C-13 CP/MAS NMR-STUDY OF THE STRUCTURAL DEPENDENCE OF UREA FORMALDEHYDE RESINS ON FORMALDEHYDE-TO-UREA MOLAR RATIOS AT DIFFERENT UREA CONCENTRATIONS AND PH VALUES

Urea-formaldehyde (UF) resins prepared at room temperature from reaction mixtures with four different formaldehyde-to-urea molar ratios (1.00, 1.50, 2.00, and 3.00), three different urea concentrations, and six pH values (1, 3, 5, 7, 9, and 12) were investigated by C-13 CP/MAS NMR spectroscopy of 50.3 MHz. The quantitation aspect of obtaining C-13 CP/MAS NMR spectra at spinning rates of about 6.5 kHz was examined. The resins prepared under acidic conditions range from a very simple structure to complicated ones, the details depending on the urea concentration, pH value, and formaldehyde-to-urea molar ratio, whereas samples prepared under neutral or basic conditions mainly consist of methylolureas and/or structures with dimethylene ether connections between two urea units. The structural variations that occur in UF resins as the formaldehyde-to-urea molar ratio, concentration, and pH value are varied are discussed in terms of structural trends in these systematically synthesized UF resin systems.