Rust Removal Process Using Removable Coatings of Maleic Acid Copolymers

The invention relates to removal of rust from metal surfaces.

Adequate removal of rust from metal surfaces in preparation for the application of paint or other protective coatings is a long standing problem. Mechanical cleaning techniques such as sand blasting, wire brush scrubbing, etc. are messy and time consuming. Previous attempts to chemically clean rusty surfaces have not been entirely satisfactory.

One particularly difficult type of metal surface to clean is the irregular surfaces found on ships, i.e., high-temperature valves, pipes, and the like. Frequently, the only cleaning method feasible is the lengthy and tiresome process of wire brushing the surface to be cleaned and then subsequently applying a solution of a wetting agent mixed with a cleaning agent to the metal surface. Such a technique suffers from the difficulty of keeping the cleaning fluid in contact with the surface to be cleaned, such as overhead objects, as well as the subsequent disposal of liquid wastes. Additionally, these solutions are often toxic, non-economical, and require large volumes of water for washing purposes.

Previous attempts to chemically remove rust have involved the use of chemicals such as inhibited hydrochloric acid, ethylenediaminetetraacetic acid (EDTA), EDTA/citric acid, etc. More recently it has been suggested that a paste of water-soluble polymer such as polyvinylpyrrolidone (PVP) and a chelating agent such as EDTA be coated onto a rusty metal surface to be cleaned. After application the paste is said to harden into a thick crust which encapsulates the rust and may be peeled from the cleaned surface and disposed of as solid waste. This process is more fully described in U.S. Pat. No. 4,325,744.

The invention is a process for removing rust from a rusty metal surface which comprises:

(a) applying to said rusty surface a layer of rust removal coating composition consisting essentially of an aqueous solution of a copolymer of maleic acid and monomer of the formula CH.sub.2 .dbd.CHR wherein R represents H, CH.sub.3, OR.sup.1 or OCOR.sup.1 and R.sup.1 represents CH.sub.3 or CH.sub.2 CH.sub.3 ; and

(b) allowing said layer of coating composition to dry whereby rust becomes incorporated into said layer and the layer containing the rust detaches itself from the surface.

Maleic acid copolymer used in the invention may be formed by hydrolysis of precursor copolymer of maleic anhydride and monomer of the formula CH.sub.2 .dbd.CHR wherein R represents H, CH.sub.3, OR.sup.1 or OCOR.sup.1 and R.sup.1 represents CH.sub.3 or CH.sub.2 CH.sub.3. The precursor copolymer may be obtained by any of the conventional methods known for making such copolymers as exemplified for instance in U.S. Pat. Nos. 3,553,183, 3,794,622 and 3,933,763 the disclosures of which are incorporated herein by reference. In practicing the invention the copolymer is used in the form of an aqueous solution generally containing between about 5 and about 60 weight percent (wt %) polymer and between about 40 and about 95 wt % water. The molecular weight of the maleic acid copolymer used may vary widely. Copolymers having K values between about 20 and about 120 or even higher are for instance generally considered suitable for use in practicing the invention.

It will be appreciated that viscosities obtainable within the preferred limits of water content and K value mentioned above may vary widely, the major variable being the amount of water used. The choice of preferred viscosity for the rust removal coating compositions of the invention will depend largely upon the intended use. For instance for lightly rusted metal surfaces it may be desired to have a thin liquid coating having a viscosity for instance between about 50 and about 50,000 centipoises (cps) such that the coating can be applied with an ordinary paint brush to a thickness between about 0.01 and about 5 mm. For many applications a relatively high viscosity, paste like coating having a viscosity e.g. between about 10,000 and about 250,000 cps may be desired. Such high viscosity coatings may be easily applied even to overhead surfaces e.g. with a putty knife to form coatings of between about 0.5 and about 20 mm or thicker as desired. The paste like form of the copolymer is especially preferable for application to vertical or overhead surfaces where excessive dripping and flowing of the coating after it is applied to the rusted surface would be undesirable.

If desired the viscosity of coating composition for use in the invention may be increased by including in the composition one or more thickening agents in an amount sufficient to increase the viscosity of the composition to the desired value. For this purpose any conventional thickening agents may be used. When used, thickening agents are frequently used in amounts between about 0.1 and about 10 wt % based on total composition. Suitable thickening agents include for instance: natural or synthetic gums such as xanthan, guar, tragacanth, etc.; cellulose derivatives such as carboxyethyl cellulose; hydrogels such as crosslinked polymers of acrylic acid; and other synthetic thickening agents such as crosslinked poly(methyl vinyl ether/maleic anhydride). Crosslinked interpolymers of the type described in U.S. Pat. No. 3,448,088, the disclosure of which is incorporated herein by reference, are for instance suitable for this purpose.

The precursors of the copolymers for use in the process of the invention are maleic anhydride copolymers of the formula ##STR1## As mentioned the copolymer is used in the form of an aqueous solution. The copolymer as used in the aqueous solution is hydrolyzed and has the general formula ##STR2##

In practicing the invention is is generally preferred that the coating composition be applied to the rusty metal surface in a thickness of at least about 0.01 mm, more preferably between about 0.5 and about 2 mm. For heavily rusted surfaces it is preferred that the coating be at least about 1 mm thick to ensure suitably complete removal of rust. Coatings applied in the preferred thicknesses mentioned will, under most normal conditions, dry in periods of time between about 0.5 and about 8 hours. Drying time depends upon a number of conditions including primarily coating thickness and viscosity and atmospheric conditions, especially temperature and humidity. If coatings are allowed to dry completely the rust becomes incorporated in the coating (assuming the coating is sufficiently thick for the amount of rust on the surface of the metal) and the dried coating containing the rust becomes detached from the metal surface in the form of e.g. flakes or small strips which may remove themselves by separating from the metal surface or may be easily removed such as by brushing or blowing. In the case of overhead surfaces the self-removing feature is such that it is usually sufficient merely to allow the flakes or strips of dried coating to fall from the surface of the metal under the influence of gravity. The self-removing property of the copolymers used is relatively insensitive with respect to variations in temperature and humidity. Under some conditions, such as when the coating is not allowed to dry completely, it may be necessary to brush or scrape the surface to completely remove the rust laden coating. While the exact mechanism by which the rust is incorporated into the coating becomes detached from the metal surface is not fully understood, it is believed that the coating composition soaks into and complexes the rust with the film forming properties of the coating being such that the coating containing rust tends as it becomes completely dry to detach spontaneously from the metal surface.

The following examples are intended to illustrate the invention without limiting the scope thereof. The material identified in the examples as VAZO 52 is azo-isobutyro valeronitrile initiator available from duPont.

Eighteen (18) grams of commercial copolymer poly(methylvinylether-co-maleic anhydride)-Gantrez AN 139, a product of GAF Corporation, was placed in a glass jar with screw-cap and 80 grams distilled water was added. The jar was placed on a shaker and was shaken at room temperature, until a clear solution was obtained, indicating complete hydrolysis.

The polymer solution was analyzed with the following results:

Solids: 20.0%

K-Value: 107.8

Acid Number: 643.43 (Theory: 648.56)

pH: 2.9

Brookfield Viscosity: 9100 cps (20%)

A 28 gauge sheet of black iron, the surface of which was covered with rust, was placed flat on a bench, and was coated with a 1.27 mm thick, 63 mm wide layer of the copolymer solution, using a doctor knife.

The coated metal was allowed to stand overnight. Next morning the brittle film was found to be separated completely from the metal substrate with the rust firmly embedded in the separated film. The surface of the metal was completely free of rust.

Thirty-five (35) grams of commercial poly(ethylene-co-maleic anhydride)-EMA 21, a product of Monsanto Chemical Company--was placed in a screw-cap jar and 65 grams distilled water was added. The jar was placed on a shaker and the mixture was shaken at room temperature, until clear solution was obtained.

The polymer solution was analyzed with the following results:

Solids: 33.34%

K-Value: 56.6

Acid Number: 972.44 (Theory: 977.60)

pH: 2.8

Brookfield Viscosity: 6920 cps (as is)

A 28 gauge sheet of black iron, the surface of which was covered with rust, was placed flat on a bench, and was coated with a 1.27 mm thick, 63 mm wide layer of the copolymer solution, using a doctor knife.

The coated metal was allowed to stand overnight. Next morning the brittle film was found to be separated completely from the metal with the rust firmly embedded in the separated film. The surface of the metal was completely free of rust.

A two liter kettle, equipped with mechanical stirrer, reflux condenser, gas inlet tube and thermometer was purged thoroughly with nitrogen. To the kettle were charged in the following sequence:

840.0 g toluene

294.0 g maleic anhydride

64.5 g vinyl acetate, and

3.0 g VAZO 52

The system was heated to 65.degree. C. and this temperature was held for 15 minutes. After that, 193.5 g vinylacetate was placed in a dropping funnel and was added to the reaction mixture in 1 hour while maintaining the temperature. After the addition was over, the temperature was held for 1 more hour, then 0.5 g VAZO 52 was added. The temperature was kept at 65.degree. C. and the addition of 0.5 g VAZO 52 was repeated twice at one hour intervals, until the test gave negative results for maleic anhydride.

The polymer slurry was filtered, then the cake was reslurried in 600 ml methylene chloride. The slurry was agitated for 1/2 hour at room temperature and then it was filtered. The filtered polymer was washed three times with 100 ml methylene chloride, then it was dried in a vacuum at 80.degree. C.

The analysis of the polymer was as follows:

Solids: 98.58%

Acid Number: 604.88 (Theory: 601.12)

Thirty-five (35) grams of this copolymer was then placed in a glass jar with screw-cap and 65 grams distilled water was added. The jar was placed on a shaker and the mixture was shaken at room temperature until a clear solution was obtained indicating complete hydrolysis. The polymer solution was analyzed with the following results:

Solids: 34.97%

K-Value: 38.1

Acid Number: 210.28

Brookfield Viscosity: 3450 cps

Relative Viscosity (1%): 1.3832

A 28 gauge sheet of black iron, the surface of which was covered with rust, was placed flat on a bench and was coated with a 1.27 mm thick 63 mm wide layer of the copolymer solution using a doctor knife.

The coated metal was allowed to stand overnight. Next morning the brittle film was found to be separated completely from the metal substrate with the rust firmly embedded in the separated film. The surface of the metal was completely free of rust.

The procedure of example 1 may be used to practice the invention using poly(ethylvinylether-co-maleic anhydride) or poly(propylvinylether-co-maleic anhydride) as the precursor copolymer rather than the poly(methylvinylether-co-maleic anhydride) of example 1. Likewise the procedures of example 3 may be followed using vinyl propionate rather than the vinyl acetate of example 3.

While the invention has been described above with respect to preferred embodiments thereof, it will be understood by those skilled the art that various changes and modifications may be made without departing from the spirit or scope of the invention.