UNIDADES IMPORTNATES EN EL PROCESO AMBIENTAL

Most frequently, for ambient curing systems, aliphatic or cycloa- liphatic polyamines or amine functional polymeric or oligomeric derivatives are the curing agents of choice. Epoxy groups can react with primary and secondary amine groups, respectively forming secondary and tertiary amine groups. The reactivity depends on type of amine. The general order of reactivity of amines is primary > secondary >> tertiary and aliphatic > cycloaliphatic > aromatic. These reactions are catalyzed by water, alcohols, tertiary amines, and weak acids (such as phenols). The generalized curing reaction of a two-component epoxy-amine system is depicted in Figure 2.44.

Figure 2.45: Aliphatic polyamines

Polyamines

The most common aliphatic polyamines belong to the homologous

series of diethylene triamine (DETA), triethylenetetramine (TETA), and tetraethylenepentamine (TEPA), which contain both primary and secondary amine groups.

Aliphatic polyamines (Figure 2.45) cure epoxy resins at a fast rate and produce a densely cross-linked network with very good chemi-

Figure 2.44: Representation of curing reaction of two-component epoxy- amine systems

cal resistance. However, they suffer from such limitations as short pot life, poor flexibility, poor impact resistance, and more impor- tantly, high volatility, toxicity and potential for skin sensitization. They also have the tendency to produce blushing (amine bloom) when used in high humidity and low temperature conditions.

Cycloaliphatic amines (Figure 2.46) are less volatile than ali-

phatic polyamines but are still considered skin sensitizing agents. Unless modified by acid accelerators such as salicylic acid, they require higher temperature for full curing. Other important cyclic curatives are N-aminoethylpiperazine and m-xylylenediamine; the latter has aliphatic amines attached to the aromatic ring and the- refore gives the typical performance advantages of aromatic and cycloaliphatic amines.

Figure 2.46: Cycloaliphatic amines: (a) isophorone diamine, (b) 1,2-diamino- cyclohexane, (c) 4,4’-diaminodicyclohexylmethane, (d) N-aminoethylpipera- zine, (e) m-xylylenediamine

Figure 2.47: Aromatic amines: (a) m-phenylenediamine, (b) 4,4’-diaminodi- phenylmethane, (c) 4,4’-diaminodiphenylsulfone

Aromatic amines (Figure 2.47) are less reactive than aliphatic

amines and require higher curing temperatures. They yield rigid networks with superior chemical and heat resistance, but their dark color limits their applications.

Polyoxyalkylene amines (polyglycol amines) are another impor-

tant group of polyamine hardeners. Chemically they are amine terminated polyethers derived from polyethylene glycols or polypro- pylene glycols. Among their unique features are flexibility, longer pot life and lighter color.

In order to address the issues of volatility and toxicity associated with low MW aliphatic amines, different chemical modifications are adopted to derive amine cross-linkers with higher MW and hence lower volatility. Some commercially important derivatives are polyamine adducts, amine terminated polyamides, amidoamines, Mannich bases, and phenalkamines.

Polyamine adducts

Polyamine adducts (Figure 2.48) are the products (adducts) prepa- red by using excess equivalents of a polyamine (such as DETA and TETA) to a standard epoxy resin. Such amine-functional products have higher MW and lower volatility, making them good candidates for amine type curing agents. Their final film properties are similar to those obtained by polyamines, but they have a reduced tendency for blushing.

Figure 2.48: Polyamine adduct

Polyamides and amidoamides:

Amine functional polyamides are prepared by reacting dimer fatty acids with excess equivalents of polyamines. The products are effec- tive epoxy curing agents, with both primary and secondary amines available for curing. Figure 2.49 shows a representative structure, though more than one dimer fatty acid segment is possible based on the ratio of reactants. The spacing between amine groups by a dimer fatty acid segment gives an open network with high flexibility and impact resistance but reduced resistance to chemicals and strong

Figure 2.49: Representative structure of reactive polyamide resin

Figure 2.50: Representative structure of phenalkamine

solvents. These agents provide better water and corrosion resistance along with improved wetting and adhesion. Their curing rate is slower but the tendency for blushing is much less.

Other curing agents analogous to polyamide are amidoamides, which are produced by reacting fatty acids with an excess of polya- mines. They have lower viscosity than polyamides. Their cured film properties are closer to those of polyamide cured systems, but their lower cross-link density results in lower corrosion resistance.

Mannich bases and phenalkamines:

Mannich bases are derived by reaction of phenol, formaldehyde and an aliphatic or cycloaliphatic polyamine. These curing agents show enhanced reactivity due to the catalytic effect of phenolic hydroxyl. Phenalkamines are similar products using alkyl phenols (Figure 2.50). Cardanol-based phenalkamines are very popular pro- ducts used in marine and protective coatings. These types of curing agents are known for their excellent low temperature curing (up to 0 °C) characteristics even in damp conditions. They exhibit better compatibility, excellent blush resistance and excellent chemical resistance along with good wetting, adhesion and surface tolerance properties.

Ketones can reversibly react with primary amines with the loss of water to give ketimines (Figure 2.51). They can be considered blocked

polyamines. Absorption of atmospheric moisture during and after

application of the coating produces a ketone and a polyamine.

Dicyandiamine

Dicyandiamine is a crystalline solid (melting point 207 °C) that is incompatible with epoxy resin at room temperature. It is used as a cross-linker for epoxy powder coatings. However, the reaction of dicyandiamide with the epoxy group proceeds differently from the classical amines, as the primary adducts undergo different che- mical rearrangements. They are frequently modified to improve solubility in epoxy resins (Figure 2.52).

Figure 2.52: Dicyandiamine and modified dicyandiamine

Figure 2.53: Curing of epoxy with thiol