Optically Clear Adhesive (OCA)
Structure And Function of OCA
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A type of Pressure Sensitive Adhesive (PSA).
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Deposited onto touch screens by Wet-Bonding (as a liquid then cured using UV light) or by Dry-Bonding (as a film).
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Holds all the materials of the touch screen together.
Dry Bonding
(+ UV Curing)
Wet Bonding
Source: http://elighttech.com/service/2
What Happens During UV Curing
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Adhere to substrates via intermolecular forces of attraction.
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Crosslinking of Polymers: Forms polymer matrix, improves structural integrity + Durability
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Adhesive strength is enhanced by:
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Close distance between substrate and adhesive.
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Liquid OCA can seep into microscopic surface irregularities to increase extent of intermolecular forces.
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E.g. Hydrogen Bonding
Attractive forces between molecules: Instantaneous dipole-induced dipole interactions between otherwise non-polar molecules, permanent dipole-permanent dipole interactions between polar molecules, and hydrogen bonding with specific types of molecules.
More about intermolecular forces: https://byjus.com/chemistry/different-types-of-intermolecular-forces/
Crosslinking: Binding two or more molecules together via covalent bonding or mechanical crosslinking where polymers cross over one another.
Other Notable Properties of OCA
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Shock Absorption
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Achieved by dissipating energy evenly.
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Molecules are mobile and shift to redistribute the impact.
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Prevents the concentration of impact.
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Red Lines/Region: Example of energy dissipating from concentrated area
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Improving Touch Sensitivity
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OCA application eliminates air gaps in the layers.
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OCA is a better transmitter of electrical signals.
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Improves signal detection.
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Improving Touch Accuracy (by user)
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OCA has a more similar refractive index to LCDs and glass, compared to air.
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Allows reduced partial reflection internally and hence better transmission of light.
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More light reaches the eyes, increases visibility of content.
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Light travelling from air (white portion) to glass (purple portion), faint red line = partial reflection
Light travelling from OCA (light purple) to glass (darker purple portion), faint red line = partial reflection
Animations Created With: https://phet.colorado.edu/sims/html/bending-light/latest/bending-light_all.html
Why OCAs Might Weaken Over Time
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Prolonged Exposure To UV Radiation:
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UV radiation cleaves bonds within the adhesive.
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This decreases the structural integrity of the material as it deforms.
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Adhesive and cohesive properties decrease.
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Extremely High Temperatures:
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Causes OCA to melt and become too viscous, losing its structural integrity.
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Lack of structural integrity.
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Crosslinking density ↓ = More "liquid-like"
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The material can no longer hold itself well enough → Cannot adhere to substrates as well.
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Extremely Low Temperature:
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Low mobility of molecules = Cannot shift to adapt to stress.
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Becomes very brittle: ↓ Durability, ↓ Cohesiveness, ↓ Adhesiveness.
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High Humidity:
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Excess moisture can also cause hydrolysis of crosslinks.
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Causes a lack of structural integrity, and a decrease in cohesiveness properties.
Hydrolysis: Break down of chemical bonds by the addition of water, to form simpler substances. https://byjus.com/chemistry/hydrolysis/#:~:text=Hydrolysis%20is%20a%20common%20form,word%20break%20or%20to%20unbind.