JPH09141734A - Uniaxially stretched molded article of polytetrafluoroethylene and production thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a uniaxially stretched molded article of polytetrafluoroethylene having strength of 1GPa or more. SOLUTION: A billet containing a fine powder of polytetrafluoroethylene with a mol.wt. of 6000000 or more is extruded in a pasty state in a reduction ratio of 600 or more to obtain a molded article. This article is heat-treated at temp. equal to or higher than the crystal m.p. of polytetrafluoroethylene in a freely stretchable state and subsequently gradually cooled until crystallization is completed and succeedingly heated until the melting of a crystal is completed. This molded article is stretched by 100 times or more at a stretching speed of 50mm/sec or more and quenched to obtain a uniaxially stretched molded article.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a uniaxially stretched molded product of polytetrafluoroethylene (hereinafter referred to as “PTFE”), particularly a tape or film of PTFE having a tensile breaking strength of 0.5 GPa or more, preferably 1 GPa or more. ,
The present invention relates to a uniaxially stretched molded product such as a fiber.

[0002]

2. Description of the Related Art PTFE is one of fluororesins, and in addition to PTFE, this fluororesin includes FEP (tetrafluoroethylene-6 fluoropropylene copolymer), PFA (4
Ethylene fluoride-perfluoroalkoxy group copolymer),
Examples include ETFE (tetrafluoroethylene-ethylene copolymer).

Each of these fluororesins has excellent heat resistance, chemical resistance, water and moist heat resistance, ultraviolet light resistance, electrical insulation, and non-adhesiveness and surface abrasion resistance that are unmatched. ing. Among these fluororesins, PTFE has the highest level of heat resistance, chemical resistance, water resistance and wet heat resistance, among others.

[0004]

However, when PTFE is processed into a molded product such as a tape, a film or a fiber and used, the mechanical strength is insufficient, and its solution is desired.

Therefore, the present invention has been made in view of such circumstances, and an object thereof is to provide a PTFE uniaxially stretched molded article such as a tape, a film and a fiber having a strength of 1 GPa or more and a method for producing the same. To do.

[0006]

In order to achieve the above-mentioned object, the uniaxially stretched molded PTFE article of the present invention has a molecular weight of 60.
Crystallized after heat treatment at a temperature above the crystal melting point of polytetrafluoroethylene in a state in which expansion and contraction are free, for a molded product obtained by paste extruding a billet containing fine powder of polytetrafluoroethylene of at least 0,000 at a reduction ratio of 600 or more. Slow cooling until the end of crystallization, followed by heating until the end of melting of the crystal, and 1 at a stretching speed of 50 mm / sec or more.
It is characterized in that it is obtained by stretching at least 100 times and then rapidly cooling.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, the billet is P
It is preferable that the TFE polymer fine powder is wet-treated with an extrusion aid, and the wet powder is compressed to form. The particle size of the fine powder is 0.1 μm
It is preferably 0.5 μm.

The PTFE polymer used in the present invention is
It is a polymer of PTFE, that is, tetrafluoroethylene, and has a molecular weight of 6,000,000 or more, preferably 8,000,000 or more. Further, a copolymer containing several% or less of a different monomer as a comonomer may be used.

This polymer fine powder has a particle size of 0.1 which is suitable for paste extrusion, as described above.
μm to 0.5 μm is preferred, which is synthesized by emulsion or radiation polymerization. As a result of the copolymerization, it is possible to increase the reduction ratio during paste extrusion. In the present invention, the reduction ratio is 600 or more,
It is preferably 800 or more. If the reduction ratio is less than this, it is impossible to achieve sufficient strength.

The extrusion aid is a PTF required for paste extrusion.
It can be used as a lubricant for E-fine powder and has been generally used conventionally in the industry. The compounding amount is usually used in the range of 15 to 25%, but it is not limited to this and it is necessary to have a large reduction ratio.

As the extrusion aid, there are generally hydrocarbon-based organic solvents or petroleum-based solvents, such as Isopar E, Isopar H, Isopar M (all are Esso chemical products), Sumoyle P-55 (Matsumura Oil). , Kerosene, naphtha, Risella # 17 oil, petroleum ether and the like, but are not limited thereto. The extrusion aid may be used alone or in combination of two or more different kinds.

In terms of material, P as a polymer mentioned above
Only two kinds of additives, TFE and an extrusion aid necessary for paste extrusion, are sufficient, and an antioxidant and other additives other than this are not necessary at all.

Next, a molding method for making a uniaxially stretched PTFE molded article using these materials will be described.

The molding method of PTFE uniaxially stretched molded article is as follows:
It consists of the following 7 steps.

1. Sieve of PTFE fine powder 2. 2. Blending of PTFE fine powder and extrusion aid. Mixing / dispersion / impregnation / sieving 4. Preform (billet molding) 5. Paste extrusion 6. Heat treatment and cooling 7. Ultra-stretching and cooling Among these 7 steps, up to 4 billet forming steps can be considered to be almost the same as the contents of the paste extruding step of PTFE fine powder which is generally performed. PTF
The most important and essential point of the present invention for controlling the fine structure of the molecular arrangement of the PTFE molecules necessary for producing the uniaxially stretched molded product of E is the latter three steps, that is, 5. Paste extrusion, 6. Heat treatment and cooling, 7. Super stretching and cooling.

The contents will be described below step by step.

1. Sieve of PTFE fine powder Since PTFE fine powder has an inherent adhesiveness, it receives a force due to vibration or its own weight during transportation and storage,
Easy to form a lump of powder. This is difficult to handle and makes uniform impregnation with the extrusion aid difficult. Also, if you apply some force to mechanically loosen this lump,
The fine powder is easily made into fibers by the shearing force at this time, which adversely affects extrusion molding. From the above, PT
It is extremely important for the FE fine powder to make the fine powder as fine and smooth as possible before compounding the extrusion aid. For this purpose, 8 mesh or 10 mesh each 2.0
It is necessary to pass through a sieve with holes of mm or 1.7 mm. Furthermore, the sieve weighing of this polymer is
It is desirable to perform it in a room whose temperature is controlled to be equal to or lower than the room temperature transition point (about 19 ° C.) of E.

2. Blending of PTFE fine powder and extrusion aid Put the required amount of polymer and extrusion aid through a sieve into a dry jar with sufficient capacity and a tight stopper. For good mixing, a space of 1/3 to 2/3 of the container volume is created. Immediately after compounding, airtightly seal the container,
Prevents volatilization of extrusion aid.

3. Mixing / dispersion / impregnation / sieving Mixing is completed, shake the container tightly closed so as to disperse the extrusion aid, and then place it on a rotating platform and rotate the container at an appropriate speed of 20 m / min or less for about 30 minutes. Let it mix and disperse. The rotation speed is sufficient for mixing and dispersion,
It should not be so strong as to cause fine powder fibrillation due to shear forces. Then, the extrusion aid is sufficiently impregnated and permeated into the fine powder secondary particles, and the surface of the primary particles is wetted with the extrusion aid at room temperature for 6 hours to 24 hours.
Let stand for hours. After this, through a sieve of appropriate size,
Remove lumps created by mixing.

4. Preform (Billet Molding) This process requires proper preform equipment.
The wet PTFE fine powder produced in the previous step is put into the cylinder of this preforming device, and compressed by a ram to form a billet. The pressure used for compression is
Corresponding to the cylinder size, 1kg / cm-10kg / cm
Is required, and a hold for several minutes is required. After forming the billet by the preform, it is necessary to subject it to paste extrusion as soon as possible in order to prevent the extrusion aid from scattering. During the paste extrusion, the billet wet-treats the PTFE-based polymer fine powder with an extrusion aid,
Since the wet powder is formed by compression, it is easy to extrude the paste from the billet to the monofilament, and the monofilament can be easily formed.

5. Paste Extrusion The temperature conditions for paste extrusion of PTFE fine powder are
It is closely related to the temperature change of the crystal structure of PTFE. As is generally known, below 19 ° C, PTFE is a triclinic system, and this crystal structure has a high deformation resistance.
It is unsuitable for plastic working at a temperature considerably lower than the melting point of E. At 19 ° C or higher, the crystal structure of PTFE takes a hexagonal system, and as the temperature rises higher, irregular portions increase along the long axis of the crystal, so that the crystal elasticity decreases and the plastic deformability decreases. To increase.

From these facts, the temperature condition for the paste extrusion of the PTFE fine powder is preferably 30 ° C. or higher, but empirically a temperature range of 40 ° C. to 60 ° C. is preferable.

Further, in order for paste extrusion to be carried out sufficiently efficiently, it is important that no force is applied to the billet before it has been sufficiently conditioned to a given temperature. If this is not observed, the non-negligible part of the billet amount will remain in the cylinder without being extruded normally, which will worsen the yield, or if it is extruded by force, it will be a normal amount for this molded product. Even if a strict heat treatment is performed, it will hinder the super stretching.

More importantly, the reduction ratio (Reduct
Ion Ratio, hereinafter referred to as "RR". ). RR is
It is the ratio of the cylinder cross section of the extruder to the cross section of the die,
Although it is important in general paste extrusion, it has a special importance in the production of PTFE uniaxially stretched molded products.

That is, the essence of increasing the strength of a polymer is that the polymer molecule is extended within the range of the bond angle between atoms constituting the main chain and the rotation angle for each bond, and the polymer molecule is extended to the limit. The ultimate is to arrange the chains in the axial direction.

The method for achieving this fine structure control differs depending on whether the polymer molecular chain is a bent chain or a rigid straight chain. Similar to polyethylene, PTFE is classified into a bent-chain polymer, but as is well known, PTFE
Since the molecule is a rod-shaped molecule having a helical structure, it has been found that, unlike polyethylene, it actually behaves as a fairly rigid linear polymer, as a result of the study of the present invention. That is, it can be said that PTFE is a polymer located between the bent chain type and the rigid straight chain type. Therefore, since PTFE is also a bent-chain type polymer like polyethylene, a super-stretching step is required for controlling the fine structure necessary for producing a high-strength uniaxially stretched molded product. Stretching of PTFE fine powder effectively begins with the paste extrusion process. The substantial stretch ratio λ is a heat treatment (Free End Anneal; hereinafter referred to as “FEA”) in a state in which the end of the paste extruded product is not bound, that is, in a stretchable state.
Then, the stretching ratio when super-stretching in a thermostat with a stretching device is λ
, Λ = RR × λ, but there is a heat treatment step between the reduction and the super-drawing, and at this time the molded product shrinks, so this formula is qualitatively correct and The inverse proportional relationship of λ can be explained, but it is not quantitatively correct.

Since the substantial stretch ratio λ required for a uniaxially stretched PTFE product is constant when the molecular weight of PTFE is constant, the stretch ratio λ in the super-stretching for a specific PTFE is the same as that of the PTFE molded product. If RR becomes large, it will decrease according to the above formula. This is one of the important ideas in strengthening the PTFE fine powder.

What is important about the concept of the reduction ratio is that even if the substantial stretching ratio λ is the same, if the reduction ratio is different, the same arrangement structure cannot be finally obtained. In order to achieve the high strength of PTFE, it is necessary to obtain a PTFE molded article having a large RR as much as possible. As a result, the strength is rather improved and stabilized even if the drawing ratio in the super drawing is decreased.

The reason for this has not been sufficiently clarified so far, but the larger the RR in the range of the FEA (free end heat treatment) condition in the present invention, the more the oriented structure of PTFE remains after FEA. It is considered to have an advantageous effect on the ultimate arrangement of the PTFE molecules by the super stretching following the process. However, the heat treatment is under stronger conditions than the present invention, for example, a high temperature of 450 ° C. or higher, or 370 ° C. × 2.
When the sintering is performed for more than a time, this orientation structure disappears. From the above, RR is at least 600 or more,
It is preferably 800 or more.

The structure of the die may be one for general PTFE paste extrusion. That is, the taper angle is 30 ° to 60 °, and the land is long enough to prevent twisting or kinking.

6. Heat Treatment and Cooling Heat treatment conditions are most important for uniaxial stretching of PTFE. Because it enables the super stretching of PTFE,
It is this heat treatment condition that determines whether or not the uniaxially stretched product E of E can provide a strength of 1 GPa or more and guarantee the uniform stabilization of the strength in the longitudinal direction.
In a sense, it is easy to super-stretch PTFE, but if the heat treatment conditions are inappropriate, super-stretching is possible but strength does not appear, or strength does not appear stable in the longitudinal direction. There are many. Strictly speaking, it is necessary to clearly define the heat treatment temperature and time, and the cooling rate and the temperature range in which the cooling rate is controlled to be constant. P
Exactly such strict heat treatment conditions are necessary for uniaxial stretching of TFE. Moreover, it is not enough to specify exactly those. The heat treatment necessary for the uniaxial stretching of PTFE requires that the PTFE monofilament should be mechanically heat-treated.

That is, the mechanical state in the heat treatment of the monofilament necessary for the uniaxial stretching of PTFE is to make the monofilament mechanically free. This is referred to as FEA in this specification as described above. As a matter of course, FEA does not hinder the expansion and contraction of the monofilament during heat treatment. Contrary to FEA, when both ends of the monofilament are completely fixed and heat treated without sagging,
It can be seen that this monofilament can hardly be drawn. Therefore, the stretching ratio will decrease in response to the binding stress or partial stress at both ends of the monofilament during heat treatment. However, even if both ends are fixed during the heat treatment, if a slack (surplus length) of about 20% or more is applied so that the monofilament is not subjected to the force due to the heat shrinkage during the heat treatment, this can be considered to be substantially FEA. This idea becomes important when considering industrial manufacturing methods.

Regarding the heat treatment temperature and time, 350
C, 30 minutes is the minimum required level. 350 ° C, 2
It is difficult for 0 minutes to perform sufficient sintering. Desirably, it requires 1.5 hours at 350 ° C. or higher. 370
A temperature of 2 hours or more at 450 ° C. or 450 ° C. or more is an unsuitable level because the oriented structure is not left after the heat treatment and cooling. The above-mentioned FEA enables super-stretching where the PTFE uniaxially stretched molded article provides the ultimate orientation of PTFE molecules required.

Finally, cooling conditions after the heat treatment of PTFE performed at the above-mentioned specific temperature and time are completed will be described.

The importance of the cooling rate has been described above, but the reason is that the cooling rate determines the crystallinity of the heat-treated PTFE. The higher the degree of crystallinity, the higher the strength of the PTFE uniaxially stretched molded article produced in the next super-stretching step, the more the defects in the longitudinal direction of the molded article are reduced, and the less the variation of the strength is surprisingly.

However, the crystallinity is not necessarily the maximum, but is determined by the balance between the molecular orientation obtained by the heat treatment (depending on the reduction ratio and the degree of slack) and the crystallinity (depending on the cooling rate). There is an optimum range, which is believed to determine the effective molecular orientation in superstretching and the high strength of uniaxially molded parts.

In the case of a crystalline polymer, it is generally known that the crystallinity depends on the cooling rate after the heat treatment at a temperature above the melting point. However, it is rare in the case of a polymer that the resulting crystallinity affects the result of the next step (super-stretching) performed again at the melting point or higher.

7. Super-stretching and cooling In order to stretch the PTFE molded product after paste extrusion, a thermostatic chamber with a stretcher is required in the laboratory.

To achieve the super-stretching of PTFE, the stretching conditions need to be strictly controlled like the heat treatment conditions.
For that purpose, the stretching device needs to have a certain level of capability or more.

In this stretching device, a PTFE monofilament is set between chucks of a stretching device, and then the stretching device is inserted into a thermostatic chamber, and when the temperature inside the chamber reaches a specified temperature, it is operated from the outside to a predetermined stretching ratio. Is stretched at a predetermined stretching speed,
A thermostatic chamber with a stretcher that can immediately release the chuck and sample to the outside at room temperature after stretching.
The thermocouple should be positioned to indicate the temperature near the PTFE monofilament held between the chucks, and the temperature should be controllable within ± 1 ° C, and preferably within ± 0.5 ° C. As for the stretching machine, the stretching speed is required to be at least 50 mm / sec, and it is preferable that the stretching speed is 10 times as high as 500 mm / sec.

Now, the FEA monofilament is sandwiched by the chuck of the stretching machine so that the monofilament axis is correctly parallel to the stretching direction, and the FEA monofilament is inserted into a constant temperature bath kept at a constant temperature, and the temperature of the sample is adjusted to the predetermined temperature. To do.

Generally, since the heat capacity of the drawing machine itself is larger than that of the FEA monofilament, it takes some time to recover the temperature drop by inserting the drawing machine, but the temperature near the FEA monofilament is a predetermined temperature. It is necessary to condition for about 5 minutes after returning to.

The stretching temperature described below is the most important one among the superstretching conditions. Generally, the temperature is 360 ° C or higher, but the most preferable range is 387 to 388 ° C, which is an extremely narrow region. The reason for this has not been elucidated so far, but the present inventors presume that it may be due to the difference in the thermal stability of the microstructure of the PTFE uniaxially stretched molded product formed by super-stretching.

Regarding the stretching speed, the upper limit of the stretching speed is not clear due to the performance limitation of the apparatus, but in general, the faster the better, the level of at least 50 mm / sec is required. The critical stretching ratio depends on the heat treatment conditions, particularly the cooling conditions: the cooling rate and the temperature range controlled by keeping the cooling rate constant. This is a low level as compared with the level of 100 to 300 times in the case of ultra stretching of ultra high molecular weight polyethylene. It is considered that this is because the PTFE molecule is a polymer belonging to an intermediate type between a bent chain and a rigid chain. Of course, when the reduction ratio RR in the paste extrusion process in the case of PTFE is taken into consideration, the substantial stretch ratio is equal to or higher than that of polyethylene.

The last important condition for super-stretching is
Immediately after the completion of stretching, it is taken out from the thermostat and cooled. This cooling condition may be air cooling, but it is preferable to use a condition closer to quenching. Immediately after the end of the super stretching, the obtained uniaxially stretched molded product should not be brought into contact with a stretcher that is still at a sufficiently high temperature. If they come into contact with each other, the molecular orientation is immediately returned, and the strength is significantly reduced.

Therefore, a molded product obtained by paste extruding a PTFE billet at a reduction ratio of 600 or more is heat-treated in a state of free expansion and contraction and then gradually cooled, and then drawn 100 times or more at a drawing speed of 50 mm / sec or more. By doing so, it becomes possible to produce a uniaxially stretched molded article of PTFE in which the molecular chains are arranged in the direction of the stretching axis, and the arrangement of the molecular chains serves to provide a strength of 1 GPa or more.

[0047]

EXAMPLES Examples of the present invention will be described in detail below.

Example 1 Polyflon TFE F-10
4 (Daikin Industrial PTFE fine powder) was first passed through a 4-mesh sieve and then through an 8.6-mesh and a 16-mesh sieve.
After blending 5% by weight, the container is tightly closed, placed on a rotary stand, and the container is rotated in the circumferential direction at a speed of 20 m / min for 30 minutes to perform mixing.
Then, after leaving it at room temperature for 24 hours in a tightly plugged state, the wet PTFE was placed in a ram extruder at an extrusion temperature of 60 ° C. and a die with a reduction ratio of 800 was used to obtain a thickness of 0.4 m.
It was extruded into a tape of m and 5 mm in width. Sagging this tape 2
After heat treatment at 350 ° C. for 1 hour with both ends fixed at 5%, it was cooled to 275 ° C. at a cooling rate of 10 ° C./min and then rapidly cooled to room temperature.

This baked tape was preheated at 388 ° C. for 5 minutes, and then at the same temperature, the stretching speed was 50 mm / sec and the magnification was 100 times and 12 times.
After being stretched 5 times, it was immediately cooled to room temperature.

When the tensile breaking strength of this stretched tape was measured (room temperature, pulling speed: 20 mm / min), it showed 1.9 GPa at 100 times stretching and 2.1 GPa at 125 times stretching.

(Example 2) A thickness of 0.52 mm and a width of 5 mm were obtained in the same manner as in Example 1 except that the reduction ratio was set to 600.
The tape of Example 1 was produced, heat-treated under the same conditions as in Example 1, and then similarly stretched and quenched.

When the tensile strength at break of this stretched tape was measured (room temperature, pulling speed: 20 mm / min), it was 0.56 GPa at 100 times stretching and 1.1 GPa at 125 times stretching.

(Example 3) A thickness of 0.52 mm and a width of 5 mm were obtained in the same manner as in Example 1 except that the reduction ratio was set to 600.
Manufactured tape. In this case, the thickness is about 5% when extruded.
Extrusion was performed with decreasing drawdown (forward tension on the tape). After heat treatment under the same conditions as in Example 1, the film was similarly stretched and quenched.

When the tensile strength at break of this stretched tape was measured (room temperature, pulling speed: 20 mm / min), it was 0.85 GPa at 100 times stretching and 1.53 GPa at 125 times stretching. (Example 4) Wet PTFE obtained in the same manner as in Example 1 was paste extruded at a reduction ratio of 600 and roll-rolled to obtain a film having a thickness of 0.1 mm, which was continuously slit in the length direction. It was made into a tape with a width of 5 mm.

This was heat-treated under the same conditions as in Example 1, then similarly stretched 125 times and quenched.

When the tensile breaking strength of this stretched tape was measured (room temperature, tensile speed 20 mm / min), it was found to be 1.3 Gpa.

[0057]

In summary, according to the present invention, 1 GPa
An excellent effect is obtained in that a PTFE uniaxially stretched molded product having the above tensile rupture strength can be obtained.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Office reference number FI technical display location // B29K 27:18 B29L 7:00

Claims (6)

[Claims] 1. A molded product obtained by paste-extruding a billet containing fine powder of polytetrafluoroethylene having a molecular weight of 6,000,000 or more at a reduction ratio of 600 or more.
Heat treatment at a temperature above the crystalline melting point of polytetrafluoroethylene in a state of free expansion and contraction, then gradually cool until the crystallization is completed, and then heat until the melting of the crystals is completed, then 50 mm
A uniaxially stretched molded product of polytetrafluoroethylene, which is obtained by stretching 100 times or more at a stretching speed of 1 / second or more and quenching. 2. The uniaxially stretched molded product of polytetrafluoroethylene according to claim 1, which has a tensile strength at break of 1 Gpa or more. 3. A molded product obtained by paste-extruding a billet containing fine powder of polytetrafluoroethylene having a molecular weight of 6,000,000 or more at a reduction ratio of 600 or more,
Heat-treated at a temperature above the crystal melting point of polytetrafluoroethylene in a state where expansion and contraction were free, then gradually cooled, and then heated until the melting of crystals was completed, and then stretched 100 times or more at a stretching speed of 50 mm / sec or more, A method for producing a uniaxially stretched molded product of polytetrafluoroethylene, which comprises quenching. 4. The method for producing a uniaxially stretched polytetrafluoroethylene molded article according to claim 3, wherein a sheet-shaped or tape-shaped molded article is obtained by paste extrusion. 5. The method for producing a uniaxially stretched polytetrafluoroethylene molded article according to claim 3, wherein the heat treatment is performed at a temperature of 340 ° C. or higher. 6. The method for producing a uniaxially stretched molded product of polytetrafluoroethylene according to claim 3, wherein gradual cooling is performed at a cooling rate of 10 ° C./min or less.