NEW STOCK FRACTIONATION CONCEPT IN RECYCLED FIBER BASED MULTI-PLY BOARD MAKING

This paper presents a new idea for recycled fiber (RCF) fractionation in multiply boardmaking. The fractionation method is based on screenin g with a continuous slot. In typical RCF-based two-ply boardmaking, th e short fiber fraction is used in the back ply and the long fiber frac tion, after being dispersed and refined, in the top ply. In this case, the purpose of the dispersion stage is to refine the stickies and not to remove them. The subsequent refining of the top ply stock is inten ded to improve sheet strength and surface smoothness. The purpose of t his work was to examine the possibility of achieving the same or bette r sheet properties than in the conventional process by using only opti mized screening and fractionation. If this is feasible, dispersion and refining of the RCF-based stock would be unnecessary. In the case of 100% recycled fiber, the target was to use the clean fine fractions in the top ply and the long fiber fractions in the back ply. The test wa s carried out by comparing a set of laboratory sheets. Recycled stock and chemical pulp taken from a linerboard mill's machine chest were us ed as reference. The reference sheets consisted of recycled fiber in t he back ply and chemical pulp in the top ply. Corresponding test sheet s were produced in which the top ply contained fractionated shout fibe rs and the base ply contained screened long fibers. These laboratory s heet test results revealed improved sheet stiffness and higher Ring Cr ush Test (RCT) and Shout Span Compression Test (SCT) values through in creased bulk at equal bonding strength. However, even though the surfa ce properties of the laboratory sheets were not measured, some improve ment in smoothness and printability can be expected, basically because the sheet in the back ply is cleaner and more even. The laboratory re sults show that, even though the strength properties of chemical pulp are roughly twice as good as those of recycled stock, it is possible t o produce a two-ply sheet with equal strength properties by employing optimized fractionation and screening of RCF-based stock. Since no dis persion and less refining is required, the new production method is co mpetitive. As the stock is cleaner, better runnability on the board ma chine is expected because of greater openness of wires and felts, and longer machine cleaning sequences.