Urethane Design Glossary

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GLOSSARY OF TERMS

ASTM TEST METHODS : You will note that we refer to ASTM tests and test results frequently. We use ASTM tests to classify properties, but the limitations on the usefulness of the standard tests are well stated in ASTM D 394 and D 1630: "No relation between this test and service performance is given or implied..." As stated previously and repeated for emphasis: NEVER by-pass the all-important prototype and field test steps. However, once performance has been proven, relevant ASTM test data serves well in specification writing.

COMPRESSION : Stress on a material tending to cause deflection, since, technically, urethane elastomers are considered incompressible. Because no volume change occurs, we prefer to use the term "deflection." Cellular products such as urethane foam do compress because of the collapse of the air-filled cells.

ELASTOMER : The word is generally applied to the man-made rubbers. It's simply the contraction of the words "elastic polymer." Each of the elastomer classes is characterized by it's own set of characteristics which make it useful.

HARDNESS : The end result of a number of properties such as modulus, state of cure, and cross-link density. It should never be used, as it often is, as a sole specification. "Hardness" as applied to elastomers, is defined as the relative resistance to indentation (not puncture) of a probe of specified dimensions under specified spring load, read on a scale of 0-100. The instrument used is called a durometer. Shore and Rex are names of manufacturers of ASTM recognized durometers. There are two generally used scales: "A" and "D". They overlap as shown in the diagram on page 8. By convention, we allow tolerance in reading of +/- 5 points on the durometer "A" up to 90, and +/- 3 points from 90 to 96 durometer A. The "A" scale is not reliable above 96A, so we use the durometer "D" scale for harder materials and a tolerance of +/- 5 points. See ASTM D-314, ASTM D-2240.

HEAT BUILD-UP : This term, peculiar to the rubber industry, means the temperature rise within an elastomer body due to hysteresis and the low thermal conductivity of elastomers. Since the physical properties of urethanes are reduced as the temperature rises above 160°F, heat build-up is to be avoided. The amount of heat liberated per deformation cycle is proportional to the amplitude of the strain, the frequency of application and the duration of the condition. Designers can avoid heat build-up by constructive use of shape factor.

HYSTERESIS : Refers to the percent energy lost per cycle of deformation or 100% minus the resilience percent. Hysteresis is the result of inter-molecular friction and is manifested by conversion of mechanical energy to heat. See viscoelastic effect.

MODULUS OF ELASTICITY : In elastomers, as in steel, this term refers to the ratio of stress to the strain, produced by that stress. Within the region of low strain (up to 15%), an elastomer's stress-strain curve is almost linear and design calculations which assume stress proportional to strain may be made with tolerable error. Strains greater than 15% are far from proportional to stress. Modulus of elasticity in this engineering sense should not be confused with "modulus" which is rubber industry jargon for tensile stress and is applied when strains are much greater than 15%. Elastomers in general have two moduli of elasticity; static and dynamic, in as much as they have the peculiar property of behaving stiffer when vibrated or impacted. The term "modulus" when applied to steel is defined as the slope of the straight line portion of the stress-strain curve. In the case of elastomers, modulus is defined as the stress required to produce a given strain of say 300%, would be called the 300% modulus, and is not useful in calculations.

NATURAL FREQUENCY : The characteristic frequency of vibration for a particular spring-mass system after a force or displacement is applied and removed.

POLYURETHANE : An alternative term for urethane.

RESILIENCE : The resilience of elastomers subjected to and relieved of stress has been defined by the ASTM as the ratio of energy given up on recovery from deformation to the energy required to produce the deformation, expressed as a percent.

RUBBER : The term embraces a large group of materials which have the ability, under certain conditions, to undergo large deformations and recover almost completely and instantaneously on release of the deforming force. This elasticity is due to the repetition of long molecular chains and cross links of the base polymer. The "first" rubber came from the tree "HEVEA BRASILIENSIS" and was called Indian or natural rubber. Its use can be traced to the Mayan Indian culture. Since the 1930's, at least 16 different man-made rubbers with different, improved and controlled molecular structures have been developed. Familiar types are neoprene, nitrile, butyl, silicone and urethane.

SHAPE FACTOR : The ratio of the area of an elastomer body subjected to a compressive load to the sum of the areas which are free to bulge. As the shape factor increases, the strain produced by given stress decreases. This is a critical consideration in avoiding heat build-up in dynamic applications. It is also important in static load bearing applications such as structural bearing pads where compressive stress relaxation versus time is to be avoided.

SHEAR MODULUS OF ELASTICITY : Closely approximates the torsional modulus of elasticity in elastomers.

TANGENT DELTA (TAN DELTA): A ratio of the loss modulus (viscous component of the elastomer) to the elastic modulus (storage component of the elastomer). A low tan delta means higher resilience and less hysteresis.

TENSILE PROPERTIES : These properties in steel are basic. They affect almost every design calculation for steel products and have a direct bearing on the product's serviceability. Tensile properties of elastomers, on the other hand, have little, if any bearing on serviceability and almost never affect a design calculation. They do have some influence in high impact studies, however.

TENSION : Stress on a material tending to cause elongation.

URETHANE : The name given to a class of NCO (isocyanate) terminated resins with cross linking or chain extension intermediates called curing agents. Urethane is often used as an alternative term for Polyurethane. There are eight major groups of urethanes:

MDI-Esters : produce FDA dry and wet food grade urethanes in the normal hardness range from 85 Durometer A to 45 Durometer D. They are tough, abrasion resistant and tear resistant.
TDI-Esters : produce urethanes from 50 Durometer A to 75 Durometer D which are tough, abrasion resistant, and with excellent oil and aliphatic solvent resistance.
MDI-Ethers : produce urethanes with higher resilience, better impingement type abrasion resistance, good dynamic performance, improved hydrolysis resistance and excellent low temperature properties. Some are adaptable to FDA and USDA application for wet and dry food contact.
TDI-Ethers : have excellent low temperature and dynamic properties, microbial resistance and long term water resistance.
PPDI : Terminated esters and ethers offer superior performance at higher temperatures.
MDI : Diphenylmethane Diisocyanate
TDI : Toluene Diisocyante
PPDI : Paraphenylene Diisocyanate

Taken together, urethanes posses:

Oil, water and weather resistance, ozone and oxidation resistance, and resistance to many chemicals. Some are radiation, fungus and bacteria resistant.
High tensile and tear strength compared to other elastomers.
Outstanding abrasion resistance compared to metals, plastics and other elastomers.
Higher load bearing capacity than other elastomers. Higher impact resistance and resilience than plastics.
Excellent retention of properties at very low temperatures and at temperatures up to 220°F. (Bonded to metal to 160°F).

YOUNG'S MODULUS : Alternative term for modulus of elasticity. It is the slope of the linear portion of the stress-strain curve of the elastomer in tension or compression.