| Abstract | Plastic components are plated with metal layers of up to one hundred microns thickness by autocatalytic (electroless) deposition from aqueous solutions. At the start of the present research programme only one plastic, namely ABS, was plated on a relatively large scale, yielding a product in which the metal plate was firmly bonded to the substrate. Lack of success with other plastics was due to the inability to produce an adherent metal coat. Most workers who have studied adhesion in plated plastics conclude that high interfacial adhesion is due to geometric keying between the metal plate and the micro-roughened plastic surface. No high adhesion values have been reported for smooth surfaces. However, in practice plate adhesion is usually limited by the presence of weak layers within the polymer sub-surface which are generally caused by orientation.
In part I of this thesis, an investigation on the influence of the polymer surface region on plate adhesion is reported. A technique is described in which the surface geometry of etched ABS may be reproduced on any polymer surface. A "standard" polymer system in which the relative roles of chemical and geometric adhesion are investigated is also described. It is concluded that the interfacial adhesion in the A.B.S. system is not entirely due to geometric keying, chemical adhesion being a substantial component. However, with injection-moulded ABS the plate adhesion is limited to less than half its ultimate value by an orientation ~- weakened sub-surface. More generally, with non-oriented systems it is found that the plate adhesion (F) is given by :
F = Geometric component x Chemical component.
F also increases with the toughness of the substrate.
In part II, the mechanism of the deposition, the nature of the deposited metal and the role played by the metal in determining the final plate adhesion is reported. Electron microscopy is largely used to investigate these topics. It is found that during early stages. of electroless deposition, copper and nickel grow as island structures from active palladium nuclei. The surface density of these nuclei, which are formed during the pretreatment cycle, depends on the surface chemistry of the polymer. The density of metal islands depends on both surface chemistry of the polymer and deposition parameters. Electroless copper deposits are highly crystalline solids, whereas the nickel deposits appear to behave as amorphous quasi-liquids. The mode of growth of the metal, which is largely dictated by the polymer surface, determines the final plate adhesion. Finally, part III of this thesis contains an account of the development work associated with this project. Specific plating processes for two polymers, namely high-impact polystyrene and polypropylene, have been developed. Etches based on chromic acid mixtures are found to be effective for producing surfaces capable of giving high plate adhesion |
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