Wood Softening and Forming with Ammonia - ACS Publications


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21 W o o d Softening and Forming with Ammonia M. BARISKA Institut fur Microtechnologische Holzforschung, Eidgenossische Technische Hochschule, CH 8006 Zurich, Switzerland

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C. SCHUERCH Department of Chemistry, S.U.N.Y. College of Environmental Science and Forestry, Syracuse, N.Y. 13210 A number of years ago, anhydrous liquid ammonia was found to be a powerful plasticizing agent for wood, which softened wood strips so that they could be formed readily into dramatic and complex shapes and caused them to develop permanent set in the new form (1). About the same time, the plasticizing action of aqueous ammonia was under investigation in Latvia (2), and although the effects were much less extreme, the softening was sufficient to serve as a basis for extensive scientific and technological work that has led to some industrial developments (3-18). The use of gaseous ammonia also proved successful when the accelerating effect of moderate moisture content on absorption was recognized (19), and the versatility of the method can now probably be greatly extended by application of the results of investigations in the low pressure range (20). The complete definition of the system wood-water-ammonia as a function of temperature, pressure and composition is still far from complete, but general trends are apparent enough to be summarized at this time. Both the processing and final properties of wood are markedly affected by the conditions of the treatment, and ammonia forming of wood is thus, in a sense, a spectrum of processes from which one can be selected to produce minor softening in the cold or, at the other extreme, one that causes greatly enhanced flexibility and stable final permanent set. During a period of concern for dwindling natural resources, a serious scientific and technological investigation of methods of wood forming could be of considerable social value. Wood has many advantages as a naturally renewable material of high strength to weight ratio, but i t also has some properties which limit its u t i l i t y and lead to waste. It has variable physical and mechanical properties within a single sample due to i t s complex microscopic structure. It is not dimensionally stable to moisture changes. Although i t can be easily worked by tools and machines, i t cannot be easily molded to a complex shape. As a result of the last, very large losses of material accrue during fabrication from tree to products with complicated forms.

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In Wood Technology: Chemical Aspects; Goldstein, I.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

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Frequently e x c e s s i v e m a t e r i a l i s r e q u i r e d i n the product i t s e l f to a l l o w f o r the weakness of c r o s s g r a i n e d s e c t i o n s and j o i n t s . These disadvantages a l l have p e r t i n e n c e to the s o f t e n i n g and forming o f wood. To the extent t h a t wood can be softened and bent, t h i n n e r stock may be used because j o i n t s and c r o s s g r a i n e d s e c t i o n s can be avoided and design r e s t r i c t i o n s can be minimized. However, wood's n a t u r a l r i g i d i t y and v a r i a b l e p r o p e r t i e s now l i m i t i t s forming t o r a t h e r h i g h grade s t o c k , and some methods — i n c l u d i n g the ammonia treatment — a l t e r somewhat the p h y s i c a l and mechanical c h a r a c t e r i s t i c s . A number o f hydrogen-bonding s o l v e n t s , other than ammonia, have a l s o been i n v e s t i g a t e d f o r wood s o f t e n i n g . These i n c l u d e (21-25), formamide (21), dimethyl s u l f o x i d e (26), phenol and urea (27). These d i f f e r i n the degree of f l e x i b i l i t y produced i n the wood and the ease w i t h which they are removed. A l l are more expensive than ammonia and most are more d i f f i c u l t to remove. Since the purpose of t h i s r e p o r t i s p r i m a r i l y to i n t e r p r e t the behavior of the wood-ammonia system and t o r e l a t e i t t o p r a c t i c a l a p p l i c a t i o n , we w i l l f i r s t b r i e f l y review c u r r e n t methods of forming wood. We w i l l d i s c u s s i n some d e t a i l methods of s o f t e n i n g wood w i t h water and heat and the molecular changes u n d e r l y i n g them and then extend these concepts to e x p l a i n the more complicated but s i m i l a r ammonia-based systems, and t h e i r p o s s i b l e p r a c t i c a l i m p l i c a t i o n s . 311111:168

A l t e r n a t i v e Forming Methods. Wood can be formed i n t o complex shapes w i t h l e a s t chemical o r molecular change by c u t t i n g t h i n s l i c e s , assembling them w i t h the g r a i n p a r a l l e l or p e r p e n d i c u l a r and g l u i n g them i n p l a c e over the d e s i r e d form. These processes o f l a m i n a t i o n and plywood moulding depend on the f a c t t h a t s t i f f n e s s of any lamella-shaped m a t e r i a l v a r i e s as the t h i r d power of i t s t h i c k n e s s , so f l e x i b i l i t y i s g r e a t l y increased as the t h i c k n e s s i s decreased. The s e p a r a t i o n of i n d i v i d u a l s l i c e s by a glue l i n e i n h i b i t s f l a w propagation, and the crossed o r i e n t a t i o n of plywood a l s o enhances dimensional s t a b i l i t y to moisture. Reasonably complex curves can thus be produced i n f u r n i t u r e manufacture, and the i n h e r e n t expense of these methods i s o f f s e t by u s i n g lower q u a l i t y veneers f o r the interior layers. Wood can a l s o be softened f o r forming by p l a s t i c i z a t i o n w i t h water. Wood shows c o l l o i d c h a r a c t e r : i t i s o f t e n d e f i n e d as a g e l , predominantly a m a t r i x of m i c r o f i b r i l s surrounded by a f l u i d medium, hydrate water. Wood substance i s generated by the l i v i n g c e l l i n a w a t e r - s a t u r a t e d m i l i e u and, t h e r e f o r e , has by nature a c e r t a i n f l e x i b i l i t y , which i s a l t e r e d even by d r y i n g and remoistening. I f r e t a i n e d i n a n e v e r - d r i e d s t a t e , t h i n s e c t i o n s are extremely supple and can be bent and woven r e a d i l y i n the c o l d . Once d r i e d , wood can be resoftened by r a i s i n g i t s water content e s p e c i a l l y w i t h i n c r e a s e i n temperature. F o r

In Wood Technology: Chemical Aspects; Goldstein, I.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

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l a r g e r dimensional s t o c k , soaking i n hot water o r p r e f e r a b l y steam treatment i s v i r t u a l l y r e q u i r e d (27). Most simple wood bending i s done by t r e a t i n g wood w i t h s a t u r a t e d steam. U s u a l l y , the steaming o f wood i s i n t e r r u p t e d before wood reaches s a t u r a t i o n o r i s h i g h l y p l a s t i c i z e d . Normally, wood w i l l be softened s u f f i c i e n t l y f o r forming by steaming one minute per mm o f t h i c k n e s s . Then the softened "work p i e c e " i s p l a c e d on a form and bent r a p i d l y by hand or machine. The bent p i e c e i s r e s t r a i n ed i n i t s new form and put aside t o s e t . Steaming a f f e c t s the compression s t r e n g t h o f the wood to a g r e a t e r extent than the t e n s i o n s t r e n g t h . During the bending procedure one t r i e s to take advantage o f t h i s f a c t . For bending, a f l e x i b l e metal s t r a p w i t h two b l o c k s attached to i t s ends i s placed on the s i d e of the wood which i s t o be convex. On bending, w i t h the b l o c k s t i g h t l y i n place on the ends of the wood, the e n t i r e wood member i s p l a c e d i n compression w h i l e the s t e e l s t r a p i s under t e n s i o n . Under these c o n d i t i o n s a minimum r a d i u s o f curvature r a t i o can be obtained. On a microscopic l e v e l , b u c k l i n g o f the p l a s t i c i z e d c e l l w a l l s occurs towards the lumena under compression. At the weakest p o i n t s l a y e r s and m i c r o f i b r i l s o f the c e l l w a l l s are p a r t i a l l y separated, w i t h p a r t i a l d e s t r u c t i o n o f the c e l l w a l l , to form s o - c a l l e d s l i p l i n e s and s l i p planes. As a r e s u l t , the u l t i m a t e compressive s t r e n g t h of the wood decreases w h i l e the t e n s i o n s t r e n g t h normally i s not a f f e c t e d . T h e r e a f t e r , the formed wood r e t a i n s a higher f l e x i b i l i t y , but a l s o greater shrinkage and s w e l l i n g behavior due to i t s l o o s e r s t r u c t u r e caused by the s l i p regions of the c e l l w a l l s . V a r i a t i o n s on the hot steam process have been reported. S e v e r a l repeated treatments of steaming and bending can permit more extreme bends than a s i n g l e o p e r a t i o n . Wood i s more p l a s t i c at n o n - e q u i l i b r i u m s t a t e s o f moisture content i f i t has been subjected to s t r e s s , e.g. to bending during water a d s o r p t i o n (28). Water a d s o r p t i o n s e t s up an a d d i t i o n a l s t r e s s which on the t e n s i o n s i d e of the wood beam shows the same d i r e c t i o n as the bending s t r e s s . Thus, i n t h i s r e g i o n of the wood, creep w i l l be a c c e l e r a t e d d u r i n g s o f t e n i n g w h i l e the compression s t r e s s has been lowered. I f the wood i s beaten p a r a l l e l to the g r a i n during steaming, c e l l s are p a r t i a l l y separated from one another and the work p i e c e can be bent t o v a r i o u s forms without s p r i n g back. Some deciduous species of wood can be steamed under compression, r e t a i n e d i n the wet c o n d i t i o n and l a t e r bent i n the c o l d ( 2 9 ) . High frequency h e a t i n g of wood provides a r a p i d p l a s t i c i z a t i o n process comparable t o conventional steam p r o c e s s i n g . While moist f i b e r b o a r d tends to d e f i b e r on forming, dry f i b e r b o a r d heated t o 400° f o r f i v e seconds can be bent to complex shapes and r a p i d l y cooled w i t h l i t t l e d i m i n u t i o n of s t r e n g t h ( 3 0 ) . I t i s thus c l e a r that both the success i n p r o c e s s i n g and the f i n a l p r o p e r t i e s o f the formed wood can depend on the p a r t i c u l a r

In Wood Technology: Chemical Aspects; Goldstein, I.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

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combination of p l a s t i c i z e r , heat, and mechanical treatment used. In the more complex ammonia-water-wood system, much more e f f o r t i s r e q u i r e d to determine optimum c o n d i t i o n s f o r p a r t i c u l a r r e s u l t s , s i n c e a much wider range of r e s u l t s i s p o s s i b l e . Steam bending has severe l i m i t a t i o n s i n the q u a l i t y and number of species of wood t h a t can be bent (27). In general s t r a i g h t grained high density northern hardwoods u s u a l l y give a minimum r a d i u s of curvature r a t i o or a minimum f a i l u r e r a t e . These v a r i a t i o n s between woods r e f l e c t d i f f e r e n c e s i n micros c o p i c s t r u c t u r e and chemical o r g a n i z a t i o n of the m a t e r i a l , f o r phase geometry can be as important as molecular s t r u c t u r e i n determining the p r o p e r t i e s of both n a t u r a l and s y n t h e t i c m u l t i phase systems (31). Therefore, i t i s c l e a r that the mechanical behavior of the wood-water system cannot be e x p l a i n e d e n t i r e l y at the molecular l e v e l or as i n t e r a c t i o n of macromolecules w i t h s o l v e n t . Nevertheless, the general trends observed do f o l l o w general p r i n c i p l e s of solvent-polymer i n t e r a c t i o n and can be so explained. Fundamentals of Wood-water I n t e r a c t i o n s . Goring has l u c i d l y described the i n f l u e n c e of heat and water on wood components (32): As temperature i s r a i s e d , s o l i d polymers absorb energy and the chains develop more v i o l e n t motion u n t i l a r a t h e r narrow temperature range i s reached at which i n t e r molecular bonds are broken and the macromolecules become capable of l a r g e s c a l e displacement w i t h respect to each other. In t h i s range the polymer p r o p e r t i e s change r a p i d l y and amorphous polymers g e n e r a l l y undergo a change from a glassy to a rubbery or p l a s t i c s t a t e . S i m i l a r s o f t e n i n g p o i n t s are observed f o r both l i g n i n and h e m i c e l l u l o s e . Furthermore, the presence of water acts as a t y p i c a l low molecular weight d i l u e n t , lowering the s o f t e n i n g p o i n t or tack temperature of l i g n i n from about 190° to 70-116° C. Very s i m i l a r behavior i s observed w i t h i s o l a t e d h e m i c e l l u l o s e . In c o n t r a s t , water cannot penetrate the c r y s t a l l a t t i c e of c e l l u l o s e and c e l l u l o s e softens at about 230° whether wet or dry. The s o f t e n i n g of wood f o r forming depends d i r e c t l y on these polymer-solvent i n t e r a c t i o n s and are a dramatic i n d i c a t i o n that much of the wood s t i f f n e s s i s due to i n t e r m o l e c u l a r a s s o c i a t i o n f o r c e s , predominantly hydrogen bonds. The e x t r a suppleness of never-dried wood can be r e l a t e d to the f a c t that n e v e r - d r i e d c e l l u l o s e has a much h i g h e r e q u i l i b r i u m moisture content at a l l r e l a t i v e h u m i d i t i e s than c e l l u l o s e a f t e r d r y i n g (33). The m u l t i p l e hydrogen bonds t h a t are formed on d r y i n g form p a r t i a l l y ordered regions t h a t cannot e n t i r e l y again be loosened w i t h water alone. P o s s i b l y the same phenomenon occurs w i t h l i g n i n and h e m i c e l l u l o s e . E f f e c t of A p p l i e d Force. In a d d i t i o n to c o n s i d e r i n g the i n f l u e n c e of water on the wood, one must consider the e f f e c t of an a p p l i e d f o r c e i n conjunction w i t h water as p l a s t i c i z e r .

In Wood Technology: Chemical Aspects; Goldstein, I.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

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I f a f o r c e i s a p p l i e d to wood w i t h i n the p r o p o r t i o n a l i t y l i m i t s , the wood w i l l bend and i f the f o r c e i s r e l e a s e d , the wood returns to i t s o r i g i n a l form w i t h an e l a s t i c recovery. I n c o n t r a s t , i f the wood i s d r i e d under s t r e s s , a s u b s t a n t i a l s u p e r p o s i t i o n o f s t r e s s e s occurs i n conjunction w i t h the drying and s h r i n k i n g process. Since the o r d e r i n g of macromolecules o r l a r g e r s t r u c t u r a l elements under t e n s i o n i s d i f f e r e n t from those under compression, as the water molecules are removed, new hydrogen bonds can form between d i f f e r e n t subunits of the s t r u c t u r e t o support the d i s t o r t e d s t r u c t u r e i n i t s new form. In t h a t case one would expect that i n t e r n a l s t r e s s e s would be present i n the wood and on resteaming the wood could recover i t s o r i g i n a l shape. On the other hand, there may be during d r y i n g some true p l a s t i c deformation, some amount o f i r r e v e r s i b l e displacement of macromolecules, f i b r i l s or f i b e r s r e l a t i v e to one another. For example, i f a beam i s d r i e d during bending under l o a d , an a d d i t i o n a l t e n s i o n s t r e s s w i l l be s e t up i n the outer zones w h i l e i n i n n e r regions of the beam compression s t r e s s e s w i l l be introduced. The r e s u l t a n t s t r e s s e s from the d r y i n g and bending can surpass the p r o p o r t i o n a l i t y region between load and corresponding deformation on the t e n s i o n s i d e of the beam and creep w i l l occur. I n that case one would expect permanent s e t not recovered on resteaming. Thus some degree of permanent set i s not l i m i t e d t o ammonia forming, and even s m a l l q u a n t i t i e s of ammonia i n aqueous systems probably f a c i l i t a t e creep. P l a s t i c i z a t i o n w i t h NH^— Processes on Molecular

Level

D i f f e r e n c e s Between Ammonia and Water. There are s i g n i f i c a n t d i f f e r e n c e s i n p h y s i c a l p r o p e r t i e s between wood f u l l y p l a s t i c i z e d by ammonia and wood p l a s t i c i z e d by steam. The ammonia-saturated wood shows comparatively l i t t l e e l a s t i c deformation w i t h s t r e s s and undergoes a l a r g e time-dependent p l a s t i c deformation and creep. Therefore, most extreme r e s u l t s are obtained i f forming i s c a r r i e d out s l o w l y o r i n some case perhaps i n t e r m i t t e n t l y . When the bending force i s r e l e a s e d , the wood does not r e t u r n to i t s o r i g i n a l shape. I f the wood i s d r i e d and then wet w i t h water i t s w e l l s more on the compression s i d e than the t e n s i o n s i d e and s t r a i g h t e n s , but on d r y i n g returns e s s e n t i a l l y to i t s formed shape. These macroscopic d i f f e r e n c e s can be r e l a t e d to the molecular i n t e r a c t i o n s between wood components and the two s o l v e n t s . Nayer and Hossfeld (34) have shown t h a t wood s w e l l i n g i n a s e r i e s of s o l v e n t s increases w i t h increase i n hydrogen bonding c a p a c i t y of the s o l v e n t and decreases w i t h increase i n molecular s i z e . Ammonia as a s o l v e n t w i t h a s i m i l a r molecular s i z e but a greater hydrogen bonding c a p a c i t y than water would be expected to s w e l l and s o f t e n wood more and

In Wood Technology: Chemical Aspects; Goldstein, I.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

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such i s the case ( l b ) . Anhydrous ammonia can even penetrate the c e l l u l o s e c r y s t a l l a t t i c e i n wood and r e l a x i n t e r - c r y s t a l l i n e f o r c e s . The s o l u b i l i t y and s w e l l i n g o f l i g n i n a l s o i n c r e a s e w i t h the hydrogen bonding c a p a c i t y o f s o l v e n t s and are a t a maximum i n s o l v e n t s o f intermediate cohesive energy density (δ = 10 - 12) (35). Ammonia i s much c l o s e r t o t h i s optimum value than i s water. These f a c t o r s undoubtedly are predominant i n making ammonia a s u p e r i o r s o f t e n i n g agent f o r wood, a l l o w i n g creep during forming and permanent s e t i n the f i n a l product. E f f e c t o f Water. Wood i s u s u a l l y t r e a t e d w i t h ammonia i n the presence o f some amount o f water. The e f f e c t o f water depends not only on the amount o f water but a l s o somewhat on the h i s t o r y o f the wood sample and the method o f treatment. Thus, when oven-dried veneer s t r i p s were t r e a t e d w i t h c o l d l i q u i d ammonia-water mixtures a t ambient p r e s s u r e , the f l e x i b i l i t y o f the t r e a t e d wood was s u b s t a n t i a l l y decreased when the moisture content o f the ammonia was much above 10% (26). Other p r o t o n i c s o l v e n t s a c t s i m i l a r l y (26) . I n apparent c o n t r a s t , the r a t e of s o r p t i o n o f ammonia from the gas phase by wood i s markedly enhanced by moisture i n the wood (19). Bone dry wood absorbs ammonia q u i t e s l o w l y a t ambient temperatures but i f the wood has ten t o twenty percent moisture content, s o r p t i o n and p l a s t i c i z a t i o n occur much more r a p i d l y . Presumably the moisture opens the pore s t r u c t u r e o f the wood and a l s o d i s s o l v e s ammonia much more r e a d i l y than bone-dry wood. On continued treatment, the water i s presumably d i s p l a c e d from the wood by the ammonia s i n c e the x-ray d i f f r a c t i o n p a t t e r n o f the wood i s u s u a l l y C e l l u l o s e I I I , a m o d i f i c a t i o n which can not be formed unless most o f the water i s d i s p l a c e d i n t o the vapor phase (26). The reverse phenomenon, displacement o f ammonia by water has been proven by chemical a n a l y s i s as w e l l as by p h y s i c o - c h e m i c a l methods (36). ( F i g . 1 ) . Non-protonic s o l v e n t s can be used i n mixtures w i t h l i q u i d ammonia, a l l o w i n g r e l a x a t i o n , and i n some cases i n h i b i t i n g checking and shrinkage (Carbowax 400) ( 2 6 ) . S o r p t i o n , K i n e t i c s and Transport. I f wood i s immersed d i r e c t l y i n l i q u i d ammonia a t ambient pressure, c o n v e c t i o n o f the l i q u i d i s i n h i b i t e d by the presence o f a i r , and i f the wood i s a t ambient temperature, i t must be cooled to l e s s than -30° C before l i q u i d can flow i n t o the pore s t r u c t u r e . N e v e r t h e l e s s , under the b e s t o f circumstances much o f the wood substance must be reached by d i f f u s i o n r a t h e r than convection because a v a r i e t y of p h y s i c a l r e s t r i c t i o n s i n h i b i t l i q u i d flow i n wood (37,38). The k i n e t i c s and thermodynamics o f gaseous ammonia s o r p t i o n and d i f f u s i o n have been s t u d i e d i n d e t a i l (36,39,40). In g e n e r a l , two stages o f the gaseous ammonia p l a s t i c i z a t i o n process can be d i s t i n g u i s h e d . I n the i n i t i a l phase the hydrate envelope o f the wood substance i n t e r a c t s w i t h ammonia, causing the formation o f ammonia-complexes, NHi» 0H~, NH^OH e t c . These +

In Wood Technology: Chemical Aspects; Goldstein, I.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

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time in hours Journal of Polymer Science

Figure 1. Water vapor adsorption kinetics of ammoniatreated ramie cellulose ( Φ first exposure to water vapor after drying from ammonia; Ο repeated exposures there­ after to water vapor after drying from 6% moisture con­ tent) (36)

echwood 1st ^H

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Figure 2. Variation of the integral net heat of sorp­ tion (—Q ), change of free energy (—AG), and change of the integral entropy ( T A S ) with surface coverage (Θ) of beechwood (39) 8

In Wood Technology: Chemical Aspects; Goldstein, I.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

ASPECTS

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are a l s o s o r p t i v e l y a c t i v e and a c c e l e r a t e ammonia a d s o r p t i o n r e l a t i v e t o the s p e c i f i c s u r f a c e area o f the wood. Therefore, a t very low vapor p r e s s u r e s , the r a t e o f d i f f u s i o n o f ammonia vapor surpasses t h a t o f the water vapor by two orders o f magnitude, (Table I ) and a t h i g h e r vapor p r e s s u r e s , by a t l e a s t one order o f magnitude. I n the course o f the d i f f u s i o n N H 3 i s a l s o sorbed on a l l wood s u r f a c e s . During s o r p t i o n o f N H 3 by c e l l u l o s i c m a t e r i a l i n the low vapor pressure range, the isotherms show a steep r i s e which i n d i c a t e s t h a t s t r o n g bonding f o r c e s a c t between sorbate and adsorbent. The i n t e g r a l s o r p t i o n heats o f ammonia c a l c u l a t e d from the measured isotherms are the same, whether the adsorbent i s beech wood, b i r c h wood o r c e l l u l o s e . The c a l c u l a t e d i n t e g r a l n e t heat o f s o r p t i o n shows that the bond between sorbate and adsorbent must be s t r o n g e r than between the N H 3 molecules i n the l i q u i d phase ( F i g . 2 ) , and corresponds (about 3Kcal/mole) t o the formation o f hydrogen bonds between wood and ammonia. The c a l c u l a t i o n of the change of entropy as a f u n c t i o n o f the coverage o f the s o r p t i o n area, r e v e a l s t h a t there i s i n a d d i t i o n a t l e a s t a second type o f N H 3 uptake. I n the middle range o f vapor pressure ( p / p = 0.5 - 0.7) a l l c e l l u l o s i c adsorbents are c h a r a c t e r i z e d by a more i n t e n s e ammonia uptake, and i n t h i s r e s p e c t , the shape of the isotherm d i f f e r s from the type I I d e s c r i b e d by Brunauer (41) and d i s p l a y s a double S-form (39) ( F i g . 3 ) . The sorbate c o n c e n t r a t i o n i n the h i g h e r range o f vapor pressure i n c r e a s e s a s y m p t o t i c a l l y , i n d i c a t i n g c a p i l l a r y condensation w i t h f i b e r s a t u r a t i o n c a p a c i t i e s c o n s i d e r a b l y h i g h e r than i n water vapor. (Table I I ) . Therefore, i t i s c l e a r t h a t the ammonia sorbate l a y e r d i r e c t l y adhering to the wood i s bound chemosorptively, and t h a t the overl a y i n g sorbate l a y e r s are accumulated by p h y s i s o r p t i o n f o l l o w e d by c a p i l l a r y condensation. The isotherms d i s c u s s e d are n o t r e p r o d u c i b l e f o r subsequent complete ad- and d e s o r p t i o n t r e a t ments lower the ammonia s o r p t i o n c a p a c i t y o f the m a t e r i a l as i s shown i n Table I I . The values presented i n Table II i n d i c a t e t h a t w i t h ammonia the s p e c i f i c s u r f a c e area o f wood i s on the average, twice o r three times l a r g e r than w i t h water ( c f . r e f s . 42,43 e t a l . f o r data on water s o r p t i o n ) and t h a t the ammonia s o r p t i o n area o f the samples decreases from one s o r p t i o n c y c l e t o the next. Thus the substance o f the adsorbent apparently undergoes a process o f d e n s i f i c a t i o n w i t h each ammonia contact ( 4 4 ) . Along w i t h t h i s process o f d e n s i f i c a t i o n , there i s a l s o a decrease i n degree o f c r y s t a l l i n i t y . The extent of change i n c r y s t a l l i n i t y i s h i g h l y dependent on the s p e c i e s o f the specimen and on the p r e v i o u s NH3-treatment but u s u a l l y l i e s between 5 and 18% (45-47). The displacement o f ammonia by water i s o f p r a c t i c a l s i g n i f i c a n c e , s i n c e some d i f f i c u l t y has been noted i n removing ammonia from t h i c k work p i e c e s . C l e a r l y a d r y i n g c y c l e a f t e r treatment w i t h a reasonably h i g h humidity might be advantageous i n d i f f i c u l t cases. Q

In Wood Technology: Chemical Aspects; Goldstein, I.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

WOOD TECHNOLOGY:

334

Table I

CHEMICAL

D i f f u s i o n c o e f f i c i e n t s of ammonia vapor (D

XTTJ

ASPECTS

) and

of water vapor (D„ _) i n wood specimen w i t h equal 112 υ

dimensions (Yellow b i r c h ) .

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Vapor pressure

D._ «10

D_

T T

steps

H2O

2

(p/po)

. - .1 .1 - .2 .2 - .3 .3 - .4 .4 - .5 .5 - .6 .6 - .7 .7 - .8

-10

T

NH3

2

(cm /sec)

(cm /sec)

390 18 14 18 13

3.4 2.0 2.3 1.4

20 12

0.7

According t o G. N. C h r i s tens en (1960), quoted only f o r q u a l i t a t i v e comparison.

Table I I Some r e s u l t s o f the a n a l y s i s o f the NH3 s o r p t i o n isotherms on beechwood.

Specimen

Beechwood

Treatment

Treating Temp. °C

1. N H 3 - a d s .

20

Specific Surface Area m g 2

Fiber Saturation Point %

3

872 772

69

1. NH -des. 2. NH -ads. 2. NH -des.

780 659

58

3

3

3. NH -ads. 3

Compared t o approximately 35% i n water.

In Wood Technology: Chemical Aspects; Goldstein, I.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

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Processes on Supramolecular and M i c r o s c o p i c L e v e l s I f ammonia pressure i s s u f f i c i e n t l y h i g h , the hydrate envelope of the wood can be exchanged f o r ammonia. T h e r e a f t e r slower processes r e s u l t i n the p e n e t r a t i o n and l o o s e n i n g of wood substance during the f i r s t hour of ammonia c o n t a c t . As a r e s u l t the d e n s i t y of the wood substance decreases t e m p o r a r i l y by approximately 10% ( F i g . 4 ) , a t f u l l tank pressure, accompanied by an excessive s w e l l i n g of the c e l l w a l l s , and the mechanical p r o p e r t i e s o f the b u l k wood change s h a r p l y (48). I f then the ammonia i s removed from the s a t u r a t e d wood, the wood substance w i l l be d e n s i f i e d to an i n c r e a s i n g degree depending on the l e n g t h of treatment. The pore volume, e s p e c i a l l y that of pores w i t h r a d i i f a l l i n g i n t o the range from the c e l l luminae down to the t o r i , i s thereby reduced by more than o n e - t h i r d ( F i g . 5) by the f o l l o w i n g mechanism. Because o f the decrease i n c a p i l l a r y r a d i u s , the m e n i s c i of the receding N H 3 - f l u i d remain a c t i v e i n the c e l l w a l l c a p i l l a r y tubes a t s t i l l lower r e l a t i v e vapor pressures of N H 3 than at that of water. These m e n i s c i e x e r c i s e s t r o n g t r a n s v e r s e t e n s i l e s t r e s s e s on the c a p i l l a r y tube w a l l s . At the same time there i s an accumulation of d r y i n g s t r e s s e s , as i n the case of evaporating water. The v e c t o r s of these forces a r e mostly i n the r a d i a l d i r e c t i o n , and e a s i l y o v e r s t r e s s the p l a s t i c i z e d c e l l w a l l . Consequently, f i r s t the pores w i t h l a r g e r diameters, then the s m a l l e r ones can p a r t l y or e n t i r e l y c o l l a p s e (44). A f t e r the complete removal of the ammonia from the wood, the extent of the c o l l a p s e can be measured. This c o l l a p s e i s e n t i r e l y caused by the decrease i n v o i d volume, and i s made up to a l a r g e r extent by the p a r t i a l c l o s u r e of the c e l l luminae and to a s m a l l e r extent by the r e d u c t i o n of the pore volume of the c e l l w a l l s . At the molecular l e v e l , wood substance does not seem to remain loosened a f t e r the NH3-treatment except f o r an i n c r e a s e i n amorphous areas. I t i s c l e a r from the preceding d i s c u s s i o n that c o l l a p s e , d e n s i f i c a t i o n and i n c r e a s e d amorphous character can be minimized by s i n g l e gaseous treatments of s h o r t d u r a t i o n . The degree of r e l a x a t i o n i s l e s s than that under f u l l s w e l l i n g c o n d i t i o n s , but can be i n the range o f p r a c t i c a l working c o n d i t i o n s as has been demonstrated i n Z u r i c h and i s d i s c u s s e d later. I t should a l s o be remembered that the s t r u c t u r e of wood substance i s not homogeneous. There a r e p h y s i c a l d i s c o n t i n u i t i e s : w e l l ordered regions o f m i c r o f i b r i l s , f i b r i l l a r s u r f a c e s , v a r y i n g o r i e n t a t i o n s of f i b r i l s i n the l a y e r s o f the c e l l w a l l s , d i f f e r e n c e s i n l i g n i n and p e c t i n content between t r a c h e i d s , middle l a m e l l a e and parenchyma c e l l s and d i f f e r e n c e s i n d e n s i t y between s p r i n g wood and summer wood. At the present time i t i s not known i n d e t a i l a t what s i t e s ammonia s o r p t i o n begins o r the minimum pressure o f ammonia r e q u i r e d to induce a p a r t i c u l a r l e v e l o f s o f t e n i n g , (although

In Wood Technology: Chemical Aspects; Goldstein, I.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

C H E M I C A L ASPECTS

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WOOD TECHNOLOGY:

ι 168

24 •

specimen

6

8 10

2



6

β 10

2

h 2-10

Time of NH -treotment 3

Wood Science and Technology

Figure 4. Density (y ) of dry beechwood cell wall substance (initial value 1.52 g/cm ) after ammonia saturation for various periods of time (in hours), followed by vacuum drying (44) H

3

In Wood Technology: Chemical Aspects; Goldstein, I.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

2

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there have been some analyses of the p r e f e r e n t i a l uptake o f ammonia i n p a r t i c u l a r t i s s u e s . ) (49) I t i s q u i t e l i k e l y that the i n t e r - c r y s t a l l i n e uptake o f ammonia which occurs above 0.5 r e l a t i v e vapor pressure (50)» and the e x c e s s i v e s w e l l i n g c h a r a c t e r i s t i c o f the use of f u l l tank pressure are unnecessary for most forming o p e r a t i o n s . T r a n s l a t i o n between supermolecular s t r u c t u r a l u n i t s may occur under s t r e s s as has, f o r example, been observed f o r aqueous ammoniacal systems (16a). Therefore, i t may be p o s s i b l e a t i n t e r m e d i a t e ammonia pressures a l s o , to o b t a i n much g r e a t e r r e l a x a t i o n and creep than can be produced i n steam forming. At the present time, there has been too l i t t l e experimental forming done at i n t e r m e d i a t e ammonia p r e s s u r e s . Processes on Macroscopic L e v e l On ammonia treatment, the gross s w e l l i n g behavior and changes i n the s t r e n g t h and the s t r u c t u r e of wood are very s i m i l a r to those observed during and a f t e r steaming of wood w i t h water vapor. The changes d e s c r i b e d below r e f e r to wood s a t u r a t e d w i t h ammonia. S w e l l i n g and S h r i n k i n g : The r a t e of s w e l l i n g o f wood o f a l l species i n ammonia i s f a s t e r than i n water (43,51). This i s understandable s i n c e the r a t e of d i f f u s i o n o f ammonia surpasses that o f water. At e q u i l i b r i u m , almost a l l species show s u p e r s w e l l i n g i n t a n g e n t i a l d i r e c t i o n d u r i n g ammoniasoaking. There are, however, a few exceptions where supers w e l l i n g occurs i n the r a d i a l d i r e c t i o n (e.g. Douglas f i r (43), hard maple (51). The s u p e r s w e l l i n g i s a s s o c i a t e d w i t h loosened wood s t r u c t u r e , w i t h the s l i p regions o f the c e l l w a l l s . Upon removal of ammonia from f u l l vapor pressure, a l l s p e c i e s undergo an e x c e s s i v e shrinkage (44,45,52) mostly f o l l o w e d by c o l l a p s e of the c e l l s t r u c t u r e . Repeated water soaking of ammonia-treated m a t e r i a l enhances t h i s dimensional c o l l a p s e (44) (see F i g . 6 ) . The s w e l l i n g and shrinkage of ammonia-treated s p e c i e s i n water i s h i g h e r than those o f untreated m a t e r i a l , but water penetrates ammonia-treated m a t e r i a l more s l o w l y than untreated * (43,51). Under most circumstances, wood w i l l not be subjected t o repeated water treatment a f t e r forming and the i n f l u e n c e of moisture vapor i s much l e s s severe. I f wood samples are subjected to ammonia treatment, then vacuum d r i e d and f i n a l l y subjected t o atmospheres of 50 to 98% r e l a t i v e humidity, the t r e a t e d wood and untreated c o n t r o l s both absorb an excess o f moisture and then on f u r t h e r standing i n the same atmosphere l o s e moisture (50). The i n i t i a l "overshoot" i s s l i g h t l y h i g h e r w i t h ammonia t r e a t e d samples but the f i n a l e q u i l i b r i u m values appear somewhat lower w i t h the t r e a t e d samples (53). S w e l l i n g measurements show s i m i l a r t r e n d s . S i m i l a r phenomena have been observed w i t h other polymer s o l v e n t w o o c

In Wood Technology: Chemical Aspects; Goldstein, I.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

WOOD TECHNOLOGY:

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338

C H E M I C A L ASPECTS

Pore-radius R Wood Science and Technology

Figure 5. Volumes of pores with radii ranging from 7.5-7,500 nm in correlation with the entire void volume of 1 g of beechwood before and after treatments of 1, 2, and 8 hr, followed by vacuum drying (Compare Fig. 4) (44)

^

_ tangential_

15

4

— w

Figure 6. Linear shrinkage and swelling measurements of beechwood after alternating treatments with ammonia and water. Arrow f symbolizes swelling: a in ammonia, w after two days of watering. Arrow I symbolizes shrinkage. Reference values are dimensions in air-dry state.

5»-

*...

-9T

1-10

time of NH - treatment

[hrs]

In Wood Technology: Chemical Aspects; Goldstein, I.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

21.

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339

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systems and can be r e l a t e d to r e c r y s t a l l i z a t i o n phenomena (53) . For p r a c t i c a l purposes, the s e n s i t i v i t y o f ammonia t r e a t e d wood to moisture vapor can probably be considered about the same as untreated wood. S p e c i f i c G r a v i t y : The s p e c i f i c g r a v i t y o f wood can be r a i s e d about 10-40% as a consequence o f a s i n g l e ammonia t r e a t ment over an extended p e r i o d (44,51). Repeated treatment enhances the d e n s i f i c a t i o n . Color: The degree of c o l o r change i n wood i s determined by the time and temperature of treatment. Often the c o l o r change i s advantageous. One method of c o n t r o l has been described (54) and apparently others are i n use i n the S o v i e t Union. Mechanical P r o p e r t i e s : Bending s t r e n g t h i s i n c r e a s e d a f t e r ammonia treatment and d r y i n g , but the amount i s dependent upon the species (51) (3% f o r beech compared to 30% f o r p o p l a r ) (_5) . The modulus of e l a s t i c i t y i s lowered t o about 1/5 - 1/10 o f the i n i t i a l v a l u e (36), but a f t e r treatment i t regains approximately i t s o r i g i n a l v a l u e (45,51). Compression and t e n s i l e s t r e n g t h s are enhanced a f t e r treatment and d r y i n g , by about 10-40%, again depending on the s p e c i e s (51). Much o f t h i s i n c r e a s e r e f l e c t s the i n c r e a s e d amount of m a t e r i a l p e r u n i t cross s e c t i o n ( 5 1 ) . The toughness of wood i s g r e a t l y reduced by 30-40% as a r e s u l t of treatment (51). A l l mechanical p r o p e r t i e s are reported to be improved by treatment w i t h aqueous ammonia, but i t i s questionable that t h i s i s true o f toughness. The o v e r a l l e f f e c t of ammonia on wood r e s u l t s from processes, some o f which lower and others enhance the s t r e n g t h p r o p e r t i e s of wood. Among the former are the f o l l o w i n g : A decrease i n the degree of c r y s t a l l i n i t y which causes the i n t r o d u c t i o n of weaker secondary bonds i n t o the substance. D i s l o c a t i o n of components i n the c e l l w a l l making the m a t e r i a l more l o o s e . Crimpings and m i c r o f a i l u r e s i n the c e l l w a l l which e l i m i n a t e primary and secondary bonds. Among the l a t t e r are: The i n c r e a s e of s p e c i f i c g r a v i t y by the c o l l a p s e of c e l l s and c e l l w a l l s , g i v i n g more substance per u n i t volume to w i t h s t a n d f o r c e s ; e l i m i n a t i o n of l o c a l s t r e s s maxima i n the t i s s u e by r e l a x a t i o n or creep. Reduced a n i s o t r o p y of the p r o p e r t i e s i n the t a n g e n t i a l and r a d i a l d i r e c t i o n s (44) as the substance becomes denser and by c o l l a p s e t a n g e n t i a l and r a d i a l c e l l w a l l p a r t s are mixed. Often the most prominent e f f e c t i s the i n c r e a s e i n s p e c i f i c g r a v i t y . I t should be emphasized that maximum e f f e c t s are observed on extended treatment times at h i g h r e l a t i v e vapor pressures. R h e o l o g i c a l P r o p e r t i e s . R h e o l o g i c a l s t u d i e s show that ammonia a f f e c t s the compressive mechanical behavior of wood to a much h i g h e r extent than the t e n s i l e behavior (55,16). Since the compression s t r e n g t h i s h i g h l y dependent on l i g n i n content,

In Wood Technology: Chemical Aspects; Goldstein, I.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

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t h i s e f f e c t probably suggests l i g n i n p l a s t i c i z a t i o n e a r l y i n the process. The question whether ammonia t r e a t e d wood shows l i n e a r o r n o n - l i n e a r v i s c o e l a s t i c behavior has not been answered so f a r . The measurements reported by Bach (56) apparently were not under constant temperature c o n d i t i o n s . S t r a i n recovery a f t e r l o a d i n g i n the p l a s t i c i z e d s t a t e i s s m a l l . The longer the l o a d i n g p e r i o d the s m a l l e r the recoverable s t r a i n . This suggests p l a s t i c f l o w under l o a d and a conversion o f delayed e l a s t i c s t r a i n i n t o an i r r e v e r s i b l e deformation.

P r a c t i c a l and P o t e n t i a l Uses o f Ammonia i n Wood Technology The use of ammonia f o r s o f t e n i n g wood has n o t been l i m i t e d to bending, compressing (56) and forming whole wood. For example, two d i f f e r e n t a p p l i c a t i o n s i n p u l p i n g have been reported (57,58)» I n one p r o p o s a l , l i g n o c e l l u l o s i c m a t e r i a l i s d e f i b e r e d by an e x p l o s i v e decompression o f wood chips which have been p l a s t i c i z e d w i t h ammonia a t e l e v a t e d temperatures and pressures. I n a second i n v e s t i g a t i o n wood chips were p l a s t i c i z ed w i t h ammonia i n an Asplund p r e s s u r i z e d r e f i n e r , during d e f i b r a t i o n . Low energy requirements appear t o be c h a r a c t e r i s t i c o f the l a t t e r process. Pulp q u a l i t y and p o t e n t i a l a p p l i c a t i o n s were described i n both cases. There are a l s o r e p o r t s o f the production o f boards and moldings from p l a s t i c i z e d wood p a r t i c l e s , pulp o r sawdust without adhesives (5,8,59,60). By compressing and h e a t i n g t o h i g h temperatures, p a r t i c l e boards can be produced which have mechanical p r o p e r t i e s comparable t o conventional r e s i n bonded boards. However, t h e i r s p e c i f i c g r a v i t y i s on the average about twice t h a t of o r d i n a r y commercial products ( 5 9 ) . The use o f aqueous ammonia i n the p r e p a r a t i o n o f wood f i b e r f i l l e d p h e n o l i c p l a s t i c molding has been i n v e s t i g a t e d . I n t h i s case ammonia both p l a s t i c i z e s the wood f i l l e r and c a t a l y z e s the p h e n o l i c methylol condensation. With proper formulations and treatments, i t i s p o s s i b l e t o maximize the q u a n t i t y o f wood f i b e r t h a t can be used and minimize the r e s i n without d e t e r i o r a t i o n o f p r o p e r t i e s o f the molded product (61). P r e l i m i n a r y i n v e s t i g a t i o n s suggest t h a t wood s l i c i n g w i t h knives can be a p p l i e d s u c c e s s f u l l y to ammonia p l a s t i c i z e d wood and t h a t t h i c k e r boards and veneers can be cut than by conventional methods. Savings o f m a t e r i a l and energy are envisioned over the use o f sawing ( 6 2 ) . Some wood species s u f f e r severe checking on k i l n d r y i n g due to i n t e r n a l s t r e s s e s . I t has been suggested that the use of ammonia gas i n the k i l n might l e a d t o s t r e s s r e l a x a t i o n and lower l o s s e s during d r y i n g . I t i s , however, c e r t a i n that the discovery of appropriate c o n d i t i o n s would r e q u i r e a systematic i n v e s t i g a t i o n f o r i n the d r y i n g of l a r g e dimensional stock f u l l y p l a s t i c i z e d w i t h ammonia, checking i s more severe under

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conventional d r y i n g c o n d i t i o n s than w i t h water-wet green wood. I n v e s t i g a t i o n s i n the low p a r t i a l pressure range would be indicated. The most e x t e n s i v e research that has been undertaken on wood s o f t e n i n g and forming i s t h a t a t the I n s t i t u t e of Wood Chemistry i n R i g a , L a t v i a n S.S.R.. T h e i r s t u d i e s have been p r i m a r i l y upon the i n f l u e n c e of aqueous ammonia on wood and have i n c l u d e d fundamental s c i e n t i f i c and engineering r e s e a r c h , and process and product development. Some o f t h e i r p u b l i c a t i o n s a l s o d i s c u s s i n v e s t i g a t i o n s of gaseous treatments. Although some of t h e i r technology i s d i r e c t e d toward bending and forming o p e r a t i o n s , the main t h r u s t of the research appears to be d i r e c t e d toward the improvement o f p h y s i c a l p r o p e r t i e s of woods by compression. T h e i r technology depends upon a more d e t a i l e d study of the rheology o f wood under v a r i e d c o n d i t i o n s o f temperature and ammonia c o n c e n t r a t i o n than has been attempted i n the West (2-18). Probably t h e i r forming o p e r a t i o n s , e s p e c i a l l y that of compression, do not r e q u i r e as complete r e l a x a t i o n as i s p o s s i b l e w i t h pure ammonia and they may use longer forming times than has been customary i n Western e x p e r i mentation. Long forming times are i n d i c a t e d i n t h e i r American patent (18), and these would a l l o w , presumably, more creep than would be expected from some r e s u l t s reported i n the West on the i n f l u e n c e of water i n the forming process. They have prepared some compressed wood samples which show, i n a d d i t i o n t o i n c r e a s e d d e n s i t y and s u r f a c e hardness, a lower moisture r e g a i n than untreated wood up t o 80% r e l a t i v e humidity. Using m a t e r i a l s such as these, they have experimented w i t h the manufacture of parquet f l o o r i n g , a r a t h e r severe t e s t o f dimensional s t a b i l i t y . A v a r i e t y o f other f i n i s h e d products have been prepared, some on p i l o t p l a n t s c a l e . However, i t i s not known t o what extent they have appeared as products i n the open market. T h e i r technology i s a v a i l a b l e through l i c e n s u r e . The o r i g i n a l suggestion that ammonia could be used to produce extreme f l e x i b i l i t y and permanent s e t i n wood was based on observations of the e f f e c t of l i q u i d ammonia a t low temperature and atmospheric pressure on w e l l d r i e d wood samples (1,63). These c o n d i t i o n s produce maximum s w e l l i n g and r e l a x a t i o n , and f i b r i l l a t i o n and d i s c o l o r a t i o n tend to be l e s s than w i t h e q u a l l y lengthy treatments at h i g h e r temperatures and p r e s s u r e s . However, s a f e t y hazards are severe and the method i s extremely w a s t e f u l o f chemical, s i n c e the l i q u i d i n the luminae does n o t c o n t r i b u t e to the s o f t e n i n g process. Most work s i n c e t h a t time has been c a r r i e d out i n pressure v e s s e l s on wood w i t h moisture content about t e n percent. This technique has been e x p l o r e d at a number of centers of i n d u s t r i a l a r t s and by s e v e r a l a r t i s t s , some of whose work has appeared i n p u b l i c e x h i b i t i o n s . Greater use o f the process i n a r t s and c r a f t s i s c l e a r l y i n d i c a t e d , f o r no other technique allows the formation of such extreme shapes or f l u i d l i n e s i n wood so e a s i l y and without l o s s o f s t r e n g t h .

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In the many examples produced t o date, dimensional s t a b i l i t y has not been a problem. A recent process development from Z u r i c h by M. B a r i s k a (20) has a number of important and i n t e r e s t i n g f e a t u r e s . The method i s s p e c i f i c a l l y designed to t e s t the a p p l i c a b i l i t y o f ammonia forming to commercial p r a c t i c e . Secondly i t c o n s t i t u t e s a lower extremum, an i n v e s t i g a t i o n of c o n d i t i o n s of minimum u s e f u l p l a s t i c i t y . Furthermore i t i s based on extensive i n v e s t i g a t i o n s of k i n e t i c s , thermodynamics, s o r p t i o n processes, and s t r u c t u r a l changes c h a r a c t e r i s t i c s of the ammonia wood system. Some of the p e r t i n e n t concepts u n d e r l y i n g the method are the f o l l o w i n g : Much of the s t i f f n e s s of wood i s c o n t r i b u t e d by l i g n i n , and a high c o n c e n t r a t i o n o f l i g n i n e x i s t s i n the middle l a m e l l a . Ammonia has a high thermodynamic a c t i v i t y at room temperature and an extremely high r a t e o f d i f f u s i o n . I t , t h e r e f o r e , appeared l i k e l y that a r a p i d b r i e f impregnation o f wood w i t h ammonia might cause s u f f i c i e n t p l a s t i c i z a t i o n of the middle l a m e l l a to permit wood forming. This conjecture was t e s t e d as f o l l o w s : Specimens of a i r - d r i e d European beech ( c a 12% moisture content, 1/2 i n . χ 1 i n . χ 36 i n . ) were p l a s t i c i z e d i n s a t u r a t e d steam and i n ammonia vapor a t room temperature (ca 23° C). The treatment time f o r the steam treatment c o n t r o l s was that t y p i c a l o f i n d u s t r i a l p r a c t i c e , about 1 minute per m i l l i m e t e r of t h i c k n e s s . The treatment p e r i o d f o r ammonia s o f t e n i n g v a r i e d from the same time down t o the s h o r t e s t p o s s i b l e blow, which c o n s i s t e d of f i l l i n g a three g a l l o n t r e a t ­ ment tank w i t h ammonia gas a t tank pressure and then decompress­ i n g , a t o t a l p e r i o d o f about 30 seconds. A f t e r s o f t e n i n g , the i n d i v i d u a l work pieces were bent to a form w i t h v a r y i n g r a d i i of curvature (30, 15, 7.5, 3.8, and 1.9 cm) and kept i n a mold f o r t h i r t y minutes. Regardless of treatment c o n d i t i o n s , compression f a i l u r e s occurred i n the range of l e s s than 7.5 cm., radius of curvature. Spring back o f the t r e a t e d and rehardened pieces that were bent to a radius of 30 cm. was measured a t d i f f e r e n t times ( F i g . 7 ) . A s t a t i s t i c a l a n a l y s i s of the data showed no d i f f e r e n c e between the two s o f t e n i n g methods at the 95% confidence l i m i t . Color change due t o ammonia treatment was i n d i s t i n g u i s h a b l e from that of steam p l a s t i c i z a t i o n to the unaided eye. Judged by these r e s u l t s , r a p i d low temperature ammonia s o f t e n i n g o f wood i s at l e a s t as e f f e c t i v e as conventional hot steam p l a s t i c i z a t i o n and the e f f e c t s of the two methods are e s s e n t i a l l y i n d i s t i n g u i s h a b l e . Ammonia s o f t e n i n g consumes l e s s energy on the s i t e and does not r e q u i r e s e t t i n g time. However p r o t e c t i o n a g a i n s t c o r r o s i o n and against i r r i t a t i o n of working personnel i s r e q u i r e d . Under some l o c a l c o n d i t i o n s and f o r s p e c i f i c a p p l i c a t i o n s t h i s method may be p r e f e r a b l e to conventional p r a c t i c e .

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-I 'If""

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Figure 7. Spring-back of treated and rehardened pieces of beechwood after treating with ammonia (I) or steam (2), bending to a 30-cm radius, and setting for various times

In Wood Technology: Chemical Aspects; Goldstein, I.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

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It is of interest that the minimum practical relaxation of wood by ammonia approximates the maximum obtainable with steam. Specific effects and applications which cannot be achieved by means of steam forming may, therefore, be obtained under the wide range of conditions possible in the wood-water-ammonia system. Abstract During sorption, softening and forming processes in wood­ -water-ammonia systems, changes occur on molecular to macroscopic levels. They are time-dependent and history-dependent functions of temperature, pressure, and system composition, and require definition in terms ranging from molecular interactions to engineering properties. The influence of ammonia is comparable to but more extreme than that of water. At room temperature its effect is comparable to the effect of water alone at 200° C. In general the rate of diffusion of ammonia in wood is much faster than water; bound water is displaced by ammonia since ammonia sorption is more powerful; the fiber saturation capacity of the wood is much higher for ammonia; the wood can be more highly swollen. Ammonia-treated wood is less elastic and can undergo plastic flow and creep. The wood tends after drying to be more dense but less dimensionally stable. The processing characteristics and final properties of ammonia­ -treated wood cover a wide range depending on treatment conditions. At various stages of development are rapid ammonia treatments for wood forming, with results comparable in effect to steam bending (Zurich), modification of wood properties by compression (Riga), defiberization and pulping, wood slicing, adhesion of particles in molding, and applications in arts and crafts. Literature Cited (1) Schuerch, C.(a) Ind. Eng. Chem., (1963), 55, 39. (b) Forest Prod. J., (1964), 14, 377-381. (2) Kalnins, A. J. and Darzins, T. A, Latv. Lauksaimn. Akad. Raksti, XI, (1962))421-2. (3) "Modification of Wood" [Modifikatsiya Drevesiny], Academy of Science Latvian SSR, Institute of Wood Chemistry, "Zinatne," Riga (1967) (Russian). This book contains thirty one articles by the Institute's staff. Abstracted by article in A.B.I.P.C. (Jan. 1970), 40 #7, 563 ff. (4) Kalnins, A. J., Darzins, T. A., Jukna, A. D., Berzins, G. V., Holztechnologie, (1967), 8, 23. (German) (5) Kalnins, A. J., Cellulose Chemistry and Technology, (1969), 3, 199· (Russian) (6) Kalnins, A. J., Berzins, G., Skrupsis, W. and Rumba, A., Holztechnologie, (1969), 10, 17. (German) (7) Berzins, V. G. and Doronin, J. G., Holztechnologie, (1970),

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11, 125. (German) (8) Vejina, L., Latv. P.S.R., Zinat. Akad. Vestis.(1970), 2, 52-54. (Russian) (9) Berzins, G., Latv. P.S.R., Zinat. Akad. Vestis., (1970), 10, 130. (Russian) (10) Berzins, G., Latv. P.S.R., Zinat. Akad. Vestis., (1970), 2, 61-69. (Russian) Doronin, Y. G., Latv.P.S.R., Zinat. Akad. Vestis., (1970), 2, 55-60. (Russian) (11) Onisko, W., Matejak, Μ., Silwan, (1971), 115, (2), 39-50. (Polish) (12) Erins, D., Karklins, J., Odincous, P., Veveris, G., Khimya Drevesiny, (1971), 7, 159-69. (Russian) (13) Erinsh, P. P., Odintsov, P. Ν., Alksne, I. Μ., Khimya Drevesiny, (1971), 9, 19-28. (Russian) (14) Erinsh, P. P., Cinite, V. Α., Khimy Drevesiny, (1971), 9, 29-38. (Russian) (15) Lielpeteris, U. U., Ziedinsh, I. O., Khimy Drevesiny, (1971), 9, 167-171. (Russian) (16) Rocens, Κ., Holztechnologie, (1976), 17, 40-45. (German) (a) Work of Erins, P. P., and Odinkov, P. Ν., herein quoted. (17) For other contributions of the Institute of Wood Chemistry Riga, see Chemical Abstracts (1971), 76, 101409q, 101410h; (1972), 77, 76925r, 166446q, 166442k; 78, 5605w, 5606x, 5607y, 17847q. (18) Various patents issued on the basis of work at the Institute of Wood Chemistry, include the following to Berzins, G. V. et al. Ger. Offen 2,020,810 (C1. B27k) 07 Sept 1972. U.S. Patent 3,646,687 Mar. 7 (1972). U.S.S.R. Patents No. 208,923; 316,309; 299,364; 3,646,687. (19) Davidson, R. W., "Plasticizing Wood with Anhydrous Ammonia," Technical Bulletin, Dept. Wood Products Engineering, SUNY College of Environmental Science and Forestry, Syracuse, NY 13210. (20) Bariska, Μ., herein reported, to be published in entirety elsewhere. (21) Sadoh, T., Yamaguchi, Ε., Bull. Kyoto Univ. Forests, (1968), 40, 276-283. (Japanese) (22) Sadoh, T., Journ. Japan Wood Research Soc., (1969), 15 (1), 29-34. (Japanese) (23) Sadoh, T., Journ. Japan Wood Research Soc., (1970), 16 (7), 334-338. (Japanese) (24) Sadoh, T., Journ. Japan Wood Research Soc., (1968), 14 (3), 175. (Japanese) (25) Beall, F. C., "Wood Forming Method" U.S. Pat. 3,717,187 (Feb. 20, 1973). (26) Schuerch, C., Burdick, M. P., Mahdalik, Μ., I. and E.C. Product Research and Development, (1966), 5, 101-105.

In Wood Technology: Chemical Aspects; Goldstein, I.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

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(27) Peck, E. C., "Bending Solid Wood to Form" Agriculture Handbook No. 125, U.S. Dept. of Agriculture Forest Service (1968). (28) Takemura, T., Memoirs College of Agriculture, Kyoto Univ., (1966), 88, 31-48. (English) (29) Stevens, W. C. and Turner, Ν., Technical Brochure, "Experiments with Flexible Wood." Forest Products Research Laboratory, Princess Risborough, April 1966. (30) Back, E. L., Didriksson, Ε. I. Ε., Johanson, F., Norberg, K. G., Forest Products Journal (1971), 21 #9, 96-100. (31) Alfrey, T., Angew. Chem. Internat. Ed., (1974), 13 (2), 105-107. (32) Goring, D. A. I., Pulp and Paper Mag. of Canada, (1963), 12, T-518-527. (33) Urquhart, A. R., Eckevall, N., J. Text. Inst., (1930), 21, 499-510. (34) Nayer, Α. Ν., Hossfeld, R. L., J. Amer. Chem. Soc., (1949), 71, 2852-2855. (35) Schuerch, C., J. Amer. Chem. Soc., (1962), 74, 5061-5067. (36) Bariska, M. and Popper, R., J. Polymer Sci., (1971), C 36, 199-212. (37) Schuerch, C., I. and E. C. Product Research and Development, (1965), 4, 61-66. (38) Schuerch, C., Forest Products Journal, (1968), 18, 47-53. (39) Bariska, M. and Popper, R., Wood Science and Technology (1975), 9, 153-163. (40) Bariska, M., Habilitationsschrift: "Physikalische and Physikalisch - Chemische Aenderungen im Holz waehrend und nach NH3-Behandlung." Eidgenossische Technische Hochschule, Zurich, October 1974 p. 93 ff. (German) (41) Brunauer, S., "The Adsorption of Gases and Vapors, (1945), Vol. 1, University Press, Princeton, N.J. (42) Spalt, H.A.,Forest Products Journal, (1958), 8, 288-295. (43) Stamm, A. J.,"Wood and Cellulose Science,"The Ronald Press Co., New York, 1964. (44) Bariska, Μ., Wood Science and Technology, (1975), 9, 293-306. (45) Pentoney, R. Ε., I. and E. C. Product Research and Develop­ ment, (1966), 5, 105-110. (46) Fukada, Ε., Wood Science and Technology, (1968), 2, 299-307. (47) Lewin, Μ., Roland, L. G., J. Polymer Sci., (1971), C 36, 213-229. (48) Bariska, M., Strasser, Ch., J. Polymer Sci., (1976), 41, in press. (49) Bariska, Μ., Bulletin, International Association of Wood Anatomists (1969) No. 2, 3-8. (50) Bariska, M., Skaar, C., Davidson, R. W., Wood Science (1969), 2 (2) 65-72.

In Wood Technology: Chemical Aspects; Goldstein, I.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

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(51) Pollisco, F. S., Dissertation, SUNY College of Environ­ mental Science and Forestry, Syracuse 1968. (52) Schuerch, C., U.S. Patent 3,282,313, (Nov. 1, 1966). (53) Pollisco, F. S., Skaar, C., Davidson, R. W., Wood Science, (1971), 4 (2) 65-70. (54) Davidson, R. W., Schuerch, C., J. Polymer Sci. (1971), C, 36, 231-239; U. S. Patent 3,642,042 (Feb. 15, 1972). (55) Bach, L. Materials Science and Engineering, (1974), 15, 211-220. (56) Bach, L., Hastrup, Κ., Materiaux et Constructions, (1973), 6, (32), 137-139. (57) O'Connor, J. J . , Tappi, (1972), 55 (3) 353-358. (58) Peterson, R. C., Strauss, R. W., J. Polymer Sci., (1971), C 36, 241-250. (59) Graf, G., et al., Holztechologie, (1971), 12, 235-238; (1972), 13, 152-155. (60) Shaines, Α., U.S. Pat. 3,514,353 (May 26, 1970). (61) Jukna, A. D., Inst. Wood Chem., Riga, personal communica­ tion. (62) Davidson, R. W., Baumgardt, W. G., Forest Products Journal (1970), 20 (3) 19-24. (63) The first observation of the plasticization of wood by ammonia was by Stamm, A. J . , Forest Products Journal, (1955), 413-416.

In Wood Technology: Chemical Aspects; Goldstein, I.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.