Relevant wood characteristics for gluing beech and ash with regard to discoloration

Relevant parameters for the hardening of acid catalyzed and water based adhesives (e.g. MUF) are certain wood characteristics such as water absorption capacity, pH-value, buffering capacity towards acid and extractive content. This study was performed to characterize these parameters for beech (Fagus sylvatica L.) and ash (Fraxinus excelsior L.) with and without coloured heartwood in comparison to Norway spruce (Picea abies (L.) Karst.). Chemical characteristics were also examined for larch (Larix decidua MILL.). The determination of water absorption capacity revealed a high water absorption coefficient for beech and ash without heartwood. While for both hardwoods with coloured heartwood significantly lower water absorption capacity could be determined, the lowest values were found for spruce. It is assumed that the distinct differences in water absorption are appointed to have an impact on the curing behaviour. Measurements of the pH were performed on fresh and aged veneer surfaces as well as in cold water extractive solutions. For beech and ash the discovered influence of discoloration on pH is significant, but assumed to be negligible for curing. The comparison of pH values measured on surfaces with pH values obtained for cold water extractive solutions showed a satisfying correlation. Measurements of the buffering capacity on cold water extractives revealed high buffer capacity of hardwoods in comparison to spruce and larch. The coloured heartwood showed a significantly minor capacity for both hardwoods.