Print PageE-readers, portable reading devices using electrophoretic displays, are becoming more and more popular. Their reliable functioning and service life depend on the display being protected against temperature and humidity by suitable adhesive products.
Some market studies predict the e-reader and e-book market to grow to 48 million units by 2018.
Conventional LCD flat screens are not suitable for such applications in most cases as they not only consume a great deal of electricity which allows only short battery life. Also strong lighting such as direct sunlight affects LCD readability to a great extent. Therefore, ereaders use electrophoretic displays. These displays use "electronic ink," or differently charged colored spheres in the electric field which migrate towards the display surface or away from it. In doing so, they generate light or dark pixels (see illustration 2). This technology works without backlight and gives a sense of reading just like with paper.
Furthermore, the device consumes electricity only when new pages are being loaded, for example when turning the page, and readability is excellent even in direct sunlight.
Illustration 2: Detailed view of an e-paper laminate
Adhesive protects against temperature and humidity
Electrophoretic displays can be used not just in e-readers, but also in many other electronic appliances, such as automatic price displays in supermarkets (electronic shelf labels), mobile phones or smart cards. While the total number of displays produced in 2008 was only 18 million, the market is predicted to reach 681 million units by 2018. Various tests have already been carried out to determine the factors impacting the functionality of the displays. One essential factor for long service life is the appropriate sealant, as e-paper laminates react sensitively to high or low temperatures and humidity. Therefore, both sides of the "electronic ink" layer must be protected by barrier films. Moreover, additional edge sealing is necessary. As shown in illustration 3, an adhesive bead is applied along the rim of the display. The adhesive's essential properties are good adhesion to the materials used and low water vapor permeation - also called WVTR value (WVTR = water vapor
transmission rate). DELO has developed special light-curing epoxy resins for this purpose, based on experience gained in the production of encapsulation compounds for the edge sealing of organic light-emitting diodes (OLEDs).
Illustration 3: Edge encapsulation of an electrophoretic display with adhesive (blue). The DELO adhesive can be cured through the UV-blocked barrier layer (glass, PET, PEN, etc.) by means of visible light.
Illustration 3 shows a section through the display. The adhesive is deposited between a nontransparent substrate and a UV-blocked cover glass or film. Therefore, curing with UV light is impossible. Accordingly, the products developed by DELO are not cured with UV light, as is usual for cationic-curing epoxy resins, but with visible light. These special light-curing adhesives bring numerous advantages in production and
processing:
- Unlike with heat-curing products, fast in-line curing is possible.
- The adhesive can be cured through PEN or UV-blocked PET or glass. This can be done using an area lamp or a spot light source that moves along the rim.
- The adhesive has a long storage life (6 months at 5°C) and a convenient processing times of several weeks.
- The material is not considered hazardous and thus does not require complicated and expensive refrigerated transport.
- Again unlike heat-curing products, the products can be filled in economical large containers as production quantities and thus the adhesive consumption rise. (Due to their high reactivity, heat-curing products can be filled in small containers only).
Adhesives for flexible displays
Electrophoretic displays are not necessarily flat and rigid. In principle, they can also be bent or even flexible, depending on the application. In these cases, the substrate and protective cover are made of flexible alternatives like plastic or metal films, unlike the glass usually used at present. A major advantage of the new adhesive is that it can also be flexibilized for these applications.
"Flexible" can be defined in different ways. For some applications, it is sufficient that the fragile glass cover is replaced by an unbreakable plastic film to make the display more rugged. In this case, flexibility is less important than a reduction of the risk of breakage, for example with mobile phones.
Applications which go one step further are displays applied to bent surfaces, for example in the automotive sector. This allows more freedom of design. Portable displays are also conceivable, for example displays for military use that might worn around the forearm or otherwise conform to different shapes. Finally, rollable or foldable displays, as might be used in future mobile phones, will require even more flexibility.
However, high flexibility has a negative effect on water vapor transmission, as the polymer network of a flexible product is not as close-meshed as that of a rigid one. Therefore, it is important to find the right adhesive as well as the optimum combination of flexibility and WVTR. Chart 1 lists some newly developed products suitable for different application areas.
From left to right, the flexibility and thus the WVTR value increase. The rightmost products show an exceptionally high flexibility for epoxy resins (see illustration 4).
Chart 1: Selection of different development products
As shown in the chart, viscosities can be tuned from medium to low. If required, viscosities down to 1000 mPas are possible. This is necessary if the adhesive is applied by capillating into the gap between substrate and cover glass/film. For this purpose, the dispensing needle is moved very close to the rim of the cover glass or protective film, so that the adhesive is sucked into the gap.
Illustration 4: Example for the flexibility of a cured adhesive film with a thickness of 150μm (no substrate, just adhesive).
The peeling resistance increases with increasing flexibility of the adhesive, as expected. However, peeling resistance is difficult to quantify because the bonding surface is seldom the substrate material like glass or PET, but rather the barrier layer applied to it. There are multiple suppliers of barrier films, and standards are unfortunately not yet available. Generally, it can be said that the new adhesives achieve very good and clearly higher strengths on many of the barrier films used, as compared to pure PET or PEN. In peel tests, the films are often torn apart before the bonding fails.
New application areas
The new adhesives can be used not only for edge sealing, but can also be laminated onto the total area. Various adhesives have already been successfully tested in roll-to-roll systems (R2R). This opens up further application areas in flexible electronics, such as for sealing flexible organic solar cells or flexible OLEDs. DELO's new flexible encapsulation materials make a substantive contribution to the development of such future technologies.
About the author:
Dr. Daniel Lenssen studied physics at the RWTH Aachen and did his doctor's degree in the field of semiconductor physics at the Research Centre Jülich. Before he started his work for DELO as a Regional Sales Manager, he had been employed as a Development Engineer by EPCOS. Today, Dr. Daniel Lenssen is the Business Development Manager Display Application at DELO. (E-Mail: 
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)Author: Dr. Daniel Lenssen, Business Development Manager
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DELO Industrial Adhesives.DELO Industrial Adhesives
is exhibiting at
Printed Electronics & Photovoltaics Europe 2010
Dresden, Germany
13 - 14 Apr 2010
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