A Lexicon of Technical Epoxy Terminology

Abrasion Resistance

Abrasion resistance is the ability of a material to resist wear, erosion, or rubbing away by frictional forces. It is important in applications where materials are exposed to harsh environments or heavy wear and tear. Several tests measure the abrasion resistance of epoxy and, thus, its suitability for specific applications. One of the most common tests is the Taber Abrasion Test, which quantifies the amount of material worn away after a set number of cycles of abrasive wheels rotating on the surface of the material.

Relevant Test Standard: ASTM D4060. Standard Test Method for Abrasion Resistance of Organic Coating by the Taber Abraser.

Compressive Strength

Compressive strength refers to a material’s capacity to withstand loads or axial stresses before failure (breaking under compression). Compressive strength is a critical property in structural materials, as it determines whether a material can support heavy loads without collapsing, deforming, or other compressive failure. Compressive strength is typically expressed as Megapascals (MPa) or pounds per square inch (psi), reflecting the force per unit area that the material can withstand without compressive failure.

Relevant Test Standard: ASTM D695. Standard Test Method for Compressive Properties of Rigid Plastics.

Elongation

Elongation is the percentage increase in length caused by tension. It is usually expressed as a ratio of the changed length to the original size. Elongation evaluates a material’s ductility, or how much it can be stretched or deformed, which determines its capacity to flex before breaking.

Relevant Test Standard: ASTM D638. Standard Test Method for Tensile Properties of Plastics.

Flexural Modulus

Flexural or bending modulus quantifies a material’s resistance to bending deformation. It indicates how much a material flexes under a certain stress or bending load before deforming permanently and is used to compare the stiffness or elasticity of different materials. Understanding flexural modulus is important as, in many applications, it is critical to quantify a material’s resistance to bending. The material’s flexural modulus is typically expressed in units of pressure, which are Megapascals (MPa) or pounds per square inch (psi). An epoxy system with a higher flexural modulus is more resistant to bending. A lower flexural modulus indicates a higher tendency to bend given a specified amount of bending stress applied.

Relevant Test Standard: ASTM D790. Standard Test Methods for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials.

Flexural Strength

Flexural or bending strength measures a material’s ability to resist deformation under bending loads. This measurement quantifies the maximum stress that a material can withstand in bending before it fails or breaks. It is crucial to assess the performance of materials in applications subjected to bending or flexing. The material’s flexural strength is typically expressed in units of pressure, which are Megapascals (MPa) or pounds per square inch (psi). Compared to flexural modulus, flexural strength indicates the maximum stress that can be tolerated before failure, not simply stress that deforms but does not lead to failure.

Relevant Test Standards: ASTM D790. Standard Test Methods for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials. ASTM D6272. Standard Test Method for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials by Four-Point Bending.

Linear Shrinkage

Linear shrinkage is the reduction in dimensions of a material along one or more axes after the cooling or curing process is complete. Epoxy increases in density and decreases in volume as it cures. Linear shrinkage is expressed as a percentage of the measurement of the original dimensions and indicates the amount of shrinkage that has occurred. The measurement is important for assessing the dimensional stability or fit of the material in many precision applications. There are epoxy formulations specially designed to minimize shrinkage.

Relevant Test Standard: ASTM D2566. Standard Test Method for Linear Shrinkage of Cured Thermosetting Casting Resins During Cure.

Modulus of Elasticity

The modulus of elasticity (Young’s modulus, elastic modulus, or modulus) measures a material’s elasticity or its stiffness or resistance to deformation during tension or compression. This measurement will indicate the point (yield point) at which a material under stress passes through an elastic region and incurs permanent deformation even after the applied stress is removed. The modulus of elasticity quantifies the relationship between a material’s stress (force per area) and strain (proportional deformation). It is fundamental for analyzing and predicting the elastic behavior of a material under tension or compression. The modulus of elasticity is typically expressed as Megapascals (MPa) or in pounds per square inch (psi).

Relevant Test Standard: ASTM D638. Standard Test Method for Tensile Properties of Plastics.

Shore “D” Hardness

Shore “D” Hardness quantifies a material’s resistance to indentation or penetration by a specified probe under standardized conditions. It is often used to assess the hardness of semi-rigid materials such as plastics, rubbers, and elastomers. Shore D hardness is measured using a durometer, a device with an indenter pressed into material under a standard force. It indicates how much force it takes to put an indentation into a material. Shore D hardness is also an easy way to determine whether an epoxy has fully cured, as any hardness measure below the known hardness for a full cure means the epoxy has not fully cured. The measurement is important to assess a material’s durability and wear resistance where there are expected mechanical stresses. Another prominent scale for measuring hardness is the Shore A scale, used for soft materials with medium to high flexibility, as well as semi-rigid plastics on the high end of the scale. The Shore A scale measures Shore A hardness, a metric that is valuable in material selection, quality control, and comparative analysis of hardness values across different products.

Relevant Test Standard: ASTM D2240. Standard Test Method for Rubber Property-Durometer Hardness.

Tensile Strength

Tensile strength is the maximum stress a stretched or pulled material can withstand before breaking or fracturing (failure). The tensile strength is when the maximum force is applied before the material breaks or fails. The measurement is important to evaluate how a material will perform in applications subjected to tensile loads. Tensile strength may be assessed at the point that the material fails (ultimate tensile strength) or enters the deformation stage (yield point). The particular material and application will inform which test is most useful. The tensile strength measurement is typically expressed in force per unit area as Megapascals (MPa) or pounds per square inch (psi).

Relevant Test Standard: ASTM D638. Standard Test Method for Tensile Properties of Plastics.

Adhesion

Adhesion is the binding or molecular force of attraction between different molecules, surfaces, atoms, or substances. The adhesion properties of an epoxy adhesive, as well as other properties and physical characteristics of the material, are determined by the ultimate form of the crosslinked polymer chain. Epoxy adhesives are known for their excellent adhesive qualities, with the ability to adhere or stick to a wide range of materials/substrates, including metals, plastics, ceramics, stone, wood, and composites. The strength of adhesion is a critical factor that will directly affect performance, longevity, and safety in many epoxy applications, including sealing and coating.

Relevant Test Standards: ASTM D1002. Standard Test Method for Apparative Shear Strength of Single-Lap-Joint Adhesively Bonded Metal Specimens by Tension Loading (Metal-to-Metal). ASTM D4541. Standard Test Method for Pull-Off Strength of Coatings Using Portable Adhesion Testers.

Chemical Resistance

Chemical resistance indicates the ability of epoxy to withstand exposure to various chemicals without undergoing degradation, deterioration, or other loss of physical and mechanical properties. Chemical resistance is critical for applications where the epoxy will come under chemical attack by solvents, acids, or other corrosive chemicals or aggressive substances. Many epoxy formulations are chemically resistant and the degree of resistance depends on the specific formulation, as well as the types of chemicals to which the material will be exposed and the nature of the chemical attack.

Relevant Test Standard: ASTM D543. Standard Practices for Evaluating the Resistance of Plastics to Chemical Reagents.

Dielectric Constant

The dielectric constant, also known as relative permittivity and designated Dk, measures a material’s ability to store electrical energy (a charge) in an electric field. It does not have any unit of measurement as it is expressed as a ratio of an insulator’s capacitance to that of a vacuum. A higher dielectric constant indicates a material can store more energy, with greater capacitance and polarization of the material.

Relevant Test Standard: ASTM D150. Standard Test Methods for AC Loss Characteristics and Permittivity (Dielectric Constant) of Solid Electrical Insulation.

Dielectric Strength

Dielectric strength measures a material’s electrical insulation properties and is the maximum electric field strength a material can endure without electrical breakdown. It indicates the epoxy’s insulation capability against electrical current. In dielectric breakdown, the material’s resistance decreases rapidly and becomes electrically conductive (short circuits). The measure of dielectric strength is expressed as Volts per unit thickness (V/mil).

Relevant Test Standard: ASTM D149. Standard Test Method for Dielectric Breakdown Voltage and Dielectric Strength of Solid Electrical Insulating Materials at Commercial Power Frequencies.

Dissipation Factor

The dissipation factor is a measure of the power dissipated (electric energy loss) by a dielectric material, and is expressed as the ratio of heat energy lost to stored energy (capacitance) in a dielectric substance. The measurement quantifies the amount of energy dissipated as heat within the material and reflects the efficiency of the dielectric material in storing and releasing energy. The higher the dissipation factor the more electrical energy is absorbed, and the lower the dissipation factor the less energy loss and the more efficient insulating qualities. The dissipation factor provides valuable information to assess energy transfer efficiency, heat generation, and power loss, in determining the suitability of material for electrical and insulating applications.

Relevant Test Standard: ASTM D150. Standard Test Methods for AC Loss Characteristics and Permittivity (Dielectric Constant) of Solid Electrical Insulation.

Electrical Conductivity

Electrical conductivity measures the ability of epoxy to conduct electrical current and form an electrical interconnection between two bonded substrates. Epoxy alone is not electrically conductive. To create electrical conductivity, different types of fillers and polymeric additives are added to the epoxy to achieve the desired conductivity. A conductive epoxy resin system consists of epoxy resin mixed with a conductive filler. The epoxy resin provides the adhesive and mechanical strength properties to the design, and the conductive filler contributes electrical conductivity. There are a variety of conductive fillers of different sizes, shapes, and types, including metallic particles (such as silver, copper, or nickel) or non-metallic particles (such as carbon or graphite). Electrical conductivity, as well as specific properties and performance of the conductive epoxy system, varies depending on the type and concentration of the conductive filler used in the system, the filler particle size and shape, and other factors. A higher proportion of conductive filler results in higher conductivity but may weaken the bond strength by displacing the adhesive.

Relevant Test Standard: ASTM D257. Standard Test Methods for DC Resistance or Conductance of Insulating Materials.

Heat Deflection Temperature (HDT)

Heat deflection temperature (or deflection temperature or heat distortion temperature) is the temperature at which a polymer starts to soften (deform) under a fixed load. It is an indicator of the stiffness of a material as temperatures increase. This measurement is critical in determining an epoxy’s suitability for applications under mechanical load or stress and exposure to heat. Heat deflection temperature is an important measure of the maximum temperature for structural applications and can be used to compare different materials. The heat deflection temperature will vary depending on the formulation of the epoxy system and the heat deflection temperature must be above the maximum application temperature for optimal performance of the epoxy system.

Relevant Test Standard: ASTM D648. Standard Test Method for Deflection Temperature of Plastics Under Flexural Load in the Edgewise Position.

Hydrophobicity

Hydrophobicity describes the tendency of a material to repel or resist and not absorb water. A hydrophobic epoxy formulation is important for applications where water resistance, moisture protection, and protection against environmental factors are required to prevent degradation, loss of performance qualities, corrosion, or application failure. An epoxy may have a low contact angle and the ability to wet the surface of a material or substrate while still being hydrophobic. The surface tension of the epoxy and the surface energy of the material or substrate are key factors in wetting.

While there is no specific ASTM standard for testing hydrophobicity of epoxy, the following standard may be informative: ASTM D7334. Standard Practice for Surface Wettability of Coatings, Substrates and Pigments by Advancing Contact Angle Measurement.

Surface Resistivity

Surface resistivity is the resistance to electrical current flow along the surface of a material under specified conditions, typically measured in ohms. The higher the value of resistivity, the better the material’s insulation properties. A low value of resistivity indicates a material that is a good conductor of electrical current. Epoxy coatings with low surface resistivity are commonly used in electronic components to ensure proper functioning and longevity. The measure of surface resistivity is important to assess the ability of a material to conduct electrical current and plays an important role in comparative analysis to select materials for electronic components or to be used in electronic systems.

Relevant Test Standard: ASTM D257. Standard Test Methods for DC Resistance or Conductance of Insulating Materials.

Thermal Conductivity

Thermal conductivity measures a material’s ability to conduct heat, expressed in watts per meter-kelvin. Due to high heat generation from many electronic devices, heat dissipation is a critical factor. Electrically insulating materials with high thermal conductivity are desirable to improve heat dissipation. Epoxy adhesives with high thermal conductivity are utilized in heat sink or potting applications to improve thermal management efficiency. Effective thermal management prevents overheating and improves the reliability, longevity and safety of electrical components and helps to maintain structural integrity of materials by managing thermal stresses.

Relevant Test Standard: ASTM E1952. Standard Test Method for Thermal Conductivity and Thermal Diffusivity by Modulated Temperature Differential Scanning Calorimetry.

UV Resistance

UV resistance is a material’s ability to withstand exposure to ultraviolet (UV) radiation from sunlight or artificial sources without degradation. Most epoxy is subject to degradation and deterioration from various sources of light, including ultraviolet (UV) light emitted by the sun or artificial light sources. UV light is harmful to polymers, in a process known as photodegradation that affects the physical, mechanical and chemical properties of polymers. Since many epoxy applications are in outdoor environments, UV degradation may lead to yellowing, cracking, and reduction of mechanical properties. To avoid or reduce photodegradation, a light stabilizer additive or UV absorber or inhibitor that resists the effects of ultraviolet rays may be added to an epoxy formulation to reduce the risk of chalking, cracking, surface yellowing, delamination, or degradation, or reduction of mechanical properties.

Relevant Test Standards: ASTM G154. Standard Practice for Operating Fluorescent Ultraviolet (UV) Lamp Apparatus for Exposure of Nonmetallic Materials. ASTM G155. Standard Practice for Operating Xenon Arc Light Apparatus for Exposure of Nonmetallic Materials.

Volume Resistivity

Volume resistivity is a material’s capacity to resist electrical current flow through its volume (the thickness of a solid specimen) under specified conditions, rather than just along its surface. It is typically measured in ohms-centimeters. High volume resistivity indicates a material with good insulation properties. This property is critical in applications for electronic components or electrical systems, as high volume resistivity will indicate that electrical currents will be prevented from passing through the material (avoiding leaks, electrical interference or short circuits).

Relevant Test Standard: ASTM D257. Standard Test Methods for DC Resistance or Conductance of Insulating Materials.

Fillers

Fillers are inert materials added as a common ingredient to epoxy formulations to modify their properties or characteristics, including viscosity, mechanical strength, thermal management or stability, electrical conductivity, corrosion and chemical resistance, flame retardancy, and dimensional stability. Fillers can provide superior performance to an epoxy system and can affect or control nearly every important property of an epoxy system. Fillers that are inexpensive can reduce the overall cost of the epoxy system. Common fillers include silica, calcium carbonate, microspheres, and fibers.

Hardener

The hardener, also known as a curing agent, is the catalyst that initiates and drives the chemical reaction that causes the epoxy resin to cure and harden. It reacts with the resin to form crosslinks between polymer chains, to create a three-dimensional polymer network of great mechanical strength, during a process called curing or polymerization. It is often a clear liquid, and the resin and hardener are mixed in a range of ratios specific to the application or product. The two mixed components create the adhesive. The hardener can be customized for specific and a wide range of applications and is therefore a critical component in achieving specific properties and performance requirements.

Pigment

Pigments are colorants in the form of powders of finely ground solid particles, pastes, or liquid added to resins to impart color, opacity, UV resistance, or other desired visual, aesthetic, or other properties to the cured product. Otherwise, epoxy resins are usually colorless or clear in their uncured state.

While there is no single standard with test methods solely for pigments in epoxy systems, there are various ASTM standards related to testing of pigments and colored coatings. These include ASTM D3022, ASTM D2244, ASTM D523, ASTM D4587, and ASTM D2805.

Resin

The resin is the main ingredient in the resin component of the epoxy system. It is a viscous liquid that is often clear or amber in color. The resin is responsible for providing the adhesive with its bonding properties and resistance to chemicals, water, heat, and other environmental challenges. The resin serves as a thermosetting polymer that initiates polymerization (also called curing) when mixed with a hardener (sometimes referred to as a catalyst). During polymerization, epoxy resins form crosslinks, creating a three-dimensional polymer of great mechanical strength.

Relevant Test Standards: ASTM D1652. Standard Test Method for Epoxy Content of Epoxy Resins and ASTM D1763. Standard Specification for Epoxy Resins. While there is no single ASTM standard that relates to all resins, there are various ASTM standards addressing properties of specific types of resins.

Substrate

Substrate is the underlying material or surface onto which epoxy is applied or bonded. Epoxy adhesives are known for their excellent adhesive qualities, with the ability to adhere or stick to a wide range of substrates, including metals, plastics, ceramics, stone, wood, and composites. The particular substrate in an application and its preparation will impact adhesion, durability, and performance of the epoxy system.

Proper preparation of the substrate, an understanding of the substrate’s characteristics, and an assessment of compatibility of the substrate and epoxy, are crucial for achieving strong adhesion and optimal performance of the epoxy system.

Relevant Test Standards: ASTM D3359. Standard Test Methods for Rating Adhesion by Tape Test, ASTM D2093. Standard Practice for Preparation of Surfaces of Plastics Prior to Adhesive Bonding, ASTM D2651. Standard Guide for Preparation of Metal Surfaces for Adhesive Bonding, ASTM D2201. Standard Practice for Preparation of Zinc-Coated and Zinc-Alloy-Coated Steel Panels for Testing Paint and Related Coating Products, ASTM D4541. Standard Test Method for Pull-Off Strength of Coatings Using Portable Adhesion Testers, ASTM D257. Standard Test Methods for DC Resistance or Conductance of Insulating Materials, and ASTM D4060. Standard Test Method for Abrasion Resistance of Organic Coatings by the Taber Abraser.

Thixotropy

Thixotropy is a property of some epoxy formulations where they become less viscous and more fluid when subjected to shear force, such as in the agitation of mixing, shaking, stirring, or other stresses. When the shear force is stopped the viscosity increases. This means that when the epoxy is stirred or disturbed, it becomes thinner and easier to work with, but when the agitation is stopped it will thicken and stay in place. The value of thixotropy is indicative of the epoxy’s ability to keep its shape. Epoxy systems may be specially formulated to be thixotropic to resist flow, sagging, or dripping, and for effective gap filling. Thixotropic epoxies offer better application control, smooth finishes, and enhanced adhesion in a variety of applications.

Relevant Test Standard: ASTM D2196. Standard Test Methods for Rheological Properties of Non-Newtonian Materials by Rotational (Brookfield) Viscometer.

ASTM

ASTM International (formerly known as American Society for Testing and Materials) is one of the world’s largest international standards organizations. It develops and publishes voluntary technical international standards covering testing procedures, specifications and standard practices for a wide range of materials, products, systems and services. Their designated numbers indicate the broad area of the standard and the year they were published. Over 12,900 ASTM standards are published each year and are updated regularly. ASTM standards include standards governing tests and procedures for epoxy. Engineers, scientists, governmental organizations, and others use ASTM standards to guarantee the reliability and consistency of materials.

Deflection Temperature or Heat Deflection Temperature

Deflection temperature (or heat deflection temperature or heat distortion temperature) is the temperature at which a polymer starts to soften (deform) under a fixed load. It is an indicator of the stiffness of a material as temperatures increase. This measurement is a critical parameter in determining an epoxy’s suitability for applications under mechanical load and exposure to heat. Heat deflection temperature is an important measure of the maximum temperature for structural applications and can be used to compare different materials. The heat deflection temperature will vary depending on the formulation of the epoxy system.

Relevant Test Standard: ASTM D648. Standard Test Method for Deflection Temperature of Plastics Under Flexural Load in the Edgewise Position.

Glass Transition Temperature (Tg)

Glass transition temperature, denominated as Tg, refers to the temperature at which an epoxy transitions from a rigid, glassy state to a more flexible, rubbery state. Although this measurement is expressed as a temperature, the transition and softening of epoxy from a solid state to a rubber-like state is a continuous process over a certain temperature range. Thus, the Tg is not a precise single temperature. When the temperature of a material is close to or exceeds the Tg, its properties, including mechanical strength, electrical properties, and chemical resistance, might change significantly, resulting in degradation or failures of the device or application. Tg is measured using a thermal mechanical analyzer. Epoxy systems with higher Tg values are typically preferred for applications requiring high-temperature resistance and structural stability.

Relevant Test Standard: ASTM E1356. Standard Test Method for Assignment of the Glass Transition Temperatures by Differential Scanning Calorimetry.

Thermoset

Thermosetting resins, including epoxy resins, are thermosetting polymers. They irreversibly cure into a solid form when exposed to heat, light, or a catalyst. During the curing process, the resin undergoes crosslinking reactions until almost all the polymer chains are crosslinked to form an entire matrix in a three-dimensional network. After curing, the chemical reactions are irreversible; the resin cannot be remelted, and the shape cannot be changed. Due to the crosslinked structure, thermosetting resins have high dimensional stability, excellent mechanical strength, high temperature resistance, and good chemical resistance.

Relevant Test Standards: ASTM D3850. Standard Test Method for Rapid Thermal Degradation of Solid Electrical Insulating Materials by Thermogravimetric Method (TGA). ASTM D256. Standard Test Methods for Determining the Izod Pendulum Impact Resistance of Plastics. There are various other ASTM standards addressing the properties, performance, and applications of thermoset resins.

UL 94 V-O Rating

UL 94 V-O refers to the UL Solutions test program described in UL 94, the Standard for Tests for Flammability of Plastic Materials for Parts in Devices and Appliances, which determines the material’s tendency to either extinguish or spread the flame once the test specimen has been ignited. The V-O rating indicates that the material was tested vertically and self-extinguished within a specified time after removing the ignition source. It also shows whether the test specimen dripped flaming particles that ignited a cotton indicator below the sample. Achieving a UL 94 V-O rating for epoxy-based products ensures compliance with certain safety and regulatory requirements and standards. The UL 94 V-O rating is an important safety standard for an epoxy system. It indicates its flame retardancy, which is critical for materials used in applications with significant fire risk.