Fast-Curing Carbohydrate-Based Adhesives - ACS Publications

Downloaded by UNIV LAVAL on September 17, 2015 | http://pubs.acs.org Publication Date: December 31, 1989 | doi: 10.1021/bk-1989-0385.ch027

Chapter 27 Fast-Curing Carbohydrate-Based Adhesives Joseph J. Karchesy, R a n d y J. C l a r k , R i c h a r d F. H e l m , V a h i d Ghodoussi, and R o b e r t L. K r a h m e r Department of Forest Products Oregon State University Corvallis, O R 97331

A new carbohydrate-phenolic-based resin was synthesized by graft ing resorcinol on a known glucose-urea-phenol-based resol. The new resin was formulated into fast-curing adhesives that were used to glue high-moisture-content veneers under hot-pressing conditions and glulam under cold-setting conditions. While generally favorable gluing results were achieved, microscopic analyses of gluelines made with veneers having 18% and higher moisture content indicate prob lems with tracheid compression. Structural elucidation of the new resin and its precursors is in progress. N-glucosylurea and Ν,Ν'diglucosylurea have been identified among the first formed reaction products in the synthesis of these resins. C a r b o h y d r a t e - p h e n o l i c - b a s e d resins have s h o w n p r o m i s e for p a r t i a l replacement of p h e n o l a n d f o r m a l d e h y d e i n exterior p l y w o o d adhesives (1,2). S u c h resins are p r o d u c e d i n a two-stage r e a c t i o n sequence. F i r s t , t h e c a r b o h y d r a t e i s reacted w i t h p h e n o l , a n d sometimes u r e a , under a c i d c a t a l y s i s a t elevated t e m p e r a t u r e s (up t o 150 ° C ) , t o p r o d u c e a n acid-stage resin. T h e acid-stage resin is t h e n m a d e basic, f o r m a l d e h y d e a d d e d , a n d the r e a c t i o n c o n t i n u e d at lower t e m p e r a t u r e s t o p r o d u c e a resol-type resin. A d h e s i v e s f o r m u l a t e d f r o m these resins have c u r i n g speeds consistent w i t h present-day p l y w o o d p r o d u c t i o n needs; i n t h e western U n i t e d States, veneers are t y p i c a l l y d r i e d t o 0 t o 7% m o i s t u r e content a n d the adhesive c u r e d b y h o t pressing t h e panels a t a p p r o x i m a t e l y 140 t o 150 ° C a n d 1.2 M P a . F u t u r e w o o d composites w i l l l i k e l y require t h e g l u i n g o f l o w - q u a l i t y w o o d w i t h higher m o i s t u r e content. Faster c u r i n g rates ( c o m p a r e d t o p h e n o l - f o r m aldehyde resins) w i l l also l i k e l y b e desirable i n n e w adhesives. E m p l o y m e n t o f faster c u r i n g adhesives t o s h o r t e n press t i m e , lower press t e m p e r a t u r e , a n d glue 0097-6156/89/0385-0387$06.00/0 © 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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h i g h - m o i s t u r e - c o n t e n t w o o d translates i n t o savings i n t i m e , energy, a n d cost. E v e n t o d a y , t h e desire t o glue h i g h m o i s t u r e content veneers i n the A m e r i c a n p l y w o o d i n d u s t r y has come a b o u t f r o m efforts t o reduce t i m e a n d energy spent i n d r y i n g veneer. I f veneers o f 15 t o 2 0 % m o i s t u r e content c o u l d be used i n s t e a d of t h e u s u a l 0 t o 7%, t h e n s u b s t a n t i a l savings c o u l d be r e a l i z e d . H o w e v e r , at such m o i s t u r e contents, present t r a d i t i o n a l h o t - p r e s s i n g , p h e n o l - f o r m a l d e h y d e adhesives f a i l because o f p a n e l blows, over pénétrât i o n , a n d other p r o b l e m s . T h e p o t e n t i a l for m o d i f i c a t i o n o f c a r b o h y d r a t e - p h e n o l i c - b a s e d resins i n t o a f a s t - c u r i n g adhesive resin has recently been d e m o n s t r a t e d b y C l a r k et a l . (3). R e s o r c i n o l was grafted o n t o a k n o w n glucose-urea-phenol-based resol (1) t o p r o duce a n e w resin ( C P R ) w i t h t h e properties s h o w n i n T a b l e I . O r i g i n a l l y , t h i s resin was synthesized u s i n g resorcinol as a m o d e l t o e s t a b l i s h r e a c t i o n c o n d i t i o n s for l a t e r research, w h i c h is a i m i n g at g r a f t i n g condensed t a n n i n derivatives o n t o the c a r b o h y d r a t e - b a s e d resin s y s t e m . However, t h e C P R resin is effective as a n adhesive itself. F u r t h e r m o r e , i t s p h e n o l a n d resorcinol contents are s o m e w h a t less t h a n t h e c u r r e n t c o m m e r c i a l p h e n o l - r e s o r c i n o l - f o r m a l d e h y d e ( P R F ) l a m i n a t i n g resins t h a t c o n t a i n a r o u n d 3 5 % p h e n o l a n d a b o u t 16 t o 2 0 % r e s o r c i n o l .

T a b l e I. C P R R e s i n P r o p e r t i e s

Percent p h e n o l Percent resorcinol M o l a r r a t i o o f reactants G / P / U / F / R Percent solids 1

1

1

Viscosity ( c P ) pH 1

2

3

2

14.3 12.6 1:1:0.5:2:0.8 60 1,950

3

8.1

3

Percent o f t o t a l resin weight. Glucose/phenol/urea/formaldehyde/resorcinol. R o o m temperature.

Experimental

Methodology

R e s i n synthesis, adhesive f o r m u l a t i o n , a n d e v a l u a t i o n techniques are d e s c r i b e d by C l a r k et a l . (3,4)- P l y w o o d shear specimens were prepared a c c o r d i n g t o U . S . P r o d u c t S t a n d a r d P S 1-83 for exterior p l y w o o d (5). G l u l a m shear specimens were tested a c c o r d i n g t o t h e A m e r i c a n I n s t i t u t e o f T i m b e r C o n s t r u c t i o n ( A I T C ) s t a n d a r d s A I T C - T 1 0 7 a n d A I T C - T 1 1 0 for d r y shear a n d vacuum-pressure soak (6). G l u l a m test specimens were also s u b j e c t e d t o a 2-hour b o i l t r e a t m e n t (not an A I T C test) p r i o r t o shear (7,8). C - N M R s p e c t r a were recorded at 100.6 M H z o n a B r u k e r A M - 4 0 0 N M R spectrometer. G e l p e r m e a t i o n c h r o m a t o g r a p h y 1 3


27.

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was c a r r i e d o u t o n a W a t e r s m o d e l 6 0 0 0 A l i q u i d c h r o m a t o g r a p h e q u i p p e d w i t h a m o d e l R 4 0 1 differential refractometer a n d a m o d e l 440 u l t r a v i o l e t detector o p e r a t i n g at 254 n m . A series o f M i c r o s t y r a g e l c o l u m n s (100 Â , 500 À , 1 0 À , a n d 1 0 A) were e l u t e d w i t h t e t r a h y d r o f u r a n at 2 m L / m i n . T h e s y s t e m was c a l i b r a t e d w i t h p o l y s t y r e n e s t a n d a r d s , a n d m o l e c u l a r weight c a l c u l a t i o n s were c a r r i e d o u t o n a S p e c t r a P h y s i c s m o d e l S P 4200 c o m p u t i n g i n t e g r a t o r e q u i p p e d w i t h a G P C + P R O M p r o g r a m . V a p o r phase o s m o m e t r y ( V P O ) o f the C P R resin peracetate was done at G a l b r a i t h L a b o r a t o r i e s , K n o x v i l l e , T N . A u t h e n t i c N - g l u c o s y l u r e a ( I ) was synthesized b y H y n d ' s procedure (9). 3


4

Results and Discussion

A d h e s i v e s m a d e f r o m the C P R resin were tested for t h e i r a b i l i t y t o b o n d h i g h m o i s t u r e - c o n t e n t veneers under hot-pressing c o n d i t i o n s as w e l l as to b o n d g l u l a m u n d e r c o l d - s e t t i n g c o n d i t i o n s (3,4)- A d h e s i v e f o r m u l a t i o n t y p i c a l l y i n v o l v e d a d j u s t m e n t o f the resin to p H 9.5 t o 10 w i t h 5 0 % s o d i u m h y d r o x i d e a n d a d d i t i o n o f a hardener c o n s i s t i n g o f w a l n u t shell flour, p a r a f o r m a l d e h y d e , a n d a s m a l l a m o u n t o f 5 0 % aqueous f o r m a l d e h y d e ( 3 ) . P r e l i m i n a r y studies w i t h these a d hesives i n d i c a t e d veneers u p t o 2 2 % m o i s t u r e content c o u l d be b o n d e d t o m a k e t w o - p l y l a m i n a t e s , p r o v i d e d t h a t the adhesive v i s c o s i t y was at least 12,000 c P a n d adhesive gel t i m e s were u n d e r 1 h o u r at r o o m t e m p e r a t u r e . M o r e d e t a i l e d studies u s i n g 18%-moisture-content veneers ( 1 / 1 0 - i n r o t a r y - p e e l e d D o u g l a s - f i r ) were c a r r i e d o u t t o m a k e t h r e e - p l y g l u e b o n d specimens. T h e t h r e e - p l y s p e c i mens were p r e p a r e d u s i n g a n average glue spread of 61 p o u n d s per 1,000 square feet of double glueline a n d pressed at 127 ° C a n d 175 p s i for 8 m i n u t e s . A v e r a g e w o o d f a i l u r e values o f 8 5 % or better were o b t a i n e d for g l u e b o n d specimens t h a t were either sheared d r y or o b t a i n e d f o l l o w i n g a v a c u u m - p r e s s u r e soak t r e a t m e n t . Specimens sheared after 8 hours of b o i l i n g (two 4-hour cycles) gave o n l y 7 5 % average w o o d f a i l u r e . T h e reason for the l o w w o o d f a i l u r e values for b o i l e d specimens is not e n t i r e l y clear at present. A n average o f 8 5 % w o o d f a i l u r e is r e q u i r e d o f these shear tests for c o m m e r c i a l e x t e r i o r p l y w o o d (5). F o r a c o n t r o l test a n d c o m p a r i s o n o f the effectiveness o f the C P R adhesives for b o n d i n g h i g h - m o i s t u r e - c o n t e n t veneers, a c o m m e r c i a l p h e n o l - f o r m a l d e h y d e p l y w o o d adhesive was tested o n 1 8 % M C veneers under the same c o n d i t i o n s . T h e c o m m e r c i a l p l y w o o d adhesive c o m p l e t e l y f a i l e d t o b o n d the h i g h M C veneers (i.e., there was 0 % w o o d f a i l u r e i n a l l test specimens) ( 3 ) . M i c r o s c o p i c e x a m i n a t i o n o f gluelines i n p l y w o o d specimens m a d e u s i n g the C P R adhesive a n d 1 8 % M C veneers showed severe t r a c h e i d compression i n the e a r l y w o o d zones of g r o w t h rings (4). T h i s o b s e r v a t i o n i n d i c a t e s t h a t 175 p s i is a n excessive pressure for such h i g h - m o i s t u r e - c o n t e n t veneers. Loss i n p a n e l thickness due t o w o o d compression d u r i n g hot pressing is o f concern i n present c o m m e r c i a l p l y w o o d p r o d u c t i o n a n d is addressed b y the use of step-pressing


390



techniques (10,11). I n the case of panels m a d e f r o m h i g h - m o i s t u r e - c o n t e n t veneers, m u c h lower pressing pressures w i l l also have t o be i n v e s t i g a t e d . T o test the c o l d - s e t t i n g a b i l i t y o f the C P R adhesives, g l u l a m specimens were prepared using vertically planed Douglas-fir boards w i t h an i n i t i a l moisture content of 1 1 % (3). A d h e s i v e was a p p l i e d at a n average rate of 70.5 p o u n d s per 1,000 square feet o f single glueline, a n d the specimens pressed at r o o m t e m p e r ature a n d 150 p s i for 24 h o u r s . W o o d f a i l u r e values above 9 0 % were o b t a i n e d for d r y , v a c u u m - p r e s s u r e soak, a n d 2-hour b o i l - t r e a t e d shear specimens. T h e A m e r i c a n I n s t i t u t e of T i m b e r C o n s t r u c t i o n S t a n d a r d s requires a m i n i m u m of 7 0 % w o o d f a i l u r e for d r y shear ( A I T C T 1 0 7 ) ; a n d i n a d d i t i o n , the adhesive has t o pass a s i x - p l y e n d d e l a m i n a t i o n test ( A I T C T 1 1 0 ) (6). T h e 2-hour b o i l i n g test is i n c r e a s i n g l y done t o test new l a m i n a t i n g adhesives (7,8). L i t t l e is k n o w n a b o u t the m o l e c u l a r s t r u c t u r e of the c a r b o h y d r a t e - u r e a p h e n o l i c - b a s e d resins, a n d t h e i r f o r m a t i o n appears c o m p l e x . M o l e c u l a r weight d i s t r i b u t i o n s o b t a i n e d for the p e r a c e t y l a t e d C P R resin b y gel p e r m e a t i o n c h r o m a t o g r a p h y ( G P C ) differ d e p e n d i n g o n the detection m e t h o d . F i g u r e 1 c o m pares d e t e c t i o n b y refractive i n d e x ( R I ) t o detection b y U V (254 n m ) . T h r e e d i s t i n c t c o m p o n e n t s seem t o be f o r m e d i n the C P R resin as i n d i c a t e d b y the R I curve. H o w e v e r , o n l y one c o m p o n e n t o f the resin is detected b y U V a b s o r p t i o n , i n d i c a t i n g i n c o r p o r a t i o n o f p h e n o l a n d resorcinol i n t o t h a t resin c o m p o n e n t . V a p o r phase o s m o m e t r y ( V P O ) o f the p e r a c e t y l a t e d C P R resin gave M „ o f 1,121, w h i c h is i n close agreement w i t h the G P C results u s i n g R I d e t e c t i o n . A l t h o u g h one m i g h t have expected furans t o be i n v o l v e d i n p o l y m e r f o r m a t i o n because of the w e l l - k n o w n d e g r a d a t i o n o f glucose u n d e r a c i d c o n d i t i o n s t o p r o d u c e 5 - h y d r o x y m e t h y l f u r f u r a l (12), C h r i s t e n s e n a n d G i l l e s p i e (2) a n d C l a r k et a l . (3) have n o t e d the absence of f u r a n resonances i n the C - N M R s p e c t r a of these resins. M o r e o v e r , the C - N M R s p e c t r a of the p e r a c e t y l a t e d C P R r e s i n , its precursor a c i d stage resin, a n d resol stage resin a l l show s t r o n g resonances i n the 60 t o 80 p p m r e g i o n , suggesting the f o r m a t i o n of g l u c o s y l u r e a derivatives (4). I n p a r t i c u l a r , the C - l or a n o m e r i c c a r b o n resonance, w h i c h occurs at 97 p p m i n /?-D-glucose (13), is shifted u p f i e l d t o a b o u t 80 p p m i n the resins ( F i g u r e 2) a n d 82 p p m i n a n a u t h e n t i c s a m p l e of N - g l u c o s y l u r e a (I). T h e resonances o c c u r r i n g between 60 a n d 75 p p m are very close t o the c h e m i c a l shifts for the C - 2 t o C - 6 c a r b o n s o f /?-D-glucose a n d N - g l u c o s y l u r e a (I). 1 3

1 3

I

II



27.


K A R C H E S Y E T A L

0 2

4

6

8

10

12

14

1 6 1 8 2 0 2 2

24

391

2628

ELUTION TIME (min.) F i g u r e 1. M o l e c u l a r weight d i s t r i b u t i o n s o f p e r a c e t y l a t e d C P R resin b y G P C .





27.

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393


M o r e recently, K a r c h e s y et a l . ( O r e g o n S t a t e U n i v e r s i t y , u n p u b l i s h e d d a t a ) have i d e n t i f i e d b o t h N - g l u c o s y l u r e a (I) a n d Ν,Ν'-diglucosylurea (II) i n the glucose-urea-phenol-based a c i d stage r e s i n . O t h e r h i g h l y colored, water-soluble r e a c t i o n p r o d u c t s are also f o r m e d . T h e i d e n t i f i c a t i o n of these colored r e a c t i o n p r o d u c t s as w e l l as the d e t e r m i n a t i o n of the role of the glucosylureas i n C P R resin f o r m a t i o n are c u r r e n t l y u n d e r i n v e s t i g a t i o n . B e n n a n d Jones have s h o w n glucosylureas t o be very stable i n aqueous s o l u t i o n s u p t o p H 11 where d e g r a d a t i o n becomes significant (14)- T h e i r o b s e r v a t i o n correlates w e l l w i t h our o b s e r v a t i o n t h a t C P R - b a s e d adhesives are c o m p l e t e l y w a t e r p r o o f w h e n cured w i t h f o r m u l a t i o n p H ' s between 7.8 a n d 10.8 ( 3 ) , b u t were n o t waterproff w h e n f o r m u l a t e d above p H 11. However, the s t a b i l i t y of glucosylureas b e l o w p H 11 ( a n d i n t o the a c i d i c range) is u n u s u a l w h e n c o m p a r e d t o other g l u c o s y l a m i n e c o m p o u n d s . T h i s m a y be due t o the low b a s i c i t y of the n i t r o g e n g r o u p at C - l , w h i c h is a t o the u r e a c a r b o n y l . F o r i n s t a n c e , b o t h g l u c o s y l u r e a a n d N - a c e t y l - D - g l u c o s y l a m i n e do not undergo m a n y of the u s u a l d e g r a d a t i o n reactions of hexoses s u b s t i t u t e d w i t h amines at C - l (14,15). W h e n glucose is reacted w i t h m e t h y l a m i n e u n d e r o n l y s l i g h t l y a c i d i c c o n d i t i o n s , a vast a r r a y of r e a c t i o n p r o d u c t s i n c l u d i n g furans a n d pyrroles is p r o d u c e d (16). T h e low b a s i c i t y of g l u c o s y l u r e a nitrogens s h o u l d not be confused w i t h t h e i r n u c l e o p h i l i c i t y , w h i c h depends m o r e o n p o l a r i z a b i l i t y (17). T h e glucosylurea nitrogens are q u i t e capable o f entering i n t o p o l y m e r i z a t i o n a n d c r o s s l i n k i n g re actions w i t h m e t h y l o l or s i m i l a r f u n c t i o n a l groups. O n a m o r e s p e c u l a t i v e note, the h y d r o x y l groups of the g l u c o s y l m o i e t y m a y also be e n t e r i n g i n t o c o n d e n s a t i o n reactions w i t h p h e n o l i c m e t h y l o l groups i n the second b a s e - c a t a l y z e d r e a c t i o n stage of resin synthesis. C o n n e r et a l . (18) have recently s h o w n ev idence t h a t h y d r o x y l groups of sugars a n d other p o l y o l s are b o n d i n g v i a a n ether linkage (to at least a l i m i t e d extent) i n c a r b o h y d r a t e extended or m o d i fied p h e n o l - f o r m a l d e h y d e - based resols. Conclusions C a r b o h y d r a t e - p h e n o l i c - b a s e d resins c a n be m o d i f i e d t o change t h e i r p h y s i c a l a n d c h e m i c a l properties, a n d faster c u r i n g adhesives c a n be m a d e f r o m these m o d i f i e d resins. However, the n a t u r e of the research presented here is ex p l o r a t o r y , a n d m u c h r e m a i n s t o be done. I n p a r t i c u l a r , the m o l e c u l a r s t r u c t u r e o f these resins needs t o be defined. A cknowledgment s T h i s w o r k was s u p p o r t e d b y grants f r o m the Forest R e s e a r c h L a b o r a t o r y , O r e gon S t a t e U n i v e r s i t y , a n d the U n i t e d States D e p a r t m e n t of A g r i c u l t u r e C o m p e t i t i v e R e s e a r c h G r a n t s P r o g r a m for Forest a n d R a n g e l a n d R e n e w a b l e Resources ( 8 6 - F S T Y - 9 - 0 1 7 5 ) . T h i s is p a p e r n u m b e r 2271 f r o m the Forest R e s e a r c h L a b o ratory, Oregon State University.


394


Literature

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1. Gibbons, J. P.; Wondolowski, L. Carbohydrate-Phenol Based Condensation Resins Incorporating Nitrogen-Containing Compounds; Canadian Patent 1 090 026, 1980. 2. Christiansen, A. W.; Gillespie, R. H. Forest Prod. J. 1986, 36, 20-28. 3. Clark, R. J.; Karchesy, J. J.; Krahmer, R. L. Forest Prod. J., submitted for publication. 4. Clark, R. J. Investigation of a New Resin as an Exterior Adhesive to Bond High Moisture Content Veneers, MS Thesis, Oregon State University, Corvallis, June 1986.


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Fast-Curing Carbohydrate-Based Adhesives - ACS Publications

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