Shear Refinement of Formaldehyde-Free Corn Starch and Mimosa Tannin (Acacia mearnsii) Wood Adhesives

The aim of this work was to reduce the viscosity of formaldehyde-free corn starch–mimosa tannin wood adhesives, without adversely affecting the mechanical properties of the product. The reduction of viscosity was achieved using shear refinement. The study focused on the physical phenomena before cross-linking of the wood adhesive. The physical (rheological characterization) and mechanical (bond strength) properties of formaldehyde-free corn starch and mimosa tannin wood adhesives were measured. The results showed that the shear refinement (290 rpm and 5 min, optimal conditions) reduced the viscosity of the corn starch–mimosa tannin wood adhesives (from 100 000 to 458 Pa s) with the advantage of being stable over time. Mechanical tests showed that the shear refinement did not influence the mechanical properties of corn starch–mimosa tannin wood adhesives.     

Wood-dowel rotation welding – a heat-transfer model

The relationship between temperature, time of friction and thermal flux for high-speed rotational wood-dowel welding has been modelled through a heat-transfer model. It was shown that the interface temperature could be estimated as a function of the friction time by the general equation T ₀ = T _i + 2βuτ√α/h√π √t, where T ₀ – temperature at the welding interface, T _i – initial temperature of the wood, t – time, τ – the friction stress, u – the rate of rotation or vibration, β – the fraction of mechanical energy convertible into thermal energy, and h and α are, respectively, the thermal conductivity and diffusivity of the wood. For both the rotation welding and linear welding systems, the value of β is found to be 0.080±0.01. The results obtained for dowel rotation welding indicate that a temperature of 180°C is optimal for rotational dowel welding. The model was validated from experimental data on rotational dowel welding for the portion of the curve in which temperature increased as a function of time. Furthermore, it was also validated from experimental data for linear vibration welding.     

Predicting the Thermal Behaviour of Wood During Linear Welding Using the Finite Element Method

A numerical model to simulate the temperature behaviour of wood welding samples during the welding process was developed to understand the influence of material parameters on the welding temperature. A finite element method and the CAST3M software were used to simulate and model the temperature changes during welding of beech wood. This model takes into account the different properties of the wood welded bondline, the geometry of the sample and the external conditions. The energy produced by the friction welding of the wood samples was determined from infrared thermography measurements for the welding process and inputted into the model. The comparison between the predicted and experimental results shows that the model is reliable. The applied pressure, the vibration, the extrusion of material and the chemical reactions, particularly exothermic reactions, are not taken into account in this model and thus probably explain the differences existing between actual and simulated values. However, this numerical simulation gives information on the distribution of the temperature in the sample. The model predicted that the temperature difference between the centre and the side of the sample is not higher than 4°C. This means that the border effects are negligible. The model was tested for different welding times. According to the model a heat flow about 70 kW/m² is necessary at the welding line to ensure a satisfactory bonding for the chosen sample geometry. Welding of large wood pieces has also been simulated in this study.     

Performance of Dowel-Welded Wood Furniture Linear Joints

Comparison of three types of furniture joints such as scarf joints, step butt joints and dovetail joints held together either by one or two welded dowels, glued dowels and steel nails showed that the dowels always gave better shear strength and greater stiffness than the steel nails. The results of welded dowels and glued dowel joints were found to be comparable. The application of the welded joint technique to joints where the number of dowels is limited by the narrow space in which they can be applied, such as in furniture, can give results comparable to those obtained by gluing the same dowels.