Mechanochemical reactions in lignocellulosic materials

In this work, industrially relevant mechanochemical processes of lignocellulosic materials were investigated. To this day, mechanochemistry remains unknown even to most chemists, and reports on its effects in the forest products industry are scarce. As many processes of the pulp and paper sector include mechanical treatments, the effect of such treatments on the chemical properties of the materials should be known. The treatments that were studied herein were: preparation of nanofibrillated cellulose (NFC) by Masuko refining, thermomechanical pulping at high temperatures (HT-TMP) and ball milling in the presence of a reactive vinyl monomer, namely, styrene. The HT-TMP was also studied as a potential reinforcing agent in biocomposites. Material properties were characterized with techniques such as electron paramagnetic resonance (EPR) and ultraviolet resonance Raman (UVRR) spectroscopies, optical analysis of fiber dimensions, tensile testing, and scanning electron microscopy (SEM). The main hypothesis of the work was that mechanoradicals are formed upon various mechanical treatments. This was confirmed for kraft pulp homogenization into NFC and reactive ball milling of cotton. With mechanical pulping, the situation is more complex: thermal and mechanical effects are simultaneous and therefore difficult to distinguish. The same holds true for composite preparation by extrusion, where both heating and shear forces are present. Mechanoradicals were shown to be able to act as starting points for cellulose copolymerization but the efficacy of these reactions was greatly affected by the presence of oxygen and water. HT-TMP fibers were shown to have potential in industrial composite reinforcement due to their hydrophobicity, ease of dispersion in the PLA matrix, and low specific energy consumption. In conclusion, acknowledging the formation of mechanoradicals in industrial processes is crucial. These radicals may participate in unwanted side reactions, such as oxidation or polymer degradation. On the other hand, mechanical processing may yield new avenues for fiber modification.