Swelling of mechanical pulp fines

The objective of the study was to determine the swelling of different types of mechanical pulp fines. The physical and chemical characteristics of the fines were also examined. It was found that the degree of swelling correlates with the proportion of fibrillar material, that is fibrillar content of the fines. The fines with the lowest fibrillar content had a swelling comparable to mechanical pulp fibres (0.69 g/g), whereas the fines with a high fibrillar content had a swelling comparable to never‐dried kraft pulp fibres (1.41 g/g). Hemicellulose content and charge could not explain the differences in swelling of different types of the mechanical pulp fines. While the lignin content appears to be an important factor in the degree of swelling of mechanical pulp fines, the results suggest that structural differences between the particles are also important. The ‘bulk elastic modulus’ was determined by measuring the change in swelling for a known change in osmotic pressure. All the mechanical pulp fines had a high bulk elastic modulus compared to kraft fines. However, fibrillar fines had a lower bulk elastic modulus than flake‐like fines. Mechanical pulp fines, both fibrillar and flake‐like varieties, did not hornify appreciably. The swelling of both the fines and the fibre fractions increased slightly with the specific energy consumption in the refining.