Behaviour of different furnish mixtures in mechanical printing papers

Mechanical printing papers consist of a mixture of mechanical pulp – groundwood or TMP − chemical reinforcing pulp and filler. They may also contain recycled fibre. The mixtures of different mechanical or chemical pulps are not used. A mixture of groundwood rich in fines and long-fibred TMP could be assumed to be an optimum mechanical pulp for high-quality mechanical printing papers, both regarding runnability and printability. The main objective of this thesis work was to research the validity of this hypothesis. The experimental part consisted of both lab trials and mill trials on modern paper machines. The lab studies showed that a mixture of fine groundwood and well-bonding TMP can have synergy in SC paper. Synergy advantage was achieved especially in tear strength and to some extent also in fracture energy, tensile energy absorption and stretch while usually no synergy was found in tensile strength or Scott-Bond. Synergy advantage could also be achieved in light scattering coefficient and calendering response in density, air permeability and pore size distribution. All these synergies found in SC paper sheets were most probable at about a 30 % chemical pulp share of fibre. In LWC base paper sheets similar synergy was not found. The mill studies also showed synergy advantage in the tear strength of SC paper but not in its tensile strength. Therefore the use of a mixture of groundwood and TMP and slightly more refined chemical pulp allowed a smaller than calculated chemical pulp share in paper, which reduces the furnish costs. In the mill trials synergy in printability was rare. In LWC paper no synergy was found. However, the use of groundwood could improve the printability of TMPbased LWC paper and the use of TMP in groundwood-based LWC paper allowed the reduction of the chemical pulp share. Synergy was found in strength properties which depend on both fibre length and bonding but not in strength properties depending primarily on bonding. Synergy advantage in tear strength seems to be possible with any mixture of paper furnishes which have their bonding degrees on the opposite sides of the optimum, i.e. tear strength maximum, depending on fibre length. In highly filled SC paper with well-bonding pulps at about a 30 % chemical pulp share of fibre the bonding level vs. fibre length seems to be most suitable for synergy in strength properties with a mixture of groundwood and TMP. With the same mixture an optimum sheet structure for synergy in the light scattering coefficient and calendering response of SC paper could be achieved. In better bonded LWC base paper no similar synergy was found. Also differences in sheet density, drying shrinkage and surface chemistry, and passing the limiting state of fines content, which all affect bonding, can be partial reasons for these synergies. This thesis showed valid the hypothesis of a mixture of groundwood and TMP being an optimum mechanical pulp for high-quality mechanical printing papers. The mixture was better than pure groundwood or TMP. However, both the lab and mill studies showed that the synergy advantages achieved with this mixture in SC paper are most sensitive both to the bonding ability and fibre length of pulps and to the need of bonding. Thus, e.g. the basis weight of paper and probably also the wood quality could affect the existence of synergy. The results of both the extensive lab studies and several mill trials were quite similar. Most of the lab testing of this research was done by an experienced laboratory assistant and the statistical reliability of most test results was checked. All this confirms the reliability of the results though the synergies found were often slight. The exploitation of a mixture of groundwood and TMP in paper is feasible only in very few mills having existing production capacity of both pulps. Thus these results should be used in the development of fine mechanical pulps to achieve the advantages with one single pulp.