Thermo-pressed specimens coated with lignin-containing thermo-mechanical pulp fines and modified enzymatically to increase hydrophobicity

This study investigates the valorization of fines derived from thermomechanical pulp and made hydrophobic by enzymatic treatment to fabricate thermo-pressed specimens with reduced surface roughness and enhanced hydrophobicity. Thermomechanical pulp was fractionated into three main fractions: (i) a coarse fraction, average fiber length 2.6 mm and a fines content of 13.8%; (ii) a middle fraction, average fiber length 1 mm and fines content of 24.5 %; and (iii) a fines fraction, average fines length 0.3 mm and fines content of 66.5 %. Scanning electron microscopy of the three fractions revealed various structural components including intact fibers, split fibers, and fibrillated objects. Chemical composition analysis confirmed differences in lignin content among the fractions, with the fines fraction exhibiting the highest lignin content (30%). Wet and dry-formed thermopressed specimens were produced from the coarse and middle fractions, which exhibited notable differences in mechanical properties, i.e., tensile strengths between 4 and 20 MPa. Application of the fines fraction as a surface layer effectively reduced both the porosity and surface roughness of the wet- and dry-formed specimens. Hydrophobized fines increased the water contact angle of the wet-formed and thermo-pressed specimens to 86.8 ± 5.3° and 99.5 ± 1.4°. The deposition of hydrophobized fines (20 wt%) onto dry-formed samples created a water barrier that lasted for more than 20 min. However, assessment over a period of 4 months showed that the water barrier was reduced to some 6 min.