Thermal stability and decomposition products of propyl pentaborane heated at 147 to 190 deg 

Mc Donald, G.

English

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J 
(HEF-2) IN THE RANGE 147O TO 190° C d 
* ..’ 
e- 
By G. McDonald 
Flight Propulsion Labor a tory 
FOR AERONAUTICS 
WASHINGTON 
November 13, 1957 
 
. ... 
https://ntrs.nasa.gov/search.jsp?R=19650017030 2020-03-24T05:12:19+00:00Z
I 
NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS 
RESEARCH MEMORANDUM 
. 
TRERMAL STABILITY OF A COMMERCIAL PROPYT, PENTABORANE (REF-2) 
I N  THE RANGE 147' TO 190' C 
By G .  McDonald 
SUMMARY 
The thermal s t a b i l i t y  of a commercial grade of propyl pentaborane 
(BEF-2) w a s  invest igated a t  temperatures of I_L47' and 190'9. 
of decomposition are hydrogen, hydrocarbons, and a s o l i d  nonvolat i le  ye l -  
low boron hydride. 
The products 
The following equation w a s  developed from which may be ca lcu la ted  
the  approximate time required t o  decompose 20 percent of propyl pentaborane 
a t  any other  temperature: 
where T i s  temperature i n  OK and tZ0% is  the  time i n  minutes required 
f o r  decomposition of 20 percent of the sample. 
INTRODUCTION 
A s tudy of t h e  thermal s t a b i l i t i e s  of decaborane and of e t h y l  
decaborane (REF-3) w a s  previously completed a t  t h e  NACA Lewis  l abora tory  
(ref.  1) i n  order  t o  fu rn i sh  data per t inent  t o  fuel-system design and 
f u e l  s torage .  The s tudy reported herein w a s  made t o  obtain knowledge of 
t he  thermal s t a b i l i t y  of commercial propyl pentaborane (BEF-2) and t o  
compare t h i s  s t a b i l i t y  with the  previously measured s t a b i l i t y  of penta- 
borane (ref.  2) and decaborane ( r e f .  1) . Measurements were made of t h e  
amount of nonvola t i le  decomposition products formed when propyl penta- 
borane w a s  heated f o r  various times a t  temperatures of 147' and 190' C .  
APPARATUS, MATERIALS, AND PROCEDURF, 
The propyl pentaborane w a s  heated i n  a cy l ind r i ca l  s t a i n l e s s - s t e e l  
bomb 3 inches long having a n  outs ide diameter of 5/8 inch and an ins ide  
2 
? -  
e. ..e .e. 
0 .  * e  . * e  e .  . .e e. . e. e .e e: : : NACA RM E57H29 
e . .  g c .  .e .e .e e.. 
.' . ;. . 
diameter of 11/32 inch. 
The bomb w a s  closed with a 1/8-inch s t a i n l e s s - s t e e l  Hoke valve and heated 
i n  an aluminum block furnace,  which f i t t e d  c lose ly  about the  bomb. 
furnace w a s  e l e c t r i c a l l y  heated and su i tab ly  insu la ted .  
wel l  extended through the  block t o  the midpoint on the  bomb. 
The volume was approximately 1/4 cubic inch. 
The 
A thermocouple 
The propyl pentaborane used w a s  of commercial grade and consis ted 
of approximately 80 percent monopropyl pentaborane, 15 percent dipropyl 
pentaborane, and 5 percent pentaborane. The mater ia l  w a s  used without 
f u r t h e r  pu r i f i ca t ion .  
The "-2 or  propyl pentaborane (1.4 g) w a s  placed d i r e c t l y  in  the  
The weighed bomb. This t r a n s f e r  w a s  performed i n  an i n e r t  atmosphere. 
bomb w a s  sealed,  evacuated, weighed, and placed in  the furnace f o r  t he  
des i red  time. Af te r  removal from t h e  furnace,  the bomb w a s  quenched and 
the  v o l a t i l e  mater ia l  w a s  removed a t  a temperature of 110' C on a vacuum 
system u n t i l  constant weight w a s  obtained. The d i s t i l l a t i o n  temperature 
w a s  below the temperature of decomposition. The bomb w a s  reweighed t o  
determine the  nonvolat i le  res idue .  The percent formation of nonvolat i le  
res idue,  f o r  use i n  f igu res  1 and 2 ,  was ca lcu la ted  by dividing the  weight 
of nonvolat i le  res idue by the weight of i n i t i a l  f u e l  sample. 
of decomposition used f o r  t he  calculat ions and f o r  f igu re  3 w a s  determined 
frm t h e  r s t i o  n f  the percent of nonvolati le res idue a t  any time t o  the  
percent of nonvolat i le  residue a t  a time s u f f i c i e n t  f o r  complete 
decomposition. 
The percent 
IiESULTS AND DISCUSSION 
A curve of the  percent of formation of nonvolat i le  res idue p l o t t e d  
aga ins t  t i m e  is  shown i n  f igu re  1 f o r  propyl pentaborane a t  a temperature 
of 147O C. 
shows a decrease i n  r a t e  of decomposition with time. This r e l a t i o n  i s  i n  
con t r a s t  t o  the  near ly  steady r a t e  of formation of nonvolat i le  res idue,  
with increase i n  time, which w a s  observed with decaborane and commercial 
e t h y l  decaborane (FiEF-3) ( r e f .  1). 
of nonvola t i le  res idue p l o t t e d  against  time at  a temperature of 190° C 
f o r  propyl pentaborane is  shown in  f igure  2 .  This curve a t  190' C shows 
a shape s i m i l a r  t o  t h a t  of t he  curve t h a t  w a s  obtained a t  147' C .  Ex- 
periments a t  both temperatures were car r ied  out f o r  a t i m e  s u f f i c i e n t  t o  
decompose the  sample completely. 
S imi la r ly  t o  pentaborane (ref. 2),  t he  propyl pentaborsne 
A curve f o r  the  percent formation 
Figure 3 shows a p l o t  of t he  reciprocal  of Kelvin temperature aga ins t  
l o g ( t  20% - 2 )  where tzo$ is the time i n  minutes required f o r  de- t he  
composition of 20 percent of t he  sample. Two minutes has been subtracted 
from t h e  time required f o r  the  decomposition of 20 percent of t h e  sample 
i n  order  t o  remove from the  calculat ions the approximate time required 
t o  b r i n g  the  sample up t o  reac t ion  temperature ( r e f .  2 ) .  
I . 
8 
4 
I+ cn cn 
P 
3 
These da t a  f o r  propyl pentaborane may 2e.represented b y  t h e  following - i d  :. e quat ion : 
A 
P 
s 
where T i s  temperature i n  OK. The percent decomposition of propyl 
pentaborane is  not a l i n e a r  func t ion  o f  t i m e ,  i n  con t r a s t  t o  t h e  decompo- 
s i t i o n  observed f o r  decaborane. However, f o r  percentages of nonvolat i le  
res idue less than about 20 percent ,  the time required t o  form any given 
percent  of nonvolat i le  res idue may be approximated by assuming a l i n e a r  
r e l a t i o n  between percent  formation of nonvolat i le  res idue  and t i m e .  
The physical  p roper t ies  of t he  residue t h a t  i s  obtained from propyl 
pentaborane are similar t o  those obtained from the  pyro lys i s  of penta- 
borane both i n  color  and l ack  of a softening or melting poin t .  While 
t h e  residue from propyl pentaborane was not  inves t iga ted  as completely 
as w a s  t h e  residue from decaborane and e t h y l  decaborane (HEF-3), it is  
expected from t h e  cha rac t e r i s t i c s  observed t h a t  t he  res idue  i s  not  ex- 
t ens ive ly  soluble  i n  boron hydrides o r  a lky la ted  boron hydrides.  The 
ex ten t  of f u e l - l i n e  clogging with propyl pentaborane might be expected 
t o  5c aomcwhat mor2 than that o5served w i t h  pentalmrane heemse  of the 
decreased s t a b i l i t y .  As observed f r o m  f i g u r e  3, t h e  s t a b i l i t y  of propyl 
pentaborane is less than t h a t  of  pentaborane, decaborane, o r  e t h y l  
decaborane. Similar  t o  the  s t a b i l i t y  r e l a t i o n  between e t h y l  decaborane 
and decaborane, propyl pentaborane i s  less s t a b l e  than pentaborane. 
Similar  t o  t h e  experience with pentaborane, pressure  buildup occurs 
with propyl pentaborane because of the l i b e r a t i o n  of a mixture of hydrogen 
and hydrocarbons. 
temperature of s torage is  s u f f i c i e n t l y  slow t o  permit s torage  t o  be a 
p r a c t i c a l  operat ion.  
Below 100' C ,  pressure buildup is  slow and a t  normal 
CONCLUSIONS 
The r e s u l t s  ind ica te  that  a commercial grade of propyl pentaborane 
(HEF-E) i s  less s t ab le  t o  pyro lys i s  than pentaborane. Physical  p roper t ies  
of t h e  product of pyrolysis  are similar t o  those of t he  product of pyroly- 
s is of pentaborane . 
L e w i s  F l i g h t  Propulsion Laboratory 
National Advisory Committee for Aeronautics 
Cleveland, Ohio, August 30, 1957 
4 
REFERENCES 
1. McDonald, G .  E . :  Thermal S t a b i l i t y  of Decaborane and of a Commercial 
E thyl  Decaborane (HEF-3) i n  t h e  Range 202' t o  252' C .  
E56124, 1956. 
NACA RM 
2 .  McDonald, G .  E .  : Thermal S t a b i l i t y  of Pentaborane i n  t h e  Range 329' 
t o  419' F.  NACA RM E54G16, 1956. 
P cn cn 
P 
c 
0 .  0 ) .  . ... . 0 . .  . 5 NACA RM E 5 7 ~ ~ 9  ..a e 
c 
6 8 10 12 14 16 18 
Figure 1. - Decomposition of propyi pentaborane a t  147' C. 
6 
0 2 4 5 8 10 12 14 16 18 
Time, hs 
Figure 2. - Decomposition of propyl pentaborane at 1900 C. - 
t 
NACA RM E57H29 
r Decaborane --- Ethyl decaborane - -  - - -  Pentaborane 
-- Propyl pentaborane 
2.0 2.1 2.2 2.3 
Reciprocal of temperature, 1/T,  OK 
Figure 3 .  - Reciprocal temperature p lo t ted  aga ins t  t i m e  
required fo r  2 0  percent decomposition. 
7 
NACA - Langley Flold, Va. 



Thermal stability of a commercial propyl pen- taborane /hef-2/ in the range 147 deg to 190 deg c

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