A morphological study of the wood/phenol-formaldehyde adhesive interphase

This paper evaluates the morphology of the wood/phenol-formaldehyde adhesive interphase by dynamic mechanical analysis and C-13 CP/MAS NMR. It is shown that a low molecular weight phenol-formaldehyde (PF) resin (M-n = 270 g/mol, M-w/M-n = 1.2) interacts with the wood cell wall polymers on the nanometer scale, as demonstrated by enhanced intermolecular coupling near the main glass transition of wood lignin. With CP/MAS NMR, T-1 rho(H) measurements indicated that the low molecular weight PF significantly increased the relaxation time of wood polymers. Nanoscale miscibility is proposed to occur for the low molecular weight PF resin and amorphous wood polymers. On the other hand, a high molecular weight PF resin with a broad distribution of molecular weights (M-n = 2840 g/mol, M-w/M-n = 5.0) did not enhance the intermolecular cooperativity around the glass transition but rather enhanced molecular motions, as indicated by a lower glass transition temperature of the composite. The high molecular weight resin did not affect the T-1 rho(H) of wood polymers, suggesting an overall phase separated system. The enhanced molecular motions in the PF-High composites were attributed to a low molecular weight fraction that effectively penetrated the wood structure while remaining uncrosslinked and thus acting as an internal plasticizer.