulcanized rubber are the same as those described above.

[0025] The method for compounding the devulcanized rubber and the above-mentioned components is not particularly limited, and any of the following may be used: a method in which the components other than the vulcanization-related components such as sulfur and vulcanization accelerator are pre-mixed to form a master batch, and the remaining components are then added and further kneaded; a method in which the components are added in any order and kneaded; and a method in which all the components are added simultaneously and kneaded.

[0026] The rubber composition containing the devulcanized rubber has excellent processability and excellent physical properties after vulcanization. Therefore, the devulcanized rubber according to the present invention and the rubber composition containing the devulcanized rubber are particularly suitable for use in pneumatic tires having a large amount of rubber. [Example]

[0027] The present invention will be explained in more detail below by way of examples.

[0028] (Preparation of vulcanized rubber as raw material for devulcanized rubber) A Daihan Lab Mixer was used to mix 100 parts by mass of rubber components according to the compounding recipe in Table 1. In the first mixing stage, all compounding ingredients except for sulfur and vulcanization accelerator were added to the rubber components and kneaded, and then in the second mixing stage, sulfur and vulcanization accelerator were added to the resulting mixture and kneaded to prepare a rubber composition.

[0029] [Table 1]

[0030] Details of each component in Table 1 are as follows: Natural rubber: STR20 Styrene butadiene rubber: Asahi Kasei "Tufden 2000" Butadiene rubber: Asahi Kasei "NF35" Carbon black: "Seast 3" manufactured by Tokai Carbon Co., Ltd. Silica: Oriental Silica "Nipsil AQ" Zinc oxide: "Zinc oxide type 1" manufactured by Mitsui Mining & Smelting Co., Ltd. Anti-aging agent: "Nocrac 6C" manufactured by Ouchi Shinko Chemical Industry Co., Ltd. Stearic acid: Kao Corporation's "Lunac S-20" Sulfur: Hosoi Chemical Industry Co., Ltd. "Powdered sulfur for rubber, 150 mesh" Vulcanization accelerator 1: "Noccela NS" manufactured by Ouchi Shinko Chemical Industry Co., Ltd. Vulcanization accelerator 2: "Noccela D" manufactured by Ouchi Shinko Chemical Industry Co., Ltd.

[0031] The rubber composition was vulcanized at 150°C for 25 minutes to produce a sheet of vulcanized rubber, and the vulcanized rubber was pulverized using a roll (6-inch roll manufactured by Kansai Roll Co., Ltd.) to prepare powdered vulcanized rubber (NR type) and powdered vulcanized rubber (mixed type).

[0032] (Devulcanized rubber manufacturing) The powdered vulcanized rubber (NR type) or vulcanized rubber (mixed type) was mixed with a devulcanizing agent in the proportions shown in Table 2, and then the mixture of vulcanized rubber powder and devulcanizing agent was cooled using a roll (6-inch roll manufactured by Kansai Roll Co., Ltd.) with a surface cooled to 25°C, and the mixture was passed through the roll repeatedly at minimum intervals for 40 minutes to produce devulcanized rubbers according to Examples 1 to 5 and Comparative Examples 1 to 5 (heating temperature during mixing: 25°C).

[0033] [Table 2]

[0034] Details of each desulfurizing agent in Table 2 are as follows: Desulfurizing agent 1: "Stearylamine" (carbon number 18) manufactured by Tokyo Chemical Industry Co., Ltd. Desulfurizing agent 2: "Acetylated stearylamine" (desulfurizing agent with alkylamide group, carbon number 20) Desulfurization agent 3: "Boc compound of hexadecylamine" (desulfurization agent with alkylamide group, carbon number 21) Desulfurizing agent 4: "Hexadecylamine" (carbon number 16) manufactured by Tokyo Chemical Industry Co., Ltd. Desulfurizing agent 5: Tokyo Chemical Industry Co., Ltd. "Dodecylamine" (carbon number 12) Desulfurizing agent 6: "Acetylated dodecylamine" (desulfurizing agent with alkylamide, carbon number 14) The desulfurizing agents 2, 3 and 6 were produced by the following method.

[0035] (Desulfurization agent 2) 10.8 g of stearylamine and 6 g of pyridine (manufactured by Tokyo Chemical Industry Co., Ltd.) were dissolved in 500 ml of ethyl acetate (manufactured by Nacalai Tesque). 5 g of acetic anhydride (manufactured by Tokyo Chemical Industry Co., Ltd.) was added dropwise to this solution while stirring at room temperature, and the mixture was then heated to 80°C and stirred for 4 hours. After stirring, the precipitate was filtered off, and the filtrate was washed three times with 100 ml of 10% aqueous citric acid solution (citric acid manufactured by Nacalai Tesque). An appropriate amount of anhydrous magnesium sulfate (manufactured by Nacalai Tesque) was then added and dehydrated. After filtering off the anhydrous magnesium sulfate, the solution was concentrated under reduced pressure to obtain a slurry. This was then filtered, and the solid was washed with hexane (manufactured by Nacalai Tesque) and then vacuum dried to obtain desulfurizing agent 2 described in the following formula (1). [ka]

[0036] (Desulfurization agent 3) 9.64 g of hexadecylamine and 3.5 g of sodium bicarbonate (manufactured by Nacalai Tesque) were dissolved in 200 ml of dichloromethane (manufactured by Nacalai Tesque). A solution of 8.72 g of di-tert-butyl dicarbamate (manufactured by Tokyo Chemical Industry Co., Ltd.) dissolved in 100 ml of dichloromethane was added dropwise to this solution while stirring at room temperature, and stirring was continued overnight. After stirring was completed, the precipitate was filtered off, and the filtrate was washed three times with 100 ml of 10% aqueous citric acid solution (citric acid manufactured by Nacalai Tesque). An appropriate amount of anhydrous magnesium sulfate (manufactured by Nacalai Tesque) was then added and dehydrated. After filtering off the anhydrous magnesium sulfate, the solution was concentrated under reduced pressure to obtain a slurry. This was filtered, and the solid was washed with hexane (manufactured by Nacalai Tesque) and then vacuum dried to obtain desulfurization agent 3 described in the following formula (2). [ka]

[0037] (Desulfurization agent 6) 5.6 g of dodecylamine and 2.4 g of pyridine (manufactured by Tokyo Chemical Industry Co., Ltd.) were dissolved in 300 ml of ethyl acetate (manufactured by Nacalai Tesque, Inc.). 3.1 g of acetic anhydride (manufactured by Tokyo Chemical Industry Co., Ltd.) was added dropwise to this solution while stirring at room temperature, and the mixture was then heated to 80°C and stirred for 4 hours. After stirring, the precipitate was filtered off, and the filtrate was washed three times with 100 ml of 10% aqueous citric acid solution (citric acid manufactured by Nacalai Tesque, Inc.). An appropriate amount of anhydrous magnesium sulfate (manufactured by Nacalai Tesque, Inc.) was then added and dehydrated. After filtering off the anhydrous magnesium sulfate, the solution was concentrated under reduced pressure to obtain a slurry. This was filtered, and the solid was washed with hexane (manufactured by Nacalai Tesque, Inc.) and then vacuum dried to obtain desulfurization agent 6 described in the following formula (3). [ka]

[0038] (Preparation of Rubber Composition) Each component was blended according to the formulation in Table 3 with respect to 100 parts by mass of the rubber component, and kneaded using a lab mixer manufactured by Daihan Co., Ltd. to prepare a rubber composition containing the devulcanized rubber produced in Examples 1 to 5 and Comparative Examples 1 to 5.

[0039] Details of each component in Table 3 are as follows: Natural rubber: STR20 Carbon black: "Seast 3" manufactured by Tokai Carbon Co., Ltd. Zinc oxide: "Zinc oxide type 1" manufactured by Mitsui Mining & Smelting Co., Ltd. Anti-aging agent: "Nocrac 6C" manufactured by Ouchi Shinko Chemical Industry Co., Ltd. Stearic acid: Kao Corporation's "Lunac S-20" Sulfur: Hosoi Chemical Industry Co., Ltd. "Powdered sulfur for rubber, 150 mesh" Vulcanization accelerator: "Noccela NS" manufactured by Ouchi Shinko Chemical Industry Co., Ltd.

[0040] (Processability of rubber compositions containing devulcanized rubber and evaluation of physical properties of vulcanized rubber after vulcanization (300% elongation stress)) The scorch time of the rubber compositions containing the devulcanized rubbers produced in Examples 1 to 4 and Comparative Examples 1 to 4 was measured by the following method to evaluate the processability. Scorch time: The scorch time (t5) of the rubber composition was measured using a Mooney viscometer conforming to JIS K6300-1. The evaluation was performed using an index where the value of Comparative Example 1 was set to 100, with a larger index indicating a longer scorch time and better processability.

[0041] Next, the obtained rubber composition was vulcanized under conditions of 150°C for 25 minutes to prepare test pieces of a predetermined shape, and the obtained test pieces were used to carry out the following tests. 300% elongation stress: The 300% elongation stress was measured using a tensile test (JIS No. 3 dumbbell) in accordance with JIS K6251. Evaluation was based on an index where the value of Comparative Example 1 was set to 100, with a larger index indicating better physical properties after vulcanization.

[0042] [Table 3]

[0043] The results in Table 3 show that the rubber compositions containing the devulcanized rubbers produced in Examples 1 to 5 have superior processability compared to the rubber compositions containing the devulcanized rubbers produced in Comparative Examples 1 to 5. In addition, it is also shown that the vulcanized rubbers of the rubber compositions containing the devulcanized rubbers produced in Examples 1 to 5 (vulcanized rubbers 3 to 6 and 10) have superior physical properties after vulcanization compared to the vulcanized rubbers of the rubber compositions containing the devulcanized rubbers produced in Comparative Examples 1 to 5 (vulcanized rubbers 1 to 2 and 7 to 9).

Claims

1. A method for producing devulcanized rubber by heating and mixing vulcanized rubber with an amide compound of an alkylamine having 12 or more carbon atoms.

2. A method for producing devulcanized rubber as described in claim 1, wherein the heating and mixing is carried out while applying shear force.

3. A method for producing devulcanized rubber as described in claim 1 or 2, wherein the heating temperature during the heating and mixing is 50°C or less.

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