Hybrid Structural Instant Adhesives Combine Strength and Speed

DN Staff

July 15, 2015

4 Min Read
Hybrid Structural Instant Adhesives Combine Strength and Speed

The greatest challenge that design, manufacturing, and production engineers face when assembling with adhesives is finding the right technology that will deliver the required bond strength and fixture speed to facilitate production. While some adhesives provide strong structural bonds, they take hours to fixture and require that parts be racked until they attain handling strength. The technologies that offer the fastest cure do not bear loads or withstand stresses.

For example, instant adhesives -- known as cyanoacrylates, or CAs -- are legendary for their ability to fixture in seconds to a wide variety of substrates and create high-strength bonds. These one-part adhesives are widely accepted as excellent tools for assembling and repairing small parts with bond line gaps of 0.010 inches or less.

Easy to use and automate, cyanoacrylates undergo cure when confined between two substrates in the presence of the microscopic surface moisture that is found in trace amounts on virtually all surfaces. The adhesives fixture to handling strength in seconds, with full cure occurring in 24 hours. Along with their rapid room-temperature cure, instant adhesives are solvent-free and offer bond strength that easily resists shear and tensile stresses. Cyanoacrylates are frequently used in high-volume production environments to bond parts made of plastic, metal, elastomers, and porous materials.

Instant adhesives are a natural choice for high-speed assembly applications, which require an extremely strong bond in seconds. In the medical industry, they are used to assemble tubing, endoscopes, and catheters. In general assembly, they reliably bond dissimilar substrates and hard-to-bond plastics and are often used for wire tacking and O-ring bonding.

The challenge of cyanoacrylate technology is its inability to provide high sheer/peel strength, durability, and toughness, as well as create bonded joints that bear heavy loads. These adhesives are unable to provide a structural bond and can be brittle. They weaken on exposure to polar solvents and can experience blooming -- a white haze or dust that settles along the bond line and creates aesthetic concerns.

By definition, a structural adhesive literally becomes part of an assembly. It will not fail when the substrates that it bonds are stressed to their yield point. As a critical part of a load-bearing joint, structural adhesives absorb strong forces and withstand large deformation without rupturing. In fact, bonded substrates will fail before the structural adhesive deteriorates. Over the last century, manufacturers and engineers have accepted many adhesives for structural design, including epoxies, acrylics, methylmethacrylates (MMAs), modified silanes, and polyurethanes. All of these technologies, however, take considerably longer than CAs to fixture and cure.

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To solve the speed-versus-strength deadlock, adhesive manufacturers have formulated hybrid adhesive technologies designed to eliminate the limitations of instant adhesives and deliver a rapid structural bond. These hybrids revolutionize cyanoacrylates by retaining the speed of the technology while expanding its capabilities to applications where traditional CAs are not durable enough.

Hybrid instant adhesives combine the strength of an accepted structural adhesive chemistry with the speed of a CA, increasing manufacturing speeds and assembly durability. The first structural instant adhesive, introduced by Henkel Corp. in 2014, combines epoxy and cyanoacrylate technology.

Epoxy was a natural choice for the first hybrid structural instant adhesive formulation because of its ability to deliver a strong structural bond on metals, plastics, and glass (a substrate with which CAs are particularly ineffective). Epoxies shrink very little upon cure and offer high cohesive strength, toughness, very good heat and environmental resistance, excellent depth of cure, and large gap-filling abilities. These adhesives are commonly used in the aerospace, electronics, automotive, and medical industries for assembly of electric motors, small engines, printed circuit boards, wire harnesses, nameplates, and speakers.

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Although epoxies are great structural adhesives, they require long fixture times, ranging from 15 minutes to two hours. Parts must be racked during this cure process until the adhesive reaches handling strength. Although heat can accelerate epoxy cure, temperature sensitivity of some substrates like plastics can limit the chemistry's suitability.

The first hybrid structural cyanoacrylate is a two-part formulation comprised of a cyanoacrylate component with a cationic catalyst and a cationic curable epoxy material. When the two parts are mixed in a 1:1 ratio, a high-viscosity gel adhesive is formed. The cationic catalyst initiates cure of the epoxy, and the CA cures on exposure to ambient moisture. Cure happens at room temperature. The cured adhesive is clear with no gap and light yellow with larger gaps.

The two-component mixture virtually eliminates the problem of cyanoacrylate blooming -- the undesirable formation of a white haze or dust around the bond line. With a three- to five-minute fixture time, the hybrid combines the most critical attributes of the cyanoacrylate -- fast fixture time and substrate versatility -- with the advantages of the structural epoxy, i.e., high bond strength, temperature/environmental/impact resistance, and the ability to fill gaps up to 0.2 inches.

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