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Philips Research
Themadag Mikrocentrum: “Verbinden van Kunststoffen”
Optical adhesive bonding
Ir. Esther Janssen
24 april 2012
2 Esther Janssen, April 24, 2012
Contents
• Function of optical bonding • Basics in optics • Optically transparent adhesives • Important properties • Optical bonding technologies • Challenges • Optical bonding applications • Summary
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Function of optical bonding
• Optically transparent adhesives are used for the following reasons:
– Extract, transmit or redirect light – Enhance brightness – Reduce glare effects and mirror images – Mechanically join optical components – Bond dissimilar materials, e.g. plastic to glass – Minimize thermal stress by matching the difference in expansion of
2 bonded materials
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Basics in optics: Light passing an interface • When light is travelling through an
interface between 2 optical materials, part of the light gets reflected and part of the light changes its direction as it enters the second medium: it is refracted
• This is caused by the difference in refractive index of the 2 materials. The larger the difference, the more reflection.
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Basics in optics: Refraction of light
• The speed at which light travels is dependent upon the density of the materials it is traveling through.
• Refractive index is defined as the relative speed (v) at which light moves through a material with respect to its speed in vacuum (c)
– n = c/v
• Snell's law of refraction: • n1 sin (θi) = n2 sin (θr)
θi
θr
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Basics in optics: Reflection of light
• Reflectivity of a surface also depends on the refractive index, as described by the Fresnel equations.
• The incident angle of light is the same as the angle of reflected light: – θincidence = θreflection
• The fraction of the intensity of perpendicular incident light that is reflected from the interface is given by reflection coefficient R.
•
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Basics in optics: Total internal reflection
• If the incident angle increases past a specific value (Brewster's angle), light is no more refracted into the medium of lower refractive index. The light is captured, which is called total internal reflection.
– sin(θc) = n1/n2
θc
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Basics in optics: Reflection loss
• So light is lost due to reflection of light at interfaces between materials with different refractive indices.
• The reflection coefficient for glass in air is for example about 4.1% • Optically transparent adhesives are used to match the refractive indices
of the bonded materials and thereby increasing brightness and contrast. • Refractive index of these adhesives range between 1.3 and 1.6.
� Air 1.00 Water 1.33 Silicone 1.4 - 1.6 Glass (typ.) 1.52 PMMA 1.49 PC 1.61 Diamond 2.42
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Optically transparent adhesives
• The following types of materials are available as optically transparent adhesive:
– Silicone – Epoxy – Acrylic – Polyurethane – Cyanoacrylate
• Liquid or tapes • Thermal, moisture or UV-curing
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Optically Clear Adhesive tapes (OCA)
• For bonding touch panels or ITO film to displays • Pressure-sensitive, acrylic adhesive • Typically 25-125 micron of adhesive thickness • Roll-lamination • 3M, Nitto Denko, …
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UV curable optical adhesives
• Fast bonding or encapsulation of optical components, like fiber optics, lenses, mirrors, prisms
• Can be cured in seconds • Acrylics, epoxies, thiolenes, • Epoxy Technology, Dymax, Delo, Henkel,
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Optical grade silicones
• Silicones are very attractive optical grade materials – High and stable optical transparency – Resistant to very high and low temperatures – Resistant to high flux (UV-)light – Properties can be engineered
• Transparent to opaque • High and low viscosity • Refractive index • Soft to hard
• Lenses and encapsulants for LED devices • Dow Corning, Wacker, Momentive,
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Important properties
• Absorption spectrum • Refractive index • Adhesion • Elasticity modulus • Resistance to
– UV – Temperature (high, low, shock) – Moisture – Vibration – Chemicals
• Outgassing
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Optical bonding technologies
• Dispensing • Screen printing • Casting • Lamination
– Vacuum – Roll
• Molding
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Challenges
• Find adhesive with optimum properties for performance, processing and cost
• Realize and maintain the optical performance • No air bubbles • Lifetime due to degradation of the adhesive joint
– Due to UV, X-ray, temperature, moisture,.. • Controlling bond line thickness • Adhesion to low energy surfaces • Dimensional stability due to curing shrinkage
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Optical bonding applications
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LEDs
Displays Photovoltaic energy panels
X-ray detector
Applications
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Displays
• Bonding a cover plate to an LCD panel improves optical performance and ruggedness:
– Improved view ability in bright ambient conditions – Decreased temperature of display – More resistant to Shock and vibration – Prevents condensation
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• Silicones are used as optical interface materials in LED-packages, because of their very high light transmission and resistance to high temperatures and UV-light.
• A phosphor plate is bonded to the LED-chip with a silicone adhesive • A silicone lens is molded on top of this LED-chip plus phosphor plate.
LEDs
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X-ray detector
• Computed Tomography (CT) is a medical imaging technology which converts X-rays into visible light and thereby provides a cross sectional image of a patient’s body.
• Optical adhesives are commonly used in the X-ray detectors to optically bond the scintillator to the detector.
Scintillator Optical adhesive Photo detector
X-ray
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Photovoltaic energy panels
• Optical bonding in solar panels is used to bond the solar cells to the front glass sheet.
• Ethyl Vinyl Acetate is often used as optical adhesive. It is applied as film and laminated and thermally cured in vacuum.
• A solar panel consists of: – Tempered glass – Encapsulant (EVA) – Interconnected solar cells – Encapsulant (EVA) – Rear-side foil
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Summary
• Optical transparent adhesives are being used to make an optical joint between two or more optical components.
• It prevents loss of light due to reflections. • Optical transparent adhesives are available in different chemistries and
forms. • Several processes can be used to create an optical bond • Transmission and refractive index are important adhesive properties • Optical bonding is used in displays, LEDs, solar panels and X-ray
detectors.
23 Esther Janssen, April 24, 2012