Impact of Type and Content of Nanocellulose on Phenol–Resorcinol–Formaldehyde Resin’s Cure Kinetics, Rheology, and Flexural Properties

Phenol–resorcinol–formaldehyde (PRF) is a cold-setting wood adhesive whose cured form is intrinsically brittle and whose bonding effectiveness deteriorates with storage age due to the loss of reactive methylol groups. This study investigated the potential of nanocellulose (NC) reinforcement to enhance toughness and offset aging-related losses. Before the hardener was added, three NC types—cellulose nanocrystals (CNC) and bleached and unbleached cellulose nanofibers (BNF, UBNF)—were homogenized into fresh (NP) and 4-year-aged (OP) PRF resin at one and three weight percent. The resin was then cast and gradually post-cured. Differential scanning calorimetry and rheometry were used to track curing activity, and flexural characteristics were assessed on cured specimens. The inclusion of NC increased the temperature at which gelation began, prolonging the working time of the resin. In comparison to neat PRF, CNF (BNF/UBNF) integration enhanced flexural strength by up to 39%, while CNC addition increased flexural modulus by up to 57%. Fibril aggregation was responsible for the 12–20% decrease in BNF/UBNF strength at three weight percent loading. Aged PRF was regularly outperformed by fresh PRF composites. These findings support the use of NC reinforcement in wood–PRF composites for extrusion-based additive manufacturing by demonstrating that it can toughen PRF and restore characteristics lost due to resin aging.