Epoxy Resins and Compounds Terminology


Abrasion Resistance
The resistance of a surface to rubbing or friction. A measure of durability as opposed to hardness.

The term “aliphatic” derives from the Greek word for oil- aleiphar. In scientific circles, aliphatic is an adjective meaning of, relating to, or being comprised of an organic compound having an open-chain structure.

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Coefficient of Thermal Expansion (CTE)

Thermal expansion is defined as the tendency of matter to change in volume in response to a change in temperature. Most matter expands as temperature increases, although there are rare examples of materials which contract as temperature rises. This effect only occurs within a limited temperature range and is limited in size. A material’s coefficient of thermal expansion is expressed as the degree of expansion divided by the change in temperature; this response to temperature change is expressed as its coefficient of thermal expansion:

The coefficient of thermal expansion is used:

  • in linear thermal expansion
  • in area thermal expansion
  • in volumetric thermal expansion

These characteristics are closely related. The volumetric thermal expansion coefficient can be measured for all substances of condensed matter (liquids and solid state). The linear thermal expansion can only be measured in the solid state and is common in engineering applications.

Compressive Strength

Compressive strength is the capacity of a material to withstand axially directed pushing forces. When the limit of compressive strength is reached, materials are crushed. Concrete can be made to have high compressive strength, e.g. many concrete structures have compressive strengths in excess of 50 MPa, whereas a material such as soft sandstone may have a compressive strength as low as 5 or 10 MPa.

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Dielectric Constant

A dielectric constant is an essential piece of information when designing capacitors, and in other circumstances where a material might be expected to introduce capacitance into a circuit. If a material with a high dielectric constant is placed in an electric field, the magnitude of that field will be measurably reduced within the volume of the dielectric. This fact is commonly used to increase the capacitance of a particular capacitor design. The layers beneath etched conductors in Printed Circuit Boards (PCBs) also act as dielectrics.

Dielectric Strength

In physics, the term dielectric strength has the following meanings:

  • Of an insulating material, the maximum electric field strength that it can withstand intrinsically without breaking down, i.e., without experiencing failure of its insulating properties.
  • For a given configuration of dielectric material and electrodes, the minimum electric field that produces breakdown.

The theoretical dielectric strength of a material is an intrinsic property of the bulk material and is dependent on the configuration of the material or the electrodes with which the field is applied. At breakdown, the electric field frees bound electrons. If the applied electric field is sufficiently high, free electrons may become accelerated to velocities that can liberate additional electrons during collisions with neutral atoms or molecules in a process called avalanche breakdown. Breakdown occurs quite abruptly (typically in nanoseconds)., resulting in the formation of an electrically conductive path and a disruptive discharge through the material. For solid materials, a breakdown event severely degrades, or even destroys, its insulating capability.

Dissipation Factor

Electric power is lost in all dielectric materials, usually in the form of heat. The dissipation factor is expressed as the ratio of the resistive power loss to the capacitive power, and is equal to the tangent of the loss angle.

It is also referenced as the loss tangent

tan d,

and approximate power factor.

In capacitors, the dissipation factor is the ratio of a capacitor's equivalent series resistance (R) to its capacitive reactance (Xc). DF is usually expressed as a percentage.


Durometer is one of several measurements for the hardness of a given material—typically soft materials such as polymers, elastomers, and rubbers. The term is also used to refer to the measurement device itself. The word “durometer” is derived from the Latin word “duro”, meaning hard.In the 1920s, Albert F. Shore—founder of the Shore Instrument Company—developed a device to measure the hardness of materials. His device was not the first of its kind and was not even the first to be called a “durometer.” Nevertheless, the term “durometer” today usually refers to “Shore hardness.”

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In the uniaxial tensile test commonly carried out to determine some properties of engineering materials, a small testpiece is stretched from an initial, undeformed length L0 to a current, deformed length L. Stretch ratio, also known as elongation is a measure of the deformation defined as:


\lambda = {L \over L_0}.

Undeformed material then has a stretch ratio of 1.

The percent elongation is the elongation of a material expressed as the percent of the initial length.


The term epoxy specifically refers to the chemical functionality provided by the chemical elements C2H4O bonded into a cycle ether arrangement.  More generally, epoxy refers to the reaction products of molecules containing multiple epoxy functionality with various chemical hardeners to from solid thermoset materials.


An exotherm is an uncontrollable reaction between a solvent-free resin and hardener. This reaction occurs when heat generated by the resin-hardener reaction cannot readily escape. The trapped heat accelerates the reaction, further generating more heat and accelerating the reaction until it becomes uncontrollable. This reaction generally only occurs in bulk mixes, since mixed resin is usually applied in a thin film (such as a coating, glue line, laminate layer or filler layer) from which heat can readily escape.

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Flexural Modulus

The stiffness of a structure is of principal importance in many engineering applications, so the modulus of elasticity is often one of the primary properties considered when selecting a material. A high modulus of elasticity is sought when deflections are undesirable, while a low modulus of elasticity is required when flexibility is needed.

Flexural Strength

Flexural strength is also known as modulus of rupture, bend strength, or fracture strength. Flexural strength is measured in terms of stress, and thus is expressed in pascals (Pa) in the SI system. The value represents the highest stress experienced within the material at its moment of rupture. In a bending test, the highest stress is reached on the surface of the sample. For a rectangular sample under a load in a 3 pt bend setup: \sigma = \frac{3FL}{2bd^2}

  • F is the load (force) at the fracture point
  • L is the length of the support span
  • b is width
  • d is thickness

For a rectangular sample under a load in a 4 pt bend setup: \sigma = \frac{FL}{bd^2}

  • F is the load (force) at the fracture point
  • L is the length of the support (outer) span
  • b is width
  • d is thickness
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Gel time

The point in time after mixing a two component material, or heating a one component material to its cure temperature, at which the material ceases to flow.  The material will change from a flowable liquid to a continuous mass at this point.

Glass Transition Temperature

The glass transition temperature, Tg, is the temperature at which an amorphous solid, such as glass or a polymer, becomes brittle on cooling, or soft on heating. More specifically, it defines a pseudo second order phase transition in which a supercooled melt yields, on cooling, a glassy structure and properties similar to those of crystalline materials e.g. of an isotropic solid material. Tg is usually applicable to wholly or partially amorphous solids such as common glasses and plastics (organic polymers).


The term “galvanization” originally referred to various electromechanical processes named after the 18th century Italian scientist Luigi Galvani. Today, however, the term typically refers to the process of adding a thin layer of metal to an item made from steel in order to prevent rust. The term has also come to encompass the application of a protective metallic coating to metals.

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Hardness refers to various properties of matter in the solid phase that give it high resistance to various kinds of shape change when force is applied. Hard matter is contrasted with soft matter.

One way of measuring hardness is with the use of Shore hardness gauges.  These gauges impress a point into a surface under spring load.  The gauge measures the rsistance to the point's penetration.  Harder materials are usually measured with a Shore Type "D" gauge, softer materials with a Shore Type "A" gauge, and very soft materials with a Shore Type "OO" gauge.

Heat Distortion Temperature

The heat distortion temperature (HDT or HDTUL) is the temperature at which a polymer or plastic sample deforms under a specified load. This property of a given plastic material is applied in many aspects of product design, engineering, and manufacture of products using thermoplastic components.

Hydrolytic Stability

Hydrolytic stability is the ability of additives and certain synthetic lubricants to resist chemical decomposition (hydrolysis) in the presence of water.

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Izod Impact Test

Izod impact strength is an ASTM standard method of determining impact strength. A notched sample is generally used to determine impact strength.

Impact is a very important phenomenon is governing the life of a structure. In the case of aircraft, impact can take place by the bird hitting the plane while it is cruising, during take off and landing there is impact by the debris present on the runway

An arm held at a specific height (constant potential energy) is released. The arm hits the sample and breaks it. From the energy absorbed by the sample, its impact strength is determined.

This test can also be used to determine the notch sensitivity.

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Linear Shrinkage/Shrinkage

All thermoset materials shrink as they cure.  The source of this shrinkage is three-fold.  First, thermoset materials tend to exotherm, or heat up as they cure.  The materials flow into their final shape as a liquid that is warm and then set to a solid while warm.  Upon cooling back to room temperature, the solid goes through normal contraction exprerienced by solids as they cool.  This results in a part that is smaller than the mold or part is was originally poured into.

A second source of shrinkage in thermoset materials is due to the molecular re-organization that results in the cure of the thermoset material.  When a mixture of molecules reacts chemically into a single, new molecule, the distances between atoms is reduced.  This decrease in molecular length results in shrinkage.

Third, in general, liquids are less dense than solids.  A thermoset material begins as a lower density liquid and ends as a higher density solid.  This change in density results in shrinkage.

Linear shrinkage specifically refers to the change in length of an extended volume of thermoset material on cure.

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Modulus of Elasticity

An elastic modulus, or modulus of elasticity, is the mathematical description of an object or substance's tendency to be deformed elastically (i.e., non-permanently) when a force is applied to it. The elastic modulus of an object is defined as the slope of its stress-strain curve in the elastic deformation region:

\lambda \ \stackrel{\text{def}}{=}\  \frac {\text{stress}} {\text{strain}}

where λ (lambda) is the elastic modulus; stress is the force causing the deformation divided by the area to which the force is applied; and strain is the ratio of the change caused by the stress to the original state of the object. If stress is measured in pascals, since strain is a unitless ratio, then the units of λ are pascals as well. An alternative definition is that the elastic modulus is the stress required to cause a sample of the material to double in length. This is not realistic for most materials because the value is far greater than the yield stress of the material or the point where elongation becomes nonlinear, but some may find this definition more intuitive.


A MSDS (Material Safety Data Sheet) is an informational sheet which provides workers with procedures for handling and working with Epic Resins’ epoxy resins and potting compounds in a safe manner. An Epic Resins’ epoxy resin MSDS provides information of physical data, storing and handling instructions, and emergency procedures to ensure maximum safety and understanding of all epoxy materials.

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Pot Life
The length of time that a catalyzed resin system retains a viscosity low enough to be used in processing.
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Refractive Index

The refractive index (or index of refraction) of a medium is a measure for how much the speed of light (or other waves such as sound waves) is reduced inside the medium. For example, typical glass has a refractive index of 1.5, which means that light travels at 1 / 1.5 = 0.67 times the speed in air or vacuum. Two common properties of glass and other transparent materials are directly related to their refractive index. First, light rays change direction when they cross the interface from air to the material, an effect that is used in lenses and glasses. Second, light reflects partially from surfaces that have a refractive index different from that of their surroundings.

Reaction Injection Molding

Reaction Injection Molding (RIM) is a process for forming products such as automotive panels, turbine housings, and anything requiring a high strength to weight ratio. The process is similar to injection molding, but uses thermosetting polymers which require a curing reaction within the mold. The mixture is injected into a mold under high pressure with an impinging mixer, then allowed to remain in the mold long enough to expand and cure.

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Shear Strength

Shear strength is the ability of a material to resist forces that can cause the internal structure of the material to slide against itself. Adhesives tend to have high shear strength. Shear strength is measured in a torsion test and is measured in pounds per square inch, based on the area of the sheared edge. The shear strength of a structural adhesive is the maximum shear stress in the adhesive before failing under torsional loading.

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Tensile Strength

Tensile strength σUTS, or SU measures the stress required to pull something such as rope, wire, or a structural beam to the point where it breaks.

The tensile strength of a material is the maximum amount of tensile stress that it can be subjected to before failure. The definition of failure can vary according to material type and design methodology. This is an important concept in engineering, especially in the fields of material science, mechanical engineering and structural engineering.

Thermal Conductivity

In physics, thermal conductivity, k, is the property of a material that indicates its ability to conduct heat. It is used primarily in Fourier's Law for heat conduction.

First, we define heat conduction by the formula:

H=\frac{\Delta Q}{\Delta t}=k\times A\times\frac{\Delta T}{x}

where \frac{\Delta Q}{\Delta t} is the rate of heat flow, k is the thermal conductivity, A is the total surface area of conducting surface, ΔT is temperature difference and x is the thickness of conducting surface separating the 2 temperatures.

Thus, rearranging the equation gives thermal conductivity,

k=\frac{\Delta Q}{\Delta t}\times\frac{1}{A}\times\frac{x}{\Delta T}

(Note: \frac{\Delta T}{x} is the temperature gradient)

In other words, it is defined as the quantity of heat, ΔQ, transmitted during time Δt through a thickness x, in a direction normal to a surface of area A, due to a temperature difference ΔT, under steady state conditions and when the heat transfer is dependent only on the temperature gradient.

Alternately, it can be thought of as a flux of heat (energy per unit area per unit time) divided by a temperature gradient (temperature difference per unit length)

k=\frac{\Delta Q}{A\times{} \Delta t}\times\frac{x}{\Delta T}

Typical units are SI: W/(m·K) and English units: Btu·ft/(h·ft²·°F). To convert between the two, use the relation 1 Btu·ft/(h·ft²·°F) = 1.730735 W/(m·K). [Perry's Chemical Engineers' Handbook, 7th Edition, Table 1-4]

Thixotropic Index

Thixotropy is the property of some non-newtonian pseudoplastic fluids to show a time-dependent change in viscosity; the longer the fluid undergoes shear stress, the lower its viscosity. A thixotropic fluid is a fluid which takes a finite amount of time to attain equilibrium viscosity when introduced to a step change in shear rate. However, this is not a universal definition; the term is sometimes applied to pseudoplastic fluids without a viscosity/time component. Many gels and colloids are thixotropic materials, exhibiting a stable form at rest but becoming fluid when agitated.

Thixotropic Index: obtain Brookfield viscosities using the same spindle at two different rotational speeds, usually a tenfold difference (e.g. 1 RPM and 10 RPM). This will provide a"thixotropic index" for the particular material. The higher the difference in viscosity at the two speeds, the more thixotropic the material is and easier to pump.

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Viscosity is a measure of the resistance of a fluid to being deformed by either shear stress or extensional stress. It is commonly perceived as "thickness", or resistance to flow. Viscosity describes a fluid's internal resistance to flow and may be thought of as a measure of fluid friction. Thus, water is "thin", having a lower viscosity, while vegetable oil is "thick" having a higher viscosity. All real fluids (except superfluids) have some resistance to stress, but a fluid which has no resistance to shear stress is known as an ideal fluid or inviscid fluid. The study of viscosity is known as rheology.

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