Inter-Ply Bond Measurement Standards in Recycled Paperboard Production
Inter-ply bond standards mandate TAPPI T 541 or ISO 16260 testing under strict ISO 187 conditioning to prevent converting delamination on recycled paperboard.
Mineral Filler Loading
Sheet Loading
Incorporating inorganic pigments into the cellulose matrix of a paper web improves optical properties and printability while reducing fibre use. The level of mineral filler loading is expressed as a percentage of total sheet weight, ranging from five to thirty percent. This practice increases brightness and opacity but reduces the strength of the paper.
Chemical Retention
Fillers such as precipitated calcium carbonate, kaolin clay or talc have no natural affinity for cellulose fibres. Therefore, retention aids such as cationic polymers must be added to the furnish to bind the fine filler particles to the larger fibres during drainage. If retention is poor, the fillers are washed out with the white water, leading to deposits on the forming wire and increased treatment costs.
Correct retention chemistry is essential for maintaining a uniform distribution of filler throughout the sheet profile.
Paper Strength
Because mineral filler particles occupy spaces that would otherwise be filled by fibre-to-fibre bonds, high loading levels weaken the tensile, burst and tear strength of the sheet. To counteract this loss, mills use dry-strength additives or increase the refining of the pulp. The trade-off between optical improvement and strength loss dictates the maximum filler loading for each paper grade.
For example, copy papers and lightweight coated grades carry high filler levels to prevent print-through, while packaging grades use minimal filler to preserve structural stiffness and tear resistance, which are necessary for outer carton performance.