Metodología adicional para la tercera versión

Amines are important curing agents for epoxy resins. Amine compounds are classified into primary, secondary, and tertiary amines, depending on whether one, two or three hydrogen molecules of ammonia (NH3) have been substituted for hydrocarbon,

respectively[14].

Depending on their number in one molecule, amines are called monoamine, diamaine, triamine, or polyamine. Amines are also classified into aliphatic, alicyclic and aromatic amines according to the types of hydrocarbons involved. These are all important curing agents for the epoxy resins [14].

Aliphatic amines can be used for room temperature cure. The heat resistance of the cured resin is generally about 1000C, and it has excellent bonding properties. Resins cured with aromatic amines have been used to attain greater heat and chemical resistance than those cured using aliphatic amines[14].

The curing of epoxy resin by amine curing agents is expressed by the formula shown in figure below. The epoxy moieties of the DGEBA can react with either the primary or secondary amine to form an OH in the main chain as in (i) and (ii) which, later on, can react with another epoxide ring to further crosslink the resin. The relative rates of those three reactions are important for the final structure and properties of the cured resin[14].

Figure 2.13 Amine curing reaction of epoxy resins

Referring to figure 2.13, in order for the cured resin to become a cross-linked polymer, the curing agent must have more than three active hydrogen atoms and two amino groups in a molecule above. When the number of moles in epoxy groups is equal to that of the active hydrogen, the loading of the curing agent in epoxy resin becomes optimal [14].

The curing speed of individual amines depends on the type and loading of amine. It also depends on the type of epoxy resin. The most commonly used glycidyl-ether type resins easily cure at room temperature, but inner epoxy types such as cyclohexene oxide and epoxidized polybutadiene hardly cure. Glycidyl-ester type cures quite faster than the glycidyl-ether type.

Diglycidyl ether of bisphenol A (DGEBA), which is a condensation product of bisphenol A and epichlorohydrin, is primarily cured by aliphatic amines at room temperature, but with aromatic amines, DGEBA is slowly cured and this requires thermal curing [14].

2.5.1 Aliphatic Amines

Aliphatic amines are room temperature curing agents. They rapidly react with epoxy resins. A large quantity of heat is generated, and they have a short pot life (usable time). If curing of tertiary amines is performed at high temperature, the properties of curing agents that cure at the room temperature are improved. The heat deformation temperature of cured object of DGEBA is 120 oC at the highest [14].

Resins cured with aliphatic amines are strong. They also have excellent bonding properties. In addition, they have resistance to alkalis and some inorganic acids, and they have good resistance to water and solvents, but they are not really good to many organic solvents[14].

Aliphatic amines irritate the skin. Aliphatic amines having high molecular weight and low vapour pressure are less toxic, but they still need to be handled with care[14].

2.5.2 Aromatic Amines

In comparison to aliphatic amines, aromatic amines are weaker bases, and they slowly cure at room temperature. This is as a result of steric hindrance caused by the aromatic ring. The curing virtually stops in the B-stage of a linear polymer solid due to the large differences in the reaction of primary and secondary amines [14].

There are usually two steps involved in the curing of aromatic amines. The first step is heating to a temperature of about 80 oC in order to lessen the heat being generated. This helps to slow cure in order to reduce the exotherm remaining. The second heating is carried out at a high temperature of about 150 to 170 oC [14].

Aromatic amines provide excellent heat resistance, they yield heat distortion temperatures of 150 to 160 oC, with good mechanical properties and the cured resins are strong. In addition, aromatic amines have good electrical properties and excellent chemical resistance, particularly against alkalis, and because of this, it is a curing agent that is highly resistant to solvents [14].

2.5.3 Tertiary and secondary amines

Tertiary amines, the active hydrogen in which has been completely replaced with carbon hydroxide, do not cause an additional reaction with epoxy resin, but work as a polymerisation catalyst [14].

The curing temperature significantly influences the curing speed, the heat generation, and the properties of the cured resin. Thus, this amine is rarely used alone, particularly in large castings, because the properties at the centre and the outer region are different due to the large quantity of heat generated. They are often used in the field of paints and adhesives where the material used is thin [14].

Although tertiary amines are less useful as a curing agent, they are very important compounds as accelerators for acid anhydrides, and they are useful as accelerators or co-curing agents for polyamine and polyamide curing agents[14].

2.5.4 Amine Hardening Systems

Polyfunctional primary RNH2 and secondary R2NH amines are widely used as curing

agent for epoxy resins. Theoretically, each primary amine group is capable of reacting with two epoxy groups. Hence a polyamine such as ethylene diamine H2N-CH2-CH2-

NH2 is capable of reacting with four epoxy groups because of the four active hydrogen

atoms attached to the nitrogen atom [14].