JP2003092202A - Polymer ptc element and manufacturing method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the adhesiveness of each electrode of a polymer PTC element having a structure with its electrodes formed on both the surfaces of the sheet-form compact of a polymer PTC composition made of conductive powder and a crystalline polymer compound, and to suppress the room- temperature resistivity of the polymer PTC element as values which are not larger than 2 Ω/cm after its initial time and after its repeated switching operations. SOLUTION: The manufacturing method of the polymer PTC element has a process for using a metal carbide as its conductive powder; a process for setting in the range of 45-60% the volumetric filling factor of the conductive powder to reduce its resistivity; a process for subjecting the surfaces of a polymer PTC composition compact to plasma processing to improve its adhesiveness with respect to metals, etc.; a process for forming each electrode by embedding each electrode in the polymer PTC composition compact to expose a portion of the conductor of each electrode to the surface of the compact; and a process for subjecting further the exposed surface of the portion of the conductor of each electrode to plating processing.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a PTC element having a positive temperature characteristic in which the resistance rapidly rises when reaching a specific temperature range, that is, a so-called PTC (Positive Temperature Coefficient) characteristic, and particularly to a crystalline polymer. The present invention relates to a polymer PTC element which has a structure in which an electrode is provided on a molded body made of a composition in which conductive powder is filled, and in which the contact resistance between the molded body and the electrode is reduced and which exhibits good ohmic characteristics.

[0002]

2. Description of the Related Art A PTC element showing a positive temperature characteristic in which electric resistance rapidly increases in a specific temperature range is often used as a heater for automatically controlling the temperature, a self-reset type overcurrent protection element, or the like. ing. As the composition used for the PTC element, a ceramic-based PTC composition such as barium titanate (BaTiO ₃ ) to which a small amount of yttrium oxide (Y ₂ O ₃ ) is added, conductive particles such as carbon black, and a crystalline polymer are used. Polymeric PTC compositions dispersed therein are known.

PT using a ceramic PTC composition
The C element uses a rapid increase in resistivity at the Curie point, but the resistivity in the steady state is about -100Ω
Since it is as high as cm, a relatively large current of about several amperes cannot flow. This means that the PTC element using the ceramic PTC composition is difficult to use as an overcurrent protection element. Further, the ceramic-based PTC composition has a problem that it requires many steps for molding and processing into a desired shape and is inferior in impact resistance.

On the other hand, the polymer PTC element using the polymer PTC composition has a low resistivity at room temperature,
Suitable for overcurrent protection element, excellent in impact resistance, molding,
Easy to process.

The operating principle of the polymer PTC element is to separate the conductive particles forming the network at room temperature by utilizing the large thermal expansion of the crystalline polymer at the crystal melting point. For this reason, when excessive heat is generated by a current of a specified value or more, the resistivity rapidly rises at a temperature near the crystal melting point, and when the temperature returns to room temperature, the network of conductive particles is reformed, and the resistivity also increases. descend.

In a general method for producing a polymer PTC element, a roll or the like is used to disperse conductive particles in a crystalline polymer to obtain a polymer PTC composition, which is then heated and rolled. There is a dry method in which a sheet is formed into a sheet, an electrode made of a metal foil or the like is pressure-bonded, and then punched into a desired shape.

Further, as a method of obtaining a sheet of a polymer PTC composition, there is a wet method of forming a film by using a slurry in which a conductive filler is dispersed in a solution of a crystalline polymer, and in this case, an electrode is constituted. There is also a method in which a film is formed on the metal foil, and the film-formed sides are made to face each other and integrated.

As mentioned above, the polymer PTC composition includes
A crystalline polymer is used, and a typical example thereof is high-density polyethylene (hereinafter, HDPE). HDPE is excellent in terms of price and moldability, and is often used in polymer PTC compositions, but since it has relatively low adhesiveness with electrodes and conductive fillers, it has a high resistivity in the steady state. However, there is a problem that the repeated operation characteristics of the polymer PTC element are deteriorated.

As a countermeasure against this, the surface of the electrode which is in contact with the polymer PTC composition is subjected to physical or chemical treatment,
A method of roughening the surface of the electrode and a method of directly metal-plating a molded body of the polymer PTC composition have been used.

However, when the surface of the electrode plate which is in contact with the polymer PTC element is roughened, the contact resistance is relatively lowered and the adhesion between the polymer PTC composition and the electrode plate is improved, but Between the molecular PTC composition and the electrode plate,
The problem remains that good ohmic contact cannot be obtained.

In particular, when the volume filling rate of the conductive powder dispersed in the polymer PTC composition is increased to 45% or more for the purpose of reducing the room temperature resistance of the polymer PTC element and improving the stability against repeated operation. However, there are problems that it is difficult to keep the room temperature resistivity below a certain value and that the increase in the resistivity after each repeated operation cannot be completely suppressed.

On the other hand, in the case where the surface of the molded product of the polymer PTC composition is directly metal-plated and this is used as an electrode,
It is difficult to provide sufficient adhesion strength between the polymer PTC composition and the plating film, and the contact resistance between the polymer PTC composition and the plating film is high, and the resistivity increases due to repeated operation. Occurs.

Further, when the polymer PTC element is repeatedly operated, there is a problem that the room temperature resistivity increases due to deterioration of the polymer PTC composition itself due to energization. It is conceivable that the cause of this is that the crystalline polymer deteriorates due to the thermal history of each repeated operation.

[0014]

SUMMARY OF THE INVENTION Therefore, the technical problem of the present invention is that the stability of repeated operation is high and the adhesion between the polymer PTC composition and the electrode is high, and the polymer PTC composition and the electrode are high. It is to provide a polymer PTC element having a low contact resistance with.

[0015]

In order to solve the above-mentioned problems, the present inventors have studied the kind of conductive powder to be used, the surface treatment of a polymer PTC composition, the method for forming an electrode, etc. By using various metal carbides as the conductive powder, it is possible to reduce the amount of the binder to reduce the influence of the deterioration of the crystalline polymer due to the repeated operation, and to perform the plasma treatment on the surface of the polymer PTC composition. In order to improve the adhesion between the polymer PTC composition and the electrode, an electrode forming method in which a conductor is embedded so that a part of the polymer PTC composition molded body is exposed, and the surface is further plated It has been found that the contact resistance can be reduced.

That is, according to the present invention, a molded body made of a composition in which a conductive powder is dispersed in a binder containing a crystalline polymer, the surface of which is subjected to plasma treatment, and at least a part of which is exposed on the surface of the molded body. A polymer PTC element comprising a conductor embedded in the molded body as described above and a plating layer formed on the surface of the molded body in which the conductor is embedded.

In the polymer PTC element according to the present invention, the conductive powder is titanium carbide (Ti).
C), Tungsten Carbide (WC, W ₂ C), Zirconium Carbide (ZrC), Vanadium Carbide (VC), Niobium Carbide (NbC), Tantalum Carbide (TaC), Molybdenum Carbide (Mo ₂ C).
It is a polymer PTC element characterized by containing at least one selected from the following.

In the polymer PTC element according to the present invention, the conductive powder has a volume filling rate of 45 to 60.
% Is a polymer PTC element.

Further, according to the present invention, a conductive polymer powder is kneaded with a binder containing a crystalline polymer to obtain a polymer PTC composition, the polymer PTC composition is molded into a desired shape, and a plasma is formed on the surface thereof. After the treatment, the conductor is pressure-bonded so that a part of the conductor is exposed on the surface of the molded body, and a plating treatment is applied to the surface of the molded body where a part of the conductor is exposed.
It is a method for producing the polymer PTC element.

Further, in the present invention, in the method for producing a polymer PTC element, the conductor is pressure-bonded by applying a conductor paste comprising a conductor powder, a binder and a solvent onto the surface of the molded body. The method for producing a polymer PTC element is characterized in that it is dried and dried, and then pressure is applied.

[0021]

The high-density polyethylene, which is often used as a binder in the polymer PTC composition, does not have a polar group, and therefore has very little adhesion to a binder having a polar group or a metal. For this purpose, it is effective to introduce a polar group into the surface of the polymer PTC composition, that is, the adhesive interface, and more specifically, it is effective to generate a carbonyl group by oxidation.

A method such as acid treatment, flame treatment, plasma treatment or the like can be applied to this. As a result of the examination, the acid treatment is basically a wet treatment, which complicates the process, and the flame treatment is difficult to control the degree of oxidation because the target is a combustible polymer compound. Therefore, the plasma treatment, which has few of these drawbacks, was most useful.

As a method for forming an electrode having high adhesion to the polymer PTC composition and low contact resistance at the interface, a conductor paste composed of a conductor powder, a binder and a solvent is used as the polymer PTC composition. It has been found that a method is useful in which a part of the conductor powder is exposed on the surface of the polymer PTC composition by applying it to the surface of the polymer, drying it and then applying pressure, and further subjecting the surface to a plating treatment. Was done. this is,
It is considered that the contact area becomes very large by providing such a structure between the polymer PTC composition and the electrode.

Further, the metal carbide is used as the conductive powder, because when the metal powder is used, a partial conductive path is formed in the polymer PTC composition due to the aggregation of the powder itself, and the withstand voltage characteristic. When carbon-based powders such as carbon black and graphite are used, the resistivity of the powder itself is higher than that of the metal carbide, and the room temperature resistivity before performing the switching operation is the case of using the metal carbide. Because it will be higher than.

Further, the volume filling rate of the conductive powder is 45 to.
The reason for limiting the range to 60% is that below this range, the resistivity at room temperature after repeated switching operation exceeds 2Ω · cm, which is a numerical value required for a PTC element. Then, it becomes difficult to knead the conductive powder and the binder, and it becomes substantially impossible to manufacture the device.

[0026]

BEST MODE FOR CARRYING OUT THE INVENTION Next, embodiments of the present invention will be described in detail with reference to specific examples.

Here, HDPE having a softening point of 130 ° C. is used as a binder, and a temperature of 140 is used for kneading with conductive powder.
It was carried out using a roll set at ° C. If the roll temperature at this time is 200 ° C. or higher, the viscosity of HDPE is significantly lowered and the workability is lowered, so it is desirable to set the roll temperature to 200 ° C. or lower.

The polymer PTC composition obtained by kneading was preformed into a sheet with a roll, and a sheet having a thickness of 250 μm was formed by using a hot press set at a temperature of 160 ° C. If the temperature at this time is 140 ° C. or lower, the viscosity of HDPE is high and the moldability is impaired, and at a temperature of 200 ° C. or higher, the workability is lowered for the same reason as above, so that the temperature is 140-20
A range of 0 ° C is desirable.

Next, the polymer PTC composition sheet was subjected to double-sided plasma treatment. The processing conditions are oxygen atmosphere,
The atmospheric pressure was 30 Pa and the irradiation power was 100 W. Then, 100 parts by weight of nickel powder having an average particle size of 5 μm,
A conductive paste prepared by the composition of 8 parts by weight of polyvinyl butyral and 30 parts by weight of a solvent obtained by mixing methyl ethyl ketone and cyclohexanone in a weight ratio of 1: 1,
It was applied to both sides of a polymer PTC composition sheet that had been plasma treated.

After the conductor paste was applied, it was dried at room temperature for 5 hours and hot pressed at a temperature of 160 ° C. for 10 minutes to press-bond nickel powder. As a result, a sheet of the polymer PTC composition in which most of the nickel powder was embedded in the sheet and a part of which was exposed on the surface of the sheet was obtained. Further, the surface of the sheet was subjected to electroless plating and electrolytic plating using nickel to form electrodes.

The sheet obtained as described above was punched out into a square having a side of 10 mm to obtain a sample for evaluation. The evaluation items are the temperature dependence of the resistivity, the resistivity at the initial stage and after 500 switching operations, and the bonding strength of the electrodes. The resistivity measurement at high temperature was performed by immersing the sample in an oil bath. A digital multimeter with a DC 4 probe was used to measure the resistivity.

The bonding strength was measured by soldering a lead wire to the electrode, coating the periphery with an epoxy resin for reinforcement, and pulling the lead wire. For this, a tensile tester was used, the tensile speed was set to 50 mm / min, and the stress when the electrodes were peeled off was taken as the bonding strength.

[0033]

EXAMPLES Next, specific examples will be described.

(Example) TiC having an average particle size of 3 μm,
WC, W ₂ C, ZrC, VC, NbC, TaC, Mo ₂
C was weighed so that the volume filling rate was 52%, kneaded with HDPE to prepare a polymer PTC composition, and thereafter, a polymer PTC element sample was obtained according to the above operation. Measurements were performed on the above items for these eight types of samples.

Table 1 summarizes the measurement results of the resistivity of these samples at 23 ° C.

[0036]

[Table 1]

As is clear from Table 1, the polymer PTC elements of this example each have a resistivity at 23 ° C. of P.
It is less than the value of 2 Ω · cm required for the TC element.

FIG. 1 is a diagram showing the results of measuring the temperature dependence of the resistivity of a sample using TiC as the conductive powder in this example. As is apparent from FIG. 1, the resistivity near room temperature is less than 1 Ω · cm and satisfies the numerical value of 2 Ω · cm or less required for the PTC element. Further, at the temperature corresponding to the softening point of HDPE, a sharp rise of the resistivity is recognized.

The ratio (R ₂ / R ₁ ) of the resistivity R ₂ after switching to the resistivity R ₁ at room temperature is much higher than 10 ⁸ . This satisfies the numerical value of 10 ⁴ or more, which is sufficient to operate as a PTC element and to be used as a planar heating element. Samples using conductive powders other than TiC also showed almost the same results as in FIG. Therefore,
Obviously, the same result can be obtained by mixing and using the above-mentioned conductive powder.

Table 2 shows the measurement results of the bonding strength of the electrodes.

[0041]

[Table 2]

According to these results, the polymer PT of this example
The minimum value of the electrode bonding strength of the C element is 85.2 Pa, which is sufficiently higher than the value of 50 Pa required to secure sufficient reliability as the PTC element. Therefore, it is clear that the electrode formed by the method of the present invention secures the necessary adhesiveness.

FIG. 2 shows the result of measuring the change in resistivity after the switching operation for the sample using TiC as the conductive powder in the present embodiment. The switching operation was performed by applying a current of 10 A (50 V). According to this result, the resistivity is 2Ω · cm even after the switching operation is repeated 1000 times.
Is below.

Here, the target of the number of switching operations that does not cause the characteristic deterioration is set to 500, but the result of FIG. 2 is a number that sufficiently satisfies this. The samples using conductive powders other than TiC also showed almost the same results as in FIG. Therefore, it can be easily estimated that the same result can be obtained by mixing and using the conductive powder.

(Comparative Example 1) Next, for comparison of electrode forming methods, TiC was used as a conductive powder, and a nickel foil having a thickness of 50 μm was press-bonded by hot pressing. A polymer PTC element was prepared, and room temperature resistivity, change in resistivity after switching operation, and electrode bonding strength were evaluated.

Each of these evaluation results is shown in Tables 1 and 2 and FIG. 2 together with the above examples. According to these results, the resistivity exceeds 100 Ω · cm both in the initial value and after the switching operation, and P
It is clear that it cannot be used as a TC element. The maximum bonding strength of the electrode is 15.0 Pa,
It is clear that the reliability as a PTC element cannot be ensured.

(Comparative Example 2) Next, for comparison of the electrode forming methods, the surface of the nickel foil on the side in contact with the polymer PTC composition was roughened with an electrolyte, and Comparative Example 1 was used. A polymer PTC element was prepared by the same method and evaluated in the same manner as in Comparative Example 1. Regarding these evaluation results,
The results are summarized in Tables 1 and 2 and FIG.

From these results, in this example, due to the effect of roughening the nickel foil used for the electrode, the bonding strength of the electrode showed a value almost equal to that of the example, and the initial value of the resistivity was also
It is less than 2 Ω · cm. However, after repeating the switching operation, an increase in resistivity was observed, and it is clear that the characteristics were inferior to those in the above-mentioned Examples.

(Comparative Example 3) Next, for comparison of electrode forming methods, TiC was used as a conductive powder, and the electrode was formed by degreasing the surface of the polymer PTC composition sheet, followed by electroless plating of nickel and nickel. A polymer PTC element was produced by the same method as in Example except that the electrolytic plating was performed, and the same evaluation as in Comparative Example 1 was performed. These evaluation results are also shown in Tables 1 and 2 and FIG.
Are summarized in.

From these results, in this example, the bonding strength of the electrodes is low, and therefore the room temperature resistivity is higher than 300 Ω · cm both at the initial value and after the switching operation, and it cannot be used as a PTC element. Is clear.

Comparative Example 4 Next, in order to verify the lower limit of the volume filling rate of the conductive powder, TiC powder was used as the conductive powder, and the volume filling rate was 43 to 60%. A polymer PTC element was manufactured by the same method as
The initial value of the resistivity and the change after the switching operation were measured. FIG. 3 shows the relationship between the volume filling rate of the conductive powder and the resistivity. The change in resistivity after the switching operation is shown in FIG. 2 together with the examples and other comparative examples when the conductive powder has a volume filling rate of 44%.

From FIG. 3, the volume filling ratio of the conductive powder is 44.
Although the resistivity is below 2 Ω · cm in the vicinity of%, it is clear that the resistivity sharply increases in the region below that. Further, according to FIG. 2, it can be seen that in the sample having the volume filling rate of the conductive powder of 44%, the resistivity increases due to the repetition of the switching operation, and the numerical value of 2Ω · cm or less cannot be maintained. Therefore, the lower limit value of the volume filling rate of the conductive powder is 45%.

[0053]

As described above, according to the present invention, the polymer PTC using the metal carbide as the conductive powder is used.
By subjecting the surface of the composition molded body to plasma treatment and press-bonding so that a part of the conductor is exposed on the surface of the molded body, a plating film is formed to improve adhesion between the electrode and the polymer PTC composition. It is possible to obtain a polymer PTC device having a high contact resistance at the interface and a very small change in resistivity after repeated switching operations.

[Brief description of drawings]

FIG. 1 is a diagram showing a result of measuring temperature dependence of resistivity of a sample using TiC as a conductive powder.

FIG. 2 is a diagram showing a result of measuring a change in resistivity after a switching operation.

FIG. 3 is a diagram showing a relationship between a volume filling rate of conductive powder and a resistivity.

[Explanation of symbols]

200 Examples 201 Comparative Example 1 202 Comparative Example 2 203 Comparative Example 3 204 Comparative Example 4

Claims (5)

[Claims] 1. A molded body comprising a composition in which a conductive powder is dispersed in a binder containing a crystalline polymer, the surface of which is subjected to plasma treatment, and the molding so that at least a part of the molded body is exposed on the surface of the molded body. A polymer PTC element comprising a conductor embedded in a body and a plating layer formed on the surface of a molded body in which the conductor is embedded. 2. The polymer PTC element according to claim 1, wherein the conductive powder is titanium carbide (Ti).
C), Tungsten Carbide (WC, W ₂ C), Zirconium Carbide (ZrC), Vanadium Carbide (VC), Niobium Carbide (NbC), Tantalum Carbide (TaC), Molybdenum Carbide (Mo ₂ C).
1. A polymer PTC element comprising at least one selected from the group consisting of: 3. The polymer PTC element according to claim 1, wherein the volume filling rate of the conductive powder is 45 to 60%. 4. A polymer PTC composition is obtained by kneading a conductive powder into a binder containing a crystalline polymer, wherein the polymer PTC is obtained.
After the composition is molded into a desired shape and subjected to plasma treatment on the surface, a conductor is pressure-bonded so that a part of the conductor is exposed on the surface of the molded body, and a part of the conductor is exposed. The method for producing a polymer PTC element according to any one of claims 1 to 3, wherein the surface is plated. 5. The method for manufacturing a polymer PTC element according to claim 4, wherein the conductor is pressure-bonded by applying a conductor paste including a conductor powder, a binder, and a solvent to the surface of the molded body. A method for producing a polymer PTC element, which comprises performing pressure drying after drying.