Claims
- 1. A process for producing a coherent protective layer on a metallic surface, said process comprising the steps of:
- forming a layer of tin oxide on the metallic surface; and
- reacting the tin oxide layer with an aqueous solution of a homo- or copolymer of polyacrylic acid) or even more highly carboxylated unsaturated-carbon-backbone acid or a hydrolyzable precursor thereof.
- 2. A process according to claim 1, wherein said tin oxide layer is deposited on the surface electrochemically.
- 3. A process according to claim 1, wherein said tin oxide is formed anodically.
- 4. A process according to claim 2, wherein said tin oxide is formed anodically in the presence of borate.
- 5. A process according to claim 2, wherein the electrolysis continues for under one minute.
- 6. A process according to claim 1, wherein the acid has a concentration of under 5%.
- 7. A process according to claim 6, wherein said concentration is under 1%.
- 8. A process according to claim 1, wherein the deposit with acid is cured from 80.degree. C. to 100.degree. C. for from 10 to 60 minutes at at least 80% relative humidity.
Priority Claims (1)
| Number |
Date |
Country |
Kind |
| 8725714 |
Nov 1987 |
GBX |
|
Parent Case Info
This is a division of application Ser. No. 257,096, filed Oct. 13, 1988.
This invention relates to coating metallic substrates, particularly to prevent their interaction with fluid media which they might otherwise contact, and to substrates so coated, particularly for packaging, e.g. for beverages.
Polyelectrolyte coatings such as poly(acrylic acid) are known for this purpose but have first to be insolubilised. Thus, Japanese Pat. Publication No. 74 31604 suggests that tinned steel sheet may be provided with scratch-resistant coatings which have good adhesion to paint by dipping the sheet into a solution containing poly(acrylic acid) and/or an acrylic acid-vinyl alcohol copolymer and then heating to a temperature below the melting point of tin. However, such coatings are found not to be acid resistant.
In an attempt to overcome this drawback, UK Pat. application No. GB2173805A describes a process for the preparation of a coherent protective layer on a metallic substrate, which process in one example comprises applying to the substrate an aqueous solution of poly(acrylic acid) partially neutralised by sodium hydroxide, and heat-curing the layer so formed in air for 10 minutes at 235.degree. C.
According to the present invention, a process for the preparation of a coherent protective layer on a metallic surface comprises (preferably electrochemically) depositing on the surface a layer of a metal capable of forming a basic oxide optionally forming said oxide optionally anodically and reacting the deposit with a (preferably electrolytically applied) aqueous solution of a homo- or copolymer of poly(acrylic acid) or an even more highly carboxylated unsaturated-carbon-backbone acid or a hydrolysable precursor thereof.
The deposited metal is preferably zinc, in which case either one of the following is not optional/preferable but essential: zinc oxide is formed, or the poly(acrylic acid) is applied to the substrate electrochemically; the latter is preferred preferably at under 1% acid concentration for under 1 minute. Aluminium does not form a satisfactory basic oxide for the purposes of the invention. The deposited metal may be tin, in which case the step of forming its oxide is not optional but is essential, and electrolytic application of the acid is not preferred.
The aqueous solution is preferably less concentrated than 5% by weight, preferably less than 1% by weight, and the acid may be encouraged to react with the deposit by making the latter the anode in the aqueous solution. This optional anodising may be additional to the optional (earlier) anodising to form the basic oxide. Electrolysis is advantageous in ensuring an even and controlled formation of the layer, and may be brief, e.g. under 1 minute.
The deposit is advantageously reacted with the acid by curing it at from 80.degree. C. to 100.degree. C. for from 10 to 60 minutes at at least 80%, (preferably 100%) relative humidity, although this can be replaced by ageing under room conditions for a week. Both the curing and the ageing can non-preferably be omitted.
The reaction product (thus, preferably zinc polyacrylate) is preferably from 1 to 50 more preferably 5 to 10, microns thick.
The invention extends to the substrate thus coated, and the containers or other artefacts made from the coated substrate.
US Referenced Citations (1)
| Number |
Name |
Date |
Kind |
|
2215165 |
Sumner |
Sep 1940 |
|
Foreign Referenced Citations (1)
| Number |
Date |
Country |
| 2173805A |
Oct 1986 |
GBX |
Non-Patent Literature Citations (1)
| Entry |
| Nishihara, T., et al., "Surface Treatment of Tinned Steel Sheets" Abstract, 159358n. |
Divisions (1)
|
Number |
Date |
Country |
| Parent |
257096 |
Oct 1988 |
|
Patent Grant
4976829
