But recent formulations are changing the properties of these venerable products. Instantly-curing, brittle, low-temperature, causing white powdery blooming and smelly: everything you thought you knew about cyanoacrylates is no longer correct. Will Dalrymple reports
Cyanoacrylate, C₅H₅NO₂, was invented by accident when Dr Harry Coover was trying to make clear plastic gun sights for Allied soldiers in World War II, explains Warren Wilkinson, joint managing director at Bondloc. While what he made then was deemed to be too sticky, nine years later he realised the potential of a product that would quickly bond a variety of substrates with only a tiny amount of moisture present. And, as its founding name suggests, just a one square inch bond of this product will hold around one tonne.
First things first, then. Why does it need moisture, and what does it do? Peter Swanson, managing director of adhesives distributor Intertronics, describes the monomer constituents in the adhesive, acrylic resins, as ‘desperate’ to bond. All that’s stopping them is an acidic stabiliser added to the liquid carrier. So when the material makes contact with a room-temperature surface, the small amount of humidity resting on that substrate neutralises the stabiliser, and the chain reaction can begin. He’s careful to point out that this is not a moisture-cure product in the way that for example an RTV adhesive might react chemically with the water. Moisture just kick-starts the process.
Since the process starts off with exposure to moisture on exposed surfaces, some of its best performance is interfacial – between surfaces. Dr Julie Joseph, senior technology specialist, general manufacturing & maintenance at Henkel, says that a dot of dispensed superglue doesn’t immediately cure. “But if you close the gap between two substrates, due to moisture, you quickly have handling strength, within seconds. With cyanoacrylates, less is more. The thinner the bond line, the faster it will cure and the stronger the bond you will achieve.”
Put another way, that means there should be no – or almost no – gap. Peter Kraushofer, a technical support specialist at Bostik brand Born2Bond, which also includes as well a CA range, suggests that the ideal distance between substrates would be 30-50micron, because that distance provides the fastest adhesion and the highest performance. But he recognises that few real-world applications are so strict, so suggests that in manual applications, tolerances in manufactured plastics might reach 50-150 micron.
Fortunately, CA formulations can help here. It’s easy to get a thin bond line with traditional CAs because of their low viscosity, as Swanson points out – at typically up to 100 centipois, they are nearly as thin as water.
GRAB STRENGTH: AN IMMEDIATE ADVANTAGE
The quickness of cure, within seconds, means that CAs are often used as tooling adhesives in assembly operations, Swanson points out, holding elements of the part immobile so it can be permanently fixed using a structural adhesive, for example. (CAs are not characterised as structural because of the brittleness of their bonds; they also traditionally have had little resistance to higher temperatures and have poor moisture resistance.) Its very rapidity could be an issue for dispensing equipment, adds Swanson; not to speak of health and safety, as the stuff bonds very readily to fingers (see box, p14). For this reason, Intertronics advises dispensing CAs with a short fluid path using inert, disposable plastics, such as PTFE or polypropylene, and limit the exposure to moisture.
Another negative is blooming, which occurs when excess adhesive has been applied. It is this excess adhesive, especially outside of the joint, which can result in formation of white powder on the bond line.
To really understand what CAs can do, and how new products are changing their performance, a few misunderstandings need to be cleared up. The first preconception to break down is CA’s reputation as an instant adhesive. Although it does bond in seconds, suppliers actually hesitate to characterise it as an instant adhesive. Joseph at Henkel suggests that a CA will provide perhaps 60% of final strength within seconds, but the rest of the crosslinking won’t be completed for 24 hours.
Another misunderstanding is that all CAs are the same. While the most popular CA base starts from an ethyl monomer (the adhesive’s chemical base), it still comes in multiple versions offering different viscosities, packaging options and cure speeds. And that’s just one monomer. In the UK, Bondloc alone formulates CAs also using methyl and alkoxy monomers, and their properties are completely different. According to Wilkinson, methyl grades are used for metal bonding and applications that require improved solvent resistance. Alkoxy grades are typically used for non-blooming applications.
NEW PRODUCTS
A recent trend in CA development has been combination products. CAs can be toughened in the same way that epoxies and other adhesives can be toughened. For example, Loctite introduced a toughened CA in 1982. Bondloc offers a rubber-toughened version of some CAs. Henkel’s toughened Loctite grades can prevent cracks from propagating (reducing brittleness) or to give good peel strengths.
A decade ago, Henkel launched Loctite 3090, a two-part adhesive containing a chemical activator that allows it to cure with a thicker bond line (suitable for filling, up to 5mm). Carrying on with that theme, a few years later it launched Loctite HY 4090, which it describes as a hybrid adhesive, also two-part, that included epoxy technology to offer a durable structural joint.
Similarly, Bostik has also launched a number of two-part CAs under the Born2Bond branding in the last couple of years. These products are not combinations of other adhesive technologies, but a new line, starting from a different monomer, methoxyethyl. The innovation is as much in the production as it is the formulation. Swanson at Intertronics, which is a distributor of the range in the UK, calls the innovative manufacturing method the ‘secret sauce’ of the product. Expanding on that, Kraushofer says: “We developed a patented synthesis procedure that reduces energy costs and offers better output.”
The products and production were originally developed years before by Afinitica, which was acquired by Bostik corporate parent Arkema in 2018. They have made waves within the business. Adds Kraushofer: “We call ourselves a start-up within Bostik. Its hot melt and sealant products are thought of in terms of tonnes of adhesives. But we develop engineering adhesives; we think not in tonnes but in grams, or milligrams. At first, everybody glanced curiously at our small bottles and fancy application equipment. But now inside Bostik we are one of the fastest-growing divisions.”
Several products were launched initially, all quite different from traditional CA performance. While the bottles of the Born2Bond Ultra look like standard CAs – small and single-part – what’s inside is low-odour and low-blooming. Fixture times increase with each of the three viscosities available, from 5-10 sec for the Ultra LV (20-50cP) to 15 seconds for Ultra HV (120-170cP). Born2Bond Flex is a two-part syringe-dispensed adhesive described as suitable for small parts applications and can gap fill up to 1cm. It offers enhanced impact protection, but its primary feature is flexibility: it offers more than 200% elongation before a break. The adhesive fixtures in 60 seconds and hardens in 5-10 minutes.
Bostik has also launched a combined formulation of a methoxyethyl CA adhesive with a UV light cure adhesive, Born2Bond Light Lock. Kraushofer says: “A disadvantage of standard light-cure is that it won’t cure between non-transparent parts, so even if it starts to cure on the side, it won’t cure into depth. What we have done is combined them, and put photo-initiators into the CA, basically, though it’s not as easy as it sounds.”
Swanson picks up the story, adding: “A good application example is medical tubing, gluing a tube into a connector. When you push the tube into the connector, the adhesive tends to push out, and there’s a fillet of adhesive outside the bond line, and the medical industry hates that; this excess adhesive doesn’t cure quickly, and could cause blooming and contamination. Adding UV for a second or two solves the problem.” (Bondloc also offers dual-setting and UV cure CAs in varying viscosities.)
Another new product, Born2Bond Structural, is quite different. It is a hybrid CA and acrylate structural adhesive with what is described as a unique feature: an open time of up to 25 minutes, but a fixture time of less than one minute. Kraushofer explains: “When you apply that adhesive in a bead on a substrate, the reaction is chemically blocked until you press the parts together. At the moment you start to assemble the parts, you have 15-60 sec until it handles like a CA. With two-part adhesives, you have only 3-5 minutes of opening time, but no longer. With this product, you can put the adhesive on a substrate and assemble immediately, or put the adhesive on one part and then some on another 20 parts and then assemble them all; it’s like curing on demand.”
Henkel, too, has been hard at work behind the scenes reformulating CAs. During 2022, it is launching reformulated CAs to remove CMRs: carcinogens, mutagens and repro-toxic chemicals. Joseph explains that, due to the reclassification of some industrial chemicals, many CAs will gain a new hazard label – the exploding chest – due to the presence of CMRs. (There are some 35 CA products in the Loctite catalogue; some of those are bound to remain the same by having received product approval by a third party). All of the replacements have been tested to confirm their performance remains as good as the previous products.
Included in this launch is a new product, Loctite 402, which also tackles another cyanoacrylate weakness: elevated temperature ageing performance. It can withstand use for sustained periods at temperatures up to 135ºC.
Adds Swanson: “All employers these days are trying to have safer products on shopfloors. The workforce is increasingly educated about what these things are. In terms of sustainability, a significant amount of companies have more ESG goals. In a world of chemistry where legislation and the regulatory environment is tighter, it’s in everyone’s interest to have more friendly chemistries.”
BOX: Cleaning up
Superglue is so good at bonding skin that some formulations are used instead of stitches. To remove it, Henkel offers several suggestions. First, soap and warm water – soak the skin in warm soapy water to loosen the glue, then attempt to peel the glue off. Second, lemon juice, diluted with the same amount of water. Third, olive oil or laundry detergent. To remove it from nails, Henkel recommends using acetone (nail polish remover). To remove it from hair, Henkel recommends washing with shampoo and conditioner, and then using a fine-toothed comb to tease out what’s left. www.is.gd/letata
BOX: Special cases
Adhesives suppliers have also developed surface primers and curing accelerators to overcome the challenges of difficult-to-bond low surface energy substrates such as polypropylene, polyethylene, PTFE, and thermoplastic rubber materials. For example, in combination with B770 polyolefin primer, Bondloc successfully bonded a special grade of CA, B406S, developed to be more reactive than standard ethyl grades, for an automotive application. The two were used to bond a silicone rubber gasket into an ABS moulding for an automotive timing belt cover application. Without any pre-treatment, the silicone gasket would not bond. But after priming, the fast-acting B406S was applied to the moulding and the components assembled.
