Adhesives from Renewable Resources - American Chemical Society

Chapter 13 Viscosity and Formaldehyde Consumption of Procyanidin Solutions Downloaded by RUTGERS UNIV on December 28, 2017 | http://pubs.acs.org Publication Date: December 31, 1989 | doi: 10.1021/bk-1989-0385.ch013

Lawrence J. P o r t e r Chemistry Division Department of Scientific and Industrial Research Petone, New Zealand

The last decade has seen quite remarkable advances in our knowledge of the structure and properties of the proanthocyanidins. Viscosity measurements were made of solutions of procyanidins isolated from Theobroma cacao and Chaenomeles speciosa with number-average degrees of polymerization of 6.1 and 11.8, respectively, in water and 1% sodium hydroxide at 25 °C. Procyanidins are apparently com pletely crosslinked by formaldehyde up to a chain length of 6 units, but few units are crosslinked in polymeric procyanidins. T h e sec ond order rate constants observed for the formaldehyde reaction with catechin or epicatechin are approximately six times higher than that observed for the C. speciosa polymer. P r o a n t h o c y a n i d i n s are p l a n t p h e n o l i c b i o p o l y m e r s t h a t consist o f flavanoid m o n o m e r u n i t s . T w o m a j o r classes o f p r o a n t h o c y a n i d i n s o c c u r : those t h a t possess a r e s o r c i n o l - p a t t e r n Α-ring ( F i g u r e 1) a n d those t h a t possess a p h l o r o g l u c i n o l p a t t e r n Α-ring. T h e l a t t e r are b y far the m o s t c o m m o n , o c c u r r i n g i n a h i g h p r o p o r t i o n o f m o n o c o t y l e d o n o u s a n d d i c o t y l e d o n o u s p l a n t s (1,2). T h e r e s o r c i n o l p a t t e r n p r o a n t h o c y a n i d i n s are confined t o a few genera o f t r o p i c a l o r s u b t r o p i c a l h a r d w o o d s a n d associated s h r u b b y species (2), b u t are e c o n o m i c a l l y i m p o r t a n t , since the i n t e r n a t i o n a l l y c o m m e r c i a l l y p r e d o m i n a n t w a t t l e (3,4) a n d q u e b r a c h o (5) t a n n i n s are o f t h i s t y p e . Together, they c o n s t i t u t e a p p r o x i m a t e l y t w o - t h i r d s (i.e., a p p r o x i m a t e l y 300,000 tons) o f the w o r l d p r o d u c t i o n o f vegetable t a n n i n s (5). T h e r e s o r c i n o l - p a t t e r n p r o a n t h o c y a n i d i n s are w i d e l y used not o n l y for leather t a n n i n g , b u t also for a range o f other c o m m e r c i a l p r o d u c t s , p a r t i c u l a r l y as adhesives f o r p l y w o o d a n d fiberboard (6,7). W a t t l e t a n n i n i s p r o d u c e d f r o m s u s t a i n e d - y i e l d forests o f Acacia meamsii. l a r g e l y i n s o u t h e r n A f r i c a (6,8). S o m e o f the i m p e t u s a t least t o develop other uses for w a t t l e t a n n i n , a p a r t f r o m 0097-6156/89/0385-0172$06.00/0 ·

1989 American Chemical Society

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

Downloaded by RUTGERS UNIV on December 28, 2017 | http://pubs.acs.org Publication Date: December 31, 1989 | doi: 10.1021/bk-1989-0385.ch013

PORTER

Viscosity and Formaldehyde

etc F i g u r e 1. S t r u c t u r e a n d n o m e n c l a t u r e o f p r o a n t h o c y a n i d i n s . P h l o r o g l u c i n o l - p a t t e r n Α-ring p r o a n t h o c y a n i d i n s : R = O H ; R = H . Procyanidins. R = R = O H . Prodelphinidins. R e s o r c i n o l - p a t t e r n Α-ring p r o a n t h o c y a n i d i n s : R = R = H . Profisetinidins. R = H ; R = O H . Prorobinetinidins. 2

2

2

2

1

1

2

2


174

ADHESIVES F R O M RENEWABLE RESOURCES

leather t a n n i n g , grew o u t o f a w o r l d w i d e t r e n d away f r o m vegetable t a n n i n s i n favor o f s y n t h e t i c a n d c h r o m e t a n n i n g m e t h o d s . Potentially, the phloroglucinol-pattern proanthocyanidins, the procyanidins, a n d p r o d e l p h i n i d i n s ( F i g u r e 1) represent a n enormous resource o f renewable i n d u s t r i a l phenolics. T h e y occur i n h i g h c o n c e n t r a t i o n i n t h e b a r k a n d needles of m o s t conifers, a n d also i n t h e m a j o r i t y o f T e m p e r a t e Zone h a r d w o o d s

(2).

L a r g e q u a n t i t i e s o f t h i s t y p e o f t a n n i n are s t i l l used i n the leather i n d u s t r y - especially i n C h i n a and R u s s i a (Sun Dawang, personal communication) and India (9). A l t h o u g h a c t u a l tonnages are difficult t o a s c e r t a i n , t h e q u a n t i t i e s used for


leather t a n n i n g i n these countries are a b o u t h a l f those o f t h e t o t a l p r o d u c t i o n o f w a t t l e a n d quebracho t a n n i n s c o m b i n e d .

A t t e m p t s t o use p r o c y a n i d i n s as

w o o d adhesives a n d other p r o d u c t s , p a r t i c u l a r l y those d e r i v e d f r o m D o u g l a s - f i r , western h e m l o c k , a n d Pinus

radiata (10,11), date back t o t h e 1950V, b u t so f a r ,

no t r u l y successful i n d u s t r i a l process has been developed, l a r g e l y because o f a n u m b e r o f p r o b l e m s associated w i t h t h e i r h i g h c h e m i c a l r e a c t i v i t y a n d r e l a t i v e instability i n solution. M u c h o f t h e p r o b l e m i n f i n d i n g successful i n d u s t r i a l a p p l i c a t i o n s h a s r e v o l v e d a r o u n d a lack o f basic u n d e r s t a n d i n g o f t h e s t r u c t u r e a n d c h e m i s t r y of p h l o r o g l u c i n o l - p a t t e r n p r o a n t h o c y a n i d i n s .

Whereas, South A f r i c a n work-

ers (especially R o u x a n d h i s colleagues) m o u n t e d a concerted

a n d successful

c a m p a i g n b e g i n i n g i n t h e 1950's t o u n d e r s t a n d t h e c h e m i s t r y o f w a t t l e a n d quebracho p r o a n t h o c y a n i d i n s (4,5), s i m i l a r advances i n o u r knowledge o f t h e p h l o r o g l u c i n o l - p a t t e r n p r o a n t h o c y a n i d i n s h a d t o w a i t for the i m p r o v e d c h e m i c a l technology o f the last decade. T h e s e advances s t a r t e d w i t h i s o l a t i o n o f p r o c y a n i d i n oligomers as t h e free phenols u s i n g d e x t r a n gels (12) a n d e l a b o r a t i o n o f t h e i r properties a n d t h e subsequent u n e q u i v o c a l d e m o n s t r a t i o n t h a t p o l y m e r i c p r o c y a n i d i n s a n d p r o d e l p h i n i d i n s consist o f extended chains o f

flavan-3-ol

u n i t s (13), t h e c o m m o n e s t

t y p e b e i n g based o n e p i c a t e c h i n u n i t s ( F i g u r e 2 ) . Subsequent t o these studies, r a p i d advances have been m a d e i n o u r u n d e r s t a n d i n g o f t h e reactions o f p r o c y a n i d i n s i n a c i d i c o r basic s o l u t i o n s a n d t h e i r r e a c t i o n w i t h sulfite, t h i o l s , a n d other phenols. A d v a n c e s i n these areas u p t o t h e present have been considered i n some d e t a i l b y H e m i n g w a y a n d his colleagues (14-17) a n d w i l l n o t be f u r t h e r discussed, except i n context. T h e c u r r e n t s t u d y seeks t o e x t e n d o u r knowledge o f t h e b e h a v i o r o f p r o c y a n i d i n s i n t w o areas i m p o r t a n t t o t h e i r i n d u s t r i a l u t i l i z a t i o n : 1) t h e v i s c o s i t y of p r o c y a n i d i n p o l y m e r s i n aqueous s o l u t i o n s , a n d 2) the s t o i c h i o m e t r y a n d rate of r e a c t i o n o f p r o c y a n i d i n s w i t h f o r m a l d e h y d e . Experimental

Methodolodgy

P r o c y a n i d i n s . C a t e c h i n ( F l u k a ) a n d e p i c a t e c h i n (ex. cacao beans) were r e c r y s t a l l i z e d a n d d r i e d before use. T h e oligomers e p i c a t e c h i n - ( 4 / ? — ^ - e p i c a t e c h i n a n d [epicatechin-(4/?—•8)]3-epicatechin were o b t a i n e d f r o m the e t h y l acetate s o l -


PORTER


175


13.

F i g u r e 2. A n e x a m p l e of a h e x a m e r i c h o m o - o l i g o m e r of e p i c a t e c h i n , c o n t a i n i n g one (4/?—»6) a n d four (4/?—»8) i n t e r f l a v a n o i d linkages, φ = 3 , 4 - d i h y d r o x y p h e n y l .


176


u b l e f r a c t i o n of cacao b e a n procyanidin*" b y c h r o m a t o g r a p h y o n Ser. adex L H 20 a n d M C I - g e l a n d identified b y c o m p a i on of t h e i r properties w i t h p u b l i s h e d d a t a (18,19). T h e p e n t a m e r i c a n d h e x a m e r i c p r o c y a n i d i n fractions were o b t a i n e d f r o m the cacao b e a n p o l y m e r f r a c t i o n (see below) b y c h r o m a t o g r a p h y o n F r a c t o g e l H W - 4 0 i n m e t h a n o l (20). T h e m o l e c u l a r weight of a l l oligomers was checked b y negative i o n F A B mass spectroscopy u s i n g a g l y c e r o l m a t r i x (21). T h e p r o c y a n i d i n p o l y m e r s were i s o l a t e d b y acetone-water e x t r a c t i o n a n d S e p h a d e x L H - 2 0 c h r o m a t o g r a p h y as described elsewhere (13). T h e polymers h a d the f o l l o w i n g properties: (1) cacao b e a n (Theobroma cacao): [ α ] § ? = + 1 5 5 ° (c 0.2, w a t e r ) . Analysis. C a l c d . for C i s H ^ O e ^ O : C , 55.4; Η, 5.0. F o u n d : C , 55.4; H , 4.6. N u m b e r - a v e r a g e m o l e c u l a r weight d e t e r m i n e d b y v a p o r pres sure o s m o m e t r y i n m e t h a n o l : 1,970; (2) j a p o n i c a fruits (Chaenomeles speciosa) p o l y m e r : [c*]i$ = +149 (1 0.2, w a t e r ) . Analysis. C a l c d . for C i 5 H i 0 6 - 3 H 0 : C , 52.6; H , 5.3. F o u n d : C , 52.9; H , 5.5. N u m b e r - a v e r a g e m o l e c u l a r weight d e t e r m i n e d b y v a p o r pressure o s m o m e t r y i n m e t h a n o l : 4,035.


8

8

2

2

V i s c o s i t y M e a s u r e m e n t s . These were c a r r i e d o u t o n s o l u t i o n s of the p r o c y a n i d i n p o l y m e r s u s i n g a F e r r a n t i - S h i r l e y cone viscometer at 25.0 ± 0.1 ° C w i t h a 3 . 5 - c m - r a d i u s cone. T h i s viscosity was measured at several r o t a t i o n rates t o check for shear dependence. T h e results were constant over the ranges used (20 t o 500 r e v o l u t i o n s per m i n u t e , d e p e n d i n g o n the v i s c o s i t y ) , a n d the r e s u l t i n g viscosity values were averaged t o o b t a i n the results i n the test. Formaldehyde Reaction Rate and Consumption Experiments. Solu t i o n s of a p p r o x i m a t e l y 1% w / v f o r m a l d e h y d e were p r e p a r e d b y s u i t a b l e d i l u t i o n of a 3 8 % w / v f o r m a l i n s o l u t i o n ( B D H ) a n d were s t a n d a r d i z e d b y the c h r o m o t r o p i c a c i d m e t h o d (22). T h e k i n e t i c a n d r e a c t i o n s t o i c h i o m e t r y e x p e r i m e n t s were c a r r i e d o u t i n a m a g n e t i c a l l y s t i r r e d , water-jacketed, 70 m L - c a p a c i t y closed glass beaker e q u i p p e d w i t h four p o r t s for a condenser, c o m b i n e d glass a n d c a l o m e l electrode, n i t r o gen i n l e t , a n d s a m p l e w i t h d r a w a l . T h e l a t t e r three were a l l sealed w i t h s e p t a . T h e k i n e t i c r u n s were c a r r i e d o u t b y t r a n s f e r r i n g a n a p p r o p r i a t e a m o u n t of a c c u r a t e l y weighed flavanoid t o the vessel together w i t h 50 m L of p H 8.0 b o r a t e / h y d r o c h l o r i c a c i d ( M e r c k ) buffer. T h e vessel was assembled a n d allowed t o come t o t e m p e r a t u r e (30 ° C m a i n t a i n e d v i a water c i r c u l a t e d f r o m a t h e r m o s t a t e d w a t e r b a t h ) u n d e r a slow s t r e a m of n i t r o g e n . T h e flavanoid caused a d o w n w a r d shift of buffer p H , a n d t h i s was carefully readjusted t o p H 8.0 b y d r o p wise a d d i t i o n of 0 . 1 M s o d i u m h y d r o x i d e . T h e n , the r e a c t i o n was i m m e d i a t e l y i n i t i a t e d b y a d d i t i o n of 0.5 m L of 1% formaldehyde s o l u t i o n , a n d the r e s i d u a l f o r m a l d e h y d e c o n c e n t r a t i o n was measured b y r e m o v a l at t i m e d i n t e r v a l s of 0.5 m L a l i q u o t s , d i l u t i o n of these a l i q u o t s to 5.0 m L w i t h p H 6.0 c i t r a t e - s o d i u m h y d r o x i d e ( M e r c k ) buffer, a n d transfer of two 1.0 m L samples o f the d i l u t e d so l u t i o n t o 8 m L - c a p a c i t y screw-top glass v i a l s e q u i p p e d w i t h teflon-lined l i d s . A 3.0 m L a l i q u o t of a s o l u t i o n of 1 m L of acetylacetone a n d 0.6 m L of g l a c i a l acetic a c i d i n 100 m L of 2 M a m m o n i u m acetate was a d d e d t o each v i a l , w h i c h were


13.

PORTER


177

c a p p e d a n d the contents a g i t a t e d o n a v o r t e x m i x e r ; the v i a l s were t h e n heated at 60 ° C for 40 m i n u t e s . T h e f o r m a l d e h y d e c o n c e n t r a t i o n was t h e n m e a s u r e d s p e c t r o p h o t o m e t r i c a l l y f r o m the i n t e n s i t y of the a b s o r p t i o n b a n d at 412 n m b y reference t o a s t a n d a r d curve (23). T h e t o t a l c o n s u m p t i o n o f f o r m a l d e h y d e for each r e a c t i o n was e s t i m a t e d b y r e m o v i n g t w o 7 m L a l i q u o t s o f the r e a c t i o n m i x t u r e a n d h e a t i n g these at 60 ° C for 2 h o u r s i n screw-top glass v i a l s a n d a n a l y z i n g r e s i d u a l f o r m a l d e h y d e b y the above m e t h o d .


Results and Discussion V i s c o s i t y M e a s u r e m e n t s . T h e viscosity results o b t a i n e d for the t w o p r o c y a n i d i n p o l y m e r s were as follows, the percentage values b e i n g the s o l u t i o n concentrations ( w / w ) :

Polymer

20%

Theobroma cacao Chaenomeles speciosa

4.5 6.4

V i s c o s i t i e s (mPa-s0 40% (NaOH) 40% 192 159 441 897

30% 18.0 45.7

T h e lower viscosities observed for the T. cacao p o l y m e r are e x p e c t e d as its number-average degree of p o l y m e r i z a t i o n is a b o u t h a l f t h a t of the C. speciosa p o l y m e r , the values b e i n g 6.1 a n d 11.8, respectively. T h e degrees o f p o l y m e r i z a t i o n were c a l c u l a t e d b y d i v i d i n g the number-average m o l e c u l a r weights b y the h y d r a t e d m o n o m e r m o l e c u l a r weight as i n d i c a t e d b y m i c r o a n a l y s i s (24). T h e s e results m a y be c o m p a r e d w i t h viscosities o b t a i n e d i n a s i m i l a r w a y f r o m conifer b a r k e x t r a c t s w h i c h , w h i l e heterogeneous, c o n t a i n p o l y m e r i c p r o c y a n i d i n s or m i x e d p o l y m e r i c p r o c y a n i d i n s a n d p r o d e l p h i n i d i n s as t h e i r p r e d o m i n a n t c o m p o n e n t s (2). F o r e x a m p l e , W e i s s m a n (25) r e p o r t e d a v i s c o s i t y of 65 m P a - s for a 3 0 % s o l u t i o n of the water e x t r a c t f r o m Pinus oocarpa b a r k , a n d D i x a n d M a r u t s k y (26) o b t a i n e d a value of 31 m P a - s for a s i m i l a r s o l u t i o n f r o m P t c e a abies b a r k . T h e s e viscosities are s i m i l a r t o those observed for the 3 0 % p r o c y a n i d i n p o l y m e r s o l u t i o n s . T h e y i n d i c a t e t h a t the viscosities of these b a r k e x t r a c t s o l u t i o n s are d o m i n a t e d b y the p r o a n t h o c y a n i d i n s a n d t h a t there is l i t t l e influence f r o m any a c c o m p a n y i n g p o l y s a c c h a r i d e s - a s a l r e a d y suggested b y W e i s s m a n n (25)-m contrast t o w a t t l e e x t r a c t s where g u m s p l a y a n i m p o r t a n t role i n d e t e r m i n i n g s o l u t i o n viscosities ( 7 ) . A y l a (27) r e p o r t e d a v i s c o s i t y of 65 m P a - s for a 4 0 % s o l u t i o n o f the b a r k e x t r a c t f r o m Pinus brutia. T h i s is very m u c h lower t h a n t h a t o b t a i n e d for the T. cacao p r o c y a n i d i n p o l y m e r , even t h o u g h A y l a ' s (27) e s t i m a t e o f 7-8 for t h e number-average degree of p o l y m e r i z a t i o n was a p p a r e n t l y higher t h a n the value of 6.1 o b t a i n e d for the T. cacao p o l y m e r . However, i t has r e c e n t l y been s h o w n t h a t the P. brutia p o l y m e r is a c t u a l l y a p r o c y a n i d i n - 0 - g l u c o s i d e (28). When


178



allowance is m a d e for t h i s , the degree of p o l y m e r i z a t i o n of the P. brutia is reduced t o 4 t o 5, w h i c h p r o b a b l y accounts for the lower viscosity.

polymer

I n c o n t r a s t , Y a z a k i a n d H i l l i s (29) o b t a i n e d a v i s c o s i t y of 8,500 m P a - s for a 4 5 % s o l u t i o n of the aqueous e x t r a c t f r o m Pinus radiata b a r k . T h i s is a l m o s t a n order of m a g n i t u d e higher t h a n t h a t expected o n the basis of the p r o c y a n i d i n p o l y m e r results. V i s c o s i t i e s of the m e t h a n o l - s o l u b l e p o r t i o n a n d the u l t r a f i l t e r e d p o r t i o n s o f t h i s e x t r a c t were 500 a n d 90 m P a - s , respectively. T h e former value is a b o u t t h a t expected for a p r o a n t h o c y a n i d i n p o l y m e r , b u t the l a t t e r i n d i c a t e s t h a t m o s t o f the p o l y m e r has been e x c l u d e d b y the filter, a n d i t f u r t h e r i m p l i e s t h a t m o l e c u l a r sizes of p r o a n t h o c y a n i d i n s based o n u l t r a f i l t r a t i o n measurements are often m i s l e a d i n g . W h e n P. radiata b a r k is e x t r a c t e d b y s u l f i t e - c a r b o n a t e , the s o l u t i o n v i s c o s i ties are m u c h lower. F o r e x a m p l e , W o o (30) r e p o r t e d a v i s c o s i t y of 1,600 m P a - s for a 4 5 % s o l u t i o n o f " T a n n a p h e n , " a c o m m e r c i a l t a n n i n e x t r a c t f r o m P. radi ata b a r k t h a t contains a p p r o x i m a t e l y 7 0 % p r o a n t h o c y a n i d i n s . W h e n e x t r a c t e d w i t h s u l f i t e - c a r b o n a t e , the p r o a n t h o c y a n i d i n s w i l l be p a r t l y d e p o l y m e r i z e d (31), w h i c h w i l l cause a f a l l i n viscosity. W h e t h e r the very h i g h viscosities observed for aqueous e x t r a c t s b y Y a z a k i a n d H i l l i s (29) are due t o the P. radiata p r o a n t h o c y a n i d i n s b e i n g of m u c h higher m o l e c u l a r weight t h a n other conifer t a n n i n s or due t o c o m p l e x a t i o n o f the p r o a n t h o c y a n i d i n s w i t h the p o l y s a c c h a r i d e f r a c t i o n (32) r e m a i n s t o be s h o w n . T h e v i s c o s i t y measurements i n alkaline s o l u t i o n are m o r e difficult t o i n terpret. H e m i n g w a y a n d his colleagues (15,17) have s h o w n t h a t , i n s t r o n g l y alkaline s o l u t i o n s , p r o c y a n i d i n p o l y m e r s are r a p i d l y converted at a m b i e n t t e m peratures t o species where most m o n o m e r s c o n t a i n a rearranged Α-ring, such as s h o w n i n F i g u r e 3. O n t h i s basis, p r o c y a n i d i n s are converted t o chains w i t h a greater degree of r i g i d i t y t h a n the o r i g i n a l p o l y m e r . It is possible t h a t t h i s m a y e x p l a i n the increased v i s c o s i t y (33). H o w e v e r , the p r o c y a n i d i n results are very different f r o m those o b t a i n e d b y W e i s s m a n n (25) for a l k a l i n e e x t r a c t s f r o m Pinus oocarpa b a r k . O n the basis o f 3 0 % w / w s o l u t i o n s at 25 ° C , the w a t e r - s o l u b l e m a t e r i a l was f o u n d t o have a v i s c o s i t y of 65 m P a - s , whereas, the m a t e r i a l soluble i n 1% s o d i u m h y d r o x i d e h a d a v i s c o s i t y of 1,294 m P a - s . I n l i g h t of the results of the c u r r e n t s t u d y , t h i s o b s e r v a t i o n is o n l y e x p l i c a b l e i f the viscosity of the s o d i u m h y d r o x i d e e x t r a c t is due t o n o n t a n n i n components. R e a c t i o n o f P r o c y a n i d i n s w i t h F o r m a l d e h y d e . O u r knowledge of the k i n e t i c s a n d s t o i c h i o m e t r y of the r e a c t i o n of p r o a n t h o c y a n i d i n p o l y m e r s w i t h f o r m a l d e h y d e t o p r o d u c e crosslinked resins is based m a i n l y o n three studies of the r e a c t i o n of m o d e l phenols or c a t e c h i n w i t h f o r m a l d e h y d e (34-36). These studies showed t h a t , at lower t e m p e r a t u r e s a n d p H values between 2 a n d 9, the s t o i c h i o m e t r y of the r e a c t i o n was near e q u i m o l a r i n c a t e c h i n a n d f o r m a l d e h y d e . K i a t g r a j a i et a l . (36) s t u d i e d the k i n e t i c s o f the r e a c t i o n o f c a t e c h i n w i t h f o r m a l d e h y d e over a range of stoichiometries, p H values, a n d t e m p e r a t u r e s , a n d o b t a i n e d the a c t i v a t i o n energies for the r e a c t i o n . T h e y i n t e r p r e t e d t h e i r d a t a i n



13.

PORTER

F i g u r e 3.

179


R e p r e s e n t a t i o n of the general s t r u c t u r e of a p r o c y a n i d i n p o l y m e r

c h a i n t h a t has undergone alkaline rearrangement. R = H , or a c o n t i n u a t i o n of the same t y p e of s t r u c t u r e [after reference

(15)].


180

ADHESIVES F R O M RENEWABLE

RESOURCES


t e r m s of first-order k i n e t i c s , w h i c h was s t a t e d to be observed over the first h a l f of the r e a c t i o n , followed b y a slower rate process. T h i s was e x p l a i n e d b y a s s u m i n g different r e a c t i v i t y of C - 6 a n d C - 8 t o w a r d f o r m a l d e h y d e . T h e s e observations were a l i t t l e s u r p r i s i n g i n v i e w of the fact t h a t M c G r a w a n d H e m i n g w a y (37) h a d observed t h a t e l e c t r o p h i l i c s u b s t i t u t i o n at C - 6 or C - 8 lacked regioselectivity for a s m a l l a t t a c k i n g s p e c i e s - s u c h as f o r m a l d e h y d e . T h e r e a c t i o n k i n e t i c s were reinvestigated, therefore, for c a t e c h i n , e p i c a t e c h i n , a n d t w o p r o c y a n i d i n s . T h e progress of the r e a c t i o n was followed b y m o n i t o r i n g the decrease i n f o r m a l d e h y d e c o n c e n t r a t i o n w i t h t i m e . P r e v i o u s studies used the h y d r o x y l a m i n e h y d r o c h l o r i d e m e t h o d o f a n a l y s i s (34-36), b u t t h i s was avoided i n the c u r r e n t s t u d y as i t requires tedious p H t i t r a t i o n s . I n s t e a d , a c o l o r i m e t r i c m e t h o d was used t h a t was first developed b y N a s h (23), i n v o l v i n g f o r m a t i o n of 3 , 5 - d i a c e t y l 1 , 4 - d i h y d r o l u t i d i n e , b y r e a c t i o n of f o r m a l d e h y d e w i t h a m m o n i a a n d acetylacetone at n e u t r a l p H . T h e cyclic p r o d u c t absorbs at 412 n m w i t h a m o l a r e x t i n c t i o n coefficient of 8,000 (23). O t h e r c o l o r i m e t r i c m e t h o d s cannot be used as they a l l involve very s t r o n g l y a c i d i c or basic m e d i a (22), w h i c h w o u l d force the phenol-formaldehyde reaction to completion. T h e reactions were c a r r i e d out i n a b o r a t e - h y d r o c h l o r i c a c i d buffer at p H 8.0, the p H b e i n g readjusted, i f necessary, b y a d d i t i o n of 1 M s o d i u m h y d r o x i d e s o l u t i o n after the a d d i t i o n of p r o c y a n i d i n . T h e above p H was chosen t o o b t a i n a convenient r e a c t i o n rate w h i l e m i n i m i z i n g catechinic a c i d f o r m a t i o n (36). It was f o u n d t h a t , i f water r a t h e r t h a n buffer was used for the r e a c t i o n , a n d the p H was s i m p l y adjusted t o p H 8.0 as described b y K i a t g r a j a i et a l . (36), the observed p H was u n s t a b l e a n d subject to d r i f t . T h e e x p e r i m e n t a l values for the k i n e t i c s a n d s t o i c h i o m e t r y of f o r m a l d e h y d e c o n s u m p t i o n are s u m m a r i z e d i n T a b l e I. T h e e x p e r i m e n t a l values for r e a c t i o n s t o i c h i o m e t r y i l l u s t r a t e t h a t , at least u p to a h e x a m e r , p r o c y a n i d i n s f o r m c o m p l e t e l y c r o s s l i n k e d p r o d u c t s i n d i l u t e s o l u t i o n , whereas, very l i t t l e c r o s s l i n k i n g occurs for the C. speciosa p r o c y a n i d i n p o l y m e r . I f the values of X for the hexamer a n d the T. cacao p o l y m e r are c o m p a r e d , i t m a y be seen t h a t , a l t h o u g h they have s i m i l a r degrees of p o l y m e r i z a t i o n , the h e x a m e r obeys the crosslinked m o d e l w e l l , whereas, the p o l y m e r consumes m o r e f o r m a l d e h y d e t h a n p r e d i c t e d by t h i s m o d e l . T h e difference is a p p a r e n t l y due t o the presence of longer c h a i n l e n g t h species i n the polydisperse (24) T. cacao p r o c y a n i d i n p o l y m e r ( w h i c h contains oligomers f r o m t r i m e r s to greater t h a n heptamers as i n d i c a t e d by c h r o m a t o g r a p h y - F A B mass s p e c t r o m e t r y ) . T h i s result i m p l i e s t h a t the a b i l i t y of p r o c y a n i d i n oligomers to f o r m s u b s t a n t i a l l y crosslinked p r o d u c t s must s t a r t to f a i l at somewhere a r o u n d a c h a i n l e n g t h o f 8 or 9 u n i t s . T h e s e results s u p p o r t the thesis t h a t t o achieve h i g h degrees of c r o s s l i n k i n g i n p r o a n t h o c y a n i d i n chains, b r i d g i n g species larger t h a n f o r m a l d e h y d e m u s t be used to s p a n the increased i n t e r m o l e c u l a r distances due to unfavorable steric d i s p o s i t i o n s ( 7 ) . However, the above results show t h a t f o r m a l d e h y d e is able to achieve s u b s t a n t i a l c r o s s l i n k i n g at a s u r p r i s i n g l y h i g h degree of p o l y m e r i z a t i o n . These results also e x p l a i n the success of u t i l i z i n g P. brutia b a r k extracts for w o o d


13.


PORTER

181

T a b l e I. S t o i c h i o m e t r y a n d K i n e t i c s of the R e a c t i o n B e t w e e n F o r m a l d e h y d e a n d Some F l a v a n - 3 - o l s ( p H 8.0 a n d 30 ° C )


(a) S t o i c h i o m e t r y Compound

X (observed)

Epicatechin Dimer Tetramer Pentamer Hexamer Theobroma cacao p o l y m e r Chaenomeles speciosa p o l y m e r

1.0 0.79 0.70 0.54 0.59 0.73 0.96

3

4

5

5

(b) K i n e t i c s

1

X

(predicted) 1.0 m o d e l 0.5 m o d e l 2

2.00 1.50

1.0 0.75 0.62 0.60 0.58 0.58 0.54

1.25 1.20 1.17 1.17 1.08

6

Compound

kziM-^ec" )

Catechin Epicatechin Dimer Chaenomeles

0.23 0.22

1

0.095 0.036

3

speciosa

polymer

X = moles of f o r m a l d e h y d e consumed per e p i c a t e c h i n u n i t . P r e d i c t e d values of X a s s u m i n g either 0.5 or 1.0 molecules of f o r m a l d e h y d e reacts w i t h each Α-ring site. I f the 0.5 m o d e l is o b e y e d , t h e n a l l h y d r o x y m e t h y lene groups w i l l have c r o s s l i n k e d . Epicatechin-(4/3—•8)-epicatechin. [Epicatechin-(4/?—•δ)]3-epicatechin. U n f r a c t i o n a t e d m i x t u r e s of e p i c a t e c h i n p e n t a m e r s or hexamers o b t a i n e d chrom a t o g r a p h i c a l l y f r o m the T. cacao p o l y m e r . C o n c e n t r a t i o n of f o r m a l d e h y d e was 3.6 χ 1 0 ~ m o l a r a n d the r a t i o of f o r m a l d e h y d e per Α-ring r e a c t i o n site was 0.5 i n each r u n . 1

2

3

4

5

6

3


182



adhesives (27) since, w i t h a degree of p o l y m e r i z a t i o n o f 4 t o 5, they are o b v i o u s l y well w i t h i n the established range for s u b s t a n t i a l c r o s s l i n k i n g . A l t e r n a t i v e l y , a p r o c y a n i d i n p o l y m e r w i t h a h i g h degree of p o l y m e r i z a t i o n m u s t be cleaved w i t h reagents s u c h as sulfite o f resorcinol to reduce the average c h a i n lengths p r i o r t o r e a c t i o n w i t h f o r m a l d e h y d e (38,39). K i n e t i c s . D a t a were collected for c a t e c h i n , e p i c a t e c h i n , epicatechin-(4/?—>8)e p i c a t e c h i n a n d the C. speciosa p o l y m e r . T h e r a t i o o f react ants was chosen so t h a t the c o n c e n t r a t i o n o f f o r m a l d e h y d e a n d r e a c t i o n sites was e q u i m o l a r , a s s u m i n g complete c r o s s l i n k i n g (i.e., 0.5 molecule of f o r m a l d e h y d e per react i o n site converts t o one molecule p e r r e a c t i o n site after c r o s s l i n k i n g , as each f o r m a l d e h y d e m o l e c u l e reacts w i t h t w o sites). T h e c o n c e n t r a t i o n o f f o r m a l d e hyde was kept the same for each c o m p o u n d so as t o p r o v i d e a consistent basis for c o m p a r i s o n . P l o t s of ( a b s o r b a n c e ) versus t i m e were l i n e a r for greater t h a n 9 0 % of the r e a c t i o n for each s y s t e m s t u d i e d . T h e s e observations m a y be i n t e r p r e t e d i n t e r m s o f the r e a c t i o n o b e y i n g second-order k i n e t i c s where the reactants are e q u i m o l a r (40). Second-order or m o r e c o m p l e x k i n e t i c s are t y p i c a l of p h e n o l f o r m a l d e h y d e condensations under alkaline c o n d i t i o n s ( 7 ) . A s expected, k2 for c a t e c h i n a n d e p i c a t e c h i n are the same, since the react i o n r a t e w o u l d be expected t o be independent o f C - r i n g stereochemistry. T h e c o m p a r a t i v e l y s m a l l e r rate constants observed for the t w o p r o c y a n i d i n s c o u l d be e x p l a i n e d b y the fact t h a t t h e y possess fewer r e a c t i o n sites p e r m o n o m e r u n i t t h a n c a t e c h i n or e p i c a t e c h i n a n d also b y steric effects. - 1

Conclusions T h e current s t u d y confirms t h a t f o r m a l d e h y d e is too s m a l l a b r i d g i n g g r o u p to s u b s t a n t i a l l y crosslink extended p r o a n t h o c y a n i d i n c h a i n s , extensive c r o s s l i n k i n g a p p a r e n t l y s t a r t i n g t o f a i l at c h a i n lengths of greater t h a n 6 u n i t s . T h e s t u d y also defines the v i s c o s i t y range c h a r a c t e r i s t i c o f p r o c y a n i d i n p o l y m e r s a n d i l l u s t r a t e s t h a t the v i s c o s i t y o f aqueous extracts f r o m some conifer b a r k s is d o m i n a t e d b y p r o a n t h o c y a n i d i n s , whereas, h i g h viscosities c h a r a c t e r i s t i c of n o n t a n n i n s or t a n n i n - p o l y s a c c h a r i d e complexes are c h a r a c t e r i s t i c o f the aqueous e x t r a c t s i n other cases. Literature Cited

1. Hemingway, R. W . In Natural Products Extraneous to the Lignocellulosic Cell Wall of Woody Plants; Rowe, J . W . , E d . ; Springer: New York, Chapter 6.6 (in press). 2. Porter, L . J. In Natural Products Extraneous to the Lignocellulosic Cell Wall of Woody Plants; Rowe, J . W . , E d . ; Springer: New York, Chapter 6.7 (in press). 3. Roux, D . G . ; Ferreira, D . ; Botha, J. J. J. Agric. Food Chem. 1980, 28, 4. Roux, D . G . ; Ferreira, D . Pure Appl. Chem. 1982, 54, 2465.


216.

13.

PORTER


183

5. Viviers, P. M.; Kolodziej, H.; Young, D. Α.; Ferreira, D.; Roux, D. G. J. Chem. Soc., Perkin Trans. 1 1983, 2555. 6. Porter, L. J.; Hemingway, R. W. In Natural Products Extraneous to the Lignocellulosic Cell Wall of Woody Plants; Rowe, J. W., Ed.; Springer: New York, Chapter 9.3 (in press). 7. Pizzi, A. In Wood Adhesives, Chemistry and Technology; Pizzi, Α., Ed.; Marcel Dekker: New York, 1983; Chapter 4, pp. 177-246. 8. Roux, D. G. Phytochemistry 1972, 11, 1 2 1 9 . 9. Santappa, M.; Sundara Rao, V. S. J. Sci. Ind. Res. 1982, 41, 7 0 5 .


10. Herrick, F. W. J. Agric. Food Chem. 1980, 28, 2 2 8 . 11. Jenkin, D. J. J. Adhesion 1984, 16, 2 9 9 . 12. Thompson, R. S.; Jacques, D.; Haslam, E.; Tanner, R.J.N. J. Chem. Soc., Perkin Trans. 1. 1972, 1 3 8 7 . 13. Czochanska, Z.; Foo, L. Y.; Newman, R. H.; Porter, L. J. J. Chem.Soc.,Perkin Trans. 1. 1 9 8 0 , 2 2 7 8 .

14. Hemingway, R. W.; Laks, P. E.; McGraw, G. W.; Kreibich, R. E . Proc. F.P.R. International-Achievements and the Future 1985, 16, 17-11. 15. Laks, P. E.; Hemingway, R. W. Holzforschung 1987, i n press. 16. Laks, P. E.; Hemingway, R. W. J. Chem.Soc.,Perkin Trans. 1 1987, 4 6 5 . 17. Laks, P. E.; Hemingway, R. W.; Conner, A. H. J.Chem.,Soc.,Perkin Trans. 1. 1 9 8 7 , 1875.

18. Hemingway, R. W.; Foo, L. Y.; Porter, L. J. J. Chem. Soc.,Perkin Trans. 1. 1 9 8 2 , 1209.

19. Morimoto, S.; Nonaka, G.; Nishioka, I. Chem. Pharm. Bull. 1986, 34, 6 3 3 . 20. Derdelinckx, G.; Jerumanis, J. J. Chromatog. 1984, 285, 2 3 1 . 21. Karchesy, J. J.; Hemingway, R. W.; Foo, L. Y.; Barofsky, E.; Barofsky, D. F. Anal. Chem. 1986, 58, 2563. 22. Pickard, A. D.; Clark, E. R. Talanta 1984, 31, 7 6 3 . 23. Nash, T. Biochem. J. 1953, 55, 4 1 6 . 24. Porter, L. J. Aust. J. Chem. 1986, 39, 557. 25. Weissmann, G. Int. J. Adhesion Adhesives. 1983, 31. 26. Dix, B.; Marutzky, R. J. Appl. Pol. Sci.: Appl. Pol. Symp. 1984, 40, 9 1 . 27. Ayla, C. J. Appl. Pol. Sci.: Appl. Pol. Symp. 1984, 40, 6 9 . 28. Porter, L. J.; Foo, L. Y.; Furneaux, R. H. Phytochemistry 1985, 24, 5 6 7 . 29. Yazaki, Y.; Hillis, W. E. Holzforschung 1980, 34, 1 2 5 . 30. Woo, J. K. Paper presented at the International Symposium on Adhesion and Adhesives for Structural Materials, 28-30 September 1982, Washington State University, Pullman, Washington. 31. Foo, L. Y.; McGraw, G. W.; Hemingway, R. W. J. Chem. Soc. Chem. Commun. 1983, 672.

32. Rahman, M. D.; Richards, G. N. Carbohydrate Res. 1987, (in press). 33. Vollmert, B. In Polymer Chemistry; Springer: New York, 1973, P 3 7 8 . 34. Hillis, W. E.; Urbach, G. J. Appl. Chem. 1957, 9, 4 7 4 .


184


35. Hemingway, R. W.; McGraw, G. W. J. Liquid Chromatog. 1978, 1, 163. 36. Kiatgrajai, P.; Wellons, J. D.; Gollob, L.; White, J. D. J. Org. Chem. 1982, 47, 2913. 37. McGraw, G. W.; Hemingway, R. W. J. Chem.Soc.,Perkin Trans. 1. 1982, 973. 38. Kreibich, R. E.; Hemingway, R. W. Forest Prod. J. 1987,37, 2, 43. 39. Kreibich, R. E.; Hemingway, R. W. Forest Prod. J. 1985,35, 3, 23. 40. Gould, E . S. In Mechanism and Structure in Organic 1959, p. 161.

Chemistry; Holt: New York,


R E C E I V E D June 2, 1988


Adhesives from Renewable Resources - American Chemical Society

Recommend Documents