Plastic article or earthenware shaping or treating: apparatus – With means applying coating material to work or work contact... – Separate means applying material directly to shaping means,...
Patent
1990-07-09
1994-06-07
Bell, James J.
Plastic article or earthenware shaping or treating: apparatus
With means applying coating material to work or work contact...
Separate means applying material directly to shaping means,...
425 90, 4251311, 4251315, 425463, A01J 2100
Patent
active
053184173
DESCRIPTION:
BRIEF SUMMARY
FIELD OF INVENTION
This invention relates to the manufacture of porous hollow fibre membranes and more particularly to an extrusion die assembly for forming such membranes.
BACKGROUND ART
Porous polymeric structures having pores in the range of 0.01 to 10 microns are commonly used for microfiltration. Such membrane structures may be prepared from thermoplastic polymers using precipitation techniques and formed into various shapes including hollow fibres or flat sheets.
The thermal precipitation technique for membrane formation commences with the formation of a solution of a thermoplastic polymer in a solvent at an elevated temperature. The solution is then cooled and, at a specific temperature which depends on the polymer, the solvent, the concentration of the polymer in the solvent and the rate of cooling, phase separation occurs and the liquid polymer separates from the solvent.
True solutions require that there be a solvent and a solute. The solvent constitutes a continuous phase, and the solute is distributed randomly in the solvent at a molecular level. Such a situation is almost unknown with polymer solutions. Long polymer chains tend to bend back on themselves and form temporary interactions or bonds with other polymer chains with which they come into contact. These interactions are continually forming and breaking, and new ones are formed. Polymer solutions are thus rarely true solutions but lie somewhere between true solutions and mixtures.
In many cases it is also difficult to state which is the solvent and which is the solute. In the art, it is accepted practice to call a mixture of polymer and a solvent a solution if it is optically clear without obvious inclusions of either phase in the other. Phase separation is usually then taken to be that point where there is an optically detectable separation.
There is yet another case where the heated mixture of polymer, solvent, and other components if present, if neither a solution nor a mixture in the usual sense of the words. This is the case where a surface-active agent is present in sufficient concentration to form ordered structures such as micelles.
A hollow fibre forming installation disclosed in U.S. Pat. No. 4,708,799 includes a vessel containing a heated solution of a polymer and a liquid which is inert relative to the polymer which is delivered to an extrusion die by a pump. The hollow fibre is extruded through a nozzle into a spinning tube containing inert liquid of the solution. The fibre and the inert liquid pass through an atmospheric gap to a spinning tube in the same direction and at substantially the same linear velocity.
In that installation, the hollow fibre is extruded in the molten state and formed in the spinning tube. The application of the inert liquid to the molten fibre has an adverse effect on the porosity of the surface of the fibre. Furthermore, the use of a spinning tube is cumbersome and costly.
DISCLOSURE OF INVENTION
According to a first aspect of the invention there is provided a process for making a polymeric, porous hollow fibre comprising the steps of: temperature and for a time for the polymer and solvent to mutually dissolve, the hollow fibre, non-equilibrium liquid-liquid phase separation takes place to form a bicontinuous matrix of the polymer and solvent in which the polymer and solvent form two intermingled separate phases of large interfacial surface area, and,
According to the invention there is also provided an extrusion die assembly for producing a hollow fibre membrane which has a quadruple co-extrusion head having four concentric passageways. The axial passageway receives a lumen forming fluid, the next outward passageway receives a homogenous mixture of the polymer and a surfactant solvent to form the membrane, the next outwardly concentric passageway receives a coating fluid and the outermost passageway receives a cold quench fluid.
Each fluid is transported to the extrusion head by means of individual metering pumps. The four fluids are individually heated and are transported along thermally i
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Khoo Paul S.
Kopp Clinton V.
Streeton Robert J. W.
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