FASTENINGS & ADHESIVES FEATURE
Superglues – or cyanoacrylates – are growing in popularity in industrial applications due to their versatility, ease of use, ability to perform multi-substrate bonding, fast fixture times, and suitability for a wide variety of materials including plastics, elastomers and metals. But how do they work? Bob Goss, senior technology specialist at Henkel, explains
THE BENEFITS OF... superglues C
yanoacrylates are kept liquid inside the bottle by an acidic stabiliser and,
when dispensed, the moisture on the surface of the substrate neutralises this stabiliser and cure is initiated. While the best results are obtained in atmospheres with about 40-60% humidity, full cure will still be obtained with less humidity but a little more time will be required. With a typical bond line thickness of
just 0.1mm, cyanoacrylates offer initial cure in seconds. In fact the joint will achieve about 50% of its total strength within 60 seconds, and 100% in 12-24 hours. Joints set into a rigid thermoplastic have an operating temperature range from -50˚C to +80-90˚C, although some offer up to +120˚C. Cyanoacrylates can join most
engineering plastics, including ABS, LCP (liquid crystal polymer), PBT, polycarbonate, PMMA (acrylic), nylon (polyamide) and polysulfone, as well as nitrile, butyl and neoprene rubbers. They are also suitable for wood, leather, fabrics, steel and aluminium. For plastics and rubbers where the surface energy is below 33mN/m – such as PEEK, POM (acetal/Delrin), polyethylene, polypropylene, silicone rubber, Viton and PTFE (Teflon) – a bond can still be achieved by ensuring that the surface is properly wetted. They are not, however, suitable for bonding glass.
RESEARCH & DEVELOPMENT Despite the resounding attributes of cyanoacrylates, technology developments are continuing. For instance, recent or current development programmes at Henkel include those targeting acidic/porous bonding, toughened grades and flexible grades – such as the recently introduced LOCTITE 4092 for medical industry applications. Further developments include low odour products and cyanoacrylates with gap filling properties. A recent introduction here is LOCTITE 3090, a two-part product that offers the capability to fill gaps up to 5mm.
To focus on a particular
development area, toughened instant adhesives feature rubber particles in an adhesive matrix that act as crack arrestors. This is because, although normal cyanoacrylates can support high tensile loads, under a peel load, joints can break relatively easily due to crack propagation. As a result, for applications where peel strength is required, toughened adhesives such as LOCTITE 480 are required. Although the cure time is a little slower at around 60 seconds, such products offer particularly good adhesion between metals, or between metal and rubber.
EPOXY ALTERNATIVES Epoxies represent a viable alternative to instant adhesives. These typically comprise a resin and a hardener which are mixed together (manually or automatically) to achieve a cured product. Cure times range from 5-10 minutes to four hours. However, as a rule of thumb, if an epoxy is required with good durability, adhesion and toughness, then a slow- curing epoxy is often the best choice. The properties of epoxies vary
enormously in terms of flexibility/ toughness, temperature resistance, viscosity, colour, cure speed and conductivity (electrical and thermal). Typically, epoxies offer temperature resistance up to 120˚C, although products up to 180˚C are available; while in terms of viscosity, these adhesives range from about 5-1000 Pa.s. Epoxies are traditionally strong performers on metals and ceramics, but less good on elastomers and some plastics. In a direct comparison between cyanoacrylates and epoxies, the former are better for small part bonding in plastics or rubber where an instant cure is required, while the latter are targeted at larger components made from composites or metals where good gap filling properties are needed.
HYBRID SOLUTIONS But what if the benefits of both cyanoacrylates and epoxies could be combined? LOCTITE 4090 is the first hybrid instant structural adhesive. It comprises part-epoxy for high bond strength and impact resistance, and part-cyanoacrylate for speed and substrate versatility. In essence, two different chemistries are taking place in a single adhesive. Within the cyanoacrylate can be found the hardener for the epoxy, and within the resin of the epoxy lies the accelerator for the cyanoacrylate. The cure time for this adhesive is
around five minutes and the product offers very good strength, not just on metals but on certain plastics such as polycarbonate and ABS acrylic. A lap shear strength (after 24 hours at 22˚C) of 17N/mm2
on grit blasted steel; 13N/mm2
can be obtained on
LOCTITE 4090 comprises part-epoxy for high bond strength and impact resistance, and part-cyanoacrylate for speed and substrate versatility
etched aluminium; and 6.9N/mm2 on polycarbonate. If users wish to bond to polyethylene or
polypropylene, then a surface treatment will be required prior to application. It also offers high impact resistance,
temperature resistance up to 150˚C, high moisture resistance and gap filling up to 5mm, as well as very good chemical/solvent resistance against motor oil, petroleum, ethanol, isopropanol and water. In one application example, TIG
The cure time for the 4090 adhesive is around five minutes and the product offers very good strength, not just on metals but on certain plastics such as polycarbonate and ABS acrylic
welded aluminium fishing rod racks were proving troublesome as the welds required grinding and polishing prior to painting. To eliminate these processes, Henkel proposed a LOCTITE two-part acrylic adhesive to bond the metal sections, and LOCTITE 4090 to bond the plastic end caps to the aluminium. The outcome was an overall cost reduction and far better joint aesthetics.
Henkel
www.loctite.co.uk
DESIGN SOLUTIONS | SEPTEMBER 2016
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