Refining systems for sack paper pulp: Part I HC refining under pressurised conditions and subsuequent LC refining

Unbleached kraft pulp for sack paper is usually refined in a high consistency process followed by low consistency refining in order to achieve the desired paper properties, such as a high strain at break and high tensile energy absorption at low airflow resistance. In this study, conventional atmospheric high consistency refining is compared to pressurised high consistency refining after preheating to 125C and 175C. After high the consistency refining stage, the pulp samples were refined in a low consistency process in a laboratory Escher-Wyss refiner. The results show that the highest levels of tensile energy absorption and strain at break were achieved with curled and kinked fibres refined in a pressurised refiner at 175C at a relatively low refining energy input, before the low consistency refining stage. These results were obtained at low airflow resistance, which is advantageous for sack paper. The disadvantage of the pressurised high consistency refining process is a loss in tensile strength. Paper properties were evaluated on freely dried as well as ISO sheets.