JPH056494B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for forming a melt-extruded crystalline thermoplastic resin sheet, and particularly to a method for cooling the resin sheet with less haze. (Prior art) The molten crystalline thermoplastic resin is closely cast on the surface of the cooling roller using a method such as the electrostatic charge application method (Japanese Patent Publication No. 37-6142), and is cooled to an appropriate temperature and solidified to form a predetermined shape. The resin sheet is biaxially stretched at a stretching ratio of , and is made into a resin sheet of a predetermined thickness for use. The cooling conditions for the crystalline plastic resin sheet (hereinafter referred to as special resin) that enters the casting process depend on the haze (degree of haze), surface undulations, and unevenness (flatness) of the resin sheet finally obtained by the special resin. ) or have a large effect on physical properties such as strength, and if the cooling rate at the crystallization temperature of the resin is slow, it will result in a resin sheet with high crystallization, and when stretched, it will be difficult to cut,
Problems such as uneven thickness, insufficient strength, and haze occur. In order to increase the cooling efficiency of the resin sheet, various proposals have been made including the above-mentioned electrostatic charge application method. For example, methods such as blowing cooling air or cooling water spray or flowing cooling water onto the surface of a characteristic resin sheet during casting are known (Japanese Patent Publication No. 44-30388). It has disadvantages that impair its character. To avoid this difficulty, JP-A-49
No.-108163 discloses a technology for slow cooling in an amorphous temperature range and then rapid cooling. In addition, when forming thick sheets that are prone to problems, parallel airflow outlets are provided at several locations facing the cooling roller, and cooling air is applied to the sheet surface that is not in contact with the cooling roller. A method of quickly passing through and escaping the temperature range where crystallization is likely to occur by simultaneously removing dust generated by solidification of a vaporized low-molecular substance and rapidly cooling the resin (Japanese Patent Application Laid-Open No. 52-93477)
has been proposed, as well as a method (Japanese Patent Application Laid-open No. 59669/1983) to increase the cooling effect by flowing the cooling air flow countercurrently in the direction of rotation of the cooling roller. The methods proposed so far have focused on quickly cooling the resin sheet as a whole, and while taking care not to damage the flatness of the resin sheet surface, the resin sheet is cooled by cooling air while it is still at a high temperature. However, the relationship between cooling conditions and haze has hardly been specifically considered. However, the haze of a resin sheet affects its commercial value, and is enough to control its life and death, especially when it is used as a photographic material. (Objective of the Invention) An object of the present invention is to provide a method for forming a melt-extruded biaxially oriented resin sheet of the above-mentioned characteristic resin, which has less haze. (Structure of the Invention) The present inventors have diligently studied the haze reduction of biaxially oriented resin sheets, and found that the haze is determined by the degree of crystallization (crystallization It is necessary to suppress the degree of crystal finalization of the surface, and it has been learned that to do this, the timing of starting air cooling can be delayed. That is, an object of the present invention is to cast a crystalline thermoplastic resin extruded in a molten state onto the surface of a cooling roller, cool it, and solidify it to form a resin sheet, by using an air cooling device provided opposite to the surface of the cooling roller. The resin sheet is air-cooled from the melting point of the resin to a temperature that gives its maximum crystallization rate, with respect to the temperature of the resin sheet on the outside air surface that is not in contact with the cooling roller surface of the resin sheet. This is achieved by a method of forming a resin sheet, which is characterized in that it starts from a temperature point within a temperature range that gives a relative crystallization rate of 0.7 to 70%. The crystallization rate of the characteristic resin according to the present invention has a relationship with the temperature of the characteristic resin as shown in FIG. 1. In Figure 1, the horizontal axis is the temperature T of the characteristic resin,
The vertical axis is the crystal velocity G. The crystallization rate is determined by the radius r of the spherulites that occur in the characteristic resin.
Directional growth rate dr/dt [micron (μ)/second (S)]
Defined by G = dr/dt [μ/S] The crystallization rate G is O at the melting point MP of the characteristic resin.
It increases as the temperature T decreases, reaches the maximum crystallization rate G _M at temperature T _M , and then decreases again.
It is almost symmetrical with respect to the vertical direction of T _M. However, the range used in the present invention is from MP to _TM . The relative crystallization rate g is defined as a percentage of the crystallization rate G at each temperature based on _GM . g=(G/G _M )×100% The characteristic resin to which the present invention is applied is preferably polyester, and more preferably polyethylene terephthalate. (Example) The present invention will be specifically described using an example (FIG. 2). In Fig. 2, 1 is an extrusion die, and 11 is an extrusion die 1.
2 is a casting cooling roller, and the roller diameter is
500〓, while circulating cooling water at a temperature of 30℃ inside the cooling roller 2, it rotates at a circumferential speed of 2m/〓. Reference numeral 3 denotes an arc-shaped air cooling device provided concentrically on the cooling roller facing the casting surface 21 thereof, and an air cooling start end 31 detects the temperature T of the outside air surface of the cooling roller made of a special resin using non-contact infrared radiation. By checking the temperature, the air-cooling device can be slid concentrically and freely installed at a temperature point P _i on the air-cooled casting surface 21 at an arbitrary relative crystallization speed gi. The resin sheet (original sheet) that has been cooled is peeled off at the take-out roller 4 and then longitudinally stretched and transversely stretched to continuously form a product sheet. In this example, polyethylene terephthalate was used.
The points P ₁ , P ₂ , ..., P ₉ (corresponding temperatures T ₁ , T ₂ , ..., T ₉ ; corresponding relative crystals rate of change g
₁ , _g2 ,..., _g9 ) to start air cooling, followed by biaxial stretching to continuously produce polyethylene terephthalate sheets with a thickness of 175μ, and measure the haze of each sample using a haze meter (Tokyo Electric Power Co., Ltd.). Color Co., Ltd.
The results are listed in Table 1.

[Table] As is clear from Table 1, the effects of the present invention can be clearly recognized.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the relationship between temperature T and crystallization rate G of a characteristic resin. FIG. 2 is a side view for explaining the casting portion of the extrusion biaxial stretching film forming machine. 1... Extrusion die, 1... Cooling roller, 3... Air cooling device.

Claims (1)

[Claims] 1. When a crystalline thermoplastic resin extruded in a molten state is cast onto the cooling roller surface and cooled and solidified to form a resin sheet, an air cooling device installed opposite to the cooling roller surface is the relative crystallization acceleration of 0.7 to 70 from the melting point of the resin to the temperature that gives its highest crystallization rate, with respect to the temperature of the resin sheet on the outside air surface facing the outside air without contact.
A method for forming a resin sheet, characterized in that air cooling of the resin sheet is started from a temperature point within a temperature range that gives %.