CN
Materials Science – Optical Adhesives and Their Aging
Selecting the right optical adhesive is one of the most critical decisions in the design of a bonded HMI. The adhesive is far more than just a “filler” between the display and the cover glass. It is a highly specialized functional material that must simultaneously meet optical, mechanical, and chemical requirements. Its properties determine not only the initial performance but, above all, the long-term stability of the entire system over many years under harsh conditions.
Material Classes: OCA and OCR and Their Applications
Optical adhesives can generally be divided into two main categories, which differ fundamentally in their physical form and processing method.
OCR (Optical Clear Resin)
Optical Clear Resin (OCR) is a liquid, optically clear encapsulation or bonding material used in display and glass lamination. OCR is applied between two optical substrates (e.g., display and cover glass or touch sensor) and then cured. In liquid form, it is precisely dispensed between the layers to be bonded. Its low viscosity allows it to completely fill even the smallest irregularities and surface structures.
Advantages of OCR
- High optical quality: Low light scattering, high transmission, and good color stability.
- Adaptability to complex geometries: Suitable for large, curved, or uneven surfaces.
- Structural bonding: After curing, a strong, permanent bond forms between the optical layers.
- Possibility of “re-workability” – repair of displays during production and in the field
OCR is ideally suited for
- Applications with increased requirements for mechanical stability, shock and impact resistance
- Optical systems where conventional OCA films reach their limits
- Displays of all sizes and shapes in combination with any combination of cover glass and touch
OCA (Optical Clear Adhesive)
OCA is a pre-cured, high-viscosity adhesive film that is typically supplied in rolls or sheets of a specified thickness (usually 50 to 250 µm) and is covered with protective liners on both sides.
Advantages of OCA:
- Easier and faster processing
- High throughput rates
- Also available in very thin layers
OCA is ideal for:
- Flat displays
- Medium-sized displays
- Lamination of film components
Challenges:
- High requirements for surface quality
- Risk of silvering on uneven surfaces
LOCA (Liquid Optical Clear Adhesive)
LOCA is a liquid adhesive that is typically cured by UV radiation. In liquid form, it is precisely dispensed between the layers to be bonded. Its low viscosity allows it to completely fill even the smallest irregularities and surface structures.
This makes LOCA particularly suitable for:
- textured surfaces
- large displays
- curved geometries
However, LOCA requires a complex process chain involving:
- dispensing systems
- vacuum lamination to prevent air bubbles and air pockets
- UV curing systems
Reworkability is severely limited after complete curing.
Overview:
- OCA (Optical Clear Adhesive): A high-viscosity, pre-cured adhesive film designed primarily for laminating films.
- OCR (Optical Clear Resin): A liquid optical adhesive designed primarily for system stability, reworkability, and high durability
The choice between OCA and OCR depends heavily on the specific application. In both cases, the bonding technique is central to the process, as only a cleanly executed bond ensures a permanently optically and mechanically stable connection, as well as long-term stability.
Chemical Basis: Silicone vs. Acrylate – A Fundamental Difference
Regardless of whether OCA or OCR is used, optical adhesives are chemically based on either silicone polymers or acrylate polymers.
Acrylate-based adhesives
Acrylate-based adhesives are widely used in applications with reduced durability requirements.
They offer:
- very good optical clarity
- transmission values above 99%
- very low haze (< 0.5%)
- high adhesion to glass and ITO surfaces
- lower costs
Disadvantages:
- limited temperature resistance
- risk of yellowing under UV radiation or at high temperatures
- tendency to become brittle at high temperatures, leading to air and water entrapment
Silicone-based adhesives
Silicone-based adhesives are the premium choice for heavy-duty applications.
In addition to the advantages of acrylate-based adhesives, they offer:
- extreme resistance to UV radiation and thus virtually no yellowing
- wide operating temperature range
- permanent elasticity
- excellent absorption of thermomechanical stresses
These properties make silicone the only safe choice for applications where reliability under extreme conditions is non-negotiable.
Aging Mechanisms and Failure Modes
The four main stress factors for optical adhesives are:
- UV radiation
- high temperatures
- moisture
- mechanical fatigue
Typical failure modes:
Delamination:
This is the separation of the adhesive from one of the surfaces (glass, film, or display). The main cause is a loss of adhesion, often due to hydrolysis (chemical breakdown of the adhesive bonds by water) at the interfaces. Inadequate surface preparation, contamination, or moisture ingress due to faulty edge sealing are the most common triggers.
Yellowing:
This is a yellowish discoloration of the adhesive caused by the degradation of the polymer due to UV radiation or high temperatures. It reduces light transmission and distorts color reproduction. This effect is particularly pronounced in acrylate-based adhesives.
Bubbling:
The formation of bubbles in the adhesive can be caused by insufficient vacuum degassing during the lamination process or by outgassing of volatile components at high temperatures. Bubbles act as lenses and lead to optical distortions and, subsequently, to delamination.
Silvering:
These are silvery-appearing areas that typically occur at the edges of displays. They are caused by microscopic air pockets when the adhesive does not completely fill the surface roughness of the glass. Insufficient adhesion during lamination or excessively rough surfaces are the main causes.
Haze:
An increase in haze can result from incompatibilities between the adhesive and other materials, from moisture absorption, or from polymer degradation. It reduces contrast and leads to a milky appearance.
Shrinkage of the bonded surface:
The shrinkage of optical adhesives during curing or after curing over the course of the product’s lifecycle can generate mechanical stresses in the bonded assembly due to insufficient flexibility of the adhesive. These stresses can strain the adhesion, cohesion, or substrates, thereby initiating potential failure mechanisms.
The selection of a high-quality adhesive qualified for the application and a precisely controlled manufacturing process are crucial to avoiding these failure modes.
Note on the technical information in this chapter
The technical values, test profiles, and performance data stated in this chapter are provided for technical classification purposes. Actual results depend on the specific display configuration, material system, bonding process, application profile, and scope of qualification. Product-specific values are validated on a project basis using data sheets, measurements, test reports, and customer specifications.