As they don’t begin curing as soon as they are dispensed, light-cure adhesives provide an unusual degree of controllability to assembly operations. That means that tricky-to-fit parts can be positioned and repositioned until perfect, since these adhesives remain liquid until exposed to ultraviolet light.
Also, they are generally single-part adhesives, which means that no mixing or dosing equipment is required to measure out a precise ratio of ingredients.
“It is by far the easiest adhesive to dispense; it doesn’t cure in the nozzle or pipe,” says Bob Goss (pictured, right), customer technical support advisor at Henkel.
When the right wavelength of light hits the adhesive, a photoinitiator molecule within reacts and generates a free radical, a high-energy particle that tends to round up molecules together and join them, says Peter Swanson, (pictured below), managing director of Intertronics. Most UV cure adhesives use acrylic technology, which consists of monomers, oligomers and additives; in UV light, they polymerise: link together, forming a strong bond.
But that doesn’t mean they will cure if left out in direct sun, explains Goss. UV radiation is often measured in mW/cm²;
a low-intensity lamp produces 20-50mW/cm²; a high-intensity lamp more than 1,000mW/cm²; the sun, less than 10mW/cm². A cure dose refers to the quantity of energy put into the material, in Joules (Watts x seconds). A 20mW lamp might take a minute, but a high-intensity lamp only seconds, for a given cure dose.
On the other hand, the optical nature of the cure limits its usefulness, points out Swanson. “In principle, you have to see the bond line to cure it. That does preclude joining two lumps of metal with UV-cure; you’d use two-part epoxy, for example, for that. But for metals to glass and plastic, it works really well,” he says.
Where the light doesn’t reach, the adhesive can’t cure, Goss adds. For that reason, when used as an encapsulant for electronics, these adhesives are often formulated to include a secondary cure mechanism: heat, moisture or anaerobic (the presence of metal) where some corners might be in shadow.
Also, the clarity of the material dictates the speed, and depth, of cure. In recent years, glass and plastic additives have posed difficulties for the cure process. Goss recalls: “A classic example is the rear mirror button on windscreens. When, many years ago, we used the UV-cure adhesive developed for toughened glass windscreens, it stopped curing on laminated glass, because it needed to transmit through the plastic layer. We had to modify it to find a different UV wavelength. That’s true also with shower doors.”
The same is true for solid plastics, points out Swanson at Intertronics, many of which now have UV inhibitors to stop them ageing in normal light. Still, many plastic bonding adhesives cure with a combination of UV and visible light, which allows an adequate transmission through UV-blocked plastics.
THE RISE OF LEDS
A technological change has been the use of LED lights, as opposed to arc lamps that have been used for 60 years. While LED output is not the same, they use less energy and are instant on-off, whereas arc lamps had to be left on all day. They are also more stable over time.
“Mercury, a constituent of arc lamps, is on its way to being banned, so we’ll be going to LED lamps whether we like it or not,” states Swanson.
Lamps come in many shapes and sizes. The biggest issue is that LED luminaires emit a narrow spectrum of light; that has to match the photoinitiator in the adhesive to cure properly. Originally, all UV adhesives cured in the long wave UV range, around 365nm (UVA). But, explains Goss, poly-carbonate filters out light below 380nm, so adhesive manufacturers were forced to develop other wavelengths. Now, common grades cure at 380, 385, 395 and 405nm as well.
Both suppliers point out the importance of matching adhesive formulation, dispensing system and light cure, so that all are engineered to perform optimally.
APPLICATIONS
Where the application involves glass and process control is vital, UV adhesives really make a difference, Goss says, even if there is a cost premium. “In some department stores, you can purchase curtain rails with finials made of glass spheres, which are between the size of a cricket and golf ball. Originally, they were using an epoxy to bond glass to metal, but they were getting lots of work in progress. Even quick-setting epoxy takes 10 minutes to develop handling strength. In that time, it is nowhere near strong enough to hold that weight. Now they are using Loctite 350, one of our standard grades. While they are perhaps paying five times as much in adhesive, their work in progress is down to almost nothing, and there is better durability with UV acrylic than epoxy.”
Another competitive advantage is in automation; because they are easy to dispense, UV cure adhesives can be applied by robot.
Putting it another way, Swanson states: “Commercially, the benefit of UV curing is in productivity. It lends itself to volume manufacture. Just to do one-off doesn’t make the initial investment worth it. If you are making millions, the initial investment is trivial. It doesn’t lend itself to small numbers of big products, but big numbers of small products.”
Those characteristics match up well with medical applications, such as bonding cannulas (needles) into syringe bodies, which are made in their millions. Suppliers can tweak the adhesive viscosity to their exact requirements.
Another application Swanson mentions is temporary masking for metal surface treatment, in which users can apply the liquid over a large area and cure it to protects the material underneath. Once finished, it is just peeled off.
BOX: Bölhoff’s Onsert: a clear advantage?
The company has developed a range of thermoplastic and hybrid thermoplastic-metal disc-shaped fasteners. Because they are at least translucent, the adhesive underneath can be cured in a matter of seconds using a high-intensity hand-held UV gun. Adhesives used include Delo Photobond and Delo Dualbond. Pull-off force was measured to be 1.75kN, and shear was about 2kN, declining to just over 1kN at 90°C in a sample using a 4mm-thick hybrid fastener that included an M5 bolt on an aluminium substrate, with 0.25g of adhesive cured with the Onsert Portable Mini gun.
Another advantage of using thermoplastic bases is that they can flex under load and substrate deformation, according to Bölhoff. And because the fasteners are cured virtually immediately, there’s no need for temporary fixturing during a long cure.
