Secondary Amine Curatives

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. 

Epichlorohydrin/diaminomethyl/cyclohexane derivative condensates act as effective epoxy adhesive curatives, even after such mild cure conditions as 20 min at 100°C. One example of such a curative is l,3-bis(4-aminomethylcyclohexanemethylamino)-2-propanol. Room temperature overlap shear strengths on steel in excess of 6,700 psi (475kg/cm ) were reported using DGEBA-type epoxy resins. 

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. 

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. 

Bis- or polyguanides have been described as DGEBA-resin curatives, alone or as mixtures with 10. The guanides form lower melting temperature eutectic mixtures with 10, thus easing homogeneous mixing. 

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Table VII. Secondary Amine Curatives for Heat Cure Epoxies...

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. 

Neoprene Type TW was shown to have low oral toxicity in rats. The LD q was found to be in excess of 20,000 mg/kg. Human patch tests with Types GN, W, WRT, and WHV showed no skin reactions (169). The FDA status of Du Pont Neoprene polymers is described (172). Although polychloroprene itself has not been shown to have potential health problems, it should be understood that many mbber chemicals that may be used with CR can be dangerous if not handled properly. This is particularly tme of ethylenethiourea curatives and, perhaps, secondary amine precursors often contained in sulfur modified polychloroprene types. Material safety data sheets should be consulted for specific information on products to be handled. 

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. 

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. 

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. 

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). 

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. 

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 ... 

The reactivity of epoxy groups toward both nucleophilic and electrophilic species can most easily be explained by the release of ring strain inherent in the three-membered oxirane group. When nucleophilic curatives, such as amines or mercaptans, are used, attack occurs at the secondary ring carbon atom of the epoxy group (Eq. 1). Electrophilic curatives, such as... 

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