Modified polytetrafluoroethylene fine powder and its manufacturing method, and electric wire and tube using it

Abstract

To optimize the primary particle size of a modified PTFE fine powder to shorten the sintering time during the extrusion molding. A modified polytetrafluoroethylene fine powder which is a fine powder of a non-melt-processable modified polytetrafluoroethylene comprising units derived from tetrafluoroethylene, units derived from hexafluoropropylene, units derived from a perfluoro(alkyl vinyl ether) represented by CF.sub.2═CFO—C.sub.nF.sub.2n+1 (n is an integer of from 1 to 6) and units derived from a (perfluoalkyl)ethylene represented by CH.sub.2═CH—C.sub.mF.sub.2m+1 (m is an integer of from 3 to 7).

Claims

1. A modified polytetrafluoroethylene fine powder which is a fine powder of a non-melt-processable modified polytetrafluoroethylene, wherein the modified polytetrafluoroethylene comprises units derived from tetrafluoroethylene, units derived from hexafluoropropylene, units derived from a perfluoro(alkyl vinyl ether) represented by the formula (I) and units derived from a (perfluoroalkyl)ethylene represented by the formula (II):
CF.sub.2═CFO—C.sub.nF.sub.2n+1  (I) wherein n is an integer of from 1 to 6,
CH.sub.2═CH—C.sub.mF.sub.2m+1  (II) wherein m is an integer of from 3 to 7, and wherein the modified polytetrafluoroethylene fine powder has a core-shell structure comprising a core part made of a modified PTFE having no unit derived from the hexafluoropropylene and a shell part made of a modified PTFE having units derived from the hexafluoropropylene.

2. The modified polytetrafluoroethylene fine powder according to claim 1, which has an average primary particle size of from 0.20 to 0.28 μm.

3. The modified polytetrafluoroethylene fine powder according to claim 1, wherein the sum of the units derived from hexafluoropropylene, the units derived from a perfluoro(alkyl vinyl ether) and the units derived from a (perfluoroalkyl)ethylene, is at most 0.3 mass %, per a total mass of the fine powder.

4. The modified polytetrafluoroethylene fine powder according to claim 1, wherein the units derived from hexafluoropropylene is from 0.005 to 0.090 mass %, the units derived from a perfluoro(alkyl vinyl ether) is from 0.01 to 0.20 mass %, and the units derived from a (perfluoroalkyl)ethylene is from 0.001 to 0.010 mass %, per a total mass of the fine powder.

5. The modified polytetrafluoroethylene fine powder according to claim 1, wherein the core part:shell part representing the mass ratio of the core part to the shell part is from 70:30 to 95:5.

6. A method for manufacturing the modified polytetrafluoroethylene fine powder according to claim 1, the method comprising a first step of polymerizing tetrafluoroethylene, the perfluoro(alkyl vinyl ether) and the (perfluoroalkyl)ethylene in the presence of an aqueous medium, a polymerization initiator and an emulsifier and a second step of adding hexafluoropropylene so as to be polymerized with tetrafluoroethylene.

7. The method according to claim 6, wherein the modified polytetrafluoroethylene fine powder has a core-shell structure and wherein in the first step, tetrafluoroethylene is continuously or intermittently supplied to a polymerization reactor, and at a time when from 70 to 95 mass % of tetrafluoroethylene in the total amount to be used in the polymerization reaction is supplied, the second step is initiated by adding hexafluoropropylene.

8. The method according to claim 6, wherein the emulsifier is at least one fluorinated emulsifier selected from the group consisting of a C.sub.5-8 fluorinated carboxylic acid having an etheric carbon atom and salts thereof.

9. A method for coating an electric wire, the method comprising: providing a mixture comprising the modified polytetrafluoroethylene fine powder according to claim 1; and coating at least a portion of the electric wire with the mixture thereby producing an electric wire having a coating layer.

10. An electric wire having a coating layer, produced by the method according to claim 9.

11. A method for producing a tube, the method comprising: providing a mixture comprising the modified polytetrafluoroethylene fine powder according to claim 1; and molding the mixture thereby producing the tube.

12. A tube, produced by the method according to claim 11.

Description

EXAMPLES

(1) Now, the present invention will be described in detail with reference to Examples. However, the present invention is by no means restricted thereto.

(2) [Measuring Methods]

(3) (A) Average Primary Particle Size of Modified PTFE Fine Powder (Unit: μm)

(4) The average primary particle size was measured by means of a laser scattering particle size distribution analyzer (tradename “LS230”, manufactured by Coulter Co., Ltd.).

(5) (B) Standard Specific Gravity (SSG) of Modified PTFE Fine Powder

(6) SSG was measured in accordance with ASTM D1457-10 and D4895-10. 12.0 g of a modified PTFE fine powder was weighed and held in a cylindrical mold having an inner diameter of 28.6 mm for two minutes under 34.5 MPa to prepare a molded sample. The molded sample was put in an oven of 290° C. and heated at a rate of 120° C./hr. After being held at 380° C. for 30 minutes, the molded sample was cooled at a rate of 60° C./hr and held at 294° C. for 24 minutes. Then, the molded sample was held in a desiccator of 23° C. for 12 hours, and the mass of the molded sample at 23° C. in air and the mass in water were measured. The specific gravity value of the molded sample to water at 23° C. was obtained. The obtained value was multiplied by the density value of water at 23° C. to obtain a value of standard specific gravity.

(7) (C) Evaluation of Extrusion Pressure

(8) The extrusion pressure was measured by the method described in ASTM4895-10, section 10.8. 200 g of a modified PTFE fine powder which was left at room temperature for more than 2 hours, was put into a glass bottle having an internal capacity of 900 cc, and 60 mL of a lubricant (trade name “Multipar H” (registered trademark), manufactured by Multisol) was added, followed by mixing for 25 minutes to obtain a PTFE fine powder mixture. The obtained PTFE fine powder mixture was left in a constant temperature chamber at 30° C. for 2 hours and preform molded at 100 psi (weight pound/inch.sup.2) and then extruded through an orifice having a diameter of 0.79 mm, a land length of 0.38 mm and a cone angle of 30°, at 25° C. under conditions of a reduction ratio (ratio of the cross-section of the inlet to the cross-section of the outlet of the die) of 1,600 and an extrusion rate of 18 mm/min, to obtain a paste extrudate (bead). The pressure required for the extrusion at that time was measured, and it was designated as the extrusion pressure.

(9) (D) Measurement of Content of Comonomers

(10) FT-IR (FT/IR4100, Manufactured by JASCO Corporation)

(11) A PTFE fine powder was left at room temperature (from 18 to 25° C.) for more than 2 hours to dry. 0.01 g of the dried PTFE fine powder was put in a mold having an inner diameter of 3 mm and a height of 3 mm and pressurized by hand press for 30 seconds to prepare a sample to be measured.

(12) The infrared absorption spectrum of the sample to be measured was measured by means of a Fourier transform infrared spectrophotometer (FT-IR, product name: FT/IR4100, manufactured by JASCO Corporation).

(13) The content of the PPVE units in the total particles is a value (mass %) given by multiplying a ratio of an absorption of infrared absorption band at 935 cm.sup.−1 to an absorption at 993 cm.sup.−1 by the coefficient of the conversion factor 0.135.

(14) The content of the HFP units in the total particles is a value (mass %) given by multiplying a ratio of an absorption of infrared absorption band at 35 cm.sup.−1 to an absorption at 985 cm.sup.−1 by the coefficient of the conversion factor 0.10.

(15) The content of the PFBE units in the total particles was obtained by a solid .sup.19F-NMR method. A 400 MHz NMR apparatus was used, the sample rotation number was set to 30 KHz, the flip angle was set to 45°, the pulse repetition waiting time was set to 4 seconds, and the accumulation number was set to 500 times or more. The compositional ratio of the TFE units to the PFBE units was calculated from the ratio of the peak intensity derived from CF.sub.3 of PFBE (detected at a near of −81 ppm) and the peak intensity derived from CF.sub.2 (detected at a near of −120 ppm) in the obtained 19.sup.F NMR spectrum.

(16) The calculation formulae are mentioned in detail below.
Content of TFE units=3(A−2B)/(3A−2B)
Content of PFBE units=4B/(3A−2B)

(17) Here, considering the chemical shift in a case where the main chain CF.sub.2 in PTFE is −120 ppm, A and B are calculated as the following integrated values.

(18) A=an integrated value in a range of from −95 to −145 ppm+an integrated value of spinning side bands

(19) The spinning side bands were integrated under a condition having no problem for calculating peaks in general.

(20) B=an integrated value within the range of from −80 to −85 ppm

(21) (E) Mass Ratio of Core:Shell

(22) The mass ratio of the core part to the shell part is a mass ratio of TFE polymerized before the time of adding HFP to TFE polymerized after adding HFP.

(23) (F) Appearance of Extruded Bead After Sintering

(24) An extrudate (bead) was produced in the same manner as in the method for measuring the extrusion pressure.

(25) 30 cm of the obtained extrudate was held in an oven of 220° C. for 2 hours to remove the lubricant and then further heated and sintered in the oven at 370° C. for 10 minutes to obtain a sintered product. Appearance of the obtained sintered product was visually observed. When the appearance was transparent and uniform, it was evaluated as ∘ (good), and when the appearance was not uniform, for example whitening was observed, it was evaluated as x (poor).

Example 1

(26) Into a 4,000 L-stainless steel autoclave equipped with a stirrer, 6.54 kg of a 30 mass % concentration-aqueous solution of C.sub.2F.sub.5OC.sub.2F.sub.4OCF.sub.2COONH.sub.4 (ammonium perfluoro-3,6-dioxaoctanoate, hereinafter referred to “APFDO”) as an emulsifier, 35 kg of paraffin wax as a stabilizing aid and 2,200 L of deionized water were charged.

(27) The inside of the autoclave was flushed with nitrogen and then depressurized, and 60 g of PFBE and 515 g of PPVE were charged. Then, the autoclave was pressurized with TFE, and the temperature was raised to 77° C. with stirring.

(28) Then, the autoclave was pressurized with TFE to 10.34 bar (1.034 MPa), and 50 L of deionized water having 13.7 g of ammonium persulfate dissolved therein as a polymerization initiator was injected. Then, the polymerization was carried out, while adding TFE so as to maintain the inner pressure of the autoclave at 10.34 bar (1.034 MPa).

(29) In the middle of the polymerization, 15.9 kg of a 30% aqueous solution of APFDO and 71.8 g of disuccinic acid peroxide (concentration 70%, and the balance is water) as a polymerization initiator were added. Further, in the middle of the polymerization, the temperature was raised to 80° C.

(30) Further, in the middle of the polymerization, 1,172 g of disuccinic acid peroxide (concentration 70%, and the balance is water) was added.

(31) Further, when the amount of added TFE became 967 kg (87.9 mass % in the total amount of TFE to be supplied), 625 g of HFP was charged. When the amount of added TFE became 1,100 kg, the reaction was terminated, and TFE in the autoclave was released. The polymerization time was 140 minutes.

(32) The obtained PTFE aqueous emulsion was cooled, and the supernatant paraffin wax was removed. The solid content concentration in the PTFE aqueous emulsion was about 30 mass %. Further, coagulum in the autoclave was in trace amounts.

(33) In this Example, the amount of the used emulsifier was 6.732 kg in total, namely about 6,082.8 ppm, per 1,100 kg of the finally obtained modified PTFE. Further, the amount of the used stabilizing assistant was 35 kg in total, namely about 1.59 mass %, per 2,200 L of deionized water.

(34) The average primary particle size of the PTFE fine particles in the obtained PTFE aqueous emulsion was measured. Result is shown in Table 1 (the same applies hereinafter).

(35) The PTFE aqueous emulsion was diluted with deionized water to be 10 mass %, controlled at 26° C. and stirred to be aggregated to obtain a wet powder. Then, the wet powder was dried at 160° C. to obtain a modified PTFE fine powder.

(36) By the above described methods, SSG, the paste extrusion pressure, the content of each comonomer, the mass ratio of the core part:the shell part and the appearance of the extruded bead after drying and sintering were evaluated. Results are shown in Table 1 (the same applies hereinafter).

Example 2

(37) The polymerization reaction was carried out in the same manner as in Example 1 to obtain a PTFE aqueous emulsion, except that the amount of added PFBE was changed to 30 g. The polymerization time was 130 minutes. The solid content concentration of the PTFE aqueous emulsion was about 30 mass %.

(38) The PTFE aqueous emulsion was treated in the same manner as in Example 1 to obtain a wet powder, and the wet powder was dried to obtain a modified PTFE fine powder.

Comparative Example 1

(39) The polymerization reaction was carried out to obtain a PTFE aqueous emulsion in the same manner as in Example 1, except that PFBE was not added. The polymerization time was 130 minutes. The solid content concentration of the PTFE aqueous emulsion was about 30 mass %.

(40) The PTFE aqueous emulsion was treated to obtain a wet powder in the same manner as in Example 1, and the wet powder was dried to obtain a modified PTFE fine powder.

(41) TABLE-US-00001 TABLE 1 Comparative Example 1 Example 2 Example 1 Content of HFP 0.014 0.014 0.014 comonomer Monomer (I) 0.028 0.028 0.028 [mass %] (PPVE) Monomer (II) 0.005 0.003 — (PFBE) Sum of comonomers 0.047 0.045 0.042 Mass ratio of core part: shell part 87.9:12.1 87.9:12.1 87.9:12.1 Primary particle size [μm] 0.234 0.237 0.284 Standard specific gravity (SSG) 2.182 2.192 2.187 Paste extrusion pressure [MPa] 44 37 35 Extruded bead appearance ∘ ∘ x after firing

(42) As shown in the results in Table 1, all of Examples 1 and 2 and Comparative Example 1 are methods of copolymerizing TFE, HFP and the monomer (I) (PPVE) to produce a modified PTFE fine powder. The modified PTFE fine powder in Comparative Example 1 where the monomer (II) (PFBE) was not added, had an average particle size of 0.284 μm, while the modified PTFE fine powders in Examples 1 and 2 where the monomer (II) (PFBE) was added had a smaller average particle size of 0.234 μm and 0.237 μm respectively.

(43) Further, the modified PTFE fine powders in Examples 1 and 2 had the almost same standard specific gravity (SSG) as in Comparative Example 1 and also had a similar paste extrusion pressure as the evaluation of the paste extrusion processability, to Comparative Example 1.

(44) Further, in the evaluation of the appearance of the extrudate (bead) after drying and sintering, it was transparent and uniform in Examples 1 and 2 and had no problem, while the extrudate (bead) in Comparative Example 1 had whitening on the appearance.

(45) This application is a continuation of PCT Application No. PCT/JP2016/059722, filed on Mar. 25, 2016, which is based upon and claims the benefit of priority from Japanese Patent Application No. 2015-087700 filed on Apr. 22, 2015. The contents of those applications are incorporated herein by reference in their entireties.