Method for Sizing and Hydrolyzing Polytetrafluoroethylene Fabrics, Fibers, Yarns, or Threads

The present invention is a method for coating polytetrafluoroethylene fabrics, fibers, yarns, or threads such that when in the form of cloth, the cloth will be sized. In particular, the invention is a method for coating polytetrafluoroethylene fabrics, fibers, yarns, or threads using a perfluorinated polymer solution or dispersion.

Polytetrafluoroethylene fabrics are woven from strands of polytetrafluoroethylene. Polytetrafluoroethylene is commonly known as Teflon and is a registered trademark of E. I. DuPont de Nemours & Company, Inc.

Polytetrafluoroethylene fabrics, fibers, yarns, or threads are available in many varieties from a variety of distributors. Typical of the polytetrafluoroethylene is a fabric sold by Stern and Stern Textiles, Incorporated, New York, N.Y., called T41-30. It is a leno weave cloth with 45.times.21 ends/inch and has a thickness of 0.010 inch. It has a weight of 4.35 ounces per square yard.

Polytetrafluoroethylene fabrics, fibers, yarns, or threads are used for a variety of purposes including such things as filters, screens, reinforcement, packing, insulation, liners and gasket materials. They are also used as supports for fluoropolymer ion exchange active films. Such films are commonly used as ion exchange membranes in electrolytic cells.

When used as filters, it is important for the polytetrafluoroethylene filters to be water wettable. Otherwise, they will not operate efficiently because of the large pressure drop when aqueous materials are being filtered. However, it is not easy to make polytetrafluoroethylene water wettable. The present invention, makes a water wettable polytetrafluoroethylene.

Polytetrafluoroethylene fabrics are limp and exhibit low friction, thread to thread. This causes fabrics made from such materials to become distorted under normal handling and causes holes to appear without breaking threads.

The prior art has attempted to coat polytetrafluoroethylene fabrics, fibers, yarns, or threads by using mechanical lamination equipment. Other prior art has attempted to attach polytetrafluoroethylene fabrics, fibers, yarns, or threads to ion exchange membrane films using heat treatment and extraction procedures rather than stabilizing the cloth itself (see U.S. Pat. No. 4,272,560).

Other known relevant art includes: U.S. Pat. Nos. 3,770,567; 3,925,135; 4,272,560; 4,324,606; 4,399,183; 4,341,605; and 4,437,951.

Burrell [J. Paint Tech., Volume 41, page 495 (1969)]predicts a non-crystalline polymer will dissolve in a solvent of similar solubility parameter without chemical similarity, association, or any intermolecular force. However, he fails to mention anything about the solubility of polymers demonstrating crystallinity.

It would be highly desirable to be able to coat polytetrafluoroethylene fabrics, fibers, yarns, or threads such that when they are woven into fabrics, the fabrics will be sized. The present invention provides such a method.

The invention is a method for coating polytetrafluoroethylene fabrics, fibers, yarns, or threads comprising:

(a) contacting polytetrafluoroethylene fabrics, fibers, yarns, or threads with a sizing composition of a perfluorinated polymer containing sites convertible to ion exchange groups and a treating agent having:

a boiling point less than about 110.degree. C.;

a density of from about 1.55 to about 2.97 grams per cubic centimeter; and

a solubility parameter of from greater than about 7.1 to about 8.2 hildebrands;

(b) removing the treating agent from the sizing composition, thereby depositing the perfluorinated polymer onto the surface of the polytetrafluoroethylene; and

(c) hydrolyzing the perfluorinated polymer to make it water wettable.

Particularly preferred as a treating agent is a compound represented by the general formula:

wherein:

X is selected from the group consisting of --F, --Cl, --Br, and --I;

X' is selected from the group consisting of --Cl, --Br, and --I;

Y and Z are independently selected from the group consisting of --H, --F, --Cl, --Br, --I and --R';

R' is selected from the group of perfluoroalkyl radicals and chloroperfluoroalkyl radicals having from 1 to 6 carbon atoms.

The most preferred treating agent is 1,2-dibromotetrafluoroethane.

"Sizing composition" is a composition containing a treating agent and a perfluorinated polymer containing sites convertible to ion exchange groups, wherein the polymer is at least partially dissolved in the treating agent.

Polytetrafluoroethylene fabrics are limp cloths before sizing. The threads usually tend to slide about during handling and are, thus, very difficult to handle without changing the shape of the fabric. The present invention provides a method for sizing polytetrafluoroethylene fabrics, fibers, yarns, or threads. Polytetrafluoroethylene in the present invention is treated as fabrics, fibers, yarns, or threads, and later woven into fabrics.

Polytetrafluoroethylene fabrics, fibers, yarns, or threads suitable for use in the present invention are those commercially available from a variety of producers. The denier of the fabric, fiber, or thread is not critical to the successful operation of the present process. Likewise, the overall physical dimensions of the fabrics, fibers, yarns, or threads are not critical.

Under the procedures of the present invention polytetrafluoroethylene fabrics, fibers, yarns, or threads are treated with a solution or dispersion of a treating agent and a perfluorinated polymer.

Non-ionic forms of perfluorinated polymers described in the following patents are suitable for use in the present invention: U.S. Pat. Nos. 3,282,875; 3,909,378; 4,025,405; 4,065,366; 4,116,888; 4,123,336; 4,126,588; 4,151,052; 4,176,215; 4,178,218; 4,192,725; 4,209,635; 4,212,713; 4,251,333; 4,270,996; 4,329,435; 4,330,654; 4,337,137; 4,337,211; 4,340,680; 4,357,218; 4,358,412; 4,358,545; 4,417,969; 4,462,877; 4,470,889; and 4,478,695; European Patent Application No. 0,027,009. Such polymers have equivalent weights from about 500 to about 2000.

Particularly preferred are copolymers of monomer I with monomer II (as defined below). Optionally, a third type of monomer may be copolymerized with I and II.

The first type of monomer is represented by the general formula:

where:

Z and Z' are independently selected from the group consisting of --H, --Cl, --F, or --CF.sub.3.

The second monomer consists of one or more monomers selected from compounds represented by the general formula:

where:

Y is selected from the group consisting of --SO.sub.2 Z, --CN, --COZ, and --C(R.sup.3 f)(R.sup.4 f)OH;

Z is I, --Br, --Cl, --F, --OR, or --NR.sub.1 R.sub.2 ;

R is a branched or linear alkyl radical having from 1 to about 10 carbon atoms or an aryl radical;

R.sup.3 f and R.sup.4 f are independently selected from the group consisting of perfluoroalkyl radicals having from 1 to about 10 carbon atoms;

R.sub.1 and R.sub.2 are independently selected from the group consisting of --H, a branched or linear alkyl radical having from 1 to about 10 carbon atoms or an aryl radical;

a is 0-6;

b is 0-6;

c is 0or 1;

provided a+b+c is not equal to 0;

X is --Cl, --Br, --F or mixtures thereof when n>1;

n is 0 to 6; and

R.sub.f and R.sub.f' are independently selected from the group consisting of --F, --Cl, perfluoroalkyl radicals having from 1 to about 10 carbon atoms and fluorochloroalkyl radicals having from 1 to about 10 carbon atoms.

Particularly preferred is when Y is --SO.sub.2 F or --COOCH.sub.3; n is 0 or 1; R.sub.f and R.sub.f' are --F; X is --Cl or --F; and a+b+c is 2 or 3.

The third and optional monomer suitable is one or more monomers selected from the compounds represented by the general formula:

where:

Y' is --F, --Cl or --Br;

a' and b' are independently 0-3;

c' is 0 or 1;

provided a'+b'+c' is not equal to 0;

n' is 0-6;

R.sub.f and R'.sub.f are independently selected from the group consisting of --Br, --Cl, --F, perfluoroalkyl radicals having from about 1 to about 10 carbon atoms, and chloroperfluoroalkyl radicals having from about 1 to about 10 carbon atoms; and

X' is --F, --Cl, --Br, or mixtures thereof when n'>1.

Hydrolysis of the polymers formed from the monomers described above may be accomplished by contacting the polymer with an alkaline solution at a temperature and for a time sufficient to convert the polymer to its ion exchange active form. Preferably, the alkaline solution is a sodium or potassium hydroxide solution which has a concentration of from about 0.2 weight percent to about 75 weight percent. Commonly, an approximately 25 weight percent sodium hydroxide solution is used. Temperatures in the range of from about 10.degree. C. to about 110.degree. C. are suitable. Preferably, the temperature is in the range of from about 70.degree. to about 80.degree. C. Minimum contact times may vary from a few minutes at higher temperatures to a few hours at lower temperatures. Preferably, however, at about 70.degree.-80.degree. C., the polymers should be contacted for about 1/2 to about 16 hours. Polymers so treated are water wettable.

In addition to the alkaline materials, described above, acids, such as HCl may be used to convert the polymers into their hydrogen form if the polymers have carboxylic-type ion exchange groups, i.e., Y in polymer II is --CN, --COZ, or --C(R.sup.3 f)(R.sup.4 f)OH. Such polymers are also water wettable.

The monomer FSO.sub.2 CF.sub.2 CF.sub.2 OCF.dbd.CF.sub.2 has a density of about 1.65 grams per cubic centimeter and polytetrafluoroethylene has a density of about 2.2 grams per cubic centimeter. A copolymer of this monomer with tetrafluoroethylene would, thus, have a density between the two values.

It has been discovered that certain perhalogenated treating agents have a surprising effect of dissolving and dispersing the polymers, especially when the polymers are in a finely divided state.

Treating agents suitable for use in the present invention to form the sizing compositions of the present invention preferably have the following characteristics:

a boiling point less than about 110.degree. C.;

a density of from about 1.55 to about 2.97 grams per cubic centimeter;

a solubility parameter of from greater than about 7.1 to about 8.2 hildebrands.

It is desirable that the treating agents have a boiling point of from about 30.degree. C. to about 110.degree. C. The ease of removal of the treating agent and the degree of treating agent removal is important in producing various films, coatings and the like, without residual treating agent; hence a reasonable boiling point at atmospheric pressure allows convenient handling at room conditions yet effective treating agent removal by atmospheric drying or mild warming.

It is desirable that the treating agent has a density of from about 1.55 to about 2.97 grams per cubic centimeter. The polymers of the present invention have densities from about 1.55 to about 2.2 grams per cubic centimeter. Primarily, the polymers have densities of from about 1.6 to about 2.2 grams per cubic centimeter. Treating agents of the present invention will therefore swell dissolve and disperse small particles of this polymer, aided by the suspending effects of the similarity in densities.

The prior art did not balance density. They were interested in forming solutions and solutions do not separate.

Solubility parameters are related to the cohesive energy density of compounds. Calculating solubility parameters is discussed in U.S. Pat. No. 4,348,310, the teachings of which are incorporated by reference for the purpose of their teachings about solubility parameters.

It is desirable that the treating agent has a solubility parameter of from greater than about 7.1 to about 8.2 hildebrands. The similarity in cohesive energy densities between the treating agent and the polymer determine the likelihood of dissolving, swelling or dispersing the polymer in the treating agent.

It is desirable that the treating agent has a vapor pressure of up to about 760 millimeters of mercury at the specified temperature limits at the point of treating agent removal. The treating agent should be conveniently removed without the necessity of higher temperatures or reduced pressures involving extended heating such as would be necessary in cases similar to U.S. 3,692,569 or the examples in British Patent 2,066,824A in which low pressures (300 millimeters) had to be employed as well as non-solvents to compensate for the higher boiling points and low vapor pressures of the complex solvents.

It has been found that treating agents represented by the following general formula are particularly preferred provided they also meet the characteristics discussed above (boiling point, density, and solubility parameter):

wherein:

X is selected from the group consisting of --F, --Cl, --Br, and --I;

X' is selected from the group consisting of --Cl, --Br, and --I;

Y and Z are independently selected from the group consisting of --H, --F, --Cl, --Br, --I and --R';

R' is selected from the group of perfluoroalkyl radicals and chloroperfluoroalkyl radicals having from 1 to 6 carbon atoms.

The most preferred treating agents are 1,2-dibromotetrafluoroethane (commonly known as Freon 114 B 2)

and 1,2,3-trichlorotrifluoroethane (commonly known as Freon 113):

Of these two treating agents, 1,2-dibromotetrafluoroethane is the most preferred treating agent. It has a boiling point of about 47.3.degree. C., a density of about 2.156 grams per cubic centimeter, and a solubility parameter of about 7.2 hildebrands.

1,2-dibromotetrafluoroethane is thought to work particularly well because, though not directly polar, it is highly polarizable. Thus, when 1,2-dibromotetrafluoroethane is associated with a polar molecule, its electron density shifts and causes it to behave as a polar molecule. Yet, when 1,2-dibromotetrafluoroethane is around a non-polar molecule, it behaves as a non-polar treating agent. Thus, 1,2-dibromotetrafluoroethane tends to dissolve the non-polar backbone of polytetrafluoroethylene and also the polar, ion-exchange-containing pendant groups. The solubility parameter of 1,2-dibromotetrafluoroethane is calculated to be from about 7.13 to about 7.28 hildebrands.

It is surprising that an off-the-shelf, readily-available compound such as 1,2-dibromotetrafluoroethane would act as a solvent for the fluoropolymers described above. It is even more surprising that 1,2-dibromotetrafluoroethane happens to have a boiling point, a density and a solubility parameter such that it is particularly suitable for use as a solvent/dispersant in the present invention.

In practicing the present invention, the polymer may be in any physical form. However, it is preferably in the form of fine particles to speed dissolution an dispersion of the particles into the treating agent. Preferably, the particle size of the polymers is from about 0.01 micron to about 840 microns. Most preferably, the particle size is less than about 250 microns.

To dissolve and disperse the polymer particles into the treating agent, the polymer particles are placed in contact with the treating agent of choice and intimately mixed. The polymer and the treating agent may be mixed by any of several means including, but not limited to, shaking, stirring, milling or ultra sonic means. Thorough, intimate contact between the polymer and the treating agent is needed for optimum dissolution and dispersion.

The polymers of the present invention are dissolved and dispersed into the treating agents at concentrations ranging from about 0.1 to about 50 weight percent of polymer in treating agent. At concentrations below about 0.1 weight percent, there is insufficient polymer dissolved and dispersed to be effective as a medium for coating articles or forming films within a reasonable number of repetitive operations. Conversely, at concentrations above about 50 weight percent there is sufficient polymer present as a separate phase such that viable, coherent films and coatings of uniform structure cannot be formed without particulate aggregates, etc.

Preferably, the concentration of the polymer in the treating agent is from about 0.1 to about 20 weight percent. More preferably, the concentration of the polymer in the treating agent is from about 0.3 to about 10 weight percent. Most preferably, it is from about 5 to about 15 weight percent.

Dispersing the polymer into the treating agent can be conducted at room temperature conditions. However, the optimum dispersing effects are best achieved at temperatures from about 10.degree. C. to about 50.degree. C. At temperatures above about 50.degree. C. the measures for dissolving and dispersing the polymer have to include pressure confinement for the preferred treating agents or method of condensing the treating agents. Conversely, at temperatures below about 10.degree. C., many of the polymers of the present invention are below their glass transition temperatures thus causing their dispersions to be difficult to form at reasonable conditions of mixing, stirring, or grinding.

Dissolving or dispersing the polymers into the treating agent are best conducted at atmospheric pressure. However, dispersing effects can be achieved at pressures from about 760 to about 15,000 millimeters of mercury or greater. At pressures below about 760 millimeters of mercury, the operation of the apparatus presents no advantage in dissolving and dispersing polymers, rather hindering permeation into the polymers and thus preventing formation of the sizing compositions.

Conversely, pressures above about 760 millimeters of mercury aid very little in dissolving and dispersing polymers compared to the difficulty and complexity of the operation. Experiments have shown that at about 20 atmospheres the amount of polymer dissolved and dispersed in the treating agent is not appreciably greater.

The fabric, fiber, or thread upon which the sizing composition is to be coated is preferably cleaned or treated in such a way as to assure uniform contact with the sizing composition. The fabric, fiber, or thread can be cleansed to remove any dust or oils from the fabric, fiber, or thread by washing with a degreaser or similar solution followed by drying.

After being cleaned, the fabrics, fibers, yarns, or threads may be pre-conditioned by heating or vacuum drying prior to contact with the sizing compositions and the coating operation. Temperatures and pressures in the following ranges are preferably used: about 20 millimeters of mercury at about 110.degree. C. or thereabout is sufficient in all cases; however, mild heat is usually adequate, on the order of about 50.degree. C. at atmospheric pressure.

The following methods are suitable for fixing the sizing composition of the present invention to a polytetrafluoroethylene fabric, fiber, or thread: (1) dipping the fabric, fiber, or thread into the sizing composition, followed by air drying and sintering at the desired temperature with sufficient repetition to build the desired thickness; (2) spraying the sizing composition onto the fabric, fiber, or thread is used to advantage for covering large or irregular shapes; (3 ) pouring the sizing composition onto the fabric, fiber, or thread is sometimes used; and (4) painting the sizing composition with brush or roller has been successfully employed. In addition, coatings may be easily applied with metering bars, knives or rods. Usually, the coatings or films are built up to the thickness desired by repetitive drying and sintering. Then the sizing composition may be evened out using scraping knives, rods, or other suitable means. The sizing composition can be applied in a single step or in several steps depending on the concentration of the polymer in the sizing composition and the desired thickness of the coating.

Following the application of the sizing composition, the treating agent is removed by any of several methods including, but not limited to, evaporation or extraction. Extraction is the use of some agent which selectively dissolves or mixes with the treating agent but not the polymer. These removal means should be employed until a uniform deposition of polymer is obtained.

The treating agent removal is typically carried out by maintaining the coated substrate at temperatures ranging from about 10.degree. C. to about 110.degree. C., with the preferred temperature range being from about 20.degree. C. to about 100.degree. C. The temperature selected depends upon the boiling point of the treating agent. The temperature is in the range of from about 20.degree. C. to about 50.degree. C. for 1,2-dibromotetrafluoroethane.

The pressures employed for the removal of the treating agent from the coated substrate can range from about 20 mm of mercury to about 760 mm of mercury depending on the nature of the treating agent, although pressures are typically in the range of from about 300 mm of mercury to about 760 mm of mercury for 1,2-dibromotetrafluoroethane.

The formation of the coating can be carried out as part of the polymer deposition and treating agent removal process or as a separate step by adjusting the thermal and pressure conditions associated with the separation of the polymer from the sizing composition. If the sizing composition is laid down in successive steps, a coating can be formed without any subsequent heating above ambient temperature by control of the rate of evaporation. This can be done by vapor/liquid equilibrium in a container or an enclosure; therefore, the treating agent removal step can be merely a drying step or a controlled process for forming a coating.

After the treating agent has been removed, the residual polymer, as a separate step, is preferably subjected to a heat source of from about 200.degree. C. to about 320.degree. C. for times ranging from about 10 seconds to about 120 minutes, depending upon the thermoplastic properties of the polymers. The polymers having melt viscosities on the order of 5.times.105 poise at about 300.degree. C. at a shear rate of 1 sec.-1 as measured by a typical capillary rheometer would require the longer times and higher temperatures within the limits of the chemical group stability. Polymers with viscosities on the order of 1 poise at ambient temperatures would require no further treatment.

The most preferred treatment temperatures are from about 220.degree. C. to about 320.degree. C. and a time of from about 0.2 to about 45 minutes for the most preferred polymers for use in the present invention. Such polymers permeate the fabric, fiber, or thread under the conditions described above.

A use for the present invention is the preparation of impregnated or polymer-permeated reinforcement media which may be used to support membrane films. Reinforcement scrims or cloths may be prepared by dipping, painting or spraying the sizing compositions onto the scrim or cloth. Then, the coated scrim or cloth is baked or sintered to fix the fluoropolymer impregnation to the scrim or cloth. The impregnated scrim or cloth is easier to handle than untreated scrims or cloths.

The coated unhydrolyzed polytetrafluoroethylene material may be reacted with amines, ammonia, or other suitable chemicals to make it dyeable or adhesive.

A sample of woven polytetrafluoroethylene cloth was used for treating. It was about 6 inches square composed of: (a) 24 leno ends per inch each is composed of two, two hundred denier polytetrafluoroethylene thread in the warp and (b) 24 fill ends per inch where each is composed of a 400 denier polytetrafluoroethylene thread. It was dipped in a copolymer of tetrafluoroethylene and CF.sub.2 .dbd.CF--O--CF.sub.2 --CF.sub.2 --SO.sub.2 F having an ion exchange an equivalent weight of 800 when hydrolyzed, was blended with 1,2,2-trichlorotrifluoroethane to form a suspension containing about 10% polymer solids by weight. It was allowed to dry. The dipping and drying process was repeated 6 times. The sample was then heated to about 440.degree. F. while being restrained from shrinkage mechanically. After this process, the sample, whose original weight was 2.34 grams, had increased to 5.67 grams. To convert the --SO.sub.2 F groups to --SO.sub.2 Na, the sample was then placed in a 25 weight percent aqueous NaOH solution and was heated to about 70.degree. Celsius overnight. To convert the --SO.sub.2 Na to the --SO.sub.2 H form, the sample was then removed from the NaOH solution, rinsed with deionized water, placed in 6 normal hydrochloric acid, and warmed gently. The acid was changed 3 times to insure complete conversion of the ionic groups to the acidic form. The sample was removed from the acid, dried overnight, and weighed. It weighed 5.40 grams. The sample was then analyzed to determine the overall equivalent weight of the coated polytetrafluoroethylene. It was found to have an overall equivalent weight of 1736 grams/eq.