Reversible thermochromic composite material based on cellulose nanocrystals and preparation method thereof

Abstract

According to the reversible thermochromic composite material based on the cellulose nanocrystals, the cellulose nanocrystals CNC serve as a base material, a molecular cross-linking reaction is generated through diammonium hydrogen phosphate, a biomass aerogel material with the high adsorption capacity and an ordered pore channel structure is successfully constructed, meanwhile, tetradecanol TD serves as a phase change material and a solvent, and the reversible thermochromic composite material based on the cellulose nanocrystals is obtained. Crystal violet lactone CVL and bisphenol A BPA are used as a color developing agent and a developing agent, and then the reversible thermochromic phase change material is prepared; meanwhile, the material has thermochromic performance, reversible performance and phase change performance. The preparation method comprises the following steps: 1, preparing CD aerogel; 2, preparing the reversible thermochromic phase change material; and 3, preparing the thermochromic phase change material with a reversible function.

Description

Technical Field

[0001] The present invention relates to the field of composite phase change materials with reversible thermochromic properties, and in particular to a reversible thermochromic composite material based on cellulose nanocrystals and a preparation method thereof. Background Art

[0002] Organic phase change materials have the characteristics of high thermal energy storage density and constant conversion temperature. By combining organic phase change materials with reversible thermochromic materials, reversible thermochromic phase change materials can be obtained. For example, existing document 1 (Li BX, Luo Z, Yang WG, et al. Adaptive and adjustable MXene / reduced graphene oxide hybrid aerogel composites integrated with phase-change material and thermochromic coating for synchronous visible / infrared camouflages[J]. ACS nano, 2023, 17 (7): 6875-6885.) MXene dispersion and GO suspension are ultrasonically mixed and ascorbic acid is added for pre-reduction and hydrothermal assembly, the obtained hydrogel is directionally frozen and then freeze-dried to obtain an aerogel, n-octadecane is adsorbed to obtain a composite phase change material, and then the composite phase change material is sprayed with thermochromic ink, and finally a thermochromic composite phase change material is obtained. Although this technical solution obtains a thermochromic composite phase change material, it has the problem of complicated preparation procedures due to the use of pre-reduction and hydrothermal assembly methods. At the same time, since Mxene and GO are used as raw materials, the raw materials are also expensive.

[0003] To address the complex preparation process and expensive raw materials, the inventors' research group conducted preliminary work. Document 2 (He Haizhi, Zhang Huanzhi. A Thermochromic Phase Change Material with Repairable Function, Preparation Method, and Application: 202411847202.X[P]. 2025-02-25) used polyvinyl alcohol (PVA) and boric acid (BA) as raw materials for cross-linking, thermochromic ink as the color-changing material, and polyethylene glycol as the phase-change material to produce a thermochromic material with repairable function through a one-step process based on physical casting. The material exhibited repairable, reversible thermochromic, and phase-change properties, with a color change temperature of 50°C and a phase change enthalpy of 55.583-78.679 J / g for reversible thermochromic performance. However, the color change temperature of the material obtained by this technical solution was 50°C, which is not suitable for room temperature conditions. Furthermore, the phase change enthalpy was low, making it difficult to ensure the material's performance stability.

[0004] In order to obtain a reversible thermochromic composite phase change material at room temperature, existing document 3 (Facile preparation of reversible thermochromic phase change materials towards temperature-controlled information storage and self-reporting, 10.1016 / j.est.2022.104292) uses TC-PCM as the core material of thermochromic organic compounds, wherein the thermochromic compound is composed of a developer, a developer and a solubility agent. In this technical solution, crystal violet lactone CVL and bisphenol A BPA are used as developer and developer, and, based on the developer, the technical solution uses ethyl stearate and butyl stearate as solubility agents. However, ethyl stearate and butyl stearate require organic solubility agents because they are not hydrophilic, that is, there is a problem of poor biocompatibility. At the same time, ethyl stearate and butyl stearate also have the problem of expensive raw material costs. Summary of the Invention

[0005] The present invention aims to provide a reversible thermochromic composite material based on cellulose nanocrystals and a method for preparing the same. In response to the technical problems existing in the prior art, cellulose nanocrystals (CNC) are used as the substrate. The basic principles involved are as follows:

[0006] 1. The micromorphology of CNC is needle-shaped, so it has a high aspect ratio and high specific surface area;

[0007] 2. CNC has functional groups represented by -OSO3- and -OH. Therefore, it has high dispersibility in aqueous solution and can undergo molecular cross-linking reaction with diammonium hydrogen phosphate to construct biomass aerogel materials;

[0008] 3. CNC can form hydrogen bonds with TD to avoid TD leakage.

[0009] 4. TD, CVL, and BPA form a uniform interpenetrating TBC through molecular chain assembly, ensuring the overall uniformity and stability of the color change. The TBC and CD aerogel are bound together by intermolecular assembly and hydrogen bonding, rather than simple porous physical adsorption. As a result, the reversible thermochromic composite material is stable and can withstand certain compression and external forces.

[0010] To achieve the above object, the present invention is implemented through the following technical solutions:

[0011] A reversible thermochromic composite material based on cellulose nanocrystals, cellulose nanocrystals CNC as a substrate, through the molecular crosslinking reaction of diammonium hydrogen phosphate, successfully constructed CD biomass aerogel material with high adsorption capacity and ordered pore structure, at the same time, with TD as the phase change material and the solvent, crystal violet lactone CVL and bisphenol A BPA as the color developing agent and the developer, and then a reversible thermochromic phase change material is prepared;

[0012] The obtained reversible thermochromic phase change material has the properties of thermochromic performance, reversibility and phase change performance;

[0013] The thermochromic performance, that is, when applied as a thermochromic material, the thermochromic phase change material is in a dark blue state at room temperature; when the temperature is higher than 38 DEG C, the color of the thermochromic phase change material gradually changes from dark blue to gray white;

[0014] The reversibility, that is, when applied as a thermochromic material, has reversibility, the thermochromic phase change material is in a dark blue state at room temperature; when the temperature is higher than 38 DEG C, the color of the thermochromic phase change material is in a gray white state, that is, when the temperature is higher than 38 DEG C, the color of the thermochromic phase change material gradually changes from dark blue to gray white state;

[0015] The phase change performance, that is, when applied as a phase change material, the phase change temperature is 29.09-37.03 DEG C, the phase change enthalpy value is 158.753-171.209 J / g, and the thermal conductivity coefficient is 0.3338-0.4339 W / m K.

[0016] A preparation method of a reversible thermochromic composite material based on cellulose nanocrystals, comprising the following steps:

[0017] Step 1, preparation of CD aerogel, first, dilute CNC, under certain conditions, dilute CNC in deionized water and stir magnetically to obtain a CNC solution, then add a certain concentration of ammonium dihydrogen phosphate to the CNC solution and continue to stir to obtain a CD solution, finally, freeze-drying under certain conditions to obtain CD aerogel;

[0018] The mass ratio of CNC, diammonium hydrogen phosphate, CVL, A BPA and TD satisfies 2.5:2.5:1:4:46-50, that is, the addition amount of TD is 82-86 wt.%;

[0019] In the step 1, the stirring conditions for preparing the CNC solution are that the stirring temperature is 20-30 DEG C and the stirring time is 8-9 h;

[0020] In step 1, the freezing conditions are a temperature of -30°C and a freezing time of 12 hours, and the freeze-drying conditions are a freeze-drying temperature of -50°C and a freeze-drying time of 72-96 hours;

[0021] Step 2, preparation of a reversible thermochromic phase change material, first, heating TD at a melting temperature of 50°C to melt it, then heating CVL and BPA in the molten TD for 30 minutes and blending them to obtain a reversible thermochromic phase change material TBC;

[0022] In step 2, the conditions for continuing stirring the CBT mixed solution are as follows: the stirring temperature is 75-80° C. and the stirring time is 1-1.5 h;

[0023] Step 3, preparation of a thermochromic phase change material with reversible function, with an immersion adsorption temperature of 75°C and an immersion adsorption time of 24 hours, the CD aerogel obtained in step 1 is placed in the reversible thermochromic phase change material TBC obtained in step 2 for vacuum immersion adsorption, and the thermochromic phase change material TBC / CD-TD with reversible function can be obtained.

[0024] The present invention has been tested and has the following characteristics:

[0025] FTIR testing showed that TBC / CD and TD had the same characteristic peaks, and TBC / CD was successfully prepared by physical casting of the solution in one step. In addition, TD and CNC were chemically bonded, and no new substances were produced.

[0026] XRD tests show that TBC / CD and TD have the same characteristic peaks, and TBC / CD has no effect on the crystallization behavior of TD;

[0027] SEM test shows that TD is evenly distributed in TBC / CD;

[0028] The leakage test showed that TBC / CD had good anti-leakage performance and no leakage occurred after 40 minutes at 80°C.

[0029] Thermochromic tests show that TBC / CD has reversible thermochromic properties. That is, when the temperature is higher than 38°C, the color of the thermochromic phase change material is grayish white. When the temperature is higher than 38°C, the color of the thermochromic phase change material gradually changes from dark blue to grayish white.

[0030] DSC tests show that the phase transition temperature of TBC / CD is 29.09-37.03°C, and the phase transition enthalpy is 158.753-171.209 J / g;

[0031] The DSC cycle test showed that TBC / CD has high cycle stability and thermal stability, and the enthalpy value of the cycle curve did not change significantly after 200 cycles;

[0032] Thermal conductivity tests show that TBC / CD has good thermal conductivity, with a thermal conductivity coefficient of 0.3338~0.4339W / (m·K).

[0033] Therefore, the present invention has the following advantages over the prior art:

[0034] 1. By using cellulose nanocrystals (CNCs) with excellent mechanical properties and biocompatibility as a substrate, a biomass aerogel material with high adsorption capacity and ordered pore structure was successfully constructed. Therefore, it has good biocompatibility. At the same time, when compounded with thermochromic compounds, no organic cosolvent is required.

[0035] 2. The reversible thermochromic phase change material prepared by the present invention has excellent thermal stability and cycle stability;

[0036] 3. The melting enthalpy of the phase change material is 159.80-171.20 J / g, and the crystallization enthalpy is 158.75-171..19 J / g, which has good heat storage performance per unit mass. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 TD, FTIR graphs of Example 1, Example 2, Example 3 and Comparative Example 1;

[0038] Figure 2 TD, XRD patterns of Example 1, Example 2, Example 3 and Comparative Example 1;

[0039] Figure 3 The SEM images of Example 1, Example 2, Example 3 and Comparative Example 1 are shown;

[0040] Figure 4 The figures are the anti-leakage test figures of Example 1, Example 2, Example 3 and Comparative Example 1;

[0041] Figure 5 is the reversible thermochromic image of Example 1;

[0042] Figure 6 TD, DSC test charts of Example 1, Example 2, Example 3 and Comparative Example 1;

[0043] Figure 7 This is the DSC 200 cycle test chart of Example 1;

[0044] Figure 8 TD is a thermal conductivity diagram of Example 1, Example 2, Example 3 and Comparative Example 1. DETAILED DESCRIPTION

[0045] The present invention is further described in detail through embodiments and in conjunction with the accompanying drawings, but the present invention is not limited thereto.

[0046] Example 1

[0047] A method for preparing a reversible thermochromic composite material based on cellulose nanocrystals adopts a one-step method based on physical casting, and the raw materials are cellulose nanocrystals CNC, ammonium dihydrogen phosphate, crystal violet lactone CVL: tetrabromobisphenol A BPA and tetradecanol TD, which meet the mass ratio of 2.5:2.5:1:4:50, that is, the addition amount of TD is 86wt.%, which specifically includes the following steps:

[0048] Step 1, preparation of CD aerogel: First, CNC is diluted in deionized water and magnetically stirred at a concentration of 2.5 wt.% at 20°C for 8 hours to obtain a CNC solution. Then, ammonium dihydrogen phosphate (2.5 wt.%) is added to the CNC solution at a stirring temperature of 20°C for 1 hour and the mixture is stirred to obtain a CD solution. Finally, the mixture is freeze-dried under certain conditions to obtain CD aerogel.

[0049] The freeze-drying method comprises the following steps: first freezing the CD solid gel at a freezing temperature of -30°C for a freezing time of 12 hours to obtain the CD solid gel; and then freeze-drying the CD solid gel at a freezing temperature of -50°C for a freezing time of 72 hours;

[0050] Step 2, preparation of a reversible thermochromic phase change material, first, heating TD at a melting temperature of 50°C to melt it, then heating CVL and BPA in the molten TD for 30 minutes and blending them to obtain a reversible thermochromic phase change material TBC;

[0051] Step 3, preparation of a reversible thermochromic phase change material, with an immersion adsorption temperature of 75°C and an immersion adsorption time of 24 hours, the CD aerogel obtained in step 1 is placed in the reversible thermochromic phase change material TBC obtained in step 2 for vacuum immersion adsorption, thereby obtaining a reversible thermochromic composite material TBC / CD-TD based on cellulose nanocrystals. Specifically, the TBC / CD-TD obtained in Example 1 is referred to as TBC / CD-86 because the addition amount of TD is 86 wt.%.

[0052] To verify the TD content, we conducted weighing tests before and after adsorption. The results showed that the CD aerogel weighed 3.71 g, the added TD weighed 22.79 g, and the resulting TBC / CD weighed 26.5 g. Therefore, calculations indicate that the added TD was completely adsorbed by the CD aerogel, with an adsorption capacity of 86 wt.%.

[0053] In order to prove that TBC / CD has thermochromic properties, temperature change test was carried out. Figure 5 As shown,

[0054] When the temperature rises above the phase transition temperature of TD, i.e., above 38 °C, the color of TBC / CD gradually changes from dark blue to grayish white;

[0055] When the temperature drops below the phase transition temperature of TD, i.e., below 38°C, the color of TBC / CD gradually changes from grayish white to dark blue;

[0056] The test results show that TBC / CD has reversible thermochromic properties.

[0057] In order to prove the composition of TBC / CD-86, XRD test and FTIR test were carried out. At the same time, TD was subjected to FTIR test and XRD test to compare with TBC / CD-86. Figure 1 The XRD test results are shown in Figure 2 As shown in the figure, the test results of TBC / CD-86 are not substantially different from those of TD. The test results show that there is no chemical reaction during the bonding process between CD aerogel and TD, that is, TBC / CD-86 has no effect on the crystallization behavior of TD, which can ensure that the composite phase change material has good heat storage capacity.

[0058] In order to verify the basic morphology of TBC / CD-86, SEM test was carried out. The test results are as follows Figure 3 As shown in the figure, the CD aerogel of TBC / CD-86 is densely and uniformly filled with TD, that is, it reaches a fully saturated state.

[0059] To demonstrate the packaging performance of TBC / CD-86, a leak test was conducted. For comparison, a leak test was also conducted on TD. The leak test involved heating the sample at 80°C for 40 minutes.

[0060] The test results of TD are as follows Figure 4 As shown, TD melts and flows to the surrounding areas;

[0061] The test results of TBC / CD-86 are as follows Figure 4 As shown, TBC / CD-86 showed no significant changes;

[0062] The test results show that the prepared CD aerogel can effectively prevent the leakage of TD.

[0063] In order to prove the phase change performance of TBC / CD-86, DSC test was carried out. At the same time, DSC test was carried out on TD for comparison.

[0064] TD test results are as follows Figure 6 As shown in Table 1, within the test temperature range of 20-100 ° C, there is an endothermic peak in the melting process of TD, the temperature of the endothermic peak is 38.05 ° C, and the enthalpy value of the endothermic peak is 196.90 J / g; there is an exothermic peak in the crystallization process, the temperature of the exothermic peak is 30.64 ° C, and the enthalpy value of the endothermic peak is 197.64 J / g;

[0065] The test results of TBC / CD-86 are as follows Figure 6 As shown in Table 1, within the test temperature range of 20-100°C, there is an endothermic peak in the melting process of TD, the temperature of the endothermic peak is 36.66°C, and the enthalpy value of the endothermic peak is 169.03 J / g; there is an exothermic peak in the crystallization process, the temperature of the exothermic peak is 29.56°C, and the enthalpy value of the endothermic peak is 168.49 J / g.

[0066] According to the addition amount of TD of 86 wt.%, the theoretical enthalpy value of TBC / CD-86 is 169.97 J / g. Therefore, there is no substantial difference between the enthalpy value of TBC / CD-86 and the theoretical enthalpy value, which indicates that CD aerogel has no negative impact on the phase transition behavior of TD.

[0067] Table 1 Phase change enthalpy and temperature of composite phase change materials with different TD addition amounts

[0068]

[0069] In order to prove the cyclic stability of TBC / CD-86, a cyclic test was conducted, and the test results are shown in the figure. Figure 7 As shown in the figure, the thermal cycling curve of TBC / CD-86 shows no significant change after 200 cycles. The test results show that the phase change performance of TBC / CD-86 does not change significantly before and after cycling, which proves that TBC / CD-86 has good cycling stability.

[0070] In order to prove the thermal conductivity of TBC / CD-86, a thermal conductivity test was conducted. At the same time, for comparison, a thermal conductivity test was conducted on TD.

[0071] TD test results are as follows Figure 8 As shown in Table 2 , the thermal conductivity of TD is 0.2703 W / m·K;

[0072] TBC / CD-86 test results are as follows Figure 8 As shown in Table 2 , the thermal conductivity of TBC / CD-86 is 0.3725 W / m·K;

[0073] Test results show that the thermal conductivity of TBC / CD-86 is 60% higher than that of TD. This is because TD itself has poor thermal conductivity. Therefore, after the introduction of CD aerogel, a three-dimensional heat conduction network is formed, which significantly improves the thermal conductivity of the composite material.

[0074] Table 2 Thermal conductivity of composite phase change materials with different TD addition amounts

[0075] Sample name TD Example 1 Example 2 Example 3 Comparative Example 1 Thermal conductivity (W / m·K) 0.2703 0.4339 0.3944 0.3725 0.3338

[0076] In order to demonstrate the effect of the addition amount of TD on the performance, comparative example 1, example 1, example 2 and example 3 are provided, in which the addition amount of TD is 88 wt.%, 86 wt.%, 84 wt.% and 82 wt.% respectively.

[0077] Comparative Example 1

[0078] A method for preparing a reversible thermochromic phase change material with a TD addition amount of 88 wt.%. The steps not specifically described are the same as those in Example 1, except that the addition amount of TD is 88 wt.%. The obtained material is named TBC / CD-88.

[0079] It should be noted that the amount of addition with the suffix TD is not necessarily the actual adsorption amount. The reason is that when the amount of TD added is excessive, supersaturation occurs. However, for ease of understanding, the material obtained in Comparative Example 1 is still named TBC / CD-88.

[0080] To verify the TD content, weighing tests were performed before and after adsorption. The results showed that when the CD aerogel mass was 3.71 grams, the mass of added TD was 22.79 grams, resulting in a final TBC / CD mass of 26.4 grams. Therefore, calculations indicate that the adsorption amount was only 87.67 wt.%, lower than the 88 wt.% added. This indicates that the added TD was not fully adsorbed by the CD aerogel, indicating oversaturation.

[0081] The FTIR test results of TBC / CD-88 are as follows Figure 1 The XRD test results are shown in Figure 2 As shown in the figure, the test results of TBC / CD-88 are not substantially different from those of TBC / CD-86. The test results show that the addition of TD has no substantial effect on the composition of TBC / CD.

[0082] In order to verify the basic morphology of TBC / CD-88, SEM test was carried out. The test results are as follows Figure 3As shown, the skeleton of the CD aerogel of TBC / CD-88 was completely covered, the pores were filled, and the surface of the composite phase change material showed a stacking phenomenon, i.e., reached a supersaturation state.

[0083] In order to prove the encapsulation performance of TBC / CD-88, a leakage prevention test was performed. The leakage prevention test results of TBC / CD-88 are shown in Table 3. Figure 4 As shown, there was liquid flow on the surface of TBC / CD-88, i.e., there was a leakage phenomenon. In combination with the test results of the SEM test, it can be known that the TD loading amount of TBC / CD-88 was in a supersaturation state.

[0084] The DSC test results of TBC / CD-88 are shown in Table 4. Figure 7 As shown in Table 1 and Table 4, within the test temperature range of 20-100℃, the melting process of TBC / CD-88 had an endothermic peak, the temperature of the endothermic peak was 37.03℃, and the enthalpy value of the endothermic peak was 171.20J / g; the crystallization process had an exothermic peak, the temperature of the exothermic peak was 29.13℃, and the enthalpy value of the endothermic peak was 171.19J / g.

[0085] According to the TD addition amount of 88wt.%, the theoretical enthalpy value of TBC / CD-88 was 173.27J / g, and therefore, the enthalpy value of TBC / CD-88 was lower than the theoretical enthalpy value. The test results showed that the excessive addition amount of TD, i.e., reaching a supersaturation state, directly led to the decrease of the enthalpy value of the composite material.

[0086] The thermal conductivity performance test results of TBC / CD-88 are shown in Table 5. Figure 7 As shown in Table 2 and Table 5, the thermal conductivity coefficient of TBC / CD-88 was 0.3338W / m·K, which was lower than the thermal conductivity coefficient of 0.4339W / m·K obtained in Example 1. The test results showed that the increase of the addition amount of TD led to the decrease of the thermal conductivity performance.

[0087] Example 2

[0088] A preparation method of a thermochromic phase change material with a TD addition amount of 84wt.%, the steps not specifically explained were the same as those of Example 1, and the difference was that the addition amount of TD was 84wt.%, and the obtained material was named as TBC / CD-84.

[0089] In order to prove the content of TD, a weighing test before and after adsorption was performed. The test results were that when the mass of the CD aerogel was 3.71 grams, the mass of the added TD was 19.47g, and the finally obtained TBC / CD was 23.18g. Therefore, it can be calculated that the added TD was completely adsorbed by the CD aerogel, i.e., the adsorption amount was 84wt.%.

[0090] The FTIR test results of TBC / CD-84 are shown in Table 6. Figure 1 As shown, the XRD test results are as follows Figure 2 As shown in the figure, the test results of TBC / CD-84 are the same as those of TBC / CD-86, that is, there is no substantial difference. The test results show that the addition of TD has no substantial effect on the composition of TBC / CD.

[0091] In order to verify the basic morphology of TBC / CD-84, SEM test was carried out. The test results are as follows Figure 3 As shown in the figure, each component is evenly dispersed. TD is evenly distributed in the CD aerogel, but does not cover the entire aerogel, that is, it has not reached the saturation state.

[0092] In order to prove the packaging performance of TBC / CD-84, a leak test was conducted. The leak test results of TBC / CD-84 are as follows: Figure 4 As shown in the figure, TBC / CD-84 has no obvious changes. The test results show that TBC / CD-84 has encapsulation properties and can effectively prevent TD leakage.

[0093] The DSC test results of TBC / CD-84 are as follows Figure 7 As shown in Table 1, within the test temperature range of 20-100°C, the melting process of TBC / CD-84 has an endothermic peak with a temperature of 36.20°C and an enthalpy of 162.59 J / g; the crystallization process has an exothermic peak with a temperature of 29.09°C and an enthalpy of 162.03 J / g.

[0094] According to the addition amount of TD of 84 wt.%, the theoretical enthalpy value of TBC / CD-88 is 165.40 J / g. Therefore, the enthalpy value of TBC / CD-84 is not substantially different from the theoretical enthalpy value, which indicates that CD aerogel has no negative impact on the phase transition behavior of TD.

[0095] The thermal conductivity test results of TBC / CD-84 are as follows: Figure 7 As shown in Table 2, the thermal conductivity of TBC / CD-84 is 0.3944 W / m·K, which is lower than the thermal conductivity of 0.4339 W / m·K obtained in Example 1. The test results show that reducing the amount of TD added also leads to a decrease in thermal conductivity.

[0096] Example 3

[0097] A method for preparing a thermochromic phase change material with a TD addition amount of 82 wt.%, wherein the steps not specifically described are the same as those in Example 1, except that the addition amount of TD is 82 wt.%, and the obtained material is named TBC / CD-82.

[0098] To prove the content of TD, the weighing test before and after adsorption was carried out. The test results are that when the CD aerogel mass is 3.71 grams, the mass of TD added is 17.31g, and the final TBC / CD is 21.02g. Therefore, it is calculated that the added TD is completely adsorbed by the CD aerogel, that is, the adsorption amount is 82wt.%.

[0099] The FTIR test results of TBC / CD-82 are shown in Figure 1 The XRD test results are shown in Figure 2 The test results of TBC / CD-82 are the same as TBC / CD-86, that is, there is no substantial difference. The test results show that the added amount of TD has no substantial effect on the composition of TBC / CD.

[0100] To prove the basic morphology of TBC / CD-82, SEM test was carried out. The test results are shown in Figure 3 The components are uniformly dispersed. TD is uniformly distributed in CD aerogel, but it does not cover the whole aerogel, that is, it does not reach the saturation state.

[0101] To prove the encapsulation performance of TBC / CD-82, the leakage prevention test was carried out. The leakage prevention test results of TBC / CD-82 are shown in Figure 4 The TBC / CD-82 has no obvious change. The test results show that TBC / CD-82 has encapsulation performance and can effectively prevent the leakage of TD.

[0102] The DSC test results of TBC / CD-82 are shown in Figure 7 and Table 1, within the test temperature range of 20-100℃, the melting process of TBC / CD-82 has an endothermic peak, the temperature of the endothermic peak is 36.13℃, and the enthalpy value of the endothermic peak is 159.80J / g; the crystallization process has an exothermic peak, the temperature of the exothermic peak is 29.16℃, and the enthalpy value of the endothermic peak is 158.75J / g;

[0103] According to the added amount of TD being 82wt.%, the theoretical enthalpy value of TBC / CD-82 is 161.46J / g, therefore, the enthalpy value of TBC / CD-84 has no substantial difference with the theoretical enthalpy value, that is, it shows that the CD aerogel has no negative effect on the phase change behavior of TD.

[0104] The thermal conductivity test results of TBC / CD-82 are shown in Figure 7 and Table 2, the thermal conductivity coefficient of TBC / CD-82 is 0.3725W / m·K, which is the same as the result of Example 2, and lower than the thermal conductivity coefficient of 0.4339W / m·K obtained in Example 1. The test results show that the further reduction of the added amount of TD leads to the further reduction of the thermal conductivity.

Claims

1. A reversible thermochromic composite material based on cellulose nanocrystals, characterized by: Cellulose nanocrystals (CNC) were used as a substrate, and molecular cross-linking reactions were generated by diammonium hydrogen phosphate to successfully construct a CD biomass aerogel material with both high adsorption capacity and an ordered pore structure. At the same time, tetradecanol (TD) was used as a phase change material and solvent, and crystal violet lactone (CVL) and bisphenol A (BPA) were used as colorants and developers to prepare a reversible thermochromic phase change material. The obtained reversible thermochromic phase change material has thermochromic performance, reversible performance, and phase change performance at the same time.

2. The reversible thermochromic composite material according to claim 1, characterized in that: The thermochromic performance means that when used as a thermochromic material, under normal temperature conditions, the thermochromic phase change material is in a dark blue state; when the temperature is higher than 38°C, the color of the thermochromic phase change material gradually changes from dark blue to grayish white.

3. The reversible thermochromic composite material according to claim 1, characterized in that: The reversible performance, that is, when used as a thermochromic material, is reversible. Under normal temperature conditions, the thermochromic phase change material is in a dark blue state; when the temperature is higher than 38°C, the color of the thermochromic phase change material is grayish white, that is, when the temperature is higher than 38°C, the color of the thermochromic phase change material gradually changes from dark blue to grayish white.

4. The reversible thermochromic composite material according to claim 1, characterized in that: The phase change temperature of the phase change material TD is consistent with the critical color change temperature of crystal violet lactone CVL. The phase transition temperature control process of TD can enhance the color change sensitivity and stability of the color-changing function of the composite material.

5. The reversible thermochromic composite material according to claim 1, characterized in that: The thermochromic performance means that when used as a thermochromic material, under normal temperature conditions, the thermochromic phase change material is in a dark blue state; when the temperature is higher than 38°C, the color of the thermochromic phase change material gradually changes from dark blue to grayish white.

6. The reversible thermochromic composite material according to claim 1, characterized in that: The reversible performance, that is, when used as a thermochromic material, is reversible. Under normal temperature conditions, the thermochromic phase change material is in a dark blue state; when the temperature is higher than 38°C, the color of the thermochromic phase change material is grayish white, that is, when the temperature is higher than 38°C, the color of the thermochromic phase change material gradually changes from dark blue to grayish white.

7. The reversible thermochromic composite material according to claim 1, characterized in that: The phase change performance, that is, when used as a phase change material, has a phase change temperature of 29.09-37.03° C., a phase change enthalpy of 158.753-171.209 J / g, and a thermal conductivity of 0.3338-0.4339 W / m·K.

8. A method for preparing a reversible thermochromic composite material based on cellulose nanocrystals, characterized in that The following steps are involved: Step 1: Preparation of CD aerogel: First, dilute CNC with CNC. Under certain conditions, dilute CNC in deionized water and magnetically stir to obtain a CNC solution. Then, add a certain concentration of ammonium dihydrogen phosphate to the CNC solution and continue stirring to obtain a CD solution. Finally, freeze-dry under certain conditions to obtain CD aerogel. Step 2, preparation of a reversible thermochromic phase change material, first, heating TD at a melting temperature of 50°C to melt it, then heating CVL and BPA in the molten TD for 30 minutes and blending them to obtain a reversible thermochromic phase change material TBC; Step 3, preparation of a thermochromic phase change material with reversible function, with an immersion adsorption temperature of 75°C and an immersion adsorption time of 24 hours, the CD aerogel obtained in step 1 is placed in the reversible thermochromic phase change material TBC obtained in step 2 for vacuum immersion adsorption, and the thermochromic phase change material TBC / CD-TD with reversible function can be obtained.

9. The preparation method according to claim 5, characterized in that: The mass ratio of CNC, diamine hydrogen phosphate, CVL, ABPA and TD is 2.5:2.5:1:4:46-50, that is, the addition amount of TD is 82-86wt.%.

10. The preparation method according to claim 5, characterized in that: In step 1, the stirring conditions for preparing the CNC solution are as follows: the stirring temperature is 20-30° C. and the stirring time is 8-9 h; In step 1, the freezing conditions are a temperature of -30°C and a freezing time of 12 hours, and the freeze-drying conditions are a freeze-drying temperature of -50°C and a freeze-drying time of 72-96 hours; In the step 2, the stirring conditions for preparing the CBT mixed solution are as follows: the stirring temperature is 75-80° C. and the stirring time is 1-1.5 h.

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