GB2205576A - Dispersion of fluororesin powder in organic liquid. - Google Patents

Description

L- - 2205576 DISPERSION OF FLUORORESIN POWDER IN ORGANIC LIQUID This

invention relates to a dispersion of a powder of a fluororesin such as, for example, polytetrafluoro ethylene in an organic liquid medium.

Polytetrafluoroethyelene (PTFE) and other fluorine- containing polymers or fluororesins are incomparably superior in many properties such as lubricity, untacki ness, weatherability and water and oil repellency.

Fluororesins are used for various purposes and in various manners. For coating and impregnating purposes it is possible to use a dispersion of a fluororesin powder in a suitable liquid medium, and at present aqueous dispersions of PTFE predominate in this branch of applications of fluororesins. For example, glass cloth base untacky tapes for electrical insulation are produced by immersing glass cloth in a PTFE dispersion and, after drying, baking the wetted glass cloth, and some oilless bearingas are produced by impregnating a porous material such as a sintered alloy with PTFE using a PTFE dispersion.

Conventional PTFE dispersions are usually prepared by forming PTFE as a powder dispersed in water by emulsion polymerization of tetrafluoroethylene, adding a surfactant to.the aqueous dispersion and suitably concentrating the dispersion. When a conventional aqueous dispersion of PTFE is coated on or impregnated into a desired material the surfactant contained in the dispersion adversely affects, for example, water and oil repellency of the deposited PTFE. For complete removal of the surfactant as well as water used as the dispers ing medium the coating or impregnating treatment has to be followed by a heat treatment, and in the case of forming a continuous coating film good in tightness of adhesion the heat treatment needs to be a baking treat ment. From a different aspect, PTFE surfaces provided by using conventional PTFE dispersions are not always fully satisfactory in some properties, particularly in lubricity since PTFE formed by emulsion polymerization is a high molecular weight polymer.

It is an object of the present invention to provide a dispersion of a fluororesin powder in a liquid medium, which does not need to contain any surfactant, is excellent in uniformity and stability and can provide fluororesin surfaces excellent in lubricity and water and oil repellency on or in various materials.

According to the invention there is provided a dispersion of a powder of a fluororesin in an organic liquid, characterized in that the fluororesin is a polymer lower than 10000 in molecular weight and that the fluororesin powder is smaller than 2 Am in mean particle size.

We have found that powders of fluororesins lower than 10000 in molecular weight can easily and uniformly be dispersed in some organic liquids commonly used as solvents without using any surfactant if the powders are smaller than 2 14m in mean particle size, and that the obtained dispersions are excellent in stability. When the molecular weight of the dispersed fluororesin is more than 10000, the fluororesin surfaces provided by using the dispersion are not always fully satisfactory in lubricity. It is preferable to use a fluororesin not higher than 2000 in molecular weight. Fluororesin powders larger than 2..,Am in mean particle size are not very good in dispersibility in organic liquids and, even though forcibly dispersed, do not provide very stable dispersions. It is preferable to use a fluororesin powder not larger than 114m in mean particle size.

A conceivable way to obtain a fine powder of a low molecular weight fluororesin, which is usually in a wax like state, is to mechanically pulverize the fluororesin.

Actually, however, by this method it is impracticable to obtain a powder smaller than about 3)(m in mean particle size.

We have already invented a method of converting an ordinary fluororesin of high molecular weight into a lower molecular weight fluororesin in the form of sub micron particles as disclosed in our earlier U.K. patent application No. 8727400 filed on 23rd November, 1987.

According to that method, a fluororesin is heated to a temperature above its melting temperature in the presence of fluorine gas or a suitable fluoride gas, and a hot reaction gas produced by reaction of fluorine with the fluororesin is extracted from the reactor and cooled to precipitate the molecular weight reduced fluororesin contained in the reaction gas. By this method the molecular weight of the fluororesin can be reduced to the extent of about 1000-3000, and the molecular weight reduced fluoropolymer in the form of submicron particles is excellent in lubricity, dispersibility, etc. In the present invention it is preferred to use a fluororesin powder obtained by this method.

For a fluororesin dispersion according to the invention it is preferred to use an organic liquid relatively low in surface tension, and more specifically an organic liquid whose surface tension is not higher than 20 dynIcm at room temperature.

A fluororesin dispersion according to the invention is excellent in stability so that the settling velocity of the dispersed resin particles is very low compared to that in conventional PTFE dispersions. Even in the dispersion according to the invention sedimentation of the resin particles occurs in a long time. However, it is a merit of the invention that the settled resin particles can easily be re-dispersed by stirring to resume the uniformly dispersed state with very good reproducibility. Such re-dispersing can be made just before using the fluororesin dispersion. In conventional PTFE dispersions it is very difficult to re-disperse once settled particles of PTFE.

Fluororesin dispersions according to the invention are useful for coating or impregnating various materials with a fluroresin to thereby afford the treated materials with lubricity, releasability, water and oil repellency and/or weatherability. Besides, the same dispersions can be used for producing composite materials by dispersing fluororesin particles in another resin or rubber, or can be added, for example, to paints and inks to improve weatherability and/or untackiness or to oils and greases to improve durability and extreme pressure resistance. Although it is difficult to uniformly introduce a fluororesin powder in dry state into another material, the-object can easily be accomplished by using a nonaqueous dispersion according to the invention and subsequently removing the organic liquid medium by simple evaporation.

Various fluororesins can be used in the present invention. Examples of particularly suitable fluoro resins are PTFE, copolymers of ethylene and tetrafluoro ethylene (TFE), copolymers of TFE and hexafluoro propylene, copolymers of TFE and a perfluoroalkoxy- ethylene, polychlorotrifluoroethylene, polyvinylidene fluoride and polyvinyl fluoride.

The fluororesin which is to be reduced in molecular weight by the method we have already invented may be in any form: not only small particles and pellets but also sheets and irregularly shaped scraps can be used. It is permissible to use a fluororesin containing a filler.

In advance of reducing molecular weight by our method, the molecular weight of the starting fluororesin may be reduced to some extent by a known method, such as thermal cracking in the presence of a fluorinating agent or radiation cracking, with a view to enhancing the rate of reaction with fluorine and increasing the yield of a desired low molecular weight fluororesin powder.

As the fluorine source gas, usually fluorine gas, nitrogen trifluoride gas or chlorine trifluoride gas 1S used together with a diluent gas such as nitrogen. The quantity of the fluorine source gas to be brought into contact with the fluororesin is variable depending on the kind and physical form of the fluororesin, but it is required that at least 0.01 part by weight of fluorine should be present in the reactor in which 100 parts by weight of the starting fluororesin is kept heated. The presence of an excessively large amount of fluorine will 2S cause an excessive reduction of the molecular weight of the fluororesin. Usually it is suitable that up to 10 parts by weight of fluorine coexists with 100 parts by weight of the fluororesin under treatment.

In carrying out the molecular weight reducing reaction the starting fluororesin is kept heated at a temperature not lower than its melting temperature and not higher than 600 0 C, and the gas phase containing the fluorine source gas is maintained at a temperature in the range from 200 to 550 0 C and preferably slightly lower than the temperature of the fluororesin. Under such conditions the-molecular weight reduced fluororesin readily vaporizes, and the vaporized fluororesin hardly decomposes. The reaction can be carried out in a reactor of any type insofar as the reactor is suited to gas-solid contact reaction. For example, it is suitable to use a reactor of a forced gas recirculation type having many shelves or trays in multi-deck arrangement or a reactor of a fluidized bed type. The rate of reaction can be enhanced by raising the gas pressure, though the reaction proceeds at a practically satis factory rate even under the atmospheric pressure.

To obtain a low molecular weight fluororesin at good yield and in a fine powder form, the hot reaction gas produced by the above gas-solid contact reaction is cooled to a temperature below 100 0 C to thereby precipitate the molecular weight reduced fluororesin.

For this purpose the reactor is connected with a cooler, which is connected with a gas-solid separator or solid matter collector. It is possible to use a single chamber both as a cooler and as a.collector. The cooling medium may be air, water, refrigerant or liquefied gas. The particle size of the precipitated low molecular weight fluororesin can be controlled over a fairly wide range such as from about 0.1 Am to about 10)Lkm by controlling the rate.of cooling the reaction gas: the particle size becomes smaller as the rate of cooling is enhanced. The separator or collector is, for example, of a settling chamber type using gravitational force, of a collision plate or guide plate type using inertial force or of a cyclon type or bug filter type using centrifugal force.

Low molecular weight fluororesins obtained by the above described method are usually in the form of very fine particles which are spherical or flaky. The molecular weight reduced fluororesins are very stable because of existence of -CF 3 group at the ends of the molecular chain as a result of carrying out the molecular weight reducing decomposition reaction in the presence of highly active fluorine radical.

In the present invention an organic liquid compound is used for dispersing therein fine particles of a low molecular weight fluororesin. In selecting the organic liquid it is important to consider not only specific gravity but also surface tension of the liquid. That is, it is important to use an organic liquid relatively low in surface tension for well wetting and dispersing the fine particles of the fluororesin without using any surfactant.

Preferred organic liquids are 1,1,2-trichloro- 1,2,2-trifluoroethane whose surface tension is 19.0 dyn/cm at 250C, n-heptanq whose surface tension is 20.3 dyn/cm at 20 0 C and n-hexane whose surface tension is 18.4 dyn/cm at 200C. When any of these preferred liquids is used a uniform and very stable dispersion of a fluororesin powder can easily be obtained. However, the same fluororesin powder exhibits inferior wetta bility and dispersibility when use is made of an organic liquid compound higher in surface tension, such as n-butyl acetate (25.2 dyn/cm at 20 0 C), methyl isobutyl nitrile (25.4 dyn/cm at 25 0 C), carbon tetrachloride (26.8 dyn/cm at 20 0 C) or trichloroethylene (29.0 dyn/cm at 300C). That is, a very good dispersion is obtained by using an organic liquid whose surface tension is not higher than 20 dyn/cm at room temperature. On this condition, the most suitable liquid is selected with consideration of boiling point and vapor pressure according to the intended use of the fluororesin dispersion. Use of a liquid having a relatively high vapor pressure is favorable for rapid evaporation of the liquid from the fluororesin dispersion coated on or impregnated into an objective material without making heat treatment. Use of a liquid low in surface tension is favorable not only for well dispersing fluororesin particles but also for infiltrating the fluororesin dispersion into deep or narrow regions of an objective material.

When the organic liquid medium is a fluorinated compound such as, for example, trichlorotrifluoroethane it is likely that a portion of the dispersed fluororesin dissolves in the liquid medium. A fluororesin coating film formed by using such a dispersion is superior in uniformity and strength of adhesion to the coated surface. This is probably because the dissolved fluoro resin acts as if it were a binder.

In a fluororesin dispersion according to the invention it is suitable that the content of the fluoro resin powder is from 0.1 to 20 wt%. If the content of the resin powder is less than 0.1 wt% the dispersion can hardly be regarded as an effective application of the fluororesin. If the content of the powder is more than wt% the dispersion becomes too thick and assumes a grease-like state and, hence, is unsuited for use as a dispersion. In this invention a preferred range of the content of the fluororesin powder in the dispersion is from 5 to 10 wt%.

The invention is further illustrated by the following nonlimitative examples.

EXAMPLE 1

A reactor made of nickel was charged with 5-mm cubic pellets of PTFE having a molecular weight of about 8500, and a mixture of 10% of fluorine gas and 90% of nitrogen gas was continuously introduced into the reactor while the temperature in the reactor was maintained at about 500 0 C. A reaction gas produced by reaction of fluorine with the PTFE pellets was continuously extracted from the reactor at a suitable rate and introduced into a cooler, wherein the gas was cooled to about 40 0 C for precipitation of PTFE of reduced molecular weight. By this operation a white and very fine powder of PTFE was obtained. The PTFE powder was 0.5Am in mean particle'size and had a melting point of 265 0 C. The molecular weight of this PTFR was calculated to be 1500 from the following relatinship between melting point (T m) and molecular weight (MW), shown in USP 3,067,262.

MW = 200 68SE1/T m ( 0 K) - 116001 To prepare a PTFE dispersion according to the invention, 10 g of the powder of the moleculare weight reduced PTFE was put into a 200-m1 beaker, and 90 g of 1,1,2-trichloro-1,2,2-trifluoroethane (R-113) was slowly added while continuously stirring the resultant mixture.

The PTFE particles exhibited very good wettability and dispersibility so that a uniform and stable dispersion was easily obtained. In a capped test tube the dispersion was left standing for more than 3 hr, but settling of the PTFE particles was imperceptible. By the next day most of the particles in the test tube had settled to the bottom, but a uniform dispersion could be resumed by light stirring.

EVALUATION TEST 1 The PTFE dispersion prepared in Example 1 was applied to an aluminum plate by immersion method (Sample A) or by spraying method (Sample B). In either case a solid coating film was formed by simply drying the dispersion on the aluminum plate.

For comparison, a commercial aqueous dispersion of PTFE was applied to the aluminum plate by immersion (Sample A') or by spraying (Sample BI). In either case the aluminum plate coated with the aqueous dispersion was heat-treated at 400 0 C to thereby form a solid coating film.

The coating film on every sample was subjected to measurement of angle of contact with water as an indication of water repellency, and, for evaluation of lubricity, to a friction.test using a friction tester of the Bowden-Leben type. In the friction test the PTFE film on each sample was rubbed against an aluminum surface polished with #400 sand paper with a steel ball having a diameter of 8 mm. The friction testing load Z5 was 1000 g, and the friction speed was 0.14 m/min. The results are shown in Table 1.

i TABLE I

Angle of Coefficient Contact of Friction (degree) Sample A (Ex. 1) 143 0.06 Sample B (Ex. 1) 143 0.07 Sample A' (comparative) 105 0.16 Sample BI (comparative) 103 0.18 Aluminum plate (uncoated) 77 0.27 EVALUATION TEST 2 To the PTFE dispersion prepared in Example 1, a PVC resin powder and an ABS resin powder were alternately added. In either case the amount of addition of the resin powder was varied such that the PTFE particles in the dispersion amounted to 5, 10 and 20 wt% of the added resin powder. After well mixing, the liquid medium of the dispersion was evaporated to thereby obtain a resin mixture powder. In every case the obtained resin mixture powder was shaped under heating into a sheet.

In the thus prepared resin sheets the fine particles of low molecular weight PTFE were uniformly dispersed in the matrix of PVC or ABS. On these resin sheets, measurements of angle of contact with water (PVC resin sheets only) and coefficient of friction were as shown in Table 2.

TABLE 2

Matrix Amount of PTFE Angle of Coefficient Resin Particles (wt%) Contact of Friction (degree) 0 87 0.18 108 0.09 PVC 10. 118 0.07 118 0.06 0 - 0.31 - 0.24 ABS - 0.22 - 0.20 EXAMPLES 2-4

The low molecular weight PTFE powder prepared in Example 1 was dispersed alternately in n-heptane, in n-hexane and in a mixture (1:1 by volume) of acetone and R-113 as Examples 2, 3 and 4, respectively. In every case the PTFE particles exhibited good wettability and dispersibility so that a uniform and stable dispersion was easily obtained, and in every case the PTFE powder occupied 10 wt% of the dispersion.

Each of the PTFE dispersions of Examples 2-4 was left standing in a capped test tube for more than 3 hr, but settling of the PTFE particles was imperceptible.

By the next day most of the PTFE particles in each test tube settled to the bottom. In the case of Example 4 a uniform dispersion could easily be resumed by light stirring, but in Examples 2 and 3 more intense stirring was needed to resume uniform dispersion.

For comparison, the same PTFE powder was alternately dispersed in carbon tetrachloride, in trichloroethan, in tetrahydrofuran, in methyl isobutyl ketone, in n-butyl acetate, in toluene and in n-butanol.

In any of these cases the wettability and dispersibility of the PTFE particles were judged to be insufficient, because the particles could not uniformly be dispersed in the organic liquid by stirring and adhesion of the particles to the test tube wall was considerable even after well stirring. - COMPARATIVE EXAMPLES 1 AND 2 As Comparative Example 1, 10 g of a PTFE powder having a mean particle size of 3 gm was dispersed in g of R-113. The PTFE had a melting point of 3150C and molecular weight of 8500. As Comparative Example 2, g of anothe PTFE powder having a mean particle size of 7 14m was dispersed in 90 g of R-113. This PTFE had a melting point of 317 0 C and molecular weight of 10000.

In both Comparative Examples 1 and 2 the PTFE particles exhibited good wettability and could be uniformly dispersed in the orgainc liquid. Each of these dispersions was left standing in a capped test tube to evaluate dispersibility of the PTFE particles and stability. of the dispersion. In both Comparative Examples 1 and 2, partial sedimentation of the particles became perceptible within 3 hr. After nearly complete sedimentation of the particles it was tried to re disperse the particles in the liquid by intense stirring, but uniform dispersion could not be resumed in either case.

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Claims (1)

1. A dispersion of a powder of a fluororesin in an organic liquid, characterized in that the fluororesin is a polymer lower than 10000 in molecular weight and that said powder is smaller than 2,Am in mean particle size.

2. A fluororesin dispersion according to Claim 1, wherein surface tension of said organic liquid is not higher than 20 dynIcm at room temperature.

3. A fluororesin dispersion according to Claim 1 or 2, wherein said organic liquid is selected from 1,1,2 trichloro-1,2,2-trifluoroethane, n-heptane and n-hexane.

4. A fluororesin dispersion according to Claim 1, 2 or 3, wherein the molecular weight of said fluororesin is not higher than 2000.

5. A fluororesin dispersion according to any of Claims 1 to 4, wherein said powder is not larger than 1,4m in mean particle size.

6. A fluororesin dispersion according to any of Claims 1 to 5, wherein the content of said powder is from 0.1 to 20 wt%.

7. A fluororesin dispersion according to Claim 6, wherein the content of said powder is in the range from to 10 wt%.

8. A fluororesin dispersion according to any of the preceding claims, wherein said fluororesin is poly tetrafluoroethylene.

9. A fluororesin dispersion according to any of the preceding claims, wherein said fluororesin is selected from polychlorotrifluoroethylene, polyvinylidene fluoride, polyvinyl fluoride, copolymers of ethylene and tetrafluoroethylene, copolymers of tetrafluoroethylene and hexafluoropropylene and copolymers of tetrafluoro ethylene and perfluoroalkoxyethylene.

10. A dispersion of a powder of a fluororesin in an orgainc liquid, substantially as hereinbefore described in any of Examples 1 to 4.

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Pubisshed 198S r TI.e Patent.a,,E- Ho-:.se. Fe, 71 F_f- Holbsn'.. London WC1R 4TP. Farther copies maybe obtained from, The Patent Office, Sn2es Branch. St Mary Cray, Orpington. Kent BA5 3RD Printed by MWt,-plex techniques Itd. S, Mary Cray. Kent. Clon. 1 87.