Dihydrazides - The Versatile Curing Agent for Solid Dispersion Systems

This paper will introduce dihydrazides, a relatively obscure class of curing agents useful in virtually all thermoset resin systems. First described in 1958, by Wear, et al. dihydrazides are useful curing agents for epoxy resins, chain extenders for polyurethanes, and excellent crosslinking agents for acrylics. In one-part epoxy systems, depending on the backbone chain of the chosen dihydrazide, Tg's over 150°C and pot life over 6 months as cure time of less than a minute can be seen. In acrylic emulsions, dihydrazides can be dissolved into the water phase and serve as an additional crosslinker, as the emulsion coalesces and cures. As a chain extender for poylurethanes, dihydrazides can be used to increase toughness and reduce yellowing. Introduction O O | | | | Dihydrazides are represented by the active group: H2NHNC(R) CNHNH2, where R is can be any polyvalent organic radical preferably derived from a carboxylic acid. Carboxylic acid esters are reacted with hydrazine hydrate in an alcohol solution using a catalyst and a water extraction step. The most common dihydrazides include adipic acid dihydrazide (ADH), derived from adipic acid, sebacic acid dihydrazide (SDH), valine dihydrazide (VDH), derived from the amino acid valine, and isophthalic dihydrazide (IDH). The aliphatic R group can be of any length. For example, when R group is just carbon, the resulting compound is carbodihydrazide (CDH), the fastest dihydrazide. Or R as long as C-18 has been reported as in icosanedioic acid dihydrazide (LDH). Some of the more common dihydrazides are represented in Figure 1. Although hydrazine is a suspected carcinogen, the safety of ADH has been well established (2) IDH is approved by the FDA for indirect food contact when used to cure epoxy resins under 21 CFR 175.300. Figure 1. (Dihydrazide Structures and Properties)