Epoxies are one of the most versatile family of structural adhesives. They bond well to many substrates and can be easily modified to achieve a wide range of properties. They provide high shear strength on a wide variety of substrates including plastics, metals, wood, ceramics and glass. When fully cured, these thermosetting adhesives offer high thermal and chemical resistance, as well as high cohesive strength and minimal shrinkage. Toughened epoxy adhesives are often an excellent choice for very damanding applications, and find extensive use in the automotive, industrial and aerospace industry.
The majority of epoxy adhesives are based on diglycidyl ether of bisphenol A (DGEBA). Other epoxy resins are also used but to a much lesser extend due to their higher price. They are typically blended with DGEBA to improve or modify the properties of the epoxy adhesive. DGEBA resins used in most adhesive formulations are derived from the reaction of bisphenol A and epichlorohydrin. When the repeating unit, n, does not exceed 1.0, the resins are usually viscous liquids and when n exceeds 2.0, the resins are solid and need to be melted prior processing or have to be dissolved in a solvent. A low viscosity of the epoxy resin is critical for dispensing and room temperature cure, and in the case of a two-part epoxy, essential for thorough mixing with the hardener. A resin of higher molecular weight and higher functionality typically improves heat and environmental resistance, but also results in a more brittle product.
Epoxy resins are very versatile structural adhesives because they can be co-reacted with many different resins and low molecular weight compounds. The curing agent affects all properties of the adhesive, like cohesive strength, hardness, toughness, flexibiltiy and resistance to chemicals and heat.
The most popular curing agents are polyamides and amidoamines. They are used in many ‘‘general-purpose’’ 2-part epoxy adhesives and bond well to many different substrates. However, they cure only slowly at room-temperature due to the high viscosity and low degree of exotherm. To speed-up cure, they are often blended with low molecular weight (cyclo-)aliphatic amines. A major drawback of many low-molecular weight curatives is the lower temperature resistance and lower strength at elevated temperatures. Furthermore, many low-molecular weight aliphatic amines are considered to be strong irritans and many aromatic amines are often toxic and proper ventilation and personal safety equipment must be employed. As a general rule, the heat resistance and cohesive strength of an epoxy adhesive increases with increasing epoxy content and cross-link density.
Besides primary and secondary amines, tertiary amines are sometimes employed. These amines do not react with the epoxy but act as a catalyst, that is, they accelerate room temperature and elevated temperature cure. They are often blended with other curatives.
Another important group of hardeners are mercaptans. This class of curatives provides very fast cure, sometimes in minutes. However, mercaptans have a very strong, unpleasant, hydrogensulfide-like odor. They also have a tendency to be more brittle and usually have lower peel-strength. To reduce the oder, high(er)-molecular weight mercaptans are employed. These systems are often used for two component, five minutes curing epoxies.
Another important class of curatives are anhydrides. These hardeners react much slower with epoxy resin than amines and are usually cured at elevated temperatures to complete the polymerization. The reaction occurs after ring opening of the anhydride by secondary hydroxyl groups in the epoxy resin and can be accelerated with tertiary amine catalysts. These epoxies have high temperature resistance, excellent electrical properties and exhibit low viscosity, long pot life, and low exotherm.
Besides two component epoxy adhesives, single component epoxy systems have been developed. In many cases, the curing agent is insoluble in the resin at room temperature and, therefore, inactive. Curing is
induced by heating the adhesive which causes the curing agent to
melt and diffuse into the resin, thereby initiating cure. The most prominent example of this type of curing agent is dicyandiamide (DICY).
These adhesives are usually cured between 250°F and 350°F. The shelf-life at room temperature is usually no more than
six month whereas many two-part epoxies have two year shelf-life. Another important curative of this type are
imidazoles. Like dicyandiamide, they are insoluble in the resin at
room temperature and react with the epoxy at elevated temperature.
Another mechanism utilized in one-part epoxy adhesive is chemical
blocking of the curing agent. An example of this type of curing
agent are cyanourea compounds.
Epoxy adhesives are rather brittle. To improve their flexibility, they are often toughened with (reactive) rubbers and thermoplastic resins. The most common tougheners for epoxy adhesives are acrylonitrile-butadiene rubbers. The nitrile rubber phase-separates during cure and forms small spherical rubber inclusions which impart toughness and higher peel strength to the structural epoxy adhesive. Often, the rubber is terminated with reactive groups. The most common reactive rubbers are carboxyl-terminated (CTBN) and amine-terminated acrylonitrile butadiene rubbers (ATBN).
Another important class of tougheners are urethane oligomers with pendant (blocked) reactive groups. During cure, the epoxy cross-links with the reactive groups of the urethane which results in a urethane-epoxy network. The combination of urethane and nitrile rubber tougheners can yield extremely tough and durable epoxy adhesives. These adhesives are often one-part adhesives. Due to their excellent corrosion resistance and bond performance, they are widely used in the automotive and aerospace industry as structural adhesives. Due to their high viscosity, they typically have to be dispensed warm. Several two part epoxy adhesives with two or more tougheners have been developed that reach high strength and high toughness as well. However, one-part epoxy adhesives still outperform these two-part epoxy systems.