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Polyester Gel Coats

Materials Used In FRP Gel Coats


White Gel Coat

Example of a container with a white marine-grade gel coat.

Todd Johnson

Traditional gel coats used in composites are pigmented ambient-cure polyester resins, formulated to use as the coating in open molding. The gel coats are made to provide a durable yet attractive finish to composite products while also guarding them against wear and tear.


A variety of materials are used in the processing of traditional gel coats. Which materials are used and their amounts have a great impact on both the gel coat's performance and the end product's capabilities. The materials that are used in FRP composite gel coats include:

  • Polymer
  • Reactive monomer
  • Pigments
  • Fillers
  • Thixotropic agents
  • Promoters
  • Inhibiters
  • Additives


The materials comprising polyester polymers are ultimately what give them their long list of properties. One ingredient you will find in all polyester polymers is an unsaturated acid component, usually maleic anhydride. It results in cross-linking. In addition, a saturated acid is needed as well, typically isophthalic. Isophthalic is found in general use gel coats which sport excellent weathering, water resistance, and chemical resistance qualities.


Gel coats require monomers for two reasons. First, they react and cross-link with sites in the polymer to create a cross-linked thermoset product. Second, they lower the gel coat's thickness or viscosity to help with the application. For example, most gel coats are sprayed and a lower viscosity allows for better and smaller particles being sprayed, and ultimately, a better end result.

Styrene and methacrylate are two commonly used monomers. Be sure to refer to your local regulations which limit the type and amount of monomers allowed for use in gel coats. The monomers' quantities and combinations affect a gel coat's:

  • Flash point
  • Evaporation rate
  • Reactivity of the system
  • Weathering properties
  • Viscosity


Fillers are needed in gel coats to reach certain desired characteristics and alter spray properties. Water and other environmental factors may affect a gel coat's color, texture, and appearance. Fillers can change physical properties of a gel coat making them more resistant to such conditions. Commonly used fillers include:

  • Calcium carbonate
  • Talc
  • Aluminum trihydrate (ATH)

Thixotropic Agents

Thixotropic means to have a viscosity that is dependent on shear rate, and gel coats are comprised to have this quality. During spraying, a high shear operation, gel coats should have a low viscosity. To prevent sag, a gel coat should return to a high viscosity once deposited onto the mold and hence under low shear. Thixotropic agents make this behavior possible. Through hydrogen bonding, these agents form a network with a polyester polymer. Networks break down, lowering the viscosity of the gel coat during high shear. The network recovers, increasing the viscosity of the gel coat, at low shear. While a quick rate of recovery decreases a risk of sag, it also raises the risk of air entrapment.

What is known as the Thixotropic Index (TI) of a gel coat can be calculated. First, the low shear and high shear viscosity of the product is measured. Next, the ratio between these two numbers determines the value.

Two main classes of thixotropic agents:

  • Fumed silica
  • Organoclays

Synergists, another type of thixotropic agent, boost the thixotropic network.

Promoters and Inhibits

Cure behavior, including gel time and lay-up time, are determined by the promoters and inhibitors used in gel coats. Gel time must be lengthy enough for spraying, air release and leveling. Lay-up time must be short enough to ensure production rates. However short gel times also have benefits including:

  • Reducing under cure issues i.e. alligatoring (caused by styrene solubility of the gel coat)
  • Improved gloss

Ambient conditions with peroxide initiators (catalysts) are used for curing most gel coats. The peroxide is split into free radicals by promoters, or accelerators. The free radicals then strike the unsaturation sites in polyester polymer, readying them for monomer reactions. Cobalt is the most used promoter in gel coat, but when used on its own, correct cure behavior is often not achieved. Co-promoters are needed to improve the cure behavior. They increase the ability promoters to split the peroxide catalyst into free radicals, and shorten the gel time while accelerating the cure rate. Inhibitors are used to:

  • Provide self life stability
  • Help control gel time

During storage of gel coats or after peroxide it added, free radicals are generated. These react preferentially with inhibitors. Once all inhibitors are used the cross-linking or curing process starts.

Specialty Additives

Additives are used in gel coats to reach specific properties. Some examples are:

  • Air release agents to decrease porosity
  • UV asorbers
  • Light stabilizers
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