Plastic metallization

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Plastic metallization is the coating of a material made of plastic with a metal layer .

Background and application

The advantages of plastics as a base material are manifold. Low weight, insensitivity to corrosion , inexpensive production of the blanks by injection molding and the elimination of mechanical surface treatment are the main reasons that make plastics interesting as a base material. For example, while previously in the automotive industry only metals (e.g. steel , brass , die-cast zinc ) were used as the basic material for galvanized exterior parts (door handles, lettering, decorative strips, radiator grills, hubcaps, etc.) , today they are almost completely replaced by galvanized plastics been. It is used in a wide variety of ways and extends to all branches of industry, not only for decorative, but also for technical purposes such as shielding the housings of electronic devices or the cheap manufacture of antennas and waveguide assemblies as well as in the sanitary area for mixer taps , shower heads and faucet handles . For purely decorative purposes, metallized plastics are used, for example, for decorative buttons, brooches and buckles.

Collective mirrors for incandescent torches and bicycle headlights have been made of aluminized polystyrene since around 1975/1980 and have replaced mirrors made of polished aluminum, previously tinplate, mirrored glass and, in the case of carbide miner's lamps, also polished brass. Break-proof mirrors made of thin plastic sheets are now often replacing mirrored glass.

Foil balloons (fair, amusement park, ...) are denser than the helium filling due to the metallization. This opaque layer can be printed with translucent printing inks. Metallized foils or papers serve as a rescue blanket, candy packaging, tinsel and confetti. Foldable reflex umbrellas and flash reflectors are used for studio photography.

Protective clothing, for example on the blast furnace tap, is aluminized to reflect heat radiation. Mirrored lenses can also be metallized.

Procedure

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In principle, various thin-film technology processes are available for plastic metallization :

Depending on the process, different plastics can be coated and different adhesive strengths can be achieved.

Sputter deposition

With sputter deposition, a target (coating material) is bombarded with particles in a high vacuum . By dissolving the coating material and accelerating it onto the substrate, layer thicknesses of 3 to 5 µm are usually deposited. Coatable plastics must above all be evacuable. This is significantly influenced by the outgassing behavior and the water absorption of the plastic.

Plasma-assisted chemical vapor deposition

Pure CVD (Chemical Vapor Deposition) processes enable materials to be deposited by chemical reactions at> 500 ° C. As a rule, plastics cannot withstand these temperatures. Combination methods of PVD and CVD processes can be used to reduce the process temperature.

Thermal spray

By heating the coating material, leaching and accelerating particles and bombarding the substrate material, the particles solidify on the surface. Layer thicknesses are usually in the range> 50 µm.

Electroplate

The galvanic deposition of metals on plastics is known as plastic electroplating. Plastics are usually not electrically conductive , so the surface must first be covered with a well-adhering, electrically conductive layer for a subsequent electrolytic coating. The aforementioned coating processes are also used for this. In detail, the following process steps are necessary for electroplating:

  • Pickling with oxidative metal salt solutions to roughen the surface,
  • Activation with metal nuclei, e.g. B. Palladium ,
  • Chemical metallization to form a conductive layer, here a thin layer (0.3 to 0.4 μm) made of copper or nickel is produced by reducing their metal salts.
  • Deposition of the actual metal layer in electroplating baths, whereby in the chrome plating example two layers of copper and nickel with a thickness of 10 to 40 μm are first deposited in order to achieve optimal adhesion.

Process description using the example of the electroplating of ABS plastics

(Industrial electroplating of ABS is acrylonitrile butadiene styrene - copolymer ) and ABS PC -Kunststoffen most widespread. Other plastics such as PA6.6 , PEI , LCP ( palladium-doped ) can also be metallized with this process.

The first step in electroplating ABS plastics is to roughen the surface. In a chromosulfuric acid pickle (400 g / l CrO 3 and 400 g / l H 2 SO 4 ), where the working temperature for ABS is 60 ° C and ABS / PC is 69 ° C, a component of ABS, butadiene , is made from The surface is oxidized and caverns in the microscopic range are created. Palladium nuclei, which are surrounded by a tin shell and form a colloid, are stored in the so-called activator in these caverns. In a further step, the tin shell, which ensures the adherence of the germ in the caverns, is removed in the accelerator ( tetrafluoroboric acid 17 g / l) temp .: 45-50 ° C so that the germ is exposed. The high standard potential of the palladium ensures the start of the reaction in the subsequent step, the chemical ( electroless ) nickel plating in a nickel bath (nickel sulfate; ammonia and sodium hypophosphite as electron suppliers ). Here a reducing agent , which is itself oxidized, emits the electrons necessary for nickel deposition . This creates the first thin, conductive nickel layer, which, due to the filling of the caverns, has a strong mechanical interlocking with the plastic and therefore adheres well.

This layer can then be further built up conventionally and, for example, a copper-nickel-chromium system, as is widespread in decorative electroplating, can be applied.

Pre-treatments

Various pretreatments are used, especially during electroplating:

  • Chemical coatings with the help of
    • Palladium activation
  • Chemical etching processes (chemical bonding forces); Not every plastic is suitable for electroplating with the aid of chemical etching processes.
  • Plasma pretreatment (physical bonding forces)
  • Mechanical roughening (mechanical bonding forces)

With plasma coating , a plasma is generated in a vacuum chamber by admitting a process gas (e.g. oxygen ) and generating an electrical discharge. This discharge creates highly reactive ions and radicals that trigger reactions in the plastic surface (e.g. oxidation ). These reactions are often associated with material removal (nanometers) and roughening. These chemical and morphological changes in the surface improve the ability to metallize.

Roughening processes such as grinding , sandblasting , polishing , honing, etc. a. the surface of the plastic can be changed mechanically to create a mechanical interlocking. A combination of these methods provides e.g. B. the META-COAT process .

literature

  • Dietrich Rathmann: Plastic electroplating . In: Chemistry in Our Time . tape 15 , no. 6 , 1981, pp. 201-207 , doi : 10.1002 / ciuz.19810150606 .