Upcycling bark into lignin- and hemicellulose containing cellulose microfibrils for high-strength composites

Abstract Microfibrillated cellulose or cellulose microfibrils (CMF) are promising materials with versatile applications. However, the utilization of CMF is limited due to the high cost of production. In the present study, CMF were prepared from spruce bark subjected to different intensities of chemical pretreatment and used for nanocomposite preparation. Spruce bark was delignified using various methods, including subcritical water extraction, neutral sulfite semi-chemical pulping, and peracetic acid treatment. The pretreated bark was mechanically fibrillated and the resulting CMF suspensions were processed into cellulose nanopapers. For composite preparation, the latter were impregnated with phenolic resin and hot-pressed to form a composite laminate. Nanopapers and composites were characterized in tensile and flexural tests, respectively. Results showed that the chemical composition, morphology, and physical properties of cellulose nanopapers were highly dependent on the pretreatment method. The thermal stability of nanopapers was found to increase with increasing lignin content. All treated samples showed nanoscale cellulose fibrils with diameters ranging from 8–16 nm with average tensile strength and Young’s modulus of 29–114 MPa and 5–12 GPa, respectively. Furthermore, the prepared composites demonstrated promising flexural properties with values of 109–195 MPa and 8–13 GPa for flexural strength and modulus, respectively. Despite the low purity of CMF used herein, the flexural properties of the resulting composites exceeded that of similar nanocomposites reported in previous studies. The findings of the present study thus indicate that a composite with favorable mechanical properties can be prepared from spruce bark, a resource typically not put to material use.