vulcanization method

Abstract

To provide a vulcanization method capable of uniformly vulcanizing a rubber composition by microwave irradiation.SOLUTION: There is provided a method for vulcanizing a rubber composition containing a diene-based rubber and carbon black by microwave irradiation, wherein the frequency of the microwaves irradiating the rubber composition is changed by a frequency change range selected from a range from more than 0 Hz and 1000 Hz or less for each passage of a time interval selected from a range from more than 0 μsec and 100 μsec or less.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a vulcanization method. Background Art

[0002] It is known to use microwave energy in the manufacture of rubber products. For example, Patent Document 1 discloses that the total heating time can be shortened by using microwave energy for at least a part of the thermal energy required in the manufacture of pneumatic tires. Prior Art Documents Patent Documents

[0003] Patent Document 1 Japanese Unexamined Patent Publication No. 8-66924 Summary of the Invention Problems to be Solved by the Invention

[0004] However, in actual fact, in Patent Document 1, heating by microwave energy is completed before the start of the vulcanization reaction. In this regard, conventionally, when vulcanization is performed by irradiating microwaves, it is extremely difficult to control the vulcanization reaction, and there have been problems such as non-uniform vulcanization causing local scorching. Due to such problems as non-uniform vulcanization, rubber vulcanization using microwaves has not yet been fully put into practical use at present, and the use of microwaves has at most been limited to preheating before vulcanization.

[0005] Accordingly, an object of the present invention is to provide a vulcanization method capable of uniformly vulcanizing a rubber composition by microwave irradiation. Means for Solving the Problems

[0006] The inventors focused on the fact that carbon black can be a heating element with excellent microwave absorption properties and conducted extensive research. They discovered that when heating (vulcanizing) a rubber composition containing carbon black by microwave irradiation, uniform vulcanization can be achieved by sweeping the microwave frequency in a predetermined manner, leading to the present invention.

[0007] In other words, the gist of the present invention for achieving the above objective is as follows.

[0008] The present invention relates to a vulcanization method in which a rubber composition containing diene rubber and carbon black is vulcanized by irradiating it with microwaves, The microwave frequency irradiated onto the rubber composition is characterized by changing the frequency by a frequency change range selected from the range of 0 Hz to 1000 Hz at each time interval selected from the range of 0 μsec to 100 μsec. According to the vulcanization method of the present invention, it is possible to uniformly vulcanize a rubber composition by microwave irradiation.

[0009] In the vulcanization method of the present invention, it is preferable that the time interval is constant each time. In this case, uniform vulcanization can be achieved more effectively.

[0010] In the vulcanization method of the present invention, it is preferable that the frequency variation range is constant each time. In this case, uniform vulcanization can be achieved more effectively. [Effects of the Invention]

[0011] According to the present invention, a vulcanization method is provided that enables uniform vulcanization of a rubber composition by microwave irradiation. [Brief explanation of the drawing]

[0012] [Figure 1] This is a schematic diagram illustrating an example of a sweep pattern of microwave frequencies irradiated onto a rubber composition according to the present invention. [Modes for carrying out the invention]

[0013] The present invention will be described in detail below based on its embodiments.

[0014] (Vulcanization method) A vulcanization method according to one embodiment of the present invention is a vulcanization method in which a rubber composition containing diene rubber and carbon black is vulcanized by irradiation with microwaves, The microwave frequency irradiated onto the rubber composition is characterized by changing the frequency by a frequency change range selected from the range of 0 Hz to 1000 Hz at each time interval selected from the range of 0 μsec to 100 μsec.

[0015] In this specification, "microwave" refers to electromagnetic waves with a frequency of 300 MHz to 300 GHz.

[0016] Figure 1 is a schematic diagram showing an example of a sweep pattern of the microwave frequency irradiated onto a rubber composition according to the vulcanization method of this embodiment. In the sweep pattern shown in Figure 1, the frequency of the microwave irradiated onto the rubber composition is increased by a constant frequency change (F) at regular intervals (T).

[0017] The inventors believe that the mechanism by which the vulcanization method of this embodiment can uniformly vulcanize rubber compositions is as follows. Normally, when a rubber composition containing carbon black is irradiated with microwaves, interference occurs between the microwaves directly irradiated onto the rubber composition and the microwaves that are reflected at least once within the microwave generator before being irradiated onto the rubber composition. In this regard, for example, if the microwave frequency is constant, areas in the rubber composition where interference is likely to occur and areas where it is not will be fixedly unevenly distributed, which may result in some parts being overheated and others underheated. This situation is particularly pronounced at temperatures above 120°C where the crosslinking reaction occurs, and is thought to result in non-uniform vulcanization. In contrast, in the vulcanization method of this embodiment, the frequency of the irradiated microwaves is swept in a predetermined manner, so that localized concentration of energy absorption in the rubber composition can be effectively avoided, and as a result, uniform heating (vulcanization) can be achieved.

[0018] Therefore, in the vulcanization method of this embodiment, it is not essential to move or rotate the object to be heated within the device (as in a turntable-type microwave oven) in order to avoid uneven heating.

[0019] In the vulcanization method of this embodiment, a variable frequency microwave generator (VFM), particularly a variable frequency microwave generator using a semiconductor oscillator or amplifier, can be used.

[0020] In the vulcanization method of the present embodiment, the time interval (T) is the time between timings at which the frequency is changed, in other words, the time for which microwaves of a constant frequency are irradiated. The time interval (T) is selected from the range of more than 0 μsec and not more than 100 μsec. If the time interval (T) exceeds 100 μsec, there is a possibility that heat concentration on a partial portion of the rubber composition cannot be sufficiently suppressed. Further, the lower limit of the time interval (T) only needs to be more than 0 μsec, and may be appropriately adjusted according to the specifications of the apparatus to be used. In particular, from the viewpoint of microwave irradiation efficiency, the time interval (T) is preferably 1 μsec or more. From the same viewpoint, the time interval (T) is preferably selected from the range of 3 to 50 μsec, more preferably selected from the range of 5 to 30 μsec, and still more preferably selected from the range of 10 to 25 μsec.

[0021] In view of the above mechanism, the time interval (T) may be constant every time as shown in FIG. 1, or may be different each time. However, in the vulcanization method of the present embodiment, from the viewpoint of more effectively achieving uniform vulcanization, it is preferable that the time interval (T) is constant every time.

[0022] Further, the frequency change width (F) is selected from the range of more than 0 Hz and not more than 1000 Hz. If the frequency does not change at all (when the frequency change width (F) is 0 Hz), heat will concentrate on a partial portion of the rubber composition. Further, if the frequency change width (F) exceeds 1000 Hz, the stability of microwave irradiation deteriorates, which may adversely affect uniform vulcanization. From the same viewpoint, the frequency change width (F) is preferably selected from the range of 10 to 500 Hz, more preferably selected from the range of 50 to 350 Hz, and still more preferably selected from the range of 100 to 260 Hz.

[0023] In view of the above mechanism, the frequency change width (F) may be constant every time as shown in FIG. 1, or may be different each time. However, in the vulcanization method of the present embodiment, from the viewpoint of more effectively achieving uniform vulcanization, it is preferable that the frequency change width (F) is constant every time.

[0024] Furthermore, in view of the mechanism described above, the way in which the frequency is changed by the frequency change range (F) may be an increase each time, or a decrease each time, or the increase or decrease may differ each time, as shown in Figure 1. However, in the vulcanization method of this embodiment, it is preferable to increase the frequency by the frequency change range (F) each time.

[0025] Note that variable frequency microwave generators typically have upper and lower limits on the variable frequency specified in their specifications. Therefore, when using such a variable frequency microwave generator and repeatedly increasing the frequency, the frequency may reach or approach the upper limit of the variable frequency. In this case, you should first switch the frequency to or near the lower limit of the variable frequency, and then resume increasing the frequency each time. The same procedure should be followed when repeatedly decreasing the frequency.

[0026] In the vulcanization method of this embodiment, the frequency band used for microwaves is not particularly limited and can be appropriately selected based on, for example, the specifications of the microwave generator used (in particular, the type of oscillator or amplifier).

[0027] In the vulcanization method of this embodiment, although not particularly limited, the duration of the operation of irradiating with microwaves while sweeping the frequency in a predetermined manner is preferably 100 seconds or more, more preferably 200 seconds or more, and even more preferably 300 seconds or more. In this case, the degree of vulcanization of the rubber composition can be increased more sufficiently.

[0028] In the vulcanization method of this embodiment, although not particularly limited, the temperature of the rubber composition may be monitored when irradiating with microwaves while sweeping the frequency in a predetermined manner. By monitoring the temperature of the rubber composition, more reliable and uniform vulcanization can be achieved. Furthermore, in the vulcanization method of this embodiment, the heating rate of the rubber composition may be controlled when irradiating with microwaves. In that case, the heating rate can be controlled, for example, by fine-tuning the microwave irradiation output (W). The method for controlling the heating rate is not particularly limited, but it can be PID control. Alternatively, the heating rate can be controlled by switching the microwave irradiation on and off. The heating rate can be selected, for example, from a range of 0.05°C / second to 0.5°C / second.

[0029] In the vulcanization method of this embodiment, the maximum temperature reached when actually vulcanizing the rubber composition (the so-called vulcanization temperature) is not particularly limited and can be appropriately selected depending on the purpose, but it is generally preferable to set it to 140°C or higher, preferably 190°C or lower, and more preferably 160°C or lower.

[0030] <Rubber composition to be vulcanized> The rubber composition to be vulcanized used in the vulcanization method of this embodiment contains at least diene rubber and carbon black. Furthermore, the rubber composition to be vulcanized may optionally contain vulcanizing agents, other components, etc.

[0031] Examples of diene rubbers include natural rubber (NR), butadiene rubber (BR), isoprene rubber (IR), styrene-butadiene rubber (SBR), ethylene-propylene-diene rubber (EPDM), chloroprene rubber (CR), halogenated butyl rubber, and acrylonitrile-butadiene rubber (NBR). These diene rubbers may be used individually or in combination of two or more. Among these, from the viewpoint of more reliably enjoying the effect of uniform vulcanization by the vulcanization method of the present invention, it is preferable to use at least one of natural rubber, butadiene rubber, isoprene rubber, and styrene-butadiene rubber as the diene rubber.

[0032] Examples of carbon black include GPF, FEF, HAF, ISAF, and SAF grades. These carbon blacks may be used individually or in combination of two or more types.

[0033] The carbon black content in the rubber composition is not particularly limited and can be appropriately selected depending on the purpose. However, from the viewpoint of more reliably enjoying the effect of uniform vulcanization by the vulcanization method of the present invention, it is preferable that the carbon black content be 10 parts by mass or more, and more preferably 30 parts by mass or more, per 100 parts by mass of diene rubber. Furthermore, from the viewpoint of maintaining the mechanical properties of the resulting vulcanized rubber composition, the carbon black content in the rubber composition is preferably 120 parts by mass or less, and more preferably 70 parts by mass or less, per 100 parts by mass of diene rubber.

[0034] The rubber composition preferably contains a vulcanizing agent. Examples of vulcanizing agents include sulfur-based vulcanizing agents such as sulfur and morpholine disulfide; organic peroxides such as benzoyl peroxide, dicumyl peroxide, di-t-butyl peroxide, t-butylcumyl peroxide, methyl ethyl ketone peroxide, cumene hydroperoxide, and 2,5-dimethyl-2,5-di(t-butylperoxy)hexane; and others. Other examples of vulcanizing agents include hexamethylene bisthiosulfate disodium salt dihydrate, 1,3-bis(citraconimidomethyl)benzene, 4,4'-diphenylmethane bismaleimide, and m-phenylene bismaleimide. These vulcanizing agents may be used individually or in combination of two or more.

[0035] The content of the vulcanizing agent in the rubber composition is not particularly limited and can be appropriately selected depending on the purpose. For example, it can be 0.1 parts by mass or more and 10 parts by mass or less per 100 parts by mass of diene rubber.

[0036] Other components include fillers other than carbon black (such as silica), vulcanization aids such as stearic acid, vulcanization accelerators, vulcanization accelerators such as zinc oxide, antioxidants, softeners, plasticizers, and processability improvers, and these can be included in appropriate amounts.

[0037] The rubber composition to be vulcanized can be obtained, for example, by mixing the above-mentioned components in accordance with conventional methods using a kneader such as a roll, internal mixer, or Banbury rotor.

[0038] The shape of the rubber composition to be vulcanized is not particularly limited. For example, the rubber composition to be vulcanized can be a pre-molded article that has been molded in advance. In particular, even pre-molded articles with complex shapes, which conventionally tended to experience uneven heating, can be uniformly vulcanized, and thus a desired vulcanized molded article can be obtained.

[0039] Furthermore, metal parts may be included in the rubber composition to be vulcanized. In the vulcanization method of this embodiment, as described above, the sweeping pattern of the frequency of the irradiated microwaves is optimized, so it is expected that defects such as arcing in metal parts will be suppressed. [Examples]

[0040] The present invention will be described in more detail below with reference to examples, but the present invention is not limited in any way to the following examples.

[0041] <Preparation of rubber composition> A rubber composition was prepared by kneading the compounds according to the conventional method using the formulations shown in Table 1.

[0042] [Table 1]

[0043] *1 Isoprene rubber: Manufactured by JSR Corporation, IR2200 *2 Carbon Black A: Manufactured by Tokai Carbon Co., Ltd., Seast 7HM, ISAF grade *3 Carbon Black B: Manufactured by Tokai Carbon Co., Ltd., Seast 3H, HAF grade *4 Anti-aging agent: N-phenyl-N'-(1,3-dimethylbutyl)-p-phenylenediamine (6PPD), manufactured by Ouchi Shinko Chemical Industry Co., Ltd. *5 Vulcanization accelerator: N-cyclohexyl-2-benzothiazolyl sulfenamide (Noxellar CZ), manufactured by Ouchi Shinko Chemical Industry Co., Ltd.

[0044] <Vulcanization of rubber composition> The above rubber composition was prepared as a sheet sample with dimensions of 80 mm x 80 mm x 2 mm thickness, and vulcanized by microwave irradiation according to the following procedure.

[0045] In Examples 1-3, a variable-frequency microwave generator (manufactured by LAMBDA Technology, product name "VariWave®") was used to heat (vulcanize) a sheet-like sample by irradiating it with microwaves while changing (increasing) the microwave frequency by a certain frequency change range shown in Table 2 at regular time intervals shown in Table 2. At that time, the sheet-like sample was sandwiched in a plastic mold, the initial vulcanization pressure was set to 0.5 MPa, and the microwave irradiation output was PID controlled so that the temperature of the sheet-like sample, as measured by an infrared thermometer, rose from approximately 30°C at a heating rate of 0.2°C / second. Once it reached 140°C, it was held at that temperature for 30 minutes. The variable-frequency microwave generator used has a variable frequency range of 5.8 to 6.6 GHz according to its specifications. Therefore, each time the microwave frequency was increased and reached 6.6 GHz, it was switched to 5.8 GHz, and then the frequency increase was resumed.

[0046] In Comparative Examples 1 to 3, sheet-shaped samples were heated (vulcanized) by irradiation with fixed-frequency (2.45 GHz) microwaves using Milestone General Co., Ltd.'s microwave synthesis reactor, "flexiWAVE". The initial vulcanization pressure and temperature control were the same as in the Examples.

[0047] For each sheet-like sample after vulcanization, the hardness was measured at 36 points at 1 cm intervals using a digital hardness tester RH 101a from Polymer Instruments Co., Ltd. The percentage (%) of all 36 measurement points where the hardness was equal to or greater than the predetermined value shown below was then calculated. Carbon Black A (ISAF) content: 25 parts... Hardness: 50 Carbon Black A (ISAF) content: 50 parts... Hardness: 60 Carbon Black B (HAF) content: 50 parts... Hardness: 60 The results are shown in Table 2 as "vulcanization uniformity." A higher percentage indicates higher vulcanization uniformity. Note that "hardness above a specified value" is based on the results of a toluene immersion test of sheet-like samples, where if the hardness was above the specified value, the material did not dissolve in toluene due to crosslinking.

[0048] [Table 2]

[0049] Table 2 shows that, in the examples, the uniformity of vulcanization of the obtained vulcanized product is high because microwave irradiation was performed while sweeping the frequency in a predetermined manner. [Industrial applicability]

[0050] According to the present invention, a vulcanization method is provided that enables uniform vulcanization of a rubber composition by microwave irradiation.

Claims

1. A vulcanization method comprising vulcanizing a rubber composition containing diene rubber and carbon black by irradiating it with microwaves, A vulcanization method characterized in that the frequency of microwaves irradiated onto the rubber composition is changed by a frequency change range selected from the range of 0 Hz to 1000 Hz at each time interval selected from the range of 0 μsec to 100 μsec.

2. The vulcanization method according to claim 1, wherein the time interval is constant each time.

3. The vulcanization method according to claim 1 or 2, wherein the frequency change range is constant each time.

4. The vulcanization method according to claim 1 or 2, wherein the duration of the microwave irradiation operation is 100 seconds or more.

5. The vulcanization method according to claim 1 or 2, wherein when irradiating the rubber composition with microwaves, the heating rate is selected and controlled from a range of 0.05°C / second or more and 0.5°C / second or less.

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