JPS5825368B2 - Method for manufacturing porous polytetrafluoroethylene film - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a fired porous film made of polytetrafluoroethylene (hereinafter referred to as PTFE).

PTFE is an industrially important material with excellent properties such as heat resistance, chemical resistance, mechanical properties, and electrical insulation, and its porous film can be used as filters for highly corrosive substances, diaphragms for batteries, Widely used in separation membranes for isotopes such as uranium hexafluoride.

Conventionally, fired porous PTFE films are produced by uniformly mixing PTFE powder and a liquid lubricant such as naphtha, and shaping this mixture into a film by extrusion or rolling.
It was manufactured by a method in which it is stretched at a temperature below the melting point of PTFE to make it porous, and then heated to a temperature above the melting point of PTFE and fired while maintaining the stretched state.

This conventional method has the problem that the stretching process for making the film porous and the heating process for firing must be carried out separately, resulting in a long manufacturing process.

This is because the conventional stretching method heats the entire unfired film, so if the temperature during stretching is set above the melting point of PTFE, the stretching will occur near both ends of the unfired film, which is the starting point of stretching. This is because the temperature cannot be raised above the melting point of PTFE because there is a risk that the film will break.

In addition, when obtaining a thick porous film by the above-mentioned conventional method, a thick unfired film is also used, but it is difficult to uniformly stretch this thick unfired film, and the resulting porous film has no pores formed. Not only does this result in large variations in pore diameter (but also variations in porosity depending on location).

Furthermore, since the manufacturing process is long, a huge amount of energy is consumed, leading to an increase in product costs.

In view of the above-mentioned current situation, the inventors of the present invention have conducted extensive studies and found that unfired P.
Both ends of the TFE film in the direction to be stretched are placed outside the heating zone, and the unfired film is stretched using the both ends as base points while heating the portion of the unfired film located inside the heating zone to a temperature equal to or higher than the melting point of PTFE. This allows the unfired film to be fired and made porous without causing breakage, and by stacking a predetermined number of unfired films on top of each other, these films, which are in a molten state during stretching, can be brought into close contact with each other and integrated. The present inventors have discovered that a thick porous film can be produced easily, and have completed the present invention.

That is, the method for producing a porous PTFE film according to the present invention involves stacking at least two sheets of unfired PTFE films such that both ends of the stacked unfired films in the direction in which they are to be stretched are located outside the heating zone. After placing the stacked unfired films in the heating zone, the portions of the stacked unfired films are heated at least once under a temperature condition higher than the melting point of PTFE.
It is characterized in that it is stretched in the axial direction to bring the unfired films into close contact with each other and to make them porous.

The unfired PTFE film used in the present invention includes:
An unfired film (hereinafter referred to as unfired A film) obtained by uniformly mixing PTFE powder and a liquid lubricant, compressing and preforming this mixture, and then forming it into a film by extrusion, rolling, etc. This can be cited as one of the specific examples.

The liquid lubricant used is one that can wet the surface of PTFE and can be removed by evaporation, extraction, etc. after forming a mixture of PTFE powder and the liquid lubricant into a film, such as liquid paraffin. , hydrocarbon oils such as naphtha and white oil, aromatic hydrocarbons such as toluene and xylene, alcohols, ketones, esters,
Examples include silicone oil, fluorochlorocarbon oil, solutions of polymers such as polyisobutylene and polyisoprene dissolved in these solvents, mixtures of two or more of these, and water or aqueous solutions containing surfactants.

The amount of this liquid lubricant mixed with the PTFE powder varies depending on the molding method used to obtain the unfired A film, the presence or absence of additives, etc., but is usually about 5 parts by weight per 100 parts by weight of the PTFE powder.
~50 parts by weight, preferably 10 to 30 parts by weight.

Furthermore, in the present invention, when the liquid lubricant is mixed with the PTFE powder, various additives such as pigments for coloring,
Carbon black, graphite, silica powder, asbestos powder, glass powder, metal powder, metal oxide powder, metal oxide powder, etc. are mixed in to improve strength against compression, improve wear resistance, prevent low temperature flow, etc. You can also do that.

The unfired film A thus obtained contains a liquid lubricant, and in the present invention, this is usually treated by a heating method, an extraction method, or a combination thereof prior to stretching to make it porous. Although the liquid lubricant is removed before use, it is also possible to use the film while containing the liquid lubricant and remove this liquid lubricant by heating during stretching.

In the present invention, a type of unfired film other than the above-mentioned unfired film A can be used. For example, if the following type of unfired film is used, a porous film with higher porosity can be obtained.

(1) A blowing agent is further mixed with the PTFE powder and liquid lubricant, and this mixture is used in the same manner as in the case of obtaining the unfired A film. (referred to as a film).

(2) An unfired film (hereinafter referred to as unfired C film) obtained by pre-stretching the unfired A film in an unfired state in a small amount (both uniaxially) to form countless fine pores.

(3) An unfired film containing a blowing agent obtained by pre-stretching the unfired film in at least one axial direction at a temperature below the decomposition temperature of the blowing agent to form countless fine pores (hereinafter referred to as unfired film). ).

(4) An unfired film (hereinafter referred to as unfired E film) obtained by heating the unfired film to a temperature above the decomposition temperature of the blowing agent and below the melting point of PTFE to form countless fine pores due to the decomposition of the blowing agent. ).

(5) The unfired film is heated to a temperature above the decomposition temperature of the blowing agent and below the melting point of PTFE, and is pre-stretched in at least one axis while decomposing and foaming the blowing agent to form countless fine pores. (hereinafter referred to as unfired F film).

As the blowing agent for obtaining the unfired films of types B, D, E and F, either an organic blowing agent or an inorganic blowing agent may be used.

Specific examples of organic blowing agents include azo blowing agents such as azodicarbonamide, azobisisobutyronitrile, barium azodicarboxylate, diethyl azodicarboxylate, diazoaminobenzene, azocyclohexylnitrile, and hydrazide blowing agents. as p-) luenesulfonyl hydrazide, pensesulfonyl hydrazide, p
p'-oxybisbenzenesulfonyl hydrazide, etc., p'-oxybisbenzenesulfonyl semicarbazide, p-)luenesulfonyl semicarbazide, etc. as a semicarbazide blowing agent, and N-N'-dinitrosopenta as a nitroso blowing agent. Methylenetetramine, N-
Examples include N'-dimethyl N-N'-dinitrosoterephthalamide and the like.

Furthermore, specific examples of inorganic blowing agents include ammonium carbonate, sodium bicarbonate, ammonium nitrite, and the like.

These blowing agents can be used alone or in combination of two or more.

The amount of these blowing agents mixed with the PTFE powder is set within the range of 0.1 to 15 parts by weight based on 100 parts by weight of the PTFE powder.

If the amount of the blowing agent mixed is less than 0.1 part by weight, the effect of the blowing agent cannot be expected, and if it is more than 15 parts by weight, the processability will decrease and it will be difficult to obtain a molded product, and a part of it will be PTFE.
It tends to remain in the porous body.

In addition, from the viewpoint of processability and the like, it is particularly preferable that the amount is 1 to 5 parts by weight.

In addition, a foaming aid such as urea, ethanolamine, oxalic acid, co-phosphoric acid, citric acid, etc. can also be used to adjust the decomposition temperature, decomposition rate, and gas generation amount of the foaming agent.

The stretching rate in the preliminary stretching when obtaining the C, D and F type unfired films used in the present invention is the stretching rate in the subsequent stretching to make the film porous, and the pore size of the target porous film. It is determined depending on the ratio and pore diameter, etc., but in the case of unfired C film, it is usually 20 to 400.
%, usually 20-300 for unfired and F films
It is about %.

The thickness of the unfired film in the present invention may be varied depending on the number of stacked sheets and the stretching rate during stretching to make it porous, but it is usually about 0.03 to 0.8 m in degree of abrasion. be.

- In the present invention, at least two of the various types of unfired films as described above are overlapped, and the overlaid unfired films are arranged such that both ends of the overlaid unfired films in the direction in which they are to be stretched are located outside the heating zone. , the portion of the stacked unfired films arranged in the heating zone is made of PTFE.
The unfired films are heated to a temperature higher than the melting point of the film and stretched in at least one axial direction starting from the two ends, and the stacked unfired films are tightly integrated and made porous, forming countless pores. A fired porous film is obtained.

In addition, in the case where a pre-stretched unfired film is used in the present invention and the stretching at the time of making it porous is performed in a direction other than the direction of the pre-stretching, the length in the direction in which the pre-stretching is performed at the time of stretching at the time of making it porous. The stretching is carried out by regulating the dimensions, and the stretching state in the preliminary stretching direction is fixed.

If this dimension control is not carried out, shrinkage will occur in the direction of pre-stretching, and the fine pores formed during pre-stretching may decrease or disappear, and the technical significance of pre-stretching will be lost, so it is preferable. do not have.

In the present invention, arranging the stacked unfired films so that both ends in the stretching direction are outside the heating zone is the most effective method when hot stretching is performed at a high temperature higher than the melting point of the unfired PTFE film. By keeping the temperature at both ends of the unfired film on which the force acts below the melting point of PTFE, preferably below the softening point, to prevent softening or melting of the both ends, the mechanical strength of the both ends can be maintained, and the mechanical strength during hot stretching can be maintained. This is to prevent the unfired film from breaking.

At this time, the portion of the unfired film placed in the heating zone is heated to a temperature higher than the melting point of PTFE;
In order to perform the firing in a short time and to prevent deterioration due to heating, it is preferable to heat and bake at about 340 to 410°C.

In this step, the portion of the unfired film disposed within the heating zone is heated and fired as described above and is stretched in at least one direction.

Stretching is to form countless pores to make it porous, and the stretching rate is determined depending on the porosity and pore diameter of the target porous film, the stretching direction, the number of stretching axes, etc. , usually about 15 to 1000%, and considering the pore formation effect due to stretching, the uniformity of the pore diameter, etc., it is about 3%.
It is suitable that it is 0 to 600%.

In addition, when using a film that has been pre-stretched as an unfired film, such as the above-mentioned C, D or F type film, and stretching for making it porous is performed in the pre-stretching direction, stretching for making it porous is carried out. In other words, the sum of the stretching ratio at the time of preliminary stretching and the stretching ratio at the time of making the film porous is within the above range, based on the unstretched unfired film. Adjust the stretching rate in the stretching process for making it porous.

In the present invention, when uniaxially stretching the stacked unfired films, both ends in the non-stretching direction are placed outside the heating zone in the same way as both ends in the direction to be stretched, or It is preferable to fix both ends in the non-stretching direction using chucks, clips, etc., and to regulate the dimensions so that the distance between the two ends does not change, since this facilitates the formation of pores during stretching.

When obtaining a porous film by the method of the present invention,
Either the same type of unfired films may be superimposed on each other, or different types of unfired films may be superimposed on each other.

Although the main objective of the present invention is to obtain a porous film with little variation in pore size, it is also possible to obtain a porous film in which the pore sizes near the surface and inside the film are different, if desired.

To achieve this purpose, when stacking unfired films, for example, a C-type film with fine pores formed by stretching may be placed between two A-type films, or a When a type A film is stretched to make it porous by arranging it, some of the stacked unfired films are formed with pores that have a larger diameter than those in other films. What is necessary is to form pores.

Namely, 1. In the former case, the resulting porous film has a larger pore diameter in its interior, that is, in a portion corresponding to the C type film, than in the surface portion, and in the latter case, the pore diameter in the surface portion is larger than that in the interior.

According to the present invention, it is also possible to obtain a porous film in which the pore diameters of the pores change continuously from one side to the other side, and gradually become larger or smaller.

In order to obtain such a porous film, type B films in which the amount of foaming agent mixed is successively changed are stacked on top of each other in the order of the amount of foaming agent mixed in, or type C films are produced in which the stretching ratio at the time of forming micropores is successively changed. It is best to stack them in the order of their stretching ratios to make them porous.

The porosity and pore diameter of the porous film thus obtained vary depending on the stretching ratio, stretching direction, number of stretching axes, etc., but usually the porosity is about 35 to 95%, and the pore diameter is about 0. It is about 01 to 50μ.

In another embodiment of the present invention, heat treatment can be performed while maintaining the stretched state of the porous film, in other words, controlling the length in the stretching direction.

By performing this heat treatment, the stretched state of the porous film can be fixed, and a film with particularly excellent dimensional stability when used at high temperatures can be obtained.

If the above-mentioned restrictions are not carried out during this heat treatment, the pores will be significantly reduced or even disappear, which is not preferable.

Incidentally, if the length of the porous film in a direction other than the stretching direction is further regulated during the heat treatment, reduction and disappearance of pores can be more effectively prevented.

As a means for controlling the size of the porous film during the above heat treatment, for example, a method of grasping both ends of the porous film in the stretching direction with chucks, clips, etc. and heating the film while maintaining the gap, or a method of heating the porous film while maintaining the gap therebetween, or a method of heating the porous film by holding the ends of the porous film in the stretching direction with a chuck or a clip, or a method of heating the porous film while maintaining the distance therebetween, or a method of heating the porous film by holding both ends of the porous film in the stretching direction with chucks, clips, etc. Examples include a method of heating between a winding roll and a winding roll.

The heat treatment in the present invention may be carried out at a temperature higher than the temperature at which the porous film is used, and when carried out, the porous film obtained by the stretching is once cooled and then reheated, or the porous film obtained by the stretching is heated continuously after the stretching is completed. Let's do it.

Therefore, in the present invention, both ends of the stacked unfired films in the direction in which they should be stretched are placed outside the heating zone, so that the unfired films, which were conventionally considered difficult to stretch at temperatures above the melting point, can be stretched at temperatures above the melting point. With the above method, uniform stretching can be achieved, and since stretching and firing can be performed simultaneously, the process can be shortened.

Further, since a predetermined number of unfired films are stacked and integrated in close contact with each other, a thick porous film can be easily obtained.

Furthermore, by changing the combination of unfired films,
In addition to porous films with uniform pore diameters, porous films with different pore diameters near the film surface and inside the film, or porous films in which the pore diameters gradually increase or decrease from one side of the film to the other. Obtainable.

Since the obtained porous film is fired, it has high mechanical strength and can stably perform its functions for a long period of time, and the heat-treated product has characteristics such as particularly excellent dimensional stability.

Hereinafter, the present invention will be explained in more detail by way of Examples with reference to the drawings, but these are not intended to limit the present invention.

In addition, all "parts" in the examples mean "parts by weight."

Example I A mixture of 100 parts of PTFE powder (product name: Teflon 6J, manufactured by 5-well Fluorochemical Co., Ltd.) and 20 parts of liquid lubricant Naphtha No. 1 was preformed under compression at a pressure of 20 kg/ffl, and then this was preformed. was extruded into a round bar shape, and this round bar was then passed between a pair of metal rolling rolls to obtain a long unfired A film with a thickness of 110μ, and this unfired A film was further extruded at 120°C for 2 minutes. Heat to remove liquid lubricant.

Next, two sheets of the unfired A film from which the liquid lubricant has been removed are stacked on top of each other and wound into a roll around the tubular core.

Thereafter, as shown in FIG. 1, the overlapping unfired A films 1 wound around a tubular core are set on the feeding side of the stretching device, and one end of the lengthwise direction is placed on the winding roll 2.
The temperature of the heating zone 3 is maintained at 350° C., and the rotation speed of the pair of pinch rolls 50 provided on the exit side is lower than the rotation speed of the pair of pinch rolls 4 provided on the entrance side of the heating zone 30. Then, the part of the unfired A film 1 disposed in the heating zone is stretched by 50% in the length direction while firing, and the two unfired A films are made porous and closely integrated to form a porous film with a thickness of 216μ. quality film (sample number 1
) was obtained.

In this case, cold air at a temperature of 20° C. was blown onto the pinch rolls 4 and 5 to prevent the base points 6 and 7 from softening or melting when stretching the portion of the overlaid unfired A film 1 disposed in the heating zone.

Note that 8 and 9 are guide rolls, and 10 is a cooling roll.

On the other hand, using the above-mentioned superimposed unfired A films 1 and setting the stretching ratios as shown in Table 1, porous films of sample numbers 2 and 3 were obtained.

Furthermore, work was carried out in the same manner as in the case of sample number 2 except that the number of stacked sheets of unfired A film was 10, and the thickness was 10.
A 50μ porous film (sample number 4) was obtained.

Table 1 shows the results of measuring the porosity, pore diameter, and tensile strength in the stretching direction of these porous films.

For comparison, data for a porous film (sample number 5) obtained by performing the same procedure as sample number 2 except that the heating zone temperature was set at 300° C. is also shown.

Example 2 The stacked unfired A film 1 used to obtain the porous film of sample number 1 was passed through a pinch roll 4 as shown in Fig. 2, and the width of the heating area was changed from the inlet side to the outlet side. is guided to a heating zone 3 having a shape where
The rotational speeds of the pinch rolls 5 on the exit side are set to be approximately the same, and both ends of the stacked unfired A film 10 in the width direction are placed in a tenter set stretching machine 11 installed outside both ends of the heating zone 3. While grasping with a chuck, the portion of the unfired A film 1 disposed in the heating zone was stretched 200% in the width direction to obtain a porous film (sample number 6) with a thickness of 105 μm.

On the other hand, the unfired A film 1 is stretched in the width direction (stretching ratio 200%) using a tenter set stretching machine 11, and is also stretched in the length direction (stretching ratio 200%) between the pinch rolls 4 and 5. An axially stretched porous film (sample number 7) was obtained.

The properties of these porous films are shown in Table 2.

For comparison, data for porous films of sample numbers 8 and 9 obtained by performing the same operation as sample numbers 6 and 7 except that the temperature of the heating zone was set at 300° C. are shown at the same time.

Example 3 The unfired A film from which the liquid lubricant in Example 1 had been removed was pre-stretched at 25° C. to a stretching ratio of 100% in the length direction to obtain a film with a thickness in which numerous fine pores were formed. An unfired C film having a size of 100 μm and a porosity of 60% is obtained, and two sheets of the unfired C film are overlapped and wound into a roll around a tubular core.

Next, the temperature of the heating zone of the stretching device shown in FIG. Stretched in the horizontal direction, sample numbers 10 and 11-2
A sheet of porous film was obtained.

Separately, the unfired film C, which has been pre-stretched in the longitudinal direction and wound around a tubular core, is kept at a temperature of 350°C in the heating zone of the stretching apparatus shown in FIG. A biaxially stretched porous film (Sample No. 12) was obtained by stretching 200% in the width direction while keeping the rotational speed approximately the same and regulating the dimension in the length direction.

Furthermore, this unfired film C was stretched by the stretching apparatus shown in FIG. 2 so that the stretching ratio in the length direction of the unfired film C was 200% while stretching 200% in the width direction, resulting in biaxial stretching. A porous film (sample number 13) was obtained.

The properties of these porous films were as shown in Table 3.

Example 4 To 100 parts of the PTFE powder used in Example 1, 1 part of an azo foaming agent azodicarbonamide (product name Cellmic C217, manufactured by Gokyo Kasei Co., Ltd., decomposition temperature 215°C) and 20 parts of liquid paraffin as a liquid lubricant were added. After uniformly mixing, this mixture is subjected to compression preforming and extrusion molding in the same manner as in Example 1, and is further passed between rolling rolls to obtain a long unfired film containing a foaming agent and having a thickness of 110 μm.

Thereafter, this film is immersed in trichlorethylene to extract and remove the liquid lubricant, and then the two films are overlapped and wound into a roll around a tubular core.

Next, this unfired film is heated at a heating zone of 350°C.
The film was stretched by 200% in the longitudinal direction while decomposing and foaming the foaming agent using the stretching apparatus shown in FIG.

Furthermore, porous films of sample numbers 15 and 16 were obtained in the same manner as sample number 14 except that the blowing agent parts were changed to 4 parts and 12 parts.

The properties of these porous films are shown in Table 4.

Example 5 Using the unfired film (containing 4 parts of blowing agent) from which the liquid lubricant in Example 4 was removed, E and F type unfired films were obtained by the following method.

(1) An unfired film was pre-stretched at 25°C to a stretching ratio of 100% in the length direction, and a green film containing a foaming agent with a thickness of 105 μm and a porosity of 60% was prepared with numerous fine pores formed. Obtain a fired film.

(2) The unfired film is heated to 300°C to decompose and foam the foaming agent to form countless microscopic pores, resulting in a thickness of 110 mm.
An unfired E film with μ and a porosity of 38% is obtained.

(3) The unfired film is heated to 250°C to decompose and foam the foaming agent while pre-stretching in the length direction to a stretching ratio of 100%, resulting in a thickness of 10% with countless micropores.
An unfired F film with a thickness of 5μ and a porosity of 68% is obtained.

Next, after stacking two of each of these D-F type unfired films and winding them in a roll around a tubular core, the process was carried out in the same manner as in the case of sample number 15, and the properties shown in Table 5 were obtained. Three porous films having sample numbers 17 to 19 were obtained.

Example 6 The porous films of sample numbers 2.6 and 7 were cut to a length and width of 20 cm, and both ends in the stretching direction were gripped with chucks to prevent the length in the stretching direction from changing. sample number 20.
．． Three heat-treated porous films No. 21 and No. 22 were obtained.

Table 6 shows the porosity of these porous films and the heat shrinkage rate in the stretching direction calculated by the following formula after leaving the film in a heat-shrinkable state in a heating furnace at 200° C. for 20 minutes.

For reference, we measured the heat shrinkage rates of sample numbers 2.6 and 7 in the stretching direction, and found that sample number 2 was 10.5% (length direction), sample number 6 was 12.8% (width direction), and sample number 6 was 12.8% (width direction). number 7
is 10.3% (length direction) and 12.7% (width direction)
Met.

According to the above examples, the porous film obtained by the present invention has high mechanical strength, small variation in pore diameter, and those subjected to heat treatment have low heat shrinkage and excellent dimensional stability when used at high temperatures. I understand.

[Brief explanation of drawings]

FIG. 1 is a side view schematically showing a porous film manufacturing apparatus used in the present invention, and FIG. 2 is a plan view showing the main parts of another apparatus. 1... Overlaid unfired films, 2...
...Heating zone, 4,5...Pinch roll.

Claims (1)

[Claims] 1. After overlapping at least two unfired polytetrafluoroethylene films and arranging the overlapping unfired films so that both ends in the direction in which they should be stretched are outside the heating zone, The portion of the stacked unfired films disposed in the heating zone is stretched in at least one axis at a temperature equal to or higher than the melting point of polytetrafluoroethylene to bring the unfired films into close contact with each other and to make them porous. A method for producing a porous polytetrafluoroethylene film. 2 At least two sheets of unfired polytetrafluoroethylene films are stacked together and arranged so that both ends of the stacked unfired films in the direction in which they should be stretched are outside the heating zone, and then the stacked unfired films are The part of the unfired film disposed in the heating zone is stretched in at least one direction under a temperature condition equal to or higher than the melting point of polytetrafluoroethylene to bring the unfired films into close contact with each other and make them porous, and then heat-treated while maintaining the stretched state. A method for producing a porous polytetrafluoroethylene film, characterized by: