Desulfurization method for sulfur-crosslinked rubber

Abstract

To provide a novel desulfurization method for a sulfur-crosslinked rubber which requires no desulfurizing agent and causes no problem of odor and which facilitates desulfurization and does not take a long period of time, so that physical properties are less likely to deteriorate.SOLUTION: Desulfurization is carried out by bringing hydrogen plasma into contact with a sulfur-crosslinked rubber. The hydrogen plasma is preferably generated by microwave. The contact is preferably carried out in a processing chamber. A pressure in the processing chamber is preferably 2000 Pa or less.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for devulcanization of sulfur-crosslinked rubber. [Background technology]

[0002] In recent years, interest in resource recycling of organic polymers such as plastics and rubber has been growing. Recycling of vulcanized rubber has been considered, but because it is economically difficult to make it viable as an industry, most of it is currently consumed as fuel. Other ways to reuse vulcanized rubber besides fuel include using it as it is, such as as a fender, powdering it and solidifying it, and using it as various fillers.

[0003] Many vulcanized rubber products are made of sulfur crosslinked rubber, and in order to recycle, reuse, and recycle them, it is necessary to cleave the characteristic sulfur crosslinks (SS bonds) and devulcanize them, and it is therefore necessary to increase the devulcanization rate. Conventional methods for devulcanizing sulfur crosslinked rubber are the shear method and the chemical method, and the plasma method is also known experimentally.

[0004] The shear method is a method of devulcanization in which sulfur crosslinked rubber is melt-kneaded and the sulfur bonds are physically cut by shear stress (Patent Document 1). Although this method is simple, it requires high temperature conditions and, if it takes a long time to complete devulcanization, there is a problem that the proportion of the rubber main chains that are partially cut increases, resulting in deterioration of physical properties.

[0005] The chemical method is a method of desulfurizing by chemically cutting the sulfur bonds of sulfur-crosslinked rubber with a desulfurizing agent (Patent Document 2). Although this method does not cause much deterioration in physical properties due to cutting of the rubber main chain, the desulfurizing agents (disulfide compounds, thiol compounds, dimethyl sulfoxide, amine compounds, etc.) are a group of compounds that emit a peculiar odor, so there is a problem that the odor is generated during desulfurization and that the odor also accompanies the regenerated rubber after desulfurization.

[0006] The plasma method is a method in which the sulfur-crosslinked rubber is brought into contact with so-called plasma to promote a devulcanization reaction, and Patent Document 3 describes air, oxygen, nitrogen, and oxygen-enriched gas as plasma generating gases. This method does not require a desulfurizing agent and does not have the problem of odor, but as described below, according to the investigations by the present inventors, there is a major problem in that devulcanization does not progress easily. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] JP 2017-019949 A [Patent Document 2] Special Publication No. 2010-535912 [Patent Document 3] China Patent Application Publication No. 111621058 Summary of the Invention [Problem to be solved by the invention]

[0008] As described above, the inventors have focused on the plasma method, which does not require a desulfurizing agent and has no odor problem, and have investigated desulfurization using oxygen plasma (which is thought to cut S-S bonds and form S-O bonds). However, there are problems in that desulfurization is difficult to proceed and takes a long time, which leads to deterioration of physical properties.

[0009] Therefore, an object of the present invention is to provide a new method for devulcanization of sulfur crosslinked rubber, which does not require a devulcanizing agent, is free from odor problems, and allows devulcanization to proceed easily and does not take a long time, so that physical properties are not easily deteriorated. [Means for solving the problem]

[0010] [1] A method for devulcanization of sulfur-crosslinked rubber, comprising contacting the sulfur-crosslinked rubber with hydrogen plasma.

[0011] [2] The method for devulcanization of sulfur crosslinked rubber according to the above [1], wherein the hydrogen plasma is generated by microwaves.

[0012] [3] The method for devulcanization of sulfur crosslinked rubber according to the above [1] or [2], wherein the contact is carried out in a treatment chamber.

[0013] [4] The method for devulcanization of sulfur crosslinked rubber according to the above [3], wherein the pressure in the treatment chamber is 2000 Pa or less.

[0014] [Effect] When sulfur cross-linked rubber is exposed to hydrogen plasma, the devulcanization of the rubber proceeds effectively. Hydrogen plasma is small and highly diffusible, so it is believed to penetrate deep into the rubber and convert the cross-linked S into H2S due to its reactivity. Furthermore, because devulcanization can be completed in a short time (for example, 5 hours or less), damage to the rubber is minimal and its physical properties are less likely to deteriorate. Effect of the Invention

[0015] The method for devulcanization of sulfur-crosslinked rubber of the present invention has the excellent effects of not requiring a devulcanizing agent, being free of odor problems, and being easy to proceed with devulcanization without taking a long time, so that physical properties are less likely to deteriorate. [Brief description of the drawings]

[0016] [Figure 1] FIG. 1 is a cross-sectional view of the processing apparatus used in the examples. [Diagram 2] FIG. 2 is a graph showing the results of the total desulfurization amount and swelling rate in treatment experiment 1. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] <1> Sulfur crosslinked rubber The type of rubber used for the sulfur crosslinked rubber is not particularly limited, but examples thereof include ethylene propylene rubber (EPDM, EPM), natural rubber (NR), isoprene rubber (IR), butyl rubber (IIR), butadiene rubber (BR), styrene butadiene rubber (SBR), chloroprene rubber (CR), and nitrile rubber (NBR). The shape of the sulfur crosslinked rubber is not particularly limited, but is preferably in the form of a plate, a thread, or a pulverized form (flakes, granules, etc.) in that the surface area that comes into contact with the hydrogen plasma is large.

[0018] <2> Hydrogen Plasma The hydrogen plasma is not particularly limited, but is preferably so-called low-temperature plasma (room temperature to 1000K), and more preferably the temperature of the ions and molecules in the plasma is 100°C or lower. This is because changes in the molecular structure of the rubber due to thermal load are suppressed, and the physical properties of the rubber are less likely to deteriorate.

[0019] The method for generating hydrogen plasma is not particularly limited, but it is preferable to generate the hydrogen plasma using microwaves as described above, because the energy efficiency for achieving a high electron density is high. The frequency of the microwave is not particularly limited, and examples include 2.45 GHz, 5 GHz, and various other frequencies in the ISM band. The microwaves may be pulsed microwaves, increasing the peak value of the microwave power while decreasing the average value of the microwave power.

[0020] The hydrogen used as the plasma generating gas is not limited to high purity hydrogen, but may be hydrogen containing small amounts of other gases (oxygen, nitrogen, etc.). The time for which the plasma is in contact varies depending on the size and shape of the sulfur crosslinked rubber and cannot be generally determined, but it is preferable in terms of efficiency to keep it, for example, for 5 hours or less.

[0021] As described above, it is preferable to perform the contact with hydrogen plasma inside the processing chamber, because this reduces loss of hydrogen plasma. The pressure inside the processing chamber is preferably 10,000 Pa or less, and more preferably 2,000 Pa or less, because this facilitates the diffusion of active species (e.g., atomic hydrogen) acting on sulfur bridges from the plasma generation region to the surface of the processing object. EXAMPLES

[0022] Next, examples of the present invention will be described. Note that the materials, conditions, structures, shapes, and dimensions in the examples are merely examples and can be appropriately changed without departing from the spirit of the invention. First, the processing equipment used will be described.

[0023] [Processing device] As shown in FIG. 1, the processing apparatus is composed of a processing vessel, an antenna, a quartz tube, a test specimen mounting tube, a high-frequency power source, a plasma generation gas supply, a pressure gauge, a quadrupole mass spectrometer (QMS), a pressure reducing device, and a leak valve.

[0024] The processing vessel is a cylindrical vessel with a bottom made of a conductive material (for example, aluminum), and has a processing chamber with an inner diameter of 170 mm and an inner height of 195 mm. The processing vessel also has a lid with a quartz window.

[0025] The antenna is attached to the bottom of the processing vessel via an insulating plate, and stands upright in the center of the processing chamber as a radiating conductor.

[0026] The quartz tube is erected so as to concentrically surround the antenna, and is fixed to the bottom of the processing vessel by a quartz presser.

[0027] The test piece mounting cylinder is a cylinder made of a conductive material (for example, aluminum), which is erected so as to concentrically surround the antenna and the quartz cylinder, and is connected to the bottom of the processing vessel.

[0028] The high frequency power source generates a high frequency signal in the quasi-microwave band (1 GHz to 3 GHz) or microwave band (3 GHz to 30 GHz) with a predetermined power, and is connected to the processing vessel via a coaxial cable, a coaxial waveguide, or other waveguide. When a coaxial cable or a coaxial waveguide is used, its inner conductor is connected to the antenna, and its outer conductor is connected to the processing vessel and the test specimen mounting tube. Microwave energy is then intensively sent to a dielectric member that covers the antenna and has a part of it exposed inside the vacuum vessel, and the microwave propagates in a surface wave mode along the boundary between the vacuum-exposed surface of the dielectric member and the plasma, so that plasma is generated from the plasma generating gas near the vacuum-exposed surface of the dielectric member.

[0029] The plasma generating gas supply unit consists of a flow rate controller such as a mass flow controller (MFC) connected to the gas inlet of the processing vessel and a reactive gas storage unit (not shown), and in this example has three switchable systems for hydrogen, argon, and oxygen.

[0030] The pressure gauge measures the pressure inside the processing chamber.

[0031] A quadrupole mass spectrometer (QMS) measures the proportion of molecular weights from 1 to 200 in a certain space by forming an electric field in the quadrupole that allows only ions of a specific mass (m / z value) to pass through without being repelled, using the partial pressure ratio between the inside of the device and the gas monitor via the ion current value. In this example, a Canon Anelva "Model M-201GA-CRMY" was used.

[0032] The pressure reducing device is composed of a mechanical booster pump and a rotary pump connected via a valve to a gas exhaust part of the processing vessel, and reduces the pressure inside the processing chamber by exhausting gas therefrom.

[0033] The leak valve is connected to a gas exhaust portion of the processing vessel, is normally closed, and is opened when it is desired to open the processing chamber to the atmosphere.

[0034] [Treatment experiment 1 (Difference in plasma generation gas)] As shown in Fig. 1, a test piece of sulfur-crosslinked EPDM rubber cut to 150 mm x 75 mm x 1 mm was attached to the inner surface of the test piece attachment tube of the treatment device. The sulfur-crosslinked EPDM rubber is white and does not contain carbon black. Treatment experiment 1 was performed in which hydrogen, argon, and oxygen were switched from the plasma generation gas supply unit to flow into the treatment chamber, and hydrogen plasma, argon plasma, and oxygen plasma were generated by microwaves as described above, and the test piece was brought into contact with the plasma to attempt desulfurization. The test piece contacted with hydrogen plasma was the example, the test piece contacted with argon plasma was the comparative example 1, the test piece contacted with oxygen plasma was the comparative example 2, and the test piece not treated with plasma was the comparative example 3.

[0035] The processing conditions are shown in Table 1. The inside of the processing chamber was depressurized by the depressurization device, and then the plasma generation gas was introduced to the chamber, which was then heated to 150 Pa at the start of processing. The pressure inside the QMS was 10 -3 The microwaves used were pulse microwaves with a pulse frequency of 2.45 GHz, a pulse duty ratio of 10%, and a pulse width of 100 s.

[0036] [Table 1]

[0037] The above treatment resulted in the following three results, as shown in Table 1. (1) Visual observation of the test piece When the test pieces were visually observed after the treatment, the test pieces of the Example were significantly changed to a dark brown color, whereas the test pieces of Comparative Examples 1 and 2 were almost unchanged from their white color.

[0038] (2) Total desulfurization amount The total amount of desulfurization measured by QMS was significantly high in the Examples, but was less than 20% of that in the Examples in Comparative Examples 1 and 2. The results are also shown in FIG.

[0039] (3) Swelling rate When rubber comes into contact with a solvent, it absorbs the solvent and swells. This swelling occurs even in rubber before devulcanization, but the more devulcanized the rubber is, the greater the swelling becomes as the solvent penetrates into the sintered sulfur bonds. Therefore, the degree of swelling is an indicator of the progress of devulcanization. Therefore, a small piece (500 mg) of the treated test piece was immersed in toluene at room temperature for 24 hours, the change in weight was measured, and the swelling ratio was calculated using the following formula. Swelling rate (%) = (weight after immersion / initial weight) × 100 (Formula 1) The swelling ratio of the Example was more than twice that of the untreated Comparative Example 3, but Comparative Examples 1 and 2 only showed a slight change from the untreated Comparative Example 3.

[0040] [Treatment experiment 2 (Difference in hydrogen plasma treatment time)] Next, a test piece of sulfur-crosslinked EPDM rubber was attached to the inner surface of the test piece attachment tube of the treatment device in the same manner as above. Hydrogen was introduced into the treatment chamber from the plasma generation gas supply unit, hydrogen plasma was generated by microwaves, and the test piece was brought into contact with the hydrogen plasma for treatment times of 30 minutes, 60 minutes, 120 minutes, and 240 minutes to attempt desulfurization, in treatment experiment 2. All of these correspond to examples.

[0041] The treatment conditions are shown in Table 2. The pressure inside the treatment chamber at the end of treatment varied depending on the treatment time.

[0042] [Table 2]

[0043] The above treatment resulted in the following two results, as shown in Table 2.

[0044] (1) Total desulfurization amount The total amount of desulfurization measured by QMS increased significantly up to 60 min of treatment, and then increased slowly. As described above, there is a possibility that the measured value of the total desulfurization amount differs due to the difference in the pressure inside the treatment chamber.

[0045] (2) Swelling rate The difference in swelling rate due to the treatment time was difficult to evaluate and was considered to be within the range of variation.

[0046] The present invention is not limited to the above-described embodiment, and can be embodied by making appropriate modifications without departing from the spirit of the invention.

Claims

1. A method for devulcanization of sulfur-crosslinked rubber, comprising contacting the sulfur-crosslinked rubber with hydrogen plasma.

2. 2. The method for devulcanizing a sulfur-crosslinked rubber according to claim 1, wherein the hydrogen plasma is generated by microwaves.

3. 3. The method for devulcanizing a sulfur-crosslinked rubber according to claim 1 or 2, wherein the contact is carried out in a treatment chamber.

4. 4. The method for devulcanizing sulfur-crosslinked rubber according to claim 3, wherein the pressure in the treatment chamber is 2000 Pa or less.

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