CN110791005A

CN110791005A - PTC composite material containing polyethylene, carbon black and conductive modified graphene and preparation method thereof

Info

Publication number: CN110791005A
Application number: CN201911170956.5A
Authority: CN
Inventors: 王庚超; 史光发; 蔡晓敏; 计成志
Original assignee: ANHUI HUANRUI ELECTROTHERMAL EQUIPMENT Co Ltd; East China University of Science and Technology
Current assignee: ANHUI HUANRUI ELECTROTHERMAL EQUIPMENT Co Ltd; East China University of Science and Technology
Priority date: 2019-11-26
Filing date: 2019-11-26
Publication date: 2020-02-14
Anticipated expiration: 2039-11-26
Also published as: CN110791005B

Abstract

The invention discloses a PTC composite material containing polyethylene, carbon black and conductive modified graphene, which is prepared from the following components in parts by weight: 50-75 parts of polyethylene, 1-8 parts of conductive modified graphene, 2-15 parts of a compatilizer and 20-40 parts of carbon black. The PTC composite material has the characteristics of low room temperature resistivity, high PTC strength, low NTC strength after peak temperature, good resistance reproducibility, low cost and the like, can be used for preparing a safe and reliable self-temperature control electric tracing device and an overcurrent and overheating protection device with low room temperature resistivity, and has wide application prospect.

Description

PTC composite material containing polyethylene, carbon black and conductive modified graphene and preparation method thereof

Technical Field

The invention belongs to the technical field of functional polymer materials, and particularly relates to a PTC composite material containing polyethylene, carbon black and conductive modified graphene and a preparation method thereof.

Background

The high molecular PTC functional material is a material with resistance positive temperature coefficient, and is compounded by components such as a crystalline polymer matrix, nano conductive particles and the like. The material has the resistivity which is rapidly increased to a limit value near the transition temperature, and reversible transition of (semi) conductor-insulator occurs, so that the material can be used for self-temperature-control electric heating belts, overcurrent protection devices, other temperature sensing devices and the like. At present, the polymer PTC material has the phenomena of low room-temperature conductivity, poor resistance stability and resistance reduction (NTC effect) after the resistance is higher than a melting point, so that the application development of the material is restricted.

Research shows that the room temperature conductivity of the PTC material can be improved by increasing the amount of conductive fillers such as carbon black and metal powder, but the PTC strength and mechanical properties of the material are sacrificed. Patents CN103304938A, CN109762277A, CN102604215A, etc. utilize the principle of phase separation and the distribution tendency of conductive fillers in different matrixes, so as to reduce the amount of fillers and simultaneously improve the conductivity at room temperature, but the PTC materials obtained by the method have poor resistance reproducibility. Patent CN 109494035A, CN 1730529A and the like improve resistance stability and effectively inhibit NTC effect by irradiation crosslinking, but higher irradiation dose can accelerate material degradation and increase manufacturing cost. In recent years, more and more reports of applying graphene to polymer PTC materials are provided, and the graphene can not only improve the room-temperature conductivity of the PTC materials, but also limit the migration of conductive particles such as carbon black and the like due to a two-dimensional lamellar structure of the graphene, so that the PTC strength is improved and the NTC effect is inhibited. In order to fully exert the function of graphene, it is important to modify the surface of graphene and maintain high conductivity after modification, but most of graphene used in many current applications is single-layer or few-layer graphene prepared by a chemical oxidation method, the preparation method is complex and the conductivity is not high, and the modification of the surface of graphene can further reduce the conductivity of graphene.

Disclosure of Invention

Aiming at the defects of the prior art, the invention aims to provide a PTC composite material which has low room-temperature resistivity, is safe and reliable and is low in price and contains polyethylene, carbon black and conductive modified graphene, and a preparation method thereof.

In order to achieve the purpose, the technical scheme adopted by the invention is as follows:

the invention provides a PTC composite material containing polyethylene, carbon black and conductive modified graphene, which is prepared from the following components in parts by weight: 50-75 parts of polyethylene, 1-8 parts of conductive modified graphene, 2-15 parts of a compatilizer and 20-40 parts of carbon black.

The polyethylene is medium density polyethylene or high density polyethylene with the crystallinity of more than 60 percent.

The weight portion of the polyethylene can also be selected from 60 parts, 58 parts and the like, namely, the weight portion of the polyethylene can be selected from one of 60-75 parts and 58-75 parts.

The compatilizer can be selected from 4, 6, 7, 8, 10, 12 parts by weight and the like, namely, the compatilizer can be selected from one of 4-15 parts, 6-15 parts, 8-15 parts, 10-15 parts, 4-12 parts, 6-12 parts, 8-12 parts and 4-7 parts by weight.

The conductive modified graphene can be selected from 3, 4, 6 and 7 parts by weight, namely, one of 3-8 parts, 3-7 parts, 4-7 parts and 3-6 parts by weight.

The weight parts of the carbon black can be 29, 33, 35, 38 parts and the like, namely, the weight parts of the carbon black can be one of 20-38 parts, 20-35 parts, 20-33 parts and 20-29 parts.

The compatilizer is one of maleic anhydride grafted polyethylene, acrylate grafted polyethylene or ethylene-vinyl acetate copolymer.

The average particle size of the carbon black is 30-100 nm.

The preparation method of the conductive modified graphene comprises the following steps:

adding multilayer graphene (MLG) and a dispersing agent into an inorganic acid solution with the concentration of 0.1-1 mol/L (M), and ultrasonically stirring and uniformly mixing to obtain a mixed solution A;

adding a conductive polymer monomer 3, 4-Ethylenedioxythiophene (EDOT) into the mixed system A, and continuing ultrasonic stirring under an ice bath condition to obtain a mixed system B;

dissolving an initiator in an inorganic acid solution with the concentration of 0.1-1 mol/L (M) to obtain an initiation system C; dropwise adding the initiation system C into the mixed system B to initiate polymerization;

the concentration of the multilayer graphene is 5-25 mg/mL; the mass ratio of the multilayer graphene to the 3, 4-ethylenedioxythiophene is 1 (0.05-0.5); the mass ratio of the 3, 4-ethylenedioxythiophene to the dispersant is 1 (1-3); the molar ratio of the 3, 4-ethylenedioxythiophene to the initiator is 1 (1.0-1.5);

and then filtering the mixed solution, washing with deionized water and ethanol, and freeze-drying to obtain the conductive modified graphene (PEDOT-MLG).

The multilayer graphene is 1-20 nm thick and 1-50 μm in diameter.

The dispersing agent is one of sodium polystyrene sulfonate, sodium dodecyl sulfate and sodium hexadecyl sulfate.

The initiator is a bi-component initiator composed of ammonium persulfate and ferric chloride, wherein the molar ratio of the ferric chloride to the ammonium persulfate is 1 (50-200).

The inorganic acid is one of hydrochloric acid, sulfuric acid and perchloric acid.

The second aspect of the invention provides a preparation method of the PTC composite material containing polyethylene, carbon black and conductive modified graphene, which comprises the following steps:

according to the proportion, polyethylene, the conductive modified graphene, carbon black and a compatilizer are placed in a torque rheometer and are mixed for 5-15min under the conditions that the temperature is 150-180 ℃ and the rotating speed is 50-80r/min, so that the PTC composite material containing the polyethylene, the carbon black and the conductive modified graphene is obtained.

Due to the adoption of the technical scheme, the invention has the following advantages and beneficial effects:

the invention adopts the high-conductivity polymer poly (3, 4-ethylenedioxythiophene) to conduct conductive modification on the multilayer graphene, thereby not only improving the dispersibility of the multilayer graphene in the polymer, but also keeping the high conductivity and the high-temperature stability of the conductivity of the multilayer graphene before modification. Therefore, the conductive modified multilayer graphene is suitable for composite processing with a polymer matrix in a melt blending mode.

According to the invention, the conductive modified graphene is introduced into a polyethylene/carbon black composite system, and the high conductivity and the length-diameter ratio of the graphene are utilized to play a role in bridging among carbon black particles, so that the room temperature resistivity and the filler amount of the PTC material are reduced. Meanwhile, the two-dimensional layered structure of the graphene can obviously improve the viscosity of the matrix, isolate agglomeration of carbon black particles, improve the resistance-temperature curve reproducibility of the PTC material system, inhibit the NTC effect above the melting point of the matrix, and ensure the requirements on the stability of low room temperature resistance and the safety of the material in the long-term use process.

The graphene adopted by the invention is multilayer graphene, and has the advantages of low price, suitability for industrialization and the like compared with single-layer or few-layer graphene.

The invention selects the medium density polyethylene or high density polyethylene with the crystallinity of more than 60 percent as the polymer matrix, and endows the composite material with high PTC strength.

The conductive modified graphene is prepared by taking multilayer graphene as a raw material and carrying out in-situ surface modification on a high-conductivity polymer, and the multilayer graphene not only keeps the high conductivity and the high-temperature stability of the conductivity before modification, but also shows good dispersibility after conductive modification. The PTC composite material has the characteristics of low room temperature resistivity, high PTC strength, low NTC strength after peak temperature, good cycle stability, low cost and the like, can be used for preparing a safe and reliable self-temperature control electric heat tracing device and an overcurrent and overheating protection device with low room temperature resistivity, and has wide application prospect.

Drawings

In fig. 1, a is multilayer graphene, and b is a transmission electron microscope image of the conductive modified graphene prepared in example 1.

Fig. 2 is a schematic diagram of a conductivity-temperature change curve of the multi-layer graphene and the conductively modified graphene prepared in example 1.

FIG. 3 is a graph showing the change of conductivity with time of PEDOT-MLG prepared in example 1 at a temperature of 180 ℃.

FIG. 4 is a scanning electron micrograph of a cross section of the composite materials prepared in comparative example 2 (a, b in the figure) and example 1 (c, d in the figure).

Fig. 5 is a graph showing resistance-temperature curves of the composite materials prepared in comparative example 1 and example 1.

FIG. 6 is a graph showing resistance-temperature curves of a cooling-heating cycle of the composite material prepared in comparative example 1.

FIG. 7 is a graphical representation of the resistance-temperature curve of the cold-heat cycle for the composite prepared in example 1.

Detailed Description

In order to more clearly illustrate the invention, the invention is further described below in connection with preferred embodiments. It is to be understood by persons skilled in the art that the following detailed description is illustrative and not restrictive, and is not to be taken as limiting the scope of the invention.

In the present invention, all parts and percentages are by weight, unless otherwise specified, and the equipment and materials used are commercially available or commonly used in the art. The methods in the following examples are conventional in the art unless otherwise specified.

Example 1

1g of sodium polystyrene sulfonate and 5g of multilayer graphene (the thickness is about 5nm, the diameter is 7-15 mu m), adding the materials into 950ml of 1M sulfuric acid solution, and ultrasonically stirring for 1h to obtain a mixed system A.

And (3) adding 0.5g of 3, 4-ethylene dioxythiophene monomer into the mixed system A, and continuing ultrasonic stirring for 1h under the ice bath condition to obtain a mixed system B.

0.8g of ammonium persulfate and 12mg of ferric chloride were dissolved in 50ml of a 1M sulfuric acid solution to obtain initiation system C.

And dropwise adding the initiation system C into the mixed system B for 30min to initiate polymerization, and stirring the reaction for 24h in ice-water bath. And filtering the product, washing the product for multiple times by deionized water and ethanol, and freeze-drying the product to obtain the conductive modified graphene.

The transmission electron microscope picture shows that polythiophene is uniformly loaded on the surface of graphene in a form of tens of nanometers (as shown in fig. 1, a in fig. 1 is multilayer graphene, and b is the transmission electron microscope picture of the conductive modified graphene prepared in example 1). Compared with untreated multi-layer graphene, the conductivity of the conductive modified graphene is reduced, but the high conductivity of 610S/cm is still maintained (Table 1). The conductivity-temperature curve shows that the conductivity of the conductive modified graphene is slightly affected by the temperature (as shown in fig. 2, fig. 2 is a schematic diagram of the conductivity-temperature change curve of the multi-layer graphene and the conductive modified graphene prepared in example 1). The conductivity of the conductive modified graphene is slightly changed when the conductive modified graphene is placed in an environment of 180 ℃ for 1 hour (fig. 3, fig. 3 is a schematic diagram of the change curve of the conductivity of the PEDOT-MLG prepared in example 1 along with time under the condition of the temperature of 180 ℃). The results show that the conductive modified graphene has excellent high-temperature stability and is very suitable for preparing the conductive composite material by a melt blending method.

A preparation method of a PTC composite material containing polyethylene, carbon black and conductive modified graphene comprises the following steps:

adding 66 parts by weight of medium density polyethylene (with the crystallinity of 60-75%), 3 parts by weight of maleic anhydride grafted polyethylene, 28 parts by weight of carbon black (with the average particle size of 50nm) and 3 parts by weight of conductive modified graphene into a torque rheometer, and mixing for 10min under the conditions that the banburying temperature is 180 ℃ and the rotating speed is 60r/min to obtain the PTC composite material containing polyethylene, carbon black and conductive modified graphene.

As shown in FIG. 4, FIG. 4 is a sectional scanning electron micrograph of the composite materials prepared in comparative example 2 (a, b in the figure) and example 1 (c, d in the figure). The section scanning electron microscope shows that the compatibility of the conductive modified graphene in the embodiment 1 and the polymer matrix and the dispersibility of the conductive modified graphene in the matrix are obviously superior to those of the comparative example 2; thus, the composite material prepared in example 1 has lower room temperature resistivity and higher mechanical strength (as shown in table 1) compared to comparative example 2. As can be seen from fig. 5 and table 1, fig. 5 is a graph illustrating resistance-temperature curves of the composite materials prepared in comparative example 1 and example 1, and the composite material prepared in example 1 has a low resistivity at room temperature, and the resistivity decrease tendency after the melting point is significantly reduced, that is, example 1 has a low NTC strength, compared to comparative example 1 in which the conductive modified graphene is not added. Meanwhile, it is found that, for comparative example 1 without the addition of the conductive modified graphene, the resistance-temperature curve reproducibility is poor after 10 times of thermal cycles (as shown in fig. 6, fig. 6 is a schematic diagram of the resistance-temperature curve of the composite material prepared in comparative example 1); in contrast, in example 1 in which the conductive modified graphene is added, the reproducibility of the resistance-temperature curve after 10 thermal cycles is significantly improved (as shown in fig. 7, fig. 7 is a schematic diagram of the resistance-temperature curve of the composite material prepared in example 1 in the cold-hot cycle).

Example 2

adding 1g of sodium polystyrene sulfonate and 20g of multilayer graphene (the thickness is about 5nm, the diameter is 5-17 mu M) into 950ml of 0.1M hydrochloric acid solution, and ultrasonically stirring for 1h to obtain a mixed system A.

And adding 1g of 3, 4-ethylene dioxythiophene monomer into the mixed system A, and continuing ultrasonic stirring for 1h under the ice bath condition to obtain a mixed system B.

Initiation system C was obtained by dissolving 1.9g of ammonium persulfate and 14mg of ferric chloride in 50ml of 0.1M hydrochloric acid solution.

And dropwise adding the initiation system C into the mixed system B for 30min to initiate polymerization, and stirring for reaction for 12h under ice bath. And filtering the product, washing the product for multiple times by deionized water and ethanol, and freeze-drying the product to obtain the conductive modified graphene.

70 parts by weight of medium density polyethylene (with the crystallinity of 60-75%), 2 parts by weight of maleic anhydride grafted polyethylene, 20 parts by weight of carbon black (with the average particle size of 50nm) and 8 parts by weight of conductive modified graphene are added into a torque rheometer and are mixed for 12min under the conditions that the banburying temperature is 170 ℃ and the rotating speed is 70r/min, so that the PTC composite material containing polyethylene, carbon black and conductive modified graphene is obtained.

Example 3

adding 3g of sodium dodecyl sulfate and 6g of multilayer graphene (the thickness is about 5nm, and the diameter is 7-15 mu m) into 950ml of 0.5M sulfuric acid solution, and ultrasonically stirring for 1h to obtain a mixed system A.

Initiation system C was obtained by dissolving 2.4g of ammonium persulfate and 8mg of ferric chloride in 50ml of 0.5M sulfuric acid solution.

And dropwise adding the initiation system C into the mixed system B for 30min to initiate polymerization reaction, and stirring the reaction for 36h in an ice bath. And filtering the product, washing the product for multiple times by deionized water and ethanol, and freeze-drying the product to obtain the conductive modified graphene.

50 parts by weight of high-density polyethylene (with the crystallinity of 80-90 percent), 15 parts by weight of maleic anhydride grafted polyethylene, 30 parts by weight of carbon black (with the average particle size of 50nm) and 5 parts by weight of conductive modified graphene are added into a torque rheometer and are mixed for 15min under the conditions that the banburying temperature is 150 ℃ and the rotating speed is 50r/min, so that the PTC composite material containing polyethylene, carbon black and conductive modified graphene is obtained.

Example 4

adding 3g of sodium hexadecylsulfate and 10g of multilayer graphene (the thickness is about 5nm, and the diameter is 7-15 mu M) into 950ml of 1M hydrochloric acid solution, and ultrasonically stirring for 1h to obtain a mixed system A.

And adding 2g of 3, 4-ethylene dioxythiophene monomer into the mixed system A, and continuing ultrasonic stirring for 1h under the ice bath condition to obtain a mixed system B.

Initiation system C was obtained by dissolving 3.85g of ammonium persulfate and 18mg of ferric chloride in 50ml of 1M hydrochloric acid solution.

And dropwise adding the initiation system C into the mixed system B for 30min to initiate polymerization reaction, and stirring the reaction for 48h in an ice bath. And filtering the product, washing the product for multiple times by deionized water and ethanol, and freeze-drying the product to obtain the conductive modified graphene.

adding 55 parts by weight of medium density polyethylene (with the crystallinity of 60-75%), 3 parts by weight of maleic anhydride grafted polyethylene, 40 parts by weight of carbon black (with the average particle size of 50nm) and 2 parts by weight of conductive modified graphene into a torque rheometer, and mixing for 15min under the conditions that the banburying temperature is 180 ℃ and the rotating speed is 60r/min to obtain the PTC composite material containing polyethylene, carbon black and conductive modified graphene.

Example 5

adding 3.8g of sodium polystyrene sulfonate and 5g of multilayer graphene (the thickness is about 5nm, the diameter is 7-15 mu M) into 950ml of 0.5M perchloric acid solution, and ultrasonically stirring for 1h to obtain a mixed system A.

And adding 2.5g of 3, 4-ethylene dioxythiophene monomer into the mixed system A, and continuing ultrasonic stirring for 1h under the ice bath condition to obtain a mixed system B.

4g of ammonium persulfate and 28mg of ferric chloride were dissolved in 50ml of a 0.5M perchloric acid solution to give initiation system C.

And dropwise adding the initiation system C into the mixed system B for 30min to initiate polymerization reaction, and stirring the reaction for 18h in an ice bath. And filtering the product, washing the product for multiple times by deionized water and ethanol, and freeze-drying the product to obtain the conductive modified graphene.

64 parts by weight of high-density polyethylene (with the crystallinity of 80-90 percent), 5 parts by weight of maleic anhydride grafted polyethylene, 30 parts by weight of carbon black (with the average particle size of 50nm) and 1 part by weight of conductive modified graphene are added into a torque rheometer and are mixed for 5min under the conditions that the banburying temperature is 160 ℃ and the rotating speed is 80r/min, so that the PTC composite material containing polyethylene, carbon black and conductive modified graphene is obtained.

Comparative example 1

69 parts by weight of medium density polyethylene (with the crystallinity of 60-75 percent), 3 parts by weight of maleic anhydride grafted polyethylene and 28 parts by weight of carbon black (with the average particle size of 50nm) are added into a torque rheometer and are mixed for 10 minutes under the conditions that the banburying temperature is 180 ℃ and the rotating speed is 60r/min, so as to obtain the polyethylene/carbon black PTC composite material.

Comparative example 2

Adding 66 parts by weight of medium density polyethylene (with the crystallinity of 60-75%), 3 parts by weight of maleic anhydride grafted polyethylene, 28 parts by weight of carbon black (with the average particle size of 50nm) and 3 parts by weight of graphene into a torque rheometer, and mixing for 10 minutes under the conditions that the banburying temperature is 180 ℃ and the rotating speed is 60r/min to obtain the polyethylene/graphene/carbon black PTC composite material.

Comparative example 3

adding 1g of sodium polystyrene sulfonate and 5g of multilayer graphene (the thickness is about 5nm, and the diameter is 7-15 microns) into 950ml of 1M sulfuric acid solution, and ultrasonically stirring for 1h to obtain a mixed system A.

Adding 57 parts by weight of medium density polyethylene (with the crystallinity of 60-75%), 3 parts by weight of maleic anhydride grafted polyethylene, 30 parts by weight of carbon black (with the average particle size of 50nm) and 10 parts by weight of conductive modified graphene into a torque rheometer, and mixing for 10min under the conditions that the banburying temperature is 180 ℃ and the rotating speed is 60r/min to obtain the PTC composite material containing polyethylene, carbon black and conductive modified graphene.

TABLE 1 comparison of physical Properties of composites of examples and comparative examples

Note:^aPTC Strength Log (Peak resistivity/Room temperature resistivity)

^bNTC strength Log (peak resistivity/resistivity 30 ℃ above peak temperature)

As can be seen from the comparison of the performances of the examples and the comparative examples in Table 1, the products prepared in examples 1 to 5 have low room temperature resistivity and high conductivity; while the graphene added in the comparative example 1 is not conductively modified, the graphene added in the comparative example 2 is not conductively modified, so that the room temperature resistivity is high, and the conductivity is relatively poor. Except that the PTC strength of the product is lower than 6 due to the fact that the carbon black is added in a large amount in the example 4, the PTC strength of the product prepared in other examples is higher than that of the product prepared in the three comparative examples, and excellent switching performance is achieved, wherein the PTC strength of the product prepared in the comparative example 3 is only 3.9 due to the fact that the conductive modified graphene is added in an excessive amount, and the PTC strength is far lower than that of the product prepared in the example, and the switching performance is poor. The NTC strength of the products prepared in the embodiments 1-5 is lower than 0.6, and the products have good safety; and the NTC strength of the products of comparative examples 1 and 2 is higher than 1, and the product safety is poor. The tensile strength of the products prepared in examples 1-5 is better than that of the three comparative examples, wherein the reduction range of the tensile strength of comparative examples 2 and 3 is larger.

Although the present invention has been described with reference to a preferred embodiment, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.

Claims

1. The PTC composite material containing polyethylene, carbon black and conductive modified graphene is characterized by being prepared from the following components in parts by weight: 50-75 parts of polyethylene, 1-8 parts of conductive modified graphene, 2-15 parts of a compatilizer and 20-40 parts of carbon black.

2. The PTC composite comprising polyethylene, carbon black and conductive modified graphene according to claim 1, wherein the polyethylene is medium density polyethylene or high density polyethylene having a crystallinity of more than 60%.

3. The PTC composite material containing polyethylene, carbon black and conductive modified graphene according to claim 1, wherein the compatibilizer is one of maleic anhydride grafted polyethylene, acrylate grafted polyethylene or ethylene-vinyl acetate copolymer.

4. The PTC composite material containing polyethylene, carbon black and conductive modified graphene according to claim 1, wherein the carbon black has an average particle size of 30 to 100 nm.

5. The PTC composite material containing polyethylene, carbon black and conductive modified graphene according to claim 1, wherein the preparation method of the conductive modified graphene comprises the following steps:

adding multilayer graphene and a dispersing agent into an inorganic acid solution with the concentration of 0.1-1 mol/L, and uniformly stirring and mixing by ultrasound to obtain a mixed solution A;

adding a conductive polymer monomer 3, 4-ethylenedioxythiophene into the mixed system A, and continuing ultrasonic stirring under an ice bath condition to obtain a mixed system B;

dissolving an initiator in an inorganic acid solution with the concentration of 0.1-1 mol/L to obtain an initiation system C; dropwise adding the initiation system C into the mixed system B to initiate polymerization;

and then filtering the mixed solution, washing with deionized water and ethanol, and freeze-drying to obtain the conductive modified graphene.

6. The PTC composite material containing polyethylene, carbon black and conductive modified graphene according to claim 5, wherein the multilayer graphene has a thickness of 1-20 nm and a diameter of 1-50 μm.

7. The PTC composite material containing polyethylene, carbon black and conductive modified graphene according to claim 5, wherein the dispersant is one of sodium polystyrene sulfonate, sodium dodecyl sulfate and sodium hexadecyl sulfate.

8. The PTC composite material containing polyethylene, carbon black and conductive modified graphene according to claim 5, wherein the initiator is a two-component initiator consisting of ammonium persulfate and ferric chloride, and the molar ratio of ferric chloride to ammonium persulfate is 1 (50-200).

9. The PTC composite material containing polyethylene, carbon black and conductive modified graphene according to claim 5, wherein the inorganic acid is one of hydrochloric acid, sulfuric acid and perchloric acid.

10. A method of preparing a PTC composite comprising polyethylene, carbon black and conductively-modified graphene as claimed in any one of claims 1 to 9, comprising the steps of: