FR2615518A1 - DISPERSION OF A FLUORINATED RESIN POWDER IN AN ORGANIC LIQUID - Google Patents

Abstract

THE INVENTION RELATES TO A DISPERSION OF A POWDER OF A FLUORINE RESIN IN AN ORGANIC LIQUID. ACCORDING TO THE INVENTION, FLUORINE RESIN IS A POLYMER WITH LESS THAN 10,000 MOLECULAR WEIGHT AND POWDER LESS THAN 2 MM OF AVERAGE SIZE. THE INVENTION APPLIES IN PARTICULAR TO THE COATING AND IMPREGNATION OF MATERIALS.

Description

The present invention relates to a dispersion of a powder of a

  fluorinated resin such as, for example, polytetrafluoroethylene in a medium

organic liquid.

  Polytetrafluoroethylene (PTFE) and other polymers containing fluorine or fluorinated resins are incomparably superior by many properties such as lubricity, non-stickiness,

  weathering and their water-repellent and oil-repellent condition.

  Fluorinated resins are used for a variety of purposes and in a variety of ways. For coating and impregnation purposes, it is possible to use a dispersion of a fluororesin powder in a suitable liquid medium, and currently, aqueous PTFE dispersions predominate in this application branch of fluorinated resins. For example, non-tacky glass fabric ribbons for electrical insulation are produced by immersing the glass fabric in a PTFE dispersion and, after drying, by baking the wet glass cloth and some self-lubricating pads are produced by impregnating a porous material, such as an alloy

  sintered, PTFE, using a PTFE dispersion.

  Conventional PTFE dispersions are usually prepared by forming PTFE into a water dispersed powder by emulsion polymerization of tetrafluoroethylene, adding a surfactant to the aqueous dispersion and concentrating the dispersion appropriately. When a conventional aqueous dispersion of PTFE is coated on or impregnated in a desired material, the surfactant contained in the dispersion adversely affects, for example, the water repellency and oil repellency of the deposited PTFE. For a complete removal of the surfactant as well as the water used as dispersion medium, the coating or impregnation treatment must be followed by a heat treatment and in the case of the formation of a continuous coating film having a good adhesion, the heat treatment must be a baking treatment. In a different aspect, the PTFE surfaces obtained by using conventional PTFE dispersions are not always completely satisfactory by certain properties, in particular in terms of lubricity, because PTFE formed by a polymerization in

  Emulsion is a high molecular weight polymer.

  The present invention relates to a dispersion of a fluororesin powder in a liquid medium, which must not contain a surfactant, which is excellent in uniformity and stability and which can produce resin surfaces. fluorinated by their smoothness and their water-repellent state and

  oil repellent on or in various materials.

  According to the invention, there is provided a dispersion of a powder of a fluorinated resin in an organic liquid, characterized in that the fluorinated resin is a polymer having a molecular weight of less than 10,000 and in that the fluororesin powder has less than 2 um * 1

of average particle size.

  We have found that fluorinated resin powders having a molecular weight of less than 10,000 can be easily and uniformly dispersed in certain organic liquids commonly used as solvents without the use of a surfactant if the powders have an average particle size of less than 10,000. 2 Hm and that the dispersions obtained have excellent stability. When the molecular weight of the dispersed fluororesin is greater than 10,000, the surfaces of fluororesin obtained using the dispersion are not always completely satisfactory by their creaminess. It is better to use a resin

  fluorinated having not more than 2000 molecular weight.

  Fluorinated resin powders having more than 2 μm mean particle size are not very good in their dispersibility in organic liquids and even if they are forcibly dispersed, do not produce very stable dispersions. It is preferable to use a fluororesin powder having not more than 1> um

average particle size.

  A conceivable way of obtaining a fine powder of a fluorinated resin of such a low molecular weight, which is usually in a state of wax,

  consists of mechanically spraying the fluororesin.

  However, in reality, by this method, it is impossible to obtain a powder having less than about

  3> m of average particle size.

  We have already invented a method for converting a high molecular weight fluorinated resin into a lower molecular weight fluorinated resin in the form of sub-micron particles, as disclosed in French Patent Application No. 87 16,657 filed December 1, 1987 which is incorporated herein by reference. According to this method, a fluororesin is heated to a temperature above its melting temperature in the presence of fluorine gas or a suitable gaseous fluoride and a hot reaction gas produced by the reaction of the fluorine with the fluorinated resin is extracted from the reactor and cooled to precipitate the fluorinated resin of reduced molecular weight contained in the reaction gas. By this method, the molecular weight of the fluororesin can be reduced to an extent of about 1000-3000, and the fluoropolymer of reduced molecular weight in the form of smaller particles than the micron is excellent in its creaminess, dispersibility and so on. In the present invention, it is preferable to use a fluororesin powder

obtained by this method.

  For a fluorinated resin dispersion according to the invention, it is preferable to use an organic liquid having a relatively low surface tension, and more particularly an organic liquid whose surface tension is not greater than

dynes / cm at room temperature.

  A fluorinated resin dispersion according to the invention is excellent in its stability, therefore the rate of release of the dispersed particles of the resin is very low in comparison with that of conventional PTFE dispersions. Even in the dispersion according to the invention, sedimentation of the resin particles occurs in a long time. However, the present invention has the merit that the deposited resin particles can easily be redispersed by stirring to resume the uniformly dispersed state with very good reproducibility. Such a redispersion can be made just before using the dispersion of the fluororesin. In conventional PTFE dispersions, it is very difficult to redisperse the particles

deposited PTFE.

  The fluororesin dispersions according to the invention are useful for coating or impregnating various materials by means of a fluororesin so as to give the treated materials lubricity, the ease of demolding, the water-repellent and oil-repellent state and / or the resistance weathering. In addition, the same dispersions can be used for the production of composite materials by dispersing the fluororesin particles in another resin or rubber, or they can be added, for example, to paints and inks to improve weather resistance and / or lack of adhesiveness or oils and greases to improve durability and extreme pressure resistance. Although it is difficult to uniformly introduce a dry fluororesin powder into another material, the objective can be easily achieved by using a non-aqueous dispersion according to the invention and subsequently removing the medium.

  organic liquid by simple evaporation.

  Various fluorinated resins can be used in the present invention. Examples of particularly suitable fluorinated resins are PTFE, copolymers of ethylene and tetrafluoroethylene (TFE), copolymers of TFE and hexafluoropropylene, copolymers of TFE and perfluoroalkoxyethylene, polychlorotrifluoroethylene, fluoride of

  polyvinylidene and polyvinyl fluoride.

  The fluororesin which is to be reduced in molecular weight by the method which has already been invented may be in any form: not only small particles and pellets but also leaves and irregularly shaped waste may be used. It is possible to use a fluorinated resin containing a filler. Before reducing the molecular weight by this method, the molecular weight of the starting fluororesin can be reduced to a certain extent by a known method, for example by thermal cracking in the presence of a fluorinating agent or by radiation cracking, so that to improve the reaction rate with fluorine and to increase the yield of an insufficiently low molecular weight fluorinated resin powder. As the fluorine source gas, gaseous fluorine, nitrogen trifluoride gas or chlorine trifluoride gas is usually used with a diluent gas such as nitrogen. The amount of the fluorine source gas to be brought into contact with the fluororesin is variable depending on the type and the physical form of the fluororesin but it is necessary that at least 0.01 part by weight of the fluorine is present in the reactor. parts by weight of the fluorinated starting resin are kept heated. The presence of an excessively large amount of fluorine will cause an excessive reduction in the molecular weight of the fluororesin. Usually, it is suitable that up to 10 parts by weight of fluorine coexist with 100 parts by weight of the fluororesin

Processing.

  By carrying out the molecular weight reduction reaction, the starting fluororesin is kept heated to a temperature not lower than its melting temperature and not more than 600 C and the gaseous phase containing the gas as Fluorine source is maintained at a temperature between 200 and 550 C and preferably slightly lower than the temperature of the fluororesin. Under these conditions, the fluorinated resin of reduced molecular weight is easily vaporized, and the vaporized fluororesin hardly breaks down. The reaction can be carried out in a reactor of any type as long as the reactor is suitable for a gas-solid contact reaction. For example, it is appropriate to use a forced recirculation type reactor having numerous shelves or trays in a multi-bridge arrangement or a fluidized bed type reactor. The reaction rate can be improved by raising the gas pressure, although the reaction proceeds at a satisfactory pace in the reaction.

  practical, even at atmospheric pressure.

  In order to obtain a low molecular weight fluorinated resin in good yield and in the form of a fine powder, the hot reaction gas produced by the above gas-solid contact reaction is cooled to a temperature below 100 ° C, thereby precipitating the reaction. fluorinated resin of reduced molecular weight. For this purpose, the reactor is connected to a cooler which is connected to a gas-solid separator or solids collector. It is possible to use a single chamber at a time as a cooler and as a collector. The cooling fluid can be air, water, a refrigerant or a liquefied gas. The particle size of the low molecular weight precipitated fluorinated resin can be controlled over a fairly wide range, for example between 0.1 μm and around 10 μm.

  controlling the cooling rate of the reaction gas

  nel: the particle size decreases as the cooling rate increases. The separator or collector is, for example, of the settling chamber type using the gravitational force, the collision plate type or the guide plate using the inertial force or the cyclone type or the filter type.

using centrifugal force.

  The low molecular weight fluorinated resins obtained by the method described above usually have the form of very fine particles which are spherical or lamellar. The fluorinated resins of reduced molecular weight are very stable thanks to the existence of the -CF3 group at the ends of the molecular chain because the molecular weight reduction decomposition reaction was carried out in the presence of a radical.

very active fluorine.

  In the present invention, an organic liquid compound is used for particle dispersion

  fine fluorinated resin of low molecular weight.

  In choosing the organic liquid, it is important to consider not only the specific gravity but also the surface tension of the liquid. Indeed, it is important to use an organic liquid having a relatively low surface tension to thoroughly wet and disperse the fine particles of the fluororesin

  without using any surfactant.

  The preferred organic liquids are 1,1,2-trichloro-1,2,2-trifluoroethane, the surface tension of which is 19.0 dynes / cm at 25 ° C., n-heptane having a surface tension of 20.3. dynes / cm at 20 ° C. and n-hexane whose surface tension is 18.4 dynes / cm at 20 ° C. When one of these preferred liquids is used, a dispersion can easily be obtained

  uniform and very stable of a fluororesin powder.

  However, the same fluororesin powder has a lower wettability and dispersibility when using an organic liquid compound having a higher surface tension such as n-butyl acetate (25.2 dynes / cm at 20 C), the methyl isobutyl nitrile (25.4 dynes / cm at 25 ° C.), carbon tetrachloride (26.8 dynes / cm at 20 ° C.) or trichlorethylene (29.0 dynes / cm at 30 ° C.). Indeed, a very good dispersion is obtained by using an organic liquid whose surface tension is not greater than dynes / cm at room temperature. With this condition, the most suitable liquid is chosen considering the boiling point and the vapor pressure according to the use

  desired dispersion of the fluorinated resin.

  The 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 in an objective material without performing heat treatment. The use of a liquid having a low surface tension is favorable not only for well dispersing the fluororesin particles but also for infiltrating the fluororesin dispersion in the deep regions.

  or narrow of a matter as objective.

  When the organic liquid medium is a

  fluorinated compound, such as, for example, trichlorotrifluoro-

  ethane, it is likely that a portion of the dispersed fluororesin will dissolve in the liquid medium. A fluororesin coating film formed using such a dispersion is superior in uniformity and strength of adhesion to the coated surface. This is probably due to the fact that the dissolved fluorine resin

acts as if she were a binder.

  In a fluororesin dispersion according to the invention, it is appropriate that the content of the fluororesin powder is between 0.1 and 20% by weight. If the content of the resin powder is less than 0.1%, the dispersion can hardly be considered as an effective application of the fluororesin. If the content of the powder is greater than 20% by weight, the dispersion becomes too thick and grease-like and is therefore unsuitable for use as a dispersion. In this invention, a preferred range of the content of the fluororesin powder in the dispersion is between

and 10% by weight.

  The invention will be better illustrated by the

  non-limiting examples that follow.

Example 1

  A reactor made of nickel was charged with -

  cubic 5 mm pellets of PTFE having a molecular weight of about 8500, and a mixture of 10% fluorine gas and 90% nitrogen gas was continuously introduced into the reactor while the temperature in the reactor was maintained at approximately 500 ° C. A reaction gas produced by the reaction of fluorine with the PTFE pellets was continuously extracted from the reactor at a suitable rate and introduced into a cooler, where the gas was cooled to about 40 ° C. for the precipitation of PTFE from reduced molecular weight. By this operation, a white and very fine powder of PTFE was obtained. The PTFE powder had an average particle size of 0.5 μm and had a melting point of 265 C. The molecular weight of this PTFE was calculated to be 1500 from the following relationship between the melting point ( Tm) and the weight m

  molecular (MW) shown in US Patent No. 3,067,262.

MW = -

  685 El / Tm (_K) - 1/60 To prepare a PTFE dispersion according to the invention, 10 g of the PTFE powder of reduced molecular weight was placed in a 200 ml beaker and

  slowly added 90 g of 1,1,2-trichloro-1,2,2-trifluoro-

  ethane (R-113) while stirring continuously the resulting mixture. The PTFE particles exhibited very good wettability and dispersibility, so a stable and uniform dispersion was easily obtained. In a capped test tube, the dispersion was allowed to stand for more than 3 hours, but decantation of the PTFE particles was imperceptible. At the next day most of the particles in the test tube had settled to the bottom but a uniform dispersion could be reformed

by slight agitation.

Evaluation test 1.

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

  dispersion on the aluminum plate.

  For comparison, a commercially available aqueous dispersion of PTFE was applied to the aluminum plate by immersion (Sample A ') or by spraying (Sample B'). In each case, the aluminum plate coated with the aqueous dispersion was heat-treated at 400 ° C to thereby form a solid coating film. The coating film on each sample was subjected to a water contact angle measurement as an indication of the water repellency and, for the lubricity evaluation, to a friction test using a medium. friction test type Bowden-Leben. In the friction test, the PTFE film on each sample was rubbed against an aluminum surface polished with No. 400 sandpaper with a steel ball having a diameter of 8 mm. The friction test load was 1000 g, and the friction rate was 0.14 m / min. The results are shown

in table 1.

Table 1.

  Friction Contact Coefficient Angle (degrees) Sample A (Example 1) 143 0.06 Sample B (Example 1) 143 0.07 Sample A '(comparison) 105 0.16 Sample B' (comparison) 103 0.18 Plate of aluminum (uncoated) 77 - 0.27

Evaluation test 2.

  To the PTFE dispersion prepared in Example 1 was alternately added a PVC resin powder (polyvinyl chloride) and an ABS resin powder (acrylonitrile butadiene styrene). In each case, the addition amount of the resin powder was changed so that the PTFE particles in the dispersion reached 5, 10 and 20% by weight of the added resin powder. After mixing well, the liquid medium was evaporated from the dispersion to thereby obtain a powder of a resin mixture. In each case, the powder of the resulting resin mixture was shaped by heating into a sheet. In the resin sheets thus prepared, the fine particles of low molecular weight PTFE were uniformly dispersed in the PVC or ABS matrix. On these resin sheets, measurements of water contact angle (PVC resin sheets only) and coefficient of friction

  were as shown in Table 2.

TABLE 2.

  Resin Quantity Angle of Particle Size Coefficient PTFE Friction Contact (degrees) (% by weight)

0 87 0.18

108 0.09

PVC

118 0.07

20 to 118 0.06

o - 0.31

- 0.24

ABS 10 - 0.22

- 0.20

Examples 2-4.

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

  PTFE powder occupied 10% by weight of the dispersion.

  Each of the PTFE dispersions of Examples 2-4 was allowed to stand in a capped test piece for more than three hours, but decantation of the PTFE particles was imperceptible. By the next day, most of the PTFE particles in each test tube had settled to the bottom. In the case of Example 4, a uniform dispersion could easily be reformed by slight stirring, but in Examples 2 and 3, more intense stirring was required to restore the

uniform dispersion.

  For comparison, the same PTFE powder was alternately dispersed in tetrachloride

  carbon, trichloroethane, tetrahydro-

  furan, in methyl isobutyl ketone, in n-butyl acetate, in toluene and in n-butanol. In each of these cases, the wettability and dispersibility of the PTFE particles was judged to be insufficient, because the particles could not be uniformly dispersed in the organic liquid by stirring and the adhesion of the particles to the the test piece was considerable, even after good agitation.

Comparison Examples 1 and 2.

  As a comparison example 1, a PTFE powder having an average particle size of 3 μm was dispersed in 90 g of R-113. PTFE had a

  melting point of 3150C and a molecular weight of 8500.

  For Comparative Example 2, 10 g of another PTFE powder having an average particle size of 7 "m was dispersed in 90 g of R-113 This PTFE had a melting point of 317 C and a molecular weight of

10000.

  In both Comparison Examples 1 and 2, the PTFE particles exhibited good wettability and could be uniformly dispersed in the organic liquid. Each of these dispersions was allowed to stand in a capped test tube to evaluate the dispersibility of the PTFE particles and the stability of the dispersion. In both comparison examples 1 and 2, partial sedimentation of the particles became noticeable within 3 hours. After nearly complete sedimentation of the particles, attempts were made to redisperse the particles in the liquid by intense agitation, but uniform dispersion was not possible.

be restored in no case.

Claims (9)

  1. Dispersion of a fluororesin powder in an organic liquid, characterized in that the fluororesin is a polymer having a molecular weight of less than 10,000 and in that said powder has less than 2 μm of average particle size.   2. Dispersion according to claim 1, characterized in that the surface tension of the organic liquid is not greater than 20 dynes / cm at ambient temperature.   3. Dispersion according to claim 2, characterized in that the organic liquid is chosen   in the group consisting of 1,1,2-trichloro-1,2,2-tri-   fluoroethane, n-heptane and n-hexane.   4. Dispersion according to claim 2, characterized in that the molecular weight of the resin   fluorinated is not greater than 2000.   5. Dispersion according to claim 4, characterized in that the powder has no more than 17 μm of mean granulometry.   Dispersion according to Claim 1, characterized in that the content of the powder is   between 0.1 and 20% by weight. -   7. Dispersion according to claim 6, characterized in that the content of the powder is between 5 and 10%.   8. Dispersion according to claim 1, characterized in that the fluorinated resin is polytetrafluoroethylene. 9. Dispersion according to claim 1, characterized in that the fluororesin is selected from the group consisting of polychlorotrifluoroethylene, polyvinylidene fluoride, polyvinyl fluoride, copolymers of ethylene and tetrafluoroethylene,   copolymers of tetrafluoroethylene and hexafluoro-   propylene and copolymers of tetrafluoroethylene and perfluoroalkoxyethylene.