Radiation curable coating composition with hydrophobic properties for optical fibers and optical fibers coated thereby

A radiation curable secondary coating composition for forming a secondary polymeric coating having good hydrophobic properties on a primary polymeric coating on an optical fiber is disclosed. The secondary coating composition is a mixture of a radiation curable composition capable of forming a polymeric coating, such as aliphatic difunctional and trifunctional urethane oligomers, and a hydrophobic agent selected from the group consisting of an aqueous dispersion of negatively charged hydrophobic resin particles, fluoropropylmethylcyclotrisiloxane, fluoropropylmethylsiloxarediol, trifluoropropylsiloxypolydimethylsiloxane, polybutadiene diacrylate and polybutadiene dimethacrylate.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates protective coatings for optical fibers. More
 particularly, the present invention relates to radiation curable,
 secondary protective liquid coating composition for optical fibers
 formulated to have hydrophobic properties and optical fibers coated
 thereby.
 2. Description of the Prior Art
 For many years now, 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. Glass fibers are inherently
 strong because the glass forming the fiber has an intrinsic strength on
 order of 3.times.10.sup.9 N/m.sup.2 (Pa). See, "Optical Fibers for
 Transmission", J. E. Midwinter, 1979, John Wiley and sons. The retention
 of such strength is largely dependent upon the application environment
 surrounding the optical fiber. For example, if the optical fiber is
 subjected to an abrasive environment, the abrasion, even with micron sized
 particles, produces microscopic flaws in the glass surface. These flaws
 propogate through the glass and eventually cause a fracture of the glass
 fiber. Glass optical fibers can also lose strength from contact with
 moisture and ions in solution. It is well known that hydrolysis of the
 surface bonds in silica glass occurs rapidly in the presence of water
 containing sodium ions. The hydrolysis of the surface bonds causes
 significant strength deterioration.
 To protect the integrity and strength of glass optical fibers from
 environmentally caused degradation, one or more protective polymeric
 coatings have been applied to the outer surface, such as the cladding, of
 such optical fibers, shortly after the optical fibers have been drawn from
 a glass preform. One purpose of the coatings is to protect the surface of
 the optical fiber from mechanical scratches and abrasions typically caused
 by subsequent handling and use. Another purpose of the coatings is to
 protect the glass from exposure to moisture and ions in solution. The
 coating or coatings may also have some influence over the fiber's optical
 characteristics because the coatings are physically responsive to external
 mechanical forces and temperature.
 The coating compositions applied to the optical fiber are typically viscous
 liquid, radiation curable compositions. Typically, the coating
 compositions are cured on the optical fiber by exposing the coating
 composition to ultraviolet radiation, electron beam radiation or ionizing
 radiation for a predetermined period of time deemed suitable for effective
 curing.
 Fluorinated UV radiation curable secondary coatings have been provided over
 optical fibers to repel water and moisture. See generally U.S. Pat. Nos.
 4,908,297, 5,298,291, and 5,567,794. The coatings disclosed in these
 patents are relatively expensive because the fluorinated raw materials
 which provide water repelling properties are relatively expensive.
 Accordingly, it is desirable to have a secondary coating having water
 repelling properties and good adhesion to the primary coating which is
 also relatively inexpensive.
 SUMMARY OF THE INVENTION
 It is an object of the present invention to provide a relatively
 inexpensive radiation curable liquid coating composition for application
 to an optical fiber which has good hydrophobic properties and good
 mechanical properties over an underlying primary coating on the optical
 fiber.
 This object is accomplished, at least in part, by providing a radiation
 curable secondary coating composition for forming a secondary coating
 having hydrophobic properties on a primary coating on an optical fiber,
 the secondary coating composition comprising a mixture of: a radiation
 curable composition capable of forming a polymeric coating, and a
 hydrophobic agent selected from the group consisting of a negatively
 charged hydrophobic colloid of resin particles suspended in water,
 fluoropropylmethylcyclotrisiloxane, fluoropropylmethylsiloxanediol,
 trifluoropropylsiloxypolydimethylsiloxane, polybutadiene diacrylate and
 polybutadiene dimethacrylate.

DETAILED DESCRIPTION OF THE PRESENT INVENTION
 Typical telecommunications elements include an elongated transmission
 medium such as a metallic wire or an optical fiber. Referring to FIG. 1, a
 typical optical fiber 10 transmission medium is shown. The typical optical
 fiber 10 is formed by a glass core 12 which is surrounded by a glass
 cladding 14. The glass cladding 14 of the optical fiber 10 is usually
 surrounded by one or more protective polymeric coatings. For example, as
 shown in FIG. 1, an inner protective polymeric coating 16 is applied over
 the cladding 14 and an outer protective polymeric coating 18 is applied
 over the inner coating 16. The inner 16 and outer 18 protective coatings
 may also be referred to as inner primary and outer primary coatings or
 primary and secondary coatings. The inner coating 16 is usually obtained
 by applying a radiation curable (polymerizable) composition over the
 cladding 14. The radiation curable composition is normally applied by
 passing the optical fiber through a first die or a coating applicator
 using techniques well known in the art, and therefore, not described
 herein. Once the radiation curable composition is applied over the
 cladding 14, the composition may be cured by exposing it to radiation,
 such as ultraviolet radiation, electron beam radiation or ionizing
 radiation, to initiate curing (polymerization) thereof. Ultraviolet
 radiation is most commonly used. The application and curing of the
 radiation curable composition to form the inner coating 16 is typically
 followed by the application and curing of a radiation curable composition
 capable of forming a polymeric coating which forms the outer or secondary
 coating 18. This sequence is known as a wet-on-dry application of the
 outer coating 18. Alternatively, the application of the radiation curable
 composition which forms the inner coating 16 may be directly followed by
 the application of the radiation curable composition forming the outer
 coating 18 prior to exposure to the curing radiation. This is known in the
 art as a wet-on-wet application. Each application technique is well known
 in the art.
 Typically, the commercially preferred radiation curable compositions for
 forming primary and secondary coatings on optical fibers include aliphatic
 difunctional and trifunctional urethane acrylates. These preferred
 radiation curable compositions may be diluted with reactive diluents such
 as tripropylene glycol diacrylate and may include photo-initiators to
 improve the curing process. There are a variety of photo-intiators
 marketed by Ciba under the trademarks Irgacure or Darocur that will work
 for the present invention.
 Those skilled in the art will appreciate that the hydrophobic or water
 repelling properties of a coating are typically determined by measuring
 the contact angle between a water drop placed on the coating and the
 coating surface with a goniometer. Basic cured secondary coatings formed
 with aliphatic urethane diacrylate and triacrylate oligomers typically
 exhibit a contact angle of about 45 degrees. According to the present
 invention, the hydrophobic properties of the basic cured secondary coating
 can be significantly improved by adding to the radiation curable
 composition a small quantity of a hydrophobic agent such as an aqueous
 dispersion of hydrophobic polymeric resin particles,
 fluoropropylmethylcyclotrisiloxane, fluoropropylmethylsiloxanediol,
 trifluoropropylsiloxypolydirnethylsiloxane, or polybutadiene
 di(meth)acrylate. With the addition of one or more of these materials, the
 hydrophobicity of the resulting cured coating can be greatly increased as
 exhibited by a larger measured contact angle. Examples of secondary
 coating composition having good hydrophobic or water repelling properties
 made according to the present invention follow below.
 EXAMPLE 1
 A basic secondary radiation curable composition capable of forming a
 polymeric secondary coating was prepared, in part, by mixing 180 grams of
 Ebecryl.TM. 4866, which is an aliphatic urethane triacrylate oligomer
 diluted with 30 weight percent tripropylene glycol diacrylate (TRPGDA)
 produced by UCB Chemicals, 33 grams of Photomer.TM. 6010, which is an
 aliphatic urethane diacrylate oligomer produced by Henkel, 65 grams of
 TRPGDA produced by UCB Chemicals, and 14 grams Darocur.TM. 4265, which is
 a 50:50 weight percent mixture of diphenyl-2,4,6-trimethylbenzoyl
 phosphine oxide and 2-hydroxy-2-methyl-1-phenylpropane-1-one photoinitator
 produced by Cib. Those skilled in the art will appreciate that the
 relative amounts of the aliphatic urethane acrylate oligomers and reactive
 diluents may be adjusted as required to obtain a basic secondary radiation
 curable composition having a viscosity which is suitable for application
 over a primary coating on an optical fiber. The basic secondary radiation
 curable composition was applied over a typical ultra violet radiation
 cured primary coating on a test piece and subjected to ultraviolet
 radiation in an Iwasaki processor with a UV radiation exposure of 1.0
 J/cm.sup.2. After curing, the secondary radiation curable composition
 provided a secondary coating in which the contact angle measured between a
 drop of water on the surface of this coating and the surface measured
 approximately 66 degrees. The basic coating exhibited a modulus of about
 30 to 60 MPa, a T.sub.g of about 30 to 60.degree. C. and an elongation to
 break of about 10 to 30 percent.
 EXAMPLE 2
 A basic secondary radiation curable composition capable of forming a
 polymeric secondary coating was prepared as described in Example 1.
 Approximately 15.5 grams (about 5 weight percent of the total composition)
 of Zonyl.TM. TE-3667N, which is an aqueous dispersion of negatively
 charged hydrophobic polymeric resin particles produced by DuPont, was
 thoroughly mixed into the above basic composition to produce a modified
 secondary coating composition according to the present invention suitable
 for application over a primary coating on an optical fiber. The polymeric
 resin particles in the Zonyl.TM. aqueous dispersion are a fluoropolymer,
 such as polytetrafluoroethylene, and range in size between 0.05 and 0.5
 microns, with an average size of about 0.22 microns. The resin particles
 comprise approximately 58 to 62 weight percent of the colloid. The
 Zonyl.TM. aqueous dispersion includes one or more surfactants such as
 octyl phenoxypolyethoxyethanol and nonyl phenoxypolyethoxyethanol and the
 surfactants make up to approximately 5 weight percent of the colloid.
 Also, the aqueous dispersion further includes ammonium perfluorooctanoate
 in an amount up to approximately 0.5 weight percent of the colloid.
 The modified secondary coating composition with the Zonyl.TM. aqueous
 dispersion was stirred with a mechanical stirrer for about 30 minutes.
 After stirring, the composition was sonicated for about 30 minutes to
 remove entrapped air bubbles. The modified secondary coating composition
 was applied over a typical radiation cured primary coating on a test piece
 and subjected to ultraviolet radiation in an Iwasaki processor with a UV
 radiation exposure of 1.0 J/cm.sup.2. After curing, the modified secondary
 coating composition provided a hydrophobic coating. The contact angle
 measured between a drop of water on the surface and the surface of this
 coating measured approximately 66 degrees. The coating exhibited a modulus
 of about 30 to 60 MPa, a T.sub.g of about 30 to 60.degree. C. and an
 elongation to break of about 10 to 30 percent. All of these properties are
 suitable for a secondary coating for an optical fiber.
 EXAMPLE 3
 A basic secondary radiation curable composition capable of forming a
 polymeric secondary coating was prepared as described in Example 1.
 Approximately 15.5 grams (about 5 weight percent of the total composition)
 of a trifluoropropyl copolymer vinyl stopped fluid marketed by General
 Electric under the product number FF160 was added to the basic secondary
 composition. The vinyl stopped fluid is a proprietary mixture containing
 approximately 80 to 99 weight percent
 trifluoropropylsiloxypolydimethylsiloxane.
 The secondary coating composition with the vinyl stopped fluid was stirred
 with a mechanical stirrer for about 30 minutes. After stirring, the
 composition was sonicated for about 30 minutes to remove entrapped air
 bubbles. The secondary coating composition was applied over a typical
 radiation cured primary coating on a test piece and subjected to
 ultraviolet radiation in an Iwasaki processor with a UV radiation exposure
 of 1.0 J/cm.sup.2. After curing, the resulting secondary coating
 composition provided a hydrophobic coating. The contact angle measured
 between a drop of water on the surface and the surface of this coating
 measured approximately 75 degrees. The coating exhibited a modulus of
 about 30 to 60 MPa, a T.sub.g of about 30 to 60.degree. C. and an
 elongation to break of about 10 to 30 percent, indicating no loss of
 mechanical properties due to the addition of the vinyl stopped fluid. As
 stated above, all of these properties are suitable for a secondary coating
 for an optical fiber.
 EXAMPLE 4
 A basic secondary radiation curable composition capable of forming a
 polymeric secondary coating was prepared as described in Example 1.
 Approximately 15.5 grams (about 5 weight percent of the total composition)
 of a trifluoropropyl silanol stopped fluid marketed by General Electric
 under the product number FF7149 was added to the basic secondary coating
 composition to form a modified secondary coating composition. The silanol
 stopped fluid is a proprietary mixture containing approximately 10 to 30
 weight percent fluoropropylmethylcyclotrisiloxane and up to 80 to 99
 weight percent fluoropropylmethylsiloxanediol.
 As in the other examples, the modified secondary coating composition with
 the silanol stopped fluid was stirred with a mechanical stirrer for about
 30 minutes. After stirring, the modified composition was sonicated for
 about 30 minutes to remove entrapped air bubbles. The resulting modified
 secondary coating composition was applied over a typical radiation cured
 primary coating on a test piece and subjected to ultraviolet radiation in
 an Iwasaki processor with a UV radiation exposure of 1.0 J/cm.sup.2. After
 curing, the modified secondary coating composition provided a hydrophobic
 coating. The contact angle measured between a drop of water on the surface
 of this coating measured approximately 71 degrees. The coating exhibited a
 modulus of about 30 to 60 MPa, a T.sub.g of about 30 to 60.degree. C. and
 an elongation to break of about 10 to 30 percent, indicating no loss of
 mechanical properties due to the addition of the silanol stopped fluid. As
 stated above, all of these properties are suitable for a secondary coating
 for an optical fiber.
 EXAMPLE 5
 A basic secondary radiation curable composition capable of forming a
 polymeric secondary coating was prepared as described in Example 1.
 Approximately 15.5 grams (about 5 weight percent of the total composition)
 of a polybutadiene di(meth)acrylate oligomer diluted with about 20 weight
 percent of 1,6 hexanediol diacrylate (HDDA) marketed by Sartomer under the
 product designation CN301 was added to the basic secondary radiation
 curable composition to form a modified secondary coating composition.
 Similar to the examples, the modified secondary coating composition with
 the polybutadiene di(meth)acrylate oligomer was stirred with a mechanical
 stirrer for about 30 minutes. After stirring, the composition was
 sonicated for about 30 minutes to remove entrapped air bubbles. The
 modified secondary coating composition was applied over a typical
 radiation cured primary coating on a test piece and subjected to
 ultraviolet radiation in an Iwasaki processor with a UV radiation exposure
 of 1.0 J/cm.sup.2. After curing, the resulting secondary coating
 composition provided a hydrophobic coating. The contact angle measured
 between a drop of water on the surface and the surface of this coating
 measured approximately 80 degrees. The coating exhibited a modulus of
 about 30 to 60 MPa, a T.sub.g of about 30 to 60.degree. C. and an
 elongation to break of about 10 to 30 percent, indicating no loss of
 mechanical properties due to the addition of the polybutadiene
 di(meth)acrylate oligomer. As stated above, all of these properties are
 suitable for a secondary coating for an optical fiber.
 EXAMPLE 6
 It was observed that oxidation products of the HDDA reactive diluent in the
 composition described in Example 5 imparted some color to the resulting
 coating. Although the physical properties of the resulting coating made
 according to composition of Example 5 were found to be satisfactory, in
 some optical fiber applications, the color imparted by the HDDA reactive
 diluent oxidation products may not be desirable. Accordingly, a modified
 coating composition using a slightly different polybutadiene based
 acrylate hydrophobic agent was formulated to overcome the coloration of
 the coating. The composition is described below.
 A basic secondary radiation curable composition capable of forming a
 polymeric secondary coating was prepared as described in Example 1.
 Approximately 15.5 grams (about 5 weight percent of the total composition)
 of a polybutadiene diacrylate oligomer marketed by Sartomer under the
 product designation CN302 was added to the basic secondary composition to
 form a modified coating composition.
 Similar to the other examples, the modified secondary coating composition
 with the polybutadiene diacrylate oligomer was stirred with a mechanical
 stirrer for about 30 minutes. After stirring, the modified composition was
 sonicated for about 30 minutes to remove entrapped air bubbles. The
 modified secondary coating composition was applied over a typical
 radiation cured primary coating on a test piece and subjected to
 ultraviolet radiation in an Iwasaki processor with a UV radiation exposure
 of 1.0 J/cm.sup.2. After curing, the resulting secondary coating
 composition provided a hydrophobic coating. The contact angle measured
 between a drop of water on the surface and the surface of this coating
 measured approximately 85 degrees. The coating exhibited a modulus of
 about 30 to 60 MPa, a T.sub.g of about 30 to 60.degree. C. and an
 elongation to break of about 10 to 30 percent, indicating no loss of
 mechanical properties due to the addition of the polybutadiene
 di(meth)acrylate oligomer. As stated above, all of these properties are
 suitable for a secondary coating for an optical fiber.
 In accordance with the present invention, the secondary coating composition
 of the present invention can be applied over a primary coating on an
 optical fiber according to well known techniques. For example, the
 secondary coating composition can be applied over a primary coating by
 passing the optical fiber with primary coating through a die or coating
 applicator. The secondary coating composition is also supplied to the die
 or coating applicator. Once the uncured secondary coating composition is
 applied over the primary coating on the optical fiber, the secondary
 coating composition is exposed to sufficient radiation, such as
 ultraviolet radiation, to cure the secondary coating composition.
 Typically, the ultraviolet radiation is provided by passing the coated
 optical fiber through one or more ultraviolet radiation lamps which
 provide ultraviolet radiation in a suitable wavelength range and at a
 suitable intensity to cure the secondary coating composition.
 It can be seen from the foregoing disclosure and example that the present
 invention offers substantial advantages over the prior art in terms of
 cost. The embodiment disclosed herein achieve the object of the invention;
 however, it should be appreciated by those skilled in the art that
 departures can be made from the following claims without parting from the
 spirit and scope of the invention.