JP2015034255A - Foam and production method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a foam which is composed of a resin material including a crystalline fluororesin and has sufficient light transmissivity and its production method.SOLUTION: A foam is composed of a resin material including a crystalline fluororesin and has a haze of 50% or smaller. A method of producing the foam comprises impregnating a non-foam molding of a resin material including the crystalline fluororesin with carbon dioxide in a supercritical state under a condition at a temperature equal to or higher 31.1°C and equal to or lower than the glass transition temperature of the crystalline fluororesin and then allowing the non-foam molding to foam in an atmosphere at a pressure lower than the pressure in the impregnation and a temperature equal to or higher than the temperature in the impregnation and equal to or lower than the glass transition temperature of the crystalline fluororesin.

Description

  The present invention relates to a foam of a resin material containing a crystalline fluororesin and a method for producing the same.

  Crystalline fluororesins have characteristics such as weather resistance, chemical resistance, heat resistance, and antifouling properties, and are therefore used as film materials for film structures and agricultural greenhouses. The film material does not have the high light transmittance required for an optical film, but the film material is required to have a light transmission level at which the light can be taken or the state of the other side can be seen through the film material. There is. Moreover, when this film | membrane material is a foam, it is anticipated that characteristics, such as heat insulation, sound insulation, and weight reduction, can be exhibited.

The light-transmitting foam made of a synthetic resin has an average bubble diameter in the range of 10 to 200 nm, a bubble number density of 10 ⁹ to 10 ¹⁵ / cm ³ , and a visible light region with a wavelength of 400 to 800 nm. A light-transmitting synthetic resin foam in which the ratio (percentage) between the light transmittance of the foam and the light transmittance of a non-foam having the same shape as the foam is 70% or more has been proposed (Patent Literature). 1). The foam is produced by impregnating a synthetic resin with an inert gas supercritical fluid under high pressure and heating conditions, and then releasing the pressure. The heating condition is a temperature lower than the melting point of the synthetic resin and higher than the melting point of the synthetic resin impregnated with the supercritical fluid of the inert gas.

JP 2003-176375 A

  However, when the present inventors manufactured a foam made of a crystalline fluororesin by the manufacturing method described in Patent Document 1, the obtained foam had a median diameter of bubbles exceeding 1 μm, and light generated by the bubbles. The haze was increased by scattering and the light transmission was not sufficient.

  The present invention provides a foam made of a resin material containing a crystalline fluororesin and having sufficient light transmission properties, and a method for producing the same.

The foam of the present invention is a foam of a resin material containing a crystalline fluororesin, and the haze of the foam is 50% or less.
The foam of the present invention preferably has a plurality of closed cells, and the median diameter of the closed cells is preferably 200 nm or less.
The crystalline fluororesin includes ethylene-tetrafluoroethylene copolymer, tetrafluoroethylene-perfluoro (alkyl vinyl ether) copolymer, tetrafluoroethylene-hexafluoropropylene copolymer, tetrafluoroethylene-perfluoro (methyl vinyl ether) copolymer. It is preferably at least one selected from the group consisting of a polymer, polyvinyl fluoride and polychlorotrifluoroethylene.

The method for producing a foam of the present invention comprises a non-foamed molded body of a resin material containing a crystalline fluororesin, and carbon dioxide in a supercritical state under conditions of 31.1 ° C. or higher and a glass transition temperature of the crystalline fluororesin or lower. After the impregnation, the non-foamed molded body is foamed in an atmosphere lower than the pressure during the impregnation and not lower than the temperature during the impregnation and not higher than the glass transition temperature of the crystalline fluororesin.
The crystallinity of the crystalline fluororesin is preferably 5 to 70%.
The heat of fusion of the crystalline fluororesin is preferably 5 J / g or more.
The storage elastic modulus of the resin material containing the crystalline fluororesin at the temperature at the time of foaming is preferably 1 × 10 ⁷ to 1 × 10 ⁹ .
The amount of carbon dioxide impregnated in the non-foamed molded article immediately after impregnating carbon dioxide in a supercritical state is preferably 0.1% by mass or more.

The foam of the present invention is made of a resin material containing a crystalline fluororesin and has sufficient light transmittance.
According to the method for producing a foam of the present invention, a foam made of a resin material containing a crystalline fluororesin and having sufficient light transmittance can be produced.

It is sectional drawing which shows the mode of arrangement | positioning of each member in a flat plate press.

The following definitions of terms apply throughout this specification and the claims.
“Fluorine resin” means a polymer compound having a fluorine atom in the molecule.
“Crystalline fluororesin” means a fluororesin having a melting point in a DSC curve obtained by measurement with a differential scanning calorimeter (DSC).
“Supercritical carbon dioxide” means carbon dioxide in a supercritical state above the critical point (critical temperature and pressure) of carbon dioxide. Combined with dissolving power.
“Main component” means that 50% by mass or more of the component is contained in a resin material or the like.
“Median diameter” means the bubble diameter (so-called 50% diameter, d50) at a point where the integration percentage is 50% in the distribution curve of the bubble diameter distribution (integration distribution).

<Foam>
The foam of the present invention is a foam of a resin material containing a crystalline fluororesin, and the haze of the foam is 50% or less.

(Resin material)
The resin material contains a crystalline fluororesin as a main component. The resin material may contain other components (other resins, additives, etc.) within a range not impairing the effects of the present invention.

(Crystalline fluororesin)
Crystalline fluororesins include ethylene-tetrafluoroethylene copolymer (hereinafter referred to as ETFE), tetrafluoroethylene-perfluoro (alkyl vinyl ether) from the viewpoint of balance of crystallinity, mechanical properties and affinity with carbon dioxide. ) Copolymer (hereinafter referred to as PFA), tetrafluoroethylene-hexafluoropropylene copolymer (hereinafter referred to as FEP), tetrafluoroethylene-perfluoro (methyl vinyl ether) copolymer (hereinafter referred to as MFA). .), At least one selected from the group consisting of polyvinylidene fluoride (hereinafter referred to as PVDF), polyvinyl fluoride and polychlorotrifluoroethylene. Of these, ETFE, PFA, FEP, MFA, polyvinyl fluoride, and polychlorotrifluoroethylene are preferable, ETFE, PFA, and FEP are more preferable, and ETFE and PFA are particularly preferable.

  The degree of crystallinity of the crystalline fluororesin is preferably 5% or more, more preferably 10% or more, and more preferably 15% or more from the necessity of suppressing deformation depending on the mechanical properties of the matrix resin and generating sufficiently small closed cells. Further preferred. The degree of crystallinity of the crystalline fluororesin is preferably 70% or less, more preferably 60% or less, and even more preferably 58% or less, from the viewpoint of ensuring light transmittance of the crystalline fluororesin.

The degree of crystallinity of the crystalline fluororesin is determined by measuring the specific heat in the temperature region sandwiching the melting point of the crystalline fluororesin using DSC, obtaining the heat of fusion from the area of the endothermic peak believed to be due to the melting point, and using the following formula: Use to calculate.
Crystallinity = 0.98 × heat of fusion (J / g)
About the measuring method of crystallinity degree, it describes in the following literature. This method has a tetrafluoroethylene (hereinafter referred to as TFE) unit and is assumed to have a crystal structure formed from the same unit, and ETFE, PFA, FEP, MFA, etc. are evaluated without problems. Is possible.
C. Nakafuku et al. "Polymer Journal", Vol. 31, 1999, p. 557

  The heat of fusion of the crystalline fluororesin is preferably 5 J / g or more, more preferably 10 J / g or more, and even more preferably 15 J / g or more for the same reason as the crystallinity described above. The heat of fusion of the crystalline fluororesin is preferably 70 J / g or less, more preferably 60 J / g or less, and still more preferably 58 J / g or less from the viewpoint of light transmittance.

  The content of the crystalline fluororesin is preferably 50% by mass or more and more than 80% by mass in the resin material (100% by mass) from the viewpoint that the foam can sufficiently exhibit the characteristics derived from the crystalline fluororesin. More preferably, 90 mass% or more is further more preferable. The upper limit of the content of the crystalline fluororesin is 100% by mass.

(ETFE)
ETFE has a structural unit based on ethylene (hereinafter referred to as E) and a structural unit based on TFE.
The proportion of the structural unit based on TFE is preferably 30 to 70 mol%, more preferably 40 to 65 mol%, of the total (100 mol%) of the structural unit based on TFE and the structural unit based on E. 60 mol% is more preferable.
The proportion of the structural unit based on E is preferably 30 to 70 mol%, more preferably 35 to 60 mol%, of the total (100 mol%) of the structural unit based on TFE and the structural unit based on E. 60 mol% is more preferable.
If the proportion of the structural unit based on TFE and the proportion of the structural unit based on E are within the above ranges, the balance of properties such as weather resistance, chemical resistance, heat resistance, and antifouling property will be good.

ETFE may have a structural unit based on another monomer. Examples of the other monomer include a monomer having a fluorine atom (excluding ETFE) and a monomer having no fluorine atom (excluding E).
Examples of the monomer having a fluorine atom include vinyl fluoride, vinylidene fluoride, trifluoroethylene, hexafluoropropylene, and perfluoro (alkyl vinyl ether).
Examples of the monomer having no fluorine atom include olefins (excluding E), vinyl esters, vinyl ethers, and the like.
The proportion of structural units based on other monomers is preferably 30 mol% or less, more preferably 0.1 to 15 mol%, of all the structural units (100 mol%) constituting ETFE, 0.2 More preferably, it is 10 mol%.

(PFA)
PFA has a structural unit based on TFE and a structural unit based on perfluoro (alkyl vinyl ether) (hereinafter referred to as PFVE).
The proportion of the structural unit based on TFE is preferably 90 to 99.8 mol%, more preferably 93 to 99.5 mol%, of the total (100 mol%) of the structural unit based on TFE and the structural unit based on PFVE. Preferably, 95 to 99 mol% is more preferable.
The proportion of the structural unit based on PFVE is preferably 0.2 to 10 mol%, more preferably 0.5 to 7 mol%, of the total (100 mol%) of the structural unit based on TFE and the structural unit based on PFVE. Preferably, 1 to 5 mol% is more preferable.
When the proportion of the structural unit based on TFE and the proportion of the structural unit based on PFVE are within the above ranges, the balance of properties such as weather resistance, chemical resistance, heat resistance, and antifouling property becomes good.

PFA may have a structural unit based on another monomer. Examples of the other monomer include monomers having a fluorine atom (excluding TFE and PFVE) and monomers not having a fluorine atom.
Examples of the monomer having a fluorine atom include vinyl fluoride, vinylidene fluoride, trifluoroethylene, and hexafluoropropylene.
Examples of the monomer having no fluorine atom include olefin, vinyl ester, vinyl ether and the like.
The proportion of structural units based on other monomers is preferably 30 mol% or less, more preferably 0.1 to 15 mol%, of all the structural units (100 mol%) constituting PFA, 0.2 More preferably, it is 10 mol%.

(FEP)
FEP has a structural unit based on TFE and a structural unit based on hexafluoropropylene (hereinafter referred to as HFP).
The proportion of the structural unit based on TFE is preferably 50 to 98 mol%, more preferably 60 to 95 mol%, of the total (100 mol%) of the structural unit based on TFE and the structural unit based on HFP. 90 mol% is more preferable.
The proportion of the structural unit based on HFP is preferably 2 to 50 mol%, more preferably 5 to 40 mol%, of the total (100 mol%) of the structural unit based on TFE and the structural unit based on HFP. 25 mol% is more preferable.
If the proportion of the structural unit based on TFE and the proportion of the structural unit based on HFP are within this range, the balance of properties such as weather resistance, chemical resistance, heat resistance, and antifouling properties will be good.

FEP may have a structural unit based on another monomer. Examples of the other monomer include a monomer having a fluorine atom (excluding TFE and HFP) and a monomer having no fluorine atom.
Examples of the monomer having a fluorine atom include vinyl fluoride, vinylidene fluoride, trifluoroethylene, PFVE, and the like, and vinylidene fluoride is preferable.
Examples of the monomer having no fluorine atom include olefin, vinyl ester, vinyl ether and the like, and E is preferable.
The proportion of structural units based on other monomers is preferably 30 mol% or less, more preferably 0.1 to 15 mol%, of all the structural units (100 mol%) constituting FEP, 0.2 More preferably, it is 10 mol%.

(Other ingredients)
Examples of other resins include fluororesins other than crystalline fluororesins, polyamide resins, polyester resins, and polyvinyl alcohol resins.
Examples of the additive include pigments, ultraviolet absorbers, light stabilizers, fillers, crosslinking agents, antioxidants, antistatic agents, flame retardants, nucleating agents, oil agents, dyes, and the like.
The total content of other resins and additives is preferably 50% by mass or less, more preferably 20% by mass or less, and still more preferably 10% by mass or more in the resin material (100% by mass).

(Haze)
The haze of the foam is 50% or less, preferably 40% or less, and more preferably 30% or less. The lower limit of haze is 0%. If the haze is within the range, the foam has a light transmission level at or above which light can be taken or through which it can be seen.
In order to keep the haze of the foam within this range, specifically, the foam may be produced by a production method described later.
The haze of the foam is measured using a haze meter according to JIS K 7136.

(Median diameter)
As the foam bubbles, closed cells are preferable from the standpoint of sufficiently exhibiting properties such as heat insulation, sound insulation and weight reduction.
The median diameter of the closed cells is preferably 200 nm or less, and more preferably 100 nm or less, from the viewpoint that the foam has sufficient light transmittance.
The median diameter of the closed cells is preferably 10 nm or more, and more preferably 30 nm or more, from the viewpoint of ease of production of the foam.
The median diameter of closed cells is determined by cooling the foam to cryogenic temperature using liquid nitrogen or the like, then breaking the observed cross section with an electron microscope, and observing 100 or more closed cells in the observed cross section. The bubble diameter is measured, and a distribution curve of the bubble diameter distribution (integrated distribution) is created and obtained.

(Function and effect)
Since the foam of the present invention described above is a foam of a resin material containing a crystalline fluororesin, it is derived from a crystalline fluororesin such as weather resistance, chemical resistance, heat resistance, and antifouling properties. In addition to having properties, it can exhibit properties derived from foams such as heat insulation, sound insulation and weight reduction. Moreover, since the haze of a foam is 50% or less, it has sufficient light transmittance.

<Method for producing foam>
In the method for producing a foam of the present invention, carbon dioxide in a supercritical state is applied to a non-foamed molded article of a resin material containing a crystalline fluororesin under conditions of 31.1 ° C. or higher and a glass transition temperature of the crystalline fluororesin or lower. In this method, after impregnation, the non-foamed molded article is foamed in an atmosphere lower than the pressure during the impregnation and not lower than the temperature during the impregnation and not higher than the glass transition temperature of the crystalline fluororesin.

Specifically, for example, the following steps (a) to (d) are included.
(A) The process of shape | molding the resin material containing crystalline fluororesin and obtaining a non-foaming molded object.
(B) A step of impregnating a non-foamed molded body with carbon dioxide in a supercritical state under conditions of 31.1 ° C. or higher and a glass transition temperature of crystalline fluororesin.
(C) After step (b), the non-foamed molded article is foamed in an atmosphere lower than the pressure in step (b) and not lower than the temperature in step (b) and not higher than the glass transition temperature of the crystalline fluororesin. And a step of obtaining a foam.
(D) After the step (c), the foam is allowed to cool or forcibly cooled and then washed as necessary.

(Process (a))
Examples of methods for molding the resin material include known molding methods (extrusion molding method, injection molding method, press molding method, etc.).
The shape of the non-foamed molded body is not particularly limited.

(Process (b))
In the present invention, carbon dioxide is used as a foaming agent. Since carbon dioxide has a high affinity with the fluororesin, it easily penetrates into the fluororesin and easily swells the fluororesin.
In the present invention, a crystalline fluororesin is used as the fluororesin. The crystalline fluororesin is composed of a crystal part having a high electron density and an amorphous part having a low electron density, and the swollen state by carbon dioxide is not uniform. It is considered that a foam having a relatively uniform cell diameter can be formed by using the heterogeneous structure.

  In the present invention, the non-foamed molded article is impregnated with carbon dioxide in a supercritical state. Carbon dioxide in a supercritical state easily penetrates into the fluororesin and easily swells the fluororesin. Further, after the impregnation, when the pressure is lower than the pressure at the time of impregnation, the carbon dioxide is vaporized and the non-foamed molded article is easily foamed.

The temperature at the time of impregnation is not lower than the critical temperature of carbon dioxide (31.1 ° C.) and preferably 40 ° C. or higher because carbon dioxide needs to be in a supercritical state.
The higher the temperature during impregnation, the faster the permeation of carbon dioxide in the supercritical state, while the density of carbon dioxide in the supercritical state decreases, so there is an upper limit for the temperature during impregnation. Further, since the volume of the crystalline fluororesin is increased by impregnation with carbon dioxide, if the crystalline fluororesin does not have sufficient hardness, foaming occurs before the step (c). For this reason, there is an upper limit for the temperature during impregnation. From the viewpoints of the above two points, the temperature during the impregnation is preferably equal to or lower than the glass transition temperature of the crystalline fluororesin.

The pressure during impregnation is not less than the critical pressure of carbon dioxide (7.4 MPa). From the viewpoint of shortening the impregnation time of carbon dioxide in a supercritical state, 7.4 MPa or more is preferable, and 10 MPa or more is more preferable.
The pressure during impregnation is not particularly limited, but is preferably 100 MPa or less, and more preferably 50 MPa or less, from the viewpoint of producing an industrial production apparatus.

The impregnation time is appropriately set depending on the kind of the crystalline fluororesin, the composition of the resin material, the size of the non-foamed molded article, the conditions during the impregnation, and the like. Usually, it is 0.1 to 24 hours, and preferably 0.1 to 1 hour.
As a container used for impregnation, a pressure resistant container is usually used.

The amount of carbon dioxide impregnated into the non-foamed molded product immediately after the step (b) is a very important process condition from the viewpoint of controlling the foaming in the step (c), but the most important is considered to be the step (c ) Is the amount of carbon dioxide impregnated in the crystalline fluororesin. In order to sufficiently generate the required closed cells having a small median diameter, the impregnation amount of carbon dioxide contained in the crystalline fluororesin during the treatment in the step (c) is preferably 0.1% by mass or more. 0.3 mass% or more is more preferable, and 0.5 mass% or more is more preferable. Further, in order to prevent the bubbles from becoming larger than the required median diameter, the impregnation amount of carbon dioxide contained in the crystalline fluororesin during the treatment of step (c) is preferably 10% by mass or less, and 8% by mass. % Or less is more preferable, and 6% by mass or less is more preferable.
The amount of carbon dioxide impregnated into the non-foamed molded body is the mass W0 of the non-foamed molded body before the step (b), the mass W1 of the non-foamed molded body just before the step (c) after taking out from the pressure vessel. Measure and obtain from the following formula.
Amount of carbon dioxide impregnated into non-foamed molded article = (W1-W0) / W0 × 100

(Process (c))
After the step (b), carbon dioxide in the container is released to the outside, and the pressure in the container is made lower than the pressure in the step (b), usually atmospheric pressure.
The non-foamed molded article is taken out of the container and immediately placed in an atmosphere lower than the pressure in step (b) and not lower than the temperature in step (b) and not higher than the glass transition temperature of the crystalline fluororesin. At this time, carbon dioxide impregnated in the non-foamed molded body is vaporized to foam the non-foamed molded body.

  From the end of step (b) (start of releasing carbon dioxide in the container) to the start of step (c) (place the non-foamed molded article in an atmosphere not lower than the temperature of step (b) and not higher than the glass transition temperature of the crystalline fluororesin. ) Time is not particularly limited. In order to obtain a foam having closed cells having the required median diameter, it is necessary to adjust the amount of carbon dioxide impregnated appropriately for the temperature of step (c). This can be realized by grasping the change over time in the amount of carbon dioxide impregnated and adjusting the required initial content and the waiting time from step (b) to step (c).

  Foaming may be performed in the air or in a liquid medium. The liquid medium preferably does not dissolve or swell the non-foamed molded article and the foam. Examples of the liquid medium include water, glycerol, silicone oil, heat transfer oil, solvent, and alternative chlorofluorocarbon.

The temperature at the time of foaming is preferably equal to or higher than the temperature at the time of impregnation because it is desirable to promote vaporization of the impregnated carbon dioxide in the crystalline fluororesin.
The temperature at the time of foaming is preferably equal to or lower than the glass transition temperature of the crystalline fluororesin from the viewpoint of suppressing the foam shape maintaining property at the time of foaming, in other words, suppressing the median diameter of the generated closed cells to be sufficiently small.

The storage elastic modulus of the resin material at the temperature at the time of foaming is preferably 1 × 10 ⁷ Pa or more from the viewpoint of suppressing the deformation of the matrix resin so that the median diameter of the generated closed cells becomes sufficiently small. 10 ⁸ Pa or more is more preferable.
The storage elastic modulus of the resin material at the temperature at the time of foaming is preferably 1 × 10 ⁹ Pa or less, since the matrix resin is required to have some flexibility in order to generate closed cells themselves, and preferably 7 × 10 ⁸ Pa. Pa or less is more preferable.
The temperature at the time of foaming is preferably set so that the storage elastic modulus of the resin material is within the above range.
The storage elastic modulus of the resin material is measured using a dynamic viscoelasticity measuring device.

  The pressure at the time of foaming should just be a pressure lower than the pressure of a process (b). When foaming is performed in the air, it is usually atmospheric pressure, and when foaming is performed in a liquid medium, it is usually a liquid pressure in the liquid medium.

  The time for placing the non-foamed molded article in the atmosphere not lower than the temperature of the step (b) and not higher than the glass transition temperature of the crystalline fluororesin is the type of the crystalline fluororesin, the composition of the resin material, the size of the non-foamed molded article, It is appropriately set depending on the magnification and the like. Usually, it is 0.1 to 60 minutes, and preferably 0.1 to 5 minutes.

(Process (d))
After step (c), the foam is allowed to cool or forcibly cooled to stop foaming.
You may wash | clean a foam as needed.

(Function and effect)
In the method for producing a foam according to the present invention described above, the non-foamed molded article of the resin material containing the crystalline fluororesin is subjected to a condition of 31.1 ° C. or higher and below the glass transition temperature of the crystalline fluororesin. After impregnating carbon dioxide in the critical state, the non-foamed molded article is foamed in an atmosphere lower than the pressure during impregnation and not lower than the temperature during impregnation and below the glass transition temperature of the crystalline fluororesin , Closed cells having a small median diameter (specifically, 200 nm or less) are formed. Therefore, a foam having a low haze and sufficient light transmission can be obtained.

EXAMPLES Hereinafter, the present invention will be specifically described with reference to examples, but the present invention is not limited to these examples.
Examples 1 to 13 are examples, and examples 14 to 25 are comparative examples.

(Glass-transition temperature)
Using a solid dynamic viscoelasticity measuring device (DVA-220, manufactured by IT Measurement Co., Ltd.), measuring the viscoelasticity of a non-foamed molded article at a frequency of 10 Hz while raising the temperature at a temperature rise rate of 2 ° C per minute. went. Tan δ, which is the ratio (G ″ / G ′) of loss elastic modulus G ″ to storage elastic modulus G ′, is plotted against temperature, and the temperature at which tan δ exhibits a maximum is taken as the glass transition temperature.

(Crystallinity, heat of fusion)
The crystallinity of the crystalline fluororesin can be measured at a scanning speed of 10 ° C./min in a sufficiently wide temperature range sandwiching the melting point of the crystalline fluororesin using a differential scanning calorimeter (DSC) (Q20, manufactured by TA instrument). The specific heat was measured, the heat of fusion was determined from the area of the endothermic peak considered to be due to the melting point, and calculated using the following formula. Data analysis was based on 2nd Heating calorimetric data.
Crystallinity = 0.98 × heat of fusion (J / g)

(Melting point)
The melting point of the crystalline fluororesin was the top temperature of the endothermic peak.

(Swelling amount)
The swelling amount (volume%) of the non-foamed molded product when the non-foamed molded product was immersed in carbon dioxide in a critical state for 60 minutes at a pressure of 20 MPa and a temperature of 60 ° C. or 75 ° C. was determined as follows.
An approximately 10 mm square non-foamed molded body (film) was prepared. This was put in a heat-resistant container with a sapphire viewing window, and supercritical carbon dioxide was introduced to impregnate the non-foamed molded article with supercritical oxygen dicarbide. The dimensional change of the non-foamed molded body at that time was evaluated only in the two-dimensional direction, and the value was set to 3/2 to obtain the volume swelling amount.

(Impregnation amount)
The amount of carbon dioxide impregnated into the non-foamed molded article immediately after the step (b) was determined as follows.
The mass W0 of the non-foamed molded body (film) before the step (b) was measured. After the step (b), the non-foamed molded article was taken out from the pressure vessel, and after a predetermined time had passed, the mass W1 was measured, and the impregnation amount was determined from the following formula.
Amount of carbon dioxide impregnated into non-foamed molded article = (W1-W0) / W0 × 100

(Storage modulus)
The storage elastic modulus of the non-foamed molded article was determined as follows.
Using a solid dynamic viscoelasticity measuring device (DVA-220, manufactured by IT Measurement Co., Ltd.), measuring the viscoelasticity of a non-foamed molded article at a frequency of 10 Hz while raising the temperature at a temperature rise rate of 2 ° C per minute. went. The storage elastic modulus at the required temperature was determined from the obtained temperature-dependent data of the storage elastic modulus.

(thickness)
The thickness of the non-foamed molded body and the foamed body was measured using a contact type micrometer (Model ID-H5030, manufactured by Mitutoyo Corporation) that contacts at a constant pressure.

(Median diameter)
The median diameter of the closed cells in the foam is such that after the foam is cooled to a cryogenic temperature using liquid nitrogen or the like, the foam is broken, and the resulting cross section is observed with an electron microscope. The bubble diameter of the closed cells was measured, and a distribution curve of the bubble diameter distribution (integrated distribution) was created and determined.

(Foaming ratio)
The foaming ratio of the foam is determined using the specific gravity D1 of the non-foamed molded body before step (b) and the specific gravity D2 of the foam after step (c) using a hydrometer (manufactured by Shimadzu Corporation, AUW220D). It measured at 25 degreeC and calculated | required from the following formula.
Foaming ratio = D1 / D2

(Haze)
The haze of the foam was measured at room temperature using a haze meter (SM color computer, manufactured by Suga Test Instruments Co., Ltd.).

(Electron microscope image)
About the foam, the cross section of the foam was observed using the scanning electron microscope (The JEOL Co., Ltd. make, JSM-6010).

(Crystalline fluororesin)
Table 1 shows the crystalline fluororesins used in the examples.

(Film production)
As shown in FIG. 1, two polyimide films 3 (manufactured by Toray DuPont Co., Ltd.) are provided between a stainless steel mirror surface plate 1 having a smooth surface and a stainless steel mirror surface plate 2 having a recess formed on the surface. The pellet-shaped resin material 4 is arranged through Kapton (thickness 75 μm), and this is set in a heating flat plate press (manufactured by Toyo Seiki Seisakusho Co., Ltd., Mini Test Press) and left for 5 minutes between heated flat plate presses. And then pressed. The press temperature was 300 ° C. for ETFE 1 to 5, 330 ° C. for PFA, 300 ° C. for FEP, and 230 ° C. for PVDF. The pressure was 3.4 MPa.

(Example 1)
Step (a):
6 g of a pellet-shaped resin material made only of a crystalline fluororesin (manufactured by Asahi Glass Co., Ltd., Fluon (registered trademark) C-88AXM) was molded by the method described above to obtain a film having a thickness of 100 μm.

Step (b):
The film was placed in a pressure vessel, high pressure carbon dioxide was injected into the pressure vessel while the pressure vessel was placed in a 40 ° C. warm bath, the internal pressure was increased to 20 MPa, and the pressure was maintained for 1.5 hours.

Step (c):
The pressure vessel was removed from the warm bath, the carbon dioxide inside was released, and the inside was returned to atmospheric pressure. Immediately after taking out the film from the pressure vessel, it was put into glycerol heated to 80 ° C. and held for 2 minutes to foam the film to obtain a foamed film.

Step (d):
The foamed film was taken out of the glycerol, cooled to room temperature, washed with ethanol, and vacuum dried at room temperature for 24 hours.
Table 4 shows the evaluation results of the foamed film.

(Examples 2-15, 17-25)
The foamed film (or non-foamed film) was the same as Example 1 except that the crystalline fluororesin used, the thickness of the film, and the conditions of steps (b) and (c) were changed as shown in Table 2 and Table 3. ) Table 4 shows the evaluation results of the foamed film.

(Example 16)
The thickness of the film and the conditions of the step (b) are changed as shown in Table 3, and after the step (b) is finished, the step (c) is not performed and the film is left in an atmospheric pressure state as it is. A foamed film (or non-foamed film) was obtained in the same manner as in Example 1 except that foaming was performed. Table 4 shows the evaluation results of the foamed film.

A film made of crystalline fluororesin is impregnated with supercritical carbon dioxide under conditions of 31.1 ° C. or higher and below the glass transition temperature of the crystalline fluororesin, and then at a pressure lower than the pressure during impregnation. The foamed films of Examples 1 to 13 obtained by foaming the non-foamed film in an atmosphere at a temperature above the glass transition temperature and below the glass transition temperature of the crystalline fluororesin have a low haze and a sufficient light transmittance. Had.
According to the cross-sectional observation of the foamed film with a scanning electron microscope, the bubble portion did not have particularly large anisotropy and directionality, and had a shape that was almost a perfect circle. The direction in which the cross-section breaks is not particularly defined, and the bubbles are considered to be generally circular in any cross section of the foamed film. As a result, it can be determined that the formed bubbles are closed cells that are hardly communicated.

In Examples 14 to 15 substantially not including the step (c), foaming was not observed.
In Example 17 using PVDF as the crystalline fluororesin, even if bubbles were formed inside by the step (c), the amorphous layer of PVDF was deformed while being left at room temperature, and foaming disappeared. Finally, no foam was obtained.
Example 18 in which the time from the end of step (b) (start of releasing carbon dioxide in the container) to the start of step (c) (place the film in an atmosphere of 80 ° C.) is too long is shown in step (c) Since most of the carbon dioxide impregnated in the film escaped from the film at the start, step (c) could not be performed.

In Examples 19 to 25, in which the temperature of the step (c) is higher than the glass transition temperature of the crystalline fluororesin, the hardness of the crystalline fluororesin is insufficient at the time of the step (c). The diameter became 1 μm or more, and as a result, the haze of the foam was remarkably increased.
In Example 16 in which the temperature of the step (b) is high and the step (c) is not performed and the film is left in an atmospheric pressure state and foamed as it is, the median diameter of the bubbles exceeds 1 μm, and the light generated by the bubbles The haze was increased by scattering and the light transmission was not sufficient. Example 16 corresponds to the manufacturing method described in Patent Document 1.

  The foam of the present invention is useful as a membrane structure, a membrane material for agricultural greenhouses, curtains, etc., and has a lighter specific gravity than conventional fluororesin films in general building material applications, improving handling. It is useful.

DESCRIPTION OF SYMBOLS 1 Mirror surface plate 2 Mirror surface plate 3 Polyimide film 4 Resin material

Claims (8)

A foam of a resin material containing a crystalline fluororesin,
A foam having a haze of the foam of 50% or less. Has multiple closed cells,
The foam according to claim 1, wherein the median diameter of the closed cells is 200 nm or less.   The crystalline fluororesin is an ethylene-tetrafluoroethylene copolymer, a tetrafluoroethylene-perfluoro (alkyl vinyl ether) copolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, a tetrafluoroethylene-perfluoro (methyl vinyl ether) copolymer. The foam according to claim 1 or 2, which is at least one selected from the group consisting of a polymer, polyvinyl fluoride, and polychlorotrifluoroethylene. A method for producing the foam according to any one of claims 1 to 3,
A non-foamed molded article of a resin material containing a crystalline fluororesin is impregnated with carbon dioxide in a supercritical state under conditions of 31.1 ° C. or more and a glass transition temperature of the crystalline fluororesin, and then the pressure during the impregnation A method for producing a foam, wherein the non-foamed molded body is foamed in an atmosphere at a lower pressure and at an impregnation temperature or more and a glass transition temperature of the crystalline fluororesin or less.   The method for producing a foam according to claim 4, wherein the crystallinity of the crystalline fluororesin is 5 to 70%.   The method for producing a foam according to claim 4 or 5, wherein the heat of fusion of the crystalline fluororesin is 5 J / g or more. Production of the foam as described in any one of Claims 4-6 whose storage elastic modulus of the resin material containing the said crystalline fluororesin in the temperature at the time of foaming is 1 * 10 < ⁷ > -1 * 10 < ⁹ > Pa. Method.   The foam according to any one of claims 4 to 7, wherein an impregnation amount of carbon dioxide in the non-foamed molded body immediately after impregnating with carbon dioxide in a supercritical state is 0.1 mass% or more. Manufacturing method.

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