INTRODUCTION 



For the past 10 years, ammonium phosphate (NHt^)2HP04 and ammonium sulfate (NHi+)2S04 

 have been used as primary ingredients in forest fire retardant formulations. These 

 chemicals, when applied to forest fuels, are known to alter or inhibit thermal degrada- 

 tion and combustion reactions. Because of the reduced flammability of chemically 

 treated fuels, the use of such fire retarding chemicals has become an important tool in 

 wildfire suppression. Knowledge of the reactions or mechanisms taking place and their 

 relation to flammability will assist in a systematic selection of fire retardant chemi- 

 cals and may lead to more effective methods of application. 



Recent studies performed at the Northern Forest Fire Laboratory have indicated 

 significant differences in the effects ammonium phosphate and ammonium sulfate have on 

 flammability as measured indirectly by such parameters as rate of spread, fire intensity, 

 radiant energy, flame height, and residue.-^ We do not know whether these differences 

 in effects are due to the chemicals' physical differences, a difference in their thermal 

 decomposition and availability, or to a difference in the actual mechanism by which, 

 they alter fuel degradation. 



Truax, Harrison, and Baechler (1956) noted that both compounds were effective in 

 retarding flaming combustion, but ammonium phosphate was superior in retarding glowing 

 combustion. Browne and Tang (1963), on the basis of thermogravimetric analyses in a 

 nitrogen atmosphere, found that the compounds have similar effects on the volatilization 

 of wood and the threshold temperature for pyrolysis, although a difference in their 

 volatilization rate was exhibited. Tang (1967) in a similar study found that ammonium 

 phosphate had the same effects as ammonium sulphate on the pyrolysis of wood and 

 cellulose but had little effect on the volatilization rate of lignin; the cellulose 

 portion of wood was found to have the highest volatilization rate. The volatilization 

 rate for wood seemed to be a combination of the effects on alpha-cellulose and lignin. 

 Tang and Eickner (1968), using differential thermal analysis, compared the effects of 

 2-percent by weight ammonium sulfate and a 2-percent by weight ammonium phosphate treat- 

 ment on pyrolysis and combustion of wood, cellulose, and lignin. Little difference was 

 noted in thermograms and relative maximum heat intensities and it was concluded that 

 these parallel results probably^ were produced by a similar mechanism. 



Past research (Shafizadeh 1968^ Kilzer and Broido 1965) has indicated that combus- 

 tion adds secondary and competitive reactions to initial degradation reactions. It is 

 likely that the occurrence of flaming combustion causes cellulose as well as the 

 retardant chemicals to undergo different reactions and at different rates. Although 

 relationships undoubtedly exist between the pyrolysis and combustion of cellulose and 

 the role retardant chemicals play in altering related reactions, it may not be possible 

 to predict combustion characteristics on the basis of pyrolysis characteristics. 

 Thermal analysis in oxygen, or in an air atmosphere, may or may not accurately represent 

 flaming and glowing combustion. ^ 



The purpose of the study was to provide extensive thermal analysis data that could 

 be used to categorize the effects of these retardant chemicals on the pyrolysis and 

 combustion of cellulose.^ 



^Charles W. George and Aylmer D. Blakely. Study of the effects of diammonium 

 phosphate and ammonium sulfate on flammability. 1968. (Unpublished report on file at 

 the Northern Forest Fire Laboratory, USDA Forest Serv. , Missoula, Montana.) 



^For the purpose of this paper, pyrolysis is defined as the degradation of a 

 material in an inert atmosphere or vacuum. Combustion refers to the process taking 

 place when the initial material, as well as its degradation products, are in contact 

 with oxygen or air. 



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