Primary Amine Curatives

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 

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

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. 

BMI/amine Michael adduct resins may be further modified and blended with other thermosets or reactive diluents to achieve either specific end-use properties or processability. Epoxy resins are very suitable for the modification of BMI/primary amine adducts, because the secondary amine functionality in the aspartimide structure is a curative for the epoxy group. 

Carboxylic acid derivatives can be converted into primary amines with loss of one carbon atom by both the Hofmann rearrangement Eind the Cur tius rearrai ement- Although the Hoimann rearrangement involves a primary amide and the Curtius reanui ement involves an acyl azide, both proceed through similar mechanisms. 

The reactivity of epoxy groups towards nucleophilic and electrophilic species can be explained through the release of ring strain in the three member oxirane group. Nucleophilic curatives such as amines or mercaptans attack the secondary ring carbon while electrophilic curatives behave as Lewis or Bronsted acids. The epoxy ring can be opened by hydroxyl or other epoxy group aided by tertiary amines, Lewis acids or coreactants such as primary amines, mercaptans and dicarboxylic acids ... 

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. 

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... 

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. 

Multifunctional primary and secondary aliphatic polyamines are used as coreactants. They add readily to the epoxy group at low temperatures to produce highly cross-linked networks. The aromatic amines are somewhat slower than the aliphatic amines, but provide higher heat stability. Examples of amine curatives include diethylenetriamine, triethyl-enetetramine, N-aminoethylpiperazine, and m-phenylenediamine. 

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. 

The 100% stoichiometric addition of a primary or secondary amine to cure an epoxy can be relatively easily calculated the amount of resin containing one epoxy group will react with the amount of hardener, which contains one active hydrogen. However, dicyandiamide is, at least in part, a catalytic curative one molecule is said to contain about 4.5 active hydrogen equivalents. 

The largest group of epoxy curatives contains the aliphatic, cycloaliphatic and aromatic primary and secondary amines. Depending on the amine chosen, cure temperatures can vary from just below ambient (generally aliphatic and cycloaliphatic) to as high as 120 to 175 °C (aromatic). 

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. 

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