Method for curing optical fiber coating material

Details Method for curing optical fiber coating material Method for curing optical fiber coating material

2001-03-15

2002-08-13

Pianalto, Bernard (Department: 1762)

Coating processes

Direct application of electrical, magnetic, wave, or...

Polymerization of coating utilizing direct application of...

C427S385500, C427S551000, C427S595000

Reexamination Certificate

active

06432489

ABSTRACT:

This invention relates to a method for curing coating materials on optical fibers, including primary and secondary coating materials and taping materials, with electron beams.
BACKGROUND OF THE INVENTION
Optical communications fibers include a variety of types such as quartz glass, multi-component glass and plastic fibers. In reality, quartz glass fibers are vastly used in a wide variety of applications because of their light weight, low loss, high durability and high transmission capacity. Since quartz glass fibers are very thin and sensitive to external factors, quartz glass fibers for optical communications are generally of the construction that a quartz glass fiber which is spun from a melt is coated with a liquid curable resin capable of curing to a soft state, the coating is cured to form a primary coating, and the primary coating is protected with a secondary coating using a liquid curable resin capable of curing to a hard state. This is generally designated a coated optical fiber or simply optical fiber. A tape element is fabricated by bundling several, typically four, coated optical fibers and coating the bundle with a taping material, followed by curing.
Typical of the coating material are urethane acrylate base ultraviolet-curable resin compositions. As disclosed in JP-B 1-19694 and Japanese Patent Nos. 2,522,663 and 2,547,021, liquid UV-curable resin compositions comprising a urethane acrylate oligomer, a reactive diluent, and a photo-polymerization initiator are known. To meet the recent demand for increasing the drawing speed of optical fibers for productivity improvement purposes, the UV curing system can find no solution other than the use of an increased number of UV lamps. This has a limit when curing is effected in a limited space. Japanese Patent No. 2,541,997 discloses electron radiation as exemplary actinic energy radiation, but does not refer to electron beam accelerating voltages. If electron beams accelerated at high voltage as used in the prior art are irradiated to optical fibers, the dopant which is added to the optical fiber core in order to provide an increased refractive index can be altered or blackened, resulting in an undesirably increased transmission loss.
SUMMARY OF THE INVENTION
An object of the invention is to provide a method for curing an optical fiber coating material with electron beams in an efficient manner to form a cured coating having improved properties.
The inventors have found that by irradiating a specific coating material on an optical fiber with electron beams, obtained by driving electrons under an accelerating voltage of 50 to 190 kV, in a low dose of 10 to 100 kGy, the coating material can be effectively cured without increasing the transmission loss of the optical fiber.
It has also been found that since the dose of electron beams is determined by the current flow through the filament and the processing speed, it is possible to keep a constant dose by controlling the current flow in proportion to the drawing speed.
In general, electron beams are produced by conducting electric current through a filament to heat the filament for emitting thermal electrons, and accelerating the thermal electrons under the impetus of a voltage (accelerating voltage) to form electron beams. The accelerating voltage affects the penetration depth of electrons when irradiated to the optical fibers. If the accelerating voltage is too low, only a surface layer of the resin coating is cured. Since electron beams have great energy, a greater dose can cause crosslinking at radical polymerizable functional groups and anywhere. In order to provide cured coatings with consistent properties, not only the electron accelerating voltage, but also the dose of electron beams must be controlled. It has been found effective that a composition comprising a polyether polyurethane bearing at least two ethylenically unsaturated groups in a molecule and having a number average molecular weight of 800 to 10,000 and a nitrogenous reactive diluent is used as an optical fiber coating material and this coating material is cured with electron beams under the above-specified conditions of accelerating voltage and dose.
Accordingly, the invention provides a method for curing an optical fiber coating material comprising (A) 100 parts by weight of a polyether polyurethane bearing at least two ethylenically unsaturated groups in a molecule and having a number average molecular weight of 800 to 10,000, and (B) 1 to 40 parts by weight of a nitrogenous reactive diluent, the method comprising the step of irradiating the coating material with electron beams, produced by accelerating electrons under a voltage of 50 to 190 kV, in a dose of 10 to 100 kGy.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The method for curing an optical fiber coating material according to the invention is characterized in that an optical fiber coating material comprising (A) a polyether polyurethane bearing at least two ethylenically unsaturated groups in a molecule and having a number average molecular weight of 800 to 10,000 and (B) a nitrogenous reactive diluent is irradiated with electron beams, produced by accelerating electrons under a voltage of 50 to 190 kV, in a dose of 10 to 100 kGy.
Component (A) is a polyether polyurethane bearing at least two ethylenically unsaturated groups in a molecule and having a number average molecular weight (Mn) of 800 to 10,000. With Mn of less than 800, the cured coating loses elongation. With Mn of more than 10,000, the cure by electron beam irradiation becomes ineffective.
The optical fiber coating material is applicable as a primary coating material, secondary coating material and taping material. Of these optical fiber coating materials, the primary coating material is a liquid curable resin to form a soft cured coating. For the primary coating material, the polyether polyurethane bearing at least two ethylenically unsaturated groups in a molecule should preferably have a number average molecular weight of 2,500 to 10,000 because a longer distance must be set between crosslinks. The secondary coating material for protecting the primary coating is a liquid curable resin to form a hard cured coating. For the secondary coating material, the polyether polyurethane bearing at least two ethylenically unsaturated groups in a molecule should preferably have a number average molecular weight of 800 to 3,500 because a relatively short distance must be set between crosslinks. For the taping material, the polyether polyurethane bearing at least two ethylenically unsaturated groups in a molecule should preferably have a number average molecular weight of 800 to 2,500.
The polyether polyurethane bearing at least two ethylenically unsaturated groups in a molecule is obtained, for example, by reacting a diol having an oxyalkylene group of 2 to 10 carbon atoms with a diisocyanate and a compound having an ethylenically unsaturated group.
Illustrative examples of the diol having an oxy-alkylene group of 2 to 10 carbon atoms include polyethylene glycol, polypropylene glycol, polytetramethylene glycol, 2-methyltetrahydrofuran glycol, 3-methyltetrahydrofuran glycol, polyheptamethylene glycol, polyhexamethylene glycol, polydecamethylene glycol, and polyalkylene oxide-added diols of bisphenol A. From the moisture absorption and viscosity standpoints, polypropylene glycol, polytetramethylene glycol, 2-methyltetrahydrofuran glycol, and 3-methyltetrahydrofuran glycol are preferable. The polyalkylene oxides used herein include homopolymers as well as random and block copolymers thereof.
Examples of the diisocyanate used herein include 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, 1,3-xylene diisocyanate, 1,4-xylene diisocyanate, 1,5-naphthalene diisocyanate, m-phenylene diisocyanate, p-phenylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, 2,2,4-trimethylhexamethylene diisocyanate, and 2,4,4-trimethylhexamethylene diisocyanate. Of these, 2,4-tolylene diisocyanate and isophorone diisocyanate are preferred. These diisocyanates may be used alone or in admixture of

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