Chemical Process
Dissolution of hemicellulose polymers into a liquid medium allows for the controlled modification of the wood cell wall structure during pulping. In the presence of cooking chemicals, xylan solvation occurs when the alkaline or acidic conditions of the cooking liquor break the hydrogen bonds and covalent links holding the hemicelluloses in the fibre matrix. This step is essential for producing high purity dissolving pulps or for improving the flexibility of fibres in packaging grades.
The rate of dissolution depends on the temperature, the concentration of the solvent, the botanical source, and the duration of the cook.
Pulp Yield
Managing the loss of these sugars is a primary concern for mill operators trying to maximize the output of the digester. Xylan solvation can lead to a significant drop in yield if the conditions are too aggressive. Some of the dissolved xylan can be re-deposited onto the fibre surfaces at the end of the cook to improve the bonding potential of the pulp.
This re-precipitation requires a careful adjustment of the pH to encourage the polymers to leave the solution and adhere to the cellulose.
Refining Efficiency
Fibres with a higher hemicellulose content swell more easily in water, which reduces the energy required for mechanical refining. This increased hydration potential leads to a more flexible fibre that conforms better during sheet formation.
