Method for preparing high-stability PPTC (Polymeric Positive Temperature Coefficient) based on polyvinylidene chloride coated modified tungsten carbide

Abstract

The invention provides a method for preparing high-stability PPTC (Polymeric Positive Temperature Coefficient) based on polyvinylidene chloride coated modified tungsten carbide, and belongs to the technical field of high-stability device preparation. Firstly, polyvinylidene chloride emulsion and water or ethanol are mixed and stirred to obtain coating agent emulsion; then coating the coating agent emulsion and tungsten carbide powder while stirring; then the tungsten carbide powder is put into a vacuum drying box to be dried, and coated modified tungsten carbide is obtained; and finally, blending the coated modified tungsten carbide and HDPE (High-Density Polyethylene) to prepare the PPTC. The surface of tungsten carbide is coated with polyvinylidene chloride, the oxidation process in which oxygen and water molecules participate is effectively blocked, and the anti-oxidation effect is achieved. Tests after treatment in a constant-temperature and constant-humidity chamber show that the coated modified tungsten carbide plays a good role in resisting oxygen and water, so that oxidation of the tungsten carbide is inhibited, and the prepared PPTC is small in resistance change and high in stability.

Description

Technical Field

[0001] The invention belongs to the technical field of high-stability device preparation, and in particular relates to a method for preparing high-stability PPTC based on polyvinylidene chloride coated modified tungsten carbide. Background Art

[0002] Tungsten carbide powder is a black hexagonal crystal with metallic luster and hardness similar to that of diamond. It is a good conductor of electricity and heat. Due to its high conductivity, it can be used as a conductive filler and mixed with HDPE (high-density polyethylene) to prepare low-resistance polymer positive temperature thermistor (PPTC), which has a wide range of application prospects.

[0003] The oxidation rate of tungsten carbide powder is slow in dry air at 20-500℃. In the range of high relative humidity, the thickness of the oxide layer increases with the increase of humidity, the degree of oxidation intensifies, the resistance of tungsten carbide powder increases, the resistance of PPTC also increases, and the stability is poor.

[0004] Research has found that oxygen and water are the key to the oxidation of tungsten carbide. Blocking its contact with oxygen and water can inhibit the low-temperature oxidation of tungsten carbide. Prior art CN117509644A discloses a method for preparing coated modified tungsten carbide powder, wherein a coating agent and a solvent are mixed and stirred to obtain a mixed solution, wherein the coating agent is a silane coupling agent or stearic acid; the mixed solution and tungsten carbide powder are placed on a magnetic stirring heating table for coating; the coated tungsten carbide powder is filtered and washed; the tungsten carbide powder is dried to obtain the coated tungsten carbide powder. In this technology, an antioxidant effect is achieved by isolating the process in which water molecules participate in the oxidation process. After research, it was found that the coating antioxidant effect in the prior art CN117509644A is not in the composite system, but is relatively exposed to tungsten carbide, so the improvement effect in the HDPE / tungsten carbide system is limited; because silane coupling agent or stearic acid are both small molecule materials, they are often used as grafting agents in the mixing process of inorganic and organic substances in the prior art to improve the bonding interface, so that the inorganic matter is evenly dispersed in the organic matter; therefore, in the HDPE / tungsten carbide system, silane coupling agent or stearic acid is used to improve the compatibility between tungsten carbide powder and HDPE, narrow the interface gap, and improve the oxidation of tungsten carbide powder to a certain extent, but silane or stearic acid is not stronger than HDPE itself in blocking oxygen and water, and the compatibility is also limited; therefore, in theory, the coating isolation effect of silane coupling agent or stearic acid in the prior art CN117509644A is very limited in the HDPE / tungsten carbide system. Therefore, it is a very valuable research to find a material that has strong isolation ability in the HDPE / tungsten carbide system and can be well combined with tungsten carbide powder, and to conduct actual verification to prepare an antioxidant HDPE / tungsten carbide system. Summary of the invention

[0005] In view of the shortcomings of the prior art, the purpose of the present invention is to provide a method for preparing highly stable PPTC based on polyvinylidene chloride coated modified tungsten carbide. The polyvinylidene chloride coated modified tungsten carbide has good interface modification ability, improves the tungsten carbide-PE (polyethylene) interface bonding, and has very good isolation for tungsten carbide. After testing, its isolation performance for oxygen and water is better than that of HDPE itself, so it effectively isolates environmental water and oxygen corrosion, and improves the stability of PPTC.

[0006] In order to solve the above technical problems, the technical solution adopted by the present invention is as follows: A method for preparing high-stability PPTC based on polyvinylidene chloride coated modified tungsten carbide, comprising the following steps: (1) mixing and stirring a coating agent and a solvent to obtain a coating agent emulsion, wherein the coating agent is a polyvinylidene chloride emulsion; (2) mixing the coating agent emulsion and tungsten carbide powder to coat; (3) drying the tungsten carbide powder to obtain coated modified tungsten carbide; (4) The coated modified tungsten carbide powder is blended with polyethylene to prepare PPTC.

[0007] In the present invention, the coating agent is configured into an emulsion, the emulsion is stirred and mixed with tungsten carbide powder, and after drying, polyvinylidene chloride is coated on the surface of tungsten carbide powder to obtain coated modified tungsten carbide, and then the coated modified tungsten carbide is blended with polyethylene to prepare PPTC; in the present invention, polyvinylidene chloride (PVDC) is a thermoplastic polymer with strong intermolecular cohesion and high crystallinity. In the molecular structure, two high-content chlorine atoms are symmetrically distributed. When the tungsten carbide powder is coated and modified, it can be firmly adsorbed on the surface of the tungsten carbide powder to provide better dispersibility; in addition, the chlorine atoms in the polyvinylidene chloride molecules have hydrophobicity. Therefore, it is difficult for oxygen and water molecules to move in its molecules, making PVDC have excellent oxygen and moisture barrier properties, and its oxygen barrier property is not affected by the humidity of the surrounding environment, which ultimately makes the isolation ability of polyvinylidene chloride better than that of HDPE itself; therefore, polyvinylidene chloride has good interface improvement and isolation effects at the same time. When it is used to coat modified tungsten carbide and make PPTC, it can avoid the tungsten carbide from contacting and oxidizing with water in the air to cause the resistance of PPTC to increase from two technical directions, and the stability is greatly improved than that of a simple interface modifier, thereby greatly improving the stability and service life of PPTC.

[0008] Preferably, the tungsten carbide powder is a micron-sized tungsten carbide powder produced by a mechanical alloying method, a direct reduction carburization method or a sol-gel method, and the particle size of the tungsten carbide powder is 1um-30um.

[0009] Preferably, in step (1), the mass concentration of the coating agent emulsion is 10-30%.

[0010] Preferably, in step (1), the solvent is water and / or anhydrous ethanol.

[0011] Preferably, in the step (2), the mass ratio of the tungsten carbide powder to the coating agent in the coating agent emulsion is 1600:1.0-5.0.

[0012] Preferably, in step (2), the stirring and mixing time is 5-20 min, and the stirring speed is 200-500 r / min.

[0013] Preferably, in step (3), the drying time is 1-2 hours and the drying temperature is 40-70°C.

[0014] Preferably, in step (4), the mass ratio of the coated modified tungsten carbide powder to polyethylene is 15-20:1.

[0015] Preferably, the polyethylene is HDPE.

[0016] More preferably, the coated modified tungsten carbide powder and HDPE are mixed at a mass ratio of 16:1 to prepare a PPTC with a side length of 10 mm (length and width are both 10 cm) and a thickness of 0.5 mm, and the PPTC resistance is tested.

[0017] Compared with the prior art, the present invention has the following beneficial effects: The method for preparing high-stable PPTC based on polyvinylidene chloride coated modified tungsten carbide provided by the present invention comprises the following steps: firstly, a coating agent is configured into an emulsion, the emulsion is stirred and mixed with tungsten carbide powder, and after drying, polyvinylidene chloride is coated on the surface of tungsten carbide powder to obtain coated modified tungsten carbide, and then the coated modified tungsten carbide is blended with polyethylene to prepare PPTC; the polyvinylidene chloride (PVDC) of the present invention is a thermoplastic polymer with strong intermolecular cohesion and high crystallinity. In the molecular structure, two high-content chlorine atoms are symmetrically distributed. When the tungsten carbide powder is coated and modified, the PVDC can be firmly adsorbed on the surface of the tungsten carbide powder, providing better dispersibility and playing a role in the tungsten carbide powder. It has a similar interface modification effect as silane. In addition, the chlorine atoms in the polyvinylidene chloride molecules are hydrophobic, so it is difficult for oxygen molecules and water molecules to move in the molecules, which makes PVDC have excellent oxygen and moisture barrier properties, and its oxygen barrier property is not affected by the humidity of the surrounding environment, which ultimately makes the isolation ability of polyvinylidene chloride better than that of HDPE itself. Therefore, it has better interface improvement and isolation effects at the same time. It avoids tungsten carbide from contacting and oxidizing with water in the air to cause higher resistance, thereby improving the stability and service life of PPTC. After accelerated oxidation experiments, the stability is greatly improved, and when the same resistance level is reached, the service life is extended several times. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is a schematic diagram of a process for preparing a coated tungsten carbide powder provided in an embodiment of the present invention; Figure 2 This is a surface morphology of tungsten carbide in PPTC provided in Comparative Example 2 of the present invention; Figure 3 is a surface morphology of tungsten carbide in PPTC provided in Example 1 of the present invention; Figure 4 is a surface morphology of tungsten carbide in PPTC provided in Example 2 of the present invention; Figure 5 The PPTC structure and resistance test principle diagram provided by the embodiment of the present invention. DETAILED DESCRIPTION

[0019] The following embodiments of the present invention are described in further detail in conjunction with the accompanying drawings and examples. The following examples are used to illustrate the present invention, but are not intended to limit the scope of the present invention.

[0020] In order to make the technical solutions and advantages of the present invention more clear and explicit, the present invention is further described in detail with reference to the following embodiments.

[0021] like Figure 1 As shown, the present invention provides a method for preparing a highly stable PPTC based on polyvinylidene chloride coated modified tungsten carbide, comprising the following steps: (1) mixing and stirring a coating agent and a solvent to obtain a coating agent emulsion, wherein the coating agent is a polyvinylidene chloride emulsion; (2) mixing the coating agent emulsion and tungsten carbide powder to coat; (3) vacuum drying the tungsten carbide powder to obtain coated modified tungsten carbide; (4) The coated modified tungsten carbide powder is blended with polyethylene to prepare PPTC.

[0022] Example 1: Step 1, the coating agent is polyvinylidene chloride emulsion, the solvent is water and ethanol in a ratio of 4:1, and the polyvinylidene chloride emulsion and the solvent are mixed and stirred to obtain a coating agent emulsion with a concentration of 20wt%; Step 2: Mix 1600 parts of tungsten carbide powder and 10 parts of the coating agent emulsion (2 parts of coating agent) by mass, and stir for 15 minutes; Step 3: Place the tungsten carbide powder obtained in step 2 in a vacuum drying oven at 45° C. and dry for 1 hour; Step 4: coated modified tungsten carbide and HDPE are mixed at a mass ratio of 16:1 to prepare PPTC with a side length of 10 mm (length and width are both 10 cm) and a thickness of 0.5 mm; Step 5: Place the PPTC prepared in step 4 into a constant temperature and humidity chamber under double 85 test conditions for accelerated oxidation experiment to detect the PPTC resistance; the principle is as follows Figure 5 As shown, the PPTC is clamped between two copper foil electrode plates, and the current I is measured by connecting the two probes of the ammeter A. g , the voltage U is measured by connecting the two probes of the voltmeter V g , through the formula Calculating the resistance of a PPTC ,in is the voltage applied to PPTC by the four-probe method, is the current passing through the PPTC using the four-probe method; the experimental conditions and test results are recorded in Table 1.

[0023] Example 2: Step 1, the coating agent is polyvinylidene chloride emulsion, the solvent is water, and the polyvinylidene chloride emulsion and the solvent are mixed and stirred to obtain a coating agent emulsion with a concentration of 25wt%; Step 2: Mix 1600 parts of tungsten carbide powder and 16 parts of the coating agent emulsion (4 parts of coating agent) by mass, and stir thoroughly for 18 minutes; Step 3: Place the tungsten carbide powder obtained in step 2 in a vacuum drying oven at 55° C. and dry for 1.5 hours; Step 4: coated modified tungsten carbide and HDPE are mixed at a mass ratio of 16:1 to prepare PPTC with a side length of 10 mm (length and width are both 10 cm) and a thickness of 0.5 mm; Step 5, placing the PPTC prepared in step 4 into a constant temperature and humidity chamber under double 85 test conditions for accelerated oxidation experiment to detect the PPTC resistance; The experimental conditions and test results are recorded in Table 1.

[0024] Embodiment 3: Step 1, the coating agent is polyvinylidene chloride emulsion, the solvent is water, and the polyvinylidene chloride emulsion and the solvent are mixed and stirred to obtain a coating agent emulsion with a concentration of 15wt%; Step 2: Mix 1600 parts of tungsten carbide powder with 10 parts of the coating agent emulsion (1.5 parts of coating agent) and stir thoroughly for 20 minutes; Step 3: Place the tungsten carbide powder obtained in step 2 in a vacuum drying oven at 55° C. and dry for 1.0 hour; Step 4: coated modified tungsten carbide and HDPE are mixed at a mass ratio of 16:1 to prepare PPTC with a side length of 10 mm (length and width are both 10 cm) and a thickness of 0.5 mm; Step 5, placing the PPTC prepared in step 4 into a constant temperature and humidity chamber under double 85 test conditions for accelerated oxidation experiment to detect the PPTC resistance; The experimental conditions and test results are recorded in Table 1.

[0025] Comparative Example 1: Step 1: Using the solution in the prior art CN117509644A, 1600 parts of tungsten carbide powder are surface treated with 1.0 part of silane; Step 2: Stearic acid-modified tungsten carbide and HDPE are mixed at a mass ratio of 16:1 to prepare PPTC with a side length of 10 mm (length and width are both 10 cm) and a thickness of 0.5 mm; Step 3: Place the PPTC prepared in step 2 in a constant temperature and humidity chamber under double 85 test conditions for accelerated oxidation experiment to detect the PPTC resistance; The experimental conditions and test results are recorded in Table 1.

[0026] Comparative Example 2: Step 1: Uncoated modified tungsten carbide and HDPE are mixed at a mass ratio of 1:16 to prepare PPTC with a side length of 10 mm and a thickness of 0.5 mm; Step 2: Prepare the PPTC and put it into a constant temperature and humidity chamber under double 85 test conditions for accelerated oxidation experiment to detect the PPTC resistance; The experimental conditions and test results are recorded in Table 1.

[0027] Table 1, PPTC accelerated oxidation test results of Examples 1-3 and Comparative Examples 1 and 2, resistance unit is (mΩ); Oxidation time (hr) 0 24 168 336 672 1008 Example 1 Resistance 2.16 2.82 5.33 7.87 10.84 13.36 Example 2 Resistance 2.22 2.68 4.76 7.22 9.56 11.48 Example 3 Resistance 2.13 2.99 5.44 7.76 11.66 14.63 Comparative Example 1 Resistance 2.12 3.13 7.25 11.67 17.39 21.67 Comparative Example 2 Resistors 2.01 3.76 10.66 18.77 26.73 33.61

[0028] It can be seen from the data in Table 1 that after the PPTC prepared in Examples 1-3 was treated in a constant temperature and humidity chamber, the resistance test results showed that the oxidation of tungsten carbide powder was greatly delayed, indicating that the coated polyvinylidene chloride played a good role in oxygen and water barrier, thereby inhibiting the oxidation of tungsten carbide and preparing PPTC with small resistance change; taking Example 1 and Comparative Example 2 as examples, the resistance of Comparative Example 2 reached 10.66 mΩ in 168 hours; Example 1 reached a similar level of 10.84 mΩ in 672 hours, indicating that under the same conditions, if the resistance reaches 10.66 mΩ, which is a performance failure resistance, then the service life of the present invention is increased by more than 3 times, greatly improving the stability. It can be seen from Comparative Example 1 that coating tungsten carbide with silane has a certain effect on stabilizing the PPTC resistor. When the oxidation time is 1008, the comparative example 1 is reduced from about 33.61 to 21.67, which is about 35% lower (compared with comparative example 2), and the improvement is limited; because the stability improvement in comparative example 1 is due to the improvement of the interface by silane, it plays a certain protective role and delays the oxidation of tungsten carbide in the system, but the improvement is limited; after adopting the technical solution of the present invention, the oxidation time is also reduced from about 33.61 to between 11 and 14 when the oxidation time is 1008, which is at least 56.6% lower (compared with comparative example 2), and the stability is greatly improved.

[0029] Figure 2-4 The liquid nitrogen brittle fracture of the PPTC material and the SEM image after acetone etching are respectively shown in Comparative Example 2, Example 1, and Example 2 after 1008 hours of accelerated oxidation treatment, which can characterize the surface morphology of tungsten carbide particles. It can be seen from the figure that tungsten carbide will oxidize after the cycle and the surface will become rough. Figure 2 Tungsten carbide surface ratio Figure 3 and Figure 4 The roughness is higher, that is, the surface of tungsten carbide treated with polyvinylidene chloride emulsion is smoother than that without modification, indicating that polyvinylidene chloride, as a coating layer that isolates oxygen and water, can inhibit the oxidation of tungsten carbide and play a good protective role, so that the prepared PPTC resistor is stable.

[0030] The above embodiments are only used to illustrate the present invention, but not to limit the present invention. Although the present invention is described in detail with reference to the embodiments, it should be understood by those skilled in the art that various combinations, modifications or equivalent substitutions of the technical solutions of the present invention do not depart from the spirit and scope of the technical solutions of the present invention, and should be included in the scope of the claims of the present invention.

Claims

1. A method for preparing highly stable PPTC based on polyvinylidene chloride coated modified tungsten carbide, characterized in that: The following steps are involved: The coating agent and the solvent are mixed and stirred to obtain a coating agent emulsion, wherein the coating agent is a polyvinylidene chloride emulsion; The coating agent emulsion and tungsten carbide powder are stirred and mixed for coating; Drying the tungsten carbide powder to obtain coated modified tungsten carbide powder; The coated modified tungsten carbide powder is blended with polyethylene to prepare PPTC.

2. The method for preparing high-stability PPTC based on polyvinylidene chloride coated modified tungsten carbide according to claim 1, characterized in that: The tungsten carbide powder is micron-sized tungsten carbide powder produced by mechanical alloying, direct reduction carburization or sol-gel method, and the particle size of the tungsten carbide powder is 1um-30um.

3. The method for preparing high-stability PPTC based on polyvinylidene chloride coated modified tungsten carbide according to claim 1, characterized in that: The mass concentration of the coating agent emulsion is 10-30%.

4. The method for preparing high-stability PPTC based on polyvinylidene chloride coated modified tungsten carbide according to claim 1, characterized in that: The solvent is water and / or anhydrous ethanol.

5. The method for preparing high-stability PPTC based on polyvinylidene chloride coated modified tungsten carbide according to claim 1, characterized in that: The mass ratio of the tungsten carbide powder to the coating agent in the coating agent emulsion is 1600:1.0-5.

0.

6. The method for preparing high-stability PPTC based on polyvinylidene chloride coated modified tungsten carbide according to claim 1, characterized in that: When the coating agent emulsion is mixed with tungsten carbide powder, the stirring time is 5-20 minutes, and the stirring speed is 200-500r / min.

7. The method for preparing high-stability PPTC based on polyvinylidene chloride coated modified tungsten carbide according to claim 1, characterized in that: When the tungsten carbide powder is dried, the drying time is 1-2 hours and the drying temperature is 40-70°C.

8. The method for preparing high-stability PPTC based on polyvinylidene chloride coated modified tungsten carbide according to claim 1, characterized in that: The mass ratio of the coated modified tungsten carbide powder to polyethylene is 15-20:1.

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