EP0340529A1 - Hard water stabilizing additive for activating agents before zinc phosphating - Google Patents

Abstract

The invention relates to hard water-stabilising additives for treatment baths for the activation of metal surfaces of iron or steel, zinc, galvanised iron or steel, aluminium or aluminised iron or steel before the step of phosphating with phosphating baths which contain zinc ions and consist of poly(aldehydocarboxylic acids).

Description

The invention relates to additives for treatment baths for the activation of metal surfaces made of iron or steel, zinc or galvanized steel as well as aluminum or aluminized steel before phosphating the surfaces mentioned 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. The present invention further relates to their use.

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 are phosphated, conventionally iron or steel Sheets, more recently also 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 corrosion protection. 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-PS 2 951 600 and DE-PS 3 213 649.

In recent times, however, so-called "low zinc phosphating processes" have increasingly been used, as specified, for example, in DE-PS 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 by the low-zinc process in one dipping process, but is also equally important for the zinc phosphating by a spray or combined spray-immersion process and immersion-spray process.

The activation of the metal surface has the following objectives: increasing the rate of nucleation 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 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.

The effects of the activating agent ultimately lead to the fact that the finely divided and dense zinc phosphate layers, which adhere well to the metal base, achieve good anchoring of the lacquer layers to be applied and thus good corrosion protection as the main goal of zinc phosphating.

In practice, products containing only polymeric titanium (IV) phosphate, 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 zinc phosphating, but can also be added beforehand to an - if necessary mildly alkaline - cleaning bath.

Since the technical production of such activating agents with a constant and high quality is difficult, there has been no shortage of attempts to develop activating agents on a basis 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. In DE-PS 29 31 712, hydrolysis-stable organic titanium compounds are used as activating agents for zinc and zinc -Manganese or manganese surfaces 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 weak etching of the iron surface; If a coherent iron oxalate layer is formed, the activation effect disappears (US Pat. No. 2,164,024, German Pat. No. 1,771,924).

EP-PS 0 056 675 describes a process for pretreating steel wire before zinc phosphating with a bath, which contains sodium salts of oxalic, tartaric or citric acid as activating agents.

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, or 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-OS 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 process delivers very satisfactory activation results with a significantly longer bath service life than phosphonic acid-free activation baths. However, it has the disadvantage that the wastewater to be disposed of is additionally contaminated with the organophosphorus compounds which are difficult to biodegrade.

DE-OS 36 15 294 describes the use of anionic copolymers of unsaturated carboxylic acids and acrylic acid derivatives, isobutylene and / or styrene, and of anionic condensation products of naphthalenesulfonic acid and formaldehyde for the "stabilization" of activation baths. This is understood to mean the delay in the "aging" of the baths, that is to say the phenomenon that activated baths rapidly lose their effectiveness even when 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 hardness formers. 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-OS 21 25 963 as an additive to cleaning baths containing grain-refining titanium compounds in order to expand the pH range necessary for the activating action.

In contrast, the present invention has as its object to stabilize the activating agents with respect to the hardness constituents of the water with complexing agents which contain only the less ecologically problematic elements carbon, hydrogen and oxygen.

Surprisingly, it was found that polymers which also carry aldehyde groups in addition to carboxyl groups cancel the sensitivity to hard water of the activating agents without adversely affecting the activating effect.

The invention relates to 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 prior to the step of phosphating with phosphating baths containing zinc ions, consisting of poly (aldehydocarboxylic acids) and / or their water-soluble alkali metal salts.

According to one embodiment of the present invention, the poly (aldehydocarboxylic acids) 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 carboxyl group content 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 consists in that the poly (aldehydocarboxylic acids) are used as alkali metal salts, the sodium salts being particularly preferred.

Furthermore, the present invention relates to the use of the additives according to the invention in treatment baths for the activation of metal surfaces in an amount of 0.05 to 3 g / l.

A preferred embodiment of the present invention consists in that the poly (aldehydocarboxylic acids) 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 in principle the additives according to the present invention can be used in all common phosphating processes, a further preferred embodiment of the present invention is that the additives are used before low-zinc phosphating.

A further preferred embodiment of the present invention consists in using the additives for treatment baths for the activation of metal surfaces in activation baths with a pH of 5 to 9.

The poly (aldehydocarboxylic acids) 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 sold. 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 a wide range of possible uses" (note: 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 solids Use rich aqueous pigment dispersions as well as builders for detergents and cleaning agents. This company publication also contains information on relevant patent literature, for example DE-PS 10 71 339 (production), DE-OS 19 04 940 (complexing agent), DE-OS 19 04 941 (polyoxycarboxylic acids), DE-PS 19 42 556 ( Complexing agent), DE-OS 21 54 737 (rust protection treatment), DE-OS 23 30 260 and DE-PS 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 poly (aldehydocarboxylic acids) linked linearly 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 2% 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 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 formic 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) according to the invention leads to area-based masses ("basis weights") of the phosphate layers, as otherwise only obtained when activating agents are used in deionized water. Under the hardness conditions examined, a polymer addition of 0.5 g / l proved to be sufficient, whereas larger amounts (2 g / l), in contrast, 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) according to the invention in the preparation of the ready-to-use activation 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 the agents produced according to the invention and to compare the 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 Table 1 (immersion phosphating, normal zinc process) 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 sheets, which were then phosphated. Initially and then after every tenth sample sheet, the average weight per unit area 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 coarsely 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-OS 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

Tabelle 2 Feststoff (%) pH-Wert Dichte (g/ml) Viskosität bei 20°C (mPa.s) Stockpunkt (°C) Farbzahl DIN 6162 Viskositätszahl* (ml/g) POC HS 5060 40 1,3 1,1 240 -9 2 28 POC HS 0010 50 1,0 1,2 115 -9 7 8 POC HS 65 120 35 1,7 1,1 380 -9 3 47 * Die Viskositätszahl wurde bezogen auf 100 % Feststoff, gemessen als 2 %ige Lösung in 0,1 N NaBr bei 25 °C und pH 10, Ubbelohde Viskosimeter, Kapillare 0a. Tabelle 3 Einfluß von Additiven zur Hartwasserstabilisierung von Aktivierbädern. Bsp. Aktivierungsmittel^a) Zusatz (1 g/l) Flächengewicht in g/m² Kapazität^b) Vgl.1 FIXODINE^R 6 ^c) ohne ^d) - Vgl.2 dto. EMA 1103 4,5-7,5^d) - 1 dto. POC HS 5060 2,9 2,0 2 dto. POC HS 0010 2,4 2,4 3 dto. POC HS 65 120 2,6 2,5 4 ^e) POC HS 5060 2,4 3,2 5 ^f) POC HS 5060 2,5 2,8 6 ^e) POC HS 5060 (0,5 g/l) 2,5 3,3 ^a) Ansatz 0,2 gew.-%ig in Stadtwasser mit 18 °d. ^b) Aktivierbare Fläche in m² pro 2 l Aktivierbad. ^c) Testsubstanz: titanphosphathaltiges Aktivierungsmittel der Firma Collardin, Köln. ^d) Grobe Kristalle wegen mangelhafter Aktivierungswirkung. ^e) Titanphosphathaltiges Aktivierungsmittel, hergestellt mit 1,7 Gew.-% POC HS 5060. ^f) Titanphosphathaltiges Aktivierungsmittel, hergestellt mit 2,4 Gew.-% 1-Amino-1-phenylmethan-1,1-diphosphonsäure. The examples demonstrate the effectiveness of the additive 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.

Table 2 Solid (%) PH value Density (g / ml) Viscosity at 20 ° C (mPa.s) Pour point (° C) Color number DIN 6162 Viscosity number * (ml / g) POC HS 5060 40 1.3 1.1 240 -9 2nd 28 POC HS 0010 50 1.0 1.2 115 -9 7 8th POC HS 65 120 35 1.7 1.1 380 -9 3rd 47 * The viscosity number was based on 100% solids, measured as a 2% solution in 0.1 N NaBr at 25 ° C. and pH 10, Ubbelohde viscometer, capillary 0a. Influence of additives for hard water stabilization of activation baths. E.g. Activation agent ^a) Additive (1 g / l) Basis weight in g / m² Capacity ^b) See 1 FIXODINE ^R 6 ^c) without ^d) - See 2 dto. EMA 1103 4.5-7.5 ^d) - 1 dto. POC HS 5060 2.9 2.0 2nd dto. POC HS 0010 2.4 2.4 3rd dto. POC HS 65 120 2.6 2.5 4th ^e) POC HS 5060 2.4 3.2 5 ^f) POC HS 5060 2.5 2.8 6 ^e) POC HS 5060 (0.5 g / l) 2.5 3.3 ^a) approach 0.2 wt .-% in city water with 18 ° d. ^b) Activatable area in m² per 2 l activating bath. ^c) Test substance: activating agent containing titanium phosphate from Collardin, Cologne. ^d) Coarse crystals due to poor activation. ^e) Activating agent containing titanium phosphate, produced with 1.7% by weight of POC HS 5060. ^f) Activating agent containing titanium phosphate, prepared with 2.4% by weight of 1-amino-1-phenylmethane-1,1-diphosphonic acid.

The titanium phosphate-containing activating agents used in Examples 4, 5 and 6 (cf. Notes e and f) are described in the unpublished German patent application P .. .. ... (D8102).

Claims (7)

1. 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 step of phosphating with phosphating baths containing zinc ions, consisting of poly (aldehydocarboxylic acids) and / or their water-soluble alkali metal salts. 2. Additive according to 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 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 carboxyl group content in the range of 55 to 90 mol% and
- A molecular weight in the range of 1,000 to 20,000. 3. Additive according to Claims 1 and 2, characterized in that they contain the sodium salts as the alkali metal salt of the poly (aldehydocarboxylic acids). 4. Use of the additives according to claims 1 to 3 in treatment baths for the activation of metal surfaces in an amount of 0.05 to 3 g / l. 5. Use according to claim 4 in an amount of 0.5 to 1 g / l. 6. Use according to claims 4 and 5 before a low zinc phosphating. 7. Use according to claims 4 to 6 in activation baths with a pH of 5 to 9.