The primary functions of the resin are to transfer stress between the reinforcing fibers, act as a glue to hold the fibers together, and protect the fibers from mechanical and environmental damage. Resins used in reinforced polymer composites are either thermoplastic or thermoset.

There are two major groups of resins that make up what we call polymer materials—thermosets and thermoplastics. These resins are made of polymers (large molecules made up of long chains of smaller molecules or monomers).

Thermoset resins are used to make most composites. They’re converted from a liquid to a solid through a process called polymerization, or cross-linking. When used to produce finished goods, thermosetting resins are “cured” by the use of a catalyst, heat or a combination of the two. Once cured, solid thermoset resins cannot be converted back to their original liquid form. Common thermosets are polyester, vinyl ester, epoxy, and polyurethane.


Thermosets cross link during the curing process to form an irreversible bond.

  • Polyester: Unsaturated polyester resins (UPR) are the workhorse of the composites industry and represent approximately 75% of the total resins used. A range of raw materials and processing techniques are available to achieve the desired properties in the formulated or processed polyester resin. Polyesters are versatile because of their capacity to be modified or tailored during the building of the polymer chains. They have been found to have almost unlimited usefulness in all segments of the composites industry. The principle advantage of these resins is a balance of properties (including mechanical, chemical, and electrical) dimensional stability, cost and ease of handling or processing. Polyester producers have proved willing and capable of supplying resins with the necessary properties to meet the requirements of specific end user applications. These resins can be formulated and chemically tailored to provide properties and process compatibility.
  • Epoxy: Epoxy resins have a well-established record in a wide range of composites parts, structures and concrete repair. The structure of the resin can be engineered to yield a number of different products with varying levels of performance. A major benefit of epoxy resins over unsaturated polyester resins is their lower shrinkage. Epoxy resins can also be formulated with different materials or blended with other epoxy resins to achieve specific performance features. Epoxies are used primarily for fabricating high performance composites with superior mechanical properties, resistance to corrosive liquids and environments, superior electrical properties, good performance at elevated temperatures, good adhesion to a substrate, or a combination of these benefits. Epoxy resins do not however, have particularly good UV resistance.
  • Vinyl Ester: Vinyl esters were developed to combine the advantages of epoxy resins with the better handling/faster cure, which are typical for unsaturated polyester resins. These resins are produced by reacting epoxy resin with acrylic or methacrylic acid. This provides an unsaturated site, much like that produced in polyester resins when maleic anhydride is used. The resulting material is dissolved in styrene to yield a liquid that is similar to polyester resin. Vinyl esters are also cured with the conventional organic peroxides used with polyester resins. Vinyl esters offer mechanical toughness and excellent corrosion resistance. These enhanced properties are obtained without complex processing, handling or special shop fabricating practices that are typical with epoxy resins.
  • Phenolic: Phenolics are a class of resins commonly based on phenol (carbolic acid). Phenolics are thermosetting resins that cure through a condensation reaction producing water that should be removed during processing. Pigmented applications are limited to red, brown or black. Phenolic composites have many desirable performance qualities including high temperature resistance, creep resistance, excellent thermal insulation and sound damping properties, corrosion resistance and excellent fire/smoke/smoke toxicity properties. Phenolics are applied as adhesives or matrix binders in engineered woods (plywood), brake linings, clutch plates, circuit boards, to name a few.
  • Polyurethane: Polyurethane is a family of polymers with widely ranging properties and uses, all based on the exothermic reaction of an organic polyisocyanates with a polyols (an alcohol containing more than one hydroxyl group). A few basic constituents of different molecular weights and functionalities are used to produce the whole spectrum of polyurethane materials. Polyurethanes appear in an amazing variety of forms. These materials are all around us, playing important roles in more facets of our daily life than perhaps any other single polymer. They are used as a coating, elastomer, foam, or adhesive. When used as a coating in exterior or interior finishes, polyurethanes are tough, flexible, chemical resistant, and fast curing. Polyurethanes as an elastomer have superior toughness and abrasion in such applications as solid tires, wheels, bumper components or insulation. There are many formulations of polyurethane foam to optimize the density for insulation, structural sandwich panels, and architectural components. Polyurethanes are often used to bond composite structures together. Benefits of polyurethane adhesive bonds are that they have good impact resistance, the resin cures rapidly and the resin bonds well to a variety of different surfaces such as concrete.

Thermoplastic resins, on the other hand, are not cross-linked and, so, can be melted, formed, re-melted and re-formed. Thermoplastic resins are characterized by materials such as ABS, polyethylene, polystyrene, and polycarbonate.


Thermoplastics form extremely strong bonds within chain molecules.

These resins are recognized by their capability to be shaped or molded while in a heated semi-fluid state and become rigid when cooled. We are surrounded by everyday household items made of thermoplastics.

Thermoset Resins In Depth


To avoid any confusion in terms, readers should be aware that there is a family of thermoplastic polyesters that are best known for their use as fibers for textiles and clothing. Thermoset polyesters are produced by the condensation polymerization of dicarboxylic acids and difunctional alcohols (glycols). In addition, unsaturated polyesters contain an unsaturated material, such as maleic anhydride or fumaric acid, as part of the dicarboxylic acid component. The finished polymer is dissolved in a reactive monomer such as styrene to give a low viscosity liquid. When this resin is cured, the monomer reacts with the unsaturated sites on the polymer converting it to a solid thermoset structure.

Unsaturated polyesters are divided into classes depending upon the structures of their basic building blocks. Some common examples would be orthophthalic (“ortho”), Isophthalic (“iso”), dicyclopentadiene (“DCPD”) and bisphenol A fumarate resins. In addition, polyester resins are classified according to end use application as either general purpose (GP) or specialty polyesters such as fire retardant (FR).


Cure rates can be controlled to match process requirements through the proper selection of hardeners and/or catalyst systems. Generally, epoxies are cured by addition of an anhydride or an amine hardener as a 2-part system. Different hardeners, as well as quantity of a hardener produce a different cure profile and give different properties to the finished composites. Since the viscosity of epoxy is much higher than most polyester resin, it requires a post-cure (elevated heat) to obtain ultimate mechanical properties making epoxies more difficult to use. However, epoxies emit little odor as compared to polyesters.

Epoxy resins are used with a number of fibrous reinforcing materials, including glass, carbon and aramid. This latter group is small in volume, comparatively high cost and is usually used to meet high strength and/or high stiffness requirements. Epoxies are compatible with most composites manufacturing processes, particularly vacuum-bag molding, autoclave molding, pressure-bag molding, compression molding, filament winding and hand lay-up.

Curing Polyester and Vinyl Ester Resins

Resins must cure in a way that is compatible with the fabrication process. Some parts are small and can be laid-up quickly. The faster a resin cures, the quicker the turnaround is on the molds and the greater the production rates. Other parts may involve large lay-ups where more time is required for the lamination process. In compression molding, pultrusion and sometimes RTM, heated molds provide rapid curing.

The physical properties of a finished part are greatly affected by its cure. The hardness of the laminate is affected by the curing process as well as the chemical resistance of the laminate surface. Flexural, compressive, and tensile properties are partially determined by the efficiency of the cure. The cure must be complete to develop the full potential of a resin. Thick laminates require special attention. Resin exotherm must be controlled in order to prevent excessive shrinkage, laminate warping, and other problems related to high exotherms during cure.