EP0783603B1 - Procede d'elaboration de filaments et de fibres de polybenzazole - Google Patents

Procede d'elaboration de filaments et de fibres de polybenzazole Download PDF

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Publication number
EP0783603B1
EP0783603B1 EP95931706A EP95931706A EP0783603B1 EP 0783603 B1 EP0783603 B1 EP 0783603B1 EP 95931706 A EP95931706 A EP 95931706A EP 95931706 A EP95931706 A EP 95931706A EP 0783603 B1 EP0783603 B1 EP 0783603B1
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EP
European Patent Office
Prior art keywords
filament
water
washing
fiber
dope
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired - Lifetime
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EP95931706A
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German (de)
English (en)
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EP0783603A1 (fr
Inventor
Ashish Sen
Yoshihiko Teramoto
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Toyobo Co Ltd
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Toyobo Co Ltd
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Publication date
Priority claimed from US08/500,651 external-priority patent/US5525638A/en
Application filed by Toyobo Co Ltd filed Critical Toyobo Co Ltd
Publication of EP0783603A1 publication Critical patent/EP0783603A1/fr
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Publication of EP0783603B1 publication Critical patent/EP0783603B1/fr
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Classifications

    • D—TEXTILES; PAPER
    • D01—NATURAL OR MAN-MADE THREADS OR FIBRES; SPINNING
    • D01F—CHEMICAL FEATURES IN THE MANUFACTURE OF ARTIFICIAL FILAMENTS, THREADS, FIBRES, BRISTLES OR RIBBONS; APPARATUS SPECIALLY ADAPTED FOR THE MANUFACTURE OF CARBON FILAMENTS
    • D01F6/00—Monocomponent artificial filaments or the like of synthetic polymers; Manufacture thereof
    • D01F6/58—Monocomponent artificial filaments or the like of synthetic polymers; Manufacture thereof from homopolycondensation products
    • D01F6/74—Monocomponent artificial filaments or the like of synthetic polymers; Manufacture thereof from homopolycondensation products from polycondensates of cyclic compounds, e.g. polyimides, polybenzimidazoles

Definitions

  • This invention relates to processes for the preparation of polybenzazole fibers and fiber filaments.
  • Fibers prepared from polybenzoxazole (PBO) and polybenzothiazole (PBT) may be prepared by first extruding a solution of polybenzazole polymer in a mineral acid (a polymer "dope") through a die or spinneret to prepare a dope filament.
  • the dope filament is then drawn across an air gap, with or without stretching, and then coagulated in a bath comprising water or a mixture of water and a mineral acid. If multiple filaments are extruded simultaneously, they may then be combined into a multifilament fiber during or after the coagulation step.
  • the filament or fiber is then washed in a washing bath to remove most of the mineral acid, and then dried.
  • the physical properties of such filaments and fibers such as tensile strength, are known to be relatively high. However, further improvement in such properties is desirable.
  • this invention is a process for removing polyphosphoric acid from a polybenzazole dope filament, which comprises:
  • Polybenzazole dope filaments for use in the process of the present invention may be prepared by the extrusion of a polybenzazole dope through an extrusion die with a small diameter or a "spinneret".
  • the polybenzazole dope comprises a solution of polybenzazole polymer in polyphosphoric acid.
  • polybenzazole refers to polybenzoxazole (“PBO”) and polybenzothiazole (“PBT”). PBO, PBT and random, sequential and block copolymers of PBO and PBT are described in references such as Wolfe et al., Liquid Crystalline Polymer Compositions, Process and Products, U.S.
  • Patent 4,703,103 (October 27, 1987); Wolfe et al., Liquid Crystalline Poly(2,6-Benzothiazole) Compositions, Process and Products, U.S. Patent 4,533,724 (August 6, 1985); Wolfe, Liquid Crystalline Polymer Compositions, Process and Products, U.S. Patent 4,533,693 (August 6, 1985); Evers, Thermo-oxidatively Stable Articulated p-Benzobisoxazole and p-Benzobisthiazole Polymers, U.S. Patent 4,359,567 (November 16, 1982); Tsai et al., Method for Making Heterocyclic Block Copolymer, U.S. Patent 4,578,432 (March 25,1986); 11 Ency.
  • the polybenzazole polymer may be rigid rod, semi-rigid rod or flexible coil. It is preferably a lyotropic liquid-crystalline polymer, which forms liquid-crystalline domains in solution when its concentration exceeds a critical concentration.
  • the intrinsic viscosity of rigid polybenzazole polymers in methanesulfonic acid at a temperature of 25°C is preferably at least about 10 dL/g, more preferably at least about 15 dL/g and most preferably at least about 20 dL/g.
  • the dope should contain a high enough concentration of polymer for the polymer to form an acceptable filament after extrusion and coagulation.
  • concentration of polymer in the dope is preferably high enough to provide a liquid-crystalline dope.
  • the concentration of the polymer is preferably at least about 7 weight percent, more preferably at least about 10 weight percent and most preferably at least about 14 weight percent. The maximum concentration is limited primarily by practical factors, such as polymer solubility and dope viscosity.
  • the concentration of polymer is preferably no more than 30 weight percent, and more preferably no more than about 20 weight percent.
  • Suitable polybenzazole polymers or copolymers and dopes can be synthesized by known procedures, such as those described in Wolfe et al., U.S. Patent 4,533,693 (August 6, 1985); Sybert et al., U.S. Patent 4,772,678 (September 20, 1988); Harris, U.S. Patent 4,847,350 (July 11,1989); and Gregory et al., U.S. Patent 5,089,591 (February 18, 1992).
  • suitable monomers are reacted in a solution of nonoxidizing and dehydrating acid under nonoxidizing atmosphere with vigorous mixing and high shear at a temperature that is increased in step-wise or ramped fashion from no more than 120°C to at least 190°C.
  • the dope may then be formed into a filament by extrusion through a spinneret, and drawing the filament across a gap. Suitable processes are described in the references previously incorporated and U.S. Patent 5,034,250.
  • the spinneret preferably contains a plurality of holes. The number of holes in the spinneret and their arrangement is not critical to the invention, but it is desirable to maximize the number of holes for economic reasons.
  • the spinneret may contain as many as 100 or 1000 or more, and they may be arranged in circles, grids, or in any other desired arrangement.
  • the spinneret may be constructed out of ordinary materials that will not be degraded by the dope, such as stainless steel.
  • Dope exiting the spinneret enters a gap between the spinneret and the coagulation bath.
  • the gap is typically called an "air gap” although it need not contain air.
  • the gap may contain any fluid that does not induce coagulation or react adversely with the dope, such as air, nitrogen, argon, helium or carbon dioxide.
  • the dope is preferably drawn to a spin-draw ratio of at least about 20, highly preferably at least about 40, more preferably at least about 50 and most preferably at least about 60.
  • the spin-draw ratio is defined in this application as the ratio between the take-up velocity of the filaments and the capillary velocity (v c ) of the dope in the spinneret.
  • the shear rate at the spinneret hole wall is preferably in the range of from 1800 to 6500 s -1 . The draw should be sufficient to provide a filament having the desired diameter.
  • step (a) of the process of the invention the dope filament is contacted with water or a mixture of water and polyphosphoric acid under conditions sufficient to reduce the phosphorous content of the filament to less than 10,000 ppm by weight. This may be carried out as a single operation in one washing apparatus, or the filament may travel through several baths or washing cabinets to reduce the phosphorous content to the desired level. If a mixture of water and polyphosphoric acid is used, the concentration of polyphosphoric acid in solution should be lower than that contained in the filament in order to effectively wash the filament. Such mixtures are preferably used in the initial stages of washing, since gradual removal of polyphosphoric acid from a multifilament fiber tends to improve its physical properties.
  • the filament is first "coagulated" in a coagulation bath containing water or a mixture of water and polyphosphoric acid, which removes enough of the solvent to prevent substantial stretching of the filament during any subsequent processing.
  • the filament may then be further washed in a multi-step process.
  • coagulation does not necessarily imply that the dope is a flowing liquid and changes into a solid phase.
  • the dope may be at a temperature low enough so that it is essentially non-flowing before the coagulation step begins.
  • the amount of solvent removed during the coagulation step will depend on the residence time of the filament in the coagulation bath, the temperature of the bath and the concentration of solvent therein. For example, using a 20 weight percent solution of polyphosphoric acid at a temperature of about 23°C, a residence time of about one second will remove about 70 percent of the solvent present in the filament.
  • washing of the filament is carried out in a continuous process by running the filament through a series of baths or washing cabinets
  • Washing cabinets typically comprise an enclosed cabinet containing one or more rolls which the filament travels around a number of times, and across, prior to exiting the cabinet. As the filament travels around the roll, it is sprayed with a washing fluid. The washing fluid is continuously collected in the bottom of the cabinet and drained therefrom.
  • the surface of the filament is not allowed to dry after the coagulation step starts and before the washing step(s) are completed. It is theorized, without intending to be bound, that the wet, never-dried surface of the filament is relatively porous and provides paths to wash residual phosphorus from inside the filament. On the other hand, it is theorized that the pores close when they become dry and do not open even when they become wet again. The closed pores trap residual phosphorus inside the filament.
  • the temperature of the coagulation bath is preferably at least about 10°C, more preferably at least about 25°C, and is preferably no greater than about 50°C, more preferably no greater than about 40°C.
  • the residence time of the filament in the coagulation bath is preferably at least about 1 second, and is preferably no more than about 5 seconds.
  • the concentration of acid in the coagulation bath is preferably at least about 0.5 percent by weight, more preferably at least about 20 percent, and is preferably no greater than about 40 percent, more preferably no greater than about 25 percent. For a continuous process, it is preferable to use as low temperature and high a solvent content as is practical, so that the solvent may be removed as slowly as possible.
  • the temperature of the washing fluid(s) are preferably at least about 25°C, more preferably at least about 50°C, and is preferably no greater than about 120°C, more preferably no greater than about 100°C.
  • the washing fluid may also be applied in vapor form (steam), but is more conveniently used in liquid form.
  • the residence time of the filament in the washing bath(s) will depend on the desired concentration of residual phosphorus in the filament, but typical residence times are in the range of from 180 seconds to 300 seconds.
  • the duration of the entire washing process utilized in the first step of the process of the invention is preferably no greater than about 200 seconds, more preferably no greater than about 160 seconds.
  • the concentration of phosphorous in the filament is preferably brought down as slowly as is practical in the coagulation and washing operations, given that for such processes, fewer steps and higher line speeds are desirable. It is believed that a slower reduction in the phosphorous concentration in the filament provides a filament which has better physical properties. It is also believed that this result is more efficiently achieved in a continuous multi-step operation, utilizing a series of baths or washing cabinets, by decreasing the concentration of acid in the washing bath as the filament proceeds down the washing line. Conveniently, the washing fluid residue collected after the last washing step may be used as the washing fluid in the next-to-last washing step, and so forth up the line, with washing fluid containing the highest acid concentration being used in the first washing step.
  • the concentration of acid in the washing baths or cabinets is preferably at least about 0.2 percent by weight, and is preferably no greater than about 40 percent by weight.
  • the residual concentration of phosphorous in the filament after step (a) of the process is preferably less than about 8,000 ppm, more preferably less than about 6,000 ppm, and most preferably less than about 4,000 ppm.
  • the residual phosphorus content of a substantially dry filament may be measured using X-ray fluorescence techniques described in E.P. Bertin, Principles and Practice of X-Ray Spectrometric Analysis - Second Ed. (Plenum Press 1984). Suitable equipment is commercially available under the trade name KEVEX 770 XRF and from Philips Electronic Instruments.
  • the filament utilized in the process of the invention may be combined into a multifilament fiber at any point during the process of the invention. Preferably, however, the filaments are combined just prior to, or during, coagulation. While the term "filament" is used throughout this application to describe the process of the invention, the process of the invention may of course also be carried out on a filament contained in a multifilament fiber, utilizing the same process parameters as described herein for use with a single filament.
  • the filament is preferably under tension during at least part of the washing process. More preferably, tension is also applied throughout the coagulation and washing process, particularly when the fluid temperature is very high. The tension is preferably sufficient to prevent the filament from shrinking or relaxing.
  • the dope filament is contacted with an aqueous solution of an inorganic base under conditions sufficient to convert at least about 50 percent of the acid groups present in the filament to the corresponding salt form (hereafter "neutralization step").
  • neutralization step may likewise be carried out in a single operation, or the filament may travel through several baths or washing cabinets to reduce the phosphorous content to the desired level. Preferably, however, this step is carried out in a single washing cabinet as described above.
  • suitable water-soluble bases include sodium hydroxide, ammonium hydroxide, sodium carbonate, and sodium bicarbonate.
  • the percentage of acid groups which have been converted may be followed by any suitable technique, such as nuclear magnetic resonance spectroscopy (NMR) or Fourier transform infrared spectroscopy (FTIR).
  • the concentration of base in the solution is preferably at least about 0.2 weight percent, more preferably at least about 0.4 weight percent, and is preferably no greater than about 1.2 weight percent, more preferably no greater than about 0.8 weight percent.
  • the duration of this second step will depend on the concentration of the base, with longer residence times required for lower concentrations, but is preferably no greater than about 120 seconds, more preferably no greater than about 60 seconds. At least 50 percent of the acid groups remaining after step (a) are converted to their salt form, preferably at least about 75 percent, and most preferably at least about 95 percent are so converted.
  • the preferred pH of the base solution used in the neutralization step will depend on the duration of the step, with a higher pH preferred with a shorter duration, but is preferably in the range of from 10 to 14, more preferably in the range of from 11 to 12.
  • the concentration of base and residence times are preferably selected to achieve a stoichiometric ratio of base:acid groups in the fiber of at least 0.5:1.0, more preferably at least 0.75:1.00, and is preferably no greater than 1.5:1.0, more preferably no greater than 1.25:1.0, but is most preferably 1:1.
  • the stoichiometry of the process can be determined by a suitable method, such as by measuring the ratio of phosphorous to the conjugate acid of the inorganic base in the fiber after the neutralization step. For example, if sodium hydroxide is used, the ratio of phosphorous:sodium in the fiber may be measured by a suitable technique such as Neutron Activation Analysis.
  • the process of the present invention is run in a continuous fashion with a line speed of at least 50 m/minute.
  • the line speed is highly preferably at least about 200 m/minute, more preferably at least about 400 m/minute and most preferably at least about 600 m/minute.
  • the fiber is preferably washed further with water for a residence time of at least about 1 second to remove most of the residual base.
  • the particular washing conditions will depend on the amount of residual base present, with longer residence times required to remove greater amounts of base.
  • the filament may be dried, heat-treated, and/or wound on rolls as desired, as described, for example, in U.S. Patent 5,296,185.
  • Multifilament fibers containing PBZ polymers may be used in ropes, cables, fiber-reinforced composites and cut-resistant clothing.
  • PBO polybenzoxazole
  • PPA polyphosphoric acid
  • PBO filaments were extruded at a temperature between 165°C out of a 180 ⁇ m spinneret with 42 holes into a coagulation bath, and combined into a multifilament fiber.
  • a glass shroud was placed in the air gap, between the spinneret face and the surface of the coagulation bath liquid in order to minimize air currents in the air gap.
  • the filaments were produced by using a shear rate at the spinneret hole wall of about 3500 s -1 .
  • the spin-draw ratio utilized is 44, with a fiber take-up speed of 200 m/minute.
  • the resulting filaments had a fineness (denier) of 1.7 dtex (1.5 denier) per filament and a diameter of 11.5 ⁇ m.
  • the fibers were coagulated in a bath of water and polyphosphoric acid having an acid content of about 20 percent by weight.
  • the residence time in the coagulation stage was about 0.5 seconds and the temperature was about 10°C.
  • the fibers were then washed off-line with water (as comparative examples), or in a three-step process using water, a 0.05 weight percent aqueous solution of sodium hydroxide, and water, using a washing temperature of about 23°C.
  • the fiber was dried under nitrogen at room temperature (23°C) for an additional 48 hours.
  • a portion of the samples were heat-set through a nitrogen-purged tube furnace with a residence time of 2 seconds at 600°C.
  • a constant tension of about 3.1 cN/dtex (3.5 g/denier) was maintained on the fiber during heat setting.
  • Residual phosphorus was measured using X-ray fluorescence on a Philips PW1404/DY685 sequential spectrometer with scandium X-ray tubes and fiber samples which had been pressed into a pellet for analysis. The tensile strength retention and intrinsic viscosity of each fiber was then measured, both before and after heat-treatment. The retention of tensile strength (TSR), defined as (photo-aged tensile strength/initial tensile strength) x 100 percent, was used for expressing the retention of tensile strength after photo-aging, although separate samples were used for each measurement. Photo-aging was carried out in an Atlas Model Ci65A weatherometer with a xenon lamp and borosilicate filter. Fiber strands were mounted on sample holders and photo-exposed in the weatherometer. The exposure was 765 doewatt/m 2 with a 300 to 800 nm wave length for a total of 100 hours.
  • the procedure used for measurement of tensile strength was as follows: Tensile properties were measured in accordance with ASTM D-2101, on an Instron 4201 universal testing machine. A 4.55 kg (10 pound) load cell was used with a crosshead speed of 2.54 cm (1.0 inches)/minute, and a gauge length of 25.4 cm (10.0 inches). Tensile data was obtained on the 42-filament fibers with a twist factor of 6 to 7. The intrinsic viscosity (IV) of the fiber samples was measured by dissolving them in methanesulfonic acid, and measuring the intrinsic viscosity at 23°C.
  • fiber samples comprised of filaments with a fineness (denier) of 1.7 dtex (1.5 denier) per filament and a diameter of 11.5 ⁇ m were prepared, coagulated in water for 1 second, washed in water for 10 minutes, and contacted with a 0.1 N aqueous solution of a base for 10 minutes.
  • Example 12 the samples were subsequently washed with water at room temperature for 24 hours. The tensile strength of the samples were measured, and heat-treatment was carried out, as described in Examples 1-10.
  • the residual sodium and phosphorous content of the fiber was also shown (Na), as measured by Neutron Activation Analysis. The data is shown in Tables IIa and llb.
  • Example Base 2nd Wash P (ppm) Na (ppm) 11 NaOH No 5000 9000 12 NaOH Yes 2400 940 13 Na 2 CO 3 No 5300 14000

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  • Chemical & Material Sciences (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • General Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Textile Engineering (AREA)
  • Artificial Filaments (AREA)
  • Spinning Methods And Devices For Manufacturing Artificial Fibers (AREA)

Claims (10)

  1. Procédé permettant d'éliminer l'acide polyphosphorique d'un filament de solution à filer contenant un polybenzazole, lequel procédé comporte :
    a) le fait de mettre le filament de solution à filer en contact avec de l'eau ou avec un mélange d'eau et d'acide polyphosphorique, dans des conditions suffisantes pour réduire à moins de 10 000 ppm en poids la teneur en phosphore du filament, puis
    b) le fait de mettre le filament de solution à filer en contact avec une solution aqueuse d'une base inorganique, dans des conditions suffisantes pour transformer au moins 50 % en poids des groupes de type acide polyphosphorique présents dans le filament en sel de cette base et de cet acide,
    ce procédé étant mis en oeuvre en mode continu à une vitesse d'avance du filament d'au moins 50 m/min.
  2. Procédé conforme à la revendication 1, dans lequel l'étape (a) comporte les étapes successives consistant
    1) à faire coaguler le filament dans un bain de coagulation, et
    2) à laver le filament dans au moins un bain de lavage distinct.
  3. Procédé conforme à la revendication 2, dans lequel la durée du séjour du filament dans le bain de coagulation vaut au moins 1 seconde et au plus 5 secondes.
  4. Procédé conforme à la revendication 2, dans lequel la durée cumulée du séjour du filament dans le ou les bains de lavage ne vaut pas plus de 200 secondes.
  5. Procédé conforme à la revendication 1, dans lequel la concentration résiduelle de phosphore dans le filament après l'étape (a) est inférieure à 8 000 ppm.
  6. Procédé conforme à la revendication 1, dans lequel au moins 75 % des groupes acides restants après l'étape (a) sont salifiés au cours de l'étape (b).
  7. Procédé conforme à la revendication 1, dans lequel au moins 95 % des groupes acides restants après l'étape (a) sont salifiés au cours de l'étape (b).
  8. Procédé conforme à la revendication 1, caractérisé en ce qu'il est mis en oeuvre en mode continu à une vitesse d'avance du filament d'au moins 200 m/min.
  9. Procédé conforme à la revendication 1, dans lequel on lave la fibre avec de l'eau, après l'étape (b), pendant une durée de séjour d'au moins 1 seconde.
  10. Procédé conforme à la revendication 1, dans lequel, au cours de l'étape (b), le rapport stoechiométrique de la base aux groupes acides présents dans la fibre vaut moins de 1,25/1,0.
EP95931706A 1994-09-30 1995-09-08 Procede d'elaboration de filaments et de fibres de polybenzazole Expired - Lifetime EP0783603B1 (fr)

Applications Claiming Priority (5)

Application Number Priority Date Filing Date Title
US500651 1990-03-28
US31626694A 1994-09-30 1994-09-30
US316266 1994-09-30
US08/500,651 US5525638A (en) 1994-09-30 1995-07-12 Process for the preparation of polybenzazole filaments and fibers
PCT/US1995/011229 WO1996010661A1 (fr) 1994-09-30 1995-09-08 Procede d'elaboration de filaments et de fibres de polybenzazole

Publications (2)

Publication Number Publication Date
EP0783603A1 EP0783603A1 (fr) 1997-07-16
EP0783603B1 true EP0783603B1 (fr) 1999-12-08

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EP95931706A Expired - Lifetime EP0783603B1 (fr) 1994-09-30 1995-09-08 Procede d'elaboration de filaments et de fibres de polybenzazole

Country Status (6)

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EP (1) EP0783603B1 (fr)
CN (1) CN1159836A (fr)
CA (1) CA2199514A1 (fr)
DE (1) DE69513844T2 (fr)
MX (1) MX9702304A (fr)
WO (1) WO1996010661A1 (fr)

Families Citing this family (10)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE60128915T2 (de) * 2000-04-28 2008-02-14 Toyo Boseki K.K. Polybenzazolfaser und ihre verwendung
ES2364969T3 (es) * 2001-11-23 2011-09-19 Inventio Ag Ascensor con medios de transmisión en forma de correa, en particular con una correa trapezoidal de dentado interior como medio portante y/o agente motor.
ATE409761T1 (de) 2002-06-26 2008-10-15 Toyo Boseki Polybenzazolfaser und deren verwendung
US7189346B2 (en) 2004-07-22 2007-03-13 E. I. Du Pont De Nemours And Company Polybenzazole fibers and processes for their preparation
KR20080034830A (ko) * 2005-03-28 2008-04-22 이 아이 듀폰 디 네모아 앤드 캄파니 측쇄 히드록실 기 및 양이온을 가진 폴리아렌아졸 중합체섬유
ATE495287T1 (de) 2005-03-28 2011-01-15 Du Pont Verfahren zur entfernung von phosphor von einer faser oder einem garn
JP4769291B2 (ja) 2005-03-28 2011-09-07 イー・アイ・デュポン・ドウ・ヌムール・アンド・カンパニー 紡績ヤーン中のポリ燐酸に加水分解を受けさせる方法
JP4769295B2 (ja) * 2005-03-28 2011-09-07 イー・アイ・デュポン・ドウ・ヌムール・アンド・カンパニー 紡績マルチフィラメントヤーン中のポリ燐酸に加水分解を融合無しに受けさせる方法
DE602006019572D1 (de) * 2005-03-28 2011-02-24 Du Pont Heisse oberflächen-hydrolyse von polyphosphorsäure in spun-garnen
US7888457B2 (en) * 2005-04-01 2011-02-15 E. I. Du Pont De Nemours And Company Process for removing phosphorous from a fiber or yarn

Family Cites Families (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5273703A (en) * 1992-08-13 1993-12-28 The Dow Chemical Company Process for post-spin finishing of polybenzoxazole fibers
US5292469A (en) * 1993-01-05 1994-03-08 The Dow Chemical Company Process for coagulation, washing and leaching of shaped polybenzazole articles
US5393478A (en) * 1993-08-20 1995-02-28 The Dow Chemical Company Process for coagulation and washing of polybenzazole fibers

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Publication number Publication date
EP0783603A1 (fr) 1997-07-16
MX9702304A (es) 1997-06-28
DE69513844D1 (de) 2000-01-13
WO1996010661A1 (fr) 1996-04-11
DE69513844T2 (de) 2000-07-27
CN1159836A (zh) 1997-09-17
CA2199514A1 (fr) 1996-04-11

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