Chemistry of Fire Retardancy - ACS Publications - American Chemical

14 Chemistry of Fire Retardancy SUSAN L. LEVAN

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on February 28, 2016 | http://pubs.acs.org Publication Date: May 5, 1984 | doi: 10.1021/ba-1984-0207.ch014

U.S. Department of Agriculture, Forest Service, Forest Products Laboratory, Madison, WI 53705

Fire retardancy of wood involves a complex series of simultaneous chemical reactions, the products of which take part in subsequent reactions. Most fire retardants used for wood increase the dehydration reactions that occur during thermal degradation so that more char and fewer combustible volatiles are produced. The mechanism by which this happens depends on the particular fire retardant and the thermal-physical environment. This chapter presents a literature review of the investigations into the mechanisms, a discussion of test methods used for determining fire retardancy, the various formulations used to make wood fire retardant, and the research needs in the field of fire retardancy.

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WAS FIRST T R E A T E D

FOR FIRE RETARDANCY

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t h e first

CentUiy

A . D . w h e n t h e R o m a n s u s e d s o l u t i o n s o f a l u m a n d v i n e g a r to p r o t e c t t h e i r b o a t s a g a i n s t fire. I n 1 8 2 0 , G a y - L u s s a c a d v o c a t e d t h e u s e o f a m m o n i u m p h o s p h a t e s a n d borax for t r e a t i n g c e l l u l o s i c m a t e r i a l . M a n y of the p r o m i s i n g inorganic chemicals used today w e r e i d e n t i fied b e t w e e n 1 8 0 0 a n d 1 8 7 0 . S i n c e t h e n , t h e d e v e l o p m e n t o f fire r e t a r d a n t s for w o o d has a c c e l e r a t e d . C o m m e r c i a l l y t r e a t e d w o o d b e c a m e a v a i l a b l e a f t e r t h e U . S . N a v y (1895) s p e c i f i e d i t s u s e i n s h i p c o n s t r u c t i o n , a n d N e w Y o r k C i t y (1899) r e q u i r e d its u s e i n b u i l d i n g s o v e r t w e l v e s t o r i e s t a l l (I). P r o d u c t i o n r e a c h e d o v e r 6 5 m i l l i o n b o a r d feet i n 1943, b u t b y 1964 o n l y 32 m i l l i o n b o a r d feet was t r e a t e d a n n u a l l y (I). I n c r e a s e d efforts t o e x p a n d t h e u s e o f w o o d p r o d u c t s i n i n s t i t u t i o n a l a n d c o m m e r c i a l s t r u c t u r e s m a y r e q u i r e w o o d to b e t r e a t e d w i t h fire r e t a r d a n t s . T h e r e f o r e , r e s e a r c h o n fire-retardant t r e a t m e n t s for w o o d has a c c e l e r a t e d . Early

Studies O n e of the earliest studies on

fire-retardant

t r e a t m e n t s for w o o d

was c o n d u c t e d b e t w e e n 1930 a n d 1935 (Forest P r o d u c t s L a b o r a t o r y ) . This chapter not subject to U.S. copyright. Published 1984, American Chemical Society In The Chemistry of Solid Wood; Rowell, Roger; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.


532

T H E CHEMISTRY OF SOLID WOOD

T h i s study r e s u l t e d i n a series of reports on a c o m p r e h e n s i v e e v a l u a t i o n of f i r e - r e t a r d a n t t r e a t m e n t s for w o o d ( 2 - 6 ) . O n e h u n d r e d a n d thirty single chemicals or combinations of chemicals in the form of v a r i o u s salts w e r e e v a l u a t e d f o r f l a m e - s p r e a d r e d u c t i o n , s m o k e , a n d c o r r o s i v i t y . D i a m m o n i u m p h o s p h a t e r a n k e d first i n r e d u c i n g f l a m e spread, followed by m o n o a m m o n i u m phosphate, a m m o n i u m chlo ride, a m m o n i u m s u l f a t e , b o r a x , a n d z i n c c h l o r i d e . Z i n c c h l o r i d e , a l t h o u g h e x c e l l e n t as a f l a m e r e t a r d a n t , p r o m o t e d s m o k e a n d g l o w i n g . A m m o n i u m sulfate was the least e x p e n s i v e , b u t u n d e r cer tain e n v i r o n m e n t a l c o n d i t i o n s it was c o r r o s i v e to m e t a l s . N o n e o f the 130 c o m p o s i t i o n s t e s t e d was c o n s i d e r e d i d e a l b e c a u s e of t h e a d v e r s e effects o n s o m e o f t h e p r o p e r t i e s o f w o o d . S e v e r a l r e v i e w s o f t h e subject are available a n d p r o v i d e additional b a c k g r o u n d material (J, 7-10).

Protection of Wood with Fire Retardants F i r e - r e t a r d a n t t r e a t m e n t s for w o o d can b e classified i n t o t w o g e n e r a l c l a s s e s : (1) t h o s e i m p r e g n a t e d i n t o t h e w o o d o r i n c o r p o r a t e d i n t o w o o d c o m p o s i t e p r o d u c t s , a n d (2) t h o s e a p p l i e d as p a i n t o r s u r face c o a t i n g s . C h e m i c a l i m p r e g n a t i o n h a s t h e g r e a t e r u s e , p r i m a r i l y f o r n e w m a t e r i a l s , w h e r e a s c o a t i n g s h a v e b e e n l i m i t e d p r i m a r i l y to materials i n e x i s t i n g constructions. T h e r e are advantages a n d d i s a d vantages to e a c h class. C o a t i n g s are a p p l i e d easily a n d t h e y are e c o nomical. C h e m i c a l impregnation usually involves full-cell pressure t r e a t m e n t a n d c a n b e c o s t l y . A c o a t i n g is s u b j e c t to a b r a s i o n o r w e a r t h a t c a n d e s t r o y t h e e f f e c t i v e n e s s o f t h e fire r e t a r d a n t . C h e m i c a l i m p r e g n a t i o n s d e p o s i t t h e fire r e t a r d a n t w i t h i n t h e w o o d , so t h a t i f t h e s u r f a c e is a b r a d e d , c h e m i c a l s a r e s t i l l p r e s e n t . O n - s i t e a p p l i c a t i o n o f s u r f a c e c o a t i n g s r e q u i r e s s t r i c t c o n t r o l o f t h e a m o u n t a p p l i e d to e n s u r e c o r r e c t l o a d i n g l e v e l s f o r a p a r t i c u l a r f l a m e - s p r e a d r a t i n g (11). B o t h coating a n d i m p r e g n a t i o n systems are based o n the same c h e m ical c o m p o u n d s , a l t h o u g h t h e f o r m u l a t i o n s for e a c h vary. M o s t o f t h e c h e m i c a l s u s e d i n fire-retardant f o r m u l a t i o n s h a v e a l o n g h i s t o r y of use for this p u r p o s e , a n d most formulations are b a s e d o n e m p i r i c a l investigations for best o v e r a l l p e r f o r m a n c e . T h e s e c h e m icals i n c l u d e the phosphates, s o m e n i t r o g e n c o m p o u n d s , s o m e b o rates, silicates, a n d m o r e recently, amino-resins. T h e s e c o m p o u n d s r e d u c e t h e f l a m e s p r e a d o f w o o d b u t h a v e d i v e r s e effects o n s t r e n g t h , hygroscopicity, durability, machinability, toxicity, gluability, and p a i n t a b i l i t y ( I , 12, 13).

Test Methods K n o w l e d g e o f v a r i o u s test m e t h o d s u s e d to e v a l u a t e t h e effec t i v e n e s s o f fire r e t a r d a n t s is n e c e s s a r y t o u n d e r s t a n d t h e m e c h a n i s m s

In The Chemistry of Solid Wood; Rowell, Roger; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.

14.

LEVAN

533

Chemistry of Fire Retardancy


o f fire r e t a r d a n c y a n d f o r m u l a t i o n s o f fire r e t a r d a n t s . S o m e o f t h e s e tests a r e u s e d b y r e g u l a t o r y a g e n c i e s t o e v a l u a t e b u i l d i n g m a t e r i a l s a n d s o m e a r e u s e d f o r research a n d d e v e l o p m e n t w o r k only. T h e c o m m o n l y u s e d test m e t h o d s a p p l i c a b l e to evaluate fire-retardant treatments i n c l u d e t h e r m o g r a v i m e t r i c analysis (TG); differential t h e r m a l analysis ( D T A ) , a n d a similar technique, differential scanning c a l o r i m e t r y ( D S C ) ; 2 - , 8-, a n d 25-ft t u n n e l f l a m e - s p r e a d tests; a n d the o x y g e n i n d e x test. O t h e r test m e t h o d s a r e u s e d t o evaluate t h e effect o f fire-retardant t r e a t m e n t s o n s u c h r e l a t e d p r o p e r t i e s as s m o k e d e v e l o p m e n t , heat release rate, a n d toxicity. Thermogravimetric Analysis (TG). T G involves weighing a sample w h i l e i t is exposed to heat. T h e c h i e f use o f this t e c h n i q u e has b e e n t o s t u d y t h e t h e r m a l d e c o m p o s i t i o n o f p o l y m e r i c m a t e r i a l s and to accumulate kinetic information about such decomposition. A sample is s u s p e n d e d o n a sensitive balance that measures the w e i g h t ( F i g u r e 1) as i t i s e x p o s e d t o a f u r n a c e . A i r , n i t r o g e n , o r a n o t h e r gas flows around the sample to remove the pyrolysis o r combustion p r o d -

SPRING-

-DEMODULATOR Χ Ύ RECORDER

TRANSDUCER ARMATURE^

TIME TEMPERATURE SWITCH

TRANSDUCER COIL

WT.

H O · + Ο ·

(D

Ο · + H

2

-» H O· + Η ·

(2)

H

T h e s e t w o equations g o v e r n t h e e x p o n e n t i a l increase i n free r a d i c a l concentration; however, these postulations are based o n p r e m i x e d


14.

LEVAN

545


gas f l a m e s w i t h e x c e s s o x y g e n a v a i l a b l e . A p p l i c a t i o n o f t h i s t h e o r y t o the c o m b u s t i o n of solids must be treated w i t h reservations because the combustion of w o o d proceeds i n oxygen-deficient diffusion flames w h o s e processes are a c o m p l e x series of simultaneous reactions d e p e n d e n t o n the m a t e r i a l a n d t h e e n v i r o n m e n t . T h e r e f o r e , the exact


r o l e f r e e r a d i c a l s p l a y i n t h e c o m b u s t i o n o f w o o d is n o t k n o w n . C e r t a i n fire r e t a r d a n t s affect v a p o r - p h a s e r e a c t i o n s b y i n h i b i t i n g t h e c h a i n r e a c t i o n s i n R e a c t i o n s 1 a n d 2. H a l o g e n s s u c h as b r o m i n e a n d c h l o r i n e are g o o d free r a d i c a l i n h i b i t o r s a n d have b e e n s t u d i e d e x t e n s i v e l y i n t h e p l a s t i c s i n d u s t r y (40, 49, 5 0 ) . G e n e r a l l y , l a r g e a m o u n t s of h a l o g e n are r e q u i r e d ( 1 5 - 3 0 % b y weight) to attain a p r a c t i c a l d e g r e e o f fire r e t a r d a n c e . T h e e f f i c i e n c y o f t h e h a l o g e n d e c r e a s e s i n t h e o r d e r B r > C l > F. A m e c h a n i s m f o r t h e i n h i b i t i o n o f t h e c h a i n b r a n c h i n g r e a c t i o n s ( u s i n g H B r as t h e h a l o g e n ) is H

· + H B r -» H

2

+ Br ·

(3)

O H · + H B r -* H 0 + Br ·

(4)

2

T h e h y d r o g e n h a l i d e c o n s u m e d i n t h e s e r e a c t i o n s is r e g e n e r a t e d t o continue the i n h i b i t i o n . A l t h o u g h this proposed m e c h a n i s m was based on experiments w i t h p r e m i x e d hydrocarbon flames, the same o r d e r of effectiveness exists w i t h w o o d . A n a l t e r n a t e m e c h a n i s m (Reactions 5 - 7 ) was suggested for h a l o g e n i n h i b i t i o n w h i c h i n v o l v e s r e c o m b i n a t i o n o f o x y g e n a t o m s (50). Ο · + B r · + M —* B r O · + M *

(5)

Ο · + Br -» BrO · + Br ·

(6)

Ο · + O B r · -* Br · + 0

(7)

2

2

T h u s t h e i n h i b i t i v e effect r e s u l t s f r o m t h e r e m o v a l o f a c t i v e o x y g e n a t o m s ( O ·) f r o m t h e v a p o r p h a s e . A d d i t i o n a l i n h i b i t i o n c a n r e s u l t from r e m o v a l of O H radicals i n the c h a i n - b r a n c h i n g reactions: BrO · + BrO · +

· O H -» H B r + 0

(8)

2

· O H —» B r · + H 0

2

(9)

Reactions 5 - 9 explain the lack of halogen inhibition i n h y d r o c a r b o n n i t r o u s o x i d e f l a m e s w h e r e t h e h y d r o g e n - o x y g e n c h a i n is n o t r e q u i r e d f o r o x i d a t i o n (50). S o m e p h o s p h o r u s c o m p o u n d s a l s o h a v e b e e n f o u n d t o i n h i b i t f l a m i n g c o m b u s t i o n b y t h i s m e c h a n i s m (51).

INCREASED CHAR/REDUCED VOLATILES THEORIES. M o s t o f t h e e v i d e n c e r e l a t i n g t o t h e m e c h a n i s m o f fire r e t a r d a n c y i n t h e b u r n i n g of w o o d indicates that retardants alter fuel p r o d u c t i o n b y increasing the amount of char a n d r e d u c i n g the amount of volatile, combustible In The Chemistry of Solid Wood; Rowell, Roger; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.

546

T H E CHEMISTRY OF SOLID W O O D


v a p o r s . M a n y f i r e r e t a r d a n t s f o r w o o d a l s o l o w e r t h e t e m p e r a t u r e at w h i c h active pyrolysis occurs. E a r l y s t u d i e s i n v o l v e d t r e a t m e n t o f w o o d s p e c i m e n s w i t h firer e t a r d a n t c h e m i c a l s , t h e n s u b j e c t i n g the t r e a t e d s p e c i m e n s to t h e r m a l a n a l y s i s b y T G . B r o w n e a n d T a n g (41) t e s t e d e i g h t c o m p o u n d s , s o m e o f w h i c h w e r e k n o w n t o b e e f f e c t i v e fire r e t a r d a n t s , and some of w h i c h w e r e not. T h e T G results (Figures 8 - 1 0 ) indicate that a l l c o m p o u n d s increased the residual char w e i g h t of the material. E x c e p t f o r s o d i u m t e t r a b o r a t e , t h e m o r e e f f e c t i v e t h e salt as a f l a m e retardant the l o w e r the t e m p e r a t u r e of active pyrolysis a n d the greater the a m o u n t of char. T h e s e results w e r e c o n f i r m e d t h r o u g h r e p e a t e d e x p e r i m e n t s (45, 46). E x p e r i m e n t s were conducted on the pyrolysis products of w o o d s a m p l e s to affirm that t h e i n c r e a s e d a m o u n t s o f c h a r i n v o l v e d a d e c r e a s e i n t h e a m o u n t o f c o m b u s t i b l e tars (52). T h e c h e m i c a l s i n c r e a s e d t h e y i e l d o f c h a r , w a t e r , a n d n o n c o n d e n s a b l e gases at t h e expense of the f l a m m a b l e tar fraction. T h e s e results c o n f i r m e d that the increased a m o u n t of r e s i d u a l char i n T G results was associated w i t h the reduction of the combustible volatiles. A p o s s i b l e c h e m i c a l m e c h a n i s m for the r e d u c t i o n of these c o m b u s t i b l e v o l a t i l e s is t h a t f i r e - r e t a r d a n t c h e m i c a l s s o m e h o w i n h i b i t e d the formation of levoglucosan (1,6-anhydroglucopyranose), a major volatile fraction obtained from the t h e r m a l degradation of cellulose (see C h a p t e r 13). T h e r e s u l t s o b t a i n e d f r o m T G p r o m p t e d m a n y r e s e a r c h e r s to i n v e s t i g a t e t h i s p o s s i b l e m e c h a n i s m . T h e a m o u n t o f l e v o glucosan p r o d u c e d b y treated and untreated specimens of cellulose w a s a n a l y z e d a n d t h e r e s u l t s c a n b e f o u n d i n T a b l e I (53). A l l t h e chemicals i n Table I r e d u c e d the percentage of levoglucosan regard less o f t h e r e l a t i v e e f f e c t i v e n e s s o f t h e fire r e t a r d a n t as d e t e r m i n e d b y t h e o x y g e n i n d e x t e s t . T h e i r findings i n c l u d e t h e effect o f a c i d i c , n e u t r a l , a n d b a s i c a d d i t i v e s o n t h e l e v o g l u c o s a n y i e l d (Table II). T h e a c i d t r e a t m e n t h a d t h e m o s t p r o n o u n c e d effect o n t h e b r e a k d o w n . T h e s e results a n d the oxygen index results suggest that alkali a n d a c i d t r e a t m e n t s i m p a r t f l a m e r e t a r d a n c y to c e l l u l o s e t h r o u g h d i f ferent chemical mechanisms. In degree of p o l y m e r i z a t i o n ( D P ) studies of borax treatments a n d a m m o n i u m d i h y d r o g e n o r t h o p h o s p h a t e (53), c e l l u l o s e t r e a t e d w i t h t h e a c i d c h a r r e d a n d d e p o l y m e r i z e d v e r y r a p i d l y . Its D P v a l u e d e c r e a s e d f r o m 1 1 1 0 to 6 5 0 a f t e r o n l y 2 m i n o f h e a t i n g at 1 5 0 ° C . C e l l u l o s e t r e a t e d w i t h b o r a x s h o w e d a D P r e d u c t i o n f r o m 1 3 0 0 to 7 0 0 a f t e r 1 h o f h e a t t r e a t m e n t at 1 5 0 ° C . B o t h t h e s e c o m p o u n d s catalyzed the suppression of levoglucosan formation but they had d i f f e r e n t effects o n t h e c h a i n d e p o l y m e r i z a t i o n r e a c t i o n (53).


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Chemistry of Fire Retardancy - ACS Publications - American Chemical

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