Wood Sci. Technol. 14 :1-7 (1980) Woocl Science ancl Technology �9 by Springer-Verlag 1980

T h e C a r b o x y m e t h y l a t i o n o f O r g a n o s o l v a n d K r a f t L i g n i n s *

W. Lange

Ordinariat fiir Holztechnologie (Holzchemie) der Universit~it Hamburg

W. Schweers t

Institut fiir Holzchemie und chemische Technologic des Holzes der Bundesforschungsanstalt ffir Forst- und Holzwirtsehaft, Hamburg-Reinbek

Summary. Kraft and ethanol organosolv lignins from Picea abies and Fagus sylvatica were car- boxymethylated with bromoacetic acid. Of the total hydroxyl groups in the lignins, 60-70 % were earboxymethylated. The reaction led to an increase of the number-average molecular weights. The moisture adsorption of the carboxymethylated products is about one and a half as high as that of the original lignins. There are no remarkable differences between the solu- bilities of the treated and untreated products.

In t roduct ion

In the past several publications have appeared concerning the use o f lignins as a

source for the manufacture o f technically useful chemicals. Among these, experi-

ments were described dealing with the preparat ion of polyurethane foams from

this material. I t was shown that lignin can serve as a raw material for this purpose,

if it has been modified by an oxyalkylat ion reaction with ethylene or propylene

oxide (Christians et al., 1970). The phenolic hydroxyl groups of lignin react quan-

t i tatively with ethylene oxide, i f the side chains do not contain C, C double bonds

or carbonyl groups, as Ishikawa and coworkers (1961) demonstrated by reacting

lignin model compounds with ethylene oxide.

Hsu and Glasser (1975) reduced the unsaturation of lignin by an addit ion re- action with maleic anhydride. The copolymer obtained was oxyalkylated with pro-

pylene oxide and yielded a polyester-polyetherpolyol suitable ~or the preparat ion

* The authors wish to express their appreciation to Mr. Dietrich Meier, Federal Research Or- ganization for Forestry and Forest Products, Hamburg, Federal Republic of Germany, for pre- paration and donation of lignin samples

They are also grateful to Mrs. Maren Albrecht and Mr. Uwe Engel, Federal Research Or- ganization for Forestry and Forest Products, Hamburg, Federal Republic of Germany, for the determination of C, H, the molecular.weights and methoxyl and hydroxyl groups

0043-7719/80/0014/0001/$1.40

2 W. Lange and W. Schweers

of a semi-rigid polyurethane foam of low density and good strength. Another

method to avoid difficulties during the oxyalkylation reaction of phenolic hydroxyl groups of lignin could be a modification of lignins by a carboxymethylation re-

action. In the case of sufficient reactivity of the phenolic hydroxyl groups with halo- genoacetic acid, carboxymethyl lignins with a low remaining hydroxyl content should be obtained. These lignin derivatives should exhibit no resistance to the

oxyalkylation. In contrast to the polyether-polyols from unmodified lignins, the oxyalkylation of carboxymethylated lignins should yield polyester-polyols or, de- pendent on the rate of the etherification of the hydroxyl groups, polyester-poly-

ether-polyols. Carboxymethylations of lignin have previously been carried out with lignosul-

fonic acid only (Tanaka et al., 1966). The objective of this study was to examine

the reactivity of other lignin preparations with halogenoacetic acid.

Results

Carboxymethylations

Since for a reaction in homogeneous phase alkali-soluble lignins should be used, kraft lignins and ethanol organosolv lignins from European beech (Fagus sylvatica L.) and

European spruce (Picea abies Karst.) were carboxymethylated. According to the

procedure used by Tanaka and coworkers (1966) for the carboxymethylation of lignosulfonic acid, the lignins were dissolved in sodium hydroxide solution and treat- ed with bromoacetic acid at 40 ~ for four hours. The carboxymethylated lignins were precipitated from the reaction mixture by acidification with sulfuric acid.

Table 1. The moisture absorption of the treated and untreated lignins in a steam saturated atmosphere

Sample Moisture absorption in 3h 24h 48h 10 days

Ethanol organosolv lignin from spruce 2.9 5.4 7.2 7.9 Kraft lignin from spruce 2.1 7.5 8.2 9.4 Carboxymethylated ethanol organosolv lignin

from spruce 3.3 10.2 10.5 13.0 Carboxymethylated kraft lignin from spruce 4.7 13.3 14.2 14.2

Ethanol organosolv lignin from beech Kraft lignin from beech Carboxymethylated ethanol organosolv lignin

from beech Carboxymethylated kraft lignin from beech

3.2 7.8 8.6 10.6 2.4 8.3 9.8 14.8

7.3 11.4 12.1 13.8 8.4 13.7 15.2 22.9

Carboxymethylation of Organosolv and Kraft Lignins 3

Properties of the Carboxymethylated Products

Solubility

There are no remarkable differences between the solubilities of the carboxymethyl- ated and original lignins. The carboxymethylated organosolv lignins as well as the organosolv lignins themselves are soluble in ethanol, propylene glycol, dioxane and dimethyl sulfoxide. The kraft lignins as well as their carboxymethylated products are only slightly soluble in methanol, ethanol and acetone. They are fully soluble in dimethyl sulfoxide and partially in dioxane. The carboxymethylated kraft lig- nins are soluble in boiling propylene glycol and in a 10:1 mixture of ethanol/ water. They precipitate partially on cooling the solutions. The original kraft lign- ins are not completely soluble even in boiling propylene glycol or ethanol/water (10 : 1). On cooling, the solutions form a precipitate.

Hygroscopicity

The moisture adsorption of the carboxymethylated products in an atmosphere sa- turated with water vapour is higher than those of the original lignins (Table 1).

Chemical Composition

The content of hydroxyl groups of all lignins investigated decline considerably dur- ing the carboxymethylation. The extent of the carboxymethylation can be re- cognized by infrared- and nmr-spectroscopy. The infrared spectra show a decrease in the absorption at 3400 cm -1 due to the OH-bonds and an increase in the ad- sorption peaks at 1690-1710 cm -1. Signals at 4.6 ppm (5) in the nmr-spectra of the carboxymethylated lignins, which are absent in the spectra of the untreated lignins, are due to the methylene protons of the -OCH2COOH group. Their relative intensity allows a rough estimation of the extent of the carboxymethylation. The

exact amount of carboxymethoxyl groups was assessed by conductometric titra- tions similar to the method described by Sarkanen and Schuerch (1955). This me- thod was quite sufficient for the determination of carboxymethylether groups but failed to give precise values for the content of phenolic hydroxyl groups. The plots of the phenolic hydroxyl titration had only very short sections that could be inter- preted as straight lines of the neutralization reaction and the surplus of hydroxide, respectively. The curved sections between them did not allow a determination of an exact point of equivalence. Results of the analyses of the lignins are compiled in Table 2.

From the analytical data the following formulas can be calculated: Ethanol organosolv lignin from spruce:

C9HT.0401.os(OCHa)o. 91(0H)1. la

4

Carboxymethylated ethanol organosolv lignin from spruce:

C9H6.9600.84(OCH3)o.96(OH)o.27(OCH2COOH)0.72

Kraft lignin from spruce:

C9Hs.4801.09(OCH3)o.84(OH)l.IO

Carboxymethylated kraft lignin from spruce:

C9H6.0500.72 (OCH3)o. 96(OH)o.48(OCH2COOH)o. 76

Ethanol organosolv lignin from beech:

C9H6.2401.35(OCH3)1.44( OH)1.28

Carboxymethylated ethanol organosolv lignin from beech:

C9H6.0100.80(OCH3) 1.5 l(OH)o. 69(OCH2COOH)o. 74

Kraft lignin from beech:

C 9H5.62 ~ 1.24( OCH 3) 1.28(OH) 1.4s

Carboxymethylated kraft lignin from beech:

C 9H4.61 O0.94(OC H 3) 1.44(OH)0.22( OCH2 COOH ) 1.02

W. Lange and W. Schweers

Table 2. Analytical data of the treated and untreated ligrtins. All values in per cent

Sample C H O OH OCH 3 OCH2COOH

Ethanol organosolv lignin from spruce 66.30 6.13 27.57 10.78 15.68 -

Kraft lignin from spruce 66.59 5.32 28.09 10.83 15.08 - Carboxymethylated ethanol

organosolv lignin from spruce 63.11 5.69 31.20 2.08 13.62 25.05

Carboxymethylated kraft lignin from spruce 62.46 5.33 32.21 3.66 13.37 26.02

Ethanol organosolv lignin from beech 61.81 5.89 32.30 10.92 21.97 -

Kraft lignin from beech 62.40 5.54 32.06 12.48 19.96 - Carboxymethylated ethanol

organosolv lignin from beech 59.72 5.63 34.65 4.86 19.62 22.90

Carboxymethylated kraft lignin from beech 59.02 5.25 35.73 1.46 17.58 30.24

Carboxymethylation of Organosolv and Kraft Lignins

Table 3. Molecular weights and number average degree of polymerisation of the lignin pre- parations

n

Sample Molecular weight DPn

Ethanol organosolv lignin from spruce Carboxymethylated ethanol organosolv lignin from spruce Kraft lignin from spruce Carboxymethylated kraft lignin from spruce Ethanol organosolv lignin from beech Carboxymethylated ethanol organosolv lignin from beech Kraft lignin from beech Carboxymethylated kraft lignin from beech

910 5.1 1560 8.2 2240 12.7 4010 18.8 1210 6.0 1920 8.0 570 2.9

2090 8.3

Molecular weights

The number-average molecular weights of the carboxymethylated products are in

all cases distinctly higher than those of the untreated lignins. Obviously, the higher molecular weights of the carboxymethylated lignins cannot be attributed to

the replacement of hydroxyl groups by carboxymethyl groups alone. As a com- parison of the number-average degree of polymerization (DPn) of the treated and

untreated lignins shows, the carboxymethylation reactions must be accompanied by

condensation reactions (Table 3).

Conclusion

It has been shown, that kraft lignins and ethanol organosolv lignins of spruce and beech are etherifiable with bromoacetic acid, but of the total hydroxyl groups, only 60 -70 % were carboxymethylated. A variation of experimental conditions

yielded no higher rates of etherification. For the use of carboxymethylated lignins as a source for the preparation of

polyurethane foams, a total carboxymethylation of the phenolic hydroxyl groups is desirable to avoid difficulties during the oxyalkylation reaction. Since the po- tentiometric titration failed to give precise values for the content of phenolic hydroxyl groups, the rate of the etherification of this groups could not be deter- mined. The suitability of the carboxymethylated lignins for the preparation of polyurethane foams therefore should be studied by oxyalkylation reactions to form the corresponding polyols.

Experimental

Preparation of Lignins

Kraft Lignins

Wood chips, corresponding to 800g o. d. wood were digested for 3 hours at 165 ~

in a laboratory digester with 3.2 l white liquor containing 187 g NaOH, 64 g Na2S

6 w. Lange and W. Sehweers

and 30 g Na2COa. The black liquor was separated from the pulp and heated to 60 ~ By introducing carbon dioxide, the lignin was precipitated. At the end of the precipitation, sulfuric acid was added up to a pH of 2.0 to improve the fdtra- bility. The lignin was recovered by filtration through a suction filter, thoroughly washed with water and dried in a vacuum dryer at 50 ~

Ethanol Organosolv Lignins

Wood chips, corresponding to 1 kg o. d. wood were digested at 195 ~ with 10 l ethanol/water 1 : 1 in a laboratory digester. The times of digestion were different for hard- and softwoods:

spruce: time of heating 75 min time of digestion 65 min time of cooling 4 -5 hours

beech: time of heating 75 rain

time of digestion 35 min time of cooling 4 -5 hours

At the end of the delignification, the lignin solution was separated from the pulp. The solution was mixed with 4.51 water to improve the precipitation of the lignin, and the ethanol was removed in a rotary evaporator. The pulp was extracted with ethanol for several hours, water was added to the ethanolic solution, and the etha- nol was removed in an evaporator as described before. Both suspensions of lignin were mixed, the lignin was recovered by filtration through a suction filter, thoroughly washed with water, and dried in a vacuum dryer at 40~

Carboxymethylation

A sample of spruce and beech kraft and organosolv lignins, respectively, correspond- ing to 10g of dry tignin, was dissolved in 100ml of a solution, containing 13.3g sodium hydroxide. The solution was stirred and 30 g bromoacetic acid was added slowly. The mixture was stirred and heated to 40 ~ for 4 hours. At the end of the reaction, the carboxymethylated lignin was precipitated by adding of 250 ml 2N sulfuric acid. The precipitate was recovered by filtration through a suction filter, thoroughly washed with water and dried in a vacuum drier at 40 ~

Analyses

Carbon and hydrogen were determined in a Carlo Erba Elemental Analyzer, type 1104; oxygen was calculated by difference. The hydroxyl groups were determined according to a method described by Verley and B61sing (1901) and methoxyl groups according to the method of Vieb6ck and Schwappach (1930).

Carboxymethylation of Organosolv and Kraft Lignins 7

The carboxymethyl groups were determined by conductometr ic ti tration.

Samples of 3--4 g carboxymethyla ted lignins were dissolved in 133 ml ethanol and

67 ml acetone and t i t rated with a 2.5 N LiOH solution. A conductivi ty bridge

(Philips PR 9501) and a conductivity cell (Philips PW 9510) were used. The mole-

cular weights were determined osmometricaUy in dioxane solution using a Knauer

vapor pressure osmometer. Because the kraft lignin from beech was not completely

soluble in dioxane, the molecular weight of this sample was determined in DMSO

solution.

References

Christians, D. T.; Leandro, S.; Look, M.; Nobell, A.; Armstrong, T. S. 1970. Process for pro- ducing polyoxyalkyler/e ether polyols from lignin. US. Patent 3 546 199

Hsu, O. H. H.; Glasser, W. G. 1975. Polyurethane foams from carboxylated lignins. J. Appl. Polym. Sci. Appl. Polym. Symp. 28:297-307

Ishikawa, H.; Oki, T.; Fugita, F. 1961. Hydroxylethylation of phenolic hydroxyl groups in hardwood lignin. Mokuzai Gakkaishi 7 :85-89

Sarkanen, K. W.; Schuerch, C. 1955. Conductometric determination of phenolic groups in mixtures such as isolated lignins. Anal. Chem. 27:1245-1250

Tanaka, Y.; Abe, H., Senju, R. 1966. The active centre for the gel conversion at the treat- ment of lignin with potassium dichromate (orig. jap.). Kogyo Kagaku Zasshi 69:1968-1970

Verley, A.: B61sing, F. 1901. ~ber quantitative Esterbildung und Bestimmung yon Alkoholen, respektive Phenolen. Bet. 34:3354-3358

Vieb6ck, F.; Schwappach, A. 1930. Eine neue Methode zur mal~analytischen Bestimmung der Methoxyl- und Xthoxylgruppe. Ber. 63:2818-2823

(Received February 7, 1979)

Dr. W. Lange Ordinariat ftir Holztechnologie (Holzchemie) der Universit~it Hamburg Prof. Dr. W. Schweers Institut ftir Holzchemie und chemische Technologie des Holzes der Bundesforschungsanstalt ftir Forst und Holzwirtschaft Leuschnerstral~e 91 D-2050 Hamburg 80 Federal Republic of Germany