Investigation of important influencing factors on the tensile shear strength of two component polyurethane with distinct foaming behaviour

In this experimental research study, the tensile shear strengths of gap-filling two-component polyurethane (PUR) bonded lap joint specimens made out of spruce wood were investigated more closely. Based on a general full factorial design, the values of various possible influencing factors were systematically varied. These factors were the ratio of isocyanate to polyol, the amount of the additional water, water available from the wood substrate (wood moisture content), the press temperature and the thickness of the bond line. Two different types of bonding were examined, the foamed and the solid wood bonding. The foam bonded lap joint specimens had gaps with defined bond line thicknesses, allowing the PUR adhesive to foam between the two adherends. Furthermore, also solid wood specimens, where this foaming behaviour was prevented by the absence of predefined cavities, were investigated. Generally, the influences of the factors on the tensile shear strength were different for each type of bonding. The highest strength values of the foam bonded specimens were achieved with a high wood moisture content (21%), a low press temperature (20 °C), a high amount of isocyanate (> 85 wt%), an average amount of additional water (5.5%) and a low foam thickness (0.5 mm).