Observations on the Microfibril Angle of Finnish Papermaking Fibres

The geographical variation of microfibril angle (MFA) in Finnish papermaking fibres was examined by measuring the microfibril angles with a confocal laser scanning microscope (CLSM). The fibres used were obtained from Finnish pulp mills, and the Scots pine raw material for them came from different geographical locations in Finland. Also the effect of beating and drying on MFA was examined. The effect of microfibril angle on paper behaviour in drying and paper strength properties was investigated by determining the final drying stress of test sheets and by testing different mechanical properties of the dried test sheets. The tests included tensile properties and bond strength analysis. The CLSM inspection of microfibril angles did not reveal any significant geographical variation in market pine kraft fibres: the average MFA varied between 3-15 degrees except for the fibres of one pulp made of both saw mill chips and 1st thinning pulpwood. In that particular pulp, the internal variation of MFA was extremely high, which can probably be best explained by the different types of raw material. Beating and drying affected the microfibril angles significantly: beating increased the MFA in all tested pulps and also increased the internal variation within pulps significantly. The MFA of beaten pulp fibres varied between 12-32 degrees. Also drying caused both increase in MFA (averages 39-48 degrees) and increase in the internal variation. A final drying stress developed into the test sheets was calculated based on dimensional changes and corresponding loads recorded during drying. Since variation in microfibril angles between the different pulps was relatively small, no conclusions on the effect of MFA on drying stress could be made. Similarly, no real differences in paper strength properties could be detected. Based on these results, it can be said that Finnish mills probably need not consider MFA as a significant fibre property in their Scots pine raw material procurement. Only a relatively small number of fibres could be measured for their MFA due to the quite time-consuming measuring technique. Furthermore, since the raw material procurement areas of pulp mills are wide, it is impossible to determine whether the fibres are exactly from a certain geographic location. These factors must be taken into account when assessing the results of this study.