Easy-to-use one component UV curable adhesives are environmentally friendly and offer high bond strengths to similar and dissimilar substrates.

Adhesives that cure under a UV light source are user friendly and versatile — if you know how to apply them.

Adhesive products have different ease-of-use characteristics. Some must be precisely weighed and mixed prior to use. Some need a carefully controlled heat cycle to cure properly. Some have a precarious pot life. Some offer a narrow window of open time. And then there are the UV curable compounds.

This diverse group of adhesives, sealants, and encapsulants cures only when activated by a suitable UV light source. As one-component materials, they don’t require labor intensive weighing or mixing steps. With the exception of special dual-cure systems, UV curable compounds require no heat cycle. These adhesives also remain stable for long periods of time, with shelf life of six months or more if kept away from UV light.

Because they cure on demand when users turn on the UV light, they offer an unlimited open time.

The UV cure mechanism provides a couple of other practical benefits as well. For one, UV curables are among the most environmentally friendly adhesives because they emit no volatile compounds as they harden. The light-initiated cure takes place both quickly and at low temperatures, so substrates experience little or no noticeable temperature change. This cool cure makes the adhesive products a natural fit for a wide range of heat-sensitive substrates and components. The characteristics put UV curable among the most user-friendly adhesive products on the market today.

UV Curable Capabilities

UV curable products come in many different formulations for use in a wide range of bonding, sealing, and encapsulation applications, but they tend to share many features and capabilities.

After exposure to UV light, these compounds form rugged thermoset materials with excellent durability, strength, hardness, impact resistance, adhesion, and electrical properties. The compounds cover a service temperature range from -80° to 350°F and offer good chemical resistance — even in the presence of moisture or heat.

Today’s second generation UV curables also exhibit a particularly low shrinkage rate, which improves dimensional stability and can even help prevent substrates from warping. It’s also an advantage when applying UV compounds in thick cross sections. With today’s formulations, thicknesses up to 1/2" are far easier to attain than with first-generation products.

In-depth curing greatly extends the range of applications — particularly to the potting and encapsulation of sensitive electronic components like integrated circuits, light-emitting diodes, high-voltage coils, and optical fibers. Additionally, in-depth cure enables bonding and potting applications involving printed circuit boards, capacitor seals, and electrical connectors.

The Right Light

Because they are 100 percent reactive and not inhibited by oxygen, UV cure reactions can take place at ambient temperatures and in the presence of air. When fully exposed to a suitable light source (without any shadowing), they can cure in a minute or less. Even after the light source has been removed, the cure reaction will continue until all the UV reacting species have been consumed. This capability allows users to make the most economical use of UV energy.

The UV energy used to cure these formulations typically falls in the range of 10 to 40 mW/cm2. A variety of lamps can generate this energy, providing their output includes UV light of 250 to 365 nm wavelengths. Cures can also be achieved with low intensity UV sources, but cure times will be significantly longer and may require several minutes or more.

In general, a thicker layer of UV curing material will require somewhat longer exposure to UV light than a thinner one, but the relationship is not directly proportional. Also, the rate of cure increases with the amount of UV intensity deposited on the surface, but again the relationship is not directly proportional. Furthermore, the rate of curing depends on the distance of the surface of the UV curing polymer system from the UV radiation source.

As with any adhesive, optimal adhesion requires clean substrates. Oils, greases, release agents, dirt, and other contaminants should be removed before adhesive application. In cases involving metals or other inorganic substrates, the degree of cleanliness can be ascertained by a simple test which involves spreading a few drops of cool water on the surface. If the water spreads over the area with a continuous film, parts are sufficiently clean for further processing; if the water beads or stays in puddles, EPA acceptable solvents such as IPA or acetone should be used for degreasing. The water test should then be repeated before applying the UV-cure compound.

Substrate Versatility

UV-cure compounds have been successfully applied on many different types of organic and inorganic substrates. Among them are difficult-to-bond materials such as pretreated polyester and polyimide films, sputtered metal films, and high-purity alumina ceramics.

UV-cure adhesives will also work with polyethylene, polypropylene, and other polyolefins, though these substrates require special surface treatments to promote adhesion. The same goes for surface-treated fluorocarbon polymers such as polytetrafluoroethylene (PTFE) and chlorinated fluorocarbon resins. Whenever these difficult-to-bond substrates are involved, it’s imperative that UV light exposure takes place without any shadowing or diminished intensity.

Successful assembly operations are today using UV-cure compounds on highly-automated continuous processing equipment. The fastest of these lines can reach speeds in 80 ft/min.

Even more applications are likely over the coming years as more engineers recognize just how easy to use and versatile these adhesives can be.