JP5954105B2 - Propylene-based resin foam stretch-molded body and method for producing the same - Google Patents

Description

  The present invention relates to a foamed stretch-molded body formed of a propylene-based resin and a method for producing the same.

  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 using sodium carbonate or azo compound as a foaming agent and foaming to carbon dioxide gas or nitrogen gas generated by thermal decomposition of the foaming agent results in a large foam cell. However, it is difficult to use in the field of packaging containers because the gas barrier property, appearance characteristics, and strength are significantly reduced due to foaming, which makes it difficult to use in the field of packaging containers. Or the distribution of the size of the foamed cells can be applied, so that it can be applied to the field of packaging containers.

  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. When a polypropylene foam is used and a foamed preform is formed according to Patent Document 1, and the foamed preform is stretch-molded to produce a foamed stretch-molded product made of polypropylene, the appearance characteristics are remarkably poor. The fact is that no product that can be put to practical use is obtained.

  In general, various proposals have been made regarding polypropylene having excellent stretch molding or blow moldability (for example, Patent Documents 2 to 5), but polypropylene suitable for foamed stretch-molded bodies by physical foaming, particularly physical foaming. There is very little examination about.

For example, Patent Document 6 discloses a foamed injection molded product of a propylene-based resin. Here, the foaming agent used in the examples is a so-called chemical foaming agent, such as carbon dioxide gas or nitrogen gas. No specific study has been made on physical foaming using an inert gas.
Further, Patent Document 7 discloses an injection-foamed molded product of polypropylene by physical foaming using an inert gas, but no consideration is given to stretch molding.

  Further, Patent Document 8 discloses a hollow foam molded body (blow molded body) of polypropylene, and discloses that polypropylene having a specific MFR and molecular weight distribution is used. However, although this patent document describes that an inert gas such as nitrogen gas can be used as a foaming agent, all of the foaming agents actually used in the examples are inorganic foaming agents ( Polyslenes), and so-called physical foaming has not been studied at all. In addition, in the technique proposed here, a non-foamed layer is formed on the surface by co-extrusion of a non-foaming thermoplastic resin on one or both sides of the hollow foam molded body in order to ensure surface smoothness. doing. As understood from this, in the case of producing a foamed stretch-molded product of polypropylene by foaming, particularly physical foaming, a laminated structure by coextrusion or the like may be used in order to ensure appearance characteristics such as surface smoothness. In fact, a stretched molded product having a single layer structure of polypropylene and having good appearance characteristics has not been obtained.

JP 2008-094495 A JP 2009-234627 A Japanese Patent No. 3641926 JP 2003-286377 A WO2008 / 032735 JP 2001-30285 A JP 2012-136633 A JP 2009-299016 A

Accordingly, the present invention is a foamed stretch-molded product comprising a single-layer structure of a propylene-based resin, obtained by physical foaming using an inert gas as a foaming agent, and foaming in which appearance defects due to foaming are effectively suppressed. By providing a stretched molded body and a method for producing the same.
Another object of the present invention is to provide a foamed stretch-molded article comprising a single-layer structure of a propylene-based resin having a shape of a container, particularly a bottle, and a method for producing the same.

  The present inventors have introduced a foam structure by physical foaming (so-called micro-cellular foaming) using an inert gas as a foaming agent when producing a stretch-molded product (for example, blow-molded product) made of a propylene resin. As a result of many experiments conducted on the above, as a result of using a propylene-based resin having a low crystallinity and a broad crystal melting peak and having an MFR in a certain range, the expansion of the foam cell is effectively suppressed. The present inventors have found that a foamed stretch-molded product having a single-layer structure of a propylene-based resin having good appearance characteristics can be obtained, thereby completing the present invention.

That is, according to the present invention, it is a foam stretch-molded product by physical foaming with a foaming ratio of 1.05 to 2.0 times, which is a single-layer structure of a propylene-based resin,
(A) 70% by weight or more of random polypropylene and MFR (230 ° C.) of 1 to 80 g / 10 min.
(B) In the melting curve obtained by DSC measurement, the maximum intensity peak temperature (Tm) is less than 150 ° C., and the difference between the melting start temperature (Ti) and the melting end temperature (Tf) of the melting peak including the maximum intensity peak (Ti-Tf) is 50 ° C. or higher,
Is provided, which is characterized by satisfying the above.

In the foam stretch molded article of the present invention,
(1) The surface roughness Ra (JIS-B-0601-1994) in the foamed region in which the foamed cells are distributed is 10 μm or less,
(2) having the form of a container (especially a bottle);
Is preferred.

According to the invention,
Preparing a propylene resin satisfying the following conditions (A) and (B);
(A) 70% by weight or more of random polypropylene and MFR (230 ° C.)
Is 1 to 80 g / 10 minutes,
(B) In the melting curve obtained by DSC measurement, the maximum intensity peak temperature (Tm) is less than 150 ° C., and the melting start temperature (Ti) of the melting peak including the maximum intensity peak
And the difference (Ti−Tf) between the melting end temperature (Tf) and 50 ° C. or more,
Preparing a resin melt in which the propylene-based resin is impregnated with an inert gas;
The resin melt is injection filled by injection filling into an injection mold so that foaming does not occur;
Removing the molded unfoamed preform from the injection mold;
Releasing inert gas from the surface of the resulting unfoamed preform;
Heating and foaming the unfoamed preform;
Then stretching the foamed preform;
A method for producing a foam stretch-molded article is provided.

  The foamed stretch-molded product of the present invention is a single-layer structure of propylene-based resin, and a laminated structure by coextrusion or the like is not introduced. Therefore, no special apparatus or resin material for lamination is required, which is extremely advantageous in avoiding an increase in production cost.

  In addition, in the present invention, by using a propylene-based resin that satisfies the above-described conditions (A) and (B), it does not have a structure in which non-foamed layers having no foamed cells are laminated. The appearance defect due to foaming is effectively suppressed, and for example, a stretched molded article having good surface smoothness can be obtained.

  Expanded stretch molded articles composed of such a single-layer structure of a propylene-based resin are suitably applied to applications that require weight reduction, high rigidity, and heat insulation properties, but have particularly good appearance characteristics and commercial value. Since it has the advantage that it is expensive and can be manufactured at low cost, it is extremely useful for applications as containers, particularly bottles.

The figure for demonstrating the principle of foaming cell (bubble) production | generation. The figure which shows the manufacturing process of the foaming extension molded object of this invention. The figure which shows the foam which is an example of the foaming expansion molding 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 cross section along the maximum extending | stretching direction of a foam extending | stretching molded object. It is a figure which shows the melting curve by DSC measurement of the propylene-type resin used in Experimental example 1 and Experimental example 2. FIG. It is a figure which shows the relationship between residual gas amount and stretch molding temperature about the foaming state in the surface layer part of the foam preform after foaming.

<Principle of foam cell generation>
In general, the generation of foam cells (bubbles) is represented by extrusion foaming performed by impregnating a foamed gas into a molten resin, keeping the inside of the molding machine at a high pressure, and releasing it to the atmospheric pressure when extruding it from the die. Thus, it is done by releasing the pressure of the molten resin. On the other hand, as shown in Patent Document 1, the present inventors have reported that the foamed cells can be made fine by cooling and solidifying the resin impregnated with the foaming gas without foaming and foaming while heating. ing.
With reference to FIG. 1, the generation principle of a foam cell is demonstrated. A polyester typified by polyethylene terephthalate (PET) can be solidified in an amorphous state by quickly cooling to a glass transition temperature (Tg) or lower. The polyester is impregnated with an inert gas (FIG. 1 (a1)), and then heated (above the glass transition temperature), the impregnated gas expands, resulting in spherical or nearly spherical bubbles ( Foamed cells) are generated and grow, but when they grow to a certain size, the surrounding resin (polyester) is cured by the stress associated with the growth of the bubbles, and the growth is suppressed. Thus, bubbles are newly generated and grow (FIG. 1 (a2)). As the foaming continues in this way, the gas concentration in the resin decreases, the generation of bubbles (that is, foaming) is completed ((a3) in FIG. 1), and then stretching is performed. As described above, in the case of polyester, since the growth of bubbles is suppressed, it is possible to obtain a foam stretch-molded product in which fine bubbles (foamed cells) are distributed.

  On the other hand, polypropylene has a characteristic that it cannot be solidified in an amorphous state because its glass transition point (Tg) is below room temperature (about −10 ° C.) and the crystallization speed is remarkably fast. The degree varies depending on the resin type, but is approximately 30 to 70%. Also in the physical foaming of polypropylene, an inert gas is impregnated (FIG. 1 (b1)), and then the impregnated gas expands by heating this to produce bubbles having a spherical shape or a shape close to a spherical shape. This is the same as polyester. In the case of polypropylene, the resin is softened by heating above the melting start temperature (Ti) of the crystal, and bubbles grow. However, polypropylene has a very high gas diffusion rate as compared with polyester resins, and stretch hardening does not work like polyester resins. For this reason, when the polypropylene impregnated with the gas is heated, the bubbles grow greatly in a short time due to the increase of the gas pressure in the foam cell. This is because there is no element that suppresses the growth of bubbles like polyester.

  Therefore, in the case of polypropylene, when the heating temperature is set to a high temperature, coarse bubbles having a spherical shape or a shape close to a spherical shape are generated at once (FIG. 1 (b2)). Therefore, in the subsequent stretch molding, such coarse bubbles are stretched in the stretching direction, so that large flat cells are formed in the surface layer portion, and the appearance characteristics of the resulting stretch molded body (uniformity of whiteness) , Smoothness) is impaired. Therefore, foaming is suppressed by lowering the heating temperature, thereby suppressing bubble coarsening (FIG. 1 (b3)). At this time, if a resin having a low melting point (Tm) or a resin having a narrow melting peak width (Tf-Ti) is used, the temperature required in the stretching step subsequent to foaming becomes high, leading to bubble coarsening. Therefore, in this process, it is important to select a resin that satisfies specific conditions (A) and (B) as a polypropylene resin.

That is, in the present invention, it is necessary not to adopt a laminated structure by co-extrusion or the like, but to avoid an appearance defect due to foamed stretch molded product and foaming by a single layer structure of propylene resin. It is indispensable to suppress the coarsening of foam cells (bubbles).
Therefore, in the present invention, a propylene resin satisfying specific conditions (A) and (B) is selected and used, and the propylene resin is expanded according to the process shown in FIG. An inert gas is impregnated, an unfoamed preform is formed by injection molding using a propylene-based resin impregnated with gas, and then the preform is heated to obtain a foamed preform. Stretch molding is performed, whereby the desired foam stretch molded article is obtained.

<Manufacture of expanded foam molding>
Taking a blow bottle as an example of the stretched molded product, as shown in FIG. 3, in the present invention, injection molding, foaming and remaining using a specific propylene resin impregnated with an inert gas are used. A foamed preform 50 for a container is produced by releasing the gas, and the foamed preform 50 is stretch-molded (blow-molded) to obtain a bottle 60 that is a stretch-molded body.

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 portion that is not stretched, and is formed with a screw portion 51a that is screw-engaged with the cap, and a support ring 51b for conveyance or the like (depending on the type of container to be molded) Some may not have a support 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. In addition, the mouth portion 61 has a screw portion 51 a and a support ring 51 b screw portion, like 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 of the sealing performance, foaming of the screw portion 51a (61a of the container 61) 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, and the foamed cells (bubbles) having the spherical shape or the shape close to the spherical shape are distributed therein, and the mouth portion Reference numeral 51 denotes a non-foamed region, and no foamed cell exists in the non-foamed region. Moreover, in the bottle 60, the trunk | drum 63 (including the bottom part 65) becomes a foaming area | region, and the foaming of the flat shape by which the foam cell of the spherical shape or the shape close | similar to a sphere was extended in the extending direction in the inside. The cells are distributed, the mouth portion 61 is a non-foamed region, and no foamed cell exists in the inside.

1. Propylene resin;
The propylene-based resin used in the present invention must satisfy the following conditions (A) and (B).
(A) 70% by weight or more, preferably 80% by weight or more of random polypropylene is contained, and MFR (230 ° C.) is in the range of 1 to 80 g / 10 minutes, particularly 5 to 50 g / 10 minutes.
(B) In the melting curve obtained by DSC measurement, the maximum strength peak temperature (Tm) is less than 150 ° C, particularly 145 ° C or less, and the melting start temperature (Ti) and the melting end temperature of the melting peak including the maximum strength peak. The difference (Ti−Tf) from (Tf) is 50 ° C. or more,
Especially 60 degrees C or more.

The above condition (A) will be explained. First, random polypropylene is propylene and other α-olefins other than propylene (for example, ethylene, butene-1,4-methyl-1-pentene, etc.) or cyclic olefins. A random copolymer having a copolymer content of α-olefin and the like such that the properties of polypropylene are not impaired, generally about 1 to 8% by weight. That is, the use of such a material containing a large amount of random copolymer means that a so-called propylene resin having low crystallinity is used.
That is, when polypropylene with high crystallinity (for example, propylene homopolymer or propylene block copolymer) is used, foaming is performed by heating a polypropylene preform (cooled solidified product) impregnated with gas. This is because the peak is sharp, so that the crystals are melted and softened with heating, and as a result, the foamed cells are coarsened at once.
Also in stretch molding, polypropylene with high crystallinity has poor moldability. That is, when polypropylene having a large crystal size and a uniform crystal melting temperature is used, the ratio of the amorphous resin is small, and breakage occurs from the crystal grain boundary. Therefore, the propylene resin used in the present invention must contain at least the amount of random polypropylene in the above-mentioned range.

  In addition, as long as the content of random polypropylene is in the above-mentioned range and the other conditions (conditions for MFR and DSC dissolution curves) are satisfied, the propylene-based resin used in the present invention is other than homopolypropylene or propylene. Not only block copolymers with α-olefins or cyclic olefins, but also other thermoplastic resins (for example, polyethylene) may be blended.

  Further, MFR (ASTM-D-1238) in the condition (A) is a condition necessary for effectively performing secondary molding such as injection molding and stretch molding described later. That is, when the MFR of the propylene-based resin used is lower than the above range, injection molding becomes difficult, and when the MFR is too high, the secondary moldability is lowered. In addition, it tends to be difficult to suppress foaming during injection molding.

  Furthermore, the above-mentioned condition (B), that is, the condition relating to the melting curve by DSC is related to the above-mentioned random polypropylene content, but during heating for foaming, the resin is gently softened and the bubble growth proceeds slowly. It is a condition for making it happen. For example, FIG. 7 is a diagram showing a melting curve by DSC measurement of the propylene-based resin used in Experimental Example 1 (invention example) and Experimental Example 2 (comparative example) to be described later. If the maximum peak temperature of the curve is higher than the above range or the difference (Ti−Tf) between the melting start temperature (Ti) and the melting end temperature (Tf) is smaller than the above range, When heated, the softening of the propylene-based resin occurs abruptly. For this reason, the bubbles rapidly become coarse, and eventually, it becomes impossible to obtain a foamed stretch molded product (for example, a bottle) having excellent appearance characteristics. End up. On the other hand, when the above-described melting curve satisfies the condition (B) as in Experimental Example 1, the softening of the propylene-based resin by heating gradually proceeds. By setting the temperature in an appropriate range, it is possible to effectively prevent the bubble from becoming coarse and to obtain a foamed stretch-molded article having excellent appearance characteristics.

  In addition to these advantages, the use of the above-mentioned polypropylene-based resin has low crystallinity, so gas permeability is high and gas can be easily released from the skin, thereby suppressing skin foaming. Is advantageous. Furthermore, such a polypropylene resin also brings an advantage that generation of cluster cells in which crazes and foamed cells are connected can be suppressed. In other words, in a propylene resin with many crystals, crazes are formed so as to sew crystal grain boundaries, whereas in such a low crystallinity polypropylene resin, there are sufficient amorphous regions between crystals. Since the foamed cells can grow into a spherical shape, the crazing of the cells can be suppressed.

2. Impregnation with inert gas;
In the present invention, the melt of the propylene resin is impregnated with an inert gas that is a foaming agent. However, such an inert gas does not react with the propylene resin used. In addition, any inert gas can be used as long as it does not adversely affect the environment and the like, and generally any inert gas can be used. From the viewpoint of cost and the like, nitrogen gas, carbon dioxide gas or the like is used.

  In order to impregnate the inert gas, a propylene-based resin that is maintained in a heated and melted state in a resin kneading part (or plasticizing part) of the injection molding machine is used in an injection molding process of the next process. By supplying an inert gas at a predetermined pressure. That is, 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 forming a preform to be subjected to stretch molding. In this case, the temperature and gas pressure of the propylene-based resin melt are set so that a sufficient amount of gas is dissolved to form a desired number of foamed cells (bubbles). For example, the higher the temperature, the smaller the amount of gas dissolved, but the faster the impregnation rate. The lower the temperature, the larger the amount of gas dissolved, but the longer the impregnation takes, and the higher the gas pressure, The amount of dissolution increases and therefore the number of foam cells also increases.

  In the above-mentioned melt of propylene-based resin, as long as the melt characteristics of the resin are not impaired, known additives for resins used depending on the use of the stretched molded article, such as colorants, antioxidants, etc. Further, an antistatic agent, a crystal nucleating agent or the like may be blended.

3. injection molding;
In the present invention, a preform, which is a precursor of the stretched molded body, is molded using a melt of a propylene-based resin 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 that the preform having such a shape is formed by injection molding, and cannot be performed by, for example, 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, remarkably coarse foam cells (bubbles) are generated, and it is impossible to avoid a significant deterioration in the appearance characteristics of the obtained molded body. On the other hand, injection molding is a closed system, and a gas-impregnated resin melt is injected and filled in the cavity (closed space) of the injection mold. This is because even when the temperature is higher than the melting point, 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 holding the pressure. By adopting such means, Thus, foaming in the molding die is suppressed, and the foamed cell (1a) generated by the subsequent foaming step can be made fine and uniform.
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.

  However, if the MFR of the resin is low, the fluidity in the mold is deteriorated, and even if the resin melt in the mold is pressed by holding pressure, it becomes difficult to suppress foaming. That is, since the flow resistance of the resin is large, the pressure loss increases, and the entire resin in the molding die cannot be maintained at an effective pressure, and foaming occurs. Accordingly, the resin used in the present invention has an MFR (230 ° C.) of 1 or more, preferably 5 or more, more preferably 10 or more. The upper limit of MFR for injection molding is not particularly limited as long as it is within the range in which normal injection molding can be performed. The following is acceptable.

  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 part broken, this state is transferred to the mold surface, and the broken foam is formed in the molded body by cooling and solidifying on the mold surface. It is fixed to the surface of a certain preform, and this is generally called a swirl mark. Such swirl marks are reflected as surface roughness on the surface of a molded body 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 molded article having good appearance characteristics can be obtained.

Referring to FIG. 4 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, the resin melt is formed by the cavity 27 by injecting and filling a propylene resin melt impregnated with an inert gas from the injection nozzle 29 into the cavity 27 and cooling and solidifying the resin melt in the cavity 27. Shaped into the shape to be made. 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 3 mm, the light transmittance for visible light having a wavelength of 500 nm is 50% or more.

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
In the formula, M ₀ represents the weight of the preform obtained by injecting under the condition setting so that there is no molding defect such as sink without impregnation with inert gas,
M ₁ represents the weight of the gas-impregnated preform obtained by impregnating 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. In addition, when the preform has a hollow shape, such as a test tube, for example, heating for foaming is performed from the inside of the preform using high-frequency heating with an iron core or the like inserted therein. It can be carried out.
Further, in the case of the container preform 50 as shown in FIG. 3, 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 as shown in FIG. 5, a foam cell 35 having a spherical shape or a shape close to a spherical shape (hereinafter referred to as a spherical foam cell). Will be generated a lot).

  By the way, in the present invention, as the propylene resin impregnated with the inert gas, those satisfying the above-mentioned conditions (A) and (B) are used. By making the region, the growth of bubbles is avoided, the formation of coarse foam cells is suppressed, and furthermore, as shown in FIG. 5, the foam cells 35 are distributed in the surface layer portion. A non-foamed thin skin layer 37 can be formed. Such a skin layer 37 is formed because the dissolved gas is released from the surface after the preform is taken out of the cavity 27. That is, in heating with an oil bath or an infrared heater, the surface of the preform is heated from the preform surface and gradually heated to the center of the preform thickness. This is effectively suppressed by releasing gas from the gas. Since the resin satisfying the conditions (A) and (B) has a low crystallinity, the gas diffusivity is high, and this gas can be released efficiently. Further, degassing can be promoted by keeping the preform at a high temperature in an oven or the like (the upper limit is the temperature at which foamed cells start to be generated).

This degassing differs depending on the crystallinity of the resin used and the gas diffusion coefficient. For example, from the experimental result (FIG. 8) of Experimental Example 10, this stretch molding temperature y (° C.) is expressed by the following formula:
y <-33x + 132
Where x is the residual gas concentration (wt%),
It is preferable to set so as to satisfy the above.

  The heating temperature for such foaming is lower than the melting point of the propylene-based resin (maximum intensity peak temperature in the DSC melting curve), and varies depending on the melting point, but usually a temperature of about 110 to 140 ° C. may be used. . By performing foaming at such a temperature and setting the heating time, the amount of gas impregnation, and the gas release time within appropriate ranges, it is possible to ensure appropriate foaming ratio, foam cell size, and the like. Furthermore, in the present invention, it is preferable to obtain the intended foamed stretch-molded product (for example, the bottle 60 described above) by directly subjecting the preform foamed by heating to the stretch-molding step. That is, by directly performing stretch molding using heat applied for foaming, a foam stretch molded product can be obtained without increasing the number of steps, which is advantageous in terms of cost.

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 2.0 times, particularly 1.1 to 1.5 times. The foaming ratio of the resulting foam stretch-molded product can be adjusted to a predetermined range. That is, when the expansion ratio becomes higher than necessary, the spherical foam cells 35 become coarse, and further, the spherical foam cells 35 grow to the surface layer portion of the preform. The skin layer 37 in which no is present is not formed, resulting in a decrease in the appearance of the foamed stretch-molded product finally obtained. On the other hand, if the expansion ratio is too low, the advantages of foaming, such as lightness, heat insulation, rigidity, and light shielding properties, are impaired. For this reason, the foaming ratio of the preform should be set within the above range.
The expansion ratio is a ratio of the volume when foamed to the volume when not foamed (the volume when foamed / the volume when not foamed) in the foamed region where the foamed cells are distributed.

In general, when the foaming ratio is adjusted as described above, the average diameter (equivalent circle diameter) of the foamed cells 35 is about 5 to 200 μm, and the cell density is about 10 ⁴ to 10 ¹⁰ cells / cm ³ . is there.
Furthermore, it is preferable that the thickness of the skin layer 37 in which the foamed cells 35 do not exist is set to be at least 10 μm or more.

  Moreover, after taking out the preform from the cavity 27, the thickness of the skin layer 37 can be increased by adjusting the time until the heat foaming and the heat foaming time. That is, the longer the time until the start of heating foaming, the greater the amount of gas released from the surface of the preform and the greater the thickness of the skin layer 37, but the longer the heating foaming time, the larger the foam cell 35 becomes. Since it grows, the thickness of the skin layer 37 becomes thin.

  Further, in the present invention, a propylene-based resin is used that has a very high gas diffusion rate and less curing of the resin by stretching than polyester such as PET, and therefore, the growth rate of the foam cell 35 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 stretch-molded product (for example, the bottle 60 described above) can be obtained by subjecting the foamed preform softened by heating and formed with the spherical foamed cells to the stretch-molding step.

  In this stretch molding, the preform is stretched by heating the preform to a temperature not lower than the melting start temperature (Ti) of the propylene resin used and lower than the maximum strength peak temperature (Tm) of the melting point, and stretched in a predetermined shape. The body 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 exceeds the melting point peak (Tm), the resin becomes viscous and unsuitable for stretching. The resin becomes brittle.

  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 stretching temperature in the same range, the foamed preform can be stretch-molded as it is, which is advantageous in terms of process. In the present invention, since the polypropylene resin satisfies the conditions (A) and (B), it can be stretch-molded in the foaming temperature range of the preform. If the melting peak width (Tf-Ti) of the polypropylene resin is small, the foaming temperature and the stretching temperature do not match, and the preform must be further heated for stretching. In that case, foam cells of the preform grow excessively, and cells exceeding the thickness of the skin layer 37 are formed, and the appearance of the stretched molded product is significantly impaired.

  That is, by adjusting the stretch molding temperature as described above, the foamed preform was stretched with spherical foam cells together with the vessel wall, and cooled and solidified by contacting with the mold on the outer surface of the vessel wall and stretched in the surface direction. The cell is fixed as it is.

  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 form of the intended stretch molded body, and is shown in FIG. 6, for example. Thus, a flat foam cell 40 having a long cell diameter in the extending direction is formed, and a skin layer 43 that is a non-foamed layer in which the foam cell 40 does not exist is formed on the surface layer portion.

The maximum stretch direction length L of the flat foam cell 40, the aspect ratio (maximum stretch direction length L / thickness t), and the thickness d of the skin layer 43 are the average cell diameter of the spherical foam cell 35 described above, and It is set in an appropriate range depending on the draw ratio.
For example, in order to obtain the bottle 60 of FIG. 3, the molding part 53 of the foam preform 50 has a draw ratio in the axial direction (height direction) and the biaxial direction in the circumferential direction by blow molding by blowing air or the like. It is stretched so as to be about 2 to 4 times, and blow molding is performed so that the thickness of the body portion 63 is about 150 to 400 μm. For example, the surface of the body portion 63 has a non-foamed skin. The layer 43 is formed with a thickness of about 5 to 100 μm.
The

  In the stretch-molded body thus stretch-molded (the barrel portion 63 in the bottle 60 of FIG. 3), the foaming ratio is 1.05 to 2.0 times depending on the foaming ratio of the foam preform described above. It becomes a range. 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, in the surface layer portion of the foam region where the flat foam cells 40 are distributed, a skin layer in which no foam cells are present is formed, so that high surface smoothness can be secured, The outer and inner surfaces are smooth surfaces with a surface roughness Ra (JIS-B-0601-1994) of 10 μm or less.

  As described above, according to the present invention, a propylene resin satisfying specific conditions is used, and a single layer of propylene resin is obtained through the steps of impregnation with inert gas, injection molding, foaming, release of residual gas, and stretch molding. A foam stretch-molded body having a structure is obtained. Such a foam stretch-molded body has a single layer structure of a propylene-based resin, but the expansion of foam cells is effectively limited. For example, the foam stretch-molded body has an appropriate foaming ratio and high surface smoothness. It effectively prevents deterioration of the appearance due to foaming, and exhibits advantages such as weight reduction, heat insulation, light shielding, and rigidity by foaming.

  The foamed stretch-molded article of the present invention is extremely useful in the field of containers, particularly as a bottle, because it effectively prevents deterioration in appearance due to foaming, has excellent surface smoothness, and has high printability. For example, when a bottle is used, the light transmittance with respect to visible light in the body portion can be set to 30% or less, high light shielding properties can be exhibited, and the bending strength in the body portion 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.

(Experimental example 1)
Random polypropylene J246M (manufactured by Prime Polymer, 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 the heating cylinder, kneaded with polypropylene resin, dissolved, and counter Resin is injected into a mold whose pressure has been increased to 6 MPa with a pressure device, and the degree of holding pressure is adjusted so as not to foam (holding pressure 60 MPa, injection holding time 19 seconds), and solidifies by cooling, and the gas is impregnated. Obtained a preform for a container having a substantially non-foamed test tube shape. 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.

  After storing the obtained preform at 25 ° C. for 6 hours, the preform body portion except the mouth portion was heated and foamed by an infrared heater and immediately blow-molded to obtain a foaming bottle having an internal volume of about 400 ml. The heating temperature of the preform was 116 ° C. on the body surface. 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 5.6 μm, and the expansion ratio of the bottle body was 1.19 times.

(Experimental example 2)
Preform molding was performed in the same manner as in Experimental Example 1 except that the supplied polypropylene resin was changed to MG03B (manufactured by Nippon Polypro Co., Ltd., MFR = 30). In the DSC measurement of the resin, the melting start temperature, maximum strength peak temperature, and melting end temperature were 113 ° C., 152 ° C., and 161 ° C., respectively, and the melting peak width (Tf—Ti) was 48 ° C. The weight reduction rate of the obtained preform was 0%.

  After storing the obtained preform at 25 ° C. for 6 hours, the preform body except the mouth was heated by an infrared heater to be foamed, and immediately blow-molded. In order to mold without rupturing the bottle, when the temperature of the preform is heated up to 128 ° C on the surface of the body, it becomes coarser to 1 mm or more so that bubbles in the skin layer can be flipped, and a foamed bottle with a good appearance is obtained I could not. In order to suppress foaming of the skin layer, when the body surface temperature of the preform is lowered to 120 ° C., the resin is insufficiently softened and cannot be molded into a bottle shape. No foaming bottle could be obtained.

(Experimental example 3)
Preform molding was performed in the same manner as in Experimental Example 1 except that the supplied polypropylene resin was changed to PM940M (manufactured by Sun Allomer, MFR = 30). In the DSC measurement of the resin, the melting start temperature, maximum strength peak temperature, and melting end temperature were 45 ° C, 131 ° C, and 154 ° C, respectively, and the melting peak width (Tf-Ti) was 109 ° C. The weight reduction rate of the obtained preform was 0%.

  After storing the obtained preform at 25 ° C. for 4 hours, the preform body except the mouth was heated and foamed with an infrared heater and immediately blow-molded to obtain a foaming bottle having an internal volume of about 400 ml. The heating temperature of the preform was 113 ° C. on the body surface. 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 4.4 μm, and the expansion ratio of the bottle body was 1.14 times.

(Experimental example 4)
Preform molding was performed in the same manner as in Experimental Example 1 except that the supplied polypropylene resin was changed to J-71GR (manufactured by Prime Polymer, MFR = 11) and the injection holding pressure was set to 70 MPa. In the DSC measurement of the resin, the melting start temperature, maximum strength peak temperature, and melting end temperature were 113 ° C., 147 ° C., and 157 ° C., respectively, and the melting peak width (Tf—Ti) was 44 ° C. The weight reduction rate of the obtained preform was 0%.

  After storing the obtained preform at 25 ° C. for 6 hours, the preform body portion except the mouth portion was heated and foamed by an infrared heater and immediately blow-molded to obtain a foaming bottle having an internal volume of about 400 ml. The heating temperature of the preform was 122 ° C. on the body surface. 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 9.0 μm, and the expansion ratio of the bottle body was 1.27 times.

(Experimental example 5)
The supplied polypropylene resin was changed to B241 (manufactured by prime polymer, MFR = 0.5), and 0.40% nitrogen gas was supplied from the middle of the heating cylinder in the same manner as in Experimental Example 1 and injection molding was performed. Even if the pressure is 100 MPa and the injection holding time is 22 seconds, foaming in the preform cannot be suppressed, the weight reduction rate of the obtained preform is 4%, and the entire body of the preform is whitened. It could not be used for the next blow process.

(Experimental example 6)
Preform molding was performed in the same manner as in Experimental Example 1 except that the polypropylene resin to be supplied was changed to PM20V (manufactured by Sun Allomer, MFR = 45) and the injection holding pressure was set to 70 MPa. In the DSC measurement of the resin, the melting start temperature, maximum strength peak temperature, and melting end temperature were 90 ° C., 146 ° C., and 159 ° C., respectively, and the melting peak width (Tf—Ti) was 69 ° C. The weight reduction rate of the obtained preform was 0%.

  After storing the obtained preform at 25 ° C. for 6 hours, the preform body portion except the mouth portion was heated and foamed by an infrared heater and immediately blow-molded to obtain a foaming bottle having an internal volume of about 400 ml. The heating temperature of the preform was 124 ° C. on the body surface. 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 7.1 μm, and the foaming ratio of the bottle body was 1.21 times.

(Experimental example 7)
Preform molding was performed in the same manner as in Experimental Example 1 except that a resin obtained by dry blending 20 parts by weight of homopolypropylene PM900A (manufactured by Sun Allomer, MFR = 30) was supplied to 80 parts by weight of J246M (made by prime polymer, MFR = 30). It was. In the DSC measurement of the molded preform, the melting start temperature, maximum strength peak temperature, and melting end temperature were 103 ° C, 141 ° C, and 174 ° C, respectively, the melting peak width (Tf-Ti) was 71 ° C, and MFR ( 230). The weight reduction rate of the obtained preform was 0%.

  After storing the obtained preform at 25 ° C. for 6 hours, the preform body portion except the mouth portion was heated and foamed by an infrared heater and immediately blow-molded to obtain a foaming bottle having an internal volume of about 400 ml. The heating temperature of the preform was 120 ° C. on the body surface. 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 7.3 μm, and the expansion ratio of the bottle body was 1.20 times.

(Experimental example 8)
Preform molding was carried out in the same manner as in Experimental Example 1 except that 50 parts by weight of J246M (manufactured by prime polymer, MFR = 30) was supplied with a resin obtained by dry blending 50 parts by weight of homopolypropylene PM900A (manufactured by Sun Allomer, MFR = 30). It was. In the DSC measurement of the molded preform, the melting start temperature, maximum strength peak temperature, and melting end temperature were 102 ° C., 165 ° C., and 174 ° C., respectively, the melting peak width (Tf-Ti) was 71 ° C., and MFR ( 230). The weight reduction rate of the obtained preform was 0%.

  After the obtained preform was stored at 25 ° C. for 6 hours, the preform body except the mouth was heated and foamed by an infrared heater, and foamed bottle molding was attempted. The temperature of the preform was 135 on the body surface. When heated to 0 ° C., it became coarser to 1 mm or more so that bubbles in the skin layer could be repelled, and a foam bottle with a good appearance could not be obtained. In order to suppress foaming of the skin layer, when the body surface temperature of the preform is stretched at 135 ° C. or less, the resin is insufficiently softened and cannot be molded into a bottle shape. No foaming bottle could be obtained.

(Experimental example 9)
Preform molding was performed in the same manner as in Experimental Example 1 except that the amount of nitrogen gas supplied was 0.80 wt% using J246M (manufactured by Prime Polymer, MFR = 30). The weight reduction rate of the obtained preform was 0%. The obtained preform is stored in a room at 25 ° C., the residual amount of nitrogen gas contained from the weight measurement of the preform is measured, and the relationship between the surface temperature when the skin starts to foam significantly by heating is further investigated. The results are shown in Table 1 and FIG. From this result, it is possible to obtain a foamed bottle with a good appearance by stretching under the conditions below the dotted line portion in FIG.

35: Spherical foam cell 37: Non-foamed skin layer 40: Flat foam cell 43: Non-foamed skin layer 50: Preform for container 60: Bottle

Claims (4)

A foam-stretched molded article by physical foaming having a foaming ratio of 1.1 to 2.0 times, which is a single-layer structure of a propylene-based resin,
(A) 70% by weight or more of random polypropylene and MFR (230 ° C.) of 1 to 80 g / 10 min.
(B) In the melting curve obtained by DSC measurement, the maximum intensity peak temperature is less than 150 ° C., and the difference between the melting start temperature (Ti) and the melting end temperature (Tf) of the melting peak including the maximum intensity peak (Ti− Tf) is 50 ° C. or higher,
A foamed stretch-molded article characterized by satisfying   The foam stretch molded article according to claim 1, wherein the surface roughness Ra (JIS-B-0601-1994) in the foamed region in which the foam cells are distributed is 10 μm or less.   The foam stretch-molded article according to claim 2, which has a form of a container. Preparing a propylene resin satisfying the following conditions (A) and (B);
(A) 70% by weight or more of random polypropylene and MFR (230 ° C.)
Is 1 to 80 g / 10 minutes,
(B) In the melting curve obtained by DSC measurement, the maximum intensity peak temperature (Tm) is less than 150 ° C., and the melting start temperature (Ti) of the melting peak including the maximum intensity peak
And the difference (Ti−Tf) between the melting end temperature (Tf) and 50 ° C. or more,
Preparing a resin melt in which the propylene-based resin is impregnated with an inert gas;
The resin melt is injection filled by injection filling into an injection mold so that foaming does not occur;
Removing the molded unfoamed preform from the injection mold;
Releasing inert gas from the surface of the resulting unfoamed preform;
Heating and foaming the unfoamed preform;
Then stretching the foamed preform;
A method for producing a foamed stretch-molded article.

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