Interest in aromatic polyamide as hardeners is largely confined to diprimary amines. Most of these are solids, and two that have assumed importance are meta-phenilenediamine and 4,4-diaminodiphenylmethane (methylene dianiline). The later is nowadays an important hardener for adhesives, but it is interesting to note that when originally reported it was described as giving such low bond strength that is could hardly be considered for adhesive use. The reason may have been that the aromatic amines in general give rather brittle cured resins which, if tested by a method that involves much bonding, can appear to have low adhesive strength. The sulfur homologue of this hardener, 4,4-diaminodiphenyl sulfone, has more recently also become fairly widely used, it has an advantage in being of low toxidity-indeed it is administered orally in the treatment of leprosy.
The aromatic polyamine hardeners are of much higher molecular weight, relative to their functionally, than the aliphatic ones and therefore the quantity added to the resin is quite large, sometimes one half the weight, (some of the aromatic amine hardeners are also much more expensive that the resin).
Generally speaking, the aromatic polyamine hardeners, unlike most aliphatic ones, are not very hygroscopic. Under normal circumstances they require heat for curing, preferably a temperature considerably above 100 oC. However, some aromatic diamines. In particular 4,4-diaminodiphenylmethane can be cured at a much lower temperature- even at room temperature –by dibutyl phthalate. This addition reduces hot strength, but other desirable properties of the aromatic amine adhesive systems are largely retained. A small amount of a monocarboxylic acid such as acetic acid will also accelerate the curing of aromatic polyamine hardeners.
Room temperature curing is claimed as one of the advantages of certain hydrogenated aromatic amines. Some of these amine (presumably not those capable of curing at room temperature) are said to produce B stage resins, and some, for example, hydrogenated meta-phenylenediamine and 4,4-diaminodiphenilmethane, with a fairly high proportion of their structure converted to cycloaliphatic, have the great advantage of being liquids.