DIELECTRIC MATERIAL AND MANUFACTURING METHOD THEREOF

Abstract

Provided are a dielectric material and a method for manufacturing the same. The dielectric material includes: subjecting a foamed sphere obtained by a primary foaming to a second foaming in a second moulding chamber filled with CO.sub.2 at a second temperature in the range of 20° C. below T.sub.m to 5° C. below T.sub.m and under a second pressure of 15-20 MPa for 30-3600 min to obtain the dielectric material, wherein the primary foaming comprises specific steps of: foaming a foaming material sphere with a diameter of 20-800 mm in a first moulding chamber filled with CO.sub.2 at a first temperature in the range of 80° C. below T.sub.m to 20° C. below T.sub.m and under a first pressure of 15-20 MPa to obtain the foamed sphere. Further provided is a dielectric material manufactured by the method above.

Claims

1. A method for manufacturing a dielectric material, comprising, subjecting a foamed sphere obtained by a primary foaming to a second foaming in a second moulding chamber filled with CO.sub.2 at a second temperature of 20° C. below T.sub.m to 5° C. below T.sub.m and under a second pressure of 15-20 MPa for 30-3600 min to obtain the dielectric material, wherein the primary foaming comprises specific steps of: foaming a foaming material sphere with a diameter of 20-800 mm in a first moulding chamber filled with CO.sub.2 at a first temperature of 80° C. below T.sub.m to 20° C. below T.sub.m and under a first pressure of 15-20 MPa to obtain the foamed sphere, wherein a foaming time for the primary foaming meets the following formula: $t\ge a\times {\left(d/2\right)}^{1.75};$ in which, t is the foaming time for the primary foaming, expressed in hour(s); a=0.07762 h/mm.sup.1.75; and D is the diameter of the foaming material sphere, expressed in mm.

2. The method of claim 1, wherein the first temperature is in the range of 50° C. below T.sub.m to 30° C. below T.sub.m; and/or the first pressure is in the range of 15-18 MPa; and/or the first and second moulding chambers used in the primary foaming and the second foaming are in a shape of sphere; and/or the second temperature is in the range of 10° C. below T.sub.m to 15° C. below T.sub.m; and/or a foaming time for the second foaming is 60-3000 min; and/or the second pressure is in the range of 15-20 MPa, preferably 15-18 MPa.

3. The method of claim 1, wherein the primary foaming further comprises, after foaming, depressurizing the first moulding chamber at a rate of preferably at least 100 MPa/s; preferably, after depressurizing, leaving the foamed sphere to stand for at least 24 hours.

4. The method of claim 1, wherein the method further comprises depressurizing the second moulding chamber after the second foaming at a rate of preferably at least 100 MPa/s.

5. The method of claim 1, wherein the foaming material sphere is a solid sphere; and/or the foaming material sphere is obtained by injection molding; and/or a foaming material of the foaming material sphere is polyolefin or polyester; and/or the diameter of the foaming material sphere is 20-200 mm.

6. The method of claim 1, wherein a foaming material of the foaming material sphere is selected from the group consisting of polyethylene, polypropylene, polybutene, and polyethylene terephthalate; preferably, the foaming material of the foaming material sphere is one or more selected from the group consisting of polypropylene homopolymer, ethylene-propylene copolymer, and ethylene-propylene-butene copolymer; more preferably, the foaming material of the foaming material sphere is selected from the group consisting of ethylene-propylene copolymer and ethylene-propylene-butene copolymer.

7. The method of claim 1, wherein the diameter of the foaming material sphere is 20-200 mm, the first temperature is in the range of 50° C. below T.sub.m to 30° C. below T.sub.m, the first pressure is in the range of 15-18 MPa, the foaming time for the primary foaming is 262-14730 min, the second temperature is in the range of 15° C. below T.sub.m to 10° C. below T.sub.m, the second pressure is in the range of 15-18 MPa, and the foaming time for the second foaming is 30-3600 min; preferably, the diameter of the foaming material sphere is 90-110 mm, the first temperature is in the range of 50° C. below T.sub.m to 40° C. below T.sub.m, the first pressure is in the range of 15-16 MPa, the foaming time for the primary foaming is 4300-4500 min, the second temperature is in the range of 12° C. below T.sub.m to 10° C. below T.sub.m, the second pressure is in the range of 15-16 MPa, and the foaming time for the second foaming is 1100-1300 min; preferably, the diameter of the foaming material sphere is 190-210 mm, the first temperature is in the range of 50° C. below T.sub.m to 40° C. below T.sub.m, the first pressure is in the range of 15-16 MPa, the foaming time for the primary foaming is 14700-14730 min, the second temperature is in the range of 12° C. below T.sub.m to 10° C. below T.sub.m, the second pressure is in the range of 15-16 MPa, and the foaming time for the second foaming is 3000-4000 min.

8. A dielectric material obtained by the method of claim 1, comprising a dielectric material body in a shape of sphere.

9. The dielectric material of claim 8, wherein a density of the dielectric material body changes from large to small in the direction from inside to outside.

10. The dielectric material of claim 8, wherein the dielectric material body has a diameter of 30 mm to 1000 mm; and/or an outer side of the dielectric material body is provided with a protective layer; preferably, the protective layer is selected from the group consisting of a polypropylene coated film, a polyethylene coated film, and a polyethylene terephthalate coated film; and/or a dielectric constant of the dielectric material body in a radial direction gradually changes from 2.08 to 1.04, and a change rule is in accordance with the following formula: ${\epsilon }_r=2-{\left(\frac{r}{R}\right)}^2,$ wherein r is a distance from a point in the radial direction of the dielectric material body to a center of the dielectric material body, ε.sub.r is the dielectric constant at a point in the radial direction of the dielectric material body, and R is a radius of the dielectric material body.

11. The method of claim 2, wherein the primary foaming further comprises, after foaming, depressurizing the first moulding chamber at a rate of preferably at least 100 MPa/s; preferably, after depressurizing, leaving the foamed sphere to stand for at least 24 hours.

12. The method of claim 2, wherein the method further comprises depressurizing the second moulding chamber after the second foaming at a rate of preferably at least 100 MPa/s.

13. The method of claim 3, wherein the method further comprises depressurizing the second moulding chamber after the second foaming at a rate of preferably at least 100 MPa/s.

14. The method of claim 2, wherein the foaming material sphere is a solid sphere; and/or the foaming material sphere is obtained by injection molding; and/or a foaming material of the foaming material sphere is polyolefin or polyester; and/or the diameter of the foaming material sphere is 20-200 mm.

15. The method of claim 3, wherein the foaming material sphere is a solid sphere; and/or the foaming material sphere is obtained by injection molding; and/or a foaming material of the foaming material sphere is polyolefin or polyester; and/or the diameter of the foaming material sphere is 20-200 mm.

16. The method of claim 4, wherein the foaming material sphere is a solid sphere; and/or the foaming material sphere is obtained by injection molding; and/or a foaming material of the foaming material sphere is polyolefin or polyester; and/or the diameter of the foaming material sphere is 20-200 mm.

17. The method of claim 2, wherein a foaming material of the foaming material sphere is selected from the group consisting of polyethylene, polypropylene, polybutene, and polyethylene terephthalate; preferably, the foaming material of the foaming material sphere is one or more selected from the group consisting of polypropylene homopolymer, ethylene-propylene copolymer, and ethylene-propylene-butene copolymer; more preferably, the foaming material of the foaming material sphere is selected from the group consisting of ethylene-propylene copolymer and ethylene-propylene-butene copolymer.

18. The method of claim 2, wherein the diameter of the foaming material sphere is 20-200 mm, the first temperature is in the range of 50° C. below T.sub.m to 30° C. below T.sub.m, the first pressure is in the range of 15-18 MPa, the foaming time for the primary foaming is 262-14730 min, the second temperature is in the range of 15° C. below T.sub.m to 10° C. below T.sub.m, the second pressure is in the range of 15-18 MPa, and the foaming time for the second foaming is 30-3600 min; preferably, the diameter of the foaming material sphere is 90-110 mm, the first temperature is in the range of 50° C. below T.sub.m to 40° C. below T.sub.m, the first pressure is in the range of 15-16 MPa, the foaming time for the primary foaming is 4300-4500 min, the second temperature is in the range of 12° C. below T.sub.m to 10° C. below T.sub.m, the second pressure is in the range of 15-16 MPa, and the foaming time for the second foaming is 1100-1300 min; preferably, the diameter of the foaming material sphere is 190-210 mm, the first temperature is in the range of 50° C. below T.sub.m to 40° C. below T.sub.m, the first pressure is in the range of 15-16 MPa, the foaming time for the primary foaming is 14700-14730 min, the second temperature is in the range of 12° C. below T.sub.m to 10° C. below T.sub.m, the second pressure is in the range of 15-16 MPa, and the foaming time for the second foaming is 3000-4000 min.

19. The method of claim 3, wherein the diameter of the foaming material sphere is 20-200 mm, the first temperature is in the range of 50° C. below T.sub.m to 30° C. below T.sub.m, the first pressure is in the range of 15-18 MPa, the foaming time for the primary foaming is 262-Use Gap Code 14730 min, the second temperature is in the range of 15° C. below T.sub.m to 10° C. below T.sub.m, the second pressure is in the range of 15-18 MPa, and the foaming time for the second foaming is 30-3600 min; preferably, the diameter of the foaming material sphere is 90-110 mm, the first temperature is in the range of 50° C. below T.sub.m to 40° C. below T.sub.m, the first pressure is in the range of 15-16 MPa, the foaming time for the primary foaming is 4300-4500 min, the second temperature is in the range of 12° C. below T.sub.m to 10° C. below T.sub.m, the second pressure is in the range of 15-16 MPa, and the foaming time for the second foaming is 1100-1300 min; preferably, the diameter of the foaming material sphere is 190-210 mm, the first temperature is in the range of 50° C. below T.sub.m to 40° C. below T.sub.m, the first pressure is in the range of 15-16 MPa, the foaming time for the primary foaming is 14700-14730 min, the second temperature is in the range of 12° C. below T.sub.m to 10° C. below T.sub.m, the second pressure is in the range of 15-16 MPa, and the foaming time for the second foaming is 3000-4000 min.

20. The dielectric material of claim 9, wherein the dielectric material body has a diameter of 30 mm to 1000 mm; and/or an outer side of the dielectric material body is provided with a protective layer; preferably, the protective layer is selected from the group consisting of a polypropylene coated film, a polyethylene coated film, and a polyethylene terephthalate coated film; and/or a dielectric constant of the dielectric material body in a radial direction gradually changes from 2.08 to 1.04, and a change rule is in accordance with the following formula: ${\epsilon }_r=2-{\left(\frac{r}{R}\right)}^2,$ wherein r is a distance from a point in the radial direction of the dielectric material body to a center of the dielectric material body, ε.sub.r is the dielectric constant at a point in the radial direction of the dielectric material body, and R is a radius of the dielectric material body.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0038] FIG. 1 shows a scanning electron microscope image of product according to Example 1.

[0039] FIG. 2 is a diagram showing the dielectric constant as a function of radius according to examples and comparative examples.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0040] The present disclosure is further described by exemplary embodiments below, but is not thus limited to the scope of the embodiments. The experimental methods that not specifies the specific conditions in the following examples are selected according to the conventional methods and conditions, or according to the commodity instructions.

[0041] In the following examples and comparative examples, the solid spheres of polypropylene homopolymer were obtained by conventional injection molding of commercially available polypropylene particles using an injection molding machine.

[0042] In the present disclosure, the melting point is measured by a differential scanning calorimeter (NETZSCHDSC 204HP, Germany). The testing steps are as follow: the temperature is raised from 25° C. to 200° C. at a heating rate of 10° C./min, and held at the raised temperature for 5 min. The temperature is then declined to 50° C. at a cooling rate of 10° C./min, and held at the declined temperature for 5 min. Finally, the temperature is raised again to 200° C. at a heating rate of 10° C./min. The peak value in the second heating curve corresponds to the melting point.

[0043] In the present disclosure, the density of the foamed sample is measured by a drainage method, and the foaming ratio is calculated by the following formula:

[00003]$\begin{array}{cc}{R}_V={\rho }_0/{\rho }_f& \left(2‐6\right)\end{array}$

wherein, ρ.sub.0 and ρ.sub.f are the sample density before and after foaming, respectively.

[0044] In the following examples and comparative examples, the dielectric constant of the dielectric materials was obtained by conversion from density, and the conversion formula is as follows:

[00004]${\epsilon }_r=\frac{{\epsilon }_{\mathrm{pp}}}{{\rho }_{\mathrm{pp}}/{\rho }_{\mathrm{foam}}}+\frac{{\epsilon }_{\mathrm{air}}\left({\rho }_{\mathrm{pp}}/{\rho }_{\mathrm{foam}}-1\right)}{{\rho }_{\mathrm{pp}}/{\rho }_{\mathrm{foam}}}$

wherein ε.sub.r is a dielectric constant at a point in the radial direction of the dielectric material body; ε.sub.pp is a dielectric constant of the propylene homopolymer solid sphere, with a value of 2.2; ρ.sub.pp is a density of the propylene homopolymer solid sphere, with a value of 900 kg/m.sup.3; ε.sub.air is a dielectric constant of air, with a value of 1; ρ.sub.foam is a density at a point in the radial direction of the dielectric material body. The density at a point in the radial direction of the dielectric material body can be measured using a conventional drainage method in the art, which may include: cutting the dielectric material body along a first spherical surface and a second spherical surface to obtain a tested part containing the point, and measuring the density of the tested part by a drainage method (that is, the density at the point in the radius direction of the dielectric material body), wherein the first spherical surface and the second spherical surface are complete spherical surface; the spherical centers of the first spherical surface and the second spherical surface coincide with the spherical center of the dielectric material body; the thickness of the tested part (that is, the difference between the radials of the first spherical surface and the second spherical surface) is 1 mm; and the point is located at the center in the thickness direction of the tested part.

[0045] Unless otherwise specified, the dielectric constant in the present disclosure refers to the relative dielectric constant.

Example 1

[0046] A polypropylene homopolymer solid sphere (having a T.sub.m of 155° C.) with a diameter of 100 mm was placed into a spherical moulding chamber (with a diameter of 300 mm).

[0047] The moulding chamber was heated to a first foaming temperature of 110° C. by programmed temperature control at a heating rate of 10° C./mm. The moulding chamber was kept at the first foaming temperature. CO.sub.2 was injected thereto until a pressure of injected CO.sub.2 reached 15 MPa. A foaming was conducted for 4400 min at this pressure.

[0048] The moulding chamber was rapidly depressurized at a rate of 400 MPa/s. The foamed sphere after a primary foaming was took out from the moulding chamber and left to stand for 24 hours. Then, the foamed sphere (with a diameter of 104 mm) was placed into the spherical moulding chamber again. The moulding chamber was heated to a second foaming temperature of 145° C. by programmed temperature control at a heating rate of 10° C./mm. The moulding chamber was kept at the second foaming temperature. CO.sub.2 was injected thereto until a pressure of injected CO.sub.2 reached 15 MPa. A second foaming was conducted for 1200 min at this pressure. The moulding chamber was rapidly depressurized at a rate of 400 MPa/s to obtain a dielectric material with gradually changing dielectric constant. The dielectric material comprises a spherical dielectric material body, wherein the density of the dielectric material body changes from large to small in the direction from inside to outside. The scanning electron microscope image of the cross section of the spherical dielectric material passing through the spherical center is shown in FIG. 1, in which the lower a position in the image is, the closer it is to the spherical center. From FIG. 1, it can be seen that there are gas pores distributed in the spherical dielectric material, and the closer they are to the spherical center, the smaller the size of the gas pores is. At the position of the spherical center, the spherical dielectric material is solid.

Example 2

[0049] A polypropylene homopolymer solid sphere (having a T.sub.m of 155° C.) with a diameter of 200 mm was placed into a spherical moulding chamber (with a diameter of 300 mm). The moulding chamber was heated to a first foaming temperature of 115° C. by programmed temperature control at a heating rate of 10° C./mm. The moulding chamber was kept at the first foaming temperature. CO.sub.2 was injected thereto until a pressure of injected CO.sub.2 reached 15 MPa. A foaming was conducted for 14730 min at this pressure. The moulding chamber was rapidly depressurized at a rate of 400 MPa/s. The foamed sphere after a primary foaming was took out from the moulding chamber and left to stand for 24 hours. Then, the foamed sphere (with a diameter of 210 mm) was placed into the spherical moulding chamber again. The moulding chamber was heated to a second foaming temperature of 145° C. by programmed temperature control at a heating rate of 10° C./mm. The moulding chamber was kept at the second foaming temperature. CO.sub.2 was injected thereto until a pressure of injected CO.sub.2 reached 15 MPa. A second foaming was conducted for 3600 min at this pressure. The moulding chamber was rapidly depressurized at a rate of 400 MPa/s to obtain a dielectric material with gradually changing dielectric constant. The dielectric material comprises a spherical dielectric material body, wherein the density of the dielectric material body changes from large to small in the direction from inside to outside.

Example 3

[0050] A polypropylene homopolymer solid sphere (having a T.sub.m of 155° C.) with a diameter of 100 mm was placed into a spherical moulding chamber. The moulding chamber was heated to a first foaming temperature of 130° C. by programmed temperature control at a heating rate of 10° C./mm. The moulding chamber was kept at the first foaming temperature. CO.sub.2 was injected thereto until a pressure of injected CO.sub.2 reached 15 MPa. A foaming was conducted for 4400 min at this pressure. The moulding chamber was rapidly depressurized at a rate of 400 MPa/s. The foamed sphere after a primary foaming was took out from the moulding chamber and left to stand for 24 hours. Then, the foamed sphere (with a diameter of 110 mm) was placed into the spherical moulding chamber again. The moulding chamber was heated to a second foaming temperature of 145° C. by programmed temperature control at a heating rate of 10° C./mm. The moulding chamber was kept at the second foaming temperature. CO.sub.2 was injected thereto until a pressure of injected CO.sub.2 reached 15 MPa. A second foaming was conducted for 1200 min at this pressure. The moulding chamber was rapidly depressurized at a rate of 400 MPa/s to obtain a dielectric material with gradually changing dielectric constant. The dielectric material comprises a spherical dielectric material body, wherein the density of the dielectric material body changes from large to small in the direction from inside to outside.

Example 4

[0051] A polypropylene homopolymer solid sphere (having a T.sub.m of 155° C.)with a diameter of 100 mm was placed into a spherical moulding chamber. The moulding chamber was heated to a first foaming temperature of 110° C. by programmed temperature control at a heating rate of 10° C./mm. The moulding chamber was kept at the first foaming temperature. CO.sub.2 was injected thereto until a pressure of injected CO.sub.2 reached 15 MPa. A foaming was conducted for 4400 min at this pressure. The moulding chamber was rapidly depressurized at a rate of 400 MPa/s. The foamed sphere after a primary foaming was took out from the moulding chamber and left to stand for 24 hours. Then, the foamed sphere (with a diameter of 104 mm) was placed into the spherical moulding chamber again. The moulding chamber was heated to a second foaming temperature of 145° C. by programmed temperature control at a heating rate of 10° C./mm. The moulding chamber was kept at the second foaming temperature. CO.sub.2 was injected thereto until a pressure of injected CO.sub.2 reached 15 MPa. A second foaming was conducted for 1200 min at this pressure. The moulding chamber was rapidly depressurized at a rate of 400 MPa/s to obtain a dielectric material with gradually changing dielectric constant. The dielectric material comprises a spherical dielectric material body, wherein the density of the dielectric material body changes from large to small in the direction from inside to outside.

Examples 5-8

[0052] These examples were conducted similar to example 1 except that the parameters in the following table are different:

TABLE-US-00001 The diameter Foaming Foaming of a time time foaming The The for the The The for the material first first primary second second second sphere temperate pressure foaming temperate pressure foaming Example 5 20 mm 110° C. 18 MPa  265 min 145° C. 15 MPa 150 min Example 6 50 mm 110° C. 18 MPa 1310 min 145° C. 15 MPa 400 min Example 7 80 mm 110° C. 18 MPa 3000 min 145° C. 15 MPa 800 min Example 8 150 mm  110° C. 18 MPa 8910 min 145° C. 15 MPa 2600 min 

[0053] Dielectric materials with gradually changing dielectric constant were obtained according to examples 5-8. The dielectric material comprises a spherical dielectric material body, wherein the density of the dielectric material body changes from large to small in the direction from inside to outside

Comparative Example 1

[0054] A polypropylene homopolymer solid sphere (having a T.sub.m of 155° C.)with a diameter of 100 mm was placed into a spherical moulding chamber. The moulding chamber was heated to a first foaming temperature of 110° C. by programmed temperature control at a heating rate of 10° C./mm. The moulding chamber was kept at the first foaming temperature. CO.sub.2 was injected thereto until a pressure of injected CO.sub.2 reached 15 MPa. The moulding chamber was kept at this pressure for 300 min. The moulding chamber was rapidly depressurized at a rate of 400 MPa/s. The foamed sphere after a primary foaming was took out from the moulding chamber and left to stand for 24 hours. And then the foamed sphere (with a diameter of 104 mm) was placed into the spherical moulding chamber again. The moulding chamber was heated to a second foaming temperature of 145° C. by programmed temperature control at a heating rate of 10° C./mm. The moulding chamber was kept at the second foaming temperature. CO.sub.2 was injected thereto until a pressure of injected CO.sub.2 reached 15 MPa. The moulding chamber was kept for 50 min at this pressure. The moulding chamber was rapidly depressurized at a rate of 400 MPa/s to obtain a spherical dielectric material of which the dielectric constant does not change gradually.

Comparative Example 2

[0055] A polypropylene homopolymer solid sphere (having a T.sub.m of 155° C.) with a diameter of 100 mm was placed into a spherical moulding chamber. The moulding chamber was heated to a first foaming temperature of 110° C. by programmed temperature control at a heating rate of 10° C./mm. The moulding chamber was kept at the first foaming temperature. CO.sub.2 was injected thereto until a pressure of injected CO.sub.2 reached 15 MPa. The moulding chamber was kept at this pressure for 300 min. The moulding chamber was rapidly depressurized at a rate of 400 MPa/s. The foamed sphere after a primary foaming was took out from the moulding chamber and left to stand for 24 hours. Then, the foamed sphere (with a diameter of 104 mm) was placed into the spherical moulding chamber again. The moulding chamber was heated to a second foaming temperature of 145° C. by programmed temperature control at a heating rate of 10° C./mm. The moulding chamber was kept at the second foaming temperature. CO.sub.2 was injected thereto until a pressure of injected CO.sub.2 reached 15 MPa. The moulding chamber was kept for 150 min at this pressure. The moulding chamber was rapidly depressurized at a rate of 400 MPa/s to obtain a spherical dielectric material of which the dielectric constant does not change gradually.

Comparative Example 3

[0056] A polypropylene homopolymer solid sphere (having a T.sub.m of 155° C.) with a diameter of 100 mm was placed into a spherical moulding chamber. The moulding chamber was heated to a foaming temperature of 145° C. by programmed temperature control at a heating rate of 10° C./mm. The moulding chamber was kept at the foaming temperature. CO.sub.2 was injected thereto until a pressure of injected CO.sub.2 reached 15 MPa. The moulding chamber was kept at this pressure for 100 min. Then, The moulding chamber was rapidly depressurized at a rate of 400 MPa/s to obtain a spherical dielectric material of which the dielectric constant does not change gradually.

Effect Example

[0057] The dielectric constant of the Luneburg lens manufactured by the methods according to Examples 1 and 2 can vary with the rule, as shown in FIG. 2. The first foaming temperature in Example 3 is too high, so that the gas pore size of the sphere obtained by the first foaming is too large, which results in that the gas pore grows too largely during the second foaming, and therefore the relative dielectric constant is low. In example 4, the standing time is too short, resulting in that CO.sub.2 can not completely escape from the sphere, which causes excessive CO.sub.2 permeation in the sphere during the second saturation, and low relative dielectric constant. In comparative example 1, the second saturation time is too short, which leads to too little CO.sub.2 permeation and high relative dielectric constant. In comparative example 2, the second saturation time is too long, which leads to too much CO.sub.2 permeation and low relative dielectric constant. In Comparative example 3, a first foaming is not performed, and therefore foaming does not exist at the core of the sphere, while complete foaming exists at the surface of the sphere, and thus the relative dielectric constant is higher near the core of the sphere and lower near the surface.

[0058] While specific embodiments have been described above in the present disclosure, it should be understood for those skilled in the art that the embodiments are exemplary illustration only and that various changes or modifications may be made to the embodiments without departing from the principles and essence of the present disclosure. Therefore, the scope of protection in the present disclosure is defined by the appended claims.