Toughening Epoxy Adhesives
Epoxy adhesives are used globally due to their high-performance properties. Epoxy adhesives are targeted towards applications that require a high modulus, high-temperature resistance, chemical resistance, and moisture resistance. Epoxies also have excellent bond strength to many metals, ceramics, glass, and even other polymers. Epoxy adhesives, however, can have too high of a hardness and can be too rigid for many applications. This can be due to dissimilar bonding substrates or large temperature cycles. Rigid epoxies tend to crack or even delaminate under these conditions. In these situations, polyurethane adhesives are considered. What if you need a high strength, crack resistant adhesive with good chemical and moisture resistance? The answer is toughened epoxy adhesives.
Toughened epoxy adhesives reduce the brittleness of most epoxy adhesives and also increases strength. Many epoxy manufacturers try to reduce the brittleness or hardness of epoxy. One way to reduce the hardness of epoxy is to simply make it more flexible. This is done by adding diluents, acrylates or plasticizers. Plasticized epoxy adhesives tend to have lower glass transition temperatures (Tg) and reduced resistance to high temperatures, chemicals, and moisture. This is due to the reduced cross-link density of the adhesive.
Another way to reduce brittleness is to introduce a second phase in the epoxy matrix. This second phase interrupts crack propagation through the brittle epoxy matrix. The second phase must be incompatible with the epoxy matrix and must phase-separate uniformly during cure. Ideally, the toughening agent that creates the second phase would react with the solid epoxy to create covalent bonds between the epoxy and toughening agent molecules. An important technical requirement for the toughener used in the epoxy adhesives is compatibility. For ease of processing and storage, the toughening agent (second phase) will stay in suspension prior to cure. A separation of the epoxy and toughening agent can create issues during use of the epoxy, usually requiring further mixing by the end user. There are several toughening agents that have been used to reduce epoxy brittleness and increase crack resistance. Toughening agents available for commercial use include thermoplastic resins, rubber particles, high molecular weight polyether amines, core-shell rubber particles, dispersed acrylic rubbers, and amine-terminated butadiene acrylonitrile resins.
A method for toughening epoxies that have been employed in the coatings industry is to react a polyurethane segment into an epoxy polymer. Epoxy coatings partially reacted with a urethane pre-polymer have lower hardness, increased adhesion to substrates, and higher impact resistance. In recent years, this same technology has been tested for adhesive applications. The addition of a polyurethane segment gives unexpected improvements in adhesion and temperature resistance. The use of urethane segments in epoxy is accomplished by using blocked polyurethane pre-polymers.
Blocked Polyurethane Pre-polymers
Commercially available polyurethane pre-polymers blocked with phenolic compounds will become unblocked in the presence of a strong amine compound. These pre-polymers do not need heat to activate. A primary amine will react with one blocked group. This reaction creates a low cross-link density polymer.
Increasing levels of the amount of blocked pre-polymer results in a decrease in gel time of the adhesive compound. This is due to the fact that phenolic compounds are catalysts for epoxy reactions. Is increased amounts of the pre-polymers become unblocked the phenolic compounds further catalyze the epoxy reducing the gel time.
Tensile strength initially increases with increasing levels of blocked prepolymer. This is most likely the result of vitrification before polymerization is complete. The incorporation of the blocked polyurethane prepolymer allows polymerization to complete before vitrification occurs. As additional blocked prepolymer is added to the formulation, strength falls as the hard block content of the polymer decreases.
Lap shear strength develops over time as polymerization begins to occur. Generally, lap shear strength begins to build rapidly after 4-5 hours. At 8-10 hours after initial polymerization, lap shear strength becomes much more evident. In many applications, it is at this point where assembled materials can be handled, moved, or further processed. There may be a slight decrease in lap shear after 24 hours as the adhesive becomes more brittle but compared to unmodified epoxy adhesives, toughened adhesives still exhibit higher shear strengths.
The properties of commercial epoxy adhesives can be improved through the addition of a blocked polyurethane prepolymer or another toughening additive. The blocked polyurethane prepolymer improves an epoxy adhesive’s performance in three ways.
The most important performance advantage of blocked polyurethane prepolymers is the increase in reactivity of the epoxy resin by the unblocking of phenolic compounds via amine interactions. These phenolic compounds act as a catalyst. This, in turn, causes an increase the adhesive’s green strength.
A phase separation of polyurethane segments from the epoxy increases overall toughness, impact resistance, and tear resistance. The final advantage is a lower glass transition which allows for fill polymerization of the epoxy system before vitrification occurs. These factors improve physical properties of the epoxy such as strength without significantly changing other characteristics such as hardness and service temperature.
Epoxy adhesives are favored over other types of adhesives mainly for its high strength to a variety of substrates as well as superior resistance to chemicals, moisture, and the environment. However, in many bonding applications, epoxy is too brittle and can crack due to temperature cycles and mismatched substrates. Toughening epoxy adhesives creates a less brittle adhesive resistant to temperature cycles and can withstand a CTE mismatch. Toughening epoxy resins can be done in a variety of ways. One of the very effective ways is the introduction of polyurethane blocked pre-polymers. Other additives can also toughen epoxy adhesives such as rubberized resins (CTBN) and polybutadiene acrylonitrile resins, core-shell rubber particles, or acrylic particles. Solids such as the core-shell and rubber particles need to be mixed into the epoxy manually by the manufacturer. This process can be difficult, expensive, and takes a great deal of time. Newer products have been introduced to the adhesive market of these solid tougheners pre-dispersed in various epoxies making it easier for the adhesive manufacturers to incorporate into new or existing formulations. When mixing manually, batch to batch variation can be very high. The use of pre-dispersed toughening particles in epoxy results in a much more consistent and uniform epoxy adhesive.