SYNTHETIC CONDITIONS AND CHEMICAL STRUCTU RES OF UREA-FORMALDEHYDE RESINS .2. SYNTHETIC PROCEDURES INVOLVING A CONDENSATION STEP UNDER STRONGLY ACIDIC CONDITIONS AND THE PROPERTIES OF THE RESINS OBTAINED

Strong-acid catalyzed urea-formaldehyde resins were prepared with comb ination variables of temperature and time of condensation reaction, pr e-methylolation, post-condensation addition of urea, and additional co ndensation under weakly acidic conditions. A resin, free from urons an d methylene bonds between tertiary nitrogen atoms, was obtained when t he condensation reaction under the conditions of pH.: 1.0-1.5 and form aldehyde/urea (F/U) molar ratio: 3.0 was conducted at a low temperatur e (65 degrees C) for a short time (45 min). A resin with a F/U molar r atio of 1.3 derived from this resin by the post-condensation addition of urea exhibited considerably high bonding strength but also high pos t-cure formaldehyde emission. In general, both formaldehyde emmision a nd bonding strength decreased with increases in the condensation tempe rature and time. Pre-methylolation under weakly alkaline conditions re sulted in a slower condensation reaction under strongly acidic conditi ons but slightly higher bonding strength. Additional condensation unde r weakly acidic conditions caused a slight increase in the bonding str ength.