Background A polymer of an acrylic, a methacrylic, or a vinyl alcohol compound having a perfluoroalkyl group or a highly fluorinated alkyl group has hitherto been used usefully as an anti-reflection agent or a protective coating agent for water and oil repellence, moisture-proofing, and stain-proofing because of its chemical and physical characteristics.

However, such a fluorinated polymer has poor scratch resistance due to low crosslinking density and low surface hardness because of low Tg.

To improve the mechanical strength, a highly fluorinated polymer having a fluorinated cyclic structure in a highly fluorinated backbone chain has been developed (see Examined Patent Publication (Kokoku) No. 63-18964, and Unexamined Patent Publication (Kokai) Nos. 63-238111 and 63-238115). This polymer is amorphous and has excellent optical characteristics such as high light transmittance and low refractive index. Polymers having a fluorinated cyclic structure has widely been used in the optical and electrical fields. For example, it is used as an anti-reflection agent in an optical fiber, optical lens, optical cell, display and solar battery, or used as a protective coating agent for water and oil repellence, moisture-proofing and stain-proofing in flexible substrate in the vicinity of nozzle of ink jet printer, solar battery, photosensitive/fixing drum, touch panel, pellicle film and semiconductor buffer coated film.

The solvents known to be capable of dissolving a fluorinated polymer were limited to perfluorocarbons (hereinafter also referred to as PFCs). Therefore, PFCs were exclusively used as the solvent to coat a substrate with the fluorinated polymer. However, PFCs have drawbacks such as long lifetime in air and high global warming effect. With a rise in consciousness of protection for global environment, it has recently been required to limit use and discharge of PFCs.

Summary of Invention The present invention provides a coating composition for an optical device part or an electrical device part, which composition is prepared by dissolving a fluorinated polymer having a fluorinated cyclic structure in a solvent which is environmentally friendly and is capable of forming an excellent coated film, and a coating method using the same. The fluorinated solvent has a global warming potential less than 1000 and a fluorinated polymer having a fluorinated cyclic structure in a backbone chain dissolved in said fluorinated solvent.

The invention also provides a method of coating an optical device part or an electrical device part, which comprises coating a surface of an optical device part or an electrical device part with the coating composition described above and drying.

When using the composition and method of the invention, there can be formed a coated film, which has a uniform thickness and is free from defects, on the surface of a substrate (optical device part or electrical device part) because of excellent solvent characteristics of a fluorinated solvent.

Detailed Description of Illustrative Embodiments The Global warming potential (GWP) is based on the definition set forth by the Intergovernmental Panel in Climate Change in Climate Change : The IPCC Scientific Assessment, Cambridge University Press (1990). According to the Panel, GWP is the integrated potential warming due to the release of 1 kilogram of sample compound relative to the warming due to 1 kilogram of C02 over a specified integration time horizon (ITH) using the following equation: where AT is the calculated change in temperature at the earth's surface due to the presence of a particular compound in the atmosphere [calculated using a spreadsheet model (using

parameters described by Fisher et al. in Nature 344, 513 (1990)) derived from Atmospheric and Environmental Research, Inc.'s more complete one-dimensional radiative-convective model (described by Wang et al. in J. Atmos. Sci. 38,1167 (1981) and J. Geophys. Res.

90, 12971 (1985)], C is the atmospheric concentration of the compound, T is the atmospheric lifetime of the compound and x designates the compound of interest. For further details of GWP, refer to WO 97/14762.

The film-forming polymers used herein are fluorinated polymers having a fluorinated cyclic structure in a backbone chain function as an anti-reflection agent and a protective coating agent for a surface of an optical device part or an electrical device part.

Fluorinated polymers which can be used in the present invention includes, for example, polymer obtained by radically polymerizing a perfluoroether having two terminal double bonds alone or radically polymerizing a perfluoroether having two terminal double bonds with another monomer which is radically copolymerizable with the perfluoroether. For example, such a polymer is disclosed in Unexamined Patent Publication (Kokai) Nos. 63- 238111 and 63-238115. According to these publications, a fluorinated polymer is obtained by cyclopolymerization, that is, it is obtained by radically polymerizing a perfluoroether having two terminal double bonds, e. g. CF2=CF (CF2) R-0-(CF2) mCF=CF2 (wherein n is from 0 to 5, m is from 0 to 5, and m + n is from 1 to 6) alone or radically polymerizing the perfluoroether having two terminal double bonds with another monomer which is radically copolymerizable with the perfluoroether. For example, by polymerizing CF2=CF-O-CF2CF=CF2, a fluorinated polymer having a cyclic structure of the following formula: in the backbone chain can be obtained. Illustrative examples of the monomers which are radically copolymerizable with perfluoroether having two terminal double bonds includes, for example, fluoroolefin such as tetrafluoroethylene; fluorovinyl ether such as perfluorovinyl ether; vinylidene fluoride, vinyl fluoride and chlorotriethylene.

The fluorinated polymer can be, for example, those disclosed in Examined Patent Publication (Kokoku) No. 63-18964. Illustrative examples thereof include an amorphous copolymer comprising a monomer unit represented by the following formula:

of perfluoro-2, 2-dimethyl-1, 3-dioxol (PDD) and a monomer unit of tetrafluoroethylene, and an amorphous terpolymer further comprising a monomer unit of an ethylenically unsaturated monomer, in addition to the monomer units. In the copolymer, the content of the monomer unit of PDD is at least 11.2 mole %. When the content is lower than 11.2 mole %, the resulting copolymer exhibits crystallinity to cause light scattering and, therefore, the copolymer is not suited for use as the anti-reflection agent for an optical device. For the same reason, the content of the monomer unit of PDD is preferably at least 12 mole % in the terpolymer. As the ethylenically unsaturated monomer for the terpolymer, for example, there can be used olefin such as ethylene and 1-butene; vinyl compound such as vinyl fluoride and vinylidene fluoride; and perfluoro compound such as perfluoropropene.

Commercially available fluorinated polymers suitable for use herein include, for example, those which are manufactured by Asahi Glass Co., Ltd. under the trade name of Cytop CTX-805 and CTX109A.

The solvent for the fluorinated polymer is, in accordance with the invention, one having a global warming potential less than 1000, and preferably less than 500. Such solvents include ethers comprising hydrogen and fluorine, i. e., hydrofluoroethers (HFE).

Useful hydrofluoroethers include the following two varieties: (1) segregated hydrofluoroethers, wherein ether-bonded alkyl or alkylene, etc., segments of the HFE are either perfluorinated (e. g., perfluorocarbon) or non-fluorinated (e. g., hydrocarbon), but not partially fluorinated; and (2) omega-hydrofluoroalkylethers, wherein ether-bonded segments can be non-fluorinated (e. g., hydrocarbon), perfluorinated (e. g., perfluorocarbon), or partially fluorinated (e. g., fluorocarbon or hydrofluorocarbon).

Illustrative examples of segregated hydrofluoroethers useful herein include hydrofluoroethers which comprise at least one mono-, di-, ortrialkoxy-substituted perfluoroalkane, perfluorocycloalkane, perfluorocycloalkyl-containing perfluoroalkane, or

perfluorocycloalkylene-containing perfluoroalkane compound. These HFEs are described, for example, in WO 96/22356, and can be represented below in Formula I : Rf-(0-Rh).

(Formula I), wherein: x is from 1 to 3, preferably 1; Rf is a perfluorinated hydrocarbon group having a valency x, which can be straight, branched, or cyclic, etc., and contains from 6 to 15 carbon atoms, wherein Rf can optionally contain one or more catenary heteroatoms, and in all cases, Rf can optionally comprise a terminal FsS-group ; and each Rh is independently a linear or branched alkyl group having from 1 to 3 carbon atoms, preferably having 1 or 2 carbon atoms, and more preferably methyl group.

Among the above HFE, it is preferable that Rf does not contain heteroatoms and does not contain a terminal FsS-group.

Representative hydrofluoroether compounds described by Formula I include the following:

C4F9CF (OC2H5) CF (CF3) 2 C3F7CF (OC2H5) CF (CF3) 2 C2F5CF (OC2H5) CF (CF3) 2 and C2F5CF (OCH3) CF (CF3) 2

wherein cyclic structures designated with an interior"F"are perfluorinated. These HFE compounds can be used alone or in admixture with another HFE.

Other useful hydrofluoroethers include omega-hydrofluoroalkyl ethers which can be described by the general structure shown in Formula II: X-Rf'-(O-Rf")y-O-R"-H (Formula II) wherein: X is either F or H; Rr is a divalent perfluorinated organic radical having from 1 to 12 carbon atoms; Ru, ils a divalent perfluorinated organic radical having from 1 to 6 carbon atoms; R"is a divalent organic radical having from 1 to 6 carbon atoms, and is preferably perfluorinated; and y is an integer from 0 to 4; wherein when X is F and y is 0, R"contains at least one F atom; provided that the total number of fluorinated carbon atoms is at least 6.

Representative compounds described by Formula II include the following compounds:

C4F9OC2F4H, HC3F60C3F6H C5F11OC2F4H C6F130CF2H C6F13OC2F4OC2F4H c - C6F11CF2OCF2H HCF20 (C2F2O)n(CF2O)mCF2H, wherein m = 0-2, and n = 0 - 3, C3F70 [C (CF3) CF20] pCFHCF2, wherein p is 1 or 2, C4F90CF2C (CF3) 2CF2H HCF2CF20CF2C (CF3) 2CF20C2F4H C7F15OCFHCF3 C8F17OCF2O (CF2) 5H and C8F17OC2F4OC2F4OC2F4OCF2H

A solvent particularly useful for a coating composition and a coating method of the present invention is one having the formula: Rf-OC2H5, wherein Rris a linear or branched perfluoroalkyl group having 6 to 15 carbon atoms. Preferably, Rrhas 6 to 8 carbon atoms, and 3-ethoxyperfluoro (2-methyl hexane) (CF3CF (CF3) CF (OC2Hs) C3F7) is the most preferable solvent. These solvents have certain similar solvent characteristics as those of a conventional PFC, and also exhibit similar high solubility to the fluorinated polymer as does a PFC. Specifically, 3-ethoxyperfluoro (2-methylhexane) has a surface tension of 1.4 X 10-2 N/m and a viscosity (25°C) of 1.2 X 10-3 Pa. s, each of which is a factor which can affect formation of a uniform and thin coated film. This solvent has a very small global warming potential as compared with a conventional PFC, and is environmentally friendly. Specifically, it is calculated that a lifetime in air of 3- ethoxyperfluoro (2-methylhexane) is no more than 2.0 years and its global warming potential is 250 by integral of 100 years. On the other hand, it is calculated that a lifetime in air of PFC having the same boiling point, N (C3F7) 3 is about 3000 years and its global warming potential is about 5400 by integral of 100 years. As described above, HFE, and particularly 3-ethoxyperfluoro (2-methylhexane) used in the present invention are solvents, which have certain similar solvent characteristics as those of a conventional PFC and are also environmentally friendly.

The composition and coating method of the present invention can be used to coat, for example, optical wave guide materials such as optical fibers ; optical cells such as optical lens, photo mask and reticle; and display and solar battery for the purpose of anti- reflection, or applied to a surface of flexible substrate in the vicinity of nozzle of ink jet printer, solar battery, photosensitive/fixing drum, touch panel and touch panel for the purpose of protective coating, or used as optical thin film such as pellicle film of excimer laser, semiconductor protective coating such as semiconductor buffer coated film and LSI interlayer insulation film.

The coating method of the present invention can be preferably used to form an anti-reflection coating on an optical wave guide material. When leaked light penetrated without reflecting at a core/clad interface reflects from an outermost surface of a clad thereby to return into a core, disturbance of a light signal passing through the core can be caused. The disturbance can be prevented by coating the outside of the clad with a fluorinated polymer. The optical waveguide includes, for example, all-quartz optical fiber

wherein both of a core and a clad are made of quartz, and plastic optical fiber (e. g. optical fiber having a core/clad structure wherein a core is made of polycarbonate or a polymer comprising a unit derived from styrol, substituted styrol, acrylate, methacrylate or fluoroacrylate, and a clad is made of a polymer comprising a unit derived from vinylidene fluoride, tetrafluoroethylene and hexafluoropropylene). In case where an optical fiber made of quartz as the substrate is coated, the substrate can be previously treated with a silane coupling agent to enhance the adhesion between the substrate and fluorinated polymer. In case where an optical fiber made of plastic as the substrate is coated, the substrate can be previously treated with a primer to enhance the adhesion between the substrate and fluorinated polymer.

The coating composition can be easily prepared by adding the polymer having a fluorinated structure into hydrofluoroether (HFE) at, for example, room temperature (e. g.

25°C), followed by stirring. The composition can be applied by known techniques such as, for example, spin coating, dip coating, potting and spraying methods. The concentration of the solution of the fluorinated polymer composition varies depending in part upon the kind of the fluorinated polymer, but is typically from about 1 to about 20% by weight. In some embodiments, the coating composition will consist essentially of the film-forming poymer and, as solvent, one or more hydrofluoroethers as described herein.

In other embodiments, the coating composition may further comprise other solvents, additives, etc.

After coating, a coated film is formed by drying. When the surface of the substrate has high heat resistance, such as quartz optical fiber, the drying can be carried out at a temperature at or above the boiling point of the solvent (e. g., 128°C) to reduce the drying time. When the surface of the optical fiber is made of more heat sensitive substrate, e. g., plastic, a film having a desired thickness is preferably obtained by repeating coating and drying at a comparatively low drying temperature (50 to 80°C) to avoid degradation of the substrate. The dry thickness of the coating is usually from 0.1 to 5 um. In case of an antireflection agent coating of the optical waveguide, the thickness is usually from 0.3 to 1 um (e. g. 0.5, um). According to the coating method of the present invention, uniform coating can be obtained even in case of such thin coating because of excellent solvent characteristics of HFE (for example, CF3CF (CF3) CF (OC2Hs) C3F7.

Example The invention will now be further explained by the following illustrative example. Using 3-ethoxyperfluoro (2-methylhexane) (CF3CF (CF3) CF (OC2Hs) C3F7) (solvent 1) as the solvent of the present invention, and perfluorooctane (CsFis) (solvent C2), dicyclopentafluoropropane (mixture of CF3CHC12 and CC1F2CF2CHC1F) (solvent C3) and 1,1,1,2,3,4,4,5,5,5-decafluoropentane (solvent C4) as the solvents of comparative examples, the solubility of the fluorinated polymer having a fluorinated cyclic structure in a fluorinated backbone chain, Cytop CTX-805 (trade name) and CTX109A (trade name) manufactured by Asahi Glass Co., Ltd. was examined.

Using a dropper, 0.03 g of Cytop was charged in a 10 ml screw glass bottle and 10 g of indicated solvent was added, followed by stirring with a stirrer (150 rpm) for three minutes. After allowing to stand for five minutes, the state of the resulting solution was visually observed. The dissolution tests were conducted at room temperature (24°C). The results are shown in Table 1 below, together with global warming potentials of the respective solvents.

Table 1 Solvent 1 C2 C3 C4 Cytop CTX-805 soluble Soluble slightly soluble slightly soluble Cytop CTX-109A soluble Soluble slightly soluble slightly soluble Global warming 210 7400 390 1300 potential In the above table,"soluble"means that no turbidity or deposit was visually observed, while"slightly soluble"means a white film-like substance is floating.

As is apparent from the above results, the polymer having a fluorinated cyclic structure is soluble in CF3CF (CF3) CF (OC2Hs) C3F7 at room temperature, like PFC. As is apparent from the table, the solvent is an environmentally friendly solvent because of its very small global warming potential such as about 1/30 as compared with PFC in which the polymer having a fluorinated cyclic structure is soluble.