\HYDROXYPROPYLATION OF LIGNIN and

LIGNIN-LIKE MODEL COMPOUNDS/

BY

Chih Fae \\Wu/ /

Thesis submitted to the Graduate Faculty of the

Virginia Polytechnic Institute and State University

in partial fulfillment of the requirements for the degree of

Master of Science

in

Forest Products

Approved

/c - M. A. ogfiaruso

September, 1982 Blacksburg, Virginia

T. c. Ward

-,

ACKNOWLEDGEMENTS

I would like to thank Dr. Wolfgang G. Glasser, my major advisor,

for all his help and encouragement through my graduate study in Virginia

Polytnchnic Institute and State University. He taught me not only how

to treat lignin as a polymer but also how to be an independent chemist.

I would also like to thank Mrs. Charlotte A. Barnett for her advice

in teaching me techniques of modern instruments and Dr. Michael A.

Ogliaruso, Dr. Thomas C. Ward, and Mrs. Frieda Bostian for taking time

to review my thesis and giving me valuable suggestions.

Most of all, my parents deserve their best respects for their love,

understanding, and support throughout my education programs. Finally,

a special appreciation is extended to my wife, without her love and

patience, this research would have never been completed.

ii

TABLE OF CONTENTS

ACKNOWLEDGEMENTS

LIST OF TABLES . .

LIST OF FIGURES.

INTRODUCTION . .

LITERATURE REVIEW.

Lignin ....

Preparation of Lignin.

Formation and Structure of Lignin.

Macromolecular Properties of Lignin.

Reaction of Alkylene Oxides

Anionic Polymerization

Cationic Polymerization .•

Oxyalkylation of Lignin. .

MATERIALS AND METHODS ... . . ~ . Preparation of Lignin Samples

Kraft Lignin .

Methylation of Kraft Lignin.

Demethylation of Kraft Lignin. .

Derivatization Methods ...

Preparation of Lignin Polyols .•.

. ii

v

vi

1

2

2

2

4

5

8

8

11

. . . . . 13

. 17

17

17

17

. 17

. . . 18

. . . . 18

Preparation and Purification of Hydroxypropylated Lignin . 18

Analysis Methods .•.

Methoxyl Content .

Total Hydroxyl Content

iii

. . . . . ~ . . 19

19

19

iv

Phenolic Hydroxyl Content.

Mole of Substitution . . .

Molecular Weights and Weight Distributions

Residual Free Syringic Acid and Benzoic Acid

Viscosity. • . .

Glass Transition Temperature •

RESULTS AND DISCUSSION .

Homopolymerization. .

Anionic Polymerization •

Hydroxypropylation of Model Compounds

Effect of Functionality ...

Effect of KOH Concentration. .

Hydroxypropylation of Lignin. .

Effect of Lignin Structure

Effect of Catalysts and Catalyst Concentrations.

Effect of Lignin Content .

Effect of Temperature.

LITERATURE CITED

Page

20

21

22

22

• • 23

• • 23

• • 24

25

• 25

• • 2s·

29

• • 32

• • 34

• . 34

• • 36

• 39

. . 40

• 74

Table

1.

LIST OF TABLES

~

Estimated values of lignin ($/T) (1)...................... 44

2. Annual production of pulp and lignin (11). . • • • . . . • • • • • . • • • -.45

3. The Structure and properties of lignin-like model compounds 46

4. The characteristics of hydroxypropylated model compounds •• 47

5. Chemical composition of several Kraft lignins and their

molar substitutions after hydroxypropylation raection ••.•• 48

6. Effect of KOH concentration on chemical structure of hydroxypropylated lignin................... • • • • • . • . . • • • . • . 49

7. Peak temperature and chemical structure parameters of several catalyzed hydroxypropyl lignins ...•.•.•.•.•••••••• 50

8. Peak temperature and chemical structure parameters of

hydroxypropyl Kraft lignins produced with varying KCl

concentrations ....................................... a.... 51

v

Figure

1.

2.

3.

4.

LIST OF FIGURES

The structure of 3 primary lignin precursors_ (12) ••••.••• 52

Mesomeric of.the phenoxyl radical formed from p-coumaryl alcohol by dehydrogenation (13) •......................... 53

Freudenberg's lignin formula (16) ..••....•••.•. ~ ......•.. 54

Structural sketch of a.computer-simulated softwood lignin (20) .. ~ ................. · .................................. 55

5. .Comparison of the molecular weight of Kraft and sulfite lignin as the delignification proceeds (26) ...••.•..•••.• 56

6. The H-NMR spectrum of hydroxypropylated lignin •.•••••.••• 57

7. Vapor pressure diagram of propylene oxide in the inert state ............ · ......................................... 58

8. Pressure-Temperature-time diagram of propylene oxide homopolymerization in relation to catalyst ••..•.••..•••.. 59

9. Concentration-time curves of propylene.oxide of homo-polymerization re.actions. • • . • . • • • • • • • • • • • • • . • • • . • • • • • • • • . 60

10. Effect of KOH concentration on the peak temperature in lignin hydroxypropylation reactions •••.•••.•.•••.•.••• 61

li. Effect of KOH concentration on viscosity of homopolymer •• 61

12. Concentration-time curVes of.KOH-catalyzed homopolymeri-zatio·n reactions at various temperatures •• ~ •.•.••••••••••. 62

13. ,Press1:1re-Temperature...,time diagram of propylene oxide in acid-catalyzed homopolymerization reactions ••••••• : •••••• 63

14. Molecular structure of lignin-like model compounds •••.•.• 64

15. Pressure-time diagram of reactions of propylene oxide

with model compounds· . ......•. -... · ; . . . . . . . . . . . . . . . . . . . . . . . . 65

vi

vii

Figure

16. Concentration-time curves of reactions of propylene _ oxide with. model compounds •...••.• .'. . . . • • • • • • • • • • • • • . • • • • 66

17. Product functionality of the reaction of syringic acid with propylene oxide in relation to reaction time ••• 67

18. Concentration-time curves. of propylene oxide in hydroxy-propylation of compound Vat various molar V/KOH ratio ••• 67

19. Pressure-time diagram of hydroxypropylation reactions of benzoic acid (I) at various I/KOH-molar ratios ••••••.••.• 68

20. GPC chromatography of homopolymer, lignin-propylene oxide

copolymer and reaction pro~uct mixture .••..•.•••••••••••• 69

21. _The typical pressure-time diagram of the hydroxypropy-lation reaction of lignin preparations •...•••••.••••••••. 70

22. Concentration-time curves of reactions of propylene

oxide with several lignin preparations in relation to different functionalities •••••••••.••.•. ·.••••••••.••••. . • • 71

23. Concentration-time curves of reactions of propylene

. oxide with Kraft lignin in relation to KOH concentration .. 72

24. Concentration-time curves of hydroxyptopylation reaction of lignin with various cationic catalysts •.•••••.•.•••.•• 73

25. Effect of lignin content on molar substitution •. !······· 73

INTRODUCTION

The utilization of lignin can generally be classified into three

areas : (a) fuel, (b) low molecular weight chemicals, and (c) high

molecular weight polymers. The estimated value of lignin for various

end uses is listed in Table 1 (1). Most of the high-valued end uses

for lignin are from its polymeric products, particularly from structural

resins such as polyurethanes. But it has also been realized that

commercial interests in lignin-based polyurethane products is restricted

by such inferior properties as high rigidity (2) and high water

sorption (3).

To enhance lignin's utility for polyurethane products, its flexi-

bility and functionality must be improved. Hydroxypropylation has been

shown to be a useful method for such purpose by virtue of its ability

to affect chemical structure, functionality, and molecular properties.

Hydroxypropylation is achieved by reacting lignin with propylene oxide

in the presence of catalyst. Although the potentiai use of hydroxy-

propylated lignin derivatives in polyurethane products has been

demonstrated previously (4,5 ), no systematical study has been con-

ducted to fully understand the reaction of lignin with propylene oxide.

1

~ITERATURE REVIEW

Knowledge of l_ignin has evolved over one hundred years •. The

importance of lignin is widely recognized by scientists in different

disciplines because of its abundance and its unique function in plants.

Lignin analysis has developed into a multi-faceted technique involving

modern chemical methods of analysis and scientific concepts relating

to its formulation, reaction behavior, and properties. Legislative

regulation of the practice of dischargi_ng lignin-containing effluents

fro~ pulp mills has encouraged a broader utilization of lignin materials

and has advanced the field of lignin chemistry from pure science to

industrially motivated practicality. Indeed, not only structure but

also preparation and properties of lignin have been extensively studied

in recent years, particularly in view of the macromolecular architecture

which influences the performance of lignin products, such as molecular

mass, molecular mass distribution, solubility, and thermal properties.

The following sections will review the current state of these

subjects.

Lignin

The most abundant component of plants other than cellulose is

lignin. Lignin is present in most vascular plants in their stems, roots,

leaves, etc. The lignin content of softwood ranges from 24 to 33%,

based on the total wood substance and that of hardwood ranges from

16 to 24% (6).

Preparation of Lignin

The isolation of lignin from plants can generally be achieved

2

3

either mechanically or chemically. Lignin prepared by grinding wood

meal in a vibratory ball mill followed by solvent extraction is gene-

rally referred as milled wood lignin. Milled wood lignin is widely used

in laboratories because there are no profound chemical modification

involved in its isolation; however, the maximum yield of only 50% even

after grinding for 30 days results in questionable representativity(7).

For chemical isolation, both organic solvents and inorganic solutions

are used to dissolve lignin from wood. Organic solvents include dioxane,

alcohols, acetone, etc., and inorganic solutions comprise a wide range

of acids and alkalis such as sodium sulfide, sodium hydroxide, hydro-

chloric acid, sulfuric acid, etc. The lignin yield from these systems

varies significantly with conditions.

For industrial scale production of lignin, pulping processes,

particularly chemical processes, are employed. The two most important

chemical processes in which lignin is a by-product are Kraft and sulfite

pulping. Kraft lignin can be recovered in reasonly high yield by acidi-

fying the spent pulping liquor ( black liquor) and filtering the preci-

pitated lignin. For complete precipitation of lignin, the pH of the

liquor must be lowered to 3 or less, and the acidification is conducted

in two steps (8). In the first step, the pH of black liquor is lowered

to 9 by flue gas carbon dioxide to precipitate a major portion of

lignin as sodium salt. The sodium salt is then suspended in water and

acidified with sulfuric acid to pH 3 or less at 80-90°c.

Ligninsulfonates which are recovered from the sulfite process

are produced either as free sulfonic acid or as calcium, magnesium,

sodium, or ammonium salts. These are commonly recovered in the form of

4

spray dried powders. However, some of the less soluble ligninsulfonates

generate scaling on heat exchange surface and an alternate method for

obtaining these ligninsulf onate is in the form of concentrated solu-

tions, i.e., 40 to 60%. This concentration may be accomplished by

reverse osmosis (9) or ultrafiltration (10).

The annual production of lignin from these processes is listed in

Table 2 (11). Over 90% of the commercially available lignin is contained

in the black liquor of the Kraft process. Lignins from pulping processes

usually contain a substantial amount of impuri~ies, since spent liquors

are not only composed of lignin but also of hemicellulose, degradation

products from carbohydrates, and extractives. Purification is sometimes

needed for special purposei.

Formation and Structure of Lignin

Lignin is composed of carbon, hydrogen, and oxygen, although the

proportion of these elements varies with respect to wood species, loca-

tion, age, and method of isolation. Lignin is believed to be formed

through an enzyme-catalyzed dehydrogenative polymerization process of

three primary p-OH cinnamyl alcohol derivatives : coniferyl, sinapyl,

and p-OH coumaryl alcohol (12). The structure of these three precusors

is shown in Figure 1.

The main mechanism of lignin formation involves phenoxy radicals

which undergo coupling processes. Phenoxy radicals have delocalized,

free, and unpaired electrons in several positions of the phenylpropane

unit. Lignin is formed by the coupling of the several mesomeric forms

of these radicals at various coupling sites depending on their relative

electron spin density concentration ( 13). Radical formation of p-cou-

5

maryl alcohol is illustrated in Figure 2.

The molecular structure of lignin has been studied by many resear-

chers (14-20). The most common methods for lignin structure analysis

include degradative depolymerization and reaction of lignin-like model

compounds. Freudenberg and coworkers degraded lignin by permanganate

oxidation (14) and alkaline nitrobenzene oxidation (15). They also

synthesized an amorphous dehydrogenation polymer of coniferyl alcohol;

it proved to be similar to coniferous lignin in many respects. This was

accomplished by treating coniferyl alcohol in aqueous solution with iso-

lated laccase or peroxidase and hydrogen peroxide (16). Later, Freuden-

berg proposed a lignin structure in whi·ch the major components were

guaiacyl propane subunits with only 25% p-hydroxy phenylpropane and

5% syringyl propane units (16). This structure is shown in Figure 3.

Lignin structures were also proposed by other investigators such as

by Sakakibara (17), Adler (18), and Glasser (19). Recently, Glasser and

Glasser (20) treated lignin as a statistical polymer and developed

lignin structures for both softwood and hardwood lignin by employing

computer techniques. The softwood lignin structure from this effort

is shown in Figure 4.

Macromolecular Properties of Lignin

Molecular mass, molecular mass distribution (MWD), solubility,

and thermal characteristics of polymers are generally closely related

to their utilization behavior. These properties of lignin will be re-

viewed in this section.

Gel permeation chromatography (GPC) is commonly used to study

molecular mass and molecular mass distribution. Solvents for GPC deter-

6

minations are dioxane, DMSO, DMF, alcohols, or sodium hydroxide solutions.

Gel types vary depending on the nature of lignins. Connors (21) used a

Sephadex LH-60 dextran gel column with DMF as solvent to chromatograph

lignins, lignin model compounds, and polyethylene standards, and he

found a close relationship between elution volume and molecular mass up

to 20,000. Huttermann (22) studied the GPC of lignins and claimed that

the MW of some lignin fractions exceeded 100,000 g/mole can be separated

on. cont~olled pore glass beads in Sepharose CL-6B columns·

There are many reports of MWs of lignin below 1,000 (23), but

values greater than 1,000,000 g/mole have also been reported for Kraft

lignin and ligninsulfonates ( 24, 25). The average MW of Kraft lignin

and ligninsulfonate in relation to degree of delignif ication is shown

in Figure 5. The low MW of Kraft lignin as compared to ligninsulfonates

must be attributed to severe hydrolysis of lignin during Kraft pulping I

(26). Analyzing average molecular weight by viscosity average and light

scattering average, Lindberg (27) found that the MW of dioxane-soluble

Kraft lignin can be calculated from the Mark-Houwink equation, a {n} = kM , where {n} is intrinsic viscosity, M is the average mole-

cular mass, and k and a are empirical constants, by using k and a values

of 0.0168 and 0.12, respectively. In the case of ligninsulfonates, the

average MW increases slightly when the pH of the cooking liquor is de-

creased from 4 to 2, and a rather large increase is observed when the

pH drops below 2 (28). This is believed to be due to condensation reac-

tions of ligninsulfonates in concentrated acid medium. Gordon and Mason

(29) fractionated spent liquor and obtained eight fractions with MW

ranging from 3,700 to 58,000 g/mole. McCarthy and coworkers (30)used a

7

column elution method to fractionate hemlock spent liquor and reported

that the MW of lignin ranges from 400 to 15,000 g/mole.

These studies indicate that most lignins are widely dispersed.

Dispersity ranges from 2.0 to 7.0 have been reported (31). The polydis-

perse character of lignins demonstrates that lignin molecules are non-

linear polymers and the polymeric network is probably made up of short

chains with branches and cross-links in a variety of ways to give an

infinite, three-dimensional structure.

Schuerch (32) investigated the solubility of lignin in a number

of solvents and found that the solvent power for lignin depends on the

solubility parameter (o) which is a function of both cohesive energy

density (c.e.d.) and hydrogen bonding capacity of the solvent. He also

found that the maximum solvent effect occurs at c.e.d. value of .11 1/2

(cal/cc) ; solvents with c.e.d. values far away from 11 do not

dissolve lignin. Lindberg (33), in his study of lignin with various or-

ganic solvents, found that the dissolved lignin performs differently in

various solvents. Bland and Menshuni (34) studied the solubility of

mill wood lignin in sodium hydroxide and showed that the amount of

lignin dissolved in mild alkali is limited by the sodium hydroxide

concentration. They found that, when the concentration of sodium hydro-

xide is high, it appears impossible to distinguish an upper limit of

solubility due to the formation of extremely viscous solutions. However,

at very high concentration of sodium hydroxide, the solubility is low

and insoluble sodium lignate is formed.

Thermal characteristics, particularly glass transition temperature,

T , of lignin are related to the bulk behavior of lignin during reac-g

8

tion. Kleinert (35) has discussed the thermoplastic behavior of alcohol

lignin in hot water and found that the reactivity of lignin in wood is

mainly limited by its interfacial area. Stone and Scallan (36) have

also demonstrated that a large increase in surface area of dioxane

lignin was observed at its softening temperature. But autoadhesion of

lignin occurs at temperatures above T due to the increased area of con-g

tact and molecular motion (37). Recently, Falkehag (38) found that the 0

Tg of unmodified and methylated pine Kraft lignin is 180 and 128 C, res-

pectively, and he pointed out that the decrease of T after methylation g

is mainly due to the masking of hydroxy groups which tend to form inter-

nal hydrogen bonds.

Reaction of Alkylene Oxides

Polymerization of alkylene oxides was first reported in 1863 by

Wurtz (39), who found that polyethylene glycol and ethylene glycol were

obtained when ethylene oxide was heated with water in a closed tube.

Since then, the polymerization of alkylene oxides has been studied ex-

tensively. In general, the polymerization of alkylene oxides can be

classified into four categories : stepwise anionic, cationic, coordinate

anionic, and radical polymerizations. The particular type and mechanism

of each reaction depends on the catalyst. In this section, only stepwise

anionic and cationic polymerization will be reviewed because they were

employed in the present study.

Anionic Polymerization

When alkali metal is used as a catalyst, stepwise anionic polymeri-

zation is induced. Boyd and Marle (40) have set up the following reac-

9

tion mechanism from their observations

y + CH2 - CHR

' I 0

where HY is a hydrogen active compound and

B is a base

----- +

They also found that the rate-determining step of this reaction is an

SN2 type reaction and epoxide-containing substitutents with greater

electron attracting power have higher reactivity although steric hind-

ranee of the substitutents may also become a factor. From this mecha-

nism it is clear that both alkylene oxide conversion and degree of

polymerization increase as alkoxide formation increases. Fife and

Roberts (41) used sodium hydroxide as catalyst and found that both

ethylene oxide and propylene oxide polymerize to give polymers of low

molecular weight. Polyethylene oxides of molecular weights between 200

and 3,000 g/mole, and polypropylene oxides of molecular weights between

500 and 3,000 g/mole were isolated and these low molecular weights were

attributed to frequent chain transfer and other side reactions. Later,

Steiner and coworkers (42) studied the kinetics of polymerization of

propylene oxide catalyzed with potassium hydroxide and they found that

reaction rate and monomer conversion are determined by the concentra-

tion of potassium hydroxide.

Perry and Hibbert (43) reacted various glycols with ethylene oxide

at serveral molar ratios and found that polyglycols as high as 18 units

10

long are still capable of reacting with ethylene oxide. They used this

multistage polymerization principle to produce polyethylene oxides of

molecular weight ranging from 387 to 3,270 g/mole. Holland (44) proposed

a method to prepare polyethylene oxide of a desired molecular weight by

multistage polymerization and later this multistage process was further

used in the preparation of various block copolymers containing

polyoxypropylene blocks and polyoxyethylene blocks ( 45, 46).

both

Copolymerization of alkylene oxides and active hydrogen compounds

such as alcohols, phenols, carboxylic acids, and amines depends not only

on the acidity of the active hydrogen compound and the basicity of the

alkylene oxide but also on the nature of solvent and catalyst; however,

no systematic study on this topic has yet been conducted. In the reac-

tion of 2-ethylhexanol witq propylene oxide catalyzed with sodium ~ydro­

xide, Ishii and coworkers ( 47) found that two reaction rates exist,

whereby the first reaction is the reaction of propylene oxide with the

primary alcohol group of 2-ethylhexanol, and the second reaction is the

reaction of propylene oxide with the secondary alcohol group which is the

product the first reaction. They also found that the reactivity of the

primary alcohol is greater than that of the secondary alcohol after con-

ducting similar studies with various alcohols (48).In the reaction of

higher alcohols with ethylene oxide using different kinds of alkoxide

catalysts, they observed various rate constants for various catalysts

and greater rate constants were obtained with more basic alkoxides, i.e.,

t-BuOK > t-BuONa >KOH> RONa > CH30Na > NaOH (49).

When an aliphatic caboxylic acid is reacted with an alkylene oxide,

a monoester is formed. The condensation reaction between carboxylic acid

11

and polyglycols is, however, preferred when ethylene oxide is used (50}.

Fraenkel-Conrat and Olcott (51) obtained rnonoesters when fatty acids

were reacted with excess propylene oxide in the presence of alkaline

catalysts. However, in the case of a reaction at high temperatures or

if benzoic acid is employed, the following equation applies representing

transesterification :

RCOOH + OH ----+

high T, OH------------ -+

It was however reported that monoesters, diesters, and glycols were

obtained when a mixture of fatty acid and 0.5-1.0% KOH catalyst was

heated in a stream of nitrogen and then reacted with ethylene oxide at 0

150 c (52).

Cationic Polymerization

Cationic polymerization of alkylene oxide occurs when acid catalysts

are employed. These catalysts include Lewis acids, inorganic acids~ etc.,

and the reaction mechanism is shown as follows (53) :

+ HX+ H + X

H+ + CH2 ~ CHR 'o/

where HX is an acid SNl + -----+ . HOCH2 CHR

In this mechanism, both SNl and SN2 reactions have been proposed and

12

.. supported by experimental data. Although an oxonium ion is formed in both

mechanisms, no decisive conclusions have been obtained.

Among acid catalysts, many inorganic halides are classified

with regard to catalytic activity toward ethylene oxide and propylene

oxide (54, 55). Some halides are listed below. It must,however, be noted

that this classification is. based on a specific set of polymerization

conditions, and some of the halides listed as inactive may prove to be

active under different reaction conditions.

Catalytically active halides are

Catalytically inactive halides are :

HgCl, MgC12' FeC12' CoC12' NiBr2' NiC12' CnClz, CdC12' HgClz, CrC13 , AsCI3 , SbCI3 , GeCI4

Nevertheless, polymerizations of ethylene oxide and propylene oxide

with metal halide catalysts are not performed commercially, although

the patent claimed by Scriba and Graulich (56) relating to cationic

polymerization seems to be of industrial interest. They claimed that

propylene oxide containing small amounts of a regulator is polymerized

by acid-activated montomorillonite to give a liquid polymer which can

be used in polyurethanes. The degree of polymerization can be controlled

by the amount of active hydrogen compound added as regulator.

Recently, Ishii and coworkers (57) studied the polymerization of pro-

pylene oxide catalyzed by various metal chlorides in the presence of

acetyl chloride. They found that monomeric and dimeric products were

obtained in polymerizations by H2so4 , KCl, NaCl, CdC12 , MgC12 , Cuc12 ,

PdC12 , ~nd AlC13 ,but oligomers from monomer to tetramer were formed

13

when reactions were catalyzed by FeC13 , TiC13 , SnC14 , and SbC14 ,

Not only types of catalyst but also catalyst concentrations affect

the polymerization. Ishii et al. (57) have polymerized propylene oxide

in the presence of a large amount of acetyl chloride and SnC14 as

0 catalyst at 25 C and found that the rate of polymerization increased

with increasing concentrations of catalyst and acetyl chloride but

decreased with increasing concentrations of propylene oxide.

Oxyalkylation of Lignin

The two main hydrogen donor components for polyurethane products

are polyether and polyester polyols. Since lignin contains hydroxyl and

carboxyl groups, both polyether and polyester polyol functionality

can be derived from lignin during oxyalkylation as was shown in the

mechanisms of the previous section.

The f~rst oxyalkylation lignin was prepared in 1948 by Ishikawa

and Senzyu (58) who prepared a hydroxyethylated lignin by reacting

extractive-free wood with ethylene oxide in sodium hydroxide solu~ion.

These authors obtained hydroxyethylated lignin with a methoxyl content

of 15.5%. indicating structural changes in lignin during hydroxyethy-

lation. Christian et al. (4 ) attempted to increase the number of

available reactive positions for reaction with isocyanates by reacting

lignin with ethylene oxide and propylene oxide. Results showed that the

performance of rigid polyurethane foams formulated from this oxyalky-

lated lignin is superior to those f 9rmulated from unmodified

lignin. Hsu and Glasser (5 ) prepared a viscous, homogeneous, and lique-

fied polyester-polyether polyol from carboxylated softwood lignin. The •.

carboxylation was carried out by reacting lignin with maleic anhydride

14

at 170°C for two hours. The carboxylated lignin was then hydroxypro-

pylated with a mixture of ethylene glycol, propylene oxide, and a 0 catalyst at 140 C for 2 hours. The properties of polyols from Kraft

lignin and carboxylated Kraft lignin were compared and results indi-

cated that both solubility and viscosity o.f polyester-polyether polyols

were higher than those of the uncarboxylated lignins. These authors

claimed that lignin is activated by carboxylation, and the reduction of

unsaturated and conjugated systems occurs during modification. Reed

(59) has conducted a similar study with various amounts of lignin,

maleic anhydride, ethylene glycol, propylene oxide, and catalyst. He

found that polyols with a wide range of Hydroxyl numbers and viscosity

can be prepared from modified Kraft lignin. This demonstrates that a

variety of polyurethane products can be derived from Kraft lignin.

However, the lignin content of these preparations remains to be limited

to 10-15% (59).

Recently, polyols with high lignin contents ranging from 37 to

60% have been successfully prepared (60). In these preparations, Kraft

lignin was directly reacted with propylene oxide in the presence of

KOH as catalyst and an inert solvent such as toluene or hexane to assure

sufficient heat and mass transfer during the reaction. Results showed

that these polyols had viscosities (Brookfield) ranging from 10 to

4,000 poises and hydroxyl numbers between 248 and 597. Polyols with

these properties are suitable for polyurethane preparations as descri-

bed by Case (61).

Previous stu.dies have mainly focused on properties and per_forma-

nces of whole hydroxypropylation mixtures. But such mixtures always

15

contain both homopolymers of propylene oxide (PO) and copolymers of

lignin and PO; the actual role lignin played during these reactions has

remained unclear. To study the effect of hydroxypropylation conditions

on the·chemical structure of lignin derivatives, Glasser et al. (62)

separated and purified lignin copolymers from the hydroxypropylation

mixture by acetonitrile-hexane extraction, precipitation, and washing.

Results showed that the total hydroxyl number of purified hydroxypro-

pylated lignins ranged from 339 to 473; phenolic hydroxyl content

ranged from 0.022 to o.98% indicating a 55 to 99% reaction completion

of phenolic hydroxyl groups. Results also showed that the molar subs-

titution of these hydroxypropylated lignins was always about 1.0, which

corresponds to a lignin content of the hydroxypropyllignin of about

60%.

OBJECTIVES

General Objectives

The overall objectives were to study the reaction of propylene

oxide and lignin to optomize reaction conditions for both high molar

substitutions and high lignin content.

Specific Objectives

The specific objectives were

1. To study the effect of anionic and cationic catalysts and their

concentrations on homopolymerization and copolymerization,

2. To explore the influence of various functional groups by using

lignin model compounds,

3. To prepare lignin derivatives by methylation and demethylation,

4. To investigate the reaction rate of lignin hydroxypropylation -

in relation to

a. the effect of catalysts,

b. the effect of catalyst concentrations,

c. the effect of temperatures,

d. the effect of lignin contents, and

e. the effect of lignin structures.

5. To examine the change in functionality of lignin products by

total hydroxyl content and phenolic hydroxyl content, and

6. To e~amine the change in structure of lignin products by T g

and molar substitution.

16

MATERIALS AND METHODS

Preparation of Lignin Samples

For the work presented in this thesis, lignin samples were Kraft

lignin, methylated Kraft lignin, and demethylated Kraft lignin.

Kraft Lignin

Conmiercial Kraft lignin, Indulin AT, which was supplied by the

Westvaco Corp., Charleston, South Carolina, was used in most reactions.

Indulin AT has a methoxyl content of 13.45% and elemental analysis val-

ues of 61.29% C, 5.18% H, 2.94% S, and 30.59% 0 (by difference). The

ash content of Indulin AT is less than 1.0%.

Methylation of Kraft Lignin

Methylation of lignin was achieved by reacting lignin with dimethyl

sulfate at 60°c for for three hours. During methylation, sodium hy-

droxide was added to maintain a pH value of 12.5. After the reaction

was completed, two fractions were recovered. One fraction containing

solid precipitates was recovered by filtration. Another fraction in

which lignin retained in solution was recovered by acidification with

HCl and filtration. Both fractions were freeze-dried.

Demethylation of Kraft Lignin

Demethylation was done by reacting lignin with concentrated hydro-

iodic acid and acetic acid at loooc for three hours. After the reaction,

two fractions were obtained. One fraction in which lignin was a preci-

pitated solid was recovered by filtration. The filtrate was then ex-

tracted with chloroform to remove the residual iodine ions. The second

fraction which was soluble in acidic solution was recovered by neutrali-

zation. Both fractions were freeze-dried.

17

18

Derivatization Methods

Preparation of Lignin Polyols

Lignin polyols were prepared by hydroxypropylation. 10 gm kraft

lignin was reacted with 50 ml (density = 0.859 g/ml) propylene oxide

in the presence of catalyst at 190°C or 250°C for 90 minutes. Both

alkaline catalyst such as KOH and acid catalysts such as KCl, Al203,

A1Cl3, H3B03, and ZnO were used. The reaction was carried out in a

300 ml Parr reactor which is equipped with a stirrer, pressure gauge,

cooling loop, safety valve, and thermocouple. The temperature program

was monitored through a potentiometric controller.

After the reaction was completed, the reactor was cooled. and the

pressure dropped to zero. The black viscous material remaining in the

reactor was collected and dissolved in methanol. The removal of excess

and unreacted inorganic substance was achieved by centrifugation. Re-

sidual methanol in the filtrate was removed by vacuum evaporation at

45°c. The final viscous product was collected as polyol.

Preparation and Purification of Hydroxypropylated Lignin

The hydroxypropylated lignin was separated from homopolymers by

liquid-liquid hot extraction and purified by water- or ether-precipi-

ta tion and washing.

The above polyol was redissolved in 150 ml acetonitrile and poured

into the liquid-liquid extractor which was then filled with hexane.

The extraction was carried out at reflux temperature for 48 hours.

After extraction, acetonitrile and hexane layers were separated by a

separatory funnel.

The acetonitrile layer which was the bottom layer in the funnel

19

was collected and precipitated into large excess of water. The preci-

pitated substance was then filtered and washed with water. The final

product was freeze-dried

The hexane layer was evaporated under vacuum at 45°C and weighed.

Analysis Methods

Methoxyl Content

The determination of methoxyl content was based on the TAPPI Stan-

<lard T2m-60. The methoxyl group was first converted to methyl iodide

by treating lignin with hydroiodic acid. Iodine, which was liberated

during reaction of bromine and hydroiodic acid, was determined by ti-

trating with standardized sodium thiosulfate. The percentage of methoxyl

content was calculated by the following equation:

% Methoxyl = N X(A-B) x 0.00517 x lOO w

where A = ml of thiosulfate required for the sample

B = ml of thiosulfate required for the blank

N = normality of sodium thiosulfate solution, and

W oven dry weight of sample

Total Hydroxyl Content

The total hydroxyl content was determined by potentiometric. titra-

tion of residual acetic acid after esterification of the free hydroxyl

groups. The procedure was as follows :

Approximately 30 mg of sample was placed into.a tared, 2 ml ampoule

and the weight was recorded. Enough reagent, which consisted of a 10:3

mixture of pyridine:acetic anhydride, was added to the ampoule so that

the ratio of reagent weight to sample weight was approximately 8:1. The

ampoule was then sealed and shaken overnight in a 60°C water bath.

20

Upon completion of reaction, the ampoule was cooled to room tern-

perature. The ampoule was rinsed with 15 ml acetone transferring the

reaction mixture quantitatively to a 100 ml beaker. An equal amount of

water was added; the beaker was covered and allowed to s'tand for one

' hour to degrade excess acetic anhydride. The acetic acid was then ti-

trated with standardized O.lN NaOH.

Duplications were made for each sample and an average value was

obtained. The total hydroxyl content was calculated by the following

equation:

mlb [(~ x mgs) - mls]N x 1.7

% OH = ~-m_g~~~~~~~~~~~ w

where mlb = ml NaOH to titrate blank

mls ml NaOH to titrate sample

x factor

mgb = mg pyridine:acetic anhydride mixture in blank

mgs = mg pyridine:acetic anhydride mixture in sample

N = normality of NaOH solution

W = sample weight

theoretical % OH for model compound factor = ~~~~~~~--~~~~~~~~~-~~­experimental % OH for model compound

Phenolic Hydroxyl Content

The phenolic hydroxyl content was determined by UV method. Appro-

ximately 3 mg sample was dissolved in 10 ml dioxane-water (9:1) solu-

tion. After lignin was completely dissolved, 3 ml of the above solu-

tion was diluted with dioxane:water (1:1) solution to total volume of

25 ml.

The phenolic hydroxyl group of sample was then ionized with a few

drops of KOH solution. This sample was scanned against an untreated

21

sample and the absorbance of the 250 nm peak was determined by drawing

a baseline from 230 to 280 nm. The phenolic hydroxyl content was cal-

culated from:

% <POH = b.A c . (0.192)

where b.A. = peak height of ~E curve

C = concentration of sample, mg/ml

The <POH content of model compound samples was determined in a

similar manner.

Molar Substitution(MS) ** The molar substitution was determined from the proton NMR spec-

trum after acetylation with acetic anhydride. The acetylation

procedure was as follows. Approximately 50 mg sample was first

dissolved in 2 ml pyridine, then 2 ml acetic anhydride were added.

The container was flushed with nitrogen and covered to stand overnight.

After the reaction was completed, the reaction mixture was precipi-

tated in either water or ethyl ether, filtered, and washed well. The

precipitate was recovered by centrifugation and freeze-dried. The pro-

ton NMR was run after the acetylated sample was completely dissolved in

0.5 ml deuterated chloroform.

A typical H-NMR spectrum of a sample is shown in Figure 6. The MS

was calculated from the ratio of integration value of region F to that

of E. Protons at these regions represent protons in methyl groups

adjacent to alkyl and ester groups as illustrated in the diagram.

** Defined according to Desmarais (63) as average number of moles of Propylene oxide per hydroxyl functional group on lignin.

22

Molecular Mass (MW) and Molecular Mass Distribution (MWD)

MWDs were determined by GPC on DuPont ZORBAX or .ALTEX µ -spherogel

columns in 0.2 N LiCl/ or LiBr/DMF solution. The procedure was as

follows

Approximately 10 mg sample was dissolved in 10 ml 0.2 N LiCl/ or

LiBr/DMF solution. After the sample was completely dissolved, about

6 µi sample was injected onto the column which had a flow rate of

1.0ml/min. at room temperature. The absorbance of the sample was

monitored by a UV detector at 280 nm.

MWD curves were obtained from the elution curves and average MWs

were calculated from the elution curves on polystyrene-calibrated

columns. · MWs were calculated by using the following equation. Nine

points were selected along the elution curve for each calculation.

(MW) = l: A.

].

l: (A. /M..) ]. ].

where (MW) A. M:-

]. ].

average molecular mass = absorbance at ith point = polystyrene mass at ith = 1,2,3, •.•••

Residual Free Syringic Acid and Benzoic Acid

point

The residual free a~id content in hydroxypropylation reaction

mixture was determined by reverse phase HPLC on HPX-87 by Bio-Rad Lab.

Syringaldehyde served as internal standard. The procedure was as follow

follows :

Approximately 7.0 mg of purified hydroxypropylated sample was

first dissolved in 5 ml 50% methanol solution. After the sample was

completely dissolved, about 6 µi of this solution was injected onto

23

the column which had a flow rate of l.Oml/min. at room temperature.

The absorbance of the sample was monitored by a UV detector at 280nm.

Free acid contents were calculated from the peak areas and

appropriate response factors. The free acid content with respect to

the original sample size was then calculated from the weight ratio

of the recovered sample and the original sample •.

Viscosity

Viscosities of samples were measured with a Brookfield visco-

meter (Brookfield Co.). The samples were not diluted. Calibration of

the viscometer was achieved by using standard solutions with known

viscosities.

Glass Transition Temperature (T )

Glass transition temperatures of samples were determined on a

differential scanning calorimeter, model DSC 2 (Perkin-Elmer Co.) in

the Chemistry Department, VPI&SU, Blacksburg, Virginia.

The freeze-dried samples were dried in a pistol dryer to constant

weight. 10.0 mg of the sample were weight accurately into the sample

pan, and the sample was prepared in pellet form by pressing it on a

crimper .•

The glass transition temperature was immediately determined on

the DSC. Scanning temperaturerange was between 300 and 500°K with a

heating rate of 10°K/min. After a scan was completed, the Tg was obtained

from the intersect of the tangents of the DSC curve.

RESULTS AND DISCUSSION

The objectives of this research were to investigate the reaction

of propylene oxide (PO) with lignin in terms of reaction mechanism and

reaction kinetics. In the absence of a convenient and experimentally

reliable procedure for sampling the reactor content at various time

intervals, and to assay the reagent composition with regard to degree

of reaction, reaction progress was monitored by means of a pressure-

temperature-time (P-T-t) diagram. The use of the P-T-t, diagram as a

direct indicator of propylene oxide concentratio~ in the reactor is an

approximation which is based on the following assumptions:

1. Measurable vapor pressure in the reactor is contributed solely

by propylene oxide (an assumption which is probably invalid for the

final reaction stage in which some propylene glycol and low molecular

weight propyl ethers may contribute to vapor pressure as well);

2. The ideal gas law applies to the conditions inside the reactor;

3. Pressure reductions are a direct indication of propylene oxide

depletion inside the reactor;

4. The effect of impurities on propylene oxide depletion is

minimum.

Under these assumptions, the reaction mechanism of the reactiofr of

PO with lignin was evaluated by determining the consumption of PO as a

consequence of reaction with lignin. All propylene oxide concentra-

tions were normalized with the inert state vapor pressure of PO at the

temperature,c0 T ,at which a reading was made, and this was represented '

as percent of initial of PO concentration. The inert state of vapor

pressure of PO at various temperatures is shown in Figure 7, During

24

25

the reaction, any reduction of PO concentration was recognized as an

indication of reaction, and the slope of the normalized concentration

versus time curves represents the reaction rate. This method of mecha-

nism evaluation was applied to the system propylene oxide and catalyst;

propylene oxide, lignin-like model compound, and catalyst; and propylene

oxide, lignin and various types of lignin derivatives, and catalyst.

Homopolymerization

Anionic Polymerization

Anionic hydroxyp~opylation occurs when PO is reacted with an alka-

line catalyst, which is coI!llilonly KOH. The effect of the presence of

KOH on the P-T-t diagram of a propylene oxide reaction is illustrated

in Figure 8. Where there is no exothermic reaction or noticeable pres-

sure drop (indicating propylene oxide consumption) without KOH catalyst,

the presence of KOH in the reactor causes the temperature to rise be-

yond control temperature, and the pressure to decline rapidly once the

exothermic reaction has conunenced. No surface area effect of the cata-

lyst was found when the KOH was added in the form of pellets or powder.

This independence of surface area effect agrees with earlier results

reported by Steiner et al. (42).

Catalyst concentration was found to have a significant influence

on reaction rate, on the peak temperature of the homopolymerization

reaction, and on the viscosity of the reaction product. This is illus-

trated in Figures 9,10, and 11. When catalyst concentration rose from

0 to 2.6 millimoles per mole of propylene oxide, the reaction rate in-

creased from close to zero to nearly instantaneous. Thus, catalyst

concentration seems to be the rate-determining factor of propylene oxide

26

homopolymerization under the reaction conditions chosen, However, no

increase in reaction rate was observed at KOH concentrations beyond 2.6

mmoles/mole PO. This suggests that the reaction rate reaches an upper

limit in relation to KOH concentrations and that it has a zero order at

higher catalyst concentrations.

Not only reaction rate but also peak temperature and homopolymer

viscosity seems to be affected by catalyst concentrations.

Hydroxypropylation is an exothermic reaction in which the rate of energy

liberation is correlated with the reaction rate and thereby catalyst

concentrations. Since high rates of energy liberation result in high

peak temperatures in the initial phase, peak temperatures and catalyst

concentrations are correlated. This is shown in Figure 10.Catalyst con-

centrations are related lineraly to the reciprocal of peak temperatures

between 0 and 3.4 mmoles KOH/ mole PO; no correlation was found for

even higher catalyst concentrations. If one assumes that (a) homopoly-

merization begins at near peak temperature and (b) the activation energy

of homopolymerization is a constant, then under the reaction conditions

chosen in this study, the reaction rate and rate constant can be estima".'.":::_

ted from the Arrhenius equation lnK=(-E/R)(l/T) + lnA,where K, E, R, T, p p

and A are rate constant, activation energy, gas constant, peak tempera-

ture, and Arrhenius constant, respectively. If the relationship between

peak temperature as l/T and catalyst concentration is substituted into p

the above equation, the new equation is E lnK= ( - R) . a • KOH + lnA + . b

where a, b, and KOH are the slope, intercept of line (Figure 10),

and KOH concentration respectively and others are the same as previous

27

equation. The reaction rate increases exponentially with KOH concentra-

tion between 0 and 3.4 millimoles per mole of propylene oxide, but this

equation is only applicable to conditions chosen in this system.

The viscosity of homopolymer reaction products was measured in rela-

tion to catalyst concentration, and the relationship is illustrated in

Figure 11, also. The viscosity increases sharply with KOH concentra-

tion increases between 0 and 2.6 millimoles per mole of propylene oxide,

and remains constant thereafter. However, it should be pointed out that

neither homopolymer product, from low or from high KOH content reactions,

exhibits high molecular weights. All measured average molecular weights

of homopolymer products, determined by polystyrene-calibrated HPLC col-

umns, were below 750 grams per mole.

The Arrhenius equation also allows the evaluation of the relation-

ship between activation energy, reaction rate, rate constant, and reac-

tion temperature. The influence of reaction temperature on PO homopoly-

merization was evaluated, but both activation ·energy and rate constant

of this reaction could not be determined from this system. Ther~

sults are illustrated in Figure 12. While the reaction rate is almost

zero for temperature of 85°c or below, the reaction is almost instanta-

neous at temperatures above 150°C. Furthermore, the similarity of reac-

tion rates of 155, 190, and 250°c cooks suggests that the reaction

rate is not dependent upon control temperatures but peak temperatures,

since these three cooks yield the same peak temperature of 326°c.

Although it is more common to perform oxyalkylation reactions in

the presence of alkaline catalysts, acidic catalysts also are sometimes

employed. The appeal of cationic reaction mechanisms lies in the

28

formation of primary hydroxyl groups from propylene oxide and related

alkylene oxides. For this reason, the use of H3B03, A1203, A1Cl3, ZnO,

and KCl were tested as potential catalysts in the PO homopolymerization.

The P-T-t diagrams .of these reactions are illustrated in Figure 13 . The

striking similarity between the P-T~t diagram of acid catalyzed PO

reactions with that of the uncatalyzed control suggests that there is

no homopolymerization under the conditions selected. Variations in the

addition of these presumed acid catalysts over a wide concentration

range also failed to result in differences in the P-T-t diagram, indi-

cating no relationship between homopolymerization behavior and acid ca-

talyst concentrations. For various concentrations of KCl in the homo-

polymerization reaction, results are similar to those in Figure 13.

Thus, acid catalysts proved to be totally ineffective for catalyzing

propylene oxide homopolymerization under conditions of 250°c or below.

Hydroxypropylation of Model Compounds

Lignin is a complex polymer containing various functional groups

which can be expected to complicate the study of the reaction of lignin

with PO. In an attempt to explore the influence of various functional

groups on the reaction behavior, model compounds were employed. These

model compounds represent a wide range of different functionalities and

are illustrated in Figure 14 and Table 3. Since these compounds contain

at least one hydroxyl group, copolymers of model compounds and PO can be

formed but homopolymers are also formed in the presence of the KOH cata-

lyst. Therefore, these two polymerizations actually compete with each

other during reactions.

29

Effect of Functionality

The pressure-time (P-t) diagram of these model compounds is shown

in Figure 15. The reaction of model comoounds with PO can be divided

into three groups. The first group shows a very slow reaction, such as

in the case of benzoic acid. The second group shows a two-phase reac-

tion involving an initial slow and subsequent rapid reaction period,

such as in the case of vanillic acid, syringic acid, and p-hydroxy-

benzoic acid. The rest of the compounds belongs to the third group,

which reacts almost instantaneously.

The normalized C-t curves of these compounds are shown in Figure

16. The very high residual PO concentration in the benzoic acid cook

indicates the lack of both homopolymerization and copolymerization.

This suggests that KOH is first neutralized by the carboxyl group of

benzoic acid and that liberation of free KOH from the carboxylate--

a consequence of esterification-- is slow. Thus, the activation energy

of the copolymerization reaction is probably high.

In contrast to benzoic acid, compounds of the third group give

almost instantaneous reactions. Since the difference in reaction rate

is related to their molecular structures, the presence of. carboxyl

groups must be held responsible for the experienced reduced reaction

rates. The fact that the reaction rates of the model compounds of the

third group are almost identical indicates, however, that both alipha-

tic and phenolic hydroxyl groups result in essentially equal reaction

rates with PO.

The reaction behavior of the compounds of the second group which

30

include vanillic, syr;Lng;i.c 1 and p-hydroxybenzoic ac;i.d is different

from the other two groups by having two distinct reaction phases.

The first reaction proceeds slowly and results in only < 10% consum-

ption of monomeric PO. Only after this initial slow reaction is

completed, the second rapid reaction which consumes > 90% of all

PO starts. The beginning of the second phase reaction seems to be

correlated with the pK value of the model compound. a ,

Peak temperatures and reaction product functionality of several

hydroxypropylated model compounds are shown in Table 4. Most compounds,

except benzoic and syringic acid, result in similar peak temperatures

suggesting similar reaction rates, which was also suggested by their

0-t curves (Figure 16). The high phenolic hydroxyl content of syringic

acid product after the reaction as compared to those of vanillic

and p-hydroxybenzoic acid indicates differences in reactivity of

phenolic hydroxy-containing compounds in relation to their substi-

tutions. Obviously, phenolic hydroxyl groups with no or one electro-

negative substitutent in ortho position are favored toward an initia-

tion reaction as compared to those compounds with two electronegative

substitutents in ortho position.

Molar substitutions of some model compounds are also shown in

Table 4. The definition of both molar substitution (MS) and degree

of substitution (DS) is the same as those for hydroxyalkylcellulose(63).

The molar substitution is the average number of moles of PO that is

attached to each hydroxyl group of each unit and the degree of

substitution is the average number of hydroxyl groups per unit sub-

31

situted in the reaction. For mono .... and d;i..-functional compounds, such

as those in this study, the highest DS values are 1 and 2, respectively,

but the MS values could be any number from 0 to very large. It is

believed that both the MS and DS are factors to control the solubility

and compatibility of the final products (63 ). In the following tables

only the MS value will be discussed and the DS value can be obtained

from the ratio between the total number of moles of PO attached per

each unit and the MS value.

From Table 4, the highest MS value is observed with 4-hydroxy-3-

methoxybenzyl alcohol in which the average hydroxyl group is copoly-

merized with 3 PO monomers. Even so, this MS value corresponds to

only 10% of the total amount of monomeric PO in the reaction. Therefore,

the bulk of the reaction as indicated by the pressure drop must be

attributed to the second and rapid reaction phase; the reaction

rate of this second reaction is probably related to homopolymerization

rather than to copolymerization.

From the results of Table 4, the MS values follow the order of

4-hydroxy-3-methoxybenzyl alcohol > 2,6-dimethoxyphenol > vanillic acid

> benzoic acid. The comparison of these values suggests that the reacti-

vity of the functional groups with PO follows the order of aliphatic

OH > phenolic OH > carboxyl group or the order of declining pK values. a

To understand the sequence of homo- and co-polymerization reactions

during the reaction, a series of reactions of PO with syringic acid

were quenched at various reaction intervals. The percent of unreacted

phenolic hydroxyl and free syringic acid content was measured and the

32

result is shown in Figure 17. They indicate a final free syringic acid

content of 0.25% and a final phenolic hydroxyl content of 27% of the

original content. The continuous decrease in both free syringic acid

and phenolic hydroxyl content during the reaction indicates the

continuity of copolymerization reaction. Since syringic acid is a

difunctional compound, the disappearance of free syringic acid may be

due to the reaction of either carboxyl group or phenolic hydroxyl groups.

The loss of these functional groups may be attributed to either copoly-

merization of PO with phenolic hydroxyl or carboxyl groups, or esteri-

fication of homopolymers with carboxyl groups. If the difference

between the two curves in Figure 17 represents the consumption of car-

boxyl groups, then the bulk reaction of carboxyl group occurs in the

e~rly reaction period wheras the bulk reaction of the phenolic hydroxyl

group reacts in the later period.

Effect of KOH Concentrations

The effect of KOH concentration on the hydroxypropylation reaction

of 2,6-dimethoxyphenol was explored and the results are shown in Figure

18. No difference could be observed for the reaction rate or the reac-

tion pattern for three different KOH concentration levels.

In contrast to 2,6-dimethoxyphenol, both the reaction rate and

the reaction pattern of hydroxypropylation of benzoic acid was found

to depend largely on KOH concentrations. The P-t diagrams of five benzoic

acid cooks are shown in Figure 19, in which three reaction patterns are

distinguished. At a molar ratio of benzoic acid to KOH of 0.99, neither

homo- nor co-polymerization is evident because no drop in pressure is

33

observed within normal reaction time limits. At molar ratio of 0.49,

a near instantaneous reaction is observed, probably due to homopoly-

merization. At this molar ratio, free KOH was present from the begin-

ning. At molar ratio of 0.68 and 0.72, the reaction is characterized

by two distinct reaction phases : the first phase is a slow reaction

with a small pressure drop, and the second phase constitutes the

usual instantaneous reaction which consumes most of the mc~omeric PO.

There is a retardation period between the first and the second phase,

and this is generally longer at high molar ratios of I to KOH.

Furthermore, since a constant amount of KOH was used in these

cooks (with concentration of model I being varied), variations in

reaction rate and reaction pattern suggest that they are not dependent

on the initial KOH concentration but rather on the actual free KOH

concentration. Products from these reactions were analyzed for residual

free benzoic acid content and all showed a positive test, indicating

that the extent of copolymerization of benzoic acid with PO or with

homopolymer is not large.

34

Hydroxypropylation of Lignin

The reaction of PO with lignin is much more complicated than homo-

polymerization or copolymerization of PO with model compounds since

lignin is a multifunctional polydisperse network polymer. The charac-

teristics of Indulin, a commercial Kraft lignin, and several Kraft

lignin derivatives are shown in Table 5. Total hydroxyl, phenolic

hydroxyl, and carboxyl contents are 11.86%, 2.13%, and 4.01%, respec- ·

tively. Since all these functional groups contain active hydrogen

atoms, a multitude of reaction between PO and lignin may take place,

and both homopolymerization and copoly:merization are possible as

shown in the GPC chromatography in conjunction with UV and RI monitoring

of reaction products ( Figure 20).

The P-t diagram of th.e reaction of PO with. lignins is shown in

Figure 21. The result indicates that there are two distinct reaction

patterns : one showing a near instantaneous reaction and the other

exhibiting two sequential reactions. The difference between the two

.reaction patterns must be attributed to differences in reaction condi-

tions and lignin structures. These parameters will be discussed

in the following section.

Effect of Lignin Structures

Various lignins were employed in the investigation of the effect

lignin structure variation on hydroxypropylation. These lignins

include Kraft lignin, methylated Kraft lignin, and demethylated

Kraft ligni..'1. Their characteristics are shown in Table 5. The methoxyl

content of these lignins ranged from 0.64% for demethylated Kraft

35

lignin to 25.6% for methylated lignin. Both total hydroxyl and pheno-

lic hydroxyl content increased by demethylation, although condensa-

tion is also likely as is evidenced by increases in the glass tran-

sition temperature (Table 5). However, the hydroxyl content declined

greatly by methylation, resulting in 1.32% and 0.09% total hydroxyl

and phenolic hydroxyl content, respectively. The glass transition

temperature declined only slightly as a result of methylation.

The C-t curves of these reactions are shown in Figure 22. Only

one rapid bulk reaction is observed for both methylated and demethylated

Kraft lignin, while two sequential reactions are observed for Kraft

lignin. The reaction rate in these reactions followed the order of

methylated Kraft lignin > demethylated Kraft lignin > Kraft lignin.

The high reaction rate in methylated Kraft lignin cook is proba-

bly due to its low total acidity. The high residual reactor pressure

of the reaction of demethylated Kraft lignin suggests either an

incomplete reaction of PO or the formation of low boiling secondary

reaction products. The reaction remains incomplete probably

because of the high glass transition temperature of the lignin prepa-

ration. Thus, there is a definite effect of the glass tr~nsition

temperature on the reaction and this indicates that the reaction has

to be carried out at temperatures above the glass transition tempera-

ture of the particular lignin preparation.

Molar substitutions(MS) of the hydroxypropyl lignin derivatives

(HPL) are also shown in Table 5. The low MS of methylated Kraft lignin

i~e. 1.76, indicated that only 1.8% of the total monomeric PO was

copolymerization with the lignin preparation. Thus, the rapid initial

36

reaction rate observed with methylated Kraft lignin cook is mainly

caused by the homopolymerization.

The low MS of demethylated Kraft lignin is probably due to

incomplete reaction. The highest MS is obtained for Kraft lignin.

Therefore, it can be concluded that lignin structure variations have

a significant effect on reaction.rate, reaction pattern, and molar

substitution.

Effect of Catalysts and Catalyst Concentrations

The P-t diagram of hydroxypropylation of lignin at various KOH

concentrations is similar to those shown in Figure 21. Their C-t

curves are shown in Figure 23. Both reaction rate and reaction pattern

are affected by KOH concentrations. The reaction rate increases

with KOH concentration until it reaches 0.89 mmole /g lignin.However,

two sequential reactions are only observed at low KOH concentrations.

The high reaction rate at high KOH concentration is probably due to

increased total alkalinity catalyzing and accelerating homopolymeri-

zation. However, no significant difference in reaction rates is

observed between cooks with KOH concentration of or greater than

O. 89 nnnole /g lignin. This suggests that the the factor controlling th.e

reaction rate of _lig~in hydroxyprQpylation is the carboxyl content of

lignin, although the phenolic hydroxyl group may play a role in the

initial stage.

The effect of KOH concentration on peak temperature is shown in

FigurelO. At low KOH concentrations, i.e. below 4.9 mmole/mole PO,

no correlation between KOH concentration and peak temperature is

37

observed. But at high KOH concentrations, the value of the reciprocal

of peak temperature (l/T ) decreases with increasing KOH concentra-p

tion. By employing the same assumptions as shown earlier, the

Arrhenius equation can be rewritten in terms of KOH concentration as

E lnk= ( - - ) • a • KOH R

where k= rate constant

E= activation energy

R= gas cons.tant

+ lnA + b

a= slope of l/Tp vs KOH concentration, Figure 9

h= intercept of l/Tp .vs KOH concentration, Figure 9

A= Arrhenius constant

KOH = KOH concentration

Thus, the reaction rate in lignin hydroxypropylation at high KOH con-

centrations is exponentially related to KOH concentration.

The value of total hydroxyl content, phenolic hydroxyl content,

and molar subsitution of these HPLs are shown in Table 6. The relatively

constant value of MS suggests that the copolymerization rate of PO

with lignin is not affected by KOH concentration under the reaction

conditions selected. This is probably because there were no free KOH

molecules present in the initial stage of reaction since the

number of moles of acidic functionality of lignin is greater than

that of KOH. However, the release of free KOH from phenolic hydroxyl

groups in the later stages of reaction causes homopolymerization.

This also suggests that the increased reaction rate at high KOH

concentrations as shown in Figure 23 is mainly caused by homopoly-

38

merization.

The phenolic hydroxyl group is quite reactive in the KOH-catalyzed

hydroxypropylation reaction since less than 5% of the original phenolic

hydroxyl group remains unreacted. The total hydroxyl content is reduced

from 11.86% to 6.71% as the result of increased unit weight by copoly-

merization with PO. Results from the total hydroxyl content determi-

n·a tion agree well with those obtained by MS determination.

The C-t curves of several hydroxypropylation reactions of lignin

in the presence of various cationic catalysts, such as H3Bo3 ,zno, and

KCl, are shown in Figure 24. The typical pressure diagram of the

reaction indicates that all cationic catalysts have the same effect,

and that the high final PO concentration represents incomplete

conversion of PO. However, the reactions result in homogeneous

products, indicating complete derivatization.

Several chemical structural parameters of cationically catalyze~

HPLs are shown in Table 7. KCl results in a slightly lower MS and

a slightly higher total hydroxyl content as compared to 'all other

HPLs. This suggests that the cationic copolymerization rate of PO

with lignin may be related to the valence of cations, since KCl is

univ~lent and the others are multivalent. No significant difference

in terms of total hydroxyl content, phenolic hydroxyl content, and

MS is observed for ZnO, Al2o3 ,and H3Bo3 catalyzed HPLs.

The characteristics of the anionically (KOH) catalyzed HPL are

also shown in Table 7. Both cationic and anionic catalysts seem to

yield similar lignin derivatives in terms of their total hydroxyl

content and MS; but cationic catalysts seem to produce reaction

•.

39

products with significantly greater phenolic hydroxyl contents.

Variations in the concentration of cationic catalysts do not

seem to matter; all C-t curves are similar to those shown in Figure

24. Therefore, neither reaction rate, nor reaction pattern, nor percent

conversion of PO is affected by variations in KCl concentration.

Total hydroxyl content, phenolic hydroxyl content, and MS of

HPLs produced from different amount of KCl reactions are shown in

Table 8. A slightincrease in MS together with decreasing total hydroxyl

and phenolic hydroxyl contents is observed when KCl concentratiorr

is increased. The actual cause of these changes remains unknown.

Effect of Lignin Contents

The C-t curves of lignin reactions in which the lignin content

was varied between 0 and 20% were similar to those shown in Figure 23.

Both reaction rate and reaction pattern were affected by lignin

content. The negative effect.of lignin content must be attributed

to increasing total acidity of the system with increasing lignin

contents. Two distinctly sequential reactions are only observed when

lignin contents exceeded 14% of total reagent weight or the molar

ratio between carboxyl group and KOH is greater than 1.0.

The relationship between MS and lignin content is shown in

Figure 25. The MS is zero for both zero and 100% lignin content. The

convex curve s_uggests that MS first increases to a peak value and then

decreases. Furthermore, increasing MS together with decreasing

reaction rate indicates that increasing acidity accelerates copoly-

merization rate. Thus, at high acidity, homopolymerization rate is

greatly reduced.

40

Effect of Temperatures

Lignin reactions at control temperatures ranging from 125 to

2S0°c were performed. The C-t curves of these reactions were found

to be similar to those shown in Figure 23. At temperatures below

190°c, the reaction between PO and Kraft lignin appears incomplete

as indicated by the presence of a heterogeneous reaction product

(partly tar and partly solid). Thus, there is a definite effect of

glass transition temperature on the reaction. This suggests that

the reaction should be carried out at temperatures above the glass

transition temperature of the particular lignin to expand its

contact surface area and to enhance its reactivity.

A similar study for KCl-catalyzed hydroxypropylation reactions 0 was conducted at temperatures above 200 C. All reactions yielded

homogeneous products but significant level of residual reactor

pressure. Molar substitutions of these HPLs were determined and

results show a fairly constant degree of derivatization. Since

high reaction rate is resulted in high temperature as shown from

their C-t curves, this observation suggests that o"nly homopolymeri-

zation rate is increased a-i: high temperatures.

CONCLUSIONS

HOMOPOLYMERIZATION

1. Homopolymerization is an exothermic reaction.

2. No homopolymerization is observed in the absence of catalyst or

in the presence of such catalysts as A12o3 , H3Bo3 , ZnO, or KCl.

3. The concentration of catalysts of (2) has no effect on reaction rate.

4. Anionic catalysts ( KOH ) may catalyze PO homopolymerization.

5. The reaction rate of homopolymerization is an exponential function

of KOH concentrations.

6. Both the peak temperature of the reaction and the viscosity of

the homopolymer are linearly related to the KOH concentration.

HYDROXYPROPYLATION OF MODEL COMPOUNDS

1. Both homopolymerization and copolymerization take place during

hydroxypropylation of lignin-like model compounds.

2. Reaction rate and reaction pattern depend on the ratio of total

acidity to to.tal alkalinity of the system.

3. The highest molar substitution is obtained with 4-hydroxy-3-methoxy-

benxyl alcohol ( 2,9 ) and the lowest molar substitution is

obtained for benzoic acid ( 0.0 ) •

4. Homopolymerization is the major reaction contributing to measu-

rable pressure drop in the reactor.

41

42

5. The molar substitution of functional groups with PO follows the

order of aliphatic OH > phenolic OH > carboxyl

although carboxyl groups seem to react first with KOH.

6. Phenolic hydroxyl groups without or with only one eletronegative

suostituterit. in ortho position respond more favorably to the

initiation reaction than those with two substitutents of this type

in ortho position.

HYDROXYPROPYLATION OF LIGNINS

1. Both homopolymerization and copolymerization take place during

hydroxypropylation of lignin samples.

2. Reaction rate and reaction pattern depend on the total acidity

vs total alkalinity of the system.

3. Reaction rate is exponentially related to KOH concentrations.

4. Reaction rate is not affected by either type or concentrations

of cationic catalysts.

5. Molar substitution of HPLs is affected by lignin structure and

lignin content in the reactor.

6. Molar substitution of HPLs is a constant over the KOH concentra-

tion range of 0.21 to 1.34 mm.oles/ g lignia and the temperature 0 range of 190 to 280 C.

7. Both cationic and anionic catalysts result in HPLs with similar

molar substitutions, but cationic catalysts yield reaction products

with higher phenolic hydroxyl content.

43

8. There is a definite effect of glass transition temperature on the

hydroxypropylation of lignin.

GENERAL CONCLUSION

It.was attelllpted to improve the flexibility and functionality of

lignin by introducing new and long side chains through hydroxypropy-

lation. It was found that molar substitution varied between zero and

three, and that it was related to the ratio of total acidity to total

alkalinity of the system. Reactions with model compounds indicated

that conjugated aliphatic hydroxyl and certain phenolic hydroxyl

groups reacted with particular ease with PO. Therefore, reaction

products from lignins rich in these types of functional groups

can be expected to produce derivatives with low glass transition

temperature and low rigidity~

Recommendations for further studies of the hydroxypropylation

of lignin include : a. to study low MW lignins.

b. to test low cross link density lignins.

c. to react with more reactive alkylene oxides

such as ethylene oxide or epichlorohydrin.

d. to employ multistage hydroxypropylation processes.

44

Table 1. Estimated values of lignin ($/T) (1).

Lignin-Derived Products

Fuel (black liquor) Vanillin Phenol/benzene PF-Resin additive Polyurethane component Dispersants

$/T

100 400 107

500 500

120-400

45

Table 2. Annual production of _pulp ·and lignin (11).

PulE, 106 tons Lignin, 106 tons Region Kraft Sulfite Kraft Sulfite

United States 35 3.5 21 2.1

World 94 9.7 57 5.8

* In spent pulping liquors.

Table 3. The structure and properties of ·lignin-like model compounds.

p-Hydroxy 2,6-Benzoic Syringic Vanillic Benzoic Dimethoxy- Vanillyl Cinnamyl

Acid Acid Acid Acid Phenol Guaiacol Alcohol Alcohol

Mal. Weight, g 122.4 198.2 168.2 138. 13 154.2 124.13 154.2 134.2

Melt Point, OC 122.4 204 214 214 56 34

Boiling Point, oc 250 264 205 114 250 ~

°' pk 4.21 4 .35* 4.52 4,58 & 9.23 9.70* 9.98 > 10* > 10* a

*Estimated values.

47

Table 4. The characteristics of hydroxypropylated model compounds.

I II III IV v VI VII VIII

TP, 0 c 195 264 372 385 385 367 372 363

cti-OH 1 ' % 0 8.58 10. 17 12.48 11.10 13.7 11. 15 0

¢-OH2 , % 0 27 0 1 2 2 2 0

MS, PO/OH o3 1.9 2. 1 2.9

. 1cti-OH content before reaction.

2cti-OH content after reaction.( of initial )

3Estirnated value.

Table 5. Chemical composition of several kraft lignins and their molar substitutions after hydroxypropylation reaction.

Liguin OCH3, % I:OH, % 4>-0H, % C=O ,% Tg, oc Tpf °C MS*

Kraft lignin 13. 70 11.86 2.13 4.01 156 197 . 2.91

Methylated 25.56 1.32 0.09 137 350 1. 76 Kraft lignin

Demethylated 0.64 15. 09 2.55 234 217 Kraft lignin 1.42

* Tp=peak temperature and MS=molar substitution; both are parameters of ~orresponding

hydroxypropyl lignins.

49

Table 6. Effect of KOH concentration on chemical structure of hydroxypropylated ligin.

KOH mmole/ g Lignin 0.21 0.36 0.54 .0.89 1.34 Avg. ± Std. Dev.

~OH, % 6.69 6. 77 6.67 6. 71 ± 0.63

<l>-OH, % 0 .15 0.14 0.12 0.14 ± 0.01

MS 2.11 1.85 2.14 2.09 1.94 2.03 ± 0.12

50

Table 7. Peak temperature and chemical structure parameters of several catalyzed hydroxypropyl lignins.

Catalyst

A1203 ZnO H3B03 KCl KOH

0 Tp, C 286 285 330 305 278

WH, % 6.64 6.68 7.51 6.71*

qi-OH, % 0.62 0.61 0.58 0.14*

MS 1.92 1.94 1.89 1. 74 2.03*

*Average value from Table 6.

51

Table 8. Peak temperature and chemical structure parameters of hydroxy-propyl lignins produced with varying KCl concentrations.

KCl, rnmole/ g Lignin

0.42 0.56 o. 71

Tp, 0 c 326 317 318

IOH, % 7.51 6.64 5.75

<I>-OH, % 0.58 0.53 0.36

MS 1. 74 1.92 2.00

p-Coumaryl

Alcohol

52

Sinapyl

Alcohol

Coniferyl

Alcohol

Figure 1, The structure of 3 primary lignin precurso~s.

CH 20H I

CH20H I

CH 20H I

CH 20H I

CH 20H I .

CH 20H I

CH CH CH CH CH • CH II II II II II I CH -H" CH CH CH CH CH

0 >-

0 <

-o~ ~o~ -o ..... .. II

YI Q lJ1 .. w II II II

OH I 0 I I 0 I I 0 I I 0 I I 0 I •

Fig. 2. Mesomeric Forms of the Phenoxyl Radical Formed from P-Coumaryl Alcohol

by Dehydrogenation (13).

54

11.<0I! !llC'Oll • I I !IC- C'O

I I

;~."Oll. ll~C(~ll -·~2. l,lli I I l'l lj

l\JcO :::-.. O\!c !I .COii I IC ~ O:'.tc 11 .. co11 - 1 1 ·

lf/~011 -c'.-o 0 . ~-~11 llC-0

- Cll lllCOll c~·11.. ll?c: '.i1 I . 1 '· ~- 1

/z 1/2 ~ ':,.. 0:0.!c

!IC- -ll4! ;,· I llCO (( .COii 15 I

~ 1 2 1 • 1 "· O.l\1c Oil (1 I J:(i)1c "'._,, 11c r1c- ,........o"'-

!\l•.·o ·,. 11.cc111 ['' I 011 iir'~ ,'.l) 11.ccm oc c11 2 . I 'J • " I () · I I o--n1 "'-: Oi\ll' 11• ~ ___!__. JI(:--' 11c---u1

I r· • ----i I I I II. 0 n11 ~"';IJ !IC:Oll !IC llC< ll!

GL.- l\lcOy-~ . .,,::J,Ji\l<~lf/\:·011 -"~' -" I L? J . r fi., I l:Ja I I-la I

11 .COii ~ O~lc O '--'( o--- <Tl l\leO~.- ~ Ol\11· ·,.. < l:\fo "t I /-..._ . I

l!ri-( -0 ) I l:;f __ ~:;; 1~~

0:---0 . Oil 0-

11<'0 C:,,11 100~ ul - (,-, J '·

1• ~1 I f'\) llJ.'o/<.lll l\l<!O '•1 11/j-'Oll

L,,.:') I !.COi i ~- ., tll! Cl I

l1---r'.·11 Jl,COll[~,~Oi\lc 11.C:Oll CJ\lc llCOll c"-11

.ioo~" ~ o ::t@o11 ~~~ ~)OMo $0>1" -$OM~ 0'1o

Oil -0 -0

Fig. 3 Freudenberg' s · Li gnin Fo.rmula (16} •

Figure 4. Structural sketch of a computer-simulated softwood lignin (revised) (20).

Lil Lil

56

-·--------71 . SULPHITE 125

100

,., ' 0

>< 7S -

50 J 25

20 40 60 80 LIGNIN REMOVED (%)

Figure 5. Comoarison of MW of Kraft lignin 1, and sulfite lignin in relation

to degree o{ delignification (26).

I -c-l -~-- c-

'©0CH3 0-

. . : A : : C :n: E: F . . . . .. I ' I I r I ' I r , I r I I I I ' I • I I I , I f I ' I I .. , • ,-, I I ' ,~, f J .. r 1 I I ' f r I I , I I , I ' r I I I rT I r I r I I I IT I I I I'' .. r{-. I rt, .,. I I : I .. r rr IT I I r r· I ' I f' rrrr r rr-i n·, I I f I. I I

10 9 s 1 6 s 4 3 2 i ·o .s ppm

Figure 6. The H-NMR spectrum of a hydroxvorooylated lignin.

\JI

""

<U ,... :: "' "' <U ,... ~

0

58

so 100 150 200 0

temperature, C

250 300 350

Figure 7. Vapor pressure diagram of propylene oxide in the

inert state.

59

900 -----r-----r"----:----.,---__,

800

700

600

500

400

300 I I

I I

I 200 I

I I

I I

100 /· I

I

" 0

0 20

Pressure 1

.. . ' : ~·. 2 : ;: \ .

; I : '·· Temperature . '. ~~-------~ ... / . . !,' ; ~ . . •/ f'f i. 11 : 'I J .

. . . .

Pressure 2

. . .. ...

40 60 80 tim~, min.

. . . . .

400

300

. 200

100

0

100

Figure 8. Pressure-Temperature-tilne (P-T-t) diagram of propylene a.xi.de homopolymerization in relation to catalyst. reaction 1 : without catalyst; reaction2 : with KOH.

60

100 -··~ ·-··-··

,_ '.,,, ""TJ - •• - .. - •• - •• -··-

o' 80 'o ~"\ u a\<> . + \ .. § 60 I ..

•r-1 I ~. .µ I ctl I .

'a_,, ~· ~ .µ. § 40 'O ~

..~

u 'D.. ~ \ 0 \ u \

' 20 ' ..,, ' ~' .... ~ ...... _~41T;g:

0 0 20 40 60 80 100 120

time, min.

Figure 9. Concentration-time (c~t) curves of propylene oxide of homopolymerization reactions.

KOH = 1. 0 ( - O-), 2. 6- ( - X-) , 1. 5 ( -V,.) , 'O. 72mmole (-a-). and 0. mmole/mole PO.

r-1

·~ 0

2.0

1. 8

C"'l 1.6 b r-1

:><:

"' p. E-1 ~ 1.4

1.2

1.0

b

\ •-o-o \ ' Qo~ \

\ -· \ \ \ e \

\ ·--- ·-

5

o~

10 15

<? ....

20

KOH, nnnole/mole PO

Figure 10, The effect of KOII concentration on the peak temperature in lignin hydroxypro-

pylation. Lignin (-0-), Blank(-8-).

800

700 v-v----v-

600 fr "' :>--.µ

•r-1 500 ' Vl 0 u Vl

·r-1 >

400

v 300

0 2 4 6 8

KOH, mmole / mole PO

Figure 11, The effect of KOH concentration on viscosity of homopolymers.

°' .....

100

... 0~ 80 u

.. ~ 60 0

•r-l .µ ell ~ .µ ~

~ 40 ~ 0 u

20

o· 0

62

.o. •..... a ............ . -V... .. o Ab c w '"Cr-z=:r-··-c···-c-··-c··· ... ~~ ~ ' ...... ' ~--- . I ~

I ........

I '~ I . I •

I ' I V,. I

x~

~I ' . A \~-o ~:--._O.;.._ __

'"' ,. .----• p---~ ·-'"'111:..---4-. -.... ta,-._

20 40 60 80 100

time, min.,

Figure 12. Concentration-time (C-t) curves of KOH-catalyze~ homopolyme.:t:ization reactions at

various temperatures.

T= 85 (-C-), 125 (-'\/-), 155 (-A-),

190 (-O-), and 25ooc (-X -) •

63

800 .. ~'-~--~- c--~- - 400

; " ii

I I v. ~

600 '9 I - 300 I

b I I r ..................... . ~

• . •

·~ . en I . · c. .

V· . .. 400 ~ ' .. - zoo 4) s.. o.· ::s L·· en en cJ Q)

"" ~ :1 .. , • ... ~

• I . .:91

200 .: I

100 ~ • p -. . . . I • . ',1

~' I

I I

0 ' I 0 0 I I

0 ::Zo 40 60 80 100 time, min.

Figure 13. P:ressure-Temperature-tiJile diagram of propylene oxide in.·acid-catalyzed hcmopolymerl.zation. catalyst :

' ' Al 0 2 3 (-0-), a3Bo3 (-t:l-), KCl (-V-),and ZnO(-)(-).

i (1) ~ $1)

E' '"i (1) . .. no

64

COOH COOH

© HfO~~ Benzoic Acid

(I)

Guaicol (VI)

p-Hydroxy-Benzoic Acid (IV)

Syringic Acid (II)

· Vanillyl Alcohol

(VII)

Vanillic Acid (III)

2, 6-Dime thoxy....; Phenol CHJ)H (V)

I

CH II

CH

© CinnamJ"l Alcohol

(VIII)

Figure l~ ·• The molecular structure of lignin model compounds.

65

800

x . :o : l ~ . .

600

400

200

i t~~l 1 . . x~~: : ~I: ~ ::"'Cl : ~=, 9; ~-c--o-·-·-o-

:\ .01 ~-A.-t;. A-A. ':: *' : :\:~, : :: I l "V

J \· 'x l : • I \ : .fl ~ \ t A

. /,~ \ : . \ : r;/ A x\ :

.. •\ 0: rt . \ . ~ \ ~.

' .. , O(~ v !'" · 0x=:-- 0--0--0-

' "-9 ·. x x I X ...._ 'V· · · · ··· V····"l··

,o :-··-~-··~··-/

A

\_ I

0

0 20 40 60 80 100 120 time, min

Figure 15. Pressure-ti.me diagram of reactions of propylene oxi.de with m::>del

compounds. c~mpoun.ds : I (-C-), II C-A-), III (-0-),

Dl (-V-), V (-X-), VI (-,4-), VII (-"f-), and VIII (-9-) ..

t: 0 u

100

80

60

40

20

0 0

66

. ! . . l . . I I l : I I I I · . I T !· i I I \

: 0 . \

' II : ! . • \ l • \ l a ~ . \: ~ \ ··~ ·. \ ~Ll 0 -~ \ v-. e":.=.<l:i-0- -c- - -0-- -dr. 'Wr . -~ •-r ·- '~· ~~~-~- Bv-.. 9 .. --q- ~ 't. .. A .. 4

" " 20 40 60 80 100 120

time, min.

Figure 16. Concentration-time curves of reactions of propylene oxide with. model compounds. Compounds : I C-A-), II {; -fl_;..), III (-Q-), IV (-y-), v c-e-), VI(-.-), VII (-CJ-),

and VIII (-~-).

100

80

r;J .~ o•i 60

•rl H 0

... .µ

g 40 .µ ~ 0 u

20

0

o~~--.~~.-~~-.--....:.

0

' ' • ' ~ \A ~ ~A

' ' \ \ \ \ \ \ \

' A \'o \ ' ', .....

0-..._ ---4o 80 120

time, min.

Figure 17. Product functionality of the reac-tion of syringic acid with propy-lene oxide in relation to reactton time. Compound II (..Q~) and

Phenolic Hydroxyl content (-~-).

100 oo~k. I- I

0 I

80 I u I I

o\<> I I ... I .

i:: I 0 60 I ·ri 'I( .µ A td 1--< ~ .µ l fil 40 i·· u i:: 0 u \b ..

20 0-0-4-0-A---i--~-ft-

"' ..._x-x-x-x-

0

0 20 40 60 80 100 time, min.

Figure 18. Concentration-time curves of propy-lene oxide in hydroxypropylation of

compound V at various molar V/KOH

ratios. V/KOH= 1.8 (-C>-), 2 • 9 (- fl.-) , and 5 • 0 (- X. - ) .

0\ ...:.

700

600

500

·r-i U1 ~ 400 (]) i-.. ::l U1 U1 (]) i-..

0... 300

200

100

0

68

/<i 0 . 20 40 60 80 100

time, min.

Figure 19. Pressure-time diagram of hydroxypropylation reactions of benzoic acid (I ) at various I/KOH ratios.

I /KOH = 0. 99 (-V-)' 0. 72 (-A-)' 0. 68 (-a-), and 0.49(-0-).

>-1-(/) z lJJ 1-z _J <!: z (.!)

en

r-, I \

I \ I \ I I I \

69

I \

RI SIGNAL UV SIGNAL

HOMOPOLJ'MER OF PROPYLENE OXIDE

(CONTROL)

REACTION PRODUCT OF LIGNIN AND PROPYLENE OXIDE

I ', I ...... ___

I I

HEXANE EXTRACT

WATER-SOLUBLE FRACTION OF ACETONITRILE

s~.

LIGNIN - PROPYLENE OXIDE COPOLYMER

ELUTION VOLUME

Figure 20. GPC chromatography of homopolymer , lignin-

propylene oxide copolymer, and reaction product mixture after hydroxypropylation.

•.-l Cll p. .. Q)

""' :J Cll Cll Q)

""' P-1

700

600

500

400

300

290

100

0

0

I I

,/

I I I I I I I I I I I

I I

I. I

i '/

20

70

40 60 80 lQO time, min

Figure 21. The typical .pressure".""t:i..IDe diagram of the hydroxypropy-lation reaction of lignin preparations.

0 ,-... 0 Pol ..._,

100

80

60

40

20

0

71

.----Ox-----------------.°'' ~ 'c:\

0

'. ~ \ "'· '0 \

20

• \ Jr

! b \ I ' . x . 0 ' I ' '\. . \, " l 'o \ . ' ·'Q >( i 'o-o'o~ . \ . ...~ -0--0--0-

. ' I X

4t "~ . · .. A · ·"-x-. ~:::...1-..:_~·-·-A--·-A-

. . ·-- ·-40 60 80 100

time, min

Figure 22. Concentration-time curves of reactions of propylene o:rlde with several lignin preparations in relation to

different functionalities.

In4ulin (- X -) , methylated Indulin (-A -) , demethylated Indulin.(-O-) and . PO (-•-).

100 l-o

u 80 "'" .. §

·r-l 60 .µ ro !--< .µ § g 40 0 u

20

0

0

72

B~AA \x\ I •\

o\ A

20

I \\ I l \c I • )(. : \ \ 0 A· . ' \. \ I \ )( . ' I \ .. I \ .. X, I A ..

tJI ' 'x 0 ·, ' \ \ A, ",x C\ "'•A -...... • "°'" -.... .. -.~ i;;J ....

"{;_ -c- ~A-... o-. -o--cJ----·O=- -~o-

40 60 80 100 time, min.

120

Figure 23. Concentraticn-time curves of reactions of propylene oxide with Kraft lignin in

relation to KOH concentration.

KOH= 1.34 (-CJ-), 0.89 (-()-), 0.54 (-~-), and 0. 21 nunole / g Lignin (- 'X-).

100

X·x·r9 t- 8i o' u 80. I ~ I

I

"' a

i:l ex 0 60 . 'ii

•rl \ .µ ro \ 1-l d .µ i:l ~ Q) 40 CJ ~v i:l 0 \ u ~v .

20 '8--i--8'-.-~-

0

0 20 40, 60 80 100 time, min.

~igure 24. Concentration-ti.lne curves of hydro-xypropylation reaction of lignin

with various cationic catalysts.

Al20 3 , (-0:-), H3Bo3 (-0-),

KCl (-)(.-), and ZnO (-9-).

6 -0 P-1

ft

~.

, /

3.0 I • I I ,.

I I 2.0 I

I I f I f

1.0 ' I

0

0 20 40 60 80 100

Lignin, % Total

Figure 25. Effect of lignin content on molar substitution.

-.,J w

REFERENCES

1. Glasser, W. G., Proceedings of the 9th Annual Hardwood Symposium of Hardwood Research Council, Pipestem; WV, May 25-28, 1981.

2. Santelli T. R. and R. T. Wallace, U.S. Pat. 3,072,634.

3. Moorer H. H., W. K. Daugherty and F. T. Ball, U.S. Pat.3,519,581.

4. Christian D. T.,M. Look and T. S. Armstrong, U.S. Pat. 3,546,199

5. Glasser W. G. and O. H.-H. Hsu, App. Polymer Symp.28,297,1975.

6. Sarkanen K. V. and H. L. Hergert, in Lignins : Occurrence, Formation, Structure and Reaction, Sarkanen K. V. and C. H. Ludwig Eds. Wiley-Interscience, New York, 1971, pp. 44-65.

7. Sarkanen K. V. and Y. z. Lai, in Lignins : Occurrence, Formation, Structure and Reaction, Sarkanen K. V. and C. H. Ludwig Eds. Wiley-Interscience, New York, 1971·, pp. 166-175.

8. Goheen D. W. and C. H. Hoyt, in Lignins : Occurrence, Foqnation, Structure and Reaction, Sarkanen K. V. and C. H. Ludwig Eds. Wiley-Interscience, New York, 1971, pp.833-840.

9• Editor, Chem. Eng. News, 41(25), 41, 1958.

10. Forss,·x. and A. Fuhrmann, For. Prod. J., 29(7), 39, 1979.

11. Goheen, D. w., Proceedings of l'iSF. Conference, Colu.'11bus, uhio,

·October, 1979.

12. Hergert, H. L., For. Prod. J., 10, 610,1960.

13. Glas$er, W. G., in Pulp and Paper:Chemistry and Chemical Technology Vol. I, Casey, J.P. Ed., Wiley-Interscience,NY,1980,pp.39-99.

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The vita has been removed from the scanned document

HYDROXYPROPYLATION OF LIGNIN AND LIGNIN-LIKE MODEL COMPOUNDS

by

CIITH. FAE WU

(ABSTRACT)

Copolymerization reaction o~ propylene oxide and Kraft lignin with

various .catalysts, catalyst concen~rations, at several temperature

levels were studied. The reaction rate of propylene oxide was esti-

mated from the pressur_e vs time diagram of the reaction and the copoly-

merization extent was evaluated on the basis of H-NMR spectroscopy in

terms of TIX)lar substitution of purified lignin/propylene oxide

copolymers.

The investigation included studies with lignin-like model compounds 0 under conditions of varying KOR concentrations at 190 C. Reaction rate

and molar substitution were determined.

Homogeneous ( completely liquefied ) Kraft lignin products were . 0 only obtained at reaction temperatures above 190 C or temperatures

above the glass transition temperature of the particul!ir lignin

preparation. Various lignins including Kraft lignin, methylated Kraft

lignin, and demethylated Kraft lignin were employed to reacted with

propylene.oxide at 190°c. The effect of lignin structure variations

on both reaction'rate and moiar substitution were determined.

Results revealed that homopolymerization was the major reaction

although copolymerization also took place during the hydroxypro-

pylation. Reactions with model compounds indicated that the reaction

rate depended on the ratio of total alkalinity to total acidity

in the system which was found to be related with KOH concentration,

lignin content, and lignin structure. Particularly, carboxyl groups

of lignin resulted in high acidity and low homopolymerization rate.

It was also found that molar substitution varied between zero

and three and results from model compound study indicated that

conjugated aliphatic hydroxyl and certain phenolic hydroxyl groups

reacted with particular ease with propylene oxide. Therefore,

reaction products from lignins rich in these types of functional

groups can be expected to produce derivatives with low glass

transition temperature and improved solubility.