Monoammonium Phosphate: 

 Effect on Flammability of 

 Excelsior and Pine Needles 



Aylmer D. Blakely 



INTRODUCTION 



Forest fire retardants were first operationally delivered by air- 

 craft in the mid-1950's, and consisted mainly of chemicals that 

 retained water and amassed thick layers on aerial fuels. The 

 first chemical to be used extensively was sodium-calcium borate 

 (commonly called "borate") that not only thickened the water, 

 but had some fire-retarding properties when dry (Miller and 

 Wilson 1957). Borate was toxic to plants, erosive on pumps, 

 and costly because of the required mix ratio. Bentonite clay 

 was introduced to overcome some of the problems caused by 

 borate, but it only thickened the water and had no long-term 

 retarding properties when dry (Phillips and Miller 1959). Borate 

 and bentonite use was phased out when chemicals (called long- 

 term retardants) were introduced that more effectively retard 

 combustion, even when completely dry. 



Experiments with long-term chemicals were performed by 

 Truax (1939) to quantify the fire-retarding abilities of water 

 solutions of several commonly used chemicals and chemical 

 combinations, and to determine the feasibility of using them on 

 wildfires. These were some of the same chemicals that had been 

 used successfully for impregnating and fireproofing building 

 materials since the early 1900's. Studies by Truax and later by 

 Tyner (1941) showed that dianmionium and monoammonium 

 phosphate water solutions were the most effective for retarding 

 combustion. Other phosphate compounds have been tested, but 

 the ammonium phosphates are the most chemically available 

 for affecting pyrolysis. George and others (1977) reflect that 

 phosphate compounds formed with Fe, Ca, or Mg usually are 

 ineffective because of their high temperature requirements for 

 decompositions and thus their unavailability in terms of altering 

 pyrolysis and combustion reactions. Operation Firestop (1955a, 

 1955b) tested some phosphate chemicals along with borate, but 

 because of the test methods and interpretation of resuhs, 

 borate was considered the superior fire-retarding chemical, and 

 thus its use as the original aerial-delivery fire retardant. 



Monoammonium phosphate (MAP) was dropped from a 

 TBM airtanker onto forest fires in Georgia (Johansen 1959) 

 with good results. Soon after, attempts were made to thicken 

 the MAP with clays or gums (Johansen and Shimmel 1%3) for 

 better adherence to aerial fuels. Use of MAP was abandoned 

 when Pyro, a liquid mixture of ammonium phosphate species, 

 was introduced in the Southeast (Johansen and Crow 1%5). 

 Pyro was comparatively inexpensive and required only dilution 

 with water and pumping into the airtanker. MAP was consid- 

 ered unhandy because it required breaking bags open and mix- 

 ing with water under agitation. Later developments of mixing 

 equipment resolved most of the limiting mixing procedures. 



In 1%1 tests were performed (Hardy and others 1%2) to 

 compare the effectiveness of several different retardant formu- 

 lations that were being suggested and introduced by firefighters 

 and chemical companies. Among those chemicals were borate, 

 algin-gel, diammonium phosphate (DAP) thickened with algin 

 and pectin, and ammonium sulfate thickened with attapulgite 

 clay. The tests showed that the sulfate- and phosphate-based 

 materials were superior to borate and water thickeners, espe- 

 cially after all the water had evaporated. Fire-Trol® (formu- 

 lated with ammonium sulfate and clay thickener) and Phos- 

 Chek® (formulated with diarnmonium phosphate and gum 

 thickener) brand fire retardants were first produced commer- 

 cially about 1%2, and formulations containing various dry 

 chemical combinations have since been the principal fire retard- 

 ants used in the United States for combating wildfires. Several 

 studies have been conducted to better quantify the combustion- 

 retarding effectiveness of sulfates and phosphates, and to iden- 

 tify their basic fire-retarding mechanisms (George and Susott 

 1971; George and Blakely 1972; Browne and Tang 1%3; and 

 Eickner 1%2). Other phosphate-based retardants have since 

 been used extensively: Pyro, previously mentioned, and Fire- 

 Trol 931, made of 10-34-0 ammonium polyphosphate (Wood 

 1970; George 1971; George and others 1977). 



In recent years, many of the chemicals used to prepare 

 retardant formulations — basic retardant chemicals as well as 

 additives for coloring and corrosion inhibition — have increased 

 severalfold in price and, in some cases, have become difficult 

 or impossible to obtain. For example, for several years the 

 DAP used in retardants was produced as a byproduct from the 

 conversion of coal to coke. Phosphoric acid (H3 PO4) was used 

 to remove (scrub) ammonia bearing-off gases from manufactur- 

 ing effiuents. In this reaction, either mono- or diammonium 

 phosphate is produced, but unfortunately, the cost of using 

 these products has become prohibitive. MAP and DAP are also 

 manufactured by bubbling ammonia gas through H3PO4. Much 

 of the high cost of combining ammonia and acid is in the costs 

 of ammonia or nitrogen (N); therefore, it is more economical 

 to add only one ammonia (NHj) to each phosphate (PO4) to 

 make NH4H2P04(MAP). In other cases, retardant users are 

 searching for chemicals that are less expensive because of their 

 formulas and/or manufacturing processes, but which are still 

 cost-effective. 



This study quantified the fire-retarding effectiveness of 

 monoammonium phosphate chemicals from different sources 

 and compared their effectiveness to diammonium phosphate 

 (the basic fire retardant chemical in some currently approved 

 retardants). Tests were performed with five MAP samples 

 manufactured by various companies and/or processes. 



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