During the assembly process, the use of a liquid optically clear adhesive (LOCA) is important for ideal visual properties in LCD screen manufacture. The assembly process has evolved and advanced, but there are still potential weaknesses or defects that must be taken into account in both adhesive selection and process design. In this blog installment, we will discuss one of these potential defects: entrapped bubbles.

What Are Liquid Optically Clear Adhesives(LOCA)?

LOCA, used during the building of LCD displays, are the industry standards for assembling displays within certain size ranges and for particular end uses. They are commonly used in high-demand, mid-sized monitors in automotive and aerospace applications, among others.

LOCA materials are utilized primarily for two purposes:

• face bonding of cover glass to LCD surface
• edge bonding and sealing

The multiple-layer assembly approach using LOCAs results in dramatic improvements in brightness and clarity of images, minimizes or eliminates issues of light refraction, and improves the durability of the display.

What Are the Factors that Can Cause Bubbles in LOCAs?

There are two categories of sources of bubble issues with LOCAs.

• adhesive material selection
  • very high viscosity of adhesive, preventing escape of bubbles
  • high shrinkage of the LOCA 

• processing of adhesive
  • missing degassing step during the bonding process
  • application of very high vacuum pressure (not well optimized)
  • manual dispensing, resulting in variation in volume and pattern method

Related Article: Significance of Accelerated Weathering Exposure in the Design of LOCA

For each of the causes noted for bubbles in LOCA, there exists a corresponding set of solutions or proper behaviors that can minimize or eliminate it.

• Proper LOCA Selection/Design
  • Appropriate viscosity range of adhesive – Typically, lower viscosity adhesives allow for more effective degassing manufacturing steps and lower bubble entrapment.
  • Choice of a low-shrinkage material -- High shrinkage of the adhesive layer after application can result in the development of voids, which can appear as bubbles.
  • Choice of proper curing mechanism -- UV curable adhesives are generally more reliable and less susceptible to bubble formation. Thermal curing materials can cause adhesive flow and air gap generation. 

• Proper Manufacturing Process Control
  • Automated dispensing process -- Manual dispensing, as noted above, is inconsistent and prone to mistakes and potential voids in coating. The use of automation is an obvious solution.
  • Autoclave or vacuum during bonding process -- The incorporation of an autoclave stage or introduction of optimized vacuum conditions will result in more consistent thickness, curing, and proper degassing of the adhesive when bonded.
  • Degassing step before dispensing -- Degassing before application remains one of the most effective and simple approaches and can often be enough to eliminate bubble issues on its own.

Chase Corporation, Westwood, Ma., founded in 1946, is a leading manufacturer of industrial products for high-reliability applications with a global customer base, operating in diverse market sectors. The Resin Designs division specializes in the manufacture of polymers for industrial uses, consumer coatings, and adhesives. Our outstanding manufacturing and technical support groups can provide your organization with a reliable global supply, unmatched quality, and superior technical support.