CN1970612A - Preparation method of electrically conductive composite material with positive temperature coefficient effect - Google Patents

Abstract

The invention relates to a preparation method of conductive composite material with positive temperature coefficient effect. It includes the following steps: (1) Put the two polymer blends together with the conductive filler in a twin-screw extruder or internal mixer for 5 to 20 minutes to melt and blend, and the melting temperature is higher than the melting point of the polymer matrix used. The melting point of the polymer is 10-50°C higher; (2) The blend is molded into a film in a flat vulcanizer. The method has simple process and low cost, and the prepared conductive composite material has good stability.

Description

A preparation method of conductive composite material with positive temperature coefficient effect

technical field

The invention belongs to the field of material processing, and in particular relates to a preparation method of a conductive composite material with a positive temperature coefficient effect.

Background technique

The characteristic of conductive composite materials with positive temperature coefficient (PTC) effect is that the resistivity of the material increases with the increase of temperature. Conductive polymer composite material is a functional polymer material formed by adding conductive substances to high molecular polymers through dispersion compounding, lamination compounding, and formation of surface conductive films. When adding conductive fillers to the polymer matrix to prepare conductive composites, the change of the conductivity of the composites is discontinuous within a certain concentration range of conductive fillers. As the filler content increases, the resistivity decreases slowly. When the filler content reaches a certain When a critical value is reached, the volume resistivity of the composite system suddenly decreases sharply, and a small increase in the filler content will cause a sudden change in the resistivity, that is, the conductive filler particles can contact each other or be close enough to form a continuous conductive layer through tunneling or electronic transition. pathways or conductive networks, at which point the critical volume fraction of conductive filler is known as the percolation threshold. At a certain transition temperature, the resistivity of conductive materials will rapidly increase to a limit value near the percolation threshold, resulting in a jump of several orders of magnitude, and the mutual transformation from (semi)conductor to insulator occurs, showing a significant PTC effect. This material is commonly used in telecommunications engineering, self-regulating heaters, current limiters, circuit overload protectors, etc. An important parameter to characterize PTC materials is PTC strength. PTC strength refers to the ratio of the maximum resistance value of the composite material changing with temperature to the resistance value at room temperature.

Conductive substances include metal fibers, carbon black, carbon nanotubes, and the like. For example: the application number is 200510055197.X, and the Chinese patent titled "conductive composite material" discloses that a metal fiber and another metal with a low melting point are added to a thermoplastic resin as a composite conductive filler to prepare a Methods for Conductive Composite Materials. The disadvantage of this method is that the metal used as the conductive filler is easily oxidized, which will make the conductivity of the conductive material and the strength of the PTC unstable after a period of time. Inorganic conductive fillers have relatively stable properties, such as carbon black and metal oxides.

For conductive composite materials, a large percolation threshold means adding more conductive fillers, which may increase the melt viscosity of the system, deteriorate the processing performance, reduce the impact strength of the material, and affect the mechanical properties. For example, when a conductive composite material with a high percolation threshold is used as a PTC material, the NTC phenomenon will occur, and the PTC material with a high percolation threshold will cause considerable wear and tear on the machine during production, thereby affecting its processing performance. Composite systems based on a single polymer are usually filled with 15% to 20% by mass of carbon black. For example: the Chinese patent titled "polyolefin/carbon black PTC conductive composite material and preparation method thereof" (application number: 03135951.5) discloses a method for preparing a conductive composite material in a certain proportion with a polyolefin and carbon black; The Chinese patent "Preparation Method of Polyethylene Positive Temperature Coefficient Material" (Application No.: 95104914.3) discloses a method for preparing PTC material with high-density polyethylene and carbon black. The disadvantage of these two methods is that the content of carbon black used exceeds 20% by mass, and the percolation threshold of the composite material system is relatively large, and the previous patent also needs microwave radiation for the composite material system. Such a high filling of carbon black will bring some adverse effects, such as increasing the melt viscosity of the system and deteriorating processing performance; reducing the impact strength of the material; carbon black particles are easy to fall off, causing pollution to the clean room, etc. Microwave radiation is performed on the system, which increases the preparation process and increases the cost of materials.

The present invention utilizes the theory of "dual conductive percolation threshold", the conductivity of the CB filled blend polymer matrix presents a double percolation effect, the conductive particles are not completely uniformly dispersed in the polymer, but are preferentially distributed through most of the conductive particles Conductive pathways are formed in one of the polymers or at the interface of two polymers, thereby lowering the percolation threshold of the composite. That is, the conductivity of the composite material depends on two factors: one is the concentration of CB in its enriched phase, and the other is the continuity of the structure of the CB-enriched phase. In other words, above the percolation threshold, the CB-rich phase transforms into a conductor, and the conductive phase forms a continuous conductive network structure in the multiphase polymer system, at which point the entire composite can transform into a conductor. Due to the concentration of the CB non-rich phase, the percolation threshold of CB in this type of system is lower than that of a single matrix. In this way, the content of carbon black will be reduced. Obviously, the double percolation effect of CB filled blend polymers can be used to fill less conductive fillers to make higher performance PTC materials, and at the same time solve the problem of high carbon black filling. The problem of decreased mechanical properties.

Contents of the invention

The object of the present invention is to provide a preparation method of a composite material with a positive temperature coefficient effect, which has simple process, low cost and good stability of the prepared conductive composite material.

One of the preparation methods of a conductive composite material with positive temperature coefficient effect of the present invention comprises the following steps:

(1) Melt and blend the two polymer blends with the conductive filler in a twin-screw extruder or internal mixer for 5 to 20 minutes. The melting temperature is higher than that of the polymer with the highest melting point in the polymer matrix used. 10-50°C higher melting point;

(2) The blend is molded into a film in a flat vulcanizer.

The polymer is a blend of two polymers with a large difference in melt index or a large difference in polarity, and these polymers include low-density polyethylene, high-density polyethylene, polypropylene, polyvinyl chloride , nylon, polyurethane, polystyrene, polymethyl methacrylate, ethylene terephthalate and polybutylene teremalonate;

Described polypropylene (PP) and low-density polyethylene (LDPE) mass ratio are 30-70: 70-30;

The mass ratio of the polymethyl methacrylate (PMMA) to polystyrene (PS) is 30-70: 70-30;

The conductive filler is conductive carbon black, conductive tin oxide or conductive zinc oxide, and the particle size range of the conductive filler is 10-100nm, or carbon nanofibers, carbon nanotubes, with a diameter of 1-50nm. The weight content in the polymer is 2%-17%.

The carbon black can be combined with carbon nanotubes, carbon fibers, etc. to form a composite conductive filler. When the carbon black mass content is 5-9%, the carbon black particle size is 29-90nm.

The blending sequence of the polymer and carbon black can be changed to obtain different types of conductive materials with different PTC strengths.

Beneficial effect

The advantage of the present invention is that: the low percolation threshold conductive composite material can be produced by the method adopted in the present invention. In addition, the method has the advantages of simple process, low cost, no pollution, safety and environmental protection. The prepared conductive composite material with positive temperature coefficient effect has low percolation threshold, good processing performance and mechanical properties, and good PTC strength, which can be used in telecommunications engineering, self-controlling heaters, current limiters, and circuit overload protection. device etc.

Detailed ways

Below in conjunction with specific embodiment, further illustrate the present invention. It should be understood that these examples are only used to illustrate the present invention and are not intended to limit the scope of the present invention. In addition, it should be understood that after reading the teachings of the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms also fall within the scope defined by the appended claims of the present application.

Example 1

When polypropylene and low-density polyethylene are 40/60 and carbon black mass content is 7%, carbon black (particle diameter is 29nm) and low-density polyethylene and polypropylene are put together in internal mixer melt blending 12 ~15 minutes, the melting temperature is 180°C, the blend is molded into a film in a flat vulcanizer, the film thickness is 150-300 μm, and the PTC strength of the obtained conductive material is 3.9×10 ³ .

Example 2

When polypropylene and low-density polyethylene were 70/30 and carbon black mass content was 5%, put carbon black (particle size is 29nm) and low-density polyethylene and polypropylene together in the internal mixer and melt blending 12 ~15 minutes, the melting temperature is 180°C, the blend is molded into a film in a flat vulcanizer, the film thickness is 150-300 μm, and the PTC strength of the obtained conductive material is 2.3×10 ⁴ .

Example 3

When polypropylene and low-density polyethylene are 70/30 and carbon black mass content is 5%, first with low-density polyethylene and particle diameter be the carbon black of 29nm melt blending 6～7 minutes in internal mixer, then Add polypropylene and melt blend for 5-6 minutes, the melting temperature is 180°C, the blend is molded in a flat vulcanizer to form a film with a film thickness of 150-300 μm, and the PTC strength of the obtained conductive material is 1.6×10 ⁴ .

Example 4

When the ratio of polypropylene and low-density polyethylene is 70/30 and the mass content of carbon black is 5%, the polypropylene and the carbon black with a particle size of 29nm are melt-blended for 6 to 7 minutes in a banbury mixer, and then low-density polyethylene is added. The density polyethylene is blended for 5-6 minutes, the melting temperature is 180°C, and the blend is molded in a flat vulcanizing machine to form a film with a film thickness of 150-300 μm. The PTC strength of the prepared conductive material was 1.5×10 ⁵ .

Example 5-12

The implementation method is the same as in Example 1, and other conditions are shown in the specific examples.

Example number PP/LDPE (mass ratio) Carbon black particle size (nm) Adding order Carbon black content (mass fraction)   Melting temperature (°C) PTC strength 5 70/30 65 PP+LDPE+CB 6% 180 3.8×10 ⁵ 6 70/30 65 (LDPE+CB)+PP 6% 180 5.6×10 ⁴ 7 70/30 65 (PP+CB)+LDPE 6% 180 3.6×10 ⁶ 8 70/30 90 PP+LDPE+CB 6% 180 1.6×10 ⁶ 9 70/30 90 (LDPE+CB)+PP 6% 180 2.5×10 ⁵ 10 70/30 90 (PP+CB)+LDPE 6% 180 2.9×10 ⁷ 11 50/50 29 PP+LDPE+CB 7% 180 5.3×10 ² 12 30/70 29 PP+LDPE+CB 7% 180 1.4×10 ²

Example 13

When polypropylene and low-density polyethylene are 70/30, replace carbon black with the mixed conductive filler of carbon nanotube and carbon black (particle diameter is 29nm) that mass ratio is 1/4, and the mass content of mixed conductive filler is 5% , melt blending in an internal mixer for 12 to 15 minutes, the melting temperature is 180°C, the blend is molded into a film in a flat vulcanizer, the film thickness is 150 to 300 μm, and the PTC strength of the obtained conductive material is 2.3×10 ⁴ .

Examples 14-21

The implementation method is the same as in Example 13, and other conditions are shown in the specific examples.

Example number PS/PMMA (mass ratio) Carbon black particle size (nm) Adding order Carbon black content (mass fraction) Melting temperature (℃) PTC Strength 14 65/35 29 PS+PMMA+CB 9% 245 1.7×10 ³ 15 65/35 29 (PMMA+CB)+PS 9% 245 9.7×10 ² 16 65/35 29 (PS+CB)+PMMA 9% 245 5.3×10 ⁴ 17 65/35 65 PS+PMMA+CB 8% 245 7.8×10 ³ 18 65/35 65 (PMMA+CB)+PS 8% 245 1.6×10 ³ 19 65/35 65 (PS+CB)+PMMA 8% 245 8.6×10 ⁴ 20 50/50 29 PS+PMMA+CB 8% 245 7.6×10 ² twenty one 40/60 29 PS+PMMA+CB 9% 245 2.4×10 ²

Example 22

When polymethyl methacrylate and polystyrene are 70/30 and conductive tin oxide (particle size is 72nm) mass content is 15%, polymethyl methacrylate and polystyrene and conductive tin oxide are together Melt and blend in an internal mixer for 12 to 15 minutes, the melting temperature is 245°C, the blend is molded into a film in a flat vulcanizer, the film thickness is 150 to 300 μm, and the PTC strength of the obtained conductive material is 5.1×10 ⁴ .

Example 23

The implementation method is the same as 22, and the conductive tin oxide is replaced by conductive zinc oxide (particle size: 36nm), and the mass content of conductive zinc oxide is 13%, and the PTC strength of the obtained conductive material is 6.8×10 ² .

Claims (8)

1. preparation method with positive temperature coefficient effect conducing composite material is characterized in that may further comprise the steps:

(1) two kinds of blend polymers were placed in twin screw extruder or the Banbury mixer melt blending 5～20 minutes with conductive filler material, melt temperature is higher 10～50 ℃ than the fusing point of the polymkeric substance that fusing point is the highest in the used polymeric matrix;

(2) blend mold pressing film forming in vulcanizing press.

2. a kind of preparation method with positive temperature coefficient effect conducing composite material according to claim 1 is characterized in that: polymkeric substance is selected from new LDPE (film grade), high density polyethylene(HDPE), polypropylene, polyvinyl chloride, nylon, urethane, polystyrene, poly-methyl methacrylate polyester, to the third dioctyl phthalate second diester, poly-a kind of in the third dioctyl phthalate fourth diester.

3. a kind of preparation method with positive temperature coefficient effect conducing composite material according to claim 2 is characterized in that: described polypropylene and new LDPE (film grade) mass ratio are 30-70: 70-30.

4. a kind of preparation method with positive temperature coefficient effect conducing composite material according to claim 2 is characterized in that: described poly-methyl methacrylate polyester and polystyrene mass ratio are 30-70: 70-30;

5. a kind of preparation method according to claim 1 with positive temperature coefficient effect conducing composite material, it is characterized in that: described conductive filler material is conductive carbon black, conductive tin oxide or conductive zinc oxide, conductive filler material particle diameter model 10-100nm, the weight content in polymkeric substance is 2%～17%.

6. a kind of preparation method with positive temperature coefficient effect conducing composite material according to claim 1 is characterized in that: described conductive filler material is carbon nano fiber or CNT (carbon nano-tube), and diameter 1-50nm, the weight content in polymkeric substance are 2%～17%.

7. a kind of preparation method according to claim 5 with positive temperature coefficient effect conducing composite material, it is characterized in that: described carbon black can cooperate the formation complex conductive fillers with carbon nanotube, carbon fiber etc., when the carbon black mass content was 5-9%, black grain diameter was 29-90nm.

8. a kind of preparation method with positive temperature coefficient effect conducing composite material according to claim 1 is characterized in that: the blend order of described polymkeric substance and carbon black can change arbitrarily.