WO1991003503A1

WO1991003503A1 - Primary coating compositions for optical glass fibers

Info

Publication number: WO1991003503A1
Application number: PCT/US1990/004976
Authority: WO
Inventors: Erwin S. Poklacki; Timothy E. Bishop
Original assignee: DeSoto Inc
Current assignee: DeSoto Inc
Priority date: 1989-09-01
Filing date: 1990-08-31
Publication date: 1991-03-21
Anticipated expiration: 1992-03-01
Also published as: AU6406490A; CA2024379A1

Abstract

Polyurethane (meth)acrylate oligomers having a (meth)acrylate functionality of about 1.9 or less and photocurable liquid coating compositions including the oligomer are disclosed. These compositions comprise the oligomer and a mono(meth)acrylate.

Description

PRIMARY COATING COMPOSITIONS FOR OPTICAL GLASS FIBERS

Technical Field

This invention relates to polyurethane (meth)acrylate oligomers and coating compositions containing the same which are suitable as primary coatings for optical glass fibers. The coatings exhibit a reduced modulus and improved adhesion to glass. Background of the Invention Optical glass fibers are frequently coated with two superposed photocured coatings. The coating which contacts the glass is a relatively soft, primary coating. The outer, exposed coating is a much harder secondary coating that provides resistance to handling forces, such as those encountered when the fiber is cabled.

The coating of optical glass fibers with photocured coating compositions, usually using ultraviolet light, is well known today. Photocuring compositions are selected because of their rapid cure speed. Faster cure speed is generally desirable to increase the production of optical glass fibers.

Important properties of the cured coating include adhesion to the glass, resistance to water absorption and resistance to microbending especially at low service temperatures.

Conventional compositions typically include an acrylate-terminated polycarbonate diol-based polyurethanes. Coatings produced from these conventional compositions have an acrylate functionality of 2 or more and are too hard to be utilized as primary coatings. These coatings can also exhibit poor adhesion and poor resistance to microbending. When the usual mono(meth)acrylate ethers having a low glass transition temperature are added to these compositions in an amount sufficient to provide adequate flexibility, the water resistance and adhesion of the coating can be undesirably reduced.

The present invention provides compositions suitable for use as a primary optical glass fiber coating and comprises a polyurethane (meth)acrylate oligomer having a (meth)acrylate functionality of about 1.9 or less and a mono(meth) crylate. Coatings produced therefrom are soft and flexible while retaining good water resistance and adhesion. Summary of the Invention

This invention provides new polyurethane (meth)acrylate oligomers and photocurable liquid coating compositions containing the same. The coating composition is suitable for use as a primary coating for optical glass fibers. The oligomer comprises the reaction product of a prepolymer or admixture of prepolymers, a diisocyanate, and a hydroxy (meth) crylate, and has a (meth)acrylate functionality of about 1.9 or less. Preferred prepolymers are polycarbonate diols that are present in an amount sufficient to provide excess hydroxy functionality as compared to the free nitrogen-carbon-oxygen group (NCO) functionality of the diisocyanate as reduced by the hydroxy functionality of the hydroxy (meth)acrylate to achieve the desired (meth)acrylate functionality. The coating composition comprises: (1) the polyurethane (meth)acrylate oligomer; and (2) a mono(meth)acrylate having a glass transition temperature (T_fl) below about -20°C. The coating composition can also include a small amount of a monoethylenically unsaturated material having a T_g greater than about 40°C. and a strong capacity for hydrogen bonding.

Conventional photoinitiators, stabilizers, adhesion promoters and the like can be present in the coating composition. Coatings produced from the present coating composition have a high adhesion to glass, low water absorption and fast cure speeds. These coatings are flexible, exhibiting a very low tensile modulus, e.g., preferably less than about 1.5 megapascals (MPa) .

As previously discussed, many conventional- coating compositions include an acrylate-terminated polyurethane having an acrylate functionality of 2 or more utilize a mono(meth)acrylate which is usually a polyether to provide, and adjust, the flexibility of the coatings made therefrom. However, these mono(meth)acrylate polyethers, which typically have a T_g of less than about - 0°C, reduce the water resistance and the adhesion of the coating. In contradistinction, the present composition utilizes a polyurethane (meth)acrylate oligomer having a (meth)acrylate functionality of about 1.9 or less which imparts flexibility to the coatings. This enables the composition to better accommodate (1) the conventional mono(meth)acrylate polyethers, as by reducing the amount utilized, and (2) a relatively higher T_g, i.e., less than about -20°C, mono(meth)acrylate without sacrificing water resistance and adhesion to obtain the desired flexibility. In one embodiment of the present invention, the mono(meth)acrylate includes a

(meth)acrylate of an alkoxylated phenol to obtain coatings that are even more flexible and maintain satisfactory water resistance and adhesion.

Coatings produced from the present composition can contain unreacted hydroxy groups if excess hydroxy functionality is utilized in the production of the oligomer. Unreacted hydroxy groups are conventionally expected to reduce the water resistance of a coating due to the group's hydrophilic nature. However, the present coatings exhibit less loss of water resistance than is expected for the amount of excess hydroxy functionality present.

Detailed Description of the Preferred Embodiments

Although this invention is susceptible to embodiments in many different forms, preferred embodiments of the invention are shown. It should be understood, however, that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the invention to embodiments illustrated.

As previously discussed, the photocurable liquid coating compositions disclosed herein are suitable for use as a primary coating for optical glass fibers. The coating composition comprises: (1) the polyurethane (meth)acrylate oligomer having a

(meth)acrylate functionality of about 1.9 or less and • (2) the mono(meth)acrylate having a glass transition temperature (T_g) below about -20°C.

The term "functionality", and various grammatical forms thereof, indicates the average number of groups present per molecule of the referred to material that are capable of participating in a chemical reaction. The number associated with the term functionality describes the average number of reactive groups present per molecule. For example, the phrase "a (meth)acrylate functionality of about 1.9 or less" means that for a given sample of the oligomer, the sum of the (meth)acrylate groups present divided by the number of oligomer molecules present is about 1.9 or less. The term "(meth)acrylate", and various grammatical forms thereof, identifies esters that are the reaction product of an acrylic or a methacrylic acid with the hydroxy group, or groups, of an alcohol or other hydroxy containing organic compound. The term "glass transition temperature", and various grammatical forms thereof, is defined as the temperature at which the homopolymer of the referred to material changes from a vitreous state to a plastic state.

The polyurethane (meth)acrylate oligomer is the reaction product of a prepolymer or mixture of prepolymers, an organic diisocyanate and a hydroxy (meth)acrylate.

The prepolymer is a carbon chain that can comprise oxygen and/or nitrogen atoms, the terminal (meth)acrylate functionality is added to the prepolymer by use of the diisocyanate. Selection of the prepolymer can affect the physical properties of the coatings produced from the oligomer-containing composition.

The prepolymer has at least one prepolymer functional group that is reactive with the isocyanate group, e.g., a hydroxy, mercapto, amine or similar group. Presently, a preferred prepolymer functional group is the hydroxy group. If the prepolymer is monofunctional it is used in admixture with other polyfunctional prepolymers in a proportion to obtain the desired (meth)acrylate functionality of the oligomer.

The prepolymer, or admixture thereof, preferably has a functionality of about 1.5 to about 3, most preferably about 1.8 to about 2.5, groups that are reactive with the isocyanate group.

The number average molecular weight of the prepolymer is about 500 to about 2,000, preferably about 800 to about 1,800, daltons.

The term "dalton", in its various grammatical forms, defines a unit of mass that is l/12th the mass of carbon-12.

Prepolymers are selected from the group consisting of polycarbonates, polyesters, polyethers and mixtures thereof. The polycarbonate diols are conventionally produced by the alcoholysis of diethylcarbonate with a diol. The diol is an alkylene diol having about 2 to about 12 carbon atoms, e.g., 1,4-butane diol, 1,6-hexane diol, 1,12-dodecane diol and the like. Preferably, the diol has about 4 to about 8 carbon atoms. Mixtures of these diols can also be utilized. The polycarbonate diol can contain ether linkages in the backbone in addition to carbonate groups. Thus, polycarbonate copolymers of alkylene oxide monomers and the previously described alkylene diols are suitable. Suitable alkylene oxide monomers include ethylene oxide, tetrahydrofuran oxide and the like. These copolymers produce cured coatings that exhibit a lower modulus and also inhibit crystallinity of the liquid coating composition, as compared to polycarbonate diol homopolymers. Admixtures of polycarbonate diols and polycarbonate copolymers can be utilized.

Suitable polycarbonate diols include Duracarb 122, commercially available from PPG Industries and Permanol KM10-1733, commercially available from Permuthane, Inc., MA. Duracarb 122 is produced by the alcoholysis of diethylcarbonate with hexane diol.

Illustrative polyesters include polybutylene adipate, polycaprolactones and the like.

Illustrative polyethers include poly(propylene oxide), poly(tetramethylene glycol) and the like.

A wide variety of diisocyanates alone or in admixture with one another can be utilized. Representative diisocyanates include isophorone diisocyanate (IPDI) , toluene diisocyanate, methylene diphenyl diisocyanate, hexamethylene diisocyanate, cyclohexylene diisocyanate, methylene dicyclohexane diisocyanate, 2,2,4-trimethyl hexamethylene diisocyanate, -phenylene diisocyanate, 4-chloro-l,3-phenylene diisocyanate, 4,4*-biphenylene diisocyanate, 1,5-naphthylene diisocyanate,

1,4-tetramethylene diisocyanate, 1,6-hexamethylene diisocyanate, 1,10-decamethylene diisocyanate, 1,4-cyclohexylene diisocyanate, and the like. A preferred diisocyanate is IPDI.

The hydroxy (meth)acrylate can be a mono(meth)acrylate or a poly(meth)acrylate. Monohydric monoacrylates are presently preferred. The (meth)acrylate terminal group in the oligomer is introduced with a urethane linkage by the reaction of the isocyanate group with a hydroxy group of the hydroxy (meth)acrylate.

Suitable monohydric acrylates are the C₂-C₄ alkyl acrylates. Illustrative of these acrylates are 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, and the like. Also suitable are reaction products of 2-hydroxylethyl acrylate with one or more molecules of caprolactone. Mixtures of these acrylates are also suitable. The methacrylate counterparts of the above acrylates can also be utilized.

The reaction to produce the oligomer is preferably conducted in a diluent which reduces the viscosity of the oligomer. This use of the diluent results in the formation of an oligomer-diluent admixture having a viscosity that facilitates production of the coating composition, albeit production can occur at an elevated temperature which further reduces the viscosity.

The diluent is preferably not reactive in the oligomer synthesis but is reactive during the cure of the coating composition. The admixing of the diluent having a

(meth)acrylate functionality of about 1 and the oligomer arithmetically results in the average (meth)acrylate functionality of the oligomer-diluent admixture being lower than the (meth)acrylate functionality of the oligomer. However, the oligomer of the present invention has a (meth)acrylate functionality of about 1.9 or less before the diluent is added due to the chemical reaction of producing the oligomer.

These diluents contribute to the low modulus of the cured film and hence typically have a T_g below about -20°C.

Illustrative of these diluents are octyl/decyl acrylate (an admixture of octyl acrylate and decyl acrylate) , polytetramethy1ene glycol diacrylate, the mono(meth) crylates illustrated hereinafter, the like, and mixtures thereof.

A minor amount of a catalyst for the urethane-for ing reaction is typically utilized, e.g., about 0.03 to about 0.1, preferably about 0.05 weight percent of dibutyl tin dilaurate. A minor amount, i.e., less than about 0.1 weight percent, of a inhibitor can be present to control the oligomer producing reaction. Illustrative inhibitors include butylated hydroxy toluene, phenothiazine, and the like. A sparge of inert gas, e.g., dry air, nitrogen, carbon dioxide or the like, is utilized to ensure that there is no moisture present which can adversely affect the production of the oligomer. The reactants for the production of the oligomer, i.e., the prepolymer, the diisocyanate, and the hydroxy (meth)acrylate, can be combined and reacted simultaneously in a suitable vessel.

A preferred method of producing the polyurethane (meth)acrylate oligomer is to introduce the diluent, the diisocyanate and the inhibitor into the vessel and elevate the temperature thereof to about 30°C. to about 50°C. while agitating and sparging with an inert gas. The agitation and the sparge are maintained throughout the reaction. Next, the catalyst can be introduced into the vessel. The hydroxy

(meth)acrylate is then slowly introduced into the vessel, typically over a time period of about 15 minutes to about one hour. After the addition of the hydroxy (meth)acrylate is completed, the reaction is maintained at an elevated temperature for a time period sufficient to consume substantially all of the hydroxy functionality of the hydroxy (meth)acrylate and produce a (meth)aerylate-terminated urethane isocyanate. For example, at a temperature of about 30°C. to about 50°C, substantially all of the hydroxy functionality is consumed in a time period of about 0.5 to about 2 hours. The temperature of the reactants can then be increased to facilitate admixing by further reducing the viscosity. Subsequently, the prepolymer is introduced into the vessel and reacted with the (meth)acrylate-terminated urethane isocyanate. After introduction of the prepolymer, the temperature of the. reactants is further elevated to about 50°C. to about 70°C. and maintained at that temperature for a time period sufficient to reduce the NCO content to less than about 0.2 percent, preferably less than about 0.1 percent of the oligomer. Typically, this reduced NCO content is achieved in a time period of about 1.5 to about 10 hours, albeit this reaction can be left to run overnight without adversely affecting the oligomer. After the desired NCO content is achieved, additional diluent can be added to produce an oligomer-diluent admixture having the desired viscosity at the temperature at which subsequent admixing of the oligomer-diluent admixture with the other constituents of the present coating composition is performed.

This method of producing the oligomer is preferred because reaction of all of the hydroxy functionality of the hydroxy (meth)acrylate is desired and this may not occur if the prepolymer is reacted with the diisocyanate first or with the diisocyanate and the hydroxy (meth)acrylate. If the prepolymer utilized is an admixture of monofunctional and polyfunctional prepolymers, the mole percent of each can be selected to enable use of a stoichiometrically correct proportion of prepolymer admixture to NCO functionality of the diisocyanate after the NCO functionality is reduced by the hydroxy functionality of the hydroxy (meth)acrylate. A stoichiometric excess supplied by the polyfunctional prepolymer can be present in the prepolymer admixture. When only a polyfunctional prepolymer is utilized, stoichiometric excess of the polyfunctional prepolymer is added to the vessel to provide an excess of prepolymer functionality as compared to the NCO functionality present after the initial NCO functionality of the diisocyanate is calculated to be, or actually, reduced by the hydroxy functionality of the hydroxy (meth)acrylate. The amount of this excess prepolymer functionality is that which is effective to provide a lowered modulus and a coating that satisfactorily protects the fiber. This use of an excess of prepolymer functionality results in the reduction of the (meth)acrylate functionality of the resultant oligomer to about 1.9 or less because functional groups from different polyfunctional prepolymer molecules are available to react with the NCO functionality. This lessens the probability that two functional groups of a single polyfunctional prepolymer molecule each will react with NCO functionality to produce an oligomer having a (meth)acrylate functionality of 2 or more. When only a polyfunctional prepolymer is utilized, the mole ratio of polyfunctional prepolymer: diisocyanate: hydroxy (meth)acrylate is in a range of about 1.4:1.8:1.0, respectively, to about 1.7:2.0:1.0, respectively. There are two preferred ways to determine the

NCO functionality present for the purpose of calculating the excess prepolymer functionality required when a polyfunctional prepolymer is utilized. One way is to subtract the hydroxy functionality of the hydroxy (meth)acrylate from the NCO functionality of the diisocyanate. Alternatively, and most preferably, the hydroxy (meth)acrylate and the diisocyanate are reacted, as described above, to consume essentially all of the hydroxy functionality of the hydroxy (meth)acrylate and the NCO functionality of the resultant (meth)acrylate-terminated urethane isocyanate reaction product is then conventionally measured. The proper amount of prepolymer can then be calculated.

Preferably, up to about 40, more preferably about 10 to about 35, percent excess prepolymer functionality is present in the oligomer when a polyfunctional prepolymer is utilized. Thus, if the prepolymer is a polyhydroxy prepolymer such as a polycarbonate diol then excess hydroxy functionality in the above amounts would be present. The (meth)acrylate functionality of the oligomer is about 1.9 or less, preferably at least about 1.5 and most preferably about 1.5 to about 1.8.

The number average molecular weight of the polycarbonate polyurethane (meth)acrylate oligomer is about 600 to about 15,000, preferably about 1,000 to about 5,000 daltons.

Although desiring not to be bound by a particular theory, it is presently believed that polymer chains, provided by the prepolymer, having unreacted terminal groups extending from the oligomer reduce the modulus of the cured coating because they remain free to move as the cured coating is flexed. Changing the length and number of these free polymer chains changes the modulus obtained. An additional constituent of the coating composition is the mono(meth)acrylate having a T_g below about -20°C. As previously discussed, the mono(meth)acrylate can function as a diluent during the production of the oligomer. The T_g of the mono(meth)acrylate can be as low as about -90°C. Suitable mono(meth)acrylates include ethoxy- ethoxyethyl (meth)acrylate, phenoxyethyl (meth)acrylate, butoxyethyl (meth)acrylate, nonyl-substituted phenoxyethyl (meth)acrylate, and the like.

Other suitable mono(meth)acrylates include the (meth)acrylate of the C₇-C₁₀, preferably C_β-C₉, alkyl substituted phenol that is alkoxylated with a C₂-C₄ alkylene oxide so that it contains about 1 to about 10 moles of the oxide per mole of the phenol. Preferably, the (meth)acrylate of the alkoxylated phenol contains about 3.5 to about 4 moles of oxide per mole of the phenol.

Suitable alkylene oxides include ethylene oxide, propylene oxide, butylene oxide, and mixtures thereof. Presently, ethylene oxide is preferred. Commercially available illustrative acrylates of the alkoxylated phenol include alkoxylated nonyl phenol acrylates such as Aronix M-lll, Aronix M-113 and Aronix M-117 from Toa Gosei, Japan.

Mixtures of the above suitable mono(meth)acrylates can also be utilized. Monoacrylates are preferred.

The coating composition can further include a monoethylenically unsaturated material having a high T_g and a strong capacity for hydrogen bonding. These monoethylenically unsaturated materials typically have a T_g greater than about 40°C. and are illustrated by N-vinyl monomers, e.g., N-vinyl pyrrolidone, N-vinyl caprolactam, mixtures thereof and the like. When the (meth)acrylate of the alkoxylated phenol is utilized, it is preferable that the monoethylenically unsaturated material is also utilized. The T_g of this monoethylenically unsaturated material can be as high as about 120°C.

The wavelength of the light utilized to cure the coating compositions of the present invention can vary somewhat depending upon the photoinitiator selected. In present practice, the light utilized is usually in the ultraviolet range which extends from about 200 to about 400 nanometers (nm) however, light of a longer wavelength, e.g., light having a wavelength of up to about 600 nm, preferably up to about 520 nm, can be utilized.

The photoinitiators utilized are conventional components of light curable ethylenically unsaturated coatings. Preferred photoinitiators are aryl ketones, e.g., benzophenone, acetophenone, diethoxy acetophenone, benzoin, benzil, anthraquinone, and the like. Commercial photoinitiators are illustrated by Irgacure 184 which is hydroxycyclohexyl phenyl ketone and is available from Ciba-Geigy Corp., Ardsley, NY, and Lucirin TPO which is available from BASF, Chattanooga, TN.

Volatile organic solvents are preferably not utilized in the present coating composition.

The polyurethane (meth)acrylate oligomer is present in the coating composition in an amount in the range of about 30 to about 80, preferably about 45 to about 70 weight percent based on the total weight of the coating composition.

The mono(meth)acrylate is present in the coating composition in an amount in the range of about 10 to about 70, preferably about 20 to about 55 weight percent based on the total weight of the coating composition. As previously discussed, an aliquot of the mono(meth)acrylate can be utilized as a diluent to reduce the viscosity of the oligomer during synthesis thereof, to produce an oligomer-diluent admixture. The remainder of the mono(meth)acrylate is admixed therewith subsequently.

The photoinitiator is present in the coating composition in a range of about 0.5 to about 10, preferably about 2 to about 6, weight percent based on the total weight of the coating composition.

The monoethylenic ^"material having a high T_g, when utilized, is preferably present in the coating composition in a range of about 1 to about 15, more preferably about 2 to about 6 weight percent based on the total weight of the coating composition.

The viscosity of the coating composition, as measured at a temperature of 25°C. using a Brookfield viscometer, is about 3,000 to about 12,000 centipoise (cp) , preferably about 4,000 to about 10,000 cp.

The coating composition can further include - conventional adhesion promoters, light stabilizers and antioxidants.

Silanes are conventional adhesion promoters which can be present typically in an amount less than about 1 weight percent. Illustrative silanes include gamma methacryloxypropyl trimethoxy silane, commercially available from Dynasylan Inc., Switzerland under the trade designation MEMO and gamma mercaptopropyl trimethoxy silane which is commercially available from Union Carbide under the designation A-189.

Conventional light stabilizers such as hindered amines which provide ultraviolet stability for the cured composition can be present in amounts less than about 1 weight percent. Illustrative commercial stabilizers include the following Ciba-Geigy Corp., Ardsley, NY products: bis(2,2,6,6,-tetramethyl- 4-piperidinyl) sebacate commercially available under the trade designation Tinuvin 770; and Tinuvin 292. An illustrative antioxidant is thiodiethylene

(3,5-di-tert-butyl-4-hydroxy) hydrocinnamate, commercially available from Ciba-Geigy Corp. under the trade designation Irganox 1035.

The present coating compositions can be applied as a primary coating to glass fibers utilizing conventional processes. The coating thickness typically is about 10 to about 40 microns.

The following Examples are presented by way of illustration and not limitation.

EXAMPLE 1: Synthesis of the Present Polyurethane

(Meth)acrylate Oliαomer A polyurethane (meth)acrylate oligomer of the present invention was prepared utilizing the reagents, and proportions thereof, of TABLE I, below. The procedure for the synthesis was as follows.

A suitable vessel having agitation, a dry air sparge, and a heat source was provided. The agitation, sparge and heat source were utilized during the entire synthesis. The isophorone diisocyanate, butylated hydroxy toluene and about 90 weight percent of the octyl/decyl acrylate of oligomer A, about 90 weight percent of the phenoxyethyl acrylate of oligomer B or about 85 weight percent of phenoxyethyl acrylate of oligomer C were introduced into the vessel and the temperature of these reagents elevated to, and maintained at, 40°C. Next, the dibutyl tin dilaurate was introduced into the vessel. The hydroxyethyl acrylate was then introduced into the vessel over a time period of about half an hour. The reagents present in the vessel were then reacted for a time period of one hour. At the end of the time period the NCO functionality was conventionally calculated. The amount of polycarbonate diol required was determined to provide 120 percent hydroxy functionality for oligomers A and B and 133 percent hydroxy functionality for oligomer C, as compared to the NCO functionality present. The temperature of the reagents present was elevated to 60°C. The polycarbonate diol was then quickly introduced into the vessel and the temperature elevated to 70°C. This temperature was maintained until the NCO content was less than 0.1 percent. The remaining proportion of the octyl/decyl acrylate, or phenoxyethyl acrylate, for oligomer A, B, or C, respectively, was then introduced into the vessel to obtain a 80 weight percent oligomer and 20 weight percent diluent admixture.

Oligomer-diluent admixture A had a theoretical functionality of about 1.7 and a viscosity of about 40,600 millipascal seconds (mPa»s) at a temperature of 25°C. The oligomer produced from admixture A had a calculated number average molecular weight of about 3800 daltons.

Oligomer-diluent admixture B had a theoretical functionality of about 1.7 and a viscosity of about 88,100 mPa s at a temperature of 25°C. The oligomer produced from admixture B had a measured number average molecular weight of about 3800 daltons.

Oligomer-diluent admixture C had a theoretical functionality of about 1.5 and a viscosity of about 81,100 mPa«_"S at a temperature of 25°C. The oligomer produced from admixture C had a calculated number average molecular weight of about 4100 daltons.

TABLE I

REAGENTS FOR THE SYNTHESIS OF POLYCARBONATE

URETHANE ACRYLATE OLIGOMERS

Oligomer-Diluent Admixture (weiσht percent) Reagents

Isophorone diisocyanate Ocytl/decyl acrylate Phenoxyethyl acrylate Butylated hydroxy toluene Dibutyl tin dilaurate Hydroxyethyl acrylate Polycarbonate diol¹

¹ Permanol KM10-1733, commercially available from Permuthane Inc., MA.

EXAMPLE 2: Coating Compositions of the Present

Invention Aliquots of the oligomer-diluent admixtures of EXAMPLE 1 were utilized to prepare coating compositions of the present invention. The constituents of these compositions, and proportions utilized, are presented in TABLE II, below. The compositions were prepared by admixing all of the constituents in a suitable vessel at an elevated temperature of 55°C. for a time period of 20 minutes. Substantially homogeneous coating compositions were produced.

¹ The oligomer-diluent admixture A of EXAMPLE 1 was utilized. ² The oligomer-diluent admixture B of EXAMPLE 1 was utilized.

³ The oligomer-diluent admixture C of EXAMPLE 1 was utilized.

⁴ A photoinitiator, commercially available from BASF, Chattanooga, TN. ⁵ A silane adhesion promoter, commercially available fro Dynasylan Inc., Switzerland.

A light stabilizer, commercially available from Ciba-Geigy, Ardsley, NY. ⁷ A antioxidant, commercially available from Ciba-Geigy Corp., Ardsley, NY.

⁸ An alkoxylated nonyl phenol acrylate, commercially available from Toa Gosei, Japan.

⁹ An aryl ketone photoinitiator, commercially available from Ciba-Geigy Corp., Ardsley, NY.

¹⁰ A light stabilizer, commercially available from Ciba-Geigy, Ardsley, NY.

¹¹ An amine stabilizer.

Some physical properties of the coating compositions D to H are presented in TABLE III, below. The procedure for determining each property is described hereinafter.

TABLE III

PHYSICAL PROPERTIES

Highest value obtained before breaking. ² ² HHiiddee vvaarriiaattiioonnss iinn imeasurements for the two wet adhesion specimens were observed. ¹ NA - not available.

The viscosity was measured using a Brookfield Model RVTDV viscometer operated in accordance with the instructions provided therewith. The temperature of each sample tested was 25°C. The cure speed [Joules/square centimeter

(J/sq cm) ] indicates the number of J/sq cm required to obtain a 95% cure of a 3 mil thick coating utilizing a "D" lamp from from Fusion Curing Systems, Rockville, MD. The "D" lamp emits radiation having a wavelength of about 200 to about 470 nanometers with the peak radiation being at about 380 nanometers and the power output thereof is about 300 watts per linear inch. Currently the optical glass fiber coating industry utilizes primary coating compositions having a cure speed of about 1.0 J/sq cm and this is considered to be adequate. Thus, all coating compositions for which cure speed was determined exceed this requirement. Coating composition D exhibited the fastest cure speed followed by coating composition H. Coating composition E did not cure as fast as coating compositions D or H but exhibited improved wet adhesion as compared thereto. The cure speed of composition E exceeds the acceptable industrial standard.

A film for determination of the modulus of the coating was prepared by drawing down a 3 mil coating on glass plates using a Bird bar, commercially available from Pacific Scientific, Silver Springs, MD. The coating was cured using the "D" lamp . The coating was cured at a dose of about 1.0 J/sq cm which provided complete cure. The film was then conditioned at

23 + 2°C. and 50 ± 3% relative humidity for a minimum time period of 16 hours.

Six, 0.5 inch wide test specimens were cut from the cured film parallel to the direction of the draw down. Triplicate measurements of the dimensions of each specimen were taken and the average utilized. The modulus of these specimens are then determined using an Instron Model 4201 from Instron Corp., Canton, MA, operated in accordance with the instructions provided therewith. A coating having a modulus of less than about 4

MPa is acceptable to the optical glass fiber industry albeit a lower modulus is desirable in many applications. The present composition satisfies this desire by producing coatings having a modulus of 1.4 MPa or less.

To determine the dry and wet adhesion of a film to glass, films were prepared by drawing down 3 mil coatings on glass plates using the Bird bar. The coatings were cured using the "D" lamp. The films were then conditioned at a temperature of 23 + 2°C. and a relative humidity of 50 ± 5% for a time period of 7 days. A portion of the film was utilized to test dry adhesion. Subsequent to dry adhesion testing, the remainder of the film to be tested for wet adhesion was further conditioned at a temperature of 23 + 2°C. and a relative humidity of 95% for a time period of 24 hours. A layer of a polyethylene wax/water slurry was applied to the surface of the further conditioned film to retain moisture. The test was performed utilizing an apparatus including a universal testing instrument, e.g., the Instron Model 4201, and a device, including a horizontal support and a pulley, positioned in the testing instrument. After conditioning, sample specimens that appeared to be uniform and free of defects were cut in the direction of the draw down. Each specimen was 6 inches long and 1 inch wide and free of tears or nicks. The first one inch of each specimen was peeled back from the glass plate. The plate was secured to the horizontal support with the affixed end of the specimen adjacent to the pulley. A wire was attached to the peeled-back end of the specimen, run parallel to the specimen and then run through the pulley in a direction perpendicular to the specimen. The free end of the wire was clamped in the upper jaw of the testing instrument which was then activated. The test was continued until the average force value became relatively constant. Currently, the optical glass fiber industry requires a dry adhesion of at least 50 grams and a wet adhesion of at least 20 grams. The present coatings greatly exceed the dry and wet adhesion requirements of the industry.

To determine the water resistance a 10 mil draw-down of the composition was made on a glass plate utilizing a Bird bar. The composition was cured utilizing the "D" lamp to provide a dose of 1.0 J/sq cm. Three test samples each having dimensions of 1/2" x 1 1/2" were cut from the cured coating. Each sample was weighed utilizing an analytical balance to obtain weight measurement J and then immersed in separate containers of deionized water. After a time period of 24 hours, the samples were removed from the water, blotted to remove excess water on the surface and reweighed to obtain weight measurement K. The samples were then placed in aluminum pans and maintained therein at ambient conditions, i.e., ambient temperature (about 20° - 30°C.) and ambient humidity, for a time period of 120 hours. The samples were then reweighed to obtain weight measurement L. The following formulations were utilized to calculate the water absorption and the extractables.

(I) % water absorption = [ (K - J)/J] x 100

(II) % extractables = [ (L - J)/J] x 100 The present coating composition provides coatings which satisfy or exceed the commercial standard of about 2 to about 5 for the difference between % water absorption and % extractables. This invention has been described in terms of specific embodiments set forth in detail, but it should be understood that these are by way of illustration only and that the invention is not necessarily limited thereto. Modifications and variations will be apparent from the disclosure and may be resorted to without departing from the spirit of the invention, as those skilled in the art will readily understand. Accordingly, such variations and modifications of the disclosed products are considered to be within the purview and scope of the invention and the following claims.

Claims

WE CLAIM:

1. A photocurable liquid coating composition suitable as a primary coating for an optical glass fiber comprising: (l) a polyurethane (meth)acrylate oligomer having a (meth)acrylate functionality of about 1.9 or less that is the reaction product of a prepolymer or admixture of prepolymers, a diisocyanate and a hydroxy (meth)acrylate; and (2) a mono(meth)acrylate having a glass transition temperature below about -20°C.

2. The coating composition in accordance with claim 1 wherein the prepolymer is selected from the group consisting of polycarbonates, polyesters and mixtures thereof having about 1.5 to about 3 functional groups that are reactive with the isocyanate group.

3. The coating composition in accordance with claim 2 wherein the prepolymer has about 1.8 to about 2.5 functional groups.

4. The coating composition in accordance with claim 1 wherein the number average^' molecular weight of the prepolymer is about 500 to about 2,000 daltons.

5. The coating composition in accordance with claim 1 wherein the number average molecular weight of the prepolymer is about 800 to about 1,800 daltons.

6. The coating composition in accordance with claim 1 wherein the prepolymer is (1) a polycarbonate diol produced from an alkylene diol having about 2 to about 12 carbon atoms, (2) a polycarbonate copolymer of an alkylene oxide and the alkylene diol or (3) an admixture of (1) and (2) wherein the prepolymer is present in an amount sufficient to provide an excess hydroxy functionality of up to about 40 percent as compared to the free nitrogen-carbon-oxygen group functionality of the diisocyanate as reduced by the hydroxy functionality of the hydroxy (meth)acrylate.

7. The coating composition in accordance with claim 1 wherein the prepolymer is (1) a polycarbonate diol produced from an alkylene diol having about 4 to about 8 carbon atoms, (2) a polycarbonate copolymer of an alkylene oxide and the alkylene diol or (3) an admixture of (1) and (2) and wherein the prepolymer is present in an amount sufficient to provide an excess hydroxy functionality of about 10 to about 35 percent as compared to the free nitrogen-carbon-oxygen group functionality of the diisocyanate as reduced by the hydroxy functionality of the hydroxy (meth)acrylate.

8. The coating composition in accordance with claim 1 wherein the reaction of the diisocyanate and the hydroxy (meth)acrylate is performed prior to reaction with the prepolymer.

9. The coating composition in accordance with claim 1 wherein the (meth)acrylate functionality of the oligomer is about 1.5 to about 1.8.

10. The coating composition in accordance with claim 1 wherein the (meth)acrylate functionality of the oligomer is at least about 1.5.

11. The coating composition in accordance with claim 1 further including a monoethylenically unsaturated material having a glass transition temperature greater than about 40°C. and a strong capacity for hydrogen bonding.

12. The coating composition in accordance with claim 1 wherein the hydroxy (meth)acrylate is a monohydric monoacrylate.

13. The coating composition in accordance with claim 1 wherein the prepolymer is polyfunctional and the mole ratio of prepolymer: diisocyanate: hydroxy acrylate is in the range of about 1.4:1.8:1.0, respectively, to about 1.7:2.0:1.0, respectively.

14. The coating composition in accordance with claim 1 wherein the number average molecular weight of the oligomer is about 600 to about 15,000 daltons.

15. The coating composition in accordance with claim 1 wherein the number average molecular weight of the oligomer is about 1,000 to about 5,000 daltons.

16. An optical glass fiber coated with the coating composition of claim 1.

17. A photocurable liquid coating composition suitable as a primary coating for an optical glass fiber comprising: (1) a polyurethane (meth)acrylate oligomer having a (meth)acrylate functionality of about 1.9 or less that is the reaction product of a prepolymer that is (a) a polycarbonate diol, (b) a polycarbonate copolymer of an alkylene oxide and an alkylene diol or (c) an admixture of (a) and (b) , a diisocyanate and a hydroxy (meth)acrylate, the prepolymer having a number average molecular weight of about 500 to about 2,000 daltons and being present in an amount sufficient to provide up to about 40 percent excess hydroxy functionality as compared to the free nitrogen-carbon-oxygen group functionality of the diisocyanate as reduced by the hydroxy functionality of the hydroxy (meth)acrylate, the mole ratio of prepolymer: diisocyanate: hydroxy (meth)acrylate being in the range of about 1.4:1.8:1.0, respectively, to about 1.7:2.0:1.0, respectively; and (2) a mono(meth)acrylate having a T below about -20°C.

18. The coating composition in accordance with claim 17 wherein the prepolymer provides about 10 to about 35 percent excess hydroxy functionality.

19. The coating composition in accordance with claim 17 wherein the number average molecular weight of the prepolymer is about 800 to about 1,800 daltons.

20. The coating composition in accordance with claim 17 wherein the reaction of the diisocyanate and the hydroxy monoacrylate is performed prior to reaction with the prepolymer.

21. The coating composition in accordance with claim 17 wherein the (meth)acrylate functionality of the oligomer is about 1.5 to about 1.8.

22. An optical glass fiber coated with the coating composition of claim 17.

23. A polyurethane (meth)acrylate oligomer suitable in a photocurable liquid coating composition comprising the reaction product of a prepolymer or an admixture of prepolymers, a diisocyanate and a hydroxy (meth)acrylate, the oligomer having a (meth)acrylate functionality of about 1.9 or less.

24. The polyurethane (meth)acrylate oligomer in accordance with claim 23 wherein the prepolymer is selected from the group of polycarbonates, polyethers, polyesters and mixtures thereof having about 1.5 to about 3 functional groups that are reactive with the isocyanate group.

25. The polyurethane (meth)acrylate oligomer in accordance with claim 23 wherein the number average molecular weight of the prepolymer is about 500 to about 2,000 daltons.

26. The polyurethane (meth)acrylate oligomer in accordance with claim 23 wherein the number average molecular weight of the prepolymer is about 800 to about 1,800 daltons.

27. The polyurethane (meth)acrylate oligomer in accordance with claim 23 wherein the prepolymer is (1) a polycarbonate diol produced from an alkylene diol having about 2 to about 12 carbon atoms, (2) a polycarbonate copolymer of an alkylene oxide and the alkylene diol or (3) an admixture of (1) and (2) and wherein the prepolymer is present in an amount sufficient to provide an excess hydroxy functionality of up to about 40 percent as compared to the free nitrogen-carbon-oxygen group functionality of the diisocyanate as reduced by the hydroxy functionality of the hydroxy (meth)acrylate.

28. The polyurethane (meth)acrylate oligomer in accordance with claim 23 wherein the prepolymer is (1) a polycarbonate diol produced from an alkylene diol having about 4 to about 8 carbon atoms, (2) a polycarbonate copolymer of an alkylene oxide and the alkylene diol or (3) an admixture of (a) and (b) and wherein the prepolymer is present in an amount sufficient to provide an excess hydroxy functionality of about 10 to about 35 percent as compared to the free nitrogen-carbon-oxygen group functionality of the diisocyanate as reduced by the hydroxy functionality of the hydroxy (meth)acrylate.

29. The polyurethane (meth)acrylate oligomer in accordance with claim 23 wherein the (meth)acrylate functionality of the oligomer is about 1.5 to about 1.8.

30. A polyurethane (meth)acrylate oligomer suitable in a photocurable liquid coating composition comprising the reaction product of a prepolymer, a diisocyanate and a hydroxy mono(meth)acrylate, wherein the prepolymer is (1) a polycarbonate diol produced from an alkylene diol having about 2 to about 12 carbon atoms, (2) a polycarbonate copolymer of an alkylene oxide and the alkylene diol or (3) an admixture of (1) and (2) and is present in an amount sufficient to provide excess hydroxy functionality as compared to the free nitrogen-carbon-oxygen group functionality of the diisocyanate as reduced by the hydroxy functionality of the hydroxy (meth)acrylate, the oligomer having a (meth)acrylate functionality of about 1.9 or less.

31. The oligomer in accordance with claim 30 wherein the mole ratio of prepolymer: diisocyanate: hydroxy mono(meth)acrylate is in the range of about 1.4:1.8:1.0, respectively, to about 1.7:2.0:1.0, respectively.

32. The oligomer in accordance with claim 30 wherein the alkylene diol has about 4 to about 8 carbon atoms.

33. The oligomer in accordance with claim 30 wherein wherein the number average molecular weight of the prepolymer is about 500 to about 2,000 daltons.

34. The oligomer in accordance with claim 30 wherein wherein the number average molecular weight of the prepolymer is about 800 to about 1,800 daltons.

35. A method of producing a polyurethane (meth)acrylate oligomer having an acrylate functionality of about 1.9 or less comprising the steps of: (a) reacting a diisocyanate with a hydroxy

(meth)acrylate at an elevated temperature for a time period effective to consume substantially all of the hydroxy functionality; and then

(b) reacting therewith a prepolymer comprising (1) a polycarbonate diol, (2) a polycarbonate copolymer of an alkylene diol and an alkylene oxide or (3) an admixture of (1) and (2) .

36. The oligomer in accordance with claim 35 wherein the mole ratio of prepolymer: diisocyanate: hydroxy (meth)acrylate is in the range of about 1.4:1.8:1.0, respectively, to about 1.7:2.0:1.0, respectively.