Liquid optically-clear adhesive (LOCA) is a liquid-based bonding technology used in touch panels and display devices to bind the cover lens, plastic or other optical materials to the main sensor unit or to each other. Adhesives are used to improve the optical characteristics of the device as well as improve other attributes such as durability.
The primary advantage of LOCA, when compared to traditional adhesives, is its re-workable property and ability to bind to non-even surfaces, while continuing to add superior optical properties and durability to the device. The technology is the next iteration of bonding technology that follows traditional die-cut film adhesives, also known as Optically Clear Adhesives (OCA Tape). Utilizing LOCA also allows for thinner designs and new technologies, such as the use of silicone, improve the properties of the final product.
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Optical bonding, including LOCA and non-liquid Optically Clear Adhesives (OCA Tape), are used in a wide range of electronic equipment – especially those with touch panels. The adhesive is used to bond the touch panel to the main liquid crystal display, and also to bond any protective cover, such as the lens, to the touch panel. The adhesive is then cured onto the device through UV, heat, moisture, or a combination of these three methods, depending on the manufacturer and specifications. Major applications include capacitive touch panels, 3D televisions (3D TV), and glass retarders.
Optical bonding improves the optical performance of the display. It eliminates the air gap between the cover glass and the LCD, and usually includes an anti-reflective (A/R) coating (as well as anti-smudge and anti-glare treatments on the cover glass). Optical bonding improves the contrast ratio by reducing the amount of reflected light, thus improving the viewability of the LCD screen. This is especially important in outdoor applications where direct sunlight can decrease the viewability of the screen.[1]
Reflection from the glass as well as the adhesive can degrade the viewability of the LCD. Reflection is caused by an impedance mismatch between air and the glass. The reflection makes the white brighter, but dilutes black and other colors - decreasing the contrast. LOCA suppliers aim to match the refractive index of glass and clear plastic PMMA (refractive index approximately 1.5) used in displays to minimize loss. LOCA supplier Henkel reports an increased contrast of 400% in sunlight through the use of LOCA with Anti-Reflective glass, vastly improving the viewing experience.[2]
Besides the optical advantages, bonding a sheet of glass to the LCD also improves the durability of the display. It can resist scratches, condensation, and has an improved range of operating temperatures. As touchpad devices become ubiquitous in consumer markets, this increased ruggedness becomes even more important. Also, by reducing the light loss due to reflection, the battery life of the device can be extended as the device does not need as much backlight to power the display.
LOCA has many advantages in comparison with traditional non-liquid film type adhesives (OCA Tape):
LOCA are generally used in displays and touch panel applications. Application is expected to increase as mobile devices, televisions, tablets, monitors and laptops migrate to touch interfaces. Although there are many varied applications, Henkel divides the applications into three categories:[4]
Touch Panel Sensor Assembly: To bond touch panel sensors that require two layers of ITO (indium-tin-oxide) coated glass. This type of bonding will eventually be replaced by direct bonding.
Cover Lens Bonding: To fill the air gap in touch panel sensors that utilize a cover lens (such as clear plastic PMMA) and the glass touch panel sensor.
Direct Bonding: To achieve the highest amount of contrast, such as in an outdoor viewing application, it is necessary to “directly bond” the cover lens to the LCD module.
While traditional chemistry is based on acrylic-based liquid, new technologies include the use of silicone. Silicone allows for new methods of curing the adhesive. The following table illustrates the difference between acrylic and silicone in single and double curing processes.
| curing process | acrylic | silicone |
|---|---|---|
| single cure | UV only | UV only |
| dual cure | UV + heat | UV + moisture |
This curing process including silicone LOCA also helps reduce the amount of MURA - the haze around the edge of large LCD screens. This light-cure silicone technology (part of the Loctite brand of LOCA developed by Henkel) reduces the shrinkage to less than 1%.
Typical workflows for LOCA include two types of equipment: Dispensing equipment and Curing equipment.
Dispensing equipment usually includes bubble free dispensers by means of a bottle or cartridge.
Curing equipment usually includes UV curing lamps or spot cure systems. Major suppliers of LOCA generally offer both dispensing and curing equipment workflow solutions.