Abstract
The reactions of di-hydroxymethylurea with phenol under alkaline (pH = 10), weak (pH = 6) and strong acidic (pH = 2) conditions were investigated via the 13C-NMR method. Based on the proposed reaction mechanisms, the variations of the structures of different condensed products were analyzed and the competitive relationship between self- and co-condensation reactions was elucidated. The required experimental conditions for co-condensations were clearly pointed out. The main conclusions include: (1) the self-condensation between urea formaldehyde (UF) or phenol formaldehyde (PF) monomers were dominant while the co-condensations were very limited under alkaline conditions. This is because the intermediates produced from urea, methylolurea and phenol are less reactive in co-condensations with respect to self-condensations; (2) under weak acidic conditions, the self-condensations occurred exclusively among the UF monomers. The co-condensation structures were not observed; and (3) the co-condensations became much more competitive under strong acidic conditions as the relative content of the co-condensed methylenic carbon accounts for 53.3%. This result can be rationalized by the high reactivity of the methylolphenol carbocation intermediate toward urea and methylolurea. The revealed reaction selectivity and mechanisms may also be applied to the synthesis of those more complex co-condensed adhesives based on natural phenolic and amino compounds.
Highlights
Urea-formaldehyde resin (UF) is one of the important wood adhesives, and it is currently the most commonly used amino resin
It is very likely that what we found in this study, the revealed reaction selectivity and mechanisms, can be applied to the synthesis of these co-condensed adhesives based on natural phenolic and amino compounds
Because thermosetting phenol formaldehyde resin is generally synthesized under alkaline conditions, the reaction medium is advantageous to the self-condensation of Phenol-formaldehyde resin (PF)
Summary
Urea-formaldehyde resin (UF) is one of the important wood adhesives, and it is currently the most commonly used amino resin. Bunichiro Tomita’s research team described the structure of PUF co-condensation resin [8,9,10] In their studies, the structure of PUF was analyzed by the 13 C-NMR method. Analyzing the structure of tannin resin with 13 C-NMR is much more difficult, due to the complex structure of tannin compounds, as well as the impurity of tannin extraction It is very likely that what we found in this study, the revealed reaction selectivity and mechanisms, can be applied to the synthesis of these co-condensed adhesives based on natural phenolic and amino compounds. The required reaction conditions for the co-condensations were clarified
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