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UV Curable Adhesives – More than the name implies

UV-curing adhesives have been around for several decades, and their use is widespread, particularly in medical device assembly and the electronics industry. Conventional UV curable adhesives (UV cure acrylates) became popular in medical device assembly in the late 1980’s when restrictions on solvent bonding drove manufacturers to find a more environmentally and user-friendly solution. The boom in the electronics industry in the 1990’s saw the introduction of many exciting new UV-curable adhesives, geared to help speed up production lines by giving instant, high-performance bonding results.

Curing UV activated adhesives through plastic substrates is challenging. Most plastics are UV-stabilised to prevent the plastic degrading, becoming brittle and turning yellow. This means they block out the UV light wavelength (which is also required to cure the adhesive). Development chemists experimented with different photoinitiators and found those with curing wavelengths more similar to that of visible light would trigger the adhesive to cure through UV-stabilised plastics, as well as allowing adhesive cure without the need of a UV lamp. The popularity of this type of light-activated adhesive in a variety of other industries is rapidly increasing. Thanks to its versatility it is now used by fabricators of a broad range of plastic items and assemblies.

Adhesives that cure with both UV and visible light are most appropriately called photo- or light-curable adhesives but most manufactures name them UV adhesives and add a note that they also cure with visible light.

Also, a misnomer is describing UV-curable as the “type” of adhesive. The name says nothing about the type of adhesive. It simply states the cure mechanism. For example, an epoxy can be a heat-cure epoxy, a two-component (mixable) epoxy or a UV-cure epoxy. Most silicones are moisture-cured, but there are UV-curable silicones and similarly UV-curable cyanoacrylates.

To further complicate the naming of adhesives, many adhesives have more than one cure mechanism. Various types of UV-curable adhesives include:

UV/Anaerobic sealant – The UV portion allows manufacturers to assemble parts with instant, on-demand handling times. The shadow-cure areas can then cure anaerobically to form high-strength, highly durable bonds with the chemical and environmental resistance of a high-strength anaerobic adhesive.
UV/Heat Cure – The UV portion allows high-speed surface mount on PC boards. The heat cure ensures cure under the components.
UV/ Moisture Cure – The UV portion allows a high-speed conformal coating on a PC board. The moisture cure ensures that shadowed areas cure.

These are routinely called dual cures…Where this name falls down is in products that have more than two cure mechanisms. Such as:
UV/Heat/Activator – The activator portion bonds the magnets into motor cans readily; the UV portion is used to instantly cure the squeezed out excess material, and the heat cure gives added assurance that if the gap was too large for the activator cure, that all product is cured.

If the names mean little, how do I choose a UV adhesive?

When choosing an adhesive, first narrow down the best technologies to suit your design, bonding and durability requirements. Then consider the manufacturing process most suitable to your assembly.

UV is often associated with high-speed automated production lines; however, glass and crystal artists use UV cure adhesives. The attraction of UV for the artist is the on-demand curing. No cure occurs until the artist has the pieces positioned exactly where they desire. Then they hit the light and cure is on demand. Often UV-curable adhesives are crystal-clear, so they offer the appearance of an “invisible” glue-line, so as not to detract from the finish of the bonded artwork.

What are the costs?

UV curable adhesives are competitive with other types of adhesives. The equipment costs are commensurate with the project scale. Sunshine is still free and a good sunny day will cure adhesive. Small handheld units cost less than a steak dinner. Conversely, industrial lamps cost thousands of dollars. The popularity of these industrial systems is proof that the time and energy savings over other cure methods, such as heat-cure ovens, is that they are worth the investment.

Adhesive Safety Consideration

Because a variety of types of adhesives are cured with UV, it is important to follow the worker health and safety recommendations of the manufacturer.
Conventional UV-curable adhesives (UV Cure acrylates) are solvent-free, 100% solid materials are easy to incorporate into almost any workplace. Other types of UV-curable adhesives such as UV Cure Cyanoacrylates (Superglue) or UV Cure Epoxy may have special storage and handling requirements.

Light Safety Considerations

To put the safety of these adhesives into perspective, it helps to know that a bright sunny day is about 3mW/cm² of UV light in the 320-380 nm wavelength. If your UV lamp is 30mW/cm², you will get sun burnt 10 times faster. If your lamp is an industrial lamp with about 3000mW/cm² you will get sun burnt 1000 times faster.

High energy lamps should be positioned over a conveyor, and the conveyor should be shielded. Use a radiometer to test the surrounding area for stray UV light to ensure worker safety. For curious workers, safety shutters are often installed so if the shutter is opened; the lamp automatically shuts down.

UV light does not bounce or reflect well. However, workers should still be shielded from light in their line of vision. Even if the radiometer shows the light is all in the visible range, not in the UV range, dark, UV blocking glasses are recommended as it is uncomfortable to look at a light bulb.

UV-curable adhesives

For further help and advice, please contact Permabond.