By Donald J. Davis, technical services, business development, Kent Displays, Inc.
Boogie Board® Writing Tablets are pressure-sensitive writing surfaces used for doodling, note-taking, lists and drawing with a pen on paper feel and electronic erase. These electronic writing tablets reflect ambient light back to the user and are bi-stable so that energy from the battery is used only to erase the displayed writing. The tablets use a liquid crystal and UV-cured polymer matrix between thin plastic films coated with transparent conductors and have been mass-produced using cleanroom-based roll-to-roll (R2R) manufacturing processes for over 10 years. Now the R2R process expertise behind these writing tablets is being offered as a technical service for other technologies, converting needs, and manufacturing scale-up.
Introduction
Roll-to-roll manufacturing covers a variety of processing techniques such as printing, coating, cutting, scoring, and lamination which continuously handle flexible substrates (e.g., plastic film, paper, metal foil). The thickness of the substrate is very small compared to its width and length so the material can be wound into a roll and processed in mass on long webs.
In contrast to other manufacturing methods, such as sheet-to-sheet processing, R2R offers significant advantages in the manufacture of products at high production rates in large volumes, resulting in lower costs because material handling between processes is eliminated due to the substrate providing the means of conveyance. In addition, the continuous nature of roll-to-roll results in a short transfer time between sequential processes. This firm has applied R2R manufacturing techniques to the production of consumer electronics, dramatically reducing manufacturing costs and enabling new products such as writing tablets.
The premier flat-panel writing tablet
In the flat-panel display industry, one of the first flexible displays to be roll-to-roll manufactured and commercially available in mass volume were this firm’s non-traditional writing tablets [1] (see Figure 1). This unusual, pioneering device is pressure-sensitive with electronic erase and marketed as a replacement for paper for note-taking, doodling and sketches. An image is written on the tablet when the user applies pressure to the top surface with either a stylus or a fingertip. The pressure creates a bright line on a dark background which can be erased with a simple voltage pulse. It is flexible, made with rugged plastic substrates, has an individual handwriting experience, and is low cost with good writing response via sharp written lines. The tablet fills the need for a lightweight, relatively inexpensive electronic device that offers a natural writing experience with the feel of paper for handwritten and drawn images on a reusable medium that also is environmentally conscious in a technologically driven world.
The appeal of this firm’s writing tablet is the natural paper-like writing experience. The writing is analog with no pixels to distort the image. The tablet has seen tremendous adoption due to the similarity with paper, ease of use, and simple yet fun operation. It quickly became popular and successful in the consumer market in simple applications such as note-taking, lists, and literally anywhere a scratch pad was used. Our tablets are a perfect fit for roll-to-roll manufacturing due to the simple structure of two flexible substrates, a liquid crystal/polymer dispersion and light absorbing backcoat. R2R processing allows for low-cost manufacturing and high volumes due to reduced substrate and materials handling and the continuous web transport of devices at a rapid rate.
Fundamental science
Cholesteric Liquid Crystal (ChLC) technology has inherent reflective color and grayscale, and requires no power to maintain a static image [2,3]. This simplifies the device structure and enables a reflective, flexible, low-power device possible which can be manufactured roll-to-roll. ChLC material is a natural reflector due to its inherent chiral structure and is bi-stable when alignment is designed appropriately and can be switched between two stable states: a reflective state and a slightly scattering transmissive state. Being bi-stable, ChLC devices require power only when being switched from one state to another. The ChLC material in our writing tablets uses pressure to write lines to the reflective texture and a small voltage pulse to switch or erase to the transmissive texture. More details of the fundamental ChLC technology can be found in the literature [4].
FIGURE 1. Photograph of a flexible writing tablet (left); photograph of this firm’s writing-tablet product that allows electronic erasing of handwritten notes
Our tablets consist of two flexible polyethylene terephthalate (PET) film substrates, each flood coated with PEDOT/PSS (poly (3,4) ethylenedioxythiophene/polystyrenesulfonic acid) as a conductor a light-cured dispersion of ChLC and polymer between the substrates, and spacer beads to maintain the cell gap. A cross section of a typical writing tablet is shown in Figure 2.
Spacer
Backcoat
FIGURE 2. Cross-section of this firm’s writing tablet. The polymer forms pillars that support the substrates but do not encapsulate the ChLC.
The back of the tablet can be flood coated with a specific opaque, absorbing color (often black) or a semi-transparent color or a patterned image [5]. The tablet’s electro-optic layer contains ChLC material and a polymer structure to create written line acuity. Figure 3 shows SEM micrographs of the polymer pillar structure confirming that the ChLC is not encapsulated by the polymer but is allowed to flow around the polymer making the device rewritable without destruction [6]. The pressure induces flow in the ChLC, resulting in a written bright line on a darker background.
Our writing tablet is very touch-sensitive, and different line widths can be created on its surface by the user varying their writing pressure and speed. The thickness and reflectivity of the written line increases with pressure. In addition, the linewidth and pressure sensitivity of the tablet can be tuned by varying materials during the manufacturing process as shown in Figure 4. In our product family, there are different designed line widths, depending on the market need for the particular device; for example, a narrow line for products for detailed drawing applications or a wider, brighter line for wall-mounted applications.
FIGURE 3. SEM micrographs of the polymer structure formed within the writing tablet. The spacers show up as solid white circles in the low magnification image on the left. In the higher-magnification image on the right, a channel structure for the liquid crystal is evident (the black regions).
FIGURE 4. Tablet line widths for different devices at the same pressure and speed. Notice that each data point shows the photograph of a line drawn on the tablet that has the line width shown for that data point. This measurement is made with a written line speed of 75 mm/sec and a 150 gF applied to the stylus.
Roll-to-roll writing-tablet manufacturing
The writing tablets are made on three separate R2R-manufacturing lines in Kent, OH. The process begins with unwinding rolls of PET film coated with PEDOT followed by the necessary process steps to effectively convert the coated film into finished tablets at the end of the manufacturing line.
The production lines do not produce wastewater or chemicals requiring treatment and use a closed-loop system for process cooling. Environmentally friendly UV-curable materials and processes are used throughout manufacturing. UV curing lowers air pollution, as well as the energy required to treat exhausts by dramatically reducing solvent-vapor production compared to more conventional thermal curing processes [7].
After coating of the ChLC-material monomer and UV curing, the device is laser-singulated from the web of films [8]. Laser singulation permits the manufacture of devices of non-conventional shapes. By carefully controlling the cutting parameters, the laser simultaneously cuts through the flood-coated conductor and substrates while welding the top and bottom substrates together without electrically shorting the conductors together from the top and bottom substrates. The laser welds add sufficient strength to the edges of the device such that it does not delaminate and can be considered sealed. The laser is fed the cut parameters from a CAD file and thus retooling to cut a different shape is not necessary.
A broad range of technical services are required for the manufacture of electronic writing tablets – from flexible-film component design and development, knowledge of the global supply chain for coated plastic films, and assets with advanced manufacturing processes such as cleanroom-based, high-volume laser and rotary diecutting of parts, coating, flexographic and screen printing of decorative and functional materials, lamination, and corona surface treatment. This R2R-manufacturing expertise is being applied to other flexible-electronics devices, such as organic solar cells, flexible batteries, and other flexible electro-optic devices, including switchable light-control products, goggles, sunroofs and window films.
Conclusion
The convergence of innovations in many technical fields were required to achieve this firm’s writing tablets, including production engineering, materials science, liquid crystal display (LCD) technology, process engineering, and product development. The roll-to-roll manufacturing processes developed here are mature and allow writing tablets and other flexible devices to be built in a significantly automated process with high throughput and volume at low cost.
References
1. E. Montbach and D. Davis, “Roll-to-Roll Manufacturing of Flexible Displays,” in Jehuda Greener, Glen Pearson, and Miko Cakmak (Ed.), Roll-to-Roll Manufacturing: Process Elements and Recent Advances, (pgs. 285-324), New York: John Wiley & Sons, (2018).
2. D.-K. Yang, J. W. Doane, “Cholesteric Liquid Crystal/Polymer Gel Dispersion: Reflective Display Application,” SID Intl. Symp. Digest Tech. Papers, 23, 759 (1992).
3. D.-K. Yang, J. L. West, L.-C. Chien, J. W. Doane, “Control of reflectivity and bistability in displays using cholesteric liquid crystals,” J. Appl. Phys., 76 (2), 1331 (1994).
4. J. William Doane and Asad Khan, “Cholesteric Liquid Crystals for Flexible Displays,” In Gregory P. Crawford (Ed.), Flexible Flat Panel Displays, (pgs. 331-352), West Sussex, England: John Wiley & Sons, Ltd., (2005).
5. E. Montbach, J. Krinock, J. Chiu, C. Braganza, M. Schlemmer, O. Pishnyak, D. Marhefka, A. Khan, and J. W. Doane “Patterned Image Flexible Reflex Displays,” SID Symposium Digest, 43, (2012).
6. T. Schneider, E. Montbach, D. Davis, S. Franklin, D. McDaniel, M. Lightfoot, N. Venkataraman, F. Nicholson, A. Khan, and J. W. Doane, “UV-Cured Flexible Cholesteric Liquid Crystal Displays,” Radtech UV&EB Curing Technology, Technical Conference Proceedings May 4-7, (2008).
7. Green, E. Montbach, N. Miller, D. Davis, A. Khan, T. Schneider, J. W. Doane, “Energy-Efficient Flexible Reflex™ Displays,” Proc. Intl. Disp. Research Conf., 28, 55 (2008).
8. T. Schneider, “Method of simultaneous singulation and edge sealing of plastic displays,” US Patent 11/756,987, June 1, 2007.
Donald J. Davis, technical services, business development at Kent Displays, Inc. (Kent, OH), holds a Bachelor of Arts in Physics from Thiel College and a Master of Science in Physics from Oklahoma State University. He has over 30 years of experience in advanced manufacturing and product development and was instrumental in developing the roll-to-roll manufacturing process for this firm’s writing tablets. Don is currently leading business-development efforts for contract manufacturing of R2R products on Kent Displays Technical Services team. He can be contacted at 814-573-8896, ddavis@kentdisplays.com, www.kentdisplays.com.
Comments