JPH0427015B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to 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, cups, and other deep-drawn containers have begun to be used for food packaging.

Although polyethylene terephthalate is a crystalline polymer, its crystallization rate is slower than that of polyethylene, polypropylene, nylon, etc., so 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 with the stretching ratio due to the molecular orientation effect and crystallization promotion effect.
Therefore, compared to amorphous polymers such as polyvinyl chloride and polystyrene, it is easier to make the wall thickness of a deep-drawn container uniform. 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 deep-drawn 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, and is extremely hygienic. It also has high gas barrier properties, high fragrance retention, and olefin, etc. Recently, there has been a great deal of interest in packaging containers made from polyethylene terephthalate because of the lack of odor found in polyethylene terephthalate.

However, deep-drawn containers made of polyethylene terephthalate have almost uniaxially stretched side walls, and although they are strong in the stretching direction, they can crack if there is misalignment in the direction perpendicular to the stretching direction or along the stretching direction. It's getting easier. These shortcomings are not a problem in normal packaging applications, but under harsh conditions, for example, when testing Japanese sake containers, they are filled at temperatures close to 70 degrees Celsius, and at sub-zero temperatures they are filled to a length of 150 cm.
Under test conditions in which the container is dropped from a height of

As a result of intensive research into deep-drawn containers that do not have such drawbacks, the inventors of the present invention found that the side walls of deep-drawn containers made of polyethylene terephthalate are also stretched in the lateral direction, parallel to the circumferential direction of the container side walls. The present inventors have discovered that when the birefringence in a cut plane perpendicular to the side wall surface is at least a specific value, vertical cracking when dropped under the above-mentioned severe conditions can be prevented, and the present invention has been achieved.

The present invention provides a deep-drawn container obtained by making the diameter or width of the plug 80% or less of the inner diameter of the mold when manufacturing a deep-drawn container from a sheet made of polyethylene terephthalate by the plug assist method. Birefringence △n = n 1 - _{n 2} , which is the difference between the refractive index n ₁ in the circumferential direction and the refractive index n ₂ in the thickness direction, on a cross section parallel to the circumferential direction of the side wall of the container and perpendicular to the _side wall surface. is at least 0.05, and is a deep-drawn polyester container with excellent impact resistance.

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 components, and polyethylene terephthalate in 80% or more by weight of other polymers. is 20% by weight or less. Further, low molecular weight substances can be added to polyethylene terephthalate for the purpose of preventing blocking, promoting crystallization, improving processability, etc.

The deep-drawn container of the present invention can be manufactured 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 container.

A plug assist forming method is generally used to perform deep drawing using this unstretched sheet.
In this case, the size of the plug that is generally used is one with a width of 85 to 90% and a penetration depth of about 95% of the mold dimensions, and currently no plugs are used under other conditions. be. 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.
In contrast, the deep-drawn container of the present invention with excellent impact resistance is obtained by reducing the size of the plug to 80% or less of the mold size, unlike the conventional plug-assisted molding method. However, reducing the size of the plug in this manner increases the difficulty of molding. Therefore, the plug temperature must be set at least within the glass transition temperature (Tg) of the sheet or Tg + 90°C.

The deep-drawn container of the present invention was obtained for the first time by the above-mentioned special plug-assisted molding method. In conventional deep-drawn containers, the side walls of the container have only longitudinal molecular orientation. In contrast, the deep-drawn container of the present invention has a biaxially oriented structure in which molecular chains are oriented also in the circumferential direction, so 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 the examples was determined by the following measuring method.

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

Birefringence △n: Cut a 5 mm square specimen from the side wall of the molded container with sides parallel and perpendicular to the circumferential direction, and measure the cross section parallel to the circumferential direction and perpendicular to the side wall using an Atsupé 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] = 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 were made into pellets with a thickness of 0.45 mm and a width of 300 mm. Got a sheet.
This sheet was thermoformed using a homemade deep drawing machine to obtain a molded container shown in FIGS. 1 and 2. FIG. 1 is a longitudinal sectional view of the molded container, and FIG. 2 is a perspective view thereof.

The width of the plug was 65% of the mold dimension, and the sheet surface temperature immediately before molding was approximately 120°C as measured with an infrared thermometer. Further, a vacuum was drawn using dry nitrogen at a pressure of about 5 kg/cm ² and at the same time, compressed air was applied.

As shown in Fig. 3, the obtained molded container 1 is fitted with a paper cup 2, filled with 67°C hot water to 80% of the total content, and the opening is immediately sealed with aluminum foil coated with hot melt adhesive. After sealing, it was left in a low temperature bath at 3°C overnight.

After being left overnight, a drop test was performed from a height of 150 cm, and only one out of 100 polyethylene terephthalate molded containers was damaged. The birefringence Δn of the cut surface of the side wall of this molded container was 0.07.

Comparative Example 1 The same sheet as in Example 1 was used and thermoformed under the same conditions as in Example 1 to obtain a molded container of the same shape.

When polyethylene terephthalate molded containers were fitted with paper cups and a drop test was conducted in the same manner as in Example 1, 70 out of 100 polyethylene terephthalate containers were damaged. The birefringence Δn of the cut surface of the side wall of this molded container was 0.03.

[Brief explanation of drawings]

FIG. 1 is a longitudinal cross-sectional view of a molded container of the present invention, FIG. 2 is a perspective view thereof, and FIG. 3 is a longitudinal cross-sectional view of a molded container 1 of the present invention and a paper cup 2 fitted together.

Claims (1)

[Claims] 1. When producing a deep-drawn container from a sheet made of polyethylene terephthalate by the plug assist method, the diameter or width of the plug is 80% or less of the inner diameter of the mold. In a cut plane parallel to the circumferential direction and perpendicular to the side wall surface, the birefringence Δn=n ₁ −n ₂ , which is the difference between the refractive index n ₁ in the circumferential direction and the refractive index n ₂ in the thickness direction, is at least 0.05. A deep-drawn polyester container with excellent impact resistance.