Preparation method of a cellulose porous membrane with high haze and high light transmittance

Abstract

The present invention discloses a method for preparing a cellulose porous membrane with high haze and high light transmittance. First, a cellulose derivative is uniformly mixed with an alkali solution, and then the mixture is dissolved at low temperature to become an ice-water mixture state; then it is melted into a homogeneous transparent liquid casting solution; then the casting solution is made into a uniform casting solution coating; then the casting solution coating is subjected to a freezing treatment to solidify it from a liquid state by condensation; then a freeze-drying treatment is carried out to sublime the solvent, obtaining a regenerated cellulose matrix with a porous structure, and then together with a flat substrate is placed in a coagulation bath to make the membrane naturally fall off, obtaining a cellulose porous membrane with high haze and high light transmittance. The present invention uses the directional freezing method to regulate the pore structure of the membrane, and then regulates the optical properties such as light transmittance and haze, and finally constructs a non-dense cellulose porous membrane with high haze and high light transmittance. The pore structure on the surface and inside of the membrane helps the mass transfer of the membrane in the aqueous phase system, and at the same time has better light management potential.

Description

Technical Field

[0001] The present invention belongs to the field of cellulose membranes for light management, and specifically relates to a method for preparing a cellulose porous membrane with high haze and high light transmittance. Background Art

[0002] In recent years, light management films based on cellulose and its derivatives have attracted extensive attention in the fields of environment and energy. On the one hand, light management films can support, protect, and prevent fouling of optical devices through good mechanical properties and controllable surface characteristics; on the other hand, light management films can also utilize their unique optical properties to regulate the light propagation path, and obtain higher photocatalytic efficiency by improving the light absorption and utilization rate of photoactive materials or devices. Light management films usually need to have both excellent light transmittance and optical haze. High light transmittance can ensure the transmission of visible light, while high haze can improve the forward scattering of light and increase the light propagation path in the material, thereby improving the photoelectric conversion efficiency and quantum yield.

[0003] Currently, cellulose and its derivatives have become preferred materials for preparing light management films due to their renewable and degradable characteristics. However, due to the binary property of cellulose with the coexistence of crystalline regions and amorphous regions, generally, its high transparency and high optical haze are mutually exclusive, that is, it is difficult to obtain both the light transmittance and optical haze of cellulose films simultaneously. With the in-depth research, transparent films with both high transparency and high haze can be prepared using multi-scale cellulose. The literature "Fang Z, Zhu H, Bao W, et al. Highly transparent paper with tunable haze for green electronics [J]. Energy & Environmental Science, 2014, 7(10): 3313 - 3319." realizes the controllable haze of cellulose transparent films by adjusting the mass ratio of nanocellulose and TEMPO (oxidant). The literature "Xu X, Zhou J, Jiang L, et al. Highly transparent, low-haze, hybrid cellulose nanopaper as electrodes for flexible electronics [J]. Nanoscale, 2016, 8(24): 12294 - 12306." realizes the adjustable haze of the film by adjusting the ratio between nanocellulose (NFC) and cellulose nanocrystals (CNC). The main characteristics of related technologies are: (1) To ensure high light transmittance, the film material is usually a dense and pore-free structure; (2) To increase the haze of the film material, a second-phase component substance usually needs to be introduced.

[0004] However, existing methods all have problems such as high cost and complex processes. More importantly, most of the existing cellulose-based light management films are non-porous dense structures, so it is impossible to obtain good gas and liquid mass transfer capabilities, which limits their application in the field of environmental photocatalysis. Therefore, it is urgent to develop a porous cellulose light management material with high mass transfer performance. Summary of the Invention

[0005] Aiming at the deficiencies of the existing technology, the technical problem to be solved by the present invention is to provide a method for preparing a cellulose porous membrane with high haze and high light transmittance.

[0006] The technical solution of the present invention to solve the above technical problem is to provide a method for preparing a cellulose porous membrane with high haze and high light transmittance, which is characterized in that the method comprises the following steps:

[0007] Step 1: Mix the cellulose derivative with the alkali solution evenly to form a mixture; then carry out low-temperature dissolution on the mixture until the mixture turns into an ice-water mixed state; then melt the ice-water mixed state into a homogeneous transparent liquid state to obtain a cellulose derivative casting solution.

[0008] Step 2: After defoaming the casting solution, pour it evenly on the flat substrate and scrape it flat to make a uniform casting solution coating.

[0009] Step 3: Construct a porous structure by directional freezing: first freeze the casting solution coating to make it solidify from the liquid state by cooling; then carry out freeze-drying treatment on the solidified casting solution coating to sublimate the solvent to obtain a regenerated cellulose matrix with a porous structure.

[0010] Step 4: Place the regenerated cellulose matrix with a porous structure together with the flat substrate in a coagulation bath to make the film fall off naturally, and obtain a cellulose porous membrane with high haze and high light transmittance.

[0011] Compared with the existing technology, the beneficial effects of the present invention are as follows:

[0012] (1) The present invention uses the directional freezing method to regulate the pore structure of the membrane, and then regulates the optical properties such as light transmittance and haze, and finally constructs a non-dense cellulose porous membrane with high haze and high light transmittance. The pore structure on the surface and inside of the membrane helps the mass transfer of the membrane in the aqueous phase system. At the same time, this cellulose porous membrane can achieve high absorption and forward scattering of visible light, has better light management potential, and is suitable for the development of visible light active photocatalytic membrane reaction systems, water purification, and membrane pollution control, etc.

[0013] (2) The directional freezing method adopted by the present invention belongs to the directional freezing film-forming technology. While preparing a highly transparent film, a porous structure can be formed inside the film. The pore cavities on the surface and inside of the membrane will increase the haze of the transparent cellulose porous membrane, which is beneficial to the generation of strong light scattering inside.

[0014] (3) This method has a simple process and low processing cost. It does not require the addition of a second-phase material component and is easy to promote industrialization and carry out industrial production.

[0015] (4) The present invention uses cellulose derivatives as raw materials, which have the advantages of being green, low-cost, renewable, and having good flexibility, meeting the environmental protection requirements. Description of the Drawings

[0016] Figure 1 SEM image of the cellulose porous membrane prepared in Example 1 of the present invention;

[0017] Figure 2 SEM image of the cellulose porous membrane prepared in Example 2 of the present invention;

[0018] Figure 3 SEM image of the cellulose membrane prepared in Comparative Example 1 of the present invention;

[0019] Figure 4 SEM image of the cellulose porous membrane prepared in Example 3 of the present invention;

[0020] Figure 5 Membrane intrinsic resistance diagram of the cellulose porous membranes prepared in Examples 1 - 3 of the present invention and the cellulose membrane prepared in Comparative Example 1;

[0021] Figure 6 Infrared spectrum diagram of the cellulose porous membranes prepared in Examples 1 - 3 of the present invention and the cellulose membrane prepared in Comparative Example 1;

[0022] Figure 7 Transmittance diagram in the visible light band of the cellulose porous membranes prepared in Examples 1 - 3 of the present invention and the cellulose membrane prepared in Comparative Example 1;

[0023] Figure 8 Haze diagram in the visible light band of the cellulose porous membranes prepared in Examples 1 - 3 of the present invention and the cellulose membrane prepared in Comparative Example 1. Detailed Embodiments

[0024] The following are specific embodiments of the present invention. The specific embodiments are only used to further illustrate the present invention in detail and do not limit the protection scope of the claims of the present invention.

[0025] The present invention provides a method for preparing a cellulose porous membrane with high haze and high light transmittance (hereinafter referred to as the method), which is characterized in that the method comprises the following steps:

[0026] Step 1: Mix the cellulose derivative evenly with the alkali solution to form a mixture; then place the mixture in a constant low-temperature environment for low-temperature dissolution until the mixture turns into an ice-water mixed state; then melt the ice-water mixed state into a homogeneous and transparent liquid to obtain the cellulose derivative casting solution.

[0027] Preferably, in Step 1, the cellulose derivative is cellulose carbamate.

[0028] Preferably, in Step 1, the alkali solution is sodium hydroxide solution or potassium hydroxide solution, preferably sodium hydroxide solution.

[0029] Preferably, in Step 1, the mass fraction of the alkali solution is 5-12 wt%, and the mass ratio of the cellulose derivative to the volume of the alkali solution is 1 g: 10-30 mL.

[0030] Preferably, in Step 1, the temperature of low-temperature dissolution is -10 to -30 °C (preferably -15 to -25 °C), the time is 30 to 60 min, and the low-temperature dissolution is carried out under stirring conditions.

[0031] Preferably, in Step 1, the melting temperature is room temperature (preferably 15-35 °C, more preferably 20-30 °C, further preferably 25 °C), the time is 5-10 min, and the melting is carried out under stirring conditions.

[0032] Preferably, in Step 1, after dissolving the inorganic salt in the alkali solution, add the cellulose derivative to form a mixture, and then carry out low-temperature dissolution; the inorganic salt is sodium iodide, sodium bromide or sodium sulfate, more preferably sodium iodide; the anions of the inorganic salt are iodide ions, bromide ions and sulfate ions; the mass ratio of the cellulose derivative to the inorganic salt is 2-5: 1-3.

[0033] Step 2: After defoaming the casting solution, pour it evenly on a flat substrate (a glass plate in this embodiment) and scrape it flat to make a uniform casting solution coating.

[0034] Preferably, in Step 2, the defoaming process is: put the casting solution into a high-speed centrifuge for centrifugal defoaming, the centrifugal speed is 2000-5000 r / min, and the centrifugal time is 5-20 min. Scraping is carried out with a scraper.

[0035] Preferably, in Step 2, the thickness of the casting solution coating is 200-1000 μm, preferably 500 μm.

[0036] Step 3. Construct a porous structure through directional freezing: First, freeze the casting solution coating so that it solidifies from a liquid state due to cooling (as the temperature decreases, the solvent gradually solidifies, and the ice crystals generated by solidification squeeze, displace, and embed the solute between the ice crystals); then, perform freeze-drying on the solidified casting solution coating to sublimate the solvent and cause phase separation (the ice crystals transform from a solid phase across the liquid phase to a gas phase), obtaining a regenerated cellulose matrix with a porous structure;

[0037] Preferably, in Step 3, the temperature of the freezing treatment is -50 to -150 °C (preferably in a liquid nitrogen environment), and the time is 0.5 to 5 min (preferably 1 min) until the casting solution coating is completely solidified.

[0038] Preferably, in Step 3, the freeze-drying time is 4 to 8 h, the temperature is 0 to -45 °C, and the vacuum degree is 20 to 200 Pa.

[0039] Step 4. Place the regenerated cellulose matrix with a porous structure together with the flat substrate in a coagulation bath to make the membrane fall off naturally, obtaining a cellulose porous membrane with high haze and high light transmittance.

[0040] Preferably, in Step 4, the coagulation bath uses an aqueous solution with a pH of 3 to 8 (preferably pure water).

[0041] Testing methods for the morphology and structure of the membrane:

[0042] 1. Morphology testing: Use a scanning electron microscope (SEM) to observe the morphology and structure of the product and obtain SEM images with a magnification of 500×.

[0043] 2. Structure testing: Use Fourier transform infrared spectroscopy (ATR-FTIR) to identify the functional groups of the membrane through characteristic peaks. The spectral recording range is 400 to 4000 cm -1 。

[0044] 3. Membrane intrinsic resistance testing: Use a dead-end filtration device to measure the flux and calculate the membrane intrinsic resistance (Rm) through the original pure water flux (J0) of the membrane.

[0045] 4. Optical property characterization: Use a LAMBDA 750 UV / vis / NIR spectrophotometer (PerkinElmer, USA) to characterize the optical properties of the membrane: transmittance and haze. The light source wavelength range of the spectrophotometer is set to 380 to 780 nm, and the membrane samples have the same thickness to exclude the influence of sample thickness on the transmittance and haze data. Cut the membrane samples with different pore structures into the same size, fix them on the clip for testing thin film samples, and place them in a UV-visible spectrophotometer equipped with an integrating sphere attachment for testing.

[0046] Example 1

[0047] (1) Mix cellulose carbamate with an aqueous NaOH solution with a mass fraction of 9% at a ratio of 1 g:14 ml, stir at room temperature to make it evenly mixed and swollen to form a mixture; then place the mixture in a low-temperature cooling circulator or a low-temperature constant-temperature reaction bath at -20 °C and stir for 60 min until the mixture becomes light yellow, transparent and in a slushy state; then take it out from the low-temperature cooling circulator and continuously stir at 25 °C to melt it, obtaining a homogeneous and transparent cellulose carbamate casting solution;

[0048] (2) Transfer the casting solution to a centrifuge tube and centrifuge at a speed of 3000 r / min in a high-speed centrifuge for 10 min to remove the bubbles from the casting solution; then evenly pour the casting solution onto a flat substrate (a glass plate in this example) and scrape it flat with a spatula to make a casting solution coating with a thickness of 500 μm coated on the flat substrate;

[0049] (3) Place the flat substrate coated with the casting solution coating in a refrigerator or an ultra-low temperature refrigerator at -100 °C for freeze treatment for 1 min until the casting solution coating is completely solidified; then place the solidified casting solution coating together with the flat substrate in a freeze dryer for freeze drying for 6 h, with the cold trap temperature set at -30 °C and the vacuum degree at 30 Pa, obtaining a regenerated cellulose matrix with a porous structure;

[0050] (4) Place the regenerated cellulose matrix with a porous structure together with the flat substrate in pure water to make the membrane naturally fall off, obtaining a cellulose porous membrane with high haze and high light transmittance, denoted as RM1.

[0051] After testing, the visible light transmittance of RM1 is 93.32%, the haze is 69.72%, and the inherent resistance of the membrane is 0.64×10 11 m -1 。

[0052] Example 2

[0053] This example is the same as Example 1, and the only difference is that: in step (1), the mass ratio of cellulose carbamate to the volume of the aqueous NaOH solution is 1 g:19 ml.

[0054] The cellulose porous membrane with high haze and high light transmittance obtained through the above steps is denoted as RM2. After testing, the visible light transmittance of RM2 is 92.12%, the haze is 74.19%, and the inherent resistance of the membrane is 0.527×10 11 m -1 。

[0055] Comparative Example 1

[0056] This comparative example is exactly the same as steps (1) and (2) of Example 2, and the only difference is that:

[0057] (3) Place the flat substrate coated with the casting solution into pure water to allow the film to naturally peel off, and obtain a cellulose film denoted as RM0.

[0058] After testing, the visible light transmittance of RM0 is 96.37%, the haze is 1.42%, and the inherent resistance of the film is 0.45×10 14 m -1 .

[0059] Example 3

[0060] This example is the same as Example 1, and the only difference is that in step (1), the mass ratio of cellulose carbamate to the volume of the NaOH aqueous solution is 1 g:24 ml.

[0061] After testing, the cellulose porous film with high haze and high transmittance obtained through the above steps is denoted as RM3. After testing, the visible light transmittance of RM3 is 91.78%, the haze is 46.15%, and the inherent resistance of the film is 0.382×10 11 m -1 .

[0062] Example 4

[0063] This example is the same as Example 2, and the only difference is that in step (3), the freezing temperature of the refrigerator or ultra-low temperature freezer is set to -150°C.

[0064] After testing, the visible light transmittance of the cellulose porous film with high haze and high transmittance obtained through the above steps is 81.44%, the haze is 75.24%, and the inherent resistance of the film is 0.417×10 11 m -1 .

[0065] Example 5

[0066] (1) Dissolve sodium iodide in a 9% NaOH aqueous solution, then add cellulose carbamate, and stir at room temperature to mix evenly and swell to form a mixture. The mass ratio of sodium iodide to cellulose carbamate is 1:1, and the mass ratio of cellulose carbamate to the volume of the NaOH aqueous solution is 1 g:14 ml; then place the mixture in a low-temperature cooling circulator (low-temperature constant-temperature reaction bath) at -20°C and stir for 60 min until the mixture becomes light yellow, transparent and in a slushy state; then take it out from the low-temperature cooling circulator and continue to stir at 25°C to melt it to obtain a homogeneous and transparent cellulose carbamate casting solution;

[0067] (2) The same as Example 1;

[0068] (3) Place the flat substrate coated with the casting solution coating in a refrigerator or ultra-low temperature freezer at -150 °C for 1 min of freezing treatment until the casting solution coating is completely solidified; then place the solidified casting solution coating together with the flat substrate in a freeze dryer for 6 h of freeze drying, with the cold trap temperature set at -30 °C and the vacuum degree at 30 Pa to obtain a regenerated cellulose matrix with a porous structure;

[0069] (4) Place the regenerated cellulose matrix with a porous structure together with the flat substrate in pure water to make the film fall off naturally to obtain a cellulose porous film with high haze and high light transmittance.

[0070] It is tested that the visible light transmittance of the cellulose porous film with high haze and high light transmittance obtained through the above steps is 85.79%, and the haze is 73.75%.

[0071] From Figure 1 、 Figure 2 and Figure 4 It can be seen that Examples 1 - 3 prepared cellulose membranes with different pore sizes, realizing the regulation of the pore size structure of the cellulose carbamate membrane. By comparing Figure 3 the dense and pore-free cellulose membrane prepared by the solution method, it also shows that the method of the present invention can prepare a porous regenerated cellulose membrane.

[0072] From Figure 5 It can be seen that the cellulose porous membranes prepared in Examples 1 - 3 have good mass transfer performance, while the cellulose membrane prepared by the solution method in Comparative Example 1 has poor mass transfer performance.

[0073] From Figure 6 It can be seen that there is almost no change in the infrared spectra of the membranes prepared in Comparative Example 1 and Examples 1 - 3, which also shows that the directional freezing method will not change the chemical properties of the membrane itself.

[0074] From Figure 7 It can be seen that the light transmittance of the cellulose porous membranes prepared in Examples 1 - 3 by the directional freezing method is all above 80%, indicating that the cellulose porous membranes prepared by the directional freezing method have a high transmittance.

[0075] From Figure 8 It can be seen that compared with the cellulose membrane prepared by the solution method, the haze of the cellulose porous membrane prepared by the directional freezing method has been greatly improved.

[0076] Comprehensively Figure 7 and Figure 8 It also shows that the present invention has successfully prepared a cellulose porous membrane with high haze and high light transmittance by the directional freezing method.

[0077] Matters not described in the present invention are applicable to the prior art.

Claims

1. A preparation method of a cellulose porous membrane with high haze and high light transmittance, characterized in that, The method comprises the following steps: Step 1: Mix the cellulose derivative with an alkali solution evenly to form a mixture; then carry out low-temperature dissolution of the mixture for 30 - 60 min under stirring conditions at a temperature of -10 to -30 °C until the mixture turns into an ice-water mixed state; then melt the ice-water mixed state into a homogeneous transparent liquid state to obtain a cellulose derivative casting solution; the melting temperature is room temperature, the time is 5 - 10 min, and the melting is carried out under stirring conditions; Step 2: After defoaming the casting solution, pour it evenly onto a flat substrate and scrape it flat to make a uniform casting solution coating; Step 3: Construct a porous structure by directional freezing: first freeze the casting solution coating at -50 to -150 °C for 0.5 - 5 min to make the solvent solidify from the liquid state by condensation; then carry out freeze-drying treatment on the solidified casting solution coating at 0 to -45 °C for 4 - 8 h to make the solvent sublimate and complete phase separation to obtain a regenerated cellulose matrix with a porous structure; Step 4: Place the regenerated cellulose matrix with a porous structure together with the flat substrate in a coagulation bath to make the membrane fall off naturally to obtain a cellulose porous membrane with high haze and high light transmittance.

2. The preparation method of the high haze and high light transmittance cellulose porous membrane according to claim 1, characterized in that, In Step 1, the cellulose derivative is cellulose carbamate; the alkali solution uses sodium hydroxide solution or potassium hydroxide solution.

3. The preparation method of the high haze and high light transmittance cellulose porous membrane according to claim 1, wherein In Step 1, the mass fraction of the alkali solution is 5 - 12 wt%, and the mass ratio of the cellulose derivative to the volume of the alkali solution is 1 g:10 - 30 mL.

4. The preparation method of the high haze and high light transmittance cellulose porous membrane according to claim 1, characterized in that, In Step 1, after dissolving the inorganic salt in the alkali solution, then add the cellulose derivative to form a mixture, and then carry out low-temperature dissolution; The inorganic salt is sodium iodide, sodium bromide or sodium sulfate; the mass ratio of the cellulose derivative to the inorganic salt is 2 - 5:1 - 3.

5. The preparation method of the high haze and high light transmittance cellulose porous membrane according to claim 1, characterized in that, In Step 2, the thickness of the casting solution coating is 200 - 1000 μm.

6. The preparation method of the high haze and high light transmittance cellulose porous membrane according to claim 1, characterized in that, In Step 3, the vacuum degree of freeze-drying is 20 - 200 Pa.

7. The preparation method of the high haze and high light transmittance cellulose porous membrane according to claim 1, wherein, In Step 4, the coagulation bath uses an aqueous solution with a pH of 3 - 8.

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