Method for preparing highly stable PPTC based on polyvinylidene chloride-coated modified tungsten carbide

Abstract

The present invention provides a method for preparing highly stable PPTC based on polyvinylidene chloride-coated modified tungsten carbide, belonging to the technical field of preparation of highly stable devices. First, a polyvinylidene chloride emulsion is mixed and stirred with water or ethanol to obtain a coating agent emulsion; then the coating agent emulsion is used to coat tungsten carbide powder during stirring; after that, the tungsten carbide powder is placed in a vacuum drying oven for drying treatment to obtain coated and modified tungsten carbide; finally, PPTC is prepared by blending the coated and modified tungsten carbide with HDPE. In the present invention, polyvinylidene dichloride is coated on the surface of tungsten carbide, effectively blocking the oxidation process involving oxygen and water molecules and playing an antioxidant role. The prepared PPTC, after being tested after treatment in a constant temperature and humidity chamber, shows that the coated and modified tungsten carbide plays a good role in oxygen and water resistance, thereby inhibiting the oxidation of tungsten carbide and preparing PPTC with small resistance change and high stability.

Description

Technical Field

[0001] The present invention belongs to the technical field of preparing high-stability devices, and particularly relates to a method for preparing a 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. Its hardness is similar to that of diamond, and it is a good conductor of electricity and heat. Due to its high conductivity, it can be used as a conductive filler. Mixing it with HDPE (high-density polyethylene) to prepare a low-resistance polymer positive temperature coefficient thermistor (PPTC) has broad application prospects.

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

[0004] Research has found that oxygen and water are the keys to the oxidation of tungsten carbide. Blocking its contact with oxygen and water can inhibit the low-temperature oxidation of tungsten carbide. The prior art CN117509644A discloses a method for preparing coated and modified tungsten carbide powder. Mixing and stirring a coating agent and a solvent to obtain a mixed solution, where the coating agent is a silane coupling agent or stearic acid; placing the mixed solution and tungsten carbide powder in a magnetic stirring heating table for coating; performing suction filtration and washing on the coated tungsten carbide powder; drying the tungsten carbide powder to obtain the coated tungsten carbide powder. In this technology, by blocking the process in which water molecules participate in the oxidation process, it plays an antioxidant role. Through research, it is found that the coating antioxidant effect in the prior art CN117509644A is not in a composite system, but relative to the relatively bare tungsten carbide. Therefore, the improvement effect in the HDPE / tungsten carbide system is limited; because both the silane coupling agent and stearic acid are small-molecule materials, they are commonly used as grafting agents in the mixing process of inorganic and organic substances to improve the bonding interface and make the inorganic substances disperse evenly in the organic substances; therefore, using a silane coupling agent or stearic acid in the HDPE / tungsten carbide system to improve the compatibility between tungsten carbide powder and HDPE and reduce the interface gap has a certain improvement effect on the oxidation of tungsten carbide powder to a certain extent, but the ability of silane or stearic acid to block oxygen and water is not stronger than that of HDPE itself, and the compatibility is also limited; therefore, theoretically, the coating isolation effect of the silane coupling agent or stearic acid in the prior art CN117509644A is very limited in the HDPE / tungsten carbide system. Therefore, finding a material that has strong isolation ability in the HDPE / tungsten carbide system, can bind well with tungsten carbide powder, and verifying it practically to prepare an antioxidant HDPE / tungsten carbide system is a very valuable research. Summary of the Invention

[0005] Aiming at the deficiencies of the prior art, the object of the present invention is to provide a method for preparing highly stable PPTC based on polyvinylidene chloride-coated modified tungsten carbide. By using polyvinylidene chloride to coat and modify tungsten carbide, it has good interfacial modification ability. While improving the interfacial bonding situation between tungsten carbide and PE (polyethylene), it has very good isolation performance for tungsten carbide. After testing, its oxygen and water isolation performance is better than that of HDPE itself. Therefore, it effectively isolates the erosion of environmental water and oxygen, and improves the use stability of PPTC.

[0006] To solve the above technical problems, the technical solution adopted by the present invention is as follows:

[0007] A method for preparing highly stable PPTC based on polyvinylidene chloride-coated modified tungsten carbide, comprising the following steps:

[0008] (1) Mix and stir the coating agent and the solvent to obtain a coating agent emulsion, and the coating agent is a polyvinylidene chloride emulsion;

[0009] (2) Stir and mix the coating agent emulsion with tungsten carbide powder for coating;

[0010] (3) Dry the tungsten carbide powder to obtain coated and modified tungsten carbide;

[0011] (4) Blend the coated and modified tungsten carbide powder with polyethylene to prepare PPTC.

[0012] In the present invention, the coating agent is configured into an emulsion, and the emulsion is stirred and mixed with tungsten carbide powder. After drying treatment, polyvinylidene chloride is coated on the surface of tungsten carbide powder to obtain coated and modified tungsten carbide. Then, the coated and modified tungsten carbide is blended with polyethylene to prepare PPTC. In the present invention, polyvinylidene dichloride (PVDC) is a thermoplastic polymer with strong intermolecular cohesion and high crystallinity. In its molecular structure, two chlorine atoms with high content are symmetrically distributed. When coating and modifying tungsten carbide powder, it can firmly adsorb on the surface of tungsten carbide powder, providing good dispersibility. In addition, the chlorine atoms in the polyvinylidene dichloride molecule have hydrophobicity, so it is difficult for oxygen molecules and water molecules to move in its molecule, making PVDC have excellent oxygen barrier property and moisture barrier property, and its oxygen barrier property is not affected by the surrounding environmental humidity. Finally, the isolation ability of polyvinylidene dichloride is better than that of HDPE itself. Therefore, polyvinylidene dichloride has both good interfacial improvement and isolation effects. When it is used to coat and modify tungsten carbide and make PPTC, it can avoid the increase of PPTC resistance caused by the contact and oxidation of tungsten carbide with water in the air from two technical directions, and the stability improvement is much better than that of a simple interfacial modifier, thus greatly improving the stability and service life of PPTC.

[0013] Preferably, the tungsten carbide powder is a micron-sized tungsten carbide powder produced by mechanical alloying, direct reduction and carbonization, or sol-gel method, and the particle size of the tungsten carbide powder is 1um-30um.

[0014] Preferably, in the step (1), the mass concentration of the coating agent emulsion is 10-30%.

[0015] Preferably, in the step (1), the solvent is water and / or absolute ethanol.

[0016] Preferably, in the step (2), the mass ratio of tungsten carbide powder to the coating agent in the coating agent emulsion is 1600:1.0-5.0.

[0017] Preferably, in the step (2), the stirring and mixing time is 5-20min, and the stirring speed is 200-500r / min.

[0018] Preferably, in the step (3), the drying time is 1-2 hours, and the drying temperature is 40-70°C.

[0019] Preferably, in the step (4), the mass ratio of the coated and modified tungsten carbide powder to polyethylene is 15-20:1.

[0020] Preferably, the polyethylene is HDPE.

[0021] More preferably, the coated and modified tungsten carbide powder and HDPE are kneaded in a mass ratio of 16:1 to prepare a PPTC with a side length of 10mm (both length and width are 10cm) and a thickness of 0.5mm, and the resistance of the PPTC is detected.

[0022] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0023] The method for preparing a highly stable PPTC based on polyvinylidene chloride-coated modified tungsten carbide provided by the present invention first prepares an emulsion of a coating agent, stirs and mixes the emulsion with tungsten carbide powder, and after drying treatment, coats polyvinylidene chloride on the surface of the tungsten carbide powder to obtain coated modified tungsten carbide. Then, the coated modified tungsten carbide is blended with polyethylene to prepare PPTC. Polyvinylidene chloride (PVDC) in the present invention is a thermoplastic polymer with strong intermolecular cohesion and high crystallinity. In its molecular structure, two chlorine atoms with high content are symmetrically distributed. When coating and modifying the tungsten carbide powder, it can firmly adsorb on the surface of the tungsten carbide powder, provide good dispersibility, and play an interface modification role similar to that of silane. In addition, the chlorine atoms in the polyvinylidene chloride molecule have hydrophobicity, so it is difficult for oxygen molecules and water molecules to move in its molecule, making PVDC have excellent oxygen barrier and moisture barrier properties, and its oxygen barrier property is not affected by the surrounding environmental humidity. Finally, the isolation ability of polyvinylidene chloride is better than that of HDPE itself. Therefore, it has both good interface improvement and isolation effects, avoids the oxidation of tungsten carbide caused by contact with water in the air, resulting in an increase in resistance, and thus improves the stability and service life of PPTC. Through actual measurement of the accelerated oxidation experiment, the stability is greatly improved. When reaching the same resistance level, the service time is extended several times. Brief Description of the Drawings

[0024] Figure 1 is a schematic flow chart of the preparation method of the coated tungsten carbide powder provided by the embodiment of the present invention;

[0025] Figure 2 is a surface morphology diagram of tungsten carbide in the PPTC provided by Comparative Example 2 of the present invention;

[0026] Figure 3 is a surface morphology diagram of tungsten carbide in the PPTC provided by Embodiment 1 of the present invention;

[0027] Figure 4 is a surface morphology diagram of tungsten carbide in the PPTC provided by Embodiment 2 of the present invention;

[0028] Figure 5 is the PPTC structure and its resistance test principle diagram provided by the embodiment of the present invention. Detailed Embodiments

[0029] The following further describes in detail the embodiments of the present invention in conjunction with the drawings and embodiments. The following embodiments are used to illustrate the present invention, but cannot be used to limit the scope of the present invention.

[0030] To make the technical solutions and advantages of the present invention clearer and more distinct, the following examples are given to further illustrate the present invention in detail.

[0031] As Figure 1As shown, the present invention provides a method for preparing highly stable PPTC based on polyvinylidene chloride-coated modified tungsten carbide, comprising the following steps:

[0032] (1) Mix and stir the coating agent and the solvent to obtain a coating agent emulsion, wherein the coating agent is a polyvinylidene chloride emulsion;

[0033] (2) Stir and mix the coating agent emulsion with tungsten carbide powder for coating;

[0034] (3) Perform vacuum drying treatment on the tungsten carbide powder to obtain coated and modified tungsten carbide;

[0035] (4) Blend the coated and modified tungsten carbide powder with polyethylene to prepare PPTC.

[0036] Example 1: Step 1, the coating agent is a polyvinylidene dichloride emulsion, and the solvent is water and ethanol in a ratio of 4:1. After mixing and stirring the polyvinylidene dichloride emulsion with the solvent, a coating agent emulsion with a concentration of 20 wt% is obtained;

[0037] Step 2, by mass, mix 1600 parts of tungsten carbide powder with 10 parts of the above coating agent emulsion (converted to 2 parts of the coating agent) and stir for 15 minutes;

[0038] Step 3, put the tungsten carbide powder obtained in Step 2 into a vacuum drying oven and dry it at 45 °C for 1 hour;

[0039] Step 4, mix the coated and modified tungsten carbide and HDPE at a mass ratio of 16:1 and knead to prepare a PPTC with a side length of 10 mm (both length and width are 10 cm) and a thickness of 0.5 mm;

[0040] Step 5, put the PPTC prepared in Step 4 into a constant temperature and humidity chamber under the double 85 test conditions for an accelerated oxidation experiment to detect the resistance of the PPTC; the principle is as Figure 5 shown, the PPTC is clamped between two electrode plates made of copper foil, and the current I is measured through two probes connected to the ammeter A g , the voltage U is measured through two probes connected to the voltmeter V g , and the resistance of the PPTC is calculated through the formula , where is the voltage applied to the PPTC by the four-probe method, is the current passing through the PPTC by the four-probe method; the experimental conditions and test results are all recorded in Table 1.

[0041] Example 2: Step 1, the coating agent is a polyvinylidene dichloride emulsion, and the solvent is water. After mixing and stirring the polyvinylidene dichloride emulsion with the solvent, a coating agent emulsion with a concentration of 25 wt% is obtained;

[0042] Step 2: Mix 1600 parts of tungsten carbide powder with 16 parts of the above coating agent emulsion (converting the coating agent to 4 parts) by mass, and stir well for 18 minutes;

[0043] Step 3: Put the tungsten carbide powder obtained in Step 2 into a vacuum drying oven and dry it at 55°C for 1.5 hours;

[0044] Step 4: Mix the coated and modified tungsten carbide and HDPE in a mass ratio of 16:1, and knead to prepare a PPTC with a side length of 10 mm (both length and width are 10 cm) and a thickness of 0.5 mm;

[0045] Step 5: Put the PPTC prepared in Step 4 into a constant temperature and humidity chamber under double 85 test conditions for an accelerated oxidation experiment, and detect the resistance of the PPTC;

[0046] The experimental conditions and test results are all recorded in Table 1.

[0047] Example 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 15 wt%;

[0048] Step 2: Mix 1600 parts of tungsten carbide powder with 10 parts of the above coating agent emulsion (converting the coating agent to 1.5 parts) by mass, and stir well for 20 minutes;

[0049] Step 3: Put the tungsten carbide powder obtained in Step 2 into a vacuum drying oven and dry it at 55°C for 1.0 hour;

[0050] Step 4: Mix the coated and modified tungsten carbide and HDPE in a mass ratio of 16:1, and knead to prepare a PPTC with a side length of 10 mm (both length and width are 10 cm) and a thickness of 0.5 mm;

[0051] Step 5: Put the PPTC prepared in Step 4 into a constant temperature and humidity chamber under double 85 test conditions for an accelerated oxidation experiment, and detect the resistance of the PPTC;

[0052] The experimental conditions and test results are all recorded in Table 1.

[0053] Comparative Example 1:

[0054] Step 1: Adopt the scheme in the prior art CN117509644A, and treat 1600 parts of tungsten carbide powder with 1.0 part of silane;

[0055] Step 2: Modify tungsten carbide and HDPE with stearic acid, and knead in a mass ratio of 16:1 to prepare a PPTC with a side length of 10 mm (both length and width are 10 cm) and a thickness of 0.5 mm;

[0056] Step 3: Put the PPTC prepared in Step 2 into a thermo-hygrostat chamber under the double 85 test conditions for an accelerated oxidation experiment to detect the resistance of the PPTC.

[0057] The experimental conditions and test results are all recorded in Table 1.

[0058] Comparative Example 2:

[0059] Step 1: Mix uncoated and modified tungsten carbide and HDPE at a mass ratio of 1:16 and knead them to prepare a PPTC with a side length of 10 mm and a thickness of 0.5 mm.

[0060] Step 2: Put the prepared PPTC into a thermo-hygrostat chamber under the double 85 test conditions for an accelerated oxidation experiment to detect the resistance of the PPTC.

[0061] The experimental conditions and test results are all recorded in Table 1.

[0062] Table 1. Results of the accelerated oxidation experiment of PPTCs in Examples 1-3 and Comparative Examples 1 and 2. The unit of resistance is (mΩ).

[0063] Oxidation time (hr) 0 24 168 336 672 1008 Resistance of Example 1 2.16 2.82 5.33 7.87 10.84 13.36 Resistance of Example 2 2.22 2.68 4.76 7.22 9.56 11.48 Resistance of Example 3 2.13 2.99 5.44 7.76 11.66 14.63 Resistance of Comparative Example 1 2.12 3.13 7.25 11.67 17.39 21.67 Resistance of Comparative Example 2 2.01 3.76 10.66 18.77 26.73 33.61 。

[0064] It can be seen from the data in Table 1 that after the PPTCs prepared in Examples 1-3 were treated in the thermo-hygrostat chamber, the resistance test results showed that the oxidation of tungsten carbide powder was greatly delayed, indicating that the coating of polyvinylidene chloride played a very good role in oxygen and water resistance, thus inhibiting the oxidation of tungsten carbide and resulting in small changes in the resistance of the prepared PPTCs. Taking Example 1 and Comparative Example 2 as examples, the resistance of Comparative Example 2 reached 10.66 mΩ at 168 hours; the resistance of Example 1 reached a similar level of 10.84 mΩ at 672 hours. This shows that under the same conditions, if the resistance reaching 10.66 mΩ is defined as the failure resistance of the performance, 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 resistance. At an oxidation time of 1008 hours, the resistance of Comparative Example 1 decreased from about 33.61 to 21.67, a decrease of about 35% (compared with Comparative Example 2), and the improvement amplitude is limited. Because the improvement in stability in Comparative Example 1 is due to the improvement of the interface by silane, which plays a certain protective role and delays the oxidation of tungsten carbide in the system, but the improvement amplitude is limited. After adopting the technical solution of the present invention, at the same oxidation time of 1008 hours, the resistance decreased from about 33.61 to between 11 and 14, with a minimum decrease of 56.6% (compared with Comparative Example 2), and the stability improvement amplitude is relatively large.

[0065] Figures 2 - 4They are SEM images of the liquid nitrogen brittle fracture surfaces of the PPTC materials after 1008 hours of accelerated oxidation treatment in Comparative Example 2, Example 1, and Example 2, respectively, after acetone etching, which can characterize the surface morphology of tungsten carbide particles. It can be seen from the figures that tungsten carbide will oxidize after cycling and the surface becomes rough. Figure 2 The surface of tungsten carbide in Figure 3 is smoother than that of Figure 4 and

[0066] has a higher roughness, that is, the surface of tungsten carbide treated with polyvinylidene chloride emulsion is smoother than that without modification, indicating that polyvinylidene chloride, as an oxygen and water barrier coating layer, can inhibit the oxidation of tungsten carbide and play a good protective role, resulting in stable PPTC resistors. The above embodiments are only used to illustrate the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the embodiments, those of ordinary skill in the art should understand that various combinations, modifications, or equivalent replacements 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 all be covered within 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; and the mass concentration of the coating agent emulsion is 10-30%; The coating agent emulsion and tungsten carbide powder are stirred and mixed for coating; the mass ratio of the tungsten carbide powder to the coating agent in the coating agent emulsion is 1600:1.0-5.0; 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; the mass ratio of the coated modified tungsten carbide powder to the polyethylene is 15-20:

1.

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 solvent is water and / or anhydrous ethanol.

4. 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.

5. 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.

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