Amines are widely used commercial monomers in a variety of chemical processes to provide cured materials that have great value in today’s modern world. Amines come in a variety of types, each with its own unique characteristics that make it a valuable tool for the formulation chemist. The versatility of amines is such that they can react with epoxies, polyols, to make a wide variety of materials suitable to many applications. We will discuss here the reactions of amines with epoxies.
Amines can be classified into a number of categories, tertiary amines, secondary amines, and primary amines. Each category can be further subdivided into aliphatic, cycloaliphatic, and aromatic types. These amines each have their own characteristics, properties, and purpose.
Aliphatic primary amines are characterized by several typical characteristics that span across the chemistries where the amine is used as a co reactant or acts as a curing agent. Primary amines are very reactive, and that allows the monomers to react at room temperature or lower, forming a thermoset. Aliphatic amines make curing of epoxies possible under very cold conditions. They produce reasonable good chemical resistant coatings compared to other lower cost coating materials in the market. Aliphatic amines are typically formulated with other amine curing agents to control formation of carbamates, [blushing] and control the reactivity to increase pot life or drying time.
Another type of primary aliphatic amines are cycloaliphatic amines. This class of aliphatic amines is characterized by the presence of an amino group on the six carbon ring structure. There are a number of advantages with this type of amine. They are not as reactive as the primary aliphatic amines, and thus they are more suitable for applications requiring a slower cure. Cycloaliphatic amines also have a lower tendency to blush upon curing. [a chemical reaction that forms carbamate species while in contact with CO2 and moisture from the air.] Blushing is a phenomenon that turn the surface of the thermoset pink and it is a degradation on the coating surface that will prevent adhesion of top coats. Cycloaliphatic amines will also produce epoxy cured coatings with better heat and chemical resistance, and improved mechanical properties.
Secondary amines are lesser known than their primary amine types. These amines typically are slower to react than the primary aliphatic amine; and can be used to modify and slow the gelation process. However, these amines have disadvantages such as the side reaction of blushing, leads to lower crosslink density which can comprise thermal, chemical, and mechanical properties.
Aromatic amines include an amine functional group attached to a benzene ring structure. The presence of the benzene ring structure greatly enhances chemical resistance, moisture absorption, and improves mechanical properties. Aromatic amines are very stable and react very slowly and therefore accelerators are needed to be added to speed up the rate of reactions. Even with accelerators an elevated temperature cure is needed when using aromatic amines. For many years the widest used aromatic amine was Methylene Dianiline (MDA) in coatings and other applications. It was used in some of the most chemically resistance coating products ever provided. It also gave great heat resistance, a long pot life, and good flexibility. Its use, however, due to toxicity issues has declined very significantly. Alternative chemistries have now been developed by coating formulators to replace the MDA-like performance.
Tertiary amines cure an epoxy resin by catalytically induced anionic polymerization. Imidazoles are one class of tertiary amines, which offer a pot life range from a few hours to six months and are excellent accelerators for other curing agents. Tertiary amines are added in small amounts because they can be very reactive at room and elevated temperatures. Since they do not become part of the molecular structure of the cured material, they are kept at low levels to prevent out gassing.
Formulators use the characteristics from each class of amines to formulate hardener packages for particular applications in coatings, composites, adhesives, and others. In our next blog we will discuss additional types of amines curing agents such as amines adducts, phenalkamines, polyetheramines and others.