JPS6328007B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing deep-drawn polyester containers having high impact strength.

Sheets of polyethylene terephthalate are
Conventionally, biaxially stretched films have been widely used in large quantities in fields such as electricity, printing, agricultural materials, and packaging.
Very recently, trays made of polyethylene terephthalate or cups, which are deep-drawn products, have begun to be used for food packaging purposes.

Although polyethylene terephthalate is a crystalline polymer, its crystallization rate is lower than that of polyethylene.
Since it is slower than polypropylene, nylon, etc., an amorphous sheet can be obtained by rapidly cooling it from a molten state. Therefore, a highly transparent sheet that is not found in polyethylene, polypropylene, nylon, etc. can be obtained.

Furthermore, since polyethylene terephthalate is a crystalline polymer, when the stretching ratio exceeds about 2 times, the toughness increases significantly as the stretching ratio increases due to the effect of molecular orientation and the effect of promoting crystallization. For this reason, it is easier to make the wall thickness of a deep-drawn container uniform compared to amorphous polymers such as polyvinyl chloride and polystyrene. Furthermore, when an amorphous transparent sheet is stretched and crystallized, the sheet does not whiten even if the degree of crystallinity is high. These reasons combine to make polyethylene terephthalate thermoformed containers highly transparent and strong.

On the other hand, polyethylene terephthalate does not contain any additives such as stabilizers that can be eluted into the contents, making it extremely hygienic, having high gas barrier properties, and high fragrance retention. Due to the absence of bromination, which is seen in
Recently, there has been a great deal of interest in packaging containers made from polyethylene terephthalate.

However, the side walls of deep-drawn containers made of polyethylene terephthalate are almost uniaxially stretched, and although they are strong in the stretching direction, that is, the longitudinal direction, they are extremely weak in the transverse direction, and are strong along the stretching direction. Cracks are likely to occur. These shortcomings are not a problem in normal packaging applications, but under harsh conditions, such as when testing sake containers, they are filled at temperatures close to 70 degrees Celsius and dropped from a height of 150 cm at sub-zero temperatures. Under such severe test conditions, vertical cracks may occur in the molded container.

As a result of intensive research into a method for manufacturing a deep-drawn container made of polyethylene terephthalate that does not have these drawbacks, the inventors of the present invention found that a plug of a specific size is used and the side wall of a deep-drawn container made of polyethylene terephthalate is stretched in the lateral direction. The present inventors have discovered that vertical cracking when dropped under the above-mentioned severe conditions can be prevented when the material is placed in the same position as above.

That is, the present invention is a method for manufacturing a deep-drawn container by deep-drawing a sheet made of polyethylene terephthalate, in which the diameter or width of the plug is set to 80% or less of the inner diameter of the mold, and when the plug is actuated, the Pressurized air is sent from below the sheet to push the plug in so that the mold does not come in contact with the sheet, and then vacuum and compressed air are applied to the container, which is parallel to the circumferential direction of the top of the side wall and perpendicular to the side wall surface. Birefringence △n= indicated by the difference between the refractive index n ₁ in the circumferential direction and the refractive index n ₂ in the thickness direction on the cut plane
This is a method for producing a deep-drawn polyester container, characterized by forming the container so that n ₁ −n ₂ is at least 0.05.

In the present invention, polyethylene terephthalate refers to homopolymers of ethylene terephthalate, copolymers in which 80% or more of the repeating units are ethylene terephthalate units and 20% or less are other copolymerizable components, and polyethylene terephthalate in an amount of 80% by weight or more. It is a polymer blend in which the amount of other polymers is 20% by weight or less. In addition, polyethylene terephthalate prevents blocking, promotes crystallization,
There are no restrictions on the addition of low-molecular substances for the purpose of improving processability, etc., as long as there is no food hygiene problem.

The polyester deep-drawn container of the present invention can be produced by deep-drawing an unstretched sheet made of polyethylene terephthalate under specific conditions.

An unstretched sheet made of polyethylene terephthalate is obtained by discharging molten polyethylene terephthalate in the form of a sheet from the mouthpiece of an extruder, and then cooling and solidifying it on the surface of a rotating quenching body. The cooling temperature is preferably 40 to 60°C. If the temperature exceeds 60°C, cooling will be insufficient and the thermoformability of the sheet will deteriorate. Furthermore, if the surface temperature of the rotary quenching body is too low than 40°C, wrinkles will occur on the sheet surface, which will impair the appearance of the molded product.

A plug assist forming method is generally used to perform deep drawing using this unstretched sheet.
In this case, the size of the plug generally used is one whose width or diameter is approximately 85 to 90% of the mold size. However, molded containers manufactured by the conventional plug-assisted molding method have disadvantages in that they are prone to vertical cracking and are weak against impact.

On the other hand, the manufacturing method of the present invention differs from the conventional plug-assisted molding method in that the first feature is that the diameter or width of the plug is 80% or less of the inner diameter of the mold. By the way, although it is possible to extend the bottom of the molded container in the lateral direction simply by reducing the size of the plug, the uppermost side wall of the molded container is hardly extended in the lateral direction.
Therefore, in the present invention, when the plug is activated during deep drawing, pressurized air is sent from below the sheet on the mold to push the plug so that the sheet does not come in contact with the sheet, as shown in Figure 1. The second feature is that the uppermost part of the obtained molded container is laterally stretched by applying a vacuum or compressed air. 1 in Figure 1 is a mold;
2 is an unstretched sheet, 3 is a compressed air feed pipe, and 4 is a vacuum drawn pipe. Also, 5 and 6 are their respective kotuku. By the way, if the size of the plug is reduced as described above, the difficulty of molding increases, so the plug temperature must be set within the temperature range of the glass transition temperature Tg of the unstretched sheet to Tg + 90°C.

In the present invention, a molded article with a high stretching ratio can be obtained by vacuum/pressure forming.

In the present invention, the refractive index in the circumferential direction is measured in a cut plane parallel to the circumferential direction and perpendicular to the side wall surface of the uppermost side wall of the container obtained by the above-mentioned molding method.
The molding is performed so that the birefringence Δn=n ₁ -n ₂ , which is the difference between n ₁ and the refractive index n ₂ in the thickness direction, is at least 0.05. Therefore, in conventional deep-drawn containers, the side walls have molecular orientation only in the vertical direction, and vertical cracks are likely to occur, whereas in the deep-drawn containers formed by the method of the present invention, horizontal cracks occur even at the top of the side walls. Because it has a biaxially oriented structure with molecules oriented in both directions, it does not suffer from vertical cracking and has excellent impact resistance.

The present invention will be explained below with reference to Examples. In addition, each characteristic value in an Example was calculated|required by the following measuring method.

Intrinsic viscosity [η]: Measured at 25°C using a mixed solvent of phenol/tetrachloroethane (50/50).

Birefringence △n: Cut out a 5 mm square specimen with sides parallel and perpendicular to the circumferential direction from the side wall of the molded container, and measure the cross section parallel to the circumferential direction and perpendicular to the side wall using an Atsube refractometer. The difference between the refractive index n ₁ in the circumferential direction and the refractive index n ₂ in the thickness direction measured with polarized light parallel and perpendicular to the side wall surface is the birefringence △ of the cross section.
Let it be n.

Example 1 Pellets of polyethylene terephthalate with an intrinsic viscosity [η] of 0.73 were discharged from a 40 mmφ extruder at a resin temperature of 290°C, cooled and solidified on the surface of a rotating quenching body at 53°C, and formed into a pellet with a thickness of 0.45 mm and a width of 300 mm. A stretched sheet was obtained. This unstretched sheet was deep drawn using a handmade deep drawing machine. At this time, the diameter of the plug is set to 65% of the mold size, and at the same time as the plug is activated, the pot 5 in Fig. 1 is opened and compressed air is sent from under the unstretched sheet, and after the plug is pushed in, the pot 5 is closed, and then The Kotoku 6 was opened and evacuated, and at the same time, compressed air with a pressure of about 9 kg/cm ² was applied from above the sheet. The thus obtained molded container 7 shown in FIG. 2 is fitted with a paper cup 8 as shown in FIG.
The container was filled with boiling water at 3°C, and the opening was immediately sealed with aluminum foil coated with hot-melt adhesive, and then left overnight in a low-temperature bath at 3°C. After being left overnight, a drop test was conducted from a height of 150 cm, and 0 out of 100 cases of polyethylene terephthalate molded containers 7 were damaged. The birefringence Δn at the top of the side wall of this molded container 7 was 0.08. Further, the birefringence Δn at the bottom of the side wall was 0.09.

Comparative Example 1 Using the same sheet as in Example 1, the diameter of the plug was set to 90% of the mold dimension using a normal deep drawing machine, and vacuum forming was performed using the plug assist molding method without sending compressed air from below the unstretched sheet. I went. The obtained molded container was fitted with a paper cup in the same manner as in Example 1, and a drop test was conducted in the same manner as in Example 1. The breakage rate of the polyethylene terephthalate molded container was 70 out of 100. Ta. The birefringence Δn at the top of the side wall of this molded container was 0.032, and the birefringence Δn at the bottom was 0.038.

[Brief explanation of the drawing]

FIG. 1 is a longitudinal cross-sectional view of a mold used in carrying out the present invention, FIG. 2 is a perspective view of a deep-drawn container according to the present invention, and FIG. 3 is a diagram showing a deep-drawn container 7 and a paper cup 8 fitted together. FIG.

Claims (1)

[Claims] 1 In a method of manufacturing deep-drawn containers by deep-drawing a sheet made of polyethylene terephthalate, the diameter or width of the plug is set to the inner diameter of the mold.
80% or less, and at the same time as the plug is activated, pressurized air is sent from under the sheet on the mold to push the plug in so that the sheet does not touch the mold, and then vacuum and pressurized air are applied to form the resulting container. Birefringence △n=n ₁ − indicated by the difference between the refractive index n ₁ in the circumferential direction and the refractive index n ₂ in the thickness direction in a cross section parallel to the circumferential direction and perpendicular to the side wall surface at the top of the side wall. A method for producing a deep-drawn polyester container, characterized by forming the container so that n ₂ is at least 0.05.