Properties of urea-formaldehyde resins for wood-based composites

This chapter reviews recent progresses on properties, chemical structure, thermalcuring behavior, hydrolytic stability, morphology, microstructure, crystalline structure,and modifications of urea-formaldehyde (UF) resin as an adhesive for wood-basedcomposite panels, particularly by focusing on the parameters related to formaldehydeemission (FE), such as synthesis reaction pH conditions, formaldehyde/urea (F/U) moleratio, and resin modifications.The reaction pH condition of UF resin synthesis showed that the amount of freeformaldehyde strongly affected the reactivity of UF resin, and also indicated that theweak acid reaction condition provided a balance between increasing resin reactivity andimproving adhesion strength of UF resins. Solid-state ¹³C-NMR spectroscopy indicatedthat the molecular mobility of cured UF resin increased with decreasing the reaction pHused during its synthesis. The ¹³C-NMR spectroscopy showed that UF resins with higherF/U mole ratios (i.e., 1.6 and 1.4) had two distinctive peaks, indicating the presence ofdimethylene ether linkages and methylene glycols, which give a greater contribution tothe FE than that of lower F/U mole ratio. However, these peaks were not detected at theUF resins with lower F/U mole ratios (i.e., 1.2 and 1.0). Lowering F/U mole ratio of UFresins as a way of abating FE consequently requires improving their reactivity. As theF/U mole ratio decreases, thermal curing behavior of these UF resins such as the gel time,onset and peak temperatures, and heat of reaction (ΔH) increased, while the activationenergy (Ea) and rate constant (k) were decreased. The results also suggested that as theF/U mole ratio decreased, the FE of particleboard (PB) was greatly reduced at theexpense of the reactivity of UF resin and slight deterioration of performance of PBprepared. Dynamic mechanical analysis (DMA) results partially explained the reasonwhy UF resin adhesives with lower F/U mole ratio resulted in relatively poor adhesionperformance. Morphological investigation on UF resins illustrated that the spherical structures incured UF resins were much more resistant to the hydrolytic degradation by the acid thanamorphous region. Atomic force microscopy (AFM) images showed two distinctiveregions, i.e., hard and soft phases in cured UF resins. The AFM study suggested that thesoft phase was much more susceptible to the hydrolysis of cured UF resin than the hardphase. The soft phase of cured UF resins by ammonium chloride was much more easilyhydrolyzed than those cured by ammonium sulfate, indicating that hardener types had agreat impact on the hydrolytic degradation behavior of cured UF resins. For the first time,the presence of thin filament-like crystalline structures on the fracture surface of curedUF resin was reported. And X-ray diffraction (XRD) results showed that the crystallineregions of cured UF resins with lower F/U mole ratio contribute partially to the improvedhydrolytic stability of the cured resin.