Lignin in Adhesives - ACS Publications - American Chemical Society

Chapter 2 Lignin in Adhesives Introduction and Historical Perspective

Downloaded by UNIV LAVAL on June 18, 2014 | http://pubs.acs.org Publication Date: December 31, 1989 | doi: 10.1021/bk-1989-0385.ch002

Norman

G.

Lewis

and

Thomas

R.

Lantzy

W o o d Chemistry and Biochemistry Department of Forest Products V i r g i n i a Polytechnic Institute and State University Blacksburg, VA 24061

Lignins, complex organic polymers produced by all vascular terrestrial plants and second in abundance only to cellulose, are the substances holding plant fibers together. They are recovered mainly as byproducts from woodpulping operations with about 75 million tons produced annually worldwide. Over the last hundred years or so, there has been an enormous effort to develop lignin-based adhesives, but this has met with no real commercial success principally due to product variability, dark color, and lack of chemical reactivity. Lignin-based adhesives normally require excessively long curing times and high curing temperatures during composite board production. However, they can be employed as extenders in diverse wood-composite resins with no significant deterioration in the mechanical properties of the board products. This is particularly true if the lignins are activated chemically (e.g., by methylolation). Several recent reports have been directed toward improving even further the lignin chemical reactivity (e.g., in the synthesis of soda bagasse lignin-formaldehyde-resorcinol and lignin-isocyanate resins). Apparently, these developments surmount previous difficulties and allow for some cautious optimism for the future. T e r r e s t r i a l vascular p l a n t s have evolved w i t h a u n i q u e c a p a c i t y t o synthesize l i g n i n , whose m a i n p h y s i o l o g i c a l functions are t o p r o v i d e r i g i d i t y a n d s t r e n g t h to p l a n t cell w a l l s a n d t o act as a b a r r i e r t o i n f e c t i o n (1). N e x t t o cellulose, l i g n i n i s n a t u r e ' s second most a b u n d a n t o r g a n i c m a t e r i a l a n d is often described as the m a t e r i a l b i n d i n g p l a n t fibers together (#). 0097-6156/89/0385-0013$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.


14

ADHESIVES F R O M RENEWABLE RESOURCES

I n g y m n o s p e r m s , l i g n i n is f o r m e d f r o m j>-coumaryl (1) a n d c o n i f e r y l (2) a l cohols, whereas, i n angiosperme, s i n a p y l a l c o h o l (3) is also i n v o l v e d (3). L i g n i f i c a t i o n is generally viewed t o occur e x c l u s i v e l y v i a the r a n d o m , d e h y d r o g e n a t i v e p o l y m e r i z a t i o n o f m o n o l i g n o l s (1) - (3) (^), a r e a c t i o n r e q u i r i n g b o t h H2O2 a n d peroxidase (5) ( F i g u r e 1). It is also w e l l k n o w n t h a t the r a t i o of m o n o l i g n o l s (1) - (3) i n v o l v e d i n the l i g n i f i c a t i o n process is species (6), t i m e (7), l i g h t (8) a n d m o r p h o l o g i c a l o r i g i n (9,10) dependent. L i g n i f i c a t i o n c a n a p p a r e n t l y also be influenced b y g r a v i t a t i o n a l forces experienced b y p l a n t s d u r i n g g r o w t h (11-19). T h e f o l l o w i n g conclusions c a n be m a d e w i t h regard t o current knowledge of the l i g n i f i c a t i o n process: 1) s t r i c t e n z y m a t i c c o n t r o l l e a d i n g t o the f i n a l p r o d u c t a p p a r e n t l y does n o t o c c u r a n d 2) b o t h r e g u l a t o r y processes c o n t r o l l i n g i t s d e p o s i t i o n a n d s t r u c t u r e in situ are p o o r l y u n d e r s t o o d . L i g n i n f o r m a t i o n i n p l a n t s of m a n y f a m i l i e s of A n g i o s p e r m a e , p a r t i c u l a r l y those i n C o m m e l i n i d a e of C r o n q u i s t (SO) (i.e., grasses), suffers f r o m a n a d d i t i o n a l c o m p l i c a t i o n due t o the presence of c e l l - w a l l - b o u n d h y d r o x y c i n n a m i c acids, s u c h as p - c o u m a r i c (4) (21), ferulic (5) (22,23), 5 - h y d r o x y f e r u l i c (6) (24), d i f e r u l i c (7) (23,25,26), a n d 4 , 4 / - d i h y d r o x y t r u x i l l i c (8) acids (27) ( F i g u r e 2). T h e s e acids have l o n g been s p e c u l a t e d t o b e i n v o l v e d d i r e c t l y i n l i g n i f i c a t i o n , a n d t h i s is o n l y n o w b e i n g c l a r i f i e d . T h i s c l a r i f i c a t i o n was achieved b y a d m i n i s t e r i n g specifically C - l a b e l l e d forms of ferulic a c i d (5) t o wheat ( Triiicum aestivum L . ) . Subsequent a n a l y s i s of the p l a n t tissue (28) a n d i s o l a t e d l i g n i n s (29) b y s o l i d a n d s o l u t i o n state C - N M R , respectively, revealed t h a t these acids were c o v a l e n t l y b o n d e d t o l i g n i n . 1 3

1 3

Lignin and Papermaking T h e discovery of l i g n i n a n d i t s subsequent use i n adhesive f o r m u l a t i o n s are i n t e r t w i n e d w i t h t e c h n o l o g i c a l developments i n the p u l p a n d p a p e r i n d u s t r y . S u r p r i s i n g l y , i t is not generally a p p r e c i a t e d t h a t the massive w o r l d w i d e p r o d u c t i o n of p u l p a n d p a p e r , u s i n g w o o d as a resource, o n l y b e g a n a b o u t 1850 or so. T h i s is i n spite of the fact t h a t f o r m a t i o n of p a p e r f r o m w o o d (e.g., b y t r e a t m e n t o f m u l b e r r y b a r k w i t h lye) h a d been developed b y T s ' a i L u n i n 105 A . D . i n C h i n a (SO) a n d perhaps even earlier b y others (31). However, b y the t i m e t h i s t e c h n o l o g y reached the W e s t e r n H e m i s p h e r e v i a the M i d d l e E a s t , w o o d h a d l o n g been replaced b y other p l a n t m a t e r i a l s o f higher cellulosic c o n tent (e.g., c o t t o n , l i n e n , flax). W o o d r e m a i n e d forgotten as a source of p a p e r u n t i l R e n e A . F . de R e a m u r (1683-1757) n o t e d t h a t wasps p r o d u c e d a p a p e r l i k e nest f r o m w o o d (30). I n 1839, P a y e n discovered t h a t w o o d was n o t homogeneous b u t c o n t a i n e d cellulose a n d a n " i n c r u s t i n g m a t e r i a F (32-34) for w h i c h S c h u l t z e coined the t e r m l i g n i n i n 1865 (35). A s s h o w n i n T a b l e I, c h e m i c a l w o o d p u l p i n g processes were developed t o dissolve away l i g n i n , hemicelluloses, a n d extractives (30). T h e s e represent the m a j o r w o o d p u l p i n g processes i n o p e r a t i o n today.


LEWIS AND LANTZY

Lignins in Adhesives: Introduction

-Η©· OH

free-radical coupling

1, R R «H r

2

2, R «OCH ,R aH 1

3

2

3, R .R «OCH


t

2

Lignin

3

F i g u r e 1. L i g n i n f o r m a t i o n f r o m m o n o l i g n o l s (1) - (3).


16


T a b l e I. M a j o r C h e m i c a l P u l p i n g Processes Type

Developer(s)

Soda Sulphite Kraft

T i l g h m a n (1857) D a h l (1884)

Burgess a n d W a t t (1854)


Sources of Lignin E s s e n t i a l l y a l l of the l i g n i n c o m m e r c i a l l y a v a i l a b l e is i s o l a t e d as b y p r o d u c t s f r o m either the s u l p h i t e or the k r a f t process. T a b l e II gives a very conservative i d e a of a n n u a l l i g n i n p r o d u c t i o n i n the U n i t e d States (2) a n d w o r l d w i d e (36). T a b l e I I . E s t i m a t e d A n n u a l P r o d u c t i o n of L i g n i n ( m i l l i o n s of tons) U n i t e d States

Worldwide

Kraft lignin

Type

20

Lignosulphonates (Sulphite pulping)

1.5

75 15

I n a d d i t i o n t o k r a f t or s u l p h i t e l i g n i n s , there are a n u m b e r of other p r o cesses w h e r e b y l i g n i n c o u l d p o t e n t i a l l y be recovered (e.g., b y a c i d o l y s i s , s t e a m e x p l o s i o n , organosolv, b i o l o g i c a l t r e a t m e n t , etc). T h e e x i s t i n g a n d p o t e n t i a l processes for l i g n i n recovery are described briefly i n the f o l l o w i n g sections. Kraft Pulping. T h i s is the m a j o r w o o d p u l p i n g process a n d p o t e n t i a l l y represents the p r i m a r y source of t e c h n i c a l l i g n i n . D u r i n g p u l p i n g , cellulosic fibers are o b t a i n e d f r o m w o o d b y t r e a t m e n t w i t h solutions of s o d i u m h y d r o x i d e a n d s u l p h i d e at elevated t e m p e r a t u r e s , pressure a n d h i g h p H . T h e hemicelluloses ( a n d some cellulose) are degraded t o give m a i n l y isosaccharinic acids, the c h e m i s t r y of w h i c h has been adequately described elsewhere (37). O n the other h a n d , the l i g n i n released f r o m the cell w a l l d u r i n g p u l p i n g increases i n m o l e c u l a r size as d e l i g n i f i c a t i o n proceeds (38) a n d is h i g h l y p o l y disperse (39). K r a f t spent p u l p i n g l i q u o r s thus consist of l i g n i n ( « 4 7 % ) , h y d r o x y acids ( « 2 8 % , e.g., i s o s a c c h a r i n i c a c i d ) , i n o r g a n i c s , a n d s m a l l q u a n t i t i e s of other organics (37). L i g n i n c a n be i s o l a t e d f r o m these spent p u l p i n g l i q u o r s as a p r e c i p i t a t e b y a c i d i f i c a t i o n . A l m o s t a l l k r a f t l i g n i n is b u r n e d for energy recovery; however, a b o u t 35,000 tons are p r o d u c e d a n n u a l l y i n the U n i t e d States for v a r i o u s c h e m i c a l b y p r o d u c t s (2). Sulphite Pulping. T h i s is a generic t e r m used t o describe various s u l p h i t e c h e m i c a l w o o d p u l p i n g processes c a r r i e d out at different pH's a n d p u l p y i e l d s (37). ( T h e t e r m y i e l d refers t o the q u a n t i t y o f fiber recovered f r o m the o r i g i n a l wood.)


2.

LEWIS AND LANTZY

17


I n the a c i d s u l p h i t e l o w - y i e l d p u l p i n g process, w o o d is t r e a t e d w i t h s o l u t i o n s of a m m o n i u m , s o d i u m , calcium, or magnesium s u l p h i t e / b i s u l p h i t e at low p H ( « 1-2) a n d a t elevated t e m p e r a t u r e s a n d pressures. D u r i n g t h i s t r e a t m e n t , ess e n t i a l l y a l l o f the l i g n i n s , hemicelluloses, a n d e x t r a c t i v e s are " d i s s o l v e d " away, l e a v i n g b e h i n d a fiber o f very h i g h cellulosic content. T h e hemicelluloses u n d e r g o severe h y d r o l y s i s , affording m a i n l y w a t e r - s o l u b l e monosaccharides i n t h e spent s u l p h i t e l i q u o r ( S S L ) . T h e s u l p h o n a t e d l i g n i n fragments released i n i t i a l l y d u r i n g d e l i g n i f i c a t i o n ( « 3 0 - 4 0 % l i g n i n removal) are m a i n l y m o n o m e r i c

compounds

c o n t a i n i n g either m o n o - , d i - , o r t r i - s u l p h o n i c acids (3,40). H o w e v e r , as d e l i g n i f i c a t i o n proceeds, t h e l i g n i n released f r o m t h e c e l l w a l l i s o f i n c r e a s i n g m o l e c u l a r


size a n d h i g h l y p o l y disperse (40). C o n s e q u e n t l y , l o w - y i e l d spent s u l p h i t e l i q u o r s ( S S L ) recovered after w o o d p u l p i n g l a r g e l y consist o f l i g n o s u l p h o n a t e s ( « 5 5 % ) , w o o d sugars ( « 2 5 - 3 0 % , e.g., glucose, m a n n o s e , etc.), i n o r g a n i c s , a n d other o r ganics i n s m a l l e r a m o u n t s (37). D e p e n d i n g u p o n the p u l p i n g process e m p l o y e d , l i g n o s u l p h o n a t e s c a n b e recovered as either a m m o n i u m , c a l c i u m , s o d i u m , o r m a g n e s i u m salts (35).

Currently, only about 2 0 % of a l l S S L materials pro-

d u c e d are used as c h e m i c a l p r o d u c t s (e.g., as i n e x p e n s i v e b i n d e r s f o r a n i m a l feed a n d d i r t r o a d s , a n d as dispersants i n o i l - w e l l d r i l l i n g a n d

cement/concrete

admixtures). Acidolysis. A c i d o l y s i s l i g n i n s are recovered f r o m processes w h e r e b y p l a n t m a t e r i a l (e.g., w o o d ) is saccharified b y t r e a t m e n t w i t h m i n e r a l acids, s u c h as s u l p h u r i c o r h y d r o c h l o r i c acids (41)· T h e l i g n i n is recovered p r i m a r i l y as a n i n s o l u b l e residue. T h i s process h a s been used i n E u r o p e i n t h e p a s t , b u t is n o t commercially important i n the Western Hemisphere. Steam Explosion. S t e a m - e x p l o s i o n l i g n i n s are o b t a i n e d f r o m w o o d (or some other p l a n t m a t e r i a l ) t h a t h a s been s u b j e c t e d briefly t o h i g h t e m p e r a t u r e s a n d pressures followed b y r a p i d decompression (42). T h i s process i s used t o a l i m i t e d extent t o d a y p a r t i c u l a r l y for t h e processing o f l o w - q u a l i t y h a r d w o o d s .

The

l i g n i n s recovered are o f r e l a t i v e l y l o w m o l e c u l a r weight ( M « 700) a n d soluble n

i n either a l k a l i o r c e r t a i n o r g a n i c solvents (42,43). Organosolv. O r g a n o s o l v l i g n i n s are o b t a i n e d as r e l a t i v e l y l o w - m o l e c u l a r - w e i g h t entities b y t r e a t m e n t o f p l a n t tissue w i t h aqueous s o l u t i o n s o f o r g a n i c solvents, n o r m a l l y c o n t a i n i n g trace a m o u n t s o f m i n e r a l acids (44)-

Solvents i n c l u d e

e t h a n o l , m e t h a n o l , b u t a n o l , acetic a c i d , e t h y l acetate, p h e n o l , e t c . T o d a t e , t h i s a p p r o a c h h a s n o t been c o m m e r c i a l i z e d d u e t o t h e q u a n t i t i e s o f o r g a n i c solvents c o n s u m e d a n d t h e l o w q u a l i t y o f the p u l p fiber o b t a i n e d . Enzymatic Liberation. T h i s is a process w h e r e b y l i g n i n - r i c h residues are o b tained b y treatment of plant material w i t h cellulases/hemicellulases/pectinases, etc.

T h e s e l i g n i n p r e p a r a t i o n s m a y m o r e closely resemble " n a t i v e " l i g n i n d u e

to t h e v e r y m i l d processing c o n d i t i o n s e m p l o y e d .


18


Lignin in Wood-Composite Adhesives


It is w o r t h w h i l e t o review the U . S . m a r k e t size for the four p r i n c i p a l resins c u r r e n t l y used i n w o o d - p a n e l p r o d u c t s t o d a y (^5). T h e s e are p h e n o l - f o r m a l d e h y d e ( P F ) , urea-formaldehyde ( U F ) , melamine-formaldehyde ( M F ) , and resorcinolf o r m a l d e h y d e ( R F ) ( T a b l e I I I ) . W h e n these p r o d u c t i o n figures are c o m p a r e d t o the q u a n t i t i e s o f l i g n i n p o t e n t i a l l y a v a i l a b l e ( T a b l e I I ) , i t is i m m e d i a t e l y o b v i o u s t h a t a l l w o o d adhesives c o u l d be replaced b y o n l y a very s m a l l f r a c t i o n o f the l i g n i n p r o d u c e d a n n u a l l y d u r i n g c h e m i c a l w o o d p u l p i n g processes.

T a b l e I I I . U . S . P r o d u c t i o n of T h e r m o s e t t i n g R e s i n s (1983) i n M e t r i c T o n s (χ 1 0 ) 3

Resin

1

RF UF MF PF Totals

Total

Wood Products

15.5 586.0 90.0 660.0

1.5 440.0 5.0 297.0

1,351.5

743.5

* R F = resorcinol-formaldehyde U F = urea-formaldehyde M F = melamine-formaldehyde P F = phenol-formaldehyde

T h e r e have been m a n y a t t e m p t s t o replace these resins w i t h l i g n i n d e r i v a tives for w o o d c o m p o s i t e adhesives s u i t a b l e for p l y w o o d , p a r t i c l e b o a r d a n d w a f e r b o a r d . M o s t of these studies have been e m p i r i c a l i n n a t u r e , a n d few have achieved f u r t h e r c o n s i d e r a t i o n for i n d u s t r i a l a p p l i c a t i o n . A s w o o d b i n d e r s , t e c h n i c a l l i g n i n s are v a r i a b l e i n q u a l i t y a n d p o o r l y reactive i n c o m p a r i s o n t o c o n v e n t i o n a l resin systems s u c h as p h e n o l - f o r m a l d e h y d e ( P F ) resins. C o n s e q u e n t l y , t h e y are n o t u t i l i z e d o n t h e i r o w n . Indeed, i f t h e y were, t h i s w o u l d adversely affect p r o d u c t i o n q u a l i t y a n d t i m e s , a n d necessitate e q u i p m e n t changes. I n the w o o d c o m p o s i t e i n d u s t r y , resins h a v i n g s u c h deleterious effects are n o t l i k e l y t o be used even if savings could be made in terms of material costs. Progress t o w a r d p r o d u c i n g i n d u s t r i a l l y acceptable l i g n i n adhesives c a n be b r o k e n d o w n i n t o t w o m a i n categories as either 1) l i g n i n - b a s e d b i n d e r s (by themselves as s u c h or i n c h e m i c a l l y m o d i f i e d f o r m ) , or 2) as c o p o l y m e r s ( w i t h other r e a c t a n t adhesives). Lignosulphonates. C r u d e S S L c a n be o b t a i n e d as a b r o w n i s h , s p r a y - d r i e d powder or as a viscous, hygroscopic, d a r k - c o l o r e d l i q u i d . S S L a n d the l i g n o s u l p h o n a t e s present i n i t have been the focus o f n u m e r o u s efforts to p r o d u c e a n


2.

LEWIS AND LANTZY

19


i n d u s t r i a l l y useful adhesive. E s s e n t i a l l y , there are t w o m e t h o d s o f c u r i n g S S L based adhesives, n a m e l y , b y t h e r m o s e t t i n g o r b y free-radical p o l y m e r i z a t i o n .


W i t h respect t o t h e r m o s e t t i n g , i t h a s been w e l l d o c u m e n t e d t h a t crude C a based S S L c a n be used as a p a r t i c l e b o a r d adhesive (46)- T h i s a p p l i c a t i o n h a s been e v a l u a t e d o n a m i l l - s c a l e basis i n D e n m a r k , F i n l a n d , a n d S w i t z e r l a n d , b u t h a s n o t been a d o p t e d o n a c o m m e r c i a l scale. T h i s is because resin c u r i n g r e q u i r e d b o t h h i g h press a n d autoclave temperatures as w e l l as l o n g h e a t i n g t i m e s . W h i l e t h i s development was a " t e c h n i c a l " success, i t was - a c c o r d i n g to N i m z (36) - never c o m m e r c i a l i z e d due t o t h e frequency o f fires experienced during mill-scale trials. Subsequent a t t e m p t s t o i m p r o v e the properties (i.e., r e a c t i v i t y ) o f S S L adhesives have essentially e m p l o y e d either a c i d i f i c a t i o n o f C a - b a s e d S S L o r m e m b r a n e filtration o f the crude p u l p i n g l i q u o r s . A c i d i f i c a t i o n o f C a - S S L w i t h concentrated H2SO4 gave a n adhesive s u i t a b l e for p a r t i c l e b o a r d a p p l i c a t i o n s (47). However, processing parameters s t i l l required h i g h press t e m p e r a t u r e ( « 204 ° C ) , press t i m e s (5-7 m i n ) , a n d pressure (400 p s i ) . T h e very l o w p H ( < 1) o f t h e resin used m a y also have a l o n g t e r m corrosive effect o n n a i l s , staples, etc., a n d engender w o o d d e t e r i o r a t i o n . M e m b r a n e filtration o f S S L shows s o m e w h a t greater p r o m i s e . T h i s a p p r o a c h was first r e p o r t e d b y Forss et a l . (48,49) * t h e development o f K A R A T E X , a P F / l i g n i n adhesive b i n d e r c o n t a i n i n g either l i g n o sulphonates o r k r a f t l i g n i n s o f n o m i n a l m o l e c u l a r weight > 5000 as d e t e r m i n e d by passage t h r o u g h a m e m b r a n e . A p p l y i n g t h i s a p p r o a c h , S h e n a n d C a l v e (50) e x a m i n e d t h e w a f e r b o a r d b i n d i n g properties o f a n a m m o n i u m - b a s e d S S L t h a t h a d been subjected t o m e m b r a n e filtration. B o a r d pressing c o n d i t i o n s were s t i l l excessive (e.g., 8 m i n at 210 ° C for 1 1 - m m - t h i c k w a f e r b o a r d ) . Best b o a r d properties were o b t a i n e d w i t h t h e l o w - m o l e c u l a r weight permeate ostensibly o f < 5,000 m o l e c u l a r weight a n d c o n t a i n i n g s u l p h o n a t e d l i g n i n s a n d m o n o s a c c h a rides. O n t h e other h a n d , t h e l i g n o s u l p h o n a t e retentate ( > 5,000 m o l w t ) gave b o a r d s o f unacceptable m e c h a n i c a l properties. n

It s h o u l d be recognized at t h i s p o i n t t h a t 1) t h e l i g n o s u l p h o n a t e s ( < 5,000 m o l w t ) l a r g e l y consist o f the m o n o m e r s c o n t a i n i n g m o n o - , d i - o r t r i - s u l p h o n i c acids as p r e v i o u s l y described (3,40), a n d 2) t h e m e c h a n i c a l properties o f t h e waferboards were d e t e r m i n e d largely b y monosaccharide, r a t h e r t h a n l i g n o s u l p h o n a t e , content (51). It was subsequently proven, under c o n t r o l l e d l a b o r a t o r y c o n d i t i o n s , t h a t the rate o f t h e r m o s e t t i n g o f b o t h h i g h a n d l o w m o l e c u l a r weight l i g n o s u l p h o n a t e s can be i m p r o v e d b y increasing the m o n o s a c c h a r i d e c o n tent o f t h e adhesive (52). I n a n analogous m a n n e r , f o r m a l d e h y d e was s h o w n to b e capable o f s l i g h t l y i n c r e a s i n g t h e rate o f t h e r m o s e t t i n g , p r e s u m a b l y b y increasing the rate of crosslinking. However, w h i l e t h e use o f f r a c t i o n a t e d NH4-SSL r e m a i n s a " t e c h n i c a l success," t h e t h e r m o s e t t i n g rates are s t i l l very l o w i n c o m p a r i s o n t o P F resins. Therefore, t h e l i k e l i h o o d o f p r o d u c i n g a c o m m e r c i a l l y acceptable t h e r m o s e t t i n g l i g n o s u l p h o n a t e - b a s e d resin is s t i l l very f a r f r o m reality.


20



L i g n o s u l p h o n a t e s i n crude S S L c a n also be o x i d a t i v e l y p o l y m e r i z e d b y either c h e m i c a l (36,53) or b i o c h e m i c a l (54) means. I n the f o r m e r process, o x i d a n t s such as h y d r o g e n p e r o x i d e , c a t a l y z e d b y s u l p h u r d i o x i d e (or p o t a s s i u m f e r r i c y a n i d e ) , c a n b e used t o cure fermented C a - b a s e d S S L . T h i s was f o u n d t o be s u i t a b l e as a n adhesive for m e d i u m - d e n s i t y i n t e r i o r - g r a d e p a r t i c l e b o a r d (36,53). O p t i m u m b o n d i n g c o n d i t i o n s used 2 0 % o f a t e c h n i c a l S S L ( 5 4 % ) , 9 % o f a 3 5 % H2O2 s o l u t i o n , 4 % a m m o n i u m c h l o r i d e a n d 0.6% SO2 for b o n d i n g p a r t i c l e b o a r d at 120 ° C w i t h a press t i m e o f 5 m i n . D i f f i c u l t i e s experienced p r e v i o u s l y i n c o n t r o l l i n g the e x o t h e r m i c n a t u r e o f t h i s c u r i n g r e a c t i o n have a p p a r e n t l y been resolved. W h e t h e r t h i s development w i l l receive c o m m e r c i a l endorsement r e m a i n s t o be seen. A n o t h e r a p p r o a c h t o c u r i n g S S L b y o x i d a t i v e p o l y m e r i z a t i o n e m p l o y s the use o f e n z y m e s (54)- I n t h a t s t u d y , S S L was t r e a t e d w i t h the w h i t e - r o t fungus Fomes annosus a n d a l a c c a s e - i n d u c i n g phenoloxidase u n t i l the s o l u t i o n reached a " h o n e y l i k e " consistency. T h i s viscous l i q u i d was t h e n s u b s e q u e n t l y used t o b o n d w o o d p a r t i c l e s together at a pressure o f 0.03 k g c m " . It now needs t o be e s t a b l i s h e d w h e t h e r the m e c h a n i c a l properties o f these b o a r d s s u r v i v e accelerated a g i n g tests. T h e a p p l i c a t i o n o f l i g n o s u l p h o n a t e s as a n extender or co-reactant i n P F or U F resins has been w e l l s t u d i e d , i n d u s t r i a l l y a p p l i e d , a n d e x t e n s i v e l y d o c u m e n t e d i n a recent comprehensive review (36). Since t h a t s t u d y , waferboards have been p r o d u c e d w i t h excellent m e c h a n i c a l properties after b e i n g b o n d e d at 204 ° C , 4 t o 5 m i n , 3375 k P a w i t h a l i g n o s u l p h o n a t e - P F resin (55). T h e p h e n o l i c r e s i n was m o d i f i e d b y use o f K 3 F e ( C N ) e s t a b i l i z e d l i g n o s u l p h o n a t e s ; b o t h p h e n o l a n d the l i g n o s u l p h o n a t e s were used i n a p p r o x i m a t e l y e q u a l p r o p o r t i o n s . W i t h r e g a r d t o p o t e n t i a l l i g n i n u t i l i z a t i o n , p e r h a p s some of the best rep o r t e d results have been e n u n c i a t e d by Forss et a l . (48,49). A s described p r e v i o u s l y , spent s u l p h i t e l i q u o r ( S S L ) was s u b j e c t e d t o m e m b r a n e filtration, a n d the l i g n o s u l p h o n a t e retentate ( > 5,000 n o m i n a l m o l e c u l a r weight cutoff) was used t o replace f r o m 40 t o 7 0 % o f P F resin w i t h no significant d e t e r i o r a t i o n i n m e c h a n i c a l b o a r d p r o p e r t i e s . P a r t i c l e b o a r d , p l y w o o d , a n d fiberboard were t h e n p r o d u c e d i n F i n l a n d u s i n g resins c o n t a i n i n g h i g h m o l e c u l a r weight l i g n o s u l p h o n a t e s a n d P F i n m i l l - s c a l e t r i a l s under n o r m a l processing c o n d i t i o n s , a n d t h i s use was a p p a r e n t l y successful (48,49). A w i d e l y s t a t e d advantage o f t h i s u l t r a f i l t r a t i o n a p p r o a c h is t h a t l i g n i n derivatives can be c o m m e r c i a l l y p r o d u c e d w i t h less p r o d u c t v a r i a b i l i t y . H o w e v e r , i t does not a p p e a r t h a t t h i s m e t h o d o l o g y is c u r r e n t l y b e i n g used c o m m e r c i a l l y t o p r o d u c e l i g n i n - b a s e d adhesives. 2

T h e use o f S S L or l i g n o s u l p h o n a t e s i n other p o l y m e r i c adhesive systems has also been e x a m i n e d [e.g., w i t h p o l y a c r y l a m i d e , p r o t e i n s / a l d e h y d e s , p o l y e t h y l e n e o x i d e , p o l y e t h y l e n e i m i n e , epoxides, m e l a m i n e , styrene o x i d e , p o l y i s o c y a n a t e s (36)]. S o f a r , these procedures, for different reasons, have n o t led t o a n y m a j o r p r a c t i c a l a p p l i c a t i o n (36). It w o u l d , however, be i n t e r e s t i n g t o r e e x a m i n e some o f these processes u s i n g n o t crude spent s u l p h i t e l i q u o r s , b u t i n s t e a d those purified by membrane filtration.


2.

LEWIS AND LANTZY

21


Kraft Lignins. K r a f t l i g n i n s are n o r m a l l y o b t a i n e d as b r o w n p o w d e r s h a v i n g very b r o a d m o l e c u l a r weight d i s t r i b u t i o n s a n d v a r i a b l e p r o p e r t i e s . B o t h color a n d p r o d u c t v a r i a b i l i t y are disadvantages t h a t do n o t l e n d themselves t o easy acceptance b y resin m a n u f a c t u r e r s . T h e y are m a i n l y u n s u i t a b l e as t h e r m o s e t t i n g adhesives since press t i m e s r e q u i r e d are t o o l o n g , press t e m p e r a t u r e s t o o h i g h , a n d b o a r d / p a n e l m e c h a n i c a l properties p o o r .

T h i s p o o r resin r e a c t i v

i t y is r e a d i l y e x p l a i n a b l e ; the p h e n o l i c content o f k r a f t l i g n i n is l o w , there are n o r m a l l y s u b s t i t u e n t s ortho a n d para t o a n y p h e n o l i c h y d r o x y l f u n c t i o n a l i t i e s , a n d i t s m o l e c u l a r size m a y prevent efficient c r o s s l i n k i n g reactions due t o steric constraints.


K r a f t l i g n i n s are e s s e n t i a l l y a c i d - i n s o l u b l e , b u t u p t o 3 5 % b y weight of l i g n i n i n s o l u t i o n c a n be achieved u n d e r a l k a l i n e c o n d i t i o n s . H o w e v e r , as s u c h , these so l u t i o n s are t o o viscous t o be of a n y p r a c t i c a l use i n l i q u i d adhesive f o r m u l a t i o n s . O n e m e a n s t o reduce v i s c o s i t y difficulties has been t o o b t a i n h i g h solids ( « 4 0 % ) , l o w - v i s c o s i t y l i g n i n s o l u t i o n s b y e m p l o y i n g p h e n o l - H 2 0 or p h e n o l : H 2 Ο : N a O H (or NH3) as solvents (56).

T h i s a p p r o a c h was used t o give a p l y w o o d adhesive

w i t h excellent b o a r d p r o p e r t i e s (57).

T h e adhesive was p r e p a r e d i n a t w o -

step process b y p a r t i a l l y c o n d e n s i n g p h e n o l a n d f o r m a l d e h y d e together u n d e r alkaline conditions.

T h e r e s u l t i n g condensate was t h e n reacted w i t h a l i g n i n

concentrate a n d f o r m a l d e h y d e i n the presence o f a l k a l i t o p r o d u c e a resin o f 3 7 . 2 % s o l i d s content, p H 11.4, a n d a v i s c o s i t y o f 460 c P . L i k e l i g n o s u l p h o n a t e s , the use o f k r a f t l i g n i n as extenders or co-react a n t s i n P F a n d U F resins has been w e l l e x p l o r e d a n d o c c a s i o n a l l y i m p l e m e n t e d (36,58-60).

R e f e r r i n g a g a i n t o the w o r k by Forss (48,49),

some o f the best

results c l a i m e d , as regards P F r e p l a c e m e n t , were o b t a i n e d u s i n g h i g h m o l e c u l a r weight k r a f t l i g n i n s . Indeed, p r o p e r t i e s a p p e a r e d s u p e r i o r t o those u s i n g lignosulphonates. I n r e a l t e r m s , t h o u g h , the p r o b l e m s t h a t k r a f t ( a n d o t h e r l i g n i n s ) face are those o f p o o r r e a c t i v i t y , p r o d u c t v a r i a b i l i t y , a n d d i s c o l o r a t i o n . I n order for m o r e reactive k r a f t l i g n i n adhesives t o be m a d e , t h e y m u s t be c h e m i c a l l y m o d i f i e d i n some m a n n e r . T h i s c a n be done b y a v a r i e t y o f means (e.g., i n t r o d u c t i o n o f reactive c r o s s l i n k i n g sites o n t o the l i g n i n molecules b y means o f h y d r o x y m e t h y l a t i o n , e p o x i d a t i o n , i s o c y a n a t i o n , a n d the l i k e ) . H y d r o x y m e t h y l a t i o n ( m e t h y l o l a t i o n ) was first r e p o r t e d as a means o f a c t i v a t i n g k r a f t l i g n i n s for c r o s s l i n k i n g reactions w i t h P F resins (61-63).

I n these

studies, i t was d e m o n s t r a t e d t h a t b a s e - c a t a l y z e d c o n d e n s a t i o n o f f o r m a l d e h y d e w i t h s o f t w o o d k r a f t l i g n i n i n t r o d u c e d C H O H groups m a i n l y at C - 5 of the a r o 2

m a t i c r i n g a n d , t o a lesser extent at Cp ( F i g u r e 3).

These authors

(61-63)

i n d i c a t e d t h a t o f a l l the C - 5 a n d C/j p o s i t i o n s i n softwood l i g n i n , o n l y 0.33 a n d 0.03, respectively, were a v a i l a b l e for c o n d e n s a t i o n w i t h f o r m a l d e h y d e . T h e Cp p o s i t i o n s require a c t i v a t i o n for c o n d e n s a t i o n b y either a n adjacent C - c a r b o n y l a

or b y the presence o f a C , / ? - d o u b l e b o n d . T h e s e findings were u n a m b i g u o u s l y a

c o n f i r m e d i n a recent i n v e s t i g a t i o n by C h e n a n d G r a t z l (64).

H o w e v e r , even

w i t h the i n t r o d u c t i o n o f these a d d i t i o n a l c r o s s l i n k i n g sites, the r a t e o f c o n -




F i g u r e 3. P r o p o s e d h y d r o x y m e t h y l a t i o n reactions of l i g n i n

(61-64)-


2.


LEWIS AND LANTZY

23

d e n s a t i o n o f m e t h y l o l a t e d k r a f t l i g n i n s is s t i l l far less t h a n t h a t for P F resins C o n s e q u e n t l y , t h e y are n o t used o n t h e i r o w n a n d need t o be a p p l i e d i n

(65).

c o m b i n a t i o n w i t h P F resins

(65-71).

Interestingly, s o d a bagasse l i g n i n is m u c h m o r e reactive t o w a r d f o r m a l d e h y d e t h a n other l i g n i n s , a feature a t t r i b u t a b l e t o fewer s u b s t i t u e n t s at C - 3 a n d C - 5 T h i s m a t e r i a l , after m e t h y l o l a t i o n , r e s o r c i n o l g r a f t i n g , a n d m i x i n g w i t h

(72).

p h e n o l i c r e s i n , p r o d u c e s c o l d - s e t t i n g w o o d adhesives s u i t a b l e for s t r u c t u r a l f i n gerjoints a n d g l u l a m . Epoxy Resins.

T h e use o f l i g n i n i n o r g a n i c p o l y i s o c y a n a t e - l i q u i d a r o m a t i c


e p o x i d e b i n d e r s has also been e v a l u a t e d ( 7 5 ) .

A c c o r d i n g t o t h e a u t h o r s , the

l i g n i n served o n l y as a d i l u e n t , a l t h o u g h f a i r l y h i g h levels o f s u b s t i t u t i o n ( «

35%)

b y l i g n i n were achieved. Lignin Isocyanates. I n the v e r y near f u t u r e , i s o c y a n a t e adhesives are l i k e l y t o grow i n i m p o r t a n c e i n the w o o d p a n e l i n d u s t r y . I n t h i s r e g a r d , a n u m b e r of i n v e s t i g a t i o n s have a t t e m p t e d t o i m p r o v e t h e w o o d - b o n d i n g p r o p e r t i e s of l i g n i n b y r e a c t i o n w i t h isocyanates, or b y i n c l u s i o n o f i s o c y a n a t e f u n c t i o n a l ities i n t o t h e l i g n i n p o l y m e r .

F o r e x a m p l e , G a m o (74)

was able t o prepare

a n adhesive s u i t a b l e for p r o d u c i n g 3-ply p l y w o o d w i t h acceptable cal p r o p e r t i e s .

mechani-

T h i s was achieved by r e a c t i n g 20 p a r t s o f i s o c y a n a t e s o l u t i o n

( m e t h y l e n e d i i s o c y a n a t e : t o l u e n e , 3:1) w i t h 100 p a r t s k r a f t l i g n i n - f o r m a l d e h y d e resin.

O t h e r approaches involve r e a c t i o n o f k r a f t l i g n i n w i t h p r o p y l e n e

oxide

t o afford h y d r o x y p r o p y l l i g n i n s t h a t c a n t h e n be reacted w i t h p o l y m e t h y l e n e p o l y p h e n y l e n e i s o c y a n a t e or h e x a m e t h y l e n e d i i s o c y a n a t e ( 7 5 ) .

In a somewhat

s i m i l a r m a n n e r , l i g n i n - m o d i f i e d p o l y u r e t h a n e adhesives have been p r e p a r e d from 4,4/-diphenylmethane-diisocyante-lignin-maleic anhydride-propylene oxide c o p o l y m e r s ( 76,77). T h e use o f l i g n i n , essentially as a d i l u e n t (extender), t i g a t e d (78).

has also been inves-

I n t h i s case, d i - or p o l y i s o c y a n a t e s were reacted w i t h ethylene

or p r o p y l e n e carbonates i n a s o l u t i o n c o n t a i n i n g l i g n i n . T h e s e m i x e d ethylene and

p r o p y l e n e c a r b o n a t e - c o n t a i n i n g o r g a n i c p o l y i s o c y a n a t e s were s u i t a b l e as

p a r t i c l e b o a r d adhesives.

S team-Explosion,

Acidolysis, Organosolv,

and Cellulase Lignins.

S t e a m - e x p l o s i o n l i g n i n s have received some a t t e n t i o n as adhesives m a i n l y because t h i s process offers some p o t e n t i a l for u t i l i z i n g l o w - q u a l i t y h a r d w o o d s .

Like

k r a f t l i g n i n , these p r e p a r a t i o n s also require a c t i v a t i o n [e.g., b y h y d r o x y m e t h y l a t i o n (79),

i s o c y a n a t i o n (75,80)]

to achieve any t y p e o f acceptable w o o d c o m -

p o s i t e adhesive p r o p e r t i e s . A c i d o l y s i s l i g n i n s , o r g a n o s o l v , a n d cellulase l i g n i n s are p r e s e n t l y l a b o r a t o r y curiosities due t o a lack of c o m m e r c i a l i z a t i o n . W h e t h e r these l i g n i n s w i l l offer any advantages at a l l w i t h respect t o resin adhesives c u r r e n t l y used r e m a i n s t o be e s t a b l i s h e d . P r e l i m i n a r y e x p e r i m e n t s [e.g., w i t h a c i d o l y s i s l i g n i n s (81)] not d e m o n s t r a t e d a n y s u p e r i o r q u a l i t i e s t o date.


have

24



Conclusions C o n s i d e r a b l e research a c t i v i t y has been directed t o w a r d p r o d u c i n g w o o d c o m posite adhesives f r o m l i g n i n , a n d t h i s has been a c c o m p a n i e d b y very l i t t l e p r a c t i c a l success i n t e r m s o f c o m m e r c i a l i m p l e m e n t a t i o n . B y themselves, a n d r e gardless o f source, l i g n i n s offer n o advantages i n t e r m s o f c h e m i c a l r e a c t i v i t y , p r o d u c t q u a l i t y , o r color w h e n c o m p a r e d t o c o n v e n t i o n a l w o o d c o m p o s i t e adhe sives. A t l o w replacement levels (10 t o 3 0 % ) , l i g n i n s c a n a n d w i l l continue t o be e m p l o y e d as extenders for U F a n d P F resins. W h e n t h e y are used as extenders, best results are o b t a i n e d w h e n c h e m i c a l l y a c t i v a t e d (e.g., b y m e t h y l o l a t i o n ) . H o w e v e r , l i g n i n m a y f i n d a p p l i c a t i o n as a s u i t a b l e w o o d - c o m p o s i t e adhesive i n adhesive f o r m u l a t i o n s i f c h e m i c a l l y m o d i f i e d i n some m a n n e r . I n t h i s respect, the recent developments w i t h s o d a bagasse l i g n i n - r e s o r c i n o l - f o r m a l d e h y d e a n d l i g n i n - i s o c y a n a t e adhesives a l l o w for some c a u t i o u s o p t i m i s m for the f u t u r e .

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Lignin in Adhesives - ACS Publications - American Chemical Society

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