Preparation method of ionic liquid phase change microcapsule

Abstract

The invention discloses a preparation method of an ionic liquid phase change microcapsule, and belongs to the technical field of microcapsule preparation. A continuous phase 1, a continuous phase 2 containing a water phase monomer and a dispersed phase containing an ionic liquid are prepared respectively, a coaxial micro-fluidic device is adopted for preparing the ionic liquid phase change microcapsule, the coaxial micro-fluidic device mainly comprises a stainless steel needle and a silica gel tube, the stainless steel needle is inserted into the center of the silica gel transparent tube to form a coaxial flow type chip, and the coaxial flow type chip is used for preparing the ionic liquid phase change microcapsule. The cost of a microfluidic device is greatly reduced, meanwhile, the reuse rate of a microchip is increased, and the method can be used for preparing the ionic liquid-based highly-monodispersed polyurea microcapsule which is uniform in particle size, higher in coating rate and higher in stability.

Description

TECHNICAL FIELD

[0001] The present application relates to a method for preparing phase change energy storage material by using microcapsule technology, and the microsphere microcapsule prepared by microfluidic chip is a new technology rising in recent years, which has the advantages of low energy consumption, high raw material utilization rate, uniform microsphere microcapsule particle size, etc. The self-made laboratory microfluidic chip is a coaxial flow type, which is composed of a stainless steel needle and a silica gel tube. The coaxial flow type chip is formed by inserting the stainless steel needle into the center of the silica gel transparent tube, which greatly reduces the cost of the microfluidic device and improves the reusability of the microchip, thereby achieving the purpose of saving energy. BACKGROUND

[0002] Ionic liquid is a multi-atomic organic salt composed of asymmetric cations and various anions, generally composed of organic cations and inorganic anions. As a new type of phase change functional material, ionic liquid has very special physical and chemical properties, such as high decomposition temperature, good volume stability, good solubility and conductivity, no volatility and flammability, and the following advantages: mild reaction conditions, stable chemical properties, negligible vapor pressure, adjustable acidity, green and environmentally friendly, recycling and other advantages, which can be widely used in catalysis, synthesis, extraction and separation fields, and can be used as green solvent, flame retardant and compatibilizer, etc.

[0003] Due to the problems of supercooling, phase separation and easy leakage of ionic liquid in use, it is greatly affected by external environmental factors, and has certain limitations. Therefore, developing a functional material that can improve the phase change stability of ionic liquid is of great significance for the application of ionic liquid in heat storage. Among them, the microfluidic technology can quickly generate monodisperse system microcapsules at room temperature, realize the controllable wall structure and particle size of microcapsules, and realize the crystallization observation, which can accurately control the influence law of different crystallization time and temperature on nucleation probability and rate, so as to achieve the ultimate goal of inhibiting or eliminating supercooling degree. SUMMARY

[0004] The purpose of the present application is to provide a preparation method of ionic liquid phase change heat storage material, which uses microsphere microcapsule prepared by microfluidic chip. The combination of microfluidics and microcapsule preparation technology can well overcome the defect of uneven particle size of microcapsule prepared by traditional method, and the supercooling degree of ionic liquid can be more effectively controlled by using microfluidics. The ionic liquid-based microcapsule is prepared by using self-made microfluidic device (the internal microchannel is a coaxial flow channel), the particle size of the microcapsule is accurately controlled by adjusting the flow rate of the dispersed phase and the continuous phase, and the ionic liquid-based highly monodisperse polyurea microcapsule with uniform particle size, higher coating rate and stronger stability can be prepared by this method.

[0005] The ionic liquid phase change microcapsule is composed of an outer wall material and an ionic liquid core material coated inside.

[0006] The outer wall material of the ionic liquid microcapsule has good thermal stability, excellent corrosion resistance and protection.

[0007] The core material of the ionic liquid phase change microcapsule has good heat storage performance, strong electrostatic field, high heat capacity, high heat energy storage density, high thermal stability and the like.

[0008] The device for preparing the ionic liquid phase change microcapsule comprises a microfluidic device, and the microfluidic device comprises three micro-injection pumps and a coaxial flow channel pipe.

[0009] The outer wall material of the ionic liquid phase change microcapsule is prepared by using a mechanical stirring method, and sodium dodecyl sulfonate is used as an emulsifier, and polyurea generated by reaction of an oil-phase monomer toluene-2,4-diisocyanate and a water-phase monomer triethylenetetramine is used as shell material.

[0010] The phase change temperature of the ionic liquid phase change microcapsule (as a whole) is in the range of -15 to -20 DEG C, the melting phase change enthalpy is greater than 62 J / g, and the freezing enthalpy is greater than 45 J / g.

[0011] The core material of the ionic liquid phase change microcapsule is 1-ethyl-3-methyl imidazole bis(trifluoromethylsulfonyl) imide.

[0012] The preparation method of the ionic liquid microcapsule comprises the following steps:

[0013] (1) Preparation of continuous phase 1: a proper amount of polyvinyl alcohol (PVA) and an emulsifier are dissolved in deionized water, and the solution is fully stirred and dissolved at 70 DEG C;

[0014] (2) Preparation of continuous phase 2: half of the continuous phase 1 solution is taken, and a certain amount of triethylenetetramine (TETA) is added and stirred to fully dissolve;

[0015] (3) Preparation of dispersed phase: a proper amount of toluene-2,4-diisocyanate (TDI) is added to 1-ethyl-3-methyl imidazole bis(trifluoromethylsulfonyl) imide [EMIM][NTF2] preheated at 40 DEG C;

[0016] (4) Using the device, the dispersed phase containing TDI monomer is dispersed into droplets under the shearing action of continuous phase 1 at the tip of the stainless steel needle in the coaxial flow channel, at this time the oil-in-water droplets (O / W droplets) are formed, and then flow to the three-way tube channel opening along the flow of continuous phase 1, and then react with TETA monomer in continuous phase 2 at the interface to form polyurea capsule wall to encapsulate [EMIM][NTF2] droplets to form polyurea phase change microcapsules, and the microcapsules finally flow into the sample bottle through the silica gel conduit;

[0017] (5) Seal the sample bottle and place it in a 40℃ oven for 24h to allow the two monomers to fully react to form a dense capsule wall. After the reaction is complete, wash the capsule surface with deionized water and anhydrous ethanol for 2 times respectively to clean the unreacted monomers on the surface of the capsules.

[0018] (6) Place the sample in a vacuum drying oven for 24h to obtain polyurea phase change microcapsules, and store the obtained microcapsules in a beaker for subsequent testing.

[0019] The continuous phase 1 is prepared by dissolving 1-2wt% polyvinyl alcohol (PVA) and 0.5-1wt% emulsifier in deionized water, heating to 70℃ and stirring to completely dissolve; the emulsifier is sodium dodecyl sulfonate.

[0020] The continuous phase 2 is prepared by taking a part of the continuous phase 1 solution and adding 2-4w% triethylenetetramine thereto, and stirring to fully dissolve;

[0021] The dispersed phase is prepared by adding an appropriate amount of toluene-2,4-diisocyanate to 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide preheated to 40℃, and the mass ratio of toluene-2,4-diisocyanate to 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide is 1:4.

[0022] By adjusting the flow rates of the continuous phase 1 and the dispersed phase 1, O / W droplets of different particle sizes can be obtained. The larger the flow rate of the continuous phase 1 and the smaller the flow rate of the dispersed phase 1, the smaller the particle size of the O / W droplets, and vice versa; the dispersed phase 1 adopts jet mode, the speed of the continuous phase is fixed at 0.2m / s, and the flow rate of the dispersed phase is gradually adjusted from 0.01m / s to 0.018m / s. The prepared capsule particle size can be 500μm-1500μm.

[0023] The radii of the needle tube and the silica gel hose are set as R 针管 =125μm and R 硅胶软管 =750μm, respectively, the whole channel length L 硅胶软管 is set as 50000μm, and the inner channel length L 针管 (representing the length of the needle tube coaxially in the silica gel hose) is set as 1500μm.

[0024] The experimental device is a self-made laboratory microfluidic chip of a coaxial flow type, a stainless steel needle and a silica gel tube are used, the stainless steel needle has good corrosion resistance and high hardness, the silica gel hose has good chemical stability and insulation, and is simple and easy to obtain, and harmless to human body.

[0025] The preparation method of the ionic liquid phase change microcapsule has uniform particle size, higher coating rate and stronger stability. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 is Microfluidic device schematic diagram

[0027] Figure 2 is Microfluidic microcapsule actual photo

[0028] Figure 3 is Pure core material ionic liquid DSC curve

[0029] Figure 4 is Microcapsule product DSC curve (1000 μm)

[0030] Figure 5[ EMIM][NTF2] non-circulation and 20 times circulation DSC diagram

[0031] Figure 6 is not [EMIM][NTF2] based microcapsule DSC diagram (heating section) under the same particle size

[0032] Figure 7 is not [EMIM][NTF2] based microcapsule DSC diagram (cooling section) under the same particle size

[0033] Figure 8[ EMIM][NTF2] based microcapsule DSC diagram of different temperature rates

[0034] Figure 9 is 1 min after microencapsulation (1000 μm)

[0035] Figure 10 is 1 h after microencapsulation (1000 μm) DETAILED DESCRIPTION

[0036] The application will be described below by examples, but the application is not limited to the following examples.

[0037] Example 1 polyurea microcapsule

[0038] The composition of the microcapsule: the microcapsule is prepared by interfacial polymerization; the specific steps are as follows: firstly, the sodium dodecyl sulfate emulsifier and polyvinyl alcohol are mixed and then added to deionized water, heated and stirred to completely dissolve, to obtain continuous phase 1, wherein the concentration of polyvinyl alcohol (PVA) is 1.5wt% and the concentration of emulsifier is 0.8wt%; then, a part of the continuous phase 1 is taken, triethylenetetramine (TETA) is added and mixed uniformly, wherein the concentration of triethylenetetramine (TETA) is 3wt%, to obtain continuous phase 2.

[0039] The toluene-2, 4-diisocyanate and 1-ethyl-3-methyl imidazole bis (trifluoromethyl sulfonyl) imine are mixed uniformly at a mass ratio of 1:4, the rotation speed is kept at 8000r / min for 5min to obtain the core material, toluene-2, 4-diisocyanate emulsion, then a proper amount of deionized water is added for dilution, and the stirring is continuously carried out at 400r / min for 5min; finally, 0.9g of triethylenetetramine is dissolved in 5g of deionized water to form the dispersed phase, and the microcapsule is prepared by using the microfluidic device prepared by the application (see Figure 1 concludes The sample is collected after the dropwise addition is completed, then the sample is reacted in an oven at 40℃ for 24h, then the sample is discharged, filtered, dried in an oven, and finally a white powder is obtained.

[0040] The size of the stainless steel needle (dispersed phase) is 0.25mm.

[0041] The size of the silica gel tube (continuous phase) is that the inner diameter is 1.5mm and the outer diameter is 2.5mm

[0042] The speed of the continuous phase is kept at 0.2m / s. The flow rate of the dispersed phase is gradually adjusted from 0.01m / s to 0.018m / s. The research results show that as the speed of the dispersed phase increases, the generation of the droplets exhibits better monodispersity, and the diameter of the droplets also presents a gradual increase, which indicates that the increase of the speed is beneficial to control the uniformity and size of the droplets, and the generation period of the droplets is also significantly reduced. For example, by adjusting, the O / W droplet particle size of 500μm-1500μm can be obtained; the microcapsule Figure 3 is 1min after the microcapsule is formed, the microcapsule presents a transparent state, Figure 4 is 1h after the microcapsule is formed, the microcapsule presents a milky white color.

[0043] The performance analysis is as follows:

[0044] (1) The supercooling analysis of the DSC image of the [EMIM][NTF2] microcapsule

[0045] The melting temperature (T m ) is defined as the starting point of the endothermic peak (the curve peak deflects downward) during heating, the freezing temperature (T f ) is defined as the starting point of the exothermic peak (the curve peak deflects upward) during cooling, and the cold crystallization temperature (T cc) as the onset of the exothermic peak when heating from supercooled liquid to crystalline solid, the glass transition temperature (T g ) as the midpoint of the small heat capacity change when heating from amorphous glass to liquid.

[0046] Figure 3- Figure 4 is [EMIM][NTF2] core material and microcapsule DSC curve, for the ionic liquid [EMIM][NTF2], Figure 3 is There is an endothermic melting peak, Tm = -17.6℃, which forms a stable crystal form III, which is very close to the literature Tm = -19.3℃, according to the experimental results after microencapsulation, after encapsulation one or two endothermic melting peaks appear, according to the literature explanation for pure ionic liquid, if two endothermic melting peaks are formed after microencapsulation, from left to right in turn Cr III [-14.3℃(-19.3℃)] crystal form and Cr II (-4.3℃) crystal form, if only one endothermic melting peak is formed after microencapsulation, it corresponds to Cr III crystal form. (2) The effect of cycle number on the supercooling degree of core material

[0047] In order to test the influence of cycle coefficient on the supercooling degree of ionic liquid [EMIM][NTF2], the ionic liquid microcapsule charging and discharging was tested by DSC, and the specific test method was as follows: in order to reduce the error, the charging and discharging experiment of phase change microcapsule should always use the same program for testing. The test is divided into six programs in total: ① from 25℃ to -80℃ (10℃ / min); ② keep at -80℃ for 5min; ③ from -80℃ to 40℃ (5℃ / min); ④ keep at 40℃ for 5min; ⑤ from 40℃ to -80℃ (5℃ / min); ⑥ -80℃ to 25℃ (10℃ / min). The whole test process is carried out in nitrogen atmosphere protection, and the second and fourth segments are kept for 5min in order to eliminate the thermal history of PCM. Before testing, two sets of aluminum flat bottom crucibles with a mass difference of not more than 0.1mg should be selected, 5mg-10mg of sample should be placed in the crucible, and the aluminum cover with holes should be covered on the crucible, and then it should be compacted to serve as the test sample.

[0048] DSC test was carried out on the core material [EMIM][NTF2], as shown in Figure 5 showsAs shown in the table, the melting enthalpy of [EMIM][NTF2] without cycling is 65.8 J / g, and the freezing enthalpy is 47.8 J / g; the data of [EMIM][NTF2] after 20 cycles show that the melting enthalpy is 62.5 J / g, and the freezing enthalpy is 46.2 J / g, which indicates that the enthalpy change of [EMIM][NTF2] after 20 cycles can be ignored, while the supercooling degree of the core material increases from 27.2 °C to 40.4 °C, which increases by 48.5%, so multiple cycles will have a worse impact on the supercooling degree.

[0049] The temperature corresponding to the focus of the initial tangent line with the maximum slope before the peak and the baseline is defined as the phase change temperature, and the latent heat of phase change is calculated by integrating the area of the phase change peak. The microcapsules with a particle size of 500 μm to 1500 μm were tested by DSC at a rate of 5 °C / min, as shown in FIGS. 6-7, the test results show that the solidification termination temperature is basically unchanged, and the melting starting point changes obviously. During the change of the particle size of 500 μm to 1000 μm, the melting starting point is constantly advanced, and the supercooling degree is constantly reduced, and the supercooling degree is the smallest at 1000 μm, which is 25.4 °C, while the supercooling degree of the uncoated core material is 27.2 °C, which is reduced by 7%, the reason may be that the encapsulation of the polyurea wall material provides a relatively large nucleation specific surface area for the core material, which changes the uniform nucleation of the uncoated core material to the double nucleation mechanism of the capsule wall heterogeneous nucleation and the core material uniform nucleation, thereby accelerating the crystallization of the core material and reducing the supercooling degree. When the capsule particle size increases from 1000 μm to 1500 μm, the supercooling degree increases to 35.6 °C, and the possible reason is that the wall layer is too thick, the heat transfer efficiency is slow, which leads to the increase of the supercooling degree.

[0050] According to the DSC image of FIGS. 6-7, the melting process of the microcapsules with a particle size of 500 μm to 750 μm shows a double-peak phenomenon, which indicates that the melting nucleation of the capsules may exist two control mechanisms, and the melting points formed under different nucleation mechanisms have different crystal structures, corresponding to different melting point temperatures.

[0051] From the macroscopic point of view, the multiple peaks of [EMM][NTF2] crystallization and melting are caused by the heterogeneous nucleation of the capsule wall in the high-temperature direction, and the uniform nucleation of the capsule core in the low-temperature direction. According to the DSC data, the heterogeneous nucleation of the capsule wall is dominant, which is greater than the uniform nucleation of the capsule core, which indicates that the use of polyurea to coat the ionic liquid is an effective means to promote the solid-liquid phase change. With the gradual increase of the capsule particle size, the core material encapsulated by the microcapsules increases, and the proportion of the uniform nucleation of the capsule core gradually increases, and when the particle size reaches 1000 μm, the melting peak in the high-temperature direction and the melting peak in the low-temperature direction are combined into one, and the melting peak at low temperature disappears, at this time the nucleation mechanism is uniform nucleation. When the particle size is 1000 μm to 1500 μm, the melting peak in the high-temperature direction appears, and the nucleation mechanism changes to the heterogeneous nucleation of the capsule wall as the main and the uniform nucleation of the core material as the auxiliary.

[0052] According to the DSC data of the microcapsules with particle sizes of 500μm to 1500μm, the enthalpy of the microcapsules is reduced compared to that of the core material. This is because the polyurea wall material accounts for a certain mass proportion. As the particle size of the microcapsules increases, the melting enthalpy and solidification enthalpy also increase. This is because the larger the particle size, the more core material is contained, and thus the larger the enthalpy value. For the encapsulation rate of microcapsules with different particle sizes, the maximum encapsulation rate is 90.8%, which is 35.8% higher than that of polyurea microcapsules prepared by traditional methods. (4) Effect of temperature change rate on microcapsule supercooling and phase change enthalpy

[0053] Microcapsules with a particle size of 1000 μm were analyzed at different cooling rates, such as... Figure 8 shows The results show that as the cooling rate increases, the crystallization initiation temperature and peak temperature of the microcapsules gradually decrease, while the supercooling of the microcapsules continuously increases. Appropriately reducing the cooling rate can improve the crystallinity of the microcapsules per unit time. The crystallization peaks shift towards lower temperatures, and in addition to gradually widening the peak shape, the peak area also increases, indicating an increase in the enthalpy of crystallization. Theoretically, the heat absorbed when a substance melts should be consistent with the heat released when it solidifies, i.e., ΔHm = ΔHc. However, in this experiment, ΔHc measured at different temperature changing rates was less than ΔHm. This is mainly because crystallization is a process from disorder to order. If there is not enough time, the crystallized substance cannot return to its original state. This is because the core material inside the microcapsule needs to undergo chain segment migration, orientation, and rearrangement during crystallization. When the cooling rate increases, the crystallization time is shortened, resulting in insufficient time for chain segments to crystallize. The formed crystals are imperfect, with deep defects, and the crystallization process is hindered. The energy required for crystallization increases, resulting in "hysteresis" and severe supercooling.

Claims

1. A method for preparing phase change microcapsules of ionic liquids, characterized in that, The device used includes a microfluidic device, the microfluidic device includes three micro-injection pumps, a coaxial flow channel pipe, the coaxial flow channel pipe includes a silica gel hose, one end of the silica gel hose is connected with the first micro-injection pump corresponding to the continuous phase 1, a stainless steel needle is horizontally inserted into the center of the silica gel hose, the needle end of the stainless steel needle is coaxially suspended in a long outer pipe, the other end of the silica gel hose is provided with a three-way pipe or connected with the three-way pipe, the second passage of the three-way pipe is connected with the third micro-injection pump corresponding to the continuous phase 2, and the third passage of the three-way pipe is connected with a sample bottle below; the rear part of the needle end of the stainless steel needle is externally provided with a microscope, and the generation of droplets is observed through the transparent silica gel hose; the other end of the stainless steel needle is connected with the second micro-injection pump. The method comprises the following steps: (1) Preparation of the continuous phase 1: a proper amount of polyvinyl alcohol (PVA) and an emulsifier are dissolved in deionized water, and the solution is fully stirred and dissolved at 70 DEG C; (2) Preparation of the continuous phase 2: half of the continuous phase 1 solution is taken, and a certain amount of triethylene tetramine (TETA) is added thereto and stirred to fully dissolve; (3) Preparation of the dispersed phase: a proper amount of toluene-2, 4-diisocyanate (TDI) is added to the preheated ionic liquid at 40 DEG C; (4) The dispersed phase containing TDI monomers is dispersed into droplets under the shearing action of the continuous phase 1 at the needle tip of the coaxial flow channel pipe, and at this time, the O / W droplets are formed, and then the O / W droplets flow to the three-way pipe passage opening along with the continuous phase 1, and then react with TETA monomers in the continuous phase 2 at the interface to form polyurea phase change microcapsules, and finally, the microcapsules flow into the sample bottle through the silica gel guide pipe; (5) The sample bottle is sealed and placed in an oven at 40 DEG C for 24 hours to fully react the two monomers to form a dense capsule wall; after the reaction is completed, the unreacted monomers on the surface of the capsule are cleaned by washing with deionized water and anhydrous ethanol twice respectively. (6) The sample is placed in a vacuum drying oven to obtain the polyurea phase change microcapsules.

2. The method of claim 1, wherein, The continuous phase 1 is prepared by dissolving 1-2wt% of polyvinyl alcohol (PVA) and 0.5-1wt% of an emulsifier in deionized water, and fully stirring and dissolving the solution at 70 DEG C; the emulsifier is sodium dodecyl sulfonate.

3. The method of claim 1, wherein, The continuous phase 2 is prepared by taking part of the continuous phase 1 solution, adding 2-4w% of triethylene tetramine thereto, and stirring to fully dissolve.

4. The method of claim 1, wherein, The dispersed phase is prepared by adding a proper amount of toluene-2, 4-diisocyanate to the preheated 1-ethyl-3-methyl imidazole bis(trifluoromethylsulfonyl) imide at 40 DEG C, and the mass ratio of toluene-2, 4-diisocyanate to 1-ethyl-3-methyl imidazole bis(trifluoromethylsulfonyl) imide is 1:

4.

5. The method of claim 1, wherein, By adjusting the flow rates of the continuous phase 1 and the dispersed phase 1, O / W droplets with different particle sizes can be obtained; the larger the flow rate of the continuous phase 1 and the smaller the flow rate of the dispersed phase 1, the smaller the particle size of the O / W droplets, and vice versa; the dispersed phase 1 adopts a jet mode, the speed of the continuous phase is fixed at 0.2m / s, and the flow rate of the dispersed phase is gradually adjusted from 0.01m / s to 0.018m / s.

6. The method of claim 1, wherein, The needle tube and silicone hose radii are set to R 针管 = 125 μm and R 硅胶软管 = 750 μm, respectively, with the overall channel length L 硅胶软管 set to 50,000 μm, with the inner channel length L 针管 set to 1,500 μm.

7. The method of claim 1, wherein, The particle size of the microcapsules is 500-1500 microns.

8. Ionic liquid phase change microcapsules prepared according to the process of any one of claims 1 to 7.

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