3 Electrophoretic displays (EPD)
In the past few decades, much attention has been paid to the EPDs over ordinary paper due to their low cost, low weight, low power consumption and safety. EPDs are reflective displays that act based on the migration of charged suspension particles in the dielectric fluid towards the oppositely charged electrode and this is known as electrophoresis [20,25,26] (Fig. 4). Recently, many displays have entered the market through companies like Amazon Kindle, Hanvon and OED Technologies. Two major companies in this field are SiPix and E-Ink which have been already merged, but these two technologies are different. The SiPix technology consists of microcaps of plastic electrophoretic display, which is very thin, lightweight and produced by the roll-to-roll process (Fig. 5) [27]. The properties of the electrophoretic display and electronic ink are explained in detail in the following.
3.1 Electrophoretic displays (EPD) function
The so-called electrophoresis principle refers to the movement of suspended charged particles in a suspension fluid under the influence of a DC electric field. Whenever the electric field between the electrodes is used in a cell, the particles migrate in relation to the electrical charge and the suspension fluid remains stable [20,28,29]. Therefore, electrophoretic particles are one of the main components of EPDs. Generally, a spherical particle, with a charge ‘q’, under an electric field ‘E’ and suspended in an electrophoretic liquid, is under the influence of four forces: the electric, buoyancy, gravity and retarding viscous forces, as it moves between the bivalent electrode and the opposite pole [30]. The Helmholtz-Smoluchowski Equation [3] (Eq. (1)) is employed to describe the electrophoretic velocity (U) of a charged particle. In this equation, the terms ε, ξEP, Ex and μ are the dielectric constant of the liquid, the zeta potential of the particle, the applied electrical field and the mobility of the particle, respectively. The electrophoretic zeta potential (ξEP) is a characteristic of the charged particle. Electrophoresis leads to the movement of charged particles through a stationary solution. Various parameters including the viscosity of the transport medium and its dielectric behaviour, the size and charge density of the black and white particles, the microcapsule shell thickness and its dielectric level can affect the function and performances of the EPD. One way to make the particles instable in the liquid medium is to compensate for the gravity between the dispersion solvent and particles, and as a result, reduce the sedimentation [31].
In general, EPDs containing colour suspensions or dispersed charged particles in a dielectric medium create contrasting colours in a cell with two conductive, transparent and parallel electrodes that have been placed at a specified distance of about a micron.
Since 1960, the EPDs (EPDs) have been developed as a type of reflective display. Their images can be repeatedly written or erased electrically. This technology has numerous advantages such as wide viewing angle and high contrast ratios that are similar to the printed papers. The EPD is the first and basic choice to make electronic papers. However, the ability to ensure image quality and longevity of particle clustering, agglomeration and aggregation are some of serious problems that limit its applications in the industry.
3.2 Effective parameters in the image quality of the EPD display with E Ink
The properties of electrophoretic particles are key to determination of the image quality. The enhanced image quality requires very small particle size with a narrow size distribution, large surface charge to accurately create and control the images, high contrast ratio, swift response to the applied voltage, transparency used in the shell, light stability and stable dispersion of ink and other parameters. Consequently, several researchers have explored the effect of modified particles, surface morphology, surface charges and special stability [32–34]. Thus, for characterization of E Ink microcapsules, various instrumental techniques including ultraviolet–visible spectroscopy (UV–Vis), Optical Image Microscopy, Fourier Transformed Infrared Spectroscopy (FTIR), Scanning Electron Microscope (SEM), Zeta Potential, Dynamic Light Scattering (DLS) and electrophoretic cell were used [34–41].
As mentioned earlier, the spatial stability of electrophoretic particles is a key factor in determining image quality, which is specified from the measurement of the zeta potential. In fact, zeta potential is a factor for potential stability of colloidal systems. If all the particles in suspension have a positive or negative charge, the particles tend to repel each other and show no tendency to integrate. The tendency of particles with similar charge to repel each other is directly related to the zeta potential. In general, stable and unstable border of the suspension can be determined by zeta potential. Suspensions containing particles with zeta potential greater than 30 mV or less than −30 mV are considered to be stable [42].
Also, coloured displays can be prepared using coloured dyes or organic pigments as coloured electrophoretic nanoparticles. Dye or pigment in electronic ink should have good brilliance, colour strength and excellent performance with light, heat and solvent resistance which can offer great potential to be proposed for wider range of applications [43–45]. Good electronic ink in EPDs can achieve long-term suspension stability and higher surface charge in electrophoretic suspension [37,46,47]. Some of the nanoparticles were even modified by some modifiers such as polyethylene [34,46,48,49] and octadecylamine [32,50,51] in EPDs application. For accurate control of the image and rapid response to the applied electric field, the particles should have a high surface charge such that, the mobility is within the range of 10-5–10-6 cm2/Vs, the density difference with the solvent is less than 0.5 g/cm3 and the appropriate diameter is about 190–500 nm [30,52].
3.3 Electronic ink (E Ink) or electrophoretic ink
E Ink is a direct result of the integration of chemistry, physics and electronics. The composition of E Ink for EPD contains electrophoresis particles such as charged coloured material or microcapsules that dispersed in a dielectric environment and charge control agent [22–24]. Based on the device and the aforementioned working principle, the important materials of this technology include the coloured particles (dyes/pigments), the microcapsule shell, the insulation oil and the charge control agents and stabilizers. The following sections explains each one of those components.
3.3.1 Dyes/pigments as coloured particles for core
As mentioned earlier, the coloured particles of the nano to micro-meter size are the key materials to evaluate the functions of electrophoretic. The pigments are needed to fulfil several requirements; decrease the amount of sedimentation, the density must be specifically compatible with the suspension solvent, the solubility in the solvent must be low enough, the brightness must be high so that ensure the effective optical performance, the surface must be capable of being charged easily, ensuring the mass production requires the pigments to be properly stable and also capable of being purified easily. The absorption of particles on the capsule surface or in the pixel must be avoided in case of their encapsulation into microcapsules or pixels. Materials of various types have been investigated for EPD applications [9,53–61]. TiO2 [38,62], carbon black [41], SiO2 [63], Al2O3 [58], yellow pigment [34,64], red pigment [32,65], ironic red and magnesium purple are the inorganic materials which have attracted much attention in research. Toluidine reds, phthalocyanine blue [66–69] and phthalocyanine green [51,70] have also been investigated as organic particles. In general, the dyes/pigments of nanometre size are dispersed in a solution in the original states, followed by coating with polymeric materials to form a core-shell structure. Materials with alkoxy group, acetyl group or halogens are typical long chain organic materials suitable as shell materials due to their hydrogen bonds. Availability in nature as well as high brightness are the reasons why EPD devices have long been manufactured by black and white particles made of black carbon and titanium dioxide respectively. Since both these materials are conductive, the desired requirements are achieved through coating polymers on them [71].
In the image quality due to contrast, the properties of white pigment are very important. Mostly, researchers used TiO2 as a common white pigment because of its whiteness and excellent optical and reflection properties. The most important problem with this pigment is its instability in the suspension due to its high density. In the past decade, researchers have tried intensively to solve this problem suggesting solutions such as hollow nanoparticles TiO2 [72], TiO2 modified with modifier [62,73] and TiO2 coated with polymer [22,43,74]. For the first time, Comiskey et al. report the E Ink microcapsules with white particles dispersed in a blue fluid that was prepared with the in situ polymerization method of urea and formaldehyde. Titanium dioxide with a specific gravity of 4.2 was used for reflection and high colour purity as a white particle [75]. The polyethylene was used as a coating on titanium dioxide in order to reduce the specific gravity and as a surface modification of particles to respond to the applied electric field. In this study, the response time was reported as 0.1 s. As demonstrated in Fig. 6(a), when a microencapsulated electrophoretic particle is placed between two electrodes with opposite charges, the charged particles are oriented by applying a current which if otherwise are orientated towards the electrode with opposite charge. In this case, when a viewer looks at the particle from above, he sees a white background with negative charge in the vicinity of positive electrode. Furthermore, part (b) shows the photomicrograph of the original examples of the electrophoretic microcapsules built-in the electric field [75].
Yang et al. modified the titanium dioxide particles with Vinyl Triethoxysilane (VTES) by Sol-Gel method through flow groups graft on the TiO2 particles surface. TiO2 particles have excellent properties in the dark surroundings for contrast and are extensively used as white electrophoretic particles in the production of E Ink. However, since this particle has high density, Van der Waals attraction is not sufficient and leads to aggregation, quick sedimentation and shows slow response to the electric field. Therefore, extensive research has been conducted on surface modification. In this study, the results of the entire FTIR have confirmed new peaks in 560 and 670 cm-1 wavelengths due to the stretching vibrations and two peaks with 12,020 and 1120 cm−1 wavelength that represents the stretching vibrations of Si-O bonds in VTES. Thus, it was shown that VTES was also grafted on the TiO2 surface. Modified particles size has reported in the range of 100–200 nm with very narrow distribution [37]. Recently, the use silica nanoparticles has been reported with response time of 180–191 ms in the prototype of electrophoretic display [30]. Currently the EPDs products can show 16 Gy levels of white to black colours with 260–300 ms and 1000 ms as response time and refresh time respectively [5]. Despite the fact that white pigments are commercialized, there is still a need to improve their properties spatially fast response to electric field.
Full-coloured display can be developed by dividing each of the image elements in the black and white EPDs and placing horizontal coloured filters as RGB (red, green, blue) and CMY (blue, red, yellow) arrays [76]. However, the coloured filter absorbs large amounts of reflected light, which leads to low contrast and brightness. Recently, studies have focused on the preparation of the tri-colour electrophoretic particles for colour displays (CEPD). The encapsulated dye and modified pigment are used for the synthesis of electrophoretic particles. Preparation of coloured ink was obtained through the placement of coloured material into the polymers such as polystyrene, poly (N vinyl pyrrolidone), poly (methyl methacrylate) and some other copolymers [23,24]. However, some drawbacks such as low visibility and poor light stability limit the use of dyes in the CEPD. In comparison, organic pigments with ultra-light resistance, better stability and higher colour strength show more suitability for CEPD [77]. Numerous methods have been employed for the preparation of applied dyes in the CEPD that are listed in the following sections.
3.3.2 The shell materials for surrounding coloured material
In this technology, microcapsules or micropixels comprise of the electrophoretic display device where the shell wall turns to a key material. The key role of the shell in electrophoretic display is to encapsulate the coloured particles as well as medium. For this purpose, it is not only required to have good transparency and low level of conductivity but also should compatible with the materials inside it. Another specification is the manner of mechanical stability while maintaining the flexibility in the same time. Hence, organic polymers such as polyamine, polyurethane, polysulfones, polyethylene acid, cellulose, gelatine, Arabic gum, etc. are considered as the most suitable choices [32,55,78-87]. According to the chosen materials, various methods have been employed for fabrication of microcapsules including in situ polymerization of formaldehyde and urea to form urea-formaldehyde resin [3,28,82,88] and composite coagulation of gelatine and Arabic gum to form composite film [79,89,90].
3.3.3 Dielectric liquid medium
There is a suspension of coloured particles in a liquid medium inside the microcapsules of electrophoretic display devices. Based on the key requirements of these devices, the medium should represent several special specifications including thermal and chemical stability, suitable insulation properties (dielectric constant larger than 2), almost identical reflectivity and density with particles as well as low resistance to their transportation, and finally, environmental-friendly nature. Application of different single organic solvents or formulated solvents such as alkylene, aromatic/aliphatic hydrocarbon, oxosilane, etc. can satisfy the requirements mentioned above [57,71,79,91,92]. One of the most widely used methods is the formulation of 2-phenylbutane-tetrachloroethylene, isopar L-tetrachloroethylene and n-haxane-tetrachloroethylene. Mixing of high and low density fluorinated solvent and hydrocarbon is a common way for proper adjustment of density. Table 1 shows some solvents used in EPDs application.