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 !!