Urea-formaldehyde Resin Curing Process Studied By Time-Domain Nuclear Magnetic Resonance

Urea-formaldehyde resin is one of the very important adhesives industries. The curing process of UF occupies an important place in field of the practical application. The curing process has a direct impaction product quality. This paper research the curing process of non-hardener urea-formaldehyde resin; research the curing process for UF resin with zinc chloride hydrolyze as curing agent and analyze changes of hardness and liquidity; research the curing process for UF resin with ammonium chloride/acetic acid as curing agent and analyze changing process of spin-spin relaxation time and molecular dynamics by TD-NMR technology. The curing process for UF resin with phthalic anhydride as curing agent is studied by differential scanning calorimetry (DSC) and thermo gravimetric (TG). Before and after curing. Structure of urea-formaldehyde resin is analyzed by fourier transform infrared spectroscopy (FTIR). Particles size of UF resin is studied by X-ray diffraction (XRD) after curing. Some conclusions as follows:1. Urea-formaldehyde resin without hardeners started curing at about 120℃ and kept curing at temperature range from 30℃ to 140℃ and ended around 180℃ .2. What is known by spin-spin relaxation time changes, spin-lattice relaxation time changes, the relationship between system quality and temperature. The total signal between T1 and T2 reduce with increasing temperature, which confirms the hydrogen content of UF gradually decreases. As water evaporates slowly lead to reduce hydrogen content at temperature range from 30℃ to 140℃ . The hydrogen content change of UF is larger when the temperature is above 140 ℃ ,which is due to that urea-formaldehyde resin system happened crosslinking condensation reaction and generated formaldehyde and water that separate from system.3. Urea-formaldehyde resin started curing at about 120 ℃ and ended around 140 ℃ with ZnCl2 as curing agent in the curing process.4. Spin-spin relaxation time changes of UF with ZnCl2 as curing agent is analyzed in the curing process. T2 gradually reduces with the rise of temperature, which lead to liquidity of UF resin gradually decreases and transform liquid into solid. Spin-lattice relaxation time changes of UF is analyzed. T1 become gradually large with increasing temperature at temperature range from 30℃ to 140℃ in the curing process, which lead to hardness of UF resin gradually increases; high temperature makes T1 reduce and aging phenomena of UF resin happen and hardness decrease when the temperature is above 120 ℃ .5. Urea-formaldehyde resin started curing at about 100℃ and ended around 120℃ with ammonium chloride/acetic acid as curing agent. Chemical activation energy of UF is 16.56 kJ/mol in the curing process. Hardener reduces reaction conditions.6. Spin-spin relaxation time of UF with hardener ammonium chloride/acetic acid is analyzed. T2 gradually reduces with increasing temperature, which lead to liquidity of UF gradually decrease. Between ammonium chloride and formaldehyde happened reaction and generated hydrochloric acid under the action of acetic acid, hydrochloric acid and acetic acid provide acidic environment together, which confirms urea-formaldehyde resin system happened condensation reaction.7. The curing process of UF with hardener o-phthalic anhydride is analyzed by differential scanning calorimetry (DSC) and thermo gravimetric (TG). Urea-formaldehyde resin curing reaction is more intense with curing agent. The best percentage of o-phthalic anhydride is 2%. UF resin is studied by X-ray diffraction (XRD) after curing. Urea-formaldehyde resin curing form more mesh structure "chain bridge" and particles is larger with hardener o-phthalic anhydride comparing with the one without hardener. Before and after curing, structure of urea-formaldehyde resin is analyzed by fourier transform infrared spectroscopy (FTIR). Condensation crosslinking reaction happen between urea-formaldehyde resin molecules and form ether and peptide bond, and thereafter a mesh structure macromolecular polymer is formed.