CN110216826B - Foamed polypropylene with oriented pores and capable of being used for piezoelectric material and preparation method thereof - Google Patents

Foamed polypropylene with oriented pores and capable of being used for piezoelectric material and preparation method thereof Download PDF

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CN110216826B
CN110216826B CN201910581575.XA CN201910581575A CN110216826B CN 110216826 B CN110216826 B CN 110216826B CN 201910581575 A CN201910581575 A CN 201910581575A CN 110216826 B CN110216826 B CN 110216826B
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polypropylene
pressure
temperature
mold
foaming
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CN110216826A (en
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曹堃
姜自印
姚臻
徐佳慧
龚昌景
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Zhejiang University ZJU
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Zhejiang University ZJU
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
    • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
    • B29C44/00—Shaping by internal pressure generated in the material, e.g. swelling or foaming ; Producing porous or cellular expanded plastics articles
    • B29C44/02—Shaping by internal pressure generated in the material, e.g. swelling or foaming ; Producing porous or cellular expanded plastics articles for articles of definite length, i.e. discrete articles
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
    • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
    • B29C44/00—Shaping by internal pressure generated in the material, e.g. swelling or foaming ; Producing porous or cellular expanded plastics articles
    • B29C44/34—Auxiliary operations
    • B29C44/36—Feeding the material to be shaped
    • B29C44/38—Feeding the material to be shaped into a closed space, i.e. to make articles of definite length
    • B29C44/42—Feeding the material to be shaped into a closed space, i.e. to make articles of definite length using pressure difference, e.g. by injection or by vacuum
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N30/00—Piezoelectric or electrostrictive devices
    • H10N30/80—Constructional details
    • H10N30/85—Piezoelectric or electrostrictive active materials
    • H10N30/857—Macromolecular compositions
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
    • B29K—INDEXING SCHEME ASSOCIATED WITH SUBCLASSES B29B, B29C OR B29D, RELATING TO MOULDING MATERIALS OR TO MATERIALS FOR MOULDS, REINFORCEMENTS, FILLERS OR PREFORMED PARTS, e.g. INSERTS
    • B29K2023/00—Use of polyalkenes or derivatives thereof as moulding material
    • B29K2023/10—Polymers of propylene
    • B29K2023/12—PP, i.e. polypropylene

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  • Physics & Mathematics (AREA)
  • Spectroscopy & Molecular Physics (AREA)
  • Manufacture Of Porous Articles, And Recovery And Treatment Of Waste Products (AREA)

Abstract

The invention discloses a preparation method of foamed polypropylene with oriented holes and capable of being used for piezoelectric materials, which comprises the steps of firstly melting polypropylene, and then uniformly mixing the polypropylene with supercritical fluid to obtain uniformly mixed melt; the mass ratio of the polypropylene to the supercritical fluid is 1: 0.005-0.05; injecting the uniformly mixed melt into a mold with an inner cavity filled with supercritical carbon dioxide, wherein the pressure in the inner cavity of the mold is 8-30 Mpa, after the uniformly mixed melt is injected, keeping the mold at 80-140 ℃ for 2-120 min, and then heating to 150-170 ℃ for 30-240 min to foam; and then releasing the pressure, cooling and shaping to obtain the foamed polypropylene with oriented pores and capable of being used for the piezoelectric material. The method can prepare the foaming material with high orientation degree, and the static pressure electric coefficient of the foaming material can reach about 590pC/N after the electric polarization treatment.

Description

Foamed polypropylene with oriented pores and capable of being used for piezoelectric material and preparation method thereof
Technical Field
The invention relates to a technology for preparing foamed polypropylene with oriented pores and capable of being used for piezoelectric materials, in particular to a technology for changing the thermal history of polypropylene in a supercritical fluid atmosphere, inducing to generate perfect crystallization with tightly piled molecular chains, foaming in a limited space to obtain polypropylene foam with an oriented cell structure, and then electrically polarizing the polypropylene foam to obtain the piezoelectric materials with excellent performance.
Background
Polypropylene of the formula (C)3H6) n, the melting point is 164-170 ℃, and the high-transparency material can be used for preparing household appliances, pipes, high-transparency materials and films.
Polypropylene foams have their own performance advantages in applications, but the preparation of polypropylene foams with ideal cells is currently the most difficult. The melt strength of common polypropylene (such as linear isotactic polypropylene) is very low, when the temperature reaches the crystallization melting temperature, the melt strength of a matrix is reduced sharply, the foaming temperature window is narrow, and the too low melt strength makes the cell wall difficult to resist external force, so that the cells collapse and break. It is necessary to improve the process for improving the foaming properties of polypropylene.
The method mainly comprises two methods for improving the foaming performance of PP and widening the foaming temperature range of PP, wherein the method comprises the steps of improving the melt strength of a PP matrix, including long-chain branch modification and blending modification; and the other is to improve the foaming technical strategy. The modification and improvement of the melt strength of the linear isotactic polypropylene (iPP) have obvious effect on the preparation of high-quality polypropylene foam, but the method improves the cost of the PP, and the introduction of the blend can influence other properties of the foaming material, so that the method is more valuable for preparing the high-quality polypropylene foam by directly utilizing the universal iPP with low melt strength. Chinese patent (publication No. CN101580599A) discloses that macromolecular polymer is used as raw material, and the defects of long saturation time and small solubility in the prior foaming technology are overcome by changing the foaming process, so that the production efficiency is greatly improved, and the cell structure is improved.
The piezoelectric material is an intelligent material which can convert mechanical energy into electric energy through a special material structure of the piezoelectric material under the action of external pressure. The piezoelectric materials used at the earliest are mainly inorganic piezoelectric materials represented by quartz crystals, inorganic ceramics, and the like, and thereafter, organic polymer piezoelectric materials represented by polyvinylidene fluoride (PVDF) have appeared. In the nineties of the twentieth century, the finnish scientist Kari Kirjavainen charged the prepared PP pores to prepare a first pore membrane with piezoelectric properties. The hole-type piezoelectric material has high piezoelectric coefficient, good flexibility and low cost, so the research heat tide of the hole-type piezoelectric material is raised after the report. Through the development of the last two decades, researchers have conducted detailed research on the preparation process and optimization method of the porous piezoelectric material, and developed novel porous piezoelectric materials such as PET, PEN, FEP, PTFE and the like and novel preparation methods represented by a die pressing method. The most widely used and commercialized materials are the foamed polypropylene piezoelectric materials. Piezoelectric polypropylene foams reported in the literature have a static piezoelectric constant (d) at ambient temperature after a long period of stabilization33) About 200 pc/N.
Disclosure of Invention
The technical problem to be solved by the invention is to provide a preparation method of foamed polypropylene with oriented holes and capable of being used for piezoelectric materials, the foamed material with high orientation degree can be prepared by the method, and the highest static pressure electric coefficient can reach about 590pC/N after electric polarization treatment.
In order to solve the above technical problems, the present invention provides a method for preparing a foamed polypropylene having oriented pores, which can be used for a piezoelectric material, comprising:
firstly, melting polypropylene, and then uniformly mixing the polypropylene with supercritical fluid to obtain uniformly mixed melt; the mass ratio of the polypropylene to the supercritical fluid is 1: 0.005-0.05;
injecting the uniformly mixed melt into a mold with an inner cavity filled with supercritical carbon dioxide, wherein the pressure in the inner cavity of the mold is 8-30 Mpa, after the uniformly mixed melt is injected, keeping the mold at 80-140 ℃ for 2-120 min, and then heating to 150-170 ℃ for 30-240 min to foam; and then releasing the pressure, cooling and shaping to obtain the foamed polypropylene with oriented pores and capable of being used for the piezoelectric material.
The mould is a limited mould, and polypropylene can only grow in the maximum two dimensions of three dimensions XYZ in the mould; that is, the restricted mold restricts the growth direction of cells at the time of foaming, so that polypropylene grows in one or two dimensions.
As an improvement of the preparation method of the foamed polypropylene with oriented pores for piezoelectric materials of the present invention:
the melting temperature is 170-220 ℃ (preferably 170-180 ℃).
Description of the drawings: the melting temperature is selected so that it is higher than the crystalline melting temperature of the polypropylene, so that the crystals in the polypropylene matrix melt and can be processed.
As a further improvement of the preparation method of the expanded polypropylene with oriented pores for piezoelectric materials of the invention:
the injection temperature is 150-180 deg.C (preferably 150-170 deg.C), and the injection pressure is 8.5-30.5 MPa.
Description of the drawings: the injection temperature and injection pressure selection criteria were as follows: the supercritical fluid can be uniformly mixed with the polypropylene matrix in a short time without phase separation. That is, the polypropylene melt has an appropriate viscosity and can flow under the plasticizing action of the supercritical fluid.
This is the temperature and pressure of the melt prior to injection.
As a further improvement of the preparation method of the expanded polypropylene with oriented pores for piezoelectric materials of the invention:
the temperature of the die is 80-140 ℃ (preferably 80-120 ℃); the pressure of the die is 8-30 MPa.
Description of the drawings: the selection principle of the original temperature and pressure of the die is as follows: under the plasticizing action of the supercritical fluid, the temperature and the pressure can induce the polypropylene to generate high-melting-point crystals. I.e. below the melting temperature of the polypropylene crystals, so that the polypropylene molecular chains can be piled up again to generate a certain amount of perfect crystals with high melting points.
The injection pressure needs to be greater than the mold pressure to complete the injection.
As a further improvement of the preparation method of the expanded polypropylene with oriented pores for piezoelectric materials of the invention: the temperature (namely, the foaming temperature) reached by raising the temperature again after the injection into the mold is 150-170 ℃ (preferably 150-160 ℃); the foaming pressure is 8-30 MPa.
Description of the drawings: the selection principle of the temperature reached by heating the uniformly mixed melt again after the melt is injected into the mold is as follows: in the temperature range, the polypropylene molecular chain has certain mobility, most of crystals are molten, but a certain amount of perfect crystals are remained, so that the melt strength of the polypropylene is enhanced. In the limited foaming process, the melt strength of the polypropylene is beneficial to cell growth and can support cell deformation.
The pressure will rise when the temperature rises, and when the foaming pressure is reached, the pressure relief valve needs to be adjusted to stabilize the foaming pressure.
As a further improvement of the preparation method of the expanded polypropylene with oriented pores for piezoelectric materials of the invention:
the supercritical fluid is supercritical CO2Supercritical N2Supercritical methanol, supercritical butane or supercritical methyl chloride.
The supercritical state fluid means that the temperature is higher than the supercritical temperature while the pressure is higher than the supercritical pressure, for example, supercritical CO2Refers to CO2The temperature of (A) is higher than 31.1 ℃ and the pressure is higher than 7.4 MPa.
In the invention, the rapid pressure relief refers to that high-pressure fluid in a supercritical state is subjected to instantaneous rapid pressure reduction through a pressure reduction device, such as a pressure reduction control valve, and the average pressure relief rate can reach 5-30 MPa/s.
The invention also provides the foamed polypropylene which is prepared by the method and has oriented holes and can be used for piezoelectric materials: volume expansion of 1.5-9 times, length diameter of pores of 20-200 μm, orientation degree of pores of 2-7, and pore density of 105～108Per cm3。
The invention also provides the application of the foamed polypropylene with oriented pores, which is prepared by the method and can be used for piezoelectric materials: firstly, coating electrodes on two sides of the foamed polypropylene, and then carrying out polarization treatment by using a high-voltage electric field, thereby obtaining the piezoelectric material.
The foamed polypropylene is first vacuum evaporated, ion sputtered or solution coated to coat electrodes on two sides of the foamed polypropylene.
The electric polarization treatment mode is a corona charging method or a contact charging method;
the corona charging method comprises the following steps: the voltage of corona charging is-150 to-5 kv or 5 to 150kv, the grid voltage is-500 to-10000 v or 500 to 10000v, the charging temperature is less than or equal to 120 ℃, the charging time is 5ms to 1h, and the distance between an electrode and a film is 1.5 to 10 cm;
the contact charging method comprises the following steps: connecting a power supply with high voltage to electrodes coated on two sides of the foamed polypropylene, and then applying a voltage of 4-10 kv or-4-10 kv to the electrodes for charging, wherein the charging time is 5 ms-1 h.
The technical conception of the invention is as follows: the invention envisages that the crystallization characteristic of polypropylene under the supercritical fluid atmosphere is utilized, the polypropylene is firstly swelled and permeated under certain pressure and temperature, most of original crystals in the polypropylene are melted, then the melted polypropylene is injected into a low-temperature limited mould for recrystallization, the generated crystals contain crystals with high melting points, so that the melt strength is improved, then the temperature required by foaming is increased for further swelling and permeation, the supercritical fluid is supersaturated through rapid pressure relief, and thus the porous foamed polypropylene with oriented pores is obtained through nucleation and foaming. And finally, plating electrodes on the prepared porous polypropylene foam, placing the porous polypropylene foam in an electric field, and applying proper voltage to carry out polarization treatment on the porous polypropylene foam so as to prepare the piezoelectric material.
The technical scheme of the invention is as follows: melting a polypropylene material in an extruder, adding a supercritical fluid, uniformly mixing the supercritical fluid and the polypropylene melt, and injecting the mixture into a limited mold with a lower temperature; cooling the melt to the temperature of the mold and recrystallizing for a certain time; and then heating to a foaming temperature for permeating and swelling for a period of time, quickly relieving the pressure to normal pressure, placing in an ice-water bath for cooling and shaping to obtain the polypropylene foaming material with uniformly oriented holes, finally plating an electrode on the polypropylene foaming material, applying a certain electric field, and taking out after charging to obtain the piezoelectric material with higher static pressure electric coefficient.
The invention takes polypropylene as raw material, overcomes the defect of melt strength of a polypropylene matrix in the foaming process by changing the foaming process and adopting a limited foaming mode, widens the foaming temperature window of the polypropylene, improves the cell structure and obtains uniform oriented cells. And then the prepared foam film with the oriented pores is polarized in an electric field, and the foam film is converted into a material with excellent piezoelectric property. After the foamed polypropylene with uniformly oriented holes is subjected to electric polarization treatment, the static pressure coefficient is 300-600 pC/N.
Compared with the existing foaming preparation method, the invention belongs to a variable-temperature limited foaming method, and overcomes the following technical defects in the existing foaming preparation method:
1. the foaming temperature operation window is narrow, and the quality of a foamed sample is unstable;
2. the foamed sample has a higher density and poor cell orientation.
Drawings
FIG. 1 is a scanning electron micrograph of a cross section of a polypropylene restricted foam sample of example 1;
FIG. 2 is a scanning electron micrograph of a cross section of a polypropylene restricted foam sample of example 2;
FIG. 3 is a scanning electron micrograph of a cross section of a polypropylene restricted foam sample of example 3;
FIG. 4 is a scanning electron micrograph of a cross section of a polypropylene restricted foam sample of example 4;
FIG. 5 is a scanning electron micrograph of a cross section of a polypropylene restricted foam sample of example 5;
FIG. 6 is a scanning electron micrograph of a cross section of a polypropylene restricted foam sample of comparative example 1.
Detailed Description
The single-screw extruder used in the following case had a nominal screw diameter of 30mm and a length-to-diameter ratio of 25. The single-screw extruder is divided into three sections, wherein the front section accounts for 1/3 of the total length; the middle section accounts for 1/3 and the rear section accounts for 1/3 of the total length; a main feed inlet is arranged at the starting position of the front section, and a lateral line feed inlet is arranged at the junction of the front section and the middle section; the polypropylene pellets enter the extruder through the main feed port and the supercritical carbon dioxide is fed into the extruder through a side feed port located at the full length of the main feed port 1/3.
The flat mold is a limited mold with limits in both XY directions, i.e., polypropylene can only be Z-long within the mold.
The thickness of the obtained polypropylene foam film is less than or equal to 500 mu m, preferably 100-200 mu m.
The following parts are all parts by weight.
Example 1
Melting 100 parts of polypropylene granules in the front section of a single-screw extruder, wherein the temperature of the front section is 190 ℃, and the retention time of materials in the front section is about 1.0 minute; adding 1.5 parts of supercritical carbon dioxide into the middle section, wherein the retention time of the materials in the middle section is 1.5 minutes; the back section is an injection section, polypropylene and supercritical carbon dioxide are uniformly mixed and then injected into a limited mould, the temperature is 175 ℃, the pressure is 10Mpa, and the retention time of the material in the injection section is 2.0 minutes; and discharging the uniformly mixed melt during injection.
Then injecting the uniformly mixed melt into a flat plate mold filled with a certain amount of supercritical carbon dioxide (filled with the supercritical carbon dioxide until the pressure meets the set pressure) at 175 ℃ and 10Mpa, keeping the mold at 100 ℃ and 8Mpa for 15min, and then heating to 165 ℃ and keeping for 100 min; finally, opening a pressure reducing valve to release pressure (the average pressure release rate is about 20Mpa/s) for foaming, and cooling the mould after pressure release to 20 ℃ so as to shape the polypropylene foam pores; obtaining the polypropylene foam film.
The resulting polypropylene foam films were subjected to the following performance tests:
1. the polypropylene foam film was coated on both sides with copper electrodes, and was electrically polarized in a contact charging manner by applying a voltage of 8kv between the electrodes, and was charged for 15 seconds, and the static pressure coefficient thereof was measured to be about 520pC/N at the highest.
2. Soaking the polypropylene foam film in liquid nitrogen for 30min, and characterizing the hole structure of the section by using a scanning electron microscope after brittle fracture, as shown in figure 1, the foaming multiplying power of a limited foaming polypropylene sample is 5.5, the length and diameter of the foam holes are 80-200 mu m, the orientation degree is 4.4, and the density of the foam holes is 8.2 × 107Per cm3。
Example 2
Melting 100 parts of polypropylene granules in the front section of a single-screw extruder, wherein the temperature of the front section is 175 ℃; adding 3.6 parts of supercritical carbon dioxide into the middle section; the back section is an injection section, polypropylene and supercritical carbon dioxide are uniformly mixed and then injected into a limited mould, the temperature is 168 ℃, and the pressure is 16 Mpa; and discharging the uniformly mixed melt during injection.
Then injecting the uniformly mixed melt into a flat plate mold filled with a certain amount of supercritical carbon dioxide (filled with the supercritical carbon dioxide until the pressure meets the set pressure) at 168 ℃ and 16Mpa, keeping the mold at 120 ℃ and 14Mpa for 4min, then heating to 163 ℃ and keeping the temperature for 240 min; finally, opening a pressure reducing valve to release pressure (the average pressure release rate is about 10Mpa/s) for foaming, and cooling the mould after pressure release to 20 ℃ so as to shape the polypropylene foam pores; obtaining the polypropylene foam film.
The resulting polypropylene foam films were subjected to the following performance tests:
1. the polypropylene foam film was coated on both sides with copper electrodes, and was electrically polarized in a contact charging manner by applying a voltage of 8kv between the electrodes, and was charged for 80 seconds, and the static pressure coefficient thereof was measured to be about 410pC/N at the highest.
2. Soaking the polypropylene foam film in liquid nitrogen for 30min, and characterizing the hole structure of the section by using a scanning electron microscope after brittle fracture, as shown in figure 2, the foaming multiplying power of a limited foaming polypropylene sample is 8.5, the length and diameter of the foam holes are 80-200 mu m, the orientation degree is 3.5, and the density of the foam holes is 6.5 × 107Per cm3。
Example 3
Melting 100 parts of polypropylene granules in the front section of a single-screw extruder, wherein the temperature of the front section is 180 ℃; adding 2.8 parts of supercritical carbon dioxide into the middle section; the back section is an injection section, polypropylene and supercritical carbon dioxide are uniformly mixed and then injected into a restricted mould, the temperature is 151 ℃, and the pressure is 28 Mpa; and discharging the uniformly mixed melt during injection.
Then injecting the uniformly mixed melt into a flat plate mold filled with a certain amount of supercritical carbon dioxide (filled with the supercritical carbon dioxide until the pressure meets the set pressure) at 151 ℃ and 28Mpa, keeping the mold temperature at 120 ℃ and the pressure at 25Mpa for 84min, then heating to 153 ℃ and keeping the temperature for 240 min; finally, opening a pressure reducing valve to release pressure (the average pressure release rate is about 15Mpa/s) for foaming, and cooling the mould after pressure release to 10 ℃ so as to shape the polypropylene foam pores; obtaining the polypropylene foam film.
The resulting polypropylene foam films were subjected to the following performance tests:
1. the polypropylene foam film was coated on both sides with platinum electrodes, and was electrically polarized in a contact charging manner by applying a voltage of 8kv between the electrodes, and was charged for 210s, and the static piezoelectric coefficient thereof was measured to be about 590pC/N at the highest.
2. Soaking the polypropylene foam film in liquid nitrogen for 30min, and characterizing the hole structure of the section by using a scanning electron microscope after brittle fracture, as shown in figure 3, the foaming multiplying power of a limited foaming polypropylene sample is 4.7, the length and diameter of the foam holes are 120-200 mu m, the orientation degree is 6.6, and the density of the foam holes is 4.0 × 107Per cm3。
Example 4
Melting 100 parts of polypropylene granules in the front section of a single-screw extruder, wherein the temperature of the front section is 220 ℃; adding 0.7 part of supercritical carbon dioxide into the middle section; the back section is an injection section, polypropylene and supercritical carbon dioxide are uniformly mixed and then injected into a restricted mould, the temperature is 159 ℃, and the pressure is 20 Mpa; and discharging the uniformly mixed melt during injection.
Then injecting the uniformly mixed melt into a flat plate mold filled with a certain amount of supercritical carbon dioxide (filled with the supercritical carbon dioxide until the pressure meets the set pressure) at 159 ℃ and 20Mpa, keeping the mold at 140 ℃ and 16Mpa for 2min, then heating to 158 ℃ and keeping the temperature for 120 min; finally, opening a pressure reducing valve to release pressure (the average pressure release rate is about 12Mpa/s) for foaming, and cooling the mould after pressure release to 20 ℃ so as to shape the polypropylene foam pores; obtaining the polypropylene foam film.
The resulting polypropylene foam films were subjected to the following performance tests:
1. the polypropylene foam film was coated on both sides with copper electrodes, and was electrically polarized in a contact charging manner by applying a voltage of 8kv between the electrodes, and was charged for 19min, and the static pressure coefficient thereof was found to be about 310pC/N at the highest.
2. Soaking the polypropylene foam film in liquid nitrogen for 30min, and characterizing the hole structure of the section by using a scanning electron microscope after brittle fracture, as shown in FIG. 4, the foaming multiplying power of a limited foaming polypropylene sample is 3.5, the length and diameter of the foam holes are 60-150 μm, the orientation degree is 3.8, and the density of the foam holes is 6.4 × 107Per cm3。
Example 5
Melting 100 parts of polypropylene granules in the front section of a single-screw extruder, wherein the temperature of the front section is 200 ℃; adding 2.0 parts of supercritical carbon dioxide into the middle section; the back section is an injection section, polypropylene and supercritical carbon dioxide are uniformly mixed and then injected into a restricted mould, the temperature is 156 ℃, and the pressure is 14 Mpa; and discharging the uniformly mixed melt during injection.
Then injecting the uniformly mixed melt into a flat plate mold filled with a certain amount of supercritical carbon dioxide (filled with the supercritical carbon dioxide until the pressure meets the set pressure) at 156 ℃ and 14Mpa, keeping the mold at 80 ℃ and 12Mpa for 10min, then heating to 157 ℃ and keeping for 240 min; finally, opening a pressure reducing valve to release pressure (the average pressure release rate is about 30Mpa/s) for foaming, and cooling the mould after pressure release to 15 ℃ to shape the polypropylene foam pores; obtaining the polypropylene foam film.
The resulting polypropylene foam films were subjected to the following performance tests:
1. the polypropylene foam film was coated on both sides with platinum electrodes, and was electrically polarized in a contact charging manner by applying a voltage of 8kv between the electrodes, and was charged for 10ms, and the highest static pressure coefficient thereof was measured to be about 480 pC/N.
2. Soaking the polypropylene foam film in liquid nitrogen for 30min, and characterizing the hole structure of the section by using a scanning electron microscope after brittle fracture, as shown in FIG. 5, the foaming multiplying power of a limited foaming polypropylene sample is 6.4, the length and diameter of the foam holes are 50-200 μm, the orientation degree is 4.1, and the density of the foam holes is 4.5 × 107Per cm3。
Comparative example 1
The raw materials described in example 3 are prepared according to the existing isothermal foaming preparation method, and the specific process parameters are as follows:
melting 100 parts of polypropylene granules in the front section of a single-screw extruder, wherein the temperature of the front section is 180 ℃; adding 2.8 parts of supercritical carbon dioxide into the middle section; the back section is an injection section, polypropylene and supercritical carbon dioxide are uniformly mixed and then injected into a restricted mould, the temperature is 151 ℃, and the pressure is 28 Mpa; and discharging the uniformly mixed melt during injection.
Then injecting the uniformly mixed melt into a flat plate mold filled with a certain amount of supercritical carbon dioxide (filled with the supercritical carbon dioxide until the pressure meets the set pressure) at 151 ℃ and 28Mpa, wherein the mold temperature is 155 ℃, the pressure is 25Mpa, and the mold temperature is kept for 324 min; finally, opening a pressure reducing valve to release pressure (the average pressure release rate is about 15Mpa/s) for foaming, and cooling the mould after pressure release to 10 ℃ so as to shape the polypropylene foam pores; obtaining the polypropylene foam film.
The resulting expanded polypropylene was prepared into a piezoelectric material according to the method described in example 3, and the static piezoelectric coefficient was found to be about 100pC/N, as shown in FIG. 6 by scanning electron microscopy.
Finally, it is also noted that the above-mentioned lists merely illustrate a few specific embodiments of the invention. It is obvious that the invention is not limited to the above embodiments, but that many variations are possible. All modifications which can be derived or suggested by a person skilled in the art from the disclosure of the present invention are to be considered within the scope of the invention.

Claims (5)

1. The preparation method of the foamed polypropylene with oriented holes and capable of being used for piezoelectric materials is characterized by comprising the following steps of:
firstly, melting polypropylene at the temperature of 170-220 ℃; then uniformly mixing the mixture with the supercritical fluid to obtain a uniformly mixed melt; the mass ratio of the polypropylene to the supercritical fluid is 1: 0.007-0.036; the supercritical fluid is supercritical CO2；
Injecting the uniformly mixed melt into a mold with an inner cavity filled with supercritical carbon dioxide, wherein the injection temperature is 150-170 ℃, and the injection pressure is 8.5-30.5 Mpa; the pressure in the inner cavity of the mold is 8-30 Mpa, after the uniformly mixed melt is injected, the mold is kept for 2-120 min at the temperature of 80-140 ℃, then the temperature is raised to 150-170 ℃ and kept for 30-240 min for foaming, and the foaming pressure is 8-30 Mpa; and then releasing the pressure, cooling and shaping to obtain the foamed polypropylene with oriented pores and capable of being used for the piezoelectric material.
2. The method for preparing the expanded polypropylene having oriented pores for piezoelectric material according to claim 1, wherein: the temperature of the die is 80-120 ℃; the pressure of the die is 8-30 MPa.
3. The expanded polypropylene having oriented pores, which is useful for piezoelectric materials, prepared by the method according to claim 1 or 2, wherein: volume expansion of 1.5-9 times, length diameter of pores of 20-200 μm, orientation degree of pores of 2-7, and pore density of 105～108Per cm3。
4. Use of the expanded polypropylene with oriented pores prepared by the method of claim 1 or 2 for piezoelectric materials, wherein: firstly, coating electrodes on two sides of the foamed polypropylene, and then carrying out polarization treatment by using a high-voltage electric field, thereby obtaining the piezoelectric material.
5. Use according to claim 4, characterized in that: the electric polarization treatment mode is a corona charging method or a contact charging method;
the corona charging method comprises the following steps: the voltage of corona charging is-150 to-5 kv or 5 to 150kv, the grid voltage is-500 to-10000 v or 500 to 10000v, the charging temperature is less than or equal to 120 ℃, the charging time is 5ms to 1h, and the distance between an electrode and a film is 1.5 to 10 cm;
the contact charging method comprises the following steps: connecting a power supply with high voltage to electrodes coated on two sides of the foamed polypropylene, and then applying a voltage of 4-10 kv or-4-10 kv to the electrodes for charging, wherein the charging time is 5 ms-1 h.
CN201910581575.XA 2019-06-30 2019-06-30 Foamed polypropylene with oriented pores and capable of being used for piezoelectric material and preparation method thereof Active CN110216826B (en)

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