JPS6342566B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a film by an inflation molding method in which a crystalline thermoplastic resin is extruded from an annular die and expanded under internal pressure to form a tubular film. One method for improving transparency in inflation film molding is to increase the temperature of the resin. This is because when the temperature of the resin is raised, the viscosity of the molten resin decreases, which reduces surface irregularities in the extrudate, and also because it becomes possible to cool the resin more rapidly, which makes it possible to reduce the spherulite size of the crystalline resin. However, in general, if the temperature of the resin is raised too high, the resin deteriorates in the extruder or die, forming a gel, which causes fish eyes to appear on the film, and the film to be colored or smelled. Further, in the process after exiting the die, there are problems in that the self-supporting nature of the molten resin deteriorates, making molding difficult and causing blocking on the inner surface of the film. The present inventors have arrived at the present invention as a result of intensive studies to solve such problems. That is, the present invention uses an inflation molding method in which a molten crystalline thermoplastic resin is extruded into a cylindrical shape from an annular die, expanded under internal pressure to form a tube, cooled, and rolled up. The present invention provides a method for producing a transparent film of thermoplastic resin, which comprises extruding the surface of the resin at a temperature of 5 to 50 degrees Celsius higher than the center of the molten resin. In the inflation molding method of the present invention, the essence of the resin is to make the surface temperature of the resin 5°C to 50°C higher than the temperature of the center when extruded into a cylindrical shape. The temperature at the center means, in a steady state, the same temperature as the average temperature of the molten resin before it is extruded from the annular die, and the surface temperature of the resin during extrusion is calculated from this.
Increasing the temperature by 50°C to 50°C means that some kind of heating means is provided at the exit of the annular die to heat the resin during extrusion to a temperature 5°C to 50°C higher from the outside. In this way, only the surface of the cylindrical molten resin extruded from the annular die is locally removed from the center of the resin.
By raising the temperature by 10°C to 30°C, it is possible to produce a film with good transparency without causing the above-mentioned molding problems. If the surface temperature of the resin during extrusion is lower than 5°C than the temperature of the center of the resin, the effect of improving transparency will be low; if the temperature is higher than 50°C, resin deterioration, blocking on the inner surface of the film, or Forming after exiting the die becomes difficult. In the present invention, the method of increasing the surface temperature is to provide a heating device near the die lip of the annular die. Specifically, as shown in FIGS. Another method is to embed an annular heater inside to heat the die lip. In this case, the position of embedding the heater must be determined by taking into account the strength of the die lip, local heating effect, heater capacity, etc., but usually it is placed at equal intervals around the die lip at a distance of about 0.1 to 3.0 cm from the die lip. will be installed. If the installation distance is less than 0.1 cm, the strength of the die slip part will decrease and the
cm or more may affect the local heating effect. Further, the depth of embedding is desirably about 5 cm at most from the die surface; if the embedding is deeper than 5 cm, the local heating effect will be weakened. In this case, since the molten resin passes through the narrow gap of the die lip at a certain speed, it is assumed that the set temperature of the heating device embedded near the die lip and the resin surface temperature usually match. Needless to say, these heaters for local heating are used in combination with general heating means provided outside the die and, if necessary, inside the die. Conventional means such as air cooling or water cooling are used to cool and solidify the cylindrical molten resin discharged from the die, but in the present invention, since the surface temperature of the resin is higher than usual, more rapid cooling is used. becomes possible. Therefore, by using any method for improving the cooling capacity, such as cooling with cold air or water, the transparency and mechanical strength of the resulting film can be further improved. Crystalline thermoplastic resins used in the present invention are crystalline resins such as polyolefin resins such as polyethylene, polypropylene, polybutene-1, and poly-4-methylpentene-1, polyamide resins, polyester resins, and polyvinyl alcohol resins. It is. To explain this in more detail, regarding polyethylene, low-density polyethylene homopolymer and ethylene produced by high pressure method are used at 70 mol%.
Copolymers with vinyl acetate, acrylic acid, methacrylic acid, etc. containing the above, high-density polyethylene produced by medium-low pressure method, and copolymers with other α-olefins ranging from low density to high density. Examples include polyethylene copolymers. Examples of polypropylene include isotactic and syndiotactic stereoregular polypropylene, as well as block or random copolymers containing propylene as a main component and ethylene or the like. Examples of polyamide resins include nylon 6, nylon 66, and nylon 11, and typical examples of polyester resins include polyethylene terephthalate and polyethylene isophthalate. These resins can be used alone or in combination. Among these crystalline thermoplastic resins, polyolefin-based resins are preferred, and one that shows remarkable effects is ethylene-α containing 80 mol% or more of ethylene.
-Low density (0.890~
0.945) There is quality. When molding the low density product,
If a die having a relatively large die lip width of 1.5 mm to 5.0 mm is used and the method of the present invention is applied, the formed film will have excellent transparency and dart impact strength. Furthermore, when the method of the present invention is applied to the crystalline thermoplastic resin in combination with a known nucleating agent such as dibenzylidene sorbitol, the effects of the present invention can be further improved. The present invention uses a heating device embedded in the vicinity of the die lip of the annular die mentioned above to keep the surface temperature of the resin extruded into a cylindrical shape within a range of 5 to 50 degrees Celsius higher than the temperature at the center of the resin at the exit of the die lip. By locally heating the surface, it is possible to prevent deterioration of the resin within the die and at the same time eliminate surface irregularities (melt fracture, etc.) in the molten resin at the exit of the die. Furthermore, in the process after exiting the die, the molten resin is heated only to the surface temperature of the resin, making it highly self-sustaining. This allows for more rapid cooling, which reduces the spherulite size, resulting in excellent transparency and dart impact strength. This makes it possible to produce films with excellent mechanical strength. The present invention will be explained in detail in the following examples. Examples 1 to 3 and Comparative Examples 1 to 3 In the inflation molding method, Example 1 and Comparative Example 1 were made using an ethylene-butene-1 copolymer (ethylene content: 96.5 mol%, melt index) produced by a medium-low pressure gas phase polymerization method. Tux (MI) 1.0,
In Example 2 and Comparative Example 2, ethylene-butene-1 copolymer (ethylene content 98.5 mol%, MI 2.0, density 0.935, n-value 1.6)
In Example 3 and Comparative Example 3, high-pressure low-density polyethylene (melt index 1.0, density 0.923) was used.
The case is shown below. Extruder each of the above resins [screw diameter 50 mm]
Φ, L/D = 25, die (diameter 75mmΦ, embedded heater installation location: distance from die lip 5.0mm,
A film with a thickness of 30 μm was formed by air cooling using a die lip with a depth of 10.0 mm (installed inside and outside the die lip) at a blow-up ratio (BUR) of 1.8 and a take-up speed of 30 m/min. Molding conditions and physical properties are the first
Shown in the table. As shown in Table 1, the method of the present invention significantly improves transparency and dart impact strength. 【table】

[Brief explanation of the drawing]

FIG. 1 is a schematic view of a die equipped with an embedded heater, viewed from directly above, and FIG. 2 is a schematic cross-sectional view of the die equipped with an embedded heater. 1...Dice, 2...Die lip, 3...Embedded heater.

Claims (1)

[Claims] 1. In an inflation molding method in which a molten crystalline thermoplastic resin is extruded into a cylindrical shape from an annular die, cooled, and rolled up, the molding process is performed within a range of 0.1 to 3.0 cm from the die lip.
A thermoplastic resin characterized in that the surface of the resin is extruded at a temperature of 5°C to 50°C higher than the center of the molten resin by a heater embedded at a distance of 5 cm from the die surface to a depth of 5 cm. A method for producing a transparent film. 2. The method according to claim 1, wherein the crystalline thermoplastic resin is a polyolefin.