A tin-free catalyst for electroless platinum deposition on polyethylene terephthalate

Abstract

The metallization of polymer materials has many applications in a range of industries for decorative and=or functional purposes, e.g., nickel plating of ABS plastics in automotive parts [1] and copper plating of polyimides in printed-circuit boards [2]. Electroless deposition has attracted a great deal of interest as an economic means of metallization of polymers. Recently, this technique has been shown to have signi®cant potential for the fabrication of a new generation of electrodes used in implantable medical applications, such as de®brillation, pacing and cardiomyoplasty [3]. Since electroless deposition takes place only on conductors, it is necessary to use a catalyst to seed the insulator surface with catalytic metal particles [1]. Traditionally, a two-step treatment is used in which a SnCl2 solution acts as a sensitizer and a PdCl2 solution as an activator to provide catalytic metallic Pd sites [4]. More recently, a one-step treatment has been introduced using a mixed SnCl2 ± PdCl2 solution to yield Pd±Sn alloy as catalyst centres [5±9]. These activation treatments are normally followed by an acceleration treatment to remove excessive Sn ions which inhibit electroless deposition. Both these methods use tin which is known to be toxic and therefore they are not suitable for medical implants. A new method for the activation of a nonconductor which eliminates the use of tin has been invented by our research group [10]. In this letter, we report the use of this new catalyst for electroless platinum deposition on polyethylene terephthalate (PET) surfaces. Platinum and PET are chosen because of their proven suitability for use in implantable medical devices. The results to be discussed were mostly generated from experiments on PET ®lms which facilitates characterization and testing. Preliminary results from PET ®bres are also brie y discussesd. PET ®lms of 0.1 mm thickness were supplied by Goodfellow and PET ®bres of diameter approximately 22 im were supplied by Akzo Fibres. Unless otherwise stated, all chemicals were supplied by Aldrich or Sigma as laboratory reagents with no further puri®cation. A tin-free catalyst was prepared by dissolving 0.2% (w=v) PdCl2 into dimethylsulphoxide (DMSO), which is referred to as DMSO±Pd catalyst in this report. A reducer was formulated with 4% (v=v) hydrazine in aqueous solution. The electroless deposition bath was a proprietary formulation and it was used as recommended, except for some adjustment of the ammonia concentration. The electroless deposition of Pt on PET using the new catalyst method involved the following four steps, with thorough rinsing in de-ionized water between each step. As-received PET ®lms or ®bres were ®rstly washed in hot detergent solution (NewBon at a strength of 0.1%) for 30 min at 80 8C to remove wax and=or oil residues present on the surface. The cleaned samples were then etched in a hot alkaline bath which consisted of NaOH (100 g ly1) and 0.1% surfactant (proprietary product) for 30 min at 80 8C. The etching provided a rough PET surface for better adhesion through the mechanical interlocking of the Pt coating and the PET substrate. Catalysis was achieved by dipping the PET samples into the DMSO±Pd catalyst for a speci®c time and then dipping the samples into the reducer for 1 min, with both the catalyst and the reducer maintained at room temperature. The catalysed samples were immersed into a pre-heated electroless deposition bath at the recommended temperature of 60 8C for the required deposition time. The above cleaning and etching steps were carried out with vigorous mechanical agitation, whereas the catalysis and electroless deposition were conducted with gentle agitation. In addition, some selected ®bre samples, treated under slightly different etching conditions, were electrolessly plated using either the new catalyst or the traditional mixed Sn±Pd catalyst to give a comparative study on the adhesion between the Pt coatings and the PET substrate.