WO1998037030A1 - Radiation-curable optical fiber compositions comprising fluorescent additive to impart color stability - Google Patents

Abstract

Radiation-curable optical fiber coatings and protective materials having fluorescent additive which retards yellowing and color degradation in cured coatings. The compositions comprise radiation-curable oligomer, reactive diluent, and optional photoinitiator. The compositions can be formulated to serve as primary coatings, secondary coatings, or matrix materials. The fluorescent additive can also increase cure speed.

Description

RADIATION-CURABLE OPTICAL FIBER COMPOSITIONS COMPRISING FLUORESCENT ADDITIVE TO IMPART COLOR STABILITY

BACKGROUND OF THE INVENTION

1. Field of the Invention The present invention relates to stabilized optical fiber coatings and protective materials, and in particular, to radiation-curable optical fiber coatings and protective materials which comprise at least one fluorescent additive which, after radiation-cure, imparts color stability even under severe radiation conditions.

2. Description of Related Art Radiation-curable coatings and other types of protective materials are extensively used in the optical fiber industry during the production of optical fibers and cables. Optical glass fibers are routinely coated with at least one radiation-curable coating immediately after the glass fiber is manufactured in a draw tower so as to preserve the pristine character of the glass fiber. Immediately after the coating is applied to the fiber, the coating is rapidly cured by exposure to radiation (commonly ultraviolet light) . The industry demands faster production speeds, and therefore, a high-speed cure of the coating composition is essential. Radiation- curable matrix materials further protect the individual strands of fiber which can be bundled together into optical fiber ribbons, optical fiber cables, and associated structures.

Because a variety of competing characteristics are desired in the coating system, multiple layers of coatings are routinely employed in optical fiber production. These typically include a soft inner primary coating, which directly contacts the glass fiber and prevents microbending, and a tougher outer primary coating (i.e., a secondary coating), which provides a more durable exterior for the glass fiber. Examples of radiation-curable primary coatings are disclosed in U.S. Pat. No. 5,336,563 to Coady et al . Additional aspects of optical fiber coating technology are disclosed in U.S. Pat. Nos. 5,199,098 to Nolan et al . ; 4,923,915 to Urruti et al.; 4,720,529 to Kimura et al . ; and 4,474,830 to Taylor et al .

A long felt need exists to improve the stability, and in particular color stability, of optical fiber coatings and protective materials. A common type of radiation-curable coating used by industry is based on a urethane (meth) acrylate oligomer (methacrylate being less common than acrylate) . Although urethane acrylate-type coatings have been quite successful, many of them tend to yellow particularly upon photoinduced aging under fluorescent or UV light. The industry increasingly demands coatings which are -- and remain -- colorless or at least substantially colorless. Substantially colorless coatings serve an important function because they facially suggest a lack of coating degradation.

Furthermore, coated optical fibers may need to be color- coded, and yellowing can change or mask the color of the coated optical fiber. Color-change is troublesome when, for example, a worker in the field later needs to repair or work on the optical fiber cable and identify individual strands of fiber. Allegedly non-yellowing optical fiber coatings are discussed in, for example, U.S. Pat. Nos. 4,962,992 to Chapin et al . and 5,146,531 to Shustack. Performance demands on optical fiber coatings increase every year, and there is a heightened need for substantially colorless coatings having reduced rates of photoinduced color degradation. In particular, there is a need for substantially colorless coatings having reduced rates of photoinduced yellowing. There is also^" a need for colored coatings (which comprise colorless coatings mixed with colorants) having reduced rates of photoinduced color degradation. This need applies to optical fiber inner and outer primary coatings, as well as to other radiation-curable protective materials in the cable structure like matrix materials.

The present invention provides advantageous color stability by use of fluorescent additives in combination with urethane acrylate compositions. These additives have not been commonly used in the optical fiber industry with urethane acrylate compositions. U.S. Pat. Nos. 5,427,862 and 5,504,830 to Ngo et al . disclose photocurable polyimide compositions which, unlike urethane acrylates, are colored. These patents mention possible use of so-called fluorescent brightener which would function as a whitener in the colored polyimide- based composition. However, polyimides are structurally different from the compositions based on urethane acrylate oligomers, and would be expected to have different photolytic aging mechanisms. Hence, this prior art does not teach or suggest how to prepare urethane acrylate types of compositions which are -- and remain -- substantially colorless or which possess a reduced rate of color degradation. What is needed in the optical fiber art, but what the art has not yet provided, is an additive which is effective in urethane acrylate types of optical fiber coatings and which delays and reduces deterioration of color particularly upon exposure to UV or fluorescent light. Additives, however, can easily disrupt the delicate balance of properties required for an optical fiber coating (e.g, fast cure speed, mechanical properties, and non-gelation) even when present in small amounts. Ideally, the additive would even impart other desirable properties in the composition besides color stability.

Prior art attempts to accomplish these objectives through the use of UV absorbing additives have not met with the success of the present invention (see e.g., U.S. Pat. No. 4,482,204 to Blyler et al) .

SUMMARY OF THE INVENTION The present invention provides a method for reducing the rate of yellowing increase in a substantially colorless radiation-cured urethane acrylate composition by use of a fluorescent additive which absorbs in the invisible UV region and emits in the blue to blue-violet light region. The present invention also provides a novel radiation-curable urethane acrylate composition which comprises this fluorescent additive and, upon radiation-cure, exhibits improved color stability and prolonged non-yellowing character.

The compositions are preferably designed for use as an optical fiber coating or related optical fiber protective material. Optical fiber applications have their own set of unique performance requirements which distinguish them from conventional applications. The present invention, however, is focused on but not limited to optical fiber applications. The invention provides a radiation-curable composition for protecting an optical fiber comprising as pre-mixture ingredients:

(A) about 10 wt . % to about 90 wt . % of at least one radiation-curable urethane (meth) acrylate oligomer which comprises an oligomer backbone, at least one urethane linking group, and at least one radiation-curable end- capping group;

(B) about 10 wt . % to about 90 wt . % of at least one reactive diluent; (C) optionally, an effective amount of at least one photoinitiator; and

(D) an effective amount of at least one fluorescent additive which absorbs in the invisible ultraviolet region and emits in the blue to blue-violet light region, wherein the composition, after radiation cure, is substantially colorless, and the effective amount of fluorescent additive reduces the rate of discoloration in the composition after radiation-cure.

In addition to improving the non-photodegradation properties of the cured coatings, the fluorescent additive can also function to increase the cure speed and increase the depth of cure. Fluorescent additives also can serve as a fluorescent marker useful in identifying and analyzing materials . The radiation-curable composition can be tailored to have properties upon cure (e.g., mechanical properties) which allow it to serve as an inner primary, an outer primary (i.e., a secondary), or a single coating. The radiation-curable compositions also can be tailored to be a matrix material or other types of materials used in optical fibers or cables.

The present invention also can be adapted for a variety of optical fiber coating technologies including both sequential and simultaneous ("wet-on-wet") cure systems. In the former, inner and outer primary coatings are applied and cured sequentially (one after the other) in separate cure steps, whereas in the latter, the coatings are each applied sequentially, before cure, but then cured simultaneously in a single cure step.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 illustrates the structure of a fluorescent additive, Uvitex OB^®, and its absorption and emission spectra .

Figures 2-3 each illustrate the reduced rate of yellowing during UV induced aging (short and long exposure times, respectively) when a radiation-cured inner primary coating composition comprises a fluorescent additive. Comparison is provided with compositions comprising blue dye additive.

Figure 4 illustrates the yellowing change induced in a radiation-cured inner primary coating at 125°C.

Figure 5 illustrates the effect of a fluorescent additive used at different concentrations on the cure speed of an inner primary coating composition.

Figure 6 illustrates the effect of a fluorescent additive used at different concentrations on the cure speed of an outer primary coating composition.

Figure 7 illustrates inner primary coating aging under UV light with a fluorescent additive used at different concentrations .

Figure 8 illustrates inner primary coating aging under fluorescent light with a fluorescent additive used at different concentrations.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The following definitions apply to the present invention:

" (Meth) acrylate" means acrylate and/or methacrylate. Acrylate is generally preferred over methacrylate, but methacrylate may also be used.

"Substantially colorless" means that the material has at most little color tint, and in particular little yellow tint, when viewed by itself with the naked eye under conventional interior lighting. There should be substantially no absorption or reflection in the visible range (about 400-750 nm) . Some coatings which are substantially colorless may have a very slight yellow tint due to blue absorption. This term excludes facially colored materials such as the colored polyimides disclosed in U.S. Pat. Nos. 5,427,862 and 5,504,830. Substantially colorless materials exhibit increased amounts of color, often yellow, when aged in accelerated aging tests. The present invention does not use the fluorescent additive simply to mask existing color in the coating, but to prevent the development of color with aging. If the substantially colorless coating is mixed with a colorant so that both colored and substantially colorless components are present, the fluorescent additive prevents development of color in the substantially colorless component.

"Pre-mixture ingredient" means the ingredient before mixing individual ingredients. Pre-mixture ingredients may have the ability to interact or react with each other after mixing. "Effective amount of at least one fluorescent additive" means that a person skilled in the art can determine the amount based on the particular need for non-yellowing in the cured composition. Non-yellowing may be balanced with other important coating properties such as cure speed and depth of cure. Also important is whether the composition is tailored to be, for example, an inner primary or outer primary coating.

"Effective amount of at least one monomer diluent" means that a person skilled in the art can determine the amount based on the particular needs for the cured composition. For example, required properties for inner and outer primary coatings differ. The amount of diluent will effect modulus, Tg, hydrophobicity, cure speed, and other important properties.

"Effective amount of at least one photoinitiator" means that a person skilled in the art can determine the amount of photoinitiator based on the particular coating system. For example, the amount of photoinitiator may depend on the activity of the photoinitiator or the norr- yellowing character of the photoinitiator. Also important may be whether the coating is an inner or outer primary coating, and whether the fiber production process is a simultaneous or sequential cure. The "optional" use of photoinitiator means that for some cure systems, such as electron beam cure, a photoinitiator is not generally needed. However, at least some photoinitiator will be required for optical fiber applications which employ a rapid UV-cure. Radiation-curable optical fiber coatings and other protective materials can be based on urethane (meth) acrylate oligomers and one or more reactive diluents. Early work in this area includes U.S. Patent No. 4,682,851 to Ansel of Desoto (now DSM Desotech) . The present invention, which relates to fluorescent additives for such coatings and protective materials, is believed to be applicable to a variety of such coatings based on a wide variety of urethane (meth) acrylate oligomers and reactive diluents. For inner primary coatings, the compositions disclosed in U.S. Pat. No. 5,336,563 are of particular interest, and the complete disclosure of this patent is hereby incorporated by reference.

The present invention is not limited to how the oligomer or the coating composition is prepared. Conventional means can be used. Besides the fluorescent additive, a wide variety of additional ingredients or additives can be used with the oligomer and reactive diluent to the extent that they do not interfere with the fluorescent additive's function of reducing the rate of yellowing and discoloration. Blends of oligomers, reactive diluents, and other ingredients can be used to tailor properties by conventional means.

•Urethane acrylate oligomers are preferred over urethane methacrylate oligomers. They can be the reaction product ^'of a polyol (which forms the oligomer backbone) , a polyisocyanate (which links the oligomer backbone with an end-capping group) , and an end-capping radiation-curable acrylate group (which polymerizes upon exposure to radiation) . Oligomer synthesis can be carried out by methods disclosed in, for example, U.S. Pat. No. 5,336,563, the complete disclosure of which is hereby incorporated by reference. Synthetic conditions are usually adjusted so that the oligomer has on average about one to two polyol backbone units, and preferably, about one polyol unit per oligomer molecule.

Conventional oligomer synthetic methods known in this art can be used.

Although the aforementioned U.S. Pat. No. 5,336,563 is directed toward oligomers for inner primary coatings, the synthetic methods disclosed therein can also be used to prepare oligomers for outer primary (secondary) coatings, matrix materials, and other protective materials. Such coatings are disclosed in, for example, U.S. Pat. Nos. 4,522,465 and 4,514,037 to Bishop et al , the complete disclosures of which are hereby incorporated by reference. Also, U.S. Pat. No. 4,806,574 to Krajewski et al . discloses methods for tailoring the molecular architecture of the oligomer by, for example, use of polyfunctional cores. U.S. Pat. No. 5,093,386 to Bishop et al. and U.S. Pat. No. 4,992,524 to Coady et al . also disclose synthetic strategies for preparing radiation- curable oligomer for materials protecting optical fibers. All of these references are hereby incorporated by reference and teach how to prepare suitable urethane acrylate oligomers .

The number average molecular weight of the oligomer is not particularly limited but can be, for example, about 750-10,000 g/mol, and preferably, about 1,000-5,000 g/mole. Molecular weight can be chosen to achieve the desired viscosity, modulus, solvent resistance, oxidative stability, and other important properties. Oligomer molecular weight and its distribution can be determined by gel permeation chromatography.

The oligomer can be present in amounts between about 10 wt . % and about 90 wt.%, and preferably, between about 40 wt.% and about 80 wt.%, and more preferably, between about 45 wt.% and about 70 wt.% of the total composition. The person skilled in the art can tailor the oligomer amount in view of the end requirements. For example, use of greater amounts of oligomer tends to increase cure speed .

The oligomer is preferably not an oligomer which must undergo reaction of a precursor oligomer, particularly when volatile by-products would be produced. For example, polyimide oligomers are usually made by a precursor polymer approach as noted above .

The urethane acrylate oligomer can be, for example, an oligomer comprising a backbone of polyether, polyester (including polycaprolactone) , polycarbonate, hydrocarbon (saturated and unsaturated) , and silicone units. Polycarbonate- and polyether-based oligomers, and mixtures of these, are preferred. The oligomer can comprise block copolymer and random copolymer structures . The oligomer can be synthesized with use of at least one polyol oligomeric compound, and preferably a diol, which serves as oligomer backbone. Amine functional group (s) also can be present in the backbone-forming oligomer. The backbone hydroxyl group, when reacted with the polyisocyanate during oligomer synthesis, will generate the urethane linkage, whereas the amine group, when reacted with the polyisocyanate, will generate the urea linkage. Urea linkages may allow for improved adhesion and greater stiffness. Therefore, the relative amounts of urethane and urea linkages can be used to tailor the properties of the oligomer and the overall coating composition.

Polyether polyols can be used to serve as oligomer backbone. For example, they can be prepared by ring- opening polymerization of cyclic ethers like THF and derivatives thereof, as discussed in, for example, U.S. Pat. No. 4,992,524 to Coady et al . Polyether backbone repeat units can be based on, for example, C2-C6 alkyleneoxy repeat structures. Representative polyether structures include ethyleneoxy, propyleneoxy, and tetramethyleneoxy repeat units. Substituents such as methyl or ethyl or other alkyl or substituted alkyl groups can be included as side groups off of the polyether backbone .

The oligomer can comprise polyether backbone as disclosed in, for example, U.S. Pat. No. 5,538,791 to Shustack. Polyether type materials including silicone modified materials are also disclosed in, for example, EP Patent Publication No. 0,407,004 (A2) to Duecker which is directed to matrix materials . These references are hereby incorporated by reference.

Polycarbonate polyols can serve as oligomer backbone, as taught by the aforementioned U.S. Pat. No. 5,336,563. The polycarbonate backbone can be tailored by inclusion of polyether units, as known in the art, to minimize crystallization and reduce modulus. Polycarbonate structures can be, for example, based on polyalkylcarbonate structures . Examples of polycarbonates include those prepared by alcoholysis of diethylene carbonate with C2-C12 alkylene diols such as, 1, 4-butanediol, 1 , 6-hexanediol , 1 , 12-dodecane diol, and the like.

In addition, the oligomer can comprise backbones based on hydrocarbon or polyolefin as disclosed in, for example, U.S. Patent Nos. 5,146,531 and 5,352,712 to Shustack, which are hereby incorporated by reference. The backbone can comprise unsaturated or saturated hydrocarbon, although saturated ones are preferred. The oligomer can comprise polybutadiene, including hydrogenated polybutadiene, types of backbone. The oligomer can also comprise polyester backbone.

Polyester diols include the reaction products of polycarboxylic acids, or their anhydrides, and diols. Acids and anhydrides thereof include, for example, phthalic acid, isophthalic acid, terephthalic acid, trimellitic acid, succinic acid, adipic acid, sebacic acid, malonic acid, and the like. Diols include, for example, 1, 4-butanediol, 1 , 8-octanediol , diethylene glycol, 1,6-hexane diol, dimethylol cyclohexane, and the like. Included in this classification are the polycaprolactones . In a preferred embodiment, particularly for an inner primary coating, the oligomer is a block copolymer oligomer and comprises both polyether and polycarbonate backbone . Each backbone block can have a molecular weight of about 750-3,000 g/mol. In one preferred embodiment, the oligomer is a block copolymer prepared from, among other components, a mixture of polyhexylcarbonate and polypropylene glycol . In another preferred embodiment, copolymeric polyether is used based on copolymerization of tetrahydrofuran and methyltetrahydrofuran .

The urethane acrylate oligomer further comprises at least one urethane linking group which can be formed with use of polyisocyanate linking group compounds. Polyisocyanate linking group compounds are well-known in the art of optical fiber coatings, and the selection thereof is not particularly limited. During oligomer synthesis, the polyisocyanate linking group compound can either link the polyol backbone compound to itself, another type of polyol backbone compound, or a radiation- curable end group compound. Preferably, the polyisocyanate linking group compound is a diisocyanate compound, although higher order isocyanates can also be used such as, for example, triisocyanates . The polyisocyanate is also preferably aliphatic although some aromatic polyisocyanates can be included. In general, aromatic isocyanate compounds have been associated with yellowing, although the person skilled in the art can determine whether relatively small amounts of aromatic groups can be tolerated in a given composition. Exclusive use of aromatic polyisocyanates is contemplated if yellowing is not generated.

Polymeric isocyanates can, in some cases, be useful. However, the polyisocyanate compound preferably has, for example, 4-20 carbon atoms. The molecular weight of the polyisocyanate can be less than about 1,000 g/mol, and preferably, less than about 500 g/mol.

Examples of diisocyanates include diphenylmethylene diisocyanate, hexamethylene diisocyanate, cyclohexylene diisocyanate, methylene dicyclohexane diisocyanate,

2 , 2 , 4-trimethyl hexamethylene diisocyanate, m-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 polyalkyloxide and polyester glycol diisocyanates such as polytetramethylene ether glycol terminated with TDI and polyethylene adipate terminated with TDI, respectively. Toluene diisocyanate is a preferred example of an inexpensive aromatic linking group compound. Isophorone diisocyanate is a preferred aliphatic linking group compound.

Urethane linkages can be generated with conventional urethanation catalysts known in the art such as, for example, dibutyltin dilaurate or diazabicyclooctane crystals .

Finally, the oligomer further comprises at least one radiation-curable end-capping group. End-capping means that the oligomer contains a terminal point on its molecular chain end which can radiation cure. The oligomer preferably has two end-capping groups which radiation cure. The selection of compounds used to form these end groups is not particularly limited, as these components are well-known in the art.

In general, the end-capping group will be formed from at least one monoethylenically unsaturated compound of relatively low molecular weight less than, for example, 500 g/mol, and preferably, less than about 300 g/mol. Methacrylate and acrylate compounds are preferred to prepare the end-capping group, and acrylate compounds are particularly preferred. Hydroxyalkyl acrylate compounds are preferred, and hydroxyethyl acrylate is a particularly preferred compound. Other preferred examples include hydroxypropyl acrylate and hydroxybutyl acrylate. End-capping compounds are further discussed in U.S. Pat. No. 5,366,563.

Oligomer components can be selected to attain the optimal balance of properties for a given application demanded by the ultimate optical fiber cable manufacturer. The particular properties of interest in the present invention, however, are non-yellowing in particular and color stability in general, and the polyurethane acrylate oligomer should be tailored with this goal in mind. Additional disclosure about suitable components useful in conventional polyurethane synthesis can be found in, for example, Polyurethane Handbook, G. Oertel (Ed.), Hanser Publishers, 1985 (e.g., Chapter 2, "Chemical and Physical-Chemical Principles of Polyurethane Chemistry," and Chapter 3, "Raw Materials"), the complete disclosure of which is hereby incorporated by reference.

The reactive diluents are well-known in the art of optical fiber coatings, and the selection thereof is not particularly limited. In many cases, mixtures of diluent compounds are needed to obtain optimal properties. Suitable diluents particularly include (meth) acrylate compounds, although acrylate compounds are preferred.

The diluent functions to decrease the viscosity of the oligomer and tailor the final coating properties like, for example, refractive index and polarity (moisture absorption) . They also function to adjust the compositions' mechanical properties and crosslink density and determine whether the compositions can serve as, for example, inner primary, outer primary, single coatings or matrix materials . Aromatic diluents like phenoxyethyl acrylate or ethoxylated nonylphenol acrylate tend to raise the refractive index of the material. Aliphatic diluents like lauryl acyrylate impart hydrophobicity, and diluents with long chain alkyl groups also tend to soften the composition. Polar diluents like N-vinyl pyrollidone can improve room temperature mechanical properties by hydrogen bonding. Multi-functional diluents like trimethylolpropane triacrylate can increase cure speed and crosslink density. Formulations can be tailored with non-polar diluents to minimize water absorption because water generally has a detrimental impact on fiber. Preferably, the functional group present in the reactive diluent is capable of copolymerizing with the radiation- curable functional group present on the radiation-curable oligomer.

The diluent compound molecular weight is not particularly limited but is generally below about 1,000 g/mol. The oligomer diluent, however, may itself contain some oligomeric character such as repeating etheric groups like ethyleneoxy or propyleneoxy in an alkoxylated alkylphenol acrylate diluent.

^•The total amount of diluent is not particularly limited, but will be selected by the person skilled in the art to be effective to achieve the advantages of the present invention for a particular application. The total amount of diluent can be, for example, between about 10 wt.% and about 90 wt.%, and preferably, between about 20 wt.% and about 60 wt.%, and more preferably, between about 25 wt.% and about 50 wt.%.

The reactive diluent can be a conventional monomer or mixture of monomers having, for example, an acrylate functionality and an C₄-C_2C alkyl or polyether moiety. Particular examples of suitable reactive diluents include: hexylacrylate, 2-ethylhexylacrylate, isobornylacrylate, decyl-acrylate, laurylacrylate, stearylacrylate, 2-ethoxyethoxy-ethylacrylate, laurylvinylether, 2-ethylhexylvinyl ether, N-vinyl formamide, isodecyl acrylate, isooctyl acrylate, vinyl- caprolactam, N-vinylpyrrolidone, and the like.

Another type of reactive diluent that can be used is a compound having an aromatic group. Particular examples of reactive diluents having an aromatic group include: ethyleneglycolphenylether-aerylate, polyethyleneglycolphenyletheracrylate , polypropyleneglycolphenylether-acrylate , and alkyl-substituted phenyl derivatives of the above monomers, such as polyethyleneglycolnonylphenyl- etheracrylate . The reactive diluent can also comprises a diluent having two or more functional groups capable of polymerization. Particular examples of such monomers include : ethoxylated bisphenol-A-diacrylate - commercially available as SR 349A monomer and supplied by Sartomer, C₂-C₁₈ hydrocarbon-dioldiacrylates,

C₃-C₁₈ hydrocarbon triacrylates, and the polyether analogues thereof, and the like, such as 1,6- hexanedioldiacrylate, trimethylolpropane tri-acrylate, hexanedioldivinylether, triethylene-glycoldiacrylate, pentaerythritol-triacrylate, and tripropyleneglycol diacrylate .

Preferred diluents for a primary coating are alkyl and aromatic acrylate diluents disclosed in the aforementioned U.S. Patent No. 5,336,563. A particularly preferred diluent system in the present invention is a mixture of isodecyl acrylate and ethoxylated nonylphenol acrylate. Other preferred examples include those listed in V.V. Krongauz and A.J. Tortorello, J. Appl. Polym. Sci. , 57 (1995) 1627-1636. Diluents such as ethoxylated bisphenol A diacrylate can be particularly useful for formulating secondary coatings. A preferred diluent system for a secondary coating is provided in Example 2. The person skilled in the art can tailor coating mechanical properties by selection of conventional diluents to prepare relatively soft or relatively hard coatings or other types of protective materials.

After dilution of oligomer with diluent, the viscosity of the uncured composition is preferably less than about 12,000 cps but greater than about 2,000 cps, and preferably, between about 3,000 and about 8,000 cps at ambient temperature. The viscosity is preferably stable over time so that long shelf life is attained. Many additives in optical fiber coatings can reduce shelf life, and additives are preferably selected to not interfere with shelf life. The person skilled in the art can tailor the viscosity to the particular application technology.

In a preferred embodiment, an inner primary coating composition is prepared comprising:

(1) a urethane acrylate oligomer which comprises a block copolymer backbone based on aliphatic polyether polyol and polycarbonate polyol (about 40-60 wt.%), and

(2) reactive diluents isodecyl acrylate (about 10-20 wt.%) and ethoxylated nonylphenol acrylate (about 20-30 wt . % ) .

In a preferred embodiment of an outer primary or secondary coating composition, the composition comprises:

(1) an oligomer which is based on a backbone of an aliphatic polyether polyol (about 20-40 wt.%), and

(2) reactive diluent, ethoxylated bisphenol -A- diacrylate (about 40-70 wt.%) and ethoxylated nonylphenol acrylate (about 5-15 wt.%) .

As disclosed further below, other ingredients such as photoinitiator and additives such as adhesion promoter can be included. Photoinitiators are well-known in the art of optical fiber coatings to increase cure speed, and the selection of photoinitiator is not particularly limited. Conventional photoinitiators can be used. Mixtures of photoinitiators can often provide the optimal amount of surface and through cure. Commonly, there will be a trade-off between rapid cure speed and other desirable properties in the composition. The person skilled in the art can determine the optimal balance of properties. Use of photoinitiator is preferred, but yet optional because it is not required for electron beam cure. Rapid optical fiber production with UV-cure, however, requires photoinitiator.

The total amount of photoinitiator is not particularly limited but will be sufficient, for a given composition and application, to accelerate cure. The amount can be, for example, about 0.1 wt.% to about 10 wt.%,. and preferably, about 0.5 wt.% to about 5 wt.%. The photoinitiator is preferably a non-yellowing typ as known in the art . Suitable examples of photoinitiators include hydroxymethylphenylpropanone , dimethoxyphenylacetophenone, 2-methyl-l- [4- ( ethylthio) - phenyl] -2-morpholino-propanone-l , 1- (4—isopropylphenyl) - 2-hydroxy-2-methylpropan-l-one, 1- (4 -dodecyl -phenyl) -2- hydroxy-2-methylpropan-l-one, diethoxyphenyl acetophenone, and the like. Phosphine oxide photoinitator types (e.g., Lucerin TPO by BASF) such as benzoyl diaryl phosphine oxide photoinitiators have become popular, particularly if pigments are present in the material. Mixtures of photoinitiators can be used. Non-yellowing photoinitiators can be used as discussed in, for example, U.S. Patent No. 5,146,531, the complete disclosure of which is hereby incorporated by reference. A preferred photoinitiator system for a primary coating is a mixture of 2-hydroxy-2-methyl-l-phenyl-l- propanone and bis (2 , 6-dimethoxybenzoyl) (2,4,4- trimethylpentyl) phosphine oxide. For a secondary coating, a preferred photoinitiator system is a mixture of diphenyl- (2 , 4 , 6-trimethylbenzoyl) phosphine oxide and 1-hydroxycyclohexylphenyl ketone. Suitable photoinitiators are also taught in U.S. Pat. No. 5,336,563.

Crucial to the function of the present invention is the use of functionally effective amounts of at least one fluorescent additive which protects the cured compositions against discoloration and yellowing during aging. The functionally effective amount of the fluorescent additive is not particularly limited, but can be readily determined by the person skilled in the art based on the present disclosure. In general, the amount can be, for example, about 0.01 wt.% to about 3 wt.%, and preferably, between about 0.1 wt.% and about 1 wt . % . In some instances, too much fluorescent additive may have detrimental impact on the coating (e.g., slow cure speed) . The person skilled in the art, therefore, can determine what is the optimum amount to achieve both non- yellowing but not overly adversely effect other important properties .

The present invention is not limited by theory, but small amounts of fluorescent additive likely can sensitize the composition to radiation-cure by light emission. However, larger amounts can retard radiation- cure by light absorption effects. Hence, balancing of these effects may be required. The fluorescent additive can be either a separate component, which is not part of a radiation-curable molecule, or can be incorporated into a radiation-curable molecule so that it is bound to the molecular network after cure. In general, optical fiber coatings and materials should contain minimal amounts of unbound material. Incorporation of the fluorescent moiety into the network can be carried out by, for example, use of an isocyanate-alcohol reaction.

The fluorescent additive can be of a relatively low or high molecular weight, but in general has a molecular weight of less than about 1,000 g/mol, and preferably less than about 500 grams/mole.

A key characteristic of the fluorescent additive of the present invention is the light absorption and emission spectra of the material. In general, the fluorescent spectrum will be shifted to higher wavelengths compared to the absorption spectrum, and the fluorescent additive has the capacity to convert invisible radiation to visible radiation. The fluorescent additive can absorb light in the near UV region and emit bands in the violet or blue regions . In particular, good results have been obtained wherein absorption can occur at wavelengths between about 300 nm and about 420 nm, whereas emission can occur at wavelengths between about 380 nm and 520 nm. Thus, the fluorescent material upon absorption of ultraviolet light not only converts absorbed energy into heat,, but also emits light. Although the invention is not limited by theory, the additional energy dissipation through light emission may prevent or delay onset of coating photodegradation and photoyellowing . The emission of blue light further serves to neutralize undesirable yellowing tints.

Mixtures of fluorescent additives can be used to achieve the aims of the present invention. For example, different fluorescent additives can have different emission spectra.

The fluorescent additive can have any of the known fluorescent structures as long as the advantages of non- yellowing are achieved. Examples include stilbene, N- heteroaromatic coumarine , benzoxazole, and derivatives thereof. Fluorescent structures can be found in, for example, The Chemistry of Synthetic Dyes, Vol. V, Chapter 8 (K. Veenkataraman) and Color Chemistry, Chapter 10 (H. Zollinger) , the complete disclosures of which are hereby incorporated by reference. The fluorescent additive is preferably a benzoxazole compound, and more preferably, a bis (benzoxazole) compound.

The fluorescent additive can comprise fluorescent moieties and other moieties which will not be involved in the fluorescent effect. In such cases, the non- fluorescing moieties can be tailored or substituted to modify the properties and effect of the fluorescent additive. For example, pendant alkyl substituents could be modified (propyl, butyl, pentyl, hexyl , and the like) without substantial change in the fluorescent character. Such alterations, however, may affect the polarity of the additive and its ability to mix with other components.

Particular examples of fluorescent additives include 2, 4-dimethoxy-6- (1' -pyrenyl) -1, 3 , 5, -triazine; N-methyl-4- methoxy-naphthalimide; 2-styryl-benzoxagole; 4,4'- diamino-stilbene-2 , 2'disulfonic acid; 1 , 3-diphenyl-2- pyrazoline. A particularly preferred example of a fluorescent additive is supplied by Ciba-Geigy and called by the trade name, Uvitex OB^® [2, 2'- (2,5- thiophenediyl) bis (5-t-butylbenzoxazole) ] , although the present invention is not limited thereto (see Example 1 and Figure 1) . Compounds which are labeled as fluorescent brighteners can be used, although fluorescent brighteners function to mask existing color, whereas the present invention primarily functions to prevent color from forming in substantially colorless coatings.

The presence of the fluorescent additive in these compositions allows for additional advantages to accrue. For example, the rate and extent of coating cure could be measured by fluorescent effects in situ. See, for example, "Kinetics of Anisotropic Photopolymerization in Polymer Matrix" by V.V. Krongauz et al . in Polymer, 1991, Vol. 32, No. 9, pg. 1654; "In Situ Monitoring of Methyl Methacrylate Polymerization by Fluorescence Quenching of Fluorene" by Miller et al . Polymer Preprints, Vol. 34(2), pg. 492 (1993). Also, diffusion and molecular transport effects can be monitored. See, for example, "Real Time Monitoring of Diffusion Between Laminated Polymer Films" by V.V. Krongauz et al . , Polymer, Vol. 35, No. 5, pg. 929 (1994) . These effects can be used to examine and improve the properties of optical fiber coatings and materials by, for example, on-line analysis of mixing of inner primary and outer primary coating layers during fiber coating. The fluorescent additive generally will impart a luminous appearance to the composition both before and after cure and can serve as a visual tag.

The fluorescent additive appears to be particularly effective when used in the relative soft inner primary coatings.

The addition of fluorescent additive to a coating formulation results in reduction in coating discoloration produced by a prolonged exposure to fluorescent or ultraviolet light. The change in color is measured on a Macbeth^® Spectrophotometer Series 1500 Color-Eye as ΔE . The color change reduction caused by the fluorescent additive can be measured as percent change in ΔE relative to the material containing no fluorescent additive. For inner primary coatings, decreases in color change have been observed:

(a) up to about 60% for coatings exposed to fluorescent light (0.03 kJ/cm²) for long exposure time (200 hours) and comprising fluorescent additive (1 wt.%);

(b) up to about 53.6% for coatings exposed to ultraviolet light (0.03 kJ/cm²) for long exposure time

(200 hours) and comprising fluorescent additive (1 wt.%) . For outer primary coatings, color change decreases up to about 34.6% have been observed under these conditions. Although fluorescent additives have found application outside of the optical fiber arts, optical fiber technology presents several unique technical challenges for the use of these compounds. Perhaps foremost is the need for very rapid cure, long term stability, and the balance of a wide variety of properties. Many additives, for example, slow down cure or generate degradation during long term aging. Many additives interfere with the use of other additives. In short, optical fiber compositions are to meet a highly complex set of demands foreign to many if not most applications. The present compositions can comprise conventional additives. Many different types of additives in optical fiber coatings are known, and the present invention is not particularly limited thereby. The additive should not unduly interfere with the effectiveness of the fluorescent additive, if possible. Relevant disclosure concerning suitable additives is provided in, for example, the aforementioned U.S. Patent Nos. 5,336,563, 5,093,386, 4,992,524, and 5,146,531. Silane adhesion promoters and thermal antioxidants are preferred additives .

Generally, additives can be present in amounts up to several percent. For example, conventional adhesion promoters such as organofunctional silanes can be used like acrylate, amino, or mercapto functional silanes.

The amounts employed can be about 0.1-5 wt.%, and preferably, between about 0.3-3 wt.% for primary coatings to increase adhesion and retain adhesion despite exposure to moisture. Use of mercaptopropyl trimethoxy silane adhesion promoter in optical fiber coatings was first disclosed in U.S. Patent No. 4,849,462 to Bishop, the complete disclosure of which is hereby incorporated by reference. Use of silanes was later claimed in, for example, U.S. Patent No. 5,146,531, which is also incorporated herein for its disclosure about silane adhesion promoter.

Other preferred additives include thermal antioxidants such as hindered phenols or hindered amine light stabilizers. A preferred type of thermal antioxidant for both primary and secondary coatings is a thiodiethylene cinnamate derivative, Irganox 1035 available from Ciba-Geigy. The thermal antioxidant can be, for example, present in about 0.1 wt.% to about 1 wt.%. Shelf stabilizers also can be important additives as noted above. Butylated hydroxy toluene (BHT) is a commonly used stabilizing additive. Additives are also useful to tailor the handling characteristics of coated optical fiber. For example, slip agents and friction adjusting additives are useful. Still other additives or components which may appear in the final coating include pigments, light sensitive and light absorbing compounds, catalysts, initiators, lubricants, wetting agents, and leveling agents. These additives may be present in an effective amount that is usual for the additive when used in optical fiber coatings or protective material. The person skilled in the art can design the use of such additives .

U.S. Patent Nos. 5,146,531; 5,352,712; 5,538,791, and EP Patent Publication No. 0,407,004 (A2) all disclose compositions comprising representative examples of oligomers (including oligomer backbone compounds, polyisocyanate linking compounds, and end-capping compounds) reactive diluents, optional photoinitiators, and additives which can be used in the present invention. Those compositions can be tailored by use of fluorescent additives as disclosed herein.

After cure, the modulus of the primary coating can be, for example, about 100-2,000 psi and is preferably, less than about 1,000 psi, and more preferably, less than about 500 psi. The modulus of the secondary coating can be, for example, about 2,000-150,000 psi. The modulus of a matrix material is generally between that of a primary coating and a secondary coating and can be, for example, about 1,000-2,000 psi. Elongation and tensile strengths of these materials can also be optimized. Thermal mechanical measurements can be used to optimize the glass transition temperature (Tg) and Tg range for these materials . Several particular properties are desirable for the present compositions. The primary coating preferably has low water sensitivity and optimized adhesion for ribbon and loose-tube fiber assembly applications. Refractive index should be preferably at least about 1.48. The secondary coating preferably has low hydrogen generation and is relatively haze free. Glass fibers will generally have diameter of about 125 microns. Coating compositions can be, for example, used at thicknesses of 10-150 microns, and preferably, 20-60 microns. Prevention of color change by use of at least one fluorescent additive is an important aspect of the present invention. In a preferred embodiment, this prevention of color change is observed during aging of substantially colorless materials. However, prevention of color change according to this invention can also be observed during aging of colored materials . In this alternative embodiment, a substantially colorless composition as described herein is formulated to be colored and comprises both at least one fluorescent additive and also a colorant like a pigment and/or dye. Mixtures of colorants can be used.

Colored UV-curable ink compositions have been employed in optical fiber technology, and cured inks can be color-stabilized with use of at least one fluorescent additive. Thin layers of these inks can be coated onto- the coated optical fiber to impart color. UV-curable inks are discussed in, for example, "Ultraviolet Color Coding of Optical Fibers - a Comprehensive Study" by S. Vannais and J. Reese in Wire Journal International, October 1991, pgs . 71-76, the complete disclosure of which is hereby incorporated by reference. In addition, color change of UV-cured inks is discussed in the publication by D. Szum in Polymers Paint Colour Journal, November 24, 1993, Vol. 183, pgs . 51-53, the complete disclosure of which is hereby incorporated by reference. Colored optical fiber materials are also disclosed in JP 64-22975 and JP-64-22976, the complete disclosures of which are hereby incorporated by reference.

In addition, colored outer primary (secondary) coatings which comprise at least one fluorescent additive can also be prepared as disclosed in, for example, WO 90/13579, the complete disclosure of which is hereby incorporated by reference. The compositions disclosed therein comprise pigment having particle size less than about one micron and acyl phosphine oxide photoinitiator. Conventional colorants, dyes, and pigments can be used having conventional colors. Pigments are preferred over dyes because dye color tends to fade with time. Colorants are preferably stable to ultraviolet radiation, and pigments are in the form of small particles. Particle size can be reduced by milling.

The colored material can comprise oligomers, monomers and diluents, photoinitiators, stabilizers, and additives, as disclosed herein for substantially colorless coatings but adapted to be a printing ink binder, a colored outer primary coating, a colored matrix material, or the like.

Pigments can be conventional inorganic or organic pigments as disclosed in, for example, Ullmann' s Encyclopedia of Industrial Chemistry, 5th Ed., Vol. A22, VCH Publishers (1993) , pages 154-155, the complete disclosure of which is hereby incorporated by reference. The pigment can be selected based on, for example, whether the composition is a printing ink or secondary coating. Printing inks will be more heavily pigmented.

General classes of suitable colorants include, among others, inorganic white pigments; black pigments; iron oxides; chromium oxide greens; iron blue and chrome green; violet pigments; ultramarine pigments; blue, green, yellow, and brown metal combinations; lead chromates and lead molybdates; cadmium pigments; titanate pigments; pearlescent pigments; metallic pigments; monoazo pigments; disazo pigments; disazo condensation pigments; quinacridone pigments; dioxazine violet pigment; vat pigments; perylene pigments; thioindigo pigments; phthalocyanine pigments; and tetrachloroisoindolinones ; azo dyes; anthraquinone dyes; xanthene dyes; and azine dyes.

More in particular, suitable inorganic pigments for printing inks include, for example, titanium dioxide, iron oxide, iron silicate, iron cyan blue (or Prussian blue), aluminum powder, cooper-zinc allow powder, and carbon black. Suitable organic pigments for printing inks include, for example, diarylide yellow, diarylide orange, naphthol AS red, Rubin 4 B calcium salt, salts of basic dyes, phthalocyanine blue, reflex blue, phthalocyanine green, and polycyclic pigments. Fluorescent pigments can be used.

Suitable pigments for an outer primary coating include titanium dioxide, carbon black (Degussa Special 4A or Columbian Raven 420) , lamp black (General carbon LB#6) , phtalo blue G (Sun 249-1282) , phtalo blue R (Cookson BT698D) , phtalo green B (Sun 264-0238) , phtalo green Y (Mobay G5420) , light chrome yellow (Cookson Y934D) , diarylide yellow (Sun 274-3954) , organic yellow (Hoechst H4g) , medium chrome yellow (Cookson Y969D) , yellow oxide (Pfizer YL02288D) , lead-free-yellow (BASF Paliotol 1770), raw umber (Hoover 195), burnt umber (Lansco 3240X) , lead free orange (Hoechst RL70) , red oxide (Pfizer R2998D) , moly orange (Cookson YL988D) , arylide red (Hoechst F5RKA) , quinacridone red (Ciba RT759D) , quinacridone violet (Ciba RT887D) , and the like. The amount of the colorant, pigment, or dye is also conventional and will be determined by such factors as the shade, coloring strength, and fastness of the colorant as well as the dispersibility, rheological properties, and transparency. Also, printing inks are generally more heavily pigmented than outer primary coatings. The amount can be that which is sufficient to impart the required color, and more than that is not generally preferred. The amount of colorant can be, for example, between about 0 wt . % and about 25 wt.%, and preferably, about 0.25 wt.% and about 15 wt.%, and more preferably, between about 0.5 wt.% and about 5 wt.%. A preferred type of ink composition is the Cablelite

LTS UV-curable ink series commercially available from DSM Desotech, Inc. (Elgin, Illinois).

Conventional fillers and extenders can be used with the colorants, pigments, and dyes. The invention will be further illustrated with the following non-limiting examples. Unless otherwise indicated, percentages are weight percent and are with respect to the weight of the total composition. All examples disclose use of coatings which are substantially colorless immediately after cure and before accelerated- aging was performed.

EXAMPLES EXAMPLE 1 AND COMPARATIVE EXAMPLES A AND B

Example 1 and Comparative Examples A and B illustrated that a fluorescent additive can reduce the rate of photo- induced yellowing more effectively than an ultraviolet light absorber in a substantially colorless urethane acrylate composition. Accelerated aging tests were used to mimic very long term aging behavior.

A radiation-curable inner primary coating composition was prepared similar to that of the inner primary coating of Example 2 and comprising the following pre-mixture ingredients: (A) an aliphatic urethane acrylate oligomer having a block polyether/polycarbonate backbone structure (57 wt.%); (B) a mixture of reactive diluents (39.5 wt.%); (C) a photoinitiator (3 wt.%); and (D) a thermal antioxidant additive (0.5 wt.%) . Either fluorescent additive or UV absorber was added to this radiation-curable composition as follows:

Example 1 : the fluorescent additive, Uvitex OB^® (Ciba Corp.), having the chemical name [2 , 2 ' - (2 , 5-thiophenediyl) bis (5-t- butylbenzoxazole) ] (1 wt.%); represented by the structure shown in Figure 1 and exhibiting a light absorption and emission specta also illustrated in Figure 1. Comparative Example A: the UV-absorbing dye, 72-510 Macrolex Blue^® RR (Miles Inc.) (0.001 wt.%); chemical name 1 , 4-Bis [2 , 6-diethyl-4- methylphenyl) amino] anthraquinone (CAC #32724- 622)- and can be obtained from Miles Inc., Pittsburgh, Pa.

Comparative Example B: no additive added to base formulation

The lower concentration of dye was selected because of its high extinction coefficient in the UV region. The three compositions were coated as 75 micron films on polished TLC glass plates and irradiated under nitrogen flow with the light of a D-lamp (Fusion) . The total radiation dose was about 1.0 J/cm². Square pieces (2 X 2 inches) of cured coating (which were substantially colorless) were cut out, placed into a cardboard frame, and aged in a QUV unit. QUV light spectra are disclosed in Handbook of Coating Additives, L.J. Calbo (Ed.), Marcel Dekker Inc., NY, Basel, (1987) at pg . 261. The color change was determined using a Macbeth^® Series 1500 Color-Eye colorimeter. Conventional methods known in the art were employed. See, for example, Heinrich Zollinger's Color Chemistry, 2nd Ed., VCH (1991), Chapter 2.7, and references cited therein. The results for each sample were recorded as a function of aging time and are reproduced in Table 1 and illustrated in Figure 2 (shorter exposure) and Figure 3 (longer exposure)

TABLE I

Discoloration and yellowing upon exposure to UV light was exhibited by all of the cured film samples. However, the samples containing the blue dye developed higher yellowing. In contrast, the films containing the fluorescent additive demonstrated a reduced rate of yellowing during the first 20 days of accelerated aging.

Analogous experiments were carried out except that a thermal stress (125°C) was applied rather than a photolytic stress. The data demonstrated that the fluorescent additive did not adversely affect the rate of yellowing. Although the fluorescent additive did not substantially reduce the rate of yellowing, it had no adverse affect after 18 days of aging, whereas the blue dye had some adverse affect (see Figure 4) .

EXAMPLE 2 This example illustrated that a fluorescent additive, which reduces yellowing, can be added to urethane acrylate compositions without significantly impairing cure speed, or at least without impairing cure speed more than other conventional additive such as adhesion promoter. Indeed, cure speed was even enhanced at low concentrations of fluorescent additive. Data were collected for both an inner primary and an outer primary urethane acrylate coating comprising the following ingredients (in addition to the fluorescent additive) :

inner primary coating (premixture ingredients) :

(A) oligomer represented by H-I-PPG-I-PC-I-H (56 wt.%)

[wherein H is 2-hydroxyethyl acrylate, I is isophorone diisocyanate, PPG is polypropylene glycol (mw 1,000), and PC is polyhexylcarbonate (MW 900)]

(B) diluent, isodecyl acrylate (14%)

(C) diluent, ethoxylated nonylphenol acrylate (25.5%)

(D) photoinitiator, 2 -hydroxy-2 -methyl- 1-phenyl-l- propanone (0.75%)

(E) photoinitiator, bis (2 , 6-dimethoxybenzoyl) (2 , 4 , 4- trimethyIpentyl) hosphine oxide (2.25%)

(F) stabilizer, hindered phenol thiodiethylene cinnamate derivative (0.5%)

(G) adhesion promoter, 3-mercaptopropyltrimethoxy silane

(1.0%)

outer primary coating (premixture ingredients) :

(A) oligomer represented by H-T-PTGL-T-H (32.3%),

[wherein T is toluene diisocyanate (80/20 2,4-/2,6- isomers) and PTGL is a copolymer of tetramethylene glycol and methyltetramethylene glycol (mw 1000)]

(B) ethoxylated nonylphenol acrylate (8.2%)

(C) ethoxylated bisphenol-A-diacrylate (56.0%) (D) diphenyl- (2 , 4 , 6-trimethylbenzoyl) phosphine oxide

(1.0%)

(E) 1-hydroxycyclohexylphenyl ketone (2.0%)

(F) hindered phenol thiodiethylene-cinnamate derivative

(0.5%)

The fluorescent additive, Uvitex OB^®, was added to radiation-curable urethane acrylate inner primary and outer primary coatings in amounts of 0.1, 0.5, and 1 wt.%. Cure speed for both coatings was measured as a function of fluorescent additive concentration with use of FTIR spectral analysis. The results were measured by relative absorbance units as a function of exposure time, and are tabulated in Table II and illustrated in Figure 5 (inner primary) and Figure 6 (outer primary) .

The data indicated for both the inner primary and outer primary coating that use of 0.1 % fluorescent additive actually accelerated the cure speed. Use of the fluorescent additive at 1%, although slightly slowing cure speed, did not reduce cure speed to an unacceptable level. In comparison, the commonly used additive mercaptopropyl trimethoxysilane adhesion promoter has also been demonstrated to reduce cure speed with increasing concentration, but the effect is not significant enough to prevent its use. TABLE II

EXAMPLE 3

In Example 3, a urethane acrylate inner primary coating (substantially colorless) similar to that of Example 2 was aged under UV light with the presence of 0.1, 0.5, and 1 wt . % fluorescent additive, Uvitex OB^®. The data are shown in Table III and illustrated in Figure 7. The data illustrated that the fluorescent additive reduces the rate of yellowing increase during UV light exposure, particularly at concentrations of 0.5 and 1.0 wt.%.

Similar non-yellowing effect was observed with urethane acrylate inner primary coating aged under fluorescent light, as demonstrated by the data of Table IV and illustrated by Figure 8.

TABLE III

TABLE IV

EXAMPLE 4

This Example demonstrated that the addition of a fluorescent additive did not adversely affect mechanical properties for both inner and outer primary coatings urethane acrylate coatings. Radiation-cured coatings similar to those in Examples 1-3 were used, and the concentration of fluorescent additive was set at 1 wt.%. Conventional dynamic mechanical spectroscopic (DMS) methods were employed to determine E' , E' ' , and tan delta values .

For inner primary coating, the additive did not materially affect the DMS curves. The tan delta Tg value remained at -16°C and the E' value at about 30°C was not materially altered (2.2 versus 2.0 MPa).

For outer primary coating, the additive also did not materially affect the DMS curves. The tan delta Tg value increased only slightly from 48 °C to 52°, and the E' value also only increased slightly (24.1 MPa versus 20.7 MPa at about 80°C) .

Example 5 A commercial UV-curable printing ink, Cablelite LTS, is mixed with one percent fluorescent additive, Uvitex OB^® by conventional means. The composition is cured with ultraviolet light by conventional means. Color change is expected to be minimal despite aging under taxing conditions including UV light, fluorescent light, high relative humidity, and elevated temperature.

All references disclosed herein are hereby incorporated by reference.

While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to those of ordinary skill in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.

Claims

WHAT IS CLAIMED IS:

1. A method for reducing the rate of discoloration in a substantially colorless, radiation-cured optical fiber protective material which comprises: providing a radiation-curable composition which, upon radiation-cure, yields said substantially colorless optical fiber protective material, wherein said radiation-curable composition, before radiation-cure, comprises as pre-mixture ingredients:

(i) about 10 wt.% to about 90 wt.% of at least one radiation-curable urethane (meth) acrylate oligomer,

(ii) an effective amount of at least one monomer diluent, and

(iii) optionally, an effective amount of at least one photoinitiator, and further providing in said radiation-curable composition

(iv) an effective amount of at least one fluorescent additive which absorbs in the invisible ultraviolet region and emits in the blue to blue-violet light region, wherein said fluorescent additive reduces the rate of discoloration in said optical fiber protective material after radiation-cure of said radiation-curable composition.

2. A method according to claim 1, wherein said fluorescent additive absorbs at wavelengths between about 300 nm and 420 nm, and emits at wavelengths between about 380 nm and 520 nm.

3. A method according to claim 1, wherein said fluorescent additive has a molecular weight of less than about 500 grams/mole.

4. A method according to claim 1, wherein said fluorescent additive is a benzoxazole compound.

5. A method according to claim 1, wherein said fluorescent additive is a bis (benzoxazole) compound.

6. A method according to claim 1, wherein said urethane (meth) acrylate oligomer comprises (i) at least one polyether, polycarbonate, hydrocarbon, or polyester backbone; (ii) (meth) acrylate end-capping groups, and

(iii) aliphatic urethane linking groups which link said- end-capping (meth) acrylate groups to said oligomer backbone .

7. A method according to claim 1, wherein said fluorescent additive is effectively used in amounts which also increases the cure speed of said composition.

8. A method according to claim 1, wherein said optional photoinitiator is a non-yellowing photoinitiator .

9. A method according to claim 1, wherein said oligomer has a molecular weight of less than 5,000 g/mol.

10. A method according to claim 1, wherein said diluent comprises an alkyl (meth) acrylate diluent, an aromatic (meth) acrylate diluent, or a mixture thereof.

11. A radiation-curable composition for protecting an optical fiber comprising as pre-mixture ingredients:

(A) about 10 wt.% to about 90 wt.% of at least one radiation-curable urethane (meth) acrylate oligomer which comprises an oligomer backbone, at least one urethane linking group, and at least one radiation- curable end- capping group;

(B) about 10 wt.% to about 90 wt.% of at least one reactive diluent;

(C) optionally, an effective amount of at least one photoinitiator; and

(D) an effective amount of at least one fluorescent additive which absorbs in the invisible ultraviolet region and emits in the blue to blue-violet light region, wherein said composition, after radiation cure, is substantially colorless, and said effective amount of fluorescent additive reduces the rate of discoloration in said composition after radiation-cure.

12. A composition according to claim 11, wherein said fluorescent additive absorbs at wavelengths between about 300 nm and 420 nm, and emits at wavelengths between about 380 nm and 520 nm.

13. A composition according to claim 11, wherein said fluorescent additive has a molecular weight of less than about 500 grams/mole.

14. A composition according to claim 11, wherein said fluorescent additive is a benzoxazole compound.

15. A composition according to claim 11, wherein said fluorescent additive is a bis (benzoxazole) compound.

16. A composition according to claim 11, wherein said urethane (meth) acrylate oligomer comprises (i) at least one polyether, polycarbonate, hydrocarbon, or polyester backbone; (ii) (meth) acrylate end-capping groups, and (iii) aliphatic urethane linking groups which link said end-capping (meth) acrylate groups to said oligomer backbone.

17. A composition according to claim 11, wherein said fluorescent additive is used in amounts which also increases cure speed.

18. A composition according to claim 11, wherein said optional photoinitiator is a non-yellowing photoinitiator .

19. A composition according to claim 11, wherein said oligomer has a molecular weight of less than 5,000 g/mol .

20. A composition according to claim 1, wherein said diluent comprises an alkyl (meth) acrylate diluent, an aromatic (meth) acrylate diluent, or a mixture thereof.

21. A radiation-curable composition for protecting an optical fiber comprising as pre-mixture ingredients:

(A) about 10 wt.% to about 90 wt.% of at least one radiation-curable urethane (meth) acrylate oligomer which comprises an oligomer backbone, at least one urethane linking group, and at least one radiation-curable end- capping group;

(B) about 10 wt.% to about 90 wt.% of at least one reactive diluent;

(C) optionally, about 0.1 wt.% to about 10 wt.% of at least one photoinitiator; and

(D) about 0.1 wt.% to about 3 wt.% of at least one fluorescent additive which absorbs in the 300-420 nm region and emits in the 380-520 nm region.

22. A method according to claim 1, wherein said fluorescent additive functions to reduce the rate of discoloration up to about 60% for materials exposed to fluorescent light, 0.03 kJ/cm², at an exposure time of 200 hours .

23. An optical fiber comprising a protective material for said fiber, wherein said protective material is a radiation-cured product of a composition according- to claim 11.

24. A radiation-curable composition for protecting an optical fiber comprising as pre-mixture ingredients: (A) about 10 wt.% to about 90 wt.% of at least one radiation-curable urethane (meth) acrylate oligomer which comprises an oligomer backbone, at least one urethane linking group, and at least one radiation-curable end- capping group;

(B) about 10 wt.% to about 70 wt.% of at least one reactive diluent;

(C) optionally, an effective amount of at least one photoinitiator;

(D) an effective amount of at least one fluorescent additive which absorbs in the invisible ultraviolet region and emits in the blue to blue-violet light region, and

(E) at least one colorant, wherein said effective amount of fluorescent additive reduces the rate of discoloration in said composition after radiation- cure .