A separation membrane used to separate gas, liquid or solid, particularly a specific component such as an ion material is designed to have selectivity to the material to be removed by appropriately combining a dense structure or a porous structure for selectively permeating and removing the specific component, and simultaneously allow a permeable material to be permeated with low resistance.
Recently, a membrane separation technology using a separation membrane having such a structure has been frequently applied even to water purification and sewage and wastewater processes. The separation membrane for water treatment is classified into a polymer membrane, a ceramic membrane, and a metal membrane according to the material, and into microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO) membranes, and among them, the ultra-filtration membrane has a characteristic of allowing the ion material to be permeated, but removing low-molecular/polymer particles or bacteria and viruses, and may vary according to the reference, but usually has a pore size in a range from 0.01 to 0.1 μm. The ultra-filtration membrane having such characteristics has a wide application range such as pretreatment of process water or ultrapure water, reuse, sewage and wastewater treatment and water purification.
In general, a separation membrane used for water treatment generates contamination on the membrane surface while filtering contaminated raw water to adsorb and grow a contamination source on the membrane surface, and when the separation membrane is severely contaminated, water permeation pressure acting during the filtration is increased, and production quantity is gradually decreased, ultimately leading to reduction in filtration function of the separation membrane.
In order to control contamination of the separation membrane, water is purified using chlorine-based, and acid and alkali materials, but since this method has a problem of shortening the service life of the separation membrane, studies using a polyvinylidene fluoride-based resin which is a material having high chemical resistance have been recently conducted.
As a method of preparing a separation membrane using a polyvinylidene fluoride-based resin as a material, there has been generally used a non-solvent induced phase separation method in which a porous structure is formed by performing casting and extrusion spinning on the polyvinylidene fluoride-based resin in a polymer solution including a good solvent and a pore former at a low temperature, at which the phase separation by heat does not occur, and then solidifying the resin in a non-solvent. The non-solvent induced phase separation method is advantageous in that the size of pores may be freely adjusted, but is disadvantageous in that finger-like macrovoids are included and mechanical strength of the separation membrane is reduced, thereby shortening the service life of the membrane.
Meanwhile, a heat-induced phase separation method is a method in which a separation membrane is prepared by using a polyvinylidene fluoride-based resin and a latent solvent, spinning the polyvinylidene fluoride-based resin at a temperature at which the phase separation by heat occurs, and cooling and solidifying the resin, and it is general to exhibit a spherical structure by crystals of a polymer, particularly, spherulite. The heat-induced phase separation method is advantageous in that a separation membrane, which is mechanically strong, is easily prepared, but it is difficult to reduce the pore size to the ultrafiltration membrane size.
A separation membrane by the non-solvent or heat-induced phase separation method has been prepared by using an apparatus in the form of a reactor equipped with a stirrer to dissolve a solvent and additives, removing bubbles, and then performing extrusion under a pressure of nitrogen or a gear pump, but the method is disadvantageous in that it requires a lot of time to dissolve the polymer and stabilize the removal of bubbles, and it is difficult to manage the process, such that it is difficult to spin a highly viscous polymer solution, and when spinning is performed by increasing the temperature of the solution, the temperature of the tubing connected to the nozzle needs to be adjusted, and the like.
Recently, attempts have been made to prepare a separation membrane using an extruder so as to compensate for the disadvantage and enable a continuous process. However, as a disadvantage, temperature needs to be increased to a melting point of the polymer or more in order to dissolve the polymer pellet or the powder, and it is difficult to control the heat-induced phase separation generated by heat during the process in which the polymer solution is cooled to room temperature. When a spherical structure is exhibited in this case as described above, it is difficult to decrease the size of pores.
Therefore, there is a need for a method of preparing a separation membrane having a pore size in a range of a ultrafiltration membrane, which may be applied to water purification, process water, reuse, and sewage and wastewater treatment while performing a continuous spinning process using an extruder.
Meanwhile, in the case of a polyvinylidene fluoride-based separation membrane generally prepared, the surface is hydrophobic and has a characteristic vulnerable to contamination, and as a general hydrophilic modification method for preventing this problem, methods of mixing a hydrophilic polymer during the preparation of a polymer solution, or performing surface modification or coating in a post-treatment process, and the like have been frequently used. However, the former has good productivity, but it is difficult to have a uniform distribution because most of the hydrophilic polymers have low compatibility with polyvinylidene fluoride, and the latter has a problem with uniformity and durability of the surface. Therefore, there is a need for consideration on a method which may maintain durability along with uniformity, and enables a continuous process.
Throughout the present specification, a plurality of papers and patent documents are referenced, and citations thereof are indicated. The disclosure of each of the cited papers and patent documents is incorporated herein by reference in its entirety to describe the level of the technical field to which the present invention pertains and the content of the present invention more apparently.