Fire retardancy theories

Coating Theory. This theory includes fire retardants which form an impervious skin on the fiber surface. This coating may be formed during normal chemical finishing, or subsequently when the fire retardant and substrate are heated. It excludes the air necessary for flame propagation and traps any tarry volatiles produced during pyrolysis of the substrate. Examples of this type of agent include the easily fusible salts such as carbonates or borates. 

Report 97 Rubber Compounding Ingredients - Need, Theory and Innovation, Part II Processing, Bonding, Fire Retardants, C. Hepburn, University of Ulster. 

C. Hepburn, Rubber compounding ingredients—need, theory and innovation Part II—Processing, bonding, fire retardants. Report 97, Rapra, 1997. 

A patent (5) describes the basic formulation and theory used in the best fire-retardant coatings developed to date however, it finds greatest use on wood for interior use. 

The most widely accepted theory of the mechanism of fire-retardant chemicals in reducing flaming combustion of wood is that the chemicals alter the pyrolysis reactions with formation of less flammable gases and tars and more char and water (4,5,8,21,24-29). Some fire retardants start and end the chemical decomposition at lower temperatures. Heat of combustion of the volatiles is reduced. Shafizadeh (21) suggests that a primary function of fire retardants is to promote dehydration and charring of cellulose. 

Several theories have been advanced to explain the effect of fire retardants. These theories include the following. 

Theories of Fire Retardancy. Several theories have been proposed for the mechanism of fire retardants. Browne (38) made a complete literature search on these theories and summarized the research in an effort to understand the mechanisms involved in the inhibition of pyrolysis and combustion of wood. These mechanisms can be categorized under several theories. 

Barrier theories. Fire-retardant chemicals prevent the escape of volatile products by forming a glassy barrier. This barrier also prevents oxygen from reaching the substrate and insulates the wood surface from high temperatures. 

Thermal theories. Fire-retardant chemicals may increase the thermal conductivity of the wood to dissipate the heat from the surface faster than it is supplied by the igniting source, or they may affect chemical and physical changes so that heat is absorbed by the chemical, preventing the wood surface from igniting. 

Dilution by noncombustible gases theories. Nonflammable gases released by the decomposition of the fire-retardant chemicals dilute the combustion gases formed by the pyrolysis of the wood and form a nonflammable gaseous mixture. 

Free radical trap theories. Fire-retardant chemicals release free radical inhibitors at pyrolytic temperatures that interrupt the chain propagation mechanism of flammability. 

Increased charireduced volatiles theories. Fire-retardant chemicals lower the temperature at which pyrolysis occurs, directing the degradation pathway toward more char production and fewer volatiles. 

Reduced heat content of volatiles theories. Fire-retardant chemicals lower the heat content of the combustible volatiles. This reduction in heat content always occurs when the amount of char is increased and the amount of volatiles is reduced. Therefore, Theories 5 and 6 function together, resulting in more char, fewer volatiles, and lower heat content of volatiles. 

In most cases, a given fire retardant operates by several of these mechanisms, and much research has been done to determine the magnitude and role of each of these mechanisms in fire retardancy. The influence of the combining effect of several of these mechanisms is illustrated by phosphorus-nitrogen synergism. The theories involved in this synergistic system are discussed to demonstrate this interaction between mechanisms. Some mechanisms apply only to... 

Thermal Theories. Researchers at Forest Products Laboratory impregnated wood with a metal alloy to determine whether change in thermal conductivity is a mechanism of fire retardants (38). The alloy was selected to melt at 105 °C. The treated and untreated specimens were subjected to a flame on one side and the temperature rise was recorded on the unexposed side. The rise of temperature was slower over the alloy-treated specimen than over the untreated specimen until the melt temperature of the alloy. Above this temperature the treated and untreated specimens then followed the same time-temperature regimes. The untreated specimen burst into... 

Increased Char/Reduced Volatiles Theories. Most of the evidence relating to the mechanism of fire retardancy in the burning of wood indicates that retardants alter fuel production by increasing the amount of char and reducing the amount of volatile, combustible... 

For example, dyes or fire retardants that are applied to fabrics must be on the Inventory if they are made in or imported into the United States. But if those dyes or fire retardants chemically react with the fabrics, the reaction products are exempt from PMN requirements. The theory is that the... 

Furthermore, from the viewpoint of the gas theory of fire retardancy, it appears that nitrogen produced during the pyrolysis of DHP, being noncombustible, alters the flammability by dilution of the flammable gases however, the contribution is less significant. 

Dehydration or Chemical Theory. In the dehydration or chemical theory, catalytic dehydration of ceUulose occurs. The decomposition path of ceUulose is altered so that flammable tars and gases are reduced and the amount of char is increased ie, upon combustion, ceUulose produces mainly carbon and water, rather than carbon dioxide and water. Because of catalytic dehydration, most fire-resistant cottons decompose at lower temperatures than do untreated cottons, eg, flame-resistant cottons decompose at 275—325°C compared with about 375°C for untreated cotton. Phosphoric acid and sulfuric acid [8014-95-7] are good examples of dehydrating agents that can act as efficient flame retardants (15—17). 

The objective of this theory in relation to fire is to dissipate the heat of the fire and retard the combustible substrate (fuel) from reaching the ignition point. 

It was found that for the charcoal based squib, the transfer time was almost double the initiation dme. This large transfer time obviously takes care of the variations in the excitation timings due to different factors in the manufacture of squibs. This ensures continuous flow of current thru all the squibs connected in series until, the time the most sensitive squib breaks the circuit. In the case of LMNR and LDNOC fuseheads the transfer time was found to be quite low and the total time of some of the fuseheads was less than or equal to the excitation times of some others in the same group. Such squibs when connected in series caused failures in r.eries firing. As a confirmation to this theory, a retardant was introduced in the LMNR fuse-heads to increase the transfer lime, and no misfires were noted... 

In spite of the fact that the phlogiston theory made it possible for a large number of facts to be coordinated into a system, it nevertheless retarded the progress of chemistry and prevented a number of the best chemists from seeing the correct explanation of the facts they uncovered. So until it, along with the four-element theory, were comprehensively rejected, modem chemistry remained unfounded for earth, air, water, and fire are not the elements, and substances do not bum because of the presence in them of a common principle of inflammability. 

---