Many types of foam burn rapidly. Some even drip burning material on to the surface below, helping a fire spread rapidly. However, there are foams formulated to provide high levels of fire resistance. Whenever possible these should be used near potential sources of ignition.
The fire-resistance of foam for use in appliances, enclosures and equipment is measured by testing to procedures set out in the UL94 standard. (Note: this does NOT address foam used in upholstery or building construction.) Flammability (the ease with which a material burns,) is shown by a UL94 rating, such as the UL94 V-0 indication carried by PORON® 4701-V0-M.
Anything composed of carbon and hydrogen will burn if conditions are right, and that means having oxygen and an ignition source. All foams are formed from these elements and oxygen is always present in the atmosphere, so to start a fire all that's needed is an ignition source. This could be provided by an electrical spark, such as when motors or relays produce arcing. High temperatures, as caused by friction between moving parts or electrical current flowing through wiring are others possible causes.
UL94 flammability ratings
The UL organization is an independent global testing organization dedicated to improving safety. Founded in 1894 as "Underwriters Electrical Bureau" it started out by testing noncombustible insulation material and grew to become first "Underwriters Laboratories" and now just UL.
UL94 is the "Standard for Tests for Flammability of Plastic Materials for Parts in Devices and Appliances". In these tests material is held either vertical or horizontal and subjected to a flame for a brief period. The vertical tests are more demanding as part of the evaluation has to do with dripping. The materials receive a rating based on how easily they light, whether they self-extinguish when the flame is removed, and the rate at which they burn.
For the vertical test, the ratings look like this:
- 5VA - After five applications of flame the material burns for less than sixty seconds but does not burn-through.
- 5VB - Like 5VA but some burn-through occurs.
- V-0 - Burning stops in less than 10 seconds after the flame is withdrawn and there are no drips.
- V-1 - Burning continues for up to 30 seconds, no drips.
- V-2 - Burning stops within 30 seconds but some dripping may occur.
UL94 also tests foamed material in a horizontal orientation. These ratings are:
- HF-1 - Less than two seconds of afterflame, afterglow less than 30 seconds, and no dripping.
- HF-2 - Afterflame less than three seconds, afterglow ≤ 30s, dripping is allowed.
- HBF - Burns at less than 40mm/min.
Testing is done on relatively small samples of material. UL advises that the results don't apply to material thicker than 13 mm (1/2") or where surface area is greater than 1 m2 (10.7 ft2.) Clearly, this excludes large blocks that might be used in applications like construction, upholstery and packaging.
Fire-resistant PORON® Urethanes
PORON® Urethanes are one of the few foam materials to achieve both HBF and V-0 ratings in the UL94 tests. PORON® 4701-V0-M comes in thicknesses of 5.0mm (13/64") and 6.0mm (1.4") and a width of 1,372 mm (54") making it suitable for a wide range of gasketing and sealing applications.
Other Characteristics of PORON®
PORON® is a family of open cell urethane foam products manufactured by Rogers Corporation. Using proprietary techniques, they're able to control the size and distribution of pores throughout the foam. This results in a very consistent material with predictable and consistent properties. Unlike some foams, for PORON® an array of ASTM test data is available.
PORON® is known for a soft feel and excellent compression set resistance. The open cell structure compresses easily so compression force deflection numbers are generally low, but recovers quickly when the load is released. It die cuts readily so is easy to work with, and the smooth edges that result eliminate the need for finishing operations.
Putting PORON® to Work
Gasketing and cushioning are ideal applications for PORON® urethane foam where softness and compression set resistance help ensure long life. As a gasket material PORON® works especially well in gaps subject to vibration or varying size, helping prevent rattles or squeaks and accommodating dimensional variations due to tolerance stack-ups.
Fire-resistant PORON® is an excellent choice in situations where an ignition source may be present. Enclosures for electrical switchgear are a good example. In these a foam gasket fills gaps around the door or opening, preventing dust from contaminating the equipment. Cabinets holding transformers are another application, as are motor housings and electrical appliances.
It is important to note that as an open cell material PORON® offers only limited resistance to liquids and gases. Compressing the foam closes-up the cells, providing a degree of resistance to fluid transmission, but gaskets made from PORON® are not suitable for immersion.
A Caution Regarding Pressure Sensitive Adhesives (PSA's)
In many applications laminating a PSA to the foam makes it much easily to fix in place. For example, an enclosure door gasket with a PSA on one side is readily mounted in place without needing a separate application of adhesive. However, many PSA's are not UL rated. Using an non-UL rated PSA in a situation where there is a risk of fire negates the benefits of using a material like PORON® 4701-V0-M. When specifying UL rated foam with a PSA, ask the supplier to use UL rated adhesive as well.
Consider the Risk
Open cell urethane foam makes an excellent cushioning and gasketing material. Unfortunately though, many foams will burn easily if exposed to an ignition source. UL94 assigns flammability ratings to plastics and foam, making it easy to identify fire resistant materials. When sourcing materials for applications where fire is possibility, always look for those proven to resist burning, such as PORON® 4701-V0-M.
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