Water Toleration of Urea Formaldehyde Adhesive

The methylol urea compounds referred to earlier are truly, if only slightly water soluble, that is to say, the more solvent (water) that is added, the greater the weight of solute dissolved. On the other hand, the resinous liquids used as adhesives are not true solutions but colloidal dispersion, and although it is possible to produce dispersion that are infinitely compatible with water, the reaction products used as adhesives are not generally of this type.

The extreme case of a resin having infinite compatibility with water at all temperatures is an indication that the resin having infinite compatibility with a high ratio of formaldehyde (and probably smells strongly of it), or that it has been relatively under reacted.

Practically all U.F. adhesives having limited water-toleration at normal temperature can be diluted at such temperature with at least an equal volume of water before showing any sign of opalescence due to incipient precipitation. Water-toleration always increases with an increase in temperature but individual behavior varies widely; some resins precipitate readily at say 20 ^oC but are infinitely water-tolerant at 40 ^oC. The physical nature of the precipitate also varies; it may be fluffy and bulky, gummy or powdery.

"Water-solubility" is a term that cannot accurately be applied to resins that precipitate (even though low molecular-weigh fraction may remain in solution): as more water is added, less resin remains in solution. The resin may, however be considered as a solvent for water, because if more resin is added to a system containing a water-precipitated fraction, the precipitate re-dissolves and homogeneity restored, but with a consequent increase in resin concentration.