Chemical Processing
Chemical wood pulp treated with whitening agents provides the primary structural basis for high-brightness printing papers. In the manufacture of bleached kraft pulp, the lignin is separated from cellulose fibres using a sodium hydroxide and sodium sulfide cooking liquor before undergoing bleaching stages with chlorine dioxide or oxygen. This process preserves the long-chain cellulose molecules that give paper its high tear strength.
Additionally, the low residual lignin prevents the paper from yellowing when exposed to ambient light over time. Unbleached alternatives lack the surface properties required for multi-colour offset printing.
Fibre Integrity
Retaining fibre length during cooking secures the tensile strength of the paperboard sheet. Although bleach sequences whiten the stock, the chemical exposure can degrade the polymer chain length of the cellulose. Processing lines manage chemical concentrations to prevent this degradation.
The resulting bleached kraft pulp yields a sheet with high burst and tensile indexes.
Process Balance
Brightness requirements must be balanced with the cost of bleach chemicals and the target yield of the cooking plant. Standard grades of bleached kraft pulp target an ISO brightness of eighty-eight percent or higher. Achieving this brightness level requires multi-stage bleaching systems that remove residual lignin without breaking down the fibre matrix.
This material allows packaging to accept fine graphics.
