The effect of fibre wall thickness and microfibril angle on TMP energy consumption, fibre development and paper properties
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The overall objective of the “Basic phenomena in mechanical pulping” project financed by NER (the Nordic Energy Research) of which the PhD study was a part, was to show a radical reduction (30%) in specific energy consumption in the production of mechanical pulp. Energy consumption is an important aspect in mechanical pulping as some 1200-2500 kWh/t is consumed during thermomechanical (TMP) refining. The specific energy consumption for producing thermomechanical pulp from Scots pine is normally 10-20 % higher than that of Norway spruce when compared at the same pulp freeness. Explanations for this difference have been searched from the higher extractives content of pine and from the differences in the physical and chemical structure of the fibre walls between pine and spruce. The main objective of this study was to obtain a better understanding of how wood raw material properties (fibre wall thickness, fibril angle) affect energy consumption and influence the fibre development in TMP pulping. The reduction in fibre wall thickness is higher for thick-walled materials than thinwalled materials. In the case of spruce, the thin-walled raw materials had lower external fibrillation and higher internal fibrillation than the thick-walled. Raw materials with thicker fibre walls produce more fines and external fibrillation and thus have low energy consumption (SEC) at a given freeness. However the effect was not strong enough to explain the difference generally observed between pine and spruce. The energy consumption to a given freeness in this study was found to be higher for Norway spruce than Scots pine due to harsher refining condition for the pine sampler. The results show a difference in the development of the fibre properties and resulting laboratory sheets with different fibre wall thickness and fibril angle, however the observed differences can not explain the difference in energy consumption generally seen between spruce and pine. Linear regression models were performed to study the effect of raw material properties on specific energy consumption. The models showed that at equal freeness, SEC was reduced by 40 kWh/t pr 0.1 μm higher fibre wall thickness. In this trial energy consumption of the spruce pulps was on average 360 kWh/t higher than pine. The fibril angle had no significant effect on SEC to a given freeness. It was shown that fibre bendability is increasing with specific energy consumption upon mechanical pulp refining. Spruce TMP fibres were more flexible than pine after refining. Local damages of the fibre wall, such as delamination, kinks, and compressions were the main effect in the increasing of the bendability with the refining. The specific surface area was found to increase with specific energy consumption, external fibrillation of long fibres and with the fines content in paper leading to dense and strong paper. A regression model showed that a reduction in fibre wall thickness will increase paper density with 67 kg /m3 pr 0.1 μm of fibre wall thickness. It was found that fibril angle has a significant influence on paper roughness. Fibres with low fibril angle will give low roughness values, probably because these fibres collapse more easily.