Figure 2: The electromagnetic spectrum organizes radiant energy of all types by wavelength.
Most UV light systems offer a broad spectrum of output from 200 nm to more than 700 nm, but they also emit infrared (IR) or heat energy. Temperatures under some UV lamps can exceed 150°F (66°C), a problem for thermally sensitive parts such as thermoplastics that can discolor and/or distort under these conditions. As a by-product of start-up, some older UV systems emit ozone, a poisonous gas that must be vented out of the plant.
Presently, traditional UV light sources can cost anywhere from several thousand dollars to well over $20,000, depending on the sophistication of the system. With each replacement lamp or bulb costing approximately $500, system maintenance costs easily exceed several thousand dollars annually for each UV curing system.
UV curing adhesives themselves have several performance limitations. Because UV light must reach the adhesive bond line to achieve full cure and the associated performance properties, transmission through substrates is critical. A variety of factors dictate the ability of the light to reach the adhesive. Most colored substrates will not transmit UV light. Many grades of clear plastics include additives such as UV inhibitors that prevent discoloration during sterilization or aging, but these additives can also prevent cure. Similarly, when curing through large volumes of adhesive in a potting or filling application, the adhesive itself can act as a transmission limiter and result in low depth of cure.
In the early 1990s, adhesive formulators introduced UV/visible (UV/V) adhesive systems that respond to 200- to 390-nm UV light as well as 400- to 410-nm light, at the cusp of the visible electromagnetic spectrum. This technology cures using many existing broadband-emitting UV light sources and takes advantage of a slightly greater portion of the spectrum.
The addition of the 400- to 410-nm visible photoinitiators improved the curing and performance characteristics of UV/V adhesives. These adhesives work with UV-blocking and slightly tinted substrates, and the cure rate and cure-through depth are also improved. In some cases, cure depths in excess of 0.25 in. are achieved with the appropriate combination of adhesive, light source, and process.
The cure rate for many visible light–cure adhesives is as fast as 10 to 20 seconds, a substantial reduction from the 45- to 60-second cure of UV/V adhesives. A beneficial process called photobleaching occurs with UV/V adhesives in which the slight discoloration of the cured material fades over time upon exposure to ambient light. Early UV adhesives did not experience photobleaching and, therefore, retained their slight yellow color for the life of the device.
The advent of the UV/V photoinitiators allowed a wide range of formulating options, ranging from hard, rigid resins to semiflexible materials.
Advancements in Visible Light–Curing Technology
The latest generation of light-cure adhesives features new photoinitiators that react solely with light in the visible wavelengths that exceed 425 nm. These new adhesives cure in less than 10 seconds and are compatible with metals, glass, and a wide array of plastics. They can be used on UV-blocking substrates and select colored materials, particularly translucent grades of purple, blue, gray, and white.
Visible light–curing materials can offer adhesion comparable to most commercially available UV/V adhesives, with particularly high adhesion on polycarbonate and polyvinylchloride (PVC). This new technology can cure to depths in excess of 0.5 in. and is, therefore, suitable for potting applications. Available in low- and high-viscosity formulations, these adhesives are suitable for bonding polycarbonate and other thermoplastics without causing stress cracking, and offer fluorescence for easy quality control inspection.
Much of the benefit of visible light–cure technology is directly tied to the efficiency of the cure equipment. An ever-growing variety of focused visible light sources provides considerable processing advantages for appliance manufacturers.
Visible light–cure systems are available in both point and flood configurations that can be lamp- or bulb-based, similar to some early UV systems. The output of these light sources is considerably narrower banded than the current commercially viable UV light systems. Typical bulb-based visible light sources provide output ranges of approximately 400–600 nm and minimize excess unusable light and IR (heat) energy output. Because of its substantial heat reduction, visible light–cure technology can be used on temperature-sensitive materials.
The initial cost and ongoing maintenance expenses for visible bulb systems are considerably less than those of traditional UV and UV/V systems. The initial cost for most commercially available visible systems is well under $2000. With bulb lives twice those of standard UV and UV/V bulbs, appliance manufacturers can realize a nearly immediate cost savings in maintenance alone.
A second category of visible light–cure equipment, light emitting diode (LED) technology, emits very focused visible light wavelengths—a significantly tighter output range than visible lamp technology. In most cases, LED curing systems emit at one primary wavelength, such as 420 nm, and offer slight amounts of residual light in nearby wavelengths (± 15 nm).
LED systems are extremely efficient and cost-effective, as excess and unnecessary broadband light and heat/IR energy are not emitted. LEDs produce higher outputs that more effectively cure the adhesives. Whereas a traditional UV light source might offer an output irradiance of 150 mW/cm2, a visible LED system offers more than 2 W/cm2.
LED curing systems, currently available as point or spot sources, are predicted to have light-output lives in excess of 10,000 hours and are typically built into solid-state housings that make them extremely durable and portable. These systems also take up less space than UV-cure equipment and are easy to automate.
Safety is perhaps the most significant benefit afforded by higher-wavelength visible light–cure systems. Because the light output is visible, UV-related system shielding and operator protective equipment can be minimized or eliminated. Safety glasses are often still recommended due to the brightness of the visible light sources. By eliminating IR and ozone, the systems do not require heat-protective equipment or ventilation systems.
A relatively limited number of visible light–cure adhesive grades are currently available. In the near future, appliance manufacturers can expect a variety of new visible formulations with enhanced physical properties and broader viscosity options. Light sources are also continuously evolving, particularly in the area of LED systems where higher output, broader cure areas, and new configurations will result.
Typical Appliance Applications
Use of visible light–cure adhesives in the appliance industry has grown as manufacturers employ new materials and adapt designs to reduce the complexity of their assemblies. The transition from metal and glass to plastic substrates such as PVC, polycarbonate, and styrene acrylonitrile (SAN) has driven the conversion from clips and mechanical fasteners to adhesives. By using adhesives, appliance designers can reduce the number of components in an appliance assembly by eliminating bracket frames, screws, and clips.
Visible light–cure adhesives are used in a range of appliance assembly applications, bonding and sealing a variety of materials on white goods such as refrigerators, washers, and dryers, as well as on smaller electrical appliances. These adhesives can help increase the reliability of appliances and greatly reduce long-term maintenance costs.
Visible light–cure adhesives are also being used to assemble spill-proof shelving designs found in refrigerators. The adhesive is applied to the lip of the shelf’s plastic frame, and the glass center is then dropped into place. The assembly is then cured in less than 40 seconds. The adhesive provides a leakproof seal that was often difficult with mechanical clips and eliminates a component from the shelf frame.
On washers and dryers, one typical application involves securing snap-fit light tubes into the control panel of the machines. In the past, some snap-fits were not strong enough, and the plastic lights would fail by slipping out of position and pushing back into the panel. By applying a visible light–cure instant adhesive to the lights, the lights are permanently secured in place. As the adhesive is applied onto at least one opaque material, the initial light cure is supplemented by a secondary moisture-cure mechanism that solidifies the adhesive in areas where light cannot be transmitted.
Visible light–cure adhesives are also providing permanent strain relief for wires and cords that can pull loose during appliance assembly or end use. On printed circuit boards, the adhesives are applied over soldered bare wires to keep them from pulling away from the board. These materials can also be used to strengthen cord ends and help resist bending.