Journal of Adhesion Science and Technology, Vol.32, No.23, 2537-2551, 2018
Comparison of the properties of urea-formaldehyde resins by the use of formalin or urea formaldehyde condensates
In industry, formalin and urea formaldehyde condensates (UFC) are used for the preparation of urea-formaldehyde adhesives. The aim of this study was to evaluate the characteristics of a UF resin prepared with formalin (42% concentration) and two kinds of UFC with various urea content (F/U similar to 5 and 4.3 malar ratio). The resins were prepared with final molar ratio F/U = 1.1. The UF resin with formalin showed better mechanical properties than those with UFC. The C-13 NMR and FT-IR results indicated that methylene, methylene ether and urons groups in a UF resin prepared with formalin were present in higher proportion than observed in industrial UF resins. The DSC (differential scanning calorimetry) results showed that the curing temperature of a UF resin prepared with formalin is lower than that of a UF resin prepared with UFC. The mechanical properties of MDFs prepared with the three types of adhesives showed that MDFs bonded with a UF prepared with formalin yields a better mechanical properties than MDFs prepared with a UF resin based on UFC. Moreover, resin synthesized with UFC (F/U = 5) result in higher IBS (internal bond strength) MOR (modulus of rupture), MOE (modulus of elasticity), TS (thickness swelling) and formaldehyde emission (FE) than resin synthesized with UFC (F/U = 4.3). The MDF bonded with the UF2 resin presented poor results, with the highest TS and the lowest IBS, MOE, MOR and FE.