Additives & Fillers

A wide variety of additives and modifier ingredients can be used to expand the usefulness of polymers, enhance their processability or extend product durability.

Although additives are generally used in relatively low quantity by weight compared to resins, reinforcements and fillers, they perform critical functions. While additives and modifiers often increase the cost of the basic material system, these materials always improve cost/performance.

Additive Functions

There are a number of additives that are used to modify and enhance resin properties that become a part of the polymer matrix. These additives include

  • Thixotropes: In some processes such as hand lay-up or spray-up, thixotropic agents may be used. When “at rest,” resins containing thixotropic agents remain at elevated viscosities. This reduces the tendency of the liquid resin to flow or drain from vertical surfaces. When the resin is subjected to shear, the viscosity is reduced and the resin can be easily sprayed or brushed on the mold. Fumed silica and certain clays are common thixotropic agents.
  • Pigments & Colorants: Pigment dispersions and color pastes can be added to resin or gel coat for cosmetic purposes or to enhance weatherability. In the case of gel coats, finely milled pigments are blended with resin using high shear mixers. Many pigments react differently in polyester resin than in paint, and various pigments may slow down or speed up resin gel time. Additives can be mixed in as part of the resin or applied as part of the molding process (as a gel coat). A wide range of coatings can be applied after molding.
  • Fire Retardants: Most thermoset resins are combustible and create toxic smoke when burned. Combustion resistance is improved by proper choice of resin, use of fillers or flame retardant additives. Included in this category are materials containing ATH (alumina trihydrate), bromine, chlorine, borate and phosphorus.
  • Suppressants: In open mold applications, styrene emission suppressants are used to block evaporation for air quality compliance. These wax-based materials form a film on the resin surface and reduce styrene emissions during curing.
  • UV Inhibitors & Stabilizers: Both thermoset and thermoplastic composites may use special materials which are added to prevent loss of gloss, crazing, chalking, discoloration, changes in electrical characteristics, embrittlement and disintegration due to ultraviolet (UV) radiation. Additives, which protect composites by absorbing the UV, are called ultraviolet absorbers. Materials, which protect the polymer in some other manner, are known as ultraviolet stabilizers. In the event that a non-gel coated resin will be exposed to sunlight, the addition of a UV stabilizer will slow the surface degradation.
  • Conductive Additives: Most composites do not conduct electricity. It is possible to obtain a degree of electrical conductivity by the addition of metal, carbon particles or conductive fibers. Electromagnetic interference shielding can be achieved by incorporating conductive materials.
  • Release Agents: Release agents facilitate removal of parts from molds. These products can be added to the resin, applied to molds, or both. Zinc stearate is a popular mold release agent that is mixed into resin for compression molding. Waxes, silicones and other release agents may be applied directly to the surface of molds.

Initiators, Promoters & Inhibitors

In polyesters, the most important additive is catalyst or initiator. Typically, organic peroxide such as methylethylketone peroxide (MEKP) is used for room temperature cured processes, or benzoyl peroxide is added to the resin for heat-cured molding. When triggered by heat, or used in conjunction with a promoter (such as cobalt napthenate), peroxides convert to a reactive state (exhibiting free radicals), causing the unsaturated resin to react (cross-link) and become solid. Some additives such as TBC (tertiary butyl catechol) are used to slow the rate of reaction and are called inhibitors. Accelerators such as DMA (dimethyl aniline) speed curing.


Fillers not only reduce the cost of composites, but also frequently impart performance improvements that might not otherwise be achieved by the reinforcement and resin ingredients alone. Fillers are often referred to as extenders. In comparison to resins and reinforcements, fillers are the least expensive of the major ingredients. Fillers can improve mechanical properties including fire and smoke performance by reducing organic content in composite laminates. Also, filled resins shrink less than unfilled resins, thereby improving the dimensional control of molded parts. Important properties, including water resistance, weathering, surface smoothness, stiffness, dimensional stability and temperature resistance, can all be improved through the proper use of fillers.

Use of inorganic fillers in composites is increasing. When used in composite laminates, inorganic fillers can account for 40 to 65% by weight. There are a number of inorganic filler materials that can be used with composites, including:

  • Calcium carbonate is the most widely used inorganic filler. It is available at low cost in a variety of particle sizes and treatments from well-established regional suppliers, especially for composite applications. Most common grades of calcium carbonate filler are derived from limestone or marble and very common in automobile parts.
  • Kaolin (hydrous aluminum silicate) is the second most commonly used filler. It is known throughout the industry by its more common material name, clay. Mined clays are processed either by air flotation or by water washing to remove impurities and to classify the product for use in composites. A wide range of particle sizes is available.
  • Alumina trihydrate is frequently used when improved fire/smoke performance is required. When exposed to high temperature, this filler gives off water (hydration), thereby reducing the flame spread and development of smoke. Composite plumbing fixture applications such as bathtubs, shower stalls and related building products often contain alumina trihydrate for this purpose.
  • Calcium sulfate is a major flame/smoke retarding filler used by the tub/shower industry. It has fewer waters of hydration, and water is released at a lower temperature. This mineral filler offers a low cost flame/smoke retarding filler.