Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
2002-06-10
2004-09-07
Cheung, William (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C526S329700, C428S373000, C264S001240, C264S001280, C264S001290
Reexamination Certificate
active
06787602
ABSTRACT:
The present invention relates to a process for the manufacture of an optical fiber made of polymers with low transmission losses, this fiber comprising a core and a sheath, the core being formed from a first polymer based on methyl methacrylate and optionally on another (meth)acrylic ester and the sheath being formed from a second polymer having a lower refractive index than that of the core.
The present invention also relates to a plant for the implementation of this process.
One of the problems encountered by manufacturers of optical fibers made of polymers is that of reducing to a minimum the amount of defects, impurities and dusts in the core polymer, as they absorb or scatter light and thus accentuate the weakening of the light transmitted in the optical fiber.
A known process for the manufacture of an optical fiber made of polymers consists first in preparing a solid cylindrical rod formed from a first polymer based on methyl methacrylate and then secondly in melt fiberizing this solid cylindrical rod by extrusion. The second polymer, which acts to form the sheath of the optical fiber, can be applied by coextrusion or coating from a solution.
Such a process is disclosed in particular in French patent No. 2 405 806. One of the difficulties of such a process is the production of a solid cylinder of the polymer of the core, which is prepared by radical bulk polymerization of purified methyl methacrylate. Complete control of the polymerization and in particular of the exchanges of heat is essential in order to prevent any formation of bubbles.
Another known process for the manufacture of an optical fiber made of polymers consists of a continuous process according to which the monomers for forming the core of the fiber, essentially methyl methacrylate, the polymerization initiator and the chain-transfer agent are distilled and purified under sealed or leaktight conditions. A polymerization vessel is subsequently filled with the materials thus distilled, and the radical bulk polymerization is carried out by heating under reduced pressure.
The resulting polymer, the temperature of which is not lowered to the glass transition temperature or less, is continuously conveyed to a spinning device in order to subsequently obtain a fiber.
Such a process is disclosed in particular in French patent No. 2 493 997.
As in the preceding process, the radical bulk polymerization has to be fully and precisely controlled. This is because the control temperature is in this instance particularly important as the radical bulk polymerization of methyl methacrylate is highly exothermic and can dangerously accelerate. At the industrial level, the exothermicity of the polymerization reaction causes safety problems which are complex to manage.
However, despite the abovementioned disadvantages of the radical bulk polymerization of methyl methacrylate, this type of polymerization is currently recommended in numerous publications and is preferred to the aqueous suspension polymerization of methyl methacrylate.
Thus, according to French patent No. 2 493 997, the preparation by suspension polymerization of the core of the optical fiber made of poly(methyl methacrylate) requires a large amount of water, the resulting polymer then being easily contaminated by optical foreign materials present in this water. Furthermore, still according to this French patent, another disadvantage of this type of polymerization results from the possibility of the contamination of the poly(methyl methacrylate) by optical foreign materials during the stage of dehydration of the polymer. Moreover, it is also specified, in this French patent, that a stage of pelletizing or of preshaping of the resulting polymer is required for melt shaping or spinning the polymer. In point of fact, there is then reason to fear that the polymer will be contaminated by optical foreign materials during the stage of pelletizing for the polymer or during a stage of feeding such a polymer in the form of pellets to a device for the manufacture of fibers, or that the polymer will be oxidized by the air, as the device for the preparation of the polymer is in the majority of cases separated from the device for the manufacture of the fibers.
According to the Japanese patent applications published before examination under the numbers 58 88,701 and 58 88,702, a significant loss of optical transmission of 890 db/km (at a wavelength of 646 nm) and of 1060 db/km (at a wavelength of 577 nm) is observed when the poly(methyl methacrylate) is prepared by aqueous suspension polymerization.
After much research by the present inventors, it has been found, surprisingly, that it is possible to manufacture an optical fiber made of polymers exhibiting low transmission losses, this fiber comprising a core and a sheath, the core being formed from a first polymer obtained by aqueous suspension polymerization of methyl methacrylate and optionally of another (meth)acrylic ester and the sheath being formed from a second polymer having a lower refractive index than that of the core.
More specifically, a subject matter of the present invention is a batchwise process for the manufacture of an optical fiber made of polymers, this fiber comprising a core and a sheath, the core being formed from a first polymer based on methyl methacrylate and optionally on a (meth)acrylic ester other than methyl methacrylate and the sheath being formed from a second polymer having a lower refractive index than that of the core.
The process according to the invention is characterized in that it is implemented in an in-line plant ranging from a device for the purification of the starting materials to a spinning device, involving the intermediacy of the various devices of the in-line plant and the various transfer means connecting the various devices of the in-line plant, this plant being leaktight to the external air and to dust and sheltered from light, in particular ultraviolet radiation.
In addition, the process according to the invention comprises the following stages:
(1) beads of the first polymer are prepared by suspension polymerization of purified methyl methacrylate and optionally of at least one purified (meth)acrylic ester other than methyl methacrylate in demineralized, filtered and deoxygenated water, the polymerization being carried out in the presence of at least one radical polymerization initiating agent, of at least one chain-transfer agent and of at least one suspending agent and in the virtually complete absence of polymerization inhibitor and of impurities, such as:
(a) biacetyl, in an amount reduced to at most 1 ppm with respect to the total amount of monomers introduced into the polymerization reactor;
(b) transition metal ions capable of giving strong light absorption in the visible region;
(c) dust and particles, the various abovementioned starting materials used in the suspension polymerization having been filtered before polymerization with a filtration threshold of 0.1 micron;
the polymerization also being carried out with stirring, under an atmosphere of an inert and dedusted gas;
(2) on conclusion of stage (1), the beads are separated and washed using demineralized and dedusted water and are dried under an atmosphere of a dedusted and preferably inert gas, and the dried beads are stored under this atmosphere in at least one intermediate tank;
(3) at least a portion of the beads obtained on conclusion of stage (2) is transferred, under an atmosphere of a dedusted and preferably inert gas, from the intermediate tank or tanks to a coextrusion device and the core of the fiber, starting from said beads, and the sheath of the fiber, starting from a polymer having a lower refractive index than that of the core, are coextruded;
(4) the fiber obtained at the outlet of the coextrusion device is cooled in a gradual and controlled fashion, so as to avoid quenching the first polymer constituting the core of the fiber, and the fiber is drawn, in order to obtain a fiber with a mean total diameter which can vary from 250 to 2 000 microns.
A first important characteristic of the process acc
Chenet Pierre
Erout Marie-Noëlle
Cheung William
McCarter & English LLP
Optectron Industries
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