Chapter 28
Estimating the Adhesive Quality of Lignins for Internal Bond Strength
Downloaded by UNIV OF SYDNEY on May 29, 2013 | http://pubs.acs.org Publication Date: July 31, 1989 | doi: 10.1021/bk-1989-0397.ch028
Natsuko Cyr and R. George S. Ritchie Alberta Research Council, P.O. Box 8330, Station F, Edmonton, Alberta T6H 5X2, Canada
Quantitative C-13 N M R spectrometry and size exclusion chromatography afforded sufficient data on lignins from steam-exploded aspen wood, steam-exploded aspen bark waste, and a Kraft hardwood to determine the number, and kind, of reactive sites which undergo reaction with phenol-formaldehyde resin. A model was proposed to predict the quality of the resin produced, when such lignins are methylolated. Board performance (as IB) was taken as a measure of the binder quality. Preliminary testing of boards made from methylolated lignin:phenol-formaldehyde compositions with up to 50% lignin content not only passed the Canadian Standards for random-oriented waferboard but agreed well with the prediction model. T h e cost of w o o d adhesives i n r e c o n s t i t u t e d b o a r d industries can be a s u b s t a n t i a l p o r t i o n of p r o d u c t i o n costs. T h e fact t h a t lignins are less expensive t h a n P F resins alone makes t h e m a t t r a c t i v e to use i n resins as extenders. H o w e v e r , the c h e m i c a l s t r u c t u r e of l i g n i n s isolated f r o m various l i g n o c e l l u losics depends o n the p l a n t species, the m e t h o d of i s o l a t i o n , a n d age of the l i g n i n f r a c t i o n (1). A d h e s i v e s w h i c h are derived i n p a r t f r o m l i g n i n tend to have a h i g h degree o f v a r i a b i l i t y i n p e r f o r m a n c e , due to s u c h l i g n i n v a r i a tions. O n e a p p r o a c h to reduce these variations is to m a s k the l i g n i n under a surface of grafted c o p o l y m e r (2-5). A n alternative is to develop a m e t h o d whereby l i g n i n s u i t a b i l i t y for a b i n d e r c a n be assessed, for the p u r p o s e of pre-screening l i g n i n samples w i t h o u t resorting to extensive f o r m u l a t i o n s a n d t e s t i n g ; such is the o b j e c t i v e o f this p a p e r . 0097-6156/89A)397-0372$06.00A) © 1989 American Chemical Society
In Lignin; Glasser, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
28.
C Y R & RITCHIE
Estimating the Adhesive Quality ofLignins
373
Experimental S t e a m - e x p l o d e d l i g n i n s were prepared f r o m aspen by the T i g n e y process (6) a n d p u r i f i e d as described (1). A c e t y l a t i o n s were a c c o m p l i s h e d i n acetic a n h y d r i d e - p y r i d i n e a n d the acetates isolated i n the u s u a l f a s h i o n . M e t h y l o l a t i o n s were p e r f o r m e d as described b y M a r t o n et al. (7). Q u a n t i t a t i v e C - 1 3 N M R s p e c t r a were o b t a i n e d u s i n g "inverse gated d e c o u p l i n g " o n a B r u k e r W P - 8 0 spectrometer at 20.1 M H z . T y p i c a l l y 10,000 scans were a c c u m u l a t e d . C h r o m i u m acetoacetonate was added to the c h l o r o f o r m - d a n d D M S O - d s o l u t i o n s to shorten T i . S p e c t r a i n D M S O were o b t a i n e d at 363 Κ to increase the c h r o m i u m salt s o l u b i l i t y a n d decrease viscosity. Size e x c l u s i o n c h r o m a t o g r a p h y was p e r f o r m e d o n a M i l t o n - R o y H P L C us i n g three W a t e r s c o l u m n s (one 1000 A , two 10,000 A ) . L i g n i n acetates were a n a l y z e d as 1% solutions i n d i c h l o r o m e t h a n e , u s i n g a flow rate of 2 m L / m i n , a n d a U V detector (254 n m ) . E l u t i o n t i m e s were c a l i b r a t e d against p o l y s t y r e n e s t a n d a r d s . C a l c u l a t i o n s o n m o l e c u l a r weights were p e r f o r m e d b y the m e t h o d of Y a u et al. (8). T h e l i g n i n s were separately m i x e d w i t h P F resin ( d r y b l e n d i n g ) at 3 0 % and 5 0 % levels p r i o r t o a p p l i c a t i o n as a b i n d e r . T h e f o r m a t i o n a n d t e s t i n g (9) of the waferboards (30 c m χ 30 cm) were done b y the A l b e r t a Research Council Panel Testing Laboratory.
Downloaded by UNIV OF SYDNEY on May 29, 2013 | http://pubs.acs.org Publication Date: July 31, 1989 | doi: 10.1021/bk-1989-0397.ch028
6
Parameters B a c k (10) has i n d i c a t e d t h a t superior b o a r d performance is achieved w i t h covalent b o n d i n g of the adhesive to the w o o d . A b i n d e r , t h e n , must have at least the m i n i m u m n u m b e r of reactive sites per molecule. If there is one or fewer such sites, t h e n the l i g n i n s h o u l d behave as a filler, w h i c h m a y or m a y not be c h e m i c a l l y b o u n d to the resin. I n the case of t w o reactive sites, a " l i n e a r " m a c r o m o l e c u l e is possible, or the l i g n i n m a y be considered to behave as a n extender for a resin. W h e n three or more sites are a v a i l a b l e , c r o s s l i n k i n g c a n o c c u r a n d the l i g n i n c o u l d t h e n b e c o m e a f u l l p a r t n e r i n the crosslinked b i n d e r . O n e m a y project h o w the l i g n i n c o u l d behave, once the reactive sites o n the l i g n i n molecule have been m a p p e d . F o r t h i s c h a p t e r , the i n t e r a c t i v e sites w i l l be alcohols a n d b e n z y l alcohols, to s i m u l a t e the r e a c t i o n of P F resins w i t h the c a r b o h y d r a t e s i n the w o o d . U n l i k e P F resins, l i g n i n s do not have m e t h y l o l groups a t t a c h e d to the a r o m a t i c r i n g s . However, m e t h y l o l s can be grafted o n t o l i g n i n at ph y d r o x y b e n z y l a n d g u a i a c y l sites C - 3 , C - 5 a n d C - 5 , respectively, p r o v i d e d these C9 u n i t s of the l i g n i n have a free p h e n o l i c O H g r o u p at C - 4 . N o r e a c t i o n occurs w i t h s y r i n g y l groups. I n h a r d w o o d s , the p r o p o r t i o n of p h y d r o x y b e n z y l groups is very s m a l l c o m p a r e d to g u a i a c y l a n d s y r i n g y l u n i t s , so C - 4 free g u a i a c y l groups measure the n u m b e r of p o t e n t i a l m e t h y l o l a t i o n sites. T h e m e t h o x y l s i g n a l i n C - 1 3 N M R s p e c t r a of l i g n i n s ( b o t h free, i n D M S O , a n d a c e t y l a t e d , i n chloroform) is u n a m b i g u o u s a n d was used as a reference for the benzene r i n g . T h e integrated m e t h o x y l s i g n a l w i l l v a r y d e p e n d i n g u p o n the a m o u n t s of s y r i n g y l , g u a i a c y l , a n d p - h y d r o x y p h e n y l
In Lignin; Glasser, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
374
LIGNIN: PROPERTIES AND MATERIALS
groups present. Since the last is ignored here, t h e r a t i o o f s y r i n g y l t o g u a i a c y l o r " s / g " r a t i o can be d e t e r m i n e d b y N M R b y a n u m b e r o f m e t h o d s . F r o m the l i g n i n s p e c t r u m i n D M S O define: a
s
g
[area i n region 100 — 109 ppm] ^ 2 [area i n region 109 — 123 ppm]
3
T h e n u m b e r o f m e t h o x y l groups p e r average benzene r i n g :
Downloaded by UNIV OF SYDNEY on May 29, 2013 | http://pubs.acs.org Publication Date: July 31, 1989 | doi: 10.1021/bk-1989-0397.ch028
"=S t
2
