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Glossary of Terms


An early stage in the reaction of thermosetting resins in which the material is fusible and still soluble in certain liquids.

Also known commonly as glue. A substance in a liquid or semi-liquid state that adheres or bonds materials together through surface attachment. Adhesives usually require a controlled temperature for curing.

The change in properties of a material over time when tested under defined environmental conditions. Aging can lead to improvement or deterioration of the material's properties.

A curing agent for epoxy resins. Amines are derivatives of ammonia (NH3)

Useful accessories used for different working steps for a variety of product applications.  Examples include; squeeze bottles, wax sheets, needles, syringes, adhesives, meter mix equipment, release agents, sealers, fillers and glass fabrics


An intermediate stage in the reaction of thermosetting resins in which the material softens when heated and swells when in contact with certain liquids, but may not entirely fuse or dissolve. The resin in an uncured thermosetting system is usually in this stage.

The adhesion of materials by adhesives.

Bond Strength
The unit load applied in tension, compression, flexure, peel, impact, cleavage or shear, required to break an adhesive assembly with failure occurring in or near the plane of the bond.

Internal void or a trapped globule of air or other gas.


The final stage in the reaction of thermosetting resins in which the material is relatively insoluble and infusible. Certain thermosetting resins in a fully cured state are in this stage.

A substance that markedly speeds up the cure of an adhesive when added in a minor quantity. May be solid or liquid in form.

The scale of temperature which features 0° and 100° as the freezing and boiling point of water respectively. To convert Centigrade to Fahrenheit multiply by 1.8 and add 32, e.g. (100° x 1.8) +32 = 212°F.

1/100th of a poise; (dyne-sec/cm2); a unit of measurement for viscosity.

Degree of clearness in a cured epoxy system or other material.

Coefficient of Linear Thermal Expansion
A measure of the change in length of an epoxy system over a given temperature range after it has been cured. It is expressed in in/in/°C (ASTM D696-44) or x10-6/K-1.

Coefficient of Thermal Expansion (CTE)
Change in a unit of length or volume that accompanies a unit change in temperature. It is measured in parts per million per °C or °K (x10-6/K)

The state in which the particles of a single substance are held together by primary or secondary valence forces observed in the tendency of the substance to stick to itself. As used in the adhesive field, the state in which the particles of the adhesive (or the adhered) are held together.

Compressive Modulus
The ratio within the elastic limit of compressive strength to compressive strain (deformation per original length). It is expressed in lbs/in2 (ASTM D695-54) or MPa (ISO 604).

Compressive Strength
Crushing load at the failure of a specimen divided by the original sectional area of the specimen. It is expressed in lbs/in2 or MPa.

Compressive Strength, Ultimate
The maximum load in pounds that can be carried per square inch of cross section of the material under test. It is expressed in lbs/in2 or MPa.

Having the quality of conducting or transmitting heat, electricity, or static electricity.

Tying together large molecules and hence changing the physical properties of a material. Cross-linking usually involves formation of three-dimensional molecular network and is customarily associated with thermosetting resins.

To change the physical properties of an epoxy by chemical reaction through polymerization, usually requiring either heat or a catalyst, sometimes in combination.

Cure Cycle
The schedule of time periods at specified temperatures to which a reacting thermosetting plastic or rubber composition is subjected in order to reach certain specified properties.

Curing Agent
A catalytic or reactive agent which when added to resin causes polymerization.


The removal of entrained air from a material by vacuum.

Separation of a tool and a model after the material has cured. If Demoulding is difficult, this can be due to small sections or in the worst case, total model/tool bonding. To ensure easy demoulding ensure all surfaces are completely sealed and released. On difficult-to-draw models, consider incorporating demoulding aids such as jackscrews and air fittings directly into the tool.

The ratio of a substance's mass to its volume at a given temperature and pressure. Example water at 25°C, 1 atmosphere of pressure = 1.0gr/cm3 = 8.35 lb. per gallon. Also expressed as g/cm3.

A reactive or non-reactive additive whose primary function is to lower the viscosity of the material to which it was added.

Any change from an initial colour possessed by a material, either due to environmental or internal conditions.

Showing a symmetrical distortion of a flat or curved section of an object so that as normally viewed. Term often used when clear "epoxy domes" are applied to labels, pins, magnets, key tags.


The property of materials whereby they tend to recover their original size and shape after deformation. Note - if the strain is proportional to the applied stress, the material is said to exhibit ideal elasticity.

A material which at room temperature can be stretched repeatedly to at least twice its original length and, upon immediate release of the stress, will return with force to its approximate original length.

Ability of a material to stretch or become longer. Unit of measure is %.

Used interchangeably with POTTING. Refers to the enclosure of an item in a substance. Sometimes used specifically in reference to the enclosure of capacitors or modules.

The liberation of heat energy during a chemical reaction. Exotherm is increased in large masses.


Failure, Adhesive
The failure at the bond line between substrates and an adhesive; the adhesive separating entirely from the substrate.

Failure, Substrate
The failure of the substrate material itself, upon subjecting bonded adhered surfaces to a stress.

A substance, often inert, added to a system to improve properties and/or decrease cost.

Small globular mass which has not blended completely into the surrounding material and is particularly evident in a transparent or translucent material.

Flame Retardance
The ability of an epoxy system to resist combustion or burning. Some materials tend to extinguish themselves when subjected to a flame. Such materials are classified as self-extinguishing. ASTM D790-63.

Flash Point
The temperature at which the material gives off flammable vapour in sufficient quantity to ignite momentarily on the application of a flame under specified conditions.

Flexural Modulus
The ratio, within the elastic limit, of flexural stress to the corresponding strain. It is expressed in lbs/in2 (ASTM D790-63) or MPa.

Flexural Strength
Ability of a material to withstand failure due to bending. Unit of measure is lbs/in2 or MPa. Flexural strength at breakage is expressed as %.

Movement of a material during the curing stages before completely cured.


The initial jelly-like solid phase that develops during the transition from a liquid to a solid. Note - in this state the epoxy is soft, flexible and has no strength.

Glass Transition Temperature (Tg)
Approximate midpoint of the temperature range over which a material undergoes a phase change from brittle to rubbery or vice versa. Unit of measure is °C.

The property of a cured material which causes it to reflect light.


A substance or mixture of substances added to an epoxy resin to promote or control the curing reaction.

Heat Distortion (Deflection Temperature)
The temperature at which a material softens enough to distort under a given load. It is not usually considered to be beyond the maximum usable temperature, but is an indication of the maximum usable temperature when the material is load bearing at a given load. As the load decreases, the maximum usable temperature will increase. Measured in °C.

Humidity (Relative)
The ratio of the amount of moisture contained in the atmosphere to the amount of moisture that can be carried in the atmosphere at a given temperature. Relative humidity is expressed in percent, e.g. 75% R.H. at a given temperature means that the air is 75% saturated with moisture.

The property of a material to absorb and hold moisture.


A manufacturing process whereby a resin system is drawn into a fabric under vacuum.

Intermittent Operating or Service Temperature
The temperature to which the epoxy system can be subjected to for short periods of time without the degradation of its properties.

Izod Impact Strength
A measure of the brittleness of a material. Brittle materials will have low Izod impact values (0.15 for example). Tough materials will have high Izod impact strengths (0.60 for example). Units of measure are usually ft-lb/in. This different to Impact Strength (Charpy) which is expressed as kJ/m2 (ISO 179).


The result of uniting together several layers typically made up from a fabric and a resin system.


Moisture Resistance
Having some resistance to high humidity. A moisture resistant adhesive will not be easily affected by moisture. Will not easily change its chemical and physical properties due to moisture. Should not be confused with "water-proof".


Any material is opaque if no light can be transmitted through it.

Operating or Service Temperature
The temperature at which an epoxy system can operate continuously without degradation of its properties.

Uneven surface somewhat resembling an orange peel.


The fine solid particle, usually inorganic, used in the preparation of coloured products, and substantially insoluble. In contrast, a dye is soluble.

Small regular or irregular crater in a surface, usually with its width approximately of the same order of magnitude as its depth

Pot Life
The length of working time of a two component reactive system from the time of the addition of the curing agent in a specific mass (i.e. 100 grams), remains workable and suitable for use. It is expressed in minutes or hours.


Release Agents
Release agents promote easier demoulding. They are available in different forms such as waxes, aerosol products or liquids that can be applied to surfaces by brushing or spraying. 


To convert an epoxy into a fixed or hardened state by chemical or physical action through polymerization.

Shear Strength
The shear force required to break a specimen divided by its cross-sectional area; the force being applied parallel to the cross-sectional area. Units of measure are lbs/in2 or MPa.

Shop Temperature
Temperature of the working environment e.g. a laboratory or working area. Shop temperature should be held between 21-25°C. Cold inhibits curing, whereas higher temperatures may cause an undesirable exothermic reaction.

Shore A Hardness
A materials hardness measured on a durometer, the scale of which is 0-100, used on elastomers and other flexible materials. Consists of a pin point depression into the material, the material being at least 100 mils thick. A Shore A reading of 80 equals a Shore D reading of 30.

Shore D Hardness
The materials hardness measured on a durometer similar to the Shore A durometer, the scale of which is 0-100, used on rigid and semi-rigid materials. Consists of a pin point depression into the material.(DIN 53505).

The decrease in volume, or contraction, of a material by the escape of any volatile substance, or by a chemical or physical change in the material.

Softening Range
The range of temperature in which a thermoplastic changes from a rigid to a soft state. Expressed as °C or °F.

Soft Spots / Incomplete Cure
Soft spots are small bubbles or streaks detected in a tool. They may appear in the tooling surface or throughout the tool laminate or casting. They can range from a slightly softer than normal section to a liquid-like totally unreacted tool mass.

Specific Gravity
The ratio of the weight of any volume of a mass or substance to the weight of an equal volume of water at given temperature. The specific gravity of a substance times the density of water equals the density of the substance.

Storage Life
The period of time during which a packaged epoxy or curing agent can be stored under specific temperature conditions and remain stable for use. Also known as the shelf life of a product.

An applied force or pressure, as tension or shear, exerted on a body which produces a resultant strain on the material. The ability of the material to withstand a stress depends on the strength of its cohesive force or molecular structure.

A material upon the surface of which an epoxy is spread for any purpose, such as bonding, coating or mechanical testing.

Surface Tension
The property of a liquid which causes the surface to pull into the smallest area for a maximum volume, hence drops are spherical. The fact that water drops on a wax surface do not spread out due to surface tension. If a wetting agent were added to the water the round droplet would spread out into a film because of the lowered surface tension.


Temperature, Curing
The temperature to which an adhesive or an assembly is subjected to cure the adhesive. Expressed in °C or °F.

Tensile Strength
The pulling force necessary to break a given specimen divided by the cross sectional area. Units of measure are lbs/in2 or MPa. It measures the resistance of a material to stretching without rupture. Normally is not used with reference to elastic materials which recover after elongation.

Thermal Conductivity
The measure of how rapidly heat is conducted through a material. Units of measure are

Thermal Shock Resistance
The ability of a cured system to resist cracking or crazing under conditions of rapid and continuous thermal change. The 1/4 inch Olyphant Washer test is cycled over a temperature range of -55°C to +125°C, this temperature is normally used as the test condition unless otherwise stated.

Describes the property of an epoxy, to set or become rigid and non-meltable when heated with or without pressure.

False body. The property of a paste or fluid to thicken or set up to a paste or semi-gel when allowed to stand. Agitation breaks it down but further standing will permit another viscosity rise.

Time, Curing
The period of time during which an assembly or part may be subjected to heat or pressure, to cure the epoxy. It is the time between the addition of curing agent to the resin, and completed polymerization. It is expressed in minutes or hours.


The property of resistance to flow exhibited within the body of a fluid. Usually measured in centipoise.
1 cps = Water, 400 cps = #10 Motor Oil, 1,000 cps = Castor Oil

Volume Shrinkage
Amount of dimensional change during cure. Expressed as %.


Water Absorption
% by weight water absorbed by material in 24 hrs @ RT

The thorough impregnation of a material by a liquid. The more viscous a fluid, and the higher its surface tension, the more difficult it is for the liquid to "wet" materials. Certain additives, for example, water softeners, reduce surface tension, or viscosity and improve wetting properties, allowing the material to flow out more.


Yield Strength
The load in lbs/in2 or MPa where the material under test begins to change dimensions and will not completely recover when the load is removed. Yield strength will normally be lower than ultimate strength. Generally speaking the more rigid a material is the closer will be the yield and ultimate strengths. Furthermore, the more resilient a material is, the greater the spread between the yield and ultimate strengths.