Thermoplastics with Very High Lignin Contents - American Chemical

Chapter 18

Thermoplastics with Very High Lignin Contents Yan Li and Simo Sarkanen

Downloaded by GRIFFITH UNIV on September 8, 2017 | http://pubs.acs.org Publication Date: November 30, 1999 | doi: 10.1021/bk-2000-0742.ch018

Department of Wood and Paper Science, University of Minnesota, St. Paul, MN 55108-6128

Annually huge quantities of kraft lignins are generated as byproducts from the chemical conversion of wood chips into pulp for making paper. Traditionally these industrial byproducts have been employed as fuel, but the imperative to maximize output can create a surplus of kraft lignin which is difficult to use profitably. A n important key to the effective utilization of kraft lignins may be discerned in their colligative behavior, which is dominated in a unique way by pronounced noncovalent interactions between the constituent molecular species; the resulting associated complexes appear to be assembled from the individual components in a specific order. The ensuing macromolecular domains in the solid state would be expected to contribute substantially to cohesiveness in lignin-based polymeric materials. Previously it had been very difficult to exceed an effective limit of 25-40% (w/w) for incorporating lignin derivatives into polymeric materials, but four or five years ago it proved possible to fabricate 85% kraft lignin containing thermoplastics that possess tensile strengths and Young's moduli which vary monotonically with the degree of intermolecular association. These could be the first authentic lignin-based plastics ever to have been made. More recently, alkylated 100% kraft lignin-based polymeric materials have been produced with tensile properties that are virtually identical to those of polystyrene. This may be the first time that substantial tensile strengths have been achieved with materials composed only of a simple lignin derivative. Efficacious plasticizers for alkylated kraft lignins could thus serve as a vital means for realizing large-scale lignin utilization.

The utilization of lignin derivatives as thermoplastics has challenged the ingenuity of lignin chemists for more than 20 years. Despite the extensive efforts that have been

© 2000 American Chemical Society Glasser et al.; Lignin: Historical, Biological, and Materials Perspectives ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

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invested in trying to incorporate lignin derivatives into polymeric materials, only limited success has been achieved for the broad variety of formulations examined (1). These attempts have been prompted by the potentially enormous quantities of industrial lignin derivatives that could be used in this way if only such uses could be reliably and profitably established. For example, the chemical pulping industry each year produces a huge amount of byproduct lignin from the conventional kraft process. It has been estimated that more than 25 million tons of kraft lignins are generated annually in the United States. However, less than 0.1 % of these byproducts are typically employed in applications other than as a relatively low value fuel in the recovery furnaces of pulp mills (2). Attempts to maximize pulp production in certain cases can result in a surplus of kraft lignin that can no longer be burned because the capacity of the recovery furnace has been exceeded (2). On the other hand, insistent demands to reduce pollution have fostered an atmosphere that is conducive to the development of environmentally friendly pulping methods; perhaps steam explosion and Organosolv pulping could have matured to the point of economic viability in this regard. The success of such new pulping methods relies heavily on the creation and marketing of high value-added lignin-based products (3). Moreover, there has been growing interest in producing bioethanol (4-6) and chemicals (7,8) from sugars; to the extent that these feedstocks are derived from lignocellulosic plant materials, such processes will generate lignin as a residual byproduct. Prospects for Lignin-Based Plastics Among all the conceivable uses to which byproduct lignins could be put, the development of lignin-based plastics with a wide range of predictable mechanical properties seems to be the most compelling. This is one of the few foreseeable ways of elevating lignin derivatives to the level of high volume commodity products. A n advantage of lignin-based polymeric materials is that they are, if suitably formulated, biodegradable by white-rot fungi (9). However, it is difficult to use lignin derivatives as polymeric materials with reproducible properties when the structures of lignin components themselves have been imperfectly elucidated: polymers with even very small variations in their chemical configurations can exhibit pronounced differences in mechanical behavior. The inexactness that has typified depictions of lignin structure has arisen from a particular assumption about the configuration of the native biopolymer to which most workers in the field have subscribed. For several decades, it was thought that lignins in wood are three-dimensional network polymers constituted from (phydroxyphenyl)propane units by random distributions of about ten different linkages (10). In the past, most work devoted to analyzing lignin structures has only provided partial characterization in terms of the relative ratios of the three monomer residues, namely the p-hydroxyphenyl, guaiacyl and syringyl units, and the frequencies of different interunit linkages determined by degradative methods (11). Analytical protocols have long been in place to estimate the contents of different functional groups in lignins (11), which are related to their reactivity when they are subjected to chemical modification before, for example, their incorporation into polymeric materials.

Glasser et al.; Lignin: Historical, Biological, and Materials Perspectives ACS Symposium Series; American Chemical Society: Washington, DC, 1999.


353 The main functionality in lignins is actually the hydroxyl group. Accordingly most of the attempts to use byproduct lignins as polymeric materials have depended on their phenolic nature or simply their hydroxyl functional group frequencies. On this basis byproduct lignins have been employed as polymeric components in phenolformaldehyde resins, polyurethanes, epoxies and acrylics (/). Alternatively they can be used as grafting backbones for the attachment of other synthetic polymer chains by means of free radical polymerization reactions (72). On the other hand, a contrasting approach has advocated the degradation of lignins into various monomeric and oligomeric components which would then be polymerized or copolymerized into a variety of different polymeric materials (75). However, the degradation reactions are usually very complicated and expensive, resulting in mixtures of many products, and so it has been difficult to justify the feasibility of such approaches. Thus no serious attempts have been made to use lignins as polymeric materials in their own right. This is due, at least in part, to the fact that the structural characteristics and the physicochemical properties of lignins are not fully understood. Owing to the lack of adequate analytical degradative methods, the primary structures of lignins, namely the sequences of interunit linkages along the macromolecular lignin chains, are still unknown. However, certain aspects of the physicochemical properties of lignins have been found to be inconsistent with what would be expected for a random threedimensional network polymer (74). Among them, the associative behavior of kraft lignin components has been found to be remarkably specific in character and, as such, has been interpreted in terms of vestiges of native macromolecular configurations nonrandomly disposed along the original polymer chains (75-27). Recent developments in the field of lignin biosynthesis have affirmed that the dehydrogenative polymerization of lignin precursors and the assembly of lignins in plant cell walls are under highly regulated biochemical control. Indeed, a mechanism for the formation and replication of macromolecular lignin primary structures has been proposed (22-24). If these ideas are correct, native lignins should possess nonrandom primary structures and well-defined configurations just like all other biopolymers. This could be critically important as far as the development of lignin-based polymeric materials is concerned. Thus a particular consequence of native macromolecular structure, namely the evident specificity in the intermolecular associative interactions between kraft lignin components (75-27), can be expected to favor the formation of cohesive domains in kraft lignin-based polymeric materials. Indeed, the idea of producing plastics with very high lignin contents was first enunciated in 1990 (7 7) on the basis of these intrinsically powerful intermolecular forces. About four years ago their impact on the mechanical properties of a series of 85% kraft lignin-based thermoplastics was revealed for the first time (25). At that point the feasibility of creating plastics with very high lignin contents was established. Thereafter thermoplastics based entirely on simple lignin derivatives, such as alkylated kraft lignins, have been in the process of being developed at the University of Minnesota. Physicochemical Properties of Kraft Lignins There is no doubt that the structures of lignin components, the nature of the intermolecular interactions between them, and the resulting associated complexes play



354 very key roles in determining the physical properties, such as chain-segmental mobility and response to deformation, of lignin-based polymeric materials. Therefore, it has become vitally important to elucidate lignin structures explicitly in order to establish meaningful structure-property relationships which could facilitate the systematic modification of lignin-based polymeric materials. Although the actual configurations and conformations of lignin components in solution and in the solid state have not been clearly established, investigations of the physicochemical properties of lignin derivatives have proven to be both informative and suggestive (15-21). Especially relevant is the finding that the associated macromolecular complexes, which are the predominant species in kraft lignin preparations, are not randomly assembled entities at all (16, 19). In solution, association between kraft lignin components takes place spontaneously and reversibly. Under aqueous alkaline conditions, dissociation of kraft lignin species occurs in dilute solutions (~0.5 g L ) , while a marked tendency to associate prevails at higher concentrations (~ 100 gL" *). The modes ofthese associative/dissociative processes were delineated through plots of M vs. 1 / M , which are characterized by slopes of -2 where is the number-average product of the molecular weights for the interacting species at any point during the process. From the observed invariance of , it was deduced that association between kraft lignin species is quite specific: not only do the higher molecular weight complexes interact selectively with lower molecular weight individual components, but each complex seems to possess a locus which is complementary to only one of the components present. Intermolecular nonbonded orbital interactions, dominated by those ofthe HOMO-L UMO type between the aromatic rings, were presumed to govern the underlying mechanism of the associative processes (16,19). These findings clearly imply that the configurations of the kraft lignin components are nonrandom; moreover, the individual molecular species must be able to adopt, at least transiently, more extended conformations that are compatible with macromolecular associated complex formation. 1

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The hydrodynamic behavior of the associated kraft lignin complexes was found to be in accord with a flexible lamellar configuration for these species (26), while the discrete kraft lignin components seemed to adopt appreciably expanded random coil conformations in aqueous alkaline solutions (20). Through an analysis that has become almost classical in its importance (26), attention was drawn to the fact that the thicknesses of monomolecular films of various lignin derivative fractions disposed on a water subphase appear to maintain constant values of about 2 nm despite significant differences in sample molecular weight (27). This observation is consistent with the working hypothesis that lignin derivatives consist of disk-like macromolecular fragments that have been cleaved from lamellar parent structures which are about 2 nm thick. Therefore it seems that lignin derivatives are predominantly composed of lamellar macromolecular complexes in which the individual molecular components are strongly held together by multiple noncovalent intermolecular interactions. Consequently, lignins may represent a class of macromolecules that are quite different from most synthetic polymers. Conventional methods for characterizing polymers cannot disclose the properties of the individual lignin components without the intrusion of the associated complexes. For example, size exclusion chromatography reveals the apparent molecular weight distributions of both associated complexes and


355

individual components (15-21). Differential scanning calorimetry (DSC) of kraft lignin fractions (isolated by successive organic solvent extractions from a parent preparation) were profoundly influenced by the effects of intermolecular noncovalent interactions: the variation of T with relative log M exhibited a marked discontinuity (17,28) unlike anything encountered with a conventional homologous polymeric series. The very low values of the Kuhn-Mark-Houwink-Sakurada exponents characterizing the variation of intrinsic viscosity with the molecular weight of kraft lignin (29) could arise from these associative phenomena and thus need to be reevaluated. Indeed, in neutral aqueous solution and in organic solvents where association is favored, the apparent average molecular weights of lignin preparations are often very large (above 10 ). Moreover, the apparent molecular weight distributions of acetylated methylated kraft lignin samples in D M F exhibit well-defined multimodal patterns of species encompassing these very high molecular weights, which indicates that the supramaeromolecular kraft lignin complexes are entities with well-defined configurations (17). The sizes of the species in electron micrographs, which are representative of acetylated methylated kraft lignin samples recovered from D M F , extend to apparent dimensions comparable to those of 20 million molecular weight polystyrene. In marked contrast, the profiles described by eluting kraft lignin samples from Sephadex G100 with 0.10 M N a O H are monomodal in shape and reflect effective molecular weight distributions which approach those of the discrete components very closely: the apparent weight-average molecular weights of kraft lignin preparations in aqueous 0.10 M N a O H seldom exceeds 5000 (16,17,19, 21). It should be borne in mind that the solubilities of lignin samples in aqueous alkaline solution and polar organic solvents such as D M F and DMSO could indicate that lignins are not highly cross-linked. In fact it has been estimated that the cross-linking density of native lignin macromolecules is only 0.052 or 1 in 19 units (29). g

n


7

Lignin-Containing Polymeric Materials Despite considerable effort, it had in the past been remarkably difficult to improve upon an effective incorporation limit of 25-40% (w/w) for lignin derivatives in polymeric materials without severely compromising their mechanical integrity (57). The major problem has been the brittleness caused by high levels of lignin components in such formulations. Blends of lignin derivatives with other synthetic polymers also have been investigated (32), but in most cases these resulted in multiphase morphologies and the corresponding lignin contents were typically quite low. Use of lignin derivatives as polyols in poiyurethanes has been the most intensively investigated application (5, 57, 33-35). This is not surprising since synthetic poiyurethanes can be strong and tough, soft and flexible, or very elastomeric, depending upon the components used and the cross-linking density. The effects of cross-linking density, the molecular weight of the incorporated lignin derivative, and the lignin content on lignin-containing poiyurethanes were all investigated through a series of materials based on a kraft lignin-polyether triol-poly(methylene diphenyldiisocyanate) system (35, 34). It was found that, at low and intermediate N C O / O H ratios, the tensile strengths and Young's moduli of the poiyurethanes derived from a particular kraft lignin fraction isolated for the purpose generally increased with



356

lignin content up to 25-33%. In contrast, at high NCO/OH ratios, the tensile strengths and Young's moduli attained their respective maximum values of -45 MPa and -1.2 GPa when the content of the kraft lignin fraction fell between 5 and 10%. When the contents exceeded 30-35%, however, the poiyurethanes obtained were always hard and brittle regardless of the NCO/OH ratios employed (33). A series of four fractions were obtained from a parent kraft lignin preparation by solvent extraction for the purpose of investigating the effects of their molecular weights upon the properties of the kraft lignin-polyether triol-polymeric MDI poiyurethanes (34). At a fixed N C O / O H ratio of 0.9, the tensile strengths and Young's moduli of the poiyurethanes generally (with one exception) increased with kraft lignin content up to 30%. Maximum values of -45 MPa and -1.5 GPa for tensile strength and Young's modulus, respectively, were obtained with particular fractions when the kraft lignin content was about 30%. On the other hand, the ultimate strains of the poiyurethanes prepared from the two higher molecular weightfractionsdecreased monotonically with kraft lignin content, while those of the poiyurethanes produced from the two lower molecular weight fractions exhibited maxima of about 180-250% at kraft lignin contents around 10%, whereafter they decreased rapidly as the lignin content increased. Overall, the tensile behavior of the poiyurethanes did not exhibit any uniformly systematic relationship with respect to the molecular weight of the kraft lignin fractions. As before, when the lignin contents exceeded 30%, the poiyurethanes obtained were glassy and brittle regardless of the molecular weights of the fractions employed (34). Poiyurethanes derived from hydroxypropyl kraft lignin-diisocyanate with and without poly(ethylene glycol) (31), and Organosolv (Alcell) lignin-poly(ethylene glycol)-polymeric M D I formulations (3) exhibited roughly comparable trends. Interestingly, lignin-derived poiyurethanes were capable of exhibiting tensile strengths between 50 and 100 M P a when the lignin contents were 25-40%. However, the ultimate elongations fell to quite low values as the lignin contents approached 40%. Chain-extended hydroxypropyl lignin-diisocyanate poiyurethanes showed -50-100% ultimate elongations with lignin contents in the 20-30% range, but these fell to low values when lignin contents exceeded 30% (31). The hydroxyl functionality also has been employed to incorporate lignins or lignin derivatives into epoxy resins (3 7-40). Hydroxyalkyl lignin derivatives were epoxidized by reaction with epichlorohydrin in aqueous alkali, and the resulting epoxy resins were cured with a suitable diamine (3 7,38). The network polymers thus produced exhibited behavior leading to ductile failure with tensile strengths and ultimate strains o f - 5 5 MPa and -8%, respectively, at lignin contents around 40%. In one case, a 57% lignincontaining thermoset material was produced with a tensile strength of 42 MPa and an ultimate strain of 6%. The tensile modulus of these cured epoxy resins increased linearly with lignin content, while the tensile strength increased with lignin content initially, but leveled off beyond 40% (38). Kraft lignins have also been employed as fillers or extenders in a diglycidyl ether of bisphenol A (DGEBA)-polyamine based epoxy adhesive resin (39). When the resins had been thermally cured, partial chemical bonding between the kraft lignin and the curing agent was supposed to have taken place. The cured lignin-containing epoxy resins were found to be homogeneous with lignin contents up to 20%, at which point


357 the shear strength also attained its maximum value. However, the latter was drastically reduced in the blend specimens having lignin contents in excess of 35% (39), a problem that probably resulted from phase separation. Another successful epoxy resin system was developed by mixing an alkaline solution of kraft lignin with the water-soluble epoxy compound, poly(ethylene glycol) diglycidyl ether, and a curing reagent, triethylene tetramine or hexamethylene diamine (40). It was found that gelation took place over a broad range of lignin contents between 20 and 80%, indicating that the functional groups of the kraft lignin reacted with the epoxide. From dynamic mechanical measurements, the peak temperature for the loss modulus, which roughly can be considered to correspond to the glass transition temperature (T ), increased uniformly from — 18 to ~85°C as the lignin content varied between 0 and 60%. Thus the glass transition temperature could be modified over a wide range by appropriately selecting the epoxy compound and the curing reagent (40).


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The First 85% Lignin-Based Thermoplastics The macromolecular domains resulting in the solid state from the strong noncovalent interactions between the individual molecular components (15,16,18-21) would be expected to contribute substantially to cohesiveness in lignin-based polymeric materials. The successful fabrication of plastics with very high lignin contents should thus be attainable (7 7). The feasibility of doing so was demonstrated for the first time more than four years ago with blends composed of 85% underivatized softwood kraft lignin with polyvinyl acetate) and two plasticizers (41). The tensile behavior of these polymeric materials depended directly upon the degree of intermolecular association between the intrinsic kraft lignin components (Figure 1 ). While the polymeric material based on the most associated kraft lignin preparation was not very deformable, the ultimate strain extended to over 65% in the case of the plastic containing the most dissociated kraft lignin preparation (25). The latter result affirms that true lignin-based materials can exhibit plastic behavior when the individual components are dissociated, i.e. separated sufficiently to allow the macromolecular chains to be drafted past one another in response to mechanical stress. In other words, it demonstrated that ligninbased materials are not inherently brittle because of their chemical structures, and they can indeed exhibit substantial flexibility. However, it is the extent to which the individual molecular species interact with one another that determines the mechanical properties of such materials. In respectively extending to 25 MPa and 1.5 GPa, both the tensile strength (o ) and Young's modulus (E) of the 85% lignin containing thermoplastics increased linearly with the degree of intermolecular association (Figure 2). Thus such polymeric materials are truly lignin-based in a very fundamental way. Moreover, these new thermoplastics are homogeneous and possess T 's near room temperature. Melt-flow index measurements have furthermore suggested that the 85% kraft lignin-based polymeric materials can be successfully extrusion-molded (25). Thus it is possible to produce plastics with mechanical properties that are based directly on the molecular structures of lignin derivatives. Now alkylated 100% kraft lignin-based polymeric materials have been created with very encouraging mechanical properties. max

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Alkylated 100% Kraft Lignin-Based Polymeric Materials Kraft lignins themselves do not exhibit melting behavior, and even their glass transition temperatures are difficult to detect clearly. Probably the melting points are so high that kraft lignins would undergo pronounced chemical changes before the onset of melting. The high melting point may be of structural origin, in that it could reflect a highly organized supramacromolecular structure characteristic of the materials in the solid state. In an attempt to obtain lignin preparations with more desirable thermal properties (and in the process generate materials that are not soluble in aqueous alkaline solution), kraft lignin preparations were alkylated, typically in the simplest way by methylation and ethylation. Quasi-melting behavior (coalescence of powdery particles into globules) was observed for the resulting alkylated kraft lignin samples, which is very important as far as the injection-molding of thermoplastics is concerned. Differential scanning calorimetric (DSC) studies of methylated kraft lignin fractions produced by ultrafiltration (before alkylation) showed broad endothermic transitions that were most prominent between 100 and 150°C (Figure 3). This indicates the onset of chain segmental motion and may correspond to a range of glass transition temperatures that could overlap with the observed quasi-melting behavior of the kraft lignin derivatives. To determine what the effects of the low molecular weight components might be on the strengths of the alkylated 100% kraft lignin-based materials, the parent kraft lignin was subjected to single-step fractionation by ultrafiltration in aqueous 0.10 M NaOH through a 10,000 nominal molecular weight cutoff membrane (Figure 4). The parent kraft lignin preparation and higher molecular weight fraction were alkylated with the corresponding dialkyl sulfates in aqueous 60% dioxane solution at pH 11-12 followed by complete methylation with diazomethane in chloroform. It was found that solvent-casting from DMSO of the ethylated methylated derivatives yielded cohesive materials. These alkylated 100% kraft ligmn-containing polymeric materials exhibited very encouraging mechanical properties (Figure 5). The Young's moduli (-1.9 GPa) and the tensile strengths (-37 MPa) are remarkably high in view of what had been previously accomplished for the mechanical properties of materials with extremely high lignin contents: this is the first time that a polymeric material composed exclusively of any simple lignin derivative has been shown to possess measurable tensile strength. Therefore these alkylated kraft lignin preparations are promising thermoplastic materials in their own right, even though their tensile energy absorption to fracture and maximum strain are still fairly low. Interestingly, the ethylated methylated higher molecular weight kraft lignin sample exhibited better tensile behavior than the parent preparation, as would be expected since polymers with higher molecular weight generally produce stronger materials. Earlier results showed that, under these casting conditions, variations in the degree of association between the individual molecular components before derivatization seem to have no systematic effect upon the Young's moduli of the corresponding alkylated kraft lignin-based polymeric materials (41). This indicated that the high concentrations in DMSO during solvent casting promote the formation of highly associated species from the alkylated kraft lignin components regardless of the degree of association of the lignin samples before derivatization. Thus, while intermolecular noncovalent


361


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362

Parent Kraft Lignin Preparation and Higher Molecular Weight Fraction

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364

Table I. Some Common Polymeric Materials: Tensile Strength, Young's Modulus and Elongation to Failure (42) Tensile Strength MPa

Young's Modulus GPa

Elongation to Failure %

Polyethylene (LDPE)

14

0.22

400

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2.1

2

Polypropylene

35

1.4

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Polymeric Material

Alkylated kraft lignin

a

a

1000

Polymeric material solely composed of ethylated methylated kraft lignin fraction.

interactions are required to maintain the essential cohesiveness of the materials, it is presumably the unusually high propensity for extensive intermolecular association that is inherently responsible for the brittleness of kraft lignin-based polymeric materials. The alkylated 100% kraft lignin-based thermoplastics are virtually identical to polystyrene as far as their tensile properties are concerned (Table I). The problem is that both polystyrene and the 100% kraft lignin-based polymeric materials are quite brittle; they therefore need to be plasticized or toughened i f the goal of injectionmolding useful components from them is to be realized. The difference is that polystyrene is extremely easy to plasticize while 100% kraft lignin-based polymeric materials had never previously been plasticized. The difficulty in achieving adequate plasticization for the latter presumably resides in the strong noncovalent forces that prevail between the individual kraft lignin components. Therefore suitable plasticizers must be able to overcome, but not totally destroy, the strong intermolecular interactions between kraft lignin components in facilitating their chain segmental mobility while preserving the integrity of the material under externally imposed tensile stresses. In this connection, it should be borne in mind that kraft lignin-based materials certainly do not have to be brittle, as was established by the tensile behavior of the thermoplastics based on highly dissociated kraft lignin preparations (25). Thus major efforts are under way at the University of Minnesota in the search for plasticizers that act effectively with alkylated 100% kraft lignin-based polymeric materials to achieve sufficient degrees of plasticization for producing tough components by injection-molding techniques.


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Conclusions Levels of incorporation for lignin derivatives in polymeric materials have been limited either by brittleness resulting from association between the lignin components, or by phase separation arising from incompatibilities within the incipient blends. However, it has now been shown that thermoplastic materials with very high lignin contents can be created. Simple kraft lignin derivatives have been found to be very promising in this regard. Alkylated 100% kraft lignin-based polymeric materials have been successfully produced with tensile properties that are virtually identical to those of polystyrene. With the development of effective plasticizers (43), entirely new types of thermoplastics possessing a broad range of mechanical properties, while embodying very high contents of simple lignin derivatives, can be anticipated in the not too distant future. Acknowledgments Acknowledgment for support of this work is made to the United States Environmental Protection Agency through the National Center for Clean Industrial and Treatment Technologies (although it does not necessarily reflect the views of the Agency or the Center, so no official endorsement should be inferred) and to the Minnesota Agricultural Experiment Station. Paper No. 984436801 of the Scientific Journal Series of the Minnesota Agricultural Experiment Station, funded through Minnesota Agricultural Experiment Station Project No. 43-68, supported by Hatch Funds. Literature Cited 1. Lignin—Properties and Materials; Glasser, W. G., Sarkanen, S., Eds.; American Chemical Society: Washington, D. C.; ACS Symp. Ser. 1989, 397. 2. Kirkman, A . G.; Gratzl, J. S.; Edwards, L. L. Tappi J. 1986, 69, 110-114. 3. Vanderlaan, M. N.; Thring, R. W. Biomass and Bioenergy 1998, 14, 525-531. 4. Gregg, D.; Saddler, J. N. Appl. Biochem. Biotechnol. 1996, 57/58, 711-727. 5. Cao, N. J.; Krishnan, M. S.; Du, J. X.; Gong, C. S.; Ho, N. W. Y . ; Chen, Z . D.; Tsao, G. T. Biotechnol. Letters 1996, 18, 1013-1018. 6. Vinzant, T. B.; Ehrman, C. I.; Adney, W. S.; Thomas, S. R.; Himmel, M. E. Appl. Biochem. Biotechnol. 1997, 62, 99-104. 7. Dorsch, R.; Nghiem, N. Appl. Biochem. Biotechnol. 1998, 70-72, 843-844. 8. Donnelly, M. I.; Millard, C. S.; Clark, D. P.; Chen, M. J.; Rathke, J. W.Appl. Biochem. Biotechnol. 1998, 70-72, 187-198. 9. Milstein, O.; Gersonde, R.; Hüttermann, Α.; Chen, M.-J.; Meister, J. J. Appl. Environ. Microbiol. 1992, 58, 3225-3232. 10. Sakakibara, A . Wood Sci. Technol. 1980, 14, 89-100. 11. Adler, E. Wood Sci. Technol. 1977, 11, 169-218. 12. Meister, J. J. In Renewable-Resource Materials—New Polymer Sources; Carraher, C. E., Jr., Sperling, L. H., Eds.; Plenum Publishing Corp.: New York, 1986; pp 305-322. 13. Lindberg, J. J.; Kuusela, Τ. Α.; Levon, K . ACS Symp. Ser. 1989, 397, 190-204. 14. Goring, D. A . I. ACS Symp. Ser. 1989, 397, 2-10.



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Thermoplastics with Very High Lignin Contents - American Chemical

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