SYNTHETIC CONDITIONS AND CHEMICAL STRICTU RES OF UREA-FORMALDEHYDE RESINS .3. MOLECULAR-STRUCTURE OF RESIN SYNTHESIZED BY CONDENSATION UNDER STRONGLY ACIDIC CONDITIONS

Fractionation with ethanol was applied to investigate the molecular st ructures of ureaformaldehyde (UF) resins. A typical resin with a forma ldehyde/urea molar ratio of 1.3 synthesized by condensation under stro ngly acidic conditions and by the post-condensation addition of urea h ad a greater amount of ethanol-soluble fraction (66% in terms of urea residues) than that of a resin synthesized by condensation under weakl y acidic conditions (56% in terms of urea residures). The ethanol-solu ble fraction of the strong-acid catalized UF resin had a combined-form aldehyde/urea molar ratio smaller than 1.0, contained a greater amount of free urea, and had a smaller amount of methylol groups. On the oth er hand, the ethanol-insoluble fraction had a greater combined-formald ehyde/urea molar ratio, contained a smaller amount of methylol groups, and had an intra-molecular ring structure. These molecular structures suggest that the strong-acid catalized UF resin forms an insufficient ly and unevenly cross-linked polymer network and little free formaldeh yde is generated in the process of resin curing. These results provide an explanation for the little formaldehyde emission and low bonding s trength of the strong-acid catalized UF resin.