GEL COAT for FRP

Functions/Significance/Applications:

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

1.      Resin

2.      Fillers

3.      Thixotropic agents

4.      Pigments

5.      Solvents

RESIN:

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. 

FILLERS:

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.

THIXOTROPIC AGENTS:

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.

PIGMENTS:

They are dispersed into the resin to act as coloring agents. Their concentration ranges from 2-10 % by weight.

SOLVENTS:

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.

RESIN MATERIAL FOR FIBER REINFORCED PLASTIC

RESIN:

Function:

The resin in RP holds the fibers together and transmits the loads from fiber to fiber.

Requirements of an ideal resin in RP:

1.      The uncured resin must wet the reinforcement well and should be cured at room temperature or by heat. Also, when cured, it should have good adherence to the fibers, good heat resistance and good electrical and chemical properties.

2.      The resin should be available in a wide range of viscosities, should be compatible with pigments, filler material and if possible, must be cured at temperature ranging from ambient to 150⁰C.

Conventional resins:

1.      Polyester: It is the 1^st resin used in the low pressure process and is the most widely used polymer because it possesses almost all the requirements of an ideal resin.

a.       Polyester resins are available in liquid forms in many viscosities, so good fiber impregnation is relatively easy.

b.      Using different catalysts the resin can be cured at room temperature or at an elevated temperature or by ultra violet (uv) light. This allows a part to be worked on all day without its setting up and then be cured overnight in an oven, or by using uv radiation from a lamp (sunlight can also be used)

c.       Cured materials have excellent electrical and chemical properties.

d.      Cured materials have good strength to weight ratios.

e.       Costs are relatively low.

f.       Disadvantages of polyester resin: (1) Relatively high shrinkage occurs when curing takes place. (2) Under high humidity or moisture condition, less adherence (than ideal) to the fibers occurs hence fibers are specially treated to upgrade the performance of laminates. (3)For thin fiberglass reinforced parts, relatively high brittleness occurs.

g.      Special formulations are produced which show increased toughness. Vinyl ester resins are tougher and have higher temperature resistance than the conventional polyester resins.

2.      Epoxies: These materials have all good properties of the polyester. The epoxy resins also have some special properties of their own, which makes them superior to the polyesters like: Low shrinkages and excellent adhesion to most material which gives higher strength to weight ratio to the laminates.

3.       Other resins: Some of the older resins are modified so that they can be used with the new low pressure technique. Some new resin types are also developed. Today there are a number of thermoset resin each having unique characteristic e.g. Phenolics, Silicones, Melamines, Polyimides, Diallyl phthalates.

CURING TECHNIQUES:

The resins were cured at pressures ranging from 0-15 psi but now pressures up to 50-100 psi are used. The use of high pressures will result in more highly densified part because no volatiles are given off in the polymer. The densification takes place by compression of the dissolved air. This results in stronger parts.