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  Radiation- or thermally-curable oxetane barrier sealantsUSPTO Application #: 20070034515Title: Radiation- or thermally-curable oxetane barrier sealants Abstract: This invention relates to cationically curable sealants that provide low moisture permeability and good adhesive strength after cure. The composition consists essentially of an electrophoretic device containing an oxetane compound and a photoinitiating system comprising and photoinitiator and optionally a photosensitizer. (end of abstract) Agent: National Starch And Chemical Company - Bridgewater, NJ, US Inventors: Shengqian Kong, Stijn Gillissen USPTO Applicaton #: 20070034515 - Class: 204627000 (USPTO) Related Patent Categories: Chemistry: Electrical And Wave Energy, Apparatus, Electrophoretic Or Electro-osmotic Apparatus, Barrier Separator (e.g., Electrodialyzer, Etc.) The Patent Description & Claims data below is from USPTO Patent Application 20070034515. Brief Patent Description - Full Patent Description - Patent Application Claims
[0001] This invention is a continuation-in-part application of co-pending U.S. patent application Ser. No. 11/098,116, filed on Apr. 5, 2005. FIELD OF THE INVENTION [0003] This invention relates to barrier sealants, adhesives, encapsulants, and coatings for use in electronic and optoelectronic devices. (As used in this specification and claims, adhesives, sealants, encapsulants, and coatings are similar materials, all having adhesive, sealant, and coating properties and functions. When any one is recited, the others are deemed to be included.) BACKGROUND [0004] Radiation curable materials have found increased use as coatings, adhesives, and sealants over the past three decades for reasons including low energy consumption during cure, rapid cure speed through both radical and cationic mechanisms, low curing temperature, wide availability of curable materials, and the availability of solvent-free products. These benefits have made such products especially suited for rapidly adhering and sealing electronic and optoelectronic devices that are temperature sensitive or cannot conveniently withstand prolonged curing times. Optoelectronic devices particularly are often thermally sensitive and may need to be optically aligned and spatially immobilized through curing in a very short time period. [0005] Numerous optoelectronic devices are also moisture or oxygen sensitive and need to be protected from exposure during their functional lifetime. A common approach is to seal the device between an impermeable substrate on which it is positioned and an impermeable glass or metal lid, and seal or adhere the perimeter of the lid to the bottom substrate using a radiation curable adhesive or sealant. [0006] A common manifestation of this package geometry is exemplified in FIG. 1, which discloses the use of a radiation curable perimeter sealant (1) to bond a metal or glass lid (2) over an organic light emitting diode (OLED) stack (3) fabricated on a glass substrate (4). Although various configurations exist, a typical device also contains an anode (5), a cathode (6), and some form of electrical interconnect between the OLED pixel/device and external circuitry (7). For the purposes of this invention, no particular device geometry is specified or required aside from one which incorporates an adhesive/sealant material such as a perimeter sealant (1). [0007] In many configurations, as for the example in FIG. 1, both the glass substrate and the metal/glass lid are essentially impermeable to oxygen and moisture, and the sealant is the only material that surrounds the device with any appreciable permeability. For electronic and optoelectronic devices, moisture permeability is very often more critical than oxygen permeability; consequently, the oxygen barrier requirements are much less stringent, and it is the moisture barrier properties of the perimeter sealant that are critical to successful performance of the device. [0008] Good barrier sealants will exhibit low bulk moisture permeability, good adhesion, and strong interfacial adhesive/substrate interactions. If the quality of the substrate to sealant interface is poor, the interface may function as a weak boundary, which allows rapid moisture ingress into the device regardless of the bulk moisture permeability of the sealant. If the interface is at least as continuous as the bulk sealant, then the permeation of moisture typically will be dominated by the bulk moisture permeability of the sealant itself. [0009] It is important to note that one must examine moisture permeability (P) as the measure of effective barrier properties and not merely water vapor transmission rate (WVTR), as the latter is not normalized to a defined path thickness or path length for permeation. Generally, permeability can be defined as WVTR multiplied by permeation path length, and is, thus, the preferred way to evaluate whether a sealant is inherently a good barrier material. [0010] The most common ways to express permeability are the permeability coefficient (e.g. gmil/(100 in.sup.2dayatm)), which applies to any set of experimental conditions, or the permeation coefficient (e.g. gmil/(100 in.sup.2day) at a given temperature and relative humidity), which must be quoted with the experimental conditions in order to define the partial pressure/concentration of permeant present in the barrier material. In general, the penetration of a permeant through some barrier material (permeability, P) can be described as the product of a diffusion term (D) and a solubility term (S): P=DS [0011] The solubility term reflects the affinity of the barrier for the permeant, and, in relation to water vapor, a low S term is obtained from hydrophobic materials. The diffusion term is a measure of the mobility of a permeant in the barrier matrix and is directly related to material properties of the barrier, such as free volume and molecular mobility. Often, a low D term is obtained from highly crosslinked or crystalline materials (in contrast to less crosslinked or amorphous analogs). Permeability will increase drastically as molecular motion increases (for example as temperature is increased, and particularly when the T.sub.g of a polymer is exceeded). [0012] Logical chemical approaches to producing improved barriers must consider these two fundamental factors (D and S) affecting the permeability of water vapor and oxygen. Superimposed on such chemical factors are physical variables: long permeation pathways and flawless adhesive bondlines (good wetting of the adhesive onto the substrate), which improve barrier performance and should be applied whenever possible. The ideal barrier sealant will exhibit low D and S terms while providing excellent adhesion to all device substrates. [0013] It is not sufficient to have only a low solubility (S) term or only a low diffusivity (D) term in order to obtain high performance barrier materials. A classic example can be found in common siloxane elastomers. Such materials are extremely hydrophobic (low solubility term, S), yet they are quite poor barriers due to their high molecular mobility due to unhindered rotation about the Si--O bonds (which produces a high diffusivity term (D). Thus, many systems that are merely hydrophobic are not good barrier materials despite the fact that they exhibit low moisture solubility. Low moisture solubility must be combined with low molecular mobility and, thus, low permeant mobility or diffusivity. [0014] For liquid materials that are radiation cured to solid sealants, such as the inventive compositions, the attainment of lower molecular mobility within the cured matrix is approached through high crosslink density, microcrystallinity, or close packing of molecular backbones between the crosslinked portions of the matrix. BRIEF DESCRIPTION OF THE DRAWING [0015] FIG. 1 is a perimeter sealed optoelectronic device. SUMMARY OF THE INVENTION [0016] The inventors have discovered that certain resin and resin/filler systems provide superior barrier performance, particularly to moisture, through the incorporation of an oxetane resin and a cationic initiator into the barrier composition. The oxetane resin in general will have the structure in which R.sup.1, R.sup.2, R.sup.3, R.sup.4, R.sup.5, and R.sup.6 are selected from the group consisting of hydrogen and alkyl, haloalkyl, alkoxy, aryloxy, aryl, ester groups. Such barrier materials may be used alone or in combination with other curable resins and various fillers. The resulting compositions exhibit a commercially acceptable cure rate, a balance of high crosslink density and molecular packing (low permeant mobility/diffusivity term, D), hydrophobicity (low water solubility term, S), and adhesion (strong adhesive/substrate interfaces) to make them effective for use in sealing and encapsulating electronic, optoelectronic, and MEMS devices. DETAILED DESCRIPTION OF THE INVENTION [0017] This invention is a cationically curable barrier sealant consisting essentially of (a) an oxetane compound and (b) a cationic initiator. The barrier adhesive or sealant optionally contains (c) one or more fillers and optionally, (d) one or more adhesion promoters or one or more epoxy resins. When one or more epoxy resins are present, they are selected from the group consisting of bisphenol F diglycidyl ether, resorcinol diglycidyl ether, novolac glycidyl ethers, halogenated glycidyl ethers, naphthalene diglycidyl ether, and cycloaliphatic epoxies. The use of the cationic initiators results in a radiation-curable formulation; however, the use of a cationic catalyst that can trigger polymerization at room or elevated temperatures may be used for thermal cure. The resulting compositions are suitable for use in sealing and encapsulating electronic and optoelectronic devices. [0018] Within this specification, the term radiation is used to describe actinic electromagnetic radiation. Actinic radiation is defined as electromagnetic radiation that induces a chemical change in a material, and for purposes within this specification will also include electron-beam curing. In most cases electromagnetic radiation with wavelengths in the ultraviolet (UV) and/or visible regions of the spectrum are most useful. [0019] For the purposes of this document optoelectronic devices are defined broadly as those which involve optical and/or electrical input or output signals. Non limiting examples of optoelectronic devices include organic light emitting diode (OLED) displays, OLED microdisplays, liquid crystal displays (LCD), electrophoretic displays, plasma displays, microelectromechanical (MEMS) devices, liquid crystal-on silicon (LCOS) devices, photovoltaic cells, charge coupled device (CCD) sensors, and ceramic-metal oxide semiconductor (CMOS) sensors. [0020] Within this specification, the term oxetane compound refers to any small molecule, oligomer, or polymer carrying an oxetane functionality. The oxetane compound in general will have the structure in which R.sup.1, R.sup.2, R.sup.3, R.sup.4, R.sup.5, and R.sup.6 are selected from the group consisting of hydrogen and alkyl, haloalkyl, alkoxy, aryloxy, aryl, ester, thio-ester, and sulfide groups. In one embodiment, the oxetane compounds are selected from the group of oxetane compounds having the structures: Continue reading... Full patent description for Radiation- or thermally-curable oxetane barrier sealants Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Radiation- or thermally-curable oxetane barrier sealants patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. 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