JPS5824162B2 - membrane composition - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a membrane composition with improved filtration performance and mechanical strength for use in ultrafiltration, reverse osmosis, and the like.

Semipermeable membranes used in ultra-Pa, reverse osmosis, etc. are obtained by removing the solvent under specific conditions from a solution in which a polymer to be formed into the membrane is dissolved.

Since these membranes have low mechanical strength when used alone, they are usually put into practical use by being directly formed on synthetic fiber cloth or nonwoven fabric.

The surface of cloth or nonwoven fabric is quite rugged, and therefore the film to be formed thereon is necessarily thick, usually several hundred microns or more.

Therefore, membrane capacity is limited due to thickness.

In addition, since cloth, nonwoven fabric, etc. have a rough structure, the semipermeable membrane formed thereon is likely to cause local distortion, which has a disadvantage in that it adversely affects the life of the membrane.

The present invention aims to improve the above-mentioned drawbacks by using a porous body consisting of a laminated locally fractured structure obtained by biaxial stretching as a base for a semipermeable membrane.

The drawing shows a simulated cross-section of an example of the film composition of the present invention in the thickness direction.

In the figure, 1 is a porous body comprising a laminated locally fractured structure obtained by biaxial stretching and constitutes a base, and 2 is a semipermeable membrane formed thereon.

The porous body 1 has a structure 3 caused by fracture of a crystalline polymer,
The semipermeable membrane 2 is composed of solid fine particles 4 and fractured holes 5, and a portion 6 of the semipermeable membrane 2 has cut into the inside of the porous body.

The underlying porous body 1 is made of a crystalline polymer compound serving as its skeleton, such as polyolefins such as polyethylene and polypropylene, polyamides, polyesters such as polyethylene terephthalate and polybutylene terephthalate, and polyhalogens such as polyvinylidene chloride and polyvinylidene fluoride. and substances that can become solid fine particles that are difficult to melt during stretching, such as finely powdered inorganic fillers such as silica, alumina, and calcium carbonate, organic pigments, and polymer compounds that have a higher melting point than the crystalline polymer compound that forms the skeleton. At least one of them, and if necessary, a lubricant to improve dispersibility, a surfactant to impart hydrophilicity, a hydrophilic polymer compound, a heat stabilizer, an antioxidant, etc. are selected as appropriate, The composition obtained by melt-mixing is mechanically deformed at a relatively low temperature to generate fine cracks, crazing, etc. inside the composition, and then the composition is placed under tension to prevent these cracks and crazing from disappearing. It can be obtained by elevating the temperature to a temperature at which the composition can be easily stretched, and then biaxially stretching to cause local breaks.

Generally, the stretching ratio is 1.5 to 10 times in one direction, and the longitudinal and lateral stretching ratio is in the range of 1:5 to 5:1.

The state of the pores in a porous body depends on the material and compounding ratio of the porous body,
Although the shape of the solid fine particles and the stretching conditions can vary considerably, those particularly suitable for the present invention have an average pore diameter of 0.1 to 100 μm and a density of 100 broken pores in the planar direction. - above, average height of fracture holes in thickness direction 0,0
It has a diameter of 1 to 10μ and a porosity of 30 to 80%.

It is also possible to modify the micropores obtained by stretching through various post-treatments such as solvent treatment, chemical crosslinking, radiation crosslinking, and grafting, and essentially the layered fractured structure obtained by biaxial stretching can be modified. Any of them can be used as the base of the present invention.

Semipermeable thin films are made by dissolving the polymer that forms the film in a solvent, adding a swelling agent and other additives, applying the solution to the above-mentioned base, allowing the solvent to partially evaporate, and then immersing it in cold water and heating it if necessary. It can be formed by a known method such as water treatment.

The features of the present invention are that the base is made of a crystalline polymer skeleton that has been strongly oriented by biaxial stretching, so it has high strength and can withstand practical use with a thickness of about 10 μm, and the base surface is smooth. Secondly, the semi-permeable membrane can be made very thin, with a thickness of several microns or less, which reduces the resistance within the membrane when it passes offshore.As is clear from the drawing, the structure of the underlying surface is flat in the plane direction. Since it is composed of holes, the semipermeable membrane digs into the base appropriately, and is firmly fixed, while at the same time preventing excessive digging, and suppressing an increase in the thickness of the semipermeable membrane.

Because of these characteristics, it is possible to improve the permeability without reducing the separation ability of the membrane, and it is possible to maintain stable membrane performance over a long period of time.

Next, examples of the present invention will be described.

A blend consisting of 100 parts by weight of polypropylene, 100 parts by weight of fine alumina powder with an average particle size of 1 μm, and 10 parts by weight of a nonionic surfactant was made cloudy by mechanical deformation at room temperature, and then heated under tension to 145 parts by weight. 100 parts by weight of cellulose acetate and 80 parts by weight of cellulose acetate were placed on a porous body with a thickness of 70 μm and a porosity of 60% obtained by biaxially stretching 4×4 times at °C.
A solution consisting of 120 parts by weight of formamide was thinly and uniformly applied, left in the air for 30 seconds, immersed in ice water, and then heat treated in water at 60°C to form a semipermeable membrane.

Microscopic observation of a cross section of the dried membrane revealed that the thickness of the semipermeable membrane on the substrate was 10 μm or less.

This is designated as sample A. Next, as a comparative example, a semipermeable membrane was formed by coating, coagulating, and heat-treating the same solution as above on a nylon cloth substrate, except for increasing the coating thickness (using the conventional method). thing) was created.

This is designated as sample B.

The film properties of the above two types of samples were as follows.

As is clear from the above examples, the membrane composition of the present invention has excellent overflow performance and mechanical strength, and is industrially very useful.

[Brief explanation of the drawing]

The drawing is a simulated cross-sectional view of a membrane composition according to the invention. 1...Porous body, 2...Semipermeable thin film, 3
... Locally fractured structure, 4 ... Solid fine particles.

Claims (1)

[Claims] , 1 A semipermeable membrane is formed on the surface of a layered porous body consisting of laminated locally fractured structures obtained by biaxially stretching a crystalline polymer composition in which solid fine particles that are difficult to soften during stretching are dispersed. Membrane composition.