1,000,000,000 900,000,000 800,000,000 700,000,000 600,000,000
The shocking price Americans are paying for carelessness with fire. The National Fire Prevention Association reports that a fire loss of slightly more than $400 million in 1943 has climbed to $ 8 9 6 million i n 1952. ( ° Estimated on basis of first five months)
ire Retardant Faints G r o w i n g out of a revised chemical theory of fire, two major approaches have been developed b y paint chemistry researchers to frustrate combustion's destructive, irreversible reaction a n d its toll on American economy I N THE CENTER of an actual fire, paint itself has a role of little signifiance. However, around the perimeter of a burning area, paint can play a major role in fire prevention. For instance, if the paint is of a flammable nature with a low ignition point, it can materially add to the spread of the fire. If the paint is inert, it may have little or n o influence on the fire. If the paint is of a heat-refiecting nature, it may keep 3730
the temperature o f the combustible substrate and the paint itself below the ignition point. This last w i l l either prevent the further spread of the fire or reduce t h e rate at which the fire does spread. Finally, if the paint is an insulator, it may keep the temperature of the substrate below the ignition point and help the substrate retain its physical properties. It is logical to think that properly d e signed paints can at least reduce the rate CHEMICAL
of fire spread. If this rate of spread can be controlled, the reduction may b e great enough so that additional fire fighting techniques will take effect more quickly in the center of the blaze and provide a fire of smaller size to combat. Wood and cloth, or more generally, cellulose, is the most common type of fuel for a fire. It is well known that the burning of cellulose is due to the formation and combustion of flammable vapors when AND
E N G I N E E R I N G
NEWS
the cellulose is subjected to high tempera tures. That is, cellulose, a solid with no appreciable vapor pressure, does not burn as such, but rather decomposes into flam mable fragments, w h i c h in burning gen erate heat, and this heat further decom poses the cellulose to carry o n the process. It has b e e n established that t h e com plete burning takes place in two stages. At first, with a source of ignition, which may b e either an open flame or a hot sur face above 300° C , thermal decomposi tion begins; the cellulose decomposes heterogeneously into gaseous, liquid, tarry, and solid products. Combustion of the flammable gases takes place. The liquids and tars partially volatilize to give more volatile fractions w h i c h burn and, in part, give a carbonized residue which does not burn readily. T h e process continues thus until only carbonaceous matter is left. Then the second or glowing stage begins. In the second stage, the residual car bonaceous matter oxidizes and glows, con tinuing until the organic matter is con sumed, leaving only a fluffy ash ( Fig. 1 ). While it is difficult, if not impossible, to duplicate experimentally the conditions of burning in air and to collect intermedi ate reaction products, it has been possible to change the conditions in such a way as to retard the process to the point where some intermediate products may be iso lated. Products from t h e destructive distil lation of cellulose have shown that the gaseous and liquid portions formed in the first stage of burning are comprised of such low molecular weight, volatile compounds as acetic acid, methyl ethyl ketone, for maldehyde, and methane. Moreover, the tars give rise to aliphatic, aromatic, and heterocyclic compounds. Paint is primarily a surface covering which : ( 1 ) aids the substrate t o resist weathering, ( 2 ) reduces the exposed sur face area ( as in t h e case of porous w o o d ) , and ( 3 ) provides decorative and sanita tion properties. It is obvious that a paint with a fourth characteristic, namely, fire protection, would b e an ideal paint. I t is seen, indirectly, that t h e first two* charac teristics do help to reduce flammability ini-
Heat Liquids · Heat
G2 + Heat
+
Heat
- > - C 0 2 4- H 2 0 Rapid k Flaming
+ Liquids Heat
Be nonflammable or of low order of flammability Smooth out the surface of the sub strate to reduce the surface area exposed to air Retain insulation as long as pos sible under direct fire attack Possess sufficient decorative and protective characteristics of the usual paints s o that they will b e practical coating materials 3 1.. N O .
3 7 »
Gases -
0 2 4- Meat
^ +
Solids
0 2 4- Heat
Solids -
- ^ - CG 2 4- H 2 0 Slow Glowing Burning Stages for Cellulose
Figure 1.
tially, in fact, once actually burning it has been found paint tends to decrease the rate of fire rather than add to it. It is ob vious, therefore, that what is desired is a paint which in and of itself will be com pletely passive to an attack of either heat or open flame and at the same time thoroughly protect the burnable surface beneath it by these properties. These would b e called the ideal features of such a paint. However, the ideal is seldom attainable and something near to the ideal would b e just as desirable for general purposes. Intumescent paints are one type of coating w h i c h come close to this ideal. Another, mastics, in one sense come even closer in their characteristics to protect steel perfectly at temperatures u p to 5 4 0 0 ° F., but in some instances they have drawbacks of their own, like weight and decorative properties.
should apply in the thermal decomposi tion of cellulose. As a polyhydric alcohol, cellulose is subject to catalytic dehydra tion and this, it is thought, is the major reaction involved in the flameproofing of cellulose. The dehydration process is thought to operate by acid or basic cataly sis through carbonium or carbonion mech anism. The broad basis for this concept lies in the reactions performed b y Lewis acids and bases. For instance, w h e n an acid is brought into the vicinity of a com pound that can furnish a pair of electrons, carbonium ion formation is possible. These reactions are the result of the complexing of the electron-poor atoms of the acid with the electron rich atoms of the base through the line pairs of t h e latter. CH3 -H
CH 3 The Revised Chemical Theory
H
