Testing, Terminology & Methods


All data is collected from unmodified, mixed and cured epoxy. Pure epoxy mixtures are used to eliminate the effect of fibers or fillers. All of the mechanical data reported is based on epoxy mixed at the target ratio and cured under the conditions specified in the top row of the table on the product technical data sheets.

The published data is based on the same test conducted several times on multiple specimens, generating average result numbers. Averages, not the highest values, are reported. These averages are rounded to an appropriate number of significant figures.

ASTM standards are followed for all testing. When comparing another manufacturer’s product to PRO-SET, be sure to note if they used the same standardized test.


Pot Life is 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.

Viscosity is 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.

Thix Index or Shear Thinning Index is a ratio determined by viscosity measurements taken at 10 and 100 RPM. The low speed reflects how epoxy will flow undisturbed. The high-speed measurement indicates how well it will flow when a shear force is applied as is often the case during processing. Mix Ratio is the target ratio of resin to hardener required to achieve published properties and may be different by weight and volume due to the differing densities of the resin and hardener.

Every resin/hardener combination has an optimal target and acceptable range. Please note that the target is often not in the middle of the acceptable range. Achieving the correct mix ratio can be simplified by using PRO-SET Dispensing Equipment.

Density is the 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.

Hardness is 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.


Compression Yield strength is the 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.

Tensile Strength is 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.

Tensile Elongation, also referred to as strain, 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.

Flexural Strength is a measurement of 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.

Flexural Modulus is calculated by measuring 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.

Lap Shear Strength measures the strength of an epoxy bonded joint when loaded in shear. The test is performed by bonding two metal coupons together with an overlap and then pulling them apart in tension in a test machine. The tensile force creates a shear force in the bond line and the resulting stress is reported as the Lap Shear strength.

Tensile Adhesion Strength is the stress required to fail a bond line with a force perpendicular to the bonded surface. An aluminum stud with a flat end is bonded to the material to be tested. A device is threaded on to the stud and applies a pulling force to the stud and against the material simultaneously. The load required to fail the bond divided by the bonded surface area and the resulting stress is reported on the technical data sheet as the Tensile Adhesion strength.


Glass Transition Temperature (Tg) is a very useful property for understanding the thermal characteristics of an epoxy resin system. The Tg is the temperature at which the epoxy changes from a glassy (solid)state to a soft, rubbery state. It can be considered the point at which a measurable reduction in physical properties occurs resulting from exposure to elevated temperatures.

Be aware that Tg values can be reported after a second heat. The second heat is the process of testing the sample after it has been exposed to an initial first heat which resulted in an elevated temperature, 392°F (200°C), post-cured sample. A second heat Tg value is not representative of your sample unless you have replicated the 392°F (200°C) cure schedule that was used for the first Tg test.


The Dynamic Mechanical Analyzer (DMA) determines the Tg using a mechanical method. The test sample is placed into a three-point bending fixture and a cyclical load is applied. The temperature of the sample is increased and the change in the deflection is measured. As the temperature is increased during the test, the response of the sample changes. The sample’s response is plotted using three different graphs based on how the bending energy is transferred into the sample: storage modulus, loss modulus, and tan delta.

Storage Modulus is the elastic response. The recovered part of the energy originally put into the sample.

Loss Modulus is the energy that is absorbed by the sample due to friction and internal motion.

Tan Delta is the ratio of loss modulus to storage modulus, the dampening character of the sample.

When the epoxy is below its Tg, the storage modulus is high and the loss modulus is low. The sample releases energy efficiently and does not absorb energy well due to its stiffness. When the sample gets closer to its Tg, the storage modulus decreases. Energy is now absorbed into the sample, driving the loss modulus higher.

Tg Onset Storage Modulus is a conservative value indicating a measured loss of stiffness.

Tg Peak Tan Delta is the highest measured Tg value. TG DSC ONSET–FIRST HEAT

While a DMA measures the thermal properties of a sample via mechanical means; a Differential Scanning Calorimeter (DSC) machine measures the heat flow in and out of a sample to determine its Tg. This test is conducted by placing a fully cured sample into a small pan in the DSC and heating it to 392°F (200°C) at a rate of 18°F (10°C) per minute. The heat flow into the sample is measured and compared to an empty reference pan. The difference in heat flow is measured and plotted. An inflection occurs in the plotted curve at the Tg; the Onset is measured at the beginning of this inflection.

TG DSC Ultimate is 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.

Heat Deflection Temperature (HDT) is 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).