Why are they so popular?
Perhaps at some point, while reading a technical datasheet or talking with a coatings industry specialist, you've heard about amines and amides. These are the most commonly used molecules commercially as hardeners, especially in the formulation of epoxy coatings. This is primarily because it is relatively simple to obtain the raw materials used to synthesize them. Compounds like ethylenediamine (EDA), diethylenetriamine (DETA), triethylenetetramine (TETA), and tetraethylenepentamine (TEPA) are common examples of polyamines used as hardeners. These compounds can be synthesized from raw materials such as ethylene, ammonia, and alcohols, through amination reactions.
On the other hand, the synthesis of polyamides typically involves the reaction between a diamine (such as EDA or DETA) and a dicarboxylic acid (such as adipic acid). These are relatively simple substances commercially available; they can be derived from sources like petrochemical products and their derivatives. Additionally, the synthesis of polyamides follows well-established standard processes in the chemical industry, facilitating large-scale production for various applications, including epoxy resins. It's not that there aren't other types of hardeners with equal or better properties than polyamines and polyamides; it's just that they are more difficult to synthesize and therefore more costly.
The good thing is that amines, amides, polyamines, and polyamides provide excellent crosslinking or curing with epoxy resin. This allows us to achieve excellent results in the properties of the final product, coupled with the accessibility in the synthesis of these molecules. Now, let's explain why these molecules work so well as hardeners for epoxy resins.
Why are they such good hardeners?
The polyamine and polyamide hardeners are excellent because they provide good mechanical strength to the cured molecular structure, good chemical resistance, and they are also compatible with the epoxy molecule's oxirane functional group, which reacts with the hardener. This oxirane group requires active hydrogens to open up and form cross-links, and amines and amides have active hydrogens.
Mechanical Resistance
Now let's see why they create paints with good mechanical resistance. The most important properties are resistance to plastic deformation and toughness. These properties depend on the structure formed between the polymeric chains of the epoxy resin and the hardeners.
For resistance to plastic deformation, we need the hardeners to create a lot of cross-linking, so the final structure will be very compact and rigid. It will take a lot of effort to plastically deform it. Hardeners can increase cross-linking through their functionality. Functionality refers to the amount of active hydrogen available for reaction. Remember I mentioned earlier that the oxirane group of the epoxy chain seeks hydrogen to react; so, if we have a hardener molecule that has 2 hydrogens ready to react at each of its two ends instead of 1, and each hydrogen forms a cross-link, then that molecule as a whole will form 4 cross-links. If in addition to that, we also have 2 active amine hydrogens not only at the ends but also in the center of the hardener molecule (as in the case of TETA), and we find a way to make them react (since it is more difficult to react functional groups located in the center due to steric hindrance), then we will have 2 more cross-links and therefore the overall structure will be more rigid, tough, and resistant.
Of course, functionality is not everything. Functionality only speaks to the possibility of a reaction occurring, not its actual occurrence. For this reaction to occur while keeping the amount of hardener used constant, we must consider the reactivity of the hardener. Here we find the sole reason why resins cured with amines are known to be stiffer and harder than those cured with amides; it's because amines are more reactive than amides. Amides have a carbonyl group in their structure that allows for chemical resonance. This resonance involves electron movement from one place to another, contributing to the molecule's stability. Stable molecules are less reactive. The fact that the molecule is less reactive means it reacts less and therefore generates fewer cross-links. It's as simple as that.
Another reason that provides greater resistance to deformation is the length of the hardener and whether it is aliphatic or aromatic. The longer the molecule, the more the linear section can bend and rotate around its axis, making the overall structure more flexible. If it's shorter, there will be less room for that. On the other hand, aromatic amine and amide molecules have a benzene ring in their structure that is bulky and hinders the structure from rotating around its bonds, reducing flexibility
Chemical Resistance
The chemical resistance of resins cured with amine and amide is very good and they are quite similar. However, amide resins are generally more chemically resistant than amine resins.
Firstly, as a common trait of both types of hardeners, they both generate a highly cross-linked and compact structure when reacting with the epoxy resin, which is impermeable and hinders the ingress of contaminants or external reactants. As we saw in the section on mechanical resistance, amines typically create more cross-linked structures and therefore more impermeable ones, but amides are not far behind. Besides the impermeable and low-porosity structure, there is also a steric effect, which naturally hinders larger external agents from reaching vulnerable points in the structure.
Secondly, both amide and amine bonds with the resin chain are strong covalent bonds. It would require high concentrations of external reactants or very aggressive conditions with elevated temperatures to degrade or break these bonds. Covalent bonds are among the strongest in chemistry. However, the reason why resins cured with amides are more chemically stable is because amides allow for resonance, making it harder for free electrons from nitrogen to react with external agents. In the case of nitrogen in amine, its free electrons are fixed and localized. If, hypothetically, an external agent manages to penetrate the impermeable labyrinth of the structure and also has sufficient energy to overcome the steric hindrance and approach the amide bond, the electrons would move to protect stability. Even so, if the external agent comes with enough energy, the reaction might still occur, but resonance is undoubtedly a third secret weapon that amide bonds possess and amines do not. This is why epoxy resins cured with amides have that extra advantage.
