Chromium-free surface-treated tinplate, production method and surface treating agent therefor

Abstract

Provided are a chromium-free surface-treated tinplate, a production method and a surface treating agent thereof. By coating, on the surface of a tinplate, an environmentally friendly aqueous surface treating agent containing 0.1-5 wt % of a zinc salt, 0.1-5 wt % of a zirconium salt and/or a molybdenum salt and 5-30 wt % of siloxane or polysiloxane, a layer of chromium-free passivation film having uniform and dense ingredients and a good performance and being stable is formed on the surface of a tin layer. The passivation film contains 0.1-20 mg/m.sup.2 of zinc, 0.1-20 mg/m.sup.2 of zirconium and/or molybdenum and 0.5-100 mg/m.sup.2 silicon. The passivation film can impart an excellent surface stability, corrosion resistance and paint film adhesion performance to the surface of the tinplate; in addition, contact with food is safe. The tinplate is comparable to chromium passivation in performance, and the production process thereof does not use a chromate, so that a truly green production process of a tinplate is achieved, complying with the requirements of increasingly strict environmental protection laws and regulations.

Claims

1. A chromium-free surface-treated tinplate, wherein a chromium-free passivation film is formed on a surface of a tin layer, wherein the chromium-free passivation film comprises 0.1-20 mg/m.sup.2 of zinc, 0.1-20 mg/m.sup.2 of zirconium and/or molybdenum, and 0.5-100 mg/m.sup.2 silicon, and wherein the chromium-free passivation film is also fluorine-free and phosphorus-free, wherein the zinc in the chromium-free passivation film is from a zinc salt, and the zinc salt is selected from the group consisting of zinc sulfate, zinc acetate, zinc nitrate, zinc gluconate, and zinc methionine; and the silicon in the chromium-free passivation film is from an organosiloxane or polysiloxane, and the organosiloxane or polysiloxane is obtained by hydrolysis of an epoxy silane coupling agent.

2. The chromium-free surface-treated tinplate of claim 1, wherein the zirconium in the chromium-free passivation film is from a zirconium salt; the molybdenum in the chromium-free passivation film is from a molybdenum salt.

3. The chromium-free surface-treated tinplate of claim 2, wherein the zirconium salt is selected from the group consisting of zirconium oxysulfate, zirconium oxynitrate, ammonium zirconium carbonate, tetrabutyl zirconate, and zirconium isopropoxide.

4. The chromium-free surface-treated tinplate of claim 2, wherein the molybdenum salt is selected from the group consisting of molybdic acid, ammonium molybdate, sodium molybdate, and potassium molybdate.

5. A method for producing the chromium-free surface-treated tinplate of claim 1, comprising: a) electrotinning process and soft melting treatment, wherein a phenolsulfonic acid tin plating or methanesulfonic acid tin plating process is used as the electrotinning process, wherein the tin layer is subjected to the soft melting treatment after the phenolsulfonic acid tin plating or methanesulfonic acid tin plating process is finished; b) washing, after the soft melting treatment, a surface of a tinplate by immersing the tinplate in water or spraying water to the surface of the tinplate, and removing redundant water on the surface of the tinplate using a wringing roll; c) coating the surface of the tinplate by spraying or rolling an aqueous surface treating agent, and removing redundant aqueous surface treating agent with a wringing roll, wherein a liquid film of the aqueous surface treating agent is coated uniformly; and d) drying the surface of the tinplate coated with the aqueous surface treating agent in hot air, wherein a temperature of the hot air is controlled between 80-120° C., and a drying time is 0.2-2 seconds, and wherein the aqueous surface treating agent is dried into a film and the chromium-free surface-treated tinplate is obtained.

6. The method of claim 5, further comprising an immersing step prior to the coating step of step c), wherein the immersing step comprises immersing the tinplate in the aqueous surface treating agent for 0.2-5 seconds.

7. The method of claim 5, wherein the aqueous surface treating agent comprises 0.1-5 wt % of the zinc salt, 0.1-5 wt % of the zirconium salt and/or the molybdenum salt, 5-30 wt % of an organosiloxane or polysiloxane and a balance of water, wherein the aqueous surface treating agent has a pH of 3-6, and wherein the zinc salt is selected from the group consisting of the zinc sulfate, zinc acetate, zinc nitrate, zinc gluconate, and zinc methionine, and the organosiloxane or polysiloxane is obtained by the hydrolysis of the epoxy silane coupling agent.

8. The method of claim 7, wherein the aqueous surface treating agent further comprises at least one of a reinforcing agent, a wetting agent and an organic acid regulator, wherein the reinforcing agent has a content of 0.1-2 wt %, the wetting agent has a content of 0.1-2 wt %, and the organic acid regulator has a content of 0.1-1 wt %.

9. The method of claim 8, wherein the reinforcing agent is polyvinyl alcohol, and the wetting agent is polyethylene glycol.

10. The method of claim 8, wherein the organic acid regulator is selected from the group consisting of citric acid, acetic acid, and fumaric acid.

11. The method of claim 7, wherein the zirconium salt is selected from the group consisting of zirconium oxysulfate, zirconium oxynitrate, ammonium zirconium carbonate, tetrabutyl zirconate, and zirconium isopropoxide.

12. The method of claim 7, wherein the molybdenum salt is selected from the group consisting of molybdic acid, ammonium molybdate, sodium molybdate, and potassium molybdate.

Description

DETAILED DESCRIPTION OF THE INVENTION

(1) The disclosure is further illustrated with reference to the following specific Examples.

(2) Table 1 lists the ingredients in the aqueous surface treating agents for the chromium-free passivated tinplates in Examples 1-10 and the treatment process according to the disclosure, wherein the contents of the various ingredients in the surface treating agents are based on mass percentage (wt %), and water makes up the balance; wherein the treating method means direct coating of an aqueous surface treating agent, or immersion plus subsequent coating, and the treating time means a total amount of time needed from immersion+coating or direct coating to completion of baking.

(3) The method for producing a chromium-free surface-treated tinplate according to the disclosure comprises the following steps:

(4) 1) a black sheet for a tinplate was subjected to an electrotinning process and then soft melting treatment of the tin layer, wherein a phenolsulfonic acid tin plating or methanesulfonic acid tin plating process was used as the electrotinning process, wherein the tin layer was subjected to the soft melting treatment after the tin plating was finished;

(5) 2) after the soft melting, the tinplate surface was washed by immersing the tinplate in distilled water or sprinkling distilled water to the tinplate surface for washing, and the redundant water on the tinplate surface was removed using a wringing roll;

(6) 3) the tinplates in Examples 1-5, 7, 9-10 were immersed in the corresponding aqueous surface treating agents for 0.2-5 seconds;

(7) 4) the aqueous surface treating agents of Examples 1-10 were coated onto the immersed or un-immersed tinplate surfaces by spraying or rolling, and a wringing roll was used to wring out the redundant aqueous surface treating agents, such that the liquid films of the aqueous surface treating agents had uniform thicknesses, wherein the film thickness could be adjusted depending on the spray amount or coating amount, and the pressure of the wringing roll;

(8) 5) the tinplate surfaces coated with the surface treating agents were dried in hot air, wherein the temperature of the hot air was controlled between 80-120° C., and the drying time was 0.2-2 seconds, wherein the aqueous surface treating agents were dried into films, so that chromium-free surface-treated tinplates were obtained.

(9) After chromium-free passivated tinplate samples were prepared according to Examples 1-10 of the disclosure, the resulting chromium-free surface-treated tinplates were evaluated for baking discoloration resistance, paint film adhesion and corrosion resistance. The evaluation results are shown in Table 2, compared with a chromium passivated comparative sample, wherein the comparative sample was a conventional tinplate sample treated by chromate electrolytic passivation, wherein the chromium content in the passivation film of the comparative sample was 5 mg/m.sup.2.

(10) The evaluation items are as follows:

(11) 1) Baking discoloration resistance

(12) Working conditions during coating of a tinplate were simulated, wherein the surface-treated tinplates obtained in the Examples were baked with hot air at 200° C. for 60 minutes. The tinplate surfaces were observed to see if baking discoloration occurred, so as to investigate their baking discoloration resistance.

(13) 2) Paint film adhesion

(14) The method for evaluating paint film adhesion made reference to the method for evaluating paint film adhesion adopted in QB/T 2763-2006 “Coating of Tin (or Chromium) Plated Thin Steel Plates”. A commercially available epoxy phenolic coating was used as a coating to coat the tinplate surfaces treated with the passivating agents of the disclosure. The dry film weight of the tinplate coating was 6-8 g/m.sup.2. After the paint film surface was scratched and peeled with adhesive tape, the degree to which the paint film was detached from the surface was inspected. The paint film adhesion was evaluated based on the area of the paint film that fell off, and compared with the chromium passivated sample.

(15) 3) Sulfide staining resistance

(16) The method for evaluating the sulfide staining resistance made reference to the method for evaluating the sulfide staining resistance in QB/T 2763-2006 “Coating of Tin (or Chromium) Plated Thin Steel Plates”. The formation of sulfide stains on the surfaces of the samples treated with the passivating agents of the disclosure was observed based on the testing results, and a comparison was made with the chromium passivated sample.

(17) 4) Acid resistance

(18) The method for evaluating the acid resistance made reference to the method for evaluating the acid resistance in QB/T 2763-2006 “Coating of Tin (or Chromium) Plated Thin Steel Plates”. The formation of acid stains on the surfaces of the samples treated with the passivating agents of the disclosure was observed based on the testing results, and a comparison was made with the chromium passivated sample.

(19) As can be seen from Table 2, the tinplates made according to the method involving the chromium-free surface treatment of the disclosure have achieved performances comparable with those of the chromium passivated comparative sample in terms of baking discoloration resistance, paint film adhesion, sulfide staining resistance and acid resistance, among which the paint film adhesion and corrosion resistance are even better.

(20) TABLE-US-00001 TABLE 1 Zn salt Mn salt Zr salt Organosiloxane or Polyvinyl Treating time No. (wt %) (wt %) (wt %) polysiloxane (wt %) alcohol (wt %) pH Treating method (s) Ex. 1 0.1 5 — 10 2 5 Immersion + coating 5 Ex. 2 0.1 — 5 10 2 5 Immersion + coating 5 Ex. 3 0.5 3 — 10 1 5 Immersion + coating 3 Ex. 4 0.5 — 3 20 1 4 Immersion + coating 3 Ex. 5 1 1 1 20 0.5 4 Immersion + coating 1 Ex. 6 1 0.5 0.5 20 0.5 4 Coating 0.5 Ex. 7 3 0.5 — 30 1 3 Immersion + coating 1 Ex. 8 3 — 0.5 30 1 3 Coating 0.5 Ex. 9 5 0.1 — 5 0.2 6 Immersion + coating 2 Ex. 10 5 — 0.1 10 0.2 6 Immersion + coating 2

(21) TABLE-US-00002 TABLE 2 Baking discoloration Paint film Sulfide staining Acid resistance adhesion resistance resistance Ex. 1 ◯ ◯ ◯ ⊚ Ex. 2 ◯ ◯ ◯ ◯ Ex. 3 ◯ ◯ ◯ ◯ Ex. 4 ◯ ⊚ ⊚ ◯ Ex. 5 ◯ ◯ ◯ ⊚ Ex. 6 ◯ ⊚ ◯ ◯ Ex. 7 ◯ ⊚ ⊚ ◯ Ex. 8 ◯ ◯ ◯ ◯ Ex. 9 ◯ ◯ ◯ ◯ Ex. 10 ◯ ◯ ◯ ◯ Comparative ◯ ◯ ◯ ◯ Example Note: ⊚—good performance, better than chromium passivation; ◯—performance comparable with chromium passivation; .circle-solid.—performance inferior to chromium passivation.