JPH0257009B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a method for producing a polypropylene sheet for thermoforming, and more specifically, a method for producing a polypropylene sheet that can be thermoformed to obtain various molded products with excellent transparency and good thermoformability. Regarding the method. [Prior art and problems to be solved by the invention] Because crystalline polypropylene has excellent strength, heat resistance, and moisture permeability, it has recently been used in the production of various molded products in place of polyvinyl chloride resin and polystyrene resin. It has been used in However, crystalline polypropylene does not have sufficient transparency due to its crystallinity, and its rigidity and thermoformability are inferior to polyvinyl chloride resins and polystyrene resins, so its use is severely limited. However, it has not reached the point where it can replace competing products. Therefore, in order to improve the transparency of polypropylene, various proposals have been made to control the crystal structure by melt-extruding polypropylene and rapidly cooling the film-like body. However, in the method using a cooling roll, water droplets adhere to the roll surface, making it impossible to cool the material to below the dew point. For this reason, there are limits to the quenching conditions, and sheets with excellent surface conditions cannot be obtained due to air entrainment. Furthermore, it is not possible to reduce the external haze on both sides of the sheet. In addition, although the water cooling method has a high rapid cooling effect, uniform cooling cannot be achieved, so it is not possible to obtain a sheet with excellent uniformity (on the surface and inside), and in particular, it has the disadvantage of high external haze. be. Furthermore, it is known to use a nucleating agent to control the crystal structure, but the haze level is limited to 15%. For this reason, these film-forming methods have not yielded polypropylene sheets that have a haze of 7% or less and can compete with polyvinyl chloride resins. In particular, no method has been proposed for producing a thick polypropylene sheet having a thickness of 0.3 mm or more with sufficient transparency. Therefore, a method has been proposed to improve the transparency of a container obtained by thermoforming a polypropylene sheet, rather than the transparency of the polypropylene sheet itself. For example, there is a method of thermoforming a polypropylene sheet under low-temperature conditions with a highly oriented surface roughness, or a method of rolling or stretching a sheet with a surface roughness of 0.7 μRMS or less to improve its transparency to some extent, and then thermoforming it. A method of obtaining transparency as a thermoformed container has been proposed. However, these methods have drawbacks such as insufficient moldability because the sheet is oriented, relatively high pressure is required, expensive molding machines and molds, and poor mold reproducibility. Moreover, since the obtained containers are highly oriented, their strength is highly directional and they tend to crack easily, and they also suffer from shrinkage and deformation when used at high temperatures, so they cannot be used as retort food containers. Additionally, rolling and stretching treatments have the disadvantage that whitening tends to occur in the resulting sheet. As mentioned above, the conventional methods have various drawbacks, and it has not yet been possible to obtain containers with excellent transparency and good thermoformability. It is an object of the present invention to provide a method for producing a polypropylene sheet, which eliminates the above-mentioned conventional drawbacks and allows various molded products with excellent transparency to be obtained with good thermoformability by thermoforming the sheet. [Means for Solving the Problems] That is, the present invention melt-extrudes crystalline polypropylene into a film, and introduces the extruded molten resin film into a slit in which the cooling water flows in the direction of flow of the cooling water. A sheet with a thickness of 0.3 to 2.0 mm that is rapidly cooled,
The melting point of the crystalline polypropylene is below 70
Characterized by heat treatment at temperatures above ℃, external haze 5% or less, tensile modulus 15000Kg/cm ²
The above provides a method for producing a substantially non-oriented polypropylene sheet for thermoforming having a thickness of 0.3 to 2.0 mm. The external haze of the polypropylene sheet for thermoforming obtained by the present invention is 5% or less, preferably 3% or less,
More preferably, it is 2% or less, and the tensile modulus is
It is 15,000 Kg/cm ² or more, preferably 17,000 Kg/cm ² or more. Furthermore, the polypropylene sheet for thermoforming obtained by the present invention is substantially unoriented, and the thickness is
It is 0.3-2.0 mm, preferably 0.4-1.2 mm. In the present invention, crystalline polypropylene is used as a raw material. Examples of the crystalline polypropylene include crystalline propylene homopolymers, random polypropylenes containing 15% by weight or less of α-olefins such as ethylene, butene-1, pentene-1, and mixtures thereof. Further, the melt index (MI) of this crystalline polypropylene is 0.5 to 20 g/10 minutes, preferably 1 to 15 g/10 minutes. If the MI is less than 0.5 g/10 minutes, the discharge rate from the extruder will decrease, resulting in poor productivity and the resulting sheet will have low rigidity, which is not preferable. Moreover, if the MI exceeds 20 g/10 minutes, it becomes difficult to form a sheet due to the low viscosity. Furthermore, a petroleum resin, a nucleating agent, etc. can be added as appropriate if necessary. The petroleum resin may be either alicyclic or aliphatic, and usually has a number average molecular weight of 500 to 1000 and a softening point of 50 to 180°C. The effect of adding these petroleum resins differs in that the crystal structure of polypropylene is usually an α to β structure, but a smectica structure is obtained. In addition, specific examples of nucleating agents include silica,
Examples include talc and dibenzylidene sorbitol. Furthermore, stabilizers such as heat stabilizers and ultraviolet absorbers, antistatic agents such as various surfactants, etc. can also be added. The raw material resin is melt-extruded into a film. Here, homopolypropylene and random polypropylene, which are raw material resins, may be coextruded in multiple layers, or crystalline polypropylene, which is raw material resin, and modified polyolefin or other easily adhesive resins such as low density polyethylene (LDPE), ethylene, etc. - Multilayer coextrusion with vinyl acetate copolymer (EVA) or the like may be performed. The melt extrusion method is usually T
- The die method etc. can be applied. In this way, a highly transparent molten resin film is extruded from the die outlet. In order to obtain a molten resin film with excellent transparency, it is necessary to extrude under conditions such that the surface of the film is as smooth as possible. Specifically, the resin temperature is kept low, and the die exit temperature is kept relatively high by heating it using a die lip heater or the like. Normally, it is sufficient to provide a difference of about 10 to 60°C between the resin temperature and the die exit temperature. It is also effective to use a die with no scratches on its surface. Next, the molten resin film extruded as described above is rapidly cooled. That is, while the surface of the molten resin film is kept smooth, the film around the melting point is rapidly cooled and fixed. Therefore, if the temperature of the film-like body has decreased, it is preferable to reheat the surface of the film-like body. It is also effective to smooth the surface of the membrane by rolling or the like. The rapid cooling is carried out by introducing the extruded molten resin film into a slit in which the cooling water flows in the direction of the flow of the cooling water. The material of the slit portion is not particularly limited and may include metal, plastic, wood, cloth, etc. Further, the slit portion may be constructed of a pair of endless belts or a pair of rolls kept at a predetermined interval. In particular, by forming the slit portion into two or more multi-stage slits, it is possible to obtain an excellent sheet with even higher productivity. In the case of multi-stage slits, the first stage slits are preferably plate-shaped slits, but the slits in the second and subsequent stages are not limited to plate-shaped slits, and may be slits of other shapes. By forming the first stage slit into a plate-like slit, an excellent sheet with high productivity can be obtained. 1st
There is no particular limit to the width of the step slit, but it is usually 20 mm.
Below, preferably 10mm or less, more preferably 6mm
It is as follows. Further, the slit height is 3 mm or more, preferably 5 mm or more. The cooling water supplied to the plate-shaped slits is usually kept at a water level as low as possible, for example, 10 mm or less, preferably 5 mm or less. Note that the flow rate of the cooling water is usually adjusted to be faster than the traveling speed of the resin film, preferably twice or more. Further, the temperature of the cooling water is not particularly limited, and is preferably -10°C to +50°C, and a temperature of 20°C or lower is particularly effective for cooling efficiency and preventing the occurrence of haze spots. Further, as the cooling water, only water or an aqueous solution containing an organic or inorganic thickener is used, but an aqueous solution containing a thickener is preferable in terms of uniform cooling and surface smoothness. Here, various organic thickeners can be used, such as natural polymer substances, semi-synthetic products, and synthetic products. Natural polymer substances include sweet starch, potato starch,
Starches such as wheat starch; mannans such as konjac; seaweeds such as agar and sodium alginate; vegetable mucilages such as gum tragacanth and gum arabic; microbial mucilages such as dextran and levan; glue, gelatin, casein, collagen, etc. There are many proteins, etc. Semi-synthetic products include cellulose-based substances such as viscose, methylcellulose, and carboxymethylcellulose; starch-based substances such as soluble starch, carboxymethyl starch, and dialdehyde starch. In addition, synthetic products include polyvinyl alcohol, sodium polyacrylate, polyethylene oxide, and the like. On the other hand, inorganic thickeners include silica sol, alumina sol, clay, water glass, and various metal salts. In addition to aqueous solutions prepared by adding these thickeners to water, viscous substances such as polyethylene glycol, polypropylene glycol, and silicone oil can also be used alone. The viscosity of the aqueous solution with added thickener should be between 2 and 3000 centipoise (cp), preferably between 3 and 1000 cp. Through this rapid cooling process, a polypropylene sheet with a thickness of 0.3 to 2.0 mm is obtained. The external haze of this polypropylene sheet is not particularly limited, but it is preferably set to 5% or less, and the closer it is, the more preferable it is. In order to keep the external haze of the polypropylene sheet obtained in this way to 5% or less, it is necessary to control the cooling conditions. To control this cooling condition, as mentioned above, cooling water is supplied to the water-cooling slits at a water level as low as possible, and the flow rate of the cooling water in the water-cooling slits is normally higher than the running speed of the resin film introduced in the same direction. At the same time, the cooling slits are arranged in multiple stages for cooling. This makes it possible to improve the uniformity of cooling and the cooling effect. In this way, a polypropylene sheet is produced by rapidly cooling the molten resin film to usually 100°C or lower, preferably 60°C or lower. In the present invention, as described above, since the die exit temperature is set slightly higher than the resin temperature, the surface condition of the extruded molten resin film can be maintained in an extremely good condition. By rapidly cooling this film-like body, a polypropylene sheet having an external haze of 5% or less can be obtained. This sheet is then heat treated. The heat treatment is
Heat rolls, heated air, inert liquid, etc. are heated at a temperature below the melting point of the raw material crystalline polypropylene and 70°C or higher, preferably at 80°C or higher and 10°C lower than the melting point of the crystalline polypropylene. This is done by heating using Although it is not particularly necessary, the shrinkage stress is 5Kg/cm ² during the above heat treatment or before and after the heat treatment.
The following moderate stretching or rolling may be performed. That is, the rapidly cooled sheet is heated to a temperature below the melting point, preferably 5 to 70°C lower than the melting point, more preferably 5 to 50°C lower than the melting point, and then rolled or rolled. Here, the sheet may be heated using a roll, an oven, or the like. Note that the stretching may be performed by biaxial stretching in addition to uniaxial stretching. This moderate stretching or rolling imparts a slight orientation to the sheet, thereby further reducing the occurrence of sagging and wrinkles due to sheet heating while maintaining good thermoformability during thermoforming. As described above, a substantially non-oriented thermoformable polypropylene sheet having an external haze of 5% or less, a tensile modulus of 15,000 Kg/cm ² or more, and a thickness of 0.3 to 2.0 mm can be produced. By thermoforming the polypropylene sheet for thermoforming thus obtained, a molded article with excellent transparency can be obtained. That is, a transparent molded product can be obtained by subjecting the sheet to thermoforming such as vacuum forming, pressure forming, plug-assisted pressure forming, matt molding, etc. at a temperature 5 to 50° C. lower than the melting point of crystalline polypropylene. The molding conditions and molding method are appropriately selected depending on the thickness of the sheet, the specifications of the container (shape, drawing ratio, etc.), and the like. However, as a forming method, pressure forming is preferable to vacuum forming, and the drawing ratio is 1/2.
In the case of the above containers, plug-assisted pressure forming is preferred. [Effects of the Invention] According to the polypropylene sheet for thermoforming obtained by the present invention, various molded products with excellent transparency can be obtained by various thermoforming processes. Moreover, since the polypropylene sheet for thermoforming obtained by the present invention is substantially non-oriented,
The molding pressure can be low, the molding temperature can be relatively high, and molded products with good mold reproducibility and high thickness unevenness accuracy can be obtained. Furthermore, it has good thermoformability and does not require the use of expensive high-pressure thermoforming equipment. Moreover, the molded product obtained by thermoforming has good heat resistance, and there is no risk of deformation due to orientation shrinkage even when used at high temperatures. Moreover, since the molded product does not have any directionality, a molded product with high strength and high rigidity can be obtained. Further, in the present invention, since no stretching or rolling steps are particularly required, the manufacturing process is simplified and the polypropylene sheet can be manufactured at a lower cost. Furthermore, the polypropylene sheet obtained by the present invention does not whiten due to external stress such as bending or punching. Therefore, the polypropylene sheet obtained by the present invention can be effectively used for manufacturing various transparent molded products including general containers and retort food containers by thermoforming. [Example] Next, an example of the present invention will be shown. Examples 1 to 7 Homopolypropylene resin (density 0.91 g/cm ³ ,
MI2.1g/10min, melting point 165℃, manufactured by Idemitsu Petrochemical Co., Ltd.
Product name: Idemitsu Polypro F200S) using a T-die extrusion device (extruder 90mmφ, L/D=28, die width 730mm,
The die lip opening is 1.5 to 3 mm, the resin temperature is 240℃, and the die lip temperature is 240℃ using a lip heater heating die.
The mixture was melted and kneaded at 280°C, and a transparent molten resin film was extruded. Next, this membrane-like body was heated to a two-stage slit type water cooling device (first stage slit: height 50 mm) as shown in Fig. 1.
mm, width 2.5mm, slit upper water tank water level 5mm, cooling water temperature 5℃; second stage slit: height 10mm, width 5mm,
The mixture was continuously introduced into a water tank above the slit (water level 10 mm, cooling water temperature 5° C.) for rapid cooling, and various extrusion conditions were changed to obtain polypropylene sheets of different thicknesses. This polypropylene sheet was subsequently heat treated using a heat treatment roll consisting of four rolls each having a diameter of 300 mm (temperature: 145°C) to obtain polypropylene sheets for thermoforming having different thicknesses as shown in Table 1. Table 1 shows the measurement results of haze and physical properties of this sheet. Next, in order to evaluate the thermoformability of this sheet and the transparency of the molded product, various approximately cylindrical containers having a diameter of 50 mmφ and different depths were molded by the various molding methods shown in Table 1. Table 1 shows the evaluation results of moldability and the measurement results of physical properties of the molded product (container). Comparative Example Example 3 was carried out in accordance with Example 3, except that the heat treatment was not performed. Results first
Shown in the table.

【table】

【table】 [Brief explanation of drawings]

FIG. 1 is an explanatory diagram showing an example of a water cooling device used in the method of the present invention.

Claims (1)

[Claims] 1 Melt extrusion of crystalline polypropylene into a film,
The extruded molten resin film is introduced into a slit in the direction of the flow of cooling water and rapidly cooled to produce a sheet with a thickness of 0.3 to 2.0 mm, which is heated at 70°C below the melting point of the crystalline polypropylene. Characterized by heat treatment under the above temperature conditions, external haze is 5% or less, tensile modulus is 15000Kg/cm2 ^or more, and thickness is 0.3
A method for producing a ~2.0 mm essentially non-oriented polypropylene sheet for thermoforming.