Method for preparing polyvinylidene fluoride hollow fiber porous membrane by using recycled membrane filaments

Abstract

The invention relates to a method for preparing a polyvinylidene fluoride hollow fiber porous membrane by using recycled membrane filaments, which comprises the following steps: 1) pretreatment of the recycled membrane filaments: cutting the recycled membrane filaments into sections, washing, soaking in liquid nitrogen, crushing and sieving to obtain recycled membrane filament powder, and testing the melt index of the recycled membrane filament powder; 2) mixing: adding the recycled membrane filament powder in the step 1) into the mixing procedure according to the melt index, the addition proportion of the recycled membrane filament powder is 10-50% of the total mass of the polyvinylidene fluoride resin, and uniformly mixing the raw materials to obtain mixed powder for membrane casting; and 3) spinning to form a membrane, namely melting and extruding the mixed powder for casting the membrane in the step 2), cooling in a coagulating bath, and then stretching, heat setting and post-processing to obtain the polyvinylidene fluoride hollow fiber porous membrane. The recycling method is simple and effective, the hollow fiber porous membrane spun from the recycled material is stable in performance, and waste of raw materials and production cost can be reduced to a great extent.

Description

Technical Field

[0001] The invention relates to a method for preparing a polyvinylidene fluoride hollow fiber porous membrane by utilizing recycled membrane fibers, and belongs to the technical field of membranes. Background Art

[0002] Polyvinylidene fluoride, abbreviated as PVDF in English, is a highly non-reactive thermoplastic fluorine-containing polymer with excellent chemical corrosion resistance, high temperature resistance, oxidation resistance and weather resistance, as well as excellent piezoelectricity, dielectricity and thermoelectricity and other special properties. It is widely used in lithium battery adhesives, diaphragm coating adhesives, fluorocarbon coatings, photovoltaic films, water treatment membranes, pipes, rods and other industries. However, due to the production of upstream basic materials, PVDF has been a product in short supply for quite a long time.

[0003] In recent years, with the rapid development of the environmental protection field, polyvinylidene fluoride (PVDF), as an excellent membrane material, has also been widely used in the preparation of industrial microfiltration membranes and ultrafiltration membranes. At present, the annual production capacity of PVDF hollow fiber membranes in the world has reached hundreds of millions of square meters. It can be seen that the annual usage of PVDF is very large and is still growing rapidly. Combined with the above-mentioned situation of PVDF in short supply and skyrocketing prices, all membrane material manufacturers are facing the problem of no goods to buy and high costs.

[0004] Due to the limitations of the ultrafiltration membrane component manufacturing process, the intermediate link of converting PVDF from raw materials to membrane components faces a certain amount of membrane fiber waste. According to the different production processes and automation levels of each company, the overall waste rate is expected to be between 5% and 15%. Among them, the component manufacturing process is the process link with the highest waste rate, which is expected to account for more than 80% of the overall waste rate. At present, the practice of various companies is either to stockpile a large amount of waste in warehouses, which not only occupies a large amount of storage space but also requires pollution isolation measures; or to pay high fees to qualified environmental protection companies for treatment. These environmental protection qualified companies mainly landfill the above-mentioned waste materials, which will cause harm to the natural environment and have an adverse impact on the sustainable development of natural resources.

[0005] In fact, the wasted membrane contains a large amount of PVDF. In the context of PVDF in short supply and high prices, if this part of PVDF can be recycled, it can not only avoid resource waste and reduce harm to the natural environment, but also improve the effective yield of PVDF into membrane to a certain extent, indirectly reducing the purchase of PVDF raw materials and the production cost of membrane companies.

[0006] At present, there are mainly two manufacturing processes for producing PVDF hollow fiber ultrafiltration membranes: the nonsolvent induced phase separation method (NIPS method), also known as the wet method, and the thermally induced phase separation method (TIPS method), also known as the thermal method. Among them, the recycling of PVDF membranes made by the wet method has been reported after engineering applications.

[0007] Chinese Patent CN101422707B discloses a method for recycling waste PVDF hollow fiber membranes with liners. The method includes the following steps: a) pretreatment of washing the waste PVDF membranes; b) dissolution; c) filtration; d) adjusting the concentration; e) adding a pore-forming agent; f) spinning hollow fiber membranes. This patent mainly solves the problem of recycling waste PVDF hollow fiber membranes with liners used in engineering. The pollutants contained in such used hollow fiber membranes are complex and generally difficult to clean completely. Even if recycled according to the method in the patent, it will lead to a decline in the performance of the newly spun hollow fiber membranes.

[0008] Chinese Patent CN114588784A discloses a comprehensive utilization method for waste reinforced hollow fiber membranes with liners. The method is to soak and clean the waste reinforced hollow fiber membrane modules separately in a mixed solution of sodium hypochlorite and sodium hydroxide and a citric acid solution, then leak-check with nitrogen in soapy water, repair the detected leakage points, and finally recycle them as membrane elements for membrane oxygenation and membrane aeration bioreactors to achieve the resource utilization and reduction of solid waste of waste reinforced hollow fiber membrane modules, so as to achieve the purpose of comprehensive utilization. This method is to reuse the waste membrane modules for other purposes, but in fact, the amount of membrane elements used for membrane oxygenation and membrane aeration bioreactors is much less than that of existing PVDF hollow fiber membranes.

[0009] Chinese Patent CN115612169A discloses a method for quickly cooling the PVDF layer on the used hollow fiber membrane with a liner by low-temperature freezing, and then breaking and detaching the brittle PVDF surface layer from the PET support layer through external physical actions, finally realizing the separation of PET and PVDF, so as to recycle the PVDF material. This method requires a large amount of chemicals such as acids and sodium hypochlorite to clean the membrane filaments. Under the physical action after rapid cooling, it is also very easy to rub off the lining material PET together with PVDF, which will ultimately result in insufficient purity of the recycled PVDF material. In addition, using physical methods such as rubbing is not easy to accurately control for such relatively thin hollow fiber membranes, with low efficiency, and the PVDF on the surface layer cannot be completely detached from the lining by 100%.

[0010] The above-mentioned patents are all aimed at solving the problem of how to recycle the PVDF hollow fiber membranes prepared by the wet method after being used in engineering projects. There is no mention of how to recycle and regenerate the PVDF hollow fiber membranes prepared by the thermal method. Summary of the Invention

[0011] In view of the above problems existing in the prior art, the present invention provides a method for preparing polyvinylidene fluoride hollow fiber porous membranes by using recycled membrane filaments. The processing process of this method is simple and effective, and physical means are adopted throughout the process without changing the structure of PVDF. The prepared PVDF hollow fiber membranes are comparable to the membrane filaments prepared with new materials in terms of basic performance, greatly reducing the waste of raw materials and production costs, significantly improving the overall yield of the membrane filaments, and having great practical application value and practical significance.

[0012] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0013] A method for preparing polyvinylidene fluoride hollow fiber porous membranes by using recycled membrane filaments, comprising the following steps:

[0014] 1) Pretreatment of recycled membrane filaments: Cut the recycled membrane filaments into segments, wash them with water, soak them in liquid nitrogen, crush and sieve them to obtain recycled membrane filament powders;

[0015] 2) Mixing: Add the recycled membrane filament powders obtained in step 1) to the mixing process. The addition ratio of the recycled membrane filament powders accounts for 10-50% of the total mass of the polyvinylidene fluoride resin, such as 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%. Mix all the raw materials evenly to obtain a mixed powder for film casting;

[0016] 3) Spinning and film formation: Melt and extrude the mixed powder for film casting obtained in step 2), cool it in a coagulation bath, then stretch it, heat-set it, and perform post-treatment to obtain polyvinylidene fluoride hollow fiber porous membranes.

[0017] In the present invention, the recycled membrane filaments in step 1) are the waste membrane filaments generated during the process of preparing polyvinylidene fluoride hollow fiber membranes by the thermal method, or the waste membrane filaments generated during the production process of converting polyvinylidene fluoride from raw materials into membrane components.

[0018] In the present invention, the cutting into segments in step 1) means cutting the recycled membrane filaments into membrane filament segments with a size of 2-5 mm, such as 2 mm, 3 mm, 4 mm, 5 mm.

[0019] In the present invention, the washing with water in step 1) means soaking the cut membrane filament segments in pure water for cleaning. Preferably, the soaking time is 1-6 h, such as 1 h, 2 h, 3 h, 4 h, 5 h, 6 h; preferably, the number of water washing times is 2-4 times, such as 2 times, 3 times, 4 times, to dissolve the water-soluble protective agent on the membrane filaments, and drain them after washing.

[0020] In the present invention, the immersion in liquid nitrogen in step 1) means immersing the membrane filament segments drained after water washing into liquid nitrogen for 20 to 60 seconds, such as 20 seconds, 30 seconds, 40 seconds, 50 seconds, 60 seconds.

[0021] In the present invention, the pulverizing and sieving in step 1) means pulverizing the membrane filament segments after immersion in liquid nitrogen into powder, and screening the powder through a 80-mesh sieve; preferably, the powder above the 80-mesh sieve is repeatedly subjected to liquid nitrogen immersion and pulverizing and sieving until it passes through the 80-mesh sieve.

[0022] In the present invention, in the mixing process of step 2), first measure the melt index of the membrane filament powder, and then determine its actual addition ratio in polyvinylidene fluoride according to the measured value of the melt index of the recycled membrane filament powder;

[0023] Preferably, when the melt index range is ≥2 g / 10 min and ≤4 g / 10 min (230 °C, 5 Kg), the addition ratio is ≥10 wt% and ≤20 wt%, such as 10 wt%, 12 wt%, 14 wt%, 16 wt%, 18 wt%, 20 wt%;

[0024] When the melt index range is >4 g / 10 min and ≤6 g / 10 min (230 °C, 5 Kg), the addition ratio is >20 wt% and ≤30 wt%, such as 20.1 wt%, 22 wt%, 24 wt%, 26 wt%, 28 wt%, 30 wt%;

[0025] When the melt index range is >6 g / 10 min and ≤8 g / 10 min (230 °C, 5 Kg), the addition ratio is >30 wt% and ≤50 wt%, such as 30.1 wt%, 35 wt%, 40 wt%, 45 wt%, 50 wt%;

[0026] Specifically, the melt index refers to measuring the melt index of a certain amount of powder at 230 °C under a pressure of 5 kg.

[0027] In the present invention, the polyvinylidene fluoride resin in step 2) includes recycled membrane filament powder and new polyvinylidene fluoride resin;

[0028] Preferably, the new polyvinylidene fluoride resin is selected from polyvinylidene fluoride homopolymers or copolymers with a weight average molecular weight of 200,000 to 700,000, such as 200,000, 300,000, 400,000, 500,000, 600,000, 700,000. Here, the new polyvinylidene fluoride resin refers to the polyvinylidene fluoride resin other than the recycled membrane filament powder.

[0029] In the present invention, the mixed powder for film casting in step 2) contains the following raw materials in mass percentage:

[0030] 25 to 60% of polyvinylidene fluoride resin, such as 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, preferably 30 to 50%;

[0031] 10 to 60% of organic pore former, such as 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, preferably 30 to 50%;

[0032] 10 to 50% of inorganic pore former, such as 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, preferably 20 to 40%;

[0033] 0 to 5% of additive, such as 0%, 0.1%, 1%, 2%, 3%, 4%, 45%, 5%, preferably 0 to 3%;

[0034] The sum of the above raw material ratios is 100%. The polyvinylidene fluoride resin includes recycled membrane fiber powder and new polyvinylidene fluoride resin.

[0035] In the present invention, the recycled membrane fiber powder is added to the mixing process in proportion, and the proportion of its addition is controlled to be 10 - 50 wt% of the total amount of polyvinylidene fluoride resin (PVDF). At the same time, the proportion of the total amount of polyvinylidene fluoride resin in the mixed powder for film casting is controlled within a certain range. When exceeding this proportion, the viscosity of the casting solution is too high, making it difficult to form a film and requiring high equipment requirements; while below this proportion range, the feeding requirements are high, and the mechanical strength of the obtained porous membrane is also weak.

[0036] Preferably, the organic pore former is selected from one or a mixture of phthalic acid esters (such as dibutyl phthalate, dioctyl phthalate, etc.), benzoic acid esters, sebacic acid esters, adipic acid esters, phosphate esters, benzophenone, tributyl acetylcitrate, soybean oil, diphenylmethane, trioctyl trimellitate, ethylene bisstearamide, γ-butyrolactone.

[0037] Preferably, the inorganic pore former is selected from one or a mixture of nano-silica, nano-calcium carbonate, nano-zinc oxide, etc.

[0038] Preferably, the additive is selected from one or several of lubricants, antioxidants, heat stabilizers, ultraviolet absorbers, anti-aging agents. The additive is a conventional selection in the field and can be added according to specific functional requirements. The present invention does not make specific limitations.

[0039] The specific operation of the spinning and film forming in step 3) of the present invention is a conventional technical means well-known in the field. Those skilled in the art can refer to the prior art and combine and screen steps and parameters according to actual needs. The present invention has no special requirements. The following content is only an example of some preferred parameters and is not used to limit the present invention.

[0040] In the present invention, the melt extrusion in step 3) is to shear and melt the mixed powder for film casting by a twin-screw extruder, and then extrude and form it through a spinneret for annular hollow fiber membranes. The melting temperature of the extruder is controlled between 220 and 280 °C, such as 220 °C, 230 °C, 240 °C, 250 °C, 260 °C, 270 °C, 280 °C.

[0041] In the present invention, the coagulation bath cooling in step 3) is to immerse the hollow fiber membrane filaments formed by melt extrusion into a coagulation bath water tank for cooling and forming. The temperature of the coagulation bath is 20 to 70 °C, such as 20 °C, 30 °C, 40 °C, 50 °C, 60 °C, 70 °C.

[0042] In the present invention, the stretching in step 3) is to stretch the hollow fiber membrane filaments obtained by coagulation bath cooling in the length direction, and the stretching ratio is 1.5 to 3 times, such as 1.5 times, 2 times, 2.5 times, 3 times.

[0043] In the present invention, the heat setting in step 3) is to shape the membrane filaments through a hot oven. The setting temperature is 90 to 150 °C, such as 90 °C, 100 °C, 110 °C, 120 °C, 130 °C, 140 °C, 150 °C, and the time is 10 to 60 minutes, such as 10 minutes, 20 minutes, 30 minutes, 40 minutes, 50 minutes, 60 minutes.

[0044] In the present invention, the post-treatment in step 3) includes organic extraction post-treatment and inorganic extraction post-treatment; preferably, after the post-treatment is completed, it also includes washing with water to obtain the membrane product;

[0045] Preferably, the extractant for the organic extraction post-treatment is selected from one or more of n-hexane, cyclohexane, gasoline, ethanol, N, N-dimethylformamide, N, N-dimethylacetamide, dichloromethane, chloroform, and isopropanol. Specifically, according to the process characteristics, an extractant that is easy to recover is preferred; more preferably, the extraction temperature is 20 to 80 °C, such as 20 °C, 30 °C, 40 °C, 50 °C, 60 °C, 70 °C, 80 °C, and the extraction time is 4 to 10 hours, such as 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, 10 hours;

[0046] Preferably, the extractant for the inorganic extraction post-treatment is selected from sodium hydroxide solution or hydrochloric acid, more preferably 5 to 10 wt% of an aqueous sodium hydroxide solution, such as 5 wt%, 7 wt%, 9 wt%, 10 wt%, or 2 to 5 wt% of hydrochloric acid, such as 2 wt%, 3 wt%, 4 wt%, 5 wt%; more preferably, the extraction temperature is 20 to 80 °C, such as 20 °C, 30 °C, 40 °C, 50 °C, 60 °C, 70 °C, 80 °C, and the extraction time is 4 to 10 hours, such as 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, 10 hours.

[0047] Compared with the prior art, the beneficial effects of the technical solution of the present invention are as follows:

[0048] The present invention provides a method for preparing a polyvinylidene fluoride hollow fiber porous membrane using recycled materials. The method is simple and effective, and the performance of the hollow fiber porous membrane spun from the recycled materials is stable, which can greatly reduce the waste of raw materials, lower the production cost and the environmental protection treatment pressure, and has important practical significance and economic benefits. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure 1 is a flow chart of the method for preparing a polyvinylidene fluoride hollow fiber porous membrane using recycled membrane filaments according to the present invention;

[0050] Figure 2 is a scanning electron microscope image of the cross-section of the hollow fiber membrane prepared in Example 1;

[0051] Figure 3 is the pore size and its distribution diagram of the hollow fiber membrane prepared in Example 1;

[0052] Figure 4 is a scanning electron microscope image of the cross-section of the hollow fiber membrane prepared in Comparative Example 1;

[0053] Figure 5 is the pore size and its distribution diagram of the hollow fiber membrane prepared in Comparative Example 1. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0054] The present invention will be further described below through specific embodiments. The embodiments described in the present invention are only for the purpose of illustrating the present invention and do not limit the scope of the present invention.

[0055] The sources of raw materials in the examples and comparative examples are as follows:

[0056] Polyvinylidene fluoride resin: The weight average molecular weight is 300,000 - 320,000, Solvay, Belgium, 6010;

[0057] Nanosilica: Evonik Degussa (China) Co., Ltd., R972;

[0058] Nanocalcium carbonate: Shanghai Yuanjiang Chemical Co., Ltd.;

[0059] Unless otherwise specified, other raw materials and reagents can be obtained through ordinary commercial channels.

[0060] The separation performance of the prepared polyvinylidene fluoride hollow fiber porous membrane is mainly characterized by three characteristic parameters, namely the average pore size of the membrane, the pure water flux and the mechanical strength.

[0061] (1) Average pore diameter of the membrane: Measured using a POROLUX 1000 pore size analyzer. The calculation formula based on the test principle is:

[0062] D = 4δcosθ / P

[0063] In the formula: D - membrane pore diameter, μm;

[0064] δ - liquid surface tension, N / m;

[0065] θ - contact angle between the liquid and the pore wall, °;

[0066] P - gas pressure, Pa;

[0067] (2) Definition of pure water flux (LMH): Under certain operating pressure conditions, it is the volume of water passing through the effective membrane area per unit time. Its calculation formula is:

[0068] J = Q / At

[0069] Where: J - flux, L / m 2 ·h@0.1 mPa;

[0070] Q - permeation amount of pure water, L;

[0071] A - filtration area of the membrane, m 2 ;

[0072] t - time for collecting the permeate, h;

[0073] (3) Mechanical strength: The breaking strength (MPa) and elongation at break (%) of the hollow fiber membrane filaments are tested using a single fiber tensile machine at a certain pulling speed (50 mm / min).

[0074]

Example 1

[0075] To prepare a polyvinylidene fluoride hollow fiber porous membrane using recycled membrane filaments, the steps are as follows:

[0076] 1) Cut the recycled membrane filaments cut during the casting process of the hollow fiber porous membrane module into membrane filament segments of 2 - 5 mm using a pulverizer. Soak the cut membrane filaments in pure water for 3 times, 1 hour each time, to dissolve the water-soluble protective agent on the membrane filaments. After draining, soak them in liquid nitrogen for 60 seconds, then take them out and quickly pulverize them into powder in the pulverizer. Sieve the powder through an 80-mesh filter screen to obtain the recycled membrane filament powder. The melt index of the powder is tested to be 5.8 g / 10 min (230 °C, 5 Kg).

[0077] 2) Weigh 38% of polyvinylidene fluoride resin by mass ratio (where 8% is recycled membrane filament powder, accounting for 21% of the total mass of polyvinylidene fluoride), 25% of nano-silica, 26% of dioctyl phthalate, 10% of dibutyl phthalate, and 1% of antioxidant 330. Mix them evenly in a high-speed mixer to obtain the mixed powder for film casting.

[0078] 3) Pass the mixed powder for film casting through a twin-screw extruder (screw diameter Φ20mm, length-diameter ratio 40:1), control the barrel temperature at 270°C for kneading and melting, then pass it through an annular spinneret with an outer diameter of 1.8mm and an inner diameter of 0.9mm, control the nozzle die temperature at 260°C, and inject air into the inner diameter of the spinneret at an air flow rate of 40 mL / min. Introduce the extruded hollow fiber membrane filaments into a pure water coagulation cooling bath with a temperature controlled at 30°C for shaping. Then, stretch them at a stretching ratio of 2 times through a stretching device, and then thermally set them in a heat setting oven at 140°C for 60 minutes. Extract the thermally set and dried membrane filaments with ethanol at 70°C for 6 hours, and then extract them with a 5wt% sodium hydroxide aqueous solution at 70°C for 6 hours. Finally, after washing with water, a polyvinylidene fluoride hollow fiber porous membrane added with recycled material is obtained.

[0079] Take a photo of the cross-section of the polyvinylidene fluoride hollow fiber porous membrane prepared in this example through a scanning electron microscope, as Figure 2 shown. Use a POROLUX1000 pore size analyzer to measure the pore size and its distribution of the membrane, as Figure 3 shown.

[0080]

Comparative Example 1

[0081] No recycled membrane filament powder is added to the polyvinylidene fluoride resin in the mixed powder for film casting, and all new polyvinylidene fluoride resin is used. Specifically, weigh 38% of polyvinylidene fluoride resin, 25% of nano-silica, 26% of dioctyl phthalate, 10% of dibutyl phthalate, and 1% of antioxidant 330 by mass percentage, and mix them evenly in a high-speed mixer.

[0082] Other spinning and post-treatment conditions are the same as those in Example 1 to obtain a polyvinylidene fluoride hollow fiber porous membrane.

[0083] Take a photo of the cross-section of the polyvinylidene fluoride hollow fiber porous membrane prepared in Comparative Example 1 through a scanning electron microscope, as Figure 4 shown. Use a POROLUX1000 pore size analyzer to measure the pore size and its distribution of the membrane, as Figure 5 shown.

[0084]

Example 2

[0085] Prepare a polyvinylidene fluoride hollow fiber porous membrane using recycled membrane filaments. The steps are as follows:

[0086] 1) Cut the recycled membrane filaments cut off during the casting process of the hollow fiber porous membrane module into membrane filament segments of 2 - 5 mm with a pulverizer. Soak the cut membrane filaments in pure water for 3 times, 1 hour each time, to dissolve the water-soluble protective agent on the membrane filaments. After draining, soak them in liquid nitrogen for 40 seconds, then take them out and quickly pulverize them into powder in the pulverizer. Screen the powder through an 80-mesh filter screen to obtain the recycled membrane filament powder. The melt index of the powder is measured to be 2.6 g / 10 min (230 °C, 5 Kg).

[0087] 2) Weigh 30% of polyvinylidene fluoride resin by mass ratio (including 4% of the recycled membrane filament powder, accounting for 13.3% of the total mass of polyvinylidene fluoride), 40% of nano calcium carbonate, 25% of tributyl acetylcitrate, and 5% of γ-butyrolactone. Mix them evenly in a high-speed mixer to obtain the mixed powder for film casting.

[0088] 3) Pass the mixed powder for film casting through a twin-screw extruder (screw diameter Φ30 mm, length-diameter ratio 50:1), control the barrel temperature at 250 °C for mixing and melting, then pass it through an annular spinneret with an outer diameter of 1.8 mm and an inner diameter of 0.9 mm, and control the nozzle temperature of the nozzle die at 240 °C. Inject air into the inner diameter of the spinneret at an air flow rate of 40 mL / min. Introduce the extruded hollow fiber membrane filaments into a pure water coagulation cooling bath with a temperature controlled at 30 °C for shaping. Then stretch them with a stretching ratio of 1.5 times through a stretching device, and then heat-set them in a heat-setting oven at 130 °C for 50 minutes. Extract the heat-set and dried membrane filaments with isopropanol at 30 °C for 6 hours, then extract them with a 2 wt% hydrochloric acid solution at 30 °C for 4 hours, and finally wash them to obtain the polyvinylidene fluoride hollow fiber porous membrane added with recycled materials.

[0089]

Comparative Example 2

[0090] No recycled membrane filament powder is added to the polyvinylidene fluoride resin in the mixed powder for film casting. All use new polyvinylidene fluoride resin. Specifically, weigh 30% of polyvinylidene fluoride resin, 40% of nano calcium carbonate, 25% of tributyl acetylcitrate, and 5% of γ-butyrolactone by mass percentage, and mix them evenly in a high-speed mixer.

[0091] Other spinning and post-treatment conditions are the same as those in Example 2 to obtain the polyvinylidene fluoride hollow fiber porous membrane.

[0092]

Example 3

[0093] The steps for preparing the polyvinylidene fluoride hollow fiber porous membrane using recycled membrane filaments are as follows:

[0094] 1) Cut the recycled membrane filaments cut off during the casting process of the hollow fiber porous membrane module into membrane filament segments of 2-5 mm with a pulverizer. Soak the cut membrane filaments in pure water and wash them twice, each time for 2 hours to dissolve the water-soluble protective agent on the membrane filaments. After draining, soak them in liquid nitrogen for 20 seconds, take them out and quickly pulverize them into powder in a pulverizer. Sieve the powder through a 80-mesh filter screen to obtain the recycled membrane filament powder. The melt index of the powder is tested to be 7.2 g / 10 min (230 °C, 5 Kg).

[0095] 2) Weigh 60% of polyvinylidene fluoride resin by mass ratio (where 30% of the recycled membrane filament powder accounts for 50% of the total mass of polyvinylidene fluoride), 10% of nano calcium carbonate, and 30% of benzophenone. Mix them evenly in a high-speed mixer to obtain the mixed powder for film casting.

[0096] 3) Pass the mixed powder for film casting through a twin-screw extruder (screw diameter Φ20 mm, length-diameter ratio 60:1), control the barrel temperature at 230 °C for mixing and melting, and then pass it through an annular spinneret with an outer diameter of 1.8 mm and an inner diameter of 0.9 mm, and control the nozzle die temperature of the spinneret at 210 °C, and inject air into the inner diameter of the spinneret at an air flow rate of 40 mL / min. Introduce the extruded hollow fiber membrane filaments into a pure water coagulation cooling bath with a temperature controlled at 30 °C for shaping. Then stretch them with a stretching ratio of 3 times through a stretching device, and then heat-set them in a heat-setting oven at 90 °C for 30 minutes. Extract the heat-set and dried membrane filaments with N,N-dimethylacetamide solution at 60 °C for 4 hours, and then extract them with 3 wt% hydrochloric acid solution at 30 °C for 5 hours. Finally, wash them with water to obtain the polyvinylidene fluoride hollow fiber porous membrane added with recycled materials.

[0097]

Comparative Example 3

[0098] No recycled membrane filament powder is added to the polyvinylidene fluoride resin in the mixed powder for film casting, and all new polyvinylidene fluoride resin materials are used. Specifically, weigh 60% of polyvinylidene fluoride resin, 10% of nano calcium carbonate, and 30% of benzophenone by mass percentage, and mix them evenly in a high-speed mixer.

[0099] Other spinning and post-treatment conditions are the same as those in Example 3 to obtain the polyvinylidene fluoride hollow fiber porous membrane.

[0100]

Example 4

[0101] Prepare the polyvinylidene fluoride hollow fiber porous membrane using recycled membrane filaments. The steps are as follows:

[0102] 1) Cut the recycled membrane filaments cut off during the casting process of the hollow fiber porous membrane module into membrane filament segments of 2-5 mm with a pulverizer. Immerse the cut membrane filaments in pure water and wash them twice, 3 hours each time, to dissolve the water-soluble protective agent on the membrane filaments. After draining, soak them in liquid nitrogen for 30 seconds. Take them out and quickly pulverize them into powder in a pulverizer. Screen the powder through an 80-mesh sieve to obtain recycled membrane filament powder. Test the melt index of the powder, which is 2 g / 10 min (230 °C, 5 Kg).

[0103] 2) Weigh 44% of polyvinylidene fluoride resin (including 4.4% of recycled membrane filament powder, accounting for 10% of the total mass of polyvinylidene fluoride), 20% of nano-zinc oxide, 27% of dioctyl phthalate, 6% of tributyl acetylcitrate, and 3% of antioxidant by mass ratio, and mix them evenly in a high-speed mixer to obtain the mixed powder for casting membranes.

[0104] 3) Pass the mixed powder for casting membranes through a twin-screw extruder (screw diameter Φ35 mm, length-diameter ratio 40:1), control the barrel temperature at 240 °C for mixing and melting, then pass through an annular spinneret with an outer diameter of 1.8 mm and an inner diameter of 0.9 mm, control the spinneret temperature at 230 °C in the nozzle die, and inject air into the inner diameter of the spinneret at an air flow rate of 40 mL / min. Introduce the extruded hollow fiber membrane filaments into a pure water coagulation cooling bath with a temperature controlled at 30 °C for shaping. Then, stretch them with a stretching ratio of 2.5 times through a stretching device, and then heat-set them in a heat-setting oven at 150 °C for 10 minutes. Extract the heat-set and dried membrane filaments with N,N-dimethylformamide solution at 70 °C for 2 hours, and then extract them with 5 wt% hydrochloric acid solution at 20 °C for 6 hours. Finally, wash them with water to obtain polyvinylidene fluoride hollow fiber porous membranes added with recycled materials.

[0105]

Comparative Example 4

[0106] Prepare polyvinylidene fluoride hollow fiber porous membranes using recycled membrane filaments. The steps refer to Example 1, with the only difference being that: in step 1), there is no water washing process for the cut membrane filaments, and other operations and conditions remain unchanged to prepare polyvinylidene fluoride hollow fiber porous membranes. Since there is a part of the residual membrane filament protective agent in the recycled powder, this part of the protective agent is easily vaporized to form bubbles under the high-temperature shear of the extruder, resulting in defects or extremely easy filament breakage in the subsequent stretching process of the hollow fiber membrane filaments, greatly reducing the yield and production efficiency.

[0107]

Comparative Example 5

[0108] Prepare a polyvinylidene fluoride hollow fiber porous membrane using recycled membrane filaments. The steps are referred to Example 1, with the difference being that in step 1), there is no liquid nitrogen immersion process after washing and draining the segmented membrane filaments. Other operations and conditions remain unchanged to prepare the polyvinylidene fluoride hollow fiber porous membrane. Since the polyvinylidene fluoride hollow fiber porous membrane itself has good toughness, it is difficult to be pulverized during the subsequent pulverization process by a pulverizer. Only a very small part of the pulverized material can pass through an 80-mesh filter screen after pulverization. This greatly affects the overall recovery rate of the pulverized material and the production efficiency.

[0109]

Comparative Example 6

[0110] Prepare a polyvinylidene fluoride hollow fiber porous membrane using recycled membrane filaments. The steps are referred to Example 1, with the difference being that in step 1), there is no pulverization and screening process by a pulverizer. Other operations and conditions remain unchanged to prepare the polyvinylidene fluoride hollow fiber porous membrane. Due to the existence of the recycled membrane filaments in segments, the recycled membrane filaments are not evenly dispersed during the mixing process, and the filaments are extremely prone to breakage during the extrusion molding and stretching processes, greatly reducing the yield and production efficiency. The prepared polyvinylidene fluoride hollow fiber porous membrane also has uneven performance.

[0111]

Comparative Example 7

[0112] Prepare a polyvinylidene fluoride hollow fiber porous membrane using recycled membrane filaments. The steps are referred to Example 1, with the difference being that in step 1), the liquid nitrogen immersion process is changed to a conventional freezing method such as general freezing. Other operations and conditions remain unchanged to prepare the polyvinylidene fluoride hollow fiber porous membrane. Since the polyvinylidene fluoride hollow fiber porous membrane itself has good toughness, the conventional freezing method has low efficiency and high energy consumption, and there are still problems of difficult pulverization and low sieve passing rate during the subsequent pulverization process by a pulverizer. This greatly affects the overall recovery rate of the pulverized material and the production efficiency.

[0113]

Comparative Example 8

[0114] Prepare a polyvinylidene fluoride hollow fiber porous membrane using recycled membrane filaments. The steps are referred to Example 1, with the difference being that in step 2), the addition ratio of the recycled membrane filament powder accounts for 60% of the total mass of polyvinylidene fluoride. Other operations and conditions remain unchanged to prepare the polyvinylidene fluoride hollow fiber porous membrane.

[0115] Similarly, if the addition ratio of the recycled membrane filament powder in step 2) is <

[0116] 10%, and other operations and conditions remain unchanged, polyvinylidene fluoride hollow fiber porous membranes with the same performance can be prepared. However, the utilization rate of the recycled powder is low, which is not conducive to improving the production efficiency. This will not be elaborated here.

[0117] Perform performance tests on the polyvinylidene fluoride hollow fiber porous membranes prepared in the above examples and comparative examples. The test data are shown in Table 1:

[0118] Table 1 Performance test results of polyvinylidene fluoride hollow fiber porous membranes

[0119]

[0120]

[0121] From the performance test results of preparing polyvinylidene fluoride hollow fiber porous membranes using the recycled membrane filaments in Examples 1 to 4 and Comparative Examples 1 to 3, it can be seen that the outer diameter of the polyvinylidene fluoride hollow fiber porous membranes prepared by the above recycling method is 1.18 - 1.22 mm, the wall thickness is 0.2 - 0.3 mm, the membrane pore size is 0.03 - 0.19 μm, the pure water flux is 1152 - 5381 L / m 2 ·h@0.1 mPa, 25°C, the tensile fracture strength is 9.97 -

[0122] 26.79 Mpa, and the tensile fracture elongation rate is 92 - 116%. The polyvinylidene fluoride hollow fiber porous membranes obtained by the recycling method and process of the present invention have no difference in performance from the membranes prepared with new raw materials, can be perfectly utilized, and greatly reduce the raw material procurement and production costs.

[0123] In addition, by comparing Figure 2 and 4 it can be known that the cross-section of the hollow fiber porous membrane prepared in Example 1 of the present invention has no defects and is consistent with the structure of the comparative example. By comparing Figure 3 and 5 it can be seen that the pore sizes of the prepared hollow fiber porous membranes are the same and the distribution is narrow.

[0124] From Comparative Examples 4 to 8, it is known that if some of the treatment steps are missing or exceed the addition amount range, problems such as filament breakage, decreased production efficiency, reduced recovery rate, and increased energy consumption will occur during the recycling process. Even if hollow fiber porous membrane filaments can be finally obtained, their performance will have great deficiencies and defects in terms of fracture strength and fracture elongation rate, which will greatly shorten the service life of the membrane in engineering applications.

[0125] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the method of the present invention, several improvements and supplements can be made, and these improvements and supplements should also be regarded as the protection scope of the present invention.

Claims

1. A method for preparing a polyvinylidene fluoride hollow fiber porous membrane using recycled membrane filaments, characterized in that, it comprises the following steps: 1) Pretreatment of recycled membrane filaments: Cut the recycled membrane filaments into segments, wash them with water, soak them in liquid nitrogen, pulverize and sieve them to obtain recycled membrane filament powders; 2) Mixing: Add the recycled membrane filament powders obtained in step 1) to the mixing process. The addition ratio of the recycled membrane filament powders accounts for 10-50% of the total mass of the polyvinylidene fluoride resin. Mix all the raw materials evenly to obtain a mixed powder for membrane casting; 3) Spinning and film formation: Melt and extrude the mixed powder for membrane casting obtained in step 2), cool it in a coagulation bath, then stretch it, thermally stabilize it, and post-treat it to obtain a polyvinylidene fluoride hollow fiber porous membrane.

2. The method according to claim 1, characterized in that, the recycled membrane filaments in step 1) are waste membrane filaments generated during the preparation of polyvinylidene fluoride hollow fiber membranes by the thermal method, or waste membrane filaments generated during the production process of converting polyvinylidene fluoride from raw materials into membrane modules.

3. The method according to claim 1, characterized in that, the cutting into segments in step 1) means cutting the recycled membrane filaments into membrane filament segments with a size of 2-5 mm; and / or the washing with water in step 1) means soaking the cut membrane filament segments in pure water for cleaning. Preferably, the soaking time is 1-6 h; preferably, the number of water washing times is 2-4 times, and drain after water washing; and / or the soaking in liquid nitrogen in step 1) means immersing the membrane filament segments drained after water washing in liquid nitrogen for 20-60 seconds; and / or the pulverizing and sieving in step 1) means pulverizing the membrane filament segments soaked in liquid nitrogen into powder, and screening the powder through a 80-mesh sieve; preferably, the powder above the 80-mesh sieve is repeatedly soaked in liquid nitrogen and pulverized and sieved until it passes through the 80-mesh sieve.

4. The method according to claim 1, characterized in that, in the mixing process of step 2), first measure the melt index of the membrane filament powder, and then determine its actual addition ratio in polyvinylidene fluoride according to the measured value of the melt index of the membrane filament powder.

5. The method according to claim 4, characterized in that, when the melt index range is ≥2 g / 10 min and ≤4 g / 10 min (230 °C, 5 Kg), the addition ratio is ≥10 wt% and ≤20 wt%; when the melt index range is >4 g / 10 min and ≤6 g / 10 min (230 °C, 5 Kg), the addition ratio is >20 wt% and ≤30 wt%; when the melt index range is >6 g / 10 min and ≤8 g / 10 min (230 °C, 5 Kg), the addition ratio is >30 wt% and ≤50 wt%.

6. The method according to claim 1, characterized in that, the polyvinylidene fluoride resin in step 2) comprises recycled membrane filament powders and new polyvinylidene fluoride resin; preferably, the new polyvinylidene fluoride resin is selected from polyvinylidene fluoride homopolymers or copolymers with a weight average molecular weight of 200,000-700,000.

7. The method according to claim 1, characterized in that, the mixed powder for membrane casting in step 2) contains the following raw materials in mass percentage: 25-60% of polyvinylidene fluoride resin, preferably 30-50%; Organic pore former: 10 - 60%, preferably 30 - 50%; Inorganic pore former: 10 - 50%, preferably 20 - 40%; Additive: 0 - 5%, preferably 0 - 3%; The sum of the above raw material ratios is 100%. The polyvinylidene fluoride resin includes recycled membrane fiber powder and new polyvinylidene fluoride resin.

8. According to the method described in claim 7, wherein, the organic pore former is selected from one or a mixture of several of phthalic acid esters, benzoic acid esters, sebacic acid esters, adipic acid esters, phosphate esters, benzophenone, tributyl acetylcitrate, soybean oil, diphenylmethane, trioctyl trimellitate, ethylene bisstearamide, γ-butyrolactone; and / or the inorganic pore former is selected from one or a mixture of several of nano-silica, nano-calcium carbonate, nano-zinc oxide.

9. According to the method described in claim 1, wherein, in step 3), the melt extrusion is to shear and melt the mixed powder for film casting using a twin-screw extruder, and then extrude and form it through a spinneret for annular hollow fiber membranes. The melt temperature of the extruder is controlled between 220 - 280°C; and / or in step 3), the coagulation bath cooling is to immerse the hollow fiber membrane filaments formed by melt extrusion into a coagulation bath water tank for cooling and forming. The temperature of the coagulation bath is 20 - 70°C; and / or in step 3), the stretching is to stretch the hollow fiber membrane filaments obtained by coagulation bath cooling in the length direction, and the stretching ratio is 1.5 - 3 times; and / or in step 3), the heat setting is to set the membrane filaments through a hot oven. The setting temperature is 90 - 150°C and the time is 10 - 60 minutes.

10. According to the method described in claim 1, wherein, the post-treatment in step 3) includes organic extraction post-treatment and inorganic extraction post-treatment; preferably, the extractant for the organic extraction post-treatment is selected from one or a mixture of several of n-hexane, cyclohexane, gasoline, ethanol, N,N-dimethylformamide, N,N-dimethylacetamide, dichloromethane, chloroform, and isopropanol; more preferably, the extraction temperature is 20 - 80°C and the extraction time is 4 - 10 hours; preferably, the extractant for the inorganic extraction post-treatment is selected from sodium hydroxide solution or hydrochloric acid, preferably 5 - 10 wt% sodium hydroxide aqueous solution or 2 - 5 wt% hydrochloric acid; more preferably, the extraction temperature is 20 - 80°C and the extraction time is 4 - 10 hours.

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