How to evaluate the kraft pulp brightness stability?
Tiina Liitiä & Tarja Tamminen
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
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
• 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
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
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
Examples on benefits of UV-VIS reflectance spectroscopy
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 !!
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)
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
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
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
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
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
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
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
Role of accelerated ageing treatment & HexA
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
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
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
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
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
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
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 !!