Foaming performance and bonding strength of a novel urea-formaldehyde foaming resin facilely prepared with thermo-expandable microspheres

A novel urea-formaldehyde (UF) foaming resin to be used in low-density wood-based materials was facilely fabricated by mixing thermo-expandable microspheres (TEMs) with UF resin. The foaming performance and bonding strength of the new resin were investigated with specially designed paper/UF samples and bonded veneers joints, respectively. The developed resin had a good expansion effect, with an apparent density of less than 300 kg/m3 and expansion ratios of 4.5–9.8, and a uniform cellular structure inside the resin was demonstrated by scanning electron microscope observations. However, the addition of TEMs resulted in a 47.5%–91.4% decrease in the shear strength of the resins. As the concentration of TEMs increased from 2.5% to 10.0%, the apparent density and shear strength showed similar decreasing trends. The introduction of glutaraldehyde (GA) increased the bonding strength of the resins to 69.8%–199.7%, while significantly increasing their apparent density. Increasing the concentration of TEMs would be an effective way to reduce the apparent density of the UF foaming resins, but there will be an accompanying degradation in their strength. GA could be a potential chemical modifier for TEMs/UF resins in terms of improving their bonding strength.