Designs that won't go up in smoke

By Cynthia W. DeMaio
Rogers Corp.
Rogers, Conn.

Susan Baushke
Rogers Corp.
Elk Grove, Ill.

Edited by Jean M. Hoffman

It's impossible to predict the characteristics of a real fire. So designers must rely on standardized flammability tests to help predict material performance. These tests help cull potentially hazardous materials early in the design.

But it can be tough for suppliers just entering the mass-transit market to interpret and meet flammability requirements. So says Eric W. Simmons, principal of Pilot Flame Consulting Inc., Coquitlam, B.C. His advice for avoiding pitfalls is to contact fire-testing laboratories well in advance, understand sample requirements, and plan adequate lead time for testing.

The U.S. Dept. of Transportation mandates that all materials applied in mass transit meet performance criteria for flammability and smoke generation. The test procedures are spelled out by American Society for Testing Materials (ASTM) standards and by key players in the transportation industry such as Bombardier Inc., Montreal.

In contrast, commercial and consumer electronics manufacturers set their own flammability standards submitting materials or products to Underwriters Laboratories (UL) for listing. These manufacturers, though not subject to federal regulation with regard to flammability issues, are legally liable. As a result, most OEMs impose in-house guidelines regarding the flame resistance of materials they use.

MASS TRANSIT
Heightened awareness of fire safety in the past decade has brought a focus on the burning behavior of materials in enclosed compartments. The U.S. Federal Railroad Administration (FRA) imposes limits on the flammability of elastomeric materials for use in passenger rail cars. One commonly used material is cellular elastomer. This class of elastomer serves in a variety of applications including seat cushions, fire barriers, floor cushioning, and panel seals.

For the transportation industry, elastomer manufacturers must compound materials to balance low flammability with low rates of smoke generation. Flame-retardant (FR) agents help inhibit combustion so materials meet imposed limits. But too much FR agent can be a problem; the agent can also promote the formation of smoke.

The FRA imposes limits that cover either flame propagation or surface flame spread and rate of smoke generation. The rail-car industry also imposes guidelines for acceptable limits for toxic gas generation under standard conditions.

Flame propagation falls under ASTM C 1166 Standard Test Method for Flame Propagation of Dense and Cellular Elastomeric Gaskets and Accessories. The test is run on elastomeric materials employed in parts having surface areas of 16 in.² (100 cm²) or more. As a minimum, the standard requires testing parts such as window gaskets, door nosings, diaphragms, and roof mats.

Flame propagation is the extent to which flame spreads along a material when exposed to heat and flame. Under ASTM C 1166 guidelines, for both cellular and dense elastomers require testing samples that are 0.5 1 18 in. The standard calls for exposing the cellular and dense elastomers to a Bunsen burner with a specified flame for 5 and 15 min, respectively. In either case, the flame must not spread more than 4.87 in. (100 mm).

Flame-spread flammability evaluates how far away from the ignition source a flame travels across a liquid or solid surface. Ninety-five percent of the materials used in rail-car interiors are tested for flame spread in accordance with ASTM E 162 Standard Test Method for Surface Flammability of Materials Using a Radiant Heat Energy Source. Only flexible cellular foams are tested using a variant of the method ASTM D 3675, Standard Test method for Surface Flammability of Flexible Cellular Materials Using a Radiant Heat Energy Source.

The Flame Spread Index (I_s) has traditionally indicated a material's surface flammability. The I_s terminology is, however, currently in the process of being changed to Radiant Panel Index. The I_s number or classification indicates a comparative measure derived from observations made as the flame front moves across the sample surface under defined test conditions.

Both ASTM E 162 and ASTM D 3675 tests use a radiant energy source. ASTM E 162 requires testing four representative samples. The 6 18-in. strips are first predried at 60°C for 24 hr, then conditioned to equilibrium at 23°C and 50% relative humidity. Each specimen is individually mounted in a holder and inclined at 30° in front of a gas-fired radiant panel.

The I_s rating is derived from measuring both the rate at which the flame-front moves down the surface of the specimen and the temperature rise indicated by an array of thermocouples located in the exhaust stack above the burning material. Specifications require flexible cellular foams to have an I_s °25. The maximum I_s requirement for other materials is 35.

The final category for flammability testing of elastomeric materials is smoke. Here, smoke is defined as carbonaceous particles or liquid droplets that are suspended in air and measure less than 0.1 µm in size. These particulates come from incomplete combustion of organic materials such as oil or coal. All materials are tested under ASTM E 662 Standard Test Method for Specific Optical Density of Smoke Generated by Solid Materials.

Specifications for ASTM E 662 are based on the maximum specific optical density (D_s) for two time intervals — 1 to 1.5 min (D_s1.5) and 4 min (D_s4.0). Smoke density is determined by the attenuation of a vertical light beam located within a chamber containing the burning specimen. The test is conducted in two modes: flaming and nonflaming. Nonflaming mode subjects samples to a specified amount of radiant heat. Whereas in flaming mode they see radiant heat plus an open flame.

A material exposed to radiant heat produces smoke before it ignites. Typically, a sample produces smoke faster when it is in flaming mode. But a sample may emit more total smoke as it smolders in the nonflame test. If a material is going to fail, concludes PCFI's Simmons, chances are it will do so during the flaming mode. Of the elastomers available, most silicones are inherently temperature resistant and consistently provide low rates of smoke generation, he contends. "At the other end of the spectrum some rubber compounds don't perform well in either area."

The smoke test generates an optical density/time curve. Results are expressed in terms of D_s. For cellular foams, the FRA specifies a maximum (D_s1.5) of 100 and a (D_s4.0) of 175. With the exception of wire and cable, the D_s4.0 value for all other materials may not exceed 200.

However, no transportation regulations address the maximum amount of smoke produced. "The key is how much smoke is generated within the first 4 minutes," reports Simmons. "Passenger safety and the firefighters' ability to attack the source of the fire will suffer if the interior of a burning rail car is smoky enough to impede visibility."

Toxic-gas generation is not often a subject that agencies such as FRA like to broach, Simmons notes. The federal government doesn't regulate toxic-gas specifications, in part because of two philosophical concerns surrounding toxic-gas tests. The first is over how the gases are generated. Factors such as sample size and geometry as well as test procedures can affect not only the concentration of the gases generated but possibly even the types of compounds produced.

The second concern centers on how the generated gases are evaluated for toxicity. An analytical approach, for example, while convenient, ignores the existence of many compounds and their synergistic effects. Conversely, an animal-testing approach is expensive and controversial.

To address this issue, in the 1970s Boeing initiated a procedure called BSS 7239 to help evaluate toxicity of materials used in designs. With BSS 7239, Boeing measures the amount of toxic gas generated from a material by sampling the atmosphere of a closed chamber where the burn test takes place.

A variation of BSS 7239, based on updated analytical procedures and on additional pass/fail criteria, was developed in the 1980s by Bombardier and the Ontario Research Foundation (Bodycote Materials Testing Canada Inc.). The Bombardier toxicity test standard, SMP 800-C, is now the most commonly specified.

Toxic gases evaluated using SMP 800-C include both carbon monoxide (CO) and carbon dioxide (CO₂); hydrogen cyanide (HCN), hydrogen chloride (HCl), hydrogen fluoride (HF) and hydrogen bromide (HBr); sulfur dioxide (SO₂); and nitrogen oxides (NO_x).

According to Simmons, the materials most likely to fail SMP 800-C testing are polychloroprenes (neoprenes) which have a high chlorine content. During the burn test these materials commonly generate HCl levels on the order of thousands of parts/million, which is well above the 500-parts/million limit imposed by the standard.

FLAME TESTING
By far the most widely accepted test for the consumer and commercial electronics industries is the Underwriters' Laboratories Standard UL 94 Tests for Flammability of Plastic Materials for Parts in Devices and Appliances.

The UL 94 standard encompasses six different flame tests. Depending on the test, specimens are placed either vertically or horizontally.

The 20-mm Vertical Burn test (UL 94) yields ratings that progress from the least to the most stringent: V-2, V-1, and V-0. To comply with the requirements for a V-0 rating, each specimen (in a set of five) must extinguish in less than 10 sec. After the application of a second flame, total flaming and glowing of the sample must not exceed 30 sec. Total flaming time for a set must be less than 50 sec total. In addition, specimens can't burn through their entire length, nor drip flaming particles.

For a material manufactured in multiple densities, UL evaluates the minimum and maximum density. "We also look at a range of thicknesses because the thickness can affect the flammability," explains Karen Dubiel, engineering group leader at UL. "To establish a flame rating at a minimum thickness, at least two samples get tested to show the trend as the material thins and thickens." Thinner materials generally burn faster than thicker ones, she notes.

Included in the horizontal tests is the HB or horizontal burn test. This takes place on solid polymers such as plastics. Cellular elastomers are evaluated according to the HF (horizontal foam) test. HBF is the minimal rating assigned in this test, followed by HF-2 and HF-1 (the most stringent).

In many applications, thick foams are used for sound insulation. According to HF standards the maximum thickness that can be tested is 0.5 in. regardless of how thick the final product will eventually be, Dubiel states.

UL 746C Standard for Polymeric Materials — Use in Electrical Equipment Evaluations, provides general guidelines (including required flammability ratings) for materials employed in electrical enclosures. However, when an end-product standard calls for more stringent requirements than those of UL 746C the more stringent requirements take precedence.

Coffee-pot manufacturers, for example, may have a completely different set of requirements from those of a company making hair dryers, Dubiel states. "Variables include product configuration, how the device is actually used, whether it's for household use as opposed to a commercial application, or if it's considered portable rather than stationary. There's a complex path you have to go through to figure out what listing to pursue."

To begin the UL rating process, a designer first contacts a UL engineer to discuss product requirements. UL then reviews drawings or examines the actual device and voices concerns about the design or selected materials that may have initially been overlooked. UL then tests the material or product.

Commercial electronics involve some ambiguity. There's no authority that governs this industry. This can lead to some confusion for designers because requirements vary from one company to the next. OEMs use UL ratings to provide competitive advantage, peace of mind, and assurance that their device will be as safe as possible.

A CAUTIONARY NOTE
Laboratory tests can't simulate real-life situations. "You have to realize this about fire tests," Simmons says. "In general, unless you're doing a mock-up simulation, none of these tests can be extrapolated to what will happen in an actual fire." Fire tests impose a specific challenge on a material. The fire laboratory measures one response to this challenge. "At no time can it be said that, ‘We did an ASTM E 162 test on this material and it has a flame spread index of five, therefore it's perfectly safe and it will not burn,'" he warns.

For example, all the materials used in an advanced rail car developed in the 1980s were tested to the various standards. A mock-up test on the car used all the materials that complied with the requirements. "Because of certain geometric quirks in the design," Simmons explains, "there was a flashover within 3 min when a specific fire scenario was simulated."

"What that experience did was identify the influence of geometry and how the problem could be addressed," he says. Subsequent tests demonstrated that the problem had been resolved with simple geometrical corrections. Simmons concludes that even if materials meet standardized requirements, there's no guarantee that other factors won't initiate a potential disaster. Unpredictable factors that contribute to fire propagation include compartment geometry and ventilation, the intensity and location of the fire source, and whether it is accessible to fire fighters.

For more information on Bisco Cellular Silicones visit www.rogerscorporation.com/bisco