Wood Technology: Chemical Aspects

7 Properties of W o o d during Carbonization under Fire Conditions

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F. C. BEALL Faculty of Forestry and Landscape Architecture, University of Toronto, 203 College St., Toronto, M5S 1A1 Canada

Most of the knowledge concerning the behavior of wood under high temperature conditions is limited to such externally-measured quantities as mass loss or gas generation. Two general approaches have been used to predict the pattern of wood deterioration during f i r e exposure. The more elegant approach is through the d i f f e r ential form of the unidirectional heat-flow equation. Thermal d i f f u s i v i t y , which is the equation "constant", has a very complex behavior with temperature, precluding a formal solution to the equation. However, numerical solutions, such as can be developed from finite-element techniques, permit a means of evaluating this "constant" for small temperature increments (1). No serious effort has been made in past research to fully evaluate thermal d i f f u s i vity of wood as i t changes with temperature. Such information is elementary for designing methods of f i r e retardation, which in the past, have been almost exclusively trial and error. It is also evident that the physical changes in wood during fire exposure should be examined in an effort to modify thermal d i f f u s i v i t y . A second approach involves the evaluation or creation of empirical equations based on experimental data. The most widely-used method relates the rate of charring to the change of wood density (2). However, just as the density factor in thermal d i f f u s i v i t y is r e l a t i v e l y undefined, so is the density change from wood to char. Fragmentary information has been published on the densi ty change (3, 4), but no systematic study has been done. The purpose of this study was to c l a r i f y the change of density with species, heating rate, and temperature under oxygen-defîcient conditions. The major constraint was an arbitrary specimen size (10-mm cube), based on the observation that thicknesses equal to or greater than about 6 mm (approximately the half-thickness of the cubes) produce consistent charring rates (5). The effect of thickness on density changes is currently being studied. Experimental Procedure Sample Preparation.

The six wood species (Table I) were

107

0.71

0.55

0.45

0.39

0.37

0.25

0.25

0.24

0.22

0.29

Hard Maple

Southern pi ne

Douglas f i r

Basswood

Redwood

0.23

0.24

0.26

1.17 1.25

0.367 0.262

0. 184 0. 180

Wood Handbook (£).

1.33

1.16

0.327

0.323

0. 188

1.38

1.44

0. 176

0.361

0. 176

0.46 0.35

0.326

0. 175

0.58

Subscript c = char; ο = ovendry c o n t r o l value

0.80

0.31

Whi te oak

0

5

Q

p

ο

1 .69

0.52

0.65

0.58

1 .58 1 .41

0.64

0.61

2 .06 1 .54

0.57

1 .87

0. 07

0. 16

0. 12

0. 12

0. 15

0. 16

Table I. Mass, d e n s i t y , and shrinkage r e l a t i o n s h i p s a t 600°C among wood species heated a t 1°C/min. a m c Δν/ν ^ ΔΤ/AR Po , c m ΔΤ/Τ AL/L (g/cm3) (g/cnv ) Species char moi sture char moisture ο


7.

BEALL

Carbonization

under Fire

Conditions

109


s e l e c t e d f o r a w i d e range o f densîty, s t r u c t u r e , and s h r i n k a g e (from moisture l o s s ) b e h a v i o r . A l 1 s p e c i m e n s were s e l e c t e d f r o m f l a t - s a w n , k i I n - d r i e d b o a r d s w h i c h were f r e e f r o m v i s i b l e d e f e c t s . S e v e r a l l e n g t h s o f n o m i n a l 10 by 10 mm s t o c k f r o m e a c h s p e c i e s wi t h good rad i a l and t a n g e n t i a l f a c e s provî ded s u f f i c i e n t specimen m a t e r i a l , e i t h e r end-matched o r c o n t a i n i n g v i r t u a l l y t h e same g r o w t h r i n g s . A f t e r t h e s t o c k was c r o s s c u t , f i n a l d i m e n s i o n s o f t h e 10 mm cubes were o b t a i n e d by sand i n g . A l 1 s p e c i m e n s were o v e n d r i e d and s t o r e d o v e r d e s i c c a n t u n t i 1 m e a s u r e d . Sample Measurements. Mass was d e t e r m i n e d u s i n g an a n a l y t i c a l b a l a n c e o f 10"i>g s e n s i t î v i t y . P r i o r r e s e a r c h on c a r b o n i zed wood has shown t h a t î t s spongy a n d / o r fragîle n a t u r e c o u l d c a u s e e r r o r s i n m e c h a n i c a l measurement o f d i m e n s i o n s . T h e r e f o r e , dime n s i o n s were o b t a i n e d u s i n g a camera mounted on an i n c i d e n t l i g h t m i c r o s c o p e t o p h o t o g r a p h s p e c i m e n s wi t h a c a l i b r a t e d g r i d ( m i c r o m e t e r e y e p i e c e ) r e s t i n g on e a c h o f t h e f a c e s . The d e v e l o p e d n e g a t i ves were mounted i n s 1i des and p r o j e c t e d t o d î r e c t i y measure e a c h f a c e . Two w i d t h s were d e t e r m i ned a t un î f o r m s p a c i n g i n e a c h a x i s, f o r a t o t a l o f h measurements p e r f a c e , o r 2k p e r c u b e . The p a i r e d measurements were l a t e r a v e r a g e d . A f t e r c a r b o n i z a t i o n , t h e cube f a c e s were p h o t o g r a p h e d i n an îdentical s e q u e n c e t o p e r m i t s h r i nkage a n a l y s î s o f i n d i v i d u a l f a c e s . Specimen e x p o s u r e t o atmospherîc c o n d i t i o n s was m i n i m i z e d . Sample Runs. A b l o c k d i a g r a m o f t h e s y s t e m i s shown i n F i g u r e 1. T h r e e s p e c i m e n s were p l a c e d i n a n i c k e l b o a t and posi t i o n e d i n the Vycor f u r n a c e tube. The s y s t e m was i n i t i a l l y f l u s h e d wi t h n i t r o g e n a t 0.8 Jtt/min t o remove o x y g e n , and r e d u c e d t o 0.2 £t/min d u r i ng t h e r u n . H e a t i n g r a t e (1, 10, 50°C/min) was preset on t h e t e m p e r a t u r e programmer whi ch m a i n t a i n e d a 1 i n e a r r a t e u s i n g a p l a t i n u m r e s i stance s e n s i n g element d i r e c t l y below the f u r n a c e t u b e and h a v i n g a p r o p o r t i o n a t i n g o u t p u t v o l t a g e t o the f u r n a c e . The sample t e m p e r a t u r e was moni t o r e d wi t h a CR/AL t h e r m o c o u p l e a d j a c e n t t o t h e f a c e o f one s a m p l e . T h e r m o c o u p l e EMF was f e d t h r o u g h an e l e c t r o n i c r e f e r e n c e j u n c t i o n t o a s t r i p c h a r t r e c o r d e r wi t h a c a l i b r a t e d s p a n . When t h e d e s i red f i n a l t e m p e r a t u r e was r e a c h e d , t h e h e a t i n g was s t o p p e d and t h e s p l i t f u r n a c e e l e m e n t opened t o expedî t e c o o l i n g . R e s u l t s and D i s c u s s i o n A typî c a l mass l o s s c u r v e f o r t h e t h r e e h e a t i n g r a t e s i s shown i n F i g u r e 2. The f r a c t i o n a l r e s i d u a l ( c h a r ) mass a t 600°C i s g i v e n f o r e a c h s p e c i e s i n T a b l e I. Considerable data are a v a i l a b l e f r o m t h e 1 i t e r a t u r e , p a r t i c u l a r l y from thermogravimetry s t u d i e s , on t h e înf1uence o f s p e c i m e n and h e a t i n g p a r a m e t e r s on mass l o s s c h a r a c t e r i s t i c s .

110

WOOD TECHNOLOGY:

-T/C

"S—®

iPROGh


CHEMICAL

R

Figure 1. Block diagram of heating system. G = gas; F = flowmeter; T/C = thermocouple; REF = reference junction; REC = recorder. Dashed line indicates boundary of furnace.

100

300

m

TEMPERATURE (0

_ 600

Figure 2. Mass loss of redwood at three heating rates to end temperatures of 250°, 300°, 350°, 400°, and 600°C

m

TBTOWUPOC)

6 0 0

Figure 3. Longitudinal, radial, and tangential shrinkage of redwood heated at 1 °C/min

ASPECTS


7.

BEALL

Carbonization

under Fire

Conditions

111

General Shrinkage Behavior. Shrinkage i n the major axes i s shown i n F i g u r e 3 f o r a t y p i c a l s p e c i e s h e a t e d a t l°C/min. C e r t a i n f e a t u r e s were common f o r a l l s p e c i e s : s i m i l a r s h r i n k a g e r a t e s i n a l l a x e s s t a r t i n g a t a b o u t 350°C, g r e a t e r t a n g e n t i a l t h a n r a d i a l s h r i n k a g e a t a l l t e m p e r a t u r e s , a l a g i n o n s e t and l o w e s t v a l u e f o r l o n g i t u d i n a l s h r i n k a g e , a n d p r a c t i c a l l y i dentî c a l l o n g i tud î n a l s h r i nkage b e h a v i o r and v a l u e s f o r a l 1 s p e c i e s ( T a b l e I ) . The T/R s h r i n k a g e r a t i o s f r o m c a r b o n i z a t i o n show a similarî t y t o t h o s e r e p o r t e d f o r moi s t u r e - l o s s s h r i nkage ( 6 ) , h o w e v e r , measurements must be made on c o n t r o l s b e f o r e t h e relatîonshî ρ can be establî s h e d . I η g e n e r a l , t h e r e i s a t e n d e n c y toward i s o t r o p i sm i n t r a n s v e r s e s h r i n k a g e ( T , R ) , p a r t i c u l a r l y f o r basswood a n d s o u t h e r n p i ne. Redwood, b e c a u s e o f î t s r e l a t i v e l y low t a n g e n t i a l s h r i n k a g e , behaves more i s o t r o p i c a l l y t h a n t h e o t h e r s . The a n a l o g y between c h a r and moi s t u r e i s l e s s c l e a r f o r volumetrî c s h r i n k a g e , a l t h o u g h a t r e n d i s o b v i o u s . As wi t h t h e T/R a n a l o g y , measurements must be made o n c o n t r o l s t o c l a r i f y any r e l a t i o n s h i p . D e n s i t y Changes. The v a r i a t i o n o f d e n s i t y wi t h t e m p e r a t u r e was t h e m a j o r relatîonshî ρ s o u g h t i n t h i s s t u d y . From t h e c h a r dens i t y v a l u e s o b t a i ned a t 600 °C, i t was p o s s i b l e t o e s t a b l i sh t h e fο 11ow î n g r e g r e s s i o n e q u a t i o n s : (1)

p

c

= -0.078 + 0.79 P

(2)

p

c

(3)

P

c

( t = 1 °C/min)

r

2

= 0.98

= -0.049 + 0.71 Po

(t = 10 °C/min)

r

2

= 0.99

= -0.006 + 0.56 p

(f = 50°C/min)

r

2

= 0.97

0

0

A l 1 o f t h e s e r e l a t i o n s h i p s a p p l y t o c o n d i t i o n s o f an o x y g e n d e f i c i e n t a t m o s p h e r e and r a p i d e s c a p e o f v o l a t i l e s . The more comp1ex dependence o f dens i t y o n b o t h t e m p e r a t u r e and heat i ng r a t e i s shown i n F i g u r e k. Hi gher r a t e s o f h e a t i n g d e l a y t h e d e n s i t y change u n t i 1 a b o u t 400 ΐ , where t h e c u r v e s c r o s s and show a d i r e c t dependence between d e n s i t y change and h e a t i n g r a t e . L o n g i tud i n a l S h r i n k a g e . Despite the possi ble analogies between c a r b o n i z a t i o n and moi s t u r e l o s s f o r t r a n s v e r s e s h r i n k a g e o f wood, l o n g i t u d i n a l s h r i n k a g e a t 600°C d i d n o t v a r y s i g n i fîc a n t l y among s p e c i e s . The mean v a l u e o f l o n g i t u d i n a l s h r i n k a g e f o r t h e s i x s p e c i e s was 18.0% wi t h a s t a n d a r d d e v i a t i o n o f 0.5%. C a l c u l a t i o n s by Bacon and Tang (7) show t h a t t h e r e d u c t i o n i n l e n g t h o f c e l 1 i b i o s e , i f i t we re t r a n s f o r m e d i n t o graphî t e , w o u l d be 17.3%. T h i s c l o s e agreement s u p p o r t s t h e c o n c e p t o f i n s i t u c e l 1 u l o s e l o s i n g oxygen and f o r m i n g a g r a p h i t i c - t y p e s t r u c t u r e d u r i n g c o n t r a c t i o n o f t h e c h a i n s . Addi t i o n a l l y , t h e t h r e e h e a t i n g r a t e s p r o d u c e d d î f f e r e n t s h r i nkage p a t h s , b u t t h e same ( s t a t i s t i c a l l y ) e n d p o i n t a t 600°C ( F i g u r e 5). A f u r t h e r d i f f e r e n c e i s o b v i o u s between l o n g i t u d i n a l and t r a n s v e r s e s h r i n k a g e when t h e s e a r e p l o t t e d a g a i n s t mass l o s s ( F i g u r e 6) i n s t e a d o f t e m p e r a t u r e ( F i g u r e 3 ) . The l a g i n l o n g i t u d i n a 1


112

WOOD TECHNOLOGY:

Figure 4. Density of southern pine as affected by heating rate and temperature. Dashed line is the probable path between points at 400° and 600°C.

IT)

Figure 5.

I o

ifl) T M M P E "(0

600

Effect of heating rate on longitudi nal shrinkage of redwood

I

20

I

I

m ω Y\SS LOSS (%)

L

at)

f

Figure 6. The rehtionship of shrinkage in the major axes to mass loss of southern pine

CHEMICAL

ASPECTS


7.

BEALL

Carbonization

100| 0

under

, 20

Fire

, «

113

Conditions

ι 50

ι 80

ι 100

TOTAL mss LOSS ω

Figure 7.

Effect of heating rate on loss of oxygen from Douglas fir

nXYPEN mss

(%)

Figure 8. Effect of heating rate on the rela tionship between longitudinal shrinkage and loss of oxygen


114

WOOD TECHNOLOGY:

CHEMICAL

ASPECTS

s h r i n k a g e i s much more p r o n o u n c e d , p a r t i c u l a r l y a t 300°C, w h i c h s t r o n g l y s u g g e s t s t h a t t h e m a j o r i n i t i a l mass l o s s ( t o a b o u t 30%) o c c u r s from v o l a t i l i z a t i o n o f c a r b o h y d r a t e s i d e g r o u p s . Shrinkage f r o m 1 i g n i n o r a n y l o s s o f backbone oxygen from t h e p o l y s a c c h a r i d e s s h o u l d c a u s e a component o f l o n g i t u d i n a l s h r i n k a g e . The r e l a t i o n s h i p s were s t u d i e d f u r t h e r by u l t i m a t e a n a l y s i s f o r o x y g e n and h y d r o g e n . Fî g u r e 7 shows t h e p e r c e n t a g e o f oxygen l o s s a s a f u n c t i o n o f t o t a l mass l o s s a t t h e t h r e e r a t e s o f h e a t ing. The c u r v e s c l e a r l y show a much g r e a t e r l o s s o f oxygen a t h i g h e r h e a t i n g r a t e s d u r i n g t h e i n i t i a l mass l o s s . By c o m b i n i n g Fi g u r e s 6 and 7 i n t o Fî g u r e 8, t h e r e l a t i o n s h i p s a p p e a r much c l e a r e r . A t l°C/min, backbone oxygen i s a p p a r e n t l y l o s t a t a s u f f i c i e n t l y s low enough r a t e t o permi t new c a r b o n - t o - c a r b o n v a l e n c e bonds t o f o r m . H i g h e r h e a t i n g r a t e s p r e f e r e n t i a l l y remove s i d e g r o u p ( h y d r o x y l ) oxygen a n d / o r c a u s e a l a g i n C-C bondi ng a t t h e n e w l y - c r e a t e d backbone s î t e s . However, t h e l o n g i t u d i n a l s h r i n k a g e a t 600°C i s r e a s o n a b l y c o n s t a n t f o r a l 1 s p e c i e s and h e a t i n g r a t e s .

Literature Cited 1. 2. 3. 4. 5. 6. 7.

Knudson, R.M. and A . P . Schniewind. For. Prod. J. (1975) 25 (2):23-32. Schaffer, E . L . U.S. For. Serv. Res. Note FPL-0145. 1966. B e a l l , F.C. P.R. Blankenhorn, and G.R. Moore. Wood Science (1975) 6 (3):212-219. McGinnes, E . A . , S.A. Kandeel, and P.S. Szopa. Wood and Fiber (1971) 3 (2):77-83. A k i t a , Κ. Report Fire Res. Inst. Japan 9 (1,2). 1959. Anon. "Wood Handbook - wood as an engineering material". Govt. Printing Office. Washington. 1974. Bacon, R. and M.M. Tang. Carbon (1964) 2:221-225.

This research was funded by National Research Council Canada and the Canadian Forestry Service. Portions of the study were in cooperation with the Universi ty of Missouri.

Wood Technology: Chemical Aspects

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