Wood-Water Relationships - Advances in Chemistry (ACS Publications)https://pubs.acs.org/doi/abs/10.1021/ba-1984-0207.ch0...
1 downloads
79 Views
5MB Size
3 Wood-Water Relationships
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
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
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
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)
In The Chemistry of Solid Wood; Rowell, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.
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
130
T H E CHEMISTRY O F SOLID W O O D
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).
In The Chemistry of Solid Wood; Rowell, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.
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).
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
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-
In The Chemistry of Solid Wood; Rowell, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.
In The Chemistry of Solid Wood; Rowell, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.
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
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
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
In The Chemistry of Solid Wood; Rowell, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.
134
T H E CHEMISTRY O F SOLID W O O D
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
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
In The Chemistry of Solid Wood; Rowell, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.
3.
SKAAR
135
Wood-Water Relationships
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
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.)
In The Chemistry of Solid Wood; Rowell, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.
136
T H E CHEMISTRY O F SOLID W O O D
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
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.)
In The Chemistry of Solid Wood; Rowell, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.
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
3.
SKAAR
137
Wood-Water Relationships
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
In The Chemistry of Solid Wood; Rowell, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.
138
T H E CHEMISTRY O F SOLID W O O D
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
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-
In The Chemistry of Solid Wood; Rowell, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.
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
3.
SKAAR
Wood-Water Relationships
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).
In The Chemistry of Solid Wood; Rowell, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.
140
T H E CHEMISTRY O F SOLID W O O D
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%.
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
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
In The Chemistry of Solid Wood; Rowell, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.
3.
SKAAR
Wood-Water
141
Relationships
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
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
In The Chemistry of Solid Wood; Rowell, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.
142
T H E CHEMISTRY O F SOLID W O O D
( 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.
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
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
In The Chemistry of Solid Wood; Rowell, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.
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 .
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
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
T H E CHEMISTRY O F SOLID W O O D
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
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
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
In The Chemistry of Solid Wood; Rowell, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.
SKAAR
3.
145
Wood-Water Relationships
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
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
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)
<
V
0
m
f
m(g/g)
Figure 14. Idealized linear curve of wood volume V vs. moisture content.
In The Chemistry of Solid Wood; Rowell, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.
146
T H E CHEMISTRY O F SOLID W O O D
V
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
(CC/g)
iTt|
0
rHj
tn^
m lg/g) Figure 15. Actual form of curve of wood volume V vs. moisture content. The value X
vo
of Χ
υ
at m = 0 is r e l a t e d t o S i t y b y X , , = Swf/Mf
S i m i l a r l y Χφ
the value of X
e
X
(12)
a t my, i s r e l a t e d t o S/iy b y
o /
= S/i/M/
(13)
B e c a u s e t h e p e r c e n t s w e l l i n g StVf i s g r e a t e r t h a n t h e p e r c e n t s h r i n k a g e Shf i t i s e v i d e n t t h a t X i s g r e a t e r t h a n Χφ a n d t h a t t h e s e values define t h e limits of X . for a given wood. vo
L
C o m p a r i s o n o f E q u a t i o n s 13 a n d 1 0 r e v e a l s t h a t , f o r t h e c a s e o f a w o o d w h o s e c e l l c a v i t i e s r e m a i n c o n s t a n t i n s i z e , X f = Gf a n d , therefore, G . Similarly, X = G u n d e r the same conditions. T h e r a t i o X JG w a s m e a s u r e d (34) f o r a n u m b e r o f w o o d s a n d w a s p l o t t e d as a f u n c t i o n o f m o i s t u r e c o n t e n t i n e a c h c a s e . T h e s l o p e dv/dm is not constant over t h e entire hygroscopic range o f moisture contents. A s i n d i c a t e d i n F i g u r e 1 5 , t h e s l o p e is l o w e r at t h e l o w e r a n d u p p e r m o i s t u r e contents b u t is essentially constant o v e r m o s t o f the h y g r o s c o p i c m o i s t u r e r a n g e . T h e m e a n v a l u e o f t h e r a t i o X /G should be unity i f the cell cavity remains constant w i t h swelling o f the cell wall. I n s o m e w o o d s t h e m e a n v a l u e o f t h e r a t i o X JG is l e s s , a n d i n others greater than unity, i n d i c a t i n g deviations from the hypothetical assumption that t h e c e l l cavity r e m a i n s constant i n size. v
g
V
vo
0
0
vo
V
ANISOTROPY IN SHRINKING A N D SWELLING.
0
0
W o o d is a n i s o t r o p i c —
that is, different i n different d i r e c t i o n s — w i t h respect to d i m e n s i o n a l changes. T h e least s h r i n k a g e occurs a l o n g t h e grain a n d t h e most s h r i n k a g e i n t h e t a n g e n t i a l d i r e c t i o n ; r a d i a l s h r i n k a g e is a b o u t h a l f that o f t a n g e n t i a l s h r i n k a g e .
In The Chemistry of Solid Wood; Rowell, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.
3.
SKAAR
147
Wood-Water Relationships
Directional dimensional changes c a n b e expressed i n terms of h y g r o e x p a n s i o n c o e f f i c i e n t s , o n e f o r e a c h o f t h e t h r e e p r i n c i p a l axes.
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
T h e s e m a y b e w r i t t e n as f o l l o w s : X , = (1/1) (dtidm)
(14)
X X
r
= (1/r) (dr/dm)
(15)
t
= (1/t) (dtldm)
(16)
w h e r e X\, X , a n d X a r e t h e l o n g i t u d i n a l , r a d i a l , a n d t a n g e n t i a l h y g r o e x p a n s i o n c o e f f i c i e n t s , r e s p e c t i v e l y a n d 1, r , a n d t a r e t h e c o r r e sponding dimensions i n the respective directions. L o n g i t u d i n a l o r axial s h r i n k a g e is almost n e g l i g i b l e i n n o r m a l mature w o o d , r a n g i n g from 0.1 to 0 . 3 % w h e n such w o o d dries from t h e g r e e n t o o v e n - d r y c o n d i t i o n . T h i s s h r i n k a g e i s so s m a l l t h a t i t causes n o p r o b l e m s i n o r d i n a r y use. H o w e v e r , r e a c t i o n w o o d ( c o m p r e s s i o n w o o d i n softwoods a n d t e n s i o n w o o d i n h a r d w o o d s ) a n d also j u v e n i l e w o o d ( w o o d from n e a r t h e p i t h ) u s u a l l y s h o w m u c h h i g h e r axial s h r i n k a g e s , w h i c h m a y cause excessive c r o o k i n g , b o w i n g , o r t w i s t i n g w h e n w o o d d r i e s . T h e r e has b e e n a n i n c r e a s i n g t r e n d t o w a r d harvesting y o u n g e r trees w h i c h contain a larger p r o p o r t i o n of j u v e n i l e w o o d t h a n d o m a t u r e trees. I t is a n t i c i p a t e d therefore that excessive longitudinal shrinkage and the related w a r p i n g problems w i l l become increasingly c o m m o n . r
t
E x c e s s i v e l o n g i t u d i n a l s h r i n k a g e u s u a l l y is associated w i t h h i g h microfibril angles i n t h e S layer, t h e thickest layer o f the secondary w a l l . T h i s a n g l e Θ , w h i c h r e f e r s t o t h e l o n g axis o f t h e c e l l , is s m a l l i n n o r m a l w o o d ( F i g u r e 13). H o w e v e r , i n j u v e n i l e w o o d a n d i n r e a c t i o n w o o d t h e a n g l e θ m a y i n c r e a s e f r o m t h e n o r m a l v a l u e s o f less t h a n 3 0 ° t o v a l u e s i n e x c e s s o f 45°. B e c a u s e d i m e n s i o n a l c h a n g e s o c c u r p r i m a r i l y at r i g h t a n g l e s t o t h e m i c r o f i b r i l s t h e c o m p o n e n t o f s h r i n k a g e a l o n g t h e c e l l axis ( a n d g r a i n d i r e c t i o n ) i n c r e a s e s as θ i n c r e a s e s . F i g u r e 1 6 s h o w s t h a t t h e o b s e r v e d s h r i n k a g e Shf o f Pinus jeffreyi i n c r e a s e s w i t h fibril a n g l e θ f o r a n g l e s g r e a t e r t h a n 3 0 ° (35). T h e figure a l s o s h o w s h o w t h e t a n g e n t i a l s h r i n k a g e d e c r e a s e s w i t h i n c r e a s i n g Θ , as e x p e c t e d . 2
Several quantitative models have been proposed for explaining the effect o f f i b r i l angle θ o n b o t h a x i a l a n d transverse w o o d s h r i n k age (10). O n l y t h e m o d e l o f B a r b e r (36) w i l l b e d i s c u s s e d h e r e . I n this m o d e l t h e t y p i c a l w o o d c e l l is a s s u m e d t o b e c i r c u l a r i n cross s e c t i o n a n d t h e c e l l w a l l is c o n s i d e r e d t o c o n s i s t o f t w o l a y e r s . O n e of these is t h e t h i c k S l a y e r w h o s e m i c r o f i b r i l angle θ is the p r i n c i p a l i n d e p e n d e n t v a r i a b l e ( F i g u r e 13). T h e s e c o n d l a y e r is t h e S l a y e r , 2
l
American Chemical Society Library 1155 t6th St.. N.W. In The Chemistry of Solid Wood; Washington, DC Rowell, 20036R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.
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
148
T H E CHEMISTRY O F SOLID WOOD
0
10 ^0 90 FIBRIL ANGLE β (DEGREES)
40
50
CO
Figure 16. Experimental points and fitted curves of longitudinal and tan gential shrinkages of Pinus jeffreyi as functions offibril angle Θ. (Adapted from Ref. 37.)
w h i c h is c o n s i d e r e d to b e a t h i n c o n s t r a i n i n g sheath, s u r r o u n d i n g t h e S l a y e r a n d h a v i n g its m i c r o f i b r i l s o r i e n t e d p e r p e n d i c u l a r t o t h e c e l l axis, thus r e s i s t i n g t h e h y g r o e x p a n s i o n o f the S layer. H a l f o f the microfibrils i n t h e S layer are assumed to b e o r i e n t e d i n a spiral w h i c h m a k e s a n a n g l e θ w i t h t h e c e l l axis. T h e o t h e r h a l f s p i r a l , i n the opposite d i r e c t i o n , has t h e same angle θ w i t h respect to t h e cell axis b u t i n t h e o p p o s i t e s e n s e . T h i s i s t o o v e r c o m e t h e t w i s t i n g t e n d e n c y associated w i t h a single direction o f orientation. I n actual w o o d the S layers o f adjacent cell walls are oriented i n opposite directions to p r e v e n t t w i s t i n g o f i n d i v i d u a l c e l l s . 2
2
2
2
T h e e q u a t i o n s d e r i v e d b y B a r b e r (36) a r e g i v e n i n h i s p a p e r a n d are n o t r e p r o d u c e d h e r e . H o w e v e r , t h e c u r v e s s h o w n i n F i g u r e 17 i l l u s t r a t e t h e p r e d i c t e d r a t i o s o f l o n g i t u d i n a l (e ), t r a n s v e r s e (e ), a n d c e l l c a v i t y (e ) s w e l l i n g s o f t h e m o d e l c e l l t o t h e u n r e s t r a i n e d i s o t r o p i c s w e l l i n g (e ) as f u n c t i o n s o f t h e a n g l e Θ . F i g u r e 1 7 s h o w s t w o c u r v e s e a c h o f e / e , € / e , a n d e^e^ e a c h c u r v e d i f f e r i n g b y a f a c t o r o f 1 0 i n t h e r e l a t i v e stiffness o f t h e r e s t r a i n i n g s h e a t h a n d t h e s w e l l i n g c e l l w a l l m a t e r i a l . A s a n t i c i p a t e d , w h e n t h i s stiffness r a t i o i s h i g h t h e r e is l e s s e x t e r n a l c e l l s w e l l i n g b o t h l o n g i t u d i n a l l y (eje ) a n d t r a n s v e r s e l y ( e j / e j a n d a l s o f o r t h e c e l l c a v i t y ( e ^ e j . I n fact f o r t h e h i g h e r fibril a n g l e s Θ , t h e r e i s s o m e n e g a t i v e s w e l l i n g o r s h r i n k a g e o f t h e c e l l c a v i t y (e /€ ) f o r t h e h i g h e r stiffness r a t i o o f 5 0 . x
2
2
0
x
0
2
0
0
2
0
In The Chemistry of Solid Wood; Rowell, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.
3.
SKAAR
Wood-Water
149
Relationships
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
E/S
Figure 17. Calculated strain ratio curves L ( e ^ e j , Τ (e^ej and C ( e / e j / o r £u;o stiffness of restraining sheath/stiffness of swelling cell wall material (E/S) ratios as given by Barber (36). 2
L o o k i n g at t h e i n d i v i d u a l c u r v e s f o r a stiffness r a t i o o f 5 0 , i t m a y b e n o t e d t h a t b o t h t h e l o n g i t u d i n a l (e /e ) a n d t r a n s v e r s e (€ /€ ) swelling curves strongly resemble the experimental curves shown i n F i g u r e 16. I n b o t h figures t h e l o n g i t u d i n a l a n d t r a n s v e r s e c u r v e s s h o w t h e s a m e s h r i n k a g e (or s w e l l i n g ) at a fibril a n g l e θ o f n e a r 4 5 ° . T h e l o n g i t u d i n a l s w e l l i n g c u r v e o f F i g u r e 17 p r e d i c t s a s l i g h t n e g a t i v e s w e l l i n g for θ b e t w e e n 2 0 a n d 35° for a stiflhess ratio o f 50. T h i s n e g a t i v e s w e l l i n g h a s b e e n o b s e r v e d i n s o m e cases (37). x
0
2
0
H y g r o e x p a n s i o n is u s u a l l y g r e a t e r t r a n s v e r s e l y t h a n l o n g i t u d i n a l l y . H o w e v e r , t h e r e is c o n s i d e r a b l e a n i s o t r o p y i n t h e t r a n s v e r s e d i r e c t i o n b e c a u s e t h e t a n g e n t i a l h y g r o e x p a n s i o n c o e f f i c i e n t X is about twice the radial coefficient X . Traditionally the ratio of these two coefficients X / X was c a l l e d t h e tangential/radial (T/R) shrinkage ratio, w i t h a m e a n value a m o n g woods of about two. t
r
t
r
T h e h i g h T / R r a t i o is t h e p r i m a r y r e a s o n f o r t h e w a r p i n g i n a c r o s s s e c t i o n o f w o o d w h i c h o c c u r s i n b o a r d s w h e n t h e y a r e first d r i e d or w h e n t h e y are subjected to m o i s t u r e changes i n use. F i g u r e
In The Chemistry of Solid Wood; Rowell, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.
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
150
T H E CHEMISTRY O F SOLID W O O D
Figure 18. Cross-sectional distortion after drying of flats, squares, and rounds from representative locations log. (Courtesy of U.S. Department of Agriculture, Forest Products Laboratory.) 18 i l l u s t r a t e s s o m e o f t h e m o r e c o m m o n k i n d s o f c r o s s - s e c t i o n a l w a r p i n g that are caused b y tangential/radial anisotropy w h e n w o o d dries from the green condition. T h e pronounced c u p p i n g i n flat-sawn b o a r d s is p r o b a b l y t h e m o s t t r o u b l e s o m e k i n d o f c r o s s - s e c t i o n a l d i s tortion. S e v e r a l t h e o r i e s h a v e b e e n p r o p o s e d to e x p l a i n t r a n s v e r s e s h r i n k a g e a n i s o t r o p y . T h e s e f a l l i n t o t w o g e n e r a l c a t e g o r i e s (JO, 2 2 , 38). O n e c a t e g o r y is b a s e d o n d i f f e r e n c e s i n t h e s t r u c t u r e o f t h e r a d i a l a n d t a n g e n t i a l w a l l s o f w o o d c e l l s . T h e o t h e r c a t e g o r y is b a s e d o n d i f f e r e n c e s i n t h e s h r i n k a g e s o f d i f f e r e n t w o o d t i s s u e s s u c h as r a y v s . l o n g i t u d i n a l tissues or of e a r l y w o o d vs. latewood tissues. T h e theories based o n c e l l w a l l structure are of two k i n d s : those that relate to f i b r i l angle differences i n t h e r a d i a l a n d t a n g e n t i a l w a l l s a n d those that r e l a t e to t h e t h i c k n e s s a n d b e h a v i o r o f t h e m i d d l e lamella b e t w e e n cells. T h e m e a n fibril angle i n the radial walls may b e g r e a t e r t h a n i n t h e t a n g e n t i a l w a l l s , w h i c h r e s u l t s i n less r a d i a l shrinkage than tangential shrinkage i n the i n d i v i d u a l cells because of t h e s t r o n g e f f e c t o f f i b r i l a n g l e o n t r a n s v e r s e s h r i n k a g e ( F i g u r e 17). B o y d (39) h a s r e v i e w e d t h e p u b l i s h e d d a t a a n d t h e o r i e s o f a n i s o t r o p i c t r a n s v e r s e s h r i n k a g e . H e has c o n c l u d e d , i n a g r e e m e n t w i t h B o s s h a r d ' s (40) c o n t e n t i o n , t h a t t h e d o m i n a n t f a c t o r is t h e g r e a t e r degree of lignification i n the radial walls. This characteristic reduces s o r p t i o n o f w a t e r ( F i g u r e 11). B o y d a l s o a t t r i b u t e s a s i g n i f i c a n t effect to t h e p r e p o n d e r a n c e of r a d i a l l y f l a t t e n e d t h i c k - w a l l e d cells i n t h e latewood of some woods, particularly conifers.
In The Chemistry of Solid Wood; Rowell, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.
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
3.
SKAAR
151
Wood-Water Relationships
T h e r e a r e at l e a s t t w o t h e o r i e s b a s e d o n s h r i n k a g e d i f f e r e n c e s a m o n g w o o d t i s s u e s . T h e m o s t g e n e r a l o f t h e s e is t h e ray restraint theory. T h i s t h e o r y p r o p o s e s t h a t t h e r a y s s h r i n k less t h a n t h e l o n g i t u d i n a l tissues i n the radial d i r e c t i o n a n d therefore r e d u c e the ex t e n t o f r a d i a l s h r i n k a g e . T h i s t h e o r y a p p l i e s to at l e a s t s o m e c a s e s . T h e p l o t t e d p o i n t s a n d c a l c u l a t e d c u r v e s h o w n i n F i g u r e 19, f o r e x a m p l e , s h o w h o w t h e r a d i a l s h r i n k a g e o f b e e c h w o o d d e c r e a s e s as t h e v o l u m e o f r a y t i s s u e s i n c r e a s e s (41). I t is p r o b a b l e t h a t a d d i t i o n a l t a n g e n t i a l s h r i n k a g e is i n d u c e d b y t h e r a d i a l r e s t r a i n t b e c a u s e o f P o i s s o n ' s r a t i o effect. T h e s e c o n d t h e o r y , b a s e d o n t i s s u e s h r i n k a g e d i f f e r e n c e s , is t h e earlywood-latewood interaction theory. This theory attributes the a n i s o t r o p y to t h e a l t e r n a t i o n o f e a r l y w o o d a n d l a t e w o o d l a y e r s i n m a n y w o o d s . L a t e w o o d is o f t e n t w o o r t h r e e t i m e s m o r e d e n s e t h a n e a r l y w o o d . I t s h r i n k s m o r e , a n d is s t r o n g e r . T h e r e f o r e , t h e t a n g e n t i a l s h r i n k a g e is h i g h b e c a u s e i t is d o m i n a t e d b y t h e h i g h l a t e w o o d s h r i n k a g e w h i c h f o r c e s t h e w e a k e a r l y w o o d to s h r i n k t a n g e n t i a l l y m o r e t h a n it w o u l d i f it w e r e a l l o w e d to s h r i n k i n d e p e n d e n t l y o f the l a t e w o o d . T h e r a d i a l s h r i n k a g e is e q u a l t o t h e e f f e c t i v e w e i g h t e d average of the t w o tissue shrinkages. H o w e v e r , the effective radial s h r i n k a g e o f e a r l y w o o d is r e d u c e d b e c a u s e o f P o i s s o n ' s r a t i o effect a n d e x c e s s i v e t a n g e n t i a l s h r i n k a g e (10).
Moisture Effects on the Physical Properties of Wood A l l o f t h e p h y s i c a l a n d m e c h a n i c a l p r o p e r t i e s o f w o o d a r e af f e c t e d b y its m o i s t u r e c o n t e n t . T h e effect o n m e c h a n i c a l p r o p e r t i e s 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 s i d e r a t i o n of s o m e other i m p o r t a n t p h y s i c a l p r o p e r t i e s . I n a l l cases t h e d i s c u s s i o n is l i m i t e d t o c l e a r w o o d , f r e e f r o m d e f e c t s s u c h as k n o t s .
10
(%) 6
2 0.0
02
0.6
0.4
0.8
1.0
Figure 19. Experimental points and fitted curve of radial shrinkage s against fraction of ray tissue V . (Adapted from Ref 41.) r
In The Chemistry of Solid Wood; Rowell, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.
r
152
T H E CHEMISTRY O F SOLID W O O D
Mechanical Properties. M a n y of the important mechanical p r o p e r t i e s o f w o o d i n c r e a s e e x p o n e n t i a l l y as t h e m o i s t u r e c o n t e n t decreases b e l o w the fiber-saturation p o i n t (28) (see C h a p t e r 5). T h i s r e l a t i o n s h i p c a n b e e x p r e s s e d as S /S 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
2
x
= exp [-(r/100) ( M
2
-
(17)
M )] x
w h e r e S χ a n d S are the m a g n i t u d e s of a particular strength p r o p e r t y at m o i s t u r e c o n t e n t s M a n d M , r e s p e c t i v e l y , a n d r is a c o e f f i c i e n t that represents t h e p e r c e n t increase i n a p a r t i c u l a r s t r e n g t h p r o p e r t y S for a 1% decrease i n w o o d m o i s t u r e c o n t e n t M . 2
x
2
T h e v a l u e o f t h e c o e f f i c i e n t r is a b o u t t w o f o r t h e m o d u l u s o f elasticity, a n d f o u r for t h e m o d u l u s o f r u p t u r e i n static b e n d i n g . T h e m a x i m u m v a l u e o f a b o u t six a p p l i e s to c o m p r e s s i v e s t r e n g t h p a r a l l e l t o t h e g r a i n (42). S o m e s t r e n g t h p r o p e r t i e s , s u c h as t o u g h n e s s a n d s h o c k r e s i s tance, may decrease w i t h decreasing w o o d moisture content because these p r o p e r t i e s are p r o p o r t i o n a l to the d e f o r m a t i o n of a w o o d m e m b e r u n d e r l o a d a n d t h e stress s u s t a i n e d . M o i s t w o o d d e f o r m s m o r e t h a n d r y w o o d a n d t h e p r o d u c t o f stress a n d d e f o r m a t i o n , w h i c h is a m e a s u r e o f t o u g h n e s s , m a y a c t u a l l y b e g r e a t e r f o r m o i s t w o o d . T h e s t r e n g t h o f w o o d is g e n e r a l l y n o t a f f e c t e d b y c h a n g e s i n m o i s t u r e c o n t e n t a b o v e fiber s a t u r a t i o n b e c a u s e t h e e x c e s s w a t e r a c c u m u l a t e s i n t h e c e l l c a v i t i e s . T h e r e f o r e , i t d o e s n o t affect t h e strength of the c e l l w a l l itself, w h i c h d e t e r m i n e s the overall w o o d strength. Other Physical Properties. I n a d d i t i o n t o i t s i m p o r t a n t effect o n t h e s t r e n g t h o f w o o d , m o i s t u r e a l s o affects w o o d ' s o t h e r p h y s i c a l p r o p e r t i e s . M o i s t u r e ' s effect o n e l e c t r i c a l p r o p e r t i e s was d e s c r i b e d i n t h e s e c t i o n o n " E l e c t r i c a l R e s i s t a n c e M o i s t u r e M e t e r s " (p. 130). O t h e r p r o p e r t i e s s u c h as s p e c i f i c g r a v i t y a n d t h e r m a l p r o p e r t i e s a r e discussed here. SPECIFIC GRAVITY A N D DENSITY. T h e decrease that occurs i n spe cific gravity w i t h an increase i n m o i s t u r e content was discussed i n connection w i t h w o o d s w e l l i n g . T h e specific gravity G decreases 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 u p t o fiber s a t u r a t i o n b u t a b o v e t h i s t h e r e is n o c h a n g e b e c a u s e t h e v o l u m e r e m a i n s c o n s t a n t a n d t h e w e i g h t is b a s e d o n t h e o v e n - d r y c o n d i t i o n . m
T h e density p of w o o d , however, always increases 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 . T h i s i n c r e a s e is b e c a u s e d e n s i t y , i n c o n t r a s t t o s p e c i f i c g r a v i t y , is a l w a y s b a s e d o n t h e w e t w e i g h t o f t h e w o o d . F i g u r e 20 illustrates h o w b o t h specific gravity a n d density change w i t h increasing moisture content based on the assumption of constant c e l l cavity v o l u m e . Because the cell cavity or pore v o l u m e m
In The Chemistry of Solid Wood; Rowell, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.
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
3.
S K A A R
153
Wood-Water Relationships
I 0
ι 10
ι 20
ι 30 M(%)
ι 40
ι 50
I 60
Figure 20. Calculated curves of specific gravity G and of density ρ (gl cm?) of wood vs. moisture content which assume a constant cell cavity volume and a 30% fiber-saturation point. (Adapted from Ref. 45.) remains approximately constant, the fraction of void volume de creases 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 d u e to s w e l l i n g of the c e l l wall. T H E R M A L PROPERTIES. Some of the important thermal properties of w o o d are affected b y its m o i s t u r e c o n t e n t . T h e s e i n c l u d e specific heat, t h e r m a l conductivity, a n d t h e r m a l diffusivity. T h e s p e c i f i c h e a t c o f d r y w o o d , w h i c h is a b o u t 0 . 2 9 5 at 2 5 ° C , i n c r e a s e s l i n e a r l y w i t h t e m p e r a t u r e (10) f r o m a b o u t 0 . 2 7 ( c a l / g ° C ) at 0 ° C t o 0 . 3 8 at 1 0 0 ° C . T h e s p e c i f i c h e a t c o f l i q u i d w a t e r is 1.0. Therefore, b y u s i n g the m e t h o d of m i x t u r e s w i t h the assumption that b o u n d w a t e r h a s t h e s a m e s p e c i f i c h e a t as l i q u i d w a t e r , t h e s p e c i f i c h e a t c f o r m o i s t w o o d c a n b e c a l c u l a t e d as 0
w
c = (c
0
+ mc )/(l w
+ m)
(18)
E x p e r i m e n t a l m e a s u r e m e n t s (43) a n d a l s o t h e o r e t i c a l c o n s i d e r a t i o n s (10, 44) h a v e y i e l d e d h i g h e r v a l u e s f o r c t h a n a r e p r e d i c t e d b a s e d o n E q u a t i o n 18. H o w e v e r , f o r m a n y p r a c t i c a l p u r p o s e s E q u a t i o n 18 is a d e q u a t e as a f i r s t a p p r o x i m a t i o n .
In The Chemistry of Solid Wood; Rowell, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.
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
154
T H E CHEMISTRY O F SOLID WOOD
T h e t h e r m a l c o n d u c t i v i t y o f w o o d a l s o i n c r e a s e s g r e a t l y w i t h its m o i s t u r e c o n t e n t , as w e l l as w i t h o t h e r f a c t o r s s u c h as t e m p e r a t u r e a n d specific gravity. A n u m b e r of proposed e m p i r i c a l equations i n dicate that the t h e r m a l c o n d u c t i v i t y of w o o d increases w i t h i n c r e a s i n g moisture content. These equations have been s u m m a r i z e d by Siau (45) w h o p r o p o s e d t h a t t h e p r i m a r y e f f e c t o f w a t e r w a s t o s w e l l t h e c e l l w a l l a n d , t h u s , to p r o v i d e a l a r g e r heat c o n d u c t i o n area for a given wood. A n e m p i r i c a l e q u a t i o n t h a t r e l a t e s t h e t h e r m a l c o n d u c t i v i t y (K ) to w o o d d e n s i t y ρ ( g / c m ) a n d m o i s t u r e c o n t e n t m is E q u a t i o n 19. h
3
K
h
= [0.60
+ p(4.1
+ 5.1m)] x
10~
4
c a y c m s °C
(19)
T h e t h e r m a l d i f f u s i v i t y (D ) is e q u a l t o t h e r a t i o K /(pc). Its v a r i a t i o n w i t h m o i s t u r e c o n t e n t c a n b e d e t e r m i n e d b y t h e effect o f m o n K , p, a n d c, w h e r e c is t h e s p e c i f i c h e a t . B e c a u s e pc i n c r e a s e s m o r e r a p i d l y w i t h m o i s t u r e c o n t e n t t h a n d o e s Kh, t h e r e is a s l i g h t d e c r e a s e in w i t h increasing moisture content, on the order of 0 . 5 % per p e r c e n t i n c r e a s e i n m o i s t u r e c o n t e n t M. h
h
h
Thermodynamics of Moisture Sorption W h e n w o o d below the fiber-saturation point interacts w i t h w a t e r , h e a t is e v o l v e d , a n d t h e r e a r e c h a n g e s i n t h e f r e e e n e r g y a n d e n t r o p y of the s o r b e d water. F u r t h e r m o r e , the w o o d exerts s w e l l i n g f o r c e s t h a t c a n b e m e a s u r e d . T h e s e effects c a n b e t r e a t e d b y c l a s s i c a l t h e r m o d y n a m i c m e t h o d s a l t h o u g h m o i s t u r e s o r p t i o n b y w o o d is n o t a p e r f e c t l y r e v e r s i b l e p r o c e s s b e c a u s e s o r p t i o n h y s t e r e s i s is i n v o l v e d , as w a s p o i n t e d o u t i n t h e s e c t i o n o n " M o i s t u r e S o r p t i o n I s o t h e r m s " (p. 136). Enthalpy Changes. T h e three forms of water found i n w o o d h a v e d i f f e r e n t e n e r g y o r e n t h a l p y l e v e l s , as s h o w n i n F i g u r e 2 1 . W a t e r v a p o r i n t h e c e l l cavities has t h e highest enthalpy. T h e e n t h a l p y o f l i q u i d w a t e r i n t h e c e l l c a v i t i e s o f g r e e n w o o d is c o n s i d e r a b l y l o w e r , e s s e n t i a l l y e q u a l t o t h a t o f f r e e l i q u i d w a t e r , i f t h e effects o f capillary forces a n d d i s s o l v e d materials are neglected. T h e difference i n e n t h a l p y b e t w e e n l i q u i d w a t e r a n d w a t e r v a p o r is t h e heat of vaporization [Q ( c a l / g w a t e r ) ] o f f r e e w a t e r . T h e b o u n d w a t e r i n t h e c e l l w a l l o f w o o d is at s t i l l l o w e r e n e r g y l e v e l , Q ( c a l / g w a t e r ) b e l o w t h a t o f l i q u i d w a t e r . T h e s u m o f t h e heat of sorption, Q a n d Q , is e q u a l t o Q ( c a l / g w a t e r ) , w h i c h is t h e h e a t r e q u i r e d to e v a p o r a t e b o u n d water from the cell wall. 0
L
L
Q
v
T h e c u r v e for the e n e r g y l e v e l of b o u n d w a t e r s h o w n i n F i g u r e 21 indicates that Q a n d Q increase w i t h d e c r e a s i n g w o o d m o i s t u r e c o n t e n t b e l o w f i b e r s a t u r a t i o n Mf. T h i s i n c r e a s e m e a n s t h a t m o r e L
v
In The Chemistry of Solid Wood; Rowell, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.
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
3.
SKAAR
Wood-Water Relationships
155
Figure 21. Diagram showing the relative energy levels, in terms of Q, for water vapor, liquid water, ice, and bound water in wood at different moisture contents. (Reproduced with permission from Ref. 10. Copyright 1972, Syracuse University Press.) e n e r g y is r e q u i r e d to e v a p o r a t e c r e a s e s ( b e l o w Mf).
1 g o f w a t e r f r o m w o o d as M
de
S t a m m a n d L o u g h b o r o u g h (46) first c a l c u l a t e d Q a n d Q as functions of w o o d moisture content M b y a p p l y i n g the C l a u s i u s C l a p e y r o n e q u a t i o n t o t h e m o i s t u r e s o r p t i o n i s o t h e r m s f o r w o o d at several temperatures. F o r example, Q can be obtained by replotting s o r p t i o n i s o t h e r m s , s u c h as a r e s h o w n i n F i g u r e 9, i n t o t h e f o r m o f i s o s t e r e s o f c o n s t a n t m o i s t u r e c o n t e n t , o f In h a g a i n s t t h e r e c i p r o c a l of K e l v i n t e m p e r a t u r e . T h e s e plots y i e l d a fa mi l y of essentially s t r a i g h t l i n e s , e a c h at a d i f f e r e n t m o i s t u r e c o n t e n t . T h e m a g n i t u d e o f Q at a n y m o i s t u r e c o n t e n t is c a l c u l a t e d f r o m v
L
L
L
Q
L
=
- ( f l / 1 8 ) d ( l n h)ld(\IT)
(20)
w h e r e R is t h e gas c o n s t a n t a n d d(ln h)ld(\IT) is t h e s l o p e o f t h e c u r v e for t h e s p e c i f i e d m o i s t u r e c o n t e n t . T h e e n t h a l p y changes associated w i t h m o i s t u r e s o r p t i o n also c a n b e m e a s u r e d c a l o r i m e t r i c a l l y . I n t h i s c a s e t h e h e a t o f w e t t i n g W (cal/ g o f w o o d ) is u s u a l l y m e a s u r e d . T h e h e a t o f w e t t i n g is d e f i n e d as t h e h e a t g e n e r a t e d w h e n w o o d at s o m e i n i t i a l m o i s t u r e c o n t e n t m is thoroughly w e t t e d b y l i q u i d water. W h e n m e a s u r i n g the heat of w e t t i n g c a l o r i m e t r i c a l l y , t h e w o o d is g r o u n d i n t o s m a l l p a r t i c l e s i n o r d e r to e x p e d i t e t h o r o u g h w e t t i n g b y w a t e r . T h e w o o d p a r t i c l e s are t h e n c o n d i t i o n e d t o a d e s i r e d i n i t i a l m o i s t u r e c o n t e n t p r i o r to i n s e r t i o n into the calorimeter. T h e h e a t o f w e t t i n g W f o r w o o d at a n y i n i t i a l m o i s t u r e c o n t e n t c a n b e c a l c u l a t e d i f Q is k n o w n as a f u n c t i o n o f m o i s t u r e c o n t e n t . L
In The Chemistry of Solid Wood; Rowell, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.
156
T H E CHEMISTRY O F SOLID W O O D
T h e h e a t o f w e t t i n g is e q u i v a l e n t t o t h e i n t e g r a l o f Q dm between the limits of initial moisture content m a n d c o m p l e t e saturation. T h u s L
W=
(21)
rQ dm L
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
Jm C o n v e r s e l y , i f W is k n o w n as a f u n c t i o n o f m o i s t u r e c o n t e n t m , c a n b e c a l c u l a t e d as i n E q u a t i o n 2 2 . Q
=
L
-dWI
Q
L
(22)
dm
T h e earliest r e c o r d e d m e a s u r e m e n t s of W vs. m w e r e g i v e n , a c c o r d i n g t o S tarn m (22), b y V o l b e h r i n 1 8 9 6 . T h e r e s u l t s o f t h e s e m e a s u r e m e n t s c a n b e r e p r e s e n t e d (47) b y W
= W e x p (-Bm)
(23)
0
w h e r e W is t h e v a l u e o f W f o r i n i t i a l l y d r y w o o d ( m = 0), a n d Β is an e m p i r i c a l constant. M o r e recent m e a s u r e m e n t s are s u m m a r i z e d in Reference 48. 0
F i g u r e 22 contains plots of Q ( l o g scale) v s . w o o d m o i s t u r e content. T h e l o g a r i t h m of Q appears to decrease l i n e a r l y w i t h i n L
L
400i
J
1
1
1
Γ
Figure 22. Curves of Q (log scale) vs. wood moisture content for Euro pean spruce and beech and for Sitka spruce. (Reproduced with permission from Ref. 10. Copyright 1972 Syracuse University Press.) L
y
In The Chemistry of Solid Wood; Rowell, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.
3.
SKAAR
157
Wood-Water Refationships
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 a n t i c i p a t e d b a s e d o n E q u a t i o n s 2 2 a n d 2 3 . E q u a t i o n s 2 2 a n d 2 3 c a n b e c o m b i n e d to g i v e
QL
=
exp
QLO
(24)
w h e r e Q L O = BW , t h e v a l u e o f Q at m = 0 . T h e heats of w e t t i n g W a n d s o r p t i o n Q are i n t e r r e l a t e d , b u t they have different interpretations i n terms of moisture sorption. F o r e x a m p l e , t h e h e a t o f s o r p t i o n is p r e s u m e d t o b e a m e a s u r e o f t h e excess e n e r g y r e q u i r e d to b r e a k t h e b o n d b e t w e e n b o u n d w a t e r a n d t h e s o r p t i o n s i t e s , a n d t h e h e a t o f w e t t i n g is a m e a s u r e o f t h e t o t a l n u m b e r o f s o r p t i o n s i t e s a c c e s s i b l e to w a t e r ( 1 0 ) . 0
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
(-Bm)
L
L
T h e total heat of w e t t i n g W generally ranges b e t w e e n 1 5 a n d 2 0 cal/g of w o o d , b u t m a y b e l o w e r for w o o d s w i t h h i g h extractive c o n t e n t . It i n c r e a s e s w i t h d e c r e a s i n g p a r t i c l e s i z e a n d g e n e r a l l y w i t h r e m o v a l o f e x t r a c t i v e s (48). A s s h o w n i n F i g u r e 2 3 , t h e h e a t o f w e t t i n g at a g i v e n m o i s t u r e c o n t e n t is h i g h e r f o r p a r t i a l d e s o r p t i o n t h a n f o r p a r t i a l a d s o r p t i o n , p o s s i b l y b e c a u s e t h e r e are m o r e s o r p t i o n sites available d u r i n g desorption. 0
Free Energy and Entropy Changes. T h e heat of sorption Q consists of t w o parts, the free e n e r g y change A G a n d the e n t r o p y c h a n g e A S . T h e loss i n f r e e e n e r g y A G ( c a l / g o f w a t e r ) c a n b e c a l c u l a t e d at a n y m o i s t u r e c o n t e n t b y L
AG
= ( R T / 1 8 ) In (l/h)
(25)
20
10
W
(cal/g)
8 6
4
2l
M(%) Figure 23. Curves of heat of wetting W (log scale) for Hinoki cypress and for cotton against the moisture content (48).
In The Chemistry of Solid Wood; Rowell, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.
158
T H E CHEMISTRY OF SOLID WOOD
w h e r e h is t h e r e l a t i v e v a p o r p r e s s u r e ( a s s u m e d t o b e e q u i v a l e n t t o t h e a c t i v i t y ) o f 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 m o i s t u r e c o n t e n t m , R is t h e gas c o n s t a n t , a n d Τ is t h e K e l v i n t e m p e r a t u r e . T h e e x c e s s e n e r g y a s s o c i a t e d w i t h t h e e n t r o p y c h a n g e A S is d e f i n e d as t h e d i f ference i n Q and A G . T h u s L
TAS = Q 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
L
-
(26)
AG
F i g u r e 24 shows c u r v e s o f Q , A G , a n d T A S p l o t t e d against w o o d m o i s t u r e c o n t e n t . A l l e n e r g y t e r m s a r e n e g a t i v e (heat is g i v e n off) w h e n w o o d takes u p w a t e r f r o m t h e l i q u i d state. T h e d e c r e a s e i n e n t r o p y i n d i c a t e s t h a t b o u n d w a t e r is m o r e o r d e r e d t h a n l i q u i d w a t e r , i n analogy to t h e g r e a t e r o r d e r o f w a t e r i n ice c o m p a r e d w i t h the l i q u i d state. A s t h e m o i s t u r e c o n t e n t a p p r o a c h e s f i b e r s a t u r a t i o n the distinction b e t w e e n l i q u i d water a n d water i n w o o d decreases toward z e r o . H o w e v e r , e v e n a b o v e fiber s a t u r a t i o n t h e w a t e r i n c e l l c a v i t i e s may be different from ordinary l i q u i d water because of capillary forces and/or d i s s o l v e d materials. L
Swelling Pressure of W o o d . W o o d s w e l l s w h e n its m o i s t u r e c o n t e n t i n c r e a s e s . I f i t is r e s t r a i n e d f r o m s w e l l i n g i t w i l l e x e r t a 30CV
200
\
\
σ CO
100
\
^ 10
15
20
25
30
M (%) Figure 24. Curves of Q , A G , and TAS when liquid water is taken up by wood at various moisture contents. (Reproduced with permission from Ref 10. Copyright 1972, Syracuse University Press.) L
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
159
Relationships
s w e l l i n g pressure against the r e s t r a i n i n g m e d i u m . nitude of the s w e l l i n g pressure Π of the cell wall, at e q u i l i b r i u m w i t h r e l a t i v e v a p o r p r e s s u r e h a n d liquid water, can be calculated from the osmotic f o r s o l u t i o n s w r i t t e n as
T h e order of mag w h e n i t is i n i t i a l l y then immersed in pressure equation
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
Π = p A G = ( p R T / 1 8 ) In (llh)
(27)
w h e r e ρ is t h e d e n s i t y o f w a t e r i n t h e c e l l w a l l a n d t h e o t h e r t e r m s a r e as d e f i n e d i n E q u a t i o n 2 5 . W h e n w o o d is r e s t r a i n e d f r o m s w e l l i n g w h i l e s o a k i n g i n w a t e r , the pressures m e a s u r e d are m u c h smaller than those p r e d i c t e d from E q u a t i o n 27 b e c a u s e s w e l l i n g takes place i n t o the c e l l cavities. T h e r e f o r e , t h e m a x i m u m s w e l l i n g p r e s s u r e d e v e l o p e d is a f u n c t i o n o f t h e s t r e n g t h o f t h e w o o d (49). T h e m a x i m u m transverse swelling pressures Ρ have been mea s u r e d (49) i n w o o d d o w e l s w h i c h h a d p r e v i o u s l y b e e n d e n s i f i e d t o different specific gravities. A f t e r densification, each sample was c o n d i t i o n e d to e q u i l i b r i u m w i t h h = 0 . 3 a n d t h e n i n s e r t e d i n t o a s t e e l r e s t r a i n i n g r i n g e q u i p p e d w i t h s t r a i n g a g e s to m e a s u r e t h e t r a n s v e r s e s w e l l i n g s t r e s s . T h e w o o d d o w e l s w e r e t h e n e x p o s e d to l i q u i d w a t e r . T h e results s h o w e d that the m a x i m u m o b s e r v e d s w e l l i n g pressure i n c r e a s e d e x p o n e n t i a l l y w i t h i n c r e a s i n g specific g r a v i t y of t h e test s a m p l e s ( F i g u r e 25). T h e e x t r a p o l a t i o n o f t h e i r c u r v e s t o t h e s p e c i f i c g r a v i t y o f t h e d r y c e l l w a l l , 1.5, y i e l d s a v a l u e o f 1 3 , 2 0 0 p s i (91 M Pa). T h e t h e o r e t i c a l v a l u e b a s e d o n E q u a t i o n 2 7 is 158 M P a at r o o m temperature. T h e discrepancy b e t w e e n these two values may be be-
4.2.
0.6
1
0.7
1
OA
1
1
1
0.9
1.0
I.I
1
1
r
1
\2
1.3
1.4
1.5
Figure 25. Curve of swelling pressure (log scale) vs. dry-volume specific gravity G (49). (Reproduced with permission from Ref. 10. Copyright 1972, Syracuse University Press.) 0
In The Chemistry of Solid Wood; Rowell, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.
160
T H E CHEMISTRY O F SOLID W O O D
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
cause t h e s w e l l i n g a l o n g t h e g r a i n w a s n e g l e c t e d a n d because o f t h e c o m p r e s s i b i l i t y o f t h e c e l l w a l l i t s e l f . T h i s d i s c r e p a n c y is t r e a t e d i n the Barkas theory o f hygroelasticity discussed next. Hygroelastic Effects. W h e n 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 w o o d are restrained from swelling freely they not only exert a s w e l l i n g p r e s s u r e b u t also c o m e to a l o w e r m o i s t u r e c o n t e n t t h a n i f u n r e s t r a i n e d . T h e r e v e r s e effect also h o l d s t r u e : w o o d r e s t r a i n e d from shrinking exhibits a higher e q u i l i b r i u m moisture content than i f u n r e s t r a i n e d . T h i s i s t h e hygroelastic effect, s o m e t i m e s c a l l e d t h e Barkas effect b e c a u s e B a r k a s w a s t h e first t o t r e a t i t q u a n t i t a t i v e l y (26). Barkas proposed a generalized osmotic pressure theory for h y g r o s c o p i c g e l s s u c h as w o o d , b a s e d o n t h e g e n e r a l i z e d P o r t e r e q u a tion i n the form (dV/dm) dP p
m
(28)
= vdp
w h e r e (dV/dm) is t h e apparent specific v o l u m e o f the sorbed w a t e r at c o n s t a n t v a p o r p r e s s u r e ρ ( b e c a u s e V i s t h e s w o l l e n v o l u m e o f t h e g e l p e r u n i t d r y m a s s , a n d m i s t h e f r a c t i o n a l m o i s t u r e c o n t e n t ) ; dp is t h e i n c r e a s e i n h y d r o s t a t i c p r e s s u r e r e q u i r e d t o r a i s e t h e v a p o r p r e s s u r e o f t h e g e l b y t h e i n c r e m e n t dp at c o n s t a n t m o i s t u r e c o n t e n t m; a n d ν i s t h e s p e c i f i c v o l u m e o f w a t e r v a p o r . p
m
E q u a t i o n 28 reduces to t h e osmotic pressure equation ( E q u a t i o n 27) i f i t i s i n t e g r a t e d b e t w e e n t h e l i m i t s o f ρ a n d p ( w h e r e ρ = hp ), a s s u m i n g t h a t (dV/dm) = 1/p a n d t h a t t h e i d e a l gas l a w a p p l i e s t o w a t e r v a p o r , w h e r e P = Π. B a r k a s o b j e c t e d t o a p p l y i n g t h e o s m o t i c p r e s s u r e e q u a t i o n t o h y g r o s c o p i c g e l s s u c h as w o o d b e c a u s e t h i s e q u a t i o n n e g l e c t s t h e p r o p e r t i e s o f t h e g e l i t s e l f , s u c h as r i g i d i t y , w h e n c o m p a r e d w i t h solutions. F u r t h e r m o r e , it does notdistinguish b e t w e e n t h e s w e l l i n g p r e s s u r e at c o n s t a n t m a n d at c o n s t a n t V . a
0
p
m
Barkas, therefore, proposed a more generalized swelling pres sure theory for gels, w h i c h uses t h e P o r t e r e q u a t i o n , i n w h i c h t h e t e r m s Ρ, V, m, a n d ρ a r e i n t e r r e l a t e d b a s e d o n e m p i r i c a l d a t a o b tained from wood. F i g u r e 2 6 shows a p r e s s u r e - v o l u m e diagram for the cell wall of Sitka spruce w h i c h was calculated b y applying the P o r t e r e q u a t i o n t o t h e d a t a o f S t a m m a n d S e b o r g (50) o n S i t k a s p r u c e . T h e d i a g r a m i n d i c a t e s t h a t t h e b u l k m o d u l u s (VdP/dV) o f t h e c e l l w a l l is g r e a t e r at c o n s t a n t m o i s t u r e c o n t e n t m ( p r o p o r t i o n a l t o t h e s l o p e s o f t h e s o l i d l i n e s o f c o n s t a n t m) t h a n a t c o n s t a n t r e l a t i v e v a p o r p r e s s u r e h ( p r o p o r t i o n a l t o t h e s l o p e s o f t h e b r o k e n l i n e s o f c o n s t a n t h). B y u s i n g t h e s i m p l e o s m o t i c p r e s s u r e e q u a t i o n ( E q u a t i o n 27) w e a s s u m e t h a t n o m o i s t u r e c h a n g e o c c u r s i n t h e w o o d w h e n i t is c o m p l e t e l y r e s t r a i n e d f r o m s w e l l i n g d u r i n g exposure to water. A c c o r d i n g
In The Chemistry of Solid Wood; Rowell, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.
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
3.
SKAAR
161
Wood-Water Relationships
0.70
0.75
0.80
0.85
0.90
VCcc/g)
Figure 26. Curves of hydrostatic pressure Ρ vs. specific volume V at con stant moisture content m or relative vapor pressure h, from Barka (26). (Reproduced with permission from Ref. 10. Copyright 1972, Syracuse University Press.) to t h e c o n s t a n t m c u r v e s o f F i g u r e 2 6 , i f t h e v o l u m e V is t o r e m a i n constant the moisture content m must increase w h e n the w o o d c e l l w a l l is at a n e q u i l i b r i u m m o i s t u r e c o n t e n t m a n d t h e r e l a t i v e v a p o r p r e s s u r e is i n c r e a s e d . T h i s i n c r e a s e i n m o i s t u r e c o n t e n t e f f e c t i v e l y i n c r e a s e s t h e i n i t i a l v a l u e o f h so t h a t t h e s w e l l i n g p r e s s u r e is l o w e r than that calculated f r o m the s i m p l e osmotic pressure e q u a t i o n . F o r e x a m p l e , i n t h e e x p e r i m e n t o f T a r k o w a n d T u r n e r (49) p r e v i o u s l y cited, the effective value of h w o u l d b e somewhat h i g h e r than 0.3 d u e to a s l i g h t i n c r e a s e i n m , a n d t h e c a l c u l a t e d v a l u e o f s w e l l i n g p r e s s u r e b y u s i n g E q u a t i o n 2 7 w o u l d b e l o w e r t h a n t h e 158 M P a w h e n calculated b y a s s u m i n g that h = 0.3.
Theories of Water Sorption M a n y t h e o r i e s h a v e b e e n p r o p o s e d to a c c o u n t for the s o r p t i o n o f w a t e r b y 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 w o o d . O n e o f t h e e a r l i e s t t h e o r i e s ( P e i r c e , R e f e r e n c e 51) s u g g e s t s t h a t w a t e r is s o r b e d b y t e x t i l e s i n t w o f o r m s , o n e s t r o n g l y a t t a c h e d to p r i m a r y s o r p t i o n s i t e s and the other b o u n d m o r e weakly.
In The Chemistry of Solid Wood; Rowell, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.
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
162
T H E CHEMISTRY O F SOLID W O O D
M o s t s u b s e q u e n t s o r p t i o n theories, i n c l u d i n g those discussed here, have followed this general approach a n d postulate two forms o f s o r b e d w a t e r . T h e s e t h e o r i e s m a y b e c l a s s i f i e d i n t o at l e a s t t w o general types based o n the sorption m e c h a n i s m assumed. O n e type a s s u m e s s o r p t i o n o n i n t e r n a l s u r f a c e s a n d is r e p r e s e n t e d b y t h e D e n t t h e o r y (52), w h i c h is a m o d i f i c a t i o n o f t h e c l a s s i c B r u n a u e r , E m m e t t , a n d T e l l e r ( B E T ) t h e o r y (53). T h e s e c o n d t y p e a s s u m e s t h a t t h e w o o d - w a t e r system forms a solution, exemplified b y the H a i l w o o d H o r r o b i n theory. T h e r e have been other theories, not discussed h e r e , that h a v e also b e e n a p p l i e d to e x p l a i n w a t e r s o r p t i o n b y h y g r o s c o p i c m a t e r i a l s (JO, 54, 55). Dent's Surface Sorption Theory. T h e D e n t sorption theory or m o d e l (52), i n t h e s i m p l e f o r m , is a m o d i f i c a t i o n o f t h e B E T m o d e l , w h i c h is i t s e l f a n e x t e n s i o n o f t h e e a r l i e r L a n g m u i r m o d e l (56). T h e L a n g m u i r m o d e l a s s u m e s t h a t a gas ( w a t e r v a p o r i n t h e case o f w o o d ) is s o r b e d o n t o s o r p t i o n s i t e s o n t h e s u b s t r a t e o r s o r b e n t i n a m o n o layer only. T h e fraction of s o r p t i o n sites o c c u p i e d b y the v a p o r or s o r b a t e is a f u n c t i o n o f t h e v a p o r p r e s s u r e o f t h e s o r b a t e a n d a p p r o a c h e s u n i t y as t h e v a p o r p r e s s u r e i n c r e a s e s . T h e B E T m o d e l (53) e x t e n d s t h e L a n g m u i r m o d e l t o p e r m i t m o r e than one layer of condensate o n any p a r t i c u l a r sorption site. F u r t h e r m o r e , i t p o s t u l a t e s t h a t t h e gas c o n d e n s e d i n l a y e r s a b o v e t h e first l a y e r h a s t h e s a m e t h e r m o d y n a m i c p r o p e r t i e s as o r d i n a r y condensate (liquid water i n the w o o d - w a t e r system). T h e basic sorp t i o n i s o t h e r m p r e d i c t e d b y t h e B E T m o d e l fits t h e i s o t h e r m f o r w o o d r e a s o n a b l y w e l l at r e l a t i v e v a p o r p r e s s u r e s less t h a n — 0 . 3 b u t n o t at h i g h e r values. A m o d i f i c a t i o n that l i m i t s the m a x i m u m n u m b e r of l a y e r s to a finite n u m b e r , say five o r s i x g i v e s a b e t t e r fit at h i g h r e l a t i v e v a p o r p r e s s u r e ( F i g u r e 27). H o w e v e r , t h e D e n t m o d e l g i v e s a m o r e s a t i s f a c t o r y fit o v e r m o s t o f t h e h y g r o s c o p i c m o i s t u r e r a n g e . F i g u r e 28 shows the w o o d substrate c o n t a i n i n g p r i m a r y sorption sites ( v e r t i c a l l i n e s ) , s o m e o c c u p i e d b y p r i m a r y w a t e r m o l e c u l e s ( d a r k circles), a n d some c o n t a i n i n g b o t h p r i m a r y and secondary (open cir cles) w a t e r m o l e c u l e s . I n b o t h t h e B E T a n d D e n t m o d e l s t h e p r i m a r y s o r p t i o n s i t e s a r e a s s u m e d t o b e h i g h e n e r g y b i n d i n g s i t e s , s u c h as accessible h y d r o x y l g r o u p s , a n d the s e c o n d a r y sites are of l o w e r b i n d i n g energy. T h e B E T m o d e l assumes that the t h e r m o d y n a m i c p r o p e r t i e s o f t h e s e c o n d a r y w a t e r m o l e c u l e s a r e t h e s a m e as t h o s e of o r d i n a r y l i q u i d water, whereas the D e n t m o d e l assumes that they are different. If the three f u n d a m e n t a l D e n t constants, m , k n o w n t h e s o r p t i o n i s o t h e r m c a n b e w r i t t e n as 0
m = mJixh/Kl
-
k h) 2
(1 + k h x
-
k
u
and k
k h)] 2
In The Chemistry of Solid Wood; Rowell, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.
2
are
(29)
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
3.
SKAAR
163
Wood-Water Relationships
h Figure 27. Sorption isotherms predicted by BET sorption theory for var ious values of η (broken lines), compared with the experimental isotherm (solid line). Also shown is the monolayer moisture content M (10). (Re produced with permission from Ref 10. Copyright 1972, Syracuse Uni versity Press.) 1
w h e r e m is 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 to c o m p l e t e m o n o l a y e r coverage (all p r i m a r y sites o c c u p i e d b y a single m o l e c u l e o n e a c h s i t e ) ; ki a n d k a r e e q u i l i b r i u m c o n s t a n t s r e l a t e d t o t h e b i n d i n g energies of the p r i m a r y a n d secondary water layers, respectively; a n d h is the r e l a t i v e v a p o r p r e s s u r e . T h e total m o i s t u r e c o n t e n t m consists of two components; the p r i m a r y water m and t h e secondary water T h e s e c a n b e c a l c u l a t e d as 0
2
l 5
m
l
= m(l
ma = ™k h 2
-
k h) = m kih/(l 2
0
= ™ &ι& ^ /[(1 ο
2
2
+ kh {
-
~ k h) (1 + k h 2
Y
(30)
k h) 2
k h)] 2
(31)
T h e f r e e e n e r g y c h a n g e AG γ ( c a l / g o f w a t e r ) a s s o c i a t e d w i t h s o r p -
/ / / / WOOD SUBSTRATE
//////
Figure 28. Schematic diagram showing sorption sites (vertical lines), some occupied by primary water molecules (dark circles) and some by sec ondary water molecules (open circles) (59).
In The Chemistry of Solid Wood; Rowell, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.
164
T H E CHEMISTRY O F SOLID W O O D
tion of p r i m a r y water, a n d A G (sorption of secondary water) can be c a l c u l a t e d f r o m t h e e q u i l i b r i u m c o n s t a n t s k a n d k ( E q u a t i o n 32). 2
x
=
-
(R7718)ln *
=
àG
2
-
(RT/18)ln k
(32)
2
T h e coefficients m , k , a n d k can be calculated from actual s o r p t i o n i s o t h e r m s b y m o d i f y i n g E q u a t i o n 2 9 to E q u a t i o n 3 3 ( w h e r e A , B , a n d C are constants d e t e r m i n e d e m p i r i c a l l y from sorption iso t h e r m data). 0
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
i ;
2
x
him
2
= A
+ Bh
-
(33)
Ch
2
W h e n A , B , a n d C are k n o w n , the f u n d a m e n t a l constants k m c a n b e e v a l u a t e d as
and
k,
u
2
0
k
= [-
2
k
x
1/[1
= m
0
=
Β + (Β
2
-
+ 4 AC) ]/(2 A )
k (B/C)] 2
l/lAftai*! +
(34)
1/2
= (B/A) 1)] =
+ 2 k
(35)
2
1/^)
(36)
T h e first s t e p i n e v a l u a t i n g t h e c o e f f i c i e n t s f o r a p a r t i c u l a r s o r p t i o n i s o t h e r m is t o c a l c u l a t e t h e r a t i o him f r o m e x p e r i m e n t a l d a t a o f h a n d m at e a c h d a t a p o i n t . T h e him v a l u e s a r e t h e n fitted t o a p a r a b o l a , u s i n g l e a s t s q u a r e s r e g r e s s i o n p r o c e d u r e s , w i t h h as t h e i n d e p e n d e n t v a r i a b l e ( E q u a t i o n 33). T h e v a l u e s o f m k a n d k are t h e n evaluated f r o m the regression coefficients A , R , a n d C . 0>
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
In The Chemistry of Solid Wood; Rowell, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.
3.
SKAAR
Wood-Water
1
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
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
In The Chemistry of Solid Wood; Rowell, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.
0
166
T H E CHEMISTRY O F SOLID W O O D
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
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
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
In The Chemistry of Solid Wood; Rowell, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.
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
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
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
In The Chemistry of Solid Wood; Rowell, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.
m
168
T H E CHEMISTRY O F SOLID W O O D
IQQxKT
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
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
In The Chemistry of Solid Wood; Rowell, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.
3.
SKAAR
VAPOR
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
169
Wood-Water Relationships
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
In The Chemistry of Solid Wood; Rowell, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.
170
T H E CHEMISTRY O F SOLID W O O D
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
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
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 .
Literature Cited 1. Morton, W. E.; Hearle, J. W. S. "Physical Properties of Textile Fibers"; 2nd ed.; William Heinemann Ltd.: London, 1975; p. 161. 2. Zimmerman, M . H . ; Brown, C. L . "Trees—Structure and Function"; Springer-Verlag: New York, 1971. 3. Peck, E . C. "The Sap or Moisture in Wood"; U.S., For. Prod. Lab., Rep. No. 768, Madison, WI, 1953. 4. Zobel, B.; Matthias, M . ; Roberts, J. H . ; Kellison. R. C. "Moisture Content of Southern Pine Trees"; N . C . Sch. For. Tech. Rep. 37, Raleigh, N C , 1968. 5. Lawrence, W. E.; Jr. "Field-Drying Logging Residue as an Industrial Fuel"; MS thesis, Virginia Polytechnic Inst. and State Univ., Blacksburg, VA, 1981. 6. Tiemann, H . D . "Effect of Moisture on the Strength and Stiffness of Wood", U.S. For. Serv. Bull. No. 70, Washington, D C , 1906. 7. Kollmann, F.; Hockele, G. Holz Roh- Werkst. 1962, 20(12), 461-73. 8. Christensen, G. N . ; Hergt, H. F. A. J. Polym. Sci., Part A-1 1969, 7(8), 2427—30. 9. Stamm, A. J. Ind. Eng. Chem. 1927, 19, 1021-25. 10. Skaar, C. "Water in Wood"; Syracuse Univ. Press: Syracuse, 1972. 11. Beall, F. C. "Relative Humidity and Moisture Content Instrumentation";
In The Chemistry of Solid Wood; Rowell, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.
3.
12. 13. 14.
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
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.
SKAAR
Wood-Water Relationships
171
Proc., Symposium on Wood Moisture Content, Temperature and H u midity Relationships, U . S . , For. Prod. Lab. Rep., Madison, WI, 1979. U.S. Forest Products Laboratory, "Wood Handbook"; U . S . D . A . Handbook No. 72, rev., 1974. Couture, R. F.; Hill, J. L . For. Prod. J. 1974, 24(4), 17-23. James, W. L . "Electric Moisture Meters for Wood"; U.S., For. Prod. Lab. Rep. F P L - 6 , Madison, WI, 1975. Sharp, A. R.; Riggin, M. T.; Kaiser, K.; Schneider, M . H . Wood Fiber 1978, 10(2), 74-81. Nanassy, A. J. Wood Sci. 1973, 5, 187-93. Hillel, D . "Fundamentals of Soil Physics"; Academic Press: New York, 1980. Spalt, H . A. For. Prod. J. 1958, 8(10), 288-95. Okoh, Κ. A. I.; Skaar, C. Wood Fiber 1980, 12(2), 98-111. Weichert, L . Holz Roh- Werkst. 1963, 21(8), 290-300. Skaar, C. "Moisture Sorption Hysteresis in Wood"; Proc., Symposium on Wood Moisture Content, Temperature and Humidity Relationships, U.S. For. Serv. For. Prod. Lab., Madison, WI, 1979. Stamm, A. J. "Wood and Cellulose Science"; Ronald Press, 1964. Simpson, W. T.; Rosen, H . N . Wood Fiber 1981, 13(3), 150-58. Christensen, G. N . ; Kelsey, Κ. E . Holz Roh- Werkst. 1959, 17(5), 178-88. Wangaard, F. F.; Granados, L . A. Wood Sci. Technol. 1967, 1(4), 253-77. Barkas, W. W. "The Swelling of Wood Under Stress"; For. Prod. Res. Bull. (G.B.) London, 1946. Mitchell, R. H . Ph.D. dissertation, Virginia Polytechnic Inst. and State Univ., Blacksburg, VA, 1981. U.S. Forest Products Laboratory. "Wood Handbook", U . S . D . A . Handbook No. 72 (rev.), U.S. G P O : Washington, D C , 1974. Kellogg, R. W.; Wangaard, F. F. Wood Fiber 1969, 1(3), 180-204. Stamm, A. J. Wood Sci. 1971, 4(2), 114-28. Tiemann, H . D . "Wood Technology"; 2nd ed., Pitman Publ. Co.: New York, 1944. Stamm, A. J.; Loughborough, W. K. "Variation in Shrinking and Swelling of Wood"; Trans. A S M E , Louisville, KY, 1941. Greenhill, W. L . "The Shrinkage of Australian Timbers, I"; Austr., CSIRO, Div. For. Prod. Technol. Pap. No. 21, Melbourne, 1936. Keylwerth, R. Holz Roh- Werkst. 1964, 22(7), 255-58. Meylan, B. A. For. Prod. J. 1968, 18(4), 75-78. Barber, Ν. F.; Meylan, B. A. Holzforsch. 1968, 22(4), 97-103. Meylan, B. A. Wood Sci. Technol. 1972, 6(4), 293-301. Kollmann, F.; Cote, W. Α., Jr. "Principles of Wood Science and Tech nology", Vol. I; Springer-Verlag: New York, 1968. Boyd, J. D . Mokuzai Gakkaishi 1974, 20(10), 473-82. Bosshard, Η. H . Holz Roh- Werkst. 1956, 14(8), 285-94. McIntosh, D . C. For. Prod. J. 1955, 5(5), 355-59. Panshin, A. J.; de Zeeuw, C. H . "Textbook of Wood Technology", 4th ed.; McGraw-Hill: New York, 1980. Hearman, R. F. S.; Burcham, J. N . Nature (London) 1955, 176,978. Kelsey, K. E.; Clarke, L . N. Austr. J. Appl. Sci. 1956, 7, 160-75. Siau, J. F. "Flow in Wood"; Syracuse Univ. Press: Syracuse, 1971. Stamm, A. J.; Loughborough, W. K. J. Phys. Chem. 1935, 39, 121-32. Cooper, D . N . E.; Ashpole, D. K. J. Text. Inst. 1959, 50 T, 223-32. Kajita, H . "The Heat of Wetting of Wood in Water (I)", Bull. No. 20 of the Kyoto Prefectural Univ. Forests, Kyoto, 1976; pp. 49-61. Tarkow, H . ; Turner, H . D . For. Prod. J. 1958, 8(7), 193-97. Stamm, A. J.; Seborg, R. M. J. Phys. Chem. 1935, 39, 133-42. Peirce, F. T. J. Text. Inst. 1929, 20, T133-50. Dent, R. W. Text. Res. J. 1977, 47(2), 145-52.
In The Chemistry of Solid Wood; Rowell, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.
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
172
T H E CHEMISTRY O F SOLID W O O D
53. Brunauer, S.; Emmett, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 30919. 54. Simpson, W. T. Wood Fiber 1973, 5(1), 41-49. 55. Chirife, J.; Iglesias, Η. Α. J. Food Technol. 1978, 13, 159-74. 56. Langmuir, I. J. Am. Chem. Soc. 1918, 40, 1361. 57. Hailwood, A. J . ; Horrobin, S. Trans. Faraday Soc. 1946, 42B, 84-92. 58. Barrie, J. A. in "Diffusion in Polymers"; Crank, J . ; Park, G. S., Eds.; Aca demic Press: London, 1968. 59. Skaar, C.; Babiak, M . Wood Sci. Technol. 1982, 16, 123-38. 60. Stamm, A. J. "Passage of Liquids, Vapors and Dissolved Materials Through Softwoods"; U . S . D . A . Tech. Bull. No. 929, U.S. Dept. Agric., Wash ington, D C , 1946. 61. Skaar, C . ; Siau, J. F. Wood Sci. Technol. 1981, 15, 105-12. 62. Smith, D . ; Lee, E . "The Longitudinal Permeability of Some Hardwoods and Softwoods"; Spec. Rep. For. Prod. Res. No. 13, London, 1958. 63. Banks, W. B. Wood Sci. Technol. 1981, 15, 171-77. 64. Hawley, L . F. "Wood-Liquid Relations"; U . S . D . A . Tech. Bull. No. 248, U.S. Dept. Agric: Washington, D C , 1931. 65. Comstock, G. L . Wood Fiber 1970, 1, 283-89. 66. Spolek, G. Α.; Plumb, O. A. Wood Sci. Technol. 1981, 15, 189-99. 67. Rosen, Η. N . Wood Sci. 1982, 14(3), 134-37. 68. Hart, C. A. Wood Sci. 1977, 9(4), 194-201. 69. Rosen, Η. N . Wood Fiber 1978, 10(3), 218-28. RECEIVED
for review May 9, 1983.
ACCEPTED
July 7, 1983.
In The Chemistry of Solid Wood; Rowell, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.
