Mechanistic Studies of Halogenated Flame Retardants: The Antimony

4 Mechanistic Studies of Halogenated Flame Retardants: The Antimony-Halogen System JOHN W. HASTIE and C. L. McBEE

Downloaded by UNIV OF AUCKLAND on May 3, 2015 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0016.ch004

Inorganic Chemistry Section, Institute for Materials Research, National Bureau of Standards, Washington, D. C. 20234

Introduction In a d d i t i o n to the prominent use o f v a r i o u s halocarbons as flame e x t i n g u i s h i n g agents there e x i s t s another c l a s s of halogenated flame r e t a r d a n t s which, a t present, i n c o r p o r a t e s p r i m a r i l y the a d d i t i o n a l elements o f Sb and P. Despite the i n c o n c l u s i v e mechanistic understanding f o r halogen induced flame i n h i b i t i o n i n general, there is reason t o b e l i e v e t h a t the mechanistic a c t i o n o f halocarbons, e.g., CF Br, and halogenated Sb- o r P-containing species, e.g., SbCl , SbBr o r P O C l , has a common molecular b a s i s (1). For both r e t a r d a n t c a t e g o r i e s , the primary f u n c t i o n o f the non-chloride o r bromide moiety appears t o be its ability to serve as a convenient c a r r i e r f o r the flame i n h i b i t i n g c h l o r i d e or bromide component t o the flame f r o n t . However, some s y n e r g i s t i c e f f e c t s are observed. For i n s t a n c e , the r e d u c t i o n i n burning v e l o c i t y is f a r g r e a t e r f o r s p e c i e s such as S b C l than f o r the e q u i v a l e n t amount o f Cl (2). S i m i l a r l y , f o r C F B r the observed degree o f flame i n h i b i t i o n exceeds t h a t expected f o r the equival e n t amount o f Br. In order to understand such s y n e r g i s t i c systems it is necessary t o separate the i n d i v i d u a l mechanistic e f f e c t s f o r each a c t i v e component. The p r e s e n t study attempts t o d e f i n e the flame i n h i b i t i n g mechanism f o r the s y n e r g i s t i c i n t e r a c t i o n o f antimony and halogens, with p a r t i c u l a r emphasis on systems i n v o l v i n g [Sb O3] and c h l o r i n a t e d hydrocarbons (square brackets denote s o l i d s t a t e compounds). There are, we b e l i e v e , two main f a c t o r s governing the flame r e t a r d i n g performance o f the halogenated Group V systems i n comm e r c i a l use. F i r s t , in practical retardancy f o r m u l a t i o n s the t h e r modynamic and chemical k i n e t i c p r o p e r t i e s o f the s u b s t r a t e , whereby volatile f u e l and i n h i b i t o r s p e c i e s are produced, are of g r e a t s i g n i f i c a n c e . Thus, f o r example, the in situ i n t e r a c t i o n o f HCl or HBr with a polymer i n c o r p o r a n t o f [Sb O ] to yield volatile species such as SbOBr, SbBr , SbCl , and SbOCl p r o v i d e s the means f o r i n t r o d u c i n g flame i n h i b i t i n g s p e c i e s t o the flame f r o n t . The thermodynamic and k i n e t i c parameters f o r such s u b s t r a t e i n t e r a c - 3

3

3

3

3

3

2

2

3

3

3

118

In Halogenated Fire Suppressants; Gann, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

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HASTTE AND M c BEE

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Antimony-Halo gen System

t i o n s w i l l t h e r e f o r e be considered i n the d i s c u s s i o n which f o l l o w s . The second factor? namely, the mechanistic a c t i o n o f these v o l a t i l e halogen-containing s p e c i e s w i t h i n the flame, w i l l a l s o be i n v e s t i gated. The l a b o r a t o r y flames o f i n t e r e s t i n c l u d e p r i m a r i l y the atmospheric premixed systems o f H2-O2-N2 and ΟΗι*-θ2-Ν2, burning under f u e l r i c h c o n d i t i o n s . Such flames a r e considered reasonable model systems o f " r e a l - f i r e " chemical k i n e t i c phenomena, as has been argued elsewhere (.1).


Apparatus

and General Experimental

Procedure

Very s p e c i a l i z e d experimental techniques a r e r e q u i r e d t o char a c t e r i z e t h e b a s i c molecular processes where both moderate temper ature p y r o l y s i s and h i g h temperature f l a m i n g combustion occur. We u t i l i z e a combination o f mass spectrometric and o p t i c a l s p e c t r o s c o p i c techniques f o r the c h a r a c t e r i z a t i o n o f flame i n h i b i t i o n phenomena. A d e t a i l e d d e s c r i p t i o n o f our mass s p e c t r o m e t r i c appar atus and procedures f o r sampling r e a c t i v e s p e c i e s from atmospheric pressure flames, has a l r e a d y been g i v e n elsewhere (3,4). F i g u r e 1 p r o v i d e s a schematic o f t h i s system i n i t s flame sampling mode o f o p e r a t i o n . A Knudsen r e a c t o r system i s used as the primary means of c h a r a c t e r i z i n g s u b s t r a t e p y r o l y s i s phenomena. F o r the present study, e s t a b l i s h e d o p t i c a l s p e c t r o s c o p i c procedures a r e used t o monitor the s p e c i e s H, OH and SbO i n atmospheric pressure flames. The Knudsen Reactor System. A convenient technique f o r the study o f p y r o l y s i s phenomena u t i l i z e s a s o - c a l l e d Knudsen e f f u s i o n r e a c t o r i n combination with t h e molecular beam mass spectrometer. A Knudsen c e l l serves as a temperature- and p r e s s u r e - c o n t r o l l e d i n e r t c o n t a i n e r f o r the r e a c t i n g system. The Knudsen c e l l i s heated r a d i a t i v e l y by a r e s i s t a n c e furnace as shown i n F i g u r e 2. Substrate samples a r e contained i n an alumina cup w i t h i n the Knudsen c e l l . Note ( i n F i g u r e 2) the presence o f a gas i n l e t l i n e to the Knudsen c e l l . T h i s allows f o r the e x t e r n a l c o n t r o l o f gaseous components such as HC1 and H2O. P r o v i d i n g the t o t a l gas pressure w i t h i n the c e l l does not exceed about 1 0 " atm and an o r i f i c e area o f < 1 0 " cm i s used, an e f f u s i n g molecular beam may be generated from such a c e l l . T h i s beam having, by d e f i n i t i o n ,a composition r e p r e s e n t a t i v e o f t h a t f o r the vapor contained by the c e l l can then be c o n v e n i e n t l y analyzed u s i n g the l i n e - o f - s i g h t mass spectrometric d e t e c t o r . The molecular beam i s d i r e c t e d along the center a x i s o f the vacuum space denoted as r e g i o n I I i n F i g u r e 1. Upon e n t e r i n g t h e mass spectrometer chamber (see F i g u r e 1) the molecular beam i s chopped by a mechanical wheel, p a r t i a l l y convert ed t o p o s i t i v e ions by e l e c t r o n impact, mass s e l e c t e d and d e t e c t e d using frequency-dependent l o c k - i n phase s e n s i t i v e d e t e c t i o n methods as d e s c r i b e d elsewhere (_3) . 3

2

2

From the theory o f Knudsen e f f u s i o n mass spectrometry e.g., see Grimley (5), the magnitude o f the d e t e c t e d i o n s i g n a l i s r e l a t e d t o the p a r t i a l pressure f o r the p r e c u r s o r s p e c i e s by


HALOGENATED FIRE SUPPRESSANTS

PHASE SENSITIVE


DETECTOR

Combustion and Flame

Figure 1. Schematic of molecular beam mass spectrometric system for sampling 1-atm flames (indicated) and effusing vapor from a Knudsen cell (not shown but normally located in region II) (3)

T O MASS FILTER

EFFUSION B E A M

SAMPLE

KNUDSEN C E L L

I

WATER C O O L E D Cu- JACKET Ta - RESISTANCE F U R N A C E & SHIELD

PUMPING PORTS WATER COOLED Cu E L E C T R O D E S

E X T E R N A L G A S INLET THERMOCOUPLE

Figure 2. Knudsen reactor assembly located in the vacuum space denoted as region II in Figure 1


4.

Antimony-Halogen System

HASTIE AND MC BEE

121

Ρ = k l T, where Ρ i s the p a r t i a l pressure f o r the species w i t h i n the Knudsen c e l l , I i s the i n t e n s i t y of the mass analyzed detected p o s i t i v e i o n s i g n a l , Τ i s the temperature o f the Knudsen c e l l , and k i s a c a l i b r a t i o n constant i n d i c a t i v e o f the instrument s e n s i t i v i t y and the nature o f the e l e c t r o n impact process. T y p i c a l values of k f o r our apparatus f a l l i n the r e g i o n of 1 0 " - 1 0 " atm/pV Κ ( f o r 1 0 Ω). In our s t u d i e s k i s determined by two separate methods. E i t h e r I i s measured f o r a s p e c i e s of known ?{e.g. SbÔg(6)}, or use i s made o f the r e l a t i o n : η

1 2

7


y

k =

2.2557x10 * χ w AMV^(ITV^t)

.„ a

t

m

/

v

W

where w i s the t o t a l weight l o s s i n gm, A i s the Knudsen o r i f i c e area i n cm , At i s the time i n t e r v a l i n s, M i s the gm molecular weight, and Τ i s the temperature i n K, e.g., see Drowart and Goldfinger ( 2 ) . A f u r t h e r c o n s i d e r a t i o n i n making the conversion of i o n i n t e n s i t y to p a r t i a l pressure data i n v o l v e s the assignment of ions t o t h e i r proper n e u t r a l p r e c u r s o r s . T h i s i s a r e l a t i v e l y s t r a i g h t foreward process f o r systems where a s i n g l e vapor species i s present, e.g., f o r Sbj+Og. But f o r a system c o n t a i n i n g both SbOCl and SbCl3, ions such as S b C l and S b could be d e r i v e d from each s p e c i e s . In order to r e s o l v e such ambiguous i o n i n t e n s i t y data we a d j u s t the r e a c t i o n c o n d i t i o n s o f HCl pressure and temperature t o favor o n l y the presence o f SbCl3, f o r i n s t a n c e . A summary o f our ion-precursor assignments i s g i v e n i n the Appendix. We assume t h a t the t o t a l i o n i z a t i o n c r o s s s e c t i o n s are e q u i v a l e n t f o r each molecular s p e c i e s o f i n t e r e s t . An u n c e r t a i n t y of up t o a f a c t o r of two i n s p e c i e s p a r t i a l pressure i s l i k e l y to r e s u l t from t h i s unavoidable s i m p l i f i c a t i o n . As the mass spectrometer serves t o i d e n t i f y the v a r i o u s gaseous components o f a r e a c t i n g system, the determination o f Ρ for each species allows e q u i l i b r i u m constants and hence r e a c t i o n f r e e - e n e r g i e s t o be c a l c u l a t e d . Furthermore, as the temperature i s a c o n t r o l l e d v a r i a b l e , the observed temperature dependence of the e q u i l i b r i u m constant allows r e a c t i o n enthalpy and entropy data to be determined. To summarize the b a s i c thermodynamic concepts, the e q u i l i b r i u m constant Κ i s a f u n c t i o n o f the r e a c t a n t and prod uct s p e c i e s p a r t i a l pressures, and: 2

+

dftnK d(l/T) AF

+

-AH

R

= -RTitnK =

AH-TAS,

where A H , A S and A F are the r e a c t i o n enthalpy, entropy and f r e e energy r e s p e c t i v e l y a t the temperature T; R i s the u n i v e r s a l gas


122

HALOGENATED

FIRE SUPPRESSANTS

constant. I t should be noted t h a t the residence time f o r a molecular s p e c i e s i n our Knudsen c e l l r e a c t o r i s t y p i c a l l y i n the r e g i o n o f 10-^-10- s. During t h i s time the gas s p e c i e s undergo approximately 10 c o l l i s i o n s with the sample s u r f a c e . Only i n e x c e p t i o n a l cases are these c o n d i t i o n s unfavorable to the attainment o f thermodynamic e q u i l i b r i u m w i t h i n the c e l l . A convenient t e s t f o r the presence o f e q u i l i b r i u m w i t h i n the c e l l i s p r o v i d e d by the o b t a i n ment o f p a r t i a l pressure and apparent e q u i l i b r i u m constant data t h a t are independent of the Knudsen o r i f i c e diameter. We use two separate hole diameters o f 0.1 cm and 0.025 cm f o r t h i s t e s t . As a f u r t h e r t e s t , the e q u i l i b r i u m constant should be independent of pressure f o r constant temperature c o n d i t i o n s . 5


9

The O p t i c a l Spectroscopic System. O p t i c a l s p e c t r o s c o p i c techniques may be used t o monitor flame temperatures and c e r t a i n atomic and diatomic species (8). Our apparatus and procedures are s i m i l a r to those used by other workers (9,10). The method o f monitoring H atom cone e n t r a t i o n s i s an i n d i r e c t although w e l l e s t a b l i s h e d one (10). B a s i c a l l y we r e l y on the presence o f the f o l l o w i n g balanced r e a c t i o n i n L i c o n t a i n i n g flames: Li + H 0 2

= LiOH + H

f o r which the e q u i l i b r i u m constant i s known, as i s the H2O concent r a t i o n . The c o n c e n t r a t i o n r a t i o Li/LiOH may be experimentally determined by comparing the resonance emission i n t e n s i t i e s f o r Na and L i where the t o t a l Na and L i c o n c e n t r a t i o n s are made equal. Hence the H atom c o n c e n t r a t i o n can be d e r i v e d . We a l s o make use of an a l t e r n a t i v e method, i n v o l v i n g the measurement of CuH emission (428.3 nm) i n t e n s i t i e s as a measure of r e l a t i v e H atom concentrat i o n s (9) . Flame temperatures are determined with the same apparatus using the Na D - l i n e r e v e r s a l technique. Flame r e t a r d a n t species are introduced t o the flame e i t h e r by t r a n s p i r a t i o n o f vapor with the premixed combustion gases, or by entrainment o f n e b u l i z e d aqueous s o l u t i o n s . The n e b u l i z e r used i s of the type d e s c r i b e d by Mavrodineanu and Boiteux (11). Experimental R e s u l t s f o r Substrate Reactions Reaction of Ethylene C h l o r i n a t e d Polymer (40% CI) w i t h [SbpQa]A powdered mixture (50/50 by weight) of an ethylene c h l o r i n a t e d polymer with [Sb 03] can be taken as a r e p r e s e n t a t i v e example of commercial r e t a r d a n t formulations i n v o l v i n g the i n t e r a c t i o n of a halogen source with [Sb2Û3] or r e l a t e d antimony compounds. S i m i l a r mixtures have been c h a r a c t e r i z e d by Touval (12), among o t h e r s , using thermogravimetric a n a l y s i s (TGA) and d i f f e r e n t i a l thermal a n a l y s i s . A l s o the e f f e c t i v e n e s s o f such mixtures as flame r e t a r d ants i s e m p i r i c a l l y w e l l known, e.g. see the review o f P i t t s (13). 2

3



4.

123


HASTiE AND MC BEE

From the TGA r e s u l t s (12), i t i s known t h a t about 76% o f the i n i t i a l [Sb203l component i s l o s t by a halogen induced v a p o r i z a t i o n p r o cess. Using the Knudsen r e a c t o r mass spectrometric technique we can e s t a b l i s h the molecular d e t a i l s o f t h i s process. T y p i c a l experimental r e s u l t s are shown i n F i g u r e 3 , together with the TGA curve o f Touval (12) f o r comparison. Note t h a t the l o s s o f [Sb2Û3] from the s u b s t r a t e mixture occurs p r i m a r i l y over the temperature i n t e r v a l o f 250-450 °C and i n the form o f SbCl3 and SbOCl molecular s p e c i e s . In t h i s experiment the i n i t i a l mixture had a composition e q u i v a l e n t to a 3 . 3 mole r a t i o o f HCl/ [Sb203], and from an a n a l y s i s o f the end-of-run r e s i d u e ( p r i m a r i l y [Sb203]) i t was found t h a t the corresponding mole r a t i o l o s t by v a p o r i z a t i o n was 5.0. By comparison, the s i m i l a r TGA data i n d i cated about a 4 . 3 mole r a t i o l o s s o f HCl t o [Sb2Û3]. From our o b s e r v a t i o n o f SbCl3 and SbOCl as the p r i n c i p a l antimony-containing vapor s p e c i e s , and from the thermodynamic prope r t i e s o f these s p e c i e s (see Appendix), the 0\)WJJUL v a p o r - f o r ming g a s - s o l i d r e a c t i o n s can be w r i t t e n as: + [Sb^]

2HC1

+ [ S b 0 ] = 2Sb0Cl + H 0 2

= 2SbCl

+ 3H 0

6HC1

3

3

2

2

(1) (la)

Thus i f these r e a c t i o n s proceeded completely to the r i g h t , r e a c t i o n s (1) and (la) would r e s u l t i n the u t i l i z a t i o n o f HCl/[Sb2(>3] mole r a t i o s o f 6 and 2, r e s p e c t i v e l y . Under c o n d i t i o n s o f comp l e t e r e a c t i o n , the observed r a t i o o f 5 would r e q u i r e r e a c t i o n (1) to be three times more e f f e c t i v e than r e a c t i o n (la) i n t r a n s p o r t i n g antimony t o the vapor phase. A check on these mass-balance cons i d e r a t i o n s i s p r o v i d e d by the areas under the SbOCl and SbCl3 pressure-temperature curves o f F i g u r e 3 . These curves i n d i c a t e an o v e r a l l r a t i o o f SbCl3/SbOCl ^2.0 which compares reasonably with the mass balance p r e d i c t i o n o f 3.0, p a r t i c u l a r l y as the r e a c t i o n s are not completely t o the r i g h t . In order t o express the pressure-temperature data, contained i n the curves o f F i g u r e 3 , i n terms t h a t may r e a d i l y be t r a n s f e r r e d to o t h e r experimental or even " r e a l - f i r e " c o n d i t i o n s i t i s necessary t o d e f i n e apparent e q u i l i b r i u m constants f o r the v a r i ous p o s s i b l e r e a c t i o n s . Thus, f o r r e a c t i o n (1) we have: 2

(PSbCl ) (PH 0) 3

3

2

K, = 1

(PHC1)

6

a[Sb 0 ] 2

3

where Ρ represents s p e c i e s p a r t i a l pressure and a[Sb2Û3] i s the thermodynamic a c t i v i t y o f condensed [Sb203] which, by d e f i n i t i o n , i s u n i t y f o r the pure [Sb2Û3] phase. For the present, we c o n s i d e r K^ as an "apparent" e q u i l i b r i u m constant s i n c e we have y e t t o e s t a b l i s h whether r e a c t i o n (1) i s a t e q u i l i b r i u m . A l s o , [Sb2Û3] i s assumed f o r the moment t o be p r e s e n t a t u n i t a c t i v i t y . Values of K^ as a f u n c t i o n o f temperature are g i v e n i n F i g u r e 4. S i m i l a r l y ,



124

H A L O G E N A T E D FIRE SUPPRESSANTS

Figure 3. Partial pressure of volatile species resulting from the thermal interaction (heating rate 2°C/min) of an ethylene chlorinated polymer (40% CI, Borden Chemical) with an equal weight of [Sb O li. Thermogravimetric analysis curve of Touval (12) is also shown for comparison. Curve for Shfie is based on absolute pressure data given by Jungermann and Plieth (6). 2

TEMPERATURE

c

e

s


4.


HASTiE AND MC BEE

125

values o f K3 f o r the homogeneous r e a c t i o n : SbCl

3

+ H0 2

= SbOCl + 2HC1

(3)

are a l s o i n d i c a t e d i n the F i g u r e 4. Data f o r K\ have not been p l o t t e d as t h i s r e a c t i o n i s merely another form of r e a c t i o n s (1) and (3). Note t h a t Κ χ i s p a r t i c u l a r l y s e n s i t i v e to the temperature and the absence o f a s i n g l e monotonie dependence of Κ χ on the r e c i p r o c a l temperature i s i n d i c a t i v e of the presence o f a d d i t i o n a l r e a c t i o n s , other than r e a c t i o n (1), i n v o l v i n g the vapor s p e c i e s S b C l , HCl and H 0. a


3

2

Reaction of HCl with [SbÔ^]. In order to determine the nature o f the a d d i t i o n a l r e a c t i o n s i n f e r r e d above, i t i s necessary to c o n t r o l the t o t a l pressure o f HCl e x t e r n a l l y r a t h e r than by the i n t e r n a l r e l e a s e mechanism provided by the p y r o l y s i s of the c h l o r i n a t e d polymer. F i g u r e 5 i n d i c a t e s the nature o f the Κ χ tempera ture dependence'for t o t a l HCl pressures i n the r e g i o n o f 10~ -10" atm over an i n i t i a l l y pure [Sb 03] s u b s t r a t e . Note t h a t Κχ reaches a maximum value i n the r e g i o n of 420°C which i s a s i m i l a r r e s u l t to t h a t obtained f o r the c h l o r i n a t e d polymer -[Sb 03] system. Again, more than one process f o r the production o f SbCl3 i s evident from the slope change of the l o g Κ χ versus T"" curve. I t i s noteworthy that the slope o f t h i s curve between temperatures o f about 420 and 520°C y i e l d s a r e a c t i o n enthalpy of -20 kcal/mol, i n agreement w i t h a value based on the l i t e r a t u r e heats o f formation f o r the v a r i o u s r e a c t i o n components (see Appendix ). F i g u r e 5 a l s o i n d i c a t e s the dependence of Κ3 on temperature f o r the same system. The molecularity#or s t o i c h i o m e t r i c dependences, of the r e a c t i o n ( s ) l e a d i n g t o the formation of SbCl3 and SbOCl can be i n f e r r e d from the dependence of the products p a r t i a l pressure on the r e a c t a n t p r e s s u r e f o r isothermal c o n d i t i o n s . For i n s t a n c e , i f r e a c t i o n (1) i s predominant i n forming SbCl3 and Κχ i s pressure independent {i.e. the system i s a t e q u i l i b r i u m ) then the pressure of SbCl3 should vary as the 6/5 power of the HCl p a r t i a l p r e s s u r e . T h i s r e s u l t assumes that H 0 i s formed p r i m a r i l y by r e a c t i o n (1) and i s not e x t e n s i v e l y i n v o l v e d i n secondary processes such as r e a c t i o n (3). As i s shown i n F i g u r e 6 the pressure o f SbCl3 shows the expected 6/5 power dependence with HCl pressure thus i n d i c a t i n g r e a c t i o n (1) t o be the main overall r e a c t i o n f o r these p a r t i c u l a r conditions. The r e a c t i o n s (1) and (3) may not be i n a s t a t e o f general thermodynamic e q u i l i b r i u m . A c o n d i t i o n o f g e n e r a l thermodynamic e q u i l i b r i u m r e q u i r e s Κ t o be pressure independent f o r constant Τ and t h a t the v a r i a t i o n o f Κ with Τ y i e l d a second law r e a c t i o n enthalpy i n agreement with the corresponding t h i r d law value. In p r a c t i c e , we f i n d t h a t Κχ decreases w i t h i n c r e a s i n g Ρ(HCl) where Τ < 420°C. The value of K3 i n c r e a s e s w i t h i n c r e a s i n g Ρ(HCl) where Τ SbOCl + 2HC1

5[SbOCl] -> S b C l

3

4.

2

H 0 + SbCl

3

3.

2

2

[T=693] 97

15.8[Τ=693]-45.8

32

4.5[T=640]^16

8.2[T=743]

8.2[T=663]

8.7[T=538]^25

18.7[T=693] 40

-14[T=700]

+ H0

2

2b. 2HC1 + 3 [ S b 0 ] -> 2 [SbOCl ' S b ^ ]

3

2

-17[T=600]

2

3

+ H0

2

-11 [T=693] -20

2a. 2HC1 + 2 [ S b 0 ] -> [2Sb0Cl - S b ^ ]

2

+ 3H 0 -26[T=500]

3

2HC1 + [ S b 0 ] -> 2 [SbOCl] + H 0

3

2.

2

6HC1 + [ S b 0 ] -> 2 S b C l

Obs.

1.

Reaction τ

63

-17

-23

-20

m/

23 [T=693]

44

41

103

33

(β/

168Î/

*

[T=298]

* 35^

41[T=693]

19[T=693]

42[T=590]

27[T=693]

m

τ

64

16

m/

36 [T=590]

44[T=693]

AS L i t . Obs. L i t .

76&30[T=526] 25

0[T=693]

-20[T=693]

Obs.

ΔΗ

Summary o f main r e a c t i o n s i n the antimony oxide-halogen s u b s t r a t e systems.

Table I


4.

HASTTE AND MC BEE

Footnotes

131


f o r Table I

1. A l l l i t e r a t u r e data are based on the thermodynamic parameters given i n the Appendix. m. From the observed Δ Η f o r r e a c t i o n s 4(a) and (b) we can show t h a t f o r these data to be s e l f c o n s i s t e n t , AH [SbOCl-Sb 0 ] = -356 kcal/mol; t h i s i s w i t h i n the u n c e r t a i n t y f o r the reported l i t e r ature value (see the Appendix). n. Based on an estimated S 98[SbOCl] = 37±5 cal/deg mol. p. By analogy with above r e a c t i o n 2. q. From S [ S b O C l ] = 71.6±0.8 cal/deg mol (cale.) and S s [ S b O C l ] = 37 cal/deg mol ( e s t . ) . r. Based on AS f o r r e a c t i o n s 2 and 2a and the l i t e r a t u r e S g8 f o r S b C l , HCl, [ S b 0 ] , and H 0. s. Based on As f o r r e a c t i o n 2a. t. Using AH [SbOCl] = -28.7 kcal/mol (see Appendix), u. Our P(Sbi 0e) data, determined using the t o t a l v a p o r i z a t i o n method o f c a l i b r a t i o n f o r the k - f a c t o r , were found to be i n s a t i s f a c t o r y agreement with the vapor pressure data g i v e n by Jungermann and P l i e t h (6) f o r senarmonite. Τ

f

2

3

2

29


2 9 8

2

3

2

3

2

f

t

Mass Transport o f Retardant

to Flame Front

C o n t r o l l i n g F a c t o r s . Under a c t u a l f i r e c o n d i t i o n s the r e lease and t r a n s p o r t o f a flame i n h i b i t i n g s p e c i e s , o r some s u i t able molecular precursor, from a decomposing polymer substrate to the flame w i l l be determined p r i m a r i l y by: - the p a r t i a l vapor pressure f o r the i n h i b i t o r a t the polymer s u r f a c e , and - the formation o f a d i f f u s i v e b a r r i e r or boundary l a y e r a t the surface-gas i n t e r f a c e which l i m i t s the t r a n s p o r t o f r e t a r d a n t to the flame. These flame i n h i b i t i o n process-determining-factors w i l l i n t u r n be dependent upon the temperature, the r a t e o f bulk gas flow across the s u r f a c e , the e f f e c t i v e thickness o f the flame f r o n t , and the thermodynamic a c t i v i t y o f r e t a r d a n t i n the polymer s u b s t r a t e . Consider f i r s t the p o s s i b l e l i m i t a t i o n o f S b C l and HCl t r a n s p o r t from the s u b s t r a t e to the flame due to boundary l a y e r forma t i o n between the v a p o r i z i n g s u r f a c e and the bulk gas, as shown s c h e m a t i c a l l y i n F i g u r e 10. The e f f e c t o f such a b a r r i e r would be to reduce the f l u x o f r e t a r d a n t reaching the bulk gas and hence the flame f r o n t . From F i c k * s law o f mass d i f f u s i o n (15) and the accepted concept of a boundary l a y e r we have: 3

J ( S b C l ) = (DM/6RT)[P(surface)-P ( b u l k ) ] , 3


132

Σ.

4

0

HALOGENATED

ο Ε

Γ

8

20h

SbCI,


(SbOCI)

FERE

SUPPRESSANTS

(4) obs

(2 SbOCl · (SbOCl* Sb,0.)

TEMPERATURE °C

Figure 9. Reaction free energy dia gram showing the main reaction products at various temperatures, based on data in Table I. Condensed phases indicated by parentheses.

VERTICAL DISTANCE FROM SUBSTRATE SURFACE

Figure 10. Idealized fire system including a boundary layer between substrate and flame


4.


HASTiE AND MC BEE

133

2 where J(SbCl3) i s the f l u x o f SbCl3 (units g/cm - s e c ) , D i s the d i f f u s i o n c o e f f i c i e n t o f S b C l i n the atmospheric gas (mainly N ) * i n u n i t s o f cm sec" , δ i s the e f f e c t i v e t h i c k n e s s o f the boundary l a y e r (cm) Ρ(surface) = P(SbCl3) a t the s u r f a c e , and Ρ (bulk) = P(SbCl3) i n the bulk gas (units atm). In the absence o f such a d i f f u s i o n l i m i t e d boundary the f l u x would be g i v e n by the Langmuir expression f o r f r e e molecular v a p o r i z a tion: J = P/[2.557x10 ( T / M ) ' ] g / c m -sec. 3

2

2

D i f f u s i o n C o e f f i c i e n t s f o r S b C l and HCl. The d i f f u s i o n c o e f f i c i e n t f o r a s p e c i e s i n a gas mixture can be estimated from the e x p r e s s i o n :


?

= 2.62xl0" [T (Mi+M )/2MiM 3 3

D

3

2

1

2

Ρ σ 12

2

1/2

c m

2

s e c

-l

2

as d e r i v e d from hard sphere ( i . e . , a d i a b a t i c e l a s t i c c o l l i s i o n s ) k i n e t i c theory, e.g., see H i r s c h f e l d e r e t a l . [p. 544 (16)]. The average c o l l i s i o n diameter 2 between u n l i k e s p e c i e s (e.g. N and SbClg) i s estimated from the e x p r e s s i o n : 2

( σ

°12 "

ΐ

+

0

2

) / 2

A

'

°

Values o f the molecular diameters

and

are taken a s :

a(SbCl~) i>7.OA ο

J

a(HCl) = 2.54 A, and σ ( Ν ) = 2.10 A, 2

using molecular parameters g i v e n by Sutton (17.) · 350 °C, D(SbCl

3

Therefore, a t

2 -1 i n N ) = 0.28 cm sec , 2

and D(HC1

2 -1 i n N ) = 1.34 cm sec . 2

Boundary Layer Thickness δ. An upper l i m i t to the value o f δ i s g i v e n by the flame s t a n d - o f f d i s t a n c e , which i s t y p i c a l l y ob served t o be o f the order o f 0.1 cm (21). A l t e r n a t i v e l y , i f we assume t h a t the common gas dynamic case o f laminar flow over a f l a t p l a t e a p p l i e s here, then δ can be estimated from the exprès6 = 1.5 L H - ^ V S R

1

7

2

'


134


where L i s the l e n g t h o f t r a v e l by the gas over the p l a t e , Ν Ν

= v/Dp

(Schmidt number),

= VLp/v

(Reynolds number), and

R


where V i s the bulk gas v e l o c i t y ν i s the c o e f f i c i e n t o f v i s c o s i t y f o r the gas, and ρ i s the gas d e n s i t y (p = PM/RT), e.g., see Graham and Davis (18) and Turkdogan e t a l . (19). To a good approximation both ν and ρ can be taken as the e s t a b l i s h e d values f o r a i r i . e . , -4 ν = 3.18x10 gm/cm-sec and -4 Q ρ = 5.6x10 gm/cm* a t 350

°C

[Handbook o f Chemistry and Physics (20)]. The bulk gas v e l o c i t y V i s an a d j u s t a b l e q u a n t i t y . For the p r e s e n t purpose we assume a value o f : V = 10 cm sec , which i s s i m i l a r t o the experimental values o f Hirano e t a l . (21) f o r a burning cardboard system. T h i s choice o f V i s not p a r t i c u l a r l y c r i t i c a l t o the arguments which f o l l o w . The e f f e c t i v e sample l e n g t h L i s a l s o an a d j u s t a b l e parameter. In more c l a s s i c a l boundary l a y e r problems, L i s d e f i n e d as the sample l e n g t h and may be e x p e r i m e n t a l l y measured. However, f o r a propagating f i r e system some g e o m e t r i c a l assumptions are r e q u i r e d i n determining L. In the absence o f d i r e c t evidence, the f o l l o w ing arguments are used to estimate l i k e l y v a l u e s f o r L. Since the phenomenon o f flame spread allows f o r a c o n t i n u a l replenishment o f f u e l and heat across the s u r f a c e , the c o n v e c t i v e movement o f gas i s maintained a t the flame f r o n t and a small e f f e c t i v e value f o r L can t h e r e f o r e be expected. Now a f i r e can be considered as an aggregation o f i n d i v i d u a l flame elements. Each flame element w i l l c o n s i s t o f a pre-flame, r e a c t i o n zone and burnt gas r e g i o n , as suggested i n F i g u r e 10. For purposes o f flame propagation and i n h i b i t i o n , the r e g i o n near and a t the r e a c t i o n zone w i l l be o f primary s i g n i f i c a n c e . T h i s i s the r e g i o n where the c o n c e n t r a t i o n o f propagating r a d i c a l s , such as H, i s i n a p p r e c i a b l e excess o f the thermodynamic e q u i l i b r i u m v a l u e . I t i s known t h a t f o r e i t h e r d i f f u s i o n o r premixed f u e l - r i c h CH^-air flames the t h i c k n e s s o f t h i s r e g i o n v a r i e s between about 0.1-0.4 cm. Likewise, the con c e n t r a t i o n o f suspected i n h i b i t o r s p e c i e s such as HCl, HBr o r SbO i s predominant i n t h i s r e g i o n , as i n d i c a t e d i n F i g u r e 10 (22). Thus as the flame f r o n t moves across the combustible s u r f a c e the passage o f an i n h i b i t o r , such as SbCl , through the boundary l a y e r


4.

HASTiE

AND M C BEE


135

i s l i k e l y t o be most s i g n i f i c a n t over a d i s t a n c e o f about 0.4 cm, c o i n c i d i n g with the passage o f the pre-flame and r e a c t i o n zone r e g i o n s . Given the non-ideal geometry o f r e a l f i r e s , t h i s d i s t a n c e c o u l d vary and an estimated f a c t o r o f three u n c e r t a i n t y i n L should be allowed f o r . From the above c o n s i d e r a t i o n s , a v a l u e o f the boundary l a y e r t h i c k n e s s δ i s c a l c u l a t e d as: δ = 0.24

cm f o r V = 10 cm/sec .


This d i s t a n c e i s o f a s i m i l a r order o f magnitude as the flame s t a n d - o f f d i s t a n c e which tends to support our i n t e r p r e t a t i o n o f L f o r flame-spread a p p l i c a t i o n s .

F l u x P r e d i c t i o n s . With these estimated data f o r D and δ, e i t h e r the mass t r a n s p o r t f l u x or the p a r t i a l pressure d i f f e r e n t i a l across the boundary l a y e r can be c a l c u l a t e d from the f l u x r e l a t i o n s h i p g i v e n above. The p a r t i a l pressure d i f f e r e n t i a l f o r an i n h i b i t o r s p e c i e s , such as SbCl3 , can be estimated Λ pKLOKL provided there are no k i n e t i c l i m i t a t i o n s t o the movement o f SbCl3 and HCl through the polymer s u b s t r a t e to the v a p o r i z i n g s u r f a c e . T h i s i n v o l v e s a determination o f the value o f P(SbCl3) a t the s u r f a c e from the r e a c t i o n f r e e energies g i v e n i n Table I. The f l u x o f r e t a r d a n t e n t e r i n g a flame f r o n t may be d e r i v e d as f o l l o w s . Consider a polymer s u b s t r a t e c o n t a i n i n g a halogen source equiva l e n t to 6 mole % HCl, together with 2 mole % [Sb2Û3]. We a l s o assume t h a t the p a r t i a l pressure o f Η 0 i n the atmosphere i s f i x e d by the a i r humidity which i s t y p i c a l l y e q u i v a l e n t to 3xl0~ atm. From the p r e v i o u s l y determined e q u i l i b r i u m constants f o r r e a c t i o n (1), the amount o f SbCl3 p r e s e n t a t the s u r f a c e o f the s u b s t r a t e can be c a l c u l a t e d . F i g u r e 11 shows the general r e l a t i o n s h i p between P(SbCl3> and the i n i t i a l unreacted Ρ(HCl) f o r v a r i o u s t y p i c a l p y r o l y s i s temperatures. Thus f o r the assumed c o n d i t i o n o f Ρ(HCl) t o t a l = 6 x l 0 " atm we have P(SbCl3) = 1.3xl0" atm and Ρ(HCl) = 2.1xl0" atm a t 350 °C, and 65% o f the HCl i s converted to SbCl3The e f f e c t o f an order o f magnitude i n c r e a s e i n P(H20) on P(SbCl3) may a l s o be noted i n F i g u r e 11. From the previous determinations o f K3, the p r o d u c t i o n o f SbOCl can be shown t o be n e g l i g i b l e f o r these c o n d i t i o n s . 2

2

2

2

2

These p a r t i a l pressures are assumed to represent the halogen r e t a r d a n t s p e c i e s c o n c e n t r a t i o n s a t the s u b s t r a t e s u r f a c e . The corresponding c o n c e n t r a t i o n s i n the bulk gas can be expected to be s i g n i f i c a n t l y reduced by the p r e d i c t e d presence o f a mass t r a n s p o r t - l i m i t i n g boundary l a y e r . An upper e f f e c t i v e l i m i t f o r P(SbCl3) i n the bulk gas can be estimated from the amount o f SbCl3 r e q u i r e d i n the flame f o r s i g n i f i c a n t flame i n h i b i t i o n to occur. From our observations o f burning v e l o c i t y and H atom c o n c e n t r a t i o n r e d u c t i o n s i n the presence o f SbCl3, t h i s pressure i s known t o




2

H 0=3 χ 10 atm 2

I

ι

0

2

ι

ι

ι

ι

4 6 8 10 TOTAL HC1 PRESSURE χ 10 ATM

ι

1

12

2

Figure 11. Dependence of SbCl partial pressure on total HCl pressure and temperature for conditions of H 0 par tial pressure equal to 3 X 10~ or 3 X 10~ atm, based on the K, values in Figure 4. Broken line indicates maximum pos sible SbCh pressure, i.e., complete conversion of HCl to SbCl . Note that SbOCl is a negligible species for these conditions. s

2

2

3

3


4.

HASTiE

AND


M C BEE

137

f a l l i n the range o f 10 -10 atm (see the f o l l o w i n g d i s c u s s i o n ) . In p r a c t i c e , t h i s i s an upper l i m i t s i n c e the flame removes SbCl3 from the bulk gas (see F i g . 10). We can t h e r e f o r e reasonably assume the p a r t i a l pressure o f SbCl3 i n the bulk gas t o be small compared with the corresponding pressure a t the s u r f a c e . From these estimates i t f o l l o w s t h a t the f l u x o f SbCl3 i s J(SbCl ) 3

-5 2 -7 2 = 6.8 χ 10 g/cm sec (=3.0x10 mole/cm s e c ) .

T h i s value may be compared with the maximum Langmuir f l u x no boundary l a y e r ) o f 0.35 g/cm sec. Thus,

(i.e.,


2

J/J(max) ^2χ1θ"

4

and a s i g n i f i c a n t r e d u c t i o n i n f l u x over the maximum allowed by f r e e molecular v a p o r i z a t i o n appears l i k e l y under a c t u a l f i r e con ditions . S i m i l a r c o n s i d e r a t i o n s t o those given above f o r sbCl3 i n d i c a t e the HCl f l u x t o the flame f r o n t t o be: J(HCl) = 0.9χ1θ"

4

2

g/cm sec a t 350 °C.

At 450 °C the f l u x e s a r e c a l c u l a t e d a s : J(SbCl ) 3

-4 2 = 0.9x10 g/cm sec,

and -4 2 J(HCl) = 0.4x10 g/cm sec. Thus the amount o f SbCl3 r e l a t i v e t o HCl i n c r e a s e s with i n c r e a s i n g p y r o l y s i s temperature. T h i s e f f e c t i s l a r g e l y a r e s u l t o f the absence o f s t a b l e antimony o x y c h l o r i d e s o l i d s a t elevated tempera tures ( i . e . , >420 °C). P r e d i c t e d Flame I n h i b i t o r Species Concentrations. From a knowledge o f the f l u x o f i n h i b i t o r e n t e r i n g a flame f r o n t we can d e r i v e i n h i b i t o r species concentrations f o r a w e l l d e f i n e d f i r e system, such as t h a t represented i n F i g u r e 10. F o r example, con s i d e r a h y p o t h e t i c a l polyethylene - t e r e p h t h a l a t e s u b s t r a t e c o n t a i n i n g [Sb2Û3] together with a halogenated hydrocarbon e q u i v a l e n t t o 6 mole % HCl. The predominant gaseous f u e l component i n t h i s system i s acetaldehyde, CH3CHO. A s t o i c h i o m e t r i c combust i o n r e a c t i o n between CH3CHO and a i r , i . e . : CH CH0 + 5/20 3

2

= 2C0

2

+ 2^0


138

HALOGENATED

FIRE SUPPRESSANTS

consumes 0.078 mole f r a c t i o n CH CH0, with r e s p e c t to the f u e l - a i r mixture. The N component i n the bulk gas mixture amounts to 0.725 mole f r a c t i o n . T h i s amount can a l s o be considered as an upper l i m i t to the N c o n c e n t r a t i o n a t the substrate surface. Based on experimental observations f o r analogous systems such as polymer candle burning, e.g., see Fenimore and Martin (23), we may c o n s i d e r 0.4 N mole f r a c t i o n as a l i k e l y lower l i m i t surface concentration. I f we assume 0 to be o f n e g l i g i b l e c o n c e n t r a t i o n a t the surface then the f u e l component c o n c e n t r a t i o n should t h e r e f o r e be i n the r e g i o n o f 0.27-0.6 mole f r a c t i o n . Consider now the e f f e c t o f the d i f f u s i o n boundary l a y e r on the f l u x o f CH3CHO e n t e r i n g the flame f r o n t . At a temperature o f 350 °C we c a l c u l a t e : 3

2

2

2


2

D(CH CH0) i n N 3

I f Ρ(CH CH0) = 0.26

2

= 0.82

cm

2

1

s e c " [ u s i n g σ(CH CH0) 2.9 3

A].

atm a t the surface then

3

-4 2 J(CH CH0) = 7.6x10 g/cm sec. 3

Comparison of t h i s f l u x with t h a t f o r S b C l ratio of: J(SbCl )/J(CH CH0) = 1.68xl0" 3

3

g i v e s a molar f l u x

2

3

Now a t the flame r e a c t i o n zone, under the c h a r a c t e r i s t i c s t o i c h i o m e t r i c r e a c t i o n c o n d i t i o n o f a d i f f u s i o n flame, 0.078 mole f r a c t i o n o f CH3CHO i s r e q u i r e d . Hence, from the above f l u x r a t i o , the amount o f SbCl3 w i t h i n the flame i s : SbCl

3

i n flame = 1.2x10

3

mole f r a c t i o n .

From the observed burning v e l o c i t y ( 2 ) — a n d Η atom c o n c e n t r a t i o n — reductions (see F i g . 12 g i v e n l a t e r ) i n premixed flames t h i s amount of SbCl3 i s more than s u f f i c i e n t to s t r o n g l y i n h i b i t a hydrocarbon — or even H — f u e l e d flame. S i m i l a r arguments t o those above i n d i c a t e : 2

-2 HCl i n flame = 1.0x10 mole f r a c t i o n . An a l t e r n a t i v e method o f c o n v e r t i n g the SbCl3 f l u x to a flame c o n c e n t r a t i o n i s to c o n s i d e r the f l u x o f a i r e n t e r i n g the flame. For example, consider a volume element determined by the boundary l a y e r t h i c k n e s s 6(= 0.24 cm), the e f f e c t i v e r e a c t i o n zone t h i c k ness L(= 0.4 cm) and a u n i t width (see F i g . 10). Let the flow o f a i r across the surface i n t o t h i s volume element be V(= 10 cm/sec). Then the molar a i r f l u x w i l l be given by: J(air) =

V/V


4.

HASTiE

139


AND M C BEE

J (air) = 2.1x10 3 where V (=22.4x10 From b e f o r e :

°C,

3 cm

m

mole/cm sec a t 350

a t N.T.P.) i s the molar volume o f a i r .

-7 2 J ( S b C l ) = 3.0x10 mole/cm sec, 3

hence the c o n c e n t r a t i o n o f SbCl^ i n a flame under these assumed c o n d i t i o n s o f a i r f l u x i s determined as:


SbClg i n flame = 1.3xl0~

3

mole f r a c t i o n ,

which i s s i m i l a r t o the p r e v i o u s estimate. We may r e c a l l t h a t macroscopic f l a m m a b i l i t y t e s t s f o r sub s t r a t e s c o n t a i n i n g a s i m i l a r chloride/antimony oxide content t o t h a t assumed here i n d i c a t e an e f f e c t i v e degree o f flame retardancy, e.g. see the F i g u r e 8 g i v e n by P i t t s (13). Note a l s o from P i t t s t h a t f o r s u b s t r a t e mole r a t i o s o f Sb:Cl > 1:3 very l i t t l e g a i n i n flame retardancy i s achieved. Our data a r e i n accord with t h i s o b s e r v a t i o n owing to the f i n i t e values o f Κ ensuring the presence o f excess s u b s t r a t e [SbÔ^], even f o r s t o i c h i o m e t r i c r e a c t i o n con ditions. Fate o f SbClg i n the Pre-Flame

Region

Once SbCl3 e n t e r s a flame i t encounters a f a r d i f f e r e n t chemical environment to t h a t a t the s u b s t r a t e s u r f a c e . In p a r t i c u l a r , the d i f f u s i o n o f H 0 as a combustion product i n t o the p r e flame region, and the i n c r e a s e d temperature, w i l l tend to favor the conversion o f SbCl3 t o SbOCl v i a r e a c t i o n (3). For i n s t a n c e , a t some p o i n t i n the pre-flame r e g i o n we can expect the f o l l o w i n g c o n d i t i o n s to occur: 2

Τ = 1200

Κ

Ρ(Η 0) = 10

atm

2

P ( S b C l ) = 5x1θ" 3

5

atm

-3 and

Ρ(HCl) = 10

atm.

From the thermodynamic data g i v e n i n the Appendix, the f r e e energy change f o r r e a c t i o n (3) a t 1200 Κ i s 19.7 kcal/mol. Hence, we c a l c u l a t e : Ρ(SbOCl) = 1 . 3 x l 0 "

3

atm.

and SbOCl becomes the predominant antimony c o n t a i n i n g s p e c i e s a t


140


the expense o f SbCl.3 f o r these c o n d i t i o n s . From premixed flame s t u d i e s ( 3 ) , we know t h a t mass d i f f u s i o n from the r e a c t i o n zone provides a s i g n i f i c a n t c o n c e n t r a t i o n o f H-atoms i n the pre-flame r e g i o n (e.g. ^ 1 0 - 1 0 atm). The exothermic bi-molecular r e a c t i o n : 3


SbOCl + H

HCl + SbO

h

(F.l)

can t h e r e f o r e be expected to occur r e a d i l y . As r e a c t i o n (3) i s c o n s i d e r a b l y endothermic i t w i l l most l i k e l y be slow as compared with r e a c t i o n F . l . I t t h e r e f o r e f o l l o w s t h a t the steady-state c o n c e n t r a t i o n o f SbOCl would be small and t h i s accounts f o r i t s non-observation i n our mass spectrometric flame sampling s t u d i e s (22). On the other hand, both HCl and SbO were observed as major products o f SbCl3 decomposition i n the pre-flame r e g i o n ( 2 2 ) . Thus SbOCl i s most l i k e l y a s i g n i f i c a n t intermediate species (but o f low steady-state concentration) i n the pre-flame conversion of SbCl3 to the flame i n h i b i t i n g moieties of SbO and HCl. These arguments do not preclude the presence o f a d d i t i o n a l r e a c t i o n s such as: SbCl

3

+ H + HCl +

which were suggested e a r l i e r

SbCl , 2

(22).

A c t i o n of SbCl3 and HCl i n Flame I n h i b i t i o n Thus f a r we have considered the chemistry o f SbCl3 from the p o i n t o f i t s i n i t i a l s u b s t r a t e production to i t s conversion to other species i n the pre-flame r e g i o n . These secondary species o f SbO and, to a l e s s e r extent, HCl are considered to be d i r e c t l y r e s p o n s i b l e f o r flame i n h i b i t i o n . As has been argued elsewhere ( 1 , 2 2 ) , we c o n s i d e r t h a t these species are c a t a l y t i c a l l y i n v o l v e d i n a c c e l e r a t i n g the recombination of flame r a d i c a l s and p a r t i c u l a r l y H atoms. For i n s t a n c e , H + HCl = H

2

+ CI

C l + RH + HCl + R CI + H 0

2

-* HCl +

0

2

C l + H + M + HCl + M where RH i s a f u e l species and M a t h i r d body such as N , 2

SbO

+ H -> SbOH

SbOH + H -> SbO

+

H2


and

4.

HASTiE

AND


M C BEE

141

These processes are l i k e l y to be more r a p i d than the normal r a d i c a l recombinations o f : H + OH + M + H 0

+ M

2

H + H + M->H +M.


2

Using the Li/Na technique f o r monitoring H atom concentrations we have demonstrated t h a t the presence o f antimony does indeed cause a r e d u c t i o n i n the steady-state flame H atom c o n c e n t r a t i o n as shown, f o r example, i n F i g u r e 12. Further evidence f o r SbO as a s i g n i f i c a n t flame species was a l s o obtained by the o b s e r v a t i o n o f known emission s p e c t r a f o r SbO (System A a t 506.2 nm)-as shown i n the F i g u r e 12. I t i s i n t e r e s t i n g to note t h a t a s i m i l a r degree o f e f f e c t i v e n e s s i n reducing H atom concentrations i s shown by HCl a t 30 times the c o n c e n t r a t i o n o f SbCl3T h i s r e s u l t i s s i m i l a r to t h a t found by burning v e l o c i t y measurements (2) . From the f l u x arguments given above an HCl/SbCl3 flame c o n c e n t r a t i o n r a t i o o f about 8 was i n d i c a t e d . Hence, f o r these c o n d i t i o n s , the HCl cont r i b u t i o n to flame i n h i b i t i o n would be n e g l i g i b l e as compared with t h a t provided by the antimony component o f SbCl3Further flame measurements are i n progress to e s t a b l i s h the molecular d e t a i l s o f the apparent c a t a l y t i c r o l e o f antimony species i n e f f e c t i n g r a d i c a l recombination. Conclusions Our s t u d i e s i n d i c a t e t h a t the e f f e c t i v e n e s s o f antimony-halogen flame r e t a r d a n t systems depends on the f o l l o w i n g p r i n c i p a l factors : a. The degree of conversion of the s u b s t r a t e halogen component to SbCl3 and SbOCl s p e c i e s , b. The f l u x o f these antimony h a l i d e species from the subs t r a t e s u r f a c e through a d i f f u s i o n boundary l a y e r to the flame front, c. The conversion i n the flame o f the species SbCl3 and SbOCl to SbO and HCl, d. The i n t e r a c t i o n o f SbO and, to a l e s s e r extent, HCl with H atoms with a r e s u l t a n t l o s s o f chain branching and reduced burning v e l o c i t y . We have shown t h a t the f i r s t f a c t o r (a) i s s t r o n g l y i n f l u e n c e d by the formation o f intermediate s o l i d o x y c h l o r i d e phases. The production o f the v o l a t i l e species SbOCl can a l s o be s i g n i f i c a n t i n the presence o f H 0 vapor. As o x y c h l o r i d e s are g e n e r a l l y more v o l a t i l e species than the corresponding halides,we are c o n s i d e r i n g t h e i r p o s s i b l e r o l e as r e t a r d a n t species f o r other metal oxide systems such as [Sn0 ], [ A l 0 3 ] , [ T i 0 ] and [ C r 0 3 ] . Here-to-fore, the l e s s than f a v o r a b l e thermodynamics f o r the conversion of HCl (or HBr) to v o l a t i l e metal c h l o r i d e s such as TiCli+ or CrCl3 has l i m i t e d c o n s i d e r a t i o n o f such systems as [Sb 03] s u b s t i t u t e s . 2

2

2

2

2

2




Figure 12. Concentration profiles for H atoms in the H /O /N flame 3/1/3.7 (1650 K) showing catalytic effects of 3.2 χ 10* mole fraction SbCl and 9.6 χ JO" mole fraction HCl on the loss of Η atoms. Additives were introduced with the premixed combustion mix ture nebulized aqueous droplets. 2

g

s

s

4


4.


H A S T i E AND MC BEE

143

With regard t o r o l e o f SbCl3 o r SbOCl i n flame i n h i b i t i o n , i t appears t h a t the halogen moiety i s o f secondary importance? i t s primary f u n c t i o n being t o t r a n s p o r t the metal component t o the flame r e a c t i o n zone.


Appendices:

B a s i c Data

Estimated Thermodynamic P r o p e r t i e s f o r the Species SbOCl. The s p e c i e s SbOCl has not been p r e v i o u s l y i d e n t i f i e d and hence no thermodynamic data e x i s t . In view o f i t s s i g n i f i c a n c e t o t h e present work we have made the f o l l o w i n g e s t i m a t i o n s f o r the s t a b i l i t y and entropy p r o p e r t i e s o f SbOCl. These estimates a r e b e l i e v e d t o be s u f f i c i e n t l y r e l i a b l e t o a l l o w c a l c u l a t i o n s o f i t s probable c o n c e n t r a t i o n i n a " r e a l f i r e " environment. The heat o f formation AHf i s estimated as f o l l o w s . Consider the a t o m i z a t i o n energy f o r the process: CI - Sb =

0

+ CI + Sb +

0,

as ΔΗ atoms = D(Sb-Cl) + D(Sb=0) . We assume D(Sb-Cl) t o be the average bond energy i n SbCl ? namely, 7 4 . 8 kcal/mol. A l s o D(Sb=0) i s taken as the bond d i s s o c i a t i o n energy f o r t h e f r e e diatomic s p e c i e s SbO. L i t e r a t u r e v a l u e s o f D(SbO) vary from about 9 0 t o 1 0 5 kcal/mol. From the known p e r i o d i c trends f o r the s i m i l a r (157±3),

s p e c i e s o f GeO

AsO

(114±3),

SeO

(100±15),

SnO

(126±2)

and TeO ( 9 2 . 5 ± 2 ) i t appears t h a t D(SbO) ^ 1 0 5 kcal/mol i s the most appropriate v a l u e . T h i s r e s u l t i s supported by a s i m i l a r value o f 1 0 4 r e p o r t e d by Labsham as c i t e d by the review o f Brewer and Rosen b l a t t ( 2 4 ) . Hence, the a t o m i z a t i o n energy f o r SbOCl i s taken as 1 7 9 . 8 ± 1 0 kcal/mol. This i s equivalent to ΔΗ 298(SbOCl) = - 2 8 . 7 kcal/mol £

which compares reasonably with the estimated value o f - 2 5 . 5 k c a l / mole g i v e n by Wagman e t a l ( 2 6 ) . The entropy o f SbOCl can be s a t i s f a c t o r i l y determined from estimated geometrical and v i b r a t i o n a l parameters u s i n g the w e l l known treatment o f s t a t i s t i c a l thermodynamics ( 2 6 ) . Based on t h e known f r e q u e n c i e s o f v i b r a t i o n f o r S b C l [see Nakamoto ( 2 7 ) ] and SbO [see Brewer and Rosenblatt (24)] we estimate f o r SbOCl f r e 3

quencies o f 7 1 0 ± 3 0

1

cm" ,

360±20

1

cm"

and

200±50

1

cm" .

An

0 =

Sb-Çl

bond angle o f 1 2 0 ° i s assumed and i n t e r n u c l e a r d i s t a n c e s o f 2 . 3 2 A and 1 . 8 5 A f o r Sb-Cl and Sb = 0 , r e s p e c t i v e l y , a r e used. I t f o l l o w s from these estimates and the s t a t i s t i c a l treatment t h a t S__ (SbOCl) = Q

71.6±0.8

cal/deg mol.

L i t e r a t u r e Thermodynamic Data. The l i t e r a t u r e thermodynamic data f o r the other s p e c i e s o f i n t e r e s t may be obtained from the compilations o f Wagman e t a l ( 2 5 ) and the JANAF Tables ( 2 8 ) ,


144


with s e v e r a l exceptions. P e r t i n e n t data are summarized i n Table A. I t should be noted t h a t Δ Η and A s can be determined to a good approximation from these Δ Η - 2 9 8 and S298 data s i n c e , f o r the r e a c t i o n s o f i n t e r e s t , the neat content and entropy changes are almost independent o f temperature. τ

T

TABLE A Summary o f B a s i c Thermodynamic Data


Species

AH

S °

f

kcal/mol

[SbOCl] [2Sb0Cl-Sb 0 ] 1

3

c

a

l

2

/

9

8

d

e

Comments g

-344.3

52.8

- 89.4

37±5

m

o

1

Senarmonite, i . e . , c u b i c form. S

°298

o

u

r

e

s

t

i

m

a

t

e

-

-346.9, -354.5*

HCl

- 22.06

H0

- 57.8

2

SbCl-

44.64 45.1

- 75.0

80.7

- 28.7 or - 25.5

71.6

or

82.1

Larger S ° 2 9 8 value i s from Wilmshurst (29).

•5

SbOCl

Larger Δ Η ^ value i s our estimate.

0.8

Mass S p e c t r a l Fragmentation Data. In order to u t i l i z e mass s p e c t r a l i o n i n t e n s i t i e s f o r species p a r t i a l pressure déterminat i o n s i t i s necessary to e s t a b l i s h the mass s p e c t r a l fragmentat i o n p a t t e r n f o r each s p e c i e s o f i n t e r e s t . These are given as f o l l o w s f o r 90 eV i o n i z i n g e l e c t r o n energy: SbCls SbOCl Sbi+Ofi

1

SbCl

+ 3

:

• Sb0Cl :

• Sb 0 4

6

(22.6), S b C l

+

(18.9), S b 0

+

(9.2), S b 0 3

+

(60.5), S b C l

(19.9), +

5

The r e s u l t s f o r S b C l and SbÔ^ l i t e r a t u r e data, i . e . : 3

+ 2

SbCl

(2.1), S b 0 3

(7.2), S b

(20.2), S b

4

+ 4

+

+

+

(17.2), S b 0

(9.7) (41.0) +

(71.5)

compare f a v o r a b l y with a v a i l a b l e


4.

AND MC BEE

HASTiE

SbCl

3


[ P r e i s s (30)] SbCl (23.2), S b C l +

3

+ 2

(58.5), S b C l

Sbi*06 [Boerboom e t a l ( 3[1) ] SbO (68), SbÔe"* (13.6), S b O i * +

145

1

3

+

+

(7.8), S b

+

(10.5)

(6.8).


I t i s p e r t i n e n t t o note t h a t , c o n t r a r y t o the observations o f Boerboom e t a l (31) and Kazanas e t a l (32), the r e l a t i v e abundances f o r the mass s p e c t r a l ions i n the Sbi*06 system were temperature independent, as i s t o be expected i f SbÔô i s the s o l e neutral precursor. Literature Cited 1. 2. 3. 4. 5.

6. 7. 8. 9. 10. 11. 12.

13.

14. 15. 16.

H a s t i e , J . W., J . Res. N.B.S. (1973) 77A, 733. Lask, G. and Wagner, H. G., E i g h t h Symposium ( I n t e r n a t i o n a l ) on Combustion (1962) W i l l i a m s and W i l k i n s Co., p. 432. H a s t i e , J. W., Comb. Flame (1973) 21, 187. H a s t i e , J . W., I n t . J. Mass Spec. Ion Phys. (1975) 16, 89. Grimley, R. Τ., Mass Spectrometry in "The C h a r a c t e r i z a t i o n o f High Temperature Vapors", 195, Ed. J. L. Margrave, John Wiley and Sons, Inc., New York (1967). Jungermann, E. and Plieth, Κ., Z. Physik. Chem. (1967) 53,1. Drowart, J . and G o l d f i n g e r , P., Angew Chem. I n t . Ed. (1967) 6, 581. Gaydon, A. G., "Spectroscopy o f Flames", Chapman and H a l l , London (1957). Bulewicz, Ε. M. and Sugden, Τ. Μ., Trans. Faraday Soc. (1956) 52, 1475. Bulewicz, Ε. Μ., James C. G., and Sugden, Τ. Μ., Proc. Roy. Soc. (1956) A255, 89. Mavrodineanu, R. and Boiteux, H., "Flame Spectroscopy", John Wiley and Sons, New York (1965). Touval, I . , "A comparison o f antimony oxide and s t a n n i c oxide hydrate as flame r e t a r d a n t s y n e r g i s t " (1973) Polymer Conf. S e r i e s , U n i v e r s i t y D e t r o i t , May 22, 1973; see a l s o J . F i r e and Flammability (1972) 3, 130. Pitts, J . J., "Inorganic flame r e t a r d a n t s and their mode o f a c t i o n " p. 133 in "Flame Retardance o f Polymeric M a t e r i a l s " , V o l . I , Eds. W. C. K u r y l a and A. J. Papa, Marcel Dekker, I n c . , New York (1973). Behrens, R. G. and Rosenblatt, G. M., J. Chem. Thermodynamics, (1973) 5, 173. J o s t , W., " D i f f u s i o n in S o l i d s , L i q u i d s , Gases", Academic Press, New York, (1960). H i r s c h f e l d e r , J . O., C u r t i s s , C. F., and B i r d , R. Β., "Molecu lar Theory o f Gases and L i q u i d s " , John Wiley and Sons, New York (1954).



146


17. Sutton, L. Ε., "Tables o f Interatomic D i s t a n c e s and C o n f i g u r a t i o n in Molecules and Ions", Spec. Publ. No. 18, Chemical S o c i e t y , Londong (1965). 18. Graham, H. C. and Davis, H. H., J. Amer. Ceram. Soc. (1971) 54, 88. 19. Turkdogan, E. T., Grieveson, P. and Darken, L. S., J. Phys. Chem. (1963) 67, 1647. 20. "Handbook o f Chemistry and P h y s i c s " , 55th E d i t i o n , CRC Press, New York (1974). 21. Hirano, T., N o r e i k i s , S. E. and Waterman, Τ. Ε., Comb. Flame (1974), 22, 353; see a l s o I n t e r i m T e c h n i c a l Report No. 2, P r o j e c t J1139 "Measured v e l o c i t y and temperature p r o f i l e s o f flames spreading over a t h i n combustible solid", I I T Research I n s t i t u t e , Chicago, Illinois, May (1973). 22. H a s t i e , J . W., Comb. Flame (1973) 21, 49. 23. Fenimore, C. P. and M a r t i n , F. J . , "The Mechanisms o f Pyroly sis, Oxidation, and Burning o f Organic M a t e r i a l s " , Nat. Bur. Stand. (U.S.) Spec. Publ. 357 (June 1972). 24. Brewer, L. and Rosenblatt, G. Μ., "Advances in High Tempera t u r e Chemistry", (L. E y r i n g , Ed.) 2, 1 Academic Press, New York (1969). 25. Wagman, D. D., Evans, W. Η., Parker, V. B., Halow, I . , B a i l e y , S. M. and Schumm, P. Η., "Selected Values o f Chemical Thermo dynamic P r o p e r t i e s " , N.B.S. Tech. Note 270-3 (U. S. Govt. P r i n t i n g O f f i c e , Washington, D. C.) (1968). 26. Herzberg, G., " I n f r a r e d and Raman Spectra o f Polyatomic Molecules", Van Nostrand, New York (1945). 27. Nakamoto, Κ., " I n f r a r e d Spectra o f Inorganic and C o o r d i n a t i o n Compounds", John Wiley & Sons, Inc., New York (1963). 28. JANAF T a b l e s , J o i n t Army Navy Air Force Thermochemical T a b l e s , 2nd Ed. NSRDS-NBS 37, U. S. Govt. P r i n t i n g O f f i c e , Washington, D. C. (1971). 29. Wilmshurst, J . K., J. Mol. Spec. (1960) 5, 343. 30. P r e i s s , Η., Z. Anorg. Allg. Chem. (1972) 389, 280. 31. Boerboom, A. J. H., Reyn. H. W., Vugts, H. F. and Kistemaker, J . , "Thermochemistry o f antimony and antimony t r i o x i d e " p. 945 in Advances in Mass Spectrometry 3, Ed. W. L. Mead (1966). 32. Kanzanas, E. K., C h i z h i k o v , D. Μ., Tsvetkov, Yu.V. and O l ' S h e v s k i i , M. V., Russ. J . Phys. Chem. (1973) 47, 871.


4.

HASTiE

AND M C BEE


147


DISCUSSION A. W. BENBOW: Perhaps t h e major d i f f i c u l t y i n h e r e n t i n t h e study o f t h e mechanism o f t h e s y n e r g i s t i c flame-retardant a c t i o n o f metal oxides ( p a r t i c u l a r l y antimony o x i d e ) w i t h h a l o g e n - c o n t a i n i n g compounds i s t h a t i t i s a m u l t i s t a g e p r o c e s s . Dr. H a s t i e ' s work and r e c e n t work c a r r i e d o u t a t t h e C i t y U n i v e r s i t y , London, have c o n c e n t r a t e d on d i f f e r e n t a s p e c t s o f t h e i n h i b i t i o n p r o c e s s and t h e r e s u l t s t e n d t o be complementary. F o l l o w i n g o u r e x p e r i m e n t a l work i n which l a r g e l y u n s u c c e s s f u l attempts were made t o i n c o r p o r a t e a wide range o f m e t a l h a l i d e s i n t o v a r i o u s polymers, i t became o b v i o u s t h a t an i m p o r t a n t s t a g e i n t h e m e t a l o x i d e / h a l o g e n compound i n t e r a c t i o n was t h e r e a c t i o n between them i n t h e condensed phase t o produce v o l a t i l e s p e c i e s such as t h o s e mentioned by Dr. H a s t i e . As an example o f o u r s t u d i e s , a h a l o g e n a t e d compound ( a c t u a l l y I . C . I . C e r e c l o r 70, a c h l o r i n a t e d wax w i t h 70% c h l o r i n e c o n t e n t ) and antimony o x i d e were h e a t e d i n a i r i n a s e n s i t i v e thermobalance, and t h e k i n e t i c s o f t h e v o l a t i l i z a t i o n r e a c t i o n were m o n i t o r e d . V a r i a t i o n o f t h e r a t i o by w e i g h t o f antimony t o h a l o g e n heated t o g e t h e r showed t h a t t h e predominant v o l a t i l i z e d s p e c i e s was t h e trihalide. T h i s was c o n f i r m e d by c h e m i c a l a n a l y s i s . Other h a l o g e n compounds, which v o l a t i l i z e d i n p r e f e r e n c e t o decomposing i n t h e condensed phase, r e q u i r e d h i g h e r than t h e o r e t i c a l r a t i o s o f h a l o g e n : antimony but t h e v o l a t i l i z e d s p e c i e s was t h e t r i h a l i d e i n each case. I was v e r y i n t e r e s t e d t o hear Dr. H a s t i e s t r e s s i n g t h e importance o f antimony o x y h a l i d e s (e.g., SbOCl) i n t h e f l a m e . Such e n t i t i e s have p r e v i o u s l y been c o n s i d e r e d i m p o r t a n t i n t h e condensed phase r e a c t i o n s between antimony o x i d e and h a l o g e n compounds. I n f a c t t h e endothermic breakdown o f antimony o x y c h l o r i d e s i n t h e condensed phase was f r e q u e n t l y p o s t u l a t e d as an i m p o r t a n t p a r t o f t h e i r mode o f a c t i o n . However, o u r p r e s e n t work has shown t h a t t h e r e a c t i o n between antimony o x i d e and h a l o g e n a t e d compounds i s i n f a c t m o d e r a t e l y e x o t h e r m i c . Thus t h e i n v o l v e m e n t o f o x y h a l i d e s i n t h e condensed phase seems u n i m p o r t a n t . When attempts a r e made t o use o t h e r m e t a l o x i d e s


148

HALOGENATED

F I R E SUPPRESSANTS


as s u b s t i t u t e s f o r antimony o x i d e i n r e a l f l a m e retardant compositions the r e s u l t s are g e n e r a l l y very poor. The o n l y e x c e p t i o n s a r e some o f t h e h y d r a t e d m e t a l o x i d e s (aluminum, s t a n n i c , z i n c ) which a r e now b e i n g brought on t o t h e market, and i t seems t h a t g e n e r a l l y t h e s e m e t a l o x i d e s have a d i f f e r e n t mode o f a c t i o n from t h a t o f antimony o x i d e . J . W. HASTIE: I agree w i t h t h e g e n e r a l comments o f Benbow w i t h t h e f o l l o w i n g e x c e p t i o n . Our r e s u l t s do show t h a t o x y c h l o r i d e s a r e p r e s e n t as condensed phase i n t e r m e d i a t e s . T h i s does n o t p r e c l u d e t h e p o s s i b i l i t y o f an o v e r a l l e x o t h e r m i c r e a c t i o n (observed by Benbow and C u l l i s ) between antimony o x i d e and t h e h a l o g e n s o u r c e s i n c e t h e condensed o x y c h l o r i d e f o r m i n g r e a c t i o n s a r e exothermic as shown i n T a b l e I o f o u r paper.


Mechanistic Studies of Halogenated Flame Retardants: The Antimony

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