Humidity's Effect on the Dynamic-mechanical Behavior of Phenol-formaldehyde Impregnated Beech Wood Veneer

The dynamic-mechanical behavior of phenol-formaldehyde-impregnated beech wood (Fagus sylvatica L.) veneers was evaluated at various relative humidities. The aim was to identify the plasticizing effects of moisture on phenol-formaldehyde-impregnated veneers. Thin beech veneer samples were impregnated with a low and a medium molecular weight phenol- formaldehyde resin. The measurements were performed with a dynamic- mechanical thermal analysis equipped with a humidity generator. The storage modulus and the loss factor of the uncured phenol-formaldehyde- impregnated and the untreated control veneers were determined at a relative humidity of 15%, 60%, and 85% and a constant temperature of 25 °C. The untreated control veneers exhibited the smallest reduction of the normalized storage modulus. In contrast, the phenol-formaldehyde- impregnated samples exhibited a more intense relative reduction of the storage modulus with increasing relative humidity. This effect was more distinct for samples impregnated with low molecular weight phenol- formaldehyde than for those impregnated with medium molecular weight phenol-formaldehyde. In addition, higher solids contents in the resins led to even smaller normalized storage modulus. Consistently, the loss factor indicated a greater softening of the samples with an increasing humidity and higher solids content of the phenol-formaldehyde.