how to evaluate the kraft pulp brightness stability? · outline introduction objectives application...
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![Page 1: How to evaluate the kraft pulp brightness stability? · Outline Introduction Objectives Application of UV-VIS reflectance spectroscopy for brightness stability studies Proper selection](https://reader031.vdocuments.mx/reader031/viewer/2022022013/5b2eb3777f8b9adc6e8c923c/html5/thumbnails/1.jpg)
How to evaluate the kraft pulp brightness stability?
Tiina Liitiä & Tarja Tamminen
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Outline
Introduction
Objectives
Application of UV-VIS reflectance spectroscopy for
brightness stability studies
Proper selection of accelerated ageing method
– Dry vs. humid heat treatment
Conclusions
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Why good brightness stability is important?
• Brightness affects pulp price and production costs
• Brightness should remain unchanged during
– transportation and storage
– further processing
– end-use of the final product, etc.
• Good brightness stability improves cost-efficiency
– reduced need for ’over-bleaching’ to compensate the possible brightness loss
– reduced use of optical brighteners and dyes
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• Hexenuronic acid (HexA) and its degradation products
• Residual lignin
• Oxidized carbohydrate structures (e.g. C=O groups)
• Metals (Fe, Mn, Cu)
• Chlorinated structures
• Extractives
Pulp components affecting thermal brightness reversion
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Role of accelerated ageing method
• Both dry and humid heat treatments are used to simulate
the natural ageing
• Humidity enhances thermally induced brightness reversion
• Humidity may also emphasize factors affecting brightness
reversion differently
– Detrimental effect of HexA is not as pronounced in dry
conditions (105°C, 4h) (Björklund et al 2005)
– Dry heat treatment may lead to erroneous conclusions
regarding the brightness reversion tendency
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Clarify the importance of proper accelerated ageing
treatment and address useful analytical methods for reliable investigation of kraft pulp brightness stability
• Applicability of UV-VIS reflectance spectroscopy for brightness stability studies
– Detection of HexA’s reactions during ageing directly from pulp handsheets
• Behaviour of HexA in dry and moist conditions
Objectives
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Examples on benefits of UV-VIS reflectance spectroscopy
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Evaluation of brightness stability
Brightness measurement before and after accelerated ageing treatment:
• Decrease of brightness (DR∞)
● Relative decrease of brightness (DR∞/R∞·100%)
• Post Color (PC) number
Reveals only brightness reversion tendency –
no information on reasons leading to discoloration !!
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UV-VIS reflectance spectroscopy
Chemical changes:
• UV-active structures, precursors to colored structures
• Color
Absorbance spectra (k/s)
2R
)R(1
s
k 2
0
1
2
3
4
5
6
7
8
200 300 400 500 600 700 800
nmk/s
Kraft
Kraft-O
Reflectance spectra (R∞)
Visible changes:
• Pulp color, i.e. brightness
0
0.2
0.4
0.6
0.8
1
200 300 400 500 600 700 800
nm
Refl
ecta
nce
Kraft
Kraft-O
R457~ISO Br.
HexA (240 nm)
Lignin (280-290 nm)
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0
2
4
6
8
10
ECF-Birch TCF-Birch
PC
-nu
mb
er
ECF Birch
TCF Birch
Brightness, % 89.7 88
Kappa number 1.0 4.7
Lignin kappa 1.0 1.6
Total lignin, % 1.5 1.1
Viscosity, ml/g 1240 700
Extractives, % 0.5 0.5
HexA, mmol/kg - 36
Brightn
ess r
evers
ion
tendency incre
ases
80°C, 65% RH, 48 h
Detection of HexA’s reactions by UV-VIS reflectance measurements
• High HexA content leads to low brightness stability of TCF pulp
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HexA’s reactions in UV-VIS absorbance spectra
0
0.5
1
1.5
2
2.5
3
3.5
200 300 400 500 600 700 800
nm
k/s
1 TCF
2 TCF Aged
3 ECF
4 ECF Aged
1
2
4
3
Higher HexA (240 nm) content in TCF Birch
(1) compared to ECF Birch (3)
HexA reacts during humid heat treatment…
… and degradation product of HexA (285 nm)
is formed together with colored structures
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Minor changes by difference absorbance spectra (D k/s)
• Aged pulp spectrum – Untreated pulp spectrum
-1
-0.5
0
0.5
1
1.5
200 300 400 500 600 700 800
nm
D k/s
HexA degrades
PC number = 100 · D k/s @ 457 nm
1 TCF
2 ECF
Str
uctu
res
form
ed
S
tru
ctu
res
rem
oved
2-formyl-5-furan carboxylic
acid (FFA) is formed
O
O
HO
O
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Degradation products of HexA
O COOH
O COOHOHC
Humid heat treatment: 70°C, 8% moisture content
(Ph.D. Thesis of Sevastyanova, 2005)
FFA can be detected from pulp sheets by UV-Vis as an indicator for reactions of HexA !!
O
O H O
O
O
O
OH
H O
H O O C
HexA
HO
HO
O
(FA)
(FFA)
(RA)
React further forming colored products
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UV-VIS in brightness stability studies of mill pulps
• Differences in brightness stability not explained by
chemical composition
• HexA content of all pulps below detection limit
(< 4.5 mmol/kg)
Brightn
ess r
evers
ion
tendency incre
ases
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
Birch E. grandis E. globulus
PC
-nu
mb
er
80°C, 65% RH, 48 h
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Sensitivity to HexA and lignin
• Even residues of HexA and lignin can be detected by
UV-VIS, explaining differences in brightness stability
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
200 250 300 350 400 450 500
nm
k/s
E. Globulus
E.Grandis
Birch
Lignin
HexA
-0.15
-0.1
-0.05
0
0.05
0.1
0.15
0.2
0.25
200 250 300 350 400 450 500
nm
E. Globulus
E. Grandis
Birch
D k
/s
Absorbance spectra (k/s) Difference
absorbance spectra (D k/s)
FFA
Degradation of HexA
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Advantages of UV-VIS reflectance measurements
• In addition to brightness drop or PC-numbers, provides
valuable information also on reasons leading to
brightness reversion
• Together with more detailed characterisation methods –
or instead of those – can be used as a fast and simple
method to reveal reactions behing yellowing
• Formation of FFA is easily detected by UV-VIS and can be
considered as an indicator of the involvement of HexA
to brightness reversion
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Role of accelerated ageing treatment & HexA
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Dry and humid heat treatments
Humid heat 80°C, 65% RH, 48 h (ISO 5630-3)
Dry heat 105 °C, 4 h (Tappi UM-200)
105 °C, 48 h
Pulps with various HexA contents
• ECF and TCF bleached laboratory kraft pulps (SW) • Fractionated ECF bleached E. grandis mill pulp
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0
1
2
3
4
5
6
7
ODEZ/QP ODEDP ODEDD OQPPaaP OQPZ/QP
PC
nu
mb
er
Humid heat treatment of ECF and TCF pulps with various HexA content
80°C, 65% RH, 48 h
PC-numbers correlate very well with HexA content
• ECF pulps have better brightness stability than TCF pulps
• Brightness reversion of TCF pulps increases with increasing
HexA content
No HexA
4.9 mmol/kg HexA
11 mmol/kg HexA
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0.0
0.2
0.4
0.6
0.8
1.0
ODEZ/QP ODEDP ODEDD OQPPaaP OQPZ/QP
PC
nu
mb
er
Dry heat treatment of ECF and TCF pulps with various HexA content
No correlation with HexA content
• Similar differences in ECF pulps due to different carbonyl
contents remain
• Detrimental effect of HexA in TCF pulps cannot be seen
105 °C, 4 h
No HexA
4.9 mmol/kg HexA
11 mmol/kg HexA
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0.0
0.4
0.8
1.2
1.6
2.0
ODEZ/QP ODEDP ODEDD OQPPaaP OQPZ/QP
PC
nu
mb
er
Prolonged dry heat treatment of ECF and TCF pulps with various HexA content
No correlation with HexA content
• Similar differences in ECF pulps due to different carbonyl
contents remain
• Detrimental effect of HexA in TCF pulps cannot be seen
105 °C, 48 h
No HexA 4.9 mmol/kg HexA
11 mmol/kg HexA
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Similar behavior of Eucalyptus grandis mill pulps
0.0
1.0
2.0
3.0
4.0
Euca 1 Euca 2 Euca 3
Humid heat Dry heat 4 h Dry heat 48 h
PC n
um
bers
H
um
id h
eat
0.0
0.2
0.4
0.6
0.8
1.0
1.2
PC n
um
bers
D
ry h
eat
7.2 mmol/kg HexA
11 mmol/kg HexA
No HexA
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Reactions of HexA and FFA in dry and moist conditions
• HexA reacts also in dry conditions forming FFA
• HexA’s degradation products are less reactive in dry
conditions..??
-0.05
0
0.05
0.1
0.15
0.2
200 300 400 500 600
nm
D k
/s
Humid heat
Dry heat, 48h
Dry heat, 4h
PA-OQPZP
FFA
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Conclusions
• UV-Vis reflectance spectroscopy can be used as a very
sensitive and simple method to follow the reactions of HexA
leading to brightness reversion
• Dry heat treatment does not fully reveal the contribution of
HexA to brightness reversion – and may thus lead to
erroneous conclusions
– HexA degrades also in dry conditions
– The color forming reaction of HexA’s degradation
product(s) probably do not take place similarly without
moisture
Thank you for your attention !!