Polymer positive temperature coefficient device based on strong water absorbent protection and preparation method thereof

Abstract

The application discloses a polymer positive temperature coefficient device based on strong water absorbent protection and a preparation method thereof, belongs to the field of fuses, and mixes calcium chloride powder and tungsten carbide powder to perform vacuum grinding, so as to obtain modified tungsten carbide powder; PPTC is prepared by blending the modified tungsten carbide and polyvinylidene fluoride (PVDF), and the resistance of the PPTC is detected. The calcium ions in the calcium chloride have strong coordination ability, can form a coordination bond with hydroxyl oxygen atoms on the surface of the tungsten carbide, and reduce the water absorption capacity of the tungsten carbide. The calcium chloride reacts with water molecules to form different forms of hydrates, so as to firmly fix the water. Strong water absorption provides a dry environment, inhibits the oxidation of the tungsten carbide, and significantly improves the resistance stability and service life of the PPTC device in a use environment.

Description

Technical Field

[0001] This invention belongs to the field of fuses and relates to a self-resetting fuse manufacturing technology, specifically to a polymer positive temperature coefficient device based on strong water-absorbing agent protection and its manufacturing method. Background Technology

[0002] Polymer positive temperature coefficient (PPTC) devices are important overcurrent protection components for circuits. Their core principle is the PTC effect of a composite material consisting of conductive filler and a polymer matrix. Under normal conditions, the device exhibits low resistance. When an overcurrent occurs, the device heats up, causing the polymer to expand, breaking the conductive path and causing a sharp increase in resistance, thus cutting off the circuit. After the fault is cleared, the device is cooled and reset.

[0003] Tungsten carbide is an ideal conductive filler for PPTC due to its good conductivity, hardness, and thermal expansion coefficient that matches that of polymers. However, micron-sized tungsten carbide powder has a large specific surface area and is prone to oxidation in high temperature and high humidity environments, forming insulating tungsten oxides on the surface. This leads to an increase in the initial resistance of PPTC devices, a decrease in the resistance recovery rate, and even failure after multiple cycles. This instability severely restricts the application of PPTC devices in fields with high reliability requirements.

[0004] Currently, most methods for improving the stability of PPTC focus on tungsten carbide coating and surface treatment, but these processes are often cumbersome and involve organic solvents, resulting in poor environmental performance. Therefore, developing a technology that can fundamentally inhibit the oxidation of conductive fillers is key to improving the long-term stability of PPTC devices. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide a polymer positive temperature coefficient device based on strong water-absorbing agent protection and its preparation method, thereby solving the problem of insufficient long-term stability of PPTC devices in existing technologies.

[0006] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0007] On one hand, the present invention provides a method for preparing a polymer positive temperature coefficient device based on a strong water-absorbing agent, comprising the following steps:

[0008] Step 1: Mechanically mix anhydrous calcium chloride powder and tungsten carbide powder to form a mixed powder;

[0009] Step 2: Vacuum ball mill the mixed powder to obtain modified tungsten carbide powder;

[0010] Step 3: The modified tungsten carbide powder is blended with PVDF (polyvinylidene fluoride) powder to obtain the polymer positive temperature coefficient device PPTC.

[0011] Preferably, in step 1, the mechanical mixing is carried out in a conventional mixer for a mixing time of 0.2-1.0 hours and a grinding speed of 50-120 r / min.

[0012] Preferably, in step 1, the mass ratio of anhydrous calcium chloride powder to tungsten carbide powder is (0.10-0.65):100; if the content of anhydrous calcium chloride powder is too small, the modification effect will be poor, and if the content of anhydrous calcium chloride powder is too large, it will affect the initial resistance of PPTC. The aforementioned mass ratio is a commonly used ratio range that can meet the requirements of use.

[0013] Preferably, in step 1, the tungsten carbide powder is micron-sized tungsten carbide powder produced by mechanical alloying, direct reduction carbide or sol-gel method, and the particle size of the tungsten carbide powder is 1um-10um;

[0014] Preferably, in step 1, the particle size of the anhydrous calcium chloride powder is less than 0.5 μm;

[0015] Preferably, in step 2, the vacuum ball milling time is 0.5-2 hours. If the vacuum ball milling time is too short, the reaction will be insufficient, affecting the long-term stability of the device obtained later. If the vacuum ball milling time is too long, it will cause unnecessary energy waste, and excessive ball milling will also affect the stability of the subsequent device.

[0016] Preferably, in step 2, the rotational speed of the vacuum ball milling process is 80-150 r / min;

[0017] Preferably, in step 2, the temperature of the vacuum ball milling process is below 80°C;

[0018] Preferably, in step 2, the vacuum degree of the vacuum ball milling process is lower than 50.0 Pa;

[0019] Preferably, in step 3, the mass ratio of PVDF to modified tungsten carbide powder is 1:7-13. Insufficient tungsten carbide powder content will result in excessively high electrical resistance and poor performance, while excessive tungsten carbide powder content will negatively impact the mechanical properties of the composite material.

[0020] On the other hand, the present invention provides a polymer positive temperature coefficient device based on a strong water absorbent agent, which is prepared by the above-described preparation method.

[0021] The principle of this invention for modifying tungsten carbide with anhydrous calcium chloride is as follows:

[0022] Anhydrous calcium chloride first passes through calcium ions (Ca) 2+The calcium chloride powder reacts with the oxygen vacancies, hydroxyl groups and other polar functional groups on the WC surface to form stable chemical bonds. This process is considered the microscopic basis of chemical anchoring, which effectively changes the surface energy and polarity of the WC filler. This allows the modified tungsten carbide powder to have chemical coordination and hygroscopic effects when used to prepare polymer positive temperature coefficient devices (PPTC), ultimately resulting in the PPTC prepared by this invention having high stability.

[0023] Chemical coordination is the role of calcium ions (Ca). 2+ The chemical coordination between WC and the WC filler surface: WC, as a transition metal carbide, has oxygen vacancies, hydroxyl groups (-OH), and other polar functional groups on its surface introduced by the preparation process or environmental exposure. These defect sites provide an ideal chemical bonding interface for calcium ions. Calcium ions, as a typical Lewis acid, have a high charge density and a strong electron-accepting tendency; they tend to form coordination bonds with electron-rich polar functional groups (such as hydroxyl groups) on the WC surface. This process is not merely a simple electrostatic attraction, but a chemical bonding involving electron transfer. Through coordination with oxygen vacancies or hydroxyl groups on the WC surface, calcium ions are anchored to the WC particle surface, forming a stable chemical bond layer. This chemical anchoring alters the surface electron cloud density and polarity of WC. The formation of coordination bonds makes the electron distribution on the WC surface uneven, endowing the originally inert surface with a certain degree of polarity, which greatly improves the wettability and dispersibility of WC in polar polymer matrices (such as PVDF). Secondly, this chemical bond establishes a robust physical connection between WC and CaCl2, ensuring that during subsequent processing and use, C... a Cl2 will not easily detach from the WC surface, thus ensuring the durability of the modification effect.

[0024] The hygroscopic effect of calcium chloride (CaCl2) stems from the extremely strong hygroscopic properties of anhydrous CaCl2. Under ambient humidity, it absorbs moisture, forming a series of stable hydrates (such as CaCl2·H2O, CaCl2·2H2O, and even CaCl2·6H2O). This process is not a static physical adsorption but is accompanied by significant chemical phase transitions and energy release, dynamically affecting the dispersibility, interfacial state, and electron transport behavior of WC packing materials.

[0025] This moisture absorption process mainly works through the following three effects:

[0026] 1. Spatial hindrance and dispersion stability: As the moisture absorption process proceeds, the hydrate layer formed by CaCl2 on the WC surface gradually thickens. Studies have shown that under specific conditions, the thickness of this hydrate layer can reach 2.1 nm. This expanding hydrate layer plays an effective role in spatial hindrance between WC particles, physically preventing particle agglomeration due to van der Waals forces or other interactions. This self-expansion effect caused by moisture absorption helps maintain the uniform dispersion of WC filler during the processing and molding stage of the composite material, avoiding the formation of conductive network islands due to filler agglomeration.

[0027] 2. Interface bonding and structural rearrangement: The formation of hydrates creates a unique interface region between WC particles and the PVDF matrix. The presence of this region may induce changes in the crystallization behavior of the PVDF matrix or generate stronger interactions with the matrix through hydrogen bonding. When the material undergoes thermal cycling or external stress, this dynamically changing interface region can absorb some energy, thereby delaying the formation and propagation of microcracks. More importantly, during the molding and cooling process, the shrinkage of the hydrate layer may generate a capillary tension, which can drive WC particles to self-assemble at the microscale, causing them to evolve from a disordered random distribution to a more ordered and compact arrangement.

[0028] 3. Dynamic Protective Layer and Ion Conductivity Pathway: The hydrate layer formed by moisture absorption constitutes a dynamic protective layer on the surface of WC particles, which can isolate oxygen and moisture from the external environment, thereby slowing down the oxidation process of WC to a certain extent; at the same time, the dissolved Ca in the hydrate layer 2+ and Cl - Ions, along with free water molecules, together form a tiny ionic conductive path. The existence of this path means that electron transport between WC particles no longer depends entirely on direct physical contact between particles, but can be assisted or supplemented by ionic conductivity. This is crucial for building a stable conductive network at low filler content.

[0029] In summary, the hygroscopic effect of calcium chloride is a dynamic physical process. It drives the rearrangement and optimization of the microstructure of WC packing on a macroscopic level through the triple effects of steric hindrance, interface regulation, and dynamic protection.

[0030] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0031] This invention directly modifies the conductive filler by ball milling anhydrous calcium chloride with tungsten carbide. The calcium ions in the calcium chloride have a strong coordination ability and can form coordination bonds with the hydroxyl oxygen atoms on the surface of tungsten carbide, reducing the water absorption of tungsten carbide. Calcium chloride can also react chemically with water molecules to form different forms of hydrates, thereby firmly fixing the moisture. The strong water absorption provides a dry environment and inhibits the oxidation of tungsten carbide. PPTC devices prepared using this method exhibit smaller resistance drift and significantly extended service life in high-temperature and high-humidity accelerated aging tests. Attached Figure Description

[0032] Figure 1 This is a schematic diagram of the fabrication process of the polymer positive temperature coefficient device of the present invention;

[0033] Figure 2 The image shows the morphology of the liquid nitrogen brittle fracture surface of the PPTC sample provided in Comparative Example 1.

[0034] Figure 3 This is a liquid nitrogen embrittlement cross-section morphology image of the PPTC sample provided in Example 1;

[0035] Figure 4 This is a morphology diagram of the liquid nitrogen brittle fracture surface of the PPTC sample provided in Example 2. Detailed Implementation

[0036] To make the technical solution and advantages of the present invention clearer and more apparent, the following embodiments are provided to further illustrate the present invention in detail.

[0037] Example 1: Step 1, weigh 100g of micron-sized tungsten carbide powder (model: GWC030) and 0.35g of chemically pure calcium chloride powder (100:0.35) and put them into a mixer. Stir at 70 r / min for 0.5 hours to obtain mixed powder;

[0038] Step 2: Transfer the mixed powder to a vacuum ball mill and ball mill it for 1.5 hours under a vacuum of 35.0 Pa and a rotation speed of 110 r / min. Keep the temperature below 55°C and allow it to cool naturally to room temperature to obtain modified tungsten carbide powder.

[0039] Step 3: PVDF and modified tungsten carbide powder are mixed at a mass ratio of 1:10 to prepare PPTC with a side length of 10mm (length and width are both 10mm) and a thickness of 0.5mm.

[0040] Step 4: Place the prepared PPTC in a constant temperature and humidity chamber under double 85 test conditions for accelerated oxidation experiment; perform resistance test on the oxidized PPTC; the experimental conditions and test results are recorded in Table 1.

[0041] Example 2: Step 1, weigh 100g of micron-sized tungsten carbide powder (model: GWC030) and 0.50g of chemically pure calcium chloride powder (100:0.50) and put them into a mixer. Stir at 80 r / min for 0.7 hours to obtain mixed powder;

[0042] Step 2: Transfer the mixed powder to a vacuum ball mill and ball mill it for 1.2 hours under a vacuum of 40.0 Pa and a rotation speed of 90 r / min. Keep the temperature below 70°C and allow it to cool naturally to room temperature to obtain modified tungsten carbide powder.

[0043] Step 3: PVDF and modified tungsten carbide powder are mixed at a mass ratio of 1:10 to prepare PPTC with a side length of 10mm (length and width are both 10mm) and a thickness of 0.5mm.

[0044] Step 4: Place the prepared PPTC in a constant temperature and humidity chamber under double 85 test conditions for accelerated oxidation experiment; perform resistance test on the oxidized PPTC; the experimental conditions and test results are recorded in Table 1.

[0045] Example 3: Step 1, weigh 100g of micron-sized tungsten carbide powder (model: GWC030) and 0.15g of chemically pure calcium chloride powder (100:0.15) and put them into a mixer. Stir at 100 r / min for 0.4 hours to obtain mixed powder;

[0046] Step 2: Transfer the mixed powder to a vacuum ball mill and ball mill it for 1.0 hour under a vacuum of 20.0 Pa and a rotation speed of 120 r / min. The temperature is below 65℃. Allow it to cool naturally to room temperature to obtain modified tungsten carbide powder.

[0047] Step 3: Mix PVDF and modified tungsten carbide powder at a mass ratio of 1:10 to prepare PPTC with a side length of 10mm (length and width are both 10mm) and a thickness of 0.5mm.

[0048] Step 4: Place the prepared PPTC in a constant temperature and humidity chamber under double 85 test conditions for accelerated oxidation experiment; perform resistance test on the oxidized PPTC; the experimental conditions and test results are recorded in Table 1.

[0049] Example 4: Step 1, weigh 100g of micron-sized tungsten carbide powder (model: GWC030) and 0.60g of chemically pure calcium chloride powder (100:0.60) and put them into a mixer. Stir at 90 r / min for 0.6 hours to obtain a mixed powder.

[0050] Step 2: Transfer the mixed powder to a vacuum ball mill and ball mill it for 1.5 hours at a vacuum of 45.0 Pa and a rotation speed of 140 r / min. Keep the temperature below 75°C and allow it to cool naturally to room temperature to obtain modified tungsten carbide powder.

[0051] Step 3: PVDF and modified tungsten carbide powder are mixed at a mass ratio of 1:10 to prepare PPTC with a side length of 10mm (length and width are both 10mm) and a thickness of 0.5mm.

[0052] Step 4: Place the prepared PPTC in a constant temperature and humidity chamber under double 85 test conditions for accelerated oxidation experiment; perform resistance test on the oxidized PPTC; the experimental conditions and test results are recorded in Table 1.

[0053] Comparative Example 1:

[0054] PPTC prepared by blending untreated tungsten carbide powder with PVDF was placed in a constant temperature and humidity chamber under double 85 conditions for accelerated oxidation; the experimental conditions and test results are recorded in Table 1.

[0055] Table 1, the unit of resistance is (mΩ);

[0056] ;

[0057] As shown in Table 1, the resistance changes of PPTC prepared from calcium chloride composite modified tungsten carbide powder obtained in Examples 1, 2, 3, and 4 after 1008 hours in a constant temperature and humidity chamber were only 17.46, 16.16, 19.08, and 14.89, respectively, while the comparative example showed an increase of 34.13. Compared with the untreated sample, this indicates that the composite modified tungsten carbide powder prepared in these examples has better antioxidant properties, and the resistance change in PPTC preparation is small. Compared with Examples 1-4, the resistance change with increasing calcium chloride content during modification is slightly smaller and not particularly significant; the initial resistance increases slightly (because calcium chloride has poor conductivity), but the impact is not significant.

[0058] Figure 2-4 These are SEM images of the liquid nitrogen brittle fracture surfaces of different PPTC samples after a 1008-hour double 85 accelerated oxidation test, used to observe the surface microstructure of tungsten carbide particles.

[0059] As can be seen from the comparison, the tungsten carbide particles in all PPTC samples underwent a certain degree of oxidation under high temperature and high humidity conditions, resulting in surface roughening; among them, the attached... Figure 2 (Comparative Example 1) shows that the surface of the unmodified tungsten carbide particles is severely oxidized and has the roughest morphology. This rough surface is caused by the oxidation reaction, which will destroy the conductive contact between particles and directly lead to the increase in resistance (as shown in Table 1).

[0060] Appendix Figure 3 With appendix Figure 4 (Examples 1 and 2) show that the tungsten carbide particles modified with calcium chloride have a significantly smoother and flatter surface under the same oxidation conditions; this indicates that the composite treatment with calcium chloride effectively absorbs moisture and significantly slows down the erosion of the particles by the oxidation reaction.

[0061] In summary, the microscopic morphology observation and macroscopic electrical performance test results are in complete agreement: calcium chloride modification protects the tungsten carbide filler and reduces oxidation damage, thereby maintaining the integrity of the conductive network. This makes the PPTC devices prepared in Examples 1-4 have an overwhelming advantage in long-term reliability compared with the unmodified system (Comparative Example 1), exhibiting small resistance drift and highly stable electrical performance, meeting the stringent requirements of high-performance PPTC devices for long-term stability.

[0062] 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 skilled in the art should understand 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 covered within the scope of the claims of the present invention.

Claims

1. A method for fabricating a polymer positive temperature coefficient device based on a strong water-absorbing agent, characterized in that, Includes the following steps: Anhydrous calcium chloride powder and tungsten carbide powder are mechanically mixed to form a mixed powder; The mixed powder was vacuum ball-milled to obtain modified tungsten carbide powder; Modified tungsten carbide powder was blended with PVDF to obtain a polymer positive temperature coefficient device.

2. The method for fabricating the polymer positive temperature coefficient device according to claim 1, characterized in that, The mechanical mixing is carried out in a mixer for 0.2-1.0 hours at a speed of 50-120 r / min.

3. The method for fabricating the polymer positive temperature coefficient device according to claim 1, characterized in that, The vacuum ball milling is carried out in a vacuum ball mill, with a stirring time of 0.5-2.0 hours, a grinding speed of 80-150 r / min, a vacuum degree of less than 50 Pa, and a controlled temperature of less than 80℃.

4. The method for fabricating the polymer positive temperature coefficient device according to claim 1, characterized in that, The mass ratio of anhydrous calcium chloride powder to tungsten carbide powder is (0.10-0.65):

100.

5. The method for fabricating the polymer positive temperature coefficient device according to claim 1, characterized in that, The tungsten carbide powder is micron-sized tungsten carbide powder produced by mechanical alloying, direct reduction carbide, or sol-gel method, and the particle size of the tungsten carbide powder is 1µm-10µm.

6. The method for fabricating the polymer positive temperature coefficient device according to claim 1, characterized in that, The anhydrous calcium chloride powder has a particle size of less than 0.5 μm.

7. The method for fabricating the polymer positive temperature coefficient device according to claim 1, characterized in that, The mass ratio of PVDF to modified tungsten carbide powder is 1:7-13.

8. A polymer positive temperature coefficient device based on strong water-absorbing agent protection, characterized in that, It is prepared by the preparation method described in any one of claims 1-7.

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