JPS6362377B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a transparent polypropylene resin molded article, and more particularly to a method for producing a molded article having excellent thermoformability and transparency. Polypropylene resins have excellent strength, rigidity, heat resistance, etc., and have recently been used in the production of various molded products in place of polyvinyl chloride resins and polystyrene resins. However, polypropylene resins have the drawback of insufficient transparency due to their crystallinity. Therefore, various methods have been proposed for producing molded articles with improved transparency using polypropylene resins. For example, after heating a crystalline plastic sheet to a temperature above its melting point and rapidly cooling it,
A method of manufacturing containers by pressure forming at a temperature below the melting point (Japanese Patent Publication No. 17689/1989). A method of manufacturing a thermoforming sheet by setting the surface roughness of both sides of a thermoplastic synthetic resin sheet to 0.7 μRMS or less, and then applying uniaxial stretching of 3 times or less to the sheet (Japanese Patent Application Laid-Open No. 128673/1983) Public bulletin). Made of resin containing 0.01-2.0wt% nucleating agent based on isotactic polypropylene, surface roughness is 0.7μRMS
A method of manufacturing a transparent container by stretching the following sheet 1.02 to 3 times in one direction at a temperature lower than its melting point, and thermoforming the resulting uniaxially oriented sheet at a temperature lower than its melting point (Japanese Patent Application Laid-Open No. 1983-1999- 94371). However, in the above method, since a preformed sheet is reheated, reheating causes deterioration of the resin and uneven heating, and increases energy costs. Moreover, uniform cooling and rapid cooling are insufficient, and the resulting sheet does not have sufficient transparency. Furthermore, there are problems such as sagging and wrinkles of the sheet during thermoforming, making it impossible to obtain molded products with excellent transparency and uniformity. In addition, in the method (2), although the original sheet itself is inferior in transparency, the transparency is attempted to be improved by secondary stretching. however,
In this method, although the stretching ratio is usually as high as 1.5 to 2.5 times, the transparency is not necessarily sufficient. Moreover, if the stretching ratio is increased to improve transparency,
The disadvantage is that thermoformability deteriorates significantly, and it tears in the MD direction, making molding impossible. Moreover, in order to obtain a transparent container, a certain degree of stretching by deep drawing is required, and uniform transparency cannot be obtained due to uneven stretching.
Moreover, there is a major drawback in that vacuum forming is not possible due to the high degree of sheet orientation and low heat resistance. Furthermore, although the stretching ratio can be made relatively lower in the method compared to the method in (2), the uniform kneading properties of the nucleating agent are not sufficient, resulting in problems such as plate-out and mucus in the die during sheet forming. Furthermore, there are health and safety issues, and transparency and uniformity are not always sufficient. As mentioned above, the conventional methods have various drawbacks, and it has not yet been possible to obtain containers with excellent transparency and good ripening properties. The present invention aims to eliminate the above-mentioned conventional drawbacks and to provide a method that can produce molded articles with excellent thermoformability and transparency. That is, the present invention melt-extrudes polypropylene resin into a film, introduces the extruded molten resin film into a slit through which cooling water flows, and rapidly cools it.
1.01~ in one direction at a temperature lower than the melting point of the resin
The present invention provides a method for producing a transparent polypropylene resin molded article, characterized in that a sheet or film obtained by stretching at a stretching ratio of 1.50 times is thermoformed at a temperature lower than the melting point of the resin. In the method of the present invention, a polypropylene resin is used as a raw material for the molded article. Examples of the polypropylene resin include propylene homopolymers, block copolymers and random copolymers of propylene and other copolymer components such as ethylene, and mixtures thereof. In particular, from the viewpoint of heat resistance, it is preferable to use a propylene homopolymer or block copolymer. In addition, the melt index of this polypropylene resin is usually 0.2 to 10.
g/10 minutes, preferably 0.3 to 8 g/10 minutes. In the method of the present invention, silica, talc, p-tert-
Nucleating agents such as butylbenzoic acid, aluminum, dibenzylidene sorbitol, and heat stabilizers such as alkylene bisphenol and β,β'-thiopropionic acid esters can also be added as appropriate. In the present invention, the raw material resin is melt-extruded into a film. As a method for melt-extruding the raw resin into a film, a T-die method, which is usually used for forming sheets, etc., can be used. The molten resin film extruded in this way is then introduced into a slit through which cooling water flows, but at this time it is preferable to extrude the film under conditions to ensure excellent transparency. In other words, it is preferable to extrude under conditions that make the surface of the film as smooth as possible. For example, extrusion can be carried out by using a die with no scratches on the surface or by extruding at a relatively high die exit temperature. preferable. The thickness of the molten resin film thus extruded is usually 3 mm or less, preferably 0.1 to 2.0 mm. Next, the transparent molten resin film extruded as described above is introduced into a slit through which cooling water flows and is rapidly cooled. That is, cooling water is made to flow through the slits, and the molten film is introduced into the slit in the direction of water flow to rapidly cool it. 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 in two or more stages, it is possible to obtain an excellent product with even higher productivity. The width of the slit is not particularly limited, but is usually 20 mm or less, preferably 10 mm or less, and more preferably 6 mm or less. 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. In addition, the temperature of the cooling water is preferably -10℃ to +50℃, and especially when manufacturing sheets with a thickness of 0.2 mm or more, the liquid temperature should be 20℃ or less, particularly preferably 10℃ or less, to prevent haze spots from occurring. Effective. In this way, a polypropylene resin sheet or film is produced by rapidly cooling the molten resin film to usually 100°C or lower, preferably 60°C or lower. Note that in order to further improve the transparency of this sheet or film, it is preferable to add an annealing step. That is, the sheet or film is heated below its melting point, usually 10 to 60°C above its melting point.
By heating with a heating roll or the like in a low temperature range, preferably in a temperature range 20 to 50°C lower than the melting point, even better transparency can be obtained. Furthermore, the transparent sheet or film obtained by rapid cooling in this manner is stretched in one direction at a stretching ratio of 2 times or less at a temperature lower than its melting point. That is, the quenched transparent sheet or film is heated to a temperature lower than its melting point, usually 100 to 160°C, preferably 120 to 150°C, and subjected to roll stretching or roll rolling. Here, the sheet or film may be heated by radiation or the like, or by using an oven, a hot roll, or the like. In addition, the stretching ratio is 1.01 to 1.50 times, and it varies depending on the type of thermoforming, molding conditions, shape and size of the molded product to be manufactured, but usually in the case of air pressure forming.
Range of 1.01~1.50 times, 1.02~ for vacuum forming
1.10 times the range. Although the shrinkage stress varies depending on the stretching conditions, 0.2 to 20 Kg/cm ² is usually sufficient. In other words, the stretching ratio has almost no effect on the transparency of the resulting sheet or film, so it depends on the conditions during thermoforming, but in general, as long as the thermoforming is carried out to the extent that no sagging or wrinkles occur. It is determined by taking into consideration the strength (degree of orientation) required for the molded product. As described above, a stretched sheet or film with excellent transparency can be produced. here,
The thickness of this sheet or film is 0.05~1.5mm,
The thickness is usually about 0.05 to 0.6 mm, and the haze at a thickness of 0.2 mm is 20% or less, preferably 15% or less, more preferably 10% or less. In the present invention, by thermoforming the stretched sheet or film obtained in this way at a temperature lower than the melting point of the polypropylene resin,
The desired transparent polypropylene resin molded product can be manufactured. That is, the stretched sheet or film is heated at a temperature lower than the melting point of the polypropylene resin, specifically 5 to 50°C below the melting point.
A transparent polypropylene resin molded product can be obtained by thermoforming such as vacuum forming, pressure forming, plug-assisted pressure forming, matt molding, etc. at a low temperature. According to the method of the present invention, molded articles with excellent transparency can be obtained by various thermoforming processes. Furthermore, the method of the present invention provides excellent transparency and thermoformability even if the degree of stretching is low. Therefore, according to the method of the present invention, it is possible to perform everything from vacuum forming to pressure forming by adjusting the stretching ratio. Furthermore, the resulting molded product has high transparency and good formability. Furthermore, since the method of the present invention can easily incorporate an annealing treatment in the stretching process, the transparency of the sheet can be increased, and therefore the molded product has high transparency and uniformity, and has a high heat distortion temperature. Therefore, it can be molded at a higher temperature, has good shapeability by low-pressure vacuum molding, and can produce excellent molded products even with complex shapes. Therefore, the method of the present invention can be applied to molded products such as various packaging containers, etc. due to the characteristics of polypropylene resin such as heat resistance, chemical resistance, moisture permeability, and hygiene, as well as its excellent transparency and formability. For example, it can be effectively used for manufacturing blister packaging, PTP containers, etc. Next, examples of the present invention will be shown. Examples 1 to 7 Isotactic polypropylene (melt index 2.0 g/10 minutes, melting point 168°C) was
T-die extrusion equipment (extruder 60mmφ, L/
A transparent molten resin film was extruded using a die (D = 28, die width 550 mm, die lip opening 1 mm). The extruded molten resin film is cooled by cooling devices A and B shown below.
and molded under the molding conditions shown in Table 1 to obtain a polypropylene sheet. Next, this sheet was heated to 130° C. using a heating roll, and then stretched at a predetermined stretching ratio shown in Table 1 to obtain a stretched polypropylene sheet. The obtained stretched polypropylene sheets were subjected to the prescribed thermoforming shown in Table 1, and the moldability and molded products were evaluated. The results are shown in Table 1. Cooling device A (single-stage slit) Slit height 50mm, width 3mm, water tank water level above the slit 5mm, cooling water temperature 2°C Cooling device B (two-stage slit) First-stage slit: height 50mm, width 2mm, water tank water level above the slit 5 mm Second stage slit: height 10 mm, width 4 mm, water tank water level above the slit 10 mm Cooling water temperature: 10°C Comparative Example 1 A sheet was manufactured according to Example 3 except that stretching was not performed in Example 3, It was thermoformed and its moldability and molded product were evaluated. The results are shown in Table 1. Reference Example 1 In Example 2, vacuum forming was performed at a forming temperature of 155° C. instead of air pressure forming, but the forming could not be completed. For this reason, when the molding temperature was raised to 165°C, the transparency and surface gloss of the molded product decreased significantly. 【table】

Claims (1)

[Claims] 1. Melt extrusion of polypropylene resin into a film,
A sheet or film obtained by introducing an extruded molten resin film into a slit through which cooling water flows, quenching it, and then stretching it in one direction at a stretching ratio of 1.01 to 1.50 times at a temperature lower than the melting point of the resin. A method for producing a transparent polypropylene resin molded article, which comprises ripening the above resin at a temperature lower than the melting point of the resin. 2. The manufacturing method according to claim 1, wherein the slit is a two-stage slit. 3. The manufacturing method according to claim 1, wherein the sheet or film obtained by stretching has a shrinkage stress of 0.2 to 20 Kg/cm ² .