Chemical fibre processing for new & improved product properties
Overall aims:
To enhance the industrial utilization and full fibre potential for paper and board products by reducing strength losses during kraft processing. To achieve full strength potential of kraft pulps by studies at the ultrastructural level of mechanisms controlling the fibre wall coherence, related to fibre strength and mechanisms involved in fibre joint strength.
Background:
The reasons for the superior strength of laboratory-cooked pulps compared to industrially produced kraft pulps are still unresolved. Since the difference in strength is in the range of 15-35%, the problem is of major economic importance to solve in order to achieve the full strength potential of the pulps. It is known that exposing chips to mixing at the end of an alkaline cook and moving parts along the fibre line decreases the strength of the pulp. It has been suggested that this is due to ultrastructural changes in the fibre wall, leading to decreased bonding between fibrils in the wall and consequently decreased fibre strength.
Our previous studies indicate that conditions during pulping as well as the chemical composition of fibres affect fibre vulnerability when exposed to mechanical stresses. It has been proposed that higher xylan content can lead to more resilient fibres. For example, it is a well known that the beatability, i.e. the increase in tensile strength upon beating, is greatly affected by the hemicellulose content in pulps. Increased amount of hemicelluloses also increases the response to beating. Additional studies on industrially produced pulps have shown that the lower tensile strength compared to a corresponding laboratory pulp is likely to be attributable to reduced fibre bonding ability owing to a more degraded xylan (i.e. hemicellulose) on the industrially produced fibres.
As a first step, two major interactive chemical pulp projects have therefore been established within the chemical TEMA area concerning xylan viz: a) Re-deposition of xylan on kraft pulp softwood fibres (KTH, Industry); and b) Effect of xylan on fibre damage sensitivity (Södra Cell, StoraEnso, EkaChemical, Smurfit Kappa, Innventia, SLU). The collaborative projects involve pilot plant pulping of selected spruce raw material and are designed in the first case to test the effects of removal and addition of xylan during the actual kraft pulping process. Studies involve the industrial testing of routine physical parameters of the pulps and paper sheets and advanced analysis by scientists at SLU, Innventia and KTH. Pilot plant trials were initiated in May 09 with the conventional testing of pulps carried out by the companies with the advanced analysis currently taking place at the universities and Innventia.