JP2006111708A - Foamed material having low dielectric constant and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a foamed material having a low dielectric constant, which is a thin film, is smooth and has heat resistance and to provide a method for producing the same. <P>SOLUTION: The method for producing a formed material comprises: a process for holding a material in a container, raising pressure in the container to fixed pressure by using inorganic gas, heating the inside of the container to a given temperature and keeping the temperature for a fixed time; and a process for reducing the pressure to atmospheric pressure after passage of a fixed time. The formed material has 1.0-3.0 dielectric constant, 10%-90% porosity, and 0.01-10 μm average pore diameter. The material is a polyamide such as a thermoplastic polyimide, a polyether imide, etc. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a low dielectric constant foam material and a method for producing the same. Specifically, the present invention relates to a foam material using polyimide and a manufacturing method thereof. A foam material is produced by impregnating polyimide with an inorganic gas in a supercritical state. The obtained material has a low dielectric constant and is used as a heat-resistant insulating material for electrical and electronic equipment, aerospace equipment, and military equipment as a high insulating material.

  With the speeding up of information processing of information / communication equipment and the higher frequency of communication radio waves in the highly information-oriented society, wiring boards on which electronic components and the like are mounted are also required to have high frequency performance. For example, an insulating layer of a wiring board is required to have a low dielectric constant (ε) and dielectric loss tangent (tan δ) at high frequencies in order to exhibit excellent high frequency transmission characteristics and low crosstalk characteristics.

  For example, an energy loss in the transmission process called dielectric loss occurs in the circuit of the wiring board, and this dielectric loss is proportional to the product f × ε × tanδ of the signal frequency f and the dielectric loss tangent tanδ of the material with relative permittivity ε. Have For this reason, in the case of a wiring board used for high frequency applications with a high frequency f, a material having a small relative dielectric constant ε and dielectric loss tangent tanδ is required. Further, since the signal transmission speed is inversely proportional to the 1/2 power of the dielectric constant ε, a material having a small dielectric constant is required for high frequency applications.

  For materials used for such high-frequency applications, it is conceivable to deal with such a method as adding an insulating layer to the material, but in general, the material itself is often used with a low dielectric constant. Has been done. Examples of the material to be used include a fluorine-containing polymer such as polytetrafluoroethylene as a low dielectric constant resin material, and a polyimide resin.

  Even polytetrafluoroethylene as a low dielectric constant resin material has a high dielectric constant of about 2.1. For this reason, as an attempt to lower the dielectric constant, attention has been paid to the fact that the dielectric constant of a plastic material is determined by the molecular skeleton, and methods for modifying the molecular skeleton have been performed. However, it is said that there is a limit to controlling the dielectric constant to be lowered by a method of modifying the molecular skeleton.

  As another method, there is a method in which the dielectric constant of the plastic material is controlled to be low by making the plastic material porous and including air by utilizing the low dielectric constant of air having a dielectric constant of 1.

  Generally, a dry method and a wet method are known as methods for producing a porous material. The dry method includes a physical foaming method and a chemical foaming method. The physical foaming method is a method in which cells are formed in a polymer by dispersing a low boiling point liquid such as chlorofluorocarbons and hydrocarbons in a polymer, and then volatilizing a foaming agent by heating. However, the physical foaming method is said to be a suitable method for obtaining a foam material having a cell diameter of several tens of μm or more, but it is difficult to control to obtain a foam material having a fine and uniform cell diameter. It is said. In addition, the effects on the environment such as the harmfulness of substances used as foaming agents and the destruction of the ozone layer caused by the substances have been pointed out.

  The chemical foaming method is a method in which a compound added in a polymer is thermally decomposed and cells are formed by a gas generated by the thermal decomposition to obtain a foamed material. However, the chemical foaming method has been pointed out that the added compound leaves a residue in the product after thermal decomposition. Especially in applications of electronic parts, etc., it is pointed out that the required level of low contamination of parts is high, which may cause problems such as effects caused by corrosive gas generated by this chemical foaming method and contamination by residues etc. Has been.

  As a method to obtain a foam material with a small cell diameter and a high cell density, a gas such as nitrogen or carbon dioxide is dissolved in the polymer at a high pressure, and then the pressure is released until the glass transition temperature or softening point of the polymer is reached. A method of forming a cell by heating is performed.

  For example, in JP-A-6-322168, a thermoplastic polymer is charged into a pressure vessel, heated to or near the softening point of the polymer, gas is charged, the pressure is adjusted to a supercritical fluid state, and the polymer is added. A method of producing a low density porous foam article by a method of saturating and rapidly releasing pressure is disclosed.

  Japanese Patent Laid-Open No. 2001-55464 discloses a heat-resistant polymer foam material in which a heat-resistant polymer is impregnated with a non-reactive gas under pressure, then the pressure is reduced, and then heated by heating at a temperature exceeding 120 ° C. It is disclosed that a heat-resistant polymer foam material having an average pore diameter of 0.01 or more and less than 10 μm is obtained by a production method.

As a wet method, there has been proposed a method of making a porous polymer by dispersing a hydrophilic polymer that can be baked or solvent-extracted in the polymer and removing the hydrophilic polymer. For example, in JP 2003-26850 A, a step of preparing a resin solution containing a polyimide resin precursor, a dispersible compound dispersible with respect to the polyimide resin precursor, and a solvent, a resin solution is applied, The step of forming a film in which the dispersible compound is dispersed in the polyimide resin precursor by drying the solvent, the step of extracting the dispersible compound from the film by extraction with an extraction solvent, and the film at a temperature range of 190 to 250 ° C It is disclosed that a porous polyimide resin is produced by a step of preheating and a step of imidizing a film.
JP-A-6-322168 JP-A-2001-55464 Japanese Patent Laid-Open No. 2003-26850

  The present invention relates to a low dielectric constant foam material and a method for producing the same. Specifically, the present invention relates to a foam material using polyimide and a manufacturing method thereof. A foam material is produced by impregnating polyimide with an inorganic gas in a supercritical state.

As a result of intensive studies, the present inventors have found that a foamed material having smoothness, excellent surface properties, low dielectric constant, excellent heat resistance and insulating performance can be obtained by the following production method.
That is, a step of keeping the material in the container, pressurizing the inside of the container to a predetermined pressure using an inorganic gas, and then heating the inside of the container to a predetermined temperature and holding it for a predetermined period of time; A method for producing a foam material, comprising a step of reducing the pressure to atmospheric pressure. Furthermore, the manufacturing method of a foam material including the process of pressurizing and heating the said foam material and shaping.

  In the manufacturing method of the said foaming material, the said pressure is 1-50 Mpa, The said temperature is room temperature-120 degreeC. Preferably, the pressure is 7 to 35 MPa, and the temperature is 50 to 100 ° C.

  The inorganic gas is selected from the group consisting of nitrogen, carbon dioxide, air, or a combination thereof. The inorganic gas is preferably in a supercritical state. The inorganic gas impregnates 1 to 5% by weight of the material.

The characteristics of the foam material obtained by the above production method are as follows.
The dielectric constant is 1.0 to 3.0, and the porosity is 10% to 90%. The average pore diameter is 0.01 to 10 μm. The thickness is 35 μm to 150 μm.

  The material is a foamed material that is polyimide, or preferably a foamed material in which the polyimide is selected from the group consisting of thermoplastic polyimide, polyetherimide, or a combination thereof. More preferred is a foamed material having a crystallinity of 0 to less than 20%.

  Specifically, in the present invention, a thermoplastic polyimide is used as a material, a supercritical inorganic gas is impregnated into the thermoplastic polyimide material under high pressure, and then the pressure is released to obtain a foamed thermoplastic polyimide material. it can.

  In this invention, the process of pressurizing and heating and molding the foam material obtained by the said method can further be included. In this case, it is preferable that the pressurization is 10 to 30 kg / cm 2, the heating is 150 to 350 ° C., and the process is 30 seconds to 3 minutes. When the material is polyimide or polyetherimide, it is preferable that the pressure is 20 kg / cm 2, the heating is 250 ° C., and the process is 1 minute.

  In the present invention, prior to the method for producing the foamed material, a step of previously forming the material into a thin film having a thickness of 35 μm to 150 μm can be further included.

Embodiment of the Invention

Form of Material As a form of the polyimide resin that can be used in the present invention, a film or a sheet is preferable. The thickness is 15 to 500 μm, preferably 35 to 150 μm. When the thickness is 150 μm or more, it is possible to produce a low dielectric constant foam material according to the present invention. However, since market needs require use in small circuits, thinning, weight reduction, and flexibility must be considered to satisfy this need. In view of such needs, it is inappropriate to provide a foam material using a material of 150 μm or more. Further, when the thickness is 35 μm or less, it is difficult to make it smooth, it becomes difficult to hold the supercritical inorganic gas in the resin, and it becomes difficult to foam.
Polyimide resin The structure of the polyimide resin that can be used in the present invention is preferably 0 to 20% or less in terms of crystallinity. When the degree of crystallinity is 20% or more, free space is reduced in the resin structure, and it becomes difficult to impregnate a sufficient supercritical inorganic gas necessary for foaming. For this reason, it becomes difficult to obtain the intended foamed material.

  This is a fact that is proved by the following theory. That is, in a two-phase model in which an amorphous region (amorphous region) and a crystalline region cannot be mixed even if a small molecule such as helium penetrates the crystalline region, the solubility coefficient S is the solubility Sa of the amorphous polymer. And the volume fraction α of the amorphous region is expressed as follows.

S = α × Sa
Further, the diffusion coefficient D is expressed by the following equation with the diffusion coefficient Da of the amorphous polymer, the geometrical inhibition factor τ, and the immobilization factor β of the polymer chain.

D = Da / τβ
Here, τ is a factor for bypassing a crystal region that cannot be diffused, and β is a factor that inhibits diffusion by forming a bridge structure in the crystal region.

  The temperature characteristics of the polyimide resin that can be used in the present invention are preferably those having a melting point Tm of 350 ° C. or higher, a glass transition temperature Tg of 200 ° C. or higher, and a crystallization temperature Tc of 320 ° C. or higher. Moreover, about polyetherimide, that whose glass transition temperature Tg is 200 degreeC or more is preferable. In particular, a suitable polyimide resin used in the present invention can raise Tc by being recrystallized by reheating after foaming, and a low dielectric constant foam material having higher heat resistance can be obtained.

  Further, as a polyimide resin having temperature characteristics other than those described above, those having no melting point Tm and crystallization temperature Tc and having a glass transition temperature Tg of 200 ° C. or more can be used.

Further, as a mechanical property of the polyimide resin that can be used in the present invention, as a physical property capable of retaining the inorganic gas without changing its shape, a storage elastic modulus G ′ = 10 ⁸ Pa or more at a temperature at which the supercritical inorganic gas is impregnated In addition, as a physical property for sufficient foaming, it is preferable that the loss elastic modulus G ″ = 10 ⁷ Pa or more at a temperature at which the inorganic gas in the supercritical state is impregnated.

  Furthermore, the electrical characteristics of the polyimide resin used in the present invention are preferably those having a dielectric constant of about 3.2 and a dielectric loss tangent of about 0.0063. However, a material having the same effect as that of the present invention can be obtained using a polyimide resin not limited to this range.

In addition, about the polyimide resin film which can be used by this invention, you may use what melt | fused or adhere | attached metal foil on the single side | surface or both surfaces.
The manufacturing method of a foam material according to this invention is shown below.

That is, a step of keeping the material in the container, pressurizing the inside of the container to a predetermined pressure using an inorganic gas, and then heating the inside of the container to a predetermined temperature and holding it for a predetermined period of time; Including reducing pressure to atmospheric pressure,
This is a foaming method of the material.

  Furthermore, it is the foaming method of the said material including the process of pressurizing and heating a foaming material and shaping. The foaming method of the material, wherein the pressure is 7 to 35 MPa, and the temperature is 50 to 100 ° C.

In the method for foaming the material, the inorganic gas is a gas selected from the group consisting of nitrogen, carbon dioxide, air, or a combination thereof.
In the method of foaming the material, the inorganic gas is in a supercritical state.

The material is foamed by impregnating the thin film material with 1 to 5% by weight of the inorganic gas.
The foaming method of the material further includes a step of previously setting the material to a thickness of 35 μm to 150 μm.

An example of the manufacturing method for obtaining the foam material in this invention is shown below.
Step a A thin polyimide resin film as shown above is put into a pressure vessel.
Step b Inorganic pressure is impregnated in the pressure vessel under a constant pressure and temperature and maintained for a predetermined time.
c process The inside of a pressure-resistant container is open | released to a normal pressure after predetermined time progress, and inorganic gas is hold | maintained in a polyimide resin film.
d process A polyimide resin film is taken out from a gas impregnation process, and the molded object impregnated with inorganic gas is obtained using the apparatus which can pressurize and heat.

Specific Elements / Conditions for Process b Specific elements / conditions for process b will be described in detail below.
Inorganic gas First, as the inorganic gas to be used, it is preferable to use an inert gas having a small environmental load, and carbon dioxide, nitrogen, or air is particularly preferable. When the polyimide resin film is impregnated with this gas, the inorganic gas is optimally in a supercritical state. The supercritical state is a non-condensed fluid that exceeds the critical temperature and pressure of the gas-liquid. In this state, since the critical temperature is exceeded, the molecule is in intense thermal motion, and since the critical pressure is exceeded, it has a high density comparable to the liquid. For example, the critical point in carbon dioxide is 31.1 ° C./7.3 MPa, and the critical point in nitrogen is −147.1 ° C./3.4 MPa. By bringing the inorganic gas into a supercritical state, the polyimide resin film can be quickly dissolved and diffused.

  The amount of the polyimide resin impregnated with the inorganic gas is preferably in the range of 1 to 5 wt% with respect to the polyimide resin. When the impregnation amount is 1 wt% or less, it is difficult to obtain a high foaming rate because the amount (saturation amount) of the inorganic gas necessary for foaming is insufficient. Further, the present inventors have confirmed from the experimental results of the present invention that foaming can be performed even with an impregnation amount of 5 wt% or more.

Measuring method of inorganic gas impregnation amount The measuring method of the inorganic gas impregnation amount with respect to the polyimide resin film is as follows.
First, the weight of the polyimide resin film before impregnating with the inorganic gas is weighed (for example, measured to 4 digits after the decimal point with an electronic balance).

  Next, after impregnating the polyimide resin with an inorganic gas, the polyimide resin is quickly taken out from the pressure vessel, and the weight of the polyimide resin impregnated with the inorganic gas is weighed (for example, measured to 4 digits after the decimal point with an electronic balance). .

The obtained measurement result is applied to the following equation to calculate the amount of impregnation of the inorganic gas.
Inorganic gas impregnation amount (wt%) = ((weight in B−weight in A) / (weight in A)) × 100
Temperature The temperature condition for impregnating the polyimide resin with an inorganic gas is suitably from room temperature to 120 ° C, more preferably from 50 to 100 ° C. Below 50 ° C., it takes a lot of time to impregnate the inorganic gas with a sufficient amount (saturation amount) necessary for foaming. For this reason, the time which a manufacturing process requires becomes long and efficiency worsens. Further, at a temperature of 100 ° C. or higher, since the intermolecular space gradually increases, it becomes difficult to hold the impregnated inorganic gas in the resin, causing a problem of causing gas escape.

Pressure About pressure, 1-50 Mpa is suitable and 7-35 Mpa is more preferable. At 7 MPa or less, when carbon dioxide is used as the inorganic gas, a supercritical state cannot be obtained, so that the polyimide resin cannot be impregnated with the inorganic gas. In addition, it is possible to increase the impregnation rate of the inorganic gas at 35 MPa or more, but since a very high pressure is required, a safety problem on the apparatus and a new apparatus are necessary to ensure the safety. Become inefficient and uneconomical.

Time The time for maintaining the high pressure state in step b may be 5 minutes to 48 hours, and more preferably 30 minutes to 24 hours. In the case of 30 minutes or less, it becomes insufficient to reach a sufficient amount (saturation amount) of the inorganic gas required for foaming, and it becomes difficult to obtain a material having a high foaming rate.

Specific elements and conditions of step c If necessary in step c, in order to retain the impregnated inorganic gas in the polyimide resin material, the material is cooled to a temperature range in which the inorganic gas can exist at least in a liquid state. Is also possible. As a result, it is possible to maintain a state in which more inorganic gas is impregnated in the polyimide resin material, and it is possible to provide a material having a high foaming rate.

Regarding the temperature dependence in that case, the following equation is established.
(Transmission coefficient P) = (Diffusion coefficient D) x (Solubility coefficient S)
The following formula is established for the solubility coefficient.

Solubility coefficient logS = -ΔH / RT + C (R is gas constant, ΔH is heat of dissolution)
That is, the solubility coefficient increases as the temperature increases. Therefore, it is theoretically consistent to store at low temperature from the viewpoint of increasing solubility.

The following formula holds for the diffusion coefficient.
Diffusion coefficient logD = -E _D / RT + C (E _D is the activation energy of diffusion)
From this, the diffusion coefficient is also logarithmically proportional to the reciprocal of the temperature, which supports the effectiveness of low-temperature storage.

The following equation holds for the gas permeability coefficient.
Gas permeability coefficient logP = -Ep / RT + C (Ep is activation energy)
Here, since it is Ep = E _D + ΔH, have an inverse proportional relationship with respect to the sum of the heat of solution and activation energy (logarithmically).

  The polymer that makes up the thin film constantly repeats the thermal vibration of micro-Brownian motion. As a result, gaps (fine holes) in the chain through which gas can permeate are formed. As the temperature rises, molecular motion increases and the vibration width of the polymer chain increases, so that gas molecules are likely to diffuse.

Diffusion is mass transfer from high to low concentration due to molecular motion
Stokes equation Diffusion coefficient D = KT / 6πrζ (ζ is the coefficient of friction and proportional to viscosity η)
Einstein's formula D = KT / 6πrζn ² (where n is the degree of polymerization) D decreases with the degree of polymerization. From these, it is considered that the viscosity of the polymer is useful for preventing diffusion. The method may further include a step of pressurizing and heating the foamed material obtained by the above. In this post-process, it is preferable that the said pressurization shall be 10-30 kg / cm <2>, the said heating will be 150-350 degreeC, and the said process shall be 30 second-3 minutes. When the material is polyimide or polyetherimide, it is preferable that the pressure is 20 kg / cm 2, the heating is 250 ° C., and the process is 1 minute.

  Suitable devices for heating and pressurizing the polyimide resin include, for example, a hot press, a hot roll, and the like. In particular, in the hot press, the pressure and temperature can be uniformly applied to the entire material, and therefore, it is more preferably used in the method of the present invention. When using a hot press, a spacer may be used between the press plates. Thereby, the material surface can be finished smoothly. When the spacer is not used, it may be difficult to maintain smoothness because the surface is uneven due to bubbles generated by foaming.

  The heating temperature is preferably between the glass transition temperature Tg and Tg + 100 ° C. of the polyimide resin. This is because in the vicinity of Tg, the molecular motion of the resin becomes active, so that it is easy to adjust the foam and the shape of the foam material. Therefore, it is easy to obtain a foam material that is thin and smooth.

  The pressurizing pressure is preferably 1 to less than 30 kgf per unit square centimeter. If the unit of square centimeters is 30 kgf or more, it is difficult to foam the material, so it is difficult to obtain a foam material having a high foaming rate.

  The pressurization time is preferably 1 to 10 seconds. If it is 1 second or less, the heat transfer to the entire inside of the polyimide resin is insufficient, and the d step ends, and there is a possibility that uneven foaming and uneven surface smoothness may occur. If it is 10 seconds or longer, heat transfer is excessively sufficient to the inside of the polyimide resin, the surface skin layer becomes thick, and there is a possibility that the foamed material has few cells inside the material.

Pretreatment process In this invention, prior to the manufacturing method of the said foam material, the pretreatment process which makes the said material a thin film with a thickness of 35 micrometers-150 micrometers beforehand can further be included.

The low dielectric constant foam material obtained by the method described in detail as an example above has a thickness of 25 to 500 μm and a thin film and smoothness. Its characteristics are an apparent specific gravity of 0.10 to 1.00 g / cm ³ , a cell diameter of 1 to 2 μm, a cell wall of 0.1 to 0.2 μm, and a surface skin layer thickness of 10 to 50% of the total thickness. It is. Furthermore, it has a dielectric constant of 1.5 to 2.7 and a dielectric loss tangent of 0.0020 to 0.0054 as electrical characteristics. The thickness tolerance of the obtained foamed material is within 10%.

Further, a metal foil may be fused or bonded to one side or both sides of the obtained foam material.
[The invention's effect]

  According to the present invention, it is possible to obtain a foamed material which is smooth and excellent in surface property, heat resistance and insulation performance. The obtained foamed material has high heat resistance and very high insulation. The material obtained from the thin film material has a smoothness with a tolerance of ± 10%. Whereas the dielectric constant of the polyimide resin before being subjected to the production method of the present invention is 3.2, the dielectric constant of the foamed material obtained by the production method of the present invention is 1.5. Furthermore, since the foamed material obtained by the present invention has a skin layer on the surface, there is an advantage that there is no intrusion of chemicals used for manufacturing circuit boards of electric and electronic equipment. This shows that the foamed material of the present invention is a material suitable for use as an insulating material for circuit boards.

Used Film Material The polyimide (PI) film and polyetherimide film (PEI) used in Examples and Comparative Examples are as follows.

Polyimide film (PI-1)
Product name "AURUM PI-PA" manufactured by Mitsui Chemicals, Inc.
75 μm thickness, Tg: 258 ° C., d = 1.33 g / cm ³ , storage elastic modulus at 250 ° C. 7.3 × 10 ⁸ Pa, loss elastic modulus at 250 ° C. 1.1 × 10 ⁸ Pa
Polyetherimide film (PEI-1)
Product name "Superio UT-F" manufactured by Mitsubishi Plastics Co., Ltd.
Thickness 75 μm and 50 μm, Tg: 226 ° C., d = 1.29 g / cm ³

Production of foam materials
(1) Impregnating the thinned polyimide resin with an inorganic gas at a predetermined pressure / temperature. The inorganic gas dissolved here is in a supercritical state.
(2) The pressure is reduced after elapse of a predetermined time, and the inorganic gas is held in the polyimide resin.
(3) Heat the polyimide resin impregnated with inorganic gas to near Tg to obtain a foamed material.
Elastic modulus measurement
Using ARES 1KFRTN1-FCO manufactured by Rheometric Scientific, the storage elastic modulus (G ′) curve of each material was obtained at a rate of temperature increase of 5 ° C./min at 1 Hz, and this was used as the elastic modulus.
Observation of Form of Sheet Section The prepared foamed sheet was cleaved after freezing in liquid nitrogen, and the section was measured using a scanning electron microscope (SEM) (JXA-6100P manufactured by JEOL) at an acceleration voltage of 5 kV.
Judgment of appearance shape (smoothness) In the material after foaming, it was judged as ○ when no warping, distortion, wrinkle, etc. were observed, and × when observed.
Measurement of thickness The thickness was measured using a MITUTOYO thickness gauge (model No. 7331).
Measurement of dielectric constant Using a PRECISION LCR METER 4285A (measurable range 75 k-30 MHz) manufactured by Agilent, dielectric constant and dielectric loss tangent measurement were performed at 1 MHz.
[Examples 1-7]

Each unfoamed polyimide film shown in Table 1 was impregnated with carbon dioxide by being immersed in a carbon dioxide atmosphere at 100 ° C. and 20 MPa in a pressure vessel for 60 minutes and then returning to atmospheric pressure. The carbon dioxide impregnation rate of the film under these conditions was 3.3 wt% in PI-1. After returning to atmospheric pressure, with a hot press set to each temperature shown in the table, heating is performed at a pressure of 20 kg / cm ^{3 for} 1 minute, and then cooled to make the polyimide smooth with a thickness tolerance of less than 10%. Foam material was obtained. The specific gravity and average pore diameter of the obtained foamed material are shown in the table.

Comparative Examples 1-4

  A polyimide film PI-1 having a size of 15 mm × 15 mm was impregnated with carbon dioxide under the same conditions as in Examples 1 to 7, and then inserted between two hot rolls having a diameter of 15 cm set to the temperatures shown in Table 1. . At this time, the gap between the two rolls was 0.2 mm, and the rotation speed was 6 rpm. Although the film foamed, it was greatly distorted, and a smooth foamed material with a thickness tolerance within 10% could not be obtained.

Comparative Examples 5-7

  After impregnating 15 mm × 15 mm size PI-1 with carbon dioxide under the same conditions as in Examples 1 to 7, heating was performed for 1 minute at the oven temperature setting shown in the table. Although the film foamed, it was greatly warped and a smooth foamed material with a thickness tolerance within 10% could not be obtained.

[Examples 8 to 10]

  The polyetherimide film PEI-1 having a size and thickness shown in Table 2 was immersed in a carbon dioxide atmosphere at 50 ° C. and 20 MPa in a pressure vessel for 60 minutes, and then returned to atmospheric pressure to impregnate carbon dioxide. The carbon dioxide impregnation rate of the film under these conditions was 3.0 wt%. After returning to atmospheric pressure, heating was performed under the respective conditions using a hot press, followed by cooling to obtain a smooth foamed material made of polyetherimide and having a thickness tolerance within 10%. The density and average pore diameter of the obtained foamed material are shown in the table.

Comparative Example 8

  After heating the polyetherimide film PEI-1 having the size and thickness shown in Table 2 impregnated with carbon dioxide under the same conditions as in Examples 8 to 10 using a hot press under the conditions shown in the table. Cooled down. The film melted and a foam material could not be obtained.

Comparative Example 9

  The polyetherimide film PEI-1 having the size and thickness shown in Table 2 impregnated with carbon dioxide under the same conditions as in Examples 8 to 10 was heated using the hot press under the conditions shown in Table 2. After cooling. Although the film foamed, it was greatly distorted, and a smooth foamed material with a thickness tolerance within 10% could not be obtained.

Comparative Examples 10-11

  After heating the polyetherimide film PEI-1 having the size and thickness shown in Table 2 impregnated with carbon dioxide under the same conditions as in Examples 8 to 10 using a hot press under the conditions shown in the table. Although cooled, the film did not foam.

[Examples 11 to 13]

Immediately after impregnating carbon dioxide with the unfoamed polyimide film PI-1 having a size of 60 mm × 60 mm under the same conditions as in Examples 1 to 7, the film was sandwiched between two metal plates into which spacers were inserted, By heating and cooling at a pressure of 20 kg / cm ^{2 for} 1 minute with a hot press set to the temperature shown in Table 3, polyimide foam materials having respective densities shown in the table were obtained. When the dielectric constant of these polyimide foam materials was measured, both the dielectric constant and dielectric loss tangent decreased as the specific gravity after foaming decreased.

Comparative Example 12

  The dielectric constant was measured for the unfoamed polyimide film PI-1 having a size of 60 mm × 60 mm, but it was not possible to obtain a dielectric constant and dielectric loss tangent higher than those after foaming.

[Examples 5 and 14]

The unfoamed polyimide film PI-22 having a size of 15 mm × 15 mm was crystallized by heating in a hot oven under the conditions shown in Table 4. The crystallinity was determined from the relationship between the specific gravity and the crystallinity shown in Table 5 (cited from Mitsui Chemicals, Inc. Technical Data H-04 “Crystallizing Conditions for AURUM Molded Products”). Carbon dioxide was impregnated under the same conditions as in Examples 1 to 7. The carbon dioxide impregnation rate of each film at this time is as shown in Table 4. After impregnation with carbon dioxide, By heating with a hot press at a pressure of 20 kg / cm ² for 1 minute and then cooling, a smooth foamed material made of polyimide and having a thickness tolerance within 10% was obtained.

Comparative Example 13

The unfoamed polyimide film shown in Table 5 was crystallized by heating in a hot oven under the conditions shown in Table 4. The crystallinity was determined from the relationship between density and crystallinity shown in Table 5. The crystallized polyimide film was immersed in a carbon dioxide atmosphere at 100 ° C. and 20 MPa in a pressure vessel for 60 minutes and then returned to atmospheric pressure to impregnate carbon dioxide. The carbon dioxide impregnation rate of each film under these conditions is as shown in the table. The sheet impregnated with gas was returned to the atmosphere, then heated for 1 minute at a pressure of 20 kg / cm ² with a hot press at 250 ° C. and then cooled, but no foamed material was obtained.

[Example 15]

A polyetherimide film PEI-1 having a size of 15 mm × 15 mm impregnated with carbon dioxide under the same conditions as in Examples 8 to 10 was stored in liquid nitrogen (atmospheric temperature −198 ° C.). Table 6 shows the carbon dioxide content when 3 minutes have elapsed after completion of the impregnation, 1 hour has elapsed, and 2 hours have elapsed. Taken out of liquid nitrogen after 5 hours from impregnation, heated with a hot press set at 275 ° C. at a pressure of 20 kg / cm ^{2 for} 10 seconds and then cooled, and made of polyetherimide, thickness tolerance within 10% A smooth thin-film foam material was obtained.

Comparative Example 14

A polyetherimide film PEI-1 having a size of 15 mm × 15 mm impregnated with carbon dioxide under the same conditions as in Examples 8 and 9 was stored in a freezer (atmosphere temperature −20 ° C.). Table 6 shows the carbon dioxide content when 3 minutes have elapsed after completion of the impregnation, 1 hour has elapsed, and 2 hours have elapsed. After 5 hours from the impregnation, it was taken out from the freezer and heated with a hot press set at 275 ° C. at a pressure of 20 kg / cm ^{2 for} 10 seconds and then cooled, but no foam material was obtained.

Comparative Example 15

A polyetherimide film PEI-1 having a size of 15 mm × 15 mm impregnated with carbon dioxide under the same conditions as in Examples 8 and 9 was stored at room temperature (25 ° C.). Table 6 shows the carbon dioxide content when 3 minutes have elapsed after completion of the impregnation, 1 hour has elapsed, and 2 hours have elapsed. After 5 hours from the impregnation, heating was performed at a pressure of 20 kg / cm ² with a hot press set at 275 ° C. for 10 seconds and then cooled, but no foam material was obtained.

[Example 16]

A polyetherimide film (PEI-1) having a sheet size of 15 mm × 15 mm was impregnated with supercritical carbon dioxide under conditions of 50 ° C. × 20 MPa × 180 min. After the elapse of time, the film was taken out, and the film sandwiched at a gap of 0.5 mm and a press plate interval of 0.3 mm / 0.2 mm was heated with a hot press heated to a predetermined temperature. At this time, the heating pressure was 20 kgf / cm ² and the heating time was 5 seconds. As a result, when the heating temperature was not lower than the glass transition temperature of the film and not higher than the melting point, a low dielectric constant foam material having a smooth and good surface could be obtained. Table 7 shows the specific gravity and appearance after foaming under these conditions.

FIG. 1 is a diagram showing the internal structure of a foam material obtained by the present invention.

Claims (19)

A foam material having a dielectric constant of 1.0 to 3.0 and a porosity of 10% to 90%. 2. The foam material according to claim 1, wherein the average pore diameter is 0.01 to 10 μm. The foam material according to claim 1 or 2, wherein the foam material is a thin film material having a thickness of 35 µm to 150 µm. The foam material according to claim 1, wherein the foam material is polyimide. The foam material according to claim 4, wherein the polyimide has a crystallinity of 0 to less than 20%. The foam material according to claim 4 or 5, wherein the polyimide is selected from the group consisting of thermoplastic polyimide, polyetherimide, or a combination thereof. Maintaining the material in the container, increasing the pressure in the container to a predetermined pressure using inorganic gas, and then heating the container to a predetermined temperature and holding it for a predetermined time; and after the predetermined time has passed, the container is brought to atmospheric pressure. Including stepping down,
A method for producing a foam material. The method for producing a foam material according to claim 7, wherein the pressure is 1 to 50 MPa, and the temperature is room temperature to 120 ° C. The method for producing a foam material according to claim 7, wherein the pressure is 7 to 35 MPa, and the temperature is 50 to 100C. The method for producing a foam material according to one of claims 7 to 9, wherein the pressurization time is 1 to 10 seconds, and the heating time is 1 to 10 seconds. The method for producing a foam material according to claim 7, wherein the inorganic gas is a gas selected from the group consisting of nitrogen, carbon dioxide, air, or a combination thereof. The method for producing a foam material according to claim 7, wherein the inorganic gas is in a supercritical state. The method for producing a foam material according to one of claims 7 to 12, wherein the inorganic gas is impregnated with 1 to 5% by weight of the material. The method for producing a foam material according to one of claims 7 to 13, wherein a storage elastic modulus G 'at a temperature at which the inorganic gas in a supercritical state is impregnated is 10 ⁸ Pa or more. The method for producing a foam material according to claim 7, wherein a loss elastic modulus G ″ at a temperature at which the inorganic gas in a supercritical state is impregnated is 10 ⁷ Pa or more. The method for producing a foam material according to claim 7, further comprising a step of pressurizing and heating the foam material. The method for producing a foam material according to claim 16, wherein the pressurization is 10 to 30 kg / cm2, the heating is 150 to 350 ° C, and the process time is 30 seconds to 3 minutes. The pressure is a pressure of less than 1 to 30 kgf per square centimeter relative to the material, the temperature is a glass transition temperature Tg to Tg + 100 ° C of the thin film material, and a time is 1 to 10 seconds. 18. A method for producing a foam material according to 17. The method according to one of claims 7 to 17, further comprising the step of previously forming the material into a thin film having a thickness of 35 µm to 150 µm.