The print quality of paper made from co-refining BCTMP and recycled fibres

Go-refining is a process whereby secondary fibres (office waste) and mechanical pulp are refined together in a secondary stage refiner as opposed to refining them separately and subsequently mixing them. Paper samples were manufactured on a pilot scale paper machine from both pulps refined together (bleached chemi-thermomechanical pulp and office waste) and the original pulp mix without co-refining. The pilot paper machine papers, calendered to the same-PPS roughness using conventional, temperature gradient, and soft nip calendering were printed on an sheet-fed offset press using a novel approach called the Normal Contrast Intensity (NCI). In NCI, the inking level is gradually increased to beyond optimum inking conditions. Samples are retrieved and analyzed at each inking level. The average print density, print-trough, variation in print density, and contrast are evaluated for all samples. It is found that increasing the secondary fibre content decreases print-through, increases contrast but also print unevenness. Refining pulps together also increases print-through but improves printing uniformity. It produces the same contrast and graininess as when pulps are refined separately. Calendering has complex effects on the printing characteristics of paper. Temperature gradient calendering improves print-through. Soft-nip calendering is best for print uniformity but worst for print-through and contrast. It is concluded that co-refining has a market potential and that pulp furnish and calendering conditions can be tailored to customer's end-use.