JP2006051831A - Method for producing hollow molded body - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing with good productivity a hollow molded body which has a surface layer composed of polypropylene, is shaped by accurately reflecting the design of a mold cavity and provides a good surface gloss and a superior appearance. <P>SOLUTION: In this hollow molded body producing method in which the molten parison of a thermoplastic resin is fed into a cavity between molding molds and clamped and a pressurized fluid is blown into the parison to force the parison to contact with a mold shaping face to solidify the thermoplastic resin, the polypropylene, in which the melt index (230°C, 2.16 kgf) is 0.5 to 20 g/minute, the molecular-weight distribution is less than 8, the spherulite diameter is 40 μm or less after an elapse of three minutes after melting at 220°C for three minutes then lowered in temperature to 130°C at 10°C/minute, and the growth rate of spherulite is less than 12 μm/minute, is used at least as the thermoplastic resin for the surface layer of the parison. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

The present invention relates to a method for producing a hollow molded body. More specifically, the present invention relates to a method for producing a hollow molded article having an excellent appearance by constituting at least the surface layer of the hollow molded article with polypropylene having specific physical properties.

  Hollow molded bodies made of thermoplastic resins are used in a wide range of industrial fields because they are lightweight and excellent in mechanical strength. In particular, hollow molded products made of polypropylene are excellent in rigidity, so they are widely used not only in various containers but also in fields such as automobile parts such as bumpers, household electrical appliances, and housing equipment. Yes.

  In these application fields, since the quality of the product surface quality is the quality of the product appearance, development of a method for producing a hollow molded body having a more excellent surface property is strongly demanded. In order to meet such a demand, for example, in Japanese Patent Publication No. 2-40498, when forming a hollow molded article having excellent surface gloss, a large number of fine irregularities having a depth of 2 to 100 microns were formed on the surface. A method has been proposed in which the parison is mirror-finished and sandwiched between molds heated to a temperature higher than the crystallization temperature of the resin, blow molded, and then the mold temperature is cooled to the crystallization temperature or lower. Yes.

  However, in this method, special control is required for forming the parison, and a large molded product has a disadvantage that a molding cycle is long and productivity is low. Further, JP-A-4-77231 discloses a hollow molding in which a parison of a crystalline resin is supplied between molds, and after clamping, a pressure fluid is injected into the parison so as to adhere to the mold surface and cool. In this method, the temperature of the mold is maintained between the vicinity of the temperature at which the crystallization speed of the crystalline resin is maximized and the melting point, a fluid serving as a refrigerant is injected into the parison, and the refrigerant is circulated under pressure. A method has been proposed.

  By adopting such a molding method, it is possible to reduce the die line and weld line on the surface of the molded product, but there is a problem that the transferability of the mold cavity design and the surface gloss are not satisfactory. .

  The present invention provides a method for producing a hollow body that can accurately produce a mold cavity design on the surface of the hollow body and that has a good surface gloss and can produce a molded product having an excellent appearance. It is for the purpose.

  In order to achieve the above object, the present inventors have conducted various studies on the physical properties to be provided as a hollow molding material that can be formed into a hollow molded article having an excellent appearance, together with the molding conditions in the hollow molding method. The present inventors have found that polypropylene having the above physical properties is suitable as a material for the surface layer of a hollow molded body, and have completed the present invention based on such knowledge.

That is, the gist of the present invention is as follows.
[1] A hollow molded body in which a molten parison of a thermoplastic resin is supplied between molds and clamped, and a pressurized fluid is blown into the parison so as to be in close contact with the mold molding surface to solidify the thermoplastic resin. In the production method, at least the thermoplastic resin of the surface layer of the parison has a melt index (230 ° C., 2.16 kgf) of 0.5 to 20 g / 10 min, and a molecular weight distribution (weight average molecular weight / number average molecular weight). Is less than 8, and after melting at 220 ° C. for 3 minutes, the temperature drop rate is 10 ° C./min and the temperature drop rate is 130 ° C., and after 3 minutes the spherulite diameter is 40 microns or less and the growth rate of the spherulite diameter is A method for producing a hollow molded article, characterized by using polypropylene of 12 microns / min or less.
[2] The method for producing a hollow molded article according to the above [1], wherein the polypropylene according to the above [1] contains 10 to 3000 ppm of a nucleating agent.
[3] The temperature of the mold molding surface in close contact with the mold molding surface is set to a temperature range between a temperature lower by 10 ° C. than the crystallization temperature of the polypropylene described in [1] above and a melting point [1]. ] Or the manufacturing method of the hollow molded object of [2].
[4] When the thermoplastic resin is solidified, the pressurized fluid blown into the parison is one having a temperature of room temperature or lower, and the pressurized fluid is circulated inside the parison to be cooled [1] to [3] The manufacturing method of the hollow molded object in any one of.

  The method for producing a hollow molded body according to the present invention includes a step of supplying a molten resin parison of a thermoplastic resin between molds that are opened, and mold clamping by blowing a pressurized fluid into the interior of the parison. It is a method of using a polypropylene having a specific physical property at least on the surface layer of the parison as a thermoplastic resin of the molding material, including a step of closely adhering to a surface and a step of solidifying a thermoplastic resin.

  Here, the thermoplastic parison of the thermoplastic resin used for the production of the hollow molded body may be formed entirely of polypropylene having specific physical properties. Moreover, you may use the parison comprised so that the polypropylene may be used as a surface layer, and it may have a two-layer structure which uses other resin as an inner layer, or a three-layer structure including the intermediate layer which consists of another resin. When these two-layer or three-layer parisons are used, the inner layer or intermediate layer is advantageously combined with a resin having excellent impact resistance such as high-density polyethylene, low-density polyethylene, or polyamide. Such a parison can be molded using a conventional multilayer hollow molding machine.

  In the first step of the method of the present invention, polypropylene or the like, which is a molding material, is melt-kneaded by an extrusion molding machine and extruded as a cylindrical molten parison from a die provided at the tip, and the molten parison is opened. The mold is clamped and the molten parison is sandwiched when it hangs down from the center of the pair of hollow molds. In the second step, a pressurized fluid injection tube is inserted into the molten parison sandwiched between the molds, and a pressurized fluid such as compressed air is blown into the entire cavity inside the mold. Inflate to adhere.

  Here, when the molten parison is brought into close contact with the entire cavity in the mold, the temperature of the mold molding surface is set to 10 ° C. lower than the crystallization temperature of polypropylene in contact with the mold, and the melting point of the polypropylene. The mold cavity design is more accurately reflected on the surface of the hollow molded body. A general heating method can be used to adjust the temperature of the molding surface of the mold. For example, the mold can be heated from the inside of the mold by circulation of a heat medium or an electric heating method, or from the outside. It may be a method.

  Furthermore, in the third step, the thermoplastic resin in a state where the cavity inner surface shape is transferred in the mold is cooled and solidified to obtain a hollow molded body. The cooling and solidification of the thermoplastic resin in this third step may be performed by cooling the mold, or a pressurized fluid for cooling is circulated in the hollow part of the parison to which the product shape is given in the mold. It is also possible to directly cool it. When a pressurized fluid for cooling is used, a pressurized fluid cooled to room temperature or lower, preferably compressed air of about 2 to 10 kg / cm 2 G cooled to −20 ° C. or lower, more preferably −30 ° C. or lower is used. Is preferred. As a method of circulating the pressurized fluid for cooling, when inserting the pressurized fluid injection pipe for parison expansion, the pressurized fluid discharge pipe is inserted at the same time, and the cooling process starts. It is only necessary to open the main plug of the pipe so that the pressurized fluid heated in contact with the high-temperature polypropylene can be discharged from the exhaust pipe. Further, in this case, the cooling rate is preferably adjusted by adjusting the flow rate of the pressurized fluid at the outlet of the exhaust pipe and keeping the pressure in the hollow portion constant.

  Furthermore, what is necessary is just to use the metal mold | die which has a gas vent hole generally used for the hollow shaping | molding as the metal mold | die for hollow shaping | molding used with the method of this invention. This vent hole has a hole diameter of about 0.2 to 0.5 mm, but in order to obtain a molded article having a better appearance, the hole diameter is 100 microns or less, and the hole bitch is 50 mm or less. It is good to use what was processed in this way. Further, the surface of the mold is preferably a mirror-finished surface of about 0.5S, but a surface that has been subjected to graining or patterning according to the purpose of use may be used.

  Next, regarding the polypropylene used in the method of the present invention, crystalline polypropylene having specific physical properties is used. That is, this crystalline polypropylene has a melt index (230 ° C., 2.16 kgf) of 0.5 to 20 g / 10 minutes, a molecular weight distribution (weight average molecular weight / number average molecular weight) of less than 8, and 220 ° C. 3 minutes after melting at 10 ° C./minute, the spherulite diameter after 3 minutes is 40 microns or less, and the growth rate of the spherulite diameter is 12 microns / minute or less. It is what you have.

  Here, when the melt index value is less than 0.5 g / 10 min, the surface roughness of the molded body due to melt fracture increases during the molding, and this exceeds 20 g / 10 min. Even when a material is used, it causes surface roughness, so that a material within the above numerical range is used. In addition, regarding the molecular weight distribution (weight average molecular weight / number average molecular weight), if this value exceeds 8, the surface gloss will be lowered, and the sharpness of the mapping onto the surface of the molded product will be reduced. That is 8 or less.

  Further, regarding the spherulite diameter and the growth rate of polypropylene under specific conditions, when the spherulite diameter exceeds 40 microns, the sharpness of the mapping onto the surface of the molded product is lowered, and the growth rate of the spherulite is reduced. Even when a material exceeding 12 microns / minute is used, the sharpness of the mapping is deteriorated, so that a material having the above-mentioned crystallization characteristics is used. Regarding the measurement of the crystallization characteristics of this polypropylene, a polypropylene pellet is sandwiched between glass plates, set on a hot plate, heated to 220 ° C. and maintained at this temperature for 3 minutes, and then the hot plate is polarized. It is set in a microscope, and the temperature is decreased at a temperature decrease rate of 10 ° C./min, and the spherulite diameter of the crystal observed 3 minutes after reaching 130 ° C. can be measured by visual observation and photography. Also, regarding the growth rate of spherulites, after reaching 130 ° C. by the above-mentioned temperature lowering operation, photographs were taken of the spherulite diameter after 1 minute, 2 minutes, 3 minutes, 4 minutes, and 5 minutes, and the elapsed time and sphere It can be determined as a linear gradient in the correlation equation with the crystal diameter.

  The polypropylene having the above-mentioned physical property values used in the present invention is obtained by using a combination of a transition metal compound such as titanium and an organoaluminum compound as a catalyst and adding an electron donating compound thereto to polymerize propylene. What is necessary is just to select and use the thing whose crystal form is a microstructure, a spherulite diameter is below a specific value, and a crystal growth rate is below a specific value.

  Furthermore, what contains 10-3000 ppm of nucleating agents can be used for the polypropylene used by this invention. As the nucleating agent used here, those having the effect of refining the crystal size of polypropylene are suitable, for example, organic systems such as 2,2-methylenebis (4,6-t-butylphenyl) sodium phosphate. Examples thereof include metal phosphates, aluminum hydroxy-diparatertiary butyl benzoate, dibenzylidene sorbitol, dimethyl benzylidene sorbitol, and magnesium silicate. If the addition amount of the nucleating agent is less than 10 ppm, the effect of the addition is not sufficiently exhibited, and if it exceeds 3000 ppm, there is a possibility of bleeding out, so that it is within the range of 10 to 3000 ppm.

In addition to the nucleating agent, the polypropylene used in the present invention may be blended with commonly used antioxidants, pigments, talc and other fillers.

Hereinafter, the present invention will be described more specifically with reference to examples.
[Example 1]
As a raw material polypropylene, it has a melt index (230 ° C., 2.16 kgf) of 3.52 g / 10 minutes, a molecular weight distribution (weight average molecular weight / number average molecular weight) of 6.0, and observation under a polarizing microscope. In this method, after melting at 220 ° C. for 3 minutes, the spherulite diameter after 30 minutes is 30 microns immediately after the temperature is lowered to 130 ° C. at a rate of 10 ° C./minute, and the growth rate of the spherulite diameter is 10 microns / minute. Of polypropylene [Idemitsu Petrochemical Co., Ltd .: Idemitsu Polypro; J400MP] was used. The crystallization temperature of this polypropylene was 114 ° C.

  This polypropylene was supplied to an extruder of a hollow molding machine and melt-kneaded, and then the parison was extruded under the conditions of a resin temperature of 220 ° C. and an extrusion speed of 185 g / sec, and the mold surface temperature was maintained at 110 ° C. After hanging down at the center of a mirror-plated mirror mold, the mold was clamped and the parison was taken into the mold. Next, a compressed air injection pipe extending from the lower part of the mold to the hollow part in the parison is inserted, and compressed air is blown from this injection pipe at room temperature so that the surface of the parison is in close contact with the entire surface of the mold cavity. Was inflated. As the compressed air, one having 7 kg / cm @ 2 G was used.

Next, after the mold cavity inner surface shape was transferred in the above process and the heat-softened polypropylene with the product shape was cooled and solidified by cooling the mold, the mold was opened and the hollow molded body was taken out It was. The hollow molded body thus obtained has a length of 320 mm and a width of 190 m.
m, a slightly flat hollow container having a height of 40 mm, and the wall thickness at the center was 3 mm.

  Next, a sample was taken out from the body of the hollow container, and the surface gloss of the molded body and the clarity of the image were evaluated. For surface gloss, glossiness (%) was measured based on JIS K7105. The clarity of the mapping was evaluated by visual observation of the clarity of the mapping of the fluorescent lamp when the fluorescent lamp was copied onto the surface of the molded article after conditioning for 24 hours at 23 ° C. These evaluation results are shown in Table 1.

[Example 2]
As a molding material, a composition in which 2,2-methylenebis (4,6-t-butylphenyl) sodium phosphate as a nucleating agent was blended so as to have a concentration of 800 ppm in the same polypropylene as in Example 1 was used. Otherwise, a hollow molded body was obtained in the same manner as in Example 1. Table 1 shows the evaluation results of the obtained hollow molded body.

Example 3
The steps until the starting polypropylene and parison were expanded were the same as in Example 1. Next, the cooling and solidification of the heat-softened parison provided with the product shape was directly cooled with a cooling medium. That is, when the compressed air injection pipe is inserted from the lower part of the mold into the hollow part of the parison, the compressed air discharge pipe is inserted at the same time, and when the parison expands, the main plug of the discharge pipe is closed and the parison starts cooling. Immediately before the operation, the main plug was opened to allow the cooling medium to flow through the hollow portion. Next, compressed air of 3 kg / cm @ 2 G cooled to minus 40 DEG C. as a cooling medium is injected into the parison hollow portion, and the compressed air heated by heat exchange while circulating in the hollow portion is discharged into the discharge pipe. As a result, the parison was directly cooled to obtain a hollow molded body. Table 1 shows the evaluation results of the obtained hollow molded body.

Example 4
The same polypropylene composition as in Example 2 was used as the molding material, and the method for producing the hollow molded body was the same as in Example 3. Table 1 shows the evaluation results of the obtained hollow molded body.

[Comparative Example 1]
As a raw material polypropylene, it has a melt index (230 ° C., 2.16 kgf) of 0.44 g / 10 minutes, a molecular weight distribution (weight average molecular weight / number average molecular weight) of 8.4, and observation under a polarizing microscope. In this method, after melting at 220 ° C. for 3 minutes, the spherulite diameter after 3 minutes from immediately after the temperature is lowered to 130 ° C. at a rate of 10 ° C./minute is 20 microns, and the growth rate of the spherulite diameter is 8 microns / minute. Polypropylene was used.
The method for producing the hollow molded body was the same as in Example 1. Table 1 shows the evaluation results of the obtained hollow molded body.

[Comparative Example 2]
Example 1 was the same as Example 1 except that the same polypropylene as that used in Comparative Example 1 was used as the molding material, and a composition in which the same nucleating agent as Example 2 was blended to a concentration of 800 ppm was used. Table 1 shows the evaluation results of the obtained hollow molded body.

  According to the method of the present invention, it is possible to produce a hollow molded article having an excellent appearance, in which the mold cavity design is accurately reflected on the surface of the hollow molded article and the surface gloss is good.

Claims (4)

  A method for producing a hollow molded body in which a molten parison of a thermoplastic resin is supplied between molding dies and the mold is clamped, and a pressurized fluid is blown into the inside of the parison to bring it into close contact with the mold molding surface to solidify the thermoplastic resin In at least the surface layer of the parison, the thermoplastic resin has a melt index (230 ° C., 2.16 kgf) of 0.5 to 20 g / 10 min and a molecular weight distribution (weight average molecular weight / number average molecular weight) of less than 8. In addition, after melting at 220 ° C. for 3 minutes, the spherulite diameter after 3 minutes from the temperature decrease rate of 10 ° C./min is 130 μm and the growth rate of the spherulite diameter is 12 μm / min. A method for producing a hollow molded body, comprising using polypropylene having a content of less than or equal to minutes.   The method for producing a hollow molded article according to claim 1, wherein the polypropylene according to claim 1 contains 10 to 3000 ppm of a nucleating agent.   The temperature of the mold molding surface when closely contacting with the mold molding surface is a temperature range between a temperature lower by 10 ° C. than the crystallization temperature of the polypropylene according to claim 1 and a melting point. A method for producing a hollow molded body.   When solidifying the thermoplastic resin, a pressurized fluid blown into the parison is used at a temperature equal to or lower than room temperature, and the pressurized fluid is circulated inside the parison for cooling. A method for producing a hollow molded body.