Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Rod – strand – filament or fiber
Reexamination Certificate
1999-06-07
2001-03-27
Edwards, Newton (Department: 1774)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Rod, strand, filament or fiber
C428S362000, C428S369000, C428S399000
Reexamination Certificate
active
06207273
ABSTRACT:
TECHNICAL FIELD
The present invention relates to processes for producing a heat-resistant filament and filament assembly of high strength and high elastic modulus each composed of a thermotropic liquid crystal polymer. Further, the invention relates to processes for producing a filament and a filament assembly in which a thermotropic liquid crystal polymer is mixed with another extrudable polymer and to the above filament assembly.
BACKGROUND ART
Since the emergence of a thermotropic liquid crystal polymer, the heat resistance thereof and the attainability of high strength and high elastic modulus thereby have been noted and thus some prior arts have been developed with respect to the process for producing the fiber (see, for example, U.S. Pat. Nos. 3,975,487, 4,468,364 and 4,161,470 and Japanese Patent Laid-Open Gazette No. 196716/1988).
It is known that the thermotropic liquid crystal polymer can have high strength and high elastic modulus only by spinning if it Is performed under appropriate conditions. Further, it is known that the heat treatment and redrawing can improve the strength and elastic modulus thereof. In particular, it is reported that some types exhibit a strength improvement to 5 to 6 times the original.
In the spinning of the above conventional thermotropic liquid crystal polymer filament, it has been necessary to conduct the spinning with the use of a nozzle having a very minute aperture for obtaining a fine denier filament because it is difficult to increase the draft ratio thereof. Further, extrusion abnormalities such as melt fracture are likely to occur. Thus, the extrusion rate cannot be made high resulting in extremely poor productivity.
Therefore, with respect to the properties of the obtained filament, no processes have been established for stably producing a filament with the ultimately high strength and elastic modulus at high productivity except on a laboratory scale.
The inventors have conducted extensive investigations into the causes of the low productivity of the prior art, the difficulty in stably producing a product with high strength and high elastic modulus and the poor spinning operation efficiency (end breakage, denier nonuniformity and product quality dispersion, etc.) in connection with the melt spinning of the thermotropic liquid crystal polymer filament. As a result, the following has been found.
(1) The thermotropic liquid crystal polymer as a starting material is not always uniform.
The cause is in an aspect an inevitable consequence of the technology of polymerizing the thermotropic liquid crystal polymer. In other aspects, the cause relates to the heat history difference and heat deterioration after polymerization and the increase of polymerization degree by heat treatment. With respect to these, a rapid product quality improvement has been attained for the recent years by virtue of, for example, polymerization and subsequent treatment technologies and post-polymerization filter technologies. However, the improvement is still not satisfactory.
(2) The melting point of the thermotropic liquid crystal polymer is so high that, after leaving the spinning nozzle, the surface of the filament is cooled by the temperature of the atmosphere with the result that the draft ratio cannot be made high.
Thus, a skin layer is formed at the surface of the filament, thereby creating a structural difference between the inner part and the surface part. This is an obstacle of the high quality (for example, high strength, elastic modulus and elongation). In the spinning of the customary thermoplastic polymers, the draft ratio can be increased in the form of a melt having left the nozzle to thereby orient the molecules. However, with respect to the thermotropic liquid crystal polymer, greater belief is in that the orientation is completed in the nozzle, and the thought has not arrived at a concept of stably increasing the draft ratio upon leaving the nozzle.
(3) The nozzle diameter is reduced for increasing the orientation in the nozzle. Further, the reduction of the nozzle diameter is inevitable for obtaining a fine denier because the above draft ratio cannot be increased.
However, the extrusion rate is proportional to fourth power of the nozzle diameter, so that the reduction of the nozzle diameter leads to extremely poor productivity. Further, the shear rate of the extrusion is inversely proportional to third power of the nozzle diameter, so that the reduction of the nozzle diameter leads to extremely high shear rate, thereby causing extrusion abnormalities such as melt fracture. With the use of a polymer having instability factors in its starting material such as the thermotropic liquid crystal polymer, stable operation cannot be conducted at shear rates close to the extremity.
(4) The thermotropic liquid crystal polymer filament is a functional fiber with high strength, elastic modulus, chemical resistance and heat resistance and further excellent electrical properties. However, it is also an industrial fiber, so that how cheaply the thermotropic liquid crystal polymer can be produced is important.
However, because of the above factors (1) to (4) affecting in combination, it has been unfeasible to stably produce a fine denier filament of thermotropic liquid crystal polymer with high strength and high elastic modulus on an industrial scale.
Often, the thermotropic liquid crystal polymer filament is used as an industrial reinforcing fiber in FRP, FRTP, concrete reinforcement and the like. In such uses, the thermotropic liquid crystal polymer must exhibit improved affinity for, adhesion to and uniform miscibility with the matrix.
The above thermotropic liquid crystal polymer has only been formed into fibers but not into a nonwoven web or a filament assembly because special spinning means and subsequent heat treatment are required therefor. In a simple assembly of rigid thermotropic liquid crystal polymer filaments, the filaments cannot be mutually entangled, so that the assembly is readily disintegrated with external force, thereby disenabling the holding of the outline as a filament assembly. Although the fixing with an adhesive can be thought of, the use of the adhesive is unfavorable because it generally degrades the heat resistance and electrical properties of the filament assembly. Further, heat-resistant adhesives are expensive.
A technique comprising shortly cutting the conventionally produced thermotropic liquid crystal polymer filament to thereby form the same into a nonwoven web or a paper has been reported (EPC Patent Laid-Open Gazette No. 167682 (A)). However, not only is the reinforcing effect of short fibers poor in the use in FRP and FRTP but also additional steps such as adhesive bonding and fibrillation for formation into a nonwoven web or a paper are inevitable. The fibrillation has a drawback of degrading the performance of the highly elastic fiber.
The heretofore proposed processes for producing a filament assembly or a nonwoven web from the thermotropic liquid crystal polymer filaments have drawbacks in that not only is a cost increase inevitable but also the production of a nonwoven web or filament assembly of long-fiber filaments is difficult and the quality of the resultant product is poor. Specifically, there are problems such that a binder is requisite for uniformity and filament integration and that the filaments are not loosened.
A filament of high strength and high elastic modulus cannot be obtained by melt spinning the thermotropic liquid crystal polymer through the melt spinning nozzle conventionally employed in the filament spinning followed by free fill and flow. Instead, the filament diameter is unfavorably large and an impracticable filament assembly (nonwoven web) results.
The reason has been revealed to be that the filament having exited the nozzle is oriented by the nozzle shear rate to thereby have increased strength and further the surface of the filament is cooled to solidify because of high melting point, so that only the weight thereof does not lead to application of a draft with the result that the filame
Kurihara Kazuhiko
Mazawa Yoichi
Oishi Toshikazu
Yazawa Hiroshi
Bucknam and Archer
Edwards Newton
Polymer Processing Research Institute Limited
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