DE69607654T2 - METHOD FOR PRODUCING POLYMERIC OPTICAL FIBERS WITH GRADUATED BREAKING INDEX - Google Patents

Description

(a) Field of the invention

The invention relates to a method for producing a polymeric optical gradient fiber according to the preamble of claim 1.

Polymeric graded index optical fibers can be used for broadband optical transmission. Given the increasing trend to replace electrical wire transmission with optical fiber transmission, it is becoming necessary to provide homes and offices with optical fiber connections ("fiber to the home"). Since broadband optical fiber connections require alignment that is too precise to be accomplished without professional skills and equipment, connectors are needed to bring the optical channel to a size suitable for home and office use. Polymeric graded index optical fibers can be very well suited for this purpose because they offer a relatively large bandwidth and can have a relatively large diameter.

(b) Technical background

Polymeric gradient optical fibers and processes for their production are known.

For example, Japanese Patent Application Laid-Open No. 94/186,441 describes a method using a two-component spinning process using a core dope and a sheath dope. An optical art A refractive index distribution optical transmission body is produced by producing a polymer-monomer mixture determined by a 50 to 90% conversion from the outside discharge port of a nozzle plate with a concentric discharge port, while simultaneously discharging a monomer mixture or a polymer-monomer mixture with a 50% or less conversion from the inside of the discharge port. The latter mixture contains a non-polymerizable colorless transparent compound with a high refractive index, such as benzyl butyl phthalate. These mixtures are applied to form a core/clad multicomponent rod. The high refractive index compound diffuses from the mixture in the inner layer (the core mixture) into the mixture in the outer layer (the cladding mixture). Further polymerization of the produced rod results in an optical transmission body with a refractive index distribution that decreases continuously from the center to the outside. To produce an optical fiber from the rod, the rod is drawn to the desired fiber diameter.

A disadvantage of this process is that it is multi-stage. The individual steps of releasing a rod, post-polymerizing the rod and drawing it into a fiber do not result in a particularly economical process. The formation of the gradient rod is also slow. It would be desirable to have a process that produces a fiber directly, i.e. without first having to form a rod that has to be drawn. It would also be desirable for such a process to be a one-step process, preferably a continuous process.

Apart from the disadvantageous multi-stage character, the known process has a further disadvantage in that the non-polymerizable compound with high refractive index, the is miscible with the cladding and core polymers, remains in the system as a plasticizer. Furthermore, the presence of such a low molecular weight compound means that the final optical fiber may be subject to temperature-induced instability as a result of further diffusion of the low molecular weight compound. A polymeric gradient optical fiber would therefore be desirable in which the refractive index distribution is fixed in the polymer.

Processes for obtaining polymeric optical fibers in which the refractive index distribution in the finally obtained fiber is not determined by the presence of a low molecular weight compound are known.

In this regard, reference is made to Japanese Laid-Open Patent Application No. 90/33,104. Therein a process is described for producing a polymeric gradient fiber, again by two-component spinning of a core and a sheath dope. The sheath is spun from a polymer blend, e.g. poly(methyl methacrylate), and a monomer, e.g. methyl methacrylate. The core is spun from a high refractive index monomer, e.g. phenyl methacrylate. After being delivered from a two-component spinneret, the spun core-sheath filament is passed through a heated tube for fusion and then exposed to UV irradiation to initiate photopolymerization. In this way, a core-clad optical fiber is formed in which a refractive index distribution is generated due to the diffusion of the methyl methacrylate and phenyl methacrylate monomers. The refractive index distribution is fixed in a polymer structure as a result of polymerization.

This process is also not carried out at the desired product speed, with typical dwell times times in the heated tube and during irradiation are each about three minutes. The process also has a disadvantage in that the core does not contribute to filament formation. In order to ensure a stable spinning process in which a filament with sufficient strength is formed, it is a basic requirement that the polymer concentration in the sheath spinning mass is sufficiently high, a typical value being 60% by weight. Too high a polymer concentration can adversely affect the diffusion rate and thus lead to a process that is too slow.

Another method is described in Japanese Patent Application Laid-Open No. 91/42,604. Here, a polymeric gradient optical fiber is produced by two-component spinning, where the core material is a polymer-monomer mixture and the cladding material is a polymer with a low refractive index, e.g. poly-(tetrafluoropropyl methacrylate). The method also requires too long processing times and again requires a heated tube and UV irradiation.

To carry out the processes according to JP 90/33,104 and JP 91/42, 604, a two-component spinning system is used in which the spinning mass is discharged from the bottom to the floor. For reasons of practical handling, it would be desirable to have a process for producing polymeric optical gradient fibers in which the refractive index distribution occurs sufficiently quickly for normal spinning (i.e. in which the spinning mass is discharged from the top to the bottom of the spinning system).

The state of the art also includes various other processes for producing polymeric gradient optical fibers. For example, JP 77/5857 relates to a process in which a rod with high refractive index (diallyl phthalate). A monomer with a low refractive index (methyl methacrylate) is diffused into the rod with simultaneous polymerization. A similar process is described in JP 81/37521. A special polymerized polymer with a high refractive index is added to a monomer that is in the gas phase, and this then diffuses into the polymer without further polymerization taking place.

In JP 79/30, 301, two different monomers are heterogeneously copolymerized so that a gradient is created in the monomer composition. In JP 86/130, 904, a mixture of monomers with different refractive indices is subjected to crosslinking from the outside. The inner monomer mixture is enriched with the monomer with the higher refractive index.

JP-A-01-265207 describes spinning a coaxial multilayer fiber from two types of spinning liquids containing a polymer, vinyl monomer and a photosensitizer and having different refractive indices. However, it is not described that the monomer in the core spinning dope has a higher refractive index than the monomer in the sheath spinning dope.

EP-A-0 615 141 describes a process for producing optical transmission bodies made of plastic, in which a transparent polymer melt and a transparent diffusible material with a refractive index that differs from that of the transparent polymer are coaxially extruded. However, this process does not use two different monomers, one of which is present in the core spinning mass and has a higher refractive index and the other monomer is contained in the sheath spinning mass.

According to EP 208 159, a polymer-monomer mixture is extruded, whereby the monomer is volatile and has a relatively high refractive index. Evaporation from the outside results in the core containing more monomer with a high refractive index than the cladding.

According to EP 496 893, a tube is made of poly(methyl methacrylate) and filled with a monomer mixture of methyl methacrylate and benzyl methacrylate. By rotating the tube (about 20 hours) under polymerization conditions, a gradient rod is formed. This rod is stretched to form a polymeric gradient optical fiber. According to EP 497 984, a polymeric tube is also made and filled with monomer. A rod with a refractive index gradient is created by diffusion. A polymeric gradient optical fiber is drawn from the rod.

All these descriptions of the state of the art concern unsuitable discontinuous processes which do not meet the above-mentioned requirements and in which only spinning speeds of less than 1 m/min are achieved.

It is the aim of the invention to provide an efficient process for the production of polymeric gradient optical fibers, which can be carried out as a continuous two-component spinning process, which is faster than the known processes (preferably even up to 1 cm/min or more).

(c) Description of the invention 1. Summary

The above object is achieved according to the invention by a method for producing a polymeric optical gradient fiber by means of a two-component spinning process, in which a core dope and a sheath dope are used, wherein the core dope and the sheath dope contain a polymer and a monomer, and the monomer in the core dope has a higher refractive index than the monomer in the sheath dope.

Particular embodiments of the invention are the subject of the dependent claims.

2. Detailed description

Two-component spinning processes are known to those skilled in the art. A description of the basic principles can be found, for example, in D. R. Paul and S. Newman, Polymer Blends, Chapter 16: Fibers from polymer blends, pages 176-177. In these processes, the core and sheath dopes are simultaneously extruded through a spinneret, with the core/sheath configuration being achieved either by the shape of the spinneret (e.g. by having it orifices with two concentric holes) or by means of a tube through which the core dope is extruded, which is arranged immediately in front of a normal spinneret.

In the process of the invention, the refractive index distribution evolves from a clear step-like distribution present immediately after the spinning dopes emerge from the spinnerets to the desired gradual decrease from the center of the core to the outside of the sheath. Immediately after passing through the spinning points (which are located immediately below the spinneret in the preferred top-down spinning process), the refractive index distribution is determined by the difference in refractive indices between the two spinning dopes. This initial difference in refractive indices of the core and sheath spinning dopes is generally in the range 0.01 to 0.10. It is preferred that this difference is relatively large, as this allows the largest numerical aperture and the largest degree of freedom in determining any steep parts of the distribution, while achieving a satisfactory processing speed.

The extrusion of the two spinning masses results in a continuous formation of a thread (in this case an optical fiber). At a certain point in the formed thread, the diffusion of the higher and lower refractive index monomers causes the core to become richer in low index monomer and poorer in higher index monomer, while the reverse is true for the cladding. As the extruded filaments continue to advance, the gradual distribution immediately after the spinning points becomes a more or less gradual distribution. If the diffusion process were allowed to run freely, the difference in refractive index between the core and the cladding would eventually be eliminated.

In order to fix the desired refractive index distribution in the polymer fiber, the monomers are each induced to copolymerize to form polymers. For this reason, it is necessary that both the core and the sheath spinning mass contain a polymerization initiator, preferably a TV initiator. Where polymerization has taken place, the diffusion of monomers is inhibited. Under practical conditions, there can be a difference in cross-linking across the fiber diameter. It is common knowledge for those skilled in the art how this can be avoided by combining the type and concentration of initiator accordingly. With a suitable selection of the spinning speed and the duration of the UV irradiation, the process can be carried out in such a way that the completion of the polymerization coincides with the point in time at which the desired refractive index distribution is achieved.

For a given system of polymers, monomers, UV initiators and wavelength of irradiation, one skilled in the art can determine, without undue experimentation, the conditions required to produce polymeric gradient optical fibers. A further description of variations that can be made within the scope of the invention follows.

An important variable is the polymer concentration in the dopes. If the core and sheath dopes have a sufficiently low polymer concentration, the diffusion rate of both the high refractive index monomer and the low refractive index monomer will be high enough not to limit the speed of the spinning process.

Surprisingly, it has been found that with the process of the invention it is possible to provide a process in which the diffusion takes place at such a high rate that the polymeric gradient optical fibers can be produced in a continuous two-component spinning process at a rate of approximately 10 m/min. For this purpose, the core and sheath spinning dope solutions of the polymer in the monomer are present at a polymer concentration of 10 to 50 wt.%. Since a high rate of diffusion is most advantageous, it is preferred that the polymer concentration is in the range of 10 to 30 wt.%.

In both the core and sheath spinning dope, the combination of a polymer and a monomer not only leads to unexpectedly high process speeds, but it is also a decisive advantage that even at high spinning speeds, filaments with sufficient strength can be formed. For this purpose, the continuous formation of the continuous bicomponent filaments is best carried out so that the core and sheath polymers each have a molecular weight of greater than 100,000. In order to achieve high diffusion rates and high spinning speeds, which require a low polymer concentration, it is preferred that the polymers each have a molecular weight of greater than 500,000.

To obtain a gradient polymer optical fiber with optimal properties in terms of optical transmission, it is desirable that the core and cladding polymers are chemically identical. Even more preferably, they are also physically identical. Apart from the positive influence on the final properties of the fiber, this also offers a surprisingly simple and convenient process. If the core and cladding polymers are the same, the refractive index differences in the core and cladding dopes (and thus in the finished gradient polymer optical fiber) are determined only by the monomer selection.

In this case, it is preferred that the refractive index difference between the high refractive index monomers contained in the core and the low refractive index monomer contained in the cladding be in the range specified above. Preferably, this difference is 0.01 to 0.1.

For the purposes of this specification, it is assumed that the refractive index of a monomer is proportional to the refractive index of the corresponding homopolymer. The refractive index of the interpolymers formed by copolymerizing the monomers with high or low refractive index in combination with the original polymers, is proportional to the proportion of each monomer incorporated into such interpolymers.

It is further preferred that the monomer in one of the two dopes corresponds to the polymer contained in the same dope. Although this does not preclude that each of the dopes of the monomer and polymer correspond to each other and that the core and sheath therefore contain a different polymer, in order to avoid a step-like refractive index distribution it is highly preferred that the polymers are the same and that either the core dope or the sheath dope contains the corresponding monomer.

For reasons of processability, inexpensive availability of high molecular weight substances and good optical properties, it is preferred that the polymer in both the core and sheath dopes is poly(methyl methacrylate). In this case, it is further preferred that the sheath monomer is methyl methacrylate and the core monomer has a higher refractive index than methyl methacrylate. The monomer in this case is preferably an acrylic monomer. Examples of high refractive index acrylic monomers are benzyl methacrylate, phenyl methacrylate, 1-phenylethyl methacrylate, 2-phenylethyl methacrylate, furfuryl methacrylate, 2-chloroethyl acrylate. Benzyl methacrylate and phenyl methacrylate are most preferred.

It should be noted that suitable optical polymers and corresponding or analogous monomers with higher and lower refractive index are known to those skilled in the art. For example, gradient index materials and components were described by Y. Koike in Polymers for lightwave and integrated optics, L. A. Hornak (ed.), Chapter 3, pages 71-104.

If the production of polymeric optical gradient fibers for use in the near infrared (near-IR) range is desired, optical attenuation caused by the absorption of light by overtones of the vibration frequencies of the bonds containing hydrogen atoms, in particular O-H, N-H, C-H, should be avoided as far as possible. This can be achieved by replacing the hydrogen atoms with heavier elements. In other wavelength ranges too, replacing hydrogen atoms with heavier elements can achieve an advantageous broadening of the frequency ranges in polymeric optical fibers where absorption minima occur.

The polymeric gradient optical fibers according to the invention are mostly used with visible light. This has advantages in the case of home-made optical fiber connections. Furthermore, the preferred material poly(methyl methacrylate) is mostly used with visible light because it has an absorption minimum in this range.

In any case, fluorine and/or chlorine are preferred as hydrogen replacement elements, as these can also be used to adjust the refractive index. F and Cl have contrasting individual effects: while fluorine can cause a sharp reduction in the refractive index, chlorine causes it to increase.

For further explanation and without limitation, a description of the best mode for carrying out the invention, an example and a comparative example follow.

Two gels are processed in a two-component spinning process to form a core-sheath fiber. The core gel (core spinning mass) consists of high molecular weight poly(methyl methacrylate) - PMMA - dissolved in monomeric methacrylate with a higher Refractive index as IC4A (methyl methacrylate), preferably benzyl methacrylate. The sheath spinning mass is a gel of PMMA in MMA.

The PMMA polymers preferably used and the minimum gel concentrations required to spin a thread with sufficient strength are listed in Table I below. Table I

It is preferred that the core polymers used are those with the highest possible molecular weight. The molecular weight of the polymer is preferably lower than that in the core, but more preferably a mixture of higher molecular weight and lower molecular weight polymers is used in the core.

The spinning masses (gels) are prepared by dissolving the polymer(s) in the respective monomers at 80ºC with stirring. To avoid thermal crosslinking, the addition of the inhibitor (e.g. benzoquinone) to the spinning masses is preferred. An initiator (e.g. Irgacure® 369) is added to carry out UV crosslinking.

The spinning system is equipped with two piston pumps, one for the core and one for the sheath. The pumps are filled with the spinning mass by pouring them in and the spinning system is then assembled. Two lines lead from the piston pumps to a two-component spinneret, through which a core-sheath fiber is formed. By adjusting the output of the piston pumps, the composition of the fiber in relation to the volume ratio between sheath and core can be selected as desired (e.g. from 20/80 to 80/20). In technical practice, continuous dosing systems are preferred over piston pumps.

During spinning, the thread formed is surrounded by a glass tube in which an inert atmosphere (e.g. nitrogen flow) prevails to exclude oxygen (which has a detrimental effect on UV cross-linking). After a predetermined distance (within which the diffusion of the monomers takes place) a cross-linking section is provided which has a number of UV sources with a wavelength of their radiation preferably in the range of 300 to 450 nm. For example, a Philips Cleo Performance (313-370 nm) is used. Other suitable lamps include Philips Color 03 (340-400 nm), 05 (370 nm), 08 (360-370 nm) and 10 (340-400 nm). Preferably, the emission is in the range of 350-400 nm, since these wavelengths have an advantageous penetration depth and suitable initiators are available for them.

It should be noted that the ideal situation is when diffusion takes place in a first stage to form the desired refractive index profile and that in a second stage this profile is fixed by UV cross-linking. In general, however, these stages overlap in practice, since the cross-linking speed is of the same order of magnitude (seconds) as the diffusion speed. Experts are able to It is possible to determine the point in time at which crosslinking should begin to achieve the desired profile without a reasonable amount of experimentation and with the usual expertise in crosslinking over relatively long distances (thick layers, large diameters, ie millimetre scale rather than thin films in the nm range). Reference is made to J. Rad.Curing, 1980, 7(2), page 20, and to JG Woods, "Radiation curable adhesives", in: Radiation Curing: Science and Technology (SP Pappas, ed.), Plenum Publishers, New York, 1992, Chapter 9.

EXAMPLE

A solution of PMMA (trade name Elvacite 2041 from ICI, It = 540.0000 measured by HPLC) MMA was prepared with a concentration of 28 wt%. The solution contained 100 ppm benzoquinone as a thermal inhibitor and 0.1 wt% (±)-2-benzyl-2-(dimethylamino)-4'-morpholinobutyrophenone (Irgacure 369) as a UV initiator.

In the same way, a second solution of PMMA (Elvacite 2041) in BzMA, also with a concentration of 28 wt.%, was prepared.

In a spinning test, two high-pressure injection pumps were filled with these solutions. The two pumps are connected to a two-component spinneret. The spinning temperature was 75ºC. A two-component fiber with the solution with MMA in the sheath and the solution with BzMA in the core is spun through a capillary with a diameter of 1 mm into a nitrogen atmosphere. The extrusion speed was 0.6 m/min. The volume ratio of sheath to core was 50:50. The take-up speed was 1.2 m/min. Below the spinneret, a diffusion section with a length of 50 cm was formed, followed by a curing section with a length of 150 cm. The drawing off takes place in the diffusion section, so that the residence time in the cross-linking section was 75 seconds. The cross-linking sections contain 12 W tubes in a circle around the spun filament with a radiation between 213 and 370 nm (Philips Cleo Performance).

The light transmission of the manufactured fiber could be easily confirmed using a light tube.

COMPARISON EXAMPLE

In a comparative spinning test, the same procedure was followed but using PMMA with a lower molecular weight, namely Elvacite 2021 with Mw = 130,000, measured by HPLC. No thread formation was observed and the solution dripped from the spinneret.

Claims (9)

1. A method for producing a polymeric optical gradient fiber using a two-component spinning process in which a core spinning mass and a sheath spinning mass are used, the core spinning mass and the sheath spinning mass containing a polymer and a monomer, characterized in that the monomer in the core spinning mass has a higher refractive index than the monomer in the sheath spinning mass. 2. Process according to claim 1, characterized in that the spinning masses contain a UV initiator and that UV crosslinking follows spinning. 3. Process according to claim 2, characterized in that the UV crosslinking is effected by irradiation with a wavelength of 350 to 400 nm. 4. Process according to one of the preceding claims, characterized in that the core and shell masses are solutions of the polymer in the monomer and have a polymer concentration of 10 to 50 wt.%. 5. Process according to one of the preceding claims, characterized in that the core and shell polymers each have a molecular weight of over 100,000. 6. Process according to claim 5, characterized in that the polymers each have a molecular weight of over 500,000. 7. Process according to one of the preceding claims, characterized in that the shell and core polymers are chemically identical. 8. Process according to claim 7, characterized in that both the shell and the core polymers are polymethyl methacrylate. 9. Process according to claim 8, characterized in that the shell monomer is methyl methacrylate and the core monomer is benzyl methacrylate.