Patent Application
20110209901
The present invention relates to improvements in polyester film to make it more suitable as a flexible substrate in electronic, photonic and optical assemblies or structures, particularly electronic displays, photovoltaic cells and semiconductor devices.
Polyester film is known for use in the manufacture of flexible electronic or opto-electronic technology, as disclosed in, for instance, WO-A-03/022575. The polyester film acts a substrate on which electronic circuitry is manufactured and mounted in order to drive the electronic operation of the flexible device. The component which comprises the flexible substrate and circuitry is often described as a backplane. The polyester film substrate must satisfy a number of requirements, including dimensional stability (particularly at high temperatures); and a high degree of surface smoothness in order that the thin conductive layer(s) disposed thereupon are as defect-free as possible. The presence of defects in the conductive layer reduces the pixel yield in the electronic display, i.e. the number of pixels in the field of view, and therefore reduces the quality of the display. The substrate, which typically has a multi-layer structure, must also exhibit good adhesion between the layers thereof, as well as good adhesion to the conductive layer disposed thereupon. Moreover, a new generation of electronic devices and displays, including rollable electronic displays, has increased the requirements of flexibility for the substrate and the assembly comprising the substrate. In these new electronic devices, it is desired to increase flexibility to the extent that the device is able to conform to a curved or cylindrical surface, and particularly is able to do so reversibly, without adversely affecting the functionality of the device. A rollable electronic display is a display which is sufficiently flexible that it can be rolled from a flat form into a substantially cylindrical form, and particularly that it can be reversibly rolled. In other electronic devices and electronic displays, it is desired to allow bending of the device or display at a specified angle. It is therefore desirable to produce a device or display in which it is possible to induce a specified curvature.
The object of the present invention is to address one or more of the afore-mentioned problems.
The present invention provides a method of improving the flexibility of a coated polyester substrate for an electronic device comprising a coated polyester substrate layer and an electrode layer comprising conductive material, said method comprising:
(a) providing a polyester film; and
(b) disposing an organic/inorganic hybrid coating on one or both surfaces of said polyester film,
wherein said coating is derived from a coating composition comprising a low molecular weight reactive component selected from monomeric acrylates and/or an unsaturated oligomeric component selected from acrylates, polyether acrylates, epoxy acrylates and polyester acrylates; a solvent; and inorganic particles, and optionally further comprising a photoinitiator.
By improving (i.e. increasing) the flexibility of the polyester substrate, the flexibility of the electronic device or electronic display is thereby improved, allowing the manufacture of, for instance, rollable electronic displays and devices.
The present invention further provides a method of improving the flexibility of an electronic device comprising a coated polyester substrate layer and an electrode layer comprising conductive material, said method comprising:
(a) providing a polyester film;
(b) disposing an organic/inorganic hybrid coating on one or both surfaces of said polyester film; and
(c) providing the coated polyester film as a substrate in the electronic device,
wherein said coating is derived from a coating composition comprising a low molecular weight reactive component selected from monomeric acrylates and/or an unsaturated oligomeric component selected from acrylates, polyether acrylates, epoxy acrylates and polyester acrylates; a solvent; and inorganic particles, and optionally further comprising a photoinitiator.
The present invention further provides a method of improving the flexibility of an electronic device comprising a coated polyester substrate layer and an electrode layer comprising conductive material, said method comprising:
(a) selecting the polyester substrate to be a polyester film coated on one or both surfaces thereof with an organic/inorganic hybrid coating which is derived from a coating composition comprising a low molecular weight reactive component selected from monomeric acrylates and/or an unsaturated oligomeric component selected from acrylates, polyether acrylates, epoxy acrylates and polyester acrylates; a solvent; and inorganic particles, and optionally further comprising a photoinitiator; and
(b) providing said film as a substrate in the electronic device.
In one embodiment, the organic/inorganic hybrid coating is present on both sides of the polyester substrate.
The term “improved flexibility” is used herein to refer to a polyester substrate coated with a coating composition as defined herein, which has a greater resistance to cracking of the coating of the coated substrate upon application of a strain and/or a bending force, when compared to a substrate coated with an alternative coating composition.
In particular, the coating compositions as defined herein improve flexibility to the extent that the polyester substrate coated with a coating composition as defined herein can be elongated in the transverse direction at a draw rate of 60 mm/min by about 3% or more, preferably about 5% or more, preferably about 8% or more, preferably about 10% or more, preferably about 12% or more, preferably about 15% or more, preferably about 20% or more, preferably about 25% or more of its original dimension before a first crack appears in the coating of the coated polyester substrate.
The term “improved flexibility” is also used herein to refer to a polyester substrate coated with a coating composition and electrode layer as defined herein or to an electronic device comprising the coated polyester substrate and electrode layer as defined herein, which has a greater resistance to cracking of the electrode layer upon application of a strain and/or a bending force, when compared to a substrate coated with an alternative composition or an electronic device including a polyester substrate coated with an alternative coating composition.
In particular, the coating compositions as defined herein improve flexibility to the extent that the polyester substrate coated with a coating composition and electrode layer as defined herein can be elongated in the transverse direction at a draw rate of 60 mm/min by about 3% or more, preferably about 5% or more, preferably about 8% or more, preferably about 10% or more, preferably about 12% or more, preferably about 15% or more of its original dimension before a first crack appears in the conductive material of the electrode layer.
This improvement in flexibility is particularly advantageous where the substrate layer is to be used in the manufacture of an electronic device which is rollable i.e. a device which can be rolled from a flat form into a substantially cylindrical form.
The “critical radius of curvature” of the polyester substrate coated with a coating composition provides a measure of the degree to which the coated polyester substrate may be deformed whilst retaining mechanical integrity, i.e. before a first crack appears in the coated substrate. Thus, the “critical radius of curvature” is the minimum radius to which the coating of the coated polyester substrate can be bent before a first crack appears in the coating of the coated substrate.
Similarly, when referring to the “critical radius of curvature” of the polyester substrate coated with a coating composition and electrode layer, it is the minimum radius to which the coated polyester substrate can be bent before a first crack appears in the conductive material of the electrode layer.
In particular, the improvement in flexibility is such that the polyester substrate coated with a coating composition, has a critical radius of curvature of about 10 mm or less, preferably about 8 mm or less, preferably about 6 mm or less, preferably about 5 mm or less, preferably about 4 mm or less, preferably about 3 mm or less, preferably about 2.5 mm or less, preferably about 2 mm or less, preferably about 1.5 mm or less, preferably about 1 mm or less, preferably about 0.75 mm or less, preferably about 0.5 mm or less.
For the polyester substrate coated with a coating composition and electrode layer, the improvement in flexibility is such that the polyester substrate coated with a coating composition and electrode layer, has a critical radius of curvature of about 10 mm or less, preferably about 8 mm or less, preferably about 6 mm or less, preferably about 5 mm or less, preferably about 4 mm or less, preferably about 3 mm or less, preferably about 2.5 mm or less, preferably about 2 mm or less, preferably about 1.5 mm or less.
The term polyester as used herein includes a polyester homopolymer in its simplest form or modified, chemically and/or physically. In particular, the film substrate is a biaxially oriented polymeric film comprising a layer of polyester or copolyester derived from:
The thickness of the biaxially oriented polyester film is preferably from about 12 to about 250 μm, more preferably from about 12 to about 150 μm, and typically is about 25-125 μm in thickness. The film is self-supporting by which is meant capable of independent existence in the absence of a supporting base.
The polyester is obtainable by condensing said dicarboxylic acids or their lower alkyl (up to 6 carbon atoms) diesters with one or more diols. The aromatic dicarboxylic acid is preferably selected from terephthalic acid, isophthalic acid, phthalic acid, 2,5-, 2,6- or 2,7-naphthalenedicarboxylic acid, and is preferably terephthalic acid or 2,6-naphthalenedicarboxylic acid, preferably 2,6-naphthalenedicarboxylic acid. The diol is preferably selected from aliphatic and cycloaliphatic glycols, e.g. ethylene glycol, 1,3-propanediol, 1,4-butanediol, neopentyl glycol and 1,4-cyclohexanedimethanol, preferably from aliphatic glycols. Preferably the copolyester contains only one glycol, preferably ethylene glycol. The aliphatic dicarboxylic acid may be succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azeleic acid or sebacic acid. Preferred homopolyesters are polyesters of 2,6-naphthalenedicarboxylic acid or terephthalic acid with ethylene glycol. A particularly preferred homopolyester is poly(ethylene naphthalate), and particularly polyesters of 2,6-naphthalenedicarboxylic acid with ethylene glycol.
Formation of the polyester is conveniently effected in a known manner by condensation or ester interchange, generally at temperatures up to about 295° C. For instance, the preferred PEN polyester can be synthesised by condensing 2,5-, 2,6- or 2,7-naphthalenedicarboxylic acid, preferably 2,6-naphthalenedicarboxylic acid, or a lower alkyl (up to 6 carbon atoms) diester thereof, with ethylene glycol. Typically, polycondensation includes a solid phase polymerisation stage. The solid phase polymerisation may be carried out on a fluidised bed, e.g. fluidised with nitrogen, or on a vacuum fluidised bed, using a rotary vacuum drier. Suitable solid phase polymerisation techniques are disclosed in, for example, EP-A-0419400 the disclosure of which is incorporated herein by reference. In one embodiment, the PEN is prepared using germanium catalysts which provide a polymeric material having a reduced level of contaminants such as catalyst residues, undesirable inorganic deposits and other byproducts of the polymer manufacture. The “cleaner” polymeric composition promotes improved optical clarity and surface smoothness. Preferably, PEN has a PET-equivalent intrinsic viscosity (IV) of 0.5-1.5, preferably 0.7-1.5, and in particular 0.79-1.0. An IV of less than 0.5 results in a polymeric film lacking desired properties such as mechanical properties whereas an IV of greater than 1.5 is difficult to achieve and would likely lead to processing difficulties of the raw material.
Formation of the film may be effected by conventional techniques well-known in the art. Conveniently, formation of the film is effected by extrusion, in accordance with the procedure described below. In general terms the process comprises the steps of extruding a layer of molten polymer, quenching the extrudate and orienting the quenched extrudate in at least one direction.
The film may be biaxially-oriented. Preferably, the film is biaxially oriented. Orientation may be effected by any process known in the art for producing an oriented film, for example a tubular or flat film process. Biaxial orientation is effected by drawing in two mutually perpendicular directions in the plane of the film to achieve a satisfactory combination of mechanical and physical properties.
In a tubular process, simultaneous biaxial orientation may be effected by extruding a thermoplastics polyester tube which is subsequently quenched, reheated and then expanded by internal gas pressure to induce transverse orientation, and withdrawn at a rate which will induce longitudinal orientation.
In the preferred flat film process, the film-forming polyester is extruded through a slot die and rapidly quenched upon a chilled casting drum to ensure that the polyester is quenched to the amorphous state. Orientation is then effected by stretching the quenched extrudate in at least one direction at a temperature above the glass transition temperature of the polyester. Sequential orientation may be effected by stretching a flat, quenched extrudate firstly in one direction, usually the longitudinal direction, i.e. the forward direction through the film stretching machine, and then in the transverse direction. Forward stretching of the extrudate is conveniently effected over a set of rotating rolls or between two pairs of nip rolls, transverse stretching then being effected in a stenter apparatus. Stretching is generally effected so that the dimension of the oriented film is from 2 to 5, more preferably 2.5 to 4.5 times its original dimension in the or each direction of stretching. Typically, stretching is effected at temperatures higher than the Tg of the polyester, preferably about 15° C. higher than the Tg. Greater draw ratios (for example, up to about 8 times) may be used if orientation in only one direction is required. It is not necessary to stretch equally in the machine and transverse directions although this is preferred if balanced properties are desired.
A stretched film may be, and preferably is, dimensionally stabilised by heat-setting under dimensional support at a temperature above the glass transition temperature of the polyester but below the melting temperature thereof, to induce crystallisation of the polyester. During the heat-setting, a small amount of dimensional relaxation may be performed in the transverse direction, TD by a procedure known as “toe-in”. Toe-in can involve dimensional shrinkage of the order 2 to 4% but an analogous dimensional relaxation in the process or machine direction, MD is difficult to achieve since low line tensions are required and film control and winding becomes problematic. The actual heat-set temperature and time will vary depending on the composition of the film and its desired final thermal shrinkage but should not be selected so as to substantially degrade the toughness properties of the film such as tear resistance. Within these constraints, a heat set temperature of about 180° to 245° C. is generally desirable.
The film may also, and indeed preferably is, further stabilized through use of an online relaxation stage. Alternatively the relaxation treatment can be performed off-line. In this additional step, the film is heated at a temperature lower than that of the heat-setting stage, and with a much reduced MD and TD tension. Film thus processed will exhibit a smaller thermal shrinkage than that produced in the absence of such post heat-setting relaxation.
In one embodiment, heat-setting and heat-stabilisation of the biaxially stretched film is conducted as follows. After the stretching steps have been completed, heat-setting is effected by dimensionally restraining the film at a tension in the range of about 19 to about 75 kg/m, preferably about 45 to about 50 kg/m of film width, using a heat-set temperature preferably from about 135° to about 250° C., more preferably 235-240° C. and a heating duration typically in the range of 5 to 40 secs, preferably 8 to 30 secs. The heat-set film is then heat-stabilised by heating it under low tension, preferably such that the tension experienced by the film is less than 5 kg/m, preferably less than 3.5 kg/m, more preferably in the range of from 1 to about 2.5 kg/m, and typically in the range of 1.5 to 2 kg/m of film width, typically using a temperature lower than that used for the heat-setting step and selected to be in the range from about 135° C. to 250° C., preferably 190 to 250° C., more preferably 200 to 230° C., and more preferably at least 215° C., typically 215 to 230° C., and for a duration of heating typically in the range of 10 to 40 sec, with a duration of 20 to 30 secs being preferred.
The heat-set, heat-stabilised film exhibits a very low residual shrinkage and consequently high dimensional stability. Preferably, the film exhibits a coefficient of linear thermal expansion (CLTE) within the temperature range from −40° C. to +100° C. of less than 40×10−6/° C., preferably less than 30×10−6/° C., more preferably less than 25×10−6/° C., more preferably less than 20×10−6/° C. Preferably, the film has a shrinkage at 30 mins at 230° C., measured as defined herein, of less than 1%, preferably less than 0.75%, preferably less than 0.5%, preferably less than 0.25%, and more preferably less than 0.1%. Preferably, the film has a residual dimensional change ΔLT measured at 25° C. before and after heating the film from 8° C. to 200° C. and then cooling to 8° C., of less than 0.75%, preferably less than 0.5%, preferably less than 0.25%, and more preferably less than 0.1%, of the original dimension. In a particularly preferred embodiment, the substrate is a heat-stabilised, heat-set biaxially oriented film comprising poly(ethylene naphthalate) having the afore-mentioned shrinkage characteristics after 30 min at 230° C., and preferably having the afore-mentioned residual dimensional change ΔLT characteristics. It will be appreciated that these dimensional stability characteristics refer to the uncoated heat-stabilised, heat-set, biaxially oriented polyester film.
The film may conveniently contain any of the additives conventionally employed in the manufacture of polyester films. Thus, agents such as cross-linking agents, pigments and voiding agents, agents such as anti-oxidants, radical scavengers, UV absorbers, thermal stabilisers, flame retardants and inhibitors, which are solid, or bound covalently to the polyester and finally agents such as optical brighteners, gloss improvers, prodegradents, viscosity modifiers and dispersion stabilisers may be incorporated as appropriate. In particular, the film may comprise a particulate filler which can improve handling and windability during manufacture. The particulate filler may, for example, be a particulate inorganic filler (e.g. voiding or non-voiding metal or metalloid oxides, such as alumina, silica and titania, calcined china clay and alkaline metal salts, such as the carbonates and sulphates of calcium and barium), or an incompatible resin filler (e.g. polyamides and olefin polymers, particularly a homo- or co-polymer of a mono-alpha-olefin containing up to 6 carbon atoms in its molecule) or a mixture of two or more such fillers.
The components of the composition of a layer may be mixed together in a conventional manner. For example, by mixing with the monomeric reactants from which the film-forming polyester is derived, or the components may be mixed with the polyester by tumble or dry blending or by compounding in an extruder, followed by cooling and, usually, commination into granules or chips. Masterbatching technology may also be employed.
In a preferred embodiment, the film is optically clear, preferably having a % of scattered visible light (haze) of <10%, preferably <6%, more preferably <3.5% and particularly <1.5%, measured according to the standard ASTM D 1003. In this embodiment, filler is typically present in only small amounts, generally not exceeding 0.5% and preferably less than 0.2% by weight of a given layer.
One or both surfaces of the polyester film has disposed thereon an organic/inorganic hybrid coating derived from a coating composition comprising a low molecular weight reactive component and/or an unsaturated oligomeric component; a solvent; and inorganic particles, and optionally further comprising a photoinitiator, as referred to hereinabove. The organic/inorganic hybrid coating layer provides a flat, planarised surface to the substrate film whose natural surface roughness may vary as a function of inorganic filler particles present in its composition. The organic/inorganic hybrid coating preferably also provides a degree of mechanical protection to the film, as judged for example by the Taber abraser test (ASTM Method D-1044). The Taber Abrasion test will typically cause controlled damage to the surface of unprotected film such that under the standard conditions of treatment, the haze of the film is seen to increase by 40-50%. The organic/inorganic hybrid coating resists the deterioration of the film surface under similar conditions and results in an increase in measured haze of the material of preferably no more than 20%, more preferably no more than 10% and most preferably no more than 5%.
The organic/inorganic hybrid coatings described herein comprise inorganic particles distributed throughout an organic polymeric matrix. The polymeric matrix is derived from a coating composition comprising (i) a low molecular weight reactive component (e.g a monomeric acrylate); and/or (ii) an unsaturated oligomeric component (e.g, acrylates, urethane acrylates, polyether acrylates, epoxy acrylates or polyester acrylates); (iii) a solvent, and optionally (iv) a photoinitiator. As used herein, the term “low molecular weight” describes a polymerisable monomeric species. The term “reactive” signifies the polymerisability of the monomeric species. The coatings are cured either thermally or by free radical reaction initiated by a photolytic route, and the presence of a photoinitiator is optional.
The inorganic phase is typically silica or metal oxide particles, and these can be dispersed in the polymerisable organic matrix by a number of strategies. In one embodiment, the inorganic particles are silica particles. The inorganic particles preferably have an average particle diameter of 0.005 to 3 μm; in one embodiment at least 0.01 μm, and in one embodiment no more than 1 μm. The inorganic particles do not substantially affect the optical properties of the substrate. In one embodiment, the inorganic particles are present in an amount of from about 5% to about 60% by weight of the solids components of the coating composition, and preferably from about 5% to about 60% by weight of the cured coating layer.
In one embodiment, the organic/inorganic hybrid coating composition is UV-curable and comprises monomeric acrylates (typically multi-functional acrylates) in combination with inorganic (preferably silica) particles in a solvent (such as methylethylketone), typically wherein the coating composition comprises the acrylates and silica at about 5 to 50 wt % solids of the total weight of the coating composition, and typically further comprising a minor amount (e.g. about 1% by weight of the solids) of photoinitiator. Multi-functional monomeric acrylates are known in the art, and examples include dipentaerythritol tetraacrylate and tris(2-acryloyloxyethyl) iso cyanurate.
Preferably the coating composition comprises at least two different multifunctional monomeric acrylates.
The coating compositions can be applied using conventional coating techniques, including continuous as well as dip coating procedures. The coatings are generally applied to a dry thickness of from about 1 to about 20 microns, preferably from about 2 to 10 microns, and particularly from about 3 to about 10 microns. The coating composition can be applied either “off-line” as a process step distinct from the film manufacture, or “in-line” as a continuation of the film manufacturing process. The coating compositions, after application to the substrate, can be cured at a temperature of from about 20 to about 200° C., preferably from about 20 to about 150° C. While ambient temperatures of 20° C. require cure times of several days, elevated temperatures of 150° C. will cure the coatings in several seconds.
The exposed surface of the film may, if desired, be subjected to a chemical or physical surface-modifying treatment to improve the bond between that surface and a subsequently applied layer. A preferred treatment, because of its simplicity and effectiveness, is to subject the exposed surface of the film to a high voltage electrical stress accompanied by corona discharge. The preferred treatment by corona discharge may be effected in air at atmospheric pressure with conventional equipment using a high frequency, high voltage generator, preferably having a power output of from 1 to 20 kW at a potential of 1 to 100 kV. Discharge is conventionally accomplished by passing the film over a dielectric support roller at the discharge station at a linear speed preferably of 1.0 to 500 m per minute. The discharge electrodes may be positioned 0.1 to 10.0 mm from the moving film surface.
In a preferred embodiment, the polyester film base is coated on one or both surfaces thereof, prior to application of the organic/inorganic hybrid coating, with a primer layer to improve adhesion of the substrate to the afore-mentioned coating composition. The primer layer may be any suitable adhesion-promoting polymeric composition known in the art, including polyester and acrylic resins. The primer composition may also be a mixture of a polyester resin with an acrylic resin. Acrylic resins may optionally comprise oxazoline groups and polyalkylene oxide chains. The polymer(s) of the primer composition is/are preferably water-soluble or water-dispersible.
Polyester primer components include those obtained from the following dicarboxylic acids and diols. Suitable di-acids include terephthalic acid, isophthalic acid, phthalic acid, phthalic anhydride, 2,6-naphthalenedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, adipic acid, sebacic acid, trimellitic acid, pyromellitic acid, a dimer acid, and 5-sodium sulfoisophthalic acid. A copolyester using two or more dicarboxylic acid components is preferred. The polyester may optionally contain a minor amount of an unsaturated di-acid component such as maleic acid or itaconic acid or a small amount of a hydroxycarboxylic acid component such as p-hydroxybenzoic acid. Suitable diols include ethylene glycol, 1,4-butanediol, diethylene glycol, dipropylene glycol, 1,6-hexanediol, 1,4-cyclohexanedimethylol, xylene glycol, dimethylolpropane, poly(ethylene oxide) glycol, and poly(tetramethylene oxide) glycol. The glass transition point of the polyester is preferably 40 to 100° C., further preferably 60 to 80° C. Suitable polyesters include copolyesters of PET or PEN with relatively minor amounts of one or more other dicarboxylic acid comonomers, particularly aromatic di-acids such as isophthalic acid and sodium sulphoisophthalic acid, and optionally relatively minor amounts of one or more glycols other than ethylene glycol, such as diethylene glycol.
In one embodiment, the primer layer comprises an acrylate or methacrylate polymer resin. The acrylic resin may comprise one or more other comonomers. Suitable comonomers include alkyl acrylates, alkyl methacrylates (where the alkyl group is preferably methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, t-butyl, 2-ethylhexyl, cyclohexyl or the like); hydroxy-containing monomers such as 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl acrylate, and 2-hydroxypropyl methacrylate; epoxy group-containing monomers such as glycidyl acrylate, glycidyl methacrylate, and allyl glycidyl ether; carboxyl group or its salt-containing monomers, such as acrylic acid, methacrylic acid, itaconic acid, maleic acid, fumaric acid, crotonic acid, styrenesulfonic acid and their salts (sodium salt, potassium salt, ammonium salt, quaternary amine salt or the like); amide group-containing monomers such as acrylamide, methacrylamide, an N-alkylacrylamide, an N-alkylmethacrylamide, an N,N-dialkylacrylamide, an N,N-dialkyl methacrylate (where the alkyl group is preferably selected from those described above), an N-alkoxyacrylamide, an N-alkoxymethacrylamide, an N,N-dialkoxyacrylamide, an N,N-dialkoxymethacrylamide (the alkoxy group is preferably methoxy, ethoxy, butoxy, isobutoxy or the like), acryloylmorpholine, N-methylolacrylamide, N-methylolmethacrylamide, N-phenylacrylamide, and N-phenylmethacrylamide; acid anhydrides such as maleic anhydride and itaconic anhydride; vinyl isocyanate, allyl isocyanate, styrene, α-methylstyrene, vinyl methyl ether, vinyl ethyl ether, a vinyltrialkoxysilane, a monoalkyl maleate, a monoalkyl fumarate, a monoalkyl itaconate, acrylonitrile, methacrylonitrile, vinylidene chloride, ethylene, propylene, vinyl chloride, vinyl acetate, and butadiene. In a preferred embodiment, the acrylic resin is copolymerised with one or more monomer(s) containing oxazoline groups and polyalkylene oxide chains. The oxazoline group-containing monomer includes 2-vinyl-2-oxazoline, 2-vinyl-4-methyl-2-oxazoline, 2-vinyl-5-methyl-2-oxazoline, 2-isopropenyl-2-oxazoline, 2-isopropenyl-4-methyl-2-oxazoline, and 2-isopropenyl-5-methyl-2-oxazoline. One or more comonomers may be used. 2-Isopropenyl-2-oxazoline is preferred. The polyalkylene oxide chain-containing monomer includes a monomer obtained by adding a polyalkylene oxide to the ester portion of acrylic acid or methacrylic acid. The polyalkylene oxide chain includes polymethylene oxide, polyethylene oxide, polypropylene oxide, and polybutylene oxide. It is preferable that the repeating units of the polyalkylene oxide chain are 3 to 100.
Where the primer composition comprises a mixture of polyester and acrylic components, particularly an acrylic resin comprising oxazoline groups and polyalkylene oxide chains, it is preferable that the content of the polyester is 5 to 95% by weight, preferably 50 to 90% by weight, and the content of the acrylic resin is 5 to 90% by weight, preferably 10 to 50% by weight.
Other suitable acrylic resins include:
(i) a copolymer of (a) 35 to 40 mole % alkyl acrylate, (b) 35 to 40% alkyl methacrylate, (c) 10 to 15 mole % of a comonomer containing a free carboxyl group such as itaconic acid, and (d) 15 to 20 mole % of an aromatic sulphonic acid and/or salt thereof such as p-styrene sulphonic acid, an example of which is a copolymer comprising ethyl acrylate/methyl methacrylate/itaconic acid/p-styrene sulphonic acid and/or a salt thereof in a ratio of 37.5/37.5/10/15 mole %, as disclosed in EP-A-0429179 the disclosure of which is incorporated herein by reference; and
(ii) an acrylic and/or methacrylic polymeric resin, an example of which is a polymer comprising about 35 to 60 mole % ethyl acrylate, about 30 to 55 mole % methyl methacrylate and about 2 to 20 mole % methacrylamide, as disclosed in EP-A-0408197 the disclosure of which is incorporated herein by reference.
The primer or adherent layer may also comprise a cross-linking agent which improves adhesion to the substrate and should also be capable of internal cross-linking. Suitable cross-linking agents include optionally alkoxylated condensation products of melamine with formaldehyde. The primer or adherent layer may also comprise a cross-linking catalyst, such as ammonium sulphate, to facilitate the cross-linking of the cross-linking agent. Other suitable cross-linking agents and catalysts are disclosed in EP-A-0429179, the disclosures of which are incorporated herein by reference.
A further suitable primer is disclosed in U.S. Pat. No. 3,443,950, the disclosure of which is incorporated herein by reference.
The coating of the primer layer onto the substrate may be performed in-line or off-line, but is preferably performed “in-line”, and preferably between the forward and sideways stretches of a biaxial stretching operation.
The coated films preferably have a % of scattered visible light (haze) of <10%, preferably <6%, more preferably <3.5% and particularly <1.5%, measured according to the standard ASTM D 1003.
In one embodiment of the present invention, the coated films exhibit a surface having an Ra value, measured as described herein, of less than 0.7 nm, preferably less than 0.6 nm, preferably less than 0.5 nm, preferably less than 0.4 nm, preferably less than 0.3 nm, and ideally less than 0.25 nm, and/or an Rq value, measured as described herein, of less than 0.9 nm, preferably less than 0.8 nm, preferably less than 0.75 nm, preferably less than 0.65 nm, preferably less than 0.6 nm, preferably less than 0.50 nm, preferably 0.45 mm or lower, preferably less than 0.35 nm, and ideally less than 0.3 nm.
According to a further aspect of the present invention, there is provided a composite film comprising:
(i) a biaxially oriented polyester substrate;
(ii) a primer layer coated on one or both surfaces of the polyester substrate;
(iii) on one or both surfaces of said primer-coated polyester substrate, an organic/inorganic hybrid coating derived from a coating composition comprising a low molecular weight reactive component selected from monomeric acrylates and/or an unsaturated oligomeric component selected from acrylates, polyether acrylates, epoxy acrylates and polyester acrylates; a solvent; and inorganic particles, and optionally further comprising a photoinitiator, wherein said inorganic particles have an average particle diameter of from about 0.005 to about 3 μm; and
(iv) optionally on a surface of the coated substrate an electrode layer comprising a conductive material.
The coated polyester film is suitable as a substrate for, and in the manufacture of, flexible electronic devices, including electronic, photonic and optical assemblies or structures, preferably electronic display devices, photovoltaic cells, sensors and semiconductor devices, particularly in the manufacture of the backplanes referred to above, and more particularly in rollable electronic displays. In one embodiment, the term “electronic device” as used herein refers to a device which contains as essential features at least a polyester substrate and electronic circuitry. Electronic and opto-electronic devices may comprise a conductive polymer. Preferably, the device is an electronic display device including, for example, an electroluminescent (EL) device (particularly an organic light emitting display (OLED)); a photovoltaic cell or a semiconductor device (such as organic field effect transistors, thin film transistors and integrated circuits generally). In one embodiment, the term “electroluminescent display device”, and particularly the term “organic light emitting display (OLED) device”, as used herein refers to a display device comprising a layer of light-emitting electroluminescent material (particularly a conductive polymeric material) disposed between two layers each of which comprises an electrode, wherein the resultant composite structure is disposed between two substrate (or support or cover) layers. In one embodiment, the term “photovoltaic cell” as used herein refers to a device comprising a layer of conductive polymeric material disposed between two layers each of which comprises an electrode, wherein the resultant composite structure is disposed between two substrate (or support or cover) layers. In one embodiment, the term “transistor” as used herein refers to a device comprising at least one layer of conductive polymer, a gate electrode, a source electrode and a drain electrode, and one or more substrate layers. Thus, in one embodiment, the method and use referred to hereinabove include the step of disposing an electrode layer on the coated substrate described hereinabove, in accordance with conventional manufacturing techniques known in the art, and the composite film referred to hereinabove further comprises an electrode layer (optionally transparent or translucent) on the coated substrate. The electrode layer may have a thickness in the range from about 5 to about 200 nm, preferably about 10 to about 100 nm, preferably from about 15 to about 50 nm and particularly from about 20 to about 30 nm. The electrode layer may be a layer, or a patterned layer, of a suitable conductive material as known in the art, for instance gold or a conductive metal oxide such as indium tin oxide, optionally doped with other metals as is known in the art. Other materials suitable as for the electrode layer are well-known to the skilled person and include, for instance, silver, aluminium platinum, palladium, nickel. In a preferred embodiment, the electrode layer comprises gold. In one embodiment, a tie layer is deposited on the coated film referred to hereinabove prior to deposition of the electrode layer. Such a tie-layer typically comprises a metallic layer deposited by conventional techniques onto a surface of the coated film, wherein the metallic layer is different to the conductive material of the electrode layer. For instance, where the electrode layer is gold, the tie layer may be a layer of metallic titanium.
In a further embodiment, the composite film described hereinabove may further comprise a layer which exhibits barrier properties to water vapour and/or oxygen transmission, particularly such that the water vapour transmission rate is less than 10−6 g/m2/day and/or the oxygen transmission rate is less than 10−5/mL/m2/day, and which is typically applied prior to application of the electrode layer. Such a barrier layer may be organic or inorganic (preferably inorganic), and is typically applied by vacuum deposition or sputtering techniques. Materials which are suitable for use to form a barrier layer are disclosed, for instance, in U.S. Pat. No. 6,198,217 and WO-A-03/087247, the disclosures of which are incorporated herein by reference.
According to a further aspect of the present invention, there is provided a flexible electronic device comprising a polyester substrate and an electrode layer comprising a conductive material, and further comprising on one or both surfaces of said polyester substrate an organic/inorganic hybrid coating derived from a coating composition comprising a low molecular weight reactive component selected from monomeric acrylates and/or an unsaturated oligomeric component selected from acrylates, polyether acrylates, epoxy acrylates and polyester acrylates; a solvent; and inorganic particles, and optionally further comprising a photoinitiator, wherein said inorganic particles have an average particle diameter of from about 0.005 to about 3 μm.
According to a further aspect of the present invention, there is provided a method of manufacture of a rollable electronic display comprising a coated polyester substrate layer and an electrode layer comprising conductive material, said method comprising:
(a) providing a polyester film; and
(b) disposing a coating on one or both surfaces of said polyester film, characterised in that said coating is an organic/inorganic hybrid coating derived from a coating composition comprising a low molecular weight reactive component selected from monomeric acrylates and/or an unsaturated oligomeric component selected from acrylates, polyether acrylates, epoxy acrylates and polyester acrylates; a solvent; and inorganic particles, and optionally further comprising a photoinitiator,
(c) disposing an electrode layer comprising conductive material on one or both surfaces of said coated polyester film;
and further characterised in that the composite structure comprising said coated polyester substrate layer and electrode layer can be elongated in the transverse direction at a draw rate of 60 mm/min by 3% or more of its original dimension before a first crack appears in the conductive material of the electrode layer, and/or in that the composite structure comprising the coated polyester substrate layer and electrode layer has a critical radius of curvature of about 10 mm or less.
As shown in
The following analyses were used to characterize the films described herein:
α=ΔL/(L×(T2−T1))
∈x=(z−T/2)R+∈x0;∈y=∈y0 (1)
∈x=1/Ei(σx−viσy);∈y=1/Ei(σy−viσx) (2)
σx=Ci((z−T/2)/R+∈x0+vi∈y0)
σy=Ci(∈y0+vi(z−T/2)R+vi∈x0) (3)
The invention is further illustrated by the following examples. The examples are not intended to limit the invention as described above. Modification of detail may be made without departing from the scope of the invention.
A commercially available biaxially oriented heat-stabilised polyethylene terephthalate (PET) film (Melinex® ST506; Dupont Teijin Films; UK) having a thickness of 175 μm and treated on both surfaces with an inter-draw in-line primer coating of an acrylic resin as described hereinabove, was used as the substrate. This film was then coated with the inorganic hardcoat disclosed in WO-A-03/087247, prepared before application by the following steps:
(i) 517 cm3 of methyltrimethoxysilane (obtained from OSi Specialities) was added to 1034 cm3 demineralised water at room temperature and stirred for 24 hours.
(ii) 54 cm3 of 3-glycidoxypropyl trimethoxysilane (obtained from Aldrich Chemical Company) was added to 108 cm3 of demineralised water at room temperature and stirred for 24 hours.
(iii) 53 cm3 of 10% aqueous acetic acid (Aldrich Chemical Company) was added to 700 cm3 of Ludox LS colloidal silica (12 nm). To this was added 162 cm3 of the hydrolysed 3-glycidoxypropyl trimethoxysilane/water mixture and 1551 cm3 of the hydrolysed methyltrimethoxysilane/water mixture. This mixture was stirred for 12 hours before coating. The final pH of the composition was 6.05.
The coating was applied to both surfaces of the polyester film and crosslinked thermally. The final dry coating thickness, after curing/drying, was 2 μm.
Comparative example 1 was repeated using an organic coating composition comprising a mixture of monomeric and polymeric acrylates (including methylmethacrylate and ethylacrylate) and a photoinitiator (Irgacure™ 2959; Ciba) in a solvent of methyl ethyl ketone (2-butanone) was prepared at 26.5 wt % solids (of which about 1% of these solids is the photoinitiator) to a viscosity of about 1.22 cP (centipoise). The coating was dried at 80° C. and then cured by UV-radiation. The coating did not include any inorganic particles.
Comparative example 1 was repeated using a hybrid organic/inorganic coating composition comprising acrylate monomers and silica particles in MEK solvent was prepared to 10% solids and a viscosity of about 1.7 cP. The coating was applied and then cured immediately by UV-radiation.
For each of the three films prepared as described above, a conductive coating comprising a first layer of titanium metal (0.075 μm) and a second layer of gold metal (0.02 μm) was vacuum-coated using conventional sputtering techniques (Ti sputtered at 7 kW; Au at 6.5 kW). Each film was then analysed using the fracture under tension test described herein. Eight samples were cut from each of the three planarised films and each of the three Au-coated planarised films, in the machine direction. They were then subjected to various levels of strain ranging from 0.5% to 17%. The data are presented in Table below.
The data in Table 1 above demonstrate that Example 1 showed surprisingly superior performance.
The critical radius of curvature for each of the above film samples in a two point bend was then calculated from the crack density data using the modelling method described hereinabove. In order to derive these, firstly the critical strain for each sample must be defined. The critical strain, ∈c is taken here to be the highest observed value of strain at which mechanical integrity is preserved (i.e. the highest strain for which no cracking is observed). These values are listed in table 2 together with corresponding values of stress derived for the particular layer where cracking was observed (i.e. either the polymeric coating layer or the gold layer). The critical stress for the respective layers was calculated using the entries for strain and values for Young's modulus of each of the materials which exhibited fracture as defined in table 3.
Values for the Young's modulus and Poisson ratio of each material are listed in table 3. These data were required in the subsequent calculation of the critical radius of curvature for each of the multilayer laminate example and comparative examples. In some cases an estimate of the material modulus was adopted, however it was established by further calculation that a tenfold variation in the value of the planariser modulus causes only around 10% variation in the predicted stress in curvature of the corresponding coating.
The critical radius of curvature (in mm) for each sample calculated in the model described herein is given in Table 4 below. The critical radii are reported below in respect of the coated film and also in respect of the coated film with the Ti/Au layer disposed thereon.
The results in Table 3 show that the coated film of Example 1 is unexpectedly more flexible since it can bend to a radius of 0.72 mm before the first crack appears. When coated with conductive material, Example 1 can bend to a radius of 2.41 mm before the first crack appears, again unexpectedly more flexible than the other examples.
The three coating compositions exemplified above were also applied to a biaxially-oriented heat-stabilised PEN substrate having a thickness of 125 μm and treated on both surfaces with an inter-draw in-line primer coating of a sulphonated polyester resin as described hereinabove. The PEN substrate was prepared as follows. A polymer composition comprising PEN was extruded and cast onto a hot rotating polished drum. The film was then fed to a forward draw unit where it was stretched over a series of temperature-controlled rollers in the direction of extrusion (machine direction; MD) to approximately 3.3 times its original dimensions. The draw temperature was approximately 130° C. At this stage, the film was treated on both surfaces with a sulphonated polyester resin primer coating in order to promote adhesion to the subsequent coating. The film was then passed into a stenter oven at a temperature of 135° C. where the film was stretched in the sideways direction to approximately 3.4 times its original dimensions. The biaxially stretched film was then heat-set at temperatures up to 235° C. by conventional means, while the transverse dimensions of the web were reduced by 4%. The film was then cooled and would onto reels. The total thickness was 125 μm. The heat-set biaxially stretched film was then unwound and then further heat-stabilised in a roll-to-roll process by passing the film through an additional set of ovens, of which the maximum temperature was 190° C. The film was unsupported at its edges and transported through the ovens under a low line tension, allowing it to relax and stabilize further. The biaxially stretched, heat-set, surface-primed and off-line-stabilized film was then unwound and further modified on both sides by the above coatings. The coated films were then sputtered and analysed in the same manner as the PET sample. Similar results were observed.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0715062.6 | Aug 2007 | GB | national |
| 0715452.9 | Aug 2007 | GB | national |
| 0716017.9 | Aug 2007 | GB | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/GB08/02633 | 8/1/2008 | WO | 00 | 2/1/2010 |
