Adhesives from Renewable Resources - American Chemical Society

Chapter 4 Cross-linking Options for Lignins Downloaded by UNIV OF MISSOURI COLUMBIA on August 4, 2013 | http://pubs.acs.org Publication Date: December 31, 1989 | doi: 10.1021/bk-1989-0385.ch004

Wolfgang G. Glasser Department of Forest Products and Polymer Materials and Interfaces Laboratory Virginia Polytechnic Institute and State University Blacksburg, VA 24061

For isolated lignins to become the primary backbone component of a network polymer, a reaction with a bifunctional or multifunctional crosslinking agent must be performed that is sufficiently reactive as well as compatible with lignin in either solvent-free or highly concentrated form. Since the essential prerequisites of compatibility and miscibility often are not met by isolated lignins, improvements may require chemical modification prior to crosslinking. Opportunities exist for preparing lignins by chemical derivative formation for incorporation into polymer networks. Modification reactions include sulfonation, methylolation, phenolation, alkoxylation, acrylation, and many others. These prepare lignins for incorporation into phenolics, epoxies, urethanes, acrylics, and several other types of thermosetting materials. T h e macromolecular, multifunctional nature o f isolated lignins invites applications i n w h i c h l i g n i n serves as the p r i n c i p a l c o m p o n e n t o f t h e r m o s e t t i n g n e t w o r k p o l y m e r s ( i ) . M o s t p r o m i n e n t ( i n terms o f m a r k e t v o l u m e ) o f the thermosett i n g m a t e r i a l s are phenolics, a n d these are j o i n e d b y p o l y u r e t h a n e s , u n s a t u r a t e d polyesters, p o l y a m i n e s , a n d epoxies w i t h s m a l l e r m a r k e t v o l u m e s (2). A l t h o u g h most efforts t o i n c o r p o r a t e l i g n i n i n t o thermosets b y c r o s s l i n k i n g w i t h other resin c o m p o n e n t s have concentrated o n phenolics ( 5 ) , other o p t i o n s exist as w e l l . T h i s paper briefly reviews alternatives for c r o s s l i n k i n g l i g n i n s i n a v a r i e t y of t h e r m o s e t t i n g network p o l y m e r systems. Gillham's T T T Diagram T h e extent t o w h i c h l i g n i n becomes a c o m p o n e n t o f a p o l y m e r network m a y be constrained either b y r e a c t i v i t y o r b y s o l u b i l i t y . T h e c o n t r i b u t i o n o f l i g n i n s t o p h e n o l i c resins is k n o w n t o be l i m i t e d b y t h e n u m b e r o f available uncondensed 0097-6156/89/0385-0043$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.

Downloaded by UNIV OF MISSOURI COLUMBIA on August 4, 2013 | http://pubs.acs.org Publication Date: December 31, 1989 | doi: 10.1021/bk-1989-0385.ch004

44

ADHESIVES F R O M RENEWABLE RESOURCES

p h e n o l i c g u a i a c y l u n i t s (i.e., r e a c t i v i t y ) , a n d t h i s m a y b e i m p r o v e d b y m o d i f i c a t i o n p r i o r t o c r o s s l i n k i n g (3,4). B y c o n t r a s t , p o o r s o l u b i l i t y i n a n d m i s c i b i l i t y w i t h t h e c o m p o n e n t s o f a l i q u i d network f o r m i n g resin were h e l d responsible for t h e c r e a t i o n o f inhomogeneous (i.e., phase separated) c u r e d p o l y u r e t h a n e s (5,6). T h e process o f g e l a t i o n is best i l l u s t r a t e d b y t h e t i m e - t e m p e r a t u r e t r a n s f o r m a t i o n ( T T T ) cure d i a g r a m o f G i l l h a m (7). T h i s is i l l u s t r a t e d i n F i g ure 1 i n s i m p l i f i e d f o r m . T h e d i a g r a m i l l u s t r a t e s t h e various t r a n s f o r m a t i o n s a resin undergoes d u r i n g i s o t h e r m a l cure i n r e l a t i o n t o t i m e . A t l o w t e m p e r a t u r e a n d short t i m e , t h e resin r e m a i n s a homogeneous (fluid) m i x t u r e t h a t expe riences d e m i x i n g (or phase separation) as a result o f r e a c t i o n advancement i f either t i m e o r t e m p e r a t u r e increases. G e l a t i o n , w h i c h a c c o r d i n g t o F l o r y (8), corresponds t o t h e f o r m a t i o n o f a molecule w i t h i n f i n i t e m o l e c u l a r weight, is seen t o o c c u r at a l a t e r p o i n t i n t i m e d u r i n g i s o t h e r m a l cure. T h e fact t h a t cure follows d e m i x i n g i m p l i e s t h a t a n inhomogeneous m a t e r i a l i s f o r m e d , since phase s e p a r a t i o n l i m i t s t h e a b i l i t y o f molecules t o undergo c r o s s l i n k i n g a n d n e t w o r k f o r m a t i o n . T h e process o f phase s e p a r a t i o n , i n t u r n , is governed b y t h e G i b b s free energy e q u a t i o n : AGmixing

=

AH mixing ~

TAS %xing m

P h a s e s e p a r a t i o n occurs w h e n A G rises above 0. T h i s m a y be t r i g g e r e d b y a rise i n e n t h a l p y (i.e., Δ Η ) o r a decline i n e n t r o p y (i.e., A S ) . T o allow for the f o r m a t i o n o f a u n i f o r m network p o l y m e r , phase s e p a r a t i o n m u s t be delayed u n t i l c r o s s l i n k i n g is w e l l enough advanced t o prevent i n d i v i d u a l molecules f r o m d e m i x i n g . T h i s delay is achieved b y either r e d u c i n g Δ Η o r b y r a i s i n g AS ( i n concert w i t h T ) . T h e e n t h a l p y f a c t o r ( Δ Η ) i s c o n t r o l l e d b y t h e difference i n H i l d e b r a n d ' s s o l u b i l i t y p a r a m e t e r (δ) between t h e v a r i o u s r e a c t i n g c o m p o n e n t s , since AH

= φ φ (δ 1

2

1

-

6) 2

2

where φ is v o l u m e f r a c t i o n ; a n d A S declines w i t h i n c r e a s i n g m o l e c u l a r weight a c c o r d i n g t o F l o r y a n d H u g g i n s (9). B o t h approaches have been e m p l o y e d w i t h l i g n i n - b a s e d network p o l y m e r s . T h e lower m o l e c u l a r weight fractions o f k r a f t l i g n i n have been used successfully for t h e f o r m u l a t i o n o f homogeneous s o l u t i o n cast p o l y u r e t h a n e films (10-12), a n d t h e s o l u b i l i t y o f l i g n i n has been increased by c h e m i c a l m o d i f i c a t i o n p r i o r t o c r o s s l i n k i n g . T h e f o l l o w i n g discussion i l l u s trates e x a m p l e s o f h o w c h e m i c a l m o d i f i c a t i o n m a d e c r o s s l i n k i n g possible b y l i m i t i n g phase s e p a r a t i o n . C h e m i c a l M o d i f i c a t i o n s . U n m o d i f i e d l i g n i n is w e l l k n o w n for i t s p o o r s o l u b i l i t y characteristics a n d i t s h i g h glass t r a n s i t i o n t e m p e r a t u r e . M e t h o d s for improving the solubility (and/or reactivity) of lignin prior to crosslinking i n specific network f o r m i n g systems are s u m m a r i z e d i n T a b l e I . S u c h systems m a y be based o n aqueous s o l u t i o n s a t p H below o r above n e u t r a l o r o n s o l u t i o n s i n p o l a r o r n o n p o l a r solvents. T y p i c a l m o d i f i c a t i o n s t h a t enhance the s o l u b i l i t y o f


4.

GLASSER

45

Cross-Linking Options for Lignins

l i g n i n i n aqueous solvent systems o f p H < 7 involve s u l f o n a t i o n , c a r b o x y l a t i o n (by o x i d a t i o n ) , a n d g r a f t i n g w i t h a s u b s t i t u e n t t h a t p r o d u c e s a w a t e r - s o l u b l e p o l y m e r c h a i n . S o l u b i l i t y i n aqueous s o l u t i o n s o f p H > 7 t y p i c a l l y concentrates o n t h e i n t r o d u c t i o n o f i o n i z a b l e f u n c t i o n a l groups.

Sulfonation, phenolation,

a n d c a r b o x y l a t i o n (by r e a c t i o n w i t h a n h y d r i d e s or chloroacetic to this group.

acid)

belong

O r g a n i c solvent s o l u b i l i t y m a y be enhanced b y r e d u c i n g the

h y d r o g e n - b o n d i n g c a p a c i t y of l i g n i n t h r o u g h e t h e r i f i c a t i o n a n d / o r t h r o u g h t h e


i n t r o d u c t i o n o f s u b s t i t u e n t s w i t h low p o l a r i t y . T h e list i n T a b l e I p r o v i d e s evidence for the o b s e r v a t i o n t h a t i s o l a t e d ( p o l y m e r i c ) l i g n i n , i n the m a j o r i t y of cases, needs t o be c h e m i c a l l y m o d i f i e d p r i o r t o n e t w o r k f o r m a t i o n .

(As men-

t i o n e d above, phase s e p a r a t i o n p r i o r t o g e l a t i o n m a y , however, also be c o n t r o l l e d a n d delayed b y the use o f low m o l e c u l a r weight l i g n i n f r a c t i o n s . ) C r o s s l i n k i n g R e a c t i o n s i n W a t e r at p H 7. p r a c t i c e for p h e n o l i c s (44)-

R e s i n cure i n aqueous a l k a l i is c o m m o n

A l t h o u g h most i s o l a t e d l i g n i n s have s a t i s f a c t o r y

s o l u b i l i t y i n aqueous a l k a l i , p h e n o l a t i o n has been f o u n d t o enhance the tolerance o f p h e n o l i c resins t o the a d d i t i o n o f m a c r o m o l e c u l a r f r a c t i o n s o f l i g n i n (4,4$-47). I m p r o v e d m i s c i b i l i t y a n d r e a c t i v i t y b o t h account for t h i s p h e n o m e n o n . T h e c r o s s l i n k i n g o f l i g n i n derivatives i n aqueous a l k a l i n e m e d i u m has been achieved w i t h several a d d i t i o n a l c r o s s l i n k i n g agents. N i m z et a l . e m p l o y e d b i f u n c t i o n a l b i s - d i a z o n i u m salts i n a n effort t o p r o d u c e a n o v e l t y p e o f azo resin f r o m l i g n i n sulfonates (24) et a l . (25)

( F i g u r e 3). T h i s technique was a d o p t e d b y P s o t t a

who found that, although sulfonated wattle extracts crosslinked well

a n d f o r m e d useful n e t w o r k p o l y m e r s , s u l f o n a t e d l i g n i n derivatives posed p r o b lems a t t r i b u t e d t o f r o t h i n g (due t o reagent d e c o m p o s i t i o n ) a n d loss o f c o n t r o l over cure r a t e . B i f u n c t i o n a l a c i d chlorides were useful i n c r o s s l i n k i n g s u l f o n a t e d l i g n i n m o d e l c o m p o u n d s i n aqueous a l k a l i ( F i g u r e 4), b u t t h e y b e h a v e d p o o r l y d u r i n g i n t e r molecular crosslinking of polymeric lignins

(26).

C y a n u r i c c h l o r i d e was d e s c r i b e d as a potent t r i f u n c t i o n a l c r o s s l i n k i n g agent for b o t h m o d e l c o m p o u n d s a n d s u l f o n a t e d l i g n i n s (27)

( F i g u r e 5).

A healthy

i n c o r p o r a t i o n o f t r i a z i n e f u n c t i o n a l i t y p r o d u c e d homogeneous n e t w o r k gels. I n t r o d u c t i o n o f c a r b o x y l i c a c i d groups enhances surface active (17)

o f l i g n i n s as w e l l as t h e i r r e a c t i v i t y w i t h p r o p y l e n e o x i d e (39).

properties (A

more

h i g h l y p r o p o x y l a t e d l i g n i n has s u p e r i o r s o l u b i l i t y a n d r e a c t i v i t y w i t h d i i s o cyanates.)

C a r b o x y l a t e d l i g n i n s have, however, n o t been the target o f n e t w o r k -

f o r m i n g reactions. C r o s s l i n k i n g i n O r g a n i c Solvents. T h e solubility of most isolated lignins i n ( p o l a r ) o r g a n i c solvents is p o o r . It needs to be i m p r o v e d before u n i f o r m netw o r k p o l y m e r s c a n be p r o d u c e d b y c r o s s l i n k i n g . T h i s m a y be achieved t h r o u g h


46


230 r

x

End of phase separation

phase separation Vitrification


Gelation

0

1

2

3

4

L o g time (min) F i g u r e 1. T i m e - t e m p e r a t u r e - t r a n s f o r m a t i o n cure d i a g r a m b y G i l l h a m [adapted from (7)].

OCH,

H C0 3

OH

OH

F i g u r e 2. C o n d e n s a t i o n r e a c t i o n o f lignosulfonates at p H < 7 [adapted f r o m ( 3 ) ] .

S0 Na 3

OH

pH 8... 14

U N©

III Ν

S0 Na 3

OL .

QH

N

Ν> ^γΌ0Η

υ τ

3

Να S0£

F i g u r e 3. R e a c t i o n o f l i g n o s u l p h o n a t e s w i t h b i s - d i a z o n i u m salts [adapted f r o m {24,25)}.


GLASSER

Cross-Linking Options for Lignins

Table I. A Non-comprehensive List of Crosslinking Reactions a n d P r e t r e a t m e n t M e t h o d s for I m p r o v i n g S o l u b i l i t y a n d / o r R e a c t i v i t y Principal Solvent Water


( P H < 7)

Water ( p H > 7)

Organic solvents

Method S S M MO G

1

Principal Crosslinking Agent

2

Network Type 3

H+ ( C ) H 0 (OC) PF NA NA

PR PR PR ΝΑ ΝΑ

S,M,P

PF

PR

S S s CM MAC MCR

BD AAC ce NA NA NA

AZR ΡΕ Resins ΝΑ ΝΑ ΝΑ

E A M+EP MA

NEH DI NEH MEA

ER PU ER AR

2

2

Reference 13,14

15 16 η 18 4,19*,2Ρ,

21 - 23 24,25

26 27 28 29 17,30 31,32 33-40 41,42 43

S = s u l f o n a t i o n , M = m e t h y l o l a t i o n , M O = miscellaneous o x i d a t i o n , G = grafting, Ρ = phenolation, C M = carboxymethylation, M A C = m a l e i c a n h y d r i d e c o p o l y m e r i z a t i o n , M C R = miscellaneous c a r b o x y l a t i o n reactions, Ε = e p i c h l o r o h y d r i n (also i n c o n j u n c t i o n w i t h p h e n o l a t e d l i g n i n ) , A = a l k o x y l a t i o n (i.e., ethylene, propylene, a n d butylène oxides), M + E P = m o d i f i c a t i o n w i t h c o m p o u n d s c o n t a i n i n g u n s a t u r a t e d e n d groups ("divalent h y d r o c a r b o n s " ) followed b y e p o x i d a t i o n w i t h peroxide, M A = m e t h a c r y l i c a c i d . X

C = condensation, O C = o x i d a t i v e c o u p l i n g , P F = p h e n o l f o r m a l d e h y d e , N A = c r o s s l i n k i n g n o t p e r f o r m e d - m o d i f i c a t i o n was a i m e d at network f o r m a t i o n , B D = b i s - d i a z o n i u m salts, A A C = a r y l a c i d chlorides, C C = c y a n u r i c chloride, N E H = n o r m a l e p o x y hardeners, M E A = methacrylate. P R = phenolic resin, A Z R = azo resin, P E = polyester, E R = e p o x y resin, P U = p o l y u r e t h a n e s , A R = a c r y l i c resin. B a s e d o n h i g h - m o l e c u l a r - w e i g h t f r a c t i o n o f u l t r a - f i l t e r e d spent sulfite or k r a f t l i q u o r . B a s e d o n a l k a l i - s o l u b l e m e t h y l o l a t e d k r a f t l i g n i n w h i c h was p r e c i p i t a t e d b y a c i d i f i c a t i o n a n d blended w i t h a n a c i d - c u r i n g phenolic resin. 2

3

4

5

American

Chemical

Library

Society

1155 16th St., N.W.

In Adhesives from Renewable Resources; Hemingway, R., et al.; Washington, O.C. 20036 ACS Symposium Series; American Chemical Society: Washington, DC, 1989.



48

'S0 No

S0 No

3

H3CO-

. Q

+

OH

3

CIOC-(Q>-COCI

pH 10.5 H3CO )

OC—^^-co

0

ο

^OCH,

NaS0

Adhesives from Renewable Resources - American Chemical Society

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