Preparation and characterization of water-based adhesive and its application in radiation-cooling coatings

In order to prepare environmentally friendly coatings with radiative cooling performance in the middle and low-temperature domain (below 150°C), this paper first prepared a water-based adhesive. The preparation and test methods and results are as follows. The siloxane-coated cadmium telluride composite particles were prepared, and the surface layer was modified by amination with silicone coupling agent KH550. The modified composite particles were used to crosslink waterborne polyurethane to prepare waterborne adhesive and waterborne radiation coating. The composite particles and their modified aqueous emulsion film were characterized by XRD, FT-IR, TEM, TGA, and universal tester. Besides that, three kinds of water-based adhesives with different cadmium telluride content were prepared into radiation-cooling coatings and coated on steel plates. The temperature was raised to 70°C together with the uncoated steel plates, and then the temperature was lowered naturally. The cooling time of 4 steel plates to 30°C was recorded. The results show that the composite particles are microspheres, uniform in size and narrow in distribution. The radiation coatings prepared with three kinds of binders have good cooling and radiation performance, and the cooling performance increases with the increase of cadmium telluride content in the coating.


Introduction
In actual operation, the overhead wire of the power network generates a lot of Joule heat through the current, so that the internal temperature of the wire increases [1] .At high temperatures, the microstructure and physical properties of wire materials will cause irreversible and harmful changes, such as mechanical and fatigue properties decline and sag increase, which directly endangers the operation safety of the line [2] .In general, the operating temperature of the overhead wire should not be higher than 80°C [3] .Therefore, the heat generated by the overhead wire during work must be dissipated in time.The heat dissipation of the existing overhead wire mainly depends on the air circulation of the working environment, but the natural heat dissipation cooling efficiency is insufficient, especially in the case of hot weather and the increase of the transmission load of the line, and the heat dissipation effect is worse.
Thermal radiation coating is a kind of coating that can effectively accelerate the heat exchange between the coating and the surrounding environment, which can save energy and reduce consumption and is widely used in high energy consumption industries such as petrochemical and metallurgy [4] .For example, Zhao et al. developed an infrared radiation coating applied to the inner wall of an industrial furnace.The test results showed that the coating had good thermal shock resistance between 0℃ and AMCE-2023 Journal of Physics: Conference Series 2713 (2024) 012080 IOP Publishing doi:10.1088/1742-6596/2713/1/012080 2 1100℃ [5] .Zhao et al. developed a kind of infrared radiation energy-saving coating based on iron, manganese, copper, and cobalt.After sintering at 1100℃, its emissivity in the 8~14 μm band exceeded 0.89 [6] .The existing technology mainly focuses on the high-temperature radiation coating, which needs to be fired at a high temperature above 1000℃ after spraying.The process is complicated, and due to the excessive use of inorganic binders, the coating is prone to cracking and falling off due to cold and heat shrinkage in the actual application process, thus affecting the service life [7] .Obviously, although the high-temperature radiation coating has a good heat exchange function, it is not suitable for the heat exchange scene in the low-temperature environment such as the overhead wire of the power grid (below 200℃).
Cadmium telluride, as a photoelectric material, its emission spectrum is close to that of sunlight [8] .In this experiment, water-based adhesive and radiation-cooling coating based on amino-siloxane-cadmium telluride composite polyurethane (PU) emulsion were prepared.The adhesive has good ductility and is suitable for low-temperature environments.The thermal radiation coating prepared by the adhesive has a good heat exchange function.
X-ray diffraction (XRD) was tested by the D/MAX-2400 instrument of Rigaku Corporation, with a scanning Angle of 2θ ranging from 10° to 35° and a scanning speed of 2°/min.The infrared spectrum (FT-IR) was tested by Nicolet-NEXUS 670 instrument of American Thermoelectric Company, using the potassium bromide wafer forming method, TGS detector, and 4 cm resolution to scan 32 times.The thermogravimetric analysis was performed by the PE-TGA7 instrument of Perkin Elmer Company in a nitrogen atmosphere with a temperature range of RT ~ 600℃ and a heating rate of 20℃/min.The adhesive sample was dispersed in N, n-dimethylformamide, and 1 drop solution was placed on a copper grid to be dried and observed under transmission electron microscopy (TEM).The mechanical properties were tested by the CMT5105 universal testing machine made by Shanghai Xinsi Company, and the tensile rate was 10 mm/min.Infrared thermal imaging was taken by testo 865 instrument of TESto GMBH.

2.2.1
Preparation of polyurethane prepolymer.200 g (0.1 mol) of molten PEG2000 was added to a four-port flask equipped with nitrogen, vacuum, and stirring device, and vacuumed at 100℃ for 2 h to remove water in the raw material.Cooling to 60℃, 88.8 g (0.4 mol) IPDI and 1 drop of dibutyltin dilaurate catalyst were added and heated up to 80℃.After reacting for 3 h, N-methylpyrrolidone solution dissolved in 13.4 g (0.1 mol) DMPA was added for 2 h, and the polyurethane prepolymer was obtained by cooling to 50℃.The preparation route is shown in Figure 1.

Preparation of aminosiloxane-cadmium telluride nanoparticles.
The single-mouth flask pumped with high-purity nitrogen three times was placed in an ice water bath at 0℃, and 11.2 g NaBH and 200 g distilled water were added and stirred.After stirring, we continued to add 15 mg (0.12 mmol) of tellurium powder for a 12 h reaction.The tellurium powder disappeared and a white precipitate appeared at the bottom of the flask, and the supernatant was taken and sealed for preservation.2.28 g (0.01mol) cadmium chloride powder with crystal water, 6.36 g (0.06mol) mercaptopropionic acid, and excess distilled water were added into a single-mouth flask and heated to 100℃.The supernatant was transferred into the flask, and stirred for 10 min, then 600 mL TEOS was added and the reflux reaction continued for 10 h.The products were precipitated with methanol and filtered, and the filtered products were dried in a vacuum oven at 60℃ for 24 h to obtain the siloxane-cadmium telluride complex, which was sealed and preserved away from light.10 g siloxane-cadmium telluride complex particles, 50 mL KH550, and 500 mL toluene were added to a round-bottom flask equipped with a condensing device and then opened for magnetic agitation.After the reaction at 95℃ for 10 h, the products were centrifuged and washed with industrial ethanol.After repeated three times, the products were dried in a vacuum oven at 60℃ for 24 h.The dried aminosiloxane-cadmium telluride nanoparticles were obtained and sealed for preservation away from light.

Preparation of amino-siloxane-cadmium telluride composite PU water-based adhesive .
The polyurethane prepolymer prepared in section 1.2.1 was heated to 45℃, 10.1 g (0.1 mol) triethylamine was added for neutralization, 200 g acetone for viscosity reduction, and the heat was held for 10 min.At a high-speed stirring of 1200 r/min, 600 g of deionized water was added for emulsification.After homogeneous dispersion, 0 g, 3 g, and 6 g amino-siloxane-cadmium telluride nanoparticles were added for chain expansion, reacting for 10 min, a sufficient amount of ethylenediamine was added for 30 min to ensure the reaction of all isocyanate groups in the system, and the acetone in the system was removed by vacuum distillation to obtain the emulsion with 30% solid content.The mass fraction of the siloxane-cadmium telluride complex in the emulsion film is about 0, 1%, and 2%.The preparation route is shown in Figure 2, and the sample ratio of different amino siloxane-cadmium telluride contents is shown in Table 1.

Preparation of radiation coatings.
The sintered and commercially available Fe-Co Ni-Cr oxide powder was used as the base material for radiation coating, and a certain amount of self-made aziridine-modified graphene and the water-based binder prepared in subsection 1.2.3 was added, with a mass ratio of 50:5:45 [9][10] .The low-temperature radiation curing coating was obtained by high-speed dispersion and applied to the steel plate.The curing temperature was room temperature, 120℃.

X-ray diffraction analysis
The crystal structures of cadmium telluride and siloxane-cadmium telluride particles were determined by X-ray diffraction, as shown in Figure 3.It can be seen from Figure 3 that the XRD peak pattern of cadmium telluride conforms to the characteristic of typical sphalerite crystals, while the wide peak of siloxane-cadmium telluride becomes narrower at about 23°.This is because cadmium telluride, after being coated with siloxane, reduces the proportion of cadmium telluride crystals in the complex on the one hand, and the coated siloxane has an amorphous structure on the other hand.

Infrared spectrum analysis
The particle samples of the siloxane-cadmium telluride complex and amino-siloxane-cadmium telluride complex were tested by infrared spectrum.The results are shown in Figure 4. Figure 4 shows the stretching vibration peaks of Si-O-Si and Si-O-H at 1100 cm and 970 cm, while the -OH vibration peaks at 3440 cm and 1632 cm reflect the presence of adsorbed water molecules on the surface of the two particles.Compared with the siloxane-cadmium telluride particles, the aminosiloxane-cadmium telluride has a -NH vibration peak at 1 565 cm, a -CH vibration peak at 2 930 cm, and a Si-O-C vibration peak at 1030 cm.These new peaks are the characteristic peaks of KH550.The results indicate that KH550 was successfully grafted onto the surface of the siloxane-Cdte particles [11] .
Figure 5 shows the FT-IR spectra of polyurethane prepolymer and polyurethane-siloxane-cadmium telluride complex.In Figure 5, the peak of the prepolymer sample at 2262 cm belongs to the unreacted -NCO group, and the peak at this position completely disappears in the spectrum of the polyurethane-siloxane-cadmium telluride complex, indicating that the prepolymer has reacted completely after chain extension and no NCO group exists.The spectra of both samples have a peak near 1700 cm, which is the strong stretching vibration peak of C=O on the carbamate group.However, the peak position of the prepolymer is near 1730 cm, while the peak position of the polyurethane complex is near 1670 cm because a large number of urea bonds are formed after the reaction of -NCO and -NH in the prepolymer.It is further confirmed that the reaction of -NH on aminosiloxane-cadmium telluride with the NCO terminal group in the prepolymer in an aqueous solution produces a urea carbonyl group.The results are consistent with the literature [12] .As can be seen from Figures 6 and 7, after the coating of the siloxane network formed by TEOS and the grafting modification of KH550, the complex particles are microspherical and uniform in size, ranging from 4 nm to 22 nm, and the distribution is narrow, indicating that the ions are well dispersed in aqueous solution or DMF solution, and no large-scale agglomeration occurs.The above results show that the growth of complex particles of siloxane in the coating process of cadmium telluride is good, and there is no side reaction affecting the coating process, which lays a foundation for its application in the industrial field.

Thermogravimetric properties of polyurethanes and polyurethane-siloxane-cadmium telluride composites
Radiation coatings are in a state of high temperature and heat exchange for a long time.In order to study the influence of the introduction of inorganic particles on the heat resistance of organic adhesives, TGA was used to analyze the thermal stability of three samples with different inorganic contents in Table 1.The results are shown in Figure 8.As can be seen in Figure 8, a slight weight loss occurred in the first stage of the 1# pure PU sample in the temperature range of 200 ~ 260℃, mainly due to the evaporation of residual water in the polyurethane and the decomposition of oligomers, and the decomposition of hard and soft segments of the polyurethane mainly occurred in the temperature range of 260~340℃ and 340~500℃.With the introduction of inorganic substances, the heat resistance of 2# and 3# samples should be improved in the temperature range of the first stage, but the actual decomposition temperature is not significantly increased.The analysis may be caused by the large surface area and high polarity of inorganic particles, which adsorb a large number of water molecules.The weight loss caused by the volatilization of water molecules offsets the ability of inorganic particles to maintain the weight of the sample at high temperatures.In the second stage, the decomposition temperature of 2# and 3# increased significantly, reaching 370℃, about 30℃ higher than that of the 1# sample.Due to the higher proportion of inorganic particles, the quality retention rate of the 3# sample was always higher than that of the 2# sample.The quality retention rates of 2# and 3# samples were also consistently higher than 1# before the third stage 450°C.On the one hand, the introduction of inorganic particles forms covalent bonds with the main chain of polyurethane prepolymers through the reaction between -NH and -NCO, limiting the movement of organic chain segments at high temperatures; on the other hand, the high polarity of inorganic particles on the surface also greatly increases the formation of non-covalent bonds such as hydrogen bonds between organic and inorganic phases, further restricting the movement of organic chain segments.

Mechanical properties analysis
The mechanical properties of the emulsions of organic-inorganic complex siloxane-cadmium telluride with particle mass fraction of 0, 1%, and 2% and their film forms were tested.The results are shown in Table 2.
Table 2 As can be seen from Table 2, compared with ordinary polyurethane emulsions of Sample 1#, the viscosity of emulsions of 2# and 3# increases with the increase of inorganic-organic complex particle content.This is because on the one hand, a large number of amino groups on the surface of the inorganic phase act as crosslinking agents, cross-linking linear polyurethane prepolymers into a network structure; on the other hand, the inorganic surface contains a large number of hydrated hydroxyl and silicon hydroxyl groups.These can form a large number of hydrogen bonds between the soft and hard segments of polyurethane, thereby increasing the viscosity of the emulsion on a macro level.
In addition, the hardness, tensile strength, and elongation at break increase with the increase of inorganic content.The increase in hardness and tensile strength can be considered as the result of the increase in crosslinking degree, hydrogen bond density, and inorganic content of emulsion film-forming material.The increase in elongation at break can be explained as the increase in hydrogen bond density causes the formation of a fuzzy interface layer between the soft and hard segments of polyurethane.The fuzzy interface layer is the transition layer between the soft and hard segments, which can effectively separate the soft and hard segments and form a strong non-covalent bond attraction between the soft and hard segments, thus increasing the tensile strength and elongation at the break of the material at the same time.This is consistent with the research results in the literature and also provides ideas for the future design and development of materials with both mechanical strength and flexibility [12] .

Radiation and cooling effects of radiation coatings
The radiation coating was prepared with 2# emulsion adhesive, which was coated on the surface of the steel plate, solidified at 120℃, and compared with the uncoated steel plate for cooling and radiation performance.The cooling performance test method was to heat two steel plates to 75℃ at an ambient temperature of 15℃, the heat source was turned off, and the time was recorded when the steel plate dropped from 75℃ to 30℃, as shown in Figure 9.As can be seen from Figure 9, the cooling time of the thermal radiation coating sample prepared by 1#~3# adhesive is 275 s, 233 s, and 215 s, indicating that the introduction of cadmium telluride has a significant improvement on the cooling effect of radiation.In addition, the cooling time of the uncoated steel plate is 388 s, the time of the 3# is shortened by more than 40% compared with the uncoated steel plate, and the heat dissipation power is increased by about 80%.
The thermal radiation performance test method is to place two steel plates of the same specification at the same time on a table at a constant temperature of 90℃, where the left one is coated with a thermal radiation coating made of 3# adhesive steel plate, and the right is an uncoated steel plate.A thermal imager was used to observe the infrared radiation effect of the two steel plates.The infrared imaging photos are shown in Figure 10.The results of Figure 10 show that the thermal radiation performance of the 3# adhesive-coated steel plate is much higher than that of the unsprayed aluminum plate.

Conclusion
In this study, the composite particles coated with siloxane cadmium telluride are prepared, the surface of the composite particles is aminated with KH550, and the modified composite particles are used for the crosslinking of waterborne polyurethane, so as to prepare the water-based adhesive of radiation coating suitable for the cooling of power grid overhead wires.The composite particles and the modified aqueous emulsion film are characterized by XRD, FT-IR, TEM, and TGA.The results show that the composite particles are micro-spherical in structure, uniform in size ranging from 4 nm to 22 nm, and narrow in distribution, and no agglomeration occurs in the solution.The heat resistance and mechanical properties of waterborne polyurethane film are obviously improved by the introduction of inorganic composite particles.The former increases by about 30℃, and the latter increases from 22 MPa to 27 MPa and 32 MPa, respectively.The water-based radiation coatings are prepared by using the above binders.The cooling and radiance properties of the coatings at the same high temperature and the same heat source are tested by the radiation cooling test and infrared thermal imaging.The results of the natural cooling test at the same high temperature show that the cooling time from 70℃ to 30℃ varies from 215 s to 388 s, and decreases with the increase of cadmium telluride content.The radiation test results of the same heat source show that the steel plate containing cadmium telluride radiation cooling coating has stronger thermal radiation performance.

Figure 6 and
Figure 7 are transmission electron microscope photos and particle size distributions of amino-modified siloxane-cadmium telluride particles, respectively.

Figure 9 .
Figure 9.Comparison of radiant cooling time of steel plate.

Figure 10 .
Figure 10.Infrared thermal imaging of steel plate without/with thermal radiation coating.

Table 1 .
The Ratio of Waterborne Polyurethane Binder with Different Amino Siloxane-Cadmium Telluride Content.
. Mechanical Properties of Polyurethanes and Polyurethane-siloxane-cadmium Telluride Composites.