Studies of the formation and properties of urea-formaldehyde resin adhesives
Urea-formaldehyde (UF) resin is used primarily as an adhesive for manufacturing wood panels (medium density fiberboard, particleboard and plywood). These are important New Zealand export products. A better understanding of the formation of UF resin will allow maximisation of its adhesive properties (high bonding strength) and minimisation of formaldehyde emissions (an adverse environmental effect). These improvements will enhance the competitive advantage of New Zealand resin products. The research reported here has been directed primarily towards monitoring the UF reaction by Nuclear Magnetic Resonance spectroscopy (NMR), preparing UF resins, determining the performance of the resins produced and characterising the resins by high performance liquid chromatography (HPLC), gel permeation chromatography (GPC) and differential scanning calorimetry (DSC) methods. The development of a dynamic (RAPID) NMR acquisition method has contributed to the identification of optimum conditions for UF resin manufacture. A series of NMR "snap-shots" of the UF reaction system over the entire reaction period has been obtained. A wide range of reaction conditions, eg formaldehyde to urea molar ratio, formaldehyde concentration, initial addition pH, condensation pH, addition reaction time and reaction temperature, have been investigated. From the species present and the changes in their relative concentrations during the reaction, optimum reaction conditions have been determined as: Formaldehyde concentration: 35-46% F/U molar ratio: 1.8-2.0 Temperature: 88°C Addition reaction time: 25-30 min Initial addition pH: 8.0-9.0 Condensation pH: 4.5-5.0 The dynamic (RAPID) NMR studies were complemented by HPLC characterisation of samples taken during the different reaction stages. The main low molecular weight species present in the UF reaction system have been identified. In general the HPLC results were consistent with those obtained by the dynamic (RAPID) NMR method. Low molecular weight species were identified by electrospray mass spectrometry. Molecular weight, molecular weight distribution and resin aging were investigated by the GPC method. The number average molecular weights of typical final UF resins were in the range of 600-800 Dalton. The weight average molecular weights of the same resins were in the range of 7000-10 000 Dalton. A series of laboratory resins were prepared to test the appropriateness of the optimum reaction conditions predicted from the NMR and HPLC studies. Resin performance in wood panels was determined by measuring internal bond, formaldehyde emission and cold water swell. The effects of F/U molar ratio, resin loading and board thickness on the panel properties have also been determined. The results showed improvement in the internal bond and formaldehyde emissions. Comparison of the experimental resin prepared under the optimum reactions with a commercial unmodified UF resin indicated that a 12% increase in internal bond. Formaldehyde emissions were similar. It is clear from the results of this study that current manufacturing conditions for the production of unmodified UF resins are close to optimum. Future improvements are likely to be based upon structural and/or non-structural additives.
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