HK1143421B

HK1143421B - Electro-optic displays, and materials for use therein

Info

Publication number: HK1143421B
Application number: HK10109952.5A
Authority: HK
Inventors: S‧N‧法泽尔; G‧M‧丹纳; 曹岚; D‧D‧米勒; E‧H‧克林根伯格
Original assignee: 伊英克公司; 空气产品和化学品有限公司
Priority date: 2007-06-26
Filing date: 2008-06-26
Publication date: 2014-09-05

Description

Electro-optic displays and materials for use therein

Technical Field

The present application relates to:

(a) U.S. patent nos. 6,831,769;

(b) U.S. patent publication nos. 2005/0122563;

(c) U.S. Pat. Nos. 7,012,735;

(d) U.S. Pat. Nos. 7,173,752;

(e) U.S. patent nos. 7,110,164; and

(f) U.S. patent No.7,349,148.

The present invention relates to electro-optic displays and materials for use therein. More particularly, the present invention relates to preventing pore growth in electro-optic displays. The invention is intended for use particularly, but not exclusively, in displays comprising encapsulated electrophoretic media. Some of the materials provided by the present invention may be used in applications other than electro-optic displays.

Background

Background terminology and status regarding the art of electro-optic displays is discussed in detail in U.S. Pat. No.7,012,600 and the aforementioned U.S. Pat. No.7,173,752, to which the reader is referred for further information. Accordingly, the following will briefly summarize the terminology and the status of the technical field.

The term "electro-optic" as used herein in the conventional sense of imaging technology for materials or displays refers to a material having first and second display states differing in at least one optical property, the material being transitioned from the first display state to the second display state by application of an electric field to the material. In the displays of the invention, the electro-optic medium is generally solid (such displays will hereinafter be referred to for convenience as "solid electro-optic displays"), by which is meant a solid in the sense that the electro-optic medium has a solid outer surface, although the medium may, and typically does, have an internal liquid or gas-filled space. Thus, the term "solid state electro-optic display" includes encapsulated electrophoretic displays, encapsulated liquid crystal displays, and other types of displays discussed below.

The term "gray state" is used herein in its conventional meaning in the imaging arts to refer to an intermediate state between the two extreme optical states of a pixel, and does not necessarily imply a black-and-white transition between the two extreme states. The terms "black" and "white" are used hereinafter to refer to the two extreme optical states of the display and should be understood to generally include extreme optical states which are not strictly black and white. The term "monochrome", as used hereinafter, is defined as a driving scheme that drives pixels only to their extreme optical states without intervening grey states.

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.

The term "pulse" is used herein in its conventional sense: integral of voltage over time. However, some bistable electro-optic media are used as charge transducers and an alternative definition of a pulse, i.e. the integral of current with respect to time (equal to the total charge applied), can be used for such media. Depending on whether the medium is used as a voltage-time pulse transducer or a charge pulse transducer, the appropriate pulse definition should be used.

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

(a) rotating bichromal member displays (see, e.g., U.S. Pat. Nos. 5,808,783; 5,777,782; 5,760,761; 6,054,0716,055,091; 6,097,531; 6,128,124; 6,137,467 and 6,147,791);

(b) electrochromic displays (see, e.g., O' Regan, B. et al, Nature 1991, 353, 737; Wood, D., Information Display, 18(3), 24 (3.2002); Bach, U. et al, adv. Mater., 2002, 14(11), 845; and U.S. Pat. Nos. 6,301,038, 6,870.657 and 6,950,220);

(c) electrowetting displays (see Hayes, r.a. et al, "Video-Speed electronic paper Based on Electrowetting" ("Video Speed electronic paper Based on Electrowetting technology"), Nature, 425, 383-;

(d) particle-based electrophoretic displays in which a plurality of charged particles move in 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 application publication No. WO00/38000; WO 00/36560; WO 00/67110 and WO 01/07961; and European patent No.1,099,207Bl; and 1,145,072 Bl; and other Patents and applications of the Massachusetts Institute of Technology (MIT) and the Inyke (InE k) company, 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; see, for gaseous fluids, for example, Kitamura, T.et al, "Electrical Toner movement for electronic Paper-like display" ("movement of electronic Toner in electronic Paper-like display"), IDWJapan, 001, Paper HCS1-I and Yamaguchi, Y.et al, "Toner display using electrostatically charged insulating particles charged three plasma display" ("Toner display using electrostatically charged insulating particles"), IDW Japan, 2001, Paper AMD 4-4); 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; 1,501,194, 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 and comprise a plurality of capsules, each of which itself comprises an internal phase comprising electrophoretically mobile particles suspended in a liquid suspending medium, and a capsule wall surrounding the internal phase. Typically, the capsules themselves are held in a polymeric binder to form an adhesive layer between two electrodes; see the aforementioned patents and applications for MIT and EInk. Alternatively, the walls surrounding the discrete microcapsules in an encapsulated electrophoretic medium comprising a plurality of discrete droplets of an electrophoretic fluid and a continuous phase of a polymeric material may be replaced with a continuous phase, thus producing a so-called polymer dispersed electrophoretic display; 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 held within a plurality of cavities formed within a carrier medium, typically a polymer film, see, for example, U.S. patent nos. 6,672,921 and 6,788,449.

Electrophoretic media may operate in a "shutter mode" in which one display state is substantially opaque and one display state is light transmissive. See, for example, U.S. Pat. Nos. 6,130,774 and 6,172,798, and U.S. Pat. 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 can also operate in the shutter mode. An electro-optic display comprises a layer of electro-optic material, which term 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 transitioned from the first display state to the second display state by application of an electric field to the material. Although this optical property is generally a color perceptible to the human eye, it may be other optical properties, such as transmission of light, reflection, fluorescence or, in the case of displays intended for machine reading, a false color in the sense of a change in the reflectance of electromagnetic wavelengths outside the visible range.

Other types of electro-optic media may also be used in the displays of the present invention.

In addition to the layer of electro-optic material, an electro-optic display typically includes at least two other layers, one of which is an electrode layer, disposed on opposite sides of the electro-optic material. The manufacture of a three-layer electro-optic display typically involves at least one lamination operation. For example, in a number of the aforementioned patents and applications by the Massachusetts Institute of Technology (MIT) and Eink corporation, a process is described for manufacturing an encapsulated electrophoretic display in which an encapsulated electrophoretic medium comprising capsules in an adhesive is coated onto a flexible substrate comprising Indium Tin Oxide (ITO) or similar conductive coating on a plastic film, which serves as one electrode of the final display, and the capsule/adhesive coating is dried to form an electrophoretic medium adhesion layer which adheres strongly to the substrate. A backplane is separately prepared, which backplane comprises an array of pixel electrodes and an appropriate conductor arrangement for connecting the pixel electrodes to a drive circuit. To form the final display, the substrate with the bladder/adhesive layer thereon is laminated to the backplane using a lamination adhesive (by replacing the backplane with a simple protective layer such as a plastic film, a very similar process can be used to prepare an electrophoretic display for use with a stylus or similar movable electrode that can slide over the protective layer).

As discussed in detail in the aforementioned us patent 6,831,769, the laminating adhesive used in electro-optic displays should meet certain electrical standards, and this presents considerable problems in the selection of the laminating adhesive. Commercial manufacturers of laminating adhesives naturally make considerable effort to ensure that the properties of such adhesives, such as adhesive strength and lamination temperature, are adjusted so that these adhesives perform well in their primary applications typically involving laminating polymeric or similar films. However, in these applications, the electrical properties of the laminating adhesive are irrelevant and therefore commercial manufacturers will not notice such electrical properties. Indeed, there is considerable variation (up to several times) in certain electrical properties between different batches of the same commercial laminated adhesive, presumably because the manufacturer has not been concerned with the ultimate variation in electrical properties at all by attempting to optimize the non-electrical properties of the laminated adhesive (e.g., against bacterial growth).

The number of commercial materials that can meet most of the previously described disparate requirements for lamination adhesives used in electro-optic displays is very small, and small amounts of water-dispersed polyurethane emulsions have been used for this purpose in practice. A similar group of materials has been used as binders for encapsulated electrophoretic media.

However, the use of such polyester-based polyurethane emulsions as laminating adhesives is still not a completely satisfactory compromise between the desired mechanical and electrical properties. Laminating adhesives such as propylene polymers and pressure sensitive adhesives are available with better mechanical properties, but the electrical properties of these materials are not suitable for use in electro-optic displays. In addition, there has not been a satisfactory way to alter the electrical properties of polyurethane emulsions to "fine tune" them to match the electrical properties of a particular electro-optic medium. It would therefore be highly advantageous if some way could be found to "decouple" the mechanical and electrical properties of the laminating adhesive so that each set of properties could be individually optimized, i.e., in practice one could select an adhesive with the desired high mechanical properties and then optimize its electrical properties for use with a particular electro-optic medium. One aspect of the present invention provides a way to alter the electrical properties of an adhesive without substantially affecting its mechanical properties. The invention can also be used to modify the electrical properties of an adhesive without substantially affecting its mechanical properties.

In addition, given the selection of lamination adhesives for use in electro-optic displays, attention must be paid to the process of assembling the display. Most prior art processes for final lamination of electrophoretic displays are essentially batch processes, where the electro-optic medium, the laminating adhesive and the backplane are only combined together prior to final assembly, and it is therefore desirable to provide a process that is more suitable for mass production. However, the aforementioned U.S. Pat. No.6,982,178 describes a method of assembling solid state electro-optic displays, including encapsulated electrophoretic displays, which is well suited for mass production. This patent essentially describes a so-called "front panel 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 and in the present patent application 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). Where the electro-optic medium exhibits a change in reflectivity at non-visible wavelengths, the term "light transmissive" should of course be construed to refer to transmission of the relevant non-visible wavelengths. Typically, the substrate is 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 or metal oxide layer, such as aluminium or 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.

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

The aforementioned U.S. patent 6,982,178 also describes a method for testing the electro-optic medium in a front plane stack prior to introducing the front plane stack 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 in the medium, thereby avoiding the ultimate cost of laminating a defective electro-optic medium into the display, and 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 2004/0155857 describes a so-called "double release panel" which is essentially a simplified version of the front panel laminate of the aforementioned U.S. Pat. No.6,982,178. One form of dual release sheet comprises a layer of solid electro-optic medium sandwiched between two adhesive layers, one or both of which are covered by a release sheet. 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 process substantially similar to that already described for assembling an electro-optic display from a front plane laminate, but which includes 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, although the order of the two laminations can be reversed if desired.

The foregoing 2007/0109219 describes a so-called "inverted front panel stack," which is a variation of the front panel stack described in the foregoing U.S. Pat. No.6,982,178. The inverted front panel stack sequentially comprises: at least one of a light-transmissive protective layer and a light-transmissive electrically-conductive layer, an adhesive layer, a layer of solid electro-optic medium, and a release sheet. The inverted front plane stack is used to form an electro-optic display having a laminate adhesive layer between the electro-optic layer and the front electrode or front substrate; there may or may not be a second, typically very thin, adhesive layer between the electro-optic layer and the backplane. The electro-optic display has both good definition and low temperature performance.

The foregoing 2007/0109219 also describes various methods designed for large-scale manufacturing of electro-optic displays employing an inverted front plane laminate. The preferred approach to these methods is a "multiple-up" method designed to allow simultaneous lamination of multiple components for multiple electro-optic displays.

In view of the advantages of the assembly methods described in the aforementioned U.S. patent 6,982,178 that employ a front plate stack, it is desirable to be able to incorporate a lamination adhesive into such a front plate stack. It is also desirable to be able to introduce a lamination adhesive into the dual release film and into the inverted front panel stack as previously described.

As already mentioned, the lamination process used to manufacture electro-optic displays imposes stringent requirements on the mechanical and electrical properties of the laminating adhesive. In the final display the laminating adhesive is located between electrodes for applying the electric field required to change the electrical state of the electro-optic medium so that the electrical properties of the adhesive become critical. It is obvious to the electrical engineer that the bulk resistivity of the lamination adhesive becomes important because the voltage drop over the electro-optic medium is substantially equal to the voltage drop over the electrodes minus the voltage drop over the lamination adhesive. If the resistivity of the laminating adhesive is too high, a considerable voltage drop will occur within the adhesive layer, requiring the voltage on the electrodes to be raised. Increasing the voltage on the electrodes in this way is undesirable because it increases the power consumption of the display and requires the use of more complex and more expensive control circuitry to handle the increased voltage involved.

However, there are other constraints that the laminating adhesive must satisfy. Void growth is encountered in many types of solid state electro-optic displays and it is therefore important to protect the final display from void growth in order to ensure a high quality display, since such voids will produce visible imperfections in the image written on the display, as explained below. To ensure that the final display is void free, it is critical that the lamination to form the front sheet stack (when implemented) and the final lamination to the back sheet be performed without forming voids. It is also desirable that the resulting display be able to withstand significant temperature changes (such as may occur when a portable computer or personal digital assistant is moved from an air-conditioned automobile to hot outdoor sunlight) without introducing or aggravating the formation of voids, since it has been found that some displays that initially appear substantially void-free can create undesirable voids when exposed to such temperature changes. This phenomenon is termed "pore regeneration".

It has been previously determined that when a display of the preferred type described in the aforementioned U.S. patent 6,982,178 and comprising an encapsulated electrophoretic medium laminated to a Thin Film Transistor (TFT) backplane by a polyurethane laminating adhesive is exposed to elevated temperatures (say 70-90 ℃) for an extended period of time (over about 10 hours), voids begin to appear at the interface between the laminating adhesive and the backplane, and grow to form air gaps between the laminating adhesive and the backplane. These air gaps cause visible imperfections in the image formed on the electrophoretic medium because the electrophoretic medium does not switch between its optical states in the areas affected by the air gaps. As a result, the aperture and associated non-switching region can be grown to a large size, typically about 1 to 5mm in diameter.

The aforementioned U.S. Pat. nos. 7,173,752 and 7,349,148 describe the use of thermal crosslinkers in the adhesion layer to reduce pore growth in electro-optic displays. The crosslinker may include an epoxy group in the form of a glycidyl group (i.e., an epoxy methyl group). The crosslinking agent may also include a tertiary amine. A particularly preferred crosslinking agent is described as N, N-diglycidylaniline which is present in the adhesive layer at a concentration of at least about 5000 parts per million by weight, and preferably at least about 10000 parts per million. Other useful classes of crosslinking agents include epoxy ethers of alkyl or cycloalkyl polyols having at least two hydroxyl groups, and polymers having a backbone and a plurality of epoxy groups depending from the backbone. Specific useful crosslinkers described include homopolymers and copolymers of 1, 4-cyclohexanedimethanol diglycidyl ether, neopentyl glycol diglycidyl ether, O-triglycidyl glycerol, and glycidyl methacrylate.

The thermal crosslinkers described in the aforementioned U.S. Pat. nos. 7,173,752 and 7,349,148 are very effective in reducing pore growth because they provide a curable adhesive layer that is sufficiently hard to resist pore growth at high temperatures and sufficiently flexible to provide good adhesion between the different layers of the electro-optic display, especially if both the front substrate and the backplane of the display are flexible. However, from a manufacturing point of view, these crosslinking agents present certain problems. Generally, in the manufacture of a front plane stack, an inverted front plane stack or a dual release plane, an electro-optical layer (e.g., a microencapsulated or polymer dispersed electrophoretic layer) is coated onto a substrate (which in some cases is simply a release plane) and the resulting substrate/electro-optical layer subassembly is then laminated, typically under heat and pressure, onto an adhesive layer that has previously been coated onto the release plane. The adhesive layer is heated during lamination to allow the adhesive material to flow and thereby eliminate any surface irregularities on the adjacent surface of the electro-optic layer.

When using the thermal crosslinkers described in the aforementioned U.S. Pat. nos. 7,173,752 and 7,349,148, lamination of the electro-optical layer to the adhesive layer must be performed before the crosslinking reaction is carried out, otherwise poor lamination results will be obtained. Thus, the adhesive/crosslinker mixture must be stored prior to application to the electro-optic layer. Since the crosslinking reaction is thermally initiated, the reaction proceeds slowly at room temperature; in the case of the preferred N, N-diglycidylaniline, the reaction proceeds to an unacceptable degree in less than one week at room temperature. Storage of the adhesive/crosslinker mixture at 5 ℃ under refrigeration can extend the shelf life of the mixture up to 8 weeks, but this is often inconvenient in a manufacturing environment. The relatively short shelf life of such adhesive/crosslinker mixtures limits certain types of manufacturing process flows and inventories; for example, it may be desirable to prepare small batches of adhesive/crosslinker mixtures at frequent intervals and apply the mixtures and use up the applied adhesive layer in a short period of time, whereas from a manufacturing standpoint it is generally more convenient to mix and apply large batches of adhesive, which may be stored and used as needed.

It has now been found that the use of epoxidized vegetable oils as thermal crosslinkers in adhesives can increase the shelf life of the adhesive/crosslinker mixture, while still allowing full cure at relatively low temperatures and good prevention of pore growth.

Disclosure of Invention

Accordingly, in one aspect, the present invention provides an electro-optic display comprising:

a layer of solid electro-optic material capable of changing at least one optical property upon application of an electric field thereto;

a backplane comprising at least one electrode arranged to apply an electric field to the layer of electro-optic material; and

an adhesive layer disposed between the layer of electro-optic material and the backplane and adhesively securing the layer of electro-optic material to the backplane,

the adhesive layer includes a thermally activated cross-linking agent capable of cross-linking the adhesive layer when exposed to an activation temperature, the cross-linking agent including an epoxidized vegetable oil fatty acid or an epoxidized ester of such a fatty acid.

Most common epoxidized vegetable oil fatty acids and esters are derived from soybean oil and linseed oil. The epoxidized fatty acid or ester may be proprietary in the United statesOf the type described in application publication No. 2006/0020062. This published application describes coating compositions comprising a latex resin and C of a vegetable oil fatty acid_l-6Alkyl esters, C_2-6Alkenyl esters, epoxidation C_2-6Alkenyl esters, monoglycerides, diglycerides, C_4-6A polyol ester or an ethylene glycol ester, wherein the ester has at least one oxirane ring formed between two adjacent carbons in a carbon chain of a fatty acid. A specific commercial epoxidized oil which has been found to be useful in the present invention is Vikoflex 7190 ("VIKOFLEX" is a registered trademark) sold by Arkema Inc, 2000 Market Street, Philadelphia PA 19103-. The manufacturer states that this material is an epoxidized linseed oil with an oxirane oxygen content of 9%, which implies an average of about 6 oxirane groups per molecule (for reasons discussed below, it is advantageous to use an epoxidized oil or ester with at least about 3 epoxide groups per molecule). The epoxidized oil or ester is present in the adhesive layer at a concentration of at least about 5000 parts per million by weight, and preferably at least about 10000 parts per million. Generally, the optimum concentration of the crosslinking agent appears to be about 20000 to about 40000 parts per million by weight. Vikoflex 4050, 5075, 7010, 7040, 7080, 7170, 9010, 9040 and 9080 from the same manufacturer have also been demonstrated to be useful in the present invention. The CAS numbers and other data for these materials provided by the manufacturer are listed in table 1 below.

TABLE 1

	Esters of phenyl or naphthyl
				Vikoflex 5075		Epoxidized propylene glycol dioleate	334	348
Vikoflex 7010	Epoxidized methyl esters of soybean fatty acids	228	240
				Vikoflex 7040		Is unknown	235	258
Vikoflex 7080	Epoxidized octyl esters of soybean fatty acids	275	285
				Vikoflex 9010		Epoxidized methyl esters of linseed oil fatty acids	185	200
Vikoflex 9040	Epoxidized butyl esters of linseed oil fatty acids	178	200
				Vikoflex 9080	71302-79-9	Epoxidized 2-ethylhexyl esters of linseed oil fatty acids	206	225

Other useful commercial epoxidized oils include those sold under the registered trademark "FLEXOL" by Dow chemical Corporation, Midland ML.

In the electro-optic displays of the present invention, the adhesive layer may comprise a polyurethane, typically formed from an aqueous polyurethane latex. Suitable aqueous polyurethane latexes are commercially available and their preparation is well known in the art. Generally, these aqueous polyurethane latexes include polyurethane-polyurea polymers, which are formed by the reaction of at least one isocyanate with a polyol and a polyamine. Various commercially available isocyanates, polyols and polyamines can be used to form the polyurethane-polyurea polymer. Nonionic or ionic groups are typically added to the polymer chain as internal emulsifiers to make the polymer water-dispersible. Since the epoxy groups on the epoxidized oil fatty acid or ester react primarily with the acid groups on the polymer backbone, it is preferred to use ionically stabilized polymers in the present invention. Carboxylates and sulfonates are mainly used as ionic emulsifiers for polyurethane dispersion.

In the electro-optic displays of the present invention, the adhesive layer may include an agent other than a crosslinking agent effective to reduce the bulk resistivity of the crosslinked adhesive layer. As described in the aforementioned U.S. patent nos. 7,173,752 and 7,349,148, the bulk resistivity reducing agent includes at least one of a salt, a polyelectrolyte, or a hydroxyl-containing polymer having a number average molecular weight of no more than about 5000. Preferred bulk resistivity reducing agents are quaternary ammonium salts and polyethylene glycols. For example, the bulk resistivity-reducing agent includes tetrabutylammonium chloride, tetrabutylammonium hexafluorophosphate, or polyethylene glycol having a number average molecular weight of no more than about 2000.

The invention also provides a method for manufacturing an electro-optic display, the method comprising:

providing an assembly comprising a layer of solid electro-optic material capable of changing at least one optical property upon application of an electric field thereto; a backplane comprising at least one electrode arranged to apply an electric field to the layer of electro-optic material; and an adhesive layer disposed between the layer of electro-optic material and the backplane and adhesively securing the layer of electro-optic material to the backplane, the adhesive layer comprising a thermally-activated cross-linking agent capable of cross-linking the adhesive layer, the cross-linking agent comprising an epoxidized vegetable oil fatty acid or an epoxidized ester of such a fatty acid; and

exposing the adhesive layer to a temperature sufficient to activate the crosslinking agent, thereby crosslinking the adhesive layer.

The invention also provides an electro-optic display comprising:

the adhesive layer has been crosslinked by a thermally activated crosslinking agent comprising an epoxidized vegetable oil fatty acid or an epoxidized ester of such a fatty acid.

The invention also provides an article of manufacture (front panel laminate) comprising, in order:

a light-transmitting conductive layer;

a layer of solid electro-optic medium in electrical contact with the electrically conductive layer;

an adhesive layer; and

the release plate is arranged on the upper surface of the shell,

The present invention also provides an article of manufacture (dual release film) comprising:

a layer of solid electro-optic medium having first and second surfaces on opposite sides thereof;

a first adhesive layer on the first surface of the layer of solid electro-optic medium;

a release sheet disposed on the opposite side of the first adhesive layer from the layer of solid electro-optic medium; to know

A second adhesive layer on said second surface of said layer of solid electro-optic medium,

wherein at least one of the first and second adhesive layers comprises a thermally activated cross-linking agent capable of cross-linking the adhesive layer in which it is located when exposed to an activation temperature, the cross-linking agent comprising an epoxidized vegetable oil fatty acid or an epoxidized ester of such a fatty acid.

The invention also provides an article of manufacture (inverted front panel laminate) comprising, in order:

a release plate;

a layer of solid electro-optic medium;

an adhesive layer; and

at least one of a light-transmissive protective layer and a light-transmissive conductive layer,

The aforementioned front plane laminate, inverted front plane laminate and dual release film of the present invention may comprise any of the optional features of such front plane laminates, inverted front plane laminates and dual release films described in the aforementioned patents and applications. Thus, for example, the front plane stack of the invention may comprise conductive vias in contact with the conductive layer of the front plane stack and extending through the electro-optical medium thereof, and contact pads in contact with the conductive vias and arranged to be in contact with electrodes provided on the back plane to which the front plane stack is to be laminated.

The invention also provides an adhesive composition comprising a polyurethane and a thermal crosslinker capable of crosslinking the polyurethane when exposed to an excitation temperature, the crosslinker comprising an epoxidized vegetable oil fatty acid or an epoxidized ester of such a fatty acid.

The present invention also provides a laminated structure comprising first and second layers of solid glass, metal or plastic, and an adhesive layer disposed between and securing the first and second layers together, wherein the adhesive layer comprises a thermally activated cross-linking agent capable of cross-linking the adhesive layer when exposed to an activation temperature, the cross-linking agent comprising an epoxidized vegetable oil fatty acid or an epoxidized ester of such a fatty acid.

In such a laminate structure, at least one of the first and second layers has at least one layer of inorganic, organic or metallized features therein or on a surface thereof. The laminated structure is in the form of a printed circuit board. The crosslinking agent has an average of at least about 3 ethylene oxide groups per molecule. The adhesive layer may comprise polyurethane. The adhesive layer may also include an agent effective to reduce the bulk resistivity of the adhesive layer after crosslinking.

In a broader aspect, the invention provides an electro-optic display comprising:

the adhesive layer includes a thermally activated cross-linking agent capable of cross-linking the adhesive layer when exposed to an activation temperature, the cross-linking agent including an epoxidized material such that at a concentration of 0.195 mole epoxide equivalents per kilogram resin:

(a) after holding at 50 ℃ for 120 hours, the adhesive layer is less than 70% cured as measured by the swell ratio; and

(b) the fully cured resin has at least about 5x10 measured at 0.1Hz and 100 deg.C⁴Pa storage modulus.

In such an electro-optic display, the adhesive layer comprises a polyurethane and the cross-linking agent comprises an epoxidized vegetable oil fatty acid or an epoxidized ester of such a fatty acid.

Finally, the present invention provides an adhesive composition comprising a polyurethane and a thermal crosslinker capable of crosslinking the polyurethane when exposed to an excitation temperature, the crosslinker comprising an epoxidized material such that at a concentration of 0.195 mole epoxide equivalents per kilogram of resin:

Drawings

FIG. 1 of the accompanying drawings is a graph showing the shelf life of an adhesive composition of the present invention as a function of the epoxy equivalent weight of the crosslinker employed, as determined in the experiment reported in example 2 below;

FIG. 2 is a graph showing the modulus of elasticity as a function of frequency for different adhesive compositions of the invention as determined in the experiment reported in example 3 below;

FIG. 3 is a graph showing the bulk resistivity of non-crosslinked and crosslinked adhesive compositions containing different degrees of doping, as determined in the experiments recorded in example 4 below;

FIGS. 4-7 are graphs showing the gel content and swell ratio of adhesive compositions of the invention as a function of cure time at varying temperatures, as determined in the experiments reported in example 5 below;

similar to fig. 2, fig. 8 is a graph showing the modulus of elasticity as a function of frequency for different adhesive compositions of the invention as determined in the experiment recorded in example 6 below;

fig. 9 is a graph showing the degree of cure of a control adhesive composition and an adhesive composition of the invention using diglycidylaniline as a crosslinker at 50 ℃ as a function of time.

Detailed Description

As indicated, the present invention relates to the use of a thermally-activated cross-linking agent capable of cross-linking an adhesive layer of an electro-optic display, the cross-linking agent being an epoxidized vegetable oil fatty acid or an epoxidized ester of such a fatty acid. The invention also provides methods for making electro-optic displays having such crosslinked adhesive layers, electro-optic displays made by such methods, and components (i.e., front plane stacks, inverted front plane stacks, and dual release films) for forming such displays. It has been found that the use of such thermally activated cross-linking agents is effective in avoiding the formation of voids in the electro-optic display.

Although epoxidized vegetable oil fatty acids and esters useful in the present invention are well known and indeed commercially available materials, the emphasis has been on these materials as yet unused for crosslinking water-based polyurethane polymers. Suggested uses for epoxidized oils and esters include plasticization of polymeric materials such as polyvinyl chloride (PVC), polyvinyl alcohol (PVA), chlorinated rubber, nitrocellulose and chloroprene rubber; thermal and light stabilization of various PVC compounds; pigment dispersants as excellent grinding fluids; acid acceptance in chlorinated hydrocarbons, phosphates and natural resins; acid scavenging in soy-based ink compounds; and a reactive diluent in the epoxy resin.

Different types of materials including aziridines, carbodiimides, polyisocyanates, blocked isocyanates, melamine formaldehyde, and various epoxy derivatives are well known for crosslinking aqueous polyurethane films. Crosslinking such adhesive films has a number of benefits, including improved mechanical integrity, roughness, and better solvent resistance. However, the epoxidized vegetable oils and derivatives used in the present invention provide a beneficial balance between good film stability at lower temperatures and yield of highly crosslinked films at moderate temperatures. In contrast, crosslinking agents such as aziridines, carbodiimides, epoxy derivatives, polyisocyanates do not have good film stability, since they are very reactive even at low temperatures, whereas other crosslinking agents such as blocked isocyanates, melamine formaldehyde have very good film stability, but require very high curing temperatures to activate the crosslinking reaction. Having good film stability at low temperatures (-10 ℃ to 30 ℃) means that only a minimal amount of crosslinking occurs in the stored film, thus facilitating the long term storage of an adhesive film coated on, for example, a release sheet prior to lamination to an electro-optic layer or other coated substrate. As explained above, it is desirable that the crosslinking reaction occur after the two substrates have been laminated together to provide void-free lamination and improved mechanical integrity. This is particularly important if one or both substrates have substantial surface irregularities ("high loft"), since in this case the substantially uncrosslinked adhesive is able to flow and cover the entire irregular surface to which it is laminated, whereas such flow would not occur if the film was crosslinked prior to lamination. Once the two substrates are laminated together, the adhesive containing the epoxidized oil or derivative can be fully crosslinked at moderate temperatures (40 ℃ to 100 ℃). This provides maximum performance in terms of adhesive and prevents thermal damage to the heat-sensitive laminated substrate, as well as reduces thermal stress during the crosslinking reaction.

For polyurethanes, the epoxidized oils and esters used in the present invention have many advantages over other known crosslinking agents. Since blocked isocyanates or melamines act as crosslinking agents when used, no Volatile Organic Compounds (VOC) are released during the crosslinking reaction. Such VOCs can remain in the adhesive layer causing undesirable performance problems. Epoxidized oils and esters do not require the addition of catalysts or other additives to promote the crosslinking reaction and are less toxic than monomeric epoxy derivatives, melamines, polyisocyanates, and aziridines. Epoxidized oils and esters have improved thermal and light stability.

It is believed (although the invention is in no way limited to this knowledge) that in the present invention, crosslinking is essentially achieved by the reaction of the epoxy groups on the vegetable oil fatty acids or esters with the carboxyl groups on the polyurethane (or other) polymer backbone. Some reaction also occurs between epoxy groups and amino groups on the polyurethane. This crosslinking improves the mechanical and chemical resistance of the adhesive.

The optimum amount of cross-linking agent to be used in any particular adhesive composition is preferably determined empirically. However, as a general guide, it may be stated that crosslinker concentrations of generally at least about 5000ppm (w/w on a solid basis) are required. In many cases, the optimum proportion of cross-linking agent is greater than about 10000ppm, and a proportion of from about 20000 to about 40000ppm is generally desirable to provide sufficient mechanical strength in the cross-linked adhesive to prevent void growth.

The epoxidized fatty acids and esters used in the present invention are oily materials which are relatively difficult to disperse in aqueous media such as aqueous polyurethane latexes. Thus, vigorous mixing, usually with a shear blade mixer, is required to ensure that the epoxidized fatty acids and esters are uniformly dispersed in the aqueous latex. If mixing is insufficient, the coated adhesive layer appears hazy or has visible droplets of crosslinker, while a suitably mixed adhesive layer is transparent.

As described in the aforementioned U.S. patent nos. 7,173,752 and 7,349,148, the adhesive compositions of the present invention may include agents effective to reduce the bulk resistivity of the adhesive layer after crosslinking. As shown in the examples below, crosslinking of a laminate adhesive according to the present invention tends to increase the bulk resistivity of the adhesive. However, as described in these patents, this increase in bulk resistivity can be offset by the addition of certain well-known resistivity-reducing agents, such as salts, polyelectrolytes, or hydroxyl-containing polymers, thereby producing a crosslinked adhesive that has substantially the same bulk resistivity as a non-crosslinked adhesive. Tetrabutylammonium hexafluorophosphate is generally preferred as the volume resistivity reducing agent.

As illustrated in the examples below, the preferred embodiments of the present invention are capable of increasing the shelf life of the crosslinkable polyurethane adhesive composition by a factor of about 8, up to about 8 weeks, with only a modest increase (on the order of about 50%) in the time required for crosslinking at 60 ℃.

In its broadest aspect, the present invention relates to the use of epoxy-containing crosslinkers that only work very slowly at low temperatures, thus providing extended shelf life at storage temperatures around room temperature while still providing high storage modulus after full cure. More specifically, it has been found that at a concentration of 0.195 mole epoxy equivalents per kilogram of adhesive (based on solid state-this corresponds to 2% of DGA by weight), the cured adhesive layer should be less than 70% when measured as the swelling ratio after 120 hours at 50 ℃. In addition, the fully cured resin should have a viscosity of at least about 5X10 when measured at 0.1Hz and 100 deg.C⁴Pa storage modulus. The former specification ensures that the adhesive has a shelf life of several weeks at 20 ℃, while the latter ensures that the crosslinked adhesive can provide a strong bond with a backplane used in electro-optic displays.

The following examples are provided merely for illustrative purposes to illustrate preferred reagents, conditions and techniques for use in the crosslinkable adhesives of the present invention.

Example 1: screening for potential cross-linking Agents

Various commercial crosslinkers including carbodiimides, melamine formaldehyde, blocked isocyanates, epoxy diluents, etc. were screened to determine membrane stability and the degree of crosslinking. Each crosslinker was slowly added to the conventional aqueous polyurethane dispersion while mixing with stirring with low to medium shear with a paddle. The polyurethane dispersion was prepared from tetramethylxylene diisocyanate and polypropylene glycol polymer as described in U.S. patent application publication No. 2005/0124751. After the addition of the crosslinker to the dispersion was complete, stirring was continued for 30 minutes and then the dispersion was allowed to sit for 1 hour before coating the film of adhesive composition onto a poly (ethylene terephthalate) (PET) substrate. To determine the stability of the coated films to crosslinking, samples of the coated films were stored at room temperature (25 ℃) and in a refrigerator (at 5-10 ℃). Periodically monitoring the film by immersing a piece of the coated substrate in acetone; if the polyurethane film did not dissolve or decompose into gel particles, it was recorded as having been crosslinked and the time at which crosslinking occurred was recorded. Longer storage times without crosslinking are desirable because they provide longer allowable times between the initial application of the adhesive layer and its subsequent lamination to the electro-optic layer or other coated substrate.

The degree of crosslinking was determined by measuring the ability of the crosslinked adhesive to swell in acetone. First, the crosslinking reaction is completed by exposing the membrane to elevated temperature (60 ℃ for 5 days under nitrogen or 85 ℃ for 50 hours). The film was then cut to a specific size and placed into a glass tray containing acetone. After one hour, the dimensions of the film are measured and the final length value Lf divided by the cube of the initial length Li, i.e. (Lf/Li) is recorded³The value of (d) is taken as the swelling ratio. Crosslink density is inversely proportional to the swell ratio, so a lower swell ratio indicates a higher crosslink density. The swelling ratio is a relative number, and thus control for comparison is required. Thus, the values for diglycidyl aniline (DGA), a difunctional monomeric aromatic epoxide, are provided in Table 2 below, which are preferred crosslinkers in the aforementioned U.S. Pat. No.7,173,752. DGA cured films exhibit very high crosslink densities, such as good properties desired as lamination adhesives. Table 2 below shows the curing times at 25 ℃ and 5-10 ℃ for various crosslinkers, and the swelling ratios after curing at the two aforementioned temperatures, which performed best in screening studies in terms of both film stability and swelling ratio.

TABLE 2

Sample baseCrosslinker in solid state%	Curing at 25 ℃ (week)	Curing at 5-10 deg.C	Curing at 85 ℃ (50 hr, N)₂) Swelling in acetone (Lf/Li)³	Curing (5 days) at 60 ℃ swelling in acetone (Lf/Li)³
					Diglycidylaniline (5%)	＜1	5	4.1	3.7
Carbodilite E-02(5％)	＞12	N/A	Dissolution	N/A
					Carbodilite E-02(10％)	1-2	N/A	5.4	5.4
Carbodilite E-02(15％)	1-2	N/A	4.1	N/A
					Witcobond-XW(2％)	＞12	N/A	Gel particles	N/A
Witcobond-XW(3.5％)	3	N/A	5.4	8
					Witcobond-XW(5％)	3	11	5.4	5.8
Vikoflex 7190(3％)	7	N/A	4.9	6.9
					Vikoflex 7190(5％)	6	N/A	4.5	4.9
Vikoflex 7190(10％)	5	＞24	3.7	4.1

Despite the high crosslink density of DGA, its membrane stability (as much as many crosslinkers are very low) is less than one week crosslinked at room temperature. Witcobond-XW, a bisphenol A epoxy emulsion from Crompton, Inc., showed better film stability than DGA, but the crosslink density was low even at high concentrations. Carbodiimide Carbodilite E-02 from Nisshinbo corporation showed similar results to DGA, but required higher concentrations of crosslinker. Compared to DGA, epoxidized linseed oil Vikoflex 7190 showed a significant improvement in film stability with similar cross-linking density at 10% concentration. Thus, the data of table 2 indicates that the use of epoxidized fatty acid esters according to the present invention can provide improved film stability of the adhesive without compromising crosslink density.

Example 2: evaluation of other epoxidized fatty acid crosslinkers

According to the results shown in the above table 2, other epoxidized fatty acid crosslinking agents were evaluated in the same manner as in example 1, and the results are shown in the following table 3.

TABLE 3

Sample (% x linker based on solids)	Epoxy equivalent wt.	Curing at 25 ℃ (week)	Curing at 85 ℃ (50 hr, N)₂) Swelling in acetone (Lf/Li)³
				Diglycidylaniline (5%)	103	< 1(6 days)	4.1
Vikoflex 7190(10％)	172	4-5	3.7

Vikoflex 4050(10％)	280	More than 8 weeks	Dehumidification
				Vikoflex 9080(10％)	216	6-7	5.4
Vikoflex 7170(10％)	229	7-8	3.7

The data in table 3 demonstrate that epoxidized fatty acids and esters provide good adhesive film stability and high crosslink density.

Further experiments were conducted to confirm that the epoxidized fatty acid ester was also found to experience an increase in the film stabilization time experienced by DGA at low temperatures. For this purpose, adhesive compositions comprising 8.6% and 14.3% by weight (based on solids) of Vikoflex 7190 were formulated, coated and tested in the same manner as in example 1 above. The days taken for the adhesive composition to gel at 25 ℃, 15 ℃ and 5 ℃ were 14, 17 and > 113 for 8.6% of the composition, respectively. For the 14.3% mixture, 22 and 59 at 25 ℃ and 15 ℃ respectively.

Figure 1 of the accompanying drawings is a graph showing data from table 3, which is re-plotted to show shelf life (i.e., cure time at 25 ℃) as a function of epoxy equivalent weight in the crosslinker. As can be seen in fig. 1, the shelf life of the adhesive composition varies substantially linearly with the equivalent weight of the epoxidized fatty acid esters, each having a similar chemical structure.

Example 3: direct mechanical analysis results

Adhesive compositions were prepared comprising the same polyurethane as in the previous examples and 3%, 5%, and 10% (solids basis) Vikoflex 7190, and 5% (solids basis) DGA. Each composition was coated onto a release sheet and dried at 25 ℃ for about 24 hours, and the coating weight of the dispersion was controlled so as to form an adhesive layer of 50 μm thickness on the release sheet. The adhesive layer was then cured in a nitrogen purge oven at 85 ℃ for 50 hours.

The cured adhesive layer was peeled from the release sheet and folded to multiple thicknesses to provide an adhesive layer thick enough for shear modulus testing on a dynamic mechanical analyzer Model RDA III. The results were converted to the corresponding values at 70 ℃ and are shown in fig. 2 of the drawings. It can be seen from fig. 2 that the shear modulus (stiffness) of the cured adhesive composition (as expected) increases with increasing amount of crosslinker Vikoflex 7190 in the composition, and the curve for the composition with 10% of this crosslinker is substantially the same as the curve for the composition containing 5% DGA.

Example 4: conductivity of adhesive composition

When the adhesive composition is used in an electro-optic display, the adhesive composition is generally between the electrodes of the display, and thus the electrical conductivity (typically measured as its inverse, bulk resistivity) of the crosslinked adhesive composition becomes important, because too high a bulk resistivity results in a large voltage drop across the adhesive layer, and a reduced voltage across the electro-optic layer. Thus, the following bulk resistivities were measured by standard techniques: (a) the polyurethanes used in the previous examples; (b) the same polyurethane with 5% DGA added cured for 50 hours at 85 ℃ in a nitrogen purge oven; and (c) the same polyurethane with 10% Vikoflex 7190 added cured under the same conditions as (b). The volume resistivity of the same three adhesive compositions after addition of 200 and 800ppm tetrabutylammonium hexafluorophosphate (abbreviated as TBAHF6 in fig. 3) which is known to reduce the volume resistivity of crosslinked polyurethane adhesive compositions was also measured. The results are shown in fig. 3.

As can be seen from fig. 3 (as expected from previous studies described in the aforementioned U.S. patent No.7,173,752), although the increase in bulk resistivity in the composition comprising Vikoflex was significantly smaller, the cross-linking of the polyurethane considerably increased the bulk resistivity of the adhesive composition in the composition comprising DGA and Vikoflex; note that the ordinate in fig. 3 is on a logarithmic scale, and the bulk resistivity of the composition comprising undoped Vikoflex is only about one third of that of the composition comprising DGA. In addition, the increase in bulk resistivity caused by crosslinking is much greater than overcome by including a modest 200ppm dopant in the composition. There was little difference between the bulk resistivity of the non-crosslinked and Vikoflex-containing compositions for the 200 and 800ppm samples.

Example 5: curing time of adhesive composition

It has been demonstrated above that the adhesive composition of the invention has a longer shelf life than prior compositions in which the crosslinker was DGA. The need for increased shelf life should be balanced with the effect of the crosslinker on the cure time (i.e., the time required to drive the crosslinking reaction to substantial completion) since it affects the manufacturing throughput of any given plant. Thus, to investigate the effect of the epoxidized oil crosslinker on cure time, adhesive compositions were prepared using the same polyurethane as in the previous examples and 5.7% by weight (based on solids) of Vikoflex 7190. The adhesive composition was cast into a film and the resulting film was cured in an oven at 40, 50, 60 and 70 ℃. Samples of the film were swollen in acetone at different intervals depending on the curing temperature employed and the swelling ratio and gel content were measured as described previously. Gel content is a measure of the amount of polymer that has been added to the swollen gel. This is measured by drying the swollen gel and using the ratio of the mass of the dried sol to the initial mass of the casting polymer. The curing time may be determined as the time for the swelling ratio and gel content to reach substantially constant values. Fig. 4 to 7 show the swelling ratio and gel content measured at curing temperatures of 40, 50, 60 and 70 ℃.

As can be seen from fig. 4 to 7, the curing of the adhesive composition is strongly temperature dependent, with the curing time decreasing from more than 300 hours at 50 ℃ to 100 to 150 hours at 60 ℃. The latter value is important because the DGA crosslinked composition takes about 100 hours to cure at 60 ℃. Thus, replacing DGA with Vikoflex 7190 according to the invention provides a several fold increase in shelf life (as demonstrated above) at the expense of only about a 50% increase in cure time.

The curing properties of the adhesive composition can be further examined by modeling the temperature-dependent behavior. The activation energy can be calculated from the data of FIGS. 4 to 7 based on the Arrhenius relationship with respect to reaction rate, which is related to exp (E)_aK T) is proportional, wherein E_aFor activation energy, k is the boltzmann constant, and T is the absolute temperature. Activation energy is generally designated as E_aK in units of DEG KThus, for the data shown in fig. 4-7, the activation energy was found to be approximately 5530 ° K. Using this value, the shelf life of the adhesive composition at different temperatures can be calculated and the results are shown in Table 4 below. These calculated values are in good agreement with the data shown previously.

TABLE 4

Storage temperature of	Time to gel, hours
		0	3757
5	2610
		15	1309
25	688

Example 6: increase in obtainable modulus

The DMA test of example 3 above was repeated with an adhesive composition comprising the same polyurethane as in the previous examples and 20000ppm DGA as control, or 15000, 22000, 30000 or 60000ppm Vikoflex 7190 as crosslinker. All compositions were cured at 60 ℃ for 125 hours. The results are shown in fig. 8.

Bearing in mind that the epoxy equivalent weight of Vikoflex 7190 is greater than about 70% of the epoxy equivalent weight of DGA, it can be seen that Vikoflex produces a substantially greater increase in shear modulus of the adhesive composition than DGA per mole of epoxy groups introduced; 22000ppm Vikoflex produces a shear modulus substantially identical to that of DGA of 20000ppm, whereas Vikoflex introduces very few epoxy groups, whereas 30000ppm Vikoflex produces a shear modulus of DGA substantially greater than 20000 ppm. In addition, the data in fig. 8 shows: with increasing Vikoflex concentration, the shear modulus continued to increase up to about 60000ppm Vikoflex, resulting in a greater shear modulus than that achieved with DGA.

It is believed (but the invention is in no way limited by this recognition) that one or two of the following reasons result in a greater "effect" of the epoxy groups in the Vikoflex crosslinker in increasing shear modulus. First, the epoxy equivalent weight of a Vikoflex crosslinker means that it contains about 6 epoxy groups per molecule and is therefore capable of acting as a multifunctional crosslinker, whereas DGA is only capable of acting as a difunctional crosslinker. It is well known to those of ordinary skill in polymer chemistry that multifunctional crosslinkers generally produce crosslinked polymers that are harder than difunctional crosslinkers. Second, in the Vikoflex crosslinker the epoxy groups appear in three long fatty acid chains that can move freely, independently of each other, so that the distance between epoxy groups is much larger than in relatively small and compact DGA molecules. Thus, after one epoxy group reacts with the polyurethane (thus fixing the crosslinker to a specific site on the polyurethane), there are many more potential reaction sites in the "effective range" of the Vikoflex crosslinker than are the DGA molecules. This may affect the proportion of epoxy groups on the crosslinker that successfully react with sites on the polyurethane, especially at high crosslinker concentrations.

Example 7: increase in curing time

An adhesive composition was prepared comprising the same custom polyurethane as in the previous examples, and 2% by weight DGA or 2.25% by weight Vikoflex 7190. The compositions were stored at 50 ℃ and samples were taken at intervals and their swelling ratios were tested in the same manner as described above. The results of the swelling test were converted into the degree of cure in a conventional manner, and fig. 9 is a graph showing the change in the degree of cure at 50 ℃ with storage time.

As can be seen in fig. 9, while the adhesive composition containing DGA contained a greater proportion of epoxy groups, the adhesive composition containing Vikoflex crosslinked more slowly than the adhesive composition containing DGA. In particular, after 5 days (120 hours) at 50 ℃, the DGA composition was about 97% cured, while the Vikoflex composition was only about 46% cured. These results indicate that at storage temperatures around room temperature, the Vikoflex composition should have a longer shelf life than the DGA composition.

Example 8: storage modulus

The same composition as in example 7 was tested for storage modulus in the same manner as previously described and the results are shown in table 5 below.

TABLE 5

Crosslinker, concentration%	G’，100℃，0.1Hz，KPa	G’，150℃，0.1Hz，KPa
			DGA，0.1	1.16	2.44×10^-4
DGA，1.0	17.2	4.19
			DGA，1.25	20.6	13.3
DGA，1.5	27.9	30.1
			DGA，2.0	49.0	90.2
DGA，6.0	230	208
			Vikoflex，3.0	88.0	77.0

Vikoflex，4.5	180	137
			Vikoflex，6.0	223	208
Vikoflex，7.5	312	341

In interpreting the data in table 5, it should be noted that greater than about 50% by weight of Vikoflex 7190 was used to produce the same epoxy concentration as compared to DGA. It can be seen that Vikoflex produces a storage modulus at a given epoxy concentration that is substantially greater than that produced by DGA. Indeed, when attaching a front plate to a back plate in an electro-optic display, it is clear that a storage modulus of at least 50KPa is desirable, and this level of storage modulus is easier to achieve with Vikoflex than with DGA.

Although the adhesive compositions of the present invention have been described primarily above with reference to their use in electro-optic displays, it will be appreciated that the compositions are not limited to any particular use and may be used in a variety of uses other than electro-optic displays. For example, the adhesive composition may be used to laminate a variety of metal, glass and plastic substrates, which may have multiple layers of inorganic, organic or metallized components in or on their surfaces. For example, the adhesive composition can be used in laminated printed circuit boards where the ability to treat the non-crosslinked form of the adhesive composition in irregular surfaces of such boards is useful.

Claims

1. An electro-optic display comprising:

an adhesive layer disposed between the layer of electro-optic material and the backplane and adhesively securing the layer of electro-optic material to the backplane, the adhesive layer comprising polyurethane,

the adhesive layer includes a thermally activated cross-linking agent capable of cross-linking the adhesive layer when exposed to an activation temperature, the cross-linking agent including an epoxidized vegetable oil fatty acid or an epoxidized ester of a vegetable oil fatty acid.

2. The electro-optic display of claim 1, wherein the cross-linking agent is derived from soybean oil or linseed oil.

3. An electro-optic display according to claim 1 wherein the cross-linking agent is C of a vegetable oil fatty acid_1-6Alkyl ester, C_2-6Alkenyl esters, epoxidised C_2-6Alkenyl esters, monoglycerides, diglycerides, C_4-6A polyol ester or an ethylene glycol ester, wherein the epoxidized ester of a vegetable oil fatty acid has at least one oxirane ring formed between two adjacent carbons in the carbon chain of the fatty acid.

4. An electro-optic display according to claim 1 wherein the cross-linking agent has an average of at least 3 ethylene oxide groups per molecule.

5. An electro-optic display according to claim 1 wherein the cross-linking agent is present in the adhesive layer at a concentration of at least 5000 parts per million by weight.

6. An electro-optic display according to claim 5 wherein the cross-linking agent is present in the adhesive layer at a concentration of at least 10000 parts per million by weight.

7. The electro-optic display of claim 6, wherein the cross-linking agent is present in the adhesion layer at a concentration of from 20000 parts per million to 40000 parts per weight.

8. An electro-optic display according to claim 1 wherein the adhesive layer comprises a polyurethane-polyurea polymer formed by the reaction of at least one isocyanate with a polyol and a polyamine.

9. An electro-optic display according to claim 1 wherein the adhesive layer further comprises an agent effective to reduce the bulk resistivity of the adhesive layer after crosslinking.

10. An electro-optic display according to claim 9 wherein the bulk resistivity reducing formulation comprises at least one of a salt, a polyelectrolyte, or a hydroxyl containing polymer having a number average molecular weight of no more than 5000.

11. An electro-optic display according to claim 10 wherein the volume resistivity reducing agent comprises a quaternary ammonium salt or polyethylene glycol.

12. An electro-optic display according to claim 11 wherein the bulk resistivity reducing agent comprises tetrabutylammonium chloride, tetrabutylammonium hexafluorophosphate, or polyethylene glycol having a number average molecular weight of no more than 2000.

13. A method for manufacturing an electro-optic display, the method comprising:

providing an assembly comprising a layer of solid electro-optic material capable of changing at least one optical property upon application of an electric field thereto; a backplane comprising at least one electrode arranged to apply an electric field to the layer of electro-optic material; and an adhesive layer disposed between and adhesively securing the layer of electro-optic material to the backplane, the adhesive layer comprising polyurethane, the adhesive layer comprising a thermally-activated cross-linking agent capable of cross-linking the adhesive layer, the cross-linking agent comprising an epoxidized vegetable oil fatty acid or an epoxidized ester of a vegetable oil fatty acid; and

14. An electro-optic display comprising:

the adhesive layer has been crosslinked by a thermally-activated crosslinker comprising an epoxidized vegetable oil fatty acid or an epoxidized ester of a vegetable oil fatty acid when exposed to an activation temperature.

15. A front panel laminate comprising, in order:

a light-transmissive conductive layer;

a layer of a solid electro-optic medium in electrical contact with the electrically conductive layer;

an adhesive layer comprising polyurethane; and

the release plate is arranged on the upper surface of the shell,

16. A dual release film comprising:

a release sheet disposed on the opposite side of the first adhesive layer from the layer of solid electro-optic medium; and

wherein at least one of the first and second adhesive layers comprises a polyurethane and a thermally activated cross-linking agent capable of cross-linking the adhesive layer in which it is located when exposed to an activation temperature, the cross-linking agent comprising an epoxidized vegetable oil fatty acid or an epoxidized ester of a vegetable oil fatty acid.

17. An inverted front plane stack comprising, in order:

a release plate;

a layer of solid electro-optic medium;

an adhesive layer comprising polyurethane; and

18. An adhesive composition comprising a polyurethane and a thermally activated cross-linking agent capable of cross-linking the polyurethane when exposed to an activation temperature, the cross-linking agent comprising an epoxidized vegetable oil fatty acid or an epoxidized ester of a vegetable oil fatty acid.

19. The adhesive composition of claim 18 wherein the cross-linking agent is derived from soybean oil or linseed oil.

20. The adhesive composition of claim 18 wherein the crosslinker is C of a vegetable oil fatty acid_1-6Alkyl ester, C_2-6Alkenyl esters, epoxidised C_2-6Alkenyl esters, monoglycerides, diglycerides, C_4-6A polyol ester or an ethylene glycol ester, wherein the epoxidized ester of a vegetable oil fatty acid has at least one oxirane ring formed between two adjacent carbons in the carbon chain of the fatty acid.

21. The adhesive composition of claim 18 wherein the crosslinker has an average of at least 3 ethylene oxide groups per molecule.

22. The adhesive composition of claim 18 wherein the crosslinker is present at a concentration of at least 5000 parts per million by weight.

23. The adhesive composition of claim 22 wherein the cross-linking agent is present at a concentration of at least 10000 parts per million by weight.

24. The adhesive composition of claim 23 wherein the cross-linking agent is present at a concentration of from 20000 parts per million to 40000 parts by weight.

25. A laminated structure comprising first and second layers of solid glass, metal or plastic, and an adhesive layer disposed between and securing the first and second layers together, the adhesive layer comprising polyurethane, wherein the adhesive layer comprises a thermally-activated cross-linking agent capable of cross-linking the adhesive layer when exposed to an activation temperature, the cross-linking agent comprising an epoxidized vegetable oil fatty acid or an epoxidized ester of a vegetable oil fatty acid.

26. The laminate structure of claim 25 wherein at least one of the first and second layers has at least one layer of inorganic, organic or metallized features therein or on a surface thereof.

27. The laminate structure of claim 25 in the form of a printed circuit board.

28. The laminate structure of claim 25 wherein the crosslinker has an average of at least 3 ethylene oxide groups per molecule.

29. The laminate structure of claim 25 wherein the adhesive layer further comprises an agent effective to reduce the bulk resistivity of the adhesive layer after crosslinking.

30. An electro-optic display comprising:

the adhesive layer includes a thermally activated cross-linking agent capable of cross-linking the adhesive layer when exposed to an activation temperature, the cross-linking agent including an epoxidized vegetable oil fatty acid or an epoxidized ester of a vegetable oil fatty acid such that at a concentration of 0.195 mole epoxy equivalents per kilogram resin:

(a) less than 70% of the adhesive layer is cured, as measured by the swell ratio after holding at 50 ℃ for 120 hours; and

(b) the fully cured resin has at least 5x10 measured at 0.1Hz and 100 deg.C⁴Pa storage modulus.

31. A method for manufacturing an electro-optic display, the method comprising:

providing an assembly comprising a layer of solid electro-optic material capable of changing at least one optical property upon application of an electric field thereto; a backplane comprising at least one electrode arranged to apply an electric field to the layer of electro-optic material; and an adhesion layer disposed between and adhesively securing the layer of electro-optic material to the backplane, the adhesion layer comprising polyurethane, the adhesion layer comprising a thermally-activated cross-linking agent capable of cross-linking the adhesion layer, the cross-linking agent comprising an epoxidized vegetable oil fatty acid or an epoxidized ester of a vegetable oil fatty acid, such that at a concentration of 0.195 mole epoxy equivalents per kilogram of adhesive (on a solids basis):

(b) the fully cured resin has at least 5x10 measured at 0.1Hz and 100 deg.C⁴A storage modulus of Pa; and

the method further includes exposing the adhesive layer to a temperature sufficient to activate the crosslinking agent, thereby crosslinking the adhesive layer.

32. An adhesive composition comprising a polyurethane and a thermally activated cross-linking agent capable of cross-linking the polyurethane when exposed to an activation temperature, the cross-linking agent comprising an epoxidized vegetable oil fatty acid or an epoxidized ester of a vegetable oil fatty acid such that at a concentration of 0.195 mole epoxy equivalents per kilogram resin:

(a) after 120 hours at 50 ℃, the adhesive composition is less than 70% cured, as measured by the swell ratio; and