Uron and uron-urea-formaldehyde resins

The favored pH ranges for the formation of urons in urea-formaldehyde (UF) resins preparation were determined, these being at pH's higher than 6 and l ower than 4 at which the equilibrium urons <-> N,N'-dimethylol ureas are sh ifted in favor of the cyclic uron species. Shifting the pH slowly during th e preparation from one favorable range to the other causes shift in the equ ilibrium and formation of a majority of methylol ureas species, whereas a r apid change in pH does not cause this to any great extent. UF resins in whi ch uron constituted as much as 60% of the resin were prepared and the proce dure to maximize the proportion of uron present at the end of the reaction is described. Uron was found to be present in these resins also as linked b y methylene bridges to urea and other urons and also as methylol urons, the reactivity of the methylol group of this latter having been shown to be mu ch lower than that of the same group in methylol ureas. Thermomechanical an alysis (TMA) tests and tests on wood particleboard prepared with uron resin s to which relatively small proportions of urea were added at the end of th e reaction were capable of gelling and yielding bonds of considerable stren gth. Equally, mixing a uron-rich resin with a low F/U molar ratio UF resin yielded resins of greater strength than a simple UF of corresponding molar ratio indicating that UF resins of lower formaldehyde emission with still a cceptable strength could be prepared with these resins. (C) 1999 John Wiley & Sons, Inc.