JP2007083717A - Multilayer foam molded body and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To manufacture a multilayer foam molded body wherein an outer layer is a foam layer in which micro foaming cells are dispersed uniformly, and an inner layer is an unfoam layer. <P>SOLUTION: An unfoam molded body P<SB>1</SB>is arranged in a cavity of both divided dies 1a, 1b and a mold bundle is carried out. The dies are heated by a heater 2a and a heater 2b, and at the same time, a valve for supply 11a is opened to charge a supercritical fluid generated at a supercritical fluid generating device 10 into the cavity through a supply port 7. By maintaining a pressure inside the cavity at a supercritical pressure or more, the desired region forming the foam layer of the unfoam molded body P<SB>1</SB>is impregnated with the supercritical fluid (gas for foaming). Then, the pressure inside the cavity is relieved, and the rapid pressure reduction makes foam cell nuclei caused by the impregnated supercritical fluid grow up, and forms the foam layer wherein the micro foam cells are dispersed to expand to a shape copied after the cavity. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a multilayer foam molded article excellent in impact resistance and a method for producing the same.

  A foamed molded body made of a thermoplastic resin can be used in various fields such as building materials, automotive parts, and packaging materials because it can reduce weight, improve heat insulation properties, improve sound absorption properties, and improve impact resistance.

  In order to improve the impact resistance of foam molded products, a large amount of foaming agent is required, and instead of foaming the entire foam molding cycle, it is a multilayer foam having a foam layer and a non-foam layer. A molded body is used. Below, the prior art example of the manufacturing method of a multilayer foaming molding is demonstrated.

  (A) A method for producing a multilayer foam sheet in which a foam layer and a non-foam layer are laminated by co-extrusion of a foam resin and a non-foam resin (see Patent Document 1 and Patent Document 2).

(B) A method for producing a multilayer foamed molded article having a foam layer and a non-foamed layer by filling the mold with a non-foamable resin and an unfoamed foamable resin and then foaming the foamable resin. (See Patent Document 3 and Patent Document 4).
JP 2003-94504 A JP 2000-263576 A JP-A-11-349719 Japanese Patent Laid-Open No. 11-188823

  However, in the above methods (a) and (b), different types of molding materials are required, and an expensive coextrusion apparatus equipped with an extruder for non-foamable resin and an extruder for foamable resin Manufacturing cost is increased. Moreover, there exists a possibility that the intensity | strength of the junction part of a non-foaming resin layer and a foaming resin layer may be low, and may peel.

  In addition, combustible gases such as chlorofluorocarbons, butane, and pentane used as physical foaming agents have a relatively large global warming potential, and it is necessary to make the molding equipment explosion-proof, or gas escape from the foam. Takes time. In addition, sodium bicarbonate, azo compounds, nitroso compounds and the like used as chemical foaming agents have low thermal stability, and degradation residues reduce the physical properties of the resin, and it is difficult to produce foamed molded products with a high expansion ratio. In addition, the chemical foaming agent has a problem that it is difficult to perform foaming while accurately controlling a foamed cell diameter by accurately selecting a foamed region or a foamed layer in an unfoamed molded body made of the same material.

  The present invention has been made in view of the above problems, and a multilayer foamed molded article having a foam layer in which fine foam cells are uniformly dispersed as an outer layer and an unfoamed layer as an inner layer, and a method for producing the same. The purpose is to provide.

  In order to solve the above unsolved problems, the multilayer foamed molded article of the present invention has an outer layer composed of a foam layer using a supercritical fluid as a foaming agent and an inner layer composed of a non-foamed layer, and the foam The body layer and the non-foamed layer are made of the same thermoplastic resin.

  In addition, the supercritical fluid is made of carbon dioxide or nitrogen.

  Further, in the foam layer, the shape of the foam cell is preferably a polyhedron or a substantially spherical body.

  In the foam layer, the cell diameter of the foam cell is preferably 50 μm or less.

  The method for producing a multilayer foamed molded article of the present invention has an outer layer composed of a foam layer using a supercritical fluid as a foaming agent and an inner layer composed of a non-foamed layer, and the foam layer and the non-foamed body In a method for producing a multilayer foamed molded body having the same layer of thermoplastic resin, a split mold having a cavity for regulating the outer surface of the multilayer foamed molded body is used, and the heat is placed in the cavity of the mold. A mold clamping step in which a non-foamed molded body made of a plastic resin is accommodated, and after the mold clamping step, the supercritical fluid is injected into the cavity and the foam layer of the unfoamed molded body Impregnation step of impregnating the supercritical fluid in the region forming the gas, and after the impregnation step, discharging the supercritical fluid in the cavity and releasing the pressure to thereby release the bubble nuclei by the impregnated supercritical fluid Grown, is characterized in that it has a, a foaming process of expanding into a shape to follow said cavity to form a finely foamed cells are distributed foam layer.

  Since the present invention is configured as described above, the following effects can be obtained.

  The multilayer foamed molded product according to the present invention can absorb impact from the outside with the foam layer as the outer layer, and can maintain tensile strength, compressive strength, etc. with the non-foam layer as the inner layer.

  Further, the bonding strength at the boundary between the foam layer and the non-foam layer is high, and there is no possibility of peeling.

  The method for producing a multilayer foamed molded product according to the present invention is a high foaming method in which fine foamed cells (bubbles) having a small cell diameter are uniformly dispersed from the outer surface of the unfoamed molded product to a predetermined depth by the impregnated supercritical fluid. A foam layer having a magnification can be reliably formed.

  Furthermore, since the multilayer foam molding apparatus is inexpensive and the molding cycle is short, the manufacturing cost can be reduced.

  Embodiments of the present invention will be described with reference to the drawings.

  Drawing 1 is an explanatory view showing an example of an apparatus used for a manufacturing method of a multilayer foaming fabrication object by one embodiment.

  As shown in FIG. 1, one split mold 1a has a heater 2a disposed on the outer surface and an internal cooling jacket 3a, and the other split mold 1b is a heater 2b disposed on the outer surface. And a cooling jacket 3b installed therein, and at the time of mold clamping, the cavity surface 4a of one split mold 1a and the cavity surface 4b of the other split mold 1b are formed into a plate-like shape that is the final molded product. A cavity having a shape that regulates the outer surface of the multilayer foamed molded article is formed.

  One split mold 1a is provided with a discharge port 6 penetrating from the outer wall to the cavity surface 4a and a supply port 7 penetrating from the outer wall to the cavity surface 4a. The discharge port 6 is connected to a discharge pipe 8 having a discharge valve 8a interposed therein. The supply port 7 is supercritical via a supply pipe 11 having a supply valve 11a interposed. A fluid generator 10 is connected.

  The other split mold 1b is provided with a discharge port 5 penetrating from the outer wall to the cavity surface 4b. The discharge port 5 is connected to a discharge conduit 9 having a discharge valve 9b interposed therebetween. .

  Then, the process of the manufacturing method of the multilayer foaming molded object by one embodiment of this invention is demonstrated.

(1) Open both split molds 1a and 1b, which are split molds having cavities that regulate the outer surface of the multilayer foamed molded article, and open the cavity surface 4a of one split mold 1a and the other split mold. to mold clamping by placing the unfoamed molded article P ₁ between the cavity surface 4b of 1b.

In this step, the temperature of the unfoamed molded article P ₁ may or even at room temperature, in order to shorten the heating time in the subsequent step, the crystallization temperature near a crystalline resin, the glass transition point in the non-crystalline resin (Tg) It is desirable to keep the temperature in the vicinity.

Incidentally, non-expanded molded article P ₁ is made of a thermoplastic resin, injection molding, extrusion molding, prepared by compression molding.

(2) After the step (1), raising the temperature by heating by a heater 2b unfoamed molded article P ₁ of the heater 2a and the other split mold blocks 1b of one of the split molds 1a to a predetermined temperature .

  In this step, in the case of a crystalline resin such as polypropylene and polyethylene, the predetermined temperature is raised to 1 to 10 ° C, preferably 1 to 3 ° C lower than the melting point (Tm) at which melting starts. The reason is that the melting of the crystalline resin starts at a melting point or higher and the predetermined shape cannot be maintained.

  In the case of an amorphous resin, the temperature is preferably near the glass transition point (Tg). This is because when the temperature is significantly higher than the glass transition point (Tg), the amorphous resin starts to soften and the predetermined shape cannot be maintained.

  (3) Simultaneously with the heating by the heaters 2a and 2b in the step (2), the supply valve 11a is opened, and the supercritical fluid is injected from the supercritical fluid generator 10 into the cavity through the supply port 7. To do.

  The supercritical fluid in this step is preferably nitrogen gas or carbon dioxide in a supercritical fluid state from the viewpoint of cost and global environment conservation. Further, from the viewpoint of miniaturizing the foam cell, the pressure in the cavity is preferably equal to or higher than the critical pressure. Incidentally, it becomes a supercritical fluid state at −147.1 ° C. or more and 3.35 MPa or more with nitrogen gas, and at 33.1 ° C. or more and 7.30 MPa with carbon dioxide.

(4) After the steps (2) and (3), the supercritical fluid (hereinafter referred to as “foaming gas”) is maintained in the unfoamed molded product P ₁ by maintaining the inside of the cavity at a critical pressure or higher. Gradually impregnate from the outer wall.

The impregnation time depends on the kind of the thermoplastic resin and the thickness of the unfoamed molded product P _1. For example, in the case of 5 mm thick polypropylene, less than 3 minutes, preferably less than 1 minute is sufficient. If the impregnation time is too long, not only will the molding cycle be prolonged, but the thickness of the foam layer will increase, and mechanical properties other than impact strength, such as tensile strength and bending strength, will be unfavorable.

  (5) After the step (4), heating by the heaters 2a and 2b of the two split molds 1a and 1b is stopped, the discharge valves 8a and 9b are opened, and the foaming gas in the cavity is discharged to the discharge port 5 , 6 to release the pressure in the cavity, and at the same time, cooling medium (cooling water) is allowed to flow through the cooling jackets 3a and 3b for cooling.

In this step, the pressure in the cavity suddenly decreases due to the discharge of the foaming gas, and fine bubble nuclei are generated by the foaming gas impregnated in a specific region of the unfoamed molded product P _1. Diffusion / movement starts toward the bubble nuclei in the vicinity, and growth from the bubble nuclei to the bubbles begins.

Over a predetermined period of time, simultaneously with the foaming of a specific region of the unfoamed molded article P ₁ (outer layer), the unfoamed molded article P ₁ is a multilayer foamed molded article shaped to follow the cavity by the volume expansion is formed. Then, in addition to the cooling by the cooling medium, the growth of bubble nuclei is stopped by the cooling by the vaporization of the foaming gas.

  The multilayer foam molded article according to the present invention has an outer layer composed of a foam layer using a supercritical fluid as a foaming agent and an inner layer composed of a non-foam layer, and the foam layer and the non-foam layer are It consists of the same thermoplastic resin. For this reason, it becomes possible to absorb the impact from the outside with an outer foam layer, and to hold | maintain tensile strength, compressive strength, etc. with an inner non-foam layer.

  Next, a reference example will be described.

  In this reference example, by changing a part of the steps (1) to (5) in the embodiment described above as follows, the temperature is changed to one split mold 1a and the other split mold 1b. By providing the difference, it is possible to produce a multilayer foam molded article having a foam layer only on one side.

For example, the mold is clamped in a state where the unfoamed molded product P ₁ is in contact with the other split mold 1b. Further, the temperature of the other split mold 1b, in the case of non-crystalline resin forming the unfoamed molded article P _1, the glass transition temperature (Tg), in the case of crystalline resin is considerably than the crystallization temperature The temperature is set to a low temperature and is set to a temperature near the glass transition temperature or the crystallization temperature of one split mold 1a.

Thus, only the right portion of the unfoamed molded article P ₁ is foamed, since the left part temperature is low, does not foam. As a result, it is possible to produce a multilayer foamed molded article in which the right half is a foam layer and the left half is an unfoamed layer.

  Next, the manufacturing method of the multilayer foaming molding by other embodiment of this invention is demonstrated.

  FIG. 2 is an explanatory view showing an example of an apparatus used in the method for producing a multilayer foam molded article according to the present embodiment.

  As shown in FIG. 2, one split mold 21a includes a cavity surface 24a that regulates the outer surface of a substantially half of the bottle-shaped multilayer foamed molded article, a heater 22a disposed on the outer wall surface, and an inner mold. A cooling jacket 23a is provided. The other split mold 21b includes a cavity surface 24b that regulates the outer surface of the remaining portion of the bottle-shaped multilayer foamed molded article, a heater 22b disposed on the outer wall surface, and an internal cooling jacket 23b. is doing.

  Further, one split mold 21a is provided with a supply port 27 having one end opened on the cavity surface 24a and the other end connected to the supercritical fluid generator 40 via a supply pipe 41. In addition, a discharge port 26 penetrating from the outer wall to the cavity surface 24a is provided at a portion spaced from the supply port 27, and a discharge conduit 29 having a discharge valve 29a interposed is provided in the discharge port 26. Yes.

  Then, the process of the manufacturing method of the multilayer foaming molding by this Embodiment is demonstrated.

Performing mold clamping by placing the bottle-like unfoamed molded article P ₂ into the cavity formed by the cavity surface 24b of the cavity surface 24a and the other split mold 21b (1) one of the split molds 21a.

(2) After the step (1) is heated by the heater 22b of the heater 22a and the other split mold 21b of one of the split molds 21a, to raise the temperature of the unfoamed molded article P ₂ to a predetermined temperature .

  In this step, in the case of crystalline resins such as polypropylene and polyethylene, the predetermined temperature is maintained at a temperature lower by 1 to 10 ° C., preferably lower by 1 to 3 ° C. than the melting point (Tm) at which melting starts. The reason is that the melting of the crystalline resin starts above the melting point and the predetermined shape cannot be maintained. In the case of an amorphous resin, the temperature is preferably near the glass transition point (Tg). This is because when the temperature is significantly higher than the glass transition point, the amorphous resin starts to soften and the predetermined shape cannot be maintained.

  (3) Simultaneously with the heating by the heaters 22a and 22b in the step (2), the supply valve 41a is opened and the supercritical fluid is injected into the cavity through the supply port 27.

  In this step, the supercritical fluid is preferably nitrogen gas or carbon dioxide in a supercritical fluid state from the viewpoint of cost and preservation of the global environment. In order to make the foam cell (bubble) finer, it is desirable that the pressure in the cavity be a pressure equal to or higher than the critical pressure of the injected supercritical fluid.

(4) After the step (3), by maintaining the inside of the cavity at a critical pressure or higher, the supercritical fluid (hereinafter referred to as “foaming gas”) is allowed to flow outside the bottle-shaped unfoamed molded product P ₂ . And gradually impregnate from the inner surface.

  Although this impregnation time depends on the type of thermoplastic resin, for example, in the case of 5 mm thick polypropylene, less than 3 minutes, preferably less than 1 minute is sufficient. When the impregnation time is increased, not only the molding cycle becomes longer, but also the foam layer becomes thicker, and mechanical properties such as rigidity other than impact strength are lowered.

  (5) After the step (4), heating by the heaters 22a and 22b of the two split molds 21a and 21b is stopped, the discharge valve 29a is opened, and the foaming gas in the cavity is passed through the discharge port 26. At the same time as releasing the pressure in the cavity, the cooling jackets 23a and 23b are cooled by flowing a cooling medium.

Origin in this step, the pressure rapidly cavity is reduced by the discharge of the foaming gas, bubble nuclei of supercritical fluids impregnated in a particular region or layer of unfoamed molded article P ₂ grows, the bubble nuclei As a result, diffusion and movement toward the bubble nuclei in the vicinity thereof are started and the bubbles grow.

Furthermore, a cooling medium is supplied into the inside of the bottle-shaped unfoamed molded product P ₂ through the nozzle 25 inserted in the mouth. The cooling medium is preferably a gaseous medium at room temperature in consideration of a post-process for separating the cooling medium from the bottle-shaped multilayer foamed article.

On the other hand, the bottle-like unfoamed molded article P _2, when the outer surface side simultaneously cavity surface 24a when foaming, volume expanded toward 24b, reaches a shape to follow the cavity surface 24a, to 24b, both split molds 21a, In addition to the cooling by 21b and the temperature decrease of the bottle-shaped multilayer foamed molded article, the foaming is stopped by the cooling by the vaporization of the foaming gas.

  Even in the case of the present embodiment, it is possible to selectively foam only the region or layer in which the foaming gas is impregnated in the unfoamed molded article by changing a part of the process. The bottle-shaped multilayer foam molded article formed by such a method can absorb impact from the outside with the foam layer, and can retain rigidity or suppress liquid leakage with the non-foam layer.

  By the way, the impact strength is improved by foaming because the bubbles play a role as a kind of cushion against the impact, but the impact strength improves as the cell diameter decreases. This is because, in the same bubble volume, the number of cells increases as the cell diameter decreases, and the number of bubbles that can directly contribute to shock absorption increases. Moreover, it is because the probability that an impact is directly applied to an unfoamed region that cannot absorb the impact is reduced. Further, depending on the type of thermoplastic resin, the molecular chain on the inner wall of the bubble is stretched by the growth of the bubble, and the strength is improved. For this reason, the smaller the cell diameter, the greater the number of cells, and the corresponding increase in the surface area of the bubble inner wall that undergoes stretching. Here, in the case of the same bubble volume, if the cell diameter is halved, the number of cells becomes 8 times, and the surface area of the bubble inner wall becomes 2 times. Similarly, if the cell diameter becomes 1/10, the number of cells becomes 1000 times, and the surface area of the bubble inner wall becomes 10 times.

  The degree of improvement in impact strength due to the decrease in cell diameter depends on the type of thermoplastic resin, but for foamed molded products having a cell diameter of 50 μm or less, 1.5 times or more of the unfoamed molded product, and for cell diameters of 10 μm or less. When the cell diameter is 2 μm or more, the cell diameter is 5 μm or less, and the cell diameter is 2.5 times or more.

  Furthermore, the cell shape of the bottle-shaped multilayer foamed article produced by the method is a polyhedral air bubble that is nearly spherical. In the case of a bubble deformed in a specific direction, the strength is different depending on the direction in which the force acts, so that the application is limited, and depending on the shape of the bubble, stress concentration occurs and becomes a starting point of crack generation. In this respect, if the shape of the bubbles is a polyhedron or a substantially spherical body, anisotropy does not occur in the strength of the multilayer foamed molded article, and it does not become a starting point of cracks.

  A strip-shaped unfoamed molded article having a width of 10 mm, a length of 80 mm, and a thickness of 2 mm by injection molding using an ethylene / propylene copolymer crystalline resin (polypropylene manufactured by Idemitsu Petrochemical Co., Ltd., grade: E-233GV) as a thermoplastic resin. Was molded. This was clamped in a state of being accommodated in a cavity of a divided mold, heated to 140 ° C., carbon dioxide in a supercritical state at a pressure of 20 MPa was injected, and impregnation was performed for 1 minute. After impregnation for 1 minute, the pressure was released at a reduced pressure rate of 10 MPa / second to obtain a multilayer foam molded article according to Example 1 in which only the surface layer was foamed.

  A multilayer foam molded article according to Example 2 was obtained in the same manner as in Example 1 except that the impregnation time was 2 minutes.

For the multilayer foamed molded products according to Example 1 and Example 2, the IZOD impact test (JIS
K 7110) and the foamed state of the surface layer were observed with a scanning electron microscope, and the cell diameter, the number of cells, and the thickness of the foamed layer were measured. In addition, peeling of the foaming area | region and the non-foaming area | region by the impact test did not occur. These results are shown in Table 1.

  As shown in Table 1, the multilayer foamed molded products according to Examples 1 and 2 according to the present invention exhibited a high impact strength about 3.3 times that of the unfoamed molded product (Comparative Example).

It is explanatory drawing which shows an example of the apparatus used for the manufacturing method of the multilayer foaming molding by one Embodiment. It is explanatory drawing which shows an example of the apparatus used for the manufacturing method of the multilayer foaming molding by other embodiment.

Explanation of symbols

1a, 1b, 21a, 21b Split mold 2a, 2b, 22a, 22b Heater 3a, 3b, 23a, 23b Cooling jacket 4a, 4b, 24a, 24b Cavity surface 5, 6, 26 Discharge port 7, 27 Supply port 8 29 Discharge pipe 10, 40 Supercritical fluid generator 11, 41 Supply pipe

Claims (5)

  It has an outer layer composed of a foam layer using a supercritical fluid as a foaming agent and an inner layer composed of a non-foam layer, and the foam layer and the non-foam layer are made of the same thermoplastic resin. A multilayer foamed molded product.   2. The multilayer foamed molded article according to claim 1, wherein the supercritical fluid comprises carbon dioxide or nitrogen.   The multilayer foamed molded article according to claim 1 or 2, wherein the foam layer has a foamed cell shape of a polyhedron or a substantially spherical body.   The multilayer foamed molded article according to any one of claims 1 to 3, wherein the foam layer has a cell diameter of the foamed cell of 50 µm or less. Multilayer foam molding having an outer layer made of a foam layer using a supercritical fluid as a foaming agent and an inner layer made of a non-foamed layer, and the foam layer and the non-foamed layer made of the same thermoplastic resin In a method of manufacturing a body,
A mold clamping step in which a mold of a split type having a cavity for regulating the outer surface of the multilayer foamed molded body is used, and the mold is clamped in a state where the unfoamed molded body made of the thermoplastic resin is accommodated in the cavity of the mold. When,
After the mold clamping step, an impregnation step of injecting the supercritical fluid into the cavity to impregnate the supercritical fluid in a region where the foam layer of the unfoamed molded body is formed;
After the impregnation step, the supercritical fluid in the cavity is discharged to release the pressure, thereby growing bubble nuclei by the impregnated supercritical fluid and forming a foam layer in which fine foam cells are dispersed. And a foaming step of expanding into a shape following the cavity.

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