EP0340529B1 - Hard water stabilizing additive for activating agents before zinc phosphating - Google Patents

Description

The invention relates to the use of additives for treatment baths for the activation of metal surfaces made of iron or steel, zinc or galvanized Sathl as well as aluminum or aluminized steel before phosphating the said surfaces with phosphating baths containing zinc ions, in particular before a so-called low zinc phosphating, in which the Ratio of zinc to phosphate ions in the treatment solution is less than 1:12.

Processes for producing phosphate layers on iron or steel surfaces with the aid of phosphoric acid solutions that contain various polyvalent metal cations as well as accelerating additives (e.g. oxidizing agents) have long been proven prior art. Such processes are used in particular in the automotive industry in order to achieve improved corrosion protection for automobile bodies. The phosphated surfaces are then painted, preferably by cathodic electrocoating.

The usual materials used for body construction, conventionally iron or steel sheets, are phosphated. more recently also increasingly galvanized or hot-dip galvanized steel or materials with a surface made of zinc alloys which contain, for example, iron, nickel, cobalt or aluminum as alloying partners. Corrosion-inhibiting phosphating of such surfaces is common not only in automobile construction, but also in the manufacture of household appliances such as washing machines or refrigerators.

Before the treatment mentioned above, the workpieces are cleaned, rinsed and activated in order to achieve a thin and uniform phosphate layer during phosphating, which is known to be a prerequisite for good protection against corrosion. In the long-established "high zinc phosphating processes" it was possible to remove adhering oils, greases and other impurities, also from mechanical processing, from the metal surface in one process step and at the same time to activate it for the subsequent step of zinc phosphating. Corresponding treatment baths are described, for example, in the context of processes for pretreating metal surfaces before phosphating in DE-A-2 951 600 and DE-A-3 213 649.

In recent times, however, so-called "low zinc phosphating processes" have increasingly been used, as are specified, for example, in DE-C-2 232 067. In conjunction with the usually subsequent electrocoating, these lead to significantly improved corrosion protection. However, these processes are much more sensitive to changes in the process parameters and to impurities that are introduced into the phosphating bath with the metal sheets to be coated. With that comes the step of activation the metal surface is much more important than before. It has proven to be particularly advantageous to follow the activation in a separate process step after the cleaning and degreasing step. This is particularly true if the phosphating is carried out in a dipping process using the low-zinc process, but is also equally important for the zinc phosphating using a spray or combined spray-immersion process and immersion-spraying process.

The activation of the metal surface has the following goals:
Increase in the nucleation rate and thus the number of crystal nuclei in the starting phase of zinc phosphating, which leads to a layer refinement; by forming crystals as close together as possible, the porosity of the desired zinc phosphate layer is reduced. This results in a uniform and closed zinc phosphate layer over the entire metal surface with a low basis weight (specified in grams of metal phosphate per m² of metal surface), whereby low basis weights have proven to be favorable as an adhesive base for paints.
Reduction of the minimum phosphating time, ie the time until the metal surface is completely covered with a closed zinc phosphating layer.

These effects of the activating agent ultimately lead to a good anchoring of the lacquer layers to be applied and thus good corrosion protection as the main goal of zinc phosphating is achieved via the finely divided and dense zinc phosphate layers adhering well to the metal grunge.

In practice, only effective polymeric titanium (IV) phosphate-containing products such as those described by Jernstedt, for example in US Pat. Nos. 2,456,947 and 2,310,239, have proven to be effective activating agents with the required properties. These activating agents are nowadays preferably used in a separate rinsing bath directly before the zinc phosphating, but can also be added beforehand to an - if necessary mid-alkaline - cleaning bath.

Since the technical production of such activating agents with constant and high quality is difficult, there has been no shortage of attempts to develop activating agents based on other than titanium phosphate.

Jernstedt describes activating agents based on zirconium phosphate or reaction products of water-soluble tin and lead compounds with disodium hydrogen phosphate in US Pat. Nos. 2,456,947 and 2,462,196. DE-C-29 31 712 describes hydrolysis-stable organic titanium compounds as activating agents for zinc , Zinc-manganese or manganese surfaces are described. They are obtained by reacting a beta-diketontitanylacetylacetonate with gluconic acid or gluconates in the presence of a hydrogen halide salt of an aliphatic amino alcohol.

Another way of increasing the nucleation rate on steel is to treat the surface with dilute aqueous copper sulfate or copper nitrite solutions and with oxalic acid. However, the latter may only cause a slight etching of the iron surface; if a coherent iron oxalate layer is formed, the activation effect disappears (US Pat. No. 2,164,024, DE-A-17 71 924).

EP-B-0 056 675 describes aqueous solutions for the treatment of iron-containing metal surfaces which contain a titanium compound, a phosphate and / or acid phosphate, pyrophosphate and carbonate and / or acid carbonate and a pH of 8.5 to 9.5. Instead of carbonate, carboxylic acids or alkali metal salts or ammonium salts of carboxylic acids can also be used. Solutions of this type are used to treat the metal surfaces before subsequent zinc phosphating.

The phosphate-containing activating agents quickly become unusable due to the hardness of the water. It is therefore necessary in practice to use the activating agents in at least partially softened, and even better in fully deionized water. This is just as costly as the alternative of adding new activation mixes to the activation baths after only a short use.

As a possible solution to the problem, EP-A-180 523 proposes the use of phosphonic acids for complexing the water hardness or for stabilizing hard water in the activating agents. In application tests, the method delivers satisfactory activation results with a significantly longer bath service life compared to activation baths free of phosphonic acid. However, it has the disadvantage that the waste water to be disposed of is additionally contaminated with the organophosphorus compounds which are difficult to biodegrade.

DE-A-36 15 294 (= EP-A-0 201 841) describes the use of water-soluble anionic copolymers of unsaturated carboxylic acids, for example acrylic acid, with acrylic acid esters, acrylic acid amide, acrylonitrile, isobutylene and / or styrene or of water-soluble, anionic condensation products from naphthalenesulfonic acid and formaldehyde for the "stabilization" of activation baths. These activating baths, which are used to pretreat metal surfaces before zinc phosphating them, are based on titanium, pyrophosphate and total phosphate and have a pH of 8 to 9.5. Stabilization is understood to mean a delay in the aging of the baths - by suppressing the coagulation of the titanium colloids - that is, the phenomenon that activated baths quickly lose their effectiveness, even if fully deionized water is used, regardless of whether they are used or not . As a side effect of the use of polymer, it is stated that the preparation of the activation baths is of less quality Water can be used as usual. The specific conductivity is used as a parameter for the water quality. However, this parameter, which is common in practice for identifying water quality, naturally does not say anything about the presence of hardening agents. Corresponding tests with hard water even proved the ineffectiveness of the polymers described for hard water stabilization. Similar copolymers, which likewise have no aldehyde groups, are claimed in DE-A-21 25 963 as cleaning baths containing titanium compounds to add grain refinement in order to expand the pH range necessary for the activating action.

Furthermore, DE-A-19 42 556 describes the use of oligomeric or polymeric aldehydo- or oxyaldehydocarboxylic acids or their salts as complexing agents and their preparation. These are rectilinear, practically uncrosslinked oligomers or polymers whose molar content of carboxyl or carboxylate groups is at least 50%, of carbonyl groups at most 50% and optionally of terminal hydroxyl groups at most 66.6% and whose degree of polymerization is between 3 and 500 . They are preferably produced by oxidative polymerization of α, β-unsaturated aldehydes, preferably acrolein, or oxidative copolymerization of α, β-unsaturated aldehydes with α, β-unsaturated mono- or dicarboxylic acids, preferably acrylic acid or maleic acid, using as the oxidizing agent and at the same time polymerization initiator is preferably used H₂O₂.

In contrast, it was the object of the present invention to achieve the stabilization of treatment baths for the activation of metal surfaces - before zinc phosphating them - against the hardness constituents of water by using additives which only contain the less ecologically problematic elements carbon, hydrogen and oxygen .

Surprisingly, it was found that by using polymers which also carry aldehyde groups in addition to carboxyl groups, the hard water sensitivity of the activating agents can be eliminated without adversely affecting their activating action.

Accordingly, the present invention relates to the use of copolymers of acrylic acid and derivatives of acrylic acid as hard water stabilizing additives for treatment baths for the activation of metal surfaces made of iron, steel, zinc, galvanized or alloy-galvanized iron or steel, aluminum or aluminized iron or steel before the phosphating step with phosphate baths containing zinc ions, which is characterized in that poly (aldehydocarboxylic acids) and / or their water-soluble alkali metal salts are used in the treatment baths as copolymers in amounts of 0.05 to 3 g / l, the pH of the treatment baths in the range from 5 to 9 lies.

• einer Viskositätszahl im Bereich von 5 bis 50 ml/g,
• einer Säurezahl im Bereich von 450 bis 670,
• einem Säureäquivalentgewicht im Bereich von 125 bis 70,
• einem Stockpunkt von weniger als 0 °C,
• einem Gehalt an Carboxylgruppen im Bereich von 55 bis 90 mol-% und
• einem Molekulargewicht im Bereich von 1 000 bis 20 000.

According to one embodiment of the present invention, the poly (aldehydocarboxylic acids) to be used or their alkali metal salts obtainable by the reaction of hydrogen peroxide, acrolein and acrylic acid with

• a viscosity number in the range from 5 to 50 ml / g,
• an acid number in the range from 450 to 670,
• an acid equivalent weight in the range from 125 to 70,
• a pour point of less than 0 ° C,
• a content of carboxyl groups in the range of 55 to 90 mol% and
• a molecular weight in the range of 1,000 to 20,000.

While according to the invention the poly (aldehydocarboxylic acids) are used in the acid form, one embodiment of the present invention is that the poly (aldehydocarboxylic acids) are used as alkali metal salts, the sodium salts being particularly preferred.

A preferred embodiment of the present invention consists in that the poly (aldehydocarboxylic acids) and / or their alkali metal salts to be used according to the invention are used in an amount of 0.5 to 1 g / l in treatment baths for the activation of metal surfaces.

While the poly- (aldehydocarboxylic acids) and / or their alkali metal salts to be used according to the invention can in principle be used before all common phosphating processes, a further preferred embodiment of the present invention is that they are used before low-zinc phosphating.

The poly (aldehydocarboxylic acids) to be used according to the invention are commercially available and are available from DEGUSSA AG, Frankfurt, for example under the names POC OS 20, POC HS 0010, POC HS 2020, POC HS 5060, POC HS 65 120 and POC AS 0010 , POC AS 2020, POC AS 5060 or POC AS 65 120. The HS designation relates to the acid form and the AS designation relates to the sodium salt form of the poly (aldehydocarboxylic acids). They can be produced using a special process developed by Degussa, the "oxidative polymerization" of acrolein. Acrolein is treated alone or as a mixture with acrylic acid in aqueous solution with hydrogen peroxide. The H₂O₂ acts as an initiator of the polymerization and as a molecular weight regulator. At the same time, some of the aldehyde groups of acrolein are oxidized to carboxyl groups by hydrogen peroxide. This creates polymers with pendant aldehyde and carboxyl groups, namely the poly (aldehydocarboxylic acids).

Information about the production of the poly (aldehydocarboxylic acids) described above and about their possible uses can be found in the company publication of DEGUSSA AG with the title "POC-environmentally friendly polycarboxylic acids with diverse application possibilities" (printed notice: CH 215-3-3-582 Vol). According to this, the poly (aldehydocarboxylic acids) can be used, for example, as hardness stabilizers with regard to the inhibition of the crystallization of calcium and other alkaline earth metal salts, as deposit inhibitors in seawater desalination, as dispersants for high solids content Use aqueous pigment dispersion and as a builder for detergents and cleaning agents. This company publication also contains information on relevant patent literature, for example DE-B-10 71 339 (production), DE-A-19 04 940 (complexing agent), DE-A-19 04 941 (polyoxycarboxylic acids), DE-B- 19 42 556 (complexing agent), DE-A-21 54 737 (rust protection treatment), DE-A-23 30 260 and DE-A-23 57 036 (production).

The free poly (aldehydocarboxylic acids) can be neutralized with lyes to the corresponding salts, e.g. with NaOH to sodium poly (aldehydocarboxylates).

The carboxyl and carbonyl content and the average molecular weight of the various poly (aldehydocarboxylic acid) qualities can be varied by selecting the reaction conditions. The general formula (I) represents the basic structure of the poly (aldehydocarboxylic acids) to be used according to the invention:

The poly (aldehydocarboxylic acids) are predominantly linear poly (aldehydocarboxylic acids) linked via carbon-carbon bonds with many carboxyl and a few carbonyl side groups and hydroxyl end groups. Their chemical constitution is characterized in particular by the general formula (I).

The average degrees of polymerization are characterized by the viscosity numbers. These are usually between 5 to 50 ml / g, based on 100% solids, measured as a 23% solution in 0.1 N NaBr at 25 ° C. and pH 10 in an Ubbelohde viscometer, capillary Oa. The spatial linkage of the monomer units can be assumed to be atactic, the order of the linkage to be statistical.

The carboxyl group content, expressed in mol% COOH, can also be calculated from the acid number (DIN 53402) of the dried polymers. The acid number of aqueous poly (aldehydocarboxylic acids) is unsuitable for calculating the molar percentages of COOH, since the technical qualities contain small amounts of amic acid, acetic acid and β-hydroxypropionic acid as by-products.

The sodium poly (aldehydocarboxylates) must be converted into the H form by ion exchange before the acid number is determined.

While when using commercially available activating agents in hard water (city water hardness 18 ° d) as a result of inadequate activation during the subsequent zinc phosphating, coarse crystals are immediately formed, the addition of the poly (aldehydocarboxylic acids) to be used according to the invention leads to area-based masses ("basis weights") Phosphate layers, as are otherwise only obtained when the activating agents are mixed in deionized water. Under the hardness conditions investigated, a polymer addition of 0.5 g / l proved to be sufficient, whereas larger amounts (2 g / l) led to the coarse appearance of the phosphate layers. The optimum amount of complexing agents will have to be determined in a test series for the local hardness conditions of the (tap) water used.

By adding the poly (aldehydocarboxylic acids) to be used according to the invention in the preparation of the ready-to-use activating bath, the amount to be used can be flexibly adapted to the water hardness at the respective place of use.

Examples

In order to determine the activating effect of agents when using the poly (aldehydocarboxylic acids) according to the invention and for the comparison of products used, the surfaces of steel coupons (material St 1405, dimensions 10 cm x 20 cm, approx. 1 mm thickness) were measured using standardized phosphating processes according to the table 1 (immersion phosphating, normal zinc method) phosphated.

"Weight per unit area" means the mass per unit area of the metal phosphate layer in grams per square meter, which is determined in accordance with DIN 50 492. To determine the bath capacity, two liters of a 0.2% strength by weight aqueous preparation of the activating agent were loaded with test panels, which were then phosphated. Initially and then after every tenth test sheet, the average basis weight of four successive sample sheets was determined. The average values calculated from this are given in Table 3. The baths were considered exhausted if ten sheets in a row showed defects or coarse-crystalline areas during zinc phosphating. The bath capacity is given in m² of activatable area per 2 l of activating bath.

Comparative Example 1

A commercially available activating agent containing titanium phosphate from Collardin, Cologne (Fixodine ^R 6) was used as the comparative product. The activation results achieved with this are shown in Table 3.

Comparative Example 2

An ethylene-maleic anhydride copolymer EMA 1103 from MONSANTO according to DE-A-36 15 294 was used in the same amount as the additive according to the invention. The result (see Table 3) shows the lack of effectiveness of this polymer for hard water stabilization.

Examples 1 to 6

The examples demonstrate the effectiveness of the additive to be used according to the invention when used together with differently formulated activating agents. The poly (aldehydocarboxylic acids) POC HS 5060, POC HS 0010 and POC HS 65 120 from DEGUSSA AG, Frankfurt, which are characterized in more detail in Table 2, were used.
The results obtained are also summarized in Table 3.

The titanium phosphate-containing activating agents used in Examples 4, 5 and 6 (see Notes e and f) are described in the unpublished German patent application DE-A-38.14.287.

Claims (5)

1. The use of copolymers of acrylic acid and derivatives of acrylic acid as hard-water-stabilizing additives for treatment baths for the activation of metal surfaces of iron, steel, zinc, galvanized or alloy-galvanized iron or steel, aluminium or aluminized iron or steel before phosphating with phosphating baths containing zinc ions, characterized in that poly(aldehydocarboxylic acids) and/or water-soluble alkali metal salts thereof are used as copolymers in the treatment baths in quantities of 0.05 to 3 g/l, the pH value of the treatment baths being in the range from 5 to 9.

2. The use claimed in claim 1, characterized in that the poly(aldehydocarboxylic acids) or their alkali metal salts are obtainable by the reaction of hydrogen peroxide, acrolein and acrylic acid with

- a viscosity number of 5 to 50 ml/g

- an acid value of 450 to 670,

- an acid equivalent weight of 125 to 70,

- a pour point below 0°C,

- a content of carboxyl groups of 55 to 90 mol-% and

- a molecular weight of 1,000 to 20,000.

3. The use claimed in claims 1 and 2, characterized in that the sodium salts are used as the alkali metal salts of the poly(aldehydocarboxylic acids).

4. The use claimed in claims 1 to 3, characterized in that the poly(aldehydocarboxylic acids) and/or their alkali metal salts are used in quantities of 0.5 to 1 g/l.

5. The use claimed in claims 1 to 4 before low-zinc phosphating.