US3520789A - Electropainting - Google Patents

Description

- July 14, 1970 T. B. LEMMON 3,520,789

ELECTROPAINTING Filed July 19, 1967 2 Sheets-Sheet l LL] Q Q E U i, LL] '8 u E s 3 2* Lu 6 z 8 LL! 8 E.

Rio 3 (Harv/ y sszw/allu mu TERENCE BRIAN LEMMON Inventor By DAVIS, HOXIE,

EAITHFULL & HAPGOOD, His Attorneys Jul y 14, 1970 T. a. LEMMON 3,52

ELEOTROPAINTING Filed July 19. 1967 2 Sheets-Sheet 2 N L% m b3 I Q U Z U .5 g I a; U

I d a Q: L I E q: r a Q Q \l w E Lu I an Q I 2 E l n 1 E 1 .N Q 8 a s (Hm/1 0/ seam/01m mu TERENCE BRIAN LEMON Inventor By DAVIS HOXIE,

FAITHFULL & HAPGOOD, His AF s 3,520,789 ELECTROPAINTING Terence B. Lemmon, Coventry, England, assignor to Courtaulds Limited, London, England, a British com- Filed July 19, 1967. Ser. No. 654,661 Claims priority, application Great Britain, July 20, 1966, 32,578/ 66 Int. Cl. B01]; 5/02; C23b 13/00 US. Cl. 204-181 4 Claims ABSTRACT OF THE DISCLGSURE This invention is concerned with improvements in electropainting which allow thin uniform surface coatings to be electrodeposited on an anode object.

We have found that electropainted coatings are initially deposited on an anode object as discrete particles of the pigmented binder. Frequently the particles coalesce imperfectly so that the coating contains pores derived from the residual interstices. The pores allow the electrophoretic process to continue for a period longer than that which a coherent non-porous coating allows, so that a generally thicker coating results. Another effect of the porous coating is that the anode is more thickly coated at parts which are nearer the cathode, the coating developing a wedge shape, the thickness of which diminishes with distance from the cathode.

We have further confirmed the observations of other workers in this field, that with commercially used binders, which are viscous liquids at ambient temperatures, increasing the temperature of the bath during the electrolytic process decreases the resistance of the coating on the anode and allows an overall thicker coating to be deposited.

This phenomenon is discussed, for example, in Electropainting by R. L. Yeates, published by Robert Draper, 1966 on page 23 and in Electrophoresis by K. H. Fragen in Farbeund Lack, 1964, 70, pp. 271279.

We have found a number of binders which are different from the binders usually employed in that they are solid at ambient temperature. These binders are employed in this invention to obtain uniform, thin coatings on anode objects.

According to the present invention a process for uniformly coating an anode object comprises electropainting with an aqueous paint embodying a polyanionic binder which is solid at ambient temperatures, the aqueous paint being at a temperature causing the binder to be deposited on the anode as particles, whilst the deposited film is allowed to attain a temperature causing the particles to coalesce forming an entire coating.

The temperature at which the particles of the electrodeposited binder coalesce to form an entire coating is not readily correlated with the simple physical properties of the binders, such as the melting points, for these properties are ambiguous; the melting point may well cover a range of temperatures and is frequently accompanied by a chemical change in the binder which affects the determination.

We propose to introduce the term coalescence temperature to describe an important property of the binders, and we have devised an apparatus and test for determining the coalescence temperature. The apparatus com- United States Patent prises an electrolytic cell constructed of an electrically insulating material, for example polymethyl-methacrylate sheet, in which electrodes of mild steel plate are placed with parallel confronting surfaces separated by 2% inches. The anode has previously been degreased in methylene chloride and coated with insulating masking tape leaving unmasked only an inch square of the surface confronting the cathode. The electrodes are coupled to an electrical supply developing a potential difference of 50 volts and to a recording ammeter.

The tests consists of (i) introducing the aqueous solution or dispersion of the binder at a predetermined temperature into the electrolytic cell as the electrolyte, (ii) energising the electrodes and allowing electrodeposition to continue until the current ceases, (iii) washing the anode in water to free it from electrolyte and measuring the thickness of the deposited film and (iv) noting the period during which current was flowing.

The test is repeated with the electrolyte at different temperatures until the results of two tests at temperaures differing by at most 5 C. are respectively outside and within the specification (a) period during which current was flowing: less than 1.5 seconds. (b) thickness of deposited film; at most 1 10- inch.

The coalescence temperature" of the binder is then bracketed by the two temperatures of the electrolyte, and is the temperature of the paint bath which causes an insulating film of approximately 1X10 inch thickness to be deposited on the anode in from 1 to 2 seconds under the conditions stipulated in the test.

The scope of the invention may be restated as electropainting an anode with an aqueous paint embodying a polyanionic binder solid at ambient temperature, whilst maintaining the aqueous paint below the coalescence temperature of the binder and allowing the paint film deposited on the anode to attain the coalescence temperature, whereupon further electrodeposition on the anode is inhibited.

The deposited film tends to have a temperature higher than the aqueous paint due to the dissipation of electrical and chemical energies which accompany the formation of the film. It is, therefore, usually unnecessary to heat the film in the course of the electropainting process. The period required for the film to attain the coalescence temperature may be adequately controlled by the temperature of the aqueous paint: increasing the difference between the coalescence temperature and the temperature of the aqueous paint results in a longer period of electrodeposition and therefore a thicker deposited film.

It is essential that the deposited film should attain the coalescence temperature to halt the electrodeposition process, even though the film is cooled by contact with the aqueous paint. Obviously, therefore, the rate of abstraction of heat from the film by the paint must not be greater than the rate of generation of heat within the film and the anode in contact therewith. This puts a practical limit on the lowest temperature of the paint, unless the film is to be heated from an auxiliary source. We prefer, in the absence of auxiliary film heating, that the aqueous paint should have a temperature less than 40 centigrade degrees and more than centigrade degrees below, the coalescence temperature of the binder, and better still within the range to centigrade degrees below the coalescence temperature.

The sharp cessation of electrodeposition of the binder which occurs when the film attains the coalescence temperature is demonstrated in the ensuing examples and this phenomenon is mainly responsible for the more uniform film deposited on the anode. A related benefit is the improved throwing power of the paint, that is the ability of the paint to deposit a film on parts of the anode distant from the cathode.

The solid binders of this invention behave quite differently from the viscous liquid binders known from the prior art, in that the increasing temperature results in a thinner coating being laid down and the coating itself has a more uniform thickness relatively independent of the distance of the coated part from the cathode. This means of controlling the thickness of the Coating in combination with good throwing power represents a significant advance in the art.

The solid binders of this invention are exemplified by quaternary or more complex copolymers containing units of one or more components from each of the following groups, the range of the percentage contribution of each group to the weight of the copolymer also being stated:

(a) styrene, vinyl acetate, and the methyl and ethyl esters of acrylic and methacrylic acids, contributing from 5 to 55 percent:

(b) alkyl acrylates and methacrylates in which the alkyl group contains from 3 to 12, preferably from 7 to 12 carbon atoms, contributing from to 60 percent:

(c) N alkoxymethyl derivatives of acrylamide and methacrylamide, contributing from 9 to 15 percent:

(d) alkali metal, ammonium and amine salts of acrylic methacrylic and itaconic acids, contributing from 1 to percent, preferably from 9 to 15 percent;

the proportions being so selected that the binder is soluble in water.

The alkyl acrylates and methacrylates of group (b) are exemplified by the Z-ethylhexyl esters, the n-octyl esters, the butyl esters and the n-hexyl esters.

The preferred substances of group (0) are the N- methoxymethyl-, N-ethoxymethyland N-propoxymethylderivatives of acrylamide and methacrylamide.

The preferred salts of group (d) are the sodium, ammonium and triethylamine salts of acrylic and methacrylic acids. The salts of other alkali metals such as potassium may be employed and in general the useful amines are secondary and tertiary momoamines for example diethylamine, trimethylamine and triethanolamine.

The viscosity of a 40 percent aqueous solution of the binder should be less than 10 poises and it may, therefore, be necessary to limit the average molecular weight of the binder copolymer by employing a chain-transfer agent in the free-radical copolymerization of the components. Mercaptans, for example Z-mercaptoethanol are useful in this respect.

The paint containing the binder preferably has a vehicle consisting of water. Other water-miscible solvents may be present, for example alcohols such as isopropanol and various butyl alcohol isomers. However, as these solvents are frequently deposited with the binder on the anode during electropainting, in a concentration sufficient to lower the coalescence temperature, the concentration of the non-aqueous solvents is preferably kept below that amount which causes the coalescence temperature to be decreased to below room temperature.

Another group of solid binders for use in carrying out this invention are the phosphate esters made by reacting (a) one or more polymers having at least one epoxy group, with (b) orthophosphoric acid, alone or in conjunction with one or more organic acids or anhydrides, particularly the fatty acids of drying oils e.g. linoleic and oleic acid. The binder is rendered soluble in water by neutralising the phosphate ester with an alkali, ammonia or an amine.

The epoxy-containing polymers are exemplified by the dihydric phenol/epichlorhydrin condensates such as those marketed as Epon in the USA. and Epikote in Great Britain.

The invention is illustrated by the following examples in which parts and percentages are by weight.

4 EXAMPLE 1 A binder solution was prepared in the following way. A charge of monomers was made up as follows:

Parts Methyl methacrylate 29 2 ethylhexylacrylate 53 Methacrylic acid 10 N methoxymethylacrylamide 8 Azobutyronitrile (1 part) was dissolved in 150 parts of the isopropanol/ water azeotrope (approximately 88 parts of isopropanol to 12 parts of water) and the solution heated to reflux at about 80 C. The charge of monomers was added slowly over 1 hour to the gently refluxing solution and refluxing was continued for a further 3 hours. The viscosity of the solution at this stage was 1.55 poises at C. and the non-volatile content of the solution 40.2 percent. A portion of the azeotrope was distilled from the solution and triethylamine and water were added respectively to neutralise the acid groups of the copolymer and to bring the solids content of the solution to 50 percent in a mixture of equal parts of isopropanol and water. This was the stock solution employed in this and the following examples.

The stock solution was let down with water until the solids content was 10 percent and then poured into an electrolytic bath equipped with anodes of the kind described earlier with reference to the determination of the coalescence temperature of binders. The electrodes were connected to a source of 50 volts and the charge of the current with time was recorded by a recording ammeter connected to the electrodes.

The differences attributable to the temperature of the solution during electropainting process are shown in the following table.

Thickness of Period of current; deposited film Temp. of solution 0.) flow (seconds) (XlO- inch) It will be seen that the film is thinner and that the time for which the current passes before the resistance of the film inhibits the current, is shorter, as the temperature of the solution is raised. The coalescence temperature of the copolymer (the binder of this invention) is between and 35 C. The thickness of the film deposited at C. was not detectable, but nevertheless inhibited the current within 1 second.

The thickness of the film does not increase beyond the time that the current is inhibited, as was shown by repeating the test with the electrolyte at 20 C. and maintaining the electrodes at a potential difference of volts for 2 minutes. The graph of film thickness versus time is shown in FIG. 1.

As a contrast to the behaviour of the binders of this invention, the performance of a commercial viscous liquid binder in the same apparatus over a period of 2 minutes and at different temperatures is given in the following Table 2.

Thickness of film Temp. C.): l0* inch) It will be seen that the deposited film is thicker when the temperature of the paint is higher, in direct contrast to what was observed in the practice of the present invention.

EXAMPLE 2 The diluted stock solution containing percent solids of Example 1, served as the electrolyte in a cell equipped with an anode bar extending at right angles to the cathode surface.

The electrodes were energised at a potential difference of 50 volts for 2 minutes and the thickness of the film at a succession of points along the anode was determined.

The same test was carried out employing a commercial, viscous liquid binder. The results of both tests are recorded in the graph of FIG. 2 and identified as A and B, respectively. It will be seen that the film has a more uniform thickness when deposited by the process according to the present invention and that a greater length of the anode bar was coated.

What I claim is:

1. Electropainting an anode wih an aqueous paint embodying a polyanionic binder solid at room temperature, whilst maintaining the aqueous paint less than 40 C. and more than 10 C. below the coalescence temperature of the binder and allowing the paint film coating the anode to attain the coalescence temperature, whereupon further electro-deposition on the coated anode is inhibited.

2. Electropainting an anode as claimed in claim 1, wherein the polyanionic binder is a copolymer containing (a) from 5 to 55 percent by weight of the units of one or more components chosen from the group consisting of styrene, vinyl acetate, methyl acrylate,

6 ethyl acrylate, methyl methacrylate (b) from 10 to 60 percent by weight of the units of one or more components chosen from the group consisting of alkyl acrylates and methacrylates of which the alkyl group contains from 3 to 12 carbon atoms (c) from 9 to 15 percent by weight of the units of one or more components chosen from the group consisting of N-alkoxy methyl derivatives of acrylamide and methacrylate and ethyl 10 methacrylamide, the alkoxy group having from 1 to 3 carbon atoms ((1) from 1 to percent by weight of the units of one or more components chosen from the group con- 15 sisting of alkali metal, ammonium or amine salts of acrylic, methacrylic and itaconic acids,

the proportions of the components being so chosen that the copolymer is soluble in water. 3. Electropainting an anode as claimed in claim 2 20 wherein the viscosity of a 40 percent aqueous solution of the copolymer is at most 10 poises at C.

4. Electropainting an anode as claimed in claim 3 wherein the paint vehicle consists essentially of water.

References Cited UNITED STATES PATENTS 3,382,165 5/1968 Gilchrist 204181 HOWARD S. WILLIAMS, Primary Examiner

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