JPS5814291B2 - Manufacturing method of polytetrafluoroethylene molding powder - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing polytetrafluoroethylene molding powder.

As is well known, polytetrafluoroethylene has poor fluidity even at temperatures above its melting point, and its molding is carried out by a method similar to the powder metallurgy method of compression preforming and sintering.

For this reason, the properties of the polytetrafluoroethylene powder itself directly affect its moldability and the physical properties of the desired molded product.

That is, the polytetrafluoroethylene molding powder must have good powder flowability from the viewpoint of ease of handling such as filling into a mold, and must have a high apparent density so that compression preforming can be performed using the smallest possible mold. It must basically satisfy the requirements that the particle size of the primary particles of the powder be as small as possible so that a dense molded product can be obtained with as little pre-molding pressure as possible.

On the other hand, polytetrafluoroethylene molding powder is obtained by crushing coarse particles after polymerization, but since polytetrafluoroethylene is a relatively soft synthetic resin, even if a crusher using impact force is used. Tearing fuzz is generated in each particle due to crushing, and therefore, the smaller the particle size is made for the purpose of improving the properties of the molded product, the lower the fluidity and apparent density of the powder become, making the powder difficult to handle.

Therefore, it is not possible to satisfy all of the above requirements at the same time by simply grinding coarse powder.

As a method to solve this problem all at once, it has been proposed to form secondary particles by granulating fine powder as primary particles that have been produced by pulverization.

The main granulation method is to granulate the polytetrafluoroethylene secondary particles, which have poor fluidity, in a type blender.
A method of forming secondary particles while applying mild mechanical stirring while spraying an organic liquid having a surface tension of 5 dynes/cm or less (% Kosho No. 43-6290), in which the above polytetrafluoroethylene secondary particles are ~90
A method of forming secondary particles while stirring in water at ℃ (
(Japanese Patent Publication No. 43-8611), or a method in which the above-mentioned polytetrafluoroethylene primary particles are added to a dispersion of water and an insoluble or water-dispersible organic liquid, and granulated with stirring at a temperature of 0 to 100°C (Japanese Patent Publication No. 43-8611). Publication No. 44-22619)
This is a method such as

By granulating polytetrafluoroethylene powder in this manner, it is possible to produce polytetrafluoroethylene powder that is significantly improved over conventional products in terms of the balance between ease of handling the powder and properties of molded products. It is not possible to satisfy the following requirements.

That is, after production, the polytetrafluoroethylene molding powder is subjected to various external forces for packaging, transportation, molding, etc., and is subjected to several deposition periods before being subjected to molding.

During this time, the granulated powder tends to form further agglomerated agglomerates.

Even if such lumps are formed, there will be no problem if they are easily broken down by a small external force during molding and immediately return to the original unit of particles with good fluidity.

In addition, in all of the conventional granulation methods mentioned above, granulation is carried out extremely excessively by making the stirring time extremely long, increasing the temperature or pressure, or stirring at low speed. It is known that formation of agglomerates can be prevented by tightly compacting polytetrafluoroethylene particles.

However, when granulation is carried out excessively in this manner, the desirable properties of the primary particles with respect to the properties of the molded article are lost, and the properties of the resulting molded article are significantly deteriorated.

Even though a method for granulating polytetrafluoroethylene fine powder has been developed, it does not mean that all the problems associated with polytetrafluoroethylene powder have been solved, and this method still There is no denying that it is destined to be a balance between ease of handling and the properties of molded products obtained from the powder.

The present inventors conducted research and completed the present invention in order to fundamentally solve the problems in the conventional polytetrafluoroethylene molding powder manufacturing method as described above.

That is, the present invention involves moistening polytetrafluoroethylene granular powder with an average particle size of 100 μm or less with a liquid having a boiling point of 200° C. or less and a surface tension of 40 dynes/cm or less at 25° C. in the presence or absence of water. After being granulated into a spherical shape by mechanical stirring, it is fluidized with an air stream at 0 to 150°C without substantially changing the average particle size, resulting in a powder flow rate of 6.
Above, the shaking mass degree is 0. The present invention relates to a method for producing polytetrafluoroethylene molding powder, which yields molded products with excellent physical properties, characterized in that the powder weighs 7 kg/cm or less.

According to the method of the present invention, a polytetrafluoroethylene granulated powder that has good moldability and provides molded articles having excellent physical properties is produced.

That is, the polytetrafluoroethylene molding powder of the present invention has excellent powder flowability, small agglomeration properties, and high apparent density, and can provide molded products with good mechanical and electrical properties. be.

The raw material coarse powder used in the present invention is produced by a conventionally known method, for example, by polymerizing tetrafluoroethylene in the presence of water containing a free radical reaction initiator, or by polymerizing tetrafluoroethylene in a gas phase or liquid. The polymer powder obtained by irradiating the phase with ionizing radiation is processed in the presence of water or in a dry state by a grinder such as a hammer mill, a grinder with bladed rotors, a liquid energy grinder, or a cutting blade. Average particle size 100μ or less,
Preferably, it is a granular fine powder with a size of 50 μm or less.

The shape of the granular powder may be fibrous or non-fibrous.

Next, the polytetrafluoroethylene fine powder thus obtained was heated to a boiling point of 200°C or less in the presence or absence of water.
Granulation is carried out by mechanical stirring while wetting with a liquid having a surface tension of 40 dynes/cm or less at 5°C.

As this granulation method, for example, Japanese Patent Publication No. 44-22619
The method described in Japanese Patent Publication No. 43-6290 can be employed.

The former method is a method in which polytetrafluoroethylene powder is stirred in a relatively large amount of water in the presence of an organic liquid. It is necessary that the surface tension at ℃ is 40 dynes/cm or less.

The amount used is 0.1 kg per 1 kg of polytetrafluoroethylene.
It is desirable to use the amount in the range of 3 to 1 liter.

Stirring is usually performed for 1 minute or more.

This granulation method is superior to other methods in that it can produce particles with a nearly spherical shape and a smooth particle surface, but it is difficult to use when stirring for a long time and at the speed of stirring. When the ratio is gradual, that is, when a relatively silent mechanical force is applied over a long period of time, the particles tend to harden, increase in particle size, and deteriorate the physical properties of the molded article, which is undesirable.

Therefore, when this method is employed in the method of the present invention, it is preferable to perform high-speed stirring for a relatively short period of time.

As the liquid used in the granulation method, a liquid having a boiling point of 200° C. or less and a surface tension of 40 dynes/cm or less at 25° C. is used.

Specifically, for example, aliphatic hydrocarbons such as hexane, heptane, cyclohexane, and gasoline, aromatic hydrocarbons such as benzene, toluene, and xylene, carbon tetrachloride, chloroform, trichloroethane, perk rotor trifluoroethane, and difluorotetrafluorocarbons. Fluorinated hydrocarbons such as ethane, fluorotoric methane, tetrafluorocarbon moethane, and other fluorinated hydrocarbons with a surface tension (at 25°C) of 40
Various alcohols, esters, ketones, etc. of dyne/cm or less may also be used.

However, when granulation is carried out in the presence of water, only those liquids that are insoluble in water or those that do not dissolve more than 15% in water are used.

Moreover, in this case, those having a boiling point of 200° C. or higher require a long time to dry and are not practical.

Also, the surface tension of the liquid is 40 dynes/cm. If the size is larger than this, the polytetrafluoroethylene powder becomes difficult to wet, and the desired effect of the present invention cannot be obtained.

The polytetrafluoroethylene particles thus obtained are then agitated with a stream of air.

That is, if the polytetrafluoroethylene particles are placed in a cylindrical or angular container and the gas is blown into the lower part through a perforated plate or the like under conditions that do not substantially change the particle size, the particles are made to flow. The polytetrafluoroethylene particles slightly rub or collide with the vessel wall or with each other.

As a result, the fine primary particles lightly attached to the surface of each particle are compressed or removed, making the outer layer of the particle smooth and dense, resulting in fluidity without causing any substantial change in grain size. It is thought that spherical particles with good properties are formed.

Since the compression and collision caused by the flow are very mild, it is thought that only the outer layer is reshaped and the interior is hardly affected.

Therefore, it is possible to obtain very small spherical particles with good physical properties, high fluidity, and agglomeration properties.

However, strictly speaking, it is unclear how fluid treatment actually affects the morphology and internal and external properties of the powder.

For example, the surface of each particle of the granulated powder obtained by the method disclosed in Japanese Patent Publication No. 44-22619 is generally very smooth and has no protrusions such as tear fluff found in primary particles.

Nevertheless, the powder fluidity of this powder is generally unsatisfactory and there is a problem of large agglomeration during handling.

This fluidization treatment may be carried out after drying the granulated powder, but it may also be carried out directly into a fluidized bed without being substantially dried after granulation, and the drying process may be completed while being fluidized by a heated drying air stream.

The higher the temperature of the gas for fluidization, the harder the particles become. However, if the temperature exceeds 150°C, the surface becomes too hard, resulting in poor fusion between particles during molding, even if the fluidity improves significantly in a short period of time. Decrease the properties of the molded product.

Further, at a temperature lower than 0°C, the particle surface becomes smooth, which is effective, but requires a long time and is not practical. Therefore, the operating range is 0 to 140°C, but preferably 30 to 100°C.

The flow time has a relationship with the flow temperature, and once the temperature is determined, the flow time is also approximately determined.

The lower limit of this time is the minimum time required to improve the flowability and agglomeration of the powder, and the upper limit is set at a point at which the physical properties of the molded article do not begin to deteriorate substantially.

These relationships can be determined experimentally, and according to the inventor's experiments, for example, at a temperature of 55°C, the flow time is about 15 minutes to about 60 minutes, and at 80°C, the flow time is about 10 minutes to 25 minutes. It is.

Furthermore, at low temperatures such as 0°C, it is necessary to flow for about 60 minutes or more, and there is almost no deterioration in physical properties even if it is allowed to flow for a longer time. The physical properties deteriorate significantly when the flow is

In the present invention, air or an inert gas is used as the carrier gas to be caused to flow, and air is preferable.

The polytetrafluoroethylene powder produced by the method of the present invention has a powder fluidity of 6 or more and a shaking agglomeration degree of 0.7 kg・c.
m or less, an apparent density of 750 g/l or more, and a particle size of about 200 to 1000 μ.

Furthermore, a molded article obtained by molding the polytetrafluoroethylene powder obtained by the method of the present invention in a conventional manner has a tensile strength of 350.
kg/cm2 or more, has an elongation of 300% or more, a dielectric breakdown voltage of 6 KV/0.1 mm or more, and has a very smooth surface.

The method of the present invention can also be applied to polytetrafluoroethylene powder containing fillers.

In this case, a uniform granulated powder containing a filler is produced in advance and then fluidized.

In this way, effects similar to those described above can be obtained.

Examples of the filler used in this case include glass fiber powder, graphite, and molybdenum disulfide.

The method of the present invention will be specifically explained below with reference to Examples.

Example 1 Polytetrafluoroethylene coarse powder obtained by suspension polymerization of tetrafluoroethylene in water in the presence of a polymerization initiator that generates free radicals was finely pulverized to give an average particle size of 40 μm and an apparent density of 300 g. /11 polytetrafluoroethylene fine powder was produced.

The molded article obtained from this powder had the following properties.

Dielectric breakdown voltage 1 0. 3KV/0. 1
mvt tensile strength 5 2 0 kg/i elongation 380% Next, 50 liters of water 1 average particle size 40 in a stirring tank with an internal volume of 100 liters
Add 5 kg of the above-mentioned polytetrafluoroethylene fine powder of μ and start stirring at 400 rpm using a sled-type stirrer whose rotating diameter is κ, which is lowered vertically into the stirring tank, and immediately add 2 liters of trichlorethylene. Preparation 1
Stirred for 0 minutes.

The polytetrafluoroethylene powder moistened with trichlorethylene was separated from the water and dried in an air permeable dryer to obtain a polytetrafluoroethylene granulated powder.

The properties of this powder and the physical properties of the molded product obtained from it are as follows.

Powder fluidity 3 Shaking mass consistency 1. OOkg・apparent density 6 7 0 9/It Average particle size 540 μ Tensile strength of molded product 4 3 0 kg/= elongation
Degree 350% Breakdown voltage 7. 8KV/0. 1 mm However, the measurement method for each of the above physical properties is as follows.

[Powder fluidity]

This is carried out using the measuring device shown in FIG.

In other words, the hopper is supported with its center line aligned with the support stand 12.
1 and 2 are used.

Hopper 1 has an inlet 3 with a diameter of 74 mm and an outlet 4 with a diameter of 12 mm.
The height from the inlet 3 to the outlet 4 is 123 mm, and there is a partition plate 5 at the outlet 4, which allows a certain amount of powder in the hopper to fall arbitrarily.

Also, the diameter of the inlet 6 of hopper 2 is 76 m4, the diameter of the outlet 7 is 12 mxs, and the height from the inlet 6 to the outlet 7 is 120 m
Similarly to the hopper 2, a partition plate 8 is provided at the hopper 2.

The distance between hopper 1 and hopper 2 is set such that the distance between each partition plate 5 and 8 is 15 cIfL.

Further, in FIG. 1, numerals 9 and 10 are outlet covers for each hopper, and numeral 11 represents a receiver for the fallen powder.

Powder fluidity measurement is performed by measuring approximately 200 g of powder to be measured at 23.5
Leave it in a room with a temperature of ~24.5℃ for more than 4 hours, and then
This is done using material that has passed through a mesh sieve.

(I) First, a cup full of powder to be measured with a capacity of 30 cc is put into hopper 1, and the partition plate 5 is immediately opened to allow the powder to fall into hopper 2.

After the powder has completely fallen into the hopper 2, leave it for 15±2 seconds, then open the partition plate 8 and observe whether the powder flows down from the outlet 7. If all the powder has flowed down within 8 seconds, then It was determined that the object had fallen.

(II) Repeat the same measurement as in (I) above three times to see if it falls. If it falls two or more times out of three, it is judged as "good" fluidity, and if it does not fall even once, it is judged as "poor." ”.

If it falls only once out of three times, the same measurement is carried out two more times, and if it falls both times it is determined to be "good", otherwise it is determined to be "poor".

(1) For the powder whose fluidity was determined to be "good" in the above measurements, the powder from the above-mentioned hopper 2 was put into hopper 1 and measured in the same manner as above, and the result was that the fluidity was "good". If the number of cups is exceeded, the number of cups is increased sequentially until it becomes "defective", and the measurement is continued up to a maximum of 8.

At the time of each measurement, the powder that flowed out from the hopper 2 in the previous measurement may be reused.

(5) According to the above measurements, the larger the amount of polytetrafluoroethylene used, that is, the larger the number of powder particles used, the more difficult it is to fall.

Therefore, the number obtained by subtracting 1 from the number when the fluidity becomes "poor" is defined as the "powder fluidity" of the powder.

[Shaking mass consistency]

As shown in Fig. 2, the measuring device includes a frame 2 vertically inserted into the box 1, a horizontal table 3 on which the box 1 is placed, a vibration device (not shown) for vibrating the box 1, and a vibration device. It consists of a shaft 4 that connects the box 1.

Box 1 is made of stainless steel with a thickness of 4 mm, and the internal dimensions are 76 x 62 x horizontal (vibration direction) and height, respectively.
It is 95mm.

The box 1 is provided with a groove 5 into which the frame 2 is inserted.

This frame 2, like box 1, is made of stainless steel, has a thickness of 1 mm, and the width of each part is l: 120, l1: in Fig. 3.
80, l2: 2.0, l34.6, l43mm.

The measurement method is as follows. First, insert the frame 2 into the box 1 all the way to the bottom, weigh out 1 g of hepolytetrafluoroethylene powder, and drive the vibration device to insert the frame 2 into the box 1 with an amplitude of 22 crrL and a frequency of 1 vibration. 5 in seconds
00 times A constant speed reciprocating motion is applied in the direction A of the arrow shown in FIG.

Since the velocity is constant, at one turning point, the powder in box 1 is
At the other turning point, the powder collides as a whole with the inner wall of the box on the opposite side, and at the end of the vibration, the powder becomes a lump.

Next, this box 1 was directly removed from the X-horizontal table 3, and the frame 2 was pulled up at a speed of 200 ii/min, and the force required at this time was recorded by a self-recorded tension test.

When the frame 2 is pulled up, the nodules of the polytetrafluoroethylene powder are cut by the frame 2, but the larger the degree of agglomeration of the powder, the stronger the nodules are, and therefore the greater the force required to pull up the frame 2.

In this way, the records by the self-recording tension testing machine are recorded as shown in Figure 4A and Figure 4G.

However, in FIG. 4, the vertical axis shows the force required to pull up the frame, and the horizontal axis shows the height of lifting the frame.

The degree of agglomeration is indicated by the area (LcrrL) surrounded by the curve thus recorded and the horizontal axis.

The unit of this shaking agglomeration degree is (g, cm), which indicates the energy when the powder agglomeration is cut.

The value of the shaking agglomeration degree determined in this way becomes larger as the powder is more easily agglomerated.

For example, FIG. 4A shows the test results of the polytetrafluoroethylene powder of the present invention produced in Example 1, and FIG. The test results are for a powder that was moistened with an organic liquid and rolled, then left to dry in a drying oven.
If you compare these, Figure 4 A, which is difficult to lump together, is frame 2.
It is known that the maximum force required to pull up is lower than in Figure 4, and the force decreases quickly after reaching the maximum point, whereas in the case of Figure 4, the large force continues for a long time. .

〔Particle size〕

Granulated powder is prepared according to JISK-6891-5-4, and in the case of fine powder with a particle size of 100μ or less, it is processed using a wet sieve using carbon tetrachloride as a cleaning agent according to ASTM D-1457-69.
Measured by the method.

[Tensile strength/elongation]

The physical properties of the obtained molded product were determined by putting 5 g of powder into a mold with a rectangular depression of 96 x 18 mm, and applying a pressure of 300 ky/
After preforming with CrlL, it was fired at 370°C for 5.5 hours, and the temperature was lowered to room temperature at a rate of 50°C/hour to produce a plate-shaped molded product, which was used as a sample and measured according to the method of JISK-6888-5-4. .

[Dielectric breakdown voltage]

190g of powder is placed in a mold with an inner diameter of 50mm, preformed and fired in the same manner as described above for [tensile strength] to produce a block-shaped molded product, which is then cut to a thickness of 9mm. 1
A tape was made in between, and measurements were made using this tape.

1 kg of the powder obtained above was transferred to a fluidized bed dryer midget dryer (made of a cylindrical body with an inner diameter of about 240 mW, and a perforated plate with holes of 3 mg in diameter at the bottom of the cylindrical body) manufactured by Fuji Powdal ■. ) and the superficial velocity is 0.
．． Air at a temperature of 30° C. was introduced at a rate of 6 m/sec and allowed to flow for 60 minutes.

A portion of the fluidized powder was also taken out as a sample 5 minutes, 10 minutes, 20 minutes, and 30 minutes after the start of fluidization.

The properties of these powders and the physical properties of molded products obtained from each powder are shown in Table 1 below.

Example 2 In Example 1 above, the solution was soaked at 70°C for 30 minutes, and
The procedure was repeated in the same manner as in Example 1, except that a portion of the fluidized powder was removed after 30 minutes, 10 minutes, 20 minutes, and 30 minutes.

The properties of this powder and its molded products are shown in Table 2 below.

Example 3 In Example 2 above, flow at 140°C for 30 minutes,
The process was carried out in the same manner as in Example 1, except that some of the powder was taken out after 5, 10 and 30 minutes.

The physical properties of this powder and its molded product are shown in Table 3 below.

Example 4 In the same manner as in Example 1, the fluid was flowed for 60 minutes at 0°C, and then for 5 minutes.
The process was carried out in the same manner as in Example 1, except that some of the powder was taken out after 20 and 30 minutes.

The physical properties of this powder and its molded product are shown in Table 4 below.

Example 5 Polytetrafluoroethylene fine powder with an average particle size of 10 μm was put into a V-type blender with an internal volume of 100 liters, which had an internal stirrer and liquid spraying mechanism whose shaft was the same as that of the main body, and a jacket on the outside. While rotating the stirrer at 400 rpm and the blender body at 20 rpm, 10 liters of methanol was sprayed at the same time, the rotation was stopped after 20 minutes, and warm water was passed through the jacket to remove methanol and dry the powder.

The powder taken out from the Type B blender was sieved through a 20 mesh (inch mesh) gold steel to remove coarse particles.

The average particle diameter of the coarse secondary particles (granulated powder) is 550μ, the apparent density is 685g/l, the powder fluidity is 2, the shaking agglomeration degree is 1.20kg・cm, and the tensile strength of the molded product is 4.5
5 kg/cr? t, elongation is 370%, dielectric breakdown voltage is 8. IKV/0. It was 1 mm.

1 kg of this powder was placed in the same fluidizer as used in Example 1, and was fluidized for 30 minutes by introducing air at 30° C. at a superficial velocity of 0.8 m/sec.

At the same time, a portion of the fluid powder was taken out as a sample after 5, 10, 20, and 30 minutes.

The properties of the powder thus obtained and the properties of the molded product obtained from the powder are shown in Table 5.

Example 6 In Example 5, air at 80° C. was used instead of air at 30° C., and all other treatments were carried out in the same manner as in Example 5.

The properties of the powder thus obtained and the properties of the molded product obtained from the powder are shown in Table 6 below.

Claims (1)

[Claims] 1 Polytetrafluoroethylene granular powder with an average particle size of 100μ or less is heated to a boiling point of 200°C in the presence or absence of water.
Hereinafter, the powder is granulated by mechanical stirring while being moistened with a liquid having a surface tension of 40 dynes/cm or less at 25°C, and then fluidized with an air stream at 0 to 150°C without substantially changing the average particle size to flow the powder. degree of 6 or more, shaking mass degree 0. A method for producing polytetrafluoroethylene molding powder, which yields a molded product with excellent physical properties, characterized in that the powder weighs 7 kg/m or less.