Method for coating an article with a ladder siloxane polymer

Abstract: The invention provides a method for coating an article with a siloxane-based coating composition that results in a coating with high thermal mechanical stability, the method also allowing commercially acceptable speeds to be employed while inhibiting beading of the coating prior to cure. In one embodiment, an article is coated with a composition containing a first methylsilsequioxane oligomer having an initial viscosity of about 2000 cp or less, a second methylsilsequioxane oligomer having an initial viscosity of about 40,000 to about 90,000 cp, a viscosity-lowering additive of a lower aliphatic ketone, and a catalyst. In another embodiment, the invention provides a method for coating an article with a modified methylsilsequioxane coating suitable for higher temperature applications than an all-methyl coating. This embodiment involves coating an article with a composition comprising an oligomer characterized by A:B:C, where A represents the number of methyl pendant groups in the oligomer and is about 13 to about 67, B represents dimethyl pendant groups and is about 0 to about 33, and C represents phenyl pendant groups and is about 10 to about 80, a lower aliphatic ketone viscosity-lowering additive, and a catalyst of a moderately strong protic acid. In a third embodiment, it is possible to attain an even higher temperature coating. The method involves a step of applying a coating containing modified methylsilsequioxane oligomer that was prepared by mixing methyltriethoxysilane monomer with dimethyldiethoxysilane monomer that has already been partially hydrolyzed and oligomerized.

Description: BACKGROUND OF THE INVENTION Field of the Invention The invention relates to coatings, in particular, coatings comprising siloxanes. Discussion of the Related Art Glass optical fibers drawn from a heated glass preform or glass melt exhibit good strength as long as the surface of the drawn fiber remains defect-free. Thus, protective coatings, typically formed from thermoplastics or liquid-applied curable polymer-based resins, are normally applied to fibers immediately after they are drawn and prior to contact between the fiber and any other surface. Typically, the fiber is passed through a fluid polymer that is diluted in monomer or in a solvent to effect the coating, e.g., by use of a draw cup, such as that disclosed in U.S. Patent No. 4,264,649. Optical fibers used in applications where temperatures are high, e.g., exceeding 175°C, for extended periods of time require a coating that is mechanically stable at such temperatures, i.e., a coating with high thermal mechanical stability. Conventional ultraviolet-cured acrylate coatings can be safely used only at temperatures below about 100°C. For higher temperatures, solvent-based polyimide coatings are often used. However, due to low polymer solubility, only thin coatings, e.g., much less than 10 µm, can be applied at each pass of the fiber through a coating composition. Since a protective coating about 10 to about 50 µm thick is generally desirable, several passes, e.g., several successive draw cups, are required to obtain a polyimide coating of adequate thickness. Moreover, the draw speed for solution-based polyimide coatings is limited to about 0.1 m/sec to allow for removal of solvent and completion of the imidization reaction, if required, before the coated fiber is able to be wound on a spool. Such limitations are not compatible with commercial requirements because slow draw speeds and a need for numerous passes through a draw cup complicate the process, reduce productivity, and increase expense. Materials having siloxane backbones, e.g., crosslinkable primarily poly(dimethyl silicones), have been used for coating optical fibers for higher temperature applications. Silicone coatings, however, are rubbery, do not adhere well to glass fiber, and have poor mechanical strength. Organosilsequioxanes, which are precursors of siloxane ladder polymers (dual siloxane chains having cross-links that resemble the rungs of a ladder), have also been used for higher temperature coatings. See, for example, U.S. Patent No. 5,052,779 to Honjo et al. and U.S. Patent No. 4,835,057 to Bagley et al. Organosilsequioxanes are made by carefully controlled hydrolysis and condensation of organotrialkoxysilanes, typically triethoxysilanes. The resulting silanol groups condense with other silanol groups or with residual silyl ethoxy groups to form a siloxane bond, with the elimination of either water or alcohol. The condensation is typically accelerated by heat, e.g. temperatures of about 100 to about 150°C, and by the addition of an acid or a base. As reflected in the Honjo et al. patent, siloxane ladder materials are normally applied to a fiber as a solution of a solid silsequioxane polymer (known in the art as flake), having a molecular weight of about 5,000 to about 10,000, in a solvent, and thermally cured after solvent evaporation. Use of a solvent with the silsequioxane is also discussed in the Bagley et al. patent. Methoxy or ethoxy substituted silanes are normally used as precursors to the silsequioxanes, with the former being more reactive. Pendant groups on the siloxane backbone are often methyl or phenyl. Silsequioxanes form tougher and more adherent coatings than silicones, and generally have desirable thermal mechanical stability. However, the silsequioxane coatings described above require slow draw speeds, e.g., less than 0.5 m/sec, to obtain an acceptable, bubble-free, non-tacky coating. In the Bagley et al. patent, for example, the draw speed was 20 m/minute (about 0.3 m/sec) to obtain a relatively thin, 12 µm coating. As stated above, such draw speeds are too slow for commercial applicability. Typically, for specialty, high temperature optical fiber, draw speeds of about 1 m/sec or higher are considered to be acceptable. Thus, siloxane-based coatings which exhibit high thermal stability, i.e., mechanical stability at high temperatures, and which are capable of being applied at acceptable draw speeds, are desired. SUMMARY OF THE INVENTION The invention provides a method for coating an article with a siloxane-based coating composition, the resultant coating exhibiting desirable thermal mechanical stability. (Thermal mechanical stability indicates the temperature at which a material maintains its mechanical integrity after exposure for an extended time, e.g., greater than one month, and shows no development of cracks when viewed through a light microscope with a magnification ranging from about 150X to about 400X.) The method also allows more commercially acceptable coating speeds to be employed, while inhibiting beading of the coating prior to cure. In one embodiment, an article is coated with a composition containing a first methylsilsequioxane oligomer having an initial viscosity of 2000 cp (2Pas) or less, a second methylsilsequioxane oligomer having an initial viscosity of 40,000 to 90,000 cp (40-90Pas), viscosity-lowering additive of a lower aliphatic ketone, and a catalyst. It was realized that, surprisingly, the presence of a small amount of the lower aliphatic ketone, e.g., about 1 to about 6 weight percent based on the total weight of the siloxane oligomer, had a unique viscosity-lowering effect on silsequioxane compositions that included, for example, a substantial amount (e.g., over 50 weight percent based on the total weight of the siloxane oligomer) of the higher viscosity, liquid methylsilsequioxane. (Oligomer, for purposes of the invention, indicates a partially reacted material that is flowable as a liquid. The term initial viscosity means the viscosity of the oligomer prior to inclusion in the mixture, and in the absence of any solvent or dispersant. Viscosity values used herein reflect viscosity at a temperature of about 25°C. Lower aliphatic ketone is defined as a ketone that contains no more than 5 carbon atoms, and for purposes of the invention also includes cyclopentane.) The invention provides desirable results in that the use of the higher-viscosity methylsilsequioxane oligomer increases the composition's resistance to beading induced by surface tension, and the presence of the lower-viscosity methylsilsequioxane oligomer and the viscosity-lowering additive allows for good coating speeds. The method is particularly useful for coating optical fibers. Optical fibers were coated with a 20-30 µm thick methylsilsequioxane coating after a single draw at a speed of over 3 m/sec, a thickness and speed acceptable for commercial applications for high temperature resistant fiber. DETAILED DESCRIPTION OF THE INVENTION In one embodiment, the invention provides a method for coating an article with a methylsilsequioxane oligomer. The method involves the steps of disposing a coating composition on an article, and curing the coating. The coating composition contains a mixture of a first methylsilsequioxane oligomer having an initial viscosity of 2000 cp (2Pas) or less, a second methylsilsequioxane oligomer having an initial viscosity of 40,000 to 90,000 cp (40-90Pas), a viscosity-lowering additive of a lower aliphatic ketone, and a catalyst. The viscosity of the overall coating composition is advantageously 2,000 to 20,000 cp (2-2.0Pas). This viscosity is useful in obtaining a coating that is capable of being applied at an acceptably fast rate and yet resists beading. Advantageously, the article is an optical fiber. Metal articles of various shapes are also capable of being coated according to the invention. The first, lower viscosity methylsilsequioxane oligomer is typically a solventless liquid. It is advantageously present in the coating composition in an amount ranging from 10 to 20 weight percent based on the weight of the total siloxane oligomer in the coating composition. The second, higher viscosity methylsilsequioxane oligomer is also typically a solventless liquid. The second methylsilsequioxane oligomer is advantageously present in the coating composition in an amount ranging from 80 to 90 weight percent based on the total weight of the siloxane oligomer in the coating composition. The lower aliphatic ketone viscosity-lowering additive is a volatile component that lowers the viscosity of the oligomers, in particular the second, higher viscosity, oligomer. (Volatile indicates that the material is easily removed from a thin polymer coating during an early stage of a thermal cure.) The additive is typically present in an amount ranging from about 1 to about 6 weight percent based on the weight of the total siloxane oligomer. Advantageously, the additive is acetone. The catalyst acts to accelerate the condensation of the methylsilsequioxane oligomer into the ladder siloxane structure. The catalyst is advantageously present in an amount ranging from 0.5 to 6 weight percent based on the weight of the total siloxane oligomer. Advantageously, the catalyst is zinc octoate. It is possible for metal soaps to be used to accelerate the condensation reactions. Examples of suitable metals include copper, cobalt, nickel, molybdenum, chromium, zinc, manganese, and vanadium. Useful catalysts generally represent a balance between shelf life and cure rate (which are normally inversely proportional). Catalysts used for latent cure of epoxies, e.g., Lewis acid-amine complexes, do not appear to offer acceptable properties in the invention. In preparing the coating composition, localized oligomer condensation is possible upon addition of a catalyst. To decrease the probability of such localized condensation, it is useful to dilute the oligomers and the catalyst with a solvent before mixing. Advantageously, the solvent is a lower aliphatic ketone that is able to function as the viscosity-lowering additive when the solvent remains at relatively low concentration after removal of the bulk solvent. One method of preparation is to (a) dilute the first oligomer to about twice its volume in a solvent, (b) separately dilute the second oligomer in the solvent (b) separately dilute the catalyst to about 20 times its volume in the solvent, and (c) mix the first oligomer solution, second oligomer solution, and catalyst solution. The solvent is then removed. If the solvent is to be used as the viscosity-lowering additive, the desired amount is advantageously left in the composition. Other processes for preparing the composition are also possible. The coating composition is applied to the article by any suitable method. Where the article is an optical fiber, the composition is advantageously applied by using a conventional draw cup, as discussed above. The coating is cured by any suitable method, generally by heat, the term cure indicating formation of an insoluble tack-free material with mechanical integrity. The methylsilsequioxane coating composition cures to a clear, adherent, ladder siloxane coating. Using the process of this embodiment, it is possible to dispense a methylsilsequioxane coating composition on an optical fiber using a draw cup, at a speed of over 1 m/sec, advantageously over 3 m/sec, and obtain a post-cure coating thickness of at least about 20 µm after a single draw. The thermal mechanical stability limit of the all-methyl silsequioxane coating is below 150°C, as reflected in Example 1 below. A draw speed of about 3 m/sec is acceptable as a compromise between the desirable temperature resistance of the fiber and the cost of a process involving such a draw speed. Example 1: Procedure Table I lists the silsequioxane oligomers evaluated, including the qualitative rate of cure for the non-catalyzed oligomer. All the oligomers were obtained from OI-NEG, Toledo, OH. Ratio of Pendant Groups (Methyl: Dimethyl: Phenyl) Rate of Cure (uncatalyzed) 100:0:0 fast 67:33:0 moderate 67:0:33 moderate 50:0:50 moderate 40:20:40 moderate 33:33:33 moderate 20:0:80 slow 13:13:75 slow 0:0:75:25 diphenyl slow 0:0:100 slow The degree of cure of an oligomer after a thermal treatment was evaluated qualitatively by noting the hardness and/or tackiness of a film on a glass slide. During initial testing, a layer of oligomer (both alone and containing a catalyst) was spread on a glass slide with a 6 mil (152 µm) doctor blade. The slide was placed in a convection oven maintained at 140 or 160°C, or, for less active catalysts, about 180°C. The degree of cure was evaluated by the ease with which the coating could be scratched with a pencil. The results are presented in Table I. Numerous catalysts were evaluated as to their effect on the cure rate of the oligomers. To decrease the probability of localized oligomer condensation during addition of the catalysts, the oligomers and catalyst were diluted in a solvent prior to mixing. Typically, for the all-methyl silsequioxane, a weighed amount of liquid, solventless oligomer (having a viscosity of about 2000 cp (2Pas) was diluted to twice its volume with acetone. About 3-6% catalyst based on the weight of the oligomer was dissolved in approximately 20 times its volume of acetone and added to the resin solution. The acetone, which was used both as a solvent and as a viscosity-lowering additive, was then removed to the desired amount using a rotary evaporator equipped with a water bath maintained at 45°C. The second, higher viscosity oligomer was also diluted with acetone before being added to the composition. The same procedure was used with modified methylsilsequioxane oligomers (except that no second, higher viscosity oligomer was added). Oligomers were applied to optical fiber in two ways. Initial evaluations were made using a conventionally coated fiber as a substrate on a bench-scale tower. After an initial screening on this apparatus, the faster-curing formulations were tested on a full-size 29-foot (8.8.4m) draw tower, air-cooled, with 50 grams of tension, modified by the addition of a heater composed of three 3-foot (0,9m), wire wound, silica tubes placed directly below the coating cup. The effect of increasing the silanol concentration in the uncured oligomer was evaluated by adding acidified water (having a pH of about 3 to about 5). The addition of water to the all-methyl oligomers resulted in a tacky coating with some oligomers, while causing a hard, bubbly coating on others, and was therefore discontinued. Coating weight loss was monitored with a DuPont 9900 Thermal Analyzer equipped with a 951 Thermogravimetric (TGA) Module. A 5-10 mg sample of cured oligomer was heated at 10°/min from 60 to 550°C, while the chamber was swept with oxygen at a flow rate of 50 cc/min. Results: 100% methylsilsequioxane: An initial experiment was performed with an ethanol solution of a methylsilsequioxane oligomer having a viscosity of about 2000, 4-5% residual ethoxy groups, and 12-14% OH groups. The ethanol was removed and the oligomer coated onto a fiber and cured. A tack-free beaded coating was obtained at a tower draw speed of 0.4 m/sec. TGA experiments showed that the all-methyl oligomer began rapidly losing weight at 369°C. To maintain at least six hours of latency at room temperature and to permit a reasonable working period, zinc octoate was found to be an acceptable catalyst with all-methyl oligomer. The higher viscosity all-methyl oligomer, the catalyst, and the viscosity-lowering additive were found to be significant factors in increasing line speed. Specifically, a composition was prepared from two methylsilsequioxane oligomers with viscosities of 64,000 cp (64 Pas) and 2,000 cp (2Pas), present in a weight ratio of 69:11, respectively, containing 3 weight percent zinc octoate, and 3 weight percent acetone, based on the weight of the siloxane oligomer in the coating composition. The composition cured to form a tack-free, bead-free 20-30 µm coating at a line speed of 3.3 m/sec. Cracking was observed in the coatings after aging for one month at 150°C.

Claims: A method for coating an article, comprising the steps of: disposing a coating composition on an article, wherein the coating composition comprises a first methylsilsequioxane oligomer having an initial viscosity of 2000 cp (2Pas) or less, a second methylsilsequioxane oligomer having an initial viscosity of 40,000 to 90,000 cp (40-90Pas), a viscosity-lowering additive of a lower aliphatic ketone, and a catalyst; and curing the coating. The method of claim 1, wherein the viscosity-lowering additive is acetone. The method of claim 1, wherein the catalyst is zinc octoate. The method of claim 1, wherein the first oligomer is present in an amount ranging from 10 to 20 weight percent and the second oligomer is present in an amount ranging from 80 to 90 weight percent, based on the weight of the total siloxane oligomer in the coating composition. The method of claim 1, wherein the viscosity-lowering additive is present in an amount of 1 to 6 weight percent, based on the weight of the total siloxane oligomer in the coating composition. The method of claim 1, wherein the catalyst is present in an amount ranging from 0.5 to 6 weight percent, based on the weight of the total siloxane oligomer in the coating composition. The method of claim 1, wherein the article is an optical fiber. The method of claim 7, wherein the coating composition is disposed on the fiber by drawing the fiber through a container that comprises the coating composition. The method of claim 8, wherein the thickness of the cured coating after a single pass through the container is at least 20 µm. The method of claim 9, wherein the fiber is drawn through the container at a speed of at least about 1 m/sec. The method of claim 10, wherein the speed is at least 3 m/sec.

Inventors: BAUTISTA JERRY RODOLFO, CHANDROSS EDWIN ARTHUR, KUCK VALERIE JEANNE, BAUTISTA, JERRY RODOLFO, CHANDROSS, EDWIN ARTHUR, KUCK, VALERIE JEANNE
Applicants: LUCENT TECHNOLOGIES INC, LUCENT TECHNOLOGIES INC.
Application year: 1998
Main classification: C09D183
IPC codes: C08K5, C03C25, C03C17, G02B6, C09D183, C08G77