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The Lignin Problem
Simo Sarkanen
Department of Bioproducts
and Biosystems
Engineering
University of Minnesota
OH
O
O
OH
OH
O
OH
OHOMe
O
OHC OH
OH
O
O
MeOHO
OMe
MeO
OMeHO
O
O
MeO
HO
O
O
OOMe
O
O
HOOH
OMe
HO
O
MeO
OMe
OMe
OMe
HO
OH
OH
OH
HO
MeO
OH
OH
O OH
OH
OMe
O
MeO
O
O
MeO
OH
HO
O-4
(carbohydrate)
CHO
HO
MeO
CHO
OHC
HO
O
MeOOH
MeOHO
OMe
OH
OOMe
O
O
OHO
OMeO
HO
HO
OMe
MeO HO
O
OH
OH
MeO
OH
O
O
OH
Me
O
Schematic Depiction
Proposed in 1979for Structural
Features ofSoftwood Lignins
Reproduced with permission from: Chen, Y.-r., and Sarkanen, S. Phytochemistry Reviews 2003, 2, 235-255 ©2004 Kluwer
Academic Publishers, which was adapted from A. Sakakibara, Wood Sci. Technol. 1980, 14, 89-100.
Schematic Depiction Proposed in 1996
for Structural Featuresof Softwood Lignins
Reproduced with permission from: Chen, Y.-r., and Sarkanen, S. Phytochemistry Reviews 2003, 2, 235-255 ©2004 Kluwer
Academic Publishers, which was adapted from G. Brunow
et al. ACS Symp. Ser. 1998, 697, 113-147.
O
HO
O
HO
MeO
OHOH
O
HO
OMeO
OMe
O
O
OH
MeOOH
OOH
HO
MeO
OMe
OMeOH
O
OH
OMe
OOMeHO
HOO
HO OH
O
MeOOH
HO
OH
OMe
OH
MeOOH
OHHO
HO
O
O
OMeO
HO
OMeO
HO
OMeOH
OH
OH
O
OHOH
O
OOH
MeOOMe
O
HO
HO
O
MeO
OHOH
O
MeO
OHC
HO
MeO
HO
O
HO
MeO MeO
OH
OH
OOHlignin
OMe
lignin
O
OMe
OH(CHO)
O
O
MeO
HOO
OH
OH
OMe
4-O-5-5'
4'-O-8
8-O-4'
8-O-4'8-O-4'
8-O-4' 8-O-4'
8-O-4'
8-O-4'
8-O-4'
8-O-4'
8-O-4'
8-O-4'
5'-O-4
8-5'
1'-8
8-5'
4'-O-8
8-8'
4-O-5-5'
OH
8-5'
8-O-4'
MeO
hypothetical 1'-8 precursor
O
1983—Original Report about Lignin Peroxidase
1986—Polymerization of Lignin Preparationby Lignin Peroxidase
in vitro
1993—Simultaneous Degradation and Polymerization of Lignin Preparation by Lignin Peroxidase
in vitro.
Without expressing lignin peroxidase, T. cingulata biodegradeslignin almost as rapidly as P. chrysosporium.
T. cingulata cDNAlibrary synthesis
mRNA isolated from fungal hyphae expressing lignin biodegrading activity inhomogeneous solution culture.
50 x 106
independent clones with average cDNA
insert
size of 1100 bp.
De novo mass spectroscopic sequencing of lignin depolymerase polypeptidesfor screening cDNA
library
Conventional Approach to Producing Lignin-Containing Polymeric Materials
Introduce a suitable lignin derivative in progressively greater
proportions into another perfectly good polymeric material
—until the mechanical properties are fatally compromised.
Examples: phenolformaldehyde resins, polyurethanes, epoxies, acrylics
—also: some polymer blends with lignin derivatives,synthetic polymer chains grafted onto lignin
derivative backbones
Incorporation limit for lignin derivative: typically 25–40%
kraft lignin content %
E G
Pa
0.0
0.5
1.0
1.5
0 5 10 15 20 25 30 350
10
20
30
40
450M 620;M 900M 1290;M 1710M 2890;M 3800M 10500;M
nw
nw
nw
nw
==
==
==
==
σ max
MP
a
Young’s moduli
(E) and tensile strengths (σmax
) of cured kraft lignin–polyether triol–polymeric MDI polyurethanes with reactant NCO/OH ratio of 0.9. Effects of softwood kraft lignin content in the form of four fractions with different molecular weights isolated from parent preparation by solvent extraction. Data from: H. Yoshida, R. Mörck, K.P. Kringstad
& H. Hatakeyama, J. Appl.
Polym. Sci., 1990, 40, 1819-1832.
lignin ester content (wt %)0 10 20 30 40 50
tens
ile s
treng
th (M
Pa)0
20
40
60
80
○
acetateΔ
butyrate◊
hexanoate□
laurate
lignin ester:
Effect of Organosolv Lignin Ester Content
on Tensile Strengths ofMultiphase Blends with
Cellulose Acetate Butyrate
Tensile strengths of multiphase blends of cellulose acetate butyrate with lignin esters. Variation with content of Organosolv lignin esters. Data from: I. Ghosh, R.K. Jain & W.G. Glasser, J. Appl. Polym. Sci. 1999, 74, 448-457.
R
280
60 30 10 6 3 1 0.6 0.3
V
A
0.0 0.5 1.0 1.5 2.0
Optima XL−Amolecular weight analyses: sedimentation equilibrium in
3-w 10x M
Paucidisperse Kraft Lignin Fractions Isolated from Parent Preparation
Sephadex
G100/aqueous 0.10 M NaOH
elution profiles.
VR
0.0 0.4 0.8 1.2 1.6
log
mol
. wt.
2.5
3.0
3.5
4.0
4.5
5.0
softwood kraft lignin componentsexhibit neither crosslinking
nor long-chain branching
zM
wM
Polydispersities ofKraft Lignin Components
with givenHydrodynamic Volume
Semilogarithmic
plots of average molecular weights versus size-exclusion chromatographic elution volume.
0.0
60 30 10 6 3 1 0.3
7
1 7
1
V R
A 280
M x 10-3w
0.0 0.5 1.0 1.5 2.0 2.5
Kraft lignins with different degrees of association
Molecular weight distributions of kraft lignin preparations differing solely in their degrees of intermolecular association: (1) = 5330, = 936; (2) = 6740, = 1300; (3) = 9670, = 1840; (4) = 12200, = 1930; (5) = 14500, = 2850; (6) = 20500, = 4640; (7) = 28300, = 10500. (Sephadex
G100/aqueous 0.10 M NaOH
elution profiles.)
nM
wM nM nMwMnMwM
nMwMnM
wMnMwM wM
0.00010.0010.010.11.01040
elution volume mL
A320
3
1
3
1
10 15 20 250
polystyrene standard mol. wt. x 10 -7
Molecular Weight Distributions of Acetylated Methylated Supramacromolecular Associated Kraft Lignin Complexes in DMF
Apparent molecular weight distributions in DMF of acetylated methylated kraft lignin preparations differing in degree of association. Elution profiles from 107
Å
pore-size poly(styrene-divinylbenzene) column monitored at 320 nm. Samples were fractionated through Sephadex
LH20 in aqueous 35% dioxane
after association at 195 gL-1
for (1) 6740 h, (2) 3910 h and (3) 1630 h in aqueous 1.0 M ionic strength 0.40 M NaOH.
Molecular weight distributions of parent kraft lignin preparation and higher molecular weight fraction (Sephadex
G100/aqueous 0.10 M NaOH
elution profiles).
Starting Materials Used in Alkylated Kraft Lignin-Based Thermoplastics
Tensile behavior to failure for polymeric materials composed entirely of ethylated methylated kraft lignin: (A) parent preparation; (B) higher molecular weight fraction. (Stress-strain σ-ε
curves delineated by Instron
model 4026 Test System employing 0.05 mm min-1
crosshead speed with 9 mm specimen gauge lengths.)
Polymeric MaterialTensile
StrengthMPa
Young’sModulus
GPa
Elongationto Failure
%Polyethylene (LDPE) 14 0.22 400Polystyrene (HI) 28 2.1 2Polypropylene 35 1.4 400Alkylated kraft lignin b 37 1.9 2Acrylonitrile-Butadiene-Styrene 38 2.0 4Poly(vinyl
chloride) (rigid) 47 1.6 60Epoxy cast 59 2.4 5Polyurethane (thermoset) 90 0.41 1000
SOME COMMON POLYMERIC MATERIALS
Tensile Strength, Young’s Modulus and Elongation to Failure a
a
Handbook of Plastic Materials and Technology; Rubin, I. I., Ed.; Wiley: New York, 1990.b
Polymeric material solely composed of ethylated methylated kraft lignin fraction.
σ M
Pa
Δε
60%80%
25%
0.0 0.2 0.4 0.6 0.8 1.0 1.2
0
10
20
30
40
50
0%
20%
30%
40%
Plasticization of Ethylated Methylated
Higher Molecular WeightKraft Lignin Fraction with
Poly(butylene adipate)
Progressive plasticization of ethylated methylated higher molecular weight kraft lignin-based polymeric material by poly(1,4-butylene adipate). (Stress-strain σ-ε
curves delineated by Instron
model 4026 Test System employing 0.05 mm min-1
crosshead speed with 9 mm specimen gauge lengths.)
σ M
Pa
Δε
30%
35%
40%
50%
60%
70%
0%
0.0 0.2 0.4 0.6 0.8 1.0
0
10
20
30
40
50
Plasticization of Methylated Higher
Molecular Weight Kraft Lignin Fraction with
Poly(trimethylene glutarate)
Progressive plasticization of methylated higher molecular weight
kraft lignin-based polymeric material by poly(trimethylene
glutarate). (Stress-strain σ-ε
curves delineated by Instron
model 4026 Test System employing 0.05 mm min-1
crosshead speed with 9 mm specimen gauge lengths.)
poly(trimethylene adipate)−methylated highermolecular weight kraft lignin fraction
0.0 0.2 0.4 0.6 0.8 1.0
-50
0
50
100
150
kraft lignin weight fraction
T g o C
poly(trimethylene glutarate)−methylated highermolecular weight kraft lignin fraction
Comparison betweenTg –Composition Curves
for Methylated Kraft Lignin-based Thermoplastics
respectively Embodying Stronger and Weaker
Interactions withAliphatic Polyesters
Dependence of Tg
on composition of blends involving the methylated higher molecular weight kraft lignin fraction and either ( ) poly(trimethylene adipate) or ( ) poly(trimethylene glutarate)
STRATEGIES FOR IMPROVING PLASTICIZER EFFICACY
The mechanical properties of kraft lignin-based thermoplastics rest upon supra-macromolecular complexes containing many thousands of individual components.
Any tendency to dismantle these huge associated entities must be minimizedwhile blend homogeneity must be preserved.
Thus (1) lignin–plasticizer interactions should be adjusted to the thresholdrequired for blend miscibility
while (2) the ability of the lignin components to interact with the plasticizerneeds to be enhanced under these less favorable circumstances.
The first condition can be simply achieved by judicious selection of plasticizer.
The second condition can be approached by introducing low molecular weight compounds that, in binding to peripheral lignin components in the complexes, synergistically enhance their
interactions with the plasticizer.
Thus the impact of the plasticizer will be accentuated without strengtheningits intermolecular attraction to the lignin components.
0.0 0.2 0.4 0.6 0.8 1.0-50
0
50
100
150
200
lignin weight fraction
T g
o C
poly(trimethylene succinate)−methylated higher molecular weight kraft lignin fractionpoly(ethylene glycol)−methylated highermolecular weight kraft lignin fraction
Variation of Tg
with composition for blends of methylated higher molecular weight kraft lignin fraction with (○) poly(trimethylene
succinate) and (●) poly(ethylene
glycol); curve fits are depicted to Gordon-Taylor equation.
Comparison between Tg –Composition Curves
for Methylated Kraft Lignin-Based Thermo-
plastics respectively Embodying Stronger
and Weaker Interactions with Polymeric Plasticizers
35% PEG
25% PEG
30% PEG
Δ ε
σ M
Pa
20% PEG
40% PEG
0%
poly(ethylene glycol)−methylated higher molecularweight kraft lignin fraction (PEG)
0.0 0.2 0.4 0.6 0.8 1.00
10
20
30
40
50 poly(butylene adipate)−methylatedhigher molecular weight
kraft lignin fraction (PBA)
20% PBA
30% PBA
35% PBA
40% PBA
50% PBA
45% PBA
Comparison between Efficacies of
Plasticization with Poly(ethylene glycol)
and Poly(butylene adipate)
Plasticization of methylated higher molecular weight kraft lignin fraction by poly(ethylene
glycol) and poly(butylene
adipate) (stress–strain σ–ε
curves delineated by Instron
Model 4026 Test System employing 0.05 mm min-1
crosshead speed with 9 mm specimen gauge lengths).
CONCLUSIONSSimple alkylated kraft lignin-based polymeric materials are plasticized inhomogeneous blends with ~30% (w/w) levels of suitable low-Tg polymers.
The kraft lignin species in such thermoplastics range from individual componentsto huge supramacromolecular complexes.
These supramacromolecular complexes tend to be dismantled counter-productively when the polymeric plasticizer interacts more strongly with the individual kraft lignin components.
Hence plasticizer efficacy can only be improved by enhancing the effect of the interactions with the kraft lignin components without increasing the actual strengths of the corresponding intermolecular forces.
Miscible low-Tg aliphatic polyesters affect spacings between pairs of edge-on aromatic rings much more than between those that are cofacially disposed.
Moreover, suitable small molecules that interact preferentially with kraft lignin components can act synergistically with polymeric plasticizers in alkylatedkraft lignin-based thermoplastic blends.
Acknowledgement for support of this work is made to the United States Department of Agriculture (Grant 98-35103-6730), to the United States Environmental Protection Agency through the
National Center for Clean Industrial and Treatment Technologies, to the Vincent Johnson Lignin Research Fund, and to the Minnesota Agricultural Experiment Station.
Acknowledgements