GB1572210A - Treating metal surfaces - Google Patents

Description

(54) TREATING METAL SURFACES (711) We, PYRENE CHEMICAL SERVICES LIMITED, a British Company, of Ridgeway, Iver, Buckinghamshire, SLO 9JJ, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: - In our Application No. 42127/75 (Serial No. 1,520,027), we have described and claimed a process for treating an uncoated metal surface by coating the surface with a substantially chromi um4ree aqueous composition comprising a melamine-formaldehyde resin and a vegetable tannin.We have found that comparable results can be achieved by carrying out the same process, but replacing the mel amine-formal dehyde resin with another water-soluble or waterdispersible organic polymer. Metals which can be treated by the process of the invention include iron, steel, galvanised steel and aluminium.

The concentration of the tannin in the aqueous composition is preferably from 0.1 to 20% by weight Amounts lower than 0.1% by weight generally give too small an effect for practical purposes; concentrations higher than 20% by weight tend to be unsatisfactory for workability and economics of the process, and do not always allow the tannin to be fully dissolved in the composition.Suitable forms of tannin for use in this invention are hydrolysed and condensed tannins, and their decomposition products, e.g. depside tannin or gallotannin, Chinese tannin, hamamelitannin, tannic acid derived from Acer Ginnala, chebulinic acid, sumac tannin, Chinese gallotannin, ellagitannin, catechin, catechin-tannin or quebracho tannin. Suitable sources of tannin are given in Table II of Application No 4Q127/ 75 (Serial No. 1,520,027).

The pH suitable for the aqueous solution or suspension containing the water-soluble or water-dispersible organic polymer and the vegetable taaain depends on the type of tannin, on the metal to be treated, and on the coating conditions, but is generally from 2 to 6.5. At a pH less than 2, the reaction may take place sufficiently violently to damage the surface appearance. At a pH higher than 6.6, the reactivity with zinc or zinc alloy thereof may be too weak to form a passive state coating having good corrosion resistance. At a pH higher than 7.0, the treating baths will be unstable owing to decompostion of the tannin component.

It is therefore necessary to use a polymer which is stable in acidic media. Neverthe less, almost all conventional watersoluble Or water-dispersible polymers may be used lin this invention. Such polymers include vinylic polymers and copolymers derived from vinyl acetate, vinyl chloride and / or vinylidene chloride; acrylic polymers and copolymers derived from acrylic acid, methacrylic acid, acrylates methacrylates hydroxyacrylic acid and/ or hydroxymethacrylic acid; polymers and copolymers of Ithe alkyd type, epoxyd type, fluorine type, polyester type, styrene type and olefin type; synthetic rubbers and naturally occurring polymer. Aqueous solutions and dispersions containing such a polymer or copolymer having a molecular weight greater than 1,000, preferably greater than 5,000, or more than one such ,(co)polymer, are suitable for use in the process according to the invention. There are preferably at least one, and more preferably at least 3,75 parts by weight of resin per part of weight of the tannin in the treatment composition.

In order Q achieve the desired pH, It may be necessary to add an acidic or alkaline substance to the treatment corn- postion. Suitable substances include inorganic acids such as phosphoric, nitric, sulphuric, hydrofluoric, hydrochloric or boric acid, or a salt thereof; and organic acids such as oxalic, citric, malic, maleic, phthalic, lactic, tartaric, choroacetic, acrylic or methacrylic acid, or a salt thereof.

Suitable alkaline materials include alkalis such as caustic soda or kali, ammonia and amines such as ethylamine, diethylamine, triethylamine or ethanolamine.

Various additional components may be added to the compositions used in this,inven- tion. These include compounds which provide metal di- and ;tri-valent cations such as those of zinc, manganese, cobalt, nickel, drown, titanium and zirconium; nitrate (as in British Patent Specification No.

1P81,826); thiourea; inert colouring agents designed to provide a specific colour in the protective coating; silicon compounds; a conductive material to improve weldability such as a pulverulent metal (zinc) as disclosed in U. S. Patent Specification No.

3,671,331 or conductive carbon, as in Britlish Patent Speoification No. 1,479,054; and stabilising agents to keep the organic components from separating.

The coatings which are formed on metal surfaces by the process of this invention serve as a base for painting without the need for further passivating treatments such as phosphating. This is the case even if the coated workpieoe is subjected to bending, pressing and/or degreasing.

The following Example illustrates the invention.

Example Hot galvanised steel sheets (100 mm X 100 mm X 0.3 mm in size) treated tem- porarily with an anti-rust oil were cleaned by spraying them with an aqueous solution containing 20 g/l sodium tertiary phosphate at 60 C for 20 seconds and were then rinsed with water.The cleaned sheets were then immersed at room temperature for one minute in a treating bath prepared by mixing 500 ml. of an aqueous solution of 50 g. of Chinese gallotannin (available from Fuji Chemicals Go. under the trade name of Tannic Acid AL) and 10 g. of thiourea in demineralised water, and 2500 ml of an aqueous suspension of an organic polymer prepared by diluting an emulsion of 100 g.

of polyacrylate ester (available from Toyo Ink Co. under the trade name of Tocryl N142) in demineralised water, and adjusting the pH to 3.5. Excess liquid was removed by passing the treated sheets through rolls and the sheets were dried in a hot circulating oven at 1200C for 1 minute to form a coating.

Comparative Example 1 Cleaned galvanised steel sheets, pretreated in the same manner as in Example 1, were immersed at room temperature for one minute in 1 litre of an aqueous solution prepared by dissolving 50 g. of Chinese gallotannin in demineralised water and adjusting the pH to 3.5, passed through rolls to remove excess liquid, and dried in hot air circulating oven at 1200C for one minute.

Comparative Example 2 Test pieces as used in Example 1, were coated with an emulsion of 100 gIl of a polyacrylate as in Example 1 in as amount of about 10 g/m2 by the bar coating procedure and dried in a hot air recycling oven at 1200C for one minute to form a coating.

Table 1 shows the results of the 24 hour salt spray test l(JIS-Z-2371) for each of the products of the Example and Comparative Examples 1 and 2.

Table 1 Specimen Appearance of Coating Salt spray test results Example Clear and colourless No abnormality Comparative 1 Milky white 80 - 90% white rust Comparative 2 Clear and colourless 100% white rust The test sheets prepared in the Example and in Comparative Examples 1 and 2 were coated with an alkyd melamine paint (available from Dai Nippon Toryo Co.

under the trade name of Ilelicon No. 700 white) by the bar coating procedure and baked in a hot air circulating oven at 140"C for 25 minutes to provide a film having a thickness of about 30 microns. Table 2 show the results of cross-hatching, Erichsen and impact tests for each painted sheet (these tests are described below).

Corrosion tests on the painted sheets were carried out by scratching the paint film to the surface of the base metal with a knife and subjecting the sheets to the salt spray test according to JIS-Z-2371 for 120 hours.

The coated surfaces were then rinsed with water and dried, and the corroded width was measured by pressing and removing rapidly a Cellophane +(Registered Trade Mark) tape applied on the cut Table 2 Adhesion of Painted film Cross-hatching Erichsen Impact Corroded with after Specimen test test test salt spray test Example ,100/100 100/100 No abnormality 3 mm Comparative 1 100/100 50/100 more than 50% stripped 10 mm Comparative 2 100/100 100/100 No abnormality 10 mm Cross-hatching test: The film was cross-hatched to form 100 1 mm X 1 mm. squares to a depth reaching to the base metal and a Cellophane tape was pressed against the cross-hatched squares and removed suddenly. The result was represented by a number of the un- stripped squares/100 squares.

Erichsen test: The film was cross-hatched to form 100 1 mm X 1 mm. squares. The sheet was then pressed with an Erichsen tester to a displacement of 5 mm and a Cellophane tape was applied to the deformed squares and then removed.

Impact test: The film was impacted by a 500 g. weight inch in diameter falling through a distance of 50 cm. A Du Pont impact tester was used. The film was then stripped off using a Cellophane tape.

The test sheets prepared according to the Example and Comparative Examples 1 and 2 were cleaned with 20 g/l of an alkaline cleaner available from Nihon Parkerizing Co. Ltd under the trade name Finecleaner 4326) at 600C by spraying for 60 seconds, rinsed with water and then dried in hot air.

The cleaned sheets were then subjected to the salt spray test according to JIJ-Z-2371 to test the corrosion resistance iof the protective coatings.

Table 3 shows the results which were obtained.

Table 3 Specimen 24 hour salt spray test Corroded width after 120 hour salt spray test Example 1Q--2046 white rust 5 mm Comparative 1 100% white rust More than 10 mm Comparative 2 100% white rust IMore than 10 mm Untreated sheet 100% white rust More than 10 mm WHAT WE CLAIM IS:- 1. A process for treating an uncoated metal surface which comprises coating the surface with a substantially chromium-free aqueous composition comprising a watersoluble or water-dispersible organic polymer (other than a mel amine- formaldehyde resin) and a vegetable tannin.

2. A process according to claim 1 in which the composition has a pH of from 2 to 6.5.

3. A process according to claim 2 in which the composition contains from 0.1 to 20% by weight d the tannin.

4. A process according to claim 1 sub- stantially as described in the Example.

5. A metal surface when treated by a process according to any preceding claim.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. Table 2 Adhesion of Painted film Cross-hatching Erichsen Impact Corroded with after Specimen test test test salt spray test Example ,100/100 100/100 No abnormality 3 mm Comparative 1 100/100 50/100 more than 50% stripped 10 mm Comparative 2 100/100 100/100 No abnormality 10 mm Cross-hatching test: The film was cross-hatched to form 100 1 mm X 1 mm. squares to a depth reaching to the base metal and a Cellophane tape was pressed against the cross-hatched squares and removed suddenly. The result was represented by a number of the un- stripped squares/100 squares. Erichsen test: The film was cross-hatched to form 100 1 mm X 1 mm. squares. The sheet was then pressed with an Erichsen tester to a displacement of 5 mm and a Cellophane tape was applied to the deformed squares and then removed. Impact test: The film was impacted by a 500 g. weight inch in diameter falling through a distance of 50 cm. A Du Pont impact tester was used. The film was then stripped off using a Cellophane tape. The test sheets prepared according to the Example and Comparative Examples 1 and 2 were cleaned with 20 g/l of an alkaline cleaner available from Nihon Parkerizing Co. Ltd under the trade name Finecleaner 4326) at 600C by spraying for 60 seconds, rinsed with water and then dried in hot air. The cleaned sheets were then subjected to the salt spray test according to JIJ-Z-2371 to test the corrosion resistance iof the protective coatings. Table 3 shows the results which were obtained. Table 3 Specimen 24 hour salt spray test Corroded width after 120 hour salt spray test Example 1Q--2046 white rust 5 mm Comparative 1 100% white rust More than 10 mm Comparative 2 100% white rust IMore than 10 mm Untreated sheet 100% white rust More than 10 mm WHAT WE CLAIM IS:-

1. A process for treating an uncoated metal surface which comprises coating the surface with a substantially chromium-free aqueous composition comprising a watersoluble or water-dispersible organic polymer (other than a mel amine- formaldehyde resin) and a vegetable tannin.

2. A process according to claim 1 in which the composition has a pH of from 2 to 6.5.

3. A process according to claim 2 in which the composition contains from 0.1 to 20% by weight d the tannin.

4. A process according to claim 1 sub- stantially as described in the Example.

5. A metal surface when treated by a process according to any preceding claim.