III. GENERAL PRINCIPLES OF MANUFACTURE AND APPLICATION
It is very important in the commercial production of UF resins to be able to control the size of the molecules by the condensation reaction, since their properties change continuously as they grow larger. The most perceptible change is the increase in viscosity. Low-viscosity
syrups are formed first. These change into high-viscosity syrups, which are clear to turbid. Molecular weight may vary from a few hundred to a few thousand, with a wide range of molecular size. These molecules are built up by water splitting off at random between reactive groups of neighboring molecules, thereby increasing their size. Once their solubi- lity, viscosity, pH, concentration, and so on, have been determined, they constitute the resins available commercially. The most important factors influencing the final properties of aminoplastic resins in industrial manufacture are the purity of the reagents, the molar proportions of the materials used, the preparation process used, and the pH variation and control.