JPS6346097B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a filled polytetrafluoroethylene fine powder containing carbon fiber powder.

Polytetrafluoroethylene (hereinafter abbreviated as PTFE) molded bodies filled with carbon fiber powder have excellent abrasion resistance and rolling properties, and have improved chemical resistance compared to those filled with glass fiber powder, so they can be coated with a special adhesive. Used for moving parts, etc.

PTFE molding powders containing fillers are already widely used, but the practical application of fine powders containing fillers is relatively new, and as the practical requirements for this resin have diversified, It has been increasing recently. In other words, such filler-containing PTFE fine powder can be used as molded products such as thin-walled tubes, pipes, and sheets, or for covering electric wires, etc. by so-called paste extrusion molding performed by adding an extrusion aid. It is useful in this respect.

There are two methods for blending the filler into the PTFE fine powder: a dry method and a wet method. The dry method is
This method involves adding a filler after coagulating the PTFE aqueous dispersion and turning it into fine powder, but when this method is adopted, the fine powder itself becomes a secondary aggregate of primary particles. Uniform mixing with the filler cannot be achieved. Furthermore, the PTFE powder is deformed by the mechanical force during mixing, making it impossible to obtain a molded product with a uniform internal structure.

Therefore, in order to produce a mixed powder free of these drawbacks, a wet method is adopted as the mixing method. The wet mixing method is a method in which filler powder is added to an aqueous PTFE dispersion as it is or in the form of the same aqueous dispersion, and the filler powder is coagulated to obtain a filler-containing fine powder. Furthermore, in general
It is also known that when coagulating a PTFE aqueous dispersion, if a water-insoluble liquid is present, the coagulated particles can be agglomerated into spheres or granules of an appropriate size.
When carbon powder is used as a filler, a method of adding an organic liquid before or after coagulation in wet mixing has been proposed (Japanese Patent Publication No. 1983-
(See Publication No. 34653).

The present inventors attempted to produce a PTFE fine powder containing carbon fiber powder as a filler according to the above-mentioned known technology, but found that the method used for carbon fillers could not be applied as is. I learned something. i.e. when using carbon fiber powder, PTFE before or after co-coagulation as when using carbon filler.
The method of adding a water-insoluble organic liquid to an aqueous dispersion also requires separation of PTFE or filler,
It was found that losses occur and the mixing state of the filler becomes non-uniform.

The present invention is a method for producing a filler-containing PTFE fine powder that does not have the above-mentioned drawbacks, and its gist is as follows:
The PTFE colloidal aqueous dispersion and the carbon fiber powder are combined and coagulated while stirring, and from the start of the coagulation to the completion of the coagulation, the boiling point is 30~30.
The present invention relates to a method for producing a PTFE fine powder containing a filler, which comprises adding a water-insoluble organic liquid having a surface tension of 35 dynes/cm or less at 150°C and 20°C.

The PTFE used in the present invention includes TFE homopolymers, and TFE copolymers copolymerized with 2% or less of olefins as modifiers, such as hexafluoropropylene, chlorotrifluoroethylene, perfluoroalkyl vinyl ether, etc. can be mentioned.

As the PTFE colloidal aqueous dispersion, it is preferable to use an aqueous dispersion containing 5 to 35% by weight of PTFE colloidal particles having an average particle size of 0.1 to 0.3 μm.

The carbon fiber powder, which is a filler used in the present invention, is made by finely pulverizing carbon fiber, and each particle has a diameter of about 3 to 30 μm and an average length of about 10 to 1000 μm. Carbon fiber powder is usually left as is.
It can be added to the PTFE aqueous dispersion, but if the particle size is small, it may be difficult to uniformly disperse it in the PTFE aqueous dispersion. In such a case, by adding water to the carbon fiber powder and sufficiently stirring and mixing to form a uniform aqueous dispersion, uniform mixing with the PTFE aqueous dispersion becomes possible. carbon fiber powder
The addition ratio to PTFE is usually about 5 to 80% by weight.

In the method of the present invention, coagulation can be caused simply by continuing stirring, as in the case of coagulation of a normal aqueous PTFE dispersion. However, coagulation can take a long time, and when coagulation is performed only by stirring, the timing is often inconsistent, so careful monitoring is necessary to detect the starting point. The operation becomes complicated. For these reasons, it is generally preferred to use a coagulant in the method of the invention.

As the coagulant, any commonly used coagulant for coagulating PTFE aqueous dispersions can be used, such as water-soluble salts such as magnesium chloride and aluminum chloride, mineral acids such as nitric acid, hydrochloric acid, and sulfuric acid, alcohols, Examples include water-soluble organic liquids such as acetone, cationic surfactants, and the like.

As a water-insoluble liquid, the boiling point is 30-150℃, 20℃
Hydrocarbons such as hexane, heptane, gasoline, kerosene, and toluene, chlorinated hydrocarbons such as carbon tetrachloride, trichloroethylene, and trichlorotrifluoro are preferred. ethane,
Examples include fluorinated hydrocarbons such as tetrachlorodifluoroethane and trichlorofluoromethane. The amount of the water-insoluble organic liquid added may generally be 30 to 100 parts per 100 parts (by weight, hereinafter the same) of the solid content.

Next, the method of the present invention will be explained in accordance with the implementation steps with reference to the attached FIG. 1.

Generally, the aqueous dispersion of carbon fiber powder is added to the aqueous PTFE dispersion while stirring;
Add coagulant after PTFE and filler are thoroughly and uniformly mixed. Of course, the mixing order of the PTFE aqueous dispersion and the filler aqueous dispersion may be reversed.

FIG. 1 schematically shows how the stirring torque of the dispersion changes over time before and after coagulation. Curves 1 and 2 in the figure are representative examples of torque changes over time. Q in the figure is the point of addition of the coagulant, [A] is the pre-coagulation zone, and [B]
is the coagulation zone and [C] is the post-coagulation zone. The state of the solid content in each zone is colloidal particles in [A], jelly-like in [B], and hydrophobic in [C]. The hydrophobic state in [C] means that the dispersed particles in the liquid are not compatible with water, and are in a state where they immediately precipitate or float to separate from the water when stirring is stopped.

In the coagulation of an aqueous PTFE dispersion, as shown in Figure 1, there are cases where one torque peak appears and cases where two torque peaks appear.
In many cases, the state of the dispersion liquid changes as described above due to the stirring torque.
It can be clearly divided into three zones: [B] and [C].

In other words, when a coagulant is added while stirring, coagulation begins immediately, the solid content becomes jelly-like, and the viscosity of the dispersion increases rapidly.When it finally reaches a peak, hydrophobicization begins and the viscosity rapidly decreases. .

In the case of curve 1, peak P is the branching point between these two points. For curve 2, peak P ₁ is the viscosity peak and P ₂ is the point where hydrophobization begins. The point R when the stirring torque becomes constant is the point R when the coagulation is completed.

Note that the stirring torque curve when the PTFE aqueous dispersion contains a filler is also almost the same as the curve in FIG. 1.

In the production method of the present invention, it is necessary to add the water-insoluble organic liquid to the aqueous dispersion in zone [B] in FIG. 1 during coagulation. [B] Even within the zone, especially the above-mentioned stirring torque peak P or P ₁
It is recommended to inject all at once before and after the above-mentioned peak P or _P1 , since the state of change in the viscosity of the liquid can be clearly determined visually in the vicinity.

If a water-insoluble organic liquid is added in the [A] zone, the PTFE particles in the dispersion will be stabilized first, leading to granulation of the filler particles, and as a result, the PTFE particles will remain in the dispersion. Non-uniform mixing of PTFE particles and filler. remained
Although PTFE particles can be coagulated by strong coagulation conditions (e.g. strong mechanical force, heat, excess coagulant, etc.), the uniformity of the mixture is further compromised.

Furthermore, when a water-insoluble organic liquid was added in the [C] zone, only the PTFE particles were granulated, and some of the carbon fiber powder, which was a filler, was granulated.
Most of it is released by simply adhering to the outside of the PTFE particles.

After coagulation, the obtained filler-containing PTFE fine powder is washed with water and dried.

Contains filler produced by the production method of the present invention
PTFE fine powder usually has a particle size of 200 to 5000μ, is easy to separate from a dispersion, has good fluidity, and does not form agglomerates even after drying, making it very easy to handle. Further, even if the amount of solid fine particles as a filler is relatively large, uniform and complete mixing is ensured, and no detachment of solid fine particles into the aqueous medium during operation is observed. In addition, this powder is uniformly mixed with carbon fiber powder as a filler, has high powder flowability, and can be easily formed into paste extrusion and calendering to form sheets, tubes, bars, etc. of any thickness. It is possible to do so.

Hereinafter, the manufacturing method of the present invention will be explained in more detail with reference to Examples and Comparative Examples. In each example, % represents weight % unless otherwise specified.

Comparative example 1 Stainless steel cylindrical coagulation tank (inner diameter 450 mm,
Pour 30°C of water into a 100°C (inner volume: 100mm), with 15mm wide bamboo boards mounted vertically on opposite sides of the inner wall, and adjust the temperature to 30±2°C using a temperature control jacket. While stirring at 200 rpm using a stirrer with stirring blades, add 0.9 kg of carbon fiber (average diameter 12.5μ, average length 130μ) and thoroughly disperse. Here PTFE aqueous dispersion with a concentration of 25% 20.4
When added and continued stirring, the viscosity of the liquid increases after a few minutes and becomes slurry-like. If the stirring is continued, the contents become hydrophobic and float, so the stirring is stopped.

The floating powder is separated from the liquid by passing through a 100-mesh wire mesh and dried. At this time, the separated liquid contained 0.56 kg of carbon fiber powder, which means that the obtained powder contained only about 0.34 kg of carbon fiber powder.

Example 1 In the procedure of Comparative Example 1, when the liquid reached a slurry state (i.e., approximately at point P in FIG. 1),
The same procedure was repeated except that 1.6 of trichlorotrifluoroethane was added to the coagulation tank. In this case, the coagulated particles were obtained by settling into the liquid, but when this was passed, no separation of the carbon fiber powder into the liquid was observed.

The average particle size of the obtained dry powder was 1000μ, and the apparent density was 600g/.

Next, an extrusion mixture prepared by mixing 84 parts by weight of the obtained powder with 16 parts by weight of naphtha was mixed with a reduction ratio (Reduction ratio).
Ratio) Under 168, extrude the paste and inner diameter 4
mm, formed into a tube with an outer diameter of 5 mm, and the extrudate
Baked at ℃ for 10 minutes. The breaking load of the obtained tube in the tensile test in the extrusion direction was 11.1 kg, and the elongation was
It was 160%.

Comparative Example 2 The same procedure as in Example 1 was followed except that trichlorotrifluoroethane was added to the coagulation tank immediately after the PTFE aqueous dispersion was added to the coagulation tank (i.e., in area [A] in Figure 1). When the process was repeated, it was observed that the carbon fiber powder alone precipitated agglomeration, so stirring was stopped.

Comparative Example 3 In Example 1, trichlorotrifluoroethane was added to the coagulation tank when the stirring torque reached a constant value after the stirring liquid had become slurry-like (i.e., [C] in Fig. 1). The same procedure was repeated except that in the area

When the powder after coagulation was over-separated from the liquid, 0.15 kg of carbon fiber powder was separated and flowed out into the liquid.

[Brief explanation of the drawing]

FIG. 1 is a diagram schematically showing the change over time in the stirring torque of the dispersion before and after coagulation. 1 and 2...Curve showing the change in torque over time, [A]...Pre-coagulation zone, [B]...During coagulation zone, [C]...Post-coagulation zone, Q...Coagulant addition Time point, P...branch point, _P1 ...viscosity peak,
_P2 ...Hydrophobization starting point, R...Coagulation completion point.

Claims (1)

[Scope of Claims] 1. When producing polytetrafluoroethylene fine powder containing carbon fiber powder as a filler from an aqueous dispersion by co-coagulation, a polytetrafluoroethylene colloidal aqueous dispersion and carbon fiber powder are co-coagulated. The boiling point is 30 to 150℃, and the surface tension at 20℃ is
A method for producing a filled polytetrafluoroethylene fine powder, characterized by adding a water-insoluble organic liquid of 35 dynes/cm or less. 2. The manufacturing method according to claim 1, wherein the carbon fiber powder is made into an aqueous dispersion in advance and combined with a polytetrafluoroethylene colloidal aqueous dispersion. 3. The water-insoluble organic liquid is added by injecting the polytetrafluoroethylene aqueous dispersion when the viscosity of the aqueous polytetrafluoroethylene dispersion reaches approximately the maximum after the start of coagulation. manufacturing method.