HK1167416B - Adhesives and binders for electro-optic displays - Google Patents

Description

Adhesives and adhesives for electro-optic displays

The present application relates to the following U.S. patents: no.6,120,588, 6,124,851, 6,312,304, 6,657,772, 6,727,881, 6,831,769, 6,982,178, 7,012,600, 7,106,296, 7,110,163, 7,110,164, 7,236,292 and 7,561,324, to which the reader is referred for background information regarding electro-optic displays.

Technical Field

The present invention relates to electro-optic displays and to materials for use therein, particularly adhesives and lamination adhesives. The present invention relates in part to adhesives and binders having electrical or other properties that make them particularly suitable for use in electro-optic displays. The present invention is also directed to providing a polyurethane that can be used in other applications besides electro-optic displays. The invention also relates to articles of manufacture useful in the manufacture of electro-optic displays.

Background

Background terminology and prior art relating to electro-optic displays is discussed in detail in U.S. Pat. No.7,012,600, to which the reader is referred for further information. Accordingly, the terminology and the prior art in the field are briefly summarized as follows.

As applied to materials or displays, the term "electro-optic" is used herein in its conventional sense in the imaging arts 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 display state to its second display state by application of an electric field to the material. Some electro-optic materials are solid, which refers to a solid in the sense that the material has a solid outer surface, although the material may, and typically does, have an interior space that is filled with a liquid or gas. For convenience, such displays employing solid electro-optic materials will be referred to hereinafter as "solid electro-optic displays". Thus, the term "solid state electro-optic display" includes rotating bichromal member displays, encapsulated electrophoretic displays, microencapsulated electrophoretic displays, and encapsulated liquid crystal displays.

The terms "bistable" and "bistability" are used herein in their conventional sense in the art to refer to displays comprising display elements having first and second display states differing in at least one optical property such that, after any given element is driven to assume its first or second display state by means of an addressing pulse having a finite duration, that state will continue for a time at least several times (e.g. at least 4 times) the minimum duration of the addressing pulse required to change the state of that display element after the addressing pulse has terminated.

Several types of electro-optic displays are known, for example:

(a) rotating two-color component displays (see, e.g., U.S. Pat. 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, Nature 1991, 353, 737 to O' Regan, b. et al; Information Display, 18(3), 24 (3.2002), d. Bach, u. et al, adv. mater, 2002, 14(11), 845 to Bach, u. et al, and U.S. patent nos. 6,301,038, 6,870,657, and 6,950,220);

(c) electrowetting displays (see, for example, Hayes, R.A. et al, Nature, 425, 383-385 (9/25 2003), entitled "Video-Speed Electronic Paper Based on electrowetting" 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. Pat. 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 application 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 patent application publication Nos. WO 00/38000; WO 00/36560; WO 00/67110 and WO 01/07961; and European patent Nos. 1,099,207B1 and 1,145,072B 1; and other MIT and Ink patents and applications discussed in the aforementioned U.S. Pat. No.7,012,600).

There are several different variations of electrophoretic media. The electrophoretic medium may use a liquid or gaseous fluid; for gaseous fluids see, for example, Kitamura, T.et al published in IDW Japan, PaperHCS1-1 in 2001 under the name "electric inside movement for electronic Paper-like display" and Yamaguchi et al published in IDW Japan, Paper AMD4-4) under the name "inside display using insulating substrates charged triangular display"; U.S. patent publication nos. 2005/0001810; european patent application 1,462,847; 1,482,354, respectively; 1,484,635, respectively; 1,500,971, respectively; 1,505,194, respectively; 1,536,271, respectively; 1,542,067, respectively; 1,577,702, respectively; 1,577,703 and 1,598,694; and international application WO 2004/090626; WO 2004/079442 and WO 2004/001498. The medium may be encapsulated, comprising a plurality of capsules (capsules), each of which itself comprises an internal phase comprising electrophoretically mobile particles suspended in a fluid suspension medium, and a capsule wall surrounding the internal phase. Typically, the capsules themselves are held within a polymeric binder to form a coherent layer between two electrodes; see the MIT and E Ink patents and applications mentioned above. Alternatively, the walls surrounding the discrete microcapsules in the encapsulated electrophoretic medium may be replaced by a continuous phase, thus producing a so-called polymer dispersed electrophoretic display, wherein 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 the purposes of this application, such polymer-dispersed electrophoretic media are considered to be a subclass of encapsulated electrophoretic media. Another variation is the so-called "microcell electrophoretic display", in which charged particles and a fluid are retained within a plurality of cavities formed within a carrier medium, typically a polymer film; see, for example, U.S. Pat. Nos. 6,672,921 and 6,788,449.

Although electrophoretic media are typically opaque (e.g., because in many electrophoretic media, the particles substantially block transmission of visible light through the display) and operate in a reflective mode, many electrophoretic displays can be operated in a so-called "shutter mode" in which one display state is substantially opaque and one display state is light transmissive. See, for example, the aforementioned U.S. patent nos. 6,130,774 and 6,172,798, and U.S. patent No.5,872,552; 6,144,361, respectively; 6,271,823, respectively; 6,225,971, respectively; and 6,184,856. A dielectrophoretic display similar to the electrophoretic display but relying on variations in electric field strength may operate in a similar mode; see U.S. patent No.4,418,346. Other types of electro-optic displays can also operate in the shutter mode.

Encapsulated electrophoretic displays are generally immune to failure modes of aggregation and settling of conventional electrophoretic devices and have additional advantages, such as the ability to print or coat the display on a variety of different flexible and rigid substrates. (the term "printing" is intended to include all forms of printing and coating including, but not limited to, kiss coating such as slot die coating, slot or extrusion coating, slide or cascade coating, curtain coating, roll coating such as knife-over-roll coating, forward and reverse roll coating, gravure coating, dip coating, spray coating, meniscus coating, spin coating, brushing, air knife coating, screen printing processes, electrostatic printing processes, thermal printing processes, ink jet printing processes, electrodeposition (see U.S. Pat. No.7,339,715), and other similar techniques.

Other types of electro-optic media, such as polymer dispersed liquid crystals, may also be used in the displays of the present invention.

When considering the choice of materials for use in electro-optic displays, care must be taken as to how the display is assembled. The vast majority of prior art processes for the final manufacture of electrophoretic displays are batch-wise processes in which the electrophoretic medium, the laminating adhesive and the backplane are brought together just prior to final assembly, and it is therefore desirable to provide processes that are more suitable for mass production. The aforementioned U.S. patent No.6,982,178 describes a method of assembling solid state electro-optic displays, including encapsulated electrophoretic displays, that is well suited for mass production. This patent essentially describes a so-called "front plane laminate" (FPL) comprising, in sequence, a light-transmissive electrically conductive layer, a layer of solid electro-optic medium in electrical contact with the electrically conductive layer, an adhesive layer and a release sheet. Generally, the light-transmissive electrically-conductive layer is carried by a light-transmissive substrate, which is preferably flexible in the sense that the substrate can be manually wound, for example, on a 10 inch (254 mm) diameter cylinder without permanent deformation. The term "light transmissive" as used herein means that the layer so designated is sufficiently light transmissive to permit a viewer looking through the layer to observe changes in the display state of the electro-optic medium, typically through the conductive layer and adjacent substrate (if any). The substrate is typically a polymer film and typically has a thickness in the range of about 1 to about 25 mils (25 to 634 μm), preferably about 2 to about 10 mils (51 to 254 μm). Suitably, the conductive layer may be a thin metal oxide layer, such as aluminium or Indium Tin Oxide (ITO), or may be a conductive polymer. Polyethylene terephthalate (PET) films coated with aluminum or ITO are commercially available, for example from dupont DE Nemours & Company, Wilmington DE, wil l.

Assembly of an electro-optic display using such front plane lamination may be achieved by: the release sheet is removed from the front plane laminate and the adhesive layer and the backplane are brought into contact under effective conditions sufficient to cause the adhesive layer to adhere to the backplane, thereby securing the adhesive layer, the layer of electro-optic medium, and the conductive layer to the backplane. Since this front plane lamination can be mass produced, typically using roll-to-roll coating techniques and then cut into sheets of any size required for use with a particular backing sheet, the process is well suited for mass production.

The aforementioned U.S. patent No.6,982,178 also describes a method for testing electro-optic media in a front plane laminate prior to introducing the front plane laminate into an electro-optic display. In the test method a release plate is provided with a conductive layer and a voltage sufficient to change the optical state of the electro-optical medium is applied between the conductive layer and the conductive layer on the opposite side of the electro-optical medium. Observation of the electro-optic medium can then reveal any imperfections present in the medium, which avoids laminating a defective electro-optic medium into the display, resulting in the ultimate cost of discarding the entire display rather than just the defective front plane laminate.

The aforementioned U.S. Pat. No.6,982,178 also describes a second method for testing electro-optic media by placing an electrostatic charge on a discharge plate to form an image on the electro-optic medium. The image is then viewed in the same manner as before to detect any imperfections in the electro-optic medium.

The aforementioned U.S. patent No.7,561,324 describes a so-called "double release panel" which is essentially a simplified version of the aforementioned front plane lamination. One form of dual release sheet comprises a layer of solid electro-optic medium sandwiched between two adhesive layers, with a release sheet overlying one or both of the adhesive layers. Another form of dual release layer comprises a layer of solid electro-optic medium sandwiched between two release plates. Both forms of the dual release film are intended for use in a method substantially similar to that already described for assembling an electro-optic display from a front plane laminate, but which comprises two separate laminates. Typically, the dual release sheet is laminated to the front electrode in a first lamination to form a front sub-assembly, and then the front sub-assembly is laminated to the backplane in a second lamination to form the final display.

The aforementioned U.S. patent No.7,110,164 describes a method of assembling an electro-optic display. Wherein a layer of electro-optic medium is coated on a first release sheet and a layer of laminating adhesive is coated on a second release sheet, and the two resulting structures are then laminated together to form a structure comprising, in order: a first release sheet, an electro-optic layer, an adhesive layer, and a second release sheet.

In view of the advantages of the assembly method described in the aforementioned U.S. patent No.6,982,178 using front plane lamination, it would be desirable to incorporate a material for use in electro-optic displays into such front plane lamination. It would also be desirable to be able to incorporate this material into the aforementioned double release panels, as well as the structure previously described in U.S. patent No.7,110,164.

As already noted, when front plane lamination or dual release films are used to make an electro-optic display, the laminated adhesive layer is typically located between the electrodes of the final display (there may be more than one adhesive layer between the electrodes; see, for example, U.S. patent publication No.2007/0109219, which describes a form of dual release film known as "inverted front plane lamination"). As discussed in the aforementioned U.S. patent No.6,831,769, the electrical properties of the lamination adhesive have a significant effect on the electro-optic performance of the display. However, the lamination adhesive need not be the only polymer component present between the electrodes of an electro-optic display. In encapsulated electrophoretic displays, for example as described in the aforementioned U.S. patent No.6,839,158, the electro-optic layer typically includes, in addition to the capsules themselves, a polymeric binder which, when dried or cured, forms the capsules into a mechanical bonding layer, particularly when the capsules are present in the desired monolayer capsule form as taught in that patent. The adhesive may also be located between the electrodes of the final display, so the adhesive also affects the electro-optic performance of the display. Under conditions where the adhesive is closer to the capsules of the encapsulated electrophoretic display than the laminating adhesive (typically separated from the interior by a thickness of adhesive), the adhesive has even a greater effect on the electro-optic performance of the display than the laminating adhesive. Similarly, the continuous substrate of a rotating bichromal member display, the continuous phase of a polymer dispersed electrophoretic display, and the wall material used in a microencapsulated display (all of which correspond essentially to the binder of an encapsulated electrophoretic display, and all of which are hereinafter referred to as binders) primarily affect the electro-optic performance of the display. One aspect of the invention relates to adhesives with improved electrical and mechanical properties for use in electro-optic displays, and to displays, front plane laminates, inverted front plane laminates and dual release films incorporating such adhesives.

U.S. patent No.7,477,444 describes an adhesive for use in electro-optic displays comprising a polyurethane made from an isocyanate and a polyester diol having a molecular weight of less than about 2000, or a polyester diol comprising two polyester diol segments linked by a steric hindrance group, wherein each polyester diol segment has a molecular weight of less than about 2000. Having the same or similar chemistry for the adhesive and lamination adhesive of an electro-optic display can provide manufacturing and storage stability benefits. However, since the aforementioned adhesive does not have sufficient adhesive properties and is too rigid, resulting in defects such as lamination voids in the lamination, the lamination quality is low and cannot be used as a lamination adhesive.

Disclosure of Invention

It has now been found that chemically modifying the aforementioned polyurethane adhesives results in less rigid materials with improved bonding properties, thus making them suitable for use as lamination adhesives in electro-optic displays. It has also been found that the resulting adhesive can be applied directly to the dried electrophoretic layer, thereby simplifying the process for making an electro-optic display.

Accordingly, the present invention provides a polyurethane formed from an isocyanate, a polyether diol and a polyester diol, the polyester diol having a molecular weight of less than 2000, or comprising two polyester diol segments linked by a steric hindrance group, each polyester diol segment having a molecular weight of less than 2000, the molar ratio of polyether diol to polyester diol being from 1:9 to 9: 1.

The term "sterically hindering group" as used herein is defined as any group that is capable of linking two polyester diol segments together and is sufficiently bulky to provide steric hindrance to crystallization of the polyester segments. The sterically hindered group may include, for example, a quaternary carbon atom; a group specifically used is the-C (CH3) 2-group.

In such polyurethanes, the molar ratio of polyether diol to polyester diol may be from 1:4 to 4: 1. The polyester diol may be a polycaprolactone diol having a molecular weight of less than 1500, or a polycaprolactone diol comprising two polycaprolactone segments linked by a bending group, each polycaprolactone segment having a molecular weight of no more than 1500. Poly (adipic acid) diols may also be used to form the polyurethanes of the present invention. The isocyanate may be 4,4' -methylene bis (cyclohexyl isocyanate), commonly known in the polyurethane industry as "H12 MDI". The polyether diol may be a poly (propylene oxide) diol, a preferred one having a molecular weight of 1000 to 3000. The molar ratio of isocyanate to hydroxyl groups in the polyurethane is preferably less than 1.3. The crossover temperature of the polyurethane at 1Hz, at which its storage modulus is equal to its loss modulus, is in the range from 10 to 90 ℃ and preferably in the range from 40 to 80 ℃. The polyurethane may be in the form of an aqueous latex.

The invention also provides a sub-assembly for use in the construction of an electro-optic display, the sub-assembly comprising a layer of solid electro-optic material, and a layer of laminating adhesive adhered to the layer of electro-optic material, the layer of laminating adhesive comprising a third polyurethane of the invention. Such a subassembly may further comprise a light-transmissive electrically-conductive layer on the side of the layer of electro-optic material opposite the lamination adhesive layer; and a release sheet (in the form of a front plane laminate) on the opposite side of the lamination adhesive layer from the layer of electro-optic material. Alternatively, such a subassembly may further comprise a second adhesive layer on the opposite side of the layer of electro-optic material from the lamination adhesive layer; and a release sheet (giving the subassembly the form of a double release film) disposed on the opposite side of the second adhesive layer from the layer of electro-optic material. The third form of the subassembly further comprising at least one of a light-transmissive protective layer and a light-transmissive electrically-conductive layer on the opposite side of the lamination adhesive layer from the electro-optic material layer; and a release sheet on the side of the layer of electro-optic material opposite the layer of laminating adhesive (giving the subassembly an inverted front plane laminate). Finally, such a subassembly may also include first and second release sheets overlying exposed surfaces of the layer of electro-optic material and the layer of laminating adhesive.

The invention may also relate to an electro-optic display comprising such a sub-assembly and at least one electrode arranged to apply an electric field to the layer of electro-optic material, and to an electronic book reader, portable computer, tablet computer, cell phone, smart card, sign, watch, shelf label or flash drive comprising such a display.

Drawings

FIG. 1 is a schematic cross-sectional view through a front plane lamination of the present invention;

FIG. 2 is a schematic cross-sectional view through a dual release membrane of the present invention;

fig. 3 shows the results of a differential scanning calorimetry test performed in example 3 below;

fig. 4 shows the results of the storage stability test performed in example 4 below;

fig. 5 shows the DSC curve of the polyurethane prepared in example 7 below.

Detailed Description

Referring now to figures 1 and 2, by way of illustration only, one way in which the polyurethanes of the present invention may be used as adhesives and laminating adhesives in the manufacture of electro-optic displays will be described. FIG. 1 is a schematic view through a subassembly (front plane laminate, or FPL) used in the process; the subassembly includes a substrate, an electrically conductive layer, an electro-optic layer, and an adhesive layer, the subassembly being illustrated at an intermediate stage in the process prior to laminating the subassembly to a second subassembly.

The front plane laminate (shown generally as 100) shown in FIG. 1 includes a light-transmissive substrate 110, a light-transmissive electrode layer 120, an electro-optic layer 130, a laminate adhesive layer 180, and a release sheet 190; the release sheet is illustrated as being removed from the lamination adhesive layer 180 in preparation for laminating the FPL 100 to a backplane.

The substrate 110 is typically a transparent plastic film, such as a 5 mil (127 μm) poly (ethylene terephthalate) (PET) sheet, although thinner plastic films, such as 3 mils (76 μm) or 0.5-1(13-25 μm) mils, may also be used. Thicker films, such as 7 mils (177 μm), may also be used when better mechanical protection of the electrode is desired. The lower surface of the substrate 110 (fig. 1) forming the viewing side of the final display may have one or more additional layers (not shown), such as a protective layer to absorb ultraviolet radiation, a barrier layer to prevent oxygen or moisture from entering the final display, and an anti-reflective coating to improve the optical performance of the display. Applied to the upper surface of the substrate 110 is a thin, light-transmissive, electrically conductive layer 120, preferably of ITO, which is commonly used as the front electrode in the final display. ITO coated PET films are commercially available.

The electro-optical layer 130 may be deposited onto the conductive layer 120, typically by slot coating, to make electrical contact between the two layers. The electro-optical layer 130 shown in fig. 1 is an encapsulated electrophoretic medium and comprises microcapsules 140, each of which comprises negatively charged white particles 150 and positively charged black particles 160 in a hydrocarbon-based fluid. The microcapsules 140 are held within a polymeric binder 170. When an electric field is applied across the electro-optic layer 130, the white particles 150 move towards the positive electrode and the black particles 160 move towards the negative electrode, so that the electro-optic layer 130 appears white or black to an observer viewing the display through the substrate 110, depending on whether the conductive layer 120 is positive or negative with respect to the adjacent pixel electrodes within the backplane.

The FPL 100 can be made by: the laminating adhesive 180 is applied to the release sheet 190 in a conventional liquid form, suitably by slot coating, dried (or otherwise cured) to form a solid layer, and then the adhesive and release sheet are laminated to the electro-optic layer 130, which has previously been applied to the substrate 110 with the electrically conductive layer 120; the lamination described above can conveniently be achieved using hot roll lamination. Alternatively, and in accordance with the present invention, particularly when the third polyurethane of the present invention is used as a laminating adhesive, the laminating adhesive may be applied over the electro-optic layer 130 and dried or otherwise cured before covering it with the release sheet 190. The release sheet 190 is suitably a 5 mil (127 μm) film; depending on the nature of the electro-optic medium used, it may be desirable to coat the film with a release agent, such as silicone. As shown in fig. 1, the release sheet 190 is peeled or otherwise removed from the lamination adhesive 180 prior to laminating the FPL 100 to a backplane (not shown) to form the final display. It will be apparent that the release sheet 190 may be omitted if the lamination adhesive 180 is applied directly to the electro-optic layer 130 and dried, and the resulting front plane laminate is immediately laminated to the backplane.

For further details regarding front plane lamination and methods of making and using the same, the reader is referred to the aforementioned U.S. Pat. No.6,982,178.

Fig. 2 shows a dual release plate (shown generally at 300) of the present invention. The dual release sheet 300 comprises an intermediate layer 302 of electro-optic material, in particular, in the layer of figure 2, comprising a capsule 304 within a polymer adhesive 306. The balloon 304 may be similar to the balloon described above with reference to fig. 1. The panel 300 also includes a first adhesive layer 308, a first release sheet 310 overlying the first adhesive layer 308, a second adhesive layer 312 disposed on an opposite side of the electro-optic layer 302 from the first adhesive layer 308, and a first release sheet 314 overlying the second adhesive layer 312.

The plate 300 may be formed by: a layer of adhesive is first applied to the release sheet 310 and then dried or cured to form the first adhesive layer 308. Next, a mixture of the bladder 304 and the adhesive 306 is printed or otherwise deposited onto the first binder layer 308, and then the mixture is dried or cured to form the adhesive layer 302. Finally, a layer of adhesive is deposited onto the layer 302, dried or cured to form a second adhesive layer 312, and then a second release sheet 314 is applied over the second adhesive layer 312.

It will be apparent to those skilled in the coating art that the above described sequence of operations to form the sheet 300 is well suited for continuous production and, with careful selection of materials and process conditions, it is possible to achieve the entire process step in a single pass through a conventional roll-to-roll coating apparatus.

To assemble a display using a dual release film such as film 300, one release sheet (typically the one on which the electro-optic material is coated) is peeled off and the remaining layers of the dual release film are then bonded to the front substrate using a lamination process such as heat, radiation, or chemical based. The front substrate typically includes a conductive layer that will form the front electrode of the final display. The front substrate may comprise additional layers, such as an ultraviolet filter layer or a protective layer intended to protect the conductive layer from mechanical damage. Thereafter, the other release sheet is peeled away, exposing a second adhesive layer that is used to bond the electro-optic material coating assembly to the backplane. Again, thermal, radiation, or chemical based lamination processes may be used. Although in practice it is almost always more convenient to laminate the order of first laminating the dual release film to the front substrate and then laminating the resulting front sub-assembly to the back sheet, it will be appreciated that the order of the two lamination described is essentially arbitrary and may therefore also be reversed.

For further details regarding the dual release film and its method of preparation and use, the reader is referred to the aforementioned U.S. patent No.7,561,324.

The display of the present invention may be used in any application to which electro-optic displays of the prior art have been applied. Thus, the displays of the present invention may be used, for example, in electronic book readers, portable computers, tablet computers, cellular phones, smart cards, signs, watches, shelf labels, and flash drives.

As already mentioned, the present invention relates to "custom" polyurethanes having properties that make them particularly useful as adhesives and laminating binders in encapsulated electrophoretic or other types of electro-optic displays. Having outlined the general reasons why polyurethanes are preferred as binders in electro-optic displays, NeoRez R9314 and R9621 are particularly useful for commercially available polyurethanes, with a preferred blend comprising 75 weight percent of the former and 25 weight percent of the latter.

However, although the 75/25 blend provides excellent overall performance for displays, there are some drawbacks associated with its use. First, due to the higher weight percentage of polyester segments in R9314, the material undergoes a melting/crystallization transition around 40 ℃. The polymer crystallization causes a change in the electrical properties of the material, which affects the performance of the display over time. Second, it is generally undesirable to use polymer blends because the polymers in the blend potentially undergo macroscopic phase separation, thereby forming a heterogeneous material. Third, commercial polyurethanes are rarely used in electro-optic displays, allowing manufacturers of such materials to modify polyurethanes to improve their performance in most applications in such a way as to compromise their effectiveness when used as adhesives for electro-optic displays. Thus, there is a need for a single component "custom" polyurethane of known composition having properties optimized for use as a binder in electro-optic displays.

In the first and second (or "binder") polyurethanes of the present invention, the length of the polyurethane segment is controlled. It is known that the crystalline transition of polyurethanes such as R9314 is due to long polyester segments located between urethane linkages on the polymer backbone. By reducing the length of the polyester segments between urethane groups (local molecular weight below 2000), the polyester segments are rendered incapable of chain folding crystallization, thereby eliminating crystallization of the polymer as a whole. To synthesize such polymers, polyester diols having relatively short chain lengths have been used. The diol may be a polycaprolactone diol having a molecular weight of no greater than 1500.

Two different types of polyester diols may be used in the binder polyurethane. The first type is generally represented by the following formula I:

wherein m + n is less than 13.

This first type of polyester diol comprises two polyester segments (polycaprolactone segments in formula I) linked by a group that does not provide any steric hindrance to the crystallization of the polyester segments. The molecular weight of this type of polyester diol should not exceed 2000.

The second type of polyester diol is generally represented by the following formula II:

wherein m < 13 and n < 13.

This second type of polyester diol comprises two polyester segments (polycaprolactone segments in formula II) linked by a steric hindrance group that is sufficiently bulky to provide sufficient steric hindrance to hinder crystallization of the polyester segments. In this type of polyester diol, the two polyester segments are practically separated from each other by a steric hindrance group and the molecular weight of each segment can be up to 2000.

As already noted, the present invention also provides a third polyurethane for use as an lamination adhesive in electro-optic displays. As discussed above, heretofore, when it was desired to provide a layer of lamination adhesive adjacent to a layer of electro-optic material, it was generally necessary to use an "indirect process" in which the lamination adhesive was applied to a release sheet, dried or cured to form an adhesive layer, and then the lamination adhesive/release sheet subassembly was laminated to the layer of electro-optic material. Although this is perceived as simpler for the first eye than the "direct process" (in which the lamination adhesive is applied directly to the layer of electro-optic material), experience has shown that many lamination adhesives that achieve better results in the indirect process face several serious problems in the direct process. For example, the polyurethane described in U.S. patent No.7,342,068 provides superior results when applied to an encapsulated electrophoretic display by an indirect method as a laminating adhesive, as described in the aforementioned U.S. patent No.7,012,735. However, experience has shown that the final electrophoretic display is very poor in switching when the same lamination adhesive is applied in a direct process. Furthermore, as already mentioned the first and second polyurethanes of the invention, although they provide good results in electrophoretic displays, are not suitable as laminating adhesives in such displays, because they lack sufficient adhesive properties and are too rigid, with a crossover temperature at 1Hz higher than 180 ℃.

It has been empirically found that for polyurethanes of constant molecular weight, the high crossover temperature can be significantly reduced by reducing the isocyanate to hydroxyl mole ratio, which in the preferred binder polyurethanes of the present invention is from about 1.4 (compare examples 1 and 2 below) to about 1.3, and preferably about 1.2; the cross-over temperature of the modified polyurethane of example 1, having a molar ratio of about 1.2, proved to be about 60 c, which is comparable to the polyurethane described in us patent No.7,342,068. (for polyurethanes with a substantially constant isocyanate to hydroxyl molar ratio, the crossover temperature tends to increase with increasing molecular weight.) the synthesis of this polyurethane, designated "H12 MDI-high PCL", is described in more detail in example 5 below. However, when the polyurethane was applied as a laminating adhesive using the direct method, it was found that the display showed poor conversion at 15V after 24 hours of storage at room temperature. With increasing applied voltage or operating temperature of the display, the switching of the display is significantly improved, indicating that the lamination adhesive may be crystallizing. The crystallization was confirmed by differential scanning calorimetry, which showed a melting/crystallization transition at about 30-40 ℃ during repeated heating/cooling cycles.

To reduce the crossover temperature and thereby reduce the tendency toward crystallization, the polyurethane was further modified with poly (propylene oxide) (PPO) diol introduced to provide a 1: 1 molar ratio of polycaprolactone to PPO (the polymer is shown as "H12 MDI-high PCL/PPO" as described in example 6 below), which effectively reduces the concentration of polyester diol and potentially improves the bonding properties. However, the material obtained still showed crystallization during DSC testing; since it shows a melting transition on the first heating and not on the second heating, the tendency of the polyurethane to crystallize is nevertheless greatly reduced compared with polyurethanes without PPO diol.

Accordingly, the polyurethane tested was further modified by replacing the polycaprolactone diol of formula I above with a neopentyl glycol-based diol of formula II above having a molecular weight of 2000 (as described in example 7 below; the polyurethane is shown as "H12 MDI-NPGD/PPO"). As can be seen from the DSC data reported in this example, there was no melt transition in both heating cycles, which suggests that polymer crystallization has been completely eliminated.

It appears that increasing the ratio of polyether polyol to polyester polyol, while maintaining the polymer molecular weight and the isocyanate to hydroxyl mole ratio constant, enhances adhesion to a given substrate (by measuring peel force) and reduces the rate of polymer hydrolysis. And in practice polyurethanes containing relatively high proportions of polyether polyols may be synthesized using relatively small amounts of N-methylpyrrolidone, a material which is prone to cause problems in some electro-optic displays and may therefore have to be removed before the polyurethane can be used as a laminating adhesive in an electro-optic display.

Some examples are given below only by way of illustration to further illustrate the preferred polyurethanes of the present invention.

Example 1: custom polyurethanes with short polyester segments

The polyurethane prepolymer was synthesized under nitrogen atmosphere in a 500 ml jacketed glass reactor equipped with a mechanical stirrer, thermometer and nitrogen inlet. 4,4' -methylenebis (cyclohexyl isocyanate) (20.99 g, Bayer Desmodur W), polycaprolactone diol (31.25 g, Aldrich, average Mn about 1250), and dibutyltin dilaurate (0.04 g, Aldrich) were charged to a reactor and the mixture was heated at 80 ℃ for 2 hours. Thereafter, a solution of 1-methyl-2-pyrrolidone (10 g, Aldrich) in 2, 2-bis (hydroxymethyl) propionic acid (3.35 g, Aldrich) was added to the reactor, and the reaction was allowed to continue at 80 ℃ for an additional 1 hour to give an isocyanate terminated prepolymer. The reactor temperature was then lowered to 60 ℃ for 30 minutes and triethylamine (2.4 g, Aldrich) was added to neutralize the carboxylic acid. The reactor temperature was then further reduced to 30 ℃ and deionized water (105 grams) was added to convert the prepolymer to an aqueous dispersion. After the dispersing step, chain extension was carried out immediately at 30 ℃ for more than 1 hour using hexamethylenediamine (3.5 g, Aldrich) dissolved in a small amount of deionized water. Finally, the dispersion was heated at 60 ℃ for 1 hour to ensure that all residual isocyanate groups had reacted.

Example 2: custom polyurethanes with polyesters containing sterically hindered groups

The polyurethane prepolymer was synthesized under nitrogen atmosphere in a jacketed glass reactor equipped with a mechanical stirrer, thermometer and nitrogen inlet. 4,4' -methylenebis (cyclohexyl isocyanate) (18.3 g) was charged to the reactor followed by 1-methyl-2-pyrrolidone (27 g), polycaprolactone diol (molecular weight 2000, 49.3 g), and 2, 2-bis (hydroxymethyl) propionic acid (3.4 g). The reactor was heated to 95 ℃ for 4 hours and then cooled to 70 ℃ to form an isocyanate terminated prepolymer. Triethylamine (2.0 g) was slowly added to the prepolymer and the resulting mixture was mixed for 30 minutes. Water (84 grams) was added to a second reactor equipped with a mechanical stirrer and thermometer. Subsequently, the prepolymer (80 g) was slowly transferred from the first reactor to the second reactor with stirring. Hexamethylene diamine (2.7 g of a 70% solution) was added to the second reactor for chain extension and the resulting dispersion was heated to 80 ℃ for 1 hour to complete the reaction.

Example 3: thermal Properties of polyurethane

The polyurethanes prepared in examples 1 and 2 above were tested using differential scanning calorimetry. To provide a control, the aforementioned 75: 25R 9314/R9621 blend (shown simply as "blend" in FIG. 3) was also tested in the same manner. The results are shown in fig. 3.

As can be seen in fig. 3, the blend shows a marked exothermic peak at about 40 ℃, corresponding to the melting of the polyester segment in R9314; when heated again, this peak disappeared. There are no comparable peaks in the curves for the polyurethanes of the invention, indicating that the use of short polyester segments with or without steric hindrance groups is indeed effective in eliminating the thermal transition seen in R9314 and ensuring long-term thermal stability of the polyurethane adhesives of the invention.

Example 4: storage stability of polyurethane

This test was conducted to determine whether the polyurethane of example 2 has better storage stability than the aforementioned R9314/R9621 blend. To this end, a dried film of each adhesive was laminated between two ITO-coated polyester poly (ethylene terephthalate) films, the ITO layer naturally being in direct contact with the polyurethane. To ensure that all samples have the same thermal history, they were heated in an oven at 70 ℃ for several hours and then held at 25 ℃ and 50% relative humidity for 5 days. In the tests described below, the time zero is taken from the end of this storage period.

The samples were then held at 25 ℃ and 50% relative humidity for 6 weeks, and the resistance of the adhesive layer was measured at intervals. The results are shown in fig. 4. As can be seen from the figure, the resistance of the blend increased by about 50 percent over the course of the experiment, while the resistance of the polyurethane of inventive example 2 changed by no more than about 10 percent from the initial value. Significant changes in the polyurethane are indeed possible as a result of experimental error; since the experiment did not control the relative humidity during the resistance measurement, some random variation, up to about 10 percent, was understood to be likely due to the above reasons.

The results in fig. 4 suggest that avoiding the polymer crystallization experienced by the prior art polyurethane blends can help stabilize the electrical properties of the adhesive. As previously noted, the electrical properties of the adhesive significantly affect the electro-optic performance of electro-optic displays, and therefore, the results in fig. 4 illustrate that the use of the polyurethane adhesive of the present invention should help provide time-stable electro-optic performance in such displays.

Example 5: synthesis of H12MDI-high PCL (control)

The polyurethane prepolymer was synthesized under a nitrogen atmosphere in a three-necked round bottom flask equipped with a mechanical stirrer, septum, and nitrogen inlet. 4,4' -methylenebis (cyclohexyl isocyanate) (31.4 g, Bayer Desmodur W), polycaprolactone diol (100 g, Aldrich, average Mn of about 2000), 2-bis (hydroxymethyl) propionic acid (6.7 g, Aldrich), 1-methyl-2-pyrrolidone (56 g, Aldrich), and dibutyltin dilaurate (0.04 g, Aldrich) were charged to a flask and the mixture was heated at 95 ℃ for 5 hours to give an isocyanate terminated prepolymer. The reactor temperature was then lowered to 70 ℃ and triethylamine (5 g, Aldrich) was added to neutralize the carboxylic acid and the reaction mixture was allowed to stabilize for 30 minutes. Water (210 g) was charged to a 500 ml jacketed reactor equipped with a mechanical stirrer, nitrogen inlet and thermometer. The prepolymer is then transferred to an aqueous reactor with stirring to form an aqueous dispersion. After the dispersion step, chain extension was performed immediately at room temperature using hexamethylenediamine (3.2 g, Aldrich) dissolved in a small amount of deionized water over 30 minutes. Finally, the dispersion was heated at 70 ℃ for 1 hour to ensure that all residual isocyanate groups had reacted.

Example 6: synthesis of H12MDI-PCL/PPO

The polyurethane prepolymer was synthesized under a nitrogen atmosphere in a three-necked round bottom flask equipped with a mechanical stirrer, septum, and nitrogen inlet. 4,4' -methylenebis (cyclohexyl isocyanate) (31.4 g, Bayer Desmodur W), polycaprolactone diol (50 g, Aldrich, average Mn of about 2000), poly (propylene oxide) diol (50 g, Aldrich, average Mn of about 2000), 2-bis (hydroxymethyl) propionic acid (6.7 g, Aldrich), 1-methyl-2-pyrrolidone (40 g, Aldrich), and dibutyl dilaurate (0.04 g, Aldrich) were charged to a flask and the mixture was heated at 95 ℃ for 6 hours to form an isocyanate terminated prepolymer. The reactor temperature was then lowered to 70 ℃, triethylamine (3.79 g, Aldrich) was added to neutralize the carboxylic acid, and the reaction mixture was allowed to stabilize for 30 minutes. Water (220 g) was charged to a 500 ml jacketed reactor equipped with a mechanical stirrer, nitrogen inlet and thermometer. The prepolymer is then transferred to an aqueous reactor with stirring to form an aqueous dispersion. After the dispersion step, chain extension was performed immediately at room temperature using hexamethylenediamine (4.6 g, Aldrich) dissolved in a small amount of deionized water over 30 minutes. Finally, the dispersion was heated at 70 ℃ for 1 hour to ensure that all residual isocyanate groups had reacted.

Example 7: synthesis of H12MDI-NPGD/PPO

The polyurethane prepolymer was synthesized under a nitrogen atmosphere in a three-necked round bottom flask equipped with a mechanical stirrer, septum, and nitrogen inlet. 4,4' -methylenebis (cyclohexyl isocyanate) (31.4 g, Bayer DesmodurW), the 2200A diol of CPAP (50 g, available from Solvay SA, Brussels, Belgium, average Mn of about 2000), poly (propylene oxide) diol (50 g, Aldrich, average Mn of about 2000), 2-bis (hydroxymethyl) propionic acid (6.7 g, Aldrich), 1-methyl-2-pyrrolidone (40 g, Aldrich), and dibutyltin dilaurate (0.04 g, Aldrich) were charged to a reactor and the mixture was heated at 95 ℃ for 6 hours to give an isocyanate terminated prepolymer. The reactor temperature was then lowered to 70 ℃ and triethylamine (3.79 g, Aldrich) was added to neutralize the carboxylic acid and the reaction mixture was allowed to stabilize for 30 minutes. Water (220 g) was charged to a 500 ml jacketed reactor equipped with a mechanical stirrer, nitrogen inlet and thermometer. The prepolymer is then transferred to an aqueous reactor with stirring to form an aqueous dispersion. After the dispersion step, chain extension was performed immediately at room temperature using hexamethylenediamine (4.6 g, Aldrich) dissolved in a small amount of deionized water over 30 minutes. Finally, the dispersion was heated at 70 ℃ for 1 hour to ensure that all residual isocyanate groups had reacted.

Fig. 5 is a DSC scan of a film of the resulting polymer after drying, showing that the film after drying is free of crystalline material.

Although the invention has been described above primarily in relation to encapsulated electrophoretic media having discrete capsules, similar advantages can be obtained when the adhesive of the invention is used in other types of electro-optic displays as discussed previously.

Claims (17)

1. A polyurethane formed from an isocyanate, a polyether diol and a polyester diol, the polyester diol having a molecular weight of less than 2000, or comprising two polyester diol segments linked by a steric hindrance group, each polyester diol segment having a molecular weight of less than 2000, the polyether diol to polyester diol molar ratio being from 1:9 to 9:1, wherein the polyether diol is a poly (propylene oxide) diol.

2. The polyurethane of claim 1, wherein the molar ratio of polyether diol to polyester diol is from 1:4 to 4: 1.

3. The polyurethane of claim 1 wherein the polyester diol is a polycaprolactone diol having a molecular weight of less than 1500.

4. The polyurethane of claim 1, wherein the polyester diol is a polycaprolactone diol comprising two polycaprolactone segments connected by a bending group, each polycaprolactone segment having a molecular weight no greater than 1500.

5. The polyurethane of claim 1 wherein the isocyanate is 4,4' -methylenebis (cyclohexyl isocyanate).

6. The polyurethane of claim 1, wherein the polyether diol has a molecular weight of 1000 to 3000.

7. The polyurethane of claim 1 having an isocyanate to hydroxyl ratio of less than 1.3.

8. The polyurethane of claim 1 having a crossover temperature in the range of 10 to 90 ℃ at 1 Hz.

9. The polyurethane of claim 8 having a crossover temperature in the range of 40 to 80 ℃ at 1 Hz.

10. The polyurethane of claim 1 in the form of an aqueous latex.

11. A sub-assembly for use in constructing an electro-optic display, the sub-assembly comprising a layer of solid electro-optic material, and a layer of lamination adhesive adhered to the layer of electro-optic material, the layer of lamination adhesive comprising the polyurethane of claim 1.

12. An electro-optic display comprising a sub-assembly according to claim 11 and at least one electrode arranged to apply an electric field to the layer of electro-optic material.

13. An electronic book reader, portable computer, tablet computer, cell phone, smart card, sign, watch, shelf label or flash drive comprising a display according to claim 12.

14. The subassembly of claim 11, further comprising:

a light-transmissive electrically-conductive layer on an opposite side of the layer of electro-optic material from the lamination adhesive layer; and

a release sheet on the side of the lamination adhesive layer opposite the layer of electro-optic material.

15. The subassembly of claim 11, further comprising:

a second adhesive layer on an opposite side of the layer of electro-optic material from the lamination adhesive layer; and

a release sheet disposed on an opposite side of the second adhesive layer from the layer of electro-optic material.

16. The subassembly of claim 11, further comprising:

at least one of a light-transmissive protective layer and a light-transmissive electrically-conductive layer on an opposite side of the lamination adhesive layer from the electro-optic material layer; and

a release sheet on an opposite side of the layer of electro-optic material from the layer of laminating adhesive.

17. The subassembly of claim 11, further comprising: first and second release sheets overlying exposed surfaces of the layer of electro-optic material and the layer of laminating adhesive.