The Chemistry of Solid Wood - American Chemical Society

3 Wood-Water Relationships

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C. SKAAR Department of Forest Products, School of Forestry and Wildlife, College of Agriculture and Life Sciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061

Wood is a hygroscopic material, and its mass, dimensions, and density, as well as its mechanical, elastic, electrical, thermal, and transport properties are affected by its moisture content. Wood is formed in a water-saturated environment in the living tree, but most of the water is removed prior to use. In use its moisture content and dependent properties change with changes in ambient conditions, particularly relative humidity. Wood is anisotropic with respect to most of its physical properties. The thermodynamics of moisture sorption, including enthalpy, free energy and entropy changes, are moisture dependent. Water sorption by wood is treated in terms of both surface and solution theories. Moisture transport in wood is also treated, particularly in relation to drying.

Wood Moisture and the Environment W o o d differs f r o m m o s t m a t e r i a l s u s e d for c o n s t r u c t i o n a n d o t h e r p u r p o s e s i n t h a t i t is c o n t i n u a l l y e x c h a n g i n g m o i s t u r e w i t h i t s s u r r o u n d i n g s . T h i s is t r u e i n b o t h t h e l i v i n g t r e e as w e l l as u n d e r c o n d i t i o n s o f final u s e . T h e m o i s t u r e c o n t e n t o f w o o d is u s u a l l y c a l c u l a t e d i n t e r m s o f its d r y w e i g h t . T h e fractional moisture content m is d e f i n e d as t h e ratio of the mass W o f r e m o v a b l e w a t e r to t h e d r y mass W of t h e w o o d ( E q u a t i o n 1). w

0

m

(1)

= WJW

0

M o i s t u r e c o n t e n t is o f t e n e x p r e s s e d i n t e r m s o f percent weight, or M

= 1 0 0 x m = 1 0 0 (WJW ) 0

0065-2393/84/0207-0127/$12.25/0 © 1984 American Chemical Society In The Chemistry of Solid Wood; Rowell, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.

of d r y

(2)

128

T H E CHEMISTRY O F SOLID W O O D

T h e d e f i n i t i o n o f M as g i v e n a b o v e is e q u i v a l e n t to t h e t e r m r e g a i n as u s e d f o r c e r t a i n o t h e r h y g r o s c o p i c m a t e r i a l s s u c h as t e x t i l e s (J). T h e t e r m m o i s t u r e c o n t e n t is d e f i n e d o n a w e t r a t h e r t h a n d r y w e i g h t b a s i s . T h e wet basis moisture content M is t h e n r e l a t e d to M by w

M Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on September 27, 2015 | http://pubs.acs.org Publication Date: May 5, 1984 | doi: 10.1021/ba-1984-0207.ch003

w

= M / ( l + M/100)

(3)

It m a y b e n o t e d that M can b e greater t h a n 1 0 0 % b u t that M is a l w a y s l e s s t h a n 1 0 0 % . T h e d r y w e i g h t b a s i s , e i t h e r m o r M , w i l l be used throughout this chapter. w

W a t e r i n the L i v i n g T r e e . W o o d i n t h e l i v i n g t r e e is f o r m e d and functions in an essentially water-saturated environment. T h e f u n c t i o n i n g s a p w o o d cells are a part of the vascular system that c o n ducts w a t e r a n d solutes f r o m t h e roots to t h e leaves t h r o u g h a c o n t i n u o u s w a t e r - s a t u r a t e d n e t w o r k o f w o o d c e l l s (2). W h e n t h e t r e e is f e l l e d t h e w a t e r i n t h e w o o d is c u t off f r o m t h e s o i l w a t e r a n d t h e w o o d c o m m e n c e s to lose m o s t o f its m o i s t u r e . Moisture Content of G r e e n W o o d . T h e moisture content of w o o d i n a f r e s h l y f e l l e d t r e e is d e s i g n a t e d as t h e green moisture content. T h e g r e e n m o i s t u r e c o n t e n t m a y v a r y c o n s i d e r a b l y a m o n g different k i n d s of trees a n d b e t w e e n h e a r t w o o d a n d sapwood w i t h i n a tree. It m a y also v a r y w i t h h e i g h t i n the tree a n d w i t h the season o f t h e y e a r i n w h i c h t h e t r e e is f e l l e d . T h e green moisture content of the heartwood of 27 different softwood species g r o w n i n the U n i t e d States, based on p e r c e n t of o v e n - d r y w e i g h t , is r e p o r t e d t o r a n g e f r o m 3 0 t o 1 2 1 % w i t h a m e a n o f 5 5 % (3). F o r s a p w o o d o f t h e s a m e s o f t w o o d s t h e m e a n w a s 1 4 9 % w i t h a range f r o m 98 to 2 4 9 % . I n contrast, for 34 h a r d w o o d s , n o consistent difference was found i n the green moisture contents of h e a r t w o o d a n d s a p w o o d . T h e m e a n h e a r t w o o d v a l u e was 8 1 % (range f r o m 44 to 162%), close to t h e m e a n o f 8 3 % (range f r o m 44 to 146%) for the s a p w o o d o f t h e same trees. S t u d i e s o n Pinus taeda (4) i n d i c a t e a s t r o n g i n c r e a s e i n g r e e n moisture content w i t h increasing height i n the tree. Similar trends w e r e o b s e r v e d a m o n g a n u m b e r o f A p p a l a c h i a n h a r d w o o d s a n d soft w o o d s (5). L o g s cut f r o m trees f e l l e d d u r i n g late w i n t e r a n d early s p r i n g i n temperate climates generally exhibit higher green moisture contents than those h a r v e s t e d d u r i n g s u m m e r a n d fall. W a t e r i n g r e e n w o o d is f o u n d i n t h r e e b a s i c f o r m s : bound w a t e r i n t h e c e l l w a l l s , free o r capillary water i n the cell cavities, a n d water vapor, a l s o i n t h e c e l l c a v i t i e s . T h e t o t a l a m o u n t o f w a t e r i n v a p o r f o r m is n o r m a l l y o n l y a s m a l l f r a c t i o n o f t h e t o t a l a n d is n e g l i g i b l e at normal temperatures and moisture contents. W h e n green w o o d dries

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

3.

Wood-Water

S K A A R

129

Relationships


t h e w a t e r l e a v e s t h e c e l l c a v i t i e s f i r s t b e c a u s e i t is h e l d w i t h s m a l l e r forces t h a n the b o u n d water. F u r t h e r m o r e , most p h y s i c a l p r o p e r t i e s , s u c h as s t r e n g t h p r o p e r t i e s a n d s h r i n k a g e , a r e u n a f f e c t e d b y r e m o v a l o f f r e e w a t e r (See C h a p t e r 5). T h e fiber-saturation point is d e f i n e d as t h e m o i s t u r e c o n t e n t at w h i c h the c e l l cavities are e m p t y of l i q u i d water b u t the c e l l walls a r e s t i l l s a t u r a t e d w i t h b o u n d w a t e r (6). T h e fiber-saturation p o i n t is d e s i g n a t e d as rrif ( f r a c t i o n o f d r y mass) o r M f ( p e r c e n t o f d r y mass). M e a s u r i n g W a t e r Content of W o o d . T h e r e a r e as m a n y as fifteen m e t h o d s that h a v e b e e n u s e d to m e a s u r e w o o d m o i s t u r e c o n t e n t (7). S o m e o f t h e m o r e c o m m o n o r u s e f u l m e t h o d s a r e d i s c u s s e d here. GRAVIMETRIC M E T H O D . T h e m o i s t s a m p l e is w e i g h e d , W , a n d t h e n d r i e d u n t i l a r e f e r e n c e w e i g h t , W , is a t t a i n e d . T h e d i f f e r e n c e is t a k e n as t h e w e i g h t o f w a t e r , W , i n the moist wood. Ordinarily w o o d is d r i e d i n a c o n v e c t i o n o v e n m a i n t a i n e d at 1 0 3 ± 2 ° C . I n t h i s c a s e , t h e a t m o s p h e r e is at a s u f f i c i e n t l y l o w r e l a t i v e v a p o r p r e s s u r e h (h = p/p ; ρ is t h e a m b i e n t w a t e r v a p o r p r e s s u r e a n d p is t h e v a p o r p r e s s u r e o f p u r e w a t e r at t h e o v e n t e m p e r a t u r e ) t h a t h is a s s u m e d to b e z e r o . M

0

w

0

0

T h e r e are several errors i n v o l v e d i n g r a v i m e t r i c m o i s t u r e m e a s u r e m e n t s . O n e e r r o r is t h e a s s u m p t i o n t h a t h is z e r o i n a n o r d i n a r y o v e n . T h i s effect c a n b e m i n i m i z e d b y u s i n g a v a c u u m o v e n o r a s t r o n g d e s i c c a n t s u c h as p h o s p h o r u s p e n t o x i d e . A n o t h e r p r o b l e m is the e v a p o r a t i o n o f v o l a t i l e w o o d c o n s t i t u e n t s , i f p r e s e n t , to g i v e a higher apparent moisture content in the wood. A third p r o b l e m in a c c u r a t e m o i s t u r e m e a s u r e m e n t is t h e effect o f s a m p l e m o i s t u r e h i s t o r y (8). A v a r i a t i o n o f t h e g r a v i m e t r i c m e t h o d is to h e a t t h e w o o d i n a d i s t i l l a t i o n a p p a r a t u s c o n t a i n i n g a w a t e r - i m m i s c i b l e l i q u i d s u c h as toluene or xylene. T h i s l i q u i d dissolves the organic volatiles a n d the w a t e r c o n d e n s e s i n a s e p a r a t e c a l i b r a t e d t r a p w h e r e i t is c o l l e c t e d and m e a s u r e d . K A R L FISCHER TITRATION M E T H O D .

In

this m e t h o d the

moisture

c o n t e n t is m e a s u r e d b y t i t r a t i o n , u s i n g t h e K a r l F i s c h e r r e a g e n t , w h i c h consists of a s o l u t i o n of p y r i d i n e ( C H N ) , sulfur dioxide, a n d i o d i n e i n m e t h a n o l ( M e O H ) . T h i s s o l u t i o n r e a c t s w i t h w a t e r as f o l lows: 5

C

A

C

c

+ SO, + c

5

c

I, +

c

c

HI

H 0 2

c

+

c

so,

c

(pyridine)



130


T h e e n d point of the titration m a y be d e t e r m i n e d either colorimetr i c a l l y (free i o d i n e p r e s e n t ) o r e l e c t r i c a l l y (free w a t e r i n c r e a s e s t h e conductivity of t h e solution). T h e K a r l F i s c h e r m e t h o d c a n b e u s e d to measure the moisture contents o f m a n y materials besides w o o d , i n c l u d i n g solids, l i q u i d s , a n d gases. I t g i v e s t h e b e s t r e s u l t s o f a n y o f t h e s t a n d a r d m e t h o d s u s e d f o r m e a s u r i n g w o o d m o i s t u r e c o n t e n t (7), b u t i s n o t p r a c t i c a l for large w o o d s a m p l e s , p a r t i c u l a r l y those w i t h h i g h m o i s t u r e c o n tents. ELECTRICAL RESISTANCE MOISTURE METERS.

The

electrical

resis

tance o f w o o d is e x t r e m e l y s e n s i t i v e to its m o i s t u r e c o n t e n t , a p p r o x imately d o u b l i n g for each 1% decrease i n moisture content over the hygroscopic range of moisture contents. T h e development of a suc cessful resistance m o i s t u r e m e t e r m a y b e a t t r i b u t e d p r i m a r i l y to t h e p i o n e e r i n g w o r k o f S t a m m (9) w h o first m e a s u r e d t h i s r e l a t i o n s h i p quantitatively. Because of the nature of electrical conduction i n w o o d t h e r e is also a s t r o n g i n c r e a s e i n r e s i s t i v i t y w i t h a decrease i n w o o d temperature. F i g u r e 1 illustrates h o w the electrical resistivity of w o o d varies w i t h both moisture content a n d temperature. M o s t resistance moisture meters are essentially megohmeters that m e a s u r e t h e resistance b e t w e e n pairs o f p i n electrodes d r i v e n into t h e w o o d to various depths. Because the p i n electrodes taper along their lengths, t h e relationship b e t w e e n a resistance reading a n d t h e r e s i s t i v i t y (resistance o f a u n i t c u b e ) is c o m p l e x . T h e r e f o r e the meters are calibrated empirically b y using data obtained o n a g i v e n s p e c i e s a t r o o m t e m p e r a t u r e ( 1 0 , 11). R e s i s t a n c e m o i s t u r e m e t e r scales m a n u f a c t u r e d for u s e i n N o r t h A m e r i c a read directly i n moisture content, based o n calibration data f o r D o u g l a s - f i r at 2 7 ° C . F i g u r e 2 s h o w s t h e r a n g e i n e l e c t r i c a l r e s i s 12,

ts C%) Figure 1. Logarithm of DC resistivity of wood as a function of moisture content (10).


3.

SKAAR

131

Wood-Water Refationships

t a n c e a m o n g d o m e s t i c U . S . w o o d s as a f u n c t i o n o f w o o d m o i s t u r e c o n t e n t b e t w e e n t h e l i m i t s o f 7 a n d 2 5 % at (27 ° C ) . T h e c a l i b r a t i o n data for Douglas-fir fall approximately m i d w a y b e t w e e n t h e u p p e r a n d l o w e r c u r v e s (12).


N o t e that t h e curves s h o w n i n F i g u r e s 1 a n d 2 are confined to the moisture content limits b e t w e e n 6 - 7 a n d 2 4 - 2 5 % . M e a s u r e ments below 6 or 7% are not reliable w i t h ordinary moisture meters b e c a u s e t h e resistance is t o o h i g h (above Ι Ο Ω ) . A t m o i s t u r e c o n t e n t s a b o v e 2 4 o r 2 5 % , r e a d i n g s a r e less r e l i a b l e than readings b e l o w 24 o r 2 5 % for t w o reasons. F i r s t , t h e rate o f c h a n g e o f resistance w i t h m o i s t u r e c o n t e n t decreases m a r k e d l y , so the sensitivity is r e d u c e d . S e c o n d , t h e m o i s t u r e content r e a d i n g d e c r e a s e s s u b s t a n t i a l l y w i t h t i m e b e c a u s e o f p o l a r i z a t i o n effects. T h e latter effect c a n b e m i n i m i z e d b y t h e u s e o f a l t e r n a t i n g c u r r e n t ( A C ) rather than the direct current ( D C ) instruments traditionally used for resistance meters. A n o t h e r m e t h o d proposed for m i n i m i z i n g polarization a n d r e l a t e d effects i s t o u s e s h o r t r e p e t i t i v e c u r r e n t p u l s e s r a t h e r t h a n c o n t i n u o u s v o l t a g e o n t h e s a m p l e (13). T h i s m e t h o d a l s o r e d u c e s t h e o h m i c h e a t i n g effect at h i g h e r m o i s t u r e c o n t e n t s . S o m e c o n t e m p o rary resistance meters have provisions for switching to t h e p u l s e d c u r r e n t m o d e for w o o d m o i s t u r e contents greater t h a n 1 2 % a n d r e t a i n the D C m o d e at l o w e r m o i s t u r e c o n t e n t s . 1 1

A resistance m e t e r reads moisture contents h i g h e r than t h e true values w h e n used o n h o t w o o d , a n d vice versa for c o l d wood. T h e r e f o r e , t h e r e a d i n g s m u s t b e a d j u s t e d f o r t h i s t e m p e r a t u r e factor. A f a m i l y o f c u r v e s u s e d to adjust m e a s u r e m e n t s m a d e o n w o o d at t e m p e r a t u r e s f r o m - 4 0 ° F ( - 4 0 ° C ) t o 1 6 0 ° F (71 ° C ) i s r e p r o d u c e d i n F i g u r e 3 (14). I t i s p r o b a b l e t h a t i n d i v i d u a l s p e c i e s , i n a d d i t i o n t o s h o w i n g variations f r o m the s t a n d a r d c u r v e o f resistance against m o i s ture c o n t e n t , also s h o w v a r i a t i o n w i t h respect to t h e t e m p e r a t u r e a d j u s t m e n t f a c t o r s (10). S o m e m o d e r n m e t e r s a r e p r o v i d e d w i t h a d j u s t a b l e m e t e r c a l i b r a t i o n f o r d i r e c t t e m p e r a t u r e c o m p e n s a t i o n (11). Resistance m o i s t u r e meters are useful for d e t e r m i n i n g the m a g nitude of moisture gradients i n wood, particularlyd u r i n g drying. This is a c c o m p l i s h e d b y m e a s u r i n g t h e m o i s t u r e c o n t e n t a t d i f f e r e n t depths f r o m the surface b e c a u s e the m e t e r readings are most affected by the wettest point o f penetration. F o r the same reason, if the w o o d surface has b e e n w e t t e d b y r a i n o r h i g h h u m i d i t y c o n d i t i o n s t h e s u r f a c e r a t h e r t h a n i n t e r i o r m o i s t u r e c o n t e n t i s m e a s u r e d . T h i s effect can b e m i n i m i z e d b y use o f probes that are insulated along their l e n g t h s , e x c e p t f o r t h e p e n e t r a t i n g t i p s t h a t s e r v e as t h e e l e c t r o d e s . DIELECTRIC MOISTURE METERS.

T h e s e m o i s t u r e meters use A C ,

u s u a l l y at r a d i o f r e q u e n c i e s . T h e r e a r e t w o g e n e r a l t y p e s : t h e capac-





3.

SKAAR

Wood-Water Relationships

133

TEMP (°F)

Figure 3. Temperature calibration curves for a DC resistance moisture meter (14). (Courtesy U.S. Department of Agriculture, Forest Products Laboratory.) itance t y p e w h i c h m e a s u r e s p r i m a r i l y t h e d i e l e c t r i c c o n s t a n t o f t h e w o o d , a n d t h e power-loss type w h i c h measures the rate of energy a b s o r p t i o n b y w o o d f r o m a n o s c i l l a t i n g e l e c t r i c field. T h e capacitance type essentially measures the dielectric constant of wood. A t a g i v e n frequency, the dielectric constant increases w i t h w o o d d e n s i t y , m o i s t u r e c o n t e n t ( F i g u r e 4), a n d i n c r e a s i n g t e m p e r a t u r e (10). T h e m o s t e f f e c t i v e e l e c t r o d e c o n f i g u r a t i o n f o r a c a p a c i t a n c e m e t e r a p p e a r s to b e a p a i r o f f l a t p a r a l l e l e l e c t r o d e s c o n t a c t i n g e a c h o f t w o o p p o s i t e faces o f t h e w o o d to b e m e a s u r e d . T h e r e is t h e n M(%)

Figure 4. Dielectric constant e vs. dry wood specific gravity G for several different moisture contents. (Reproduced with permission from Ref. 10. Copyright 1972, Syracuse University Press.) 0


134



a simple geometrical relationship b e t w e e n the measured capacitance and the dielectric constant o f the wood. H o w e v e r , most meters of t h e c a p a c i t a n c e t y p e , as w e l l as o f t h e p o w e r - l o s s t y p e u s e c o n c e n t r i c e l e c t r o d e s p l a c e d o n o n e w o o d s u r f a c e (14). T h i s t y p e o f e l e c t r o d e is m o r e p r a c t i c a l for use b u t t e n d s to r e a d t h e m o i s t u r e c o n t e n t n e a r the w o o d surface rather than i n t h e interior. T h e p o w e r - l o s s m e t e r is t h e m o s t c o m m o n t y p e o f d i e l e c t r i c m o i s t u r e meter. It senses t h e p r o d u c t o f t h e d i e l e c t r i c constant a n d loss factor. G e n e r a l l y , t h e l o s s f a c t o r i n c r e a s e s w i t h w o o d m o i s t u r e content b u t m a y exhibit variations f r o m this b e h a v i o r d e p e n d i n g o n t h e f r e q u e n c y o f m e a s u r e m e n t (JO, I I , 14). A n i n c r e a s e i n t e m p e r a t u r e p r o d u c e s effects s i m i l a r t o i n c r e a s i n g m o i s t u r e c o n t e n t , w i t h interaction b e t w e e n these t w o parameters. Therefore, temperature adjustments o f meter readings are c o m p l e x , sometimes increasing a n d s o m e t i m e s d e c r e a s i n g t h e s c a l e r e a d i n g as t e m p e r a t u r e i n c r e a s e s (14) . MISCELLANEOUS METHODS. Several other methods have been explored for m e a s u r i n g w o o d moisture content, some of w h i c h are discussed briefly. Nuclear Magnetic Resonance (NMR). N M R techniques have b e e n a p p l i e d t o w o o d m o i s t u r e m e a s u r e m e n t s i n t h e l a b o r a t o r y (15). T h i s t e c h n i q u e is b a s e d o n t h e fact t h a t t h e h y d r o g e n n u c l e u s is a n u c l e a r m a g n e t i c d i p o l e d u e t o i t s c h a r a c t e r i s t i c s p i n . W h e n i t is s u b j e c t e d t o a s t a t i c m a g n e t i c field o f s t r e n g t h , H , t h e m a g n e t i c dipole precesses about the direction of H w i t h a frequency y w h i c h is d i r e c t l y p r o p o r t i o n a l t o H . F o r t h e b a s i c h y d r o g e n n u c l e u s ( p r o t o n ) y = 4 . 2 5 7 H w h e r e y is i n k H z w h e n H is m e a s u r e d i n G a u s s 0

0

0

0

0

0

0

0

(15) . T w o different techniques of N M R have b e e n a p p l i e d to measure w o o d m o i s t u r e c o n t e n t b a s e d o n t h e p r e s e n c e o f the h y d r o g e n n u c l e i i n w a t e r . I n o n e o f these, d e s i g n a t e d as a steady-state m e t h o d , t h e w o o d is s u b j e c t e d t o a n a l t e r n a t i n g m a g n e t i c f i e l d o f c o n s t a n t f r e q u e n c y , w i t h H v a r i e d s l o w l y so as t o r e s o n a t e y w i t h r e s p e c t t o the applied frequency. A t resonance a strong absorption of energy o c c u r s , a n d t h e w i d t h a n d i n t e n s i t y o f this a b s o r p t i o n c u r v e g i v e i n f o r m a t i o n o n t h e m o i s t u r e c o n t e n t o f t h e w o o d (16). 0

0

T h e s e c o n d g e n e r a l N M R t e c h n i q u e a p p l i e d t o w o o d (15) i s t h e p u l s e d N M R m e t h o d . I n this case " a short i n t e n s e b u r s t o f a m a g n e t i c field o s c i l l a t i n g i n r e s o n a n c e w i t h t h e s p i n p r e c e s s i o n f r e q u e n c y is a p p l i e d at r i g h t a n g l e s t o H " (15). A v o l t a g e i s i n d u c e d b y t h e p u l s e in a coil s u r r o u n d i n g t h e sample. This voltage decays exponentially, a n d a n a n a l y s i s o f t h i s free induction decay g i v e s i n f o r m a t i o n o n t h e n a t u r e o f t h e m o l e c u l e s c o n t a i n i n g t h e h y d r o g e n n u c l e i , as w e l l as to t h e i r n u m b e r . F i g u r e 5 s h o w s a p l o t o f t h e a m p l i t u d e o f t h e f r e e 0


3.

SKAAR

135



1.0,

0

50

100

150

200

M(%)

Figure 5. Free induction decay (FID) voltage vs. moisture content. (Reproduced with permission from Ref 15. Copyright 1978, Wood Fiber.) induction decay voltage 50 a f t e r p u l s i n g as a f u n c t i o n o f w o o d m o i s t u r e c o n t e n t f o r s p r u c e a n d m a p l e w o o d (15). Neutron Moisture Meter. A n e u t r o n m o i s t u r e m e t e r c a n also b e u s e d t o m e a s u r e w o o d m o i s t u r e c o n t e n t (JO). T h i s c o n s i s t s o f a fast n e u t r o n g e n e r a t o r w h i c h i s a s o u r c e o f h i g h - e n e r g y n e u t r o n s . T h e s e a r e d i r e c t e d i n t o t h e w o o d ( F i g u r e 6) w h e r e s o m e a r e m o d erated into slow neutrons b y the hydrogen atoms a n d scattered back toward a slow-neutron detector. T h e n u m b e r moderated a n d d e t e c t e d is p r o p o r t i o n a l t o t h e a m o u n t o f w a t e r i n w o o d b e c a u s e o f t h e high content of hydrogen i n water. S u c h neutron meters have b e e n d e v e l o p e d f o r f i e l d u s e i n m e a s u r i n g s o i l m o i s t u r e c o n t e n t (17). T h e n e u t r o n moisture m e a s u r e m e n t t e c h n i q u e gives information o n t h e a m o u n t o f w a t e r p e r u n i t v o l u m e o f the w o o d . To r e d u c e this to a w e i g h t b a s i s t h e d e n s i t y o f t h e w o o d m u s t a l s o b e k n o w n . T h i s DETECTOR \ (NEUTRONS) / i - v SOURCE

DETECTOR \ (GAM M A-RAYS) / ^ \ SOURCE

Figure 6. Schematic diagram of a nuclear gauge for moisture measurement of hulk materials. (Adapted from Nuclear-Chicago Corporation.)


136



m a y b e a c c o m p l i s h e d b y u s i n g a s u p p l e m e n t a l s y s t e m s u c h as a 7r a d i a t i o n a n d d e t e c t i o n s y s t e m ( F i g u r e 6). A b e a m o f 7 r a y s d i r e c t e d i n t o t h e w o o d is a b s o r b e d i n p r o p o r t i o n t o t h e w o o d d e n s i t y . T h e 7 rays not a b s o r b e d are d e t e c t e d a n d are i n v e r s e l y p r o p o r t i o n a l to the density of the wood. T h e output data from the neutron and 7 detec tors can b e c o m b i n e d to o b t a i n t h e m o i s t u r e c o n t e n t o n a w e i g h t basis. Moisture Sorption Isotherms. G r e e n w o o d l o s e s m o i s t u r e to t h e a t m o s p h e r e a n d a p p r o a c h e s a m o i s t u r e c o n t e n t d e s i g n a t e d as t h e equilibrium moisture content ( E M C ) f o r t h e p a r t i c u l a r a t m o s p h e r i c c o n d i t i o n s . T h e E M C is a f u n c t i o n o f r e l a t i v e h u m i d i t y , t e m p e r a t u r e , p r e v i o u s exposure history (hysteresis), species, a n d other m i s c e l l a neous factors. E F F E C T O F R E L A T I V E H U M I D I T Y A N D SORPTION

HISTORY.

An

indi

r e c t m e t h o d f o r e s t i m a t i n g w o o d m o i s t u r e c o n t e n t is to m e a s u r e i t s e q u i l i b r i u m r e l a t i v e v a p o r p r e s s u r e h. T h i s is r e l a t e d to w o o d m o i s ture content by a sorption isotherm. T h e percent relative h u m i d i t y (H) o r r e l a t i v e v a p o r p r e s s u r e (h) (H = 1 0 0 h) is t h e m o s t i m p o r t a n t f a c t o r i n d e t e r m i n i n g t h e E M C f o r w o o d . A c u r v e s h o w i n g E M C as a f u n c t i o n o f p e r c e n t r e l a t i v e h u m i d i t y o r r e l a t i v e v a p o r p r e s s u r e at c o n s t a n t t e m p e r a t u r e is c a l l e d a moisture sorption isotherm. F i g u r e 7 s h o w s t h r e e t y p i c a l s o r p t i o n i s o t h e r m s for D o u g l a s - f i r at 9 0 ° F (32 ° C ) (18). T h e g e n e r a l s i g m o i d s h a p e s f o r a l l t h r e e c u r v e s is a p p a r e n t , b u t e a c h c u r v e r e p r e s e n t s t h e i s o t h e r m f o r a d i f f e r e n t

Figure 7. Initial desorption (IN DES), adsorption (ADS), and secondary desorption (SEC DES) isotherms for Doughs-fir. (Adapted from Ref. 18.)



3.

SKAAR

137


s o r p t i o n e x p o s u r e h i s t o r y . T h e u p p e r m o s t c u r v e is t h a t for t h e i n i t i a l d e s o r p t i o n o r d r y i n g f r o m t h e g r e e n c o n d i t i o n . T h e l o w e s t c u r v e is the a d s o r p t i o n i s o t h e r m o b t a i n e d b y e x p o s i n g the w o o d , after v a c u u m d r y i n g , to s u c c e s s i v e l y h i g h e r r e l a t i v e h u m i d i t i e s . T h e i n t e r m e d i a t e c u r v e is t h e s e c o n d a r y d e s o r p t i o n i s o t h e r m o b t a i n e d b y r e - e x p o s i n g the s a m p l e to s u c c e s s i v e l y l o w e r h u m i d i t i e s after first e q u i l i b r a t i n g it t o e s s e n t i a l l y 1 0 0 % r e l a t i v e h u m i d i t y . A sample taken through repetitive cycles of relative h u m i d i t y e x p o s u r e b e t w e e n 0 a n d 1 0 0 % t e n d s to f o l l o w t h e a d s o r p t i o n a n d secondary desorption curves repetitively. T h e adsorption isotherm (A) is a l w a y s l o w e r t h a n t h e c o r r e s p o n d i n g d e s o r p t i o n i s o t h e r m ( D ) a n d t h e i r r a t i o , d e s i g n a t e d as t h e A / D r a t i o , c a n n o t e x c e e d u n i t y . T h e A / D ratio varies w i t h relative h u m i d i t y a n d different kinds o f w o o d (19) ( F i g u r e 8). A t r o o m t e m p e r a t u r e i t g e n e r a l l y r a n g e s b e t w e e n 0.8 a n d 0.9, a n d t e n d s to decrease w i t h i n c r e a s i n g t e m p e r a t u r e (20). S o r p t i o n h y s t e r e s i s i n w o o d is b e n e f i c i a l f r o m t h e v i e w p o i n t o f w o o d u t i l i z a t i o n . T h i s is b e c a u s e w o o d e x p o s e d to c y c l i c h u m i d i t y c o n d i t i o n s shows s m a l l e r changes i n m o i s t u r e c o n t e n t for g i v e n h u m i d i t y changes t h a n w o u l d b e the case i f t h e r e w e r e n o hysteresis (21). S o r p t i o n h y s t e r e s i s r e d u c e s t h e e f f e c t i v e s l o p e dMIdH of the sorption i s o t h e r m a n d the d i m e n s i o n a l changes associated w i t h h u midity changes. E F F E C T OF TEMPERATURE. T h e sorption isotherms for w o o d g e n e r a l l y d e c r e a s e w i t h i n c r e a s i n g t e m p e r a t u r e ( F i g u r e 9) a b o v e 0 ° C .

0.951

r

_l

I

50

60

1

70

L_

80

Figure 8. Representative A / D (M /M ) ratios as functions of relative hu midity Η for different woods and bark (19). a

d


138



TEMP (°C)

h

Figure 9. Sorption isotherms as affected by temperature. T h i s r e s u l t is as e x p e c t e d b a s e d o n t h e r m o d y n a m i c c o n s i d e r a t i o n s a n d is d i s c u s s e d l a t e r i n t h i s c h a p t e r . T h e a p p a r e n t fiber-saturation p o i n t M y , w h i c h is o b t a i n e d b y e x t r a p o l a t i n g t h e s o r p t i o n i s o t h e r m to 1 0 0 % r e l a t i v e h u m i d i t y , d e c r e a s e s a p p r o x i m a t e l y 0 . 1 % / ° C r i s e i n t e m p e r a t u r e (22). Above the boiling point of water the sorption isotherms appar e n t l y c o n t i n u e t o d e c r e a s e w i t h i n c r e a s i n g t e m p e r a t u r e (23). I t i s difficult to measure isotherms above 100 °C because t h e vapor pres sure o f w a t e r is greater than a t m o s p h e r i c pressure. T h e r e f o r e , to a t t a i n r e l a t i v e h u m i d i t i e s n e a r 1 0 0 % i t is n e c e s s a r y t o c a r r y o u t t h e measurements i n a pressurized system. I f m e a s u r e m e n t s a r e m a d e at a t m o s p h e r i c p r e s s u r e t h e m a x i m u m relative h u m i d i t i e s that c a n b e attained decrease w i t h i n c r e a s i n g t e m p e r a t u r e ( F i g u r e 10). T h e m a x i m u m r e l a t i v e h u m i d i t y p o s s i b l e at a n y t e m p e r a t u r e i s e q u i v a l e n t t o t h e r a t i o o f t h e p r e v a i l i n g a t m o s p h e r i c p r e s s u r e t o t h e v a p o r p r e s s u r e o f w a t e r at that t e m p e r a t u r e , e x p r e s s e d i n p e r c e n t . T h e p r a c t i c e o f d r y i n g l u m b e r at h i g h temperatures (above 100 °C) has created a r e n e w e d interest i n t h e s o r p t i o n i s o t h e r m s o f w o o d a t t h e s e t e m p e r a t u r e s (23). B e l o w 0 °C the hygroscopicity of w o o d decreases w i t h decreasing t e m p e r a t u r e , t h e o p p o s i t e o f t h e t r e n d a b o v e 0 ° C (10). EFFECT

OF W O O D

SPECIES

A N D EXTRACTIVES.

The

sorption

iso

therms of all woods are generally similar i n shape. H o w e v e r , there may b e considerable variations a m o n g t h e m w i t h respect to t h e a b solute values o f hygroscopicity. T h i s variation m a y b e because o f differences i n t h e p r o p o r t i o n o f the p r i m a r y w o o d constituents, such as c e l l u l o s e , h e m i c e l l u l o s e , a n d l i g n i n i n d i f f e r e n t w o o d s ; o r m o r e importantly, because o f differences i n the k i n d a n d quantity of ex-



3.

SKAAR


139

120 TEMP(°C) Figure 10. Maximum possible relative humidities at atmospheric pressure and temperatures above 100 °C (23). t r a c t i v e s . T h e a d s o r p t i o n i s o t h e r m s s h o w n i n F i g u r e 11 i n d i c a t e t h a t h e m i c e l l u l o s e s are t h e m o s t h y g r o s c o p i c , a n d l i g n i n t h e least h y g r o s c o p i c , o f t h e p r i m a r y c h e m i c a l c o n s t i t u e n t s o f w o o d (24). T h e hygroscopicities of w o o d s w i t h h i g h extractive contents are generally l o w e r than those w i t h o u t extractives. F o r example, the heartwood of n i n e tropical woods showed an increase i n apparent

HEMI

H0L0 //WOOD

Figure 11. Adsorption isotherms for wood hemicellulose (HEMI), holocellulose (HOLO), Khson lignin (KLIG), and wood at 25 °C(24).


140


fiber saturation, based o n t h e adsorption isotherm, from a mean of 2 1 . 9 % for unextracted w o o d to 2 7 . 6 % following successive extractions w i t h b e n z e n e - a l c o h o l , 9 5 % alcohol a n d water, for 1 0 - 2 0 d , using a S o x h l e t a p p a r a t u s (25). T h e c o r r e s p o n d i n g m e a n d e s o r p t i o n f i b e r saturation point increased from 2 8 . 3 to 33.7%.


OTHER

FACTORS

AFFECTING

HYGROSCOPICITY.

Several

other

fac

t o r s affect t h e h y g r o s c o p i c i t y o f w o o d . O n e o f t h e s e factors i s t h e effect o f m e c h a n i c a l s t r e s s (26). C o m p r e s s i v e s t r e s s e s d e c r e a s e t h e m o i s t u r e c o n t e n t o f w o o d , a n d t e n s i l e s t r e s s e s i n c r e a s e i t . T h i s effect is r e l a t e d t o t h e s w e l l i n g p r e s s u r e o f w o o d . The hygroscopicity of wood m a y be reduced appreciably by h e a t i n g (22), t h e e f f e c t i n c r e a s i n g w i t h i n c r e a s i n g t e m p e r a t u r e a n d t i m e o f h e a t i n g (27). M o i s t u r e C o n t e n t o f W o o d i n U s e . W o o d retains its h y g r o s c o p i c c h a r a c t e r i s t i c s after i t is p u t i n t o u s e . I t is t h e n s u b j e c t e d to f l u c t u a t i n g h u m i d i t y , t h e d o m i n a n t factor i n d e t e r m i n i n g its E M C . T h e s e f l u c t u a t i o n s m a y b e m o r e o r l e s s c y c l i c a l s u c h as t h e 2 4 - h d i u r n a l changes o r t h e a n n u a l seasonal changes. In order to m i n i m i z e t h e changes i n w o o d moisture content i n s e r v i c e , w o o d is u s u a l l y d r i e d to a m o i s t u r e c o n t e n t that a p p r o x i m a t e s the average E M C conditions to w h i c h it w i l l b e exposed. T h e s e c o n ditions vary w i t h respect to w o o d i n t e n d e d for interior c o m p a r e d w i t h e x t e r i o r u s e i n a g i v e n g e o g r a p h i c l o c a t i o n . T h e y also v a r y w i t h geographical location. F o r e x a m p l e , t h e target m o i s t u r e contents o f 8% for w o o d i n t e n d e d for interior u s e a n d 1 1 % for w o o d i n t e n d e d for e x t e r i o r u s e a r e r e c o m m e n d e d (28) i n m o s t o f t h e c o n t i n e n t a l U n i t e d S t a t e s . C o r r e s p o n d i n g f i g u r e s f o r t h e d r y s o u t h w e s t e r n states are 6 a n d 9 % , r e s p e c t i v e l y , a n d those f o r t h e d a m p coastal areas o f t h e s o u t h e a s t a r e 11 a n d 1 2 % , r e s p e c t i v e l y . T h e p r i m a r y reason for d r y i n g w o o d to a moisture content e q u i v alent to its m e a n E M C u n d e r u s e conditions is to m i n i m i z e d i m e n sional changes i n t h e final product.

Shrinking and Swelling of Wood T h e m o i s t u r e c o n t e n t o f w o o d i n t h e l i v i n g t r e e is always a b o v e the fiber-saturation point. Therefore, t h e changes i n w o o d moisture content that o c c u r d u r i n g t h e life o f t h e tree a r e essentially l i m i t e d to c h a n g e s i n t h e l e v e l s o f w a t e r i n t h e c e l l c a v i t i e s , t h a t i s , t o t h e s o - c a l l e d free w a t e r . T h e c e l l w a l l s i n g r e e n w o o d a r e , t h e r e f o r e , i n the fully saturated condition and n o hygroscopic shrinking or swelling occurs, except that r e s u l t i n g f r o m changes i n fiber-saturation points already referred to, w h i c h are a function of temperature. H o w e v e r , w h e n trees a r e f e l l e d a n d t h e c e l l walls lose m o i s t u r e , s h r i n k a g e o c c u r s i n p r o p o r t i o n t o t h e e x t e n t o f loss o f t h i s bound water. B e c a u s e w o o d i n u s e is g e n e r a l l y e x p o s e d t o c y c l i n g r e l a t i v e


3.

SKAAR

Wood-Water

141

Relationships


h u m i d i t y , s w e l l i n g also o c c u r s d u r i n g t h e a d s o r p t i o n of w a t e r b y the cell w a l l of w o o d . The Cell Wall. Before considering the d i m e n s i o n a l changes i n t h e c e l l w a l l o f w o o d a s s o c i a t e d w i t h g a i n o r loss o f m o i s t u r e i t is d e s i r a b l e t o first c o n s i d e r t h e d e n s i t y p' o f t h e c e l l w a l l a n d h o w i t varies w i t h moisture content. DENSITY OF THE D R Y C E L L W A L L . T h e d r y c e l l w a l l of w o o d has a d e n s i t y o f a p p r o x i m a t e l y 1.5 g / c m w h e n m e a s u r e d b y p y c n o m e t r i c or v o l u m e - d i s p l a c e m e n t methods. S o m e w h a t h i g h e r values are ob t a i n e d w h e n u s i n g w a t e r as o p p o s e d to n o n s w e l l i n g d i s p l a c e m e n t m e d i a s u c h as t o l u e n e o r b e n z e n e (22). 3

T h e a p p a r e n t d e n s i t y p ' o f t h e c e l l w a l l o f w o o d has a l s o b e e n m e a s u r e d b y o p t i c a l m e t h o d s . I n this case the r e l a t i v e fractions of v o i d a n d c e l l - w a l l v o l u m e s are d e t e r m i n e d optically b y u s i n g thin m i c r o t o m e d s e c t i o n s o f w o o d (29). T h e s e d a t a a r e t h e n c o m b i n e d w i t h m e a s u r e m e n t s o f t h e d r y w o o d d e n s i t y p to g i v e p ' , b a s e d o n E q u a t i o n 4. 0

0

Po'

= Po(V ' + V 0

0

0

W

(4)

w h e r e V ' a n d V " are the c e l l w a l l a n d v o i d v o l u m e s , optically on the m i c r o t o m e d w o o d sections. 0

0

measured

M e a s u r e m e n t s of the d r y cell wall density based on microscopic observations g e n e r a l l y g i v e l o w e r v a l u e s (1.42 g / c m ) t h a n those o b t a i n e d u s i n g p y c n o m e t r i c a l l y ( 1 . 4 7 g / c m ) w i t h t o l u e n e as a d i s p l a c e m e n t m e d i u m (29). T h i s d i s c r e p a n c y is a t t r i b u t e d t o v a r i o u s u n c o n t r o l l a b l e f a c t o r s s u c h as c e l l - w a l l r u p t u r e s p r o d u c e d d u r i n g p r e p a ration of the m i c r o t o m e d sections. 3

3

For the purpose of the discussion that follows the density of the d r y c e l l w a l l w i l l b e t a k e n as 1.5 g / c m , a n d its s p e c i f i c g r a v i t y G J as 1.5. 3

MAXIMUM

SHRINKING

AND SWELLING

OF THE C E L L

WALL.

When

d r y w o o d is i m m e r s e d i n w a t e r t h e c e l l w a l l s w e l l s i n p r o p o r t i o n to t h e v o l u m e o f w a t e r a d s o r b e d . I f i t is a s s u m e d t h a t t h e s o r b e d w a t e r has t h e s a m e d e n s i t y as f r e e l i q u i d w a t e r , t h e p e r c e n t s w e l l i n g Sw ' o f t h e c e l l w a l l c a n b e a p p r o x i m a t e d b y E q u a t i o n 5. m

Sw

f

m

= MG ' 0

(5)

T h u s , w i t h G ' t a k e n as 1.5, t h e p e r c e n t v o l u m e t r i c s w e l l i n g o f t h e c e l l w a l l f r o m t h e d r y c o n d i t i o n is 1.5 t i m e s t h e p e r c e n t m o i s t u r e content M . 0

T h e maximum possible swelling Sw ^ o f t h e c e l l w a l l is o b t a i n e d w h e n t h e c e l l w a l l is s a t u r a t e d , t h a t is w h e n M = M y . T h e f i b e r saturation point can be m e a s u r e d i n a n u m b e r of different ways ma


142


( S t a m m (30) h a s l i s t e d n i n e s u c h m e t h o d s ) . S o m e w h a t d i f f e r e n t v a l u e s are o b t a i n e d u s i n g different m e t h o d s . T h e r e also a p p e a r to b e v a r i a t i o n s a m o n g w o o d s . A m e a n v a l u e o f a p p r o x i m a t e l y 3 5 % f o r Sw ^ w a s c a l c u l a t e d (29) b a s e d o n m e a s u r e m e n t s o f 18 w o o d s n a t i v e t o t h e c o n t i n e n t a l U n i t e d S t a t e s . L o w e r v a l u e s h a v e a l s o b e e n f o u n d (JO, 18, 22, 30) a n d 3 0 % w i l l b e t a k e n h e r e t o b e t h e n o m i n a l v a l u e o f M y at r o o m t e m p e r a t u r e f o r t h e p u r p o s e o f c a l c u l a t i n g t h e m a x i m u m possible s w e l l i n g of the c e l l w a l l of wood.


ma

Sw ^. m

T h e s w e l l i n g o f t h e c e l l w a l l at fiber s a t u r a t i o n Sw'f is e q u a l to T h e r e f o r e , f r o m E q u a t i o n 5 it can be w r i t t e n that: = MG '

Sw'f

f

(6)

0

T a k i n g M y as 3 0 % a n d G ' as 1.5, t h e m a x i m u m v o l u m e t r i c s w e l l i n g o f t h e c e l l w a l l is 4 5 % , b a s e d o n t h e a s s u m p t i o n s g i v e n a b o v e . 0

C o n v e r s e l y i t c a n b e s h o w n t h a t t h e p e r c e n t s h r i n k a g e Sh^ t h e c e l l w a l l is g i v e n b y E q u a t i o n 7 SK

of

(7)

= (M -M)G/ f

for a p e r c e n t m o i s t u r e c o n t e n t c h a n g e f r o m M y to t h e l o w e r m o i s t u r e c o n t e n t M w h e r e G'f is t h e s p e c i f i c g r a v i t y o f t h e c e l l w a l l b a s e d o n o v e n - d r y w e i g h t W a n d a f u l l y s w o l l e n v o l u m e Vy. T h e m a x i m u m s h r i n k a g e Shf f r o m M y t o M = 0 is t h e r e f o r e g i v e n b y E q u a t i o n 8. 0

= MfG/

Sh'f

(8)

T h e r a t i o Sw'flSh'f t h e r e f o r e is e q u a l t o t h e r a t i o G'jIGJ, based on E q u a t i o n s 6 a n d 8. T h e s p e c i f i c g r a v i t y G ' o f t h e c e l l w a l l at a n y m o i s t u r e c o n t e n t M is g i v e n b y m

G ' m

= G 7 ( l + G 'm) 0

0

(9)

w h e r e m = M / 1 0 0 . A t M = M y t h e s p e c i f i c g r a v i t y Gf is g i v e n b y Gf = G 7 ( l + G 'mf). T a k i n g G ' as 1.5 a n d my as 0 . 3 0 , G/ = 1.5/ [1 + 1.5 (0.3)] = 1 . 0 3 5 . The Gross Wood. T h e d i m e n s i o n a l changes i n t h e gross w o o d a r e n o t g e n e r a l l y t h e s a m e as t h o s e f o r t h e c e l l w a l l m a t e r i a l f o r s e v e r a l r e a s o n s . F i r s t , t h e c e l l c a v i t i e s affect t h e s h r i n k a g e o f t h e gross w o o d . S e c o n d , t h e c e l l w a l l s t r u c t u r e is a n i s o t r o p i c , r e s u l t i n g in differences i n s w e l l i n g a n d shrinkage i n different directions i n the cell wall. T h i r d , the c e l l structure varies a m o n g different kinds of w o o d y t i s s u e , s u c h as r a y t i s s u e c o m p a r e d w i t h l o n g i t u d i n a l t i s s u e . F i n a l l y , m e c h a n i c a l s t r e s s e s affect t h e e x t e n t a n d d i r e c t i o n o f d i m e n 0

0

0


3.

SKAAR

143

Wood-Water Rehtionships

sional changes. T h e s e factors a l l c o n t r i b u t e to t h e o v e r a l l d i m e n s i o n a l instability of w o o d associated w i t h moisture changes. In t h e discussion that follows, t h e v o l u m e t r i c s h r i n k i n g a n d s w e l l i n g o f the gross w o o d w i l l b e t r e a t e d first, f o l l o w e d b y d i s c u s s i o n o f a n i s o t r o p y , a n d f i n a l l y t h e effect o f s t r e s s .


VOLUMETRIC SHRINKING A N D SWELLING.

The volumetric

swelling

o f t h e c e l l w a l l o f w o o d is p r o p o r t i o n a l t o t h e v o l u m e o f w a t e r a b s o r b e d . T h e gross w o o d h o w e v e r contains a i r spaces; therefore, its v o l u m e t r i c s w e l l i n g d e p e n d s o n what happens to t h e a i r spaces during water sorption by the cell wall. T i e m a n n (31) h a s i n d i c a t e d t h a t t h e r e a r e t h r e e p o s s i b i l i t i e s f o r these air spaces d u r i n g w a t e r s o r p t i o n , s h o w n schematically i n F i g u r e 12. F i r s t , a l l o r p a r t o f t h e s w e l l i n g m a y t a k e p l a c e i n t o t h e c e l l cavities ( F i g u r e 12b) w i t h r e d u c t i o n i n l u m e n v o l u m e . I f a l l o f t h e s w e l l i n g takes p l a c e i n t o t h e c e l l cavities t h e r e w o u l d b e n o e x t e r n a l s w e l l i n g i n t h e gross w o o d . S e c o n d , t h e c e l l cavities m a y b e unaf fected b y t h e c e l l w a l l s w e l l i n g a n d r e m a i n t h e same size ( F i g u r e 12c). T h i r d , t h e c e l l c a v i t y m a y s w e l l t o a l e s s e r o r g r e a t e r e x t e n t than t h e c e l l w a l l i t s e l f ( F i g u r e 12d). I f i t is h y p o t h e s i z e d t h a t t h e c e l l c a v i t y r e m a i n s c o n s t a n t i n s i z e as w o o d c h a n g e s m o i s t u r e c o n t e n t i t c a n b e s h o w n (10) t h a t t h e v o l u m e t r i c s h r i n k a g e Shf o f a w o o d o f s w o l l e n v o l u m e s p e c i f i c g r a v i t y Gf c a n b e p r e d i c t e d , b a s e d o n a m o d i f i c a t i o n o f E q u a t i o n 8, as i n E q u a t i o n 10. Sh

f

(10)

= MfGf

S t a m m a n d L o u g h b o r o u g h (32) f i r s t r e p o r t e d t h a t t h i s r e l a t i o n s h i p has b e e n r e p o r t e d (32) t o b e a p p r o x i m a t e l y v a l i d f o r w o o d s o f t h e c o n t i n e n t a l U n i t e d S t a t e s . T h e m e a n v a l u e o f t h e r a t i o Shf/G w a s 27 for 107 h a r d w o o d species a n d 26 for 52 softwood species of t h e U n i t e d States. T h e s e ratios s h o u l d b e e q u i v a l e n t to t h e fiber-satu r a t i o n p o i n t Mf i f t h e g r e e n v o l u m e s p e c i f i c g r a v i t y G is t a k e n t o b e g

g

Figure 12. Volumetric swelling of a single cell showing the cell. Key: a, bel*Ore swelling; b, all swelling into cell cavity; c, all swelling external; d, both cavity and external swelling (10). In The Chemistry of Solid Wood; Rowell, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.

144


e q u a l t o Gf at t h e f i b e r - s a t u r a t i o n p o i n t . T h i s is a v a l i d a s s u m p t i o n i f t h e r e is n o s h r i n k a g e a b o v e Mf w i t h a c h a n g e o f w o o d m o i s t u r e c o n t e n t . T h i s is g e n e r a l l y t r u e u n l e s s c o l l a p s e o f c e l l c a v i t i e s o c c u r s d u r i n g r e m o v a l of free water. V o l u m e t r i c s h r i n k a g e d a t a o n o t h e r w o o d s h a v e also i n d i c a t e d t h a t t h e r a t i o Shf/G t e n d s t o a p p r o x i m a t e t h e fiber-saturation mois t u r e c o n t e n t Mf. F o r e x a m p l e , a m e a n r a t i o w a s f o u n d f o r Shf/G of 2 7 f o r 1 7 0 A u s t r a l i a n w o o d s (33). D a t a o n t r o p i c a l w o o d s s u g g e s t s o m e w h a t l o w e r v a l u e s f o r t h i s s a m e r a t i o . T h e m e a n v a l u e f o r 140 Indian woods was approximately 20, considerably lower than the v a l u e s f o r U . S . w o o d s . T h i s m a y i n d i c a t e t h a t t r o p i c a l w o o d s a r e less hygroscopic than temperate-zone woods, possibly because of their higher mean extractive contents. T h e r e a s o n t h e c e l l c a v i t y t e n d s to c h a n g e o n l y a s m a l l extent i f at a l l d u r i n g m o i s t u r e c h a n g e s is p r o b a b l y r e s i d e n t i n t h e m i c r o f i b r i l o r i e n t a t i o n i n t h e t y p i c a l c e l l w a l l o f w o o d (32). F i g u r e 13 is a s i m p l i f i e d d i a g r a m o f t h e w o o d y c e l l w a l l . T h e c e n t r a l o r S l a y e r is t h e t h i c k e s t l a y e r . Its m i c r o f i b r i l s a r e n e a r l y p a r a l l e l to t h e c e l l axis a n d t e n d t o s w e l l t r a n s v e r s e l y as m o i s t u r e c o n t e n t i n c r e a s e s . T h e m i c r o fibrils i n t h e Si a n d S l a y e r s h o w e v e r a r e o r i e n t e d n e a r l y p e r p e n d i c u l a r to t h e c e l l a x i s . T h e r e f o r e , a l t h o u g h t h e y a r e t h i n , t h e y t e n d to r e s t r a i n s w e l l i n g o f t h e c e l l w a l l b e c a u s e o f t h e h i g h s t r e n g t h o f microfibrils along t h e i r l e n g t h . Transverse s w e l l i n g a n d s h r i n k i n g of i n d i v i d u a l c e l l s a n d , t h e r e f o r e , o f t h e g r o s s w o o d a r e also r e d u c e d . g


g

2

3

Figure 13. Cell wall schematic diagram showing S , S , and S of sec ondary wall, primary wall, and their fibril orientations θ with respect to the cell axis (10). ;

2

3


SKAAR

3.

145


T h u s t h e c e l l c a v i t y t e n d s t o r e m a i n n e a r l y c o n s t a n t as t h e c e l l w a l l shrinks o r swells. Fortunately, from the utilization standpoint, wood

does not

s h r i n k a n d s w e l l t o t h e s a m e e x t e n t as d o e s t h e c e l l w a l l . I f t h i s w e r e not so, a l l w o o d s w o u l d s h r i n k a n d s w e l l v o l u m e t r i c a l l y , for a g i v e n m o i s t u r e c h a n g e , as m u c h as t h e c e l l w a l l , r a t h e r t h a n i n p r o p o r t i o n


to t h e i r s p e c i f i c g r a v i t i e s . T h e r e f o r e , t h e y w o u l d s h r i n k o r s w e l l m o r e than t h e y a c t u a l l y d o . It s h o u l d also b e n o t e d that t h e m a g n i t u d e o f the

fiber-saturation

sional changes.

p o i n t o f a g i v e n w o o d d i r e c t l y affects i t s d i m e n

The

fiber-saturation

point m a y be reduced b y the

r e s t r a i n i n g effects o f t h e c e l l w a l l l a y e r s b e c a u s e o f h y g r o e l a s t i c ef f e c t s , as i s d i s c u s s e d l a t e r . M o i s t u r e - i n d u c e d d i m e n s i o n a l changes i n w o o d have b e e n d e s c r i b e d t r a d i t i o n a l l y i n t e r m s o f s h r i n k a g e Sh ( b a s e d o n g r e e n d i mensions)

o r o f s w e l l i n g Sw ( b a s e d

o n d r y d i m e n s i o n s ) , as g i v e n

above. H o w e v e r , i t is s o m e t i m e s m o r e a p p r o p r i a t e t o d e s c r i b e these changes i n t e r m s o f t h e d i m e n s i o n s at s o m e i n t e r m e d i a t e m o i s t u r e content. F o r v o l u m e changes a hygroexpansion coefficient X

v

may be

d e f i n e d as f o l l o w s , X , = (l/v)(dv/dm)

(11)

w h e r e ν i s t h e w o o d v o l u m e a t m o i s t u r e c o n t e n t m a n d dv/dm change of v o l u m e p e r unit moisture content

is t h e

change.

F i g u r e 14 s h o w s t h e l i n e a r i d e a l i z e d i n c r e a s e i n v o l u m e ν o f w o o d as i t s m o i s t u r e c o n t e n t ra i n c r e a s e s f r o m z e r o t o a m o i s t u r e c o n t e n t g r e a t e r t h a n f i b e r s a t u r a t i o n ray. I n t h e i d e a l i z e d c a s e s h o w n h e r e t h e v o l u m e i n c r e a s e s l i n e a r l y w i t h m f r o m z e r o t o ray, w i t h a c o n s t a n t s l o p e dv/dm. increases, because

T h e magnitude of X

v

h o w e v e r d e c r e a s e s as m

t h e v o l u m e ν i n E q u a t i o n 11 i n c r e a s e s w i t h ra.

SLOPE * dv/dm ν (cc)

h

2

F i g u r e 2 9 s h o w s c u r v e s o f HIM ( = him) v s . H ( = 100/i) c a l c u l a t e d f r o m e x p e r i m e n t a l s o r p t i o n d a t a (also p l o t t e d ) o n w o o d a n d b a r k at 25 °C, for b o t h a d s o r p t i o n a n d d e s o r p t i o n . F i g u r e 30 shows the curves of the total moisture content Μ , a n d of M a n d M , all ex p r e s s e d i n p e r c e n t ( M = 100 m). T h e s e curves w e r e o b t a i n e d u s i n g values of m , k , a n d k c a l c u l a t e d f r o m t h e c u r v e i n F i g u r e 29 for the adsorption isotherm of wood. T h e curves labelled a n d M are derived from the H a i l w o o d — H o r r o b i n sorption isotherm model. x

0

x

2

2

s

H a i l w o o d - H o r r o b i n Solution Sorption Theory. The Hail w o o d - H o r r o b i n (57) m o d e l t r e a t s m o i s t u r e s o r p t i o n as h y d r a t i o n o f the p o l y m e r , t a k e n h e r e to b e d r y w o o d , b y s o m e o f t h e s o r b e d w a t e r c a l l e d w a t e r o f h y d r a t i o n , m/,. T h e h y d r a t e f o r m s a p a r t i a l s o l u t i o n with the r e m a i n i n g sorbed water, called water of solution, m. A n e q u i l i b r i u m is a s s u m e d t o e x i s t b e t w e e n t h e d r y w o o d a n d w a t e r a n d the h y d r a t e d w o o d w i t h an e q u i l i b r i u m constant K E q u i l i b r i u m is also a s s u m e d to exist b e t w e e n t h e h y d r a t e d w o o d a n d w a t e r v a p o r at r e l a t i v e v a p o r p r e s s u r e h w i t h e q u i l i b r i u m c o n s t a n t K . A t h i r d c o n s t a n t m is d e f i n e d as t h e m o i s t u r e c o n t e n t c o r r e s p o n d i n g t o c o m s

v

y

2

0


3.

SKAAR

Wood-Water

1


6.0|

Rehtionships

165

r

Figure 29. Plotted points and fitted curves of the ratios H / M ( = h/m) vs. relative vapor pressure h for mean adsorption and desorption data on 10 woods and barks (19).

20

I

I

ι

ι

t>

/

/ '! WOOD (ADS)

1 / /

/

-

15

/

/

M (%)

H.

f

/

it

f I'

t

!: '/

10

/

//

Λ

.^;LA . M

5

0

0

H(%) 201

40I

60I

801

100

Figure 30. Mean adsorption isotherms calculated from uppermost curve of Figure 29 and curves of M M«, M , and M vs. H. Also shown is M 1 ?

h

s


0

166


p l e t e h y d r a t i o n of t h e w o o d . T h e m o d e l also assumes that t h e s o l u t i o n of h y d r a t e d w o o d a n d dissolved water behaves ideally, an assumption t h a t has b e e n c r i t i c i z e d (58). T h e H a i l w o o d - H o r r o b i n single hydrate m o d e l predicts a sorp t i o n i s o t h e r m o f t h e s a m e f o r m as t h e D e n t m o d e l , t h a t i s , a p a r a b o l i c r e l a t i o n s h i p b e t w e e n him a n d h as g i v e n i n E q u a t i o n 3 3 . F u r t h e r more, two of the f u n d a m e n t a l constants, m a n d K are identical w i t h t h e D e n t c o n s t a n t s m a n d k . T h e t h i r d c o n s t a n t Κχ is a n a l o g o u s to ki o f t h e D e n t m o d e l b u t n o t i d e n t i c a l . T h e y a r e r e l a t e d b y


0

0

2

2

K,

= Kgfa +

1)

(37)

T h e w a t e r o f h y d r a t i o n m^ a n d o f s o l u t i o n m a r e a n a l o g o u s a n d a l m o s t e q u a l t o t h e p r i m a r y m a n d s e c o n d a r y m^ m o i s t u r e c o n t e n t s , r e s p e c t i v e l y ( F i g u r e 30). E q u a t i o n s f o r t h e f r e e e n e r g y c h a n g e s as sociated w i t h the water of h y d r a t i o n a n d of solution, i n analogy w i t h E q u a t i o n 32 are g i v e n b y s

x

AG

h

=

-

(ΛΓ/18)1η K

i ;

AG

S

=

-

(ΒΓ/18)1η

K

2

(38)

Moisture Transport W a t e r i n w o o d is r a r e l y i n s t a t i c e q u i l i b r i u m . I t is c o n t i n u a l l y a d j u s t i n g to changes i n its e n v i r o n m e n t . T h e m o s t d r a m a t i c change o c c u r s w h e n g r e e n w o o d is first d r i e d . H o w e v e r , e v e n i n u s e w o o d is e x p o s e d t o c y c l e s o f c h a n g i n g h u m i d i t y , b o t h d a i l y a n d s e a s o n a l l y . T h e r a t e o f c h a n g e o f w o o d m o i s t u r e c o n t e n t is d e t e r m i n e d b y several factors. T h e s e factors i n c l u d e t h e c u r r e n t m o i s t u r e c o n t e n t and gradients, specific gravity, dimensions a n d grain orientation of the wood, a n d the temperature, relative humidity, and air velocity s u r r o u n d i n g t h e w o o d . I t is c o n v e n i e n t t o d i s c u s s t h e s e p a r a m e t e r s i n t e r m s o f t h e i r effects o n t w o p h e n o m e n o l o g i c a l c o e f f i c i e n t s t h a t h a v e b e e n c u s t o m a r i l y u s e d to express m o i s t u r e t r a n s p o r t i n w o o d a n d o t h e r materials. T h e s e coefficients are the m o i s t u r e diffusion coefficient w h i c h d e t e r m i n e s the rate of m o v e m e n t i n t e r n a l l y t h r o u g h the w o o d a n d the surface e m i s s i o n coefficient w h i c h d e t e r m i n e s the rate of t r a n s p o r t b e t w e e n t h e w o o d surface a n d its s u r r o u n d i n g s . T h e i n t e r n a l t r a n s p o r t c o e f f i c i e n t w i l l b e d i s c u s s e d first, f o l l o w e d b y c o n sideration of the surface coefficient. The Moisture Diffusion Coefficient. The one-dimensional m o i s t u r e f l u x (J) ( g / c m s) o f w a t e r t h r o u g h w o o d c u s t o m a r i l y is g i v e n as t h e p r o d u c t o f t h e m o i s t u r e d i f f u s i o n c o e f f i c i e n t D ( c m / s ) a n d t h e g r a d i e n t dcjdx o f m o i s t u r e c o n c e n t r a t i o n c ( g / c m ) i n t h e d i r e c t i o n of flow, or 2

2

m

3


3.

SKAAR

Wood-Water

167

Relationships D

=

(39)

-J/(dc /dx) m

T h e d r i v i n g p o t e n t i a l a s s u m e d for m o i s t u r e m o v e m e n t b a s e d o n E q u a t i o n 3 9 is t h e m o i s t u r e c o n c e n t r a t i o n c . O t h e r d r i v i n g p o t e n tials m a y a l s o b e a s s u m e d . T a b l e I l i s t s t h e p o t e n t i a l s t h a t h a v e b e e n proposed, the r e s u l t i n g transport coefficients, a n d t h e i r relationships to D i n e a c h c a s e (59). A l t h o u g h o n e o r m o r e o f t h e s e o t h e r p o t e n t i a l s m a y b e m o r e d e s c r i p t i v e o f t h e d r i v i n g force for m o i s t u r e m o v e m e n t , the d i s c u s s i o n that follows w i l l b e r e s t r i c t e d to t h e diffusion coeffi c i e n t b e c a u s e i t is so w e l l e s t a b l i s h e d i n t h e l i t e r a t u r e , a n d c a n b e r e l a t e d to a n y o f t h e o t h e r s . F u r t h e r m o r e , it appears u n c h a n g e d i n the unsteady-state diffusion equation (Fiek's second law), u n l i k e any of the o t h e r coefficients. T h u s F i c k ' s s e c o n d l a w m a y b e w r i t t e n , for o n e d i m e n s i o n , as


m

dcjdt

(40)

= d(Ddc /dx)/dx m

T h e c o e f f i c i e n t D is a f f e c t e d b y m a n y f a c t o r s , t h e m o s t i m p o r t a n t of w h i c h are t e m p e r a t u r e , m o i s t u r e content, specific gravity, a n d g r a i n o r i e n t a t i o n w i t h r e s p e c t to d i r e c t i o n o f f l o w as f i r s t c a l c u l a t e d q u a n t i t a t i v e l y b y S t a m m (60). F o r e x a m p l e , F i g u r e 3 1 s h o w s t h e s t r o n g i n c r e a s e i n D w i t h t e m p e r a t u r e as w e l l as t h e m u c h h i g h e r v a l u e a l o n g (D/) t h a n a c r o s s (D ) t h e g r a i n f o r w o o d o f 0 . 5 s p e c i f i c g r a v i t y (45). F i g u r e 3 1 a l s o i n d i c a t e s t h e c o m p l e x effect o f w o o d m o i s t

T a b l e I. Some M o i s t u r e T r a n s p o r t Coefficients U s e d for W o o d , T h e i r A s s u m e d Potentials, a n d T h e i r Relationships to the D i f f u s i o n Coefficient D Refotion Assumed Potential Moisture Concentration Fractional Moisture Content Percent Moisture Content Vapor Pressure Relative Vapor Pressure Osmotic Pressure Spreading Pressure

Transport (cgs

Symbol (cgs units)

Cm

D = -J/(dcJdx) (cm /s) Κ, = -J/(dmJdx) (g/cm s) K = -J/(dM/dx)

(g/cm ) m 3

to

Diffusion

Coefficient units)

Coefficient D =

D

2

(g/g) M

M

(g/ioog) Ρ (dyne/cm ) h (ratio) Π (dyne/cm ) 2

2

Φ (dyne/cm)

(g/100 c m s) Kp = -J/(dp/dx) (g c m / d y n e s) K = -J/(dh/dx) (g/cm s) K = -J/(dWdx) (g c m / d y n e s) Κφ = -//(θφ/θχ) (g/dyne s) H

n

Km = K

M

=

D(dc /dm) m

D(dc /dM) m

Kp =

D(dc /dp)

KH =

D(dcjdh)

K

m

=

D(dc /dU)

Κφ =

D(dcjd4>)

n


m

168


IQQxKT


I

I

0

5

1—\"Ί

ι

5

Zli

10

1

ι

M(%)

15

1

Γ

ι

I

20

25

I

Figure 31. Curves of Dj and Ό* vs. wood moisture content for various temperatures. (Reproduced with permission from Ref. 45. Copyright 1971, Syracuse University Press.)

t u r e c o n t e n t , w i t h D / g e n e r a l l y d e c r e a s i n g a n d D i n c r e a s i n g , as m o i s t u r e c o n t e n t increases o v e r t h e h y g r o s c o p i c range f r o m 5 to 2 5 % . T h i s d i f f e r e n c e is b e c a u s e t h e r a t e o f v a p o r f l o w t h r o u g h t h e e l o n gated c e l l cavities l i m i t s l o n g i t u d i n a l diffusion a n d the rate of b o u n d w a t e r f l o w t h r o u g h the c e l l walls d e t e r m i n e s the rate of transverse d i f f u s i o n (60). t

T h e strong increase i n the diffusion coefficient D w i t h increasing m o i s t u r e c o n t e n t m a y b e r e l a t e d to the decrease i n the activation e n e r g y E for m o i s t u r e d i f f u s i o n i n t h e c e l l w a l l w i t h i n c r e a s i n g m o i s t u r e c o n t e n t , as is s h o w n i n F i g u r e 3 2 . A c c o r d i n g t o t h e d i a g r a m t h e e n e r g y E is l e s s t h a n t h e e n e r g y E r e q u i r e d t o v a p o r i z e t h e w a t e r f r o m t h e b o u n d w a t e r l e v e l t o t h e v a p o r state (61). T h i s d i a g r a m is s i m i l a r to F i g u r e 2 1 e x c e p t that t h e e n e r g y l e v e l s for t h e a c t i v a t e d m o l e c u l e s are also s h o w n . B

B

v

A b o v e f i b e r s a t u r a t i o n , t h e effect o f m o i s t u r e c o n t e n t o n D is e v e n m o r e c o m p l e x because of the great variability i n capillary f l o w


3.

SKAAR

VAPOR


169


WATER VAPOR

POTENTIAL ENERGY LIQUID

20

30

50

Μ , PERCENT Figure 32. Curves showing relative energy levels of water vapor, acti vated molecules, and liquid and bound water (61).

t h r o u g h a n d p a r t i c u l a r l y b e t w e e n w o o d cells. T h e s e cells are c o n nected by pores whose dimensions and numbers vary by several o r d e r s o f m a g n i t u d e b e t w e e n a n d e v e n w i t h i n w o o d s (62). T h i s e x treme variability makes quantitative estimates of D virtually impos s i b l e f o r m o i s t u r e m o v e m e n t a b o v e f i b e r s a t u r a t i o n (63). H a w l e y (64) f i r s t d e m o n s t r a t e d t h e c o m p l e x n a t u r e o f m o i s t u r e flow through wood above the fiber-saturation point resulting from capillary forces associated w i t h air b u b b l e s a n d pores of variable r a d i i i n t e r c o n n e c t i n g c e l l s . U s i n g C o m s t o c k ' s (65) s i m p l i f i e d s t r u c t u r a l m o d e l f o r s o f t w o o d s , S p o l e k a n d P l u m b (66), h o w e v e r , w e r e a b l e t o p r e d i c t t h e c a p i l l a r y p r e s s u r e s i n s o u t h e r n y e l l o w p i n e as a f u n c t i o n of percent of water saturation of the cell cavities. S u c h a quantitative analysis w o u l d b e m o r e difficult to i m p l e m e n t i n the case o f w o o d s other than southern yellow pine because their structures and p e r m e abilities are m o r e v a r i a b l e i n m o s t cases. H o w e v e r , c o m p u t e r m o d e l i n g t e c h n i q u e s a r e d e v e l o p i n g to t h e p o i n t w h e r e m o r e g e n e r a l models m a y b e c o m e feasible. T h e Surface Emission Coefficient. D u r i n g wood drying, par t i c u l a r l y o f t h i n w o o d s u c h as v e n e e r s , f l a k e s , a n d c h i p s , t h e l i m i t i n g r a t e f a c t o r m a y b e t h e r a t e at w h i c h m o i s t u r e c a n b e r e m o v e d f r o m


170


t h e w o o d s u r f a c e . T h i s is p r o p o r t i o n a l t o t h e s u r f a c e e m i s s i o n c o e f ficient

S , d e f i n e d as e

S = J/(Cm ~ C ) e

s

(41)

where c is t h e m o i s t u r e c o n t e n t ( g / c m ) at t h e w o o d s u r f a c e , a n d c is t h e v a l u e f o r t h e w o o d at e q u i l i b r i u m w i t h t h e d r y i n g a i r . 3

Ms


m

m&

A s is t h e c a s e w i t h t h e d i f f u s i o n c o e f f i c i e n t D, S c a n b e r e l a t e d to s i m i l a r c o e f f i c i e n t s b a s e d o n a s s u m e d p o t e n t i a l s o t h e r t h a n c . T h e s e c o e f f i c i e n t s a r e r e l a t e d t o S as d e f i n e d a b o v e i n t h e s a m e w a y that the alternate coefficients s h o w n i n Table I are r e l a t e d to D . I n t h e c a s e o f S , h o w e v e r , t h e f u n d a m e n t a l p o t e n t i a l is p r o b a b l y t h e v a p o r p r e s s u r e d i f f e r e n c e (p — p ) b e c a u s e v a p o r m o v e s e s s e n t i a l l y i n r e s p o n s e to v a p o r p r e s s u r e differences. m

s

e

R o s e n (67) h a s g i v e n s o l u t i o n s o f t h e d i f f u s i o n e q u a t i o n f o r w o o d f r o m w h i c h t h e s u r f a c e m o i s t u r e c o n t e n t c a n b e p r e d i c t e d at v a r i o u s stages o f w o o d d r y i n g as a f u n c t i o n o f t h e t r a n s p o r t r a t i o L , d e f i n e d as S a / D , w h e r e a is h a l f t h e t h i c k n e s s o f t h e w o o d . B a s e d o n t h e s e solutions a n d o n e x p e r i m e n t a l d r y i n g data, R o s e n s h o w e d that the s u r f a c e m o i s t u r e c o n t e n t s c a l c u l a t e d at v a r i o u s stages o f d r y i n g w e r e essentially e q u i v a l e n t to t h e values o b t a i n e d b y u s i n g the p s y c h r o m e t r i c a p p r o a c h g i v e n b y H a r t (68). R o s e n (69) s h o w e d t h a t t h e c o e f f i c i e n t S i n c r e a s e s w i t h i n c r e a s i n g a i r v e l o c i t y o v e r t h e r a n g e f r o m 1 t o 12 m / s . T h e r a t e o f i n c r e a s e b e c a m e l e s s p r o n o u n c e d at t h e h i g h e r a i r v e l o c i t i e s .

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3.

12. 13. 14.


15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52.

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for review May 9, 1983.

ACCEPTED

July 7, 1983.


The Chemistry of Solid Wood - American Chemical Society

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