(424h) Furnish Optimization for a Multilayered Paperboard
Multi-layered symmetrical paperboard sheets were made in a laboratory sheet former from old newsprint (ONP) in the middle layer and old corrugated container (OCC), Indian mixed recycled paper IMR), and virgin bleached softwood kraft (BSK) pulp in surface layers. The proportions of ONP and surface layer pulp were varied. Cationic starch in different proportions was added to the pulps used. The board sheets were surface sized in a laboratory coating unit with a mixture of oxidized starch and sodium silicate in varying proportions.
Different strength properties such as tensile strength, burst strength, tear strength, bending stiffness, compression strength (ring crush test), and ply-bond strength (Scott Bond test) of these board sheets were determined using standard testing procedures. In the pulps used, the BSK had much higher tensile, burst, and tear strength than those of OCC, IMR, and ONP. The ONP was the bulkiest of all the pulps and produced the sheets with maximum bending stiffness index. Tensile strength of OCC, IMR, and ONP were nearly equal, the OCC and IMR had slightly higher tear strength than the ONP. The major observations were:
- Tensile, burst, tear, and compression strength of the board varied almost linearly with the proportions of the pulps used irrespective of the construction of the board, whether multilayered or single-layered form the mixture of the pulp.
- Bending stiffness passed through a maximum for symmetrical multilayered board when about 70% ONP was used in the middle layer and 30% of either, BSK, OCC, or IMR was used in the surface layers. The improvement achieved in bending stiffness was nearly identical for either of BSK, OCC, and OMR in the surface layers.
- The Scott Bond values were also depended on the proportions of the pulps used in the board in a manner similar to tensile and tear strength. However, for a given proportions of pulps, Scott Bond values were slightly higher for multilayered board than for the single-layered board.
- Addition of cationic starch to the stock resulted in a significant improvement in Scott Bond values of multilayered board.
- Surface sizing of the board with oxidized starch resulted in a significant improvement of RCT values of the board. The increased proportion of sodium silicate in the sizing solution further improved the compression strength.
Relationships between the physical and morphological characteristics of the component pulps and the board properties have been established to enable optimization of the board furnish for a given end use.