LIQUID RADIATION-CURABLE COATING COMPOSITIONS AND OPTICAL GLASS FIBERS COATED THEREWITH

Abstract: A liquid radiation-curable coating composition is dis­ closed which is adapted to deposit, after radiation curing, an adherent cured coating on a glass surface. The coating composition comprises a radiation-curable polyethylenic polymeric organic compound, and from about 0.5% to about 5% of the coating composition of a polyalkoxy silane containing an organic substituent which carries a single active hydrogen atom which is capable of reacting with ethylenic unsaturation by Michael adduction and which is selected from the amino hydrogen atom and the mercaptyl hydrogen atom. Polymethoxy silanes are particularly con­ templated, and the coating of glass optical fibers is especially intended. The compositions of this invention provide im­ proved adhesion to glass, and retention of adhesion upon exposure to high humidity atmospheres is also improved.

Description: Liquid Radiation-Curable Coating Compositions and Optical Glass Fibers Coated Therewiti DESCRIPTION Technical Field This invention relates to radiation-curable coatings for optical glass fibers to improve adhesion to the glass, particularly after exposure to high humidity atmospheres. Ultraviolet radiation is particularly contemplated. Background Art The coating of optical glass fibers with low tensile modulus resin coatings to protect these fibers against abrasion is known, as is the desirability of using coating compositions which cure rapidly on exposure to ultraviolet radiation for this purpose. In many instances it is desired to have the coating adhere strongly to the glass fiber surface, and to maintain this adhesion despite exposure to high humidity atmospheres. However, the ultraviolet-curing coatings which provide appropriate coatings for the optical glass fiber are not strongly adherent to the glass surface. Moreover, the limited adhesion which does exist is markedly impaired when moisture penetrates the coating. It is desired to provide ultraviolet-curing coatings which cure to possess improved adhesion to the optical glass fiber surface, and which maintain the desired adhesion despite exposure to moisture. Disclosure of Invention In accordance with this invention, a liquid radiation-curable polyethylenically unsaturated coating composition has incorporated therein from 0.5% to 5% of a polyalkoxy silane containing an organic substituent which carries a single active hydrogen atom which is capable of reacting with ethylenic unsaturation by Michael adduction, and which is selected from the amino hydrogen atom and the mercaptyl hydrogen atom. A single mercaptyl hydrogen atom is particularly preferred, as in the preferred gamma-mercaptopropyl trimethoxy silane. For purposes of comparison, amino silanes with more than one active hydrogen atom are illustrated by gamma-aminopropyl triethoxy silane and N-beta-(N-vinylbenzylaminopropyl) trimethoxy silane. The amino silanes may be used as such, or provided as the hydrogen chloride or similar salt. Coating the glass surface of optical fibers is particularly intended, so the coating composition should cure on radiation exposure to have a room temperature tensile modulus which is low enough (less than about 103 000 N/cm2 = 150 000 psi) to minimize the danger of microbending. Acrylate unsaturation and ultraviolet cure are preferred. The polyalkoxy silanes are preferably methoxy-substituted, most preferably trimethoxy substituted, with a single aliphatic substituent having only one amino or mercaptyl hydrogen atom. These apparently react with the ethylenic unsaturation, preferably acrylic in nature, either during or before radiation exposure. Regardless of when the reaction occurs, the limited coupling of the silane compound with the available ethylenic unsaturation provides superior adhesion and adhesion retention on humidity exposure. Polyalkoxy silanes having other reactive groups have been tried, but these do not perform nearly as well as those employed herein. When very low service temperatures are contemplated, optical glass fiber is presently ptotected by applying to theTreshly formed glass fiber a buffer coating of a resin which preferably possesses an appropriate refractive index, e.g., in excess of 1.48, and a low tensile modulus (below 6900/cm2= 10,000 psi. preferably below 1380 N/cm2 =2000 psi.) which remains low at very low temperature (-60"C.) to minimize the danger of microbending when such low temperatures are encountered. These low tensile modulus buffer coatings are overcoated with a topcoat to provide enhanced moisture resistance and abrasion resistance. When higher service temperatures are contemplated, economy suggests a single coating, and these are selected to have a somewhat higher tensile modulus so that the coating on the glass will better resist abrasion and moisture. Regardless of which type of coating is applied to the glass, one still needs to minimize the tensile modulus while enhancing adhesion to the glass and while maintaining this adhesion despite exposure to humid atmospheres. For rapid application and cure, coatings which cure on exposure to radiation, preferably ultraviolet radiation, are needed. As a result, the resistance to moisture and the capacity to maintain adhesion on exposure to moisture must be built into an ultraviolet curable coating composition which adheres to glass and which has ancillary properties appropriate to the coating of optical glass fibers. Radiation-curable coatingsfor optical glass fiber may be of various types, but it is always necessary, in order to provide the low to moderate tensile modulus which is necessary in a coating which will contact the glass, to employ a polyethylenic polymeric organic compound. Many appropriate polyethylenic organic compounds are known, and those which are not liquid can be combined with unsaturated liquids to assi¯st¯application, but it has been difficult to achieve preferred properties. Coatings of relatively low tensile modulus which are usually overcoated by coatings developed by Robert E.-Ansel, see Japanese Laid-Open (Kokai) No. 223638/83. These can be described as ethylenic-terminated polyurethane, polyamide or polyurea oligomers having a molecular weght in the range of about 2000 to about 8000 and containing one amide, urea or urethane group for every 300 to 900 units of molecular weight, and containing polyalkylene polyether, polyalkylene polythiol or polyalkylene polyester in which the alkylene moiety contains 2 to 6 carbon atoms. These are preferably diacrylate polyurethane-ureas, and they are combined with 20% to 50% of the composition of a radiation-curable monoethylenic monomer having a Tg below about -10 C., such as phenoxyethyl acrylate, and with 2% to 20% of a monoethylenic monomer having a strong capacity for hydrogen bonding, such as N-vinyl pyrrolidone. An illustrative coating of the above type is made by reacting 4 moles of 5, 5'-methylene bis(cyclohexyl isocyanate) with 2 moles of polyoxypropylene glycol of molecular weight 230 in the presence of 3.4 moles of N-vinyl pyrrolidone and 9.7 moles of phenoxyethyl acrylate. This mixture, with 3% of diethoxy acetophenone as photoinitiator, can be ultraviolet-cured on freshly-drawn optical glass fiber, and it can be modified to enhance retention of adhesion to the glass on humidity exposure in accordance with this invention. This composition is used as the control and in the tabulated modifications of the control which are evaluated hereinafter in Example 3. Another type of very low tensile modulus coatings which is usuallyovercoated is described in Ansel et al. European publication No. 0113779 published July 25, 1984 (see also PCT International Publication No. W084/00424. These coatings can be described as consisting essentially of an organic polysiloxane having from 2 to 6 reactive side chains each of which carry a functional group providing one reactive site which has been reacted to provide a single radiation-curable monoethylenically unsaturated side chain. There is about one such side chain for every 500 to 5000 units of molecular weight. This preferably provides a prime coating which has a tensile modulus at room temperature (250C.) and also at -600C. which is below 2072.X/cm2(3000 psi}. These coatings can be illustrated by reacting one mole of 2-hydroxyethyl acrylate with one mole of isophorone diisocyanate to produce an acrylate-functional urethane monoisocyanate and then reacting three moles of this reaction product with one mole of tris(2-hydroxy polyoxypropyl) polysiloxane of average molecular weight of 6000 and equivalent weight of 2000 (80% methyl substituted and 20% polyoxypropyl substituted). Upon the addition of 3% of diethoxy acetophenone photoinitiator, the mixture can be ultraviolet-cured on freshly drawn optical glass fiber, and it can be modified to enhance adhesion on exposure to humid atmospheres in accordance with this invention. Moderate modulus coatings which are useful without topcoating can be provided by combining: (1) from 65% to 85% of a diethylenic-terminated polyurethane, which may contain urea groups, this polyurethane being based on a diisocyanate having an average molecular weight of from 400 to 40000; and (2) from 5% to 25% of a radiation-curable monoethylenically unsaturated liquid monomer having a Tg below about -20 C. These are illustrated by an adduct of two moles of 2-hydroxyethyl acrylate with one mole of a diisocyanate-terminated polyurethane made by adducting toluene diisocyanate with a polytetramethylene glycol formed by polymerizing tetrahydrofuran to a molecular weight of 600-800. The polyurethaneformed by acrylating this diisocyanate has a molecular weight of about 1900. 75.4 parts of this product mixed with 2.5 parts of benzophenone, 0.4 parts of benzil dimethyl ketal photoinitiator, 9 parts cf trimethylol propane triacrylate and 12.4 parts of ethoxy ethoxyethyl acrylate provides a coating composition which can be coated upon optical glass fiber and ultraviolet cured to provide an effective single-coated product which is used as the control in the Table presented hereinafter. This coating composition can also be modified in accordance with this invention to increase its capacity to retain adhesion to the glass upon high humidity exposure. All proportions herein are by weight, unless otherwise specified, and Tg denotes the glass transition temperature which, for a monomer, is the glass transition temperature of a homopolymer made from that monomer. Since ultraviolet cure is preferred, acrylic unsaturation is best, but since the radiation can vary, electron beam radiation is useful, though less preferred, so the character of the ethylenic unsaturation can vary. Other useful unsaturations are illustrated by methacrylic, itaconic, crotonic, allylic, vinylic, etc. These can be provided (using methacrylic unsaturation as illustrative) by reaction of isocyanate functionality with 2-hydroxyethyl methacrylate. Al-Iylic unsaturation nay be introduced using allyl alcohol in place of 2-hydroxyethyl acrylate. Vinylic unsaturation may be introduced using hydroxy butyl vinyl ether. Accordingly, while acrylate unsaturation has been referred to previously as illustrative and preferred, other radiation curable monoethylenically unsaturated compounds may be used, though more powerful radiation would be necessary, as illustrated by electron beam radiation. The radiation which effects cure will vary with the photoinitiator used. Even visible light may be employed using appropriate photoinitiators. These are illustrated by camphor quinone and coumarin which are used together with a tertiary amine, such as triethyl amine. Diphenylbenzoylphosphine oxide is useful in and near the ultraviolet range. When ultraviolet light is employed, the coating composition will usually contain a ketonic photoinitiator, such as about 3% of diethoxy acetophenone. Other photoinitiators are also known, such as acetophenone, benzophenone, m-chloro-acetophenone, propiophenone, thioxanthone, benzoin, benzil, anthraquinone, and the like. The photoinitiators may be used singly or in mixtures, and are present in an amount up to about 10% of the coating, usually 1-5%. Various amines may also be added, like diethyl amine, but are not needed in many instances. The invention is illustrated by a series of coating compositions formulated by simple mixing of the components tabulated below. The mixture is warmed to about 55"C. for 1 hour to dissolve all of the components. TABLE Component Control Ex. 1 Ex. 2 l-Urethane acrylate oligomer 75.7 -- -2-Trimethylolpropane triacrylate 9.0 -3-Ethoxy ethoxyethyl acrylate 12.4 -- -4-Benzyl dimethyl ketal 0.4 - S -Benzophenone 2.5 -- - items 1-5 combined -- 99.0 99.0 7-Gaema-aminopropyl triethoxy silane -- 1.0 -8-N-Beta-(N-vinylbenzylamino ethyl) -gamma-aminopropyl trimethoxy silane.hydrogen chloride (40% in methanol) -- -- 1.0 Adhesion (g/25.4 uErVsee note 1/ 24/11 123/71 131/49 Note 1: This is a 1800 peel test on a 0.076 mn(3 mil) glass. The film is pulled back upon itself to measure the force which removes the film from the glass. The first number is the initial adhesion. The second number is the adhesion after 24 hours exposure of the coated glass to 95% relative humidity at 25 C. As can be seen, polyalkoxy silanes improve adhesion and retention of adhesion on humidity exposure, but further improvement is provided herein. Example 3. To illustrate this invention, an ultraviolet curable optical fiber coating composition which cured to provide excellent initial adhesion of 442 gm/25.4 mm was greatly damaged by 24 hours exposure to 95% relative humidity at 25 C. such that the adhesion after exposure was only 9 gm,25.QThis composition was modified to include 0.6% of diphenyl amine and 0.3t of dilauryl thiodipropionate and 1% of gamma aminopropyl triethoxysilane. The initial adhesion was improved to 561 gm/25.4mm. After humidity exposure adhesion had only decreased to 86.8 gm/25.4mm. Replacing the aminosilane with 1% of gamma-mercaptopropyl trimethoxy silane provided an initial adhesion of 604 gm/25.4mmIhis initial adhesion only decreased to 245 2 25.4nmafter the specified humidity exposure.

Claims: CLAIMS 1. A liquid radiation-curable coating composition adapted to deposit, after radiation curing, an adherent cured coating on a glass surface, comprising a radiation-curable polyethylenic polymeric organic compound, and from about 0,5 % to about 5% of the coating composition of a polyalkoxy silane containing an organic substituent which carries a single active hydrogen atom which is capable of reacting with ethylenic unsaturation by Michael adduction and which is selected from the amino hydrogen atom and the mercaptyl hydrogen atom. 2. A coating composition as recited in claim 1 in which said active hydrogen atom is an amino hydrogen atom or a mercaptyl hydrogen atom. 3. A coating composition as recited in claim 1 in which said radiation-curable polyethylenic polymeric organic compound is acrylate-functional. 4. A coating composition as recited in claim 3 in which said radiation-curable acrylate-functional polymeric organic compound is a polyurethane or a polyurethane urea. 5. A coating composition as recited in claim 1 in which sald composition cures to provide--a film having a tensile modulus less than about 103 000 N/cm2 (150,000 psi). 6. A coating composition as recited in claim 1 in which said composition cures to provide a film having a tensile modulus less than about 6900 N/cm2 (10 000 psi). 7. A coating composition as recited in claim 1 in which said radiation--curable -polyethylenic polymeric -organic compound is acrylate-functional, and a photoinitiator is present to render said composition curable with ultraviolet radiation. 8. An optical glass fixer coated with a radiation-cured coating of the composition of claim 1. 9. An optical glass fiber coated with an ultraviolettcured coating of the composition of claim 7. 10. A coated optical glass fiber as recited in claim 9 in which said polyalkoxy silane is gamma-mercaptopropyl trimethoxy silane.

Inventors: BISHOP, TIMOTHY E.
Applicants: DESOTO, INC.
Application year: 1984
Main classification: C03C25
IPC codes: C03C25, C09D4, C09D183, C08G18, C09D133, C09D175, C08G77