Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Rod – strand – filament or fiber
Patent
1998-04-06
2000-11-07
Edwards, N.
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Rod, strand, filament or fiber
428394, D01F 618
Patent
active
061434116
DESCRIPTION:
BRIEF SUMMARY
The invention relates to methods of manufacture of hollow polymeric fibres by wet spinning, to a multi-hole spinneret for use in such manufacture, and to a method of production of hollow carbon fibre from hollow polymeric fibre, specifically polyacrylonitrile.
BACKGROUND OF THE INVENTION
Spinning has been defined as the transformation of a liquid material into a solid fibre. There are three main methods for spinning fibres: melt spinning, dry spinning and wet spinning. These methods can be combined depending on the final properties required of the material (such as a polymer) being spun.
Melt spinning is preferred if the polymer can be melted without degradation and is a common method for spinning thermoplastics such as polypropylene and nylon. The molten polymer is extruded through a spinneret into a gaseous medium such as air where the fibre cools producing solid, non-porous fibre. The filament is usually then drawn to orientate the polymer molecules which also improves the tensile properties of the fibre.
Dry spinning involves the extrusion of a polymer dope (polymer dissolved in an appropriate solvent) into a heated zone where the solvent evaporates. This is a slower process than the cooling of melt spun fibres and, as a result tends to produce fibres with non-uniform properties and a less circular cross section.
Wet spinning is identical to dry spinning except in the way the solvent is removed from the extruded filaments. Instead of evaporating the solvent, the fibre is spun into a liquid bath containing a solvent
on-solvent mixture called the coagulant. The solvent is nearly always the same as that used in the dope and the non-solvent is usually water.
Dry and wet spinning can be combined to form a process known as dry jet wet spinning. Polymer dissolved in a suitable solvent is extruded into a gap before entering a coagulation bath containing a coagulant that is miscible with the solvent but not with the polymer. A phase inversion process takes place producing a solid fibre. The bath can contain a mixture of solvent and non-solvent. This method helps prevent blockage of the spinneret and also allows some drawing of the fibre prior to coagulation, increasing orientation of the polymer molecules. The air gap has been shown to produce fibres that are stronger and more extensible than fibres produced from an immersed jet.
The fibre microstructure is established in the coagulation bath and requires optimisation of conditions. The critical process is the transition from a liquid to a solid phase within the fibrils and there are two possible such transitions. One is phase inversion--the precipitation of polymer to form a solid phase, the other is gelation. The former yields fibre of poor mechanical properties where as the latter produces an elastic gel giving rise to a fine microstructure once the solvent is removed. For membrane-type fibres phase inversion is preferable. For fibres with the appearance of a solid wall phase inversion should be slowed down so that gelation precedes phase inversion. Conditions in the coagulation bath have, therefore, to be optimised so that gelation precedes phase inversion. It has been shown that gelation occurs more rapidly at lower temperatures and at higher solid concentration in the dope.
The concentration of solvent in the coagulation bath can also be adjusted to obtain the desired microstructure. A low solvent concentration promotes rapid solvent extraction although this results in a thick skin on each filament which ultimately reduces the rate of solvent extraction and can lead to the formation of macrovoids. A high concentration of solvent in the coagulant gives a denser microstructure but solvent extraction is low. Temperature of the coagulation bath, jet stretch and immersion bath can similarly affect coagulation and microstructure. The fibre produced is essentially a swollen gel and is unoriented. The microstructure consists of a fibrilar network with the spaces in-between called macrovoids.
The invention is directed towards an improved spinning method of dry
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Patent Abstracts of Japan vol. 011, No. 167 (C-425), May 28, 1987 & JP, A,61 296115 (Asahi Medical KK), 26 see abstract.
Edwards N.
The Secretary of State for Defence in Her Britannic Majesty's Go
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