WO2022118215A1

WO2022118215A1 - White polytetrafluoroethylene powders

Info

Publication number: WO2022118215A1
Application number: PCT/IB2021/061183
Authority: WO
Inventors: Michael C. Dadalas; Gabriele H. GOTTSCHALK-GAUDIG; Klaus Hintzer
Original assignee: 3M Innovative Properties Company
Priority date: 2020-12-02
Filing date: 2021-12-01
Publication date: 2022-06-09

Abstract

Methods of preparing a white polytetrafluoroethylene micropowder are described. The methods include thermally degrading a high molecular weight polytetrafluoroethylene powder and chemically bleaching the resulting thermally degraded powder. The methods may also include processing the powder to achieve the desired particle size. White polytetrafluoroethylene powders prepared by thermal degradation are also described.

Description

WHITE POLYTETRAFLUOROETHYLENE POWDERS

FIELD

[0001] The present disclosure relates to micronized polytetrafluorethylene powders prepared by thermal degradation, including treatments to remove discoloration and the resulting white PTFE micropowders.

SUMMARY

[0002] Briefly, in one aspect, the present disclosure provides methods of preparing a white polytetrafluoroethylene micropowder comprising thermally degrading a polytetrafluoroethylene to form a thermally degraded PTFE; and chemically bleaching the thermally degraded PTFE to form a white thermally degraded PTFE having an L* value of at least 98.

[0003] In another aspect, the present disclosure provides white, thermally degraded polytetrafluoroethylene micropowders .

DETAILED DESCRIPTION

[0004] Homopolymers of tetrafluoroethylene (TFE) or copolymers of TFE with less than one weight percent of other copolymerizable fluorinated monomers are referred to herein and in the art as polytetrafluoroethylene (PTFE). Exemplary comonomers include perfluorinated vinyl/allylethers of the formula CF2=CF-(CF2)_X-O-Rf. Generally, Rf is a fluorinated or perfluorinated group with one to ten carbon atoms. Rf may be linear or branched and may be interrupted by one or two oxygen atoms.

Exemplary comonomers can also include olefins of the formula CX2=CY-R; X = H, F; Y = H, F, Cl; and R = F or an alkyl group with one to five carbon atoms, which may be partially or fully fluorinated.

[0005] PTFE can be distinguished by the polymerization method used to form it. One method to produce PTFE is emulsion polymerization followed by coagulation (E-PTFE). Alternatively, suspension PTFE (S-PTFE) is made without the addition of emulsifiers during polymerization. Both PTFE-families can be homopolymers of TFE or can be modified with up to 1 wt.% of perfluorinated comonomers.

[0006] PTFE is typically polymerized to high molecular weights. For example, the molecular weight (MW) of PTFE used in film, tape, and wire coating is typically more than 5 million grams per mole, while the MW of PTFE used for molding and sintering billets may be up to 50 million grams per mole or even greater. Such high molecular weights result in high melt viscosities, e.g., in the range of 10^ to 10^2 Pa«s, and corresponding low melt flow rates or melt flow indices.

[0007] Due to its high melting temperature and low melt flow rates, high molecular weight PTFE is not processable by commonly used technologies used in plastic industries, e.g. injection molding and extrusion. Although specialized equipment to process these materials have been developed, these high molecular PTFE materials are commonly referred to as “non melt-processible.” As a result, such high molecular weight PTFE is difficult or impractical to compound into other resins as a filler.

[0008] Polytetrafluoroethylene micropowders comprise small particles of low molecular weight polytetrafluoroethylene (PTFE). The PTFE is generally milled or ground to produce a micropowder having an average particle size of 1 to 30 microns, e.g., 1 to 20 microns, however submicron average particle sizes are also known. These micronized, low molecular weight powders can be compounded into other polymers or resins to, e.g., reduce the coefficient of friction, improve wear resistance, and provide non-stick properties. They may also be used as rheology modifiers in fluid systems.

[0009] Generally, the difference between high MW non-melt processible PTFE and low molecular weight PTFE useful in micronized powders may be characterized by their melt flow index (MFI), also referred to as the Melt Flow Rate (MFR). As used herein, the MFI (or MFR) refers to the values measured according MFI Test Method described in the Example section of the present disclosure. MFI is expressed as a mass flow rate per unit time, e.g., grams per 10 minutes.

[0010] As is well-known, the measured value of MFI is a function of three operating conditions that must be specified, i.e., the load applied, the temperature and the diameter of the orifice. Generally, the MFI increases with increasing load, temperature and orifice diameter. In order to obtain reliable data, it is often necessary to select these parameters based on the MW of the sample. For example, higher loads and temperatures, and larger orifices may be required for high MW PTFE. However, due to equipment and test method constraints, lower loads and temperatures or smaller orifices may be required for lower MW materials.

[0011] As used herein, high molecular weight PTFE refers to PTFE with an MFI (as measured according to the MFI Test Method) of no greater than 1 gram per 10 minutes when tested at 372 °C with a load of 10 kg and an orifice diameter of 2 mm, i.e. an MFI (10/372/2) of no greater than 1 g/10 mins. In some embodiments, the high MW PTFE has an MFI (10/372/2) of no greater than 0. 1 g/10 mins, or even no greater than 0.01 g/10 mins.

[0012] Alternatively, as it can be difficult or impractical to measure the melt flow rate of high MW PTFE, they are often characterized by their specific standard gravity (SSG). Typical values of SSG range from 2.13 to 2.26 g/crrP. High MW PTFE has also been characterized by its amorphous content after a melting process, which is typically greater than 15% (see Modem Fluoropolymers Chapter 12; ISBN 0- 471-97055-7).

[0013] Low molecular weight PTFE can be prepared by direct polymerization, i.e., by controlling the polymerization conditions to achieve the desired low MW. However, PTFE micropowders are more commonly prepared by degrading high molecular weight PTFE to convert it to the desired lower molecular weight PTFE. The most common approach is radiation degradation where the high molecular weight PTFE is exposed to radiation, e.g., gamma irradiation, electron beam or cobalt irradiation, resulting in lower molecular weight materials. These low MW materials can then be milled or ground to the desired particle size resulting in a white PTFE micropowder. [0014] However, during the irradiation process a considerable amount of CF3(CF2)_n-COOH can be produced. Such low molecular weight perfluorinated alkanoic acids are the subject of increased regulation. For example, at least perfluorooctanoic acid has to be removed due to regulations; but this results in additional efforts and costs.

[0015] Thermal degradation processes can also be used to form low molecular weight PTFE micropowders. Such processes are described in GB 1 035 566 or US 5634963. For example, PTFE parts/pieces/powder can be placed in an oven with temperatures of, e.g., above 400 °C but lower than 650 °C to degrade the PTFE to obtain the desired molecular weight. Alternatively, extruders can be used to thermally degrade high molecular weight PTFE at, e.g., 500 to 600 °C. The thermally degraded PTFE is then milled to the desired particle sizes.

[0016] Generally, the high MW PTFE is thermally degraded to produce a low MW PTFE having an MFI (as measured according to the MFI Test Method) of at least 0. 1 grams per 10 minutes when tested at 372 °C with a load of 2.16 kg and an orifice diameter of 1 mm, i.e. an MFI (2. 16/372/1) of at least 0.1 g/10 mins. In some embodiments, the thermally degraded PTFE has an MFI (2.16/372/1) of at least 1 g/10 mins, e.g., at least 5 g/10 mins, or even at least 10 g/10 mins. It is important to note that both the load and the orifice diameter used to measure the MFI of the thermally degraded PTFE are less than the load and orifice diameter used to measure the MFI of high MW PTFE. Thus, a thermally-degraded PTFE with an MFI of 0.1 g/10 mins at 2.16 kg, 372 °C and a 1 mm orifice has a substantially higher MFI (and substantially lower MW) than a high MW PTFE with an MFI of 0. 1 g/10 mins at 10 kg, 372°C and a 2 mm orifice.

[0017] The use of thermal degradation to convert high molecular weight PTFE to the desired lower molecular weights offers several advantages over radiation techniques including minimizing or eliminating the formation of low molecular weight perfluorinated alkanoic acids. In comparison to irradiated micropowders, thermally degraded PTFE does not show low molecular weight CF3(CF2)_n- COOH compounds (n = 1 - 14) in amounts greater than 500 ppb. In some embodiments, the content of C8 to C14 (i.e., n = 6 to 12) perfluorinated carboxylic acids is less than 400 ppb, e.g., less than 200 ppb, less than 100 ppb, or even less than 50 ppb by weight based on the weight of the PTFE. Thermally decomposed PTFE also contains low levels of carboxylic groups, i.e., COOH, COF and CONH2 groups. In some embodiments, the thermally degraded PTFE comprises fewer than 100, or even fewer than 50 carboxylic groups per 10^ carbon atoms. Additional benefits of thermal degradation include lower costs, simpler processes, and the elimination of radiation sources. In some embodiments, thermal degradation can produce higher melt flow index materials as compared to low MW PTFE obtained by direct polymerization. Also, thermal degradation may be used to convert excess or waste high MW PTFE into low MW PTFE suitable for use in micronized powders.

[0018] Despite these advantages, during thermal degradation or the subsequent grinding and milling operations, thermally degraded PTFE micropowders become discolored, taking on a dark gray appearance. Although the physical properties may still be acceptable, the appearance of thermally degraded PTFE micropowders limits their application.

[0019] It is believed that TFE or other gases formed during the thermal degradation process are the root cause for the greyish color. Extraction or washing with solvents or surfactants were not efficient to reduce the greyish color (see comparative examples). Instead, the present inventors discovered that the high melt flow index PTFE materials produced by thermal degradation could be chemically bleached to achieve the desired white color. This bleaching step can occur before or after the milling/grinding operations.

[0020] In the chemical bleaching processes of the present disclosure, the thermally degraded PTFE is treated with an oxidizing gas, e.g., air, oxygen or ozone. This can be done at temperatures of 100 to 310 °C, preferably at 200 to 300 °C; with a bleaching time of 10 to 12 hours. Chemical bleaching can also be performed with fluorine gas (F2), chlorine gas (CI2) or nitrous oxide (N2O). In some embodiments, the oxidizing gas is F2, for example 5 to 20 wt. % of F2 in nitrogen (N2). Bleaching treatments with such gases can be done at 50 to 290 °C, e.g., 50 to 200 °C; with a bleaching time of 5 to 10 hours. The chemical bleaching processes can be continuous or batch wise. For example, batchwise on trays, or batchwise or continuous using tumbling devices, rotary kilns or in fixed beds or fluidized beds.

[0021] Examples

Table 1: Summary of materials used in the preparation of the examples.

[0022] MFI TEST METHOD. The melt flow index was measured according to DIN EN ISO 1133-1 (March 2012). The measurement was done with an MFI measurement system from Goettfert Werkstoff- Prufinaschinen GmbH, Typ MI-4, 011.01.6. PTFE powder was placed into the system and pressed with a 10 kg weight. After this, additional powder was added to guarantee a useful amount of PTFE in the filling pipe. The powder was preheated to 372 °C and held for 10 minutes at this temperature. After this holding time, samples were tested at 372 °C, using a load of 2.16 kg and an orifice diameter of 1 mm. The outcoming molten micronized powder was collected and weighed.

[0023] CIE L*a*b* TEST METHOD. CIE L*a*b* tests were conducted according to DIN5033 using a Chroma Meter CR-5, from Konica-Minolta. For measuring the L*a*b value, the CM-A203 orifice, diameter 30 mm, and the CM-A124 zero-calibration box were used. The PTFE micropowder was placed into a Petri dish (CM-A128, diameter 45 mm, 15 mm depth) and compacted to about 75% volume. [0024] PARTICLE SIZE METHOD. The particle size of the thermally degraded PTFE was measured by laser diffraction, following ISO 13320, using a Helos BFS-Magic from Sympatec. The dry dispenser used was a Rodos M (4 mm Injector) with a Vibri Nr. 1130 from Sympatec.

[0025] FLUORINATED ACID CONTENT METHOD. The extractable carboxylic acid content was measured according to the procedure described in WO 2019/215636 Al, “Fluoropolymers with Very Low Amounts of a Fluorinated Alkanoic Acid or Its Salts,” Hintzer et al., published 14 November 2019.

[0026] DETERMINATION OF CABOXYL END GROUPS METHOD. The amount of carboxy acid end groups was measured according to the procedure described in WO 2019/055793 Al, “Fluoropolymer Dispersion, Method for Making the Fluoropolymer Dispersion, Catalyst Ink and Polymer Electrolyte Membrane,” Chen et al., published 21 March 2019.

[0027] SSG MEASUREMENT METHOD. The standard specific gravity (SSG) was measured following DIN EN ISO 12086-1 and -2. A Typ ME254S analytical balance from Sartorius with the added density set was used. The parts to be measured were stored at 23 °C for two hours. A first measurement was taken in air, with a second measurement taken with the sample submerged in butyl acetate. The analytical balance reported the density directly.

[0028] The melt flow indices of PTFE micropowders prepared by direct polymerization and thermal degradation were measured according to the MFI TEST METHOD. The results are shown in Table 2. Table 2: MFI of low MW PTFE prepared by direct polymerization and thermal degradation.

[0029] The white-black value based on the measured L* value of three materials prepared by direct polymerization were evaluated according to the CIE L*a*b* TEST METHOD to determine the benchmark color for white PTFE micropowders. The L* values are summarized in Table 3.

Table 3: CIE L* values for PTFE micropowders prepared by direct polymerization.

[0030] A sample of PTFE micropowder was prepared by thermal degradation as described in US 5,634,963 (PTFE TD-1). The MFI of the starting high MW PTFE could not be measured at 372 °C, even with a 10 kg load and a 2 mm orifice, showing that the MFI (10/372/2) was less than 0.1 gm/10 minutes. The SSG of the high MW PTFE was between 2.15 and 2.17 g/cm^, as measured according to the SSG MEASUREMENT METHOD.

[0031] This thermally degraded PTFE was milled to an average particle size of 8 microns (as measured according to the PARTICLE SIZE METHOD. The low MW PTFE had 20 ppb of Cg to C 14 carboxylic acids (as measured according to the FLUORINATED ACID CONTENT METHOD) and 30 carboxyl end groups per 10^ carbon atoms (as measured according to the DETERMINATION OF CABOXYL END GROUPS METHOD.)

[0032] As measured according to the CIE L*a*b* TEST METHOD, this thermally degraded PTFE had L* = 94.7. This L* value is too low, indicating a gray color that is not acceptable for many applications.

[0033] Washing Tests. In an attempt to improve the discoloration after thermal degradation, two washing procedures were tried. First, 20 g of PTFE-TD-1 were combined with 80 g of a 1 wt.% solution of TERIGITOL TMN 100X surfactant from Dow Chemical in water. This PTFE/surfactant suspension was mixed for 10 minutes at 500 rpm with a propeller mixer. After this procedure the PTFE solid was settled down via running the suspension at a centrifuge at 2000 rpm. The resulting water phase on top of the mixture was clear and showed no discoloration. As a result, no improvement in the color of the thermally degraded PTFE could be measured.

[0034] A second trial to improve discoloration via a washing procedure was done, by mixing 20 g of PTFE-TD-1 with 80 g of isopropanol. This PTFE/solvent suspension was mixed for 10 minutes at 500 rpm with a propeller mixer. After stopping mixing, PTFE settled out and the upper isopropanol phase was clear. As a result, no improvement in the color of the thermally degraded PTFE could be measured. [0035] Chemical Bleaching - Oxygen-Containing Gas (Example 1).

[0036] Samples of the thermally degraded PTFE micropowder (PTFE-TD-1) were placed in an oven and thermally treated at temperature from 240 to 280 °C in air for up to six hours. The chemically bleached samples were evaluated with the CIE L*a*b* TEST METHOD and the results are summarized in Table 4.

Table 4: CIE L* value for Oxygen-Treated PTFE-TD-1 micropowder (EX-1).

[0037] Chemical treatment with air led to an increase in L* indicating a reduction in the gray color. Sample S-6 had a L* value of greater than 98; however, the time and temperature required to achieve this level of bleaching using air (approximately 21 vol% oxygen) as the oxidizing agent may be excessive for practical applications.

[0038] Chemical Bleaching - Fluorine-Containing Gas. (Example 2). [0039] A sample of thermally degraded PTFE was chemically treated with fluorine as follows. PTFE- TD-1 (400 grams) was treated at 200 °C for 30 minutes with 10 wt.% F2 in N2 at 90 KPa (0.9 bar). The system was flushed, and the process was repeated a total of ten times, using fresh fluorine -containing gas each time. The resulting material was measured according the methods described above and had an MFI (2.16 kg/372°C /I mm) of 12.0 g/10 min, 18 ppb of Cg to C 4 carboxylic acids and 25 carboxyl end groups per 10^ carbon atoms.

[0040] The sample was evaluated using the CIE L*a*b* TEST METHOD. As shown in Table 5, the chemically bleached sample of thermally degraded PTFE micropowder was significantly whiter (higher L* value) than the unbleached sample. Also, the chemically bleached sample was comparable to the benchmark value established from the PTFE micropowders prepared by direct polymerization (as shown in Table 2). Thus, the white thermally degraded PTFE micropowders prepared by the methods of the present disclosure are suitable for applications requiring white materials.

Table 5: CIE L* color values for Fluorine-treated PTFE micropowder.

(*) See Table 3

[0041] In the following claims, the MFI TEST METHOD, the CIE L*a*b* TEST METHOD, the PARTICLE SIZE METHOD, the FLUORINATED ACID CONTENT METHOD, the DETERMINATION OF CABOXYL END GROUPS METHOD, and the SSG MEASUREMENT METHOD refer to the methods described in the Example section.

Claims

What is Claimed is:

1. A method of preparing a white polytetrafluoroethylene micropowder comprising

(i) thermally degrading a polytetrafluoroethylene (PTFE) having a melt flow index of no greater than 0. 1 grams per 10 minutes at 372 °C when using a 10 kg load and a 2 mm orifice to form a thermally degraded PTFE having a melt flow index of at least 0.1 grams per 10 minutes at 372 °C when using a 2.16 kg load and a 1 mm orifice, wherein the melt flow indices are measured according to the MFI TEST METHOD; and

(ii) chemically bleaching the thermally degraded PTFE by reaction with an oxidizing agent to form a white thermally degraded PTFE having an L* value of at least 98, as measured according to the CIE L*a*b* TEST METHOD.

2. The method of claim 1, wherein the white, thermally degraded PTFE has an L* value of at least 99.

3. The method of claim 1 or 2, wherein the thermally degraded PTFE has a melt flow index of at least 1 gram per 10 minutes at 372 °C when using a 2.16 kg load and a 1 mm orifice, as measured according to the MFI TEST METHOD.

4. The method according to any one of claims 1 to 3, wherein the oxidizing agent is a gas comprising at least one of oxygen, ozone, and fluorine (F2).

5. The method of claim 4, wherein chemically bleaching the thermally degraded PTFE to from the white thermally degraded PTFE comprises treating the thermally degraded PTFE with the gas at 50 to 290 °C for 5 to 10 hours, wherein the gas comprises 5 to 20 wt. % of F2.

6. The method of claim 4, wherein chemically bleaching the thermally degraded PTFE to from the white thermally degraded PTFE comprises treating the thermally degraded PTFE with the gas at 100 to 310 °C for a bleaching time of 10 to 12 hours, wherein the gas comprises oxygen or ozone.

7. The method of any one of the preceding claims, further comprising (iii) grinding or milling the thermally degraded PTFE to form a micropowder have an average particle size of no greater than 20 microns as measured according to PARTICLE SIZE METHOD.

8. The method of claim 7, wherein grinding or milling the thermally degraded PTFE occurs after chemically bleaching the thermally degraded PTFE to form the white thermally degraded PTFE.

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9. The method of any one of the preceding claims, wherein the polytetrafluoroethylene is a homopolymer of tetrafluoroethylene.

10. The method of any one of claims 1 to 8, wherein the polytetrafluoroethylene is a copolymer comprising greater than 99 weight percent of tetrafluoroethylene repeat units and less than 1 weight percent of at least one perfluorinated comonomer.

11. A white polytetrafluoroethylene micropowder made by the method of any one of the preceding claims.

12. A white thermally degraded polytetrafluoroethylene micropowder having a melt flow index of at least 0. 1 grams per 10 minutes at 372 °C when using a 2. 16 kg load and a 1 mm orifice, was measured according to the MFI TEST METHOD; an L* value of at least 98, as measured according to the CIE L*a*b* TEST METHOD; and an average particle size of no greater than 20 microns as measured according to PARTICLE SIZE METHOD.

13. The white thermally degraded polytetrafluoroethylene micropowder of claim 12, wherein the melt flow index is at least 1 gram per 10 minutes at 372 °C when using a 2. 16 kg load and a 1 mm orifice, as measured according to the MFI TEST METHOD.

14. The white thermally degraded polytetrafluoroethylene micropowder of claim 12 or 13, wherein the L* value is at least 99, as measured according to the CIE L*a*b* TEST METHOD.

15. The white thermally degraded polytetrafluoroethylene micropowder according to any one of claims 12 to 14, comprising less than 50 ppb by weight based on the weight of the PTFE of Cg to C ] 4 carboxylic acids as measured according to the FLUORINATED ACID CONTENT METHOD and fewer than 50 carboxylic groups per 10^ carbon atoms, as measured according to the DETERMINATION OF CABOXYL END GROUPS METHOD, wherein the carboxylic groups consist of COOH, COF and CONH2 groups.

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