Wood Adhesives from Phenolysis Lignin - American Chemical Society

Chapter 25

Wood Adhesives from Phenolysis Lignin A Way To Use Lignin from Steam-Explosion Process

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Hiro-Kuni Ono and Kenichi Sudo Forestry and Forest Products Research Institute, Ministry of Agriculture, Forestry, and Fisheries, P.O. Box 16, Tsukuba Norin, Ibaraki 305, Japan

The lignin extracted from steam exploded pulp was phenolated in the presence of sulfuric acid. The degree of phenolation was calculated to be in excess of one mole/lignin (C ) unit on the basis of C N M R measurements. The phenolated lignin was methylolated in order to prepare adhesive resins. The cure behavior of the adhesive resins was examined by Torsional Braid Analysis (TBA). Results revealed that the phenolated steam explosion lignin-based resins had intrinsic retardation in cure as compared to a commercial phenolic resin. This defect, however, was partly overcome by increasing their pH values. The adhesives from these resins generally provide excellent bond strength comparable to phenolic resin. 9

13

The steam explosion process is a recent development in wood processing ( 1 , 2 ) . Much attention has been paid to this process from the viewpoint of total wood utilization. Cellulose and hemicellulose from this process can be converted into sugars of commercial value by enzymatic methods (3). However, the conversion of lignin from this process (steam explosion lignin) into useful materials continues to present difficulties. Preparation of adhesives from it is considered to be a feasible way to solve this problem. Steam explosion lignin is reported to have more reactive functional groups (4), and to contain no sulfur, as compared to kraft (thio) lignin and lignin sulfonates. The absence of blocked reactive functional groups in the lignin must be an advantage to various chemical modifications by which the lignin would be utilized as an adhesive of commercial use. The addition of phenol-formaldehyde precondensate to lignin or methylolated lignin has been known as a way to introduce phenolic reactivity into lignin, and this is usually applied to the preparation of lignin-based 0097-6156/89A)397-0334$06.00A) © 1989 American Chemical Society

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


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adhesives (5-10). In this case, all the lignin does not combine with the precondensate. Some part of it is considered to function as filler. T h e amount of lignin added is usually limited by some strength requirement. In order to make lignin molecules contribute to bond strength, lignin should be incorporated into the phenolic main chain structure by a phenolysis reaction, for instance. There have been two kinds of lignin phenolysis reactions reported. Muller and Glasser have prepared phenolated lignins by a two-step reaction (11). Kraft, acid hydrolysis, and steam explosion lignins were allowed to react with formaldehyde before the methylolated lignins were combined with phenol. The adhesion quality of the pheno lated lignin/phenol-formaldehyde resins was investigated, and the pheno lated steam explosion lignin/phenol-formaldehyde resin was found to have slightly lower bond strength than the kraft lignin/phenol-formaldehyde and a neat phenolic resin. It was suggested that a considerable amount of syringyl propane units in the steam explosion aspen lignin was partly respon sible for the lower bond strength as compared to the neat phenolic resin (12). The other phenolysis reaction is the direct reaction of phenol with the substituted propyl side chain of lignin. Wacek et al. elucidated the chem istry of this reaction by oxidation (13). They concluded that condensation occurred between the o- or p-position of phenol and lignin's α-position sub stituted by O H , O - R i , = 0 or = C - R 2 ( R i and R 2 : lignin residue). This mechanism has been confirmed by Kratzl et al. by using radioactive C labeled lignin (14). Kobayashi et al. have prepared phenolated lignin by applying this phenolysis method in order to obtain thiolignin-based mold ing compounds (15). This method is suitable for hardwood lignins so far as they have sufficient functional groups at their α-positions of the propyl side chain. Since the steam explosion process has usually been applied to hardwoods, the latter phenolysis method is of interest from the viewpoint of adhesive preparation as compared to the former phenolysis. 1 4

Experimental Materials. Lignin (SEL) was extracted with dilute alkali from steam ex ploded white birch (Betula plaiyphylla) pulp. T h e pulp was prepared at 200°C for 10 minutes. The C N M R measurement of its acetylated prod uct provided a methoxy:aryl ratio of 1.48, and this is in good agreement with Obst and Landucci's value of 1.44 (16). The molecular weight of C6-C3 units of S E L was calculated to be about 180. Chemical reagents were the first grade in the Japanese industrial standard. They were used as received. A commercial phenolic resin (D-17 of Ohshika Co.) was used in order to compare with bond strength of lignin-based adhesives. A commercial ad ditive (Hot Ρ of Ohshika Co.) and an extender (wheat flour: Akahana of Nisshin Co.) were used for the adhesive formulation. 1 3

Phenolysis of SEL. Phenol (120 g) was charged into a 300 ml round bot tom flask equipped with thermometer, stirrer and cooler. S E L (60 g) was added slowly so that it could dissolve thoroughly. Sulfuric acid (0.5 ml) was then added as a catalyst. T h e typical sulfuric acid:SEL charge ratio


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was 1.5 m M o l e / g . T h e r e a c t i o n schedule consisted of 1 h r of w a r m - u p p e r i o d a n d 3 hrs of phenolysis at 170° C , followed b y r e m o v a l of u n r e a c t e d p h e n o l under reduced pressure. T h e p h e n o l a t e d S E L was a b l a c k substance w h i c h dissolved more easily i n organic solvents, especially i n alcohols, t h a n d i d S E L . T h e softening p o i n t of the p h e n o l a t e d S E L was a b o u t 140°C. O t h e r phenolysis l i g n i n s , w i t h different charge ratios of s u l f u r i c a c i d , were p r e p a r e d to c o m p a r e w i t h the a m o u n t of b o u n d p h e n o l . Preparation of Phenolated SEL-Formaldehyde Resins. Phenolated S E L f o r m a l d e h y d e resins ( L P ' s ) were prepared i n a m a n n e r s i m i l a r t o resolet y p e p h e n o l - f o r m a l d e h y d e resin b y u s i n g a n alkaline c a t a l y s t . I n a 100 m l flask e q u i p p e d w i t h t h e r m o m e t e r , stirrer a n d cooler, a n a p p r o p r i a t e a m o u n t of f o r m a l i n was c o m b i n e d w i t h the equivalent a m o u n t of m e t h a n o l as a d i s s o l v i n g a i d , a n d 5 0 % aqueous N a O H s o l u t i o n (3.2 g). T h e p o w d e r o f the p h e n o l a t e d S E L (20 g) was t h e n s l o w l y added to the m i x t u r e , a n d it was forced to dissolve t h o r o u g h l y b y a g i t a t i o n . T h e r e a c t i o n schedule consisted of 15 m i n u t e s of w a r m - u p p e r i o d , 60 minutes of m e t h y l o l a t i o n at 6 0 ° C a n d 10 m i n u t e s of w a r m - u p p e r i o d to 8 0 ° C , followed b y c o n d e n s a t i o n at 8 0 ° C . A f t e r the reaction was complete, m e t h a n o l a n d some water were removed under reduced pressure, r e s u l t i n g i n a black viscous resin. T h r e e f o r m a l d e h y d e charge levels of 1.5, 3 a n d 5 moles were e m p l o y e d based o n f u n c t i o n a l i t y of the p h e n o l a t e d S E L for the resin p r e p a r a t i o n . S o l i d content of the three resins was adjusted w i t h water to a r o u n d 4 6 % . P r e p a r a t i o n of the three resins was carefully c o n t r o l l e d i n order to adjust t h e i r viscosities to a r o u n d 0.3 P a s , w h i c h is supposed to be i n a desirable range for adhesive a p p l i c a t i o n . L P ' s f r o m formaldehyde charge ratios of 1.5, 3 a n d 5 were n a m e d as L P - Α , L P - B , a n d L P - C , respectively. M e t h y l o l a t i o n of S E L i t s e l f was also carried out i n the same m a n n e r as L P p r e p a r a t i o n i n order to c o m p a r e their b o n d s t r e n g t h w i t h those of L P ' s . Adhesive Formulation. A d h e s i v e s for t e s t i n g consisted of 100 parts resin, 10 p a r t s wheat flour as extender, 4 parts c o m m e r c i a l a d d i t i v e , a n d 2 p a r t s w a t e r . T h i s f o r m u l a t i o n is r e c o m m e n d e d b y Japanese b o a r d m a n u f a c t u r e r s . A d h e s i v e s w i t h o u t wheat flour were also f o r m u l a t e d to e x a m i n e the b o n d s t r e n g t h of the neat l i g n i n based resins. Specimen for Tensile Shear Adhesion Test. S i n g l e l a p specimens for shear s t r e n g t h tests were m a d e f r o m b i r c h test panels ( l e n g t h : 80 m m , w i d t h : 25 m m , d e p t h : 3 m m ) . T w o of the panels were g l u e d w i t h the adhesives i n a s p r e a d i n g rate of 1.5 g / 1 0 0 c m for single glue l i n e . B o n d i n g area of the specimens was 25 x 13 m m . T h e specimens were first pressed i n a n a m b i e n t t e m p e r a t u r e for a n h o u r under a pressure o f 0.78 M P a a n d t h e n hot-pressed at 140°C under a pressure of 0.98 M P a for 6 m i n u t e s . 2

C NMR Spectroscopy. C N M R measurements were c a r r i e d out u s i n g a J E O L J M N - G S X 400 spectrometer for q u a n t i t a t i v e a n a l y s i s i n order to e x a m i n e the a m o u n t of b o u n d p h e n o l i n the p h e n o l a t e d S E L ' s . T h e a n a l y s i s was c o n d u c t e d i n D M S O - d b y u s i n g gated d e c o u p l i n g technique. 1 3

13

6

Gel Permeation

Chromatography

(GPC).

M o l e c u l a r weight d i s t r i b u t i o n s of


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p h e n o l y s i s l i g n i s d u r i n g the phenolysis were e x a m i n e d at 60° C i n T H F as m o b i l e phase b y u s i n g a T o y o S o d a H P L C - 8 0 2 U R gel p e r m e a t i o n c h r o m a t o g r a p h e q u i p p e d w i t h t w o 60 c m p o l y s t y r e n e gel c o l u m n s i n series ( T S K - G E L H G 2 5 0 0 a n d G 1 0 0 0 ) . T h e c h r o m a t o g r a m s were m o n i t o r e d b y refractometer. Torsional Braid Analysis. L P - Α , L P - B , a n d L P - C were coated o n glass b r a i d s a n d t h e i r cure b e h a v i o r s were e x a m i n e d b y u s i n g a R H E S C A R D 1 1 0 0 A t o r s i o n a l b r a i d a n a l y z e r . T h e i r r e l a t i v e r i g i d i t y changes a l o n g w i t h cure t e m p e r a t u r e ( h e a t i n g rate was l ° C / m i n ) were m o n i t o r e d i n order to e x a m i n e the cure speed of the L P resins. T h e r e l a t i v e r i g i d i t y changes d u r i n g cure at 140°C were also m e a s u r e d to e x a m i n e the cure speed dependence o n resin p H . Tensile Shear Strength Measurement. T h e tensile shear b o n d s t r e n g t h was d e t e r m i n e d i n accordance w i t h Japanese s t a n d a r d J I S Κ 6851 ( n o r m a l a n d repeated b o i l i n g test) b y u s i n g a T o y o S e i k i S t r o g r a p h W tensile tester. I n the n o r m a l test, the test specimens were c o n d i t i o n e d for at least 48 hrs at 2 0 ± 5 ° C under a relative h u m i d i t y of 65 ± 2 0 % before tensile shear s t r e n g t h was m e a s u r e d . I n the repeated b o i l i n g test, the specimens were i m m e r s e d i n b o i l i n g water for 4 hrs, d r i e d at 6 0 ± 3 ° C for 20 hrs a n d i m m e r s e d a g a i n i n b o i l i n g water for 4 h r s , followed b y i m m e r s i o n i n water at r o o m t e m p e r a t u r e u n t i l cooled. T h e specimens were tested i n wet state. S i x specimens were tested for each resin. Results and Discussion Optimum Reaction Period between Phenol and Lignin. T h e e n d p o i n t of the p h e n o l y s i s r e a c t i o n c a n be e s t i m a t e d b y the p e r i o d w h e n the c o n s u m p t i o n of p h e n o l reaches e q u i l i b r i u m . P h e n o l y s i s p r o d u c t s were m o n i t o r e d b y G P C d u r i n g the r e a c t i o n . T h e change i n m o l e c u l a r weight d i s t r i b u t i o n d u r i n g the p h e n o l y s i s is s h o w n i n F i g u r e 1. T h e increase o f the peak at 19 m i n clearly demonstrates t h a t p o l y m e r i z a t i o n of S E L occurs as the r e a c t i o n proceeds. T h e r a t i o of peak area of p h e n o l a t e d l i g n i n (16 to 28.7 m i n i n e l u t i o n t i m e ) to t h a t of u n r e a c t e d p h e n o l (27.9 m i n ) vs. r e a c t i o n t i m e is s h o w n i n F i g u r e 2. U n r e a c t e d p h e n o l d i m i n i s h e s r a p i d l y at the e a r l y stage of the r e a c t i o n before r e a c h i n g e q u i l i b r i u m w i t h i n 3 h r s . T h i s was defined as o p t i m u m p h e n o l y s i s r e a c t i o n p e r i o d . Functionality Measurement of Phenolated Lignin. It is i m p o r t a n t to have knowledge of the f u n c t i o n a l i t y of the p h e n o l a t e d l i g n i n f r o m the p o i n t of v i e w of f u r t h e r c h e m i c a l m o d i f i c a t i o n . T h e a m o u n t of b o u n d p h e n o l i n the p h e n o l y s i s r e a c t i o n has been measured b y t i t r a t i n g the p h e n o l e x t r a c t e d f r o m the r e a c t i o n m i x t u r e (15). T h i s i n d i r e c t m e t h o d measures the u n r e acted p h e n o l a n d determines b o u n d p h e n o l as the difference between the i n i t i a l charge a n d the t i t r a t e d p h e n o l . T h i s is sometimes m i s l e a d i n g . * H N M R spectroscopy is another c a n d i d a t e for the d e t e r m i n a t i o n of the a m o u n t of b o u n d p h e n o l . H o w e v e r , t h i s c a l c u l a t i o n is difficult since the n u m b e r of p r o t o n s before a n d after the phenolysis r e a c t i o n is u n k n o w n .


338


400

5

H

208


I

I

18.θ

I

-

\s

1

y

0.6

ο pH

9.9

pH

10.5

• pH

11.6

Δ

3 0.4 ~~

343

Wood Adhesives from Phenolysis Lignin

ι

/

LU

• Phenolic Resin

0.2

10.8)

(pH 1

1

1

1

ι

ι

20 40 60 80 100 CURING PERIOD (min)

I

120

F i g u r e 5. C u r e rate dependence o n p H for the p h e n o l a t e d s t e a m e x p l o s i o n l i g n i n - b a s e d resin L P - B .

T h i s m i g h t be e x p l a i n e d w i t h f a i l u r e t o crosslink sufficiently. S a n o et a l . have also r e p o r t e d t h a t phenolysis of h a r d w o o d l i g n i n sulfonates enhances adhesion properties (22). It is t h u s clearly d e m o n s t r a t e d t h a t the i n t r o d u c t i o n o f p h e n o l i n t o l i g n i n improves adhesion properties.



344

T a b l e I I . Tensile Shear B o n d S t r e n g t h of A d h e s i v e s f r o m P h e n o l y s i s L i g n i n i n N o r m a l Test a n d after R e p e a t e d B o i l T r e a t m e n t B o n d Strength (10 Pa) (Standard Deviation) 5

Normal

Repeated B o i l

Adhesives from L P - B w i t h extender w i t h o u t extender

47.2 (8.8) 65.3 (9.6)

52.9 (6.1) 43.5 (3.3)

Adhesives from L P - C w i t h extender w i t h o u t extender

41.5 (9.6) 68.4 (7.8)

55.6 (7.9) 51.4 (6.7


Adhesives

Adhesives from methylolated S E L w i t h extender 45.9 (9.2) w i t h o u t extender 32.0 (10.1)

0 0

A d h e s i v e f r o m c o m m e r c i a l phenolic resin w i t h extender 62.6 (6.6)

47.3 (11.5)

T h e two p h e n o l a t e d S E L adhesives w i t h wheat flour do not p r o v i d e g o o d b o n d s t r e n g t h i n the n o r m a l test, b u t p r o v i d e excellent s t r e n g t h after repeated b o i l . T h e b o n d s t r e n g t h increment after repeated b o i l i n d i c a t e s the occurrence of post-cure i n the p h e n o l a t e d S E L adhesives. T h e a c i d i t y of the wheat flour m i g h t r e t a r d resin cure b y r e d u c i n g the p H of the adhesives (23). L P - C p r o v i d e d better b o n d q u a l i t y as c o m p a r e d to L P - B . Since the difference between L P - B a n d L P - C lies i n the f o r m a l d e h y d e r a t i o c h a r g e d , t h i s f i n d i n g suggests t h a t a s t o i c h i o m e t r i c charge m i g h t not be adequate for the f o r m a t i o n o f m e t h y l o l groups i n L P ' s . T h e r e m i g h t be a p r o b l e m w i t h the r e a c t i o n of f o r m a l d e h y d e w i t h the p h e n o l a t e d S E L as c o m p a r e d to p h e n o l . G e n e r a l l y s p e a k i n g , the p h e n o l a t e d S E L adhesives w i t h o u t wheat flour p r o v i d e b o n d strengths c o m p a r a b l e t o the p h e n o l i c resin i n n o r m a l a n d r e p e a t e d - b o i l tests. It has been r e p o r t e d t h a t the resin f r o m the r e a c t i o n of f o r m a l d e h y d e w i t h the m i x t u r e of the two-step p h e n o l a t e d s t e a m e x p l o s i o n l i g n i n a n d p h e n o l has also p r o v i d e d c o m p a r a b l e b o n d s t r e n g t h (12). It is n o t e w o r t h y t h a t the p h e n o l a t e d S E L l i g n i n was d i r e c t l y m e t h y l o l a t e d w i t h o u t any a d d i t i o n of p h e n o l . A l t h o u g h there are s t i l l some p r o b l e m s , such as the ineffective m e t h y l o l a t i o n o f the p h e n o l a t e d S E L a n d the selection o f a s u i t a b l e extender, i t c a n generally be concluded t h a t phenolysis is a p r o m i s i n g m e t h o d to develop s t e a m e x p l o s i o n l i g n i n i n t o a t t r a c t i v e adhesives c o m p a r a b l e t o c o m m e r c i a l p h e n o l i c resin. Conclusions Q u a n t i t a t i v e a n a l y s i s o f phenolysis l i g n i n b y C N M R has i n d i c a t e d t h a t the a m o u n t o f p h e n o l b o u n d to the s t e a m e x p l o d e d l i g n i n is u n e x p e c t e d l y 1 3


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large as c o m p a r e d t o t h i o l i g n i n . T h e s t e a m explosion l i g n i n itself c o u l d re act w i t h f o r m a l d e h y d e , r e s u l t i n g i n t h e r m o s e t t i n g resins. T h i s r e s i n , h o w ever, d i s p l a y e d p o o r adhesion properties, especially i n t h e repeated b o i l test. T h e p h e n o l a t e d l i g n i n - f o r m a l d e h y d e resins p r o v i d e d excellent adhe sives c o m p a r a b l e t o a c o m m e r c i a l phenolic resin. P h e n o l y s i s appears t o b e a p r o m i s i n g m e t h o d t o u t i l i z e s t e a m e x p l o s i o n l i g n i n s as adhesives.


Acknowledgments T h i s research was funded b y the B i o m a s s Transfer P r o g r a m o f the Japanese M i n i s t r y o f A g r i c u l t u r e , F o r e s t r y a n d Fisheries. T h e a u t h o r s w o u l d like t o express their t h a n k s t o M r . H . O h a r a for e x t r a c t i n g l i g n i n a n d t o M s . Y a o X i n o f the Forest P r o d u c t s Research I n s t i t u t e o f H e i l o n g i a n g P r o v i n c e i n C h i n a for p r e p a r i n g the p h e n o l a t e d l i g n i n d u r i n g her ten m o n t h s leave a t their I n s t i t u t e . Literature C i t e d 1. Dietrichs, Η. H.; Sinner, M . ; Puls, J . Holzforschung 1978, 32, 193. 2. Marchessault, R. H . ; St.-Pierre, J . In Future Sources of Organic Raw Materials-CHEMRAWN I; St.-Pierre, L . E.; Brown, G . R., Eds.; Pergamon: New York, 1980; p. 613. 3. Shimizu, K . ; Sudo, K . ; Nagasawa, S.; Ishihara, M . Mokuzai Gakkaishi 1983, 29, 428. 4. Sudo, K . ; Shimizu, K . ; Sakurai, K. Holzforschung 1985, 39, 281. 5. Johansson, I. J . Ger. Offen. 2 624 673, 1976. 6. Forss, K . J.; Fuhrmann, A . G . M . Ger. Offen. 2 601 600, 1976. 7. Adams, J . W.; Schoenherr, M . W . U.S. Pat. 4 306 999, 1981. 8. Enkvist, T . U . E . U.S. Pat. 3 864 291, 1975. 9. Sakakibara, A . Japanese Pat. 51-22497, 1976. 10. Clarke, M . R.; Dolenko, A . J. U.S. Pat. 4 113 675, 1978. 11. Muller, P. C . ; Glasser, W . G . J. Adhesion 1984, 17, 159. 12. Muller, P. C . ; Kelley, S. S.; Glasser, W . G . J. Adhesion 1984, 17, 185206. 13. von Wacek, Α.; Daubner-Rettenbacher, H . Monatsch. Chem. 1950, 81, 266. 14. Kratzl, K . ; Buchtela, K.; Gratzl, J . ; Zauner, J . ; Ettingshausen, O. Tappi 1962, 45, 113. 15. Kobayashi, Α.; Haga, T . ; Sato, K. Mokuzai Gakkaishi 1966, 12, 305. 16. Obst, J . R.; Landucci, L . L . Holzforschung 1986, 40, 87. 17. Robert, D. R.; Bardet, M . ; Gagnaire, D. Proc. Inter. Symp. Wood and Pulping Chem., Tsukuba Science City, Japan, 1983, Supp. Vol., p. 1. 18. Landucci, L . L . Holzforschung 1985, 39, 355. 19. Lapierre, C . ; Monties, B. Holzforschung 1984, 38, 333. 20. Fujita, N.; Ogasawara, K . Netsukoukasei Jyushi 1982, 3, 31. 21. Steiner, P. R.; Warren, S. R. Holzforschung 1981, 35, 273. 22. Sano, Y . ; Ichikawa, A . Mokuzai Gakkaishi 1987, 33, 47. 23. Sellers, T . , Jr. Plywood and Adhesive Technology; Marcell Dekker: New York, 1985; C h . 19. RECEIVED February 27,1989 In Lignin; Glasser, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

Wood Adhesives from Phenolysis Lignin - American Chemical Society

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