a website collaboration between Science Service and the Smithsonian Institution


E&MP 90.005a thru 90.005f

Noise Control

November 25, 1964


cover - 1 - 90.005a:

Open-pore Polyurethane Foam and Felt
Design News
NOVEMBER 25, 1964

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STRUCTURE of open-pore polyurethane foam is 3-dimensional network of interwoven strands and interconnected air cells. Random orientation of strands and air cells minimizes possibility of open flow through channels. There are no connecting membranes between strands as in other foam materials.

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Open-pores polyurethane foam is made by a process that provides an open mesh-like skeletal structure containing a high percentage of void space. As a result, the material is extremely porous and permeable, has a low resistance to flow and is capable of entrapping and holding large amounts of liquid and solid matter.

Good chemical and physical properties make the material well-suited to a variety of applications. These include air and liquid filtration, humidification, demisting, sound absorption and abrasive polishing and cleaning.

Open-pore polyurethane felt is a recently introduced, compressed variation of the standard foam material. The felt provides greater density, smaller pore sizes, finer textures and much higher mechanical strength. To date, primary applications of open-pore felt have been in the field of sound absorption, capillary conveyance of liquids and extremely fine particle filtration.

Porosity and texture

Open-pore foam is produced in a number of standard porosity grades. Rated in terms of pores per lineal inch (ppi), these range from 10 to 100 ppi. Tolerance on porosity is closely controlled and is generally + 5 ppi in most grades.
Texture of the open-pore foam ranges from coarse and abrasive in the 10 ppi grades to soft and downy in the 100 ppi grade. Void volume is constant at 97 percent for all grades. Density is also constant at a nominal 2 lb. per cu ft.
Porosity, density and texture of the open-pore felt depend on the porosity of the original standard foam and on the extent

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show open-pore polyurethane foam well-suited for air filter applications. Filters have received widespread acceptance in heating and air-conditioning fields and are gaining increasing acceptance as carburetor filters for automotive and small engines.

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1 CARBURETOR AIR CLEANER made of open-pore foam is designed to be washable. Filter element has self-sealing lips and at top and bottom.

2 CENTRIFUGAL OIL-MIST COLLECTOR employs open-pore foam filter to condense or remove oil droplets from air stream. Air containing oil vapor or mist is drawn by centrifugal impeller from vicinity of grinding machines, impinged against internal baffle and then swirled through foam filter. Units are designed by Dustrol Div., Robbins & Myers, Inc., Springfield, Ohio, and employ zippers to allow easy removal for cleaning in common detergent.

3 HIGH-EFFICIENCY FILTER for gas-fired furnaces and central air-conditioning system uses pleated construction to double foam surface area. Pleating is achieved by curving open-pore sheets back and forth around light wire rods fixed to frame. Filter is designed to be permanent and may be washed, hosed or vacuumed without damage.

4 AEROSOL UPHOLSTERY SHAMPOO DEVICE uses coarse open-pore foam to hold and diffuse shampoo and to provide scrubbing action. Brush applicator is reusable.

5 HUMIDIFIER uses band of coarse, open-pore foam to carry water droplets from supply pan to heated air stream. Material is easy to fabricate and is stretched over vapor-wheel frame.

6 MICROPHONE COVERS of fine-pore foam (60 ppi or more) are used to screen out extraneous background sound and noise generated by breath or wind. Other acoustic applications include use as sound-absorption material within enclosures.

7 FILTERS OF OPEN-PORE FOAM protect sensitive memory-unit components of IBM data-processing systems from airborne contaminants. Filter pads are epoxy-bonded to aluminum screen, can be removed for cleaning by washing or vacuuming. Material is used dry and does not require oil impregnation. It also contains no metallic particles that might interfere with equipment operation.

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[pictures of uses of open-pore polyurethane]
1 copyright A.C.Spark Plug Div., General Motors Corp.
2 copyright Barden Corp., Danbury, Conn.
3 copyright American Standard, Air Conditioning Div., Elyria, Ohio.
4 copyright Bissell, Inc., Grand Rapids, Mich.
5 copyright Viking Div., Lau Blower Co., Cleveland, Ohio
6 copyright Electro-Voice, Inc., Buchanan, Mich.
7 copyright Data Systems Div., IBM, Poughkeepsie, N. Y.

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to which thickness is reduced. Standard reductions in thicknesses of 3, 5,10 and 15 firmnesses are presently available. (Intermediate firmnesses and firmnesses as high as 25 also can be obtained.) "Firmness" indicates the relationship between original and final thickness.

For example, a 1/4-inch-thick piece of 15 firmness material would have been reduced by 3-4/4 inches (15 one-quarter-inch increments) from its original 4-inch thickness.

Texture of the felts depends on pore size and firmness of the original foam material as well as heat and pressure required to set the felt. It ranges from soft and velvety to smooth and leathery.

Flow and Filtering Characteristics
In tests of foam made at an air velocity of 350 fpm, 1/2-inch-thick pads of the previously mentioned 20-100 ppi porosity grades cause pressure drops ranging from 0.045 to 1.0 inch of water. For thicknesses other than 1/2 inch, pressure drop can be computed by multiplying the drop at 1/2

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capable of holding from 140 to 300 grams.
Filtering characteristics of the felted material have not yet been fully evaluated. Felted materials will arrest smaller particles than standard foam because of the structure's smaller-sized air cells; their capacity, however, will be less. Typical flow characteristics of the felt range from 175 fpm at 0.5 inch of water for 1/8 inch of firmness, 3[sic] to 6 fpm at 0.5 inch of water for 1/8 inch of firmness 15.

Physical Properties
Like other polyurethane materials, open-pore foam is relatively strong, has good flex resistance and has high resistance to tear abrasion and impact.

Tensile strength varies with porosity and is higher for the smaller-pore-sized materials, ranging from an average of 15 psi for 10 ppi foam to an average of 35 psi for 100 ppi foam. Tear strength averages around 3.5 to 4 lb. for all pore sizes. Elongation is approximately 450 percent for grades of 60 to 100 ppi. Below 60 ppi, elongation is somewhat less. Resiliency is also good and recovery of the material is virtually completed in 1 minute.

The felted material provides high mechanical properties. Depending on initial pore size and firmness, tensile strength varies from 50 to 270 psi while tear strengths of 7 to 35 psi are achieved.

Environmental Resistance
Chemically, the material is an ester-type polyurethane and has good resistance to most chemical solutions, cleaning solvents and biological agents. Exceptions are chlorine, strong acids and strong caustics. Aliphatic and aromatic hydrocarbons are also problems, since they can cause the material to swell and lose strength. Protection against these agents can be provided by coatings.

The material is also color fast; however, light colors tend to fade when subjected to ultraviolet light.
In regard to aging, samples have shown no signs of deterioration after 5 years of normal use. However, prolonged exposure to sunlight and weather will cause serious loss of physical properties.
The foam can be used without ill effects at temperatures to 250F. It can thus be washed in hot water and even sterilized with boiling water or steam without deterioration.

Low-temperature properties are also good. Although the material slightly stiffens at very low temperatures, it can be used to -50F with out danger of embrittlement.

Design and Fabrication
Open-pore foam and felt may be shaped by reciprocating or rotary saws and knives, shearing, die-cutting and hot-wire cutting devices. Foams and felts also may be nailed, stapled, sewn, glued, laminated, taped and, in some cases, heat-sealed.

Small areas of the material can be made self-supporting while large areas require only minimum support (i.e., a 2-ft square filter may only require diagonal wires to prevent displacement under airflow). It can also be made self-gasketing when clamped in compression between adjacent surfaces. Because of its good tear strength, it can be edge-fastened.

Applications and Availability
Typical applications of foam materials are shown in illustrations.

Both felts and foams are produced by proprietary processes. The foam is available in a number of porosity grades in thicknesses from 1/4 to 12 inches (in 1/16-inch increments) under the registered trade name of "SCOTT Industrial Foam". Standard grades and thicknesses of foam are available compressed to firmnesses of 3, 5, 10 and 15 under the registered trade name of "SCOTTFELT".

Technical information for this article was provided by the Foam Div., Scott Paper Co., Chester, Pa.

Original Caption by Science Service w/Design News Report

National Museum of American History


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