CN116144127A - Low dielectric loss PTFE in-situ fiber FEP microporous material, preparation method and application thereof in wave-transparent material - Google Patents

Abstract

The invention relates to a low dielectric loss PTFE in-situ fiber FEP microporous material, a preparation method and application thereof in a wave-transparent material, and belongs to the technical field of porous foaming materials. The in situ fibrillated polytetrafluoroethylene reinforced nanocomposite is prepared by mixing perfluoropropylene (FEP) with Polytetrafluoroethylene (PTFE). The addition of PTFE nanofibers can significantly improve the rheological properties of the material. In supercritical CO ₂ The obvious change of strain hardening effect and melt viscoelasticity response in the foaming process for forming the foaming agent obviously improves the foaming behavior of the material. FEP nanofibers have a higher expansion ratio and a more uniform cell structure. FEP/PTFE microporous material relatively solid plateIs better hydrophobic and does not affect its excellent flame retardant properties. The dielectric constant and dielectric loss of the composite material at 10GHz were 1.13 and 0.00015, respectively.

Description

Low dielectric loss PTFE in-situ fiber FEP microporous material, preparation method and application thereof in wave-transparent material

Technical Field

The invention relates to a low dielectric loss PTFE in-situ fiber FEP microporous material, a preparation method and application thereof in a wave-transparent material, and belongs to the technical field of porous foaming materials.

Background

With the advent of the 5G age, large-scale integrated circuits have rapidly progressed, and electronic devices have necessarily progressed toward miniaturization, high speed, high integration, and low power consumption, but the development of very large-scale integrated circuits has been limited by signal delay effects due to interlayer capacitance, metal wire resistance, and inter-wire capacitance. It is generally believed that signals reaching GHz levels and above, the skin effect of the conductors is more pronounced, dielectric leakage effects begin to stand out, transmission line losses become non-negligible, the main sources of losses are conductor losses and dielectric losses, and other loss types such as radiation losses are generally smaller. The reduction of conductor loss can be achieved from two points, one is to increase the line width of the wiring and the other is to reduce the roughness of the copper foil. In a high-frequency high-speed PCB, devices need high integration and miniaturization, so that the increase amplitude of line width is limited; the roughness of the copper foil is also becoming smaller and smaller. Therefore, there is a need to reduce dielectric loss, which is independent of the structure of the transmission line, and the only way to do this is to use materials with lower loss angles. The PCB substrates currently used in industry are FR-4 epoxy glass fiber board (epoxy board), glass fiber cloth reinforced Polytetrafluoroethylene (PTFE), olefin polymer, polyimide resin (PI) and the like, but the materials currently used are not suitable for high frequency signal transmission because of either poor processability or too high dielectric loss. Therefore, development of a high-performance material with an ultra-low dielectric constant and dielectric loss is urgent.

The poly (perfluoroethylene propylene) (FEP, F46) is a copolymer of Tetrafluoroethylene (TFE) and Hexafluoropropylene (HFP), and has a molecular structure corresponding to a structure in which a part of fluorine atoms in a PTFE molecular chain are replaced with trifluoromethyl groups. Due to the introduction of HEP monomer units, symmetry of PTFE molecular chains is broken, crystallinity, melting point and viscosity of the HEP monomer units are equivalent, so that the FEP is made into fluoroplastic with excellent comprehensive performance, the fluoroplastic can be plasticized and processed, the performance of the fluoroplastic is 50 ℃ lower than that of PTFE at the highest use temperature, the other performance of the fluoroplastic is equivalent to that of PTFE, the working temperature range of the HEP monomer units is-190-205 ℃, the FEP can conveniently replace PTFE, and the fluoroplastic can be applied to fields of high performance, low dielectric property and the like, such as communication, aerospace, high-frequency electronic equipment and the like. However, when the FEP material is applied to wave-transparent equipment products, the FEP material still has the problems of poor dielectric property and high wave-transparent loss.

Disclosure of Invention

The technical problems to be solved by the invention are as follows: when the FEP material in the prior art is directly used in the wave-transmitting equipment material, the problems of low dielectric property and high wave-transmitting loss exist. The invention provides an FEP supercritical foaming material added by PTFE, which remarkably improves rheological processing performance, increases foaming ratio and reduces dielectric constant of the material by using PTFE in FEP.

The technical proposal is as follows:

the low dielectric loss PTFE in-situ fiber FEP microporous material is characterized in that the main material is FEP and PTFE fiber is contained in the material; the PTFE content in the FEP ranges from 0.1 to 10wt%; PTFE fiber diameter is 100-500nm; the microporous material is prepared by a supercritical foaming method, the pore diameter of the cells of the microporous material is 20-200 mu m, and the density of the cells is 1 multiplied by 10 ⁶ ～1×10 ¹⁰ Individual/cm ³ 。

The microporous material has a dielectric constant of 0.00015-0.00050 under the condition of 2.5GHz to 10 GHz.

The microporous material has a water drop contact angle of 120-140 degrees on the surface.

The preparation method of the low dielectric loss PTFE in-situ fiber FEP microporous material comprises the following steps:

step 1, mixing PTFE fibers with FEP master batch, and then performing vulcanization treatment to obtain an FEP/PTFE nanocomposite;

and 2, treating the FEP/PTFE nanocomposite material by a supercritical foaming mode to obtain the microporous material.

In the step 1, the temperature of the mixing process is 270-320 ℃; the temperature in the vulcanization process is 300-350 ℃ and the pressure is 5-15MPa.

In the step 2, the supercritical foaming condition is that the temperature is 235-265 ℃, the pressure is 15-20MPa, and the time is 50-200min.

The application of the microporous material in the wave-transparent material.

The wave-transmitting material is a PCB board; in the application, the PTFE fiber is used for improving the foaming ratio and dielectric property of the microporous material.

The application also comprises a method for predicting the wave-transparent performance, which comprises the following steps:

calculating the wave transmittance T of the whole foaming material considering the cell structure ₁ The method comprises the steps of carrying out a first treatment on the surface of the Calculating the equivalent wave-transmitting rate T of the material under the condition that the visual material is uniform medium ₂ Calculating the precision alpha of the data set, and calculating the minimum value of the alpha through a genetic algorithm, wherein the corresponding numerical values of the real part and the imaginary part of the equivalent dielectric constant are calculated values;

wherein ε' _f Is the real part of the equivalent dielectric constant, epsilon' _f Is the imaginary part of the equivalent dielectric constant.

Wave transmission rate T ₁ The method is calculated by the following steps:

θ ₁ for incident angle, n _c D is the number of cell walls encountered when electromagnetic waves penetrate the foam material _c Is the pore size of the cells, d _w Is the average thickness of the walls of the bubble; l is the thickness of the material flat plate;

T _p and T _s The wave transmission coefficients of the TM mode and the TE mode are respectively represented;

T _p/s ＝|t| ² the method comprises the steps of carrying out a first treatment on the surface of the Subscripts represent p or s;

/>

i imaginary symbol, β is 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 transmission coefficient of the medium 1-2 and the medium 2-3 respectively;

for TE mode, it is calculated by:

for the TM mode, it is calculated by the following formula:

x and y respectively represent the medium before the incident interface of the electromagnetic wave and the medium after the interface, and when the values are 1,2 and 3, respectively represent the air medium on the path when the electromagnetic wave comes, the foam material flat plate medium and the air medium on the path when the electromagnetic wave comes; n is complex refractive index N _j ＝n _j +ik _j The real part of refractive index and extinction coefficient are respectively n _j 、k _j J is the medium represented, j=1, 2 or 3.

Lambda is the wavelength, theta ₂ U is the angle of refraction ₂ 、v ₂ The intermediate variables set for simplifying the calculation satisfy the following formula:

ρ ₁₂ 、ρ ₂₃ 、τ ₁₂ 、τ ₂₃ respectively expressed as the reflection coefficient and the transmission coefficient of medium 1- & gt medium 2 and medium 2- & gt medium 3,

representing the reflection phase, calculated by:

p ₁ ＝n ₁ cosθ ₁ ；

n ₁ 、n ₂ and k ₂ Real and imaginary parts obtained by calculating ε', tan δ are determined:

when the subscript j takes the values of 1,2 and 3 respectively, the subscript j represents an air medium on the electromagnetic wave arrival time path, a foaming material flat plate medium and an air medium on the electromagnetic wave arrival time path respectively; epsilon' is the real part of the dielectric constant; tan delta is the dielectric loss tangent.

Advantageous effects

1. The invention adopts an in-situ fiberizing process to prepare FEP/PTFE blend, supercritical CO ₂ The microporous foam is obtained by a mould pressing foaming method.

FEP has very low dielectric constant (2.1) and dielectric loss (0.00085), has better processability relative to Polytetrafluoroethylene (PTFE), and does not affect other excellent properties such as flame retardance, hydrophobic oleophobic properties, etc.

3. The added PTFE microspheres are stretched into fibers and even entangled into a net structure through the strong shearing force of the torque rheometer, so that the viscoelasticity of the FEP matrix is changed, the foaming of the nanocomposite is further promoted, the hole breaking behavior in the subsequent foaming process is inhibited, and the foaming multiplying power is improved. After the micropores are introduced, the contact angle of the FEP microporous material is greatly increased, the dielectric constant and dielectric loss are greatly reduced, and the excellent corrosion resistance and flame retardance of the FEP microporous material are not affected. The method is suitable for the application of the high-frequency high-speed PCB.

Drawings

Fig. 1: scanning Electron Microscopy (SEM) of PTFE fiber morphology in FEP matrix: a) 0-PTFE, b) 0.5-PTFE, c) 1-PTFE, d) 3-PTFE, e) 5-PTFE

Fig. 2: DMA and DSC curves for FEP/PTFE composites

Fig. 3: effect of PTFE nanofibers on shear rheological properties at 290 ℃ (a) storage modulus (G'), (b) loss modulus (G "), (C) complex viscosity (|η|) and (d) tan delta

Fig. 4: elongational viscosity of FEP/PTFE nanocomposite at 290 ℃ at different elongational rates: (a: 0-PTFE, b:0.5-PTFE, c:1-PTFE, d:3-PTFE and e: 5-PTFE) and a tensile hardening index χ _E

Fig. 5: SEM image of FEP/PTFE nanocomposite at 248 ℃: (a: 0-PTFE, b:0.5-PTFE, c:1-PTFE, d:3-PTFE and e: 5-PTFE)

Fig. 6: foaming ratio, cell density and cell diameter of FEP/PTFE nanocomposite

Fig. 7: measurement and calculation of different expansion ratio dielectric constants of FEP/PTFE foam at different frequencies (a) 2.5GHz (b) 5GHz (c) 10GHz

Fig. 8: measurement and calculation of dielectric loss at different expansion ratios of FEP/PTFE foam at different frequencies (a) 2.5GHz (b) 5GHz (c) 10GHz

Fig. 9: wettability of PMI and FEP/PTFE nanocomposite foam Water

Fig. 10: wettability of FEP/PTFE nanocomposite foaming material soybean oil

Fig. 11: PMI foam and FEP foam are soaked in alkali liquor

Fig. 12: vertical burning test of PMI foam and FEP/PTFE foam (a-d are PMI foam burning patterns, e are patterns of melting and dripping of test pieces of FEP/PTFE foam, f-i are burning patterns of FEP/PTFE foam, and j is a pattern of FEP/PTFE foam after burning)

FIG. 13 is a schematic view showing the wave-transparent process of the foaming material

FIG. 14 is a flow chart for inversion of equivalent dielectric constant

Detailed Description

Development of a polymer dielectric material with ultralow dielectric constant and dielectric loss, high temperature resistance, corrosion resistance, hydrophobicity and flame retardance is of great significance for meeting the requirements of the 5G field on low-delay and high-transmission dielectric materials. The in situ fibrillated polytetrafluoroethylene reinforced nanocomposite is prepared by mixing the perfluorinated propylene (FEP) with Polytetrafluoroethylene (PTFE) using a torque rheometer. PTFE nanofibers with the diameter smaller than 500nm and the larger length-diameter ratio are prepared. Experimental results of a shear rheometer and an extensional rheometer show that the addition of PTFE nanofibers can significantly improve the rheological properties of the material. In supercritical CO ₂ The foaming behavior of the material is obviously improved by the obvious change of the strain hardening effect and the melt viscoelasticity response in the molding foaming process of the foaming agent. FEP nanofibers have a higher expansion ratio and a more uniform cell structure. When the polytetrafluoroethylene fiber content is 0.5%, the foaming performance is optimal. The FEP/PTFE microporous material is better hydrophobic than the solid plate and does not affect its excellent flame retardant properties. In addition, the dielectric constant and dielectric loss of the FEP/PTFE composite material at 10GHz are respectively 1.13 and 0.00015, so that the requirements of a low-delay and high-transmission PCB board are met.

Example 1 preparation of FEP/PTFE nanocomposite

And mixing quantitative PTFE with FEP by a torque rheometer, mixing at 290 ℃ for 12min at a mixing speed of 60rpm to obtain an FEP/PETE nanocomposite, and then pressing into various plates by a flat vulcanizing machine at 330 ℃ and 10MPa for later use. Table 1 shows the composition of the nanocomposite.

TABLE 1 composition of FEP/PTFE nanocomposite

EXAMPLE 2 supercritical carbon dioxide foam

The foaming sample adopts CO ₂ As a physical blowing agent. First 10X 5mm ³ Is placed in a high pressure foaming kettle to dissolve CO ₂ . The autoclave was heated to a temperature and three times CO was used before pressurization ₂ Purging to remove air from the autoclave. The autoclave was then pressurized to the desired pressure using an ISCO high pressure pump (260D, usa). The samples were saturated in an autoclave at a temperature of 245℃and 248℃and 254℃and 257℃and a pressure of 18MPa, respectively, for 90min. Then, foaming was performed by rapid decompression for 2 s.

Characterization method

1. Measurement of foaming Rate

Foam density was determined by water displacement method (ASTM D792). The Expansion Ratio (ER) was measured after foaming for 3 days. ER calculation formula is:

wherein ρ is _solid And ρ _foam The densities of the solid and foam samples, respectively.

2. Scanning electron microscope

Scanning electron microscopy was used to observe the cross section of the FEP/PTFE nanocomposite and the cell morphology of the microporous foam at an accelerating voltage of 10 kV. Nanocomposite materials and foams are brittle broken at low temperatures in liquid nitrogen, and then gold is coated on their fracture cross sections. Using Image J-Pro software, one can go from SThe average cell diameter size (D) and the average cell density (N) were further obtained in the EM micrograph _f ). At least 3 micrographs were taken for each sample to calculate the average, as follows

Where N is the number of cells in the SEM micrograph and A is the area of the micrograph (cm) at the actual measurement location ² )。

3. Differential scanning calorimetry

Samples (4-10 mg) were subjected to differential scanning calorimetry (DSC, TA Q250) experiments under nitrogen atmosphere, heating from 40℃to 350℃at a heating rate of 10℃per minute, to eliminate the heat history, then cooling to 40℃at a cooling rate of 10℃per minute, and finally heating from 40℃to 350℃at a heating rate of 10℃per minute. Having thermal properties that enable us to determine the foaming temperature range of the FEP/PTFE nanocomposite, the miscibility of FEP and PTFE.

4. Thermogravimetric analysis

To measure the degradation temperature of FEP and FEP/PTFE nanocomposites to investigate the molecular chain structure change before and after blending, thermogravimetric analysis (TGA) was performed using a TGA550 instrument. The program temperature range was set at 800℃at room temperature and the heating rate was 10℃per minute under a nitrogen atmosphere.

5. Dynamic mechanical analysis

Dynamic mechanical analysis testing was performed in tensile mode. The sample was trimmed to a sheet of 20 mm long, 5mm wide and 1 mm thick. The loss tangent (tan delta) of all samples was measured at a heating rate of 3 deg.c/min, a frequency of 1Hz, in a temperature range of-140 deg.c to 150 deg.c.

6. Analysis of dielectric Properties

The vector network analyzer is used for measuring by connecting a single-frequency separation medium resonant cavity clamp, and dielectric constants and dielectric losses of samples with flatness at 2.5GHZ, 5GHz and 10GHz are respectively measured.

7. Contact angle test

Contact angle systems were used to measure the contact angle of water and soybean oil on FEP and FEP/PTFE nanocomposite films and foams and polymethacrylimide foams (PMIs). The contact angle was measured by dropping water onto the sample surface.

8. Analysis of flame retardant Properties

Vertical burn tests were performed on PMI, FEP and FEP/PTFE composite foams using a horizontal vertical burn meter according to the UL94 standard. All specimens were cut at a size of 125X 15X 3 mm. The total time for the test piece to burn out was recorded. The Limiting Oxygen Index (LOI) of PMI foams was determined using a limiting oxygen index meter according to ASTM D2863.

Morphology analysis of FEP/PTFE nanocomposite

As shown in fig. 1, which is an SEM image of the FEP/PTFE nanofiber composite, as the PTFE content increases, the number and diameter of the fibers increases, and the diameter of the fibers is less than 500nm, so the aspect ratio can be very high. Many fibers are entangled together to form a network structure that increases the matrix melt strength and effectively prevents cell breakage during foaming. When the content of the dispersed phase is low, the probability that molecules of the dispersed phase meet each other is low. Under the action of tensile stress, the diameters of the separated fibers gradually decrease, and along with the increase of the content of the dispersed phase, in the process of forming the fibers, the ellipsoids and the ellipsoids, the ellipsoids and the fibers, and the fibers collide with each other, coagulate and intertwine to form a fiber network structure.

Thermodynamic analysis of FEP/PTFE nanocomposites

The thermal behavior of FEP/PTFE nanocomposites was studied using Differential Scanning Calorimetry (DSC), while taking into account the melting and crystallization processes. Region a of fig. 2 is the Tan delta plot of the DMA tested nanocomposite, from Tg temperatures it can be seen that the FEP and PTFE thermodynamically incompatible region b of fig. 2 is the different content PTFE/FEP blend, pure FEP and pure PTFE quadratic temperature rise curve, the lower melting point (Tm 1) corresponding to the melting point temperature of pure FEP; the high melting point (Tm 2) corresponds to polytetrafluoroethylene. The melting temperature of FEP in the mixture (255-257 ℃) is seen to be lower than that of pure FEP (259 ℃). The enthalpy of fusion of the sample incorporating polytetrafluoroethylene nanofibers was also slightly lower, indicating lower crystallinity due to the barrier effect of the nanofibers.

TABLE 1 melting temperatures (Tm 1, tm 2) glass transition temperature (Tg), and secondary transition (Tβ) of samples

*https://www.sigmaaldrich.cn/CN/zh/product/aldrich/182478

Analysis of rheological Properties

In the compression molding foaming process, the rheological properties of the melt play a crucial role. To investigate the effect of polytetrafluoroethylene nanofibers on the shear rheological properties of nanocomposites, a Small Amplitude Oscillatory Shear (SAOS) test was performed in the linear viscoelastic region at 290 ℃. The storage modulus (G ') and loss modulus (G') are plotted against shear rate (ω) as shown in regions a, b of FIG. 3. With the increase of Polytetrafluoroethylene (PTFE) content, the values of G 'and G' in the low frequency region tend to increase, while the viscosity in the high frequency region decreases with the increase of PTFE content, which is beneficial to the processing of the composite material, and the viscosity in the low frequency region increases to stabilize and inhibit cracking of the foam cells in the foaming process. G' is more sensitive than G "when structural changes occur. As shown in FIG. 3 a, the G 'values of the samples 0.5-PTFE, 1-PTFE, 3-PTFE and 5-PTFE deviate from the G' value of sample 0-PTFE, indicating a strong interaction between the resulting PTFE and FEP matrix. In addition, the PTFE-added sample can observe a plateau region in the low frequency region. Considering that polytetrafluoroethylene nanofibers can be interwoven into a network or mesh structure, the strong interaction between the dispersed phase and the continuous phase causes the polymer melt to exhibit a solid or gel-like viscoelastic response. The change in melt elastic response of the PTFE nanofiber reinforced FEP is expressed as tan delta, delta being the loss angle calculated as G 'divided by G'. It can be seen from fig. 3 d that all samples with PTFE nanofibers have a significant decrease in tan delta compared to the 0-PTFE sample, especially in the low frequency region, which means that these melts have a higher elastic response. The existence of the nanofiber and the existence of the network structure can greatly enhance the viscoelasticity of the nanocomposite melt, and can effectively prevent the combination and rupture of cells in the cell growth stage.

As the cells undergo biaxial stretching during growth, the cell walls, and therefore the elongational viscosity of the melt, plays an integral role in cell growth. Based on the shear rheological test, different stretching rates (0.01-1 s ^-1 ) Elongational viscosity η of lower sample ⁺ (t, ε). Fig. 4 shows the elongational viscosity curves of FEP of different PTFE contents. As shown in FIG. 4 a, sample 0-PTFE was free of strain hardening due to the lack of branching and polytetrafluoroethylene nanofibers. Other PTFE-added samples were at 0.01-1 s ^-1 The PTFE nanofiber composites all exhibited significant strain hardening at the strain rate.

The change in elongational viscosity is described in detail using quantitative analysis methods. Strain hardening factor χ _E Is defined as follows. η (eta) ⁺ (t) is the transient shear viscosity, η in this study ⁺ (t, ε) was a value at which the Hencky strain reached 1.0.

0.01～1s ^-1 Is of a pure PTFE sample at the strain rate _E Less than 1, exhibits strain softening behavior. This phenomenon is due to the unbranched structure of FEP, where all samples containing PTFE nanofibers have χ at all rates employed _E The values are very high. As discussed in the shear rheology test, the fiber network may result in a longer relaxation process. Existing nanofibers can impede stretching and flow of the FEP matrix. Significant strain hardening indicates that the incorporation of PTFE nanofibers can alter the extensional rheological behavior of FEP.

Forming and foaming of FEP/PTFE nanocomposite

Foaming experiments were performed on a compression foaming apparatus equipped with a gas filling pump. The pressure of the die cavity is 18MPa, the saturation time is 90min, and the temperature is 245-257 ℃. The foaming temperature is one of the important parameters for regulating and controlling the foaming rate and the cell structure of the polymer, and the FEP shows that with the increase of the foaming temperatureThe properties of the product are closer to those of the amorphous polymer, the crystals in the FEP/PTFE sample are partially or completely melted with increasing foaming temperature, the cell size is gradually increased, as expected, the cell diameter is increased with increasing foaming multiplying power, the cell diameter is reduced with increasing foaming multiplying power, but when the temperature is increased to 257 ℃, compared with 254 ℃, the foaming multiplying power is not obviously increased, even reduced, and the corresponding foaming multiplying power and cell diameter are correspondingly changed. With the rising of the foaming temperature, on one hand, the degree of freedom of the FEP molecules is increased, and the deformation energy of the FEP molecules is greatly enhanced, so that the growth and the coalescence of cells are more facilitated; on the other hand, CO ₂ The reduced solubility of the molecules in the polymer matrix clearly adversely affects cell growth and growth. Obviously, when the temperature reaches 257 ℃, the melt viscosity of the FEP and the composite sample thereof becomes low, even the FEP cannot cover gas, collapse of cells and the like occur, and the macroscopic appearance of the reduction of foaming multiplying power is realized. Below 254℃however, with increasing temperature, although CO ₂ The solubility of the molecules in the polymer matrix is reduced, but the deformability change caused by the temperature rise is more superior in competition, and the macroscopic appearance of the rise of foaming multiplying power is not like the phenomenon of massive hole breaking and collapse under the condition of 257 ℃. Compared with the pure FEP sample, the PTFE is added, the foaming ratio is increased, probably because the crystal area structure of the FEP molecular chain is destroyed after the PTFE is added, so that the CO dissolved under the corresponding condition ₂ More, the fiber network formed after PTFE is added can effectively inhibit broken holes without covering gas, so that the foaming ratio is macroscopically increased, the density of corresponding cells is reduced, and the diameter of cells is increased.

Relationship between dielectric behavior and Expansion Ratio (ER) of FEP/PTFE foam

To date, several theoretical models have been developed for predicting the effective dielectric constant of a dielectric material, including modified clausius-Mo Suodi expression (1), li Xiteng inner gram equation (2), maxwell-ganett theory (3) and jayasunde-Smith model (4).

As regards the FEP/PTFE microcellular foam, it comprises two parts: air filled pores and cell walls composed of FEP/PTFE matrix. To accurately investigate the relationship between dielectric constant and foaming, dielectric constants at 2.5GHz, 5GHz and 10GHz (for FEP or FEP/PTFE ε ₁ =2.1 and epsilon for dry air ₂ =1.0) theoretical dielectric constant at the foaming magnification was examined (fig. 7). In addition, the dielectric constant values measured at 2.5GHz, 5GHz and 10GHz for FEP/PTFE foam, respectively, are also shown. Clearly, experimental results for FEP/PTFE foams show that the above model can be used to predict the dielectric constant of FEP/PTFE foams. Based on this result, if a 10-fold expansion ratio can be obtained, the dielectric constant of the FEP/PTFE foam would theoretically be further reduced to 1.1 or less. As for dielectric loss, the overall also decreases with increasing foaming ratio of the FEP/PTFE foam. In addition, with the change of frequency, the dielectric constant and dielectric loss were very stable at 2.5GHz to 10 GHz. More importantly, the FEP/PTFE foam achieved an ultra-low dielectric constant as low as 1.13 (Table 2) below the lowest value of most reported low-k dielectrics and a dielectric loss of 0.00015 (FIG. 8), which is the lowest value of the low dielectric loss materials reported so far. PEN foams have great potential for use in 5G communication integrated circuits, combining ultra-low dielectric constants and dielectric losses.

Hydrophobic and oleophobic Properties

Hydrophobicity is a more important parameter for the application of high performance materials in electronics and microelectronics because it controls the adsorption of moisture and deterioration of dielectric properties, the wettability of FEP/PTFE foam with water is shown in fig. 9, and it can be seen that the contact angle of FEP foam with water is about 30 ℃ greater, from 110 ° to 140 °, the surface roughness of the foamed material increases, so that the hydrophobic properties of FEP/PTFE foam are further improved. The comparative PMI foam exhibited hydrophilic properties, with a contact angle of water as low as 77.4 °. As shown in fig. 10, the FEP/PTFE foam also exhibits oleophobic properties corresponding to low surface energy soybean oil, with oil contact angles as high as 115.6 ° and with the contact angle remaining unchanged. And PMI foam oil contact angle is as low as 31.6 °. The FEP/PTFE foam can be better applied to the environment of extremely humid oil-containing media by combining the excellent hydrophobic and oleophobic properties.

Corrosion resistance

The circuit board circuit manufacturing process generally relates to a copper etching process, and is based on a copper-clad plate, after pattern transfer, copper outside a circuit area is etched by pattern electroplating or dry film hole covering, and finally dry film or corrosion-resistant metal is removed to form a circuit. Alkaline etching is a common etching method. Therefore, the copper-clad plate is required to have certain alkali corrosion resistance. After the PMI foam is soaked in 10% NaOH solution at 70 ℃ for 10 hours, the PMI foam macroscopically shows obvious swelling phenomenon. Since PMI molecules and alkali liquor are hydrolyzed, the reaction equation is shown below.

While FEP foam has little change in appearance. Fig. 11 is an SEM image before and after immersing in alkali solution, and it can be seen that the cell wall portion of the PMI foam has been corroded to become macropores and broken pores, while the cell structure of the FEP foam is hardly changed. Corrosion is one of the most serious problems in the industry world, while the excellent corrosion resistance of FEP foam makes it more competitive in the PCB field.

Flame retardant Properties

Vertical burn tests for PMI foam and FEP/PTFE foam are shown in FIG. 12, and when run video is taken on SIVIDEO1, it can be seen that PMI foam is easily ignited (FIGS. 12 a, b) and the dripped melt ignites the underlying cotton layer (FIGS. 12 c, d). The FEP/PTFE foam burned under the bunsen burner flame (f of fig. 12), self-extinguished off-fire (g of fig. 12), and also self-extinguished off-fire after the second ignition for 10s (h, f of fig. 12), and some of the melt was dropped, failing to ignite the cotton layer (e of fig. 12), and almost white from above the post-combustion FEP/PTFE foam (J of fig. 12), demonstrating excellent low smoke flame retardant properties of the FEP/PTFE foam.

Performance prediction calculation of PTFE in-situ fiber-forming FEP microporous material

The invention also predicts and calculates the electromagnetic wave-transmitting performance of the composite foaming material, and the specific process is as follows:

as shown in fig. 13, if the whole foam material is a flat plate with a thickness of L, air on the electromagnetic wave arrival path is defined as medium 1, the foam material flat plate is medium 2, air on the electromagnetic wave arrival path is medium 3, and the refractive index real part and extinction coefficient are n respectively _j 、k _j J is the medium represented (j=1, 2, 3), the complex refractive index is N _j ＝n _j +ik _j . By inputting the real part (epsilon '), imaginary part (epsilon') or tangent (tan delta) of the dielectric constant (which can be obtained by test), and the foaming ratio (ER), pore size (d) _c ) The wave transmittance (T) and the reflectance (R) of the foaming material are obtained.

For a uniform dielectric sheet, this model only considers the vector addition of the first reflection (transmission) and the second reflection (transmission), and the calculation formula of the reflection coefficient and the transmission coefficient is as follows:

r ₁₂ 、r ₂₃ 、t ₁₂ 、t ₂₃ the reflection coefficient and the wave transmission coefficient of the medium 1- & gt 2 and 2- & gt 3 are respectively divided,wherein r is ₁₂ 、r ₂₃ 、t ₁₂ 、t ₂₃ The method can be solved by using a Fresnel formula; beta is the phase thickness, which has the value:

d _w is the average thickness of the walls of the bubble; lambda is the wavelength of the electromagnetic wave, its value in the medium is inversely proportional to the refractive index (which can be calculated here by means of a complex number), c is the speed of light, f is the frequency, n _n For the refractive index of the medium of this layer, n1, n2 and n3 are defined as the refractive indices of the cell wall upper wall, cell wall and cell wall lower wall, respectively.

The total reflectance R and transmittance T are as shown in formula (3):

θ ₁ for incident angle, theta ₂ Is the angle of refraction;

wherein the p value corresponds to the q value, which is an intermediate variable corresponding to the characteristic impedance of the dielectric layer, and the vertical polarization corresponding to p and the horizontal polarization corresponding to q are represented by the formula (4), wherein the p value is substituted into the formula (3) when the vertical polarization is calculated, and the q value is substituted into the formula (3) when the horizontal polarization is calculated, wherein the air p is around the cell wall in the foaming material ₁ ＝p ₃ ，q ₁ ＝q ₃ 。

The horizontal polarization is represented in the formula (5), which can be used for calculating TM mode, and the vertical polarization is represented in the formula (6), which can be used for calculating TE mode, wherein x and y respectively represent the medium before the incident interface and the medium after the interface of the electromagnetic wave:

when x=1, y=2, t can be calculated respectively ₁₂ And r ₁₂ And so on … …

The computation of TM mode (p-wave) and TE mode (s-wave) are independent of each other, and do not affect each other, N ₁ And N ₂ In order to have a complex refractive index,

A＝1-T-R

the wave-transparent rate (T) of TE mode can be obtained _TE ) And reflectivity (R) _TE ) Wave transmittance (T) of TM mode _TM ) And reflectivity (R) _TM )。

After addition of the cell structure, the overall reflectance was R+RT ² +…+RT ^2m . The transmittance and reflectance of the total material were calculated by the formula (5):

in n _c For the number of cell walls encountered when electromagnetic waves penetrate the foam material, n is calculated _c It is assumed that cells are uniformly arranged on an electromagnetic wave transmission line.

The dielectric constant and dielectric loss are indexes which can represent the dielectric performance of the material most, and the inversion method is taken as the thought to provide a method for solving the equivalent dielectric constant of the foaming material. The specific flowchart is shown in fig. 5, and the basic steps can be summarized as follows:

(1) Inputting the counted dielectric constant and dielectric loss of the material before foaming, and inputting the parameters of the foamed material such as pore diameter, foaming multiplying power and the likeObtaining the calculated value T of the wave transmission rate of the foaming material by combining the number with the electromagnetic wave frequency ₁ Calculated by the formula (6).

(2) Then inputting dielectric constant epsilon 'of the unfoamed material with the same thickness as the foamed material' _f And dielectric constant imaginary part epsilon' _f Solution regions (e.g.. Epsilon' _f 1-2 epsilon' _f 0 to 0.01) as a solution interval to directly calculate the wave transmittance T thereof ₂ ；

Lambda is the wavelength, theta ₂ U is the angle of refraction ₂ 、v ₂ The intermediate variables set for simplifying the calculation satisfy the following formula:

ρ ₁₂ 、ρ ₂₃ 、τ ₁₂ 、τ ₂₃ respectively expressed as the reflection coefficient and the transmission coefficient of medium 1- & gt medium 2 and medium 2- & gt medium 3,

representing the reflection phase, calculated by:

p ₁ ＝n ₁ cosθ ₁ ；

n ₁ 、n ₂ and k ₂ Real and imaginary parts obtained by calculating ε', tan δ are determined:

when the subscript j takes the values of 1,2 and 3 respectively, the subscript j represents an air medium on the electromagnetic wave arrival path, a foaming material flat plate medium and an air medium on the electromagnetic wave arrival path respectively; epsilon' is the real part of the dielectric constant; tan delta is the dielectric loss tangent.

Alpha is the analog precision and represents T ₁ And T is ₂ Is a close proximity to (a) to (b).

In the calculation, α is minimized, and at this time, ε 'of the uniform dielectric sheet' _f And epsilon' _f The real part and the imaginary part of the dielectric constant of the foaming material are calculated.

(3) In the calculation, the adopted genetic algorithm continuously approximates alpha to the minimum value by a method of crossing, mutation and iteration, and the value of alpha is 10 when the iteration is performed for about 50 times ^-8 Left and right are approximately 0, and the calculation speed and accuracy are improved through algorithm optimization. Table 2 shows the actual measurement values and the simulation values of the foam materials.

TABLE 2 measured dielectric properties and simulation values for PTFE in situ fiber FEP microporous materials

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Claims (11)

1. The low dielectric loss PTFE in-situ fiber FEP microporous material is characterized in that the main material is FEP and also contains PTFE fibers; the PTFE content in the FEP ranges from 0.1 to 10wt%; PTFE fiber diameter is 100-500nm; the microporous material is prepared by a supercritical foaming method, the pore diameter of the cells of the microporous material is 20-200 mu m, and the density of the cells is 1 multiplied by 10 ⁶ ～1×10 ¹⁰ Individual/cm ³ 。

2. The low dielectric loss PTFE in-situ fiber FEP microporous material according to claim 1, wherein said microporous material has a dielectric constant of 0.00015 to 0.00050 at 2.5GHz to 10 GHz.

3. The low dielectric loss PTFE in-situ fiber FEP microporous material of claim 1, wherein said microporous material has a water drop contact angle of 120-140 ° on the surface.

4. The method of preparing a low dielectric loss PTFE in situ fiber FEP microporous material according to claim 1, comprising the steps of:

step 1, mixing PTFE fibers with FEP master batch, and then performing vulcanization treatment to obtain an FEP/PTFE nanocomposite;

and 2, treating the FEP/PTFE nanocomposite material by a supercritical foaming mode to obtain the microporous material.

5. The method for preparing the low dielectric loss PTFE in-situ fiber-forming FEP microporous material according to claim 1, wherein in said step 1, the temperature of the mixing process is 270-320 ℃; the temperature in the vulcanization process is 300-350 ℃ and the pressure is 5-15MPa.

6. The method for preparing the in-situ fiber-forming FEP microporous material with low dielectric loss PTFE according to claim 1, wherein in the step 2, the supercritical foaming condition is that the temperature is 235-265 ℃, the pressure is 15-20MPa, and the time is 50-200min.

7. Use of the low dielectric loss PTFE in situ fiber-forming FEP microporous material of claim 1 in a wave transparent material.

8. The use of claim 7, wherein the wave-transparent material is a PCB board; in such applications, the PTFE fibers are used to increase the expansion ratio and dielectric properties of the microporous material.

9. The use of claim 7, further comprising a method of predicting wave-transparent performance, comprising the steps of:

calculating the wave transmittance T of the whole foaming material considering the cell structure ₁ The method comprises the steps of carrying out a first treatment on the surface of the Calculating the equivalent wave-transmitting rate T of the material under the condition that the visual material is uniform medium ₂ Calculating the precision alpha of the data set, and calculating the minimum value of the alpha through a genetic algorithm, wherein the corresponding values of the real part and the imaginary part of the equivalent dielectric constant are calculated values;

wherein ε' _f Is the real part of the equivalent dielectric constant, epsilon' _f Is the imaginary part of the equivalent dielectric constant.

10. The use according to claim 9, characterized in that the wave transmission rate T ₁ The method is calculated by the following steps:

θ ₁ for incident angle, n _c D is the number of cell walls encountered when electromagnetic waves penetrate the foam material _c Is the pore size of the cells, d _w Is the average thickness of the walls of the bubble; l is the thickness of the material flat plate;

T _p and T _s The wave transmission coefficients of the TM mode and the TE mode are respectively represented;

T _p/s ＝|t| ² the method comprises the steps of carrying out a first treatment on the surface of the Subscripts represent p or s;

。

imaginary symbol of i, beta is phase thickness, where lambda 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 transmission coefficient of the medium 1-2 and the medium 2-3 respectively;

for TE mode, it is calculated by:

for the TM mode, it is calculated by the following formula:

x and y respectively represent the medium before the incident interface of the electromagnetic wave and the medium after the interface, and when the values are 1,2 and 3, respectively represent the air medium on the path when the electromagnetic wave comes, the foam material flat plate medium and the air medium on the path when the electromagnetic wave comes; n is complex refractive index N _j ＝n _j +ik _j The real part of refractive index and extinction coefficient are respectively n _j 、k _j J is the medium represented, j=1, 2 or 3.

Lambda is the wavelength, theta ₂ U is the angle of refraction ₂ 、v ₂ The intermediate variables set for simplifying the calculation satisfy the following formula:

ρ ₁₂ 、ρ ₂₃ 、τ ₁₂ 、τ ₂₃ respectively expressed as the reflection coefficient and the transmission coefficient of medium 1- & gt medium 2 and medium 2- & gt medium 3,

representing the reflection phase, calculated by: />

p ₁ ＝n ₁ cosθ ₁ ；

n ₁ 、n ₂ And k ₂ Real and imaginary parts obtained by calculating ε', tan δ are determined:

when the subscript j takes the values of 1,2 and 3 respectively, the subscript j represents an air medium on the electromagnetic wave arrival time path, a foaming material flat plate medium and an air medium on the electromagnetic wave arrival time path respectively; epsilon' is the real part of the dielectric constant; tan delta is the dielectric loss tangent.

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