Learn more about epoxy

There is a special vocabulary used for epoxy formulation, chemistry, and processing. To learn more about epoxy, it’s a good idea to become familiar with those terms and their definitions. We’ve put together a glossary epoxy-related terminology as used throughout this website and on our Entropy Resins technical datasheets, rack cards, and product labels. While many of these words are no doubt familiar to you already, they can sometimes take on a different nuance when used specifically in reference to epoxy resins, hardeners, and processing.


Ambient Temperature: The temperature of the air or material in contact with the epoxy. Epoxy will be affected more by the surface temp than the air temperature.

Amine Blush: A waxy byproduct of the epoxy curing process that may appear on a recently cured epoxy surface under certain conditions. Amine blush is very easily removed with plain water and a scrubbing pad. If amine blush appears, always remove it before applying any subsequent coatings as it can interfere with adhesion.

Colorants: Materials that change the color of the epoxy it is added to. Colorants can be divided into 2 sub-categories: Dyes and Pigments.

Compression Yield: The amount of stress required to cause plastic deformation. Plastic deformation is the permanent change in the shape or size of a solid body without fracture, resulting from sustained stress beyond the elastic limit. Cylinder shaped specimens are placed in a test machine that applies an increasing compressive force until plastic deformation weakens the sample. The highest force recorded prior to deformation is the Compression Yield Strength.

Cure time: The amount of time it takes for the epoxy to solidify to a state that is hard enough to sand to a fine dust and has the majority of its full properties. Cure times are given at 77°F. Increasing the temperature will reduce the cure time and conversely lowering the temperature will increase the cure time. The cure time can be divided into 3 sub-categories: open time, initial cure, and final cure.

Density: Mass divided by volume. We conduct these tests at 77°F (25°C) so that the density measurement in grams per cubic centimeter(g/cc) is also equal to the specific gravity.

Dyes: Liquid colorants. Dyes can be used to achieve nearly transparent to opaque transparencies. Since they are liquids they tend to disperse in the epoxy evenly with relatively little effort. The thicker the dye the more effort will be needed for even dispersion.

Epoxy: A thermoset plastic that is formed when mixing a resin and hardener at a given ratio. Epoxies generally have good moisture resistance and durability. Learn more about selecting epoxy resin and hardener.

Exothermic/Exotherm: A reaction that generates its own heat. Uncontrolled exotherm is when the generated heat is not dispersed, causing the epoxy to cure faster. When the epoxy cures faster it generates the heat at a higher rate therefore adding to the problem. Epoxy that undergoes uncontrolled exotherm can suffer adverse side effects. See Understanding Volume and Effect on Exothermic Reactions for more information.

Flexural Modulus: The deflection of a beam during the Flexural Strength test. In a manner similar to the calculation of Tensile Modulus, the deflection and stress are used to determine the Flexural Modulus.

Flexural Strength: The maximum amount of bending stress a sample can withstand before fracturing. The sample is simply supported at each end and an increasing load is applied in the center. The stress caused by bending is calculated and the amount that results in failure is recorded.

Fiberglass: A type of reinforcement commonly used with epoxy. Strands of glass can be woven together to create different types of fabric styles or chopped into short strands and bound together with a chemical binder. When working with epoxy we recommend working with woven glass as it has more strength and does not contain a chemical binder. Learn more about applying fiberglass.

Gel – Initial Cure: The cure phase when the epoxy begins to gel, or “kick-off.” The epoxy is no longer workable and will progress from a tacky, gel consistency to the firmness of hard rubber, which you will be able to dent with your thumbnail. The mixture will become tack free about midway through the initial cure phase. While it is still tacky (about like masking tape), a new application of epoxy will still form a primary bond with it without surface prep. However, this ability diminishes as the mixture loses its tack.

Hardener: One of the two components that reacts to form the solid epoxy. Hardeners differ from activators or catalysts because hardeners must be used at the recommended ratio. Altering the ratio of resin to hardener doesn’t change cure speed, it simply prevents the epoxy from ever curing. Learn more about selecting resin and hardener.

Hardness: A material’s resistance to deformation. This test is conducted with a Durometer utilizing the D scale. A Durometer forces a metal point into the material and provides a numerical reading which corresponds to the resistance at the point. The results of a hardness test are important for comparative purposes and determining the degree of cure.

Heat Deflection Temperature or HDT: The temperature at which the epoxy will deform under constant load. A sample is submerged in oil at a carefully calibrated temperature and subjected to 264 psi of bending stress in the center. The temperature of the oil is then gradually raised until the bar deflects .01 inches in the center. This temperature is considered to be the heat deflection temperature. HDT of Laminate is the temperature at which a typical 1/8″epoxy/fiberglass laminate will deform under constant load with the same test parameters as above. The HDT of a laminate is so much higher than a neat resin that it will not deform even at the test’s maximum temperature of 572°F (300°C).

Hot Coating: When the second layer of epoxy is thick enough to fill the weave of the fiberglass cloth and provide a smooth surface in one application. Hot coating is frequently done when building surfboards. Hot coating must be done on a horizontal surface for the thick coating of epoxy to not run off the surface. Learn more about hot coating.

Minimum Cure Temperature: This is the minimum temperature at which the epoxy will cure. If the ambient or surface temperature drops below this temperature the epoxy will pause its cure and will not begin to cure again until the temperature rises about 10°F above the minimum temperature. The additional heat is needed to activate the curing process; once the curing process is initiated again it can continue to cure until the temperature drops below the minimum cure temperature or until the epoxy reaches full cure. Note: moisture absorption and surface deformation can happen if the epoxy is below the minimum cure temperature for a long period of time. Unmixed components do not need to be stored above the minimum cure temperature but it is recommended.

Mold Surface: This is the surface from which you want the epoxy to release. A smooth, clean, and well-sealed mold surface should give you a very nice epoxy surface that needs little to no work before prepping for a topcoat. Mold surfaces are especially important when casting.  Learn more about mold surface preparation for casting projects.

Outgassing: When air trapped in a substrate, such as wood, leaves the substrate as it is warmed. This can result in bubbles appearing in an epoxy coating or casting. Learn more about outgassing in Bubble-Free Coating & Casting.

Open Time: The portion of the cure time, after mixing, that the resin/hardener mixture remains a liquid and is workable and suitable for the application.

Pigments: Solid colorants. Sometimes the solids will be suspended in a liquid carrier. Since pigments are solids they will typically give more opaque transparencies but at low loadings, they can be translucent. Pigments may take more effort to evenly disperse in epoxy and can settle out of the epoxy if the open time is long enough.

Primary/Chemical Bond: A bond that relies on the ability of the new epoxy application to react with the previous application of epoxy. This allows all the layers of adhesive to cure together and fuse into a single layer, chemically speaking. Therefore, epoxy applied over partially cured epoxy will chemically bond with it and is considered a primary bond where no surface prep is needed. Learn more about bonding types in Basic Instructions.

Pot Life: The amount of time a mixture of resin and hardener has a workable viscosity while in the mixing container. Pot life is determined using150 gram and 500-gram samples in a standardized container at 72°F (22°C),77°F (25°C) and 85°F (29°C). Both mass and ambient temperature affect the rate at which an epoxy system will cure. Pot life should be used only for comparative purposes when evaluating an epoxy system’s cure time. Working Time is the amount of time the viscosity of the epoxy remains low enough to be processed. It is determined using a Gel Timer which employs a spindle traveling through a 1/8″ thick volume of liquid epoxy. Working time is the amount of time the spindle can travel through the epoxy without leaving an indent in the curing epoxy.

Reinforcement Fibers: Materials that can be used in conjunction with epoxy to make a part strong, durable, and light. The most common types of reinforcement fibers. Learn more about reinforcement fibers in Applying Fiberglass.

Resin: One of the 2 components that gets combined to form solid epoxy. The resin component will have the epoxide backbone that will react with the hardener to polymerize and make the solid epoxy thermoset plastic. Learn more about selecting resin and hardener.

Secondary/Mechanical Bond: Secondary bonding relies on mechanical, rather than chemical, bonding of an adhesive to a material or cured epoxy surface. The adhesive must “key” into pores or scratches on the surface. Learn more about bonding types in Basic Instructions.

Solid – Final Cure: The epoxy mixture has cured to a solid-state and can be dry sanded. You will no longer be able to dent it with your thumbnail. At this point, the epoxy has reached most of its ultimate strength and clamps can be removed.

Solvent: A substance that can dissolve another substance.

Stringer (surfboard): A thin strip of wood that runs from nose to tail down the center of a foam blank, adding rigidity to the surfboard.

Tack-Free Time: The time it takes until an epoxy surface is no longer sticky and cannot pick up dirt and debris. Learn more about tack-free time in Hot Coating Surfboards.

Technical Datasheet/TDS: A document describing the properties of a raw material. In the case of epoxies, TDS may also include cured properties. Entropy Resins TDS can be found on product pages on the PDFs tab.

Tensile Elongation: Also referred to as strain, Tensile Elongation indicates how much the material can “stretch” before it fails. Dog bone shaped samples are placed in a test machine that applies an increasing tensile force until failure. The change in sample length is measured with an extensometer. The point at which the sample fails is the Tensile Elongation.

Tensile Modulus: Describes the amount of elongation (strain) that results from a specific amount of stress. This property is essentially the stiffness of the material. During the Tensile Strength test, elongation is measured and recorded at the corresponding stress before the material yields. The stress divided by the strain, in the elastic region, equals the modulus or the slope of the stress/strain curve.

Tensile Strength: The stress that is required to fracture the epoxy and cause a failure. Dog bone-shaped specimens are placed in a test machine that applies an increasing tensile force until failure. The highest stress recorded prior to failure is the Tensile Strength.

Tg DSC Ulitmate: The highest Tg value that can be attained for a particular epoxy system. In order to achieve this temperature resistance in an application, the epoxy must be post-cured at a pre-defined elevated temperature for a specific amount of time. See the Technical Data Sheet for a specific resin/hardener combination, or contact our Technical Department, 888-377-6738.

Vacuum Bagging: A clamping technique that harnesses atmospheric pressure to hold epoxy coated components together in a lamination until the epoxy cures. Learn more about vacuum bagging.

Viscosity: A fluid’s resistance to a shear force and can be thought of as how easily a fluid flows. A Rotational Viscometer is used to measure viscosity. A spindle rotates in the epoxy to measure its resistance. A thicker fluid will give the spindle more resistance, indicating a higher viscosity. Since temperature will affect the viscosity, we provide data points at different temperatures as well as graphs that provide viscosity data over a wide range of temperatures. The manufacturing process and processing temperature are important considerations when determining the required mixed epoxy viscosity. Infusion processes often require a very low viscosity to enable good flow whereas a wet layup may require a higher viscosity that allows thorough fabric wet out yet prevents drain out.