CN111234429A - PTFE/boron nitride composite material and preparation method thereof - Google Patents

Abstract

The invention discloses a PTFE/boron nitride composite material and a preparation method thereof. The mixed powder is first obtained by liquid phase mixing, and then cold-pressed and sintered to obtain a composite material. The following steps are included in sequence: 1) Stripping of boron nitride (BN): the hexagonal boron nitride powder is treated with molten citric acid for 24 hours, washed with water until neutral, and then filtered and dried to obtain boron nitride nanosheets (n-BN ); 2) Mixing of polytetrafluoroethylene (PTFE) and n-BN: Disperse PTFE powder and boron nitride nanosheets (n-BN) in silicone oil to form a stable emulsion, and then mix them evenly by vigorous stirring, filter , washed with dichloromethane, and dried to obtain mixed powder; 3) Cold pressing and sintering: put the mixed powder into a molding die, cold-press under a certain pressure, take out the parison, place it in a sintering furnace, press A certain temperature curve is sintered to finally obtain a PTFE/boron nitride composite material. The liquid phase mixing method adopted in the present invention effectively improves the mixing uniformity, and the prepared PTFE/boron nitride composite material has excellent wear resistance and thermal conductivity.

Description

A kind of PTFE/boron nitride composite material and preparation method thereof

technical field

The invention relates to a PTFE/boron nitride composite material and a preparation method thereof, in particular to a method for preparing a PTFE/boron nitride composite material by liquid phase mixing combined with cold pressing and sintering.

Background technique

Polytetrafluoroethylene (PTFE) has excellent chemical stability, radiation resistance, dielectric properties, and extremely low coefficient of friction and self-lubrication, making it one of the most advanced and widely used resins in fluoroplastics. It not only has a wide range of applications in machinery, chemical engineering and electrical and electronic fields, but also has great application value in aerospace and military fields. However, due to the structural characteristics of PTFE molecules, the attraction between macromolecules is small, and the band-shaped crystals are easily peeled off by flakes, thus showing poor mechanical properties, poor creep resistance, easy cold flow, poor resilience, no wear resistance, no wear resistance. The shortcomings of electrical conductivity, non-thermal conductivity, and large coefficient of linear expansion greatly limit its wide application in some industries. Therefore, in-depth research on the structure and physicochemical properties of PTFE, especially the development of new PTFE materials with excellent comprehensive properties through chemical and physical modification, has become the main direction of current PTFE research and development. Inorganic nanofiller filling modification is one of the most commonly used modification methods of PTFE.

The inorganic nanofillers currently used to fill modified PTFE mainly include nanoscale diamond, CaCO ₃ , CaTiO ₃ , carbon nanotubes, and graphene. Marcus et al. filled PTFE with glass fiber, and the friction and wear properties of the composite were improved. However, Lim et al. added 2% nanodiamond to PTFE to obtain the smallest friction coefficient and wear damage. Cai Xiong et al. used modified _CaCO3 to fill PTFE in order to improve its mechanical properties. However, these reports all have the problem that the filler and the PTFE matrix are not mixed uniformly enough. Song Yongyao et al. improved the mixing uniformity of filler and matrix by adding coupling agent, but the addition of coupling agent complicates the preparation process and aggravates environmental pollution, and in the subsequent PTFE sintering process, high temperature conditions may lead to coupling decomposition of the agent. Jian Xuezhen et al. used graphene to fill PTFE. First, graphene is carboxylated and aminated, and then compounded with PTFE. The electron cloud on the surface of the power supply group will be biased towards fluorine atoms, that is, towards PTFE. Under the action of van der Waals force, the electron cloud flow generated by the induced effect strengthens the interfacial adhesion between graphene and PTFE, which can improve the performance of PTFE to a certain extent. But this intermolecular force is relatively small, so the performance improvement is limited.

As a two-dimensional material with a structure similar to graphene, hexagonal boron nitride nanosheets (n-BN) are a very promising filler material with good thermal conductivity, which is superior to most metals and ceramics Material. And with the decrease of sheet thickness, the value of thermal conductivity increases, and the thermal conductivity of single-layer BN nanosheets is greater than that of multilayer and bulk BN. In addition, like graphene, n-BN is also a nanomaterial with very good mechanical strength. It is precisely because of its extremely high thermal conductivity and good mechanical properties that n-BN has been widely used as a two-dimensional inorganic filler for organic polymers to improve the thermal conductivity and mechanical properties of organic polymers.

Hexagonal boron nitride can have a good modification effect on PTFE, but at present, the main use is bulk BN, which is easy to cause the uneven structure of the composite material to become a stress concentration point, which reduces the performance of the material. In addition, the reported BN-filled modified PTFE basically uses high-speed mechanical mixing of two raw materials, and then cold-presses and sinters the mixed powder. Mechanical mixing is difficult to be uniform, and during the sintering process, the PTFE molecular chain will undergo thermal motion, which may cause the aggregation of BN, resulting in the inhomogeneity of the composite structure and affecting the performance of the composite.

According to the existing problems in the prior art, in order to improve the mechanical properties, friction and wear properties and thermal conductivity of PTFE, this project plans to use BN nanosheets to modify PTFE: (1) Using molten citric acid to heat treat block BN, it can be The breaking of the B-N bond results in a large number of B and N vacancy defects, which is conducive to the formation of -OH and -NH- functional groups at the defects and edges of the boron nitride nanosheets, and the spacing between the boron nitride layers is stretched. Nanosheets. (2) The large electronegativity of fluorine atoms in PTFE and hydrogen atoms in silicone oil is poor, causing strong dipole interaction, so that PTFE powder can be dispersed in silicone oil to form a stable emulsion. Boron nitride can also be dispersed in silicone oil to form an emulsion. Liquid phase mixing is more conducive to uniform dispersion than solid phase mixing. In addition, the B atoms on the exfoliated n-BN surface have empty orbitals, and the fluorine atoms of PTFE have unpaired electrons, so the two can produce coordination, which is also conducive to the uniform mixing of PTFE and n-BN. (3) Due to the very high viscosity of PTFE when it is melted, the composite material is prepared by the molding process of cold pressing and sintering. In the composite material thus obtained, the dispersion effect of PTFE and n-BN is better, and a composite material with excellent performance can be obtained.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a method for preparing PTFE/boron nitride composite material with liquid phase mixing combined with cold pressing and sintering, which is made up of the following components by mass percentage:

PTFE: 90-99 wt%;

Boron Nitride: 1-10 wt%.

It is characterized in that, the preparation method of described PTFE/boron nitride composite material comprises:

(1) Stripping of boron nitride (BN)

The hexagonal boron nitride powder was treated with molten citric acid for 24h, washed with water until neutral, filtered and dried to obtain boron nitride nanosheets (n-BN);

(2) Liquid phase mixing of PTFE and n-BN

Weigh PTFE and n-BN in proportion, respectively disperse PTFE powder and boron nitride nanosheets (n-BN) in silicone oil to form a stable emulsion, then uniformly mix, filter, wash with dichloromethane, and dry to obtain mixed powder;

(3) Cold pressing and sintering

Put the mixed powder into the mold, carry out cold pressing under pressure, and hold the pressure for a period of time; then put the blank into the sintering furnace, and sinter it according to a certain temperature control program. After the sintering is completed, the product is obtained by cooling with the furnace.

The above-mentioned PTFE/boron nitride composite material is characterized in that: the particle size of the hexagonal boron nitride is 1-10 μm.

The above-mentioned PTFE/boron nitride composite material is characterized in that: the polytetrafluoroethylene is a suspended fine powder with a particle size of 20 μm-80 μm.

The above-mentioned PTFE/boron nitride composite material is characterized in that: in the step 1), the reaction temperature is 160-180°C.

The above-mentioned PTFE/boron nitride composite material is characterized in that: in the step 2), the mass fraction of n-BN is 1-10 wt%.

The above-mentioned PTFE/boron nitride composite material is characterized in that: in the step 3), the cold pressing pressure is 100-140MPa.

The above-mentioned PTFE/boron nitride composite material is characterized in that: in the step 3), during the sintering process, the temperature control procedure is as follows:

I. The heating rate is 1.5°C/min, the target temperature is 130°C, and the temperature is kept for 1h;

II. The heating rate is 1.4°C/min, the target temperature is 250°C, and the temperature is kept for 1h;

III. The heating rate is 1.1°C/min, the target temperature is 327°C, and the temperature is kept for 2h;

IV. The heating rate is 1°C/min, the target temperature is 345°C, and the temperature is kept for 1h;

V. The heating rate is 0.4°C/min, the target temperature is 375°C, and the temperature is kept for 1-5h;

Ⅵ. Cool with the furnace.

Detailed ways

The present invention is further described below with reference to specific embodiments, but the protection scope of the present invention is not limited thereto.

Examples 1-4, its components and process parameters are shown in Table 1.

The preparation method of the composite material is as follows:

(1) Stripping of boron nitride (BN)

The hexagonal boron nitride powder was treated with molten citric acid for 24 hours, washed with water until neutral, then filtered and dried to obtain hexagonal boron nitride nanosheets (n-BN);

(2) Liquid phase mixing of PTFE and n-BN

Weigh a certain amount of PTFE powder, mix it with silicone oil, and stir it mechanically for about 1 hour to make the PTFE evenly dispersed in the silicone oil; also weigh a certain amount of n-BN powder, and use silicone oil as a medium to form a good dispersion under the action of mechanical stirring. liquid. Finally, the two dispersions are mixed together to form a homogeneously dispersed mixed liquid. The uniformly stirred dispersion liquid is subjected to suction filtration, after suction filtration to a certain extent, the residual silicone oil in the powder is washed with dichloromethane, and after the mixed powder is basically suction filtered and dried, it is then placed in an iron pan for further drying. Bake in an oven at 150 °C for 4-5 hours;

(3) Cold pressing and sintering

Weigh a certain amount of completely dried mixed powder and spread it evenly in the mold. The purpose is to hope that the sample will be uniformly stressed during the pressing process and the green body density will be uniform, and then press it on a flat vulcanizer with a certain pressure. After the molding is finished, take out the sample from the mold. As the most critical step in the whole sample preparation process is sintering, the sintering process is the most direct factor affecting the properties of composite materials. After many experiments, the following sintering process is more appropriate:

1) Due to the large circular holes on the iron plate of the sintering furnace, in order to avoid affecting the sintering process of the sample, an iron sheet with a diameter of 0.8 mm and a thickness of 0.8 mm was placed under the sample. Turn on the sintering furnace and turn on the turntable and blast at the same time to ensure that the sample is heated evenly in the sintering furnace;

2) Because of the poor thermal conductivity of PTFE, if the heating rate during sintering is not strictly controlled, it will have a greater impact on the sample. Therefore, during the whole experimental sintering process, the temperature control procedure is as follows:

I. The heating rate is 1.5°C/min, the target temperature is 130°C, and the temperature is kept for 1h;

II. The heating rate is 1.4°C/min, the target temperature is 250°C, and the temperature is kept for 1h;

III. The heating rate is 1.1°C/min, the target temperature is 327°C, and the temperature is kept for 2h;

IV. The heating rate is 1°C/min, the target temperature is 345°C, and the temperature is kept for 1h;

V. The heating rate is 0.4°C/min, the target temperature is 375°C, and the temperature is kept for 1-5h;

Ⅵ. Cool with the furnace.

(4) The performance test of the finished product is carried out, and the test results are shown in Table 2. It can be concluded that the prepared PTFE/boron nitride composite material has excellent wear resistance and thermal conductivity.

Table 1 The components and process parameters of the implementation case

project Example 1 Example 2 Example 3 Example 3 PTFE (wt%) 99 90 92 95 n-BN (wt%) 1 10 8 5 Molding pressure (MPa) 130 140 130 120 375℃ holding time (h) 2 2 2 5

Table 2 Implementation case product performance test

project Example 1 Example 2 Example 3 Example 4 Tensile strength (MPa) 24.75 22.75 23.25 24.50 Impact strength (MPa) 15.1 14.5 14.8 15.8 Wear performance (g) 0.0253 0.0059 0.0136 0.0150 Thermal conductivity (W·m^-1·K^-1) 0.4277 0.4478 0.4413 0.4436

Claims (7)

1. a PTFE/boron nitride composite material and preparation method thereof, is characterized in that: adopt the method of liquid phase mixing to obtain uniform mixture, and then carry out cold pressing and sintering to obtain composite material. The specific method steps are as follows: 1) Stripping of boron nitride (BN) The hexagonal boron nitride powder was treated with molten citric acid for 24 hours, washed with water until neutral, filtered and dried to obtain boron nitride nanosheets (n-BN); 2) Liquid phase mixing of polytetrafluoroethylene (PTFE) and boron nitride nanosheets (n-BN) Disperse polyPTFE powder and boron nitride nanosheets (n-BN) in silicone oil respectively to form a stable emulsion, then uniformly mix, filter, wash with dichloromethane, and dry to obtain mixed powder; 3) Cold pressing and sintering Put the mixed powder into a mold and press under a certain pressure to obtain a blank; then put the blank into a sintering furnace, and sinter the temperature according to a certain program to finally obtain a PTFE/boron nitride composite material. 2 . The PTFE/boron nitride composite material according to claim 1 , wherein the particle size of the boron nitride is 1 μm-10 μm. 3 . 3 . The PTFE/boron nitride composite material according to claim 1 , wherein the PTFE is a suspended fine powder with a particle size of 20 μm-80 μm. 4 . 4 . The PTFE/boron nitride composite material according to claim 1 , wherein in the step 1), the reaction temperature is 160-180° C. 5 . 5 . The PTFE/boron nitride composite material according to claim 1 , wherein in the step 2), the mass fraction of n-BN is 1-10 wt %. 6 . 6 . The PTFE/boron nitride composite material according to claim 1 , wherein in the step 3), the cold pressing pressure is 100-140 MPa. 7 . 7. The PTFE/boron nitride composite material according to claim 1, characterized in that: in the step 3), during the sintering process, the temperature control procedure is as follows: I. The heating rate is 1.5°C/min, the target temperature is 130°C, and the temperature is kept for 1h; II. The heating rate is 1.4°C/min, the target temperature is 250°C, and the temperature is kept for 1h; III. The heating rate is 1.1°C/min, the target temperature is 327°C, and the temperature is kept for 2h; IV. The heating rate is 1°C/min, the target temperature is 345°C, and the temperature is kept for 1h; V. The heating rate is 0.4°C/min, the target temperature is 375°C, and the temperature is kept for 1-5h; Ⅵ. Cool with the furnace.