Effects of nanocrystalline cellulose on the micro-creep properties of phenol formaldehyde resin

The use of nanocrystalline cellulose (NCC) has become increasingly prevalent in recent years, with limited studies on the the time-dependent characteristics of wood-adhesive composite reinforced by it. To analyze the viscoelasticity of control phenol formaldehyde (PF) resin and wood-resin composites added with three types of NCC, the micro-creep properties of control phenol formaldehyde (PF) resin and PF resin with cellulose nanofibrils (CNF), cellulose microfibrils (CMF), and cellulose nanocrystals (CNC) were investigated by nanoindentation (NI). The sample creep curves proved that adding CNC, CMF, and CNF into the PF resin remarkably impacted the creep properties of the wood-resin sample interface. The cured resin sample had the lowest strength and the most deformation, and the pure wood sample performed slightly better. Among the three NCC-added wood-resin samples, the CNF-added wood-resin sample has the least deformation in the same load and the least permanent deformation after unloading followed by CMF-added and CNC-added wood-resin samples. In the derivative of creep curve function, the depth tendency of CNC-added and CNF-added wood-resin samples showed a gently increasing slope with time in contrast to CMF-added ones. Furthermore, scanning probe microscopy (SPM) images supported that PF resin had filled up the pores of the wood microstructures and strengthened the wood-resin sample in the company of NCC, which was then able to carry the load.