Energy efficient high quality CTMP for paperboard (2007)

This thesis is intended to contribute to an increased knowledge about the influence of fibre characteristics on bulk and strength properties of thermomechanical pulp (TMP) and chemithermomechanical pulp (CTMP) from spruce. It deals with laboratory sheet properties and how they are affected by the conditions during pressing and drying, e.g. pressure, temperature and what dryness level the sheets have once pressing and drying is terminated. Further on it deals with how sheet properties depend on fibre properties, such as fibre length, fibre flexibility and fibre surface characteristics. The thesis is part of a long term project with the goal of increasing bending stiffness of paperboard, hence bulk and internal bond strength properties are of main interest. Apart from standard methods (ISO, TAPPI and Rapid Kothen), sheets have been pressed and dried in a modified Rapid Kothen dryer which has the capacity to press the sheets at higher pressure compared to a standard Rapid Kothen dryer. The results illustrated that there are large differences in mechanical pulp sheet properties depending on how the sheets have been pressed and dried. The main factors contributing to the bulk and strength levels achieved are a combination of pressure, temperature and to what dryness level the sheets are pressed. Sheets made from stiff fibres sprung back more when only wet pressed, and appeared to be less sensitive to pressure than sheets made from flexible fibres. The situation was the other way around when sheets were pressed and dried until dry at high temperature; pulps with stiff fibres were affected more by temperature and pressure than pulps with flexible fibres. When looking at strength development with respect to what dryness level the sheets had been pressed at high temperature, the most interesting finding was that the increase in strength was not continuous, especially when looking at the Z-strength development for high freeness pulps and long fibre fractions. There was a distinct inflection of the strength-dryness curve when dryness reached a level of ~50% and the most important dryness interval for internal strength development was found between 50 and 80%. This result combined with the fact that most paper and board machines only press the sheet to ~50% dryness, before the sheet is fed into the drying section, show that much of the inherent strength potential of mechanical pulps is unexploited. There are commercial techniques for pressing to higher dryness levels available, such as Condebelt drying and press drying. These techniques have however only been implemented to a limited extent. Further research on pressing to higher dryness levels will in the future be continued at FSCN at Mid Sweden University. Pilot refining trials with HTCTMP from spruce showed that densification and strength development were achieved by two different mechanisms: by making fibres flexible with gentle high consistency refining (HC refining) or by reducing fibre length with intense low consistency refining (LC refining). It was found that a high bulk at a very high Z-strength was achieved with LC refining even though the fibre length was reduced and at extremely low energy input. The results showed that fibres with extremely high content of sulphonated lignin on surfaces with low degree of fibrillation bond well as long as the surfaces get into contact during pressing and drying. This can be achieved by either making fibres flexible or by reducing fibre length. LC post-refining of spruce HTCTMP was found to be a very interesting process concept for production of high quality pulps intended for paperboard at a very low total energy input of ~800 kWh/admt.