CN113975976A

CN113975976A - Nano-scale porous filter membrane for liquid filtration and preparation method thereof

Info

Publication number: CN113975976A
Application number: CN202111274084.4A
Authority: CN
Inventors: 蔡海锋; 周万根; 王魁星
Original assignee: Suzhou Youkefa New Material Technology Co ltd
Current assignee: Suzhou Youkefa New Material Technology Co ltd
Priority date: 2021-10-29
Filing date: 2021-10-29
Publication date: 2022-01-28

Abstract

The invention provides a nano-scale porous filter membrane for liquid filtration and a preparation method thereof. The preparation method of the nano-scale porous filter membrane comprises the following steps: s1, providing polytetrafluoroethylene dispersing materials and auxiliary oil, compacting powder formed by mixing the polytetrafluoroethylene dispersing materials and the auxiliary oil, extruding the powder through a flat-opening die, and rolling to obtain a polytetrafluoroethylene base belt; s2, drying and degreasing the polytetrafluoroethylene tape; and S3, stretching the dried polytetrafluoroethylene tape to form a nano-scale porous structure suitable for liquid filtration. The invention uses the flat die extrusion process to ensure that the PTFE base band obtains high strength in the longitudinal and transverse directions, and further ensures that the pore-forming process is limited in the stretching process, thereby obtaining the microporous membrane with smaller pore diameter.

Description

Nano-scale porous filter membrane for liquid filtration and preparation method thereof

Technical Field

The invention belongs to the technical field of polytetrafluoroethylene microporous membrane preparation, and particularly relates to a polytetrafluoroethylene microporous membrane with small pore diameter, high bubble pressure and high strength and a preparation method thereof.

Background

Polytetrafluoroethylene (hereinafter abbreviated as PTFE) microporous membranes have since appeared and are increasingly used in the fields of household filters, medical filters, industrial filter bags, garment materials, sealing components and the like due to their excellent filtration performance and unique chemical properties.

In industrial production, filter materials play an indispensable important role in various fields. For example, in the production of electronic devices, electroplating solution, chip treatment solution, high-purity water prefiltering, cleaning solution recovery and the like all need to be subjected to precise filtration, and the filter material used is required to have micro-scale or even nano-scale micropores so as to meet the production requirements. In the field of sewage treatment, filter materials are required to have excellent contamination resistance and chemical resistance, and filter elements are required to be able to withstand the corrosive action of alkaline, acidic and corrosive cleaning agents during cleaning.

PTFE has excellent chemical stability and thermal stability, so that the PTFE has good service performance and longer service life under severe environments such as strong acid, strong alkali and the like and high-temperature working conditions; PTFE also has low surface energy and non-polarity, and therefore possesses strong hydrophobicity and moisture resistance, and has a certain self-cleaning ability, making PTFE very suitable for filter materials. The PTFE dispersion resin is prepared into a film with a unique micropore structure through paste extrusion and biaxial stretching processes, the pore diameter is generally 0.1-10 mu m, and the porosity is about 80-90%.

In some liquid filtration fields with higher precision requirements, such as the purification and recovery of etching liquid used in semiconductor manufacturing, the pore diameter of a filtration membrane needs to be controlled below 100nm, but the pore diameter of a PTFE microporous membrane produced by the conventional process is generally 0.1-10 microns; in addition, when the filtration flux is larger, the breaking strength of the microporous membrane is generally required to be more than 20Mpa, and the method is difficult to realize by using the conventional process.

As described above, the microporous polytetrafluoroethylene membrane prepared by the conventional process has certain disadvantages in terms of pore size and strength And (5) sinking.Therefore, it is necessary to provide a further solution to the above problems.

Disclosure of Invention

The invention aims to provide a nano-scale porous filter membrane for liquid filtration and a preparation method thereof, so as to overcome the defects in the prior art.

In order to achieve the above object, the present invention provides a method for preparing a nano-scale porous filter membrane for liquid filtration, comprising the steps of:

s1, providing polytetrafluoroethylene dispersing materials and auxiliary oil, compacting powder formed by mixing the polytetrafluoroethylene dispersing materials and the auxiliary oil, extruding the powder through a flat-opening die, and rolling to obtain a polytetrafluoroethylene base belt;

s2, drying and degreasing the polytetrafluoroethylene tape;

and S3, stretching the dried polytetrafluoroethylene tape to form a nano-scale porous structure suitable for liquid filtration.

As an improvement of the preparation method of the nano-scale porous filter membrane for liquid filtration, the flat-mouth die is provided with a feeding channel and an extrusion channel, one end of the feeding channel forms a feeding hole, the other end of the feeding channel is communicated with one end of the extrusion channel, and the other end of the extrusion channel forms a discharging hole;

the distance between the left side wall and the right side wall of the extrusion channel is gradually increased along the discharging direction, and the distance between the upper side wall and the lower side wall of the extrusion channel is gradually decreased along the discharging direction.

As an improvement of the preparation method of the nano-scale porous filter membrane for liquid filtration, the length of the discharge hole is 185mm, and the width of the discharge hole is 1 mm.

As an improvement of the preparation method of the nano-scale porous filtration membrane for liquid filtration of the present invention, the step S2 includes:

drying in a single-roll mode or in a multi-layer overlapping mode to form a multi-layer structure formed by overlapping and bonding more than two layers of polytetrafluoroethylene tapes.

As an improvement of the preparation method of the nano-scale porous filter membrane for liquid filtration, the stretching treatment of the dried polytetrafluoroethylene tape comprises the following steps:

s31, longitudinally stretching the dried polytetrafluoroethylene tape;

s32, pre-transversely drawing the longitudinally stretched polytetrafluoroethylene tape;

and S33, after the pre-transverse drawing is finished, transversely drawing and heat setting the polytetrafluoroethylene base band to obtain the nano-scale porous filter membrane.

As the improvement of the preparation method of the nano-scale porous filter membrane for liquid filtration, the stretching multiplying power of the longitudinal stretching is 2-5 times; the stretching ratio of the pre-transverse drawing is 1.8 times; the stretching magnification of the transverse stretching is 4.5 to 15 times.

As an improvement of the preparation method of the nano-scale porous filter membrane for liquid filtration, the average thickness of the nano-scale porous filter membrane is 15-45 μm.

As an improvement of the preparation method of the nano-scale porous filter membrane for liquid filtration, the average pore diameter of the nano-scale porous filter membrane is 40-100 nm.

As an improvement of the preparation method of the nano-scale porous filter membrane for liquid filtration, the bubble point pressure of the nano-scale porous filter membrane is 0.2-0.35 MPa.

To achieve the above object, the present invention provides a nanoporous filtration membrane obtained by the above preparation method.

Compared with the prior art, the invention has the beneficial effects that:

the invention uses the flat die extrusion process to ensure that the PTFE base band obtains high strength in the longitudinal and transverse directions, and further ensures that the pore-forming process is limited in the stretching process, thereby obtaining the microporous membrane with smaller pore diameter.

Through the multilayer stacking process, the layers of the film are mutually compensated in the biaxial stretching process, thin blocks or holes caused by factors such as impurities on the finished film can be avoided, and the reliability of the product is ensured.

Because the PTFE base band prepared by the process has large thickness and high hardness, the PTFE base band is difficult to stretch uniformly in the transverse stretching process, so that the invention creatively adds a 'pre-transverse stretching' process after longitudinal stretching, enlarges the transverse width of the base band by a certain multiplying power and simultaneously obtains a special shape with a thin middle and thick two sides. The thickness unevenness caused by the stress unevenness can be compensated in the process of transverse drawing.

Therefore, the polytetrafluoroethylene microporous membrane prepared by the invention has the advantages of high bubble point pressure, small aperture, high membrane strength and uniform thickness, and overcomes the defects in the prior art.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, it is obvious that the drawings in the following description are only some embodiments described in the present invention, and for those skilled in the art, other drawings can be obtained according to the drawings without creative efforts.

FIG. 1 is a longitudinal sectional view of a flat-mouth mold used in a method for preparing a nano-scale porous filtration membrane for liquid filtration according to the present invention;

FIG. 2 is a schematic structural diagram of a half-mold in a flat-mouth mold used in the method for preparing a nano-scale porous filter membrane for liquid filtration according to the present invention;

FIG. 3 is a schematic structural diagram of an embodiment of a multilayer stacking process in a method for preparing a nanoporous filtration membrane for liquid filtration;

fig. 4 is a microscopic SEM photograph of the nanoporous filter membrane.

Detailed Description

The present invention is described in detail with reference to the embodiments shown in the drawings, but it should be understood that these embodiments are not intended to limit the present invention, and those skilled in the art should understand that functional, methodological, or structural equivalents or substitutions made by these embodiments are within the scope of the present invention.

In the PTFE porous membrane prepared by the prior art, because the blank pressure delay of a round mouth mold extrusion rod generates large expansion in the longitudinal and transverse directions, the PTFE calendering base band has large-degree fibrosis, so that the pore of the microporous membrane is not uniform and the pore diameter is overlarge in the subsequent stretching process, and the obtained membrane has poor breaking strength.

In the field of fine filtration, it is necessary to ensure that the filter material cannot have weak points such as thin blocks or broken holes. In addition, the microporous membrane for liquid filtration is easy to stretch unevenly in the transverse stretching process due to large thickness, so that the middle of the finished membrane is thick, and the two sides of the finished membrane are thin.

In view of the above problems, an embodiment of the present invention provides a method for preparing a nano-scale porous filter membrane for liquid filtration, which includes the following steps:

s1, providing polytetrafluoroethylene dispersing materials and auxiliary oil, pressing the powder formed by mixing the polytetrafluoroethylene dispersing materials and the auxiliary oil into a compact blank, extruding the compact blank through a flat-opening die, and rolling and drying to obtain the PTFE base band.

Specifically, the PTFE dispersion materials with small standard relative density (SSG) and high molecular weight are selected, such as 601A manufactured by dupont, F106C manufactured by dakun, 216M manufactured by zhonghao chenguang chemical industry, and the like. On one hand, raw materials with higher molecular weight have the advantages that fibrils are easier to pull out from nodes, the stacking density of fibers is higher, the membrane pore diameter is smaller, on the other hand, the fibril strength is higher, and the strength of the prepared PTFE microporous membrane is also higher. In addition, the invention selects the auxiliary oil with solubility parameter close to that of PTFE, lower initial boiling point and narrower distillation range, such as white oil, aviation kerosene, degreased kerosene, synthetic alkane, paraffin oil and the like.

In one embodiment, the step S1 prepares the teflon-based tape as follows:

the polytetrafluoroethylene dispersing material and the auxiliary oil are fully and uniformly mixed according to a certain proportion, and then are placed in a constant-temperature oven for curing for a certain time, so that the auxiliary oil is fully absorbed and swelled by the polytetrafluoroethylene powder. Pouring the completely cured polytetrafluoroethylene mixed powder into pre-pressing equipment, pre-pressing the mixture into a cylindrical material blank, putting the cylindrical material blank into an extruder, and extruding the cylindrical material blank into a strip-shaped thin film with the thickness of 1mm and the width of 185mm through a flat-mouth die.

The structure of the flat-mouth mold is shown in fig. 1 and fig. 2, and the flat-mouth mold is formed by attaching two symmetrical half molds 10, and a cavity formed by the two half molds is provided with an approximately conical inclined plane extrusion channel.

Specifically, the flat die has a feed channel 11 and an extrusion channel 12. Wherein, feed channel 11 one end forms feed inlet 110, and the other end is linked together with the one end of extruding passageway 12, extrudes passageway 12's the other end and forms discharge gate 120. The distance between the left and

right side walls

121, 122 of the extrusion channel 12 gradually increases in the discharge direction, and the distance between the upper and

lower side walls

123, 124 of the extrusion channel 12 gradually decreases in the discharge direction.

In this way, the blank enters the extrusion channel 12 through the feeding channel 11, and under the action of pressure, the blank gradually spreads and disperses along the extrusion channel 12, and a strip-shaped film with the thickness of 1mm and the width of 185mm can be formed at the discharge port 120. Accordingly, the discharge hole 120 has a length of 185mm and a width of 1 mm.

In one embodiment, the inner diameter of the feed channel 11 decreases gradually in the feeding direction, which facilitates the accumulation of the blanks and the subsequent extrusion. Specifically, the feed channel 11 may be a conical channel, one end of which is further communicated with one end of the extrusion channel 12 through a straight pipe 13.

In addition, the mold halves 10 are secured together by an outer sleeve 14. Correspondingly, the two mold halves 10 are provided with a protruding structure 101 for cooperating with the outer sleeve 14 at a position where the outer sleeve 14 is sleeved.

When the extruder works, the temperature of a cylinder barrel and a neck ring die of the extruder is heated to 70-100 ℃, polytetrafluoroethylene paste is extruded into a round rod with the diameter of 17mm before entering a channel and then enters a conical extrusion channel, the paste is gradually widened in width and gradually thinned in thickness along with the change of the shape of the extrusion channel in the advancing process, and finally the paste is extruded after the tail end is slightly shaped.

Subsequently, the PTFE extruded tape was rolled into a base tape 150-. And simultaneously drying two or more rolls of the rolled base bands. The combination of multiple rolls is realized by a multilayer overlapping technology, two or more layers of rolling base bands are overlapped and bonded together under the action of a compression roller, and when drying and degreasing are carried out, the multiple layers of base bands are simultaneously sent to be dried, so that the base bands are bonded into a roll.

As shown in fig. 3. And under the action of the compression roller, multiple rolls of base bands are adhered together and enter a drying oven at the same time, and under the high-temperature action of a plurality of hot rollers, the auxiliary oil in the base bands is volatilized and then removed by a smoke exhaust fan. The set temperature of the oven is 100-140 ℃, and the set temperature of the hot roller is 170-200 ℃. In the process that the base band is tightly attached to the hot roller for drying, the microcrystals among all layers are adhered and embedded to form a compact base band.

And S2, stretching the dried and degreased polytetrafluoroethylene tape.

When the longitudinal stretching is carried out, the stretching temperature is required to be ensured to be above the glass transition temperature of PTFE, and the stretching temperature is preferably 200-380 ℃. The fibers are pulled out of the nodes by the speed difference between the front and rear rolls, and the longitudinal length of the PTFE base tape is elongated 2 to 5 times the original length.

Because the stress is transmitted from two sides to the middle during transverse stretching, the PTFE base band manufactured by the process is thick and hard, and the thick two sides of the obtained finished film are always thin when the transverse stretching is directly carried out after longitudinal stretching. Therefore, the transverse drawing is carried out after the one-step pre-transverse drawing process. The pre-transverse drawing process is completed under the heat preservation condition of 150-350 ℃, the transverse width of the PTFE longitudinal drawing belt is stretched by 1.8 times, a structure with a thin middle part and two thick sides is formed, and then transverse drawing and heat setting are carried out.

In the transverse stretching process, the temperature is controlled at 150 ℃ and 350 ℃, and the width of the PTFE base band is expanded by 4.5 to 15 times. The fibers are pulled out of the transverse nodes, the nodes are split into smaller nodes, and the longitudinal and transverse fibers are interlaced with each other to form a net structure, as shown in fig. 4. The transversely stretched film needs to be subjected to heat setting treatment, the process is generally carried out at the temperature of more than 350 ℃, and the aim is to eliminate internal stress generated by stretching, to recrystallize molecular chains, to prevent the film from shrinking and to improve the strength of the film.

The nano-scale porous filtration membrane for liquid filtration of the present invention can be prepared based on the preparation method as described above.

Specifically, in the nano-scale porous filter membrane, fibrils are pulled out from nodes and are distributed in a staggered mode in the transverse and longitudinal directions to form small and uniform pore sizes, and the average pore size is 40-100 nm. The thickness of the nano-scale porous filter membrane is 15-45 μm. The bubble point pressure of the nano-scale porous filter membrane is 0.20-0.35 MPa. The breaking strength of the nano-scale porous filter membrane is 22-35 MPa. The air permeability of the nano-scale porous filter membrane under the pressure difference of 500Pa is 10-30 mm/s.

The technical solution of the present invention will be illustrated below with reference to specific examples.

Example 1

PTFE raw material	601A
		Auxiliary oil	Mofu ISOPAR H
Extrusion die type	Flat-mouth die
		Thickness of rolling	200μm
Drying mode	Single layer
		Longitudinal drawing magnification	2 times of
Pre-transverse drawing ratio	1.8 times of
		Transverse drawing magnification	4.5 times of

Example 2

PTFE raw material	CGF216M
		Auxiliary oil	Mofu ISOPAR M
Extrusion die type	Flat-mouth die
		Thickness of rolling	200μm
Drying mode	Single layer
		Longitudinal drawing magnification	3.5 times of
Pre-transverse drawing ratio	1.8 times of
		Transverse drawing magnification	10 times of

Example 3

PTFE raw material	CGF216M
		Auxiliary oil	Mofu ISOPAR M
Extrusion die type	Flat-mouth die
		Thickness of rolling	150μm
Drying mode	Two layers of
		Longitudinal drawing magnification	3.5 times of
Pre-transverse drawing ratio	1.8 times of
		Transverse drawing magnification	10 times of

Example 4

PTFE raw material	CGF216M
		Auxiliary oil	Mofu ISOPAR M
Extrusion die type	Flat-mouth die
		Thickness of rolling	150μm
Drying mode	Two layers of
		Longitudinal drawing magnification	5 times of
Pre-transverse drawing ratio	1.8 times of
		Transverse drawing magnification	15 times of

Example 5

PTFE raw material	CGF216M
		Auxiliary oil	Mofu ISOPAR M
Extrusion die type	Flat-mouth die
		Thickness of rolling	150μm
Drying mode	Three layers
		Longitudinal drawing magnification	3.5 times of
Pre-transverse drawing ratio	1.8 times of
		Transverse drawing magnification	10 times of

Comparative example 1

PTFE raw material	CGF216M
		Auxiliary oil	Mofu ISOPAR M
Extrusion die type	Round mouth mold
		Thickness of rolling	200μm
Drying mode	Single layer
		Longitudinal drawing magnification	3.5 times of
Pre-transverse drawing ratio	1.8 times of
		Transverse drawing magnification	10 times of

In order to prove the excellent performances of the nano-scale porous filter membrane. The invention adopts the following test method to test various performances of the nano-scale porous filter membrane.

1. Microstructure

The microstructure of the film, including the shape and size of the nodes, and the direction and size of the fibers, was observed using a KYKY-6900 model Scanning Electron Microscope (SEM). The microstructure is shown in fig. 4.

2. Determination of breaking Strength

The breaking strength of the film was measured by a model YG026D electron intensity tester: 3 samples with the width of 50mm and the length of 250mm are respectively taken in the longitudinal direction and the transverse direction of the PTFE film, the stretching speed of an electronic strength machine is set to be 100mm/min, the clamping length is set to be 200mm, the samples are fixed on a clamp and screwed, and the test is started. And recording and storing the data after the test is finished. The fracture strength of the nano-scale porous filter membrane is 22-35MPa through determination.

3. Thickness measurement

The film thickness was measured using a CH-1-S micrometer caliper. The thickness of the nano-scale porous filter membrane is measured to be 15-45 mu m.

4. Bubble point pressure measurement

Measuring the bubble point pressure of the film by using a filter film bubble pressure tester: firstly, putting the membrane into alcohol to be fully soaked, then installing the membrane below a mesh plate and sealing and fixing the membrane, then pouring the alcohol on the mesh plate and slowly introducing air, and measuring the pressure when the first bubble point appears, namely the bubble point pressure. The bubble point pressure of the nano-scale porous filter membrane is 0.20-0.35MPa through measurement.

5. Liquid flow rate determination

The liquid flow rate of the membrane is measured by a filter membrane flow rate tester: firstly, the membrane is put into alcohol to be fully soaked, then the membrane is arranged below the measuring cylinder and sealed and fixed, and the time of 50ml of water passing through the membrane under the action of one atmosphere, namely the liquid flow rate, is measured. The liquid flow rate of the nano-scale porous filter membrane is measured to be 5-15 s.

6. Air permeability measurement

The air permeability of the film is measured by a full-automatic air permeability tester produced by force obligue, and the pressure difference is set to be 500 Pa. The air permeability of the high-bubble pressure polytetrafluoroethylene microporous membrane is measured to be 10-30 mm/s.

Based on the above test methods, the performance parameters of the polytetrafluoroethylene microporous membranes of the examples are set forth in Table 1.

TABLE 1 Performance parameters of microporous polytetrafluoroethylene membranes

According to the performance parameters of the PTFE membranes of the embodiments in the table 1, the invention effectively reduces the pore diameter of the PTFE microporous membrane and obviously improves the bubble point pressure and the breaking strength of the microporous membrane by changing the extrusion process and the drying mode. Flat die extruded tapes do not develop excessive fiber orientation during calendering compared to round dies, while achieving higher strength. The fiber length is limited during the drawing process, so that the pore size of the obtained finished membrane is small and uniform. In addition, the fibers of each layer of the multilayer PTFE base tape are mutually staggered and covered in the biaxial stretching process, so that smaller pore diameters are obtained. Therefore, the PTFE microporous membrane prepared by the invention has the advantages of high bubble point pressure, small aperture and high breaking strength, and overcomes the defects in the prior art.

It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrative embodiments, and that the present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. Any reference sign in a claim should not be construed as limiting the claim concerned.

Furthermore, it should be understood that although the present description refers to embodiments, not every embodiment may contain only a single embodiment, and such description is for clarity only, and those skilled in the art should integrate the description, and the embodiments may be combined as appropriate to form other embodiments understood by those skilled in the art.

Claims

1. A preparation method of a nano-scale porous filter membrane for liquid filtration is characterized by comprising the following steps:

s2, drying and degreasing the polytetrafluoroethylene tape;

2. The method of preparing a nano-scale porous filtration membrane for liquid filtration according to claim 1, wherein the flat-mouth die has a feed channel and an extrusion channel, one end of the feed channel forms a feed inlet, the other end of the feed channel is communicated with one end of the extrusion channel, and the other end of the extrusion channel forms a discharge outlet;

3. The method of preparing a nano-scale porous filtration membrane for liquid filtration according to claim 2, wherein the discharge hole has a length of 185mm and a width of 1 mm.

4. The method for preparing a nano-scale porous filtration membrane for liquid filtration according to claim 1, wherein the step S2 comprises:

5. The method of claim 1, wherein the stretching the dried ptfe tape comprises:

s31, longitudinally stretching the dried polytetrafluoroethylene tape;

6. The method of preparing a nano-sized porous filtration membrane for liquid filtration according to claim 5, wherein the stretching magnification of the longitudinal stretching is 2 to 5 times; the stretching ratio of the pre-transverse drawing is 1.8 times; the stretching magnification of the transverse stretching is 4.5 to 15 times.

7. The method of preparing a nanoporous filtration membrane for liquid filtration according to claim 1, wherein the average thickness of the nanoporous filtration membrane is 15-45 μm.

8. The method of preparing a nano-sized porous filtration membrane for liquid filtration according to claim 1, wherein the average pore size of the nano-sized porous filtration membrane is 40 to 100 nm.

9. The method of preparing a nanoporous filtration membrane for liquid filtration according to claim 5, wherein the bubble point pressure of the nanoporous filtration membrane is 0.2-0.35 MPa.

10. A nanoporous filtration membrane for liquid filtration obtainable by the preparation process according to any one of claims 1 to 9.