Cyanoacrylates for Assembly

If the adhesives world had a track team, cyanoacrylate would be Usain Bolt, the three-time Olympic sprinting champion. That’s because it is a versatile, super-fast material that cures much quicker than other types of adhesive.

Cyanoacrylates are one-part, room temperature-curing adhesives available in viscosities ranging from water-thin liquids to thixotropic gels. They are designed for assembly applications where speed and strength are essential.

When a cyanoacrylate (CA) comes in contact with a weak base, such as moisture present on most surfaces, acidic stabilizers in the adhesive are neutralized and the cyanoacrylate hardens rapidly to form a rigid thermoplastic that adheres to most substrates. However, these fickle adhesives are sensitive to moisture and temperature.

“We’re seeing steady demand for CA, with global market growth averaging more than 7 percent annually,” says Christine Salerni Marotta, global strategic application manager for general manufacturing and maintenance adhesive technologies at Henkel Corp. “That’s because they are well-suited for small part assembly, an ever-increasing need as devices are getting smaller.

“Cyanoacrylates are also conducive to applications that use a lot of different materials,” Marotta points out. “This includes harder-to-bond materials such as ethylenes and polypropylenes (when a surface primer is used) where some temperature resistance or fluid resistance is required.

“Glass is the only substrate that we don’t recommend CAs for,” notes Marotta. “While they have a good initial bond strength, long-term strength is a challenge.”

“The best part about CAs is fast curing, which many engineers prefer,” adds Venkat Nandivada, manager of technical support at Master Bond Inc., which offers products such as MB297MED and MB250NT.  “For high-throughput applications, they serve a purpose and are convenient to use. Most CAs are one-component adhesives that don’t require any precise mixing, heat or light to cure.”

Numerous Advantages

Cyanoacrylates are easy to use and automate, because they cure and bond well to metal, plastic, rubber, wood and other materials. In addition, they fixture rapidly and deliver high-strength bonds when fully cured. This speed of cure increases throughput and reduces processing costs.

“Cyanoacrylates have a shorter working life and often have a shelf life of around a year,” explains Drew Richards, marketing manager at Hernon Manufacturing Inc. “This short working life requires precise and quick placement of the substrates, but it also correlates to a lower set time, allowing faster throughput and high-speed assembly.

“More importantly, CA adhesives simply bond differently than epoxies or elastomeric adhesives,” Richards points out. “There are some applications for which CAs are clearly better.”

Despite those advantages, some engineers are reluctant to use CAs, because they cure so quickly. “There are several limitations,” warns Nandivada. “For instance, CAs tend to be more rigid or brittle than some other types of adhesives. Also, if you have a need for special properties, such as high electrical or thermal conductivity, other chemistries like epoxies or silicones will be more suitable.

“And, CA cannot be used for potting and encapsulation; typically, applications that require thin bond lines work the best,” explains Nandivada. “Any application that requires time to align parts or move components around is not ideal for CA. Dispensing can also be a challenge, especially for manual applications that use squeeze bottles, where it can be difficult to achieve consistency.”

“I believe the biggest concern for most users is sticking their fingers together, because they are so familiar with household use of Super Glue,” adds Mark Tansky, senior sales manager for Aron Alpha Industrial Krazy Glue at Toagosei America Inc. “But, in general, cyanoacrylates are an easy technology to utilize and adopt in manufacturing, with simple hand application of the adhesive directly from small bottles or via semi- or fully-automated dispensing systems.

“The preconceived notion of how ‘instant’ glues have been marketed over the past few decades at retail have crept into the mindset of some of today’s engineers,” says Tansky. “They may believe that CAs are not easy to use in a production environment or consider them to be too brittle once cured. But, the fact is that CA has been safely and successfully used for a variety of assembly applications for more than 50 years.

“Cyanoacrylate is an effective assembly method that is used to bond wiper blades to metal splines and small wigs to doll heads,” notes Tansky. “Other commercial applications include everything from making dental crowns to bonding incisions in the mortuary embalming trade.”

Automotive and medical device manufacturers are heavy users of CA. In the auto industry, it’s widely used to assemble plastic interior trim components, sensors and under-the-hood hoses. Medical device engineers rely on CA to mass-produce catheters, diagnostic test kits and surgical instruments. Cyanoacrylate is also used to assemble products such as electric guitars, furniture, power tools and speakers.

“Demand for the most part is stable, but certain areas, such as automotive part manufacturing, is showing strong growth,” says Tansky. “In particular, there is increased opportunity because of electric vehicles.” EV applications include assembling batteries and bonding magnets that are used in electric motors.

Dispensing CA

Cyanoacrylates can be dispensed both manually and with automated equipment.

“There are a lot of applications that don’t lend themselves to automation,” says Tom Muccino, sales manager at Nordson EFD, which specializes in adhesive dispensing systems. “While we’ve seen more automation in recent years, about 50 percent of applications today still require an operator to manually dispense the adhesive.

“One reason to dispense manually is because a company is producing something in low volumes,” explains Muccino. “Also, some parts just don’t lend themselves to automation, such as a component that is flexible or flimsy. For instance, coils or wires sometimes require some form of manual manipulation.

“Despite what some engineers think, CAs are not difficult to dispense,” claims Muccino. “In fact, it’s much less challenging than two-part epoxies. However, gel CA is easier to dispense than watery variants; it’s more forgiving and easy to work with.

“With benchtop dispensers, if you don’t use the right kind of tip with CA, you’ll end up with clouding issues,” warns Muccino. “Whenever you have downtime, there can be challenges when a dispensing tip is exposed to air.”

When using a benchtop unit for low- to medium-volume production applications, Muccino recommends Nordson EFD’s UltimusPlus II or Performus X15, which enable engineers to set air pressure from 0 to 1 bar for greater control of low-viscosity CAs.

“Our PTFE-lined dispense tips prevent premature curing and clogging,” explains Muccino. “They feature a unique crimp to control the flow of watery CAs. And, SmoothFlow tapered tips provide the best results for gel CAs. In addition, our blue LV Barrier piston is designed specifically for watery CAs and other thin fluids, while our white SmoothFlow piston works great with gel CAs.”

New Products

While the base monomers used in CAs have primarily remained ethyl-2, adhesive suppliers have recently tinkered with the chemistry to improve characteristics such as temperature resistance, impact resistance, flexibility and fluorescence. There have also been improvements in shelf-stabilization chemistry and plastic packaging to extend the life of adhesives in storage.

“The latest trends we have seen are the development of dual-cure CAs,” says Richards. “These adhesives cure using UV light, allowing an even faster cure that can penetrate some substrates. Also, some manufacturers are developing moisture-resistant CA formulas.

“One of [our] most interesting products is our Quantum line,” explains Richards. “This is a series of rubberized and toughened CA adhesives that are more flexible and can be significantly stronger that traditional cyanoacrylates.”

“Light curing has emerged as a new innovation that blends traditional anionic curing with the ability to flash-cure CA monomer with LED ultraviolet light,” adds Tansky. “However, the substrate requires transparency for the UV light to be effective.”

Toagosei America’s newest CA product is Aron Alpha 900 Series, a family of adhesives that are suited for flexible materials such as rubber, film and leather. “Traditional cyanoacrylates cure hard and brittle, but have a strong tensile shear strength,” claims Tansky. “Aron Alpha 900 Series offers a soft and pliable bond line, while maintaining a strong tensile shear strength bond.”

Henkel has recently introduced two new products that provide improved flexibility. “Our focus has been on how to overcome some of the challenges of traditional CAs, such as brittleness and rigidity,” says Marotta. “Loctite 4314 is a flexible light-curing CA, while 4902FL is a room temperature CA. Both products are tested to the ISO 10993 biocompatibility standard.

“The fluorescence in 4902FL allows users to verify that the adhesive has been applied to the assembly correctly, and determine if there are any adhesive voids in the bond line,” Marotta points out. “A simple black light can be used to detect the fluorescent adhesive.

“Loctite 4902FL is designed for use on close-fitting, flexible parts and can easily bond dissimilar substrates from plastics to elastomers, making it suitable for the assembly of flexible medical devices,” adds Marotta. “Its flexibility rivals that of select acrylic and urethane adhesives.”

Hybrid adhesives are another new development that Henkel pioneered several years ago. 

They combine key features of structural and instant adhesives—bond strength and speed. Hybrids deliver the speed of a cyanoacrylate and the strength of an epoxy. This combination provides high impact resistance, high-strength bonding on a variety of substrates and high temperature resistance.

“We’re seeing more demand for hybrid adhesives that combine CAs with other types of chemistries, such as acrylics and epoxies,” says Chris Martino, senior application engineer at Henkel. “They are used in structural bonding applications that need the speed of a CA backed up with other physical properties.

“HY 4090 is designed for assembly applications, while HY 4070 is a faster product primarily aimed at the repair market,” explains Martino. “Both products feature unprecedented strength, a faster cure and multi-surface adhesion.”