Adhesives from Renewable Resources - ACS Publications - American

Chapter 12 Condensed Tannins in Adhesives Introduction and Historical Perspectives

Downloaded by UNIV OF IOWA on May 22, 2016 | http://pubs.acs.org Publication Date: December 31, 1989 | doi: 10.1021/bk-1989-0385.ch012

Herbert L . Hergert Repap Technologies Inc. P . O . Box 766 2650 Eisenhower Avenue Valley Forge, P A 19482

Sequential extraction of the bark of most species of conifers and deciduous trees, using water or polar organic solvents followed by aqueous alkali, yields two fractions of polymeric polyphenols, respectively referred to as condensed tannins and phenolic acids. These materials react with formaldehyde or phenol-formaldehyde prepolymers to make suitable resins for cold-setting waterproof adhesives for wood lamination or thermosets for exterior-grade plywood. Commercial production of bark extracts from western hemlock, Douglas-fir, and redwood in North America found limited application for adhesives during the period 1955-1975. Wattle (Acacia mearnsii) bark tannins, produced in South Africa, are currently used in adhesive formulations. The availability of large quantities of pine bark residues from pulping operations in the Americas, Australia, and New Zealand located near waferboard, plywood, and wood-laminating producers who consume substantial amounts of phenolic adhesives suggests that the time is ripe for production of bark-based adhesives. Lower cost, higher yield isolation techniques, in particular, may still be needed for this objective to become a commercial reality. T h e conversion o f a n i m a l hides i n t o leather b y t r e a t m e n t w i t h water-soluble p l a n t e x t r a c t i v e s has been p r a c t i c e d since a n t i q u i t y . T h i s process became k n o w n as t a n n i n g a n d o b v i o u s l y i n v o l v e d t h e r e a c t i o n o f a n a t u r a l l y o c c u r r i n g e x t r a c tive, t a n n i n , w i t h t h e p r o t e i n i n t h e h i d e . W e n o w k n o w , o f course, t h a t t a n n i n s comprise a whole s p e c t r u m o f c h e m i c a l c o m p o u n d s , b u t generally t h e y are p o l y p h e n o l i c a n d p o l y m e r i c . T a n n i n s have been isolated f r o m a w i d e v a r i e t y of r a w m a t e r i a l s , i n c l u d i n g insect galls, f r u i t s k i n s , seed h u l l s , leaves, b a r k , a n d h e a r t w o o d . Indeed, t a n n i n s are o f nearly u b i q u i t o u s occurrence i n higher orders 0097-6156/89/0385-0155$06.00/0 « 1989 American Chemical Society

Hemingway et al.; Adhesives from Renewable Resources ACS Symposium Series; American Chemical Society: Washington, DC, 1989.


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of the p l a n t k i n g d o m . Since t h e y occur i n highest concentrations i n those tissues of a p l a n t exposed t o a i r , i t is generally agreed t h a t t h e i r f u n c t i o n i n a p l a n t is t o h e l p i t resist t h e i n v a s i o n o f pathogens. D u r i n g t h e e a r l y a n d m i d d l e stages o f t h e i n d u s t r i a l r e v o l u t i o n , r a p i d l y i n creasing q u a n t i t i e s o f leather were r e q u i r e d for factories a n d t r a n s p o r t a t i o n as w e l l as for f o o d p r o d u c t i o n t o s u p p l y t h e e x p l o d i n g p o p u l a t i o n s s p a w n e d b y t h i s u p h e a v a l . B y t h e b e g i n n i n g o f t h e t w e n t i e t h century, leather t a n n i n g was one o f t h e m a j o r i n d u s t r i e s i n N o r t h A m e r i c a . Since t h e i n d u s t r y r e q u i r e d r e l a t i v e l y c o n c e n t r a t e d sources o f t a n n i n s , m u c h t e c h n i c a l a t t e n t i o n w a s devoted t o analyses o f p l a n t m a t e r i a l s for t a n n i n content. O n l y t h e b a r k a n d heart w o o d of a l i m i t e d n u m b e r o f w o o d species were f o u n d t o c o n t a i n sufficient q u a n t i t i e s of w a t e r - s o l u b l e t a n n i n s t o m a k e t h e m a t t r a c t i v e for c o m m e r c i a l e x t r a c t i o n . I n the U n i t e d S t a t e s , t h e b a r k o f eastern h e m l o c k a n d some o a k species w a s t h e m a i n source o f t a n n i n s a t t h e t u r n o f t h e c e n t u r y (1). Subsequently, chestnut w o o d b e c a m e t h e m a i n source o f t a n n i n u n t i l i t l a r g e l y d i s a p p e a r e d because o f chestnut b l i g h t . I m p o r t e d e x t r a c t s f r o m quebracho a n d , t o a lesser extent f r o m m i m o s a (Acacia Sp.), g r a d u a l l y d i s p l a c e d domestic t a n n i n p r o d u c t i o n . T h e r a p i d g r o w t h o f the p u l p a n d p a p e r i n d u s t r y f o l l o w i n g W o r l d W a r I I c o u p l e d w i t h a renewed scientific interest i n u t i l i z a t i o n o f b a r k a n d w o o d residues l e d t o investigative p r o g r a m s o n b a r k a n d w o o d t a n n i n s . T h e leather i n d u s t r y was c o n t i n u i n g t o decline i n i m p o r t a n c e , so other a l t e r n a t i v e s were needed. O n e of these w a s replacement o f p h e n o l i n whole or i n p a r t i n p h e n o l - f o r m a l d e h y d e adhesive f o r m u l a t i o n s . T h i s w o r k progressed t o t h e p o i n t where c o m m e r c i a l q u a n t i t i e s o f p o l y p h e n o l i c e x t r a c t i v e s were m a d e a n d s o l d for adhesive a p p l i c a t i o n . E x c e s s i v e c a p a c i t y a n d l o w p e t r o c h e m i c a l l y derived p h e n o l prices i n t h e 1960's l e d t o t h e demise o f t h i s effort i n t h e U n i t e d States (2,3). M o r e recently, there has been a renewed r e c o g n i t i o n o f the p o t e n t i a l o f b a r k d e r i v e d p o l y p h e n o l s for adhesives as a result o f i m p r o v e d u n d e r s t a n d i n g o f t h e c h e m i c a l s t r u c t u r e o f these m a t e r i a l s (4,5), n e w types o f f o r m u l a t i o n s (6), a n d the fact t h a t t a n n i n s are b e i n g c o m m e r c i a l l y used i n adhesives i n S o u t h A f r i c a ( 7 ) , t h u s s e r v i n g as a p r o t o t y p e for u t i l i z a t i o n i n other p a r t s o f t h e w o r l d . I n order t o p r o p e r l y assess t h e current developments i n t h i s field, t h i s overview w i l l p r o v i d e a h i s t o r i c a l perspective o n adhesives based o n t a n n i n s as w e l l as a s u m m a r y o f t h e e x t r a c t i o n techniques a n d c h e m i c a l s t r u c t u r e s . F i n a l l y , areas where a d d i t i o n a l w o r k c o u l d be f r u i t f u l w i l l be suggested. Raw

Materials

T a n n i n s generally c a n be classified i n t o t w o b r o a d categories: h y d r o l y z a b l e a n d condensed. T h e h y d r o l y z a b l e t a n n i n s consist o f m a n y i n d i v i d u a l c o m p o u n d s a n d oligomers,' b u t a l l are based o n c o m b i n a t i o n s o f g a l l i c a c i d or i t s derivatives a n d s i m p l e sugars such as glucose o r r h a m n o s e . H y d r o l y s i s w i t h d i l u t e m i n e r a l a c i d or enzymes results i n d e g r a d a t i o n t o g a l l i c o r ellagic a c i d ( a d i m e r f o r m e d f r o m g a l l i c acid) a n d t h e sugar c o m p o n e n t , hence t h e n a m e , h y d r o l y z a b l e t a n n i n s .


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For e x a m p l e , c o r a l a g i n (1), a c o m p o n e n t of e u c a l y p t u s t a n n i n s , y i e l d s g a l l i c a c i d ( 2 ) , ellagic a c i d (3) a n d glucose w h e n h y d r o l y z e d i n d i l u t e s u l f u r i c a c i d ( F i g u r e 1). M o s t of the c h a r a c t e r i z a t i o n studies have been done o n those t a n n i n s i s o l a t e d f r o m f r u i t s , leaves, or g a l l s . V e r y l i t t l e is k n o w n a b o u t the h y d r o l y z a b l e t a n n i n s f r o m the w o o d a n d b a r k of h a r d w o o d s except for e u c a l y p t u s , o a k , a n d m a p l e (4 S). A l t h o u g h the studies have not been extensive, i t seems q u i t e l i k e l y t h a t h y d r o l y z a b l e t a n n i n s do not occur i n conifers, b u t t h e y often co-occur w i t h condensed t a n n i n s i n h a r d w o o d s . T h e r e are n o l i t e r a t u r e references t h a t suggest u t i l i t y for h y d r o l y z a b l e t a n n i n s i n adhesive f o r m u l a t i o n s , so n o f u r t h e r m e n t i o n w i l l be m a d e of t h e m i n t h i s overview. Downloaded by UNIV OF IOWA on May 22, 2016 | http://pubs.acs.org Publication Date: December 31, 1989 | doi: 10.1021/bk-1989-0385.ch012

f

C o n d e n s e d t a n n i n s , o n the other h a n d , occur i n the b a r k of a l l conifers a n d h a r d w o o d s e x a m i n e d t o date, a n d they are frequently present i n the w o o d . T h e y are p r i m a r i l y responsible for the t a n t o b r o w n color of w o o d after i t is exposed t o a i r . I n t h e i r purest f o r m , condensed t a n n i n s are colorless, b u t they become colored very r e a d i l y once i s o l a t e d because of t h e i r p r o p e n s i t y t o o x i d i z e to quinones. T h e p r i m a r y characteristic of the water-soluble condensed t a n n i n s (4) is d e h y d r a t i o n / o x i d a t i o n t o intensely colored a n t h o c y a n i d i n p i g m e n t s (5) w h e n refluxed i n b u t a n o l a n d h y d r o c h l o r i c a c i d ( F i g u r e 2). F o r t h i s reason, there has been a tendency t o refer t o these c o m p o u n d s as " p r o a n t h o c y a n i d i n s " i n the last few years. P r i o r t o t h a t , t h e y were referred t o as " l e u c o a n t h o c y a n i d i n s " (i.e., the colorless c h e m i c a l f o r m of a n t h o c y a n i d i n s ) . A l l references earlier t h a n the l a t e 1950's, w h e n the s t r u c t u r e of these substances was j u s t b e g i n n i n g t o be u n d e r s t o o d , used the t e r m "condensed" t a n n i n . A n o t h e r characteristic of the condensed t a n n i n s was u s u a l l y observed d u r i n g leather tannage w i t h these m a t e r i a l s . A q u e o u s suspensions of t a n n i n t h a t were a c i d i c f r o m the t a n n i n g process g r a d u a l l y p r e c i p i t a t e d i n s o l u b l e m a t e r i a l s k n o w n as " t a n n e r ' s r e d s " o r phlobaphenes. These substances, d e r i v e d f r o m the t a n n i n , were n o longer soluble i n w a t e r , b u t they c o u l d be dissolved i n p o l a r solvents such as e t h a n o l or acetone or i n aqueous base. Since most species of b a r k c o n t a i n a n e x t r a c t i v e f r a c t i o n t h a t p h y s i c a l l y resembles the t a n n e r ' s reds, t h e y are referred t o as phlobaphenes i n the l i t e r a t u r e . V e r y l i t t l e c h a r a c t e r i z a t i o n w o r k has been done o n t h i s f r a c t i o n , a n d there is s u b s t a n t i a l reason t o believe t h a t the b a r k p h l o b a p h e n e f r a c t i o n contains a variety of w a t e r - i n s o l u b l e p o l y m e r s , some of t h e m t o t a l l y u n r e l a t e d to the condensed t a n n i n f a m i l y . I n a d d i t i o n t o the water-soluble a n d i n s o l u b l e members of the condensed t a n n i n f a m i l y , b o t h o f w h i c h are soluble i n p o l a r o r g a n i c solvents, there is a t h i r d related fraction, usually called a "phenolic acid." T h i s material can only be i s o l a t e d b y e x t r a c t i o n w i t h aqueous a l k a l i n e s o l u t i o n s or w i t h s o d i u m sulfite or bisulfite s o l u t i o n at elevated t e m p e r a t u r e s a n d pressures. Since the t a n n i n s a n d phlobaphenes are also soluble i n aqueous base, t h e y w i l l b e c o e x t r a c t e d f r o m b a r k a l o n g w i t h the p h e n o l i c acids w h e n b a r k is e x t r a c t e d w i t h base. T h i s is i m p o r t a n t t o r e m e m b e r i n the subsequent discussion o n b a r k - b a s e d adhesives.




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159

160



Isolation of C o n d e n s e d

Tannins

B a r k or h e a r t w o o d , the p r i m a r y r a w m a t e r i a l s for t a n n i n p r o d u c t i o n , c o n t a i n m a n y extraneous substances. T h e b r o a d categories of c o m p o u n d s are l i s t e d i n T a b l e I, a n d , as c a n be seen, n o single category is i s o l a t a b l e b y s i m p l e solvent e x t r a c t i o n . M o s t w o r k o n the use of t a n n i n s for adhesives has i n v o l v e d a hot water or m i l d a l k a l i n e e x t r a c t . T h e reason for t h i s is q u i t e s i m p l e : the value of the p r o d u c t for the i n t e n d e d e n d use c o u l d not s u p p o r t the cost of subsequent r e f r a c t i o n a t i o n or p u r i f i c a t i o n . C a r b o h y d r a t e i m p u r i t i e s are p a r t i c u l a r l y u n d e s i r a b l e , so investigators over the years have u s u a l l y e x p e n d e d m u c h effort i n t r y i n g t o find a p a r t i c u l a r p l a n t species t h a t w o u l d y i e l d a n e x t r a c t h i g h i n t a n n i n content a n d low i n the offending c a r b o h y d r a t e " c o n t a m i n a n t s . " Insofar as c h e m i c a l s t r u c t u r e studies are concerned, progress was slow u n t i l c h r o m a t o g r a p h i c techniques were devised i n the last decade (9-11) for s e p a r a t i o n of the d i m e r i c a n d t r i m e r i c p r o a n t h o c y a n i d i n s f r o m the higher p o l y m e r s , sugars, etc., present i n a n e x t r a c t . T h i s p e r m i t t e d i n d i r e c t d e d u c t i o n of the s t r u c t u r e of the higher p o l y m e r i c fractions (e.g., the true t a n n i n s ) .

T a b l e I. M a t e r i a l s E x t r a c t a b l e f r o m B a r k or H e a r t w o o d w i t h V a r i o u s T y p e s of Solvents Solvent T y p e of E x t r a c t i v e W a x , f a t s , terpenes Flavones Flavanols Stilbenes Proanthocyanidins Solvent-soluble lignin S i m p l e sugars Pectins Arabinogalactans Xylans A s h (oxalates, etc.) "Polyphenolic acids" "Lignin"

Petroleum Ether

Ethyl Ether

Ethanol

X

X X X X

X X X X X X X

Hot Water

X X X X X X X


Aqueous Alkali X X X X X X X X X X X X X

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161


C h e m i c a l Structure of Condensed

Tannins

T h r o u g h a n i m p r e s s i v e a r r a y o f e x p e r i m e n t s c o n d u c t e d over the last 15 years, i t has been s h o w n t h a t the aqueous e x t r a c t s c o n t a i n a series o f c o m p o u n d s based o n a s u b s t i t u t e d flavonoid s t r u c t u r e . F o r purposes o f t h i s o v e r v i e w , i t does not a p p e a r t o b e a p p r o p r i a t e t o go i n t o a l o t of d e t a i l . M a i n l y , i t is i m p o r t a n t t o keep i n m i n d t h a t : 1) the

flavonoid

s u b s t i t u t i o n p a t t e r n seems t o be c h a r a c t e r i s t i c

for a g i v e n species; 2) s t e r e o c h e m i s t r y c a n v a r y at the 2, 3, a n d 4 p o s i t i o n s i n the C - r i n g ( F i g u r e 3); 3) the B - r i n g can have a v a r i a b l e p a t t e r n o f one, two or


three h y d r o x y l g r o u p s ; 4) the b o n d j o i n i n g the m o n o m e r i c u n i t s extends

from

the 4 p o s i t i o n i n the C - r i n g t o the 6 or 8 p o s i t i o n of the Α-ring; a n d 5) the t e r m i n a t i n g u n i t m a y differ i n s u b s t i t u t i o n p a t t e r n o r s t e r e o c h e m i s t r y f r o m a l l the o t h e r u n i t s i n t h e c h a i n . W a t e r s o l u b i l i t y seems t o be p r i m a r i l y a f u n c t i o n of c h a i n l e n g t h .

I n the

p r o c y a n i d i n p o l y m e r s ( F i g u r e 3, 6 ) , chains w i t h a D P greater t h a n a b o u t 8 are not s o l u b l e i n w a t e r . T h i s suggests t h a t the " p h l o b a p h e n e " f r a c t i o n m a y s i m p l y be a h i g h e r m o l e c u l a r weight v e r s i o n of the w a t e r - s o l u b l e t a n n i n s , w h i l e t h e soc a l l e d p h e n o l i c acids are of even h i g h e r m o l e c u l a r weight, c r o s s l i n k e d , or t i g h t l y complexed to cell w a l l carbohydrates through an alkali-labile linkage. M i m o s a ( F i g u r e 3, 7) a n d quebracho t a n n i n s ( F i g u r e 3, 8) differ f r o m the conifer b a r k t a n n i n s i n t h a t t h e Α-ring is based o n r e s o r c i n o l r a t h e r t h a n p h l o r o g l u c i n o l . T h e s e t a n n i n s also t e n d to have a higher degree o f water s o l u b i l i t y a n d less l i k e l i h o o d o f r e a r r a n g i n g t o h i g h l y colored b y p r o d u c t s , t h u s m a k i n g t h e m m o r e a t t r a c t i v e for leather t a n n i n g . G e n e r a l l y , the h o t - w a t e r e x t r a c t of a b a r k or h e a r t w o o d source o f t a n n i n w i l l c o m p r i s e a b o u t 60 t o 6 5 % t a n n i n p o l y m e r s as m e a s u r e d b y a s t a n d a r d h i d e powder

a b s o r p t i o n test.

T h e r e m a i n d e r w i l l be a m i x t u r e o f sugars, p e c t i n ,

h e m i c e l l u l o s e , a n d lower m o l e c u l a r weight ( < 3 0 0 ) p o l y p h e n o l s . C o l d - w a t e r ex traction w i l l y i e l d a somewhat higher p u r i t y t a n n i n extract, but overall yields based o n the weight o f s t a r t i n g m a t e r i a l w i l l be lower.

Quebracho wood and

m i m o s a b a r k are u n u s u a l i n t h a t t h e i r w a t e r e x t r a c t has a m e a s u r e d " p u r i t y " u p t o 75 t o 8 0 % .

T h e " t a n n i n s " i n each case consist o f a series o f

oligomers

w i t h a D P o f 2 t o a b o u t 7 t o 9, d e p e n d i n g u p o n e x t r a c t i o n c o n d i t i o n s . Cleavage Reactions a n d

Rearrangement

T o b e t t e r u n d e r s t a n d the r e a c t i v i t y o f the condensed t a n n i n s i n adhesive f o r m u l a t i o n s , i t is i m p o r t a n t t o recognize m o l e c u l a r changes a c c o m p a n y i n g e x t r a c t i o n o f the t a n n i n s w h e n sulfites or a l k a l i are used t o enhance y i e l d . A m o d e l p o u n d s t u d y o n c a t e c h i n ( F i g u r e 4,11)

com

(27) showed t h a t t r e a t m e n t w i t h s o d i u m

b i s u l f i t e s o l u t i o n at 170 ° C for 30 m i n o p e n e d the flavan r i n g w i t h f o r m a t i o n of a sulfonate g r o u p o n the 2 p o s i t i o n (12).

T h i s was b e l i e v e d t o e x p l a i n the

f o r m a t i o n o f t a n n i n sulfonates d u r i n g a c o m m e r c i a l process i n v o l v i n g bisulfite e x t r a c t i o n o f western h e m l o c k b a r k (3,13). M o r e recently, i t has been f o u n d


162


oh oh

mimosa

bark

( 2 0 - 23)

6 w e s t e r n h e m l o c k (3.12-14) r|«oh; r , r 3 « h r2«0h; r|,r3«h


2

pine bark (j5-.i9) r,«oh; r ,r3«h r , , r 3 «0h, r «h 2

quebracho

wood

(24-26)

2

F i g u r e 3. P r o a n t h o c y a n i d i n s e x t r a c t a b l e f r o m b a r k or h e a r t w o o d w i t h v a r i o u s types o f solvents.

F i g u r e 4. S u l f o n a t i o n reactions o f condensed t a n n i n s .



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(28) t h a t the i n t e r f l a v o n o i d b o n d is m u c h m o r e l i k e l y t o be cleaved t h a n the p y r a n r i n g . T h e 4 —• 6 or 4 —• 8 b o n d s are b r o k e n w i t h c o n c o m i t a n t s u l f o n a t i o n at the 4 p o s i t i o n ( 1 0 ) . W h e n the t a n n i n does not have a h y d r o x y l g r o u p i n the 5 - p o s i t i o n , as is the case w i t h m i m o s a a n d quebracho t a n n i n s , the i n t e r f l a v o n o i d b o n d is m u c h m o r e stable. I n t h i s instance, s u l f o n a t i o n p r i m a r i l y leads t o the f o r m a t i o n of s u b s t i t u t i o n at the C - 2 a t o m (29). E v e n t h o u g h the precise de t a i l s of these s u l f o n a t i o n reactions need further c l a r i f i c a t i o n , i t seems reasonable t o conclude t h a t w a t e r - i n s o l u b l e t a n n i n s c a n be converted t o lower m o l e c u l a r weight fractions w i t h s o l u b i l i z i n g sulfonate groups i n the 2 a n d / o r 4 p o s i t i o n s by t r e a t m e n t w i t h s o d i u m sulfite or bisulfite at elevated t e m p e r a t u r e s . T h i s re a c t i o n is s u i t a b l e , therefore, for e x t r a c t i o n of phlobaphenes a n d " p h e n o l i c a c i d s " f r o m b a r k as w e l l as the water-soluble t a n n i n s . D u r i n g early phases of s t r u c t u r a l studies o n b a r k p o l y p h e n o l s (15), i t was n o t e d t h a t a l k a l i t r e a t m e n t of isolated t a n n i n p o l y m e r s resulted i n the f o r m a t i o n of a n acidic g r o u p , presumed to be a c a r b o x y l , a n d the loss of f o r m a l d e h y d e r e a c t i v i t y . It was s u r m i s e d t h a t the so-called b a r k phenolic acids were, i n fact, higher m o l e c u l a r weight t a n n i n s t h a t underwent rearrangement d u r i n g i s o l a t i o n . S u p p o r t for t h i s i d e a was g r e a t l y strengthened b y e l u c i d a t i o n of the a l k a l i n e re arrangement of catechin to catechinic a c i d (13) —• (14) ( F i g u r e 5) b y Sears, et al. (30). L a k s a n d H e m i n g w a y (31) have c o n t i n u e d to s t u d y the reactions of t a n n i n s w i t h a l k a l i u s i n g p h l o r o g l u c i n o l or p h e n y l m e t h a n e t h i o l as a n u c l e o p h i l e . T h i s w o r k clearly shows o p e n i n g of the p y r a n r i n g , f o r m a t i o n of a reactive site i n the 2 p o s i t i o n , cleavage of interflavonoid b o n d s , a n d f o r m a t i o n of c a r b o n y l groups i n the Α-ring. It is not at a l l s u r p r i s i n g t h a t earlier workers h a d great difficulties i n t r y i n g t o determine b a r k t a n n i n s t r u c t u r e w h e n they used a l k a l i n e e x t r a c t i o n m e t h o d s . F u r t h e r m o r e , the rearrangement of the Α-ring w i t h c o n sequent loss of formaldehyde r e a c t i v i t y s h o u l d be kept i n m i n d w h e n adhesive f o r m u l a t i o n s are m a d e u p f r o m a l k a l i n e e x t r a c t s .

Preparation of Tannin-Based

Adhesives

Interest i n use of condensed t a n n i n s as c o m p o n e n t s of adhesive f o r m u l a t i o n s began a b o u t three decades ago. W h i l e research studies have been c a r r i e d out i n w i d e l y scattered laboratories a r o u n d the w o r l d , three m a j o r areas of a c t i v i t y c a n be d i s t i n g u i s h e d . These are: 1) development of b a r k extracts a n d c o m m e r c i a l p r o d u c t i o n facilities o n the west coast of N o r t h A m e r i c a , 1953 t o 1975; 2) a p p l i c a t i o n of t a n n i n s i n adhesive f o r m u l a t i o n s i n S o u t h A f r i c a based o n i n digenously p r o d u c e d m i m o s a (wattle) t a n n i n , early 1970's t o the present; a n d 3) a resurgence of interest i n pine b a r k as raw m a t e r i a l for t a n n i n - b a s e d adhesives, b e g i n n i n g i n the m i d d l e 1970's. E a c h of these a c t i v i t i e s has been characterized by p a r a l l e l efforts o n s t r u c t u r a l i d e n t i f i c a t i o n of the t a n n i n s a n d development of u n i q u e m e t h o d s for i n c o r p o r a t i n g the isolated t a n n i n s i n t o adhesives.



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F i g u r e 5. A l k a l i n e rearrangement reactions of

(+)-catechin.


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Western Conifer B a r k Polyphenols


F o l l o w i n g W o r l d W a r I I , extensive investigative effort was i n i t i a t e d t o f i n d new uses for the m o u n t a i n s of b a r k generated as b y p r o d u c t s of the W e s t C o a s t forest p r o d u c t s i n d u s t r y i n the U n i t e d States a n d C a n a d a . T h i s w o r k was c a r r i e d o u t at the I n s t i t u t e of P a p e r C h e m i s t r y (sponsored b y the P a c i f i c L u m b e r C o m p a n y of S c o t i a , C a l i f o r n i a ) , the O r e g o n Forest P r o d u c t s L a b o r a t o r y , R a y o n i e r Incorp o r a t e d , the Weyerhaueser C o m p a n y , a n d the Forest P r o d u c t s L a b o r a t o r y o n the U n i v e r s i t y of B r i t i s h C o l u m b i a c a m p u s . W e s t e r n h e m l o c k b a r k was considered t o be a n a t t r a c t i v e m a t e r i a l for t a n n i n p r o d u c t i o n (32), since i t s c h e m i c a l c o m p o s i t i o n was t h o u g h t t o closely p a r a l l e l t h a t of eastern h e m l o c k b a r k , the m a j o r r a w m a t e r i a l for t a n n i n g i n the U n i t e d States at the b e g i n n i n g of the century. M u c h of the h e m l o c k b a r k i n the W e s t was recovered f r o m seawater-floated logs a n d / o r the use of h y d r a u l i c debarkers t h a t d i m i n i s h e d the y i e l d of water-soluble t a n n i n s . Nevertheless, such t a n n i n as c o u l d be o b t a i n e d was f o u n d t o be useful as a n o i l w e l l - d r i l l i n g dispersant or as a n adhesive w h e n m i x e d w i t h f o r m a l d e h y d e , a c c o r d i n g t o the B r i t i s h C o l u m b i a research t e a m (33). Subsequent w o r k showed t h a t m u c h higher y i e l d s of p o l y p h e n o l i c e x t r a c t s c o u l d be o b t a i n e d b y elevated t e m p e r a t u r e e x t r a c t i o n s w i t h s o d i u m b i s u l f i t e , s o d i u m h y d r o x i d e , or a m m o n i a (34-36). Rayonier built two c o m m e r c i a l p l a n t s for the p r o d u c t i o n of such e x t r a c t s , one at H o q i u m , W a s h i n g t o n , a n d the other at V a n c o u v e r , B r i t i s h C o l u m b i a (13). A t t e m p t s t o m a k e adhesive f o r m u l a t i o n s b y direct r e a c t i o n of f o r m a l d e h y d e or i t s equivalent resulted i n p r o d u c t s t h a t were excessively viscous, a n d the w o r k i n g t i m e was t o o short for c o m m e r c i a l a p p l i c a t i o n (37). It was c o n c l u d e d t h a t f o r m a l d e h y d e , a l t h o u g h r e a d i l y reactive w i t h the t a n n i n m o l e c u l e , p r o v i d e d m u c h t o o short linkages t o connect the b u l k y t a n n i n molecules. T h i s p r o b l e m was c i r c u m v e n t e d by the p r e p a r a t i o n of a p o l y m e t h y l o l p h e n o l reagent t h a t , w h e n p u t i n s o l u t i o n w i t h the b a r k e x t r a c t , f o r m e d a c o m b i n a t i o n t h a t was s t a ble for several weeks at r o o m t e m p e r a t u r e . W h e n heated, the p o l y m e t h y l o l p h e n o l a n d b a r k e x t r a c t reacted r a p i d l y t o f o r m a n i n f u s i b l e r e s i n . C o m m e r c i a l t r i a l s were m a d e t o p r o d u c e exterior-grade D o u g l a s - f i r p l y w o o d . W i d e s p r e a d use of the extracts for t h i s purpose, however, was i n h i b i t e d b y a d r o p i n the price of p h e n o l below w h a t the b a r k extracts c o u l d be m a n u f a c t u r e d for. ( T h e best e x t r a c t for adhesive purposes was a n a m m o n i a e x t r a c t of h e m l o c k b a r k converted t o a s o d i u m d e r i v a t i v e p r i o r to s p r a y d r y i n g , a m o r e costly e x t r a c t i o n procedure t h a n s i m p l e s o d i u m h y d r o x i d e e x t r a c t i o n of b a r k . ) Subsequent work was, therefore, directed t o s u b s t i t u t i o n of r e s o r c i n o l , a m u c h m o r e expensive p h e n o l , i n c o l d - s e t t i n g , w a t e r p r o o f adhesives (38). Form u l a t i o n s based o n 30 t o 6 0 % of e x t r a c t m i x e d w i t h a r e s o r c i n o l - f o r m a l d e h y d e condensate a n d a d d i t i o n a l f o r m a l d e h y d e m e t p o t - l i f e a n d assembly t i m e requirements for t i m b e r l a m i n a t i o n . Test b o n d s passed requirements of the m a j o r performance s t a n d a r d s i n the U n i t e d States, b u t c o m p e t i t i o n f r o m lower cost, p h e n o l - m o d i f i e d resorcinol resins a n d the lack o f l o n g t e r m c o m m e r c i a l perfor-


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m a n c e records prevented significant acceptance of the p r o d u c t s . Sales o f 300 tons per year d i d not j u s t i f y c o n t i n u e d p r o d u c t i o n , so the effort was t e r m i n a t e d i n 1972. B a r k e x t r a c t i o n b y R a y o n i e r (now I T T R a y o n i e r ) was s t o p p e d i n 1976. P a r t s of the e x t r a c t i o n p l a n t at H o q i u m were subsequently used for m a k i n g products from lignin. A b o u t the same t i m e R a y o n i e r b e g a n m a r k e t i n g b a r k e x t r a c t s f r o m western h e m l o c k , the P a c i f i c L u m b e r C o m p a n y at S c o t i a , C a l i f o r n i a , b u i l t a n e x t r a c t i o n p l a n t t o p r o d u c e a l k a l i n e e x t r a c t s f r o m r e d w o o d b a r k . T h e p r o d u c t s were c a l l e d P a l c o t a n a n d S o d i u m P a l c o n a t e . T h e y m a i n l y f o u n d uses i n o i l w e l l d r i l l i n g . A t t e m p t s t o use the p r o d u c t i n adhesive f o r m u l a t i o n s do not seem t o have been successful, m o s t l y because of v i s c o s i t y p r o b l e m s associated w i t h c a r b o h y d r a t e s c o e x t r a c t e d f r o m the b a r k a l o n g w i t h the p o l y p h e n o l s . W e y e r haeuser p r o d u c e d a l k a l i n e e x t r a c t s of D o u g l a s - f i r b a r k d u r i n g the 1960's. T h e y were s a i d t o be a p p l i c a b l e i n adhesive f o r m u l a t i o n s , b u t there are n o l i t e r a t u r e references a t t e s t i n g t o t h e i r c o m m e r c i a l success. T h e m o s t recent a t t e m p t at t h i s m a r k e t was the p r e p a r a t i o n of finely d i v i d e d D o u g l a s - f i r b a r k , w h i c h h a d been p r e e x t r a c t e d w i t h n o n p o l a r solvents t o remove w a x , as a reactive extender for p h e n o l - f o r m a l d e h y d e adhesives. T h e B o h e m i a L u m b e r C o m p a n y ' s p l a n t for s u c h p r o d u c t s s h u t d o w n i n the e a r l y 1980's. A t t e m p t s were m a d e t o revive interest i n western h e m l o c k b a r k p o l y p h e nols t h r o u g h a research p r o g r a m at the W e s t e r n Forest P r o d u c t s L a b o r a t o r y , F o r i n t e k C a n a d a C o r p . I n t h i s w o r k , i t was c o n c l u d e d (39) t h a t the p h e n o l i c a c i d f r a c t i o n of the b a r k , w h e n i s o l a t e d b y a l k a l i n e e x t r a c t i o n , was insufficiently reactive w i t h f o r m a l d e h y d e t o be o f c o m m e r c i a l value. T h i s i n v e s t i g a t i o n s u g gested e t h a n o l e x t r a c t i o n as the preferred route t o t a n n i n s for adhesive a p p l i c a t i o n . S i n c e e n v i r o n m e n t a l r e s t r i c t i o n s i n the U n i t e d States have l a r g e l y e l i m i n a t e d w a t e r storage of h e m l o c k logs a n d h y d r a u l i c d e b a r k i n g d u r i n g the last decade, q u a n t i t i e s of h e m l o c k b a r k are n o w available t h a t have not been water leached. T h e b a r k is used as f u e l , b u t i t m a y be t i m e l y t o reconsider e x t r a c t i o n of p o l y p h e n o l s p r i o r t o the b u r n i n g process as a higher f o r m o f u t i l i z a t i o n t h a n simple combustion. M i m o s a T a n n i n Adhesives T h e o n l y t a n n i n s i n the w o r l d c u r r e n t l y b e i n g c o m m e r c i a l l y e x p l o i t e d for a d hesive a p p l i c a t i o n s are those i s o l a t e d b y h o t - (or cold-) water e x t r a c t i o n of Acacia meamsii b a r k i n the province of N a t a l , S o u t h A f r i c a . A p p r o x i m a t e l y 100,000 tons of m i m o s a t a n n i n were b e i n g p r o d u c e d a n n u a l l y as r e p o r t e d i n 1980, the latest year for w h i c h p r o d u c t i o n figures were a v a i l a b l e (41)- O f t h i s a m o u n t , a b o u t 10,000 tons were used i n adhesive a p p l i c a t i o n s m a i n l y i n S o u t h A f r i c a , A u s t r a l i a , a n d N e w Z e a l a n d . W h i l e t h i s n u m b e r is n o t large i n l i g h t of the 300,000 t o 400,000 tons o f p h e n o l used a n n u a l l y i n resins, i t does p r o v i d e evidence t h a t b a r k t a n n i n s c a n be e c o n o m i c a l l y used for adhesives. T h i s a p p l i c a t i o n is f a c i l i t a t e d b y the r e l a t i v e l y h i g h cost of p h e n o l a n d resorcinol i n


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S o u t h A f r i c a a n d the p o l i t i c a l strategy t o reduce or e l i m i n a t e i m p o r t s o f strategic m a t e r i a l s such a p e t r o c h e m i c a l s as m u c h as possible. E a r l i e s t a p p l i c a t i o n of m i m o s a t a n n i n s for adhesives came i n the field of p l y w o o d a n d c h i p b o a r d s (41)- E x t e r i o r p l y w o o d adhesives were based o n c o n d e n s a t i o n o f w a t t l e t a n n i n w i t h u r e a f o r m a l d e h y d e c r o s s l i n k i n g agents, a p h e n o l f o r m a l d e h y d e resol, or a p h e n o l - r e s o r c i n o l - f o r m a l d e h y d e p o l y m e r t o w h i c h p a r a f o r m a l d e h y d e was a d d e d to p r o v i d e c r o s s l i n k i n g between the r e s o r c i n o l a n d the Α-ring o f the t a n n i n . F a s t - c u r i n g rates a n d g o o d tolerance t o h i g h m o i s t u r e content veneers characterize these systems. M e t a l c a t a l y s t s s u c h as z i n c acetate have been f o u n d t o be useful for speeding the cure r a t e . P r o b l e m s w i t h viscos i t y c a n be a m e l i o r a t e d b y cold-water e x t r a c t i o n o f the b a r k t o reduce p o l y m e r i c c a r b o h y d r a t e content i n the e x t r a c t or b y m i l d s u l f o n a t i o n o f the t a n n i n s . T h e l i t e r a t u r e does n o t i n d i c a t e w h e t h e r these t w o procedures are c u r r e n t l y b e i n g practiced i n mimosa tannin production. E a r l y a t t e m p t s t o use m i m o s a t a n n i n i n p a r t i c l e b o a r d adhesives i n v o l v e d h i g h - t e m p e r a t u r e a l k a l i n e t r e a t m e n t o f the e x t r a c t t o reduce v i s c o s i t y o f the 4 0 % solids level needed (43,44)- Subsequent i m p r o v e m e n t s followed the same course as w i t h p l y w o o d , n a m e l y the use o f p h e n o l - f o r m a l d e h y d e or p h e n o l - r e s o r c i n o l f o r m a l d e h y d e as c r o s s l i n k i n g agents (45) a n d the use of c a t a l y s t s or m i x m o d i f i cations t o reduce press t e m p e r a t u r e requirements a n d t o e x t e n d p o t life. R e c e n t work (46) has s h o w n t h a t exterior c h i p b o a r d adhesives c a n also be p r e p a r e d b y c r o s s l i n k i n g of m i m o s a t a n n i n s w i t h 4 , 4 - d i p h e n y l m e t h a n e d i i s o c y a n a t e . A p p l i c a t i o n s for c o l d - s e t t i n g , w o o d - l a m i n a t i n g adhesives i n i t i a l l y followed the s a m e a p p r o a c h (47) used for l a m i n a t i n g resins f r o m western h e m l o c k (38) (i.e., r e a c t i o n o f t a n n i n w i t h p h e n o l - r e s o r c i n o l - f o r m a l d e h y d e p r e p o l y m e r s ) . I m p r o v e m e n t s resulted t h r o u g h the a p p l i c a t i o n o f K r e i b i c h ' s " H o n e y m o o n " t e c h n i q u e (48 ) w h e r e i n one side o f the m a t e r i a l t o be b o n d e d is t r e a t e d w i t h resin a n d the other w i t h c a t a l y s t . O n e o f the preferred systems (49) was p h e n o l r e s o r c i n o l - f o r m a l d e h y d e or t a n n i n - r e s o r c i n o l - f o r m a l d e h y d e at p H 8 w i t h e x t r a p a r a f o r m a l d e h y d e o n the Α-side a n d t a n n i n at 5 3 % solids or t a n n i n - r e s o r c i n o l f o r m a l d e h y d e at p H 12 o n the B - s i d e . S u c h resin systems are c u r r e n t l y used to l a m i n a t e e u c a l y p t u s or pine i n m o s t S o u t h A f r i c a n t i m b e r - l a m i n a t i n g p l a n t s . Pine B a r k T a n n i n Adhesive Formulations Interest i n p i n e b a r k as a source o f adhesive c o m p o n e n t s b e g a n t o accelerate f o l l o w i n g the o i l crisis o f 1973. S o d i u m h y d r o x i d e e x t r a c t s o f s o u t h e r n p i n e b a r k were successfully used i n r e p l a c i n g u p t o 4 0 % o f the p h e n o l i c resin for b o n d i n g of p a r t i c l e b o a r d s , oriented s t r a n d b o a r d s , a n d composites w i t h a flakeboard core a n d veneer f a c i n g (50,51). S i m i l a r results were o b t a i n e d w i t h e x t r a c t s f r o m p a t u l a p i n e (52). E n c o u r a g e d b y results o f t h i s t y p e , t h e N e w Z e a l a n d Forest Products L t d . C o r p o r a t i o n expanded their r a d i a t a pine bark tannin pilot plant to f u l l - s c a l e o p e r a t i o n i n 1981 t o p r o d u c e a n e x t r a c t t r a d e m a r k e d T a n n a p h e n . T h i s m a t e r i a l was crosslinked w i t h p a r a f o r m a l d e h y d e a n d used as a n adhesive



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for c h i p b o a r d panels m a r k e t e d p r i m a r i l y as flooring m a t e r i a l i n N e w Z e a l a n d . T h e e x t r a c t i o n p l a n t a n d adhesive f o r m u l a t i o n were d i s c o n t i n u e d recently w i t h out e x p l a n a t i o n . B o a r d s m a d e w i t h T a n n a p h e n were k n o w n t o be m a r g i n a l l y deficient i n m o i s t u r e d u r a b i l i t y . B o a r d s m e e t i n g e x t e r i o r - q u a l i t y s t a n d a r d s c a n be f o r m u l a t e d , however, u s i n g p i n e b a r k e x t r a c t s c o m b i n e d w i t h isocyanate resins (52). S a t i s f a c t o r y exterior p l y w o o d adhesive f o r m u l a t i o n s have also been prepared by combining bark extracts w i t h phenol-formaldehyde prepolymers followed b y the a d d i t i o n o f p a r a f o r m a l d e h y d e . R e c e n t efforts t o develop c o l d - s e t t i n g p h e n o l i c resins f r o m p i n e b a r k have followed p a t h s s i m i l a r t o those used for w a t t l e t a n n i n s . O n e o f these has i n volved the a c i d - c a t a l y z e d cleavage of s o u t h e r n p i n e b a r k u s i n g resorcinol as a n u c l e o p h i l e (53). A second a p p r o a c h has been t o p u r i f y t a n n i n s of c o - o c c u r r i n g c a r b o h y d r a t e a n d use t h e m as resorcinol replacements i n the H o n e y m o o n s y s t e m (48)- O n e surface was s p r e a d w i t h c o m m e r c i a l p h e n o l - r e s o r c i n o l - f o r m a l d e h y d e l a m i n a t i n g adhesive w i t h a d d e d f o r m a l d e h y d e , a n d the other was spread w i t h p i n e t a n n i n i n s o d i u m h y d r o x i d e s o l u t i o n . T h e m o s t recent ( a n d the m o s t econ o m i c a l t o date) a p p r o a c h involves resorcinol replacement b y u s i n g b a r k t a n n i n s e x t r a c t e d w i t h s o d i u m sulfite. M o l e c u l a r weights are reduced d u r i n g the course of the e x t r a c t i o n , a n d the sulfonate f u n c t i o n is a g o o d l e a v i n g group u n d e r c o n d i t i o n s of l a m i n a t i o n . T h e sulfite e x t r a c t s c a n be a p p l i e d as m i x e d syst e m adhesives, w h e r e i n they are c o m b i n e d w i t h c o n v e n t i o n a l p h e n o l - r e s o r c i n o l f o r m a l d e h y d e resins, or i n the H o n e y m o o n s y s t e m . U p t o 5 0 % replacement of s y n t h e t i c resin b y s u l f o n a t e d b a r k t a n n i n s is envisioned b y t h i s process (54). Future Outlook A n u m b e r o f q u i t e s a t i s f a c t o r y adhesive f o r m u l a t i o n techniques i n c o r p o r a t i n g b a r k t a n n i n s have been developed, especially d u r i n g the last decade. U n f o r t u nately, i n m a n y instances, t h e y i n t r o d u c e degrees of c o m p l e x i t y i n t o the b o a r d or l a m i n a t e p r o d u c t i o n l i n e not otherwise experienced w h e n 1 0 0 % p h e n o l or resorcinol-based adhesives are used. I n d u c e m e n t of the b o a r d m a n u f a c t u r e r t o s w i t c h t o b a r k - b a s e d adhesives m u s t involve significant e c o n o m i c incentives (e.g., the use o f b a r k - b a s e d adhesives m u s t be s u b s t a n t i a l l y cheaper t h a n the corres p o n d i n g phenol-based f o r m u l a t i o n ) . T h u s , i t seems a d d i t i o n a l a t t e n t i o n needs t o be p a i d t o finding lower cost, higher y i e l d i s o l a t i o n techniques. P i n e b a r k , i n p a r t i c u l a r , has the p o t e n t i a l t o give higher y i e l d extracts t h a n are r e p o r t e d i n most of the e x p e r i m e n t s m e n t i o n e d i n t h i s review. P o s s i b l y , s u l f o n a t i o n is the answer, b u t m o r e work is r e q u i r e d t o e s t a b l i s h t h i s a p p r o a c h . I n the m e a n t i m e , R & D a n d c a p i t a l c o m m i t m e n t s are needed i n N o r t h A m e r i c a a n d other p i n e g r o w i n g areas f r o m a c o n s o r t i u m of p o t e n t i a l e x t r a c t p r o d u c e r s a n d users s u c h as exists i n S o u t h A f r i c a . T h i s c o u l d w e l l lead t o a m a j o r new business a c t i v i t y d u r i n g the next decade.


12.

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169


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