Method for manufacturing a gradient foam molded body

Abstract

To optimally produce a gradient foamed molding with a foaming rate to change in a gradient manner between opposite principal faces, while controlling its gradient foaming.SOLUTION: A foamer-impregnated molding is prepared that has been impregnated with a foamer. The foamer-impregnated molding has opposite principal faces. The foamer-impregnated molding is heated, except for its peripheral regions, so that there is a temperature difference between the principal faces. As a result of heating, the foamer-impregnated molding is foamed in a gradient manner and then solidified by cooling.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for producing a gradient foamed molded article in which the cellular content changes gradiently between opposing main surfaces. [Background technology]

[0002] For example, Patent Document 1 discloses a gradient foam plastic sheet that is produced by dissolving a gas that is gaseous at room temperature and pressure in plastic at high temperature and pressure, exposing the plastic to an atmosphere under a pressure lower than the pressure at which the gas was dissolved, and then exposing both sides of the sheet to atmospheres with different temperatures. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-363324 Summary of the Invention [Problem to be solved by the invention]

[0004] The present inventors have conducted extensive research focusing on this background art, and have come to the following findings. Specifically, in Patent Document 1, a sheet is foamed by extrusion foam molding. Because foaming occurs immediately after extrusion from a die, even if both sides of the sheet are subsequently exposed to atmospheres with different temperatures, it is difficult to gradually change the foam content, i.e., it is difficult to increase the degree of the gradient. Furthermore, Patent Document 1 also shows an example in which the sheet is passed through temperature-controllable rolls to expose both sides to atmospheres with different temperatures, but sandwiching the sheet between the rolls makes it difficult to achieve high foaming. In addition, the bubbles formed in the sheet tend to be large and elongated in the extrusion direction (plane direction). Furthermore, due to the difference in foam content, i.e., the expansion coefficient of the material, between the two sides of the sheet, the sheet is prone to undulations and wrinkles. This makes it difficult to ensure flatness, and defects such as undulations and wrinkles become more pronounced when high foaming is achieved.

[0005] Therefore, in view of the above background technology, the inventors of the present invention have conducted further intensive research to successfully produce a gradient foamed molded article in which the foam content changes gradiently between opposing main surfaces while controlling the progress of gradient foaming, and as a result have completed the present invention. [Means for solving the problem]

[0006] The method for producing a gradient foamed molded body according to the present invention involves preparing a foaming-agent-impregnated molded body impregnated with a foaming agent, heating the foaming-agent-impregnated molded body except for its peripheral edges so that there is a temperature difference between the opposing main surfaces of the foaming-agent-impregnated molded body to cause gradient foaming, and then cooling and solidifying the foam. [Effects of the Invention]

[0007] According to the present invention, a gradient foamed molded article in which the cell content changes gradiently between opposing main surfaces can be successfully produced while controlling the progress of gradient foaming. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is an explanatory diagram showing an outline of a method for producing a gradient foam molded article according to an embodiment of the present invention. [Figure 2] 1 shows cross-sectional CT images of Example 1. [Figure 3] 10 is a cross-sectional CT image of Example 2. [Figure 4] 10 is a cross-sectional CT image of Example 3. [Figure 5] 10 is a cross-sectional CT image of Example 4. [Figure 6] 10 is a cross-sectional CT image of Comparative Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings.

[0010] In this embodiment, first, a foaming-agent-impregnated molded article 1 impregnated with a foaming agent is prepared. The foaming-agent-impregnated molded article 1 is preferably in the form of a sheet having a thickness of 0.3 to 10 mm, more preferably 0.5 to 3 mm, but is not limited thereto. For example, depending on the application of the gradient foamed molded article 2 to be produced, the foaming-agent-impregnated molded article 1 can be molded into a film having a thickness thinner than the above range or into a plate having a thickness thicker than the above range.

[0011] Examples of resin materials used for the foaming-agent-impregnated molded article 1 include polyethylene, polypropylene, polystyrene, polyester, polyamide, polyvinyl chloride, polyvinylidene chloride, polybutene, polyacetal, polyphenylene oxide, polymethyl methacrylate, polysulfone, polyethersulfone, polyetherketone, polyetheretherketone, polyamideimide, polycarbonate, polyarylate, polyimide, fluororesin, ethylene-propylene resin, ethylene-ethyl acrylate, epoxy resin, urethane resin, imide resin, acrylic resin, and norbornene-based resin. These may be used alone or in combination, and may be polymer alloys or composite materials of a resin matrix and a filler. However, the material is not limited to these and can be selected appropriately depending on the application of the gradient foam molded article 2 to be produced.

[0012] Examples of foaming agents include physical foaming agents such as carbon dioxide and nitrogen, or organic thermal decomposition type chemical foaming agents such as azodicarbonamide or inorganic thermal decomposition type chemical foaming agents such as sodium bicarbonate, as well as thermally expandable capsules containing foaming components. However, the foaming agent can be selected appropriately without being limited to these, depending on factors such as the solubility in the resin material used for the foaming agent-impregnated molded body 1.

[0013] Furthermore, as long as a foaming-agent-impregnated molded body 1 impregnated with a foaming agent can be finally prepared, there is no particular limitation on the means for producing the foaming-agent-impregnated molded body 1. One example is to produce the foaming-agent-impregnated molded body 1 by dissolving a foaming agent in a resin material melt-kneaded in a plasticizing unit of an injection molding machine, injection-molding the resin material into a predetermined shape while applying a dwell pressure to suppress foaming, and then removing the resin material from the mold after cooling it to a temperature lower than the cooling and solidification temperature of the resin material. Alternatively, the foaming-agent-impregnated molded body 1 may be produced by, for example, charging a resin material preformed into a predetermined shape into an autoclave and dissolving a foaming agent (preferably a physical foaming agent) in the resin material under high pressure. The foaming-agent-impregnated molded body 1 is not limited to being prepared in a state in which foaming is completely suppressed, and even if slight foaming occurs during the preparation process of the foaming-agent-impregnated molded body 1, resulting in a slightly foamed foamed foam, it is sufficient that the foaming is suppressed to an extent that the progress of gradient foaming can be controlled as described below. In other words, the foaming-agent-impregnated molded body 1 may be prepared in an unfoamed or slightly foamed state.

[0014] 1 , for example, a foaming-agent-impregnated molded body 1 formed into a predetermined shape is placed on a mounting table 10, and the portion of the foaming-agent-impregnated molded body 1, excluding its peripheral edge, is heated so as to create a temperature difference between the opposing main surfaces of the foaming-agent-impregnated molded body 1. In the illustrated example, a heat source 20 is disposed above the mounting table 10, and a shielding member 30 is provided around the heat source 20. This adjusts the irradiation range of the radiant heat from the heat source 20, thereby enabling the portion of the foaming-agent-impregnated molded body 1, excluding its peripheral edge, to be selectively heated from one direction. This makes it possible to heat the area irradiated with radiant heat from the heat source 20 so as to create a temperature difference between the opposing main surfaces of the foaming-agent-impregnated molded body 1. The top surface of the table 10 on which the foaming-agent-impregnated molded article 1 is placed is not limited to a flat surface as shown in the figure, and can be changed appropriately depending on the shape of the gradient foamed molded article 2 to be produced.

[0015] By heating the foaming-agent-impregnated molded body 1 in this manner, the bubbles grow in the heated portion of the foaming-agent-impregnated molded body 1 so that they are relatively large on the high-temperature side and relatively small on the low-temperature side, thereby allowing gradient foaming to proceed so that the bubble size and bubble rate change gradiently according to the temperature gradient between the main surfaces.

[0016] In this process, the peripheral portion of the foaming-agent-impregnated molded body 1 is not heated, and therefore remains an unfoamed portion, or a low-foamed portion (including an extremely low-foamed portion) that has unexpectedly foamed due to heat conduction from the heated portion. As a result, the heated portion of the foaming-agent-impregnated molded body 1 is surrounded by the non-foamed or low-foamed portions, and gradient foaming proceeds while being constrained in the planar direction (the direction perpendicular to the thickness direction), so that the shape change due to gradient foaming occurs preferentially in the thickness direction.

[0017] This makes it possible to control the progress of inclined foaming so that the bubbles extend in the thickness direction while suppressing the growth of bubbles in the planar direction when the inclined foaming progresses in the heated portion of the foaming-agent-impregnated molded body 1. As a result, undulations and wrinkles are less likely to occur during the process of inclined foaming, and a gradient foamed molded body 2 with good flatness can be produced.

[0018] After the desired bubble rate has been achieved, the foam is cooled and solidified to stop the growth of the bubbles and complete the process.

[0019] As described above, in this embodiment, the progress of gradient foaming can be controlled by heating the portion of the foaming-agent-impregnated molded body 1 excluding the peripheral edge portion so as to create a temperature difference between the opposing main surfaces of the foaming-agent-impregnated molded body 1. However, when heating the foaming-agent-impregnated molded body 1, it is preferable to restrain one side of the opposing main surfaces of the foaming-agent-impregnated molded body 1 (in the illustrated example, the side that contacts the mounting table 10) so as to restrict the movement of the foaming-agent-impregnated molded body 1 in the planar direction. To achieve this, for example, as shown in FIG. 1 , depending on the shape and size of the foaming-agent-impregnated molded body 1, a suction groove 11 may be carved in the top surface of the mounting table 10, and the entire periphery of the foaming-agent-impregnated molded body 1 may be vacuum-suctioned using a vacuum pump P, or, although not particularly illustrated, the foaming-agent-impregnated molded body 1 may be releasably adhered to the top surface of the mounting table 10, so that one side of the opposing main surfaces of the foaming-agent-impregnated molded body 1 may be restrained on the mounting table 10.

[0020] This allows the shape change due to the gradient foaming to occur more preferentially in the thickness direction, which in turn effectively suppresses the growth of bubbles in the planar direction, making it possible to control the progress of the gradient foaming so as to prevent undulations and wrinkles from occurring, thereby improving the flatness of the gradient foamed molded article 2 produced.

[0021] In this embodiment, radiant heat is irradiated from the heat source 20 disposed on the other of the opposing main surfaces of the foaming-agent-impregnated molded body 1 toward the foaming-agent-impregnated molded body 1 to heat only the other side, but it is also possible to heat or cool the foaming-agent-impregnated molded body 1 from the side of the mounting table 10, for example, by arranging a resistance heating element or a temperature control circuit in which a temperature control medium circulates inside the mounting table 10. In this way, the range in which the temperature difference between the main surfaces can be adjusted can be wider.

[0022] Although not specifically shown, by adjusting the arrangement of the resistance heating element and temperature control circuit arranged inside the mounting table 10 and heating only from the mounting table 10 side, it is possible to heat the area other than the peripheral portion of the foaming agent-impregnated molded body 1 so that there is a temperature difference between the main surfaces.

[0023] The temperature difference between the principal surfaces can be adjusted as appropriate depending on the application of the gradient foam molded article 2 to be produced. For example, in producing a gradient foam molded article 2 in which the progress of gradient foaming is more controlled, it is preferable to set the temperature difference between the principal surfaces so that the gradient B / b is 1.3 or greater, where b is the cellular fraction after cooling and solidification in the near-surface region on one of the opposing principal surfaces of the foaming-agent-impregnated molded article 1 (low-expansion side) and B is the cellular fraction after cooling and solidification in the near-surface region on the other (high-expansion side). When setting the temperature difference between the principal surfaces in this manner, it is preferable to adjust the amount of heat used when heating the foaming-agent-impregnated molded article 1 so that bubble growth is suppressed in the near-surface region on the low-expansion side so that the cellular fraction b after cooling and solidification is preferably 40% or less, and bubble growth is promoted in the near-surface region on the high-expansion side so that the cellular fraction B after cooling and solidification is preferably 25% or greater, more preferably 50% or greater.

[0024] Here, the near-surface region refers to a region extending from the surface to a depth of 20% of the thickness of the gradient foam molded article 2.

[0025] Furthermore, the temperature difference between the principal surfaces is more preferably adjusted so that foaming does not occur in the surface layer portion on one side (lower temperature side) of the opposing principal surfaces of the foaming-agent-impregnated molded body 1. This forms a non-foamed layer in the surface layer portion on that side, making it more difficult for undulations and wrinkles to occur as the inclined foaming progresses, and making it possible to produce an inclined foamed molded body 2 with better flatness. In addition, if a non-foamed layer is formed on the surface layer portion of the one side, it is preferable that the thickness of the non-foamed layer is 500 μm or less, and on the one side, the non-foamed layer is not included in the near-surface region defined as above.

[0026] In this embodiment, the gradient foamed molded product 2 can be produced as described above. After cooling and solidifying the product to stop the growth of the bubbles, the product can be further processed into a product shape by, for example, thermoforming, or the peripheral portions can be trimmed depending on the intended use.

[0027] The produced gradient foam molded article 2 can be used in a wide range of applications in various fields. In addition to being applicable to civil engineering and construction materials, automotive materials, packaging materials, and other applications that utilize its properties such as heat insulation, shock absorption, and sound absorption, the gradient change in the cell content between the opposing main surfaces also results in a gradient change in the dielectric constant, making it applicable, for example, to applications such as a millimeter wave low reflection sheet that improves the stability of a millimeter wave radar system. [Example]

[0028] The present invention will be described in more detail below with reference to specific examples.

[0029] [Example 1] Polyethylene terephthalate was used as the resin material, and nitrogen was dissolved in it as a foaming agent to prepare a foaming agent-impregnated molded body that was injection molded into a sheet measuring 90 mm in length, 90 mm in width, and 1.25 mm in thickness.

[0030] Next, the foaming-agent-impregnated molded body was placed on a temperature-controlled mounting table, and radiant heat was applied from a heat source arranged above the mounting table so as not to heat the peripheral portion of the foaming-agent-impregnated molded body, thereby heating the portion of the foaming-agent-impregnated molded body excluding the peripheral portion so as to create a temperature difference between the opposing main surfaces of the foaming-agent-impregnated molded body. During this process, one of the opposing main surfaces of the foaming-agent-impregnated molded body that was in contact with the mounting table was restrained. This allowed gradient foaming to proceed, and then the foam was cooled and solidified to stop the growth of bubbles.

[0031] A CT image of the cross section of the gradient foamed molded article produced in this way is shown in Figure 2. From this CT image, the aspect ratio of the cell diameter (the ratio of the cell diameter in the thickness direction to the cell diameter in the surface direction) was measured and the average value was calculated to be approximately 1.92, confirming that the foaming was gradient and the cells were elongated in the thickness direction.

[0032] The void content B in the near-surface region on the highly expanded side was 57.3%, and the void content b in the near-surface region on the low expanded side was 3.2%, with the gradient B / b being 18.1. The gradient foam molded article was cut into 5 cm square pieces and placed on a flat surface to measure its maximum height H. The ratio H / t of the maximum height H to the thickness t of the gradient foam molded article was evaluated as an index of flatness, and the value was 1.08.

[0033] [Example 2] A gradient foamed molded article was produced in the same manner as in Example 1, except that the amount of heat applied when heating the foaming-agent-impregnated molded article was increased by 7.5 times. A CT image of the cross section is shown in Figure 3. From this CT image, the aspect ratio of the cell diameters was measured and the average value was calculated to be approximately 1.44, confirming that the foaming was gradient and the cells were elongated in the thickness direction.

[0034] The void ratio B in the near-surface region on the highly expanded side was 51.2%, and the void ratio b in the near-surface region on the low expanded side was 35.6%, with the gradient B / b being 1.4. In addition, the produced gradient foam molding was cut into 5 cm square pieces and placed on a flat surface to determine its maximum height H. The ratio H / t of the maximum height H to the thickness t of the produced gradient foam molding was evaluated as an index of flatness, and the value was 1.28.

[0035] [Example 3] A gradient foamed molded body was produced in the same manner as in Example 1, except that the foaming-agent-impregnated molded body was placed on a temperature-uncontrolled mounting table, and when the foaming-agent-impregnated molded body was heated, one of the opposing main surfaces of the foaming-agent-impregnated molded body 1 that contacted the mounting table was not restrained, and both main surfaces were unrestrained, and the amount of heat applied was increased by 25%. A CT image of the cross section is shown in Figure 4. From the CT image, the aspect ratio of the cell diameters was measured and the average value was calculated to be approximately 1.58, confirming that gradient foaming occurred so that the cells were elongated in the thickness direction.

[0036] The void content B in the near-surface region on the highly expanded side was 28.5%, and the void content b in the near-surface region on the low expanded side was 1.5%, with the gradient B / b being 19.5. In addition, the produced gradient foam molding was cut into 5 cm square pieces and placed on a flat surface to determine its maximum height H. The ratio H / t of the maximum height H to the thickness t of the produced gradient foam molding was evaluated as an index of flatness, and the value was 1.89.

[0037] [Example 4] A gradient foamed molded article was produced in the same manner as in Example 1, except that an olefin resin was used as the resin material and the amount of heat used to heat the foaming-agent-impregnated molded article was reduced by 50%. A CT image of the cross section is shown in Figure 5. From this CT image, the aspect ratio of the cell diameters was measured and the average value was found to be approximately 2.68, confirming that the foamed article was gradient foamed so that the cells extended in the thickness direction.

[0038] The void content B in the near-surface region on the highly expanded side was 50.2%, and the void content b in the near-surface region on the low expanded side was 5.1%, with the gradient B / b being 9.9. In addition, the produced gradient foam molding was cut into 5 cm square pieces and placed on a flat surface to determine its maximum height H. The ratio H / t of the maximum height H to the thickness t of the produced gradient foam molding was evaluated as an index of flatness, and the value was 2.02.

[0039] [Comparative Example 1] A gradient foamed molded body was produced in the same manner as in Example 1, except that one of the opposing main surfaces of the foaming agent-impregnated molded body that was in contact with the mounting table was not restrained, and both main surfaces were unrestrained, and radiant heat was irradiated to the entire surface of the foaming agent-impregnated molded body. The gradient foamed molded article thus produced was largely distorted as a whole, and the ratio H / t determined in the same manner as in Example 1 was 5.17. The void ratio B of the near-surface region on the highly expanded side was 60.3%, and the void ratio b of the near-surface region on the low expanded side was 43.7%, with the gradient B / b being 1.4. In addition, a CT image of the cross section is shown in Figure 6. When the aspect ratio of the bubble diameter was measured from this CT image, the average value was approximately 1.25, which confirms that the growth of bubbles in the planar direction was not suppressed compared to Examples 1 to 4.

[0040] The present invention has been described above by showing preferred embodiments, but it goes without saying that the present invention is not limited to the above-described embodiments, and various modifications can be made within the scope of the present invention. [Explanation of symbols]

[0041] 1. Foaming agent impregnated molding 2. Gradient foam molding

Claims

1. A foaming agent-impregnated molded body impregnated with a foaming agent is prepared, A method for producing a gradient foamed molded body, comprising heating the foaming-agent-impregnated molded body except for its peripheral portion so that there is a temperature difference between the opposing main surfaces of the foaming-agent-impregnated molded body to cause gradient foaming, and then cooling and solidifying the foam.

2. 2. The method for producing a gradient foamed molded body according to claim 1, wherein the temperature difference between the main surfaces is set so that the gradient B / b is 1.3 or more, where b is the porosity after cooling and solidifying in the surface near-region on one side of the main surfaces, and B is the porosity after cooling and solidifying in the surface near-region on the other side of the main surfaces, which is heated at a higher temperature than the one side of the main surfaces.

3. 3. The method for producing a gradient foamed molded body according to claim 2, wherein the amount of heat when heating the foaming-agent-impregnated molded body is adjusted so that the porosity b of the surface-near region on one side of the main surface after cooling and solidifying is 40% or less.

4. 3. The method for producing a gradient foamed molded body according to claim 2, wherein the amount of heat when heating the foaming-agent-impregnated molded body is adjusted so that the porosity B of the surface-near region on the other side of the main surface after cooling and solidifying is 25% or more.

5. 3. The method for producing a gradient foamed molded body according to claim 2, wherein the amount of heat when heating the foaming-agent-impregnated molded body is adjusted so that the porosity B of the surface-near region on the other side of the main surface after cooling and solidifying is 50% or more.

6. The method for producing a gradient foam molded body according to any one of claims 1 to 5, wherein a temperature difference between the main surfaces is set so that a non-foamed layer is formed on a surface portion on one side of the main surfaces.

7. The method for producing a gradient foamed molded article according to any one of claims 1 to 5, wherein one of the opposing main surfaces of the foaming-agent-impregnated molded article is restrained when the foaming-agent-impregnated molded article is heated.

8. The method for producing a gradient foam molded article according to any one of claims 1 to 5, wherein the gradient foamed molded article is subjected to secondary molding after cooling and solidifying.

9. The method for producing a gradient foam molded article according to any one of claims 1 to 5, wherein after cooling and solidifying, a peripheral portion of the gradient foamed molded article is trimmed.

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