Due to better refractive index matching between optical film and OCA, optically clear adhesive, compared to only air gap, OCA dramatically enhances display performances; there it have been broadly applied in the displays, solar cells, automobiles, buildings, et. al. In addition to provide suitable adhesion, it is also required to be optically transparent, colorless, weathering/aging resistant (no yellowing, no bubbles), and sometimes it requires to able to sustain high temperature, dissipate stress and more as required by specific applications.
The focus of this post is to introduce the general rules for design OCA PSA films, not liquid OCA. A typical OCA film includes three layers: an OCA transfer film sandwiched with two release liners. OCA is coated on one release liner; after dried, another release liner with different release force is laminated on OCA stack.
OCA formulation design can be shown as in the following figure.
monomer selection
Monomers are selected based on the following considerations:
- Adhesion to adherent
- Polar groups, e.g. acid, enhance the adhesion to high-surface-energy substrate.
- Low-surface-energy monomers, e.g. IBOA, are good for low surface energy adherent
- Glass Transition, Tg
- Composition and molecular weight, cross-linking density, etc determines the Tg value
- Refractive Index
- Typical acrylics have RI of 1.46-1.48
- Benzene ring containing monomers have higher RI of over 1.5 as shown in the figures below
- Functionality
- Amount of functional groups
- Type: -COOH, -OH, Epoxy, etc.
Lorentz-Lorenz equation is a good tool to estimate OCA’s refractive index in the design stage.
Initiator
Initiators include thermal based, AIBN, BPO, and their derivatives; and photo-initiators
Solvents
Solvents are selected based on the following concerns
- Toxicity: ethyl acetate is environmentally friendly than Toluene
- Chain transfer to modify molecular weight: Ketone contains active hydrogen, leading to chain transfer and reduce MW.
- Safety. Needs special attention on the dealing with low boiling point solvents
stabilizers
Stabilizers can be divided into two groups: thermal stabilizers, or antioxidants, and UV stablizers.
Their stabilizing mechanism can be shown schematically as
