We already learned about the basic resin system, and will now discuss the resin additives.

Additives are used to modify and enhance resin properties for specific applications.

The first additives under discussion will be thixotropes. This additive is used to modify the flow and viscosity properties of the resin to gain processing advantages. Especially important on vertical surfaces, the liquid resin must be viscous enough to stay in place while the resin moves into the cure cycle. Polyester resin commonly uses fumed silica.

Fillers are inert additives for resin that are mainly used to reduce cost.

Some are mixed in during processing, some come loaded into the mix already. Mixing the resin to keep fillers in suspension is very important. Types of fillers include: Mineral fillers, Calcium Carbonate, Calcium Sulfate, Talc, Mica, Wood Flour, Walnut Shells, Corn Cobs, Microspheres, Ceramic spheres, etc.

Pigments and colorants can be added to resins and gelcoats for cosmetic purposes and for weatherability. High sheer mixers blend resin and fine pigments to create pigmented gelcoat.

Fire Retardants are important for resin systems depending upon their application. Additives such as alumina trihydrate and antimony trioxide reduce flame spread and smoke generation during flame conditions. Fire retardants are required for applications where reducing fire hazards is important, and I have worked with this for applications in public buildings. Other applications include train and airplane parts as well as mining equipment.

The job of suppressant additives is to help block the evaporation of styrene. This is a wax-based material that work by creating a thin film across the resin surface. Using paraffin wax is the common way to accomplish this as well as allowing for a non-tacky sandable surface.

UV inhibitors are added to some resins to slow the degradation of the surface if it should be exposed to sunlight.
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Conductive addtives such as carbon black, carbon fibers, metallic fibers, and metallized glass are added when a laminate needs to be conductive in nature. This is commonly done for parts that need to be electrostatically painted or need a reduction in static charge.

That seems to cover most of the resin additives under my awareness. Most of the above come mixed into the resin by the resin manufacturer, but some must be done at the fabrication site. Awareness of what they are and why they are important is critical to the proper use of these resin systems.