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MODIFICATION OF LIGNIN*John J. Meistera

a Forest Products Research Center, Albuquerque, NM, U.S.A.

Online publication date: 24 June 2002

To cite this Article Meister, John J.(2002) 'MODIFICATION OF LIGNIN*', Polymer Reviews, 42: 2, 235 — 289To link to this Article: DOI: 10.1081/MC-120004764URL: http://dx.doi.org/10.1081/MC-120004764

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MODIFICATION OF LIGNIN*

John J. Meister

Forest Products Research Center, 2008 Hendola Dr., NE,Albuquerque, New Mexico, 87110-4808

E-mail: [email protected]

CONTENTS

I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235

A. The Nature of Lignin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237

B. Recovery of Lignin. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241

C. Uses of Extracted Lignin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 242

II. Modification of Lignin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243

A. Decomposition of Lignin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243

B. Using Lignin as Extracted from the Plant. . . . . . . . . . . . . . . . . . . 245

C. Adding to the Lignin Biopolymer . . . . . . . . . . . . . . . . . . . . . . . . 251

III. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 280

Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 281

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 282

I. INTRODUCTION

Lignin [8068-00-6] is a natural product produced by all woody plants. Itis second only to cellulose in mass of the natural polymer formed perannum.[1] Lignin constitutes between 15 and 40 percent of the dry weightof wood with variation in lignin content being caused by species type,

235

Copyright # 2002 by Marcel Dekker, Inc. www.dekker.com

*Reprinted from Polymer Modification: Principles, Techniques, and Applications; Meister, J. J.,Ed.; Marcel Dekker, Inc.: New York; 2000, 67–144.

J. MACROMOL. SCI.—POLYMER REVIEWS, C42(2), 235–289 (2002)

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growing conditions, the parts of the plant tested, and numerous other fac-tors.[2] The data of Table 1 show the variation of lignin content by speciestype. Plants use lignin to (1) add strength and structure to their cellularcomposites; (2) control fluid flow; (3) protect against attack by micro-organisms; (4) act as an antioxidant, a UV absorber, and possibly a flameretardant; and (5) store energy.[3] When considering the present and futureuse of this biopolymer, it is important to realize that any archeological age,such as the iron age, starts and also ends before the participants realize it.We are currently at the end of the age of oil. The slow decline in available oilreserves during the early 21st century will make lignin a more important

236 MEISTER

Table 1. Lignin Content of U.S. Woods as Determined at U.S. ForestProducts Laboratory from 1927 to 1968

Scientific Name/Common Name Klason Lignin

A. HardwoodsAcer macrophyllum Pursh/Bigleaf maple 25Betula alleghaniensis Britton/Yellow birch 21 (2)a

Carya Cordiformus (Wangenh.)K. Koch/Bitternut hickory

25

Populus tremoides Michx./Quaking aspen 19 (22)Quercus falcata Michx./Southern red oak 25

Quercus rubra L./Northern red oak 24Fagus Grandifolia Ehrh./American beech 22 (2)Gleditsia tricanthos L./Honey locust 21

Liriodendron tulipifera L./Yellow-poplar 20Populus deletoides Bartr. Ex Marsh./Eastern Cottonwood

23 (3)

Salix nigra March./Black willow 21 (2)B. SoftwoodsAbies balsamea (L.) Mill./Balsam fir 29 (16)Larix occidentalis Nutt./Western larch 27 (3)

Picea glauca (Moench) Voss./White spruce 29 (8)Pinus banksiana Lamb./Jack pine 27 (27)Pinus elliottii Engelm./Slash pine 27 (15)

Pinus strobus L./Eastern white pine 27 (5)Sequoia sempervirens (D. Don) Endl./RedwoodOld growth 33

Second growth 33Tsuga canadensis (L.)Carr./Eastern hemlock

33 (7)

aNumbers in parenthesis are number of independent determinations for thecomponent. In some cases, the trees are from different locations. Values areweight percent contained in moisture-free wood. Data are from Table 3,

p. 76, Chapter 2, Chemical Composition of Wood, By R.C. Pettersen, inThe Chemistry of Solid Wood by R. Rowell, Ed., Advances in ChemistrySeries, Vol. 207, Amer. Chem. Soc., 1984, ISBN 0-8412-0796-8.


source of chemicals for our future society. When fundamental technologywithin a society changes, decades of work preceding the change must haveoccurred to develop new technologies to replace those that are obsolete. Asthe age of oil changes to the age of biomass, some of the chemical modifica-tions described below will become important industrial processes for pro-ducing the chemicals and materials that society needs.

A. The Nature of Lignin

Woody plants synthesize lignin from 3-(4-hydroxyphenyl)-2-propenol(trans-4-coumaryl alcohol¼ 1.1, grasses), 3-(4-hydroxy-3-methoxyphenyl)-2-propenol (trans-coniferyl alcohol¼ 1.2, pines), and 3-(4-hydroxy-3,5-dimethoxyphenyl)-2-propenol (trans-sinapyl alcohol¼ 1.3, deciduous) byfree radical crosslinking initiated by enzymatic dehydrogenation.[4]

Structures of these alcohols and the notation for the carbon atoms of theC9 repeat unit of lignin are given in Figure 1. As indicated in the naming ofthe alcohols, each class of plants, grasses, softwoods, and hardwoods, pro-duces a lignin rich in one type of alcohol repeat unit. That lignin is producedby free radical reaction of the alcohol mixture induced by enzymatic dehy-drogenation of a C9 alcohol.

[5]

MODIFICATION OF LIGNIN 237

Figure 1. Structures of the alcohols that form lignin and the notation for the carbon atoms inthe monomers.


The polymerization of the alkene-substituted phenols can produce anumber of bond structures by delocalization of and reaction at, the freeradical site.[6] The lignin produced by a plant is a species and plant-partspecific compound that has different composition and structure even withinthe same plant. This means that the lignin recovered from a woody plant willbe a mixture of structures and repeat unit compositions that will vary withthe source wood, species, and growing pattern of the wood.[7] ‘‘Lignin’’ then,identifies a class of C9-repeat unit, alkylphenol, network polymers formedwith the repeat unit bondings shown in Table 2, where R1 and R2 are hydro-gen or methoxyl groups. The location of the bond between the repeat units isspecified by listing the carbon atom label or heteroatom element symbol foreach atom encountered while moving from one repeat unit to the next.The most common bond in lignin, the b–O–4, is a bond starting at themiddle carbon atom (b) of the propyl sidechain on one repeat unit, linkingthrough the oxygen of the next repeat unit to the number 4, carbon atom ofthe aromatic ring of that repeat unit. One structure, g2 of Table 2, violates thenotation pattern usually used for these bonds but this label for the bi(cyclicether) structure is not common. This knowledge of the frequency of thesebonding structures in natural and synthetic lignin is based on assays andcalculations by several authors. The results of these analyses[8–11] are sum-marized in Table 3. Adler has recalculated the data of Table 3 to express it aspercent of all repeat unit bonds that are of a given type but all of this data issubject to error introduced by extraction method, processing of the lignin,and the digestion of the lignin to monomers. Despite the limitations of thedata, it does show clearly that hardwood (Beech) and softwood (Spruce)lignin differ in bonding structure.

As the lignin monomers react, structures and functional groups notpresent in the original alcohols are formed. A tabulation of functionalgroups found in milled wood lignin and kraft pine lignin is given in Table 4.The three-dimensional networks forming lignin are distributed in andbetween the plant cells.

The number of repeat units bound together in a lignin agglomeration ina plant would give a measure of the molecular weight of natural lignin. Thisnumber tells how many aromatic alcohol units are connected to one another,with the only other bonds outside the network being bonds to pectins, hemi-cellulose, or cellulose residues. The best value for the lower limit for thisnumber in softwoods is approximately 60 with a weight average molecularweight[13] for the network fragment of 11,000. This value is probably lowsince the milling process used to recover the lignin may have broken bonds inthe network and formed lower molecular weight oligomers of the polyaro-matic. The actual structure of the three-dimensional network that is the ligninmolecule in the plant has long been assumed[14] to be a randomly linked, C9

lattice as would be produced by a free radical reaction of a resonance-stabilized, aromatic alcohol.[15,16] However, ultraviolet microprobe analysis

238 MEISTER



Table 2. Repeat Unit Bondings in Lignin


240 MEISTER

Table 4. Functional Groups Found in 100, C9 Repeat Units of Lignin

Functional Group

Spruce, Milled

Wood Lignin

Pine Lignin,

Kraft Process

Hydroxyl, –OHTotal: 120 120#4-phenolic 30 60

1,2-benzenediol — 12alkyl –OHa 90 48

Carboxylic acid, –CO2H 5 16

Aldehyde, –C¼OTotal: 20 15a-propyl 7 5b-propyla 10 10

g-propyl 3 —Phenylmethanol and etherNoncyclic 42 <6

Cyclicoxacyclopenteneb 11 3b–b, bi(cyclic ether)c 10 5

Ethene double bond, >C¼C<a-phenyl-b, g-ethene 7 þ

d

1,2-diphenylethene — 7

Other — þd

aCalculated by difference.bThese ring ethers are also called coumaran or benzofuran structures.[12]cSee structure g2 of Table 2.dDetected, not quantified.

Table 3. Number of Different Bonding Linkages Between 100Lignin Repeat Units

Type of

Lignin

Loblolly

Pine, MWL

Spruce,

Oxidation

Beech,

Thioacetolysis

Bondb–O–4 55 49–51 65

a–O–4 6–8b—5 16 9–15 6b—1 9 2 155—5 9 9.5 2.3

4–O–4 3 3.5 1.5b—b 2 2 5.5b—b* — — 2

a/g–O–g 10 — —a—b 11 — 2.5b—6, 6—5 2 4.5–5a 0a

1–O–4, 1—5 1–O–4, not seen

aCombined b—6, 6—5, 1–O–4, and 1—5 content.


of loblolly pine (Pinus taeda L.) thin sections indicates that the aromaticrings of sizable portions of the lignin are preferentially oriented in theplane of the cell wall.[17,18] These observations would suggest that lignin isnot randomly crosslinked into a naturally occurring, phenolic adhesive (SeeChapter 12) but is built up as an organized, binding structure in the plant.This view of lignin as a aromatic alcohol truss is supported by the presence ofvesicules[19–21] in zones of lignification in the cell. A vesicule could allow thepreparation of structured lignin in a specific sequence of enzymaticallymediated steps. Further work is needed to confirm if lignin is an ordered,network polymer in the plant; determine what fraction of the lignin is orderedin the loblolly pine; and confirm if this order is common in the rest of theplant kingdom.

B. Recovery of Lignin

For lignin to be used as the class of chemicals it is, it must be removedfrom the plant. Added to the diversity of repeat units and bonding patternswhich characterize natural lignin is the chemical alteration introduced byeach means of removing lignin from wood. Lignin recovery processeswhich extract lignin from wood change the chemical and functional groupcomposition of lignin and make this material extremely heterogeneous.

Methods for recovering lignin are the alkali process, the sulfite process,ball milling, enzymatic release, hydrochloric acid digestion, and organic sol-vent extraction. Alkali lignins are produced by the kraft[22] and soda[23]

methods for wood pulping. These processes are based on sodium sulfateplus sodium hydroxide or just sodium hydroxide, respectively. They havelow sulfur content (<1.6wt.%), contain sulfur contamination present asthioether linkages, and are water-insoluble, nonionic polymers of 2,000 to15,000, molecular weight. Over 20 million megagrams of kraft lignin areproduced in the United States each year.[4]

The sulfite process for separating lignin from plant biomass produces aclass of lignin derivatives called lignosulfonates by attacking the biomasswith an aqueous solution of sulfur dioxide and calcium, magnesium, ammo-nium or sodium base. Lignosulfonates contain approximately 6.5 weightpercent sulfur present as ionic sulfonate groups on the alkyl chains of thelignin but are also heavily contaminated with sugars, sugar acids, terpenes,lignans, and salts. These materials are very water-soluble and commonly havemolecular weights between 10,000 and 40,000. However, molecular weightsup to 150,000 have been obtained for high mass, isolated fractions.[24] Lessthan 1 teragram (1012 g) of lignosulfonate are produced in the United Stateseach year and production from sulfite pulping operations is declining year toyear. Environmental restrictions are putting the sulfite pulp mills that pro-duce lignosulfonates out of business. Lignosulfonates for modification or use



are being imported into the United States and pulp mills based on anthra-quinone digestion are beginning to produce lignosulfonates for sale.

A final extraction process that could become a commercial source oflow molecular weight lignins is The Repap Organosolv Process for removinglignin in hot ethanol.[25] A pilot plant to manufacture 10 megagrams of lignina day by this method has been operating in Nova Scotia[26] since 1992.However, the plant, which produced lignin with no sulfur content, anincreased frequency of alkyl groups, and molecular weights of the order of600 and 1,000,[27] has been scheduled for closure in 1997. This leaves onlyalkali lignin, sulfite lignin, and lignin byproduct of ethanol from biomassoperations available for bulk modification. All other lignins are producedby processes run on laboratory scale only.

Milled wood lignin (MWL) is produced by grinding wood in a rotary orvibratory ball mill. Lignin can be extracted from the resulting powder usingsolvents such as methylbenzene or 1,4-dioxacyclohexane.[28] Milling onlyreleases 60 weight percent or less of the lignin in wood, disrupts the morphol-ogy of lignin in wood, and may cause the formation of some functionalgroups on the produced lignin.[29] Despite these limitations, milling appearsto be an effective way of recovering lignin from plants with only slightalteration. Enzymes which hydrolyze polysaccharides can be used to digestplant fibers and release lignin. After digestion, the lignin is solubilized inethanol.[30] Extensive analytical studies support the idea that enzymaticallyproduced lignin has undergone no major modification in removal from plantmaterial.[31–35]

Acid hydrolysis of the polysaccharide portion of wood releases ligninbut also causes major condensation reactions[36] that remove many etherbonds in the lignin and replace them with carbon–carbon bonds. Thesereactions can be minimized by using 41 weight percent hydrochloric acid inplace of other mineral acids but some condensation reactions still occur.[37]

This is not an effective method by which to obtain unaltered lignin. On theother hand, lignin can be solvent extracted from wood at temperatures of175�C using solvent mixtures such as 50/50 (by volume) water/1,4-dioxacy-clohexane.[38] Changes in lignin under these conditions appear to be minor.

C. Uses of Extracted Lignin

Outside of the plant, lignin is useful as a component in the diet of rumi-nant mammals; a soil property improver in the process of natural decay; and asource of peat, lignite, and coal. As a commodity forest product, however, ithas a long history as a waste product for which functional uses are sought. Thismeans that when a woody plant is rendered for its chemical content, about 25percent of the dry weight of the plant has little or no economic value. For thisreason, the most common use of lignin from pulping operations or ethanol

242 MEISTER


from biomass processes is as a fuel. The lignin produced is burned for its26.5 kJ/g of energy, 40 percent of the solar energy[4] stored by the plant.

II. MODIFICATION OF LIGNIN

Other uses for lignin can be broken into three large groups, two of whichrequire chemicalmodificationof thebiopolymer.Thesegroupsare, (1)breakinglignin down into component aromatics or repeat units, (2) using the biopoly-mer as extracted from the wood, or (3) adding to the lignin biopolymer,treating it as a starting material to be built upon to make useful materials.

A. Decomposition of Lignin

Pyrolysis

The decomposition of lignin into aromatic repeat units is a long prac-ticed art which reached its zenith around 1800 A.D. Production of chemicalsby wood pyrolysis was extensively practiced until, between 1750 and 1850A.D., coal slowly displaced wood as the major chemical source available toman. Wood is usually pyrolyzed at 260 to 410�C and lignin at 300 to 440�C toproduce 50 weight percent charcoal, 10 to 15 percent tar, and lesser amountsof 2-propanone, ethanoic acid, and methanol.[39–41] The tar is often calledwood creosote, and is a complex mixture of substituted phenols and aro-matics. It contains phenol, 2- and 4-methylphenol, 2,4-dimethylphenol,2-methoxyphenol, 4-methyl-2-methoxyphenol, and 4-ethyl-2-methoxyphe-nol.[42] Modifications of pyrolysis have been used to convert up to 23weight percent of starting lignin to ethyne, H–C�C–H, by rapid heatingof the lignin to 1,200�C and rapid quenching of the produced gas.[43,44]

Ethene, H–C¼C–H, is produced in lesser amounts but this synthesis hassignificant potential to produce the largest commodity, organic chemicalmade in the world today, ethene, for use in polymerization and synthesis.This process is not commercialized as of 1997.

Lower temperature pyrolysis has led to lignin-based surfactants. A groupat Texaco, Inc., has shown that after retorting the lignin, the phenols can beethoxylated to form nonionic surfactants that are both inexpensive and highlyuseful in industrial processes such as oil recovery.[45–47] Alternatively, pyrolysisin a reducing atmosphere of hydrogen can be used to make cresylic acid inyields of 35[48] to 40[49] weight percent of the lignin charged to the reactor.Cresylic acid is a mixture of alkyl phenols that boils between 180 and 240�C.Freudenberg and Adam[50] were able to convert 35 weight percent of theirstarting lignin to a mixture of 10, low concentration, identified phenols anda large fraction of higher molecular weight, unidentified phenolic products



using hydrogen and catalysts to promote the decomposition. Yields have sincebeen improved[51] on reactions on sulfite lignin, kraft lignin, soda lignin, andespecially from organosolv lignin. The problem of using such a mixture forfurther synthesis of commercial products is the chemical complexity of themixture. The majority of all chemicals are used for polymer synthesis andmust be 98%þ pure. The purification costs for such complex phenol mixtureswill limit their application until catalysts are found that produce 75%þ,single-compound products directly from the decomposition reaction.

Reduction with Hydrogen

Hydrogenation over Raney nickel is the most common laboratoryreduction of the structure of lignin into monomeric units. The best conditionsfor high yields of aromatic products are 160 to 170�C in 50/50 by volumeaqueous: 1,4-dioxacyclohexane for four to five hours. This reaction was usedto provide strong support for the C9 structure of lignin.[52] Longer times orhigher temperatures will hydrogenate the aromatic rings of the reaction pro-duct mixture and/or remove higher levels of methoxyl groups.

Hydrogenation has been used to convert lignin into a liquid that mightbe an additive or component of fuels. This or the production of alkyl phenolsis the goal of all efforts to create a commercial process to hydrogenate lignininto useful products. Two research groups, Inventa A. G. fur Forschung andPatent Verwertung[53,54] and Noguchi Institute of Japan[55] have developedmethods to ‘‘crack’’ lignin to alkanes and phenols. Of these two efforts, thework done in Japan was carried closest to commercialization by continuedresearch by Crown Zellerbach Corporation of the United States.[56] TheNoguchi process uses a catalyst suspended in phenol or heavy oil under10.0 to 20.0MPa of hydrogen pressure at 370 to 430�C to convert 21weight percent of the original lignin to monophenols. Subsequent work atCrown Zellerbach increased this yield to 38 weight percent. The product is,however, a cresylic acid and the preparation of any pure compound by thisprocess is still highly uneconomical because of costs of separation. Crudephenolic mixtures produced this way are more economical[57] and will prob-ably be commercialized first.

Oxidation

The oxidation of lignin[58–60] in oxygen–calcium oxide–sodium carbo-nate, nitric acid, or chrome oxide/ethanoic acid is known to produce 4-hydroxy-3-methoxybenzaldehyde (vanillin, [121-33-5], equation 1) in amountsup to 9 weight percent of the lignin. Optimization of the chrome oxide/etha-noic acid method is claimed[61] to produce 4-hydroxy-3-methoxybenzaldehyde

244 MEISTER


in 33 weight percent yield. This is one of the few commercial utilizations ofdegraded lignin. This food additive and scent is currently produced fromlignosulfonates by Borregaard in Norway and Sanyo in Japan. Organosulfurchemicals are also produced by retorting lignosulfonates. Dimethylsulfide andmethylthiol are produced by this reaction[62] with the dimethylsulfide beingfurther oxidized to dimethylsulfoxide (DMSO) for use as a solvent.

This decomposition technology for lignin is emphasized in the literatureas a major means of lignin utilization but it makes little thermodynamic oreconomic sense. With two-fifths of the plants absorbed energy being used tomake the one quarter of its dry mass that is lignin, lignin represents a largeinvestment of biochemical effort by the plant. Reducing this macromolecule toCO2 or aromatic fragments destroys much of that investment. Keeping themolecule as extracted from the wood or adding to it are thermodynamicallypreferred approaches to lignin utilization but these approaches face signifi-cant, practical barriers. Lignin is a deep brown, fluffy powder which can bethermoformed into hard, brittle solids when heated above its glass transitiontemperature. This transition from a brittle, amorphous solid to a ductile ther-moplastic occurs when lignin is heated above 90�C when it contains 13 weightpercent moisture or up to 195�C when it contains 0 weight percent moisture.Lignin thus changes its properties sharply when relative humidity changes and,once thermoformed, is a brittle glass at common application temperatures, 20to 25�C. Further, the deep brown color is a product of free radicals in the ligninwhich, if bleached away, will slowly reform by thermal and photo-absorptionmechanisms. These radicals will then react with atmospheric oxygen.[63] Thisbehavior can be a major drawback for applications of lignin to consumerproducts. Added to these difficulties are the variations in lignin produced bydifferent sources and extraction processes and the chemical complexity alreadydescribed. Despite these difficulties, the enormous amount of lignin availableat low cost has driven numerous efforts to utilize it.

B. Using Lignin as Extracted from the Plant

Using lignin in the form obtained when it is extracted from the plant doesnot mean that the lignin exists in the application just the way it does whenwithdrawn from the plant. It means that the lignin enters the applicationprocess as a reagent and is often reacted with other components of the


(1)


formulation as product is produced. This is definitely the case in the largestcurrent application for unaltered lignin, its use as a replacement for phenol inphenol–methanal (formaldehyde) adhesives.

1. Lignin in Phenol–Methanal Adhesives

Phenol–methanal adhesives are currently used in about one tenth of allplywood and particle board. The binding technology represented by these‘‘Bakelite’’ resins would be more widely applied if the reagents, particularlythe phenol, were cheaper. Using lignin in place of phenol will sharply reducethe cost of the binder.[64] Unfortunately, lignin is not structurally equivalentto phenol. Phenol has 5 hydrogen sites on the aromatic ring and no non-hydrogen, ortho or para substituents around the hydroxyl group. Kraft pinelignin has only 72 phenolic hydroxyl groups per 100 C9 repeat units and 48aliphatic hydroxyl groups per 100 C9 repeat units.

[65] For virtually all ligninphenolic hydroxyl groups, the aromatic ring is para substituted by the propylchain of the 1-propylphen-4-ol (coumaryl) structural unit. In softwoods, thehydroxyl group is often next to a methoxyl group in the number 3 position onthe ring while in hardwoods, it is completely ortho substituted by methoxylgroups. This leaves only the meta position open for reactions on the aromaticring of a lignin phenol. The implications of this structure on lignin reactivityin phenol/methanal crosslinking polymerizations can be seen from themechanism of the phenol–methanal reaction, shown in Figure 2.

In crosslinking with methanal, an aromatic hydroxyl group ionizes toform ortho [2,6] and para [4] anionic sites through which to react with apositively charged, methylene group. Lignin has most sites ortho and para toits aromatic hydroxyl groups blocked by organic, functional groups. This isthe reason why lignin reacts slower with methanal than does phenol and whylignin can only be used to replace between 40 and 70 weight percent of thephenol in an adhesive formulation. Lignin simply has too few, highly reactivesites to create a high density of crosslinks without at least 30 weight percentphenol being present. The rates of reactions determined by Dr. DouglasGardner are compared between hardwood, steam-exploded lignin; softwood,kraft lignin; and phenol[66] in Table 5-A. The rate of the hardwood lignin/methanal reaction is, as would be expected from the dimethoxyl substitutionon the ring, only 46 percent as fast as phenol at 30�C and only 12 percent as fastat 60�C. Softwood lignin has, under the same reaction conditions, a rate that is68 percent as fast as phenol at 30�C and 14 percent as fast at 60�C. The openortho positions on softwood lignin obviously allows the softwood lignin toreact more readily with methanal and should lead to more extensive cross-linking of the softwood lignin as compared to the hardwood lignin. This isconfirmed by the data of Table 5-B. Here the number of methanal groupsadded to each C9 repeat unit of the two lignins is determined by three different

246 MEISTER



Figure 2. Mechanism of the phenol–methanal reaction.

Table 5. Kinetic Parameters for the Phenol/Methanal or Lignin/Methanal Reactions Rate

Constant

k(10�2 M�1min�1)Temperature, �C Pre-exponential

FactorA (min�1)

ActivationEnergy

Ea (kJ/mole)A: Component 30 40 50 60

Phenol 2.17 6.65 24.9 79.4 5.8� 1015 101.3

Kraft lignin 1.44 1.83 5.50 11.3 3.25� 108 60.7Steam-exploded lignin 0.98 1.60 4.37 9.44 1.32� 109 64.9

B: Degree of Methanal (HCHO) Substitution per C9 Unit of Lignin by Various Methodsa

Degree of Methanal Substitution

Method Kraft lignin Steam-exploded lignin

HCHO uptake 0.39 0.251H-NMRb 0.38 (0.35) 0.18 (0.15–0.20)13C-NMR 0.42 0.27

aData from Reference 49. Formula for the rate of reaction, k, is K¼A * e�(Ea/RT).bValues in parenthesis from Reference 66.


methods. The data show that hardwood lignin only reacts with 0.23 methanalunits per C9 while softwood lignin reacts with 0.40 methanal units.

With adjustments in composition to compensate for the chemical fea-tures of each aromatic hydroxyl source, a wood binder formulated withhardwood lignin, softwood lignin, or phenol[67] will be deemed highly effec-tive if it can be: (1) formulated at lower cost, (2) applied with conventionalequipment, (3) reacted under the same process conditions, and (4) an adhe-sive that is so strong that wood parts formed with it fail in the wood phasemost of the time and not in the adhesive phase.

Adams and Schoenherr[68] achieved most of these bench marks by for-mulating an adhesive consisting of a 40 weight percent solids solution of kraftlignin in phenol/methanal/sodium hydroxide. This fluid had a viscosity of10 Pa�s and thus was a very thick and energy-consuming adhesive to spread.However, when this binder was used in the manufacture of three ply panels ofDouglas fir, destructive testing of the plywood showed failure in the woodphase 92 percent of the time. A more easily applied adhesive can be preparedby blending 37 weight percent lignin in phenol/methanal/sodium hydroxide[69]

and only partially crosslinking the mixture. This blend has a viscosity of0.46 Pa�s but sets into an adhesive layer under 1.2MPa pressure for 6minutesat 140�C that breaks in the wood phase 94 percent of the time. These data showthat, despite its chemical deficiencies, lignin is a functional replacement formuch of the phenol in ‘‘Bakelite’’ adhesives. Appropriately blended, lignin-containing adhesives will, under common treatment conditions for bindingplywood or particle board, set into an adhesive that is stronger than thewood[70] and therefore, capable of producing bonds that will be the last partof the structure to fail. As of 1991, lignin constitutes 17 percent of the resinsolids in phenol-methanal adhesives used to make exterior-grade plywood.[71]

The lignin used is generally kraft lignin. This technology is providing a smallbut stable market for the lignin fraction of wood. Growth areas of adhesivebound, wood composites; oriented strandboard; oriented waferboard;medium density fiberboard; and laminated veneer lumber will provide a grow-ing market for lignin as a partial phenol replacement. Organosolv lignin is alsobeing used as a resin extender for high performance markets of phenol/methanal resins. Organosolv lignins are used in brake pad and foundrybinder resin formulations of phenol and methanal.

2. Lignin Photostabilizers

DePaoli and Furlan have studied the use of lignin from sugar canebagasse as a photostabilizer for butadiene rubber.[73] The logic for this appli-cation is that the phenol-containing repeat units of lignin have structuresproximate to compounds currently known to act as photostabilizers inrubber. Hindered alkyl phenols with long chains para to the hydroxyl

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group, structure 1 in Figure 3, are known to inhibit photo-induced bondcleavage in rubbers by forming stable phenoxyl radicals.[74] This stable, hin-dered radical prohibits the formation of a peroxide radical on the rubberbackbone, thereby preserving the structural integrity of the elastomer.

Bagasse lignin contains approximately 2 weight percent of structures2 and 3 with the frequency ratio between them being 4 of structure 2 to 1 ofstructure 3. These structures are not only similar to those of common photo-stabilizers, these repeat units appear in a lignin chain. Polymer-bound,hindered phenols are more effective than free, molecular phenols becausethe polymer chain restricts migration and dimerization of the formed radi-cals.[75] Bagasse lignin was tested as a mixture of 90 weight percent lignin and10 weight percent N0,N-bis(1-ethyl-3-methylpentyl)-p-phenylenediamine inbutadiene rubber. Diamines are commonly used in conjunction with hinderedphenols to inhibit photodegradation in rubber. Rubber samples containingthe lignin blend and commercial stabilizers were irradiated at 350 20 nm inair and rates of photodegradation were measured. The data showed that 0.37weight percent diamine could be replaced by 2.25 weight percent lignin with-out affecting photostability of the blend. The lignin stabilized the rubber byboth capturing radicals and absorbing the ultraviolet light directly. The effecton physical properties of compounding butadiene rubber with over 2 weightpercent lignin was not investigated. While these data are positive, they fail toverify that the photostabilized rubber possesses all of the application proper-ties that the rubber must have to be used as a commercial product. Lignin haslong been known to be an excellent reinforcing agent for rubber if the low


Figure 3. Base structure of photoinhibitor (1) and common bagasse lignin repeat units (2,3).


molecular weight lignin and non-lignin constituents of industrial byproductlignin are removed.[76] If contaminants are not removed from industriallignin, they promote clustering of the lignin particles,[77] lowering of the soft-ening temperature of the rubber, and lowering of the reinforcing ability of thelignin. Unfortunately, the bagasse lignin used in the photostability studieswas only 93 weight percent lignin and was not fractionated to remove lowmolecular weight portions of the blend. Because of these deficiencies, thephotostability data are of limited significance.

3. Lignin in Electrodes and Lignin Electrodes

Lignosulfonates are used in every lead/acid battery to inhibit crystalliza-tion of the negative electrode during recharging of the battery.[78] Without theaddition of a purified, modified lignosulfonate to the negative terminal of thebattery, the battery would fail to charge after only a few discharge cycles.[79]

This lignosulfonate is commonly called an ‘‘expander’’ and is thought to con-trol the formation of lead sulfate[80] on the surface of the sponge lead in thenegative terminal.

Lignin pyrolyzed at 700�C under nitrogen forms a cohesive, conductingsolid which can act as an electrode in a storage battery. This modification todehydrogenate and deoxygenate the lignin forms a charcoal with the capacityto absorb or donate 6mmoles of electrons per gram.[81] Batteries have beenformed from these electrodes and the cells have produced 45W-hr/kg perelectrode at 70 percent efficiency (charge recovered/charge put in). While thewatt-hours/kg rating of these electrodes is about two thirds of the value of alead oxide plate, the lignin-based electrodes polarize rapidly and suffer arapid drop in discharge voltage.[82] These two performance properties workagainst effective use of the electrodes. Further, the redox capacity of carbonstructures is quite limited when compared to metals so the utility of thismodification has yet to be verified. However, since the internal structureand composition of these electrodes are unknown and, under current tech-nology, controlled solely by pyrolysis conditions, there is extensive room forimprovement of these biomass electrodes. The drive to improve these elec-trodes will be promoted by the fact that carbon is only 18 percent as heavy aslead, the common electrode for transportation batteries.

4. Construction Binders

A limited amount of lignin is used as the binder for ‘‘glass wool’’ build-ing insulation. It is applied to the hot glass as the ammonium salt solidrecovered from the kraft paper production process and allows the glassfibers to bind to one another when the fiber pads are spun or formed.

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5. Food Additives

A possible use for lignin in the future is the addition of alkali lignins topet and human food as a roughage or fiber source.[83–85] Extensive researchhas shown that high dietary fiber correlates with low incidents of coloncancer.[86,87] The lignin in biomass is known to be a significant source ofthis cancer protection,[88] possibly producing this effect by trapping free radi-cals while in the colon.[89,90] If further work shows no adverse effects fromlignin, it may be added to food to increase long term, public health.

6. Fat Purification

Small amounts of ‘‘free acid lignin’’, a kraft lignin with its anionicgroups neutralized with hydrogen ions, are used to purify recycled animalfat. The ‘‘yellow grease’’ from food preparation is purified by filtrationthrough the lignin and the contaminated lignin is sold for use in animal feed.

C. Adding to the Lignin Biopolymer

The reactions used to modify polymeric lignin are:

. Alkylation and dealkylation . Sulfomethylation

. Methylolation . Sulfonation

. Amination . Nitroxide formation

. Carboxylation and acylation . Silylation

. Halogenation and nitration . Phosphorylation

. Hydrogenolysis . Grafting and oxyalkylation

. Oxidation and reduction

A large number of lab scale, lignin modifications were performed solelyto determine the structure of lignin. Most of this work was done before 1945and represents a ‘‘classic’’ approach to the verification of structure by synthe-sis of an identifiable adduct or derivative of the structure being sought. Thereactions run to verify structure were: formation of ethanoate esters to iden-tify hydroxyl groups, capping of hydroxyl groups with dimethylsulfate toform methoxyl groups, demethylating methoxyl groups with hydrogeniodide to form hydroxyl groups, oxidation with nitric acid to decomposeand/or add to double bonds with halogens, and reacting with copper[1]

oxide to identify aldehydes.The investigation of flax lignin by Powell and Whittaker[91] is a strong

example of many efforts in this type of search for structure through modifica-tion.[92–97] While these efforts did identify the C9 repeat unit of lignin, theessential goal of this work to identify a repeat unit structure common to alllignins is now known to be spurious. The work did help to establish that



different species and plant parts have lignins containing different amounts offunctional groups and different repeat unit bonding. Laboratory-scale mod-ifications of lignin will be cited repeatedly in the discussion of modificationwhich follows.

1. Alkylation

The three methods of alkylating lignin are:

a. reaction with diazoalkanes,b. reaction with alcohols in the presence of a catalyst, usually hydro-

chloric acid,c. use of alkylsulfates and sodium hydroxide.

The sites of alkylation are the oxygen atoms of the hydroxyl, carbonyl, andcarboxyl groups in lignin. Reactions (a) and (b) are selective with diazoalkanes(a) reacting under anhydrous conditions to alkylate, chiefly, the slightly acidichydroxyl groups[98] of the phenolic, enolic,[99–102] and carboxylic[103–107] unitsto form ethers. The diazoalkane reaction with carboxylic acids only occurs insolvents in which the acid is deprotonated to an enolate anion. The RN2

reactions are shown in equation 2.A catalyzed alcohol reaction (b) on lignin takes place at a-hydroxyke-

tones, carbonyl, and carboxyl groups. The reactions give different productsand are shown in equation 3. Since alcohol/acid alkylation does not alkylatethe aromatic hydroxyl groups,[108] alkyllignins from these reactions havesharply different solubility and physical properties than those from diazoalk-

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ane alkylation. They are, for example, soluble in dilute, aqueous sodiumhydroxide and are generallymore hydrophilic than diazoalkane-treated lignins.

The use of dialkylsulfate, R2SO4, and base to alkylate lignin producesa product in which the aromatic and primary and secondary, aliphaticalcohols[109,110] are alkylated. Some tertiary alcohol groups may escapealkylation.[111] These products are extensively hydrophobic since almostall of the acidic alcohol groups are now capped as ethers and all carboxylicacid groups have been converted to esters. Unless other ionic functionalgroups are present, these products dissolve only in nonpolar or organicsolvents and have the physical properties of a thermoplastic.

Dealkylation is a common byproduct of a number of reactions on ligninand will be treated under those substitution reactions.

2. Methylolation

The addition of a methylol group, –CH2OH, is a result of the additionof methanal, H2C¼O, to the aromatic rings of the polymer. This reactionhas been investigated as a stepping stone to the use of lignin in place ofphenol[112,113] in phenol/methanal resins (See Chapter 12). As detailed inSection II.B.1, the reaction of methanal on lignin is hindered by the presenceof ortho [#3-position] methoxyl groups and para [#1-position] alkyl groupson the only aromatic rings that will engage in this addition reaction, those



with a free phenolic group. These phenomena are illustrated in Figure 2, themechanism of the methanolylation.

The level of methanolyl group addition depends on reaction pH and thesource and extraction process used to recover the lignin.Morton andAdler[114]

show that under alkaline (pH>7) conditions, milled wood spruce lignin takesup 0.15 –CH2–OH groups per methoxyl group while kraft pine lignin takes up0.5 –CH2–OH groups per methoxyl group. Without free phenol present, Allanand Halabisky[115] claim that this reaction does not lead to the increase of thelimiting viscosity number, [�], of the lignin during reactions to extensivelymethanolate lignosulfonates. The failure to increase [�] would mean that themolecular weight of the lignin sample is stable and that intermolecular methy-lene bridges, shown in equation 4, fail to form. In equation 4, R3 andR4 are thelinkages to the rest of two distinct, lignin molecules. However, both researchand process work at the Georgia Pacific Corporation[116] proves that ligno-sulfonates can be crosslinked into a solid by methanal under both acid andalkaline pH. It would appear likely, therefore, that the studies by Allan andHalabisky[115] are wrong and methylene bridges between lignosulfonate mole-cules can be made in alkaline solutions by methanal.

3. Amination

Amination, the creation of a –NR02 group on lignin where R0

¼H or anorganic unit, is a reaction used to identify structures and create emulsion sta-bilizers. The reaction is used to prove the presence of aromatic nitro groups,–NO2, in lignins treated with nitric acid.[117] The nitrolignin has been treatedthrough reduction with Raney nickel,[118] zinc and acetic acid,[119] and sodiumamalgam[120] to form an amine; the amine has been converted to a diazo group,¼ N ¼ N; and the diazo group has been reacted with substituted phenols toform dyes. There is no commercial utilization of the dyes that can be formed bythis sequence of reactions but the formation of diazo dyes does verify thepresence of nitro groups in the material treated to form the amine.

Reactions have been run to make amine-group-containing lignin[121–123]

and free, amine groups which can accept a proton have been quantified[118] inthe lignin products. The use of amination to form commercial products oflignin requires that beta amino ketones be formed on the aliphatic chains inthe polyalkylaromatic. The acid catalyzed addition of methanal and a primaryor secondary amine (Mannach reaction[124]) is conducted on alkali lignin inaqueous suspension. The reaction is shown in equation 5. In a typical synthesisreaction[125,126] with all percents by weight, 19 percent alkali lignin, 77 percentwater, 2.3 percent dimethylamine hydrochloride, and 1.4 percent methanal areslurried at a pH of 2 to 3 and heated to 80 to 85�C for 4 hours. The reactionmixture is diluted with an equal volume of water and pH is reduced below 1before the mixture is boiled briefly. The product is a water soluble, beta ketoamine derivative of lignin.

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The resulting lignin with beta keto amine groups in it is used in con-centrations of 0.2 to 2.0 weight percent to form thermally stable, aqueousemulsions of 55 to 65 percent asphalt for use in road repair. The pH of theemulsion is between 9.5 and 10.5. It can be used as a wetting binder witheither siliceous or limestone aggregates to form bituminous slurry seal coat-ing in the stabilization treatment of paving bases or can be used alone in themanufacture of floor mastics.

4. Carboxylation and Acylation

Carboxyl and acyl groups; a and b, respectively in equation 6, arecommon components of lignin, as shown by the data of Table 4. The fre-quency of both groups is increased by oxidation of lignin and strongerhydrophilic and polyelectrolyte behavior can be produced in lignin bythese reactions. Typical oxidations that produce sharp increases in the diva-lent oxygen bond content (oxo units) of lignin are treatments with chlorine,



nitric acid, and peroxides. These treatments are usually associated withextensive dealkylation. Shorygina treated hydrolysis and hydrochloric acidlignin with gaseous chlorine in 10 percent, aqueous hydrochloric acid tointroduce approximately two chlorines per C9 repeat unit in the lignin.However, the carboxyl content of the lignin increased by a factor of between10 and 20 and 66 percent of all methoxyl groups[127] were removed from thelignin. Similarly, after treating fir hydrochloric acid lignin with nitric acid intetrachloromethane, Shorygina found a 10 to 20 fold increase in the carbonylcontent of the lignin[128] and numerous authors have documented a drop inmethoxyl content of nitric acid treated lignin.[119,120,127]

By use of a peroxide under alkaline conditions, Bailey and Dence[129]

produced a sharp increase in the carbonyl content of lignin but also induced a16 to 17 percent loss of methoxyl groups as methanol. Further, production ofethanedioic, 1,3-propanedioic, 1,4-butenedioic, and 2-methoxy-1,4-butane-dioic acids verified extensive degradation of the lignin. This degradationoccurred at phenolic units, ether bonds, carbonyl units, and phenylprop-2-enal units in the lignin, which is structure c in equation 6. In aqueous, alka-line peroxides,[130] hydroperoxy anion, HOO�, attacks phenylprop-2-enal toform phenylmethanal and a 2-hydroxyethanal fragment. Such a reaction onphenylprop-2-enal units in lignin would produce a significant increase incarbonyl group content.

As of 1997, only carboxyl and acyl group formation on lignin by chlorineor alkaline peroxide has become a commercial process[131] and that only as aside effect of chlorine and peroxide bleaching of pulp for paper and packagingapplications. The use of chlorine in this process is declining because it alsoproduces chlorinated, organic byproducts. The critical reaction that thesebleaching operations produce is not the formation of oxo groups, >C¼O,but the removal of the approximately 1018 free radicals per gram of lignin[132]

in mechanically or chemically rendered, wood pulp. Terminating these freeradicals removes the intense, broad absorbance band centered at 284 nm thatgives a brown color to the pulp. The removal of these free radical absorbersthereby brightens (whitens) the pulp. There is some research underway tocreate oxo groups in lignin by reaction with ozone, O3. Ozonation to inducecarbonyl or acyl groups is not a commercial process in 1997, however.

5. Halogenation and Nitration

One of the simplest addition reactions to lignin is the addition of a halideto the alkylaromatic backbone of lignin. The reaction is given in Figure 4. Thereaction is run by bubbling chlorine into spent, aqueous pulping liquor andfollowing that additionwith additions of bromine and chlorine.[133] The weightpercent halide in the product is raised to between 20 and 40 percent. Since thehalogenated alkylaromatic is hydrophobic, it precipitates. The molecular

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weight of several halolignins has been determined by collegative methods(cryoscopically[134]; ebulliometrically[127]) and found to be between 1,000and 6,000. The halogination reaction is accompanied by extensive dealkylationand breaking of ether links in the lignin.[127,135] This second reaction wouldsharply lower the molecular weight of the treated lignin.

Previously, the halogenated lignin has been recovered as a fire retardantfor use in building materials and consumer goods. By 1997 standards, how-ever, this halogenated organic presents very significant environmental prob-lems for virtually any application and it is very improbable that thischemistry will continue to be used today or in the future. Indeed, severallignin producers specifically avoid deliberate or unintended halogenation ofany wood product just to avoid environmental problems that these com-pounds could produce.[136]

Nitration is another fairly simple reaction that is typically run in non-aqueous solvents to nitrate lignin in wood meal and run in suspension orsolution for isolated lignins.[137,138] Typical nitrating agents are nitric acid inconcentrated ethanoic acid[139–141] and nitric acid with sulfuric acid or oleum(fuming sulfuric acid). The resulting amorphous, yellow to brown powdershave molecular weights of 600 to 2,000 and are extensively dealkylated anddegraded.[142–145] Nitrogen content of the product may be as high as 6.7 weightpercent.[146] The nitrogen-containing structures that are produced in lignin areshown in equation 7. The nitro groups, 7a., are usually attached to aromaticrings and often constitute 70 to 80 percent of the nitrogen.[139] The nitrosogroups, 7b., are uncommon and seldom represent more than 2 percent of the


Figure 4. Formation of halolignin.


organically bound nitrogen.[147] In the presence of excess nitric acid[148] or inacidic, anhydrous, nitrating conditions,[142] nitrate ester groups, 7c., canbe added to lignin. There is no commercial application for nitrated ligninsas of 1997.

6. Hydrogenolysis

The hydrogen in lignin is increased by a spectrum of reactions rangingfrom catalytic reaction with hydrogen gas, H2 (g), to reaction with sodiumborohydride, NaBH4. These reactions are conducted for one of two reasons, todetermine the structure of lignin or to convert lignin to a liquid. Hydrogenaddition to determine structure is a research process and has been conductedon a lab scale for 70 years. Hydrogen addition to liquefy lignin is an effort toconvert lignin to products that could be added to fuels or be a petroleumreplacement. It is covered under the topic ‘‘Reduction with Hydrogen’’ inSection II.A. Here, a representative group of hydrogenation reactions whichprovide structural insight and specific alterations in lignin will be described.For a more detailed analysis of hydrogen addition to lignin, see Reference [150].

Typical hydrogen reductions for laboratory structure evaluation wouldbe reaction of lignin with lithium aluminum hydride; sodium borohydride; orhydrogen over a catalyst, such as Raney nickel in an aqueous-organic solventmixture. Lithium aluminum hydride is a strong but selective hydrogenatingagent and will reduce aldehydes, ketones, acids, and esters to alcohols with-out reacting with carbon–carbon double bonds in the lignin. It is reacted withlignin in anhydrous ether at room to elevated temperature. Its reaction (8a.)and that of the other lab hydrogenating agents are shown in equation 8.Sodium borohydride is a weaker hydrogenating agent and will reduce alde-hydes and ketones to alcohols without reacting with acids, esters, andcarbon–carbon double bonds in the lignin. It is reacted with lignin in alkalinesolution at room to elevated temperature. Since both ring and aliphaticcarbon–carbon double bonds and ether bonds are spared reaction withthese two agents, the three dimensional structure of lignin is not effectedby the reduction and important structural features, such as the frequencyof conjugated, carbonyl bondings, can be measured. Adler and Marton[151]

used this feature of the chemistry of these hydrogenating agents to calculatethe frequency of carbonyl groups adjacent to aromatic and ethene groups.This data is shown in Table 6.

7. Oxidation and Reduction

Oxidation, the loss of electrons, and reduction, the gain of electrons, arelabels for two major classes of reactions. The reactions described here that

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are oxidations are alkylation, sulfomethylation, methylolation, sulfonation,amination, nitroxide formation, carboxylation, acylation, halogenation,nitration, phosphorylation, oxyalkylation, and, generally, grafting. The reac-tions that are reductions are dealkylation, silylation, and hydrogenolysis.


Table 6. Estimated Conjugated Carbonyl Groups inSpruce, Milled Wood Lignin

Functional Group Structure

Percent of all C9

with This Structure

<1

3

<1

5 to 6


8. Sulfomethylation and Sulfonation

Sulfomethylation and sulfonation are reactions to add the methylenesulfonate, –CH2SO

�3 , and sulfonate, –SO�

3 , group, respectively, to lignin.The addition of methylene sulfonate groups to lignin has been proposed as ameans of improving the tanning capacity of lignin.[152] The reaction is runwith equal moles of methanal, alkali metal sulfite salt, and reactable phenolicrepeat units in water under neutral to basic pH and 100�C temperature.Repeat units of lignin which can react must have either a hydroxyl groupon the aromatic ring or a carbonyl group alpha to a double bond or thearomatic ring. These conjugated carbonyl structures are shown as structuresa and d of Table 6. These repeat units are common in lignin and producepolyelectrolyte products with some repeat units in the lignin having thestructures shown in equation 9a for aromatic hydroxyl groups and 9b forconjugated carbonyl groups, respectively. Sulfomethylation, as opposed tosulfonation, is only used when an increased content of sulfonate groups isneeded in the lignin product. It is applied to kraft lignins by the WestvacoCorporation to form dye dispersants that are marketed under the Reax tradename. The presence of base in the reaction will lower the yield of sulfonategroups and increase the number of hydroxymethyl groups[153,154] added tothe lignin product.

Lignin sulfonation is the most studied reaction in lignin chemistry sinceit was the earliest and cheapest way to make commodity, acidic cellulose forpaper. The sulfite process, described in Section I.B, was patented[155] in 1866.Research on the reaction began in the late 1880’s and produced the conclu-sion, by 1892, that the process incorporated sulfur into the lignin and thatsulfur was present as sulfonic acid groups.[156–158] Holmberg showed in 1935that lignosulfonates[159,160] were made by reactions at hydroxyl groups alpha

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to aromatic rings, as shown in equation 10a, with these reactions beingpromoted by free phenolic hydroxyl groups, R4¼H. Sulfonation occursrapidly at ether linkages alpha to a phenolic ring, equation 10b; hydroxylgroups gamma to a phenyl carbonyl pair, equation 10c; and slowly at alpha-beta diethers off the aromatic ring, equation 10d. The lignosulfonates are themost extensively used lignin product with a teragram (1012 g¼ 109 kg) of thismaterial sold in the United States every year. Industries which use lignosul-fonates are well drilling; cement manufacture, formulation, and pouring;ceramics manufacturing; and construction materials.

Both lignins and lignosulfonates are used to prepare oil well drillingmuds but lignosulfonates control the majority of the market. The lignosul-fonate must disperse and hydrate the clay in the mud but not perform theseactions on the formation being drilled. The drilled solid must be maintainedas particles and suspended in the mud until it can be filtered out on thesurface before the mud is recycled into the well.[161] This requires that themud be a gel when static but a mobile fluid when the drill bit is moving. Thismeans that the lignosulfonate must make the mud thixotropic (shear thin-ning). The mud must be stable to all formation contaminates and brines andat temperatures up to 200�C. Lignosulfonates oxidized by chromate ordichromate ion or mixtures of these ions and iron salts perform all of thesefunctions cost effectively.[162] The metal ions oxidize the lignosulfonate mole-cules to produce more carbonyl and acyl groups and to provide di- and tri-valent cations to temporarily crosslink carboxylate anions into molecularbridges to create transitory molecular weight increases in the polymer inthe static mud. These materials are marketed as chrome or ferrochromelignosulfonates.

The biggest industrial user of lignosulfonates or sulfonated lignins is thecement industry, which uses these compounds as grinding aids, air entrainingagents, concrete additives, and grout hydrating agents to control the settingand hydration rate of cement. When cement is produced from a kiln, the‘‘clinker’’ must be ground into the fine powder that will be sold.Lignosulfonate and sulfonated kraft lignin are added to the clinker duringgrinding to inhibit reagglomeration of the ground particles. The polyaro-matic, sulfonated polymer apparently reacts with bonds broken in the grind-ing to inhibit rebonding between particles. As little as 200 ppm by weight canprohibit reagglomeration of the cement.[163,164]

When cement is poured, it must contain a certain amount of air toprevent spalling (large particle formation) and cracking. The air bubblesact as internal interfaces to inhibit the propagation of cracks.Lignosulfonates are used as air entraining agents to form these bubblesand increase[165] the strength of finished concrete. Adding as little as 0.3weight percent of a mixture of lignosulfonate and cement to concrete andcement as it is being mixed or placed allows a reduction in water use in thecement of up to 20 percent. The cement has a higher compressive strength



and durability than non-lignosulfonated cement for the first several yearsafter pouring,[166] so the additive induces quick strength and durability inthe solid.

Sulfonated lignins are dispersants for the preparation of pastes andslurries of solids in water. Since the typical lignosulfonate contains onlyone sulfonate group per two to three C9 repeat units, the molecule is asurfactant structure with a 15 to 25 carbon atom, hydrophobic section.This alkylaromatic section of the molecule will plate onto solid surfaces ina slurry and by so doing, will impart a negative charge to the particles. Thelike-charged particles of the slurry or paste now become electrostaticallyrepulsive to one another and the suspension is not only more stable, it has

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significantly lower viscosity.[167] Lignosulfonates for this application havemolecular weights of between 10,000 to 40,000[168] and are most commonlyused in the ceramics and clay pottery industry at concentrations of 0.05 to 0.4weight percent of the clay solids used in a formulation. The lowered viscosityof the clay slip allows reduced levels of water to be used in the clay formula-tion, reduces energy costs in firing, and produces objects with increased greenand fired strength.[169] In the formation of building brick, the dispersingproperties of lignosulfonates are applied in the same way they are in otherceramic applications and with the same effects. However, the lignosulfonatesalso lubricate the plug mill and reduce energy costs for brick molding.Application levels for lignosulfonates in brick are between 0.5 and 2 weightpercent of the unfired clay formulation.[170] Formation of ‘‘wallboard’’ for



construction uses 0.05 to 0.16 weight percent lignosulfonate to disperse thegypsum or its partial hydrates in the slurry from which the panels aremade.[167]

The largest single use of lignosulfonates is their application to dirt roadsfor dust control and road stabilization.[171] The lignosulfonate liquor con-tains, on average, 10 weight percent hexose sugar, 10 percent pentose sugar,and 10 percent non-cellulosic carbohydrate. It is this fraction of the byprod-uct lignosulfonate from paper manufacture that acts as a binder in the roadsurface, reducing dust and increasing the road capacity to withstand heavytraffic, resist erosion, and minimize frost heave.[172]

Agricultural slurries such as pesticides, herbicides, and fertilizers aredispersed with 2 to 5 weight percent lignosulfonates.[173,174] Similarly,carbon black slurries in latex rubber are dispersed to produce a more uni-form spread of the carbon particles in the precipitated rubber used intires.[164]

Sulfonated lignin is used in the mining industry to refine ore by oreflotation. The lignosulfonates depress the entrainment of minerals such ascalcite, barite, and talc and produce a higher metal content extract from theore.[167]

Lignosulfonates were added to water in cooling towers or heating/airconditioning systems and were also used to treat boiler water. The low cost oflignosulfonates plus their ability to sequester calcium or magnesium ion[175]

or disperse heat coagulable particles[176] produced a significant market forthese anionic polymers that peaked in market share and volume in the 1960’s.Applications levels were 0.1 weight percent or less. Lignosulfonates havebeen almost completely replaced in these applications by copolymers basedon 2-propenoic acid or amide. If lignosulfonates are to be used again incooling towers or boilers, the materials must be cheaply modified to powerfulscale, corrosion, and deposition inhibitors.

With further modification, lignosulfonates are the principle primarydispersant, extenders, and grinding aids for the dye industry. Lignosulfonatesreacted with 1-methanolylphenol (second reagent, equation 10e) have[177]

increased thermal stability, good grinding aid performance, high dispersionefficiency, low fiber staining behavior, and a minimal tendency to chemicallyreduce the azo group in azo dyes. For these reasons and their low cost,methanolylphenol-lignosulfonate esters are extensively used in the dye indus-try. The modified lignosulfonate is made by reacting 0.5mmoles of 1-metha-nolylphenol per gram of lignosulfonate in a 50 weight percent, aqueoussolution at a pH of 10 and a temperature of 100�C for 5 hours to form anesterified lignosulfonate.[72] The reaction is illustrated in equation 10e.During this reaction, aldehyde groups in the lignosulfonate may be oxidizedto carboxylic acid groups, a transformation illustrated by having structure6b, with R¼H, of equation 6 become structure 6a. This removes ‘‘reducinggroups’’ from the lignosulfonate.

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9. Nitroxide Formation, Silylation, and Phosphorylation

These three modifications are all research techniques or laboratorychemistry that have been done on lignin. The methods have no commercialuse as of 1998. Nitroxide addition to lignin is a reaction done solely to elicitstructural information about lignin by introducing free radicals into thepolymer. The functional group added to lignin is shown in equation 11a

where R1 is often a phenyl group of lignin, ester-linked to the nitroxide, andthe period superscript represents the unpaired electron on the nitroxide. Asthe structure implies, nitroxide groups are often added to molecules contain-ing hydroxyl groups through esterification reactions using the carboxyl groupon the most common reagent used for nitroxide addition, 3-carboxy-2,2,5,5-tetramethylpyrroline-l-oxyl radical, equation 11b. The addition of 11b. tothree lignin model compounds; 2-methoxyphenol, 4-(1-ethanalyl)-2-methoxy-phenol, and 4-(1-methanolyl)-2-methoxyphenol; to make structures of type11a. showed[178] that the electron spin resonance spectra of the three, radical-containing molecules was essentially the same. This removes the capacity toform these radicals on lignin and deduce structure and bonding in the naturalpolymer. The authors try to promote the usefulness of this synthesis and ESRtest because diradicals on the same lignin repeat unit show ESR signals thatare characteristic of the distance between the radicals. Unfortunately,1,n-benzenediol groups, where n ranges from 2 to 4, are infrequent in ligninand the frequency of these groups and the distance between the hydroxylgroups on them does not constitute critical structural information. Thisexplains why this method has not been applied again to examine ligninstructure. Nitroxide adducts of lignin have been formed[179] to examine thecoil behavior of lignin in solution but the study provided minimal technicalinsight. While the free radical probes on solvated lignin molecules did showdifferences in different solvents, radical concentrations, and solution concen-trations, the differences were not readily interpreted in terms of the structureof the coil and the interactions between the molecules. The dispersion andsolvation of lignin in solvents, particularly aqueous base, is of general impor-tance in lignin chemistry and the digestion of wood pulp. However, the study



of solvated nitroxide radicals performed by Lindberg has not been repeatedand, when used with 1998 technology, this method is apparently incapable ofanswering questions about aggregation, coil dispersal, and polymer-solventinteraction parameter that are of interest in lignin chemistry.

Silylation requires that bonds be formed between lignin and a siliconatom. This type of reaction has been claimed in the literature[180] but thelabels applied and chemistry assumed are invalid. In a patent, Blount claimedthat a reaction between sodium silicate, Na2SiO3, sulfuric acid, and ligninproduced gray, granular solids labeled lignin silicoformate and lignin silicate.The reaction is conducted by mixing sodium silicate and sulfuric acid inapproximately stoichiometric quantities to form silicic acid,[181] a metastable,aqueous solution of Si(OH)4 which will react into colloidal silicates andpolymerize at a rate dependent on pH.[182] If lignin were introduced to thissolution, the forming silica network would entrap the lignin macromoleculesand form, depending on lignin concentration, anything from an adsorbedlignin in a silicate gel to an interpenetrating network of silicate gel and lignin.If this forming or formed gel were heated to evaporate water, the dehydratingmixture of a silicate lattice and an alkylaromatic polyalcohol would react torelease water by forming alkoxide bonds to silicon, as shown in equation 12.

In isolation, this alkoxide bond would form rapidly with a phenolic hydroxylgroup but hydrolyze equally rapidly when re-exposed to water. Aliphatichydroxyl groups would form a more stable alkoxide bond but would alsohydrolyze in water. Hydrolytically stable bonds could be formed betweenlignin and the silicate network through 1,2-benzenediol (13a) and 2-hydroxy-benzoic acid (13b) groups, which can form the divalent linkages to siliconshown in equation 13. Since there are 12 benzenediol groups per 100 repeatunits in kraft pine lignin, water-stable bonds can form between kraft ligninand silicate.

A critical provision needs to be emphasized in this discussion on stabilityof lignin–silicate structures, however. The discussion on alkoxide–silicatebond stability was prefaced by ‘‘in isolation’’, meaning a single bond in solu-tion or a swollen gel. If the alkoxide–silicate bond is formed in a sample whichultimately loses all water, the sensitivity of the carbon–oxygen–silicon bond tohydrolysis, while it still exists, is irrelevant because no water will ever reachthat bond to lyse and separate it. Once a collapsed, hydrophobic aggregate ofsilicate and lignin–silicate bonds forms, the capacity of water to penetrate intothe hydrophobic lattice from its surface, break bonds in the lattice, separateparts of the lattice from one another, and do these steps on any reasonable timescale, such as the human lifetime, drops essentially to zero. Unless theentire aggregate is dissolved in strong base; a dried, aggregate of silicate and

266 MEISTER


lignin–silicate bonds will remain stable and combined despite the fact that themajority of its individual bonds are hydrolyzable.

The products claimed by Blount[180] are actually chemisorbed aggre-gates of lignin in a silicate network with the degree of bonding and dispersionin the lignin depending on the concentration of lignin in the silicate and thepresence or absence of water in the reaction mixture. Blount formed a lignin–silicate adduct by baking a ground mixture of 50 to 75 weight percent lignin,25 to 35 weight percent silica gel, and 2.5 to 15 weight percent strong basetogether for an hour or by combining these reagents in water and heating todryness. The solution reaction will give a more dispersed and uniform prod-uct, all of the products will have some carbon–oxygen–silicon bonds, and allwill be increasingly stable with higher uniform dispersion of lignin in thesilicate and more complete dehydration of the formed aggregate. None ofthese products have a specific chemical formula or structure, none merit thenomenclature applied to them by Blount, and none have any commercial usein 1997.

Phosphorus can be added to lignin by reaction of an alkali lignin withphosphorus halides, oxyhalides, thiohalides, oxides, sulfides, and trivalentphosphorus esters or amides.[183] A representative reaction[184] is shown inequation 14. The action of these strong electrophiles is to halogenate alkylgroups at the hydroxyl substituent and produce a phosphorus bound tooxygen, as shown in equation 14a. The reagents can only attack aromatichydroxyl groups to form a phosphate ester,[184] as shown in equation 14b.

As implied by equation 14b, a polymer with multiple hydroxyl groupswill react with pentavalent or trivalent phosphorous electrophiles to crosslinkthe polymer if a molar excess of the phosphorus compound is present.This was verified by Doughty, who found that phosphorous halide or sul-fide/lignin reactions with a greater than 1/1 mole ratio produced insolubleproducts.[183] If the number of moles of phosphorous compound are equal orless than the moles of lignin in the reaction, the soluble reaction product can



be further reacted with mono and polyhydroxy compounds to produce flameretardant resins with film-forming capacity.[185] These types of products, bothsoluble and insoluble, can be produced in a number of ways[186–192] withdegrees of oxygen and alkyl group loss increasing with the severity of thereaction conditions. However, there is no current use for these compoundsand no production of them. Further, if these materials are to be used as flameretardants, the halogenation that is coincident with phosphorylation bystrong electrophiles will have to be avoided and other, potentially less cost-effective, synthetic routes to these materials must be developed.

10. Grafting by Free Radical Polymerization

Once lignin is separated from other plant products, it can be grafted.Extensive studies on the modification of lignin by graft copolymerizationhave been made[193] because of the enormous mass of kraft lignin producedeach year by the pulp and paper industry. Graft copolymerization sharplychanges the properties of lignin and allows useful products to be made fromthis underutilized portion of biomass.[194]

Lignin has been grafted with ethenylbenzene (Refs. [195,196]; styrene),4-methyl-2-oxy-3-oxopent-4-ene (Refs. [197,198]; methylmethacrylate), 2-pro-penamide (acrylamide), 2-propene nitrile (Ref. [199]; acrylonitrile), cationicmonomers, anionic monomers, and propenoic acid ethoxylates. An index ofcompounds listing structure, product name, and trivial name is given inTable 7. Two types of methods, radiation or chemical, have been used toattach sidechains of these repeat units to lignin. The radiation methods have

268 MEISTER


used both electromagnetic and particle radiation to produce grafting. Low-energy, electromagnetic irradiation based on visible or ultraviolet light relieson exciting or decomposing a particular bond either in lignin or in an initiatorpresent in the reaction mixture. This method, photoinitiation, has not beenused to graft lignin. High energy radiation grafting using either electromag-netic or particle beams proceeds by ionization and excitation reactions thatproduce anionic, cationic, and free-radical sites. Radiation-initiated, ionicgrafting reactions have not been conducted on lignin and therefore, onlyfree radical polymerization is known to contribute to grafting. The frequencyof active site formation can be expressed as a ‘‘G’’ value, where G is thenumber of molecules reacted or produced by 100 eV of absorbed energy.Lignin is quite stable to ionizing radiation[200,201] having a GR value of 0.6to 0.718. This stability makes lignin a poor candidate for radiation graftingsince in the presence of neat monomer or in solution, initiation will occur farmore readily to form homopolymer than it will to form graft copolymer.Homopolymer contains only polymerized monomer. Because of these


Table 7. Trivial Names for Compounds and Copolymers

Name Trivial name Structure

poly(1-phenylethylene) polystyrene

ethenylbenzene styrene

poly(1-(1-oxo-2-oxypropyl)-

1-methylethylene)

poly(methylmethacrylate)

2-oxy-3-oxo-4-methylpent-4-ene

methylmethacrylate

poly(1-amidoethylene) polyacrylamide

2-propenamide acrylamide

poly(1-cyanoethylene) polyacrylonitrile

2-propene nitrile acrylonitrile


deficiencies, radiation grafting of lignin will not be discussed further. If addi-tional information on radiation grafting is needed, references are providedabove to radiation grafting studies. Those are the references cited in the listof monomers grafted to lignin.

The major deficiency of radiation grafting is production of homopolymerinstead of graft copolymer. Some reduction in the amount of homopolymerproduced can be achieved by initiating the grafting reaction by chemicalmethods. Indeed, Stannett showed that chemical initiation of grafting isover 8 times more effective in converting monomer to graft copolymerthan is radiation when both are applied to hydrochloric acid lignin. Henotes[196] that ‘‘chemically initiated grafting at 60�C was more effectivethan radiation-induced grafting at room temperature.’’ Chemical initiationcan be applied in two ways. First, a reagent which attacks functional groupson the lignin backbone to produce a grafting site can be used as a graftinginitiator. Alternatively, a reagent which reacts with lignin to form a reactablefunctional group is used to form a derivative of lignin. The added groups arestructures such as peroxide or ethene bonds and are then treated or reacted toinitiate grafting. The chemical method can be used to initiate all polymeriza-tion reactions that do not require a solid supported catalyst[202] but only stepand free radical chain reactions have been conducted on lignin.

Unfortunately, for most of the products reported in the literature,neither of these methods have been used to make the material claimed tobe a graft copolymer. If the polymerization reaction does not start off of thematerial to be grafted, then almost all of the monomer is polymerized intohomopolymer with no lignin in the chain. This is a ‘‘polymerization in thepresence of’’ and it wastes the monomer that was supposed to be used in agrafting reaction. Thus, a key and often overlooked point in conductinggrafting reactions is to insure that the initiation of the polymerizationoccurs on the backbone to be grafted. This requires a special chemistry toinitiate the polymerization. Polymerization methods without such chemistriesmerely make homopolymer.

Meister et al. have achieved the grafting of lignin-containing materialsby developing an initiation system that preferentially attacks repeat units inlignin to create a site for polymer chain growth. The reaction appears generaland works on almost all ethene monomers. By this reaction, the researchersconvert lignin into process polymers for industrial use or thermoplastics foruse in consumer items. Figure 5 is a general diagram showing the materialsreacted with lignin and the applications for the resulting products.

A typical grafting reaction mixes monomer in nitrogen-saturated,organic or aqueous/organic solvent containing lignin, calcium chloride, anda hydroperoxide.[203] A research group headed by Meister has developed thisreaction chemistry over the past 15 years and have shown that it almost quan-titatively converts lignin into graft copolymer. The group first synthesized,characterized, and tested a spectrum of water soluble, lignin copolymers

270 MEISTER


that were nonionic, anionic, or cationic[204] and then showed that the watersoluble polymers were effective dispersing, flocculating, and surface activeagents. The nonionic polymers and their hydrolysis products are effectivethinners and suspending agents for drilling mud formulations,[204] as shownby the data of Table 8. These test samples compare poly(lignin-g-((1-ami-doethylene)-r-(1-carboxylatoethylene)), poly((1-amidoethylene)-r-(1-carbox-ylatoethylene)), and chrome lignosulfonate as aqueous drilling muddispersants. The copolymer performs as well as the homopolymer and ismore thermally stable than the lignosulfonate. The anionic polymers productsare thickening agents for fluid flow control,[205] as shownby the data of Table 9.The high limiting viscosity numbers of these copolymers cause rapid viscosityincrease in water as a function of copolymer concentration. The cationic poly-mers products are dewatering aids for sewage treatment,[206] as shown by thedata of Table 10.

The ethoxylate esters of propenoic acid are useful as prepolymers forurethane formation but these materials are not as effective as those preparedby Wolfgang Glasser’s research group. Therefore, only Dr. Glasser’s prod-ucts will be discussed.

In reactions with ethenylbenzene, lignin was used to make thermoplasticmaterials. Data for a spectrum of reactions run to optimize yield and createsamples of different molecular weight and composition[207] are given inTable 11. Samples PE 1 to PE 10 were made with ethenylbenzene as monomer.These products have been shown to be poly(lignin-g-(1-phenylethylene))-containing materials by a series of solubility and extraction tests and are


Figure 5. Monomers that may be grafted onto lignin and uses of the resulting products.


272 MEISTER

Table 8. Properties of Test Muds Before and After Hot Rollinga

Property

Base Mud

Graft

CopolymerFraction

HomopolymerFraction

ChromeLignosulfonate

Before After Before After Before After Before After

Viscosity in centipoise at a shear rate of:1020 s�1 52 69 74 42 74 42 29 50

510 s�1 36 47 49 24 49 24 16 33340 s�1 30 29 40 17 40 17 12 26170 s�1 21 28 27 10 27 10 7 18

Gel strength in lb/100 ft2 mud has set for:

10 sec 6 12 5 3 5 2 2 1110min 25 35 20 3 20 3 9 24Apparent Viscosity (cp) 26 33 37 21 37 22 15 25

Plastic Viscosity (cp) 16 22 25 18 25 19 13 17Yield Point (lb/100 ft2) 20 25 24 6 24 5 3 16API Filtrate Volume (mL) 12.0 13.8 7.8 8.0 7.8 8.4 11.6 14.0pH 9.1 8.6 9.0 8.0 9.0 8.0 9.5 8.2

High Pressure,High TemperatureFiltrate (mL)

— 62.8 — 52.4 — 46.0 — 64.0

aHot rolling is agitating the mud at 121�C for 16 hours.

Table 9. Synthesis Data and Physical Characteristics of Graft Terpolymera

SampleNumber

Reactants

2-Propenamide (g) A (g)

Dimethylsulfoxide

(mL)CaCl2(g) B (g)

Yield(wt%)

[�](dL/g)

1 1.60 4.66 20 0.50 0.15 70.12 10.52

2 1.60 4.66 50 0.50 0.15 86.98 11.403 1.60 4.66 50 0.50 0.25 78.40 7.404 1.60 4.66 40 0.50 0.40 69.82 9.30

5 1.60 5.16 30 0.50 0.15 78.79 12.596 1.60 5.16 30 0.50 0.15 77.27 6.817 1.60 4.66 30 0.50 0.15 87.28 10.46

8 2.58 1.87 30 0.50 0.39 67.89 0.9539 2.56 1.86 30 0.53 0.39 79.49 2.4610 21.98 15.99 219 4.35 3.35 91.02 1.97

aAll reactions, save #10, contained 0.50 g of lignin and 0.15mL of cerium (þ 4)ion. Reaction #10 contained 4.39 g of lignin and 1.28mL of cerium ion.(A) 2,2-Dimethyl-3-imino-4-oxohex-5-ene-1-sulfonic acid.(B) Hydroperoxide. Samples 1 to 7: Values are weight of 1,4-dioxa-2-hydroper-

oxycyclohexane in g. Samples 8 to 22: Values are amount of aqueous solution of1,2-dioxy-3,3-dimethylbutane in mL. Equivalents/mL¼ 7.23� 10�3.


formed with 90% or more grafting efficiency for lignin. These materials havebeen shown to be thermoplastics,[207] coupling agents for wood and plastic,[208]

and biodegradable plastics.[209]

The thermoplasticity of the graft copolymers can be verified by measure-ments of the glass transition temperature of the new solids. The glass transitiontemperature is the temperature at which an amorphous solid becomes ductileand is a characteristic of thermoplastic materials. Samples of 5 to 10mg ofreaction product were heated at 10�C per minute in a differential scanningcalorimeter to monitor heat capacity as a function of temperature. The tem-perature of each transition produced by each copolymer is shown in Table 12.

These products also occupy surfaces on wood and act to alter the wettingproperties of the plant material. The lignin and copolymer solutions gavesmooth, adherent surface coatings on the wood with contact angles againstwater of 90 to 110� while plastics and plastic–lignin mixtures did not giveadherent coatings. These data show that copolymers of lignin are surfaceactive, preferentially orienting the lignin portion of the product towardswood while the plastic sidechain is oriented outward to create a new surfacewith different wetting properties. Thus, these copolymers are surface-active,


Table 10. Synthesis and Application Data for Catinoic Graft Copolymer

A: Synthesis Data

Reactant

SampleNumber

Lignin(g)

CaCl2(g)

A(g)

C(g)

DMSO(g)

E(mL)

Yield%

1 0.50 0.93 1.92 5.15 30.72 0.50 80.182 0.50 0.99 1.29 8.36 29.28 0.50 70.44

3 0.50 1.02 0.64 10.21 31.40 0.50 91.464 0.50 1.07 — 12.77 30.71 0.50 70.91

B: Dewatering Data of Cationic Lignin Graft Copolymer with Methylsulfate Anion

SampleNumber

Concentrationin Sludge (ppm)

Filtrate Volume After Being on the Filterfor the Given Number of Minutes

1 2 3 4 5 10

BLANK 00.0 <10 10

1* 300 35 45 55 60 70 —1* 450 30 45 55 65 70 —3 150 — — — — 28 46

4 150 — — — — 52 74CommPol. 150 54 72a

aPerformance data for the commercial polymer in use at the Detroit Water and Sewerage

Works in 1991. (A) 2-Propenamide. (C) Cationic monomer. 2-Methyl-N7,N7-trimethyl-7–ammonium-3-oxo-4-oxyoct-1-ene chloride or 2-methyl-N7,N7-trimethyl-7-ammonium-3-oxo-4-oxyoct-1-ene methylsulfate. (E) 30% Hydrogen peroxide (equivalent weight: 8.383

meq/mL). *Cloudy filtrate.


coupling agents which can bind wood to hydrophobic phases such as plastic.This coupling process works best when the wetting agent has been synthesizedso that the sidechain attached to the lignin during the preparation ofthe macromolecular, surface active agent is chemically identical to the plastichydrophobic phase that is to be bound or connected to the wood. Thus, to bindpoly(1-phenylethylene) [Trivial name¼ polystyrene] to wood, coat the woodwith poly(lignin-g-(1-phenylethylene)) and to bind poly(1-cyanoethylene)

274 MEISTER

Table 11. Copolymerization Reactions of Lignin and Ethene Monomers

Reactants (g)

Sample

Number Lignin Monomer CaCl2

H2O2

(mL) Solvent

Yield

(g/wt.%)

PE 1 2.00 18.76 2.02 1.0 20.04 17.80/85.7PE 2 2.01 18.78 2.02 5.0 20.02 18.53/89.1PE 3 3.03 18.78 2.00 2.0 20.00 19.14/87.8

PE 4 2.00 18.76 1.01 2.0 20.10 18.84/90.8PE 5 2.01 4.69 2.04 2.0 20.01 5.68/84.8PE 6 2.01 9.39 2.02 2.0 20.00 10.42/91.4

PE 7 2.01 14.07 2.03 2.0 20.10 14.95/92.8PE 8 2.02 23.45 2.04 2.0 20.07 23.76/93.3PE 9 8.00 28.15 8.00 8.0 40.02 33.16/91.7PE 10 8.04 18.76 8.00 8.0 40.03 24.14/90.1

CY-AM 1 4.10 9.35/0.0* 3.05 4.00 20.04 13.86/103.0CY-AM 2 0.50 0.35/4.17 0.5 0.53 28.91 4.82/96.4CY-AM 3 0.50 0.78/3.74 0.5 0.53 28.91 4.91/97.8CY-AM 4 0.50 1.19/3.33 0.5 0.53 28.91 3.50/69.7

CY-AM 5 0.50 0.79/3.75 0.5 0.53 28.91 3.28/65.1

CY 1 4.10 9.35/0.0 3.05 4.0 20.04 13.86/103.0CY 2 3.95 6.08/0.0 3.07 4.0 20.07 9.83/98.0CY 3 4.05 3.13/0.0 3.02 4.0 20.57 6.34/88.3

CY 4 4.02 6.15/0.0 3.05 4.0 25.02 10.14/99.7CY 5 4.00 6.16/0.0 2.51 5.0 20.01 9.29/94.3

MBuD 1 4.11 9.32 3.02 4.0 20.29 4.21/31.3MBuD 1 4.01 3.14 3.05 4.0 20.32 4.39/61.4

MBuD 1 3.98 6.27 3.07 4.0 21.36 3.71/36.2

MPrPe 1 4.00 17.17 3.03 4.48 20.04 19.54/92.30MPrPe 2 4.03 11.57 3.09 4.48 20.11 15.29/98.01MPrPe 3 3.97 6.37 3.06 4.48 20.27 9.69/93.71

MPrPe 4 4.00 11.78 2.98 4.48 20.06 16.2/102.67MPrPe 5 4.05 11.53 2.57 5.60 20.35 15.69/100.71MPrPe 6 8.00 6.1 3.03 4.70 30.42 13.53/95.96MPrPe 7 8.1 2.41 3.08 4.48 30.21 9.86/93.82

MPrPe 8 7.99 17.52 3.11 8.96 30.23 24.22/94.94MPrPe 10 2.00 18.07 2.06 4.48 20.11 20.63/102.79

*First number¼weight of 2-propene nitrile added and second number¼weight of2-propenamide added.


[Trivial name¼ polyacrylonitrile or orlon] to wood, coat the wood withpoly(lignin-g-(1-cyanoethylene)).

The coupling of wood by a grafted plant part to a plastic, that has thesame or similar composition as the sidechain on the plant part, increasesthe binding strength of the plastic[210] to the wood. This was proven byperforming lap shear tensile strength tests on birch strips onto which wereinjection molded blocks of plastic. The samples with a grafted productcoating with the same repeat units in the sidechain as in the plastic gave20 to 50 percent higher tensile strength. The coupling experiments wereperformed as follows. Birch tongue depressors, a medical product of1.75mm thickness were cut into suitable sizes to match an injectionmold. Kraft pine lignin was reacted into a graft copolymer as previouslydescribed. The homopoly(1-phenylethylene) used in coupling tests is arecovered fraction of the reaction product of mechanical pulp and ethenyl-benzene. Coatings of copolymer or any comparison mixtures or ‘‘control’’blanks were prepared as a 10 weight percent solution in dimethylformamideand spread on the wood. The plastic phase was Amoco RIPO, from AmocoChemical Company. Injection molding was done on a Milberry, Model 50Mini-Jector. Experimental conditions were: cylinder temperature, 288�C;nozzle temperature, 172–176�C; pressure, 3450 kPa; pressure holding time,12 seconds; and chilling time, 1–2minute.

Lap shear strength of the pieces of wood with plastic injection moldedto them[211] was tested on a Instron, Model 4200, Universal testing instru-ment USDA. Experimental conditions were: room temperature, 23�C; roomrelative humidity: 50 percent; crosshead speed, 2.54mm/min; with the sample


Table 12. Differential Scanning Calorimetry Data for Lignin, Poly(1-phenylethylene),and Graft Copolymer

SampleNumber

Lignin

Percent inReaction

Lignin

Percent inA Peak(s, �C)

Ramp(�C/min)

Amoco RIPO#

(pure poly(1-phenylethylene))

0 0 102.6 10

Kraft Pine Lignin(pure lignin)

100.0 100.0 116.17–130.09 10

Copolymer 1 9.6 10.3 94.82 (114.62)* 10

Copolymer 2 22.0 27.3 98.43 (133.97)* 10Copolymer 3 30.0 32.2 98.23 (124.10)** 10Copolymer 4 30.0 34.5 102.35 (144.48)** 20

Copolymer 5 30.0 32.3 95.73 133.25 10Copolymer 6 46.0 50.5 94.11 125.12 10Copolymer 7 46.0 51.8 101.63 143.27 20

(A) Copolymerization reaction product. *Very small peak. **Small peak. # A commer-cial poly(1-phenylethylene) from Amoco Chemical Company.


in hand-fastened grips and an aluminum specimen holder. The lap shearstrengths of the wood–plastic samples are summarized in Table 13. Thecopolymer samples were fractionated by benzene extraction. The reactionproduct was labeled Product A. The benzene-soluble extract of the productbecame ‘‘Ben. Ex.’’ while the benzene insoluble portion of the product waslabeled Product B.

In almost all cases, coating the wood with any of the three fractions ofthe graft copolymer of lignin and ethenylbenzene (Product A, Product B, andProduct Ben. Ex.) provides stronger adhesion between wood and commercialpoly(1-phenylethylene) than coating the wood with mechanical mixtures,pure poly(1-phenylethylene), pure lignin, or nothing (blank). The datashow the copolymers to be effective coupling agents.

White rot Basidiomycetes were able to biodegrade graft copolymers oflignin and ethenylbenzene containing different proportions of lignin and

276 MEISTER

Table 13. Summarized Adhesion Strength Results

Coating Material Adhesion Strength (kPa)

30-151-2A (10.45% lignin) 2422.1 219.3 (3)*30-151-2B 2313.9 488.2 (3)30-151-2Ben.Ex. 2126.4 44.1 (3)10% Lig.þ 90% PS 2209.1 251.0 (5)

35-120-1A (24.23% lignin) 2313.9 81.4 (3)35-120-1B 2278.7 294.4 (2)

35-120-1Ben.Ex. 2094.0 213.7 (3)24% Lig.þ 76% PS 2027.1 185.5 (4)

35-110-3A (32.17% lignin) 1930.5 304.1 (3)

35-110-3B 1911.2 184.8 (3)35-110-3Ben.Ex. 2670.4 207.5 (3)32% Lig.þ 68% PS 1949.2 265.4 (5)

35-115-3A (51.70% lignin) 2838.6 60.0 (3)35-115-3B 2723.4 328.9 (3)35-115-3Ben.Ex. 2707.6 70.3 (3)

50% Lig.þ 50% PS 1843.0 91.0 (5)

Poly(1-phenylethylene) (PS) 2040.9 206.8 (5)

10% Lig.þ 90% PS 2209.1 251.0 (5)24% Lig.þ 76% PS 2027.1 185.5 (4)32% Lig.þ 68% PS 1949.2 265.4 (5)50% Lig.þ 50% PS 1843.0 91.0 (5)

Lignin 2123.6 164.1 (5)Blank (treated with DMF) 2022.2 120.7 (2)Blank (treated with nothing) 1825.7 165.5 (4)

*Number of valid repetitions of the tensile strength test in parentheses. Lig.¼Lignin.PS¼ polystyrene. DMF¼ dimethylformamide.


poly(1-phenylethylene).[212] The biodegradation tests were run on lignin/ethe-nylbenzene copolymerization products which contained 10.3, 32.2, and 50.4weight percent lignin. The polymer samples were incubated with white rot,lignin-degrading organismsPleurotus ostreatus,Phanerochaete chrysosporium,Trametes versicolor, and brown rot, cellulose-degrading organismGleophyllum trabeum. Over a 68 day period, white rot fungi degraded theplastic samples at a rate which increased with increasing lignin content inthe copolymer sample. Both poly(1-phenylethylene) and lignin componentsof the copolymer were readily degraded. Pure poly(1-phenylethylene) pelletswere not degradable in these tests. Observation by scanning electron micros-copy of incubated copolymers showed a deterioration of the plastic surface.Brown rot fungus did not affect any of these plastics.White rot fungi producedand secreted oxidative enzymes associated with lignin degradation in liquidmedia during incubation with lignin-poly(1-phenylethylene) copolymer. All ofthese applications represent significant markets for modified lignin.

In examples Cy-Am 1 to 5 of Table 11 the monomers used were2-propene nitrile [107-13-1] and 2-propenamide [79-06-1]. In examples Cy1 to 5 of Table 11, the monomer used was 2-propene nitrile [107-13-1]. Thecompounds in the first group are water absorbing agents while those in thesecond group are thermoplastics and biodegradable plastics.[213] In examplesMBuD 1 to 3 of Table 11, the monomer used was 2-methyl-1,3-butadiene[78-79-5]. These materials are uncrosslinked elastomers and potential rubberadditives. In examples MPrPe 1 to 9 of Table 11, the monomer used was2-methyl-2-oxy-3-oxopent-4-ene [80-62-6]. These materials are thermoplas-tics and biodegradable plastics.[214]

11. Grafting by Anionic Chain Polymerization

Lignin can be grafted with alkane epoxides to form polyether adductsand this reaction has been extensively studied by the research group ofDr. Wolfgang Glasser at Virginia State University and PolytechnicInstitute. The reaction is initiated by base attack on the phenolic hydroxylgroups to form phenoxide groups. The phenoxide groups then attack thepolar epoxide ring. A significant side reaction in this ring opening polymer-ization is a nucleophilic attack on the ring by hydroxyl anion. These reactionsare shown in Figure 6.

Alkoxylation is critically important as a precursor reaction whichchanges both the physical properties of the lignin and its chemistry. Themost important physical property changed is the glass transition temperatureof the lignin. As the weight fraction of ethylene or propylene oxide in theproduct increases, the glass transition temperature of the product falls. Sincea reduction in Tg is synonymous with a lowering of viscosity at any tempera-ture and a lowering of the temperature at which flow starts, this change



makes lignin a flowable liquid at temperatures 100 to 200�C below the usualtemperatures for viscous lignin flow.

Simultaneously, Figure 6 shows that alkoxylation leaves lignin‘‘capped’’ with chains ending in primary hydroxyl groups. These alcoholsare more reactive in some chemistries than the phenolic hydroxyl groupsthat they have ‘‘capped’’. A particularly important example of one suchchemistry is the reaction of the alkoxylated lignin with an organic isocyanateor diisocyanate. The reaction of the alkoxylated lignin with an isocyanate-terminated polymer allows Glasser et al. to add polymeric sidechains ofcellulose[215] or polycaprolactam[216] to the lignin graft copolymer.

Reactions of alkoxylated lignin with diisocyanates produce thermosetmaterials because the lignin polyol is always polyfunctional with a function-ality greater than 2. The isocyanate-alcohol reaction produces a urethanelinkage that, when repeated, creates a crosslinked, non-reformable polyur-ethane. This is shown in Figure 7. A broad spectrum of lignin-based urethaneshave been made and tested. The data show that these materials match if notexceed the properties of synthetic polyurethanes made without lignin.[217]

The alkoxylated lignin requires an isocyanate to hydroxyl group ratio of1.5 to form effective networks. This is a significantly higher ratio of expensiveisocyanate to hydroxyl units than is used in commercial practice. The work ofGlasser et al. shows that the glass transition temperature, Tg, of a alkoxylatedlignin–diisocyanate network depends on the weight percent lignin in the net-work, the chemical structure of the alkoxide, and the chemical structure ofthe diisocyanate.[217] As lignin content increases, Tg of the network increases.Networks made with oxyprop-1,2-ylene repeat units, –OCH2CH(CH3)–, have

278 MEISTER

Figure 6. Alkoxylation of a lignin repeat unit.


lower Tg’s than networks made with oxyeth-1,2-ylene repeat units when thediisocyanate used in the network is 1,6-diisocyanaohexane, an aliphaticdiisocyanate. When the diisocyanate is aromatic, however, there is no differ-ence between the Tg’s of the materials made with oxypropyl or oxyethylrepeat units. Networks made with aliphatic diisocyanate generally havelower Tg’s than networks made with aromatic diisocyanate.

The mechanical properties of these networks, modulus of elasticity andultimate strength, are dependent on the method of preparing the network.[217]

While the chemical composition and repeat units of the network are impor-tant factors that influence the properties of the crosslinked product, the mostimportant influence on physical properties of the network is the way thepolymeric, alkoxide chain is introduced into the network. Products madeby adding a pure polyether that has not been grafted onto lignin havehigher modulus of elasticity and ultimate strength than products madewith alkoxylated lignin copolymer. This difference in physical propertiesprobably reflects the steric hindrance and reduced diffusion produced bygrafting a polyether onto lignin. The copolymer produces a less uniformdistribution of crosslinks and a greater segregation of network parts thandoes the blend of pure polyether with the other reagents.

A key feature in forming this material that is not widely recognized out-side of the polyurethane industry is that the reactants must be fluids at 60�C,the common temperature for conducting this reaction. This is why the reduc-tion in glass transition temperature with increasing degree of alkoxylation inlignin is so important. By decreasing the temperature at which modified ligninwill flow, the alkoxylation reaction allows lignin to not only be a uniformlyreactive, poly primary alcohol, but also a fluid polyol that can be processed by


Figure 7. Reaction of ethoxylated lignin with a diisocyanate.


procedures such as reaction injection molding into thermoset solids. The pros-pects for application of this chemistry to foams, networks, and consumergoods is great since the materials match the properties of existing products,the area of application is high-profit-margin, specialty chemicals, and ligninsharply reduces cost by replacing more expensive polyol.

III. CONCLUSIONS

The utilization of lignin will increase in the next quarter century asdemand for aromatic carbon exceeds the supply available from a decreasinginventory of oil. Lignin is a more homogeneous material than petroleum thatcan provide a relatively uniform supply of alkyloxyaromatics for decomposi-tion, utilization as extracted from the plant, and use as a backbone to createlarger, altered polymers. The market that not only has the highest rate ofgrowth in 1998 but also promises the largest increases in utilization of ligninis the use of lignin as an adhesive in wood composites. Lignin constitutes 17weight percent of the solids in most exterior-grade plywood and will becomea progressively larger fraction of the binder in laminates and fiber, strand, orwafer board. Lignin has a potential to become a functional photostabilizerand free radical trap because of its high molar absorptivity at ultravioletwavelengths below 300 nm and its ability to trap and maintain free radicals.However, these applications will require careful formulation of a productthat will include a chemically altered lignin in place of the product thatcan be extracted from the plant. A possible use for lignin in the future isthe addition of alkali lignins to pet and human food as roughage, a fibersource, or a cancer protection agent.

Thermal or chemical decomposition of lignin to produce oxyaromaticswill grow when the depletion of low cost petroleum becomes pronouncedbetween 2025 and 2035 A.D. Examples of this technology would be nonionicsurfactants containing retorted, ethoxylated lignin or cresylic acid producedby pyrolysis. While this is a market for a small mass of lignin, it is a highprofit margin market with specialty chemical applications stretching fromadhesives to xylene manufacture.

The commonmodification reactionsof lignin are alkylation, dealkylation,sulfomethylation,methylolation, sulfonation, amination, nitroxide formation,carboxylation, acylation, silylation, halogenation, nitration, phosphorylation,hydrogenolysis, grafting, and oxyalkylation. Of these reactions, nitroxideformation, carboxylation, acylation, silylation, halogenation, nitration, andphosphorylation are laboratory processes that have no commercial applica-tion in 1998. Alkylation and dealkylation are common side reactions duringlignin extraction and recovery. Alkylation forms a lignin thermoplastic but thisbrittle material has no current utility. Sulfomethylation and sulfonation formlignosulfonates which have a broad market as tanning agents, electrode

280 MEISTER


stabilizers, suspending agents, and dispersing agents. They are the most widelyused, modified lignin. Sulfomethylation is only usedwhen an increased contentof sulfonate groups is needed in the lignin product. It is applied to kraft ligninsby theWestvaco Corporation to form dye dispersants that are marketed underthe Reax trade name. The current markets for lignosulfonates, a moistureretention agent in cement grouts, a dust suppressor for road treatments, anadditive for battery electrodes, or a thickening agent in inks, are small butstable markets. Methylolation is a pre-reaction to prepare lignins for use as aphenol extender in phenol–methanal resins. It is used commercially in theforest products industry. Amination is used to form lignin with beta ketoamine groups in it. This amine lignin is used to form thermally stable, aqueousemulsions of asphalt for use in road repair.

Graft copolymerized lignin is a research material being tested for indus-trial and consumer applications. Copolymerization of lignin with polarmonomers to create process polymers will permit the use of lignin in watertreatment, sewage dewatering, thickening, and dispersion. These nonionic,anionic, and cationic, water soluble polymers will be industrial process poly-mers which allow the production of consumer and manufactured products.Copolymerization of lignin with nonpolar monomers to create thermoplas-tics will permit the use of lignin in the commodity plastics market. Thesematerials are biodegradable thermoplastics, thermoplastic composites, andcoupling agents to incorporate wood and plastic into a single phase.

The alkoxylation of lignin will permit its use in processes requiring afluid reagent for further modification. The largest market for this new mate-rial will be the production of urethane solids and foams from the reaction ofthe alkoxylignin with a diisocyanate. This high value material will haveapplications as an engineering plastic or as insulation. Alkoxylated lignin isalso useful as an intermediate to be grafted into new materials by reactionwith step-reaction polymers terminated with a group that creates covalentbonds with hydroxyl groups. While carboxylic acid-terminated, step poly-mers can be reacted with alkoxylignin to form alkoxylignin esters, thermo-dynamic and equilibrium forces dictate that the best products from thischemistry will come from step-reaction polymers capped with isocyanategroups. New products of alkoxylated lignin covalently bonded to celluloseacetate or caprolactam are known and are available for utilization.

QUESTIONS

1. What is the source of lignin?2. Why is lignin made and to what use is it put when it is first manufactured?3. What are the three monomers that are used to make lignin?4. Name three methods of recovering lignin once it has been synthesized.5. Name three products from the pyrolysis of lignin.



6. Describe the use of lignin in the production of Bakelite polymers,phenol–methanal, network polymers.

7. What are the potential uses for lignin as a food additive?8. Name two of the three common methods for adding alkyl groups to

lignin.9. A common chemical reaction based on acid catalyzed addition of

methanal and a primary or secondary amine is conducted on alkalilignin in aqueous suspension to convert it into a commercial product.What is the name of this reaction, what product is formed by it, andwhat is the use of that product?

10. The study of lignin sulfonation dates to 1866. Why does this reactionhave sucha longhistoryof extended investigationsand forwhat is it used?

11. What is a graft copolymer of lignin and how is it made?

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