Journal of Polymers and the Environment, Vol. 10, Nos. 1/2, April 2002 (q 2002)

Recent Industrial Applications of Lignin: A SustainableAlternative to Nonrenewable Materials

Jairo H. Lora1 and Wolfgang G. Glasser2,3

Lignin represents a vastly under-utilized natural polymer co-generated during papermaking andbiomass fractionation. Different types of lignin exist, and these differ with regard to isolationprotocol and plant resource (i.e., wood type or agricultural harvesting residue). The incorporationof lignin into polymeric systems has been demonstrated, and this depends on solubility and reactivitycharacteristics. Several industrial utilization examples are presented for sulfur-free, water-insolublelignins. These include materials for automotive brakes, wood panel products, biodispersants, polyure-thane foams, and epoxy resins for printed circuit boards.

KEY WORDS: Lignin; biopolymers; adhesives; biodispersants; OSB; polyurethane foams; epoxy resins; wood;pulp and paper.

INTRODUCTION crops are different again. However, the differences areminor as far as most applications are concerned. Major

Lignin is nature’s most abundant aromatic (phenolic) differences exist between lignins derived from differentpolymer. Its principle function is to provide trees with pulping processes. The traditional sulfite process gener-the ability to raise a crown above ground level [1, 2]. ates a water-soluble polymeric derivative in admixtureLignin is separated from wood during pulp and papermak- with degraded carbohydrates. An aliphatic sulfonic aciding operations, where it serves primarily as fuel. Only a (SO3H) function becomes part of the lignin backbonesmall amount (ca. 1–2%) is isolated from spent pulping ensuring ready water-solubility in the presence of a suit-liquors and employed in a wide range of specialty prod- able counter ion (Na, Ca, Mg, NH4, etc.). “Kraft” or “thio”ucts. These, however, amount to 1 million tons per year lignins are generated during kraft pulping in alkalineworldwide [3, 4]. medium. They contain a small number of aliphatic thiol

groups that give the isolated product a characteristic odor,especially during heat-treatment. A very small amountTYPES OF LIGNINS AND THEIR PROPERTIESof kraft lignin is isolated from pulping liquors in theUnited States and Europe [4]; the vast majority is usedLignins vary in structure according to their methodas in-house fuel required for the recovery of chemicals.of isolation and their plant source [2, 5–7]. HardwoodsKraft lignins are dark-colored and water- and solvent-develop a different lignin than softwoods, and annualinsoluble products that dissolve in alkali owing to theirhigh concentration of phenolic hydroxy groups. Neither

1 Granit S. A, Media, Pennsylvania 19063. lignin sulfonates nor kraft lignins undergo a distinctive2 Virginia Polytechnic Institute and State University, Department of

glass-to-rubber (or fluid) transition when heated, despiteWood Science and Forest Products, Blacksburg, Virginia 24061.the fact that lignin in wood has a Tg of under 1008C [8].3 To whom all correspondence should be addressed. E-mail: wglasser@

vt.edu Numerous other lignin products and derivatives have been

391566-2543/02/0400-0039/0 q 2002 Plenum Publishing Corporation

40 Lora and Glasser

described that originated from unusual plant sources or produced on pilot scale from residues of the industrialproduction of furfural from sugar cane bagasse, and that isexperimental pulping processes [6, 7].commercially available on the specialty chemicals market(see Table I, footnotes). Angiolin was the designation

SULFUR-FREE LIGNINS: TYPES AND used for a steam explosion lignin from hardwoods. TheseSOURCES lignins have some similarities in structure, but they differ

in solubility, reactivity, and molecular weight [14]. Cur-Sulfur-free lignins are an emerging class of lignin rently there are a number of commercial biomass to etha-

products [9, 10]. Having no sulfur and being of moderate nol projects in the planning stages. When implemented,macromolecular size, these lignins resemble more closely these projects have the potential of becoming commercialthe structure of native lignin, and they exhibit distinct sources of sulfur-free lignins.properties relative to kraft and sulfite lignins. This opensup new avenues for utilization.

Lignins from Solvent PulpingSulfur-free lignins come from principally threesources: Solvent pulping is an alternative to kraft and sulfite

pulping that is claimed to be more environmentally● Biomass conversion technologies (mainly ori-acceptable and less capital-intensive. In addition to cellu-ented toward alcohol production),lose pulp, solvent pulping permits the recovery of organo-● Solvent pulping (“organosolv” processes), andsolv lignin and other chemical co-products. Lignin is● Soda pulping, particularly of alternative biomassrecovered from the spent solvent by precipitation, whichresources, such as agricultural harvesting residuestypically involves adjusting concentration, pH, and tem-and non-wood fiber crops.perature. Organosolv lignins are usually high-purity, lowmolecular weight products with relatively narrow molec-ular weight distributions [6, 7, 14]. These lignins showLignins from Biomass Conversion Technologiesa low glass transition temperature and exhibit flow whenheated. They have high solubility in organic solvents andThe production of liquid fuels, such as ethanol, from

biomass has been proposed as a strategy to alleviate are very hydrophobic and practically insoluble in water.The properties of lignins from alcohol pulping of variousdependency on foreign energy sources [3]. The biomass

conversion technologies used typically involve a raw materials are illustrated in Table II.In the 1990s, lignin from solvent pulping of hard-hydrolytic pretreatment. The pretreatment is either cata-

lyzed by added mineral acid or autocatalyzed by biomass- woods was available in industrial quantities from a Cana-dian demonstration plant that used the ethanol-basedderived organic acids, as occurs in steam explosion or

autohydrolysis [11–13]. Such treatments not only make AlcellT process. Although the AlcellT technology wasdemonstrated to successfully produce pulp for papermak-the carbohydrate component susceptible to saccharifica-

tion and/or fermentation, but also have the potential of ing plus lignin, and although many novel lignin applica-tions were industrially implemented, further developmentgenerating sulfur-free lignin. From the pretreated biomass

the lignin must be extracted (for instance with an organic of the process was suspended because of financial diffi-culties of the company supporting this endeavor. Cur-solvent or with aqueous alkali), and from the resulting

solution, lignin is recovered normally by precipitation. rently, several private and public sector ventures continueefforts in many parts of the world to introduce eitherThe lignin recovered is largely insoluble in water under

neutral or acidic conditions. It is soluble in organic sol- variations of the AlcellT process or processes using sol-vents other than ethanol. Such projects may eventuallyvents and in aqueous alkali; and it can be recovered with

a low content of contaminants such as sugars and ash. lead to the commercial availability of organosolv ligninsin the not-too-distant future.Table I shows typical properties of lignins recovered from

several biomass conversion processes [6, 14].Various biomass conversion technologies have

Lignins from Soda Pulpingadvanced to demonstration and pilot scale, and somelignins originating from such efforts have been available The fact that neither liquid fuel production from

biomass nor solvent pulping have reached commercialfor development purposes. Among these were ligninsfrom acid hydrolysis and steam explosion (see Table I). maturity has severely limited the market penetration of

sulfur-free lignins. Sulfur-free soda pulping of non-woodIn the 1980s a South African company (C. G. SmithSugar, Ltd.) offered a lignin called Sucrolin, which was feedstocks such as straw, sugar cane bagasse, flax, etc., is

Recent Industrial Applications of Lignin 41

Table I. Properties of Lignins from Biomass Conversion Processes (from Refs. 6 and 14)

Acid Hydrolysisa

TVA NYU C. G. Smith Steam Explosionb

Property HWc SWc HW Bagasse (Sucrolin)d HW (Angiolin)e Straw

Total Hydroxyl/C9 1.2 1.2 1.1 0.8 1.1 1.1% 10.5 11.2 9.3 9.6–9.8 8.5

Phenolic OH/C9 0.5 0.5 0.6 0.4 0.5 0.6% 4.7 4.1 5.4 4.3 5.4

Carboxyl/C9 0.15% 3.6

Carbonyl/C9 0.2 0.2 0.2 0.3% 2.8 2.6 2.1–2.9 4.7

Methoxyl/C9 1.2 0.6 1.3 0.6 1.0–1.1 0.6% 19.3 9.3 22.8 15.0–18.2 10.3

Tg (8C) NA 96 95 113–139 125Mn (3103)e 0.4–0.9 0.8 0.7 0.9 0.4Mw (3103)e 1.3–4.8 40.0 10.1 2.3–3.0 1.1

aAcid hydrolysis methods involve the use of catalytic amounts of mineral acids (usually H2SO4) under various conditions. The process advocatedby the Tennessee Valley Authority (TVA) of Muscle Shoals, AL employs a two-stage hydrolysis with 1 and 3% H2SO4 at steam pressures of 1.2and 1.9 MPa, respectively. The lignin is subsequently isolated by solvent or alkali extracting as described by Glasser and Strickland [13]. The datagiven are those of a typical ethanol extract. The process of New York University (NYU) achieves hydrolytic biomass pretreatment with 2% H2SO4

at 2208C in a twin-screw extruder. The lignin represents an alkali-soluble fraction of the combined residue [6]. The process of C. G. Smith Ltd.of South Africa involves the acid hydrolysis of sugar cane harvesting residue (bagasse) under conditions favoring the formation of furfural. Thelignin has become comercially available under the trade name Sucrolin; it is marketed by the Aldrich Chemical Comp. (cat. no. 37, 107-6); itrepresents the alkali-soluble residue of the combined insoluble hydrolyzate.

bSteam explosion is carried out on a continuous Stake Technology machine (Norval, Ontario). Conditions of steam explosion and lignin isolationhave been described by Wright and coworkers [11]. The lignin from Liriodendron tulipifera has been marketed under the trade name Angiolin.

cHW and SW designates hardwood and softwood resources, respectively.dSucrolin is commercially available from Aldrich Chemical Company (cat. no. 37, 107-6).eAngiolin has been available from the Biobased Materials Center of Virginia Tech, 1650 Ramble Road, Blacksburg, VA 24061-0503.For more details of the molecular weight determination, see Glasser and coworkers [14].

those used in wood pulping operations for recovery ofpracticed widely around the world and offers a potentiallyenergy and recycling of cooking chemicals. Conse-more readily available source of such lignins. Becausequently, many small nonwood mills are forced to dis-of their small size and economic constraints, many of thecharge their pulping effluents with little or no treatmentpulp mills processing nonwood resources cannot affordinto the environment. Lignin recovery from these efflu-the capital-intensive system developed for processingents can significantly reduce the environmental impactmuch larger quantities of spent pulping liquors, such asof these operations. Furthermore, the remaining effluent(after lignin removal) can be more easily purified by

Table II. Properties of Organosolv AlcellT Lignins from Different biological treatment. Another source of nonwood ligninsGenetic Origins

may come from larger nonwood mills that may have aconventional recovery process but want to incrementallyMixed Wheat

Property Hardwooda Straw Reed Kenaf expand their pulping capacity and need an economicallyand environmentally acceptable way to handle the excess

Total OH/C9 1.1–1.4 1.2 1.2 1.2liquor generated.Phenolic OH/C9 0.3–0.6 0.4–0.6 0.5–0.6 0.5

Recovery of these lignins is based on precipitation,Carboxyl/C9 0.1 0.1 ,0.1followed by liquid/solid separation and drying. Table IIIMethoxyl/C9 1.0–1.3 0.8 1.0 1.0

Tg (8C) 97 106–122 97 66–70 lists the properties of two of these products. A comprehen-Mn (3103) 0.6 0.7 sive review of the sources, properties, and uses of non-Mw (3103) 2.1–8.0 1.5 wood lignins has been published elsewhere [9, 10].

One of the difficulties of this type of process technol-aThis lignin is commercially available from Aldrich Chemical Company(cat. no. 37, 101-7). ogy when applied to nonwood fiber sources is that the

42 Lora and Glasser

Table III. Properties of Nonwood Soda Lignins of Different Origins antiplasticizers. Some lignins do not exhibit flow withtemperature, whereas others readily undergo fluid flow

Property Wheat Straw Hempwhen heated. In general, nonwood soda lignins may have

Total OH/C9 1.6 1.1 a wider range of properties than have been observed withPhenolic Oh/C9 0.8–0.9 0.6 wood lignins.Carboxyl/C9 0.1 This type of characterization of lignins in terms ofMethoxyl/C9 1.0 0.9

structure and functionality reveals relatively little aboutTg (8C) 160–185 158those differences that are hard to assess quantitativelyMn (3103) 1.8

Mw (3103) 3.3 [8]. Among the latter are the questions of solubility inorganic solvents, thermal deformability or flowability,and reactivity under specific reaction conditions. How-ever, these characteristics can be remedied by one orsoda spent pulping liquors often contain silica, which

may co-precipitate with the lignin, rendering it of lower several modification (derivatization) options [5, 15]. Thisleaves supply and variability questions as sole impedi-quality. Soda lignins have been available in Asia in the

past, but they are normally of low and variable quality. ments to industrial lignin utilization in structural mate-rials.The LPST precipitation process recently introduced com-

mercially in Europe (Granit S.A. of Lausanne, VD, Swit- The following provides information about a poten-tial, industrially practical method for the improvementzerland) is claimed to have solved this problem and to

have obtained soda lignins with relatively low ash and of solubility and thermal flow constraints by chemicalmodification, and it provides several examples of the usesilica content. As in other recovery processes, in the LPST

process there is a pH adjustment to bring the alkaline of lignins in industrial materials.liquor to an acidic environment. This forms a dispersion,which is treated in a carefully controlled maturation step

ASPECTS OF LIGNIN USE AS A POLYMERICunder a temperature and time schedule specific to theMATERIALtype of liquor being processed. In the maturation step, a

filterable slurry is formed. The cake obtained after filtra-Polymeric lignins can be used in thermosetting res-tion is washed and dried. LPST lignins recently became

ins, in thermoplastic blends, and in surfactant applicationscommercially available in powder and solution form from[3, 5]. The performance in thermosetting resins requiresa small flax specialty pulping commercial operation insufficient functionality of lignin in combination with ade-France. The mill produces about 40 M 3 per day of blackquate compatibility with cross-linking agents to preventliquor, which is processed to generate about 250,000 kg/phase separation [16]. Heat deformability and compatibil-year of lignin. Larger volumes of this product are expectedity with other polymers represents the major obstacle forto become available as lignin recovery is adopted by otherthe formulation of thermoplastic blends. Some of thesenonwood pulp mills.obstacles may be overcome by a variety of derivatizationIn summary, sulfur-free lignins, although known forreactions [15].many years, are gaining new interest as a result of a

diversification of biomass processing schemes. With thedisappearance of the popular organosolv lignins from Chemical Modification Examplescommercial availability, nonwood alkali extraction lig-nins are beginning to receive industrial attention. Soda By virtue of the inherent functionality of isolated

lignins, almost endless opportunities exist for their chemi-lignins may originate from many different plant sourcesand process variations. The analytical data obtained with cal modification. However, because of the propensity of

hydroxyl groups, esterification and etherification rankthese lignins that are focused on structure and functional-ity often reveal remarkably few differences that are of most prominent among the modification alternatives.

These and other modification examples have beensignificant consequence on generic uses. These soda lig-nins all have in common that they are of (1) low molecular reviewed elsewhere [15, 17–29]. Etherification with alky-

lene oxides (especially ethylene oxide) has been carriedweight, (2) high phenolic hydroxy content, and (3) rela-tively low (but variable) glass transition temperature. out on pilot plant scale, and hydroxyalkyl lignins have

been test-marketed (WESTVACO Corp., North Charles-However, their properties sometimes vary significantly.For instance, the thermal behavior of these lignins seems ton, SC, Hydroxyethyl Lignin, RLX 5044-21A).

Most result-oriented lignin derivatizations haveto depend on type (process and feedstock), and on thepresence of contaminants that may act as plasticizers or focused on improvements in solubility and glassiness or

Recent Industrial Applications of Lignin 43

brittleness. It is well recognized that solubility generallyfollows glass transition temperature (Tg), with solubility(in organic solvents) increasing as Tg declines. Becausethere are not many universally accepted quantitative toolsavailable to model solubility, but the methodology formeasuring glass transition temperatures is well estab-lished, most research has focused on relating glass transi-tion temperatures to derivatizations [30].

A previous article on this topic has employed theFujita-Kishimoto equation that relates Tg to plasticization[30]. Because derivatization amounts to internal plastici-zation, this equation can be employed for predictingchanges in glass transition temperature (and solubility)of modified lignins.

DTg 5ba2

C (1)

where DTg is the difference in glass transition temperaturebetween the unplasticized and the plasticized polymer, bis a parameter representing the plasticizer’s contributionto increased free volume, a2 represents the difference inthermal expansion below and above Tg (i.e., 4.8 3 1024 Fig. 1. Effect of (H2O) and internal alkylene, ethylene (EO), propylene

oxide (PO), and butylene oxide (BO) plasticization of lignin on theC21 and constant for most glasses), and C is plasticizerreduction of glass transition temperature DTg [30].content. Whereas the important b -parameter has been

found to be in the order of 0.27 for the plasticizationof lignin with water, the corresponding parameter forhydroxyalkly lignin derivatives has been given as being ble. Nonphenolic, powderous hydroxypropyl lignins arein the range of 0.05 to 0.15 [30]. However, it must be commercially available on the specialty chemicals marketrecognized that plasticization with water is limited to 5% (Aldrich Chemical Company, cat. no. 37,096-7).or less in the case of lignin, and that internal plasticization Hydroxyalkyl lignin derivatives with higher degreesby modification with alkylene oxides avoids this limita- of molar substitution, with extended alkyl ether chains,tion to narrow ranges of effectiveness. The effect of inter- can be prepared, but their synthesis is significantly morenal plasticization by reaction with ethylene oxide (EO), complicated [21, 25–27, 29].*propylene oxide (PO), and butylene oxide (BO) is illus- In summary, lignin modification by derivatizing sub-trated in Fig. 1 [30]. The reaction of lignin with alkylene

stituents offers opportunities for tayloring properties ofoxides has been studied extensively [19–29], and it

sulfur-free lignins to specific end uses. Many derivatiza-remains one of the most attractive etherification alterna-tion reactions have been reported in the past, and thesetives for the following reasons [31]: (1) it can be limitedderivatives can be expected to follow similar plasticiza-to weight gains of less than 25–30% by limiting thetion principles as those observed with alkylene oxides.modification reaction to only phenolic hydroxy groups;

(2) it can be carried out in aqueous alkali at room tempera-ture (if it is to be limited to phenolic hydroxy groups); (3) * The propoxylation procedure initially disclosed in an 1975 article

[19], which involves the heterogeneous reaction of lignin in propyleneit produces a unifunctional derivative with only aliphaticoxide suspension in the presence of solid KOH as catalyst and withhydroxy groups [28]; (4) the hydroxyalkyl lignin deriva-agitation at 1408C, has subsequently been found to be unpredictable

tive separates spontaneously from the alkaline, homoge- and dangerous. With insufficient agitation and with the catalystneous reaction mixture as the reaction progresses, because sequestered by lignin in its solid state, the reaction may lead to an

exothermic propylene oxide decomposition (or homopolymerization)the derivative becomes insoluble in aqueous alkali whenevent instead of the intended formation of a lignin-propylene oxideits acidic functionality (phenolic hydroxy groups) iscopolymer. Such a runaway reaction (with an amount of propylenedepleted (limited to alkyl groups larger than ethyl); (5)oxide of ,100 g) has resulted in a violent explosion that destroyed

the derivative is easily analyzed (by NMR spectroscopy) a laboratory used by Professor Knut Lundquist of Chalmers University[28]; and (6) the reaction product is highly soluble in a of Technology in Goteborg, Sweden in the late 1970’s. It was only

by coincidence that nobody was injured in the incident.variety of common organic solvents, and it is melt flowa-

44 Lora and Glasser

Table IV. Wear Characteristics of Brake Pads Produced with Combina- either powder or liquid form) or isocyanates. Organosolvtions of Organosolv Lignin and Phenolic Resin (from Nehez, Ref. 33) lignin has been found to be beneficial in all of these types

of binders [15, 34].Inner Outer Improve-In phenolic powder resin systems used for OSB,wear wear Average ment

Binder (mm/g) (mm/g) (mm/g) (%) lignin can be used as a direct partial replacement for thephenolic component, and the resulting boards are equal

100% phenolic 0.102 0.142 0.122 –to or better than the controls (Table V) [34, 35]. A North90% phenolic/10% lignin 0.071 0.122 0.096 21.3American producer of OSB successfully used this80% phenolic/20% lignin 0.090 0.139 0.114 6.6approach in the early 1990s to industrially produce com-mercial construction panels that met the requirements forexterior use. Lignin was blended on-site as a 5–25%

Examples of Industrial Utilizationreplacement of the powder phenolic resin normally usedas the binder in the face layers of the OSB panels. InConventional kraft and sulfite lignins are mature

products that have traditionally been used as dispersants addition to the cost benefits, the use of lignin led to animprovement of the environmental conditions in the paneland binders. Most interesting new applications are related

to sulfur-free lignins, because they offer greater versatility manufacturing facility, such as lower levels of dust in thearea around the blenders where the resin is applied to theand can be heat-processed without the irritating odor-

release commonly observed with commercial kraft lig- wood strands before board forming and pressing.Organosolv lignin can also be used with liquid phe-nins. Sulfur-free lignins are phenolic polymeric products

that can be used in many thermosetting applications, for nolic resins for OSB-manufacture, either by incorporationduring phenolic resin synthesis [36, 37] or by additionexample, in conjunction with phenolic, epoxy, or isocya-

nate resins [15, 32]. In addition to the cost and life cycle as a formulated dispersion. In this case, boards compara-ble to the control have been obtained (see Table V).advantages derived from using lignin, some environmen-

tal advantages can be obtained as well. For instance, The use of lignin in isocyanate binders for wood hasbeen explored on several occasions in the past [38–40].reduced formaldehyde emissions have been reported for

both the resin and the finished product when using lignin Industrial tests of isocyanate binders with lignin for OSB-production were carried out by ICI Polyurethanes (nowin phenol-formaldehyde resin systems.

Lignin in nature plays important biological roles, part of Huntsman Inc.) [40]. Results have shown that,when binding lignocellulosic materials (i.e., wood) withand its properties, such as antioxidant and antimicrobial

activity, among others, are well-documented. Sulfur-free isocyanates, the use of lignin in combination with specificlignin solvents results in improved performance relativelignins, being closer in structure to native lignin, are more

likely to capitalize on those properties. The following to lignin-free isocyanates used at the same binder level.provides a summary of recent industrial lignin applica-tions.

Table V. Strength Properties of OSB-Panels Produced with Combina-tions of Phenolic Resin and Lignin

Use of Lignin as Substitute for Phenolic Powder ResinsBinder

Phenolic powder resins are used as the binder in theControl Ligninmanufacture of friction products. The use of organosolv

Property 100% Phenolic 80% Phenolic/20% Ligninlignin as a partial replacement for these phenolic resinswas proposed by Nehez [33], and it was later successfully A. Powder Resinimplemented by a North American manufacturer of auto- MOR (psi)a 3456 3654

IB (psi)b 59.4 60.1motive brake pads and moldings on commercial scale.D-4 (lbs)c 144 143The use of 20% lignin/80% phenolic resin resulted inD-5 (lbs)d 115 141competitive advantages relative to controls prepared withB. Liquid Resin

100% phenolic resin. Improvements included the stability MOR (psi)a 5204 4866of the friction coefficient to temperature variations and IB (psi)b 83 295

D-4 (lbs)c 276 97improved wear behavior (Table IV).Oriented strand board (OSB) has become the domi-

aModulus of rupture.nant construction wood panel product in North America. bInternal bond.In its manufacture, wood strands are bonded with a binder cSingle cycle bending test—American Plywood Association.

dSix cycle bending test—American Plywood Association.under heat and pressure. The binders used are phenolic (in

Recent Industrial Applications of Lignin 45

Table VI. Strength Characteristics of OSB Panels Produced with Com- the purpose of achieving desired viscosity levels. Thebinations of Isocyanates and Organosolv Lignin [40] isocyanurate foams were prepared either with a commer-

cial polyol (control), or with the lignin polyol alone. TheSwelling IBb

isocyanate component constituted 40–55% of the foamsBinder (%) (kPa)on a weight basis. Other components, blowing agent,

Isocyanate 42.5 655 water, surfactants, etc., were added to the formulation inIsocyanate 1 2 pbwa lignin 1 1 pbw

accordance with industrial practice. Some lignin polyolsdimethylethyleneurea 32.5 977were incompatible with blowing agents; these samples

aPbw, parts by weight based on isocyanate. were not tested.bInternal bond. The density of all foams ranged from 1.5–2 pounds

per cubic foot (pcf) (Table VII). The compression strengthof the lignin-derived foam products was found to be

As a result it is possible to reduce isocyanate requirements adequate and in line with control materials. Dimensionalto achieve equal or better mechanical properties (Table stability at elevated temperature and moisture conditionsVI). The use of lignin also results in improved release determines the loss of foam substance during prolongedof the product from the press plattens, alleviating a prob- exposure to elevated temperature and moisture condi-lem that has hindered the more widespread use of isocya-nate binders [40].

Table VII. Composition and Properties of Industrial Foam Samplesa

Use of Lignin in Polyurethane Foams Foam Composition and Property Control Lignin Foamb

Lignins may readily become liquid fluids when mod- Polyurethane Foams:Lignin polyol (%) – 16–17ified with alkylene oxides [25]. These products consistCommercial polyol (%)c – 24–26of either or both (1) hydroxy alkyl lignins dissolved inIsocyanate (%) – 42–44poly(oxyalkylene glycols) of low molecular weight and/Additives (%)d – 15

or (2) chain-extended lignin-poly(oxyalkylene ether) Density (pcf) – 1.8–2.1copolymers [21–29]. These products differ in viscosity, Dimensional stability, DV (%)e 26–29whereby the solutions typically have lower overall shear

Isocyanurate Foams:resistance. Liquid alkoxylated lignin products are viewedLignin polyol (%) 0.0 26as potentially useful polyols for use in rigid polyurethaneCommercial polyol (%)f 35 0

foams [3, 19, 20, 23]. Their beneficial thermal degrada- Isocyanate (%) 54 54tion (fire resistant) properties have been revealed pre- Additives (%) 11.3 20

Density (pcf) 1.7 1.6–1.9viously [41]. Industrial tests of two types of foam productsDimensional stability, DV (%)e 21.1 5–15were conducted by ARCO Chemical Company in the lateCompression strength (psi)h

1980s, and the results are summarized in Table VII [31].P 23 18–24

The two different types of foam products investi- ' 7.7 9–13gated represent a polyurethane and a polyisocyanurate- Friability (%) 3.4 17–82type. Whereas polyurethanes constitute materials cross-

aResults of an Industry-University Cooperative Project between ARCOlinked with diisocyantes, isocyanurate foams contain anChemical Company, Newtown Square, PA, and Virginia Tech, Blacks-

isocyante component that has undergone some degree of burg, VA [31].homopolymerization and cyclization. Isocyanurate foams bFoams contained Angiolin, steam explosion soda-lignin from yellow

poplar (Liriodendron tulipifera), reacted with 30 moles of propylenecontaining lignin appear to be holding greater promiseoxide per kg of lignin in a pressure reactor using conditions describedthan polyurethane foams because of their tolerance toelsewhere [22]. The resulting polyols typically had homopolymer con-the darker color inherent to most lignin-based materials.tents of 19–22% and hydroxyl numbers of 120–200.

(However, it should be pointed out that nonphenolic lignin cThanol R-470X.derivatives can easily be bleached to a high brightness dContaining surfactant (0.5%), catalyst (0.2%), fire retardant (3%),

water (0.3%), and blowing agent (fluorocarbon) (1–11%).level by the use of such oxidants as hydrogen peroxideeMeasured at 1588F and .95% relative humidity for 28 days.[17, 42]). Isocyanurate foams are typically used in hiddenfChardol 336A.insulation panels.gContaining sulfonic N-95 (8.7%), surfactant, (0.5%), catalyst (1–

The results summarized in Table VII reveal that the.2%), water (0.4%), and blowing agent (fluorocarbon) (9.2%).

polyol component of all polyurethane foams was prepared hMeasured in parallel (II) or crosswise (') direction to continuousfoams (belt) former.by blending a lignin polyol with a commercial polyol for

46 Lora and Glasser

Table VIII. Performance Comparison of Printed Circuit Boards (PCBs)tions. The results determined for the isocyanurate foamsbetween Conventional FR4-Epoxy Resins and Epoxy Resins Formu-(see Table VII) reveal that all lignin-containing foamslated with ca. 50% Lignin Content (According to Kosbar et al.,

perform significantly better than the control products. ref. 49).Weight loss during exposure to elevated temperatures

FR 4 Control Lignin/Epoxyrepresents the ability of a foam to generate volatile, possi-Parameter Resin Laminatebly toxic, gaseous degradation products; therefore any

reduction in weight loss (i.e., improvement in dimen- Tg after lamination (8C) 128.3 140.4sional stability at elevated temperature) represents a sig- Min. viscosity at 120–1308C (Pa-s) 450 1250

Dielectric constant at 1 MHz 4 3.98nificant advantage. Lignin-based foams were sometimesDecomposition Temp. (8C) 312.6 309.7three or four times better than control foams. Friability,Moisture absorption (%) after 16 hhowever, which measures foam response to mechanical

boiling in water 2.18 1.76abrasion, was consistently high with all lignin-based poly- Total energy requirements for wiringols. This is of concern to the foam manufacturer, and board systems (GJ/100 kg

resin solids) 17.8a 11.0ait is unknown whether this parameter can be improvedNon-fossil per total energyby optimization.

requirements for wiring boardIn conclusion, the use of lignin in foam productssystems (%) 3.9a 18.1a,b

seems to hold some promise, especially regarding theexposure response to elevated temperature and humidity aData given are for the case of 100% metal recovery.

bData given are for kraft instead of organosolv lignin.conditions; but some handicaps (friability) are recognizedas well. Because aromatic polyols are currently basedon low-value by-products of the manufacture of PET

based wiring board was almost 40% less than that of acommodity plastics, lignin may not offer a substantialcontrol board, and (2) the nonfossil energy requirementscost advantage.increased nearly five-fold when lignin was used as acomponent (see Table VIII) [49].

Use of Lignin in Epoxy Resins

Lignin has also been used in epoxy resins, and many Lignin-Based Biodispersantsdifferent formulation approaches have been explored [15,

Soda lignins recovered by the LPST process of21, 43–46]. Marginally soluble lignins were formulatedGranit S.A. from flax pulping in France have been foundwith epoxy resins and cross-linked by heat; and ligninsto possess dispersant properties. They also have beenwere derivatized with epoxy functional groups followedfound to exhibit bacteriostatic and biocidal propertiesby cross-linking with anhydrides or diamines. A “drop-(Fig. 2).in” approach has most recently been taken by IBM for the

An Austrian company (Bioconsult Gesellschaft fuerproduction of printed circuit board-resins (PCB) [47–49].Biotechnologie mbH, Hallein, Austria) has recentlyWhereas conventional kraft and soda lignins proved todeveloped a lignin-based formulation for control ofbe hampered by high ionic contents, requiring extensivemicrobial populations in industrial water circuits, suchpostisolation purification protocols, organosolv lignin

could be used directly. All lignins created problems withorganic solvent–solubility, and they required special sol-vent mixtures for formulation. However, epoxy resins thatcontained approximately 50% lignin were successfullyprepared and applied to PCB production. These PCBsheld considerable promise, and test boards passed in allproperty categories (Table VIII). Epoxy-functional lig-nins were also prepared, and they could be used as solid,uncured films and powders formulated with amine cross-linkers in place and curable by heating without solvent[44]. Epoxy resins on lignin basis hold yet unexploredpromise for use in printed circuit boards and many otherapplications where epoxy resins excel.

The results of an extensive life cycle assessment Fig. 2. Inhibition of bacterial growth (Staphylococcus aureus) by differ-ent amounts of soda lignin from flax.revealed that (1) the total energy requirements for a lignin-

Recent Industrial Applications of Lignin 47

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3. H. L. Chum, S. K. Parker, D. A. Feinberg, J. D. Wright, P. A. Rice,S. A. Sinclair, and W. G. Glasser (1985) SERI/TR 231–488, 1–86.

4. J. D. Gargulak and S. E. Lebo (2000) ACS Symp. Ser. No. 742,304–320.

5. W. G. Glasser (2001) in F. C. Beall (ed.) The Encyclopedia ofFig. 3. Biocidal activity of various white water sludge formulations. Materials: Science and Technology, Elsevier Science Oxford, UK.(Data based on treatment of 5-L batches of white water containing 4.6 6. W. G. Glasser, C. A. Barnett, P. C. Muller, and K. V. Sarkaneng/L total solids of which 65% were CaCO3. The bars represent (A) (1983) J. Agri. Food Chem. 31, 921—930.control (blank); (B) soda straw lignin; (C) soda flax lignin; (D) a 7. W. G. Glasser, C. A. Barnett, and Y. Sano (1983) Appl. Polymercommercial lignin solfonate solution as used conventionally for sludge Symp. 37, 441–460.

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12. M. Ibrahim and W. G. Glasser (1999) Bioresource Technol. 70,is generally prevented by the addition of biocides of181–192.

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Technol. 13, 545–559.are critical parameters. The latter relates to the ease of15. W. G. Glasser and S. Sarkanen (Eds.) (1989) ACS Symp. Ser. No.resuspension of deposited slime via the shear forces of 397, 546.

the circulating fluid when the turbulence of the flow 16. W. G. Glasser (1989) ACS Symp. Ser. No. 385, 43–54.17. W. G. Glasser and R. K. Jain (1993) Holzforschung 47, 225–233.is adjusted.18. R. K. Jain and W. G. Glasser (1993) Holzforschung 47(3), 325–332.Published data obtained with industrial white water 19. O. H.-H. Hsu and W. G. Glasser (1975) Appl. Polymer Symp.

samples (5-L batches) treated with different biocidal mix- 28, 297–307.20. W. G. Glasser and O. H.-H. Hsu (1977) “Polyurethane intermediatestures revealed high effectiveness of nonwood soda lignins

and products and methods of producing same from lignin”. U.S.(Fig. 3). The product is currently used in several paperPatent 4,017,474; and Canadian Patent #1,097,617 (1981).

machines in Europe, where it is displacing more toxic, 21. W. G. Glasser, W. Nieh, S. S. Kelley, and W. de Oliveira (1990)“Method of producing prepolymers from hydroxyalkyl lignin deriv-less-environmentally friendly products. The product isatives”. U.S. Patent #4,918,167.dosed at the level of 100–200 ppm (based on weight of

22. W. G. Glasser, W. de Oliveira, S. S. Kelley, and L. S. Nieh (1992)paper produced) at a location of the water circuit where “Method of producing prepolymers from hydroxyalkyl lignin deriv-

atives”. U.S. Patent #5,102,992.good agitation and mixing are promoted.23. W. G. Glasser, O. H.-H. Hsu, D. L. Reed, R. C. Forte, and

L. C.-F. Wu (1981) ACS Symp. Ser. No. 172, 311–338.24. W. G. Glasser, L. C.-F. Wu, and J.-F. Selin (1985) in E. J. Soltes,

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J. Appl. Polymer Sci. 29, 1815–1830.component of thermoplastic materials [51]. The latter29. S. S. Kelley, W. G. Glasser, and T. C. Ward (1988) J. Wood Chem.

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