Change search
ReferencesLink to record
Permanent link

Direct link
The influence of process conditions during pulp storage on the optical properties of Norway spruce mechanical pulps
Mid Sweden University, Faculty of Science, Technology and Media, Department of Chemical Engineering. Norske Skog Saugbrugs AS, NO-1756 Halden, Norway . (FSCN)
Mid Sweden University, Faculty of Science, Technology and Media, Department of Chemical Engineering. Capisco Science and Art, SE-602 34 Norrköping, Sweden.
Mid Sweden University, Faculty of Science, Technology and Media, Department of Chemical Engineering. AkzoNobel Pulp and Performance Chemicals, SE-445 80 Bohus, Sweden.
Paper and Fibre Institute, NO-7491 Trondheim, Norway .
Show others and affiliations
2013 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, Vol. 28, no 2, 203-210 p.Article in journal (Refereed) Published
Abstract [en]

The aim of this work was to study the influence of process conditions (time, pH, temperature and consistency) on the optical properties of mechanical pulps during storage in a clean system as a reference for further work. Laboratory storage trials were performed with unbleached and hydrogen-peroxide bleached well-washed Norway spruce pulps. In general, the pulp darkened during storage due to an increase in light absorption, especially at shorter wavelengths. After long storage times, the light absorption coefficient, k had increased also at longer wavelengths. No specific peaks were seen in Delta k-spectra. The increase in light absorption was most rapid initially, during the first four hours, for all pulps when stored at high temperature (80 degrees C), and then proceeded more slowly. The corresponding change in colour, measured as a* and b*, was shifted towards red and somewhat towards yellow, and over longer periods of storage, the shift towards yellow became greater. Time and temperature were found to have the largest impact. The effects were similar regardless of the starting pH (4.3-9.7) and pulp consistency (5%-25%). The hydrogen-peroxide bleached pulps were more sensitive to storage compared to the unbleached pulp at temperatures above 50 degrees C. At storage times of up to four hours, the unbleached pulp showed no loss of brightness at either of the storage temperatures. A slightly less bleached pulp darkened more than a highly bleached pulp at all wavelengths. The only difference measured between the two pulps was that the less bleached pulp had a higher content of iron. This higher iron content may be at least part of the reason for the more extensive darkening.

Place, publisher, year, edition, pages
2013. Vol. 28, no 2, 203-210 p.
Keyword [en]
Colour, High-yield pulp, Optical properties, Process conditions, Pulp storage, Spectral data
National Category
Chemical Engineering
URN: urn:nbn:se:miun:diva-19921ISI: 000321291900005ScopusID: 2-s2.0-84882994958OAI: diva2:655412
Available from: 2013-10-11 Created: 2013-09-25 Last updated: 2014-12-16Bibliographically approved
In thesis
1. Storage of Hydrogen Peroxide Bleached Mechanical Pulp: Reduction in Reflectance over the Visible Spectrum
Open this publication in new window or tab >>Storage of Hydrogen Peroxide Bleached Mechanical Pulp: Reduction in Reflectance over the Visible Spectrum
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The objective of this thesis is to determine possible causes of the darkening of hydrogen peroxide bleached mechanical pulp over the visible spectrum and their relative contributions. It focuses on both process conditions and the composition of the pulp and the dilution water, including additions or losses of material along the process line from the bleach tower to the paper machine.

A mapping of the optical properties of the pulp along the process showed that the fine fraction of the pulp darkened more than the long fibre fraction. Simulation of retention times of different fractions showed that the main part of the fine material is retained in the paper within a few hours, a small part might circulate for considerably longer time and may therefore be strongly coloured.

Storage trials were mainly performed using a hydrogen peroxide bleached mechanical pulp intended for SC paper made of Norway spruce (Picea abies), sampled on one occasion and stored in a freezer. Unwashed or well-washed pulp was stored in distilled water or in different process waters. Some complementary trials were included, e.g. unbleached pulp.

Time and temperature were the process variables that gave the strongest darkening of the pulp, as expected, both in a clean and a more process-like system, whereas pH only had an effect in the presence of process waters; the highest brightness stability was seen at a pH around 5.5–6.0.

The darkening was due to an increase in the light absorption coefficient (k) beginning at short wavelengths, but after longer storage times the increase in kλ also became noticeable at longer wavelengths. The colour (CIE L*, a*, b*) of the pulp changed towards red and yellow, initially more towards red and then more towards yellow. These changes were clearly visible.

Washing of the bleached pulp made it less sensitive to storage; possibly due to the removal of extractives, lignin-like substances, metals and pulp fines. This washing had little effect before storage and the amount of material removed was small.

The pulp darkened more when stored in process waters compared to distilled water. Apart from fibres, most of the colour was associated with pulp fines or filler but some colour was also found in the dissolved and colloidal fractions. At an increased pulp consistency, the increase in k460 was smaller.

Storage in white water from the paper machine gave extensive discolouration with a shoulder in the absorption spectrum around 550–650 nm, which increased with time. The addition of ferric ions increased the light absorption coefficient during storage, but could not explain the increased absorption at 550–650 nm nor could it be the only cause of the darkening in the mill system. A cationic basic violet dye gave a shoulder in the absorption spectrum similar to that of the mill system, but the absorption of the dye did not increase during storage. Model calculations indicate, but do not prove, that ferric ions together with violet and red dyes could have played a major, but not exclusive role in the colour observed in the mill system after storage. The darkening not accounted for, at longer wavelengths and around 550–650 nm, is suggested to be related to fines and fillers including dissolved and colloidal substances associated with these particles.

A method to produce representative sheets for determination of optical properties of mechanical pulps was developed. The new method makes it possible to follow changes in light absorption and light scattering coefficients over the visible range of wavelengths. It is approximately six times faster than standard methods, reduces the risk of additional darkening of the sample and can be used with small pulp quantities.

The deviation from the expected linear behaviour of the light scattering coefficient, s, at wavelengths corresponding to strong light absorption has been studied using the Kubelka-Munk model and the angular resolved DORT2002 radiative transfer solution method. The decrease in s could not be explained by errors introduced in the Kubelka-Munk modelling by anisotropic scattering.

Place, publisher, year, edition, pages
Sundsvall: Mid Sweden University, 2014. 93 p.
Mid Sweden University doctoral thesis, ISSN 1652-893X ; 207
National Category
Paper, Pulp and Fiber Technology
urn:nbn:se:miun:diva-23806 (URN)978-91-87557-91-0 (ISBN)
Public defence
2014-11-21, M 102, Mittuniversitetet, Åkroken, Sundsvall, 10:46 (English)
Available from: 2014-12-16 Created: 2014-12-16 Last updated: 2015-03-13Bibliographically approved

Open Access in DiVA

Enberg_The_influence_of_process_conditions(409 kB)334 downloads
File information
File name FULLTEXT01.pdfFile size 409 kBChecksum SHA-512
Type fulltextMimetype application/pdf


Search in DiVA

By author/editor
Enberg, SofiaRundlöf, MatsPaulsson, Magnus
By organisation
Department of Chemical Engineering
In the same journal
Nordic Pulp & Paper Research Journal
Chemical Engineering

Search outside of DiVA

GoogleGoogle Scholar
Total: 334 downloads
The number of downloads is the sum of all downloads of full texts. It may include eg previous versions that are now no longer available

Total: 519 hits
ReferencesLink to record
Permanent link

Direct link