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The aim of this study was to evaluate changes in fibre properties with high (HC)- and low consistency (LC) refining of TMP and determine how these contribute to tensile index. Two process configurations, one with only HC refining and another with HC refining followed by LC refining were evaluated in three TMP mainline processes in two mills using Norway spruce. An increase in tensile index for a given applied specific energy was similar for all LC refiners in the three lines, despite differences in the fibre property profiles of the feed pulps. Compared with only HC refined pulps at a given tensile index, HC+LC refined pulps had greater fibre wall thickness, similar fibre length, strain at break and freeness, but lower light scattering coefficient, fibre curl and external fibrillation. The degree of internal fibrillation, determined by Simons’ stain measurements, was similar for both configurations at a given tensile index. The results indicate that the increase in tensile index in LC refining is largely influenced by a decrease in fibre curl and in HC refining by peeling of the fibre walls. Compared at a given tensile index, the shive content (Somerville mass fraction) was similar for both HC+LC and HC refining.

Primary refined softwood was subjected to high-consistency (HC) or low-consistency (LC) secondary refining, and the nature of the development of the internal and external fiber microstructure and ultrastructure has been compared. The primary refining of mixed softwood as a raw material was performed in pilot scale by the advanced thermomechanical pulp process. The study was aiming at the comparative characterization of LC and HC pulps at the fiber level when produced with similar and well-characterized handsheet properties. The formerly described Simons’ staining method was applied. A significant degree of fiber wall delamination/internal fibrillation (D/IF) was observed during both LC and HC refining. Both the energy input and the refining consistency had a significant impact on elevating the degree of fiber wall D/IF. The statistical evaluation of internal fiber development indicated that the fiber populations in LC- and HC-refined pulps had a similar degree of fiber wall D/IF despite having a large difference in refining energy input (420 kW h odt-1), confirming that D/IF was promoted more energy-efficiently in LC than in HC refining. The characteristic of the external fiber development from HC and LC refining was very different. Secondary LC refining promoted fiber surfaces with ribbons of thin hairlike threads arising from the inner secondary S2 layer that occasionally developed along the whole fiber length. Broad sheet- and lamellae-type external fibrillation from the S2 was typical for HC refining, and these characteristics were rarely observed in the LC pulps. The mechanisms for LC and HC fiber development are proposed. The cell wall characteristics (internal and external) of the pulp fibers appear to govern most of the physical and optical properties in handsheets.

The electrical energy consumption in thermomechanical pulping (TMP) is very high, in the range of 2 – 3 MWh/adt depending on process solution and on the product quality specifications for the paper product. Both pulpwood and energy prices have increased rapidly for some time. Due to this, the main focus of the research and development is on ways to reduce the electrical energy consumption in wood chip refining. As a step towards a more energy and cost ‐ effective refining process, Holmen Paper AB has invested in a new mechanical pulping process at its Braviken mill. In this case the primary refining stage consists of high consistency (HC) double disc refiners ‐ RGP68DD (machines with two counter rotating discs). Earlier studies on the refining conditions, such as intensity and temperature, have indicated that it should be possible to improve the energy efficiency in double disc refining while maintaining the functional pulp properties such as tensile index. The main goal of this project was to improve the energy efficiency in double disc chip refining with 150 kWh/adt to corresponding pulp properties as measured on pulp samples after refiner. In order to further improve the basic understanding of what happens to the wood fibre material when changing the process conditions, the morphological and ultrastructural changes of fibres were also studied. This part of the research work was performed in cooperation with the research program; Collaborative Research on the Ultrastructure of Wood Fibres (CRUW). This licentiate project is a part of a large development project where different techniques to improve the energy efficiency has been evaluated by means of mill scale trials at the Holmen Paper Braviken Mill. The high consistency double disc chip refining part of the project was financed by The Swedish Knowledge Foundation, Metso Paper and Holmen Paper, in cooperation with FSCN (Fiber Science & Communication Network) at Mid Sweden University. The trials were made on one of the TMP lines at the Holmen Paper Braviken mill with Norway spruce as raw material. The influence of increased specific refining energy on pulp properties were studied at different refining temperatures, refining intensity, pulp consistency and production rate. Results from these trials were later validated by means of long term trials. Intensity models and simulations for intensity changes by new segment design were made by Juha‐ Pekka Huhtanen from Tampere University of Technology, Finland. The results show that the specific energy consumption to same tensile index can be improved by means of increasing the refining pressure/temperature. The energy efficiency was improved by 80 ‐150 kWh/adt depending on load and the inlet‐ and housing pressure. The largest relative specific energy efficiency improvement was reached at low specific energy consumption levels. Similar fibre surface ultrastructure characteristics are gained by pulps with high pressure/temperature and low specific energy consumption compared to low pressure/temperature and high specific energy consumption pulps. High pressure/temperature and high specific energy consumption resulted in significantly increase in the delamination/internal fibrillation of pulp fibres. The surface ultrastructure of these fibres exhibited exposed S2 layer with long ribbontype fibrillation compared to pulps produced with lower temperature and lower specific energy consumption. When the refiner was operated at high pressure, the tensile index was preserved over the whole plate life. The specific light scattering coefficient increased with increasing pressure/temperature. A reason for this could be increased intensity caused by decreased plate gap. Increased intensity by means of refiner segment design changes resulted in large specific light scattering coefficient increase at similar tensile index, lower shives content, lower average fibre length and lower CSF at same specific energy consumption. The fresh steam consumption was reduced by the increased refiner ressure/temperature.

A study was carried out aiming at understanding the fundamental reasons for different fibre behaviour exhibited by Norway spruce and Scots pine causing large energy consumption differences during thermomechanical pulping (TMP). Ultrastructural characterization of TMP fibres and shives, which were sampled from the two wood species after primary refining, was performed using scanning electron microscopy, transmission electron microscopy (TEM) and TEM-immunogold labelling for their morphological and topochemical properties. As expected, pine wood chips needed higher electrical energy consumption to be refined to a given Canadian Standard Freeness and it produced inferior strength properties compared to spruce. Electron microscopy (EM) observations indicated that the mechanisms of fibre development during primary refining of pine and spruce were different. The two stages of pine fibre separation and development were not concurrent. Results indicated that pine fibre defibration/fracture occurred predominantly through the compound middle lamella/S1 interphase or through the S1 layer producing lesser amounts of shives during the primary refining stage than spruce. In contrast, spruce fibres defibrated mainly through the S2 layer. Detailed EM observations on shives and pulp fibres from TMP revealed the ultrastructural characteristics associated with pine fibre cell walls. Morphological and topochemical features of the S1 layer, S1/S2 interphase, such as lignin and galactoglucomannan distribution across cell walls were explored. The ultrastructural properties are discussed in relation to the TMP parameters (i.e., electrical energy consumption) and strength data. It is concluded that ultrastructural characteristics of Scots pine fibre cell walls govern the different fibre development mechanisms and explain the negative response of this wood during TMP processing.

A study was performed on stone groundwood (SGW) pulps produced on a pilot scale. The behaviour of selected juvenile and mature Norway spruce wood samples was investigated. As revealed by standard tests, sheets formed from juvenile wood showed improved light scattering properties, improved tear and tensile strength, and higher sheet density compared to those formed from mature wood. Scanning electron microscopy indicated that the differences are likely related to the manner of fibre processing and development at the ultrastructural level. Mature wood fibres showed greater fibre end breakage, a smaller long-fibre fraction, enhanced S1 fibrillation and frequently open fibres. In contrast, juvenile fibres had a 14% higher long-fibre fraction and showed typical S2 fibrillation. Fibre development of juvenile wood showed fibrillation features similar to those previously reported for thermomechanical pulp fibres. In both cases, the structural hierarchy of the wood fibre cell wall and the microfibril angle of S2 and S1 layers govern cell-wall splitting and fibrillation progression. The superior quality of the fibre furnish prepared from juvenile fibres compared to mature fibres with SGW pulping may offer an alternative process for more effective utilisation of raw materials such as top logs rich in juvenile wood.

Refining during mechanical pulping causes delamination and internal fibrillation (D/IF) of the fibre wall and changes the surface ultrastructure. Fundamental knowledge about these phenomena at the fibre cell wall level helps our understanding of the development of pulp and paper properties, which in turn facilitates the optimization of processes, helping to save energy and improve the characteristics of final products. In the present study, pulps were produced by double-disc (DD) refined thermomechanical processes (DD-TMP) and have been characterized at the fibre cell wall micro/ultrastructural level based on Fernando and Daniel’s method (2010) of Simons’ staining and scanning electron microscopy (SEM). The pulps studied were never-dried Norway spruce DD-TMP from a full-scale mill trial running under different process conditions; a) varying refining intensity, achieved by using a high-intensity turbine segment (HTS) and a standard (Ref) segment from Metso, and b) varying specific energy consumption (SEC). Improved energy efficiency was obtained with HTS segments, giving adequate or better pulp properties with respect to elongation, light scattering, Canadian Standard Freeness (CSF) at a similar tensile index level and lower energy consumption. Energy efficiency was gained through an elevated degree of D/IF and S2 fibrillation with low energy input. Both the SEC and segment designs had a significant impact on elevating the degree of D/IF. Statistical evaluation of fibre development with respect to D/IF gave evidence for improved energy efficiency in HTS refining. Ultrastructural studies on fibre surfaces showed that HTS refining produced better external fibrillation and leads to exposing the secondary S2 wall as fibre outer layer with different ribbon-type S2 fibrillation. Information obtained at the fibre wall level concerning D/IF and surface ultrastructure contribute to the fundamental knowledge about the pulp and handsheet properties and the energy efficiency of TMP processing.