The effects of cellulose nanofibers compounded in water-based undercoat paint on the discoloration and deterioration of painted wood products

AbstractCellulose nanofibers (CNFs) have many potentials as filler to improve the properties of the other materials. We have developed the novel paints containing CNFs, and which controlled the discoloration of wood products. To clarify the discoloration mechanism of wood panels using an undercoat paint containing CNFs, prepared by an integrated process from Cryptomeria japonica, the composites and films made of CNFs and acryl resin that was a raw material for the paints were prepared. Observation of the surface of the CNFs/acryl resin composite film by atomic force microscopy showed that the fibers and the resin were uniformly mixed. The composite film prevented light transmittance in the ultraviolet (UV) light region, as well as oxygen gas permeation. The permeability coefficient of the oxygen gas decreased to 60% with the addition of 1.5 wt% of CNFs to the acryl resin. The addition of CNFs also increased the breaking stress by approximately 1.5 times compared with the acryl resin film. Electron spin resonance (ESR) analysis after UV irradiation resulted in the lowest radical formation of a piece of wood wrapped in the CNFs/acryl resin composite compared with the acryl-coated specimen and the wood as it was. Therefore, the CNFs composite film shielded the UV rays and oxygen more effectively than the original acryl resin, making it difficult for these factors to reach the wood’s surface, and thus, perhaps suppressing the generation of radicals from the wood. These actions would suppress the production of coloring substances caused by the radicals, resulting in the suppression of discoloration. Furthermore, the increase in the film’s strength by the addition of CNFs would have enhanced the stability of overall the paints with a CNF-containing undercoat. These effects might have contributed not only to the prevention of discoloration but also to the prevention of the occurrence of minute cracks caused by various weather deterioration factors.