Adhesives from Renewable Resources - American Chemical Society

Downloaded by NORTH CAROLINA STATE UNIV on May 3, 2015 | http://pubs.acs.org Publication Date: December 31, 1989 | doi: 10.1021/bk-1989-0385.ch026

Chapter 26 A Glucose, Urea, and Phenol-Based Adhesive for Bonding Wood A l f r e d W. Christiansen Forest Products Laboratory Forest Service U . S. Department of Agriculture Madison, WI 53705

Earlier studies have reported on adhesive development with a carbohydrate/urea/phenol/formaldehyde resin system. Building on those results, this study shows how the adhesive shear strength of bonded wood panels is decreased by using much lower levels of urea in the synthesis. Weight loss studies reveal that urea itself plays a role in dehydrating sugars during heating. Products of resin syntheses and model compound reactions were analyzed by chromatography and C-NMR to clarify the sequence of reactions leading to polymeric resins. 13

Since t h e o i l shortages o f the 1970s, there has been a s u s t a i n e d search for m a t e rials t o replace t h e p e t r o l e u m - b a s e d resins used as d u r a b l e adhesives for e x t e r i o r w o o d p r o d u c t s . S u c h alternatives are considered i m p o r t a n t , because supplies of p e t r o c h e m i c a l s for use i n the w o o d i n d u s t r y c o u l d a g a i n become u n d e p e n d able. Ideally, t h e source o f m a t e r i a l for a n adhesive w o u l d b e r e a d i l y a v a i l a b l e , p o s s i b l y f r o m m a t e r i a l s already f o u n d near o r used b y w o o d processing p l a n t s , for e x a m p l e , a g r i c u l t u r a l o r w o o d - b a s e d renewable resources. T h e p u r p o s e o f t h i s i n v e s t i g a t i o n was t o explore t h e use o f c a r b o h y d r a t e s as c o n s t i t u e n t s i n water-resistant adhesives. P h e n o l - f o r m a l d e h y d e t y p e p o l y m e r s h a d been t h e o n l y e x t e r i o r - d u r a b l e a d hesives for w o o d b o n d i n g , u n t i l t h e recent l i m i t e d use o f isocyanates. B o t h systems are p e t r o c h e m i c a l - b a s e d . Several researchers s u b s t i t u t e d c a r b o h y d r a t e s for p a r t o f phenolic adhesives (1-4), p r o d u c i n g s o l i d , fusible novolak resins. Recently, r e a c t i o n o f c a r b o h y d r a t e a c i d - d e g r a d a t i o n p r o d u c t s w i t h p h e n o l a n d f o r m a l d e h y d e has p r o d u c e d l i q u i d resols ( 5 ) . G i b b o n s a n d W o n d o l o w s k i (6,7) replaced a considerable a m o u n t o f p h e n o l w i t h c a r b o h y d r a t e a n d u r e a t o p r o This chapter not subject to U.S. copyright Published 1989 American Chemical Society

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

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duce s o l i d , fusible resins crosslinkable w i t h h e x a m e t h y l e n e t e t r a m i n e . I n a subsequent p a t e n t (8), a few examples show how t o synthesize l i q u i d resol resins a n d cite i n i t i a l results s h o w i n g w o o d b o n d s of p r o m i s i n g d u r a b i l i t y . T h e procedure consists o f r e a c t i n g the c a r b o h y d r a t e w i t h u r e a or d i a m i n e s i n a c i d i c s o l u t i o n ( w i t h or w i t h o u t p h e n o l present), n e u t r a l i z i n g , a n d r e a c t i n g w i t h f o r m a l i n . T h e adhesive is f o r m u l a t e d by a d d i n g caustic, w a t e r , a n d fillers. E x p l o r i n g t h a t s y s t e m , t h i s i n v e s t i g a t o r (9,10) showed t h a t the c a r b o h y d r a t e , essentially i n the r e d u c i n g m o n o s a c c h a r i d e f o r m , reacted u n d e r a c i d c a t a l ysis o n l y w i t h u r e a . A n y p h e n o l i n the m i x t u r e does not react u n t i l m i x e d w i t h f o r m a l d e h y d e a n d heated f u r t h e r . B o t h hexose a n d pentose sugars w o r k e d i n the s y s t e m , p r o v i d i n g b o n d strengths t h a t were the e q u a l o f neat p h e n o l i c resins. D u r a b i l i t y after 2 h o u r s b o i l i n g was essentially equivalent t o t h a t o f p h e n o l i c resins, b u t w o o d f a i l u r e readings were generally m u c h lower for the t r i a l resins. T h e b o n d i n g t e m p e r a t u r e s for the c a r b o h y d r a t e - c o n t a i n i n g resins were higher t h a n for the p h e n o l i c resins b y 10 t o 20 ° C , d e p e n d i n g o n the s a c c h a r i d e . C o n t r a r y t o a n earlier h y p o t h e s i s (6,8), t h i s s t u d y f o u n d t h a t t h e r e a c t i o n between the saccharide a n d u r e a showed no signs o f p r o c e e d i n g t h r o u g h a f u r f u r y l t y p e s t r u c t u r e , b u t first p r o d u c e d a glycose ureide i n t e r m e d i a t e , w h i c h f u r t h e r reacted t o f o r m a m u l t i t u d e o f p r o d u c t s h a v i n g a l i p h a t i c character. T h i s chapter r e p o r t s w o r k o n t w o aspects o f t h i s adhesive s y s t e m : 1) tests o n the s t r e n g t h o f panels b o n d e d w i t h p h e n o l / c a r b o h y d r a t e / u r e a / f o r m a l d e h y d e ( P / C / U / F ) adhesive c o m p o s i t i o n s outside the ranges p r e v i o u s l y r e p o r t e d (9,10) a n d 2) a n a l y s i s o f c h e m i c a l reactions i n t h i s resin s y s t e m . Experimental

Methodology

M a t e r i a l s . T h e f o l l o w i n g c h e m i c a l s were used i n the syntheses: a - D - g l u c o s e ( 2 % /3-anomer); D - x y l o s e ( a p p r o x i m a t e l y 9 8 % ) ; p h e n o l (reagent grade); u r e a ( m p 132.1-132.8 ° C ) ; f o r m a l d e h y d e ( 3 7 % w i t h 5 % m e t h a n o l ) ; x y l i t o l ; c y c l o h e x a n o l ( m . p . 22-23 ° C ) ; t e t r a h y d r o - 2 H - p y r a n - 2 - o l (tech. grade, b p 115-122 ° C / 1 5 m m ) ; 2,4-dimethylphenol (97%); 1,1-dimethylurea; and furfural (99%, a n a l y t i c a l reagent). D e h y d r a t e d sulfolane ( t e t r a m e t h y l e n e sulfone), a h i g h - b o i l i n g (bp 285 ° C ) p o l a r c o m p o u n d i n e r t t o most hot acids a n d bases, was often a d d e d t o syntheses as a reference c o m p o u n d for C - N M R a n d h i g h - p e r f o r m a n c e l i q u i d chromatography ( H P L C ) . 1 3

T h e p h e n o l - f o r m a l d e h y d e resin used as a c o n t r o l adhesive was a c o m m e r c i a l resin ( c o n t r o l P ) characterized p r e v i o u s l y (9). A second p h e n o l i c resin ( c o n t r o l C ) , used once, is r e p o r t e d to have 4 0 . 1 % n o n v o l a t i l e s , a v i s c o s i t y o f 0.42 P a s , a n d a specific g r a v i t y of 1.180 at 25 ° C . Its measured p H was 11. F o r use, i t was m i x e d w i t h 1 5 % w a l n u t shell flour. T h e w o o d adherends were clear, flat-grained, r o t a r y - p e e l e d yellow b i r c h veneer w i t h m o i s t u r e content e q u i l i b r a t e d at 27 ° C a n d 3 0 % relative h u m i d i t y ( R H ) . T h e pieces were 3.2 m m t h i c k a n d 150 m m b y 150 m m i n a r e a .


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E x p e r i m e n t a l A p p r o a c h . T h e first two e x p e r i m e n t s e x a m i n e d the effects o n s t r e n g t h o f r e d u c i n g the adhesive's u r e a or caustic contents. T h e rest o f the e x p e r i m e n t s were r u n t o c l a r i f y the reactions i n v o l v e d i n p r o d u c i n g the r e s i n . A m o n g these, t w o e x p e r i m e n t s d e t e r m i n e d weight losses o n h e a t i n g c a r b o h y drates, w i t h or w i t h o u t u r e a a n d a c i d c a t a l y s t present. I n the final e x p e r i m e n t s , samples t a k e n f r o m resin a n d m o d e l c o m p o u n d syntheses were e x a m i n e d b y H P L C a n d C - N M R t o find w h a t p r o d u c t s were f o r m e d a n d i n w h a t sequence.


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S h e a r S t r e n g t h T e s t s o f R e s i n - B o n d e d P a n e l s . T h e s t a n d a r d procedures for s y n t h e s i z i n g the P / C / U / F types of resins, f o r m u l a t i n g the resins i n t o fi n a l adhesives, a n d p r e p a r i n g a n d t e s t i n g specimens were d e s c r i b e d p r e v i o u s l y (9,10). T w o e x p e r i m e n t s t h a t differed f r o m the s t a n d a r d follow. I n t h e first e x p e r i m e n t , resin was m a d e w i t h a P / C / U / F m o l a r r a t i o o f 1:1:0.125:2 t o evaluate r e d u c i n g the a m o u n t o f u r e a by h a l f o f the p r e v i o u s m i n i m u m ( C / U = 1:O.25)(0). P a n e l s were pressed at 150 a n d 160 ° C for the t r i a l adhesive, a n d 140 a n d 150 ° C for the c o n t r o l Ρ adhesive. T h e other e x p e r i m e n t tested the effect o n b o n d s t r e n g t h o f r e d u c i n g the levels o f s o d i u m h y d r o x i d e a n d s o d i u m c a r b o n a t e a d d e d d u r i n g adhesive f o r m u l a t i o n . T h i s e x p e r i m e n t used a resin w i t h P / C / U / F m o l a r r a t i o s o f 1:1:0.25:2, w h i c h gave g o o d strengths i n the past. T h e resin was d i v i d e d i n t o four p o r t i o n s t o be f o r m u l a t e d w i t h different a m o u n t s of the a l k a l i n e c o m p o n e n t s : the n o r m a l level (6.6 w t % s o d i u m h y d r o x i d e a n d 2.2 w t % s o d i u m c a r b o n a t e , based o n l i q u i d resin w e i g h t ) , t w o - t h i r d s o f n o r m a l , o n e - t h i r d of n o r m a l , a n d none. I n each case, 8 t o 10 m L o f water was a d d e d t o b r i n g the v i s c o s i t y d o w n t o desirable levels. F o r each o f the four f o r m u l a t e d adhesives, two p a r a l l e l - l a m i n a t e d veneer panels were b o n d e d at 160 ° C . T w o p h e n o l i c resins, c o n t r o l Ρ a n d c o n t r o l C , were used as c o n t r o l resins; c o n t r o l panels were b o n d e d at 150 ° C . W e i g h t - L o s s R a t e E x p e r i m e n t s . A large a m o u n t of condensate is generated d u r i n g the a c i d - c a t a l y z e d r e a c t i o n between u r e a a n d c a r b o h y d r a t e . T h e relative m o l a r r a t i o o f u r e a t o c a r b o h y d r a t e h a d p r e v i o u s l y been v a r i e d between 0.50:1 a n d 0.25:1 (9,10). T h e effect o f v a r i o u s u r e a - c a r b o h y d r a t e r a t i o s o n the weight loss o f c a r b o h y d r a t e at elevated t e m p e r a t u r e was d e t e r m i n e d . T w o g r a m s o f either glucose, x y l o s e , three r a t i o s o f glucose a n d u r e a m i x t u r e s (1:0.5, at t w o s l i g h t l y different i n i t i a l weights; 1:0.25; a n d 1:0.125), or a x y l o s e a n d u r e a m i x t u r e (1:0.5) were p u t i n t o separate w e i g h i n g b o t t l e s , each w i t h 3 m L o f w a t e r . E a c h b o t t l e was p a r t i a l l y i m m e r s e d i n a n u l t r a s o n i c b a t h for 1 m i n u t e t o h a s t e n dissolution. F o r the d r y i n g stage o f t h i s e x p e r i m e n t , the w e i g h i n g b o t t l e s , g a t h e r e d i n light a l u m i n u m t r a y s , were p u t i n t o a n oven at 108 ° C . A t i n t e r v a l s , the s a m ples were r e t r i e v e d f r o m the oven, cooled i n a desiccator, closed, a n d weighed for r e s i d u a l weight. T h e y were t h e n reopened a n d p u t back i n t o the oven for a d d i t i o n a l d r y i n g . T h e objective was to have t h e m a l l come t o a constant d r y weight before the a c i d - c a t a l y z e d r e a c t i o n process was s t a r t e d . A f t e r 18 h o u r s , w h e n the glucose s a m p l e finally came t o i t s o r i g i n a l d r y weight, s u l f u r i c a c i d (40 m e q



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per m o l e c a r b o h y d r a t e ) was a d d e d t o each b o t t l e t o b e g i n the a c i d - c a t a l y z e d stage o f the process. T h e b o t t l e s were reweighed a n d p u t i n t o the oven, now at 120 ° C . A t i n t e r v a l s , the bottles were reweighed as before. A f t e r 30 h o u r s , the weights were constant w i t h i n 0.01 g r a m (5 percent), a n d the e x p e r i m e n t was ended. A s a reference p o i n t for u r e a weight losses, three 0 . 5 - g r a m samples o f u r e a were heated at 108 ° C for 6 hours after b e i n g m i x e d w i t h one o f three levels o f s u l f u r i c a c i d : no a c i d , 0.66 m e q , a n d 2.13 m e q . T h e t w o h i g h e r a c i d levels represent a b o u t w h a t w o u l d be seen above i n c a r b o h y d r a t e / u r e a m o l a r r a t i o s of 1:0.125 a n d 1:0.5 respectively. E a c h w e i g h i n g b o t t l e c o n t a i n e d 3 m L o f water, i n c l u d i n g t h a t i n the a c i d . T o d e t e r m i n e the i m p o r t a n c e of the g l y c o s i d i c h e m i a c e t a l g r o u p t o the d e h y d r a t i o n , a p o l y o l a n a l o g of a glycose was used. X y l i t o l c o n t a i n s the same n u m b e r o f h y d r o x y l groups as glucose b u t no a l d e h y d e / h e m i a c e t a l g r o u p . T h e effects of u r e a o n the weight loss rate o f x y l i t o l versus glucose was d e t e r m i n e d . B o t t l e s of carbohydrate plus acid, carbohydrate plus urea, and carbohydrate plus urea a n d a c i d were used, f o r m u l a t e d as described i n the p r e v i o u s e x p e r i m e n t . M o l a r r a t i o s o f c a r b o h y d r a t e t o u r e a were a l l 1:0.5. T h r e e m L o f d i s t i l l e d w a t e r was a d d e d t o dissolve the ingredients where no a c i d c a t a l y s t was used, a n d a p p r o p r i a t e l y less where the a c i d w o u l d m a k e u p the difference i n f l u i d v o l u m e . E l a p s e d h e a t i n g periods i n the 108 ° C ovens were 2, 3, 4, 6, 8, 2 1 , a n d 28 h o u r s . A n a l y s i s o f R e a c t i o n P r o d u c t s . T h e m a j o r reactions i n the a c i d - c a t a l y z e d state were f o u n d (9) t o be those between u r e a a n d the c a r b o h y d r a t e . P h e n o l d i d not p a r t i c i p a t e as a r e a c t a n t . T o follow the sequence o f r e a c t i o n steps, samples were t a k e n f r o m a g l u c o s e / u r e a / s u l f o l a n e m i x t u r e ( m o l a r r a t i o s of 2:1:1) at v a r ious t i m e s for a n a l y s i s b y C - N M R spectroscopy a n d H P L C . S u l f o l a n e acted as the d i s p e r s i n g m e d i u m a n d a reference for c o n c e n t r a t i o n changes. A c i d i c s a m ples were n e u t r a l i z e d t o prevent further r e a c t i o n , here a n d i n l a t e r e x p e r i m e n t s (except the l a s t ) . 1 3

T o determine whether x y l o s e p a r a l l e l s the b e h a v i o r o f glucose i n r e a c t i n g t o f o r m ureides a n d other p r o d u c t s , analyses were done o n samples t a k e n f r o m a p r e v i o u s synthesis. T h a t synthesis, u s i n g x y l o s e a n d P / C / U / F m o l a r r a t i o s of 1:1:0.5:2.5, h a d y i e l d e d a s t r o n g resin ( e x p e r i m e n t 16 i n references 9,10). T h e r e a c t i o n o f f o r m a l d e h y d e w i t h a m i x t u r e was i n v e s t i g a t e d b y C-NMR analyses o f samples t a k e n f r o m a synthesis w i t h P / C / U / F m o l a r r a t i o s o f 0.75:1:0.5:2.5. T h e c a r b o h y d r a t e was glucose. T h e s t a b i l i t y of the f u r a n r i n g i n the e n v i r o n m e n t o f a c i d - c a t a l y z e d m i x tures was i n v e s t i g a t e d by a n e x p e r i m e n t where 1 m o l e of f u r f u r a l was a d d e d t o a m i x t u r e c o n t a i n i n g 0.67 m o l e 1 , 1 - d i m e t h y l u r e a ( D M U ) , 1.33 moles 2,4d i m e t h y l p h e n o l , a n d a c i d c a t a l y s t . T h e h i n d e r e d u r e a a n d p h e n o l were chosen t o l i m i t p r o d u c t s t o s i m p l e , s m a l l c o m p o u n d s . T o l i m i t losses o f the s o m e w h a t v o l a t i l e f u r f u r a l (bp 162 ° C ) over the 3-hour r e a c t i o n t i m e , the r e a c t i o n flask was heated b y a s t e a m b a t h , u s i n g v a p o r t e m p e r a t u r e s (36 t o 69 ° C ) m u c h lower t h a n i n previous e x p e r i m e n t s . 1 3


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T o test w h e t h e r u r e a c o u l d condense w i t h a s i m p l e secondary a l c o h o l g r o u p u n d e r a c i d - c a t a l y s i s c o n d i t i o n s , c y c l o h e x a n o l , a s i m p l e h i g h - b o i l i n g (bp 161 ° C ) a l c o h o l , was s u b s t i t u t e d for c a r b o h y d r a t e i n a r e a c t i o n flask w i t h u r e a (1:0.5 m o l e r a t i o ) a n d a c i d . T h e m i x t u r e was heated at 65 to 122 ° C for 2.5 h o u r s . A l t e r n a t e l y , the i m p o r t a n c e o f the h e m i a c e t a l group c a n be s t u d i e d b y u s i n g a c o m p o u n d t h a t is a n a n a l o g for a monosaccharide w i t h o u t the rest o f the h y d r o x y l groups. T e t r a h y d r o - 2 H - p y r a n - 2 - o l ( T H P ) is such a n a n a l o g for x y l o s e . T H P was reacted w i t h a n e q u i m o l a r a m o u n t of D M U , i n the absence of phe n o l , t o follow the course o f the h e m i a c e t a l - u r e a reactions. T h e a c i d - c a t a l y z e d m i x t u r e was heated between 81 a n d 116 ° C over a 2-hour p e r i o d . S a m p l e s were frozen t o prevent f u r t h e r r e a c t i o n p r i o r t o a n a l y s i s . A n a l y t i c a l M e t h o d s . S a m p l e s were a n a l y z e d b y H P L C u s i n g one of t w o types o f c o l u m n : a B i o - R a d H P X 8 7 H + (acid) c o l u m n or a B i o - R a d M i c r o - G u a r d cation-Η c a r t r i d g e , w i t h 0 . 0 1 5 N p h o s p h o r i c a c i d eluant a n d a n U V absorbance detector. T h e cation-Η cartridges were used t o get e l u t i o n o f s i m p l e u r e a c o m p o u n d s i n less t h a n 15 m i n u t e s , c o m p a r e d t o over 2 hours o n the acids c o l u m n . S a m p l e s were filtered (0.45 m i c r o n pores) before i n j e c t i o n . C - N M R s p e c t r a were collected o n a B r u k e r W M - 2 5 0 F T - N M R s p e c t r o m e ter (62.89 M H z ) at 30 or 37 ° C . N o r m a l s p e c t r a were o b t a i n e d w i t h b r o a d b a n d d e c o u p l i n g . A q u a n t i t a t i v e C - N M R s p e c t r u m was t a k e n u s i n g a n inversegated pulse sequence w i t h a 45° angle, 0.27-s a c q u i s i t i o n t i m e , 30-s delay t i m e , 91 scans, a n d a s p e c t r a l w i d t h o f 30 χ 1 0 H z . * H - N M R s p e c t r a were collected at 250.13 M H z , u s i n g a 45° pulse a n d a 2-second delay. F o r m o s t samples for w h i c h H - N M R s p e c t r a were t a k e n , a n a d d i t i o n a l s p e c t r u m was r u n after D2O was a d d e d t o i d e n t i f y exchangeable h y d r o g e n a t o m s . T h e c h e m i c a l shifts for a l l N M R s p e c t r a were measured relative t o i n t e r n a l t e t r a m e t h y l s i l a n e . D M S O - d e was the solvent. 1 3

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3

1

Results and Discussion S h e a r S t r e n g t h T e s t s o f A d h e s i v e B o n d e d P a n e l s . Results from previous work (9,10) showed t h a t the m o s t water-resistant b o n d s were f o r m e d w h e n the p h e n o l - t o - c a r b o h y d r a t e m o l e r a t i o was at least 1:1 a n d the f o r m a l d e h y d e - t o p h e n o l m o l e r a t i o was at least 1:0.5, whereas, the c a r b o h y d r a t e - t o - u r e a m o l e r a t i o r a n g e d f r o m 1:0.5 t o 1:0.25. F r o m those a c i d - c a t a l y z e d r e a c t i o n m i x t u r e s , i t was necessary t o d r i v e off at least 2 moles o f p r o d u c t water per m o l e o f car b o h y d r a t e d u r i n g resin f o r m a t i o n . S i x c a r b o h y d r a t e s - g l u c o s e , fructose, sucrose, xylose, corn syrup, and m e t h y l glucoside-had a l l performed satisfactorily, i n d i c a t i n g t h a t a w i d e v a r i e t y o f c a r b o h y d r a t e sources w o u l d be useable for s u c h adhesives. F o r c o m p a r i s o n w i t h work b e i n g presented here, b o n d s t r e n g t h re sults o n glucose-based adhesives f r o m a m o n g those p r e v i o u s e x p e r i m e n t s are given i n T a b l e I. I n the e x p e r i m e n t t e s t i n g a lower u r e a / c a r b o h y d r a t e m o l a r r a t i o (0.125:1), the r e a c t i o n was s t o p p e d w h e n d i s t i l l a t e s t o p p e d b e i n g p r o d u c e d , a n d o n l y h a l f


26.

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C H R I S T I A N S E N

as m u c h condensate was collected as h a d been o b t a i n e d w i t h U / C m o l e r a t i o s of 0.25:1. T h e d r y b o n d strengths of t r i a l panels pressed at 150 t o 160 ° C were equivalent to strengths for the c o n t r o l panels pressed at 140 t o 150 ° C ( T a b l e I I ) . B u t as p o s t - b o n d i n g c o n d i t i o n i n g became m o r e s e v e r e - g o i n g t o v a c u u m pressure soak ( V P S ) a n d t o 2-hour b o i l t r e a t m e n t s - t h e s t r e n g t h of the t r i a l panels generally weakened m o r e t h a n t h a t of the controls.


T a b l e I. Shear S t r e n g t h P r o p e r t i e s of Y e l l o w B i r c h P a n e l s B o n d e d with Glucose-Based Adhesives

M o l a r R a t i o Reactants Condensate P/C/U/F H 0/C Phenol 1

1.0:1.0:0.5:2.0

2

2

3

3.1

0.12

11.0

6.9

5.3

(3.6)

—

8.9

6.9

6.0

1.0:1.0:0.25:3.6

(3.6)

—

12.6

8.3

7.8

0.75:1.0:0.5:2.5

3.0

0.12

11.3

4.0

4.3

0.75:1.0:0.25:1.88

2.5

0.12

7.6

4.5

3.7

0.5:1.0:0.5:2.35

(0.9)

—

3.8

0

0

0.5:1.0:0.25:1.0

(2.8)

—

7.2

2.7

1.9

0.5:1.0:1.0:2.5

(1.4)

—

4.4

0.4

0

0.25:1.0:0.25:0.75

(2.0)

—

0.8

0

—

12.5

7.1

6.6

1.0:1.0:0.5:2.0

4

P h e n o l i c adhesive 1

Shear S t r e n g t h Wet Dry V P S 2-Hour B o i l (MPa) (MPa) (MPa)

'

Phenol/carbohydrate/urea/formaldehyde.

M o l e s w a t e r / m o l e c a r b o h y d r a t e ; n u m b e r s i n parenthesis w h e n t o t a l condensate collected is considered as o n l y w a t e r . W e i g h t f r a c t i o n o f o r i g i n a l p h e n o l recovered. P h e n o l a d d e d 140 m i n u t e s after r e a c t i o n s t a r t e d . T h e s t a n d a r d errors of the m e a n for the strengths of the 23 c o n t r o l panels were ± 0 . 3 M P a for d r y , ± 0 . 2 M P a for V P S / w e t , a n d ± 0 . 2 for 2-hour b o i l / w e t specimens. 2

3

4

5

S O U R C E :

Reprinted from ref. 9.


376



T h e adhesive resins r e s u l t i n g f r o m f o r m u l a t i o n s w i t h four levels of the caustic c a t a l y s t s p e r f o r m e d w e l l c o m p a r e d t o the controls, b o t h i n d r y tests a n d the two k i n d s o f wet tests. T h e d r y s t r e n g t h of the t r i a l panels (overall average 11.1 M P a ) was 10 percent lower t h a n t h a t of the c o n t r o l panels ( T a b l e H I ) . I n the wet tests, the t r i a l panels were s l i g h t l y (insignificant s t a t i s t i c a l l y ) stronger t h a n the c o n t r o l panels, averaging 6 a n d 2 percent h i g h e r , respectively, for the V P S a n d 2-hour b o i l tests. T h e strengths o f the t r i a l panels were, however, m o r e v a r i a b l e i n the wet tests.

T a b l e I I . Effect of U r e a / C a r b o h y d r a t e M o l e R a t i o (0.125:1) o n A d h e s i v e Shear S t r e n g t h of Y e l l o w B i r c h P a n e l s

Resin

1

Control Ρ

Tfial

Press A v e r a g e Shear S t r e n g t h Temperature Dry Wet Boil 2

(°C) 140 150 150 150 160 160

(MPa) (MPa)

3

(MPa)

12.4 12.7 10.4

7.9 8.0 7.4

6.9 7.0 6.3

12.7 11.1 14.1

4.8 5.4 7.6

3.0 4.6 7.9

P h e n o l / c a r b o h y d r a t e / u r e a / f o r m a l d e h y d e resin w i t h m o l a r r a t i o s o f 1:1:0.125:2, 2.09 P a s viscos ity, b o n d e d between t w o p a r a l l e l - l a m i n a t e d yellow b i r c h veneers for 5 m i n u t e s . V P S - c o n d i t i o n e d a n d tested wet. B o i l e d 2 hours a n d tested wet. 1

2

3

E f f e c t o f U r e a o n C a r b o h y d r a t e W e i g h t L o s s . D r y i n g u n c a t a l y z e d glucose or x y l o s e samples caused essentially no solids weight loss, as expected. B u t the m i x t u r e s of glucose or x y l o s e w i t h v a r i o u s a m o u n t s o f u r e a experienced weight losses o f f r o m 10 t o 2 3 % ( T a b l e I V ) . A p p a r e n t l y , a r e a c t i o n occurs i n the u n c a t a l y z e d state once the saccharide a n d u r e a are i n t i m a t e l y m i x e d a n d h e a t e d . A s the g l u c o s e / u r e a m o l e r a t i o changed f r o m 1:0.125 t o 1:0.25 t o 1:0.5, the weight loss at the e n d o f the d r y i n g stage increased f r o m 10 t o 17 to 23 percent, suggesting a n o n l i n e a r r e l a t i o n s h i p between c o m p o s i t i o n a n d weight loss. T h e s e weight losses are r o u g h l y twice the weight o f u r e a i n the i n i t i a l samples. M i c r o K j e l d a h l a n a l y s i s of the glucose m i x t u r e s w i t h the highest a n d lowest levels of u r e a i n d i c a t e d no loss o f n i t r o g e n (even t h r o u g h the a c i d - c a t a l y z e d stage). T h e a c t u a l weight lost goes b e y o n d w h a t w o u l d be p r e d i c t e d by loss o f the water d u r i n g f o r m a t i o n o f d i g l u c o s y l u r e a . F o r t w o glucose molecules r e a c t i n g


26.

377




w i t h each u r e a m o l e c u l e , one w o u l d expect a loss of one molecule o f water per m o l e c u l e of glucose. F o r d r y i n g of the higher urea-content m i x t u r e s , the a c t u a l weight losses c o r r e s p o n d to over 2.64 moles of water per m o l e glucose. It seems the presence of u r e a hastens the d e h y d r a t i o n or other d e g r a d a t i o n of glucose ( a n d x y l o s e ) . T h e m i x t u r e s c o n t a i n i n g u r e a were r e d d i s h - b r o w n at t h i s stage, c o m p a r e d to w h i t e for glucose a n d a y e l l o w i s h - w h i t e for x y l o s e .

T a b l e I I I . Effect of C a t a l y s t R e d u c t i o n s o n A d h e s i v e Shear S t r e n g t h of Y e l l o w B i r c h P a n e l s Catalyst level %of Normal

Resin

1

Trial

100 67 33 0

DSS WSS BSS Ave Min Ave Min Ave Min (MPa) (MPa) (MPa) (MPa) (MPa) (MPa) 2

pH

3

12.0 11.0 11.7 9.7

9.2 9.2 9.4

Control C Control Ρ

5

11.0 10.3 9.7 8.2

4

7.5 7.2

6.5 4.2

5.8

6.8 6.3

3.6 4.7

7.6

6.9

7.3

5.9

7.2

4.9

6.3

5.4

6.7 5.9 4.2

8.7

7.8 7.7 7.6 8.4

12.7

11.0

11.9

10.5

N o r m a l levels are 6.6% s o d i u m h y d r o x i d e a n d 2 . 2 % s o d i u m c a r b o n a t e . A v e r a g e ( A v e ) a n d m i n i m u m ( M i n ) d r y shear s t r e n g t h ( D S S ) . A v e r a g e ( A v e ) a n d m i n i m u m ( M i n ) wet shear s t r e n g t h ( W S S ) . V P S c o n d i t i o n e d a n d tested wet. 1

2

3

A v e r a g e ( A v e ) a n d m i n i m u m ( M i n ) b o i l shear s t r e n g t h ( B S S ) . 2-hour b o i l c o n d i t i o n e d a n d tested wet. P h e n o l / c a r b o h y d r a t e / u r e a / f o r m a l d e h y d e m o l a r r a t i o s = 1:1:0.25:2.

4

5

T h e n , d u r i n g the a c i d - c a t a l y z e d r e a c t i o n , the weight equivalent o f another 0.75 moles of water per m o l e of glucose was lost. A f t e r t h i s stage, a l l samples were d a r k b r o w n . Feather a n d H a r r i s (11) describe a n u m b e r o f i n t r a - a n d i n t e r m o l e c u l a r d e h y d r a t i o n m e c h a n i s m s for c a r b o h y d r a t e s i n a c i d i c s o l u t i o n . I n t h a t s t u d y , some o f those m e c h a n i s m s were a i d e d b y the presence of a m i n e s . Because of large weight losses, samples c o n t a i n i n g g l u c o s e / u r e a m o l e r a t i o s of 1:0.5 a n d 1:0.125 were a n a l y z e d for r e s i d u a l n i t r o g e n content to see i f n i t r o g e n c o m p o u n d s were lost d i s p r o p o r t i o n a t e l y fast. T h e solids were f o u n d t o c o n t a i n 8.84% a n d 2 . 3 5 % n i t r o g e n , respectively, versus 6.67% a n d 1.84% c a l c u l a t e d for the i n i t i a l u n c a t a l y z e d samples. B a s e d o n knowledge o f 1) the weights of nonaqueous ingredients i n the o r i g i n a l m i x t u r e s , 2) the m o l e c u l a r weights of these, 3) the f i n a l weights of the p r o d u c t s i n the bottles, a n d 4) f i n a l n i t r o g e n content of these p r o d u c t s , a n d the a s s u m p t i o n s t h a t 1) t h e nonaqueous a c i d



378

ADHESIVES F R O M R E N E W A B L E RESOURCES

was a l l r e t a i n e d a n d 2) the ureide N C O N u n i t s r e m a i n as a u n i t , the a m o u n t of the o r i g i n a l h y d r o x y l content of the i n i t i a l glucose t h a t c o u l d have been lost ( a s s u m i n g n o c a r b o n - c o n t a i n i n g glucose p r o d u c t s were lost) c o u l d be c a l c u l a t e d . F o r the G / U = 1:0.125 m i x t u r e , 4 0 % of the h y d r o x y l weight c o u l d have been lost, a n d for the G / U = 1:0.5 m i x t u r e , 8 2 % of the h y d r o x y l weight c o u l d have been l o s t . O f a l l the weight lost f r o m the G / U = 1:0.5 m i x t u r e , 7 8 % was lost d u r i n g the n o n a c i d i c d r y i n g step. T h e o r e t i c a l l y , for the G / U = 1:0.5 m i x t u r e , i f o n l y d i g l u c o s y l u r e a were f o r m e d , 2 0 % of the glucose h y d r o x y l weight w o u l d be lost. F r o m these considerations a n d the o b s e r v a t i o n of significant c o l o r i n g i n the c a r b o h y d r a t e - u r e a p r o d u c t s , i t seems t h a t reactions other t h a n s i m p l e c o n d e n s a t i o n m u s t have o c c u r r e d . T a b l e I V . Effect of U r e a o n C a r b o h y d r a t e W e i g h t L o s s Carbohydrate — •

c/u — 1

U r e a ( W e i g h t %)

—•

Glucose 1:0.5 14.3

Xylose

1:0.5 1:0.25 1:0.125 14.3 7.7 4.0

1:0 0

1:0.5 16.7

Percentage S o l i d s W e i g h t C h a n g e EDS

3

EACS

4

-22.7 -22.3

-16.7

-10.0

-28.2 -27.9

-27.5

-24.7 -17.0

0.4

1:0 0

2

-23.1

-1.0

-28.0 -20.9

Moles W a t e r / M o l e Carbohydrate EDS

2.6

1.8

1.0

1.0

2.3

0.1

3.3 3.3 M o l e r a t i o of c a r b o h y d r a t e / u r e a .

3.0

2.6

1.7

2.8

1.7

EACS

x

2.7

3

4

S t a r t i n g w i t h s o l u t i o n s c o n t a i n i n g a p p r o x i m a t e l y 2.00 g c a r b o h y d r a t e , proportionate urea, and 3 m L distilled water. A t the e n d of the d r y i n g stage (18 hours at 100 t o 106 ° C ) . A t t h e e n d of t h e a c i d - c a t a l y s t stage. 0.09 m L of 5 N s u l f u r i c a c i d a d d e d at b e g i n n i n g ; t h i s stage t o o k 30 hours at 120 ° C . 2

3

4

U r e a affects weight loss of glucose differently t h a n i t affects x y l i t o l , a p o l y h y d r o x y c o m p o u n d w i t h o u t a h e m i a c e t a l f u n c t i o n a l g r o u p . F o r glucose-based s a m ples, a c i d - c a t a l y z e d glucose w i t h u r e a lost m o r e weight t h a n d i d a c i d - c a t a l y z e d glucose w i t h o u t u r e a u n t i l 22 h o u r s o f h e a t i n g h a d passed ( F i g u r e 1). S u r p r i s ingly, u n c a t a l y z e d glucose w i t h u r e a lost m o r e weight t h a n either of those t w o samples, between 4 hours a n d at least 28 h o u r s . B y m i c r o - K j e l d a h l a n a l y s i s of the u n c a t a l y z e d glucose-urea p r o d u c t s , 1 3 % of the o r i g i n a l n i t r o g e n was determ i n e d t o have been lost d u r i n g the 28 h o u r s . If t h i s was lost o n l y as gaseous


26.



379


u r e a d e c o m p o s i t i o n p r o d u c t s , t h e n 2 5 % of the o r i g i n a l glucose was also lost, p r e s u m a b l y as water (2.5 moles of water per m o l e o f glucose). F o r the c a t a l y z e d glucose w i t h o u t u r e a , 2 3 % was lost d u r i n g d e h y d r a t i o n (2.3 moles o f water per m o l e o f glucose). F o r reference, a s a m p l e o f u n c a t a l y z e d u r e a s o l u t i o n heated at 108 ° C lost 1.2% o f i t s weight i n 3 h o u r s a n d 3.2% i n 6 h o u r s . T h e less a c i d i f i e d u r e a s o l u t i o n ( s i m u l a t i n g c o n d i t i o n s for G / U = 8:1) lost 6 . 2 % i n 2 hours a n d 1 3 . 8 % i n 6 h o u r s . T h e h i g h l y acidified u r e a s o l u t i o n (as i n G / U = 2:1) lost 6 . 7 % i n 2 hours a n d , surprisingly, only 10.3% i n 6 hours. A l l retained their original white color. A separate 2 - g r a m s a m p l e o f d r y u r e a solids heated i n a 110 ° C oven showed 1% weight loss i n the first 1.5 hours a n d 7 . 1 % weight loss after 17 h o u r s . T h u s , w i t h o u t a c i d c a t a l y s i s , u r e a weight loss was slow at these t e m p e r a t u r e s , a n d the rate increased b y a factor o f a b o u t 4 to 5 w i t h a d d i t i o n o f a c i d . T h e x y l i t o l - b a s e d samples e x h i b i t e d a m u c h different b e h a v i o r ( F i g u r e 2). T h e a c i d - c a t a l y z e d x y l i t o l lost weight m u c h faster i n i t i a l l y t h a n d i d glucose, t h o u g h i t seemed t o level o u t w i t h less t o t a l weight loss ( 1 8 % , or 1.5 o f moles o f w a t e r per m o l e o f x y l o s e ) . T h e m i x t u r e o f u n c a t a l y z e d x y l i t o l a n d u r e a showed a m u c h slower rate o f weight loss a n d h a d a different t y p e o f b e h a v i o r , n a m e l y , l i n e a r w i t h t i m e . H e r e , the 7 2 % n i t r o g e n weight loss c a l c u l a t e d f r o m a n i t r o g e n a n a l y s i s , i f considered as u r e a weight loss by d e c o m p o s i t i o n , accounts for the t o t a l s a m p l e weight loss. A d d i n g a c i d c a t a l y s t to the x y l i t o l - u r e a m i x t u r e p r o d u c e d b e h a v i o r s i m i l a r t o t h a t o f the u n c a t a l y z e d m i x t u r e , b u t slowed the loss o f weight. T h u s , for glucose, the a c i d seems to slow d o w n the m o r e severe u r e a - l i n k e d d e g r a d a t i o n i n the l o n g e a r l y stage, whereas, for x y l i t o l , u r e a stops the severe acid-catalyzed degradation i n this period. N M R A n a l y s e s o f R e a c t i o n P r o d u c t s . Interpretation of mixtures of mater i a l s b y C - N M R is c o m p l i c a t e d , a n d such i n t e r p r e t a t i o n was a t t e m p t e d o n l y w h e n the s p e c t r a were f a i r l y s i m p l e or w h e n k n o w n species c o u l d be p i c k e d out readily. 1 3

T h e progression o f c a r b o h y d r a t e a n d u r e a p r o d u c t s f r o m a r e a c t i o n c o n d u c t e d i n sulfolane was followed b y H P L C a n d C - N M R (10). S o m e o f the glucose converted to fructose, as expected, b u t b o t h d e c l i n e d as other r e a c t i o n p r o d u c t s f o r m e d . T h e e a r l y p r e d o m i n a n t p r o d u c t s were g l u c o s y l ureides, c o n firmed earlier (10) b y c o m p a r i s o n w i t h synthesized g l u c o s y l ureides a n a l y z e d by H P L C a n d C - N M R . T h e g l u c o s y l ureides subsequently decreased as a few other s i m p l e species ( u n i d e n t i f i e d at present) a n d p o l y m e r i c species increased. T h i s same sequence o f p r o d u c t s was n o t e d i n other resin syntheses, w h e t h e r or n o t t h e u n r e a c t i n g p h e n o l was present. F o r e x a m p l e , the progression o f the x y l o s e - u r e a r e a c t i o n m i x t u r e also showed t h i s b e h a v i o r , a n d a species believed t o be a x y l o s y l ureide was observed. T h e C - N M R s i g n a l s for the g l u c o s y l a n d assumed x y l o s y l ureide species are given i n T a b l e V , a l o n g w i t h the values o b t a i n e d for u r e a , glucose, a n d xylose m o n o m e r s . T h e values for these last t w o c o m p o u n d s agree well w i t h l i t e r a t u r e values (12). T h e shifts for the x y l o s y l 1 3

1 3

1 3



380

ADHESIVES F R O M R E N E W A B L ERESOURCES

F i g u r e 1. W e i g h t loss f r o m glucose a n d glucose-urea (2:1) m i x t u r e s at 110 ° C : a c i d - c a t a l y z e d glucose — - ; u n c a t a l y z e d glucose-urea ; and acid-catalyzed glucose-urea .

Total oven time (hours) F i g u r e 2. W e i g h t loss f r o m x y l i t o l a n d x y l i t o l - u r e a (2:1) m i x t u r e s at 110 ° C : acid-catalyzed xylitol — - ; uncatalyzed xylitol-urea ; a n d acid-catalyzed xylitol-urea .



26.

381



ureide C - l a n d C - 2 carbons are i n l i n e w i t h s u b s t i t u t i o n of h y d r o x y l groups w i t h a n a m i d e l i n k a g e , a n d the other three signals are close t o the o r i g i n a l βx y l o p y r a n o s e values. T h e H P X 8 7 H + c o l u m n c h r o m a t o g r a m s for the first s a m p l e f r o m t h i s synthesis ( t a k e n j u s t after a c i d was a d d e d t o the r e a c t i o n m i x t u r e ) showed r e t e n t i o n t i m e s for xylose at 11.1 m i n , p h e n o l at 53 m i n , a n d u r e a at 180 m i n . A t t h i s p o i n t , a c o m p o u n d e l u t i n g at 14.7 m i n , assumed t o be m o n o x y l o s y l ureide based o n i t s e a r l y appearance a n d i t s e l u t i o n t i m e r e l a t i v e t o x y l o s e , gave the t h i r d largest peak, a n d a c o m p o u n d e l u t i n g at 8.9 m i n , s i m i l a r l y assumed t o be d i x y l o s y l ureide, was fifth largest. A f t e r 30 m i n of r e a c t i o n , the supposed m o n o x y l o s y l ureide peak was not v i s i b l e , whereas, the s u p p o s e d d i x y l o s y l ureide peak persisted t o become the strongest peak after 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 the s y s t e m . T h u s , the c o m p o u n d identified as a 3 0 - m i n u t e r e a c t i o n p r o d u c t b y C - N M R is p r o b a b l y the d i x y l o s y l ureide. 1 3

Table V .

1 3

C - N M R of T h r e e C a r b o h y d r a t e - U r e a P r o d u c t s a n d T h e i r Precursors ( p p m ) 1

Carbon Position C=0 C-l C-2 C-3 C-4 C-5 C-6

Glucose a - p y r /?-pyr 2

Urea

2

160.1 92.2 72.4 73.1 70.6 71.9 61.3

96.8 74.8 76.7 70.4 76.6 61.2

Glucosyl Ureide monodi-

Xylose

3

158.2 81.3 72.9 77.7 70.3 77.6 61.2

156.3 80.8 72.9 77.7 70.3 77.6 61.2

2

Xylosyl Ureide 4

a - p y r /?-pyr 92.5 72.4 73.3 70.2 61.7

97.7 74.8 76.7 69.9 65.7

156.7 81.8 72.7 77.4 69.8 62.6

R e l a t i v e t o i n t e r n a l T M S , i n D M S O - d e solvent. C o m m e r c i a l l y o b t a i n e d m a t e r i a l , α-pyr = a - p y r a n o s e , /?-pyr = /?-pyranose. C o m p o u n d s synthesized p r e v i o u s l y (10). A s s u m e d i d e n t i t y o f m a t e r i a l i n a resin m i x t u r e ; p o s s i b l y the d i x y l o s y l ureide. 1

3

4

T h e p h e n o l / g l u c o s e / u r e a r e s i n , t o w h i c h f o r m a l d e h y d e was l a t e r t o be a d d e d , showed the u s u a l sequence o f p r o d u c t s f o r m a t i o n , b u t b y the e n d o f t h e a c i d i c r e a c t i o n stage, there were n o d i s t i n c t glucose or g l u c o s y l ureide species i d e n tifiable by C - N M R or H P L C . A f t e r n e u t r a l i z a t i o n a n d a d d i t i o n of f o r m a l i n , signals for f o r m a l i n i m m e d i a t e l y showed i n the 49 t o 55 p p m ( m e t h o x y ) a n d 82 t o 90 p p m ( h e m i f o r m a l a n d h e m i a c e t a l ) regions of the s p e c t r u m . A f t e r the e n d of a n h o u r of r e a c t i o n w i t h f o r m a l d e h y d e , the C - N M R s p e c t r u m o f the r e a c t i o n m i x t u r e showed t h a t the signals due to p h e n o l i c species h a d reduced i n size r e l a t i v e t o the sulfolane i n t e r n a l reference, b u t t h e i r n u m b e r h a d m u l t i p l i e d 1 3

1 3


382



greatly, i n d i c a t i n g t h a t m a n y p h e n o l - f o r m a l d e h y d e species h a d been created. S o m e d i s t i n c t peaks appeared i n t h e 58 t o 70 p p m r e g i o n , where one expects t o find signals for h y d r o x y m e t h y l groups a t t a c h e d t o p h e n o l , 58 t o 65 p p m (13,14), a n d t o u r e a , 64 t o 72 p p m (13,15). T h e xylose-based resin showed t h e same effect o n i t s s p e c t r u m because o f f o r m a l d e h y d e a d d i t i o n . S t a b i l i t y o f t h e F u r a n R i n g . I n the e x p e r i m e n t to d e t e r m i n e the s t a b i l i t y o f f u r a n r i n g s i n t h e presence o f a c i d , u r e a , a n d p h e n o l i c species, i n c r e m e n t a l a d d i t i o n s o f f u r f u r a l t o a n i n i t i a l D M U a n d D M P m i x t u r e resulted i n progressively m o r e intense signals at 106.8 a n d 110.3 p p m i n the C - N M R s p e c t r u m . T h e s e are n o t the 122.3 a n d 112.8 p p m signals for t h e C - 3 a n d C - 4 p o s i t i o n s i n f u r f u r a l , whose signals were present at v e r y low levels for m u c h o f the t i m e . However, those former signals are i n a region consistent w i t h C - 3 a n d C - 4 c a r b o n s o f a f u r y l species a t t a c h e d t o a n a l k y l c a r b o n (16,17). A c t i n g as a n aldehyde, f u r f u r a l c o u l d lose i t s a l d e h y d i c o x y g e n a n d combine w i t h t w o a r o m a t i c r i n g s , c r e a t i n g a t e r t i a r y a l k y l c a r b o n i n the process. P e r h a p s a t e r t i a r y a l k y l c a r b o n is the source o f a n i n c r e a s i n g l y intense s i g n a l at 48.6 p p m , a n a p p r o p r i a t e v a l u e b y o u r c a l c u l a t i o n s . S e v e r a l other signals increased w i t h each a d d i t i o n of f u r f u r a l a n d m a y b e r e l a t e d t o other f u r y l c a r b o n species. S i g n a l s t h a t appeared t o change d u r i n g the a d d i t i o n o f b o t h f u r f u r a l a n d f o r m a l d e h y d e were p r e d o m i n a n t l y near those for t h e o r i g i n a l p h e n o l i c r i n g a n d i t s m e t h y l s u b s t i t u e n t c a r b o n s . 1 3

R e a c t i o n s o f U r e a w i t h M o d e l C o m p o u n d s . U r e a d i d n o t n o t i c e a b l y react w i t h the s i m p l e secondary a l c o h o l o f c y c l o h e x a n o l d u r i n g 2.5 hours o f h e a t i n g . T h e m a j o r c o m p o n e n t s r e m a i n i n g i n the flask were f o u n d t o be c y c l o h e x a n o l , u r e a , a n d a c o m p o u n d whose C - N M R s p e c t r a l lines seem t o c o r r e s p o n d t o d i c y c l o h e x y l ether. E t h e r s are often d e h y d r a t i o n p r o d u c t s o f alcohols. T h e condensate c o n t a i n e d c y c l o h e x a n o l a n d cyclohexanone, t h e l a t t e r a p p a r e n t l y a n i m p u r i t y i n the c y c l o h e x a n o l . T h e s i g n a l for u r e a decreased w i t h t i m e , a n d t w o s m a l l , s l i g h t l y lower frequency signals (154 t o 156.5 p p m ) t h a t a p p e a r e d m a y be due t o u r e a d e g r a d a t i o n p r o d u c t s , such as b i u r e t . I f a n y c y c l o h e x y l u r e a was f o r m e d , i t w a s present at v e r y l o w levels. T h i s leaves t h e m e c h a n i s m o f s u b s t a n t i a l weight loss for n o n - a c i d - c a t a l y z e d urea-saccharide m i x t u r e s a m y s t e r y . 1 3

T H P , a s i m p l e c y c l i c h e m i a c e t a l , was reacted w i t h D M U i n the final s y n t h e sis. E v e n t h o u g h a l i q u i d c h r o m a t o g r a m o f T H P showed one peak, t h e signals i n a q u a n t i t a t i v e C - N M R s p e c t r u m appeared t o group i n t o three species, based o n t h e r e l a t i v e intensities ( T a b l e V I ) . T h e three assumed T H P species appear to be close v a r i a t i o n s o f the t e t r a h y d r o p y r a n r i n g f o r m . T h e assignments o f signals follow the trends o f values given b y J i et a l . (18) for one species o f t h i s same c o m p o u n d a n d b y r o u g h , c a l c u l a t e d values based o n t h e s p e c t r a l s i m u l a t i o n a p p r o a c h o f C h e n g (19). T H P a n d sulfolane e l u t e d together f r o m the cation-Η c a r t r i d g e 0.75 m i n u t e s after i n j e c t i o n , w h i l e t h e D M U r e t e n t i o n t i m e v a r i e d f r o m 12.5 t o 13.0 m i n u t e s . 1 3

A s a m p l e was t a k e n f r o m t h e a c i d - c a t a l y z e d r e a c t i o n o f T H P w i t h D M U after 30 m i n u t e s . T h e C - N M R s p e c t r u m o f t h i s s a m p l e showed d e p l e t i o n 1 3


26.


383


o f the m o s t a b u n d a n t T H P species, relative t o the second, a n d f o r m a t i o n of a m a j o r p r o d u c t . F r o m the m a j o r p r o d u c t ' s C - N M R signals ( T a b l e V I ) , i t appears t o be l , l - d i m e t h y l - 3 - ( 2 - t e t r a h y d r o p y r a n o s y l ) u r e a , ( D M T H P U ) . I n a l i q u i d c h r o m a t o g r a m of t h i s s a m p l e , o n l y one new peak was e v i d e n t , at 1.91 minutes elution time. 1 3

F o r a s a m p l e t a k e n after 80 m i n u t e s , a C - N M R s p e c t r u m showed l i t t l e D M T H P U left, b u t the second T H P species was s t i l l e v i d e n t . M a n y new signals a p p e a r e d i n the 18 t o 35 p p m a n d 59 t o 70 p p m regions. N e w signals also a p p e a r e d at 136.4 a n d 146.1 p p m , w h i c h c o u l d be p r o d u c e d b y a r o m a t i c C = C a n d C = N b o n d s or alkene c a r b o n s , for the 136.4 p p m value. A l i q u i d c h r o m a t o g r a m for the (filtered) s a m p l e also showed l i t t l e D M T H P U , b u t no other new d i s t i n c t peaks.


1 3

Table V I .

1 3

C - N M R C h e m i c a l S h i f t V a l u e s for T H P a n d P r o p o s e d D e r i v a t i v e s 1

Designation: Rel. Area : 4

Ring Carbon No. 2 3 4 5 6

Species o f T H P Present W o r k R e p o r t e d (18) 1 2 3 4.0 1.9 0.7

Proposed Derivatives DMTHPU DMTHPA 2

5

93.5 93.3 97.3 32.4 30.3 30.6 20.4 19.2 19.2 25.3 25.1 25.1 62.6 61.7 62.1

92.99 29.62 18.42 24.52 61.19 CH C=0 3

1

2

3

4

5

3

79.7 30.6 23.0 25.0 66.1 35.8 157.1

93.4 29.7 23.6 26.0 66.6 39.6

T e t r a h y d r o - 2 H - p y r a n - 2 - o l ; c o m m e r c i a l m a t e r i a l w i t h o n l y one H P L C peak. l,l-Dimethyl-3-(2-tetrahydropyranyl)urea, i n a product mixture. D i m e t h y l - ( 2 - t e t r a h y d r o p y r a n y l ) a m i n e , i n the lighter phase o f the d i s t i l l a t e . R e l a t i v e area b y q u a n t i t a t i v e C - N M R i n t e g r a t i o n . R e l a t i v e t o r i n g o x y g e n n u m b e r e d 1. 1 3

A C - N M R s p e c t r u m o f the f i n a l p r o d u c t showed the a d d i t i o n o f a n u m b e r o f d i s t i n c t peaks i n the 129 t o 152 p p m u n s a t u r a t e d c a r b o n r e g i o n . A n H N M R s p e c t r u m o f the s a m p l e showed nonexchangeable h y d r o g e n s i g n a l s i n the 7.3 t o 8.7 p p m region ( a b o u t 4 % o f the t o t a l integrated h y d r o g e n s i g n a l ) , w h i c h are m o s t l i k e l y associated w i t h h e t e r o a r o m a t i c r i n g hydrogens. T h e r e were also some nonexchangeable h y d r o g e n signals i n the 4.5 to 4.8 r e g i o n , w h i c h c o u l d be associated w i t h a n o n c o n j u g a t e d alkene CH2 g r o u p . 1 3

1


384


A s a m p l e o f the u p p e r phase o f condensate d i s t i l l e d f r o m the r e a c t i o n m i x t u r e gave a c o m p a r a t i v e l y s i m p l e C - N M R s p e c t r u m . T h i s s p e c t r u m ' s signals i n d i c a t e d the presence o f three species: the second species o f T H P , a s m a l l a m o u n t o f the t h i r d T H P species, a n d a new c o m p o u n d . C a l c u l a t i o n s [accordi n g t o the a p p r o a c h used b y C h e n g (19)] i n d i c a t e t h a t the new c o m p o u n d , whose signals are given i n T a b l e V I , c o u l d be d i m e t h y l - ( 2 - t e t r a h y d r o p y r a n o s y l ) a m i n e . T h i s w o u l d i n d i c a t e a d i m e t h y l a m i n e g r o u p s p l i t off f r o m the d i m e t h y l u r e a , as a m m o n i a c a n s p l i t off f r o m amides r e a c t i n g w i t h alcohols at elevated t e m p e r atures u n d e r a c i d c o n d i t i o n s (20). T h e lower phase o f the l i q u i d condensate h a d a C - N M R s p e c t r u m , w h i c h i n d i c a t e d t h a t i t m i g h t c o n t a i n at least t w o u n i d e n t i f i e d c o m p o u n d s , i n c l u d i n g u r e a - t y p e c a r b o n y l groups, d i m e t h y l a m i n o g r o u p s , a n d p y r a n r i n g groups.


1 3

1 3

T h e w o r k presented t h u s far represents the reactions o c c u r r i n g m a i n l y i n the a c i d - c a t a l y z e d stage o f the synthesis. T h e u r e a a n d saccharide first f o r m g l y c o s y l ureides, w h i c h decompose. Subsequent p r o d u c t s are n o t i d e n t i f i e d , b u t seem t o be n u m e r o u s . A l t h o u g h these later p r o d u c t s are m o s t l y a l i p h a t i c , there are i n d i c a t i o n s of possible m i n o r a m o u n t s of h e t e r o a r o m a t i c s t r u c t u r e s . I n t h i s respect, t h i s s y s t e m is s o m e w h a t s i m i l a r t o p r o d u c t s f r o m M a i l l a r d reactions o f r e d u c i n g saccharides a n d a m i n o acids (21 22) a n d i n m e l a n o i d i n s a n d o r g a n i c s o i l (23). M o s t w o r k o n M a i l l a r d p r o d u c t s has been done at near n e u t r a l or a l k a l i n e p H values. H o w e v e r , S a m u e l y (24) heated glucose w i t h acids i n the presence o f v a r y i n g a m o u n t s of u r e a a n d other nitrogenous c o m p o u n d s a n d f o u n d m e l a n o i d i n s . A review o f m o d e l reactions for a m i n e c o m p o u n d s a n d r e d u c i n g sugars (25) notes t h a t a w i d e v a r i e t y o f reactions m a y o c c u r , d e p e n d i n g o n the t e m p e r a t u r e , p H , specific r e a c t a n t s , a n d the presence o f w a t e r . T h e review a r t i c l e f u r t h e r notes t h a t i n the n e a r l y d r y state, the s u g a r - a m i n e c o n d e n s a t i o n a n d A m a d o r i rearrangement are the key reactions l e a d i n g to b r o w n nitrogenous p o l y m e r s b y a v a r i e t y o f p a t h w a y s . A difficulty c o m m o n to these studies has been the w i d e v a r i e t y o f p r o d u c t s f o r m e d . t

O n e unanswered q u e s t i o n is whether the s a c c h a r i d e - u r e a p r o d u c t s a c t u a l l y react w i t h p h e n o l t h r o u g h the i n t e r m e d i a r y o f h y d r o x y m e t h y l groups t h a t are a d d e d t o b o t h p h e n o l a n d u r e a d u r i n g the n e u t r a l r e a c t i o n stage. T o m i t a a n d M a t s u z a k i (26) have s h o w n t h a t h y d r o x y m e t h y l p h e n o l s c a n condense w i t h u r e a at a p H range o f 4.8 to 10.0, a key sign b e i n g a C - N M R s i g n a l at 44.2 p p m . T h e y note t h a t p h e n o l self-condensation c a n be suppressed b y excess u r e a at the a c i d i c e n d , b u t not at the n e u t r a l a n d basic c o n d i t i o n s t h a t occur i n the f o r m a l d e h y d e r e a c t i o n stages i n the present s t u d y ' s syntheses. A l t e r n a t e l y , there are i n d i c a t i o n s t h a t p o l y o l s c a n react to a l i m i t e d extent w i t h phenols under alkaline c o n d i t i o n s t o f o r m ether l i n k s (27). If such reactions do not f r e q u e n t l y occur, the d u r a b l e adhesive m a y be the result o f a n i n t e r p e n e t r a t i n g n e t w o r k o f the urea-saccharide a n d p h e n o l - f o r m a l d e h y d e p o l y m e r s . T h e need for as m a n y moles o f p h e n o l as monosaccharide a n d the need for at least 2 moles o f f o r m a l d e h y d e p e r m o l e o f p h e n o l to achieve the best properties (10) p o i n t to 1 3


26.



385

t h i s as a d i s t i n c t p o s s i b i l i t y . L o w e r m o l e c u l a r weight p h e n o l i c resin molecules m a y be needed for p e n e t r a t i o n of the w o o d to give g o o d l o n g t e r m d u r a b i l i t y .


Conclusions I n e x p e r i m e n t s w i t h b o n d e d w o o d panels, a u r e a / c a r b o h y d r a t e m o l e r a t i o of 0.125:1 i n a c a r b o h y d r a t e / u r e a / p h e n o l / f o r m a l d e h y d e resin gave s l i g h t l y lower strengths t h a n p r e v i o u s resins t h a t h a d u r e a levels at least twice as h i g h . D r a s t i c a l l y r e d u c i n g the a m o u n t s of s o d i u m h y d r o x i d e a n d s o d i u m c a r b o n a t e a d d e d d u r i n g adhesive f o r m u l a t i o n p r o d u c e d w i d e r s t r e n g t h v a r i a b i l i t y t h a n for c o n t r o l resins, b u t d i d n o t reduce average wet shear strengths of b o n d e d panels. E x p e r i m e n t s o n r e a c t i o n m e c h a n i s m s showed t h a t u r e a enhanced the dehyd r a t i o n (weight loss) reactions of monosaccharides i n n o n a c i d i c e n v i r o n m e n t s . A d d i t i o n of a c i d t o a glucose-urea m i x t u r e slowed its d e h y d r a t i o n at 108 ° C . T h e a d d i t i o n of sufficient u r e a to a n a c i d - c a t a l y z e d p o l y o l s o l u t i o n appears to prevent, or at least delay considerably, the n o r m a l a c i d d e h y d r a t i o n r e a c t i o n of the p o l y o l . T h e first step i n the a c i d - c a t a l y z e d resin f o r m a t i o n is p r o d u c t i o n of g l y cosidic m o n o - a n d diureides, b u t subsequent p r o d u c t s are n u m e r o u s a n d s t i l l unidentified. F u r a n r i n g s , i f f o r m e d i n the syntheses, c a n s u r v i v e under the a c i d i c c o n d i t i o n s of the i n i t i a l r e a c t i o n stage w i t h u r e a a n d p h e n o l c o m p o u n d s . T e t r a h y d r o - 2 H - p y r a n - 2 - o l , a m o d e l for the xylose r i n g h e m i a c e t a l w i t h o u t other h y d r o x y l groups, reacted w i t h a m o n o f u n c t i o n a l u r e a t o p r o d u c e the expected ureide. C o n t i n u e d r e a c t i o n resulted, as for n o r m a l saccharides, i n a m u l t i t u d e of p r o d u c t s , p r e d o m i n a n t l y a l i p h a t i c , i n m u c h lower c o n c e n t r a t i o n . U r e a d i d not react w i t h c y c l o h e x a n o l (a s i m p l e secondary alcohol) u n d e r a c i d c a t a l y s i s t o produce a p r o d u c t c o n t a i n i n g u r e a fragments. I n s u m m a r y , the s t r e n g t h of w o o d panels tested here d i d not h o l d u p w h e n the u r e a / c a r b o h y d r a t e m o l a r r a t i o was lowered to 0.125:1, b u t d i d not suffer d r a s t i c a l l y w h e n the caustic components of the adhesive were reduced b y a t h i r d . G l y c o s y l ureides were the o n l y intermediates i d e n t i f i e d i n t h e sequence of reactions l e a d i n g f r o m monosaccharide to p o l y m e r , because of the q u i c k l y e s c a l a t i n g m u l t i p l i c i t y of p r o d u c t s . A cknowledgment s T h e a u t h o r expresses a p p r e c i a t i o n to the f o l l o w i n g Forest P r o d u c t s L a b o r a t o r y employees for t h e i r help i n t h i s s t u d y : M a r t i n F . W e s o l o w s k i , C h e m i s t , for N M R spectroscopic analyses; V i r g i l H . S c h w a n d t , C h e m i s t , for the H P L C analyses; a n d Wesley L . R o r k , P h y s i c a l Science T e c h n i c i a n , for s y n t h e s i z i n g resins a n d e v a l u a t i n g their b o n d strengths.


386


Literature Cited 1. Meigs, J. V. U.S. Patent 1 593 342, 1926. 2. Meigs, J. V. U.S. Patent 1 801 053, 1931 3. Meigs, J. V. U.S. Patent 1 868 216, 1932. 4. Mudde, J. P. Modern Plastics 1980, 57(2), 69. 5. Koch, F.; Krause, R.; Steffan, R.; Woelk, H. V. Starch/Staerke 1983, 35, 304. Downloaded by NORTH CAROLINA STATE UNIV on May 3, 2015 | http://pubs.acs.org Publication Date: December 31, 1989 | doi: 10.1021/bk-1989-0385.ch026

6. Gibbons, J. P.; Wondolowski, L. U.S. Patent 4 048 127, 1977. 7. Gibbons, J. P.; Wondolowski, L. U.S. Patent 4 085 075, 1978. 8. Gibbons, J. P.; Wondolowski, L. Canadian Patent 1 090 026, 1980. 9. Christiansen, A. W.; Gillespie, R. H. Forest Prod. J. 1986, 36(7/8), 20. 10. Christiansen, A. W. Proc. Wood Adhesives in 1985: Status and Needs, 1986, p 211. 11. Feather, M. S.; Harris, J. F. Adv. Carbohydr. Chem. Biochem. 1973, 28, 161. 12. Perlin, A. S.; Casu, B.; Koch, H. J. Can. J. Chem. 1970, 48, 2596. 13. de Breet, A. J. J.; Dankelman, W.; Huysman, W. G. B.; de Wit, J. Angew. Makromol. Chem. 1977, 62, 7. 14. Kim, M.G.; Tiedeman, G.T.; Amos, L.W. Proc. Weyerhaeuser Sci. Symp. 1979 2. Phenolic Resins: Chemistry and Applications, 1980, p 263. 15. Tomita, B.; Hatano, S. J. Polym. Sci. Polym. Chem. Ed. 1978, 16, 2509. 16. Fringuelli, F.; Gronowitz, S.; Hornfeldt, A.-B.; Johnson, I.; Taticchi, A. Acta Chem. Scand. Ser. Β 1974, 28, 175. 17. Kiewiet, Α.; de Wit, J.; Weringa, W.D. Org. Magn. Reson. 1974, 6, 461. 18. Ji, T.; Han, X.; Cheng, G.; Hu, J.; Yang, Z. Shiyou Huagong 1986, 15, 314; Chem. Abst. 1986, 105, 193289u. 19. Cheng, H. N. Ind. Chem. News 1984, 5(5), 24. 20. Streitwieser, Α., Jr.; Heathcock, C. H. Introduction to Organic Chemistry; Macmillan: New York, 1981; p 548. 21. Feather, M. S.; Nelson, D. J. Agric. Food Chem. 1984, 32, 1428. 22. Olsson, K.; Pernemalm, P. Α.; Theander, O. Prog. Food Nutr. Sci. 1981, 5, 47. 23. Benzing-Purdie, L.; Ripmeester, J. A. Soil Sci. Soc. Am. J. 1983, 47, 56. 24. Samuely, F. Beitr. Chem. Physiol. Pathol. 1902, 2, 355, as cited in Danehy, J. P.; Pigman, W. W. Adv. Food Res. 1951, 3, 241. 25. Hodge, J. E . Agric. Food Chem. 1953, 1, 928. 26. Tomita, B.; Matsuzaki, Τ. Ind. Eng. Chem. Prod. Res. Dev. 1985, 24, 1. 27. Conner, A. H.; River, B. H.; Lorenz, L. F. J. Wood Chem Technol. 1986, 6, 591. RECEIVED September 22, 1988


Adhesives from Renewable Resources - American Chemical Society

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