JP2002348384A - Cross-linking method for fluororesin powder - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a cross-linking method for a fluororesin powder whereby a fine and homogeneous cross-linked powder can be obtained. SOLUTION: A fluororesin powder is dispersed in a liquid, and the resultant dispersion 24 is irradiated with an electron beam by using a radiation apparatus 11. Preferably, the irradiation with an electron beam is conducted while the fluororesin powder is kept moving by causing the liquid to flow by agitation or the like.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of crosslinking a fluororesin powder, and more particularly to a method of crosslinking a fluororesin powder capable of obtaining a uniform and fine crosslinked powder.

[0002]

2. Description of the Related Art In order to improve the properties such as strength and elongation of a fluororesin, a powdery fluororesin is baked by heating and solidified into a mat having a small bulk density.
After irradiation, the fired body is pulverized to return to the original powder state, and this is used as a molding material.

FIG. 4 shows an outline of an apparatus used in this method, wherein 1 is an irradiation apparatus for ionizing radiation such as an electron beam, and 2 is installed below the irradiation apparatus 1 through an irradiation window 3. Irradiated chamber 4 indicates a transport body arranged to circulate in the irradiation chamber 2, and includes a conveyor 5 and a tray 6 suspended therefrom.
It is composed of

[0004] On the tray 6, a fired body 7 of fine powder of a fluororesin hardened in a mat shape is placed, and the fired body 7 is irradiated with radiation when passing through the lower part of the irradiation window 3. . The interior of the irradiation chamber 2 is set at a temperature slightly higher than the melting point of the fluororesin, and is placed in an oxygen-free state by nitrogen gas or the like to eliminate the influence of oxygen. Is configured to be cooled to the original temperature by circulation of nitrogen gas by a fan.

[0005] In the above configuration, the fired body 7 is repeatedly circulated in the irradiation chamber 2, and is irradiated each time it passes through the position of the irradiation window 3, thereby being irradiated with a predetermined dose of radiation. This method of integrating a predetermined dose by repeating irradiation has been evaluated and utilized as a stable method of crosslinking fluororesin powder.

[0006]

However, according to the conventional method for crosslinking fluororesin powder, the fluororesin powder is coarsened by fusion and the ductility of each fused powder group is large. However, there is a problem that it is difficult to return the powder to a homogenous and fine original powder upon pulverization after crosslinking.

Accordingly, an object of the present invention is to provide a method for crosslinking a fluororesin powder which can obtain a uniform and fine crosslinked powder.

[0008]

In order to achieve the above object, the present invention provides a method for crosslinking a fluororesin powder by irradiating the fluororesin powder with radiation to disperse the fluororesin powder. The present invention provides a method for cross-linking a fluororesin powder, which comprises irradiating a predetermined dose of radiation to a liquid in a liquid state to cross-link the fluororesin powder.

The irradiation of the radiation in the present invention is preferably carried out in a state where the powder of the fluororesin is caused to perform a motion such as rotation in the liquid by giving a fluid action to the liquid. If you appear
Radiation can be uniformly applied to the entire circumference of the fluororesin powder, and the crosslinking becomes more uniform.

As means for causing the liquid to flow, use of a stirrer or flowing of the liquid through a predetermined flow path can be considered. Particularly, in the latter method, the flow of the liquid is set to a turbulent state. Thus, it is preferable that the movement of the powder of the fluororesin be increased. In addition, as the radiation, ionizing radiation such as an electron beam is preferable because it is easy to handle.

As the liquid for dispersing the fluororesin powder, it is preferable to use an azeotropic mixture of biphenyl (C12 H10) and diphenyl ether (C12 H10 O). This azeotropic mixture has the following excellent characteristics and is a liquid material suitable for the present invention.

Permissible radiation absorption The radiation required for cross-linking by irradiating a fluororesin such as polytetrafluoroethylene (PTFE) with radiation such as an electron beam is about 100 kGy (= 10 7 rad). The liquid in which the fluororesin powder is dispersed also absorbs this amount of radiation. However, the allowable radiation absorption of the azeotropic mixture is 1010 rad or more, so that it can be used 103 times or more, and the economy can be reduced. Advantageously.

Maximum service temperature The maximum service temperature of the azeotrope is 400 ° C., which is sufficient for the 340 ° C. crosslinking temperature of PTFE classified as high temperature.

The vapor pressure at the crosslinking temperature The liquid in which the fluororesin powder is dispersed is closed, and the irradiation window of this closed system (usually 30 to 50 mm wide × 1500 to 2 × 2).
The cross-linking temperature is 340 ° C.
The internal pressure due to the vapor pressure of the azeotropic mixture below acts, but the vapor pressure of this mixture at 340 ° C. is 0.37
MPa, so the tension applied to the edge of the irradiation window is also 1
Since the pressure is limited to about 40 MPa, the pressure resistance of the irradiation window formed of a Ti foil having a thickness of 40 μm is 160 M in many cases.
Pa becomes a level that can be sufficiently cleared.

Viscosity at crosslinking temperature The viscosity of an azeotropic mixture at crosslinking temperature is about 1 at room temperature.
/ 5 of about 0.17 mPa · s, so that it can easily penetrate between the particles of the fluororesin powder to create a good dispersion state, and further, this low viscosity is
It is possible to configure a small power source for stirring or flowing.

Density at crosslinking temperature 0.76 g for PTFE having a density of 2.17 g / cm 3
/ Cm3, so that the dispersed fluororesin powder does not precipitate in a short time.

[0017]

Next, an embodiment of a method for crosslinking a fluororesin powder according to the present invention will be described. In FIG. 1A, 11 is an electron beam irradiation device, 12 is its power supply device,
13 is an accelerating tube, 14 is an electron beam deflecting device, 15 is a scanning horn, 16 is an electron beam penetrating and scanning horn 1
5 shows an irradiation window of a 30 mm effective width Ti foil configuration provided at the tip of a horn 15 for maintaining a vacuum in 5.

Reference numeral 17 denotes a circulation circuit for circulating a liquid in which a fluororesin powder is dispersed (hereinafter referred to as a dispersion).
A dispersion flow path 18 disposed immediately below the scanning horn 15, a cooling heat exchanger 19 for cooling the dispersion, a closed dispersion liquid storage tank 20 serving as a supply of the dispersion, and a power source for circulating the dispersion. Pump 21, a liquid / powder separation device 22, and a pipe 23 connecting them.

FIG. 1B shows the cross-sectional structure of the dispersion liquid flow path 18, wherein the groove 25 for flowing the dispersion liquid 24 has a width of 30 m.
m and a radiation window 27 of 40 μm thick Ti foil provided so as to cover the upper part of the groove 25. Reference numeral 28 denotes a metal fitting for fixing the irradiation window 27 to the metal gutter 26, and reference numeral 38 denotes an outlet of the dispersion liquid 24.

The dispersion liquid storage tank 20 has a stirrer 29 for stirring the dispersion liquid 24, an electric heater 30 for keeping the dispersion liquid 24 at a constant temperature, a power supply 31, and a material inlet and a vacuum port serving as a vacuum evacuation unit. 32, a heat insulating material 33, and a forward path 34 and a return path 35 of the circulating dispersion liquid 24.

The crosslinking of the fluororesin powder in this embodiment is performed as follows. First, Dow Chemical's trade name “DOWTHERM-A” (azeotropic mixture of biphenyl and biphenyl ether) and PTFE powder are supplied from the port 32 into the dispersion liquid storage tank 20.
More evacuation is performed, thereby eliminating air.

Next, the dispersion liquid 24 is uniformly prepared by stirring with a stirrer 29, and the temperature of the dispersion liquid 24 is reduced with PTF by heating with an electric heater 30.
The temperature is raised to 340 ° C., which is the crosslinking temperature of E, and the valve 21 is closed and the valve 37 is opened to operate the pump 21.

As a result of the operation of the pump 21, the dispersion liquid 24 becomes
The liquid is supplied into the dispersion liquid flow path 18 through the pipe 23, flows in a turbulent state, and is irradiated with a predetermined dose of electron beam from the electron beam irradiation device 11 in the flowing process. The PTFE powder present in the turbulent dispersion liquid 24 and thus rotating randomly receives the electron beam uniformly over the entire circumference by this irradiation, and is contained in the dispersion liquid 24 and discharged from the flow path 18. You.

After the discharged dispersion liquid 24 is cooled by the heat exchanger 19 by the temperature rise due to the electron beam irradiation, it is stored in the storage tank 2.
0, and after being stirred by the stirrer 29, again
The liquid is supplied to the dispersion liquid channel 18 through the above route, and is irradiated with an electron beam. Thereafter, this is repeated until the irradiation amount reaches a predetermined dose, and the irradiation device 11 is stopped when a predetermined cross-linking level is reached.

After the irradiation device 11 is stopped, the valve 37 is closed and the valve 36 is opened, and P
The dispersion 24 containing the TFE powder is supplied to the liquid / powder separation device 22.
And the pump 21 and the electric heater 30 are stopped to complete the operation.

In the separation device 22, the crosslinked PTFE powder is separated from the dispersion liquid 24, and is subjected to washing and drying here or in another step thereafter, whereby a predetermined crosslinked PTFE powder is produced. The storage tank 20 has a new DOWTHERM-A and PT
The FE powder is supplied by a predetermined amount, and the next crosslinking operation is performed.

The crosslinked PTFE powder obtained as described above is
It has the fineness of the raw material powder and does not require any pre-firing and post-crushing steps as in the conventional cross-linking method, and is therefore uniform and fine without coarsening of the powder. High quality is maintained.

FIG. 2 (a) shows the relationship between the mixing ratio of DOWTHERM-A and PTFE powder and the density of the resulting dispersion near the cross-linking temperature. Assuming that the mixing amount of the powder is set to 10% by weight, the density at that time is:
0.81 g / cm3.

FIG. 2B shows that the density of the dispersion is 0.81 g.
/ Cm3 shows the relationship between the voltage of the electron beam and the effective range of the electron beam perpendicularly incident on the liquid surface when the voltage is fixed at, for example, 1 MeV. ing. Therefore, when the cross-linking operation shown in FIG. 1 is performed, it is preferable to set the voltage of the electron beam, the irradiation distance, and the like based on this relationship, so that the cross-linking operation can be performed efficiently and reliably.

In the embodiment based on FIG. 1 described above, an example in which electron beam irradiation is repeatedly performed has been described. However, ultra-fine PTFE powder, an increase in the output of an irradiation device, and a reduction in the irradiation distance are performed. By carrying out the above-mentioned measure, it is also possible to carry out a crosslinking operation with one irradiation.

FIG. 3 shows another embodiment of the method for crosslinking a fluororesin powder according to the present invention. 41 is an electron beam irradiation device, 42 is its power supply device, 43 is an accelerating tube, 44 is an electron beam deflector, 45 is a scanning horn, 46 is an irradiation window of a Ti foil configuration provided at the tip of the scanning horn 45, 47 is Irradiation window 4
6 shows a cross-linking device in which an irradiation window 48 also made of Ti foil is arranged at a position facing 6.

The cross-linking device 47 has a top plate 4
9 and a bottom plate 50, a cylindrical dispersion liquid storage tank 52 containing a predetermined amount of dispersion liquid 51 therein as a basic component, and further, an electric heater 53 is arranged on the lower surface of the bottom plate 50, and a top plate is provided. 49, a stirrer 54, a material input section 55, an inert gas injection section 56 such as nitrogen gas, and a heat exchanger 57 for recovering liquid vapor.
And a temperature sensor 58 are attached. 5
9 denotes a blade of the stirrer 54, and 60 denotes a dispersion liquid discharge section provided on the bottom plate 50.

The cross-linking operation in this embodiment proceeds as follows. First, a predetermined amount of DOWTHER
Dispersion 5 in which a predetermined amount of PTFE powder is dispersed in MA
1 is injected into the storage tank 52 from the material charging section 55, and the dispersion liquid 51 is heated by the heat from the electric heater 53 while being stirred by the stirrer 54, and the temperature is raised to 340 ° C., which is the crosslinking temperature of PTFE.

The temperature of the dispersion liquid 51 is maintained by the automatic temperature control effect of the combination of the temperature sensor 58 and the electric heater 53, while the scanning horn 45 emits an electron beam when the irradiation device 41 is activated. During this time, the dispersion liquid 51 is placed in a fluidized state by stirring by the stirrer 54, and the PTFE powder dispersed in the liquid is irradiated with a predetermined dose of electron beam under this state.

Further, the PTFE powder makes a random rotation as a result of the stirring of the dispersion liquid 51. Therefore, an even dose of electron beam is uniformly applied to the entire circumference. become. In the meantime, in the heat exchanger 57, the liquid vapor containing the non-condensable gas generated by the irradiation of the electron beam is condensed, whereby the DOWTHERM-A
While discharging non-condensable gas to the outside. From the injection part 56, a nitrogen gas is supplied little by little.

After irradiating a predetermined dose of electron beam as described above, the irradiation device 41, the stirrer 54 and the electric heater 5
3 is stopped, and then the dispersion liquid 51 is discharged from the discharge part 60 to the outside by the pressure of the nitrogen gas applied from the injection part 56, whereby the crosslinking operation is completed. After the discharge, the PTFE powder is collected from the dispersion liquid 51, and is washed and dried to be a predetermined crosslinked powder.

The obtained crosslinked PTFE powder has homogeneity and fineness as raw material, and does not require prior firing and subsequent pulverization unlike conventional crosslinking methods, so that the powder becomes coarse. There is no such problem, and a high quality crosslinked powder can be obtained.

[0038]

As described above, according to the method for cross-linking a fluororesin powder according to the present invention, the fluororesin powder is dispersed in a liquid and the liquid is irradiated with a predetermined dose of radiation. , A homogeneous and fine crosslinked powder can be obtained.

[Brief description of the drawings]

FIG. 1 is an explanatory view showing an embodiment of a method for crosslinking a fluororesin powder according to the present invention, in which (a) is a schematic view of an apparatus used, and (b) is a cross-sectional view taken along line AA of (a). Show.

FIG. 2 is an explanatory view showing the characteristics of a dispersion composed of DOWTHERM-A and PTFE powder. FIG. 2 (a) shows the relationship between the mixing ratio of these two and the density of the dispersion near the crosslinking temperature, (B) shows that the density of the dispersion is 0.
The relationship between the electron beam voltage and the effective range of the electron beam at 81 g / cm3 is shown.

FIG. 3 is an explanatory view showing another embodiment of the method for crosslinking a fluororesin powder according to the present invention, wherein (a) is a schematic view of an apparatus used, (b) is a side view thereof, and (c) is a side view thereof. B of (a)
FIG.

4A and 4B are explanatory views showing a conventional method for crosslinking a fluororesin powder, wherein FIG. 4A is a front view and FIG. 4B is a side view.

[Explanation of symbols]

 11, 41 Electron beam irradiation device 12, 42 Power supply device 13, 43 Acceleration tube 14, 44 Line deflector 15, 45 Scanning horn 16, 27, 46, 48 Irradiation window 17 Circulation circuit 18 Dispersion liquid channel 19 Cooling heat exchange Apparatus 20, 52 Dispersion liquid storage tank 21 Pump 22 Liquid / powder separation device 23 Piping 24, 51 Dispersion liquid 25 Groove 26 Metal gutter 28 Hardware 29, 54 Stirrer 30, 53 Electric heater 31 Electric heater power supply 32 Pipe opening 33 Thermal insulation 34 Outgoing path 35 Return path 36, 37 Valve 38 Outlet 49 Top plate 50 Bottom plate 55 Material input section 56 Inert gas injection section 57 Heat exchanger for liquid vapor recovery 58 Temperature sensor 59 Blade 60 Dispersion liquid discharge section

Claims (3)

[Claims] 1. A method of cross-linking a fluororesin powder by irradiating the fluororesin powder with radiation, wherein the liquid in which the fluororesin powder is dispersed is irradiated with a predetermined dose of radiation. A method of crosslinking a fluororesin powder, comprising crosslinking a fluororesin powder. 2. The step of irradiating the radiation is performed in a state where the fluororesin powder is caused to move in the liquid by giving a fluid action such as stirring to the liquid. The method for crosslinking a fluororesin powder according to claim 1. 3. The method according to claim 1, wherein the step of irradiating the radiation is performed using an azeotropic mixture of biphenyl and biphenyl ether as the liquid.