EP0991483B1 - Application of plastic on individual metal parts - Google Patents

Abstract

The invention relates to a method for applying plastic or plastic coatings to metal parts. Said method consists of the following steps: a) the individual parts are brought into contact with a solution or dispersion containing 5 to 50 wt. % organic polymer; b) the solution or dispersion is dried; c) the individual parts are brought into contact with a qualitatively substantially identical solution or dispersion according to step a), containing, however, 0.5 to 5 wt. % organic polymer, and d) finally dried. Preferably, the solution or dispersion contains a chrome VI or chrome III compound and or adhesion-promoting pigments, flow-control agents and/or defoamers (additives). Other preferred embodiments of the invention consist in brining the individual parts into contact with a solution or dispersion having a temperature of 5 to 40 °C, drying as in step b) at a temperature ranging from 40 to 60 °C and final drying in step d) at a high temperature of up to 200 °C. The main advantage of the invention is that the method can be used in the pre-treatment of individual parts before powder coating.

Description

The invention relates to a method for applying plastic or plastic-containing layers to metallic individual parts and its use as pretreatment of the individual parts for the subsequent powder coating.

A method of the type mentioned above is disclosed in DE-A-196 42 970.

A wide variety of processes are used in the industrial sector to apply plastic or plastic-containing layers to metallic workpieces. Depending on the type of workpieces, the application can be carried out without difficulty or is associated with considerable disadvantages. For example, the treatment of tape is possible in a simple manner by using the roller coating method. By suitably adjusting the concentration of the plastic solution or dispersion to be applied and the application rollers, a plastic layer of the required thickness can be produced in one operation.

The treatment of relatively large, spherical workpieces is also comparatively simple, which is usually done using the electro-dip process or the spray process.

On the other hand, the coating of individual parts presents difficulties, particularly if they have a spatial extension. The term individual parts describes, for example, radiator convection surfaces, shelves, fence elements and guardrails. In this case, roll request processes are ruled out. Spray processes are particularly uneconomical because of the high proportion of over-spray. Dipping processes have the disadvantage that drops of the plastic solution or dispersion remain at the drainage points and are visible as a thickening after the subsequent drying. This thickening has a layer thickness that is approximately 30 to 50 times as high as that of the other surface areas. Even after subsequent painting, these thickenings are clearly recognizable. In addition to the unattractive appearance, the treated parts are - depending on the intended use - practically unusable due to the lack of accuracy of fit.

The object of the invention is to provide a method for applying plastic or plastic-containing layers to metallic individual parts that avoids the known, in particular the aforementioned disadvantages and also creates layers at the drainage point with a layer thickness that matches the layer thickness of the other surface areas coated with plastic essentially matches.

• a) die Einzelteile mit einer 5 bis 50 Gew. % organisches Polymer enthaltenden Lösung oder Dispersion in Kontakt bringt,
• b) die Lösung oder Dispersion antrocknet,
• c) die Einzelteile mit einer qualitativ im wesentlichen identischen Lösung oder Dispersion gemäß Stufe a), die jedoch nur 0,5 bis 5 Gew.% organisches Polymer enthält, in Kontakt bringt,
• d) und endtrocknet.

The object is achieved in that the method of the type mentioned is designed according to the invention in such a way that

• a) bringing the individual parts into contact with a solution or dispersion containing 5 to 50% by weight of organic polymer,
• b) the solution or dispersion dries,
• c) the individual parts are brought into contact with a qualitatively essentially identical solution or dispersion according to stage a), but which contains only 0.5 to 5% by weight of organic polymer,
• d) and dries up.

Drying on is understood here to mean that the liquid film produced by treatment according to process step a) has completely dried on the surface areas different from the drainage points. In contrast, the polymer solution or dispersion present at the outflow points is slightly concentrated, but is soluble or dispersible in the subsequent process step c).

The treatment corresponding to process stage c) in turn results in a drop of liquid which is greatly thickened at the discharge point (s) compared to the other surface areas, but which is due to the considerably lower concentration of the polymer solution or dispersion after the final drying in accordance with process stage d) leads a layer which has substantially the same thickness as the layer of the other surface areas.

The initially available water rinsing to remove the thickened liquid film at the discharge point (s) instead of process step c) is not practical, since the bare metal surface would then be exposed at these points.

The solutions or dispersions used contain the usual polymers. Polyacrylates, polyurethanes, polyesters, polystyrene and epoxy, phenol, silicone, urea and / or melamine resins and copolymers thereof are particularly suitable.

According to a preferred embodiment of the invention, the polymer solution or dispersion should additionally contain a chromium VI and / or a chromium III compound, preferably in water-soluble form.

Another preferred embodiment of the invention provides for the individual parts to be brought into contact with a solution or dispersion which additionally contains adhesion-promoting pigments, leveling agents and / or defoamers. The content of the aforementioned additives should be about 0.5 to 10% by weight of the polymer content amount (dry substance of the polymers = 100%). Additives are used which are commonly used in plastic coating technology.

The pH of the solution or dispersion used is expediently in the range from 1.5 to 10. When using acidic polymer concentrates, the preferred pH range is 2 to 4, when alkaline polymer concentrates are used, 7.5 to 9.

The way in which the individual parts are brought into contact with the solution or dispersion is largely arbitrary. Immersion or flood processes are particularly suitable. It is advisable to circulate the baths. The duration of the contact is to be dimensioned such that complete wetting is ensured. It is usually in the range of a few seconds to a minute. The temperature of the solution or dispersion used can be between 5 and 40 ° C.

The drying step according to b) following process step a) can take place at room temperature. Then the drying time is 5 to 60 minutes depending on the thickness of the liquid film. According to a preferred embodiment of the invention, however, the drying is carried out at a temperature in the range from 40 to 60 ° C. Then a drying time of 1 to 3 minutes is usually sufficient. In any case, the above comments on the term drying used here should be observed.

For the treatment of the individual parts in process stage c), the information given for process stage a) applies with regard to temperature, duration of treatment, etc.

The final drying can take place at room temperature. Because of the increased drying speed, however, the use of elevated temperatures reaching up to 200 ° C. is of particular advantage. Depending on the intended temperature, the duration for the final drying is 5 seconds to 60 minutes.

The prerequisite for the perfect coating of the individual parts is their purity. Unless it is given by an immediately preceding treatment, such as electrolytic or hot-dip galvanizing, thorough cleaning / water rinsing according to known methods is required. The cleaned individual parts can be treated with a wet surface or else dried using the method of the invention.

The method according to the invention is intended in particular for the coating of individual parts made of steel, galvanized steel, aluminum, alloys thereof, as well as zinc and its alloys or of individual parts which have such surfaces. It can be used for sole treatment for permanent corrosion protection, as well as for pre-treatment before any painting. The process is particularly important for the pretreatment of individual parts for the subsequent powder coating.

Depending on the concentration of the solution or dispersion used, the layer thicknesses achieved with the method according to the invention are approximately 0.5 to 3 μm (based on the surface areas different from the runoff points).

In addition to solving the problem set by the invention, the present method has further significant advantages. It works wastewater-free. Because of the qualitatively essentially identical nature of the solution or dispersion used in process stage c), it can be added continuously or discontinuously to the solution or dispersion according to stage a) as a result of the concentration taking place by detaching the drops located at the outflow points, or for their preparation with the help the commercially available polymer concentrates, which usually contain 30 to 50% by weight of polymer, are used.

The invention is illustrated by way of example and in more detail using the examples below.

Examples

For treatment, previously cleaned and rinsed shelves made of galvanized steel with a still wet surface were used.

The plastic-containing layers were produced using dispersions which contained a copolymer of acrylic acid, acrylic acid ester, styrene and urethane as the polymer. The concentration of the polymer and the further contents of ammonium dichromate and conventional additives are listed in Table 1. Table 1: polymer dispersion 1 2 3 4 copolymer 13.35 wt. TI. 11.12 wt. TI 6.67 wt. TI 0.89 wt. TI ammonium dichromate 0.45 wt TI 0.38 wt TI 0.23 wt. TI 0.03 wt. TI additives 1.20 wt. TI 1.00 part by weight 0.60 wt. TI 0.08 wt. TI water 85.00 wt. TI 87.50 wt. TI 92.50 wt. TI 99.00 wt. TI Tl. = Parts

The shelves were treated in diving with a diving duration of 10 seconds at a bath temperature of 20 ° C.

In the examples according to the invention (Examples 1 to 3), the polymer dispersions were carried out for 2 minutes after the immersion treatment. Dried at 50 ° C, then was immersed in the polymer dispersion 4 according to the invention. Here, too, the immersion time was 10 seconds and the bath temperature was 20 ° C. The final drying was then carried out at 150 ° C. within 2 minutes.

In comparative experiments 1 to 4, only one immersion treatment with polymer dispersions 1, 2, 3 and 4 was carried out and then the final drying. In comparative example 5, polymer dispersion 4 was used in two immersion treatments used. The treatment conditions were identical to those of Examples 1 to 3. The results obtained are shown in Table 2. Table 2: 1. Diving treatment 2. Diving treatment Layer thickness at the drain edge Layer thickness of other surface example 1 Polymer dispersion 1 Polymer dispersion 4 3 µm 1.8 µm Example 2 Polymer dispersion 2 Polymer dispersion 4 3 um 1.5 µm Example 3 Polymer dispersion 3 Polymer dispersion 4 3 µm 1.0 µm Comparative Example 1 Polymer dispersion 1 - 95 µm 1.8 µm Comparative Example 2 Polymer dispersion 2 - 70 µm 1.5 µm Comparative Example 3 Polymer dispersion 3 - 35 µm 1.0 µm Comparative Example 4 Polymer dispersion 4 - 3 µm 0.1 µm Comparative Example 5 Polymer dispersion 4 Polymer dispersion 4 3 µm 0.2 µm

It follows from this that in Examples 1 to 3 according to the invention, the layer thicknesses largely correspond both at the drainage points and at the surface areas different from the drainage points. In comparison examples 1 to 4, on the other hand, the layer thicknesses at the drainage points are 25 to 52 times greater than at the other surface areas. In Comparative Example 4, the layer thickness at the surface areas different from the drain point is also insufficiently small.

Comparative Example 5 makes it particularly clear that a two-stage immersion treatment with a polymer dispersion, the concentration of which is too low in the first stage, is insufficient with regard to the layer thickness at the surface areas different from the runoff points.

Claims (7)

1. A process for applying plastics or plastics-containing layers onto metal components, characterised in that

   a) the components are brought into contact with a solution or dispersion containing 5 to 50 wt.% of organic polymer,

   b) the solution or dispersion is surface-dried,

   c) the components are brought into contact with a solution or dispersion according to stage a) which is substantially qualitatively identical, but contains only 0.5 to 5 wt.% of organic polymer and

   d) there is final drying.
2. A process according to claim 1, characterised in that the components are brought into contact with a solution or dispersion which additionally contains a chromium(IV) and/or chromium(III) compound, preferably in water-soluble form.
3. A process according to claim 1 or 2, characterised in that the components are brought into contact with a solution or dispersion which additionally contains adhesion-promoting pigments, levelling agents and/or defoaming agents (additives).
4. A process according to claim 1, 2 or 3, characterised in that the components are brought into contact with a solution or dispersion which has a temperature of 5 to 40°C.
5. A process according to 1 or more of claims 1 to 4, characterised in that the surface-drying in stage b) is performed at a temperature in the range from 40 to 60°C.
6. A process according to one or more of claims 1 to 5, characterised in that the final drying in stage d) is performed at an elevated temperature extending up to 200°C.
7. Use of the process according to one or more of claims 1 to 6 as a pretreatment of the components for subsequent powder coating.