Optical waveguides – Optical fiber waveguide with cladding – Utilizing multiple core or cladding
Reexamination Certificate
2001-03-30
2002-10-22
Palmer, Phan T. H. (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
Utilizing multiple core or cladding
C385S123000, C385S141000, C428S378000, C522S096000
Reexamination Certificate
active
06470128
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to compositions for coating glass surfaces and, more specifically, coating compositions for optical waveguides such as optical fibers, the coating compositions being fast curing and having improved adhesion to glass.
2. Description of the Related Prior Art
Optical fibers made from drawn glass have been used as a reliable transmission medium in telecommunications cables. Glass optical fibers are widely used because they have the ability to carry large amounts of information over long distances.
To facilitate these long-distance transmissions, optical fiber waveguides have been coated with plastic compositions of various materials in order to protect the fiber and increase its tensile strength. Generally, to accomplish this, optical glass fibers are frequently coated with two superposed coatings. The coating which contacts the glass is a relatively soft, primary coating that must satisfactorily adhere to the fiber and be soft enough to resist microbending especially at low service temperatures. The outer, exposed coating is a much harder secondary coating that provides the desired resistance to handling forces yet must be flexible enough to enable the coated fiber to withstand repeated bending without cracking the coating.
Coatings are applied to the fiber in-line during fiber drawing. As the state of fiber drawing technology has allowed for increased draw speeds to effectuate longer and thinner optical fibers, however, the need for coating compositions that can cure at faster rates coincident with the faster draw speeds has become more urgent. Therefore, it is highly desired for an uncured primary coating to be capable of a high cure speed.
Further, the cured primary coating should exhibit good adhesion to glass to prevent delamination of the coating from the optical fiber. Delamination can be caused by excessive moisture. This is particularly devastating to optical fibers as delamination can weaken the optical glass fiber because the delaminated coating can slide against the optical glass fiber causing microscopic scratches. Ultimately, such delamination can result in a loss of optical fiber transmission.
To reduce such delamination, adhesion promoting additives have been incorporated into coating compositions. Typically silane adhesion promoters have been used. However, it is known in the field that the use of adhesion promoters can reduce the speed of cure of a coating composition.
While the conventional coating compositions have been adequate for most applications, it would be desirable to be able to formulate coating compositions for optical glass fibers that not only possess good adhesion to glass, but also maintain a high rate of cure.
The use of acrylate oligomers with a polyester or polyether backbone for primary optical fiber coating is known in the field (see U.S. Pat. Nos. 5,418,016; 5,650,231; 6,042,943 and 6,048,911). Further, such acrylate oligomers are readily commercially available from a number of manufacturers.
The use of free radical photoinitiators for coating compositions for optical fibers is known in the field (see U.S. Pat. No. 6,042,943) and many such photoinitiators are readily commercially available from a number of manufacturers.
Radiation curable optical glass fiber coating compositions containing adhesion promoters are known. U.S. Pat. No. 4,849,462 discloses a UV-curable polyurethane polyacrylate containing about 0.5 to 5.0% by weight of a mercapto polyalkoxysilane.
U.S. Pat. No. 5,812,725 discloses increasing the adhesion of a radiation cured coating for optical glass fibers by the use of an electron beam to adjust the level of adhesion between the primary coating and a surface treated glass optical fiber. This patent generally discloses the use of a reactive diluent which can be a low viscosity acrylate monomer containing at least one functional group capable of polymerization when exposed to actinic radiation. Further the invention provides for an additional adhesion promoter, but does discuss that such may be unnecessary because of the use of the electron beam to pre-treat the glass fiber. However, there is no disclosure of vinyl acrylate specifically.
U.S. Pat. No. 4,824,919 discloses that a curable composition can include from about 4.5 to 60 part by weight of a long list of vinyl monomers, including, inter alia, vinyl acrylate, and a vinyl ether, such as the 3:1 adduct of methacrylic acid with tris(4-glycidyloxyphenyl)methane. The curable composition may include a minor amount of a toughness-imparting oligomeric urethane.
U.S. Pat. No. 5,981,738 discloses the use of ethylenically unsaturated solvents, including vinyl acrylates, capable of free radical addition with maleate or fumarate pendant groups on modified cellulose esters to prepare a UV-curable coating. The amounts of said ethylenically unsaturated solvents used are from between 50 to 90 wt % of the total coating composition.
U.S. Pat. No. 5,418,016 discloses the use of a N-vinylformamide (NVF) monomer as a reactive diluent, blended with oligomer systems and a photoinitiator to form a radiation curable composition.
U.S. Pat. No. 6,171,698 B1 discloses a radiation curable coating composition having good adhesion. The composition includes a hydrolyzed coupling agent mixture.
SUMMARY OF THE INVENTION
The present invention provides a new coating composition for optical fibers and optical fibers coated therewith, whereby the coating composition is fast curing and has improved adhesion to glass.
The coating composition comprises a radiation curable oligomer and a vinyl acrylate compound that can act as both a reactive diluent and an adhesion promoter, the vinyl acrylate compound having the following formula
wherein R is hydrogen or an alkyl group having 1 to 10 carbon atoms. Vinyl acrylate compounds in accordance with the present invention have good miscibility and low viscosity and impart to the cured primary coating a high adhesion to glass, therefore reducing and in some instances even eliminating the need for additional reactive diluent and/or adhesion promoter. With the use of a free radical photoinitiator, the vinyl acrylate compounds increase the speed of cure of the uncured coating in comparison with conventional coating compositions without a vinyl acrylate compound. Cured coating prepared from coating compositions in accordance with the present invention have good adhesion to glass and good stress characteristics.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention provides a UV-curable coating formulation for optical fiber. The coating includes a radiation curable oligomer, preferably a urethane acrylate and more preferably a difunctional urethane acrylate with a polyester, polyether, polycarbonate, polybutadience, hydrogenated polybutadience, hydrocarbon or silicone backbone, and a vinyl acrylate compound. Applicants have discovered that vinyl acrylate compounds as defined herein, are very good reactive diluents, as they are very miscible and have low viscosity (less than 1 cP). Further, during homopolymerization or copolymerization, the vinyl acrylate compound reacts as an acrylate but with the vinyl ether group staying intact. Applicants have also discovered that the addition of a vinyl acrylate compound imparts very good properties to coatings, namely good adhesion to glass. While not wishing to be bound by theory, it is believed that this is due to the presence of non-reacted double bonds in the polymerized vinyl acrylate compound that are capable of forming &pgr;-bonds with active atoms of a glass surface.
Applicants have discovered that the addition of a vinyl acrylate compound to a coating composition allows not only to dilute it, but also helps to expedite curing. Vinyl acrylate, for example, is photosensitive and when irradiated with UV light produces polymer, i.e., polyvinyl acrylate with, as mentioned above, the vinyl ether group staying intact, as can be appreciated from the following:
Wherein:
VA=vinyl acrylate; and
PVA=polyvinyl acrylate.
In a coating
Khudyakov Igor
Purvis Michael
Alcatel
Palmer Phan T. H.
Sugrue Mion, PLLC
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