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.
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.
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.
Fire-Resistant Function Fillers
Traditionally, fire-retardant additives in organic resins contain halogens. That is, the additive compounds contain either fluorine, chlorine, bromine, iodine or astatine. Brominated resins, which are the most common, are strong oxidizers. When exposed to heat, the weak bonds between the bromine and the rest of the resin's atomic structure are replaced with covalent bonds, which means the resin cannot be further reduced by flame. While it is this reaction that gives brominated resins their fire retardant properties, the bromine that is removed by this reaction then reacts with hydrogen to form hydrogen bromide (HBr). If the brominated resin is exposed to more and/or hotter flames, HBr density can increase. The resulting smoke is considerably more toxic than smoke from nonhalogenated resins. In mass transit applications, therefore, where smoke toxicity is of paramount concern, halogen use is declining.
The common nonhalogenated replacement is alumina trihydrate (ATH, Al[OH]3) When an ATH-filled composite is subjected to temperatures above 230oC/446oF, the ATH exfoliates 35 percent of its weight as water while the remainder becomes inert (noncombustible) aluminum oxide (Al2O3). This reaction is endothermic, or heat absorbing; the water acts as a heat reducing agent to hinder fire spread. ATH suppliers include Franklin Industrial Minerals (Nashville, Tenn.), Gruber Systems (Valencia, Calif.), Huber Engineered Materials (Fairmount, Ga.) and The R.J. Marshall Co. (Southfield, Mich).
While ATH functions effectively to control FST, it also is a filler, which, like other fillers, can have an undesirable impact on resin processability. "Generally, ATH products are differentiated by the median particle size," says Gary Rex, Huber's thermoset technology manager, explaining that a decrease the median size of ATH particles results in an increase in total particle surface area, and vice versa. For instance, a 9-micron particle has a surface area of about 2.1 m2/g. A 2-micron particle has a surface area of about 13.0 m2/g. The increased area promotes greater interaction between the ATH and the resin, resulting in comparatively better flame retardant properties on a constant weight percent (wt-%) basis. However, the greater particle surface area also increases resin viscosity, making fiber wetout more difficult, especially in infusion molding.
To avoid viscosity-based processing problems, Huber recommends "particle packing." Particle packing minimizes viscosity increase but maximizes beneficial resin/ATH surface area interaction by adding to the resin two ATH products, one with coarse grain size (particles greater than 9-micron diameter) and one with a fine grain (less than 3.5-micron diameter). The fine particles tend to "pack" into the small spaces between larger particles, thus displacing resin. The formerly "trapped" resin molecules are free to flow, and there is a corresponding reduction in resin viscosity. Since selection of particle sizes and weight ratios depends on the type of resin and fiber used to manufacture the product, Huber offers a range of "SB" (coarse) and "Micral" (fine) products separately, to permit custom mixing.
ATH-Based Resin Systems
While composite manufacturers commonly buy ATH in bulk and add it to organic resins prior to processing, the performance of the filled resin depends on the molder's mixing, measuring and recordkeeping skills. For that reason, some resin suppliers have incorporated ATH into factory formulations. Ashland Specialty Chemical, for example, developed MODAR (modified acrylic resin), an ATH-based thermoset system, more than 10 years ago. Modified and improved since then, the resin today, Ashland contends, surpasses current ASTM requirements, with a flame-spread index of 13. It also has a flaming smoke density of 8.1 at 1.5 minutes, 24 at 4 minutes with a maximum of 157, while the nonflaming values are 0, 2 and 155, respectively. Since it is halogen-free, the only toxic gases produced are carbon monoxide, nitrogen oxides and hydrogen chloride in volumes of 100, 4 and 0.5 ppm, respectively. MODAR variants have been validated for most molding processes, including resin transfer molding (RTM), pultrusion and filament winding. They are used in mass transit exteriors, interior panels and seating as well as third-rail covers and cable trays for in-tunnel applications, the latter providing FST protection where rodents may chew through high-voltage cables.
Among the suppliers of ATH-filled polyester, epoxy and/or acrylic resin systems are Interplastic Corp. (St Paul, Minn.), Poliya (Istanbul, Turkey), Scott Bader Ltd. (Wellingsborough, U.K.), AOC (Collierville, Tenn.) and Reichhold Inc. (Durham, N.C.). Magnolia Plastics' (Chamblee, Ga.) two-part, room-temperature-cure epoxy resins are designed to replace brominated systems. Filled with ATH, glass spheres and a proprietary mix of other additives, they meet U.S. Federal Aviation Regulations (FAR) 25.853 flame and smoke requirements for in-cabin use on passenger aircraft, which are similar to the ASTM standards.
Developed early in the 20th century, phenolics are based on an inherently FST-resistant chemistry, of which phenol (C6H6O) is the simplest member, says Chris Lee, composites market manager for Georgia Pacific Resins (GP, Atlanta, Ga.). While they are organic compounds, phenolics are nonetheless highly resistant to heat and flames. According to Mektech's Mekjian the resins outperform unsaturated polyester, vinyl ester and epoxy in FST control and far surpass ASTM flame spread requirements, with a index of less than 1. Flaming smoke density is less than 1 at 1.5 minutes, less than 20 at 4 minutes, with a maximum of approximately 50, while nonflaming density is less than 1, 10 and about 30, respectively. Toxicity concerns are minimal -- carbon monoxide and sulfur dioxide are generated at maximum values of 100 and 80 ppm, respectively.
Phenolics, however, can be difficult to process. When they cure, they outgas water, which can create voids and surface pinholes. They contain an acid catalyst, so tooling must be made of epoxy or nickel-coated metals to prevent corrosion. Additionally, phenolics are opaque and therefore cannot be successfully pigmented, and they crosslink so thoroughly that that there are few reactive sites available for proper bonding of standard gel coats. Components with an aesthetic function must be secondarily primed and painted.
Borden Chemicals, now Hexion Specialty Chemical (Columbus, Ohio), developed patented Cellobond phenolic resins, which reportedly process similarly to the resin systems they replace and can be used in RTM, spray-up, vacuum infusion, filament winding, pultrusion and compression molding. BART specified the product for replacement passenger railcars constructed after the 1979 fire. It has been used similarly in Britain, Germany and other European countries where requirements are more stringent.
To mitigate aesthetic drawbacks, Hexion developed a polyester gel coat, specially modified for optimum adhesion to phenolics. Although the gel coat/resin combination tests slightly higher for flame spread and smoke density, the results still fall well under ASTM limits.
Georgia-Pacific Resins offers phenolics for most composite processes and its resins have been used to fabricate composite parts in mass transit vehicles. The company recently released its patented, third-generation GP-274G38 resin with GP-4826 catalyst, which reportedly improves handling and wetout and reduces emissions without loss of fire-resistant and mechanical properties achieved in previous generations. The key is GP's range of proprietary catalyst systems, used to adjust viscosity for FRP processes. A latent acid catalyst is used in filament winding, vacuum infusion and hand layup processes that require lower viscosity (50 to 2,000 times the viscosity of water). A nonacid catalyst is used for sheet molding compound (SMC), and a low-emission waterborne catalyst is used for phenolic prepregs.
Phenolics for composite applications also are supplied by Plastics Engineering Co. (Sheboygan, Wis.) and Huntsman Structural Composites (Basel, Switzerland).
Inorganic resin systems also have inherent fire-resistance, because they are formulated without a fire-susceptible carbon component. Goodrich Corp.'s Engineered Polymer Div. (Jacksonville, Fla.) recently developed its trademarked FyreRoc resins, made from a metallo-silicate material. When cured and exposed to flame, the metallo-silicate creates a fire barrier, preventing protected materials from ignition. The result is an extremely limited flame spread (rated in tests at 1) and a maximum smoke density of 0.2, well under ASTM limits.
FyreRoc, however, experiences a pH-driven cure. As temperature increases during cure, water is driven off, decreasing the pH level to near neutral as the material cures. The high pH level of the uncured liquid resin damages glass fibers; therefore, more expensive silicon carbide, carbon and steel fibers are currently used. Goodrich is working on a less costly pH-resistant glass solution that will make FyreRoc-based composites affordable for mass transit applications.
Intumescent flame-retardant systems have been used for many years, usually in the form of paintable coatings. Intumescence describes the phenomenon of swelling. Intumescent systems with flame-retarding capacity usually contain three components -- an acid, carbon and a catalyst -- that interact to form a char layer, which then effectively eliminates an at-risk coated material (e.g., a balsa-cored sandwich panel) as a fuel source for the flame. Recently, intumescent materials have been incorporated into the composites themselves, to take advantage of a char barrier without resorting to secondary coating operations.
In mass transit applications, Technical Fibre Products' (TFP, Newburgh, N.Y.) Tecnofire Intumescent glass fiber mats can be used to reinforce composite constructions to mitigate the need for fire-resistant additives, says John Haaland, TFP's VP of sales and marketing. Tecnofire mats are wet-laid nonwovens combined with exfoliating graphite flakes, mineral fiber and binding agents. Exposed to flame, the flakes (modified carbon particulate in which carbon atoms, treated under heat and pressure, reorganize to become more stable graphite) expand to form an insulating char, which reduces the heat flux to the core material and inhibits combustion.
The thin, infusible mats are available in thicknesses ranging from 0.48 mm to 6.73 mm (0.019 inches to 0.265 inches). The resulting range in density provides a choice of expansion rates and activation temperatures. TFP's Tecnofire mat can be found on North American Bus Industries' (Anniston, Ala.) composite bus, the new people mover at the Dallas Airport and BART trains.
Among the companies researching and offering graphite flake-based intumescent materials is Nyacol Nano Technologies (Ashland, Mass.). In addition, Hollinee Glass Fibers (Shawnee, Ohio) and Sloss Industries Corp. (Birmingham, Ala.) offer intumescent mat.