Amine curatives

Only a few commercial uses for TDA per se have been found. In epoxy curing appHcations, 2,4- I DA has been used as a component of a eutectic mixture with short chain aUphatic glycidal ether resins (46) as well as by itself (46,47) TDA (46) and single isomers (47) are also used as amine curatives. TDA can be used as a chain extender in polyurethanes (48,49). TDA is cited as a monomer in making aromatic polymers with unique properties, eg, amorphous polyamides (50), powdered polyamides (51), and low melting, whoUy aromatic polyamides (52). 

Amine Cross-Linking. Two commercially important, high performance elastomers which are not normally sulfur-cured are the fluoroelastomers (FKM) and the polyacrylates (ACM). Polyacrylates typically contain a small percent of a reactive monomer designed to react with amine curatives such as hexamethylene-diamine carbamate (Diak 1). Because the type and level of reactive monomer varies with ACM type, it is important to match the curative type to the particular ACM ia questioa. Sulfur and sulfur-beating materials can be used as cure retarders they also serve as age resistors (22). Fluoroelastomer cure systems typically utilize amines as the primary cross-linking agent and metal oxides as acid acceptors. 

Amine curatives tend to give too fast a cure rate when used with MDI-based prepolymers and give the best results when used with castable systems based on TDI. 

Chronic exposure refers to repeated or continuous exposure over long periods of time. Amine-based materials, such as amine curatives and catalysts, have a potential for problems to occur after a number of years. The highest-profile potential problem material is MOCA, which has been subject to numerous studies. There have been very few cases of fatalities due to exposure to MOCA. The initial casualties also had exposure to precursor materials that were known carcinogens. Further details are very scant. 

Amine-based curatives are a popular choice when working with TDI-terminated prepolymers. The two most used amine-based curatives are MOCA and Ethacure 300. Other amine curatives used include Versalink 740M, MCEDA, and Cyanacure. 

Amine curatives that are solid at room temperature need to be melted prior to use. The hot molten material must be handled with care, as it can cause severe burns and may be absorbed into the skin. 

Polyurethanes are made by extending chains of a prepolymer made from a macro diol and a diisocyanate. The prepolymer is further extended with a diol or an amine curative. The long chains form a solid which is relatively weak. When the part is given a longer heat treatment, the molecules align themselves and intermolecular bonds (hydrogen bonding) are formed. At this point the full mechanical properties are established and the material, if suitably formulated, has excellent mechanical and chemical properties. 

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. 

Like the testing variables just described, material variables can influence the fracture behavior of epoxy thermosets. Material variables discussed herein include the types of epoxy resins and amine curatives. 

Since the hydrophobic-hydrophilic balance of amine-cured resins was so sensitive to alkyl substituents on the hydantoin ring, it is not surprising that it was also sensitive to the hydrocarbon moieties of the amine curatives. The range of behavior depended on the resin substituents. For example, the already hydrophobic ethyl amyl substituted Resin Ilk showed moderate but significant increases in hydrophobicity when cured with cycloaliphatic, highly branched aliphatic, or formulated aromatic amines. See Table V. 

At the other extreme, as shown in Table VI, a resin mixture containing only dimethylhydantoin (DMH) rings was quite hydrophilic with all room temperature amine curatives except the formulated aromatic amine mixture based on methylene-dianiline. (See Table VII for identification of the amines in Table V and subsequent tables.) This resin mixture was obtained by hydroxypropylating a portion of the DMH, la. 

With all the amine curatives, the stoichiometry used was one amine hydrogen/epoxide e.g., TETA, having two primary and two secondary amines/molecule, would have six equivalents/mole. 

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. 

However, the inconvenience of the customer having to both meter mix and heat cure has limited their acceptance as adhesives. The amine curatives discussed can react both by addition of an N—H to an oxirane group and by causing homopolymerization of remaining oxirane groups. 

Additives containing the phenol group accelerate many amine-epoxy reactions and phenol is often added to amine curatives such as the aromatic amines. Such mixtures sometimes have an objectionable pungent odor and are quite corrosive to the skin. Other accelerators include furfuryl alcohol and salts specific to each type of amine. 

One of the more useful benefits of the higher molecular weight amine curatives is their ability to act as tougheners, provided that the appropriate choice of backbone structure is made. Relatively low molecular weight... 

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... 

Table VL Primary Amine Curatives for Heat Cure Epoxies...

Secondary amine curatives can also be prepared in situ via reaction of primary amines with epoxy resins in an excess of epoxy resin. For instance. 

Combined tertiary/secondary amine curatives, prepared from ter-tiary/primary diamines and epoxy resins, have been used as accelerators for primary aromatic amine cures of DGEBA-type epoxy resins. For example, diethylaminopropylamine/DGEBA resin adducts have been used in m-phenylenediamine-cured epoxy resin systems to provide relatively low temperature cures (50-160°C). The cured resin systems have good toughness and heat distortion temperatures near 130°C. 

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. 

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