JP5239479B2 - Method for producing partially foamed co-injection molded body and partially foamed co-injection molded body - Google Patents

Description

  The present invention relates to a partially foamed co-injection molded article having a foamed resin layer and a non-foamed resin layer, and in particular, a method for producing the partially foamed co-injection molded article and a partially foamed co-injection molded article obtained by the method. It is about.

  Currently, polyester containers represented by polyethylene terephthalate (PET) are excellent in properties such as transparency, heat resistance and gas barrier properties, and are widely used in various applications.

  On the other hand, in recent years, the reuse of resources has been strongly demanded, and with respect to the polyester container as described above, a used container is collected and reused for various purposes as a recycled resin. By the way, as for the contents stored in the packaging container, those that are easily altered by light, for example, certain beverages, pharmaceuticals, cosmetics, etc., are molded using a resin composition in which a colorant such as a pigment is blended in a resin. Provided in a sealed opaque container. However, from the viewpoint of resource reuse, it is not desirable to add a colorant (it would be difficult to ensure transparency in the recycled resin). For this reason, the use of a transparent container is required. Therefore, it is necessary to improve recyclability of opaque containers suitable for accommodating photo-altered contents.

In order to impart light-shielding properties (opacity) without blending a colorant, it is conceivable that bubbles are present in the container wall to form a foam container, and various such foam containers have been proposed. Patent Document 1 uses a foaming extrusion blow technique using a chemical foaming agent, and Patent Documents 2 to 3 use a microcellular technique in which foaming is performed by heating after impregnating an inert gas into a resin. Foam molded products have been proposed.
JP 2003-26137 A JP-A-2005-246822 JP 2006-321887 A

  The microcellular technology used in Patent Documents 2 to 3 and the like can form extremely fine foamed cells as compared with the so-called chemical foaming used in Patent Document 1 and so on in the field of containers and the like. However, there are still problems to be solved in practical use. When the technique proposed in Patent Document 2 is applied to a stretch-molded body, the obtained foamed molded product (preform) is subjected to stretch molding. In other words, a large number of large cells are formed, so that there is a problem that strength and gas barrier properties are deteriorated and sufficient light shielding properties (light transmittance) cannot be imparted. In the case of separation using a difference in specific gravity from a polyolefin resin having a property of floating in water during recycling, such as polyethylene terephthalate having a property of sinking in water with a specific gravity of about 1.35, foaming If the cell becomes too large and becomes too light (decrease in specific gravity), there is a problem that separation of specific gravity becomes difficult.

  Patent Document 3 discloses a partially foamed molded article having a multilayer structure in which a skin layer and a core layer are formed of a non-foamed resin in which a foamed resin layer and foamed cells are not distributed. In the body, the formation of the non-foamed resin layer can avoid deterioration of various properties due to foaming, but there is a limit to avoiding the deterioration of properties. That is, the partially foamed molded article of Patent Document 3 is obtained by impregnating a resin with an inert gas as a foaming agent and then releasing the inert gas from the surface or adjusting the heating conditions for foaming. This is because the thickness and the like of the non-foamed resin layer are limited. For example, in a non-foamed resin layer, it is difficult to form a skin layer having a certain thickness or more. In addition, the number of foamed resin layers and non-foamed resin layers is limited, and the present situation is that the problem of deterioration of various characteristics due to foaming has not been solved.

Accordingly, an object of the present invention is to have a multilayer structure having a foamed resin layer in which fine and uniform foam cells are distributed and a non-foamed resin layer in which foam cells are not distributed. An object of the present invention is to provide a method for producing a partially foamed molded article having a high degree of freedom in design without limitation of the thickness of the non-foamed resin layer and a partially foamed molded article obtained by the method.
Another object of the present invention is to provide a method for producing a partially foamed molded article having a foamed resin layer in which foamed cells are uniformly distributed even when subjected to stretch molding.

According to the present invention,
Preparing a foamable resin in which a foaming agent is dissolved and a non-foamable resin in which the foaming agent is not dissolved;
The foamable resin layer and the non-foamable resin, at least for the foamable resin, are cooled and solidified by sequential or simultaneous injection while suppressing foaming by holding pressure, so that the foamable resin layer and the non-foamable resin A co-injection step of forming a multilayer primary molded body having a layer;
A foaming step in which the foamable resin layer is made into a foamed resin layer by heating the multilayer primary molded body to foam the foamable resin;
A method for producing a partially foamed co-injection molded article is provided.
In such a production method, stretch molding can be performed after the foaming step.

According to the present invention, in the partially foamed co-injection molded article having a foamed resin layer in which the foamed cells obtained by the production method are distributed and a non-foamed resin layer in which the foamed cells are not distributed,
The foamed cell has an average cell diameter in the range of 5 to 50 μm and a particle size distribution with a standard deviation of 40 μm or less.
Such a partially foamed co-injection molded article is preferably a container preform.

  In the method for producing a partially foamed molded article of the present invention, sequential or simultaneous injection using a foamable resin in which a foaming agent is dissolved and a non-foamable resin not using a foaming agent (hereinafter simply referred to as co-injection). Therefore, the thickness of the non-foamed resin layer to be formed is not limited, and the number of foamed resin layers and non-foamed resin layers is not limited. A layer structure corresponding to the intended characteristics can be employed.

  In addition, since injection is performed while pressure is applied, foaming during co-injection is effectively suppressed. Therefore, in the foamed resin layer in the partially foamed co-injection molded product, fine foam cells are uniformly distributed. For example, the average cell diameter is in the range of 5 to 50 μm and the standard deviation is 40 μm or less, and the particle size distribution is extremely sharp. If injection is performed without applying pressure, a multilayer primary molded body foamed after injection is formed, and when the molded body is heated, the cells grow secondary and become large. Accordingly, the partially foamed co-injection molded article as in the present invention is extremely useful as a container preform, and in a container obtained by subjecting this to stretch molding, the distribution degree and shape of the foamed cells in the foamed resin layer In a region where the foamed resin layer is distributed, there is no variation in light transmittance, bubble rate, strength, gas permeability, etc., and stable characteristics are exhibited. In addition, since the thickness of the non-foaming resin layer is set according to the purpose, for example, by increasing the thickness of the non-foaming resin layer, the weight reduction due to foaming can be effectively reduced, and the used containers can be separated. In addition, the gas barrier property can be prevented from being lowered by foaming, and high gas barrier property can be secured.

<Manufacture of partially foamed co-injection molded products>
In the method for producing a partially foamed co-injection molded article of the present invention, first, a foamable resin in which a foaming agent is dissolved and a non-foamable resin in which the foaming agent is not dissolved are prepared.

  The resin used for the foamable resin is not particularly limited as long as it can be impregnated with an inert gas, and a known thermoplastic resin can be used. For example, low density polyethylene, high density polyethylene, polypropylene, poly 1-butene, poly 4-methyl-1-pentene or random of α-olefins such as ethylene, propylene, 1-butene, 4-methyl-1-pentene Olefin resins such as block copolymers and cyclic olefin copolymers; ethylene / vinyl acetate copolymers, ethylene / vinyl alcohol copolymers, ethylene / vinyl chloride copolymers and other ethylene / vinyl copolymers; polystyrene Styrene resins such as acrylonitrile / styrene copolymer, ABS, α-methylstyrene / styrene copolymer; polyvinyl chloride, polyvinylidene chloride, vinyl chloride / vinylidene chloride copolymer, polymethyl acrylate, polymethacrylic acid Vinyl resins such as methyl; nylon 6, nylon Polyamide resins such as Ron 6-6, Nylon 6-10, Nylon 11 and Nylon 12; Polyester resins such as polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, and their copolyesters; Polycarbonate resins; Polyphenylene oxide resins A biodegradable resin such as polylactic acid, or the like can be used alone or in a blend of two or more. In particular, when this molded body is used for molding a container, an olefin resin or a polyester resin is suitable, and among these, a polyester resin is optimal in order to maximize the advantages of the present invention.

  As the foaming agent that dissolves in the resin, a chemical foaming agent such as sodium carbonate or an inert gas can be used. In particular, an inert gas is used from the viewpoint of generating fine and uniform foamed cells. The In general, nitrogen gas or carbon dioxide gas is used as such an inert gas.

  Dissolution of the foaming agent into the resin, for example, dissolution of the inert gas is performed by impregnating the above-described resin melt with an inert gas. Such impregnation is performed by a resin kneading part (or an injection molding machine) (or This is carried out by supplying an inert gas at a predetermined pressure to the resin held in the heated and melted state in the plasticizing part). By adjusting the amount of gas impregnation at this time, the number of foam cells generated by heating can be adjusted. For example, as the gas pressure is increased and the kneading time under the gas pressure is increased, the amount of impregnation of the gas can be increased and the number of foam cells can be increased.

In addition, as the non-foamable resin in which the foaming agent is not dissolved, the same type of resin used for the foamable resin is usually used as it is without dissolving the foaming agent. These resins can also be used. For example, a gas barrier resin such as an ethylene-vinyl alcohol copolymer (ethylene-vinyl acetate copolymer saponified product) or an aromatic polyamide can be used in order to enhance the gas barrier property. Furthermore, when other resins are used, in order to improve the adhesion to the foamable resin layer, blends obtained by blending foamable resins with other resins and olefins graft-modified with unsaturated carboxylic acids such as maleic anhydride Adhesive resins such as resins can also be used to form the adhesive layer.
As described above, as the non-foamable resin, various resins can be used without dissolving the foaming agent according to the layer structure of the target molded body.

  In the present invention, the foamable resin and the non-foamable resin are sequentially or simultaneously injected into an injection mold using a known co-injection molding machine having an injection cylinder having the number of resin layers to be formed. The mold is injection filled. In the present invention, at the time of this injection, at least with respect to the foamable resin, holding pressure is applied by continuously injecting the resin melt. In this case, it is possible to apply a holding pressure to the foamed resin injected and filled by continuous injection of another resin melt (non-foamable resin melt).

The degree of holding pressure (holding pressure and time) is appropriately set according to the amount of impregnation of the inert gas, the resin temperature, etc., so that foaming can be effectively suppressed. What is necessary is just to set so that a rate may be 5% or less. The weight reduction rate can be obtained experimentally by the following formula.
Weight reduction rate = [(M ₀ −M ₁ ) / M ₀ ] × 100
In the formula, M ₀ represents the weight of a molded body (for example, a preform) obtained by injecting with setting the conditions so that there is no molding defect such as sink without impregnation with an inert gas,
M ₁ represents the weight of a gas-impregnated preform obtained by impregnating with an inert gas,
It is represented by 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%.

  Further, upon injection, nitrogen gas, carbon dioxide gas, air or the like is supplied in advance into the injection mold, and the inside of the mold is maintained at a high pressure of, for example, 1.0 MPa or more, and thus maintained at such a high pressure. It is also possible to fill the mold with a melt of foamable resin and non-foamable resin. By injection-filling the mold held at a high pressure in this way, it is possible to effectively suppress bubble breakage when the foamable resin melt flows, and to prevent the occurrence of swirl marks. For example, even in a layer structure in which a foamable resin layer is located on the surface, a molded article having a highly smooth surface can be obtained. That is, when the foamable resin melt flows in the mold, foam breakage occurs due to the expansion of the inert gas dissolved as a foaming agent, which appears as a swirl mark on the surface of the molded body. The surface smoothness may be impaired, but such inconvenience can be surely avoided by injecting into a mold held at a high pressure.

  As described above, in the present invention, a co-injection molded body in which foaming is effectively suppressed can be obtained by co-injection molding with pressure retention while using a foamable resin containing a foaming agent. In addition, in the layer structure in which the foamable resin forms a surface layer, if necessary, co-injection is performed in a mold held under high pressure, so that there is no swirl mark and high surface smoothness is achieved. A molded body is obtained. For example, since this co-injection molded article has foaming effectively suppressed, the total light transmittance for visible light having a wavelength of 500 nm is 75% or more, and is extremely excellent in transparency. Further, even in a layer configuration in which a foamable resin layer is formed on the surface, the maximum height Pt (JIS B 0601) of the obtained molded body is 10 μm or less, and the smoothness is remarkably excellent.

  In the present invention, by heating the co-injection molded body obtained as described above, foam cells are generated in the foamable resin layer in the molded body, and the foamable resin layer is formed of foam cells. A distributed foamed resin layer is obtained, whereby a partially foamed co-injection molded product is obtained. An example of the layer structure of such a partially foamed molded product is shown in FIGS.

  That is, in the example of FIG. 1, the wall portion of the co-injection molded body indicated by 20 as a whole is formed with the skin layers 21 and 21 made of non-foamed resin on the surface, and the foamed resin in which the foamed cells A are distributed therebetween. Layer 22 is formed. In such a layer structure, since the foamed resin layer is not exposed on the surface, the smoothness is extremely high and the printing suitability is also good. In the example of FIG. 2, the skin portion 21, 21 made of non-foamed resin is formed on the surface of the wall portion 20 of the co-injection molded body, and the core layer 23 made of non-foamed resin is also formed in the center portion. A foamed resin layer 22 in which foam cells are distributed is formed between the skin layer 21 and the core layer 23. In such a layer structure, since the core layer 23 made of non-foamed resin is formed at the center, the gas barrier property is prevented from being lowered due to the formation of the foamed resin layer 22 (foamed cell A), and further the strength is reduced. This is suitable for avoidance.

  Of course, in the present invention, the layer structure of the co-injection molded body wall portion 20 is arbitrary as long as the foamed resin layer and the non-foamed resin layer are formed. For example, the non-foamed surface is formed only on one surface. A skin layer 21 made of a resin can be formed, such a skin layer 21 can be formed at all, or three or more foamed resin layers 22 can be formed.

  In general, the same resin (for example, a polyester resin such as polyethylene terephthalate or a polyolefin resin such as polyethylene) is preferably used as the resin in the foamed resin layer and the resin in the non-foamed resin layer. However, the non-foamed resin layer may be a plurality of layers made of different resins. For example, the central portion of the core layer 23 in FIG. 2 is a layer made of a gas barrier resin, and a layer made of an adhesive resin between the gas barrier resin layer and the foamed resin layer 22 further increases the gas barrier property. It can also be increased.

  Further, the layer structure itself shown in FIGS. 1 and 2 is known as shown in Patent Document 3 described above, but in the present invention, such a layer structure is formed by co-injection. The degree of freedom is high, the thickness of the non-foamed resin layer is not limited, the thickness can be increased, and the position and number of the foamed resin layer and the non-foamed resin layer are also arbitrary. That is, in the layer structure shown in the prior art, the entire wall is impregnated with an inert gas that is a foaming agent, and a part of this gas is released or the heating conditions are adjusted, Since it is formed by heating only a part of the thickness so that foaming occurs, the thickness of the non-foamed resin layer is limited, the thickness is limited, and the position and number of the foamed resin layer and the non-foamed resin layer Are also restricted. However, in the present invention, molding is performed by co-injection of a foamable resin in which the foaming agent is dissolved and a non-foamable resin not impregnated with the foaming agent. Depending on the, any thickness and layer structure can be employed.

  Further, the heating for forming the foamed layer 22 in which the foamed cells are distributed is not less than the glass transition point of the resin used for the foamable resin. By such heating, for example, the inert dissolved in the resin. A sudden change in the internal energy (free energy) of the gas is brought about, phase separation is caused, and foaming occurs due to separation from the resin body as bubbles. Of course, this heating temperature should be below the melting point of the foamable resin and non-foamable resin, preferably below 200 ° C., in order to prevent deformation of the molded product. If the heating temperature is too high, the cell diameter is difficult to control because of the rapid foaming after the heating, the appearance is deteriorated, and further, the crystallization of the body portion proceeds and the secondary formability is lowered.

  The heating as described above can be easily performed using, for example, an oil bath or an infrared heater. For example, the selective heating is performed by performing selective heating as shown in Patent Document 3 described above. The foam cell A can be generated only in the region that is heated automatically. That is, when a co-injection molded body is used as a preform for forming a container such as a bottle, only the body (and bottom) of the finally obtained bottle is heated, and the foam cell is formed only in the portion corresponding to the body. It is preferable that the generated foamed resin layer 22 is formed and the portion corresponding to the mouth portion is not heated so that the foamed cells are not generated. Of course, it is also possible to mold the portion corresponding to the mouth portion to be a non-foaming resin layer. In addition, if necessary, the foamed resin layer 22 in which the foamed cells are generated by heating only a part of the portion corresponding to the body portion can be formed. Further, the foamed resin layer 22 is formed such that the size of the foam cell A varies depending on the position by heating while moving the molded body or the heating means, or by giving the heating means such as a heater a temperature distribution. It is also possible.

  By the way, as shown in FIGS. 1 and 2, the foam cell A formed inside the foam resin layer 22 (hereinafter sometimes referred to as a spherical foam cell) is substantially spherical, Isotropically distributed. In particular, in the present invention, foaming at the time of molding is suppressed by the above-described holding pressure at the time of injection molding, so that the diameter of the foamed cell A is particularly uniform and has a sharp particle size distribution.

In addition, the cell density of the spherical foam cell A (density in the foamed resin layer 22) depends on the dissolved amount of the inert gas described above. The greater the dissolved amount, the higher the cell density, The diameter can be reduced, and the smaller the dissolved amount, the smaller the cell density and the larger the diameter of the foam cell A. The diameter of the spherical foamed cell A can be adjusted by the above heating time. For example, the longer the heating time for foaming, the larger the diameter of the spherical foamed cell A, and the shorter the heating time, the spherical foamed cell. Cell A has a small diameter. In the present invention, the above conditions are adjusted, and for example, the cell density of the spherical foam cell A in the foam layer 22 is set to about 10 ^{5 to} 10 ¹⁰ cells / cm ³ and the average diameter is set to about 3 to 50 μm. It is preferable to provide high light-shielding properties by stretch molding described later. Furthermore, when the average diameter of the spherical foam cell A is set in such a range, in the present invention, foaming during molding is prevented by holding pressure during injection molding, and a sharp particle size distribution is ensured. The standard deviation is 40 μm or less, in particular 20 μm or less.

<Extension molding>
The partially foamed co-injection molded product obtained above has a light-shielding property by foaming and can have various forms as well as a film or a sheet, but further enhances the light-shielding property by stretch molding. Therefore, it is suitable for use as a container preform.

  That is, as understood from FIGS. 1 and 2, in the molded body obtained above, the foamed cells A generated in the foamed resin layer 22 are spherical, and thus the degree of overlap in the thickness direction is low. The degree of light transmission is large. For this purpose, the foamed cell A is made flat by stretch molding, which causes multiple scattering and reflection of light in the foamed region, lowering the light transmittance, and providing higher light shielding performance per thickness. It can be done.

  Stretch molding is performed by a method known per se, for example, stretched by blow molding by heating the preform to a temperature not lower than the glass transition temperature of the resin and lower than the melting point, or vacuum molding typified by plug assist molding. Thereby, a bottle or a cup-shaped container having a foamed region (for example, a trunk) in which flat foamed cells are distributed is obtained. Alternatively, a stretched film can be produced by stretching a sheet-shaped foamed molded article, and a bag-like container can be obtained using the stretched film.

  That is, refer to FIG. 3 showing a cross section along the maximum stretching direction of the wall portion in the foaming region of the stretched molded body. A layer structure corresponding to the co-injection molded body of FIG. 1 is formed on the wall portion 30 of the stretched molded body, and a large number of flat foam cells B are distributed corresponding to the foamed resin layer 22 of FIG. The foamed layer 31 is formed, and on the surface, a non-foamed skin layer 33 in which the foamed cells B are not distributed with an appropriate thickness is formed corresponding to the skin layer 21 of FIG. It is stretched along the maximum stretching direction. By making the spherical foam cell A into such a flat foam cell B by stretch molding, a certain degree of overlap can be secured over the entire foam region, and high light-shielding properties can be expressed. .

  For example, in a container such as a bottle, the average long diameter L (length along the maximum stretching direction) of the flat foam cell B is 400 μm or less, particularly 200 μm or less, and the thickness t is 50 μm or less, particularly 30 μm or less. As described above, it is preferable to perform stretching by setting stretching conditions such as a stretching ratio in accordance with the size of the spherical foam cell A formed in the co-injection molded body. That is, by setting the size of the flat foam cell B within the above range, a high light-shielding property is exhibited over the entire region where the foam layer is present, and a reduction in strength and gas barrier properties due to foaming are effectively avoided. be able to. Moreover, it is advantageous also in reducing the weight reduction by foaming. For example, in blow molding that is stretched in the biaxial direction of the axial direction (height direction) and the circumferential direction, depending on the size of the spherical foam cell A formed on the foamed molded body (preform), etc. In plug-assist molding or the like in which stretching is performed in such a direction that the stretching ratio is about 2 to 4 times and stretching is performed in only one axial direction, the stretching in this direction becomes the maximum stretching direction, and It is preferable to perform stretching at the same stretching ratio so that the flat foam cell B having the above size is formed.

  In addition, when stretch-molding a partially foamed co-injection molded body having a foamed resin layer exposed on the surface, in particular, the average major axis L of the foam cell on the surface side is long, and the slope distribution is such that the average major axis L becomes shorter toward the inside. There is a tendency to show. This tendency is due to the fact that during the stretch molding, the growth of the foamed cells is suppressed by cooling and solidifying by contact with the mold on the surface side, but internally, the blow pressure is applied to the cells distributed in the resin having a high temperature. For this reason, the blow pressure causes the cells to be crushed.

  In the separate collection of plastic molded products, the difference in specific gravity is used for each plastic type, but the weight reduction rate can be controlled in the same way as foamed molded products. There is an advantage that fractional collection can be performed using the difference in specific gravity.

  In the stretched molded article, the thickness of the wall 30 is adjusted according to the layer structure by adjusting the amount of gas impregnation described above, the heating conditions for foaming, the time from injection molding to heat foaming, and the like. It is preferable to set the number of flat foam cells B that overlap with each other to 17 or more, preferably 30 or more, and most preferably 50 or more, thereby amplifying light scattering and multiple reflection. For example, the total light transmittance for visible light having a wavelength of 500 nm is 15% or less, particularly 10% or less, and most preferably 5% or less. For example, when the total light transmittance is 5% or less, the light-shielding property is equivalent to that of a paper container, and it is extremely advantageous for accommodating contents that are likely to be altered by light, particularly in the field of containers. .

  In producing a stretched molded article by the above-described method, the glass transition point of the resin decreases linearly or exponentially as the amount of inert gas dissolved increases. Further, the viscoelasticity of the resin also changes due to the dissolution of the gas. For example, the viscosity of the resin decreases due to an increase in the amount of dissolved gas. Accordingly, in consideration of the amount of inert gas dissolved, various conditions should be set so that a predetermined number of flat cells B overlap in the thickness direction.

  According to the present invention described above, by employing co-injection molding under predetermined conditions, a partially foamed molded body having a layer structure according to the purpose, such as the thickness and number of foamed resin layers and non-foamed resin layers, is produced. Therefore, the degree of freedom of design is higher than that of a known technique, and it is possible to effectively avoid a decrease in strength and a decrease in gas barrier property due to foaming while ensuring light-shielding properties due to foaming.

The figure which shows an example of the layer structure of the partial foaming co-injection molding manufactured according to this invention. The figure which shows the other example of the layer structure of the partial foaming co-injection molding manufactured according to this invention. The figure which shows the layer structure of the extending | stretching molded object obtained by extending | stretching the partial foaming co-injection molded object of FIG.

Explanation of symbols

21: Skin layer (non-foamed resin layer)
22: Foamed resin layer 23: Core layer (non-foamed resin layer)
A: Spherical foam cell B: Flat foam cell

Claims (4)

Preparing a foamable resin in which a foaming agent is dissolved and a non-foamable resin in which the foaming agent is not dissolved;
The foamable resin layer and the non-foamable resin, at least for the foamable resin, are cooled and solidified by sequential or simultaneous injection while suppressing foaming by holding pressure, so that the foamable resin layer and the non-foamable resin A co-injection step of forming a multilayer primary molded body having a layer;
A foaming step in which the foamable resin layer is made into a foamed resin layer by heating the multilayer primary molded body to foam the foamable resin;
A method for producing a partially foamed co-injection molded article.   The production method according to claim 1, wherein stretch molding is performed after the foaming step. In the partially foamed co-injection molded article obtained by the production method of claim 1 , having a foamed resin layer in which foam cells are distributed and a non-foamed resin layer in which foam cells are not distributed.
The partially foamed co-injection molded article, wherein the foamed cell has a particle size distribution with an average cell diameter in the range of 5 to 50 µm and a standard deviation of 40 µm or less.   The partially foamed co-injection molded article according to claim 3, which is a container preform.

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