UV-Cure Adhesives 101

UV-curing adhesives can help get manufacturers out of sticky situations.

Walter Brenner
Technical Director
Master Bond Inc.
Hackensack, N.J.

A technician uses a UV curable adhesive for perimeter bonding an optic component to a metallic part.

Organic adhesives usually contain solvents and various diluents or are composed of two components which must be carefully weighed and mixed prior to use. After these adhesives are applied, it takes heat to drive off solvents and other volatiles and speed the cure or hardening, especially for two-component mixtures. This thermal curing puts as many as 6 gallons of solvent into the air for every gallon of solid adhesive left. Such pollution is regulated by the EPA and companies are often forced to buy expensive recovery systems with afterburners.

UV-curing adhesives and compounds, on the other hand, avoid most of these problems while providing top-notch bonding and performance. For example, UV adhesives use only one component, so there's no weighing or mixing before use. This simplifies production schedules by eliminating pot-life problems. UV adhesives are also 100% reactive, so there are no volatile losses during curing and the process is nonpolluting.

UV adhesives remain stable and usable — even after storage at ambient temperatures for six months — until activated by UV light. Activation occurs only when exposed to UV light of 250 to 350 nm, and then the materials cure on demand, letting companies get rid of lengthy ambient-temperature cures or ovens needed for elevated temperature cures.

HOW THEY WORK
UV adhesives absorb radiant energy from a UV light source and convert it to chemical energy so quickly that curing is practically instantaneous. So quick, in fact, that substrates experience only a brief, superficial temperature change. This lets companies use UV adhesives on heat-sensitive materials including plastic films, moldings, and synthetic fibers, as well as elastomers and paper products.

The lack of heat is particularly valuable in electrical and electronic industries where transient thermal changes can degrade a component's performance. In addition, the low heat reduces substrate shrinkage and warpage, and permits additional on-line processing and off-line handling. This eliminates having to move parts to cooling racks before they go down the line, along with the space and labor requirements.

Completely reactive UV adhesives are not oxygen-inhibited and have fast curing rates at ambient temperatures and atmospheres, though curing is faster if heat is applied. This eliminates the need for atmospheric control — a nitrogen atmosphere, for example — to get tack-free cures. Equally important, cure continues in the dark after UV exposure until all UV reacting species are consumed, thus making economical use of UV energy.

UV compounds can cure despite cross-section thicknesses up to 0.5 in. and more for specific formulations. Maximum dimensional accuracy is assured because the compounds cure with minimum shrinkage.

UV-cured bonds remain intact over temperatures ranging from –80 to 350°F. The bonds also stay intact for a long time because they resist most chemicals even in the presence of moisture and heat.

For best adhesion, substrates must be carefully cleaned of oils, greases, release agents, dirt, and other contaminants. In many cases, such as with metals and other inorganics, a simple test determines if the surface is clean. The test involves spreading a few drops of cool water on the surface. If water spreads over the area with a continuous film, parts are clean enough. But if water beads or stays in puddles, degrease the surface with an EPA-acceptable solvents such as IPA or acetone. Repeat the water test before applying UV-cure adhesives. Polyolefins, such as polyethylene and polypropylene, as well as fluorocarbon polymers such as polytetrafluoroethylene and various chlorinated fluorocarbon resins, require special surface treatments for adequate adhesion.