Adhesives from Renewable Resources - American Chemical Society

Chapter 9 Durable Wood Adhesives from Kraft Lignin Downloaded by IOWA STATE UNIV on March 28, 2017 | http://pubs.acs.org Publication Date: December 31, 1989 | doi: 10.1021/bk-1989-0385.ch009

R o b e r t H. Gillespie Forest Products Laboratory Forest Service U . S . Department of Agriculture One Gifford Pinchot Drive Madison, WI 53705

Kraft lignin having a high reactivity with formaldehyde is the basis for two new durable wood adhesives. Bonds formed under moderate acid-catalysis of a dispersion of hydroxymethylated lignin had high strength and were water resistant. However, they required longer press times than those needed for bonding with conventional phenolic adhesives at comparable temperatures. A n alternative formulation involved the dispersed hydroxymethylated lignin reacting with a blocked diisocyanate. This formulation required less energy for bonding and yielded high-quality bonds. These formulations use little or no petrochemicals and produce energy efficient bonding. Further work on formulation variables and bonding conditions should result in commercial adhesive systems for flakeboard, oriented strandboard, and plywood manufacture. It has been d e m o n s t r a t e d (1) t h a t a t least one k r a f t l i g n i n possessed sufficient f o r m a l d e h y d e r e a c t i v i t y t o y i e l d a cured adhesive w i t h h i g h i n s o l u b i l i t y i n b o t h d i l u t e acids a n d bases. T h i s l i g n i n , after h y d r o x y m e t h y l a t i o n a n d c u r i n g b y a c i d c a t a l y s i s , p r o d u c e d w o o d b o n d s h a v i n g h i g h water resistance w i t h o u t r e q u i r i n g c o p o l y m e r i z a t i o n w i t h p h e n o l . A satisfactory level o f a c i d i t y for c a t a l y s i s c o u l d be a t t a i n e d u s i n g o r g a n i c acids a t p H levels s i m i l a r t o those f o u n d i n n a t u r a l w o o d . T h e i n c o r p o r a t i o n o f p o l y h y d r o x y c o m p o u n d s t o suppress c o n d e n s a t i o n reactions d u r i n g h y d r o x y m e t h y l a t i o n p e r m i t t e d a n element o f c o n t r o l over a d hesive w o r k i n g properties a n d b o n d performance. T h e adhesive c o u l d be r e a d i l y a p p l i e d t o w o o d surfaces, possessed g o o d storage s t a b i l i t y , a n d h a d w i d e l a t i t u d e i n w o r k i n g p r o p e r t i e s . It was r e a d i l y cleaned f r o m o p e r a t i n g e q u i p m e n t , created m i n i m a l waste, a n d posed n o special p r o b l e m s w i t h waste d i s p o s a l . W h i l e This chapter not subject to U.S. copyright Published 1989 American Chemical Society

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

Downloaded by IOWA STATE UNIV on March 28, 2017 | http://pubs.acs.org Publication Date: December 31, 1989 | doi: 10.1021/bk-1989-0385.ch009

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these results were q u i t e e n c o u r a g i n g , i t was recognized t h a t a d d i t i o n a l research was r e q u i r e d t o : 1) o p t i m i z e f o r m u l a t i o n a n d b o n d i n g c o n d i t i o n s , 2) e l u c i d a t e the r e a c t i o n m e c h a n i s m s i n v o l v e d , 3) develop s a t i s f a c t o r y q u a l i t y - c o n t r o l p r o cedures, 4) assess the cost of b o n d i n g i n c o m p a r i s o n w i t h t h a t r e q u i r e d for p h e n o l i c adhesives, 5) evaluate the l o n g t e r m performance properties, a n d 6) determine the a p p l i c a b i l i t y of the l i g n i n s y s t e m for b o n d i n g flakeboards, o r i ented s t r a n d b o a r d s , a n d p l y w o o d f r o m b o t h s o f t w o o d a n d h a r d w o o d species. A s a step t o w a r d f u l f i l l i n g these needs, t h i s chapter describes the e v a l u a t i o n of f o r m u l a t i o n variables t o help define the useful range of ingredient c o n c e n t r a t i o n s , p r a c t i c a l c o n d i t i o n s for a c h i e v i n g the h y d r o x y m e t h y l a t i o n r e a c t i o n , a n d the a c i d i f i c a t i o n c o n d i t i o n s r e q u i r e d for p r o d u c i n g g o o d w o r k i n g properties i n the adhesive so t h a t r e p r o d u c i b l e h i g h - q u a l i t y b o n d s c o u l d be f o r m e d . Background

Information

T h e earlier r e p o r t j u s t referred to described how l i g n i n m a t e r i a l s were e v a l u a t e d for f o r m a l d e h y d e r e a c t i v i t y a n d for t h e i r a b i l i t y t o f o r m solids t h a t were i n s o l u b l e i n d i l u t e acids a n d b a s e s - a requirement t h a t m u s t be m e t b y a n adhesive i f h i g h performance i n exterior e n v i r o n m e n t s is t o be expected. A l i g n i n m a t e r i a l t h a t y i e l d e d e n c o u r a g i n g results u n d e r the c o n d i t i o n s used was a p u r i f i e d k r a f t l i g n i n available c o m m e r c i a l l y i n r e p r o d u c i b l e q u a l i t y , I n d u l i n A T . It was r e p o r t e d to have a n ash content of 1% a n d a c a r b o h y d r a t e content of less t h a n 1%. It was f o u n d t h a t o n e - h a l f a m o l e of f o r m a l d e h y d e reacted w i t h each m o l e of l i g n i n , a s s u m i n g a m o l e c u l a r weight of 180 for l i g n i n . T h i s is r o u g h l y the weight of a n average C 9 u n i t , the basic r e p e a t i n g s t r u c t u r e i n l i g n i n . O n l y every other C 9 u n i t , o n the average, was able t o react w i t h f o r m a l d e h y d e . T h i s l i g n i n p r o v e d capable of f o r m i n g w a t e r p r o o f b o n d s w i t h w o o d after h y d r o x y m e t h y l a t i o n , p r e c i p i t a t i o n at p H 5.5, c a t a l y s i s w i t h o x a l i c a c i d , a n d hot pressing for 10 m i n u t e s at 150 p s i a n d 150 ° C . T h e f o r m u l a t i o n consisted of 1) r e a c t i n g (7 days, r o o m t e m p e r a t u r e ) the f o l l o w i n g m o l a r q u a n t i t i e s of m a t e r i a l s : l i g n i n 1.0, f o r m a l d e h y d e 2.0, water 20.0, t r i e t h y l e n e g l y c o l ( T E G ) 0.5, s o d i u m h y d r o x i d e 0.5; followed by 2) the a d d i t i o n a l m o l a r q u a n t i t i e s of m a t e r i a l s : acetic a c i d 0.5, a n d o x a l i c a c i d 0.4. T h i s f o r m u l a t i o n evolved b y t r i a l a n d error w i t h the deliberate use of f o r m a l d e h y d e i n large excess d u r i n g hydroxymethylation. O n e of the p r o b l e m s t h a t arose d u r i n g the previous e x p e r i m e n t a t i o n c o n cerned the character of the s o l i d p r o d u c e d b y the acetic a c i d a d d i t i o n after h y d r o x y m e t h y l a t i o n . D o l e n k o a n d C l a r k e (2) described the p h y s i c a l changes t h a t t o o k place u p o n a c i d i f i c a t i o n of h y d r o x y m e t h y l a t e d k r a f t black l i q u o r as s t a r t i n g w i t h a homogeneous black s o l u t i o n at h i g h p H a n d c h a n g i n g u p o n a red u c t i o n i n p H t o a b l a c k , s e m i s o l i d m i x t u r e difficult t o s t i r , t h e n t o a p r e c i p i t a t e v i s i b l e as a c h o c o l a t e - b r o w n d i s p e r s i o n , a n d finally at a c i d p H t o a t a n - c o l o r e d , fine p a r t i c l e size t h i x o t r o p i c d i s p e r s i o n . T h e p u r i f i e d l i g n i n e v a l u a t e d here also went t h r o u g h m u c h the same k i n d of changes, w i t h the character of the a c i d i -



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ADHESIVES F R O M RENEWABLE RESOURCES

fied d i s p e r s i o n d e p e n d i n g u p o n the l i g n i n - t o - w a t e r r a t i o , t e m p e r a t u r e , a n d rate of a c i d i f i c a t i o n . R o o m t e m p e r a t u r e a c i d i f i c a t i o n at slow rates of a d d i t i o n often l e d t o dispersions t h a t r e q u i r e d d i l u t i o n s t o as low as 2 0 % solids before a b r u s h a b l e consistency was o b t a i n e d . T h e r e are m a t e r i a l - h a n d l i n g p r o b l e m s associated w i t h the p r e c i p i t a t i o n of solids b y a c i d i f i c a t i o n of k r a f t b l a c k l i q u o r w h e t h e r or not i t has been h y d r o x y m e t h y l a t e d . I n cases where the o b j e c t i v e was t o p r o d u c e a filterable s o l i d , M e r e wether (3) f o u n d t h a t a c i d i f i c a t i o n at e l evated t e m p e r a t u r e s was effective, w h i l e W h a l e n (4) f o u n d a c i d i f i c a t i o n i n the presence of o r g a n i c l i q u i d s s u c h as c h l o r o f o r m or m e t h y l e n e chloride resulted i n a g r a n u l a r f o r m of s o l i d easily filtered, w a s h e d , a n d d r i e d . A s o l u t i o n t o t h i s p r o b l e m was needed so t h a t a n adhesive o f reasonably h i g h t o t a l solids c o u l d be p r e p a r e d t h a t r e t a i n e d a b r u s h a b l e or s p r a y a b l e consistency. C o n d i t i o n s for h y d r o x y m e t h y l a t i o n of l i g n i n have been r e p o r t e d by other investigators. However, these results are often difficult t o interpret because the l i g n i n - s t a r t i n g m a t e r i a l s lack d e f i n i t i o n , p a r t i c u l a r l y f r o m a s t o i c h i o m e t r i c s t a n d p o i n t . T h e w o r k at the C a n a d i a n L a b o r a t o r y i n O t t a w a , as d e s c r i b e d b y D o l e n k o a n d C l a r k e (2), C l a r k e a n d D o l e n k o ( 5 ) , a n d C a l v e a n d S h i e l d s ( 0 ) , i n v o l v e d crude k r a f t b l a c k l i q u o r as d i d the w o r k of E n k v i s t ( 7 ) . T h e s t a r t i n g m a t e r i a l s c o u l d be described b y s u c h parameters as percent solids content, specific g r a v i t y , p H , a n d percent of free f o r m a l d e h y d e i n the l i q u o r . T h e m o l e r a t i o of react ants c o u l d not be d e t e r m i n e d i n some of these cases. However, i n r e p o r t s where the c o n c e n t r a t i o n o f react ants c o u l d be d e t e r m i n e d , m o l e r a t i o s were c a l c u l a t e d w i t h the assumed m o l e c u l a r weight of l i g n i n as 180. R e p r e s e n t a t i v e d a t a are s u m m a r i z e d i n T a b l e I.

T a b l e I. C o n d i t i o n s for H y d r o x y m e t h y l a t i o n of L i g n i n

Lignin

Moles of Reactants HCHO NaOH

1.0

2.1

1.0 1.0

1.26 2.16

1.0

2.16

0.5 0.9 0.9 0.9

Reaction H 0

Temperature

Time

30.5 33.3 48.0 48.0

Room 80 ° C 70 ° C Room

3 days 1 hour 20 h o u r s 3 days

2

Reference {5) (8) (9) (9)

T h e h y d r o x y m e t h y l a t i o n c o n d i t i o n s used i n t h e research r e p o r t e d here i n v o l v e d efforts t o define p r a c t i c a l ranges r a t h e r t h a n o p t i m i z e c o n d i t i o n s . Experimental

Methodology

T h e h y d r o x y m e t h y l a t i o n r e a c t i o n was c a r r i e d o u t u n d e r several t i m e - t e m p e r a t u r e c o n d i t i o n s u s i n g different m o l e r a t i o s o f reactants i n a search for c o n d i t i o n s t h a t w o u l d m a x i m i z e the extent of h y d r o x y m e t h y l a t i o n . T h e h y d r o x y m e t h y l a t e d l i g n i n ( H M L ) r e a c t i o n p r o d u c t was t h e n e v a l u a t e d for i t s a b i l i t y t o f o r m


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w o o d b o n d s of h i g h q u a l i t y w i t h regard t o s t r e n g t h a n d water resistance. d e t a i l e d d e s c r i p t i o n of e x a m p l e adhesive f o r m u l a t i o n s is g i v e n i n T a b l e I I .

A

Table II. Adhesive Formulations Molecular Moles

Total

Solids

Volatiles

Ingredient

(g)

(g)

(g)

I n d u l i n A T (95%) N a O H (50%) F o r m a l i n (37%)

189.5

Weight


B a s i c components: 1.0 0.5 1.0 0.5 20.0 0.5

180 40 30 150 18 60

TEG Water Acetic acid

40.0 81.1 75.0 261.4 30.0

180.0 20.0 12.0 75.0

21.0

9.5 20.0 69.1

1

-

261.4 9.0

2

A d d i t i o n a l component: ALTERNATIVE l 0.4

126

Oxalic acid

50.4

ALTERNATIVE 2 -

Isoset C X - 1 1 crosslinking agent

3

36.0

14.4

4

5

6

67.7

67.7

A s s u m i n g 4 0 % of f o r m a l d e h y d e a d d e d t o l i g n i n w i t h r e m a i n d e r v o l a t i l i z i n g i n one f o r m or a n o t h e r - a n e s t i m a t e for c a l c u l a t i o n purposes only. 1

2

3

4

5

6

R e a c t i o n w i t h s o d i u m h y d r o x i d e f o r m i n g s o d i u m acetate a n d w a t e r . T o t a l solids = 4 7 . 3 % . L i g n i n is 5 2 . 3 % of the solids. D u e t o 2 moles of water of c r y s t a l i z a t i o n . T o t a l solids = 5 0 . 4 % . L i g n i n is 4 7 . 9 % of the solids. A s h l a n d O i l , Inc.

T h e h y d r o x y m e t h y l a t i o n r e a c t i o n was c a r r i e d out b y c o m b i n i n g w a t e r , t r i ethylene g l y c o l , a n d f o r m a l i n , a n d d i s p e r s i n g the I n d u l i n A T p o w d e r i n the l i q u i d m i x t u r e w i t h m e c h a n i c a l s t i r r i n g at r o o m t e m p e r a t u r e . T h e s o d i u m h y d r o x i d e s o l u t i o n was t h e n a d d e d w i t h a d d i t i o n a l s t i r r i n g u n t i l the dispersed s o l i d dissolved. T h e m i x t u r e was t h e n heated for a specified t i m e at a selected t e m p e r a t u r e . T h e r e a c t i o n was t e r m i n a t e d u p o n the a d d i t i o n of the g l a c i a l acetic a c i d w i t h the m i x t u r e a t 50 t o 80 ° C a n d the p r e c i p i t a t i o n o f the r e a c t i o n p r o d u c t . A f t e r c o o l i n g , the o x a l i c a c i d p o w d e r was a d d e d a n d the d i s p e r s i o n h o m o g e n i z e d t o a s m o o t h , b r u s h a b l e consistency.


114



D i f f e r e n t i a l S c a n n i n g C a l o r i m e t r y . T h e r m o g r a m s of samples were o b t a i n e d w i t h a P e r k i n - E l m e r D S C - 2 differential s c a n n i n g c a l o r i m e t e r . Stainless steel capsules sealed w i t h V i t o n O - r i n g s allowed r o u t i n e o p e r a t i o n s t o 200 ° C . I n d i u m was used t o e s t a b l i s h t e m p e r a t u r e c a l i b r a t i o n s . Baselines were established b y c o o l i n g capsules a n d r e s c a n n i n g the t e m p e r a t u r e range after o b t a i n i n g t h e i n i t i a l thermogram. S p e c i m e n p r e p a r a t i o n a n d t e s t i n g are described i n d e t a i l i n the earlier r e p o r t (1). I n general, the adhesive was used to b o n d yellow b i r c h r o t a r y - c u t veneers i n panels w i t h t w o - p l y l a m i n a t i o n s w i t h g r a i n p a r a l l e l . B o n d i n g was c a r r i e d out by hot pressing at 150 p s i , 150 ° C for either 5, 10, or 15 m i n u t e s . M i n i a t u r e specimens were cut 15 m m w i d e w i t h 1 0 - m m overlap. Specimens were tested for shear s t r e n g t h i n the d r y c o n d i t i o n a n d i n the wet c o n d i t i o n after either a v a c u u m - p r e s s u r e soak procedure or after b o i l i n g i n water for 4 h o u r s . T h e shear s t r e n g t h values r e p o r t e d are the average of five specimens tested, r o u n d e d t o the closest 5 p s i . Results and Discussion C o n t r o l o f P r e c i p i t a t e F o r m a t i o n . T h e p r o b l e m w i t h the character of the p r e c i p i t a t e d H M L p r o v e d t o be easily resolved w i t h the p a r t i c u l a r l i g n i n u n d e r s t u d y . W h e n the a d d i t i o n of the g l a c i a l acetic a c i d was c a r r i e d out at t e m p e r atures o n l y as h i g h as 50 ° C , the r e a c t i o n p r o d u c t was g r a n u l a r a n d d i d not r e t a i n large q u a n t i t i e s o f water i n a s e m i s o l i d mass. H o w e v e r , t h i s c o n d i t i o n req u i r e d a redispersion of the s o l i d after the o x a l i c a c i d c a t a l y s t h a d been a d d e d . T h i s c o u l d be a c c o m p l i s h e d w i t h m i x e r s designed to h o m o g e n i z e a n d disperse materials i n liquids. Effect of F o r m u l a t i o n a n d Processing V a r i a t i o n s o n Shear S t r e n g t h . T h e c o n d i t i o n s s t u d i e d for t h e h y d r o x y m e t h y l a t i o n r e a c t i o n i n c l u d e d 23 ° C , 50 ° C , a n d 80 ° C . T h e c r i t e r i o n used for d e t e r m i n i n g the extent of the r e a c t i o n was the a b i l i t y of the reacted l i g n i n t o develop h i g h - q u a l i t y b o n d s w i t h w o o d - i n t h i s case, b o n d s w i t h wet shear s t r e n g t h o f 900 p s i or greater. T h e development of shear s t r e n g t h as a f u n c t i o n of t i m e at each of the three t e m p e r a t u r e s is s h o w n i n F i g u r e s 1, 2, a n d 3. F i g u r e 1 shows results for r o o m t e m p e r a t u r e reactions i n v o l v i n g m o l e r a t i o s of l i g n i n 1.0, s o d i u m h y d r o x i d e 0.5, f o r m a l d e h y d e 1.25, t r i e t h y l e n e g l y c o l 0.5, a n d water 20.0. It is apparent f r o m F i g u r e 1 t h a t d a t a f r o m wet t e s t i n g s h o w b o n d - q u a l i t y differences m u c h better t h a n d o the values f r o m d r y t e s t i n g . A l s o , 5-minute press t i m e does not p r o v i d e adequate cure, w h i l e 10- a n d 15-minute press t i m e s developed wet shear s t r e n g t h greater t h a n 900 p s i at a l l t i m e s after 2 days of h y d r o x y m e t h y l a t i o n r e a c t i o n . T h e r e was a suggested t r e n d of i n c r e a s i n g wet shear s t r e n g t h development f r o m 2 t o 19 d a y s ' r e a c t i o n , b u t the increase was s l i g h t . T h u s , h y d r o x y m e t h y l a t i o n at r o o m t e m p e r a t u r e y i e l d e d h i g h - q u a l i t y b o n d s w i t h r e a c t i o n t i m e s r a n g i n g f r o m 2 to 19 days.



9.

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115

F i g u r e 1. T h e effect o f h y d r o x y m e t h y l a t i o n t i m e at 23 ° C o n shear s t r e n g t h development.


116


12 VPS 10


e

8+

„ 4 +

Press Time ! •

·

*—

A

5 min 10 min

• — • 15 min •

Boil

0

I

f

1400

^

1200

S

1000

£

800

I

600

I

400 200

1 1 1 1 1 1 M «

1 1 1 1 11

+ 1600

1 1 1 1 1 -

0

I 2 3 4 622461 2 3 4622461 2 3 4 622 46 Time (hours)

12 - D r y

10

Boil

VPS

8 ο

6

CL

S £

4 2

S ο ιο

Press Time • — · 5 min A — • |0 min •—• 15 min

î

ο

X ο - I bo

I

8

ο

_ ο 4 2 0

.— Γ OtO

2

4

6

2

4

6

2

4

6

Time (hours)

F i g u r e 2 A a n d 2B. T h e effect of h y d r o x y m e t h y l a t i o n t i m e at 50 ° C o n shear strength development.



9.

GILLESPIE

117


.5

I

1.5

0

.5

I

1.5

0

.5

I

1.5

2

Time (hours)

F i g u r e 3. T h e effect o f h y d r o x y m e t h y l a t i o n t i m e at 80 ° C o n shear s t r e n g t h development.



118


H y d r o x y m e t h y l a t i o n at 50 ° C t o o k place i n a m a t t e r o f h o u r s w i t h the same f o r m u l a t i o n . D a t a covering the t i m e s p a n f r o m 1 t o 46 hours are s h o w n i n F i g u r e 2 A w i t h wet shear s t r e n g t h above 900 p s i d e v e l o p i n g after 3 h o u r s ' r e a c t i o n . A g a i n , there was a l o n g r e a c t i o n p e r i o d d u r i n g w h i c h adequate wet shear s t r e n g t h was developed. A d d i t i o n a l d a t a were o b t a i n e d at 3, 4, 5, a n d 6 h o u r s o f r e a c t i o n t i m e , w h i c h is s h o w n i n F i g u r e 2 B for t w o f o r m a l d e h y d e t o - l i g n i n m o l e r a t i o s , 1.0 a n d 1.25 t o 1.0. I n t h i s case, the 1:1 m o l e r a t i o of f o r m a l d e h y d e t o l i g n i n appeared t o develop higher wet shear s t r e n g t h t h a n d i d the 1.25:1. A g a i n , 10- to 15-minute press t i m e s were r e q u i r e d for adequate cure. T h e d a t a suggest t h a t a r e a c t i o n t i m e of at least 5 hours at 50 ° C s h o u l d p r o v i d e high-quality bonds. T h e d a t a o b t a i n e d w i t h r e a c t i o n at 80 ° C are s h o w n i n F i g u r e 3 for a for m u l a t i o n w i t h a f o r m a l d e h y d e - t o - l i g n i n r a t i o of 1:1. T h e s e d a t a show t h a t a r e a c t i o n t i m e o f 1 h o u r p r o v i d e d h i g h wet shear s t r e n g t h . T h e effect o f v a r y i n g the s o d i u m h y d r o x i d e - t o - l i g n i n r a t i o is s h o w n i n F i g u r e 4 A where the r a t i o e v a l u a t e d was 0.25, 0.5, 0.75, a n d 1.0 t o 1.0. T h e c o n d i t i o n s used for h y d r o x y m e t h y l a t i o n were those f o u n d earlier t o p r o d u c e a complete r e a c t i o n ( w h e n 0.5 m o l e o f s o d i u m h y d r o x i d e a n d 1.15 moles o f f o r m a l d e h y d e h a d been used). O n l y t w o press t i m e s were e v a l u a t e d , 10 a n d 15 m i n u t e s , since shorter t i m e s h a d p r o v e d t o be i n a d e q u a t e for f u l l cure. T h e lowest a n d highest c o n c e n t r a t i o n s o f s o d i u m h y d r o x i d e y i e l d e d p o o r b o n d q u a l i t y , w h i l e the 0.5 a n d 0.75 levels p r o d u c e d h i g h wet shear s t r e n g t h . A d d i t i o n a l d a t a were o b t a i n e d at r a t i o s o f 0.4, 0.5, a n d 0.6 t o 1.0 as s h o w n i n F i g u r e 4 B w i t h t w o f o r m a l d e h y d e t o - l i g n i n r a t i o s , 1.0 a n d 1.25 t o 1.0. H i g h - q u a l i t y b o n d s , b o t h wet a n d d r y , were p r o d u c e d b y a l l three s o d i u m h y d r o x i d e c o n c e n t r a t i o n s a n d l i t t l e , i f any, difference resulted f r o m r e d u c i n g the f o r m a l d e h y d e content b y o n l y 0.25 m o l e per m o l e o f l i g n i n . It w o u l d be m o s t desirable t o use as l i t t l e f o r m a l d e h y d e as possible a n d s t i l l achieve complete h y d r o x y m e t h y l a t i o n u n d e r reasonable t i m e - t e m p e r a t u r e c o n d i t i o n s for r e a c t i o n . S o m e losses w o u l d be expected b y the C a n n i z a r o r e a c t i o n . T h e r a t i o o f f o r m a l d e h y d e t o l i g n i n w o u l d need t o be greater t h a n 0.5 t o 1.0. W h e n the r a t i o was increased t o 1.0, 1.5, a n d 2.0 t o 1.0, the shear s t r e n g t h d a t a s h o w n i n F i g u r e 5 were o b t a i n e d . W i t h h y d r o x y m e t h y l a t i o n t a k i n g place at r o o m t e m p e r a t u r e for 7 days, h i g h wet a n d d r y shear strengths were o b t a i n e d at a l l three r a t i o s . W h e n the h y d r o x y m e t h y l a t i o n was c a r r i e d out for 3 h o u r s at 50 ° C , shear s t r e n g t h appeared t o increase w i t h i n c r e a s i n g f o r m a l d e h y d e c o n tent, suggesting t h a t the r e a c t i o n t i m e m a y have been insufficient for complete r e a c t i o n except w h e n a large excess o f f o r m a l d e h y d e was present to advance the extent o f r e a c t i o n b y mass a c t i o n . These d a t a i n d i c a t e t h a t a f o r m a l d e h y d e t o - l i g n i n r a t i o of 1: Γ w o u l d be satisfactory, p r o v i d i n g the r e a c t i o n t i m e was adequate for complete r e a c t i o n t o occur. T h e fact t h a t a 10-minute press t i m e at 150 ° C a n d 150 p s i was r e q u i r e d before h i g h wet s t r e n g t h was o b t a i n e d i n d i c a t e d the need for m o r e energy to cure the l i g n i n adhesive t h a n was r e q u i r e d for cure o f a p h e n o l i c adhesive. T h e


9.

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2 Downloaded by IOWA STATE UNIV on March 28, 2017 | http://pubs.acs.org Publication Date: December 31, 1989 | doi: 10.1021/bk-1989-0385.ch009

119


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