CHANGE OF MECHANICAL PROPERTIES OF NORWAY SPRUCE WOOD DUE TO DEGRADATION CAUSED BY FIRE RETARDANTS

The degradation of surface layers of wood can be caused by chemical reactions of compounds contained in fire retardants which were applied to historic wooden structures in the past. Among such chemicals are e.g. ammonium sulphate and ammonium phosphate. The paper presents the experimentally established mechanical properties measured on the surface of fibrillated wood of Norway Spruce (Picea abies (L.) Karst.), and their comparison with values measured on undamaged wood. The results proved a loss of cohesion in the damaged surface layer accompanied with a decrease in tensile strength by up to 50% when compared to the values ascertained in undamaged wood. The impact of corrosion was also studied using hardness tests based on ball and pin indentation. Electron microscopy investigation revealed both the damage to the middle lamella, which mainly consists of lignin, and the damage to the secondary cell wall primarily formed by cellulose. The increasing degree of fibrillation is accompanied by an increasing proportion of cellulose in the wood, due to an appreciable loss of lignin, and an increasing proportion of carboxyl groups, presumably due to the oxidation of hydroxyl groups in cellulose and lignin. The crystalline-to-amorphous cellulose ratio also increases. Lignin oxidation appears to be the major mechanism of wood fibrillation.