Project 15: Mechanical pulp

Background: Mechanical pulp fibres have a very heterogeneous structure, and the surface structure and composition depends on both the wood species and the processing method adopted. The quality of mechanical pulps is determined by the inherent properties of the pulp fibres, such as surface ultrastructure and chemical composition, as well as physical characteristics like shape. Project 15 aims at studying the fibre-surface structure and chemistry, fibre collapsibility, and their subsequent effects on final paper quality.

1)      1)  Collapsibility-structure
Previous studies have shown that compression wood fibres may contribute to surface roughness of printing papers based on thermomechanical pulp.^1,2 A pilot plant study was conducted to evaluate the effects of a compression wood rich assortment (i.e. knots) on pulp and paper properties. Preliminary results indicate that the compression wood rich assortment has inferior strength properties compared to the reference assortment. Work in progress strives to elucidate the qualitative and quantitative effects of compression wood on paper surface properties. Results from these studies are expected during autumn 2004.

Fig. 1. Part of a compression wood fibre selected from a Norway spruce mechanical pulp and observed using scanning electron microscopy. The white arrow indicates the abrupt transition from the outer (i.e. S1) to the middle (i.e. S2) secondary cell wall layers. The cell corners (CC) are smooth due to the presence of intercellular spaces in native compression wood (from Brändström 2004).

  2)  Fibre surface
After mechanical pulping, residues of compound middle lamellae containing pectin which resists degradation – remains on the pulp-fibre surface. Pectin has a high negative charge density affecting the fibre-surface chemistry and pulp properties. Both TMP and CTMP pulps have localised patches of high concentrations of pectin (Fig. 2) on the fibre surface.³ A bioassay, developed for detecting pectin on pulp fibre surfaces has shown TMP to contain more methyl esterified galacturoanan (the main pectic substance in wood) than CTMP, ⁴ which may be attributed to the chemical treatment affecting the fibre surface in the case of CTMP processing. The presence and degree of esterification of fibre-surface pectin has recently been correlated to fibre charge in CTMP,⁵ and the colorimetric bioassay for pectin has been further developed, and used to show the de-esterification of surface-localised pectin during alkaline hydrogen peroxide bleaching of CTMP.⁶

 Fig. 2. Immuno-fluorescence labelled pectin on mechanical pulp surface.

 3)  Ph.D project
Studies conducted on characterizing the cell wall damages of spruce TMP fibre fractions showed that fibre splitting and fibrillation do not occur in a random process, but rather are related to the original native structure/ultrastructure of the fibre cell wall. As a result, S1 and S2 secondary wall layers generate two different types of fibrillation namely “flake-like” and “sheet and/or ribbon-like respectively, which contribute to the particle size and shape distribution of the pulp. In both cases, fibrillation developed from the initial cracking at sites of weakness present on the cell wall and subsequent splitting of individual fibre wall layers along the orientation of the native cellulose microfibrils (i.e. along MFA) for both S1 and S2 layers.⁷

 

 

 

 

 

 

 

 

 

 Fig. 3. Fibre fibrillation by the initiation of splits near sites of weakness (arrows) within the cell wall of pulp fibres such as cross-field (a, b.) and bordered pits (c.).

Industrial Practice

As part of the Ph.D. work, industrial practice was commenced in collaboration with Kappa Kraftliner, Piteå where there is a considerable problem with printed top birch kraftliner paper, which is thought to be due to wood extractives responsible for pitch problems in the pulp and paper. 

A study has been conducted on birch aimed at localizing the extractives involved in pitch deposition in the native wood raw materials and at different stages of the kraft pulping process to understand the nature and behavior of its redistribution using variety of microscopical methods (LM, FM, SEM and TEM) in combination with chemical analysis.

 Preliminary results have shown that the polyene antibiotic Filipin can be successfully applied for localizing sterols in both wood raw materials and the pulps at different stages during the pulping process as well to understand the redistribution of wood extractives. Other major birch wood extractive constituents, i.e. fats (neutral fats and fatty acids) that are also thought to be substantially responsible for the above problem, were successfully localized and their redistribution studied during different stages of the pulping process using both osmium tetroxide and Nile blue histochemical staining techniques.

Project group: Dinesh Fernando (Ph.D student), Jonas Brändström (project leader), Jonas Hafrén (project leader), Geoffrey Daniel (project leader), Hans Höglund (scientific advisor), Peter Sandström (industrial representative), Lennart salmén (STFI), Lars Ödberg (Sveaskog), Erik Persson (Holmen) and Magnus Paulsson (Stora-Enso).

Publications and manuscripts:

¹Brändström, J. 2004. Ultrastructure of compression wood fibres in fractions of a thermomechanical pulp. Nord. Pulp Paper Res. J., 19, 13-18

²Brändström, J. 2004. Microfibril angle of the S1 cell wall layer of Norway spruce compression wood tracheids. IAWA J. In press.

³Hafrén, J. and G. Daniel. 2003. Distribution of methyl-esterified galacturonan in chemical and mechanical pulp fibers. Journal of Wood Science 49, 361-365.

⁴Hafrén, J. and G. Daniel. 2003. A bioassay for methylated galacturonan on pulp-fiber surfaces. Biotechnology Letters 25, 859-862.

⁵Hafrén, J. and G. Daniel. 2004. Chemoenzymatic modifications of charge in chemithermomechanical wood pulp (submitted to Journal of Biotechnology).

⁶Hafrén, J. 2004. Antibody-based assay for galacturonan de-esterification on wood-pulp fibers during bleaching (manuscript).

⁷Fernando, D. and G. Daniel. 2004. Micro-morphological observations on spruce TMP fibre fractions with emphasis on fibre cell wall fibrillation and splitting.  Nord. Pulp Paper Res. J. (In press).