Process for preparing optical fiber cladding solutions

A process for preparing optical fiber cladding solutions starting with a monomer having the formula EQU CH.sub.2 .dbd.C(R)COOCHXY where PA1 R is H or methyl; PA1 X is H or CF.sub.3, PA1 Y is H or CF.sub.3 provided that when X is H, Y is --CF.sub.3, --CF(CF.sub.3)OCF.sub.2 CF(CF.sub.3)OC.sub.4 F.sub.9 or --(CF.sub.2).sub.n Z, PA1 Z is F or H; and PA1 n is 1 to 8. The monomers are subjected to UV light at about 1-400 nm for one to four hours until a cladding solution of desired viscosity is obtained.

FIELD OF THE INVENTION 
This invention relates to curable compositions for coating optical fibers. 
In particular, it refers to 100% UV curable optical fiber cladding 
solutions produced by a one step polymerization process. 
BACKGROUND OF THE INVENTION 
Step index silica/glass as well as plastic optical fibers are generally 
composed of two or more materials, but primarily composed of a core having 
a higher refractive index than an outer, transparent, lower refractive 
index cladding material. This low refractive index cladding material 
improves the light carrying ability or efficiency of the fiber by 
preventing the escape of light from the core. The larger the difference in 
the refractive index between the core and the outer coating the greater 
the luminous properties of the fiber. 
A measure of this light trapping efficiency is called the "Numerical 
Aperture", NA of a coated optical fiber. The angle at which a light ray 
may enter a fiber core and be propagated down the fiber without 
penetrating the surface of the core is termed the critical angle. This 
angle, and the fiber/cladding light trapping efficiency may be calculated 
from the respective refractive indices of the core and the cladding 
material as (n.sup.2.sub.1 -n.sup.2.sub.2).sup.1/2 =NA, where n.sub.1 and 
n.sub.2 are the refractive indices of the core and cladding respectively 
and NA is the Numerical Aperture. The Sine.sup.-1 NA is the critical 
angle, A.sub.c, and the angle of the cone of light that may enter a fiber 
without penetrating the surface of the core is the acceptance angle 
2A.sub.c. 
Thus, the larger the difference in the refractive index of the core versus 
the cladding material the greater the light gathering and trapping 
efficiency of the optical fiber. 
In arriving at a suitable polymer for cladding an optical fiber it is 
readily apparent, from a summation of the atomic contribution to molar 
refraction of organic compounds that polymers containing fluorine, in 
place of hydrogen, would yield the lowest attainable refractive index. For 
example, fluorine has a molar refraction of 0.81 and hydrogen 1.028, see 
"Handbook of Chemistry and Physics, Chemical Rubber Publishing Co." From 
this type of calculation one would assume that commercially available 
polyfluorovinyl polymers would be the polymers of choice for low 
refractive index cladding materials. However, it has been found that the 
commercially available fluoro-polymers in general have high scattering 
losses. These losses are generally attributable to polymer crystallinity. 
For example, semi-crystalline fluoropolymers are typically translucent to 
opaque solids with scattering losses near 10.sup.6 dB/km. (L. Blyer Et. 
al. "Optical Fiber Telecommunications". E. S. Miller and A. Chynoweth, 
Academic Press, Inc. New York, 1979, 300-339). 
Although numerous fluorine-containing polymer systems have been reported 
and/or patented as low refractive index cladding polymers the predominant, 
and most useful polymers to date are acrylate and methacrylate esters of 
fluorine-containing alcohols. 
A secondary, but very important, purpose served by a cladding composition 
is to act as a protective coating on the surface of the optical fiber core 
to prevent silica-silica fiber abrasion. This is especially true for pure 
silica fibers which deteriorate rapidly when exposed to atmospheric 
moisture after being drawn from molten silica. It is important then to 
provide a protective cladding, which is most conveniently done with 
polymeric materials. In addition to lowering the refractive index, 
incorporation of fluorine into a cladding polymer structure also imparts 
an additional desirable, moisture barrier property, (L. Klinger; J. Mater. 
Res 2(6), Nov./Dec. 1987). 
Because of the low viscosity and high volatility of the lower molecular 
weight fluorocarbon acrylate or methacrylate monomers, under normal 
circumstances, they cannot be conveniently used directly for cladding 
purposes. As a consequence, higher boiling acrylate esters of polyfluoro 
alcohols are currently used, such as in the Skutnik U.S. Pat. No. 
4,511,209, where the higher boiling, 1H, 1H, 
11H-eicosafluoroundecylacrylate, boiling at .about.290.degree. C. at 
atmospheric pressure, is used in UV curable cladding compositions. Because 
of the low Tg of the corresponding homopolymer a considerable amount of a 
trifunctional cross linking monomer is required to obtain a rigid polymer. 
Trimethylolpropyltriacrylate is recommended, at from 5.7 to 26.7%, in 
order to produce a hard cladding polymer, as claimed in the patent. The 
polymer containing 26.7% cross linker had a NA of 0.2 when polymerized on 
a quartz fiber. Comparatively, in this current invention, pure cladding 
polymers and copolymers can be produced from even the lowest boiling 
monomers, such as 2,2,2-trifluoroethyl acrylate, bp.about.100.degree. C. 
at atmospheric pressure and 2,2,2-trifluoroethyl methacrylate, 
bp.about.112.degree. C. at atmospheric pressure and the Polymer has a 
refractive index of 1.418 with a corresponding NA on a quartz fiber of 
0.34. 
U.S. Pat. No. 5,024,507, Jun. 18, 1991, to R. Minns, describes the use of 
copolymers of vinylidene fluoride and hexafluoropropylene (Fluorel, from 
3M Co., U.S. Pat. No. 2,968,649, Jan. 16, 1961) and ter-polymers of 
vinylidene fluoride, hexafluoropropylene and tetrafluoroethylene, also 
from 3M, as viscosity modifying polymers. These polymers are apparently 
soluble in the cladding composition of fluorocarbon acrylate and 
diacrylate monomers used in their UV curable cladding solutions. 
In the above mentioned Minns' patent and similar to the Skutnik patent the 
composition is composed of a 10% concentration of a cross linking monomer 
of a diacrylate ester of a long chain fluorocarbon diol, having low 
volatility, obtainable from the 3M Co. The resulting polymer is still 
elastomeric. A need exists for a simple low cost process for making 
optical fiber cladding solutions with an easily adjustable viscosity and 
refractive index. 
SUMMARY OF THE INVENTION 
Cladding solutions are prepared in this invention in a one step process 
directly from a fluorocarbon or hydrocarbon acrylate or a methacrylate 
monomer to give an easily controllable viscosity and easily controllable 
low refractive index between 1.3 to 1.5. These solutions are 100% UV 
photocurable cladding polymer solutions. The monomers are irradiated in a 
nitrogen purged UV transparent chamber with UV light of approximately 2500 
micro watts/cm.sup.2 at between 1-400 nm until a desired solution 
viscosity is obtained. The UV cured cladding polymers made from the 
solution are crystal clear, having a predesigned shore A or D hardness and 
toughness obtainable through acrylate/methacrylate copolymerization. 
Hydrocarbon cross linking monomers used at 0.5 to 3.0% by weight in 
cladding solution made in accordance with the process are sufficient to 
provide adequate crosslinked polymers without any significant change in 
refractive index. 
DETAILED DESCRIPTION OF THE INVENTION 
The process for manufacturing 100% UV curable cladding polymer solutions 
according to the present invention are prepared directly from degassed, 
inhibitor free monomer(s) with the addition of a UV initiator, and if 
required, the addition of an adhesion promoter to the monomer(s) mixture 
to promote adhesion to glass/quartz. This mixture is irradiated, in a 
nitrogen purged, suitable UV transparent container, with low intensity UV 
light, of approximately 2,500 micro Watts/cm.sup.2, at between 1 to 400 nm 
but preferably at about 350 nm. 
Irradiation may be intermittent or continuous and is terminated when a 
desired viscosity is attained. At this time the final polymer properties 
may be modified by the addition of a cross linking monomer such as a di- 
or polyfunctional hydrocarbon or polyfluorocarbon di-acrylate or 
di-methacrylate ester(s), to produce an infusible cladding polymer or a 
thermoplastic cladding polymer may be produced by omitting the cross 
linking monomers. 
After application to an optical fiber or other surface and exposed to a 
high intensity UV source the cladding solution will rapidly polymerize to 
a solid polymer having a specifically designed modulus, refractive index 
and shore A or D hardness. 
The fluorocarbon monomers employed have the general formula CH.sub.2 
.dbd.C(R)COOCHXY where: 
R can be hydrogen or methyl; 
X can be --CF.sub.3 or hydrogen; 
Y can be hydrogen or CF.sub.3 and when X is hydrogen then Y can be 
CF.sub.3, --CF(CF.sub.3)OCF.sub.2 CF(CF.sub.3)O(CF.sub.2).sub.4 F or 
--(CF.sub.2).sub.n Z; 
Z can be fluorine or hydrogen; 
and n can be 1 to 8. 
The preferred fluorocarbon acrylate and methacrylate monomer(s) used in the 
process of this invention are: 
__________________________________________________________________________ 
REFRACTIVE 
INDEX OF 
CHEMICAL STRUCTURE CHEMICAL NAME MONOMER 
__________________________________________________________________________ 
CH.sub.2 .dbd.C(H)COOCH.sub.2 CF.sub.3 
1H,1H-TRIFLUOROETHYL ACRYLATE 1.3506 
CH.sub.2 .dbd.C(CH.sub.3)COOCH.sub.2 CF.sub.3 
1H,1H-TRIFLUOROETHYL METHACRYLATE 
1.3624 
CH.sub.2 .dbd.C(H)COOCH.sub.2 CF.sub.2 CF.sub.2 H 
1H,1H,3H-TETRAFLUOROPROPYL ACRYLATE 
1.3629 
CH.sub.2 .dbd.C(CH.sub.3)COOCH.sub.2 CF.sub.2 CF.sub.2 H 
1H,1H,3H-TETRAFLUOROPROPYL METHACRYLATE 
1.3738 
CH.sub.2 .dbd.C(H)COOCH(CF.sub.3).sub.2 
2H-HEXAFLUOROISOPROPYL ACRYLATE 
1.3164 
CH.sub.2 .dbd.C(CH.sub.3)COOCH(CF.sub.3).sub.2 
2H-HEXAFLUOROISOPROPYL METHACRYLATE 
1.3295 
CH.sub.2 .dbd.C(H)COOCH.sub.2 (CF.sub.2).sub.3 F 
1H,1H,HEPTAFLUOROBUTYL ACRYLATE 
1.3289 
CH.sub.2 .dbd.C(CH.sub.3)COOCH.sub.2 (CF.sub.2).sub.3 F 
1H,1H-HEPTAFLUOROBUTYL METHACRYLATE 
1.3407 
CH.sub.2 .dbd.C(H)COOCH.sub.2 (CF.sub.2).sub.4 H 
1H,1H,5H-OCTAFLUOROPENTYL ACRYLATE 
1.3467 
CH.sub.2 .dbd.C(CH.sub.3)COOCH.sub.2 (CF.sub.2).sub.4 H 
1H,1H,5H-OCTAFLUOROPENTYL METHACRYLATE 
1.3559 
CH.sub.2 .dbd.C(H)COOCH.sub.2 (CF.sub.2).sub.7 F 
1H,1H-PENTADECAFLUOROOCTYL ACRYLATE 
1.328 
CH.sub.2 .dbd.C(CH.sub.3)COOCH.sub.2 (CF.sub.2).sub.7 F 
1H,1H-PENTADECAFLUOROOCTYLMETHACRYLATE 
1.332 
CH.sub.2 .dbd.C(H)COO(CH.sub.2).sub.2 (CF.sub.2).sub.8 F 
1H,1H,2H,2H-HEPTADECAFLUORODECYL 
1.3380TE 
CH.sub.2 .dbd.C(CH.sub.3)COO(CH.sub.2).sub.2 (CF.sub.2).sub.8 F 
1H,1H,2H,2H-HEPTADECAFLUORODECYL 
1.3435RYLATE 
CH.sub.2 .dbd.C(H)COOCH.sub.2 CF(CF.sub.3)OCF.sub.2 CF(CF.sub.3)O(CF.sub.2 
).sub.4 F From 3M L-12044 1.311 
CH.sub.2 .dbd.C(CH.sub.3)COOCH.sub.2 CF(CF.sub.3)OCF.sub.2 CF(CF.sub.3)O(C 
F.sub.2).sub.4 F From 3M L-12043 1.315 
__________________________________________________________________________ 
If desired, the refractive index of the above polymers may be increased by 
blending in monomers having a higher refractive index. Useful hydrocarbon 
monomers and comonomers used to modify the refractive index as well as 
physical properties of optical cladding polymers are: 
methylmethacrylate 
ethylmethacrylate 
ethylacrylate 
isopropylmethacrylate 
isopropylacrylate 
n-propylmethacrylate 
n-propylacrylate 
isobutylmethacrylate 
isobutylacrylate 
t-butylmethacrylate 
t-butylacrylate 
n-butylmethacrylate 
n-butylacrylate 
If the formulation is to be used for application to glass or quartz an 
adhesion promoter is added to the initial formulation. There are numerous 
adhesion promoters commonly used in the glass composite industry that are 
commercially available from PCR Gainesville, Fla. and Huls America, 
Summerset, N.J. and many others. Typical adhesion promoters for glass are 
acrylic acid and methacrylic acid, glycidyl methacrylate and 
3-(trimethoxysilyl) propyl methacrylate. Cladding bond strength is 
determined by cementing, with the cladding composition, the ends of two 
1.times.0.25 inch heat treated silica rods, in an inert atmosphere, 
followed by UV cure and determining tensile strength of the bond. Typical 
bond strength is 1900 lbs. per square inch. 
Photoinitiators for use in this process for UV curing are well known in the 
art. Typical examples of initiators that can be used are 2-hydroxy- 
2-methylpropiophenone; 1-hydroxycyclohexyl phenyl ketone and p-xylene bis 
(N,N-diethyldithiocarbamate). The UV initiator is dissolved in the monomer 
solution followed by UV irradiation to polymerize the mixture rapidly in 
accordance with this inventive process. 
Any monomer initiated with the UV initiator that results in a soft, tacky 
elastomer can be cross linked to increase the toughness of the elastomer. 
Cross linking agents that are useful in this process are 
ethyleneglycoldiacrylate, ethyleneglycol dimethacrylate, 
tetra(ethyleneglycol)dimethacrylate, tetra(ethyleneglycol)diacrylate, 
trimethylolpropane trimethacrylate, trimethylolpropane triacrylate, 
2,2,3,4,4-hexafluoropentandiyl-1,5-bis(methacrylate), 
2,2,3,4,4-hexafluoropentandiyl-1,5-bis(acrylate), CH.sub.2 
.dbd.CHOCOCH.sub.2 --(C.sub.2 F.sub.4 O).sub.m --(CF.sub.2 O).sub.n 
CH.sub.2 OCOCH.dbd.CH.sub.2 where m/n=0.8, and CH.sub.2 
.dbd.CHOCOCH.sub.2 --CF(CF.sub.3) O{CF(CF.sub.3)CF.sub.2 O}.sub.2 C.sub.2 
F.sub.4 !.sub.2. 
Cladding solutions of this process are prepared in a one step process 
directly from the monomer resulting in an easily controllable viscosity 
and easily controllable refractive index with a result of 100% 
photocurable cladding polymer solution. The UV cladding polymers made from 
these solutions are crystal clear having a predesigned A or D hardness and 
toughness obtainable through acrylate/methacrylate copolymerization.