Moisture resistant optical fiber coatings with improved stability

Moisture-resistant UV-curable acrylate resin coated optical fibers are described, the coating compositions comprising a phosphite stabilizing agent in combination with an acrylate-terminated polyurethane oligomer wherein the urethane groups are the reaction product of an aliphatic isocyanate and a predominantly saturated, predominantly nonpolar aliphatic diol. The coating compositions offer improved curing speed while maintaining excellent thermal stability.

BACKGROUND OF THE INVENTION 
The present invention relates to polymeric protective coatings for optical 
fibers, and more particularly to improved radiation-curable acrylate 
optical fiber coatings offering excellent moisture resistance and 
stability. 
Glass optical fibers are a relatively recent innovation in the field of 
telecommunications. As is well known, protective coatings are customarily 
applied to these fibers at the time of manufacture, both to preserve the 
inherent strength of the fibers and to protect them from certain types of 
bending which can induce signal loss in telecommunication cables. 
These requirements dictate that the coating applied to the optical fiber 
have both substantial toughness and yet be soft enough to distribute 
transverse strain applied to the fiber. An additional requirement for 
optical fiber coating materials derives from the fact that very high 
optical fiber drawing speeds are now being employed in the industry for 
reasons of manufacturing efficiency. For economic production, therefore, 
only coating materials which can be rapidly applied to and cured on the 
surface of the optical fiber are useful. 
Currently, the preferred coating materials for rapid optic fiber production 
are radiation-curable coating formulations which can be very rapidly cured 
by UV irradiation. One family of particularly preferred coatings of this 
type comprises the radiation-curable polyurethane or polyurethane-polyurea 
acrylate materials. These are typically ultraviolet-curable formulations 
comprising acrylated polyurethane oligomers, available in liquid form, 
which can be cured to films which exhibit good softness over a very broad 
temperature range, good tensile strength and toughness, and rapid UV 
curing characteristics. 
Specific examples of UV-curable acrylate compositions such as described are 
reported in published European patent applications EP 0204160 and EP 
0204161. The compositions disclosed in these applications are based on 
resins more specifically designated acrylate-terminated polyurethane, 
polyurea, or polyurethane/polyurea oligomers. 
It is of course important that the resin formulations selected for 
application to these optical fibers exhibit excellent resistance to 
moisture penetration and/or chemical breakdown over an extended period of 
time. Thus minimal interaction with water or water vapor and a polymer 
structure which resists chemical breakdown over a wide range of ambient 
conditions for the entire anticipated service life of the coated fiber are 
considered essential requisites of these coatings. 
Existing urethane acrylate coating systems can be judged to exhibit 
excessively high water absorption for use in environments high in moisture 
content. Water absorption in these environments can be a critical problem 
since it can affect coating geometry and thereby cause increased fiber 
attenuation over time. 
Notwithstanding the recognition of this problem, changes in acrylate 
coating composition to solve the problem have to date not been universally 
accepted in the industry. This is due in part to the fact that any changes 
in coating formulation which result in significant reductions in curing 
speed, or which degrade the chemical stability of the cured coating, are 
not commercially acceptable. 
Cured coating stability is a factor of increasing importance in the 
formulation of new coating compositions. K. Ohashi et. al., in "Mechanisms 
of Hydrogen Evolution and Stabilization of UV-Cured Urethane Acrylate 
Resin for Coating of Optical Fiber", Polymer Degradation and Stability, 22 
(1988) 223-232, review the problem of hydrogen evolution in cured coatings 
and some approaches to the solution of those problems. Y. Ohashi et. al., 
in Chemical Abstracts, 110 (4):25492q, disclose the utility of phosphite 
compounds for the suppression of hydrogen generation in polyether-based 
urethane acrylates, although a great many other formulation changes have 
of course also been proposed. Again, however, these proposed changes in 
composition for improved coating stability are rarely commercially 
successful, unless the "improved" composition also exhibits curing speed 
and moisture resistance which are at least equivalent to the coatings 
currently in commercial use. 
It is therefore a principal object of the present invention to provide 
improved UV-curable acrylate coating formulations offering superior 
resistance to moisture attack, excellent curing speed, and excellent 
long-term chemical stability. 
It is a further object of the invention to provide a coated optical fiber 
incorporating an improved urethane acrylate coating which offers better 
lifetime optical transmission characteristics in both moist and dry 
environments than prior art optical fibers. 
Other objects and advantages of the invention will become apparent from the 
following description thereof. 
SUMMARY OF THE INVENTION 
The present invention achieves substantial improvements in moisture 
protection in an optical fiber protective coating without sacrificing the 
thermal stability of the coating system, and with curing speed equivalent 
to or better than existing coatings. These improvements are derived from a 
careful selection of coating system components for the purpose of reducing 
the content of polar species in the coating polymer, and increasing the 
saturation of carbon chain components therein. At the same time, 
compensation for the decreased thermal stability of the selected species 
is made through the addition of appropriate stabilizers to the 
formulations. 
Coating compositions provided in accordance with the invention offer an 
excellent combination of flexibility and toughness such that they are 
ideally suited for service as a primary or first-layer protective coatings 
for optical fiber. However, modifications to the composition to provide 
increased hardness for use as a secondary or other coating layer on such 
fibers may readily be made. In either case, a substantial reduction in 
water absorption by the coating, when compared with conventional UV-curing 
urethane acrylate coatings is provided. 
In a first aspect, then, the invention includes a fast-curing but stable, 
moisture-resistant UV-curable acrylate resin coating composition. The 
principal component of the composition is a moisture-resistant 
acrylate-terminated polyurethane oligomer. Of course, as in conventional 
UV-curable acrylate coating systems, other components such as acrylate 
monomers, reactive diluents, photoinitiators, and chemical stabilizers may 
also be present. However, in the present coatings these must be carefully 
selected to maintain compatibility with the selected oligomers, and to 
preserve the moisture resistant properties of the final composition. 
Analogous to known coatings, the moisture-resistant polyurethane oligomers 
used in the coating compositions of the invention are acrylate-terminated 
polyurethanes; these result from the reaction of isocyanates and alcohols 
to form multiple urethane linkages following general synthesis procedures 
well known in the art. 
In the formulations of the present invention, however, the urethane groups 
provided in the oligomers are characterized as reaction products of an 
aliphatic isocyanate and a predominantly saturated and predominantly 
nonpolar aliphatic diol. Hence, the presence in the oligomer of polar 
components, such as the polyester and polyether glycols conventionally 
present in prior art optical fiber coatings, is minimized or avoided. 
The selection of moisture-resistant oligomers alone, however, does not 
provide an acceptable optical fiber coating. One difficulty is that such 
selection generally affects coating stability in an adverse manner, 
particularly with respect to the evolution of hydrogen from the coating 
which can be deleterious to fiber performance. This problem is solved in 
the coating compositions of the invention by additionally including in the 
composition at least one compatible chemical stabilizing agent which is 
effective to control harmful depolymerization or other interactions which 
might degrade fiber or coating performance over time. The effective 
stabilizers for this purpose have been found to be phosphite stabilizers 
or antioxidants. These markedly reduce hydrogen evolution from acrylate 
coating systems while at the same time being compatible with the 
moisture-resistant oligomers and acrylated monomers employed. 
In a second aspect, the invention comprises a coated optical fiber 
exhibiting excellent resistance to optical degradation in a moist 
environment. The coated fiber comprises at least one conventional glass 
optical fiber of single-mode or multimode type, and further incorporates 
at least one moisture-resistant polymer coating provided from a coating 
composition such as above described. Most preferably, the polymer coating 
will comprise the primary or base coating on the fiber. 
Coating compositions such as above described may be formulated to provide 
not only substantially reduced water absorption, but also curing speed 
superior to that of current commercial coatings, and with chemical 
stability which is at least equivalent thereto. Hence these coatings 
represent a significant advance in optical fiber coating technology which 
is particularly valuable in optical cabling applications such as submarine 
applications wherein high moisture resistance is a critical coating 
requirement.

DETAILED DESCRIPTION 
As previously noted, the major component of the acrylate coating 
compositions of the invention, comprising 50% or more by weight of the 
composition, is a moisture-resistant acrylate-terminated polyurethane 
oligomer. The acrylate terminal groups in such oligomers may be provided 
by a monohydric polyacrylate capping component, or by a monoacrylate 
capping component such as 2-hydroxyethyl acrylate, in the known manner. 
As previously noted, polyurethane oligomers are conventionally provided by 
reacting an aliphatic diisocyanate with a dihydric polyether or polyester, 
most typically a polyoxyalkalene glycol such as a polyethylene glycol. 
Such oligomers typically comprises 4-10 urethane groups and may be of high 
molecular weight, e.g., 2000-8000, although lower molecular weight 
oligomers, having molecular weights in the 500-2000 range, may also be 
used. Published European Patent Applications Nos. EP 0204160 and EP 
0204161 describe such syntheses in detail and are expressly incorporated 
herein by reference for a further description of these known procedures. 
The synthesis of the moisture-resistant oligomers used in the invention is 
analogous to the above-described synthesis except that the polar polyether 
or polyester glycols are avoided in favor of predominantly saturated and 
predominantly nonpolar aliphatic diols. These diols are preferably alkane 
or alkylene diols of from 2-250 carbon atoms; most preferably they are 
substantially free of ether or ester groups. 
The ranges of oligomer viscosity and molecular weight obtainable in these 
systems are similar to the molecular weights obtainable in unsaturated, 
polar oligomer systems such as described in the aforementioned European 
patent applications, such that the viscosity and coating characteristics 
thereof can be kept substantially unchanged. And, advantageously and 
unexpectedly, the reduced oxygen content of these coatings has been found 
not to unacceptably degrade the adherence characteristics of the coatings 
to the surfaces of the optical fibers being coated. 
While any of the isocyanates known for use in these polyurethane oligomer 
preparations could be used to develop acrylate oligomers from nonpolar 
aliphatic diols, the preferred isocyanates for this purpose are 
branched-chain, aliphatic diisocyanates. The most preferred isocyanates 
are branched-chain trifunctional diisocyanates comprising both isocyanate 
and additional functionality. 
Examples of such trifunctional species include trifunctional diisocyanates 
produced by the condensation of two or more alkylene diisocyanate 
molecules to produce a longer-chain diisocyanate comprising one or more 
central allophanate functional groups. Advantageously, the use of these 
branched trifunctional diisocyanates increases the crosslink density of 
the polymerization products without seriously sacrificing the aliphatic 
nature of the polymer coatings. 
As is well known, polyurea components may be incorporated in oligomers 
prepared by these methods, simply by substituting diamines or polyamines 
for diols or polyols in the course of synthesis. The presence of minor 
proportions of polyurea components in the present coating systems is not 
considered detrimental to coating performance, provided only that the 
diamines or polyamines employed in the synthesis are sufficiently 
non-polar and saturated as to avoid compromising the moisture resistance 
of the system. 
As previously indicated, the coating compositions of the invention will 
typically also comprise, in addition to the oligomer component, a lower 
molecular weight liquid acrylate-functional monomer component. This 
component is typically added to the formulation to provide the liquidity 
needed to apply the coating composition with liquid coating equipment. 
Typical acrylate-functional liquids in these systems have comprised 
monoacrylate monomers and linear aliphatic diacrylates. These have 
included diacrylates such as the polyalkylene (e.g., polypropylene or 
polyethylene) glycol diacrylates. 
In the present formulations, the requirement to utilize moisture-resistant 
components places significant selection limitations on the 
acrylate-functional liquid monomers which may be employed to control the 
viscosity of the coating systems. Not all such liquids may be successfully 
blended and copolymerized with highly non-polar oligomers. Further, many 
of the prior art monomers employed for this purpose, e.g., ethoxyethyl 
ethoxyethyl acrylate, phenoxyethyl acrylate, and butoxyethyl acrylate, 
comprise unwanted ether or ester groups. 
For adequate coating compatibility and best moisture resistance, therefore, 
we prefer the use of a liquid acrylate monomer component comprising a 
predominantly saturated aliphatic mono- or di-acrylate monomer. Saturated 
acrylate monomers of from 6-18 carbon atoms are preferred, examples of 
such monomers including isodecyl acrylate, lauryl acrylate, octyl 
acrylate, and octadecyl acrylate. Particularly preferred are the branched 
monomers such as isodecyl acrylate. Small quantities of the alkoxy 
acrylate monomers used in prior art formulations may be included if 
desired, but these are not preferred. 
As is well known, polyurethane acrylate coating formulations generally 
comprise conventional photoinitiators, such as the known ketonic 
photoinitiating additives, these being present in amounts sufficient to 
provide rapid ultraviolet curing. The present formulations will also 
comprise such photoinitiators, which may be selected and utilized in the 
same proportions as found effective for the polymerization of the known 
conventional acrylate coating compositions. 
Finally, the coating formulations of the invention will additionally 
comprise selected stabilization agents, most preferably one or more 
phosphite stabilization agents, in order to control hydrogen evolution 
from the cured coatings. Somewhat surprisingly, notwithstanding the 
avoidance of polyester- and polyether-based oligomer constituents in our 
coatings, we have found that significant increases in H.sub.2 evolution 
from the levels exhibited by present commercial coatings can be incurred 
unless provisions for adequate stabilization of the cured coating are 
made. Illustrative examples of specific phosphite stabilizers which may be 
used are diphenylisodecyl phosphite and tris-(2,4-di-tert-butyl phenyl) 
phosphite, although other phosphite stabilizers or antioxidants known to 
the art may alternatively be employed. 
The invention may be further understood from the following Example 
illustrating the formulation and curing of an optical fiber coating in 
accordance therewith. 
EXAMPLE 
A batch for a UV-curing urethane acrylate coating composition is first 
prepared. To 70 parts by weight of a moisture-resistant polyurethane 
oligomer (the reaction product of a saturated nonpolar aliphatic diol and 
a trifunctional isocyanate comprising isocyanate and allophanate 
functional groups) are added 27 parts by weight of isodecyl acrylate 
monomer, 3 parts by weight of a 2,2-dimethoxy-2-phenyl acetophenone 
photoinitiator, and 0.5 parts by weight of tris-(2,4-di-tert-butyl phenyl) 
phosphite stabilizer. The oligomer is Uvithane.TM. ZL-1365 oligomer from 
Morton Thiokol, Inc. of Moss Point, Miss., the monomer is Sartomer 395 
acrylate commercially available from the Sartomer Company, Inc., West 
Chester, Pa., the photoinitiator is Irgacure 651 photoinitiator from Ciba 
Geigy of Hawthorne, New York, and the stabilizer is Naugard 524 stabilizer 
from the Uniroyal Chemical Company, Middlebury, Conn., USA. 
The batch thus prepared is thoroughly blended by mechanical mixing at 
55.degree. C. to yield a coating formulation substantially equal in 
viscosity to a standard commercial coating. It is readily applied to 
optical fibers by standard liquid coating techniques, rapidly cures to a 
tough, resilient coating upon UV-irradiation at cure rates above those of 
a standard commercial coating, and exhibits much lower moisture absorption 
than the standard commercial coating. Also, the hydrogen evolution of the 
coating is at least as low as that of the standard coating. 
A comparison of the curing and water absorption characteristics of the 
coating of the Example with the standard commercial coating is made by 
evaluating the cure rates and water absorption characteristics of acrylate 
films of the two coating compositions on flat support substrates. For the 
determinations of water absorption and water extraction, the coating 
formulations were applied to flat glass at equivalent thicknesses, and 
each coating was cured by the application 3.5 J/cm.sup.2 of UV light 
energy. Relative cure rates were determined by the curing of coatings of 
equivalent thickness on flat substrates using a relatively low power UV 
light source, so that the level of curing over time could be accurately 
tracked. A comparison of the characteristics of the cured films thus 
determined is given in Table I below: 
TABLE 1 
______________________________________ 
Fiber Coating Tests 
Standard 
Example 
Coating 
Coating 
______________________________________ 
Cure Rate 17 9 
(seconds)* 
Water 3.08 0.516 
Absorption (%) 
Water 1.76 0.452 
Extraction (%) 
______________________________________ 
*seconds to achieve 95% of the Emodulus of the fully cured polymer coatin 
 
As is evident from a study of the data in Table 1, the acrylate films 
provided in accordance with the invention provide not only reduced water 
absorption (24-hour immersion at ambient temperature), but also reduced 
levels of water-extractable coating components as well as somewhat faster 
curing performance than the standard commercial coating. 
When applied to optical fibers and overcoated with a higher-modulus outer 
buffer coatings, the acrylate coatings of the invention provided somewhat 
lower but still acceptable stripping force (264 grams v. 349 grams) than 
the standard commercial coating. And, although the susceptibility of the 
coated fibers to thermally induced optical absorption losses at low 
temperatures (down to -60.degree. C.) were somewhat higher for the 
acrylate coatings of the Example, these were still well below the 
commercially acceptable level of 0.05 db/km. 
The stability of the cured acrylate coatings against long-term hydrogen 
evolution is evaluated by heating coating samples to elevated temperatures 
to accelerate coating breakdown and hydrogen release, with analysis of the 
evolved gas composition being carried out by gas chromatograph after a 
fixed time interval. Under a standardized set of time/temperature 
conditions, hydrogen evolution from the standard commercial coating 
referenced above generally will not exceed about 0.14 .mu.L of H.sub.2 per 
gram of coating film. 
As previously noted, hydrogen evolution from moisture-resistant coatings 
analogous to those of the invention, but not incorporating phosphite 
stabilizers, was unexpectedly found to be much higher than for the 
standard commercial coatings, with H.sub.2 release values in excess of 8 
.mu.L/gram being recorded in some tests. However, with the addition of 
appropriate phosphite stabilizers, release values in the range below 0.5 
.mu.L/gram, and into the range of 0.11-0.13 .mu.L/gram, are readily 
achievable. Other known stabilizers or antioxidants, including 
sulfur-containing (thiodipropionate) stabilizers and hindered amine or 
hindered phenolic antioxidants, were found to be relatively ineffective 
for this use. 
While the invention has been particularly described above with respect to 
specific materials and specific procedures, it will be recognized that 
those materials and procedures are presented for purposes of illustration 
only and are not intended to be limiting. Thus numerous modifications and 
variations upon the compositions and processes specifically described 
herein may be resorted to by those skilled in the art within the scope of 
the appended claims.