When the parts are molded at 0-15 psi, their surface showed up a distinct 3-D fiber pattern due to the shrinkage of the resin during curing. This condition cannot be tolerated for aerodynamic and aesthetic reasons. Use of gel coat is developed as remedy.
The usual gel coat is polyester resin heavily filled with mineral filler and a pigment. This shows very little shrinkage due to its very high filler-to-resin ratio.
Also the gel coat, when properly applied and cured, results in a hard, smooth colored coating, 10-30 mils thick throughout. This minimizes the effect of scratches.
Sometimes, a clear gel coat is used to improve the stain resistance, gloss and depth of coating when applied over cultured marble.
A chemical resistant clear gel coat is used over chemical resistant laminates to improve the resistance to corrosive environments.
Epoxy gel coats are also used to improve the surface quality of the cured reinforced plastic part.
Polyester gel coat formulation:
Gel coat consists of
3. Thixotropic agents
A low viscosity resin is used so it can be easily and will allow entrapped air to escape. Usually a high impact grade is preferred to insure freedom from chipping caused by impact thermal stresses. Concentration of the resin in the complete formulation is 25-95% by weight with lower concentration preferable.
Usually CaCO3 (calcium carbonate-marble) and hydrated Aluminum silicate are used. Other fillers are metal powders, carbides, oxides, silicates, sulfides, carbon and graphite depending on the physical, chemical or electrical properties desired. The fillers are used to reduce the resin shrinkage; lower the exothermal heat; increase the hardness and thermal conductivity; and change the density and opacity of the resin. Their concentration ranges from 5-75% by weight.
Colloidal silica (Cab-O-Sil) and Mg Al silicate clays (Bentonite) are used as thixotropic agents, to prevent sagging and running of the gel coat when applied to vertical surfaces. They also minimize filler settling and increase efficiency. Their concentration ranges from 2-15 % by weight.
They are dispersed into the resin to act as coloring agents. Their concentration ranges from 2-10 % by weight.
They are added to the formulation to thin the material to spraying consistency. Minimum amount of solvent should be used to prevent attack on the mold release which in turn results in part sticking, poor surface etc. Usually acetone is used as solvent.
TECHNIQUES FOR APPLICATION OF GEL COAT:
The catalyzed gel coats are applied to the release coated mold surface by brush, roller coat or spray. The spray coating technique is most widely used. The coat is allowed to gel and then the reinforcing fiber and resin are applied while it is still tacky. If done correctly, the bond between the gel coat and the reinforcing fiber laminate will be excellent but if the reinforcing mat is applied before gelation of the gel coat, fiber exposure or the impression of the fiber or fabric may result. However, most gel coats are made using air inhibited resin, so the gel coat will automatically cure with a tacky surface.Gel coats are used in parts where a smooth, hard, tough and colored surface is required.