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Wednesday, April 16, 2008

Hardening of Melamine Adhesive and The Properties

The fundamental difference in the curing of melamine resins and urea resins is that, whereas the latter can be cured at room temperature, melamine resins required a minimum temperature of about 65 oC. A melamine resin can in fact be hardened (imperfectly) at a much lower temperature by lowering the pH sufficiently, but the cured product has poor cohesive and adhesive melamine resins since this requirement inevitably limits their usefulness as adhesives.

Melamine resins, being more heat-reactive than urea resins, will cure in a reasonable time at a high temperature, for example, 120 – 130oC without addition of a hardener, but for curing at say 100 oC hardeners are generally added. Acid or ammonium salt of strong acids are used. The properties of melamine adhesives cured at high temperatures without a hardener are not inferior to those obtained by curing at a lower temperature with a hardener.

The properties

In the field of adhesives the principle use of melamine resins is in upgrading urea resins as described before.

As glue by themselves they have a limited use in making plywood. The wet strength falls between that of plywood made with phenolic and with urea glues, although flying much nearer to the former. Melamine adhesives are, for example resistant to boiling water, but if the immersion period is prolonged the retention of strength of glued joints is inferior to that of joints made with phenolic adhesives. In spite of this, the opinion has been expressed in the USA that under severe conditions of exposure to heat and moisture, the durability of melamine adhesives is similar to that of moderately alkaline phenol and resorsinol glues, but result of durability test made in England tend not to confirm this.