DD283165A5 - METHOD FOR THE SURFACE PRE-TREATMENT OF METALS FOR CATALYSIC ELECTROCOAT LACQUERING - Google Patents

Abstract

The inventive method relates to a method of surface pretreatment of metals prior to cathodic electrodeposition by applying a Silicoaterschicht. The Silicoaterverfahren consists of the sub-steps applying a SiOx layer and a Silanhaftvermittlers. In contrast to all KETL pretreatment processes known hitherto, these conversion layers do not undergo phosphating, so that the phosphate disposal problems are eliminated. The Silicoater treatment does not have to be directly followed by the coating, it is possible to insert an intermediate storage or open time. In addition, the silicoating of the metal surface has a very favorable effect on the long-term stability of the applied coatings, especially in the case of moisturizing. {Surface pretreatment; Metal; cathodic electrodeposition coating; Silicoater method; SiOx layer; silane coupling agent; Conversion layer; Phosphate layer; Open time; Durable resistance; Environmental friendliness}

Description

Field of application of the invention

The invention relates to a method for surface pretreatment of metals, wherein by means of the Silicoaterverfahrens a conversion layer 8 jf the metal is produced, which allows the cathodic electrodeposition coating, replaces the phosphating and has a favorable effect on the durability of the paint.

Characteristic of the known state of the art

In the cathodic electrodeposition coating (KETL), the metal parts to be coated are known to be immersed in a bath containing suitable polymer particles of general formula NR ₃ after partial neutralization with a low molecular weight carboxylic acid as positively charged particles NR ₃ H ⁺ in aqueous dispersion. Their coagulation on the material polarized as cathode is due to the sudden increase in the pH in the phase boundary region associated with the evolution of hydrogen:

2H ₂ O + 2e "-> H, + 2OH 'NR ₃ H ⁺ + OH"-> NR ₃ (Me) + H ₂ O

The final formation of the then corrosion-protecting polymer film β finally follows by thermal treatment of the coated with the wet coagulum metal parts at temperatures around 180 ° C.

Today, virtually all metal parts made of Fe, Zn and Al base materials are coated with a phosphate layer before the KETL.

This is done with the aim to present a residual rust, scale and dirt-free surface on which to proceed ₂ evolution and the coagulation of the polymer particles during the KETL strictly localized processes of H - the former to the Porten the phosphate layer reactive generated and the latter on Crystalline surfaces of the electrically insulating phosphate. It is required that the phosphate layers be extremely thin (basis weight below 2 g / m ² ), have a stochastically uniform distribution of very small pores with a relative pore area below 1%, and remain stable both mechanically and chemically under the conditions of KETL.

The cathodically generated, gaseous hydrogen must not lead to the mechanical spread of parts of the phosphate layer, because both the properties of the resulting polymer film and the bath parameters of the KETL are impaired.

The same applies if, as a result of the Akalisierung of the aqueous medium in the pores of the phosphate layer, a partial resolution about after

Zn ₃ (PO ₄ I ₂ + 60H "+ 3e"-> 3HZnO ₂ - + 1.5H ₂ + 2PO ₄ ³ ").

In the monograph by W. Rausch: "The Phosphatization of Metals", Eugen Leuze-Verlag Saulgau 1974, the problems arising in connection with a subsequent electrodeposition coating are discussed in detail The inventions appearing below deal with the dosage of so-called activators for phosphating baths (see for example DE-OS Π232067, DE-OS 3408577) and to extremely fine-crystalline hopeite (Zn ₃ PO ₄ ) ₂ layers of surface masses below 1.8 g / m 'lead, or the formation of phosphophyllite layers (Zn ₂ Fe (PO ₄ I ₂ or Zn ₂ Mn (PO ₄ I ₂ ), which are said to be more alkali-resistant than hopeite layers) (eg Eroprop Pat.0224190, Europ. Pat.0141341, Europ. Pat. 0228151) have not yet fulfilled all of the stated requirements The known pollution of the wastewater and environment by phosphates has been reduced in all previous modifications of the phosphating processes.

Object of the invention

The aim of the invention was to provide for the surface pretreatment of metal parts for subsequent cathodic electrocoating a method that leads to a thin, uniform conversion layer on the substrates, which is characterized by low porosity and high resistance under the conditions of KETL and is also phosphate-free.

Explanation of the essence of the invention

This conversion layer is produced by means of the silicoater process. These are silicate layers with an additional organic content, which on metals such as iron, zinc, aluminum u. a. and have the following properties important for cathodic electrocoating:

• extraordinarily small layer thickness (order of magnitude <0.1 pm) with high uniformity

• even distribution of finest pores

• Significant improvement in the adhesion of the protective layer to the metal, which is particularly noticeable when the system is exposed to water.

The silicoater process has hitherto only been used to improve the adhesion of organic polymers to metal substrates. First, dental prosthetics were solved by this method (DE 3403894 C1), then it was extended to the technical fields of application coating (DD 253257 A1 and DD 253259 A1) and gluing (patent application numbers 3125753 and 3125737)

 The silicoating is done in two steps:

• By flame pyrolytic decomposition of an organosilicon compound ent teht on the metal surface, a very thin, adherent, microporous SiO "layer in which the ratio O: Si> 2 r '.. In addition, this layer has exceptionally good wetting behavior.

In the second step, a silane coupling agent having the following chemical structure is applied to this SiO _x layer:

OR '

RO-Si-CH ₂ -CH ₂ -CHj-R OR '

In slightly acidic solution, the Si-OR 'groups are saponified and react with the SiOx-SchicM to form solid Si-O-Si bonds. On the other hand, the reactive radical R of the silane coupling agent reacts with the subsequently applied organic layer.

In this way, the surface of the metal for the cathodic electrocoating is specifically prepared and, on the other hand, adhesion-passing mechanisms are provided which provide a durable, water-resistant adhesive strength of the organic protective layer on the metal substrate. The remainder R ί.τι silane coupling agent is variable and must be adapted to the chemical structure of the organic layer to achieve maximum effect.

The silicoate coating carried out in this way represents a pretreatment process which does not have to be followed immediately by the coating. It is possible to switch on an open time, in which the prepared sheets are stored dry, but also temporarily protected with corrosion protection oils and then degreased again with organic solvents.

The silicoater process is therefore a pretreatment process prior to cathodic electrocoating in which work is carried out without the usual phosphate coatings and, consequently, the problem of disposal of the phosphate baths does not occur. In addition, phosphating not only prepares sewage problems, but the phosphate baths are also not unbepro.iii usable because deposited in them reaction products settle as sludge. In contrast, the silicostatic process works extremely low-waste and environmentally friendly.

embodiments

1. Implementation of the silicoater process

The pretreatment of the metal sheets by Silicoatern fo ^r the cathodic electrodeposition coating (experiment III in Example 2) in two steps.

The thoroughly degreased sheets of cold-rolled steel measuring 150mm x 70 mm are fired with a liquid gas burner. To produce the SiO 2 layer, 2% tetraethoxysilane is added to the catalytic gasoline in the LPG burner. This percentage is variable within certain limits, over 5% of the organosilicon compound should not be added, since otherwise too thick and less well adhering SiO.-layers arise. Flaming must be done in such a way that local overheating of the sheet is avoided (eg rhythmic movements of the flame treatment device), a few seconds of flaming are sufficient. The sheet metal can also be preheated, the most favorable Befismmungstemperatur of the sheet is about 110 ° C. Heating to more than 300 ⁰ C are unfavorable, since changes in the metal surface must be expected.

After cooling the sheets to about 30 ° C, the Silanhaftvermittler is applied. This is done from a weakly acidified alcoholic solution, acid can be used depending on the nature of the silane coupling agent acetic acid or 0.01 η hydrochloric acid, as the solvent are isopropanol-water, and ethanol-water mixtures are suitable. The proportion of silane coupling agent in the solution should be between 1% and 3%. In the present case, a 2% solution of Glycidoxipropyltriethoxysilan, acidified with 1 ml of 0.01 η HCl, worked in isopropanol. This solution was brushed thin with the brush, but also the application by spraying, dipping or similar procedures is conceivable. This concludes the silicoating. The blocks are now fed to the coating. They can be kept dry for weeks in this state, without the effectiveness of Silicoaterschicht is lowered. Also, the application of anticorrosive oils as a temporary corrosion inhibitor (e.g., KMO 16) and their thorough removal with suitable solvents (e.g., perchlorethylene) does not compromise the effectiveness of the silicoate pretreatment.

2. Cathodic electrodeposition coating and proof of the effectiveness of the pretreatment process Samples of steel sheets 150 mm × 70 mm in size were either degreased I only with a known industrial cleaner, II treated with a phosphating agent customary for KETL, so that a zinc phosphate layer with the main component Zn ₃ ( PO ₄ )? a surface weight of (1.8 + 0.2) g / m ² is formed, and III is treated by silicoater coating according to the method of the invention (description of the silicoatern see Example 1). All sample lots were then coated by KETL under the same conditions. For this a technical coating agent was used. The bath was characterized by χ = 1.3mS / cm, pH = 5.6. The deposition of the wet coagulum was carried out at 240 V, 2.5 min at 27 ⁰ C under constant convection of the bath. It followed the thermal treatment for 20 min at 180 ° C.

The magnetically inductive layer thickness measurement showed more pronounced local differences in the samples of batch I, while in central areas of the sample sheets values of about 15 μm were registered. Values up to 35 μm were observed at the lower edge zones. The samples of batches II and III were characterized by uniform layer thicknesses with (17 ± 2) [im with no differences between areas and edge sections.

The coated test panels were subjected to condensation (A) according to TGL18754 / 04 and the salt spray test

(B) exposed to TGL 18754/03. Before these stresses and at various time intervals during the stress, the samples were characterized by impedance measurements without destruction and then exposed to the appropriate medium again. Intact polymer layers on metal substrates are known for impedance measurements by very high ohmic resistance values and extremely small capacitances.

As soon as the aqueous medium in pores, microcracking structure-related cavities has penetrated to the metal substrate and the latter is also electrochemically effective, there is a marked decrease in the electrical resistance, while the capacitance values increase more strongly. This is a sure indication of the beginning failure of anti-corrosion coatings, even before visible signs of rust occur. The exposure duration given in Table 1 concerns precisely the situation where the penetrating water becomes undesirably effective and the local (but still submicroscopic) destruction is signaled by means of impedance measurement.

The results clearly show that the phosphate-free surface pretreatment of the present invention enables the deposition of organic overcoats by KETL, which are equivalent in their layer parameters and anti-corrosive properties to those deposited on substrates pretreated with a premium grade phosphating agent.

The advantage of the novel surface pretreatment is, above all, that no phosphate-containing waste products occur.

3. Improvement of the adhesion of organic protective layers on metal substrates by the silicoater method

Since in principle only relatively thin organic cover layers can be produced by the method of KETL whose adhesive strength on the pretreated metal substrates is not characterized by the usual measuring methods, the influence of the pretreatment process according to the invention on the improvement of the adhesive bond with the aid of an alkyd resin, which in his chemical composition similar to the binder used for KETL. For this purpose, hot dip galvanized steel plates measuring 250mm x 50mm were thoroughly degreased with perchlorethylene (I), in the second step only coated with a silane coupling agent (II), and in variant III silicoatert (SiO _x layer + silane coupling agent), the open time (time between silicates and paint coatings), 1 day open time (purple)

Table 1; Selected results on the corrosion protection properties of the coating systems produced

Sample No .: I (degreased) II (phosphated) III (silicoated)

Batch no .: 123412341234

d / μηι 15 ± 11 15 ± 10 13 ± 10 9 + 6 15 ± 2 18 ± 2 17 ± 2 17 ± 2 17 ± 2 18 ± 2 17 ± 2 18 + 2 t _E (A) / h 210 96 610 680 650 650 t _E (B | / h 4 β 1f) 6 188 204 220

and 8 days open time (III b).

The silicoating is carried out as described in Application Example 1. After the various pretreatments, the galvanized sheets were provided with two layers of alkyd resin topcoat and stored for about 4 weeks at 25 ° C and 60% relative humidity air-conditioning. Thereafter, the adhesive strength was measured by the tear-off method (TGL 27608), the rest of the samples were subjected to a continuous exchange test in distilled water at 2O ⁰ C ± 3 K according to TGL18754 / 01. At certain intervals, the water storage samples were taken and after 60 hours of air conditioning at 25 ° C and 60% rel. Humidity also supplied to the adhesion measurement.

The adhesive strength index is determined in N mm " ² according to TGL 27608 using the tear-off method by sticking test stamps centrically to the test specimens of 50 mm x 50 mm coated on both sides and tearing them off with a tensile tester after a bonding time of about 24 hours average value of the adhesion strength (1) of five individual measurements is given, the percentage error was usually well below 15%. in addition to the actual Haftfestiakeitskennwert is not specified if the typical ^p ruclibild (2) identifying the vulnerability in the composite system. It also occur mixed fractions that Order of the characteristics indicate the relevance of the fracture images For the sake of simplicity, the fracture image characteristics are differentiated in A / B = adhesion break interface metal / paint

B = cohesive failure in the paint.

Further, as the third technological adhesive characteristic value in Table 2, the cross-cut characteristic value (3) according to TGL 14302/05 is given. It represents a grading from 1 (very good) to 4 (poor).

The test results of Table 2 show that only degreasing (I) of the zinc surface after water storage very quickly leads to failure of the composite, but that the application of the Silanhaftvermittlers alone (II) is not sufficient as a surface pretreatment. Only by silicoaters (Ilia, HIb) to achieve the desired durability, even under extreme stress conditions. It is also typical that the silicoated samples barely tear at the zinc / paint interface (fracture pattern A / b) but predominantly in the layer (fracture image B). It can be concluded from the exemplary embodiment that the surface pretreatment according to the invention can also be used beyond the KETL as an effective means for improving the adhesion of organic cover layers to metal substrates.

Table 2: Adhesive strength of alkyd resin on hot-dipped galvanized steel without and with silicates as a function of the duration of use Characteristic values: Adhesive strength in N mm " ¹ (1), character, fracture pattern (2), cross-cut characteristic value (3)

Pretreatment stress 1 I (defatted) 2 3 IMSilanhaftverm.) 1 2 3 FROM 3 1 purple (silicoatert 1 day open time) 2 3 1 lllbfsilicoatert 8 days open time) 2 3 4 weeks room storage 16 B, A / B 1 8th FROM 3 21 B 1 18 B, A / B 1 20 days water storage 20 ⁰ C 1 FROM 3 2 FROM 4 20 B 1 19 B, A / B 1 SOTageWater storage20 ° C 0 FROM 4 0 FROM 4 16 B, A / B 2 15 B, A / B 2 160 days water storage 20 ° C 0 FROM 4 0 15 B, A / B 2 17 B, A / B 2

Claims (6)

1. A process for the surface pretreatment of metals, in particular iron, zinc, aluminum and their alloys for cathodic electrocoating, characterized in that on the metal an SiO _x layer and a Silanhaftvermittler layer is deposited (silicoater method). 2. The method according to claim 1, characterized in that the SiO _x layer by flame pyrolysis of an organosilicon compound, preferably . 10.5 to 5% concentration in Katalytbonzin o. Ä., Is generated on the metal surface. 3. The method according to claim 1, characterized in that applied to the SiO _x layer ~ a conventional Silanhaftvern.ittler, preferably in 0.5 to 5% concentration in alcoholic and / or alcoholic-aqueous solution, by brushing, spraying or dipping becomes. 4. The method according to claim 1, characterized in that the Silicoater layer produced in the two-step process according to claim 2 and 3 can be stored dry for a long time (open time), or in the open time with corrosion protection oils temporarily protected from corrosion and the oil can be thoroughly removed again with appropriate solvents without the effectiveness of the silicoate layer on the metal being reduced. 5. The method according to claim 1,2,3 and 4, characterized in that the silicoate layer not only enables the cutting of a uniform organic cover layer by cathodic electrodeposition en; t, but that by the silicoate layer, the adhesion of the cover layer, before all under wet conditions, so improved that overall advantageous corrosion protection properties result. 6. A process for the surface pretreatment of metals for the cathodic electrodeposition coating according to claim 1 to 4, characterized in that instead of the usual phosphate layers, the silicoate layer is applied and thus account for Entsorgungäprobleme with phosphate-containing waste products and wastewater.