CN115093644A - Polypropylene composite foam material, preparation method and wave-transparent performance prediction method - Google Patents
Polypropylene composite foam material, preparation method and wave-transparent performance prediction method Download PDFInfo
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- 239000004743 Polypropylene Substances 0.000 title claims abstract description 51
- 229920001155 polypropylene Polymers 0.000 title claims abstract description 51
- 239000002131 composite material Substances 0.000 title claims abstract description 43
- 238000000034 method Methods 0.000 title claims abstract description 37
- -1 Polypropylene Polymers 0.000 title claims abstract description 33
- 238000002360 preparation method Methods 0.000 title claims abstract description 10
- 239000006261 foam material Substances 0.000 title claims description 21
- 239000000463 material Substances 0.000 claims abstract description 90
- 238000005187 foaming Methods 0.000 claims abstract description 70
- 238000002834 transmittance Methods 0.000 claims abstract description 16
- JRPBQTZRNDNNOP-UHFFFAOYSA-N barium titanate Chemical compound [Ba+2].[Ba+2].[O-][Ti]([O-])([O-])[O-] JRPBQTZRNDNNOP-UHFFFAOYSA-N 0.000 claims abstract description 15
- 229910002113 barium titanate Inorganic materials 0.000 claims abstract description 15
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- 230000005540 biological transmission Effects 0.000 claims abstract description 9
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- 238000004364 calculation method Methods 0.000 claims description 13
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- 239000002245 particle Substances 0.000 claims description 2
- 239000012780 transparent material Substances 0.000 claims 1
- 238000002310 reflectometry Methods 0.000 abstract description 12
- 238000010521 absorption reaction Methods 0.000 abstract description 10
- 239000003989 dielectric material Substances 0.000 abstract description 5
- CURLTUGMZLYLDI-UHFFFAOYSA-N Carbon dioxide Chemical compound O=C=O CURLTUGMZLYLDI-UHFFFAOYSA-N 0.000 abstract description 4
- 229910002092 carbon dioxide Inorganic materials 0.000 abstract description 2
- 239000001569 carbon dioxide Substances 0.000 abstract description 2
- 239000007787 solid Substances 0.000 abstract 1
- 238000012360 testing method Methods 0.000 description 21
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- C08J9/00—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof
- C08J9/04—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof using blowing gases generated by a previously added blowing agent
- C08J9/12—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof using blowing gases generated by a previously added blowing agent by a physical blowing agent
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- C08J5/18—Manufacture of films or sheets
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- C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
- C08J9/00—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof
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- C08J2205/00—Foams characterised by their properties
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- C08J2205/044—Micropores, i.e. average diameter being between 0,1 micrometer and 0,1 millimeter
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- C08J2323/02—Characterised by the use of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Derivatives of such polymers not modified by chemical after treatment
- C08J2323/10—Homopolymers or copolymers of propene
- C08J2323/12—Polypropene
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Abstract
The invention relates to a polypropylene composite foaming material, a preparation method and a wave-transparent performance prediction method, wherein the dielectric constant of the polypropylene composite foaming material is in gradient distribution between 1 and 2, and the dielectric constant is adjustable. The preparation process comprises melt blending polypropylene and Barium Titanate (BT), and then using supercritical carbon dioxide (scCO) 2 ) As a physical foaming agent, the matrix is foamed in a solid state, the cell structure and the foaming multiplying power of the material are changed by adjusting the content of barium titanate and the foaming temperature, and the dielectric constant of the foaming material can be adjusted under the condition of keeping extremely low dielectric loss. This applicationThe obtained polypropylene composite foamed dielectric material has the characteristics of light weight, low dielectric, low loss, adjustable dielectric and broadband wave transmission under high frequency. Meanwhile, the invention also constructs a parallel absorption plate model calculated aiming at the TE mode and a uniform medium plate model calculated aiming at the TEM mode for calculating the wave transmittance, the reflectivity and the absorptivity of the composite foaming material.
Description
Technical Field
The invention relates to a polypropylene composite foaming material, a preparation method and a wave-transparent performance prediction method, and belongs to the technical field of preparation of supercritical foaming materials.
Background
The importance of the PCB is the parent of the electronic industry, and it is self-evident that with the development of 5G in recent years, low dielectric and low loss materials become the bottleneck of the technological progress of the PCB industry in China. The lunbo lens is an electronic device which can parallelize electromagnetic waves and is proposed by german scientists lunbo in the 40 th century in the 20 th century based on maxwell proposed fish eye model, and requires material layers with dielectric constant of 1-2 to be stacked layer by layer. When the electromagnetic wave crosses the interface, the reflectivity will increase if the difference between the dielectric constants of the two phases is large. The dielectric layers inside electronic devices such as PCBs, lunbo lenses, etc. often require impedance matching between materials, i.e., the materials have a certain dielectric constant and low loss according to the device design requirements. The dielectric constant of the material after foaming is usually between 1 and 2, and the material has broadband wave-transmitting performance, and the dielectric loss is lower than that of the solid material before foaming, so that the material is an ideal Longbo lens candidate material. Therefore, how to prepare the foaming material with the dielectric constant distributed in a gradient manner between 1 and 2 becomes an important scientific problem.
The addition of the filler has an important influence on the dielectric performance, and is a common dielectric regulation method: (1) the filler can be introduced into an interface in the material to cause interface polarization; (2) fillers also change the type and number of dipoles within the matrix; (3) the dielectric confinement effect caused by the filler affects the mobility of the dipoles in the matrix. The dielectric constant of air is approximate to 1, the dielectric constant and loss of the material can be obviously reduced by introducing the air, according to the existing research, the thickness of the hole wall of each cell, the aperture of each cell and the foaming ratio are mutually influenced, and the penetrating performance of the cells is influenced by the number and the thickness of the cell walls through which electromagnetic waves penetrate. Therefore, the dielectric constant of the material can be regulated by adjusting the proportion of the air phase, the cell structure and the content of the filler.
According to the effective medium theory, the influence factors of the equivalent dielectric constant of the composite material comprise the dielectric properties and the contents of the components. Similarly, the matrix, the filler and the air phase in the composite foaming material form the main influence factors of the dielectric property of the composite foaming material. Equivalent dielectric prediction models of several composite foam materials are as follows:
linear model:
ε m ＝f 1 ε 1 +f 2 ε 2 +f 3 ε 3 (1)
root mean square model:
rayleigh model:
lichtenecker model:
lnε m ＝f 1 lnε 1 +f 2 lnε 2 +f 3 lnε 3 (5)
for the composite foaming material, because of the difference of interface polarization brought by the intermediate phase of the matrix and the filler and the penetration behavior of electromagnetic waves caused by the change of the cellular structure of the foaming material, the experimental value and the calculated value obtained by the test have errors. Generally, the gradient dielectric constant distribution refers to the distribution of the dielectric constant in the high dielectric material varying with the filler component, and the material with the gradient distribution of the dielectric constant between 1 and 2 is rarely studied.
Disclosure of Invention
The invention provides a polypropylene composite foaming dielectric material with gradient distribution of dielectric constant between 1 and 2 and adjustable dielectric and a preparation method thereof. Meanwhile, the invention also constructs a prediction model of the dielectric property and the wave-transmitting behavior of the composite foaming material, and can provide theoretical guidance for the foaming material used for the low-dielectric-constant material and the wave-transmitting material.
A polypropylene composite foam comprising: polypropylene and barium titanate powder, wherein the mass percent of the barium titanate powder in the foam material is 1-45%; the foaming material is prepared by a supercritical foaming method; the foam material has the cell range of 20-200 mu m, the wall thickness of 0.05-2 mu m and the cell density of 10 7 -10 11 Per cm 3 (ii) a The polypropylene composite foaming material has a dielectric constant of 1-2 within the temperature of 100-160 ℃ and is in gradient distribution.
The melting point of the polypropylene is 145-155 ℃, and the melt flow index is 1.2-1.8g/10 min.
The particle size of the barium titanate powder is 100-300 nm.
The preparation method of the polypropylene composite foaming material comprises the following steps:
melting and blending polypropylene and barium titanate powder to obtain a blended material; and then the blending material is subjected to supercritical foaming to prepare the polypropylene composite foaming material.
The operation condition of melt blending is 170-210 ℃.
The blending material is a plate which is obtained by hot pressing for 1-20min under the pressure of 5-15 MPa.
Supercritical foaming with CO 2 The foaming agent has the temperature of 130 ℃ and 150 ℃ and the pressure of 10-20 MPa.
The method for predicting the dielectric property of the polypropylene composite foaming material comprises the following steps:
calculating the wave transmittance T of the whole foaming material considering the cell structure 1 (ii) a Calculating the equivalent wave-transparent rate T of the material under the condition that the apparent material is a uniform medium 2 The following matrix is constructed:
in the formula (II), epsilon' f1 ……ε’ fm Is a series of real parts of the equivalent dielectric constant, the values of the numbers in the series are sequentially increased within the range of the equivalent dielectric constant, epsilon " f1 ……ε” f1n The values of all the numbers in the number array are sequentially increased within the range of the imaginary part of the equivalent dielectric constant;
obtaining alpha with the minimum absolute value; the corresponding values of the real part of the equivalent dielectric constant and the imaginary part of the equivalent dielectric constant are the calculated values.
The wave-transparent rate T of the whole material 1 The material is determined according to the wave-transparent property of the material, and specifically, the material can be selected from any one of the wave-transparent rates of the TE mode and the TEM mode.
The wave transmittance T of the TE mode 1 Calculated by the following steps:
n c the number of cell walls traversed by the electromagnetic waves; d is a radical of c Is the cell diameter, d w Is the cell wall thickness, L is the material plate thickness, θ 1 The incident angle of the electromagnetic wave to the wall of the bubble hole is indicated; a. b and c are parameters;
λ is the wavelength, θ 2 Angle of refraction, u 2 、v 2 Intermediate variables set for simplifying the calculation satisfy the following equation:
ρ 12 、ρ 23 、τ 12 、τ 23 expressed as the reflection coefficient and the transmission coefficient of medium 1 → medium 2 and medium 2 → medium 3, respectively,
representing the reflection phase, calculated by:
p 1 ＝n 1 cosθ 1 ；
n 1 、n 2 and k 2 The real and imaginary parts obtained by calculating ε', tan δ are determined:
when the subscript j takes values of 1,2 and 3 respectively, the subscript j represents an air medium on a path when the electromagnetic wave comes, a foam material flat medium and an air medium on a path when the electromagnetic wave comes; ε' is the real part of the dielectric constant; tan δ is a dielectric loss tangent value.
a is 1.038X, X is a correction factor, and 36 is less than or equal to X and less than or equal to 42.
The wave transmittance T of the TEM mode 1 Calculated by the following steps:
n c the number of cell walls traversed by the electromagnetic waves; d is a radical of c Is the cell diameter, d w Is the cell wall thickness, L is the material plate thickness, θ 1 The incident angle of the electromagnetic wave to the wall of the bubble hole is indicated; a. b and c are parameters;
i imaginary sign, β phase thickness, where λ is wavelength, c is speed of light, f is frequency, θ 2 Is the angle of refraction; t is t 12 、t 23 The reflection coefficient and the wave-transparent coefficient of the media 1 → 2 and 2 → 3 respectively;
for TE mode, it is calculated by:
for TM mode, it is calculated by:
x and y respectively represent a medium before an incident interface and a medium after the incident interface of the electromagnetic wave, and when the values of 1,2 and 3 are taken, respectively represent an air medium on a path when the electromagnetic wave comes, a foam material flat medium and an air medium on a path when the electromagnetic wave goes;
n is complex refractive index N j ＝n j +ik j The real part of the refractive index and the extinction coefficient of the optical fiber are n j 、k j J is a representative medium, j is 1,2 or 3.
Advantageous effects
The invention adjusts the dielectric constant of the foaming material through the high dielectric property of barium titanate and the antagonism of the low dielectric property brought by introducing air phase, so that the dielectric constant is in gradient distribution between 1 and 2;
the polypropylene composite foamed dielectric material prepared by the invention belongs to a microporous foamed material, and has extremely high air content, micron-scale pore diameter and nanometer-scale pore wall, and extremely low dielectric loss which is 10 DEG -4 . The dielectric constant is stable in dielectric property at an X wave band, and the wave-transmitting rate is higher, so that the broadband wave-transmitting performance is realized;
compared with a 3D printed Longbo lens, the polypropylene composite foamed dielectric material prepared by the invention has the advantages that the dielectric is adjusted through the internal closed structure, the hydrophobic property is realized, and the equipment in the lens can be better protected.
According to the invention, based on a Fresnel formula of electromagnetic waves at a two-phase interface, a parallel absorption plate model calculated for a TE mode and a uniform medium plate model calculated for a TEM mode are constructed for calculating the wave transmittance, reflectivity and absorptivity of the composite foaming material. The equivalent dielectric constant and loss of the foaming material are obtained by an exhaustion method. In the experimental verification: for the polypropylene and barium titanate composite foaming material prepared by supercritical carbon dioxide foaming, the model correctness is verified through the dielectric test results of a coaxial method and a resonant cavity method. Therefore, the model can be used for predicting the dielectric property and wave-transparent behavior of the composite foaming material and designing the material composition and structure.
Drawings
FIG. 1 is an electron microscope image and a cell distribution diagram of a PP/BT foam material: a. b, c and d are foaming temperatures; the BT content at 135 deg.C, 137 deg.C, 140 deg.C, 143 deg.C is 0 wt%, 10 wt%, 20 wt%, 30 wt%, 40 wt%
FIG. 2 is the cell parameters of PP/BT at different BT contents and different foaming temperatures: (a) foaming ratio; (b) the pore diameter of the cells;
FIG. 3 shows the variation trend of dielectric constant and dielectric loss factor with frequency, foaming temperature and BT content measured by the resonant cavity method of PP/BT foaming material: (a) 1 ) And (a) 2 ) The foaming temperature; (b) 1 ) And (b) 2 ) BT content
FIG. 4 is a resonant cavity test result analysis of PP/BT foams: (a) influence of expansion ratio on dielectric constant (@5.8 GHz); (b) influence of expansion ratio on dielectric loss factor (@5.8 GHz);
FIG. 5 is a flow chart of a computing method;
FIG. 6 shows the results and simulations of the co-axial measurements of wave transmittance, reflectance and absorption of PP and 30 wt% BT composite foams;
FIG. 7 shows the variation of wave transmittance, reflectivity and absorptivity of PP/BT composite foaming material with porosity, and the comparison of coaxial method test and simulation value (@10GHz)
Detailed Description
Raw materials and preparation method
Polypropylene (PP), density 0.9g/cm 3 Melting point 150 ℃, melt flow index 1.5g/10min (230 ℃ under 2.16kg load), Zhenhai refining chemical company, China petrochemical industry, Inc.; nano barium titanate (BT, XF117), purity 99 wt%, jiangsu xiaofeng nano materials science and technology limited; CO 2 2 Is of technical grade and purity>99.5%, linde gas limited.
Drying PP raw material in vacuum oven (DZF-6050AB, Tianjin constantan Weiwei apparatus)Drying for 2h, and mixing PP with 10 wt%, 20 wt%, 30 wt% and 40 wt% BaTiO respectively 3 And (5) carrying out melt blending (190 ℃, 60r/min and 5min) in a torque rheometer (ARES and TA) to obtain the PP/BT composite material. Preheating in vacuum laminator (450 type, Beijing fumaric technology) at 190 deg.C for 5min, and hot pressing at 9MPa for 5min to obtain the final product. Swelling in a high-pressure foaming kettle at 135 deg.C, 137 deg.C, 140 deg.C, 143 deg.C, 15MPa for 2 hr 2 And releasing pressure and foaming to obtain a sample.
FIG. 1 is SEM bubble picture of PP/BT, which is foamed at 135 deg.C under a 1 (0wt％)、a 2 (10% by weight) a large and small pore phenomenon, a 4 、a 5 Wherein filler agglomeration is observed. Foaming at 137 ℃b 1 -b 5 The holes are broken, and the degree of the holes is more obvious as the BT content rises. Cells foamed at 140 ℃ and 145 ℃ (c) 1 -c 5 And d 5 -d 5 ) The method is complete and compact, and the pore size distribution meets the Gaussian function. FIG. 2 is a statistical chart of PP/BT cell structure, wherein the foaming ratio is generally reduced (a in FIG. 2) and the pore diameter is increased (b in FIG. 2) along with the increase of BT content.
FIG. 3 shows the dielectric properties of PP/BT foams, FIG. 3-a 1 、a 2 At the same filler content and at different foaming temperatures k And D f . As the foaming temperature rises, the expansion ratio of the material rises, and the corresponding D k And D f And decreases. A of FIG. 3 1 After the medium foaming temperature has risen from 137 ℃ to 143 ℃, D k From 1.29(5.8GHz) to 1.15(5.8 GHz). B of FIG. 3 1 The addition of medium BT remarkably improves the dielectric constant, and the principle is the high dielectric property of BT and the interface polarization generated by the phase interface between BT and PP. B of FIG. 3 2 When the BT content is increased from 0 wt% to 40 wt%, D f From 7.53X 10 -5 Rise to 7.25X 10 -4 (5.8GHz), the losses are still in the lower interval.
In order to further explore the dielectric property of PP/BT at high frequency and obtain parameters such as wave transmittance and reflectivity of PP/BT, the PP/BT foam material is subjected to coaxial method test. The test band of FIG. 4 covers S, C, X three radar bands, D of material k Remains substantially stable over a wide frequency band, D f The numerical value fluctuates, the dielectric constants of the foaming material with different filler contents and different foaming temperatures under 10GHz are shown in table 1, and the requirement of realizing the gradient dielectric constant within the range of the dielectric constant of 1-2 is met.
TABLE 1 coaxial method for testing dielectric constant of polypropylene/barium titanate foaming material with different components and foaming temperature at 10GHz
The results show that materials with a gradient dielectric constant at 10GHz were successfully prepared.
The invention also carries out prediction calculation on the electromagnetic wave-transmitting performance of the composite foaming material, and the specific process is as follows:
first, a parallel absorption plate model for calculating the wave-transparent performance of the electromagnetic wave in the TE mode is established.
If the whole foam material is a flat plate with the thickness of L, the air on the path when the electromagnetic wave comes is defined as a medium 1, the foam material flat plate is a medium 2, the air on the path when the electromagnetic wave comes is a medium 3, and the real part of the refractive index and the extinction coefficient of the air are respectively n j 、k j J is a representative medium ( j 1,2,3), the complex refractive index is N j ＝n j +ik j . The real part of dielectric constant (epsilon '), the imaginary part of dielectric constant (epsilon') or the dielectric loss tangent (tan delta) (which can be obtained by experiments) of the matrix of the composite material, and the Expansion Ratio (ER) and the cell diameter (d) of the foaming material are input c ) The wave transmittance (T) and reflectance (R) of the foam material were obtained.
Total wave-transparent rate T of foaming material n Reflectivity R n Solving the formula:
n in the formula (1) c The number of the walls of the bubble hole for the penetration of the electromagnetic wave is shown as formula (2), wherein d is shown as formula (2) c Is the pore diameter of the cells， d w Cell wall thickness:
f (theta) in the formula (1) 1 ) Theta is a function of the distribution of electromagnetic waves to the cell walls 1 The method refers to f (theta) of electromagnetic wave to the wall of the bubble hole, which is calculated by a Monte Carlo Voronoi method and fitted by a Sum of sine method 1 ) The function is:
f(x)＝asin(bx+c) (3)
in the formula (3), a is 1.038X, b is 0.828, and c is 0.06875; wherein X is defined as a correction factor of the model, and the value range of X is (36-42).
In the formula (1), T and R are the wave transmittance and the reflectivity of a single-layer cell respectively, and the calculation method comprises the following steps:
in the formula (4), λ is the wavelength, θ 2 Angle of refraction, where k is k 2 ，u 2 、v 2 The intermediate variables set for simplifying the calculation satisfy the following equation:
ρ 12 、ρ 23 、τ 12 、τ 23 expressed as the reflection coefficient and the transmission coefficient of medium 1 → medium 2 and medium 2 → medium 3, respectively,
representing the reflected phase, the solution is:
the p value is an intermediate variable for distinguishing a TE mode from a TM mode (p in the TE mode) i ＝n i cosθ i In the TM mode is q i ＝cosθ i /n i Here, only the TE mode, which is p in formula (6), will be discussed 1 ＝n 1 cosθ 1 )，
The dielectric constant and the dielectric loss tangent of the input composite material can be converted into refractive indexes according to the Maxwell relation: n is a radical of an alkyl radical 1 、n 2 And k 2 Determining a real part and an imaginary part obtained by calculation of the formula (7);
when n1 and k1 are calculated for formula (7), the following are obtained:
when n2 and k2 are calculated for equation (7), the following equation is obtained:
uniform medium plate model
The uniform dielectric slab model is mainly used for calculating a TEM model, and the flow is similar to that of a parallel absorption slab model.
Total wave-transparent rate T of foaming material n Reflectivity R n Solving the equation (1);
t and R are the wave transmittance and the reflectivity of the single-layer foam hole respectively. In the electromagnetic wave, a TM mode (p wave) and a TE mode (s wave) are mutually incoherent and need to be added during calculation, so the total single-layer wave transmittance (T) and the total single-layer wave reflectance (R) are calculated by the following method:
wherein the wave-transparent rate (T) of TM mode and TE mode TE/TM ) And reflectance (R) TE/TM ) Can be calculated using the following formula:
t and r are wave-transparent coefficient and reflection coefficient respectively:
i imaginary sign, β phase thickness, where λ is wavelength, c is speed of light, f is frequency:
r 12 、r 23 、t 12 、t 23 the reflection coefficient and the wave-transparent coefficient of the medium 1 → 2 and the medium 2 → 3 respectively are divided, the corresponding parameters are substituted into the following Fresnel formula to solve, the horizontal polarization is represented in the formula (12) and can be used for calculating the TM mode, the vertical polarization is represented in the formula (13) and can be used for calculating the TE mode, and x and y respectively represent the medium in front of an incident interface and the medium behind the interface of the electromagnetic wave:
when x is 1 and y is 2, t can be calculated respectively 12 And r 12 By analogy … …
The calculation of TM mode and TE mode are independent and not influenced mutually, N 1 And N 2 The wave transmittance (T) of the TE mode can be solved for complex refractive indexes by combining the vertical type (8) - (13) TE ) And reflectance (R) TE ) And the wave-transparent rate (T) of TM mode TM ) And reflectance (R) TM )。
Equivalent dielectric model of exhaustion method
Derivation principle of equivalent dielectric model: for the foaming material, the wave transmittance T of the whole material can be solved based on the parallel absorption plate model and the uniform medium plate model 1 (it can be selected from any one of TE, TEM and TM molds according to actual conditions, and is calculated by a parallel absorption plate model in the verification process of the patent), then, assuming that the foaming material is an isotropic uniform flat plate, the flat plate has an equivalent dielectric constant (epsilon' f ) And the imaginary part of the equivalent dielectric constant (. epsilon.) " f )(ε” f And tan delta f May be computationally interconverted), there is also a corresponding equivalent wave-transparent rate T 2 (T in the formula (4)). Represents T 1 And T 2 The calculation formula of the difference calculation accuracy α is shown in fig. 5. Clearly, there are countless possible ε's to the hypothetical uniform plate' f And epsilon' f So need to be within its possible range (e.g., ∈' f Is 1-2, epsilon' f 0 to 0.01) provided that m ε's are present' f And n epsilon " f Solving m x n T by exhaustion method 2 And corresponding alpha, and sorting the elements in the result matrix from small to large according to the absolute value of alpha in sequence, and corresponding epsilon' f And epsilon' f The matrix is changed in rows and columns in the same way, and three numbers with the minimum alpha and corresponding epsilon 'are selected' f And epsilon' f Namely the equivalent dielectric constant and the imaginary part corresponding to the foaming material. Model calculation of T 2 Season d w The overall exhaustive calculation is as follows:
verification of prediction results
In the following validation process, the following test methods were used to determine the materials:
1. coaxial dielectric property test
The sample size required concentric rings with an inner diameter of 3mm and an outer diameter of 7 mm. The test was previously calibrated with a special fixture. During the test, the S parameter is saved in a vector network analyzer (N5247A, Agilent, USA), and then the dielectric constant (epsilon') and the dielectric loss tangent (tan delta) of the test material are calculated. S parameters derived by a vector network analyzer in the coaxial method test comprise amplitude (dB) and phase, and numerical values of the amplitude part of the S parameters are selected. S 21 And S 11 Respectively, the transmission and reflection coefficients when port 2 is mated to port 1, so the transmission (T) and reflection (R) are calculated as:
2. resonant cavity method dielectric property test
The sample requires an area exceeding 1X 1cm 2 The plane of (2) is calibrated by using a standard sample before the experiment, the test surface is tested under the vacuum-pumping condition during the experiment, and the tested frequency intervals are 0.8GHz, 2.45GHz, 4.1GHz and 5.8 GHz. The dielectric constant (. epsilon.') and the dielectric loss tangent (tan. delta.) of the material were obtained.
3. Cell structure testing
The foaming ratio (ER) and the cell diameter (d) of the foamed material were measured by a combination of a drainage density test and a scanning electron microscope (SEM, S3400A, Hitachi, Japan) c ) Wall thickness (d) w ) And cell density.
Dielectric and wave-transparent properties
The porosity (ER-1)/ER represents the air content in the composite foam. In fig. 6, BT30 wt% composite foam is taken as an example, and the influence of frequency and porosity on wave transmissivity, reflectivity and absorptivity is shown. The value range along with the frequency is 0.5-12 GHz, the radar signal spans L (1-2 GHz), S (2-4 GHz), C (4-8 GHz) and X wave bands (8-12 GHz), and the wave transmission rate is reduced and the reflectivity and the absorption rate are increased along with the increase of the frequency. Because the coaxial method test is a TEM mode, a uniform medium plate method model is adopted for simulation, wherein the error of the simulation values of the wave transmittance, the reflectivity and the absorptivity relative to the experimental value is less than 5%.
10GHz is the middle frequency of X wave band, and the wavelength is 30mm, lies between millimeter wave and centimetre wave. c-absorptivity the wave transmission, reflection and absorptivity of the composite foams at different porosities and BT contents are shown in fig. 7, which is 10GHz as an example. In the c-absorption rate graph 7, as the BT content increases, the wave-transmitting rate decreases, the reverse ratio increases, and the absorption rate increases. The error of the simulation result and the experimental value is less than 5%.
The resonant cavity method is one of the most accurate methods for testing the dielectric properties of the low-loss material, and is shown in table 2 as being under 5.8GHz (5.8GHz is one of the frequencies expected to replace 2.4GHz and is important in the field of communication), and the resonant cavity method is used for testing epsilon' and tan delta of the PP/BT composite foaming material and is used for testing an equivalent dielectric model (T) by utilizing an exhaustion method 2 Calculated for parallel absorber plate model) of ∈' f And tan delta f The equivalent dielectric constant and the dielectric loss tangent corresponding to three values of the smallest α are calculated as 1,2, and 3. The experimental and simulated values in table 2 are relatively close.
TABLE 1PP/BT foam exhaustion method equivalent dielectric calculation results and resonant cavity method test results (@5.8GHz)
Claims (10)
1. A polypropylene composite foam material, which is characterized by comprising: polypropylene and barium titanate powder, wherein the mass percent of the barium titanate powder in the foam material is 1-45%; the foaming material is prepared by a supercritical foaming method; the foam material has the cell range of 20-200 mu m, the wall thickness of 0.05-2 mu m and the cell density of 10 7 -10 11 Per cm 3 (ii) a The polypropylene composite foaming material is as followsA dielectric constant of 1-2 at 100-160 ℃ and a gradient distribution.
2. The polypropylene composite foam material as set forth in claim 1, wherein the melting point of polypropylene is 145-155 ℃, and the melt flow index is 1.2-1.8g/10 min; the particle size of the barium titanate powder is 100-300 nm.
3. The method for preparing the polypropylene composite foaming material of claim 1, which is characterized by comprising the following steps: melting and blending polypropylene and barium titanate powder to obtain a blended material; and then the blending material is subjected to supercritical foaming to prepare the polypropylene composite foaming material.
4. The method for preparing polypropylene composite foam material as claimed in claim 3, wherein the melt blending operation condition is 170 ℃ and 210 ℃; the blending material is a plate which is obtained by hot pressing for 1-20min under the pressure of 5-15 MPa.
5. The method for preparing polypropylene composite foaming material according to claim 3, wherein the supercritical foaming adopts CO 2 The foaming agent has the temperature of 130 ℃ and 150 ℃ and the pressure of 10-20 MPa.
6. Use of the polypropylene composite foam material as defined in claim 1 in the preparation of wave-transparent material.
7. The use according to claim 6, further comprising the step of predicting the dielectric properties of the polypropylene composite foam material, comprising:
calculating the wave-transparent rate T of the whole foaming material considering the cellular structure 1 (ii) a Calculating the equivalent wave-transparent rate T of the material under the condition that the apparent material is a uniform medium 2 The following matrix is constructed:
in the formula (II) is epsilon' f1 ……ε’ fm Is a number sequence of real parts of the equivalent dielectric constant, and the values of all numbers in the number sequence are sequentially increased within the range of the equivalent dielectric constant, epsilon' f1 ……ε” f1n The number sequence is the imaginary part of the equivalent dielectric constant, and the values of all the numbers in the number sequence are sequentially increased in the imaginary part range of the equivalent dielectric constant;
obtaining alpha with the minimum absolute value; the corresponding numerical values of the real part and the imaginary part of the equivalent dielectric constant are the calculated values.
8. Use according to claim 7, characterized in that the material as a whole has a wave-transparency T 1 The material is determined according to the wave-transparent characteristic of the material, and the material can be specifically selected from any one wave-transparent rate of a TE mode and a TEM mode;
the wave transmittance T of the TE mode 1 Calculated by the following steps:
f(x)＝asin(bx+c)；
n c the number of cell walls traversed by the electromagnetic waves; d c Is the cell diameter, d w Is the cell wall thickness, L is the material plate thickness, θ 1 The incident angle of the electromagnetic wave to the wall of the bubble hole is indicated; a. b and c are parameters;
λ is the wavelength, θ 2 Angle of refraction, u 2 、v 2 The intermediate variables set for simplifying the calculation satisfy the following equation:
ρ 12 、ρ 23 、τ 12 、τ 23 expressed as the reflection coefficient and the transmission coefficient of medium 1 → medium 2 and medium 2 → medium 3, respectively,
representing the reflection phase, calculated by:
p 1 ＝n 1 cosθ 1 ；
n 1 、n 2 and k 2 The real and imaginary parts obtained by calculating ε', tan δ are determined:
when the subscript j takes values of 1,2 and 3 respectively, the subscript j represents an air medium on a path when the electromagnetic waves come, a foam material flat medium and an air medium on a path when the electromagnetic waves go; ε' is the real part of the dielectric constant; tan δ is a dielectric loss tangent value.
9. Use according to claim 8, wherein a is 1.038X, X is a correction factor, 36 ≦ X ≦ 42.
10. Use according to claim 8, wherein the TEM mode has a wave transmission T 1 Calculated by the following steps:
f(x)＝asin(bx+c)；
n c the number of cell walls traversed by the electromagnetic waves; d is a radical of c Is the cell diameter, d w Is the cell wall thickness, L is the material plate thickness, θ 1 The incident angle of the electromagnetic wave to the wall of the bubble hole is indicated; a. b and c are parameters;
i imaginary sign, β phase thickness, where λ is wavelength, c is speed of light, f is frequency, θ 2 Is the angle of refraction; t is t 12 、t 23 The reflection coefficient and the wave-transparent coefficient of the media 1 → 2 and 2 → 3 respectively;
for TE mode, it is calculated by:
for TM mode, calculated by:
x and y respectively represent a medium before an incident interface and a medium after the incident interface of the electromagnetic wave, and when values are 1,2 and 3, respectively represent an air medium on a path when the electromagnetic wave comes, a foam material flat medium and an air medium on a path when the electromagnetic wave comes; n is complex refractive index N j ＝n j +ik j The real part of the refractive index and the extinction coefficient of the optical fiber are n j 、k j J is a representative medium, j is 1,2 or 3.
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