FIRE PERFORMANCE OF WOOD: TEST METHODS AND FIRE RETARDANT TREATMENTS Mitchell S. Sweet Fire Safety of Wood Products, USDA Forest Service Forest Products Laboratory 1 One Gifford Pinchot Drive Madison, WI 53705-2398 Abstract This report describes the fire performance characteristics of wood and wood products, along with test methods for evaluating the fire properties of these materials. Wood treated with fire retardants may provide a code-approved alternative to noncombustible materials. Fire performance and problems associated with thermal degradation are discussed in terms of the mechanisms of fire retardancy. In: Lewin, Menachem, ed. Recent advances in flame retardancy of polymeric materials. Proceedings of the 4th annual BCC conference on flame retadancy; 1993 May 18 20; Stamford, CT. Norwalk, CT: Business Communications Co. Inc.: [1993]: 36-43. 1 The Forest Products Laboratory is maintained in cooperation with the University of Wisconsin. This article was written and prepared by U.S. Government employees on official time, and it is therefore in the public domain and not subject to copyright. 36
Introduction In 1991 in the United States, approximately 3,800 people died as a result of fire in structures, compared to 7,600 deaths in 1972 (Karter, 1992; NFPA, 1973). Property loss to structural fires amounted to over $8.3 billion in 1991, an increase from $2.4 billion in 1972. In the past 20 years, fatalities caused by fires have been reduced by 50%, but even when adjusted for inflation, property losses have increased by 25%. It is generally recognized that the combustible contents of buildings often initiate fires and are usually responsible for fire-related deaths. Nevertheless, building codes place considerable emphasis on wood and wood products, which are used extensively for structural and interior components of buildings. This report describes the fire performance characteristics of wood, methods for evaluating fire properties, and fire performance of fire-retardant treated wood. Ignition Fire Performance of Wood Wood, like other cellulosic materials, undergoes thermal degradation at elevated temperatures. This phenomenon, pyrolysis, follows a low-temperature pathway (below 300 C (570 F)) and a high-temperature pathway (above 300 C) (Shafizadeh, 1984). At low temperatures, the pyrolysis of wood favors the formation of char and evolution of noncombustible gases such as H 2 O and CO 2. At high temperatures, tars and combustible gases are produced. These combustible volatiles combine with oxygen to burn. In the presence of a pilot flame, the volatiles can ignite with a minimum rate of heating of 13 kw/m 2 (1.1 Btu/ft 2 s). When no pilot flame is present, spontaneous ignition can occur, although roughly twice the heat flux is required about 25 kw/m 2 (2.2 Btu/ft 2 s) (Lawson and Simms, 1952). The surface temperature for the rapid spontaneous ignition of wood has been determined to be between 330 and 600 C (626 and 1,112 F) (Beall and Eickner, 1970; Kanury, 1972). Forest Products Laboratory (FPL) is currently using ASTM Test Method E1321-90 (ASTM, 1990) for measure-merit of both ignition and flame spread. The heat flux of the radiant panel is adjusted until the minimum flux required to ignite the specimen, using piloted ignition, is determined. Fire Resistance Wood has excellent natural fire resistance as a result of low thermal conductivity and the fact that char is formed as wood burns. This layer of char prevents further fire penetration, while the wood beneath the char retains most of its original strength properties. In the standard fire conditions for ASTM E119 (ASTM, 1988), a thick piece of wood is exposed to fire conditions between 815 C and 1,038 C (1,500 F and 1,900 F). The outermost layer of wood is charred, but deeper into the wood, the temperature drops. The active char area is found to correspond to the depth at which the wood temperature reaches 290 C (550 F). Beyond that depth, the temperature drops further; at the depth at which the temperature falls to 175 C (347 F), the wood is no longer darkened. At a depth of 13 mm (0.5 in.) from the active char zone, the wood is only 105 C (220 F). Table I contains some typical 37
values of char rates for various softwoods and hardwoods under ASTM E119 testing conditions. Flame Spread Table I Average char rates and flame spread indices for selected wood species Species Char rate (mm/h) a FSI b Softwoods Western hemlock Douglas-fir Engelmann spruce Western redcedar Southern pine Redwood Hardwoods Hard maple Yellow-poplar Red oak Basswood 37 49 49 46 42 45 36 55 60-75 70-100 69-73 130-195 70 104 170-185 100 130-140 a 1 in. = 25.4 mm. Data from White and Nordheim (1992). b Flame spread index (UL, 1971). Flame spread is the propagation of a flame along the surface of wood. ASTM Test Method E84 (ASTM, 1991) involves exposure of a specimen with dimensions of 7.32 m (24 ft) by 0.51 m (20 in. ) to a gas burner. In the past, red oak was taken to be the standard for the test and was arbitrarily assigned a value of 100. Red oak was used because of its availability and fairly uniform properties. Today, red oak is still used as a calibration material for the test method, with a flame spread index (FSI) near 100. The FSIs are now calculated from the time necessary for the flame front to advance the length of the test specimen. In general, untreated natural wood products (25 mm (1 in.) lumber) have FSIs of 90-160; a few species have flame indices below 75 (western hemlock, western redcedar, redwood) and others have values above 170 (yellow- poplar) (Table I). Building Codes have defined classifications for flame spread from the FSI of a material. Class A or I materials have indices of 0-25; Class B or II, 26-75; and Class C or III, 76-200. Heat Release Rate The rate of heat release (HRR) can be determined by several approaches, including the sensible heat and oxygen consumption methods. Based on studies performed at FPL (Tran, 1988), the oxygen consumption method was determined to be superior to the thermal method. As a result, only the oxygen method is currently used at FPL. The heat release rate quantifies the amount of heat being 38
given off by a specimen exposed to a radiant heat source, It is generally believed that a lower HRR leads to better fire performance. A higher HRR means that a specimen could contribute more energy to a growing fire and produce a larger, hotter fire than a material with a lower HRR. ASTM Test Method E906 (ASTM, 1983) is not used as a basis for ratings or regulatory purposes but rather as a research and development tool. Table II contains some typical values for HRRs of various softwoods and hardwoods. smoke Table II-Average HRR for selected wood species a Species Softwoods Douglas-fir Redwood Hardwoods Red maple Sugar maple Cottonwood Yellow-poplar Red oak White oak Hickory HRR (kw/m 2 ) b 73 81 110 98 131 73 128 107 120 afive-minute average HRR at 40 kw/m 2 (3.5 Btu/ft 2 s)) (Tran, 1992). 1 Btu/ft 2 s) = 11.35 kw/m 2. b Most fire deaths are caused not by flame and heat but by gases and smoke (NFPA, 1986). Since smoke obscures vision and makes egress more difficult, the optical density of smoke is important. The smoke density is measured by light obscuration techniques. ASTM Test Method E662 (ASTM, 1992) is used to determine the specific optical density of smoke by measuring the reduction in intensity of a light beam under flaming or nonflaming conditions. A smokedeveloped index is also determined in ASTM Test Method E84. This index is relative to that of red oak. While the toxicity of smoke is also of interest, currently there is no nationally recognized standardized test method for measuring the toxicological effect of the combustion products of wood products. Mechanisms of Fire Retardancy Browne (1958) wrote a comprehensive literature review of various theories of fire retardancy for wood. The predominant theories are that the fire retardant (1) reduces the flow of heat to prevent further combustion, (2) quenches the flame, or (3) modifies the thermal degradation process. 39
The first theory has several variations. Afire retardant may work by forming a glaze or foam that serves to insulate the wood surface, such as an intumescent coating. Some fire retardants may actually increase the thermal conductivity of the wood to such an extent that the heat is quickly dissipated, preventing ignition. Another variation of the theory states that the fire retardant may undergo a highly endothermic reaction that absorbs sufficient heat to prevent the surface of the material from igniting. The flame quenching theory proposes that fire retardants release radicals at pyrolytic temperatures that scavenge hydrogen and hydroxyl radicals from the combustion gas mixture, inhibiting the propagation of the flaming combustion. The mechanism of thermal degradation may be modified by the formation of a physical barrier, which prevents oxygen from the atmosphere from reaching the pyrolyzed wood or prevents volatile gases from escaping from the wood. The mechanism of thermal degradation may be modified in a different way. The fire retardant chemicals may lower the temperature at which pyrolysis occurs, forcing a greater proportion of the thermal degradation to follow the lowtemperature pathway. This increases the amount of char and decreases the amount of volatile gases. The effective heat content of the material is therefore lowered as a result of less combustible volatiles. In all probability, fire retardant formulations utilize several of these mechanisms. However, most commercial fire retardants for wood seem to follow the increased char/reduced volatiles theory. The increase in char is due to an acid-catalyzed hydrolysis of the cellulose and hemicellulose, which not only dehydrates the wood but also increases the condensation and cross-linking of the carbon skeleton (Browne, 1958; LeVan, 1984). Within the last few years, much attention has been focused on the strength loss associated with fire-retardant-treated plywood. Strength loss is believed to be a result of the fire retardant lowering the temperature at which pyrolysis is occurring. Instead of catalyzing the acid hydrolysis during actual fire conditions, the polysaccharides in the wood may be dehydrated prematurely in-service, under the influence of solar radiation. The degradation of the polysaccharides appears to manifest itself as a reduction in strength properties of the solid wood (LeVan and others, 1990). Fire Performance of Treated Wood Fire-retardant-treated wood usually increases the temperature at which ignition occurs. Since the fire retardant tends to increase the amount of char formed and decrease the amount of volatile gases evolved, the treated wood resists ignition under the same conditions when compared to untreated wood. When treated wood is exposed to severe fire conditions, the charring rate is not significantly different from that of untreated wood. Southern Pine was treated with a number of different fire retardants and exposed to ASTM E119 fire conditions (Schaffer, 1974). Monoammonium phosphate, zinc chloride, sodium 40
chloride, dicyandiamide with phosphoric acid, and tetrakis (hydroxymethyl) phosphonium chloride with urea had no effect on charring rate. Boric acid, borax, ammonium sulfate, monosodium phosphate, potassium carbonate and sodium hydroxide reduced the char rate by about 20 percent. Fire retardants slow down the spread of flame by altering ignition and heat release characteristics, but generally do not improve the fire endurance of wood. The property that shows the greatest improvement with fire-retardant treatment is flame spread. While untreated wood commonly has a FSI of 90-160, wood well treated with present commercial formulations has an FSI of < 25. Treating wood with fire retardants can substantially reduce HRR. LeVan and Tran (1990), treated Southern Pine plywood with borax-boric acid and measured HRR according to ASTM E906. The HRR could be lowered from a 5-min. average of 140 kw/m 2 (12.3 Btu/ft 2 s) for untreated plywood to <25 kw/m 2 (2.2 Btu/ft 2 s) for plywood treated at a 15% loading level. While this HRR reduction was only demonstrated for borax-boric acid, it is believed that other fire retardants exhibit similar behaviors. The smoke yield from treated wood is also lower than for untreated material. Brenden (1975) found that treating Douglas-fir plywood with inorganic salt fire retardants decreased smoke yield under nonflaming conditions. Sodium bichromate decreased smoke by as much as 90%. Under flaming conditions, sodium bichromate, boric acid, ammonium sulfate, and borax also reduced the smoke yield. Concluding Remarks Wood and wood products will ignite when a heat flux causes sufficient evolution of flammable gases. It is these flammable volatiles that actually ignite, rather than the wood itself. The wood components that are not volatilized become char. The char will thermally and physically insulate the remaining wood, which gives wood excellent natural fire resistance to fire penetration. Most natural wood products have flame spread properties that give ratings of Class III. Some species of wood with inherently low flame spread have a Class II rating. Fire retardant treatment will greatly improve flame spread, but will not generally improve burnthrough resistance. Many mechanisms of fire retardancy have been proposed over time, and many fire retardants utilize several of these mechanisms. However, most commercial fire retardants for wood function by reducing the amount of flammable gases and increasing the amount to char formed. There is, and will continue to be, a need for building materials that meet a minimum level of fire performance. In many instances, untreated wood and wood products provide that performance. For other uses, a higher level of fire performance is required and can be attained by treating wood with fire retardants. However, the current generation of fire retardants used for wood needs to be improved. New fire retardants that are as inexpensive and effective as the ones used today, but are able to avoid the problems associated with strength degradation, should be able to make great advances on the market. 41
References 42
on recycled paper 43