Systematic assessment of wheat extenders in formaldehyde-condensation plywood resins: part II – mechanical properties of plywood panels

Wheat flour is used as an extender for urea (UF) and phenol formaldehyde (PF) resins in plywood production. The addition of wheat flour is usually based on operational experience. Whereas flour quality parameters are important predictors for the bread production, fundamental scientific knowledge is hardly available for adhesive applications. Therefore, this work focusses on the separate investigation of the main components of wheat flour: starch and protein, as additives to UF and PF resins in different amounts. The resin batches produced were used for manufacturing 9-layered birch plywood panels. While part I focused on physico-chemical properties of the adhesives, in this present part panel performance was tested in accordance with EN 314-1 (2005) and by evaluating formaldehyde emissions using micro-chamber/thermal extractor. The UF-based panels were tested for class 2 covered exterior use and the PF panels for class 3 non-covered exterior use according to EN 314-2 (1997). Wheat protein showed the most promising wet shear strength results for both resins, the resulting high viscosity limits its addition to 30%. An addition up to 100% wheat flour and wheat starch to UF resins provided sufficient wet shear strengths. None of the extenders had an effect on formaldehyde emissions.