The influence of formation on tensile strength of papers made from mechanical pulps

The importance of paper formation is widely appreciated amongst papermakers in relation to sheet appearance, but much remains unclear about its role in paper strength, particularly for paper made from mechanical pulps. This paper addressed the question of how formation affects the tensile strength of such pulps. Two softwoods mechanical pulps from species of differing fibre length and coarseness were fractionated by fibre length and formed into handsheets at standard handsheet consistency and at a headbox consistency. As found by previous workers, handsheets from the shorter fibre fractions gave higher strength. These handsheets also had better formation, as expected from shorter fibres. To isolate the influence of formation from other factors which differ between species and fractions and may cause strength differences, we compared one fraction of one species at differing forming consistencies. This showed that the direct contribution of formation to tensile strength was significant, with poor formation giving lower strength for all fractions. In addition, we compared tensile strengths of whole pulps formed at standard handsheet consistency and headbox consistency at reduced grammages over the range 60-30 gsm.