Aqueous dispersion of low molecular weight polytetrafluoroethylene, low molecular weight polytetrafluoroethylene powder, and method for producing low molecular weight polytetrafluoroethylene

Abstract

The present invention provides an aqueous dispersion of a low-molecular-weight polytetrafluoroethylene (PTFE), which contains an easily removable surfactant, and has a good dispersion stability. Specifically, the present invention provides an aqueous dispersion of a low-molecular-weight PTFE comprising tetrafluoroethylene (TFE) units, or TFE units and modifying monomer units which are copolymerizable with the TFE units, wherein the aqueous dispersion contains from 70 to 9,000 ppm, based on the aqueous dispersion, of a fluorine-containing compound of the formula (1) below:
X(CF.sub.2).sub.mY(1)
where X is H or F; m is an integer from 3 to 5; and Y is SO.sub.3M, SO.sub.4M, SO.sub.3R, SO.sub.4R, COOM, PO.sub.3M.sub.2 or PO.sub.4M.sub.2, M being H, NH.sub.4 or an alkali metal, and R being an alkyl group having 1 to 12 carbons, and the low-molecular-weight PTFE has an average primary particle size of from 100 to 350 nm.

Claims

1. A method of producing a low-molecular-weight polytetrafluoroethylene, comprising: emulsion-polymerizing, in an aqueous medium, tetrafluoroethylene, or tetrafluoroethylene and a modifying monomer copolymerizable with the tetrafluoroethylene in the presence of a chain transfer agent, wherein the emulsion polymerization is carried out in the presence of a fluorine-containing compound of the formula (1) below:
X(CF.sub.2).sub.mY(1) where X is H or F; m is an integer from 3 to 5; and Y is SO.sub.3M, SO.sub.4M, SO.sub.3R, SO.sub.4R, COOM, PO.sub.3M.sub.2 or PO.sub.4M.sub.2, M being H, NH.sub.4 or an alkali metal, and R being an alkyl group having 1 to 12 carbons, wherein the emulsion polymerization is further carried out in the presence of a reactive compound which has a functional group capable of reacting in radical polymerization and a hydrophilic group, and wherein the reactive compound is used in an amount which corresponds to from 100 ppb to 20 ppm of the aqueous medium.

2. The method of producing a low-molecular-weight polytetrafluoroethylene according to claim 1, wherein the reactive compound has an unsaturated bond.

3. The method of producing a low-molecular-weight polytetrafluoroethylene according to claim 1, wherein the reactive compound is a perfluorovinylalkyl compound of the formula (1a) below:
CF.sub.2CF(CF.sub.2).sub.n1Y.sup.1(1a) where n.sub.1 is an integer from 1 to 10; and Y.sup.1 is SO.sub.3M.sup.1 or COOM.sup.1, M.sup.1 being H, NH.sub.4 or an alkali metal), a perfluorovinylalkyl compound of the formula (1b) below:
CF.sub.2CF(CF.sub.2C(CF.sub.3)F).sub.n2Y.sup.1(1b) where n.sub.2 is an integer from 1 to 5; and Y.sup.1 is as defined above, a perfluorovinyl ether compound of the formula (1c) below:
CF.sub.2CFO(CFX.sup.1).sub.n3Y.sup.1(1c) where X.sup.1 is F or CF.sub.3; n.sub.3 is an integer from 1 to 10; and Y.sup.1 is as defined above, a perfluorovinyl ether compound of the formula (1d) below:
CF.sub.2CFO(CF.sub.2CFX.sup.1O).sub.n4CF.sub.2CF.sub.2Y.sup.1(1d) where n.sub.4 is an integer from 1 to 10; and Y.sup.1 and X.sup.1 are as defined above, or a fluoroalkyl ether compound of the formula (1e) below:
CX.sup.2.sub.2CFCF.sub.2O(CF(CF.sub.3)CF.sub.2O).sub.n5CF(CF.sub.3)Y.sup.1(1e) where each X.sup.2 is the same and represents F or H; n.sub.5 is 0 or an integer from 1 to 10; and Y.sup.1 is as defined above.

4. The method of producing a low-molecular-weight polytetrafluoroethylene according to claim 1, wherein the reactive compound is used in an amount which corresponds to from 100 ppb to 10 ppm of the aqueous medium.

5. The method of producing a low-molecular-weight polytetrafluoroethylene according to claim 1, wherein the fluorine-containing compound is used in an amount which corresponds to from 100 to 10,000 ppm of the aqueous medium.

6. The method of producing a low-molecular-weight polytetrafluoroethylene according to claim 1, wherein the modifying monomer which is copolymerizable with tetrafluoroethylene is hexafluoropropylene, perfluoropropyl vinyl ether or vinylidene fluoride.

Description

BEST MODES FOR CARRYING OUT THE INVENTION

(1) The present invention is described more fully below by way of examples, although the invention is not limited by these examples. In each working example of the invention and comparative example, unless noted otherwise, parts and % refer respectively to parts by mass and percent by mass.

Comparative Example 1

(2) A stainless steel autoclave having an internal capacity of 6 liters and equipped with a stainless steel anchor-shaped agitation blade and a jacket for temperature regulation was charged with 3.3 L of deionized water and 5.0 g of ammonium perfluorooctanoate (APFO) as the fluorine-containing surfactant, then tightly closed. Oxygen within the system was removed by repeatedly introducing nitrogen gas under pressure and degassing the system a plurality of times, after which 70 mg of propane as a chain transfer agent was introduced under pressure with tetrafluoroethylene (TFE) and the pressure within the reactor was set to 0.10 MPa. The temperature within the reactor was raised under stirring at 500 rpm. Once the temperature within the reactor had reached 55 C., TFE was again introduced under pressure, and the pressure within the reactor was adjusted to 0.75 MPa.

(3) An aqueous solution prepared by dissolving 850 mg of ammonium persulfate (APS) as a polymerization initiator in 20 mL of deionized water was introduced under pressure with TFE into the reactor, and the pressure within the reactor was set to 0.80 MPa. Because the pressure within the reactor falls owing to decomposition of the polymerization initiator, TFE is continuously fed into the reactor so as to maintain the pressure within the reactor at 0.800.05 MPa. Throughout the reaction, the temperature within the reactor was regulated at 551, and a stirring rate was controlled to 500 rpm. When TFE consumption reached 850 g, stirring was stopped and the pressure within the reactor was released to ordinary pressure, after which the gas phase was substituted with nitrogen, thereby giving an aqueous dispersion of the low-molecular-weight PTFE.

(4) Twenty grams of nitric acid was added to 3,000 g of the above aqueous dispersion of the low-molecular-weight PTFE, coagulation was induced by applying a strong mechanical shear force, and then 20 g of a 24% by mass aqueous solution of sodium hydroxide was added. The wet powder thus obtained was filtered, and then freshly washed with 1,800 g of purified water. This washing operation was repeated three times, following which 18 hours of drying was carried out in a 160 C. hot-air circulation oven, thereby giving a low-molecular-weight PTFE powder.

Example 1

(5) Aside from charging 5.0 g of ammonium perfluorohexanoate (APFH) in place of APFO into the system, an aqueous dispersion of a low-molecular-weight PTFE was obtained in the same way as in Comparative Example 1. Coagulation, washing and drying steps were carried out on the above aqueous dispersion in the same way as in Comparative Example 1, thereby giving a target low-molecular-weight PTFE powder.

(6) TDS-80C (Dai-ichi Kogyo Seiyaku Co., Ltd.) was added as a nonionic surfactant to the above aqueous dispersion in an amount of 6.0% by mass, based on the mass of the polymer included, and the pH was adjusted to 9.0 with ammonia water, following which the dispersion was concentrated to a PTFE solid content of 60% by mass by being held at ordinary pressure and 65 C. so as to cause water to evaporate off. The average particle size of the PTEF primary particles in the concentrated aqueous dispersion is the same as that in the pre-concentration aqueous dispersion.

Example 2

(7) Aside from changing the amount of APFH charged to 10.0 g, an aqueous dispersion of a low-molecular-weight PTFE was obtained in the same way as in Example 1. The same coagulation, washing and drying steps were carried out on the aqueous dispersion as in Example 1, thereby giving the target low-molecular-weight PTFE powder.

Example 3

(8) Aside from changing the polymerization temperature to 85 C., and the amount of propane charged to 35 mg, an aqueous dispersion of a low-molecular-weight PTFE was obtained in the same way as in Example 1. The same coagulation, washing and drying steps were carried out on the aqueous dispersion as in Example 1, thereby giving the target low-molecular-weight PTFE powder.

Example 4

(9) Aside from charging 66 mg of a 50% aqueous solution of CH.sub.2CFCF.sub.2OCF(CF.sub.3)CF.sub.2OCF(CF.sub.3)COONH.sub.4 (reactive compound A) into the system, an aqueous dispersion of a low-molecular-weight PTFE was obtained in the same way as in Example 1. Coagulation, washing and drying steps were carried out on the above aqueous dispersion in the same way as in Example 1, thereby giving the target low-molecular-weight PTFE powder.

(10) TDS-80C (Dai-ichi Kogyo Seiyaku Co., Ltd.) was added as a nonionic surfactant to the above aqueous dispersion in an amount of 6.0% by mass, based on the mass of the polymer included, and the pH was adjusted to 9.0 with ammonia water, following which the system was concentrated to a PTFE solid content of 60% by mass by being held at ordinary pressure and 65 C. so as to cause water to evaporate off. The average particle size of PTEF primary particles in the concentrated aqueous dispersion is the same as that in the pre-concentration aqueous dispersion.

Example 5

(11) Aside from changing the amount of the 50% aqueous solution of the reactive compound A charged to 33 mg, an aqueous dispersion of a low-molecular-weight PTFE was obtained in the same way as in Example 4. The same coagulation, washing and drying steps were carried out on the aqueous dispersion as in Example 4, thereby giving a target low-molecular-weight PTFE powder.

Example 6

(12) Aside from changing the amount of the 50% aqueous solution of the reactive compound A charged to 6.6 mg, an aqueous dispersion of a low-molecular-weight PTFE was obtained in the same way as in Example 4. The same coagulation, washing and drying steps were carried out on the aqueous dispersion as in Example 4, thereby giving a target low-molecular-weight PTFE powder.

Example 7

(13) Aside from changing the amount of APFH charged to 1.7 g, an aqueous dispersion of a low-molecular-weight PTFE was obtained in the same way as in Example 5. The same coagulation, washing and drying steps were carried out on the aqueous dispersion as in Example 4, thereby giving a target low-molecular-weight PTFE powder.

(14) Physical properties (1) and (2) below were measured for the aqueous dispersions of the low-molecular-weight PTFE obtained in each of the Examples and in Comparative Example 1, and physical properties (3) to (8) below were measured for the powders obtained in the respective Examples and comparative examples.

(15) (1) Solid Content Concentration in Aqueous Dispersion (P %)

(16) The solid content concentration in each aqueous dispersion (P %) was determined, based on the mass of an ignition residue (Z g) obtained by heating the aqueous dispersion (X g) for 3 hours at 150 C., using the formula: P=Z/X100(%).

(17) (2) Average Primary Particle Size

(18) A working curve of the transmittance of 550 nm incident light through a unit length of an aqueous dispersion adjusted to a low-molecular-weight PTFE concentration of 0.22% by mass versus an average primary particle size determined by measuring the particle diameter in a certain specific direction on a transmission electron micrograph was constructed. The above transmittance through the aqueous dispersion being assayed was then measured, and the average primary particle size was determined based on the working curve.

(19) (3) Apparent Density

(20) Measured in accordance with JIS K-6891.

(21) (4) Average Particle Size

(22) The above average particle size was equivalent to the particle size corresponding to 50% of the cumulative particle size distribution obtained by using a laser diffraction-type particle size distribution analyzer (JEOL Ltd.) to measure the particle size distribution at a pressure of 0.1 MPa for a measurement time of 3 seconds without the use of a cascade.

(23) (5) Melt Viscosity

(24) The melt viscosity was measured in accordance with ASTM D-1238 by using a Flow Tester (Shimadzu Corporation) and a 2-8L die and holding, under a load of 0.7 MPa and at a measurement temperature (380 C.), a 2 g sample which was pre-heated at the above temperature for 5 minutes.

(25) (6) Melting Point

(26) Using a differential scanning calorimeter RDC 220 (DSC) manufactured by SII Nanotechnology Inc., temperature calibration was carried out beforehand using indium and lead as standard samples. Next, about 3 mg of the low-molecular-weight PTFE powder being assayed was placed in an aluminum pan (crimped container), the temperature was raised at a rate of 10 C./min from 250 C. to 380 C. under a stream of air having a flow rate of 200 mL/min, and the heat of fusion minimum in the above range was taken as the melting point.

(27) (7) Specific Surface Area

(28) The specific surface area was measured by BET method with a surface analyzer (available from QUANTA CHLROME under the trade name MONQSORB). A mixed gas (30% nitrogen, 70% helium) was used as the carrier gas, and liquid nitrogen was used for cooling.

(29) (8) Fluorine-Containing Compound Concentration in Powder

(30) A methanol solution of a fluorine-containing compound having a known concentration was measured using a Quattro micro API (Waters Corporation) under the following conditions: column, Atlantis dC18 (Waters Corporation); developing solution, acetonitrile/0.15% aqueous acetic acid=45/55 (vol/vol %); flow rate, 0.15 mL/min; sample volume, 5 L; column temperature, 40 C.; monitored mass, m/z=313>269 or m/z=413>369. A working curve was constructed from the concentration of the fluorine-containing compound versus the surface area of the fluorine-containing compound peak.

(31) Thirty milliliters of methanol was added as an extracting solvent to 3 g of the powder being assayed, and extraction was carried out at 150 C. for 60 minutes using Microwave Assisted Solvent Extraction MARS5 (CEM Corporation). The methanol solution following extraction was measured under the above measurement conditions, and the fluorine-containing compound concentration in the powder was determined, based on the above working curve, from the resulting fluorine-containing compound peak area. The lower limit of detection for the fluorine-containing compound concentration in the above powder was 0.01 ppm.

(32) The above results are shown in Table 1.

(33) TABLE-US-00001 TABLE 1 Comp. Ex. Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 7 Fluorine-containig APFO/5.0 APFH/5.0 APFH/10.0 APFH/5.0 APFH/5.0 APFH/5.0 APFH/5.0 APFH/1.7 compound/charged amount (g) Charged amount of 10 5 1 5 reactive compound A (ppm) Reaction time (h) 3.5 4.5 4.0 1.5 3.6 3.7 4.3 4.2 Solid content concentration 20.4 20.0 20.4 20.4 20.5 20.4 20.3 20.0 (% by mass) Average primary particle 180 220 190 190 130 180 210 180 size (nm) Apparent density (g/cc) 0.36 0.34 0.36 0.36 0.35 0.35 0.35 0.35 Average particle size (m) 5 4 5 5 5 5 5 5 Melt viscosity (Pa .Math. S) 17000 15000 18000 7500 16000 16000 16000 14000 Melting point ( C.) 329 329 329 328 328 329 329 329 Specific surface area (m.sup.2/g) 10.5 8.5 10.0 10.0 13.0 11.5 8.5 11.0 Fluorine-containing 12.47 0.13 0.17 0.13 0.11 0.11 0.17 0.10 compound concentration of powder (ppm)

(34) The above results show that, in each of Examples, the aqueous dispersion containing an emulsified particle of the low-molecular-weight PTFE was obtained in the same way as in Comparative Example 1.

INDUSTRIAL APPLICABILITY

(35) The aqueous dispersions of the low-molecular-weight PTFE and the low-molecular-weight PTFE powder of the invention are well-suited for use as an additive for modifying a molding material, an ink, a cosmetic, a paint, a grease, an office automation equipment component and a toner. The production method of the invention may be employed in the production of a low-molecular-weight PTFE which is particularly suitable as an additive for improving lubricity and a coat-surface texture.