Statistical evaluation of methylol‐free melamine‐formaldehyde oligomer distributions: Empirical formulae from elemental analysis

Abstract

AbstractThe problem of measuring quantitatively and reliably the proportions of methylene‐ and ether bridges in melamine‐formaldehyde resins has resisted chemical and spectroscopic approaches. Such measurements are specially required for refining statistical models of the overall crosslinking reaction. Pregel samples contain oligomer mixtures which can be freed from pendent methylols by treatment with ammonia. They obey the empirical formula (C3N6H6)1(CH2)R(H2O)z, where R and z can be found by optimising over measurements of %C, %N and %H. The accepted structures lead to prediction of z = −0.5R if only ether bridges are present or z= −R if only methylene bridges are present. In principle, intermediate values of −0.5R>z>‐R could allow the proportions of the two kinds of bridges to be determined. However, over a wide range of ratios R of formaldehyde to melamine, results are found to obey z = −0.2R, indicating retention of free water in the samples. Prolonged vacuum drying at room temperature may overcome the problem.