Finishing Surface Treatment

The surfaces of plastics are sometimes additionally treated or covered by a coating of metal or glass, for technical (surface hardness, friction) or aesthetic reasons (gloss). 

Layers of metal up to thicknesses of /urn can be applied by chemical methods or by vacuum evaporation. The layers have a high gloss. For thicker layers, these methods are not economical, and as far as it is possible with the plastic, coating is performed with galvanizing methods. In all these methods, the plastic surfaces must first be thoroughly degassed, degreased, or dried. 

Almost all plastics can be coated by metals under vacuum. Car hoods thus produced from high-impact plastics with a metal finish are cheaper, even today, than metal ones. The disadvantage of this method is the low adhesive strength, particularly with thicker metal layers. 

Chemical metallization is, in practice, used only for silvering. The plastic surfaces are dipped in solutions of silver salts and the silver is then reduced chemically. Here again the adhesive strength is low. 

So far, only ABS polymers have been galvanized on a commercial scale, since only in this case can a sufficiently high adhesive strength be obtained. When the plastic surfaces are pickled, the rubber elastic components are anodized. This produces pores and channels in which, for example, silver can be deposited chemically. The silver then forms the adhesive base for the copper layers subsequently deposited electrochemically, and these layers are then reinforced by the galvanized coating. Here, too, it is difficult to manufacture metal layer thicknesses of more than 10 jum because the different thermal coefficients of expansion of plastics and metals can easily lead to stresses, and thence to bubbles or cracks. 

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Fluorotitanic acid is used as a metal surface cleaning agent, as a catalyst, and as an aluminum finishing solvent (see Metal surface treatments). Fluorotitanates are used in abrasive grinding wheels and for incorporating titanium into aluminum aHoys (see Abrasives Aluminumand aluminum alloys). 

The phases and their proportions present ia hardened amalgam are controlled by many factors. The composition of the alloy the size, shape, and size distribution of the particles the thermal history of the cast ingot and the comminuted alloy and the surface treatment of the particles are some of the factors for which the manufacturer is responsible. The tooth cavity preparation and the mixing, compacting, and finishing techniques of the dentist can make the difference between satisfactory and unsatisfactory restorations, even with the best of alloys. A minimal amount of residual mercury and porosity are needed to obtain the most serviceable restorations (138). 

Primers are required to be resistant to all of the same fluids and environments as the adhesive, and are in addition expected to be compatible with secondary finishes such as corrosion and fluid resistant primers applied to cured bond assemblies. The most commonly used primers for 250°F cured epoxy adhesives also have active corrosion inhibitors themselves to combat corrosion at bondlines. This last requirement is somewhat dated, evolving from the severe corrosion and delamination problems experienced before U.S. airframe manufacturers adopted durable surface treatments. 

CP30I2 1972, Cleaning and preparation of metal surfaces , B.S.I., London Wernick, S. and Pinner, R., The Surface Treatment and Finishing of Aluminium and Its Alloys, Robert Draper, London, 3rd edn (1964)... 

Since most fabricated products are attractive as well as inherently corrosion and rust resistant when fabricated they usually do not require any finishing or decoration. For others there are paints, coatings, and other surface treatments that usually are used mainly to enhance eye appeal. Tables 8-35 to 8-37 provide some guidelines. 

Roy, C.H., Operation and Maintenance of Surface Finishing Wastewater Treatment Systems, American Electroplaters and Surface Finishers Society, Orlando, FL, 1988. 

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