JPH068344A - Production of polytetrafluoroethylene continuous molded object having high degree of crystallization - Google Patents

Abstract

(57) [Abstract] [PROBLEMS] To provide an industrially practicable method for producing a PTFE continuous molding having a high crystallinity. [Structure] A polytetrafluoroethylene continuous molded body is sintered at a temperature equal to or higher than the melting point of the resin, and then the sintered continuous molded body is sequentially set in a temperature range of 350 to 290 ° C. from a high temperature region to a low temperature region. At least 2
A method for producing a high crystallinity polytetrafluoroethylene continuous molded product, which comprises cooling while passing through two different substantially constant temperature zones within a time period of 0.5 to 10 minutes.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a continuous molded article such as a sheet or a tube made of polytetrafluoroethylene (hereinafter abbreviated as "PTFE"), and more specifically, a highly crystallized PTFE continuous molded article. Regarding the method. The highly crystallized PTFE continuous molded body of the present invention is particularly suitable for producing a porous body having a high porosity.

[0002]

2. Description of the Related Art PTFE has characteristics such as heat resistance, chemical resistance, weather resistance, and nonflammability, and further has surface characteristics such as low friction coefficient, water / oil repellency, and non-adhesiveness. It is an attractive material used in many fields. Also, PTF
In addition to the above-mentioned properties, the porous body made of E has the properties of flexibility, fluid permeability, collection / filtration of fine particles, low dielectric constant / dielectric loss tangent, etc. Applications are expanding not only in the field but also in the medical field. In the field of separation membranes, porous tubes and membranes made of PTFE are manufactured.

Conventionally, a typical method for producing a PTFE porous body is a method in which an unsintered PTFE compact containing a liquid lubricant is stretched in at least one direction and heated to about 327 ° C. or higher to sinter. It is (Japanese Patent Publication Sho 42-135)
No. 60 publication). According to this method, the porosity can be adjusted by changing the stretch ratio, but there is a limit to the stretch ratio for stretching the unsintered PTFE molded product. On the other hand, after the PTFE molded body obtained by paste extrusion is sintered and then heat treated to increase the crystallinity,
A method has been proposed for producing a PTFE porous body having a small pore size, a high porosity, and an excellent permeability by stretching at least uniaxially (Japanese Patent Laid-Open No. 64-7882).
3 gazette).

In the PTFE porous body, not only the pore size and the pore size distribution but also the proportion of pores, that is, the porosity is related to the flow rate. The size of the porosity is very important for the performance as a separation membrane. Is. In the case of a PTFE porous body, as described above, the porosity is closely related to the crystallinity of the molded body, and even with the same draw ratio, the higher the crystallinity of the PTFE molded body, the higher the porosity. Of porous material is obtained.

In the field of separation membranes, it is considered that a porous material having a smaller pore size will be required for the purpose of microfiltration in the future, but P having a higher porosity can be obtained even with a small stretching ratio.
If a TFE porous body can be produced, a separation membrane having excellent permeability even with a small pore size can be obtained. For that purpose, a technique for increasing the crystallinity of the PTFE molded product is important.

By the way, conventionally, in order to increase the crystallinity of a PTFE molded article, for example, in the case of a compression molded article, after the molded article is sintered, in order to avoid distortion or cracking of the molded article,
It is slowly cooled over a long period of time. In particular, in order to cool as slowly as possible around 327 ° C., which is the melting point at which shrinkage abruptly occurs, the temperature is kept constant for a considerable time. PTF
Also in the molding of the E porous body, a method has been proposed in which a sintered PTFE molded body is gradually cooled to increase the crystallinity and then uniaxially stretched (Japanese Patent Publication No. 53-42794).

As described above, conventionally, in order to improve the crystallinity of the PTFE molded product, a long cooling period is required. Therefore, in the case of a continuous molded product such as a tube or a sheet, a very long heat treatment tank ( A heating furnace is required, which makes industrial implementation difficult. In addition, in order to gradually cool these continuous molded bodies, it is necessary to provide a temperature gradient in a long heat treatment tank to control the temperature, which is technically very difficult to implement. On the other hand, there is a method in which a PTFE molded article is crystallized by holding it at an isothermal temperature near the crystallization temperature of the resin for a long time without slow cooling. Not suitable for continuous molding such as tubes and sheets. Therefore, there is a demand for a practical method for increasing the crystallinity of the PTFE continuous molded body.

[0008]

SUMMARY OF THE INVENTION An object of the present invention is to provide an industrially practicable method for producing a PTFE continuous molded article having a high crystallinity. The present inventor has conducted diligent research to overcome the above-mentioned problems of the prior art, and as a result, after sintering a PTFE continuous molded body by a conventional method, PT
By cooling while maintaining a temperature of at least two points near the FE crystallization temperature for a certain period of time, the advantages of both isothermal treatment and gradual cooling can be brought out, and a continuous molded body with a high degree of crystallinity can be comparatively obtained. It has been found that it can be easily manufactured in a short time. The present invention has been completed based on these findings.

[0009]

Thus, according to the present invention, a polytetrafluoroethylene continuous molded body is sintered at a temperature equal to or higher than the melting point of the resin, and then the sintered continuous molded body is heated at 350 to 290 ° C. High crystallization characterized by cooling while passing through at least two different substantially constant temperature zones set sequentially from a high temperature region to a low temperature region within a temperature range within a time period of 0.5 to 10 minutes. A method for producing a continuous polytetrafluoroethylene molded article is provided.

The present invention will be described in detail below. PTFE
The continuous molded body can be obtained by various molding methods, but a molding method by paste extrusion is particularly preferable. As the PTFE used for paste extrusion, fine powder having a number average molecular weight of usually 500,000 or more, preferably 1,000,000 or more, more preferably 20 to 20,000,000 is suitable. Two or more kinds of fine powders having different number average molecular weights may be mixed and used.

In the paste extrusion method, usually PTFE10 is used.
Extruding is carried out by mixing 15 to 40 parts by weight of a liquid lubricant with 0 parts by weight. As the liquid lubricant, general-purpose ones can be used, and examples thereof include petroleum-based solvents such as solvent naphtha and white oil, hydrocarbon oils, toluols, ketones, esters, silicone oils, fluorocarbon oils, and polysiloxanes for these solvents. Examples thereof include a solution in which a polymer such as isobutylene and polyisoprene is dissolved, a mixture of two or more of these, water or an aqueous solution containing a surfactant, and the like.

Molding by paste extrusion is carried out by molding a mixture containing PTFE fine powder and a liquid lubricant into a predetermined shape at a temperature not higher than the sintering temperature of PTFE (327 ° C. or lower), usually around room temperature. Prior to paste extrusion, preforming may be performed by a conventional method. Generally, the above mixture is preformed at a pressure of, for example, about 1 to 50 kg / cm ² , and then extruded by a paste extruder,
Alternatively, it is rolled by a calendar roll or the like, or is extruded and then rolled to produce a molded product having a predetermined shape. The shape of the molded body is a continuous molded body such as a sheet, a tube, a rod, a strip and a film, and a thin sheet can be obtained by rolling.

The liquid lubricant is removed from the molded body by subjecting the molded body by paste extrusion to a treatment such as extraction, dissolution or heat evaporation. Liquid lubricants having a low boiling point are preferably removed by heating and drying, and liquid lubricants having a relatively high boiling point such as silicone oil and fluorocarbon are preferably removed by extraction.

In addition to the liquid lubricant, PTFE may contain other substances depending on the purpose. For example, pigments for coloring, carbon black, graphite, silica powder, asbestos powder, glass powder, glass fiber, silicates for improving wear resistance, preventing cold flow and facilitating generation of pores. Inorganic fillers such as carbonates and the like, metal powder, metal oxide powder, metal sulfide powder and the like can be appropriately added.

Further, in the case of producing a porous body, substances that can be removed or decomposed by heating, extraction, dissolution, etc. in order to assist the formation of a porous structure, such as ammonium chloride, sodium chloride, other plastics, Rubber or the like can be added to PTFE in the form of powder or solution.

The continuous PTFE molded article is sintered at a temperature of 327 ° C. or higher, which is the transition point of PTFE. The sintering process is
Generally, it is carried out by passing the PTFE molded body through a furnace kept at 350 to 550 ° C. for several minutes to several tens of minutes, and in some cases, for a longer time. In the paste extrusion method, usually, a heating furnace is provided under a die of an extruder, an upper part is a liquid lubricant drying furnace (about 100 to 300 ° C.), and a lower part is a firing furnace (sintering zone), and the paste is extruded from the die. The continuous molded body is dried and then sintered.

The melt-recrystallized PTFE has many crystal defects, the crystallinity is lowered to about 45 to 70%, and the melting point is 327.degree. Therefore, the quenched PTFE molded product has a low crystallinity. On the other hand, in the present invention, the sintered PTFE continuous molded body is heat-treated under specific conditions to increase its crystallinity.

In the heat treatment method for increasing the crystallinity, the sintered PTFE continuous molded body is set at least from the high temperature region to the low temperature region in the temperature range of 350 to 290 ° C. near the crystallization temperature of PTFE. Two
The cooling is performed by passing through three different substantially constant temperature zones (isothermal zones) within a time period of 0.5 to 10 minutes. Specifically, for example, PT exiting the firing furnace
A heating furnace that holds the FE continuous molded body at 340 ° C .;
By successively passing through the heating furnace kept at 0 ° C. for each short time, both the isothermal crystallization method and the slow cooling crystallization method can be skillfully combined to achieve a high crystallinity in a short time.

For example, when the temperature of the heating furnace is set to 340 ° C., the substantially constant temperature is 340 ± 5 ° C., preferably 340 ± 3 ° C. near the set temperature during the heat treatment.
It means being kept isothermal within a certain range.
Further, the temperature zone usually means the inside of a heating furnace maintained at a substantially constant temperature.

The heat treatment of the sintered PTFE continuous molding is
In the temperature range of 350 to 290 ° C., at least two different substantially constant temperatures are set from the high temperature region to the low temperature region, and the temperature bands are sequentially passed. If there is only one constant temperature band, so-called isothermal processing is performed, which requires a long time. Two or more different constant temperature zones are sequentially provided from the high temperature side to the low temperature side. When the heat treatment temperature exceeds 350 ° C., the treatment time becomes long, and when the heat treatment temperature is below 290 ° C., crystallization becomes difficult to proceed. The heat treatment temperature is preferably in the range of 340 to 300 ° C.

After the sintering, the continuous PTFE molded body is gradually cooled from the high temperature side to the low temperature side by passing through the zone in which the temperature is kept constant. This cooling process is a kind of slow cooling process. Simply sintered PT
When the FE molded body is slowly cooled, high crystallinity cannot be obtained unless it is slowly cooled at a temperature lowering rate slower than 10 ° C./hour. High crystallinity can be obtained in a relatively short time by performing two or more isothermal treatments in stepwise. The passage time (residence time) in each constant temperature zone depends on the shape and size of the molded product, but is 0.5 to 10 minutes, preferably 1 to 8 minutes.
Minutes, more preferably about 1.5 to 5 minutes.

These two or more constant temperature zones can be successively provided in the firing furnace. The heat treatment may be performed after re-heating the PTFE continuous molded body that has been sintered in advance so as to reach the crystallization temperature or higher. Also,
After the heat treatment for increasing the crystallinity, the cooling rate may be increased by air cooling or the like. According to the method of the present invention, the crystallinity is about 80% or 80% by using the heating furnace for a relatively short time, and thus for a relatively short time.
% Or more of the PTFE continuous molded body can be manufactured.

In a preferred embodiment of the production method of the present invention, the continuous molded body is sintered in a temperature range of 400 ° C. or lower and 350 ° C. or higher, and then set substantially within a temperature range of 350 ° C. or lower and 320 ° C. or higher. And a substantially constant temperature band set within a temperature range of less than 320 ° C. and 290 ° C. or higher.

As a more specific embodiment, for example,
After sintering the continuous molded body within a temperature range of 350 to 400 ° C., a temperature band within a temperature range of 325 ° C. ± 5 ° C.
A method of sequentially passing temperature bands within a temperature range of 0 ° C. ± 5 ° C., a temperature band within a temperature range of 335 ° C. ± 5 ° C., 31
A method of sequentially passing temperature bands within a temperature range of 0 ° C. ± 5 ° C., a temperature band within a temperature range of 335 ° C. ± 5 ° C., 31
A method of sequentially passing temperature bands within a temperature range of 5 ° C. ± 5 ° C., a temperature band within a temperature range of 325 ° C. ± 5 ° C., and 30
There is a method of sequentially passing through temperature bands within a temperature range of 0 ° C ± 5 ° C.

The high crystallinity PTFE continuous molded product obtained by the production method of the present invention may be used as it is if desired, but a sheet-shaped or tube-shaped continuous molded product is stretched in at least one axial direction. As a result, a PTFE porous body can be obtained. Stretching is a sheet, tube,
It can be carried out by mechanically stretching a continuous molded body in the shape of a rod or the like by a usual method. This PTFE porous body has a higher porosity than a porous body obtained by stretching a molded body having a low degree of crystallinity at the same stretch ratio, and thus has excellent permeability.

[0026]

EXAMPLES The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[Examples 1 to 3] Fine powder of polytetrafluoroethylene (F-10 manufactured by Daikin Industries, Ltd.)
4) 100 parts by weight of drysol as a liquid lubricant was mixed with 18 parts by weight, and the mixture was allowed to stand at room temperature for one day. 1300 kg of this mixture was preformed in 3 minutes and then extruded by an extruder at an extrusion speed of 1.75 m / min, then at 50 ° C.,
Rolled to a thickness of 300 μm with a roller of 900 rpm. After rolling, it was passed through a drying furnace maintained at 130 to 15 ° C at a linear velocity of about 8 m to be dried.

The dried sheet was sequentially sintered and crystallized by a roll stretching machine having four continuous sintering zones. The first and second zones of the roll stretching machine are 385 ° C, 370 ° C,
And 350 ° C., the dry sheet was set to a linear velocity of 0.
It was passed at 8 m / min and sintered. Then, the third and fourth zones of the roll stretching machine are set at two constant temperatures between 350 and 290 ° C., and the sintered sheet is drawn at a linear velocity of 0.5.
It was processed by a negative stretching of m / min. Table 1 collectively shows each set temperature, each residence time, and the crystallinity.

[Comparative Examples 1 to 3] For comparison, a part of the dried sheet was cut into a size of 5 cm × 5 cm, and this sample was kept in a constant temperature bath at 370 ° C. for 10 minutes.
It was gradually cooled to 300 ° C. in 1 hour (Comparative Example 1). The same sample was held at 370 ° C. for 10 minutes, then rapidly cooled to 320 ° C. and then left for 1 hour (Comparative Example 2). The same sample was held at 370 ° C. for 10 minutes and then rapidly cooled to room temperature (Comparative Example 3). The results are collectively shown in Table 1.

[0030]

[Table 1]

(Footnote) Crystallinity: Crystallinity was evaluated by the amount of heat per gram using a differential scanning calorimeter (DSC). As is clear from Table 1, according to the production method of the present invention,
Due to the short residence time in the furnace, it is possible to obtain a PTFE continuous molded body having high crystallinity.

[0032]

According to the present invention, PTFE having a high crystallinity is obtained.
An industrially practicable method for producing a continuous compact is provided. The high-crystallinity PTFE continuous molded body obtained by the manufacturing method of the present invention can be particularly suitably used for manufacturing a porous body having a high porosity.

Claims (8)

[Claims] 1. A polytetrafluoroethylene continuous molded body is sintered at a temperature equal to or higher than the melting point of the resin, and then the sintered continuous molded body is successively heated in a temperature range of 350 to 290 ° C. from a high temperature region to a low temperature region. Each of the set at least two different substantially constant temperature zones is
A method for producing a high-crystallinity polytetrafluoroethylene continuous molded article, which comprises cooling while passing the material within 10 minutes. 2. A substantially constant temperature band set within a temperature range of less than 350 ° C. and 320 ° C. or more after sintering the continuous molded product in a temperature range of 400 ° C. or less and 350 ° C. or more, and 3
The manufacturing method according to claim 1, wherein a substantially constant temperature band set within a temperature range of less than 20 ° C. and 290 ° C. or higher is successively passed. 3. After sintering the continuous molded body at 370 ° C., 3
Temperature band within the temperature range of 25 ° C ± 5 ° C, 310 ° C ± 5
The manufacturing method according to claim 1, wherein the temperature bands in the temperature range of ° C are sequentially passed. 4. After sintering a continuous molded body at 385 ° C., 3
Temperature band within the temperature range of 25 ° C ± 5 ° C, 310 ° C ± 5
The manufacturing method according to claim 1, wherein the temperature bands in the temperature range of ° C are sequentially passed. 5. After sintering the continuous molded body at 350 ° C., 3
Temperature band within the temperature range of 25 ° C ± 5 ° C, 310 ° C ± 5
The manufacturing method according to claim 1, wherein the temperature bands in the temperature range of ° C are sequentially passed. 6. After sintering the continuous molded body at 370 ° C., 3
Temperature band within the temperature range of 35 ° C ± 5 ° C and 310 ° C ± 5
The manufacturing method according to claim 1, wherein the temperature bands in the temperature range of ° C are sequentially passed. 7. After sintering the continuous molded body at 370 ° C., 3
Temperature band within the temperature range of 35 ° C ± 5 ° C and 315 ° C ± 5
The manufacturing method according to claim 1, wherein the temperature bands in the temperature range of ° C are sequentially passed. 8. After sintering the continuous molded body at 370 ° C., 3
Temperature band within the temperature range of 25 ° C ± 5 ° C and 300 ° C ± 5
The manufacturing method according to claim 1, wherein the temperature bands in the temperature range of ° C are sequentially passed.