Fraunhofer Institute for Applied Polymer Research
Fraunhofer Institute for Applied Polymer Research

Pretreatment technology for metallization of plastics

Metallized plastics are used to replace metal objects by plastics for example to reduce the weight of a product of both the conductive metal and the insulating plastic form a functional unit as in the case of a printed circuit board or film.

Since metals generally bond poorly to polymers, the plastic surface has to be treated. In this regard, the highly versatile ABS (acrylonitrile-butadiene-styrene terpolymer) is widely pretreated by etching with chromic acid. There are alternatives for the Cr(VI) containing agent. We dealt extensively with pretreatment technologies based on plasma and VUV radiation and with the application of functional polymers as molecular primers.

ABS and ABS/PC

SEM ABS
© Fraunhofer IAP
Scanning electron micrograph of an ABS surface after VUV treatment.
© Fraunhofer IAP
Injection molded ABS part metallized after pretreatment with VUV irradiation.

While the adhesion of the metal on an ABS surface which was treated with chromic acid is largely based on mechanical interlocking, alternative technologies rely more on chemical interactions between the metal and the polymer. With the VUV pretreatment of ABS we are able to have a good adhesion on a flat surface with peel forces > 1 N/mm.

In such kind of technologies the chemical structure of a surface is crucial for a good adhesion. We can tailor the surface chemistry to the needs. (XPS)

 

Polyolefins

© Fraunhofer IAP, Andreas Holländer
Sintered PE nickel plated (left) after plasma treatment.
© Fraunhofer IAP, Andreas Holländer
Palladium electrodes on COC sheet in microscopy slide format.

Polyolefins as polyethylene (PE) or polypropylene (PP) are very inert materials and require a pretreatment for a good adhesion of inks or adhesives. For the electroless plating of sintered PE powder with copper and nickel we used a low pressure oxygen plasma for the activation and coated with a polymeric molecular (poly(ethylene imine)) primer before the plating. The metal layer was resistant in a tape test.

A cyclic olefin copolymer (COC) was used as a substrate for gel electrophoresis. For this purpose the COC was activated with plasma, coated with a primer and then plated with nickel (electroless) and palladium (galvanic). On these electrodes the hydrogel was coated and freeze dried in liquid nitrogen.

 

Polyimide

© Fraunhofer IAP, Andreas Holländer
Peel force of copper on PI. (VUV1: VUV in ammonia + sputtered copper Cu, VUV2: VUV in ammonia + electroless Cu, Tie-Coat: results of St. Günther, Fraunhofer FEP; Cr: results from I. Ashmanis et al.: 39th SVC, Conference Proceedings, 1996)

For the application in electronics polyimide (PI) is used as a base material for PCBs. There are various technologies for the pretreatment to achieve a good adhesion. We developed a vacuum-free process which includes a VUV treatment in ammonia and an electroless copper plating. The peel forces for a 30 µm galvanic copper on various intermediate layers and pretreatments are shown in the figure.

 

Metal plating on 3D printed parts

Ni plated SLS part
© Fraunhofer IAP, Andreas Holländer
Nickel plated part made by selective laser sintering. The part was supplied by sirris, Belgium.

The application areas of parts prepared by additive manufacturing (AM) technologies for polymers can be extended remarkably by surface functionalization. Metals can be plated directly on surface activated polymer parts produced by any AM technology. A lacquering can smoothen the surface before metal plating.