1. Field of the Invention
The present invention relates to a porous membrane and a method for manufacturing the same, and more particularly, to a porous membrane with high tensile strength, good elongation at break, and good water permeability, and a method for manufacturing the same.
2. Discussion of the Related Art
A separation method using a membrane has lots of advantages over the method based on heating or phase-changing. Among the advantages is high reliability of water treatment since the water purity required can be easily and stably satisfied by adjusting the size of the pores of a membrane. Furthermore, since the separation method using a membrane does not require a heating process, a membrane can be used with microorganism which is useful for separation process but may be adversely affected by heat.
The separation membrane may comprise a flat-type membrane and a hollow fiber membrane. A hollow fiber membrane module performs a separation process by the use of a bundle of hollow fiber membranes. Thus, in consideration of an effective area used for the separation process, the hollow fiber membrane is more advantageous than the flat-type membrane.
Typically, the hollow fiber membrane has been widely used in the field of microfiltration and ultrafiltration for obtaining axenic water, drinking water, super pure water, and so on. Recently, however, application of the separation method using the hollow fiber membrane is being expanded to include wastewater treatment, solid-liquid separation in a septic tank, removal of suspended solid (SS) from industrial wastewater, filtration of river, filtration of industrial water, and filtration of swimming pool water.
The hollow fiber membrane may be classified into a composite membrane which is manufactured by coating a tubular braid woven by polyester or polyamide fiber with a polymer resin film; and a singular membrane which is manufactured only by polymer resin without using a reinforcing member such as a tubular braid.
Since the composite membrane uses the tubular braid as the reinforcing member, the composite membrane has a good mechanical property (strength and elongation). However, the tubular braid is different in material from the polymer resin film coated thereon so that an adhesive strength is weak between the tubular braid and the polymer resin film. Also, if a physical impact, for example, aeration to prevent contamination of the composite membrane, is applied to the composite membrane continuously, the tubular braid and the polymer resin coated thereon may be separated from each other, whereby the quality of permeates may be lowered. Also, due to a thickness of the tubular braid, it is impossible to reduce a total thickness of the composite membrane below a predetermined value, whereby the composite membrane is disadvantageous in consideration of the effective area. For these reasons, there is the recent trend toward the active studies and researches for the singular membrane rather than the composite membrane.
Generally, the singular membrane may be manufactured by NIPS (Non-solvent Induced Phase Separation) or TIPS (Thermally Induced Phase Separation).
In case of the NIPS, the singular membrane may be manufactured by the following steps: preparing a spinning solution by dissolving polymer resin in a good solvent; extruding the prepared spinning solution through a spinneret; and inducing a coagulation of the spinning solution by bringing the extruded spinning solution into contact with a solution including a non-solvent.
Meanwhile, in case of the TIPS, the singular membrane may be manufactured by the following steps: preparing a spinning solution by forcibly dissolving polymer resin in a poor solvent above a phase-separation temperature; extruding the prepared spinning solution through a spinneret; and coagulating the spinning solution by bringing the extruded spinning solution into contact with a cooling solution below the phase-separation temperature.
The TIPS has disadvantageous of fastidious process control, high energy consumption, and high manufacturing cost since the polymer resin has to be forcibly dissolved in the poor solvent at high temperature of about 120˜150° C., and the spinning solution has to be maintained at about 120˜150° C. temperature until being extruded through the spinneret.
Also, since a porous membrane prepared by the TIPS includes no macro voids and has a bead structure symmetric in a membrane-thickness direction, water permeability and filtration property of the membrane is relatively low although its mechanical strength is relatively high. Also, since the porous membrane has low elongation at break, it might be damaged during an aeration process for preventing fouling.
In the NIPS, since the polymer resin is dissolved in the good solvent for preparing the spinning solution, there is no need to forcibly dissolve the polymer resin by raising the temperature, whereby the NIPS consumes less energy than the TIPS.
However, the singular membrane prepared by the NIPS has no bead structure, wherein the bead structure is made by the TIPS, but has asymmetric sponge structure including the macro voids, whereby a tensile strength of the singular membrane prepared by the NIPS is insufficient. Thus, the singular membrane prepared by the NIPS cannot satisfy the compaction index below 0.5, which is generally required in this technical field. That is, if a predetermined-level pressure is applied to the singular membrane, the singular membrane is seriously shrunk and distorted so that pores included in the singular membrane are stopped, whereby water permeability of the singular membrane is considerably lowered. Also, the singular membrane prepared by the NIPS has a problem of low rejection rate to impurities due to a large nominal pore size.