Top coats

Vinyl resins ie, copolymers of vinyl chloride and vinyl acetate which contain hydroxyl groups from the partial hydrolysis of vinyl acetate and/or carboxyl groups, eg, from copolymerized maleic anhydride, may be formulated with alkyd resins to improve their appHcation properties and adhesion. The blends are primarily used in making marine top-coat paints. 

Primers for Metal. If reasonably high performance is required ia the end product and unless cost is of paramount importance, a minimum of two coats, usually a primer and a top coat, should be appHed to metal. For highest performance, primer vehicles should provide good wet adhesion, be saponification resistant, and have low viscosity to permit penetration of the vehicle iato microsurface irregularities ia the substrate. Color, color retention, exterior durabiHty, and other such properties are generally not important ia primers. Resia systems such as those including bisphenol A epoxy resias which provide superior wet adhesion can thus be used ia spite of their poor exterior durabiHty. 

Top Goats. The selection of a top coat depends on cost, method of appHcation, and product use and performance requirements, among other factors. As a result of increasingly stringent air quaHty standards and increased solvent costs, approaches to reduction of solvent emissions are being sought. 

It is also common to distress the surface for resemblance to antique furniture. For example, spots of black pigment stain can simulate India ink spots dropped from quiH pens, and dark stains carefully appHed into corners simulate dirt accumulated over many years. When this "art work" is completed, a sanding sealer is appHed to immobilize the lower layers of the finish and to provide a surface that can be sanded smooth. Finally a top coat is appHed and poHshed smooth. 

Many alternatives to nitrocellulose lacquers for top coats have been investigated and some are used commercially. Especially for lower cost furniture exposed to hard use such as motel and institutional furniture, alkyd—urea top coats are used. Urea—formaldehyde resins, in contrast to... 

AcryHc lacquers also have disadvantages the gloss of the air-dry film is too low and the surface must be mbbed to achieve the high gloss necessary for automotive top coats and most importantly, VOC emissions are very high. AcryHc lacquers are expected to be phased out when controls become more restrictive. 

Other decoratively plated coatings produced in quantity are copper and copper alloys. Copper-plated hardware for cabinetry, lighting fixtures, household trim, and other uses is protected using a top coat of clear lacquer. Copper plating finishes are available with a variety of post-treatments to simulate antique or mstic appearances. 

Zinc—Nickel. Steel has the best salt spray resistance when the nickel is 12—13% of the alloy. At increasing nickel contents, the deposit becomes more difficult to chromate and more noble, eventually becoming cathodic to steel. At those levels and above, corrosion resistance usually decreases and is dependent on a complete lack of porosity for protection of the steel. In efforts to replace cadmium and nickel—ca dmium diffused coatings in the aircraft industry, 2inc—nickel has insufficient wear properties for some appHcation, but is under study as an undercoat to various electroless nickel top coats (153). 

Silicon—Ca.rbon Thermoset. The Sycar resins of Hercules are sihcon—carbon thermosets cured through the hydrosilation of sihcon hydride and sihcon vinyl groups with a trace amount of platinum catalyst. The material is a fast-cure system (<15 min at 180°C) and shows low moisture absorption that outperforms conventional thermosets such as polyimides and epoxies. Furthermore, the Sycar material provides excellent mechanical and physical properties used in printed wiring board (PWB) laminates and encapsulants such as flow coatable or glob-top coating of chip-on-board type apphcations. 

A frequently cited example of protection from atmospheric corrosion is the Eiffel Tower. The narrow and, for that age, thin sections required a good priming of red lead for protection against corrosion. The top coat was linseed oil with white lead, and later coatings of ochre, iron oxide, and micaceous iron oxide were added. Since its constmction the coating has been renewed several times [29]. Modern atmospheric corrosion protection uses quick-drying nitrocellulose, synthetic resins, and reaction resins (two-component mixes). The chemist Leo Baekeland discovered the synthetic material named after him, Bakelite, in 1907. Three years later the first synthetic resin (phenol formaldehyde) proved itself in a protective paint. A new materials era had dawned. 

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