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Polyurethane Glossary

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Acoustical Flexible Polyurethane Foam

Foam designed to be attenuated (dampen sounds), or be non-attenuated (transmit sound as in loud speakers).


A material used to modify the properties, processing, or end use of a base polymer. The amount of additive used is usually expressed in parts per hundred (by weight) of the total resin in the polymer formulation.

Air Flow

Amount of air expressed in cubic feet per minute, that can be drawn through a 2"x 2"x 1" foam sample at .5-inch water pressure differential. Air Flow is measured by a test (ASTM D3574).

Air Traps

Voids in molded foam parts caused by encapsulation of air pockets during mold fill-out. These voids have smooth, shiny surfaces.


A class of compounds used as catalysts in polyurethane foam reactions. Amines are characterized by having N, NH or NH2 groups in the molecule.


Materials which when added to a flexible polyurethane foam formulation improve the resistance of the foam to oxidative type reactions, such as scorch resulting from high exothermic temperatures.

Anti-Static Flexible Polyurethane Foam

Foam that contains electrically conductive materials to prevent static electricity buildup or promote static discharge. It is used primarily in packaging applications, such as for electronic components.

A-Side (T)

The isocyanate portion of a foam formulation. The opposite applies in Europe.

ASTM – American Society for Testing and Materials

An organization devoted to the establishment of standard methods and procedures for testing materials.

Auxiliary Blowing
Agent (ABA)

An additive used in the production of foam which supplements the primary blowing agent (water), and can be used to make foam softer or lighter. Compounds used to produce gases to expand, or blow, flexible polyurethane foam during production.  Auxiliary blowing agents are low temperature boiling solvents, such as methylene chloride, acetone, hydrochlorofluorocarbons, and isopentane.

Back Rind

Encountered with the soft rubber compounds.  When this occurs it appears as a slight undercut on the part at the mold parting line.

Ball Rebound

A test procedure (ASTM D3574) used to measure the surface resilience of flexible polyurethane foam. The test involves dropping a steel ball of known mass from a predetermined height onto a foam sample.  The rebound height attained by the steel ball, expressed as a percentage of the original drop height, is the ball rebound resilience value.


The process by which flexible polyurethane is foamed during production.  In all cases, blowing occurs when water and TDI (or MDI) react to form CO2.  (Also see Auxiliary Blowing Agent (ABA)).

Blowing Agent  

A gas or substance capable of producing a gas, used in making foamed materials.

Board Foot

A unit of foam measurement equal to a square foot of material one inch in thickness.

Bonded Foam

Flexible polyurethane foam particles or shredded flexible polyurethane foam (often manufacturing scrap) that has been glued to form a useful product. The resultant foam block is "peeled" into the desired thickness. Largest use is for carpet cushion. (Also see Rebonded Foam.)


The combination of two or more components into a composite. Foam is often adhered to other foam grades or to polyester fiber.

Bottom Out

Lack of support under full weight load. This characteristic is often found in low-density foam.  This term is very subjective, as a foam may bottom out with a heavy person, and be very comfortable to a lighter-weight individual.  The problem can be reduced by specifying foam with higher density and/or greater compression modulus value.

B-Side (R) 

In North America, the active hydrogen (polyol, water, amine, etc.) portion of a foam formulation.  (In Europe, the isocyanate portion of a foam formulation).


Flexible polyurethane foam pieces that have been shaped or contoured by removal of foam using abrasives.


A segment of foam cut off from continuously produced slab stock type of foam.


The weighing of carefully timed dispenses of chemicals from the metering ports of the mixing head in order to set an exact component ratio or an exact throughput of all chemicals


A chemical that changes the rate of reaction of a chemical process, but is not consumed or produced during the reaction. (Catalysts are required for foam production to balance rates of competing reactions and to attain desired physical properties.)


The cavity remaining in the structure of flexible polyurethane foam surrounded by polymer membranes or the polymer skeleton after blowing is complete.

Cell Count

The number of cells per linear inch or centimeter, expressed as pores per inch or pores per centimeter.

Cell Opening

In foamed materials, the breaking of membranes within the cell structure, permitting flow of air through the material.

Cell Size

The average diameter of the cells in the final flexible polyurethane foam product, often measured in micron units.

CFC-Free Foam

Flexible polyurethane foams that have been made without the use of chlorofluorocarbons as auxiliary blowing agents.

CFD (Compression Force Deflection)

A measure of the load bearing ability of a foam. It is the force exerted against a flat compression foot larger than the specimen to be tested. The value can be expressed at 25%, 40%, 50% and/or 65% compression (ASTM D3574). Note: previously called "CLD (Compression Load Deflection)"

Chain Extenders

Short-chain reactive molecules joining diisocyanates in a linear fashion to form crystalline hard segments that modify the properties of a polyurethane.


The ability of a flexible polyurethane foam to recover from the pinching effects of die-cutting.

Closed Cells

Foam cells having intact cell membranes thereby reducing or eliminating passageways for airflow.

Closed Pour

The case in molded foam production in which the mold lid is closed and locked and the foaming mixture is introduced through one or more special ports in the lid of the mold.

CO2 Blown Foam

Foam in which all the gas for expanding the reaction mix comes from the reaction of water with isocyanate. Sometimes called an all-water blown foam.

Cold Molding

Molding process for the production of high-resiliency foam in which the foam is cured at or near room (ambient) temperature. Pouring is carried out without adding heat.


Dyes or pigments added to impart color to the final foam.

Combustion Modified Foam

Flexible polyurethane foams manufactured by using additives based on chlorine, bromine, phosphorus chemistry to reduce ease of ignition. Hydrated alumina or melamine is also used.

Combustion Modifying Additives

A material that, when added to flexible polyurethane foam, will cause the foam to be more difficult to ignite or burn less rapidly or lose less weight during a fire than without that material. (Also known as Fire Retardants.)


The ability of the cushioning structure to deflect at the surface and to conform to body shape, preventing a concentration of pressure on the body without bottoming out.

Compression Set

A permanent partial loss of initial height of a flexible polyurethane foam sample after compression due to a bending or collapse of the cell framework within the foam sample. A high value of compression set will cause a flexible polyurethane foam cushion to quickly lose its original appearance with use, leaving its surface depressed or "hollowed out." Compression set is measured in the lab by compressing a foam sample 90% of its thickness (or down to 10% of its original thickness) and holding it at 70 degrees C (or 158 degrees F) for 22 hours. Compression set is most commonly expressed as a percentage of original compression. Other deflections, times and temperatures can be used.

Compression Set -
ASTM D 395

The amount in percent by which a standard test piece fails to return to its original thickness after being subjected to a standard compressive load for a fixed period of time and temperature. It is sometimes referred to as the elastomer's memory.


The internal portion of foam, free of any skin.

Core Density

The density of the foam sampled without skin, glue lines or compressed sections at or near the center of the final foamed shape.

Cream Time (Rise Initiation)

The time between the discharge of the foam ingredients from the mixing head and the beginning of the foam rise.  At this point, the surface of the liquid will change color, usually turning lighter as a result of saturation of the liquid with evolving gas.


Usually a mechanical or vacuum-assisted procedure to open the closed cells of a high resilience slab stock or molded foam (see degassing.)


A term referring to the process whereby chemical reactions approach completion.  At 100% completion, a foam should have 100% of the physical properties attainable with that particular formulation.

Cure Time

The length of time required for sufficient reaction completion to develop a desired level of polymer strength and dimensional stability and to attain ultimate physical properties.

Dead Foam

Foam that has a low resiliency and only slowly regains its original shape after deformation.


To compress, usually by a specified amount or percentage.


To release molding pressure on tools.  Allows breaking of foam cell windows and relieves internal gas pressures.

Demold Time

The time between the discharge of the foam ingredients from the mixing head and the time at which a molded object may be removed readily from the mold without tearing or altering its shape and without post-expansion.


A measurement of the mass per unit volume. It is measured and expressed in pounds per cubic foot (pcf) or kilograms per cubic meter (kg/m3). (Test Method ASTM D3547) Also see www.pfa.org/jifsg/jifsgs1.html

Die Cutting

The "stamping out" of foam into parts, useful for long runs of cut parts requiring consistency in size.  Also method of trimming polyurethane parts.


The gradual yellowing of foam due to a photochemical reaction. It is faster in sunlight than in artificial light, although it occurs in both. Fresh foam may discolor in the center of the block as a result of thermal or chemical events. (Does not affect physical properties.)


As applied to flexible foams, the term refers to how well a foam retains its load bearing capacity and shape with use. Most measures of durability are done with laboratory-scale tests.

Durometer (Hardness)

The resistance to indentation under conditions which do not puncture the elastomer surface. The most frequently used device is the spring loaded Shore Durometer. The scale runs from zero hardness for a liquid, to 100 for a hard plane surface such as glass.


Polymers which resist and recover from deformation produced by force, similar in behavior to natural rubber. Commonly called rubber, polymer, co-polymer and vulcanizates.


The percent that a specially shaped sample will stretch from its original length before breaking. (Test Method ASTM D3574)


The heat released as a by product of some chemical reactions.All flexible polyurethane foam production reactions are exothermic.

Fatigue (Flex Fatigue)

A softening or loss of firmness.  Fatigue can be measured in the laboratory by repeatedly compressing a foam sample and measuring the change in IFD.

Filled Foam

The addition of inorganic materials such as marble dust, barium sulfate, silica or clay, in foam to increase the density. Fillers are often added to increase the Support Factor.  Filled polyurethane foam may be inappropriate for some applications, if the polymer content of the filled foam is not sufficient for the intended application before fillers are added.  Fillers are not chemically bonded into the foam polymer. They are instead mechanically trapped within the molecular structure of the polymer. Filled foams with low polymer content or a high percentage of filler material tend to have less strength and durability.

Fire Retardants

A material that, when added to flexible polyurethane foam, will cause the foam to be more difficult to ignite, burn less rapidly or lose less weight during a fire. (Also known as "Combustion Modifying Additives".)

Flame Lamination

The practice of bonding flexible foam to a fabric or other material (film, etc.) by melting one surface of the foam with a flame source and quickly pressing it to the other substrate before the melted material re-solidifies. (Also called Flame Bonding.)


A rubber residue that clings to the part at the mold parting line. Also frequently encountered as an axial flash bead on the end of parts with small inside diameters.

Flex Fatigue

The loss of foam firmness after flexing the foam a predetermined number of cycles.

Flow Marks

A surface pattern on the part. Commonly found with the harder rubber compounds.


A lightweight cellular material resulting from the introduction of gas bubbles into a reacting polymer.


The list of chemicals and their relative amounts to be used in the preparation of a foam.

Free NCO

Isocyanate content of a substance that is available for further reaction.  This can be either free monomer, as TDI or MDI or as un-reacted isocyanate groups in a polymer or prepolymer.


A foaming technique in which air or other gases that are mechanically whipped into the polyurethane mixture using a high-shear mixer prior to the foam reaction.

Gate or Sprue

A mark on the surface of a part where material was fed into the mold cavity. It may be raised or depressed into the surface. On materials of 60 durometer hardness and harder - will be up to .055 inch in diameter - .007 height or depth.

Gel Hysteresis

The ability of foam to maintain original support characteristics after flexing. Hysteresis is the percent of 25% IFD loss measured as a compression tester returns to the normal (25% IFD) position after measuring 65% compression. Lower hysteresis values, or less IFD loss are desirable. Current research indicates that hysteresis values may provide a good indication of overall flexible foam durability. Low hysteresis in conventional foam is equal to less IFD loss.


The feel of the foam as the hand is rubbed lightly over the surface. For most furniture, bedding and textile applications, foam having a stiff or hard feel to the touch is described as having poor hand. In home furnishings, foam with a good hand has a springy, velvet feel, while abrasive pads, firm filler, and some packaging require a hand that is quite the opposite.

Heat Aging Test

Samples that are aged at an elevated temperature and retested for deterioration of original properties.

High Resilience (HR) Foam

A variety of polyurethane foam produced using a blend of polymer or graft polyols. High resilience foam has a less uniform (more random) cell structure different from conventional products. The different cell structure helps add support, comfort, and resilience or bounce. High resilience foams have a high support factor and greater surface resilience than conventional foams and are defined in ASTM D3770.

Hot Molding

A flexible molded foam production process in which high oven temperatures are used to drive the curing reaction in foams made from relatively low-reactivity polyols.

Humid Aging

An accelerated aging test method under conditions of high humidity and temperature. (ASTM D3547)


An affinity for water.


A repellency for water.

Hydroxyl Group

The combined oxygen and hydrogen radical (ľOH) that forms the reactive group on polyol molecules.

Immersion Test

A test used to measure the resistance to a fluid.  The test specimen is immersed into specific fluid for a specific time and temperature.  Properties are then retested for volume change or swelling.

Indentation Force Deflection (IFD)

A measure of the load bearing capacity of flexible polyurethane foam. IFD is generally measured as the force (in pounds) required to compress a 50 square inch circular indentor foot into a four inch thick sample, typically 15 inches square or larger, to a stated percentage of the sample's initial height. Common IFD values are generated at 25 and 65 percent of initial height.  (Test Method ASTM D3574)  Note:  Previously called "ILD – Indentation Load Deflection".

Integral Skin Foam

A molded foam having a dense, tough outer skin and a relatively lower density core. The product is achieved in a single pour using a combination of chemical and mechanical aids.


A shorthand name for the family of diisocyanates which are one of the two major ingredients in the chemical process by which polyurethane foam is made.

Knit Line

Sometimes confused with flow marks. Most often found at the two points where material was fed into the mold cavity.


The bonding of layers of foam and/or other materials together into a single composite. This may be accomplished through adhesives or through heat processes like flame lamination.


In the United States, usually the abbreviation for pure diphenylmethane diisocyanate.  In other countries, it can be the un-distilled mixture of diphenylmethane diisocyanate and higher molecular weight fractions.

Mixing Head

The device that mixes two or more component streams before dispensing the foam-producing mixture to the foam production surface or mold.


The force required in pounds per square inch to stretch the test piece to a given elongation. It expresses resistance to extension in the vulcanizates.

Mold Packing

The practice of purposely adding more material to the mold than is actually required to just fill it. The extra material serves to accommodate slight changes in material temperatures, mold temperatures and pour patterns.  It is also a way to improve load-bearing properties without changing the foam formulation.

Mold Release Agent

Any material which when applied to the mold surface serves to prevent sticking of the foamed part to the mold.

Molded Foam

A cellular foam product having the shape of the mold cavity in which it was produced.

Open Cell Structure

A permeable structure in flexible foam in which there is no barrier between cells, and gases or liquids can pass through the foam. Most cell walls have been ruptured to varying extents.

Parting Line

A ridge on the part where mold plates meet. On 0-rings and similar molded parts parting line will be .003 inch thick extending out from the part .010 inch. For compression molded parts, parting line ridge will be .004 x .015 inch.


he undesirable formation of large cavities or pockets in the foam structure. Pocketing is usually caused by rapid formation and/or release of the blowing agent before the polymer structure has gained sufficient strength to contain the gas.


A polymeric polyol containing ester groups in the main molecular chain or in side chains.


A polymeric polyol containing ether linkages (carbon-oxygen-carbon links) in the main molecular chain or in side chains.


An organic substance composed of repeating chemical units built up into large molecules.

Polymer Density

The density of the material made up strictly by the foam chemistry without fillers or reinforcements included.


A key chemical in foam formulation which, when mixed with diisocyanates and other specific ingredients, produces the reaction that causes flexible polyurethane foam to form.


Generally, a polymer connected by urethane groups. Urethane linkage and its supplements result from the reaction of polyol with isocyanate.

Pore Size

The number of cells per linear inch.


A reacted, but not completely polymerized product. In the polyurethane industry, this is usually a pre-reacted product formed by reacting polyol(s) or water with diisocyanate(s). The materials normally contain residual free isocyanate groups for further reaction with more polyol(s) or water to produce the final polymer.

Pressure Release Collapse

Shallow surface voids and/or shear collapse on molded foam parts caused by the sudden release of internal mold pressure or failure of the mold to seal. Usually seen with shiny surfaces and a thin surface skin similar to cold collapse.


Short for polyurethane

Rebonded Foam

That foam resulting from a process of adhering small particles of foam back together again to make a usable cushioning product. Various adhesives and bonding processes are used. A typical application for rebonded foam is as carpet underlay.  (Also see Bonded Foam.)


The return to original dimension and properties of a flexible polyurethane foam sample after a deforming force is removed.


An indicator of the surface elasticity or "springiness" of foam. It is measured by dropping a steel ball onto the foam cushion and measuring how high the ball rebounds.

Resiliency - ASTM D 945

The ratio of returned energy to impressed energy. A number of tests have been devised for comparing these and related properties.

Reticulated Foam

Flexible polyurethane foams characterized by a three-dimensional skeletal structure with few or no membranes between strands. Reticulated foams are generally used as filters, acoustical panels, and for controlled liquid delivery.


The dimensional change in a rubber part as it cures in the molding operations. May also refer to a specific section of the part which changes dimension more radically than other part sections.


The higher-density outer surface of a foam usually occurring when the foam surface cools more rapidly than the core.


A section of foam cut from the interior of a large bun.

Slab Stock

Flexible polyurethane foam made by the continuous pouring of mixed liquids onto a conveyor, creating a continuous loaf of foam.

Slab Stock Production Process

One of the two basic procedures used to manufacture foam. A continuous process in which the reacting foam chemical mix is dispensed on to a moving conveyor belt where the foaming process is completed. This foam is subsequently fabricated into useful shapes.

Static Fatigue

The loss in load bearing properties of a flexible polyurethane foam sample under constant compression of 75% for 17 hours at room temperature.  (ASTM D3574)


The structural members of a foam material. These roughly triangular features contain most of the solid polymer and form the cell shape.

Supersoft Foam

Foams that have an IFD measurement within the 4 to 10 pound range having a comparable feel to fiber.

Surface Firmness

The number of pounds of force necessary to indent a foam sample by 25% of its original height.


A term to describe substances that provide resiliency and stability to thin films and that markedly lower the surface tension of liquids, thus permitting easier bubble formation. An integral part of the foam manufacturing chemistry.


In polyurethane or polyisocyanurate technology, the two or more substances or materials which, when mixed together, react to form a polyurethane or polyisocyanurate polymer.


Toluene Diissocyanate.

Tear Strength

A measure of the force required to continue a tear in a foam after a split or break has been started and expressed in pounds per inch (lbs./in.)  This property is important in determining suitability of foam in applications where the material is sewn, stapled, or otherwise anchored to a solid substrate.  Also important in demoldability (ASTM D3574).

Tensile Strength

The pounds per square inch of force required to stretch a material to the breaking point. (Reference ASTM D3574.)

Tensile Strength - ASTM D412

The force required to break a dumbbell test specimen. It is calculated by dividing the breaking force in pounds by the cross section of the unstressed specimen in square inches.

Tension Set

The increase in length expressed as a percent of the original length, when a vulcanizate is stretched and released. It depends on time, load and temperature defined for the test.


A material that is cured by temperature and decomposes rather than melts upon application of elevated temperatures.  Most PUs are thermoset materials.

Tight Foam

Flexible polyurethane foam with many closed cells, resulting in low air flow measurements.

Total Vertical Motion (TVM)

The deflection of a seating system during sitting.


Actually a misnomer as applied to polyurethane foam. A colorless, crystalline substance used primarily in medicines, pesticides, and fungicides. Urethane is not used in the production of urethane polymers or foams. The urethanes of the plastics industry are so named because the repeating units of their structures resemble the chemical urethane.

Vapor Pressure

The pressure of a vapor above the liquid from which it formed.  Vapor pressure is temperature dependent.

Virgin Foam

Unfilled flexible slab stock foam that has not been processed in any manner other than cutting to shape.


The undesirable formation of large cavities or pockets in a foam structure. Voids are usually caused by poor moldability or incorrect mold filling.

Water Blown Foam

Flexible polyurethane foam in which the gas for expansion is carbon dioxide generated by the reaction between water and an isocyanate material.  All flexible polyurethane foam is water blown, although auxiliary blowing agents are often used to obtain special physical properties.


The thin membranes formed between cell struts. Windows may be present (a closed-cell foam) or absent (an open-cell foam) depending on the particular foam chemistry used.


Why does snow-white foam turn yellow over time? Yellowing is caused by an oxidation reaction of the polymer to UV light.

Informative links

  • Alliance for the polyurethanes industry, www.polyurethane.org - information about polyurethane and its applications
  • Polyurethane foam association - www.pfa.org