Resin-rich surfaces

Corrosion resistant FRP structures and also resin linings have resin or resin-rich surface layer to protect the structures from corrosive attack. Therefore, the study of corrosion behavior of resin is essentially important. 

Frequently, to provide resistance to weathering, erosion, or chemical attack, a resin-rich surface coat (gel coat) is added. This surface layer is applied first and then allowed to stiffen into a tough layer (not cured) before additional layers are applied. It usually is reinforced with a surface veil using C-glass (rather than the usual E-glass) if chemical resistance is required, or a synthetic fiber veil may be used for resistance to weather, particularly sunlight. The resin may be a special formulation that includes TP to improve the surface appearance. Subsequent layers of mat, fabric, or combinations are then applied. 

With high quality sheeting a surfacing tissue may be used to ensure a resin-rich finish. Such sheeting must be installed with the resin-rich surface exposed to the weather. 

Veils can be used to control the thickness of a resin rich surface layer. They may be used in conjunction with gel coats and top coals to provide reinforcement support and increased retention of resin. 

Some plastics may need to be surface roughened or chemically treated to promote adhesion of the decorating medium. Molded and extruded plastic parts tend to have glossy resin-rich surfaces. This is desirable if the part is used in the as-is molded... 

The gel coat is often made of the same primary resin as the molded part on which it is applied. A gel coat sometimes contains a pigment. The gel coat does not have the concentration of fillers or reinforcement that the molding compound has. Thus the gel coat provides an attractive resin-rich surface layer. The gel coat can be either applied to the surface of the mold and gelled before layup or applied as a coating after the part is fully processed. 

An exttemely smooth surface can be obtained on a reinforced plastic molding by placing a thin overlay sheet on the exposed surface of the composite. This type of mat provides little actual reinforcement. It does, however, facilitates a resin-rich surface layer to be incorporated into the molded part. 

Surfacing mat, or veil, is used in conjunction with reinforcing mats and fabrics to provide good surface finish. It is effective in blocking out the fiber pattern of the underlying mat or fabric. Surfacing mats are also used as the inside layers of corrosion resistant composites, providing a smooth, resin-rich surface. 

Roughening of a smooth surface can improve bonding by a combination of cleaning and mechanical keying. Some adherends may have lower surface properties compared with their bulk as a result of their manufacturing method, e.g. oxidation of metals or the resin-rich surface iayers of fibre composites. Mechanicai treatments can be used to remove these external layers. 

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