Lignin Epoxide - ACS Symposium Series (ACS Publications)

Chapter 40

Lignin Epoxide Synthesis and Characterization World L i - S h i h Nieh

Downloaded by JOHNS HOPKINS UNIV on September 23, 2013 | http://pubs.acs.org Publication Date: July 31, 1989 | doi: 10.1021/bk-1989-0397.ch040

1-3

1

and Wolfgang G .

Glasser

1,2

Department of Wood Science and Forest Products, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061 Polymeric Materials and Interfaces Laboratory, Virginia Polytechnic Institute and State University, Blacksburg, V A 24061 2

An epoxide resin was synthesized on the basis of lignin by reaction of epichlorohydrin (ECH) with hydroxypropyl lignin. A mixture of a quarternary ammonium salt (QAS) and potassium hydroxide (KOH) was used as catalyst. Additional KOH was added stepwise at a rate which compensated for KOH consumption during dechlorohydrogenation. The epoxidation reaction was first studied using hydroxypropyl guaiacol (HPG) as a lignin-like model compound. At room temperature, and when ECH was in excess, the reaction was completed in five days. The reaction was found to be highly dependent on the stepwise addition of KOH, and it was independent of ECH concentration. The maximum conversion of hydroxy to epoxy functionality was found to be 100 and 50% for model compound and lignin derivative, respectively. The lignin epoxide resin was crosslinked with diethylenetriamine (DETA), amine terminated poly (butadiene-co-arylonitrile) (ATBN) and phthalic anhydride (PA). Sol fraction and swelling behavior, and dynamic mechanical characteristics of the cured lignin epoxides were studied in relation to cure conditions. The major functional groups in lignin are methoxy, phenolic hydroxy, aliphatic hydroxy, and carboxy groups. During reaction with alkylene oxide, most of the functional groups of lignin, besides methoxy groups, become Current address: Mississippi State Forest Products Utilization Laboratory, Mississippi State University, Mississippi State, MS 39762

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0097-6156/89A)397-0506$06.00/0 © 1989 American Chemical Society

In Lignin; Glasser, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.


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s u b s t i t u t e d b y h y d r o x y a l k y l groups (1). R e a c t i o n s o f a l i p h a t i c h y d r o x y groups w i t h c o m p o u n d s h a v i n g α - e p o x y g r o u p s , s u c h as e p i c h l o r o h y d r i n ( E C H ) , have been i n v e s t i g a t e d . W i t h acids as c a t a l y s t , the r e a c t i o n p r o d uct consists o f a m i x t u r e of the 1,2- a n d the 1 , 3 - c h l o r o h y d r i n d e r i v a t i v e o f the parent c o m p o u n d . W h e n base is used as c a t a l y s t , the 1 , 2 - c h l o r o h y d r i n is the o n l y p r o d u c t a n d u p o n d e c h l o r o h y d r o g e n a t i o n , the α - e p o x y d e r i v a tive o f the parent c o m p o u n d is f o r m e d . Bases s u c h as l i t h i u m h y d r o x i d e , s o d i u m h y d r o x i d e , s o d i u m a l k o x i d e , a n d L e w i s bases such as t r i b u t y l a m i n e a n d t r i i s o p r o p y l o l a m i n e have a l l been used i n t h i s r e a c t i o n . T h i s s y n t h e t i c route t o e p o x y - f u n c t i o n a l i z e d derivatives o f O H - f u n c t i o n a l p o l y m e r s has been successfully a p p l i e d t o cellulose, a m o n g others ( 2 , 3 ) . E p o x y resins f r o m l i g n i n a n d l i g n i n derivatives have been e x p l o r e d as well (4-10). I n some cases, k r a f t l i g n i n e p o x i d i z e d w i t h E C H i n the presence o f s o d i u m h y d r o x i d e as c a t a l y s t u n d e r w e n t irreversible h a r d e n i n g (4,5) at 100°C since epoxides are capable o f u n d e r g o i n g h o m o p o l y m e r i z a tion. Alternatively, phenolated kraft lignin, mixtures of phenol w i t h kraft l i g n i n , a n d m i x t u r e s o f p h e n o l w i t h p h e n o l a t e d k r a f t l i g n i n were a l l e p o x idized w i t h E C H i n 25-40% N a O H which formed a solid epoxy polymer t h a t softened at 72-95°C (4-7). B o t h the degree o f e p o x i d a t i o n a n d the extent o f side reactions increased w i t h i n c r e a s i n g a m o u n t o f N a O H . W h e n tested as adhesive, the l i g n i n / p h e n o l - b a s e d e p o x y resin gave " g o o d " a d h e s i o n t o w o o d . H o w e v e r , w h e n tested as c o a t i n g m a t e r i a l t o steel, the best results came f r o m l i g n i n / p h e n o l - b a s e d e p o x y resins m o d i f i e d w i t h u r e a f o r m a l d e h y d e a n d m e l a m i n e - f o r m a l d e h y d e varnishes. I n a s t u d y b y T a i et al. ( 8 , 9 ) , l i g n i n - b a s e d epoxy resins were synthesized f r o m k r a f t l i g n i n , b i s g u a i a c y l a t e d k r a f t l i g n i n a n d p h e n o l a t e d k r a f t l i g n i n . T h e e p o x i d a t i o n was c o n d u c t e d at 97 t o 117°C w i t h the a d d i t i o n o f 7-20 moles o f E C H p e r m o l e o f l i g n i n repeat u n i t (i.e., C 9 - u n i t ) u s i n g N a O H as c a t a l y s t . T h e e p o x y content o f the r e s u l t i n g l i g n i n derivatives was f o u n d t o be insensitive t o E C H c o n c e n t r a t i o n , b u t dependent o n the a m o u n t o f N a O H a d d e d . T h e e p o x y content was f o u n d t o increase to 0.16 e q / 1 0 0 g w i t h N a O H content i n c r e a s i n g t o 4 0 % . A t higher N a O H contents, the e p o x y content declined due to h o m o p o l y m e r i z a t i o n . M a x i m u m degree o f f u n c t i o n a l i z a t i o n w i t h epoxide groups was never achieved. T h e s o l u b i l i t y o f the l i g n i n epoxides i n o r g a n i c solvents was r e p o r t e d t o be i n the order o f p h e n o l a t e d l i g n i n e p o x ide > l i g n i n epoxide > b i s g u a i a c y l a t e d l i g n i n epoxide. T h e l i g n i n epoxides were tested as adhesives o n a l u m i n u m a n d beech w o o d u s i n g a n h y d r i d e a n d d i a m i n e as c u r i n g agents. T h e l i g n i n - b a s e d e p o x y resins showed a n adhe sive s t r e n g t h equivalent t o resol resin w h e n w o o d was used as the adherent. D ' A l e l i o synthesized a series o f l i g n i n epoxides f r o m l i g n i n s t h a t h a d t h e i r f u n c t i o n a l groups p a r t i a l l y b l o c k e d b y esterification w i t h c a r b o x y l i c acids (10). T h e e p o x i d i z i n g agent was E C H u s i n g N a O H as c a t a l y s t a n d d i m e t h y l sulfoxide ( D M S O ) as solvent at 90-100°C for 14-24 h o u r s . T h e degree o f e p o x i d a t i o n was c o n t r o l l e d b y the a m o u n t o f N a O H a d d e d . It is obvious t h a t several o p t i o n s exist for the synthesis o f l i g n i n - b a s e d epoxide resins. T h e use o f E C H under a l k a l i n e c o n d i t i o n s seems t o b e favored. T h e degree o f e p o x i d a t i o n is c o n t r o l l e d b y the a m o u n t o f c a t a l y s t


LIGNIN: PROPERTIES AND MATERIALS

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a d d e d since t h e d e c h l o r o h y d r o g e n a t i o n step consumes t h e c a t a l y s t . T h e degree o f e p o x i d a t i o n is independent o f t h e a m o u n t o f E C H as l o n g as t h i s is present i n excess. C r o s s l i n k i n g t h r o u g h h o m o p o l y m e r i z a t i o n is a p o t e n t i a l p r o b l e m w h i c h l i m i t s t h e degree o f e p o x i d a t i o n achievable.


Experimental Synthesis and Characterization of Lignin-like Model Epoxide. A series o f e p o x i d a t i o n reactions were p e r f o r m e d w i t h h y d r o x y p r o p y l g u a i a c o l ( H P G ) as m o d e l c o m p o u n d a n d E C H . M o l a r r a t i o s o f E C H r H P G v a r i e d f r o m 1 t o 10. P e l l e t i z e d K O H a n d a q u a r t e r n a r y a m m o n i u m salt ( Q A S ) served as c a t a l y s t a n d reagent. T o l u e n e w a s the solvent. C a t a l y s t c o n c e n t r a t i o n a n d m e t h o d o f a d d i t i o n were v a r i e d . E x p e r i m e n t a l details are given elsewhere (11). T h e epoxidation reaction was monitored b y high performance liquid c h r o m a t o g r a p h ( H P C L ) o n a reverse phase c o l u m n . T h e r e a c t i o n p r o d u c t was i d e n t i f i e d b y I R , p r o t o n N M R a n d carbon-13 N M R spectroscopy. I n the I R s p e c t r u m , a b s o r p t i o n b a n d s at 904 c m " a n d 842 c m " were a t t r i b u t e d t o t h e e p o x y groups (12); peaks at 2.60, 2.78, a n d 3.15 p p m o n the H - N M R s p e c t r u m were assigned t o t h e three protons o f the epoxide g r o u p (13); a n d i n t h e C - N M R s p e c t r u m , t h e three g l y c i d y l c a r b o n s were identified at 4 4 . 5 , 50.6, a n d 70.6 p p m (14). 1

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Synthesis and Characterization of Lignin-Based Epoxide. Lignin-based epoxides were prepared b y r e a c t i n g H P L f r o m a m i x e d h a r d w o o d o r g a n o solv l i g n i n w i t h E C H f o l l o w i n g t h e procedure developed w i t h t h e m o d e l c o m p o u n d . D e t a i l s o f the r e a c t i o n have been disclosed elsewhere (15). T h e r e a c t i o n was m o n i t o r e d b y t i t r a t i n g the epoxide groups w i t h H B r a c c o r d i n g t o D u r b e t a k i (16). S a m p l e s were t a k e n f r o m t h e r e a c t i o n m i x t u r e at threed a y i n t e r v a l s , a n d they were t i t r a t e d for t h e i r e p o x y content. T h e f r a c t i o n o f t h e t o t a l h y d r o x y groups t h a t were converted t o epoxide groups w a s defined as degree of conversion. P r o t o n s o n t h e epoxy r i n g were i d e n t i f i e d b y H - N M R , a n d g l y c i d y l carbons were detected b y C - N M R spectroscopy. 1

1 3

Epoxy Resin Characterization. E p o x y films were p r e p a r e d b y c r o s s l i n k i n g 2 g o f l i g n i n - b a s e d epoxide h a v i n g a n epoxy content o f 0.11 e q / 1 0 0 g w i t h stoichiometric amounts of diethylenetriamine ( D E T A ) a n d aminet e r m i n a t e d p o l y ( b u t a d i e n e - c o - a c r y l o n i t r i l e ) ( A T B N ) as c r o s s l i n k i n g agents. T h e films were solvent cast (methylene chloride) a n d cured at 105°C for 24 hours f o l l o w i n g solvent e v a p o r a t i o n . T o determine sol f r a c t i o n a n d degree o f s w e l l i n g , films were oven d r i e d a n d swollen t o e q u i l i b r i u m i n d i m e t h y l f o r m a m i d e ( D M F ) . W e i g h t loss w a s t a k e n as sol fraction, a n d weight increase (due t o swelling) d e t e r m i n e d t h e degree of swelling. Dynamic mechanical a n a l y s i s was p e r f o r m e d o n a P o l y m e r L a b o r a t o r i e s D y n a m i c M e c h a n i c a l T h e r m a l A n a l y z e r ( D M T A ) . Solvent-cast l i g n i n epoxide films were scanned at a h e a t i n g rate o f 4 ° C m i n " . T h e frequency w a s 1 H z a n d t h e s t r a i n level was 4 % . U n i a x i a l s t r e s s - s t r a i n t e s t i n g w a s p e r f o r m e d o n a s t a n d a r d I n s t r o n t e s t i n g m a c h i n e e m p l o y i n g a cross-head speed o f 1 m m m i n " . S a m 1

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pies were cut w i t h a die i n a dog bone shape, f r o m solvent cast film. T e n s i l e characteristics were c a l c u l a t e d o n the basis o f i n i t i a l d i m e n s i o n s .


Results and Discussion Epoxidation of HPG. T h e r e a c t i o n scheme for the e p o x i d a t i o n o f H P G is s h o w n i n F i g u r e 1. T h e generation of o x y a n i o n I I i n toluene is assisted b y (solid) K O H a n d Q A S . II reacts r a p i d l y w i t h E C H t o p r o d u c e the 1,2c h l o r o h y d r i n o f H P G , I I I . D e c h l o r o h y d r o g e n a t i o n o f I I I proceeds i m m e d i a t e l y under the p r e v a i l i n g r e a c t i o n c o n d i t i o n s w i t h f o r m a t i o n o f epoxide I V a n d K C 1 . T h e process c a n conveniently be m o n i t o r e d b y H P L C . C o m p l e t i o n is i n d i c a t e d w h e n I is depleted. T h e degree o f conversion o f I t o I V was followed i n r e l a t i o n t o t i m e a n d c a t a l y s t a d d i t i o n i n a series o f e x p e r i m e n t s w h i c h e m p l o y e d toluene as solvent at r o o m t e m p e r a t u r e . T h e results are s u m m a r i z e d i n F i g u r e 2. T h e r e a c t i o n o f I w i t h excess E C H i n the presence o f K O H (1 eq m o l o f I) a n d a n equivalent a m o u n t (by weight) o f Q A S p r o d u c e d I V i n 6 5 % y i e l d i n about 5 days ( e x p e r i m e n t A ) . F a i l u r e t o a d d a d d i t i o n a l K O H is o b v i o u s l y responsible for the low degree o f conversion. B y r a i s i n g the K O H content t o 1.25 eq m o l " o f I a n d i n c l u d i n g Q A S i n a n a m o u n t equivalent ( b y weight) t o the ( i n i t i a l ) K O H content, the r e a c t i o n becomes 7 5 % c o m plete i n a b o u t 24 hours ( e x p e r i m e n t C ) . Subsequent a n d repeated a d d i t i o n s o f K O H (1.25 eq m o l of I, each) help the degree o f conversion t o reach 1 0 0 % i n another two d a y s . Presence o f Q A S i n the secondary K O H charges w a s f o u n d t o be unnecessary. H o w e v e r , t o t a l absence o f Q A S f r o m the r e a c t i o n m e d i u m ( e x p e r i m e n t B ) prevented the r e a c t i o n f r o m ever r e a c h i n g c o m p l e t i o n b y r e d u c i n g the degree o f conversion at each step. S i n g l e a d d i t i o n s o f large excesses o f K O H t o the reagent m i x t u r e ( e x p e r i m e n t B ) f a i l e d likewise t o produce h i g h degrees o f conversion. Stepwise a d d i t i o n o f K O H ( w i t h or w i t h o u t Q A S ) is clearly seen as necessary for c o m p e n s a t i n g for K O H c o n s u m p t i o n d u r i n g d e c h l o r o h y d r o g e n a t i o n . T h e K O H / Q A S m i x t u r e was f o u n d to be m o r e efficient t h a n K O H w i t h o u t Q A S i n genera t i n g the n u c l e o p h i l i c h y d r o x y a n i o n o f H P G . Q A S d i d not interfere w i t h the d e c h l o r o h y d r o g e n a t i o n step as l o n g as K O H was c o m p e n s a t e d for. I n e x p e r i m e n t C ( F i g . 2), the e p o x i d a t i o n r e a c t i o n reached 1 0 0 % conversion o n the fifth day, t w o days after the a d d i t i o n o f the first a d d i t i o n a l a m o u n t o f K O H . A l l r o o m t e m p e r a t u r e reactions h a d higher degrees o f conversion as c o m p a r e d t o the elevated t e m p e r a t u r e e p o x i d a t i o n reactions o f H P G . T h e d e c h l o r o h y d r o g e n a t i o n step (i.e., I l l t o I V i n F i g u r e 1) was s t i l l f o u n d t o be r a p i d , even at the m i l d e r r e a c t i o n t e m p e r a t u r e . - 1

1

- 1

Epoxidation of HPL. R e s u l t s f r o m the e p o x i d a t i o n o f H P L w i t h E C H i n the presence o f K O H a n d Q A S u s i n g m e t h y l e n e chloride (at r o o m t e m p e r a t u r e ) as solvent are s h o w n i n F i g u r e 3. T h e degree o f conversion of ( a l i p h a t i c ) h y d r o x y groups of H P L to epoxide f u n c t i o n a l i t y was m o n i t o r e d b y t i t r a t i o n . P a r a m e t e r s i m p o r t a n t to the success o f t h i s r e a c t i o n i n c l u d e d (a) stepwise a d d i t i o n o f K O H , a p p r o x i m a t e l y p a r a l l e l i n g the f o r m a t i o n of K C 1 b y d e c h l o r o h y d r o g e n a t i o n ; (b) presence o f Q A S i n the r e a c t i o n m i x t u r e ; (c) a n at least five-fold s t o i c h i o m e t r i c excess o f E C H over a v a i l a b l e


510


Ο

H,C0'

Ο

KOH/QAS

2

OCH I HC-O' I CH 3

0CH - toluene HC-OH I CH 3

2

toluene

2

2


A

CH -CHCH CI OCH

HC0CH CHCH | 2

CH

J

+KOH OCH I „

2

3

2

OH CI.

-KCI

2

HCOCH CH-CH 2

C H ,

2

0

F i g u r e 1. R e a c t i o n scheme of the e p o x i d a t i o n o f H P G .

F i g u r e 2. Degree o f conversion o f I t o I V i n r e l a t i o n t o t i m e a n d K O H a d d i t i o n . E x p e r i m e n t Β e m p l o y e d K O H alone; a n d e x p e r i m e n t s A a n d C employed K O H and Q A S .


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Lignin Epoxide

o x y a n i o n f u n c t i o n a l groups; (d) m i l d r e a c t i o n t e m p e r a t u r e ; a n d (e) absence o f m o i s t u r e f r o m the r e a c t i o n m i x t u r e . F i g u r e 3 i n d i c a t e s t h a t e p o x i d a t i o n of H P L m a y reach a degree of conversion of c a . 5 0 % after 12 d a y s . A s s u m i n g t h a t H P L has a h y d r o x y content of a b o u t 6-8% a n d a n u m b e r average m o l e c u l a r weight of 1500 g m o l " * , a 5 0 % conversion o f h y d r o x y groups t o epoxide f u n c t i o n a l i t y c a n be expected t o p r o d u c e a l i g n i n - b a s e d e p o x y resin w i t h a n epoxide f u n c t i o n a l i t y of about 0.2 e q / 1 0 0 g ; w i t h a n e p o x y e q u i v a lent weight of 400 t o 600 g; a n d w i t h a n average o f three f u n c t i o n a l groups per ( n u m b e r average) m o l e c u l a r fragment. H i g h h y d r o x y f u n c t i o n a l i t y a n d degree o f conversion, a n d absence of low m o l e c u l a r weight c o m p o n e n t s (be low t r i - or t e t r a m e r i c l i g n i n structures) are c r i t i c a l p a r a m e t e r s for l i g n i n selection for successful e p o x i d a t i o n .


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Network Characterization of Lignin-Based Epoxides. T h e formation of a p o l y m e r network requires t h a t m u l t i f u n c t i o n a l l i g n i n epoxide molecules are reacted w i t h d i - or m u l t i f u n c t i o n a l c r o s s l i n k i n g agents, such as a m i n e s or a n h y d r i d e s . M o n o f u n c t i o n a l components become c h a i n ends t h a t c o n t r i b u t e t o brittleness a n d reduce s t r e n g t h ; a n d fragments w i t h o u t a n y reac tive f u n c t i o n a l site become fillers, plasticizers or a n t i - p l a s t i c i z e r s . W h e n a h y d r o x y b u t y l l i g n i n ( H B L ) - b a s e d epoxide w i t h a n e p o x y content of 0.11 e q / 1 0 0 g was crosslinked w i t h (a) a m i n e t e r m i n a t e d p o l y ( b u t a d i e n e c o - a c r y l o n i t r i l e ) ( A T B N ) a n d (b) p h t h a l i c a n h y d r i d e ( P A ) , insoluble m a terials were f o r m e d w h i c h h a d average sol fractions ( i n D M F ) o f 10 a n d 1 4 % , respectively. T h i s indicates t h a t c r o s s l i n k i n g was q u i t e complete, a n d that little lignin remained unincorporated. Weight gained by swelling i n creased l i n e a r l y w i t h A T B N content, a n d t h i s was not s u r p r i s i n g since the A T B N used was of r e l a t i v e l y h i g h m o l e c u l a r weight (6400 g m o l " " ) w h i c h i n t r o d u c e d larger free spaces i n t o the film. 1

P r e l i m i n a r y results w i t h e p o x y networks cured w i t h c o m b i n a t i o n s o f D E T A a n d A T B N are g i v e n i n T a b l e I a n d F i g u r e 4. T a b l e I. C o m p o s i t i o n of L i g n i n E p o x i d e s Composition of Crosslinking Agent by Functionality

Sample No. A Β C D Ε

1

by Weight

ATBN (%)

DETA (%)

ATBN (%)

DETA (%)

0 10 25 50 100

100 90 75 50 0

0.00 14.3 38.3 46.1 67.9

5.4 4.2 3.7 1.5 0.0

Lignin-Epoxide Content ( w t . %) 94.6 81.5 58.0 52.4 32.1

D i s t r i b u t i o n of a m i n e f u n c t i o n a l groups equivalent t o epoxide f u n c t i o n ality. U s i n g a low m o l e c u l a r weight c r o s s l i n k i n g agent s u c h as D E T A , a m a -



512


TIME (days) F i g u r e 3. Degree o f conversion o f h y d r o x y groups o f H P L t o epoxide f u n c tionality i n relation to reaction time a n d K O H addition.

TEMPERATURE (°C) F i g u r e 4. D M T A curves o f l i g n i n epoxides cured w i t h c o m b i n a t i o n s o f D E T A a n d A T B N . (Samples identified i n T a b l e I.)



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terial with nearly 95% lignin derivative content is produced. Crosslinking epoxide functionality with amine-terminated rubber segments (i.e., ATBN) reduces the lignin derivative content to about 30% (Table I). Whereas the ATBN-free cured epoxy network forms a glassy, high-modulus material at room temperature (Fig. 4), rising rubber component introduces a secondary thermal transition at about —45°C which serves to reduce the glassy modulus at ambient temperatures. This amounts to a rubber-toughening effect with two-phase morphology. Whether a third transition at around 0°C, between the rubber transition at —45°C and that signifying lignin at 90°C, can be attributed to a mixed interphase between lignin and A T B N remains the subject of future studies. The mechanical properties of the cured lignin epoxides will be discussed elsewhere. Conclusions A lignin epoxide resin was synthesized from hydroxyalkyl lignin and E C H using K O H and a QAS mixture as catalyst at room temperature. Since rapid dechlorohydrogenation under reaction conditions consumes K O H , stepwise addition of K O H during epoxidation is necessary. A five-fold excess of ECH over stoichiometric requirements was required in order to prevent intramolecular crosslinking of multifunctional hydroxyalkyl lignin. The degree of conversion of hydroxy to epoxide groups was controlled by the stepwise addition of KOH rather than by E C H concentration. Swelling experiments showed that a lignin epoxide resin of 0.11 epoxy equivalents per 100g formed a network polymer when cured with DETA, PA, or A T B N . Phase separation was observed in the rubber-toughened lignin epoxide network. Cured epoxides had lignin derivative contents of up to 95%. This is part 19 of a publication series dealing with engineering plastics from lignin. Earlier parts have been published in J. Appl Polym. Sci.,J. Wood Chem. Technol., and Holzforschung.

Literature C i t e d 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

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Lignin Epoxide - ACS Symposium Series (ACS Publications)

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