JP2017179181A - Resin foam and production method of same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a production method of a resin foam which, when obtaining a resin foam comprising a core layer of a fine air bubble and a skin layer formed thereon by foaming a thermoplastic resin impregnated with supercritical fluid and/or subcritical fluid, has a very low abundance ratio of large air bubbles in a region between the skin layer and the core layer and which is prevented from deterioration of mechanical strength and light transmission compared to the resin before foaming, the production method preventing generation of large air bubbles in the vicinity of the skin layer.SOLUTION: Provided is a production method of a resin foam, comprising: an impregnation step of impregnating a thermoplastic resin with supercritical fluid and/or subcritical fluid under pressurized conditions; and a foaming step of obtaining a resin foam by foaming the thermoplastic resin impregnated with supercritical fluid and/or subcritical fluid by reducing the pressure, where the thermoplastic resin is foamed in the foaming step while being irradiated with ultrasonic waves. Also provided is the method for producing a foam, where in the foaming step the thermoplastic resin is irradiated with ultrasonic waves while being immersed in a liquid.SELECTED DRAWING: None

Description

  The present invention relates to a resin foam and a method for producing the same.

Resin foams are widely used as thermal insulation materials, but many of them have an average bubble diameter of micron order, are opaque and have low mechanical strength, and are not suitable for various structural materials. Was common. Therefore, in order to use a resin foam as a structural material, etc., miniaturization and high density of bubbles have become issues, and in recent years, the resin is impregnated with a supercritical fluid under pressure conditions and then the pressure is reduced. A method for obtaining a resin foam has been developed. For example, Japanese Patent Application Laid-Open No. 2008-127467 (Patent Document 1) uses a supercritical fluid and is composed of a composition containing a thermoplastic resin and a surfactant, and the average length diameter of the bubbles is 0.01 to A method for obtaining a resin foam having a cell number density of 1 μm and a range of 10 ¹¹ to 10 ¹⁶ cells / cm ³ is disclosed.

  However, in the conventional method for producing a resin foam using a supercritical fluid as described in Patent Document 1, a so-called skin layer made of a non-foamed bulk resin is always formed on the outermost surface. The skin layer suppresses the diffusion of the gas remaining inside the resin and promotes bubble growth in the vicinity of the skin layer to generate large bubbles, thereby causing mechanical strength (elastic modulus, tensile strength, bending strength, impact). There has been a problem that the intensity and the like are reduced and the transmittance of light (particularly visible light) is reduced.

JP 2008-127467 A

  The present invention has been made in view of the problems of the prior art, and a core layer in which fine bubbles are formed by foaming a thermoplastic resin impregnated with a supercritical fluid and / or a subcritical fluid, and the core layer. When a resin foam comprising a skin layer formed on the surface is obtained, a method for producing a resin foam capable of sufficiently preventing the generation of large bubbles in the vicinity of the skin layer, and to be obtained thereby Resin foam that has a very low presence of large bubbles in the area between the skin layer and the core layer, and has greatly reduced mechanical strength and light transmission compared to the resin before foaming. The purpose is to provide.

  As a result of intensive studies to achieve the above object, the present inventors have made a core layer in which fine bubbles are formed by foaming a thermoplastic resin impregnated with a supercritical fluid and / or a subcritical fluid, and a core layer thereof. When obtaining a resin foam comprising a skin layer formed on the surface, foaming of the thermoplastic resin while irradiating ultrasonic waves can sufficiently prevent the generation of large bubbles in the vicinity of the skin layer. As a result, the presence rate of large bubbles in the region between the skin layer and the core layer is extremely low, and the decrease in mechanical strength and light transmittance are greatly suppressed compared to the resin before foaming. The present inventors have found that a resin foam can be obtained and have completed the present invention.

That is, the method for producing the resin foam of the present invention includes:
An impregnation step of impregnating a thermoplastic resin with a supercritical fluid and / or a subcritical fluid under pressurized conditions;
A foaming step of obtaining a resin foam by foaming the thermoplastic resin impregnated with the supercritical fluid and / or subcritical fluid under reduced pressure;
And foaming the thermoplastic resin while irradiating with ultrasonic waves in the foaming step.

  In the method for producing a resin foam of the present invention, in the foaming step, the thermoplastic resin is preferably immersed in a liquid and foamed while being irradiated with ultrasonic waves. Moreover, it is preferable that the frequency of the ultrasonic wave is 20 to 100 kHz.

  The following resin foam of the present invention can be obtained by such a method for producing a resin foam of the present invention.

The resin foam of the present invention is
A foam made of a thermoplastic resin, wherein the presence rate of bubbles having a diameter of 10 μm or less is 20 to 80% by volume on the basis of the apparent volume, and the presence rate of bubbles having a diameter exceeding 10 μm is 0.1 volume on the basis of the apparent volume. % Core layer or less,
A resin layer made of the thermoplastic resin integrally formed on the surface of the core layer, wherein the abundance ratio of bubbles having a diameter of more than 0.01 μm is 1.0% by volume or less based on the apparent volume; A skin layer having an average thickness of 0.01 to 0.5 mm;
The resin foam is characterized in that there is no layer region between the skin layer and the core layer in which the presence rate of bubbles having a diameter exceeding 10 μm exceeds 0.1% by volume on the apparent volume basis Is the body.

  In the resin foam of the present invention, the average diameter based on the number of bubbles in the core layer is preferably 0.01 to 1 μm. The thermoplastic resin is preferably at least one selected from the group consisting of an acrylic resin and a polycarbonate resin.

  In the present invention, a resin foam comprising a core layer in which fine bubbles are formed by foaming a thermoplastic resin impregnated with a supercritical fluid and / or a subcritical fluid, and a skin layer formed on the surface thereof. The reason why the generation of large bubbles in the vicinity of the skin layer is sufficiently prevented by foaming the thermoplastic resin while irradiating ultrasonic waves is not necessarily known. Guesses as follows. That is, in a method of producing a resin foam by decompressing and foaming a thermoplastic resin impregnated with a supercritical fluid and / or subcritical fluid under a pressurized condition, gas is usually diffused from the surface, so Since the gas concentration on the surface becomes dilute and the plasticization of the resin is suppressed, the apparent glass transition temperature does not decrease, and a so-called skin layer made of a non-foamed bulk resin is always formed on the outermost surface. When such a skin layer is formed, conventionally, diffusion of gas remaining inside the resin is suppressed, and bubble growth in the vicinity of the skin layer is promoted to generate large bubbles. On the other hand, in the production method of the present invention, by reducing the pressure and applying ultrasonic waves, gas diffusion from the inside of the resin to the vicinity of the surface is induced and promoted, and the gas quickly escapes to the outside of the resin. Since the gas concentration gradient in the direction is relaxed, foaming proceeds promptly and uniformly before the skin layer is formed. As a result, the growth of bubbles in the vicinity of the skin layer is suppressed, and large bubbles are sufficiently generated. In addition, the present inventors infer that the average thickness of the skin layer becomes thinner and the size of bubbles in the core layer becomes smaller and the uniformity becomes higher.

  In addition, in the resin foam of the present invention obtained by such a production method of the present invention, the presence of large bubbles in the region between the skin layer and the core layer is extremely low, so the resin of the present invention Since the foam is a so-called microcellular, which is a foam in which only bubbles having a diameter of 10 μm or less exist substantially, the elastic modulus, tensile strength, bending strength, impact strength, etc., compared to the bulk resin before foaming The present inventors infer that the decrease in the mechanical strength of the film is greatly suppressed and the decrease in the light transmittance is also largely suppressed.

  According to the present invention, a resin foam comprising a core layer in which fine bubbles are formed by foaming a thermoplastic resin impregnated with a supercritical fluid and / or a subcritical fluid, and a skin layer formed on the surface thereof. A resin foam manufacturing method capable of sufficiently preventing the generation of large bubbles in the vicinity of the skin layer, and a large area in the region between the skin layer and the core layer that can be obtained thereby. It is possible to provide a resin foam in which the presence rate of bubbles is extremely low, and the mechanical strength and the light transmittance are significantly reduced compared to the resin before foaming.

2 is a scanning electron micrograph of a fractured surface near the surface (skin layer) of the resin foam obtained in Example 1. FIG. 2 is a scanning electron micrograph of a fracture surface inside (core layer) of the resin foam obtained in Example 1. FIG. 2 is a scanning electron micrograph of a fractured surface near the surface (skin layer) of the resin foam obtained in Comparative Example 1. FIG. 4 is a scanning electron micrograph of a fracture surface inside (core layer) of the resin foam obtained in Comparative Example 1. FIG. 4 is a scanning electron micrograph of a fractured surface near the surface (skin layer) of a resin foam obtained in Comparative Example 2. FIG. 4 is a scanning electron micrograph of a fracture surface inside (core layer) of the resin foam obtained in Comparative Example 2. FIG. 4 is a scanning electron micrograph of a fracture surface near the surface (skin layer) of a resin foam obtained in Comparative Example 3. FIG. 4 is a scanning electron micrograph of a fracture surface inside (core layer) of the resin foam obtained in Comparative Example 3. FIG.

  Hereinafter, the present invention will be described in detail with reference to preferred embodiments thereof.

(Method for producing resin foam)
First, the manufacturing method of the resin foam of this invention is demonstrated. The method for producing the resin foam of the present invention comprises:
An impregnation step of impregnating a thermoplastic resin with a supercritical fluid and / or a subcritical fluid under pressurized conditions;
A foaming step of obtaining a resin foam by foaming the thermoplastic resin impregnated with the supercritical fluid and / or subcritical fluid under reduced pressure;
And foaming the thermoplastic resin while irradiating with ultrasonic waves in the foaming step.

<Impregnation step>
In the method for producing a resin foam of the present invention, first, a thermoplastic resin is impregnated with a supercritical fluid and / or a subcritical fluid under pressure (impregnation treatment).

  The thermoplastic resin that can be used in the present invention is not particularly limited, and is an acrylic resin, polycarbonate resin, polyolefin resin, polyester resin, polystyrene resin, polyamide resin, polyalkylene ether resin, polyurethane resin, polyphenylene ether type. Resin, polyarylate resin, polyphenylene sulfide resin, polysulfone resin, polyethersulfone resin, cyclic polyolefin resin, polyetherimide resin, polyamideimide resin, polyimide resin, polyaminobismaleimide resin, etc. From acrylic resin (polymethyl methacrylate resin, etc.) and polycarbonate resin (bisphenol A type polycarbonate resin, etc.) from the viewpoint of mechanical strength and light transmittance It is preferably at least one selected from the. In addition, in the said thermoplastic resin, well-known various additives, for example, a foam stabilizer, a heat stabilizer, antioxidant, a ultraviolet absorber, a mold release agent, a coloring agent, etc. may contain.

  Further, the supercritical fluid and / or subcritical fluid used in the present invention is not particularly limited as long as it is a fluid that becomes a supercritical fluid or a subcritical fluid under a pressurized condition. Carbon dioxide), nitrogen, rare gas (such as argon), propane, water and the like. Among them, carbon dioxide or nitrogen is preferable from the viewpoint of operability in view of pressure and temperature and solubility in a resin. For example, the temperature and pressure at which carbon dioxide and nitrogen become supercritical fluids are as shown in Table 1 below, and the lower limit (critical temperature) and lower limit of pressure (critical pressure) at which such a supercritical fluid is obtained. ) Is the critical point. A subcritical fluid is either a temperature or a pressure, or both a temperature and a pressure are below a critical point (critical temperature and critical pressure), but the molecular state becomes a molecular state in a supercritical fluid. It is a close state. In the present invention, it is more preferable that the impregnation treatment is performed in a supercritical fluid, but the impregnation treatment may be performed in a subcritical fluid. The relationship between temperature and pressure at which various fluids become supercritical fluids or subcritical fluids is well known.

  The pressure condition (pressure) in the treatment (impregnation treatment) for impregnating the thermoplastic resin with the supercritical fluid and / or the subcritical fluid may be a pressure at which the fluid to be used can maintain the supercritical state or the subcritical state. What is necessary is just to select suitably according to the fluid to be used, the temperature of an impregnation process, etc., but when using carbon dioxide, for example, it is preferable to select within the range of 2-50 MPa, and select within the range of 5-40 MPa. More preferably. If the pressure is less than the lower limit, the impregnation rate and the amount of impregnation of the fluid into the thermoplastic resin tend to be insufficient.On the other hand, if the pressure exceeds the upper limit, the impregnation rate and the amount of impregnation of the resin are excessive. The foam diameter of the resin foam to be obtained tends to be excessive.

  Further, the temperature of the impregnation treatment is not particularly limited, and a temperature at which the fluid to be used can maintain a supercritical state or a subcritical state is appropriately selected. For example, when carbon dioxide is used, -20 to 100 ° C. Is preferably selected within the range of 0 to 80 ° C. If the temperature is less than the lower limit, the impregnation rate and the amount of impregnation of the fluid into the thermoplastic resin tend to be insufficient.On the other hand, if the upper limit is exceeded, the impregnation rate and the amount of impregnation of the resin are excessive. The foam diameter of the resin foam to be obtained tends to be excessive.

  Further, the time for the impregnation treatment is not particularly limited, and is appropriately selected so as to obtain a desired amount of impregnation, but is preferably selected within a range of 0.1 to 200 hours, More preferably, it is selected within a range of ˜100 hours. If the time is less than the lower limit, the amount of impregnation of the fluid into the thermoplastic resin tends to be insufficient. On the other hand, if the upper limit is exceeded, the impregnation amount is maintained in a state close to saturation, resulting in production efficiency. Tend to decrease.

  An apparatus for performing such impregnation treatment is not particularly limited, and is a batch type apparatus using a sealed container for enclosing and maintaining the thermoplastic resin together with the fluid under a pressurized condition. Alternatively, it may be a flow type apparatus using a flow-type container for continuously supplying the fluid under pressure under a condition where the thermoplastic resin is disposed.

  The amount of impregnation of the fluid in the thermoplastic resin impregnated with the supercritical fluid and / or subcritical fluid by the impregnation treatment described above is not particularly limited, and is within a range until the impregnation amount is close to saturation. The amount of the fluid impregnated with respect to 100 parts by mass of the thermoplastic resin is preferably selected within the range of 0.5 to 50 parts by mass, and the range of 1.0 to 30 parts by mass. More preferably, it is selected within. If the impregnation amount is less than the lower limit, the foaming ratio of the obtained resin foam tends to be insufficient. On the other hand, if the upper limit is exceeded, the fluid concentration gradient in the sample thickness direction increases in the foaming process, and the skin layer becomes thicker. Or the variation in the foam diameter tends to increase.

<Foaming process>
Next, in the method for producing the resin foam of the present invention, the pressure is reduced from the pressure state in the impregnation treatment, and the thermoplastic resin impregnated with the supercritical fluid and / or subcritical fluid is foamed (foaming). processing).

  The pressure during the foaming treatment is not particularly limited as long as the pressure used is lower than the lower limit pressure at which the fluid used can maintain the subcritical state, but the fluid used is subcritical. The pressure is preferably ½ or less of the lower limit pressure that can be maintained. For example, when carbon dioxide is used, it is preferably selected within the range of 0.01 to 1 MPa, and 0.1 to 0.5 MPa. More preferably, it is selected within the range. If the pressure is less than the lower limit, a device for reducing the pressure is required and the process tends to be complicated. On the other hand, if the pressure exceeds the upper limit, the difference from the time of fluid impregnation becomes small and foaming becomes insufficient. There is a tendency.

  Further, the pressure reduction from the pressurized state in the impregnation treatment to the pressure of the foaming treatment is preferably rapid decompression, the decompression speed is more preferably 0.05 MPa / sec or more, and 0.2 MPa / sec or more. It is particularly preferred. By rapidly depressurizing in this way, the size of the bubbles in the obtained resin foam tends to be smaller and the uniformity is higher.

  In the method for producing a resin foam of the present invention, when the thermoplastic resin is foamed by reducing the pressure as described above, the thermoplastic resin is irradiated with ultrasonic waves. By foaming the thermoplastic resin while reducing the pressure and irradiating with ultrasonic waves in this way, gas diffusion from the inside of the resin to the vicinity of the surface is promoted, and the gas can quickly escape to the outside of the resin. Foaming proceeds promptly and uniformly before formation, and the growth of bubbles in the vicinity of the skin layer is suppressed, and the generation of large bubbles is sufficiently prevented.

  The ultrasonic wave used here should just have a frequency as what is called an ultrasonic wave, and as such a frequency, 20-100 kHz is preferred. If the ultrasonic frequency is less than the lower limit, the ultrasonic irradiation effect tends to be insufficient. On the other hand, if the upper limit is exceeded, a general ultrasonic cleaning device cannot be used, and the cost of the apparatus increases. There is a tendency.

  Moreover, it is preferable that the ultrasonic irradiation to the thermoplastic resin is started as soon as possible after starting the decompression, and the ultrasonic irradiation is started within 30 seconds after reaching the foaming pressure. It is particularly preferable to start ultrasonic irradiation before reaching the pressure of the foaming treatment or simultaneously with reaching the pressure.

  Furthermore, it is preferable that the thermoplastic resin is foamed while being irradiated with ultrasonic waves in a state where the thermoplastic resin is immersed in a liquid. Thus, by irradiating an ultrasonic wave in a liquid, there exists a tendency for the irradiation effect of the said ultrasonic wave to increase more. Such a liquid is not particularly limited, but water is preferable.

  Also, the temperature of the foaming treatment is not particularly limited. For example, when impregnated with carbon dioxide, it is preferably selected within the range of -20 to 100 ° C, and within the range of 0 to 80 ° C. More preferably, it is selected. If the temperature is less than the lower limit, a cooling device is required and the process tends to be complicated.On the other hand, if the temperature exceeds the upper limit, the difference from the time of fluid impregnation tends to be small and foaming tends to be insufficient. is there.

  Further, the time for the foaming treatment is not particularly limited, and is appropriately selected so that foaming proceeds sufficiently or has a desired foaming ratio, but is selected within a range of 1 to 120 minutes. It is preferable that it is selected within a range of 2 to 30 minutes. If the time is less than the lower limit, the progress of foaming tends to be insufficient. On the other hand, if the upper limit is exceeded, the foaming is held and the production efficiency tends to decrease.

  By the foaming process described above, the resin foam of the present invention described in detail below is obtained. The expansion ratio in the foaming process and the relative density of the resulting resin foam (based on the bulk resin before foaming) vary depending on the resin used and the amount of the fluid impregnated in the thermoplastic resin. Although not particularly limited, the expansion ratio is preferably 1.3 to 2.5 times (relative density is 0.4 to 0.8), and the expansion ratio is 1.6 to 2 More preferably, it is 5 times (relative density is 0.4 to 0.6). If the expansion ratio is less than the lower limit, effects such as weight reduction due to foaming tend to be insufficient. On the other hand, if the expansion ratio exceeds the upper limit, the mechanical strength of the resulting resin foam tends to be insufficient.

(Resin foam)
Next, the resin foam of the present invention will be described. The resin foam of the present invention is
A foam made of a thermoplastic resin, wherein the presence rate of bubbles having a diameter of 10 μm or less is 20 to 80% by volume on the basis of the apparent volume, and the presence rate of bubbles having a diameter exceeding 10 μm is 0.1 volume on the basis of the apparent volume. % Core layer or less,
A resin layer made of the thermoplastic resin integrally formed on the surface of the core layer, wherein the abundance ratio of bubbles having a diameter of more than 0.01 μm is 1.0% by volume or less based on the apparent volume; A skin layer having an average thickness of 0.01 to 0.5 mm;
There is no layer region between the skin layer and the core layer where the abundance ratio of bubbles having a diameter exceeding 10 μm exceeds 0.1% by volume on the apparent volume basis. is there. In addition, the thermoplastic resin which comprises the resin foam of this invention is synonymous with the thermoplastic resin which can be used for the manufacturing method of the resin foam of this invention mentioned above.

  The abundance ratio of bubbles having a diameter exceeding 10 μm in the core layer needs to be 0.1% by volume or less on the basis of the apparent volume (core layer volume), and more preferably 0.02% by volume or less. preferable. If the abundance ratio of bubbles having a diameter exceeding 10 μm exceeds 0.1% by volume, it does not function as a so-called microcellular, resulting in a significant decrease in mechanical strength and a significant decrease in light transmittance.

  The abundance ratio of bubbles having a diameter of 10 μm or less in the core layer needs to be 20 to 80% by volume on the basis of the apparent volume (core layer volume), and more preferably 30 to 60% by volume. preferable. If the abundance of bubbles having a diameter of 10 μm or less is less than 20% by volume, the effect of weight reduction by foaming is insufficient, while if it exceeds 80% by volume, the mechanical strength of the resin foam becomes insufficient.

  Furthermore, the average diameter based on the number of bubbles in the core layer is preferably 0.01 to 1 μm, more preferably 0.03 to 0.5 μm, and 0.05 to 0.3 μm. Particularly preferred. If the average diameter of the bubbles in the core layer is less than the lower limit, it is difficult to obtain a sufficient foaming ratio, and the effects such as lightening by foaming tend to be insufficient. On the other hand, if the upper limit is exceeded, the resin foam The mechanical strength of the steel tends to decrease.

  In the present invention, since the thermoplastic resin is foamed while being irradiated with ultrasonic waves as described above, the size of bubbles in the core layer tends to be smaller and the uniformity is higher. The diameter of the bubbles in the core layer is preferably in the range of 0.01 to 1.0 μm, and more preferably in the range of 0.05 to 0.5 μm. Thus, since the size of the bubbles in the core layer is smaller and the uniformity is higher, the mechanical strength and the light transmittance tend to be further suppressed.

  In the resin foam of the present invention, a skin layer made of an unfoamed bulk resin made of the thermoplastic resin is integrally formed on the surface of the core layer. Such a skin layer is formed by depressurizing and foaming a thermoplastic resin impregnated with a supercritical fluid and / or a subcritical fluid under pressure as in the method for producing a resin molded body of the present invention. In the method of manufacturing a body, it is always formed on the outermost surface. In such a skin layer, the abundance ratio of bubbles having a diameter exceeding 0.01 μm needs to be 1.0% by volume or less on the basis of the apparent volume (volume of the skin layer), and the diameter is 0.01 μm. More preferably, there are no more bubbles.

  In the present invention, since the thermoplastic resin is foamed while being irradiated with ultrasonic waves as described above, the average thickness of the skin layer tends to be thinner. The skin layer needs to have an average thickness of 0.01 to 0.5 mm, and more preferably 0.10 to 0.15 mm. If the average thickness of the skin layer is less than 0.01 mm, the mechanical strength of the resin foam becomes insufficient. On the other hand, if it exceeds 0.5 mm, the effect of reducing the weight of the entire resin foam becomes insufficient.

  Furthermore, in the present invention, since the thermoplastic resin is foamed while being irradiated with ultrasonic waves as described above, there is an apparent presence rate of bubbles having a diameter exceeding 10 μm between the skin layer and the core layer. There is no layer region exceeding 0.1% by volume on a volume basis. That is, conventionally, the formation of the skin layer suppresses the diffusion of the gas remaining inside the resin and promotes the bubble growth in the vicinity of the skin layer. Therefore, the presence rate of bubbles having a diameter exceeding 10 μm is 0 on the apparent volume basis. A layer region exceeding 1% by volume is formed. On the other hand, in the production method of the present invention, by spreading the thermoplastic resin while irradiating ultrasonic waves, gas diffusion from the inside of the resin to the vicinity of the surface is promoted, and the gas quickly escapes to the outside of the resin. Thus, foaming proceeds promptly and uniformly before the skin layer is formed, and the growth of bubbles in the vicinity of the skin layer is suppressed, so that the layer region as described above is not formed. Therefore, since the resin foam of the present invention is a so-called microcellular, which is a foam having substantially only 10 μm or less in diameter as a whole, the elastic modulus is higher than that of the bulk resin before foaming. In addition, a decrease in mechanical strength such as tensile strength, bending strength, and impact strength is significantly suppressed, and a decrease in light transmittance is also greatly suppressed.

  In addition, the bubble diameter (the diameter and average diameter of each bubble), the bubble presence rate, and the average thickness of the skin layer in the resin foam of the present invention are determined in the vicinity of the surface of the resin foam and the internal fracture surface (respectively By observing 3 points or more) with a SEM (scanning electron microscope), it can be determined by a conventional method. At that time, if the cross section of the bubble is not circular, the minimum circumscribed circle of the cross section of the bubble is assumed, and the diameter is set as the diameter of the bubble. Furthermore, the relative density and expansion ratio based on the bulk resin before foaming in the resin foam of the present invention can be determined by a so-called Archimedes method by a conventional method.

  EXAMPLES Hereinafter, although this invention is demonstrated more concretely based on an Example and a comparative example, this invention is not limited to a following example.

Example 1
As a thermoplastic resin test piece, a polymethyl methacrylate resin (PMMA) plate (manufactured by Kuraray Co., Ltd., thickness: 1 mm) cut into a size of 20 mm in length and 20 mm in width was used as the test piece. Then, a test piece is put in a pressure vessel, carbon dioxide (carbon dioxide) is purged and sealed, and the inside of the vessel is maintained at 32 ° C. and 20 MPa for 3 hours, so that it becomes a supercritical fluid. The test piece was impregnated with carbon. The amount of impregnation of the supercritical fluid (carbon dioxide) in the obtained test piece was 30 parts by mass with respect to 100 parts by mass of the test piece before impregnation.

  Next, the pressure vessel is opened and the pressure is rapidly reduced from 20 MPa to atmospheric pressure (0.1 MPa) (pressure reduction speed is 0.2 MPa / sec or more), and at the same time, the test piece is subjected to an ultrasonic treatment device (manufactured by AS ONE, product Name: ASU-2)) in the water tank (temperature in the water tank is 20 ° C.), and the test piece is maintained for 30 minutes while being irradiated with ultrasonic waves (frequency: 42 kHz) in water at 20 ° C. Was foamed to obtain a resin foam.

The foaming ratio and relative density (based on the bulk resin before foaming) of the obtained resin foam were determined by the Archimedes method by a conventional method. In addition, the surface of the obtained resin foam (skin layer) and the inside (core layer) fractured surfaces (arbitrary three locations, respectively) were observed by SEM (scanning electron microscope) as follows, and the core layer The following numerical values were obtained for the skin layer and the region between the core layer and the skin layer. The obtained results are shown in Table 2. An example of an SEM image of a fracture surface near the surface (skin layer) of the obtained resin foam is shown in FIG. 1, and an example of an SEM image of an internal (core layer) fracture surface is shown in FIG.
<Core layer>
-Abundance ratio [volume%] based on the apparent volume of bubbles of 10 μmφ or less
-Abundance based on the apparent volume of bubbles exceeding 10 μmφ [volume%]
・ Average diameter [μm] based on the number of bubbles
・ Bubble diameter distribution range [μm]
<Skin layer>
-Abundance based on the apparent volume of bubbles exceeding 0.01 μmφ [volume%]
・ Average thickness [mm]
<Area between skin layer and core layer>
-Presence / absence of a layer region where the presence rate of bubbles exceeding 10 μmφ (apparent volume basis) exceeds 0.1% by volume • Average of the layer region where the presence rate of bubbles exceeding 10 μmφ (based on apparent volume) exceeds 0.1% by volume Thickness [mm]
-Abundance based on the apparent volume of bubbles exceeding 10 μmφ [volume%]
-Distribution range [μm] of bubble diameter exceeding 10 μmφ.

[SEM observation]
The obtained resin foam was submerged in liquid nitrogen and allowed to stand for 0.2 hours. Thereafter, the resin foam was taken out from liquid nitrogen and broken. Subsequently, platinum was vapor-deposited on the frozen fracture surface to obtain a sample for SEM observation, and the frozen fracture surface was observed with a scanning electron microscope (SEM, manufactured by Hitachi High-Technologies Corporation, trade name: SU3500).

(Comparative Example 1)
The pressure vessel is opened and the pressure is rapidly reduced from 20 MPa to atmospheric pressure (0.1 MPa) (pressure reduction rate is 0.2 MPa / sec or more), and in the atmosphere of 20 ° C. without ultrasonic irradiation in water. For 30 minutes to obtain a resin foam in the same manner as in Example 1 except that the test piece was allowed to foam.

  The obtained resin foam was analyzed in the same manner as in Example 1, and the obtained results are shown in Table 2. An example of a SEM image of the fracture surface near the surface (skin layer) of the obtained resin foam is shown in FIG. 3, and an example of a SEM image of the fracture surface inside (core layer) is shown in FIG.

(Comparative Example 2)
The pressure vessel is opened and the pressure is rapidly reduced from 20 MPa to atmospheric pressure (0.1 MPa) (decompression rate is 0.2 MPa / sec or more), and in the atmosphere of 35 ° C. without ultrasonic irradiation in water. For 30 minutes to obtain a resin foam in the same manner as in Example 1 except that the test piece was allowed to foam.

  The obtained resin foam was analyzed in the same manner as in Example 1, and the obtained results are shown in Table 2. An example of an SEM image of a fracture surface near the surface (skin layer) of the obtained resin foam is shown in FIG. 5, and an example of an SEM image of an internal (core layer) fracture surface is shown in FIG.

(Comparative Example 3)
The pressure vessel is opened and the pressure is rapidly reduced from 20 MPa to atmospheric pressure (0.1 MPa) (decompression rate is 0.2 MPa / sec or more), and in the atmosphere of 50 ° C. without ultrasonic irradiation in water. For 30 minutes to obtain a resin foam in the same manner as in Example 1 except that the test piece was allowed to foam.

  The obtained resin foam was analyzed in the same manner as in Example 1, and the obtained results are shown in Table 2. An example of the SEM image of the fracture surface near the surface (skin layer) of the obtained resin foam is shown in FIG. 7, and an example of the SEM image of the fracture surface inside (core layer) is shown in FIG.

  As is clear from the results shown in Table 2, in the resin foam of Example 1 obtained by foaming the thermoplastic resin while irradiating ultrasonic waves, Comparative Examples 1 to 3 obtained by the conventional method were used. Compared to the resin foam of this type, the growth of bubbles in the vicinity of the skin layer is greatly suppressed, and the generation of large bubbles is completely prevented, and the average thickness of the skin layer is further reduced and the bubbles in the core layer are It was confirmed that the size was smaller and the uniformity was higher.

  In addition, the resin foam of Example 1 is a foam in which the presence rate of large bubbles in the region between the skin layer and the core layer is extremely low, and substantially only bubbles having a diameter of 10 μm or less are present. It is a so-called microcellular, and has a significant reduction in mechanical strength such as elastic modulus, tensile strength, bending strength, and impact strength compared to bulk resin before foaming. It was confirmed that the decrease in the transmittance was also significantly suppressed.

  As described above, according to the present invention, the core layer in which fine bubbles are formed by foaming the thermoplastic resin impregnated with the supercritical fluid and / or the subcritical fluid, and the skin layer formed on the surface thereof And a method for producing a resin foam capable of sufficiently preventing the generation of large bubbles in the vicinity of the skin layer, and a skin layer and a core layer to be obtained thereby. It is possible to provide a resin foam in which the presence of large bubbles in the region between the layers is extremely low, and the decrease in mechanical strength and light transmittance is significantly suppressed compared to the resin before foaming. . Therefore, the present invention is a very useful technique for using the resin foam as various structural materials.

Claims (7)

An impregnation step of impregnating a thermoplastic resin with a supercritical fluid and / or a subcritical fluid under pressurized conditions;
A foaming step of obtaining a resin foam by foaming the thermoplastic resin impregnated with the supercritical fluid and / or subcritical fluid under reduced pressure;
A method for producing a resin foam, comprising: foaming the thermoplastic resin while irradiating ultrasonic waves in the foaming step.   The method for producing a resin foam according to claim 1, wherein in the foaming step, the thermoplastic resin is immersed in a liquid and foamed while being irradiated with ultrasonic waves.   The method for producing a resin foam according to claim 1 or 2, wherein the ultrasonic frequency is 20 to 100 kHz. The resin foam is
A foam made of the thermoplastic resin, wherein the presence rate of bubbles having a diameter of 10 μm or less is 20 to 80% by volume based on the apparent volume, and the presence rate of bubbles exceeding 10 μm in diameter is 0.1 based on the apparent volume. A core layer that is less than or equal to volume percent;
A resin layer made of the thermoplastic resin integrally formed on the surface of the core layer, wherein the abundance ratio of bubbles having a diameter of more than 0.01 μm is 1.0% by volume or less based on the apparent volume; A skin layer having an average thickness of 0.01 to 0.5 mm;
A resin foam in which there is no layer region in which the presence rate of bubbles having a diameter exceeding 10 μm exceeds 0.1% by volume on the basis of the apparent volume between the skin layer and the core layer. The manufacturing method of the resin foam as described in any one of Claims 1-3 characterized by the above-mentioned. A foam made of a thermoplastic resin, wherein the presence rate of bubbles having a diameter of 10 μm or less is 20 to 80% by volume on the basis of the apparent volume, and the presence rate of bubbles having a diameter exceeding 10 μm is 0.1 volume on the basis of the apparent volume. % Core layer or less,
A resin layer made of the thermoplastic resin integrally formed on the surface of the core layer, wherein the abundance ratio of bubbles having a diameter of more than 0.01 μm is 1.0% by volume or less based on the apparent volume; A skin layer having an average thickness of 0.01 to 0.5 mm;
The resin foam is characterized in that there is no layer region between the skin layer and the core layer in which the presence rate of bubbles having a diameter exceeding 10 μm exceeds 0.1% by volume on the apparent volume basis body.   6. The resin foam according to claim 5, wherein an average diameter based on the number of bubbles in the core layer is 0.01 to 1 [mu] m.   The resin foam according to claim 5 or 6, wherein the thermoplastic resin is at least one selected from the group consisting of an acrylic resin and a polycarbonate resin.