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Urea Formaldehyde Adhesive

Thermosetting Resin Adhesive Article Contents: Thermosetting Resin Adhesive Melamine Formaldehyde Adhesives Urea Formaldehyde Adhesiv...

Tuesday, April 8, 2008

Incorporation of the Hardener in the Resin

Pre-catalyst urea glue has been used to describe glues which are mixture of powder urea formaldehyde hardener and a powder resin. Such a mixture can have an adequate storage-like provide the moisture content is low. Among hardening agents claimed to give satisfactory storage-life are ammonium salts of non-volatile acids, and aluminum sulfate. Although these glues lack the flexibility of usage which separate hardener bestow, they have the advantage that they can be made ready for use simply by adding water. When they have been dissolved in water, the pre-catalyst urea glues have the same pot-life as if the hardener were added by the user, there is no latent hardening effect.

The idea of providing a latent hardening effect has always appeared attractive, (by latent hardener is meant one that is without effect until the mixture is heated). A satisfactory latent hardener would constitute an important development because it would remove the difficulties associated with a limited pot-life. The problem is, however a difficult one solve with a U.F. glue, but easier with a U.F. molding powder. In a Japanese study it has been reported that "pyridine-mono acetic chloride" (presumably the pyridine salt of mono chloroacetic acid), and also a mixture of pyridine and di-ammonium imidosulfamate, prolong the pot-life and cause rapid curing at high temperature.

This implies a high ratio of pot-life to curing time –perhaps above the average- but not necessarily truly latent hardening. A somewhat cumbersome but probably effective approach is that of coating the hardener particles with a wax that melts just before the curing temperature is reached. The complete coating of solid particles of a hardener is however not easy, a recently published method describes the use of a solution of an acid amide which crystallizes on to the hardener particles.