1. Field of the Invention
The present invention relates to a curable coating composition for an optical fiber, an optical fiber prepared with such cured coating composition and a method for making an optical fiber that contains such coating.
2. Technology Review
Optical fibers have acquired an increasingly important role in the field of communications, frequently replacing existing copper wires. This trend has had a significant impact in the local area networks (i.e., for fiber-to-home uses), which has seen a vast increase in the usage of optical fibers. Further increases in the use of optical fibers in local loop telephone and cable TV service are expected, as local fiber networks are established to deliver ever greater volumes of information in the form of data, audio, and video signals to residential and commercial users. In addition, use of optical fibers in home and commercial business for internal data, voice, and video communications has begun and is expected to increase.
Optical fibers typically contain a glass core, a cladding, and at least two coatings, i.e., a primary (or inner) coating and a secondary (or outer) coating. The primary coating is applied directly to the cladding and, when cured, forms a soft, elastic, and compliant material which encapsulates the glass fiber. The primary coating serves as a buffer to cushion and protect the glass fiber core when the fiber is bent, cabled, or spooled. Stresses placed upon the optical fiber during handling may induce microbending of the fibers and cause attenuation of the light which is intended to pass through them, resulting in inefficient signal transmission. The secondary coating is applied over the primary coating and functions as a tough, protective outer layer that prevents damage to the glass fiber during processing and use.
Certain characteristics are desirable for the primary coating, and others for the secondary coating. The modulus of the primary coating is preferably sufficiently low to cushion and protect the fiber by readily relieving stresses on the fiber, which can induce microbending and degrade signal transmission. This cushioning effect is preferably maintained throughout the fiber""s lifetime.
The primary coating preferably has a glass transition temperature (Tg) which is lower than the foreseeable lowest use temperature. This enables the primary coating to remain soft throughout the temperature range of use.
It is preferred for the primary coating to have a refractive index which is different from (i.e., higher than) the refractive index of the cladding. This refractive index differential between the cladding and the primary coating allows errant light signals to be refracted away from the glass core.
Finally, the primary coating must maintain adequate adhesion to the glass fiber during thermal and hydrolytic aging, yet be strippable therefrom for splicing purposes. Moisture resistance is important, because moisture also affects the adhesion of the primary coating to the glass. Poor adhesion can result in various sized delaminations which may lead to glass damage which can be significant sources of attenuation in the optical fiber.
Physical and chemical properties of polymer coatings contribute significantly to the overall performance of optical fibers. Among those are mechanical properties such as Young""s modulus, tensile strength, toughness, percent elongation, glass adhesion, hydrophobicity, and low temperature sensitivity. Strong glass to primary coating adhesion and low water absorption are desired for strength and fatigue testing, as well as for field performance in general. The coating typically makes up 75% of optical fiber by volume, so the cost factor is also of concern.
One prior coating relates to a radiation curable primary coating composition for coating an optical fiber. In one embodiment, the coating composition includes a saturated aliphatic backbone having a first end and a second end. At least one epoxide group is at the first end and at least one reactive functional group is at the second end. The composition includes a mixture of acrylate monomers, composed of a first monomer having one acrylate group, and a second monomer having at least two functional groups. The composition further includes a photoinitiator.
Other prior coatings include the use of non-chemically crosslinked thermoplastic elastomers as part of a hot melt system for coating optical fibers.
Further prior coatings use liquid polymers as part of a UV curable primary coating composition for an optical fiber. These polymers are viscous liquids at room temperature and are of low molecular weight. They also have reactive functional groups, including epoxide, hydroxyl or acrylate groups, which allow for further chemical reactions. In addition, they require a cationic photoinitiator.
In accordance with one aspect of the present invention there is provided a curable coating composition for coating an optical fiber. The composition includes a block copolymer having at least one hard block and at least one soft block, wherein the hard block has a Tg greater than the Tg of the soft block. The composition also includes at least one reactive monomer.
In accordance with another aspect of the present invention there is provided a curable coating composition for coating an optical fiber. The composition includes a block copolymer including at least one hard block having a Tg greater than about 20xc2x0 C. and at least one soft block having a Tg of less than about 20xc2x0 C. The copolymer has a molecular weight of more than about 20,000. The composition also includes at least one reactive monomer.
In accordance with a further aspect of the present invention there is provided a coated optical fiber including an optical fiber having at least one cured coating layer thereon. The coating layer has a first component which includes a block copolymer having at least one hard block and at least one soft block, wherein the hard block has a Tg greater than the Tg of the soft block and a second component.
In accordance with an additional aspect of the present invention there is provided a method for making a coated optical fiber, including providing an optical fiber; coating the optical fiber with a polymerizable composition. The composition includes a polymer component having a block copolymer having at least one hard block and at least one soft block, wherein the hard block has a Tg greater than the Tg of the soft block. The composition also includes at least one reactive monomer. The method further includes polymerizing the composition under conditions effective to form a cured coating over the optical fiber.
It is an advantage of the present invention to provide a curable coating composition that possesses enhanced or increased thermal and hydrolytic stability.
Another advantage of the present invention is to produce a coated optical fiber having increased or enhanced mechanical properties such as modulus, toughness, tensile strength, and percent elongation.
Finally, a further advantage of the present invention is to provide a coating having strong glass to primary coating adhesion and low water absorption.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described in the written description and claims hereof, as well as the appended drawings. It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework to understanding the nature and character of the invention as it is claimed.