JP5971131B2 - Polyolefin resin expanded foam container - Google Patents

Description

  The present invention relates to a foamed stretch container formed of a polyolefin resin.

  At present, plastic foam moldings are excellent in light weight and heat insulation properties, and have improved mechanical properties such as rigidity, and are applied to various applications. In particular, recently, it has become possible to form fine foamed cells inside the molded body by physical foaming using an inert gas as a foaming agent (foaming by so-called microcellular technology), and its use has further expanded. For example, it came to be applied also to field | areas, such as a packaging container (refer patent document 1). In other words, so-called chemical foaming, in which sodium carbonate or an azo compound is used as a foaming agent, and carbon dioxide gas or nitrogen gas generated by thermal decomposition of the foaming agent is foamed, the foam cell becomes coarse, and as a result It has been difficult to use in the field of packaging containers, for example, the gas barrier property, appearance characteristics, and strength are significantly reduced by foaming, and the residue of the chemical foaming agent impairs hygiene. However, in physical foaming, it is possible to make the size of the foam cell fine or to distribute the size of the foam cell, and no residue of foaming agent is generated, so it can be applied to the field of packaging containers. It has become.

By the way, although it has been described that a foamed stretch molded body formed by physical foaming, for example, a container such as a bottle, many thermoplastic resins can be used, what is actually studied and used is polyethylene terephthalate. In fact, there are few proposals using physical foaming for stretch-molded containers made of polyester such as (PET) and polyolefins such as polypropylene.
In foaming by microcellular technology, it is necessary to adjust the impregnation amount of inert gas, foaming temperature (heating temperature), heating time, etc. in order to satisfy various conditions such as the size and density of the foamed cell. Is considered to be because the permeability of nonpolar gases such as nitrogen gas and carbon dioxide gas is extremely high, so that it is extremely difficult to control foaming.

  For example, when polypropylene, which is one of polyolefins, is used, a foamed preform is formed according to Patent Document 1, and this foamed preform is stretch-molded to produce a foamed stretch-molded product made of polypropylene. Therefore, no product that can be put to practical use has been obtained at all because of the reason that sufficient light-shielding properties cannot be obtained and the appearance characteristics are remarkably poor.

JP 2008-094495 A

  Accordingly, an object of the present invention is to provide a polyolefin resin foam stretched container obtained by physical foaming using an inert gas as a foaming agent.

  The inventors of the present invention have conducted a lot of experiments on the foaming of polyolefin resin stretch-molded containers, and as a result, when appropriate foaming conditions are selected according to the amount of inert gas impregnation, etc. A unique foam cell that is not seen is formed, and in this case, the inventors have found a new finding that high light-shielding properties can be obtained even when the expansion ratio is lowered, and the present invention has been completed.

  According to the present invention, in a foamed stretched container made of polyolefin having a foamed region in which foamed cells are distributed in the container wall, in the foamed region, 10 or more foamed cells are stretched when viewed in a section along the stretched direction. There is provided a foamed and expanded polyolefin container characterized in that cell clusters are formed continuously in the direction.

In the expanded foam container of the present invention,
(1) The foam region is formed inside the container wall, and a skin layer in which foam cells do not exist is formed on at least the outer surface side of the container wall;
(2) The skin layer has a thickness of at least 5 μm or more,
(3) The container wall has a foaming ratio of 1.5 times or less and a light transmittance of visible light of 25% or less,
Is desirable.

  In the present invention, the cell cluster means an aggregate in which the foamed cells are in close contact with each other in the shape of a bunch of grapes, and the foamed cells in the cell cluster may have internal spaces communicating with each other. . In the foamed stretch container made of polyester produced by physical foaming, the foam cell exists as an independent single cell, but in the foamed stretch container made of polyolefin of the present invention, the cell cluster which is an aggregate as described above. It is a great feature that is formed.

The polyolefin expanded foam container of the present invention has a cell cluster which is an aggregate of expanded cells, and therefore exhibits high light shielding performance even at a low expansion ratio. For example, as will be understood from the examples described later, even when the foaming ratio is as low as 1.5 times or less, the light transmittance in visible light can be suppressed to 25% or less. The light shielding property can be improved while suppressing the deterioration of the property.
Further, in such a polyolefin foamed container of the present invention, since the light-shielding property is imparted without blending the color pigment, there is no need to separate the color pigment during recycling, and there is an advantage that it is excellent in recyclability. is there.
Furthermore, the formation of cell clusters also has the advantage that rigidity can be imparted to the container wall. For example, as shown in the examples described later, the expansion ratio is 1.5 times or less, particularly Even if it is about 1.01 to 1.5 times, and further about 1.05 to 1.5 times, the bending strength is 1.5 times or more.

It is a figure for demonstrating the principle of a cell cluster production | generation. It is a figure which shows the SEM photograph of the cross section along the maximum extending | stretching direction of the foam extending | stretching container of Example 1. FIG. It is a figure which shows the manufacturing process of the foaming extending | stretching container of this invention. It is a figure which shows the foam which is an example of the foaming stretch container of this invention, and the foam preform for manufacturing this bottle. It is a figure which shows an injection die. It is a figure which shows the cross section in the foaming area | region of a foam preform. It is a figure which shows the SEM photograph of the cross section along the maximum extending | stretching direction of the foam extending | stretching container of Example 3. FIG.

<Cell cluster generation principle>
The generation principle of the cell cluster will be described with reference to FIG. 1 and Experiment 1 described later. In the production of the expanded foam container of the present invention, first, a polyolefin resin as a base resin is impregnated with an inert gas, and then cooled and solidified. When the resin is heated, as the temperature rises, the amount of saturated inert gas in the resin decreases, and when the inert gas becomes supersaturated, fine bubble nuclei are formed. At this time, bubble nuclei are formed at a lower temperature as the amount of impregnated inert gas increases, and bubble nucleation temperature increases as the amount of inert gas decreases. On the other hand, the olefin resin is crystallized at the stage of cooling and solidification, and gradually softens with heating. When the temperature exceeds the crystal melting temperature of the resin, the crystal structure collapses and causes significant softening. When the amount of inert gas impregnated is small and the fine bubble nuclei are generated at a temperature higher than the crystal melting temperature, the bubbles grow in a spherical shape while pushing away the surrounding resin. On the other hand, if the amount of impregnated inert gas is large and the fine bubble nuclei are generated below the crystal melting temperature, the crystal structure cannot push the rigid resin and grow, so select the crystal grain boundaries, etc. Bubbles grow like cracks and become crazes. Further, in the stretching step that is subsequently performed after the resin is heated to a temperature equal to or higher than the crystal melting temperature, the craze is stretched in the stretching direction, and the craze is partially omnidirectional by the pressure of the inert gas that flows into the craze. It grows, and a large number of foam cells appear to be dense, forming a large number of foam cells. As a result, the foam cells are in close contact with each other in the shape of a bunch of grapes, and a cell cluster is formed in which foam cells having a shape other than a spherical shape such as a prismatic shape are gathered.
In the cell cluster, it is observed that a part of the walls of adjacent cells communicate with each other. This is because the cells are connected at the Craze stage.

<Polyolefin foam stretch container>
In the expanded foamed container made of polyolefin of the present invention, the foam cells are distributed in the container wall with reference to FIG. 2 showing a cross section of the container wall along the maximum extending direction. Such a foam cell mainly has a prismatic shape oriented in the maximum stretching direction. Further, 10 or more of the foamed cells are connected in the stretching direction to form a cell cluster.
Therefore, in the container of the present invention, the foamed cells are closely present in a complicated manner in the cell cluster. Therefore, when the light incident on the container is sufficiently reflected and refracted and the foaming ratio is set low. Can also exhibit high light shielding properties. For example, when the foam cells exist independently and no cell cluster is formed, there is a thick resin region made of polyolefin resin between the foam cell and the foam cell, and such a resin region exists independently. Since light passes through the foamed cell, sufficient light shielding properties cannot be obtained without much reflection or refraction.

  Furthermore, even if stress is applied from the outside, the container of the present invention exists while forming foamed cells with a complicated structure between the foamed cells, so that the applied force is dispersed and excellent rigidity is achieved. Indicates. In the present invention, cell clusters and independent foamed cells may be mixed as long as they are within a range that does not affect the light shielding rate, intensity, and the like.

In the present specification, the expansion ratio means a value obtained by dividing the specific gravity before foaming by the specific gravity after foaming, that is, (specific gravity before foaming) / (specific gravity after foaming). Is in the range of 1.01 to 1.5.
Further, from the viewpoint of preventing the formation of a thick region wall due to polyolefin between cell clusters, the number of foam cells constituting one cell cluster is 10 or more in the stretching direction, and preferably 20 or more. .

  Furthermore, in this invention, it is suitable to form the skin layer in which the foam cell does not exist in the surface layer part on at least the outer surface side of the container wall. By forming such a skin layer, the outer surface of the vessel wall can be a smooth surface, for example, a smooth surface having an average surface roughness Ra (JIS B 0601) of 5 μm or less. This is because characteristics can be obtained. The skin layer may be formed by extruding a resin that forms the skin layer separately, but from the viewpoint of avoiding an increase in production cost without requiring a special device or resin material for lamination. It is preferable to form by carrying out heat foaming after releasing an inert gas from the surface of the foamed preform.

  It should be noted that the formation of the skin layer in which the foamed cells do not exist is effective in enhancing the printability of the container and at the same time reducing the strength reduction and gas barrier property due to the foamed cell formation.

  The thickness of the skin layer is not particularly limited, but in general, it is preferably at least 5 μm or more, particularly 5 to 200 μm. If the thickness of the skin layer is too thin, thickness unevenness is likely to occur, and it may be difficult to stably develop the aesthetic improvement effect of the skin layer. Moreover, it is not preferable to make it thicker than the above-mentioned range from the viewpoint of economy, since it does not bring about further improvement in appearance characteristics.

  In addition, although the above skin layers should just be formed in the outer surface side of a container wall, it is still more preferable if it is formed also in the inner surface side.

<Manufacture of expanded foam container>
Taking a blow bottle as an example of a stretching container, as shown in FIG. 4, in the present invention, by injection molding using a polyolefin resin impregnated with an inert gas, foaming and release of residual gas. By producing a foamed preform 50 for a container and subjecting the foamed preform 50 to stretch molding (blow molding), a bottle 60 which is a stretched container is obtained.

The foamed preform 50 has a test tube shape as a whole, and includes a mouth part 51 and a molding part 53 (a part to be stretch-molded), and the lower end of the molding part 53 is closed to form a bottom part 55. doing.
In the foamed preform 50, the mouth portion 51 is a non-stretched portion, and is formed with a screw portion 51a that is screw-engaged with a cap and a support ring 51b for conveyance or the like (depending on the type of container to be molded). Some may have no ring 51b). Accordingly, the bottle 60 obtained by blow molding the foamed preform 50 has a mouth portion 61 corresponding to the mouth portion of the preform 50 and a body portion 63 corresponding to the preform forming portion 53. The end portion of the body portion 63 is closed to form a bottom portion 65. Further, the mouth portion 61 has a screw portion 61 a and a support ring 61 b, similarly to the mouth portion 51 of the preform 50.

As understood from such a shape, in the present invention, when the bottle 60 is manufactured, foaming at the mouth 51 (the mouth 61 of the container 60) of the preform 50 (and the container 60) must be avoided. I must. This is because a decrease in strength, dimensional stability and surface smoothness due to foaming cause poor engagement with the cap and a decrease in sealing performance. In order to avoid poor engagement with the cap and deterioration in sealing performance, foaming of the screw portion 51a (61a of the container 60) should be particularly avoided. The same applies to the case where a cup-shaped container or the like is manufactured by stretch molding instead of a bottle.
Therefore, in the foamed preform 50, the molding part 53 is a foaming region, in which crazes and fine cell nuclei that grow into foamed cells are distributed, and the mouth part 51 is It is a non-foaming region, in which no crazes and fine bubble nuclei are present. Moreover, in the bottle 60, the trunk | drum 63 (including the bottom part 65) becomes a foaming area | region, The foaming cell made by growing Craze forms the cell cluster in the inside, The mouth part 61 is This is a non-foamed region, and no foamed cells are present inside the non-foamed region.

1. Polyolefin resin;
A polyolefin resin is used in the production of the expanded foam container of the present invention. Polyolefin resins have a very high gas diffusion rate compared to other resins, such as polyester resins, and do not stretch and cure like polyester resins, so heat the polyolefin resin impregnated with gas. Then, the bubble grows greatly in a short time due to the increase in the gas pressure in the foam cell.

  The polyolefin resin used in the present invention is not particularly limited except that it is a crystalline resin in order to form crazes from fine cell nuclei. For example, low density polyethylene (LDPE), medium density polyethylene (MDPE), high density polyethylene ( HDPE), linear low density polyethylene (LLDPE), linear very low density polyethylene (LVLDPE), polyethylene such as high melt tension polyethylene (HMS-PE), polypropylene, ethylene-propylene copolymer, polybutene-1, ethylene -Butene-1 copolymer, propylene-butene-1 copolymer, ethylene-propylene-butene-1 copolymer, ethylene-vinyl acetate copolymer, ion-crosslinked olefin copolymer (ionomer) and the like.

2. Impregnation with inert gas;
In the present invention, the melt of the polyolefin resin is impregnated with an inert gas that is a foaming agent. Such an inert gas does not react with the polyolefin resin used, and Any inert gas can be used as long as it does not adversely affect the environment, etc., but any inert gas can be used, but in general, availability, impact on the environment, safety, cost, etc. From this point of view, nitrogen gas or carbon dioxide gas is used.

  In order to impregnate the inert gas, an injection molding machine used in the next injection molding process is used, and the polyolefin resin held in a heated and melted state in the resin kneading part (or plasticizing part) of this injection molding machine is used. An inert gas is supplied at a predetermined pressure. According to this method, the gas can be impregnated in the injection molding machine, and the inert gas can be efficiently impregnated in the process of molding the preform to be subjected to stretch molding. In this case, the temperature and gas pressure of the polyolefin resin melt are set so that a sufficient amount of gas is dissolved to form a desired number of foamed cells (bubbles). The higher the temperature, the smaller the amount of gas dissolved, but the faster the impregnation rate. The lower the temperature, the more gas dissolved, but the impregnation takes time. The higher the gas pressure, the higher the gas dissolved amount. Therefore, the number of foam cells also increases.

  In the expanded foam container of the present invention, it is better that the amount of dissolved gas is larger than that of conventional physical foaming from the viewpoint of generating fine bubble nuclei below the crystal melting temperature of the resin. The specific gas dissolution amount varies depending on the melting point of the resin, the type of gas, and the like, and thus cannot be specified at a glance. It is preferable to set it to -1.2 weight%.

  In addition, in the melt of the polyolefin resin, as long as the light-shielding property and rigidity of the obtained stretched foam container are not impaired, a known resin additive used according to the use of the stretch container, such as a colorant, Antioxidants, antistatic agents, crystal nucleating agents and the like may be blended.

3. injection molding;
In the present invention, a preform, which is a precursor of the stretching container, is molded using a polyolefin resin melt impregnated with the above-described inert gas. The shape of the preform may be, for example, a test tube shape as described above (in the case of producing a bottle by blow molding) or a sheet shape, depending on the intended form of stretch molding. (When producing cup-shaped containers by plug-assist molding).
It is important to perform the preform having such a shape by injection molding, and for example, it cannot be performed by extrusion molding. That is, such molding is performed by heating the resin to a temperature equal to or higher than the melting point, but the extrusion molded body is positioned in an open system by extrusion from the die, and foaming during molding can be prevented. As a result, extremely coarse foamed cells are generated, and it is impossible to avoid a significant deterioration in the appearance characteristics of the resulting container. On the other hand, injection molding is a closed system, and since a gas-impregnated resin melt is injected and filled in the cavity (closed space) of the injection mold, the polyolefin resin has a melting point at the time of molding. This is because, even at the above temperature, the foaming can be effectively avoided by keeping the inside of the system at a high pressure.

In the injection molding process, it is important to inject and fill the resin melt impregnated with the inert gas into the cavity of the molding die while sufficiently holding the pressure. Thus, foaming in the molding die is suppressed, and the size of the foamed cell generated by the subsequent foaming process can be controlled.
Injection filling with holding pressure is performed by filling a predetermined amount of resin melt into the mold cavity and then continuing the injection to compensate for the volume shrinkage due to thermal shrinkage or resin crystallization. Thus, the resin melt in the molding die is pressurized, and foaming can be effectively suppressed.

  In the present invention, during the injection molding, the inside of the mold cavity is kept at a high pressure (called counter pressure), and in this state, the gas-impregnated resin melt is subjected to holding pressure. Then, it is injected and filled into the cavity of the molding die and cooled and solidified in the cavity to form a preform having a predetermined shape. That is, such counter pressure and holding pressure effectively suppress foaming during injection molding, which is basically the same as the technique proposed by the present applicant for polyester.

  That is, the resin melt filled in the cavity of the molding die can be prevented from foaming by applying pressure. The expansion of the inert gas in which the resin pressure is dissolved is suppressed, thereby preventing foaming.

On the other hand, foaming when the resin melt flows in the mold cavity cannot be prevented by holding pressure. For example, when a gas-impregnated resin melt is filled in a mold maintained at atmospheric pressure, the gas impregnated in the resin melt at the tip of the resin melt flowing in the mold Expands due to a pressure difference from the mold inner space, and bubbles are broken. That is, since the resin melt flows in the mold with the tip portion broken, this state is transferred to the mold surface, and the broken foam is a container by cooling and solidifying on the mold surface. It will be fixed to the preform surface. This is generally called a swirl mark. Such swirl marks are reflected as surface roughness on the surface of the container obtained by stretch molding described later.
However, the occurrence of such swirl marks is effectively prevented by injecting the gas-impregnated resin melt into a mold held in a pressurized state. This is because the bubble breakage during the flow of the resin melt in the mold is effectively suppressed by the pressure in the mold.

  In this way, foaming during injection molding can be effectively suppressed, and a preform having an extremely smooth surface can be obtained, whereby a stretched container with good appearance characteristics can be obtained.

Referring to FIG. 5 for explaining the above injection process, an injection mold indicated by 20 as a whole has a shell mold 23 and a core mold 25 which are cooled and held. A cavity 27 is formed by 23 and 25, and the cavity 27 is filled with a resin melt from an injection nozzle 29. In addition, a gas port 30 communicates with the cavity 27 via a gas vent.
Such a cavity 27 corresponds to the shape of the preform to be molded, and in the example shown in the figure, corresponds to the container preform 50 shown in FIG.

  That is, a polyolefin resin melt impregnated with an inert gas is injected and filled into the cavity 27 from the injection nozzle 29, and the resin melt in the cavity 27 is cooled and solidified, whereby the resin melt is formed by the cavity 27. It is shaped into a shape. When injecting the resin melt in this manner, nitrogen gas, carbon dioxide gas, air, or the like is supplied from the gas port 30 into the cavity 27 to keep the cavity 27 at a high pressure. By filling the resin melt impregnated with the gas into the cavity 27 held at a high pressure in this way, it is possible to effectively suppress bubble breakage when the molten resin flows in the cavity 27. The occurrence of swirl marks can be prevented, and a preform having a highly smooth surface can be obtained. Here, the surface of the mold cavity 27 is a surface having high smoothness due to mirror finishing or the like. However, in a portion where smoothness is not particularly required (for example, a portion corresponding to the bottom portion of the container), sand blasting is performed as necessary. It may be partially roughened in advance using a method such as processing.

  In addition, the gas in the cavity 27 is discharged from the gas port 30 as the resin melt is injected and filled. Even after the gas is discharged, the resin melt is continuously injected to add a holding pressure. It is done. By this holding pressure, foaming in the cavity 27 is effectively prevented.

  That is, the preform obtained by the above method has high transparency while being impregnated with a gas having a function as a foaming agent because foaming after injection filling is effectively suppressed by holding pressure. For example, in a molded product having a thickness of 1.5 mm, the light transmittance with respect to visible light having a wavelength of 500 nm is equivalent to that in the case where the foaming gas is not impregnated.

When performing injection molding as described above, the degree of pressure retention (pressure retention time and time) is appropriately set according to the amount of inert gas impregnation, resin temperature, etc. so that foaming can be effectively suppressed. However, in general, the weight reduction rate may be set to 3% or less. The weight reduction rate of the preform can be experimentally obtained by the following formula.
Weight reduction rate = [(M ₀ −M ₁ ) / M ₀ ] × 100
Where
M ₀ represents the weight of the preform obtained by injecting under conditions that are not impregnated with inert gas and have no molding defects such as sink marks,
M ₁ represents the weight of the resulting gas-impregnated preform is impregnated with an inert gas.
That is, the weight reduction rate decreases as the holding pressure increases, and the weight reduction rate decreases as the holding time increases. In the present invention, it is most preferable to set the pressure holding condition so that the weight reduction rate is 0%.

  The pressure in the cavity 27 is not particularly limited, but is generally maintained in a range of 1.0 MPa or more, and a resin melt is injected and filled into the cavity 27 held at such pressure. It is preferable. If this pressure is small, foam breakage during the flow of the resin melt cannot be effectively suppressed, swirl marks are generated, and the surface smoothness is low.

4). Foaming;
In the foaming step, foaming is performed by heating the non-foamed preform obtained above using an oil bath or an infrared heater. Further, when the preform has a hollow shape like a test tube, for example, heating by heat radiation with a heated iron core or the like inserted therein can be performed.
Further, in the case of the container preform 50 as shown in FIG. 4, the molding part 53 is selectively heated in order to make the mouth part 51 non-foamed. Of course, it is also possible to selectively heat a part of the molding part 53 to make a part of the molding part 53 a foamed region.

  By such heating, foaming occurs inside the preform in which the inert gas remains, and a large number of fine bubble nuclei are generated as shown in FIG.

  The heating temperature for foaming is not higher than the melting start temperature of the polyolefin resin and varies depending on the resin, but is usually about 60 to 130 ° C. As described above, the formation of bubble nuclei with the resin crystal structure maintained maintains crazes. In the present invention, it is preferable to obtain the intended foamed stretched container (for example, the bottle 60 described above) by directly subjecting the preform foamed by heating to the stretch molding step. That is, by directly stretching and forming using heat applied for foaming, a foamed stretched container can be obtained without increasing the number of steps, which is advantageous in terms of cost. Since the temperature suitable for stretching is usually equal to or higher than the melting start temperature of the resin, in the present invention, the craze is formed by heating and then further heated to reach the stretch molding temperature. Due to the softening and stretching of the resin accompanying crystal melting, foam cells grow from the craze and form cell clusters. If the stretching temperature is too high, the cells formed in the resin are coarsened, and the appearance and surface smoothness of the foamed container after stretching are impaired. Therefore, the stretching temperature is the maximum strength melting point peak temperature (Tm) measured by DSC. The following is preferable. The stretching temperature varies depending on the resin type, crystal structure, etc., but is preferably 110 to 140 ° C., for example.

  In the present invention, it is essential to form fine bubble nuclei at a crystal melting start temperature or lower to cause craze, but bubbles generated at a crystal melt temperature or higher may be present at the same time. By adjusting the time until the preform is taken out from the cavity 27 and heated, the foaming gas can be released from the preform surface, and a gas concentration distribution can be formed in the thickness direction of the preform. I can control it. That is, the center of the preform thickness has the highest gas concentration, and therefore the bubble nucleation temperature is low, so crazes are formed, and the portion near the preform surface has a low gas concentration, the bubble nucleation temperature is high, and the foam cell is It grows in a spherical shape (hereinafter sometimes referred to as a spherical foam cell). Bubble nuclei are not generated in the portion closest to the surface, and a preform skin layer is formed.

  Particularly in the present invention, the foaming ratio in the foaming region of such a preform is preferably in the range of 1.05 to 1.5 times, particularly 1.1 to 1.3 times. The expansion ratio of the resulting expanded foam container can be adjusted to a predetermined range. That is, if the expansion ratio becomes higher than necessary, spherical foam cells grow up to the surface portion of the preform, and a preform skin layer in which no spherical foam cells are present is not formed. The appearance of the resulting expanded foam container will be reduced. On the other hand, if the gas concentration is excessively low, the bubble generation temperature rises, and as a result, no crazes are formed and cell clusters cannot be formed in the stretching container. Further, the advantages due to foaming, such as lightness, heat insulation, rigidity, light shielding, etc., are impaired. For this reason, the foaming ratio of the preform should be set within the above range.

  In addition, the thickness of the preform skin layer in which the spherical foam cells do not exist is, specifically, at least 5 μm or more so that the thickness of the skin layer of the foamed stretched container of the present invention falls within an appropriate range. It is preferable that it is set.

  Moreover, after taking out preform from the cavity 27, the thickness of a preform skin layer can be adjusted with the time to heat foaming and heat foaming time. That is, the longer the time until the start of heating foaming, the more gas is released from the surface of the preform, and the thickness of the preform skin layer increases, but the longer the heating foaming time, the larger the foam cell. As it grows, the thickness of the preform skin layer becomes thinner.

  In the present invention, since a polyolefin resin is used, which has a very high gas diffusion rate and less curing of the resin by stretching, compared with polyester such as PET, the growth rate of spherical foamed cells is considerably high. Accordingly, the heating time for foaming is considerably short, and is generally about 20 seconds to 2 minutes, although it varies depending on the size (thickness) of the preform and the amount of gas impregnation.

5. Stretch molding;
In the present invention, the intended foamed stretched container (for example, the bottle 60 described above) can be obtained by subjecting the foamed preform softened by heating and formed with foamed cells to the stretch molding process.

  This stretch molding is stretched by stretching the preform by heating and molding the preform to a temperature not lower than the melting start temperature (Ti) of the polyolefin resin used and lower than the maximum strength peak temperature (Tm) of the melting point. Is obtained. That is, when the melting start temperature (Ti) or higher is reached, the resin begins to soften and stretch blow molding becomes possible, but when the crystal melting proceeds and the melting point peak (Tm) is exceeded, the resin becomes viscous and the foam cell becomes coarse. The appearance of the stretching container is deteriorated, and further, a predetermined stretching ratio cannot be obtained, and the performance as a container is not satisfied.

  In this stretch molding, it is preferable to process the preform heated to the aforementioned foaming temperature as it is. That is, by setting the foaming temperature and the subsequent stretching temperature continuously, the foamed preform can be stretch-molded as it is, which is advantageous in terms of process.

  By adjusting the stretch molding temperature as described above, the foam preform is stretched together with the container wall along with the craze and foam cells, and the outer surface of the container wall is cooled and solidified in contact with the molding die, and the cells stretched in the plane direction are A cell cluster is formed and fixed in the same shape.

  Such stretch molding can be performed by a method known per se, and is performed by blow molding, plug assist molding, or the like depending on the shape of the intended stretching container. Cell clusters formed continuously exist, and a skin layer, which is a non-foamed layer in which no foamed cells are present, is formed on the surface layer portion.

  The thickness of the skin layer is preferably at least 5 μm or more, particularly 5 to 200 μm. Further, the maximum length in the stretching direction and the aspect ratio (maximum length in the stretching direction / thickness) of the size of the cell cluster are set in an appropriate range depending on the stretching ratio.

  In the stretch container thus stretch-molded (the barrel portion 63 in the bottle 60 of FIG. 4), the expansion ratio is 1.5 times or less, particularly 1.01 depending on the expansion ratio of the foam preform described above. It becomes the range of -1.5 times, Furthermore, 1.05-1.5 times. If the expansion ratio is too large, it is impossible to avoid a decrease in appearance characteristics due to the coarsening of the expanded cells, and if the expansion ratio is too small, the advantage of foaming is impaired.

  In addition, since a skin layer in which no foam cells are present is formed on the surface layer portion of the foamed region, high surface smoothness can be ensured, and the outer surface and inner surface have a surface roughness Ra (JIS-B). −0601-1994) becomes a smooth surface of 5 μm or less.

  As described above, according to the present invention, a polyolefin resin satisfying specific conditions is used, and through a process of impregnation with an inert gas, injection molding, foaming, release of residual gas, and stretch molding, a polyolefin resin single-layer structure is formed. A foam stretch container is obtained. Such a foam-stretched container is a single layer structure of polyolefin resin, but is derived from a complicated structure of cell clusters, exhibits high light shielding performance even at low foaming ratio, and has excellent rigidity. .

  The foamed stretch container of the present invention is extremely useful as a field of containers, particularly as a bottle, because the appearance deterioration due to foaming is effectively prevented, for example, it has excellent surface smoothness and high printability. For example, when it is used as a bottle, the light transmittance for visible light in the body portion is 25% or less, and high light-shielding properties can be exhibited. Further, the bending strength in the body portion is also random polypropylene for stretch blow applications, for example. Even in the case where is used, the pressure can be set to 1500 Pa or more.

The excellent effects of the present invention will be described in the following experimental examples.
The DSC used in the experiment (Diamond DSC: manufactured by PerkinElmer) was measured using pellets at a temperature rising rate of 10 ° C./min. Surfcom 2000SD3 (manufactured by Tokyo Seimitsu) was used as the roughness meter. As an evaluation of rigidity, compression specific strength was measured. The compression specific strength is the ratio of the compression strength of the foaming bottle to the compression strength of the non-foaming bottle having the same weight and shape. The measurement was calculated by compressing the center of the cylindrical portion of the bottle body using a universal testing machine and measuring the load when pushed in by 1 mm.

Example 1
First, a preform was formed. In other words, random polypropylene (MFR = 30) used for container molding is supplied to an injection molding machine, 0.40% by weight of nitrogen gas is supplied from the middle of a heating cylinder, kneaded with polypropylene resin and dissolved, and then supplied to a counter pressure device. The resin was injected into a mold whose pressure was increased to 5 MPa, and the degree of holding pressure was adjusted so as not to foam (holding pressure 60 MPa, injection holding time 19 seconds) to solidify by cooling. As a result, a test tube-shaped container preform which was impregnated with a gas having a weight of 12.3 g but was substantially non-foamed was obtained. The obtained preform had a weight reduction rate of 0% compared to the case where no foaming gas was added. In the DSC measurement of the resin, the melting start temperature, maximum strength peak temperature, and melting end temperature were 102 ° C., 140 ° C., and 163 ° C., respectively, and the melting peak width (Tf—Ti) was 61 ° C.

  The obtained preform was stored at 20 ° C. for 6 hours. The weight of the preform before and after storage was measured, and the nitrogen gas remaining in the preform was calculated to be 0.22% by weight. With respect to this preform, the preform body part excluding the mouth part was foamed by adjusting the heating so that the surface temperature of the preform body part was 115 ° C. with an infrared heater, and immediately blow-molded. As a result, a cylindrical foaming bottle having an internal volume of about 400 ml, a height of 195 mm (cylindrical part 135 mm), and a maximum diameter (cylindrical part) of 60 mm was obtained. The obtained bottle maintained a non-foamed mouth portion, and air bubbles were dispersed throughout the bottle body. The bottle surface maintained smoothness, the surface roughness (Ra) was 1.7 μm, and the expansion ratio of the bottle body was 1.25 times. The result of having observed the bottle cross section using SEM is shown in FIG. In the cross section of the bottle body, a cell cluster in which cells such as prismatic cylinders are gathered is formed transversely in the circumferential direction, and a series of foamed cells along the extending direction estimated from the image is 100 or more. The skin layer thickness was 70 μm, and the total light transmission amount at a wavelength of 500 nm was 14%. The compression specific strength was 2.0, indicating good strength characteristics.

(Example 2)
Bottle molding was performed in the same manner as in Example 1 except that the nitrogen gas supplied from the heating cylinder of the injection molding machine was 0.50% by weight. The nitrogen gas remaining in the preform before blowing was 0.28 wt%, and the body surface temperature of the preform heated by the infrared heater was 117 ° C. The obtained bottle maintained a non-foamed state at the mouth, and air bubbles were dispersed throughout the bottle body. The surface roughness (Ra) of the bottle was 1.9 μm, and the expansion ratio of the bottle body was 1.29 times. From the SEM image of the bottle body section, a cell cluster in which cells such as prismatic cylinders are gathered is formed transversely in the circumferential direction, and the series of foamed cells along the extending direction estimated from the image is 100 or more. The side skin layer thickness was 12 μm, and the total light transmission amount at a wavelength of 500 nm was 12%. The compression specific strength was 2.0, indicating good strength characteristics.

(Comparative Example 1)
Bottle molding was performed in the same manner as in Example 2 except that the storage time of the preform obtained by injection molding was 24 hours. The nitrogen gas remaining in the preform before blowing was 0.09 wt%, and the body surface temperature of the preform heated by the infrared heater was 115 ° C. The obtained bottle maintained a non-foamed state at the mouth, and air bubbles were dispersed throughout the bottle body. The surface roughness (Ra) of the bottle was 0.39 μm, and the expansion ratio of the bottle body was 1.19 times. From the bottle body section SEM image, the cells were arranged almost independently, and 10 or more cell clusters were not observed. The outer skin layer thickness was 75 μm, the total light transmission amount at a wavelength of 500 nm was 30%, and the compression specific strength was 1.2.

(Comparative Example 2)
Bottle molding was performed in the same manner as in Example 1 except that the nitrogen gas supplied from the heating cylinder of the injection molding machine was 0.30 wt% and the storage time of the preform was 4 hours. The nitrogen gas remaining in the preform before blowing was 0.19 wt%, and the body surface temperature of the preform heated by the infrared heater was 117 ° C. The obtained bottle maintained a non-foamed state at the mouth, and air bubbles were dispersed throughout the bottle body. The surface roughness (Ra) of the bottle was 2.3 μm, and the expansion ratio of the bottle body was 1.6 times. From the bottle body cross-sectional SEM image, coarse cells were arranged almost independently, and no cell cluster consisting of 10 or more foamed cells was observed. The thickness of the outer skin layer was 80 μm, and the total light transmittance at a wavelength of 500 nm was 25%.

(Example 3)
Bottle molding was performed in the same manner as in Example 1 except that the nitrogen gas supplied from the heating cylinder of the injection molding machine was 1.1 wt% and the storage time of the preform was 10 hours. The nitrogen gas remaining in the preform before blowing was 0.41 wt%, and the body surface temperature of the preform heated by the infrared heater was 112 ° C. The obtained bottle maintained a non-foamed state at the mouth, and air bubbles were dispersed throughout the bottle body. The surface roughness (Ra) of the bottle was 0.35 μm, and the expansion ratio of the bottle body was 1.26 times. A bottle body section SEM image is shown in FIG. From FIG. 7, a large number of cell clusters consisting of 10 or more continuous foamed cells were observed. The outer skin layer thickness was 40 μm, and the total light transmission at a wavelength of 500 nm was 8%.

(Experiment 1)
<Cell cluster generation conditions>
Random polypropylene (MFR = 30) is supplied to the injection molding machine, nitrogen gas is supplied from the middle of the heating cylinder, kneaded with polypropylene resin and dissolved, and the resin is injected into a mold whose pressure is increased to 5 MPa with a counter pressure device. As a result, a plate having a thickness of 1.5 mm was obtained. This plate was immediately sandwiched between hot plates that had been heated in advance, and heated for 15 seconds to observe the temperature at which foaming started and the shape of the foamed cells. Table 1 shows the results of testing with different amounts of nitrogen gas supplied. When the amount of nitrogen gas was increased, the foaming temperature was lowered, and it was observed that the cell shape changed from spherical to craze at the resin melting start temperature (102 ° C.).

50: Preform for container 60: Bottle

Claims (4)

  In a foamed stretched container made of polyolefin having a foamed region in which foamed cells are distributed in the container wall, in the foamed region, when viewed in a cross section along the stretched direction, 10 or more foamed cells are connected in the stretched direction to form a cell cluster. A foamed stretched container made of polyolefin, characterized in that is formed.   2. The polyolefin according to claim 1, wherein the foamed region is formed inside the container wall, and a skin layer in which foamed cells do not exist is formed at least on a surface layer portion on an outer surface side of the container wall. Foamed container.   The polyolefin foam container according to claim 2, wherein the skin layer has a thickness of at least 5 μm.   The polyolefin foamed stretched container according to any one of claims 1 to 3, wherein the container wall has a foaming ratio of 1.5 times or less and a light transmittance of visible light of 25% or less. .