Chromium-free Etching and Palladium-free Plating of Plastics
Plastics are replacing metals in the manufacture of many parts, and quite often there is a need for metallic coatings on the plastics and other non-conductors. This paper will describe new processes of preparing ABS plastic substrates for subsequent metallization.
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By Vijaykumar Ijeri, Komal Shah and Snehal Bane, Grauer & Weil (India) Ltd., Mumbai, India
As plastics are replacing many of the metal parts, there is often a need to have metallic coatings on plastics and other non-conductors. This paper will describe new processes of preparing ABS plastic substrates for subsequent metallization. A process replacing the traditional chromic acid etching step with a room temperature, environmentally-friendly etchant is described. The etchant gives the plastic substrate an increased hydrophilic surface and a microroughened, microporous resin topography.
Further, due to the high cost of palladium there is a need for a palladium-free activator. A non-precious metal activator useful for obtaining uniform, adherent coatings of metal layers on plastic substrates has been developed. The resultant colloidal solution is active and believed to adsorb on the hydrophilic surfaces of the etched plastic. A specific reducing agent is used to transform the adsorbed complex into a conductive layer which allows the plating of subsequent conducting metal layers. The possibilities of direct electroplating (without an electroless step) are also discussed.
Keywords: Plating on plastics, chromium-free etchant, palladium-free activation, hexavalent chromium substitutes
Rapid advances in polymer chemistry and production technology have resulted in the replacement of metals by plastics in a wide range of applications. Plastics provide the advantages of weight reduction, ease of fabrication and design flexibility. The electroplating of plastics was introduced commercially in Europe and North America in the early 1960s with limited acceptance. The major breakthrough that led to growth in the industry was the development of chemical processes for the surface preparation of acrylonitrile-butadiene-styrene (ABS), which enabled good bonding to be obtained between the plastic substrate and the metallic coating. The major driving force for plating-on-plastics (POP) was the automotive industry. The need for increased fuel efficiency called for a reduction in vehicle weight. Consequently, plastics were found to be useful in automotive components. Bright finishes have a popular aesthetic appeal, which is achieved by nickel/chromium plating. ABS remains the most widely used plastic for plating applications and excellent quality is obtained routinely in the industry. Metal-plated plastics find use in the automotive industry, hardware, plumbing fixtures, knobs, RFI/EMI-shielding and electronic applications.
Prior to electroplating, the plastic surface needs to be treated and made conductive. This generally involves the following steps:
- Cleaning in mild alkaline solution with surfactants, which removes any dirt accumulated on the parts during handling.
- Etching of the plastic to provide hydrophilic and micro-roughened surface that is useful for adhesion of subsequent layers of coatings.
- Activation of the etched surface to produce catalytic sites that will enable chemical deposition of nickel or copper to form a thin conductive top layer.
Once this conductive layer is formed, the parts can be electroplated with copper, nickel and chromium. This paper will discuss the developments in etching and activation processes leading to an overall economical and environmentally-friendly plating system.
ABS is a thermoplastic terpolymer that has an acrylonitrile-styrene matrix with butadiene rubber uniformly distributed in it. This quality makes it unique for plating, as the butadiene can be selectively etched out of the matrix (Fig. 1), leaving microscopic holes that are used as bonding sites for activation and electroless deposits. Other factors influencing the choice are low cost, low thermal expansion, ease of molding, good metal adhesion to the substrate and good appearance after plating.