Crest Foam Industries Inc.
Edited by Jean M. Hoffman
Reticulated polyurethane foam may appear to the uninitiated as nothing more than a synthetic sponge. But this lightweight material is strong enough to handle industrial applications ranging from sound and vibration absorption to sparkproofing fuel tanks in aircraft and race cars.
The flexible foams feature an open-pore structure with pore sizes and densities engineered specifically to suppress explosions and mitigate noise. They can prevent catastrophic explosions caused by incendiary arms fire. They can also squelch the static spark that can ignite fuel vapor during over-the-wing refueling or a race-car crash. The material resists most fuels, dissipates static, and can be manufactured with repeatable pore sizes.
What is reticulated foam?
Reticulated polyurethane foams feature a pentagonal dodecahdron geometric shape with 12 plane faces — do’ dec•a•he’dron from the Greek (dyo) two, deka (10), and hedra (base). A thermal reticulation process creates this unique cell-shape structure.
A variety of polyester and polyether polyurethanes get modified under pressure to create a flexible network of open cells. Thermal reticulation produces foams that look smooth, have high tear and tensile strength, and repeatable pore structure from batch to batch. The process parameters control cell size for specific applications.
Foams are classified as having discrete pore sizes measured as pores per inch (ppi) or as a ratio of the number of voids per solid material per linear inch. Materials with a higher ppi value contain less solid material and weigh less. Yet, they maintain a high percentage of the strength and chemical resistance present in the original material. Foam under compression, as in gasketing applications, can be made with a void volume (or ratio of total voids to solid material) as high as 98%. It can have surface areas as large as 200 ft2/ft3. The production process for polyurethane used as a filter or gasket can be tweaked to optimize properties for a particular application. The number of ppis and the size of the pores determine permeability. Typical pore sizes for reticulated polyurethane foams used industrially range from 4 to 120 ppi.
Reticulated polyurethane foam is essentially inert and is thus a candidate for filtering media. Humidifier filters, vehicle air cleaners, bacteria filters, and engine inlet prefilters are all examples. They handle compression ratios as high as 20:1, resulting in void volumes as high as 95% for filtering applications.
For gasketing, the force deflection needed to compress a square-inch section of the material 25% ranges from 2 to 7 psi depending on the pore sizes. Unlike many gasket materials, reticulated polyurethane foams suffer little compression set or “memory” effect once compression forces are removed, even at elevated temperatures. Reticulated polyurethane foams also work as gasket materials where it’s important to have flame resistance per Underwriters Laboratories’ (UL) UP94 flammability ratings. They also can be certified to meet CAL-117 fire protection standards. They are inherently impervious to breakdown from microbial organisms and can be manufactured with permanent fungicidal/bactericidal additives to help control microbial activity.
In addition to use as sealants or gaskets, reticulated polyurethane foams can be spec’d as dielectric insulators. They can be embedded with conductive metal particles in cases where a gasket or seal must also serve as a conductive material, as when providing a ground connection.
Polyurethane foams become ceramic
An interesting industrial use for reticulated polyurethane foam is in the production of ceramic filters for eliminating impurities in metal castings. Ceramic filters remove impurities by presenting a circuitous path for the molten metal poured through them during the casting process. In this application, the foam serves a sacrificial catalytic substrate. The foam is coated with ceramic slurry and dried. It then is fired in a kiln where it burns away leaving a fused ceramic filter with pore sizes ranging from 4 to 50 ppi (pores/in.).