Passenger Safety: Flame, Smoke & Toxicity Control
Tried, true -- and some new -- FST-resistant products for manufacturers of composites in rail- and road-based people movers.
By Jared Nelson, Contributing Writer | December 2005
As fuel prices and population densities increase in urban centers, mass transit use is on the rise. The Los Angeles Metropolitan Transportation Authority, for example, projects that the population of Los Angeles county will increase 33 percent by 2020. Corresponding increases in ridership will propel safety concerns to the forefront, especially in the wake of recent terrorist subway bombings in Madrid and London. Thomas Johnson, corrosion-resistant/fire-resistant industry manager for Ashland Composite Polymers (Columbus, Ohio), says the use of fiber-reinforced polymers will continue to grow as transit authorities seek to optimize transport efficiency by cutting empty vehicle weight, but the need to ensure public safety will grow along with it.
While composites are an excellent technology for weight reduction, most thermoset resins carry some safety risk in mass transit applications. Aram Mekjian, president of Mektech Composites (Hillsdale, N.J.) a supplier of Hexion Specialty Chemicals (Carpentersville, Ill.) products, explains that unmodified polyester and vinyl ester resins, unlike metals, can burn because they are organic polymers; that is, their chemistry consists of at least one carbon compound. The combustion reaction, especially in the presence of other resin components, can produce toxic byproducts (e.g., carbon monoxide, nitrogen oxide), which contribute to failure when these resins are tested against federal standards.
Over time, resin additives and fire-resistant thermoset resin formulations have been developed to meet federal and local safety requirements for control of fire, smoke and toxicity (FST). Today these well-established solutions are being joined by emerging alternatives, such as inorganic resins and specially developed fiber forms.
Source: Ashland Specialty Chemical
The Atlantique, an SNCF- French National Railway TVG ("Train a Grande" or high-speed train), can travel at a top speed of 515 kmh (320 mph). Ashland's HETRON, a halogenated fire retardant vinyl ester resin, was used to mold its its exterior nosecone. For interior use, nonhalogenated fire retardants are coming to the forefront.
Regulations and Test Methods
In the U.S., the Federal Railroad Admin. of the Department of Transportation (DoT) regulates the safety of passenger trains, buses and other "people movers." Fire safety requirements are found in Title 49, Chapter II, Part 238.103 of the Code of Federal Regulations (CFR), with Appendix B dictating flame spread and smoke requirements in areas where composites are used, based on two standards developed by ASTM International (W. Conshohocken, Pa.).
ASTM E162 "Standard Test Method for Surface Flammability of Materials Using a Radiant Heat Energy Source" measures flame spread. This test is performed using a 15.2 cm by 45.6 cm (6 inch by 18 inch) panel tilted at a 30o angle. The panel's top edge is exposed to a 670oC/1238oF heat source placed at a distance of 11.9 cm/4.7 inches. The test is run either until the flame reaches the bottom edge or (if it does not) for 15 minutes. The flame spread index (Is) is calculated, based on the distance the flame traveled and the amount of heat generated from the material. According to ASTM E162's Appendix B, the Is must be less than or equal to 35.
ASTM E662 "Standard Test Method for Specific Optical Density of Smoke Generated by Solid Materials" determines smoke density (Ds). A 7.6-cm/3-inch square test sample is placed vertically in a smoke chamber with a heat source of 2.5 W/cm2. The test is run both with and without a flame to determine which results in greater smoke density. The test is run for a maximum of 20 minutes, with optical smoke density measurements taken at specified intervals to determine the maximum density. The performance criteria specify that smoke density values must be less than or equal to 100 and 200 at time intervals of 1.5 and 4 minutes, respectively. Composites that perform to ASTM FST flame and smoke standards are considered to have flame spread and smoke concentration rates slow enough to permit passengers sufficient time to disembark.
Currently, toxicity is not regulated in 29 CFR II, Part 238.103, but can be measured using either the Boeing Specification Support Standard or the Bombardier SMP 800-C test. Both tests use a smoke chamber. The Boeing test method employs a flamed heat source, to gauge toxic fume concentration. The Bombardier method, however, uses colorimetric tubes or absorptive sampling. In the first instance, colorimetric tubes are placed into the smoke chamber. Each tube contains a specific fluid that reacts with a specific gas, resulting in a change of fluid color. Gas concentration is determined using a color band scale. Alternatively, absorptive sampling uses light spectroscopy. Each gas has a unique spectroscopic signature, which permits technicians to identify the gas and determine its level of concentration.
Both tests gauge the concentrations of six gases -- carbon monoxide (CO), hydrogen fluoride (HF), hydrogen chloride (HCl), hydrogen cyanide (HCN), nitrogen oxides (NOx) and sulfur dioxide (SO2). The Bombardier test also tests for carbon dioxide (CO2) and hydrogen bromide (HBr). As in the smoke density test, concentration levels are recorded at specified time intervals. For the Boeing test, the maximum allowable levels for the six gases are 3,500, 200, 500, 100, 100 and 100 parts per million (ppm), respectively. For the Bombardier test, carbon dioxide and hydrogen bromide are specified at 90,000 and 100 ppm, respectively. Otherwise, permitted maximums are identical to the Boeing test, with the exception of hydrogen flouride, which is reduced to 100 ppm. Ultimately, end-users and local municipalities must determine acceptable toxicity levels.
Mekjian notes that ASTM E162 and ASTM E662 criteria were originally developed using unsaturated polyester resin as a baseline. Since then, some local municipalities have developed more stringent criteria. After a 1979 passenger railcar fire in San Francisco's Bay Area Rapid Transit (BART) system, it was determined that many interior components (of composite and other materials) -- the seats, in particular -- did not meet applicable ASTM requirements. Since that time, BART has replaced seating with nonflammable, nontoxic materials, and its cars not only meet but beat ASTM criteria. Its suppliers now must conform to standards that exceed federal requirements. For example, where ASTM E662 specifies the four-minute smoke density at less than 200, BART dictates that the level must remain at 100 or below.
Source: Maktech
Ceiling panels over an escalator at Liverpool Station in the London Underground were constructed using a phenolic resin-based composite.
Outside the U.S., most countries developed FST limits similar to U.S. limits. However, following a fire in the Kings Cross station of the London Underground in 1987, the British government implemented much more stringent flame spread and smoke density requirements, which are outlined in British Standard (BS) 6853 and BS 476, respectively. Unlike the U.S. requirements, British requirements specify different criteria depending on the transportation environment. For instance, trains that travel underground have more stringent smoke density requirements than trains that travel above ground. Since the establishment of ASTM standards, fire-retardant-filled resin systems have been developed that are far more flame/smoke resistant than the polyesters used to establish ASTM flame/smoke criteria. A common solution is the addition of fire retarding materials to neat resins to improve FST performance.
