Saturday, September 27, 2008

Hot Water Effect to Resistance

The hot water (90oC) resistance of the same adhesive system has been shown to fall rapidly during the first month but to retain at least one-half of its original strength during six months. In a test of this sort, the adherent is important because of the effect of the water at the adhesive/metal interface, and consequently the surface treatment of the adherent is also important.

Melamine, like dicyandiamide a polymer of cyanamide, can be used as a curing agent rather on the lines of dicyandiamide; it has, however, a higher melting point and is less compatible and not soluble in any convenient solvent. Etherified melamine and urea formaldehyde resins, much used in epoxy coating, are not much interest adhesive compositions.

In N.America, more than in Europe, N,N-diallyl melamine is used as a hardener. Its method of use is similar to that of dicyandiamide: it can be mixed in powdered form with the resin, and it dissolves and reacts on heating.

Metal chelate compounds are of interest as curing agents partly because of the improved heat stability to be expected by including inorganic atoms in the structure of the cured resin. A part from this, chelates may also behave as catalyst at elevated temperatures. Triethanolamine borate (m.p. 2360C) is an important tertiary amine chelate that has latent catalytic properties. This compound becomes reactive through dissociation at a temperature much below its melting point, its threshold curing temperature being about 105oC. Other boron containing compound have also been suggested such as aluminum acetoacetic ester have been patented as latent hardeners. And another type of chelate with latent curing properties is a polyamine metallic salt co-ordination complex such as cadmium or zinc bromide diethylenetriamine; with these hardeners the threshold curing temperature is about 60oC

Other chelates that have recently been proposed as curing agents are certain boron difluoride complexes.

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