Epoxy Terminology

Epoxy adhesives and encapsulants come in a wide variety of styles and formulations, with complex chemical characteristics. Browse our epoxy terminology guide below to learn more about common terms used in the industry. Questions? Explore our lab services and custom formulation capabilities, or contact an experienced ResinLab representative.



A-Stage – The early stage in the reaction of some thermosetting resins in which the material is fusible and soluble in certain liquids.

Adhesive – A substance that can hold materials together by surface attachment.

Aggregate – Hard fragmented material used with an epoxy binder as a flooring or surfacing medium.

Aging – Material changes occurring with time under defined environmental conditions. Aging can improve or deteriorate the product properties.

Amine – The curing agent for epoxy resins. Amines are derived from Ammonia (NH3)

Artificial Weathering – Lab created conditions that mimic continuous outdoor exposure, involving temperature, humidity, and UV exposure (sunlight). Direct water contact may also be utilized. Lab conditions are usually more intense than normal outdoor exposure to accelerate the effect.

ASTM – American Society for Testing Materials


B-Stage – The intermediate reaction stage of some thermosetting resins where material softens when heat is applied and expands when in contact with certain liquids. This stage may not entirely dissolve or fuse. A thermosetting resin when uncatalyzed is typically in B-stage.

Baking Temperature – Curing epoxy materials at elevated temperatures about 150 °F (65.6 °C).

Bond – The mating of two or more materials by use of an adhesive.

Bond or Adherence Strength – The load applied in tension, compression, flexure, peel, impact, shear or cleavage required to break an adhesive bond. Failure occurs near or in the plane of the bond.

Brookfield Viscometer – An instrument for measuring the viscosity of adhesives under standard temperature conditions.

Bubble – An internal void or trapped air or gas.


C-Stage – The final reaction stage of some thermosetting resins where the epoxy is mostly insoluble and infusible. Some thermosetting systems in a full cured state are in C-stage.

Calorie – The amount of heat required to raise the temperature of a gram of water by 1 °C.

Catalyst – A material that quickly speeds up the cure of an adhesive when added in a minor ratio as compared to the primary reactant amount.

Celsius – A temperature scale when water freezes at 0 °C and boils at 100 °C.

Chalking – A dry powdering film that appears on a material surface. It is a breaking down of the material after being exposed to UV light or weathering conditions.

Clarity – Clearness rating in a cured epoxy.

Closed Cell Foamed Plastic – A dominance of non-interconnecting cells in a cellular plastic.

Coefficient of Linear Thermal Expansion – The measure of length change of a cured epoxy system at a specified temperature. Given in inches (per inch/per °C) ASTM D696-44

Coefficient of Thermal Expansion (CTE) – Length or volume change that accompanies a change in temperature.

Cohesion – The state in which particles of the adhesive or the adhered are held together.

Compressive Modulus – Expressed in pounds per square inch, the ratio within the elastic limit of compressive stress to corresponding compressive strain (original length deformed).

Compressive Strength – Crush load at specimen failure divided by original sectional area of specimen.

Compressive Strength (Ultimate) – Maximum load in pounds that can be carried per square inch of cross section of a test material.

Conductive – The ability to conduct or transmit heat, electricity or static electricity.

Conductivity – The degree to which an object conducts electricity. Reciprocal of volume resistivity.

Cross-Link – Changing the physical property of a material by tying together large molecules. Cross-linking is typically associate with thermosetting resins and involves building a three-dimensional molecular structure.

Cure – The physical property change of an epoxy through polymerization with the use of heat and/or a catalyst.

Cure Cycle – Time schedule with specified temperatures to which a material is subjected to reach a specified property.

Curing Agent – A reactive or catalytic component when added to a resin causes polymerization.


Degassing / Evacuation – The removal of entrapped air from an epoxy system by use of a vacuum.

Density – Ratio of a substance’s mass to its volume at a given temperature and pressure. Density of H20 at 25 °C = 8.35 lb/gallon.

Dielectric Constant – Electrical energy applied in a circuit, the insulating material of the electric charge and is released when the current is broken. Capacitance is ratio of charge absorbed to the potential (voltage applied). Dielectric constant is ratio of an insulator’s capacitance to that of dry air. A dielectric constant of 10 means the insulator will absorb 10x more electrical energy than air.

Diluent – A reactive or non-reactive additive used to lower viscosity and extend the material that it was added to.

Discoloration – The change of a material’s initial color due to internal or environmental conditions.

Dissipation Factor – The measure of electrical energy absorbed and lost in the insulating material when energy is applied in a circuit. 20-3060 has a dissipation factor of .01. This means that .01% of the energy being stored (capacitance) is lost. Dissipation factor is the ratio of the resistive component of a capacitor to the capacitive reactance of the capacitor. Also referred to as Loss Tangent. ASTM D150-59T.

Domed – Visual appearance of a symmetrical distortion of a flat or curved section of an object versus normal appearance.


Elasticity – Material properties by which they recover to their original size and shape after deformation (if a strain is proportional to applied stress it is termed/exhibits ideal elasticity or Hookean).

Elastomer – A material at 25 °C that can be repeatedly stretched to at least double its original length and upon immediate stress release returns with force to approximately its original length.

Elongation (%) – Ability of material to stretch or elongate.

Encapsulation / Potting – Referring to the enclosure of an item in a substance such as a circuit board or module.

Exotherm – The freeing of heat energy during a chemical reaction. Exotherm increases in large masses.

Extender – An inexpensive, readily available similar ingredient that can be added to a more expensive product to increase the amount of material in useful form.


Failure (Adhesive) – The complete separation of the bond line between the substrates and adhesive.

Failure (Cohesive) – Adhesive failure under stress. Results in broken bond and substrates still covered in adhesive.

Failure (Substrate) – Failure of the adhered substrate after stress application.

Filler – Often an inert substance added to an epoxy system to improve properties and reduce cost.

Fish Eye – Small globular masses not blended fully in a mixture. Most evident in clear materials.

Flame Retardant – The ability of an epoxy to resist burning or combustion. Some materials are classified as self-extinguishing when subjected to a flame. ASTM D790-63

Flash Point – The temperature at which the epoxy gives off flammable vapor in quantity enough to ignite momentarily with the application of a flame under specified conditions.

Flexural Modulus – The ratio in the elastic limit, of flexural stress to corresponding strain (pounds per square inch). ASTM D790-63

Flow – The movement of the material during the cure stages prior to full cure.

Forced Drying Temperatures – Room temperature 25 °C (72 °C) to 65 °C (150 °F)

Fusible – Capable of melting or bonding together by melting.


Gallon (US) – Volume = 231 cubic inches at 25 °C

Gardner Color Scale – System of color standards based upon stable solutions of Ferric Chloride used in evaluating materials.

Gel – The transition from liquid to solid but the epoxy is still soft, flexible, and has no strength.

Glass Transition Temperature (Tg) – The midpoint of the temperature range where the material undergoes change from brittle to rubbery or vice versa.

Gloss – The ability of a cured material to reflect light.


Hardener – An ingredient or mixture that is added to an epoxy resin to promote or control the cure reaction.

Heat Distortion (Deflection) Temperature – The temperature at which a material softens and can distort under a given load. It is not beyond maximum usable temperature but indicates the maximum usable temperature when load bearing at a given load. Load decrease equals a usable temperature increase.

Heat Sink – Any device that absorbs heat from an object. This neutralizes the extreme temperature.

Humidity, Absolute – Moisture present in the air expressed in grams per cubic meter.

Humidity, Relative – Ratio of amount of moisture contained in the atmosphere to the amount of moisture that can be carried in the atmosphere at a given temperature. Expressed in percentage.

Hygroscopic – The ability to absorb and hold moisture.


Impregnate – To fill spaces/voids with a material in an electrical unit. This does not mean complete fill or surface coating.

Insulation Resistance – The ratio of direct voltage applied to electrodes in contact with an epoxy to the total current between them. Dependent on volume and surface resistance of the system. ASTM D257-61

Izod Impact Strength – Brittleness measurement of a material. Brittle = low values, touch = high values.


Modifier – An ingredient added to a mixture to create a property change.

Moisture Resistance – Resistance to high humidity. Material properties will not be easily changed. This does not mean waterproof.


OHM Unit – Unit of electrical resistance. Resistance of a circuit in where a potential difference of one volt = a current of one ampere.

Opaque – No light can be transmitted through.

Orange Peel – Uneven surface with pits, like the skin of a lemon or orange.


Pigment – Usually an inorganic fine solid color particle and is substantially insoluble.

Pit – Small regular or irregular craters in a surface. Width and depth of the pit are similar.

Pot Life – The time from when mixing a resin and curing agent results in doubling of the mixed initial viscosity.

Pounds per square inch (PSI) – A measure of pressure.


Resistivity – Electrical current passage resistance of a material (ohm-cm).


Service Temperature (Intermittent Operating) – Subjecting to temperatures for short periods of time without degradation of chemical properties.

Service Temperature (Maximum) – The highest maximum temperature based on thermal endurance and product data.

Service Temperature (Operating) – The continuous temperature at which an epoxy can function without degrading chemical properties.

Set – The fixed or hardened state of an epoxy system.

Shear Strength – The shear force required to break a specimen divided by the cross-sectional area. The force being applied parallel to the cross-sectional area.

Shore A Hardness – Durometer reading of a material hardness. The scale is 0-100. A pin point depression into 100 mil thick material. A reading of 80 = Shore D 30. Used for elastomers and flexible materials.

Shore D Hardness – Durometer reading for hardness of a rigid/semi-rigid materials. Same scale/measuring as Shore A.

Shrinkage – Escape of volatile substances or chemical changes that result in volume decrease or contraction of a material. The amount of dimensional change during the cure process.

Softening Range – The temperature range where material changes from a rigid to soft state.

Specific Gravity (S/G) – The weight ratio of a material volume to the weight of an equal volume of water at a given temperature. The S/G of a material times the density of water = the density of the material.

Shelf life / Storage Temperature – The usable life of a product when stored at a specific temperature where the product will remain stable/unchanged.

Stress – Applied pressure/force as tension or shear exerted on a mass which results in material strain. The material ability to withstand stress is dependent on its cohesive force strength or molecular resistance.

Stripping – The removal of substance via abrasion or chemical action.

Substrate – The surface to which an epoxy is used for bonding or coating.

Surface Resistivity – The current flow resistance along the surface of an insulating material. Ohms-cm measurement.

Surface Tension – A liquids property which causes the surface to pull into small areas creating a maximum volume, hence drops are spherical.


Temperature (Curing) – The temperature at which an epoxy cures. Product dependent, room temp, with higher temperatures.

Tensile Strength – The pull force needed to break a specimen divided by the cross-sectional area. Not typically used with elastic materials which recover after elongation. (lbs/in/2 = pounds per square inch)

Thermal Conductivity – Measure of how fast heat is conducted through a material.

Thermal Shock Resistance – The ability of a cured system to resist cracking under continuous rapid thermal changes.

Thermosetting – The property of an epoxy system with or without pressure to set or become rigid and non-melting when heated.

Thixotropy – The ability of a material to thicken to a paste or semi-gel when allowed to stand.

Time, Curing – Length of time during which a substrate may be subjected to heat or pressure to cure the epoxy system.


Viscosity – Measured in centipoise, the property of resistance to flow in a material (ex: water = 1 cps, peanut butter – 250,000 cps).

Volume Resistivity / Shrinkage % – Ratio of electrical resistance through a cross-section divided by length through which current flows. (ohms-cm).


Water Absorption % – Percentage by weight of water absorbed in 24 hours at 25 °C.

Waviness, Surface – Unevenness in a surface.

Wetting – Fully impregnating a material by a liquid. A more viscous fluid and a higher surface tension will create difficulty for the liquid to “wet” material.

Work life – The length of time that a mixed epoxy system is suitable/workable for use.


Yield Strength – Load in pounds per square inch where the material changes dimensions and does not completely recover when load is removed.