Epoxy adhesives amine curatives

Whereas most room temperature curing epoxy adhesives are cured with aliphatic amines, polyamides, or amidoamines, most elevated-temperature curing epoxy adhesives are cured with aromatic amines, modified aliphatic amines, alcoholic and phenolic hydroxyls, acid anhydrides, Lewis acids, and a host of other curatives. Latent curing agents, such as dicyan-diamide and imidazoles, are typically used in one-component epoxy adhesives systems. 

These polyamidoamines are available from several suppliers worldwide (Cognis (Veramide ), Arizona Chemical (Uni-Rez ), Air Products (Ancamide ), and others) and are among the most common curatives in the general-purpose, do-it-yourself two-pack-age epoxy adhesives. They have a distinctive odor somewhat like popcorn and are easily recognized in adhesive formulations. The polyamide backbone does contribute to the overall good mechanical properties of the polyamide amine cured adhesives. 

A variety of aromatic polyfunctional amines is also used in curing epoxy adhesives. They generally are slower to react than the aliphatic amines and require a heat cure to be practical. They do provide generally better high-temperature properties than the aliphatic amines. The most commonly used aromatic amine curatives are m-phenylenediamine (MPDA, DuPont), methylenedianiline (MDA, Bayer), and diaminodiphenyl sulfone (DDS, Aceto). Albemarle Corporation s Ethacure 100 is claimed to be a more user-friendly... 

It is possible to make one-package epoxy adhesives that can have very long shelf lives at room temperature but cure rapidly when heated. One amine curative widely used to make single-package heat-cured epoxy adhesives is dicyandiamide (cyanoguanadine), commonly known as dicy. 

Uses Corrosion inhibitor raw material when reacted with amines curative for powd. coatings used in adhesives, syn. lubricants, fibers, plasticizers, polyester coatings, epoxy and polyamide resins... 

Aromatic amines react slowly with epoxies due to reduced basicity hence activators such as phenols and catalysts such as magnesium nitrate are added to the amine curative portion of the adhesive. Such cured adhesives have improved solvent resistance. 

Amine curatives useful in imparting chemical resistance to epoxy adhesives have been described in a recent patent issued to Takeda Chemical Industries, Ltd. These curatives are aldimized or ketimized derivatives of l,3,5-tris(aminomethyl)benezene or cyclohexane, prepared by condensation of the polyamine with an appropriate aldehyde or ketone (Eq. 10) and are... 

Primary amine curatives are most commonly used for room temperature curing of epoxy adhesives. However, aromatic primary amines can be used as latent catalysts for one-part heat curing products (Table VI). Mixtures of bis(aminopropyl)tetraoxaspiroundecane and m-phenylenediamine or bis(aminophenyl)ether, amine, sulfoxide, or thioether have been used to cure DGEBA-type resins at two temperatures (40-100°C and 120-160°C) to give cured specimens with good fracture toughness. Aromatic primary diamines can offer improved adhesion of epoxy adhesives to metals and... 

Secondary amines can successfully be used as heat cure catalysts if sufficient steric stabilization and/or chemical deactivation of the amine functional group occurs to promote room temperature stability. Several examples are shown in Table VII. Crystalline bis(N-cyclohexyl-3-aminopropyl) amine tetrahydrate has been used in epoxy adhesive and coating formulations as a stable curative. Interestingly, the water of hydration was found to act both as a cure accelerator and a flow stabilizer. 

One of the most commonly used amine curatives in one-part heat cure paste and film epoxy adhesives is dicyandiamide, 10, prepared by dimerization of cyanamide under relatively mild conditions (Eq. 20). Its utility is... 

Another class of widely used amine curatives for heat curing epoxy adhesives are the imidazoles. Imidazole (33), like 10, is a crystalline material which only shows appreciable solubility in common epoxy resins at elevated temperatures. There are many derivatives of 33 available, but improvements in room temperature latency and speed of cure, in addition to final adhesive properties, are always desired. 

Various poly amines also are useful as curatives for epoxy adhesives. An example of a simple polyamine is diethylenetriamine (DETA) ... 

Suitable curatives for the polysulfide-epoxy reaction include liquid aliphatic amines, liquid aliphatic amine adducts, solid amine adducts, liquid cycloaliphatic amines, liquid amide-amines, liquid aromatic amines, polyamides, and tertiary amines. Primary and secondary amines are preferred for thermal stability and low-temperature performance. Not all amines are completely compatible with polysulfide resins. The incompatible amines may require a three-part adhesive system. The liquid polysulfides are generally added to the liquid epoxy resin component because of possible compatibility problems. Optimum elevated-temperature performance is obtained with either an elevated-temperature cure or a postcure. 

Adhesives which are meant to cure at temperatures of 120 or 171°C require curatives which are latent at room temperature, but react quickly at the cure temperatures. Dicyanodiamide [461-58-5], (TH INI is one such latent curative for epoxy resins. It is insoluble in the epoxy at room temperature but rapidly solubilizes at elevated temperatures. Other latent curatives for 171°C are complexes of imidazoles with transition metals, complexes of Lewis acids (eg, boron trifluoride and amines), and diaminodiphenylsulfone, which is also used as a curing agent in high performance composites. For materials which cure at lower temperatures (120°C), these curing agents can be made more soluble by alkylation of dicyanodiamide. Other materials providing latency at room temperature but rapid cure at 120°C are the blocked isocyanates, such as the reaction products of toluene diisocyanate and amines. At 120°C the blocked isocyanate decomposes to regenerate the isocyanate and liberate an amine which can initiate polymerization of the epoxy resin. Materials such as Monuron can also be used to accelerate the cure of dicyanodiamide so that it takes place at 120°C. 

More recent efforts have focused on developments that create true hybrids. For example, blocked isocyanate prepolymers have been mixed with epoxy resins and cured with amines [68-70]. These blocked prepolymers will react initially with the amines to form amine-terminated prepolymers that cross-link the epoxy resin. Several blocked isocyanates are commercially available. The DESMOCAP (Bayer) llA and 12A products are isocyanates (believed to be blocked with nonylphenol) used as flexibilizing agents for epoxy resins. ANCAREZ (trademark. Pacific Anchor, Inc.) 2150 is a blocked isocyanate epoxy blend used as an adhesion promoter for vinyl plastisols. A one-package, heat-cured hybrid adhesive was reported consisting of isophorone diisocyanate, epoxy resin, and a dispersed solid curative based on the salt of ethylenediamine and bisphenol A [71]. Urethane amines are offered commercially that can be used with epoxy resins to develop hybrid adhesive systems [72]. 

Chem. Descrip. Mercaptan polymer precatalyzed, accelerated Uses Epoxy hardener, accelerator in concrete adhesives, grouts, mortars, and coatings co-curative improves bond to glass and metal Features Fast cure eliminates induction times of polyamides and amido-amines... 

Epoxide adhesives comprise epoxy resin, many of which are prepared from phenols and epichlorohydrin, for example, the diglycidyl ether of bis-phenol A or bis-phenol F usually, these resins are a mixtnre of molecular weights blended to fit the applications. The most-common cnratives for epoxy resins are polyanfines (used in stoichiometric amounts), usually a chain-extended primary aliphatic amine, for example, diethylene triamine or triethylene tetraamine or chain-extended equivalents, which react rapidly with the epoxy resin at room temperature. Aromatic amines react slowly at room temperature but rapidly at higher temperatures. Most epoxide adhesives also contain catalysts, typically, tertiary amines. Dicyanimide is the most-common curative for one-component high-temperature-cured epoxide adhesives. Mercaptans or anhydrides are used as curatives for epoxide adhesives for specialist applications, for example, for high-speed room-temperature cures or for electronic applications. A smaller number of epoxide adhesive are cured by cationic polymerization catalysed by Lewis acids photogenerated at the point of application. Lewis acid photoinitiators include diaryliodonium and triarly sulphonium salts. See Radiation-cured adhesives. 

Lewis and Bronsted acids, activate the epoxy ring toward ring opening by various nucleophilic species, most often hydroxyl or other epoxy groups (Eqs. 2-4). Both types of curatives can take the form of catalytic species, such as tertiary amines and Lewis acids, or coreactants, such as primary amines, mercaptans, and dicarboxylic acids. When the curatives are catalytic species, the properties of the cured adhesive are due primarily to the epoxy resin and the stability/activity of the catalyst in the resin under cure conditions. Coreactant curatives offer much greater latitude in choosing the final cured adhesive properties because the physical characteristics of the... 

Adhesives for use at 350°F to 450°F contain polyfunctional epoxies and usually a latent curative with an aromatic diamine structure such as methylene dianiline, mcta-phenylenediamine, or diaminodiphenylsulfone. Heat distortion temperatures of up to 325°F can be obtained with these aromatic amine hardeners. Toughness has now been incorporated into these high-temperature adhesives (Table I). 

Curatives have been prepared via condensation of (meth)acrylonitrile with various hydrazides such as carbohydrazide, oxalyl dihydrazide, and succinyl dihydrazide (Eq. 16). When these cyanoethylated and cyanopropylated hydrazides are mixed with DGEBA-type resins, viscosity increases are noted in 4-5 days at room temperature. However, these partially reacted mixtures are still uncrosslinked and flowable at temperatures near their final cure temperature of around 120°C. At this temperature reaction occurs very rapidly for a variety of epoxide-to-amine ratios. Adhesive compositions of DGEBA-type epoxy resins and the cyanoalkylated hydrazides give extremely strong bonds on substrates such as sheet moulding compound (SMC) and steel. 

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