Quantifying the contribution of fines production during refining to the resulting paper strength

Pulp refining is an essential process step prior to paper production. The contribution of fines production during refining to the resulting paper strength so far has mostly been considered qualitatively. A quantitative and experimental evaluation regarding their effect has not yet been published. Unbleached softwood kraft pulp was refined using a PFI mill and a disc refiner at different refining intensities. Prior to handsheet forming, fines were removed in a lab scale pressure screen from one part of the refined and unrefined samples in order to investigate the difference in tensile strength between sheets with and without fines, which were furthermore produced with and without additional wet pressing. It was found, that fines formed in a disc refiner at 250 kWh/t are responsible for up to 25% of the breaking length increase, while the PFI mill at 10,000 revolutions fines only contribute to a maximum of 12%. In terms of fines efficiency, the disc refiner was able to achieve higher results compared to the PFI mill, which however might be attributed to the higher fibre flexibilization in the PFI mill. Thus fines formed in the refining process are of high importance for strength development especially for the disc refiner