WO2008070830A2

WO2008070830A2 - Components for use in electro-optic displays

Info

Publication number: WO2008070830A2
Application number: PCT/US2007/086770
Authority: WO
Inventors: Norifusa Isobe; Steven Joseph Battista; Erika D. A. Hanna
Original assignee: E Ink Corporation
Priority date: 2006-12-07
Filing date: 2007-12-07
Publication date: 2008-06-12
Also published as: US20080137176A1; WO2008070830A3; US7649666B2

Abstract

An electro-optic display (102), or a sub-assembly of such a display, comprising a layer of electro-optic material (108) on a backplane (100) is assembled using at least one release sheet (112, 116) or masking film having a resistivity not greater than 1013 Ω square. The use of such a release sheet or masking film helps to avoid damage to transistors in the backplane during the assembly process.

Description

COMPONENTS FOR USE IN ELECTRO-OPTIC DISPLAYS

[Para 1 ] This application is related to:

(a) U.S. Patent No. 6,982,178;

(b) U.S. Patent No. 7,236,292;

(c) U.S. Patent Publication No. 2004/0155857;

(d) U.S. Patent No. 7,110,164;

(e) U.S. Patent Publication No. 2007/0109219;

(f) U.S. Patent Publication No. 2007/0152956;

(g) U.S. Patent Publication No. 2007/0211331; and (h) U.S. Patent Publication No. 2007/0153361.

[Para 2] This invention relates to components and methods for use in electro-optic displays. More specifically, this invention relates to such components and methods in which a release sheet (a term which is used herein in a broad sense to mean any sheet which is peeled from a sub-assembly during manufacture of an electro-optic display, and thus includes materials sometimes referred to as "masking films") having a relatively high conductivity is used. This invention primarily relates to such components and methods containing an electro-optic medium which is a solid (such displays may hereinafter for convenience be referred to as "solid electro-optic displays"), in the sense that the electro-optic medium has solid external surfaces, although the medium may, and often does, have internal liquid- or gas-filled spaces. Thus, the term "solid electro-optic displays" includes encapsulated electrophoretic displays, encapsulated liquid crystal displays, and other types of displays discussed below.

[Para 3] The term "electro-optic" as applied to a material or a display, is used herein in its conventional meaning in the imaging art to refer to a material having first and second display states differing in at least one optical property, the material being changed from its first to its second display state by application of an electric field to the material.

[Para 4] The terms "bistable" and "bistability" are used herein in their conventional meaning in the art to refer to displays comprising display elements having first and second display states differing in at least one optical property, and such that after any given element has been driven, by means of an addressing pulse of finite duration, to assume either its first or second display state, after the addressing pulse has terminated, that state will persist for at least several times, for example at least four times, the minimum duration of the addressing pulse required to change the state of the display element. [Para 5] Several types of electro-optic displays are known, for example:

(a) rotating bichromal member displays (see, for example, U.S. Patents Nos. 5,808,783; 5,777,782; 5,760,761; 6,054,071 6,055,091; 6,097,531; 6,128,124; 6,137,467; and 6,147,791);

(b) electrochromic displays (see, for example, O'Regan, B., et al, Nature 1991, 353, 737; Wood, D., Information Display, 18(3), 24 (March 2002); Bach, U., et al., Adv. Mater., 2002, 14(11), 845; and U.S. Patents Nos. 6,301,038; 6,870.657; and 6,950,220);

(c) electro-wetting displays (see Hayes, R.A., et al., "Video-Speed Electronic Paper Based on Electrowetting", Nature, 425, 383-385 (25 September 2003) and U.S. Patent Publication No. 2005/0151709);

(d) particle-based electrophoretic displays, in which a plurality of charged particles move through a fluid under the influence of an electric field (see U.S. Patents Nos. 5,930,026; 5,961,804; 6,017,584; 6,067,185; 6,118,426; 6,120,588; 6,120,839; 6,124,851; 6,130,773; and 6,130,774; U.S. Patent Applications Publication Nos. 2002/0060321; 2002/0090980; 2003/0011560; 2003/0102858; 2003/0151702; 2003/0222315; 2004/0014265; 2004/0075634; 2004/0094422; 2004/0105036; 2005/0062714; and 2005/0270261; and International Applications Publication Nos. WO 00/38000; WO 00/36560; WO 00/67110; and WO 01/07961; and European Patents Nos. 1,099,207 Bl; and 1,145,072 Bl; and the other MIT and E Ink patents and applications discussed in the aforementioned U.S. Patent No. 7,012,600).

[Para 6] There are several different variants of electrophoretic media. Electrophoretic media can use liquid or gaseous fluids; for gaseous fluids see, for example, Kitamura, T., et al., "Electrical toner movement for electronic paper-like display", IDW Japan, 2001, Paper HCSl-I, and Yamaguchi, Y., et al., "Toner display using insulative particles charged triboelectrically", IDW Japan, 2001, Paper AMD4- 4); U.S. Patent Publication No. 2005/0001810; European Patent Applications 1,462,847; 1,482,354; 1,484,635; 1,500,971; 1,501,194; 1,536,271; 1,542,067; 1,577,702; 1,577,703; and 1,598,694; and International Applications WO 2004/090626; WO 2004/079442; and WO 2004/001498. The media may be encapsulated, comprising numerous small capsules, each of which itself comprises an internal phase containing electrophoretically-mobile particles suspended in a liquid suspending medium, and a capsule wall surrounding the internal phase. Typically, the capsules are themselves held within a polymeric binder to form a coherent layer positioned between two electrodes; see the aforementioned MIT and E Ink patents and applications. Alternatively, the walls surrounding the discrete microcapsules in an encapsulated electrophoretic medium may be replaced by a continuous phase, thus producing a so-called polymer-dispersed electrophoretic display, in which the electrophoretic medium comprises a plurality of discrete droplets of an electrophoretic fluid and a continuous phase of a polymeric material; see for example, U.S. Patent No. 6,866,760. For purposes of the present application, such polymer-dispersed electrophoretic media are regarded as sub-species of encapsulated electrophoretic media. Another variant is a so-called "microcell electrophoretic display" in which the charged particles and the fluid are retained within a plurality of cavities formed within a carrier medium, typically a polymeric film; see, for example, U.S. Patents Nos. 6,672,921 and 6,788,449.

[Para 7] Electrophoretic media can operate in a "shutter mode" in which one display state is substantially opaque and one is light-transmissive. See, for example, U.S. Patents Nos. 6,130,774 and 6,172,798, and U.S. Patents Nos. 5,872,552; 6,144,361; 6,271,823; 6,225,971; and 6,184,856. Dielectrophoretic displays can operate in a similar mode; see U.S. Patent No. 4,418,346. Other types of electro-optic displays may also be capable of operating in shutter mode.

[Para 8] Other types of electro-optic media, for example encapsulated liquid crystal media, may also be used in the components and methods of the present invention. [Para 9] Most prior art methods for the production of electrophoretic displays are essentially batch methods in which the electro-optic medium, the lamination adhesive and the backplane are only brought together immediately prior to final assembly, and it is desirable to provide methods better adapted for mass production. [Para 1 0] The aforementioned U.S. Patent No. 6,982,178 describes a method of assembling a solid electro-optic display (including a particle-based electrophoretic display) which is well adapted for mass production. Essentially, this patent describes a so-called "front plane laminate" ("FPL") which comprises, in order, a light- transmissive electrically-conductive layer; a layer of a solid electro-optic medium in electrical contact with the electrically-conductive layer; an adhesive layer; and a release sheet. Typically, the light-transmissive electrically-conductive layer will be carried on a light-transmissive substrate, which is preferably flexible, in the sense that the substrate can be manually wrapped around a drum (say) 10 inches (254 mm) in diameter without permanent deformation. The term "light-transmissive" is used in this patent and herein to mean that the layer thus designated transmits sufficient light to enable an observer, looking through that layer, to observe the change in display states of the electro-optic medium, which will be normally be viewed through the electrically-conductive layer and adjacent substrate (if present). The substrate will be typically be a polymeric film, and will normally have a thickness in the range of about 1 to about 25 mil (25 to 634 μm), preferably about 2 to about 10 mil (51 to 254 μm). The electrically-conductive layer is conveniently a thin metal layer of, for example, aluminum or ITO, or may be a conductive polymer. Poly(ethylene terephthalate) (PET) films coated with aluminum or ITO are available commercially, for example as "aluminized Mylar" ("Mylar" is a Registered Trade Mark) from E.I. du Pont de Nemours & Company, Wilmington DE, and such commercial materials may be used with good results in the front plane laminate.

[Para 1 l ] The aforementioned 2004/0155857 describes a so-called "double release film" which is essentially a simplified version of the front plane laminate of the aforementioned U.S. Patent No. 6,982,178. One form of the double release sheet comprises a layer of a solid electro-optic medium sandwiched between two adhesive layers, one or both of the adhesive layers being covered by a release sheet. Another form of the double release sheet comprises a layer of a solid electro-optic medium sandwiched between two release sheets. Both forms of the double release film are intended for use in a process generally similar to the process for assembling an electro-optic display from a front plane laminate already described, but involving two separate laminations; typically, in a first lamination the double release sheet is laminated to a front electrode to form a front sub-assembly, and then in a second lamination the front sub-assembly is laminated to a backplane to form the final display, although the order of these two laminations could be reversed if desired. [Para 1 2] The aforementioned 2007/0109219 describes a so-called "inverted front plane laminate", which is a variant of the front plane laminate described in the aforementioned U.S. Patent No. 6,982,178. This inverted front plane laminate comprises, in order, at least one of a light-transmissive protective layer and a light- transmissive electrically-conductive layer; an adhesive layer; a layer of a solid electro-optic medium; and a release sheet. This inverted front plane laminate is used to form an electro-optic display having a layer of lamination adhesive between the electro-optic layer and the front electrode or front substrate; a second, typically thin layer of adhesive may or may not be present between the electro-optic layer and a backplane. Such electro-optic displays can combine good resolution with good low temperature performance.

[Para 1 3] The aforementioned 2007/0109219 describes various methods designed for high volume manufacture of electro-optic displays using inverted front plane laminates; preferred forms of these methods are "multi-up" methods designed to allow lamination of components for a plurality of electro-optic displays at one time. [Para 1 4] In practice, it has been found that the methods described above, using front plane laminates, inverted front plane laminates and double release films, can sometimes result in damage to the transistors or other non-linear devices typically present in the backplane of the electro-optic display. Such damage can cause pixels of the display to cease to switch between their various optical states, or to switch more slowly or incompletely. Such damage is of course highly undesirable since it adversely affects the quality of images written on the display.

[Para 1 5] It has now been discovered (although this information is not available in the published literature) that one major cause of damage to transistors or other nonlinear devices in display backplanes is discharge through the backplane of electrostatic charges generated during peeling of a release sheet from a sub-assembly used to form the display. It has also been discovered that such damage can be avoided, or at least greatly reduced, if the conductivity of the release sheet is within an appropriate range. Accordingly, the present invention relates to components comprising such release sheets for use in the manufacture of electro-optic displays, and to methods for the use of such components.

[Para 1 6] The present invention provides a sub-assembly useful in the manufacture of an electro-optic display, the sub-assembly comprising a layer of an electro-optic material and a release sheet capable of being peeled from the sub-assembly, the release sheet having a resistivity not greater than about 10¹³ Ω square. Typically, the resistivity of the release sheet will be not greater than about 10¹² Ω square, and not less than about 10² Ω square.

[Para 1 7] The sub-assembly of the present invention may further comprise a backplane comprising at least one electrode, the backplane being disposed on the opposed side of the layer of electro-optic material from the release sheet. In addition, the sub-assembly may comprise a front substrate disposed between the layer of electro-optic material and the release sheet.

[Para 1 8] The sub-assembly of the present invention may be in the form of a front plane laminate comprising, in order: a light-transmissive electrically-conductive layer; the layer of an electro-optic material, this layer being of a solid electro-optic material and being in electrical contact with the electrically-conductive layer; an adhesive layer; and the release sheet. Alternatively, the sub-assembly may be in the form of a double release sheet comprising the layer of an electro-optic material, this layer being of a solid electro-optic material and sandwiched between two adhesive layers, one or both of the adhesive layers being covered by a release sheet. The sub-assembly may also be in the form of a double release sheet comprising the layer of an electro-optic material, this layer being of a solid electro-optic material and sandwiched between two release sheets. The sub-assembly may also be in the form of an inverted front plane laminate comprising, in order: at least one of a light-transmissive protective layer and a light-transmissive electrically-conductive layer; an adhesive layer; the layer of an electro-optic material, this layer being of a solid electro-optic material; and the release sheet.

[Para 1 9] The sub-assembly of the present invention may make use of any of the various types of electro-optic media discussed above. Thus, in this sub-assembly, the electro-optic material may comprise a rotating bichromal member or electrochromic material. Alternatively, the electro-optic material may comprise an electrophoretic material comprising a plurality of electrically charged particles disposed in a fluid and capable of moving through the fluid under the influence of an electric field. The electrically charged particles and the fluid may be confined within a plurality of capsules or microcells, or may be present as a plurality of discrete droplets surrounded by a continuous phase comprising a polymeric material. The fluid may be liquid or gaseous.

[Para 20] This invention also provides a method for assembling a layer of an electro- optic material on a backplane, the method comprising: providing a sub-assembly comprising a layer of an electro-optic material and a release sheet capable of being peeled from the sub-assembly, the release sheet having a resistivity not greater than about 10¹³ Ω square; providing a backplane comprising at least one electrode and at least one non-linear device connected to the electrode; peeling the release sheet from the layer of electro-optic material; and laminating the layer of electro-optic material to the backplane.

[Para 21 ] In this method, the sub-assembly may comprise an adhesive layer disposed between the layer of electro-optic material and the release sheet, so that after the removal of the release sheet the adhesive layer is contacted with the backplane. The sub-assembly may further comprise, on the opposed side of the layer of electro-optic material from the release sheet and in order, a front substrate, a second adhesive layer and a second release sheet, the second release sheet having a resistivity not greater than about 10¹³ Ω square, the method further comprising peeling the second release sheet from the second adhesive layer and contacting the second adhesive layer with a second sub-assembly comprising at least one of a barrier layer and a hard coat, thereby securing the at least one of a barrier layer and a hard coat to the front substrate by means of the second adhesive layer. The second sub-assembly may comprise a third release sheet having a resistivity not greater than about 10¹³ Ω square, and the method may further comprise peeling the third release sheet from the second sub-assembly prior to contacting the second adhesive layer with the second sub-assembly.

[Para 22] As discussed in more detail below, it has been found that damage to transistors or similar non-linear devices in a backplane is especially likely where an electro-optic layer is already present on a backplane and an additional layer is to be secured to the opposed side of the electro-optic layer from the backplane. In these circumstances, if a release sheet is removed either from the backplane/electro-optic layer sub-assembly or from the additional layer so as to generate electrostatic charge on either the sub-assembly or the additional layer, damage to non-linear devices on the backplane is likely. Accordingly, in such a process, it is highly desirable that any release sheet peeled from the sub-assembly or the additional layer have a resistivity not greater than about 10¹³ Ω square.

[Para 23] Figures IA to ID are schematic side elevations showing various stages in the assembly of an electro-optic display by the method of the present invention.

[Para 24] Figure 2 is a schematic side elevation of a testing apparatus used in the experimental tests described below.

[Para 25] Figures 3 and 4 show respectively the variation of surface voltage and peel force with peel speed in experimental tests carried out using the apparatus of Figure 2, as described below.

[Para 26] Figure 5 shows the variation of surface voltage with masking film resistivity in experimental tests carried out using the apparatus of Figure 2, as described below.

[Para 27] As already mentioned, the present invention relates to sub-assemblies and methods for use in the manufacture of electro-optic displays. To avoid damage to transistors or other non-linear devices present in the backplane of the display, at least one release film used in the sub-assembly or method of the present invention has a resistivity not greater than about 10¹³ Ω square. All resistivities quoted herein are measured after the relevant material has been stored at 25°C and 50 per cent relative humidity for a period such that the resistivity of the material becomes stable. [Para 28] Although the release film used in the sub-assembly or method of the present invention has a resistivity up to about 10¹³ Ω square, it is generally preferred that the resistivity of the release film not exceed about 10¹² Ω square. As shown empirically below, release films having such a resistivity do produce significant reductions in the electrostatic charge generated during peeling of the release film. [Para 29] The sub-assembly of the present invention may be any sub-assembly comprising a layer of an electro-optic material and a release sheet useful in the manufacture of an electro-optic display. Thus, the sub-assembly may be a front plane laminate, double release film or inverted front plane laminate of any of the types described above. However, the sub-assembly can also take other forms. In particular, the "sub-assembly" of the present invention may comprise an electro-optic display which is itself complete and functional but which is designed to have further layers added, for example for increased protection against environmental contaminants, radiation etc.

[Para 30] Figures 1A-1D of the accompanying drawings illustrate a process for the manufacture of an electro-optic display using a sub-assembly of this type. Figures IA- ID are not to scale; the illustrated thicknesses of the various layers do not necessarily correspond to their actual thicknesses, and in all cases the thicknesses of the layers are greatly exaggerated relative to their lateral dimensions. Figure IA illustrates a backplane (generally designated 100) comprising pixel electrodes, thin film transistors and associated circuitry, all of which are omitted from Figures IA- ID for the sake of clarity. Disposed adjacent the backplane 100 is a front plane laminate (generally designated 102) of the type described in the aforementioned U.S. Patent No. 6,982,178 and comprising a substantially transparent front substrate 104 (in the form of a poly(ethylene terephthalate) (PET) film), a substantially transparent electrically- conductive layer 106 (in the form of a thin layer of indium tin oxide, ITO), a layer of electro-optic material 108, a lamination adhesive layer 110 and a first release sheet 112, all of which are as described in the patent. However, for reasons explained below, the front plane laminate 102 further comprises a layer of optically clear adhesive 114, on the opposed side of the front substrate 104 from the conductive layer 106, and a second release sheet 116 covering the optically clear adhesive 114. [Para 31 ] In the first step of the process for forming an electro-optic display, the first release sheet 112 is peeled from the front plane laminate 102 and the remaining layers of the front plane laminate are laminated to the backplane 100 under heat and pressure, as described in the aforementioned U.S. Patent No. 6,982,178, to produce the structure shown in Figure IB. To prevent damage to the backplane 100 during this step, the first release sheet has a resistivity not greater than about 10¹² Ω square. The removal of the release sheet 112 can result in the development of a substantial electrostatic charge on the remaining layers of the front plane laminate 102 and this electrostatic charge could discharge through and damage the transistors in the backplane. However, using a release sheet having a resistivity not greater than about 10¹² Ω square keeps the electrostatic charge low enough to ensure that peeling of the release sheet does not damage the transistors on the backplane. Although not shown in Figure IB, the front plane laminate 102 is provided with a conductive via which connects the conductive layer 106 to a connection provided on the backplane 100. Accordingly, the structure shown in Figure IB is in fact a fully functional electro- optic display. However, for reasons discussed in several of the aforementioned E Ink and MIT patents and applications, it is desirable to provide additional protection for the display against ingress of oxygen, moisture, radiation and other potential problem materials into the electro-optic layer itself, and the further steps described below provide such additional protection.

[Para 32] As also illustrated in Figure IB, the next step of the process uses a protective sheet (generally designated 120) which comprises, in order, a masking film 122, an anti-glare hard coat 124, a barrier layer 126 and a third release sheet 128. The barrier layer 126, which is designed to prevent penetration of moisture and ultra-violet radiation into the final display, is itself typically a complex multi-layer structure, but details of its internal construction are irrelevant to the present invention and will not be described herein. As shown in Figure IB, the protective sheet 120 is disposed adjacent the structure formed from the front plane laminate and the backplane, with the third release sheet 128 of the protective sheet 120 facing the second release sheet 116. [Para 33] In the next step of the process, the second release sheet 116 is peeled from the adhesive 114 and the third release sheet 128 is peeled from the barrier layer 126. Either of these peeling operations could result in the generation of electrostatic charge sufficient to damage the transistors of the backplane 100, and accordingly both the second and third release sheets have resistivities not greater than about 10¹² Ω square. (Although any charge generated by peeling of the third release sheet 128 will of course reside only on the protective sheet 120, this charge can discharge through and damage the backplane when the protective sheet 120 is affixed to the backplane as described below.) The protective sheet is then laminated, typically under heat and pressure, to the adhesive 114, thus producing the structure shown in Figure 1C. [Para 34] At this point the electro-optic display is essentially complete and the only remaining step is peeling of the masking film 122 from the hard coat 124 to produce the final structure shown in Figure ID. The time when this is done may vary with the application of the display. In some cases, removal of the masking film is effected when the display is incorporated into a chassis (for example the chassis of an electronic book reader, the chassis comprising a casing and electronics needed for driving the display, and other associated circuitry, for example solid state memory for storing books to be displayed); in other cases, the masking film 122 could be removed by the final customer, so that the masking film can serve to prevent damage to the hard coat 124 during delivery of the display. Regardless of the exact point at which the masking film 122 is removed to provide the final display structure shown in Figure ID, peeling of the masking film could result in the generation of electrostatic charge sufficient to damage the transistors of the backplane 100, and accordingly the masking film 122 has a resistivity not greater than about 10¹² Ω square. [Para 35] In many cases, release sheets or masking films having resistivities suitable for use in the present invention will be available commercially. However, in some cases it may be difficult to find a commercial material having a suitable resistivity and all the other properties which may be required in a release sheet or masking film used in the present invention. In such circumstances, it will typically be necessary to select a film having all the desirable properties except the desired resistivity and then to adjust the resistivity to the desired value. Such resistivity adjustment can be effected in several different ways. For example, a surface active agent may be coated on to the film. Such a surface active agent may absorb moisture from the air, thus neutralizing and reducing surface electrical charge. Alternatively, a material of appropriate resistivity may be coated on to the release sheet, or incorporated therein. In the common case where a release sheet comprises a base (typically polymeric) layer and an adhesive layer, it may often be more convenient to incorporate the surface active agent or the material of appropriate conductivity into the adhesive layer rather than the base layer.

[Para 36] Those familiar with the problems caused by electrostatic discharges during manufacturing processes will be aware of techniques which have been used to reduce such problems in other industries, including the use of high humidity environments and so-called "ion blowers" which blow a stream of air containing ions on to any area where it is desired to avoid charge build-up. Neither approach appears to give good results when dealing with the problems caused by peeling release sheets or masking films during the production of electro-optic displays. Many types of electro-optic media are sensitive to moisture (i.e., the electro-optic properties of the material are affected by moisture) so that using a high humidity environment may cause undesirable changes in the electro-optic medium itself. In addition, there are obvious problems in ensuring a high humidity environment throughout large scale, multi-stage production of electro-optic displays in a factory setting. Although ion blowers can reduce electrostatic charges, they typically act too slowly to be convenient for use when release sheets are peeled; leaving the sub-assembly from which the release sheet has been peeled exposed for the period of (say) 30 or more seconds necessary for an ion blower to remove the substantial charge generated by peeling is at least awkward in a manufacturing environment, especially since some of the thin sub-assemblies involved may move around in the air flow from the ion blower. Accordingly, use of relatively conductive release sheets in accordance with the present invention is the preferred approach.

[Para 37] The following experimental results are given, though by way of illustration only, to show that the use of relatively conductive release sheets in accordance with the present invention is effective in reducing static discharge during the manufacture of electro-optic displays. [Para 381 Experimental tests

[Para 39] Figure 2 of the accompanying drawings is a schematic side elevation of an experimental apparatus (generally designated 200) used in these experiments. The apparatus 200 comprises a base 202 from which extends upwardly an aluminum plate 204. One surface of the upper portion of plate 204 is covered by a double-sided adhesive tape 206, to which is adhered a glass plate 208. The surface of glass plate 208 remote from the tape 206 is covered by a second double-sided adhesive tape 210, to which is adhered a protective sheet 212, of the type shown in Figure IB. The protective sheet 212 is itself covered by a masking film 214.

[Para 40] The apparatus 200 further comprises a vertical bar 216 extending upwardly from the base 202 substantially parallel to the plate 204. The upper end of bar 216 carries a horizontal member 218, which carries, at its end remote from the bar 216, a descending member 220. The descending member 220 carries a clamping member 222, in which is clamped the lower end of masking film 214. The horizontal member 218, together with the associated descending member 220 and clamping member 222, can be moved vertically by an electric motor (not shown) at a constant velocity relative to the base 202, so that the masking film 214 is peeled at a constant rate from the underlying protective sheet 212. The apparatus is provided with means for measuring the force necessary to peel the masking film from the protective sheet. This peeling causes the development of electrostatic charges on both the protective sheet 212 and the masking film 214 and these charges are measured by a meter 224. [Para 41 ] Combinations of three different commercial protective sheets (PS) and two different masking films (MF) were tested using the apparatus 200. One masking film, designated simply "PET" below, was a simple PET film having no anti-electrostatic coating and a resistivity greater than 10¹⁴ Ω square. The second masking film, designated "PET/AES" below, was a similar PET film but provided with an anti- electrostatic coating and had a resistivity of about 10¹² Ω square. The meter used was a SIMCO Model FMV-003 ESD meter, and the peeling speed was 25 mm/second. The results are shown in Table 1 below. All measurements in this series of experiments, and the later series summarized in Table 2 below, were taken at 23°C and 43 per cent relative humidity. The rows labeled "|Δ|" show the absolute value of the difference between the voltages measured on the protective sheet and masking film in each test; it is this absolute voltage difference which is the best predictor of possible harmful current flows through the backplane which can damage thin film transistors.

[Para 42] Table 1

[Para 43] From the data in Table 1, it will be seen that the presence of the anti- electrostatic coating on the masking film reduced the voltage differences developed during peeling by about an order of magnitude; in neither case did the voltage difference obtained with the PET/AES films useful in the present invention exceed 1 kV. The presence of the anti-electrostatic coating on the masking film also significantly reduced the peeling force required.

[Para 44] A further series of experiments were conducted to determine whether the rate of peeling significantly affected the surface voltages and the peeling force. For this purpose, the experiment with Protective Sheet A and Masking Film PET was repeated using peel speeds of 2.1, 6.4. 13 and 25 mm/second. The results are shown in Table 2 below. Also, the results for the surface voltage on the masking film at the midpoint of the peel are plotted in Figure 3 of the accompanying drawings, and the results for the peeling force are plotted in Figure 4.

[Para 46] From the data in Table 2, and from Figures 3 and 4, it will be seen that the magnitude of both the surface voltage on the masking film and the peel force increase rapidly with peel speed. Hence, the importance of provisions for charge dissipation increases when high peel speeds are likely, for example where large displays are being manufactured, or at high production rates.

[Para 47] In a further series of experiments, various combinations of protective sheet and masking film were peeled from each other at a constant peeling force of 0.15 Kg; this resulted in different peeling speeds in the various tests. The results obtained are shown in Table 3 below and Figure 5 plots the voltage difference between the masking film and the protective sheet measured at the peeling midpoint against the resistivity of the masking film; note that both axes in Figure 5 are plotted logarithmically. [Para 48] Table 3

[Para 49] From Table 3 and Figure 5, it will be seen that the surface voltage at the peel midpoint correlated very strongly with the resistivity of the masking film; the masking films having resistivities of 10¹⁴ ohm or more produced voltage differences of the order of 10 kV, whereas the masking films having resistivities of 10¹² ohm or less produced voltage differences not greater than about 1 kV. As may be seen from Table 3, exactly the same tendency is apparent in the data from the peel endpoints. [Para 50] It will be apparent from the preceding discussion that the sub-assemblies and methods of the present invention can make use of any electro-optic layer which has solid external surfaces to which adhesive layers and/or release sheets can adhere and sufficient mechanical cohesion to permit the necessary manipulation of films containing the electro-optic layer. Accordingly, the present methods can be carried out using any of the types of electro-optic media described above. For example, the present methods can make use of rotating bichromal member, electrochromic or electrophoretic media, and in the last case the electrophoretic media may be of the encapsulated, polymer-dispersed or microcell types.

Claims

1. A sub-assembly (102) useful in the manufacture of an electro- optic display, the sub-assembly (102) comprising a layer (108) of an electro-optic material and a release sheet (112, 116) capable of being peeled from the sub-assembly (102), the subassembly (102) being characterized in that the release sheet (112, 116) has a resistivity not greater than 10¹³ Ω square.

2. A sub-assembly according to claim 1 characterized in that the resistivity of the release sheet is not greater than 10¹² Ω square.

3. A sub-assembly according to claim 1 characterized in that the resistivity of the release sheet is not less than 10² Ω square.

4. A sub-assembly according to claim 1 characterized by a backplane (100) comprising at least one electrode, the backplane (100) being disposed on the opposed side of the layer (108) of electro-optic material from the release sheet (116).

5. A sub-assembly according to claim 4 characterized by a front substrate (104) disposed between the layer (108) of electro-optic material and the release sheet (116).

6. A sub-assembly according to claim 1 in the form of a front plane laminate characterized by, in order: a light-transmissive electrically-conductive layer (106); the layer (108) of an electro-optic material, this layer (108) being of a solid electro-optic material and being in electrical contact with the electrically- conductive layer (106); an adhesive layer (110); and the release sheet (112).

7. A sub-assembly according to claim 1 in the form of a double release sheet characterized in that the layer of an electro-optic material is of a solid electro-optic material and is sandwiched between two adhesive layers, one or both of the adhesive layers being covered by a release sheet.

8. A sub-assembly according to claim 1 in the form of a double release sheet characterized in that the layer of an electro-optic material is of a solid electro-optic material sandwiched between two release sheets.

9. A sub-assembly according to claim 1 in the form of an inverted front plane laminate characterized by, in order: at least one of a light-transmissive protective layer and a light- transmissive electrically-conductive layer; an adhesive layer; the layer of an electro-optic material, this layer being of a solid electro- optic material; and the release sheet.

10. A method for assembling a layer (106) of an electro-optic material on a backplane (100), the method comprising: providing a sub-assembly (102) comprising the layer (108) of electro- optic material and a release sheet (112) capable of being peeled from the sub- assembly (102); providing a backplane (100) comprising at least one electrode and at least one non-linear device connected to the electrode; peeling the release sheet (112) from the layer (108) of electro-optic material; and laminating the layer (108) of electro-optic material to the backplane (100), the method being characterized in that the release sheet (112) has a resistivity not greater than 10¹³ Ω square.

11. A method according to claim 10 characterized in that the resistivity of the release sheet is not greater than 10¹² Ω square.

12. A method according to claim 10 characterized in that the resistivity of the release sheet is not less than 10² Ω square.

13. A method according to claim 10 characterized in that the sub- assembly (102) comprises an adhesive layer (110) disposed between the layer (108) of electro-optic material and the release sheet (112), and after the removal of the release sheet (112) the adhesive layer (110) is contacted with the backplane (100).

14. A method according to claim 13 characterized in that the sub- assembly (102) further comprises, on the opposed side of the layer (108) of electro- optic material from the release sheet (112) and in order, a front substrate (104), a second adhesive layer (114) and a second release sheet (116), the second release sheet (116) having a resistivity not greater than 10¹³ Ω square, the method further comprising peeling the second release sheet (116) from the second adhesive layer (114) and contacting the second adhesive layer (114) with a second sub-assembly (120) comprising at least one of a barrier layer (126) and a hard coat (124), thereby securing the at least one of a barrier layer (126) and a hard coat (124) to the front substrate (104) by means of the second adhesive layer (114).

15. A method according to claim 14 characterized in that the second sub-assembly (122) comprises a third release sheet (128) having a resistivity not greater than 10¹³ Ω square, and the method further comprises peeling the third release sheet (128) from the second sub-assembly (120) prior to contacting the second adhesive layer (114) with the second sub-assembly (120).