US3506555A - Electrodeposition of polytetrafluoroethylene on metals - Google Patents

Description

nited States Patent 3,506,555 ELE'CTRODEPOSITION OF POLYTETRA- FLUOROETHYLENE ON METALS Friedrich Stadler, Altotting, and Herbert Fitz, Bnrgkirchen (Alz), Germany, assignors to Farbwerke Hoechst Aktiengesellschaft vormals Meister Lucius & Brnning, Frankfurt am Main, Germany, a corporation of Germany No Drawing. Filed Oct. 10, 1967, Ser. No. 674,088 Claims priority, application Germany, Oct. 17, 1966,

50,459; Nov. 24, 1966, F 50,752

Int. Cl. C2311 13/00; B01k 5/02 US. Cl. 204-181 12 Claims ABSTRACT OF THE DISCLOSURE Process for providing metal surfaces with a coating of polytetrafluoroethylene which is free from water and, therefore, need not be dried prior to sintering, the coating being applied by electrodeposition of polytetrafluoroethylene from an aqueous dispersion to which low amounts of fluorinated surfactants, such as perfluorocaprylic acid, have been added.

This invention relates to a process for providing metals with a coating of polytetrafluoroethylene which is free from water and free from cracks by electrodeposition of polytetrafluoroethylene from an aqueous dispersion.

' It is knownthat particles suspended in water, which are electrically charged, migrate on applying an electric field of direct current voltage and are finally deposited, While being discharged on the electrodes by which the electric field is applied; dependent on the particles suspended in the water being negatively or positively charged, deposition sets in on the anode or on the cathode. This process which is called electrodeposition is used in industry, for example, for coating metal surfaces with plastics particles and thereby to attain adherent coatings. Thus the method of electro-coating is already a prior art method.

This method can also be used for coating metal surfaces with polytetrafluoroethylene since polytetrafluoroethylene particles suspended in water have a negative potential similarly to other finely dispersed plastics particles, for example urea resins.

If two electrodes are immersed in a dispersion containing polytetrafluoroethylene and if a direct current is applied to them, the negatively charged polytetrafluoroethylene particles migrate to the positively charged electrode (anode), Where they are discharged and deposited. If a polytetrafluoroethylene coating, which has been electro-deposited in this manner and which always still con tains water, is dried, coatings containing cracks are obtained which, on account of the extremely high melt viscosity of polytetrafluoroethylene, can no more coalesce into crack-free coatings during the sintering process above the melting point of polytetrafluoroethylene (327 C.). To overcome these undesirable phenomena, it has already been proposed in US. Patent 2,820,752 to electrodeposit a mixture of polytetrafluoroethylene and polyisobutylene, dispersed in water, on metal surfaces. Theplastics coating obtained which contains water to a higher or lesser extent, is dried and finally sintered at temperatures of from 300 to 500 C. In this process the polyisobutylene which, according to theprocess of the US. patent cited above, acts as a binder for the individual polytetrafluoroethylene particles, is thermally decomposed. Although it is possible in this manner to avoid the aforesaid formation of cracks in the plastics coating and thus to obtain improved coatings, this process is not 3,506,555 Patented Apr. 14, 1970 yet satisfactory both as regards the method and the desired quality of the coatings produced since the polyisobutylene or the decomposition products thereof have to be entirely removed thermally in the sintering process of the polytetrafluoroethylene. In contradistinction to the method described in the first place, this process requires longer periods of sintering which is economically unfavorable in the operation of continuous installations. Moreover, it cannot be avoided that voids are formed due to the decomposition of the polyisobutylene during the sintering process of the polytetrafluoroethylene at temperatures about 327 C., which voids can no more be closed by the molten polytetrafluoroethylene particles.

Now we have found that the aforesaid disadvantages can be avoided and polytetrafluoroethylene-containing coatings on metallic surfaces which are free from water and free from cracks, be obtained by electrodeposition of polytetrafluoroethylene from an aqueous dispersion and subsequent sintering of the deposited material, if necessary in an inert atmosphere, by adding to the aqueous dispersion of the polytetrafluoroethylene fluorinated surfactants and, if necessary, a substance which anodically by the process described in German Patent 813,462 with a content of from 5 to 250 grams, preferably 50 to grams, of polytetrafluoroethylene per liter. Then the aforesaid fluorinated surfactants are added at the rate of 0.001 to 1 gram, preferably 0.01 to 0.1 gram per liter of the aqueous polytetrafluoroethylene dispersion, i.e. also in those cases where these fluorine compounds have already been added prior to or during the polymerization (for example according to German Patent 916,470).

Metals which have to be electrodeposited in the presence of an anodically passivating substance are brass and copper. If the dispersion does not contain a passivating additive in these cases, the copper ions passing into solution from the anode bring about a salting out effect of the polytetrafluoroethylene particles which thus coagulate so that the polytetrafluoroethylene coating deposited contains a large amount of water.

As substances which anodically passivate copper or brass and thus prevent the coagulation described above, there may be used above all the complex potassium-iron cyanides, preferably potassium ferricyanide or potassium ferrocyanide, in an ammoniacal solution. The desired effect is brought about already by using very slight amounts of this additive, i.e. 0.1 to 10, preferably 0.5 to 2 grams per liter of a polytetrafluoroethylene dispersion whose pH value is adjusted to 10.5 to 11.5 by means of an aqueous ammonium hydroxide solution.

According to the process of the present invention coatings which are practically free from water can be pro-'.

duced on metal surfaces, for example aluminum, steel, chromium or nickel which, after the termination of the coating process, can be sintered without an intermediate drying process at temperatures above the melting point of polytetrafluoroethylene to form crack-free coatings.

It has unexpectedly been found that if higher concentrations of fluorinated surfactants are employed, the polytetrafluoroethylene is not deposited in a form that is free from water but in the form of larger coagulated particles which may contain up to 25 per cent by weight of water as shown in the table following hereunder.

The dispersion contained, respectively, 100 grams per liter of polytetrafluoroethylene and 0.5 to 2 grams per liter of perfiuorocaprylic acid added prior to the polymerization, as well as additional amounts of perfluorocaprylic acid in the concentrations indicated which, according to the process of the invention, were added only prior to the process of electrode-position. Aluminum plate, which had been chromated according to the Tridur process of Messrs. Schering, Berlin, was used as the anode material. The process was carried out with a direct current voltage of 100 volts.

TABLE 1 Water content of the deposited polytetra- Dry deposit Pertluorofiuoroethylfluoroethyl- Test caprylie cue, percent one, g./sq. N 0. acid g./1. by weight dm.

For comparison according to the invention 1 0. 3. 2 0. 60 2 0. 001 2. 0 0. 60 3 0. 003 1.0 0. 55 1 4 0. 01 0. 0 0. 55 1 5 0. 03 0. 0 0. 55 l 6 0. 1 0. 5 0.70 7 0.3 18 1. 40 8 1. 0 23 l. 20 For comparison- 9 3.0 11 0. 80 10. 0 2 0. 30

1 Preferred amount of fluorinated compound.

In those tests where the deposited polytetrafluoroethylene has a high content of water, the coating becomes cracked after the dryingand sintering processes so that the advantage of a substant al freedom from voids is n longer given.

There are fundamentally obtained similar results with other fluorinated surfactants in which case the most favorable ranges of concentration of surface-active, fluorinated compounds may, however, vary slightly.

If two electrodes of refined steel are immersed in an aqueous polytetrafiuoroethylene dispersion containing, according to the invention, one of the aforesaid fluorinated compounds while applying a direct current to the electrodes, a current flow sets in between the electrodes until a continuous coating of polytetrafluoroethylene has formed on the electrode which has been made the anode. The polytetrafluoroethylene coating deposited on the refined steel in this manner is practically free from water; the water content if generally less than 0.1 percent. It is not necessary, therefore, to dry the deposited polytetrafluoroethylene coatings prior to the sintering process at temperatures above 327 C. The sintered polytetrafluoroethylene then forms a homogeneous coating on the metal, which coating contains voids to a small degree only.

If the polytetrafiuoroethylene dispersion is to be used for the coating of copper and brass by a process of electrodeposition, there is added to the dispersion, in addition to the surface-active fiuorinated compound, a potassium/ iron cyanide complex compound at the rate of 0.5 to 2 grams per liter of the dispersion as an anodic passivating agent, and the pH value of the dispersion is adjusted to 10.5 to 11.5 by the addition of an aqueous ammonium hydroxide solution.

Also the polytetrafluoroethylene deposited on copper or brass according to the process of the present invention has a water content of less than 0.1 percent and is thus practically free from water. It is, therefore, not necessary to dry the coating; sintering can immediately be carried out at temperatures above 327 C, advantageously at 340 to 380 C. It has been found that the sintering process should advantageously be carried out in an atmosphere free from O in order to obtain a homogeneous and crack-free coating of polytetrafiuoroethylene on copper or brass which shows good adhesion to the substrate. If the sintering process is carried out in the presence of oxygen, for example in the air, the metal is oxidized on the surface. The oxide film forming substantially reduces the bond strength of the polytetrafloroethylene.

The direct current applied in the process of electrodepositing polytetrafluoroethylene generally ranges from 50 to 250 volts. The current density initially amounts to up to 3 amps/sq. dm. while it decreases to 0 after the coating has been terminated. However, it has been found to be more advantageous to operate with a constant voltage than with a constant current intensity, as shown in the table following hereunder.

In the following test series, the current density was 0.5 amp/sq. dm., the voltage required therefore was correspondingly regulated within a range of from 0 to volts. The other test conditions were the same as those in tests 1 to 10.

These results show that when the operation is carried out with a constant voltage, the range of concentration for the fiuorinated surfactant, within which a satisfactory coating can be obtained, is wider (cf. Table 1).

The period required for electro-coating depends on the current intensity and on the voltage. However, the periods are generally very short ranging from 5 to 15 seconds. The period required for the final sintering process is determined primarily by the fact that the coated metal object has to be brought to a temperature exceeding 327 C. The sintering process proper, which is carried out at temperatures of from 340 to 380 C., requires a period of 30 seconds.

The thickness attained of the sintered coating generally ranges from 10 to 25 microns which is sufiicient for many purposes in which only the anti-adhesive properties of the coating are of import. Moreover, there exists a good insulating action against potential differences that are not too high; the dielectric strength of the coating amounts to approx. 200 to 300 volts.

The coatings on copper and brass produced by the process of the present invention have a thickness of from 101 to 40 microns and a dielectric strength of up to 500 V0 ts.

As is usual in electro-coating processes, the metal surfaces to be coated must be subjected to a careful preliminary treatment. Suitable therefor are the mechanic and/or chemical methods known from electro-plating, for example sandblasting, degreasing and/or pickling. Aluminum and its alloys can, moreover, also chemically be modified on the surface by oxidizing, by a phosphate or chromate treatment. Prior to carrying out the electrocoating process with polytetrafiuoroethylene there must, in all cases, be obtained an unobjectionable metal surface which is free from impurities.

The process described above for coating metals with polytetrafluoroethylene by electrodeposiiton affords a wide range of application, for example above all for coating apparatuses used in the chemical industry, for insulations in the filed of electro-engineering, including the coating of wires.

The following examples are presented to illustrate many of the features of this invention without any intent that this invention be limited thereby:

EXAMPLE 1 0.01 gram per a liter of perfluorocaprylic acid, CF (CF COOH,. was dissolved in anaqueous dispersion containing 50 grams per liter of polytetrafluoroethylene; Three plates of refined steel measuring 150 mm. by 50 mm; by 1 mm., which had previously carefully been degreased with trichloroethylene and which had been pickled in a mixed acid with 100 grams per liter of nitric acid and 20 grams per liter of hydrofluoric acid at 40 C. for approx. one minute, were suspended in the dispersion described above parallel to one another at a distance of 20 mm. These three plates were connected to a source of direct current in such a fashion that the two outer plates were connected to the negative pole and the center plate was connected to the positive pole. The electric current was regulated such that a voltage of 100 volts was impressed between the anode and the cathode. The passage of current decreased in the course of 10 seconds from 3 amperes to 0, the process of electro-deposition being terminated, The anodic plates was removed from the bath, freed from possibly adhering drops of liquid by rinising with water or by blowing oif, and sintered in a sinten'ng oven at 380 C.

The polytetrafluoroethylene forms a uniform, smooth coating (thickness 20 microns) on the plate of refined steel.

EXAMPLE 2 In an aqueous dispersion containing 100 grams per liter of polytetrafluoroethylene, 0.05 gram per liter of the ammonium salt of perfluorocaprylic acid,

CF CF COONH was dissolved. For the process of electrodeposition, the same experimental arrangement as that of Example 1 was used, two plates of refined steel serving as cathodes, the anodic plate to be coated being an aluminum plate, which had previously been pickled in a 10% sodium hydroxide solution and treated in a solution containing 50 grams per liter of sodium carbonate and 30 grams per liter of sodium chromate. With an impressed voltage of 100 volts, there first flowed a current of 1.5 amperes, which decreased to in the course of 20 seconds. The subsequent sintering process was carried out at 380 C.

EXAMPLE 3 The process was carried out as described in Example 2 except that instead of the ammonium salt of perfiuorocaprylic acid a compound of the formula CF3 5SO3Na was used at the rate of 0.03 gram per liter.

EXAMPLE 4 In an aqueous dispersion containing 200 grams per liter of polytetrafluoroethylene, 0.1 gram per liter of perfluorocaprylic acid (CF (CF COOH) was dissolved. 1000 cc. of this dispersion were mixed with 1000 cc. of an aqueous solution of 2 grams per liter of potassium ferricyanide K (Fe(CN) while stirring thoroughly, and the pH value of the mixture obtained in this manner was adjusted to 10.5 by adding aqueous ammonia.

In the aqueous dispersion of polytetrafiuoroethylene obtained in this manner, which now contained approx. 100 grams per liter of polytetrafluoroethylene, 0.05 gram per liter of perfluorocaprylic acid, 1 gram per liter of potassium ferricyanide and ammonia, 3 copper plates measuring 200 mm. by 100 mm. by 1 mm. were suspended parallel to one another. The distance of the plates from one another was 25 mm.

These three plates were connected to a source of direct current in such a fashion that the two outer plates were connected to the negative pole, and the center plate was connected to the positive pole. The electric current was regulated such that a voltage of volts was impressed between the anode and the cathode. Under these conditions of electro-phoresis, a continuous, compact coating of polytetrafluoroethylene, which was practically free from water, was deposited on the anodic copper plate in the course of 20 seconds, the current intensity decreasing from 3.5 amperes to 0 ampere. The anodic plate was then removed from the bath, freed from water drops which possibly adhered to. the surface by blowing off and sintered in a suitable sintering oven at 380 C. in an atmosphere of oxygen-free nitrogen for a period of 10 minutes. The polytetrafluoroethylene formed on the copper surface a smooth coating (thickness: approx. 30 microns) which was free from voids and had a very good adherence to the substrate.

We claim:

1. A process for producing polytetrafluoroethylene-containing coatings on metallic surfaces, other than copper and brass, by a process of electrodeposition of polytetrafluoroethylene from an aqueous dispersion and a subsequent sintering process of the polytetrafluoroethylene deposited, which comprises adding to the aqueous polytetrafiuoroethylene dispersion perfluorinated carboxylic acids of the formula CF (CF ),,COOH or perfluorinated organic sulfonic acids of the formula CF (CF SO H or the water-soluble salts thereof at the rate of from 0.001 to 1 gram per liter.

2. The process as claimed in claim 1, wherein alkalior ammonium salts are used as the water-soluble salts of the perfluorinated carboxylic acids or of the perfluorinated organic sulfonic acids.

3. The process as claimed in claim 1, wherein the aqueous dispersion contains 5 to 250, preferably 50 to grams per liter of polytetrafluoroethylene.

4. The process as claimed in claim 1, wherein-the process of electrodepositing polytetrafluoroethylene is carried out with a constant voltage of from 50 to 250 volts.

5. The process for producing polytetrafluoroethylenecontaining coatings on surfaces of brass and copper by a process electrodeposition of polytetrafluoroethylene from an aqueous dispersion and a subsequent sintering process of the polytetrafluoroethylene deposited, which comprises adding to the aqueous polytetrafluoroethylene dispersion substances which anodically passivate the metal, and perfluorinated carboxylic acids of the formula CF CF ),,COOH or perfluorinated organic sulfonic acids of the formula CF (CF ),,SO H or the Water-soluble salts thereof at the rate of from 0.001 to 1 gram per liter.

6. A process for producing polytetrafluoroethylene-containing coatings on copper or brass as claimed in claim 5 wherein, as anodic passivating agent, potassium ferricyanide or potassium ferrocyanide in an ammoniacal solution in a concentration of from 0.1 to 10, preferably 0.5 to 2 grams per liter of the polytetrafluoroethylene dispersion, is added to the aqueous polytetrafiuoroethylene dispersion and that the pH value of the dispersion is maintained within the range of 10.5 to 11.5 by the addition of ammonia and that the coating is sintered in an inert atmosphere.

7. The process as claimed in claim 5, wherein the coating is sintered in an atmosphere, of nitrogen.

8. The process as claimed in claim 5, wherein alkalior ammonium salts are used as the water-soluble salts of the perfluorinated carboxylic acids or of the perfluorinated organic sulfonic acids.

9. The process as claimed in claim 5, wherein the aqueous dispersion contains 5 to 250, preferably 50 to 150 grams per liter of polytetrafluoroethylene.

10. The process as claimed in claim 5, wherein the process of electrodepositing polytetrafluoroethylene is carried out with a constant voltage of from 50 to 250 volts.

11. The application of the electrodeposited coating of metallic surfaces, other than copper and brass, with polytetrafluoroethylene from an aqueous dispersion to which, prior to the process of electrodeposition, perfluorinated carboxylic acids of the formula CF (CF ),,COOH or perfluorinated organic sulfonic acids of the formula CF (CF ),,SO H or the water-soluble salts thereof have been added at the rate of 0.001 to 1 gram per liter.

12. The application of the electrodeposited coating of surfaces of copper and brass with polytetrafluoroethylene from an aqueous dispersion to which, prior to the process of electrodeposition, substances which anodically passivate the metal and perfiuorinated carboxylic acids of the formula CF (CF COOH or perfluorinated organic sulv 8 I I fonic acids of the formula CF (CF SO H or the watersoluble salts thereof have been added at the rate of 0.001 to 1 gram per liter.

References Cited UNITED STATES PATENTS 2,800,447 7/1957 Graham 204-181 2,865,795 12/1958 Morrison 20418l FOREIGN PATENTS 1,033,953 6/1966 Great Britain.

HOWARD S. WILLIAMS, Primary Examiner