JPS5894416A - Thermoplastic container with good shock resistance made of polyethylene terephtalate - Google Patents

Abstract

PURPOSE:To attain a deep-drawn thermoplastic container fit for food packing or so with uniform wall thickness and without any vertical crack by a method wherein the difference between refractive indexes in the circumferential direction and in the depthwise direction has a specified value in the sectional plane parallel to the circumferential direction of the side wall of the formed container and perpendicular to the side wall surface thereof. CONSTITUTION:A pellet of polyethylene terephtalate is discharged by an extruder and cooled and solidified with the aid of the rotary surface of a rapidly cooled member or the like to attain a sheet. This sheet is thermally formed by a manual deep-drawing machine or so to attain a formed container 1 in which a double refractive index DELTAn=n1-n2, i.e., the difference between a refractive index n1 in the circumferential direction and a refractive index n2 in the depthwise direction, is at least 0.05 in the sectional plane parallel to the circumferential direction of the side wall of the formed container and perpendicular to the side wall surface thereof. The container thus attained is used with it being fitted to a paper cup 2.

Description

[Detailed description of the invention] The present invention relates to thermoformed containers with high impact strength.

BACKGROUND OF THE INVENTION Conventionally, polyethylene terephthalate sheets have been widely used in large quantities as biaxially oriented films in the fields of electricity, printing, agricultural materials, packaging, etc.

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 lower than that of polyethylene, polypropylene,
Since it is slower than nylon or the like, 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 a 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, high fragrance retention, and olefins. 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, the side walls of deep-drawn containers made of polyethylene terephthalate are almost uniaxially stretched.
Although it is strong in the stretching direction, it is susceptible to cracking if there is misalignment strain in the direction perpendicular to the stretching direction or along the stretching direction.

These shortcomings are not a problem in normal packaging applications, but under harsh conditions, such as testing sake containers, 7.
Under test conditions such as filling at a temperature close to 0° C. and dropping from a height of 150 crn at a temperature below freezing, vertical cracks may occur in the molded container.

As a result of intensive research into deep-drawn containers that do not have these drawbacks, the inventors of the present invention have found that they are made from polyethylene terephthalate.
If the side wall of a deep-drawn container is also stretched in the lateral direction, and the birefringence in the circumferential direction (parallel and perpendicular to the side wall surface) of the container side wall is above a certain value, the above-mentioned severe The present invention was achieved by discovering that it is possible to prevent vertical cracking when dropped under certain conditions.

In a thermoformed container made of polyethylene terephthalate, the present invention is expressed by the difference between the refractive index n1 in the circumferential direction and the refractive index n2 in the thickness direction in a cut plane parallel to the circumferential direction of the side wall of the molded container and perpendicular to the side wall surface. Birefringence Δn = n
, -n2 is at least 0.05. A thermoformed container made of polyethylene terephthalate with good impact resistance.

The thermoformed container of the present invention preferably has continuous or discontinuous protrusions on the side wall. It is particularly preferable that these protrusions be parallel to the circumferential direction.

Polyethylene terephthalate in the present invention 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 components. % or less by weight of the polymer. In addition, polyethylene terephthalate can prevent blocking, promote crystallization,
Low molecular weight substances can also be added for the purpose of improving processability.

The thermoformed container of the present invention can be manufactured by deep drawing under specific conditions using an unstretched sheet made of polyethylene terephthalate.

The unstretched sheet made of polyethylene terephthalate is
It is obtained by discharging molten polyethylene terephthalate in the form of a sheet from the die 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 should be 85 to 90% of the width of the mold size.
Those with a penetration depth of about 95% are generally used,
At present, this has not been done under any other conditions. However, molded containers manufactured by the conventional plug-assisted molding method had the disadvantage of being prone to vertical cracking (and being weak against impact.In contrast, the thermoformed container of the present invention, which has excellent impact resistance, Unlike the conventional plug-assist molding method, this can be achieved by reducing the size of the plug to 80% or less of the mold size.By the way, reducing the size of the plug in this way increases the difficulty of molding. Therefore, the plug temperature is at least the glass transition temperature (Tg) or T of the sheet.
It is necessary to set the temperature range to g+90°C.

In order to obtain a thermoformed container having protrusions on the side wall by the plug assist molding method, a suitable mold may be used. At this time, if it is difficult to form protrusions on the side wall by vacuum molding alone, it is preferable to use compressed air as an auxiliary aid. Examples of the shape of the protrusion on the side wall of the container include a continuous rib structure, a discontinuous rib structure, and a shape in which the diameter of the side wall decreases in a stepwise manner (see FIGS. 4 to 6).

The thermoformed 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 thermoformed container of the present invention has a biaxially oriented structure in which the molecular chains are oriented also in the circumferential direction.
No vertical cracking (and has excellent impact resistance).

In particular, thermoformed containers having continuous or discontinuous protrusions on the side walls have a higher impact strength or less.The present invention will be described by way of examples. In addition, each characteristic value in an Example was calculated|required by the following measuring method.

Intrinsic viscosity [η]: Phenol/tetrachloroethane (5
0150) at 25°C.

Birefringence △n: Cut out a 5-sided specimen with sides parallel and perpendicular to the circumferential direction from the side wall of the molded container. The difference between the refractive index n1 in the circumferential direction and the refractive index n2 in the thickness direction measured using polarized light parallel and perpendicular to the side wall surface is defined as the birefringence Δn of the cross section.

Example 1 Pellets of polyethylene terephthalate with an intrinsic viscosity [η)=[1.75] 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 56°C, and had a thickness of 0. A sheet with a thickness of 451 nm and a width of 600 u was obtained.

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 is 65% of the mold dimension, and the sheet surface temperature just before molding was measured with an infrared thermometer and was approximately 120%.
It was ℃. In addition, a vacuum was drawn using dry nitrogen at a pressure of about 9/crIL2, and at the same time, compressed air was applied.

As shown in FIG.
and fill 80% of the total capacity with hot water at 67℃.
Immediately, the opening was 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 performed from a height of 150 cm, and the damage of the polyethylene terephthalate molded container was 1.
It was 1 out of 00. 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.

A polyethylene terephthalate molded container was fitted with a paper cup, and a drop test was conducted in the same manner as in Example 1.
Breakage of polyethylene terephthalate containers occurred in 7 out of 100 containers.

The birefringence Δn of the cut surface of the side wall of this molded container was 0°03.

Example 2 Polyethylene terephthalate pellets with intrinsic viscosity [η] = 0.73 were heated to a resin temperature of 290 using a 40 mmφ extruder.
The mixture was discharged at 53°C and cooled and solidified on the surface of a rotating quenching body at 53°C to obtain a sheet with a thickness of 0.45 mm and a width of 300 mm. This sheet was thermoformed using a homemade deep drawing machine to obtain a molded container as shown in FIG.

The width of the plug was 70% of the mold dimension, and the temperature of the plug was 95°C. The sheet surface temperature immediately before molding was measured with an infrared thermometer and was approximately 120°C. Vacuum suction was also applied using dry nitrogen at a pressure of about 7 kg/crn 2 followed by air pressure.

The obtained molded container 1 was fitted with a paper cup 2 as shown in Fig. 7, and 80% of the total content was filled with hot water at 67°C, and immediately sealed with aluminum foil coated with hot melt adhesive. , and left overnight in a 3°C cold bath.

After being left overnight, a drop test was conducted from a height of 150 m, and the breakage of the polyethylene terephthalate molded container was 10.
There were 0 out of 5 items. The birefringence Δn of the cut surface of the side wall of this molded container was 0.07.

Comparative Example 2 Using the same sheet as in Example 2, the plug width mold dimension was 9.
A molded container made of polyethylene terephthalate with the same strips as in Example 2 was fitted with a paper cup, except that the drop test was performed in the same manner as in Example 2. [This occurred in 60 pieces per strip. The birefringence Δn of the cut surface of the side wall of this molded container is 0.
It was 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, Fig. 3 is a longitudinal cross-sectional view of a molded container 1 of the present invention and a paper cup 2 fitted together, and Fig. 4 is a continuous cross-sectional view. FIG. 5 is a perspective view of a molded container with discontinuous step-like projections; FIG. 6 is a perspective view of a molded container with continuous rib-like projections. Figure 7 shows a molded container 1 and a paper cup 2 having continuous step-like protrusions according to the present invention.
It is a longitudinal cross-sectional view of what fitted together. '4-1 Figure! 2Eil! ] Vertical 3
Eye rate 4 figure Homare 5 button

Claims (1)

[Claims] 1. In a thermoformed container made of polyethylene terephthalate, a refractive index n1 in the circumferential direction and a refractive index n in the thickness direction in a cut plane parallel to the circumferential direction of the side wall of the molded container and perpendicular to the side wall surface. , the birefringence index △n == n,
- A thermoformed polyethylene terephthalate container with excellent impact resistance, characterized in that n2 is at least 0.05. 2. The thermoformed container according to claim 1, which has continuous or discontinuous protrusions on the side wall of the container.