JPS6375116A - Production of conjugated hollow yarn membrane - Google Patents

Abstract

PURPOSE:To produce fiber for fine filtration, having improved back wash effects and long life, spinning two polyethylenes having different melt indexes under specific conditions in such a way that the polyethylenes are set at the inside and outside of hollow fiber membrane. CONSTITUTION:Two polyethylenes having >=0.95g/cm<3> density, 0.8-15 melt indexes (MI) and different melt indexes are heated at >=200 deg.C and higher than the melting point of the polymers, melted, the polyethylene having lower MI is extruded from the outside of a concentrically set ring-shaped nozzle and the polyethylene having higher MI from the inside. The extrusion ratio of both the polyethylenes is limited to a range of 20/1-1/20. The hollow fiber membrane is cooled by a gas flow and the completing point of thinning is at a position 20-150cm from the nozzle face.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for manufacturing a composite hollow fiber membrane for precision filtration, and more specifically, a method for manufacturing a composite hollow fiber membrane for precision filtration that has excellent backwashing effects and has a long life. Regarding the law.

[Background Art] In recent years, large amounts of highly purified water have been required in various fields such as semiconductor integrated circuit manufacturing processes in the electronic industry, water for boilers, and water for pharmaceutical and medical purposes. Precision filtration using membranes is becoming widely used as one way to meet these demands.

A hollow fiber membrane is preferably used as the membrane used here because it can obtain a large filtration area per unit area, and various membrane materials are used, among which polyethylene is chemically stable. Therefore, it is a preferable material.

In precision filtration using a membrane, the membrane becomes clogged with fine particles contained in the water to be filtered during use.

Since there is a large economic loss if a clogged membrane is thrown away as is, the life of the membrane is usually extended by repeating the filtration-backwashing cycle.

[Problems to be Solved by the Invention] The applicant has commercially produced a porous hollow fiber membrane by cold-stretching a polyethylene hollow system to generate crazes between crystal lamellae and further stretching the craze. Although it has been well received due to its high rejection rate and high permeability, depending on the application, fine particles may penetrate deep into the pores of the membrane, making it difficult to restore permeation performance by backwashing. There is a growing demand for membranes that improve backwashing performance while maintaining high permeability.

In response to such demands, the present invention provides a method for stably and efficiently producing a polyethylene hollow fiber membrane for precision filtration, which has excellent permeation performance and good recovery of permeation performance by backwashing.

[Means for solving the problem] The sheath-tight filtration membrane that is responsible for this kind of performance must have a membrane on the inlet side of the water to be filtered (for example, on the outside of the hollow fiber) that is small enough to block fine particles on the surface. A two-layer structure consisting of a thin film layer having pores and a thicker support layer having larger pores on the outflow side of the filtrate (for example, inside the hollow fibers) can be mentioned. The object of the present invention is to provide a method for manufacturing such a hollow fiber membrane.

That is, the gist of the present invention is that the density is 0.95H/cm" or more and the melt index value (hereinafter referred to as MI) is 0.
．． Two types of polyethylene having different MIs from 8 to 15 are heated and melted to a temperature of 200°C or less and 10°C higher than the melting point of polyethylene, and has annular discharge ports arranged concentrically. Using a hollow fiber manufacturing nozzle, polyethylene with a low MI is discharged from the outside of the nozzle, and polyethylene with a high MI is discharged from the inside at a discharge ratio of 20/1 to 1/20. The present invention relates to a method for producing a composite hollow fiber membrane, which is characterized in that the composite hollow fiber membrane is removed while being cooled.

When composite spinning is performed using the older type of polymer, delamination between layers tends to occur due to lack of compatibility, but in the present invention, two types of polyethylene with different MI are used, so the adhesion between the layers is good and peeling occurs. Never.

In addition, in the present invention, the polyethylene hollow fiber obtained by the above method is stretched to separate the crystal lamellae, and this is further stretched to obtain a porous hollow fiber membrane for precision filtration. The density of the polyethylene used needs to be 0.95 g/cm" or higher because the higher the porous structure, the better the porous structure can be obtained.

MI is one of the indicators showing the fluidity of melt, and A37M1
This is a value measured according to the method described in No. 238.

The higher the MI, the higher the fluidity of the polymer melt, and the lower the MI, the lower the fluidity. The MI of polyethylene used in the present invention is 0.8
It is necessary that the number is between 15 and 15. If it is lower than this, the fluidity is too low and melt spinning is impossible, and if it is larger than 15, a porous structure cannot be obtained even if the spun hollow fiber is drawn.

This is because the method of the present invention obtains a yarn having a laminated lamellar structure as a spun yarn, which is stretched to separate the crystals, and then further stretched.
A method in which a large number of fibrils are formed by stretching folded molecular chains with a lamellar structure, and a porous structure is formed in which the spaces between the fibrils extend from one surface of the hollow fiber membrane to the other surface. This is because there is.

In order for the yarn to have a laminated lamellar structure, it is important that sufficient shear stress be applied to the yarn during crystallization, otherwise spherulites will tend to form. Factors that determine the shear stress during crystallization in melt spinning include the viscosity of the melt from the nozzle, the sliding speed at which the melt is discharged, and the winding speed. When the shear rate is the same, the magnitude of shear stress depends on the viscosity of the melt;
In other words, if the MI value is too high, the shear stress becomes small, making it difficult to form an h1 layer lamellar structure. On the other hand, the size of the pores after the porous structure is formed depends on the size of the laminated lamellar structure, that is, the folding length of the folded molecule in there. Furthermore, the folded length becomes shorter as the shear stress during crystallization increases.

The purpose of the present invention is to efficiently and stably obtain composite hollow fibers with different pore sizes on the surfaces of the inner and outer layers. However, if the same polymer is used for both the inner and outer layers, crystallization during spinning It is difficult to achieve the above objective because the environment is a shared environment between the inside and outside layers.

Therefore, in the present invention, polyethylenes having different MI values are used in combination so that the shear stress differs between the inner and outer layers.

The purpose of the present invention is to obtain a hollow fiber manufacturing nozzle having annular discharge ports arranged concentrically. Using the nozzle, polyethylene with low MI is heated and melted from the outside of the nozzle, and polyethylene with high MI is heated and melted from the inside at a temperature of 200°C or less and 10°C higher than the melting point of polyethylene, and extruded at the same time. .

If the melting temperature is less than 10° C. higher than the melting point of polyethylene, the fluidity will be poor and spinning will be difficult, and if it exceeds 200° C., the polymer will deteriorate, which is undesirable.

Furthermore, it is necessary to discharge polyethylene with a low MI and polyethylene with a high MI so that the discharge ratio is 20/1 to 1/20. This is to ensure that the inner and outer layers are laminated stably. If this lamination is not performed stably, the thickness of each layer or the fiber length direction may fluctuate, or the interface between both layers may change in the fiber cross-section circumferential direction or fiber length direction. It will be confused by the direction. This not only causes yarn breakage because the tension during spinning concentrates on distorted areas due to unevenness and disorder, but also causes non-uniform crystallization due to non-uniform shear stress during crystallization. Alternatively, the folded length of the molecular chains becomes non-uniform, resulting in variations in pore diameter. Furthermore, these irregularities and disturbances also affect the stretching process, causing fiber breakage and variations in hollow fiber membrane performance. In order to stabilize this lamination, it is ideal that at the point where the inner and outer layers are laminated, the melting speeds of the polymers forming both layers are the same. However, in reality, the ejection linear velocity has a distribution in the hollow fiber thickness direction, so it is sufficient that the ejection linear velocities of both layers at the interface match to such an extent that the above-mentioned problems do not occur; It is not necessary that the average discharge speed of the liquid be the same.

According to research conducted by the present inventors, it has been found that the above problem does not occur if the ratio of the average discharge speeds of the polymer melts forming both layers is 1/20 to 20/1.

By taking the polymer MfL discharged under the above-mentioned conditions while cooling it, the polymer is solidified and simultaneously crystallized to obtain hollow fibers.

Although various methods can be used for this cooling, a method in which a cooling gas, for example, air at a temperature of 10 to 30° C. is caused to flow in a direction opposite to the direction in which the yarn is taken off, is preferably used.

Note that if the shear stress at the time of crystallization is appropriate, a good porous hollow fiber can be obtained by going through the steps described below. Generally, the melt coming out of the nozzle falls while decreasing in diameter (fine) until it solidifies, and once the thinning is completed and solidifies, shear stress is applied thereafter. If the thinning end point is too close to the nozzle surface, the shear stress applied to the yarn will increase, and in particular, polyethylene with a low MI value will have high molecular orientation due to excessive shear stress, resulting in yarn breakage during the subsequent drawing process. is more likely to occur. On the other hand, if it does not reach from the nozzle surface, the laminated lamellar structure will not develop sufficiently due to insufficient shear stress, and necking will occur during stretching, making it difficult to become porous. Therefore, the point at which the thinning ends is 20 to 15 mm from the nozzle surface.
It is preferable that it be at a position of 0 cm. The end point of the thinning can be set depending on the cooling conditions, and specifically, it can be set at an appropriate position by adjusting the temperature and speed of the cooling air.

Note that when using cooling air for cooling, if the wind speed of the cooling air is too high, turbulence tends to occur in the air flow, which tends to cause unevenness in the yarn diameter. Concentrates and can cause thread breakage. Furthermore, if the wind speed is too low, cooling will be insufficient and fluctuations in yarn diameter will become large. From this point of view, the speed of the cooling air is preferably 0.5 to 2.0 m/sec.

The hollow fiber thus obtained was first heated at 5 to 20°C to 1.5 to 2
．． By stretching the film approximately 5 times, the crystal layers are peeled off. If the temperature is less than 5°C, the polymer chains will become rigid and easily break during stretching, whereas if the temperature is higher than 20°C, the crystals will stretch, making it difficult for crystals to separate from each other. If the stretching ratio is less than 1.5 times, peeling will be insufficient, and if the stretching ratio is more than 2.5 times, breakage will easily occur, which is not preferable. Next, the hollow fiber is made porous by further stretching at a temperature higher than the above-mentioned stretching temperature to stretch the polymer fibers from the lamellae and generate a large number of fibrils. The stretching temperature at this time is preferably 50 to 140°C, and the stretching ratio is preferably 2 to 7.5 times.

The porous hollow fibers thus obtained were heated at a temperature of 120°C to 140°C.
It is preferable to perform constant length or relaxation heat treatment for shape stabilization.

[Example] The present invention will be explained in more detail below using Examples.

In the examples, the pore diameter was determined by measuring the pore area of 100 pores observed with a scanning electron microscope, and was converted to the diameter of a circle corresponding to the same area.

Example 1 Polyethylene with a density of 0.965 g/cc and MI 5.5 and polyethylene with a density of 0.965 g/cc and MI 1.0 were heated and melted at 180°C, and double annular spinning nozzles were arranged concentrically. The polymer was extruded from the inner and outer rings at a discharge linear velocity of 3.5 cm/min and 2.4 cm/min, respectively, and 15°C cooling air was applied countercurrently to the extrusion direction of the polymer at a wind speed of 0.8 m/sec. By flowing and cooling, the atomization end point is 35 cm from the nozzle surface, and it is wound up at a speed of 180 m/min.
A hollow fiber was obtained. (This is called an undrawn yarn.) After spinning for 30 minutes in this manner, the outer diameter of the undrawn yarn was measured at 25 points every 200 m. When I added 1111, the average value was 5
It was 22 μm, and the coefficient of variation was 3.0%. This undrawn yarn was fed at 2 m/min and heat treated at 115°C for 3 minutes, then stretched to 1.8 times its original length at 22°C and then 5.5 times its original length at 105°C. Then, it was relaxed at 115° C. and heat set to five times its original length. As a result, 1200 m of drawn yarn was obtained in 2 hours. This quantity system was not cut at all, the porosity of the drawn fiber was 67%, the average pore diameter on the inner surface was 1.2 μm, and the average pore diameter on the outer surface was 0.3 μm.
A composite membrane with an asymmetric structure was stably produced.

Example 2 By flowing cooling air at 18°C countercurrently to the extrusion direction of the polymer at a wind speed of 1.0 m/sec,
An undrawn yarn was obtained in the same manner as in Example 1, except that the thinning end point was set at 27 cm from the nozzle surface. After spinning for 30 minutes in this manner, the outer diameter of the undrawn yarn was measured at 25 locations every 200 m, and the average value was 535 μm, with a coefficient of variation of 3.5%.

120 in 2 hours from this undrawn yarn in the same manner as in Example 1.
A drawn yarn of 0 m was obtained. This quantity system does not cut at all, the porosity of the drawn fibers is 65%, the average pore size on the inner surface is 1.2 μm, and the average pore size on the outer surface is 0.3 μm, making it possible to stably manufacture composite membranes with an asymmetric structure. It was done.

Example 3 By cooling by flowing cooling air at 20°C countercurrently to the extrusion direction of the polymer at a wind speed of 1.6 m/sec,
An undrawn yarn was obtained in the same manner as in Example 1 except that the thinning end point was set at 22 cm from the nozzle surface. After spinning for 30 minutes in this manner, the outer diameter of the undrawn yarn was measured at 25 locations every 200 m, and the average value was 542 μm, with a coefficient of variation of 5.0%.

120 in 2 hours from this undrawn yarn in the same manner as in Example 1.
A drawn yarn of 0 m was obtained. This amount system was not cut at all, the porosity of the drawn fiber was 65%, the average pore diameter on the inner surface was 1.2 μm, the average pore diameter on the outer surface was 0.3 μm, and the composite membrane had an asymmetric structure. or stably manufactured.

Example 4 By cooling by flowing cooling air at 14°C countercurrently to the extrusion direction of the polymer at a wind speed of 0.6 m/sec,
An undrawn yarn was obtained in the same manner as in Example 1 except that the thinning end point was set at 125 cm from the nozzle surface. After spinning for 30 minutes in this manner, the outer diameter of the undrawn yarn was measured at 25 locations every 200 m, and the average value was 515 μm and the coefficient of variation was 3.8%.

120 in 2 hours from this undrawn yarn in the same manner as in Example 1.
A drawn yarn of 0 m was obtained. This quantity system does not cut at all, the porosity of the drawn fibers is 64%, the average pore size on the inner surface is 1.2 μm, and the average pore size on the outer surface is 0.3 μm, and a composite membrane with an asymmetric structure can be stably produced. It was done.

Example 5 Polyethylene with a density of 0.961 g/cc and MI 18.0 and polyethylene with a density of 0.965 g/cc and MIo of 8 were heated and melted at 190°C, and a double annular spinning nozzle arranged concentrically was used. The polymer was extruded from the inner and outer rings at a discharge linear velocity of 5.0 cm/min and 0.4 cm/min, respectively, and cooling air at 15°C was flowed at a flow rate of 1.1 m/sec in the direction of extrusion of the polymer. By cooling, the thinning end point was set at 95 cm from the nozzle surface, and the fiber was wound at a speed of 180 m/min to obtain a hollow fiber. Spun in this way for 30 minutes, with 25
When the outer diameter of the undrawn yarn was measured at the tube station, the average value was 514 μm and the coefficient of variation was 4.0%. Using this undrawn yarn, a drawn yarn of 1200 m was obtained in 2 hours in the same manner as in Example 1. This quantity system did not break at all, the porosity of the drawn fiber was 63%, the average pore diameter on the inner surface was 1.8 μm, and the average pore diameter on the outer surface was 0.1 μm, making the composite membrane with an asymmetric structure stable. manufactured.

Example 6 Polyethylene with a density of 0-9658/cc and MI 13 and polyethylene with a density of 0.965 g/cc and MI 3.5 were heated and melted at 155°C, and each of the double annular spinning nozzles was arranged concentrically. The polymer is extruded from the inner and outer rings at linear discharge speeds of 4.4 cm/min and 1.1 cm/min, and cooled by flowing cooling air at 22°C countercurrently to the polymer extrusion direction at a wind speed of 0.8 m/see. By doing so, the thinning end γ point was set at 40 cm from the nozzle surface, and the fiber was wound at a speed of 180 m/min to obtain a hollow fiber.

After spinning for 30 minutes in this way, the outer diameter of the undrawn yarn was measured at 25 points every 200 m, and the average value was 5.
It was 35 μm, and the coefficient of variation was 3.3%. Using this undrawn yarn, a drawn yarn of 1200 m was obtained in 2 hours in the same manner as in Example 1. This quantity system was not cut at all, the porosity of the drawn yarn was 59%, the average pore diameter on the inner surface was 2.1 A1 m, and the average pore diameter on the outer surface was 0.5 μm.
A composite membrane with an asymmetric structure was stably produced.

Comparative Example 1 By cooling by flowing cooling air at 22°C at a wind speed of 2.3 m/sec in the extrusion direction of the polymer,
An undrawn yarn was obtained in the same manner as in Example 1 except that the thinning end point was set at 15 cm from the nozzle surface. After spinning for 30 minutes in this way, the outer diameter of the undrawn yarn was measured at 5 locations at 200mwk and 25r, and the average value was 550μ.
m, and the coefficient of variation was 12%.

When an attempt was made to stretch the film in the same manner as in Example 1, the stretched yarn was cut at 15 places with a length of 200 m, making it impossible to stretch in terms of tτ.

Comparative Example 2 By cooling by flowing cooling air at 15°C at a wind speed of 0.4 m/sec in the extrusion direction of the polymer,
Except that I made it 170cm from the thinning end point or the nozzle surface! Example 1 An undrawn yarn was obtained in the same manner as in Example 1. After spinning for 30 minutes in this way, the outer diameter of the undrawn yarn was measured at 25 points at 200 mji, and the average value was 49.
0 μm, and the coefficient of variation was 15%.

When an attempt was made to stretch the film in the same manner as in Example 1, the stretched yarn was cut at 22 locations with a length of 200 m, making it virtually impossible to stretch.

Claims (1)

[Claims] 1) Two types of polyethylene with different MIs having a density of 0.95 g/cm^3 or more and a melt index value (hereinafter referred to as MI) of 0.8 to 15 are heated at 200°C.
MI is melted by heating to a temperature of 10° C. higher than the melting point of polyethylene, and the MI is Low polyethylene from the inside
A method for producing a composite hollow fiber membrane, which comprises discharging polyethylene with a high I content at a discharge ratio of 20/1 to 1/20, and withdrawing the polyethylene while cooling it. 2) The method for producing a composite hollow fiber membrane according to claim 1, characterized in that the cooling is performed by a gas flow, and the atomization end point is located at a distance of 20 to 150 cm from the nozzle surface. 3) The method for producing a composite hollow fiber membrane according to claim 1, wherein the cooling is performed by a gas flow, and the flow rate of the cooling gas is 0.5 to 2.0 m/sec.