AQUEOUS DISPERSION FOR ACTIVATING A METAL SURFACE AND METHOD FOR THE PHOSPHATING THEREOF

Abstract

The present invention relates to an aqueous dispersion as a concentrate for the activation stage of phosphating of metal surfaces, containing a dispersed particulate constituent and a thickener, the particulate constituent containing, in addition to dispersed inorganic compounds of polyvalent metal cations, polymeric organic compounds as dispersing agents which are composed at least partially of styrene and/or an -olefin having no more than 5 carbon atoms and maleic acid, its anhydride and/or its imide, and which additionally comprise polyoxyalkylene units. The aqueous dispersion is further characterized by a D50 value above 10 m. The present invention also relates to a method for anti-corrosion pretreatment of the surfaces of a metal material, in particular for zinc phosphating.

Claims

1. An aqueous dispersion having a D50 value of more than 10 m, containing: (a) at least 5 wt. % of a dispersed particulate constituent comprising (a1) at least one particulate inorganic compound of a polyvalent metal cation; and (a2) at least one polymeric organic compound composed at least partially of styrene and/or an -olefin having no more than 5 carbon atoms and maleic acid, its anhydride and/or its imide and which additionally comprises polyoxyalkylene units; and (b) at least one thickener.

2. The dispersion according to claim 1, wherein the at least one particulate inorganic compound (a1) of the dispersed particulate constituent (a) is composed at least partially of phosphates, present in an amount of at least 25 wt. % based on the dispersed inorganic particulate constituent calculated as PO.sub.4.

3. The dispersion according to claim 2, wherein the phosphates present in the at least one particulate inorganic compound (a1) are composed at least partially of one or more of hopeite, phosphophyllite, scholzite and hureaulite.

4. The dispersion according to claim 1, wherein the aqueous dispersion has a D90 value of less than 150 m.

5. The dispersion according to claim 1, wherein the aqueous dispersion has a bimodal particle size distribution and optionally one distribution maximum for a particle size below 1 m and another distribution maximum for a particle size above 10 m.

6. The dispersion according to claim 1, wherein the polymeric organic compounds (a2) contain the polyoxyalkylene units in their side chains, and total content of the polyoxyalkylene units in the polymeric organic compounds (a2) is at least 40 wt. %, but does not exceed 70 wt. %.

7. The dispersion according to claim 1, wherein the polymeric organic compounds (a2) further comprise imidazole units.

8. The dispersion according to claim 7, wherein the polyoxyalkylene units of the polymeric organic compounds (a2) are at least partially end-capped with an imidazole group.

9. The dispersion according to claim 1, wherein the polymeric organic compounds in the particulate constituent (a) have a total amine value of at least 25 mg KOH/g, but less than 125 mg KOH/g.

10. The dispersion according to claim 1, wherein the polymeric organic compounds in and based on the particulate constituent (a) is at least 3 wt. % and does not exceed 15 wt. %.

11. The dispersion according to claim 1, wherein the at least one thickener according to component (b) is selected from urea urethane resins which have an amine value of less than 8 mg KOH/g.

12. The dispersion according to claim 1, wherein the thickeners according to component (b) are present in an amount of at least 0.5 wt. %, the total content of polymeric organic compounds in the non-particulate constituent does not exceed 4 wt. %.

13. The dispersion according to claim 1, wherein the aqueous dispersion has a pH of above 7.2, but below 10.0.

14. The dispersion according to claim 1, wherein the aqueous dispersion has a maximum dynamic viscosity at a temperature of 25 C. in a shear rate range of from 0.001 to 0.25 reciprocal seconds of at least 1000 Pa.Math.s and below 5000 Pa.Math.s.

15. A method for anti-corrosion pretreatment of a metal material selected from zinc, iron or aluminum or of a component which is composed at least partially of such metal materials, comprising: (i) activating the metal material or the component by contact with a colloidal aqueous solution comprising the aqueous dispersion of claim 1 diluted by a factor of from 20 to 100,000; (ii) then phosphating, optionally zinc phosphating, the metal material or the component.

16. The method according to claim 15, wherein the components are made at least partially of zinc and aluminum, and are pretreated in series, and (i) the activating step is carried out by: contact with a colloidal aqueous solution prepared by diluting the aqueous dispersion according to claim 1, wherein the total content of particulate inorganic compounds (a1) is composed at least partially of phosphates selected from the group consisting of hopeite, phosphophyllite, scholzite, hureaulite and combinations thereof; and (ii) the phosphating step is carried out by: contact with an acidic aqueous composition containing 5-50 g/kg of phosphates dissolved in water, calculated as PO.sub.4, 0.3-3 g/kg of zinc ions and an amount of free fluoride, measured per total acidic aqueous composition; wherein a quotient of the concentration of the phosphates in the inorganic particulate constituent of the colloidal aqueous solution of the activating step (i), in mmol/kg, based on PO.sub.4; with respect to a sum of the amount of free fluoride and an amount of silicon in the acidic aqueous composition of the phosphating step, in each case in mmol/kg, is greater than 0.2.

Description

EMBODIMENTS

[0064] The properties of a dispersion according to the invention with regard to stability, flow behavior and suitability for activation in zinc phosphating are set out below.

[0065] Preparation of the Pigment Paste

[0066] To prepare a pigment paste for providing a dispersion according to the invention, 15 parts by mass of Edaplan 490 (Mnzing Chemie GmbH) were predispersed as dispersing agents in 25 parts by weight of fully deionized water (<1 Scm.sup.1) and then mixed with 60 parts by mass of zinc phosphate of quality level PZ 20. This phase was transferred to a KDL type Dyno-Mill bead mill and the zinc phosphate particles were continuously milled for two hours (milling parameters: 75% bead fill level, 2000 revolutions per minute, 20 L volumetric flow per hour, temperature of the milled material 40-45 C.). The result was an average particle size of approximately 0.35 m determined using a Zetasizer Nano ZS from Malvern. On the basis of the dispersing agents to be used according to the invention, in this case Edaplan 490, an optimal primary particle size for activation can therefore be achieved with conventional acceptable mechanical or time expenditure.

[0067] Preparation of the Dispersion According to the Invention

[0068] 2.5 parts by mass of a urea urethane resin solution containing 40 wt. % of the resin based on an amine-modified prepolymer of TDI XDI and PEG-16 (amine value<1 mg KOH/g; hydroxyl number approximately 40 mg KOH) in approximately 64 parts by mass of fully deionized water (<1 Scm.sup.1) was then supplied as a thickener, homogenized, and adjusted to pH 9 using 10% sodium hydroxide solution. Then, approximately 33 parts by mass of the pigment paste were added while stirring, adjusted to pH 9 using 1 wt. % NaOH solution and stirred to the point of complete homogenization. A sample of the dispersion according to the invention produced in this way was analyzed by laser diffraction in accordance with ISO 13320:2009 as specified in the description. For this purpose, 110 mg of the dispersion was added to 200 mL of fully deionized water (<1 Scm.sup.1) and the sample volume provided in this way was placed into the Retsch Horiba LA-950 particle analyzer, and after 60 seconds the particle size distribution curve in the sample volume was determined by laser diffraction. After evaluation as specified in the description a D50 value of 29 m was obtained (D10 value: 0.4 m; D90 value: 57 m).

[0069] The dispersion according to the invention has a pronounced thixotropic flow behavior with a maximum viscosity of 2200 Pas at a shear rate of 0.002 s.sup.1 and a dynamic viscosity below 100 Pas at a shear rate of 0.1 s.sup.1, in each case determined at 25 C. using a cone and plate viscometer with a cone diameter of 35 mm and a gap width of 0.047 mm. This is advantageous for preventing sedimentation during storage of the dispersion, and also facilitates pumpability and thus the provision and readjustment of activation baths during zinc phosphating.

[0070] Preparation of an Activation Solution for Zinc Phosphating According to the Invention

[0071] 5 liters of fully deionized water (<1 Scm.sup.1) was provided in a 5 L beaker and mixed with 3 grams of an additive solution containing 20.4 wt. % potassium pyrophosphate and 28 wt. % potassium phosphate and brought to pH 10.5 using phosphoric acid while stirring, and 10 grams of the dispersion according to the invention was added. The pH was then adjusted to 10.5 using 1% sodium hydroxide solution while stirring.

[0072] Use of the Activation Solution in Zinc Phosphating According to the Invention

[0073] For layer-forming phosphating by activation based on the dispersion according to the invention, sheets of cold-rolled steel (CRS), hot-dip galvanized steel (HDG) and aluminum (AA6014) were: [0074] a) firstly alkaline cleaned while stirring in service water (pH: 10.2-10.9; 55 C.) by being immersed for 5 minutes in a degreasing bath containing 4 wt. % of Bonderite C-AK 1565 A and 0.6 wt. % of Bonderite C-AD 1561, each of which is available from Henkel AG & Co. KGaA; [0075] b) subjected to rinsing under service water and then under fully deionized water (<1 Scm.sup.1) for approximately 30 seconds in each case; [0076] c) in a water-wetted state, brought into contact with the activation solution by immersion for 60 seconds; [0077] d) and immediately afterwards, and without further rinsing steps, immersed into a hydroxylamine-accelerated phosphating bath having a free acid content of 0.9-1.4 points (titrated to a pH of 3.6), a total acid content of 25-30 points (titrated to a pH of 8.5) and a free fluoride content of approximately 150 mg/kg, containing 4.6 wt. % of Bonderite M-ZN 1994, 0.8 wt. % of Bonderite M-AD 565, 0.24 wt. % of Bonderite M-AD 338 and 0.38 wt. % of Bonderite M-AD 110, each of which is available from Henkel AG & Co. KGaA, in fully deionized water (<1 Scm.sup.1), for 3 min while stirring at 52 C.; [0078] e) subjected to rinsing with fully deionized water (<1 Scm.sup.1) for approximately 30 s; and [0079] f) provided with an approximately 20 m thick layer of an electrocoat of the type Cathoguard 800 (BASF SE) and then cured at 180 C. for 35 min.

[0080] Table 1 summarizes the results of zinc phosphating with regard to layer weight and after aging in the corrosion test. It is apparent that homogeneous, closed zinc phosphate coatings are always produced and excellent anti-corrosion results are achieved at a relatively low layer weight.

TABLE-US-00001 TABLE 1 Substrate Layer weight.sup.1/gm.sup.2 Corrosion.sup.2,3/mm CRS 1.6 0.5.sup.2 HDG 2.1 1.8.sup.2 AA6014 1.6 3.5.sup.3 .sup.1differential gravimetric determination after detaching the phosphate layer in aqueous 5 wt. % chromic acid solution. .sup.2delamination at the scratch after aging in a VW PV1210 alternating climate test for 6 weeks over 30 cycles .sup.3longest filiform corrosion thread according to DIN EN 3665