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The Tincture of Kraft Pulps Art J. Ragauskas, Tom J. Dyer Institute of Paper Science and Technology Georgia Institute of Technology
The Tincture of Kraft Pulps
Art J. Ragauskas, Tom J. Dyer Institute of Paper Science and Technology
Georgia Institute of Technology
Overview of Kraft Pulping• + 200 year old technology• Insensitive to wood species, relatively rapid • Integrated system is conservative on chemicals and energy• Yields a high quality product• Withstands numerous challenges, mechanical pulping, soda, soda/AQ, organo-solv• Limitations:
• Yield•AQ, H-factor, EA/Sulfidity
• Odor• Chemical scrubber technology
• Pulp color• $40 – 60 x 106 bleach plant????????????• WHY
The Problem…• Holzer/1934: Presence of sulfur darkens the color of
kraft pulp more than that of a comparable soda pulp
• Bard/1941: Color may be produced by adsorption or absorption of colored material from the black liquor
• Pigman and Csellak/1948: Among the first to pin-point lignin and its degradation products as responsible for the bulk of the color found in kraft pulps, possible carbohydrate contribution
• Hartler and Norrström/1960, 70’s: Overall, the contribution from carbohydrates is low throughout the cook
Possible Chromophoric StructuresL
O
O
O
O
OCH3L OM
O
L
L
O
O
HO
L
OH
O
O
L
OH
OCH3
Ortho-Quinone Para-Quinone Catechol-Metal Complex
Hydroxy-Quinone
Stilbene or Enol EtherL O
OCH3
OHO
O
L
Alpha-Carbonyl Stilbene-Quinone Carbohydrate Derived
(Conjugated Carbonyl, Aromatic, Furan Derivatives)
Research Goals
• To explore the fundamental nature of chromophore formation during kraft pulping
• To obtain a better understanding of and define the particular lignin functionalities that are responsible for color formation in kraft pulps
• To determine the kraft pulping parameters having the greatest impact on color formation during kraft cooking
• Chart color formation through the kraft pulping process
Experimental – Kraft Pulping
• Added 100 g southern pine wood chips to individual vessels
Experimental – Kraft Pulping
• Added 100 g southern pine wood chips to individual vessels
• Added mixture of NaOH, Na2S (4:1 L/W)
NaOH + Na2S
Experimental – Kraft Pulping
• Added 100 g southern pine wood chips to individual vessels
• Added mixture of NaOH, Na2S (4:1 L/W)
• Vessels placed in rotating digester
Experimental – Kraft Pulping
• Added 100 g southern pine wood chips to individual vessels
• Added mixture of NaOH, Na2S (4:1 L/W)
• Vessels placed in rotating digester
• 90 min. ramp to maximum temperature
020406080
100120140160180
0 50 100 150
Time (minutes)
Tem
pera
ture
(o C)
Experimental – Kraft Pulping
• Added 100 g wood chips to individual vessels
• Added mixture of NaOH, Na2S (4:1 L/W)
• Vessels placed in rotating digester
• 90 min. ramp to 170oC• Cooled in water bath
Experimental – Kraft Pulping
• Disintegrated wood chips
Experimental – Kraft Pulping
• Disintegrated wood chips• Thoroughly washed and
screened pulps
Experimental Design
• Central composite design– Regression equation
• Constant lignin content– Three process variables
• % EA (14-21%)• % Sulfidity (23-57%)• Maximum temperature (162-178°C)
– 20 experiments
x3
x2
x1
806170401820806170401819806170401818806170401817806170401816806170401815806178.4401814806161.640181364617056.81812121717023.218115651704021.41013521704014.69522175502089291755016779017530206129417530165570165502049291655016379016530202134916530161
H-FactorTemperatureSulfidity, %EA, %Sample
Pulping Results
• Lignin Content– Target 4.5% lignin
– Confidence interval• 6 replicates
– Samples 15-20• ± 0.32%
– All samples are statistically the same
• At 95% C.I.
3.75
3.95
4.15
4.35
4.55
4.75
4.95
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Sample
Lig
nin
Con
tent
, %
0
0.5
1
1.5
2
2.5
3
350 400 450 500 550 600 650 700 750
Wavelength (nm)
K-M Remission Function
• Use optically thick handsheets
• Diffuse reflectance– Integrate k/s curve over
visible region– Chromophore Index
Area under Curve =Chromophore Index
k/s
Results – Chromophore Index
• Chromophore Index– % EA, % Sulfidity
• Significant parameters
– Max. Temperature• Not significant
– Curvature• Due to quadratic
relationship
160 150 140 130 120 110
Results – Chromophore Index
• Chromophore Index
– Minimal color• ↑ % EA, ↓ % Sulfidity
– Maximum color• ↓ % EA, ↑ % Sulfidity 160
150 140 130 120 110
Pulp Color Formation vs. Cooking Time
50
100
150
200
250
300
50 70 90 110 130
Cooking Time (minutes after 100oC)
Chro
mop
hore
Inde
x
21.4% EA, 23.2% Sulfidity 14.6% EA, 56.8% Sulfidity
Color Formation vs. Lignin Content
50
100
150
200
250
300
0 5 10 15 20 25Klason Lignin Content (%)
Chro
mop
hore
Inde
x
21.4% EA, 23.2% Sulfidity 14.6% EA, 56.8% Sulfidity
What is the Fundamental ComponentContributing to the Difference in Color for These Pulps?
Surface Lignin ESCA
• Electron Spectroscopy for Chemical Analysis– Bombard surface with x-rays
• Substrate ejects electrons– Specific binding energy– Depends on type of atom
• Measures 2-9 nm into surface
• Treated paper samples– Mercuric acetate
• Specific for lignin– Westermark (1999)– Heijnesson et al. (2003)
X-raysource
Sample
Channeltrondetector
Electrons
Analyser
Surface Lignin Content vs. Bulk Lignin
R2 = 0.99
01020304050607080
0 5 10 15 20 25Klason Lignin Content (%)
Surfa
ce L
igni
n Co
nten
t (%
)
21.4% EA, 23.2% Sulfidity 14.6% EA, 56.8% Sulfidity
Conclusion: Color Differences are NOT Due to Difference in Surface Lignin Content
Other Parameters Must Be Involved!
Color Formation vs. Surface Lignin
50
100
150
200
250
300
0 10 20 30 40 50 60 70 80Surface Lignin Content (%)
Chro
mop
hore
Inde
x
21.4% EA, 23.2% Sulfidity 14.6% EA, 56.8% Sulfidity
Residual Lignin Studies
Isolation of Residual Lignins
Pulp Reflux for 2 hrs (4% cons.)
in 0.1N 9:1 dioxane:HCl
Filter (coarse) Filter (fine) Neutralize
Remove dioxane under reduced
pressure
Precipitate lignin wash lignin x3
Lyophilize
Spectral Characterization
Color Formation vs. Lignin Content
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0 5 10 15 20 25
Klason Lignin (%)
Abs
orpt
ivity
(l g
-1cm
-1)
at 4
30 n
m
Condition A Condition B
Condition A: Pulping with high effective alkali/low sulfidity. Condition B: Pulping with low effective alkali/high sulfidity
(darker)
0
50
100
150
200
250
300
0 0.2 0.4 0.6 0.8 1 1.2
Visible Absorbance from Lignin * klason
Chr
omop
hore
Inde
x
Residual lignin UV/Vis results parallelpulp results, suggesting that isolationprocedure has not influenced chromophore propertiesLow Sulfidity
Characterization of Residual Lignins UV/Vis Ionization Difference
-0.2
-0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
230 280 330 380 430 480 530
Wavelength (nm)
∆ε i
A-1 A-4 A-7 B-1 B-4 B-9
Condition A: Pulping with high effective alkali/low sulfidity. Kappa # 163, 69, 30Condition B: Pulping with low effective alkali/high sulfidity Kappa # 152, 69, 30
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
A-1 A-4 A-7 B-1 B-4 B-9
Sample
∆ε i
250 nm 300 nm 350 nm
370 nm: Phenolic stilbenes350 nm: Phenolic α-carbonyl units 250 nm Unconjugated Phenolics
Color differences in pulps A vs. B can not be due to phenolic stilbenes
O
O
P(OCH3)3O
O
P(OCH3)3
OP
OOCH3
OCH3
OCH3
I II III
Trimethylphosphite ChemistryReaction with Ortho-Quinone Structures
H2O
OR
OR
R = PO(OCH3)(OH) or H
Typical 31P NMR-TMP Spectrum
0 -5 -10 -15 -20 -25PPM
Ortho-Para Quinone Adduct
Internal Standard:tri-m-tolylphosphate
O P
O
OO
Quinones
Quinones Could Be a Component in Color Difference in these PulpsBut Extinction Coefficients Must Differ or Not a Key Factor
0.00
0.08
0.16
0.24
0.32
0.40
0 5 10 15 20 25
Klason Lignin (%)
Qui
none
(mm
ol/g
lign
in)
Condition A Condition B
Condition A: Pulping with high effective alkali/low sulfidity/BrighterKappa # 163, 69, 30
Condition B: Pulping with low effective alkali/high sulfidity/Darker Kappa # 152, 69, 30
Lighter
19F-NMR Aliphatic Carbonyls
0.0
0.2
0.4
0.6
0.8
1.0
0 5 10 15 20 25
Klason Lignin (%)
Alip
hatic
Car
bony
ls (m
mol
/g li
gnin
)
Condition A Condition B
Aliphatic carbonyl do not Contribute to the Observed Optical Differences in Pulp A & B
FT-IR Carbonyls
3.50
3.70
3.90
4.10
4.30
4.50
4.70
4.90
5.10
5.30
0 5 10 15 20 25
Klason Lignin (%)
% C
arbo
nyl i
n L
igni
n
Condition A Condition B
Characterization of Residual Lignins 31P NMR
+O
O
CH3CH3
CH3
CH3P Cl Lignin-OH
OMeO
POO
O
O
CH3CH3
CH3
CH3P O Lignin
OMeO
POO
O
OP O R
O
ROPO
O
Residual Lignin Analysis 31P NMR
1.01.21.41.61.82.02.22.4
0 5 10 15 20 25
Klason Lignin (%)
Alip
hatic
OH
(mm
ol/g
ligni
n)21.4% EA, 23.2% Sulfidity 14.6% EA, 56.8% Sulfidity
Differences exist!
Residual Lignin Analysis 31P NMR
0.5
0.6
0.7
0.8
0.9
1.0
0 5 10 15 20 25Klason Lignin (%)
Phen
olic
OH (m
mol
/g lig
nin)
21.4% EA, 23.2% Sulfidity 14.6% EA, 56.8% Sulfidity
Condensed Phenolics Appear to Contribute to the Observed Optical Differences in Pulp A & B
0.30
0.35
0.40
0.45
0.50
0.55
0.60
0 5 10 15 20 25
Klason Lignin (%)
Bip
heny
l OH
(mm
ol/g
lign
in)
Condition A Condition B
Catechol In Pulp
0.08
0.10
0.12
0.14
0.16
0.18
0 5 10 15 20 25
Klason Lignin (%)
Cat
echo
l OH
(mm
ol/g
lign
in)
Condition A Condition B
Catechols do not appear to contribute to optical differences for pulps A and B
Conclusions
Conclusions
Overall color of kraft pulp– Influenced by pulping parameters
• % EA, % Sulfidity are significant• Maximum temperature not significant within
experimental limitations
Chromophore content– Changes with pulping, depending on conditions– More surface lignin needed for light colored
pulp to obtain same chromophore content
Conclusions• Lignin functional groups
– Provide further evidence that lignins from two different conditions are very different
• Quinones, Condensed Phenolics, Aliphatic hydroxyls– Appear to be contributors to the color difference of kraft pulps
studied
• Aliphatic carbonyl, Noncondensed phenolics, Catechols– Do not Appear to be Important contributors to the color difference
of kraft pulps studied
• Differences in optical properties can not be attributed to surface lignin concentration for pulps studied
Acknowledgments
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