Materials

Property Definitions

Mechanical and physical property tests are performed using the same size, shape, and/or weight of substances being compared.

Arc Resistance measures electrical-breakdown conditions. Arc resistance is measured as time, in seconds that it takes for an electrical current to arc across a distance along the insulating surface. The higher the value, the greater the resistance to break down.

Compressive Strength describes how much weight a non-moving, specified size and shape of a materiel can withstand before crushed. Compressive strength is measured in thousands of pounds per square inch. Higher numbers indicate stronger materials.

Deflection Temperature Under Load (DTUL) measures the temperature at which a material deflects a given amount under a given load. It was developed for thermoplastic materials which soften considerably when heated. It has relatively little value as a design figure for reinforced thermosetting polymers.

Density is the weight of a material per unit volume. It is measured in pounds per cubic foot (lbs/cu.ft.). Higher numbers indicate heavier materials.

Dielectric Strength is an indication of the electrical strength of a material as an insulator the specimen is placed between heavy cylindrical brass electrodes that carry electrical current. For short-term tests, the voltage is increased from zero to breakdown at a uniform rate. Breakdown by these tests means passage of sudden excessive current through the specimen; it can be verified by instruments and visible damage. The dielectric strength of an insulating material is the voltage gradient at which electric failure or breakdown occurs. The dielectric strengths of materials vary greatly with several conditions such as humidity and geometry. It is not possible to apply the standard test values directly to field use unless all conditions, including specimen dimension, are the same. Because of this, the dielectric-strength test results are of relative rather than absolute value as specification guides.

Flammability is a variable property determined by the specific test method employed in its evaluation. The test method employed usually depends upon the application of the end product. Tests can be as simple as a burner applied to a horizontal bar of given dimensions (ASTM D-635) to as complicated as a foot long tunnel to test surface flame propagation (UM E-84).

Flexural Strength is also known as bending strength. It describes how of a non-moving weight can be applied before a material yields or breaks. It is measured in thousands of pounds per square inch. Higher numbers mean the material is stronger and can withstand a heavier load.

Flexural Modulus measures the resistance of materials to bending loads. It is used to determine how far a material will bend when a given weight is applied across a given space. It is measured in millions of pounds per square inch. Higher numbers mean that there is more resistance to deflection.

(IZOD) Impact Strength Testing is a measure of how much energy is absorbed by the material when it is broken by a moving weight. There are many different test methods for measuring impact. IZOD is but one of these methods. IZOD is measured in foot pounds per inch of width (This is sometimes given as foot pounds per inch of notch.). Higher numbers mean that the material will absorb more energy before it is broken by a moving weight.

Hardness describes the ability of a material to be indented. The Rockwell method for measuring hardness forces a steel point or ball into the material and then measures penetration of the point. Letters (R, E, M, etc.) in the Rockwell reading describe the shape of the point and the load applied during the test. A number is also used. High numbers for materials with the same letter indicate harder materials. Harder materials have more resistance to penetration by another substance.

Mold Shrinkage is the difference in dimensions between the cold mold and the cold part.

Relative Permittivity is the ratio of the capacitance of a particular material to the capacitance of air. The relative permittivity of most insulating materials varies from 2 to 10, air having 1. Higher values indicate greater insulating qualities.

Specific Gravity is the ratio of the density of a material to the density of water. It is a unit less number. It can be obtained by dividing a material’s density in lbs/ft3 by 62.36.

Specific Heat (Thermal Capacity) defines how much heat is needed to raise the temperature of one pound of material one degree F. IT is measured in BTUs per pound per degree Fahrenheit (BTU/lb/~). Higher numbers mean that it takes more heat energy to raise the temperature of a material.

Tensile Elongation is when a specified size and shape bar of a material is pulled, it gets longer. Elongation tells how much longer it gets before it breaks.

Tensile Strength describes how much of a non-moving load a material can withstand before it no longer returns to its original length upon removal of the load. Tensile strength is measured in thousand s of pounds per square inch. Higher numbers indicate materials which can withstand a stronger pull before failure.

Tensile Modulus measures the ability of a material to withstand load without permanent deformation. It is normally measured as the slope of the straight line portion of a plot of stress vs. strain. It is measured in millions of pounds per square inch.

Thermal Coefficient of Expansion measures how much the length of a material will change when the material is heated or cooled. The value given is based on the inch as a unit. The number given shows how much this material will increase in length if the temperature of the material is raised one degree Fahrenheit. It is measured in inches per inch per degree Fahrenheit (in/in/ºF). Higher numbers mean that the material will expand or lengthen more for each degree rise in temperature. Smaller numbers indicate relative stability to change no matter what temperature.

Thermal Conductivity known as the K factor. It measures the transfer of heat from one side of a material to the other side. It is measured as BTUs per hour per unit area (square feet) for a thickness of one inch and a temperature difference of one degree Fahrenheit between both sides of the material (BTU/hr/sq ft/ºF/in.). Higher numbers indicate that more heat is transferred through a material in the same amount of time.

Volume Resistivity is the electrical resistance between, opposite aces of a unit cube (1cm3) of a given material. This resistance is expressed in ohms. Higher values indicate greater insulating qualities.

Water Absorption is the amount of water absorbed by a given material. Usual immersion time is 24 hours at 78ºF. Water absorbed is specified as the percent weight gained by the material.