Haze and lack of clarity are considered among the most vexing and stubborn of defects in the assembly of LCD screens and surfaces. Significant advances have been made in the optical quality and capability of liquid crystals and LEDs; however, these advances can be stunted by choices made further along in the assembly process. Specifically, liquid optically clear adhesives (LOCAs) used in the bonding of assembly layers can cause haze and light refraction either immediately or under certain aging conditions. The proper design of LOCA materials must take this risk of haze development into consideration. There are a number of effective approaches to minimizing or eliminating haze.

Previous Designs and Masking of Haze Factors

The use of LOCA adhesives in the assembly and enhancement of displays has been the industry standard for many years. These materials are utilized primarily for two purposes:

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

LOCA adhesives are generally two-component chemically-cure materials. Early iterations were designed with a focus mainly on adhesion and protection properties and were often subjected to a lack of clarity and development of haze under certain conditions. To combat this situation, many suppliers were forced to add additional active ingredients to deal with excess reactants, or masking agents to absorb or mask hazing agents. This approach was at times expensive, inefficient, and inconsistent in the control of clarity and haze issues.

What Are the Factors That Can Cause Haze in LOCAs?

There are a number of factors that have been found to result in the potential for haze in LOCA screen assemblies.

• Interaction between moisture ingress and incomplete processing (cure)

In this situation, if the LOCA adhesive is inadequately cured, any subsequent ingress of humidity and/or moisture can lead to side reactions and the formation of haze.

• LOCA and sealant mismatch at the interface (index of refraction)

If the LOCA adhesive is not fully tested for compatibility with any edge sealing chemistries, it is not uncommon to result in the formation of a hazy appearance especially near the edges of the display.

• Complex chemical interaction creating excess foreign species

LOCA adhesives must be carefully designed and all reactants balanced exactly to avoid the existence of excess remaining reactants. If not properly balanced, these remaining materials can react over time and form unwanted haze.

Learn more about L.O.C.A (Liquid Optically Clear Adhesive)

What Are the Solutions to Eliminating Haze?

For each of the above-mentioned causes for haze in LOCA display assemblies, it is possible to make use of solutions or proper behaviors that can minimize or eliminate them.

• Interaction between moisture ingress and incomplete processing (cure)

In order to avoid moisture reactions with inadequately cured LOCAs, the obvious solution is to make certain of adequate LOCA cures. This can be accomplished by careful adherence to energy and requirements for the individual adhesive in question. HumiSeal polymer chemists and process engineering teams are experienced in customizing LOCA products that are suitable for a wide range of manufacturing equipment, which can help to minimize the risk of curing inadequacies. It is also important to regularly measure and adjust UV equipment in manufacturing situations as it does not remain static and will tend to drift lower over time.

• LOCA and sealant mismatch at the interface

LOCA screen adhesives are generally paired with matching edge bonding adhesives. The selections of these two materials cannot be done independently or in a vacuum but must be treated as a “system” solution. HumiSeal, as an example, designs both LOCA screen and edge bonding materials as systems that are compatible and are based on the same LOCA chemistries to avoid undesired interactions. It is also critical that the index of refraction of these two materials be properly matched.

• Complex chemical interaction creating excess foreign species

As mentioned above, a correct and exact balance of all reactants is critical to avoid the formation of haze due to further reactions. As a result, the solution is to work closely with LOCA chemists and be certain that there are both the right type and exact amounts within very tight tolerance of all functional ingredients.

The polymer chemists at HumiSeal are experts in collaborating with LCD assemblers, providing custom or individualized solutions as required. We offer a complete solution through our outstanding manufacturing and technical support, resulting in the reliable global supply and unmatched quality required in the ever-evolving display assembly industry.

Please contact us today to discuss your application.