EP1023371A1

EP1023371A1 - Thermohardening plastic moulding materials for moulded parts for direct electrophoretic coating, method for producing the moulded parts, and their use

Info

Publication number: EP1023371A1
Application number: EP98948968A
Authority: EP
Inventors: Reinert Hennersdorf; Edmund Heinrich; Erri Caleffi
Original assignee: Raschig GmbH; Mas Engineering Srl
Current assignee: Raschig GmbH; Mas Engineering Srl
Priority date: 1997-09-29
Filing date: 1998-09-12
Publication date: 2000-08-02
Also published as: WO1999016820A1; DE19742865C1

Abstract

The invention relates to thermohardening plastic moulding materials for producing moulded parts for direct electrophoretic coating, consisting of hardenable resins, fillers and auxiliary agents. The moulding materials contain 5-25 wt. % coke with a particle size </=0.3 mm and 3-20 wt. % carbon black. The invention also relates to a method for producing moulded parts which can be directly electrophoretically coated.

Description

Thermosetting molding compounds for molded parts for direct electrophoretic coating, process for their production and their use

The present invention relates to thermosetting molding compositions for the production of moldings for direct electrophoretic coating, their production and their use.

Electrophoretic coating, especially electrocoating, is a long-known technology. For this purpose, the body that is to be painted and conducts the electrical current is placed in a paint bath and switched as the cathode or anode of an electrical field of approx. 50-400 V, the pool wall or an additional conductor serving as a counter electrode. The electrically charged or electrostatically charged paint particles migrate in the electric field to the body to be painted, the amount separated out being proportional to the amount of electricity. There are no spray losses with this process and the coatings obtained are very uniform even in hard-to-reach places (Römpp's Chemie-Lexikon, 9th edition, 1996, p. 1135). A prerequisite for the process is a sufficient electrical conductivity of the body to be painted, so that this process is mainly used for painting metals.

To paint plastics using this process, they must first be made conductive, which can be done by coating them with a thin conductive layer of nickel or copper or a conductive lacquer.

For this purpose, it is customary that the plastic surface is roughened mechanically or with a chemical pickling solution, metal nuclei are deposited on the roughened surface from a silver or palladium salt solution, on which in turn, after activation and sensitization by an electroless metal deposition, a thin layer of copper or nickel is applied, so that this surface becomes sufficiently conductive that it can be painted electrophoretically. The entire process proves to be extremely complex and requires a large number of chemical reagents are aggressive, harmful to the environment or toxic. Metal vapor deposition, which was also used, is just as complex.

Attempts have therefore been made to produce conductive plastics which can be directly electrophoretically coated by adding conductive fillers (carbon black, carbon fibers, metal fibers or powder or mica platelets coated with metals) to the plastic molding compound. The charge is then transported via a continuous network formed from the conductive fillers in the polymer.

GB 1, 077.088 describes a plastic molding composition which contains conductive metal particles and conductive non-metallic particles, in particular graphite. Since in the production of molded parts from such a mixture the upper layer accumulates in plastic and in particular contains only a few conductive metal particles and graphite, it is necessary for such molded parts to have an outer skin with a thickness of approx. 25 μm etched or ground off, to expose the conductive inner mass. In order to generate sufficient conductivity, it is proposed that 20 to 70% of the molding composition consist of conductive particles and at least 1%, preferably 35%, of which consist of metal particles. In this process, too, a complex pretreatment of the molded parts is necessary.

DE 28 25 735 C2 discloses a process for producing galvanized moldings which are free of metal particles and which consist of a soot-containing thermoplastic synthetic resin. The carbon black used must have an oil absorption of not less than 200 ml / 100 g and a surface area of not less than 500 m ² / g. The amount of the carbon black is between 3 and 100 parts by weight per 100 parts by weight of thermoplastic synthetic resin. If these regulations are not observed, a direct galvanic coating cannot be obtained. With this method, too, it is necessary, after degreasing, to activate the surfaces in a complex manner by etching with sodium hydroxide solution or chromic acid. After appropriate surface treatment, plastics of this type are also suitable for electrophoretic painting, but are uneconomical due to the necessary surface treatment. It was therefore the task of finding molding compositions for the production of moldings for direct electrophoretic coating which, without etching and other cumbersome pretreatment, enable them to be used for direct electrophoretic coating. A further task was to find a process for the production of such molding compositions.

The object is achieved by the features of the main claim and the secondary claims and promoted by the features of the subclaims.

Surprisingly, it was found that the combination of conductive carbon black in the amounts of 3 to 20% by weight and coke in the amounts of 5 to 25% by weight when used in thermosetting molding compositions and the molded parts produced therefrom provides the so-called press skin that after degreasing without mechanical or chemical removal of the press skin, electrophoretic painting is possible.

Although the effect of the coke additive has not yet been conclusively clarified, it is assumed that the granular and porous structure counteracts segregation and that a continuous conductivity of the coke grains in connection with the soot is also retained in the surface layer.

Coke in the sense of this invention is understood to mean products obtained in high-temperature coking (900-1400 ° C.), which are also referred to as metallurgical coke, foundry coke or metallurgical coke, gas flame coal or also calcined petroleum dust coke. Depending on their origin, these products contain 0.2 to 10% ash and 90 to 99% carbon. The coke, which is mostly coarse during coking, is ground to grain sizes of 0.01 to 0.3 mm, preferably 0.05 to 0.1 mm.

Carbon black is preferably the so-called conductivity carbon black with a specific surface area of approximately 80 to 1200 n Vg and an intrinsic conductivity of 0.01 to 0.5 S / cm, preferably 0.05 to 0.1 S / cm, which of the method of production is also called flame black or furnace black. Soot with a higher rate of superficial oxygen and thus lower conductivity are less suitable. The carbon black is composed of primary particles of approximately 30 to 100 nm in size, which in turn are combined to form primary aggregates of approximately 10 to 100 μm, which hold together relatively tightly and form the secondary aggregates of 0.1 to 2 mm in diameter from the commercially available carbon black powder. The secondary units are easily separated again during processing.

Instead of soot, coke dust with an average particle size of less than 50 μm, preferably less than 10 μm, in particular 1 to 5 μm, can also be used. This should also fall under the term "soot" in the following.

In order to ensure a uniform and quick mixing with the hardenable resins and good processability, the coke and soot, as well as possibly also mixed organic or inorganic fillers and, if necessary, fibrous reinforcing materials are mixed with up to 1% by weight of conventional lubricants such as waxes, Sodium or magnesium stearates or palmitates added. To increase the hydrophilicity, from 0.5 to 3% by weight of a solid polyethylene glycol can be added to the mixture.

The mixture of the starting materials is then prepared in a manner known per se by means of calenders, kneaders or extruders to give molding compositions and then granulated.

These molding compositions are further processed in a known manner by pressing, spraying, injection molding or injection molding to give molded parts, the method of processing being practically unchanged compared to thermosetting compositions not doped with coke or soot. The processability of the molding compositions for the individual molding processes is adjusted via the degree of precondensation of the curable resins, which, for. B. is measured with a torque rheometer.

The compositions usually contain hardeners or inhibitors, stabilizers against UV light and temperature loads or other processing aids. Organic and inorganic fillers are in particular additives of wood or cellulose flour, inorganic fillers or extenders, in particular calcium carbonate, aluminum oxide, silicon oxide, kaolin, talc or the like, and organic or inorganic pigments. In order to increase the strength, the masses can also be mixed in a known manner with fibrous materials, in particular glass fibers, carbon fibers but also natural fibers such as hemp or jute, in order to increase the mechanical strength.

The coke and soot content should be such that the molded parts have a surface resistance of <2000 Ω, so that they can be directly electrostatically coated without prior electroless metal deposition. Amounts of 5 to 25%, preferably 15 to 20% coke have been found to be sufficient. The proportion of carbon black should be 3 to 20% by weight, preferably 5 to 15% by weight, in order to enable direct electrophoretic painting. In special cases, up to 10% by weight, preferably 1 to 5% by weight, of metal powder with grain sizes of 0.01 to 0.3 mm, preferably 0.05 to 0.1 mm, can also be added. Copper, aluminum and iron powder are preferred. Too high admixtures have a negative impact on the strength of the molded parts, the total amount of admixtures should not exceed 80% depending on the type of resin.

The molded parts produced according to the invention only need to be degreased before electrophoretic painting in order to remove residues of the mold release agent. An etching of the surface or the application of a conductive coating has proven to be superfluous, since the molding compositions according to the invention do not produce an insulating press skin on the molded parts produced during processing.

In principle, all technically important, customary thermosetting molding compounds are suitable as thermosetting plastics, for example diallyl phthalate resins (DAP), epoxy resins (EP), urea formaldehyde resins (UF), melamine formaldehyde resins (MF), melamine phenol formaldehyde resins (MP), phenol formaldehyde unsaturated resins (PF) . Especially Phenol formaldehyde resins are preferred because of their inexpensive nature, easy processability and favorable product properties.

The invention is described in more detail in the accompanying examples, without however being regarded as a limitation.

Examples

Example 1 (comparison)

A molded part was produced from a phenolic resin molding compound of type PF 31 (according to DIN 7708). After degreasing, it was subjected to electrophoretic painting (cataphoresis) without further pretreatment. Electrophoretic paint was not deposited.

Example 2 (comparison)

A usual composition of a phenolic resin molding compound (42 parts by weight of phenolic resin novolak, 7 parts by weight of hexamethylenetetramine, 8 parts by weight of wood flour, 36 parts by weight of chalk, 0.7 parts by weight of MgO, 0.6 Parts by weight of metal stearates, 0.7 parts by weight of wax-like lubricant) were intimately mixed with 5 parts by weight of conductive carbon black and then processed on a heatable calender or extruder to form a homogeneous molding compound. After the molding compound had been granulated, so-called multi-purpose test specimens (type A - ISO 3167; by injection molding according to ISO 10724; conditions according to DIN 7708 part 8) were produced on an injection molding machine. With a measuring distance of 160 mm, these molded parts had an electrical resistance of approx. 5000 Ω. The parts were degreased and directly electrophoretically varnished, a defective varnish layer developed.

Example 3

The usual phenolic resin molding compound used in Example 2 was changed so that the total amount of lubricant (total metal stearates and wax-like lubricants) was <1 part by weight and the amount of chalk was 16 parts by weight. This mixture was homogenized well with 6 parts by weight of conductive carbon black and 20 parts by weight of coke dust and finally treated as in Example 2. The in Multi-purpose test specimens produced by injection molding had a measured electrical resistance (conditions as in Example 2) of approx. 1900 Ω and, after degreasing, could be coated electrophoretically (cataphoresis) with good success.

Example 4

A conventional composition of a melamine-phenolic resin molding composition (50 parts by weight of melamine-phenolic resin, 30 parts by weight of cell powder, 0.4 parts by weight of metal stearate, 0.6 parts by weight of waxy lubricants) was mixed with 11 parts by weight. Parts of conductive carbon black, 5 parts of coke dust and 8 parts by weight of copper powder are intimately mixed and processed as in Example 2. The multipurpose test specimens produced by the injection molding process had a measured electrical resistance of approx. 800 Ω and were degreased with good results after degreasing.

Example 5

A usual composition of a free-flowing UP molding compound (20 parts by weight of crystalline unsaturated polyester resin, 2.5 parts by weight of diallyl phthalate, 33 parts by weight of chalk, 15 parts by weight of 6 mm short glass fiber, 0.5 parts by weight of dicumyl - Peroxide, 1 part by weight of metal stearate) were well homogenized with 8 parts by weight of conductive carbon black and 20 parts by weight of metallurgical coke and treated as in Example 2. The multipurpose test specimens produced in the injection molding process had an electrical resistance of approx. 600 Ω over a measuring distance of 160 mm and could be painted well electrophoretically after degreasing.

Example 6

A common composition of an epoxy resin molding compound (20 parts by weight of epoxy resin, 10 parts by weight of epoxy resin hardener, 1 part by weight of hardness catalyst, 43 parts by weight of mineral fillers, 0.4 parts by weight of metal stearate, 0.6 parts by weight) . Parts of wax-like lubricants) were intimately mixed with 10 parts by weight of conductive carbon black and 15 parts by weight of coke dust and treated as in Example 2. The multipurpose test specimens produced by injection molding had an electrical resistance of approx. 500 Ω over a measuring distance of 160 mm and, after degreasing, could be painted electrophoretically with good results.

Claims

claims

1. Thermosetting molding compositions for the production of moldings for direct electrophoretic coating, containing curable resins, carbon black, fillers and auxiliaries, characterized in that they contain 5-25 wt .-% coke with a grain size <0.3 mm and 3 to 20 wt .-% contain soot.

2. Thermosetting molding compositions according to claim 1, comprising:

20-60% by weight curable resins

5-25% by weight coke

5-20% by weight of carbon black

5 - 50% by weight fillers and reinforcing materials

0.1 - 5 wt .-% excipients

3. Thermosetting molding compositions according to claim 1, characterized in that they contain up to 1 wt .-% lubricant and / or 0.5 to 3 wt .-% polyethylene glycol.

4. Thermosetting molding compositions according to claim 1 to 3, characterized in that they contain up to 10, preferably 1 to 5 wt .-% metal powder.

5. Thermosetting molding compositions according to claim 1 to 4, characterized in that they contain 15 to 20 wt .-% coke and 5 to 15, preferably 10 wt .-% carbon black.

6. A process for the production of electrophoretically coated molded parts from thermosetting molding compositions according to claims 1 to 5, wherein the curable resins are mixed with the fillers, auxiliaries and the carbon powders to a uniform mass and molded and cured in a conventional manner, characterized in that the Molded parts are directly electrophoretically coated after degreasing.

7. The method according to claim 6, characterized in that the curable resins are prepolymerized.

8. Use of thermosetting molding compositions according to claims 1 to 5 for the production of directly electrophoretically coatable molded parts.