Curing aliphatic amines

Polystyrene insulation on magnet wire 0.29 Encapsulated with B-staged aromatic amine cured aliphatic amine cured bisphenol A-epichlorohydrin epoxide (epoxy transfer moulded). Impregnate. 

In adhesive formulations, aliphatic amines are most commonly used to cure the DGEBA type of epoxy resin. Aliphatic amines are not widely used with the non-glycidyl ether resins, since the amine-epoxy reaction is slow at low temperatures. The reaction usually requires heat and accelerators for an acceptable rate of cure. Aliphatic amines are primarily used with lower-viscosity DGEBA resins because of the difficulty in mixing such low-viscosity curing agents with the more viscous epoxy resins. 

Table 11.8 presents a typical triethylenetetramine cured epoxy adhesive formulated with selected fillers. In this formulation the use of aluminum powder and alumina increases substantially the resistance of the adhesive to boiling water.7 This is also true when DETA is used as the curing agent.8 A typical room temperature cured aliphatic amine cured epoxy adhesive for general-purpose use is shown in Table 11.9. This shows the difference that is achieved in shear strength by curing at elevated temperatures versus room temperature.

Several types of hardeners are used in the curing Aliphatic amines Amidoamines Aromatic amines Cycloaliphatic amines Imidazoles Polyamides Anhydrides Polysulfides Mercaptans Dicyandiamide (DiCy)... 

Polyfunctional aliphatic resins have exhibited high reactivity and degrees of cure with amines but problems of toxicity have diminished their usehilness and commercial interest. SoHd epoxy resins can be prepared by the taffy process or the advancement process. 

It is thus seen that as a class the primarily aliphatic amines provide fast-curing hardeners for use at room temperatures. With certain exceptions they are skin sensitisers. The chemical resistance of the hardened resins varies according to the hardener used but in the case of the unmodified amines is quite good. The hardened resins have quite low heat distortion temperatures and except with diethylenetriamine seldom exceed 100°C. The number of variations in the properties obtainable may be increased by using blends of hardeners. 

In addition to the considerable difference of the properties of the cured resins with different hardeners it must also be stressed that the time and temperatures of cure will also have an important effect on properties. As a very general rule, with increasing aliphatic amines and their adducts the time of cure and temperature of cure (up to 120°C at least) will improve most properties . ... 

Aliphatic aldehydes, 13 571 physical properties of, 2 60t syntheses of, 12 187 Aliphatic a-hydroxy acids, 14 130 Aliphatic amine/polysulfide co-curing agent systems, 10 410... 

The best performing coatings were the vinyl ester, the bisphenol A epoxy cured with an aliphatic amine, and a novolac epoxy cured with a mixed aromatic/cycl oal i phati c amine. The saturated polyester, and a bisphenol A epoxy cured with a polyamide amine showed significant deterioration of the coating material in the acid, and corrosion of the underlying steel. Two types of novolac epoxies cured with aromatic amines showed intermediate performance. 

Curing agents account for much of the potential hazard associated with use of epoxy resins. There are several major types of curing agents aliphatic amines, aromatic amines, cycloaliphatic amines, acid anhydrides, polyamides, and catalytic curing agents. The latter two types are true catalysts, in that they do not participate in the curing process. 

Asa rule of thumb, epoxies cured with aliphatic amines, cause a majority of explosives and propellants to be excessively reactive. Epoxies cured... 

A tertiary amino group formed in curing with aliphatic amines can sometimes catalyze the epoxy group polymerization. When aromatic amines are used as curing agents, such reactions do not take place at all. 

The hisphenol A-derived epoxy resins are most frequently cured with anhydrides-, aliphatic amines, or polyamides. 

The bisphenol A-derived epoxy resins are most frequently cured with anhydrides, aliphatic amines, or polyamides, depending on desired end properties. Some of the outstanding properties are superior electrical properties, chemical resistance, heat resistance, and adhesion. Conventional epoxy resins range from low viscosity liquids to solid resins. 

Isocyanates and amines react together to form ureas. Primary aliphatic amines react very quickly at temperatures down to ambient, whereas secondary aliphatic and primary aromatic amines react less quickly. The reaction rate of secondary aromatic amines is the slowest. The speed of the reaction can further be modified by the addition of substitutes near the amine group. The control of the speed can either be electronic, as illustrated by the effect of the chlorine in the MOCA ring, or by stereo chemical influences where the groups next to the amine group have a very strong hindrance to the curing. This is... 

The mechanisms of radiation damage and effects of hardeners were studied recently by pulse radiolysis [89], The epoxy resins require a relatively large amount of curing agents (hardeners), most of them are aromatic and aliphatic amines such as diamino diphenyl methane or triethylene tetramine. On the basis of the emission spectra and kinetic behavior of excited states observed, the radiation resistance of aromatic and aliphatic amine curing epoxy resin was explained by internal radiation protection effects due to energy transfer. 

Aliphatic amines Convenience Room temperature cure, fast elevated-temperature cure Low viscosity Low formulation cost Moderate chemical resistance Critical mix ratios Strong skin irritant High vapor pressure Short working life, exothermic Poor bond strength above 80°C Rigid, poor peel and impact properties Adhesives and sealants Casting and encapsulation Coatings... 

Primary and secondary aliphatic amines react relatively rapidly with epoxy groups at room or lower temperature to form three-dimensional crosslinked structures. The resulting cured epoxies have relatively high moisture resistance and good chemical resistance, particularly to solvents. They also have moderate heat resistance with a heat distortion temperature in the range of 70 to 110°C. Thus, short-term exposures of cured adhesive joints at temperatures up to 100°C can generally be tolerated. 

These aliphatic amines can also be cured at elevated temperatures to provide a more densely crosslinked structure with better mechanical properties, elevated-temperature performance, and chemical resistance. Table 5.3 illustrates the effect of curing temperature on the bond strength of DGEB A epoxy with two different aliphatic amines. 

Other amines, such as aromatic or cycloaliphatic, are less reactive and generally require elevated-temperature cures that result in higher heat distortion temperatures (140 to 150°C). However, aromatic amine adducts of liquid epoxies can be accelerated to cure at room temperature. Aliphatic amines can also be accelerated. 

The fast reaction rate of aliphatic amines at room temperature can, however, lead to various problems. Pot life is short, which can lead to an unacceptable working life. The cured resin is relatively brittle due to its high crosslink density. High exotherms in thick sections or large masses can lead to thermal decomposition of the resin. 

In the well-recognized epoxy-aliphatic amine reaction, the primary or secondary amine adds to the epoxy ring, forming a tertiary amine, as shown in Fig. 5.2 (top). The formed hydroxyl groups accelerate the amine curing, and with excess epoxy present, the secondary hydroxyl groups can also add to the epoxy ring, as shown in Fig. 5.2 (bottom). 

There are several reasons why unmodified aliphatic amines, such as those described above, are less widely used than other curing agents in epoxy adhesive systems. These include... 

Two curing agents that have found their way into many epoxy adhesive formulations are the polyamides and amidoamines. These are commonly used in the hardware store variety two-part epoxy resins that cure at room temperature. Both are reaction products of aliphatic amines, such as diethylenetriamine, and should be included under the subclassification of modified amines. However, these products have such widespread and popular use, they are addressed here as a separate classification. 

Polyamide curing agents are the reaction products of dimerized fatty acids and aliphatic amines such as diethylenetriamine. This introduces a bulky, oil-compatible, C36 carbon group between the amine sites. Similar to the diglycidyl ether adducts of aliphatic amine, they are manufactured by adding the fatty acid to an excess of amine. They are available in a range of viscosities that can be achieved by varying the amine/acid molar ratio in the reaction. 

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