Adipic acid dihydrazide (ADH) is an effective crosslinking agent, curative and hardener. It is the most common dihydrazide crosslinking agent within a series of dihydrazides such as sebacic dihydrazide (SDH) and isophthalic dihydrazide (IDH). ADH’s has a melting point of 180 °C and a molecular weight of 174; both are lower than the alternative dihydrazides SDH and IDH.
Adipic dihydrazide is used as a difunctional crosslinking agent in paints and coatings for certain water-based acrylic emulsions. It is used as a hardener for epoxy resins and a chain extender for polyurethanes. A small use is as a formaldehyde scavenger preventing the liberation of formaldehyde.
The chemical structure of Adipic acid dihydrazide (ADH) and the reactive sites are shown below.
The applications of ADH are facilitated by the nucleophilicity of the amine function (good reaction characteristics), the good overall properties and weatherability of cured systems. The moderate solubility of ADH in water (50 g./liter) and common organic solvents facilitates the use of ADH in aqueous and solvent based systems. Important application areas are described below.
The first thing to note with ADH in epoxy formulations is that each of the primary amine end groups has a functionality of two, so the ADH molecule has an equivalency of four per epoxy moiety. Accordingly, the active hydrogen equivalent weight of ADH is 43.5. When formulated with epoxy resins, the ADH index can range between 0.85-1.15 of stoichiometric proportions, without a significant effect on mechanical properties.
The cure temperature for epoxy resins (glycidyl types) formulated with ADH is influenced by the melt-out temperature of the ADH, which allows an extended pot life at low temperatures. Storage stability can be up to six-months at room temperature, with cure times of about one-hour at 130 °C. Cure rates can be accelerated using tin or titanate catalysts, or imidazoles.
One-component ADH epoxy systems can be partially cured or “B-staged”, and later fully cured. B-staging provides handling, processing, and fabrication advantages. One component epoxy resins are used in coatings such as powder coatings, adhesives including hot melt adhesives, molding compounds and in fiber reinforced composites. Glass and carbon fiber prepreg obtained by a hot melt impregnation method are used in the fabrication of sporting goods, wind turbine blades and aircraft/aerospace components. With ADH cure, epoxy resins exhibit excellent toughness, flexibility, and adhesive properties. Tg’s of 140-160 °C are achievable using a standard liquid bisphenol A epoxy resin (DGEBA) with ADH as the hardener.
Rigid and flexible epoxy adhesives have been formulated as one component systems that can be stored at room temperature using ADH as a latent curing agent. Rigid epoxy adhesives are based on bisphenol A and novolac epoxides. These rigid adhesives exhibit excellent cohesive and adhesive properties to a wide variety of surfaces. Flexible epoxy adhesives produce more pliable bonds which better accommodate bond line stresses or differential substrate expansion rates. Flexible epoxy resins include aliphatic di- and tri-epoxy resins such as hexanediol diglycidyl ether and poly(oxypropylene) diglycidyl ethers. Semi-rigid epoxy-based adhesives utilize mixtures of both classes of epoxy resins or rigid formulations using flexibilizers.
Polyurethane Dispersions (PUDs)
ADH is an effective room temperature curative for aqueous PUDs and solution polyurethanes. In this capacity, ADH provides polyurea coatings with higher hardness, toughness and adhesion properties, excellent mechanical properties, abrasion and chemical resistance. ADH cured polyurethane coatings exhibit good color stability and weathering properties, which is not observed with standard amine curatives.
ADH is added to the water phase in a PUD. Crosslinking occurs during the drying and film coalescence process which is ideal for maximizing the film properties including gloss, scrub, stain and wear resistance and durability. Other crosslinking methods where crosslinking occurs prior to film coalescence exhibit reduced performance properties including poor flow and leveling. The full reactivity characteristics of ADH are ideal for PUR systems. Alternative curatives which show incomplete crosslinking due to slow reactivity and the lack of curative mobility in a dry film will also compromise performance.
The leading crosslinking technology for acrylic emulsion polymers is ambient temperature crosslinking chemistry based on diacetone acrylamide (DAAM) or acetoacetoxyethyl methacrylate (AAEM) and adipic acid dihydrazide (ADH) monomers. This technology, known as “keto-hydrazide crosslinking,” involves the direct reaction of the pendant ketone moiety on the DAAM-acrylic or AAEM acrylic polymer segment and the hydrazide moiety of the ADH, with the evaporation of water in the film-forming process.
This self-crosslinking technology has been adopted in high-durability paints and coatings for architecture, wood, and concrete surfaces, and more. The DAAM/ADH pair is also used in crosslinkable sizing agents, thickeners, adhesives, and sealants.
The DAAM/ADH and AAEM/ADH crosslinking system provides several key benefits for formulators and consumers, from reducing safety concerns to improving performance properties and much more. These benefits include:
- Diacetone acrylamide and acetoacetoxyethyl methacrylate uniformly copolymerize within acrylic copolymers, creating well-dispersed pendant ketone crosslinking sites.
- Wet acrylic emulsions based on DAAM or AAEM with ADH in the aqueous phase are initially non-reactive and afford emulsions with good long-term stability during shipping and storage in retail containers (also known as "in-can" stability).
- After film coalescence, crosslinking becomes rapid at ambient temperatures, thanks to water evaporation in the drying process and a reduction in pH from the loss of ammonia.
- Because crosslinking is post-coalescence, the resulting three-dimensional polymer network exhibits enhanced mechanical strength and durability as well as maximum film cohesive properties.
- Crosslinking with the keto-hydrazide chemistry enhances abrasion, scrub, stain, and blocking resistance; moisture and solvent resistance; and substrate adhesion.
With other crosslinking chemistries, premature crosslinking occurs within the emulsion particles prior to coalescence, thus retarding intermolecular diffusion between emulsion particles and resulting in weaker film products and coatings. This dynamic occurs especially when diacrylates are used in the copolymerization recipe to produce crosslinks.
Both intermediates are formaldehyde free, unlike melamine crosslinking chemistries. DAAM and ADH are easily dissolved in warm water and in many other monomers. Finally, the by-product of the crosslinking reaction is water.
Gantrade’s sales specification for ADH are given below.
|Appearance||White Crystalline Solid|
|Purity (HPLC)||≥ 99%|
177 - 184 oC
≤ 0.5 %
Adipic dihydrazide is a unique crosslinking agent and curative, offering controlled reactivity and performance enhancements in epoxy resins, polyurethane dispersions (PUDs), solvent based PURs and emulsion acrylic resins. The major applications for ADH are a latent curing agent for B-stageable epoxy resins and an ambient temperature crosslinking agent for high performance acrylic emulsion architectural coatings. Systems crosslinked or cured with ADH exhibit good color stability and weathering characteristics, adhesion, durability, hardness, and toughness.
If you are ready to leverage ADH in your technology, Gantrade maintains good, high-quality inventories of this intermediates. In addition, we readily provide product information, technical support, and product sales information, so you can make an informed decision. Contact Gantrade to speak with one of our team members to learn more about the potential of ADH in your applications, or download a TDS to learn more about adipic acid dihydrazide solubility.